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

Abstract

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 LHβ-subunit (LHβ) gene expression; however, continuous prolonged GnRH exposure leads to repression of LHβ gene transcription. Although in part, long-term repression may be due to receptor down-regulation, the molecular mechanisms of this differential regulation of LHβ transcription are unknown. Using transfection into the LH-secreting immortalized mouse gonadotrope cell line (LβT4), we have demonstrated that LHβ 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 LHβ transcription. Whereas blockade of PKC prevented the acute action of GnRH and unmasked an acute repression of LHβ 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 LHβ 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 LHβ gene expression by GnRH. Furthermore, analysis of the responsiveness of truncated and mutated LHβ 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 LHβ gene.