Mechanism of tumor-suppressive cell competition in flies

Flamingo participates in multiple models of cell competition

The growth and survival of cells with different fitness, such as those with a proliferative advantage or a deleterious mutation, is controlled through cell competition. During development, cell competition enables healthy cells to eliminate less fit cells that could jeopardize tissue integrity, and facilitates the elimination of pre-malignant cells by healthy cells as a surveillance mechanism to prevent oncogenesis. Malignant cells also benefit from cell competition to promote their expansion. Despite its ubiquitous presence, the mechanisms governing cell competition, particularly those common to developmental competition and tumorigenesis, are poorly understood. Here, we show that in Drosophila, the planar cell polarity (PCP) protein Flamingo (Fmi) is required by winners to maintain their status during cell competition in malignant tumors to overtake healthy tissue, in early pre-malignant cells when they overproliferate among wildtype cells, in healthy cells when they later eliminate pre-malignant cells, and by supercompetitors as they compete to occupy excessive territory within wildtype tissues. "Would-be" winners that lack Fmi are unable to over-proliferate, and instead become losers. We demonstrate that the role of Fmi in cell competition is independent of PCP, and that it uses a distinct mechanism that may more closely resemble one used in other less well-defined functions of Fmi.

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.

The Role of LMP1 in Epstein-Barr Virus-associated Gastric Cancer

EBV promotes many cancers such as lymphoma, nasopharyngeal carcinoma, and gastric; Latent Membrane Protein 1 (LMP1) is considered to be a major oncogenic protein encoded by Epstein- Barr virus (EBV). LMP1 functions as a carcinogen in lymphoma and nasopharyngeal carcinoma, and LMP1 may also promote gastric cancer. The expression level of LMP1 in host cells is a key determinant in tumorigenesis and maintenance of virus specificity. By promoting cell immortalization and cell transformation, promoting cell proliferation, affecting immunity, and regulating cell apoptosis, LMP1 plays a crucial tumorigenic role in epithelial cancers. However, very little is currently known about LMP1 in Epstein-Barr virus-associated gastric cancer (EBVaGC); the main reason is that the expression level of LMP1 in EBVaGC is comparatively lower than other EBV-encoded proteins, such as The Latent Membrane Protein 2A (LMP2A), Epstein-Barr nuclear antigen 1 (EBNA1) and BamHI-A rightward frame 1 (BARF1), to date, there are few studies related to LMP1 in EBVaGC. Recent studies have demonstrated that LMP1 promotes EBVaGC by affecting The phosphatidylinositol 3-kinase- Akt (PI3K-Akt), Nuclear factor-kappa B (NF-κB), and other signaling pathways to regulate many downstream targets such as Forkhead box class O (FOXO), C-X-C-motif chemokine receptor (CXCR), COX-2 (Cyclooxygenase-2); moreover, the gene methylation induced by LMP1 in EBVaGC has become one of the characteristics that distinguish this gastric cancer (GC) from other types of gastric cancer and LMP1 also promotes the formation of the tumor microenvironment (TME) of EBVaGC in several ways. This review synthesizes previous relevant literature, aiming to highlight the latest findings on the mechanism of action of LMP1 in EBVaGC, summarize the function of LMP1 in EBVaGC, lay the theoretical foundation for subsequent new research on LMP1 in EBVaGC, and contribute to the development of novel LMP1-targeted drugs.

Coordination of tissue homeostasis and growth by the Scribble-α-Catenin-Septate junction complex

Maintaining apicobasal polarity (ABP) is crucial for epithelial integrity and homeostasis during tissue development. Although intracellular mechanisms underlying ABP establishment have been well studied, it remains to be addressed how the ABP coordinates tissue growth and homeostasis. By studying Scribble, a key ABP determinant, we address molecular mechanisms underlying ABP-mediated growth control in the Drosophila wing imaginal disc. Our data reveal that genetic and physical interactions between Scribble, Septate junction complex and α-Catenin appear to be key for sustaining ABP-mediated growth control. Cells with conditional scribble knockdown instigate the loss of α-Catenin, ultimately leading to the formation of neoplasia accompanying with activation of Yorkie. In contrast, cells expressing wild type scribble progressively restore ABP in scribble hypomorphic mutant cells in a non-autonomous manner. Our findings provide unique insights into cellular communication among optimal and sub-optimal cells to regulate epithelial homeostasis and growth.

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.

WASH activation controls endosomal recycling and EGFR and Hippo signaling during tumor-suppressive cell competition

Cell competition is a conserved homeostatic mechanism whereby epithelial cells eliminate neighbors with lower fitness. Cell communication at the interface of wild-type "winner" cells and polarity-deficient (scrib^-/-) "losers" is established through Sas-mediated Ptp10D activation in polarity-deficient cells. This tumor-suppressive cell competition restrains EGFR and Hippo signaling and enables Eiger-JNK mediated apoptosis in scrib^-/- clones. Here, we show that the activation state of the endosomal actin regulator WASH is a central node linking EGFR and Hippo signaling activation. The tyrosine kinase Btk29A and its substrate WASH are required downstream of Ptp10D for "loser" cell elimination. Constitutively active, phosphomimetic WASH is sufficient to induce both EGFR and Yki activation leading to overgrowth. On the mechanistic level we show that Ptp10D is recycled by the WASH/retromer complex, while EGFR is recycled by the WASH/retriever complex. Constitutive WASH activation selectively interferes with retromer function leading to Ptp10D mistargeting while promoting EGFR recycling and signaling activation. Phospho-WASH also activates aberrant Arp2/3 actin polymerization, leading to cytoskeletal imbalance, Yki activation and reduced apoptosis. Selective manipulation of WASH phosphorylation on sorting endosomes may restrict epithelial tumorous growth.

Scribble and α-Catenin cooperatively regulate epithelial homeostasis and growth

Epithelial homeostasis is an emergent property of both physical and biochemical signals emanating from neighboring cells and across tissue. A recent study reveals that Scribble, an apico-basal polarity determinant, cooperates with α-Catenin, an adherens junction component, to regulate tissue homeostasis in the Drosophila wing imaginal disc. However, it remains to be addressed whether similar mechanisms are utilized in vertebrates. In this study, we first address how α-Catenin cooperates with Scribble to regulate epithelial homeostasis and growth in mammalian cells. Our data show that α-Catenin and Scribble interact physically in mammalian cells. We then found that both α-Catenin and Scribble are required for regulating nuclear translocation of YAP, an effector of the Hippo signaling pathway. Furthermore, ectopic Scribble suffices to suppress YAP in an α-Catenin-dependent manner. Then, to test our hypothesis that Scribble amounts impact epithelial growth, we use the Drosophila wing imaginal disc. We show that Scribble expression is complementary to Yorkie signal, the Drosophila ortholog of YAP. Ectopic expression of full-length Scribble or Scribble Leucine Rich Region (LRR):α-Catenin chimera sufficiently down-regulates Yorkie signal, leading to smaller wing size. Moreover, Scribble LRR:α-Catenin chimera rescues scribble mutant clones in the wing imaginal disc to maintain tissue homeostasis. Taken together, our studies suggest that the association of cell polarity component Scribble with α-Catenin plays a conserved role in epithelial homeostasis and growth.

PI3K-activated MSC proteomes inhibit mammary tumors via Hsp90ab1 and Myh9

Despite the advance in medications in the past decade, aggressive breast cancer such as triple-negative breast cancer is difficult to treat. Here, we examined a counter-intuitive approach to converting human bone marrow-derived mesenchymal stem cells (MSCs) into induced tumor-suppressing cells by administering YS49, a PI3K/Akt activator. Notably, PI3K-activated MSCs generated tumor-suppressive proteomes, while PI3K-inactivated MSCs tumor-promotive proteomes. In a mouse model, the daily administration of YS49-treated MSC-derived CM decreased the progression of primary mammary tumors as well as the colonization of tumor cells in the lung. In the ex vivo assay, the size of freshly isolated human breast cancer tissues, including estrogen receptor positive and negative as well as human epidermal growth factor receptor 2 (HER2) positive and negative, was decreased by YS49-treated MSC-derived CM. Hsp90ab1 was enriched in CM as an atypical tumor-suppressing protein and immunoprecipitated a non-muscle myosin, Myh9. Extracellular Hsp90ab1 and Myh9 exerted the anti-tumor action and inhibited the maturation of bone-resorbing osteoclasts. Collectively, this study demonstrated that the activation of PI3K generated tumor-suppressive proteomes in MSCs and supported the possibility of using patient-derived MSCs for the treatment of breast cancer and bone metastasis.

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.

Fat body-derived Spz5 remotely facilitates tumor-suppressive cell competition through Toll-6-α-Spectrin axis-mediated Hippo activation

Tumor-suppressive cell competition is an evolutionarily conserved process that selectively removes precancerous cells to maintain tissue homeostasis. Using the polarity-deficiency-induced cell competition model in Drosophila, we identify Toll-6, a Toll-like receptor family member, as a driver of tension-mediated cell competition through α-Spectrin (α-Spec)-Yorkie (Yki) cascade. Toll-6 aggregates along the boundary between wild-type and polarity-deficient clones, where Toll-6 physically interacts with the cytoskeleton network protein α-Spec to increase mechanical tension, resulting in actomyosin-dependent Hippo pathway activation and the elimination of scrib mutant cells. Furthermore, we show that Spz5 secreted from fat body, the key innate organ in fly, facilitates the elimination of scrib clones by binding to Toll-6. These findings uncover mechanisms by which fat bodies remotely regulate tumor-suppressive cell competition of polarity-deficient tumors through inter-organ crosstalk and identified the Toll-6-α-Spec axis as an essential guardian that prevents tumorigenesis via tension-mediated cell elimination.

The sculpting of somatic mutational landscapes by evolutionary forces and their impacts on aging-related disease

Aging represents the major risk factor for the development of cancer and many other diseases. Recent findings show that normal tissues become riddled with expanded clones that are frequently driven by cancer‐associated mutations in an aging‐dependent fashion. Additional studies show how aged tissue microenvironments promote the initiation and progression of malignancies, while young healthy tissues actively suppress the outgrowth of malignant clones. Here, we discuss conserved mechanisms that eliminate poorly functioning or potentially malignant cells from our tissues to maintain organismal health and fitness. Natural selection acts to preserve tissue function and prevent disease to maximize reproductive success but these mechanisms wane as reproduction becomes less likely. The ensuing age‐dependent tissue decline can impact the shape and direction of clonal somatic evolution, with lifestyle and exposures influencing its pace and intensity. We also consider how aging‐ and exposure‐dependent clonal expansions of “oncogenic” mutations might both increase cancer risk late in life and contribute to tissue decline and non‐malignant disease. Still, we can marvel at the ability of our bodies to avoid cancers and other diseases despite the accumulation of billions of cells with cancer‐associated mutations.

Cell competition, cooperation, and cancer

Complex multicellular organisms require quantitative and qualitative assessments on each of their constitutive cell types to ensure coordinated and cooperative behavior towards overall functional proficiency. Cell competition represents one of the operating arms of such quality control mechanisms and relies on fitness comparison among individual cells. However, what is exactly included in the fitness equation for each cell type is still uncertain. Evidence will be discussed to suggest that the ability of the cell to integrate and collaborate within the organismal community represents an integral part of the best fitness phenotype. Thus, under normal conditions, cell competition will select against the emergence of altered cells with disruptive behavior towards tissue integrity and/or tissue pattern formation. On the other hand, the winner phenotype prevailing as a result of cell competition does not entail, by itself, any degree of growth autonomy. While cell competition per se should not be considered as a biological driving force towards the emergence of the neoplastic phenotype, it is possible that the molecular machinery involved in the winner/loser interaction could be hijacked by evolving cancer cell populations.

Aging and Cancer: The Waning of Community Bonds

Cancer often arises in the context of an altered tissue landscape. We argue that a major contribution of aging towards increasing the risk of neoplastic disease is conveyed through effects on the microenvironment. It is now firmly established that aged tissues are prone to develop clones of altered cells, most of which are compatible with a normal histological appearance. Such increased clonogenic potential results in part from a generalized decrease in proliferative fitness, favoring the emergence of more competitive variant clones. However, specific cellular genotypes can emerge with reduced cooperative and integrative capacity, leading to disruption of tissue architecture and paving the way towards progression to overt neoplastic phenotypes.

The Duality of Caspases in Cancer, as Told through the Fly

Caspases, a family of cysteine-aspartic proteases, have an established role as critical components in the activation and initiation of apoptosis. Alongside this a variety of non-apoptotic caspase functions in proliferation, differentiation, cellular plasticity and cell migration have been reported. The activity level and context are important factors in determining caspase function. As a consequence of their critical role in apoptosis and beyond, caspases are uniquely situated to have pathological roles, including in cancer. Altered caspase function is a common trait in a variety of cancers, with apoptotic evasion defined as a "hallmark of cancer". However, the role that caspases play in cancer is much more complex, acting both to prevent and to promote tumourigenesis. This review focuses on the major findings in Drosophila on the dual role of caspases in tumourigenesis. This has major implications for cancer treatments, including chemotherapy and radiotherapy, with the activation of apoptosis being the end goal. However, such treatments may inadvertently have adverse effects on promoting tumour progression and acerbating the cancer. A comprehensive understanding of the dual role of caspases will aid in the development of successful cancer therapeutic approaches.

YAP drives cell competition by activating choline metabolism

Cell competition is a phenomenon that eliminates unfit cells from cell society, a function vital for maintaining cellular and organismal homeostasis. We previously showed that Madin-Darby canine kidney (MDCK) epithelial cells expressing the active form of the transcriptional coactivator Yes-associated protein (YAP) are apically extruded when surrounded by normal MDCK cells. Although we demonstrated that the arachidonic acid (AA) cascade is involved in YAP-dependent apical extrusion, the metabolic events leading to this outcome remained unclear. Here, we present the results of metabolomic analysis that identified phosphatidylcholine (PC) biosynthesis as the most significant player in this process. Removal of the PC biosynthetic components choline and methionine from culture medium inhibited YAP-dependent apical extrusion. Inhibition of either choline uptake or metabolic cycles involving choline or methionine also decreased YAP-dependent apical extrusion. At the molecular level, active YAP induced expression of the genes encoding glycerophosphocholine phosphodiesterase 1 (GPCPD1) and lecithin-cholesterol acyltransferase (LCAT), which are involved in choline metabolism. Our results indicate that YAP-dependent cell competition depends on YAP-mediated activation of the choline metabolic cycle.

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.