Tunicamycin-Induced ER Stress is Accompanied with Oxidative Stress via Abrogation of Sulfur Amino Acids Metabolism in the Liver.

Sou Kim,Young-Suk Jung,Jae-Hwan Kwak,Seunghyun Lee,Jieun Yun,Do-Young Kwon,Tae Son,Yun-Hee Lee

doi:10.3390/ijms19124114

Abstract

Endoplasmic reticulum (ER) stress is involved in non-alcoholic fatty liver disease (NAFLD), but the relationship between oxidative stress, another well-known risk factor of NAFLD, and ER stress has yet to be elucidated. In this study, we treated mice with tunicamycin (TM) (2 mg/kg body weight) for 48 h to induce ER stress in the liver and examined the metabolic pathway that synthesizes the endogenous antioxidant, glutathione (GSH). Tunicamycin (TM) treatment significantly increased mRNA levels of CHOP and GRP78, and induced lipid accumulation in the liver. Lipid peroxidation in the liver tissue also increased from TM treatment (CON vs. TM; 3.0 ± 1.8 vs. 11.1 ± 0.8 nmol MDA/g liver, p < 0.001), which reflects an imbalance between the generation of reactive substances and antioxidant capacity. To examine the involvement of GSH synthetic pathway, we determined the metabolomic changes of sulfur amino acids in the liver. TM significantly decreased hepatic S-adenosylmethionine concentration in the methionine cycle. The levels of cysteine in the liver were increased, while taurine concentration was maintained and GSH levels profoundly decreased (CON vs. TM; 8.7 ± 1.5 vs. 5.4 ± 0.9 µmol GSH/g liver, p < 0.001). These results suggest that abnormal cysteine metabolism by TM treatment resulted in a decrease in GSH, followed by an increase in oxidative stress in the liver. In HepG2 cells, decreased GSH levels were examined by TM treatment in a dose dependent manner. Furthermore, pretreatment with TM in HepG2 cells potentiated oxidative cell death, by exacerbating the effects of tert-butyl hydroperoxide. In conclusion, TM-induced ER stress was accompanied by oxidative stress by reducing the GSH synthesis, which made the liver more susceptible to oxidative stress.

Highlights

Non-alcoholic fatty liver disease (NAFLD) represents a spectrum of liver conditions from simple lipid accumulation to steatohepatitis, liver fibrosis, and hepatocellular carcinoma [1,2,3]
Excessive lipid accumulation in the liver, namely fatty liver or hepatic steatosis, is caused by abnormal lipid metabolism, and subsequent progression is exasperated by inflammatory cytokines and oxidative stress [4,5,6]
endoplasmic reticulum (ER) stress is considered a major contributor to the induction of fatty liver disease, accumulating studies suggest that oxidative stress is another critical factor in the development of NAFLD [9,10,11]

Summary

Introduction

Non-alcoholic fatty liver disease (NAFLD) represents a spectrum of liver conditions from simple lipid accumulation to steatohepatitis, liver fibrosis, and hepatocellular carcinoma [1,2,3]. Excessive lipid accumulation in the liver, namely fatty liver or hepatic steatosis, is caused by abnormal lipid metabolism, and subsequent progression is exasperated by inflammatory cytokines and oxidative stress [4,5,6]. The endoplasmic reticulum (ER) is the major site of lipid metabolism in hepatocytes, and disruption of homeostasis in the ER, referred to as ER stress, plays a critical role in the progression of fatty liver and hepatic lipid accumulation [7,8]. Oxidative stress is a primary cause of fat accumulation in the liver and is involved in the development of fibrosis in NAFLD patients [12,13,14]. The functional interactions between ER stress and oxidative stress in the development and progression of NAFLD remain unclear

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