Axelson M, Larsson O
Department of Clinical Chemistry, Karolinska Hospital, Stockholm, Sweden.
J Biol Chem. 1995 Jun 23;270(25):15102-10. doi: 10.1074/jbc.270.25.15102.
The formation of oxysterols in cultured human fibroblasts and their physiological roles as intracellular regulators of cholesterol production have been investigated. In the presence of low density lipoproteins (LDL) normal fibroblasts converted LDL cholesterol to the oxysterol 27-hydroxycholesterol in quantities apparently sufficient to down-regulate 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. The production of 27-hydroxycholesterol started 3-8 h after the addition of LDL to the incubation medium, and during this time the activity of HMG-CoA reductase decreased by 73%. Formation of other biologically active oxysterols such as 7 alpha-hydroxycholesterol, 24-hydroxycholesterol, and 25-hydroxycholesterol was not observed. When 27-hydroxylation of LDL cholesterol in mitochondria was selectively prevented by treating normal fibroblasts with cyclosporin A or by using fibroblasts genetically lacking sterol 27-hydroxylase, the suppressive effect of LDL on HMG-CoA reductase was reduced by a factor of about 10. In the absence of LDL or when the fibroblasts lacked LDL-receptors, the cells did not produce detectable amounts of 27-hydroxycholesterol, and HMG-CoA reductase was up-regulated. The results provide strong evidence that 27-hydroxycholesterol is an important intracellular mediator between LDL and the suppression of HMG-CoA reductase in human fibroblasts. The mitochondrial uptake/metabolism of LDL cholesterol seems to be a biologically important extension of the LDL pathway in human cells, since the mitochondrial products of LDL cholesterol may regulate cholesterol homeostasis or be precursors of steroid hormones or bile acids. This pathway, which has now been demonstrated in fibroblasts, may link together the two 22-year-old observations that LDL as well as oxysterols can down-regulate HMG-CoA reductase in cells.
已对培养的人成纤维细胞中氧化甾醇的形成及其作为胆固醇生成细胞内调节剂的生理作用进行了研究。在低密度脂蛋白（LDL）存在的情况下，正常成纤维细胞将LDL胆固醇转化为氧化甾醇27-羟基胆固醇，其数量显然足以下调3-羟基-3-甲基戊二酰辅酶A（HMG-CoA）还原酶。在向孵育培养基中添加LDL后3-8小时开始产生27-羟基胆固醇，在此期间HMG-CoA还原酶的活性下降了73%。未观察到其他生物活性氧化甾醇如7α-羟基胆固醇、24-羟基胆固醇和25-羟基胆固醇的形成。当用环孢素A处理正常成纤维细胞或使用遗传上缺乏甾醇27-羟化酶的成纤维细胞选择性地阻止线粒体中LDL胆固醇的27-羟化时，LDL对HMG-CoA还原酶的抑制作用降低了约10倍。在没有LDL的情况下或当成纤维细胞缺乏LDL受体时，细胞不会产生可检测量的27-羟基胆固醇，并且HMG-CoA还原酶会上调。结果提供了强有力的证据，表明27-羟基胆固醇是LDL与人类成纤维细胞中HMG-CoA还原酶抑制之间的重要细胞内介质。LDL胆固醇的线粒体摄取/代谢似乎是人类细胞中LDL途径在生物学上的重要延伸，因为LDL胆固醇的线粒体产物可能调节胆固醇稳态或作为类固醇激素或胆汁酸的前体。现在已在成纤维细胞中证明的这条途径，可能将两项已有22年历史的观察结果联系在一起，即LDL以及氧化甾醇均可下调细胞中的HMG-CoA还原酶。
