Medh J D, Fry G L, Bowen S L, Ruben S, Wong H, Chappell D A
Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, IA 52242, USA.
J Lipid Res. 2000 Nov;41(11):1858-71.
Apolipoprotein E (apoE) is the primary recognition signal on triglyceride-rich lipoproteins responsible for interacting with low density lipoprotein (LDL) receptors and LDL receptor-related protein (LRP). It has been shown that lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) promote receptor-mediated uptake and degradation of very low density lipoproteins (VLDL) and remnant particles, possibly by directly binding to lipoprotein receptors. In this study we have investigated the requirement for apoE in lipase-stimulated VLDL degradation. We compared binding and degradation of normal and apoE-depleted human VLDL and apoE knockout mouse VLDL in human foreskin fibroblasts. Surface binding at 37 degrees C of apoE knockout VLDL was greater than that of normal VLDL by 3- and 40-fold, respectively, in the presence of LPL and HTGL. In spite of the greater stimulation of surface binding, lipase-stimulated degradation of apoE knockout mouse VLDL was significantly lower than that of normal VLDL (30, 30, and 80%, respectively, for control, LPL, and HTGL treatments). In the presence of LPL and HTGL, surface binding of apoE-depleted human VLDL was, respectively, 40 and 200% of normal VLDL whereas degradation was, respectively, 25 and 50% of normal VLDL. LPL and HTGL stimulated degradation of normal VLDL in a dose-dependent manner and by a LDL receptor-mediated pathway. Maximum stimulation (4-fold) was seen in the presence LPL (1 microgram/ml) or HTGL (3 microgram/ml) in lovastatin-treated cells. On the other hand, degradation of apoE-depleted VLDL was not significantly increased by the presence of lipases even in lovastatin-treated cells. Surface binding of apoE-depleted VLDL to metabolically inactive cells at 4 degrees C was higher in control and HTGL-treated cells, but unchanged in the presence of LPL. Degradation of prebound apoE-depleted VLDL was only 35% as efficient as that of normal VLDL. Surface binding of apoE knockout or apoE-depleted VLDL was to heparin sulfate proteoglycans because it was completely abolished by heparinase treatment. However, apoE appears to be a primary determinant for receptor-mediated VLDL degradation. Our studies suggest that overexpression of LPL or HTGL may not protect against lipoprotein accumulation seen in apoE deficiency.
载脂蛋白E（apoE）是富含甘油三酯的脂蛋白上的主要识别信号，负责与低密度脂蛋白（LDL）受体和LDL受体相关蛋白（LRP）相互作用。研究表明，脂蛋白脂肪酶（LPL）和肝甘油三酯脂肪酶（HTGL）可能通过直接结合脂蛋白受体来促进极低密度脂蛋白（VLDL）和残余颗粒的受体介导摄取及降解。在本研究中，我们调查了apoE在脂肪酶刺激的VLDL降解中的必要性。我们比较了正常和apoE缺失的人VLDL以及apoE基因敲除小鼠VLDL在人包皮成纤维细胞中的结合及降解情况。在LPL和HTGL存在的情况下，apoE基因敲除VLDL在37℃时的表面结合分别比正常VLDL高3倍和40倍。尽管表面结合受到更大刺激，但脂肪酶刺激的apoE基因敲除小鼠VLDL降解显著低于正常VLDL（对照、LPL和HTGL处理分别为30%、30%和80%）。在LPL和HTGL存在时，apoE缺失的人VLDL的表面结合分别为正常VLDL的40%和200%，而降解分别为正常VLDL的25%和50%。LPL和HTGL以剂量依赖方式并通过LDL受体介导的途径刺激正常VLDL的降解。在洛伐他汀处理的细胞中，存在LPL（1微克/毫升）或HTGL（3微克/毫升）时可见最大刺激（4倍）。另一方面，即使在洛伐他汀处理的细胞中，脂肪酶的存在也未显著增加apoE缺失的VLDL的降解。在4℃时，apoE缺失的VLDL与代谢不活跃细胞的表面结合在对照和HTGL处理的细胞中较高，但在LPL存在时不变。预先结合的apoE缺失的VLDL的降解效率仅为正常VLDL的35%。apoE基因敲除或apoE缺失的VLDL的表面结合是与硫酸乙酰肝素蛋白聚糖结合，因为用肝素酶处理可使其完全消除。然而，apoE似乎是受体介导的VLDL降解的主要决定因素。我们的研究表明，LPL或HTGL的过表达可能无法预防apoE缺乏时出现的脂蛋白蓄积。
