Competition and compensation: coupled to death in development and cancer

Cell competition in primary and metastatic colorectal cancer

Adult tissues set the scene for a continuous battle between cells, where a comparison of cellular fitness results in the elimination of weaker "loser" cells. This phenomenon, named cell competition, is beneficial for tissue integrity and homeostasis. In fact, cell competition plays a crucial role in tumor suppression, through elimination of early malignant cells, as part of Epithelial Defense Against Cancer. However, it is increasingly apparent that cell competition doubles as a tumor-promoting mechanism. The comparative nature of cell competition means that mutational background, proliferation rate and polarity all factor in to determine the outcome of these processes. In this review, we explore the intricate and context-dependent involvement of cell competition in homeostasis and regeneration, as well as during initiation and progression of primary and metastasized colorectal cancer. We provide a comprehensive overview of molecular and cellular mechanisms governing cell competition and its parallels with regeneration.

RF, Mohan H, Annaert W, Chaudhary V. Maintenance of proteostasis by Drosophila Rer1 is essential for competitive cell survival and Myc-driven overgrowth

Defects in protein homeostasis can induce proteotoxic stress, affecting cellular fitness and, consequently, overall tissue health. In various growing tissues, cell competition based mechanisms facilitate detection and elimination of these compromised, often referred to as 'loser', cells by the healthier neighbors. The precise connection between proteotoxic stress and competitive cell survival remains largely elusive. Here, we reveal the function of an endoplasmic reticulum (ER) and Golgi localized protein Rer1 in the regulation of protein homeostasis in the developing Drosophila wing epithelium. Our results show that loss of Rer1 leads to proteotoxic stress and PERK-mediated phosphorylation of eukaryotic initiation factor 2α. Clonal analysis showed that rer1 mutant cells are identified as losers and eliminated through cell competition. Interestingly, we find that Rer1 levels are upregulated upon Myc-overexpression that causes overgrowth, albeit under high proteotoxic stress. Our results suggest that increased levels of Rer1 provide cytoprotection to Myc-overexpressing cells by alleviating the proteotoxic stress and thereby supporting Myc-driven overgrowth. In summary, these observations demonstrate that Rer1 acts as a novel regulator of proteostasis in Drosophila and reveal its role in competitive cell survival.

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.

Naples Prognostic Score as an Independent Predictor of Survival Outcomes for Resected Locally Advanced Non-Small Cell Lung Cancer Patients After Neoadjuvant Treatment

Background

The Naples Prognostic Score (NPS) can reflect patient's nutritional and inflammatory status, which is identified as a prognostic indicator for various malignant tumors. However, its significance in patients with resected locally advanced non-small cell lung cancer (LA-NSCLC) patients who receive neoadjuvant treatment remains unclear so far.

Methods

A total of 165 LA-NSCLC patients surgically treated from May 2012 to November 2017 were retrospectively investigated. The LA-NSCLC patients were divided into three groups according to NPS scores. The receiver operating curve (ROC) analysis was performed to reveal the discriminatory ability of NPS and other indicators for predicting the survival. The NPS and clinicopathological variables were further evaluated the prognostic value by univariate and multivariate Cox analysis.

Results

The NPS was related to age (P = 0.046), smoking history (P = 0.004), Eastern Cooperative Oncology Group (ECOG) score (P = 0.005), and adjuvant treatment (P = 0.017). Patients with high NPS scores had worse overall survival (OS) (group 1 vs 0, P = 0.006; group 2 vs 0, P < 0.001) and disease-free survival (DFS) (group 1 vs 0, P < 0.001; group 2 vs 0, P < 0.001). The ROC analysis demonstrated that NPS had better predictive ability than other prognostic indicators. Multivariate analysis revealed that NPS was independent prognostic indicator of OS (group 1 vs 0, hazard ratio [HR] =2.591, P = 0.023; group 2 vs 0, HR = 8.744, P = 0.001) and DFS (group 1 vs 0, HR =3.754, P < 0.001; group 2 vs 0, HR = 9.673, P < 0.001).

Conclusion

The NPS could be an independent prognostic indicator in patients with resected LA-NSCLC receiving neoadjuvant treatment and more reliable than the other nutritional and inflammatory indicators.

Cell competition: A historical perspective

Cell competition is a homeostatic process designed to remove from animal tissues viable cells that are unfit, abnormal or malignant and that may compromise the general fitness or the viability of the organism. Originally discovered in Drosophila in the mid-seventies of last century, there is strong evidence that it also occurs in other metazoans, where cell competition appears to play a similar surveillance role. In this review I summarize the field of cell competition, with special emphasis in the history of the phenomenon within the general frame of Developmental Biology in the second half of the XX century, pointing out the key observations and the evolution of ideas that have led to the current understanding.

Drosophila Models of Cell Polarity and Cell Competition in Tumourigenesis

Cell competition is an important surveillance mechanism that measures relative fitness between cells in a tissue during development, homeostasis, and disease. Specifically, cells that are "less fit" (losers) are actively eliminated by relatively "more fit" (winners) neighbours, despite the less fit cells otherwise being able to survive in a genetically uniform tissue. Originally described in the epithelial tissues of Drosophila larval imaginal discs, cell competition has since been shown to occur in other epithelial and non-epithelial Drosophila tissues, as well as in mammalian model systems. Many genes and signalling pathways have been identified as playing conserved roles in the mechanisms of cell competition. Among them are genes required for the establishment and maintenance of apico-basal cell polarity: the Crumbs/Stardust/Patj (Crb/Sdt/Patj), Bazooka/Par-6/atypical Protein Kinase C (Baz/Par-6/aPKC), and Scribbled/Discs large 1/Lethal (2) giant larvae (Scrib/Dlg1/L(2)gl) modules. In this chapter, we describe the concepts and mechanisms of cell competition, with emphasis on the relationship between cell polarity proteins and cell competition, particularly the Scrib/Dlg1/L(2)gl module, since this is the best described module in this emerging field.

The retinoblastoma homolog RBR1 mediates localization of the repair protein RAD51 to DNA lesions in Arabidopsis

The retinoblastoma protein (Rb), which typically functions as a transcriptional repressor of E2F‐regulated genes, represents a major control hub of the cell cycle. Here, we show that loss of the Arabidopsis Rb homolog RETINOBLASTOMA‐RELATED 1 (RBR1) leads to cell death, especially upon exposure to genotoxic drugs such as the environmental toxin aluminum. While cell death can be suppressed by reduced cell‐proliferation rates, rbr1 mutant cells exhibit elevated levels of DNA lesions, indicating a direct role of RBR1 in the DNA‐damage response (DDR). Consistent with its role as a transcriptional repressor, we find that RBR1 directly binds to and represses key DDR genes such as RADIATION SENSITIVE 51 (RAD51), leaving it unclear why rbr1 mutants are hypersensitive to DNA damage. However, we find that RBR1 is also required for RAD51 localization to DNA lesions. We further show that RBR1 is itself targeted to DNA break sites in a CDKB1 activity‐dependent manner and partially co‐localizes with RAD51 at damage sites. Taken together, these results implicate RBR1 in the assembly of DNA‐bound repair complexes, in addition to its canonical function as a transcriptional regulator.

A platform for interrogating cancer-associated p53 alleles

p53 is the most frequently mutated gene in human cancer. Compelling evidence argues that full transformation involves loss of growth suppression encoded by wild-type p53 together with poorly understood oncogenic activity encoded by missense mutations. Furthermore, distinguishing disease alleles from natural polymorphisms is an important clinical challenge. To interrogate the genetic activity of human p53 variants, we leveraged the Drosophila model as an in vivo platform. We engineered strains that replace the fly p53 gene with human alleles, producing a collection of stocks that are, in effect, 'humanized' for p53 variants. Like the fly counterpart, human p53 transcriptionally activated a biosensor and induced apoptosis after DNA damage. However, all humanized strains representing common alleles found in cancer patients failed to complement in these assays. Surprisingly, stimulus-dependent activation of hp53 occurred without stabilization, demonstrating that these two processes can be uncoupled. Like its fly counterpart, hp53 formed prominent nuclear foci in germline cells but cancer-associated p53 variants did not. Moreover, these same mutant alleles disrupted hp53 foci and inhibited biosensor activity, suggesting that these properties are functionally linked. Together these findings establish a functional platform for interrogating human p53 alleles and suggest that simple phenotypes could be used to stratify disease variants.

Cell Competition During Growth and Regeneration

Tissue growth and regeneration are autonomous, stem-cell-mediated processes in which stem cells within the organ self-renew and differentiate to create new cells, leading to new tissue. The processes of growth and regeneration require communication and interplay between neighboring cells. In particular, cell competition, which is a process in which viable cells are actively eliminated by more competitive cells, has been increasingly implicated to play an important role. Here, we discuss the existing literature regarding the current landscape of cell competition, including classical pathways and models, fitness fingerprint mechanisms, and immune system mechanisms of cell competition. We further discuss the clinical relevance of cell competition in the physiological processes of tissue growth and regeneration, highlighting studies in clinically important disease models, including oncological, neurological, and cardiovascular diseases.

Effects of Ulmi Pumilae Cortex on AGS Gastric Cancer Cells

OBJECTIVE

Ulmi Pumilae Cortex(UPC) is a deciduous tree with uneven pinnate leaves and is classified as a subfamily of Ulmuceae and contains many pharmacologically active constituents. The aim of this study was to investigate the effects of UPC on the growth and survival of AGS cells, the most common human gastric adenocarcinoma cell lines.

METHODS

The AGS cells were treated with varying concentrations of UPC. Analyses of the sub G1, caspase-3 activity, and mitochondrial depolarization were conducted to determine whether AGS cell death occured by apoptosis. Furthermore, to identify the role of the transient receptor potential melastatin (TRPM) 7 channels in AGS cell growth and survival, we used human embryonic kidney (HEK) 293 cells overexpressed with TRPM7 channels.

RESULTS

The addition of UPC to a culture medium inhibited AGS cell growth and survival. Experimental results showed that the sub G1, caspase-3 activity, and mitochondrial depolarization were increased. Furthermore, TRPM7 channel overexpression in HEK 293 cells exacerbated UPC-induced cell death.

CONCLUSION

These findings indicate that UPC inhibits the growth and survival of gastric cancer cells due to a blockade of the TRPM7 channel activity. Therefore, UPC is a potential drug for treatment of gastric cancer, and TRPM7 channels may play an important role in survival in cases of gastric cancer.

Cell competition: how to eliminate your neighbours

A conventional view of development is that cells cooperate to build an organism. However, based on studies of Drosophila, it has been known for years that viable cells can be eliminated by their neighbours through a process termed cell competition. New studies in mammals have revealed that this process is universal and that many factors and mechanisms are conserved. During cell competition, cells with lower translation rates or those with lower levels of proteins involved in signal transduction, polarity and cellular growth can survive in a homogenous environment but are killed when surrounded by cells of higher fitness. Here, we discuss recent advances in the field as well as the mechanistic steps involved in this phenomenon, which have shed light on how and why cell competition exists in developing and adult organisms.

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

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

Mechanisms of cell competition: themes and variations

Cell competition is the short-range elimination of slow-dividing cells through apoptosis when confronted with a faster growing population. It is based on the comparison of relative cell fitness between neighboring cells and is a striking example of tissue adaptability that could play a central role in developmental error correction and cancer progression in both Drosophila melanogaster and mammals. Cell competition has led to the discovery of multiple pathways that affect cell fitness and drive cell elimination. The diversity of these pathways could reflect unrelated phenomena, yet recent evidence suggests some common wiring and the existence of a bona fide fitness comparison pathway.

Cell wars: regulation of cell survival and proliferation by cell competition

During cell competition fitter cells take over the tissue at the expense of viable, but less fit, cells, which are eliminated by induction of apoptosis or senescence. This probably acts as a quality-control mechanism to eliminate suboptimal cells and safeguard organ function. Several experimental conditions have been shown to trigger cell competition, including differential levels in ribosomal activity or in signalling pathway activation between cells, although it is unclear how those differences are sensed and translated into fitness levels. Many of the pathways implicated in cell competition have been previously linked with cancer, and this has led to the hypothesis that cell competition could play a role in tumour formation. Cell competition could be co-opted by cancer cells to kill surrounding normal cells and boost their own tissue colonization. However, in some cases, cell competition could have a tumour suppressor role, as cells harbouring mutations in a subset of tumour suppressor genes are killed by wild-type cells. Originally described in developing epithelia, competitive interactions have also been observed in some stem cell niches, where they play a role in regulating stem cell selection, maintenance and tissue repopulation. Thus competitive interactions could be relevant to the maintenance of tissue fitness and have a protective role against aging.

New frontiers in cell competition

Cellular communication is at the heart of animal development, and guides the specification of cell fates, the movement of cells within and between tissues, and the coordinated arrangement of different body parts. During organ and tissue growth, cell-cell communication plays a critical role in decisions that determine whether cells survive to contribute to the organism. In this review, we discuss recent insights into cell competition, a social cellular phenomenon that selects the fittest cells in a tissue, and as such potentially contributes to the regulation of its growth and final size. The field of cell competition has seen a huge explosion in its study in the last several years, facilitated by the increasingly sophisticated genetic and molecular technology available in Drosophila and driven by its relevance to stem cell biology and human cancer.

Differential gene expression of RAW 264.7 macrophages in response to the RGD peptide lunasin with and without lipopolysaccharide stimulation

Lunasin is a novel peptide from soybean with demonstrated chemopreventive property. We compared the effect of lunasin on gene expression of RAW 264.7 macrophages with and without lipopolysaccharide (LPS)-stimulation. Our hypothesis was that lunasin will have a differential effect in RAW 264.7 gene expression in a normal and challenged state. Analysis of the microarray data using False Discovery Rate (FDR) method resulted in the identification of 340 up-regulated and 162 down-regulated genes (FDR p-value <0.05) associated with simultaneous treatment of lunasin and LPS for 24h. Treatment of lunasin with no LPS for 24h resulted in the up-regulation of 855 genes and down-regulation of 397 genes. Pre-treatment of lunasin for 24h resulted in the up-regulation of 35 genes and down-regulation of 65 genes in LPS-stimulated RAW 264.7 macrophages. GeneVenn analysis of these three sets of genes showed that there are 66 genes common among the three groups which are mostly associated with regulation of cell death, ion binding and transcription as datamined by DAVID. Analysis of the 838 genes unique to lunasin alone by functional annotation clustering tool showed that lunasin mostly affected genes associated with RNA processing, apoptosis and protein kinase activity. Further datamining of these genes by ingenuity pathway analysis (IPA) showed that lunasin affected genes involved in cellular growth and proliferation, cellular function and maintenance, and cell to cell signaling and interaction. These findings support the potential chemopreventive and chemotherapeutic use of lunasin against cancer.

IGFBP-rP1, a potential molecule associated with colon cancer differentiation

BACKGROUND

In our previous studies, we have demonstrated that insulin-like growth factor binding protein-related protein1 (IGFBP-rP1) played its potential tumor suppressor role in colon cancer cells through apoptosis and senescence induction. In this study, we will further uncover the role of IGFBP-rP1 in colon cancer differentiation and a possible mechanism by revealing responsible genes.

RESULTS

In normal colon epithelium, immunohistochemistry staining detected a gradient IGFBP-rP1 expression along the axis of the crypt. IGFBP-rP1 strongly expressed in the differentiated cells at the surface of the colon epithelium, while weakly expressed at the crypt base. In colon cancer tissues, the expression of IGFBP-rP1 correlated positively with the differentiation status. IGFBP-rP1 strongly expressed in low grade colorectal carcinoma and weakly expressed in high grade colorectal carcinoma. In vitro, transfection of PcDNA3.1(IGFBP-rP1) into RKO, SW620 and CW2 cells induced a more pronounced anterior-posterior polarity morphology, accompanied by upregulation with alkaline phosphatase (AKP) activity. Upregulation of carcino-embryonic antigen (CEA) was also observed in SW620 and CW2 transfectants. The addition of IGFBP-rP1 protein into the medium could mimic most but not all effects of IGFBP-rP1 cDNA transfection. Seventy-eight reproducibly differentially expressed genes were detected in PcDNA3.1(IGFBP-rP1)-RKO transfectants, using Affymetrix 133 plus 2.0 expression chip platform. Directed Acyclic Graph (DAG) of the enriched GO categories demonstrated that differential expression of the enzyme regulator activity genes together with cytoskeleton and actin binding genes were significant. IGFBP-rP1 could upreguate Transgelin (TAGLN), downregulate SRY (sex determining region Y)-box 9(campomelic dysplasia, autosomal sex-reversal) (SOX9), insulin receptor substrate 1(IRS1), cyclin-dependent kinase inhibitor 2B (p15, inhibits CDK4) (CDKN2B), amphiregulin(schwannoma-derived growth factor) (AREG) and immediate early response 5-like(IER5L) in RKO, SW620 and CW2 colon cancer cells, verified by Real time Reverse Transcription Polymerase Chain Reaction (rtRT-PCR). During sodium butyrate-induced Caco2 cell differentiation, IGFBP-rP1 was upregulated and the expression showed significant correlation with the AKP activity. The downregulation of IRS1 and SOX9 were also induced by sodium butyrate.

CONCLUSION

IGFBP-rP1 was a potential key molecule associated with colon cancer differentiation. Downregulation of IRS1 and SOX9 may the possible key downstream genes involved in the process.

Coupling of the cell cycle and apoptotic machineries in developing T cells

Proliferation and apoptosis are diametrically opposite processes. Expression of certain genes like c-Myc, however, can induce both, pointing to a possible linkage between them. Developing CD4(+)CD8(+) thymocytes are intrinsically sensitive to apoptosis, but the molecular basis is not known. We have found that these noncycling cells surprisingly express many cell cycle proteins. We generated transgenic mice expressing a CDK2 kinase-dead (CDK2-DN) protein in the T cell compartment. Analysis of these mice showed that the CDK2-DN protein acts as a dominant negative mutant in mature T cells as expected, but surprisingly, it acts as a dominant active protein in CD4(+)CD8(+) thymocytes. The levels of CDK2 kinase activity, cyclin E, cyclin A, and other cell cycle proteins in transgenic CD4(+)CD8(+) thymocytes are increased. Concurrently, caspase levels are elevated, and apoptosis is significantly enhanced in vitro and in vivo. E2F-1, the unique E2F member capable of inducing apoptosis when overexpressed, is specifically up-regulated in transgenic CD4(+)CD8(+) thymocytes but not in other T cell populations. These results demonstrate that the cell cycle and apoptotic machineries are normally linked, and expression of cell cycle proteins in developing T cells contributes to their inherent 1sensitivity to apoptosis.

Cell rearrangements, cell divisions and cell death in a migrating epithelial sheet in the abdomen of Drosophila

During morphogenesis, cell movements, cell divisions and cell death work together to form complex patterns and to shape organs. These events are the outcome of decisions made by many individual cells, but how these decisions are controlled and coordinated is elusive. The adult abdominal epidermis of Drosophila is formed during metamorphosis by divisions and extensive cell migrations of the diploid histoblasts, which replace the polyploid larval cells. Using in vivo 4D microscopy, we have studied the behaviour of the histoblasts and analysed in detail how they reach their final position and to what extent they rearrange during their spreading. Tracking individual cells, we show that the cells migrate in two phases that differ in speed, direction and amount of cellular rearrangement. Cells of the anterior (A) and posterior (P) compartments differ in their behaviour. Cells near the A/P border are more likely to change their neighbours during migration. The mitoses do not show any preferential orientation. After mitosis, the sisters become preferentially aligned with the direction of movement. Thus, in the abdomen, it is the extensive cell migrations that appear to contribute most to morphogenesis. This contrasts with other developing epithelia, such as the wing imaginal disc and the embryonic germband in Drosophila, where oriented mitoses and local cell rearrangements appear to direct morphogenesis. Furthermore, our results suggest that an active force created by the histoblasts contributes to the formation of the adult epidermis. Finally, we show that histoblasts occasionally undergo apoptosis.

Mitochondria As A Target For Early Detection and Diagnosis of Cancer

Mitochondrial dysfunction and mutations in mitochondrial DNA (mtDNA) have been frequently reported in cancer, neurodegenerative diseases, diabetes, and aging syndromes. The mitochondrion genome (16.5 Kb) codes only for a small fraction (estimated to be 1%) of the proteins housed within this organelle. The other proteins are encoded by the nuclear DNA (nDNA) and transported into the mitochondria. The identification of mitochondrial proteins that are aberrantly expressed in cancer cells and other diseases is now possible through recent developments in proteomic and bioinformatic technologies. These developments set the stage for a comprehensive organelle-based proteomic approach for the identification of new markers for the early detection, risk assessment, and diagnosis of cancer, and other diseases and for the identification of new targets for therapeutic prevention and intervention.

Drosophila myt1 is the major cdk1 inhibitory kinase for wing imaginal disc development

Mitosis is triggered by activation of Cdk1, a cyclin-dependent kinase. Conserved checkpoint mechanisms normally inhibit Cdk1 by inhibitory phosphorylation during interphase, ensuring that DNA replication and repair is completed before cells begin mitosis. In metazoans, this regulatory mechanism is also used to coordinate cell division with critical developmental processes, such as cell invagination. Two types of Cdk1 inhibitory kinases have been found in metazoans. They differ in subcellular localization and Cdk1 target-site specificity: one (Wee1) being nuclear and the other (Myt1), membrane-associated and cytoplasmic. Drosophila has one representative of each: dMyt1 and dWee1. Although dWee1 and dMyt1 are not essential for zygotic viability, loss of both resulted in synthetic lethality, indicating that they are partially functionally redundant. Bristle defects in myt1 mutant adult flies prompted a phenotypic analysis that revealed cell-cycle defects, ectopic apoptosis, and abnormal responses to ionizing radiation in the myt1 mutant imaginal wing discs that give rise to these mechanosensory organs. Cdk1 inhibitory phosphorylation was also aberrant in these myt1 mutant imaginal wing discs, indicating that dMyt1 serves Cdk1 regulatory functions that are important both for normal cell-cycle progression and for coordinating mitosis with critical developmental processes.

Normal development is an integral part of tumorigenesis in T cell-specific PTEN-deficient mice

PTEN is a tumor suppressor gene but whether cancer can develop in all PTEN-deficient cells is not known. In T cell-specific PTEN-deficient (tPTEN-/-) mice, which suffer from mature T cell lymphomas, we found that premalignancy, as defined by elevated AKT and senescence pathways, starts in immature T cell precursors and surprisingly not in mature T cells. Premalignancy only starts in 6-week-old mice and becomes much stronger in 9-week-old mice although PTEN is lost since birth. tPTEN-/- immature T cells do not become tumors, and senescence has no role in this model because these cells exist in a novel cell cycle state, expressing proliferating proteins but not proliferating to any significant degree. Instead, the levels of p27(kip1), which is lower in tPTEN-/- immature T cells and almost nonexistent in tPTEN-/- mature T cells, correlate with the proliferation capability of these cells. Interestingly, transient reduction of these cancer precursor cells in adult tPTEN-/- mice within a crucial time window significantly delayed lymphomas and mouse lethality. Thus, loss of PTEN alone is not sufficient for cells to become cancerous, therefore other developmental events are necessary for tumor formation.

Declining cellular fitness with age promotes cancer initiation by selecting for adaptive oncogenic mutations

Age is the single most important prognostic factor in the development of many cancers. The major reason for this age-dependence is thought to be the progressive accumulation of oncogenic mutations and epigenetic changes. Similarly, mutagens are thought to be carcinogenic primarily by engendering oncogenic mutations. Yet while the accumulation of heritable somatic changes is expected to augment the incidence of oncogenic mutations, a major effect of increased mutation load is reduced fitness. We propose that the fitness of progenitor cell compartments substantially impacts on the selective advantage conferred by particular mutations. We hypothesize that reduced cellular fitness within aged stem cell pools can select for adaptive oncogenic events and thereby promote the initiation of cancer. Thus, certain oncogenic mutations may be adaptive within aged but not young stem cell pools. We further argue that accumulating genetic alterations with age or mutagen exposure might promote cancer not only by causing oncogenic hits within cells but also by leading to eventual reduction in stem cell fitness, which then selects for oncogenic events. Therefore, initial stages of cancer development may not be limited by the incidence of initiating oncogenic changes, but instead by contexts of reduced cellular fitness that select for these changes.

Compartmentalized phosphorylation of IAP by protein kinase A regulates cytoprotection

Cell death pathways are likely regulated in specialized subcellular microdomains, but how this occurs is not understood. Here, we show that cyclic AMP-dependent protein kinase A (PKA) phosphorylates the inhibitor of apoptosis (IAP) protein survivin on Ser20 in the cytosol, but not in mitochondria. This phosphorylation event disrupts the binding interface between survivin and its antiapoptotic cofactor, XIAP. Conversely, mitochondrial survivin or a non-PKA phosphorylatable survivin mutant binds XIAP avidly, enhances XIAP stability, synergistically inhibits apoptosis, and accelerates tumor growth, in vivo. Therefore, differential phosphorylation of survivin by PKA in subcellular microdomains regulates tumor cell apoptosis via its interaction with XIAP.

Drosophila T-box transcription factor Optomotor-blind prevents pathological folding and local overgrowth in wing epithelium through confining Hh signal

Aberration of morphogen signaling leads directly to inappropriate cell differentiation and secondarily causes various pathological phenotypes such as abnormal morphogenesis and tumorigenesis. However, mechanisms for linking morphogen signaling and the higher order phenotypes have not been fully elucidated. Here we focus on the Drosophila T-box gene optomotor-blind (omb), a transcriptional target of a long-range morphogen Decapentaplegic (Dpp). Genetic analyses of omb function revealed that a negative feedback loop, where omb plays a crucial role, exists between Dpp and its upstream regulator Hedgehog (Hh), a short-range morphogen. Consequently, dysfunction of omb elicits hyperactivation of Hh signaling that causes an ectopic folding and local overgrowth in the wing columnar epithelium, neither of which are the direct results of reduced Dpp response. In the case of the local overgrowth, it was never seen in mutants for thick veins (tkv) encoding a Dpp receptor, suggesting that the Dpp signaling pathway is divided into two antagonistic branches, one of which contains Omb. Thus defect in feedback between the two morphogens explains both phenotypes, and disruption of a balance between the morphogen targets further accounts for the local overgrowth. These are the mechanisms for generating secondary phenotypes when a single signaling factor Omb fails to function.

Constitutive phosphorylation mutation in Fas-associated death domain (FADD) results in early cell cycle defects

Fas-associated death domain (FADD) is an adaptor molecule for the death receptor subfamily of the tumor necrosis factor receptor superfamily, but it is also required for cell proliferation. Cell cycle-specific regulation of FADD phosphorylation plays an important role in FADD proliferative function since mice with a mutant form of FADD mimicking constitutive phosphorylation at serine 191 (FADD-D) exhibit defective T cell proliferation. Here we characterized these mice in detail and found that T cell development in 2-4-week-old mice is relatively normal, although mature FADD-D T cells manifest defective G(0) and G(1) to S transition with abnormalities in regulation of p130, p27 degradation, retinoblastoma protein phosphorylation, and CDK2 kinase activity. These downstream defects are further associated with the failure to up-regulate the forkhead box M1 cell cycle transcription factor, FoxM1. FADD-D protein is also mislocalized during cell cycle progression. Thus, regulation of FADD phosphorylation is crucial for proper cell cycle entry.

Cyclooxygenase-2 induction by arsenite through the IKKbeta/NFkappaB pathway exerts an antiapoptotic effect in mouse epidermal Cl41 cells

BACKGROUND

Arsenic contamination has become a major public health concern worldwide. Epidemiologic data show that long-term arsenic exposure results in the risk of skin cancer. However, the mechanisms underlying carcinogenic effects of arsenite on skin remain to be studied.

OBJECTIVES

In the present study we evaluated cyclooxygenase-2 (COX-2) expression, the signaling pathways leading to COX-2 induction, and its antiapoptotic function in the response to arsenite exposure in mouse epidermal JB6 Cl41 cells.

METHODS

We used the luciferase reporter assay and Western blots to determine COX-2 induction by arsenite. We utilized dominant negative mutant, genetic knockout, gene knockdown, and gene overexpression approaches to elucidate the signaling pathway involved in COX-2 induction and its protective effect on cell apoptosis.

RESULTS

The induction of COX-2 by arsenite was inhibited in Cl41 cells transfected with IKKbeta-KM, a dominant mutant inhibitor of kbeta (Ikbeta) kinase (IKKbeta), and in IKKbeta-knockout (IKKbeta(-/-)) mouse embryonic fibroblasts (MEFs). IKKbeta/nuclear factor kappaB (NFkappaB) pathway-mediated COX-2 induction exerted an antiapoptotic effect on the cells exposed to arsenite because cell apoptosis was significantly enhanced in the Cl41 cells transfected with IKKbeta-KM or COX-2 small interference RNA (siCOX-2). In addition, IKKbeta(-/-) MEFs stably transfected with COX-2 showed more resistance to arsenite-induced apoptosis compared with the same control vector-transfected cells.

CONCLUSIONS

These results demonstrate that arsenite exposure can induce COX-2 expression through the IKKbeta/NFkappaB pathway, which thereby exerts an antiapoptotic effect in response to arsenite. In light of the importance of apoptosis evasion during carcinogenesis, we anticipate that COX-2 induction may be at least partially responsible for the carcinogenic effect of arsenite on skin.

The Drosophila casein kinase Iepsilon/delta Discs overgrown promotes cell survival via activation of DIAP1 expression

The proper number of cells in developing tissues is achieved by coordinating cell division with apoptosis. In Drosophila, the adult wing is derived from wing imaginal discs, which undergo a period of growth and proliferation during larval stages without much programmed cell death. In this report, we demonstrate that the Drosophila casein kinase Iepsilon/delta, known as Discs overgrown (Dco), is required for maintaining this low level of apoptosis. Expression of dco can suppress the apoptotic activity of Head involution defective (Hid) in the developing eye. Loss of dco in the wing disc results in a dramatic reduction in expression of the caspase inhibitor DIAP1 and a concomitant activation of caspases. The regulation of DIAP1 by Dco occurs by a post-transcriptional mechanism that is independent of hid. Mutant clones of dco are considerably smaller than controls even when apoptosis is inhibited, suggesting that Dco promotes cell division/growth in addition to its role in cell survival. The dco phenotype cannot be explained by defects Wingless (Wg) signaling. We propose that Dco coordinates tissue size by stimulating cell division/growth and blocking apoptosis via activation of DIAP1 expression.

First among equals: competition between genetically identical cells

Competition between genetically identical organisms is considered insignificant in evolutionary theory because it is presumed to have little selective consequence. We argue that competition between genetically identical cells could improve the fitness of a multicellular organism by directing fitter cells to the germ line or by eliminating unfit cells, and that cell-competition mechanisms have been conserved in multicellular organisms. We propose that competition between genetically identical or highly similar units could have similar selective advantages at higher organizational levels, such as societies.

vps25 mosaics display non-autonomous cell survival and overgrowth, and autonomous apoptosis

Appropriate cell-cell signaling is crucial for proper tissue homeostasis. Protein sorting of cell surface receptors at the early endosome is important for both the delivery of the signal and the inactivation of the receptor, and its alteration can cause malignancies including cancer. In a genetic screen for suppressors of the pro-apoptotic gene hid in Drosophila, we identified two alleles of vps25, a component of the ESCRT machinery required for protein sorting at the early endosome. Paradoxically, although vps25 mosaics were identified as suppressors of hid-induced apoptosis, vps25 mutant cells die. However, we provide evidence that a non-autonomous increase of Diap1 protein levels, an inhibitor of apoptosis, accounts for the suppression of hid. Furthermore, before they die, vps25 mutant clones trigger non-autonomous proliferation through a failure to downregulate Notch signaling, which activates the mitogenic JAK/STAT pathway. Hid and JNK contribute to apoptosis of vps25 mutant cells. Inhibition of cell death in vps25 clones causes dramatic overgrowth phenotypes. In addition, Hippo signaling is increased in vps25 clones, and hippo mutants block apoptosis in vps25 clones. In summary, the phenotypic analysis of vps25 mutants highlights the importance of receptor downregulation by endosomal protein sorting for appropriate tissue homeostasis, and may serve as a model for human cancer.

Tumor suppressor properties of the ESCRT-II complex component Vps25 in Drosophila

We have found that the Drosophila gene vps25 possesses several properties of a tumor suppressor. First, vps25 mutant cells activate Notch and Dpp receptor signaling, inducing ectopic organizers in developing eyes and limbs and consequent overproliferation of both mutant and nearby wild-type cells. Second, as the mutant cells proliferate, they lose their epithelial organization and undergo apoptosis. Strikingly, when apoptosis of mutant cells is blocked, tumor-like overgrowths are formed that are capable of metastasis. vps25 encodes a component of the ESCRT-II complex, which sorts membrane proteins into multivesicular bodies during endocytic trafficking to the lysosome. Activation of Notch and Dpp receptor signaling in mutant cells results from an endocytic blockage that causes accumulation of these receptors and other signaling components in endosomes. These results highlight the importance of endocytic trafficking in regulating signaling and epithelial organization and suggest a possible role for ESCRT components in human cancer.

Cell death in early neural life

Programmed cell death is a relevant process in the physiology and pathology of the nervous system. Neuronal cell death during development is well characterized, and studies of this process have provided valuable information regarding the regulatory mechanisms of cell death in the nervous system. In the last few years, cell death occurring at earlier developmental stages and affecting proliferating neuroepithelial cells and recently born neuroblasts has been recognized. In this review we cover the observations on cell death in the early, proliferating stages of vertebrate neural development. Genetically modified mouse model systems and complementary in vivo approaches in other vertebrates have provided a solid basis for its relevance and contribution to normal neural development, as well as for the pathological consequences of its deregulation. However, the precise functional role of cell death remains a topic of debate.

The tumor suppressor WARTS activates the Omi / HtrA2-dependent pathway of cell death

Drosophila tumor suppressor WARTS (Wts) is an evolutionally conserved serine / threonine kinase and participates in a signaling complex that regulates both proliferation and apoptosis to ensure the proper size and shape of the fly. Human counterparts of this complex have been found to be frequently downregulated or mutated in cancers. WARTS, a human homolog of Wts, is also known as tumor suppressor and mitotic regulator, but its molecular implications in tumorigenesis are still obscure. Here, we show that WARTS binds via its C-terminus to the PDZ domain of a proapoptotic serine protease Omi / HtrA2. Depletion of WARTS inhibited Omi / HtrA2-mediated cell death, whereas overexpression of WARTS promoted this process. Furthermore, WARTS can enhance the protease activity of Omi / HtrA2 both in vivo and in vitro. Activation of Omi / HtrA2-mediated cell death is thus a potential mechanism for the tumor suppressive activity of WARTS.

Wing-to-Leg homeosis by spineless causes apoptosis regulated by Fish-lips, a novel leucine-rich repeat transmembrane protein

Growth, patterning, and apoptosis are mutually interactive during development. For example, cells that select an abnormal fate in a developing field are frequently removed by apoptosis. An important issue in this process that needs to be resolved is the mechanism used by cells to discern their correct fate from an abnormal fate. In order to examine this issue, we developed an animal model that expresses the dioxin receptor homolog Spineless (Ss) ectopically in the Drosophila wing. The presence of mosaic clones ectopically expressing ss results in a local transformation of organ identity, homeosis, from wing into a leg or antenna. The cells with misspecified fates subsequently activate c-Jun N-terminal kinase to undergo apoptosis in an autonomous or nonautonomous manner depending on their position within the wing, suggesting that a cell-cell interaction is, at least in some cases, involved in the detection of misspecified cells. Similar position dependence is commonly observed when various homeotic genes controlling the body segments are ectopically expressed. The autonomous and nonautonomous apoptosis caused by ss is regulated by a novel leucine-rich repeat family transmembrane protein, Fish-lips (Fili) that interacts with surrounding normal cells. These data support a mechanism in which the lack of some membrane proteins helps to recognize the presence of different cell types and direct these cells to an apoptotic fate in order to exclude them from the normal developing field.

Coordination of cell death and the cell cycle: linking proliferation to death through private and communal couplers

In development and in the adult, complex signaling pathways operate within and between cells to coordinate proliferation and cell death. These networks can be viewed as coupling devices that link engines driving the cell cycle and the initiation of apoptosis. We propose three simple frameworks for modeling the effects of proliferative drive on apoptotic propensity. This perspective offers a potentially useful foundation for predicting group behaviors of cells in normal and pathological settings.

The protein structures that shape caspase activity, specificity, activation and inhibition

The death morphology commonly known as apoptosis results from a post-translational pathway driven largely by specific limited proteolysis. In the last decade the structural basis for apoptosis regulation has moved from nothing to 'quite good', and we now know the fundamental structures of examples from the initiator phase, the pre-mitochondrial regulator phase, the executioner phase, inhibitors and their antagonists, and even the structures of some substrates. The field is as well advanced as the best known of proteolytic pathways, the coagulation cascade. Fundamentally new mechanisms in protease regulation have been disclosed. Structural evidence suggests that caspases have an unusual catalytic mechanism, and that they are activated by apparently unrelated events, depending on which position in the apoptotic pathway they occupy. Some naturally occurring caspase inhibitors have adopted classic inhibition strategies, but other have revealed completely novel mechanisms. All of the structural and mechanistic information can, and is, being applied to drive therapeutic strategies to combat overactivation of apoptosis in degenerative disease, and underactivation in neoplasia. We present a comprehensive review of the caspases, their regulators and inhibitors from a structural and mechanistic point of view, and with an aim to consolidate the many threads that define the rapid growth of this field.

Ribosomal protein L24 defect in belly spot and tail (Bst), a mouse Minute

Ribosomal protein mutations, termed Minutes, have been instrumental in studying the coordination of cell and tissue growth in Drosophila. Although abundant in flies, equivalent defects in mammals are relatively unknown. Belly spot and tail (Bst) is a semidominant mouse mutation that disrupts pigmentation, somitogenesis and retinal cell fate determination. Here, we identify Bst as a deletion within the Rpl24 riboprotein gene. Bst significantly impairs Rpl24 splicing and ribosome biogenesis. Bst/+ cells have decreased rates of protein synthesis and proliferation, and are outcompeted by wild-type cells in C57BLKS<-->ROSA26 chimeras. Bacterial artificial chromosome (BAC) and cDNA transgenes correct the mutant phenotypes. Our findings establish Bst as a mouse Minute and provide the first detailed characterization of a mammalian ribosomal protein mutation.

Role of cell cycle regulator E2F1 in regulating CD8 T cell responses during acute and chronic viral infection

To determine the role of cell cycle regulatory protein E2F1 in T cell immunity, we compared antigen-specific CD8 T cell responses between wild type (+/+) and E2F1-deficient (E2F1-/-) mice following an acute and chronic infection with lymphocytic choriomeningitis virus (LCMV). During an acute LCMV infection, although LCMV-specific effector CD8 T cells from E2F1-/- mice were less susceptible to activation-induced cell death (AICD) in vitro, E2F1 deficiency had no significant effect on the: (1) expansion or contraction of virus-specific CD8 T cell responses; (2) proliferative renewal of memory CD8 T cells in both lymphoid and non-lymphoid organs. Importantly, under conditions of repeated antigenic stimulation in the setting of a chronic LCMV infection, E2F1 deficiency did not preclude the exhaustion of CD8 T cells specific to the immunodominant epitope nucleoprotein 396-404 (NP396-404). Taken together, our studies show that E2F1, an important tumor suppressor and cell cycle regulator, may not have a non-redundant role in regulating CD8 T cell responses in acute and chronic LCMV infections.

The PDGF/VEGF Receptor Controls Blood Cell Survival in Drosophila

The Drosophila PDGF/VEGF receptor (PVR) has known functions in the guidance of cell migration. We now demonstrate that during embryonic hematopoiesis, PVR has a role in the control of antiapoptotic cell survival. In Pvr mutants, a large fraction of the embryonic hemocyte population undergoes apoptosis, and the remaining blood cells cannibalistically phagocytose their dying peers. Consequently, total hemocyte numbers drop dramatically during embryogenesis, and large aggregates of engorged macrophages carrying multiple apoptotic corpses form. Hemocyte-specific expression of the pan-caspase inhibitor p35 in Pvr mutants eliminates hemocyte aggregates and restores blood cell counts and morphology. Additional rescue experiments suggest involvement of the Ras pathway in PVR-mediated blood cell survival. In cell culture, we demonstrate that PVR directly controls survival of a hemocyte cell line. This function of PVR shows striking conservation with mammalian hematopoiesis and establishes Drosophila as a model to study hematopoietic cell survival in development and disease.

GW112, a novel antiapoptotic protein that promotes tumor growth

GW112 is a novel gene that has little homology to other known genes. It is overexpressed in a number of human tumor types, especially in those of the digestive system. We show here that GW112 is associated with GRIM-19, a protein known to be involved in regulating cellular apoptosis. Functionally, GW112 could significantly attenuate the ability of GRIM19 to mediate retinoic acid-IFN-beta-mediated cellular apoptosis and apoptosis-related gene expression. In addition, GW112 demonstrated strong antiapoptotic effects in tumor cells treated with other stress exposures such as hydrogen peroxide. Finally, forced overexpression of GW112 in murine prostate tumor cells led to more rapid tumor formation in a syngeneic host. Taken together, our data suggest that GW112 is an important regulator of cell death that plays important roles in tumor cell survival and tumor growth.

dMyc transforms cells into super-competitors

Overexpression of myc protooncogenes has been implicated in the genesis of many human tumors. Myc proteins seem to regulate diverse biological processes, but their role in tumorigenesis remains enigmatic. Here we use Drosophila imaginal discs to mimic situations in which cells with unequal levels of Myc protein are apposed and show that this invariably elicits a win/lose situation reminiscent of cell competition; cells with lower levels of dMyc are eliminated by apoptosis whereas cells with higher levels of dMyc over-proliferate. We find that this competitive behavior correlates with, and can be corrected by, the activation of the BMP/Dpp survival signaling pathway. Hence the heritable increase in dMyc levels causes cells to behave as "super-competitors" and reveals a novel mode of clonal expansion that causes, but also relies on, the killing of surrounding cells.

Caspase activation - stepping on the gas or releasing the brakes? Lessons from humans and flies

The central components of the execution phase of apoptosis in worms, flies, and humans are members of the caspase protease family. Work in Drosophila and mammalian systems has revealed a web of interactions that govern the activity of these proteases, and two fundamental control points have been identified. These are zymogen activation - the process that converts a latent caspase into its active form, and inhibition of the resulting active protease. In humans, the driving force for caspase activity is activation of the zymogens, but in Drosophila, a major thrust is derepression of caspase inhibitors. In this review, we consider evidence for these two distinct events in terms of the regulation of caspase activity. This sets the scene for therapy to reinstate the normal death mechanisms that have been overcome in a cancer cell's quest for immortality.

Nuclear translocation of cytochrome c during apoptosis

Release of cytochrome c from mitochondria is a major event during apoptosis. Released cytochrome c has been shown to activate caspase-dependent apoptotic signals. In this report, we provide evidence for a novel role of cytochrome c in caspase-independent nuclear apoptosis. We showed that cytochrome c, released from mitochondria upon apoptosis induction, gradually accumulates in the nucleus as evidenced by both immunofluorescence and subcellular fractionation. Parallel to nuclear accumulation of cytochrome c, acetylated histone H2A, but not unmodified H2A, was released from the nucleus to the cytoplasm. Addition of purified cytochrome c to isolated nuclei recapitulated the preferential release of acetylated, but not deacetylated, histone H2A. Cytochrome c was also found to induce chromatin condensation. These results suggest that the nuclear accumulation of cytochrome c may be directly involved in the remodeling of chromatin. Our results provide evidence of a novel role for cytochrome c in inducing nuclear apoptosis.

Essential role of survivin, an inhibitor of apoptosis protein, in T cell development, maturation, and homeostasis

Survivin is an inhibitor of apoptosis protein that also functions during mitosis. It is expressed in all common tumors and tissues with proliferating cells, including thymus. To examine its role in apoptosis and proliferation, we generated two T cell–specific survivin-deficient mouse lines with deletion occurring at different developmental stages. Analysis of early deleting survivin mice showed arrest at the pre–T cell receptor proliferating checkpoint. Loss of survivin at a later stage resulted in normal thymic development, but peripheral T cells were immature and significantly reduced in number. In contrast to in vitro studies, loss of survivin does not lead to increased apoptosis. However, newborn thymocyte homeostatic and mitogen-induced proliferation of survivin-deficient T cells were greatly impaired. These data suggest that survivin is not essential for T cell apoptosis but is crucial for T cell maturation and proliferation, and survivin-mediated homeostatic expansion is an important physiological process of T cell development.

Bcl-2 as a target for overcoming chemoresistance in small-cell lung cancer

Small-cell lung cancer (SCLC) is an aggressive malignancy that is frequently metastatic at presentation and has a poor prognosis. Although initially sensitive to primary therapy, acquisition of apoptosis resistance is typical, resulting in failure of secondary chemotherapy following relapse. Expression of the antiapoptosis protein Bcl-2 is prevalent in SCLC. The understanding of this oncoprotein's function has increased dramatically over the past decade. In vitro and in vivo evidence supports a role for overexpression of Bcl-2 in SCLC and supports the notion that it is a major factor contributing to apoptosis resistance. Targeting Bcl-2 may provide a novel therapeutic approach to overcoming chemoresistance in SCLC. This article discusses the relevance of Bcl-2 to apoptosis susceptibility in SCLC and its exploitation using gene silencing to improve the clinical outcome in this disease.

Macrophage-mediated corpse engulfment is required for normal Drosophila CNS morphogenesis

Cell death plays an essential role in development, and the removal of cell corpses presents an important challenge for the developing organism. Macrophages are largely responsible for the clearance of cell corpses in Drosophila melanogaster and mammalian systems. We have examined the developmental requirement for macrophages in Drosophila and find that macrophage function is essential for central nervous system (CNS) morphogenesis. We generate and analyze mutations in the Pvr locus, which encodes a receptor tyrosine kinase of the PDGF/VEGF family that is required for hemocyte migration. We find that loss of Pvr function causes the mispositioning of glia within the CNS and the disruption of the CNS axon scaffold. We further find that inhibition of hemocyte development or of Croquemort, a receptor required for macrophage-mediated corpse engulfment, causes similar CNS defects. These data indicate that macrophage-mediated clearance of cell corpses is required for proper morphogenesis of the Drosophila CNS.