Rinninger F, Pittman R C
Department of Medicine, University of California, San Diego, La Jolla 92093.
J Lipid Res. 1988 Sep;29(9):1179-94.
We have previously shown that the liver and steroidogenic tissues of rats in vivo and a wider range of cells in vitro, including human cells, selectively take up high density lipoprotein (HDL) cholesteryl esters without parallel uptake of HDL particles. This process is regulated in tissues of rats and in cultured rat cells according to their cholesterol status. In the present study, we examined regulation of HDL selective uptake in cultured human fibroblasts and Hep G2 hepatoma cells. The cholesterol content of these cells was modified by a 20-hr incubation with either low density lipoprotein (LDL) or free cholesterol. Uptake of HDL components was examined in a subsequent 4-6-hr assay using intracellularly trapped tracers: 125I-labeled N-methyl-tyramine-cellobiose-apoA-I (125I-NMTC-apoA-I) to trace apoA-I, and [3H]cholesteryl oleyl ether to trace cholesteryl esters. In the case of fibroblasts, pretreatment with either LDL or free cholesterol resulted in decreased selective uptake (total [3H]cholesteryl ether uptake minus that due to particle uptake as measured by 125I-NMTC-apoA-I). In contrast, HDL particle uptake increased with either form of cholesterol loading. The amount of HDL that was reversibly cell-associated (bound) was increased by prior exposure to free cholesterol, but was decreased by prior exposure to LDL. In the case of Hep G2 cells, exposure to free cholesterol only slightly increased HDL particle uptake; selective uptake decreased after both forms of cholesterol loading, and reversibly bound HDL increased after exposure to free cholesterol, but either did not change or decreased after exposure to LDL. It was excluded that either LDL carried over into the HDL uptake assay or that products secreted by the cultured cells influenced these results. Thus, selective uptake by cells of both hepatic and extrahepatic origin was down-regulated by cholesterol loading, under which conditions HDL particle uptake increased. Total HDL binding was not directly correlated with either the rate of selective uptake or the rate of HDL particle uptake or the cholesterol status of the cells, suggesting more than one type of HDL binding site.
我们之前已经表明，大鼠体内的肝脏和类固醇生成组织以及体外更广泛的细胞类型（包括人类细胞）会选择性摄取高密度脂蛋白（HDL）胆固醇酯，而不会同时摄取HDL颗粒。这一过程在大鼠组织和培养的大鼠细胞中会根据其胆固醇状态受到调节。在本研究中，我们检测了培养的人成纤维细胞和Hep G2肝癌细胞中HDL选择性摄取的调节情况。通过与低密度脂蛋白（LDL）或游离胆固醇孵育20小时来改变这些细胞的胆固醇含量。在随后4 - 6小时的检测中，使用细胞内捕获的示踪剂检测HDL成分的摄取：用125I标记的N - 甲基酪胺 - 纤维二糖 - 载脂蛋白A - I（125I - NMTC - apoA - I）追踪载脂蛋白A - I，用[3H]胆固醇油醚追踪胆固醇酯。就成纤维细胞而言，用LDL或游离胆固醇预处理会导致选择性摄取减少（总[3H]胆固醇醚摄取量减去通过125I - NMTC - apoA - I测量的因颗粒摄取导致的摄取量）。相反，HDL颗粒摄取量会随着两种形式的胆固醇加载而增加。预先暴露于游离胆固醇会增加可逆性细胞相关（结合）的HDL量，但预先暴露于LDL会使其减少。就Hep G2细胞而言，暴露于游离胆固醇只会略微增加HDL颗粒摄取；两种形式的胆固醇加载后选择性摄取均减少，暴露于游离胆固醇后可逆性结合的HDL增加，但暴露于LDL后要么不变要么减少。排除了LDL残留于HDL摄取检测中或培养细胞分泌的产物影响这些结果的可能性。因此，肝源性和肝外源性细胞的选择性摄取均会因胆固醇加载而被下调，在此条件下HDL颗粒摄取增加。HDL总结合量与选择性摄取速率、HDL颗粒摄取速率或细胞的胆固醇状态均无直接相关性，这表明存在不止一种类型的HDL结合位点。
