ROLE OF CHOLESTEROL ESTER TRANSFER PROTEIN AND APOLIPOPROTEIN E IN DEVELOPMENT OF ATHEROSCLEROSIS

为了探讨载脂蛋白E在胆固醇逆向转运中的调节作用，我们研究了载脂蛋白E对血小板功能(1)和胆固醇酯转移活性(2)的影响。(1)1例载脂蛋白E缺乏患者表现出富含血小板的血浆中血小板聚集性降低，而洗涤后的血小板聚集性正常。患者血浆和高密度脂蛋白组分均抑制正常人的血小板聚集。重组载脂蛋白E重组患者的高密度脂蛋白对血小板功能有进一步的抑制作用。这些结果表明，载脂蛋白E是一个有效的，但不是唯一的，高密度脂蛋白的抑制因子。(2)以载脂蛋白E缺乏症患者的脂蛋白为模型，研究载脂蛋白E对胆固醇酯转运蛋白活性的直接影响。通过在极低密度脂蛋白(VLDL)中掺入载脂蛋白E，促进了胆固醇酯(CE)从盘状双层颗粒(DBP)到极低密度脂蛋白(VLDL)的转移。这种增强只有在CETP活性存在的情况下才能诱导。此外，CETP与VLDL孵育后，CETP活性和免疫反应性与载脂蛋白E结合的极低密度脂蛋白(E-VLDL)在凝胶过滤柱上共洗脱，而与不含载脂蛋白E的极低密度脂蛋白(C-VLDL)几乎不同，这表明E-VLDL与CETP的亲和力高于C-VLDL。补充载脂蛋白E可增强CETP引起的高密度脂蛋白组分中CE和甘油三酯(TG)含量的变化，这种变化可被CETP单抗阻断CETP活性所完全抑制。我们的结果提示：1)载脂蛋白E通过CETP促进CE和TG在极低密度脂蛋白和高密度脂蛋白之间的转移；2)载脂蛋白E的这种作用可能是通过增强CETP对极低密度脂蛋白的亲和力来实现的。

In order to explore the regulatory role of apolipoprotein E in cholesterol reverse transport, we studied the effect of apo E on platelet function(1) and on cholesterol ester transfer activity(2).(1) A patient with apo E deficiency showed evidence of decreased platelet aggregativity in platelet-rich plasma, but normal aggregativity in washed platelets. Both patient's plasma and HDL fraction inhibited platelet aggregation of normal subjects. Patient's HDL reconstituted with recombinant apo E showed further inhibitory effects on platelet function. These results suggest that apo E is a potent, but not unique, inhibitory factor of HDL. (2) The direct effect of apolipoprotein (apo) E on cholesteryl ester transfer protein (CETP) activity was studied, using lipoproteins from an apo E deficiency as a model system. The transfer of cholesteryl ester (CE) from discoidal bilayer particle (DBP) to very low density lipoprotein (VLDL) was enhanced by incorporation of apo E into VLDL. This enhancement was induced only in the presence of CETP activity. Moreover, after incubation of CETP with VLDL, CETP activity and immunoreactivity were co-eluted with apo E incorporated VLDL (E-VLDL) on a gel filtration column, but little with apo E-free VLDL (C-VLDL), suggesting that E-VLDL had higher affinity for CETP compared to C-VLDL. The supplementation with apo E of the apo E deficient serum enhanced the CETP-mediated changes of amount of CE and triglyceride (TG) in HDL fraction, which were completely inhibited by the addition of the monoclonal antibody against CETP that blocks CETP activity. Our results suggest that 1) apo E enhances the CE and TG transfer between VLDL and HDL via CETP and 2) this effect of apo E may be mediated by enhancing the affinity of CETP for VLDL.