PKCε contributes to lipid-induced insulin resistance through cross talk with p70S6K and through previously unknown regulators of insulin signaling

Insulin resistance drives the development of type 2 diabetes (T2D). In liver, diacylglycerol (DAG) is a key mediator of lipid-induced Insulin resistance. DAG activates protein kinase C ε (PKCε), which phosphorylates and inhibits the Insulin Receptor. In rats, a 3-day high-fat diet produces hepatic Insulin resistance through this mechanism, and knockdown of hepatic PKCε protects against high-fat diet-induced hepatic Insulin resistance. Here, we employed a systems-level approach to uncover additional signaling pathways involved in high-fat diet-induced hepatic Insulin resistance. We used quantitative phosphoproteomics to map global in vivo changes in hepatic protein phosphorylation in chow-fed, high-fat-fed, and high-fat-fed with PKCε knockdown rats to distinguish the impact of lipid- and PKCε-induced protein phosphorylation. This was followed by a functional siRNA-based screen to determine which dynamically regulated phosphoproteins may be involved in canonical Insulin signaling. Direct PKCε substrates were identified by motif analysis of phosphoproteomics data and validated using a large-scale in vitro kinase assay. These substrates included the p70S6K substrates RPS6 and IRS1, which suggested cross talk between PKCε and p70S6K in high-fat diet-induced hepatic Insulin resistance. These results identify an expanded set of proteins through which PKCε may drive high-fat diet-induced hepatic Insulin resistance that may direct new therapeutic approaches for T2D.