The cell biology of cell-in-cell structures

Cancer cell survival during detachment from the ECM: multiple barriers to tumour progression

Epithelial cells require attachment to the extracellular matrix (ECM) for survival. However, during tumour progression and metastasis, cancerous epithelial cells must adapt to and survive in the absence of ECM. During the past 20 years, several cellular changes, including anoikis, have been shown to regulate cell viability when cells become detached from the ECM. In this Opinion article, we review in detail how cancer cells can overcome or take advantage of these specific processes. Gaining a better understanding of how cancer cells survive during detachment from the ECM will be instrumental in designing chemotherapeutic strategies that aim to eliminate ECM-detached metastatic cells.

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.

Manipulation of autophagy in cancer cells: an innovative strategy to fight drug resistance

Autophagy is a catabolic process activated by stress conditions and nutrient deprivation, to which it reacts by promoting the degradation of damaged organelles and misfolded/aggregated proteins, as well as generating new energetic pools. Paradoxically, in cancer cells, which signal the dangerous microenvironment occurring during clinical therapies, autophagy could promote their proliferation and sustain drug resistance. Special attention is given to autophagy manipulation in order to counteract drug resistance of cancer cells. This article describes the basic properties of autophagy and focuses on the strategies of manipulating it.

Rapid reuptake of granzyme B leads to emperitosis: an apoptotic cell-in-cell death of immune killer cells inside tumor cells

A cell-in-cell process refers to the invasion of one living cell into another homotypic or heterotypic cell. Different from non-apoptotic death processes of internalized cells termed entosis or cannibalism, we previously reported an apoptotic cell-in-cell death occurring during heterotypic cell-in-cell formation. In this study, we further demonstrated that the apoptotic cell-in-cell death occurred only in internalized immune killer cells expressing granzyme B (GzmB). Vacuole wrapping around the internalized cells inside the target cells was the common hallmark during the early stage of all cell-in-cell processes, which resulted in the accumulation of reactive oxygen species and subsequent mitochondrial injury of encapsulated killer or non-cytotoxic immune cells. However, internalized killer cells mediated rapid bubbling of the vacuoles with the subsequent degranulation of GzmB inside the vacuole of the target cells and underwent the reuptake of GzmB by killer cells themselves. The confinement of GzmB inside the vacuole surpassed the lysosome-mediated cell death occurring in heterotypic or homotypic entosis processes, resulting in a GzmB-triggered caspase-dependent apoptotic cell-in-cell death of internalized killer cells. On the contrary, internalized killer cells from GzmB-deficient mice underwent a typical non-apoptotic entotic cell-in-cell death similar to that of non-cytotoxic immune cells or tumor cells. Our results thus demonstrated the critical involvement of immune cells with cytotoxic property in apoptotic cell-in-cell death, which we termed as emperitosis taken from emperipolesis and apoptosis. Whereas entosis or cannibalism may serve as a feed-on mechanism to exacerbate and nourish tumor cells, emperitosis of immune killer cells inside tumor cells may serve as an in-cell danger sensation model to prevent the killing of target cells from inside, implying a unique mechanism for tumor cells to escape from immune surveillance.

mTOR regulates phagosome and entotic vacuole fission

Phagosomes and entotic vacuoles harboring engulfed cells undergo an mTOR-regulated process of vacuolar fission that distributes engulfed contents throughout lysosome networks. Amino acid recovery from engulfed cells activates mTORC1 and rescues engulfing cells from the effects of amino acid starvation.

Macroendocytic vacuoles formed by phagocytosis, or the live-cell engulfment program entosis, undergo sequential steps of maturation, leading to the fusion of lysosomes that digest internalized cargo. After cargo digestion, nutrients must be exported to the cytosol, and vacuole membranes must be processed by mechanisms that remain poorly defined. Here we find that phagosomes and entotic vacuoles undergo a late maturation step characterized by fission, which redistributes vacuolar contents into lysosomal networks. Vacuole fission is regulated by the serine/threonine protein kinase mammalian target of rapamycin complex 1 (mTORC1), which localizes to vacuole membranes surrounding engulfed cells. Degrading engulfed cells supply engulfing cells with amino acids that are used in translation, and rescue cell survival and mTORC1 activity in starved macrophages and tumor cells. These data identify a late stage of phagocytosis and entosis that involves processing of large vacuoles by mTOR-regulated membrane fission.

Establishment and characterization of a mutagenized cell line exhibiting the 'cell-in-cell' phenotype at a high frequency

Cell-in-cell structures represent live cell events in which one cell internalizes another. Because formation of cell-in-cell structures is a rare event in most cell types and the event is associated with cell death, there has been limited clarification of this phenomenon, and its physiological role and molecular mechanism are yet to be precisely elucidated. In this study, we established a mutagenized cell line that exhibited cell-in-cell structures at a more than 10-fold higher frequency as compared to the parent cells. Interestingly, both engulfment and invasion were increased in the mutagenized cell line as compared with that in the parent cell line in the suspension culture condition. This finding indicates that this mutagenized cell line showed an interchangeable status in terms of its ability to form cell-in-cell structures, and the system described here could be useful for elucidation of the mechanisms regulating the formation of cell-in-cell structures, including engulfment and invasion, in a given cellular environment. Further studies using this cell line are warranted to understand the mechanism of formation and biological significance of the cell-in-cell formation.

Autophagy proteins regulate cell engulfment mechanisms that participate in cancer

Recent evidence has uncovered cross-regulation of mechanisms of cell engulfment by proteins of the autophagy pathway, in what is called LC3-Associated Phagocytosis, or LAP. By LAP, lysosome fusion to phagosomes and the degradation of engulfed extracellular cargo are facilitated by autophagy proteins that lipidate LC3 onto phagosome membranes. Here we discuss the contexts where LAP is known to occur by focusing on potential roles in tumorigenesis, including predicted consequences of LAP inhibition.

Modeling cell-in-cell structure into its biological significance

Although cell-in-cell structure was noted 100 years ago, the molecular mechanisms of ‘entering' and the destination of cell-in-cell remain largely unclear. It takes place among the same type of cells (homotypic cell-in-cell) or different types of cells (heterotypic cell-in-cell). Cell-in-cell formation affects both effector cells and their host cells in multiple aspects, while cell-in-cell death is under more intensive investigation. Given that cell-in-cell has an important role in maintaining homeostasis, aberrant cell-in-cell process contributes to the etiopathology in humans. Indeed, cell-in-cell is observed in many pathological processes of human diseases. In this review, we intend to discuss the biological models of cell-in-cell structures under physiological and pathological status.

Prevalence of heterotypic tumor/immune cell-in-cell structure in vitro and in vivo leading to formation of aneuploidy

Cell-in-cell structures refer to a unique phenomenon that one living cell enters into another living cell intactly, occurring between homotypic tumor cells or tumor (or other tissue cells) and immune cells (named as heterotypic cell-in-cell structure). In the present study, through a large scale of survey we observed that heterotypic cell-in-cell structure formation occurred commonly in vitro with host cells derived from different human carcinomas as well as xenotypic mouse tumor cell lines. Most of the lineages of human immune cells, including T, B, NK cells, monocytes as well as in vitro activated LAK cells, were able to invade tumor cell lines. Poorly differentiated stem cells were capable of internalizing immune cells as well. More significantly, heterotypic tumor/immune cell-in-cell structures were observed in a higher frequency in tumor-derived tissues than those in adjacent tissues. In mouse hepatitis models, heterotypic immune cell/hepatocyte cell-in-cell structures were also formed in a higher frequency than in normal controls. After in vitro culture, different forms of internalized immune cells in heterotypic cell-in-cell structures were observed, with one or multiple immune cells inside host cells undergoing resting, degradation or mitosis. More strikingly, some internalized immune cells penetrated directly into the nucleus of target cells. Multinuclear cells with aneuploid nucleus were formed in target tumor cells after internalizing immune cells as well as in situ tumor regions. Therefore, with the prevalence of heterotypic cell-in-cell structures observed, we suggest that shielding of immune cells inside tumor or inflammatory tissue cells implies the formation of aneuploidy with the increased multinucleation as well as fine-tuning of microenvironment under pathological status, which may define distinct mechanisms to influence the etiology and progress of tumors.

On the origin of cancer metastasis

Metastasis involves the spread of cancer cells from the primary tumor to surrounding tissues and to distant organs and is the primary cause of cancer morbidity and mortality. In order to complete the metastatic cascade, cancer cells must detach from the primary tumor, intravasate into the circulatory and lymphatic systems, evade immune attack, extravasate at distant capillary beds, and invade and proliferate in distant organs. Currently, several hypotheses have been advanced to explain the origin of cancer metastasis. These involve an epithelial mesenchymal transition, an accumulation of mutations in stem cells, a macrophage facilitation process, and a macrophage origin involving either transformation or fusion hybridization with neoplastic cells. Many of the properties of metastatic cancer cells are also seen in normal macrophages. A macrophage origin of metastasis can also explain the long-standing "seed and soil" hypothesis and the absence of metastasis in plant cancers. The view of metastasis as a macrophage metabolic disease can provide novel insight for therapeutic management.

Methods for the study of entosis

Entosis is a recently described nonapoptotic cell death mechanism that is initiated by the engulfment of live epithelial cells, leading to the formation of "cell-in-cell" structures. Entotic cell engulfment is induced by matrix detachment, and is driven by imbalances in actomyosin contraction between neighboring cells. Here we describe methods to quantify the formation of cell-in-cell structures by entosis, for cells cultured in suspension or in soft agar, by fluorescence imaging and time-lapse microscopy.

Apoptosis induction of K562 cells by lymphocytes: an AFM study

Antitumor immunotherapies, as a prospective approach for local cancer treatment, are attracting increasing interests. To detect the reacting course of immune and tumor cells, we have observed the process of K562 cells (a human erythroleukemic cell line) coculturing with peripheral lymphocytes, and the morphological and ultrastructural alterations of K562 cells and lymphocytes were investigated as well using atomic force microscopy (AFM). AFM morphological imaging revealed that after coculture the apoptosis-like structures such as blebbing, pores, and apoptotic bodies were observed on the K562 cells. Also, the cell-surface roughness decreased significantly, which implied the changes in chemical composition of cell membranes. Moreover, the lymphocytes were damaged to some extent induced by the coculture. The data demonstrated that K562 cells could be attacked and induced apoptosis by lymphocytes, and they would make damages to lymphocytes to escape the surveillance of immune system.

Genetic instability: tipping the balance

Tumor cells typically contain a genome that is highly divergent from the genome of normal, non-transformed cells. This genetic divergence is caused by a number of distinct changes that the tumor cell acquires during its transformation from a normal cell into a tumorigenic counterpart. Changes to the genome include mutations, deletions, insertions, and also gross chromosomal aberrations, such as chromosome translocations and whole chromosome gains or losses. This genetic disorder of the tumor cell has complicated the identification of crucial driver mutations that cause cancer. Moreover, the large genetic divergence between different tumors causes them to behave very differently, and makes it difficult to predict response to therapy. In addition, tumor cells are genetically unstable and frequently acquire new mutations and/or gross chromosomal aberrations as they divide. This is beneficial for the overall capacity of a tumor to adapt to changes in its environment, but newly acquired genetic alterations can also compromise the genetic dominance of the tumor cell and thus affect tumor cell viability. Here, we review the mechanisms that can cause gross chromosomal aberrations, and discuss how these affect tumor cell viability.

Survival features of EBV-stabilized cells from centenarians: morpho-functional and transcriptomic analyses

In the present work, we analyzed the survival features of six different Epstein-Barr virus (EBV)-stabilized lymphoid cell lines obtained from adult subjects and from subjects of more than 95 years. For the first, we found that lymphoid B cells from centenarians were more resistant to apoptosis induction and displayed a more developed lysosomal compartment, the most critical component of phagic machinery, in comparison with lymphoid B cells from adult subjects. In addition, cells from centenarians were capable of engulfing and digesting other cells, i.e., their siblings (even entire cells), whereas lymphoid cells from "control samples", i.e., from adults, did not. This behavior was improved by nutrient deprivation but, strikingly, it was unaffected by the autophagy-modulating drug, rapamycin, an autophagy inducer, and 3-methyladenine, an autophagy inhibitor. Transcriptomic analyses indicated that: (1) aspartyl proteases, (2) cell surface molecules such as integrins and cadherins, and (3) some components of cytoskeletal network could contribute to establish this survival phenotype. Also, Kyoto Encyclopedia of Genes and Genomes pathways such as Wnt signaling pathway, an essential contributor to cell migration and actin cytoskeleton remodeling, appeared as prominent. Although we cannot rule out the possibility that EBV-immortalization could play a role, since we observed this phagic behavior in cells from centenarians but not in those from adults, we hypothesize that it may represent an important survival determinant in cells from centenarians.

Cells expressing the C/EBPbeta isoform, LIP, engulf their neighbors

Descriptions of various processes that lead to cell-in-cell structures have been reported for decades. The exact molecular mechanism(s) of their formation and the physiological significance of cell-in-cell structures remain poorly understood. We had previously shown that an isoform of the CCAAT/enhancer-binding protein beta (C/EBPbeta) transcription factor, liver-enriched inhibitory protein (LIP), induces cell death in human breast cancer cells and stimulates autophagy. Here we describe a non-apoptotic cell death process where LIP mediates the engulfment of neighboring cells. We provide evidence of LIP-mediated engulfment via DNA profiling, fluorescent imaging and cell sorting studies, as well as ultrastructure analysis of LIP-expressing MDA-MB-468 breast cancer cells. Our work illustrates that expression of a specific transcription factor, LIP, can mediate cell engulfment.

Homotypic cell cannibalism, a cell-death process regulated by the nuclear protein 1, opposes to metastasis in pancreatic cancer

Pancreatic adenocarcinoma (PDAC) is an extremely deadly disease for which all treatments available have failed to improve life expectancy significantly. This may be explained by the high metastatic potential of PDAC cells, which results from their dedifferentiation towards a mesenchymal phenotype. Some PDAC present cell-in-cell structures whose origin and significance are currently unknown. We show here that cell-in-cells form after homotypic cell cannibalism (HoCC). We found PDAC patients whose tumours display HoCC develop less metastasis than those without. In vitro, HoCC was promoted by inactivation of the nuclear protein 1 (Nupr1), and was enhanced by treatment with transforming growth factor β. HoCC ends with death of PDAC cells, consistent with a metastasis suppressor role for this phenomenon. Hence, our data indicates a protective role for HoCC in PDAC and identifies Nupr1 as a molecular regulator of this process.

Information processing during phagocytosis

Phagocytosis - the process by which macrophages, dendritic cells and other myeloid phagocytes internalize diverse particulate targets - is a key mechanism of innate immunity. The molecular and cellular events that underlie the binding of targets to a phagocyte and their engulfment into phagosomes have been extensively studied. More recent data suggest that the process of phagocytosis itself provides information to myeloid phagocytes about the nature of the targets they are engulfing and that this helps to tailor inflammatory responses. In this Review, we discuss how such information is acquired during phagocytosis and how it is processed to coordinate an immune response.

Sarcomas as a mise en abyme of mesenchymal stem cells: exploiting interrelationships for cell mediated anticancer therapy

Mise en abyme meaning "placed into abyss or infinite recurrence" is an apt paradigm for the relentless growth of sarcoma cells. Its alternative meaning, "self-reflexive embedding" fits the central role attributed to cancer stem cells (CSCs). Diversely sourced and defined, mesenchymal stem cells (MSCs) may be the cells of sarcoma origin, evolve a CSC phenotype and/or contribute to tumor growth through inherent qualities for homing, neovascularization, paracrine cross-feeding, microvesicle secretion, cell fusion, entosis and immune modulation. Exploiting these qualities, MSC expressing modified forms of the TNF-related apoptosis-inducing ligand (Apo2L/TRAIL) are being developed to complement more conventional radiation and chemotherapy.

Autophagy proteins in macroendocytic engulfment

Eukaryotic cells must constantly degrade both intracellular and extracellular material to maintain cellular and organismal homeostasis. Two engulfment pathways, autophagy and phagocytosis, contribute to the turnover of intracellular and extracellular substrates by delivering material to the lysosome. Historically these are thought to be separate pathways, but recent studies have revealed the direct participation of autophagy proteins in phagocytosis. Autophagy proteins lipidate LC3 onto phagosomes and other macroendocytic vacuole membranes, and are required for lysosomal degradation of engulfed cargo, demonstrating an autophagosome-independent role for autophagy proteins in mediating the turnover of extracellular substrates. This review discusses the biological systems in which autophagy proteins have been found to regulate lysosome fusion to non-autophagic membranes.

Regulation of myosin activation during cell-cell contact formation by Par3-Lgl antagonism: entosis without matrix detachment

Two polarity proteins, partitioning defective 3 homologue (Par3) and mammalian homologues of Drosophila lethal(2)giant larvae (Lgl1/2), antagonize each other in modulating myosin II activation during cell–cell contact formation in Madin-Darby canine kidney cells. Altering the counteraction between Par3 and Lgl1/2 leads to entosis without matrix detachment.

Cell–cell contact formation following cadherin engagement requires actomyosin contraction along the periphery of cell–cell contact. The molecular mechanisms that regulate myosin activation during this process are not clear. In this paper, we show that two polarity proteins, partitioning defective 3 homologue (Par3) and mammalian homologues of Drosophila Lethal (2) Giant Larvae (Lgl1/2), antagonize each other in modulating myosin II activation during cell–cell contact formation in Madin-Darby canine kidney cells. While overexpression of Lgl1/2 or depletion of endogenous Par3 leads to enhanced myosin II activation, knockdown of Lgl1/2 does the opposite. Intriguingly, altering the counteraction between Par3 and Lgl1/2 induces cell–cell internalization during early cell–cell contact formation, which involves active invasion of the lateral cell–cell contact underneath the apical-junctional complexes and requires activation of the Rho–Rho-associated, coiled-coil containing protein kinase (ROCK)–myosin pathway. This is followed by predominantly nonapoptotic cell-in-cell death of the internalized cells and frequent aneuploidy of the host cells. Such effects are reminiscent of entosis, a recently described process observed when mammary gland epithelial cells were cultured in suspension. We propose that entosis could occur without matrix detachment and that overactivation of myosin or unbalanced myosin activation between contacting cells may be the driving force for entosis in epithelial cells.

Mechanisms of ploidy increase in human cancers: a new role for cell cannibalism

Aneuploidy is a hallmark of human cancers originating from abnormal mitoses. Many aneuploid cancer cells also have greater-than-diploid DNA content, suggesting that polyploidy is a common precursor to aneuploidy during tumor progression. Polyploid cells can originate from cell fusion, endoreplication, and cytokinesis failure. Recently we found that cell cannibalism by entosis, a form of cell engulfment involving live cells, also leads to polyploidy, as internalized cells disrupt cytokinesis of their engulfing cell hosts. By this mechanism, cannibalistic cell behavior could promote tumor progression by leading to aneuploidy. Here, we discuss cell cannibalism in cancer and other mechanisms that result in the formation of polyploid cancer cells.

Michael Overholtzer: answering existential questions on entosis

Overholtzer is working to understand how and why tumor cells invade one another.

Progress of research in cell-in-cell phenomena

The discovery of a nonphagocytotic process of cell-in-cell phenomena can be traced to over a century ago. However, its biological significance remains poorly understood. Three types of cell-in-cell phenomena have been described so far, termed "cannibalism," "emperipolesis," and "entosis." These three kinds of cell-in-cell phenomena, apart from a common feature of one cell internal to another, are distinct both cytologically and biologically. In this review, we discussed them in their morphology, cell recognition, penetration mechanisms, and physiological roles, respectively.

Autophagy machinery mediates macroendocytic processing and entotic cell death by targeting single membranes

Autophagy normally involves the formation of double-membrane autophagosomes that mediate bulk cytoplasmic and organelle degradation. Here we report the modification of single-membrane vacuoles in cells by autophagy proteins. LC3 (Light chain 3) a component of autophagosomes, is recruited to single-membrane entotic vacuoles, macropinosomes and phagosomes harbouring apoptotic cells, in a manner dependent on the lipidation machinery including ATG5 and ATG7, and the class III phosphatidylinositol-3-kinase VPS34. These downstream components of the autophagy machinery, but not the upstream mammalian Tor (mTor)-regulated ULK-ATG13-FIP200 complex, facilitate lysosome fusion to single membranes and the degradation of internalized cargo. For entosis, a live-cell-engulfment program, the autophagy-protein-dependent fusion of lysosomes to vacuolar membranes leads to the death of internalized cells. As pathogen-containing phagosomes can be targeted in a similar manner, the death of epithelial cells by this mechanism mimics pathogen destruction. These data demonstrate that proteins of the autophagy pathway can target single-membrane vacuoles in cells in the absence of pathogenic organisms.

Hepatocyte entry leads to degradation of autoreactive CD8 T cells

Although most self-reactive T cells are eliminated in the thymus, mechanisms to inactivate or control T cells specific for extrathymic antigens are required and exist in the periphery. By investigating the site in which autoreactive T cells are tolerized, we identify a unique mechanism of peripheral deletion in which naïve autoreactive CD8 T cells are rapidly eliminated in the liver after intrahepatic activation. T cells actively invade hepatocytes, enter endosomal/lysosomal compartments, and are degraded. Blockade of this process leads to accumulation of autoreactive CD8 T cells in the liver and breach of tolerance, with the development of autoimmune hepatitis. Cell into cell invasion, or emperipolesis, is a long-observed phenomenon for which a physiological role has not been previously demonstrated. We propose that this "suicidal emperipolesis" is a unique mechanism of autoreactive T-cell deletion, a process critical for the maintenance of tolerance.

Entosis: aneuploidy by invasion

Aneuploidy is one of the most prevalent phenotypes of human tumours, but the underlying cause of this phenomenon remains highly debated. Entosis, the invasion of a living cell into another cell's cytoplasm, is now shown to perturb cytokinesis and induce the formation of aneuploid cells.

A non-genetic route to aneuploidy in human cancers

Aneuploidy is common in human tumours and is often indicative of aggressive disease. Aneuploidy can result from cytokinesis failure, which produces binucleate cells that generate aneuploid offspring with subsequent divisions. In cancers, disruption of cytokinesis is known to result from genetic perturbations to mitotic pathways or checkpoints. Here we describe a non-genetic mechanism of cytokinesis failure that occurs as a direct result of cell-in-cell formation by entosis. Live cells internalized by entosis, which can persist through the cell cycle of host cells, disrupt formation of the contractile ring during host cell division. As a result, cytokinesis frequently fails, generating binucleate cells that produce aneuploid cell lineages. In human breast tumours, multinucleation is associated with cell-in-cell structures. These data define a previously unknown mechanism of cytokinesis failure and aneuploid cell formation that operates in human cancers.

Perspectives on the mesenchymal origin of metastatic cancer

Emerging evidence suggests that many metastatic cancers arise from cells of the myeloid/macrophage lineage regardless of the primary tissue of origin. A myeloid origin of metastatic cancer stands apart from origins involving clonal evolution or epithelial-mesenchymal transitions. Evidence is reviewed demonstrating that numerous human cancers express multiple properties of macrophages including phagocytosis, fusogenicity, and gene/protein expression. It is unlikely that the macrophage properties expressed in metastatic cancers arise from sporadic random mutations in epithelial cells, but rather from damage to an already existing mesenchymal cell, e.g., a myeloid/macrophage-type cell. Such cells would naturally embody the capacity to express the multiple behaviors of metastatic cells. The view of metastasis as a myeloid/macrophage disease will impact future cancer research and anti-metastatic therapies.

Neutrophils cross the BBB primarily on transcellular pathways: an in vitro study

The cerebral microcapillary endothelium forms a highly important barrier between the blood and the interstitial fluid of the brain (blood-brain barrier) that controls the passage of molecules and cells in and out of the CNS. Several CNS diseases include leukocyte extravasation through the endothelium via two mechanistically distinct routes, the paracellular and the transcellular pathway. We established a new in vitro model of the inflamed blood-brain barrier consisting of primary cultured porcine brain capillary endothelial cells which express a tight endothelial barrier even under inflammatory conditions. By means of this specialized blood-brain barrier model we extensively studied the transmigration of neutrophils. Electron and scanning force microscopy as well as immunofluorescence imaging captured the penetrating neutrophil on the endothelial cellular body in between the junctions clearly suggesting a transcellular migration pathway. Electric cell-substrate impedance sensing and transendothelial electrical resistance measurements in combination with expression analysis of tight junction proteins demonstrate that the neutrophil-endothelial interaction does not disrupt the barrier. In conclusion, this study, based on an in vitro model of the blood-brain barrier under inflammatory conditions, evidently implicates that neutrophils preferentially migrate across the BBB via the transcellular route without impairing endothelial barrier function whereas paracellular transmigration plays only a minor role if the barrier is strongly expressed.

Autophagic cell death in Dictyostelium requires the receptor histidine kinase DhkM

Through random mutagenesis, the receptor histidine kinase DhkM was found essential for autophagic cell death (ACD) in Dictyostelium. DhkM is the most downstream known molecule required for this model ACD. Different DhkM mutants showed distinct non-vacuolizing ACD phenotypes and genetically separated previously undissociated late cell death events.

Dictyostelium constitutes a genetically tractable model for the analysis of autophagic cell death (ACD). During ACD, Dictyostelium cells first transform into paddle cells and then become round, synthesize cellulose, vacuolize, and die. Through random insertional mutagenesis, we identified the receptor histidine kinase DhkM as being essential for ACD. Surprisingly, different DhkM mutants showed distinct nonvacuolizing ACD phenotypes. One class of mutants arrested ACD at the paddle cell stage, perhaps through a dominant-negative effect. Other mutants, however, progressed further in the ACD program. They underwent rounding and cellulose synthesis but stopped before vacuolization. Moreover, they underwent clonogenic but not morphological cell death. Exogenous 8-bromo-cAMP restored vacuolization and death. A role for a membrane receptor at a late stage of the ACD pathway is puzzling, raising questions as to which ligand it is a receptor for and which moieties it phosphorylates. Together, DhkM is the most downstream-known molecule required for this model ACD, and its distinct mutants genetically separate previously undissociated late cell death events.

Cell-in-cell structures formed between human cancer cell lines and the cytotoxic regulatory T-cell line HOZOT

We previously established a novel cell line, termed HOZOT, derived from umbilical cord blood mononuclear cells that is characterized as a human cytotoxic regulatory T (Treg) cell line with a FOXP3(+)CD4(+)CD8(+)CD25(+) phenotype. Here, we describe a new property of HOZOT cells: they actively penetrate into a variety of human cancer cell lines, but not into normal cell lines, and form apparent cell-in-cell structures. In the process of cell penetration, we observed that HOZOT cells adhered to target cells seemed to first insert their nuclei into the cytoplasm of target cells, distinct from the process of phagocytosis. In addition, blocking experiments showed that major histocompatibility complex class I is one of the target cell recognition molecules for HOZOT cells. Furthermore, we propose that cell-in-cell structures between HOZOT cells and target cancer cells could be one of the cytotoxic mechanisms of HOZOT cells.

Dose-dependent induction of distinct phenotypic responses to Notch pathway activation in mammary epithelial cells

Aberrant activation of Notch receptors has been implicated in breast cancer; however, the mechanisms contributing to Notch-dependent transformation remain elusive because Notch displays dichotomous functional activities, promoting both proliferation and growth arrest. We investigated the cellular basis for the heterogeneous responses to Notch pathway activation in 3D cultures of MCF-10A mammary epithelial cells. Expression of a constitutively active Notch-1 intracellular domain (NICD) was found to induce two distinct types of 3D structures: large, hyperproliferative structures and small, growth-arrested structures with reduced cell-to-matrix adhesion. Interestingly, we found that these heterogeneous phenotypes reflect differences in Notch pathway activation levels; high Notch activity caused down-regulation of multiple matrix-adhesion genes and inhibition of proliferation, whereas low Notch activity maintained matrix adhesion and provoked a strong hyperproliferative response. Moreover, microarray analyses implicated NICD-induced p63 down-regulation in loss of matrix adhesion. In addition, a reverse-phase protein array-based analysis and subsequent loss-of-function studies identified STAT3 as a dominant downstream mediator of the NICD-induced outgrowth. These results indicate that the phenotypic responses to Notch are determined by the dose of pathway activation; and this dose affects the balance between growth-stimulative and growth-suppressive effects. This unique feature of Notch signaling provides insights into mechanisms that contribute to the dichotomous effects of Notch during development and tumorigenesis.

What makes cells move: requirements and obstacles for spontaneous cell motility

Movement of individual cells and of cellular cohorts, chains or sheets requires physical forces that are established through interactions of cells with their environment. In vivo, migration occurs extensively during embryonic development and in adults during wound healing and tumorigenesis. In order to identify the molecular events involved in cell movement, in vitro systems have been developed. These have contributed to the definition of a number of molecular pathways put into play in the course of migratory behaviours, such as mesenchymal and amoeboid movement. More recently, our knowledge of migratory modes has been enriched by analyses of cells exploring and moving through three-dimensional (3D) matrices. While the cells' morphologies differ in 2D and 3D environments, the basic mechanisms that put a cellular body into motion are remarkably similar. Thus, in both 2D and 3D, the polarity of the migrating cell is initially defined by a specific subcellular localization of signalling molecules and components of molecular machines required for motion. While the polarization can be initiated either in response to extracellular signalling or be a chance occurrence, it is reinforced and sustained by positive feedback loops of signalling molecules. Second, adhesion to a substratum is necessary to generate forces that will propel the cell engaged in either mesenchymal or ameboid migration. For collective cell movement, intercellular coordination constitutes an additional requirement: a cell cohort remains stationary if individual cells pull in opposite directions. Finally, the availability of space to move into is a general requirement to set cells into motion. Lack of free space is probably the main obstacle for migration of most healthy cells in an adult multicellular organism. Thus, the requirements for cell movement are both intrinsic to the cell, involving coordinated signalling and interactions with molecular machines, and extrinsic, imposed by the physicochemical nature of the environment. In particular, the geometry and stiffness of the support act on a range of signalling pathways that induce specific cell migratory responses. These issues are discussed in the present review in the context of published work and our own data on collective migration of hepatocyte cohorts.

Thymic nurse cells exhibit epithelial progenitor phenotype and create unique extra-cytoplasmic membrane space for thymocyte selection

Thymic nurse cells (TNCs) are epithelial cells in the thymic cortex that contain as many as 50 thymocytes within specialized cytoplasmic vacuoles. The function of this cell-in-cell interaction has created controversy since their discovery in 1980. Further, some skepticism exists about the idea that apoptotic thymocytes within the TNC complex result from negative selection, a process believed to occur exclusively within the medulla. In this report, we have microscopic evidence that defines a unique membranous environment wherein lipid raft aggregates around the alphabetaTCR expressed on captured thymocytes and class II MHC molecules expressed on TNCs. Further, immunohistological examination of thymic sections show TNCs located within the cortico-medullary junction to express cytokeratins five and eight (K5 and K8), and the transcription factor Trp-63, the phenotype defined elsewhere as the thymic epithelial progenitor subset. Our results suggest that the microenvironment provided by TNCs plays an important role in thymocyte selection as well as the potential for TNCs to be involved in the maintenance of thymic epithelia.

Cell-cell channels, viruses, and evolution: via infection, parasitism, and symbiosis toward higher levels of biological complexity

Between prokaryotic cells and eukaryotic cells there is dramatic difference in complexity which represents a problem for the current version of the cell theory, as well as for the current version of evolution theory. In the past few decades, the serial endosymbiotic theory of Lynn Margulis has been confirmed. This results in a radical departure from our understanding of living systems: the eukaryotic cell represents de facto"cells-within-cell." Higher order "cells-within-cell" situations are obvious at the eukaryotic cell level in the form of secondary and tertiary endosymbiosis, or in the male and female gametophytes of higher plants. The next challenge of the current version of the cell theory is represented by the fact that the multicellular fungi and plants are, in fact, supracellular assemblies as their cells are not physically separated from each other. Moreover, there are also examples of alliances and mergings between multicellular organisms. Infection, especially the viral one, but also bacterial and fungal infections, followed by symbiosis, is proposed to act as the major force that drives the biological evolution toward higher complexity.

Mechanisms for transcellular diapedesis: probing and pathfinding by 'invadosome-like protrusions'

Immune-system functions require that blood leukocytes continuously traffic throughout the body and repeatedly cross endothelial barriers (i.e. diapedese) as they enter (intravasate) and exit (extravasate) the circulation. The very earliest studies to characterize diapedesis directly in vivo suggested the coexistence of two distinct migratory pathways of leukocytes: between (paracellular pathway) and directly through (transcellular pathway) individual endothelial cells. In vivo studies over the past 50 years have demonstrated significant use of the transcellular diapedesis pathway in bone marrow, thymus, secondary lymphoid organs, various lymphatic structures and peripheral tissues during inflammation and across the blood-brain barrier and blood-retinal barrier during inflammatory pathology. Recently, the first in vitro reports of transcellular diapedesis have emerged. Together, these in vitro and in vivo observations suggest a model of migratory pathfinding in which dynamic 'invadosome-like protrusions' formed by leukocytes have a central role in both identifying and exploiting endothelial locations that are permissive for transcellular diapedesis. Such 'probing' activity might have additional roles in this and other settings.

Role of polymorphonuclear neutrophils in atherosclerosis: current state and future perspectives

Contrary to the long-standing and widely accepted belief that polymorphonuclear neutrophils (PMN) are of marginal relevance in atherosclerosis, evidence revealing a previously unappreciated role of PMN in the process of atherosclerosis is being accumulating. Systemic inflammation involving activated PMN is clearly associated with unstable conditions of coronary artery disease and an increased number of circulating neutrophils is a well-known risk indicator of future cardiovascular outcomes. Furthermore, PMN are activated in a number of clinical conditions associated with high risk of developing atherosclerosis and are detectable into culprit lesions of patients with coronary artery disease. At present, pharmacological interventions aimed at blocking neutrophil emigration from the blood into the arterial wall and/or inhibiting neutrophil-mediated inflammatory functions are not an option for treating atherosclerosis. Nevertheless, several lines of evidence suggest that part of the atheroprotective effects of statins as well as HDL and HDL apolipoproteins may be related to their ability to modulate neutrophilic inflammation in the arterial wall. These hypotheses are not definitely established and warrant for further study. This Review describes the evidence suggesting that PMN may have a causative role in atherogenesis and atheroprogression and discusses the potential importance of modulating neutrophilic inflammation as part of a novel, improved strategy for preventing and treating atherosclerosis.

Dual requirement for the ETS transcription factors Fli-1 and Erg in hematopoietic stem cells and the megakaryocyte lineage

Fli-1 and Erg are closely related members of the Ets family of transcription factors. Both genes are translocated in human cancers, including Ewing's sarcoma, leukemia, and in the case of Erg, more than half of all prostate cancers. Although evidence from mice and humans suggests that Fli-1 is required for megakaryopoiesis, and that Erg is required for normal adult hematopoietic stem cell (HSC) regulation, their precise physiological roles remain to be defined. To elucidate the relationship between Fli-1 and Erg in hematopoiesis, we conducted an analysis of mice carrying mutations in both genes. Our results demonstrate that there is a profound genetic interaction between Fli-1 and Erg. Double heterozygotes displayed phenotypes more dramatic than single heterozygotes: severe thrombocytopenia, with a significant deficit in megakaryocyte numbers and evidence of megakaryocyte dysmorphogenesis, and loss of HSCs accompanied by a reduction in the number of committed hematopoietic progenitor cells. These results illustrate an indispensable requirement for both Fli-1 and Erg in normal HSC and megakaryocyte homeostasis, and suggest these transcription factors may coregulate common target genes.

Adhesion and transcellular migration of neutrophils and B lymphocytes on fibroblasts

During tissue inflammation, infiltrated leukocytes may have physical contacts with fibroblasts. We observed that neutrophils and B lymphocytes adhered in a larger proportion than T cells on cultured fibroblasts. Microscopy showed that adhesion was also characterized by leukocyte engulfment by the fibroblasts. In migration assays, only neutrophils and B lymphocytes were selectively able to migrate through a fibroblast barrier. Adhesion and migration were increased by stimulation with tumor necrosis factor-alpha (TNF-alpha) and phorbol-12-myristate-13-acetate (PMA). Antibodies against ICAM-1/beta2 integrin blocked the interaction of neutrophils to fibroblasts. For B lymphocytes the couple VCAM-1/alpha4 integrin was also involved in this interaction. Human skin fibroblasts presented similar adhesion characteristics as rat cardiac fibroblasts. By measuring the distance between the border of migration holes and cadherin-positive adherens junctions, more than 65% of the holes correspond to the transcellular route over the paracellular route. Furthermore, vimentin staining revealed that the migration holes were highly nested by intermediate filaments in accordance with the transcellular route. Our results demonstrated that engulfment of neutrophils and B lymphocytes by fibroblasts resulted in selective passage by a transcellular route.