Gonadotropin-releasing hormone activates the equine luteinizing hormone β promoter through a protein kinase C/mitogen-activated protein kinase pathway

Abstract

GnRH regulation of LH secretion is well understood and involves Ca2+ mobilization. However, the mechanism by which GnRH activates transcription of the LHβ gene is controversial. GnRH is known to elevate intracellular calcium and activate the protein kinase C (PKC) pathway. The present study evaluated the pathway(s) involved in GnRH induction of LHβ transcription. We have previously reported that the equine LHβ (eLHβ -448/+60) promoter is active in αT3-1 cells. Therefore, we created a clonal, stably transfected αT3-1 gonadotroph cell line harboring the eLHβ promoter (-448/+60) fused to the luciferase reporter gene. Administration of a GnRH agonist resulted in induction of promoter activity that was completely inhibited by the antagonist antide. Various calcium-affecting drugs had no effect on the promoter. Administration of phorbol 12-myristate 13-acetate (PMA) elicited an activation similar to, albeit lower than, that with GnRH. Down-regulation or pharmacological inhibition of PKC completely blocked PMA's induction of the promoter, while GnRH induction was only partly attenuated. Treatment with the mitogen-activated protein kinase (MAPK) kinase inhibitor, PD98059, completely inhibited the activation of eLHβ by PMA but only partly diminished GnRH's induction. Expression of the transcription factor, early growth response protein 1 (Egr1), correlated completely with activation of MAPK, suggesting that Egr1 is the factor through which PKC/MAPK acts. Our data suggest that GnRH induces activity of the eLHβ promoter by activating a signal transduction cascade involving PKC-MAPK-Egr1 but that has no significant requirement for calcium.