Phosphorylation of Gα11 Protein Contributes to Agonist-Induced Desensitization of 5-HT2A Receptor Signaling

Agonist treatment causes desensitization of many G proteincoupled receptor systems. Recent advances have delineated changes in receptors in the desensitization response; however, the role of G proteins remains unclear. We investigated the role of phosphorylation of Gαq/11 proteins in agonist-induced desensitization of serotonin 2A (5-HT2A) receptors. In an embryonic rat cortical cell line (A1A1v), 24-h treatment with 100 nM (-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI), a 5-HT2A/2C receptor agonist, decreased DOI-stimulated inositol phosphate accumulation and increased the phosphorylation of Gαq/11 proteins, as demonstrated by immunoprecipitation of Gαq/11 and both incorporation of 32P phosphate and labeling with a S/T/Y phosphorylation-dependent antibody. Treatment with DOI for 30 min induced desensitization but did not increase phosphorylation of Gαq/11 proteins, suggesting that different mechanisms are involved in desensitization after short- and long-term treatments. Mutation of S154A in a protein kinase C (PKC) and calcium/calmodulin dependent kinase (CaMK) consensus site in Gα11 significantly reduced DOI-stimulated phosphorylation of Gα11 and DOI-induced desensitization of 5-HT2A receptor signaling. Inhibition of PKC and CaMK attenuated phosphorylation of Gαq/11 proteins and DOI-induced desensitization of 5-HT2A receptors. Expression of Gα11 S154D, a phosphorylation mimic, reduced DOI-stimulated inositol phosphate accumulation. DOI treatment for 24 h also produced heterologous desensitization, as indicated by decreased bradykinin-stimulated inositol phosphate accumulation. These data suggest that phosphorylation of Gα11 protein by PKC and CaMK contributes to agonistinduced homologous desensitization of 5-HT2A receptor signaling as well as heterologous desensitization. The phosphorylation of Gα protein represents a novel mechanism involved in regulation of receptor signaling and agonist-induced desensitization of G protein-coupled receptors.