ANTI-NEOPLASTIC COMPOSITIONS AND USES THEREOF

Abstract: Disclosed is a nucleic acid composition consisting essentially of a first nucleic acid sequence encoding a chimeric CDKi protein and a second nucleic acid sequence encoding an adenovirus E4 protein, wherein the first and second nucleic acid sequences are operably linked to at least one regulatory sequence.

Description: ANTI-NEOPLASTTC COMPOSITIONS AND USES THEREOF BACKGROUND OF THE INVENTION The invention relates to therapy for treating neoplasia, particularly cancer. In America, half of all men and one-third of all women will develop some form of neoplasia during their lifetimes. For example, millions of Americans are currently living with cancer or have been cured of the disease. Although cancer is frequently fatal, rapid treatment of the disease results in a better prognosis for recovery. Neoplasia is caused by the abnormal growth and proliferation of cells in the body. While normal body cells grow, divide, and die in an orderly fashion, neoplastic cells grow and divide in a disorderly fashion. Such disorderly growth during neoplasia may be caused by abnormal dysregulation of the cell cycle leading to the hyper-proliferation of cells (Sherr (1996) Science 274: 1672-1677). In eukaryotic cells, progression through the cell cycle is orchestrated by functionally distinct cyclin-dependent kinases (CDK's). Complexes are formed via the association of different catalytic CDK domains and regulatory cyclin subunits. CDK4/ cyclin D and CDK6/cyclin D complexes regulate progression through G j phase; CDK2/cyclin E kinase regulates the Gj/S transition; CDK2/cyclin A complex drives the cells through S-phase; and CDC2/ cyclin B complex controls the entry, and exit from mitosis (Sherr (1996) Science 274: 1672- 1677). The activity of the CDKs is tightly regulated in response to a variety of extra- and intracellular signals, and is mediated through a combination of phosphorylation events and associations with cyclin-dependent kinase inhibitors (CDKi's) (Morgan (1995) Nature 374: 131-134). The redistribution of CDKi's between the different CDK/cyclin complexes during the cell cycle coordinates the timing of activation and de-activation of the kinase activities (Sherr and Roberts (1995) Genes & Dev. 9: 1149-1163). Two classes of structurally distinct CDKi's, the CIP/KIP and the INK4 families, have been identified in mammalian cells. The CIP/KIP family includes p21/CIPl/WAFl, p27/KIPl, and p57/KIP2 (Sherr and Roberts, supra). CIP/KIP family members share a conserved N-terminal region of an inhibitory domain having a length of approximately 60 amino acids. The members of the CIP/KIP family are potent inhibitors of the CDK4, CDK2, and CDC2 kinases, although their over-expression in vivo predominantly leads to arrest in the G λ phase of the cell cycle. The members of the INK4 family include pl5/INK4a, pl6/INK4b, pl8/INK4c, and pl9/INK4d (Serrano et al. (1993) Nature 366: 704-707; Harmon and Beach (1994) Nature 371: 257-261; Hirai et al. (1995) Mol. Cell. Biol. 15: 2672-2681; Chan et al. (1995) Mol. Cell. Biol. 15: 2682-2688; Guan et al. (1996) Mol. Biol. Cell. 7: 57-70). The INK4 proteins are comprised almost exclusively of the repetitions of a common structural motif, the ankyrin repeat (Bork (1993) Proteins 17: 363-374), and are highly specific inhibitors of the CDK4 and CDK6 associated kinases. Biochemical analysis of the members of the CIP/KIP and INK4 families of inhibitors has suggested that their modes of action are different. pl6, the prototypic member of the INK4 family, binds to both monomeric CDK4 and to the assembled, fully active CDK4/cyclin Dl kinase. The binding of pl6 to CDK4 appears to prevent the formation of CDK4/cyclin D complexes, whereas binding of pl6 to assembled CDK4/cyclin D complex produces a catalyticaliy inactive ternary complex (Parry et al. (1995) EMBO }. 14: 503-511). p27 binds primarily to the active CDK2/cyclinA and CDK2/cyclinE complexes, which leads to the formation of a catalyticaliy inactive ternary complex (Pol ak et al. (1994) Cell 78:1156-66; Polyak et al. (1994) Genes & Deυ. 8: 9-22). To inhibit cell cycle progression, the levels of the individual CDKi's must exceed the concentration of the active CDK/cyclin complexes. Control of cell proliferation has been shown to act through the regulation of the levels of various CDKi's. For example, under growth conditions that induce arrest of cell proliferation, the amount of p27 increases (Firpo et al. (1994) Mol. Cell. Biol. 14: 4889-4901; Kato et al. (1994) Cell 79: 487-496; Nourse et al. (1994) Nature 372: 570-573, Slingerland et al. (1994) Mol. Cell. Biol. 14: 3683-3694). As the cells are stimulated to reenter the cell cycle, the level of p27 is promptly reduced. Similarly, as cells become senescent, pl6 gradually increases (Alcorta et al. (1996) Proc. Natl. Acad. Sci. USA 93: 13742-13747). Given the large numbers of human deaths attributable to neoplasia, there is a need to develop useful methods and compositions to treat or prevent neoplasia, including hyperplasia and cancer. Moreover, given the important role that the cell cycle plays in several disease phenotypes, reagents that arrest cell growth are useful in allowing further study of the cell cycle for synchronizing and controlling growth of cells in culture. SUMMARY OF THE INVENTION The present features novel chimeric cyclin dependent kinase inhibitors (chimeric CDKi's) which, when combined with an adenovirus E4 protein (or active fragment thereof), not only inhibit neoplastic cell growth, but surprisingly also induce apoptosis in neoplastic cells. Using the chimeric CDKi protein, W9, apoptosis was induced in neoplastic cells regardless of their p53, Rb, p27, or pl6 status. The combination of a chimeric CDKi with an adenovirus E4 protein does not induce apoptosis in most normal cells, but rather only induces cell growth arrest. These surprising discoveries have been exploited to provide the anti-neoplastic compositions and methods of the present invention. Accordingly, in a first aspect, the invention features a nucleic acid composition consisting essentially of a first nucleic acid sequence encoding a chimeric CDKi protein and a second nucleic acid sequence encoding an adenovirus E4 protein, wherein the first and second nucleic acid sequences are operably linked to at least one regulatory sequence. Such nucleic acid sequences are capable of being expressed in a cell. In certain embodiments, the chimeric CDKi protein is a W9 protein. In certain embodiments, the adenovirus E4 protein is encoded by E4orf6. In another aspect, the invention features a composition comprising the chimeric CDKi protein /adenovirus E4 protein-encoding nucleic acid composition according to the first aspect of the invention and a pharmaceutically acceptable carrier. In one embodiment, the composition further comprises a delivery system that facilitates the internalization of the composition by a cell. Preferably, the delivery system is a recombinant virus particle. In other embodiments, the recombinant virus particle is selected from the group consisting of an adenovirus, a lentivirus, an adeno-associated virus, a retrovirus, a herpesvirus, and a vaccinia virus. Preferably, the recombinant virus particle is an adenovirus (e.g., an adenovirus lacking an entire E4 region). In another embodiment, the delivery system is a liposome. In yet another aspect, the invention features a nucleic acid composition comprising a first nucleic acid sequence encoding a secretable, internalizable form of a chimeric CDKi protein and a second nucleic acid sequence encoding a secretable, internalizable form of an adenovirus E4 protein, wherein the first and second nucleic acid sequences are operably linked to at least one regulatory sequence. In certain embodiments, the chimeric CDKi protein is a W9 protein. In certain embodiments, the adenovirus E4 protein is encoded by E4orf 6. In another aspect, the invention features a composition comprising a nucleic acid composition comprising a first nucleic acid sequence encoding a secretable, internalizable form of a chimeric CDKi protein and a second nucleic acid sequence encoding a secretable, internalizable form of an adenovirus E4 protein, and a pharmaceutically acceptable carrier, wherein the first and second nucleic acid sequences are operably linked to at least one regulatory sequence. In certain embodiments, the composition further comprises a delivery system that facilitates the internalization of the composition by a cell. Preferably, the delivery system is a recombinant virus particle. In other embodiments, the recombinant virus particle is selected from the group consisting of an adenovirus, a lentivirus, an adeno-associated virus, a retrovirus, a herpesvirus, and a vaccinia virus. Preferably, the recombinant virus particle is an adenovirus. In another embodiment, the delivery system is a liposome. In another aspect, the invention features a protein composition comprising a purified chimeric CDKi protein and a purified adenovirus E4 protein. In certain embodiments, the chimeric CDKi protein is a W9 protein. In certain embodiments, the adenovirus E4 protein is encoded by E4orf6. In yet another aspect, the invention features a composition comprising a protein composition comprising a purified chimeric CDKi protein and a purified adenovirus E4 protein, and a pharmaceutically acceptable carrier. In one embodiment, the composition further comprises a delivery system that facilitates the internalization of the composition by a cell. In still another aspect, the invention features a protein composition comprising a purified, internalizable form of a chimeric CDKi protein and a purified, internalizable form of an adenovirus E4 protein. In certain embodiments, the chimeric CDKi protein is a W9 protein. In certain embodiments, the adenovirus E4 protein is encoded by E4orf6. In another aspect, the invention features a composition comprising a protein composition comprising a purified, internalizable form of a chimeric CDKi protein and a purified, internalizable form of an adenovirus E4 protein, and a pharmaceutically acceptable carrier. In yet another aspect, the invention features a method for treating an animal with a neoplasm. The method includes administering to the animal a therapeutically effective amount of a composition according to the invention. In another aspect, the invention features a cell containing a nucleic acid composition comprising a first nucleic acid sequence encoding a secretable, internalizable form of a chimeric CDKi protein and a second nucleic acid sequence encoding a secretable, internalizable form of an adenovirus E4 protein, wherein the first and second nucleic acid sequences are operably linked to at least one regulatory sequence. In certain embodiments, the chimeric CDKi protein is a W9 protein. In certain embodiments, the adenovirus E4 protein is encoded by E4orf6. In yet another aspect, the invention features a method for treating an animal with a neoplasm, comprising introducing the nucleic acid composition comprising a first nucleic acid sequence encoding a secretable, internalizable form of a chimeric CDKi protein and a second nucleic acid sequence encoding a secretable, internalizable form of an adenovirus E4 protein, wherein the first and second nucleic acid sequences are operably linked to at least one regulatory sequence into a cell of the animal, wherein the introduced cell secretes the secretable, internalizable form of the chimeric CDKi protein and secretes the secretable, internalizable form of the adenovirus E4 protein. In final aspect, the invention features a method for treating an animal with a neoplasm, comprising introducing the nucleic acid composition comprising a first nucleic acid sequence encoding a secretable, internalizable form of a W9 protein and a second nucleic acid sequence encoding a secretable, internalizable form of an adenovirus E4 protein, wherein the first and second nucleic acid sequences are operably linked to at least one regulatory sequence into a cell of the animal, wherein the introduced cell secretes the secretable, internalizable form of the W9 protein and secretes the secretable, internalizable form of the adenovirus E4 protein. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects of the present invention, the various features thereof, as well as the invention itself may be more fully understood from the following description, when read together with the accompanying drawings in which: Figure 1 is a diagrammatic representation of a nucleic acid sequence encoding a secreted, internalizable form of a W9 protein, a non- limiting, representative chimeric CDKi of the invention. This sequence, when positioned for expression in a vector and introduced into a cell, will encode a protein that is secreted. The secreted protein is then internalized by another cell or by the cell in which it was expressed and secreted; Figure 2A is a diagrammatic representation showing non-limiting, representative recombinant CDK inhibitors of the invention tested in in vitro kinase assays, and the results of these assays. The pl6 molecule is indicated by the open box; the p27 molecule and its derivatives are indicated by the hatched boxes; and the 15 amino acids long (Gly 4 Ser) 3 linker between the pl6 and p27 moieties is indicated by the black boxes. Above the schematic for each molecule is the corresponding 5' and 3' amino acid from the parental molecule. The table in the middle shows the IC 50 's (in nM) of the purified inhibitors as determined by in vitro kinase assays that utilized CDK4/cyclin Dl, CDK2/ cyclin E, and CDC2/cyclin B kinases. At the right of Fig. 2A is listed the estimated half-life expression of the adenovirus encoded CDKi protein (in hours) as measured by pulse- chase experiments (see Example IV); Figure 2B is a schematic representation of the nucleic acid sequence (above) and amino acid sequence (below) of the W9 protein, a non- limiting, representative chimeric CDKi protein of the invention; Figure 3 is a diagrammatic representation showing the structure of non-limiting, representative replication-deficient recombinant adenoviruses of the invention that encode CDKi's (AV-CDKi's). These recombinant adenoviruses contain the entire E4 region, which is located between the deleted E3 region (ΔE3) and the left ITR; thus these adenoviruses also encode all of the E4 region proteins. The expression of the CDKi's is regulated by the CMV enhancer and promoter and the SV40 poly A sequence. Each CDKi was fused to the HA epitope tag. The ATG is in the context of the optimal Kozak sequence. Clal is the restriction site used for construction of the recombinant viruses; Figure 4A is a representation of a line graph showing the ability of non-limiting, representative AV-CDKi's of the invention to inhibit proliferation of SW480 colon carcinoma cells. Shown is the percent inhibition of cell growth following transduction with adenovirus containing an entire E4 region and a CMV promoter operably linked to no insert (Null, blue circles), pl6 (purple squares), p27 (yellow triangles), W7 (red squares), and W9 (green diamonds); Figure 4B is a representation of a line graph showing the ability of non-limiting, representative AV-CDKi's of the invention to inhibit proliferation of A549 lung carcinoma cells. Shown is the percent inhibition of cell growth following transduction with adenovirus containing an entire E4 region (AV) and a CMV promoter operably linked to no insert (Null, blue circles), pl6 (purple squares), p27 (yellow triangles), W7 (red squares), and W9 (green diamonds); Figure 5A is a representation of a FACS histogram showing the lack of induction of apoptosis in A549 lung carcinoma cells by an adenovirus containing the entire E4 region and encoding β-gal (AV-β-gal). A549 cells were transduced with AV-β-gal, harvested 2 days later, and the level of apoptosis ascertained by Annexin-PI staining, followed by FACS analysis; Figure 5B is a representation of a FACS histogram showing the induction of apoptosis in A549 lung carcinoma cells by a non-limiting, representative adenovirus of the invention containing the entire E4 region and encoding W9 (AV-W9). A549 cells were transduced AV-W9, harvested 2 days later, and the level of apoptosis ascertained by Annexin-PI staining, followed by FACS analysis; Figure 6A is a representation of a line graph showing that a non- limiting, representative adenovirus of the invention, AV-W9, is more effective than AV-pl6, AV-p27, or AV-W7 in inducing apoptosis in PC3 prostate tumor cells. Apoptosis was measured three days following transduction with the indicated AV-CDKi at equivalent MOL Shown are the percentage of cells induced to undergo apoptosis following transduction by adenoviruses containing the entire E4 region (AV) and encoding pl6 (purple squares), p27 (yellow triangles), W7 (red squares), W9 (green diamonds), or CMV promoter with no insert (Null; blue circles); Figure 6B is a representation of a line graph showing that a non- limiting, representative adenovirus of the invention, AV-W9, is more effective than similarly delivered pl6, p27, or W7 in inducing apoptosis in SW480 colon carcinoma cells. Apoptosis was measured three days following transduction with the indicated AV-CDKi at equivalent MOL Shown are the percentage of cells induced to undergo apoptosis following transduction by adenoviruses containing the entire E4 region and encoding pl6 (purple squares), p27 (yellow triangles), W7 (red squares), W9 (green diamonds), or CMV promoter with no insert (Null; blue circles); Figure 6C is a representation of a line graph showing that a non- limiting, representative adenovirus of the invention, AV-W9, is more effective than similarly delivered pl6, p27, or W7 in inducing apoptosis in A549 lung carcinoma cells. Apoptosis was measured three days following transduction with the indicated AV-CDKi at equivalent MOL Shown are the percentage of cells induced to undergo apoptosis following transduction by adenoviruses containing the entire E4 region and encoding pl6 (purple squares), p27 (yellow triangles), W7 (red squares), W9 (green diamonds), or CMV promoter with no insert (Null; blue circles); Figure 7A is a representation of a line graph showing that a non- limiting, representative adenovirus of the invention, AV-W9, is expressed at similar levels in neoplastic cells compared to similarly delivered parental pl6 and p27. DU145 prostate tumor cells were transduced with AV-W9 (green diamonds), AV-pl6 (purple squares), AV-p27 (yellow triangle) and AV-no insert (Null; blue circles). After two days, the cells were assessed for expression by intracellular staining with anti-pl6 and anti-p27 antibodies; Figure 7B is a representation of a line graph showing that a non- limiting, representative adenovirus of the invention, AV-W9, is more efficacious at inducing apoptosis in neoplastic cells than similarly delivered parental pi 6 and p27. The same DU145 prostate tumor cells transduced and assessed for intracellular expression of W9, pl6, and p27 from Fig. 7A were also assessed for apoptosis by FACS-based TdT TUNEL assay two days following transduction; Figure 7C is a representation of a line /bar graph showing a time course of W9 and pl6 expression efficiency and level of expression in AV- W9 and AV-pl6 transduced A549 cells, respectively. A549 cells were transduced with equivalent amounts of AV-W9 (green circles) or AV-pl6 (purple squares) and assessed for CDKi expression over a period of 4 days using an anti-pl6 antibody. The line graph shows the number of transduced cells expressing either W9 or pl6, while the bar graph shows the levels of expression of W9 and pl6 in the transduced cells; Figure 8A is a representation of a survival chart showing that a non-limiting, representative adenovirus of the invention, AV-W9, prevented tumor formation in a lung carcinoma xenograft model. 1 x 10 A549 cells were transduced at a MOI of 50 with AV-no insert (PBS; blue circles), and the different AV-CDKi (AV-p27, yellow triangles; AV-pl6, purple squares, AV-W7, red squares; AV-W9, green diamonds). The transduced cells were introduced subcutaneously into Balb-nude mice (n=10), and tumor formation was tracked for 12 weeks; Figure 8B is a representation of a survival chart showing that a non- limiting, representative adenovirus of the invention, AV-W9, prevented tumor formation in a prostate tumor xenograft model. 1 x 10 6 PC3 cells were transduced at a MOI of 50 with AV-no insert (PBS; blue circles), and the different AV-CDKi (AV-p27, yellow triangles; AV-pl6, purple squares, AV-W7, red squares; AV-W9, green diamonds). The transduced cells were introduced subcutaneously into Balb-nude mice (n=10), and tumor formation was tracked for 13 weeks; Figure 9 is a representation of a line graph showing that a non- limiting, representative adenovirus of the invention, AV-W9, slows tumor growth in a prostate xenograft tumor model. Subcutaneous DU145 prostate tumor xenografts (induced using one million cells) were treated with AV-W9, AV-pl6, or AV-no insert (PBS) by intratumoral injections at day 1, 3, and 5 using 1.25 X 10 u virus particles per injection. The average starting volumes of the tumors was 40 mm 3 (n=10). Tumor size was followed for a 10 week period; and Figure 10 is a representation of a series of FACS histograms showing the apoptosis-inducing ability of the entire adenovirus E4 region (or just the adenovirus E4orf6 protein alone) expressed with the W9 protein. "No drug" indicates untransduced cells with no added drug (negative control); "+ drug" indicates untransduced cells with the addition of dexamethasone (to MMTV cells) or IMX (to 293-E4 cells and to 34X cells); "W9" indicates transduction of the cells with an adenovirus lacking an entire E4 region and encoding W9 in the absence of drug; "W9 + drug" indicates transduction of the cells with an adenovirus lacking an entire E4 region and encoding W9 in the presence of dexamethasone (MMTV cells) or IMX (293-E4 cells and 34X cells). DESCRIPTION OF THE PREFERRED EMBODIMENTS The patent and scientific literature referred to herein establishes knowledge that is available to those with skill in the art. The issued U.S. patents, allowed applications, published foreign applications, and references, including GenBank database sequences, that are cited herein are hereby incorporated by reference to the same extent as if each was specifically and individually indicated to be incorporated by reference. The present invention provides compositions and methods for inhibiting the proliferation of, and inducing apoptosis in, neoplastic cells. By "neoplastic cell" is meant a cell that shows aberrant cell growth, such as increased cell growth. A neoplastic cell may be a hyperplastic cell, a cell that shows a lack of contact inhibition of growth in vitro, a tumor cell that is incapable of metastasis in vivo, or a cancer cell that is capable of metastasis in vivo. The compositions and methods of the invention are based on the discovery that a chimeric CDKi protein (e.g., W9), when combined with an adenovirus E4 protein acts synergistically to induce apoptosis selectively in neoplastic cells, as well as to inhibit cell proliferation in these cells. Preferably, a chimeric CDKi protein, when combined with an adenovirus E4 protein or active fragment thereof, induces apoptosis in neoplastic cells at lower concentrations than concentrations required to induce apoptosis in non-neoplastic cells. According to the invention, compositions are provided which include nucleic acids or proteins. The invention further provides methods for the in vitro and in vivo use of the compositions of the invention. In one embodiment, the invention provides a nucleic acid composition consisting essentially of a first nucleic acid sequence encoding a chimeric cyclin dependent kinase inhibitor (chimeric CDKi) protein, and a second nucleic acid sequence encoding an adenovirus E4 protein. In accordance with the invention, by "consisting essentially of" is meant that the indicated composition excludes ingredients that materially affect the characteristics of the composition. Thus, the composition of this embodiment of the invention does not, for example, comprise an adenovirus protein that materially affects the characteristics of the composition. An example of an adenovirus protein that materially affects the characteristics of the composition is the adenovirus El protein. A "nucleic acid" refers to single-stranded or double-stranded polynucleotides, such as deoxyribonucleic acid (DNA), ribonucleic acid (RNA), or analogs of either DNA or RNA. The first and second nucleic acid sequences are operably linked to regulatory sequences to ensure their transcription and translation into chimeric CDKi protein and adenovirus E4 protein, respectively, in a cell introduced with these nucleic acid sequences. By "introduction" is meant the introduction of exogenous nucleic acid into a cell by any means, including, without limitation, transfection, transduction, infection, and tranformation. By the term "transfection" is meant the introduction of exogenous nucleic acid into a cell by physical means. As used herein, by "transduction" is meant the introduction of exogenous nucleic acid into a cell using a viral particle, preferably a genetically modified (i.e., recombinant) viral particle. For various techniques for manipulating mammalian cells, see Keown et al. (1990) Methods of Enzymolog 185: 527-537. As used herein, by "operably linked" is meant that a nucleic acid sequence encoding a polypeptide and transcriptional regulatory sequences are connected in such a way as to permit expression of the nucleic acid sequence when introduced into a cell. By "regulatory sequence" is meant nucleic acid sequences, such as initiation signals, IRES sequences, polyadenylation (poly A) signals, promoters, and enhancers which control cellular expression of protein coding sequences with which they are operably linked. By "expression" of a nucleic acid sequence encoding a protein or polypeptide fragment is meant expression in a cell of that nucleic acid as mRNA and production of that protein or polypeptide fragment. The first and second nucleic acid sequences may be on a viral vector used to make a recombinant infectious virus particle. Useful recombinant infectious viruses include, without limitation, adenovirus, adenovirus lacking an entire E4 region, lentivirus, retrovirus, adeno- associated virus, herpesvirus, vaccinia virus, and RNA virus. The first and second nucleic acid sequences of the invention may also be on plasmids, such as an Epstein-Barr virus-based plasmid. The nucleic acid sequences of the invention may alternatively be combined with a non- nucleic acid delivery system, such as a liposome. The nucleic acid sequences are operably linked to regulatory sequences such that they are positioned for expression. The nucleic acid sequences may then be incorporated into a plasmid, which is internalized by a cell. The nucleic acid sequences operably linked to regulatory sequences may also be incorporated into a liposome, whose membrane can fuse with a cell such that the nucleic acid sequence is deposited into the cell's cytoplasm, and so internalized by the cell. The nucleic acid sequences internalized by a cell will then enter the nucleus, where they will be expressed as chimeric CDKi protein (such as W9) and adenovirus E4 proteins. Thus, by "positioned for expression" is meant that a nucleic acid sequence encoding a polypeptide is operably linked to a regulatory sequence (e.g., a promoter), such that the nucleic acid sequence is transcribed and translated in a cell to produce the polypeptide. In an additional aspect, the invention provides a nucleic acid composition comprising a first nucleic acid sequence encoding a secretable, internalizable form of a chimeric CDKi protein and a second nucleic acid sequence encoding a secretable, internalizable form of an adenovirus E4 protein, wherein the first and second nucleic acid sequences are operably linked to at least one regulatory sequence. By "secretable, internalizable form" is meant a form of a protein that is modified using standard molecular biology techniques such that it can be discharged or released by the cell which produces it and such that it can be taken up by a cell into the cell's cytoplasm and/or nucleus, where the protein can participate in intracellular functions. In other aspects, the invention provides a protein composition is provided which includes a purified chimeric CDKi protein and a purified adenovirus E4 protein. Preferably, the chimeric CDKi protein and the purified adenovirus E4 protein are modified such that they can be taken up by a cell into the cell's cytoplasm and/or nucleus, where the proteins can participate in intracellular functions. As used herein, by "purified" is meant a protein that has been separated from components which naturally accompany it or, in the case of a protein generated by recombinant biology techniques, components that accompany it in the modified cell or virus. Of course, those of ordinary skill in protein chemistry will understand that water, buffers, and other small molecules may additionally be present in a purified protein preparation. Typically, a purified protein is pure when it is at least 60%, by weight, free from other accompanying proteins and organic molecules (e.g., nucleic acid). Preferably, a purified protein is at least 75%, more preferably, at least 90%, even more preferably, at least 95%, and most preferably at least 99% by weight, free from accompanying proteins and organic molecules. Preferably, a purified protein of the invention is obtained by expression in a cell of a nucleic acid sequence encoding the protein. Purity can be measured by any appropriate method including, without limitation, column chromatography, polyacrylamide gel electrophoresis, and HPLC analysis. The invention further provides compositions for use in treating or preventing neoplasia. These compositions comprise nucleic acid compositions or protein compositions according to the invention in combination with a pharmaceutically acceptable carrier. A "pharmaceutically acceptable carrier" includes, without limitation, any and all solvents, dispersion media, coatings, antibacterial and antifungal agents, isotonic and absorption-delaying agents, and agents which improve composition internalization by a cell which are non-toxic to the cell, and which do not reduce the therapeutic activity of the nucleic acid or protein. Except insofar as any conventional medium or agent is incompatible with the active ingredient, its use as a pharmaceutically acceptable carrier in the therapeutic compositions of the invention is contemplated. The nucleic acid compositions and protein compositions described above find use in vitro, for example, in functional assays where apoptosis is used as a read-out. In another example, these compositions also find use in vitro in methods to synchronize the cell cycle of normal cultured cells. In vivo, the compositions of the invention find use as anti-neoplasia therapeutics. For example, a composition containing the nucleic acid composition described above combined with a pharmaceutically acceptable carrier may be used in a gene therapy approach to inhibit the growth of neoplastic cells. The protein composition described above may also be combined with a pharmaceutically acceptable carrier and may be delivered to a tumor to inhibit its growth. In yet another embodiment, the invention features a cell that contains nucleic acid sequences encoding secretable, internalizable W9 protein and adenovirus E4 protein, where these sequences are operably linked to regulatory sequences such that they will be expressed in the cell. This cell finds use as a vehicle to deliver paracrine-acting proteins of the invention to cells in vivo. Preferably, the cell is derived from the animal which is being treated with the proteins of the invention. In accordance with the present invention, the compositions (and components thereof) are described in detail below. Additionally, methods are provided which can be used to prepare the compositions (and components thereof) of the invention. I. Preparation of Components of the Compositions of the Invention The compositions of the invention require as one component a chimeric CDki protein, or a nucleic acid sequence encoding a chimeric CDKi protein. As a second component, the compositions of the invention require an adenovirus E4 protein, or a nucleic acid sequence encoding an adenovirus E4 protein. A. Chimeric CDKi Proteins By "chimeric CDKi protein" is meant a fusion protein comprising at least an active fragment of a protein from the INK4 family of CDKi protein and at least an active fragment of a protein from the CIP/KIP family of CDKi proteins. A "fusion protein" of the invention is a single polypeptide chain that comprises at least a active fragment of a first protein and at least a active fragment of a second protein, wherein the two fragments are joined either directly or indirectly with a peptide bond. Proteins from the CIP/KIP family of CDKi proteins include, without limitation, human p27 kipl (GenBank Accession No. U10906, Polyak et al. (1994) Cell 78:56-66); murine p27 kipl (GenBank Accession No. U09968, Polyak et al. (1994) Cell 78:56-66); rat p27 kipl (GenBank Accession Nos. D86924 and D83792, Nomura et al. (1997) Gene 191(2):211-218); human p57 KIP2 (GenBank Accession No. NM_000076, Matsuoka et al. (1995) Genes Dev. 9(6):650-662); murine p57 KIP2 (GenBank Accession No. U20553, Lee et al. (1995) Genes Dev. 9(6):639-649); canine p 21 Wafl/CiP1 (GenBank Accession No. AF076469); and human p21 Wafl / CiP ^ (GenBank Accession No. L25610; Harper et al. (1993) Cell 75:806-816, 1993). The INK4 family of CDKi proteins which includes, without limitation, human pl8 CDKN2C (GenBank Accession Nos. AF041248 and NM_001262, Blais et al. (1998) Biochem. Biophys. Res. Commun. 247(1): 146-153); human Cdil (GenBank Accession No. NM_005192, Gyuris et al. (1993) Cell 75(4):791-803); human p 19 INK4d (GenBank Accession No. NM_001800, Guan et al. (1996) Mol. Biol. Cell 7(l):57-70); human pl5 (GenBank Accession No. S75756, Jen et al. (1994) Cancer Res. 54(24):6353-6358); murine pl5 INK4b (GenBank Accession Nos. U80415, U79634, and U79639); murine p ι ό^ 4 ^ 151 (GenBank Accession Nos. AF044336 and AF044335, Zhang et al. (1998) Proc. Natl. Acad. Sci. USA 95(5):2429-2434); and human pl6 iNK4 ( GenBank Accession No. NM_000077; Serrano et al. (1993) Nature 366(6456):704-707 and Okamoto et al. (1994) Proc. Natl. Acad. Sci. USA 91(23):11045-H049). Exemplary chimeric CDKi proteins according to the invention are the fusion proteins described herein and described in PCT Publication No. W099/ 06540, hereby incorporated by reference. Preferably, a chimeric CDKi is selected from one of the following proteins described below: W3, W4, W5, W6, W7, W8, W9, W10, or Wll, as described below in Example I. Most preferably, the chimeric CDKi protein of the invention is W9. As used herein, by "active fragment" is meant a polypeptide that encompasses at least the amino acid sequence required for inhibition of the appropriate cyclin dependent kinase which is targeted by the indicated CDKi (e.g., for human p27, see, Russo et. al. (1998) Nature 395:237-243). In a preferred embodiment, the chimeric cyclin dependent kinase inhibitor is derived from a mammal ( e.g., a human). As used herein, by "W9 protein" is meant the W9 protein which consists of an N-terminal portion containing the CDK inhibitory domain (amino acid residues 25-93) of human p27 covalently linked via a peptide bond to a C-terminal portion containing the entire human pl6 (INK4) protein, as well as any equivalent protein that has either an activity equivalent to the W9 protein or has an amino acid sequence having at least one conservative amino acid substitution when compared to the amino acid sequence of the W9 protein. The W9 protein preferably has the amino acid sequence provided in Fig. 2B. The nucleic acid sequence and amino acid sequence of W9 (without the HA tag and six histidine residues) are provided in SEQ ID NOs: 23 and 24, respectively. In the presence of an adenovirus E4 protein, a chimeric CDKi protein induces an arrest in cell proliferation in neoplastic cells, and induces neoplastic cells to apoptose. Preferably, in the presence of an adenovirus E4 protein, a chimeric CDKi protein kills neoplastic cells, but does not kill non-neoplastic cells. The nucleic acid sequence and amino acid sequence of the chimeric CDKi proteins of the invention are provided in the Examples below. However, given the degenerative nature of the genetic code, any nucleic acid sequence encoding a chimeric CDKi protein is within the definition of a chimeric CDKi-encoding nucleic acid sequence. Nucleic acid sequences encoding a chimeric CDKi protein can also be made from the nucleic acid and amino acid sequences and descriptions provided herein according to standard techniques. The nucleic acid and amino acid sequence of, for example, human p27, is available as GenBank Accession No. U10906 (Polyak et al. (1994) Cell 78: 56-66). The nucleic acid and amino acid sequence of human pi 6 is available as GenBank Accession No. L27211 (Serrano et al. (1993) Nature 366: 704-707; Okamoto et al., (1994) Proc. Natl. Acad. Sci. USA 91: 11045-11049). Techniques to make a chimeric CDKi protein-encoding nucleic acid sequence include, without limitation, amplification by PCR of the nucleic acid sequences encoding a member of the CIP/KIP family of CDKi's and amplification by PCR of the nucleic acid sequences encoding a member of the INK4 family of CDKi's, identification of cDNA clones containing CDKi-encoding nucleic acid sequences using probes generated from the sequences provided herein followed by appropriate in-frame ligation, and PCR to generate the chimeric CDKi-encoding nucleic acid sequences using overlapping oli go nucleotides. All of these techniques are well known (see, for example, Ausubel et al. (1994) Current Protocols in Molecular Biology, John Wiley & Sons, New York, NY). A chimeric CDKi protein may be made by any standard method well-known to one with skill in the art. Preferably, the chimeric CDKi protein is purified. Purified chimeric CDKi protein may be produced by chemical synthesis (see, e.g., Solid Phase Synthesis, 2nd ed. (1984) The Pierce Chemical Co., Rockford, IL). The chimeric CDKi protein can be made by subcloning a chimeric CDKi-encoding sequence into an expression plasmid, transfecting that plasmid into an appropriate cell, and purifying the synthesized chimeric CDKi protein from the cell using anti- p27 or anti-pl6 antibodies. Alternative methods for protein purification include high performance liquid chromatography (HPLC; see, e.g., Fisher (1980) Laboratory Techniques in Biochemistry and Molecular Biology , Work and Burdon (eds.), Elsevier). Chimeric CDKi proteins can be synthesized in any appropriate cell type including bacteria, insect, or mammalian, by recombinant techniques well known to those with skill in the art. For example, a chimeric CDKi- encoding nucleic acid sequence can be inserted into a baculovirus vector such that it is positioned for expression in the vector. The vector may then be used to generate recombinant baculovirus particles. Insect cells (e.g., Sf9 cells) transduced with the recombinant baculovirus will express the chimeric CDKi protein. Following lysis, the chimeric CDKi protein can be purified. Where the chimeric CDKi-encoding nucleic acid sequence has been modified using standard molecular biology techniques to encode a secreted protein (see below), the secreted chimeric CDKi protein can be isolated from the conditioned growth media of the transduced insect cells and purified. "Chimeric CDKi protein," in accordance with the present invention, additionally encompasses all equivalents of a chimeric CDKi protein. A chimeric CDKi protein equivalent has an activity equivalent to that of the chimeric CDKi protein of which it is an equivalent or has an amino acid sequence having at least one conservative amino acid substitution when compared to the amino acid sequence of the chimeric CDKi protein of which it is an equivalent. Such conservative substitutions typically include substitutions within the following groups: glycine, alanine, valine, isoleucine, leucine; aspartic acid, glutamic acid, asparagine, glutamine; serine, threonine; lysine, arginine; and phenylalanine, tyrosine. A chimeric CDKi protein equivalent may be a fusion protein of a CDK inhibitory domain bonded to an INK4 family member CDKi ( e.g., p!6). By "CDK inhibitory domain" is meant a domain of a CIP/KIP family member cyclin-dependent kinase inhibitor that contacts both the kinase and the cyclin binding subunit and is sufficient for kinase inhibition. Preferably, the CDK inhibitory domain lacks the nuclear localization signal and the QT domain. One or both of the CDK inhibitory domain and INK4 may be from a mammal, and preferably both are derived from the mammal that is being treated (e.g., a mouse is treated with a chimeric CDKi protein equivalent consisting of murine protein sequences). Where the chimeric CDKi protein is W9, preferably, a W9 protein equivalent contains at its N-terminal portion the CDK inhibitory domain and at its C-terminal portion the INK4 family member protein, or a fragment thereof. Nucleic acid sequences encoding chimeric CDKi protein equivalents can be made by standard techniques including the in-frame ligation of known nucleic acid sequences encoding the CDK inhibitory domains of the various CIP/KIP family member CDKi's with the nucleic acid sequences encoding the INK4 proteins. The nucleic acid sequences encoding the human versions of these proteins is known (e.g., human pl5, GenBank Accession No. S757756; human pl6, GenBank Accession No.27211; human pl8, GenBank Accession No. AF041248; human pl9, GenBank Accession No. U20498; human p21, GenBank Accession No. L47232; human p27, GenBank Accession No. U10906). Where a chimeric CDKi protein equivalent consists of non-human protein sequences, nucleic acid sequences encoding a chimeric CDKi protein equivalent can be made by using gene sequences from non-human animals. For example, a chimeric CDKi protein equivalent may consist of a CDK inhibitory domain from a murine CIP/KIP family member fused to full length murine pl6. Transfection of these sequences into a cell, where the sequences are positioned for expression in that cell, allow for the production and purification of the encoded chimeric CDKi protein equivalent. In addition, a chimeric CDKi protein equivalent of the invention can be made by fusing proteins, or fragments thereof (or encoding nucleic acids), from two different species (e.g., a murine p27 CDK inhibitory domain joined to a human pl6). Preferably, the chimeric CDKi protein equivalent used to treat an animal will be derived from proteins from that animal. For example, a nucleic acid sequence encoding a canine chimeric CDKi protein equivalent made by fusing a nucleic acid sequence encoding canine CDK inhibitory domain to a nucleic acid sequence encoding a canine pl6 molecule. This nucleic acid sequence, when expressed in a cell, generates a chimeric CDKi protein equivalent that may be used, for example, to treat a neoplasia in a domestic dog. A chimeric CDKi protein equivalent may also be synthetic. In addition, the CDK inhibitory domain may be joined to the INK4 family member protein via a covalent peptide bond or a chemical crosslinker. Preferably, the chimeric CDKi protein equivalent is a fusion protein of the CDK inhibitory domain of a p27 protein and a pl6 protein or fragment thereof. More preferably, the chimeric CDKi protein equivalent (or encoding nucleic acid), when administered in the presence of an adenovirus E4 protein, has an anti-neoplastic activity that is comparable to that of the chimeric CDKi protein (or of the chimeric CDKi protein- encoding nucleic acid) to which it is equivalent administered under the same conditions (for example, a W9 protein equivalent has an anti- neoplastic activity comparable to that of W9 protein). Most preferably, a chimeric CDKi protein equivalent will, when combined with an adenovirus E4 protein or active fragment thereof, induce apoptosis in neoplastic cells at lower concentrations than concentrations required to induce apoptosis in non-neoplastic cells. Apoptosis may be readily assessed by, for example, staining the cells with Annexin and/or propidium iodide (PI), or by using the Tdt TUNEL assay. These apoptosis assays are well known (see, for example, Kishimoto et al. (1995) /. Exp. Med. 181: 649-655; Pepper et al. (1998) Leuk Res. 22(5):439-44; and Walsh et al., (1998) /. Immunol. Methods 217(1-2): 153-63). B. Adenovirus E4 Protein As used herein, the term "adenovirus E4 protein" encompasses at least one protein encoded by the E4 region of adenovirus, or an active fragment of an adenovirus E4 protein, or an equivalent of an adenovirus E4 protein. There are at least six different open reading frames (orfs) in the E4 region of adenovirus, of which there are at least 42 serotypes. Hence, an adenovirus E4 protein may be any one or more but not all of the adenovirus E4 orf proteins from any serotype of adenovirus. An active fragment of an adenovirus E4 protein is a fragment of an adenovirus E4 protein that, when administered with a W9 protein, provides an anti-neoplastic activity that is greater than the anti-neoplastic activity of a chimeric CDKi protein protein administered alone. An equivalent of an adenovirus E4 protein or active fragment thereof is meant a chemical (for example, without limitation, a protein, lipid, or carbohydrate) that, when administered with a W9 protein, provides an anti-neoplastic activity comparable to that provided by an adenovirus E4 protein or an active fragment of an adenovirus E4 protein. One exemplary adenovirus E4 protein is the E4orf6 protein encoded by adenovirus E4orf6. A second exemplary adenovirus E4 protein is the E4orf3 protein encoded by adenovirus E4orf3. The adenovirus E4 protein may be from any serotype of adenovirus and may be from an adenovirus isolated from any species of animal ( e.g., human, bovine, avian). The sequences of the E4 region of various adenovirus serotypes are publicly available, for example, from the GenBank database, and include, without limitation, GenBank Accession Nos. J01966, J01980, K02368, X02998, and Y09598 (type 5); AF030154 and AF083132 (type 3); L19443 (type 40); Y09598 (avian EDS); X73487 and X51800 (type 12); U77082 and J01917 (type 2); S82508 (type 9); AF108105 (type 17); and Y07760 and U55001 (type 1), each of which is hereby incorporated by reference. These adenovirus E4 region-containing sequences can be inserted with the E4 promoter into a vector or plasmid of choice (e.g., adenovirus transfer vector lacking the entire E4 region). Alternatively, adenovirus E4 region- containing sequences can be inserted in a vector or plasmid of choice operably linked to an exogenous promoter, such as a constitutively active promoter (e.g., CMV or SV40 early), or an inducible promoter (e.g., the MMTV promoter). Nucleic acid sequences containing at least part of the E4 region of adenovirus can be introduced into a cell to generate E4 proteins. Since over-expression of the adenovirus E4 proteins is often toxic to the cell, it may be desirable to operably link adenovirus E4 region-containing nucleic acid sequences to an inducible promoter prior to transfection into a cell. Any cell type may be used to express the adenovirus E4 proteins; however, preferably, the cell chosen is one in which the adenovirus E4 proteins may be appropriately folded and post-translationally modified. Adenovirus E4 proteins produced by recombinant methods may be purified according to standard methods (e.g., HPLC). Various methods may be employed to identify active fragments and/or equivalents of the adenovirus E4orf6 protein, or other E4 orf proteins, that are sufficient to synergize with a chimeric CDKi protein to induce apoptosis in neoplastic cells. By "an active fragment of an adenovirus E4 protein" is meant a fragment of an adenovirus E4 protein that, when administered with a chimeric CDKi protein, provides an anti- neoplastic activity that is greater than the anti-neoplastic activity of a chimeric CDKi protein administered alone. Preferably, the active fragment of an adenovirus E4 protein is purified. Deletional analysis may be employed to identify an adenovirus E4 protein, or active fragment thereof, capable of synergizing with a specific chimeric CDKi protein (e.g., W9) to induce apoptosis in neoplastic cells. In a non-limiting method, recombinant adenovirus encoding a specific chimeric CDKi protein may be modified such that portions of the E4 region are deleted. In this way, not all of the E4 orf proteins are produced and the deleted E4 orf protein may be identified by deletional mapping. For example, one recombinant adenovirus encoding a chimeric CDKi protein encodes only the E4orf6 protein, and none of the other E4orf proteins. Another recombinant adenovirus encoding a chimeric CDKi protein encodes only the E4orf3 and E4orf6 proteins, but does not encode any of the other E4orf proteins. Yet another recombinant adenovirus encoding a chimeric CDKi protein encodes only a fragment of E4orf6, and none of the other E4orf proteins. The E4orf6 fragment of this virus contains, of course, an ATG and a stop sequence, and produces a polypeptide fragment. To determine which portions(s) of the E4 region are required for apoptosis in neoplastic cells, the different recombinant adenoviruses encoding a specific chimeric CDKi protein (e.g., W9) and the various fragments of E4 region proteins are titered, and the same MOI unit of each recombinant adenovirus is used to transduce a variety of normal and neoplastic cells, as described below in Examples IV and VI. The ability of these recombinant adenoviruses to induce apoptosis in neoplastic cells is compared to the adenovirus encoding W9 protein and containing the entire E4 region (AV-W9) described below in Example III. Thus, where a recombinant adenovirus encoding the specific chimeric CDKi protein (e.g., W9) and fewer than all of the E4 orf proteins (or fragments thereof) is sufficient to induce apoptosis in neoplastic cells, the E4 or proteins (or fragments thereof) encoded by that recombinant adenovirus identifies those E4 proteins (or active fragments thereof) that are sufficient to synergize with that specific chimeric CDKi protein (in this case W9) to selectively induce apoptosis in neoplastic cells. In an alternate method to identify an adenovirus E4 protein, or active fragment thereof, required to enhance the apoptosis-inducing ability of a chimeric CDKi protein, an overlapping peptide library derived from all of the adenovirus E4 orf proteins may be produced. Pools of library members are then introduced into a normal or neoplastic cell expressing the chimeric CDKi protein to determine if a peptide within the pool induced apoptosis in that cell. Where apoptosis is induced, the pool is subdivided, such that eventually the peptide or peptides that are able to act synergistically with that specific CDKi protein to induce apoptosis in a cell are identified. It will be understood that a polypeptide or chemical having an ability to synergize with a chimeric CDKi protein to produce the anti- neoplastic compositions of the invention need not be derived from an adenovirus. An equivalent of an adenovirus E4 protein, or active fragment thereof, may be a protein, lipid, or carbohydrate from another virus, from any type of cell (e.g., bacterial, insect, avian, mammalian), from a fungus, or may be a synthetic chemical. For example, to identify an adenovirus E4 protein (or active fragment thereof) equivalent, a nucleic acid sequence encoding a chimeric CDKi protein and not encoding an adenovirus E4 protein is transfected into a neoplastic cell, such that the nucleic acid sequence is positioned for expression in the cell. Preferably, the nucleic acid sequence encoding the chimeric CDKi protein is stably transfected into the neoplastic cell. Pools of compounds thought to contain an adenovirus E4 protein (or active fragment) equivalent are then administered to the transfected neoplastic cell. A pool of compounds that induces apoptosis in the transfected neoplastic cell is subdivided, and the subdivided pools administered to transfected neoplastic cells until a purified compound (or mixture) able to induce the chimeric CDKi protein-transfected neoplastic cells to apoptose is identified. As positive controls, nucleic acid sequences comprising the entire adenovirus E4 region, an adenovirus E4 protein, or an active fragment of an adenovirus E4 protein, are transfected into the chimeric CDKi protein- expressing cell, such that these sequences are positioned for expression in the cell. A compound that induces a chimeric CDKi protein-expressing neoplastic cell to undergo apoptosis at a rate comparable to that observed with chimeric CDKi protein-expressing cells transfected with nucleic acid sequences comprising the entire adenovirus E4 region, an adenovirus E4 protein, or an active fragment of an adenovirus E4 protein, is an equivalent of an adenovirus E4 protein. II. Preparation of Protein Compositions of the Invention A chimeric CDKi protein and an adenovirus E4 protein may be generated in vitro, purified, and administered either systemically or locally to the affected area (e.g., administered directly into the tumor). Preferably, the chimeric CDKi protein and the adenovirus E4 protein contain translocation sequences such that they are internalizable. An "internalizable" form of a protein is modified such that it will be internalized by a cell (e.g., modified to contain a translocation sequence), as described above in the first aspect of the invention. For example, nucleic acid encoding an internalizable form of a chimeric CDKi protein may be transfected into a prokaryotic or eukaryotic cell in vitro, such that the protein is positioned for expression in that cell. The protein may also be secreted (i.e., a secretable), in which case it may be purified from the conditioned culture media of the transfected cell. Alternatively, if the internalizable form of the protein is not secreted, but is, rather, an internal protein, in which case the transfected cell is first lysed to release the internalizable chimeric CDKi protein, which can then be purified according to standard methods. Recombinant virus encoding a chimeric CDKi protein (e.g., encoding a internalizable form of a chimeric CDKi) may also be used to transduce eukaryotic cells. The chimeric CDKi protein may then be purified from the transduced cell either from the conditioned culture media (for secreted protein) or by lysis of the cell (for non-secreted protein). As described herein, the nucleic acid sequence encoding a chimeric CDKi protein may be readily modified through the attachment of a signal sequence to the protein-encoding sequences to encode a secretable chimeric CDKi protein. Likewise, the nucleic acid sequence encoding a chimeric CDKi protein may be modified through the attachment of a translocation sequence to the protein-encoding sequences to encode an internalizable chimeric CDKi protein. An adenovirus E4 protein may be similarly expressed in vitro by the same cell that expresses the chimeric CDKi protein or by a different cell. The cell expressing the adenovirus E4 protein and the cell expressing the chimeric CDKi protein need not be the same type of cell. For example, the adenovirus E4 protein may be expressed by and purified from a bacterial cell, and the chimeric CDKi protein may be expressed by and purified from an insect cell. It will be understood that the components of the protein compositions of the invention (e.g., chimeric CDKi and adenovirus E4 proteins) may be completely synthetic and produced on a peptide synthesizer. For example, if a small adenovirus E4 protein, or an active fragment thereof, is found to complement the anti-neoplastic effects of a chimeric CDKi protein, such a small protein or polypeptide fragment may be chemically synthesized by a peptide synthesizer. Purified protein compositions of the invention may be administered to target cells in vitro or in vivo by any suitable method. For example, compositions that comprise internalizable proteins may be diluted in culture media and added to cells cultured in vitro. Similarly, for in vivo delivery, compositions comprising internalizable proteins may be combined with a pharmaceutically acceptable carrier, such as buffered saline (see below), and administered (e.g., to the site of the tumor) by injection with a syringe or catheter. For the delivery of protein compositions comprising proteins lacking translocation sequences such that they will not be internalized by target cells, the composition is preferably first combined with a delivery system that will facilitate the internalization of the composition by cells. For example, the composition may first be packaged in a liposome that bears a surface positive charge. Upon delivery to a cell either in vitro or in vivo, the liposome will fuse with the cell membrane and deposit the protein composition contained within the liposome into the cytoplasm of the cell. III. Preparation of Nucleic Acid Compositions of the Invention The invention provides a nucleic acid composition comprising a nucleic acid sequence encoding a chimeric CDKi protein and a second nucleic acid sequence encoding an adenovirus E4 protein. A. Virus Based Nucleic Acid Compositions The nucleic acid sequences encoding a chimeric CDKi protein may be incorporated into a viral vector that can be used to generate recombinant virus particles (such as an adenovirus) that contains at least part of the adenovirus E4 region and encodes a chimeric CDKi protein and at least one adenovirus E4 protein (e.g., E4orf6). Where the virus is a recombinant adenovirus, the adenovirus may be of any serotype. Preferably, the adenovirus lacks the entire E4 region such that not all of the E4 orf proteins are encoded by the adenovirus. The chimeric CDKi protein-encoding sequences may be inserted into one of the regions of the adenovirus genome whose removal is not lethal. One known region of the adenovirus genome that may be non-lethally removed is the El region, which controls adenovirus replication. Into this region may be inserted promoter and/ or enhancer sequences using standard molecular biology techniques (see, for example, Ausubel et al. (1994) Current Protocols in Molecular Biology, John Wiley & Sons, New York, NY). Such promoter /enhancer sequences may be constitutively active (e.g., the CMV promoter or the EFlα promoter), cell-type specific (e.g., the SM22α gene promoter that is specifically expressed in smooth muscle cells), or inducible (e.g., the cytokine-stimulated inducible nitric oxide synthase (iNOS) gene promoter). Numerous promoter /enhancer sequences are well known and their sequences available, for example, in the GenBank database (National Center for Biotechnology Information, National Institutes of Health, Bethesda, MD). The inserted promoter (e.g., the CMV promoter) may then be operably linked to the nucleic acid sequence encoding a chimeric CDKi protein and to a poly A signal sequence (from, e.g., SV40 virus) to create a transgene. By "transgene" is meant a nucleic acid sequence encoding a desired protein or polypeptide fragment operably linked to regulatory sequences such that the nucleic acid sequence is transcribed and translated when the transgene is introduced into a cell. Transgenes typically comprise a promoter/ enhancer: :protein-encoding nucleic acid sequence::polyA signal. A polycistronic transgene comprising two protein encoding nucleic acid sequences separated by an IRES sequence is also within this definition; consequently an IRES is also a "regulatory sequence" as used herein. In an embodiment in which the delivery virus is a recombinant adenovirus, while the transgene may be inserted into the El region of an adenovirus vector containing at least part of the E4 region, it will be understood that the transgene may be inserted at any non-lethal position in the adenovirus genome to encode for a chimeric CDKi protein and at least one adenovirus E4 protein or active fragment thereof (e.g., the chimeric CDKi protein-encoding transgene may be inserted into the E3 region). One non-limiting way to make a recombinant adenovirus encoding a chimeric CDKi protein and an adenovirus E4 protein is to insert a chimeric CDKi protein-encoding transgene comprising a chimeric CDKi protein-encoding nucleic acid sequence operably linked to a CMV promoter /enhancer and an SV40 poly A signal and an adenovirus E4 protein-encoding transgene comprising an adenovirus E4 protein- encoding nucleic acid sequence operably linked to a CMV promoter /enhancer and an SV40 poly A signal into the El region of a recombinant replication-deficient adenovirus type 5 (Ad5) vector lacking both the El and E4 regions. The recombinant retrovirus vector containing transgenes encoding a chimeric CDKi protein and an adenovirus E4 protein is then packaged in 293 cells to produce infectious recombinant adenovirus particles. The recombinant adenoviruses encoding a chimeric CDKi protein and at least one E4 protein may be used to transduce cells in vivo or in vitro. Such administration may be standardized by determining the multiplicity of infection (MOI) of the recombinant adenovirus, or by determining the actual number of viral particles based on the amount of viral DNA. Such standardization of viral particles is routine and is generally described in Phillipson et al. (1975) Molecular Biology of Adenoviruses, Virology Monograph, Springer Verlag, New York. In another embodiment, the virus which does not contain any adenovirus sequences. For example, if the virus is a lentivirus, viral vectors that are used to generate recombinant lentivirus may be genetically modified to contain nucleic acid sequences encoding a chimeric CDKi protein and at least one adenovirus E4 region-encoded protein (e.g., E4orf6). The use of recombinant lentivirus as gene therapy delivery vehicles is well known in the art (see, e.g., Dull et al. (1998) /. Virol. 72: 8463-8471). One or both of the inserted nucleic acid sequences may be operably linked to exogenous promoter /enhancer and poly A signal sequences (e.g., the E4 region promoter or the CMV promoter). Alternatively, the nucleic acid sequences can be inserted such that their expression is directed by a lentivirus promoter and polyA signal. A standard lentivirus transfer vector has a capacity of 7 kB. Thus, to generate a recombinant lentivirus encoding both a chimeric CDKi protein and an adenovirus E4 protein, both nucleic acid sequences ( i.e., the sequence encoding a chimeric CDKi protein and the sequence encoding an adenovirus E4 protein) can be separated by an IRES sequence (Jang and Wimmer (1990) Genes & Dev. 4: 1560-1572) such that a polycistronic mRNA encoding both proteins (i.e., chimeric CDKi protein and adenovirus E4 protein) may be generated by the same lentivirus or exogenous promoter/enhancer. Such a polycistronic mRNA will reduce the size of the inserted transgene (i.e., the inserted nucleic acid sequence containing a promoter /enhancer and poly A signal operably linked to a chimeric CDKi protein-encoding nucleic acid sequence and an adenovirus E4 protein-encoding nucleic acid sequence). Following appropriate packaging and titering, infectious recombinant lentivirus encoding at least one adenovirus E4 protein and a chimeric CDKi protein may be used to transduce cells. The anti-neoplastic efficacy of the chimeric CDKi protein paired with an adenovirus E4 protein may be tested in in vivo or in in vitro model systems. Where the virus lacking any adenovirus sequences is an adeno- associated virus (AAV), standard recombinant DNA techniques may be employed to generate recombinant AAV encoding an adenovirus E4 protein and a chimeric CDKi protein (such as W9). Recombinant AAV can be made by transfecting a producer cell with two trans- complementing plasmids, one plasmid encoding the rep and cap proteins, and the other plasmid encoding the transgene with the AAV inverted terminal repeat (ITR) sequences. The transfected producer cell line then produces recombinant AAV infectious viral particles, which can be used to transduce cells. The transgene size capacity of an AAV transgene-ITR plasmid is typically approximately 4.5 kB. The preferred chimeric CDKi protein of the invention, W9, is encoded by a nucleic sequence that is less than 0.7 kB. Thus, a transgene encompassing, for example, a CMV promoter /enhancer: :W9-encoding nucleic acid sequence::IRES sequence::adenovirus E4 protein-encoding nucleic acid sequence: :SV40 polyA signal may be readily accommodated by a standard AAV transgene-ITR plasmid and may be used to generate recombinant AAV particles encoding both an adenovirus E4 protein and W9 protein. Where the virus lacking any adenovirus sequences is a standard retrovirus, such as a Moloney murine leukemia virus (MMLV), recombinant MMLV encoding a chimeric CDKi protein and an adenovirus E4 protein may be generated. In a standard MMLV transfer vector, such as the rkat43.3 vector (Finer et al. (1994) Blood 83: 43-50), the transgene(s) are inserted between the gag-encoding region and the 3' LTR. A standard MMLV transfer vector has a 7 kB transgene(s) capacity. Other types of viruses that may be used to package and deliver the nucleic acid compositions of the invention include, without limitation, vaccinia virus, herpes virus, and various RNA based viruses. Because recombinant viruses are used to facilitate the internalization of the nucleic acids they carry by a transduced cell, these recombinant viruses are referred to as delivery systems. In all of the above methods for packaging the nucleic acid compositions of the invention in recombinant viruses, infectious viral particles are harvested from packaging cell lines (or the medium in which these cells are cultured) transfected with viral vectors modified to contain the nucleic acid sequences encoding W9 and an adenovirus E4 protein. First, the nucleic acid sequences encoding a chimeric CDKi protein (such as W9) and an adenovirus E4 protein are inserted into the viral vector in such a manner as to be positioned for expression in the viral vector. An appropriate packaging cell is then transfected with the viral vector. The transfected packaging cell then produces infectious recombinant viral particles carrying the inserted nucleic acid sequences. The infectious viral particles may be purified from the conditioned medium in which the transfected packaging cells are cultured. Alternatively, the infectious recombinant viral particles may be purified by lysing the cells, generally with repeated rapid freeze/thaw cycles, and centrifugation to remove the cellular debris. Following separation from cellular debris, viral particles are isolated, titered according to standard methods (Phillipson et al., supra; Synder et al., (1996) Current Protocols in Human Genetics, John Wiley & Sons, New York, NY), and stored at minus 70 °C. To use, the viral particles are thawed, diluted (if appropriate) with a pharmaceutically acceptable carrier (e.g., buffered saline), and administered. B. Non-Virus Based Nucleic Acid Compositions In another embodiment, a nucleic acid composition of the invention may feature a non-virus vector, such as a plasmid comprising chimeric CDKi protein-encoding nucleic acid sequences and the adenovirus E4 protein-encoding nucleic acid sequences. Here, the delivered sequences are positioned for expression in the plasmid such that they will be expressed in a cell transfected with the plasmid. In one non-limiting example, the non-virus vector is an Epstein Barr Virus (EBV) based plasmid. If the EBV-based plasmid also contains the EBV origin of replication (oriP) and encodes the EBV nuclear protein, EBNA-1, the resulting plasmid will be capable of autonomous replication within a transfected cell. Such autonomous replication-capability will serve to enhance the amount of chimeric CDKi protein and adenovirus E4 protein expressed by a cell transfected with the EBV plasmid. If desired, a transgene(s) encoding the chimeric CDKi protein and an adenovirus E4 protein may be separated from other plasmid sequences and administered. For example, the transgenes may be modified to be flanked with the recognition sites of a rare-cutting restriction endonuclease. Following digestion with the rare-cutting endonuclease, the transgene(s) encoding a chimeric CDKi protein and an adenovirus E4 protein may be separated from the remaining plasmid sequences by electrophoretic separation on an agarose gel, purified according to standard techniques, and delivered to a neoplastic cell to induce apoptosis. In the above methods to deliver the nucleic acid compositions of the invention in non-virus plasmids or on transgene(s), the plasmid or transgene is preferably combined with a delivery system to facilitate its internalization by a neoplastic cell. Preferably, the delivery system will rely on the endocytic pathways for the internalization of the nucleic acid composition by the target cell. Exemplary delivery systems include liposomal-derived systems, poly-lysine conjugates, and artificial viral envelopes. The nucleic acid compositions of the invention may be delivered to cells or tissues via a number of different delivery systems. Such systems are generally described in PCT Publication No. W099/ 06540 (herein incorporated by reference) In one preferred, non-limiting example, a transgene consisting of: CMVpromoter/enhancer::W9-encoding nucleic acid sequence::IRES::adenovirus E4orf6-encoding nucleic acid sequence::SV40 polyA signal is entrapped in a liposome bearing positive charges on its surface (e.g., lipofectins). Such a liposome containing a nucleic acid composition of the invention may also be tagged with antibodies that specifically find a cell surface antigen of the target tissue. For example, where a B cell lymphoma is known to express a B cell receptor having a certain idiotype, a liposome may be coated with anti-idiotype antibodies, enabling the antibody-coated liposome to bind to cells of the B cell lymphoma, and, thus, be internalized. C. Nucleic Acid Compositions for Paracrine Delivery The virus vectors and non-viral plasmids encoding a chimeric CDKi protein and an adenovirus E4 protein may be used to introduce virus or plasmid sequences into cells, such that the cells express and secrete chimeric CDKi and adenovirus E4 proteins which act in a paracrine fashion by being internalized by surrounding cells. By "paracrine" is meant action by a protein on a cell in which that protein was not synthesized. For example, using a virus-based gene therapy approach, a recombinant virus may be modified to encode two different forms of a chimeric CDKi protein and an adenovirus E4 protein. In one form, the chimeric CDKi protein and adenovirus E4 protein lack sequences enabling their secretion by the cell. These proteins, therefore, act in an autocrine fashion in the cell in which they are generated. By "autocrine" is meant action by a protein on a cell in which that protein was synthesized. In a second form, both the chimeric CDKi protein and the adenovirus E4 protein are modified to contain a leader (secretion) sequence directing their localization to the endoplasmic reticulum, Golgi apparatus, and eventual secretion from the cell. The leader sequence can be any leader sequence from any nucleic acid sequence encoding a secreted protein. In addition, these secreted forms of the chimeric CDKi protein and adenovirus E4 proteins require translocation sequences that facilitate the translocation of the proteins across the cell membrane of a second cell, allowing the proteins to be internalized by that cell. These proteins are called "secretable, internalizable forms" of the proteins. Such translocation sequences sufficient to direct protein internalization are known and include portion of the Drosophila antepennepedia protein (GenBank Accession No. E01911; Asato et al. (1989) Japanese Patent No. 1989085092-A1; Derossi et al. (1996) /. Biol. Chem. 271: 18188-18193; Derossi et al. (1994) /. Biol. Chem. 269: 10444-10450; Perez et al. (1992) /. Cell. Sci. 102: 717-722) the HIV tiansactivator (TAT) protein (Kuppuswamy et al. (1989) Nucl. Acids Res. 17: 3551-3561; Frankel and Pabo (1989) Cell 55: 1189-1193; Green and Lowenstein (1989) Cell 55: 1179-1188), and mastoparan (Higashijima et al. (1990) /. Biol. Chem. 265: 14176-141860). Preferably, the translocation sequence is the sequence encoding amino acids 48-60 or amino acids 47-72 of the full length HIV tat protein. Fig. 1 is a schematic diagram showing a non-limiting example of a paracrine-acting W9 encoding nucleic acid sequence. The W9-encoding nucleic acid sequence of Fig. 1 is modified to have a leader (secretion) sequence from any source and a translocation sequence from the HIV tat sequence. The leader sequence allows the encoded W9 protein to be secreted, while the HIV tat translocation sequence allows the internalization of the W9 protein by a neighboring cell and, further, guides the W9 protein into the nucleus of the cell. In one embodiment, the secretable, internalizable form of a chimeric CDKi protein or the secretable, internalizable form of an adenovirus E4 proteins of the invention has the following sequence: N terminus-(Leader peptide) bonded to (HIV tat amino acids 1-72 peptide) bonded to (chimeric CDKi protein or adenovirus E4 protein) -C terminus. In yet additional embodiments, where the secretable, internalizable form of a CDKi protein or an adenovirus E4 protein of the invention has a signal (leader) sequence from proinsulin, and a translocation sequence from HIV tat amino acids 1-72. For example, a secretable, internalizable form of a W9 protein having a signal (leader) sequence from proinsulin, and a translocation sequence from HIV tat amino acids 1-72 has the nucleic acid sequence provided in SEQ ID NO:29, and the amino acid sequence provided in SEQ ID NO:30. In addition, the signal peptide and translocation sequences may be separated from a chimeric CDKi protein or an adenovirus E4 protein with a linker, such as a (Gly 4 Ser) 3 linker. In the case of a secretable, internalizable form of a W9 protein, where the linker is the (Gly 4 Ser) 3 linker, the translocation sequence is HIV tat amino acids 1-72, and the signal (leader) sequence is from proinsulin, the nucleic acid sequence is provided in SEQ ID NO:31, and the amino acid sequence is provided in SEQ ID NO:32. In a non-limiting example of a gene therapy approach to deliver paracrine-acting chimeric CDKi and adenovirus E4 proteins to a neoplastic cell, a non-viral replication-deficient plasmid encoding secretable, internalizable adenovirus E4 protein and secretable, internalizable W9 protein is used to transfect non-neoplastic cells. The plasmid may contain a selectable marker gene (e.g., neo) to allow its stable integration into transfected cells exposed to that marker (e.g., G418). To reduce the size of the plasmid, the two protein-encoding nucleic acid sequences may be positioned bi-directionally on the plasmid, each having its own promoter while sharing the same poly A signal. The transfected non-neoplastic cell then secretes secretable, internalizable adenovirus E4 protein and secretable, internalizable W9 protein, which are internalized by neoplastic cells and thereby act in a paracrine fashion to induce apoptosis in the non- transfected neoplastic cells. IV. Preparation of Therapeutic Compositions of the Invention The nucleic acid and protein compositions of the invention may be combined with pharmaceutically acceptable carriers to generate therapeutic compositions for use in vivo. Accordingly, the nucleic acid and protein compositions of the invention may be formulated for administration with pharmaceutically acceptable carriers such as water, buffered saline, polyol (e.g., glycerol, propylene glycol, liquid polyethylene glycol), or suitable mixtures thereof. In one embodiment, the nucleic acids or proteins of the invention are dispersed in liquid formulations, such as micelles or liposomes, which closely resemble the lipid composition of natural cell membranes. Methods for making pharmaceutically acceptable carriers and formulations thereof are found, for example, in Remington's Pharmaceutical Sciences (18th edition), ed. A. Gennaro (1990) Mack Publishing Company, Easton, PA. Formulations for parenteral administration may, for example, contain sterile water, or saline, polyalk lene glycols such as polyethylene glycol, oils of vegetable origin, or hydrogenated napthalenes. Biocompatible, biodegradable lactide polymer, lactide/glycolide copolymer, or polyoxyethylene-poloxypropylene copolymers may be used to control the release of the chimeric CDKi and adenovirus E4 proteins. Other potentially useful parenteral delivery systems include ethylene- vinyl acetate copolymer particles, osmotic pumps, implantable infusion system, and liposomes. V. Preparation of Cells of the Invention Any cell, but preferably an animal cell, can be transfected or transduced with the nucleic acid sequences of the invention which encode secretable, internalizable forms of chimeric CDKi protein and adenovirus E4 protein. Cells transfected or transduced in vitro may be implanted back into the animal from which they were collected. In vivo, the cells containing the nucleic acid sequences of the invention will express and secrete chimeric CDKi and adenovirus E4 proteins, which are internalized by surrounding cells. If the cell internalizing the secretable, internalizable forms of the chimeric CDKi and adenovirus E4 proteins is neoplastic, apoptosis will be induced in that cell. If the cell internalizing the secretable, internalizable forms of the chimeric CDKi and adenovirus E4 proteins is non-neoplastic, the proteins will not induce apoptosis, but may induce growth arrest. Eventually, the cell secreting the secretable, internalizable forms of the chimeric CDKi and adenovirus E4 proteins may internalize enough of these proteins to itself apoptose or be arrested in growth. In one non-limiting example, an animal cell may be transfected with a linearized plasmid. The plasmid comprises nucleic acid sequences encoding secretable, internalizable form of a W9 protein and secretable, internalizable form of an adenovirus E4 protein, where the sequences are positioned for expression in the cell. The plasmid may also contain a transgene encoding a selectable marker (e.g., resistance to a drug). Following transfection, the cells are incubated in culture medium containing an amount of drug sufficient to kill non-plasmid containing cells. The plasmid-containing cells are thus selected for. Because the cells secrete an internalizable form of the W9 protein and an internalizable form of an adenovirus E4 protein, these proteins can be readily obtained from the conditioned medium of the cells. Preferably, the cell to be stably transfected is a non-neoplastic cell, so that if the transfected cell should internalize any of the W9 protein and adenovirus E4 protein it has secreted, it will be growth arrested, but will not be induced to undergo apoptosis unless it internalizes a concentration of proteins much higher than that required to induce apoptosis in neoplastic cells. The plasmid used to generate the cells of the invention may comprise nucleic acid sequences encoding a secretable, internalizable form of a chimeric CDKi protein and a secretable, internalizable form of an adenovirus E4 protein, where the sequences are operably linked to a constitutive promoter or an inducible promoter. It will be understood that the secretable, internalizable form of a chimeric CDKi protein and secretable, internalizable form of an adenovirus E4 protein need not share the same type of regulatory sequences. For example, the nucleic acid sequence encoding a secretable, internalizable form of a chimeric CDKi protein may be operably linked to a constitutive promoter and the nucleic acid sequence encoding a secretable, internalizable form of an adenovirus E4 protein may be operably linked to an inducible promoter. It will also be understood that the cells of the invention need not be transfected or transduced with only one plasmid and/or recombinant virus. For example, a cell may first be transfected with a plasmid encoding a secretable, internalizable form of a chimeric CDKi protein and a hygromycin resistance marker. After a stable cell line expressing and secreting a secretable, internalizable form of a chimeric CDKi protein is established, that cell line may be transduced with a replication-deficient virus encoding a secretable, internalizable form of an adenovirus E4 protein and a neomycin resistance marker. In addition, the cells of the invention need not be transfected with an entire plasmid. For example, transgenes comprising nucleic acid sequences encoding a chimeric CDKi protein, an adenovirus E4 protein, and a resistance marker gene may be excised from plasmids prior to their transfection into a cell. The transgenes alone will then be incorporated into a transfected cell, which will express the chimeric CDKi protein, the adenovirus E4 protein, and the resistance marker gene. In accordance with the present invention, the compositions of the invention may be used in the following in vitro and in vivo methods. I. in vitro Use of the Compositions of the Invention The nucleic acid and protein compositions of the invention find use in a variety of in vitro methodologies. For example, the compositions may be used in assays to eliminate a specific subpopulation of cultured cells. Additionally, the compositions of the invention may be used in assays to determine which type of neoplastic cell is susceptible to treatment with the compositions of the invention. The compositions of the invention also find use in assays to synchronize cell growth in cultured cells. The nucleic acid compositions can be used, for example, in vitro in cell-based assays used to eliminate a subpopulation of cells by apoptosis. These assays may be used, for example, to screen for reagents that inhibit a particular signal transduction pathway. In this example, it may be desired to identify molecules that block a signal transduction pathway induced by a hormone. A549 cells may be stably transfected with plasmids comprising both a chimeric CDKi protein-encoding nucleic acid sequence and an adenovirus E4 protein-encoding nucleic acid sequence, where the sequences are operably linked to a promoter inducible by the hormone. When the transfected cells are treated with the hormone, they will be induced to undergo apoptosis. A retroviral or plasmid library encoding peptides may be generated and introduced into the stably transfected A549 cells. When these cells are treated with the hormone, the majority of the cells will die. However, the cells containing a peptide from the library that inhibits the signal transduction pathway induced by the hormone will not die. These cells, thus isolated by their survival, can be expanded, and the library member rescued to isolate the peptide capable of inhibiting the signal transduction pathway induced by the hormone. In another example, an in vitro assay may be used to determine the genotype of neoplastic cells most susceptible to induction of apoptosis following the in vitro administration of the nucleic acid compositions of the invention. Following methods similar to those described below in Examples IV and V, a number of different normal and neoplastic cells were transduced with an adenovirus (AV-W9) containing the entire E4 region and encoding a specific chimeric CDKi protein, namely W9, and three days post-transduction, apoptosis was assessed using a an Annexin/pl apoptosis-detection assay. The different neoplastic cells were selected for transduction based on the different tissues from which they arose. Moreover, because a majority of human tumors lack normal gene expression of the tumor suppressor gene, p53, and inactivation of the retinoblastoma- susceptibility gene RB is associated with the pathogenesis of several human cancers, the p53 and Rb status of these cells was considered. Also considered was the status of the endogenous pl6 and p27 genes in these cells. Tables IA and IB below show the ability of AV-W9 to induce apoptosis in neoplastic and normal cells, respectively. Table IA As shown in Tables IA and IB above, in addition to inducing cell growth arrest in both normal and neoplastic cells, AV-W9 induced apoptosis in a variety of tumor derived cell lines. However, in a variety of non-tumorigenic primary cell types, little to no AV-W9 mediated cytopathology was observed. The non-limiting adenovirus of the invention containing the entire E4 region and encoding the chimeric CDKi, W9 (AV-W9) induced apoptosis in neoplastic cells derived from a variety of tissues, including colon, lung, liver, pancreas and prostate tumors. AV-W9 did not induce (or minimally induced) apoptosis in non-neoplastic cells derived from arteries, veins, lung, liver, pancreas and prostate. Surprisingly, the induction of tumor cell apoptosis by AV-W9 was independent of the pl6, p27, p53, and Rb status of the target neoplastic cell. For example, A549 lung carcinoma cells were induced to apoptose by AV-W9 despite having normal Rb, p53, pl6, and p27. PC3 prostate tumor cells, which have a functionally inactive p53 gene and normal Rb, pl6, and p27 genes, were killed by AV-W9, as were Y79 retinoblastoma cells, which have homozygous deletion of the Rb gene, and normal p53, pl6, and p27 genes. In another example, the protein compositions of the invention may be used to synchronize normal cultured cells. Such synchronization may be useful, for example, to study DNA replication. In this example, a purified internalizable form of a chimeric CDKi protein and a purified internalizable form of an adenovirus E4 protein are added to cultured cells. Following administration, the cells are growth arrested, and can, thus, be synchronized at a specific point in the cell cycle upon restimulation for example, by addition of serum-rich media or by removal of the added protein with, e.g., neutralizing antibodies. II. in vivo Use of the Compositions of the Invention The nucleic acid and protein compositions of the invention find use in vivo as anti-neoplastic therapeutics. Gene therapy approaches may be employed to introduce nucleic acid compositions of the invention to neoplastic cells in vivo. These gene therapy approaches may utilize viruses, such as adenovirus, retrovirus, adeno-associated virus, lentivirus, vaccinia virus, RNA virus, or herpesvirus, or may utilize non-viral nucleic acids, such as plasmids, to introduce nucleic acid sequences encoding a chimeric CDKi protein and an adenovirus E4 protein into a cell. The preferred gene therapy approach for delivering the nucleic acid composition to an animal having or suspected of having a neoplasm is by the packaging of the composition within a recombinant infectious virus, such as an adenovirus. Once delivered into the body ( e.g., by injection), the infectious virus particles are able to transduce a cell and deliver into the transduced cell the nucleic acid composition of the invention. Viral particles may be harvested from a packaging cell following its transfection with the appropriate viral vectors. Non-viral plasmids or transgenes comprising the nucleic acid compositions of the invention can be delivered to cells in vitro or in vivo, preferably combined with a delivery system which, for example, exploits the endocytic pathways for internalization by the target cell. Exemplary gene delivery systems of this type include liposome-derived systems, poly-lysine systems, and artificial viral envelopes. Standard transfection protocols (e.g., electroporation, calcium phosphate precipitation, biolistic transformation, and DEAE-dextran precipitation) may be used to deliver the nucleic acid composition of the invention to cells in vitro. For in vivo delivery of non-viral plasmids or transgenes, preferably, the nucleic acid is combined with a pharmaceutically acceptable carrier and combined with a gene delivery system, such as that described in Wu and Wu, U.S. Patent No. 5,166,320, to allow internalization of the nucleic acid by a targeted cell. Protein compositions preferably are administered in combination with a delivery system that facilitates the internalization of the composition by a target cell. Therapeutic protein compositions that are not combined with a delivery system facilitating the internalization of the composition preferably comprise purified internalizable chimeric CDKi protein (e.g. W9) and purified internalizable adenovirus E4 protein. The protein composition may be diluted in, for example, buffered saline, and administered i.v. to the patient. The anti-neoplastic compositions of the invention, protein compositions comprising a chimeric CDKi protein and an adenovirus E4 protein, or the nucleic acid compositions comprising a chimeric CDKi protein-encoding nucleic acid and an adenovirus E4 protein-encoding nucleic acid, may be administered by any appropriate means. For example, the anti-neoplastic compositions of the invention may be administered to an animal within a pharmaceutically-acceptable diluent, carrier, or excipient, in unit dosage form according to conventional pharmaceutical practice. Administration may begin before the animal is symptomatic. Any appropriate route of administration may be employed, including, without limitation, parenteral intravenous, intra-arterial, subcutaneous, intramuscular, intraperitoneal, intranasal, aerosol, by suppositories, or oral administration. Therapeutics may be in the form of liquid solutions or suspensions; for oral administration, formulations may be in the form of tablets or capsules; and for intranasal formulations, in the form of powders, nasal drops, or aerosols. The anti-neoplastic compositions may be administered locally to the affected area (e.g., directly into the tumor mass), or may be administered systemically. Since, with a few exceptions, the anti-neoplastic compositions of the invention kill neoplastic cells, but do not kill normal cells, the compositions of the invention may be administered systemically in situations where, for example, the cancer has metastasized throughout the body. For example, where the anti-neoplastic compositions of the invention are nucleic acid sequences (e.g., nucleic acid sequences encoding a chimeric CDKi protein and at least one E4 protein), a plasmid encoding these sequences may be locally administered by a velocity-driven method, such as biolistic transformation to introduce foreign molecules into a cell using velocity driven microprojectiles, such as tungsten or gold particles. Other velocity-driven methods originate from pressure bursts which include, without limitation, helium-driven, air-driven, and gunpowder- driven techniques. Biolistic transformation may be applied to intact tissues, such as an intact tumor within a cancer patient. Similarly, a chimeric CDKi protein (e.g., W9) and an adenovirus protein may be administered locally by direct injection into a tumor mass. Preferably, where the proteins themselves are injected, the proteins are purified, administered with a pharmaceutically acceptable carrier ( e.g., physiological saline), and are internalizable by tumor cells. Viral particles, such as recombinant retroviruses, carrying transgenes encoding a chimeric CDKi protein and an adenovirus E4 protein may be administered in a pharmaceutically acceptable formulation systemically via the blood stream or lymphatic system, or may be administered locally to the tumor site using a programmable pump micro-syringe, such as the micro-syringes commercially available from KD Scientific (Boston, MA). Other devices, such as cannulas and i.v. lines supplying virus particles to an artery supplying blood to the targeted tumor mass, may also be used for local administration of an anti- neoplastic viral particle of the invention. If desired, treatment of a cancer patient with the anti-neoplastic compositions of the invention may be combined with more traditional therapies such as surgery, steroid therapy, radiation therapy, or chemotherapy. Below are listed methods for delivering the compositions of the invention to the liver, the pancreas, and to the head and neck. Of course, compositions of the invention may be delivered to other organs (e.g., the kidney), and may be delivered to, for example, the liver, via the hepatic artery or vein, or systemically via any major blood vessel of the body. For patients with hepatocellular carcinomas or colon tumor metastases, the compositions of the invention may be administered in combination with an intrahepatic delivery system. In a non-limiting example, where the reagent of the invention is an adenovirus containing the all or part of the E4 region and encoding the W9 protein (such as the AV-W9 recombinant adenovirus described below), approximately 5 x 10 19 virus particles per dose are delivered through a pump connected hepatic cannula, such as those commercially available from SIMS Deltec Inc., St. Paul, MN, in a volume of about 100 ml over 30 to 60 minutes. Administration of doses may be repeated as necessary. For patients with pancreatic cancer, where the composition of the invention is an adenovirus encoding at least one E4 protein and the W9 protein, a therapeutically effective amount of the recombinant adenovirus may be delivered via CT-guided administration into the pancreatic vasculature. Preferably, the adenovirus lacks the entire E4 region such that not all adenovirus E4 proteins are encoded and/ or expressed in an adenovirus-infected cell. For patients with solid tumors of the head and/or neck, where the reagent of the invention is, for example, AV-W9, a therapeutically effective amount of the recombinant adenovirus may be delivered via direct injection into the tumor at multiple points. When administered to an animal having, or suspected of having, a neoplasm, the anti-neoplastic compositions of the invention are delivered in a therapeutically effective amount. By "animal" is meant any animal, preferably a warm-blooded animal, more preferably a mammal, and most preferably a human. As used herein, the term "therapeutically effective amount" means the total amount of each active component of a therapeutic composition that is sufficient to show a meaningful patient benefit. When administered to an animal having a solid tumor, a therapeutically effective amount is an amount sufficient to slow tumor growth, more preferably, to arrest tumor growth, and, most preferably, to diminish tumor size. Where the neoplasm is a non-solid tumor, the neoplastic cells may be counted, and a therapeutically effective amount of the compositions of the invention will slow the increase in number of neoplastic cells, more preferably, prevent an increase in the number of neoplastic cells, and, most preferably, reduce the number of neoplastic cells. When applied to an individual active component, administered alone, a therapeutically effective amount refers to that component alone. When applied in combination, the term refers to the combined amounts of the active components that result in the therapeutic effect, whether the components are administered in combination, serially, or simultaneously. What constitutes a therapeutically effective amount is within the skill of one of ordinarily skill, and can be readily determined in non-human mammals prior to use in human patients. Where the anti-neoplastic compositions of the invention are purified chimeric CDKi protein and purified adenovirus E4 protein, a therapeutically effective amount will differ depending on a number of factors, including, without limitation, whether the proteins are internalizable by a cell, and whether the proteins are injected directly into a solid tumor or delivered systemically. It will be understood that a therapeutically effective amount does not necessarily consist of equal amounts of chimeric CDKi protein and adenovirus E4 protein. For example, in a therapeutically effective amount, there may be two chimeric CDKi protein molecules for every one adenovirus E4 protein molecule. Where the anti-neoplastic reagent of the invention is a recombinant virus particle encoding a chimeric CDKi protein and an adenovirus E4 protein, the therapeutically effective amount will depend on, for example, whether the virus is delivered systemically or directly into a tumor, and whether the virus is replication-deficient or replication-capable. Also, the therapeutically effective amount of the recombinant virus may differ if the proteins act within the transduced cell only, or whether they also comprise a leader sequence and translocation sequence, thus allowing them to be secreted and act in a paracrine-fashion on surrounding cells. In one non-limiting example of a therapeutically effective amount, where the reagent of the invention is the replication-deficient adenovirus containing an entire E4 region and encoding the chimeric CDKi protein, W9 (AV-W9 described below), and is administered directly into a solid tumor of approximately 40 mm , approximately 1 x 10 viral particles per injection are administered per day for 3 days. A larger number of viral particles and/or administration for more than 3 days is used for solid tumors larger than approximately 40 mm . III. Use for the Cells of the Invention Cells containing introduced nucleic acid encoding secretable, internalizable forms of chimeric CDKi protein and adenovirus E4 protein may be used in vitro as a permanent source for secretable, internalizable chimeric CDKi protein and secretable, internalizable adenovirus E4 protein. Since the proteins are secreted, they may be purified from the conditioned medium of the cells without having to kill the cells. Where nucleic acid sequences encoding the proteins are operably linked to a constitutive promoter, it may be advisable to harvest the media frequently, to prevent internalization of the proteins by the cells. The cells of the invention introduced with nucleic acids encoding a secretable, internalizable form of a chimeric CDKi protein and a secretable, internalizable form of an adenovirus E4 protein also find use in vivo, for example, as delivery vehicles for paracrine-acting secretable, internalizable chimeric CDKi protein and secretable, internalizable adenovirus E4 protein in an animal with a neoplasm. Preferably, the cell has the same MHC genotype as the animal being treated. Less preferably, the cell is of the same species as the animal being treated. The use of the cells of the invention may be exemplified with the case of a patient diagnosed with prostate cancer. While the patient is being treated with other therapies, such as gene therapy with the nucleic acid compositions of the invention, a normal epithelial cell may be taken from the patient, and expanded in vitro. The expanded cells may then be stably transfected with nucleic acid sequences encoding secretable, internalizable forms of chimeric CDKi protein and adenovirus E4 protein, where the sequences are operably linked to drug-inducible promoters and are positioned for expression in the transfected cells. After a sufficient number of stably transfected cells expressing secretable, internalizable chimeric CDKi protein and adenovirus E4 protein are obtained, these cells are implanted into the patient, preferably near sites of cancerous cells. The inducing drug is then administered to the patient, and the stably transfected cells express proteins which act in a paracrine fashion and serve to induce apoptosis in the nearby cancerous cells. Once the cancer has subsided, the patient is no longer administered the inducing drug. In a second example, a patient diagnosed with cancer given an implant of stably transfected cells lacking any MHC expression. The stably transfected cells contain nucleic acid sequences encoding secretable, internalizable forms of chimeric CDKi protein and adenovirus E4 protein, where the sequences are operably linked to constitutive promoters and are positioned for expression in the transfected cells. Once implanted, the stably transfected cells express and secrete paracrine-acting chimeric CDKi and adenovirus E4 proteins. Once the cancer has subsided, the stably transfected cells may be surgically removed. The following examples illustrate the preferred modes of making and practicing the present invention, but are not meant to limit the scope of the invention since alternative methods may be utilized to obtain similar results. EXAMPLE I Generation of p27/p!6 Fusion Proteins To create more potent anti-proliferative molecules that possess the activities of both the 1NK4 (pl6) and CIP/KIP (p27) families, a number of recombinant CDKi's were created that fused the parental human pi 6 and p27 molecules, or their derivatives. The modified CDKi's included fusion proteins of pl6 fused to 5' and 3' truncated p27 molecules. These fusion proteins were designed to increase the protein's half-life and eliminate potential phosphorylation sites involved in the negative regulation of CDKi activity. The p27-pl6 fusion proteins interacted with the CDK4/cyclinD, CDK2/cyclinA, and CDK2/cyclinE complexes and inhibited cell cycle progression at multiple points. To generate the following non-limiting, representative CDKi fusion proteins of the present invention (and nucleic acid sequences encoding these proteins), the published sequences of the human pl6 and p27 molecules were utilized. The nucleic acid (SEQ ID NO: 25) and amino acid (SEQ ID NO: 26) sequence of human p27 is available as GenBank Accession No. U10906 (Polyak et al. (1994) Cell 78: 56-66). The nucleic acid (SEQ ID NO: 27) and amino acid (SEQ ID NO: 28) sequence of human pl6 is available as GenBank Accession No. L27211 (Serrano et al. (1993) Nature 366: 704-707; Okamoto et al. (1994) Proc. Natl. Acad. Sci. USA 91: 11045-11049). To construct the representative, non-limiting CDKi fusion proteins of the invention, in general, PCR primers were used to insert a Ndel cloning site followed by sequence encoding 6xHis and an epitope tag from the influenza virus hemagglutinin protein (HA tag) at the 5' end of full length p27 or truncated p27 25 . 93 and p27 12 _ 178 . In each instance, the CDKi gene was followed by an amber stop codon with Sail cloning site. The Ndel-Sall fragments were amplified by Deep Vent polymerase (commercially available from New England Biolabs, Beverly, MA) and cloned into plasmid pT7-7 (commercially available from US Biological (USB), Swampscott, MA) to yield pT7-His6-HA-p27, pT7-His6-HA-p27 25 _ 93 and pT7-His6-HA-p27 12 . 178 . To generate fragments without a stop codon and a (Gly 4 Ser) 3 linker, an alternate set of 3' PCR primers were used to insert sequence coding for a (Gly 4 Ser) 3 linker in place of the stop codon with Sail cloning site at the 3' end. These Ndel-Sall amplified fragments were then subcloned into a Ndel and Xhol digested pKS plasmid containing full length pi 6, with the initiating ATG removed, generating an open reading frame in which the various p27 derivatives and full length pl6 are linked by (Gly 4 Ser) 3 , histidine, and aspartic acid. The representative, non-limiting CDKi proteins of this example are schematically depicted in Fig. 2A, and were constructed as follows: To construct the nucleic acid sequence encoding the p27-pl6 fusion protein (i.e., N-terminal p27 and C-terminal pl6) having a (Gly 4 Ser) 3 hinge region between the p27 and pl6 portions (W3), the p27 coding sequence was first PCT amplified using the following primers: N-terminal primer, which carries an Ndel site and 6 histidine codons that are inserted between the ATG and the second amino acid of p27 (SEQ ID NO: 1): 5'-GCGGCCGGTCATATGCACCACCATCACCA TCACTCAAACGTGCGAGTGTCT-3'; and C-terminal primer, which carries the (Gly 4 Ser) 3 repeat and EcoRI, Sail, and Hindlll restriction sites and eliminates the stop codon of p27 (SEQ ID NO: 2): 5'-GCCGCCGG CGTCGACTCGGCCGAATTCGGATCCACCCCCGCCGGAACCGCCACC CCCGCTGCCCCCGCCACCCGTTTGACGTCTTCTGAGGCCAGG-3'. The p27 PCR product was digested with Ndel and Hindlll and inserted into pT7-7 linearized with Ndel and Hindlll. The resulted construct was digested with EcoRI and Sail and a full length pl6 PCR product was inserted as an EcoRI-XhoI fragment. The position of the EcoRI site allows the in-frame insertion of pl6. The rest of the hinge region between the p27 and pl6 coding sequences is derived from the 5' end of the pl6 cDNA. The nucleic acid and amino acid sequence of W3 are provided, respectively, in SEQ ID NO: 3 and SEQ ID NO: 4. A nucleic acid sequence encoding a second p27-pl6 fusion protein, W4, was generated, where the p27 and pl6 portions were not separated by a (Gly 4 Ser) 3 hinge region. The W4-encoding nucleic acid sequence construct includes a 5' EcoRI site, along with the coding sequence for a N- terminal HA tag, and a 3' Notl site. The nucleic acid and amino acid sequence of W4 are provided, respectively, in SEQ ID NO: 5 and SEQ ID NO: 6. Two pl6-p27 fusion proteins (i.e., N-terminal pl6 and C-terminal p27), W5 (having a (Gly 4 Ser) 3 hinge region located between the pl6 and p27 portions) and W6 (not having a a (Gly 4 Ser) 3 hinge region) were similarly generated. The nucleic acid and amino acid sequence of W5 are provided, respectively, in SEQ ID NO: 7 and SEQ ID NO: 8. The nucleic acid and amino acid sequence of W6 are provided, respectively, in SEQ ID NO: 9 and SEQ ID NO: 10. In addition, a series of a series of truncated versions of p27 designed to increase the protein half-life were fused to full-length pi 6 at the N-terminus. In one p27 truncation, p27 12 . 178 , the first 12 N-terminal and the last 20 C-terminal amino acids were removed from full length p27 to remove a CDK consensus phosphorylation site (TPKK) at amino acids 187-190, two other potential phosphorylation sites for proline directed kinases, at amino acids 178-181 (SPN), and a weak CDK phosphorylation site (SPSL) at amino acids 10-13 (Sheaff et al. (1997) Genes & Dev. 11: 1464- 1478; Morisaki et al. (1997) Biochem. Biophys. Res. Commun. 240: 386-390). The nucleic acid and amino acid sequences of this truncated p27 protein (12aa-178aa) are shown in SEQ ID NO: 11 and SEQ ID NO: 12, respectively, which provide a polypeptide of the formula EcoRI- ATG-HA epitope-p27 (12-178aa)-Stop-NotI. W7 comprises amino acids 12-178 of p27 fused to full length pl6, where the p27 and pl6 portions are separated by a (Gly 4 Ser) 3 hinge region. The nucleic acid and amino acid sequence of W7 are provided, respectively, in SEQ ID NO: 13 and SEQ ID NO: 14. W8 comprises amino acids 12-178 of p27 fused to full length pl6, where the p27 and pl6 portions are not separated by a (Gly 4 Ser) 3 hinge region. The nucleic acid and amino acid sequence of W8 are provided, respectively, in SEQ ID NO: 15 and SEQ ID NO: 16. In a second truncation of p27, p27 25 . 93 , only the CDK inhibitory domain of p27 (amino acids 25-93) was retained. This domain contacts both the CDK and cyclin binding subunits and is sufficient for kinase inhibition, while lacking the nuclear localization signal at amino acids 152-166 and the QT domain, a potential site for protein interactions, at amino acids 144-194 (Russo et al. (1998) Nature 395: 237-243). Thus, the p27 25 _ 93 CDKi was created to eliminate amino acid residues that may play a role in targeting the parental p27 molecule to the ubiquitin- proteosome degradation pathway or may play a role in p27 phosphorylation. The nucleic acid and amino acid sequences of this truncated p27 protein (25aa- 93aa) are shown in SEQ ID NO. 17 and SEQ ID NO: 18, respectively, which provide a polypeptide of the formula EcoRI- ATG-HA epitope-p27 (25-93aa)-Stop-NotI. The p27 25 . 93 fragment were fused to the N-terminus of pl6 with (W10) or without (W9) the (Gly 4 Ser) 3 hinge (Fig. 2A). The nucleic acid and amino acid sequence of W9 are provided, respectively, in SEQ ID NO: 19 and SEQ ID NO: 20. The nucleic acid and amino acid sequence of W10 are provided, respectively, in SEQ ID NO: 21 and SEQ ID NO: 22. W3, W8, and W10 were further subcloned into a modified pGEX4T-l plasmid (Pharmacia Biotech, Uppsala, Sweden) (where a Ndel cloning site was inserted between the BamHI and EcoRI sites) as Ndel-Notl fragments to generate glutathione S-transferase (GST) tagged fusion proteins. A similar strategy was used to generate fusion proteins without the (Gly 4 Ser) 3 linker (i.e., W4 (p27-pl6), W7 (p27 12 . 178 -pl6), and W9 (p27 25 _ 93 -pl6)). The nucleic acid and amino acid sequences of the p27 25 _ 93 -pl6 fusion CD i, W9, without the HA tag and six histidine residues are provided in SEQ ID NO: 23 and SEQ ID NO: 24, respectively, and in Fig. 2B. p27, p27 25 , 93 , and p27 12 78 proteins were expressed in E. coli BL21 strain using the pT7 plasmids described above. For protein expression, cells were grown in LB + 50mg/ml ampicillin at 37 °C to OD 600 =0.8 and protein expression was induced by IPTG (final; cone: 20mM) for 4 hours as 37°C. Cells were collected and the pellet was frozen at -80 °C. The preparation of the cell lysate and binding to a Ni 2+ charged sepharose resin (Invitrogen Corp, San Diego, CA; Catalog no. R801) was done according to the manufacturer's instruction (Invitrogen; see also Hochuli et al. (1987) /. Chromatography 411:177-184; Janknecht et al. (1991) Proc. Natl. Acad. Sci. USA 88: 8972-8976). The bound proteins were eluted with 50 mM, 200 mM, 350 mM, and 500 mM imidazol and the fractions were analyzed on SDS/PAGE. The 200 mM, 350 mM, and 500 mM imidazol fractions were collected, dialysed against lxPBS (1 mM KH 2 P0 4 , 10 mM Na 2 HP0 4 , 137 mM NaCl, 2.7 mM KC1, pH 7.4) + 10% glycerol and stored at-80°C in aliquots. Approximately 25% of the prep was the protein. p27 25 _ 93 and p27 12 _ 17S were further purified by gel filtration column chromatography using a Superdex 75 FPLC column equilibrated with 10% glycerol in PBS. Expression and purification of the GST-tagged W3, W4, W7, W8, W9, and W10 fusion proteins was essentially as described (Gyuris et al. (1993) Cell 75: 791-803). The purified GST-fusion proteins were then buffer exchanged by dialysis into 50 mM Tris-HCl pH 8.0, 150 mM NaCl, 2 mM CaCl 2 . The GST domain was removed from the fusion proteins by enzymatic cleavage with 1 unit (USB units) of thrombin/mg of protein/hour (thrombin commercially available from USB). Following cleavage, the thrombin was inactivated with 2 fold molar excess of PPACK (USB). The cleaved GST moiety was then removed by passing the protein solution over a column of glutathione-Sepharose. Protein concentration was determined using a protein assay (BioRad, Cambridge, MA) with bovine serum albumin (BSA) as a standard. In order to more accurately determine the concentration and purity of the specific proteins in each of the preparations, the protein samples were subjected to SDS-PAGE, and stained with coomassie blue. The stained gels were analyzed using the Gel Doc 1000 image analysis system and Molecular Analyst software (BioRad). The p27 and pl6 CDKi's appear to fold correctly in all of the fusion protein CDKi's, as the biochemical data indicates that the p27 moieties were functional and intra-cellular staining with anti-pl6 antibodies indicate that at least at a gross level, the pl6 molecules were folded correctly. EXAMPLE II in vitro Kinase-Inhibiting Activities of the CDKi Proteins The natural substrates for p27 and pl6 CDKi's are cyclin-dependent kinase (CDK) complexes that are formed via the association of different catalytic CDK and regulatory cyclin subunits. The CDK4/ cyclin D and CDK6/cyclin D complexes regulate progression through Gj phase, the CDK2/cyclin E kinase regulates the G 1 /S transition, the CDK2/cyclin A complex drives the cells through S-phase, and the entry and exit from mitosis is controlled by the CDC2/cyclin B complex (Sherr, C.J. (1996) Science 274: 1672-1677). CDKi's regulate the activity of the CDK complexes through a combination of phosphorylation events and physical association (Morgan, M. (1995) Nature 374: 131-134). The redistribution of CDKi's between the different CDK/cyclin complexes during the cell cycle coordinates the timing of activation and de-activation of their kinase activity (Sherr and Roberts (1995) Genes and Dev. 9: 1149-1163). To determine the ability of the CDKi proteins of the invention, their abilities to inhibit the in vitro kinase activity of CDK4/ cyclin Dl, CDK2/cyclin E, and CDC2/cyclin B complexes was determined. The purity of the various p27-pl6 fusion proteins, p27, and pl6 preparations were normalized using pl6 and p27 specific antibodies. Active CDK4/cyclin Dl, CDK2/cyclin E, and CDC2/cyclin B complexes were obtained from Sf9 insect cells transfected with baculoviruses expressing recombinant cyclins and CDK's. Briefly, the assay employed Sf9 cell extracts that were made from cells that were coinfected with the proper CDK and cyclin expression constructs. Typically, 44 mg of Sf9 extract in 50 ml of 50 mM Tris-HCl pH 7.6, 10 mM MgCl 2 , 1 mM DTT, 25 mM ATP, 10 mCi 32 P-γ-ATP was used in the absence of the presence of the particular inhibitor (inhibitor concentration was between 25 nM to 1 mM). Partial purification of CDK4/cyclin Dl was achieved by a 20-40% ammonium sulfate preparation of the cell lysate and was used in the assays. CDK2/cyclin E was purified to greater than 90% and pretreated with CDK-activating kinase (CAK) (Morgan, M., supra) for full activation. CDC2/cyclin B was expressed as a GST fusion protein (CDC2/GST-cyclin B) and purified on glutathione-Sepharose column, cleaved by thrombin, and followed by another glutathione- Sepharose separation for the removal of the cleaved GST. GST-fused Rb (glutathione S-transferase fusion with amino acids 379-928 from the C terminus of pRB; GST-Rb) was used as a substrate for the CDK4/cyclin Dl and CDK2/cyclin E assays; histone HI was the substrate for CDC2/cyclin B. The reaction was carried out at 30 °C for 30 minutes using 2 mg of substrate. These assays were carried out in 96 well plates (Nunc, no Naperville, IL) and monitored by γ- P-ATP incorporation. The reactions were initiated by addition of the insect cell-expressed CDK (e.g., CDC2/cyclin B) and the E. coZz ' -expressed CDKi (e.g., p27 and W9). The concentrations of GST-Rb and histone HI were 4.4 mM and 19 mM, respectively, and the concentration of ATP was 50-60 mM. The reaction mixtures contained 50 mM Tris-Cl pH 7.5, 10 mM MgCl 2 , and 1 mM DTT in a total volume of 50 or 100 μl. After incubation at 30 °C for 10- 20 minutes, the reaction was terminated by the addition of a stop solution containing EDTA. The phosphorylated substrates were captured either by GST-Sepharose or TCA precipitation and then monitored for radioactivity (Microplate Scintillation Counter, Packard, Meriden, CT). The concentration of CDKi protein at which 50% of the kinase activity was blocked (IC 50 ) was calculated for various cyclin/CDK pairs. The results are indicated in Table II (below) and in Fig. 2A (three columns labeled CDK4/cyclin Dl (nM), CDK2/cyclin E (nM), and CDK2/cyclin B (nM)). Moreover, the inhibition constant, Kj for the inhibition of CDK4/cyclin Dl by p27/pl6 fusion protein was determined to be 23 nM, compared to a Kj of 75 nM for pl6 inhibition of the same CDK4 complex. Table π Inhibition of cyclin dependent kinase complexes by p27-p!6 fusion protein CDK4/ CDK2/ CDK2/ cdc2/ inhibitor cyclin Dl cyclin E cyclin A cyclin B nM=nanomolar As shown in Table II and Fig. 2 A, pl6 was a potent inhibitor of the CDK4/cyclin Dl kinase. In contrast, p27 was a powerful inhibitor of all three kinase complexes. The various p27 modifications did not positively impact the monomeric or fusion protein CDKi's inhibitory activity in vitro (see Fig. 2A). In general, the order of the pl6 and p27 CDK in the fusion CDKi does not appear to impact the activity of the fusion CDKi. Moreover, the (Gly 4 Ser) 3 hinge region is not necessary to retain p27 function in the fusion CDKi. Thus, in vitro kinase inhibition experiments indicated that the potency of the purified p27 12 . 178 , p27 25 _ 93 or the fusion p27/pl6 proteins (W3, W4, W7, W8, W9, and W10) were not appreciably different from that of full-length p27 or an equimolar mixture of pl6 and p27. The activity of the CDK4/cyclin Dl complex was inhibited by both pl6 and p27. EXAMPLE III Construction of Recombinant Adenoviruses Encoding p!6, p27, and the p27/p!6 Fusion Proteins The adenovirus vector system used for the construction of replication deficient, El region- and E3 region-deleted, E4 region- containing adenovirus 5 (Ad5) recombinants was purchased from Microbix Biosystems Inc. (Toronto, Ontario, Canada). The six-his residue, HA-tagged inhibitors were positioned for expression under the control of the CMV promoter/enhancer and the SV40 polyA signal. The adenovirus containing the E4 region and encoding p27 (AV-p27) was constructed by in vivo recombination in 293 cells following the manufacturer's instructions (Microbix). AV-p27 DNA was isolated from the amplified virus and digested with Clal. This digest removed the p27 expression cassette and the left inverted terminal repeat (ITR) and packaging signals of Ad5 (see Fig. 3). HA-tagged pl6, p27 25 _ 93 , p27 12-178 , W3, W7, and W9 molecules were cloned into a plasmid, pKS-ITR-CMV, which contains the expression cassette as well as the left ITR and packaging signals with flanking EcoRV and Clal restriction sites. The order of the functional elements is the following from 5' to 3': EcoRV-left ITR-packaging signal-CMV enhancer/promoter-CDKi insert-SV40 polyA- Clal. The EcoRV-Clal fragments containing the CDKi inserts were ligated to the deleted, large Ad5 DNA in vitro and the ligated DNA was transfected into 293 cells (commercially available from the American Type Culture Collection (ATCC), Manassas, VA). Infectious, recombinant virus particles were rescued from 293 cells. The unligated, large Ad5 fragment was unable to generate infectious viruses alone because of the lack of the left ITR and packaging signal that are essential for virus replication. Infectious recombinants formed only when the small EcoRV-Clal fragment containing the left ITR, packaging signal, the expression cassette and the CDKi insert was ligated to the Clal digested end of the Ad5 DNA re-creating an infectious Ad5 recombinant virus DNA. EXAMPLE IV Stability of pl6, p27,and the p27/pl6 Fusion Proteins Delivered by an Adenovirus Containing the Entire E4 region p27 has been reported to be a short lived protein in proliferating cells (Pagano et al. (1995) Science 269: 682-685; Hengst and Reed (1996) Science 271: 1861-1864). To determine the half-life of the various CDKi in CASMC's, pulse-chase experiments were performed using CASMC's transduced with the adenoviruses containing the entire E4 region and encoding the various CDKi (AV-CDKi's). Human coronary artery smooth muscle cell (CASMCs) were obtained from Clonetics (Walkers ville, MD). Low passage CASMC (less than passage 10) were plated at 3500 cells /cm in complete SMC media (Clonetics, plus 5% FBS and growth factors) and allowed to recover overnight. For proliferating cells, cultures were maintained throughout in complete SMC. For quiescent cells, cultures were serum starved for 48 hours in low serum media (SMC media with 0.05% FBS and 1:100 growth factors). Growth arrested (G 0 ) and proliferating (A s ) CASMC were transduced at a multiplicity of infection (MOI) of 50 with the various recombinant adenoviruses containing the E4 region and encoding CDKi's. Twenty-four hours later, the cells were radiolabeled ("pulsed") for 2 hours in media containing 35 S-methionine. The ^S-methionine containing media was then removed and replaced with media containing an excess of non-radiolabeled amino acids, and the cells "chased" for 0, 1, 3, 9, 18 hours and 0, 1, 2, 3, 4, and 5 days. Cell pellets were lysed in 50 mM Tris-Cl pH 7.5, 250 mM NaCl, 0.5% NP-40, 50 mM NaF, 5mM EDTA, ImM PMSF, 1 mM Sodium Vanadate, and protease inhibitors. Protein concentrations were determined using a protein assay (Biorad) with bovine serum albumin (BSA) as a standard. Equivalent amounts of total protein from the cells were then immunoprecipitated using antibodies bound to protein A-sepharose. The antibodies used were p27 (Kipl, commercially available from Transduction Laboratories, Lexington, KY), and pl6-C20 (commercially available from Santa Cruz Biotech., Santa Cruz, CA). The immunoprecipitates were separated by SDS-PAGE, and the gels vacuum dried and exposed to film. The radiolabeled proteins on the autoradiographs were analyzed using the Gel Doc 1000 image analysis system and Molecular Analysts software (Biorad). The observed molecular weights of the expressed, HA epitope tagged proteins corresponded to the expected sizes: p27, approximately 30kD; p27 12 . 178 , approximately 28kD; p27 25 . 93 , approximately lOkD; pl6, approximately 19kD; W3, approximately 48kD; W7, approximately 46kD; and W9, approximately 30kD (data not shown). The kinetics of signal decay from the immunoprecipitated CDKi's was assessed by autoradiography at specific time-points (see Fig. 2A). The half-life of the CDKi's was estimated as the time-point at which half the original CDKi protein signal remained. In quiescent CASMC (G 0 cells), the pl6, p27, W3, W7, and W9 proteins all had half-lives of 2 to 3 hours (Fig. 2A). The only significant difference was observed in proliferating cells(A s ), where the W7 protein demonstrated a half-life of 20 hours. EXAMPLE V AV-W9 Has a Potent Anti-Proliferative Activity in Neoplastic Cells AV-W9 inhibited cell proliferation of SW480 colon carcinoma cells (Fig. 4A) and A549 lung carcinoma cells (Fig. 4B) at lower MOI than AV- delivered parental molecules pl6 or p27, AV-W7, and AV-Null. The ratio of virus particle/plaque forming units for the pl6, p27, W7, and W9 were similar (305 vp/pfu, 267 vp/pfu, 141 vp/pfu, and 197 vp/pfu, respectively). For these experiments, 1 x 10 cells were plated per well in a 6 well plate. Following cell adherence, the media was partially aspirated, and virus was added at the indicate MOI's. After 2 hours, fresh media was added to each well, and the cells were incubated for 1-3 days. For analysis, cells were harvested, washed with PBS, and assessed for apoptosis using the TdT TUNEL assay (Pheonix Flow Systems, San Diego, CA) and/or the Annexin binding assay (R&D Systems, Minneapolis, MN) according to manufacturers' instructions. Cells were also analyzed for viability using Trypan blue labelling and cell cycle arrest by counting the number of viable cells to estimate cell growth and proliferation. EXAMPLE VI AV-W9 Induced Apoptosis in Neoplastic Cells The ability of AV-W9 to induce apoptosis in neoplastic cells was examined. First, A549 lung carcinoma cells were transduced with equivalent amounts of adenovirus containing the E4 region and encoding either β-gal or W9 (Figs. 5A and 5B, respectively). Transduced cells were harvested 2 days later and the level of apoptosis ascertained by Annexin-PI staining. With this double staining, non-apoptotic cells appear in the lower left quadrant of the histogram; nuclei from dead cells appear in the upper left quadrant of the histogram. The apoptotic cells appear in the right half of the histogram, with late apoptotic/necrotic cells appearing at the top right corner of the histogram, mid-stage apoptotic cells appearing in the middle of the right half of the histogram, and early stage apoptotic cells appearing at the lower right corner of the histogram. Although there was some necrosis in cells transduced with AV- β- gal (Fig. 5A), transduction with AV-W9 induced early, middle, and late stage apoptosis in lung carcinoma cells (Fig. 5B). The ability of AV-W9 to induce apoptosis in PC3 prostate tumor cells, SW480 colon carcinoma cells, and A549 lung carcinoma cells was also examined. To do this, PC-3, SW480, or A549 cells were transduced as before with the indicated AV-CDKi at multiplicity of infection (MOI) of 1, 10, 50, and 100 and apoptosis was assessed after 3 days using a TdT TUNEL based apoptosis assay according to manufacturer's instructions (Pheonix Flow Systems, San Diego, CA). As can be seen in Fig.6A, AV-W9 was more effective than AV-pl6, AV-p27, or AV-W7 in inducing apoptosis in PC3 prostate tumor cells. It should be noted that because the apoptosis data shown in Fig. 6A was measured three days following transduction, at MOI of AV-W9 of greater than 10, fewer apoptotic cells were detected using the TdT TUNEL method because, as assessed by microscope analysis, many of the cells were dead and had started to degrade. Fig. 6B shows that AV-W9 was more effective than AV-pl6, AV-p27, or AV-W7 in inducing apoptosis in SW480 colon carcinoma cells. Fig. 6C shows that in A549 lung carcinoma cells, AV-W9 was more effective in inducing apoptosis than AV-pl6, AV- p27, or AV-W7. EXAMPLE VII Efficacy of AV-W9 To determine whether or not the potent apoptosis-inducing ability of AV-W9 was due to an enhanced level of expression in a cell, adenoviruses having the E4 region and encoding W9 (AV-W9), pl6 (AV- pl6), p27 (AV-p27), or no insert (AV-Null) were titered. Then DU145 prostate tumor cells were transduced with equivalent amounts of each virus. Following transduction, the levels of expression of each of these recombinant adenoviruses and their abilities to induce apoptosis in DU145 cells were examined. To measure expression, transduced cells were permeabilized, and intracellularly stained with anti-pl6 or anti-p27 antibodies (commercially available from Pharmingen). Apoptosis was measured using a TdT TUNEL FACS-based assay according to manufacturer's instructions (Pheonix Flow Systems). Approximately the same amount of viral units was required to achieve the same level of expression in DU145 cells (Fig. 7A); however, a lower amount of viral units of AV-W9 was required to induce apoptosis in DU145 cells as compared to the parental strains (AV-pl6 and AV-p27) (Fig. 7B). In Figs. 7A and 7B, the same DU145 cells were transduced with AV-W9, AV-pl6, and AV-p27. Two days following transduction, the level of apoptosis (Fig. 7B) and the level of expression of the delivered CDKi (Fig. 7A) was assessed. AV-W9 elicited apoptosis required threefold less virus than AV-pl6 or AV-p27 demonstrating that AV-W9 is more efficient in eliciting apoptosis than either parental molecule. Thus, AV-W9 at a lower dosage is a more potent inducer of apoptosis than either AV-pl6 or AV-p27. Expression levels of AV-W9 and AV-pl6 were also assessed in A549 lung cells. A549 cells were transduced with equivalent amounts of AV-W9 or AV-pl6, and the level of expression was assessed with an anti- pl6 antibody (which binds to the C-terminal pl6 portion of the W9 fusion protein). As can be seen in Fig. 7C, although 80% of the transduced cells expressed pl6 and W9 after 2 days (compare the lines on the graph), the number of W9-expressing cells was significantly higher than pi 6- expressing cells by day 3, demonstrating that W9 is more stable than the parental pl6 molecule. Moreover, the level of expression of W9 in the transduced cells was higher than the level of pl6 (compare bars on the graphs). Thus, although AV-W9 and AV-pl6 had an equal ability to transduce A549 cells, the expression level of the W9 protein was higher in these cells, indicating that W9 was more stable than pl6 in these cells. EXAMPLE VIII Tumor Cells Transduced in vitro With AV-W9 Failed to Form Tumors in vivo The tumor-forming ability of human tumor cells transduced in vitro with the AV-CDKi prior to implantation into immunodeficient mice nude mice was also studied. 1 x 10 A549 lung carcinoma cells or PC3 prostate tumor cells were transduced with 50 million infectious units of adenovirus encoding W9, pl6, p27, W7, or no insert (PBS; adenovirus bearing only the CMV promoter, but no insert). The cells were then harvested and washed. Sixteen hours following transduction, the one million pretreated cells were implanted subcutaneously into 6-8 weeks old, BALB/c nude mice of mixed genders (commercially available from Taconic Labs, Germantown, NY). Tumor formation was tracked over a 2- 3 month period using calipers. Following implantation, the AV-W9 transduced human tumor cells did not produce tumors while mice that received tumor cells transduced with AV encoding the p27, pl6, or an alternative p27-pl6 fusion molecule, W7, did develop tumors. Thus, as shown on Fig. 8A, after 12 weeks, no animals developed tumors from A549 cells transduced in vitro with AV- W9. Similarly, following implantation with PC3 cells transduced with AV-W9, no animals developed tumors at 13 weeks (Fig. 8B). Thus, only W9 delivered by an adenovirus containing the E4 region (AV-W9) completely prevented tumor formation in immunodeficient mice by both PC3 and A549 cells. AV-pl6 prevented formation in 80% of the mice, while at least 70% of the mice treated with AV-W7, AV-p27, or AV- null (i.e., PBS) succumbed to tumors (Figs. 8A and 8B). EXAMPLE IX Direct Injection of AV-W9 Into Pre-Existing Tumors in vivo Inhibited Tumor Growth The ability of AV-W9 to inhibit tumor growth was next examined following direct injection into a pre-existing tumor in vivo. To do this, 1 x 10 6 DU145 human prostate tumor cells were injected subcutaneously into Balb/c nude mice. Seven to ten days later, established tumors were observed and measured. The tumors measured approximately 40 mm . The tumors were treated with 1.25 X 10 n particles of AV-W9, AV-pl6, or no insert (PBS; adenovirus containing the E4 region and bearing the CMV promoter with no insert) introduced intratumorally by direct injection on days 1, 3, and 5 using a programmable pump micro-syringe (the KDS 100 infusion pump, KD Scientific, Boston, MA) formatted to inject 30 microliters per minute. Tumors were tracked over 2-3 months. As shown in Fig. 9, delivery of AV-W9 to pre-existing tumors resulted in the regression or suppression of tumor growth, compared to tumors injected with "PBS" (i.e., adenovirus with no insert). Treatment of pre-existing tumors with the AV-pl6 was less efficacious. In a second experiment, twenty animals were implanted with one million A549 tumor cells, and five animals per group were injected with 1.25 X 10 π particles of AV-W9, AV-pl6, AV-p27, and AV-Null on days 1, 3, and 7. Table III shows the overall results from the ability of injected AV-CDKi to inhibit tumor growth of an existing tumor, where "Complete Response" means a complete remission of the tumor following injection, "Partial Response" means a retardation in the growth of the tumor compared to the Null tumor, and "No Response" means the tumor continued to grow as well as the Null injected tumor. Table m Thus, as shown in Fig. 9 and Table III, the AV-W9 anti-neoplastic reagent of the invention was able to inhibit tumor growth following localized administration in vivo, and was superior than either similarly delivered parental molecule (i.e., AV-pl6 and AV-p27). EXAMPLE X A Recombinant Lentivirus Encoding the W9 Fusion Protein in the Absence of Any Adenovirus E4 Proteins Failed to Induce Apoptosis in Neoplastic Cells A second virus-based delivery vehicle for the W9 fusion protein was generated. Here, a lentivirus vector previously described (see Dull et al. (1998) /. Virol. 72: 8463-8471) was used to generate recombinant lentiviruses encoding W9, W7, pl6, and p27. Transgenes similar to those described in Example I (i.e., a transgene consisting of CMV enhancer/ promoter-CDKi insert-SV40 poly A) were inserted into the lentivirus transfer vector, pRRL.sin-18, between the splice acceptor cite and the 3' LTR (Dull et al, supra). It should be noted, however, that these lentivirus-encoded CDKi did not have the 6 His, HA tag. The recombinant lentiviruses were packaged essentially as described in Dull et al., supra. The recombinant lentiviruses were used to transduce neoplastic cells according to the methods generally described above for the recombinant adenoviruses. For example, for A549 cells, the cells were seeded at 10 5 cells/ well in a 12 well plate in DMEM high supplemented with 10% FBS. The following day, the media was aspirated, and new media (1 ml /well) was added with or without polybrene at a final concentration of 8pg/ml. The cells were then transduced with an equivalent number of viral particles of lentiviruses encoding W7, W9, pl6, or p27. Twenty-four hours following transduction, the media of the cells was changed, and apoptosis was tested forty-eight hours later (i.e., three days post-transduction). These studies showed that W9 delivered by lentivirus in the absence of any adenovirus E4 region-encoded proteins did not induce apoptosis in neoplastic cells, but did inhibit cell proliferation of these cells (data not shown). EXAMPLE XI Adenovirus E4 Region is Required to Facilitate The Apoptosis-inducing Ability of W9 Because W9 encoded by lentivirus did not induce apoptosis when expressed in neoplastic cells, replication-defective recombinant adenoviruses encoding W9, but lacking not only the El region, but also the E4 region, were generated. The only adenovirus E4 region protein encoded by these E4 region-deleted adenoviruses was the E4orf4 protein, and that was expressed in cells infected with the E4 region-deleted adenoviruses in very low amounts. These recombinant adenoviruses were generated using the methods and cells generally described in McArthur et al., "Adenoviral Delivery of Novel CDK Inhibitors for Preventing Proliferation of Smooth Muscle Cells," U.S. patent application serial no. 60/122,974, filed March 1, 1999 (herein incorporated by reference). Because deletion of the adenovirus E4 region is a lethal mutation to adenoviruses, these recombinant adenoviruses were packaged in a cell, 293-E4, that was stably transfected with nucleic acid comprising the entire adenovirus E4 region under the control of the inducible promoter. When this W9-encoding adenovirus, lacking both the El region and the E4 region, was used to transduce cells, no induction of apoptosis occurred (Fig. 10, W9). Accordingly, the E4 region of adenovirus, or a protein (or active fragment) encoded thereby, is the region of adenovirus necessary and sufficient to synergize with the W9 protein to produce an anti-neoplastic reagent of the invention. Moreover, since the E4 region- deleted adenovirus expressed small amounts of E4orf4 protein in infected cells, the adenovirus E4 protein necessary and sufficient to synergize with W9 to induce apoptosis in neoplastic cells is not likely to be E4orf4. To determine the fragment within the adenovirus E4 region that is sufficient to facilitate the apoptosis-inducing ability of W9, an adenovirus lacking both the El region and E4 region and encoding W9 was constructed and used to transduce three cell lines: MMTV, a cell which expresses the adenovirus E4orf6 protein under the control of the MMTV promoter, inducible by the drug dexamethasone; 293-E4, a 293 cell which expresses the entire adenovirus E4 region under the control of a promoter, inducible by the drug 3-isobutyl-l-methylxanthine (IMX, Catalog No. I- 7018, Sigma Chemical Co., St. Louis, MO); and 34X, a 293 cell that expresses the adenovirus E4orf6 under the control of a promoter inducible by IMX. In each of these cell lines, some low level of E4 or E4orf6 expression can occur without drug induction due to the leakiness of the MMTV promoter and the IMX-inducible promoter. For this study, 1 x 10 cells were transduced with lysate from an El region-deleted, E4 region-deleted adenovirus containing the W9-encoding nucleic acid sequence operably linked to the CMV promoter. The cells were transduced in the presence (+drug) or absence (no drug) of either IMX or dexamethasone. IMX was prepared as a 45 mM stock solution in water titrated with ION NaOH drop-wise until dissolved. At the time of transduction of 293-E4 and 34X cells, virus was added in the presence of IMX (final concentration of 0.5 mM). For MMTV cells (293 MMTV orf6), dexamethasone (1000X=3 mM; Catalog No. D-2915, Sigma) was added at the time of infection at a final concentration of 3 μM. Cells were harvested two days later and analyzed by Annexin/pl double staining to assess the level of apoptosis. In the absence of transduction with the El region-deleted, E4 region-deleted adenovirus encoding W9, 293-E4 cells showed no apoptosis, even with the addition of the drug (Fig. 10, 293-E4). However, when the cells were transduced, apoptosis was seen even in the absence of the drug, which was presumably due to the low level of leaky E4 expression. The level of apoptosis in the transduced cells, however, increased in the presence of the drug, demonstrating that the W9 and the adenovirus E4 proteins synergistically induce apoptosis. In the case of the adenovirus E4orf6-expressing lines, MMTV and 34X, there were low levels of apoptosis in the absence of transduction of the cells by the El region-deleted, E4 region-deleted adenovirus encoding W9. This apoptosis was likely due to the leakiness of the promoters, and the known ability of E4orf6 to elicit apoptosis. However, when the cells were transduced, the level of apoptosis rose, with a dramatic increase in the level of late apoptotic cells (Fig. 10, MMTV and 34X). Since the cell lines tested are immortalized, they are analogous to neoplastic cells in vivo. These results demonstrate that the adenovirus E4orf6 protein acts synergistically with W9 to induce apoptosis in neoplastic cells. EQUIVALENTS Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific embodiments described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims.

Claims: What is claimed is: CLAIMS 1. A nucleic acid composition consisting essentially of a first nucleic acid sequence encoding a chimeric CDKi protein and a second nucleic acid sequence encoding an adenovirus E4 protein, wherein the first and second nucleic acid sequences are operably linked to at least one regulatory sequence. 2. The nucleic acid composition of claim 1, wherein the chimeric CDKi protein is a W9 protein. 3. The nucleic acid composition of claim 1, wherein the adenovirus E4 protein is encoded by E4orf6. 4. A composition comprising the nucleic acid composition of claim 1 and a pharmaceutically acceptable carrier. 5. The composition of claim 4, further comprising a delivery system that facilitates the internalization of the composition by a cell. 6. The composition of claim 5, wherein the delivery system is a recombinant virus particle. 7. The composition of claim 6, wherein the recombinant virus particle is selected from the group consisting of an adenovirus, a lentivirus, an adeno-associated virus, a retrovirus, a herpesvirus, and a vaccinia virus. 8. The composition of claim 7, wherein the recombinant virus particle is an adenovirus. 9. A nucleic acid composition comprising a first nucleic acid sequence encoding a secretable, internalizable form of a chimeric CDKi protein and a second nucleic acid sequence encoding a secretable, internalizable form of an adenovirus E4 protein, wherein the first and second nucleic acid sequences are operably linked to at least one regulatory sequence. 10. The nucleic acid composition of claim 9, wherein the chimeric CDKi protein is a W9 protein. 11. The nucleic acid composition of claim 9, wherein the adenovirus E4 protein is encoded by E4orf6. 12. A composition comprising the nucleic acid composition of claim 9 and a pharmaceutically acceptable carrier. 13. The composition of claim 12, further comprising a delivery system that facilitates the internalization of the composition by a cell. 14. The composition of claim 13, wherein the delivery system is a recombinant virus particle. 15. The composition of claim 14, wherein the recombinant virus particle is selected from the group consisting of an adenovirus, a lentivirus, an adeno-associated virus, a retrovirus, a herpesvirus, and a vaccinia virus. 16. The composition of claim 15, wherein the recombinant virus particle is an adenovirus. 17. A protein composition comprising a purified chimeric CDKi protein and a purified adenovirus E4 protein. 18. The protein composition of claim 17, wherein the chimeric CDKi protein is a W9 protein. 19. The protein composition of claim 17, wherein the adenovirus E4 protein is encoded by E4orf6. 20. A composition comprising the protein composition of claim 17 and a pharmaceutically acceptable carrier. 21. The composition of claim 20, further comprising a delivery system that facilitates the internalization of the composition by a cell. 22. A protein composition comprising a purified, internalizable form of a chimeric CDKi protein and a purified, internalizable form of an adenovirus E4 protein. 23. The protein composition of claim 22, wherein the chimeric CDKi protein is a W9 protein. 24. The protein composition of claim 22, wherein the adenovirus E4 protein is encoded by E4orf6. 25. A composition comprising the protein composition of claim 22 and a pharmaceutically acceptable carrier. 26. A method for treating an animal with a neoplasm, the method comprising administering to the animal a therapeutically effective amount of the composition of claim 4. 27. A method for treating an animal with a neoplasm, the method comprising administering to the animal a therapeutically effective amount of the composition of claim 12. 28. A method for treating an animal with a neoplasm, the method comprising administering to the animal a therapeutically effective amount of the composition of claim 20. 29. A method for treating an animal with a neoplasm, the method comprising administering to the animal a therapeutically effective amount of the composition of claim 25. 30. A method for treating an animal with a neoplasm, comprising introducing the nucleic acid composition of claim 9 into a cell of the animal, wherein the introduced cell secretes the secretable, internalizable form of the chimeric CDKi protein and secretes the secretable, internalizable form of the adenovirus E4 protein. 31. A method for treating an animal with a neoplasm, comprising introducing the nucleic acid composition of claim 10 into a cell of the animal, wherein the introduced cell secretes the secretable, internalizable form of the W9 protein and secretes the secretable, internalizable form of the adenovirus E4 protein.

Inventors: FINER MITCHELL, GYURIS JENO, MCARTHUR JAMES, MENDEZ MICHAEL J, PATEL SALIL, FINER, MITCHELL, GYURIS, JENO, MCARTHUR, JAMES, MENDEZ, MICHAEL, J., PATEL, SALIL
Applicants: CELL GENESYS INC, MITOTIX INC, CELL GENESYS, INC., MITOTIX, INC.
Application year: 2000
Main classification: C12N15
IPC codes: A61K35, A61K38, A61K48, A61P35, C07K14, C07K19, C12N15