Supercompetitor status of Drosophila Myc cells requires p53 as a fitness sensor to reprogram metabolism and promote viability

Tumor suppressor gene OSCP1/NOR1 regulates apoptosis, proliferation, differentiation, and ROS generation during eye development of Drosophila melanogaster

OSCP1/NOR1 (organic solute carrier partner 1/oxidored nitrodomain-containing protein 1) is a known tumor suppressor protein. OSCP1 has been reported to mediate transport of various organic solutes into cells; however, its role during development has not yet been addressed. Here we report the results of studies on dOSCP1 (the Drosophila ortholog of hOSCP1) to elucidate the role of OSCP1/NOR1 during development. Knockdown of dOSCP1 in the eye imaginal discs induced a rough-eye phenotype in adult flies. This phenotype resulted from induction of caspase-dependent apoptosis followed by a compensatory cell proliferation and generation of reactive oxygen species in eye imaginal discs. The induction of apoptosis appears to be associated with down-regulation of the anti-apoptotic Buffy gene and up-regulation of the pro-apoptotic Debcl gene. These effects of knockdown of dOSCP1 lead to mitochondrial fragmentation, degradation, and a shortfall in ATP production. We also found that knockdown of dOSCP1 causes a defect in cone cell and pigment cell differentiation in pupal retinae. Moreover, mutations in epidermal growth factor receptor pathway-related genes, such as Spitz and Drk, enhanced the rough-eye phenotype induced by dOSCP1 knockdown. These results suggest that dOSCP1 positively regulates the epidermal growth factor receptor signaling pathway. Overall, our findings indicate that dOSCP1 plays multiple roles during eye development in Drosophila.

Non-autonomous overgrowth by oncogenic niche cells: Cellular cooperation and competition in tumorigenesis

Tumor progression is classically viewed as the Darwinian evolution of subclones that sequentially acquire genetic mutations and autonomously overproliferate. However, growing evidence suggests that tumor microenvironment and subclone heterogeneity contribute to non‐autonomous tumor progression. Recent Drosophila studies revealed a common mechanism by which clones of genetically altered cells trigger non‐autonomous overgrowth. Such “oncogenic niche cells” (ONCs) do not overgrow but instead stimulate neighbor overgrowth and metastasis. Establishment of ONCs depends on competition and cooperation between heterogeneous cell populations. This review characterizes diverse ONCs identified in Drosophila and describes the genetic basis of non‐autonomous tumor progression. Similar mechanisms may contribute to mammalian cancer progression and recurrence.

Cell Competition Modifies Adult Stem Cell and Tissue Population Dynamics in a JAK-STAT-Dependent Manner

Throughout their lifetime, cells may suffer insults that reduce their fitness and disrupt their function, and it is unclear how these potentially harmful cells are managed in adult tissues. We address this question using the adult Drosophila posterior midgut as a model of homeostatic tissue and ribosomal Minute mutations to reduce fitness in groups of cells. We take a quantitative approach combining lineage tracing and biophysical modeling and address how cell competition affects stem cell and tissue population dynamics. We show that healthy cells induce clonal extinction in weak tissues, targeting both stem and differentiated cells for elimination. We also find that competition induces stem cell proliferation and self-renewal in healthy tissue, promoting selective advantage and tissue colonization. Finally, we show that winner cell proliferation is fueled by the JAK-STAT ligand Unpaired-3, produced by Minute^−/+ cells in response to chronic JNK stress signaling.

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In the adult fly gut, wild-type cells outcompete subfit Minute^−/+ cells

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Both stem and differentiated Minute^−/+ cells are eliminated by cell competition

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Cell competition promotes proliferation and self-renewal of normal stem cells

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The growth of healthy cells is boosted by JAK-STAT signaling

Spontaneous Cell Competition in Immortalized Mammalian Cell Lines

Cell competition is a form of cell-cell interaction by which cells compare relative levels of fitness, resulting in the active elimination of less-fit cells, “losers,” by more-fit cells, “winners.” Here, we show that in three routinely-used mammalian cell lines – U2OS, 3T3, and MDCK cells – sub-clones arise stochastically that exhibit context-dependent competitive behavior. Specifically, cell death is elicited when winner and loser sub-clones are cultured together but not alone. Cell competition and elimination in these cell lines is caspase-dependent and requires cell-cell contact but does not require de novo RNA synthesis. Moreover, we show that the phenomenon involves differences in cellular metabolism. Hence, our study demonstrates that cell competition is a common feature of immortalized mammalian cells in vitro and implicates cellular metabolism as a mechanism by which cells sense relative levels of “fitness.”

p53 in survival, death and metabolic health: a lifeguard with a licence to kill

The function of p53 as a tumour suppressor has been attributed to its ability to promote cell death or permanently inhibit cell proliferation. However, in recent years, it has become clear that p53 can also contribute to cell survival. p53 regulates various metabolic pathways, helping to balance glycolysis and oxidative phosphorylation, limiting the production of reactive oxygen species, and contributing to the ability of cells to adapt to and survive mild metabolic stresses. Although these activities may be integrated into the tumour suppressive functions of p53, deregulation of some elements of the p53-induced response might also provide tumours with a survival advantage.

The p53 control of apoptosis and proliferation: lessons from Drosophila

The canonical role of p53 in preserving genome integrity and limiting carcinogenesis has been well established. In the presence of acute DNA-damage, oncogene deregulation and other forms of cellular stress, p53 orchestrates a myriad of pleiotropic processes to repair cellular damages and maintain homeostasis. Beside these well-studied functions of p53, recent studies in Drosophila have unraveled intriguing roles of Dmp53 in promoting cell division in apoptosis-induced proliferation, enhancing fitness and proliferation of the winner cell in cell competition and coordinating growth at the organ and organismal level in the presence of stress. In this review, we describe these new functions of Dmp53 and discuss their relevance in the context of carcinogenesis.

Cardiac deficiency of single cytochrome oxidase assembly factor scox induces p53-dependent apoptosis in a Drosophila cardiomyopathy model

The heart is a muscle with high energy demands. Hence, most patients with mitochondrial disease produced by defects in the oxidative phosphorylation (OXPHOS) system are susceptible to cardiac involvement. The presentation of mitochondrial cardiomyopathy includes hypertrophic, dilated and left ventricular noncompaction, but the molecular mechanisms involved in cardiac impairment are unknown. One of the most frequent OXPHOS defects in humans frequently associated with cardiomyopathy is cytochrome c oxidase (COX) deficiency caused by mutations in COX assembly factors such as Sco1 and Sco2. To investigate the molecular mechanisms that underlie the cardiomyopathy associated with Sco deficiency, we have heart specifically interfered scox expression, the single Drosophila Sco orthologue. Cardiac-specific knockdown of scox reduces fly lifespan, and it severely compromises heart function and structure, producing dilated cardiomyopathy. Cardiomyocytes with low levels of scox have a significant reduction in COX activity and they undergo a metabolic switch from OXPHOS to glycolysis, mimicking the clinical features found in patients harbouring Sco mutations. The major cardiac defects observed are produced by a significant increase in apoptosis, which is dp53-dependent. Genetic and molecular evidence strongly suggest that dp53 is directly involved in the development of the cardiomyopathy induced by scox deficiency. Remarkably, apoptosis is enhanced in the muscle and liver of Sco2 knock-out mice, clearly suggesting that cell death is a key feature of the COX deficiencies produced by mutations in Sco genes in humans.

Apoptotic mechanisms during competition of ribosomal protein mutant cells: roles of the initiator caspases Dronc and Dream/Strica

Heterozygosity for mutations in ribosomal protein genes frequently leads to a dominant phenotype of retarded growth and small adult bristles in Drosophila (the Minute phenotype). Cells with Minute genotypes are subject to cell competition, characterized by their selective apoptosis and removal in mosaic tissues that contain wild-type cells. Competitive apoptosis was found to depend on the pro-apoptotic reaper, grim and head involution defective genes but was independent of p53. Rp/+ cells are protected by anti-apoptotic baculovirus p35 expression but lacked the usual hallmarks of 'undead' cells. They lacked Dronc activity, and neither expression of dominant-negative Dronc nor dronc knockdown by dsRNA prevented competitive apoptosis, which also continued in dronc null mutant cells or in the absence of the initiator caspases dredd and dream/strica. Only simultaneous knockdown of dronc and dream/strica by dsRNA was sufficient to protect Rp/+ cells from competition. By contrast, Rp/Rp cells were also protected by baculovirus p35, but Rp/Rp death was dronc-dependent, and undead Rp/Rp cells exhibited typical dronc-dependent expression of Wingless. Independence of p53 and unusual dependence on Dream/Strica distinguish competitive cell death from noncompetitive apoptosis of Rp/Rp cells and from many other examples of cell death.

Of flies, mice, and men: evolutionarily conserved tissue damage responses and aging

Studies in flies, mice, and human models have provided a conceptual framework for how paracrine interactions between damaged cells and the surrounding tissue control tissue repair. These studies have amassed evidence for an evolutionarily conserved secretory program that regulates tissue homeostasis. This program coordinates cell survival and proliferation during tissue regeneration and repair in young animals. By virtue of chronic engagement, however, it also contributes to the age-related decline of tissue homeostasis leading to degeneration, metabolic dysfunction, and cancer. Here, we review recent studies that shed light on the nature and regulation of this evolutionarily conserved secretory program.

Cell competition: winning out by losing notch

Cell competition where 'loser' cells are eliminated by neighbors with higher fitness is a widespread phenomenon in development. However, a growing body of evidence argues cells with somatic mutations compete with their wild type counterparts in the earliest stages of cancer development. Recent studies have begun to shed light on the molecular and cellular mechanisms that alter the competitiveness of cells carrying somatic mutations in adult tissues. Cells with a 'winner' phenotype create clones which may expand into extensive fields of mutant cells within normal appearing epithelium, favoring the accumulation of further genetic alterations and the evolution of cancer. Here we focus on how mutations which disrupt the Notch signaling pathway confer a 'super competitor' status on cells in squamous epithelia and consider the broader implications for cancer evolution.

An ancient defense system eliminates unfit cells from developing tissues during cell competition

Developing tissues that contain mutant or compromised cells present risks to animal health. Accordingly, the appearance of a population of suboptimal cells in a tissue elicits cellular interactions that prevent their contribution to the adult. Here we report that this quality control process, cell competition, uses specific components of the evolutionarily ancient and conserved innate immune system to eliminate Drosophila cells perceived as unfit. We find that Toll-related receptors (TRRs) and the cytokine Spätzle (Spz) lead to NFκB-dependent apoptosis. Diverse "loser" cells require different TRRs and NFκB factors and activate distinct pro-death genes, implying that the particular response is stipulated by the competitive context. Our findings demonstrate a functional repurposing of components of TRRs and NFκB signaling modules in the surveillance of cell fitness during development.

Cell competition in vertebrate organ size regulation

The study of animal organ size determination has provided evidence of the existence of organ-intrinsic mechanisms that 'sense' and adjust organ growth. Cell competition, a form of cell interaction that equalizes cell population growth, has been proposed to play a role in organ size regulation. Cell competition involves a cell-context dependent response triggered by perceived differences in cell growth and/or proliferation rates, resulting in apoptosis in growth-disadvantaged cells and compensatory expansion of the more 'fit' cells. The mechanisms that allow cells to compare growth are not yet understood, but a number of genes and pathways have been implicated in cell competition. These include Myc, the members of the Hippo, JAK/STAT and WNT signaling pathways, and the Dlg/Lgl/Scrib and the Crb/Std/PatJ membrane protein complexes. Cell competition was initially characterized in the Drosophila imaginal disc, but several recent studies have shown that cell competition occurs in mouse embryonic stem cells and in the embryonic epiblast, where it plays a role in the regulation of early embryo size. In addition, competition-like behavior has been described in the adult mouse liver and the hematopoietic stem cell compartment. These data indicate that cell competition plays a more universal role in organ size regulation. In addition, as some authors have suggested that similar types of competitive behavior may operate in during tumorigenesis, there may be additional practical reasons for understanding this fundamental process of intercellular communication.

Drosophila Myc: A master regulator of cellular performance

The identification of the Drosophila homolog of the human MYC oncogene has fostered a series of studies aimed to address its functions in development and cancer biology. Due to its essential roles in many fundamental biological processes it is hard to imagine a molecular mechanism in which MYC function is not required. For this reason, the easily manipulated Drosophila system has greatly helped in the dissection of the genetic and molecular pathways that regulate and are regulated by MYC function. In this review, we focus on studies of MYC in the fruitfly with particular emphasis on metabolism and cell competition, highlighting the contributions of this model system in the last decade to our understanding of MYC's complex biological nature. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Socializing with MYC: cell competition in development and as a model for premalignant cancer

Studies in Drosophila and mammals have made it clear that genetic mutations that arise in somatic tissues are rapidly recognized and eliminated, suggesting that cellular fitness is tightly monitored. During development, damaged, mutant, or otherwise unfit cells are prevented from contributing to the tissue and are instructed to die, whereas healthy cells benefit and populate the animal. This cell selection process, known as cell competition, eliminates somatic genetic heterogeneity and promotes tissue fitness during development. Yet cell competition also has a dark side. Super competition can be exploited by incipient cancers to subvert cellular cooperation and promote selfish behavior. Evidence is accumulating that MYC plays a key role in regulation of social behavior within tissues. Given the high number of tumors with deregulated MYC, studies of cell competition promise to yield insight into how the local environment yields to and participates in the early stages of tumor formation.