Phosphatidylinositol 3-Kinase Activity in 3T3-L1 Adipocytes: Effects of Pioglitazone on Insulin- and IGF-1-Stimulated Cells
Sizer, Karen M.
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Non-insulin dependent diabetes mellitus (NIDDM) is characterized by insulin resistance in which insulin production may be near normal but there is decreased insulin sensitivity in target tissues. Pioglitazone is a drug recently shown to improve insulin sensitivity in insulin resistant cells most likely by affecting an early event in insulin signal transduction. An obligatory early event in insulin signal transduction is the activation of the tyrosine kinase function of the insulin receptor. Substrates for the receptor tyrosine kinase are not well characterized but appear to include phosphatidylinositol 3-kinase (PI 3-kinase). In this study, the effects of pioglitazone on insulin- and insulin-like growth factor-l (IGF-l)-induced changes in PI 3-kinase activity in an adipocyte-like cell line (3T3-Ll) were investigated. Adipocytes were treated with pioglitazone for 24 h before stimulation with hormone and then PI 3-kinase activity was measured in anti-phosphotyrosine immunoprecipitates prepared from cell lysates, using phosphatidylinositol as substrate. Immunoprecipitable PI 3-kinase activity increased 3 to l0-fold within 1 min of exposure to either insulin or IGF-l, and remained elevated for at least 40 min. Half-maximal activity was induced by both insulin or IGF-l at concentrations of about 5 nM. Maximal effects of insulin and IGF-l were not additive. Pre-treatment of cells with pioglitazone did not alter either basal PI 3-kinase activity or the sensitivity and kinetics of increases induced by either insulin or IGF-l. In contrast, pioglitazone pre-treatment reduced the attenuation by isoproterenol of insulin-induced PI 3-kinase activity. These data suggest that pioglitazone has no direct effect on insulin-stimulated PI 3-kinase, but may indirectly facilitate this action of insulin by interfering with a mechanism which normally antagonizes insulin-induced changes in PI 3-kinase.With honors.