Cdc42 and Rac1 are major contributors to the saturated fatty acid-stimulated JNK pathway in hepatocytes

Background & Aims: Saturated free fatty acid (SFA)-stimulated c-Jun NH2-terminal kinase (JNK) activation is associated with the pathogenesis of non-alcoholic fatty liver disease (NAFLD). However, the mechanisms responsible for the effects of SFA are incompletely understood. The goal of this study was to determine the molecular mechanisms by which SFA induce JNK activation in hepatocytes. Methods: We used siRNA-mediated knockdown in Hepa1c1c7 and AML12 cell lines, as well as primary mouse hepatocytes for these studies. Results: The current model for JNK activation by SFA involves endoplasmic reticulum (ER) stress, which induces JNK activation through an inositol requiring enzyme 1 (IRE1 alpha) Apoptosis Regulating Kinase 1 (ASK1)-dependent mechanism. Here, we find that SFA-induced JNK activation is not inhibited in the absence of IRE1 alpha and ASK1. Instead we show that activation of the small GTP-binding proteins Cdc42 and Rac1 is required for SFA-stimulated MLK3-dependent activation of JNK in hepatocytes. In addition, we demonstrate that SFA-induced cell death in hepatocytes is independent of IRE1 alpha, but dependent on Cdc42, Rac1, and MLK3. Conclusions: Our results demonstrate that Cdc42 and Rac1, rather than ER stress, are important components of a SFA-stimulated signaling pathway that regulates MLK3-dependent activation of JNK in hepatocytes. (C) 2011 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved.