Linking Endoplasmic Reticulum Stress to Cell Death in Hepatocytes: Roles of C/EBP Homologous

16 Prolonged endoplasmic reticulum (ER) stress and activation of the unfolded protein response 17 (UPR) have been linked to apoptosis via several mechanisms including increased expression of 18 C/EBP homologous protein (Chop). Increased long-chain fatty acids, in particular saturated fatty 19 acids, induce ER stress, Chop expression and apoptosis in liver cells. The first aim of the present 20 study was to determine the role of Chop in lipid-induced hepatocyte cell death and liver injury 21 induced by a methionine-choline deficient diet. Albumin-bound palmitate increased Chop gene 22 and protein expression in a dose-dependent fashion in H4IIE liver cells. SiRNA-mediated 23 silencing of Chop in H4IIE liver cells reduced thapsigargin-mediated cell death by ~40% and 24 delayed palmitate-mediated cell death but only at high concentrations of palmitate (400-500 25 μM). Similar results were observed in primary hepatocytes isolated from Chop knockout mice. 26 Indices of liver injury were also not reduced in Chop knockout mice provided a methionine27 choline deficient diet. To ascertain whether ER stress was linked to palmitate-induced cell 28 death, primary hepatocytes were incubated in the absence or presence of the chemical 29 chaperones taurine-conjugated ursodeoxycholic acid or 4-phenyl butyric acid. The presence of 30 either of these chemical chaperones protected liver cells from palmitate-mediated ER stress 31 and cell death, in part, via inhibition of JNK activation. These data suggest that ER stress is 32 linked to palmitate-mediated cell death via mechanisms that include JNK activation. 33