Progesterone regulation of the progesterone receptor in rat gonadotropes

For rat pituitary cells, progesterone receptor (PR) protein localizes to go nadotropes and PR messenger RNA is induced by E-2 and rapidly but transient ly down-regulated by progesterone. Here we quantitatively establish the dow n-regulatory effect of progesterone on PR protein and evaluate possible mec hanisms. Nuclear PR-immunoreactivity (PR-IR) in gonadotropes, identified by dual immunofluorescence, was analyzed by quantitative confocal microscopy. Pituitary cells from female rate were cultured +/- 0.2 nM E-2 for 3 days. We confirmed the E-2 requirement for PR induction in gonadotropes and deter mined that the increase in PR-IR required about 24 h. After removal of E-2, PR-IR decreases were not found until 24-36 h. Addition of progesterone (40 nM) to E-2-treated cells led to a dramatic loss in PR-IR by 9 h (26% of co ntrol); by 24 h, PR-IR was barely detectable. Reappearance of nuclear PR-IR required progesterone removal (8-fold increase by 12 h after progesterone removal) and protein synthesis (cycloheximide inhibited the reappearance of PR-IR). Although progesterone decreased PR-IR whether or not E-2 was prese nt concurrent with progesterone, the recovery of PR-IR required E-2. RU486 completely blocked progesterone-induced PR down-regulation. Because the sus tained progesterone-induced loss of PR protein did not correlate with previ ously reported temporal changes in PR messenger RNA levels, we examined a r ole for protein degradation. When cells were coincubated with progesterone and the proteasome inhibitor, MG132 (1 mu M), the expected decrease in PR p rotein was abrogated. In summary, progesterone leads to a rapid and extensi ve reduction in nuclear PR protein in gonadotropes. The progesterone-depend ent downregulation of PR occurs, at least in. part, by a proteasome-mediate d pathway. Recovery of PR protein requires removal of progesterone, the pre sence of E-2, and protein synthesis. These dynamic changes in nuclear PR le vels coincide with the temporal extent of the preovulatory LH surge in rats and could provide a basis for progesterone's biphasic action on LH secreti on.