Inhibited insulin signaling in mouse hepatocytes is associated with increased phosphatidic acid but not diacylglycerol [Signal Transduction]

December 15th, 2014 by Zhang, C., Hwarng, G., Cooper, D. E., Grevengoed, T. J., Eaton, J. M., Natarajan, V., Harris, T. E., Coleman, R. A.

Although an elevated triacylglycerol (TAG) content in non-adipose tissues is often associated with insulin resistance, the mechanistic relationship remains unclear. Data support roles for intermediates in the glycerol-3-phosphate pathway of TAG synthesis: diacylglycerol (DAG), which may cause insulin resistance in liver by activating PKCϵ,and phosphatidic acid (PA), which inhibits insulin action in hepatocytes by disrupting the assembly of mTOR and rictor. To determine whether increases in DAG and PA impair insulin signaling when produced by pathways other than that of de novo synthesis, we examined primary mouse hepatocytes after enzymatically manipulating the cellular content of DAG or PA. Over-expressing phospholipase D-1 or -2 inhibited insulin signaling and was accompanied by an elevated cellular content of total PA, without a change in total DAG. Over-expression of diacylglycerol kinase-θ inhibited insulin signaling, and was accompanied by an elevated cellular content of total PA and a decreased cellular content of total DAG. Over-expressing glycerol-3-phosphate acyltransferase-1 or -4 inhibited insulin signaling and increased the cellular content of both PA and DAG. Insulin signaling impairment caused by over-expression of phospholipase D-1/2 or diacylglycerol kinase-θ was always accompanied by disassociation of mTOR/rictor and reduction of mTORC2 kinase activity. However, although the protein ratio of membrane to cytosolic PKCϵ increased, PKC activity itself was unaltered. These data suggest that PA, but not DAG, is associated with impaired insulin action in mouse hepatocytes.