Fatty acid-induced endoplasmic reticulum stress promoted lipid accumulation in calf hepatocytes, and endoplasmic reticulum stress existed in the liver of severe fatty liver cows

Abstract

Disruption of endoplasmic reticulum (ER) homeostasis, often termed ER stress, is intrinsically linked with perturbation of lipid metabolism in humans and mice. Whether ER homeostasis is affected in cows experiencing fatty liver is unknown. The aim of this study was to investigate the potential role of ER stress in hepatic lipid accumulation in calf hepatocytes and ER stress status in dairy cows with severe fatty liver. In vitro experiments were conducted in which hepatocytes were isolated from calves and treated with different concentrations of fatty acids, tauroursodeoxycholic acid (TUDCA; a canonical inhibitor of ER stress), or both. The increase in phosphorylation level of protein kinase RNA-like ER kinase (PERK) and inositol requiring protein-1α (IRE1α) proteins, and the cleavage of activating transcription factor-6 (ATF6) protein in response to increasing doses of fatty acids (which were reversed by TUDCA treatment) in primary hepatocytes underscored a mechanistic link between fatty acids and ER stress. In addition, fatty acid treatment increased the abundance of sterol regulatory element-binding protein 1c, acetyl-CoA carboxylase-α, fatty acid synthase, and diacylglycerol acyltransferase 1, and lipid accumulation in calf primary hepatocytes, whereas inhibition of ER stress by incubating with TUDCA significantly weakened these effects. Overall, results in vitro indicate that inhibition of ER stress in calf hepatocytes alleviates fatty acid-induced lipid accumulation by downregulating the expression of lipogenic genes. In vivo experiments, liver and blood samples were collected from cows diagnosed as healthy (n = 15) or with severe fatty liver (n = 15). The phosphorylation level of PERK and IRE1α, the cleavage of ATF6 protein, and the abundance of several unfolded protein response genes (78 kDa glucose-regulated protein, AMP-dependent transcription factor 4, and spliced X-box binding protein 1) were greater in liver of cows with severe fatty liver. The present in vivo study confirms the occurrence of ER stress in dairy cows with severe fatty liver. Considering the causative role of fatty acid-induced ER stress in hepatic lipid accumulation in calf hepatocytes, the existence of ER stress in the liver of severe fatty liver cows may presage its participation in fatty liver progression in dairy cows. However, the mechanistic relationship between ER stress and fatty liver in dairy cows remain to be determined.

Keywords: endoplasmic reticulum stress; fatty acid; fatty liver; unfolded protein response.