| Activator Protein-1 (AP-1) family members are the only transcription factors induced by gonadotropin-releasing hormone (GnRH) that are known to directly regulate follicle-stimulating hormone (FSH) beta-subunit gene expression and, of these, c-Fos is the most highly GnRH-induced. Thus, understanding the molecular mechanisms of c-Fos induction by GnRH will provide insight into GnRH regulation of FSH. GnRH induction of the c-Fos promoter maps between -400 and -200 bp from the transcriptional start site, a region that contains Ets and Serum-Response Factor (SRF) binding elements. Mutation of the SRF-binding site in the c-Fos promoter eliminates GnRH induction. However, in contrast to the induction by TPA, which mimics growth-factor treatment, mutation of the Ets site does not eliminate GnRH induction. Furthermore, the SRF site is sufficient for induction by GnRH, while induction by TPA requires both Ets and SRF sites. GnRH induces the MAPK pathway, which phosphorylates Elk-1, causing it to complex with SRF to allow binding to the SRF site. GnRH also induces phosphorylation of SRF, which leads to increased DNA-binding affinity. Phosphorylation of SRF occurs through the CamKII and PKA pathways, the same pathways necessary for c-Fos and FSHa induction by GnRH. A dominant-negative SRF expression vector reduces induction of c-Fos by GnRH, while CamKII activation is sufficient for phosphorylation of SRF and for induction of the c-Fos gene through the SRF site. Thus, GnRH activates the MAPK pathway to phosphorylate Elk-1 and induces CaMKII to phosphorylate SRF and the resulting Elk-1/SRF complex binds and activates the c-Fos promoter. |
