Medh J D, Bowen S L, Fry G L, Ruben S, Andracki M, Inoue I, Lalouel J M, Strickland D K, Chappell D A
Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, Iowa 52242, USA.
J Biol Chem. 1996 Jul 19;271(29):17073-80. doi: 10.1074/jbc.271.29.17073.
Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of plasma triglycerides, promotes binding and catabolism of triglyceride-rich lipoproteins by various cultured cells. Recent studies demonstrate that LPL binds to three members of the low density lipoprotein (LDL) receptor family, including the LDL receptor-related protein (LRP), GP330/LRP-2, and very low density lipoprotein (VLDL) receptors and induces receptor-mediated lipoprotein catabolism. We show here that LDL receptors also bind LPL and mediate LPL-dependent catabolism of large VLDL with Sf 100-400. Up-regulation of LDL receptors by lovastatin treatment of normal human foreskin fibroblasts (FSF cells) resulted in an increase in LPL-induced VLDL binding and catabolism to a level that was 10-15-fold greater than in LDL receptor-negative fibroblasts, despite similar LRP activity in both cell lines. This indicates that the contribution of LRP to LPL-dependent degradation of VLDL is small when LDL receptors are maximally up-regulated. Furthermore studies in LRP-deficient murine embryonic fibroblasts showed that the level of LPL-dependent degradation of VLDL was similar to that in normal murine embryonic fibroblasts. LPL also promoted the internalization of protein-free triglyceride emulsions; lovastatin-treatment resulted in 2-fold higher uptake in FSF cells, indicating that LPL itself could bind to LDL receptors. However, the lower induction of emulsion catabolism as compared with native VLDL suggests that LPL-induced catabolism via LDL receptors is only partially dependent on receptor binding by LPL and instead is primarily due to activation of apolipoproteins such as apoE. A fusion protein between glutathione S-transferase and the catalytically inactive carboxyl-terminal domain of LPL (GST-LPLC) also induced binding and catabolism of VLDL. However GST-LPLC was not as active as native LPL, indicating that lipolysis is required for a maximal LPL effect. Mutations of critical tryptophan residues in GST-LPLC that abolished binding to VLDL converted the protein to an inhibitor of lipoprotein binding to LDL receptors. In solid-phase assays using immobilized receptors, LDL receptors bound to LPL in a dose-dependent manner. Both LPL and GST-LPLC promoted binding of VLDL to LDL receptor-coated wells. These results indicate that LPL binds to LDL receptors and suggest that the carboxyl-terminal domain of LPL contributes to this interaction.
脂蛋白脂肪酶（LPL）是负责水解血浆甘油三酯的主要酶，可促进各种培养细胞对富含甘油三酯脂蛋白的结合和分解代谢。最近的研究表明，LPL与低密度脂蛋白（LDL）受体家族的三个成员结合，包括LDL受体相关蛋白（LRP）、GP330/LRP-2和极低密度脂蛋白（VLDL）受体，并诱导受体介导的脂蛋白分解代谢。我们在此表明，LDL受体也结合LPL，并介导LPL依赖的Sf 100 - 400大VLDL的分解代谢。用洛伐他汀处理正常人包皮成纤维细胞（FSF细胞）使LDL受体上调，导致LPL诱导的VLDL结合和分解代谢增加，其水平比LDL受体阴性成纤维细胞高10 - 15倍，尽管两种细胞系中的LRP活性相似。这表明当LDL受体最大程度上调时，LRP对LPL依赖的VLDL降解的贡献很小。此外，在LRP缺陷的小鼠胚胎成纤维细胞中的研究表明，LPL依赖的VLDL降解水平与正常小鼠胚胎成纤维细胞中的相似。LPL还促进无蛋白甘油三酯乳剂的内化；洛伐他汀处理导致FSF细胞中的摄取增加2倍，表明LPL本身可与LDL受体结合。然而，与天然VLDL相比，乳剂分解代谢的诱导较低，这表明LPL通过LDL受体诱导的分解代谢仅部分依赖于LPL与受体的结合，而是主要由于载脂蛋白如载脂蛋白E的激活。谷胱甘肽S-转移酶与LPL催化无活性的羧基末端结构域之间的融合蛋白（GST-LPLC）也诱导VLDL的结合和分解代谢。然而，GST-LPLC的活性不如天然LPL，表明脂解作用是LPL发挥最大作用所必需的。GST-LPLC中关键色氨酸残基的突变消除了与VLDL的结合，使该蛋白转变为脂蛋白与LDL受体结合的抑制剂。在使用固定化受体的固相测定中，LDL受体以剂量依赖方式与LPL结合。LPL和GST-LPLC均促进VLDL与LDL受体包被孔的结合。这些结果表明LPL与LDL受体结合，并提示LPL的羧基末端结构域有助于这种相互作用。
