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

Abstract

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 states of insulin resistance and non-insulin-dependent diabetes mellitus. The purpose of this study was to characterise tissue-and isoform-selective differences 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 after baseline measurements of fasting serum glucose, triglycerides, insulin and oral glucose tolerance tests. Liver, epididymal fat and soleus muscle samples were obtained from fed and overnight-fasted rats for measurements of DAG, PKC activity and individual PKC isoforms in cytosol and membrane fractions. Obese rats were heavier (488 7 vs 315 9 g) with fasting hyperglycaemia (10·5 0·8 vs 7·7 0·1 mM) and hyperinsulinaemia (7167 363 vs 251 62 pM) relative to lean controls. In fasted rats, PKC activity 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 fat. The fed state was associated with increased DAG levels and threefold higher PKC activity in muscle tissue of obese rats, and increased expression of the major muscle isoforms, PKC-and PKC-: 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) compared 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 signalling occurs selectively in muscle of obese (fa/fa) rats due to increased DAG-mediated activation and/or synthesis of PKC-and PKC-. These changes in PKC are likely to exacerbate the hyperglycaemia and hypertriglyceridaemia associated with obesity-induced diabetes.

Protein Kinase C Function in Muscle, Liver, and β-Cells and Its Therapeutic Implications for Type 2 Diabetes

“…PKC redistribution was reversed upon treatment with the insulin sensitizer rosiglitazone (12). Similar alterations in nPKC isoforms were also observed in genetic models of obesity and diabetes (13,14). In more acute models of insulin resistance, PKC translocation was also observed after 5-h lipid infusion (15), whereas 1-or 4-day infusion of glucose, which increased muscle lipid content, promoted activation of PKCε (16).…”

Cellular mechanism of insulin resistance in nonalcoholic fatty liver disease

“…Membrane translocation for the different PKC isoforms (PKC-α, -β, -ε, -δ, -θ, -η, -λ, -ι, -ζ, and -γ) was performed as described previously (47). Both membrane and cytosol proteins were detected on the same film with enhanced chemiluminescense at the same exposure time.…”

TANK-binding kinase 1 mediates phosphorylation of insulin receptor at serine residue 994: a potential link between inflammation and insulin resistance

“…Among the serine kinases implicated in Ser phosphorylation of the IR and the IRSs, protein kinase C (PKC) is one candidate with potential pathophysiological relevance. Numerous studies have linked excessive PKC activity to diminished insulin sensitivity, especially to that occurring together with increased lipid availability (Considine et al 1995, Qu et al 1999, Itani et al 2000. We have previously reported that IR Ser994 is an in vitro phosphorylation target for PKC (Coba et al 2003).…”