Steroid regulation of progesterone receptor expression in cultured rat gonadotropes

During the preovulatory period. the pituitary action of progesterone is bip hasic, moving from a severalfold augmentation of the gonadotropin release a ction of GnRH to a suppression of GnRH efficacy, which occurs in rats over a period of about 12 h, but the extent to which these biphasic effects are dependent on alterations in progesterone receptor (PR) expression is not kn own. To address this, as well as the localization of PR in cultured rat pit uitary cells, we used cells from ovariectomized rats cultured +/- 0.2 nM E- 2 with acute progesterone treatment on day 3. Northern blot of poly(A(+)) R NA extracts showed multiple PR messenger RNA (mRNA) transcripts between 4.8 -10.2 kb; E-2 treatment led to a 5- to g-fold increase in the predominant P R mRNA, transcripts !5.1 and 110.1 kb. In the presence of E-2, 200 nM proge sterone resulted in a decrease in steady-state PR mRNA levels by 3 h of exp osure, with the greatest decrease around 6 h (50% of E-2, control) and reco very by 12 h. Similarly treated pituitary cultures were subjected to dual i mmunofluorescence staining for LH and PR. In the absence of E-2, PR was und etectable. In the presence of E-2 essentially all LH-positive cells were po sitive for PR and only 1-2% of PR-immunopositive cells were negative for LH , possibly reflecting FSH-exclusive gonadotropes, PR staining was predomina ntly nuclear, but 20 nM progesterone led to a gradual increase in cytoplasm ic staining, with the nuclear-to-cytoplasmic ratio decreasing to near unity by 9-12 h of exposure. In summary, we show for the first time, that PR col ocalizes with LH in cultured female rat pituitary cells and that E-2 induce s expression of PR mRNA, as well as PR protein, in rat gonadotropes. In the presence of E-2, progesterone causes a rapid but transient down-regulation of PR message; recovery of PR mRNA is accompanied by an increase in cytopl asmic PR, suggestive of an increase in synthesis. These dynamic changes imp licate the gonadotrope PR as having a significant role within the temporal context of the rat preovulatory period.