Involvement of both Gq/11 and Gs proteins in gonadotropin-releasing hormone receptor-mediated signaling in LβT2 cells

The hypothalamic hormone gonadotropin-releasing hormone (GnRH) stimulates the synthesis and release of the pituitary gonadotropins. GnRH acts through a plasma membrane receptor that is a member of the G protein-coupled receptor (GPCR) family. These receptors interact with heterotrimeric G proteins to initiate downstream signaling. In this study, we have investigated which G proteins are involved in GnRH receptor-mediated signaling in LbetaT2 pituitary gonadotrope cells. We have shown previously that GnRH activates ERK and induces the c-fos and LHbeta genes in these cells. Signaling via the G(i) subfamily of G proteins was excluded, as neither ERK activation nor c-Fos and LHbeta induction was impaired by treatment with pertussis toxin or a cell-permeable peptide that sequesters Gbetagamma-subunits. GnRH signaling was partially mimicked by adenoviral expression of a constitutively active mutant of Galpha(q) (Q209L) and was blocked by a cell-permeable peptide that uncouples Galpha(q) from GPCRs. Furthermore, chronic activation of Gaq signaling induced a state of GnRH resistance. A cell-permeable peptide that uncouples Galpha(s) from receptors was also able to inhibit ERK, c-Fos, and LHbeta, indicating that both G(q/11) and G(s) proteins are involved in signaling. Consistent with this, GnRH caused GTP loading on G(s) and G(q/11) and increased intracellular cAMP. Artificial elevation of cAMP with forskolin activated ERK and caused a partial induction of c-Fos. Finally, treatment of Galpha(q) (Q209L)-infected cells with forskolin enhanced the induction of c-Fos showing that the two pathways are independent and additive. Taken together, these results indicate that the GnRH receptor activates both G(q/11) and G(s) signaling to regulate gene expression in LbetaT2 cells.