Mechanisms of TNF-α-induced insulin resistance

There is now substantial evidence linking TNF-alpha to the presentation of insulin resistance in humans, animals and in vitro systems. We explored the relationship between TNF-alpha and insulin resistance using knockout mice deficient for either TNF-alpha or one or both of its receptors, p55 and p75. In studies of TNF-alpha-deficient knockout mice with diet-induced obesity, obese TNF-alpha knockouts responded to an exogenous dose of insulin or glucose much more efficiently than TNF-alpha wild-type animals. This finding suggests that deletion of TNF-alpha leads to increased insulin sensitivity, ie decreased insulin resistance. In studies using genetically obese ob/ob mice, TNF-alpha receptor wild-type and p75 receptor knockout animals developed a pronounced hyperinsulinemia and transient hyperglycaemia, whereas p55 receptor and double-knockout animals did not. Moreover, in glucose and insulin tolerance tests, we found that p75 knockout animals exhibited profiles identical to those of the wild-type animals, but that p55 knockout animals and double mutants showed a mild improvement in insulin sensitivity, relative to the wild type. Since the improvement in sensitivity was slightly greater with double mutants, p55 alone cannot be responsible for TNF-alpha's promotion of insulin resistance in obese mice, despite the likelihood that it is more important than p75. How TNF-alpha-related insulin resistance is mediated is not fully clear, although phosphorylation of serine residues on IRS-I has previously been shown to be important. When we monitored Glut 4 expression in obese TNF-alpha wild-type and knockout mice, we found no convincing evidence that TNF-alpha mediation of the downregulation of Glut 4 mRNA expression is responsible for insulin resistance. However, we found an approximately 2-fold increase in insulin-stimulated tyrosine phosphorylation of the insulin receptor in the muscle and adipose tissue of TNF-alpha knockout mice, suggesting that insulin receptor signalling is an important target for TNF-alpha. Other possible mediators of TNF-alpha-induced insulin resistance include circulating free fatty acids (FFAs) and leptin.