Saxena U, Ferguson E, Bisgaier C L
Department of Pharmacology, Parke-Davis Pharmaceutical Research Division, Warner-Lambert Company, Ann Arbor, Michigan 48105.
J Biol Chem. 1993 Jul 15;268(20):14812-9.
Lipoprotein lipase (lipase), a key enzyme in lipoprotein triglyceride metabolism, has been shown to markedly increase low density lipoprotein (LDL) retention by subendothelial matrix. In the present study we assessed the role that lipoprotein and matrix components play in retention of LDL by lipase anchored to the subendothelial matrix. Lipase addition to subendothelial matrix increased LDL retention by 66-fold. Scatchard analysis of LDL binding to lipase-containing matrix yielded an association constant of 12 nM. Exogenous addition of the matrix components, heparan sulfate and dermatan sulfate (i.e. chondroitin sulfate B), reduced LDL retention by greater than 90%. These glycosaminoglycans (GAGs) also reduced lipolytic activity associated with the matrix, suggesting that lipase was released from its binding sites on the matrix. In contrast, other matrix components (collagen, fibronectin, vitronectin, and chondroitin sulfate A) neither affected LDL release nor matrix lipolytic activity. Thus, heparan sulfate and dermatan sulfate function to anchor lipase to the subendothelial cell matrix. The effects of apolipoprotein E (apoE) and apoA-I were also examined. Preincubation of the subendothelial matrix with apoE, followed by washing, did not affect subsequent lipase binding to the matrix nor its ability to retain LDL. However, the direct addition of apoE alone or in combination with phospholipid liposomes decreased lipase-mediated LDL retention in a concentration-dependent fashion. Addition of apoA-I had no effect. Thus, in these studies apoE functions to displace LDL bound to lipase, but not lipase anchored to the matrix. To further examine the physiologic implications of this process, we assessed the ability of human apoE-rich and apoE-poor high density lipoproteins (HDL) to displace LDL from matrix-anchored lipase. ApoE-rich HDL reduced LDL retention dramatically (86% at 2.5 micrograms/ml). In contrast, apoE-poor HDL, at the highest concentration evaluated (400 micrograms/ml), decreased LDL retention by only 32%. Overall, these data suggest apoE and specifically apoE-containing HDL reduce the lipase-mediated retention of LDL by subendothelial matrix. This observation, in part could explain the protective effects of apoE and apoE-containing HDL against atherosclerosis.
脂蛋白脂肪酶（脂肪酶）是脂蛋白甘油三酯代谢中的关键酶，已被证明可显著增加低密度脂蛋白（LDL）在内皮下基质中的潴留。在本研究中，我们评估了脂蛋白和基质成分在脂肪酶锚定在内皮下基质时对LDL潴留所起的作用。向在内皮下基质中添加脂肪酶可使LDL潴留增加66倍。对LDL与含脂肪酶基质的结合进行Scatchard分析，得出的结合常数为12 nM。外源性添加基质成分硫酸乙酰肝素和硫酸皮肤素（即硫酸软骨素B）可使LDL潴留减少90%以上。这些糖胺聚糖（GAGs）还降低了与基质相关的脂解活性，提示脂肪酶从其在基质上的结合位点释放。相反，其他基质成分（胶原蛋白、纤连蛋白、玻连蛋白和硫酸软骨素A）既不影响LDL的释放，也不影响基质的脂解活性。因此，硫酸乙酰肝素和硫酸皮肤素起到将脂肪酶锚定在内皮细胞基质上的作用。我们还研究了载脂蛋白E（apoE）和载脂蛋白A-I（apoA-I）的作用。在内皮下基质中预先与apoE孵育，然后洗涤，并不影响随后脂肪酶与基质的结合及其潴留LDL的能力。然而，单独直接添加apoE或与磷脂脂质体联合添加，均以浓度依赖方式降低脂肪酶介导的LDL潴留。添加apoA-I则无影响。因此，在这些研究中，apoE起到置换与脂肪酶结合的LDL的作用，但对锚定在基质上的脂肪酶无此作用。为进一步研究该过程的生理意义，我们评估了富含人类apoE和缺乏apoE的高密度脂蛋白（HDL）将LDL从基质锚定的脂肪酶上置换下来的能力。富含apoE的HDL显著降低LDL潴留（2.5微克/毫升时降低86%）。相反，缺乏apoE的HDL在评估的最高浓度（400微克/毫升）时，仅使LDL潴留降低32%。总体而言，这些数据表明apoE，特别是含apoE的HDL可减少脂肪酶介导的LDL在内皮下基质中的潴留。这一观察结果部分可以解释apoE和含apoE的HDL对动脉粥样硬化的保护作用。
