$\alpha$-Lipoic acid is known to increase insulin sensitivity in vivo and to stimulate glucose uptake into adipose and muscle cells in vitro. To elucidate the mechanism of this effect, the role of protein thiol groups and $H_2O_2$ production was investigated in α-lipoic acid-stimulated insulin receptor autophosphorylation and glucose uptake into 3T3-L1 adipocytes. α-Lipoic acid or insulin treatment of adipocytes increased intracellular level of oxidants, decreased thiol reactivity of the insulin receptor β-subunit, increased tyrosine phosphorylation of the insulin receptor, and enhanced glucose uptake. α-Lipoic acid or insulin-stimulated glucose uptake was inhibited i) by alkylation of intracellular, but not extracellular, thiol groups downstream to insulin receptor activation, and ii) by diphenylene iodonium at the level of the insulin receptor autophosphorylation. α-Lipoic acid also inhibited protein tyrosine phosphatase activity and decreased thiol reactivity of protein tyrosine phosphatase 1B. These findings indicate that oxidants produced by α-lipoic acid or insulin treatment are involved in activation of IR and in inactivation of protein tyrosine phosphatases, which eventually result in elevated glucose uptake into 3T3-L1 adipocytes.
Obesity is closely associated with a number of pathological disorders, including non-insulin-dependent diabetes, hypertension, hyperlipidemia, and cardiovascular diseases. With regard to this wide range of health implications, the need to develop new and effective strategies in controlling obesity has become more acute. However, little is known about the regulation of adipogenesis by $\alpha$-lipoic acid. Here, we demonstrated that co-treatment or post-treatment of 3T3-L1 cells with $\alpha$-lipoic acid down-regulated adipogenesis caused by a cocktail of differentiation inducers and troglitazone. Northern blot and electrophoretic mobility shift assay showed that $\alpha$-lipoic acid regulation of adipogenesis was caused by its down-regulation of adipocyte-specific gene expression such as adipocyte-specific fatty acid binding protein and lipoprotein lipase through regulation of the activity of anti- or pro-adipogenic transcription factors. $\alpha$-Lipoic acid could activate major mitogen-activated protein kinase (MAPK) pathways, while had no effect on Akt phosphorylation in the early phase of 3T3-L1 adipocytes as well as preadipocytes. These results suggest that $\alpha$-lipoic acid can down-regulate 3T3-L1 adipocyte differentiation by modulating pro- or anti-adipogenic transcription factors especially through activation of major MAPK pathways. With the direct regulation of glucose uptake, this anti-obesity activity of $\alpha$-lipoic acid can be another important mechanism for the treatment of type 2 diabetes.
