Intrinsic tumor suppression and epithelial maintenance by endocytic activation of Eiger/TNF signaling in Drosophila

Drosophila cancer models

Cancer is driven by complex genetic and cellular mechanisms. Recently, the Drosophila community has become increasingly interested in exploring cancer issues. The Drosophila field has made seminal contributions to many of the mechanisms that are fundamental to the cancer process; several of these mechanisms have already been validated in vertebrates. Less well known are the Drosophila field's early direct contributions to the cancer field: some of the earliest tumor suppressors were identified in flies. In this review, we identify major contributions that Drosophila studies have made toward dissecting the pathways and mechanisms underlying tumor progression. We also highlight areas, such as drug discovery, where we expect Drosophila studies to make a major scientific impact in the future.

Cell competition and its implications for development and cancer

Cell competition is a struggle for existence between cells in heterogeneous tissues of multicellular organisms. Loser cells, which die during cell competition, are normally viable when grown only with other loser cells, but when mixed with winner cells, they are at a growth disadvantage and undergo apoptosis. Intriguingly, several recent studies have revealed that cells bearing mutant tumor-suppressor genes, which show overgrowth and tumorigenesis in a homotypic situation, are frequently eliminated, through cell competition, from tissues in which they are surrounded by wild-type cells. Here, we focus on the regulation of cellular competitiveness and the mechanism of cell competition as inferred from two different categories of mutant cells: (1) slower-growing cells and (2) structurally defective cells. We also discuss the possible role of cell competition as an intrinsic homeostasis system through which normal cells sense and remove aberrant cells, such as precancerous cells, to maintain the integrity and normal development of tissues and organs.

Modeling tumor invasion and metastasis in Drosophila

Conservation of major signaling pathways between humans and flies has made Drosophila a useful model organism for cancer research. Our understanding of the mechanisms regulating cell growth, differentiation and development has been considerably advanced by studies in Drosophila. Several recent high profile studies have examined the processes constraining the metastatic growth of tumor cells in fruit fly models. Cell invasion can be studied in the context of an in vivo setting in flies, enabling the genetic requirements of the microenvironment of tumor cells undergoing metastasis to be analyzed. This Perspective discusses the strengths and limitations of Drosophila models of cancer invasion and the unique tools that have enabled these studies. It also highlights several recent reports that together make a strong case for Drosophila as a system with the potential for both testing novel concepts in tumor progression and cell invasion, and for uncovering players in metastasis.

Deciphering tumor-suppressor signaling in flies: genetic link between Scribble/Dlg/Lgl and the Hippo pathways

Loss of apico-basal polarity is one of the crucial factors that drives epithelial tumor progression. scribble/discs large/lethal giant larvae (scrib/dlg/lgl), a group of apico-basal polarity genes, were initially identified as members of "neoplastic" tumor-suppressors in flies. The components of the Hippo signaling pathway, which is crucial for organ size control and cancer development, were also identified through Drosophila genetic screens as members of "hyperplastic" tumor-suppressors. Accumulating evidence in recent studies implies that these two tumor-suppressor signaling pathways are not mutually exclusive but rather cooperatively act to give rise to highly malignant tumors. The interaction of these tumor-suppressor pathways could include deregulations of actin cytoskeleton, cell-cell contact, and apical-domain size of the epithelial cell.

Tumor invasion and metastasis in Drosophila: a bold past, a bright future

Invasion and metastasis are the most deadly hallmarks of cancer. Once a cancer has acquired the ability to colonize new sites in the body it becomes dramatically more difficult to treat. This has made it a focus of much of cancer research. The humble fruit fly, Drosophila melanogaster, has despite its relative simplicity, made significant contributions to the understanding of tumor progression. In this review we outline and highlight those with an emphasis on modeling the genetic and epigenetic changes required for invasion and metastasis. We will revisit the early years of cancer modeling in Drosophila where the first parallels were drawn between Drosophila and vertebrate neoplasms and highlight recent advances using genetic screens and interactions with the epithelial microenvironment and innate immune system. We focus on the power and limitations of current fly models of metastasis.

NOPO modulates Egr-induced JNK-independent cell death in Drosophila

Tumor necrosis factor (TNF) family ligands play essential roles in regulating a variety of cellular processes including proliferation, differentiation and survival. Expression of Drosophila TNF ortholog Eiger (Egr) induces JNK-dependent cell death, while the roles of caspases in this process remain elusive. To further delineate the Egr-triggered cell death pathway, we performed a genetic screen to identify dominant modifiers of the Egr-induced cell death phenotype. Here we report that Egr elicits a caspase-mediated cell death pathway independent of JNK signaling. Furthermore, we show NOPO, the Drosophila ortholog of TRIP (TRAF interacting protein) encoding an E3 ubiquitin ligase, modulates Egr-induced Caspase-mediated cell death through transcriptional activation of pro-apoptotic genes reaper and hid. Finally, we found Bendless and dUEV1a, an ubiquitin-conjugating E2 enzyme complex, regulates NOPO-triggered cell death. Our results indicate that the Ben-dUEV1a complex constitutes a molecular switch that bifurcates the Egr-induced cell death signaling into two pathways mediated by JNK and caspases respectively.

Mitogenic signaling from apoptotic cells in Drosophila

Apoptotic cells of Drosophila not only activate caspases, but also are able to secrete developmental signals like Hedgehog (Hh), Decapentaplegic (Dpp) and Wingless (Wg) before dying. Since Dpp and Wg are secreted in growing tissues and behave as growth factors, it was proposed that they play a role in compensatory proliferation, the process by which a growing blastema can restore normal size after massive apoptosis. We discuss recent results showing that there is normal compensatory proliferation in the absence of Dpp/Wg signaling, thus indicating it has no significant role in the process. Furthermore, we argue that Dpp/Wg signaling is not a resident feature of apoptotic cells, but a side effect of the necessary activation of the JNK pathway. Nevertheless, the ectopic JNK/Dpp/Wg signaling may have an important role in tissue regeneration. Recent work in other organisms suggests that paracrine signaling from apoptotic cells may be of general significance in wound healing and tissue regeneration in metazoans.

Epithelial delamination and migration: lessons from Drosophila

Metastasis is the most deadly phase of cancer progression, during which cells detach from their original niche to invade distant tissues, yet the biological processes underlying the spread of cancer are still poorly understood. The fruit fly Drosophila melanogaster provides important insights in our understanding of how epithelial cells migrate from their original location and find their way into surrounding and distant tissues in the metastatic process. Here we review recent studies on the mechanisms of migration of embryonic haemocytes, the macrophage-like immuno-surveillance cells, during normal development and wound healing. We highlight the interesting finding that hydrogen peroxide (H₂O₂) has been identified as the driving force for haemocyte chemotaxis. We also give a special emphasis to studies suggesting the concept that haemocytes, together with the tumor microenvironment, act as potential inducers of the epithelial de-lamination required for tumor invasion. We propose that cell delamination and migration could be uncoupled from loss of cell polarity via a tumor-related inflammatory response.

Elimination of oncogenic neighbors by JNK-mediated engulfment in Drosophila

A newly emerged oncogenic cell in the epithelial population has to confront antitumor selective pressures in the host tissue. However, the mechanisms by which surrounding normal tissue exerts antitumor effects against oncogenically transformed cells are poorly understood. In Drosophila imaginal epithelia, clones of cells mutant for evolutionarily conserved tumor suppressor genes such as scrib or dlg lose their epithelial integrity and are eliminated from epithelia when surrounded by wild-type tissue. Here, we show that surrounding normal cells activate nonapoptotic JNK signaling in response to the emergence of oncogenic mutant cells. This JNK activation leads to upregulation of PVR, the Drosophila PDGF/VEGF receptor. Genetic and time-lapse imaging analyses reveal that PVR expression in surrounding cells activates the ELMO/Mbc-mediated phagocytic pathway, thereby eliminating oncogenic neighbors by engulfment. Our data indicate that JNK-mediated cell engulfment could be an evolutionarily conserved intrinsic tumor-suppression mechanism that eliminates premalignant cells from epithelia.

The flower code and cancer development

It has been postulated that the preliminary steps of cancer known as "cancerization field" could be mediated by a competitive mechanism among mutated and wild-type cells. Cell competition is a process of selection among populations of cells with different fitness: the best adapted cells (winners) survive and proliferate in the tissue at the expense of the less well adapted cells (losers), and these loser cells are eliminated from the tissue by apoptosis. However, the molecular mechanisms mediating this process and the genes involved are still unknown. A mechanism of cell-to-cell communication during cell competition known as the "flower code" has been recently proposed to distinguish loser from winner cells: fwe(ubi) isoform is expressed ubiquitously in the imaginal disc while fwe(Lose) isoforms are expressed specifically during cell competition in the cells to be eliminated. Cell competition has been postulated to have implications in development, tissue homeostasis, regeneration and tumour development; the process of cell competition does not affect the total cell number and organ morphology is maintained because winner cells compensate for the loss. A role of cell competition as the mechanism occurring during initial stages of tumour formation is currently under study.

Identification of novel Ras-cooperating oncogenes in Drosophila melanogaster: a RhoGEF/Rho-family/JNK pathway is a central driver of tumorigenesis

We have shown previously that mutations in the apico-basal cell polarity regulators cooperate with oncogenic Ras (Ras(ACT)) to promote tumorigenesis in Drosophila melanogaster and mammalian cells. To identify novel genes that cooperate with Ras(ACT) in tumorigenesis, we carried out a genome-wide screen for genes that when overexpressed throughout the developing Drosophila eye enhance Ras(ACT)-driven hyperplasia. Ras(ACT)-cooperating genes identified were Rac1 Rho1, RhoGEF2, pbl, rib, and east, which encode cell morphology regulators. In a clonal setting, which reveals genes conferring a competitive advantage over wild-type cells, only Rac1, an activated allele of Rho1 (Rho1(ACT)), RhoGEF2, and pbl cooperated with Ras(ACT), resulting in reduced differentiation and large invasive tumors. Expression of RhoGEF2 or Rac1 with Ras(ACT) upregulated Jun kinase (JNK) activity, and JNK upregulation was essential for cooperation. However, in the whole-tissue system, upregulation of JNK alone was not sufficient for cooperation with Ras(ACT), while in the clonal setting, JNK upregulation was sufficient for Ras(ACT)-mediated tumorigenesis. JNK upregulation was also sufficient to confer invasive growth of Ras(V12)-expressing mammalian MCF10A breast epithelial cells. Consistent with this, HER2(+) human breast cancers (where human epidermal growth factor 2 is overexpressed and Ras signaling upregulated) show a significant correlation with a signature representing JNK pathway activation. Moreover, our genetic analysis in Drosophila revealed that Rho1 and Rac are important for the cooperation of RhoGEF2 or Pbl overexpression and of mutants in polarity regulators, Dlg and aPKC, with Ras(ACT) in the whole-tissue context. Collectively our analysis reveals the importance of the RhoGEF/Rho-family/JNK pathway in cooperative tumorigenesis with Ras(ACT).

Regulation of Hippo signaling by Jun kinase signaling during compensatory cell proliferation and regeneration, and in neoplastic tumors

When cells undergo apoptosis, they can stimulate the proliferation of nearby cells, a process referred to as compensatory cell proliferation. The stimulation of proliferation in response to tissue damage or removal is also central to epimorphic regeneration. The Hippo signaling pathway has emerged as an important regulator of growth during normal development and oncogenesis from Drosophila to humans. Here we show that induction of apoptosis in the Drosophila wing imaginal disc stimulates activation of the Hippo pathway transcription factor Yorkie in surviving and nearby cells, and that Yorkie is required for the ability of the wing to regenerate after genetic ablation of the wing primordia. Induction of apoptosis activates Yorkie through the Jun kinase pathway, and direct activation of Jun kinase signaling also promotes Yorkie activation in the wing disc. We also show that depletion of neoplastic tumor suppressor genes, including lethal giant larvae and discs large, or activation of aPKC, activates Yorkie through Jun kinase signaling, and that Jun kinase activation is necessary, but not sufficient, for the disruption of apical-basal polarity associated with loss of lethal giant larvae. Our observations identify Jnk signaling as a modulator of Hippo pathway activity in wing imaginal discs, and implicate Yorkie activation in compensatory cell proliferation and disc regeneration.

Response to Dengue virus infections altered by cytokine-like substances from mosquito cell cultures

Background

With both shrimp and commercial insects such as honey bees, it is known that stable, persistent viral infections characterized by absence of disease can sometimes shift to overt disease states as a result of various stress triggers and that this can result in serious economic losses. The main research interest of our group is to understand the dynamics of stable viral infections in shrimp and how they can be destabilized by stress. Since there are no continuous cell lines for crustaceans, we have used a C6/36 mosquito cell line infected with Dengue virus to test hypotheses regarding these interactions. As a result, we accidentally discovered two new cytokine-like substances in 5 kDa extracts from supernatant solutions of acutely and persistently infected mosquito cells.

Results

Naïve C6/36 cells were exposed for 48 h to 5 kDa membrane filtrates prepared from the supernatant medium of stable C6/36 mosquito cell cultures persistently-infected with Dengue virus. Subsequent challenge of naïve cells with a virulent stock of Dengue virus 2 (DEN-2) and analysis by confocal immunofluorescence microscopy using anti-DEN-2 antibody revealed a dramatic reduction in the percentage of DEN-2 infected cells when compared to control cells. Similar filtrates prepared from C6/36 cells with acute DEN-2 infections were used to treat stable C6/36 mosquito cell cultures persistently-infected with Dengue virus. Confocal immunofluorescence microscopy revealed destabilization in the form of an apoptosis-like response. Proteinase K treatment removed the cell-altering activities indicating that they were caused by small polypeptides similar to those previously reported from insects.

Conclusions

This is the first report of cytokine-like substances that can alter the responses of mosquito cells to Dengue virus. This simple model system allows detailed molecular studies on insect cytokine production and on cytokine activity in a standard insect cell line.

The dark side of fly TNF: an ancient developmental proof reading mechanism turned into tumor promoter

The fruit fly Drosophila is an important model for biological research; however, due to its relatively short lifespan its relevance in cancer research is often questioned. Nevertheless, among many other intriguing Drosophila models, scribble group mutants provided early evidence for the existence of tumor suppressor genes and their importance in mammalian systems is beginning to emerge. In this review, I discuss recent advances in our understanding of the phenotypes of scrib group mutants, in which the activation of JNK signaling plays a crucial role. Several mechanisms can account for the activation of JNK within scrib group mutant cells, including a mechanical stress triggered by the loss of polarity, cell competition, intrinsic tumor suppression by autonomous production of Eiger, and an inflammatory response mediated by Eiger-producing haemocytes. Eiger, the sole Drosophila homolog of tumor necrosis factor, is emerging as a 'danger signal' initiated upon the presence of external pathogens, damaged tissues and the appearance of pre-malignant cells. Remarkably, in the presence of the Ras oncoprotein Eiger can act as a tumor promoter by stimulating invasive migration and delaying the onset of metamorphosis.

A tumor-suppressing mechanism in Drosophila involving cell competition and the Hippo pathway

Mutant larvae for the Drosophila gene lethal giant larva (lgl) develop neoplastic tumors in imaginal discs. However, lgl mutant clones do not form tumors when surrounded by wild-type tissue, suggesting the existence of a tumor-suppressing mechanism. We have investigated the tumorigenic potential of lgl mutant cells by generating wing compartments that are entirely mutant for lgl and also inducing clones of various genetic combinations of lgl(-) cells. We find that lgl(-) compartments can grow indefinitely but lgl(-) clones are eliminated by cell competition. lgl mutant cells may form tumors if they acquire constitutive activity of the Ras pathway (lgl(-) UAS-ras(V12)), which confers proliferation advantage through inhibition of the Hippo pathway. Yet, the majority of lgl(-) UAS-ras(V12) clones are eliminated in spite of their high proliferation rate. The formation of a tumor requires in addition the formation of a microenvironment that allows mutant cells to evade cell competition.

Oncogenic Ras diverts a host TNF tumor suppressor activity into tumor promoter

The roles of inflammatory cytokines and the immune response in cancer remain paradoxical. In the case of tumor necrosis factor (TNF), there is undisputed evidence indicating both protumor and antitumor activities. Recent work in Drosophila indicated that a TNF-dependent mechanism eliminates cells deficient for the polarity tumor suppressors dlg or scrib. In this study, however, we show that in tumors deficient for scrib that also expressed the Ras oncoprotein, the TNF signal was diverted into a protumor signal that enhanced tumor growth through larval arrest and stimulated invasive migration. In this case, TNF promoted malignancy and was detrimental to host survival. TNF was expressed at high levels by tumor-associated hemocytes recruited from the circulation. The expression of TNF by hemocytes was both necessary and sufficient to trigger TNF signaling in tumor cells. Our evidence suggests that tumors can evolve into malignancy through oncogenic Ras activation and the hijacking of TNF signaling.

The Cdc42/Par6/aPKC polarity complex regulates apoptosis-induced compensatory proliferation in epithelia

BACKGROUND

In response to stress- or tissue-damage-induced apoptosis, unaffected epithelial cells undergo compensatory proliferation to maintain the integrity of the epithelium. Proximal signals regulating this response are not fully understood, but c-Jun N-terminal kinase (JNK) activity appears to be critical for both apoptosis and compensatory proliferation. Disruption of epithelial cell apical-basal polarity occurs in early cancer development and is often correlated with increased proliferation by means not fully characterized. We considered whether disruption of the various polarity complexes could provide signals identifying damaged epithelial cells and thus lead to apoptosis-induced compensatory proliferation.

RESULTS

We identify the Cdc42/Par6/atypical protein kinase C (aPKC) Par polarity complex as uniquely and specifically regulating apoptosis-induced compensatory proliferation in Drosophila epithelia. Genetic depletion of individual components or disruption of formation and localization of this complex, but not other polarity complexes, induces JNK-dependent apoptosis and JNK-dependent compensatory proliferation following radiation injury. When apoptosis execution is blocked, by p35 expression, Cdc42/Par6/aPKC-depleted tissues uniquely hyperproliferate, leading to tissue and organ overgrowth. Disruption of Cdc42/Par6/aPKC leads to activation of JNK through increased Rho1 and Rok activity and Rok's capacity to activate myosin but not F-actin.

CONCLUSIONS

We show that the Cdc42/Par6/aPKC polarity complex influences both a physiologic compensatory proliferation response after irradiation injury and a contrived compensatory non-cell-autonomous hyperproliferation response when cell-autonomous apoptosis, resulting from Cdc42/Par6/aPKC disruption, is inhibited. These results suggest the possibility that in cancer where apoptotic regulation is disrupted, loss of Cdc42/Par6/aPKC polarity complex organization or localization could contribute to tumor hyperproliferation and explain how polarity disruption contributes to tumor development.

Disruption of lysosome function promotes tumor growth and metastasis in Drosophila

Lysosome function is essential to many physiological processes. It has been suggested that deregulation of lysosome function could contribute to cancer. Through a genetic screen in Drosophila, we have discovered that mutations disrupting lysosomal degradation pathway components contribute to tumor development and progression. Loss-of-function mutations in the Class C vacuolar protein sorting (VPS) gene, deep orange (dor), dramatically promote tumor overgrowth and invasion of the Ras^V12 cells. Knocking down either of the two other components of the Class C VPS complex, carnation (car) and vps16A, also renders Ras^V12 cells capable for uncontrolled growth and metastatic behavior. Finally, chemical disruption of the lysosomal function by feeding animals with antimalarial drugs, chloroquine or monensin, leads to malignant tumor growth of the Ras^V12 cells. Taken together, our data provide evidence for a causative role of lysosome dysfunction in tumor growth and invasion and indicate that members of the Class C VPS complex behave as tumor suppressors.

The lethal giant larvae tumour suppressor mutation requires dMyc oncoprotein to promote clonal malignancy

Background

Neoplastic overgrowth depends on the cooperation of several mutations ultimately leading to major rearrangements in cellular behaviour. Precancerous cells are often removed by cell death from normal tissues in the early steps of the tumourigenic process, but the molecules responsible for such a fundamental safeguard process remain in part elusive. With the aim to investigate the molecular crosstalk occurring between precancerous and normal cells in vivo, we took advantage of the clonal analysis methods that are available in Drosophila for studying the phenotypes due to lethal giant larvae (lgl) neoplastic mutation induced in different backgrounds and tissues.

Results

We observed that lgl mutant cells growing in wild-type imaginal wing discs show poor viability and are eliminated by Jun N-terminal Kinase (JNK)-dependent cell death. Furthermore, they express very low levels of dMyc oncoprotein compared with those found in the surrounding normal tissue. Evidence that this is a cause of lgl mutant cells elimination was obtained by increasing dMyc levels in lgl mutant clones: their overgrowth potential was indeed re-established, with mutant cells overwhelming the neighbouring tissue and forming tumourous masses displaying several cancer hallmarks. Moreover, when lgl mutant clones were induced in backgrounds of slow-dividing cells, they upregulated dMyc, lost apical-basal cell polarity and were able to overgrow. Those phenotypes were abolished by reducing dMyc levels in the mutant clones, thereby confirming its key role in lgl-induced tumourigenesis. Furthermore, we show that the eiger-dependent Intrinsic Tumour Suppressor pathway plays only a minor role in eliminating lgl mutant cells in the wing pouch; lgl^-/- clonal death in this region is instead driven mainly by dMyc-induced Cell Competition.

Conclusions

Our results provide the first evidence that dMyc oncoprotein is required in lgl tumour suppressor mutant tissue to promote invasive overgrowth in larval and adult epithelial tissues. Moreover, we show that dMyc abundance inside versus outside the mutant clones plays a key role in driving neoplastic overgrowth.

Metabolism, cell surface organization, and disease

Genetic information flows from DNA to macromolecular structures-the dominant force in the molecular organization of life. However, recent work suggests that metabolite availability to the hexosamine and Golgi N-glycosylation pathways exerts control over the assembly of macromolecular complexes on the cell surface and, in this capacity, acts upstream of signaling and gene expression. The structure and number of N-glycans per protein molecule cooperate to regulate lectin binding and thereby the distribution of glycoproteins at the cell surface. Congenital disorders of glycosylation provide insight as extreme hypomorphisms, whereas milder deficiencies may encompass many common chronic conditions, including autoimmunity, metabolic syndrome, and aging.

Interaction between Ras(V12) and scribbled clones induces tumour growth and invasion

Human tumours exhibit a large degree of cellular and genetic heterogeneity 1. Complex cell interactions in the tumour and its microenvironment are thought to play a significant role in tumourigenesis and cancer progression 2. It is also known that cooperation between oncogenic genetic lesions is required for tumour development 3. However, it is not known how cell interactions contribute to oncogenic cooperation. The genetic techniques available in the fruit fly Drosophila melanogaster allow analysis of the behavior of cells with distinct mutations 4, giving this model organism a privileged position to study cell interactions and oncogenic cooperation. In Drosophila eye-antennal discs, cooperation between the oncogenic protein Ras^V125 and loss-of-function mutations in the conserved tumour suppressor scribble (scrib)6,7 gives rise to metastatic tumours that display many characteristics observed in human cancers 8-11. Here we show that clones of cells bearing different mutations can cooperate to promote tumour growth and invasion in Drosophila. We found that the Ras^V12 and scrib⁻ mutations can also cause tumours when they affect different adjacent epithelial cells. We show that this interaction between Ras^V12 and scrib⁻ clones involves JNK signaling propagation and JNK-induced upregulation of JAK/STAT-activating cytokines, a compensatory growth mechanism for tissue homeostasis. The development of Ras^V12 tumours can also be triggered by tissue damage, a stress condition that activates JNK signaling. Given the conservation of the pathways examined here, similar cooperative mechanisms could play a role in the development of human cancers.

Scribble mutants promote aPKC and JNK-dependent epithelial neoplasia independently of Crumbs

BACKGROUND

Metastatic neoplasias are characterized by excessive cell proliferation and disruptions to apico-basal cell polarity and tissue architecture. Understanding how alterations in cell polarity can impact upon tumour development is, therefore, a central issue in cancer biology. The Drosophila gene scribble (scrib) encodes a PDZ-domain scaffolding protein that regulates cell polarity and acts as a tumour suppressor in flies. Increasing evidence also implicates the loss of human Scrib in cancer. In this report, we investigate how loss of Scrib promotes epithelial tumourigenesis in Drosophila, both alone and in cooperation with oncogenic mutations.

RESULTS

We find that genetically distinct atypical protein kinase C (aPKC)-dependent and Jun N-terminal kinase (JNK)-dependent alterations in scrib mutants drive epithelial tumourigenesis. First, we show that over-expression of the apical cell polarity determinants Crumbs (Crb) or aPKC induces similar cell morphology defects and over-proliferation phenotypes as scrib loss-of-function. However, the morphological and proliferative defects in scrib mutants are independent of Crb function, and instead can be rescued by a dominant negative (kinase dead) aPKC transgene. Secondly, we demonstrate that loss of Scrib promotes oncogene-mediated transformation through both aPKC and JNK-dependent pathways. JNK normally promotes apoptosis of scrib mutant cells. However, in cooperation with oncogenic activated Ras or Notch signalling, JNK becomes an essential driver of tumour overgrowth and invasion. aPKC-dependent signalling in scrib mutants cooperates with JNK to significantly enhance oncogene-mediated tumour overgrowth.

CONCLUSION

These results demonstrate distinct aPKC and JNK-dependent pathways through which loss of Scrib promotes tumourigenesis in Drosophila. This is likely to have a direct relevance to the way in which human Scrib can similarly restrain an oncogene-mediated transformation and, more generally, on how the outcome of oncogenic signalling can be profoundly perturbed by defects in apico-basal epithelial cell polarity.

Correcting developmental errors by apoptosis: lessons from Drosophila JNK signaling

Spatio-temporal regulation of the cell death machinery is essential for normal development and homeostasis of multicellular organisms. While the molecular basis for the central cell death machinery driven by caspases is now well documented, its regulatory mechanisms, especially in the context of living animals, remain to be clarified. The c-Jun N-terminal kinase (JNK) pathway is an evolutionarily conserved kinase cascade that regulates the apoptotic machinery. In mammals, JNK signaling has been implicated in stress-induced apoptosis. Drosophila genetics has now provided evidence of a novel role for JNK-mediated cell death signaling in eliminating developmentally aberrant cells from a tissue. The JNK-dependent cell-elimination system is orchestrated by cell-cell communication between normal and aberrant cells and is essential for ensuring developmental robustness, as well as for protecting organisms against fatal abnormalities such as neoplastic development. These processes are mediated by cell competition, morphogenetic apoptosis, and intrinsic tumor suppression. A combinatorial approach using both genetic and live-imaging systems in Drosophila will be extremely powerful to decipher how JNK-mediated apoptosis works within multicellular communities.

Adaptive regulation at the cell surface by N-glycosylation

The association of receptors and solute transporters with components of the endocytic machinery regulates their surface levels, and thereby cellular sensitivity to cytokines, ligands and nutrients in the extracellular environment. Most transmembrane receptors and solute transporters are glycoproteins, and the Asn (N)-linked oligosaccharides (N-glycans) can bind animal lectins, forming multivalent lattices or microdomains that regulate glycoprotein mobility in the plane of membrane. The N-glycan number (sequence-encoded NXS/T) and context-dependent Golgi N-glycan branching cooperate to regulate glycoprotein affinities for the galectin family of lectins. Galectin-3 binding reduces EGF receptor trafficking into clathrin-coated pits and caveolae lipid rafts, decreases ligand-independent receptor activation and promotes alpha5beta1 integrin remodelling in focal adhesions. N-glycan branching in the medial Golgi increases glycan affinity for galectins, and the Golgi pathway is sensitive to uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) supply, in turn hexosamine pathway metabolites (fructose-6-P, glutamine and acetyl-CoA). Thus, lattice avidity and cellular responsiveness to extracellular cues are regulated in an adaptive manner by metabolism and Golgi modification to glycoproteins. Computational modelling of the hexosamine/Golgi/lattice has provided new insight on cell surface adaptation in cancer and autoimmune disease.

At the crossroads of polarity, proliferation and apoptosis: the use of Drosophila to unravel the multifaceted role of endocytosis in tumor suppression

Endocytosis is an important regulator of cell-cell signaling and endocytic trafficking has been increasingly implicated in control of tumor suppression. Recent insights from Drosophila indicate that impairment of multiple trafficking steps which lead to receptor degradation can cause tumor formation in epithelial organs. These tumors are characterized by sustained activation of a number of mitogenic signaling pathways, and by subversion of epithelial polarity and the apoptotic response. Cooperation between such alterations, as well as tumor-host interactions, is also observed. The recapitulation of several hallmarks of human cancers in fly tumors provides a framework to understand the role of defective endocytosis in cancer.