Protein kinase C mediates phosphorylation, desensitization, and trafficking of the D2 dopamine receptor

Abstract

Previously, D2 dopamine receptors (D2 DARs) have been shown to undergo G-protein-coupled receptor kinase phosphorylation in an agonist-specific fashion. We have now investigated the ability of the second messenger-activated protein kinases, protein kinase A (PKA) and protein kinase C (PKC), to mediate phosphorylation and desensitization of the D2 DAR. HEK293T cells were transiently transfected with the D2 DAR and then treated with intracellular activators and inhibitors of PKA or PKC. Treatment with agents that increase cAMP, and activate PKA, had no effect on the phosphorylation state of the D2 DAR, suggesting that PKA does not phosphorylate the D2 DAR in HEK293T cells. In contrast, cellular treatment with phorbol 12-myristate 13-acetate (PMA), a PKC activator, resulted in an ∼3-fold increase in D2 DAR phosphorylation. The phosphorylation was specific for PKC as the PMA effect was mimicked by phorbol 12,13-dibutyrate, but not by 4α-phorbol 12,13-didecanoate, active and inactive, phorbol diesters, respectively. The PMA-mediated D2 DAR phosphorylation was completely blocked by co-treatment with the PKC inhibitor, bisindolylmaleimide II, and augmented by co-transfection with PKCβI. In contrast, PKC inhibition had no effect on agonist-promoted phosphorylation, suggesting that PKC is not involved in this response. PKC phosphorylation of the D2 DAR was found to promote receptor desensitization as reflected by a decrease in agonist potency for inhibiting cAMP accumulation. Most interestingly, PKC phosphorylation also promoted internalization of the D 2 DAR through a β-arrestin- and dynamin-dependent pathway, a response not usually associated with PKC phosphorylation of G-protein-coupled receptors. Site-directed mutagenesis experiments resulted in the identification of two domains of PKC phosphorylation sites within the third intracellular loop of the receptor. Both of these domains are involved in regulating sequestration of the D2 DAR, whereas only one domain is involved in receptor desensitization. These results indicate that PKC can mediate phosphorylation of the D2 DAR, resulting in both functional desensitization and receptor internalization.