Lipoic acid protects against oxidative stress induced impairment in insulin stimulation of protein kinase B and glucose transport in 3T3-L1 adipocytes.

Abstract

Oxidative stress has been shown to impair insulin-stimulated glucose transporter 4 translocation in 3T3-L1 adipocytes. This study explores the potential of the antioxidant lipoic acid to protect the cells against the induction of insulin resistance when given before exposure to oxidative stress. 3T3-L1 were exposed for 16 h to lipoic acid after which cells were exposed for 2 h to continuous production of H2O2 by adding glucose oxidase to the culture medium. These conditions resulted in a 50-70% reduction in insulin-stimulated glucose transport activity associated with a decrease in reduced glutathione content from 37.4 +/- 3.1 to 26.4 +/- 4.9 nmol/mg protein, (p < 0.005). Lipoic acid pretreatment increased insulin-stimulated glucose transport following oxidative stress, reaching 84.8 +/- 4.4% of the control, associated with an increase in reduced glutathione content. Oxidation impaired the 4.89 +/- 0.36-fold insulin-stimulated increase in glucose transporter 4 content in plasma membrane lawns of control cells. Lipoic acid pretreatment was, however, associated with preserved insulin-induced glucose transporter 4 translocation in cells exposed to oxidation, yielding 80% of its content in controls. Although tyrosine phosphorylation patterns were not affected by lipoic acid pretreatment, insulin-stimulated protein kinase B/Akt serine 473 phosphorylation and activity were considerably impaired by oxidation but protected by lipoic acid pretreatment. A protective effect was not observed with either troglitazone, its isolated vitamin E moiety, or with vitamin C. This study shows the ability of lipoic acid to provide partial protection against the impaired insulin-stimulated glucose transporter 4 translocation and protein kinase B/Akt activation induced by oxidative stress, potentially by its capacity to maintain intracellular redox state.