Activation of the progesterone receptor by the gonadotropin-releasing hormone self-priming signaling pathway.

Signal amplification is fundamental to the normal operation of the preovulatory LH surge and is achieved through processes such as GnRH self-priming and augmentation of stimulated LH secretion by progesterone. We have proposed a model for GnRH self-priming that requires cross-communication between a GnRH receptor-activated protein kinase A pathway and the progesterone receptor (PR) to achieve amplification of the GnRH signal. We found that a pulse of GnRH administered to gonadotrope-enriched pituitary cells cultured in medium containing charcoal-treated serum plus estradiol (E2) potentiated the LH secretory response to subsequent GnRH pulses, and this potentiation could be blocked by a PR antagonist, RU486, in the absence of progesterone. Similarly, exposure of gonadotrope-enriched cultures to forskolin augmented the response to a pulse of GnRH, and the augmentation due to cAMP elevation could be reduced by RU486 in the absence of progesterone. To directly test whether stimulation with either GnRH or a cAMP analog results in transactivation of the endogenous PR, we used rat anterior pituitary cells cultured in the presence of E2 and transfected with reporter plasmids containing progesterone-responsive elements (PRE) and either a E1b or a thymidine kinase (tk) promoter linked to the chloramphenicol acetyltransferase (CAT) gene. For pituitary cells transfected with the PRE-E1b-CAT plasmid, exposure to either progesterone, GnRH, or 8-bromo-cAMP (8BrcAMP) for 6 h resulted in an induction of CAT activity which could be suppressed by coincubation with RU486. RU486 by itself had no effect on CAT activity. Similar results were obtained when a plasmid containing a different promoter (PRE-tk-CAT) was used. For cells transfected with a construct lacking a PRE (pSV2CAT), 8BrcAMP was without effect on CAT expression. When cells were made PR-deficient by omission of E2 from the incubation medium and transfected with PRE-E1b-CAT, neither progesterone, GnRH, nor 8BrcAMP was able to induce CAT activity. In summary we found that either GnRH or 8BrcAMP is able to stimulate transcription of reporter genes linked to two different PRE-containing promoters in anterior pituitary cells that contain endogenous PR; this occurred in the absence of progesterone and was suppressed by a PR antagonist. A simple interpretation of these data is that a GnRH-triggered signaling cascade can result in progesterone-independent transactivation of the PR. We propose that, in the normal operation of the preovulatory LH surge, the pathways for GnRH self-priming and progesterone augmentation converge at the PR and that the pathways serve as physiological redundancies to ensure the LH surge.