Elk-1, CREB, and MKP-1 Regulate Egr-1 Expression in Gonadotropin-Releasing Hormone Stimulated Gonadotrophs

Stimulation of gonadotropin-releasing hormone (GnRH) receptors with the GnRH analogue buserelin enhances expression of the zinc finger transcription factor Egr-1 in a pituitary gonadotroph cell line. The signaling cascade is blocked by overexpression of MAP kinase phosphatase-1 that dephosphorylates extracellular signal-regulated protein kinase in the nucleus. Chromatin immunoprecipitation experiments revealed that the phosphorylated form of Elk-1, a key regulator of gene transcription driven by serum response element (SRE), binds to the 5'-upstream region of the Egr-1 gene in buserelin-stimulated gonadotrophs. Expression of a dominant-negative mutant of Elk-1 completely blocked Egr-1 expression, indicating that Elk-1 connects the intracellular signaling cascade elicited by activation of GnRH receptors with transcription of the Egr-1 gene. GnRH receptor activation additionally induced the phosphorylation of CREB, which in its phosphorylated form bound to the Egr-1 gene. Expression of a dominant-negative mutant of CREB reduced GnRH receptor-induced upregulation of Egr-1 expression, indicating that CREB plays a role in the signaling pathway that regulates Egr-1 expression in gonadotrophs. We further identified the genes encoding basic fibroblast growth factor, tumor necrosis factor a, and transforming growth factor beta as bona fide target genes of Egr-1 in gonadotrophs. The analysis of gonadotroph cells that express-in addition to GnRH receptors-muscarinic M-3 acetylcholine receptors revealed that the nuclear events connecting GnRH receptors and muscarinic M-3 acetylcholine receptors with the Egr-1 gene are indistinguishable. J. Cell. Biochem. 105: 1267-1278, 2008. (c) 2008 Wiley-Liss, Inc.