Abstract

Peroxovanadate (PVN) is an insulin-like agent that inhibits the dephosphorylation of the insulin receptor kinase. PVN inhibited the lipolytic action of 0.1 microM isoproterenol by 88%, which is a relatively specific beta 1 catecholamine agonist at this concentration, but was largely ineffective against beta 3 agonists or forskolin. To determine whether PVN-mediated desensitization of the beta 1 AR was associated with enhanced phosphorylation, we immunoprecipitated the beta 1 AR from rat adipocytes that were metabolically labeled with 32PO4. Isoproterenol enhanced the net phosphorylation of the beta 1 AR by 8 +/- 2-fold over control. PVN increased the net phosphorylation of the beta 1 AR by 5 +/- 0.5-fold, and together with isoproterenol, they enhanced the phosphorylation of the beta 1 AR by 2-fold over isoproterenol alone. Phosphoamino acid analysis of the phosphorylated receptor revealed phosphate incorporation into serine that was proportional to the radioactivity incorporated into the immunoprecipitated receptor. PVN inhibited the serine/threonine phosphatase calcineurin, suggesting that inhibition of receptor dephosphorylation may play a role in the actions of PVN. Cyanogen bromide cleavage of the phosphorylated beta 1 AR generated a phosphoprotein with a molecular mass consistent with carboxyl-terminal phosphorylation. Furthermore, the magnitude of receptor phosphorylation by isoproterenol was 3-fold larger than that due to forskolin, suggesting that beta 1 AR is a substrate for the beta AR kinase that phosphorylates carboxyl-terminal residues in the beta(2) AR. Our findings suggest that PVN may be a powerful new tool with which to study the phosphorylation of other G protein-coupled receptors.