Diczfalusy U, Lund E, Lütjohann D, Björkhem I
Division of Clinical Chemistry, Karolinska Institute, Huddinge Hospital, Sweden.
Scand J Clin Lab Invest Suppl. 1996;226:9-17.
Recently, we described a new pathway whereby peripheral cells can eliminate intracellular cholesterol by conversion into the more polar oxysterols 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid. The latter steroids are easily excreted from the cells and transported to the liver for conversion into bile acids. Our attempts to evaluate the importance of this new mechanism are reviewed here and also our investigations on the possible presence of additional similar pathways for removal of extrahepatic cholesterol. Human alveolar macrophages in culture were shown to have a high capacity to convert cholesterol into 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid and to excrete these steroids into the culture medium. Treatment of the macrophages with cyclosporin A, an inhibitor of sterol 27-hydroxylase, reduced the excretion of the 27-hydroxylated products by more than 90%, with a concomitant accumulation of intracellular cholesterol. The quantitative importance of the mechanism in relation to reverse cholesterol transport was investigated in 14C-cholesterol labelled macrophages exposed to HDL. At very low concentrations of HDL, possibly similar to those present in tissues, the two pathways were about equally effective. At optimal concentrations of HDL, however, reverse cholesterol transport was about 10-fold more effective than the sterol 27-hydroxylase pathway. The net uptake of 27-oxygenated steroids by the liver was measured in volunteers by comparison of the levels in the hepatic vein with those in a peripheral artery. Approximately 20 mg of 27-oxygenated oxysterols was taken up by the liver during 24 hours. Quantitative conversion of these oxysterols into bile acids would correspond to 4% of the total bile acid formation. It is evident that this new pathway contributes significantly to cholesterol elimination. The possibility that the sterol 27-hydroxylase pathway is of importance for cholesterol homeostasis in the brain was investigated by measuring oxysterols in the internal jugular vein and in an artery of healthy volunteers. There was no net flux of 27-hydroxycholesterol from the brain into the circulation. There was, however, a significant flux of 24-hydroxycholesterol, corresponding to elimination of about 4 mg cholesterol/24 hours. This flux is higher than the estimated rate of synthesis of cholesterol in the human brain. To summarize, we have demonstrated two mechanisms for cholesterol elimination from extrahepatic cells by specific oxygenases capable of oxidizing the steroid side-chain. The efficiency of these mechanisms is based on the fact that side-chain hydroxylated cholesterol species are both translocated through lipophilic membranes and converted into bile acids at a much faster rate than cholesterol itself. The importance of the sterol 27-hydroxylase-mediated mechanism is illustrated by the fact that patients who lack this enzyme develop xanthomas and premature atherosclerosis in spite of normal levels of circulating cholesterol.
最近，我们描述了一种新途径，通过该途径外周细胞可将细胞内胆固醇转化为极性更强的氧化甾醇27-羟基胆固醇和3β-羟基-5-胆甾烯酸，从而消除细胞内胆固醇。后一种类固醇很容易从细胞中排出，并转运至肝脏转化为胆汁酸。本文回顾了我们评估这一新机制重要性的尝试，以及我们对可能存在的其他类似肝外胆固醇清除途径的研究。培养的人肺泡巨噬细胞显示出具有将胆固醇转化为27-羟基胆固醇和3β-羟基-5-胆甾烯酸并将这些类固醇分泌到培养基中的高能力。用环孢素A（一种甾醇27-羟化酶抑制剂）处理巨噬细胞，可使27-羟基化产物的分泌减少90%以上，同时细胞内胆固醇积累。在暴露于高密度脂蛋白（HDL）的14C标记巨噬细胞中研究了该机制在逆向胆固醇转运方面的定量重要性。在非常低浓度的HDL（可能类似于组织中存在的浓度）下，这两种途径的效果大致相同。然而，在HDL的最佳浓度下，逆向胆固醇转运比甾醇27-羟化酶途径有效约10倍。通过比较肝静脉和外周动脉中的水平，测量了志愿者肝脏对27-氧化类固醇的净摄取量。在24小时内，肝脏摄取了约20毫克的27-氧化甾醇。这些氧化甾醇定量转化为胆汁酸相当于总胆汁酸形成量的4%。显然，这一新途径对胆固醇清除有显著贡献。通过测量健康志愿者颈内静脉和动脉中的氧化甾醇，研究了甾醇27-羟化酶途径对大脑胆固醇稳态是否重要。没有2�-羟基胆固醇从大脑向循环系统的净通量。然而，有显著的24-羟基胆固醇通量，相当于每24小时消除约4毫克胆固醇。该通量高于人脑中胆固醇合成的估计速率。总之，我们已经证明了两种通过能够氧化类固醇侧链 的特定加氧酶从肝外细胞清除胆固醇的机制。这些机制的效率基于这样一个事实，即侧链羟基化的胆固醇种类既能通过亲脂性膜转运，又能以比胆固醇本身快得多的速率转化为胆汁酸。甾醇27-羟化酶介导的机制的重要性体现在缺乏这种酶的患者尽管循环胆固醇水平正常，但仍会出现黄色瘤和早发性动脉粥样硬化这一事实上。
