Flood Christofer, Gustafsson Maria, Richardson Paul E, Harvey Stephen C, Segrest Jere P, Borén Jan
Wallenberg Laboratory for Cardiovascular Research, The Sahlgrenska Academy at Göteborg University, S-413 45 Göteborg, Sweden.
J Biol Chem. 2002 Aug 30;277(35):32228-33. doi: 10.1074/jbc.M204053200. Epub 2002 Jun 17.
An initial event in atherosclerosis is the retention of lipoproteins within the intima of the vessel wall. Previously we identified Site B (residues 3359-3369) in apolipoprotein (apo) B100 as the proteoglycan binding sequence in low density lipoproteins (LDLs) and showed that the atherogenicity of apoB-containing lipoproteins is linked to their affinity for artery wall proteoglycans. However, both apoB100- and apoB48-containing lipoproteins are equally atherogenic even though Site B lies in the carboxyl-terminal half of apoB100 and is absent in apoB48. If binding to proteoglycans is a key step in atherogenesis, apoB48-containing lipoproteins must bind to proteoglycans via other proteoglycan binding sites in the amino-terminal 48% of apoB. In vitro studies have identified five clusters of basic amino acids in delipidated apoB48 that bind negatively charged glycosaminoglycans. To determine which of these sites is functional on LDL particles, we analyzed the proteoglycan binding activity of recombinant human LDLs from transgenic mice or rat hepatoma cells. Substitution of neutral amino acids for the basic amino acids in Site B-Ib (residues 84-94) abolished the proteoglycan binding activity of recombinant apoB53. Carboxyl-truncated apoB80 bound biglycan with higher affinity than apoB100 and apoB48. ApoB80 in which Site B was mutated had the same affinity for proteoglycans as apoB48. These data support the hypothesis that the carboxyl terminus of apoB100 "masks" Site B-Ib, the amino-terminal proteoglycan binding site, and that this site is exposed in carboxyl-truncated forms of apoB. The presence of a proteoglycan binding site in the amino-terminal region of apoB may explain why apoB48- and apoB100-containing lipoproteins are equally atherogenic.
动脉粥样硬化的起始事件是脂蛋白在血管壁内膜中的潴留。此前我们确定载脂蛋白（apo）B100中的位点B（残基3359 - 3369）为低密度脂蛋白（LDL）中的蛋白聚糖结合序列，并表明含apoB的脂蛋白的致动脉粥样硬化性与其对动脉壁蛋白聚糖的亲和力相关。然而，尽管位点B位于apoB100的羧基末端一半且在apoB48中不存在，但含apoB100和含apoB48的脂蛋白具有同等的致动脉粥样硬化性。如果与蛋白聚糖结合是动脉粥样硬化发生的关键步骤，那么含apoB48的脂蛋白必定通过apoB氨基末端48%中的其他蛋白聚糖结合位点与蛋白聚糖结合。体外研究已在脱脂的apoB48中确定了五个与带负电荷的糖胺聚糖结合的碱性氨基酸簇。为确定这些位点中哪一个在LDL颗粒上具有功能，我们分析了来自转基因小鼠或大鼠肝癌细胞的重组人LDL的蛋白聚糖结合活性。用中性氨基酸取代位点B - Ib（残基84 - 94）中的碱性氨基酸消除了重组apoB53的蛋白聚糖结合活性。羧基截短的apoB80与双糖链蛋白聚糖的结合亲和力高于apoB100和apoB48。位点B发生突变的apoB80与蛋白聚糖的亲和力与apoB48相同。这些数据支持以下假说：apoB100的羧基末端“掩盖”了氨基末端蛋白聚糖结合位点位点B - Ib，并且该位点在apoB的羧基截短形式中暴露。apoB氨基末端区域存在蛋白聚糖结合位点可能解释了为何含apoB48和含apoB100的脂蛋白具有同等的致动脉粥样硬化性。
