Role of antioxidants and tumor necrosis factor (TNF)-alpha in acetaminophen-induced hepatotoxicity

Inflammatory mediators and oxidative stress contribute to pathogenesis of acetaminophen-induced liver injury. We speculate that antioxidants such heme oxygenase (HO)-1 and tumor necrosis factor (TNF)-α play important roles in protecting against acetaminophen-induced hepatotoxicity. Treatment of rats with acetaminophen resulted in a time-dependent induction of HO-1 in centrilobular regions of the liver. Pretreatment of rats with hemin, a potent inducer of HO-1, prevented acetaminophen-induced hepatotoxicity. This was associated with increased expression of ferritin and manganese superoxide dismutase (MnSOD). These findings suggest that HO-1 may limit acetaminophen-induced hepatotoxicity by up regulating additional antioxidants. Hepatic necrosis induced by acetaminophen is associated with a rapid depletion of glutathione (GSH), which returns towards control levels within 6 hours. In mice lacking TNF receptor 1 (TNFR1), recovery of GSH and induction of HO-1 and MnSOD was delayed or reduced. This was associated with an exaggerated hepatotoxicity. TNF-α binding to its receptor leads to activation of nuclear factor-κB (NF-κB) and activator protein-1 (AP-1) which regulate the production of antioxidants. In wild type mice, acetaminophen administration causes a time-dependent increase in AP-1 nuclear binding activity. This was significantly attenuated in TNFR1−/− mice. Similarly, in TNFR1−/− mice, there was a prolonged decrease in NF-κB activity. These studies show that TNF-α signaling through TNFR1 plays a critical role in activation of transcription factors and induction of antioxidants. TNF-α also plays a role in initiating hepatocyte proliferation and tissue repair. In TNFR1−/− mice, significantly reduced hepatocyte proliferation was observed 24 hours post acetaminophen when compared to wild type mice. This was correlated with decreased expression of cell cycle-associated proteins Cdk2, Cdk4, cyclin D 1 and cyclin A. PI-3K and its downstream target Akt are signaling molecules activated by TNF-α. Acetaminophen administration resulted in a rapid increase in expression of total and phospho-PI-3K, however this was delayed 6–12 hours in TNFR1−/− mice. Acetaminophen-induced increases in Akt expression were also greater in wild type mice. Taken together, the present studies demonstrate that HO-1 and TNFR1 play an important hepatoprotective role in acetaminophen-induced liver injury.