METHOD FOR MODULATING PROOPIOMELANOCORTIN SYNTHESIS IN A MAMMAL

Abstract: This invention provides a novel method for decreasing proopiomelanocortin (POMC) synthesis in a mammal such as a human comprising administering to the mammal an effective amount of a molecule capable of binding to a mammalian POMC mRNA stem loop structure. The invention further provides a method for increasing POMC synthesis in a mammal comprising administering to the mammal an effective amount of an mRNA POMC stem-loop structure. A method of identifying compounds capable of modulating POMC synthesis in a mammal is also provided. The method comprises identifying compounds which bind to a proopiomelanocortin mRNA stem-loop structure.

Description: METHOD FOR MODULATING PROOPIOMELANOCORTIN SYNTHESIS IN A MAMMAL FIELD OF THE INVENTION This invention relates to the field of mammalian therapeutics. More particularly, methods of modulating proopiomelanocortin (POMC) synthesis in a mammal are provided involving administering to the mammal either proteins which bind to the stem-loop structure of the POMC mRNA or the stem- loop structures themselves. GOVERNMENT RIGHTS The work presented herein may have been supported in part by National Institute of Health grant DK36054. The United States Government may have certain rights in the invention. BACKGROUND OF THE INVENTION Proopiomelanocortin (POMC) , the precursor protein for adrenocorticotropin (ACTH) , /3-endorphin and melanocyte stimulating hormones (MSH) , is synthesized in brain, pituitary and peripheral tissues. Regulation of POMC gene expression is tissue-specific. In the anterior pituitary, gene expression is stimulated by corticotropin releasing hormone (CRH) [Fleischer, N. et al. (1970), Am . J. Physiol . 219, 445-452, Gilles, G. et al. (1978). Endocrinology. 103, 521-529] and inhibited by glucocorticoids [Fleischer, N. et al. supra (1970) , Buckingham, J. et al. (1977) . J. Endocrinol . 72, 187-193, Birnberg, N. , et al. (1983). Proc . Natl - Acad. Sci . USA. 80, 6982-6986, Eberwine, J. et al. (1984). J. Biol . Chem. 259, 2166-2170, Schachter, B. et al. (1982). Endocrinology. 110, 1442-1444], while in the intermediate lobe, dopamine serves as the primary inhibitor [Hollt, V. et al. (1982), Endocrinology, 110, 1885-1894, Chen, C. , et al. (1984). Proc. Natl . Acad. Sci . USA 80, 2211- 2215] . The time-course of these responses are such that changes in protein synthesis and secretion are apparent within a few minutes of stimulation, while changes in mRNA levels are observed after 6 hours of treatment. These temporal differences suggest that protein synthesis is regulated differently from transcription. The translation of mRNA can be directly altered by changes in the secondary structure of the mRNA or by protein- RNA or RNA-RNA interactions. The process of protein synthesis requires the interplay of nearly 100 different molecules, some of which interact to form protein-mRNA complexes, resulting in an alteration in the secondary and tertiary structure of the mRNA, thereby facilitating or hindering the translational process. Translation is dependent upon the rate of ribosome binding to mRNA, the number of ribosomes attached to a single mRNA molecule, the rate of translocation of the ribosome along the mRNA and the relative proportion of mRNA that exists in the actively translating fraction of mRNA (polysomes) and in the inactive fraction (monosomes) . Polysome profiles, a traditional method for examining translational control, have been performed on high-abundance mRNA in tissues which are relatively homogeneous in cell type, such as the liver. Since this methodology requires disruption of the tissue, it is not possible to localize translational control in a tissue which is composed of multiple cell types. The secondary structure of the mRNA caused by intramolecular base pairing is an important factor in determining the ability of ribosomes to initiate protein synthesis [Kozak, M. (1986) . Proc. Natl . Acad. Sci . USA 83, 2850-2854, Chitpatima, S. et al. (1988). J. Biol . Chem. 263, 7164-7169]. Both the 5'-end and other regions of the mRNA have been implicated in the modulation of translation [Kleene, K. et al. (1984) . Dev. Biol . 105, 71-79, Dickey, L. et al. (1988). J. Biol . Chem . 263, 3071-3074]. Since mRNA structure is likely to be involved in controlling the translational state of mRNAs a technique that will determine mRNA structure in situ may be predictive of translational control. In situ transcription (1ST) is a novel technique originally devised to decrease the exposure time for detection of mRNAs in situ [Tecott, L. et al. (1988) . Science . 240, 1661-1664; Longley, J. (1989). J. Inves . Derm . 93, 432-435; and U.S. Patent 5,021,335]. In 1ST, oligonucleotides are hybridized to mRNA in a tissue section, as is done with in situ hybridization, and reverse transcriptase and radiolabeled nucleotides are then added, resulting in the synthesis of cDNA using the oligonucleotide as a primer. This in situ reverse transcription copies endogenous mRNA into cDNA while preserving the anatomical distribution of the cDNA transcripts. The in situ transcribed radiolabeled cDNAs can be removed from the tissue section by alkaline denaturation of mRNA-cDNA hybrids and be visualized by autoradiography after they have been electrophoresed on denaturing DNA sequencing gels. The banding pattern is characteristic of changes that occur resulting from translational control of mRNAs. Thus, regulation of translation of mRNAs as a controlling point in gene expression is believed important in the acute and chronic responsiveness of a cell to external challenge. SUMMARY OF THE INVENTION This invention provides novel methods for decreasing proopiomelanocortin (POMC) synthesis in a mammal such as a human comprising administering to the mammal an effective amount of a molecule capable of binding to a mammalian POMC mRNA stem loop structure. The invention further provides methods for increasing POMC synthesis in a mammal comprising-administering to the mammal an effective amount of a mRNA POMC stem-loop structure. Methods of identifying compounds capable of modulating POMC synthesis in a mammal are also provided. These methods comprise identifying compounds which bind to a proopiomelanocortin mRNA stem-loop structure. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1. POMC Oligonucleotide Primers The oligonucleotides (PI, P2, P3, P4) are each 36 bases long and hybridize to specific regions of POMC mRNA: PI (nucleotides 383-419), P2 (205-240) , P3 (132-167) and P4 (-58 to -23) . Numbering of the nucleotide sequence is relative to the codon for the initiator methionine. The 1ST autoradiographics signal generated in rat pituitaries for each oligonucleotide is shown below their respective positions. Similar specific activities (about 5X10 8 cpm/μg) were generated in the kinase reaction and comparable amounts of radioactivity (about 1X10 cpm) were added during each hybridization. The dark, semicircular structure is the intermediate lobe. Figure 2. POMC IST-cDNA Banding Pattern Sequence specificity of the 1ST banding pattern using primer PI on mouse AtT20 cells (M) and rat pituitary (R) . The mRNA sequence is shown between the two rows. The bands correspond predominantly to the position of guanosine residues in the mRNA sequence. Differences between sequences are indicated by nucleotide,differences on either side of the common sequence. Arrows and stars highlight differences between the rat and mouse sequences. Figure 3. Modulators of POMC Translation alter the IST- Banding Pattern from AtT20 Cells NaF treatment of AtT20 cells. Panel A: polysome profile. Panel B: IST-cDNA banding patterns for NaF treatment (F) and control (C) . The shift in autoradiographic intensity of the banding pattern is best visualized by comparing the band at position a, which is more intense in lane F than in the lane C, to the band in position b, which is more intense in lane C than in lane F. Dexamethasone (D) treatment of AtT20 cells. Panel C: polysome profiles. Panel D: IST-cDNA banding pattern (band at position e is more intense in lane D relative to band g which is relatively more intense in lane (C) . Forskolin treatment (S) of AtT20 cells. Panel E: polysome profiles. Panel F: IST-cDNA banding pattern (band at position v is more intense in lane C relative to band w than in lane S) . The arrow in panel A and the dashed lines in panels C and E show the position of POMC mRNA in the polysome fractions. This was determined by hybridization of a radiolabeled POMC cDNA probe to a slot blot containing mRNA isolated from the polysome fractions. POMC mRNA was present in fractions 22-38 of the control polysome profile. In panel E, polysome fractions 30-39 had more POMC mRNA than the same fractions for the control profiles. 1ST experiments that are shown in this figure were performed on different days so the controls for panels D and F (gel lanes on the right hand portion of the figure labeled, C) are the same while the control shown in panel B is different. Since all the polysome profiles were performed on the same day, controls for panels A, C and E (open circles) are the same; experimental conditions are shown in closed squares. The arrows in panels B, D and F indicate the same band position in the different gels. Figure 4. POMC IST-Banding Pattern from the Rat Pituitary The banding patterns generated for PI, P3, and P4 in sham operated rats [Fleisher, N. et al. supra (1970) ; Birnberg, N. , et al. supra (1983); Eberwine, J. et al. supra (1984) ] and adrenalectomized rats [Gilles, G. et al supra (1978) ; Buckingham, J. et al. supra (1977) ; Schachter, B. et al. supra (1982)]. The points of reference for intensity comparison are indicated with letters A-I. Stars serve as reference marks for comparing the IST-banding pattern for PI between Figures 5 and 4 and while the o serves the same marker function for P4. Figure 5. IST-Banding Pattern Generated at Presumptive Stem-loop Structure This autoradiogram shows the IST-banding pattern generated in rat pituitary sections using PI (Lane A) , P2 (Lane B) and P3 (Lane C) which hybridize to different regions of the POMC mRNA. The 11 bases between arrows 1 and 2 delimit the putative loop region of the stem-loop structure. PI hybridizes distant from the stem-loop structure. P2 hybridizes 30 bases 3' to the presumptive stem-loop structure. P3 hybridizes to the 3'-side of the stem of the presumptive stem-loop structure. Stars serve as reference marks for comparing the IST-banding pattern for PI between Figures 5 and 4 while the o serves the same marker function for P3. Figure 6. GEL Shift of AtT20 Cell Extract with the Stem- loop Structure of POMC mRNA In lane 1, the sense riboprobe containing the POMC stem-loop structure (196 base fragment) is shown. Lanes 2 and 3 show the shifts that occur when 100,000 cpm of the riboprobe was mixed with 10 and 20 μg, respectively of AtT20 cytoplasmic extract. The shifted bands are labelled bands 1, 2, 3 and 4. The samples were electrophoresed on a 4% polyacrylamide gel. DETAILED DESCRIPTION OF THE INVENTION In one aspect of this invention, proopiomelanocortin synthesis in a mammal, preferably a human, is decreased by administering to the mammal an effective amount of a molecule capable of binding to a mammalian POMC mRNA stem loop structure, preferably SEQ ID NO: 5. Preferably such moleucles may be antisense oligonucleotides or proteins. Antisense oligonucleotides of the present invention are hybridizable with at least a portion of the stem loop structure, preferably SEQ ID NO: 5., thereby decreasing the synthesis of POMC. Such oligonucleotide may range in length from about 6 to about 100 bases. Preferably oligonucleotides of the present invention are about 10 to about 50 bases in length and most preferably are about 10 to about 25 nucleotides in length. Proteins which bind to the stem loop structure are readily identified using competition studies. An effective amount of molecules of the present invention is that amount which decreases POMC synthesis in the mammal. Molecules of the present invention may be delivered intracellularly by those methods known to those skilled in the art. For example, proteins may be delivered intracellularly by liposome loading and fusion with cells or by any method which can deliver proteins into cells. In another aspect of this invention, a method for increasing proopiomelanocortin synthesis in a mammal, Such as a human is provided comprising administering to the mammal an effective amount of the stem-loop sequence selected from the group consisting of SEQ ID NO: 1, 2, 3, 4, and 5. An effective amount of the stem loop sequence is that amount which increases POMC synthesis in the mammal. TABLE I ORIGIN OF SEQ ID NO. SEQUENCE MRNA SEQUENCE rat and human CACCACGGAA POMC stem loop mouse POMC stem UACCACGGAG loop rat CRH stem loop 3 CAGCACGGAG rat POMC stem loop 4 CACGGA rat POMC stem loop 5 GACCTCCATA GACGTGTGGA GCTGGTGCCT GGAGAGCAGC CAGTGCCAGG ACCTCACCAC GGAAAGCAAC CTGCTGGCTT GCATCCGGGC CTGCAGACTC GACCTCTCGG CG Screening methods for identifying compounds capable of modulating proopiomelanocortin synthesis in a mammal such as a human are also within the scope of the invention. The method comprises identifying compounds which bind to a proopiomelanocortin mRNA stem-loop structure. "Binding to the stem-loop structure" as used herein refers to a compound that when coupled to the mRNA stem-loop structure prohibits the complete translation of the mRNA sequence such that the corresponding protein is not synthesized as the product of translation. Compounds that bind to the mRNA could be conveniently identified using 1ST as described and exemplified herein and as further described in U.S. Patent No. 5,021,335 which patent is incorporated by reference as if fully set forth herein. Also within the purvue of this invention are molecules that will alter the functioning of the stem-loop binding proteins. Such molecules could either increase or decrease the ability of the stem-loop binding proteins to interact with the stem-loop sequence. METHODS AND MATERIALS CELL CULTURE AtT20 cells, a homogeneous line of POMC-producing cells, were grown on poly-lysine coated glass cover slips in Delbecco's modified Eagle's medium (GIBCO, Grand Island, New York) containing 4.5 g/1 glucose, penicillin, streptomycin and 15% fetal bovine serum at 37°C with 5% C02. Initial density was 5 x 10 cells per well in 24-well multi-well plates. Cells were grown for 48 h, reaching a cellular density of approximately 70% confluency. Medium was aspirated and replaced with fresh medium containing NaF (lOmM) or dexamethasone (1 μM) for 1 hour, or forskolin (10 μM) for 4 hr. After treatment, cells were washed twice with phosphate buffered saline (PBS) , fixed for 20 min in 3% neutral buffered paraformaldehyde, washed with 2 x SSC, and then stored in 70% ethanol at 4°C. TISSUE PREPARATION Adult male Sprague-Dawley rats were used for analysis of IST-banding patterns in vivo. Adrenalectomy was performed under ether anaesthesia, and the rats were allowed to recuperate for 2 weeks; they were maintained on 0.7% saline in the drinking water. Dexamethasone (100 μg/kg) was administered daily for three days (at 2:00 p.m.) via intraperitoneal injection. The rats were killed by decapitation, and the pituitary glands were removed, quick- frozen in Cryoquick (International Equipment Co. , Needham Heights, MA) and stored at -70°C until sectioning. For sectioning, the pituitaries were embedded in OCT embedding compound (Miles, Inc., Elkhart, IN), sectioned with a cryostat at -14°C, and thaw-mounted on gelatin-coated microscope slides. These slides were washed in PBS, dehydrated through graded ethanol washes, and stored at -70°C until used for 1ST. OLIGONUCLEOTIDE PROBES Oligonucleotide probes (P1-P4; see Fig.l) complementary to portions of the POMC mRNA were synthesized on a Biosearch Model 8600 DNA synthesizer (Milligon Biosearch, Burlington, MA) . These oligonucleotides were 5'- end-labelled with polynucleotide kinase (Boehringer-Mannheim) and γ 32 P-ATP (6000 Ci/mM, New England Nuclear) to a specific activity of about 5 X 10 8 cpm/μg. Cover slips and pituitary tissue sections were hybridized with 1.8 x 10 cpm/100 μl of probe in IX hybridization buffer consisting of 4 x 10xSSC(.15M sodium acetate pH 7.0, 1.5M sodium chloride) in 40% formamide and incubated at room temperature for 4 h. The sections were then washed in 2 X SSC at room temperature for 30 min followed by two 0.5 X SSC at room temperature for two hours each. The sections were then rinsed briefly in distilled water. Reverse transcription was performed by addition of 150 μl of reverse transcription buffer (50 mM Tris pH 8.3; 6mM MgCl 2 ; lOOmM KC1; 7.5mM DTT(Dithiothreitol) ; 120 U/ml RNAsin (RNAase inhibitor available commercially from Life Technologies, Inc., Gaithersburg, MD) ; 250 μM dNTPs; 600 U/ml of AMV reverse transcriptase (Avian myeloblastosis virus reverse transcriptase available commercially from Seikagaku, Bethesda, MD) and incubation at 37°C for one hour. The samples were washed in 2 X SSC at room temperature for 30 minutes and then in 0.5 X SSC at 40°C for 6 hours. ST CDNA TRANSCRIPT ISOLATION AND GEL ANALYSIS Cellular material was removed from cover slips and slides with a minimum volume of 1 N NaOH. Material from 3-8 cover slips was pooled for each treatment, while two tissue sections were pooled for each treatment. The solution containing the transcripts was neutralized with 1 N HC1. Volume was brought to 400 μl with water, NaCl was added to 0.5 M, and the solution was extracted in phenol-chloroform twice and precipitated with ethanol using 10 μg of glycogen as carrier. Alternatively, transcripts were removed by dispersion in 0.1N NaOH/1% SDS, followed by addition of 0.75 • volumes of 5M potassium acetate, centrifugation at 4°C for 5 min and ethanol precipitation of the cDNAs. Gel analysis of the cDNA transcripts was performed on 6% polyacrylamide /7 M urea gels. Gels were transferred to 3MM chromatography grade filter paper, vacuum dried and then exposed for 2-7 days with an intensifying screen at -70°C. I. IN SITU TRANSCRIPTION When POMC mRNA from rat pituitary or mouse AtT20 cells was transcribed in situ using oligonucleotide PI, two distinct transcript banding patterns were observed on DNA sequencing gels (Figure 2A) . The specific activity of each band is the same because the transcript was labeled once at the 5'-end of the oligonucleotide primer; therefore, different intensities of the cDNA bands indicate different molar amounts of cDNA. Comparison of the mRNA base sequence with the 1ST bands reveals that the bands correspond primarily with the presence of G residues. The banding pattern is not generated by alkaline hydrolysis of cDNA because several different denaturing agents, including KOH, NaOH, guanidinium hydrochloride, and guanidinium isothiocyanate have been used to remove cDNA from the tissue sections and yield identical banding patterns (Zangger, I. et al. (1989) Technique 1:108-117; Eberwine, unpublished results) . To ensure that these bands did not result from an artifact due to cross-linking of the mRNA to other molecules by the paraformaldehyde treatment of the section, unfixed rat pituitary tissue was subjected to the identical treatment, and the same banding pattern as was seen in the fixed tissue was observed (data not shown) . The banding pattern is not a random pattern generated solely by the interaction of the primer with the mRNA. When primer extension was performed on rat pituitary mRNA in solution and then electrophoresed on a DNA sequencing gel, a smear of termination sites appeared. Most bands did not correspond to the bands observed from the POMC mRNA derived from tissue sections (data not shown) . This disparity in banding pattern suggests that POMC IST-generated cDNA results from an interaction of POMC mRNA in situ with cellular structures. Specific termination of cDNA synthesis may be due to steric interference, perhaps through an interaction of mRNA in situ , with itself or with other molecules that inhibited the procession of reverse transcriptase in the assay system. Agents which enhance POMC production via translation may cause increased loading of ribosomes on POMC mRNA, resulting in a shift in the relative intensities of the banding pattern such that the longer cDNAs (high molecular weight bands) are lower in abundance than are the shorter cDNA (low molecular weight bands) . Thus, after treating AtT20 cells with factors which increase POMC synthesis, the ratio of the intensities of the high- molecular-weight bands to the intensities of the lower- molecular-weight bands should be decreased. The opposite effect would be expected to occur when an inhibitor of POMC synthesis is used. NaF decreases POMC-peptide secretion and has been reported to inhibit ribosome binding to mRNA. NaF treatment of AtT20 cells decreased the number of POMC mRNAs in the polysome fractions of the gradient (Figure 3, panel A) , and increased the intensity of the high molecular weight IST-derived cDNA bands compared to the low molecular weight bands within a gel lane (Figure 3, panel B) . Densitometric examination of the integrated areas of the autoradiographic bands showed that the high to low molecular weight band (arbitrary midpoint on the gel with the bands above the midpoint being high molecular weight and the bands below the midpoint being low molecular weight bands) ratio changes with pharmacological manipulation. The high to low band molecular weight ratios were calculated by determining the total intensity above the midpoint divided by the total intensity below the midpoint. Comparisons between the different pharmacological states were made by dividing the experimental ratio of band intensities by the control ratio. The ratio increases with NaF treatment (autoradiographic ratio = 1.42; scanning profiles not shown) . This confirms the visual suggestion that there is an increase in the molar amounts of higher molecular weight bands relative to lower molecular weight bands with NaF treatment. These data suggest that longer 1ST transcripts increase in abundance relative to shorter transcripts when there are fewer ribosomes associated with the POMC mRNA. Treatment of AtT20 cells with the POMC-peptide secretagogue, forskolin, lowered the ratio of high-molecular- weight to low molecular weight 1ST bands compared to control (Figure 3, panel F, autoradiographic ratio = 0.02). The polysome profile showed that forskolin increased the amount of POMC mRNA in the polysome fraction (Figure 3, panel E) . Dexamethasone, which decreases POMC synthesis, caused a reversal of the ratio of the autoradiographic intensity of cDNA bands, with the intensity of longer cDNAs increasing relative to the shorter cDNAs (Figure 3, panel D, autoradiographic ratio=3.1) . The polysome profile showed that dexamethasone decreased the amount of POMC mRNA in the polysome fraction (Figure 3, panel C) . The limited time- course of these manipulations was too short to elicit changes in POMC mRNA levels. II. POMC IST-BANDING PATTERN IN THE RAT PITUITARY In the following in vivo studies, whether the position of the oligonucleotide primer within POMC mRNA would modulate the alterations in band intensity was examined. Regions of POMC mRNA which were of particular interest were those predicted by Gibbs free energy calculations to be involved in forming mRNA secondary structure. The IST-signal generated in the tissue sections using all four oligonucleotides as primers for in situ cDNA synthesis is shown in Figure 1. PI, P2 and P4 gave approximately the same autoradiographic intensity in the intermediate lobe of the pituitary, while P3 gave a signal which was far less intense. The IST-cDNA banding patterns in pituitaries derived from intact and adrenalectomized rats are easily discernable in Figure 4. Adrenalectomy, which increases POMC protein synthesis and secretion [Roberts, J. et al. (1979) . Biochemistry. 28, 4907-4915, Bruhn, T., et al. (1984). Neuroendocrinology. 39, 170-175] caused a shift of band intensities, such that the higher molecular weight bands were relatively less abundant than the lower molecular weight bands using all three oligonucleotide primers. For oligonucleotide PI, twice the amount of radioactivity was applied to the sequencing gel (lane 2) to accentuate the differences in the relative intensity of individual bands within lane 2. Bands labeled A and B are of nearly equal intensity in both groups while the lower molecular weight bands (C, D and E) are of higher intensity in the adrenalectomized animals. Since the banding pattern depends upon the position of priming, distinct reference points must be used for comparison of the IST-banding patterns. Band F is lower in intensity in the adrenalectomy specimen (lane 3) than in sham-operated pituitary (lane 4) , while band G is of nearly equal intensity. Band H is lower in intensity in adrenalectomized animals (lane 6) than sham-operated rats (lane 5) while and I is higher in intensity in lane 6 than in lane 5. III. EXISTENCE OF STEM-LOOP STRUCTURE FOR POMC mRNA in situ Since stem-loop structures (intramolecular base paired sequences) are thought to inhibit the translation of mRNA because of the steric hindrance of proteins bound to the loop, an experiment to determine whether the stem-loop structure exists in situ was designed . The rationale for this experiment rests on the observation that reverse transcriptase does not melt "strongly" base-paired sequences. It was predicted that the 1ST pattern through this region would be distinctive, reflecting the presence of a structure that inhibits the activity of reverse transcriptase. The oligonucleotide P2 was chosen for priming because it is complementary to a region approximately 30 bases away from a region of the POMC mRNA which is predicted by Gibbs free energy calculations to contain a stable (about 45kCal) stem- loop structure. The banding pattern (Figure 5, lane B) shows a single-base ladder that decreases in intensity for 11 bases (between arrows 1 and 2) and then increases again. This decreased intensity in the banding pattern corresponds to the site at which the loop of the stem-loop structure would exist. The low molecular weight single base ladder (below arrow 2) corresponds to the 3'-end of the stem-loop structure. Immediately after the 11 base decrease in intensity which is presumably contained in the loop, there is a high molecular weight single base ladder (immediately above arrow 1) which corresponds to the 5'-end of the stem. The banding pattern presented in Figure 5 suggests that such stem-loop structures exist in situ. This interpretation is supported by the fact that upon overexposure of the autoradiogram the single base pattern reverts to the 1ST banding pattern of the POMC mRNA 18 bases away from the presumed 5'-end of the predicted stem-loop structure (immediately above arrow 1, data not shown) . Additional evidence in support of this interpretation is provided by the autoradiographic intensity of the tissue section after 1ST for the primer P3, which always appears weaker than either PI or P2 (Figure 1) . Primer P3 was synthesized so that it would hybridize to the 3'-portion of the presumed stem-loop structure. The lower signal suggests that hybridization of the oligonucleotide was hindered because the priming site was already base-paired with the 5'-end of the stem-loop structure for some of the POMC mRNAs. The ability of AtT20 cytoplasmic proteins to bind specifically to this putative RNA structure was assessed by using an RNA gel shift assay [Rouault, T. et al. (1990) Proc. Natl. Acad. Sci. USA 87, 7958-7962]. A 196 base region of the POMC cDNA [Uhler, M. et al. (1983). J. Biol. Chem. 258, 257-261.] containing the putative stem-loop structure subcloned into the Bluescript cRNA vector. Radiolabeled sense cRNA was synthesized and mixed with S100 cytoplasmic extract [Dignam, J. et al. (1983) . Nuc. Acids Res. 11, 1475- 1489] from AtT20 cells. The mixture was electrophoresed on a 4% polyacrylamide gel. The shift in mobility of free cRNA upon binding of protein extracts to the mRNA is dramatic (Fig. 6) . Four bands appear upon binding of the protein to the radiolabeled cRNA. As a control, cRNAs made to other regions of the POMC precursor which are not involved with the stem-loop structure did not alter the banding pattern when they were added with the specific cRNA (data not shown) . By themselves, these non-specific RNAs did not produce this specific gel shift banding pattern. These gel shift experiments indicate that AtT20 cells contain cytoplasmic proteins which specifically bind to the putative stem-loop structure in POMC mRNA. POMC gene expression is regulated by transcriptional, translational and post-translational mechanisms. Acute foot-shock stress in the rat increased the rate of POMC-derived peptide synthesis without changing POMC mRNA concentration. During chronic stress, however, POMC mRNA levels increased but the rate of translation was no longer enhanced [Shiomi, H. , et al. (1986) Endocrinology. 119, 1793-1798]. In cultured pituitary cells, prolonged exposure to phorbel ester increased POMC-derived peptide synthesis despite a decrease in steady state POMC mRNA levels, suggesting an activation of translational control mechanisms independent of gene transcription [Wand, G.S., et al. (1988). Endocrinology 123, 1153-1159.] Thus, the regulation of translation of mRNAs as a controlling point in gene expression is potentially important in the acute and chronic responsiveness of a cell to external challenge. Stem-loop structures are believed responsible for regulating the translation of POMC mRNA. It is possible to use the IST-cDNA banding pattern as an indicator of secondary structure to determine if the stem-loop structure actually exists within the cell. The presence of a specific banding pattern after reverse transcription of POMC mRNA in situ indicates that the enzyme activity terminates at specific sites along the length of the mRNA. If the termination sites were randomly distributed, a smear of cDNA bands would be seen. These termination sites are, in fact, sequence- dependent, as demonstrated by the correlation of the banding pattern with the POMC sequence for each of the oligonucleotides used in the study. The shift in the ratio of autoradiographic intensities of the higher molecular weight bands relative to the lower molecular weight bands can be interpreted in two ways. First, trivial explanations for termination, including limiting amounts of substrate, fixation conditions, RNA degradation and method of transcript removal, have been tested and eliminated. Second, the termination sites may occur because the sequence surrounding the termination site is involved in forming secondary or tertiary structure that reverse transcriptase has difficulty traversing. These local sequences could be stabilized by association with cytoplasmic structures such as ribosomes, proteins, or RNA. Since a ladder of bands exists within the loop, reverse transcriptase must occasionally penetrate presumed secondary structures so that longer cDNAs are synthesized, or there must be distinct populations of POMC mRNA differing in local structure in this region so that reverse transcriptase traverses these regions without terminating. The banding pattern in Figure 5, which shows a single-base ladder that decreases in intensity for 11 bases corresponding to a predicted stem-loop structure, may represent cDNAs transcribed from a population of mRNAs which have a secondary or tertiary structure that impedes the activity of the reverse transcriptase enzyme. Such a structure is postulated to exist by calculation of the free energy of intramolecular base pairing which predicts that a stable stem-loop structure can form between these sequences. This interpretation is supported by the fact that the single base pattern changes to a "regular" 1ST banding pattern 18 bases away from the intensity shift which would be at the 5'- end of the presumed stem-loop structure. The presence of the single base ladder on the sequencing gel corresponding to the stem-loop structure suggests several alternative possibilities for how such a pattern would be generated. The autoradiographic bands result from the mixing of IST-cDNAs from all of the POMC producing cell types on the tissue sections. It is possible that all mRNAs contain the stem- loop sequence and that reverse transcriptase can partially displace the hybrid as it synthesizes cDNA, or there may be different populations of mRNAs, each in a distinct conformation and each inhibiting progression of reverse transcriptase over the template at a different position along the mRNA. If there are populations of POMC mRNA in different conformational states, then they would be mixed in the cDNA isolation procedure and appear as an average abundance of the different cDNAs on the section rather than an abundance ratio indicative of the single cell type from which the mRNA was produced. Such a scenario could mean that each cell contains mRNA from a single class of structures containing a complete stem-loop structure. It is also possible that populations of cells contain a single, yet distinct, population of POMC mRNA conformations, or that each cell contains all of the distinct populations of POMC mRNA. If the free energies of stem-loop formation are calculated for POMC mRNA, there are several potential structures which are predicted around this region of the mRNA. Stem-loop structures of free energy greater than -30 Kcal are thought to be unstable, yet the existence of these structure in situ and their potential interconversion to other structures remains to be proven. It may be possible to search for these structures using 1ST, employing oligonucleotides corresponding to different regions of predicted stem-loops. The ability of AtT20 cytosol to bind specifically to the region of RNA which is predicted to exist as a stem- loop structure provides further evidence that this unique portion of the POMC mRNA is subject to regulatory factors which can modulate the translatability into POMC peptides. Once the structure of this cytosolic factor is elucidated as has been recently done for an iron-responsive element-binding protein [Rouault, T. , et al. supra (1990)], it may be possible to test its role in regulating protein synthesis directly. Tissue and cell-specific regulation of the POMC gene is well recognized. Individual cells contain varying amounts of POMC mRNA [Chronwall, B., et al. (1987) Endocrinology. 120, 1201-1211.], and secrete disparate amounts of hormone [Childs, G.V. et al. (1987) Endocrinology, 120, 439-445]. The existence of populations of cells containing distinct POMC mRNA structures would suggest that different cells may respond differentially to hormonal challenges, i.e., those cells containing mRNA in an "easily" translatable conformational state may respond more rapidly than other POMC containing cells with mRNA in a different structural state. Such a result would have implication for acute versus longer term effects of modulators of POMC expression including physiological modulators such as stress. SEQUENCE LISTING (1) GENERAL INFORMATION: (i) APPLICANT: Eberwine, James H. , Spencer, Corinne (ii) TITLE OF INVENTION: Method for Modulating Proopiomelanocortin Synthesis in a Mammal (iii) NUMBER OF SEQUENCES: 5 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Woodcock Washburn Kurtz Mackiewicz and Norris (B) STREET: One Liberty Place - 46th Floor (C) CITY: Philadelphia (D) STATE: PA (E) COUNTRY: U.S.A. (F) ZIP: 19103 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: PatentIn Release #1.0, Version #1.25 (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: (B) FILING DATE: (C) CLASSIFICATION: (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: Caldwell, John W. (B) REGISTRATION NUMBER: 28,937 (C) REFERENCE/DOCKET NUMBER: UPN-0688 (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: 215-568-3100 (B) TELEFAX: 215-568-3439 (2) INFORMATION FOR SEQ ID NO:l: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 10 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: unknown (ii) MOLECULE TYPE: mRNA (Xi) SEQUENCE DESCRIPTION: SEQ ID NO:l: CACCACGGAA 10 (2) INFORMATION FOR SEQ ID NO:2: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 10 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: unknown (ii) MOLECULE TYPE: mRNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2: UACCACGGAG 10 (2) INFORMATION FOR SEQ ID NO:3: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 10 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: unknown (ii) MOLECULE TYPE: mRNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3: CAGCACGGAG 10 (2) INFORMATION FOR SEQ ID NO:4: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 6 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: unknown (ii) MOLECULE TYPE: mRNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4: CACGGA (2) INFORMATION FOR SEQ ID NO:5: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 112 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: unknown (ii) MOLECULE TYPE: cDNA to mRNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5: GACCTCCATA GACGTGTGGA GCTGGTGCCT GGAGAGCAGC CAGTGCCAGG ACCTCACCAC 60 GGAAAGCAAC CTGCTGGCTT GCATCCGGGC CTGCAGACTC GACCTCTCGG CG 112

Claims: I claim: 1. A method for decreasing proopiomelanocortin synthesis in a mammal comprising administering to said mammal an effective amount of a molecule capable of binding to a mammalian POMC mRNA stem loop structure. 2. The method of claim 1 wherein the molecule is a protein. 3. The method of claim 2 wherein the mammal is a human. 4. The method of claim 2 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 1. 5. The method of claim 2 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 2. 6. The method of claim 2 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 3. 7. The method of claim 2 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 4. 8. The method of claim 2 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 5. 9. The method of claim 1 wherein the molecule is an oligonucleotide. 10. The method of claim 9 wherein the mammal is a human. 11. The method of claim 9 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 1. 12. The method of claim 9 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 2. 13. The method of claim 9 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 3. 14. The method of claim 9 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 4. 15. The method of claim 9 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 5. 16. A method for increasing proopiomelanocortin synthesis in a mammal comprising administering to said mammal an effective amount of the stem-loop structure substantially as shown in SEQ ID NO: 1, 2, 3, 4 or 5. 17. The method of claim 16 wherein the mammal is a human. 18. A method of identifying compounds capable of modulating proopiomelanocortin synthesis in a mammal comprising identifying compounds which bind to a proopiomelanocortin mRNA stem-loop structure. 19. The method of claim 18 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 1. 20. The method of claim 18 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 2. 21. The method of claim 18 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 3. 22. The method of claim 18 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 4. 23. The method of claim 18 wherein the stem loop structure has the sequence substantially as shown in SEQ ID NO: 5.

Inventors: EBERWINE JAMES H, SPENCER CORINNE, EBERWINE, JAMES, H., SPENCER, CORINNE
Applicants: UNIV PENNSYLVANIA, THE TRUSTEES OF THE UNIVERSITY OF PENNSYLVANIA
Application year: 1993
Main classification: G01N33
IPC codes: A61K38, C07K14, C12Q1