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Hepatocyte proliferation represents an important part of tissue repair. In these studies, TNF receptor 1 (TNFR1) knockout mice were used to analyze the role of TNF-α in hepatocyte proliferation during acetaminophen-induced hepatotoxicity. Treatment of wild-type (WT) mice with acetaminophen (300 mg/kg) resulted in centrilobular hepatic necrosis. This was associated with proliferation of hepatocytes surrounding the damaged areas, which was evident at 24 h. The cell cycle regulatory proteins, cyclin D1 and cyclin A, were also up regulated in hepatocytes. In contrast, in TNFR1−/− mice, which exhibit exaggerated acetaminophen hepatotoxicity, hepatocyte proliferation, and expression of cyclin D1 and cyclin A, as well as the cyclin dependent kinases, Cdk4 and Cdk2, were reduced. The cyclin-dependent kinase inhibitor p21 was also induced in the liver following acetaminophen administration. This was greater in TNFR1−/− mice compared to WT mice. To investigate mechanisms mediating the reduced hepatic proliferative response of TNFR1−/− mice, we analyzed phosphatidyl inositol-3-kinase (PI-3K) signaling. In both WT and TNFR1−/− mice, acetaminophen caused a rapid increase in total PI-3K within 3 h. Acetaminophen also increased phosphorylated PI-3K, but this was delayed 6–12 h in TNFR1−/− mice. Expression of Akt, a downstream target of PI-3K, was increased in both WT and TNFR1−/− mice in response to acetaminophen. However, the increase was greater in WT mice. Acetaminophen-induced expression of phosphorylated STAT3, a key regulator of cytokine-induced hepatocyte proliferation, was also delayed in TNFR1−/− mice relative to WT. These data suggest that TNF-α signaling through TNFR1 is important in regulating hepatocyte proliferation following acetaminophen-induced tissue injury. Delayed cytokine signaling may account for reduced hepatocyte proliferation and contribute to exaggerated acetaminophen-induced hepatotoxicity in TNFR1−/− mice.