Constitutively Active Gq Impairs Gonadotropin-Releasing Hormone-Induced Intracellular Signaling and Luteinizing Hormone Secretion in LβT2 Cells

Chronic GnRH treatment causes homologous desensitization by reducing GnRH receptor and Gq/11 expression and by down-regulating protein kinase C (PKC), cAMP, and calcium-dependent signaling. It also causes heterologous desensitization of other Gq-coupled receptors, but the mechanisms involved remain elusive. In this study, we investigated the effect of constitutive activation of Gq signaling on GnRH-induced signaling and LH secretion. We show that adenoviral expression of a constitutively active mutant Gq(Q209L) results in a state of GnRH resistance but does not alter GnRH receptor expression. We observed that Gq(Q209L) reduced expression of phospholipase C (PLC)beta1, a target of Gq in these cells, but not PLCbeta3 or PLCgamma1. Downstream of PLCbeta1, expression of novel PKC isoforms (delta and epsilon) was reduced. Adenoviral expression of a kinase-inactive, dominant-negative version of PKCdelta impaired GnRH activation of ERK, but not induction of c-Fos and LHbeta proteins, indicating that the novel PKCs signal to the ERK cascade. Despite reductions in PLCbeta1, calcium responses to GnRH were elevated in Gq(Q209L)-infected cells due to increased calcium influx through L-type calcium channels. Paradoxically, downstream calcium-dependent signaling and LH secretion were impaired. Taken together, these data demonstrate that prolonged activation of the Gq pathway desensitizes GnRH-induced signaling by selectively down-regulating the PLC-PKC-Ca2+ pathway, leading to reduced LHbeta synthesis and LH secretion.