Impaired insulin signaling in adipose tissue and skeletal muscle causes insulin resistance associated with the development of type 2 diabetes. However, the molecular mechanisms underlying insulin resistance remain to be elucidated. In this review, we describe the current understanding of the effects of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) and tumor necrosis factor (TNF)-α on insulin signal transduction in adipocytes. First, we determined that atorvastatin inhibits the tyrosine phosphorylation of insulin receptor substrate (IRS)-1 through a decrease in the RhoA-Rho-kinase pathway, resulting in the inhibition of glucose uptake. Second, we found that TNF-α induces IRS-1 phosphorylation at serine residues 636/639 and inhibits the tyrosine phosphorylation of IRS-1 through the increase in both extracellular signal-regulated kinase (ERK) and c-jun N-terminal kinase (JNK) phosphorylation. Interestingly, 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside, an AMP-activated protein kinase activator, suppresses TNF-α-induced IRS-1 serine phosphorylation at 636/639 and the phosphorylation of ERK by enhancing interactions between ERK and dual-specificity phosphatase-9. These results may be helpful in understanding the mechanisms underlying insulin resistance.
Keywords: AMP-activated protein kinase; adipocyte; cytokine; insulin receptor substrate; insulin resistance; statin.