Cytoplasmic recruitment of Mdm2 as a common characteristic of G protein-coupled receptors that undergo desensitization

Abstract

Desensitization of G protein-coupled receptors (GPCRs) represents a gradual attenuation of receptor responsiveness by continuous or repeated exposure to agonists. The most widely accepted molecular mechanism responsible for desensitization is that of GRK2-mediated receptor phosphorylation followed by association with β-arrestins. However, in most cases, this mechanism cannot explain the desensitization of GPCRs. In this study, we investigated whether there exists a direct correlation between desensitization and certain cellular events that commonly observed with desensitizing receptors. Our study showed that constitutive ubiquitination of β-arrestin, accompanied by nuclear to cytoplasmic translocation of Mdm2, was observed in cells expressing desensitizing GPCRs (dopamine D₃ receptor, K149C-dopamine D₂ receptor, β₂ adrenoceptor, and lysophosphatidic acid receptor 1). In contrast, Mdm2 was observed in the nucleus in cells expressing non-desensitizing GPCRs (dopamine D₂ receptor, C147K-dopamine D₃ receptor, and dopamine D₄ receptor). Molecular manipulation to convert the characteristics of the dopamine D₄ receptor from non-desensitizing to desensitizing changed the status of subcellular localization of Mdm2 from nuclear to cytoplasmic. With repeated agonist treatments of desensitizing receptors, Mdm2 translocated from cytoplasm to nucleus, resulting in the deubiquitination of β-arrestins. This study suggests that the property of a receptor that causes a change in subcellular localization of Mdm2, from the nuclear to cytoplasmic, could be used as a biomarker to predict the desensitization of a receptor.

Keywords: GPCR; Gβγ; Mdm2; Ubiquitination; β-arrestin.