Endocrine regulation of G-protein subunit production in an animal model oftype 2 diabetes mellitus

Adipocyte beta -adrenergic sensitivity is compromised in animal models of o besity and type 2 diabetes. Although changes in the membrane concentrations of G-protein alpha subunits (G alpha) have been implicated, it remains to be determined how these changes are affected by insulin resistance in the d ifferent animal models. Because previous studies used young animals, we mea sured the concentrations of G alpha and G beta subunits in epididymal fat f rom aged (48 weeks old) db/db mice and from their lean littermates to more closely reproduce the model of type 2 diabetes mellitus. Levels of immunore active G alphas, G alphai(1/2), G alphao and G alphaq/11 were all significa ntly greater in adipocyte membranes from the db/db mice than in membranes f rom their lean non-diabetic littermate controls. Levels of G alphai(1) and G alphai(2) were also individually determined and although they appeared to be slightly higher in db/db membranes, these differences were not signific ant. Although the levels of both G alphas isoforms were elevated, levels of the 42 and 46 kDa proteins rose by approximately 42% and 20% respectively, indicating differential protein processing of G alphas. By contrast, level s of G alpha i3 were similar in the two groups. The levels of common G beta and G beta2 were also elevated in db/db mice, whereas G beta1 and G beta4 levels were not different. To determine whether these changes were due to i nsulin resistance per se or to elevated glucocorticoid production, G-protei n subunit levels were quantified in whole cell lysates from 3T3-L1 adipocyt es that were stimulated with different concentrations of either insulin or corticosterone. Although none of the subunit levels was affected by insulin , the levels of both G alphas isoforms were increased equally by corticoste rone in a concentration-dependent manner. Since glucocorticoids are known r egulators of G alphas gene expression in many cell types and in adipocytes from diabetic rodents, the results presented herein appear to more accurate ly reflect diabetic pathophysiology than do those of previous studies which report a decrease in G alphas levels. Taken together, these results indica te that most of the selective changes in G-protein subunit production in ad ipocytes from this animal model of type 2 diabetes may not be due to dimini shed insulin sensitivity, but may be due to other endocrine or metabolic ab normalities associated with the diabetic phenotype.