Dory L
Department of Pharmacology, University of Tennessee, Memphis 38163.
J Lipid Res. 1991 May;32(5):783-92.
Recent reports from this laboratory indicate that exposure of cholesterol-loaded macrophages to high density lipoprotein 3 (HDL3) stimulates not only cholesterol efflux, but also results in a two- to threefold increase in apoE accumulation in the media (Dory, L., 1989. J. Lipid Res. 30: 809-816). The present experiments demonstrate that the effect of HDL3, and to a lesser extent HDL2, on apoE secretion is specific, concentration-dependent, and may require interaction with the HDL receptor. Very low density lipoproteins (VLDL) and low-density lipoproteins (LDL) fail to specifically stimulate apoE secretion by cholesterol-loaded macrophages. The effect of HLD3 is maximal at 25-50 micrograms/ml (0.26-0.52 microM) and can be totally abolished by mild nitrosylation (with 3 mM tetranitromethane (TNM)). Data are also presented to indicate that the increased rate of apoE secretion in the presence of HDL3 is not due to a "protective" effect of this lipoprotein on possible proteolytic degradation or cellular reuptake of apoE secreted into the media. The stimulatory effect of HDL on apoE secretion can be clearly dissociated from cholesterol efflux; HDL stimulates apoE secretion from oxysterol-treated cells in the absence of measurable cholesterol efflux, while TNM-HDL promotes substantial cholesterol efflux from cholesterol-loaded cells but has no effect on apoE secretion. The kinetics of apoE synthesis and secretion, determined in short-term labeling studies, demonstrate that under all experimental conditions examined a substantial portion of cellular apoE is not secreted. Furthermore, in cholesterol-loaded cells HDL3 increases apoE secretion essentially by diversion of a greater portion of cellular apoE pool for secretion. While HDL3 has no effect on the rate of apoE synthesis, cellular apoE turns over two-fold faster in cells incubated in the presence of HDL3 than in its absence (t 1/2 = 11 +/- 2 and 22 +/- 4 min, respectively), an observation corresponding well with the changes in the rates of apoE secretion under similar conditions. The HDL3-mediated increase in apoE secretion by cholesterol-loaded macrophages suggests another mechanism by which HDL exerts a protective effect in the development of atherosclerosis; increased contribution to the metabolic pool of apoE by peripheral tissues may lead to a more effective clearance of peripheral cholesterol by the liver (reverse cholesterol transport).
本实验室最近的报告表明，将载有胆固醇的巨噬细胞暴露于高密度脂蛋白3（HDL3）不仅能刺激胆固醇流出，还会导致培养基中载脂蛋白E（apoE）的积累增加两到三倍（多里，L.，1989年。《脂质研究杂志》30：809 - 816）。目前的实验表明，HDL3以及在较小程度上HDL2对apoE分泌的作用具有特异性、浓度依赖性，并且可能需要与HDL受体相互作用。极低密度脂蛋白（VLDL）和低密度脂蛋白（LDL）不能特异性地刺激载有胆固醇的巨噬细胞分泌apoE。HLD3在25 - 50微克/毫升（0.26 - 0.52微摩尔）时作用最大，并且可以通过温和的亚硝基化（用3毫摩尔四硝基甲烷（TNM））完全消除。还提供了数据表明，在HDL3存在下apoE分泌速率的增加并非由于这种脂蛋白对分泌到培养基中的apoE可能的蛋白水解降解或细胞再摄取具有“保护”作用。HDL对apoE分泌的刺激作用可以明显地与胆固醇流出区分开；在没有可测量的胆固醇流出的情况下，HDL能刺激经氧化甾醇处理的细胞分泌apoE，而TNM - HDL能促进载有胆固醇的细胞大量流出胆固醇，但对apoE分泌没有影响。在短期标记研究中确定的apoE合成和分泌动力学表明，在所研究的所有实验条件下，细胞内相当一部分apoE没有分泌。此外，在载有胆固醇的细胞中，HDL3基本上通过将细胞内apoE池的更大一部分转向分泌来增加apoE分泌。虽然HDL3对apoE合成速率没有影响，但在有HDL3存在的情况下培养的细胞中，细胞内apoE的周转速度比没有HDL3时快两倍（半衰期分别为11±2分钟和22±4分钟），这一观察结果与相似条件下apoE分泌速率的变化非常吻合。HDL3介导的载有胆固醇的巨噬细胞中apoE分泌增加提示HDL在动脉粥样硬化发展过程中发挥保护作用的另一种机制；外周组织对apoE代谢池的贡献增加可能导致肝脏更有效地清除外周胆固醇（逆向胆固醇转运）。
