Epithelial neoplasia in Drosophila entails switch to primitive cell states

Oncogenic signaling in the adult Drosophila prostate-like accessory gland leads to activation of a conserved pro-tumorigenic program, in the absence of proliferation

Drosophila models for tumorigenesis and metastasis have revealed conserved mechanisms of signaling that are also involved in mammalian cancer. Many of these models use the proliferating tissues of the larval stages of Drosophila development, when tissues are highly mitotically active, or stem cells are abundant. Fewer Drosophila tumorigenesis models use adult animals to initiate tumor formation when many tissues are largely terminally differentiated and postmitotic. The Drosophila accessory glands are prostate-like tissues and a model for some aspects of prostate tumorigenesis using this tissue has been explored. In this model, oncogenic signaling was induced during the proliferative stage of accessory gland development, raising the question of how oncogenic activity would impact the terminally differentiated and postmitotic adult tissue. Here, we show that oncogenic signaling in the adult Drosophila accessory gland leads to activation of a conserved pro-tumorigenic program, similar to that observed in mitotic larval tissues, but in the absence of proliferation. Oncogenic signaling in the adult postmitotic gland leads to tissue hyperplasia with nuclear anaplasia and aneuploidy through endoreduplication, which increases polyploidy and occasionally results in non-mitotic neoplastic-like extrusions. We compare gene expression changes in our Drosophila model with that of endocycling prostate cancer cells induced by chemotherapy, which potentially mediate tumor recurrence after treatment. Similar signaling pathways are activated in the Drosophila gland and endocycling cancer cells, suggesting the adult accessory glands provide a useful model for aspects of prostate cancer progression that do not involve cellular proliferation.

Conditional knockdown protocol for studying cellular communication using Drosophila melanogaster wing imaginal disc

Apicobasal polarity determinants are potential tumor suppressors that have been extensively studied. However, the precise mechanisms by which their misregulation disrupts tissue homeostasis are not fully understood. Here, we present a comprehensive protocol for establishing a conditional RNAi knockdown of scribble in Drosophila wing imaginal disc. We describe steps for generating fly lines, conditional knockdown in host stocks, and sample preparation. We then detail procedures for imaging, image analysis, and verification of wing disc phenotypes by various antibodies. For complete details on the use and execution of this protocol, please refer to Huang et al.1.

Human ERG oncoprotein represses a Drosophila LIM domain binding protein-coding gene Chip

Human ETS Related Gene, ERG, a master transcription factor, turns oncogenic upon its out-of-context activation in diverse developmental lineages. However, the mechanism underlying its lineage-specific activation of Notch (N), Wnt, or EZH2-three well-characterized oncogenic targets of ERG-remains elusive. We reasoned that deep homology in genetic tool kits might help uncover such elusive cancer mechanisms in Drosophila. By heterologous gain of human ERG in Drosophila, here we reveal Chip, which codes for a transcriptional coactivator, LIM-domain-binding (LDB) protein, as its novel target. ERG represses Drosophila Chip via its direct binding and, indirectly, via E(z)-mediated silencing of its promoter. Downregulation of Chip disrupts LIM-HD complex formed between Chip and Tailup (Tup)-a LIM-HD transcription factor-in the developing notum. A consequent activation of N-driven Wg signaling leads to notum-to-wing transdetermination. These fallouts of ERG gain are arrested upon a simultaneous gain of Chip, sequestration of Wg ligand, and, alternatively, loss of N signaling or E(z) activity. Finally, we show that the human LDB1, a homolog of Drosophila Chip, is repressed in ERG-positive prostate cancer cells. Besides identifying an elusive target of human ERG, our study unravels an underpinning of its lineage-specific carcinogenesis.

Oxidative Stress Is Associated with Overgrowth in Drosophila l(3)mbt Mutant Imaginal Discs

The loss-of-function conditions for an l(3)malignant brain tumour (l(3)mbt) in larvae reared at 29 °C results in malignant brain tumours and hyperplastic imaginal discs. Unlike the former that have been extensively characterised, little is known about the latter. Here we report the results of a study of the hyperplastic l(3)mbt mutant wing imaginal discs. We identify the l(3)mbt wing disc tumour transcriptome and find it to include genes involved in reactive oxygen species (ROS) metabolism. Furthermore, we show the presence of oxidative stress in l(3)mbt hyperplastic discs, even in apoptosis-blocked conditions, but not in l(3)mbt brain tumours. We also find that chemically blocking oxidative stress in l(3)mbt wing discs reduces the incidence of wing disc overgrowths. Our results reveal the involvement of oxidative stress in l(3)mbt wing discs hyperplastic growth.

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.

A Drosophila model of oral peptide therapeutics for adult intestinal stem cell tumors

Peptide therapeutics, unlike small-molecule drugs, display crucial advantages of target specificity and the ability to block large interacting interfaces, such as those of transcription factors. The transcription co-factor of the Hippo pathway, YAP/Yorkie (Yki), has been implicated in many cancers, and is dependent on its interaction with the DNA-binding TEAD/Sd proteins via a large Ω-loop. In addition, the mammalian vestigial-like (VGLL) proteins, specifically their TONDU domain, competitively inhibit YAP-TEAD interaction, resulting in arrest of tumor growth. Here, we show that overexpression of the TONDU peptide or its oral uptake leads to suppression of Yki-driven intestinal stem cell tumors in the adult Drosophila midgut. In addition, comparative proteomic analyses of peptide-treated and untreated tumors, together with chromatin immunoprecipitation analysis, reveal that integrin pathway members are part of the Yki-oncogenic network. Collectively, our findings establish Drosophila as a reliable in vivo platform to screen for cancer oral therapeutic peptides and reveal a tumor suppressive role for integrins in Yki-driven tumors.This article has an associated First Person interview with the first author of the paper.

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

Hh signaling from de novo organizers drive lgl neoplasia in Drosophila epithelium

The Hedgehog (Hh) morphogen regulates growth and patterning. Since Hh signaling is also implicated in carcinogenesis, it is conceivable that de novo Hh-secreting organizers, if formed in association with oncogenic hit could be tumor-cooperative. Here we validate this hypothesis using the Drosophila model of cooperative epithelial carcinogenesis. We generate somatic clones with simultaneous loss of tumor suppressor, Lgl, and gain of the posterior compartment selector, Engrailed (En), known to induce synthesis of Hh. We show that lgl UAS-en clones in the anterior wing compartment trigger Hh signaling cascade via cross-talk with their Ci-expressing wild type cell neighbors. Hh-Dpp signaling from clone boundaries of such ectopically formed de novo organizers in turn drive lgl carcinogenesis. By contrast, Ci-expressing lgl clones transform by autocrine and/or juxtracine activation of Hh signaling in only the posterior compartment. We further show that sequestration of the Hh ligand or loss of Dpp receptor, Tkv, in these Hh-sending or -receiving lgl clones arrested their carcinogenesis. Our results therefore reveal a hitherto unrecognized mechanism of tumor cooperation by developmental organizers, which are induced fortuitously by oncogenic hits.

The Initial Stage of Tumorigenesis in Drosophila Epithelial Tissues

Cancer development originates in a single mutant cell transformed from a normal cell, including further evolution of pro-tumor cells through additional mutations into malignant cancer tissues. Data from recent studies, however, suggest that most pro-tumor cells do not develop into tumors but remain dormant within or are prophylactically eliminated from tissues unless bestowed with additional driver mutations. Drosophila melanogaster has provided very efficient model systems, such as imaginal discs and ovarian follicular epithelia, to study the initial stage of tumorigenesis. This review will focus on the behaviors of emerging pro-tumor cells surrounded by normal cells and situations where they initiate tumor development.

Inducing cancer indolence by targeting mitochondrial Complex I is potentiated by blocking macrophage-mediated adaptive responses

Converting carcinomas in benign oncocytomas has been suggested as a potential anti-cancer strategy. One of the oncocytoma hallmarks is the lack of respiratory complex I (CI). Here we use genetic ablation of this enzyme to induce indolence in two cancer types, and show this is reversed by allowing the stabilization of Hypoxia Inducible Factor-1 alpha (HIF-1α). We further show that on the long run CI-deficient tumors re-adapt to their inability to respond to hypoxia, concordantly with the persistence of human oncocytomas. We demonstrate that CI-deficient tumors survive and carry out angiogenesis, despite their inability to stabilize HIF-1α. Such adaptive response is mediated by tumor associated macrophages, whose blockage improves the effect of CI ablation. Additionally, the simultaneous pharmacological inhibition of CI function through metformin and macrophage infiltration through PLX-3397 impairs tumor growth in vivo in a synergistic manner, setting the basis for an efficient combinatorial adjuvant therapy in clinical trials.

A genetic mosaic screen identifies genes modulating Notch signaling in Drosophila

Notch signaling is conserved in most multicellular organisms and plays critical roles during animal development. The core components and major signal transduction mechanism of Notch signaling have been extensively studied. However, our understanding of how Notch signaling activity is regulated in diverse developmental processes still remains incomplete. Here, we report a genetic mosaic screen in Drosophila melanogaster that leads to identification of Notch signali ng modulators during wing development. We discovered a group of genes required for the formation of the fly wing margin, a developmental process that is strictly dependent on the balanced Notch signaling activity. These genes encode transcription factors, protein phosphatases, vacuolar ATPases and factors required for RNA transport, stability, and translation. Our data support the view that Notch signaling is controlled through a wide range of molecular processes. These results also provide foundations for further study by showing that Me31B and Wdr62 function as two novel modulators of Notch signaling activity.

A switch in transcription and cell fate governs the onset of an epigenetically-deregulated tumor in Drosophila

Tumor initiation is often linked to a loss of cellular identity. Transcriptional programs determining cellular identity are preserved by epigenetically-acting chromatin factors. Although such regulators are among the most frequently mutated genes in cancer, it is not well understood how an abnormal epigenetic condition contributes to tumor onset. In this work, we investigated the gene signature of tumors caused by disruption of the Drosophila epigenetic regulator, polyhomeotic (ph). In larval tissue ph mutant cells show a shift towards an embryonic-like signature. Using loss- and gain-of-function experiments we uncovered the embryonic transcription factor knirps (kni) as a new oncogene. The oncogenic potential of kni lies in its ability to activate JAK/STAT signaling and block differentiation. Conversely, tumor growth in ph mutant cells can be substantially reduced by overexpressing a differentiation factor. This demonstrates that epigenetically derailed tumor conditions can be reversed when targeting key players in the transcriptional network.

When an animal is developing as an embryo, different cells start to specialize into the specific cell types needed to form the tissues and organs of the body. How an individual cell commits to become a certain type of cell is mostly determined by which of the genes in its DNA are active. In animal cells, DNA is wrapped around proteins called histones, and one way that cells can maintain their distinct pattern of gene activity is via chemical tags on the histones. These tags can switch nearby genes on or off, and are added or removed by other proteins called epigenetic regulators. The epigenetic tags are also stably inherited when the cell divides, meaning that a cell’s identity can be maintained over many cell generations.

If epigenetic regulators fail to work properly or get disrupted, the pattern of gene activity in a cell becomes altered. As a consequence, that cell can lose its identity and will often turn into a cancer cell. In fact, mutations in epigenetic regulators are found in several human cancers. It is not yet understood how these changes in gene expression lead cells to become cancerous.

Torres et al. have now analyzed an epigenetic regulator called Polyhomeotic in developing larvae of the fruit fly, Drosophila melanogaster. The results show that when Polyhomeotic is not produced the fly larvae develop tumors. Moreover, the mutant cells without Polyhomeotic had different gene expression profiles compared to normal cells. This in turn caused the mutant cells, which had previously committed to a certain fate, to become more like the unspecialized cells found in early embryos.

Torres et al. next showed that, among the genes that were incorrectly regulated when Polyhomeotic’s activity was compromised, one gene called knirps was switched on by mistake, which led the mutant cells to become tumor cells. When the activity of knirps was reduced instead, almost no tumors grew. Additionally, Torres et al. found that the protein encoded by knirps activates a signaling pathway that keeps tumor cells unspecialized by blocking their normal progress to a more mature and specialized state – a process known as differentiation. Experimentally raising the levels of a different molecule that ultimately promotes differentiation caused the tumor cells to grow less.

These findings suggest that tumors caused when epigenetic regulation goes awry may be reversed by targeting key genes such as knirps. Further work is now needed to test whether these findings will also extend to humans. Forcing cancer cells from a highly dividing, non-specialized state into a dead-end, mature state may lead to new ways to treat cancer.

Modelling Cooperative Tumorigenesis in Drosophila

The development of human metastatic cancer is a multistep process, involving the acquisition of several genetic mutations, tumour heterogeneity, and interactions with the surrounding microenvironment. Due to the complexity of cancer development in mammals, simpler model organisms, such as the vinegar fly, Drosophila melanogaster, are being utilized to provide novel insights into the molecular mechanisms involved. In this review, we highlight recent advances in modelling tumorigenesis using the Drosophila model, focusing on the cooperation of oncogenes or tumour suppressors, and the interaction of mutant cells with the surrounding tissue in epithelial tumour initiation and progression.

The Scribble Cell Polarity Module in the Regulation of Cell Signaling in Tissue Development and Tumorigenesis

The Scribble cell polarity module, comprising Scribbled (Scrib), Discs-large (Dlg) and Lethal-2-giant larvae (Lgl), has a tumor suppressive role in mammalian epithelial cancers. The Scribble module proteins play key functions in the establishment and maintenance of different modes of cell polarity, as well as in the control of tissue growth, differentiation and directed cell migration, and therefore are major regulators of tissue development and homeostasis. Whilst molecular details are known regarding the roles of Scribble module proteins in cell polarity regulation, their precise mode of action in the regulation of other key cellular processes remains enigmatic. An accumulating body of evidence indicates that Scribble module proteins play scaffolding roles in the control of various signaling pathways, which are linked to the control of tissue growth, differentiation and cell migration. Multiple Scrib, Dlg and Lgl interacting proteins have been discovered, which are involved in diverse processes, however many function in the regulation of cellular signaling. Herein, we review the components of the Scrib, Dlg and Lgl protein interactomes, and focus on the mechanism by which they regulate cellular signaling pathways in metazoans, and how their disruption leads to cancer.

miR-9a mediates the role of Lethal giant larvae as an epithelial growth inhibitor in Drosophila

Drosophila lethal giant larvae (lgl) encodes a conserved tumor suppressor with established roles in cell polarity, asymmetric division, and proliferation control. Lgl's human orthologs, HUGL1 and HUGL2, are altered in human cancers, however, its mechanistic role as a tumor suppressor remains poorly understood. Based on a previously established connection between Lgl and Fragile X protein (FMRP), a miRNA-associated translational regulator, we hypothesized that Lgl may exert its role as a tumor suppressor by interacting with the miRNA pathway. Consistent with this model, we found that lgl is a dominant modifier of Argonaute1 overexpression in the eye neuroepithelium. Using microarray profiling we identified a core set of ten miRNAs that are altered throughout tumorigenesis in Drosophila lgl mutants. Among these are several miRNAs previously linked to human cancers including miR-9a, which we found to be downregulated in lgl neuroepithelial tissues. To determine whether miR-9a can act as an effector of Lgl in vivo, we overexpressed it in the context of lgl knock-down by RNAi and found it able to reduce the overgrowth phenotype caused by Lgl loss in epithelia. Furthermore, cross-comparisons between miRNA and mRNA profiling in lgl mutant tissues and human breast cancer cells identified thrombospondin (tsp) as a common factor altered in both fly and human breast cancer tumorigenesis models. Our work provides the first evidence of a functional connection between Lgl and the miRNA pathway, demonstrates that miR-9a mediates Lgl's role in restricting epithelial proliferation, and provides novel insights into pathways controlled by Lgl during tumor progression.

Summary: Mir-9a overexpression can suppress the overgrowth phenotype caused by Lgl knock-down in epithelia. Gene profiling identifies pathways dysregulated in lgl mutants and shared features between flies and human cancer cells.

Oncogenic cooperation between Yorkie and the conserved microRNA miR-8 in the wing disc of Drosophila

Tissue growth has to be carefully controlled to generate well-functioning organs. microRNAs are small noncoding RNAs that modulate the activity of target genes and play a pivotal role in animal development. Understanding the functions of microRNAs in development requires the identification of their target genes. Here, we find that the conserved microRNA miR-8/miR-200 controls tissue growth and homeostasis in the Drosophila wing imaginal disc. Upregulation of miR-8 causes the repression of Yorkie, the effector of the Hippo pathway in Drosophila, and reduces tissue size. Remarkably, coexpression of Yorkie and miR-8 causes the formation of neoplastic tumors. We show that upregulation of miR-8 represses the growth inhibitor brinker, and depletion of brinker cooperates with Yorkie in the formation of neoplastic tumors. Hence, miR-8 modulates a positive growth regulator, Yorkie, and a negative growth regulator, brinker. Deregulation of this network can result in the loss of tissue homeostasis and the formation of tumors.

Selector genes display tumor cooperation and inhibition in Drosophila epithelium in a developmental context-dependent manner

During animal development, selector genes determine identities of body segments and those of individual organs. Selector genes are also misexpressed in cancers, although their contributions to tumor progression per se remain poorly understood. Using a model of cooperative tumorigenesis, we show that gain of selector genes results in tumor cooperation, but in only select developmental domains of the wing, haltere and eye-antennal imaginal discs of Drosophila larva. Thus, the field selector, Eyeless (Ey), and the segment selector, Ultrabithorax (Ubx), readily cooperate to bring about neoplastic transformation of cells displaying somatic loss of the tumor suppressor, Lgl, but in only those developmental domains that express the homeo-box protein, Homothorax (Hth), and/or the Zinc-finger protein, Teashirt (Tsh). In non-Hth/Tsh-expressing domains of these imaginal discs, however, gain of Ey in lgl^- somatic clones induces neoplastic transformation in the distal wing disc and haltere, but not in the eye imaginal disc. Likewise, gain of Ubx in lgl^- somatic clones induces transformation in the eye imaginal disc but not in its endogenous domain, namely, the haltere imaginal disc. Our results reveal that selector genes could behave as tumor drivers or inhibitors depending on the tissue contexts of their gains.

Human Cancer Cells Signal Their Competitive Fitness Through MYC Activity

MYC-mediated cell competition is a cell-cell interaction mechanism known to play an evolutionary role during development from Drosophila to mammals. Cells expressing low levels of MYC, called losers, are committed to die by nearby cells with high MYC activity, called winners, that overproliferate to compensate for cell loss, so that the fittest cells be selected for organ formation. Given MYC's consolidated role in oncogenesis, cell competition is supposed to be relevant to cancer, but its significance in human malignant contexts is largely uncharacterised. Here we show stereotypical patterns of MYC-mediated cell competition in human cancers: MYC-upregulating cells and apoptotic cells were indeed repeatedly found at the tumour-stroma interface and within the tumour parenchyma. Cell death amount in the stromal compartment and MYC protein level in the tumour were highly correlated regardless of tumour type and stage. Moreover, we show that MYC modulation in heterotypic co-cultures of human cancer cells is sufficient as to subvert their competitive state, regardless of genetic heterogeneity. Altogether, our findings suggest that the innate role of MYC-mediated cell competition in development is conserved in human cancer, with malignant cells using MYC activity to colonise the organ at the expense of less performant neighbours.

Induction and Diagnosis of Tumors in Drosophila Imaginal Disc Epithelia

In the early stages of cancer, transformed mutant cells show cytological abnormalities, begin uncontrolled overgrowth, and progressively disrupt tissue organization. Drosophila melanogaster has emerged as a popular experimental model system in cancer biology to study the genetic and cellular mechanisms of tumorigenesis. In particular, genetic tools for Drosophila imaginal discs (developing epithelia in larvae) enable the creation of transformed pro-tumor cells within a normal epithelial tissue, a situation similar to the initial stages of human cancer. A recent study of tumorigenesis in Drosophila wing imaginal discs, however, showed that tumor initiation depends on the tissue-intrinsic cytoarchitecture and the local microenvironment, suggesting that it is important to consider the region-specific susceptibility to tumorigenic stimuli in evaluating tumor phenotypes in imaginal discs. To facilitate phenotypic analysis of tumor progression in imaginal discs, here we describe a protocol for genetic experiments using the GAL4-UAS system to induce neoplastic tumors in wing imaginal discs. We further introduce a diagnosis method to classify the phenotypes of clonal lesions induced in imaginal epithelia, as a clear classification method to discriminate various stages of tumor progression (such as hyperplasia, dysplasia, or neoplasia) had not been described before. These methods might be broadly applicable to the clonal analysis of tumor phenotypes in various organs in Drosophila.

MYC, Cell Competition, and Cell Death in Cancer: The Inseparable Triad

Deregulation of MYC family proteins in cancer is associated with a global reprogramming of gene expression, ultimately promoting glycolytic pathways, cell growth, and proliferation. It is well known that MYC upregulation triggers cell-autonomous apoptosis in normal tissues, while frankly malignant cells develop resistance to apoptotic stimuli, partly resulting from MYC addiction. As well as inducing cell-autonomous apoptosis, MYC upregulation is able to trigger non cell-autonomous apoptotic death through an evolutionarily conserved mechanism known as “cell competition”. With regard to this intimate and dual relationship between MYC and cell death, recent evidence obtained in Drosophila models of cancer has revealed that, in early tumourigenesis, MYC upregulation guides the clonal expansion of mutant cells, while the surrounding tissue undergoes non-cell autonomous death. Apoptosis inhibition in this context was shown to restrain tumour growth and to restore a wild-type phenotype. This suggests that cell-autonomous and non cell-autonomous apoptosis dependent on MYC upregulation may shape tumour growth in different ways, soliciting the need to reconsider the role of cell death in cancer in the light of this new level of complexity. Here we review recent literature about MYC and cell competition obtained in Drosophila, with a particular emphasis on the relevance of cell death to cell competition and, more generally, to cancer. Possible implications of these findings for the understanding of mammalian cancers are also discussed.

Epithelial Tumors Originate in Tumor Hotspots, a Tissue-Intrinsic Microenvironment

Malignant tumors are caused by uncontrolled proliferation of transformed mutant cells that have lost the ability to maintain tissue integrity. Although a number of causative genetic backgrounds for tumor development have been discovered, the initial steps mutant cells take to escape tissue integrity and trigger tumorigenesis remain elusive. Here, we show through analysis of conserved neoplastic tumor-suppressor genes (nTSGs) in Drosophila wing imaginal disc epithelia that tumor initiation depends on tissue-intrinsic local cytoarchitectures, causing tumors to consistently originate in a specific region of the tissue. In this “tumor hotspot” where cells constitute a network of robust structures on their basal side, nTSG-deficient cells delaminate from the apical side of the epithelium and begin tumorigenic overgrowth by exploiting endogenous Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling activity. Conversely, in other regions, the “tumor coldspot” nTSG-deficient cells are extruded toward the basal side and undergo apoptosis. When the direction of delamination is reversed through suppression of RhoGEF2, an activator of the Rho family small GTPases, and JAK/STAT is activated ectopically in these coldspot nTSG-deficient cells, tumorigenesis is induced. These data indicate that two independent processes, apical delamination and JAK/STAT activation, are concurrently required for the initiation of nTSG-deficient-induced tumorigenesis. Given the conservation of the epithelial cytoarchitecture, tumorigenesis may be generally initiated from tumor hotspots by a similar mechanism.

A genetic study in Drosophila reveals a mechanism of tumorigenesis by which pro-tumor cells initiate dysplastic tumor growth at specific "hotspot" locations in epithelial tissues.

Transformed mutant cells (pro-tumor cells) can evolve through a multistep process in which they become tumorigenic and invasive. Many genes that are involved in the different steps towards cancer development have been identified; however, how certain mutant cells destroy normal tissue organization and undergo uncontrolled proliferation during the initial stages of this process remains largely unclear. Using the epithelial tissue of the wing imaginal discs of the fruit fly (Drosophila melanogaster) larvae as a model system, we have analyzed these initial stages of inducing tumors by depletion of a neoplastic tumor suppressor gene (nTSG). We discovered that these tumors always originate from specific regions of the epithelial tissue of the wing disc. We show that in other regions that we dubbed “tumor coldspots” and that lack specific cellular structures, pro-tumor cells are eliminated from the epithelial tissue by the surrounding cells. However, in “tumor hotspots,” cells constitute specific structures in their basal side, and we found that pro-tumor cells successfully avoid potential elimination and deviate from the apical side of the tissue, initiating tumorous overgrowth. Our findings reveal the molecular and cellular mechanisms underlying the initial steps of tumorigenesis at tumor hotspots in the Drosophila imaginal wing discs.

Localized epigenetic silencing of a damage-activated WNT enhancer limits regeneration in mature Drosophila imaginal discs

Many organisms lose the capacity to regenerate damaged tissues as they mature. Damaged Drosophila imaginal discs regenerate efficiently early in the third larval instar (L3) but progressively lose this ability. This correlates with reduced damage-responsive expression of multiple genes, including the WNT genes wingless (wg) and Wnt6. We demonstrate that damage-responsive expression of both genes requires a bipartite enhancer whose activity declines during L3. Within this enhancer, a damage-responsive module stays active throughout L3, while an adjacent silencing element nucleates increasing levels of epigenetic silencing restricted to this enhancer. Cas9-mediated deletion of the silencing element alleviates WNT repression, but is, in itself, insufficient to promote regeneration. However, directing Myc expression to the blastema overcomes repression of multiple genes, including wg, and restores cellular responses necessary for regeneration. Localized epigenetic silencing of damage-responsive enhancers can therefore restrict regenerative capacity in maturing organisms without compromising gene functions regulated by developmental signals.

Aerobic glycolysis tunes YAP/TAZ transcriptional activity

Increased glucose metabolism and reprogramming toward aerobic glycolysis are a hallmark of cancer cells, meeting their metabolic needs for sustained cell proliferation. Metabolic reprogramming is usually considered as a downstream consequence of tumor development and oncogene activation; growing evidence indicates, however, that metabolism on its turn can support oncogenic signaling to foster tumor malignancy. Here, we explored how glucose metabolism regulates gene transcription and found an unexpected link with YAP/TAZ, key transcription factors regulating organ growth, tumor cell proliferation and aggressiveness. When cells actively incorporate glucose and route it through glycolysis, YAP/TAZ are fully active; when glucose metabolism is blocked, or glycolysis is reduced, YAP/TAZ transcriptional activity is decreased. Accordingly, glycolysis is required to sustain YAP/TAZ pro-tumorigenic functions, and YAP/TAZ are required for the full deployment of glucose growth-promoting activity. Mechanistically we found that phosphofructokinase (PFK1), the enzyme regulating the first committed step of glycolysis, binds the YAP/TAZ transcriptional cofactors TEADs and promotes their functional and biochemical cooperation with YAP/TAZ. Strikingly, this regulation is conserved in Drosophila, where phosphofructokinase is required for tissue overgrowth promoted by Yki, the fly homologue of YAP. Moreover, gene expression regulated by glucose metabolism in breast cancer cells is strongly associated in a large dataset of primary human mammary tumors with YAP/TAZ activation and with the progression toward more advanced and malignant stages. These findings suggest that aerobic glycolysis endows cancer cells with particular metabolic properties and at the same time sustains transcription factors with potent pro-tumorigenic activities such as YAP/TAZ.

Multiple strategies of oxygen supply in Drosophila malignancies identify tracheogenesis as a novel cancer hallmark

Angiogenesis is the term used to describe all the alterations in blood vessel growth induced by a tumour mass following hypoxic stress. The occurrence of multiple strategies of vessel recruitment favours drug resistance, greatly complicating the treatment of certain tumours. In Drosophila, oxygen is conveyed to the internal organs by the tracheal system, a closed tubular network whose role in cancer growth is so far unexplored. We found that, as observed in human cancers, Drosophila malignant cells suffer from oxygen shortage, release pro-tracheogenic factors, co-opt nearby vessels and get incorporated into the tracheal walls. We also found that the parallelisms observed in cellular behaviours are supported by genetic and molecular conservation. Finally, we identified a molecular circuitry associated with the differentiation of cancer cells into tracheal cells. In summary, our findings identify tracheogenesis as a novel cancer hallmark in Drosophila, further expanding the power of the fly model in cancer research.

The transcriptional response to tumorigenic polarity loss in Drosophila

Loss of polarity correlates with progression of epithelial cancers, but how plasma membrane misorganization drives oncogenic transcriptional events remains unclear. The polarity regulators of the Drosophila Scribble (Scrib) module are potent tumor suppressors and provide a model for mechanistic investigation. RNA profiling of Scrib mutant tumors reveals multiple signatures of neoplasia, including altered metabolism and dedifferentiation. Prominent among these is upregulation of cytokine-like Unpaired (Upd) ligands, which drive tumor overgrowth. We identified a polarity-responsive enhancer in upd3, which is activated in a coincident manner by both JNK-dependent Fos and aPKC-mediated Yki transcription. This enhancer, and Scrib mutant overgrowth in general, are also sensitive to activity of the Polycomb Group (PcG), suggesting that PcG attenuation upon polarity loss potentiates select targets for activation by JNK and Yki. Our results link epithelial organization to signaling and epigenetic regulators that control tissue repair programs, and provide insight into why epithelial polarity is tumor-suppressive.

DOI:http://dx.doi.org/10.7554/eLife.03189.001

The cavities and organs within our body are lined with epithelial cells, which connect to each other to form continuous barriers. These cells have a highly polarized structure in which different components are found at the top and bottom of cells. In the fruit fly and most other animals, three genes known as the Scribble module control the polarity of epithelial cells. If these genes are faulty, the cells lose their polarity, break the epithelial barrier, and grow rapidly to form a tumor. Most malignant tumors that form from epithelial cells have lost normal cell polarity, so understanding how the organization and growth of epithelial cells are linked is a critical question.

It is not clear how the loss of cell polarity can drive tumor formation. Here, Bunker et al. used a technique called RNA sequencing to study the expression of genes in tumor cells that have mutations in the Scribble module. Hundreds of genes in the tumor cells had different levels of expression from the levels seen in normal fly cells. One of these is a gene called upd3, which was expressed much more highly in tumor cells than in normal cells. This gene activates a signaling pathway—called the JAK/STAT pathway—that promotes cell growth and division in many animals. Bunker et al. found that experimentally lowering the activity of the JAK/STAT pathway reduced the growth of the tumor cells that had lost normal polarity.

Further experiments show that disrupting the layer of epithelial cells activates two other signaling pathways that work together to switch on the upd3 gene when cell polarity is lost. Proteins belonging to the Polycomb Group also control the expression of upd3 and other genes involved in cell growth by altering how genetic material is packaged in cells.

The similarities between this response and the response to tissue damage suggest that the loss of polarity drives tumor formation through an unstoppable wound-healing reaction. Therefore, Bunker et al.'s findings link the formation of epithelial tumors to the signaling pathways that control the repair of damaged tissues.

DOI:http://dx.doi.org/10.7554/eLife.03189.002

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.