The Changes Of SR-BI Expression And Cholesterol Trafficking Mediated By SR-BI In Peritoneal Macrophages From ApoE～(-/-) Mice

Lots of epidemiological investigations, basic researches and clinical observations have manifested that high density lipoprotein-cholesterol is inverse correlated to the incidence of atherosclerosis. Scavenger receptor class B type I (SR-BI) was the first molecularly well-defined cell surface receptor for high density lipoprotein (HDL). The primary function of SR-BI is to mediate the selective intake of high density lipoprotein-cholesterol ester (HDL-CE) and the effusion of cellular unesterified cholesterol. Apolipoprotein E (ApoE) is one of the protein components of triglyceride-rich lipoprotein, plays an important part in the metabolism of lipoprotein remnant as the ligand of the receptors for the chylomicron remnants and low density lipoprotein. Although many researches related to SR-BI and apoE have been reported, the relation of the genes is not demonstrated completely. In these studies, we compared the differences of cholesterol trafficking and SR-BI expression in peritoneal macrophages between ApoE-/- and C57BL/6J mice. To illustrate the role of SR-BI in cholesterol trafficking, we subsequently explored the effects of block lipid transport 4 (BLT4), a relatively specific blocker to SR-BI, on cellular cholesterol transport in macrophages derived from ApoE-/- and C57BL/6J mice.Part I: Comparisons of cholesterol trafficking and SR-BI expression in peritoneal macrophages from ApoE-/- mice with that from C57BL/6J miceAIM: To study differences of SR-BI expression and cholesterol transport in peritoneal macrophages derived from ApoE-/- and C57BL/6J mice. METHODS: The peritoneal macrophages were prepared from ApoE-/- mice and its genetic background mice, C57BL/6J mice, respectively. RT-PCR, Western blotting were applied to detect the SR-BI mRNA and protein expression levels, respectively. Cellular lipid accumulation was determined and analyzed by high performance liquid chromatography. Cholesterol efflux rate was quantified by liquid scintillator.RESULTS: The results of RT-PCR and Western blotting showed that SR-BI mRNA transcription and protein expression level in peritoneal macrophages derived from ApoE-/- mice were decreased significantly, compared to that of C57BL/6J mice. High performance liquid chromatography analysis demonstrated that the cellular total cholesterol (TC), cholesterl ester (CE) and CE/TC ratio were increased in macrophages from ApoE-/- mice. Liquid scintillator evaluation demonstrated that the cholesterol efflux was lowered in macrophages from ApoE-/- mice.CONCLUSION: SR-BI expresstion of peritoneal macrophages from ApoE-/- mice is downregulated, which may be involved in the increase of cellular cholesterol accumulation and the decrease of cellular cholesterol efflux.Part II: The effect of BLT4 on cholesterol transport mediated by SR-BI in mouse peritoneal macrophagesAIM: To study the effect of BLT4 on cholesterol transport mediated by SR-BI in mouse peritoneal macrophages.METHODS: The peritoneal macrophages from ApoE-/- and C57 BL/6J mice were treated with 100umol/L BLT4 and without BLT4 (only solvent DMSO), respectively. RT-PCR, Western blotting were applied to detect the SR-BI mRNA and protein expression levels, respectively. Cellular lipid accumulation was determined and analyzed by high performance liquid chromatography. Cholesterol efflux rate was quantified by liquid scintillator. RESULTS: The results showed that no significant decrease of SR-BI expression level was observed in macrophages treated with BLT4. But, the accumulation of cholesterol in macrophges treated with BLT4 was illustrated by analysis of high performance liquid chromatography, and the decrease of cellular cholesterol efflux in macrophages treated with BLT4 was significant.CONCLUSIONS: SR-BI mediates the cellular cholesterol trafficking in peritoneal macrophages derived from both ApoE-/- mice and C57 BL/6J mice.