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.     

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.     

Induction of entosis by epithelial cadherin expression

Cell engulfment typically targets dead or dying cells for clearance from metazoan tissues. However, recent evidence demonstrates that live cells can also be targeted and that engulfment can cause cell death. Entosis is one mechanism proposed to mediate the engulfment and killing of live tumor cells by their neighbors, an activity often referred to as cell cannibalism. Here we report that the expression of exogenous epithelial cadherin proteins (E- or P-cadherin) in human breast tumor cells lacking endogenous expression of epithelial cadherins induces entosis and inhibits transformed growth. Entosis induced by cadherin expression is associated with the polarized distribution of Rho and Rho-kinase (ROCK) activity within entotic cells, which is dependent on p190A RhoGAP activity. ROCK inhibition or downregulation of p190A RhoGAP expression reduces entosis and increases the transformed growth of epithelial cadherin-expressing tumor cells. These data define new cell systems for the study of entosis, and identify entosis as a mechanism of cell cannibalism that is induced by the establishment of epithelial adhesion and inhibits transformed growth.     

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.     

Entosis Controls a Developmental Cell Clearance in C. elegans

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Trans‐sialidase Stimulates Eat Me Response from Epithelial Cells

Epithelial cell invasion by the protozoan parasite Trypanosoma cruzi is enhanced by the presence of an enzyme expressed on its cell surface during the trypomastigote life cycle stage. The enzyme, trans‐sialidase (TS), is a member of one of the largest gene families expressed by the parasite and the role of its activity in mediating epithelial cell entry has not hitherto been understood. Here we show that the T. cruzi TS generates an eat me signal which is capable of enabling epithelial cell entry. We have utilized purified, recombinant, active (TcTS) and inactive (TcTS2V0) TS coated onto beads to challenge an epithelial cell line. We find that TS activity acts upon G protein coupled receptors present at the epithelial cell synapse with the coated bead, thereby enhancing cell entry. By so doing, we provide evidence that TS proteins bind glycans, mediate the formation of distinct synaptic domains and promote macropinocytotic uptake of microparticles into a perinuclear compartment in a manner which may emulate entosis .      

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.     

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

Entosis, a ceU-in-ceU process, has been implicated in the formation of aneuploidy associated with an aberrant cell division control. Microtubule plus-end-tracking protein TI P150 facilitates the loading of MCAK onto the microtubule plus ends and orchestrates micro- tubule plus-end dynamics during cell division. Here we show that TIP150 cooperates with MCAK to govern entosis via a regulatory cir- cuitry 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-mo- lecular association, which perturbs the MCAK-TI P150 interaction in vitro and inhibits entosis in vivo. To probe if MCAK-TIP150 inter- action regulates microtubule plasticity to affect the mechanical properties of ceUs during entosis, we used an optical trap to measure the mechanical rigidity of live MCF7 ceils. 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-TI P150 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 ceU-in-ceU pro- cesses.     

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 matrixdetached 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.