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, p85α, or expression of S6K1 kinase using small interfering RNA transfections, which led to a reduction in p85α expression of 70% at 48 h (P < 0.05) and S6K1 of 49% (P < 0.05). Reduction in expression of either p85α 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 p85α or S6K1 could represent therapeutic targets to mitigate insulin resistance.

Reduced expression of p85α or p70 S6 kinase rescues adipocytes from insulin resistance induced by serine phosphorylation of insulin receptor substrate-1 and restores phosphatidylinositol 3-kinase activity and glucose uptake