Role of serum amyloid A during metabolism of acute-phase HDL by macrophages
Role of serum amyloid A during metabolism of acute-phase HDL by macrophages
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DOI：
10.1161/01.atv.20.3.763
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发表时间：
2000-03-01
影响因子：
8.7
通讯作者：
Malle, E
中科院分区：
文献类型：
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作者：
Artl, A;Marsche, G;Malle, E
血清淀粉样蛋白A(SAA)家族由多种基因编码，在哺乳动物中表现出等位基因变异和高度同源性。在淋巴因子介导的过程刺激肝细胞后，由炎症触发，SAA浓度在急性时相反应期间可能增加到非炎症状态下的1000倍。除了作为急性期反应物的作用外，SAA(104个氨基酸，12 kDa)被认为是继发性反应性淀粉样变性的前体蛋白，在这种情况下，N-末端部分被结合到大量的淀粉样纤维中。然而，SAA与高密度脂蛋白的结合以及随后对其生理载体代谢特性的调节似乎是SAA的主要作用。由于SAA可能在急性时相反应中取代天然高密度脂蛋白(HDL)的主要蛋白质成分载脂蛋白A-I，因此本研究旨在(1)研究J774巨噬细胞对天然和急性时相(SAA富集型)高密度脂蛋白的结合特性，(2)阐明SAA在高密度脂蛋白颗粒上的存在是否影响对高密度脂蛋白相关胆固醇酯的选择性摄取，以及(3)比较天然和急性时相高密度脂蛋白介导的细胞胆固醇外流。兔急性时相高密度脂蛋白在4℃时的总结合量和特异性结合量都比天然高密度脂蛋白高近2倍。非线性回归分析得到7.0X10(-7)摩尔/L(天然高密度脂蛋白)和3.1X10(-7)摩尔/L(急性期高密度脂蛋白)的K-d值。相应的B-max值为每毫克细胞蛋白(天然高密度脂蛋白)203 ng总脂蛋白和每毫克细胞蛋白(急性时相高密度脂蛋白)250 ng总脂蛋白。在37摄氏度时，急性期高密度脂蛋白的全息物周转略有增加，这一事实反映在降解率高出2倍。相反，SAA在高密度脂蛋白上的存在使J774巨噬细胞选择性地从急性时相高密度脂蛋白中摄取高密度脂蛋白胆固醇酯的能力显著增加(1.7倍)，这是一种广泛用于研究泡沫细胞形成和胆固醇外流特性的体外模型。用溶解的J774膜蛋白进行的配基印迹实验未能确定清道夫受体BI是天然和急性时相高密度脂蛋白的结合蛋白，但鉴定出两种结合蛋白，分子量分别为100和72 kDa，后者与CD55(也称为衰变加速因子)融合。在胆固醇外流研究中，很明显，急性时相高密度脂蛋白对细胞胆固醇的去除能力明显低于天然高密度脂蛋白。这反映在tau/2值增加了1.7倍(22小时比36小时；自然高密度脂蛋白比急性时相高密度脂蛋白)。我们对高密度脂蛋白胆固醇酯摄取增加和细胞胆固醇流出减少(急性期与天然高密度脂蛋白相比)的观察表明，SAA取代载脂蛋白A-I导致其主要生理载体的代谢特性发生显著变化。载脂蛋白部分的这些变化似乎(至少在体外测试的系统中)将最初的抗动脉粥样硬化转变为促动脉粥样硬化的脂蛋白颗粒。
The serum amyloid A (SAA) family of proteins is encoded by multiple genes that display allelic variation and a high degree of homology in mammals. Triggered by inflammation after stimulation of hepatocytes by lymphokine mediated processes, the concentrations of SAA may increase during the acute-phase reaction to levels 1000-fold greater than those found in the noninflammatory state. In addition to its role as an acute-phase reactant, SAA (104 amino acids, 12 kDa) is considered to be the precursor protein of secondary reactive amyloidosis, in which the N-terminal portion is incorporated into the bulk of amyloid fibrils. However, the association with lipoproteins of the high-density range and subsequent modulation of the metabolic properties of its physiological carrier appear to be the principal role of SAA. Because SAA may displace apolipoprotein A-I, the major protein component of native high density lipoprotein (HDL), during the acute-phase reaction, the present study was aimed at (1) investigating binding properties of native and acute-phase (SAA-enriched) HDL by J774 macrophages, (2) elucidating whether the presence of SAA on HDL particles affects selective uptake of HDL-associated cholesteryl esters, and (3) comparing cellular cholesterol efflux mediated by native and acute-phase HDL. Both the total and the specific binding at 4 degrees C of rabbit acute-phase HDL were approximate to 2-fold higher than for native HDL. Nonlinear regression analysis revealed K-d values of 7.0X10(-7) moL/L (native HDL) and 3.1X10(-7) mol/L (acute-phase HDL), respectively. The corresponding B-max values were 203 ng of total lipoprotein per milligram of cell protein (native HDL) and 250 ng of total lipoprotein per milligram of cell protein (acute-phase HDL). At 37 degrees C, holoparticle turnover was slightly enhanced for acute-phase HDL, a fact reflected by 2-fold higher degradation rates. In contrast, the presence of SAA on HDL specifically increased (1.7-fold) the selective uptake of HDL cholesteryl esters from acute-phase HDL by J774 macrophages, a widely used in vitro model to study foam cell formation and cholesterol efflux properties. Although ligand blotting experiments with solubilized J774 membrane proteins failed to identify the scavenger receptor-BI as a binding protein for both native and acute-phase HDL, 2 binding proteins with molecular masses of 100 and 72 kDa, the latter comigrating with CD55 (also termed decay-accelerating factor), were identified. During cholesterol efflux studies, it became apparent that the ability of acute-phase HDL with regard to cellular cholesterol removal was considerably lower than that for native HDL. This was reflected by a 1.7-fold increase in tau/2 values (22 versus 36 hours; native versus acute-phase HDL). Our observations of increased HDL cholesteryl ester uptake and reduced cellular cholesterol efflux (acute-phase versus native HDL) suggest that displacement of apolipoprotein A-I by SAA results in considerable altered metabolic properties of its main physiological carrier. These changes in the apolipoprotein moieties appear (at least in the in vitro system tested) to transform an originally antiatherogenic into a proatherogenic lipoprotein particle.