Regulation And Novelty Of Functional Roles Of P53 Gene In Proliferating Cells

As one of the most important tumor suppressors, the key role of p53 in tumor suppression is dramatically illustrated by the prevalence of TP53 mutations in cancer: it is estimated that more than 50% of all human tumors carry a TP53 mutation1,2. p53 gene encodes a 53KD transcription factor regulating hundreds of target genes in mammalian genomes3'4, and playing critical roles via inducing cell cycle arrest, apoptosis and senescence in response to various stress5,6.Extensive studies have been carried out on the regulation of p53. The post-translational regulation is believed to play a dominant role in p53 activation7-9. MDM2 and M DM4 are two critical negative regulators essential for suppression of p53 activity. However, the regulation of p53 gene expression at transcriptional level and its functional significance were not well understood especially in vivo.Here, we generated a BAC transgenic p53-lacZ reporter mouse model to examine the expression pattern of p53 in vivo. We found that ?-gal was highly expressed in proliferating compartments of multiple tissues in vivo. This observation suggested elevated p53 gene promoter activity in proliferating cells. To study whether the endogenous p53 gene is also expressed as the same pattern uncovered by the reporter mice, we examined p53 expression in vitro and in vivo.We first examined p53 expression in cultured cells. Consistent with previous studies, p53 mRNA was highly expressed in cycling cells in vitro. In addition, proliferating cells exhibited enhanced response upon stress. To study the activity of p53 gene promoter in vivo, p53 expression in multiple tissues of neonatal mice and adult mice was examined. Then, positive correlation between p53 gene promoter activity and cellular proliferation was suggested in vivo. To investigate the correlation between cell proliferation and p53 expression more exactly in vivo, we carried out partial hepatoctomy in mice. We found that p53 gene promoter was highly active in hepatocytes during liver regeneration. Interestingly and importantly, we found that cells with BrdU incorporation possess higher level of p53 after irradiation. Collectively, this strongly suggested that p53 is highly expressed in proliferating cells, which contributes to robust activation of p53 pathway upon stress.The robust activation of p53 may cause cell cycle arrest even cell death via cell-autonomous manner, especially in cycling cells. However, given the little chance for healthy people to get in touch with the reagent such as radioactive substance and chemo-therapy drugs, it may be much meaningful to ask the question what will happen if p53 is activated mildly.To study the physiology of mild activation of p53 in vivo, we established a pancreas-specific haplo-insufficiency of Mdm2 and Mdm4 mouse model by use of Ipf1-cre, a pancreas-specific cre line. It was found that Mdm2+/- Mdm4+/- cells were dramatically eliminated while the size, histology and functions of the pancreas is normal. This resembles a phenomenon first identified in flies called 'cell competition'.'Cell competition' refers more specifically to a phenomenon in which the growth rate of cells is altered by having different neighbors10. It is suggested to play a critical role in selecting the fittest cells and determining whether cells survive to contribute to the organism11. It is also suggested to be involved in tumorigenesis12,13. As a very important tumor suppressor and the central of stress-response network, whether p53 is involved in cellular competition in solid tissues especially during embryogenesis is unknown.Here, we found normal embryogenesis of Mdm2+/- Mdm4+/- embryos though they exhibited slight p53 pathway activation. However, with use of different assays, we found extensive elimination of Mdm2+/- Mdm4+/- cells in mosaic embryos during development. Our data here uncovered cell competition mediated by haplo-insufficiency of Mdm2 and Mdm4. This suggested selection of cells exhibiting lower level of p53 activity during mammalian embryogenesis.Collectively, for the first time it was suggested that p53 gene promoter was highly active in cycling cells, which contributes to robust activation of these cells upon stress. This robust activation of p53 pathway in cycling cells may induce cell cycle arrest and apoptosis via cell-autonomous manner, which may be critical for p53's functions as tumor suppressor. In another way, our finding here maybe helpful to understand why tissues undergoing rapid hemostasis are much sensitive to stress such as radio-therapy and chemo-therapy.In addition, we found that p53 was involved in cell competition during mouse embryogenesis. Firstly, we uncovered an example of cell competition in mammals here. Secondly, our finding here provided a novel model of p53 gene function, in which acute p53 activation may induce cell cycle arrest and apoptosis in cycling cells via cell-autonomous manner while mildactivation of p53 may induce cell competition through non-cell-autonomous manner. This phenomenon uncovered here suggested selective breeding of cells harboring lower p53 activity during embryogenesis and may be useful to explain the prefer selection of p53 mutated clones in human tumors.