AbstractIn the present study, experiments using presynchronization culture cells demonstrated that benzyl ITC (BITC), previously isolated from a tropical papaya fruit extract, induced the cytotoxic effect preferentially in the proliferating human colon CCD‐18Co cells to the quiescent ones. Quiescent CCD‐18Co cells were virtually unaffected by BITC and marginal cytotoxicity was observed at 15 μM. We observed that BITC dramatically induced the p53 phosphorylation and stabilization only in the quiescent (G0/G1 phase‐arrested) cells, but not significantly in the proliferating human colon CCD‐18Co cells when compared with quiescent ones. We also observed ataxia telangiectasia‐mutated (ATM) phosphorylation in the quiescent cells. The BITC‐induced p53 phosphorylation was counteracted by caffeine treatment, implying the involvement of an ATM/ataxia telangiectasia and Rad3‐related kinase signaling pathway. Moreover, downregulation of p53 by a siRNA resulted in the enhancement of susceptibility to undergo apoptosis by BITC. We also showed here that depletion of p53 abrogated G0/G1 arrest accompanied by the declined expression of p21waf1/cip1 and p27kip1 in CCD‐18Co cells. In conclusion, we identified p53 as a potential negative regulator of the apoptosis induction by BITC in the normal colon CCD‐18Co cells through the inhibition of cell‐cycle progression at the G0/G1 phase. © 2006 Wiley‐Liss, Inc.