Gonadotropin-releasing hormone induction of extracellular-signal regulated kinase is blocked by inhibition of calmodulin

Abstract

Our previous studies demonstrate that GnRH-induced ERK activation required influx of extracellular Ca2+ in alphaT3-1 and rat pituitary cells. In the present studies, we examined the hypothesis that calmodulin (Cam) plays a fundamental role in mediating the effects of Ca2+ on ERK activation. Cam inhibition using W7 was sufficient to block GnRH-induced reporter gene activity for the c-Fos, murine glycoprotein hormone alpha-subunit, and MAPK phosphatase (MKP)-2 promoters, all shown to require ERK activation. Inhibition of Cam (using a dominant negative) was sufficient to block GnRH-induced ERK but not c-Jun N-terminal kinase activity activation. The Cam-dependent protein kinase (CamK) II inhibitor KN62 did not recapitulate these findings. GnRH-induced phosphorylation of MAPK/ERK kinase 1 and c-Raf kinase was blocked by Cam inhibition, whereas activity of phospholipase C was unaffected, suggesting that Ca2+/Cam modulation of the ERK cascade potentially at the level of c-Raf kinase. Enrichment of Cam-interacting proteins using a Cam agarose column revealed that c-Raf kinase forms a complex with Cam. Reconstitution studies reveal that recombinant c-Raf kinase can associate directly with Cam in a Ca2+-dependent manner and this interaction is reduced in vitro by addition of W7. Cam was localized in lipid rafts consistent with the formation of a Ca2+-sensitive signaling platform including the GnRH receptor and c-Raf kinase. These data support the conclusion that Cam may have a critical role as a Ca2+ sensor in specifically linking Ca2+ flux with ERK activation within the GnRH signaling pathway.