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 control of GnRH. Physiological changes in the frequency of pulsatile GnRH release from the hypothalamus result in differential stimulation of alpha-, LHbeta-, and FSHbeta-gene expression. Previous studies indicate that the GnRH receptor couples to G proteins of the G(q/11) family, with phosphoinositide turnover and its resultant increase in intracellular calcium concentration and protein kinase C (PKC) activation, to stimulate secretion of LH and FSH. However, the molecular mechanisms by which GnRH mediates its transcriptional effects remain largely unknown. We used GH3 cells, constitutively expressing the rat GnRH receptor (GGH(3)-1' cells) and transiently transfected with a luciferase reporter gene controlled by either the alpha, LHbeta, or FSHbeta gene regulatory region (alphaLUC, LHbetaLUC, and FSHbetaLUC, respectively), to examine the roles of several signal transduction pathways in the GnRH-mediated stimulation of gonadotropin subunit gene expression. Activation of PKC by phorbol, 12-myristate, 13-acetate resulted in an increase in the luciferase activity of all three gonadotropin subunit gene reporter constructs. Phorbol, 12-myristate, 13-acetate had a greater stimulatory effect, relative to the maximal stimulation with GnRH, for the beta-subunit genes than for the alpha-subunit gene. Depletion of PKC, or inhibition of PKC by GF109203X, demonstrated that PKC-dependent pathways play a larger role in the GnRH-mediated transcriptional control of the LHbeta- and FSHbeta-genes than the alpha-subunit gene. In contrast, an L-type calcium channel agonist, Bay K 8644, was able to stimulate alphaLUC but not LHbetaLUC or FSHbetaLUC. Nimodipine, an L-type calcium channel antagonist, had a larger inhibitory effect on the GnRH response of alphaLUC, relative to LHbetaLUC or FSHbetaLUC. 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.