Rescuing 3T3-L1 Adipocytes from Insulin Resistance Induced by Stimulation of Akt-Mammalian Target of Rapamycin/p70 S6 Kinase (S6K1) Pathway and Serine Phosphorylation of Insulin Receptor Substrate-1: Effect of Reduced Expression of p85α Subunit of Phosphatidylinositol 3-Kinase and S6K1 Kinase

Phosphorylation of insulin receptor substrate-1 (IRS-1) on serine residues has been recognized as a mechanism responsible for a diminution of insulin action and insulin resistance. Potential approaches to improve insulin sensitivity may include interference with and/or reduction in expression of certain signaling intermediates that participate in the pathogenesis of insulin resistance. In this study, we transduced fully differentiated 3T3-L1 adipocytes with a constitutively active myristoylated Akt that led to hyperactivation of mammalian target of rapamycin and p70 S6 kinase (S6K1), increased serine phosphorylation of IRS-1, and reduction in insulin-stimulated phosphatidylinositol (PI) 3-kinase activity and glucose transport. We then reduced expression of the PI 3-kinase regulatory subunit, p85 alpha, or expression of S6K1 kinase using small interfering RNA transfections, which led to a reduction in p85 alpha expression of 70% at 48 h (P < 0.05) and S6K1 of 49% (P < 0.05). Reduction in expression of either p85 alpha or S6K1 achieved with small interfering RNA in the presence of myristoylated Akt rescued 3T3-L1 adipocytes from the insulin resistance induced by serine phosphorylation of IRS-1 and completely restored insulin-stimulated activation of PI 3-kinase and glucose uptake. We conclude that reduction in expression of p85 alpha or S6K1 could represent therapeutic targets to mitigate insulin resistance. (Endocrinology 150: 1165-1173, 2009)