Divergent signaling pathways requiring discrete calcium signals mediate concurrent activation of two mitogen-activated protein kinases by gonadotropin-releasing hormone

Receptors coupled to heterotrimeric G proteins are linked to activation of mitogen-activated protein kinases (MAPKs) via receptor- and cell-specific m echanisms. We have demonstrated recently that gonadotropin-releasing hormon e (GnRH) receptor occupancy results in activation of extracellular signal-r egulated kinase (ERK) through a mechanism requiring calcium influx through L-type calcium channels in alpha T3-1 cells and primary rat gonadotropes, F urther studies were undertaken to explore the signaling mechanisms by which the GnRH receptor is coupled to activation of another member of the MAPK f amily, c-Jun N-terminal kinase (JNK). GnRH induces activation of the JNK ca scade in a dose-, time-, and receptor-dependent manner in clonal alpha T3-1 cells and primary rat pituitary gonadotrophs, Coexpression of dominant neg ative Cdc42 and kinase-defective p21-activated kinase 1 and MAPK kinase 7 w ith JNK and ERK indicated that specific activation of JNK by GnRH appears t o involve these signaling molecules. Unlike ERK activation, GnRH-stimulated JNK activity does not require activation of protein kinase C and is not bl ocked after chelation of extracellular calcium with EGTA. GnRH-induced JNK activity was reduced after treatment with the intracellular calcium chelato r BAPTA-AM (1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid acet oxymethyl eater), whereas activation of ERK was not affected. Chelation of intracellular calcium also reduced GnRH-induced activation of JNK in rat pi tuitary cells in primary culture. GnRH-induced induction and activation of the JNK target c-Jun was inhibited after chelation of intracellular calcium , whereas induction of c-Fos, a known target of ERK, was unaffected. Theref ore, although activation of ERK by GnRH requires a specific influx of calci um through L-type calcium channels, JNK activation is independent of extrac ellular calcium but sensitive to chelation of intracellular calcium. Our re sults provide novel evidence that GnRH activates two MAPK superfamily membe rs via strikingly divergent signaling pathways with differential sensitivit y to activation of protein kinase C and mobilization of discrete pools of c alcium.