XIAP IRES AND USES THEREOF

Abstract: The invention features purified nucleic acid encoding a novel internal ribosome entry site (IRES) sequence from the X-linked inhibitor of apoptosis (XIAP) gene. The invention also features methods for using the XIAP IRES to increase cap-independent translation of polypeptide coding sequences linked to the XIAP IRES, and methods for isolating compounds that modulate cap-independent translation.

Description: XIAP IRES AND USES THEREOF Field of the Invention The field of the invention is regulation of protein translation. Background of the Invention Programmed cell death plays a critical role in regulating cell turnover during embryogenesis, metamorphosis, tissue homeostasis, viral infections, and cancer. Previously, we identified and cloned three mammalian genes encoding inhibitor of apoptosis proteins (IAPs): HIAP1, HIAP2, and XIAP (Farahani, R., et al, Genomics, 42:514-8, 1997; Liston, P., et al., Genomics, 46:495-503, 1997a; Liston, P., et al., Nature, 379:349-53, 1996). While the IAP genes were initially discovered in baculoviruses, their homologues have since been identified in other viruses, insects, birds, and mammals,suggesting a common evolutionary origin. X-linked IAP (XIAP) is a member of the mammalian IAP gene family. The anti-apoptotic function of XIAP is executed, at least in part, by inhibition of caspase-3 and caspase-7, two principal effectors of apoptosis. Interestingly, XIAP mRNAs are present in all human and murine fetal and adult tissues examined. Most eukaryotic mRNAs are translated primarily by ribosome scanning. First, the 40S ribosomal subunit with its associated initiation factors binds to the 5' 7- methylguanosine (m 7 G)-cap structure of the mRNA to be translated. The complex then scans in the 3' direction until an initiation codon in a favorable context is encountered, at which point protein translation is initiated. According to this model, the presence of a 5' untranslated region (UTR) with strong secondary structure and numerous initiation codons would present a significant obstacle, leading to inefficient translation by ribosome scanning. Ribosome reinitiation, shunting, and internal ribosome binding are secondary mechanisms of translation initiation that alleviate the requirement for ribosome scanning and allow translation to proceed in a cap- independent manner. Internal ribosome entry site (IRES) elements, which were first identified in picornaviruses, are considered the paradigm for cap-independent translation. The 5' UTRs of all picornaviruses are long and mediate translational initiation by directly recruiting and binding ribosomes, thereby circumventing the initial cap-binding step. Although IRES elements are frequently found in viral mRNAs, they are rarely found in non- viral mRNAs. To date, the non- viral mRNAs shown to contain functional IRES elements in their respective 5' UTRs include those encoding immunoglobulin heavy chain binding protein (BiP) (Macejak, D.G., et al. Nature, 35390-4, 1991); Drosophila Antennapedia (Oh, S.K., et al., Genes Dev, 6:1643-53, 1992) and Ultrabithorax (Ye, X., et al., Mol. Cell Biol., 17:1714-21, 1997); fibroblast growth factor 2 (Vagner, S., et al., Mol. Cell Biol., 15:35-44, 1995); initiation factor eIF4G (Gan, et al., J. Biol. Chem., 273:5006-12, 1998); proto-oncogene c-myc (Nanbru, et al., J. Biol. Chem., 272:32061-6, 1995; Stoneley, M., Oncogene, 16:423-8, 1998); and vascular endothelial growth factor (VEGF) (Stein, I., et al., Mol. Cell Biol., 18:3112-9, 1998). Cellular IRES elements have no obvious sequence or structural similarity to picornavirus IRES sequences, or to each other. Moreover, the mechanism for the regulation of IRES-directed translation is not understood. An understanding of the mechanism by which IRES elements direct cap-independent translation of cellular mRNAs and characterization of novel IRES sequences will provide new approaches for regulating the intracellular levels of both endogenously- and exogenously-encoded proteins. Summary of the Invention XIAP protein plays a critical role in regulating programmed cell death by suppressing activation of downstream caspase-3 and caspase-7. We have identified an IRES that mediates XIAP translation. The XIAP IRES element is located within a 265 nucleotide (nt) region of the XIAP 5' untranslated region (UTR). IRES-directed translation of XIAP is resistant to the repression of protein synthesis during serum deprivation-induced apoptosis. Furthermore, IRES-mediated translation of XIAP offers enhanced protection against apoptosis induced by serum deprivation in cultured HeLa cells. These studies demonstrate that the presence of an IRES element in mRNA allows a linked protein- encoding sequence to be selectively translated following the repression of cap-dependent translation. The XIAP IRES may be included in a recombinant transcription unit (e.g., a vector) to regulate the level of recombinant protein in a cell, particularly a cell under environmental stress. Furthermore, XIAP IRES antisense nucleic acid may be used to decrease a cell's resistance to apoptosis (e.g., a cancer cell). The XIAP IRES also may be used to identify compounds that modulate cap-independent protein translation. In a first aspect, the invention features a purified nucleic acid comprising or encoding a XIAP IRES, wherein, if nucleotides are present 5' or 3' to the XIAP IRES, the nucleic acid comprises at least one variant nucleotide within a 500 nucleotide region 5' or 3' to the XIAP IRES. The variant nucleotide is a nucleotide that is not present at the position of the variant nucleotide in a naturally occurring XIAP gene or XIAP mRNA, relative to the position of the XIAP IRES, and the XIAP IRES increases cap-independent translation of a cistron when the XIAP IRES is located upstream from the cistron within a messenger RNA molecule. In a preferred embodiment of the first aspect of the invention, the XIAP IRES increases stress- induced cap-independent translation. The nucleic acid may be in an expression vector. In a second, related aspect, the invention features purified nucleic acid comprising or encoding a XIAP IRES, the IRES being 5' to a coding sequence that encodes a polypeptide that is not XIAP. The nucleic acid may be in an expression vector. In a third, related aspect, the invention features a purified nucleic acid comprising or encoding a XIAP IRES, wherein the XIAP IRES has a nucleotide sequence substantially identical to a nucleotide sequence set forth in SEQ ID NOs: 1, 2,19-30. If nucleotides are present 5' or 3' to said XIAP IRES, the nucleic acid comprises at least one variant nucleotide within a 500 nucleotide region 5' or 3' to the XIAP IRES, the variant nucleotide being a nucleotide that is not present at the position of the variant nucleotide in a naturally occurring XIAP gene or XIAP mRNA, relative to the position of the XIAP IRES. In a preferred embodiment of the third aspect of the invention, the nucleic acid is in an expression vector, wherein the expression vector encodes a transcription unit comprising a XIAP IRES and a coding sequence for a polypeptide. In a further embodiment, the coding sequence may encode a polypeptide that is not a XIAP polypeptide. In yet another embodiment, the expression vector may be a gene therapy vector, and the gene therapy vector may have a tissue-specific promoter. In other embodiments, the polypeptide encoded by the gene therapy vector may be selected from XIAP, NAIP, TIAP, HIAP1, HIAP2, VEGF, BCL-2, BDNF, GDNF, PDGF-B, IGF-2, NGF, CTNF, NT-3, NT-4/5, EPO, insulin, TPO, p53, VHL, XAF, BAX, BCL- X L1 , BAD, BCL-X S , and caspases 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. In a fourth aspect, the invention features a method for increasing the level of a protein in a cell, comprising introducing into a cell an expression vector comprising a promoter operably linked to a DNA sequence encoding a transcription unit. The transcription unit comprises a XIAP IRES sequence and a coding sequence for a protein, and the presence of the XIAP IRES sequence increases the level of cap- independent translation of the protein. In various embodiments of the fourth aspect of the invention, the cell may be at risk for undergoing apoptosis, or may be undergoing apoptosis. The risk may, e.g., be due to an autoimmune disease, a degenerative disease, or an immunorejection reaction. In other embodiments of the fourth aspect of the invention, the cell may be at risk for or undergoing a heat shock response, or may be growth-arrested or may be a cancer cell, or the cell may be under environmental stress, such as hypoxic stress, osmotic stress, oxidative stress, radiation-induced stress, or toxin-induced stress. In yet other embodiments of the fourth aspect of the invention, the method may be used for gene therapy, or for inhibiting apoptosis in a cell in need thereof. The protein may be selected from XIAP, NAIP, TIAP, HIAP 1 , HIAP2, VEGF, BCL-2, BDNF, GDNF, PDGF-B, IGF-2, NGF, CTNF, NT3, NT-4/5, EPO, insulin, TPO, p53, or BCL-X L1 . In still other embodiments of the fourth aspect of the invention, the cell may be selected from the group including but not limited to: a neuron (e.g., a dopaminergic neuron), a cardiomyocyte, a skeletal myoblast, a skeletal myofiber, a hair follicle cell, an ovarian follicle cell, a retinal photoreceptor cell an oligodendrocyte, an astrocyte, and a pancreatic islet cell. Moreover, the method of the fourth aspect of the invention may used for methods including but not limited to reducing hypoxic stress in a tissue under hypoxic stress, wherein the protein may be VEGF or b-FGF, wherein expression of the protein is sufficient to reduce hypoxic stress in the tissue. Preferably, the tissue is cardiac tissue or brain tissue. In addition, the method of the fourth aspect of the invention may be used for stimulating apoptosis in a cell in need thereof. The protein may be selected from the group consisting of: caspases 1-10, BAX, BAD, BCL-X S , TRADD, FADD, XAF, VHL, and p53. In one preferred embodiment, the cell may be a cancer cell. In a fifth aspect, the invention features a method for identifying a compound that modulates protein translation comprising: a) providing a reporter cistron that is under the translational regulation of a XIAP IRES (a XIAP IRES reporter cistron); b) exposing the XIAP IRES reporter cistron to a test compound; and c) determining the amount of translation from the XIAP IRES reporter cistron exposed to the compound, relative to the amount of translation from the XIAP IRES reporter cistron not exposed to the compound. A relative increase in translation from the XIAP IRES reporter cistron exposed to the compound indicates a compound that increases XIAP IRES- dependent protein translation, and a relative decrease in translation from the XIAP IRES reporter cistron exposed to the compound indicates a compound that decreases XIAP IRES-dependent protein translation. In a preferred embodiment of the fifth aspect of the invention, the method may further include a reporter cistron that is not under the translational regulation of the XIAP IRES (an "internal control" reporter cistron), wherein: a) the amount of translation from the XIAP IRES reporter cistron exposed to the compound is normalized relative to the amount of translation of the internal control reporter cistron exposed to the compound, and b) the amount of translation from the XIAP IRES reporter cistron not exposed to the compound is normalized relative to the amount of translation of the internal control reporter cistron not exposed to the compound, and c) the amount of normalized translation from the XIAP IRES reporter cistron exposed to the compound, relative to the amount of normalized translation from the XIAP IRES reporter cistron not exposed to the compound is determined, wherein a relative increase in normalized translation from the XIAP IRES reporter cistron exposed to the compound indicates a compound that increases XIAP IRES-dependent protein translation, and wherein a relative decrease in translation from the XIAP IRES reporter cistron exposed to the compound indicates a compound that decreases XIAP IRES-dependent protein translation. In other preferred embodiments of the fifth aspect of the invention, the XIAP IRES reporter cistron is exposed to a cell extract prior to being exposed to the test compound, or after being exposed to the test compound, and the cell extract is capable of translating the XIAP IRES reporter cistron. In another preferred embodiment of the fifth aspect of the invention, the XIAP IRES reporter cistron is within a cell, and the cell is exposed to the test compound. The cell may further include a cistron that is not under the translational regulation of the XIAP IRES (i.e., an "internal control" cistron), and a) the amount of translation from the reporter cisfron in the cell exposed to the compound is normalized relative to the amount of translation of the internal control cistron in the cell exposed to the compound, and b) the amount of translation from the reporter cistron in the cell not exposed to the compound is normalized relative to the amount of translation of the internal confrol cistron in the cell not exposed to the compound, and c) the amount of normalized translation from the reporter cistron in the cell exposed to the compound, relative to the amount of normalized translation from the reporter cisfron in the cell not exposed to the compound is determined, wherein a relative increase in normalized translation from the reporter cistron in the cell exposed to the compound indicates a compound that increases XIAP IRES-dependent protein translation, and wherein a relative decrease in translation from the reporter cisfron in the cell exposed to the compound indicates a compound that decreases XIAP IRES-dependent protein translation. In a sixth aspect, the invention features a method for identifying a compound that modulates protein translation comprising: a) providing at least two reporter cistrons, wherein the reporter cistrons comprise a reporter cistron that is not under the translational regulation of a XIAP IRES (an "internal control" reporter cistron), and a reporter cistron that is under the translational regulation of a XIAP IRES (a "XIAP IRES" reporter cisfron); b) exposing the reporter cistrons to the compound; c) determining the amount of translation from the internal control reporter cisfron and the XIAP IRES reporter cistron; d) calculating the ratio of the amount of translation from the XIAP IRES reporter cistron to the amount of translation from the internal confrol reporter cistron (translation clsXI/clsIC ); and e) comparing translation clsXI/cιsIC in a sample exposed to the compound to translation clsX1/clsIC in a sample not exposed to the compound. An increase in franslation clsXI/cιsIC indicates a compound that increases XIAP IRES-dependent translation and a decrease in translation CIsX ι /c , sIC indicates a compound that decreases XIAP IRES-dependent translation. In preferred embodiments of the sixth aspect of the invention, the reporter cistrons are exposed to a cell extract prior to being exposed to the test compound, or are exposed to a cell extract after being exposed to the test compound, and the cell extract is capable of translating the XIAP IRES reporter cistron. In another preferred embodiment of the sixth aspect, the reporter cistrons are within a cell, and the cell is exposed to the test compound. In other embodiments of the sixth aspect, the reporter cistrons may comprise a single transcription unit, and the internal control reporter cistron may be located upstream from the XIAP IRES reporter cisfron. In still other embodiments of the sixth aspect of the invention, the method may be used for identifying a compound that decreases (or increases) XIAP IRES- dependent translation, wherein translation clsXI/cιslc in a cell exposed to the compound is decreased (or increased) relative to translation clsXI cιsIC in a cell not exposed to the compound. A compound that decreases the XIAP IRES-dependent franslation may be useful for treating cancer, and a compound that increases XIAP IRES-dependent translation may be useful for freating diseases or conditions that involve increased cell death, relative to normal conditions. Such disease or conditions may include, for example, myocardial infarction, neurodegenerative disease, organ loss or rejection, hair loss, or infertility. In other embodiments of the sixth aspect of the invention, the method may further comprise a third reporter cistron (a "non-XIAP IRES" reporter cistron), wherein the non-XIAP IRES reporter cistron is under the translational regulation of an IRES that is not a XIAP IRES. In a preferred embodiment, the IRES that is not a XIAP IRES may be a VEGF IRES. In another preferred embodiment of the sixth aspect, the reporter cistrons may comprise a single transcription unit and the internal control reporter gene may be located upstream from the XIAP IRES reporter cisfron and the non-XIAP IRES reporter cistron. In yet another preferred embodiment of the sixth aspect, the method further comprises: f) calculating the ratio of the amount of translation from the non-XIAP IRES reporter cisfron to the amount of franslation from the internal confrol reporter cistron (translation clsNX/CISIC ); and g) comparing translation clsNX/cιsIC in a sample exposed to the compound to translation clsNX/cιsIC in a sample not exposed to the compound. An increase in translation clsNX/cιsIC indicates a compound that increases non-XIAP IRES- dependent translation and a decrease in translation clsNX/cιsIC indicates a compound that decreases non-XIAP IRES-dependent franslation. Moreover, in a further embodiment of the sixth aspect of the invention the method may be used for identifying a compound for treating cancer, wherein: a) translation clsX1/clsIC in a sample exposed to the compound is decreased relative to translation clsXI/clsIC in a cell not exposed to the compound, and b) translation clsNX/clslc in a sample exposed to the compound is decreased relative to franslation clsNX cιsIC in a sample not exposed to the compound, wherein the compound is useful for treating cancer. In addition, in another embodiment of the sixth aspect of the invention, the method may be used for identifying a compound that inhibits apoptosis, wherein: a) franslation clsXI/cιsIC in a sample exposed to the compound is increased relative to translation clsXI/cιsIC in a sample not exposed to the compound, and b) translation clsNX/cιsIC in a sample exposed to the compound is increased relative to translation clsNX/clsIC in a sample not exposed to the compound, wherein the compound is useful for inhibiting apoptosis in a cell. In a seventh aspect, the invention features a purified nucleic acid comprising a nucleotide sequence complementary to the nucleotide sequence of a XIAP IRES (XIAP IRES antisense nucleic acid). The XIAP IRES antisense nucleic acid may be complementary to nucleotide sequences of any of the XIAP IRES nucleic acids described in the first, second, or third aspects of the invention. The XIAP IRES antisense nucleic acid is at least 10 bases long, preferably, at least 18 bases long, more preferably, at least 25 bases long, even more preferably, at least 40, 60, 85, or 120 bases long, or even as long as a full-length IRES. The XIAP IRES antisense nucleic acid may be used as a probe for detecting a XIAP IRES nucleic acid, or may be used to inhibit the activity (e.g., regulation of translation) of a XIAP IRES. In an eight aspect, the invention features a method for decreasing a cell's resistance to apoptosis, by introducing a purified XIAP IRES antisense nucleic acid into the cell, wherein the XIAP IRES antisense nucleic acid inhibits the translation of XIAP in the cell. In a preferred embodiment, the method is used to decrease a cancer cell's resistance to apoptosis. In another preferred embodiment, the cell is subjected to an apoptotic stimulus, such as a toxin or gamma irradiation after introduction of the XIAP antisense nucleic acid into the cell. In other preferred embodiments, the antisense nucleic acid comprises the sequence set forth in SEQ ID NOs: 6, 8, or 9. In a ninth aspect, the invention features a method for stimulating apoptosis in a cell. The method includes introducing into the cell a purified nucleic acid encoding a polypeptide that stimulates apoptosis in the cell, wherein the coding region for the polypeptide is under the translational regulation of a XIAP IRES. In preferred embodiments of the ninth aspect of the invention, the cell is a cancer cell and the polypeptide is XIAP. In a tenth aspect, the invention features a purified nucleic acid that hybridizes to a probe comprising at least ten consecutive nucleotides from the XIAP IRES sequence or a sequence complementary to a XIAP IRES, wherein the nucleic acid does not include the full XIAP-encoding cDNA sequence. In an eleventh aspect, the invention features a purified nucleic acid comprising a region that hybridizes to a probe comprising at least ten consecutive nucleotides from the XIAP IRES sequence or a sequence complementary to a XIAP IRES, said nucleic acid not being the full length murine or human XIAP gene or mRNA. In a twelfth aspect, the invention features a method for detecting a compound that modulates XIAP IRES-dependent translation. The method includes: (a) providing a sample comprising La autoantigen; (b) exposing the sample to a test compound; (c) contacting the La autoantigen with a XIAP IRES or an endogenous XIAP IRES; and (d) measuring the amount of binding of La autoantigen to an endogenous XIAP IRES, wherein a decrease in the binding indicates a compound that decreases XIAP IRES-dependent translation, and wherein an increase in the binding indicates a compound that increases XIAP IRES-dependent translation. In preferred embodiments of the twelfth aspect of the invention, the La autoantigen may be contacted with the XIAP IRES or the endogenous XIAP IRES prior to exposing said sample to said test compound. In a thirteenth aspect, the invention features a method for decreasing a cell's resistance to apoptosis. The method includes exposing the cell to a compound that decreases the binding of La autoantigen to an endogenous XIAP IRES, wherein a decrease in the binding is sufficient to decrease franslation of XIAP in the cell. In a preferred embodiment of the thirteenth aspect of the invention, the cell is a tumor cell or is at risk for becoming a tumor cell. In a fourteenth aspect, the invention features a method for increasing a cell's resistance to apoptosis. The method includes exposing the cell to a compound that increases the binding of La autoantigen to an endogenous XIAP IRES, wherein an increase in the binding is sufficient to increase translation of XIAP in the cell. In a preferred embodiment of the fourteenth aspect of the invention, the cell has an increased risk for undergoing apoptosis. "Cap-dependent translation" means that a 7-methylguanosine cap must be present at the 5' end of an mRNA molecule in order to initiate translation of the mRNA into protein. "Cap-independent franslation" means that a 7-methylguanosine cap is not required for translation of an mRNA molecule. Cap-independent translation initiation mechanisms include ribosome re-initiation, ribosome shunting, and internal ribosome binding. By "XIAP IRES-dependent translation" is meant cap-independent translation that occurs in the presence of a XIAP IRES or an endogenous XIAP IRES, but that does not occur in the absence of a XIAP IRES or endogenous XIAP IRES. "IRES" means a region of a nucleic acid molecule, e.g., an mRNA molecule, that allows internal ribosome entry sufficient to initiate translation in an assay for cap- independent translation, such as the bicistronic reporter assay described herein. The presence of an IRES within an mRNA molecule allows cap-independent translation of a linked protein-encoding sequence that otherwise would not be translated. "Sufficient to initiate franslation" means that the presence of an IRES increases cap-independent franslation by at least 10% (preferably by at least 20%, more preferably by at least 40%, and most preferably, by at least 60%), relative to cap- independent translation in the absence of an IRES. "XIAP IRES" means a nucleic acid that has at least 60% (preferably at least 70%, more preferably at least 80%, even more preferably at least 90%, still more preferably at least 95%, yet more preferably at least 98%, and most preferably, 100%) sequence identity to a XIAP mRNA sequence, and, furthermore, is adjacent at its 5' or 3' end, to at least one nucleotide (a "variant" nucleotide) that is not present at that position in a naturally occurring XIAP gene or XIAP mRNA. A variant nucleotide must be positioned within 500 nucleotides of the 5' or 3' end of a XIAP IRES. XIAP IRES nucleotide sequences may be found upstream from mammalian (e.g., human or mouse) XIAP coding regions in naturally occurring XIAP genes or mRNAs. Examples of XIAP IRES nucleotide sequences are the nucleotide sequences found within the region between approximately -265 and -1 relative to the human and mouse XIAP start codons. These human (SEQ ID NO: 2) and mouse (SEQ ID NO: 1) XIAP IRES sequences are shown in Fig. 4. Other preferred XIAP IRES nucleotide sequences include: the sequence from - 268 and -1 (human, SEQ ID NO: 19; mouse, SEQ ID NO: 20) relative to the human and mouse XIAP start codons; the sequence from -162 through -1 (human, SEQ ID NO: 21); the sequence from -161 through -1 (mouse, SEQ ID NO: 22); the sequence from -103 through -1 (human, SEQ ID NO: 23); the sequence from -102 through -1 (mouse, SEQ ID NO: 24); the sequence from -83 through -1 (human, SEQ ID NO: 25; mouse, SEQ ID NO: 26); the sequence from -162 through -35 (human, SEQ ID NO: 27); the sequence from -161 through -35 (mouse, SEQ ID NO: 28); the sequence from -268 through -35 (human, SEQ ID NO: 29); the sequence from -268 through -35 (mouse, SEQ ID NO: 30). "Endogenous XIAP IRES" means an IRES that is upstream of the translational start site in a XIAP-encoding mRNA that has been transcribed from an naturally occurring, endogenous XIAP gene. "Nucleic acid encoding a XIAP IRES" means nucleic acid that is template for franscription of a XIAP IRES. "Decreases the binding to a XIAP IRES" means that a compound inhibits the binding of La antigen to a XIAP IRES (e.g., by physically interacting with or chemically modifying La or another molecule that participates in the interaction between La and a XIAP IRES) or that a compound decreases the level of La within a cell (e.g., by inhibiting franscription or translation, or by stimulating mRNA or protein degradation). "Increases the binding to a XIAP IRES" means that a compound stimulates the binding of La antigen to a XIAP IRES (e.g., by physically interacting with or chemically modifying La or another molecule that participates in the interaction between La and a XIAP IRES) or that a compound increases the level of La within a cell (e.g., by stimulating transcription or franslation, or by increasing mRNA or protein half-life). "XIAP IRES antisense nucleic acid" means a nucleic acid complementary to a XIAP IRES nucleic acid sequence. Preferably, the antisense nucleic acid decreases cap-independent translation by at least 5%, more preferably by at least 10%, still more preferably by at least 20% or even 30%, and most preferably by at least 50%. "Non-XIAP IRES" means an IRES that has less than 60% identity to the XIAP IRES, for example (but not limited to), a VEGF IRES, a c-myc IRES, an FGF-2 IRES, or a BiP IRES, all of which are known in the art. "XIAP gene" means a genomic DNA or cDNA sequence that encodes XIAP. "Cisfron" means a "coding region," or segment of nucleic acid that encodes a single protein. Reporter cisfron, as used within, means a segment of nucleic acid (an mRNA or a DNA molecule) that encodes a reporter gene product (see below). The reporter cistron may be under the translational control of an IRES, for example, the XIAP IRES or the VEGF IRES. A reporter cistron may be used as an internal confrol, according to which franslation levels of other reporter genes or reporter cistrons are normalized. For example, Fig. IB shows the results from an experiment in which translation of a CAT reporter cistron under the confrol of the XIAP IRES is normalized with respect to the β-gal reporter cistron that is not under the control of the XIAP IRES. An internal confrol reporter cistron, as used within, means a reporter cisfron that is not under the control of a XIAP IRES. "VEGF IRES" means an IRES encoded by the upstream region (i.e., upstream from the coding region) of a mammalian VEGF gene, which when present within an mRNA molecule, enhances translation of a downstream cisfron in cells that are under conditions of hypoxia. The VEGF IRES is fully described in Stein, I., et al. Mol. Cell. Biol. 18:3112-9 (1998), hereby incorporated by reference. "Transcription unit" means an mRNA molecule. A franscription unit contains at least one cistron (sequence that encodes a protein), and may contain two or more cistrons (i.e., the franscription unit may encode two or more proteins). "Reporter gene" (herein used interchangeably with "reporter cistron") means any gene or translatable nucleotide sequence that encodes a product whose expression is detectable and/or quantitatable by immunological, chemical, biochemical or biological assays. A reporter gene product may, for example, have one of the following attributes, without restriction: fluorescence (e.g., green fluorescent protein), enzymatic activity (e.g., lacZ/β-galactosidase, luciferase, chloramphenicol acetylfransferase), toxicity (e.g., ricin), or an ability to be specifically bound by a second molecule (e.g., biotin or a detectably labelled antibody). It is understood that any engineered variants of reporter genes, which are readily available to one skilled in the art, are also included, without restriction, in the foregoing definition. A reporter gene or reporter cisfron, as used herein, may be a DNA or mRNA molecule. "XIAP IRES reporter cistron" means a reporter cistron that is under the franslational regulation of a XIAP IRES. "Internal confrol reporter cisfron" means a reporter cisfron that is not under the translational regulation of a XIAP IRES. "Non-XIAP IRES reporter cistron" means a reporter cistron that is under the franslational regulation of an IRES that is not a XIAP IRES. "Translation cisXI/cisIC " means the ratio of the amount of franslation from a XIAP IRES reporter cistron to the amount of translation from an internal confrol reporter cistron. "Translation cisNX/cisIC "means the ratio of the amount of franslation from a non- XIAP IRES reporter cistron to the amount of translation from an internal control reporter cisfron. "Reporter plasmid" means a DNA construct that carries a reporter gene or cistron under the transcriptional regulation of an operably linked promoter. Translation of the reporter gene or cistron may be under the confrol of a translational control element, for example, a XIAP IRES. If the reporter gene is linked to a franslational control element, the level of reporter gene activity reflects translational control of the reporter gene or cisfron. "Bicisfronic reporter plasmid" means a plasmid that contains two reporter genes or cistrons (e.g., β-gal and CAT) under the transcriptional confrol of a single promoter (e.g., the CMV promoter), such that transcriptional activation of the promoter results in the production of a single, bi-cisfronic mRNA molecule encoding both the β-gal and CAT gene products. "Modulating" means changing cap-independent translation, either by decrease or increase. "A decrease" means a lowering in the level of franslation, as measured by a decrease in reporter gene activity using a reporter gene assay, for example, lacZ/β- galactosidase, CAT, green fluorescent protein, luciferase, etc. The decrease is preferably at least 30%, more preferably 40%, and even more preferably 70%. For example, a decrease in cap-independent franslation may be detected using ELISA to measure the level of protein translated from a given cistron. Analogous methods for measuring protein levels (or relative protein levels) also may be used. "An increase" means a rise in the level of translation, as measured by an increase of reporter gene activity using a reporter gene assay, for example, lacZ/β- galactosidase, CAT, green fluorescent protein, luciferase, etc. Preferably, the increase is by at least 30%, more preferably by 40%, still more preferably by 70%, even more preferably by at least 2-fold, and most preferably by at least 3-fold. For example, an increase in cap-independent translation may be detected using ELISA to measure the level of protein translated from a given cisfron. Analogous methods for measuring protein levels (or relative protein levels) also may be used. "Promoter" means a minimal sequence sufficient to direct transcription. Also included in the invention are those promoter elements which are sufficient to render promoter-dependent gene expression controllable in a cell type-specific, tissue- specific, or temporal-specific manner, or inducible by external signals or agents; such elements may be located in the 5' or 3' or intron sequence regions of the native gene. "Operably linked" means that a gene and one or more regulatory sequences are connected in such a way as to permit gene expression when the appropriate molecules (e.g., transcriptional activator proteins) are bound to the regulatory sequences. "Expression vector" means a DNA construct that contains a promoter operably linked to a downstream gene, cistron, or RNA coding region (e.g., an antisense RNA coding region). Transfection of the expression vector into a recipient cell allows the cell to express RNA encoded by the expression vector. An expression vector may be a genetically engineered plasmid or virus, derived from, for example, a bacteriophage, adenovirus, refrovirus, poxvirus, herpesvirus, or artificial chromosome. "Gene therapy vector" means an expression vector introduced into cells for the purpose of gene therapy. "Expose" means to allow contact between an animal, cell, lysate or extract derived from a cell, or molecule derived from a cell, and a test compound. "Test compound" means a chemical, be it naturally-occurring or artificially- derived, that is surveyed for its ability to modulate an alteration in reporter gene activity or protein levels, by employing one of the assay methods described herein. Test compounds may include, for example, peptides, polypeptides, synthesized organic molecules, naturally occurring organic molecules, nucleic acid molecules, and components thereof. A "cell comprising a tissue" means a cell that is naturally a component of the tissue of interest, or a cell from an exogenous source that has been introduced into said tissue; for example, an angiogenic factor- secreting cell that is implanted into the heart for the purpose of increasing angiogenesis in the region of tissue into which the cell has been implanted. "Substantially identical" means a nucleic acid exhibiting at least 50%, preferably 60%, more preferably 70%, still more preferably 80%, and most preferably 85% identity to a reference nucleic acid sequence. The length of sequences for comparison will generally be at least 50 nucleotides, preferably at least 60 nucleotides, more preferably at least 75 nucleotides, and most preferably at least 110 nucleotides. Sequence identity is typically measured using sequence analysis software with the default parameters specified therein (e.g., Sequence Analysis Software Package of the Genetics Computer Group, University of Wisconsin Biotechnology Center, 1710 University Avenue, Madison, WI 53705). This software program matches similar sequences by assigning degrees of homology to various substitutions, deletions, and other modifications. 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. "Substantially pure DNA" means DNA that is free of the genes which, in the naturally-occurring genome of the organism from which the DNA of the invention is derived, flank the gene. The term therefore includes, for example, a recombinant DNA which is incorporated into a vector; into an autonomously replicating plasmid or virus; or into the genomic DNA of a prokaryote or eukaryote; or which exists as a separate molecule (e.g., a cDNA or a genomic or cDNA fragment produced by PCR or restriction endonuclease digestion) independent of other sequences. It also includes a recombinant DNA which is part of a hybrid gene encoding additional polypeptide sequence. "High stringency conditions" means conditions that allow hybridization comparable with that found using a DNA probe of at least 40 nucleotides in length, in a buffer containing 0.5 M NaHPO 4 , pH 7.2, 7% SDS, 1 mM EDTA, and 1 % BSA (fraction V), at a temperature of 65° C, or a buffer containing 48% formamide, 4.8X SSC, 0.2 M Tris-Cl, pH 7.6, IX Denhardt's solution, 10% dextran sulfate, and 0.1% SDS, at a temperature of 42° C. Other conditions for high stringency hybridization, such as for PCR, Northern, Southern, or in situ hybridization, DNA sequencing, etc., are well-known by those skilled in the art of molecular biology. See, e.g., F. Ausubel et al., Current Protocols in Molecular Biology, John Wiley & Sons, New York, NY, 1994, hereby incorporated by reference. "Transformation" or "transfection" means any method for introducing foreign molecules into a cell (e.g., a bacterial, yeast, fungal, algal, plant, insect, or animal cell, particularly a mammalian cell). Lipofection, DEAE-dextran-mediated transfection, microinjection, protoplast fusion, calcium phosphate precipitation, retro viral delivery, electroporation, and biolistic transformation are just a few of the methods known to those skilled in the art which may be used. "Transformed cell" or "transfected cell" means a cell (or a descendent of a cell) into which a DNA molecule comprising an IRES and/or encoding a polypeptide of the invention has been introduced, by means of recombinant DNA techniques. Such cells may be either stably or transiently transfected. "Protein" or "polypeptide" or "polypeptide fragment" means any chain of more than two amino acids, regardless of post-franslational modification (e.g., glycosylation or phosphorylation), constituting all or part of a naturally-occurring polypeptide or peptide, or constituting a non-naturally occurring polypeptide or peptide. "Apoptosis" means the process of cell death wherein a dying cell displays a set of well-characterized biochemical hallmarks which include cell membrane blebbing, cell soma shrinkage, chromatin condensation, and DNA laddering. Cells that die by apoptosis include neurons (e.g., during the course of neurodegenerative diseases such as stroke, Parkinson's disease, and Alzheimer's disease), cardiomyocytes (e.g., after myocardial infarction or over the course of congestive heart failure), and cancer cells (e.g., after exposure to radiation or chemotherapeutic agents). Environmental stress (e.g., hypoxic stress) that is not alleviated may cause a cell to enter the early phase of the apoptotic pathway, which is reversible (i.e., cells at the early stage of the apoptotic pathway can be rescued). At a later phase of apoptosis (the commitment phase), cells cannot be rescued, and, as a result, are committed to die. The above conditions and diseases place a cell at an increased risk for undergoing apoptosis. Proteins and compounds known to stimulate and inhibit apoptosis in a diverse variety of cells are well-known in the art. For example, intracellular expression and activation of the caspase (ICE) family induces or stimulates apoptotic cell death, whereas expression of the Bcl-2 family inhibits apoptotic cell death. In addition, there are survival factors that inhibit cell death in specific cell types. For example, neurotrophic factors such as NGF inhibit neuronal apoptosis. In some situations it may be desirable to artificially stimulate or inhibit apoptotic cell death by gene therapy or by a compound that mimics a gene therapeutic effect. For example, a cell that is susceptible to apoptosis induced by disease or environmental stress may be made more resistant to apoptosis by introducing an expression vector encoding an anti-apoptotic protein (such as a Bcl-2 family member or a neurofrophin) into the cell. Conversely, a cancer cell may be made less resistant to apoptosis by introducing into it an expression vector encoding a pro-apoptotic protein (such as a caspase). Placement of the encoded protein of interest under the franslational regulation of a XIAP IRES ensures that copious quantities of the protein are produced, especially under cellular conditions during which most protein translation (i.e., cap-dependent protein translation) is down-regulated, e.g., when a cell is under environmental stress, and when a cell is at a threshold for entering the apoptotic pathway. "Cell exfract" means a preparation containing the contents of cells. The exfract may be prepared simply by lysing cells (a cell lysate), or may involve additional purification steps, such as the elimination of membrane components or organelles, or enrichment of particular components of the cell lysate by methods known to those skilled in the art, such as centrifugation, differential precipitation, or chromatography. A cell exfract, as used herein, is capable of cap-dependent and cap-independent (e.g., IRES-dependent) translation of a reporter cisfron. Furthermore, a cell exfract also may be capable of transcribing a reporter cisfron prior to franslation. Therefore, a reporter protein encoded by a reporter cisfron may be translated by mixing either mRNA encoding the reporter cistron, or DNA having a promoter operably linked to the reporter cistron (e.g., an expression plasmid), with cell extract. Coupled transcription/translation systems are known in the art and are commercially available. "Translation" as used herein and as used by those of skill in the art, refers the process of generating a polypeptide that has an amino acid sequence dictated by the codon sequence of an mRNA that encodes the polypeptide. "Transcription" as used herein and as used by those of skill in the art, refers the process of using a DNA sequence as a template to generate a messenger RNA (mRNA) molecule of given nucleotide sequence. "La antigen" or "La autoantigen" means the autoimmune RNA-binding protein described in Chambers and Keene, Proc. Natl. Acad. Sci. USA 82:2115-2119, 1985 and Chambers et al., J. Biol. Chem. 263: 18043-18051, 1998. Brief Description of the Drawings Fig. 1 A is a diagram of bicisfronic reporter gene constructs containing human or mouse XIAP 5' UTRs inserted upstream from the CAT gene. Fig. IB is a graph showing the relative CAT activity resulting from transfection of the constructs shown in Fig. 1 A into HeLa cells. Fig. 2 is a diagram of a bicisfronic reporter gene construct containing the human XIAP 1-kb 5 1 UTR inserted upstream from the CAT coding region. Fig. 3 is a graph showing β-gal and CAT activity after transfection of the construct shown in Fig. 2, plus and minus a protease 2A expression plasmid. Fig. 4 is a diagram of a nucleotide sequence alignment of the mouse (SEQ ID NO: 1) and human (SEQ ID NO: 2) XIAP IRES elements and XIAP franslation start sites. Fig. 5A is a diagram showing bicisfronic and monocisfronic reporter gene constructs used for deletion and mutation analyses of the human XIAP 5' UTR. Fig. 5B is a graph showing the relative CAT activity resulting from transfection of the deletion constructs shown in Fig. 5 A. Fig. 6 is a diagram of a bicisfronic reporter gene construct containing the human XIAP 265-nucleotide 5' UTR upstream from the CAT coding region (pβgal/MRES 265/CAT). Fig. 7 is a graph showing β-gal and CAT activities in cells that were serum- starved after transfection. Fig. 8 is a graph showing the survival of serum-deprived cells transfected with an expression plasmid encoding either XIAP alone or XIAP under the translational regulation of the XIAP IRES. Fig. 9 is a graph showing that exposure to radiation up-regulates the XIAP protein level in H661 non-small cell lung carcinoma cells. Fig. 10 is a graph showing that the XIAP IRES-dependent translation is resistant to cellular stresses that inhibit cap-dependent franslation, such as expression of poliovirus protease 2 A, serum starvation, and exposure to radiation. Fig. 11 is a graph showing that XIAP mRNA containing a XIAP IRES is more efficiently translated and results in higher resistance to serum-induced apoptosis than does XIAP mRNA lacking the XIAP IRES. Fig. 12 is a diagram showing various assays (elecfrophoretic mobility shift, Northwestern analysis, ultraviolet (UV)-crosslinking, and immunoprecipitation) demonstrating that La autoantigen and other proteins bind to the XIAP IRES. Fig. 13A-13B are graphs of reporter gene assays showing that La autoantigen modulates XIAP IRES activity. Detailed Description of the Invention We have discovered a novel genetic element for the regulation of protein expression, particularly under conditions of cell stress. The X-linked inhibitor of apoptosis protein (XIAP) plays a critical role in regulating cell death by inhibiting apoptosis. Several features of the XIAP mRNA suggested to us that XIAP may not be efficiently translated by a traditional, cap-dependent mechanism. These are: i) the presence of an unusually long 5' untranslated region (UTR) (>5.5 kb for murine, >1.6 kb for human XIAP transcripts), ii) the presence of numerous potential franslation initiation sites upstream of the authentic initiation AUG codon, and iii) the high degree of secondary structure predicted for the 5' UTR. Despite these characteristics, we have found the XIAP protein in abundance in all tissues examined, indicating that XIAP mRNA is efficiently translated. Cap-dependent franslation is partially or completely inhibited under certain conditions, such as during certain phases of the cell cycle, during growth arrest, following viral infection of a cell, and following the exposure of cells to environmental stress, such as hypoxic stress, induction of a heat shock response, or entry into the early, reversible phase of apoptosis. In contrast, cap-independent translation is not inhibited by factors that inhibit cap-dependent franslation, and is often induced or enhanced under conditions that inhibit cap-dependent franslation. We hypothesized that expression of XIAP via cap-independent franslation enhances survival of a cell under stress. Accordingly, we tested whether the 5' UTR of XIAP mRNA regulates franslation initiation in a manner independent of a cap-dependent, ribosome scanning mechanism. Using bi-cisfronic mRNA reporter constructs, we show that a 265 nt region from the 5' UTR of XIAP mRNA mediates initiation of translation by an internal ribosome entry site (IRES). Several lines of evidence indicate that the 5' UTR of XIAP functions via the IRES, as opposed to mediating ribosome readthrough and re-initiation between the two cistrons in bi-cisfronic constructs. First, the 5' UTRs of both human and mouse XIAP mRNA, which promote efficient franslation of the second cistron in bicisfronic constructs, are both large (1 kb and 1.4 kb respectively) and contain numerous initiation codons. We believe that efficient readthrough and re-initiation through such sequence would be unlikely. Second, a truncated 5' UTR segment inserted upstream of the second cisfron (in a bicisfronic construct) in the reverse orientation fails to direct translation initiation of the second cisfron. If enhanced readthrough and re-initiation (as opposed to specific binding of a ribosome to an IRES) were responsible for franslation of the second cisfron, then we should have observed increased reporter gene activity from reporter gene constructs containing a second cistron preceded by the reverse-oriented truncated (265 nt) 5' UTR, because readthrough and re-initiation would be more efficient with the short 265 nt 5' UTR segment than with the longer 1 kb and 1.4 kb 5' UTR segments. The third and the most evidence for the existence of a XIAP IRES is our observation that franslation of the second cisfron in the bicisfronic construct is resistant to the overexpression of the poliovirus protease 2 A. The poliovirus protease 2 A is known to cleave and inactivate the initiation factor eIF4G subunit of eIF4F, thereby inhibiting cap-dependent franslation. Our results show that the 5' UTR of XIAP mRNA contains a functional IRES element. Translation by cap-dependent scanning is known to be inhibited at specific stages of the cell cycle and by environmental insults that can lead to heat shock or growth arrest. Therefore, a cap-independent, IRES-directed, mechanism of translation is physiologically appropriate for synthesis of a protein that regulates apoptosis, since the presence of an IRES in the XIAP transcript allows for continuous production of the XIAP protein, thereby increasing protection against apoptosis following an initial insult. This selective production of XIAP is likely to be important for the biochemical decision-making process of survival versus apoptosis. As evidence of this, HeLa cells transfected with a construct expressing XIAP under the translational control of the XIAP 5' UTR displayed 30% higher survival following serum deprivation than did cells transfected with an expression construct containing the XIAP coding region alone. Our data demonstrate that franslation of a critical regulator of apoptosis, XIAP, is mediated by internal initiation. This mode of franslation appears to be crucial for the appropriate expression of XIAP protein, and is correlated with increased survival following an initial apoptotic insult. The presence of an IRES element in the 5' UTR of XIAP, and possibly other mRNAs that encode anti-apoptotic proteins, may enhance cellular survival during exposure to transient apoptotic stimuli. The finding that a XIAP IRES enhances cap-independent franslation of a linked protein-encoding sequence suggests possible uses for the XIAP IRES in regulating protein translation. For example, XIAP IRES-encoding DNA may be used to increase the efficacy of gene therapy; this may be achieved by inserting the XIAP IRES- encoding DNA into a gene therapy vector, thereby enhancing cap-independent franslation of a linked therapeutic protein. Moreover, intracellular delivery of XIAP IRES antisense nucleic acid to unwanted cells, such as cancer cells, may be used to inhibit translation of endogenous XIAP, thereby increasing cellular susceptibility to apoptosis. In addition, reporter gene constructs comprising a XIAP IRES linked to a reporter cisfron may be used for the discovery of compounds that modulate cap- independent franslation. Such compounds may be useful for stimulating or inhibiting apoptosis. Uses for the XIAP IRES are more fully described below. Gene Therapy The XIAP IRES sequence can be inserted into gene therapy vectors such that an encoded mRNA contains a protein-coding region under the translational control of the XIAP IRES. Proteins encoded by such XIAP IRES-containing vectors are more abundantly produced under cellular conditions that favor cap-independent franslation, relative to proteins that are not under cap-independent translational control. For example, cap-dependent translation is decreased in cells (such as cardiomyocytes) subjected to environmental sfress (e.g. hypoxic stress in a failing heart); such cells are relatively more susceptible to apoptotic cell death. To render cardiomyocytes in a failing heart more resistant to hypoxia-induced cell death, a vector encoding a therapeutic protein such as an anti-apoptotic protein or an angiogenesis factor could be introduced into the cardiomyocytes. However, a vector encoding a therapeutic protein that is franslated by a cap-dependent mechanism may not be fully effective, because the mRNA encoding the protein will not be efficiently translated in cells subjected to hypoxic stress, and the resulting protein levels would be low. A more effective gene therapy would take advantage of a mechanism that allows efficient franslation of a therapeutic protein under environmental conditions that necessitate the presence of the therapeutic protein. Inclusion of a XIAP IRES sequence into the mRNA that encodes a therapeutic protein ensures that the protein is efficiently franslated when required (e.g., under conditions of cellular stress). Gene therapy vectors that encode therapeutic proteins under the translational regulation of a XIAP IRES are useful for expressing proteins in cells under conditions in which cap-independent translation is enhanced, as well as in cells under normal conditions. Cells in which cap-independent protein is enhanced include cells under environmental sfress, cells undergoing a heat shock response, cells exposed to radiation, cells that are in the early, reversible phase of apoptosis, growth-arrested cells, and cells at the Go-Gj phase of the cell cycle. Therefore, vectors encoding therapeutic proteins under the translational confrol of a XIAP IRES may be useful for increasing survival in cells at risk for harmful apoptosis, such as in patients suffering from or at risk for autoimmune diseases or other degenerative diseases. Examples of cells and situations in which it would be desirable to inhibit apoptosis include, but are not limited to: neurons (e.g., in degenerative and autoimmune diseases of the central or peripheral nervous system, such as stroke, Alzheimer's disease, Parkinson's disease, multiple sclerosis, and amyofrophic lateral sclerosis) cardiomyocytes (e.g., in heart disease or post-myocardial infarction), skeletal myocytes (e.g., in muscular degenerative disease, such as Duchenne's muscular dystrophy), kidney and liver cells (e.g., in early stages of progressive organ failure from disease or exposure to toxins), hair follicle cells (e.g., in hair loss), ovarian follicle cells, ova, sperm cells (e.g., in infertility), pancreatic islet cells, e.g., beta cells (e.g., in autoimmune diabetes) or retinal photoreceptor cells (e.g., in retinal degenerative conditions such as those resulting from retinitis pigmentosa, chemical toxicity, retinal detachment, glaucoma, diabetes, and axotomy). For example, retinitis pigmentosa is a heterogeneous collection of diseases characterized by progressive, irreversible loss of photoreceptors, which eventually leads to partial or complete blindness. Over 66 genes or genetic loci that can trigger retinal degeneration have been identified. Apoptosis has been demonstrated to occur in virtually all retinal pathologies, including retinitis pigmentosa, chemical toxicity, retinal detachment, glaucoma, diabetes, and axotomy. Suppression of apoptosis is expected to be of enormous clinical significance in treatment of retinal degenerative disease; moreover, the accessibility of the eye makes it relatively amenable to gene therapy. An appropriate expression vector (or mRNA) encoding an anti-apoptotic protein under the translational regulation of the XIAP IRES is administered to the eye such that it enters the photoreceptor cells and its encoded protein is expressed. The relatively high levels of anti-apoptotic protein resulting from XIAP IRES-regulated expression in cells under apoptotic stress decreases the susceptibility of the cells to apoptosis and thus slows disease progression. One example of such an anti-apoptotic protein is XIAP, which functions at the convergence of all apoptotic signaling pathways and therefore protects against a broad variety of cell death triggers. Expression vectors encoding an anti-apoptotic protein (such as XIAP) or other therapeutic protein (e.g., a growth or angiogenesis factor) under the franslational confrol of the XIAP IRES may also be infroduced into cells ex vivo in order to enhance the survival of cell or organ transplants. For example, the vectors may be infroduced into pancreatic beta cells prior to fransplantion into diabetic patients or into dopaminergic neurons prior to fransplantation into Parkinson's patients. Transplanted cells containing the expression vectors of the invention are more likely to survive in the patient after fransplantation than cells not containing such vectors. Vectors that encode polypeptides under the franslational regulation of a XIAP IRES may also be useful for inducing growth-arrested cells to divide in a regulated manner; for example, enhancing translation, under hypoxic conditions, of a vector- encoded growth factor that induces cells to divide in culture, which would then provide sufficient cells for implantation into patients in need of such treatment. In some situations it may be desirable to stimulate apoptosis, for example, in cancer cells or undesirable cells such as excess adipocytes. In such cases, gene therapy vectors that encode cell death-inducing proteins (e.g., caspase, p53, or bad) under the franslational regulation of a XIAP IRES may be administered to enhance a cell's sensitivity to apoptosis. Gene therapy vectors may employ tissue-specific promoters (e.g., a cardiomyocyte-, skeletal myocyte-, pancreatic beta cell-, or neuron-specific promoter, as a means of more precisely targeting expression of the therapeutic protein to the desired cell type. Polypeptides that may be expressed under the translational regulation of the XIAP IRES include, but are not limited to, those shown in Table 1 below. Table 1 Protein Genbank Accession Number XIAP U45880 NAIP U19251 TIAP SEQ ID NOS: 3 and 4 HI API U45878 HIAP2 U45879 VEGF M63971 BCL-2 M13995 BDNF M61181 NGF X52599 CNTF X55890 EPO M11319 Insulin J00265 TPO S76771 p53 U94788 VHL AF010238 XAF X99699 BAX L22474 BCL-X L1 Z23115 BAD AF031523 BCL-X S Z23116 CASPASE- 1 U13698 CASPASE-2 U13021 CASPASE-3 U26943 CASPASE-4 Z48810 CASPASE-5 U28015 CASPASE-6 U20536 CASPASE-7 U37488 CASPASE-8 U58143 CASPASE-9 U60521 CASPASE- 10 U60519 b-FGF M27968 (basic Fibroblast Growth Factor) TRADD L41690 FADD U24231 NT-3 M61180 NT-4/5 M86528 GDNF LI 9063 (Glial cell line Derived Neurotrophic Factor) PDGF-B X02811 (Platelet Derived Growth Factor B) IGF2 X03562 (Insulin-like Growth Factor II) Antisense Therapy We have observed that apoptotic stimuli increase intracellular XIAP protein levels in a XIAP IRES-dependent manner, thereby providing increased resistance to apoptosis. However, in some cases (e.g., in cancer cells) it would be desirable to counteract this enhanced XIAP IRES-dependent apoptotic resistance. Inhibition of XIAP IRES-dependent franslation using XIAP IRES-specific antisense therapy may be used to increase a cell's susceptibility to apoptosis. Antisense therapy is based on the well-known principle of suppressing gene expression by infracellularly hybridization of endogenous nucleic acid (genomic DNA or mRNA) molecules encoding the protein of interest with a complementary antisense nucleic acid, such as an antisense oligonucleotide or antisense RNA; therefore, antisense nucleic acids inhibit protein expression at the transcriptional and/or at the translational level. Antisense oligonucleotides or antisense RNA, generated by well- known methods, may be administered to patients by conventional drug delivery techniques; the antisense nucleic acids enter the appropriate cell type and hybridize with the endogenous target nucleic acid to inhibit franscription or translation of the target protein. Antisense mRNA may also be provided intracellularly to a patient by adminisfration of a gene therapy vector encoding an antisense RNA of interest. Expression of the antisense RNA may limited to a particular cell type, for example, by placing a cistron encoding the antisense RNA under the transcriptional regulation of a tissue-specific promoter. XIAP IRES antisense nucleic acids contain at least 10 consecutive nucleotides that are complementary to a XIAP IRES mRNA or DNA sequence, and preferably contain at least 14-18 consecutive nucleotides that are complementary to a XIAP IRES mRNA or DNA. XIAP IRES antisense nucleic acids may contain at least 25, 40, 60, 85, 120, or more consecutive nucleotides that are complementary to a XIAP IRES mRNA or DNA, and may be as long as a full-length XIAP IRES. Any region of the human XIAP IRES may be used as a target for antisense inhibition of XIAP translation, and particular sequences for XIAP IRES antisense nucleic acids may be selected by well-known approaches. For example, if desired, computer algorithms may be used to identify sequences that form the most stable hybridization duplexes. Computer algorithms may also be used to identify regions of the XIAP IRES that are relatively accessible within a folded mRNA molecule; antisense nucleic acids against such regions are more likely to effectively inhibit franslation of XIAP mRNA. For example, the sequence at -153 through -139 of the human XIAP IRES DNA sequence (5'-GTTTCTTAGCGGTCG-3'; SEQ ID NO: 7; see Fig. 4) is predicted to be accessible for hybridization within endogenous XIAP mRNA; therefore, an antisense nucleic acid that is complementary this sequence, e.g., 5'-CGACCGCTAAGAAAC-3' (SEQ ID NO: 8) or 5'-CGACCGCUAAGAAAC-3' (SEQ ID NO: 9) is useful for decreasing endogenous XIAP levels, thereby increasing the sensitivity of a target cell to an apoptotic stimulus. Computer algorithms that may be used to identify optimal XIAP IRES sequences for generating antisense nucleic acids include, but are not limited to, OLIGO 5.0 from National Biosciences Inc. (http://www.sxst.it/nbi olg.htm) and MFOLD (http://mfold2.wustl.edu/~mfold/rna/forml.cgi). References describing algorithms for predicting secondary structure are described in M. Zuker et al. "Algorithms and Thermodynamics for RNA Secondary Structure Prediction: A Practical Guide." in: RNA Biochemistry and Biotechnology. J. Barciszewski & B.F.C. Clark, eds., NATO ASI Series, Kluwer Academic Publishers (1999) and in Mathews et al. J. Mol. Biol. 288:911-940 (1999). In addition, specific functional regions of the XIAP IRES may be targeted for antisense therapy. For example, we have found that the polypyrimidine tract at position -46 through -35 (5'-TGTTCTCTTTTT-3'; SEQ ID NO: 5; see Fig. 4) of the human XIAP IRES DNA sequence is necessary for XIAP IRES-dependent translation. Therefore, antisense nucleic acids, e.g., 5'-AAAAAGAGAACA-3' (SEQ ID NO: 6) targeting this region of the XIAP IRES may be used to inhibit XIAP IRES-dependent franslation of endogenous XIAP. Intracellular XIAP IRES antisense nucleic acid prevents franscription and/or franslation of endogenous XIAP, thereby increasing a cell's susceptibility to apoptosis. Therefore, XIAP IRES antisense therapies are likely to prove useful in combination with traditional cancer therapy approaches, such as chemotherapy and radiation therapy, or in combination with other gene therapy approaches, e.g., expression of therapeutic anti-tumor proteins or other antisense tumor therapies. Therapy Nucleic acids of the invention and compounds identified using any of the methods disclosed herein may be administered to patients or experimental animals with a pharmaceutically-acceptable diluent, carrier, or excipient, in unit dosage form. Conventional pharmaceutical practice may be employed to provide suitable formulations or compositions to administer such compositions to patients or experimental animals. Although intravenous administration is preferred, any appropriate route of administration may be employed, for example, parenteral, subcutaneous, intramuscular, intracranial, intraorbital, ophthalmic, mfraventricular, intracapsular, intraspinal, infracisternal, intraperitoneal, infranasal, aerosol, or oral adminisfration. Therapeutic formulations may be in the form of liquid solutions or suspensions; for oral adminisfration, formulations may be in the form of tablets or capsules; and for infranasal formulations, in the form of powders, nasal drops, or aerosols. Methods well known in the art for making formulations are found in, for example, "Remington's Pharmaceutical Sciences." Formulations for parenteral administration may, for example, contain excipients, sterile water, or saline, polyalkylene glycols such as polyethylene glycol, oils of vegetable origin, or hydrogenated napthalenes. Biocompatible, biodegradable lactide polymer, lactide/glycolide copolymer, or polyoxyethylene-polyoxypropylene copolymers may be used to control the release of the compounds. Other potentially useful parenteral delivery systems for molecules of the invention include ethylene- vinyl acetate copolymer particles, osmotic pumps, implantable infusion systems, and liposomes. Formulations for inhalation may contain excipients, for example, lactose, or may be aqueous solutions containing, for example, polyoxyethylene-9-lauryl ether, glycocholate and deoxycholate, or may be oily solutions for adminisfration in the form of nasal drops, or as a gel. Test Compounds In general, novel drugs for modulation of XIAP IRES-dependent translation are identified from large libraries of both natural products or synthetic (or semi-synthetic) exfracts or chemical libraries according to methods known in the art. Those skilled in the field of drug discovery and development will understand that the precise source of test extracts or compounds is not critical to the screening procedure(s) of the invention. Accordingly, virtually any number of chemical exfracts or compounds can be screened using the exemplary methods described herein. Examples of such exfracts or compounds include, but are not limited to, plant-, fungal-, prokaryotic- or animal- based exfracts, fermentation broths, and synthetic compounds, as well as modification of existing compounds. Numerous methods are also available for generating random or directed synthesis (e.g., semi-synthesis or total synthesis) of any number of chemical compounds, including, but not limited to, saccharide-, lipid-, peptide-, and nucleic acid-based compounds. Synthetic compound libraries are commercially available from Brandon Associates (Merrimack, NH) and Aldrich Chemical (Milwaukee, WI). Alternatively, libraries of natural compounds in the form of bacterial, fungal, plant, and animal extracts are commercially available from a number of sources, including Biotics (Sussex, UK), Xenova (Slough, UK), Harbor Branch Oceangraphics Institute (Ft. Pierce, FL), and PharmaMar, U.S.A. (Cambridge, MA). In addition, natural and synthetically produced libraries are generated, if desired, according to methods known in the art, e.g., by standard extraction and fractionation methods. Furthermore, if desired, any library or compound is readily modified using standard chemical, physical, or biochemical methods. In addition, those skilled in the art of drug discovery and development readily understand that methods for dereplication (e.g., taxonomic dereplication, biological dereplication, and chemical dereplication, or any combination thereof) or the elimination of replicates or repeats of materials already known for their translation- modulatory activities should be employed whenever possible. When a crude extract is found to modulate (i.e., increase or decrease) XIAP IRES-dependent translation, further fractionation of the positive lead extract is necessary to isolate chemical constituents responsible for the observed effect. Thus, the goal of the extraction, fractionation, and purification process is the careful characterization and identification of a chemical entity within the crude extract having an activity that stimulates or inhibits XIAP IRES-dependent translation. The same assays described herein for the detection of activities in mixtures of compounds can be used to purify the active component and to test derivatives thereof. Methods of fractionation and purification of such heterogenous exfracts are known in the art. If

Claims: desired, compounds shown to be useful agents for treatment are chemically modified according to methods known in the art. Compounds identified as being of therapeutic value may be subsequently analyzed using mammalian models in which it is desirable to increase XIAP IRES-dependent franslation (for example, to increase XIAP levels in cells that are susceptible to apoptosis, such as cardiomyocytes of an animal prone to myocardial infarctions), or decrease XIAP IRES-dependent franslation (for example, to decrease XIAP levels in cancer cells, thus rendering them more susceptible to apoptosis induced by radiation or chemotherapeutic drugs). Below are examples of high-throughput systems useful for evaluating the efficacy of a molecule or compound for increasing or decreasing XIAP IRES- dependent translation. Primary screens for compounds that modulate IRES-dependent. cap-independent franslation. The presence of a XIAP IRES within a franscription unit enables the cap- independent expression of a downstream reporter cisfron. This finding allows us to provide assays for drugs that modulate XIAP IRES-dependent franslation. For example, the amount of cap-independent translation of a reporter cistron under the franslational regulation of a XIAP IRES may be determined from the amount of reporter protein (e.g., CAT, β-gal) activity encoded by the XIAP IRES-dependent reporter cistron. Relative franslation of the XIAP IRES-dependent reporter cisfron is measured in the presence and absence of a test compound, in comparison to a reference reporter cisfron that is not under the franslational confrol of a XIAP IRES. A reference reporter cistron may be (but is not necessarily) under the franslational confrol of a non-XIAP IRES, such as a VEGF IRES, c-myc IRES, FGF-2 IRES, or another IRES known in the art. Assays analogous to the one described above are readily adapted to high- throughput screens, such as those conducted in a 96-well microtiter plate or other high-throughput format, and reporter genes may be chosen specifically for their adaptability to a high-throughput format. For example, chemiluminescent assays for β-gal, CAT, and luciferase are commercially available and are easily used in high- throughput screening experiments, as reporter gene activity may be detected using a luminometer that accepts microtiter plates. Similarly, high-throughput reporter gene assays for GFP (green fluorescent protein) may be conducted using a fluorimeter with high-throughput capability. Screens can be performed using virtually any cell type, or extracts derived therefrom, subject to the particular demands of each assay. For example, an assay involving whole cells requires that reporter gene cistrons (DNA or mRNA) may be readily infroduced into the cells, e.g., by transfection or by microinjection. Selection of a particular type of cell to be used in a screening assay will also depend upon the ultimate goal of the assay. For example, cardiomyocyte extracts are useful for assays in which the goal is to identify compounds that modulate XIAP IRES activity in heart cells. And cells that undergo apoptosis in response to a given stimulus are useful for screening for a compound that stimulates IRES activity in cells on the verge of entering the apoptotic pathway. The best cell type for a particular screening assay will be apparent to one of skill in the art. Compounds that are found to modulate XIAP IRES activity may be subjected to secondary screens as outlined below. Secondary screens for compounds that modulate IRES-dependent. cap-independent translation. After test compounds that appear modulate IRES-dependent translation are identified, it may be necessary or desirable to subject these compounds to further testing. The invention provides such secondary confirmatory assays. For example, a compound that appears to modulate XIAP IRES-dependent translation can be tested for its effect on cap-independent franslational regulation by other IRES elements, such as the VEGF, c-myc, or FGF-2 IRES elements. A compound may preferentially modulate XIAP IRES-dependent translation, or instead, may have additional effects on cap-independent protein franslation in cells, by also modulating the activity of non-XIAP IRES elements. Such a compound may have the overall effect of inhibiting or stimulating apoptosis by modulating the franslation levels of a group of proteins that are under the franslational regulation of XIAP-like IRES elements. A compound that enhances XIAP IRES-dependent translation can be tested to determine whether the compound inhibits apoptosis in various cells under various conditions. There are many in vitro and in vivo apoptosis induction models known in the art. Such assays can be used for testing the anti-apoptotic potential of a compound that enhances XIAP IRES-dependent franslation. For example, cultured cells treated with a test compound can be tested for their relative resistance to apoptosis induced by hypoxia, growth factor withdrawal, or the addition of chemicals or cytokines (such as TNF). At later stages of testing, animals treated with a test compound can be tested for their resistance to tissue damage induced by myocardial infarction or stroke. A potential anti-apoptotic compound can be tested in animal models for other uses as well, such as for enhancing fertility or decreasing hair loss. Likewise, a compound that decreases XIAP IRES-dependent franslation may be useful in stimulating apoptosis of undesirable cells, such as cancer cells or excess adipocytes. A compound that is found to have apoptosis-stimulatory properties may be useful as a cancer treatment, either alone, or in conjunction with other cancer therapies. Additional uses for compounds that modulate XIAP IRES-dependent franslation will be apparent from the Gene Therapy section. A therapeutically useful compound is administered by one of the means described in the Therapy section. The following examples are to illustrate the invention. They are not meant to limit the invention in any way. Example I: General Methods Construction of bicistronic expression plasmids. The basic bicisfronic vector pβgal/CAT was constructed by inserting the β-galactosidase gene (Notl fragment) from plasmid pCMVβ (CLONTECH, Palo Alto, CA) and the chloramphenicol acetylfransferase gene (Xbal-BamHI fragment) from plasmid pCATbasic (Promega, Madison, WI) into the linker region of plasmid pcDNA3 (Invifrogen, Carlsbad, CA). The two cistrons are separated by a 100 bp intercisfronic linker region containing a unique Xhol site. The expression of bi-cisfronic mRNA is driven by a CMV promoter. The expression plasmid pCI-IRES/XIAP was constructed by inserting the 1 kb 5' UTR region of XIAP upstream of the XIAP coding region in the plasmid pCI (Invifrogen, Carlsbad, CA). The XIAP 5' UTR elements of human and mouse XIAP were obtained by RT-PCR using human and mouse fetal liver Marathon-Ready cDNAs (CLONTECH, Palo Alto, CA) and XIAP primers containing an Xhol site. 5' UTR clones were inserted into the Xhol site of the intercisfronic linker region of plasmid pβgal/CAT. The orientation and sequence of the 5' UTR fragments were confirmed by sequencing. The promoterless CAT reporter plasmid pCATbasic/UTR was constructed by inserting the -1007 to -1 human XIAP 5' UTR into the pCATbasic vector (Promega, Madison, WI) upstream of the CAT gene, as indicated in Fig. 1A. Cell culture and transient DNA transfections. NIH 3T3 and HeLa cells were cultivated in Dulbecco modified Eagle medium (DMEM) supplemented with 10% fetal calf serum (FCS) and antibiotics. Transient DNA transfections were done using Lipofectamine (GIBCO BRL, Gaithersburg, MD) according to the procedure recommended by the manufacturer. Briefly, cells were seeded at a density of 1 x 10 5 per 35 mm well and transfected 24 hours later in serum-free OPTI-MEM medium with 2 μg of DNA and 10 μl of Lipofectamine per well. The transfection mixture was replaced 4 hours later with DMEM supplemented with 10% FCS. For serum deprivation experiments, the cells were washed with PBS 24 hours post-transfection and then cultured inserum-free DMEM. Cells extracts were prepared 24 hours post-fransfection and β-gal and CAT activities were measured. β-gal and CAT analysis. Transiently transfected cells were harvested in PBS 24 hr post-fransfection and cell extracts were prepared by the freeze-thaw method as described (MacGregor, G.R., et al., "Gene Transfer and Expression Protocols," pp. 217-235. In: Methods in Molecular Biology, Vol. 7, E. J. Murray and J. M. Walker (eds.), Humana Press Inc., Clifton, N.J., 1991). β-gal enzymatic activity in cell exfracts was determined by spectrophotometric assay using ONPG (MacGregor et al., supra). CAT activity was determined by liquid scintillation method as described (Seed, B. and Sheen, J.Y., Gene, 67:271-7, 1998). Cell death assays. HeLa cells were seeded at a density of 6x10 4 cells per 1 cm well and transfected 24 hours later as described above. Cells were washed with serum- free DMEM 24 hours post-fransfection and were subsequently grown in serum- free DMEM. Cell viability was assessed at various time intervals using a colorimetric assay that measures cleavage of the tefrazolium salt WST-1 (Boehringer Mannheim, Indianapolis, IN) by mitochondrial dehydrogenases in viable cells. Assays were performed according to the procedure recommended by the supplier. The fraction of surviving cells was calculated from three separate experiments performed in triplicate. RNA isolation and Northern blot analysis. Total RNA was prepared by guanidine isothiocyanate/phenol-chloroform extraction method using the TRIZOL reagent (GIBCO BRL, Gaithersburg, MD) according to the procedure recommended by the supplier. RNA was denatured in formamide and separated on a 0.8% agarose gel. RNA was then transferred onto a Biodyne nylon membrane (GIBCO BRL, Gaithersburg, MD) and was hybridized with CAT or lacZ DNA probes labeled with 32 P using a Rediprime random primer labeling kit (Amersham, Malvern, PA). Membranes were exposed to X-ray film (Kodak, Rochester, NY) overnight using an intensifying screen (Amersham, Malvern, PA). Example II: The 5' UTR of XIAP mRNA mediates franslation of the second open reading frame in bi-cisfronic mRNAs. The 5' UTRs of both human and murine XIAP mRNAs are unusually long (> 1.6 kb and >5.5 kb respectively). Both contain a polypyrimidine tract about 30 nucleotides (nt) upsfream of the initiation AUG codon and contain numerous upstream AUG codons. To test whether regions in the 5' UTR could initiate franslation by an internal ribosome entry mechanism, we constructed bi-cisfronic mRNA transcripts (Fig. 1 A) similar to those reported elsewhere (Pelletier, J., et al., Nature, 334:320-5, 1988). The vector pβgal/CAT directs transcription of bi-cisfronic mRNA in which the first cisfron, encoding β galactosidase (β-gal), is translated by a conventional cap-dependent mechanism. The second cistron encoding chloramphenicol acetylfransferase (CAT), however, can be translated only if the preceding linker region contains an internal ribosome entry site (IRES). Plasmid constructs containing 1 kb of human or 1.4 kb of mouse XIAP 5' UTR sequence (Fig. 1 A) were transfected into HeLa and NIH 3T3 cell lines, and the expression of both CAT and β-gal reporter genes was monitored. Briefly, DNA segments corresponding to indicated regions of the 5' UTR of human (h) or mouse (m) XIAP transcript were inserted into the Xhol site of the linker region (LR) of the bi-cisfronic plasmid pβgal/CAT. HeLa cells were transfected with the plasmids shown in Fig. 1 A using Lipofectamine, and after 24 hours, cell exfracts were prepared and β- galactosidase and CAT activities were determined (Fig. IB). All constructs produced comparable amounts of β-gal activity, which was then used as in internal confrol to normalize for transfection efficiency of different plasmids. The translation of the second cistron was then determined by measuring CAT activity. The bars represent the average ±SD of five independent transfections. Constructs containing the 5' UTR of either human or mouse XIAP mRNA directed translation of CAT reporter mRNA in both cell lines at 150-fold higher levels than constructs lacking the 5' UTR, or having the 5' UTR in the reverse orientation (Fig. IB). Identical results were obtained from transfection of NIH 3T3 cells. To eliminate the possibility that CAT translation was enhanced because the 5' UTR regions contained cryptic promoters, both 5' UTRs were cloned into the promoter-less CAT reporter plasmid pCATbasic (Promega, Madison, WI) and CAT activity was assayed; no CAT activity was detected in either case. We conclude that RNA sequences in the 5' UTR of the human and murine XIAP genes are capable of directing cap-independent translation of a downstream mRNA. Example III: Translation directed by the XIAP 5' UTR element is not inhibited by poliovirus protease 2A. Following poliovirus infection, a rapid inhibition of host-cell protein synthesis is observed. This inhibition is mediated largely by the expression of viral protease 2A, which cleaves the eIF4G subunit of the cap-binding initiation factor eIF4F complex. Although cap-dependent cellular protein synthesis is inhibited, viral proteins are efficiently synthesized by an IRES-directed, cap-independent franslation initiation. We tested whether expression of the 2 A protease would affect franslation initiation directed by the XIAP 5' UTR by co-expressing a bi-cisfronic expression plasmid containing 1 kb of human XIAP 5' UTR (Fig 2) with an expression plasmid encoding 2 A protease in HeLa cells. HeLa cells were co-fransfected with plasmids pβgal/hUTR/CAT (2μg) and ρCMV-2A (2 μg) or the confrol plasmid pcDNA3 (2 μg) using 10 μl of Lipofectamine, and β-galactosidase and CAT activities were determined 48 hours post-fransfection. Expression of each reporter cisfron assayed in control transfections was set at 100%. In the presence of 2A protease, translation of the first cisfron, β-galactosidase, was reduced to 33% of control levels, whereas translation of the second, XIAP 5' UTR-directed CAT cisfron remained unchanged (Fig. 3). These results indicate that the XIAP 5' UTR mediates true cap-independent translation, which is independent of the presence of intact eIF4G. This result confirms the hypothesis that the 5' UTR contains an internal ribosome binding site (IRES), as opposed to mediating franslation by splicing or ribosome reinitiation, both of which are inhibited by protease 2A expression. Example IV: The XIAP IRES element is within a 265 nt fragment of conserved 5' UTR. We noted that although the homology of human and mouse XIAP genes within the coding region is 87%, the homology outside the coding region is confined to the region extending 270 nt upstream of the initiation codon. Fig. 4 shows a sequence comparison of the 5' UTR of mouse and human XIAP immediately upstream of the initiation AUG codon (underlined; position +1). The critical polypyrimidine tract is boxed, and deletions made to define the boundaries of the IRES element are indicated by arrows. Numbering is relative to the initiation codon (AUG). To determine the portion of the XIAP 5' UTR responsible for franslation initiation, we generated constructs containing defined deletions of the human XIAP 5' UTR. DNA segments corresponding to indicated regions of the XIAP 5' UTR were cloned into the linker region (LR) of the bi-cisfronic reporter plasmid pβ-gal/CAT (Fig. 5A; black box indicates the polypyrimidine tract) and tested their ability to initiate franslation of the second cistron, represented by relative CAT activity (Fig. 5B). Plasmids with mutated PPT were constructed by PCR-directed mutagenesis. The RNA sequence of the PPT in each construct is indicated in Fig. 5 A and base substitutions are underlined (SEQ ID NOs: 10-18). Monocistronic plasmids were constructed by deleting the βgal cisfron (cloned into the Not I restriction site) from respective plasmids. Briefly, HeLa cells were Lipofectamine-fransfected with the plasmids shown in Fig. 5A. After 24 hours, cell extracts were prepared and β-galactosidase and CAT activities were determined. Relative CAT activity was calculated by normalizing with β-gal activity. The bars represent the average ±SD of three independent transfections. To determine which part of the 5' UTR of XIAP mRNA is responsible for translation initiation, constructs containing defined deletions of the human XIAP 5' UTR were generated (Fig. 5 A). The region that retained full IRES activity was the -162 to -1 nt segment upstream of the initiation codon which was as effective as the larger 5' UTR. The smallest construct that we made contained only 83 nt of the 5' UTR. While this construct had only 25% activity of the full length XIAP UTR, this activity was still 30-fold higher than the bi-cisfronic mRNA containing no IRES. In monocisfronic plasmids, the presence of a 5' UTR in the sense orientation did not reduce franslation of the reporter gene, whereas in the antisense orientation, or with the mutation in the IRES (see below) franslation was substantially reduced, implying that the translation of XIAP is fully dependent on the IRES (Fig. 5B, bottom). There is a polypyrimidine tract (PPT) located 34 nt (i.e., from nt -35 through nt -46; see Fig. 4) upsfream of the XIAP initiation codon. With the exception of the IRES element of eIF4G, this sequence is not present in any cellular IRES described so far. Therefore, we wished to determine if the PPT is important for XIAP IRES function. While the sequence between the initiation codon and the PPT is dispensable for XIAP IRES function, deletion of the PPT abolished IRES activity completely (Fig. 5B). Specifically, pβgal/3'(-34)/CAT, which contains -1007 through -35 of the human XIAP IRES, maintains full IRES activity (Figs. 5A and 5B), as does a construct containing -268 through -35 of the human XIAP IRES. A construct containing -162 through -35 of the human XIAP IRES shows significant, but less that 100%, IRES activity. We next deleted the -162 to -47 nt segment from the 5' UTR to test whether the PPT itself is sufficient for directing translation in bi-cistronic constructs. The -162 to -47 nt deletion, however, completely abolished IRES activity demonstrating that while the PPT sequence is necessary, it is not sufficient for internal ribosome entry. To determine sequence specificity of the PPT we constructed several base substitution mutants (Fig. 5B). The substitution of pyrimi dines for purines drastically reduced IRES activity indicating the need for the PPT. When the PPT of XIAP IRES was replaced with the PPT of eIF4G IRES element, IRES activity was reduced to less than 2%. Base substitutions in every position in the PPT completely abolished IRES activity, except in positions -46 and -45 which retained some activity (38% of wild type sequence). These results suggest that a specific sequence within the PPT is critical for XIAP IRES activity. This specificity is in marked contrast to the IRES of eIF4G, the only other cellular IRES with polypyrimidine tract, where the wild-type sequence of the PPT is not critical. Example V: The XIAP IRES element mediates franslation during serum starvation. Translation of capped mRNAs is known to be inhibited following growth arrest, heat shock or during mitosis. In some cases, growth arrest induced by serum deprivation is followed by induction of apoptosis. We have shown previously that XIAP is a potent inhibitor of apoptosis induced by variety of signals including serum deprivation (Liston, et al., Nature, 379:349-353, 1996). We tested whether the XIAP IRES element could direct franslation during the onset of apoptosis triggered by serum deprivation. A bi-cisfronic reporter construct (pβgal/hlRES 265/CAT; Fig. 6) containing a 265 nt human IRES element upstream from the CAT (downstream) cisfron was Lipofectamine-transfected into HeLa cells. After 24 hr of serum deprivation the relative levels of translation of both cistrons was measured by reporter gene assay. Expression of each reporter cisfron assayed in control transfections was set at 100%. The experiment was carried out three times, with <10% variation between transfections. Translation of the first cistron was reduced to 58% of control (non-starved cells), whereas translation of the second, IRES-directed cistron remained unchanged (Fig. 7). This indicates that the XIAP IRES element enhances franslation in cells under environmental stress, such as serum deprivation. Example VI: XIAP-mediated inhibition of apoptosis is enhanced by the XIAP IRES element. We and others have demonstrated that the overexpression of XIAP protects cells against apoptosis triggered by various stimuli (Liston, P., et al., Nature, 379:349-53, 1996; Uren, A.G., et al., Proc. Natl. Acad. Sci. USA, 93:4974-8, 1996). In these experiments, however, only the coding region of the XIAP transcript was used. If the franslation of XIAP is mediated by the IRES element located within its 5' UTR, then overexpression of a transcript containing the XIAP IRES should offer increased protection following an apoptotic trigger relative to the protection conferred by the XIAP coding region alone. To test this hypothesis, we transfected HeLa cells with either an expression plasmid containing the XIAP coding region plus 1 kb of 5' UTR, or a similar construct lacking the 5' UTR region, and tested the ability of both constructs to suppress apoptosis triggered by serum starvation. HeLa cells were Lipofectamine-fransfected with expression plasmid pCI-lacZ, pCI-XIAP or pCI-IRES/XIAP. After 24 hours, the cells were washed with PBS and cultured in fresh serum- free medium. Cell viability at various time intervals was assessed by colorimetric assay using the WST-1 reagent (Boehringer Mannheim). The fraction of surviving cells was calculated from three separate experiment performed in triplicate. At all time points following transfection, IRES-XIAP protected cells from apoptosis more efficiently than did the XIAP coding region alone (Fig. 8). These results clearly demonstrate that translation of XIAP is mediated by internal ribosome entry and that this mechanism is critical for XIAP function in vivo. Example VII: The XIAP IRES element mediates translation in cells exposed to ionizing radiation and apoptotic stress. Our research has focused upon the physiological relevance of IRES-mediated XIAP franslation under different paradigms of cellular stress. First, to determine whether endogenous XIAP IRES-directed translation is affected by cellular sfress, we exposed cells of the non-small cell lung carcinoma cell line H661 to low-dose gamma irradiation and analyzed the resulting XIAP mRNA and protein levels by Northern and Western blot analysis. Changes in XIAP levels were analyzed densitometrically and normalized for levels of GAPDH (mRNA) or total protein loaded (average ± SD of three experiments; Fig. 9 inset shows representative blots). We observed that the low dose irradiation resulted in a 3.5 fold up-regulation of XIAP protein in the non-small cell lung carcinoma cell line H661, whereas the level of XIAP mRNA remained unchanged (Fig. 9). The expression pattern of other IAP genes (HIAP1, HIAP2, and NAIP) did not change. We hypothesized that the increase in XIAP protein observed after low-dose irradiation resulted from up-regulation of XIAP IRES-dependent franslation. To test this hypothesis, we transfected H661 cells with the bi-cisfronic XIAP IRES construct pβgal5'(268)/CAT (5 μg) using the SuperFect Transfection Reagent (30 μl; QIAGEN, Los Angeles, CA), incubated the cells for 12 hours after the transfection procedure, irradiated the cell with 60 Co gamma rays at dose rate of approximately 1.5 Gy/min, and measured the relative translation of both cistrons at 12 hours post-irradiation. While the expression of the cap-dependent β-galactosidase was reduced to 51% of non-irradiated cells, the expression of the IRES-driven CAT reporter was increased (Fig. 10, right), supporting the hypothesis that the up-regulation of endogenous XIAP protein observed in irradiated cells is mediated by the IRES sequence. Other cellular stresses such as poliovirus infection, growth arrest, hypoxia or heat shock are also known to inhibit cap-dependent, but not IRES-dependent, translation. We asked whether the XIAP IRES element is resistant to protein synthesis shut-off triggered by an overexpression of poliovirus protease 2 A or serum withdrawal (Fig. 10, left). For the protease 2A overexpression experiment, HeLa cells were co-transfected with plasmids pβgal/5'(-268)/CAT (2 μg) and pCMV-2A (2 μg) or the control plasmid pcDNA3 (2 μg) as described in Example I, and β-galactosidase and CAT activities were determined 48 hours post-fransfection. For the serum deprivation experiment, HeLa cells were transfected with the pβgal/5'(-268)/CAT (2 μg), the transfected cells were deprived of serum for 24 hours, and the β-galactosidase and CAT activities were determined. Expression of each reporter cisfron assayed in the confrol transfection (Fig. 10, far left) was set at 100%. In both cases the translation of the cap-dependent reporter was reduced, whereas the translation of the XIAP IRES-driven reporter remained unchanged (Fig. 10, left; bars represent the average ± SD of three independent transfections). Overexpression of XIAP has been clearly shown to protect cells against apoptosis triggered by various stimuli including serum withdrawal. In these experiments, however, only the coding region of the xiap transcript was used. We then reasoned that if the translation of XIAP is mediated by the IRES element located within its 5' UTR, the overexpression of a XIAP transcript containing the XIAP IRES should offer greater protection following an apoptotic trigger than that seen with a transcript containing the XIAP coding region alone. To test this hypothesis, HeLa cells were transfected with either pCI-XIAP, pCI-IRES/XIAP, or pCI-lacZ (negative control) as described in Example I. After 24 hours, the cells were washed with PBS and grown in fresh serum- free medium. The viability of the cells at 0, 24, 48, and 72 hours of serum deprivation was assessed by a colorimetric assay that measures cleavage of the tefrazolium salt WST-1. At all time points following transfection and serum starvation of HeLa cells, the IRES-XIAP construct protected cells from apoptosis more efficiently than did the XIAP coding region alone (Fig. 11 ; bars represent the fraction of surviving cells ± SD of three independent experiments performed in triplicate). The inset at the top of Fig. 11 shows a representative Western blot demonstrating the relative XIAP protein levels in cells transfected with the various constructs (10 μg of total protein were loaded per lane). By 72 hours of serum starvation it was clear that the amount of XIAP protein produced from the IRES-XIAP construct exceeded that produced from the construct containing the XIAP coding region alone, although the levels of xiap mRNA transcribed from both plasmids were the same. In order to initiate studies addressing the biological significance of the internal initiation of XIAP, we tested several cell lines for their ability to franslationally up- regulate XIAP in response to cellular stress. Our results show that IRES-mediated franslational up-regulation of endogenous XIAP without concomitant upregulation of XIAP mRNA varies in response to different triggers of cellular sfress in different cell lines (Table 2). While serum deprivation upregulated XIAP protein in all cell lines tested (HeLa, HEL, 293, SHY5Y), low dose irradiation induced XIAP translation only in one (H661), possibly two (H520) cell lines. Notably the H661 cell line is most resistant of the four cell lines to radiation-induced apoptosis. A general response of the cell to apoptotic stress is the shut-off of cap-dependent protein synthesis. The presence of IRES would, therefore, allow for the continuous production of XIAP, even under conditions of sfress. The degree of responsiveness of the IRES element would dictate the cellular threshold to apoptotic stimuli. This threshold level would be set according to the intrinsic properties of the cell or could be manipulated by the external stimulation of IRES-mediated XIAP translation. In either case, the induction of XIAP protein would be of potential benefit in the survival of the cell under acute but transient apoptotic conditions. Table 2. Relative levels of XIAP protein and mRNA in response to cellular stress. Relative levels of XIAP* Cell line Stress trigger Protein mRNA H661 (non-small cell carcinoma, lung) 1.0 Gy 3.33 ± 0.05 0.85 ± 0.07 H460 (non-small cell carcinoma, lung) 1.0 Gy 0.34 ± 0.14 0.76 ± 0.11 H520 (squamous carcinoma, lung) 1.0 Gy 1.41 ± 0.25 0.76 ± 0.06 SKOV3 (adenocarcinoma, ovary) 1.0 Gy 0.73 ± 0.07 0.83 ± 0.19 HeLa (cervical carcinoma) 72 hr SW 2.03 ± 0.10 0.91 ± 0.07 293 (human embryonic kidney) 72 hr SW 2.17 ± 0.32 0.80 ± 0.16 HEL (human embryonic lung) 72 hr SW 1.95 ± 0.04 0.83 ± 0.09 SHY5Y (neuroblastoma) 72 hr SW 1.75 ± 0.32 0.78 ± 0.16 * endogenous levels of XIAP protein (Western blot) and mRNA (Northern blot or Ribonuclease Protection Assay) were analyzed following either 1.0 Gy ionizing irradiation, or serum withdrawal (SW) for 72 hours. Changes in XIAP levels were analyzed densitometrically, normalized for levels of GAPDH (mRNA) or total protein loaded and compared to untreated controls which were set as 1 (average ± SD). Example VIII: The La autoantigen binds the XIAP IRES element and is necessary for XIAP franslation. As described above in the previous Examples, we have shown that the translation of XIAP is mediated by an internal ribosome binding mechanism involving an IRES element that is located within the -1 to -162 region (relative to the franslation start site) of the XIAP 5' UTR (Holcik et al, Nature Cell Biol, 1 :190-192, 1999). Our studies demonstrated that the presence of the IRES element within XIAP mRNA allows XIAP protein to be selectively franslated under conditions that repress cap- dependent franslation. It appears that this mechanism is exploited by cancer cells to survive under conditions of physiological sfress, such as hypoxia or therapy-induced damage, which would otherwise induce apoptosis. Thus, inhibition of IRES translation could be used as a therapeutic approach for inhibiting oncogenesis. Prior to our work, there has been no available information regarding proteins that are necessary for IRES-dependent translation of cellular mRNAs. Using established methods such as elecfrophoretic mobility shift assays (EMSA), Northwestern analysis, and UV-crosslinking (see, e.g., Ausubel et al., Current Protocols in Molecular Biology, John Wiley and Sons, Inc., New York, NY, 1998; Sambrook et al., Molecular Cloning: A Laboratory Manual, 2 nd Ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, 1989; or Chen et al., Biochem. Biophys. Res. Comm., 191 :18-25, 1993), we now show that a set of cytoplasmic proteins forms a specific ribonucleoprotein (RNP) complex on the XIAP IRES located between -1 and -162 of the XIAP 5' UTR (denoted fragment "A" in Fig. 12); in contrast, no RNP complex is formed on a control mRNA fragment consisting of the - 162 to -268 region of the XIAP 5' UTR (denoted fragment "B" in Fig. 12). Using UV- crosslinking combined with immunoprecipitation we have also identified one component of the XIAP-IRES RNP complex as La autoantigen (Fig. 12). La (also known as SS-B or hnRNPI; see, e.g., Chambers and Keene, Proc. Natl. Acad. Sci. USA 82:2115-2119, 1985 and Chambers et al., J. Biol Chem. 263:18043- 18051, 1998), an RNA binding protein involved in RNA polymerase III franscription termination, has been shown to bind the poliovirus IRES elements (Ehrenfeld, in: Translational Control, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, pp. 549-573, 1996). However, the involvement of La in the cap-independent franslation of cellular mRNAs containing IRES elements has not been reported. La is one of three cellular proteins (the other two being PCBP and PTB) that have been shown to be necessary for franslation of viral mRNAs containing an IRES (Blyn et al, Proc Natl Acad Sci USA, 93: 11115-11120, 1996; Blyn et al, J Viwl 71:6243-6246, 1997; Hellen et al, Proc Natl Acad Sci USA 90:7642-7646, 1993; Meerovitch et al, J Virol, 67:3798-3807, 1993). In addition to La, our Northwestern blotting and UV- crossing experiments have shown that at least three additional cellular proteins bind to the XIAP IRES (Fig. 12), the identity of which remains to be determined. Based on the apparent molecular weights of these proteins, it seems unlikely that they are either PCBP or PTB. We have established that a dominant-negative variant of La interferes with the translation of XIAP both in vitro and in vivo. Fig. 13 A is a graph showing the relative β-galactosidase and CAT activities resulting from in vitro transcription/translation (using a TNT® T7 Quick Coupled Transcription/Translation System containing rabbit reticulocyte lysate, according to the manufacturer's instructions) of the dicisfronic construct pβgal/IRES/CAT in the absence (confrol) or presence (+La DN ) of purified recombinant dominant-negative La (consisting of the dimerization domain, amino acids 226-348, of full length La, as described in Craig et al. (Mol. Cell Biol 17:163- 169, 1997). Fig. 13A shows that in vitro franslation of CAT, which is under the franslational regulation of the XIAP IRES, is markedly inhibited by dominant- negative La. Fig. 13B is a graph showing the relative β-galactosidase and CAT activities in HeLa cells co-franfected with the dicisfronic construct pβgal/IRES/CAT plus either a control expression plasmid (pCI) or an expression plasmid encoding dominant- negative La (pCI-La DN ). Fig. 13B shows that the overexpression of dominant-negative La down-regulates franslation of the XIAP IRES-linked CAT reporter mRNA. The results shown in Figs. 13A-13B demonstrate that the IRES-driven franslation of XIAP can be regulated by trans-acting factors. Therefore, it is likely that the La autoantigen and other XIAP IRES binding proteins may be therapeutically targeted to modulate levels of XIAP protein. For example, inhibiting the interaction between the endogenous XIAP IRES and La (e.g., by decreasing the level of La protein or the binding activity of La) would result in decreased intracellular levels of XIAP; such cells should be more susceptible to apoptotic stimuli. This strategy could be used, either alone or in combination with other therapies (e.g., radiation, chemotherapy, and surgery), to treat cancer. Conversely, increasing the level of La protein or the ability of La to bind the XIAP IRES would increase the intracellular XIAP level, thereby decreasing the susceptibility of a cell to apoptotic stimuli by increasing the franslation of XIAP. This therapeutic strategy may be used in the treatment of degenerative diseases or to minimize cell death in situations in which cells are exposed to physiological stressors that stimulate apoptosis. Detection of the interaction between the XIAP IRES with La or other proteins may form the basis of high- throughput screens for identifying therapeutic compounds that modulate XIAP IRES-dependent translation of XIAP. Such compounds may be used to modulate the relative apoptotic susceptibility of a cell. Any appropriate screen that is known in the art may be used. In one example, an RNA molecule containing a XIAP IRES is immobilized on a solid surface (e.g., a filter, a bead, or the inside of a microtiter well). A sample containing La or any other protein that binds the XIAP IRES, and is involved in positively regulating XIAP IRES activity, is allowed to contact the XIAP IRES, and a test compound is added. After washing away non-specifically bound protein, the relative amount of protein bound to the XIAP IRES is then measured by any known method. For example, if the protein is radiolabeled, the amount of radioactivity adhering to the XIAP IRES may be determined. Alternatively, an antibody that specifically recognizes the protein (e.g., La) may be used to label the amount of bound protein in order to determine the relative level of protein binding to the XIAP IRES in the presence and absence of the test compound. For a protein that binds to the XIAP IRES and positively regulates XIAP IRES activity (e.g., increases XIAP franslation), an increase in binding indicates a compound that increases XIAP IRES activity; such a compound may be useful for decreasing a cell's susceptibility to apoptosis. Conversely, a decrease in binding indicates a compound that decreases XIAP IRES activity; such a compound may be useful for increasing a cell's susceptibility to apoptosis. The reverse is true of a protein that binds to a XIAP IRES and negatively regulates XIAP IRES activity (e.g., decreases XIAP franslation). Numerous variations of the above-described assay, which will be apparent to those of ordinary skill in the art, may be employed to discover compounds that modulate XIAP IRES activity. For example, instead of immobilizing the XIAP IRES on a solid support, a XIAP IRES-binding protein (such as La) may be immobilized and the relative amount of bound XIAP IRES (e.g., radioactively- or fluorescently- labeled) may be measured. Other Embodiments All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each independent publication or patent application was specifically and individually indicated to be incorporated by reference. While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure come within known or customary practice within the art to which the invention pertains and may be applied to the essential features hereinbefore set forth, and follows in the scope of the appended claims. What is claimed is: 1. A method for identifying a compound that modulates protein franslation, said method comprising: a) providing a XIAP IRES reporter cistron; b) exposing said XIAP IRES reporter cisfron to a test compound; c) determining the amount of franslation from said XIAP IRES reporter cistron exposed to said compound, relative to the amount of translation from said XIAP IRES reporter cisfron not exposed to said compound, wherein a relative increase in franslation from said XIAP IRES reporter cisfron exposed to said compound indicates a compound that increases XIAP IRES-dependent protein translation, and wherein a relative decrease in translation from said XIAP IRES reporter cisfron exposed to said compound indicates a compound that decreases XIAP IRES-dependent protein translation. 2. The method of claim 1, wherein said XIAP IRES reporter cistron is exposed to a cell exfract prior to being exposed to said test compound, said cell exfract being capable of translating said XIAP IRES reporter cisfron. 3. The method of claim 1, wherein said XIAP IRES reporter cisfron is exposed to a cell extract after being exposed to said test compound, said cell exfract being capable of translating said XIAP IRES reporter cistron. 4. The method of claim 1, further comprising providing an internal confrol reporter cisfron, wherein: a) the amount of translation from said XIAP IRES reporter cisfron exposed to said compound is normalized relative to the amount of translation of said internal confrol reporter cistron exposed to said compound, and b) the amount of franslation from said XIAP IRES reporter cistron not exposed to said compound is normalized relative to the amount of translation of said internal control reporter cisfron not exposed to said compound, and c) the amount of normalized translation from said XIAP IRES reporter cistron exposed to said compound, relative to the amount of normalized franslation from said XIAP IRES reporter cisfron not exposed to said compound is determined, wherein a relative increase in normalized franslation from said XIAP IRES reporter cistron exposed to said compound indicates a compound that increases XIAP IRES-dependent protein translation, and wherein a relative decrease in translation from said XIAP IRES reporter cisfron exposed to said compound indicates a compound that decreases XIAP IRES-dependent protein translation. 5. The method of claim 1, wherein said XIAP IRES reporter cisfron is within a cell, and wherein said cell is exposed to said test compound. 6. The method of claim 5, wherein said cell further comprises an internal confrol cisfron, and wherein: a) the amount of franslation from said reporter cisfron in said cell exposed to said compound is normalized relative to the amount of translation of said internal confrol cistron in said cell exposed to said compound, and b) the amount of franslation from said reporter cisfron in said cell not exposed to said compound is normalized relative to the amount of franslation of said internal confrol cisfron in said cell not exposed to said compound, and c) the amount of normalized franslation from said reporter cistron in said cell exposed to said compound, relative to the amount of normalized franslation from said reporter cistron in said cell not exposed to said compound is determined, wherein a relative increase in normalized franslation from said reporter cistron in said cell exposed to said compound indicates a compound that increases XIAP IRES-dependent protein franslation, and wherein a relative decrease in translation from said reporter cistron in said cell exposed to said compound indicates a compound that decreases XIAP IRES-dependent protein franslation. 7. A method for identifying a compound that modulates protein franslation, said method comprising: a) providing at least two reporter cistrons, wherein said reporter cistrons comprise an internal control reporter cistron and a XIAP IRES reporter cistron; b) exposing said internal confrol reporter cisfron and said XIAP IRES reporter cisfron to said compound; c) determining the amount of franslation from said internal control reporter cisfron and said XIAP IRES reporter cisfron; d) calculating the franslation clsXI/cιs ι C ; e) comparing translation clsXI/clsIC in a sample exposed to said compound to franslation clsXI cιsIC in a sample not exposed to said compound, wherein an increase in franslation clsXI cιsIC indicates a compound that increases XIAP IRES-dependent translation and wherein a decrease in translation clsXI/cιsIC indicates a compound that decreases XIAP IRES-dependent translation. 8. The method of claim 7, wherein said reporter cistrons are exposed to a cell extract prior to being exposed to said test compound, said cell exfract being capable of translating said XIAP IRES reporter cisfron. 9. The method of claim 7, wherein said reporter cistrons are exposed to a cell extract after being exposed to said test compound, said cell exfract being capable of translating said XIAP IRES reporter cisfron. 10. The method of claim 7, wherein said reporter cistrons are within a cell, and wherein said cell is exposed to said test compound. 11. The method of claim 7, wherein said reporter cistrons comprise a single franscription unit. 12. The method of claim 7, wherein said internal control reporter cisfron is located upsfream from said XIAP IRES reporter cistron. 13. The method of claim 7, wherein said method is for identifying a compound that decreases XIAP IRES-dependent franslation, wherein franslation clsXI/clsIC in a cell exposed to said compound is decreased relative to franslation clsXI/clsIC in a cell not exposed to said compound. 14. The method of claim 13, wherein said method is for identifying a compound useful for freating cancer. 15. The method of claim 7, wherein said method is for identifying a compound that increases XIAP IRES-dependent franslation, wherein franslation clsXI/clsIC in a cell exposed to said compound is increased relative to franslation CIsXI/cιsIC in a cell not exposed to said compound. 16. The method of claim 7, further comprising a non-XIAP IRES reporter cisfron, wherein said non-XIAP IRES reporter cisfron is under the franslational regulation of an IRES that is not a XIAP IRES. 17. The method of claim 16, wherein said IRES that is not a XIAP IRES is a VEGF IRES. 18. The method of claim 16, wherein said reporter cistrons comprise a single transcription unit and wherein said internal confrol reporter gene is located upstream from said XIAP IRES reporter cisfron and said non-XIAP IRES reporter cisfron. 19. The method of claim 16, wherein said method further comprises: f) calculating the franslation clsNX/clsIC ; g) comparing translation clsN /cιsIC in a sample exposed to said compound to translation clsNX/cιslc in a sample not exposed to said compound, wherein an increase in franslation clsNX/clsIC indicates a compound that increases non-XIAP IRES-dependent franslation and wherein a decrease in franslation clsNX/cιsIC indicates a compound that decreases non-XIAP IRES-dependent franslation. 20. The method of claim 19, wherein said method is for identifying a compound for treating cancer, wherein: a) franslation clsXI/CISIC in a sample exposed to said compound is decreased relative to franslation clsXI cιsIC in a cell not exposed to said compound, and b) translation clsNX/cιsIC in a sample exposed to said compound is decreased relative to franslation clsNX/cιsIC in a sample not exposed to said compound, wherein said compound is useful for treating cancer. 21. The method of claim 19, wherein said method is for identifying a compound that inhibits apoptosis, wherein: a) franslation C]sX1 clsIC in a sample exposed to said compound is increased relative to franslation clsXI cιsIC in a sample not exposed to said compound, and b) franslation clsNX/clsIC in a sample exposed to said compound is increased relative to franslation clsNX/cιsIC in a sample not exposed to said compound, wherein said compound is useful for inhibiting apoptosis in a cell in need thereof. 22. A method for stimulating apoptosis in a cell, said method comprising infroducing into said cell a purified nucleic acid encoding a polypeptide that stimulates apoptosis in said cell, wherein the coding region for said polypeptide is under the franslational regulation of a XIAP IRES, wherein the presence of said XIAP IRES sequence increases the level of cap-independent translation of said polypeptide. 23. The method of claim 22, wherein said polypeptide is XIAP. 24. A method for decreasing a cell's resistance to apoptosis, said method comprising, introducing into said cell, an antisense nucleic acid having a sequence complementary to a XIAP IRES, wherein said antisense nucleic acid inhibits translation of XIAP in said cell. 25. The method of claim 24, wherein said cell is exposed to an apoptotic stimulus, wherein said apoptotic stimulus is gamma irradiation or a toxin. 26. The method of claim 22 or 24, wherein said cell is a cancer cell. 27. A method for regulating the level of a protein in a cell, said method comprising introducing into said cell a nucleic acid sequence, said nucleic acid sequence encoding a XIAP IRES sequence. 28. The method of claim 27, wherein said nucleic acid sequence further comprises a polypeptide coding sequence, wherein said polypeptide coding sequence for said polypeptide is under the translational confrol of said XIAP IRES sequence. 29. The method of claim 28, wherein the presence of said XIAP IRES sequence increases the level of cap-independent translation of said protein. 30. The method of claim 28, wherein said cell is a cell at risk for undergoing apoptosis. 31. The method of claim 30, wherein said cell is at risk for undergoing apoptosis due to an autoimmune disease, a degenerative disease, or immunorejection. 32. The method of claim 30, wherein said cell is selected from the group consisting of: a neuron, a cardiomyocyte, a skeletal myoblast, a skeletal myofiber, a hair follicle cell, an ovarian follicle cell, a retinal photoreceptor cell, an oligodendrocyte, an astrocyte, and a pancreatic islet cell. 33. The method of claim 28, wherein said cell is undergoing a heat shock response. 34. The method of claim 28, wherein said cell is under environmental stress. 35. The method of claim 34, wherein said environmental stress is selected from the group consisting of: hypoxic stress, osmotic sfress, oxidative sfress, radiation-induced stress, or toxin-induced sfress. 36. The method of claim 28, wherein said cell is growth-arrested. 37. The method of claim 28, wherein said cell is a cancer cell. 38. The method of claim 28, wherein said method is for inhibiting apoptosis in a cell in need thereof. 39. The method of claim 38, wherein said protein is selected from the group consisting of: XIAP, NAIP, TIAP, HIAP1, HIAP2, VEGF, BCL-2, BDNF, GDNF, PDGF-B, IGF-2, NGF, CTNF, NT-3, NT-4/5, EPO, insulin, TPO, and BCL-X L1 . 40. The method of claim 28, wherein said method is for reducing hypoxic sfress in a tissue under hypoxic sfress, wherein said protein is selected from the group consisting of: VEGF-1, VEGF-2, and b-FGF, wherein expression of said protein is sufficient to reduce hypoxic sfress in said tissue. 41. The method of claim 40, wherein said tissue is cardiac tissue or brain tissue. 42. The method of claim 28, wherein said method is for stimulating apoptosis in a cell in need thereof. 43. The method of claim 42, wherein said protein is selected from the group consisting of: caspases 1-10, BAX, BAD, BCL-X S , TRADD, FADD, XAF, VHL, and p53. 44. The cell of claim 43, wherein said cell is a cancer cell. 45. A purified nucleic acid comprising a XIAP IRES, wherein, if nucleotides are present 5' or 3', said nucleic acid comprises at least one variant nucleotide within a 500 nucleotide region 5' or 3' to said XIAP IRES, said variant nucleotide being a nucleotide that is not present at the position of said variant nucleotide in a naturally occurring XIAP gene or XIAP mRNA, relative to the position of said XIAP IRES, wherein said XIAP IRES increases cap-independent franslation of a cisfron when located upstream from said cistron within a messenger RNA molecule. 46. The nucleic acid of claim 45, wherein said XIAP IRES increases stress- induced cap-independent franslation. 47. A purified nucleic acid comprising a XIAP IRES-encoding nucleic acid, wherein, if nucleotides are present 5' or 3' to said XIAP IRES-encoding nucleic acid, said nucleic acid comprises at least one variant nucleotide within a 500 nucleotide region 5' or 3' to said XIAP IRES, said variant nucleotide being a nucleotide that is not present at the position of said variant nucleotide in a naturally occurring XIAP gene or XIAP mRNA, relative to the position of said XIAP IRES, wherein said XIAP IRES increases cap-independent translation of a cistron when located upsfream from said cistron within a messenger RNA molecule. 48. A purified nucleic acid comprising a XIAP IRES, said XIAP IRES being 5 * to a coding sequence that encodes a polypeptide other than a human or murine XIAP. 49. A purified nucleic acid encoding a mammalian XIAP IRES, said IRES being 5' to a coding sequence that encodes a polypeptide other than mammalian XIAP. 50. A purified nucleic acid that hybridizes to a probe comprising at least ten nucleic acids from the XIAP IRES, said nucleic acid not including the full XIAP- encoding cDNA sequence. 51. A purified nucleic acid comprising a mammalian XIAP IRES, wherein, if nucleotides are present 5' or 3' to said XIAP IRES, said XIAP IRES has a nucleotide sequence substantially identical to a nucleotide sequence set forth in SEQ ID NOs: 19- 30, wherein said nucleic acid comprises at least one variant nucleotide within a 500 nucleotide region 5' or 3' to said XIAP IRES, said variant nucleotide being a nucleotide that is not present at the position of said variant nucleotide in a naturally occurring XIAP mRNA, relative to the position of said XIAP IRES. 52. A purified nucleic acid encoding a mammalian XIAP IRES, wherein said XIAP IRES has a nucleotide sequence substantially identical to a nucleotide sequence set forth in SEQ ID NOs 19-30, wherein said nucleic acid comprises at least one variant nucleotide within a 500 nucleotide region 5' or 3' to said XIAP IRES, said variant nucleotide being a nucleotide that is not present at the position of said variant nucleotide in a naturally occurring XIAP mRNA, relative to the position of said XIAP IRES. 53. A purified nucleic acid comprising a nucleotide sequence complementary to at least 14 nucleotides of a nucleotide sequence of a nucleic acid selected from the group consisting of: a nucleic acid comprising a XIAP IRES, wherein, if nucleotides are present 5' or 3' to said XIAP IRES, said nucleic acid comprises at least one variant nucleotide within a 500 nucleotide region 5' or 3' to said XIAP IRES, said variant nucleotide being a nucleotide that is not present at the position of said variant nucleotide in a naturally occurring XIAP gene or XIAP mRNA, relative to the position of said XIAP IRES, wherein said XIAP IRES increases cap-independent franslation of a cisfron when located upsfream from said cisfron within a messenger RNA molecule; a nucleic acid encoding a mammalian XIAP IRES, said IRES being 5' to a coding sequence that encodes a polypeptide other than mammalian XIAP; and a nucleic acid encoding a mammalian XIAP IRES, wherein said XIAP IRES has a nucleotide sequence substantially identical to a nucleotide sequence set forth in SEQ ID NOs: 19-30, wherein said nucleic acid comprises at least one variant nucleotide within a 500 nucleotide region 5' or 3' to said XIAP IRES, said variant nucleotide being a nucleotide that is not present at the position of said variant nucleotide in a naturally occurring XIAP mRNA, relative to the position of said XIAP IRES. 54. The nucleic acid of claim 47, 49, 52, or 53, wherein said nucleic acid is contained within an expression vector and wherein said expression vector encodes a transcription unit comprising a XIAP IRES and a coding sequence for a polypeptide. 55. The expression vector of claim 54, wherein said coding sequence encodes a polypeptide that is not a XIAP polypeptide. 56. The expression vector of claim 54, wherein said expression vector is a gene therapy vector. 57. A vector containing nucleic acid encoding a XIAP IRES, wherein said nucleic acid encoding a XIAP IRES is 5' to nucleic acid encoding a polypeptide, wherein said polypeptide is selected from the group consisting of: XIAP, NAIP, TIAP, HIAPl, HIAP2, VEGF, BCL-2, BDNF, GDNF, PDGF-B, IGF-2, NGF, CTNF, NT-3, NT-4/5, EPO, insulin, TPO, p53, VHL, XAF, BAX, BCL-X L1 , BAD, BCL-X S , and caspases 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. 58. The vector of claim 57, wherein said vector further comprises a promoter, wherein said promoter is a tissue-specific promoter. 59. A method for detecting a compound that modulates XIAP IRES-dependent translation, said method comprising: (a) providing a sample comprising La autoantigen; (b) exposing said sample to a test compound; (c) contacting said La autoantigen with a XIAP IRES or an endogenous XIAP IRES; and (d) measuring the amount of binding of La autoantigen to said XIAP IRES or said endogenous XIAP IRES, wherein a decrease in said binding indicates a compound that decreases XIAP IRES-dependent translation, and wherein an increase in said binding indicates a compound that increases XIAP IRES-dependent translation. 60. The method of claim 59, wherein said La autoantigen is contacted with said XIAP IRES or said endogenous XIAP IRES prior to exposing said sample to said test compound. 61. The method of claim 59, wherein said La autoantigen is contacted with said XIAP IRES or said endogenous XIAP IRES after exposing said sample to said test compound. 62. A method for decreasing a cell's resistance to apoptosis, said method comprising exposing said cell to a compound that decreases the binding of La autoantigen to an endogenous XIAP IRES, wherein a decrease in said binding is sufficient to decrease translation of XIAP in said cell. 63. The method of claim 62, wherein said cell is a tumor cell or is at risk for becoming a tumor cell. 64. A method for increasing a cell's resistance to apoptosis, said method comprising exposing said cell to a compound that increases the binding of La autoantigen to an endogenous XIAP IRES, wherein an increase in said binding is sufficient to increase translation of XIAP in said cell. 65. The method of claim 64, wherein said cell has an increased risk for undergoing apoptosis.

Inventors: HOLCIK MARTIN, KORNELUK ROBERT G, LISTON PETER, HOLCIK, MARTIN, KORNELUK, ROBERT, G., LISTON, PETER
Applicants: HOLCIK MARTIN, KORNELUK ROBERT G, LISTON PETER, UNIV OTTAWA, HOLCIK, MARTIN, KORNELUK, ROBERT, G., LISTON, PETER, UNIVERSITY OF OTTAWA
Application year: 1999
Main classification: C12N15
IPC codes: C07K14, C12N15, C12Q1