Department of Biology, Amherst College, Amherst, Massachusetts, United States of America.
PLoS One. 2007 Aug 15;2(8):e729. doi: 10.1371/journal.pone.0000729.
Tangier disease is an inherited disorder that results in a deficiency in circulating levels of HDL. Although the disease is known to be caused by mutations in the ABCA1 gene, the mechanism by which lesions in the ABCA1 ATPase effect this outcome is not known. The inability of ABCA1 knockout mice (ABCA1-/-) to load cholesterol and phospholipids onto apoA1 led to a proposal that ABCA1 mediates the transbilayer externalization of phospholipids, an activity integral not only to the formation of HDL particles but also to another, distinct process: the recognition and clearance of apoptotic cells by macrophages. Expression of phosphatidylserine (PS) on the surface of both macrophages and their apoptotic targets is required for efficient engulfment of the apoptotic cells, and it has been proposed that ABCA1 is required for transbilayer externalization of PS to the surface of both cell types. To determine whether ABCA1 is responsible for any of the catalytic activities known to control transbilayer phospholipid movements, these activities were measured in cells from ABCA1-/- mice and from Tangier individuals as well as ABCA1-expressing HeLa cells. Phospholipid movements in either normal or apoptotic lymphocytes or in macrophages were not inhibited when cells from knockout and wildtype mice or immortalized cells from Tangier individuals vs normal individuals were compared. Exposure of PS on the surface of normal thymocytes, apoptotic thymocytes and elicited peritoneal macrophages from wildtype and knockout mice or B lymphocytes from normal and Tangier individuals, as measured by annexin V binding, was also unchanged. No evidence was found of ABCA1-stimulated active PS export, and spontaneous PS movement to the outer leaflet in the presence or absence of apoA1 was unaffected by the presence or absence of ABCA1. Normal or Tangier B lymphocytes and macrophages were also identical in their ability to serve as targets or phagocytes, respectively, in apoptotic cell clearance assays. No evidence was found to support the suggestion that ABCA1 is involved in transport to the macrophage cell surface of annexins I and II, known to enhance phagocytosis of apoptotic cells. These results show that mutations in ABCA1 do not measurably reduce the rate of transbilayer movements of phospholipids in either the engulfing macrophage or the apoptotic target, thus discounting catalysis of transbilayer movements of phospholipids as the mechanism by which ABCA1 facilitates loading of phospholipids and cholesterol onto apoA1.
Tangier 病是一种遗传性疾病，导致循环中 HDL 水平降低。尽管已知该疾病是由 ABCA1 基因的突变引起的，但 ABCA1 ATP 酶的病变如何导致这种结果的机制尚不清楚。ABCA1 基因敲除小鼠（ABCA1-/-）无法将胆固醇和磷脂加载到 apoA1 上，这导致了一种假设，即 ABCA1 介导磷脂的跨膜外排，这种活性不仅对 HDL 颗粒的形成至关重要，而且对另一个不同的过程也至关重要：巨噬细胞对凋亡细胞的识别和清除。巨噬细胞及其凋亡靶细胞表面的磷脂酰丝氨酸（PS）的表达是有效吞噬凋亡细胞所必需的，并且已经提出 ABCA1 是将 PS 跨膜外排到两种细胞类型表面所必需的。为了确定 ABCA1 是否负责控制跨膜磷脂运动的任何已知催化活性，在 ABCA1-/- 小鼠和 Tangier 个体的细胞以及 ABCA1 表达的 HeLa 细胞中测量了这些活性。当比较敲除和野生型小鼠或 Tangier 个体的永生化细胞与正常个体的细胞时，正常或凋亡淋巴细胞或巨噬细胞中的磷脂运动均未受到抑制。用膜联蛋白 V 结合测量，正常胸腺细胞、凋亡胸腺细胞和从野生型和敲除小鼠或正常和 Tangier 个体的 B 淋巴细胞诱导的腹腔巨噬细胞表面 PS 的暴露也没有改变。没有发现 ABCA1 刺激的活性 PS 外排的证据，并且在存在或不存在 apoA1 的情况下，PS 自发向外层的运动不受 ABCA1 的存在或不存在的影响。正常或 Tangier B 淋巴细胞和巨噬细胞在作为凋亡细胞清除测定中的靶细胞或吞噬细胞的能力方面也相同。没有发现证据支持 ABCA1 参与到巨噬细胞表面的 annexin I 和 II 的转运的假设，已知 annexin I 和 II 增强对凋亡细胞的吞噬作用。这些结果表明，ABCA1 中的突变不会显著降低吞噬细胞或凋亡靶细胞中磷脂的跨膜运动速率，因此排除了磷脂的跨膜运动的催化作用作为 ABCA1 促进磷脂和胆固醇加载到 apoA1 上的机制。
