Tumor necrosis factor (TNF)- is a macrophage-derived proinflammatory cytokine implicated in hepatotoxicity. In the present studies, p55 TNF receptor 1 (TNFR1) knockout mice were used to assess the role of TNF- in acetaminophen-induced antioxidant defense. Treatment of wild type (WT) mice with acetaminophen (300 mg/kg) resulted in centrilobular hepatic necrosis and increased serum alanine transaminases. This was correlated with a rapid depletion of hepatic glutathione (GSH). Whereas in WT mice GSH levels returned to control after 6-12 h, in TNFR1-/- mice recovery was delayed for 48 h. Delayed induction of heme oxygenase-1 and reduced expression of CuZn superoxide dismutase were also observed in TNFR1-/- when compared to WT mice. This was associated with exaggerated hepatotoxicity. In WT mice, acetaminophen caused a time-dependent increase in AP-1 nuclear binding activity and in c-Jun expression. This response was significantly attenuated in TNFR1-/- mice. Constitutive NF-B binding activity was detectable in livers of both WT and TNFR1-/- mice. A transient decrease in this activity was observed 3 h after acetaminophen in WT mice, followed by an increase that was maximal after 6-12 h. In contrast, in TNFR1-/- mice, acetaminophen-induced decreases in NF-B activity were prolonged and did not return to control levels for 24 h. These data indicate that TNF- signaling through TNFR1 plays an important role in regulating the expression of antioxidants in this model. Reduced generation of antioxidants may contribute to the increased sensitivity of TNFR1-/- mice to acetaminophen.