Apoptosis is a morphologically distinct form of programmed cell death that plays a major role in cancer treatments. This cellular suicide program is known to be regulated by many di erent signals from both intracellular and extracellular stimuli. Here we report that p53 suppressed expression of the cellular FLICE-inhibitory protein (FLIP) that potentially blocks apoptotic signaling in human colon cancer cell lines expressing mutated and wild-type p53. In contrast, the expression of the death receptor KILLER/DR5 (TRAIL-R2) had no e ect on FLIP expression, although exogenous p53 is known to induce KILLER/DR5 expression. In line with these observations, FLIP-negative cancer cells were sensitive to both p53-and KILLER/DR5-mediated apoptosis, whereas cells containing high levels of FLIP underwent apoptotic cell death when triggered by ectopic p53 expression but not by KILLER/DR5 expression. Treating the cells with a speci®c inhibitor of the proteasome inhibited the decrease of FLIP by p53, suggesting that p53 enhances the degradation of FLIP via a ubiquitinproteasome pathway. Thus, the data indicate that p53-mediated downregulation of FLIP may explain the potent sensitization of human cancer cells to the apoptotic suicide program induced by wild-type p53 gene transfer. Oncogene (2001) 20, 5225 ± 5231.

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 kB (NF-kB) plays an essential role in preventing apoptotic cell death. In this study, to better understand the mechanism reponsible for the p53-mediated apoptosis, the eect of wild-type p53 (wt-p53) gene transfer on nuclear expression of NF-kB was determined in human colon cancer cell lines. A Western blot analysis of nuclear extracts demonstrated that NF-kB 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 IkBa as well as its binding ability to NF-kB, thus markedly reducing the amount of NF-kB that translocated to the nucleus. The decrease in nuclear NF-kB protein correlated with the decreased NF-kB constitutive activity measured by electrophoretic mobility shift assay. Furthermore, parental cells transfected with NF-kB were better protected from cell death induced by the wt-p53 gene transfer. We also found that the wtp53 gene transfer was synergistic with aspirin (acetylsalicylic acid) in inhibiting NF-kB constitutive activity, resulting in enhanced apoptotic cell death. These results suggest that the inhibition of NF-kB activity is a plausible mechanism for apoptosis induced by the wt-p53 gene transfer in human colon cancer cells and that anti-NF-kB reagent aspirin could make these cells more susceptible to apoptosis. Oncogene (2000) 19, 726 ± 736.

No abstract

Clinical trials of adenoviral p53 gene therapy provide the evidence that the bystander effect induced by the wild-type p53 gene transfer on adjacent tumor cells contributes to tumor progression; its mechanism, however, remains uncharacterized. We report in this work that injection of adenovirus expressing the human wild-type p53 gene (Ad5CMVp53) into established human colorectal tumors in nu/nu mice resulted in CD95 ligand (CD95L) overexpression, followed by a massive neutrophil infiltration. Culture supernatants of human colorectal cancer cells infected with Ad5CMVp53 exhibited a potent chemotactic activity against murine polymorphonuclear neutrophils, which could be abolished by the anti-CD95L mAb (NOK-1). In vivo cell depletion experiments indicated that neutrophils were in part responsible for the antitumor effect of the Ad5CMVp53 infection. Our data directly suggest that overexpression of CD95L by the wild-type p53 gene transfer induces neutrophil infiltration into human colorectal tumors, which may play a critical role in the bystander effect of p53 gene therapy.