title: An amino-terminal truncated progesterone receptor isoform, PRc, enhances progestin-induced transcriptional activity.
abstract: Previously we reported the identification of two unique progesterone receptor (PR) messenger RNA transcripts that encode a smaller PR isoform, termed the C-receptor (PRc). These two PR transcripts encode a protein that is N-terminally truncated, so that it lacks the first zinc finger of the DNA binding domain, but still contains a complete hormone binding region with sequences for dimerization and nuclear localization. We also have demonstrated the existence of a 60-kDa progestin-specific binding protein in progestin target cells using a monoclonal antibody directed to the C-terminus of PRs, suggesting that these two novel transcripts generate a truncated form of PR. In this paper, we address the hypothesis that the C-receptor arises from the initiation of translation of a methionine C-terminal to the methionine start sites that generate the larger 94-kDa A and 116-kDa B human PR isoforms. The studies shown here support the postulate that another downstream in-frame methionine within the PR-coding region can serve as a translation initiation site for the generation of a third PR protein. A partial PR complementary DNA, lacking the translation start sites for B- and A-receptors was translated in vitro. The synthetic protein product bound [3H]progestins and unlabeled progestins. The antiprogestin RU486 also competed for this binding. Transfection of this partial PR complementary DNA into PR-negative HeLa cells resulted in progestin-specific binding activity. Because the third PR isoform lacks the first zinc finger of the DNA binding domain, but contains sequences for dimerization, we reasoned that the C-receptor isoform would be transcriptionally in-active and not bind DNA directly. Surprisingly, however, in the presence of A- and/or B-receptors, we found that C-receptors can modulate the transcriptional activity of A- and/or B-receptors using a reporter gene. These studies emphasize that multiple receptor isoforms may have distinct biological properties, and that the truncated C-receptor may play a role in explaining some of the pleiotropic effects of progestins.
authors.affiliation: Department of Physiology and Biophysics, Georgetown University/Lombardi Cancer Center, Washington, D.C. 20007, USA.
authors.forename: L L
authors.initials: LL
authors.lastname: Wei
authors.forename: P
authors.initials: P
authors.lastname: Hawkins
authors.forename: C
authors.initials: C
authors.lastname: Baker
authors.forename: B
authors.initials: B
authors.lastname: Norris
authors.forename: P L
authors.initials: PL
authors.lastname: Sheridan
authors.forename: P G
authors.initials: PG
authors.lastname: Quinn
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology; D001943:Breast Neoplasms / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism; D004742:Enhancer Elements, Genetic; D006067:Gonanes / Q000494:pharmacology; D006367:HeLa Cells / Q000187:drug effects / Q000378:metabolism; D006727:Hormone Antagonists / Q000494:pharmacology; D006801:Humans; D008715:Methionine / Q000235:genetics / Q000378:metabolism; D015735:Mifepristone / Q000031:analogs & derivatives / Q000494:pharmacology; D045167:Progesterone Congeners / Q000494:pharmacology; D011372:Progestins / Q000235:genetics* / Q000378:metabolism*; D011397:Promegestone / Q000378:metabolism / Q000494:pharmacology; D014176:Protein Biosynthesis; D011980:Receptors, Progesterone / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic; D014162:Transfection; D014407:Tumor Cells, Cultured
chemical_list: D000970:Antineoplastic Agents; D006067:Gonanes; D006727:Hormone Antagonists; D045167:Progesterone Congeners; D011372:Progestins; D011980:Receptors, Progesterone; C064879:ZK 112993; D015735:Mifepristone; D011397:Promegestone; D008715:Methionine; C053238:onapristone