Molecular Mechanism Of Lipids Metabolic Disorder, Hepar And Aorta Impairment Under Chronic Nflammation Condition

Objective The low chronic systematic inflammatory response is thecore and junction point of several composition ingredients(such as bloodglucose, blood lipid and uric acid) about metabolism syndrome (MS).Inflammatory mediators participate in whole process of lipid metabolicdisorder. The Sterol Regulatory Element Binding Proteins (SREBPs) arethe nuclear factors. SREBP Cleavage-activating Protein (SCAP) ismembrane protein on the endoplasmic reticulum. SCAP plays animportant role in regulating SREBPs activity. Intracelluar cholesterolhomeostasis is regulated by a negative feedback system which isdependent on intracellular cholesterol concentration. We havedemonstrated in vitro that inflammatory mediators increased LDL receptor(LDLR)-mediated cholesterol uptake and promoted foam cell formationby disrupting cholesterol-mediated LDLR feedback regulation inperipheral cells, such as vascular smooth muscle cells (VSMCs). Acetylcoenzyme A carboxylase (ACC) and fatty acid synzyme (FAS) are keyenzymes which control triglyceride (TG) synthesis. Both ACC and FAS are regulated by SCAP-SREBP1. The lipid metabolic disorder in liver ismainly characterized by large droplets of fat accumulation, accompaniedby inflammatory cell infiltration and/or fibroplasias. But the mechanismsby which inflammatory mediators disturb ACC and FAS and result in TGmetabolic disorder in liver are still unclear.By using an inflamed mouse model, this project was designed toinvestigate whether inflammatory mediators (i) caused dyslipidaemia;(ii)promoted lipid uptake via LDLR by disrupting SCAP-SREBPs mediatednegative feedback regulation of LDLR, thereby resulting lipidaccumulation in aorta and liver.Materials and methods Using8-week age C57BL/6J mice fed withnormal or atherogenic diet for20weeks respectively, we induced achronic inflammation in mice by injecting10%of casein subcutaneously.The experiments were terminated after20weeks. Plasma samples,collected at the time of sacrifice, were evaluated for lipid profiles (totalserum cholesterol, triglycerides, LDL, HDL, and VLDL). Activation of theinflammatory response was assessed by measurement of serumInterleukin-6(IL-6). Maximal plaque cross-section, plaque area per aorticcircumference, aortic wall thickness, and all other vascular beds includingrenal artery were determined with oil red O staining (ORO). Massontrichrome staining was used to observe the morphological change of liverand aorta. Histological Criteria for NAFLD/NASH was used to assess the grade and stage of fatty liver disease. Total RNA was isolated from theliver and the aorta, and mRNA of LDLR, SCAP, SREBP1,2, ACC andFAS from the liver and the aorta were examined by real-time quantitativePCR. The protein expression of LDLR, SCAP and SREBP1,2in the liveror the aorta was examined by western blotting.Results Serum IL-6was significantly elevated by the caseinsubcutaneous injection in the mice fed by both normal and atherogenic diet.In the mice fed with the normal diet, casein injection slightly decreasedTC level and obviously decreased LDL, HDL and TG levels. In the micefed with the atherogenic diet, all lipids in serum (such as TC, LDL, HDLand TG) are obviously decreased by the casein injection. These datasuggest that chronic inflammatory stress induces a metabolic disorder oflipids.Comparing to the normal diet, the atherogenic diet significantly inhibitedmRNA and protein expression of LDLR, SCAP and SREBP2in the liverand the aorta, however chronic inflammation induced by the caseininjection overrode the suppression of LDLR, SCAP and SREBPs inducedby high cholesterol diet. These data show IM can interfere withintracellular cholesterol metabolism.In the mice fed with atherogenic diet, SREBP1, ACC and FAS mRNAlevels in liver were obviously increased, and these mRNA levels furtherenhanced under the chronic inflammatory condition. Hepatocytes uptaked much TG, resulted in hepatic steatosis, ballooning, cellular necrosis,inflammatory cell infiltration and fibroplasia, namely NAFLD or NASH.Morphological figure showed mice aortic root tunica intima thickened,many foam cell and atherosclerotic plaque formation in the mice fed withatherogenic diet. The atherosclerotic plaque of aortic root were furtherthickened and widened in the mice under chronic inflammatory condition.The data suggest that IM aggravates aorta impairment.Conclusion Casein injection successfully induced a chronicinflammatory stress in the mice by showing that serum IL-6, aninflammatory marker, was significantly increased. The chronicinflammatory stress induced by casein injection promoted lipidaccumulation in aorta and liver by increasing LDLR mediated cholesteroluptake by disrupting SCAP-SREBP2-LDLR mediated feedback regulation.Meanwhile, inflammatory mediators also reduced blood lipid levels.These results suggest that there is not a safe blood plasma cholesterolconcentration under chronic inflammatory condition since the plasmacholesterol level is not associated with intracellular cholesterol extentunder inflammatory stress.The inflammatory mediators caused TG accumulation in liver byincreasing SREBP1, ACC and the FAS gene expression, thereby resultingin liver fat degeneration,accompanied by inflammatory cell infiltrationand/or fibroplasias. These data suggest that inflammatory mediators aggravate the liver injury.