Increased levels of diacylglycerol (DAG) in the liver are associated with hepatic insulin resistance. Now, new research by Brandon Gassaway, Varman Samuel, Gerald Shulman and Jesse Rinehart reports that protein kinase Cε (PKCε) is a key contributor in a network of proteins that might drive hepatic insulin resistance, particularly by interacting with 70 kDa ribosomal protein S6 kinase (P70S6K), a key kinase in the insulin signalling pathway.

Previous studies in humans and rodents showed that increased levels of DAG activate PKCε, which then impairs the insulin receptor kinase activity. “As PKC activity has been connected to insulin resistance for decades, we hypothesized that a DAG-activated PKCε would target a network of proteins and that some of these targets might contribute to the complex phenotype that is insulin resistance,” explains Rinehart.

To study the signalling pathways affected by PKCε, the researchers fed two groups of rats a high-fat diet (HFD): normal rats and rats with PKCε knocked down. Liver samples were taken and analysed using phosphoproteomic methods to compare sites of protein phosphorylation between the two groups. Surprisingly, many of the proteins affected by PKCε had not previously been associated with insulin signalling.

Next, the researchers tested a selection of candidate proteins with a small interference RNA-based screen to identify which proteins have novel interactions with insulin signalling. The authors showed that many of the proteins did regulate insulin signalling and that PKCε itself was an activator of the insulin signalling pathway. Other novel interactors with insulin signalling included 40S ribosomal protein S6 (RPS6) and neogenin 1 (NEO1).

Finally, Rinehart and colleagues found that IRS1 and RPS6 are also substrates of PKCε and suggested that PKCε crosstalked with P70S6K via the two kinases. The authors concluded that this crosstalk is involved in HFD-induced insulin resistance. “We now need to understand how each of these PKCε targets might contribute to hepatic insulin resistance,” concludes Samuel.