Insulin resistance in adipocytes after downregulation of G(i) subtypes

To determine whether downregulation of G(i) proteins is associated with insulin resistance, we incubated isolated adipocytes with N-6-(2-phenylisopropyl)adenosine (PIA; an A(1)-adenosine receptor agonist; 300 nM), prostaglandin E-1 (PGE(1); 3 mu M), or nicotinic acid (1 mM) for 4 days in primary culture. The cells were washed, and the rate of glucose transport (2-deoxy-[H-3]glucose uptake) was measured after incubation with various concentrations of insulin for 45 min. Both PIA and PGE(1) (which downregulate G(i)) decreased the maximal responsiveness of the cells to insulin by similar to 30% and caused a rightward shift in the dose-response curve. By contrast, nicotinic acid (which does not downregulate G(i)) did not alter the insulin sensitivity of the cells. Prolonged treatment of adipocytes with either PW or PGE(1) (but not nicotinic acid) rendered the cells completely resistant to the antilipolytic effect of insulin. The ability of insulin to stimulate autophosphorylation of the beta-subunit of the insulin receptor was decreased by similar to 30% in PIA-treated cells, and the dose-response curve was shifted to the right. Similarly, the ability of the receptor to phosphorylate poly(Glu(4)-Tyr(1)) was decreased by similar to 35%. This decrease in tyrosine kinase activity of the receptor may account for the decrease in insulin sensitivity of glucose transport but cannot account for the complete loss of antilipolysis. The findings suggest both a direct and indirect involvement of G(i) proteins in insulin action.