Abstract
The identification of ABCA1 as a key transporter responsible for cellular lipid efflux has led to considerable interest in defining its role in cholesterol metabolism and atherosclerosis. In this study, the effect of overexpressing ABCA1 in the liver of LDLr-KO mice was investigated. Compared with LDLr-KO mice, ABCA1-Tg x LDLr-KO (ABCA1-Tg) mice had significantly increased plasma cholesterol levels, mostly because of a 2.8-fold increase in cholesterol associated with a large pool of apoB-lipoproteins. ApoB synthesis was unchanged but the catabolism of (125)I-apoB-VLDL and -LDL were significantly delayed, accounting for the 1.35-fold increase in plasma apoB levels in ABCA1-Tg mice. We also found rapid in vivo transfer of free cholesterol from HDL to apoB-lipoproteins in ABCA1-Tg mice, associated with a significant 2.7-fold increase in the LCAT-derived cholesteryl linoleate content found primarily in apoB-lipoproteins. ABCA1-Tg mice had 1.4-fold increased hepatic cholesterol concentrations, leading to a compensatory 71% decrease in de novo hepatic cholesterol synthesis, as well as enhanced biliary cholesterol, and bile acid secretion. CAV-1, CYP2b10, and ABCG1 were significantly induced in ABCA1-overexpressing livers; however, no differences were observed in the hepatic expression of CYP7alpha1, CYP27alpha1, or ABCG5/G8 between ABCA1-Tg and control mice. As expected from the pro-atherogenic plasma lipid profile, aortic atherosclerosis was increased 10-fold in ABCA1-Tg mice. In summary, hepatic overexpression of ABCA1 in LDLr-KO mice leads to: 1) expansion of the pro-atherogenic apoB-lipoprotein cholesterol pool size via enhanced transfer of HDL-cholesterol to apoB-lipoproteins and delayed catabolism of cholesterol-enriched apoB-lipoproteins; 2) increased cholesterol concentration in the liver, resulting in up-regulated hepatobiliary sterol secretion; and 3) significantly enhanced aortic atherosclerotic lesions.
Highlights
The inverse relationship between plasma high density lipoprotein-cholesterol (HDL-C)3 concentrations and the incidence of coronary artery disease is well established in humans [1,2,3,4,5]
Hepatic Overexpression of ATP-binding cassette transporter A1 (ABCA1) in LDLr-KO Mice—To further elucidate the role of hepatic ABCA1 in lipid metabolism and atherosclerosis, we studied the effect of ABCA1 overexpression in mice lacking the LDL-receptor (LDLr) consuming a rodent chow diet
Human ABCA1 mRNA was found to be overexpressed in livers of ABCA1-Tg mice, leading to a 2-fold increase of ABCA1 mRNA levels (Fig. 1A), while human ABCA1 expression was very low in all other investigated tissues, including macrophages (Fig. 1A)
Summary
Animals—Human ABCA1-Tg mice were produced with a construct containing the human ABCA1 cDNA driven by the murine apoE promoter, as previously described [18]. Expression of human ABCA1 was determined by dot blot hybridization, as previously described [19]. Total RNA from peritoneal macrophages was extracted from a pool of macrophages (n ϭ 6 male mice per genotype) after allowing the cells to attach for 3 h on Primaria cell culture dishes (BD Biosciences, Franklin Lakes, NJ) in DMEM containing 10% fetal bovine serum, 4.5 g of glucose/liter and penicillin/ streptomycin/L-glutamine (Sigma). Macrophage protein was isolated from pools of macrophages of six male mice from each group, after allowing the cells to attach to Primaria cell culture dishes (BD Biosciences) for 3 h in DMEM containing 10% fetal bovine serum, 4.5g glucose/liter, and penicillin/streptomycin/L-glutamine (Sigma). Non-parametric data were analyzed by the Mann-Whitney test (Instat Software, Graphpad Inc, San Diego, CA)
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