Activation of Transcription by Progesterone Receptor Involves Derepression of Activation Functions by a Cofactor

Abstract

Hormone-induced progesterone receptors (PR) bound to response elements stimulate transcription initiation at target promoters through a mechanism that presumably involves cofactors or coactivators. To allow identification of such cofactors of transcriptional activation in a functional assay, we have established a reconstituted transcription system that is characterized by a specific loss of responsiveness to purified baculovirus-expressed wild type PR. In contrast to wild type PR, a C-terminally truncated PR mutant displayed strong activation potential in this system. As the purified recombinant full-length PR is capable of DNA binding, our results suggest that C-terminal sequences of PR mediate a cis-repression of N-terminal activation functions. Moreover, using this PR-nonresponsive transcription system, we identified and partially purified an activity from rat liver, termed COPRA (cofactor of PR activation), that restores transactivation by full-length PR. Characterization of COPRA revealed that this cofactor exhibits activator specificity and is not involved in basal transcription. We postulate that COPRA acts by relieving the repression of activation functions mediated by C-terminal sequences.

Atomic structure of progesterone complexed with its receptor

“…Although the overall fold of the progesterone receptor is similar to that found in related receptors 3-6 , the progesterone receptor has a quite different mode of dimerization 3,6 . A hormone-induced stabilization of the carboxy-terminal secondary structure of the ligand-binding domain of the progesterone receptor accounts for the stereochemistry of this distinctive dimer, explains the receptor's characteristic pattern of ligand-dependent protease resistance and its loss of repression 7,8 , and indicates how the anti-progestin RU486 might work in birth control. The structure also indicates that the analogous 3-keto-steroid receptors may have a similar mechanism of action.…”

“…The N,N-dimethyl-aminophenyl group attached to C12 in RU486 collides severely with helix 12 and with Trp 755, which contacts helix 12. There are limitations to modelling, but these clashes would probably force a displacement of helix 12 and the C-terminal extension, accounting for the protease sensitivity 12 and increased accessibility of the PR to a presumed repressor 7,8 . The oestrogen antagonist raloxifene also displaces helix 12 in the ER 6 , indicating a common mechanism among many antagonists of the nuclear/ steroid receptors.…”

Aromatic β-amino-ketone derivatives as novel selective non-steroidal progesterone receptor antagonists

“…All the compounds (27-55) showed high PR binding affinity (K i <100 nM). The trend observed for the b-(3-fluorophenyl) analogues (21,22,(27)(28)(29)(30)(31)(32)(33)(34)(35) was that the substituted group (R 1 ) at the 3 0 -position was preferred to 4 0 -position (e.g., 21 vs 30, 27 vs 31) for PR binding potency and selectivity against AR. The data of b-(3-fluorophenyl) analogues 31-33 clearly demonstrated that a large steric group (R 1 = Ph, Cy, morpholino) at the 4 0 -position was tolerated for PR binding activity and good for PR selectivity.…”

“…Second, N,N-dimethyl-aminophenyl group attached to C12 in mifepristone would probably force a displacement of helix 12 and the C-terminal extension, accounting for the protease sensitivity 19 and increased accessibility of the PR to a presumed repressor. [20][21][22] The docking of (À)-(R)-22 into the ligand-binding domain of PR together with mifepristone is shown in Figure 6B. It is interesting to note that the p-nitro group of (À)-(R)-22 is networked to Gln725 and Arg766, and the m-fluorophenyl group and the p-methylphenyl group in (À)-(R)-22 are superimposed with the D ring and N,N-dimethyl-aminophenyl group of mifepristone, respectively.…”