Oleate-induced decrease in hepatocyte insulin binding is mediated by PKC-δ

We have previously shown that free fatty acids (FFA) impair hepatic insulin extraction in vivo and thus generate hyperinsulinemia, a suspected risk factor for atherosclerosis and cancer. Hepatic insulin extraction is a receptor-mediated event, which is initiated by hepatocyte insulin binding. In the present study, we investigated the effect of FFA on insulin binding in freshly isolated rat hepatocytes maintained at 10 mM glucose. Hepatocyte insulin binding decreased after 1 h exposure to oleate in a concentration-dependent manner reaching a maximum (35–40%) at 125 μM. Inhibition of FFA oxidation by >90% with the carnitine palmitoyltransferase I (CPT-I) inhibitor methylpalmoxirate (MP, 30 μM) did not prevent the effect of oleate. However, when hepatocytes were treated with the PKC inhibitor bisindolylmaleimide (BIM, 1 μM) the effect of oleate was abolished. Subcellular fractionation and immunoblotting of specific PKC isoforms revealed that oleate-induced hepatic PKC-δ membrane translocation, but did not translocate-ε, -θ, -α, -βI and -βII. These results indicate that PKC-δ activation mediated the FFA-induced decrease in hepatocyte insulin binding under our conditions, and thus provides a mechanistic basis for FFA-induced hyperinsulinemia.