Homocysteine Induces Hepatic Steatosis Involving ER Stress Response in High Methionine Diet-Fed Mice

Abstract

Elevated circulating homocysteine (Hcy) has been proposed to be associated with non-alcoholic fatty liver disease (NAFLD). It is also reported that Hcy causes protein misfolding in the endoplasmic reticulum (ER). In this study, we used a high methionine diet (HMD)-fed mouse model and cultured primary hepatocytes to investigate the effects of Hcy on hepatic lipids metabolism. C57BL/6J mice received either standard chow diet (CT, n = 10) or diet supplemented with 2% methionine (MET, n = 10) for 16 weeks. In in vitro experiments, cultured mouse primary hepatocytes were treated with Hcy, or Hcy combined with 4-phenylbutyric acid (4-PBA), or tunicamycin (TM), respectively. HMD-fed mice exhibited a mild increase in plasma Hcy level. There was no significant difference of body weight gain between the two groups. Nevertheless, HMD feeding increased epididymal fat/body weight ratio, elevated plasma triglyceride (TG) level, and decreased high-density lipoprotein cholesterol (HDL) level. Similarly, mice on HMD displayed higher liver/body weight ratio, plasma aspartate aminotransferase (AST) and its ratio to alanine aminotransferase (ALT), which was supported by the morphological observations of hepatic triglyceride accumulation in liver tissue as well as primary hepatocytes. Activation of the sterol response element-binding protein 1c (SREBP1c) in Hcy-treated hepatocytes with increased expression of genes involved in hepatic de novo lipogenesis was partially reduced by pretreatment of 4-PBA. Hcy-induced ER stress was also ameliorated by 4-PBA pretreatment, thus demonstrating an important role of Hcy-induced ER stress response in hepatic steatosis. These findings suggest that elevated Hcy was a critical factor in the pathogenesis of NAFLD. Activation of the ER stress response may be involved in Hcy-induced hepatic steatosis.

Homocysteine supplementation ameliorates steatohepatitis induced by a choline‐deficient diet in mice

“…Although ER stress can contribute to the development of steatohepatitis by increasing lipid synthesis and hepatocyte injury a physiological concentration of homocysteine showed distinct functions in hepatocytes. We assessed the effect of homocysteine at concentrations of 10 and 100 μM in in vitro experiments, which reflect healthy and hyperhomocystinuria‐inducible conditions, respectively.…”

“…Before we undertook our experiments, we initially expected that homocysteine supplementation would induce more severe steatohepatitis, as previous reports have shown that homocysteine induces lipid synthesis and cell injury through ER stress . However, homocysteine supplementation instead improved steatohepatitis by reducing the ER stress in mice on a choline‐deficient diet.…”

High levels of dietary methionine improves sitagliptin-induced hepatotoxicity by attenuating oxidative stress in hypercholesterolemic rats

“…In another study, it was demonstrated that excess SAM triggered the synthesis of triglycerides, ultimately leading to hepatic steatosis and the development of NAFLD [15]. Additionally, it was reported that elevated levels of Hcy induce hepatic steatosis by activating ER stress responses [16]. ER stress responses in turn can result in oxidative stress, inflammatory cell infiltration and fibrosis [17].…”

“…In contrast, reduction in intake of Met or Met restriction is reported to result in beneficial effects like increase in energy expenditure, longevity, improvement of insulin sensitivity and reduction in adiposity [26][27][28]. There are animal studies documenting toxic effects of diets supplemented with high levels of Met (2% and above) [14,16]. Importantly, Met has been reported to induce hypercholesterolemia (HChol) by enhancing Cho synthesis in the liver [29].…”

Hyperhomocysteinemia in association with male gender, advanced age, uric acid and metabolic syndrome: A retrospective cross-sectional study

“…Experimental studies have suggested that Hcy-related endoplasmic reticulum stress increases the expression of sterol regulatory element-binding proteins (SREBPs), which further promote cholesterol and triglyceride synthesis [11]. The result was reproduced in an animal experiment by using methionine diet-fed mice, revealing elevated plasma triglyceride in increased plasma Hcy mice [12]. However, the relation between Hcy and triglyceride is not so consistent.…”