DIFFERENTIAL USE OF SIGNAL-TRANSDUCTION PATHWAYS IN THE GONADOTROPIN-RELEASING HORMONE-MEDIATED REGULATION OF GONADOTROPIN SUBUNIT GENE-EXPRESSION

The regulation of LH and FSH subunit gene expression is under the cont rol of GnRH. Physiological changes in the frequency of pulsatile GnRH release from the hypothalamus result in differential stimulation of al pha-, LH beta-, and FSH beta-gene expression. Previous studies indicat e that the GnRH receptor couples to G proteins of the G(q/11) family, with phosphoinositide turnover and its resultant increase in intracell ular calcium concentration and protein kinase C (PKC) activation, to s timulate secretion of LH and FSH. However, the molecular mechanisms by which GnRH mediates its transcriptional effects remain largely unknow n. We used GH, cells, constitutively expressing the rat GnRH receptor (GGH(3)-1' cells) and transiently transfected with a luciferase report er gene controlled by either the alpha, LH beta, or FSH beta gene regu latory region (alpha LUC, LH beta LUC, and FSH beta LUC, respectively) , to examine the roles of several signal transduction pathways in the GnRH-mediated stimulation of gonadotropin subunit gene expression. Act ivation of PKC by phorbol, 12-myristate, 13-acetate resulted in an inc rease in the luciferase activity of all three gonadotropin subunit gen e reporter constructs. Phorbol, 12-myristate, 13-acetate had a greater stimulatory effect, relative to the maximal stimulation with GnRH, fo r the beta-subunit genes than for the alpha-subunit gene. Depletion of PKC, or inhibition of PKC by GF109203X, demonstrated that PKC-depende nt pathways play a larger role in the GnRH-mediated transcriptional co ntrol of the LH beta- and FSH beta-genes than the alpha-subunit gene. In contrast, an L-type calcium channel agonist, Bay K 8644, was able t o stimulate alpha LUC but not LH beta LUC or FSH beta LUC. Nimodipine, an L-type calcium channel antagonist, had a larger inhibitory effect on the GnRH response of alpha LUC, relative to LH beta LUC or FSH beta LUC. We conclude from these results that the differential regulation of gonadotropin subunit gene expression by GnRH is caused, in part, by differential use of signal transduction pathways, activated upon GnRH binding.