Down-regulation of the gonadotropin-releasing hormone receptor messenger ribonucleic acid by activation of adenylyl cyclase in alpha T3-1 pituitary gonadotrope cells.

Pituitary regulation of reproductive processes depends on the sensitivity of gonadotrope cells to both positive and negative regulators. Hypothalamic GnRH is the primary stimulus for gonadotropin synthesis and secretion. Therefore, the ability of the gonadotrope to respond to GnRH and the status of GnRH receptors (GnRH-R) are critical in the control of reproduction. In the present study, we address the role of GnRH and two second messenger activators, a phorbol ester (12-O-tetradecanoylphorbol-13-acetate) and forskolin, in the regulation of GnRH-R gene expression in the alpha T3-1 gonadotrope cell line. Using Northern blot analysis to monitor endogenous GnRH-R steady state messenger RNA (mRNA) levels, we found that although GnRH and 12-O-tetradecanoylphorbol-13-acetate do not change GnRH-R mRNA levels, forskolin causes a time-dependent decline. All three treatments stimulate glycoprotein alpha-subunit gene expression. To dissect the molecular mechanism of forskolin action on GnRH-R gene expression, de novo RNA synthesis was inhibited with the transcription inhibitor, actinomycin-D (act-D). Act-D alone does not change GnRH-R message levels. However, in the presence of both act-D and forskolin, GnRH-R mRNA levels decline dramatically. These data demonstrate that forskolin alters GnRH-R posttranscriptionally by destabilizing its mRNA. Our data do not, however, exclude possible direct transcriptional effects. This study suggests that activators of the protein kinase-A pathway may alter gonadotrope sensitivity to GnRH by decreasing GnRH-R gene expression and, therefore, indirectly affect reproductive status.