Cell competition: Emerging mechanisms to eliminate neighbors

Autocrine glutamate signaling drives cell competition in Drosophila

Cell competition is an evolutionarily conserved quality control process that eliminates suboptimal or potentially dangerous cells. Although differential metabolic states act as direct drivers of competition, how these are measured across tissues is not understood. Here, we demonstrate that vesicular glutamate transporter (VGlut) and autocrine glutamate signaling are required for cell competition and Myc-driven super-competition in the Drosophila epithelia. We find that the loss of glutamate-stimulated VGlut>NMDAR>CaMKII>CrebB signaling triggers loser status and cell death under competitive settings via the autocrine induction of TNF. This in turn drives TNFR>JNK activation, triggering loser cell elimination and PDK/LDH-dependent metabolic reprogramming. Inhibiting caspases or preventing loser cells from transferring lactate to their neighbors nullifies cell competition. Further, in a Drosophila model for premalignancy, Myc-overexpressing clones co-opt this signaling circuit to acquire super-competitor status. Targeting glutamate signaling converts Myc "super-competitor" clones into "losers," highlighting new therapeutic opportunities to restrict the evolution of fitter clones.

Peritumoral SPARC expression induced by exosomes from nasopharyngeal carcinoma infected Epstein-Barr virus: A poor prognostic marker

Epstein-Barr virus (EBV)-associated nasopharyngeal carcinoma (NPC) cells have high metastatic potential. Recent research has revealed that the interaction of between tumor cells and the surrounding stroma plays an important role in tumor invasion and metastasis. In this study, we showed the prognostic value of expression of SPARC, an extracellular matrix protein with multiple cellular functions, in normal adjacent tissues (NAT) surrounding NPC. In the immunohistochemical analysis of 51 NPC biopsy specimens, SPARC expression levels were significantly elevated in the NAT of EBER (EBV-encoded small RNA)-positive NPC compared to that in the NAT of EBER-negative NPC. Moreover, increased SPARC expression in NAT was associated with a worsening of overall survival. The enrichment analysis of RNA-seq of publicly available NPC and NAT surrounding NPC data showed that high SPARC expression in NPC was associated with epithelial mesenchymal transition promotion, and there was a dynamic change in the gene expression profile associated with interference of cellular proliferation in NAT, including SPARC expression. Furthermore, EBV-positive NPC cells induce SPARC expression in normal nasopharyngeal cells via exosomes. Induction of SPARC in cancer-surrounding NAT cells reduced intercellular adhesion in normal nasopharyngeal structures and promoted cell competition between cancer cells and normal epithelial cells. These results suggest that epithelial cells loosen their own binding with the extracellular matrix as well as stromal cells, facilitating the invasion of tumor cells into the adjacent stroma by activating cell competition. Our findings reveal a new mechanism by which EBV creates a pro-metastatic microenvironment by upregulating SPARC expression in NPC.

In vivo evidence for homeostatic regulation of ribosomal protein levels in Drosophila

The ribosome is a molecular machine essential for protein synthesis, which is composed of approximately 80 different ribosomal proteins (Rps). Studies in yeast and cell culture systems have revealed that the intracellular level of Rps is finely regulated by negative feedback mechanisms or ubiquitin-proteasome system, which prevents over- or under-abundance of Rps in the cell. However, in vivo evidence for the homeostatic regulation of intracellular Rp levels has been poor. Here, using Drosophila genetics, we show that intracellular Rp levels are regulated by proteasomal degradation of excess Rps that are not incorporated into the ribosome. By establishing an EGFP-fused Rp gene system that can monitor endogenously expressed Rp levels, we found that endogenously expressed EGFP-RpS20 or -RpL5 is eliminated from the cell when RpS20 or RpL5 is exogenously expressed. Notably, the level of endogenously expressed Hsp83, a housekeeping gene, was not affected by exogenous expression of Hsp83, suggesting that the strict negative regulation of excess protein is specific for intracellular Rps. Further analyses revealed that the maintenance of cellular Rp levels is not regulated at the transcriptional level but by proteasomal degradation of excess free Rps as a protein quality control mechanism. Our observations provide not only the in vivo evidence for the homeostatic regulation of Rp levels but also a novel genetic strategy to study in vivo regulation of intracellular Rp levels and its role in tissue homeostasis via cell competition.Key words: ribosomal protein, proteasomal degradation, Drosophila.

Cell competition: emerging signaling and unsolved questions

Multicellular communities have an intrinsic mechanism that optimizes their structure and function via cell-cell communication. One of the driving forces for such self-organization of the multicellular system is cell competition, the elimination of viable unfit or deleterious cells via cell-cell interaction. Studies in Drosophila and mammals have identified multiple mechanisms of cell competition caused by different types of mutations or cellular changes. Intriguingly, recent studies have found that different types of "losers" of cell competition commonly show reduced protein synthesis. In Drosophila, the reduction in protein synthesis levels in loser cells is caused by phosphorylation of the translation initiation factor eIF2α via a bZip transcription factor Xrp1. Given that a variety of cellular stresses converge on eIF2α phosphorylation and thus global inhibition of protein synthesis, cell competition may be a machinery that optimizes multicellular fitness by removing stressed cells. In this review, we summarize and discuss emerging signaling mechanisms and critical unsolved questions, as well as the role of protein synthesis in cell competition.

Fibroblasts inhibit osteogenesis by regulating nuclear-cytoplasmic shuttling of YAP in mesenchymal stem cells and secreting DKK1

BACKGROUND

Fibrous scars frequently form at the sites of bone nonunion when attempts to repair bone fractures have failed. However, the detailed mechanism by which fibroblasts, which are the main components of fibrous scars, impede osteogenesis remains largely unknown.

RESULTS

In this study, we found that fibroblasts compete with osteogenesis in both human bone nonunion tissues and BMP2-induced ectopic osteogenesis in a mouse model. Fibroblasts could inhibit the osteoblastic differentiation of mesenchymal stem cells (MSCs) via direct and indirect cell competition. During this process, fibroblasts modulated the nuclear-cytoplasmic shuttling of YAP in MSCs. Knocking down YAP could inhibit osteoblast differentiation of MSCs, while overexpression of nuclear-localized YAP-5SA could reverse the inhibition of osteoblast differentiation of MSCs caused by fibroblasts. Furthermore, fibroblasts secreted DKK1, which further inhibited the formation of calcium nodules during the late stage of osteogenesis but did not affect the early stage of osteogenesis. Thus, fibroblasts could inhibit osteogenesis by regulating YAP localization in MSCs and secreting DKK1.

CONCLUSIONS

Our research revealed that fibroblasts could modulate the nuclear-cytoplasmic shuttling of YAP in MSCs, thereby inhibiting their osteoblast differentiation. Fibroblasts could also secrete DKK1, which inhibited calcium nodule formation at the late stage of osteogenesis.

Epithelial apoptotic pattern emerges from global and local regulation by cell apical area

Geometry is a fundamental attribute of biological systems, and it underlies cell and tissue dynamics. Cell geometry controls cell-cycle progression and mitosis and thus modulates tissue development and homeostasis. In sharp contrast and despite the extensive characterization of the genetic mechanisms of caspase activation, we know little about whether and how cell geometry controls apoptosis commitment in developing tissues. Here, we combined multiscale time-lapse microscopy of developing Drosophila epithelium, quantitative characterization of cell behaviors, and genetic and mechanical perturbations to determine how apoptosis is controlled during epithelial tissue development. We found that early in cell lives and well before extrusion, apoptosis commitment is linked to two distinct geometric features: a small apical area compared with other cells within the tissue and a small relative apical area with respect to the immediate neighboring cells. We showed that these global and local geometric characteristics are sufficient to recapitulate the tissue-scale apoptotic pattern. Furthermore, we established that the coupling between these two geometric features and apoptotic cells is dependent on the Hippo/YAP and Notch pathways. Overall, by exploring the links between cell geometry and apoptosis commitment, our work provides important insights into the spatial regulation of cell death in tissues and improves our understanding of the mechanisms that control cell number and tissue size.

Wnt activation disturbs cell competition and causes diffuse invasion of transformed cells through NF-κB-MMP21 pathway

Normal epithelial cells exert their competitive advantage over RasV12-transformed cells and eliminate them into the apical lumen via cell competition. However, the internal or external factors that compromise cell competition and provoke carcinogenesis remain elusive. In this study, we examine the effect of sequential accumulation of gene mutations, mimicking multi-sequential carcinogenesis on RasV12-induced cell competition in intestinal epithelial tissues. Consequently, we find that the directionality of RasV12-cell extrusion in Wnt-activated epithelia is reversed, and transformed cells are delaminated into the basal lamina via non-cell autonomous MMP21 upregulation. Subsequently, diffusively infiltrating, transformed cells develop into highly invasive carcinomas. The elevated production of MMP21 is elicited partly through NF-κB signaling, blockage of which restores apical elimination of RasV12 cells. We further demonstrate that the NF-κB-MMP21 axis is significantly bolstered in early colorectal carcinoma in humans. Collectively, this study shows that cells with high mutational burdens exploit cell competition for their benefit by behaving as unfit cells, endowing them with an invasion advantage.

The cell adhesion molecule Echinoid promotes tissue survival and separately restricts tissue overgrowth in Drosophila imaginal discs

The interactions that cells in Drosophila imaginal discs have with their neighbors are known to regulate their ability to survive. In a screen of genes encoding cell surface proteins for gene knockdowns that affect the size or shape of mutant clones, we found that clones of cells with reduced levels of echinoid (ed) are fewer, smaller, and can be eliminated during development. In contrast, discs composed mostly of ed mutant tissue are overgrown. We find that ed mutant tissue has lower levels of the anti-apoptotic protein Diap1 and has increased levels of apoptosis which is consistent with the observed underrepresentation of ed mutant clones and the slow growth of ed mutant tissue. The eventual overgrowth of ed mutant tissue results not from accelerated growth, but from prolonged growth resulting from a failure to arrest growth at the appropriate final size. Ed has previously been shown to physically interact with multiple Hippo-pathway components and it has been proposed to promote Hippo pathway signaling, to exclude Yorkie (Yki) from the nucleus, and restrain the expression of Yki-target genes. We did not observe changes in Yki localization in ed mutant tissue and found decreased levels of expression of several Yorkie-target genes, findings inconsistent with the proposed effect of Ed on Yki. We did, however, observe increased expression of several Yki-target genes in wild-type cells neighboring ed mutant cells, which may contribute to elimination of ed mutant clones. Thus, ed has two distinct functions: an anti-apoptotic function by maintaining Diap1 levels, and a function to arrest growth at the appropriate final size. Both of these are unlikely to be explained by a simple effect on the Hippo pathway.

Sas-Ptp10D shapes germ-line stem cell niche by facilitating JNK-mediated apoptosis

The function of the stem cell system is supported by a stereotypical shape of the niche structure. In Drosophila ovarian germarium, somatic cap cells form a dish-like niche structure that allows only two or three germ-line stem cells (GSCs) reside in the niche. Despite extensive studies on the mechanism of stem cell maintenance, the mechanisms of how the dish-like niche structure is shaped and how this structure contributes to the stem cell system have been elusive. Here, we show that a transmembrane protein Stranded at second (Sas) and its receptor Protein tyrosine phosphatase 10D (Ptp10D), effectors of axon guidance and cell competition via epidermal growth factor receptor (Egfr) inhibition, shape the dish-like niche structure by facilitating c-Jun N-terminal kinase (JNK)-mediated apoptosis. Loss of Sas or Ptp10D in gonadal apical cells, but not in GSCs or cap cells, during the pre-pupal stage results in abnormal shaping of the niche structure in the adult, which allows excessive, four to six GSCs reside in the niche. Mechanistically, loss of Sas-Ptp10D elevates Egfr signaling in the gonadal apical cells, thereby suppressing their naturally-occurring JNK-mediated apoptosis that is essential for the shaping of the dish-like niche structure by neighboring cap cells. Notably, the abnormal niche shape and resulting excessive GSCs lead to diminished egg production. Our data propose a concept that the stereotypical shaping of the niche structure optimizes the stem cell system, thereby maximizing the reproductive capacity.

Stemming Tumoral Growth: A Matter of Grotesque Organogenesis

The earliest metazoans probably evolved from single-celled organisms which found the colonial system to be a beneficial organization. Over the course of their evolution, these primary colonial organisms increased in size, and division of labour among the cells became a remarkable feature, leading to a higher level of organization: the biological organs. Primitive metazoans were the first organisms in evolution to show organ-type structures, which set the grounds for complex organs to evolve. Throughout evolution, and concomitant with organogenesis, is the appearance of tissue-specific stem cells. Tissue-specific stem cells gave rise to multicellular living systems with distinct organs which perform specific physiological functions. This setting is a constructive role of evolution; however, rebel cells can take over the molecular mechanisms for other purposes: nowadays we know that cancer stem cells, which generate aberrant organ-like structures, are at the top of a hierarchy. Furthermore, cancer stem cells are the root of metastasis, therapy resistance, and relapse. At present, most therapeutic drugs are unable to target cancer stem cells and therefore, treatment becomes a challenging issue. We expect that future research will uncover the mechanistic “forces” driving organ growth, paving the way to the implementation of new strategies to impair human tumorigenesis.

Azot expression in the Drosophila gut modulates organismal lifespan

Cell Competition emerged in Drosophila as an unexpected phenomenon, when confronted clones of fit vs unfit cells genetically induced. During the last decade, it has been shown that this mechanism is physiologically active in Drosophila and higher organisms. In Drosophila, Flower (Fwe) eliminates unfit cells during development, regeneration and disease states. Furthermore, studies suggest that Fwe signaling is required to eliminate accumulated unfit cells during adulthood extending Drosophila lifespan. Indeed, ahuizotl (azot) mutants accumulate unfit cells during adulthood and after physical insults in the brain and other epithelial tissues, showing a decrease in organismal lifespan. On the contrary, flies carrying three functional copies of the gene, unfit cell culling seems to be more efficient and show an increase in lifespan. During aging, Azot is required for the elimination of unfit cells, however, the specific organs modulating organismal lifespan by Azot remain unknown. Here we found a potential connection between gut-specific Azot expression and lifespan which may uncover a more widespread organ-specific mechanism modulating organismal survival.

Cell-cell interactions that drive tumorigenesis in Drosophila

Cell-cell interactions within tumour microenvironment play crucial roles in tumorigenesis. Genetic mosaic techniques available in Drosophila have provided a powerful platform to study the basic principles of tumour growth and progression via cell-cell communications. This led to the identification of oncogenic cell-cell interactions triggered by endocytic dysregulation, mitochondrial dysfunction, cell polarity defects, or Src activation in Drosophila imaginal epithelia. Such oncogenic cooperations can be caused by interactions among epithelial cells, mesenchymal cells, and immune cells. Moreover, microenvironmental factors such as nutrients, local tissue structures, and endogenous growth signalling activities critically affect tumorigenesis. Dissecting various types of oncogenic cell-cell interactions at the single-cell level in Drosophila will greatly increase our understanding of how tumours progress in living animals.

The CRL4 E3 ligase Mahjong/DCAF1 controls cell competition through the transcription factor Xrp1, independently of polarity genes

Cell competition, the elimination of cells surrounded by more fit neighbors, is proposed to suppress tumorigenesis. Mahjong (Mahj), a ubiquitin E3 ligase substrate receptor, has been thought to mediate competition of cells mutated for lethal giant larvae (lgl), a neoplastic tumor suppressor that defines apical-basal polarity of epithelial cells. Here, we show that Drosophila cells mutated for mahjong, but not for lgl [l(2)gl], are competed because they express the bZip-domain transcription factor Xrp1, already known to eliminate cells heterozygous for ribosomal protein gene mutations (Rp/+ cells). Xrp1 expression in mahj mutant cells results in activation of JNK signaling, autophagosome accumulation, eIF2α phosphorylation and lower translation, just as in Rp/+ cells. Cells mutated for damage DNA binding-protein 1 (ddb1; pic) or cullin 4 (cul4), which encode E3 ligase partners of Mahj, also display Xrp1-dependent phenotypes, as does knockdown of proteasome subunits. Our data suggest a new model of mahj-mediated cell competition that is independent of apical-basal polarity and couples Xrp1 to protein turnover.

PTP10D-mediated cell competition is not obligately required for elimination of polarity-deficient clones

Animal organs maintain tissue integrity and ensure removal of aberrant cells through several types of surveillance mechanisms. One prominent example is the elimination of polarity-deficient mutant cells within developing Drosophila imaginal discs. This has been proposed to require heterotypic cell competition dependent on the receptor tyrosine phosphatase PTP10D within the mutant cells. We report here experiments to test this requirement in various contexts and find that PTP10D is not obligately required for the removal of scribble (scrib) mutant and similar polarity-deficient cells. Our experiments used identical stocks with which another group can detect the PTP10D requirement, and our results do not vary under several husbandry conditions including high and low protein food diets. Although we are unable to identify the source of the discrepant results, we suggest that the role of PTP10D in polarity-deficient cell elimination may not be absolute.

Elevation of major constitutive heat shock proteins is heat shock factor independent and essential for establishment and growth of Lgl loss and Yorkie gain-mediated tumors in Drosophila

Cancer cells generally overexpress heat shock proteins (Hsps), the major components of cellular stress response, to overcome and survive the diverse stresses. However, the specific roles of Hsps in initiation and establishment of cancers remain unclear. Using loss of Lgl-mediated epithelial tumorigenesis in Drosophila, we induced tumorigenic somatic clones of different genetic backgrounds to examine the temporal and spatial expression and roles of major heat shock proteins in tumor growth. The constitutively expressed Hsp83, Hsc70 (heat shock cognate), Hsp60 and Hsp27 show elevated levels in all cells of the tumorigenic clone since early stages, which persists till their transformation. However, the stress-inducible Hsp70 is expressd only in a few cells at later stage of established tumorous clones that show high F-actin aggregation. Intriguingly, levels of heat shock factor (HSF), the master regulator of Hsps, remain unaltered in these tumorous cells and its down-regulation does not affect tumorigenic growth of lgl- clones overexpressing Yorkie, although down-regulation of Hsp83 prevents their survival and growth. Interestingly, overexpression of HSF or Hsp83 in lgl- cells makes them competitively successful in establishing tumorous clones. These results show that the major constitutively expressed Hsps, but not the stress-inducible Hsp70, are involved in early as well as late stages of epithelial tumors and their elevated expression in lgl- clones co-overexpressing Yorkie is independent of HSF.

Single Cell Biology: Exploring Somatic Cell Behaviors, Competition and Selection in Chronic Disease

The full range of cell functions is under-determined in most human diseases. The evidence that somatic cell competition and clonal imbalance play a role in non-neoplastic chronic disease reveal a need for a dedicated effort to explore single cell function if we are to understand the mechanisms by which cell population behaviors influence disease. It will be vital to document not only the prevalent pathologic behaviors but also those beneficial functions eliminated or suppressed by competition. An improved mechanistic understanding of the role of somatic cell biology will help to stratify chronic disease, define more precisely at an individual level the role of environmental factors and establish principles for prevention and potential intervention throughout the life course and across the trajectory from wellness to disease.

Cell mechanics and cell-cell recognition controls by Toll-like receptors in tissue morphogenesis and homeostasis

Signal transduction by the Toll-like receptors (TLRs) is conserved and essential for innate immunity in metazoans. The founding member of the TLR family, Drosophila Toll-1, was initially identified for its role in dorsoventral axis formation in early embryogenesis. The Drosophila genome encodes nine TLRs that display dynamic expression patterns during development, suggesting their involvement in tissue morphogenesis and homeostasis. Recent progress on the developmental functions of TLRs beyond dorsoventral patterning has revealed not only their diverse functions in various biological processes, but also unprecedented molecular mechanisms in directly regulating cell mechanics and cell-cell recognition independent of the canonical signal transduction pathway involving transcriptional regulation of target genes. In this review, I feature and discuss the non-immune functions of TLRs in the control of epithelial tissue homeostasis, tissue morphogenesis, and cell-cell recognition between cell populations with different cell identities.

The Field of Cell Competition Comes of Age: Semantics and Technological Synergy

Stem cells experience many selective pressures which shape their cellular populations, potentially pushing them to skew towards dominance of a few break-through clones. An evolutionarily conserved answer to curb these aberrant selective pressures is cell competition, the elimination of a subset of cells by their neighbours in a seemingly homogenous population. Cell competition in mammalian systems is a relatively recent discovery that has now been observed across many tissue systems, such as embryonic, haematopoietic, intestinal, and epithelial compartments. With this rapidly growing field, there is a need to revisit and standardize the terminology used, much of which has been co-opted from evolutionary biology. Further, the implications of cell competition across biological scales in organisms have been difficult to capture. In this review, we make three key points. One, we propose new nomenclature to standardize concepts across dispersed studies of different types of competition, each of which currently use the same terminology to describe different phenomena. Second, we highlight the challenges in capturing information flow across biological scales. Third, we challenge the field to incorporate next generation technologies into the cell competition toolkit to bridge these gaps. As the field of cell competition matures, synergy between cutting edge tools will help elucidate the molecular events which shape cellular growth and death dynamics, allowing a deeper examination of this evolutionarily conserved mechanism at the core of multicellularity.

Epithelial monitoring through ligand-receptor segregation ensures malignant cell elimination

Animals have evolved mechanisms, such as cell competition, to remove dangerous or nonfunctional cells from a tissue. Tumor necrosis factor signaling can eliminate clonal malignancies from Drosophila imaginal epithelia, but why this pathway is activated in tumor cells but not normal tissue is unknown. We show that the ligand that drives elimination is present in basolateral circulation but remains latent because it is spatially segregated from its apically localized receptor. Polarity defects associated with malignant transformation cause receptor mislocalization, allowing ligand binding and subsequent apoptotic signaling. This process occurs irrespective of the neighboring cells' genotype and is thus distinct from cell competition. Related phenomena at epithelial wound sites are required for efficient repair. This mechanism of polarized compartmentalization of ligand and receptor can generally monitor epithelial integrity to promote tissue homeostasis.

Human papillomavirus type 16 E6 induces cell competition

High-risk human papillomavirus (HPV) infections induce squamous epithelial tumors in which the virus replicates. Initially, the virus-infected cells are untransformed, but expand in both number and area at the expense of uninfected squamous epithelial cells. We have developed an in vitro assay in which colonies of post-confluent HPV16 expressing cells outcompete and displace confluent surrounding uninfected keratinocytes. The enhanced colony competition induced by the complete HPV16 genome is conferred by E6 expression alone, not by individual expression of E5 or E7, and requires E6 interaction with p53. E6-expressing keratinocytes undermine and displace adjacent normal keratinocytes from contact with the attachment substrate, thereby expanding the area of the E6-expressing colony at the expense of normal keratinocytes. These new results separate classic oncogenicity that is primarily conferred by HPV16 E7 from cell competition that we show is primarily conferred by E6 and provides a new biological role for E6 oncoproteins from high-risk human papillomaviruses.

Microbial infections can change the fate and behavior of normal vertebrate cells to resemble oncogenic cells. High-risk papillomaviruses induce infected squamous epithelial cells to form tumors, some of which evolve into malignancies. The present work shows that the enhanced competitiveness of HPV16-infected cells for the basal cell surface is primarily due to the expression of the E6 oncoprotein and not the E7 or E5 oncoproteins. Compared to normal keratinocytes, E6 induces a super-competitor phenotype while E5 and E7 do not. This work shows the importance of measuring oncoprotein traits not only as cell autonomous traits, but in the context of competition with uninfected cells and shows the potential of papillomavirus oncoproteins to be novel genetic probes for the analysis of cell competition.

Cell competition is driven by Xrp1-mediated phosphorylation of eukaryotic initiation factor 2α

Cell competition is a context-dependent cell elimination via cell-cell interaction whereby unfit cells ('losers') are eliminated from the tissue when confronted with fitter cells ('winners'). Despite extensive studies, the mechanism that drives loser's death and its physiological triggers remained elusive. Here, through a genetic screen in Drosophila, we find that endoplasmic reticulum (ER) stress causes cell competition. Mechanistically, ER stress upregulates the bZIP transcription factor Xrp1, which promotes phosphorylation of the eukaryotic translation initiation factor eIF2α via the kinase PERK, leading to cell elimination. Surprisingly, our genetic data show that different cell competition triggers such as ribosomal protein mutations or RNA helicase Hel25E mutations converge on upregulation of Xrp1, which leads to phosphorylation of eIF2α and thus causes reduction in global protein synthesis and apoptosis when confronted with wild-type cells. These findings not only uncover a core pathway of cell competition but also open the way to understanding the physiological triggers of cell competition.

Tumour-host interactions through the lens of Drosophila

There is a large gap between the deep understanding of mechanisms driving tumour growth and the reasons why patients ultimately die of cancer. It is now appreciated that interactions between the tumour and surrounding non-tumour (sometimes referred to as host) cells play critical roles in mortality as well as tumour progression, but much remains unknown about the underlying molecular mechanisms, especially those that act beyond the tumour microenvironment. Drosophila has a track record of high-impact discoveries about cell-autonomous growth regulation, and is well suited to now probe mysteries of tumour - host interactions. Here, we review current knowledge about how fly tumours interact with microenvironmental stroma, circulating innate immune cells and distant organs to influence disease progression. We also discuss reciprocal regulation between tumours and host physiology, with a particular focus on paraneoplasias. The fly's simplicity along with the ability to study lethality directly provide an opportunity to shed new light on how cancer actually kills.

Mechano-chemical feedback mediated competition for BMP signalling leads to pattern formation

Developmental patterning is thought to be regulated by conserved signalling pathways. Initial patterns are often broad before refining to only those cells that commit to a particular fate. However, the mechanisms by which pattern refinement takes place remain to be addressed. Using the posterior crossvein (PCV) of the Drosophila pupal wing as a model, into which bone morphogenetic protein (BMP) ligand is extracellularly transported to instruct vein patterning, we investigate how pattern refinement is regulated. We found that BMP signalling induces apical enrichment of Myosin II in developing crossvein cells to regulate apical constriction. Live imaging of cellular behaviour indicates that changes in cell shape are dynamic and transient, only being maintained in those cells that retain vein fate competence after refinement. Disrupting cell shape changes throughout the PCV inhibits pattern refinement. In contrast, disrupting cell shape in only a subset of vein cells can result in a loss of BMP signalling. We propose that mechano-chemical feedback leads to competition for the developmental signal which plays a critical role in pattern refinement.

Interaction between Ras and Src clones causes interdependent tumor malignancy via Notch signaling in Drosophila

Cancer tissue often comprises multiple tumor clones with distinct oncogenic alterations such as Ras or Src activation, yet the mechanism by which tumor heterogeneity drives cancer progression remains elusive. Here, we show in Drosophila imaginal epithelium that clones of Ras- or Src-activated benign tumors interact with each other to mutually promote tumor malignancy. Mechanistically, Ras-activated cells upregulate the cell-surface ligand Delta while Src-activated cells upregulate its receptor Notch, leading to Notch activation in Src cells. Elevated Notch signaling induces the transcriptional repressor Zfh1/ZEB1, which downregulates E-cadherin and cell death gene hid, leading to Src-activated invasive tumors. Simultaneously, Notch activation in Src cells upregulates the cytokine Unpaired/IL-6, which activates JAK-STAT signaling in neighboring Ras cells. Elevated JAK-STAT signaling upregulates the BTB-zinc-finger protein Chinmo, which downregulates E-cadherin and thus generates Ras-activated invasive tumors. Our findings provide a mechanistic explanation for how tumor heterogeneity triggers tumor progression via cell-cell interactions.

The hippo pathway: A master regulator of liver metabolism, regeneration, and disease

The liver is the only visceral organ in the body with a tremendous capacity to regenerate in response to insults that induce inflammation, cell death, and injury. Liver regeneration is a complicated process involving a well-orchestrated activation of non-parenchymal cells in the injured area and proliferation of undamaged hepatocytes. Furthermore, the liver has a Hepatostat, defined as adjustment of its volume to that required for homeostasis. Understanding the mechanisms that control different steps of liver regeneration is critical to informing therapies for liver repair, to help patients with liver disease. The Hippo signaling pathway is well known for playing an essential role in the control and regulation of liver size, regeneration, stem cell self-renewal, and liver cancer. Thus, the Hippo pathway regulates dynamic cell fates in liver, and in absence of its downstream effectors YAP and TAZ, liver regeneration is severely impaired, and the proliferative expansion of liver cells blocked. We will mainly review upstream mechanisms activating the Hippo signaling pathway following partial hepatectomy in mouse model and patients, its roles during different steps of liver regeneration, metabolism, and cancer. We will also discuss how targeting the Hippo signaling cascade might improve liver regeneration and suppress liver tumorigenesis.

Cell competition from development to neurodegeneration

Cell competition is a process by which suboptimal cells are eliminated to the benefit of cells with higher fitness. It is a surveillance mechanism that senses differences in the fitness status by several modes, such as expression of fitness fingerprints, survival factor uptake rate and resistance to mechanical stress. Fitness fingerprints-mediated cell competition recognizes isoforms of the transmembrane protein Flower, and translates the relative fitness of cells into distinct fates through the Flower code. Impairments in cell competition potentiate the development of diseases like cancer and ageing-related pathologies. In cancer, malignant cells acquire a supercompetitor behaviour, killing the neighbouring cells and overtaking the tissue, thus avoiding elimination. Neurodegenerative disorders affect millions of people and are characterized by cognitive decline and locomotor deficits. Alzheimer's disease is the most common form of dementia, and one of the largely studied diseases. However, the cellular processes taking place remain unclear. Drosophila melanogaster is an emerging neurodegeneration model due to its versatility as a tool for genetic studies. Research in a Drosophila Alzheimer's disease model detected fitness markers in the suboptimal and hyperactive neurons, thus establishing a link between cell competition and Alzheimer's disease. In this Review, we overview cell competition and the new insights related to neurodegenerative disorders, and discuss how research in the field might contribute to the development of new therapeutic targets for these diseases.

Misshapen Disruption Cooperates with RasV12 to Drive Tumorigenesis

Although RAS family genes play essential roles in tumorigenesis, effective treatments targeting RAS-related tumors are lacking, partly because of an incomplete understanding of the complex signaling crosstalk within RAS-related tumors. Here, we performed a large-scale genetic screen in Drosophila eye imaginal discs and identified Misshapen (Msn) as a tumor suppressor that synergizes with oncogenic Ras (Ras^V12) to induce c-Jun N-terminal kinase (JNK) activation and Hippo inactivation, then subsequently leads to tumor overgrowth and invasion. Moreover, ectopic Msn expression activates Hippo signaling pathway and suppresses Hippo signaling disruption-induced overgrowth. Importantly, we further found that Msn acts downstream of protocadherin Fat (Ft) to regulate Hippo signaling. Finally, we identified msn as a Yki/Sd target gene that regulates Hippo pathway in a negative feedback manner. Together, our findings identified Msn as a tumor suppressor and provide a novel insight into RAS-related tumorigenesis that may be relevant to human cancer biology.

Cell competition between anaplastic thyroid cancer and normal thyroid follicular cells exerts reciprocal stress response defining tumor suppressive effects of normal epithelial tissue

The microenvironment of an early-stage tumor, in which a small number of cancer cells is surrounded by a normal counterpart milieu, plays a crucial role in determining the fate of initiated cells. Here, we examined cell competition between anaplastic thyroid cancer cells and normal thyroid follicular cells using co-culture method. Cancer cells were grown until they formed small clusters, to which normal cells were added to create high-density co-culture condition. We found that co-culture with normal cells significantly suppressed the growth of cancer cell clusters through the activation of Akt-Skp2 pathway. In turn, cancer cells triggered apoptosis in the neighboring normal cells through local activation of ERK1/2. A bi-directional cell competition provides a suppressive mechanism of anaplastic thyroid cancer progression. Since the competitive effect was negated by terminal growth arrest caused by radiation exposure to normal cells, modulation of reciprocal stress response in vivo could be an intrinsic mechanism associated with tumor initiation, propagation, and metastasis.

Application of CRISPR screens to investigate mammalian cell competition

Cell competition is defined as the context-dependent elimination of cells that is mediated by intercellular communication, such as paracrine or contact-dependent cell signaling, and/or mechanical stresses. It is considered to be a quality control mechanism that facilitates the removal of suboptimal cells from both adult and embryonic tissues. Cell competition, however, can also be hijacked by transformed cells to acquire a 'super-competitor' status and outcompete the normal epithelium to establish a precancerous field. To date, many genetic drivers of cell competition have been identified predominately through studies in Drosophila. Especially during the last couple of years, ethylmethanesulfonate-based genetic screens have been instrumental to our understanding of the molecular regulators behind some of the most common competition mechanisms in Drosophila, namely competition due to impaired ribosomal function (or anabolism) and mechanical sensitivity. Despite recent findings in Drosophila and in mammalian models of cell competition, the drivers of mammalian cell competition remain largely elusive. Since the discovery of CRISPR/Cas9, its use in functional genomics has been indispensable to uncover novel cancer vulnerabilities. We envision that CRISPR/Cas9 screens will enable systematic, genome-scale probing of mammalian cell competition to discover novel mutations that not only trigger cell competition but also identify novel molecular components that are essential for the recognition and elimination of less fit cells. In this review, we summarize recent contributions that further our understanding of the molecular mechanisms of cell competition by genetic screening in Drosophila, and provide our perspective on how similar and novel screening strategies made possible by whole-genome CRISPR/Cas9 screening can advance our understanding of mammalian cell competition in the future.

Health and Fitness at the Single-Cell Level

Genetically identical cells in a tissue can respond differently to perturbations in their environment or "stress." Such stresses can be physicochemical, mechanical, or infectious or may come from competition with other cells in the tissue. Here, I discuss how the varying responses to stress influence the decision of a cell to repair or die, and how one cell's response can have effects on surrounding cells. Such responses control the health and fitness of single cells and how they compete with other genetically identical cells.See related article on p. 129.

Mechanism of tumor-suppressive cell competition in flies

Oncogenic mutations often trigger antitumor cellular response such as induction of apoptosis or cellular senescence. Studies in the last decade have identified the presence of the third guardian against mutation‐induced tumorigenesis, namely “cell competition.” Cell competition is a context‐dependent cell elimination whereby cells with higher fitness eliminate neighboring cells with lower fitness by inducing cell death. While oncogene‐induced apoptosis or oncogene‐induced senescence acts as a cell‐autonomous tumor suppressor, cell competition protects the tissue from tumorigenesis via cell‐cell communication. For instance, in Drosophila epithelium, oncogenic cells with cell polarity mutations overproliferate and develop into tumors on their own but are eliminated from the tissue when surrounded by wild‐type cells. Genetic studies in flies have unraveled that such tumor‐suppressive cell competition is regulated by at least three mechanisms: direct cell‐cell interaction between polarity‐deficient cells and wild‐type cells, secreted factors from epithelial cells, and systemic factors from distant organs. Molecular manipulation of tumor‐suppressive cell competition could provide a novel therapeutic strategy against human cancers.

Myc as a Regulator of Ribosome Biogenesis and Cell Competition: A Link to Cancer

The biogenesis of ribosomes is a finely regulated multistep process linked to cell proliferation and growth-processes which require a high rate of protein synthesis. One of the master regulators of ribosome biogenesis is Myc, a well-known proto-oncogene that has an important role in ribosomal function and in the regulation of protein synthesis. The relationship between Myc and the ribosomes was first highlighted in Drosophila, where Myc's role in controlling Pol-I, II and III was evidenced by both microarrays data, and by the ability of Myc to control growth (mass), and cellular and animal size. Moreover, Myc can induce cell competition, a physiological mechanism through which cells with greater fitness grow better and thereby prevail over less competitive cells, which are actively eliminated by apoptosis. Myc-induced cell competition was shown to regulate both vertebrate development and tumor promotion; however, how these functions are linked to Myc's control of ribosome biogenesis, protein synthesis and growth is not clear yet. In this review, we will discuss the major pathways that link Myc to ribosomal biogenesis, also in light of its function in cell competition, and how these mechanisms may reflect its role in favoring tumor promotion.

A Genetic Analysis of Tumor Progression in Drosophila Identifies the Cohesin Complex as a Suppressor of Individual and Collective Cell Invasion

Metastasis is the leading cause of death for patients with cancer. Consequently it is imperative that we improve our understanding of the molecular mechanisms that underlie progression of tumor growth toward malignancy. Advances in genome characterization technologies have been very successful in identifying commonly mutated or misregulated genes in a variety of human cancers. However, the difficulty in evaluating whether these candidates drive tumor progression remains a major challenge. Using the genetic amenability of Drosophila melanogaster we generated tumors with specific genotypes in the living animal and carried out a detailed systematic loss-of-function analysis to identify conserved genes that enhance or suppress epithelial tumor progression. This enabled the discovery of functional cooperative regulators of invasion and the establishment of a network of conserved invasion suppressors. This includes constituents of the cohesin complex, whose loss of function either promotes individual or collective cell invasion, depending on the severity of effect on cohesin complex function.

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Screen identifies genes that affect tumor behavior in a wide variety of ways

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A functionally validated network of invasion-suppressor genes was generated

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Loss of cohesin complex function can promote individual or collective cell invasion

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The fly pupal notum is an excellent in vivo system to study tumor progression

Peritumoral activation of the Hippo pathway effectors YAP and TAZ suppresses liver cancer in mice

The Hippo signaling pathway and its two downstream effectors, the YAP and TAZ transcriptional coactivators, are drivers of tumor growth in experimental models. Studying mouse models, we show that YAP and TAZ can also exert a tumor-suppressive function. We found that normal hepatocytes surrounding liver tumors displayed activation of YAP and TAZ and that deletion of Yap and Taz in these peritumoral hepatocytes accelerated tumor growth. Conversely, experimental hyperactivation of YAP in peritumoral hepatocytes triggered regression of primary liver tumors and melanoma-derived liver metastases. Furthermore, whereas tumor cells growing in wild-type livers required YAP and TAZ for their survival, those surrounded by Yap- and Taz-deficient hepatocytes were not dependent on YAP and TAZ. Tumor cell survival thus depends on the relative activity of YAP and TAZ in tumor cells and their surrounding tissue, suggesting that YAP and TAZ act through a mechanism of cell competition to eliminate tumor cells.

Tools and Concepts for Interrogating and Defining Cellular Identity

Defining the mechanisms that generate specialized cell types and coordinate their functions is critical for understanding organ development and renewal. New tools and discoveries are challenging and refining our definitions of a cell type. A rapidly growing toolkit for single-cell analyses has expanded the number of markers that can be assigned to a cell simultaneously, revealing heterogeneity within cell types that were previously regarded as homogeneous populations. Additionally, cell types defined by specific molecular markers can exhibit distinct, context-dependent functions; for example, between tissues in homeostasis and those responding to damage. Here we review the current technologies used to identify and characterize cells, and we discuss how experimental and pathological perturbations are adding increasing complexity to our definitions of cell identity.

Hyperinsulinemia Drives Epithelial Tumorigenesis by Abrogating Cell Competition

Metabolic diseases such as type 2 diabetes are associated with increased cancer incidence. Here, we show that hyperinsulinemia promotes epithelial tumorigenesis by abrogating cell competition. In Drosophila eye imaginal epithelium, oncogenic scribble (scrib) mutant cells are eliminated by cell competition when surrounded by wild-type cells. Through a genetic screen, we find that flies heterozygous for the insulin receptor substrate chico allow scrib cells to evade cell competition and develop into tumors. Intriguingly, chico is required in the brain's insulin-producing cells (IPCs) to execute cell competition remotely. Mechanistically, chico downregulation in IPCs causes hyperinsulinemia by upregulating a Drosophila insulin Dilp2, which activates insulin-mTOR signaling and thus boosts protein synthesis in scrib cells. A diet-induced increase in insulin levels also triggers scrib tumorigenesis, and pharmacological repression of protein synthesis prevents hyperinsulinemia-induced scrib overgrowth. Our findings provide an in vivo mechanistic link between metabolic disease and cancer risk via systemic regulation of cell competition.

The transcription factor Spalt and human homologue SALL4 induce cell invasion via the dMyc-JNK pathway in Drosophila

Cancer cell metastasis is a leading cause of mortality in cancer patients. Therefore, revealing the molecular mechanism of cancer cell invasion is of great significance for the treatment of cancer. In human patients, the hyperactivity of transcription factor Spalt-like 4 (SALL4) is sufficient to induce malignant tumorigenesis and metastasis. Here, we found that when ectopically expressing the Drosophila homologue spalt (sal) or human SALL4 in Drosophila, epithelial cells delaminated basally with penetration of the basal lamina and degradation of the extracellular matrix, which are essential properties of cell invasion. Further assay found that sal/SALL4 promoted cell invasion via dMyc-JNK signaling. Inhibition of the c-Jun N-terminal kinase (JNK) signaling pathway through suppressing matrix metalloprotease 1, or basket can achieve suppression of cell invasion. Moreover, expression of dMyc, a suppressor of JNK signaling, dramatically blocked cell invasion induced by sal/SALL4 in the wing disc. These findings reveal a conserved role of sal/SALL4 in invasive cell movement and link the crucial mediator of tumor invasion, the JNK pathway, to SALL4-mediated cancer progression.This article has an associated First Person interview with the first author of the paper.

Prostaglandin E2 and its receptor EP2 trigger signaling that contributes to YAP-mediated cell competition

Cell competition is a biological process by which unfit cells are eliminated from “cell society.” We previously showed that cultured mammalian epithelial Madin‐Darby canine kidney (MDCK) cells expressing constitutively active YAP were eliminated by apical extrusion when surrounded by “normal” MDCK cells. However, the molecular mechanism underlying the elimination of active YAP‐expressing cells was unknown. Here, we used high‐throughput chemical compound screening to identify cyclooxygenase‐2 (COX‐2) as a key molecule triggering cell competition. Our work shows that COX‐2‐mediated PGE₂ secretion engages its receptor EP2 on abnormal and nearby normal cells. This engagement of EP2 triggers downstream signaling via an adenylyl cyclase‐cyclic AMP‐PKA pathway that, in the presence of active YAP, induces E‐cadherin internalization leading to apical extrusion. Thus, COX‐2‐induced PGE₂ appears a warning signal to both abnormal and surrounding normal cells to drive cell competition.

Roles of C/EBP class bZip proteins in the growth and cell competition of Rp ('Minute') mutants in Drosophila

Reduced copy number of ribosomal protein (Rp) genes adversely affects both flies and mammals. Xrp1 encodes a reportedly Drosophila-specific AT-hook, bZIP protein responsible for many of the effects including the elimination of Rp mutant cells by competition with wild type cells. Irbp18, an evolutionarily conserved bZIP gene, heterodimerizes with Xrp1 and with another bZip protein, dATF4. We show that Irbp18 is required for the effects of Xrp1, whereas dATF4 does not share the same phenotype, indicating that Xrp1/Irbp18 is the complex active in Rp mutant cells, independently of other complexes that share Irbp18. Xrp1 and Irbp18 transcripts and proteins are upregulated in Rp mutant cells by auto-regulatory expression that depends on the Xrp1 DNA binding domains and is necessary for cell competition. We show that Xrp1 is conserved beyond Drosophila, although under positive selection for rapid evolution, and that at least one human bZip protein can similarly affect Drosophila development.

Cell Competition Is Driven by Autophagy

Cell competition is a quality control process that selectively eliminates unfit cells from the growing tissue via cell-cell interaction. Despite extensive mechanistic studies, the mechanism by which cell elimination is triggered has been elusive. Here, through a genetic screen in Drosophila, we discover that V-ATPase, an essential factor for autophagy, is required for triggering cell competition. Strikingly, autophagy is specifically elevated in prospective "loser" cells nearby wild-type "winner" cells, and blocking autophagy in loser cells abolishes their elimination. Mechanistically, elevated autophagy upregulates a proapoptotic gene hid through NFκB, and the elevated hid cooperates with JNK signaling to effectively induce loser's death. Crucially, this mechanism generally applies to cell competition caused by differences in protein synthesis between cells. Our findings establish a common mechanism of cell competition whereby cells with higher protein synthesis induce autophagy in their neighboring cells, leading to elimination of unfit cells.

Next-Generation Sequencing Reveals Increased Anti-oxidant Response and Ecdysone Signaling in STAT Supercompetitors in Drosophila

Cell competition is the elimination of one viable population of cells (the losers) by a neighboring fitter population (the winners) and was discovered by studies in the Drosophila melanogaster wing imaginal disc. Supercompetition is a process in which cells with elevated JAK/STAT signaling or increased Myc become winners and outcompete wild-type neighbors. To identify the genes that are differentially regulated in STAT supercompetitors, we purified these cells from Drosophila wing imaginal discs and performed next-generation sequencing. Their transcriptome was compared to those of control wing disc cells and Myc supercompetitors. Bioinformatics revealed that STAT and Myc supercompetitors have distinct transcriptomes with only 41 common differentially regulated genes. Furthermore, STAT supercompetitors have elevated reactive oxygen species, an anti-oxidant response and increased ecdysone signaling. Using a combination of methods, we validated 13 differentially expressed genes. These data sets will be useful resources to the community.

Harnessing epithelial homeostatic mechanisms to fight cancer

Cancer treatments have, in general, targeted the cancer cell itself. This approach has often been unsuccessful in the long term, especially for solid tumors. Even targeted therapies based on sequencing cancer genomes can be thwarted by genetic heterogeneity within tumors. Furthermore, genomic instability in cancer cells accelerates the generation of variants that are resistant to the treatment. Immunotherapies and anti-angiogenic treatments, which target the tumor-interacting and tumor-adjacent cells, have overcome some of these challenges, suggesting that other methods that target wild-type cells could be valuable in arresting tumor progression. Studies in Drosophila have uncovered mechanisms by which cells within an epithelium can react to neighboring cells that have genetic differences, resulting in the elimination of one population at the expense of another. Some of these mechanisms are now known to be conserved in mammals. The possibility of harnessing such mechanisms to empower normal epithelial cells to eliminate their precancerous neighbors before they develop into fully fledged cancers is an area of research that merits more attention.

Exploring MYC relevance to cancer biology from the perspective of cell competition

Cancer has long been regarded and treated as a foreign body appearing by mistake inside a living organism. However, now we know that cancer cells communicate with neighbours, thereby creating modified environments able to support their unusual need for nutrients and space. Understanding the molecular basis of these bi-directional interactions is thus mandatory to approach the complex nature of cancer. Since their discovery, MYC proteins have been showing to regulate a steadily increasing number of processes impacting cell fitness, and are consistently found upregulated in almost all human tumours. Of interest, MYC takes part in cell competition, an evolutionarily conserved fitness comparison strategy aimed at detecting weakened cells, which are then committed to death, removed from the tissue and replaced by fitter neighbours. During physiological development, MYC-mediated cell competition is engaged to eliminate cells with suboptimal MYC levels, so as to guarantee selective growth of the fittest and proper homeostasis, while transformed cells expressing high levels of MYC coopt cell competition to subvert tissue constraints, ultimately disrupting homeostasis. Therefore, the interplay between cells with different MYC levels may result in opposite functional outcomes, depending on the nature of the players. In the present review, we describe the most recent findings on the role of MYC-mediated cell competition in different contexts, with a special emphasis on its impact on cancer initiation and progression. We also discuss the relevance of competition-associated cell death to cancer disease.

Solid stress, competition for space and cancer: The opposing roles of mechanical cell competition in tumour initiation and growth

The regulation of cell growth, cell proliferation and cell death is at the basis of the homeostasis of tissues. While they can be regulated by intrinsic and genetic factors, their response to external signals emanating from the local environment is also essential for tissue homeostasis. Tumour initiation and progression is based on the misregulation of growth, proliferation and death mostly through the accumulation of genetic mutations. Yet, there is an increasing body of evidences showing that tumour microenvironment also has a strong impact on cancer initiation and progression. This includes the mechanical constrains and the compressive forces generated by the resistance of the surrounding tissue/matrix to tumour expansion. Recently, mechanical stress has been proposed to promote competitive interactions between cells through a process called mechanical cell competition. Cell population with a high proliferative rate can compact and eliminate the neighbouring cells which are more sensitive to compaction. While this emerging concept has been recently validated in vivo, the relevance of this process during tumour progression has never been discussed extensively. In this review, I will first describe the phenomenology of mechanical cell competition focusing on the main parameters and the pathways regulating cell elimination. I will then discuss the relevance of mechanical cell competition in tumour initiation and expansion while emphasizing its potential opposing contributions to tumourogenesis.

Elimination of oncogenic cells that regulate epithelial homeostasis in Drosophila

Normal epithelial tissues often put anti‐tumorigenic pressure on newly emerged oncogenic cells through cell–cell communications. In Drosophila epithelium, clones of oncogenic cells mutant for evolutionarily conserved apico‐basal polarity genes such as scribble (scrib) and discs large (dlg) are actively eliminated when surrounded by normal cells. It has been reported that c‐Jun N‐terminal kinase (JNK) signaling in polarity‐deficient cells is crucial for their cell death. However, the mechanism by which normal epithelial tissues exert anti‐tumorigenic effects on polarity‐deficient cells had been elusive. Here, I describe our genetic studies in Drosophila epithelium especially focused on the role of surrounding normal epithelial cells in response to the emergence of polarity‐deficient cells. Furthermore, I also describe recent studies regarding the mechanism by which polarity‐deficient cells are extruded from the tissue, and discuss future perspectives on the study of cell–cell communications in epithelial homeostasis.