Abstract
Scavenger receptor class B, type I (SRBI) is a key regulator of high density lipoprotein (HDL) metabolism. It facilitates the efflux of cholesterol from cells in peripheral tissues to HDL and mediates the selective uptake of cholesteryl esters from HDL in the liver. We investigated the effects of SRBI deficiency in the arterial wall and in the liver using SRBI-deficient mice and wild-type littermates fed a Western-type diet. The SRBI-deficient mice showed massive accumulation of cholesterol-rich HDL in the circulation, reflecting impaired delivery to the liver. Strikingly, SRBI deficiency did not alter hepatic cholesterol (ester) content nor did it affect the expression of key regulators of hepatic cholesterol homeostasis, including HMG-CoA reductase, the low density lipoprotein receptor, and cholesterol 7alpha-hydroxylase. However, a approximately 40% reduction in biliary cholesterol content was observed, and the expression of ABCG8 and ABCG5, ATP half-transporters implicated in the transport of sterols from the liver to the bile, was attenuated by 70 and 35%, respectively. In contrast to the situation in the liver, SRBI deficiency did result in lipid deposition in the aorta and atherosclerosis. Vascular mRNA analysis showed increased expression of inflammatory markers as well as of genes involved in cellular cholesterol homeostasis. Our data show that, although hepatic cholesterol homeostasis is maintained upon feeding a Western-type diet, SRBI deficiency is associated with de-regulation of cholesterol homeostasis in the arterial wall that results in an increased susceptibility to atherosclerosis.
Highlights
Scavenger receptor class B, type I (SRBI) is a key regulator of high density lipoprotein (HDL) metabolism
Hepatic cholesterol homeostasis is maintained upon feeding a Western-type diet, SRBI deficiency is associated with de-regulation of cholesterol homeostasis in the arterial wall that results in an increased susceptibility to atherosclerosis
The availability of gene-targeted mice that lack functional SRBI expression enabled us to study its role in the maintenance of cellular cholesterol homeostasis in the liver, the site of cholesterol disposal, and in the arterial wall, the site where pathological cholesterol accumulation occurs
Summary
Mice—SRBI-deficient mice were kindly provided by Dr M. After dissolving the lipids in 2% Triton X-100, the contents of cholesterol, cholesteryl ester, and triglycerides in liver tissue were determined as described above and expressed as nmol/mg of protein. The level of mRNA expression for each gene of interest was calculated using the threshold cycle (Ct) value, i.e. the number of PCR cycles at which the fluorescent signal during the PCR reaches a fixed threshold For each sample, both the Ct for the gene of interest and for the housekeeping gene hypoxanthine-guanine phosphoribosyltransferase were determined to calculate ⌬Ct,sample (Ct,target gene Ϫ Ct,housekeeping gene), normalizing the data and correcting for differences in amount and/or quality between the different RNA samples. ⌬⌬Ct (⌬Ct,sample Ϫ ⌬Ct,calibrator) was determined, and the relative expression levels were calculated from 2Ϫ⌬⌬Ct, according to the manufacturer’s instructions (Applied Biosystems). Statistical Analyses—Statistical significant differences among the means of the different groups of mice were tested using analysis of variance and the Student-Newman-Keuls multicomparison test (Graphpad Instat software, San Diego, CA)
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