Hakala J K, Oörni K, Ala-Korpela M, Kovanen P T
Wihuri Research Institute, Kalliolinnantie 4, FIN-00140 Helsinki, Finland.
Arterioscler Thromb Vasc Biol. 1999 May;19(5):1276-83. doi: 10.1161/01.atv.19.5.1276.
One of the first events in atherogenesis is modification of low density lipoprotein (LDL) particles in the arterial wall with ensuing formation of aggregated and fused lipid droplets. The accumulating particles are relatively depleted in phosphatidylcholine (PC). Recently, secretory phospholipase A2 (PLA2), an enzyme capable of hydrolyzing LDL PC into fatty acid and lysoPC molecules, has been found in atherosclerotic arteries. There is also evidence that both LDL and PLA2 bind to the glycosaminoglycan (GAG) chains of extracellular proteoglycans in the arterial wall. Here we studied the effect of heparin GAG on the lipolytic modification of LDL by PLA2. Untreated LDL, heparin-treated LDL, and heparin-bound LDL were lipolyzed with bee venom PLA2. In the presence of albumin, lipolysis resulted in aggregation in all 3 preparations of the LDL particles. Lipolysis of untreated LDL did not result in aggregation if albumin was absent from the reaction medium, and the lipolytic products accumulated in the particles rendering them negatively charged. However, heparin-treated and heparin-bound lipolyzed LDL particles aggregated even in the absence of albumin. Importantly, in the presence of albumin, some of the heparin-treated and heparin-bound lipolyzed LDL particles fused, the proportion of fused particles being substantially greater when LDL was bound to heparin during lipolysis. In summary, lipolysis of LDL PC by PLA2 under physiological conditions, which allow transfer of the lipolytic degradation products to albumin, leads to fusion of LDL particles in the presence, but not in the absence, of heparin. Thus, it is possible that within the GAG meshwork of the arterial intima, PLA2-induced modification of LDL is one source of the lipid droplets during atherogenesis.
动脉粥样硬化形成过程中最早发生的事件之一是动脉壁中的低密度脂蛋白（LDL）颗粒发生修饰，随后形成聚集和融合的脂滴。积累的颗粒中磷脂酰胆碱（PC）相对减少。最近，在动脉粥样硬化动脉中发现了分泌型磷脂酶A2（PLA2），一种能够将LDL PC水解为脂肪酸和溶血磷脂酰胆碱分子的酶。也有证据表明LDL和PLA2都与动脉壁细胞外蛋白聚糖的糖胺聚糖（GAG）链结合。在此，我们研究了肝素GAG对PLA2介导的LDL脂解修饰的影响。用蜂毒PLA2对未处理的LDL、肝素处理的LDL和肝素结合的LDL进行脂解。在白蛋白存在的情况下，脂解导致所有3种LDL颗粒制剂发生聚集。如果反应介质中不存在白蛋白，未处理的LDL的脂解不会导致聚集，脂解产物在颗粒中积累，使其带负电荷。然而，即使在没有白蛋白的情况下，肝素处理和肝素结合的脂解LDL颗粒也会聚集。重要的是，在白蛋白存在的情况下，一些肝素处理和肝素结合的脂解LDL颗粒会融合，当LDL在脂解过程中与肝素结合时，融合颗粒的比例会显著增加。总之，在生理条件下，PLA2对LDL PC的脂解作用（这种条件允许脂解降解产物转移到白蛋白上），在有肝素存在但无肝素时不存在的情况下会导致LDL颗粒融合。因此，在动脉内膜的GAG网络中，PLA2诱导的LDL修饰可能是动脉粥样硬化形成过程中脂滴的来源之一。
