Mechanisms and consequences of entosis.

Multiple mechanisms have emerged where the engulfment of whole live cells, leading to the formation of what are called 'cell-in-cell' structures, induces cell death. Entosis is one such mechanism that drives cell-in-cell formation during carcinogenesis and development. Curiously, entotic cells participate actively in their own engulfment, by invading into their hosts, and are then killed non-cell-autonomously. Here we review the mechanisms of entosis and entotic cell death and the consequences of entosis on cell populations.

Regulation of mitochondrial metabolism: yet another facet in the biology of the oncoprotein Bcl-2.

The Bcl-2 (Bcl is B-cell lymphocytic-leukaemia proto-oncogene) family comprises two groups of proteins with distinct functional biology in cell-fate signalling. Bcl-2 protein was the first member to be discovered and associated with drug resistance in human lymphomas. Since then a host of other proteins such as Bcl-xL, Bcl-2A1 and Mcl-1 with similar anti-apoptotic functions have been identified. In contrast, the pro-apoptotic Bcl-2 proteins contain prototypic effector proteins such as Bax and Bak, and the BH3 (Bcl-2 homology)-only proteins comprising Bak, Bid, Bim, Puma and Noxa. A complex interplay between the association of pro-apoptotic and anti-apoptotic proteins with each other determines the sensitivity of cancer cells to drug-induced apoptosis. The canonical functional of Bcl-2 in terms of apoptosis inhibition is its ability to prevent mitochondrial permeabilization via inhibiting the translocation and oligomerization of pro-apoptotic proteins such as Bax; however, more recent evidence points to a novel mechanism of the anti-apoptotic activity of Bcl-2. Overexpression of Bcl-2 increases mitochondrial oxygen consumption and in doing so generates a slight pro-oxidant intracellular milieu, which promotes genomic instability and blocks death signalling. However, in the wake of overt oxidative stress, Bcl-2 regulates cellular redox status thereby preventing excessive build-up of ROS (reactive oxygen species), which is detrimental to cells and tissues. Taken together, the canonical and non-canonical activities of Bcl-2 imply a critical involvement of this protein in the processes of tumour initiation and progression. In the present paper we review these functionally distinct outcomes of Bcl-2 expression with implications for the chemotherapeutic management of cancers.

Identification of long-lived proteins in the mitochondria reveals increased stability of the electron transport chain.

In order to combat molecular damage, most cellular proteins undergo rapid turnover. We have previously identified large nuclear protein assemblies that can persist for years in post-mitotic tissues and are subject to age-related decline. Here, we report that mitochondria can be long lived in the mouse brain and reveal that specific mitochondrial proteins have half-lives longer than the average proteome. These mitochondrial long-lived proteins (mitoLLPs) are core components of the electron transport chain (ETC) and display increased longevity in respiratory supercomplexes. We find that COX7C, a mitoLLP that forms a stable contact site between complexes I and IV, is required for complex IV and supercomplex assembly. Remarkably, even upon depletion of COX7C transcripts, ETC function is maintained for days, effectively uncoupling mitochondrial function from ongoing transcription of its mitoLLPs. Our results suggest that modulating protein longevity within the ETC is critical for mitochondrial proteome maintenance and the robustness of mitochondrial function.

Diverse roles of guanine nucleotide exchange factors in regulating collective cell migration.

Efficient collective migration depends on a balance between contractility and cytoskeletal rearrangements, adhesion, and mechanical cell-cell communication, all controlled by GTPases of the RHO family. By comprehensive screening of guanine nucleotide exchange factors (GEFs) in human bronchial epithelial cell monolayers, we identified GEFs that are required for collective migration at large, such as SOS1 and ß-PIX, and RHOA GEFs that are implicated in intercellular communication. Down-regulation of the latter GEFs differentially enhanced front-to-back propagation of guidance cues through the monolayer and was mirrored by down-regulation of RHOA expression and myosin II activity. Phenotype-based clustering of knockdown behaviors identified RHOA-ARHGEF18 and ARHGEF3-ARHGEF28-ARHGEF11 clusters, indicating that the latter may signal through other RHO-family GTPases. Indeed, knockdown of RHOC produced an intermediate between the two phenotypes. We conclude that for effective collective migration, the RHOA-GEFs → RHOA/C → actomyosin pathways must be optimally tuned to compromise between generation of motility forces and restriction of intercellular communication.

Germ Cells Get by with a Little Cannibalistic Help from Their Friends.

During development, primordial germ cells (PGCs) navigate a complex journey to generate the germline. In a recent paper in Nature Cell Biology, Nance and colleagues (Abdu et al., 2016) have discovered an unexpected step along the way: PGCs get cut in half by endodermal cells.

PIKfyve Regulates Vacuole Maturation and Nutrient Recovery following Engulfment.

The scavenging of extracellular macromolecules by engulfment can sustain cell growth in a nutrient-depleted environment. Engulfed macromolecules are contained within vacuoles that are targeted for lysosome fusion to initiate degradation and nutrient export. We have shown that vacuoles containing engulfed material undergo mTORC1-dependent fission that redistributes degraded cargo back into the endosomal network. Here we identify the lipid kinase PIKfyve as a regulator of an alternative pathway that distributes engulfed contents in support of intracellular macromolecular synthesis during macropinocytosis, entosis, and phagocytosis. We find that PIKfyve regulates vacuole size in part through its downstream effector, the cationic transporter TRPML1. Furthermore, PIKfyve promotes recovery of nutrients from vacuoles, suggesting a potential link between PIKfyve activity and lysosomal nutrient export. During nutrient depletion, PIKfyve activity protects Ras-mutant cells from starvation-induced cell death and supports their proliferation. These data identify PIKfyve as a critical regulator of vacuole maturation and nutrient recovery during engulfment.

Milk makes lysosomes lethal.

Stat3 has been shown to regulate lysosome membrane permeabilization and cell death in vivo during post-lactation mammary gland involution. It is now revealed that Stat3 induces lysosome membrane permeabilization by causing phagocytosis of milk fat globules, which destabilize the lysosome membrane leading to leakage of cathepsin proteases.

mTOR regulates phagosome and entotic vacuole fission.

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.