Function of p53 and its target genes

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1. Novel p53-target genes : Tumor suppressor p53 is a transcription factor that induces growth arrest and/or apoptosis in response to cellular stress. To identify novel p53-inducible genes, we compared the expression of genes in normal mouse embryo fibroblasts to p53-null cells by cDNA representational difference analysis (RDA). We have identified that expression of endogenous SCN3B (sodium channel subunit beta 3) and OPN (osteopontin) are upregulated in mouse embryonic fibroblasts by DNA damage in a p53-dependent manner. The p53-directed regulation of OPN expression suggests a novel model of p53 participation in immunosurveillance, involving interaction with the host immune system to prevent damaged cells from undergoing malignant transformation. The results presented above also suggest that SCN3B mediates a p53-dependent apoptotic pathway and may be a candidate for gene therapy combined with anticancer drugs.2. Biological functions of the p53 family member genes : p63 and p73 were re … More cently identified as members of the p53 gene family. In contrast to p53 however, p63 and p73 are rarely mutated in human cancers. To determine how p63 and p73 are involved in carcinogenesis and normal development, we attempted to identify target genes that are specifically regulated by p63 and/or p73 but not p53. We identified the JAG1 and JAG2 genes, encoding ligands for the Notch receptors, and the PEDF gene are direct target of p63 and p73. We also found that IL4 receptor alpha is upregulated by p73. These findings show an association between the p53 family genes and Notch signaling and suggest a potential molecular mechanism for the involvement of the p53 family genes in normal development. Our data also suggest that IL-4Ralpha could mediate, in part, certain immune responses and p73-dependent cell death.3. Adenovirus-mediated transfer of the p53 family genes, p73 and p63 induces cell cycle arrest and apoptosis in human cancer cell lines : p53 gene therapy is being tested clinically for the treatment of human cancer, however, some cancer models (in vivo and in vitro) are resistant to p53. To explore the potential use of two p53 homologues, p73 and p63, in cancer gene therapy, we introduced p53, p73 and p63 into colorectal cancer cell lines via adenoviral vectors, and compared their effects on cell growth. Among 10 cell lines tested, six cell lines displayed a similar response following transduction of p53, p73beta or p63gamma ; two lines underwent cell-cycle arrest, three lines exhibited apoptosis and one line showed no-effect following transduction. The effect on cell-cycle progression was variable in the other four cell lines. Interestingly, three cell lines were resistant to p53-mediated apoptosis, including two lines having endogenous wild-type p53 alleles, but underwent apoptosis after transduction of p73beta or p63gamma. Transduction of p73beta and p63gamma also reduced the tumorigenicity of two colorectal cancer cells in vivo. These results suggest that adenovirus-mediated p73beta and p63gamma transfer are potential novel approaches for the treatment of human cancers, particularly for tumors that are resistant to p53 gene therapy. Less