Toll Like Receptor 3 Plays a Critical Role in the Progression and Severity of Acetaminophen-Induced Hepatotoxicity

Toll-like receptor (TLR) activation has been implicated in acetaminophen (APAP)-induced hepatotoxicity. Herein, we hypothesize that TLR3 activation significantly contributed to APAP-induced liver injury. In fasted wildtype (WT) mice, APAP caused significant cellular necrosis, edema, and inflammation in the liver, and the de novo expression and activation of TLR3 was found to be necessary for APAP-induced liver failure. Specifically, liver tissues from similarly fasted TLR3-deficient (tlr3−/−) mice exhibited significantly less histological and biochemical evidence of injury after APAP challenge. Similar protective effects were observed in WT mice in which TLR3 was targeted through immunoneutralization at 3 h post-APAP challenge. Among three important death ligands (i.e. TNFα, TRAIL, and FASL) known to promote hepatocyte death after APAP challenge, TNFα was the only ligand that was significantly reduced in APAP-challenged tlr3−/− mice compared with APAP-challenged WT controls. In vivo studies demonstrated that TLR3 activation contributed to TNFα production in the liver presumably via F4/80+ and CD11c+ immune cells. In vitro studies indicated that there was cooperation between TNFα and TLR3 in the activation of JNK signaling in isolated and cultured liver epithelial cells (i.e. nMuLi). Moreover, TLR3 activation enhanced the expression of phosphorylated JNK in APAP injured livers. Thus, the current study demonstrates that TLR3 activation contributes to APAP-induced hepatotoxicity.

Toll样受体（Toll-like receptor, TLR）的激活已被证实参与对乙酰氨基酚（acetaminophen, APAP）诱导的肝毒性过程。本研究提出假说：Toll样受体3（Toll-like receptor 3, TLR3）的激活在对乙酰氨基酚诱导的肝损伤中发挥了关键作用。在禁食的野生型（wildtype, WT）小鼠中，对乙酰氨基酚可诱导其肝脏出现显著的细胞坏死、水肿与炎症反应，且研究发现，Toll样受体3的从头表达与激活是对乙酰氨基酚诱导肝衰竭的必要前提。具体而言，经同等条件禁食的Toll样受体3基因敲除（tlr3−/−）小鼠，在接受对乙酰氨基酚攻毒后，其肝脏组织在组织学与生化指标上的损伤证据显著更少。在对乙酰氨基酚攻毒后3小时，通过免疫中和手段抑制Toll样受体3活性的野生型小鼠中，同样观察到了此类保护性效应。已知在对乙酰氨基酚攻毒后可促进肝细胞死亡的三类重要死亡配体（即肿瘤坏死因子α（tumor necrosis factor alpha, TNFα）、肿瘤坏死因子相关凋亡诱导配体（TNF-related apoptosis-inducing ligand, TRAIL）与Fas配体（Fas ligand, FASL））中，与经对乙酰氨基酚攻毒的野生型对照组小鼠相比，攻毒后的tlr3−/−小鼠体内仅肿瘤坏死因子α的表达水平出现显著下调。体内实验证实，Toll样受体3的激活可促进肝脏内肿瘤坏死因子α的产生，这一过程推测通过F4/80阳性与CD11c阳性免疫细胞介导。体外实验显示，在分离培养的肝脏上皮细胞（即nMuLi细胞）中，肿瘤坏死因子α与Toll样受体3可协同激活c-Jun氨基末端激酶（c-Jun N-terminal kinase, JNK）信号通路。此外，在对乙酰氨基酚损伤的肝脏组织中，Toll样受体3的激活可增强磷酸化JNK的表达水平。综上，本研究证实Toll样受体3的激活参与了对乙酰氨基酚诱导的肝毒性过程。