Macroautophagy-aided elimination of chromatin: sorting of waste, sorting of fate?

The Role of Mitotic Slippage in Creating a "Female Pregnancy-like System" in a Single Polyploid Giant Cancer Cell

In our recent work, we observed that triple-negative breast cancer MDA-MB-231 cells respond to doxorubicin (DOX) via "mitotic slippage" (MS), discarding cytosolic damaged DNA during the process that provides their resistance to this genotoxic treatment. We also noted two populations of polyploid giant cells: those budding surviving offspring, versus those reaching huge ploidy by repeated MS and persisting for several weeks. Their separate roles in the recovery from treatment remained unclear. The current study was devoted to characterising the origin and relationship of these two sub-populations in the context of MS. MS was hallmarked by the emergence of nuclear YAP1/OCT4A/MOS/EMI2-positivity featuring a soma-germ transition to the meiotic-metaphase-arrested "maternal germ cell". In silico, the link between modules identified in the inflammatory innate immune response to cytosolic DNA and the reproductive module of female pregnancy (upregulating placenta developmental genes) was observed in polyploid giant cells. Asymmetry of the two subnuclei types, one repairing DNA and releasing buds enriched by CDC42/ACTIN/TUBULIN and the other persisting and degrading DNA in a polyploid giant cell, was revealed. We propose that when arrested in MS, a "maternal cancer germ cell" may be parthenogenetically stimulated by the placental proto-oncogene parathyroid-hormone-like-hormone, increasing calcium, thus creating a "female pregnancy-like" system within a single polyploid giant cancer cell.

The necessity of nucleophagic modality

Nucleophagy, the selective subtype of autophagy that predominantly targets only a selected and (nonessential) portion of the nucleus, and rarely the nucleus in its entirety, for degradation, reinforces the paradigm that nucleophagy recycling is a meticulous and highly delicate process guarded by fail-safe mechanisms. Our goal in this commentary is to encourage autophagy researchers and other scientists to explore nucleophagy blind spots and gain advanced insights into the diverse roles of this process and its selective modality as they pertain to intranuclear quality control and cellular homeostasis. Identifying and deciphering nucleophagic signaling, regulation, molecular mechanism(s) and its mediators, cargo composition and nuclear membrane dynamics under numerous physiological and/or pathological settings will provide important advances in our understanding of this critical type of organelle-selective autophagy.Abbreviations: INM, inner nuclear membrane; LN, late nucleophagy; mRNA, messenger RNA; NE, nuclear envelope; NL, nuclear lamina; NPC(s), nuclear pore complex(es); NVJ(s), nucleus-vacuole junction(s); ONM, outer nuclear membrane; PMN, piecemeal microautophagy of the nucleus; PND, programmed nuclear death; PNuD, programmed nuclear destruction; rDNA/rRNA, ribosomal DNA/RNA.

Anastasis: Return Journey from Cell Death

For over 20 years, it has been a dogma that once the integrity of mitochondria is disrupted and proapoptotic proteins that are normally located in the intermembrane space of mitochondria appeared in the cytoplasm, the process of cell death becomes inevitable. However, it has been recently shown that upon removal of the death signal, even at the stage of disturbance in the mitochondria, cells can recover and continue to grow. This phenomenon was named anastasis. Here, we will critically discuss the present knowledge concerning the mechanisms of cell death reversal, or development of anastasis, methods for its detection, and what role signaling from different intracellular compartments plays in anastasis stimulation.

The Contribution of Lysosomes to DNA Replication

Lysosomes, acidic, membrane-bound organelles, are not only the core of the cellular recycling machinery, but they also serve as signaling hubs regulating various metabolic pathways. Lysosomes maintain energy homeostasis and provide pivotal substrates for anabolic processes, such as DNA replication. Every time the cell divides, its genome needs to be correctly duplicated; therefore, DNA replication requires rigorous regulation. Challenges that negatively affect DNA synthesis, such as nucleotide imbalance, result in replication stress with severe consequences for genome integrity. The lysosomal complex mTORC1 is directly involved in the synthesis of purines and pyrimidines to support DNA replication. Numerous drugs have been shown to target lysosomal function, opening an attractive avenue for new treatment strategies against various pathologies, including cancer. In this review, we focus on the interplay between lysosomal function and DNA replication through nucleic acid degradation and nucleotide biosynthesis and how these could be exploited for therapeutic purposes.

The role of autophagy in escaping therapy-induced polyploidy/senescence

Autophagy is an evolutionarily conserved process necessary to maintain cell homeostasis in response to various forms of stress such as nutrient deprivation and hypoxia as well as functioning to remove damaged molecules and organelles. The role of autophagy in cancer varies depending on the stage of cancer. Cancer therapeutics can also simultaneously evoke cancer cell senescence and ploidy increase. Both cancer cell senescence and polyploidization are reversible by depolyploidization giving rise to the progeny. Autophagy activation may be indispensable for cancer cell escape from senescence/polyploidy. As cancer cell polyploidy is proposed to be involved in cancer origin, the role of autophagy in polyploidization/depolyploidization of senescent cancer cells seems to be crucial. Accordingly, this review is an attempt to understand the complicated interrelationships between reversible cell senescence/polyploidy and autophagy.

Improved Autophagic Flux in Escapers from Doxorubicin-Induced Senescence/Polyploidy of Breast Cancer Cells

The induction of senescence/polyploidization and their role in cancer recurrence is still a poorly explored issue. We showed that MDA-MB-231 and MCF-7 breast cancer cells underwent reversible senescence/polyploidization upon pulse treatment with doxorubicin (dox). Subsequently, senescent/polyploid cells produced progeny (escapers) that possessed the same amount of DNA as parental cells. In a dox-induced senescence/polyploidization state, the accumulation of autophagy protein markers, such as LC3B II and p62/SQSTM1, was observed. However, the senescent cells were characterized by a very low rate of new autophagosome formation and degradation, estimated by autophagic index. In contrast to senescent cells, escapers had a substantially increased autophagic index and transcription factor EB activation, but a decreased level of an autophagy inhibitor, Rubicon, and autophagic vesicles with non-degraded cargo. These results strongly suggested that autophagy in escapers was improved, especially in MDA-MB-231 cells. The escapers of both cell lines were also susceptible to dox-induced senescence. However, MDA-MB-231 cells which escaped from senescence were characterized by a lower number of γH2AX foci and a different pattern of interleukin synthesis than senescent cells. Thus, our studies showed that breast cancer cells can undergo senescence uncoupled from autophagy status, but autophagic flux resumption may be indispensable in cancer cell escape from senescence/polyploidy.

Micronucleus Assay in Environmental Biomonitoring

Nowadays many chemicals are widely used in agriculture to ensure high crop yields or in veterinary/human medicine to cure diseases. After their improper usage they may contaminate the environment, persist in it and adversely affect both the target and/or the non-target organisms. One of the ways to detect the occurrence of chemicals in the environment is to assess their impact on aquatic and farm animals; both are directly or indirectly exposed via their feed and water. The micronucleus assay is a standardly used cytogenetic test for the simultaneous detection of clastogenic and aneugenic agents. Additionally, cytotoxic effects are also assessed by analysing the proliferation changes using the cytokinesis-blocked proliferation index. The occurrence of micronuclei is analysed in many types of cells like the peripheral blood cells, bone marrow or cell lines according to standards for micronuclei detection. The analysis of published results has shown that the micronucleus assay is, together with the chromosomal aberration test, one of the most often used test in genotoxicity assessment. Its results have contributed to reassessing the use of multiple chemicals available on the market. Moreover, it is a compulsory test before approving the chemical/ pesticide for the market.

INDUCED POLYPLOIDY AND SORTING OF DAMAGED DNA BY MICRONUCLEATION IN RADIORESISTANT RAT LIVER EPITHELIAL STEM-LIKE CELLS EXPOSED TO X-RAYS

OBJECTIVE

Rat liver stem-like epithelial cells (WB-F344) that under certain conditions may differentiate into hepa- tocyte and biliary lineages were subjected to acute X-irradiation with the aim to examine cell cycle peculiarities dur- ing the course of survival.

MATERIALS AND METHODS

Suspensions of WB-F344 cells that grew as a monolayer and reached sub-confluence were irradiated with 1, 5, and 10 Gy of X-rays (2 Gy/min). As an intact control, sham-irradiated cells were used. After irra- diation, cells were plated into 25-cm2 tissue culture flasks to culture them for over several days without reaching contact inhibition. On days 1, 2, 3, and 5 post-irradiation, cells were harvested and examined for nuclear morpholo- gy and DNA ploidy by stoichiometric toluidine blue reaction and image cytometry. On days 7 and 9 post-irradiation, only heavily irradiated (10 Gy) cells were examined. Also, 10 Gy-irradiated cells were chosen for immunofluorescence staining to monitor persistence of DNA lesions (γ-H2AX), cell proliferation (Ki-67), and self-renewal factors charac- teristic for stem cells (OCT4 and NANOG).

RESULTS

Radioresistance of WB-F344 cells was evidenced by the findings that they do not undergo rapid and mas- sive cell death that in fact was weakly manifested as apoptotic even in heavily irradiated cells. Instead, there was cell cycle progression delay accompanied by polyploidization (via Ki-67-positive mitotic slippage or via impaired cytokinesis) and micronucleation in a dose-dependent manner, although micronucleation to some extent went ahead of polyploidization. Polyploid cells amenable for recovering from DNA damage can mitotically depolyploidize. Many micronuclei contained γ-H2AX clusters, suggesting isolation of severely damaged DNA fragments. Both factors, OCT4 and NANOG, were expressed in the intact control, but became enhanced after irradiation.

CONCLUSIONS

Although the fact of micronucleation is indicative of genotoxic effect, WB-F344 cells can probably escape cell death via sorting of damaged DNA by micronuclei. Induction of polyploidy in these cells can be adaptive to promote cell survival and tissue regeneration with possible involvement of self-renewal mechanism.

Mechanistic dissection of macro- and micronucleophagy

Nucleophagy, the mechanism for autophagic degradation of nuclear material, occurs in both a macro- and micronucleophagic manner. Upon nitrogen deprivation, we observed, in an in-depth fluorescence microscopy study, the formation of micronuclei: small parts of superfluous nuclear components surrounded by perinuclear ER. We identified two types of micronuclei associated with a corresponding autophagic mode. Our results showed that macronucleophagy degraded these smaller micronuclei. Engulfed in Atg8-positive phagophores and containing cargo receptor Atg39, macronucleophagic structures revealed finger-like extensions when observed in 3-dimensional reconstitutions of fluorescence microscopy images, suggesting directional growth. Interestingly, in the late stages of phagophore elongation, the adjacent vacuolar membrane showed a reduction of integral membrane protein Pho8. This change in membrane composition could indicate the formation of a specialized vacuolar domain, required for autophagosomal fusion. Significantly larger micronuclei formed at nucleus vacuole junctions and were identified as a substrate of piecemeal microautophagy of the nucleus (PMN), by the presence of the integral membrane protein Nvj1. Micronuclei sequestered by vacuolar invaginations also contained Atg39. A detailed investigation revealed that both Atg39 and Atg8 accumulated between the vacuolar tips. These findings suggest a role for Atg39 in micronucleophagy. Indeed, following the degradation of Nvj1, an exclusive substrate of PMN, in immunoblots, we could confirm the essential role of Atg39 for PMN. Our study thus details the involvement of Atg8 in both macronucleophagy and PMN and identifies Atg39 as the general cargo receptor for nucleophagic processes.Abbreviations: DIC: Differential interference contrast, FWHM: Full width at half maximum, IQR: Interquartile range, MIPA: Micropexophagy-specific membrane apparatus, NLS: Nuclear localization signal, NVJ: Nucleus vacuole junction, PMN: Piecemeal microautophagy of the nucleus, pnER: Perinuclear ER.

The molecular origins and pathophysiological consequences of micronuclei: New insights into an age-old problem

Micronuclei (MN), the small nucleus-like bodies separated from the primary nucleus, can exist in cells with numerical and/or structural chromosomal aberrations in apparently normal tissues and more so in tumors in humans. While MN have been observed for over 100 years, they were merely and constantly considered as passive indicators of chromosome instability (CIN) for a long time. Relatively little is known about the molecular origins and biological consequences of MN. Rapid technological advances are helping to close these gaps. Very recent studies provide exciting evidence that MN act as key platform for chromothripsis and a trigger of innate immune response, suggesting that MN could affect cellular functions by both genetic and nongenetic means. These previously unappreciated findings have reawakened widespread interests in MN. In this review, the diverse mechanisms leading to MN generation and the complex fate profiles of MN are discussed, together with the evidence for their contribution to CIN, inflammation, senescence and cell death. Moreover, we put this knowledge together into a speculative perspective on how MN may be responsible for cancer development and how their presence may influence the choice of treatment. We suggest that the heterogeneous responses to MN may function physiological to ensure the arrestment, elimination and immune clearance of damaged cells, but pathologically, may enable the survival and oncogenic transformation of cells bearing CIN. These insights not only underscore the complexity of MN biology, but also raise a host of new questions and provide fertile ground for future research.

Interstitial telomeric repeats-associated DNA breaks

During a cell's lifespan, DNA break formation is a common event, associated with many processes, from replication to apoptosis. Most of DNA breaks are readily repaired, but some are meant to persist in time, such as the chromosome ends, protected by telomeres. Besides them, eukaryotic genomes comprise shorter stretches of interstitial telomeric repeats. We assumed that the latter may also be associated with the formation of DNA breaks meant to persist in time. In zebrafish and mouse embryos, cells containing numerous breakage foci were identified. These breaks were not associated with apoptosis or replication, nor did they seem to activate DNA damage response machinery. Unlike short-living, accidental sparse breaks, the ones we found seem to be closely associated, forming discrete break foci. A PCR-based method was developed, allowing specific amplification of DNA regions located between inverted telomeric repeats associated with breaks. The cloning and sequencing of such DNA fragments were found to denote some specificity in their distribution for different tissue types and development stages.

Carbon and nitrogen depletion-induced nucleophagy and selective autophagic sequestration of a whole nucleus in multinucleate cells of the filamentous fungus Aspergillus oryzae

Autophagy is a conserved cellular degradation process in eukaryotes, in which cytoplasmic components and organelles are digested in vacuoles/lysosomes. Recently, autophagic degradation of nuclear materials, termed "nucleophagy", has been reported. In the multinucleate filamentous fungus Aspergillus oryzae, a whole nucleus is degraded by nucleophagy after prolonged culture. While developing an H2B-EGFP processing assay for the evaluation of nucleophagy in A. oryzae, we found that nucleophagy is efficiently induced by carbon or nitrogen depletion. Microscopic observations in a carbon depletion condition clearly demonstrated that autophagosomes selectively sequester a particular nucleus, despite the presence of multiple nuclei in the same cell. Furthermore, AoNsp1, the A. oryzae homolog of the yeast nucleoporin Nsp1p, mainly localized at the nuclear periphery, but its localization was restricted to the opposite side of the autophagosome being formed around a nucleus. In contrast, the perinuclear ER visualized with the calnexin AoClxA was not morphologically affected by nucleophagy. The findings of nucleophagy-inducing conditions enabled us to characterize the morphological process of autophagic degradation of a whole nucleus in multinucleate cells.

Fate of micronuclei and micronucleated cells

The present review describes available evidence about the fate of micronuclei and micronucleated cells. Micronuclei are small, extranuclear chromatin bodies surrounded by a nuclear envelope. The mechanisms underlying the formation of micronuclei are well understood but not much is known about the potential fate of micronuclei and micronucleated cells. Many studies with different experimental approaches addressed the various aspects of the post-mitotic fate of micronuclei and micronucleated cells. These studies are reviewed here considering four basic possibilities for potential fates of micronuclei: degradation of the micronucleus or the micronucleated cell, reincorporation into the main nucleus, extrusion from the cell, and persistence in the cytoplasm. Two additional fates need to be considered: premature chromosome condensation/chromothripsis and the elimination of micronucleated cells by apoptosis, yielding six potential fates for micronuclei and/or micronucleated cells. The available data is still limited, but it can be concluded that degradation and extrusion of micronuclei might occur in rare cases under specific conditions, reincorporation during the next mitosis occurs more frequently, and the majority of the micronuclei persist without alteration at least until the next mitosis, possibly much longer. Overall, the consequences of micronucleus formation on the cellular level are still far from clear, but they should be investigated further because micronucleus formation may contribute to the initial and later steps of malignant cell transformation, by causing gain or loss of genetic material in the daughter cells and by the possibility of massive chromosome rearrangement in chromosomes entrapped within a micronucleus by the mechanisms of chromothripsis and chromoanagenesis.

Biogenesis of Micronuclei

The presence of micronuclei in a cell is an indicator of DNA damage and genetic instability. In this review, mechanisms of emergence of micronuclei, their functional activity, and pathways of elimination are discussed. It is supposed that morphological and functional varieties of micronuclei as well as their degradation pathways can be determined by the chromosomal material localized inside these cell structures.

Nucleolar aggresomes mediate release of pericentric heterochromatin and nuclear destruction of genotoxically treated cancer cells

The role of the nucleolus and autophagy in maintenance of nuclear integrity is poorly understood. In addition, the mechanisms of nuclear destruction in cancer cells senesced after conventional chemotherapy are unclear. In an attempt to elucidate these issues, we studied teratocarcinoma PA1 cells treated with Etoposide (ETO), focusing on the nucleolus. Following treatment, most cells enter G2 arrest, display persistent DNA damage and activate p53, senescence, and macroautophagy markers. 2-5 µm sized nucleolar aggresomes (NoA) containing fibrillarin (FIB) and damaged rDNA, colocalized with ubiquitin, pAMPK, and LC3-II emerge, accompanied by heterochromatin fragments, when translocated perinuclearly. Microscopic counts following application of specific inhibitors revealed that formation of FIB-NoA is dependent on deficiency of the ubiquitin proteasome system coupled to functional autophagy. In contrast, the accompanying NoAs release of pericentric heterochromatin, which exceeds their frequency, is favored by debilitation of autophagic flux. Potential survivors release NoA in the cytoplasm during rare mitoses, while exit of pericentric fragments often depleted of H3K9Me3, with or without encompassing by NoA, occurs through the nucleolar protrusions and defects of the nuclear envelope. Foci of LC3-II are accumulated in the nucleoli undergoing cessation of rDNA transcription. As an origin of heterochromatin fragmentation, the unscheduled DNA synthesis and circular DNAs were found in the perinucleolar heterochromatin shell, along with activation and retrotransposition of ALU elements, colocalized with 45S rDNA in NoAs. The data indicate coordination of the basic nucleolar function with autophagy regulation in maintenance of the integrity of the nucleolus associated domains secured by inactivity of retrotransposons.

Nuclear autophagy: An evolutionarily conserved mechanism of nuclear degradation in the cytoplasm

Macroautophagy/autophagy is a catabolic process that is essential for cellular homeostasis. Studies on autophagic degradation of cytoplasmic components have generated interest in nuclear autophagy. Although its mechanisms and roles have remained elusive, tremendous progress has been made toward understanding nuclear autophagy. Nuclear autophagy is evolutionarily conserved in eukaryotes that may target various nuclear components through a series of processes, including nuclear sensing, nuclear export, autophagic substrate encapsulation and autophagic degradation in the cytoplasm. However, the molecular processes and regulatory mechanisms involved in nuclear autophagy remain largely unknown. Numerous studies have highlighted the importance of nuclear autophagy in physiological and pathological processes such as cancer. This review focuses on current advances in nuclear autophagy and provides a summary of its research history and landmark discoveries to offer new perspectives.

Macrophage-tumor cell fusions from peripheral blood of melanoma patients

Background

While the morbidity and mortality from cancer are largely attributable to its metastatic dissemination, the integral features of the cascade are not well understood. The widely accepted hypothesis is that the primary tumor microenvironment induces the epithelial-to-mesenchymal transition in cancer cells, facilitating their escape into the bloodstream, possibly accompanied by cancer stem cells. An alternative theory for metastasis involves fusion of macrophages with tumor cells (MTFs). Here we culture and characterize apparent MTFs from blood of melanoma patients.

Methods

We isolated enriched CTC populations from peripheral blood samples from melanoma patients, and cultured them. We interrogated these cultured cells for characteristic BRAF mutations, and used confocal microscopy for immunophenotyping, motility, DNA content and chromatin texture analyses, and then conducted xenograft studies using nude mice.

Findings

Morphologically, the cultured MTFs were generally large with many pseudopod extensions and lamellipodia. Ultrastructurally, the cultured MTFs appeared to be macrophages. They were rich in mitochondria and lysosomes, as well as apparent melanosomes. The cultured MTF populations were all heterogeneous with regard to DNA content, containing aneuploid and/or high-ploidy cells, and they typically showed large sheets (and/or clumps) of cytoplasmic chromatin. This cytoplasmic DNA was found within heterogeneously-sized autophagic vacuoles, which prominently contained chromatin and micronuclei. Cultured MTFs uniformly expressed pan-macrophage markers (CD14, CD68) and macrophage markers indicative of M2 polarization (CD163, CD204, CD206). They also expressed melanocyte-specific markers (ALCAM, MLANA), epithelial biomarkers (KRT, EpCAM), as well as the pro-carcinogenic cytokine MIF along with functionally related stem cell markers (CXCR4, CD44). MTF cultures from individual patients (5 of 8) contained melanoma-specific BRAF activating mutations. Chromatin texture analysis of deconvoluted images showed condensed DNA (DAPI-intense) regions similar to focal regions described in stem cell fusions. MTFs were readily apparent in vivo in all human melanomas examined, often exhibiting even higher DNA content than the cultured MTFs. When cultured MTFs were transplanted subcutaneously in nude mice, they disseminated and produced metastatic lesions at distant sites.

Conclusions and Hypothesis

Apparent MTFs are present in peripheral blood of patients with cutaneous melanomas, and they possess the ability to form metastatic lesions when transplanted into mice. We hypothesize that these MTFs arise at the periphery of primary tumors in vivo, that they readily enter the bloodstream and invade distant tissues, secreting cytokines (such as MIF) to prepare “niches” for colonization by metastasis initiating cells.

Organellophagy: eliminating cellular building blocks via selective autophagy

Maintenance of organellar quality and quantity is critical for cellular homeostasis and adaptation to variable environments. Emerging evidence demonstrates that this kind of control is achieved by selective elimination of organelles via autophagy, termed organellophagy. Organellophagy consists of three key steps: induction, cargo tagging, and sequestration, which involve signaling pathways, organellar landmark molecules, and core autophagy-related proteins, respectively. In addition, posttranslational modifications such as phosphorylation and ubiquitination play important roles in recruiting and tailoring the autophagy machinery to each organelle. The basic principles underlying organellophagy are conserved from yeast to mammals, highlighting its biological relevance in eukaryotic cells.

Long-term artificial selection reveals a role of TCTP in autophagy in mammalian cells

Understanding genomic variation and detecting selection signatures in a genome under selection have been great challenges for a century. Activation, development/exhaustion of primordial follicles in mammalian ovary determines reproductive success, menopause/end of female reproductive life. However, molecular mechanisms underlying oogenesis, particularly under artificial selection, are largely unknown. We report that a proteome-wide scan for selection signatures in the genome over 9,000 years of artificial pressure on the ovary revealed a general picture of selection signatures in the genome, especially genomic variations through artificial selection were detected in promoter and intron regions. Crossbreeding between domestic and wild species results in more than half of the protein spots exhibiting heterosis. Translationally controlled tumor protein (TCTP) is upregulated by artificial selection and positively regulates autophagy through the AMP-activated protein kinase pathway. Notably, TCTP interacts with ATG16 complex. In addition to cytoplasmic autophagy, nucleophagy occurs in the nuclei of granulosa and cumulus cells in ovaries, indicating an importance of the nuclear material for degradation by nucleophagy. Our findings provide insight into cellular and molecular mechanisms relevant for improvement of ovary functions, and identify selection signatures in the genome for ovary function over long-term artificial selection pressure.

Ionizing radiation-induced responses in human cells with differing TP53 status

Ionizing radiation triggers diverse responses in human cells encompassing apoptosis, necrosis, stress-induced premature senescence (SIPS), autophagy, and endopolyploidy (e.g., multinucleation). Most of these responses result in loss of colony-forming ability in the clonogenic survival assay. However, not all modes of so-called clonogenic cell "death" are necessarily advantageous for therapeutic outcome in cancer radiotherapy. For example, the crosstalk between SIPS and autophagy is considered to influence the capacity of the tumor cells to maintain a prolonged state of growth inhibition that unfortunately can be succeeded by tumor regrowth and disease recurrence. Likewise, endopolyploid giant cells are able to segregate into near diploid descendants that continue mitotic activities. Herein we review the current knowledge on the roles that the p53 and p21(WAF1) tumor suppressors play in determining the fate of human fibroblasts (normal and Li-Fraumeni syndrome) and solid tumor-derived cells after exposure to ionizing radiation. In addition, we discuss the important role of WIP1, a p53-regulated oncogene, in the temporal regulation of the DNA damage response and its contribution to p53 dynamics post-irradiation. This article highlights the complexity of the DNA damage response and provides an impetus for rethinking the nature of cancer cell resistance to therapeutic agents.

Three steps to the immortality of cancer cells: senescence, polyploidy and self-renewal

Metastatic cancer is rarely cured by current DNA damaging treatments, apparently due to the development of resistance. However, recent data indicates that tumour cells can elicit the opposing processes of senescence and stemness in response to these treatments, the biological significance and molecular regulation of which is currently poorly understood. Although cellular senescence is typically considered a terminal cell fate, it was recently shown to be reversible in a small population of polyploid cancer cells induced after DNA damage. Overcoming genotoxic insults is associated with reversible polyploidy, which itself is associated with the induction of a stemness phenotype, thereby providing a framework linking these separate phenomena. In keeping with this suggestion, senescence and autophagy are clearly intimately involved in the emergence of self-renewal potential in the surviving cells that result from de-polyploidisation. Moreover, subsequent analysis indicates that senescence may paradoxically be actually required to rejuvenate cancer cells after genotoxic treatments. We propose that genotoxic resistance is thereby afforded through a programmed life-cycle-like process which intimately unites senescence, polyploidy and stemness.

Nucleophagy at a glance

Under certain circumstances, the removal of damaged or non-essential parts of the nucleus, or even an entire nucleus, is crucial in order to promote cell longevity and enable proper function. A selective form of autophagy, known as nucleophagy, can be used to accomplish the degradation of nucleus-derived material. In this Cell Science at a Glance article and the accompanying poster, we summarize the similarities and differences between the divergent modes of nucleophagy that have been described to date, emphasizing, where possible, the molecular mechanism, the membrane interactions and rearrangements, and the nature of the nucleus-derived material that is degraded. In turn, we will consider nucleophagy processes in the lower eukaryotes, the budding yeast Saccharomyces cerevisiae, filamentous fungi Aspergillus and Magnaporthe oryzae and the ciliated protozoan Tetrahymena thermophila, and finally in mammalian cells. We will also briefly discuss the emerging links between nucleophagy and human disease.