Differential Activation of the Luteinizing Hormone (beta)-Subunit Promoter by Activin and Gonadotropin-Releasing Hormone: A Role for the Mitogen-Activated Protein Kinase Signaling Pathway in L(beta)T2 Gonadotrophs

LH consists of (alpha)- and (beta)-subunits, and synthesis of the (beta)-subunit has been reported to be the rate-limiting step in LH production. In this study, we found that activin A increased both the LH(beta) mRNA level and LH content in cells of the gonadotroph cell line, L(beta)T2. We next examined the effects of activin A and GnRH on LH(beta) promoter activity by reporter gene assay and compared the signal transduction pathways. Activin A and GnRH activated the LH(beta) promoter, and the response to a combination of activin A and GnRH was higher than that to activin A or GnRH alone. The effects of activin A and GnRH were specifically inhibited by inhibin-like peptide and antide, a GnRH antagonist, respectively. The activation of the LH(beta) promoter by GnRH was inhibited by PD098059 and U0126, MAP kinase kinase (MEK) inhibitors. In contrast, these protein kinase inhibitors did not inhibit the activin A-induced activation. GnRH, but not activin A, activated MAP kinase in L(beta)T2 cells. Overexpression of constitutively active MEK1 or MEK kinase activated both MAP kinase and the LH(beta) promoter. Furthermore, GnRH, but not activin A, strongly induced SRE-mediated transcription, a known target of the MAP kinase pathway. These results suggest that GnRH activates the LH(beta) promoter via the MAP kinase pathway and that activin A-induced activation of the LH(beta) promoter is independent of the MAP kinase pathway.