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Mechanism of oleic acid induced insulin resistance in cultured rat hepatocytes

Aims: The aim of this study was to determine the possible role of oleic acid in insulin signalling and in induction of insulin resistance. Methods: Hepatocytes were isolated by a modified collagenase perfusion technique of Berry and Friend and cultured for 24 h in M199 serum-free medium. To assess amino acid transport hepatocytes were incubated in Hanks-Hepes medium containing ?-amino14C-isobutyric acid (AIB) to the final concentration of 1 mmol/l. Results: In hepatocytes obtained from rats on standard diet insulin produced 86% increase of AIB transport. Oleic acid did not change basal AIB transport but strongly reduced insulin effect (> 50%). Reduction of insulin-stimulated transport was achieved one hour after addition of oleic acid. On the other side removing of oleic acid also very quickly after 3 hours restore normal condition. All of these suggest that oleic acid produce post binding perturbations affecting insulin signalling, which finally lead to reduction of insulin effect. The biological insulin effects are mediated by the membrane-spanning insulin receptor, which on binding of insulin to its extracellular domain is autophosphorylated on tyrosine residue and activation tyrosine kinase of the insulin receptor is absolutely essential for insulin signalling. This activation leads to the generation of signals that determine the cellular response. A number of different factors can modulate the kinase activity and protein kinase C (PKC) has been shown to play a pivotal role. Pretreatment with phorbol 12- myristate 13-acetate (TPA) as well as oleic acid induced insulin resistance and PKC inhibitor polymyxin B mostly antagonises the oleic acid induced reduction of AIB transport. Conclusions: Although classical paradigm defining the pathophysiology of insulin resistance have focused on the abnormal regulation, our data also support a role for fatty acids as an primary physiological regulator of insulin action, as in a switching metabolism from anabolic to catabolic mode.

amino acid transport; insulin insulin resistance; oleic acid; protein kinase C

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