ANTAGONIST-OCCUPIED HUMAN PROGESTERONE RECEPTORS BOUND TO DNA ARE FUNCTIONALLY SWITCHED TO TRANSCRIPTIONAL AGONISTS BY CAMP

When steroid hormone antagonists have inappropriate agonist effects, t he clinical consequences are grave. Progesterone antagonists bind to t wo naturally occurring isoforms of human progesterone receptors (hPR), hPR(B) and the NH2-terminally truncated hPR(A), and usually inhibit a gonist-stimulated transcription. It is shown here that elevation of cA MP levels in a human breast cancer cell line leads to the functional r eversal of progesterone antagonist action. While hPR occupied by the a ntagonists RU486 and ZK112993 are transcriptionally inactive, the anta gonist-occupied receptors become strong activators of transcription in the presence of 8-Br-cAMP. However, this functional switch does not o ccur with the progesterone antagonist ZK98299, which, unlike RU486 and ZK112993, is unable to induce hPR binding to DNA. This suggests that the 8-Br-cAMP-induced transcriptional reversal requires that the antag onist-occupied receptors be bound to DNA. Even with agonist-occupied h PR, addition of 8-Br-cAMP results in a synergistic increase in transcr iptional activity. When hPR(A) alone are transiently expressed in COS- 1 cells, transcription of a reporter gene is stimulated by the agonist R5020 and by 8-Br-cAMP and is synergistic when both are present; but the 8-Br-cAMP-dependent component of transcription proceeds in the abs ence of hPR(A), in the absence of the progesterone response element, a nd in the presence of a DNA-binding domain mutant of hPR(A) that canno t bind to the progesterone response element. Additionally, under the i ntracellular conditions in which 8-Br-cAMP activates antagonist-hPR co mplexes, there is no protein kinase A-mediated phosphorylation of the receptors. We discuss a model in which a gene that is independently tr anscribed by cAMP-responsive factors and by hPR can be selected for po sitive or negative regulation on the transcription complex due to addi tive or cooperative interactions between the two DNA-bound factors.