Progesterone Receptors (PR)-B and -A Regulate Transcription by Different Mechanisms: AF-3 Exerts Regulatory Control over Coactivator Binding to PR-B

Abstract

The two, nearly identical, isoforms of human progesterone receptors (PR), PR-B and -A, share activation functions (AF) 1 and 2, yet they possess markedly different transcriptional profiles, with PR-B being much stronger transactivators. Their differences map to a unique AF3 in the B-upstream segment (BUS), at the far N terminus of PR-B, which is missing in PR-A. Combined mutation of two LXXLL motifs plus tryptophan 140 in BUS, to yield PR-BdL140, completely destroys PR-B activity, because strong AF3 synergism with downstream AF1 and AF2 is eliminated. This synergism involves cooperative interactions among receptor multimers bound at tandem hormone response elements and is transferable to AFs of other nuclear receptors. Other PR-B functions-N-/C-terminal interactions, steroid receptor coactivator-1 coactivation, ligand-dependent down-regulation-also require an intact BUS. All three are autonomous in PR-A, and map to N-terminal regions common to both PR. This suggests that the N-terminal structure adopted by the two PR is different, and that for PR-B, this is controlled by BUS. Indeed, gene expression profiling of breast cancer cells stably expressing PR-B, PR-BdL140, or PR-A shows that mutation of AF3 destroys PR-B-dependent gene transcription without converting PR-B into PR-A. In sum, AF3 in BUS plays a critical modulatory role in PR-B, and in doing so, defines a mechanism for PR-B function that is fundamentally distinct from that of PR-A.

Regulation of the Structurally Dynamic N-terminal Domain of Progesterone Receptor by Protein-induced Folding

“…Distinct structural conformations of the NTD have been implicated to be a contributing factor for the different activities of PR-A and PR-B (9). A separate transcription activation function (termed AF3) was mapped to sequences in the unique N-terminal extension of PR-B that has the potential to exert allosteric effects on the NTD or AF2 in the LBD (27,67). In this study, no differences were detected with isolated NTDs from PR-A or PR-B with respect to osmolyte or TBP-induced protein folding.…”

Progesterone Resistance and Targeting the Progesterone Receptors: A Therapeutic Approach to Endometriosis

“…Both isoforms have two activation function domains, AF-1 proximal to the DNA-binding domain, and a ligand-dependent AF-2 domain in the C terminus (Tetel et al, 1999). By virtue of the longer N-terminus, PR-B also has a unique AF-3 domain that may contribute to its differential trans-activation properties compared with PR-A (Tung et al, 2006).…”

“…Early classes of anti-progestagens were poorly selective, yet some, such as gestrinone, still found clinical utility in the treatment of endometriosis (Cornillie et al, 1986;Coutinho, 1982). Furthermore, the ligand binding domains of PR-A and PR-B are identical and yet several in vitro and in vivo lines of evidence suggest that the effects of progesterone on transcriptional activation and repression by PR-A and PR-B are different (Conneely et al, 2001;Tung et al, 2006). To date, however, there are no agonist or antagonist agents that have been characterised with selectivity for PR-A over PR-B or vice versa.…”

Differential and Interactive Effects of Ligand-Bound Progesterone Receptor A and B Isoforms on Tyrosine Hydroxylase Promoter Activity.

“…These two isoforms are expressed from a single gene through two distinct promoters (30). The PR-A isoform lacks the activation function-3 domain and can inhibit PR-B transactivation of specific promoters (31)(32)(33). The ratio of PR-A and PR-B will often determine progesterone-mediated actions (33,34).…”