Tissue and isoform-selective activation of protein kinase C in insulin-resistant obese Zucker rats - effects of feeding

The mechanisms of insulin resistance in the obese Zucker rat have not been clearly established but increased diacylglycerol-protein kinase C (DAG-PKC) signalling has been associated with decreased glucose utilisation in state s of insulin resistance and non-insulin-dependent diabetes mellitus. The pu rpose of this study was to characterise tissue- and isoform-selective diffe rences in DAG-PKC signalling in insulin-sensitive tissues from obese Zucker rats, and to assess the effects of feeding on DAG-PKC pathways. Groups of male obese fa/fa, n=24) and lean (fa/-, n=24) Zucker rats were studied afte r baseline measurements of fasting serum glucose, triglycerides, insulin an d oral glucose tolerance tests. Liver, epididymal fat and soleus muscle sam ples were obtained from fed and overnight-fasted rats for measurements of D AG, PKC activity and individual PKC isoforms in cytosol and membrane fracti ons. Obese rats were heavier (488 +/- 7 vs 315 +/- 9 g) with fasting hyperg lycaemia (10.5 +/- 0.8 vs 7.7 +/- 0.1 mM) and hyperinsulinaemia (7167 +/- 3 63 vs 251 +/- 62 pM) relative to lean controls. In fasted rats, PKC activit y in the membrane fraction of liver was significantly :higher in the obese group (174 +/- 16 vs 108 +/- 12 pmol/min/mg protein, P<0.05) but there were no differences in muscle and ht. The fed state was associated with increas ed DAG levels and threefold higher PKC activity in muscle tissue of obese r ats, and increased expression of the major muscle isoforms, PKC-theta and P KC-epsilon: e.g. PKC activity in the membrane fraction of muscle from obese animals was 283 +/- 42 (fed) vs 107 +/- 20 pmol/min/mg protein (fasting) c ompared with. 197 +/- 27 (fed) and 154 +/- 21 pmol/min/mg protein (fasting) in lean rats. In conclusion, hepatic PKC activity is higher in obese rats under basal fasting conditions and feeding-induced activation of DAG-PKC si gnalling occurs selectively in muscle of obese fa/fa) rats due to increased DAG-mediated activation and/or synthesis of PKC-theta and PKC-epsilon. The se changes in PKC are likely to exacerbate the hyperglycaemia and hypertrig lyceridaemia associated with obesity-induced diabetes.