GnRH pulse frequency control of fshb gene expression is mediated via ERK1/2 regulation of ICER

Abstract

The pulsatile release of GnRH regulates the synthesis and secretion of pituitary FSH and LH. Two transcription factors, cAMP-response element-binding protein (CREB) and inducible cAMP early repressor (ICER), have been implicated in the regulation of rat Fshb gene expression. We previously showed that the protein kinase A pathway mediates GnRH-stimulated CREB activation. We hypothesized that CREB and ICER are activated by distinct signaling pathways in response to pulsatile GnRH to modulate Fshb gene expression, which is preferentially stimulated at low vs high pulse frequencies. In the LβT2 gonadotrope-derived cell line, GnRH stimulation increased ICERmRNAand protein. Blockade of ERK activation with mitogen-activated protein kinase kinase I/II (MEKI/II) inhibitors significantly attenuatedGnRHinduction of ICERmRNAand protein, whereas protein kinase C, calcium/calmodulindependent protein kinase II, and protein kinaseAinhibitors had minimal effects.GnRHalso stimulated ICER in primary mouse pituitary cultures, attenuated similarly by a MEKI/II inhibitor. In a perifusion paradigm, MEKI/II inhibition in LβT2 cells stimulated with pulsatile GnRH abrogated ICER induction at highGnRHpulse frequencies, with minimal effect at low frequencies. MEKI/II inhibition reducedGnRH stimulation of Fshb at high and low pulse frequencies, suggesting that theERKpathway has additional effects on GnRH regulation of Fshb, beyond those mediated by ICER. Indeed, induction of the activating protein 1 proteins, cFos and cJun, positive modulators of Fshb transcription, by pulsatile GnRH was also abrogated by inhibition of the MEK/ERK signaling pathway. Collectively, these studies indicate that the signaling pathways mediating GnRH activation of CREB and ICER are distinct, contributing to the decoding of the pulsatile GnRH to regulate FSHβ expression.