Entosis: the core mechanism and crosstalk with other cell death programs

Cell death pathways play critical roles in organism development and homeostasis as well as in the pathogenesis of various diseases. While studies over the last decade have elucidated numerous different forms of cell death that can eliminate cells in various contexts, how certain mechanisms impact physiology is still not well understood. Moreover, recent studies have shown that multiple forms cell death can occur in a cell population, with different forms of death eliminating individual cells. Here, we aim to describe the known molecular mechanisms of entosis, a non-apoptotic cell engulfment process, and discuss signaling mechanisms that control its induction as well as its possible crosstalk with other cell death mechanisms.

Mechanisms and significance of entosis for tumour growth and progression

To date, numerous mechanisms have been identified in which one cell engulfs another, resulting in the creation of 'cell-in-cell' (CIC) structures, which subsequently cause cell death. One of the mechanisms of formation of these structures is entosis, which is presumably associated with possible carcinogenesis and tumour progression. The peculiarity of the process is that entotic cells themselves actively invade the host cell, and afterwards have several possible variants of fate. Entotic formations are structures where one cell is engulfed by another cell, creating a cell-in-cell structure. The nucleus of the outer cell has a crescent shape, while the inner cell is surrounded by a large entotic vacuole. These characteristics differentiate entosis from cell cannibalism. It's worth noting that entotic formations are not necessarily harmful and may even be beneficial in some cases. In this article we will consider the mechanism of entosis and variants of entotic cell death, and also put forward hypothesis about possible variants of participation of this process on the formation and progression of cancer. This article also presents our proposed classification of functional forms of entosis.

Loss of RND3/RHOE controls entosis through LAMP1 expression in hepatocellular carcinoma

Entosis is a process that leads to the formation of cell-in-cell structures commonly found in cancers. Here, we identified entosis in hepatocellular carcinoma and the loss of Rnd3 (also known as RhoE) as an efficient inducer of this mechanism. We characterized the different stages and the molecular regulators of entosis induced after Rnd3 silencing. We demonstrated that this process depends on the RhoA/ROCK pathway, but not on E-cadherin. The proteomic profiling of entotic cells allowed us to identify LAMP1 as a protein upregulated by Rnd3 silencing and implicated not only in the degradation final stage of entosis, but also in the full mechanism. Moreover, we found a positive correlation between the presence of entotic cells and the metastatic potential of tumors in human patient samples. Altogether, these data suggest the involvement of entosis in liver tumor progression and highlight a new perspective for entosis analysis in medicine research as a novel therapeutic target.

Metformin potentiates nephrotoxicity by promoting NETosis in response to renal ferroptosis

Given the rapidly aging population, aging-related diseases are becoming an excessive burden on the global healthcare system. Metformin has been shown to be beneficial to many age-related disorders, as well as increase lifespan in preclinical animal models. During the aging process, kidney function progressively declines. Currently, whether and how metformin protects the kidney remains unclear. In this study, among longevity drugs, including metformin, nicotinamide, resveratrol, rapamycin, and senolytics, we unexpectedly found that metformin, even at low doses, exacerbated experimentally-induced acute kidney injury (AKI) and increased mortality in mice. By single-cell transcriptomics analysis, we found that death of renal parenchymal cells together with an expansion of neutrophils occurs upon metformin treatment after AKI. We identified programmed cell death by ferroptosis in renal parenchymal cells and blocking ferroptosis, or depleting neutrophils protects against metformin-induced nephrotoxicity. Mechanistically, upon induction of AKI, ferroptosis in renal parenchymal cells initiates the migration of neutrophils to the site of injury via the surface receptor CXCR4-bound to metformin-iron-NGAL complex, which results in NETosis aggravated AKI. Finally, we demonstrated that reducing iron showed protective effects on kidney injury, which supports the notion that iron plays an important role in metformin-triggered AKI. Taken together, these findings delineate a novel mechanism underlying metformin-aggravated nephropathy and highlight the mechanistic relationship between iron, ferroptosis, and NETosis in the resulting AKI.

Cell-in-cell structure in cancer: evading strategies from anti-cancer therapies

One of the regulated forms of cell death is the cell-in-cell (CIC) structure, in which a surviving cell is engulfed by another cell, a mechanism that causes the death of the engulfed cell by an adjacent cell. Several investigators have previously shown that the presence of CICs is an independent risk factor significantly associated with decreased survival in patients with various types of cancer. In this review, we summarize the role of CIC in the tumor microenvironment (TME), including changes and crosstalk of molecules and proteins in the surrounding CIC, and the role of these factors in contributing to therapeutic resistance acquisition. Moreover, CIC structure formation is influenced by the modulation of TME, which may lead to changes in cellular properties. Future use of CIC as a clinical diagnostic tool will require a better understanding of the effects of chemotherapy on CIC, biomarkers for each CIC formation process, and the development of automated CIC detection methods in tissue sections of tumor specimens.

Diversity and complexity of cell death: a historical review

Death is the inevitable fate of all living organisms, whether at the individual or cellular level. For a long time, cell death was believed to be an undesirable but unavoidable final outcome of nonfunctioning cells, as inflammation was inevitably triggered in response to damage. However, experimental evidence accumulated over the past few decades has revealed different types of cell death that are genetically programmed to eliminate unnecessary or severely damaged cells that may damage surrounding tissues. Several types of cell death, including apoptosis, necrosis, autophagic cell death, and lysosomal cell death, which are classified as programmed cell death, and pyroptosis, necroptosis, and NETosis, which are classified as inflammatory cell death, have been described over the years. Recently, several novel forms of cell death, namely, mitoptosis, paraptosis, immunogenic cell death, entosis, methuosis, parthanatos, ferroptosis, autosis, alkaliptosis, oxeiptosis, cuproptosis, and erebosis, have been discovered and advanced our understanding of cell death and its complexity. In this review, we provide a historical overview of the discovery and characterization of different forms of cell death and highlight their diversity and complexity. We also briefly discuss the regulatory mechanisms underlying each type of cell death and the implications of cell death in various physiological and pathological contexts. This review provides a comprehensive understanding of different mechanisms of cell death that can be leveraged to develop novel therapeutic strategies for various diseases.

Cell-in-Cell Structures in Gastrointestinal Tumors: Biological Relevance and Clinical Applications

This review summarizes information about cell-in-cell (CIC) structures with a focus on gastrointestinal tumors. The phenomenon when one cell lives in another one has attracted an attention of researchers over the past decades. We briefly discuss types of CIC structures and mechanisms of its formation, as well as the biological basis and consequences of the cell-engulfing process. Numerous clinico-histopathological studies demonstrate the significance of these structures as prognostic factors, mainly correlated with negative prognosis. The presence of CIC structures has been identified in all gastrointestinal tumors. However, the majority of studies concern pancreatic cancer. In this field, in addition to the assessment of the prognostic markers, the attempts to manipulate the ability of cells to form CISs have been done in order to stimulate the death of the inner cell. Number of CIC structures also correlates with genetic features for some gastrointestinal tu-mors. The role of CIC structures in the responses of tumors to therapies, both chemotherapy and immunotherapy, seems to be the most poorly studied. However, there is some evidence of involvement of CIC structures in treatment failure. Here, we summarized the current literature on CIC structures in cancer with a focus on gastrointestinal tumors and specified future perspectives for investigation.

LncRNAs and regulated cell death in tumor cells

Regulated Cell Death (RCD) is a mode of cell death that occurs through drug or genetic intervention. The regulation of RCDs is one of the significant reasons for the long survival time of tumor cells and poor prognosis of patients. Long non-coding RNAs (lncRNAs) which are involved in the regulation of tumor biological processes, including RCDs occurring on tumor cells, are closely related to tumor progression. In this review, we describe the mechanisms of eight different RCDs which contain apoptosis, necroptosis, pyroptosis, NETosis, entosis, ferroptosis, autosis and cuproptosis. Meanwhile, their respective roles in the tumor are aggregated. In addition, we outline the literature that is related to the regulatory relationships between lncRNAs and RCDs in tumor cells, which is expected to provide new ideas for tumor diagnosis and treatment.

Orai1 is an Entotic Ca2+ Channel for Non-Apoptotic Cell Death, Entosis in Cancer Development

Entosis is a non-apoptotic cell death process that forms characteristic cell-in-cell structures in cancers, killing invading cells. Intracellular Ca²⁺ dynamics are essential for cellular processes, including actomyosin contractility, migration, and autophagy. However, the significance of Ca²⁺ and Ca²⁺ channels participating in entosis is unclear. Here, it is shown that intracellular Ca²⁺ signaling regulates entosis via SEPTIN-Orai1-Ca²⁺ /CaM-MLCK-actomyosin axis. Intracellular Ca²⁺ oscillations in entotic cells show spatiotemporal variations during engulfment, mediated by Orai1 Ca²⁺ channels in plasma membranes. SEPTIN controlled polarized distribution of Orai1 for local MLCK activation, resulting in MLC phosphorylation and actomyosin contraction, leads to internalization of invasive cells. Ca²⁺ chelators and SEPTIN, Orai1, and MLCK inhibitors suppress entosis. This study identifies potential targets for treating entosis-associated tumors, showing that Orai1 is an entotic Ca²⁺ channel that provides essential Ca²⁺ signaling and sheds light on the molecular mechanism underlying entosis that involves SEPTIN filaments, Orai1, and MLCK.

An entosis-like process induces mitotic disruption in Pals1 microcephaly pathogenesis

Entosis is cell cannibalism utilized by tumor cells to engulf live neighboring cells for pro- or anti-tumorigenic purposes. It is unknown whether this extraordinary cellular event can be pathogenic in other diseases such as microcephaly, a condition characterized by a smaller than normal brain at birth. We find that mice mutant for the human microcephaly-causing gene Pals1, which exhibit diminished cortices due to massive cell death, also exhibit nuclei enveloped by plasma membranes inside of dividing cells. These cell-in-cell (CIC) structures represent a dynamic process accompanied by lengthened mitosis and cytokinesis abnormalities. As shown in tumor cells, ROCK inhibition completely abrogates CIC structures and restores the normal length of mitosis. Moreover, genetic elimination of Trp53 produces a remarkable rescue of cortical size along with substantial reductions of CIC structures and cell death. These results provide a novel pathogenic mechanism by which microcephaly is produced through entotic cell cannibalism.

The lysophosphatidic acid-regulated signal transduction network in ovarian cancer cells and its role in actomyosin dynamics, cell migration and entosis

Lysophosphatidic acid (LPA) species accumulate in the ascites of ovarian high-grade serous cancer (HGSC) and are associated with short relapse-free survival. LPA is known to support metastatic spread of cancer cells by activating a multitude of signaling pathways via G-protein-coupled receptors of the LPAR family. Systematic unbiased analyses of the LPA-regulated signal transduction network in ovarian cancer cells have, however, not been reported to date. Methods: LPA-induced signaling pathways were identified by phosphoproteomics of both patient-derived and OVCAR8 cells, RNA sequencing, measurements of intracellular Ca²⁺ and cAMP as well as cell imaging. The function of LPARs and downstream signaling components in migration and entosis were analyzed by selective pharmacological inhibitors and RNA interference. Results: Phosphoproteomic analyses identified > 1100 LPA-regulated sites in > 800 proteins and revealed interconnected LPAR1, ROCK/RAC, PKC/D and ERK pathways to play a prominent role within a comprehensive signaling network. These pathways regulate essential processes, including transcriptional responses, actomyosin dynamics, cell migration and entosis. A critical component of this signaling network is MYPT1, a stimulatory subunit of protein phosphatase 1 (PP1), which in turn is a negative regulator of myosin light chain 2 (MLC2). LPA induces phosphorylation of MYPT1 through ROCK (T853) and PKC/ERK (S507), which is majorly driven by LPAR1. Inhibition of MYPT1, PKC or ERK impedes both LPA-induced cell migration and entosis, while interference with ROCK activity and MLC2 phosphorylation selectively blocks entosis, suggesting that MYPT1 figures in both ROCK/MLC2-dependent and -independent pathways. We finally show a novel pathway governed by LPAR2 and the RAC-GEF DOCK7 to be indispensable for the induction of entosis. Conclusion: We have identified a comprehensive LPA-induced signal transduction network controlling LPA-triggered cytoskeletal changes, cell migration and entosis in HGSC cells. Due to its pivotal role in this network, MYPT1 may represent a promising target for interfering with specific functions of PP1 essential for HGSC progression.

Phenotypic Characteristics of Dormant Human Non-Small Cell Lung Cancer Cells Surviving Multifraction X-Ray Irradiation

Phenotypic characteristics of human non-small cell lung cancer cells, A549 (p53 wild-type) and H1299 (p53-deficient) as well as their descendants surviving after multifraction X-ray irradiation at a cumulative dose of 60 Gy (sublines A549HR and H1299HR, respectively) were studied before and after additional 2 Gy single dose irradiation. In 24 h after the additional irradiation, we observed a significant increase in the proportion of cells with signs of entosis (by 5 times, p<0.05) and SA-β-gal⁺ cells (by 1.6 times, p<0.01) in the general population of A549HR cells. In contrast, a significant increase in the proportion of only SA-β-gal⁺ multinucleated giant cancer cells was revealed in the parental A549 cells. Additional single dose irradiation resulted in a significant (by 1.8 times, p<0.05) increase in the proportion of multinucleated giant cancer cells in H1299HR cells in comparison with their parental H1299 cells. These changes did not correlate with changes in the proportion of entotic cells, because their high basal content in the absence of functional p53 did not change in response to additional single dose irradiation. At the same time, both p53-deficient non-small cell lung cancer cell lines showed a significant (2.9-fold for H1299 and 5.5-fold for H1299HR cells, p<0.001) increase in the proportion of SA-β-gal⁺ cells in the general population, but not in the multinucleated giant cancer cells population.

Glycosylation defects, offset by PEPCK-M, drive entosis in breast carcinoma cells

On glucose restriction, epithelial cells can undergo entosis, a cell-in-cell cannibalistic process, to allow considerable withstanding to this metabolic stress. Thus, we hypothesized that reduced protein glycosylation might participate in the activation of this cell survival pathway. Glucose deprivation promoted entosis in an MCF7 breast carcinoma model, as evaluated by direct inspection under the microscope, or revealed by a shift to apoptosis + necrosis in cells undergoing entosis treated with a Rho-GTPase kinase inhibitor (ROCKi). In this context, curbing protein glycosylation defects with N-acetyl-glucosamine partially rescued entosis, whereas limiting glycosylation in the presence of glucose with tunicamycin or NGI-1, but not with other unrelated ER-stress inducers such as thapsigargin or amino-acid limitation, stimulated entosis. Mitochondrial phosphoenolpyruvate carboxykinase (PEPCK-M; PCK2) is upregulated by glucose deprivation, thereby enhancing cell survival. Therefore, we presumed that PEPCK-M could play a role in this process by offsetting key metabolites into glycosyl moieties using alternative substrates. PEPCK-M inhibition using iPEPCK-2 promoted entosis in the absence of glucose, whereas its overexpression inhibited entosis. PEPCK-M inhibition had a direct role on total protein glycosylation as determined by Concanavalin A binding, and the specific ratio of fully glycosylated LAMP1 or E-cadherin. The content of metabolites, and the fluxes from ¹³C-glutamine label into glycolytic intermediates up to glucose-6-phosphate, and ribose- and ribulose-5-phosphate, was dependent on PEPCK-M content as measured by GC/MS. All in all, we demonstrate for the first time that protein glycosylation defects precede and initiate the entosis process and implicates PEPCK-M in this survival program to dampen the consequences of glucose deprivation. These results have broad implications to our understanding of tumor metabolism and treatment strategies.

AIM-CICs: an automatic identification method for cell-in-cell structures based on convolutional neural network

Edited by Luonan Chen Whereas biochemical markers are available for most types of cell death, current studies on non-autonomous cell death by entosis rely strictly on the identification of cell-in-cell structures (CICs), a unique morphological readout that can only be quantified manually at present. Moreover, the manual CIC quantification is generally over-simplified as CIC counts, which represents a major hurdle against profound mechanistic investigations. In this study, we take advantage of artificial intelligence technology to develop an automatic identification method for CICs (AIM-CICs), which performs comprehensive CIC analysis in an automated and efficient way. The AIM-CICs, developed on the algorithm of convolutional neural network, can not only differentiate between CICs and non-CICs (the area under the receiver operating characteristic curve (AUC) > 0.99), but also accurately categorize CICs into five subclasses based on CIC stages and cell number involved (AUC > 0.97 for all subclasses). The application of AIM-CICs would systemically fuel research on CIC-mediated cell death, such as high-throughput screening.

Cell Death by Entosis: Triggers, Molecular Mechanisms and Clinical Significance

Entosis—a homotypic insertion of one cell into another, resulting in a death of the invading cell—has been described in many reports, but crucial aspects of its molecular mechanisms and clinical significance still remain controversial. While actomyosin contractility of the invading cell is very well established as a driving force in the initial phase, and autophagy induced in the outer cell is determined as the main mechanism of degradation of the inner cell, many details remain unresolved. The multitude of triggering factors and crisscrossing molecular pathways described in entosis regulation make interpretations difficult. The question of the physiological role of entosis also remains unanswered. In this review, we summarize the knowledge of molecular mechanisms and clinical data concerning entosis accumulated so far, highlighting both coherent explanations and controversies.

Classification of Cell-in-Cell Structures: Different Phenomena with Similar Appearance

A phenomenon known for over 100 years named “cell-in-cell” (CIC) is now undergoing its renaissance, mostly due to modern cell visualization techniques. It is no longer an esoteric process studied by a few cell biologists, as there is increasing evidence that CICs may have prognostic and diagnostic value for cancer patients. There are many unresolved questions stemming from the difficulties in studying CICs and the limitations of current molecular techniques. CIC formation involves a dynamic interaction between an outer or engulfing cell and an inner or engulfed cell, which can be of the same (homotypic) or different kind (heterotypic). Either one of those cells appears to be able to initiate this process, which involves signaling through cell–cell adhesion, followed by cytoskeleton activation, leading to the deformation of the cellular membrane and movements of both cells that subsequently result in CICs. This review focuses on the distinction of five known forms of CIC (cell cannibalism, phagoptosis, enclysis, entosis, and emperipolesis), their unique features, characteristics, and underlying molecular mechanisms.

Amino acids and mechanistic target of rapamycin regulate the fate of live engulfed cells

Metabolic stress contributes to the regulation of cell death in normal and diseased tissues. While different forms of cell death are known to be regulated by metabolic stress, how the cell engulfment and killing mechanism entosis is regulated is not well understood. Here we find that the death of entotic cells is regulated by the presence of amino acids and activity of the mechanistic target of rapamycin (mTOR). Amino acid withdrawal or mTOR inhibition induces apoptosis of engulfed cells and blocks entotic cell death that is associated with the lipidation of the autophagy protein microtubule‐associated protein light chain 3 (LC3) to entotic vacuoles. Two other live cell engulfment programs, homotypic cell cannibalism (HoCC) and anti‐CD47 antibody‐mediated phagocytosis, known as phagoptosis, also undergo a similar vacuole maturation sequence involving LC3 lipidation and lysosome fusion, but only HoCC involves mTOR‐dependent regulation of vacuole maturation and engulfed cell death similar to entosis. We further find that the regulation of cell death by mTOR is independent of autophagy activation and instead involves the 4E‐BP1/2 proteins that are known regulators of mRNA translation. Depletion of 4E‐BP1/2 proteins can restore the mTOR‐regulated changes of entotic death and apoptosis rates of engulfed cells. These results identify amino acid signaling and the mTOR‐4E‐BP1/2 pathway as an upstream regulation mechanism for the fate of live engulfed cells formed by entosis and HoCC.

Sequential oncogenic mutations influence cell competition

At the initial stage of carcinogenesis, newly emerging transformed cells are often eliminated from epithelial layers via cell competition with the surrounding normal cells. For instance, when surrounded by normal cells, oncoprotein RasV12-transformed cells are extruded into the apical lumen of epithelia. During cancer development, multiple oncogenic mutations accumulate within epithelial tissues. However, it remains elusive whether and how cell competition is also involved in this process. In this study, using a mammalian cell culture model system, we have investigated what happens upon the consecutive mutations of Ras and tumor suppressor protein Scribble. When Ras mutation occurs under the Scribble-knockdown background, apical extrusion of Scribble/Ras double-mutant cells is strongly diminished. In addition, at the boundary with Scribble/Ras cells, Scribble-knockdown cells frequently undergo apoptosis and are actively engulfed by the neighboring Scribble/Ras cells. The comparable apoptosis and engulfment phenotypes are also observed in Drosophila epithelial tissues between Scribble/Ras double-mutant and Scribble single-mutant cells. Furthermore, mitochondrial membrane potential is enhanced in Scribble/Ras cells, causing the increased mitochondrial reactive oxygen species (ROS). Suppression of mitochondrial membrane potential or ROS production diminishes apoptosis and engulfment of the surrounding Scribble-knockdown cells, indicating that mitochondrial metabolism plays a key role in the competitive interaction between double- and single-mutant cells. Moreover, mTOR (mechanistic target of rapamycin kinase) acts downstream of these processes. These results imply that sequential oncogenic mutations can profoundly influence cell competition, a transition from loser to winner. Further studies would open new avenues for cell competition-based cancer treatment, thereby blocking clonal expansion of more malignant populations within tumors.

Subtype-Based Analysis of Cell-in-Cell Structures in Esophageal Squamous Cell Carcinoma

Cell-in-cell (CIC) structures are defined as the special structures with one or more cells enclosed inside another one. Increasing data indicated that CIC structures were functional surrogates of complicated cell behaviors and prognosis predictor in heterogeneous cancers. However, the CIC structure profiling and its prognostic value have not been reported in human esophageal squamous cell Carcinoma (ESCC). We conducted the analysis of subtyped CIC-based profiling in ESCC using "epithelium-macrophage-leukocyte" (EML) multiplex staining and examined the prognostic value of CIC structure profiling through Kaplan-Meier plotting and Cox regression model. Totally, five CIC structure subtypes were identified in ESCC tissue and the majority of them was homotypic CIC (hoCIC) with tumor cells inside tumor cells (TiT). By univariate and multivariate analyses, TiT was shown to be an independent prognostic factor for resectable ESCC, and patients with higher density of TiT tended to have longer post-operational survival time. Furthermore, in subpopulation analysis stratified by TNM stage, high TiT density was associated with longer overall survival (OS) in patients of TNM stages III and IV as compared with patients with low TiT density (mean OS: 51 vs 15 months, P = 0.04) and T3 stage (mean OS: 57 vs 17 months, P=0.024). Together, we reported the first CIC structure profiling in ESCC and explored the prognostic value of subtyped CIC structures, which supported the notion that functional pathology with CIC structure profiling is an emerging prognostic factor for human cancers, such as ESCC.

Mechanical Ring Interfaces between Adherens Junction and Contractile Actomyosin to Coordinate Entotic Cell-in-Cell Formation

Entosis is a cell-in-cell (CIC)-mediated death program. Contractile actomyosin (CA) and the adherens junction (AJ) are two core elements essential for entotic CIC formation, but the molecular structures interfacing them remain poorly understood. Here, we report the characterization of a ring-like structure interfacing between the peripheries of invading and engulfing cells. The ring-like structure is a multi-molecular complex consisting of adhesive and cytoskeletal proteins, in which the mechanical sensor vinculin is highly enriched. The vinculin-enriched structure senses mechanical force imposed on cells, as indicated by fluorescence resonance energy transfer (FRET) analysis, and is thus termed the mechanical ring (MR). The MR actively interacts with CA and the AJ to help establish and maintain polarized actomyosin that drives cell internalization. Vinculin depletion leads to compromised MR formation, CA depolarization, and subsequent CIC failure. In summary, we suggest that the vinculin-enriched MR, in addition to CA and AJ, is another core element essential for entosis.

Mechanisms of Regulated Cell Death: Current Perspectives

Balancing cell survival and cell death is fundamental to development and homeostasis. Cell death is regulated by multiple interconnected signaling pathways and molecular mechanisms. Regulated cell death (RCD) is implicated in fundamental processes such as organogenesis and tissue remodeling, removal of unnecessary structures or cells, and regulation of cell numbers. RCD can also be triggered by exogenous perturbations of the intracellular or extracellular microenvironment when the adaptive processes that respond to stress fail. During the past few years, many novel forms of non-apoptotic RCD have been identified, and the characterization of RCD mechanisms at a molecular level has deepened our understanding of diseases encountered in human and veterinary medicine. Given the complexity of these processes, it has become clear that the identification of RCD cannot be based simply on morphologic characteristics and that descriptive and diagnostic terms presently used by pathologists-such as individual cell apoptosis or necrosis-appear inadequate and possibly misleading. In this review, the current understanding of the molecular machinery of each type of non-apoptotic RCD mechanisms is outlined. Due to the continuous discovery of new mechanisms or nuances of previously described processes, the limitations of the terms apoptosis and necrosis to indicate microscopic findings are also reported. In addition, the need for a standard panel of biomarkers and functional tests to adequately characterize the underlying RCD and its role as a mechanism of disease is considered.

Role and dynamics of vacuolar pH during cell-in-cell mediated death

The nonautonomous cell death by entosis was mediated by the so-called cell-in-cell structures, which were believed to kill the internalized cells by a mechanism dependent on acidified lysosomes. However, the precise values and roles of pH critical for the death of the internalized cells remained undetermined yet. We creatively employed keima, a fluorescent protein that displays different excitation spectra in responding to pH changes, to monitor the pH dynamics of the entotic vacuoles during cell-in-cell mediated death. We found that different cells varied in their basal intracellular pH, and the pH was relatively stable for entotic vacuoles containing live cells, but sharply dropped to a narrow range along with the inner cell death. In contrast, the lipidation of entotic vacuoles by LC3 displayed previously underappreciated complex patterns associated with entotic and apoptotic death, respectively. The pH decline seemed to play distinct roles in the two types of inner cell deaths, where apoptosis is preceded with moderate pH decline while a profound pH decline is likely to be determinate for entotic death. Whereas the cancer cells seemed to be lesser tolerant to acidified environments than noncancerous cells, manipulating vacuolar pH could effectively control inner cell fates and switch the ways whereby inner cell die. Together, this study demonstrated for the first time the pH dynamics of entotic vacuoles that dictate the fates of internalized cells, providing a rationale for tuning cellular pH as a potential way to treat cell-in-cell associated diseases such as cancer.

Entosis: From Cell Biology to Clinical Cancer Pathology

Simple Summary

We review published clinico-histopathological studies establishing entosis an important prognostic and predictor factor in various cancer types. We also propose a new model to study this phenomenon, which involves active entry of one cell into another one. The internalized cell can remain viable and leave the host cell after a long time, potentially leading to cancer recurrence. Entotic figures are cell in cell structures, in which the nucleus of external cell is crescent-shaped, and the inner cell is surrounded by the extensive space entotic vacuole, distinguishing entosis from cell cannibalism. Entosis correlates with cancer worse prognosis in head and neck squamous cell carcinoma, anal carcinoma, lung adenocarcinoma, pancreatic ductal carcinoma, and some breast ductal carcinoma. The BxPC-3 pancreatic cancer cells provide a new, more convenient model for entosis research in comparison to the previously described semidherent MCF7 model. BxPC-3 cells undergo and survive spontaneous entosis in normal adherent culture conditions.

Abstract

Entosis is a phenomenon, in which one cell enters a second one. New clinico-histopathological studies of entosis prompted us to summarize its significance in cancer. It appears that entosis might be a novel, independent prognostic predictor factor in cancer histopathology. We briefly discuss the biological basis of entosis, followed by a summary of published clinico-histopathological studies on entosis significance in cancer prognosis. The correlation of entosis with cancer prognosis in head and neck squamous cell carcinoma, anal carcinoma, lung adenocarcinoma, pancreatic ductal carcinoma and breast ductal carcinoma, is shown. Numerous entotic figures are associated with a more malignant cancer phenotype and poor prognosis in many cancers. We also showed that some anticancer drugs could induce entosis in cell culture, even as an escape mechanism. Thus, entosis is likely beneficial for survival of malignant cells, i.e., an entotic cell can hide from unfavourable factors in another cell and subsequently leave the host cell remaining intact, leading to failure in therapy or cancer recurrence. Finally, we highlight the potential relationship of cell adhesion with entosis in vitro, based on the model of the BxPc3 cells cultured in full adhesive conditions, comparing them to a commonly used MCF7 semiadhesive model of entosis.

Loss of 4.1N in epithelial ovarian cancer results in EMT and matrix-detached cell death resistance

Epithelial ovarian cancer (EOC) is one of the leading causes of death from gynecologic cancers and peritoneal dissemination is the major cause of death in patients with EOC. Although the loss of 4.1N is associated with increased risk of malignancy, its association with EOC remains unclear. To explore the underlying mechanism of the loss of 4.1N in constitutive activation of epithelial-mesenchymal transition (EMT) and matrix-detached cell death resistance, we investigated samples from 268 formalin-fixed EOC tissues and performed various in vitro and in vivo assays. We report that the loss of 4.1N correlated with progress in clinical stage, as well as poor survival in EOC patients. The loss of 4.1N induces EMT in adherent EOC cells and its expression inhibits anoikis resistance and EMT by directly binding and accelerating the degradation of 14-3-3 in suspension EOC cells. Furthermore, the loss of 4.1N could increase the rate of entosis, which aggravates cell death resistance in suspension EOC cells. Moreover, xenograft tumors in nude mice also show that the loss of 4.1N can aggravate peritoneal dissemination of EOC cells. Single-agent and combination therapy with a ROCK inhibitor and a 14-3-3 antagonist can reduce tumor spread to varying degrees. Our results not only define the vital role of 4.1N loss in inducing EMT, anoikis resistance, and entosis-induced cell death resistance in EOC, but also suggest that individual or combined application of 4.1N, 14-3-3 antagonists, and entosis inhibitors may be a promising therapeutic approach for the treatment of EOC.

Cell-in-Cell Structures in the Liver: A Tale of Four E's

The liver is our largest internal organ and it plays major roles in drug detoxification and immunity, where the ingestion of extracellular material through phagocytosis is a critical pathway. Phagocytosis is the deliberate endocytosis of large particles, microbes, dead cells or cell debris and can lead to cell-in-cell structures. Various types of cell endocytosis have been recently described for hepatic epithelia (hepatocytes), which are non-professional phagocytes. Given that up to 80% of the liver comprises hepatocytes, the biological impact of cell-in-cell structures in the liver can have profound effects in liver regeneration, inflammation and cancer. This review brings together the latest reports on four types of endocytosis in the liver -efferocytosis, entosis, emperipolesis and enclysis, with a focus on hepatocyte biology.

Distinct Types of Cell Death and the Implication in Diabetic Cardiomyopathy

Diabetic cardiomyopathy (DCM) is a chronic complication of diabetes mellitus, characterized by abnormalities of myocardial structure and function. Researches on the models of type 1 and type 2 diabetes mellitus as well as the application of genetic engineering technology help in understanding the molecular mechanism of DCM. DCM has multiple hallmarks, including hyperglycemia, insulin resistance, increased free radical production, lipid peroxidation, mitochondrial dysfunction, endothelial dysfunction, and cell death. Essentially, cell death is considered to be the terminal pathway of cardiomyocytes during DCM. Morphologically, cell death can be classified into four different forms: apoptosis, autophagy, necrosis, and entosis. Apoptosis, as type I cell death, is the fastest form of cell death and mainly occurs depending on the caspase proteolytic cascade. Autophagy, as type II cell death, is a degradation process to remove damaged proteins, dysfunctional organelles and commences by the formation of autophagosome. Necrosis is type III cell death, which contains a great diversity of cell death processes, such as necroptosis and pyroptosis. Entosis is type IV cell death, displaying “cell-in-cell” cytological features and requires the engulfing cells to execute. There are also some other types of cell death such as ferroptosis, parthanatos, netotic cell death, lysosomal dependent cell death, alkaliptosis or oxeiptosis, which are possibly involved in DCM. Drugs or compounds targeting the signals involved in cell death have been used in clinics or experiments to treat DCM. This review briefly summarizes the mechanisms and implications of cell death in DCM, which is beneficial to improve the understanding of cell death in DCM and may propose novel and ideal strategies in future.

After-Death Functions of Cell Death

Cell death can occur through numerous regulated mechanisms, from apoptosis to necrosis, entosis, and others. Each has a distinct mode of regulation and effect on tissue homeostasis. While the elimination of individual cells is typically considered the relevant physiologic endpoint of cell death, in some cases the remnants left behind by death can also function to support tissue homeostasis. Here we discuss specific functions of the end products of cell death, and how "after-death" functions may contribute to the roles of programmed cell death in physiology.

High Frequency of Cell-in-Cell Formation in Heterogeneous Human Breast Cancer Tissue in a Patient With Poor Prognosis: A Case Report and Literature Review

Cell cannibalism is a unique pathological phenomenon that has been observed at low frequency in a variety of human tumor samples (<0.5%), including breast cancer. Cannibalistic cells typically form cell-in-cell (CIC) structures characterized by enclosure of one cell or more by another, mediating a novel type of cell death "entosis," which was proposed as the type IV cell death. A large number of CIC structures are generally associated with malignant transformation and progression, and they are believed to be primed by and form among heterogeneous cells. However, there is currently no in vivo evidence from human tumor samples. In this case report, covering a 37-year-old female breast cancer patient, we observed considerable heterogeneity and proliferative activity (>70% Ki-67 positivity) in her breast cancer cells, accompanied by high frequency of CIC formation (~6%) and poor prognosis. We consider this a typical example of cell cannibalism, supporting a role of heterogeneity in cell-in-cell formation and malignant progression. It may serve as a pretest basis for further investigations of cell-in-cell biology and breast cancer treatment.

Cell-in-Cell Phenomenon and Its Relationship With Tumor Microenvironment and Tumor Progression: A Review

The term cell-in-cell, morphologically, refers to the presence of one cell within another. This phenomenon can occur in tumors but also among non-tumor cells. The cell-in-cell phenomenon was first observed 100 years ago, and it has since been found in a variety of tumor types. Recently, increasing attention has been paid to this phenomenon and the underlying mechanism has gradually been elucidated. There are three main related process: cannibalism, emperipolesis, and entosis. These processes are affected by many factors, including the tumor microenvironment, mitosis, and genetic factors. There is considerable evidence to suggest that the cell-in-cell phenomenon is associated with the prognosis of cancers, and it promotes tumor progression in most situations. Notably, in pancreatic cancer, the cell-in-cell phenomenon is associated with reduced metastasis, which is the opposite of what happens in other tumor types. Thus, it can also inhibit tumor progression. Studies show that cell-in-cell structure formation is affected by the tumor microenvironment, and that it may lead to changes in cellular characteristics. In this review, we summarize the different cell-in-cell processes and discuss their role in tumor progression and how they are regulated by different mechanisms.

Hepatocytes Delete Regulatory T Cells by Enclysis, a CD4+ T Cell Engulfment Process

CD4+ T cells play critical roles in directing immunity, both as T helper and as regulatory T (Treg) cells. Here, we demonstrate that hepatocytes can modulate T cell populations through engulfment of live CD4+ lymphocytes. We term this phenomenon enclysis to reflect the specific enclosure of CD4+ T cells in hepatocytes. Enclysis is selective for CD4+ but not CD8+ cells, independent of antigen-specific activation, and occurs in human hepatocytes in vitro, ex vivo, and in vivo. Intercellular adhesion molecule 1 (ICAM-1) facilitates T cell early adhesion and internalization, whereas hepatocytes form membrane lamellipodia or blebs to mediate engulfment. T cell internalization is unaffected by wortmannin and Rho kinase inhibition. Hepatocytes engulf Treg cells more efficiently than non-Treg cells, but Treg cell-containing vesicles preferentially acidify overnight. Thus, enclysis is a biological process with potential effects on immunomodulation and opens a new field for research to fully understand CD4+ T cell dynamics in liver inflammation.

Population Dynamics in Cell Death: Mechanisms of Propagation

Cell death can occur through numerous regulated mechanisms that are categorized by their molecular machineries and differing effects on physiology. Apoptosis and necrosis, for example, have opposite effects on tissue inflammation due to their different modes of execution. Another feature that can distinguish different forms of cell death is that they have distinct intrinsic effects on the cell populations in which they occur. For example, a regulated mechanism of necrosis called ferroptosis has the unusual ability to spread between cells in a wave-like manner, thereby eliminating entire cell populations. Here we discuss the ways in which cell death can propagate between cells in normal physiology and disease, as well as the potential exploitation of cell death propagation for cancer therapy.

Entosis Controls a Developmental Cell Clearance in C. elegans

Metazoan cell death mechanisms are diverse and include numerous non-apoptotic programs. One program called entosis involves the invasion of live cells into their neighbors and is known to occur in cancers. Here, we identify a developmental function for entosis: to clear the male-specific linker cell in C. elegans. The linker cell leads migration to shape the gonad and is removed to facilitate fusion of the gonad to the cloaca. We find that the linker cell is cleared in a manner involving cell-cell adhesions and cell-autonomous control of uptake through linker cell actin. Linker cell entosis generates a lobe structure that is deposited at the site of gonad-to-cloaca fusion and is removed during mating. Inhibition of lobe scission inhibits linker cell death, demonstrating that the linker cell invades its host while alive. Our findings demonstrate a developmental function for entosis: to eliminate a migrating cell and facilitate gonad-to-cloaca fusion, which is required for fertility.

Biological relevance of cell-in-cell in cancers

Cell-in-cell (CIC) is a term used to describe the presence of one, usually living, cell inside another cell that is typically considered non-phagocytic. Examples of this include tumour cells inside tumour cells (homotypic), mesenchymal stem cells inside tumour cells (heterotypic) or immune cells inside tumour cells (heterotypic). CIC formation can occur in cell lines and in tissues and it has been most frequently observed during inflammation and in cancers. Over the past 10 years, many researchers have studied CIC structures and a few different models have been proposed through which they can be formed, including entosis, cannibalism and emperipolesis among others. Recently, our laboratory discovered a role for mutant p53 in facilitating the formation of CIC and promoting genomic instability. These data and research by many others have uncovered a variety of molecules involved in CIC formation and have started to give us an idea of why they are formed and how they could contribute to oncogenic processes. In this perspective, we summarise current literature and speculate on the role of CIC in cancer biology.

Induction of entosis in prostate cancer cells by nintedanib and its therapeutic implications

Entosis is a homogeneous cell-in-cell phenomenon and a non-apoptotic cell death process. Tyrosine kinase inhibitors have been used in the treatment of prostate cancer and have already demonstrated efficacy in a clinical setting. The present study investigated the role of entosis in prostate cancer treated with the tyrosine kinase inhibitor nintedanib. Prostate cancer cells were treated with nintedanib in vitro and entosis was observed. Mice xenografts were created to evaluate whether nintedanib is able to induce entosis in vivo. The reverse transcription-quantitative polymerase chain reaction, western blotting and immunofluorescence were performed to investigate whether the entosis pathway is induced by nintedanib. It was also investigated whether entosis can contribute to cell survival and progression under nintedanib stress, and nintedanib was revealed to enhance prostate cancer cell entosis. Nintedanib-induced entosis in prostate cancer cells occurred through phosphoinositide 3-kinase/cell division cycle 42 (CDC42) inhibition, followed by the upregulation of epithelial (E-)cadherin and components of the Rho kinase (ROCK) signaling pathway. In addition, nintedanib-resistant cells exhibiting entosis had a higher invasive ability. In addition, in vivo treatment of mice xenografts with nintedanib also increased the expression of E-cadherin and components of the ROCK signaling pathway. Nintedanib can promote entosis during prostate cancer treatment by modulating the CDC42 pathway. Furthermore, prostate cancer cells acquired nintedanib resistance and survived by activating entosis.

Phospholipid flippases enable precursor B cells to flee engulfment by macrophages

ATP11A and ATP11C, members of the P4-ATPases, are flippases that translocate phosphatidylserine (PtdSer) from the outer to inner leaflet of the plasma membrane. Using the W3 T lymphoma cell line, we found that Ca²⁺ ionophore-induced phospholipid scrambling caused prolonged PtdSer exposure in cells lacking both the ATP11A and ATP11C genes. ATP11C-null (ATP11C ^-/y ) mutant mice exhibit severe B-cell deficiency. In wild-type mice, ATP11C was expressed at all B-cell developmental stages, while ATP11A was not expressed after pro-B-cell stages, indicating that ATP11C ^-/y early B-cell progenitors lacked plasma membrane flippases. The receptor kinases MerTK and Axl are known to be essential for the PtdSer-mediated engulfment of apoptotic cells by macrophages. MerTK ^-/- and Axl ^-/- double deficiency fully rescued the lymphopenia in the ATP11C ^-/y bone marrow. Many of the rescued ATP11C ^-/y pre-B and immature B cells exposed PtdSer, and these cells were engulfed alive by wild-type peritoneal macrophages, in a PtdSer-dependent manner. These results indicate that ATP11A and ATP11C in precursor B cells are essential for rapidly internalizing PtdSer from the cell surface to prevent the cells' engulfment by macrophages.

Novel Methylselenoesters Induce Programed Cell Death via Entosis in Pancreatic Cancer Cells

Redox active selenium (Se) compounds have gained substantial attention in the last decade as potential cancer therapeutic agents. Several Se compounds have shown high selectivity and sensitivity against malignant cells. The cytotoxic effects are exerted by their biologically active metabolites, with methylselenol (CH₃SeH) being one of the key executors. In search of novel CH₃SeH precursors, we previously synthesized a series of methylselenoesters that were active (GI₅₀ < 10 µM at 72 h) against a panel of cancer cell lines. Herein, we refined the mechanism of action of the two lead compounds with the additional synthesis of new analogs (ethyl, pentyl, and benzyl derivatives). A novel mechanism for the programmed cell death mechanism for Se-compounds was identified. Both methylseleninic acid and the novel CH₃SeH precursors induced entosis by cell detachment through downregulation of cell division control protein 42 homolog (CDC42) and its downstream effector β1-integrin (CD29). To our knowledge, this is the first time that Se compounds have been reported to induce this type of cell death and is of importance in the characterization of the anticancerogenic properties of these compounds.

Entosis Is Induced by Glucose Starvation

Entosis is a mechanism of cell death that involves neighbor cell ingestion. This process occurs in cancers and promotes a form of cell competition, where winner cells engulf and kill losers. Entosis is driven by a mechanical differential that allows softer cells to eliminate stiffer cells. While this process can be induced by matrix detachment, whether other stressors can activate entosis is unknown. Here, we find that entosis is induced in adherent cells by glucose withdrawal. Glucose withdrawal leads to a bimodal distribution of cells based on their deformability, where stiffer cells appear in a manner requiring the energy-sensing AMP-activated protein kinase (AMPK). We show that loser cells with high levels of AMPK activity are eliminated by winners through entosis, which supports winner cell proliferation under nutrient-deprived conditions. Our findings demonstrate that entosis serves as a cellular response to metabolic stress that enables nutrient recovery through neighbor cell ingestion.

In the Hunger Games, the Winner Takes Everything

Entosis is an atypical form of cell death that occurs when a cell engulfs and kills another cell. A recent article by Overholtzer and colleagues indicates that glucose deprivation promotes entosis. AMP-activated protein kinase (AMPK) activation in the loser cells triggers their engulfment and elimination by winner cells, which endure starvation.

Mitosis can drive cell cannibalism through entosis

Entosis is a form of epithelial cell cannibalism that is prevalent in human cancer, typically triggered by loss of matrix adhesion. Here, we report an alternative mechanism for entosis in human epithelial cells, driven by mitosis. Mitotic entosis is regulated by Cdc42, which controls mitotic morphology. Cdc42 depletion enhances mitotic deadhesion and rounding, and these biophysical changes, which depend on RhoA activation and are phenocopied by Rap1 inhibition, permit subsequent entosis. Mitotic entosis occurs constitutively in some human cancer cell lines and mitotic index correlates with cell cannibalism in primary human breast tumours. Adherent, wild-type cells can act efficiently as entotic hosts, suggesting that normal epithelia may engulf and kill aberrantly dividing neighbours. Finally, we report that Paclitaxel/taxol promotes mitotic rounding and subsequent entosis, revealing an unconventional activity of this drug. Together, our data uncover an intriguing link between cell division and cannibalism, of significance to both cancer and chemotherapy.

Emperipolesis, entosis and cell cannibalism: Demystifying the cloud

There are intense published data in literature related to cell engulfment phenomena such as emperipolesis, entosis and cell cannibalism. All these are closely related phenomena with a very fine line of differences. Its correct identification has a significant diagnostic and prognostic value. After extensive literature search, a gap of knowledge was found in concept designing and clarity about understanding of aforementioned terminologies. The authors have attempted to review data of these closely knit terminologies and further organize its characteristic appearances, pathogenetic aspects and prognostic implications. The data published in English Language, from 1925 to 2015, were collected using keywords such as emperipolesis, entosis and cell cannibalism through scientific database systems such as MEDLINE, Science Direct, Cochrane Library and Google Scholar. Articles were selected which have focused to explain the phenomenon, presentation and pathogenesis of one or more of this phenomenon. A total of 48 articles were retrieved, thirty of which were selected. The various cell engulfment phenomena are very similar looking but operate through entirely different pathways.

Foci of Entotic Nuclei in Different Grades of Noninherited Renal Cell Cancers

We report here an intriguing pattern in nuclear appearance of renal clear cell cancer. In low grade clear cell cancer, detailed examination showed that in many cells, two or more nuclei were within the confines of a single cell membrane. This likely resulted from a cell being contained within its neighboring cell. Consequently, this resulted in appearance of multicellularity. This appearance of the nuclei were not associated with mitotic figures, suggesting that these did not result from nuclear fission. Additionally, the cells containing this nuclei did not show any evidence of cytokinesis including equatorial tapering, suggesting that the process may have resulted from cytokinesis failure. In some sections of higher grade clear cell cancer, these appearance were higher, though we did not observe any frank syncytium formation. On careful observation, there were isolated events of fusion of nuclei within a single cell in different grades of renal cell cancers. There occurrence was more frequent in higher grades of clear cell renal cancer and metastatic clear cell carcinoma. These features were also demonstrable in multiple fields of lower grades of clear cell carcinoma. This phenomenon of entosis may contribute to aneuploidy and tumor progression to dysplastic stages and genomic instability in renal cancers. Future studies are aimed at delineating the cell-cell boundaries and the mechanism contributing to this observation, either from peripheral cell engulfing or failure of cytosolic division for cell separation.

Cell-in-cell structures are involved in the competition between cells in human tumors

The engulfment of live cells may represent a mechanism of cell death. We reported that E-cadherin (epithelial cadherin) expression in human cancer cells favors the formation of cell-in-cell structures through the mechanism known as entosis, and that entosis contributes to a form of cellular competition in heterogeneous cancer cell populations.

V-ATPase and osmotic imbalances activate endolysosomal LC3 lipidation

Recently a noncanonical activity of autophagy proteins has been discovered that targets lipidation of microtubule-associated protein 1 light chain 3 (LC3) onto macroendocytic vacuoles, including macropinosomes, phagosomes, and entotic vacuoles. While this pathway is distinct from canonical autophagy, the mechanism of how these nonautophagic membranes are targeted for LC3 lipidation remains unclear. Here we present evidence that this pathway requires activity of the vacuolar-type H(+)-ATPase (V-ATPase) and is induced by osmotic imbalances within endolysosomal compartments. LC3 lipidation by this mechanism is induced by treatment of cells with the lysosomotropic agent chloroquine, and through exposure to the Heliobacter pylori pore-forming toxin VacA. These data add novel mechanistic insights into the regulation of noncanonical LC3 lipidation and its associated processes, including LC3-associated phagocytosis (LAP), and demonstrate that the widely and therapeutically used drug chloroquine, which is conventionally used to inhibit autophagy flux, is an inducer of LC3 lipidation.

Competition between human cells by entosis

Human carcinomas are comprised of complex mixtures of tumor cells that are known to compete indirectly for nutrients and growth factors. Whether tumor cells could also compete directly, for example by elimination of rivals, is not known. Here we show that human cells can directly compete by a mechanism of engulfment called entosis. By entosis, cells are engulfed, or cannibalized while alive, and subsequently undergo cell death. We find that the identity of engulfing ("winner") and engulfed ("loser") cells is dictated by mechanical deformability controlled by RhoA and actomyosin, where tumor cells with high deformability preferentially engulf and outcompete neighboring cells with low deformability in heterogeneous populations. We further find that activated Kras and Rac signaling impart winner status to cells by downregulating contractile myosin, allowing for the internalization of neighboring cells that eventually undergo cell death. Finally, we compute the energy landscape of cell-in-cell formation, demonstrating that a mechanical differential between winner and loser cells is required for entosis to proceed. These data define a mechanism of competition in mammalian cells that occurs in human tumors.

G-protein-coupled receptor signaling and polarized actin dynamics drive cell-in-cell invasion

Homotypic or entotic cell-in-cell invasion is an integrin-independent process observed in carcinoma cells exposed during conditions of low adhesion such as in exudates of malignant disease. Although active cell-in-cell invasion depends on RhoA and actin, the precise mechanism as well as the underlying actin structures and assembly factors driving the process are unknown. Furthermore, whether specific cell surface receptors trigger entotic invasion in a signal-dependent fashion has not been investigated. In this study, we identify the G-protein-coupled LPA receptor 2 (LPAR2) as a signal transducer specifically required for the actively invading cell during entosis. We find that G_12/13 and PDZ-RhoGEF are required for entotic invasion, which is driven by blebbing and a uropod-like actin structure at the rear of the invading cell. Finally, we provide evidence for an involvement of the RhoA-regulated formin Dia1 for entosis downstream of LPAR2. Thus, we delineate a signaling process that regulates actin dynamics during cell-in-cell invasion.

DOI:http://dx.doi.org/10.7554/eLife.02786.001

Entosis is the invasion of one cell by another and can be observed in aggressive cancers. Although the invading cell is usually killed, the surviving cell is sometimes left with the wrong number of chromosomes. This suggests that entosis may help cancer to progress because cells with an abnormal number of chromosomes are common in cancers.

For entosis to occur, the invading cell must be released from the tissue that surrounds it, so it can move towards and attach to the cell it is about to invade. Very little is currently known about the cellular and molecular events that enable these processes to occur.

Purvanov et al. studied entosis in cells grown in the laboratory and observed that invading cells produce bulges and projections at their rear end for invasion. These projections contain a protein called mDia1. This protein is involved in controlling the growth of the cytoskeleton—the structure that helps cells to both maintain their shape and to move.

Adding the signaling molecule lysophosphatidic acid, which is present in human serum, increased the likelihood that cells would invade others. From this, Purvanov et al. established the identities of the proteins involved in transmitting the lysophosphatidic acid signal that controls mDia1 activity during entosis. Changes to this signaling pathway have been associated with cancer and how it spreads between different organs and its involvement in entosis lends further support to the notion that there may be a link between cell-in-cell invasion and the advancement of cancer.

DOI:http://dx.doi.org/10.7554/eLife.02786.002

Aurora A orchestrates entosis by regulating a dynamic MCAK-TIP150 interaction

Entosis, a cell-in-cell process, has been implicated in the formation of aneuploidy associated with an aberrant cell division control. Microtubule plus-end-tracking protein TIP150 facilitates the loading of MCAK onto the microtubule plus ends and orchestrates microtubule plus-end dynamics during cell division. Here we show that TIP150 cooperates with MCAK to govern entosis via a regulatory circuitry that involves Aurora A-mediated phosphorylation of MCAK. Our biochemical analyses show that MCAK forms an intra-molecular association, which is essential for TIP150 binding. Interestingly, Aurora A-mediated phosphorylation of MCAK modulates its intra-molecular association, which perturbs the MCAK-TIP150 interaction in vitro and inhibits entosis in vivo. To probe if MCAK-TIP150 interaction regulates microtubule plasticity to affect the mechanical properties of cells during entosis, we used an optical trap to measure the mechanical rigidity of live MCF7 cells. We find that the MCAK cooperates with TIP150 to promote microtubule dynamics and modulate the mechanical rigidity of the cells during entosis. Our results show that a dynamic interaction of MCAK-TIP150 orchestrated by Aurora A-mediated phosphorylation governs entosis via regulating microtubule plus-end dynamics and cell rigidity. These data reveal a previously unknown mechanism of Aurora A regulation in the control of microtubule plasticity during cell-in-cell processes.

Androgen receptor enhances entosis, a non-apoptotic cell death, through modulation of Rho/ROCK pathway in prostate cancer cells

BACKGROUND

Cell-in-cell phenomenon has been found for more than a century. Entosis, which is a newly found homogeneous cell-in-cell phenomenon and a non-apoptosis cell death progress, has unclear function in prostate cancer progression. Here, we dissected mechanism of AR signaling related to entosis incidence in PCa progression.

METHODS

Two stable PCa cell lines, named LNCaP-ARsi and C4-2-ARsi were established with stably transfected AR-shRNA to knockdown AR mRNA expression in LNCaP and C4-2 cells, respectively. PC3-AR9 cell line was also established after stably transfecting full-length AR-cDNA into PC3 cells. All these cells were cultured in poly-HEME-coated plates to induce entosis, which is demonstrated via double staining.

RESULTS

Androgen-DHT could enhance entosis in LNCaP, C4-2 and PC3-AR9 PCa cells in a dose dependent manner. Knock-down of AR in LNCaP and C4-2 significantly suppressed entosis as compared to LNCaP-ARsc and C4-2-ARsc cells at both 1 and 10 nM DHT condition (P < 0.05). And suppression of Rho/ROCK expression resulted in interruption of AR-mediated entosis. Human PCa samples surveys demonstrated that entosis was found only in CRPC but not in BPH and ADPC where AR was less expressed as compared to CRPC.

CONCLUSIONS

These results indicated that AR might play a negative role during PCa progression via influencing entosis by modulating Rho/ROCK pathway. This newly identified AR role of enhancing entosis might help us to better understand the multiple and opposite roles of AR, which could either promote or suppress PCa cell progression via different mechanisms.