Defective isoforms of ApoE induce atherogenesis via unfolded protein responses

描述(申请人提供)：人载脂蛋白E包括三种常见的亚型，即载脂蛋白2、载脂蛋白E3和载脂蛋白E4。APOE3被认为是正常的异构体，而APOE2和ApoE4功能失调。携带ApoE缺陷亚型的个体会患上高胆固醇血症和动脉粥样硬化。同样，ApoE基因敲除(ApoE-/-)小鼠也患有类似于人类疾病的高胆固醇血症和动脉粥样硬化。载脂蛋白E缺陷患者和小鼠模型中的高胆固醇血症主要是由于含有载脂蛋白B48的血浆残余脂蛋白水平增加所致。本实验室前期的研究表明，从ApoE-/-小鼠体内获得的残余脂蛋白能够将巨噬细胞转化为泡沫细胞，而不含ApoE的残余脂蛋白诱导的泡沫细胞的形成与真核细胞翻译起始因子21(EIF-21)的磷酸化增强相吻合，这是与内质网(ER)应激有关的细胞事件。我们还观察到，抑制eIF-21的磷酸化减少了ApoE-Free残余脂蛋白诱导的泡沫细胞的形成。此外，我们观察到富含人载脂蛋白E缺陷异构体的小鼠残存脂蛋白，特别是富含载脂蛋白4的残余脂蛋白，可诱导巨噬细胞中胆固醇积聚和eIF-21磷酸化。很可能在ApoE缺失或缺失的情况下，残留脂蛋白与巨噬细胞的相互作用激活了内质网应激相关信号通路，进而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，我们将检验一个假设，即含有人类载脂蛋白E缺陷亚型的残留脂蛋白通过诱导内质网应激的机制诱导泡沫细胞的形成。我们将通过三个具体目标来检验这一假设。具体目标1将确定携带有缺陷的人载脂蛋白E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。具体目的2将确定携带人类载脂蛋白E缺陷亚型的残留脂蛋白是否通过激活内质网应激相关信号通路来调节基因表达。具体目标3将确定携带人类载脂蛋白E缺陷亚型的残留脂蛋白是否通过激活内质网应激相关信号通路来诱导泡沫细胞的形成。如果我们的假设是正确的，抑制内质网应激相关信号通路将减弱ApoE缺陷残余脂蛋白诱导的巨噬细胞脂质分解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：这项建议研究未折叠蛋白反应在泡沫细胞形成中的参与，泡沫细胞形成是动脉粥样硬化的早期阶段。本项目的数据将有助于了解动脉粥样硬化的机制，并为动脉粥样硬化导致的心肌梗死和中风提供治疗策略。

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.