Molecular mechanisms of insulin action in normal and insulin-resistant states

Insulin resistance is central to the pathophysiology of type 2 diabetes. It has been known for some time that down-regulation and reduced kinase activity of the insulin receptor play a role in insulin resistance; however, it has recently emerged that defects in the intracellular responses to insulin are also very important. We studied the molecular basis of insulin resistance in mice in which injection with gold thioglucose led to the development of hyperphagia, obesity and insulin resistance over a 4-month period. We found that the insulin-stimulated activation of MAP kinase was defective in obese, insulin-resistant mice. Similarly, we investigated insulin-stimulated PI3-kinase activation in the isolated soleus muscle of lean and obese mice, and found a marked reduction in the PI3-kinase activation of obese animals. The magnitude of the effect was greater than the reduction in insulin receptor activation, suggesting that impairment of PI3-kinase activation is a very important element in the development of insulin resistance in obese mice. In keeping with this, we found that the defect in PI3-kinase activation developed in young obese mice before the emergence of overt insulin resistance. We investigated different mechanisms by which defects in the components of the insulin signalling cascade could emerge, including down-regulation and abnormal phosphorylation of signal molecules. In adipocytes from young obese mice in which insulin resistance had not yet developed, we found that there were already marked defects in IRS-I tyrosine phosphorylation. Increased IRS-1 phosphorylation on serine and threonine residues affects tyrosine phosphorylation. Such a process could contribute to the defective IRS-I tyrosine phosphorylation in insulin-resistant animals. We found that brief exposure of 3T3-L1 adipocytes to platelet-derived growth factor led to IRS-I serine/threonine phosphorylation through a PI3-kinase-dependent pathway, and that this prevented phosphorylation of the tyrosine residues of IRS-1. Such a mechanism, induced by growth factors, TNF-alpha or some other agent, may play an important role in the development of insulin resistance in obese mice.