ROLE OF SPHINGOMYELIN IN LIPOPROTEIN METABOLISM

描述（由申请人提供）：本提案的长期目标是研究鞘氨醇（SPH）在血浆中的生理作用，其中它是仅次于磷脂酰胆碱（PC）的最丰富的磷脂。与其在膜胆固醇代谢和信号转导中的功能相反，SPH在血浆脂蛋白代谢中的作用很少受到关注，尽管其浓度在动脉粥样硬化和衰老中显着增加。基于初步数据，PI提出SPH调节血浆脂解活性、逆转胆固醇转运途径和脂质过氧化反应，从而防止PC和胆固醇的过度周转和过早降解。PI将检验以下假设：SPH凭借其与PC的结构相似性，不仅竞争性抑制LCAT（如他先前所示），还竞争性抑制其他水解PC的脂解酶，如肝脂肪酶和分泌型磷脂酶A2。他还将通过采用单层技术、酶动力学和SPH分子的结构修饰来研究抑制这些活性的机制。 SPH在脂蛋白之间的游离胆固醇和胆固醇酯（CE）的交换中的作用，以及在培养中的各种细胞选择性摄取HDL CE将通过操纵天然脂蛋白和重组HDL（rHDL）颗粒的SPH浓度进行研究。由于CE的表面浓度决定其转移速率，因此将通过[13 C] NMR研究SPH对CE分配到rHDL表面脂质中的影响。膜SPH在SR-B1受体功能中的可能作用也将被调查。PI提出测试新的假设，即SPH抑制脂质过氧化通过延缓脂质过氧自由基的传播，并认为增加的氧化敏感性的小致密LDL，相比浮力LDL，是由于低SPH/PC的比例在前者。他将把各种LDL亚组分和合成脂质体的氧化性与它们的SPH/PC比率联系起来。通过SPH的结构类似物、特异性定位于脂蛋白表面或核心的氧化荧光探针、SPH对脂蛋白膜流动性和侧向扩散速率的影响等研究SPH抑制脂质过氧化作用的机制，为进一步了解SPH在脂蛋白中的生理功能提供新的思路。及其与动脉粥样硬化和炎症的可能相关性。

DESCRIPTION (provided by applicant): The long term objective of this proposal is to investigate the physiological role of sphingomyeIin (SPH) in plasma, where it is the most abundant phospholipid next to phosphatidylcholine (PC). In contrast to its functions in membrane cholesterol metabolism and signal transduction, the role of SPH in the metabolism of plasma lipoproteins has received little attention, although its concentration is significantly increased in atherosclerosis and aging. Based on preliminary data, the PI proposes that SPH modulates plasma lipolytic activities, reverse cholesterol transport pathways, and lipid peroxidation reactions, thereby preventing excessive turnover and premature degradation of PC and cholesterol.The PI will test the hypothesis that SPH, by virtue of its structural similarities to PC, competitively inhibits not only LCAT, as he previously showed, but also other lipolytic enzymes that hydrolyze PC, such as hepatic lipase and secretory phospholipases A2. He will also investigate the mechanism(s) involved in the inhibition of these activities, by employing monolayer techniques, enzyme kinetics, and structural modifications of the SPH molecule. The role of SPH in the exchange of free cholesterol and cholesteryl ester (CE) between lipoproteins, and in the selective uptake of HDL CE by various cells in culture will be studied by manipulating the SPH concentration of native lipoproteins and recombinant HDL (rHDL) particles. Since the surface concentration of CE determines its transfer rate, the effect of SPH on the partitioning of CE into surface lipids of rHDL will be investigated by [13C] NMR. The possible role of membrane SPH in the function of SR-B1 receptor will also be investigated.The PI proposes to test the novel hypothesis that SPH inhibits lipid peroxidation by retarding the propagation of lipid peroxy radicals and that the increased oxidative susceptibility of small dense LDL, compared to buoyant LDL, is due to the low SPH/PC ratio in the former. He will correlate the oxidizability of various LDL subfractions and synthetic liposomes with their SPH/PC ratios. The mechanism by which SPH inhibits lipid peroxidation will be investigated by using structural analogs of SPH, by oxidizing fluorescent probes which are specifically localized in the surface or core of lipoproteins, and by studying the effects of SPH on membrane fluidity and lateral diffusion rates.The results from these studies should provide new insights into the physiological functions of SPH in lipoproteins, and its possible relevance to atherosclerosis and inflammation.