The tumor suppressor gene p53 is a potent transcriptional regulator of genes which are involved in many cellular activities including cell cycle arrest, apoptosis, and angiogenesis. Recent studies have demonstrated that the activation of the transcriptional factor nuclear factor kappaB (NF-kappaB) plays an essential role in preventing apoptotic cell death. In this study, to better understand the mechanism responsible for the p53-mediated apoptosis, the effect of wild-type p53 (wt-p53) gene transfer on nuclear expression of NF-kappaB was determined in human colon cancer cell lines. A Western blot analysis of nuclear extracts demonstrated that NF-kappaB protein levels in the nuclei were suppressed by the transient expression of the wt-p53 in a dose-dependent manner. Transduced wt-p53 expression increased the cytoplasmic expression of I kappaB alpha as well as its binding ability to NF-kappaB, thus markedly reducing the amount of NF-kappaB that translocated to the nucleus. The decrease in nuclear NF-kappaB protein correlated with the decreased NF-kappaB constitutive activity measured by electrophoretic mobility shift assay. Furthermore, parental cells transfected with NF-kappaB were better protected from cell death induced by the wt-p53 gene transfer. We also found that the wt-p53 gene transfer was synergistic with aspirin (acetylsalicylic acid) in inhibiting NF-kappaB constitutive activity, resulting in enhanced apoptotic cell death. These results suggest that the inhibition of NF-kappaB activity is a plausible mechanism for apoptosis induced by the wt-p53 gene transfer in human colon cancer cells and that anti-NF-kappaB reagent aspirin could make these cells more susceptible to apoptosis.