Human ApoE4 and Foam Cell Formation

描述（由申请方提供）：本项目的总体目标是确定导致富含载脂蛋白E4（ApoE4）的脂蛋白处理的巨噬细胞中胆固醇过量蓄积的机制，并阐明未折叠蛋白反应（UPR）在该过程中的作用。载脂蛋白5（Apo5）有三种主要的人类等位基因，即，52、53和54。54产物ApoE4与动脉粥样硬化性心血管疾病的风险增加有关，而ApoE3是所谓的正常亚型。Apo54在非洲裔美国人中普遍存在，与其他种族群体相比，非洲裔美国人的心血管疾病发病率高。ZhongMao Guo博士实验室先前的工作表明，增加的真核起始因子21（eIF-21）磷酸化（UPR的信号传导）是用来自ApoB48/48/ApoE-/-（E>/B48）小鼠的脂蛋白处理的巨噬细胞中胆固醇蓄积和泡沫细胞形成的潜在机制。最近，我们发现富含人ApoE4的E-/B48脂蛋白（E4/B48）在巨噬细胞中比富含相同量的人ApoE3的等浓度的E-/B48脂蛋白（E3/B48）诱导更大的胆固醇积累和增强的eIF-21磷酸化。此外，E4/B48脂蛋白比E3/B48脂蛋白更不能够诱导巨噬细胞中胆固醇流出转运蛋白ATP结合盒转运蛋白A1（ABCA 1）的表达。这种转运蛋白的缺乏与广泛的泡沫细胞形成和严重的动脉粥样硬化有关。该项目将测试假设UPR途径的激活是介导胆固醇流出的转运蛋白表达减少的机制，导致E4/B48脂蛋白处理的巨噬细胞中过量胆固醇的积累。为了检验这一假设，将进行两个具体的目标。具体目标1将研究E4/B48-脂蛋白对介导胆固醇流出的转运蛋白的巨噬细胞表达、胆固醇流出本身和泡沫细胞形成的影响。这一目的将通过测量胆固醇流出转运蛋白（ABCA 1、ABCG 1和清道夫受体-B1 [SR-B1]）的表达水平、评价胆固醇流出程度以及确定巨噬细胞经E4/B48和E3/B48脂蛋白处理后细胞胆固醇蓄积程度来实现。具体目标2将确定UPR信号通路在巨噬细胞表达介导胆固醇流出的转运蛋白、胆固醇流出本身和泡沫细胞形成中的作用。普遍定期审议有三种途径。E4/B48脂蛋白可激活eIF21相关通路，如果我们的假设是正确的，那么当E4/B48脂蛋白抑制该通路时，胆固醇外排转运蛋白表达的变化以及E4/B48脂蛋白诱导的胆固醇外排和蓄积将减弱。 公共卫生相关性：载脂蛋白5（Apo5）有三种主要的人类等位基因，即，52，53和54，54亚型与动脉粥样硬化性心血管疾病的风险增加相关联。与其他种族相比，非洲裔美国人具有更高的54等位基因频率，更容易患与动脉粥样硬化相关的心血管疾病。通过阐明含ApoE4脂蛋白在泡沫细胞形成中的作用，这是动脉粥样硬化发展的早期事件，我们将提供治疗或预防这种疾病的策略。

DESCRIPTION (provided by applicant): The overall goal of this project is to determine the mechanism(s) leading to excess cholesterol accumulation in macrophages treated with apolipoprotein E4 (ApoE4)-enriched lipoproteins, and to elucidate the role of unfolded protein response (UPR) in this process. There are three major human alleles of apolipoprotein 5 (Apo5), i.e., 52, 53, and 54. The 54 product, ApoE4, is associated with increased risk of atherosclerotic cardiovascular disease compared to ApoE3, the so-called normal isoform. Apo54 is prevalent in African-Americans, for whom cardiovascular disease is disproportionally high compared to other ethnic groups. Previous work in Dr. ZhongMao Guo"s laboratory demonstrated that increased eukaryotic initiation factor 21 (eIF-21) phosphorylation, a signaling of UPR, is a mechanism underlying cholesterol accumulation and foam cell formation in macrophages treated with lipoproteins derived from ApoB48/48/ApoE-/- (E>/B48) mice. Recently, we found that E>/B48 lipoproteins enriched with human ApoE4 (E4/B48) induce greater cholesterol accumulation and enhanced eIF-21 phosphorylation in macrophages than an equal concentration of E-/B48 lipoproteins enriched with the same amount of human ApoE3 (E3/B48). In addition, E4/B48 lipoproteins are less able than E3/B48 lipoproteins to induce the expression of the cholesterol efflux transporter, ATP-binding cassette transporter A1 (ABCA1), in macrophages. Deficiency of this transporter is associated with widespread foam cell formation and severe atherosclerosis. This project will test the hypothesis that activation of UPR pathways is a mechanism behind the reduction in transporter expression mediating cholesterol efflux, leading to accumulation of excess cholesterol in macrophages treated with E4/B48 lipoproteins. Two specific aims will be carried out to test this hypothesis. Specific Aim 1 will study the effect of E4/B48-lipoproteins on macrophage expression of transporters mediating cholesterol efflux, cholesterol efflux itself, and foam cell formation. This aim will be accomplished by measuring the expression levels of cholesterol efflux transporters (ABCA1, ABCG1, and scavenger receptor-B1 [SR-B1]), evaluating the degree of cholesterol efflux, and determining the extent of cellular cholesterol accumulation after macrophages are treated with E4/B48 and E3/B48 lipoproteins. Specific Aim 2 will determine the role that UPR signaling pathways play in macrophage expression of transporters mediating cholesterol efflux, cholesterol efflux itself, and foam cell formation. There are three UPR pathways. This project will focus on the eIF21-associated pathway because E4/B48 lipoproteins have been shown to activate it. If our hypothesis is correct, changes in expression of cholesterol efflux transporters, and cholesterol efflux and accumulation induced by E4/B48 lipoproteins will be attenuated when this pathway is inhibited. PUBLIC HEALTH RELEVANCE: There are three major human alleles of apolipoprotein 5 (Apo5), i.e., 52, 53, and 54, and the 54 isoform is associated with increased risk of atherosclerotic cardiovascular disease compared to the normal 53. African-Americans have higher 54 allelic frequency and are more prone to cardiovascular diseases related to atherosclerosis compared to other ethnic groups. By elucidating the role that ApoE4-containing lipoproteins play in the formation of foam cells, an early event in the development of atherosclerosis, we will provide strategies for treatment or prevention of this disease.