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The insulin signaling pathway has been reported to mediate R-␣-lipoic acid-(R-LA-)-stimulated glucose uptake into 3T3-L1 adipocytes and L6 myotubes. We investigated the role of the thiol antioxidant dihydrolipoic acid (DHLA) and intracellular glutathione (GSH) in R-LA-stimulated glucose transport and explored the hypothesis that R-LA could increase glucose uptake into 3T3-L1 adipocytes in an oxidant-mimetic manner. R-LA pretreatment of 3T3-L1 cells stimulated glucose transport at early time points (30 min-6 h), whereas it inhibited glucose uptake at later time points. Analysis of the oxidized and reduced content of LA in cells and medium showed that >90% of lipoic acid present was in its oxidized form. Furthermore, all oxidized forms of LA (S-, R-, and racemic LA) stimulated glucose uptake, whereas the reduced form, dihydrolipoic acid, was ineffective. Intracellular GSH levels were not changed at the early time points (before 12 h), while longer preincubation (24 -48 h) of cells with R-LA significantly increased intracellular GSH. Pretreatment of adipocytes with R-LA increased intracellular peroxide levels at early time points (30 min-6 h), after which it was decreased (12-48 h). R-LA also increased tyrosine phosphorylation of immunoprecipitated insulin receptors from 3T3-L1 adipocytes. These results indicate that (i) 3T3-L1 adipocytes have a low capacity to reduce R-LA and the oxidized form of lipoic acid is responsible for stimulating glucose uptake, (ii) R-LA modulates glucose uptake by changing the intracellular redox status, and (iii) the insulin receptor is a potential cellular target for R-LA action.

Obesity is associated with a number of pathological disorders such as non-insulin-dependent diabetes, hypertension, hyperlipidemia, and cardiovascular diseases. ␣-Lipoic acid (LA) has been demonstrated to activate the insulin signaling pathway and to exert insulinlike actions in adipose and muscle cells. Based on this similarity LA is expected to promote adipogenesis in pre-adipocytes. Here, however, we report that LA inhibited differentiation of 3T3-L1 pre-adipocytes induced by a hormonal mixture or troglitazone. Northern blot analysis of cells demonstrated that this inhibition was accompanied with attenuated expression of adipocytespecific fatty acid-binding protein and lipoprotein lipase. Electrophoretic mobility shift assay and Western blot analysis of cells demonstrated that LA modulates transcriptional activity and/or expression of a set of anti-or pro-adipogenic transcription factors. LA treatment of 3T3-L1 pre-adipocytes also resulted in prolonged activation of major mitogen-activated protein kinase signaling pathways but showed little or no effect on the activity of the insulin receptor/Akt signaling pathway. These findings suggest that LA inhibits insulin or the hormonal mixture-induced differentiation of 3T3-L1 pre-adipocytes by modulating activity and/or expression of pro-or anti-adipogenic transcription factors mainly through activating the MAPK pathways.

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