Abstract：
The liver is exposed to gut-derived bacterial endotoxin via portal circulation, and recognizes it through toll-like receptor 4 (TLR4). Endotoxin lipopolysaccharide (LPS) stimulates the self-ubiquitination of ubiquitin ligase TRAF6, which is linked to scaffold with protein kinase TAK1 for auto-phosphorylation and subsequent activation. TAK1 activity is a signal transducer in the activating pathways of transcription factors NF-κB and AP-1 for production of various cytokines. Here, we hypothesized that TRAF6-TAK1 axis would be implicated in endotoxin-induced liver disease. Following exposure to endotoxin LPS, TLR4-mediated phosphorylation of TAK1 and transcription of cell-death cytokine TNF-α were triggered in Kupffer cells but not in hepatocytes as well as TNF receptor-mediated and caspase-3-executed apoptosis was occurred in D-galactosamine (GalN)-sensitized hepatocytes under co-culture with Kupffer cells. Treatment with pyridinylmethylene benzothiophene (PMBT) improved endotoxin LPS-induced hepatocyte apoptosis in GalN-sensitized C57BL/6 mice via suppressing NF-κB- and AP-1-regulated expression of TNF-α in Kupffer cells, and rescued the mice from hepatic damage-associated bleeding and death. As a mechanism, PMBT directly inhibited Lys 63-linked ubiquitination of TRAF6, and mitigated scaffold assembly between TRAF6 and the TAK1-activator adaptors TAB1 and TAB2 complex in Kupffer cells. Thereby, PMBT interrupted TRAF6 ubiquitination-induced activation of TAK1 activity in the TLR4-mediated signal cascade leading to TNF-α production. However, PMBT did not directly affect the apoptotic activity of TNF-α on GalN-sensitized hepatocytes. Finally, we propose chemical inhibition of TRAF6-TAK1 axis in Kupffer cells as a strategy for treating liver disease due to gut-derived endotoxin or Gram-negative bacterial infection.
摘要：
肝脏通过门静脉循环暴露于肠道来源的细菌内毒素，并通过toll样受体4（TLR4）识别它。内毒素脂多糖（LPS）刺激泛素连接酶TRAF6的自泛素化，该酶与蛋白激酶TAK1连接到支架上，以进行自磷酸化和随后的激活。TAK1活性是转录因子NF-κB和AP-1激活途径中的信号转导，用于产生各种细胞因子。这里,我们假设TRAF6-TAK1轴可能与内毒素诱导的肝脏疾病有关.暴露于内毒素LPS后，TLR4介导的TAK1磷酸化和细胞死亡细胞因子TNF-α的转录在Kupffer细胞中被触发，但在肝细胞中未被触发，并且在D-半乳糖胺（GalN）中发生TNF受体介导和caspase-3执行的凋亡。与Kupffer细胞共培养的致敏肝细胞。吡啶基亚甲基苯并噻吩（PMBT）治疗通过抑制NF-κB-和AP-1调节TNF-α在Kupffer细胞中的表达，改善了内毒素LPS诱导的GalN致敏C57BL/6小鼠肝细胞凋亡，并从肝损伤相关的出血和死亡中拯救了小鼠。作为一种机制,PMBT直接抑制TRAF6的Lys63连接的泛素化，并减轻了TRAF6与Kupffer细胞中TAK1激活剂衔接子TAB1和TAB2复合物之间的支架组装。因此，PMBT中断了TLR4介导的信号级联中TRAF6泛素化诱导的TAK1活性激活，导致TNF-α产生。然而,PMBT不会直接影响TNF-α对GalN致敏肝细胞的凋亡活性。最后,我们建议化学抑制Kupffer细胞中的TRAF6-TAK1轴,作为治疗由肠源性内毒素或革兰氏阴性菌感染引起的肝病的策略.
