Gladstone Institute of Cardiovascular Disease, University of California, San Francisco, California 94158; Liver Center, University of California, San Francisco, California 94158; Department of Medicine, University of California, San Francisco, California 94158.
Liver Center, University of California, San Francisco, California 94158; Department of Medicine, University of California, San Francisco, California 94158; Gladstone Institute of Virology and Immunology, University of California, San Francisco, California 94158.
J Biol Chem. 2011 Dec 9;286(49):42615-42625. doi: 10.1074/jbc.M111.285148. Epub 2011 Oct 7.
Steatosis is a frequent complication of hepatitis C virus infection. In mice, this condition is recapitulated by the expression of a single viral protein, the nucleocapsid core. Core localizes to the surface of lipid droplets (LDs) in infected liver cells through a process dependent on host diacylglycerol acyltransferase 1 (DGAT1), an enzyme that synthesizes triglycerides in the endoplasmic reticulum. Whether DGAT1 also plays a role in core-induced steatosis is uncertain. Here, we show that mouse embryonic fibroblasts isolated from DGAT1(-/-) mice are protected from core-induced steatosis, as are livers of DGAT1(-/-) mice expressing core, demonstrating that the steatosis is DGAT1-dependent. Surprisingly, core expression did not increase DGAT1 activity or triglyceride synthesis, thus excluding the possibility that core activates DGAT1 to cause steatosis. Instead, we find that DGAT1-dependent localization of core to LDs is a prerequisite for the steatogenic properties of the core. Using biochemical and immunofluorescence microscopy techniques, we show that the turnover of lipids in core-coated droplets is decreased, providing a physiological mechanism for core-induced steatosis. Our results support a bipartite model in which core first requires DGAT1 to gain access to LDs, and then LD-localized core interferes with triglyceride turnover, thus stabilizing lipid droplets and leading to steatosis.
脂肪变性是丙型肝炎病毒感染的常见并发症。在小鼠中，这种情况通过表达一种病毒蛋白——核衣壳核心蛋白来重现。核心蛋白通过一种依赖于宿主二酰基甘油酰基转移酶 1（DGAT1）的过程定位于感染肝细胞的脂滴（LDs）表面，DGAT1 是在内质网中合成甘油三酯的酶。DGAT1 是否也在核心诱导的脂肪变性中发挥作用尚不确定。在这里，我们表明，从 DGAT1(-/-) 小鼠中分离的胚胎成纤维细胞免受核心诱导的脂肪变性的影响，以及表达核心的 DGAT1(-/-) 小鼠的肝脏也免受影响，这表明脂肪变性是 DGAT1 依赖性的。令人惊讶的是，核心表达并没有增加 DGAT1 的活性或甘油三酯合成，从而排除了核心激活 DGAT1 导致脂肪变性的可能性。相反，我们发现核心依赖于 DGAT1 将其定位于 LD 是其产生脂肪变性特性的前提。使用生化和免疫荧光显微镜技术，我们发现核心包被的液滴中脂质的周转率降低，为核心诱导的脂肪变性提供了一种生理机制。我们的结果支持一个二分模型，其中核心首先需要 DGAT1 进入 LD，然后 LD 定位的核心干扰甘油三酯的周转，从而稳定脂质滴并导致脂肪变性。
