Inflammatory Stress Increases Hepatic CD36 Translational Efficiency via Activation of the mTOR Signalling Pathway

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.

Interleukin-6 influences stress-signalling by reducing the expression of the mTOR-inhibitor REDD1 in a STAT3-dependent manner

“…mTOR is essential for IL-6-induced tumor growth [43], it mediates IL-6-induced hepatic insulin resistance [44] and contributes to the development of non-alcoholic fatty liver disease in inflammatory conditions [45]. Interestingly, mTOR activity triggers shedding of the IL-6Rα, thereby increasing pro-inflammatory IL-6 trans-signalling hence building a positive feedback loop [46].…”

Inhibition of Glutathione Production Induces Macrophage CD36 Expression and Enhances Cellular-oxidized Low Density Lipoprotein (oxLDL) Uptake

“…In hepatocytes, activation of mTOR signaling pathway is involved in regulation of CD36 translation. Induction of inflammatory stress in HepG2 cells by TNF-␣ and IL-6 treatment or in mice by administration of casein increases cellular lipid accumulation and CD36 protein expression through enhancement of CD36 translational efficiency, which is blocked by rapamycin, an mTOR-specific inhibitor (47). The induction of hepatic CD36 translation by palmitate is also completed through activation of the mTOR The cells were then used to prepare polysomal RNA fractions followed by determination of CD36 and GAPDH mRNA expression by Northern blot analysis.…”

MicroRNA-101 overexpression by IL-6 and TNF-α inhibits cholesterol efflux by suppressing ATP-binding cassette transporter A1 expression

“…Then, HepG2 cells and THP-1 macrophages at 50-70% confluence were transfected with either miR-101, Con-miR, anti-miR-101, or Con-anti-miR (1 Â 10 8 TU/ml lentivirus) using the manufacturers' protocols (Shanghai Ji Kai Gene Chemical Technology Co., Ltd., China). Cells were transfected for 48 h and treated with or without LDL (150 mg/ml) and with either IL-6 (20 ng/ml) or TNF-α (25 ng/ml) for an additional 24 h [17,18]. Then, Western blotting was conducted to assess ABCA1 protein expression with β-actin used as an internal control.…”