Differential Gonadotropin-Releasing Hormone Stimulation of Rat Luteinizing Hormone Subunit Gene Transcription by Calcium Influx and Mitogen-Activated Protein Kinase-Signaling Pathways

Abstract

Gonadotropin secretion and gene expression are differentially regulated by hypothalamic GnRH pulses by unknown mechanisms. GnRH stimulates calcium influx through L-type voltage-gated channels and activates phospholipase C, leading to increased protein kinase C (PKC) and mitogen-activated protein kinase activity. We found differential contributions of these pathways to GnRH-stimulated rat LH subunit transcription in pituitary gonadotropes and cell lines. Endogenous transcription of the alpha- and LHbeta-subunits in rat pituitary cells was stimulated by GnRH. Independent PKC activation by phorbol myristate acid stimulated only the alpha-subunit gene. In contrast, an L-channel antagonist (nimodipine) inhibited only LHbeta stimulation by GnRH, and an L-channel agonist (BayK 8644) stimulated only basal LHbeta transcription. GnRH induction of a rat alpha-subunit promoter construct in alphaT3 cells was unaffected by nimodipine or elimination of external calcium, while both treatments eliminated the LHbeta response. Application of a mitogen-activated kinase kinase (MEK) inhibitor (PD098059) decreased basal and GnRH-stimulated alpha-subunit promoter activity and had no effect on LHbeta promoter activity. In pituitary cells from mice bearing an LHbeta promoter-luciferase reporter transgene, GnRH stimulation was inhibited by nimodipine but not by PD098059. Thus, GnRH induction and basal control of the alpha-subunit gene seem to occur through the PKC/mitogen-activated protein kinase pathway, while induction of the LHbeta gene is dependent on calcium influx. Differential signaling from the same receptor may be a mechanism for preferential regulation of transcription.