Schissel S L, Jiang X, Tweedie-Hardman J, Jeong T, Camejo E H, Najib J, Rapp J H, Williams K J, Tabas I
Department of Anatomy, College of Physicians and Surgeons, Columbia University, New York, New York 10032, USA.
J Biol Chem. 1998 Jan 30;273(5):2738-46. doi: 10.1074/jbc.273.5.2738.
The subendothelial aggregation and retention of low density lipoprotein (LDL) are key events in atherogenesis, but the mechanisms in vivo are not known. Previous studies have shown that treatment of LDL with bacterial sphingomyelinase (SMase) in vitro leads to the formation of lesion-like LDL aggregates that become retained on extracellular matrix and stimulate macrophage foam cell formation. In addition, aggregated human lesional LDL, but not unaggregated lesional LDL or plasma LDL, shows evidence of hydrolysis by an arterial wall SMase in vivo, and several arterial wall cell types secrete a SMase (S-SMase). S-SMase, however, has a sharp acid pH optimum using a standard in vitro SM-micelle assay. Thus, a critical issue regarding the potential role of S-SMase in atherogenesis is whether the enzyme can hydrolyze lipoprotein-SM, particularly at neutral pH. We now show that S-SMase can hydrolyze and aggregate native plasma LDL at pH 5.5 but not at pH 7.4. Remarkably, LDL modified by oxidation, treatment with phospholipase A2, or enrichment with apolipoprotein CIII, which are modifications associated with increased atherogenesis, is hydrolyzed readily by S-SMase at pH 7.4. In addition, lipoproteins from the plasma of apolipoprotein E knock-out mice, which develop extensive atherosclerosis, are highly susceptible to hydrolysis and aggregation by S-SMase at pH 7.4; a high SM:PC ratio in these lipoproteins appears to be an important factor in their susceptibility to S-SMase. Most importantly, LDL extracted from human atherosclerotic lesions, which is enriched in sphingomyelin compared with plasma LDL, is hydrolyzed by S-SMase at pH 7.4 10-fold more than same donor plasma LDL, suggesting that LDL is modified in the arterial wall to increase its susceptibility to S-SMase. In summary, atherogenic lipoproteins are excellent substrates for S-SMase, even at neutral pH, making this enzyme a leading candidate for the arterial wall SMase that hydrolyzes LDL-SM and causes subendothelial LDL aggregation.
低密度脂蛋白（LDL）在内皮下的聚集和潴留是动脉粥样硬化发生过程中的关键事件，但其体内机制尚不清楚。以往研究表明，体外使用细菌鞘磷脂酶（SMase）处理LDL可导致形成类似病变的LDL聚集体，这些聚集体会潴留在细胞外基质上并刺激巨噬细胞泡沫细胞形成。此外，聚集的人病变LDL，而非未聚集的病变LDL或血浆LDL，在体内显示出被动脉壁SMase水解的证据，并且几种动脉壁细胞类型可分泌一种SMase（S-SMase）。然而，使用标准的体外SM-微团测定法时，S-SMase具有尖锐的酸性pH最佳值。因此，关于S-SMase在动脉粥样硬化发生中潜在作用的一个关键问题是该酶是否能够水解脂蛋白-SM，尤其是在中性pH条件下。我们现在表明，S-SMase在pH 5.5时可水解并聚集天然血浆LDL，但在pH 7.4时则不能。值得注意的是，经氧化、用磷脂酶A2处理或富含载脂蛋白CIII修饰的LDL，这些修饰与动脉粥样硬化发生增加相关，在pH 7.4时很容易被S-SMase水解。此外，载脂蛋白E基因敲除小鼠血浆中的脂蛋白，其会发生广泛的动脉粥样硬化，在pH 7.4时极易被S-SMase水解和聚集；这些脂蛋白中高SM:PC比值似乎是其对S-SMase易感性的一个重要因素。最重要的是，从人动脉粥样硬化病变中提取的LDL，与血浆LDL相比富含鞘磷脂，在pH 7.4时被S-SMase水解的程度比同一供体的血浆LDL高10倍，这表明LDL在动脉壁中发生修饰以增加其对S-SMase的易感性。总之，致动脉粥样硬化的脂蛋白即使在中性pH条件下也是S-SMase的优良底物，这使得该酶成为水解LDL-SM并导致内皮下LDL聚集的动脉壁SMase的主要候选者。
