Defective isoforms of ApoE induce atherogenesis via unfolded protein responses

描述（由申请方提供）：人载脂蛋白（Apo）E包括三种常见的亚型，称为ApoE 2、ApoE 3和ApoE 4。ApoE 3被认为是正常的同种型，而ApoE 2和ApoE 4是功能障碍的。携带缺陷型ApoE亚型的个体发展为高胆固醇血症和动脉粥样硬化。类似地，ApoE无效敲除（ApoE-/-）小鼠患有类似于人类疾病的高胆固醇血症和动脉粥样硬化。ApoE缺陷患者和小鼠模型中的高胆固醇血症主要由含有ApoB 48的残余脂蛋白的血浆水平增加引起。我们实验室先前的研究表明，从ApoE-/-小鼠获得的残余脂蛋白能够将巨噬细胞转化为泡沫细胞，并且由无ApoE的残余脂蛋白诱导的泡沫细胞形成与真核翻译起始因子21（eIF-21）的增强的磷酸化一致，这是与内质网（ER）应激相关的细胞事件。我们还观察到，抑制eIF-21磷酸化减弱了无ApoE残余脂蛋白诱导的泡沫细胞形成。此外，我们观察到富含人ApoE缺陷型的小鼠残余脂蛋白，特别是富含ApoE 4的那些，诱导巨噬细胞中胆固醇积累和eIF-21磷酸化。很有可能在ApoE缺乏或缺乏的情况下，残余脂蛋白与巨噬细胞的相互作用激活ER应激相关的信号传导途径，这反过来调节其编码产物有助于泡沫细胞形成的基因的表达。在这个项目中，我们将测试一个假设，残余的脂蛋白，含有缺陷亚型的人载脂蛋白E诱导泡沫细胞形成通过一种机制，涉及诱导ER应激。我们将以三个具体目标来检验这一假设。具体目标1将确定携带人类ApoE缺陷亚型的残余脂蛋白是否调节与泡沫细胞形成相关的基因的表达。具体目标2将确定携带缺陷型人ApoE的残余脂蛋白是否通过激活ER应激相关信号通路来调节基因表达。具体目标3将确定携带有缺陷的人ApoE亚型的残余脂蛋白是否通过激活ER应激相关信号传导途径诱导泡沫细胞形成。如果我们的假设是正确的，ER应激相关信号通路的抑制将减弱ApoE缺陷的残余脂蛋白诱导的巨噬细胞脂质代谢和泡沫细胞形成相关基因表达的变化，并抑制泡沫细胞的形成。 公共卫生相关性：这项提案研究了未折叠蛋白反应在泡沫细胞形成（动脉粥样硬化的早期阶段）中的作用。本课题的研究结果将有助于进一步了解动脉粥样硬化的发病机制，并为动脉粥样硬化引起的心肌梗死和脑卒中提供治疗策略。

DESCRIPTION (provided by applicant): Human apolipoprotein (Apo) E includes three common isoforms known as ApoE2, ApoE3 and ApoE4. ApoE3 is considered to be the normal isoform, while ApoE2 and ApoE4 are dysfunctional. Individuals carrying defective isoforms of ApoE develop hypercholesterolemia and atherosclerosis. Similarly, ApoE null knockout (ApoE-/-) mice suffer from hypercholesterolemia and atherosclerosis resembling the human disease. The hypercholesterolemia in ApoE-deficient patients and mouse models results mainly from an increased plasma level of remnant lipoproteins that contain ApoB48. Previous studies from our laboratory demonstrate that the remnant lipoproteins obtained from ApoE-/- mice is able to transform macrophages into foam cells, and that foam cell formation induced by ApoE-free remnant lipoproteins coincides with an enhanced phosphorylation of eukaryotic translation initiation factor 21 (eIF-21), which is a cellular event related to endoplasmic reticulum (ER) stress. We also observed that inhibition of eIF-21 phosphorylation attenuated ApoE-free remnant lipoprotein-induced foam cell formation. Moreover, we observed that mouse remnant lipoproteins enriched with defective isofoms of human ApoE, especially those enriched with ApoE4, induced cholesterol accumulation and eIF-21 phosphorylation in macrophages. It is highly likely that in the absence of, or deficiency in, ApoE, interaction of remnant lipoproteins with macrophages activates ER stress-related signaling pathways, which in turn regulate the expression of genes whose encoded products contribute to foam cell formation. In this project, we will test a hypothesis that the remnant lipoproteins that contain defective isoforms of human ApoE induce foam cell formation via a mechanism involving induction of ER stress. We will test this hypothesis with three specific aims. Specific aim 1 will determine whether the remnant lipoproteins carrying defective isoforms of human ApoE regulate the expression of genes related to foam cell formation. Specific aim 2 will determine whether the remnant lipoproteins carrying defective isoforms of human ApoE regulate gene expression by activation of ER stress-related signaling pathways. Specific aim 3 will determine whether the remnant lipoproteins carrying defective isoforms of human ApoE induce foam cell formation by activation of ER stress-related signaling pathways. If our hypothesis is correct, inhibition of ER stress-related signaling pathways would attenuate ApoE-deficient remnant lipoprotein-induced changes in macrophage lipid catabolism and foam cell formation-related gene expression, and suppress foam cell formation. PUBLIC HEALTH RELEVANCE: This proposal studies the involvement of unfolded protein response in foam cell formation, an early stage of atherosclerosis. Data derived from this project will contribute to understanding of the mechanism of atherosclerosis, and provide therapeutic strategies for myocardial infarction and stroke induced by atherosclerosis.