The role of FIT2 in VLDL assembly, hepatic triglyceride homeostasis, and lipoprotein atherogenicity

R01的应用重点是控制极低密度脂蛋白生成的机制 (极低密度脂蛋白)在肝细胞。MPIS之前的工作已经导致了15份出版物，确定了一个主要的 控制极低密度脂蛋白分泌和循环的机制是载脂蛋白的调节降解 (载脂蛋白B)在内质网。这一代谢协调的事件需要与ER相关的 降解途径(ERAD)，由MPI Brodsky命名并首先阐明。相反，当空档时 脂类(主要是三酰甘油；TGS)是足够的，载脂蛋白B是由MTP共同翻译的脂类，而新生的 极低密度脂蛋白颗粒由内质网驻留的脂滴(LDS)中的脂类膨胀。重塑的极低密度脂蛋白 颗粒随后被包装成COPII囊泡，用于输送到高尔基体，然后从 肝细胞(体外)或从肝细胞进入循环(体内)。与这些路径形成对比的是 控制极低密度脂蛋白的水平，极低密度脂蛋白是调节聚集在 内质网中载脂蛋白B的特征性很差。然而，MPI和同事们最近的结果表明 内质网驻留的膜蛋白FIT2在控制TG装配到apoB上起重要作用 体外和体内。具体地说，本申请中概述的初步数据-由 新型啮齿动物模型和工程化极低密度脂蛋白分泌细胞系的建立-强烈提示FIT2 调节载脂蛋白B相关脂类的浓度和组成，以及 极低密度脂蛋白。基于这些和其他新的数据，将检验以下假设：1)FIT2缺陷将 随着甘油三酯耗尽的极低密度脂蛋白的产生而增加内质网膜脂含量；2)FIT2将LDS传递到 ER，然后以MTP依赖或独立的方式整合到极低密度脂蛋白颗粒中；以及， 3)FIT2活性水平是此前未被认识到的脂肪肝严重程度的决定因素 (NAFLD)、脂肪性肝炎(NASH)和动脉粥样硬化。 由于FIT2介导的甘油三酯负载到极低密度脂蛋白上的效率也影响内质网中的肝脂水平， FIT2缺乏也可能导致内质网中的脂质失调和毒性应激反应。此外，交付 富含脂质的极低密度脂蛋白可能受到另一种ER相关因子KLHL12的限制，KLHL12有助于形成特化的极低密度脂蛋白- 常驻COPII囊泡。因此，另一种假设是，天然的KLHL12水平限制了FIT2- 支持从肝细胞运送极低密度脂蛋白，从而加剧内质网应激。总体而言，除了剖析 FIT2调节NASH、NAFLD、极低密度脂蛋白的生物发生、肝脏甘油三酯的动态平衡和脂蛋白的致动脉粥样硬化 该项目的临床相关性还包括整合最近生成的转录数据 从人类肝脏和动脉粥样硬化斑块获得的具有类似数据库的模型。对于所有这些 为了实现目标，该项目受益于MPI深厚和互补的专业知识，以及 长期的合作者，在研究脂蛋白新陈代谢方面有良好的记录。

This R01 application focuses on the mechanisms that control the generation of very low density lipoproteins (VLDLs) in liver cells. Prior work from the MPIs, which has resulted in 15 publications, established that a major mechanism controlling the secretion and circulation of VLDL is the regulated degradation of apolipoprotein (apoB) in the endoplasmic reticulum (ER). This metabolically orchestrated event requires the ER-associated degradation (ERAD) pathway, which was named and first elucidated by MPI Brodsky. In contrast, when neutral lipids (primarily triacylglycerols; TGs) are sufficient, apoB is co-translationally lipidated by MTP, and the nascent VLDL particles are expanded by lipids sourced from ER-resident lipid droplets (LDs). The re-modelled VLDL particles are next packaged into COPII vesicles for delivery to the Golgi and then secreted into the medium from hepatic cells (in vitro) or from liver hepatocytes into the circulation (in vivo). In contrast to these pathways that control the levels of VLDL, the factors that regulate the concentration and composition of lipids assembled onto apoB in the ER are poorly characterized. However, recent results generated by the MPIs and colleagues indicate that an ER-resident membrane protein, FIT2, plays a significant role in controlling TG assembly onto apoB in vitro and in vivo. Specifically, the preliminary data outlined in this application—which were made possible by the generation of novel rodent models and engineered VLDL secreting cell lines—strongly suggest that FIT2 regulates both the concentration and composition of apoB-associated lipids, as well as the atherogenicity of VLDL. Based on these and other new data, the following hypotheses will be tested: 1) FIT2 deficiency will increase ER membrane lipid content along with the generation of TG-depleted VLDL; 2) FIT2 delivers LDs into the ER, which are then integrated into VLDL particles in either an MTP-dependent or independent manner; and, 3) the level of FIT2 activity is a previously unappreciated determinant for the severity of fatty liver disease (NAFLD), steatohepatitis (NASH), and atherosclerosis. Because the efficiency of FIT2-mediated loading of TG onto VLDL also impacts hepatic lipid levels in the ER, FIT2 deficiency may also lead to lipid dysregulation in the ER and toxic stress responses. Moreover, the delivery of lipid-rich VLDLs could be limited by another ER-associated factor, KLHL12, that helps form specialized VLDL- resident COPII vesicles. Thus, another hypothesis is that native KLHL12 levels constrain the capacity of FIT2- supported delivery of VLDL from hepatic cells, thus exacerbating ER stress. Overall, besides dissecting how FIT2 regulates NASH, NAFLD, VLDL biogenesis, hepatic TG homeostasis, and lipoprotein atherogenicity, the clinical relevance of this project also includes the integration of transcriptomic data from the recently generated models with analogous databases obtained from human livers and atherosclerotic plaques. Toward all of these goals, the project benefits from deep and complementary expertise of the MPIs along with the experience of long-term collaborators with a substantial track record in studying lipoprotein metabolism.