The transcription factor p53 has been most extensively studied in its capacity to mediate tumor suppression.(27–29) Expression of p53 becomes upregulated upon various stress responses including irradiation-mediated DNA damage. To investigate the involvement of p53 in cell competition in mammals, Bondar and Medzhitov(30) used genetic mosaic mouse models and bone marrow chimeras with different levels of p53 activity. First, the authors examined the effect of stress on the competitive status of cells in the hematopoietic system using a low dose of ionizing radiation (IR). The bone marrow from irradiated mice was mixed with that from untreated mice, and the mixture of bone marrow was transferred into lethally irradiated recipient mice. Sixteen weeks later, the total number and the percentage of hematopoietic stem and progenitor cells (HSPCs) were analyzed, and the result showed that hematopoietic cells from untreated mice outcompeted those from irradiated mice. The authors further showed that HSPCs from the untreated mice replaced those from irradiated wild-type mice, but not from irradiated p53+/− mice, suggesting that IR-induced competition depends on the p53 level in the competing cells. To further characterize p53-mediated cell competition, the authors created an inducible genetic mouse model expressing the oncogenic p53 mutant R172H (mp53) in a mosaic manner. The R172H mutant suppresses endogenous p53 activity in a dominant-negative fashion by forming mixed hetero-tetramers with wild-type p53 and reducing p53 binding to the p53-responsive element in its target genes.(31,32) mp53 mice were crossed to Cre-ER mice, in which Cre can be inducibly activated by tamoxifen injection. Because the Cre-mediated recombination in the Cre-ER mice occurred only in a fraction of cells, tamoxifen injection resulted in the generation of genetic mosaic. Analyses of the percentage of mp53-expressing HSPCs showed that expression of mp53 does not confer a selective advantage under homeostatic conditions. In contrast, after IR, the percentage of mp53-expressing HSPCs substantially increased. There was no significant difference in total HSPC numbers between wild-type and mosaic mice after IR, suggesting that mp53-expressing cells replaced wild-type cells, rather than simply expanding. The authors also showed that this IR-induced, p53-mediated cell competition occurs predominantly in the HSPC compartment, not in differentiated lymphocyte compartments in the hematopoietic system. Interestingly, the classical p53-mediated DNA damage response did not contribute to the cell competition, and apoptosis was not involved in the process, which is different from cell competition in Drosophila where apoptosis plays a crucial role. Instead, several positive markers of cell proliferation, such as Ki67, cyclin B1, and cyclin A2, were expressed at higher levels in mp53 cells than in wild-type cells from irradiated mosaic mice. In contrast, genes encoding negative cell proliferation regulators, p57kip and necdin, displayed the opposite pattern of expression. Importantly, these gene expression changes were dependent on the presence of the competitor cells, suggesting that one of the underlying mechanisms on p53-mediated cell competition is through the non-cell-autonomous effect of p53 on cell proliferation. In addition, the authors observed senescence-like phenotype in outcompeted HSPCs. p16INK4a is a marker and mediator of senescence of hematopoietic stem cells.(33,34) Expression of p16INK4a was higher in outcompeted wild-type cells than in mp53 cells from irradiated mosaic mice. Similarly, P-selectin and Sdpr, which are upregulated in aged hematopoietic stem cells,(35,36) were upregulated in outcompeted wild-type cells and decreased in mp53 cells. These data suggest that induction of senescence also plays a role in p53-mediated cell competition.
In addition to this study, there are other reports showing the occurrence of cell competition in mice.(37,38) Thus, taken together with data from cultured cells, it is likely that cell competition is a general cellular process in mammals. To examine whether cell competition occurs during various stages of carcinogenesis, further in vivo studies need to be carried out. In conventional mouse model systems, tissue-specific promoters are used to knock-in or knock-out specific genes within the entire tissues. These methods are suitable to examine the effect of genetic changes on cell-autonomous processes, but not to analyze interactions between normal and transformed cells. Therefore, novel mouse model systems need to be established to induce genetic changes in a mosaic manner within the epithelium. In addition, apical extrusion or cell death of transformed cells at the earlier stage of carcinogenesis might have been overlooked in previous studies, as cells extruded into the apical epithelial lumen encounter physically harsh environments such as flow of urine or stool and are likely to leave the epithelium shortly after extrusion. Thus, in vivo live image analyses are also required to carefully examine these phenomena.