FGF21 Induced by the ASK1-p38 Pathway Promotes Mechanical Cell Competition by Attracting Cells

Flamingo participates in multiple models of cell competition

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

Toward a predictive understanding of epithelial cell death

Epithelial cell death is highly prevalent during development and tissue homeostasis. While we have a rather good understanding of the molecular regulators of programmed cell death, especially for apoptosis, we still fail to predict when, where, how many and which specific cells will die in a tissue. This likely relies on the much more complex picture of apoptosis regulation in a tissular and epithelial context, which entails cell autonomous but also non-cell autonomous factors, diverse feedback and multiple layers of regulation of the commitment to apoptosis. In this review, we illustrate this complexity of epithelial apoptosis regulation by describing these different layers of control, all demonstrating that local cell death probability is a complex emerging feature. We first focus on non-cell autonomous factors that can locally modulate the rate of cell death, including cell competition, mechanical input and geometry as well as systemic effects. We then describe the multiple feedback mechanisms generated by cell death itself. We also outline the multiple layers of regulation of epithelial cell death, including the coordination of extrusion and regulation occurring downstream of effector caspases. Eventually, we propose a roadmap to reach a more predictive understanding of cell death regulation in an epithelial context.

Epithelial recognition and elimination against aberrant cells

Epithelial cells, which are non-immune cells, not only function as a physical defence barrier but also continuously monitor and eliminate aberrant epithelial cells in their vicinity. In other words, it has become evident that epithelial cells possess immune cell-like functions. In fact, recent research has revealed that epithelial cells recognise the Major Histocompatibility Complex I (MHC-I) of aberrant cells as a mechanism for surveillance. This cellular defence mechanism of epithelial cells probably detects aberrant cells more promptly than the conventional immune response, making it a novel and primary biological defence. Furthermore, there is the potential for this new immune-like biological defence mechanism to establish innovative treatment for disease prevention, leading to increasing anticipation for its future medical applications. In this review, we aim to summarise the recognition and attack mechanisms of aberrant cells by epithelial cells in mammals, with a particular focus on the field of cancer. Additionally, we discuss the potential therapeutic applications of epithelial cell-based defence against cancer, including novel prophylactic treatment methods based on molecular mechanisms.

Scribble and E-cadherin cooperate to control symmetric daughter cell positioning by multiple mechanisms

The fate of the two daughter cells is intimately connected to their positioning, which is in turn regulated by cell junction remodelling and orientation of the mitotic spindle. How multiple cues are integrated to dictate the ultimate positioning of daughters is not clear. Here, we identify novel mechanisms of regulation of daughter positioning in single MCF10A cells. The polarity protein, Scribble cooperates with E-cadherin for sequential roles in daughter positioning. First Scribble stabilises E-cadherin at the mitotic cortex as well as the retraction fibres, to mediate spindle orientation. Second, Scribble re-locates to the junction between the two daughters to allow a new E-cadherin-based-interface to form between them, influencing the width of the nascent daughter-daughter junction and subsequent cell positioning. Thus, E-cadherin and Scribble dynamically relocate to different intracellular sites during cell division to orient the mitotic spindle and control placement of the daughter cells after cell division. This article has an associated First Person interview with the first author of the paper.

Cell polarity and extrusion: How to polarize extrusion and extrude misspolarized cells?

The barrier function of epithelia is one of the cornerstones of the body plan organization of metazoans. It relies on the polarity of epithelial cells which organizes along the apico-basal axis the mechanical properties, signaling as well as transport. This barrier function is however constantly challenged by the fast turnover of epithelia occurring during morphogenesis or adult tissue homeostasis. Yet, the sealing property of the tissue can be maintained thanks to cell extrusion: a series of remodeling steps involving the dying cell and its neighbors leading to seamless cell expulsion. Alternatively, the tissue architecture can also be challenged by local damages or the emergence of mutant cells that may alter its organization. This includes mutants of the polarity complexes which can generate neoplastic overgrowths or be eliminated by cell competition when surrounded by wild type cells. In this review, we will provide an overview of the regulation of cell extrusion in various tissues focusing on the relationship between cell polarity, cell organization and the direction of cell expulsion. We will then describe how local perturbations of polarity can also trigger cell elimination either by apoptosis or by cell exclusion, focusing specifically on how polarity defects can be directly causal to cell elimination. Overall, we propose a general framework connecting the influence of polarity on cell extrusion and its contribution to aberrant cell elimination.

Cell-cell interactions that drive tumorigenesis in Drosophila

Cell-cell interactions within tumour microenvironment play crucial roles in tumorigenesis. Genetic mosaic techniques available in Drosophila have provided a powerful platform to study the basic principles of tumour growth and progression via cell-cell communications. This led to the identification of oncogenic cell-cell interactions triggered by endocytic dysregulation, mitochondrial dysfunction, cell polarity defects, or Src activation in Drosophila imaginal epithelia. Such oncogenic cooperations can be caused by interactions among epithelial cells, mesenchymal cells, and immune cells. Moreover, microenvironmental factors such as nutrients, local tissue structures, and endogenous growth signalling activities critically affect tumorigenesis. Dissecting various types of oncogenic cell-cell interactions at the single-cell level in Drosophila will greatly increase our understanding of how tumours progress in living animals.

Photoactivatable Materials for Versatile Single-Cell Patterning Based on the Photocaging of Cell-Anchoring Moieties through Lipid Self-Assembly

Versatile methods for patterning multiple types of cells with single-cell resolution have become an increasingly important technology for cell analysis, cell-based device construction, and tissue engineering. Here, we present a photoactivatable material based on poly(ethylene glycol) (PEG)-lipids for patterning a variety of cells, regardless of their adhesion abilities. In this study, PEG-lipids bearing dual fatty acid chains were first shown to perfectly suppress cell anchoring on their coated substrate surfaces whereas those with single-chain lipids stably anchored cells through lipid-cell membrane interactions. From this finding, a PEG-lipid with one each of both normal and photocleavable fatty acid chains was synthesized as a material that could convert the chain number from two to one by exposure to light. On the photoconvertible PEG-lipid surface, cell anchoring was activated by light exposure. High-speed atomic force microscopy measurements revealed that this photocaging of the lipid-cell membrane interaction occurs because the hydrophobic dual chains self-assemble into nanoscale structures and cooperatively inhibit the anchoring. Light-induced dissociation of the lipid assembly achieved the light-guided fine patterning of multiple cells through local photoactivation of the anchoring interactions. Using this surface, human natural killer cells and leukemia cells could be positioned to interact one-by-one. The cytotoxic capacity of single immune cells was then monitored via microscopy, showing the proof-of-principle for applications in the high-throughput analysis of the heterogeneity in individual cell-cell communications. Thus, the substrate coated with our photoactivatable material can serve as a versatile platform for the accurate and rapid patterning of multiple-element cells for intercellular communication-based diagnostics.

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

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

Extracellular ATP facilitates cell extrusion from epithelial layers mediated by cell competition or apoptosis

For the maintenance of epithelial homeostasis, various aberrant or dysfunctional cells are actively eliminated from epithelial layers. This cell extrusion process mainly falls into two modes: cell-competition-mediated extrusion and apoptotic extrusion. However, it is not clearly understood whether and how these processes are governed by common molecular mechanisms. In this study, we demonstrate that the reactive oxygen species (ROS) levels are elevated within a wide range of epithelial layers around extruding transformed or apoptotic cells. The downregulation of ROS suppresses the extrusion process. Furthermore, ATP is extracellularly secreted from extruding cells, which promotes the ROS level and cell extrusion. Moreover, the extracellular ATP and ROS pathways positively regulate the polarized movements of surrounding cells toward extruding cells in both cell-competition-mediated and apoptotic extrusion. Hence, extracellular ATP acts as an "extrude me" signal and plays a prevalent role in cell extrusion, thereby sustaining epithelial homeostasis and preventing pathological conditions or disorders.

E3 ubiquitin ligase mind bomb 1 overexpression reduces apoptosis and inflammation of cardiac microvascular endothelial cells in coronary microvascular dysfunction

BACKGROUND

The apoptosis and inflammation in cardiac microvascular endothelial cells (CMECs) promote the development of coronary microvascular dysfunction (CMD). The present study aimed to explore the role of E3 ubiquitin ligase mind bomb 1 (MIB1) in the apoptosis and inflammation in CMECs during CMD.

METHODS

In vivo, CMD in rats was induced by sodium laurate injection. In vitro, rat primary CMECs were stimulated by homocysteine (Hcy). The apoptosis of CMECs was measured using flow cytometry. The inflammation of CMECs was evaluated by the level of tumor necrosis factor alpha (TNF-α) and interleukin 1 beta (IL-1β). The interplay between MIB1 and mitogen-activated protein kinase kinase kinase 5 (map3k5, also called ASK1) was measured using Co-immunoprecipitation.

RESULTS

MIB1 expression was decreased and ASK1 expression was increased in the heart tissues of CMD rats and Hcy-treated CMECs. MIB1 overexpression decreased fibrinogen-like protein 2 (FGL2) secretion, inflammation, and apoptosis induced by Hcy in CMECs. Meanwhile, MIB1 overexpression decreased the protein levels of ASK1 and p38, while not affected ASK1 mRNA levels. The following mechanism experiments revealed that MIB1 downregulated ASK1 expression by increasing its ubiquitination. ASK1 overexpression reversed the inhibitory effect of MIB1 on FGL2 secretion, apoptosis, inflammation, and p38 activation in Hcy-treated CMECs. In CMD rats, MIB1 overexpression partly retarded CMD progression, manifesting as increased coronary capillary density and decreased microthrombi formation.

CONCLUSION

MIB1 overexpression relieved apoptosis and inflammation of CMECs during CMD by targeting the ASK1/p38 pathway.

Collective effects in epithelial cell death and cell extrusion

Programmed cell death, notably apoptosis, is an essential guardian of tissue homeostasis and an active contributor of organ shaping. While the regulation of apoptosis has been mostly analysed in the framework of a cell autonomous process, recent works highlighted important collective effects which can tune cell elimination. This is particularly relevant for epithelial cell death, which requires fine coordination with the neighbours in order to maintain tissue sealing during cell expulsion. In this review, we will focus on the recent advances which outline the complex multicellular communications at play during epithelial cell death and cell extrusion. We will first focus on the new unanticipated functions of neighbouring cells during extrusion, discuss the contribution of distant neighbours, and finally highlight the complex feedbacks generated by cell elimination on neighbouring cell death.

Cell competition, cooperation, and cancer

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

Transmembrane G protein-coupled receptor 5 signaling stimulates fibroblast growth factor 21 expression concomitant with up-regulation of the transcription factor nuclear receptor Nr4a1

Fibroblast growth factor 21 (FGF21) acts as an endocrine factor, playing important roles in the regulation of energy homeostasis, glucose and lipid metabolism. It is induced by diverse metabolic and cellular stresses, such as starvation and cold challenge, which in turn facilitate adaptation to the stress environment. The pharmacological action of FGF21 has received much attention, because the administration of FGF21 or its analogs has been shown to have an anti-obesity effect in rodent models. In the present study, we found that 3-O-acetyloleanolic acid, an active constituent isolated from the fruits of Forsythia suspensa, stimulated FGF21 production concomitant with the up-regulation of a transcription factor, nuclear receptor Nr4a1, in C2C12 myotubes. Additionally, significant increases in mFgf21 promoter activity were observed in C2C12 cells overexpressing TGR5 receptor in response to 3-O-acetyloleanolic acid treatment. Treatment with the p38 MAPK inhibitor SB203580 was effective at suppressing these stimulatory effects of 3-O-acetyloleanolic acid. Pretreatment with SB203580 also significantly repressed FGF21 mRNA abundance and FGF21 secretion in C2C12 myotubes after 3-O-acetyloleanolic acid stimulation, suggesting that p38 activation is required for the induction of FGF21 by ligand-activated TGR5 in C2C12 myotubes. These findings collectively indicated that TGR5 receptor signaling drives FGF21 expression via p38 activation, at least partly, by mediating Nr4a1 expression. Thus, the novel biological function of 3-O-acetyloleanolic acid as an agent having anti-obesity effects is likely to be mediated through the activation of TGR5 receptors.

Cell competition: Bridging the scales through cell-based modeling

Cell competition is a context-dependent, cell-elimination process that has been proposed to rely on several overlapping mechanisms. A new study combining cell-based modeling and quantitative microscopy data helps to evaluate the main contributors of mutant cell elimination.

Cell Competition: A Choreographed Dance of Death

During cell competition fitter cells eliminate the weaker ones. New work identifies FGF21 as a factor that is secreted by the prospective loser cells of this competition and that acts to attract the winners towards them.