Interplay between oncogene-induced DNA damage response and heterochromatin in senescence and cancer. Nat Cell Biol. 2011;13:292-302. [PMID: 21336312 DOI: 10.1038/ncb2170]

Epigenomic Regulation of Smad1 Signaling During Cellular Senescence Induced by Ras Activation

Epigenomic modification plays important roles in regulating gene expression during development, differentiation, and cellular senescence. When oncogenes are activated, cells fall into stable growth arrest to block cellular proliferation, which is called oncogene-induced senescence. We recently identified through genome-wide analyses that Bmp2-Smad1 signal and its regulation by harmonized epigenomic alteration play an important role in Ras-induced senescence of mouse embryonic fibroblasts. We describe in this chapter the methods for analyses of epigenomic alteration and Smad1 targets on genome-wide scale.

Controlled DNA double-strand break induction in mice reveals post-damage transcriptome stability

DNA double-strand breaks (DSBs) and their repair can cause extensive epigenetic changes. As a result, DSBs have been proposed to promote transcriptional and, ultimately, physiological dysfunction via both cell-intrinsic and cell-non-autonomous pathways. Studying the consequences of DSBs in higher organisms has, however, been hindered by a scarcity of tools for controlled DSB induction. Here, we describe a mouse model that allows for both tissue-specific and temporally controlled DSB formation at ∼140 defined genomic loci. Using this model, we show that DSBs promote a DNA damage signaling-dependent decrease in gene expression in primary cells specifically at break-bearing genes, which is reversed upon DSB repair. Importantly, we demonstrate that restoration of gene expression can occur independently of cell cycle progression, underlining its relevance for normal tissue maintenance. Consistent with this, we observe no evidence for persistent transcriptional repression in response to a multi-day course of continuous DSB formation and repair in mouse lymphocytes in vivo Together, our findings reveal an unexpected capacity of primary cells to maintain transcriptome integrity in response to DSBs, pointing to a limited role for DNA damage as a mediator of cell-autonomous epigenetic dysfunction.

Linking replication stress with heterochromatin formation

The eukaryotic genome can be roughly divided into euchromatin and heterochromatin domains that are structurally and functionally distinct. Heterochromatin is characterized by its high compaction that impedes DNA transactions such as gene transcription, replication, or recombination. Beyond its role in regulating DNA accessibility, heterochromatin plays essential roles in nuclear architecture, chromosome segregation, and genome stability. The formation of heterochromatin involves special histone modifications and the recruitment and spreading of silencing complexes that impact the higher-order structures of chromatin; however, its molecular nature varies between different chromosomal regions and between species. Although heterochromatin has been extensively characterized, its formation and maintenance throughout the cell cycle are not yet fully understood. The biggest challenge for the faithful transmission of chromatin domains is the destabilization of chromatin structures followed by their reassembly on a novel DNA template during genomic replication. This destabilizing event also provides a window of opportunity for the de novo establishment of heterochromatin. In recent years, it has become clear that different types of obstacles such as tight protein-DNA complexes, highly transcribed genes, and secondary DNA structures could impede the normal progression of the replisome and thus have the potential to endanger the integrity of the genome. Multiple studies carried out in different model organisms have demonstrated the capacity of such replisome impediments to favor the formation of heterochromatin. Our review summarizes these reports and discusses the potential role of replication stress in the formation and maintenance of heterochromatin and the role that silencing proteins could play at sites where the integrity of the genome is compromised.

Two distinct modes for propagation of histone PTMs across the cell cycle

Epigenetic states defined by chromatin can be maintained through mitotic cell division. However, it remains unknown how histone-based information is transmitted. Here we combine nascent chromatin capture (NCC) and triple-SILAC (stable isotope labeling with amino acids in cell culture) labeling to track histone modifications and histone variants during DNA replication and across the cell cycle. We show that post-translational modifications (PTMs) are transmitted with parental histones to newly replicated DNA. Di- and trimethylation marks are diluted twofold upon DNA replication, as a consequence of new histone deposition. Importantly, within one cell cycle, all PTMs are restored. In general, new histones are modified to mirror the parental histones. However, H3K9 trimethylation (H3K9me3) and H3K27me3 are propagated by continuous modification of parental and new histones because the establishment of these marks extends over several cell generations. Together, our results reveal how histone marks propagate and demonstrate that chromatin states oscillate within the cell cycle.

Gallotannin is a DNA damaging compound that induces senescence independently of p53 and p21 in human colon cancer cells

The plant secondary metabolite gallotannin (GT) is the simplest hydrolyzable tannin shown to have anti-carcinogenic properties in several cell lines and to inhibit tumor development in different animal models. Here, we determined if GT induces senescence and DNA damage and investigated the involvement of p53 and p21 in this response. Using HCT116 human colon cancer cells wildtype for p53(+/+) /p21(+/+) and null for p53(+/+) /p21(-/-) or p53(-/-) /p21(+/+) , we found that GT induces senescence independently of p21 and p53. GT was found to increase the production of reactive oxygen species (ROS) by altering the redox balance in the cell, mainly by reducing the levels of glutathione and superoxide dismutase (SOD). Using the key antioxidants N-acetyl cysteine, dithiothreitol, SOD, and catalase, we showed that ROS were partially involved in the senescence response. Furthermore, GT-induced cell cycle arrest in S-phase in all HCT116 cell lines. At later time points, we noticed that p53 and p21 null cells escaped complete arrest and re-entered cell cycle provoking higher rates of multinucleation. The senescence induction by GT was irreversible and was accompanied by significant DNA damage as evidenced by p-H2AX staining. Our findings indicate that GT is an interesting anti colon cancer agent which warrants further study.

Alterations of Nuclear Architecture and Epigenetic Signatures during African Swine Fever Virus Infection

Viral interactions with host nucleus have been thoroughly studied, clarifying molecular mechanisms and providing new antiviral targets. Considering that African swine fever virus (ASFV) intranuclear phase of infection is poorly understood, viral interplay with subnuclear domains and chromatin architecture were addressed. Nuclear speckles, Cajal bodies, and promyelocytic leukaemia nuclear bodies (PML-NBs) were evaluated by immunofluorescence microscopy and Western blot. Further, efficient PML protein knockdown by shRNA lentiviral transduction was used to determine PML-NBs relevance during infection. Nuclear distribution of different histone H3 methylation marks at lysine’s 9, 27 and 36, heterochromatin protein 1 isoforms (HP1α, HPβ and HPγ) and several histone deacetylases (HDACs) were also evaluated to assess chromatin status of the host. Our results reveal morphological disruption of all studied subnuclear domains and severe reduction of viral progeny in PML-knockdown cells. ASFV promotes H3K9me3 and HP1β foci formation from early infection, followed by HP1α and HDAC2 nuclear enrichment, suggesting heterochromatinization of host genome. Finally, closeness between DNA damage response factors, disrupted PML-NBs, and virus-induced heterochromatic regions were identified. In sum, our results demonstrate that ASFV orchestrates spatio-temporal nuclear rearrangements, changing subnuclear domains, relocating Ataxia Telangiectasia Mutated Rad-3 related (ATR)-related factors and promoting heterochromatinization, probably controlling transcription, repressing host gene expression, and favouring viral replication.

Replication Stress: A Lifetime of Epigenetic Change

DNA replication is essential for cell division. Challenges to the progression of DNA polymerase can result in replication stress, promoting the stalling and ultimately collapse of replication forks. The latter involves the formation of DNA double-strand breaks (DSBs) and has been linked to both genome instability and irreversible cell cycle arrest (senescence). Recent technological advances have elucidated many of the factors that contribute to the sensing and repair of stalled or broken replication forks. In addition to bona fide repair factors, these efforts highlight a range of chromatin-associated changes at and near sites of replication stress, suggesting defects in epigenome maintenance as a potential outcome of aberrant DNA replication. Here, we will summarize recent insight into replication stress-induced chromatin-reorganization and will speculate on possible adverse effects for gene expression, nuclear integrity and, ultimately, cell function.

Progerin reduces LAP2α-telomere association in Hutchinson-Gilford progeria

Hutchinson-Gilford progeria (HGPS) is a premature ageing syndrome caused by a mutation in LMNA, resulting in a truncated form of lamin A called progerin. Progerin triggers loss of the heterochromatic marker H3K27me3, and premature senescence, which is prevented by telomerase. However, the mechanism how progerin causes disease remains unclear. Here, we describe an inducible cellular system to model HGPS and find that LAP2α (lamina-associated polypeptide-α) interacts with lamin A, while its interaction with progerin is significantly reduced. Super-resolution microscopy revealed that over 50% of telomeres localize to the lamina and that LAP2α association with telomeres is impaired in HGPS. This impaired interaction is central to HGPS since increasing LAP2α levels rescues progerin-induced proliferation defects and loss of H3K27me3, whereas lowering LAP2 levels exacerbates progerin-induced defects. These findings provide novel insights into the pathophysiology underlying HGPS, and how the nuclear lamina regulates proliferation and chromatin organization.

DOI:http://dx.doi.org/10.7554/eLife.07759.001

Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare genetic disease in which individuals age prematurely. Newborns appear normal at birth, but start ageing rapidly when they are around a year old. Symptoms of the disease include stunted growth and joint stiffness, and individuals often die of heart failure during their teens.

A mutated version of a protein called lamin A causes HGPS; this mutant is known as progerin. In cells that produce progerin, the ‘telomeres’ that protect the ends of chromosomes (the structures that contain most of the cell's DNA) from damage, are unusually short. Every time a cell divides, the telomeres get shorter. If they get too short, the DNA is damaged and the cell stops dividing and enters a state known as senescence.

HGPS affects some of the tissues in the body more severely than others, and these tissues tend to produce high levels of progerin. By gradually raising the levels of progerin in human cells, Chojnowski et al. found that DNA damage and cell senescence only occur when the amount of progerin in a cell exceeds a particular threshold. Moreover, the expression of telomerase—a complex that can elongate telomeres—prevented progerin-induced DNA damage and premature senescence.

To find out how progerin affects cells, Chojnowski et al. compared how lamin A and progerin interact with other proteins. This revealed that progerin interacts with a protein called LAP2α more weakly than lamin A. LAP2α normally associates with telomeres, but using super-high resolution microscopy, Chojnowski et al. observed that this association is less likely to occur in the cells of people with HGPS. Importantly, increasing the amount of LAP2α in progerin-expressing cells prevented DNA damage and senescence and enabled these cells to continue dividing.

Chojnowski et al. propose that in HGPS, the weak interaction between LAP2α and progerin disrupts how LAP2α interacts with telomeres, which prevents cells from dividing. Understanding this process may help to design new ways of treating HGPS, and may also help us to understand other diseases that are caused by mutations in lamin proteins.

DOI:http://dx.doi.org/10.7554/eLife.07759.002

Diverse lamin-dependent mechanisms interact to control chromatin dynamics. Focus on laminopathies

Interconnected functional strategies govern chromatin dynamics in eukaryotic cells. In this context, A and B type lamins, the nuclear intermediate filaments, act on diverse platforms involved in tissue homeostasis. On the nuclear side, lamins elicit large scale or fine chromatin conformational changes, affect DNA damage response factors and transcription factor shuttling. On the cytoplasmic side, bridging-molecules, the LINC complex, associate with lamins to coordinate chromatin dynamics with cytoskeleton and extra-cellular signals.

 

Consistent with such a fine tuning, lamin mutations and/or defects in their expression or post-translational processing, as well as mutations in lamin partner genes, cause a heterogeneous group of diseases known as laminopathies. They include muscular dystrophies, cardiomyopathy, lipodystrophies, neuropathies, and progeroid syndromes. The study of chromatin dynamics under pathological conditions, which is summarized in this review, is shedding light on the complex and fascinating role of the nuclear lamina in chromatin regulation.

Forging a signature of in vivo senescence

'Cellular senescence', a term originally defining the characteristics of cultured cells that exceed their replicative limit, has been broadened to describe durable states of proliferative arrest induced by disparate stress factors. Proposed relationships between cellular senescence, tumour suppression, loss of tissue regenerative capacity and ageing suffer from lack of uniform definition and consistently applied criteria. Here, we highlight caveats in interpreting the importance of suboptimal senescence-associated biomarkers, expressed either alone or in combination. We advocate that more-specific descriptors be substituted for the now broadly applied umbrella term 'senescence' in defining the suite of diverse physiological responses to cellular stress.

Unfolding the story of chromatin organization in senescent cells

Cell senescence, the permanent withdrawal of a cell from the cell cycle, is characterized by dramatic, cytological scale changes to DNA condensation throughout the genome. While prior emphasis has been placed on increases in heterochromatin, such as the formation of compact Senescent Associated Heterochromatin Foci (SAHF) structures, our recent findings showed that SAHF formation is preceded by the unravelling of constitutive heterochromatin into visibly extended structures, which we have termed Senescent Associated Distension of Satellites or SADS. Interestingly, neither of these marked changes in DNA condensation appear to be mediated by changes in canonical, heterochromatin-associated histone modifications. Rather, several observations suggest that these events may be facilitated by changes in LaminB1 levels and/or other factors that control higher-order chromatin architecture. Here, we review what is known about senescence-associated chromatin reorganization and present preliminary results using high-resolution microscopy techniques to show that each peri/centromeric satellite in senescent cells is comprised of several condensed domains connected by thin fibrils of satellite DNA. We then discuss the potential importance of these striking changes in chromatin condensation for cell senescence, and also as a model to provide a needed window into the higher-order packaging of the genome.

Relationship between Obesity, Gut Microbiome and Hepatocellular Carcinoma Development

During the past several decades, the percentage of excess bodyweight and obese adults and children has increased dramatically, and is becoming one of the most serious public health problems worldwide. Extensive epidemiological studies have revealed that there is a strong link between obesity and some common cancers. However, the exact molecular mechanisms linking obesity and cancer are not fully understood yet. Recently, we have reported that dietary or genetic obesity provokes alterations of gut microbiota profile, thereby increasing the levels of deoxycholic acid (DCA), a secondary bile acid produced solely by the 7α-dehydroxylation of primary bile acids carried out by gut bacteria. The enterohepatic circulation of DCA provokes DNA damage and consequent cellular senescence in hepatic stellate cells (HSCs) which, in turn, secrete various inflammatory and tumor-promoting factors in the liver, thus facilitating hepatocellular carcinoma (HCC) development in mice. Interestingly, signs of senescence-associated secretory phenotypes were also observed in the HSCs in the area of HCC arising in patients with nonalcoholic steatohepatitis, implying that a similar pathway is likely to contribute to at least certain aspects of obesity-associated HCC development in humans as well. In this review, I will provide an overview of our recent work and discuss the next steps, focusing on the potential clinical implications of our findings.

Senescence-associated inflammatory responses: aging and cancer perspectives

Senescent cells, albeit not proliferating, are metabolically and transcriptionally active, thereby capable of affecting their microenvironment, notably via the production of inflammatory mediators. These mediators maintain and propagate the senescence process to neighboring cells, and then recruit immune cells for clearing senescent cells. Among the inflammatory cues are molecules with pronounced tumor-controlling properties, both growth and invasion factors and inhibitory factors, working directly or via recruited immune cells. These senescence-inflammatory effects also prevail within tumors, mediated by the senescent tumor cells and the senescent tumor stroma. Here, we review the course and impact of senescence-associated inflammatory responses in aging and cancer. We propose that controlling senescence-associated inflammation by targeting specific inflammatory mediators may have a beneficial therapeutic effect in treatment of cancer and aging-related diseases.

Inhibition of vascular endothelial growth factor A and hypoxia-inducible factor 1α maximizes the effects of radiation in sarcoma mouse models through destruction of tumor vasculature

PURPOSE

To examine the addition of genetic or pharmacologic inhibition of hypoxia-inducible factor 1α (HIF-1α) to radiation therapy (RT) and vascular endothelial growth factor A (VEGF-A) inhibition (ie trimodality therapy) for soft-tissue sarcoma.

METHODS AND MATERIALS

Hypoxia-inducible factor 1α was inhibited using short hairpin RNA or low metronomic doses of doxorubicin, which blocks HIF-1α binding to DNA. Trimodality therapy was examined in a mouse xenograft model and a genetically engineered mouse model of sarcoma, as well as in vitro in tumor endothelial cells (ECs) and 4 sarcoma cell lines.

RESULTS

In both mouse models, any monotherapy or bimodality therapy resulted in tumor growth beyond 250 mm(3) within the 12-day treatment period, but trimodality therapy with RT, VEGF-A inhibition, and HIF-1α inhibition kept tumors at <250 mm(3) for up to 30 days. Trimodality therapy on tumors reduced HIF-1α activity as measured by expression of nuclear HIF-1α by 87% to 95% compared with RT alone, and cytoplasmic carbonic anhydrase 9 by 79% to 82%. Trimodality therapy also increased EC-specific apoptosis 2- to 4-fold more than RT alone and reduced microvessel density by 75% to 82%. When tumor ECs were treated in vitro with trimodality therapy under hypoxia, there were significant decreases in proliferation and colony formation and increases in DNA damage (as measured by Comet assay and γH2AX expression) and apoptosis (as measured by cleaved caspase 3 expression). Trimodality therapy had much less pronounced effects when 4 sarcoma cell lines were examined in these same assays.

CONCLUSIONS

Inhibition of HIF-1α is highly effective when combined with RT and VEGF-A inhibition in blocking sarcoma growth by maximizing DNA damage and apoptosis in tumor ECs, leading to loss of tumor vasculature.

Global reorganization of the nuclear landscape in senescent cells

Cellular senescence has been implicated in tumor suppression, development, and aging and is accompanied by large-scale chromatin rearrangements, forming senescence-associated heterochromatic foci (SAHF). However, how the chromatin is reorganized during SAHF formation is poorly understood. Furthermore, heterochromatin formation in senescence appears to contrast with loss of heterochromatin in Hutchinson-Gilford progeria. We mapped architectural changes in genome organization in cellular senescence using Hi-C. Unexpectedly, we find a dramatic sequence- and lamin-dependent loss of local interactions in heterochromatin. This change in local connectivity resolves the paradox of opposing chromatin changes in senescence and progeria. In addition, we observe a senescence-specific spatial clustering of heterochromatic regions, suggesting a unique second step required for SAHF formation. Comparison of embryonic stem cells (ESCs), somatic cells, and senescent cells shows a unidirectional loss in local chromatin connectivity, suggesting that senescence is an endpoint of the continuous nuclear remodelling process during differentiation.

Selective class IIa HDAC inhibitors: myth or reality

The prospect of intervening, through the use of a specific molecule, with a cellular alteration responsible for a disease, is a fundamental ambition of biomedical science. Epigenetic-based therapies appear as a remarkable opportunity to impact on several disorders, including cancer. Many efforts have been made to develop small molecules acting as inhibitors of histone deacetylases (HDACs). These enzymes are key targets to reset altered genetic programs and thus to restore normal cellular activities, including drug responsiveness. Several classes of HDAC inhibitors (HDACis) have been generated, characterized and, in certain cases, approved for the use in clinic. A new frontier is the generation of subtype-specific inhibitors, to increase selectivity and to manage general toxicity. Here we will discuss about a set of molecules, which can interfere with the activity of a specific subclass of HDACs: the class IIa.

Carbon-ion beam irradiation kills X-ray-resistant p53-null cancer cells by inducing mitotic catastrophe

Background and Purpose

To understand the mechanisms involved in the strong killing effect of carbon-ion beam irradiation on cancer cells with TP53 tumor suppressor gene deficiencies.

Materials and Methods

DNA damage responses after carbon-ion beam or X-ray irradiation in isogenic HCT116 colorectal cancer cell lines with and without TP53 (p53^+/+ and p53^-/-, respectively) were analyzed as follows: cell survival by clonogenic assay, cell death modes by morphologic observation of DAPI-stained nuclei, DNA double-strand breaks (DSBs) by immunostaining of phosphorylated H2AX (γH2AX), and cell cycle by flow cytometry and immunostaining of Ser10-phosphorylated histone H3.

Results

The p53^-/- cells were more resistant than the p53^+/+ cells to X-ray irradiation, while the sensitivities of the p53^+/+ and p53^-/- cells to carbon-ion beam irradiation were comparable. X-ray and carbon-ion beam irradiations predominantly induced apoptosis of the p53^+/+ cells but not the p53^-/- cells. In the p53^-/- cells, carbon-ion beam irradiation, but not X-ray irradiation, markedly induced mitotic catastrophe that was associated with premature mitotic entry with harboring long-retained DSBs at 24 h post-irradiation.

Conclusions

Efficient induction of mitotic catastrophe in apoptosis-resistant p53-deficient cells implies a strong cancer cell-killing effect of carbon-ion beam irradiation that is independent of the p53 status, suggesting its biological advantage over X-ray treatment.

HIRA orchestrates a dynamic chromatin landscape in senescence and is required for suppression of neoplasia

Cellular senescence is a stable proliferation arrest that suppresses tumorigenesis. Cellular senescence and associated tumor suppression depend on control of chromatin. Histone chaperone HIRA deposits variant histone H3.3 and histone H4 into chromatin in a DNA replication-independent manner. Appropriately for a DNA replication-independent chaperone, HIRA is involved in control of chromatin in nonproliferating senescent cells, although its role is poorly defined. Here, we show that nonproliferating senescent cells express and incorporate histone H3.3 and other canonical core histones into a dynamic chromatin landscape. Expression of canonical histones is linked to alternative mRNA splicing to eliminate signals that confer mRNA instability in nonproliferating cells. Deposition of newly synthesized histones H3.3 and H4 into chromatin of senescent cells depends on HIRA. HIRA and newly deposited H3.3 colocalize at promoters of expressed genes, partially redistributing between proliferating and senescent cells to parallel changes in expression. In senescent cells, but not proliferating cells, promoters of active genes are exceptionally enriched in H4K16ac, and HIRA is required for retention of H4K16ac. HIRA is also required for retention of H4K16ac in vivo and suppression of oncogene-induced neoplasia. These results show that HIRA controls a specialized, dynamic H4K16ac-decorated chromatin landscape in senescent cells and enforces tumor suppression.

Towards selective inhibition of histone deacetylase isoforms: what has been achieved, where we are and what will be next

Histone deacetylases (HDACs) are widely studied targets for the treatment of cancer and other diseases. Up to now, over twenty HDAC inhibitors have entered clinical studies and two of them have already reached the market, namely the hydroxamic acid derivative SAHA (vorinostat, Zolinza) and the cyclic depsipeptide FK228 (romidepsin, Istodax) that have been approved for the treatment of cutaneous T-cell lymphoma (CTCL). A common aspect of the first HDAC inhibitors is the absence of any particular selectivity towards specific isozymes. Some of molecules resulted to be “pan”-HDAC inhibitors, while others are class I selective. In the meantime, the knowledge of HDAC biology has continuously progressed. Key advances in the structural biology of various isozymes, reliable molecular homology models as well as suitable biological assays have provided new tools for drug discovery activities. This Minireview aims at surveying these recent developments as well as the design, synthesis and biological characterization of isoform-selective derivatives.

Stress induced premature senescence: a new culprit in ovarian tumorigenesis?

Stress induced premature senescence (SIPS) is a relative extension to the concept of exogenous cellular insult. Besides persistent double strand (ds) DNA breaks and increased β-galactosidase activity, biological significance of telomeric attrition in conjunction with senescence associated secretory phenotype (SASP) has been highlighted in SIPS. To gain insight on the potential role of this unique phenomenon invoked upon environmental stress, we sequentially validated the molecular repercussions of this event in ovarian epithelial cells after exposure to methyl isocyanate, an elegant regulator of cellular biotransformation. Persistent accumulation of DNA damage response factors phospho-ATM/γ-H2AX, morphological changes with increased cell size and early yet incremental β-gal staining, imply the inception of premature senescence. Advent of SASP is attributed by prolonged secretion of pro-inflammatory cytokines along with untimely but significant G1/S cell cycle arrest. Telomeric dysfunction associated with premature senescence is indicative of early loss of TRF2 (telomeric repeat binding factor 2) protein and resultant multiple translocations. Induction of senescence-associated heterochromatic foci formation showcases the chromatin alterations in form of trimethylated H3K9me3 in conjunction with H4 hypoacetylation and altered miRNA expression. Anchorage-independent neoplastic growth observed in treated cells reaffirms the oncogenic transformation following the exposure. Collectively, we infer the possible role of SIPS, as a central phenomenon, to perturbed genomic integrity in ovarian surface epithelium, orchestrated through SASP and chromatin level alterations, a hitherto unknown molecular paradigm. Although translational utility of SIPS as a biomarker for estimating ovarian cancer risk seems evident, further investigations will be imperative to provide a tangible way for its precise validation in clinical settings.

Stable cellular senescence is associated with persistent DDR activation

The DNA damage response (DDR) is activated upon DNA damage generation to promote DNA repair and inhibit cell cycle progression in the presence of a lesion. Cellular senescence is a permanent cell cycle arrest characterized by persistent DDR activation. However, some reports suggest that DDR activation is a feature only of early cellular senescence that is then lost with time. This challenges the hypothesis that cellular senescence is caused by persistent DDR activation. To address this issue, we studied DDR activation dynamics in senescent cells. Here we show that normal human fibroblasts retain DDR markers months after replicative senescence establishment. Consistently, human fibroblasts from healthy aged donors display markers of DDR activation even three years in culture after entry into replicative cellular senescence. However, by extending our analyses to different human cell strains, we also observed an apparent DDR loss with time following entry into cellular senescence. This though correlates with the inability of these cell strains to survive in culture upon replicative or irradiation-induced cellular senescence. We propose a model to reconcile these results. Cell strains not suffering the prolonged in vitro culture stress retain robust DDR activation that persists for years, indicating that under physiological conditions persistent DDR is causally involved in senescence establishment and maintenance. However, cell strains unable to maintain cell viability in vitro, due to their inability to cope with prolonged cell culture-associated stress, show an only-apparent reduction in DDR foci which is in fact due to selective loss of the most damaged cells.

Histone deacetylases inhibition by SAHA/Vorinostat normalizes the glioma microenvironment via xCT equilibration

Malignant gliomas are characterized by neurodegenerative actions leading to the destruction of surrounding brain parenchyma. The disturbance in glutamate homeostasis caused by increased expression of the glutamate transporter xCT plays a key role in glioma progression. We demonstrate that the HDAC-inhibitor SAHA specifically inhibits the xCT-transporter expression. Thereby, tumor cell stress is engendered, marked by increase in ROS. Moreover, SAHA dependent xCT-reduction correlates with the inhibition of ATF4-expression, a factor known to foster xCT expression. Since xCT/system Xc- is pivotal for the brain tumor microenvironment, normalization of this system is a key in the management of malignant gliomas. To date, the problem lay in the inability to specifically target xCT due to the ubiquitous expression of the xCT-transporter—i.e. in non-cancerously transformed cells too—as well as its essential role in physiological CNS processes. Here, we show xCT-transporter equilibration through SAHA is specific for malignant brain tumors whereas SAHA does not affect the physiological xCT levels in healthy brain parenchyma. Our data indicate that SAHA operates on gliomas specifically via normalizing xCT expression which in consequence leads to reduced extracellular glutamate levels. This in turn causes a marked reduction in neuronal cell death and normalized tumor microenvironment.

DNA double strand break responses and chromatin alterations within the aging cell

Cellular senescence is a state of permanent replicative arrest that allows cells to stay viable and metabolically active but resistant to apoptotic and mitogenic stimuli. Specific, validated markers can identify senescent cells, including senescence-associated β galactosidase activity, chromatin alterations, cell morphology changes, activated p16- and p53-dependent signaling and permanent cell cycle arrest. Senescence is a natural consequence of DNA replication-associated telomere erosion, but can also be induced prematurely by telomere-independent events such as failure to repair DNA double strand breaks. Here, we review the molecular pathways of senescence onset, focussing on the changes in chromatin organization that are associated with cellular senescence, particularly senescence-associated heterochromatin foci formation. We also discuss the altered dynamics of the DNA double strand break response within the context of aging cells. Appreciating how, mechanistically, cellular senescence is induced, and how changes to chromatin organization and DNA repair contributes to this, is fundamental to our understanding of the normal and premature human aging processes associated with loss of organ and tissue function in humans.

Assembly of telomeric chromatin to create ALTernative endings

Circumvention of the telomere length-dependent mechanisms that control the upper boundaries of cellular proliferation is necessary for the unlimited growth of cancer. Most cancer cells achieve cellular immortality by up-regulating the expression of telomerase to extend and maintain their telomere length. However, a small but significant number of cancers do so via the exchange of telomeric DNA between chromosomes in a pathway termed alternative lengthening of telomeres, or ALT. Although it remains to be clarified why a cell chooses the ALT pathway and how ALT is initiated, recently identified mutations in factors that shape the chromatin and epigenetic landscape of ALT telomeres are shedding light on these mechanisms. In this review, we examine these recent findings and integrate them into the current models of the ALT mechanism.

TRIM28/KAP1 regulates senescence

Senescence is a highly stable cell cycle arrest which limits the replication of cells with damaged genomes. The senescence program is activated during aging or in response to insults like DNA damage or oncogenic signaling. Upon induction of senescence, cells undergo profound changes on their transcription program, chromatin organization, and they secrete a complex mixture of mainly pro-inflammatory components termed the senescence-associated secretory phenotype (SASP). The SASP mediates multiple effects, including reinforcing senescence and activating immune surveillance responses. Given the important role that senescence has in aging, cancer and other pathologies, identifying mechanisms regulating senescence has therapeutic potential. Here we describe a role for TRIM28 (also known as KRAB-associated protein 1, KAP1) on mediating oncogene-induced senescence (OIS). TRIM28 accumulates during OIS becoming phosphorylated on serine 824. To investigate the role of TRIM28, we knocked down its expression and observed that the depletion of TRIM28 partially prevented cell arrest during OIS. While induction of p53 and p21 during OIS, was not affected by TRIM28 depletion, p16(INK4a) induction was partially prevented. Finally, we observed that the induction of IL8, IL6 and other SASP components were strongly suppressed upon TRIM28 depletion. In conclusion, the above-described results show that TRIM28 regulates senescence and affects the induction of the senescence-associated secretory phenotype.

Cellular senescence: from physiology to pathology

Recent discoveries are redefining our view of cellular senescence as a trigger of tissue remodelling that acts during normal embryonic development and upon tissue damage. To achieve this, senescent cells arrest their own proliferation, recruit phagocytic immune cells and promote tissue renewal. This sequence of events - senescence, followed by clearance and then regeneration - may not be efficiently completed in aged tissues or in pathological contexts, thereby resulting in the accumulation of senescent cells. Increasing evidence indicates that both pro-senescent therapies and antisenescent therapies can be beneficial. In cancer and during active tissue repair, pro-senescent therapies contribute to minimize the damage by limiting proliferation and fibrosis, respectively. Conversely, antisenescent therapies may help to eliminate accumulated senescent cells and to recover tissue function.

ATM activation in hypoxia - causes and consequences

The DNA damage response is a complex signaling cascade that is triggered by cellular stress. This response is essential for the maintenance of genomic integrity and is considered to act as a barrier to the early stages of tumorigenesis. The integral role of ataxia telangiectasia mutated (ATM) kinase in the response to DNA damaging agents is well characterized; however, ATM can also be activated by non-DNA damaging agents. In fact, much has been learnt recently about the mechanism of ATM activation in response to physiologic stresses such as hypoxia that do not induce DNA damage. Regions of low oxygen concentrations that occur in solid tumors are associated with a poor prognostic outcome irrespective of treatment modality. Severe levels of hypoxia induce replication stress and trigger the activation of DNA damage response pathways including ataxia telangiectasia and Rad3-related (ATR)- and ATM-mediated signaling. In this review, we discuss hypoxia-driven ATM signaling and the possible contribution of ATM activation in this context to tumorigenesis.

Immortalised breast epithelia survive prolonged DNA replication stress and return to cycle from a senescent-like state

Mammalian cells have mechanisms to counteract the effects of metabolic and exogenous stresses, many of that would be mutagenic if ignored. Damage arising during DNA replication is a major source of mutagenesis. The extent of damage dictates whether cells undergo transient cell cycle arrest and damage repair, senescence or apoptosis. Existing dogma defines these alternative fates as distinct choices. Here we show that immortalised breast epithelial cells are able to survive prolonged S phase arrest and subsequently re-enter cycle after many days of being in an arrested, senescence-like state. Prolonged cell cycle inhibition in fibroblasts induced DNA damage response and cell death. However, in immortalised breast epithelia, efficient S phase arrest minimised chromosome damage and protected sufficient chromatin-bound replication licensing complexes to allow cell cycle re-entry. We propose that our observation could have implications for the design of drug therapies for breast cancer.

Metabolic alterations accompanying oncogene-induced senescence

Senescence is defined as a stable cell growth arrest. Oncogene-induced senescence (OIS) occurs in normal primary human cells after activation of an oncogene in the absence of other cooperating oncogenic stimuli. OIS is therefore considered a bona fide tumor suppression mechanism in vivo. Indeed, overcoming OIS-associated stable cell growth arrest can lead to tumorigenesis. Although cells that have undergone OIS do not replicate their DNA, they remain metabolically active. A number of recent studies report significant changes in cellular metabolism during OIS, including alterations in nucleotide, glucose, and mitochondrial metabolism and autophagy. These alterations may be necessary for stable senescence-associated cell growth arrest, and overcoming these shifts in metabolism may lead to tumorigenesis. This review highlights what is currently known about alterations in cellular metabolism during OIS and the implication of OIS-associated metabolic changes in cellular transformation and the development of cancer therapeutic strategies.

ATM signalling and cancer

ATM, the protein kinase mutated in the rare human disease ataxia telangiectasia (A-T), has been the focus of intense scrutiny over the past two decades. Initially this was because of the unusual radiosensitive phenotype of cells from A-T patients, and latterly because investigating ATM signalling has yielded valuable insights into the DNA damage response, redox signalling and cancer. With the recent explosion in genomic data, ATM alterations have been revealed both in the germline as a predisposing factor for cancer and as somatic changes in tumours themselves. Here we review these findings, as well as advances in the understanding of ATM signalling mechanisms in cancer and ATM inhibition as a strategy for cancer treatment.

ATM regulates insulin-like growth factor 1-secretory clusterin (IGF-1-sCLU) expression that protects cells against senescence

Downstream factors that regulate the decision between senescence and cell death have not been elucidated. Cells undergo senescence through three pathways, replicative senescence (RS), stress-induced premature senescence (SIPS) and oncogene-induced senescence. Recent studies suggest that the ataxia telangiectasia mutant (ATM) kinase is not only a key protein mediating cellular responses to DNA damage, but also regulates cellular senescence induced by telomere end exposure (in RS) or persistent DNA damage (in SIPS). Here, we show that expression of secretory clusterin (sCLU), a known pro-survival extracellular chaperone, is transcriptionally up-regulated during both RS and SIPS, but not in oncogene-induced senescence, consistent with a DNA damage-inducible mechanism. We demonstrate that ATM plays an important role in insulin-like growth factor 1 (IGF-1) expression, that in turn, regulates downstream sCLU induction during senescence. Loss of ATM activity, either by genomic mutation (ATM-deficient fibroblasts from an ataxia telangiectasia patient) or by administration of a chemical inhibitor (AAI, an inhibitor of ATM and ATR), blocks IGF-1-sCLU expression in senescent cells. Downstream, sCLU induction during senescence is mediated by IGF-1R/MAPK/Egr-1 signaling, identical to its induction after DNA damage. In contrast, administration of an IGF-1 inhibitor caused apoptosis of senescent cells. Thus, IGF-1 signaling is required for survival, whereas sCLU appears to protect cells from premature senescence, as IMR-90 cells with sCLU knockdown undergo senescence faster than control cells. Thus, the ATM-IGF-1-sCLU pathway protects cells from lethality and suspends senescence.

Chromatin maintenance and dynamics in senescence: a spotlight on SAHF formation and the epigenome of senescent cells

Senescence is a stable proliferation arrest characterized by profound changes in cellular morphology and metabolism as well as by extensive chromatin reorganization in the nucleus. One particular hallmark of chromatin changes during senescence is the formation of punctate DNA foci in DAPI-stained senescent cells that have been called senescence-associated heterochromatin foci (SAHF). While many advances have been made concerning our understanding of the effectors of senescence, how chromatin is reorganized and maintained in senescent cells has remained largely elusive. Because chromatin structure is inherently dynamic, senescent cells face the challenge of developing chromatin maintenance mechanisms in the absence of DNA replication in order to maintain the senescent phenotype. Here, we summarize and review recent findings shedding light on SAHF composition and formation via spatial repositioning of chromatin, with a specific focus on the role of lamin B1 for this process. In addition, we discuss the physiological implication of SAHF formation, the role of histone variants, and histone chaperones during senescence and also elaborate on the more general changes observed in the epigenome of the senescent cells.

Inhibition of histone deacetylases enhances DNA damage repair in SCNT embryos

Recent studies have shown that DNA damage affects embryo development and also somatic cell reprogramming into induced pluripotent stem (iPS) cells. It has been also shown that treatment with histone deacetylase inhibitors (HDACi) improves development of embryos produced by somatic cell nuclear transfer (SCNT) and enhances somatic cell reprogramming. There is evidence that increasing histone acetylation at the sites of DNA double-strand breaks (DSBs) is critical for DNA damage repair. Therefore, we hypothesized that HDACi treatment enhances cell programming and embryo development by facilitating DNA damage repair. To test this hypothesis, we first established a DNA damage model wherein exposure of nuclear donor cells to ultraviolet (UV) light prior to nuclear transfer reduced the development of SCNT embryos proportional to the length of UV exposure. Detection of phosphorylated histone H2A.x (H2AX139ph) foci confirmed that exposure of nuclear donor cells to UV light for 10 s was sufficient to increase DSBs in SCNT embryos. Treatment with HDACi during embryo culture increased development and reduced DSBs in SCNT embryos produced from UV-treated cells. Transcript abundance of genes involved in either the homologous recombination (HR) or nonhomologous end-joining (NHEJ) pathways for DSBs repair was reduced by HDACi treatment in developing embryos at day 5 after SCNT. Interestingly, expression of HR and NHEJ genes was similar between HDACi-treated and control SCNT embryos that developed to the blastocyst stage. This suggested that the increased number of embryos that could achieve the blastocyst stage in response to HDACi treatment have repaired DNA damage. These results demonstrate that DNA damage in nuclear donor cells is an important component affecting development of SCNT embryos, and that HDACi treatment after nuclear transfer enhances DSBs repair and development of SCNT embryos.

The causes of replication stress and their consequences on genome stability and cell fate

Alterations of the dynamics of DNA replication cause genome instability. These alterations known as "replication stress" have emerged as a major source of genomic instability in pre-neoplasic lesions, contributing to cancer development. The concept of replication stress covers a wide variety of events that distort the temporal and spatial DNA replication program. These events have endogenous or exogenous origins and impact globally or locally on the dynamics of DNA replication. They may arise within a short window of time (acute stress) or during each S phase (chronic stress). Here, we review the known situations in which the dynamics of DNA replication is distorted. We have united them in four main categories: (i) inadequate firing of replication origins (deficiency or excess), (ii) obstacles to fork progression, (iii) conflicts between replication and transcription and (iv) DNA replication under inappropriate metabolic conditions (unbalanced DNA replication). Because the DNA replication program is a process tightly regulated by many factors, replication stress often appears as a cascade of events. A local stress may prevent the completion of DNA replication at a single locus and subsequently compromise chromosome segregation in mitosis and therefore have a global effect on genome integrity. Finally, we discuss how replication stress drives genome instability and to what extent it is relevant to cancer biology.

Tumor-specific activation of the C-JUN/MELK pathway regulates glioma stem cell growth in a p53-dependent manner

Accumulated evidence suggests that glioma stem cells (GSCs) may contribute to therapy resistance in high-grade glioma (HGG). Although recent studies have shown that the serine/threonine kinase maternal embryonic leucine-zipper kinase (MELK) is abundantly expressed in various cancers, the function and mechanism of MELK remain elusive. Here, we demonstrate that MELK depletion by shRNA diminishes the growth of GSC-derived mouse intracranial tumors in vivo, induces glial fibrillary acidic protein (+) glial differentiation of GSCs leading to decreased malignancy of the resulting tumors, and prolongs survival periods of tumor-bearing mice. Tissue microarray analysis with 91 HGG tumors demonstrates that the proportion of MELK (+) cells is a statistically significant indicator of postsurgical survival periods. Mechanistically, MELK is regulated by the c-Jun NH(2)-terminal kinase (JNK) signaling and forms a complex with the oncoprotein c-JUN in GSCs but not in normal progenitors. MELK silencing induces p53 expression, whereas p53 inhibition induces MELK expression, indicating that MELK and p53 expression are mutually exclusive. Additionally, MELK silencing-mediated GSC apoptosis is partially rescued by both pharmacological p53 inhibition and p53 gene silencing, indicating that MELK action in GSCs is p53 dependent. Furthermore, irradiation of GSCs markedly elevates MELK mRNA and protein expression both in vitro and in vivo. Clinically, recurrent HGG tumors following the failure of radiation and chemotherapy exhibit a statistically significant elevation of MELK protein compared with untreated newly diagnosed HGG tumors. Together, our data indicate that GSCs, but not normal cells, depend on JNK-driven MELK/c-JUN signaling to regulate their survival, maintain GSCs in an immature state, and facilitate tumor radioresistance in a p53-dependent manner.

Higher-order unfolding of satellite heterochromatin is a consistent and early event in cell senescence

Higher-order unfolding of peri/centromeric satellite DNA is a consistent and early event in senescence of cultured normal human and mouse cells, progeria cells, and a senescent tumor.

Epigenetic changes to chromatin are thought to be essential to cell senescence, which is key to tumorigenesis and aging. Although many studies focus on heterochromatin gain, this work demonstrates large-scale unraveling of peri/centromeric satellites, which occurs in all models of human and mouse senescence examined. This was not seen in cancer cells, except in a benign senescent tumor in vivo. Senescence-associated distension of satellites (SADS) occurs earlier and more consistently than heterochromatin foci formation, and SADS is not exclusive to either the p16 or p21 pathways. Because Hutchinson Guilford progeria syndrome patient cells do not form excess heterochromatin, the question remained whether or not proliferative arrest in this aging syndrome involved distinct epigenetic mechanisms. Here, we show that SADS provides a unifying event in both progeria and normal senescence. Additionally, SADS represents a novel, cytological-scale unfolding of chromatin, which is not concomitant with change to several canonical histone marks nor a result of DNA hypomethylation. Rather, SADS is likely mediated by changes to higher-order nuclear structural proteins, such as LaminB1.

Acute dyskerin depletion triggers cellular senescence and renders osteosarcoma cells resistant to genotoxic stress-induced apoptosis

Dyskerin is a conserved, nucleolar RNA-binding protein implicated in an increasing array of fundamental cellular processes. Germline mutation in the dyskerin gene (DKC1) is the cause of X-linked dyskeratosis congenita (DC). Conversely, wild-type dyskerin is overexpressed in sporadic cancers, and high-levels may be associated with poor prognosis. It was previously reported that acute loss of dyskerin function via siRNA-mediated depletion slowed the proliferation of transformed cell lines. However, the mechanisms remained unclear. Using human U2OS osteosarcoma cells, we show that siRNA-mediated dyskerin depletion induced cellular senescence as evidenced by proliferative arrest, senescence-associated heterochromatinization and a senescence-associated molecular profile. Senescence can render cells resistant to apoptosis. Conversely, chromatin relaxation can reverse the repressive effects of senescence-associated heterochromatinization on apoptosis. To this end, genotoxic stress-induced apoptosis was suppressed in dyskerin-depleted cells. In contrast, agents that induce chromatin relaxation, including histone deacetylase inhibitors and the DNA intercalator chloroquine, sensitized dyskerin-depleted cells to apoptosis. Dyskerin is a core component of the telomerase complex and plays an important role in telomere homeostasis. Defective telomere maintenance resulting in premature senescence is thought to primarily underlie the pathogenesis of X-linked DC. Since U2OS cells are telomerase-negative, this leads us to conclude that loss of dyskerin function can also induce cellular senescence via mechanisms independent of telomere shortening.

p19INK4d is involved in the cellular senescence mechanism contributing to heterochromatin formation

BACKGROUND

During evolution, organisms with renewable tissues have developed mechanisms to prevent tumorigenesis, including cellular senescence and apoptosis. Cellular senescence is characterized by a permanent cell cycle arrest triggered by both endogenous stress and exogenous stress. The p19INK4d, a member of the family of cyclin-dependent kinase inhibitors (INK4), plays an important role on cell cycle regulation and in the cellular DNA damage response. We hypothesize that p19INK4d is a potential factor involved in the onset and/or maintenance of the senescent state.

METHODS

Senescence was confirmed by measuring the cell cycle arrest and the senescence-associated β-galactosidase activity. Changes in p19INK4d expression and localization during senescence were determined by Western blot and immunofluorescence assays. Chromatin condensation was measured by microccocal nuclease digestion and histone salt extraction.

RESULTS

The data presented here show for the first time that p19INK4d expression is up-regulated by different types of senescence. Changes in senescence-associated hallmarks were driven by modulation of p19 expression indicating a direct link between p19INK4d induction and the establishment of cellular senescence. Following a senescence stimulus, p19INK4d translocates to the nucleus and tightly associates with chromatin. Moreover, reduced levels of p19INK4d impair senescence-related global genomic heterochromatinization. Analysis of p19INK4d mRNA and protein levels in tissues from differently aged mice revealed an up-regulation of p19INK4d that correlates with age.

CONCLUSION

We propose that p19INK4d participates in the cellular mechanisms that trigger senescence by contributing to chromatin compaction.

GENERAL SIGNIFICANCE

This study provides novel insights into the dynamics process of cellular senescence, a central tumor suppressive mechanism.

Obesity and cancer: a gut microbial connection

Multiple epidemiological studies have revealed that excess bodyweight, such as in people who are overweight or obese (defined by a body mass index higher than 25 kg/m(2)), is a major risk factor for not only diabetes and cardiovascular diseases but also cancer. Effective strategies for obesity prevention are therefore needed for cancer prevention. However, because the prevalence of excess bodyweight in most developed countries has been increasing markedly over the past several decades, with no signs of abating, alternative approaches are also required to conquer obesity-associated cancer. Therefore, we sought to understand the molecular mechanisms underlying obesity-associated cancer. Although several phenomena have been proposed to explain how obesity increases cancer risk, the exact molecular mechanisms that integrate these phenomena have remained largely obscure. Recently, we have traced the association between obesity and increased cancer risk to gut microbiota communities that produce a DNA-damaging bile acid. The analyses also revealed the role of cellular senescence in cancer, which we have been studying for the past few decades. In this review, we provide an overview of our work and discuss the next steps, focusing on the potential clinical implications of these findings.

UHRF1 overexpression drives DNA hypomethylation and hepatocellular carcinoma

Ubiquitin-like with PHD and RING finger domains 1 (UHRF1) is an essential regulator of DNA methylation that is highly expressed in many cancers. Here, we use transgenic zebrafish, cultured cells, and human tumors to demonstrate that UHRF1 is an oncogene. UHRF1 overexpression in zebrafish hepatocytes destabilizes and delocalizes Dnmt1 and causes DNA hypomethylation and Tp53-mediated senescence. Hepatocellular carcinoma (HCC) emerges when senescence is bypassed. tp53 mutation both alleviates senescence and accelerates tumor onset. Human HCCs recapitulate this paradigm, as UHRF1 overexpression defines a subclass of aggressive HCCs characterized by genomic instability, TP53 mutation, and abrogation of the TP53-mediated senescence program. We propose that UHRF1 overexpression is a mechanism underlying DNA hypomethylation in cancer cells and that senescence is a primary means of restricting tumorigenesis due to epigenetic disruption.

Mitogen-induced B-cell proliferation activates Chk2-dependent G1/S cell cycle arrest

B-cell activation and proliferation can be induced by a variety of extracellular stimuli. The fate of an activated B cell following mitogen stimulation can be dictated by the strength or duration of the signal, the expression of downstream signaling components necessary to promote proliferation, and the cell intrinsic sensors and regulators of the proliferative program. Previously we have identified the DNA damage response (DDR) signaling pathway as a cell intrinsic sensor that is activated upon latent infection of primary human B cells by Epstein-Barr virus (EBV). Here we have assessed the role of the DDR as a limiting factor in the proliferative response to non-viral B-cell mitogens. We report that TLR9 activation through CpG-rich oligonucleotides induced B-cell hyper-proliferation and an ATM/Chk2 downstream signaling pathway. However, B-cell activation through the CD40 pathway coupled with interleukin-4 (IL-4) promoted proliferation less robustly and only a modest DDR. These two mitogens, but not EBV, modestly induced intrinsic apoptosis that was independent from the DDR. However, all three mitogens triggered a DDR-dependent G1/S phase cell cycle arrest preventing B-cell proliferation. The extent of G1/S arrest, as evidenced by release through Chk2 inhibition, correlated with B-cell proliferation rates. These findings have implications for the regulation of extra-follicular B-cell activation as it may pertain to the development of auto-immune diseases or lymphoma.

Cellular senescence and its effector programs

Cellular senescence is a stress response that accompanies stable exit from the cell cycle. Classically, senescence, particularly in human cells, involves the p53 and p16/Rb pathways, and often both of these tumor suppressor pathways need to be abrogated to bypass senescence. In parallel, a number of effector mechanisms of senescence have been identified and characterized. These studies suggest that senescence is a collective phenotype of these multiple effectors, and their intensity and combination can be different depending on triggers and cell types, conferring a complex and diverse nature to senescence. Series of studies on senescence-associated secretory phenotype (SASP) in particular have revealed various layers of functionality of senescent cells in vivo. Here we discuss some key features of senescence effectors and attempt to functionally link them when it is possible.

Detecting senescence: methods and approaches

The detection of senescent cells has become an important area of research in the aging field. Due to the complexity of the senescence program and the lack of a unique signature for senescence, the detection of these cells remains problematic. This is especially true for in vivo detection in aged or diseased tissue samples. This chapter outlines approaches for the detection of senescent cells based upon methods established for mesenchymal cells in culture. A stepwise approach to the detection of senescent cells using multiple techniques is provided.

Dynamics of histone H3.3 deposition in proliferating and senescent cells reveals a DAXX-dependent targeting to PML-NBs important for pericentromeric heterochromatin organization

Oncogene-induced senescence is a permanent cell cycle arrest characterized by extensive chromatin reorganization. Here, we investigated the specific targeting and dynamics of histone H3 variants in human primary senescent cells. We show that newly synthesized epitope-tagged H3.3 is incorporated in senescent cells but does not accumulate in senescence-associated heterochromatin foci (SAHF). Instead, we observe that new H3.3 colocalizes with its specific histone chaperones within the promyelocytic leukemia nuclear bodies (PML-NBs) and is targeted to PML-NBs in a DAXX-dependent manner both in proliferating and senescent cells. We further show that overexpression of DAXX enhances targeting of H3.3 in large PML-NBs devoid of transcriptional activity and promotes the accumulation of HP1, independently of H3K9me3. Loss of H3.3 from pericentromeric heterochromatin upon DAXX or PML depletion suggests that the targeting of H3.3 to PML-NBs is implicated in pericentromeric heterochromatin organization. Together, our results underline the importance of the replication-independent chromatin assembly pathway for histone replacement in non-dividing senescent cells and establish PML-NBs as important regulatory sites for the incorporation of new H3.3 into chromatin.

Lamins are rapamycin targets that impact human longevity: a study in centenarians

The dynamic organisation of the cell nucleus is profoundly modified during growth, development and senescence as a result of changes in chromatin arrangement and gene transcription. A plethora of data suggests that the nuclear lamina is a key player in chromatin dynamics and argues in favour of a major involvement of prelamin A in fundamental mechanisms regulating cellular senescence and organism ageing. As the best model to analyse the role of prelamin A in normal ageing, we used cells from centenarian subjects. We show that prelamin A is accumulated in fibroblasts from centenarians owing to downregulation of its specific endoprotease ZMPSTE24, whereas other nuclear envelope constituents are mostly unaffected and cells do not enter senescence. Accumulation of prelamin A in nuclei of cells from centenarians elicits loss of heterochromatin, as well as recruitment of the inactive form of 53BP1, associated with rapid response to oxidative stress. These effects, including the prelamin-A-mediated increase of nuclear 53BP1, can be reproduced by rapamycin treatment of cells from younger individuals. These data identify prelamin A and 53BP1 as new targets of rapamycin that are associated with human longevity. We propose that the reported mechanisms safeguard healthy ageing in humans through adaptation of the nuclear environment to stress stimuli.

Cold atmospheric plasma, a new strategy to induce senescence in melanoma cells

Over the past few years, the application of cold atmospheric plasma (CAP) in medicine has developed into an innovative field of research of rapidly growing importance. One promising new medical application of CAP is cancer treatment. Different studies revealed that CAP may potentially affect the cell cycle and cause cell apoptosis or necrosis in tumor cells dependent on the CAP device and doses. In this study, we used a novel hand-held and battery-operated CAP device utilizing the surface micro discharge (SMD) technology for plasma production in air and consequently analysed dose-dependent CAP treatment effects on melanoma cells. After 2 min of CAP treatment, we observed irreversible cell inactivation. Phospho-H2AX immunofluorescence staining and Flow cytometric analysis demonstrated that 2 min of CAP treatment induces DNA damage, promotes induction of Sub-G1 phase and strongly increases apoptosis. Further, protein array technology revealed induction of pro-apoptotic events like p53 and Rad17 phosphorylation of Cytochrome c release and activation of Caspase-3. Interestingly, using lower CAP doses with 1 min of treatment, almost no apoptosis was observed but long-term inhibition of proliferation. H3K9 immunofluorescence, SA-ß-Gal staining and p21 expression revealed that especially these low CAP doses induce senescence in melanoma cells. In summary, we observed differences in induction of apoptosis or senescence of tumor cells in respond to different CAP doses using a new CAP device. The mechanism of senescence with regard to plasma therapy was so far not described previously and is of great importance for therapeutic application of CAP.