MECHANISMS AND REGULATION OF INTESTINAL CHOLESTEROL TRANSPORT

描述(由申请人提供)： 抑制肠道Niemann-Pick C1L1转运蛋白(NPC1L1)是肠道胆固醇吸收的重要介质，已成为治疗高胆固醇血症的重要靶点。尽管在治疗方式上取得了这些进展，但将冠心病高危患者的血浆胆固醇降低到低水平仍然很差。因此，识别针对NPC1L1的新机制对于更有效地治疗高胆固醇血症是必要的。然而，抑制NPC1L1会触发适应性细胞通路的激活，如斯特罗反应元件结合蛋白-2(SREBP2)，这可能会阻碍降胆固醇药物的有效性。在设计更有效的高胆固醇血症治疗方法时，也应考虑抑制这些代偿机制。我们的工作集中在研究NPC1L1抑制的新机制以及肠道SREBP2在体内胆固醇稳态中的作用。我们的初步数据表明，蛋白酶体和溶酶体依赖的途径参与了NPC1L1蛋白的正常周转。此外，抑制MAPK通路导致NPC1L1的细胞水平下降，从而降低其蛋白质稳定性。我们的发现表明，DNA甲基化是参与控制NPC1L1在肠上皮细胞表达的主要表观遗传决定因素。为了了解代偿途径的作用，我们最近培育了一种新型的肠道特异过表达SREBP2的转基因小鼠(命名为ISR2小鼠)。我们的数据显示，在ISR2小鼠的低密度脂蛋白和极低密度脂蛋白中，血浆胆固醇水平增加，这表明仅肠道SREBP2的激活就足以增加血浆胆固醇。我们推测，抑制MAPK通路通过蛋白酶体和/或溶酶体降解降低了NPC1L1蛋白的稳定性，DNA甲基化参与了NPC1L1基因的沉默。我们进一步假设，活动性肠道SREBP2增加了肠道脂肪合成/吸收和乳糜粒的产生，导致了血浆胆固醇的增加。我们的研究旨在利用体外和体内模型来解决这些假设。针对特定目的1的研究将探讨MAPK依赖通路调节NPC1L1蛋白稳定性的机制，并确定NPC1L1的结构域。特定目的2将系统地研究DNA甲基化导致NPC1L1基因沉默的机制。针对特定目标3设计的研究将集中于研究肠道特异过度表达活性SREBP2的小鼠(ISR2小鼠)高胆固醇血症的代偿机制。我们提出的研究对于阐明肠道在维持胆固醇稳态中的关键作用以及揭示更有效地治疗高胆固醇血症的潜在靶点具有重要意义。

DESCRIPTION (provided by applicant): Inhibition of intestinal Niemann-Pick type C1 Like 1 (NPC1L1) transporter, an essential mediator of intestinal cholesterol absorption, has emerged as an important target for management of hypercholesterolemia. Despite such advances in the therapeutic modalities, reducing plasma cholesterol to low levels in patients with high risk for coronary heart disease remains poor. Therefore, identifying novel mechanisms targeting NPC1L1 is warranted for more effective treatment of hypercholesterolemia. The inhibition of NPC1L1, however, triggers the activation of adaptive cellular pathways such as the Sterol Response Element Binding Protein-2 (SREBP2) that could hinder the efficiency of cholesterol lowering drugs. Suppressing these compensatory mechanisms should also be taken into consideration when designing more effective treatment of hypercholesterolemia. Our efforts are focused at investigating the novel mechanisms involved in the inhibition of NPC1L1 as well as delineating roles of intestinal SREBP2 in body cholesterol homeostasis. Our preliminary data showed that proteasomal and lysosomal-dependent pathways are involved in the normal turnover of NPC1L1 protein. Also, inhibition of MAPK pathway caused a decrease in the cellular level of NPC1L1 by decreasing its protein stability. Our findings showed that DNA methylation is a major epigenetic determinant involved in controlling NPC1L1 expression in intestinal epithelia cells. To understand the role of compensatory pathways, we have recently generated novel transgenic mice with intestine- specific overexpression of constitutively active SREBP2 (designated as ISR2 mice). Our data showed an increase in levels of plasma cholesterol in the LDL and VLDL fractions in ISR2 mice indicating that the activation of intestinal SREBP2 alone is sufficient to increase plasma cholesterol. We hypothesized that inhibition of MAPK pathway decreases NPC1L1 protein stability via proteasomal and/or lysosomal degradation, and that DNA methylation is involved in NPC1L1 gene silencing. We further hypothesize that the increase in intestinal lipid synthesis/absorption and chylomicron production by active intestinal SREBP2 lead to an increase in plasma cholesterol. Our studies are designed to address these hypotheses utilizing both in vitro and in vivo models. Studies designed for Specific Aim 1 will investigate the mechanisms involved in the regulation of NPC1L1 protein stability by the MAPK dependent pathway and determine the structural domains of NPC1L1 involved. Specific Aim 2 will systematically investigate mechanisms of NPC1L1 gene silencing by DNA methylation. Studies designed for Specific Aim 3 will focus on investigating the compensatory mechanisms underlying hypercholesterolemia in mice with intestine specific-overexpression of active SREBP2 (ISR2 mice). Our proposed studies are important to elucidate the critical role of the intestine in the maintenance of cholesterol homeostasis and to unravel potential targets for more effective treatment of hypercholesterolemia.