Endoplasmic reticulum stress and foam cell formation

描述(由申请人提供)：有载脂蛋白E缺乏的人会患上高胆固醇血症和动脉粥样硬化。同样，载脂蛋白E缺陷(ApoE-/-)小鼠血浆中携带载脂蛋白B48的水平升高，并以类似人类疾病的方式发展成动脉粥样硬化。在载脂蛋白E缺陷患者和小鼠模型中，动脉粥样硬化的主要原因是载脂蛋白E缺陷的载脂蛋白(E-/B48)在动脉壁沉积。沉积的脂蛋白将单核细胞聚集到动脉内膜，并将其转化为巨噬细胞，巨噬细胞主要通过两种机制参与动脉粥样硬化的发病：1)释放炎症介质；2)形成泡沫细胞。巨噬细胞对E-/B48脂蛋白的摄取失控可能是ApoE-/-小鼠泡沫细胞形成的机制之一。本实验室最近证实，E-/B48脂蛋白可减少巨噬细胞的胆固醇外流，并下调溶酶体水解酶的表达。此外，巨噬细胞对E-/B48脂蛋白的降解随着时间的推移而下降。这些新的发现表明，胆固醇外流减少和E-/B48脂蛋白降解减少都有助于泡沫细胞的形成。我们的初步研究还发现，巨噬细胞与E-/B48脂蛋白孵育后，真核细胞翻译起始因子21(eIF-2a)的磷酸化水平增加，这与未折叠蛋白反应(UPR)信号通路之一有关。因此，E-/B48脂蛋白与巨噬细胞的相互作用可能激活UPR信号通路，进而调节基因表达并诱导动脉粥样硬化事件，如触发泡沫细胞的形成。在拟议的研究中，我们将检验UPR信号通路的激活是E-/B48脂蛋白调节基因表达、诱导泡沫细胞形成和促进动脉粥样硬化的机制。本项目包括四个具体目标：1)确定E-/B48脂蛋白诱导的基因表达变化是由于小鼠巨噬细胞转录或翻译改变，还是两者兼而有之；2)确定UPR信号通路的激活是否是E-/B48脂蛋白诱导基因表达变化的机制；3)确定UPR信号通路的激活是否是E-/B48脂蛋白诱导泡沫细胞形成的机制；以及4)确定抑制eIF-2a磷酸化对ApoE-/-小鼠动脉粥样硬化的影响。如果我们的假设是正确的，UPR信号通路的失活将减弱E-/B48脂蛋白诱导的基因表达，并抑制泡沫细胞的形成和动脉粥样硬化的发展。公共卫生相关性：在美国，心肌梗塞和中风是主要的死亡原因。动脉粥样硬化是心肌梗死和中风的主要原因。动脉粥样硬化的早期阶段是动脉壁内脂质泡沫细胞的形成。本研究的目的是确定内质网应激在泡沫细胞形成中的作用。内质网应激是一种细胞事件，它降低了细胞中某些蛋白质的水平，但增加了其他蛋白质的水平。我们的初步研究表明，从载脂蛋白E缺陷的小鼠中获得的脂蛋白可以引起内质网应激并诱导泡沫细胞的形成。本方案中设计的实验将研究内质网应激是否是泡沫细胞形成的原因。这项工作的发现将有助于理解泡沫细胞形成的分子机制，并为通过抑制内质网应激来治疗或预防动脉粥样硬化提供策略。

DESCRIPTION (provided by applicant): Individuals with apolipoprotein (Apo) E deficiency develop hypercholesterolemia and atherosclerosis. Similarly, ApoE-deficient (ApoE-/-) mice elevate plasma ApoB48-carrying lipoproteins and develop atherosclerosis in a manner that resembles the human disease. The primary cause of atherosclerosis in ApoE-deficient patients and mouse models is the deposition of ApoE-deficient, ApoB48-carrying (E-/B48) lipoproteins in the arterial wall. The deposited lipoproteins recruit monocytes into the arterial intima and transform them into macrophages, which participate in the pathogenesis of atherosclerosis mainly through two mechanisms: 1) releasing inflammatory mediators, and 2) forming foam cells. An uncontrolled macrophage uptake of E-/B48 lipoproteins could be a mechanism underlying foam cell formation in ApoE-/- mice. Our laboratory recently demonstrated that E-/B48 lipoproteins reduce cellular cholesterol efflux from macrophages and down-regulates lysosomal hydrolase expression. In addition, the degradation of E-/B48 lipoproteins by macrophages declined over time. These novel findings indicate that both reduced cholesterol efflux and decreased degradation of E-/B48 lipoproteins could contribute to foam cell formation. Our preliminary studies also revealed that incubation of macrophages with E-/B48 lipoproteins enhanced eukaryotic translation initiationfactor 21 (eIF-2a) phosphorylation, which is linked to one of the unfolded protein response (UPR) signaling pathways. Thus, interaction of E-/B48 lipoproteins with macrophages may activate UPR signaling pathways, which in turn regulate gene expression and induce atherogenic events, such as triggering foam cell formation. In the proposed studies, we will test the hypothesis that activation of UPR signaling pathways is a mechanism by which E-/B48 lipoproteins regulate gene expression, induce foam cell formation and promote atherosclerosis. This project includes four specific aims: 1) to determine whether E-/B48 lipoprotein-induced changes in gene expression result from altered transcription or translation or both in mouse macrophages; 2) to determine whether activation of UPR signaling pathways is a mechanism underlying E-/B48 lipoprotein-induced gene expression changes; 3) to determine whether activation of UPR pathways is a mechanism underlying E-/B48 lipoprotein-induced foam cell formation; and 4) to determine the effect of inhibiting eIF-2a phosphorylation on atherosclerosis in ApoE-/- mice. If our hypothesis is correct, inactivation of UPR signaling pathways will attenuate E-/B48 lipoprotein-induced gene expression, and suppress foam cell formation and atherosclerosis development. PUBLIC HEALTH RELEVANCE: Myocardial infarction and stroke are the leading caused of death in the United Sates. Atherosclerosis is the primary cause of myocardial infarction and stroke. Formation of lipid-laden foam cells in the vessel wall is the early stage of atherosclerosis. The goal of this proposal is to determine the involvement of endoplasmic reticulum stress in foam cell formation. Endoplasmic reticulum stress is a cellular event that reduces the level of some proteins but increases the level of other proteins in cells. Our preliminary studies indicate that lipoproteins obtained from mice deficient in apolipoprotein E can cause endoplasmic reticulum stress and induce foam cell formation. The experiments designed in this proposal will study whether endoplasmic reticulum stress is the cause of foam cell formation. Finding from this work should contribute to understanding of the molecular mechanism of foam cell formation, and provide strategies for treatment or prevention of atherosclerosis by inhibition of endoplasmic reticulum stress.