Received for publication June 22, 2006.
Revised October 20, 2006.
Accepted for publication October 20, 2006.

Phosphorylation of G₁₁ protein contributes to agonist-induced desensitization of 5-HT_2A receptor signaling

Abstract

Agonist treatment causes desensitization of many G protein-coupled 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 G_q/11 proteins in agonist-induced desensitization of serotonin 2A (5-HT_2A) receptors. In an embryonic rat cortical cell line (A1A1v), 24 hour treatment with 100 nM of a 5-HT_2A/2C receptor agonist DOI ((-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl) decreased DOI-stimulated inositol phosphate accumulation and increased the phosphorylation of G_q/11 proteins as demonstrated by immunoprecipitation of G_q/11 and both incorporation of ³²P phosphate and labeling with a S/T/Y phosphorylation-dependent antibody. 30 min treatment with DOI induced desensitization but did not increase phosphorylation of G_q/11 proteins suggesting that different mechanisms are involved in desensitization following acute and chronic treatments. Mutation of S154A in a protein kinase C (PKC) and calcium/calmodulin dependent kinase (CaMK) consensus site in G₁₁ significantly reduced DOI-stimulated phosphorylation of G₁₁ and DOI-induced desensitization of 5-HT_2A receptor signaling. Inhibition of PKC and CaMK attenuated phosphorylation of G_q/11 proteins and DOI-induced desensitization of 5-HT_2A receptors. Expression of G₁₁ 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 IP accumulation. These data suggest that phosphorylation of G₁₁ protein by PKC and CaMK contributes to agonist-induced homologous desensitization of 5-HT_2A receptor signaling as well as heterologous desensitization. The phosphorylation of G protein represents a novel mechanism involved in regulation of receptor signaling and agonist-induced desensitization of G protein-coupled receptors.

Key words: Serotonin, Gq/11 family, Phospholipase C's, Protein Kinase C, G protein regulation, Desensitization/uncoupling

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