PIOGLITAZONE PROMOTES INSULIN-INDUCED ACTIVATION OF PHOSPHOINOSITIDE 3-KINASE IN 3T3-L1 ADIPOCYTES BY INHIBITING A NEGATIVE CONTROL MECHANISM

Activation of phosphoinositide 3-kinase (PI 3-kinase) is an early even t in insulin signal transduction that is blocked completely in adipocy tes from insulin-resistant KKA(y) mice. Treatment of KKA(y) mice with pioglitazone, an anti-diabetic thiazolidinedione, partially restores i nsulin-dependent changes in PI 3-kinase. The mechanism of this effect of pioglitazone was investigated, using murine 3T3-L1 cells as an expe rimental model. Insulin and insulin-like growth factor I(IGF-I) each e licited rapid (within 2 min) and large (2- to 5-fold) increases in PI 3-kinase activity that could be immunoprecipitated using anti-phosphot yrosine (pY) antibodies. Maximal insulin-induced activity of PI 3-kina se in pY-immunoprecipitates was similar in 3T3-L1 adipocytes and mouse adipocytes, but the kinetics of activation differed. Insulin- and IGF -I-induced changes in PI 3-kinase were each half-maximal at 3-5 nM of hormone and were not additive. Increases in both insulin-induced and I GF-I-induced pY-immunoprecipitable PI 3-kinase activity were observed when 3T3-L1 fibroblasts became confluent and when they adopted the adi pocyte phenotype. Pioglitazone (10 mu M), administered either acutely or chronically to either 3T3-L1 adipocytes or 3T3-L1 fibroblasts, did not alter greatly the kinetics, magnitude or sensitivity of changes in PI 3-kinase elicited by either insulin or IGF-I. In contrast, the att enuation by isoproterenol of insulin-induced changes in PI 3-kinase wa s prevented in cells pretreated with pioglitazone. This effect of piog litazone did not involve inhibition of isoproterenol-elicited accumula tion of cyclic AMP. Pioglitazone also prevented attenuation of insulin induced changes in PI 3-kinase by cell penetrating analogs of cyclic AMP. Pioglitazone, therefore, has no direct effect on insulin-stimulat ed PI 3-kinase activity, but interferes with a cyclic AMP-dependent me chanism that normally antagonizes this action of insulin. These data s upport the proposition that the facilitation of insulin action by piog litazone involves, at least in part, an inhibition of a negative contr ol mechanism.