Fong B S, Angel A
Institute of Medical Science, University of Toronto, Ontario, Canada.
Biochim Biophys Acta. 1989 Jul 17;1004(1):53-60. doi: 10.1016/0005-2760(89)90212-9.
Cholesterol stored in human adipose tissue is derived from circulating lipoproteins. To delineate the cholesterol transport function of LDL and HDL, the movement of radiolabelled esterified cholesterol and free cholesterol from labelled LDL and HDL to human adipocytes was examined in the present study. LDL and HDL were enriched and labelled in esterified cholesterol with [14C]cholesterol by the action of plasma lipid transfer proteins and lecithin-cholesterol acyltransferase. Doubly labelled (3H,14C) LDL and HDL were prepared by exchanging free [3H]cholesterol into the 14C-labelled lipoproteins. 14C-labelled lipoprotein and 3H-labelled lipoprotein were also prepared separately and mixed to yield a mixed doubly labelled lipoprotein. Relative to the total amount added, proportionally more free than esterified cholesterol was transferred to the adipocytes upon incubation with any doubly labelled LDL and HDL. The calculated mass of free and esterified cholesterol transferred, however, varied with different labelled lipoproteins. 3H- and 14C-labelled LDL or HDL transferred 2-3-fold more esterified than free cholesterol while the reverse occurred with the mixed doubly labelled LDL or HDL. Thus, free cholesterol-depleted particles preferentially transferred cholesterol ester to the fat cells. In the presence of the homologous unlabelled native lipoprotein, the transfers of free and esterified cholesterol from labelled LDL or HDL were specifically inhibited. Selective transfer of esterified cholesterol relative to apoprotein was also observed when esterified cholesterol uptake from both LDL and HDL was assayed along with the binding of 125I-labelled lipoprotein. The cellular accumulation of cholesterol ether-labelled HDL (a non-hydrolyzable analogue of cholesterol ester) exceeded that of cholesterol ester consistent with significant hydrolysis of the latter physiological substrate. These results demonstrate preferential transfer of free cholesterol and esterified cholesterol over apoprotein for both LDL and HDL in human adipocytes. Furthermore, the data suggest that the cholesterol ester transport function of LDL and HDL can be enhanced by free cholesterol depletion and cholesterol ester enrichment of the particles, and affirms a role for adipose tissue in the metabolism of lipid-modified lipoproteins.
储存在人体脂肪组织中的胆固醇源自循环脂蛋白。为了阐明低密度脂蛋白（LDL）和高密度脂蛋白（HDL）的胆固醇转运功能，本研究检测了放射性标记的酯化胆固醇和游离胆固醇从标记的LDL和HDL向人脂肪细胞的转运情况。通过血浆脂质转运蛋白和卵磷脂胆固醇酰基转移酶的作用，用[14C]胆固醇使LDL和HDL中的酯化胆固醇富集并标记。通过将游离的[3H]胆固醇交换到14C标记的脂蛋白中，制备了双标记（3H，14C）LDL和HDL。还分别制备了14C标记的脂蛋白和3H标记的脂蛋白并混合，以产生混合双标记脂蛋白。与添加的总量相比，与任何双标记的LDL和HDL孵育时，转移到脂肪细胞中的游离胆固醇比酯化胆固醇在比例上更多。然而，计算得出的转移的游离胆固醇和酯化胆固醇的量因不同的标记脂蛋白而异。3H和14C标记的LDL或HDL转移的酯化胆固醇比游离胆固醇多2至3倍，而混合双标记的LDL或HDL则相反。因此，游离胆固醇耗尽的颗粒优先将胆固醇酯转移到脂肪细胞中。在同源未标记的天然脂蛋白存在下，从标记的LDL或HDL中转移游离胆固醇和酯化胆固醇的过程受到特异性抑制。当检测来自LDL和HDL的酯化胆固醇摄取以及125I标记脂蛋白的结合时，还观察到相对于载脂蛋白的酯化胆固醇的选择性转移。胆固醇醚标记的HDL（胆固醇酯的一种不可水解类似物）在细胞内的积累超过了胆固醇酯，这与后者生理底物的显著水解一致。这些结果表明，在人脂肪细胞中，LDL和HDL的游离胆固醇和酯化胆固醇相对于载脂蛋白具有优先转移。此外，数据表明，LDL和HDL的胆固醇酯转运功能可通过颗粒的游离胆固醇耗尽和胆固醇酯富集来增强，并证实了脂肪组织在脂质修饰脂蛋白代谢中的作用。
