Background Low-density lipoprotein （LDL） receptor is normally regulated via a feedback system that is dependent on intracellular cholesterol levels. We have demonstrated that cytokines disrupt cholesterol-mediated LDL receptor feedback regulation causing intracellular accumulation of unmodified LDL in peripheral cells. Liver is the central organ for lipid homeostasis. The aim of this study was to investigate the regulation of cholesterol exogenous uptake via LDL receptor and its underlying mechanisms in human hepatic cell line （HepG2） cells under physiological and inflammatory conditions. Methods Intracellular total cholesterol （TC）, free cholesterol （FC） and cholesterol ester （CE） were measured by an enzymic assay. Oil Red O staining was used to visualize lipid droplet accumulation in cells. Total cellular RNA was isolated from cells for detecting LDL receptor, sterol regulatory element binding protein （SREBP）-2 and SREBP cleavage-activating protein （SCAP） mRNA levels using real-time quantitative PCR. LDL receptor and SREBP-2 protein expression were examined by Western blotting. Confocal microscopy was used to investigate the translocation of SCAP-SREBP complex from the endoplasmic reticulum （ER） to the Golgi by dual staining with anti-human SCAP and anti-Golgin antibodies. Results LDL loading increased intracellular cholesterol level, thereby reduced LDL receptor mRNA and protein expression in HepG2 cells under physiological conditions. However, interleukin 1β （IL-1β） further increased intracellular cholesterol level in the presence of LDL by increasing both LDL receptor mRNA and protein expression in HepG2. LDL also reduced the SREBP and SCAP mRNA level under physiological conditions. Exposure to IL-1β caused over-expression of SREBP-2 and also disrupted normal distribution of SCAP-SREBP complex in HepG2 by enhancing translocation of SCAP-SREBP from the ER to the Golgi despite a high concentration of LDL in the culture medium. Conclusions IL-1β disrupts cholesterol-media
CHEN Ya-xiRUAN Xiong-zhongHUANG Ai-longLI QiuJohn F. MoorheadZac Varghese
The liver plays a major role in the regulation , glucose, lipid and energy metabolism. Increasd hepatic fat deposit is a very common feature in obese, insulin-resistant patients, in metabolic syndrome, alcoholic steatohepatitis （ASH） and nonalchoholic fatty liver disaseas （NAFLD）. As a central organ for whole body lipid metabolism, disruption of the normal mechanisms for synthesis, transport and removal/ metabolism of long-chain fatty acids and triglycerides are the basis for the development of fatty liver. The exact mechanisms that link hepatic lipid accumulation, impaired glucose metabolism, and insulin resistance are unknown, but increasing evidence suggest that nuclear transcription factors play important roles. Members of the nuclear receptor superfamily, especially the peroxisome proliferator-activated receptors （PPARs） and the liver X receptor （LXR）, other factors such as sterol regulatory element binding proteins （SREBPs）, carbohydrate- response element-binding protein （ChREBP）, and nuclear transcription fator-κB （NF-κB） have emerged as dominant regulators of these processes, but the relative role of each of these factors in fatty liver disease is still undefined.
CHEN Ya-xi HUANG Ai-long RUAN Xiong-zhong Centre for Lipid Research,Key Laboratory of Molecular Biology on Infectious Diseases,Ministry of Education
