Expression of progesterone receptor (PR) mRNA in granulosa cells of ovarian preovulatory follicles is induced by LH (1, 2) and is essential for ovulation (3). Although 17β-estradiol (E) can induce PR mRNA and activate PR promoter-reporter constructs in other cell types, the effects of E in granulosa cells appear to be indirect. We show herein that E alone does not induce the expression of PR mRNA in preovulatory granulosa cells. Rather, induction of PR mRNA depends on the differentiation of granulosa cells in response to E and a physiological amount of FSH followed by exposure to agonists (elevated levels of LH, FSH, and forskolin) that markedly increase cAMP. Induction of PR mRNA by forskolin is blocked by the A-kinase inhibitor H89 and cycloheximide but not by the E antagonist, ICI 164,384. These results indicate that phosphorylation and synthesis of some regulatory factor(s) other than or in addition to the estrogen receptor (ER) are essential for transactivation of the PR gene. When distal and proximal PR promoter-reporter constructs that are responsive to E in other cell types were transiently transfected into differentiated granulosa cells, forskolin, but not E, induced activity. Likewise, when a vector containing the consensus vitellogenin B1 gene estrogen response element (ERE) was transfected into differentiated granulosa cells, forskolin, but not E, induced activity. Using electrophoretic mobility shift assays, the consensus ERE was shown to bind ERβ, the predominant subtype present in rat granulosa cells, and ERα, the predominant subtype present in luteal cells, whereas the putative ERE-like region (ERE3) of the proximal PR promoter did not bind either ER subtype. Although the identity of the specific factors binding to the ERE3 site remain to be determined, mutation of this region abolished forskolin-induced activity of ERE3-PR-CAT constructs. The GC-rich region of the distal PR promoter bound Sp1 and Sp3 but not C/EBPα/β, indicating that factors binding to ERE3 interact synergistically with Sp1/Sp3 to confer increased responsiveness of the distal promoter to forskolin. Taken together, these results indicate that activation of the A-kinase pathway leads to the phosphorylation of some transcription factor(s) other than or in addition to ER that is (are) critical for the transactivation of the PR gene and that this mechanism is selectively activated in differentiated granulosa cells possessing a preovulatory phenotype.
|Number of pages||14|
|Publication status||Published - 1 Dec 1998|