| Scavenger receptor class A (SR-A), the member of macrophage scavenger receptor family, is expressed predominantly by macrophages, vascular smooth muscle cells, and endothelial cells. CD36, a member of scavenger receptor B family, is a transmembrane glycoprotein receptor that is expressed in a variety of cells, including monocytes and macrophages. SR-A and CD36 have been implicated in not only mediating lipid internalization, but also playing a pivotal role in host defense, phagocytizing apoptotic cells and promoting cell adhesion. In addition, it was demonstrated that SR-A and CD36 can bind modified forms of low density lipoprotein (LDL) and enhance the accumulation of excessive cholesterol in the macrophages so as to promote the foam cell formation, which is one of the early pathological characters in atherosclerosis.LDL receptor is of primary importance in binding and internalization of plasma-derived LDL-cholesterol. LDLr expression is under control via a negative feedback system that is dependent on intracellular cholesterol concentration. The previous studies in our group have demonstrated that inflammation promote foam cell formation by increasing cholesterol uptake through disrupting cholesterol-mediated LDLr feedback regulation.It is well known that abnormity of the level of lipid in the blood is one of the most important risk factors. Although SR-A and CD36 might be involved in the development of AS through the mechanism not regulated by cellular cholesterol content resulting in cholesterol accumulation and foam cell formation, the mechanism of lipid metabolism and the formation of atherosclerosis in the mice with SR double knockout (DKO) in the inflammatory stress remain unclear.The current study aims to investigate inflammatory stress disrupts LDLr-mediated feedback regulation in the liver or the aorta of SR DKO mice, leading to excess uptake of native LDL and leading to foam cell formation and atherosclerosis. Further it would be make sure that there are differences of the lipid accumulation or atherosclerosis in the inflammation induced by between physical stimulation and murine herpes virus-68 infection regarding regulation of exogenous cholesterol uptake through LDLr.Male SR-A and CD36 KO mice in C57BL/6 genetic background were studied.Six-eight weeks old mice were fed with high fat diet assigned randomly (n=6/group) by injections of either subcutaneous injection with casein every two days or by intraperitoneal injection with MHV-68 every two weeks for up to 14 weeks. At termination, blood samples were taken for serum cytokines and histological assessments.To test the impact of deleting these receptors on lipid-mediated injuries in the aortas or livers under the inflammation stress induced by physical stimulation or infection , serum amyloid A (SAA), TNF-α, IL-6 were determined by Enzyme-Linked Immunosorbent Assay. Lipid levels were measured by enzymatic assay. Lipid accumulation in aortas or livers of inflammatory mice induced by casein injection and murine herpes virus-68 infection, assessed by morphologic and quantitative RT-PCR assay of cholesterol ester in livers, showed significantly increase lipid accumulation which were compared with control mice. However, the two groups of SR DKO mice under inflammatory stress showed increased level of SAA and no significant increased level of TNF-αand IL-6. The concentrations of lipid in serum of SR DKO mice injected by casein showed that there were a significantly decreased, while those of infected with MHV-68 showed there were a significantly increased, compared with control animal.SR DKO mice with a combination of a high fat diet and subcutaneous injection of casein for 14 weeks, displayed an increase of lipid droplets in livers and aortas while SR DKO mice infected with MHV-68 showed an increase of lipid accumulation in livers and the formation of atherosclerotic plaque in the aortas. The gene and protein expressions of molecules involved in cholesterol trafficking were significantly increased in the physical stimulation group, while those were significantly decreased in the infected group.Taking into consideration that hypocholesterolemia and increased lipid accumulation in aorta or liver can also be due to up-regulation of SREBP2 and its target gene (LDLr) and its regulatory gene (SCAP) under inflammatory stress by physical stimulation, our results indicated that mRNA expression and mature protein in both livers and aortas of SR DKO mice were significantly higher than that found in control animals. On the other hand, the infected mice showed that the hypercholesterolemia and lipid accumulation in aortas and livers contribution to down-regulation of the expression of genes and proteins.
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