Chroni Angeliki, Nieland Thomas J F, Kypreos Kyriakos E, Krieger Monty, Zannis Vassilis I
Molecular Genetics, Whitaker Cardiovascular Institute, Department of Medicine, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Biochemistry. 2005 Oct 4;44(39):13132-43. doi: 10.1021/bi051029o.
Apolipoprotein E (apoE) and the lipoprotein receptor SR-BI play critical roles in lipid and lipoprotein metabolism. We have examined the cholesterol efflux from wild-type (WT) and mutant forms of SR-BI expressed in ldlA-7 cells using reconstituted discoidal particles consisting of apoE, 1-palmitoyl-2-oleoyl-l-phospatidylcholine (POPC), and cholesterol (C) as acceptors. POPC/C-apoE particles generated using apoE2, apoE3, apoE4, or carboxy-terminally truncated forms apoE4-165, apoE4-202, apoE4-229, and apoE4-259 caused similar (20-25%) cholesterol efflux from WT SR-BI. Cholesterol efflux mediated by POPC/C-apoE was not enhanced in the presence of lipid-free apoE. The rate of cholesterol efflux mediated by particles containing the WT or carboxy-terminally truncated forms of apoE was decreased to approximately 30% of the WT control with the Q402R/Q418R mutant SR-BI form that is unable to bind native HDL normally but binds LDL. The rate of cholesterol efflux was further decreased to approximately 7% of the WT control with another SR-BI mutant (M158R) that binds neither HDL nor LDL. The level of binding of POPC/C-apoE particles (150 microg/mL) to SR-BI mutant forms Q402R/Q418R and M158R was 70 and 8% of the WT control, respectively. SR-BI-dependent binding of lipid-free apoE to cells was undetectable, and cholesterol efflux was less than 0.5%. The findings establish that only lipid-bound apoE promotes SR-BI-mediated cholesterol efflux and that the amino-terminal region of residues 1-165 of apoE is sufficient for both receptor binding and cholesterol efflux. The SR-BI-apoE interactions may contribute to overall cholesterol homeostasis in cells and tissues that express SR-BI and apoE.
载脂蛋白E（apoE）和脂蛋白受体SR-BI在脂质和脂蛋白代谢中发挥着关键作用。我们使用由apoE、1-棕榈酰-2-油酰-1-磷脂酰胆碱（POPC）和胆固醇（C）组成的重组盘状颗粒作为受体，检测了在ldlA-7细胞中表达的野生型（WT）和突变形式的SR-BI的胆固醇流出情况。使用apoE2、apoE3、apoE4或羧基末端截短形式的apoE4-165、apoE4-202、apoE4-229和apoE4-259生成的POPC/C-apoE颗粒，从WT SR-BI引起的胆固醇流出相似（20%-25%）。在无脂质apoE存在的情况下，POPC/C-apoE介导的胆固醇流出并未增强。含有WT或羧基末端截短形式apoE的颗粒介导的胆固醇流出率，在Q402R/Q418R突变型SR-BI形式（其不能正常结合天然HDL但能结合LDL）的情况下，降至WT对照的约30%。在另一种既不结合HDL也不结合LDL的SR-BI突变体（M158R）的情况下，胆固醇流出率进一步降至WT对照的约7%。POPC/C-apoE颗粒（150μg/mL）与SR-BI突变形式Q402R/Q418R和M158R的结合水平，分别为WT对照的70%和8%。未检测到无脂质apoE与细胞的SR-BI依赖性结合；胆固醇流出率小于0.5%。这些发现表明，只有脂质结合的apoE才能促进SR-BI介导的胆固醇流出，并且apoE的1-165位残基的氨基末端区域对于受体结合和胆固醇流出均足够。SR-BI-apoE相互作用可能有助于表达SR-BI和apoE的细胞和组织中的整体胆固醇稳态。
