Regulation of Gonadotropin-Releasing Hormone Receptors by Protein Kinase C: Inside Out Signalling and Evidence for Multiple Active Conformations

Desensitization and internalization of G protein-coupled receptors can be mediated by phosphorylation within the C-terminal tail, facilitating β-arrestin binding and targeting the receptor for internalization. Type II GnRH receptors (GnRH-Rs) show such regulation, but type I GnRH-Rs lack C-tails and are not rapidly desensitized or internalized. Here we show contrasting susceptibility of type I (human and sheep) and II (Xenopus) GnRH-Rs to regulation by protein kinase C (PKC). When human (h) or Xenopus (X) GnRH-Rs were expressed using recombinant adenovirus, PKC activation increased radioligand binding to XGnRH-Rs but not to hGnRH-Rs. A dominant-negative dynamin mutant (K44A) inhibited internalization of XGnRH-Rs (but not hGnRH-Rs) without influencing PKC regulation of XGnRH-R binding. PKC activation increased the affinity of XGnRH-Rs for the type II GnRH ligand and increased effects of low concentrations of GnRH-II on the [Ca2+]i but had no effect on type I ligand binding to hGnRH-Rs, sGnRH-Rs or XGnRH-Rs, or to chimeric receptors with the XGnRH-R C-tail added to a type I receptor. Binding of type II ligand to human or sheep receptors was also unaffected but was increased in the chimeras. Mutation of both PKC-phosphorylation consensus sites in the XGnRH-R tail did not prevent the PKC-mediated increases in binding or alter agonist-induced translocation of β-arrestin2/green fluorescent protein or inhibition of inositol phosphate accumulation by β-arrestin2/green fluorescent protein. Thus, it appears that there are two distinct active conformations of XGnRH-Rs (differing in affinity for type I and II ligands) and that these cells exhibit a novel form of inside-out signaling in which PKC feeds back to influence receptor affinity.