Ethanol-induced Stress Leads to Apoptosis Via Endoplasmic Reticulum Stress in SK-Hep1 Cells

Alcoholic liver disease causes oxidative stress and induces apoptosis during alcohol metabolism. Ethanol causes endoplasmic reticulum (ER) stress in hepatocytes, stimulating the unfolded protein response (UPR) pathway and/or Ca2+-dependent calpain and caspase-4 activities. However, it is poorly understood whether ethanol-induced oxidative stress directly leads to apoptosis promoted by ER stress-associated pathways. This study investigated this question in human liver adenocarcinoma (SK-Hep1) cells, which were treated with 200 mM ethanol for 5 hours in the presence or absence of the antioxidant N-acetyl-cysteine (NAC). We found that treatment with ethanol significantly increased ROS production and cellular apoptosis in the SK-Hep1 cells, and that this response was significantly suppressed by pretreatment with NAC. Furthermore, pretreatment with NAC significantly reduced the observed increases in the mRNA expressions of Bip, Chop, and sXbp-1, and the activity of caspase-3 in ethanol-induced apoptotic cells. However, pretreatment with NAC did not attenuate the transient rise in cytosolic Ca2+ nor the activities of caspase-4 and calpain induced by ethanol. Together, these results revealed that ethanol-induced stress promotes apoptosis not only through mitochondria-mediated pathways, but also via ER stress. The findings further suggested that ethanol-induced oxidative stress and non-oxidative stress both stimulate the pathway regulating ER stress-mediated apoptosis.