Heterologous desensitization of response mediated by selective PKC-dependent phosphorylation of Gi-1 and Gi-2

This study examined the ability of protein kinase C (PKC) to induce heterologous desensitization by targeting specific G proteins and limiting their ability to transduce signals in smooth muscle. Activation of PKC by pretreatment of intestinal smooth muscle cells with phorbol 12-myristate 13-acetate, cholecystokinin octapeptide, or the phosphatase 1 and phosphatase 2A inhibitor, calyculin A, selectively phosphorylated G alpha(i-1) and G alpha(i-2), but not G alpha(i-3) or G alpha(o), and blocked inhibition of adenylyl cyclase mediated by somatostatin receptors coupled to G(i-1) and opioid receptors coupled to G(i-2), but not by muscarinic M-2 and adenosine A(1) receptors coupled to G(i-3). Phosphorylation of G alpha(i-1) and G alpha(i-2) and blockade of cyclase inhibition were reversed by calphostin C and bisindolylmaleimide, and additively by selective inhibitors of PKC alpha and PKC epsilon. Blockade of inhibition was prevented by downregulation of PKC. Phosphorylation of G alpha-subunits by PKC also affected responses mediated by beta gamma-subunits. Pretreatment of muscle cells with cANP-(4-23), a selective agonist of the natriuretic peptide clearance receptor, NPR-C, which activates phospholipase C (PLC)-beta 3 via the beta gamma-subunits of G(i-1) and G(i-2), inhibited the PLC-beta response to somatostatin and [D-Pen(2,5)]enkephalin. The inhibition was partly reversed by calphostin C. Short-term activation of PKC had no effect on receptor binding or effector enzyme (adenylyl cyclase or PLC-beta) activity. We conclude that selective phosphorylation of G alpha(i-1) and G alpha(i-2) by PKC partly accounts for heterologous desensitization of responses mediated by the alpha- and beta gamma-subunits of both G proteins. The desensitization reflects a decrease in reassociation and thus availability of heterotrimeric G proteins.