Aneuploidy-induced delaminating cells drive tumorigenesis in Drosophila epithelia.

Genomic instability has been observed in essentially all sporadic carcinomas. Here we use Drosophila epithelial cells to address the role of chromosomal instability in cancer development as they have proved useful for elucidating the molecular mechanisms underlying tumorigenic growth. We first show that chromosomal instability leads to an apoptotic response. Interestingly, this response is p53 independent, as opposed to mammalian cells, and depends on the activation of the c-Jun N-terminal kinase (JNK) signaling cascade. When prevented from undergoing programmed cell death (PCD), chromosomal instability induces neoplasic overgrowth. These tumor-like tissues are able to grow extensively and metastasize when transplanted into the abdomen of adult hosts. Detailed analysis of the tumors allows us to identify a delaminating cell population as the critical one in driving tumorigenesis. Cells loose their apical-basal polarity, mislocalize DE-cadherin, and delaminate from the main epithelium. A JNK-dependent transcriptional program is activated specifically in delaminating cells and drives nonautonomous tissue overgrowth, basement membrane degradation, and invasiveness. These findings unravel a general and rapid tumorigenic potential of genomic instability, as opposed to its proposed role as a source of mutability to select specific tumor-prone aneuploid cells, and open unique avenues toward the understanding of the role of genomic instability in human cancer.

Enhancer-PRE communication contributes to the expansion of gene expression domains in proliferating primordia.

Trithorax-group and Polycomb-group proteins interact with chromosomal elements, termed PRE/TREs, to ensure stable heritable maintenance of the transcriptional state of nearby genes. Regulatory elements that bind both groups of proteins are termed maintenance elements (MEs). Some of these MEs maintain the initial activated transcriptional state of a nearby reporter gene through several rounds of mitosis during development. Here, we show that expression of hedgehog in the posterior compartment of the Drosophila wing results from the communication between a previously defined ME and a nearby cis-regulatory element termed the C enhancer. The C enhancer integrates the activities of the Notch and Hedgehog signalling pathways and, from the early wing primordium stage, drives expression to a thin stripe in the posterior compartment that corresponds to the dorsal-ventral compartment boundary. The ME maintains the initial activated transcriptional state conferred by the C enhancer and contributes to the expansion, by growth, of its expression domain throughout the posterior compartment. Communication between the ME and the C enhancer also contributes to repression of gene expression in anterior cells. Most interestingly, we present evidence that enhancers and MEs of different genes are interchangeable modules whose communication is involved in restricting and expanding the domains of gene expression. Our results emphasize the modular role of MEs in regulation of gene expression within growing tissues.

Chromosomal instability-induced cell invasion through caspase-driven DNA damage.

Chromosomal instability (CIN), an increased rate of changes in chromosome structure and number, is observed in most sporadic human carcinomas with high metastatic activity. Here, we use a Drosophila epithelial model to show that DNA damage, as a result of the production of lagging chromosomes during mitosis and aneuploidy-induced replicative stress, contributes to CIN-induced invasiveness. We unravel a sub-lethal role of effector caspases in invasiveness by enhancing CIN-induced DNA damage and identify the JAK/STAT signaling pathway as an activator of apoptotic caspases through transcriptional induction of pro-apoptotic genes. We provide evidence that an autocrine feedforward amplification loop mediated by Upd3-a cytokine with homology to interleukin-6 and a ligand of the JAK/STAT signaling pathway-contributes to amplifying the activation levels of the apoptotic pathway in migrating cells, thus promoting CIN-induced invasiveness. This work sheds new light on the chromosome-signature-independent effects of CIN in metastasis.

The Upd3 cytokine couples inflammation to maturation defects in Drosophila.

Developmental transitions, such as puberty or metamorphosis, are tightly controlled by steroid hormones and can be delayed by the appearance of growth abnormalities, developmental tumors, or inflammatory disorders such as inflammatory bowel disease or cystic fibrosis.1-4 Here, we used a highly inflammatory epithelial model of malignant transformation in Drosophila5,6 to unravel the role of Upd3-a cytokine with homology to interleukin-6-and the JAK/STAT signaling pathway in coupling inflammation to a delay in metamorphosis. We present evidence that Upd3 produced by malignant and nearby cell populations signals to the prothoracic gland-an endocrine tissue primarily dedicated to the production of the steroid hormone ecdysone-to activate JAK/STAT and bantam microRNA (miRNA) and to delay metamorphosis. Upd cytokines produced by the tumor site contribute to increasing the systemic levels of Upd3 by amplifying its expression levels in a cell-autonomous manner and by inducing Upd3 expression in neighboring tissues in a non-autonomous manner, culminating in a major systemic response to prevent larvae from initiating pupa transition. Our results identify a new regulatory network impacting on ecdysone biosynthesis and provide new insights into the potential role of inflammatory cytokines and the JAK/STAT signaling pathway in coupling inflammation to delays in puberty.

Proteostasis failure and mitochondrial dysfunction leads to aneuploidy-induced senescence.

Aneuploidy, an unbalanced number of chromosomes, is highly deleterious at the cellular level and leads to senescence, a stress-induced response characterized by permanent cell-cycle arrest and a well-defined associated secretory phenotype. Here, we use a Drosophila epithelial model to delineate the pathway that leads to the induction of senescence as a consequence of the acquisition of an aneuploid karyotype. Whereas aneuploidy induces, as a result of gene dosage imbalance, proteotoxic stress and activation of the major protein quality control mechanisms, near-saturation functioning of autophagy leads to compromised mitophagy, accumulation of dysfunctional mitochondria, and the production of radical oxygen species (ROS). We uncovered a role of c-Jun N-terminal kinase (JNK) in driving senescence as a consequence of dysfunctional mitochondria and ROS. We show that activation of the major protein quality control mechanisms and mitophagy dampens the deleterious effects of aneuploidy, and we identify a role of senescence in proteostasis and compensatory proliferation for tissue repair.

Regulation of Anisotropic Tissue Growth by Two Orthogonal Signaling Centers.

The Drosophila wing has served as a paradigm to mechanistically characterize the role of morphogens in patterning and growth. Wingless (Wg) and Decapentaplegic (Dpp) are expressed in two orthogonal signaling centers, and their gradients organize patterning by regulating the expression of well-defined target genes. By contrast, graded activity of these morphogens is not an absolute requirement for wing growth. Despite their permissive role in regulating growth, here we show that Wg and Dpp are utilized in a non-interchangeable manner by the two existing orthogonal signaling centers to promote preferential growth along the two different axes of the developing wing. Our data indicate that these morphogens promote anisotropic growth by making use of distinct and non-interchangeable molecular mechanisms. Whereas Dpp drives growth along the anterior-posterior axis by maintaining Brinker levels below a growth-repressing threshold, Wg exerts its action along the proximal-distal axis through a double repression mechanism involving T cell factor (TCF).

Chromosomal Instability Induces Cellular Invasion in Epithelial Tissues.

Most sporadic carcinomas with high metastatic activity show an increased rate of changes in chromosome structure and number, known as chromosomal instability (CIN). However, the role of CIN in driving invasiveness remains unclear. Using an epithelial model in Drosophila, we present evidence that CIN promotes a rapid and general invasive behavior. Cells with an abnormal number of chromosomes delaminate from the epithelium, extend actin-based cellular protrusions, form membrane blebs, and invade neighboring tissues. This behavior is governed by the activation of non-muscle Myosin II by Rho kinase and by the expression of the secreted EGF/Spitz ligand. We unravel fundamental roles of the mitogen-activated protein kinase pathways mediated by the Fos proto-oncogene and the Capicua tumor suppressor gene in the invasive behavior of CIN-induced aneuploid cells. Our results support the proposal that the simple production of unbalanced karyotypes contributes to CIN-induced metastatic progression.

Boundary Dpp promotes growth of medial and lateral regions of the Drosophila wing.

The gradient of Decapentaplegic (Dpp) in the Drosophila wing has served as a paradigm to characterize the role of morphogens in regulating patterning. However, the role of this gradient in regulating tissue size is a topic of intense debate as proliferative growth is homogenous. Here, we combined the Gal4/UAS system and a temperature-sensitive Gal80 molecule to induce RNAi-mediated depletion of dpp and characterise the spatial and temporal requirement of Dpp in promoting growth. We show that Dpp emanating from the AP compartment boundary is required throughout development to promote growth by regulating cell proliferation and tissue size. Dpp regulates growth and proliferation rates equally in central and lateral regions of the developing wing appendage and reduced levels of Dpp affects similarly the width and length of the resulting wing. We also present evidence supporting the proposal that graded activity of Dpp is not an absolute requirement for wing growth.

Gene Dosage Imbalance Contributes to Chromosomal Instability-Induced Tumorigenesis.

Chromosomal instability (CIN) is thought to be a source of mutability in cancer. However, CIN often results in aneuploidy, which compromises cell fitness. Here, we used the dosage compensation mechanism (DCM) of Drosophila to demonstrate that chromosome-wide gene dosage imbalance contributes to the deleterious effects of CIN-induced aneuploidy and its pro-tumorigenic action. We present evidence that resetting of the DCM counterbalances the damaging effects caused by CIN-induced changes in X chromosome number. Importantly, interfering with the DCM suffices to mimic the cellular effects of aneuploidy in terms of reactive oxygen species (ROS) production, JNK-dependent cell death, and tumorigenesis upon apoptosis inhibition. We unveil a role of ROS in JNK activation and a variety of cellular and tissue-wide mechanisms that buffer the deleterious effects of CIN, including DNA-damage repair, activation of the p38 pathway, and cytokine induction to promote compensatory proliferation. Our data reveal the existence of robust compensatory mechanisms that counteract CIN-induced cell death and tumorigenesis.

MicroRNA-mediated regulation of Dp53 in the Drosophila fat body contributes to metabolic adaptation to nutrient deprivation.

Multiple conserved mechanisms sense nutritional conditions and coordinate metabolic changes in the whole organism. We unravel a role for the Drosophila homolog of p53 (Dp53) in the fat body (FB; a functional analog of vertebrate adipose and hepatic tissues) in starvation adaptation. Under nutrient deprivation, FB-specific depletion of Dp53 accelerates consumption of major energy stores and reduces survival rates of adult flies. We show that Dp53 is regulated by the microRNA (miRNA) machinery and miR-305 in a nutrition-dependent manner. In well-fed animals, TOR signaling contributes to miR-305-mediated inhibition of Dp53. Nutrient deprivation reduces the levels of miRNA machinery components and leads to Dp53 derepression. Our results uncover an organism-wide role for Dp53 in nutrient sensing and metabolic adaptation and open up avenues toward understanding the molecular mechanisms underlying p53 activation under nutrient deprivation.