Different signaling pathways control acute induction versus long-term repression of LHbeta transcription by GnRH.

GnRH regulates pituitary gonadotropin gene expression through GnRH receptor activation of the protein kinase C (PKC) and calcium signaling cascades. The pulsatile pattern of GnRH release is crucial for induction of LHbeta-subunit (LHbeta) gene expression; however, continuous prolonged GnRH exposure leads to repression of LHbeta gene transcription. Although in part, long-term repression may be due to receptor down-regulation, the molecular mechanisms of this differential regulation of LHbeta transcription are unknown. Using transfection into the LH-secreting immortalized mouse gonadotrope cell line (LbetaT4), we have demonstrated that LHbeta gene transcription is increased by acute activation (6 h) of GnRH receptor or PKC but not calcium influx; in contrast long-term activation (24 h) of GnRH receptor, PKC, or calcium influx each repress LHbeta transcription. Whereas blockade of PKC prevented the acute action of GnRH and unmasked an acute repression of LHbeta transcription by calcium, it did not prevent long-term repression by GnRH or calcium. Removal of calcium resulted in potentiation of acute GnRH and PKC induction of LHbeta gene expression but prevented long-term repression by GnRH and reduced long-term repression by either calcium or 12-O-tetradecanoyl-phorbol-13-acetate (TPA). We conclude that GnRH uses PKC for acute induction, and calcium signaling is responsible for long-term repression of LHbeta gene expression by GnRH. Furthermore, analysis of the responsiveness of truncated and mutated LHbeta promoter regions demonstrated that not only do acute induction and long-term repression use different signaling systems, but they also use different target sequences for regulating the LHbeta gene.