REGULATION OF CETP BY LIPID TRANSFER INHIBITOR PROTEIN

人血浆中循环脂蛋白的组成和浓度受胆固醇酯转运蛋白(CETP)的强烈影响。鉴于CETP在脂蛋白代谢中的重要性和对动脉粥样硬化的潜在影响，CETP活性的调节一直是广泛研究的主题。我们已经确定了一种调节CETP活性的血浆蛋白，脂转移抑制蛋白(LTIP)。最近，我们纯化、克隆和表达了LTIP，并证明了其与载脂蛋白F的同源性，载脂蛋白F是一种功能未知的蛋白质。我们认为LTIP是一种调节CETP的脂流开关，它不是通过抑制CETP的活性，而是通过选择性地干扰与低密度脂蛋白之间的脂转移事件。这种对低密度脂蛋白的选择性似乎是通过LTIP与血浆中低密度脂蛋白的优先结合来调节的。一些来自体外和体内研究的证据有力地表明，LTIP是脂蛋白浓度和组成的生理调节器。我们的长期目标是确定LTIP在调节脂蛋白代谢中的作用，并确定其对动脉粥样硬化形成的影响。在这项建议中，我们进一步定义了LTIP在体内的作用，确定了其作用机制，并表征了调节其合成和分泌的机制。体外和体内研究验证了这一假设，即极低密度脂蛋白分解代谢的残留物在大小接近低密度脂蛋白时获得LTIP，这改变了它们的CETP底物状态，并决定了极低密度脂蛋白分解代谢最终产物的性质(目标1)。重组LTIP的动力学和脂类单层研究，以及脂蛋白修饰和LTIP突变研究将确定LTIP的作用机制，并确定活性所需的脂蛋白的性质和LTIP的结构特征(目标2)。最后，培养细胞的生化和分子研究以及组织特异性表达分析将被用来检验LTIP和CETP共同表达和共同调节以响应细胞胆固醇稳态的假设(目标3)。总体而言，这些研究将为人类血浆中一种特征不佳的胆固醇流量调节器提供新的见解。

In human plasma, the composition and concentration of circulating lipoproteins is strongly influenced by cholesteryl ester transfer protein (CETP). Given its importance in lipoprotein metabolism and potential influence on atherosclerosis, the regulation of CETP activity has been the subject of extensive study. We have identified a plasma protein, lipid transfer inhibitor protein (LTIP), that regulates CETP activity. Recently, we purified, cloned and expressed LTIP and demonstrated its identity with apolipoprotein F, a protein with no known function. We propose that LTIP is a lipid flux switch that regulates CETP, not by suppressing CETP activity generally, but by selectively interfering with lipid transfer events to and from LDL. This selectivity toward LDL appears to be mediated by the preferential association of LTIP with LDL in plasma. Several lines of evidence from in vitro and in vivo studies strongly suggest that LTIP is a physiological modulator of lipoprotein concentration and composition. Our long-term objective is to define the role of LTIP in regulating lipoprotein metabolism and to define its impact on atherogenesis. In this proposal we further define the role of LTIP in vivo, identify its mechanism of action, and characterize mechanisms regulating its synthesis and secretion. In vitro and in vivo studies test the hypothesis that remnants of VLDL catabolism acquire LTIP as they near LDL in size, which alters their CETP-substrate status and determines the nature of the end- products of VLDL catabolism (Aim 1). Kinetic and lipid monolayer studies with recombinant LTIP, and lipoprotein modification and LTIP mutagenesis studies will determine the mechanism of LTIP's action and define the properties of lipoproteins and the structural features of LTIP that are required for activity (Aim 2). Finally, biochemical and molecular studies with cultured cells and tissue-specific expression analyses will be used to test the hypothesis that LTIP and CETP are co-expressed and co- regulated in response to cellular cholesterol homeostasis (Aim 3). Overall, these studies will provide novel insights into a poorly characterized modulator of cholesterol flux in human plasma.