Role of Sphingomyelin in Lipoprotein Metabolism

说明(申请人提供)：鞘磷脂(SM)是仅次于磷脂酰胆碱(PC)的血浆中含量最丰富的磷脂，是细胞膜筏的重要组成部分。虽然最近的流行病学研究表明，血浆中高水平的SM增加了动脉粥样硬化的风险，但其潜在的机制尚不清楚，因为SM的正常功能尚未阐明。我们认为，由于其独特的结构和定位于细胞外表面，SM通过抑制磷脂酶和脂质过氧化反应来保护细胞膜的完整性。此外，我们认为，由于SM对胆固醇的亲和力，SM调节细胞胆固醇的动态平衡和反向胆固醇运输。这些功能的失调可能会导致炎症和促进动脉粥样硬化。在目标1中，我们建议检验SM调节胆固醇反向运输的假设，重点是它在巨噬细胞胆固醇外流和LCAT使胆固醇酯化中的作用。在Abcg1转运蛋白介导的外排过程中，SM作为胆固醇的伴侣这一新的假设将被探索。我们将研究SM在生理系统中对LCAT的调节作用。在目标2中，我们将探讨SM通过抑制促炎症脂质的形成而发挥抗炎作用的假说，如溶血磷脂、花生四烯酸、氧化磷脂和氧化甾醇。SM竞争性地抑制所有以PC为底物的酶的假设将在分泌型磷脂酶和内皮脂肪酶方面得到验证。将在脂蛋白和细胞膜中测试SM在促炎氧化PC和氧固醇生成中的抑制作用。SM缺乏对巨噬细胞和中性粒细胞功能的影响，包括细胞因子的产生和超氧化物的产生，将在髓系特异性SM缺陷小鼠中进行研究。在目标3中，SM和神经酰胺将通过确定它们对细胞间以及细胞与环境之间的胆固醇运输的影响来研究它们在细胞胆固醇动态平衡中的作用。这些研究将为这种重要的磷脂的生理作用提供新的见解，并可能确定抗炎和动脉粥样硬化的新治疗靶点。与公共健康相关：这里提出的研究将调查鞘磷脂(SM)的生理作用，这是一种普遍存在于血浆和外细胞膜的特殊脂质分子。他们将特别关注SM作为一种抗炎分子的作用，保护细胞免受环境侮辱。这些研究不仅可能导致对炎症和心脏病机制的新理解，而且还可能带来更好的治疗策略。 首先，有没有任何巨噬细胞途径的胆固醇代谢是上调或下调的abcg1消融？细胞胆固醇含量、胆固醇酯酶、ACAT、ABCA1或SR-B1的变化可能会影响外流。其次，还有其他影响外流的机制，特别是自发转移，Rothblat等人已证明在某些情况下很重要(ATVB 2006 26：541-7)和由载脂蛋白E介导的外流(ATVB 2006 26：157-62)。最后，通过一种非放射性示踪方法来比较胆固醇流出和净变化，这将是有用的，该方法将同时测量介质和细胞胆固醇含量。放射性示踪剂显示FC离开细胞的速度，但不能揭示有多少未标记的胆固醇从介质中的捐赠者重新进入细胞。

DESCRIPTION (provided by applicant): Sphingomyelin (SM) is the most abundant phospholipid in plasma next to phosphatidylcholine (PC), and is an essential component of cell membrane rafts. Although recent epidemiologic studies suggest that high SM levels in plasma increase the risk of atherosclerosis, the underlying mechanisms are unknown, because the normal functions of SM have not been elucidated. We propose that, because of its unique structure, and localization in the outer surface of cells, SM protects the integrity of cell membranes by inhibiting the phospholipase and lipid peroxidation reactions. Furthermore, we propose that because of its affinity to cholesterol, SM regulates cell cholesterol homeostasis and reverse cholesterol transport. Dysregulation of these functions could lead to inflammation and promote atherosclerosis. In Aim 1, we propose to test the hypothesis that SM regulates reverse cholesterol transport, focusing on its role in the efflux of cholesterol from macrophages, and in the esterification of cholesterol by LCAT. The novel hypothesis that SM acts as a chaperone for cholesterol during ABCG1 transporter-mediated efflux will be explored. The role of SM in the regulation of LCAT in physiological systems will be studied. In Aim 2, we will investigate the hypothesis that SM acts as an anti-inflammatory lipid by inhibiting the formation of pro-inflammatory lipids such as lyso PC, arachidonate, oxidized phospholipids and oxysterols. The hypothesis that SM competitively inhibits all enzymes that utilize PC as substrate will be tested with respect to secretory phospholipases and endothelial lipase. The inhibitory role of SM in the generation of pro-inflammatory oxidized PCs and oxysterols will be tested in lipoproteins and cell membranes. The effect of SM deficiency on the macrophage and neutrophil function, including cytokine production and superoxide generation, will be studied in myeloid-specific SM-deficient mice. In Aim 3, the role of SM and ceramide in cellular cholesterol homeostasis will be studied by determining their effects on cholesterol trafficking between cellular compartments and between cells and their environment. These studies will provide novel insights into the physiological role of this important phospholipid, and could possibly identify novel therapeutic targets against inflammation and atherosclerosis. PUBLIC HEALTH RELEVANCE: The studies proposed here will investigate the physiological role of sphingomyelin (SM) a special lipid molecule prevalent in plasma and in outer cell membrane. They will specifically focus on the role of SM as an ant-inflammatory molecule that protects cells against environmental insults. These studies could lead not only to new understanding of mechanisms of inflammation and heart disease, but also to better therapeutic strategies. First, are there any macrophage pathways for cholesterol metabolism that are up- or down regulated by ablation of ABCG1? There could be changes in cellular cholesterol content, cholesterol esterases, ACAT, ABCA1, or possibly SR-B1 that could affect efflux. Second, there are other mechanisms that effect efflux, particularly spontaneous transfer, which Rothblat et al have shown to be important in some contexts (ATVB 2006 26:541-7) and efflux mediated by apo E (ATVB 2006 26:157-62). Lastly, it would be useful to compare efflux with net change in cholesterol by a non radio tracer method that would measure both medium and cellular cholesterol content. Radio tracers reveal how fast FC leaves the cell but not how much unlabeled cholesterol re enters the cell from donors in the medium.