Constitutively active Gq impairs gonadotropin-releasing hormone-induced intracellular signaling and luteinizing hormone secretion in LbetaT2 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.
Liu
Fujun
F
Department of Medicine, University of California, San Diego, La Jolla, California 92093, USA.
Ruiz
Maribeth S
MS
Austin
Darrell A
DA
Webster
Nicholas J G
NJ
D021581:Active Transport, Cell Nucleus; D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D015536:Down-Regulation; D004789:Enzyme Activation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D043802:GTP-Binding Protein alpha Subunits, Gq-G11 / Q000378:metabolism / Q000502:physiology*; D007987:Gonadotropin-Releasing Hormone / Q000378:metabolism*; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D007986:Luteinizing Hormone / Q000378:metabolism*; D051379:Mice; D009154:Mutation; D054799:Phospholipase C beta; D010766:Phosphorylation; D020033:Protein Isoforms; D011493:Protein Kinase C / Q000378:metabolism; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism; D015398:Signal Transduction; D013997:Time Factors; D010738:Type C Phospholipases / Q000378:metabolism
D007527:Isoenzymes; D020033:Protein Isoforms; D016760:Proto-Oncogene Proteins c-fos; D007987:Gonadotropin-Releasing Hormone; D007986:Luteinizing Hormone; D011493:Protein Kinase C; D048049:Extracellular Signal-Regulated MAP Kinases; D010738:Type C Phospholipases; D054799:Phospholipase C beta; C518034:Plcb1 protein, mouse; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D002118:Calcium