Acetaminophen (APAP) overdose is the leading cause of acute liver failure (ALF) 
with limited treatment options. It is known that liver regeneration following 
APAP-induced ALF is a deciding factor in the final outcome. Previous studies 
from our laboratory using an incremental dose model involving a regenerating 
(300 mg/kg, APAP300) and a nonregenerating (600 mg/kg, APAP600) dose of APAP in 
mice have revealed several proregenerative pathways that regulate regeneration 
after APAP overdose. Here we report that DNA damage and repair mechanisms 
regulate initiation of liver regeneration following APAP overdose. Mice treated 
with nonregenerating APAP600 dose showed prolonged expression of pH2AX, a marker 
of the DNA double-strand break (DSB), compared with APAP300. In regenerating 
APAP300 dose-treated mice, H2AX was rapidly dephosphorylated at Tyr142, 
indicating timely DNA repair. Expression of several DNA repair proteins was 
substantially lower with APAP600. Poly(ADP) ribose polymerase (PARP) activation, 
involved in DNA repair, was significantly higher in the APAP300 group compared 
to the APAP600 group. Activation of p53, the major cell cycle checkpoint 
protein, was significantly higher with APAP600 as demonstrated by substantially 
higher expression of its target genes. Taken together, these data show that 
massive DNA DSB occurs in high-dose APAP toxicity, and lack of prompt DSB repair 
after APAP overdose leads to prolonged growth arrest and proliferative 
senescence, resulting in inhibited liver regeneration.