Multimodal control of pituitary gonadotropin biosynthesis. Effects of gonadal steroids, GnRH, cyclic AMP and diacylglycerols.

Abstract

We and others have previously shown that in the rat and the sheep gonadectomy increases the translational capacity of mRNAs encoding gonadotropin subunits alpha, LH beta and FSH beta. Injection of estradiol or of testosterone or dihydrotestosterone depresses the translational capacity of the mRNAs. After using estradiol to induce progesterone receptors in the pituitary of castrated animals, it was determined that progesterone does enhance the inhibitory effect of estradiol. We have also observed this inhibitory effect of gonadal steroids in vitro, suggesting that at least part of the steroid action is exerted at the pituitary level. Hybridization assay (Northern blot) using oligodeoxynucleotide probes complementary to a short portion in the cDNA strand of each subunit, showed gonadal steroids to act by decreasing the number of copies of mRNAs encoding LH and FSH subunits. Using anterior pituitary cells in culture, incubated in the presence of labeled methionine, we have confirmed our previous observation that GnRH stimulates the biosynthesis of the polypeptide chains of LH. This effect is not secondary due to LH release. It is not inhibited when incubation is performed in the presence of tunicamycin, an inhibitor of glycosylation. SDS-polyacrylamide gel electrophoresis of specific immunoprecipitates of polypeptides immunologically related to alpha allowed us to identify 3 forms of alpha-polypeptide differing in their apparent Mr:21K (partially glycosylated), 23K (authentic) and 25K (hyperglycosylated). Besides its stimulatory effect on the release and synthesis of LH, GnRH also stimulated the release of the 23K and 25K forms of alpha. In the presence of tunicamycin an additional 16K form of apoprotein-alpha was detected which accumulated within the cells. A cAMP analogue (8-Br-cAMP), intracellular cAMP generators (choleragen, forskolin), as well as an analogue of diacylglycerols (TPA) mimic the stimulatory action of GnRH. However, although no evidence has been obtained at present that either cAMP or diacylglycerols mediate the GnRH effect on the biosynthesis of the polypeptide chains of LH, our data suggest that phosphorylation of intracellular phosphoproteins plays a major role in this process.