Role of Sphingomyelin in Lipoprotein Metabolism

描述（申请人提供）：鞘磷脂（SM）是血浆中含量最多的磷脂，仅次于磷脂酰胆碱（PC），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会增加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，我们提出，由于其与胆固醇的亲和力，SM调节细胞胆固醇稳态和逆转胆固醇运输。这些功能失调可能导致炎症并促进动脉粥样硬化。在Aim 1中，我们提出验证SM调节胆固醇逆向转运的假设，重点关注其在巨噬细胞胆固醇外排和LCAT胆固醇酯化中的作用。将探讨SM在ABCG1转运体介导的外排过程中作为胆固醇伴侣的新假设。SM在LCAT生理系统中的调节作用将被研究。在Aim 2中，我们将研究SM作为一种抗炎脂质的假设，通过抑制促炎脂质的形成，如lyso PC、花生四烯酸酯、氧化磷脂和氧化固醇。SM竞争性地抑制所有利用PC作为底物的酶的假设将在分泌磷脂酶和内皮脂肪酶方面进行测试。SM在促炎氧化pc和氧甾醇生成中的抑制作用将在脂蛋白和细胞膜中进行测试。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，将在骨髓特异性SM缺乏小鼠中进行研究。在Aim 3中，SM和神经酰胺在细胞胆固醇稳态中的作用将通过确定它们对细胞间和细胞与环境之间胆固醇运输的影响来研究。这些研究将为这种重要磷脂的生理作用提供新的见解，并可能确定抗炎症和动脉粥样硬化的新治疗靶点。公共卫生相关性：本文提出的研究将探讨鞘磷脂（SM）的生理作用，SM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，这种分子可以保护细胞免受环境损害。这些研究不仅会导致对炎症和心脏病机制的新认识，而且还会带来更好的治疗策略。

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.