Insulin signalling in hepatocytes of humans with type 2 diabetes: excessive production and activity of protein kinase C-ι (PKC-ι) and dependent processes and reversal by PKC-ι inhibitors

Aims/hypothesis We examined the role of protein kinase C-iota (PKC-iota) in mediating alterations in the abundance of enzymes in hepatocytes of type 2 diabetic humans that contribute importantly to the development of lipid and carbohydrate abnormalities in type 2 diabetes. Methods We examined (1) insulin signalling in isolated hepatocytes of non-diabetic and type 2 diabetic humans and (2) the effects of two newly developed small molecule PKC-iota inhibitors on aberrant signalling and downstream processes. Results In contrast with PKC-iota deficiency in diabetic muscle, which diminishes glucose transport, PKC-iota in diabetic hepatocytes was overproduced and overactive, basally and after insulin treatment, and, moreover, was accompanied by increased abundance of PKC-iota-dependent lipogenic, proinflammatory and gluconeogenic enzymes. Heightened PKC-iota activity most likely reflected heightened activity of IRS-2-dependent phosphatidylinositol 3-kinase (PI3K), as IRS-1 levels and IRS-1/PI3K activity were markedly diminished. Importantly, insulin-stimulated PKC-iota abundance and its over-abundance in diabetic hepatocytes was reversed in vitro by both insulin deprivation and PKC-iota inhibitors; this suggested operation of an insulin-driven, feed-forward/positive-feedback mechanism. In contrast with PKC-iota, protein kinase B (Akt2) activity and activation by insulin was diminished, apparently reflecting IRS-1 deficiency. Treatment of diabetic hepatocytes with PKC-iota/lambda inhibitors diminished abundance of lipogenic, proinflammatory and gluconeogenic enzymes. Conclusions/interpretation Our findings suggest that a vicious cycle of PKC-iota overactivity and overproduction exists in hepatocytes of humans with type 2 diabetes and contributes importantly to maintaining overactivity of lipogenic, proinflammatory and gluconeogenic pathways, which underlies the lipid and carbohydrate abnormalities in type 2 diabetes.