Cell competition has been implicated in tissue development and homeostasis in several model organisms (Johnston, 2009
). Here, we describe a form of cell competition in the mammalian hematopoietic system that appears to be based on the different levels of stress accrued in individual cells within a population. This stress-induced competition is mediated by p53, and at least in the hematopoietic system, it is restricted to the HSPC compartment where outcompeted cells appear to acquire a senescence-like phenotype. p53 is known to function as a sensor for multiple forms of cell stress and DNA damage and because p53 activity is a graded function, it is well suited for reporting on different relative levels of cellular stress or damage.
Although both DDR and stress-induced competition are mediated by p53, our data suggest that they are separate phenomena. Classical p53-mediated DDR is a cell-autonomous process that is triggered based on the absolute level of DNA damage. In contrast, p53-mediated cell competition is based on the relative level of cell stress or DNA damage, and on the presence of competitor cells. In addition, the p53-DDR and stress-induced competition differ in other ways: the dose sensitivity and timing of stress-induced competition are distinct from the p53-DDR (for timing, compare to Figure S1A,B
; for dose sensitivity, compare to Figure S3C
). Furthermore, p53 function in stress-induced competition is predominant in HSPCs (), whereas p53-DDR (cell cycle arrest and apoptosis) in the hematopoietic system is much more pronounced in differentiated cells such as lymphocytes (Figure S3C
); (Anderson et al., 1977
; Down et al., 1995
; Fujikawa et al., 2000
; Kataoka and Sado, 1975
; Meijne et al., 1991
; Ploemacher et al., 1992
). Stress-based cell competition may be particularly relevant for stem cells, which contribute to differentiated cell pool and are long-lived to experience multiple insults. Recent evidence suggests that hematopoietic stem cells are inherently heterogeneous in terms of their life span, proliferation, and bias towards particular lineage, even when identified by the most up-to-date assays (Schroeder, 2010
). Our conclusion that p53-mediated competition involves stem cells is based not only on stringent surface phenotype but also on the long-term persistence (up to 6 months after IR; ) of the “winners” within multiple lineages and phenotypic HSC pool. In addition, it is possible that competition affects less primitive populations as well.
DNA damage is a type of stress that has important implications for HSPC activity, as it is heritable, often irreversible, and plays a central role in the decline of stem cell function over time (Marusyk and DeGregori, 2008
; Nijnik et al., 2007
; Reese et al., 2003
; Rossi et al., 2007a
; Sharpless and DePinho, 2007
), likely due to both accumulation of mutations (Marusyk and DeGregori, 2008
) and constraints on cell proliferation (Rossi et al., 2007b
How does p53 mediate stress-induced competition? In the best characterized case, in Drosophila
wing development, cell competition proceeds by killing of the neighboring cells (de la Cova et al., 2004
; Moreno and Basler, 2004
). Based on this evidence, it has been suggested that induction of death in the neighboring cells is a definitive feature of cell competition (Adachi-Yamada and O’Connor, 2004; Baker and Li, 2008
; Morata and Martin, 2007). However, in other systems where cell competition occurs, other outcomes may be more physiologically relevant than apoptosis, as is illustrated in recently found cases of stem cell competition in the Drosophila
germline (Issigonis et al., 2009; Jin et al., 2008; Rhiner et al., 2009; Sheng et al., 2009).
Consistent with this notion, during stress-induced competition, “loser” HSPCs underwent long-lasting, senescence-like changes, rather than apoptosis. Senescence induced in vitro
is characterized by permanent growth arrest and unresponsiveness to growth factors. However this may be an extreme case, whereas in vivo
cells may experience different degrees of senescence characterized by long-term growth inhibition. For example, naturally aged HSCs display signs of senescence while retaining the ability to proliferate (Rossi et al., 2007a
). We propose that p53-mediated stem cell-specific, senescence-like response to DNA damage operates as a “memory of past damage”: while still compatible with proliferation, it permanently marks HSPCs that have experienced DNA damage and thus promotes their gradual replacement by undamaged cells over time, if such cells are available.
HSPCs with a given level of p53 activity either “win” or “lose”, depending on p53 status of the competitor cells (). Moreover, the extent of “winning” depends on the ratio of the competitors to one another (), which does not affect the p53-DDR. This function of p53 is non-cell autonomous in a sense that it depends on the relative p53 activity in competing cells. It is likely to be based on p53-induced changes in expression of genes that control interactions of the cell with its environment (, “Group B” genes). Indeed, p53 regulates expression of a number of growth factors, cytokines, and genes involved in inflammation, cell adhesion, migration and angiogenesis (Levine et al., 2006
; Menendez et al., 2009
; Vousden and Prives, 2009
), which can impact the outcome of p53 activation. For example, regulation of expression of PAI-1 by p53 is essential for p53-mediated senescence (Kortlever et al., 2006
). More generally, non-autonomous role of p53 has been demonstrated in tumor-stroma interactions (Bar et al.
). However in most cases, the role of p53-mediated control of “Group B” genes in normal physiology is not well understood. We found that many genes in this category are expressed in a p53- and competitor-dependent manner in HSPCs (), implicating them in competitive interactions during stress response. Interestingly, these genes were expressed much later than the classical p53-DDR targets. Their regulation by p53 thus is likely to have distinct characteristics in terms of the dose and type of stress, cell-type specificity and kinetics, compared to the classical p53-DDR targets. Since many p53-DDR target genes encode basal components of apoptosis or cell cycle arrest, p53-DDR is more relevant immediately upon exposure to high levels of stress, as it will dominate over other events (). If the level of stress is insufficient to trigger irreversible cell cycle arrest or apoptosis, however, graded activities of the “Group B” p53-regulated genes () may influence cell fate through competitive interactions with other cells.
While selective expansion of HSCs with relatively lower p53 activity is beneficial for optimal tissue composition and therefore fitness, cells with p53 mutations can gain selective advantage using the same mechanism. Since p53-mediated competition occurs in long-lived HSCs, it may play an important role in the early stages of cancer development. Furthermore, as DNA damage levels increase with age (Sharpless and DePinho, 2007
), particularly in HSCs (Rossi et al., 2007a
), p53-mediated competition might also contribute to a sharp increase in human cancer incidence with age (Armitage and Doll, 1954
). If so, minimizing exposure to conditions that create selective pressure, increasing competitive status of normal cells, or otherwise blocking unwanted cancer cell competition, might provide new possibilities for tumor suppression.
Multiple steps of acquisition of mutations in tumor suppressors and oncogenes, each followed by clonal expansion, is a widely accepted model of tumorigenesis, which is thought to explain how rare and independent mutations can accumulate in a single clone (Rangarajan et al., 2004
). A physiological counterpart of clonal expansion, if there is one, has not been characterized. It is possible that both physiological and pathological clonal expansion is mediated by cell competition. Thus, understanding mechanisms of cell competition in mammals may provide new insights into cancer initiation.
Note added in proof
While this Article was in press, a related paper (Marusyk et al. 2010
) was published. The authors report irradiation induced long-term expansion of p53-deficient cells in BM chimeras, which can be blocked by addition of unirradiated WT competitors. The selection within HSPC compartment occurs in the absence of direct and immediate cytotoxicity.