Hepatocyte apoptosis is a hallmark of nonalcoholic steatohepatitis. We have previously observed that the saturated free fatty acids (FFAs) induce hepatocyte apoptosis in part via a death receptor 5 (DR5)-mediated signaling pathway. Cellular inhibitor of apoptosis protein 1 and 2 (cIAP-1 and cIAP-2) proteins are potent inhibitors of death receptor-mediated apoptosis. However, the role of the cIAPs in FFA-mediated hepatocyte apoptosis is unexplored. Our aim was to determine whether cIAPs are dysregulated during hepatocyte lipoapoptosis. cIAP proteins underwent rapid cellular elimination following treatment with the saturated FFAs palmitate (PA) and stearate. In contrast, PA did not decrease cIAP-1 and cIAP-2 mRNA expression in the cells. Degradation of cIAPs was dependent on their E3-ligase activity, suggesting that cIAPs undergo autoubiquitination that leads to proteasomal degradation. Huh-7 cells stably expressing shRNA targeting cIAP-1, but not cIAP-2, displayed enhanced sensitivity to PA-mediated apoptosis. Incubation with the SMAC mimetic JP1584, which induces rapid degradation of cIAPs, also enhanced PA-mediated apoptosis. Hepatocytes isolated from DR5 knockout mice exhibited reduced apoptosis following treatment with PA plus JP1584, implying that degradation of cIAPs sensitizes to DR5-mediated cell death pathways. A decrease of cIAP-1 was also observed in tissue from patients with nonalcoholic steatohepatitis compared with normal obese subjects. Collectively, these results implicate proteasomal degradation of cIAPs by FFA as a mechanism contributing to hepatocyte lipoapoptosis.

译文

:肝细胞凋亡是非酒精性脂肪性肝炎的标志。我们以前已经观察到饱和的游离脂肪酸(FFA)部分通过死亡受体5(DR5)介导的信号传导途径诱导肝细胞凋亡。细胞凋亡蛋白1和2(cIAP-1和cIAP-2)蛋白的细胞抑制剂是死亡受体介导的细胞凋亡的有效抑制剂。但是,尚未探讨cIAP在FFA介导的肝细胞凋亡中的作用。我们的目的是确定肝细胞脂肪凋亡过程中cIAP是否失调。用饱和FFA棕榈酸酯(PA)和硬脂酸酯处理后,cIAP蛋白经历了快速的细胞消除。相反,PA不会降低细胞中cIAP-1和cIAP-2 mRNA的表达。 cIAP的降解取决于其E3-连接酶的活性,这表明cIAP经历了自身泛素化,导致蛋白酶体降解。稳定表达靶向cIAP-1而不是cIAP-2的shRNA的Huh-7细胞显示出增强的对PA介导的细胞凋亡的敏感性。与SMAC模拟物JP1584一起孵育可以诱导cIAP的快速降解,也可以增强PA介导的细胞凋亡。从PA5加上JP1584处理后,从DR5基因敲除小鼠中分离出的肝细胞显示出降低的凋亡,这暗示cIAP的降解对DR5介导的细胞死亡途径敏感。与正常肥胖受试者相比,在非酒精性脂肪性肝炎患者的组织中也观察到cIAP-1的减少。总的来说,这些结果暗示FFA将蛋白酶体降解为IAA,这是促成肝细胞脂凋亡的一种机制。

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