Lipoic acid increases heat shock protein expression and inhibits stress kinase activation to improve insulin signaling in skeletal muscle from high-fat-fed rats

Abstract

The antioxidant α-lipoic acid (LA) has been shown to improve insulin action in high-fat (HF)-fed animal models, yet little is known about its underlying mechanisms of action. We hypothesize that LA acts by inducing heat shock proteins (HSPs), which then inhibit stress kinases known to interfere with insulin signaling intermediates. Male Wistar rats were fed a HF diet (60% calories from fat) for 6 wk, while controls received a chow diet (10% calories from fat). One-half of the rats in each group received daily LA injections (30 mg/kg body wt). In rats fed a HF diet, LA increased expression of HSP72 and activation of HSP25 in soleus muscle, but it had no effect on HSPs in muscle from chow-fed rats. LA treatment reduced phosphorylation of c-Jun NH2-terminal kinase (JNK) and inhibitor of κB kinase-β (IKKβ) activity (IκBα protein levels) in rats fed a HF diet and effectively restored insulin responsiveness, as seen by insulin-stimulated phosphorylated Akt/Akt and 2-deoxyglucose uptake in soleus muscle. LA also induced activation of p38 MAPK and AMP-activated protein kinase, proteins previously implicated in insulin-independent glucose uptake. In addition, acute LA treatment induced HSPs in vitro in L6 muscle cells and prevented the activation of JNK and IKKβ with stimulants such as anisomycin and TNF-α, respectively. In conclusion, our results suggest chronic LA treatment results in stress kinase inhibition and improved insulin signaling through a HSP-mediated mechanism.