Inflammatory stress increases hepatic CD36 translational efficiency via activation of the mTOR signalling pathway.

Lei Zhao,John F Moorhead,Yaxi Chen,Lin Hu,Xiong Z Ruan,Ping Yang,Chuan Wang,Silvia C Sookoian,Zac Varghese

doi:10.1371/journal.pone.0103071

Abstract

Inflammatory stress is an independent risk factor for the development of non-alcoholic fatty liver disease (NAFLD). Although CD36 is known to facilitate long-chain fatty acid uptake and contributes to NAFLD progression, the mechanisms that link inflammatory stress to hepatic CD36 expression and steatosis remain unclear. As the mammalian target of rapamycin (mTOR) signalling pathway is involved in CD36 translational activation, this study was undertaken to investigate whether inflammatory stress enhances hepatic CD36 expression via mTOR signalling pathway and the underlying mechanisms. To induce inflammatory stress, we used tumour necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) stimulation of the human hepatoblastoma HepG2 cells in vitro and casein injection in C57BL/6J mice in vivo. The data showed that inflammatory stress increased hepatic CD36 protein levels but had no effect on mRNA expression. A protein degradation assay revealed that CD36 protein stability was not different between HepG2 cells treated with or without TNF-α or IL-6. A polysomal analysis indicated that CD36 translational efficiency was significantly increased by inflammatory stress. Additionally, inflammatory stress enhanced the phosphorylation of mTOR and its downstream translational regulators including p70S6K, 4E-BP1 and eIF4E. Rapamycin, an mTOR-specific inhibitor, reduced the phosphorylation of mTOR signalling pathway and decreased the CD36 translational efficiency and protein level even under inflammatory stress resulting in the alleviation of inflammatory stress-induced hepatic lipid accumulation. This study demonstrates that the activation of the mTOR signalling pathway increases hepatic CD36 translational efficiency, resulting in increased CD36 protein expression under inflammatory stress.

Highlights

Non-alcoholic fatty liver disease (NAFLD) encompasses a spectrum of liver diseases without significant alcohol consumption, ranging from non-alcoholic simple fatty liver (NAFL) to nonalcoholic steatohepatitis (NASH), hepatic fibrosis and cirrhosis
There was no obvious difference in the CD36 protein half-life in HepG2 cells in the presence or absence of inflammatory cytokines (Figure 2), indicating that inflammatory stress had no effect on the degradation of the CD36 protein
The amount of Free Fatty Acid (FFA) uptake by HepG2 cells was quantified by flow cytometry, we found that inflammatory stress significantly increased the fluorescein isothiocyanate (FITC) fluorescence intensities of HepG2 cells, whereas rapamycin reduced the enhanced FITC fluorescence intensities induced by inflammatory stress (Figure 6B)

Summary

Introduction

Non-alcoholic fatty liver disease (NAFLD) encompasses a spectrum of liver diseases without significant alcohol consumption, ranging from non-alcoholic simple fatty liver (NAFL) to nonalcoholic steatohepatitis (NASH), hepatic fibrosis and cirrhosis. When insulin does not suppress triglyceride (TG) hydrolysis in adipose tissue, fatty acids are released into the circulation to form albumin/fatty acid complexes, which transport into hepatocytes by either passive transport or fatty acid transporting proteins (FATPs) including CD36 ( called ‘fatty acid translocase (FAT)’), plasma membrane fatty acid binding protein (FABPpm) and caveolin Of these CD36 plays an important role in hepatic fatty acids transport. CD36 is a transmembrane glycoprotein which is a type B scavenger receptor expressed in various cells associated with energy metabolism, including adipocytes [8], pancreatic beta cells [9], skeletal myocytes and hepatocytes [10] This multifunctional receptor has been studied extensively with regard to its role in facilitating the uptake of long-chain fatty acids and oxidised low-density lipoproteins, which are involved in the aetiology of such metabolic disorders as diabetes [11], atherosclerosis [12] and NAFLD [13]. These findings suggest that hepatic CD36 expression is closely related to hepatic steatosis

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Results

Conclusion