Our previous high fat diet studies comparing mice deficient in either protein kinase c (PKC) δ or PKCε revealed a similar role for the kinases in the generation of insulin resistance, yet opposing roles in hepatic triglyceride (TG) accumulation. To investigate the mechanisms involved, we conducted quantitative hepatic proteomics. Of the >3000 proteins detected in liver from fat-fed mice, 1% were upregulated or downregulated >1.5-fold by PKC deletion. We validated and further studied candidate proteins which were significantly regulated by both PKC isoforms, either in a similar or converse fashion.
The flavin-containing monooxygenases (FMOs), which are linked to redox stress and exhibited similar upregulation in both fat-fed PKC KO mice, may be involved in protection against insulin resistance. Therefore FMO3 was overexpressed in hepatocytes to determine whether it exerts beneficial effects. FMO3 overexpression resulted in a 2-fold reduction in the palmitate-induced elevation of ER stress and apoptosis markers, including phlda1, phospho-JNK, CHOP and cleaved caspase 3, which have been implicated in the induction of insulin resistance.
On the other hand, the HIV-TAT-interacting protein 2 (Htatip2) displayed converse regulation by PKCδ and PKCε, and could therefore be associated with altered lipid metabolism mediated by deletion of the kinases. Following the overexpression of Htatip2 in hepatocytes, [14C]palmitate tracer was employed to monitor the incorporation of fatty acid into TG. We observed 50% greater TG labelling in Htatip2-overexpressing cells over 1-2 h. This was consistent with the inverse level of TG accumulation in PKCδ and PKCε KO mice and the differential protein level from the proteomics analyses, suggesting a novel role in lipid metabolism. Further will determine the specific role of Htatip2 in lipogenesis, esterification and/or β-oxidation.
These studies have therefore identified novel candidates that contribute to the PKC-mediated regulation of lipid metabolism and insulin action, which appear not to be causally linked.