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.

Fate of Entosis: From the Beginning to the End in Untreated Advanced Breast Cancer

Homotypic entosis is a phenomenon in which one cancer cell invades a neighboring cancer cell and is closed entirely within its entotic vacuole. The fate of entosis can lead to inner cell death or survival. Recent evidence draws attention to entosis as a novel prognostic marker in breast cancer. Nevertheless, little is known about the quantity and quality of the process of entosis in human cancer specimens. Here, for the first time, we analyze the frequency of entotic figures in a case of NOS (Non-Other Specified) breast cancer with regard to location: the primary tumor, regional lymph node, and distant metastasis. For the identification of entotic figures, cells were stained using hematoxylin/eosin and assessed using criteria proposed by Mackay. The majority of entotic figures (65%) were found in the lymph node, 27% were found in the primary tumor, and 8% were found in the far metastasis. In the far metastases, entotic figures demonstrated an altered, atypic morphology. Interestingly, in all locations, entosis did not show any signs of cell death. Moreover, the slides were stained for E-cadherin or Ki67, and we identified proliferating (Ki67-positive) inner and outer entotic cells. Therefore, we propose additional criteria for the identification of pro-survival entotic structures in diagnostic histopathology.

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.

Genetic and clinical correlates of entosis in pancreatic ductal adenocarcinoma

Entosis is a type of regulated cell death that promotes cancer cell competition. Though several studies have revealed the molecular mechanisms that govern entosis, the clinical and genetic correlates of entosis in human tumors is less well understood. Here we reviewed entotic cell-in-cell (CIC) patterns in a large single institution sequencing cohort (MSK IMPACT clinical sequencing cohort) of more than 1600 human pancreatic ductal adenocarcinoma (PDAC) samples to identify the genetic and clinical correlates of this cellular feature. After case selection, 516 conventional PDACs and 21 ASCs entered this study and ~45,000 HPFs (median 80 HPFs per sample) were reviewed; 549 entotic-CICs were detected through our cohort. We observed that entotic-CIC occurred more frequently in liver metastasis compared with primary in PDAC. Moreover, poorly differentiated adenocarcinoma or adenosquamous carcinoma had more entotic-CIC than well or moderately differentiated adenocarcinoma. With respect to genetic features TP53 mutations, KRAS amplification, and MYC amplification were significantly associated with entosis in PDAC tissues. From a clinical standpoint entotic CICs were independently associated with a poor prognosis by multivariate Cox regression analysis when considering all cases or primary PDACs specifically. These results provide a contextual basis for understanding entosis in PDAC, a highly aggressive cancer for which molecular insights are needed to improve survival.

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.

Entosis: Cell-in-Cell Formation that Kills Through Entotic Cell Death

Entosis is a cell-in-cell formation mechanism that targets viable cells for uptake in epithelial cell cultures and human tumors. Entotic cells control their own engulfment, by invading into their hosts in a Rho-GTPase and actomyosin-dependent manner. Although entotic cells are internalized while alive, most eventually undergo a non-apoptotic form of cell death, called entotic cell death, that is executed non-cell-autonomously by autophagy proteins and lysosomes. Here we review the current understanding of entosis and entotic cell death and discuss the potential roles of this process in cancer.

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.

Neutrophil-tumor cell phagocytosis (cannibalism) in human tumors: an update and literature review

The recognition and removal of apoptotic cells by tissue macrophages and nonprofessional phagocytes, in a process called efferocytosis, is critical for development, tissue homeostasis and resolution of inflammation. Apoptotic bodies arising in tumor tissue are ingested by viable neoplastic cells and by resident macrophages. We described tumor cell phagocytosis of apoptotic neutrophils in human gastric carcinomas. This phenomenon is analogous to neutrophil efferocytosis performed by macrophages and by nonprofessional phagocytes during inflammatory reaction but is distinct by other types of cell-in-cell phenomena including emperipolesis and entosis both cytologically and biologically. In this review, we discussed them in their ultrastructural morphology, physiological roles, and clinicopathologic implications. This article is part of a Special Issue entitled "Apoptosis: Four Decades Later".

Alternative cell death mechanisms in development and beyond

A canonical regulatory pathway involving the members of the Bcl-2 and caspase families has been established to regulate developmental apoptosis in nematodes and flies. However, mutant mice that have major deficiencies in this apoptosis pathway show only relatively minor developmental defects. Recent revelations indicate that multiple mechanisms are involved in regulating cell death during mammalian development, tissue homeostasis, and pathological cell loss. Here, we critically evaluate the evidence demonstrating the existence of alternative cell death mechanisms, including apoptosis of lower organisms in the absence of canonical apoptosis mediators, autophagic cell death, necroptosis, elimination by shedding, keratinocyte death by cornification, and cell-cell cannibalism by entosis. The physiological relevance of alternative cell death mechanisms as primary and backup mechanisms is discussed.