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1.  The Role of Cellular Senescence in the Gastrointestinal Mucosa 
Gut and Liver  2013;7(3):270-277.
Cellular senescence is a biologically irreversible state of cell-growth arrest that occurs following either a replicative or an oncogenic stimulus. This phenomenon occurs as a response to the presence of premalignant cells and appears to be an important anticancer mechanism that keeps these transformed cells at bay. Many exogenous and endogenous triggers for senescence have been recognized to act via genomic or epigenomic pathways. The most common stimulus for senescence is progressive loss of telomeric DNA, which results in the loss of chromosomal stability and eventual unregulated growth and malignancy. Senescence is activated through an interaction between the p16 and p53 tumor-suppressor genes. Senescent cells can be identified in vitro because they express senescence-associated β-galactosidase, a marker of increased lysosomal activity. Cellular senescence plays an integral role in the prevention and development of both benign and malignant gastrointestinal diseases. The senescence cascade and the cell-cycle checkpoints that dictate the progression and maintenance of senescence are important in all types of gastrointestinal cancers, including pancreatic, liver, gastric, colon, and esophageal cancers. Understanding the pathogenic mechanisms involved in cellular senescence is important for the development of agents targeted toward the treatment of gastrointestinal tumors.
PMCID: PMC3661957  PMID: 23710306
Cell aging; Gastrointestinal neoplasms; Aging; Gastrointestinal mucosa
2.  Significance of Cellular Senescence in Aging and Cancer 
Cellular senescence is a mechanism that induces an irreversible growth arrest in all somatic cells. Senescent cells are metabolically active but lack the capacity to replicate. Evolutionary theories suggest that cellular senescence is related to the organismal decline occurring in aging organisms. Also, such theories describe senescence as an antagonistically pleiotropic process that can have beneficial or detrimental effect on the organism. Cellular senescence is believed to be involved in the cellular changes observed as aging progresses. Accumulation of senescent cells appears to occur widely as the organism ages. Furthermore, senescence is a key element of the tumor suppressor pathways. Therefore, it is part of the natural barrier against the uncontrolled proliferation observed in cellular development of malignancies in multicellular organisms. Activation of the senescence process guarantees a limited number of cellular replications. The genetic network led by p53 is responsible for activation of senescence in response to DNA damage and genomic instability that could lead to cancer. A better comprehension of the genetic networks that control the cell cycle and induce senescence is important to analyze the association of senescence to longevity and diseases related to aging. For these reasons, experimental research both in vitro and in vivo aims to develop anticancer therapies based on senescence activation. The last decade of research on role and function of senescence in aging and cancer are discussed in this paper.
PMCID: PMC2802848  PMID: 20057963
Aging; Neoplasms; Pleiotropy; Telomere; Genetic pathways
3.  Telomeres, oxidative stress and inflammatory factors: partners in cellular senescence? 
Senescence, the state of irreversible cell-cycle arrest, plays paradoxical albeit important roles in vivo: it protects organisms against cancer but also contributes to age-related loss of tissue function. The DNA damage response (DDR) has a central role in cellular senescence. Not only does it contribute to the irreversible loss of replicative capacity but also to the production and secretion of reactive oxygen species (ROS), and bioactive peptides collectively known as the senescence-associated secretory phenotype (SASP). Both ROS and the SASP have been shown to impact on senescence in an autocrine as well as paracrine fashion; however, the underlying mechanisms are not well understood. In this review we describe our current understanding of cellular senescence, examine in detail the intricate pathways linking the DDR, ROS and SASP, and evaluate their impact on the stability of the senescent phenotype.
PMCID: PMC3922784  PMID: 24472138
DNA damage response; Telomeres; Senescence; Ageing; Oxidative stress; Inflammation
4.  Insulin-like growth factor binding proteins 4 and 7 released by senescent cells promote premature senescence in mesenchymal stem cells 
Cell Death & Disease  2013;4(11):e911-.
Cellular senescence is the permanent arrest of cell cycle, physiologically related to aging and aging-associated diseases. Senescence is also recognized as a mechanism for limiting the regenerative potential of stem cells and to protect cells from cancer development. The senescence program is realized through autocrine/paracrine pathways based on the activation of a peculiar senescence-associated secretory phenotype (SASP). We show here that conditioned media (CM) of senescent mesenchymal stem cells (MSCs) contain a set of secreted factors that are able to induce a full senescence response in young cells. To delineate a hallmark of stem cells SASP, we have characterized the factors secreted by senescent MSC identifying insulin-like growth factor binding proteins 4 and 7 (IGFBP4 and IGFBP7) as key components needed for triggering senescence in young MSC. The pro-senescent effects of IGFBP4 and IGFBP7 are reversed by single or simultaneous immunodepletion of either proteins from senescent-CM. The blocking of IGFBP4/7 also reduces apoptosis and promotes cell growth, suggesting that they may have a pleiotropic effect on MSC biology. Furthermore, the simultaneous addition of rIGFBP4/7 increased senescence and induced apoptosis in young MSC. Collectively, these results suggest the occurrence of novel-secreted factors regulating MSC cellular senescence of potential importance for regenerative medicine and cancer therapy.
PMCID: PMC3847322  PMID: 24201810
senescence; mesenchymal stem cells; IGFBP4; IGFBP7; mass spectrometry; secretome
5.  Colorimetric Detection of Senescence-Associated β Galactosidase 
Most normal human cells have a finite replicative capacity and eventually undergo cellular senescence, whereby cells cease to proliferate. Cellular senescence is also induced by various stress signals, such as those generated by oncogenes, DNA damage, hyperproliferation, and an oxidative environment. Cellular senescence is well established as an intrinsic tumor suppressive mechanism. Recent progress concerning senescence research has revealed that cellular senescence occurs in vivo and that, unexpectedly, it has a very complex role in tissue repair, promoting tumor progression and aging via the secretion of various cytokines, growth factors, and enzymes. Therefore, the importance of biomarkers for cellular senescence has greatly increased. In 1995, we described the “senescence-associated β galactosidase” (SA-βgal) biomarker, which conveniently identifies individual senescent cells in vitro and in vivo. Here, we describe an updated protocol for the detection of cell senescence based on this widely used biomarker, which contributed to recent advances in senescence, aging and cancer research. We provide an example of detecting SA-βgal together with other senescence markers and a proliferation marker, EdU, in single cells.
PMCID: PMC3769963  PMID: 23296655
Aging; Biomarker; Cellular senescence; EdU labeling; SA-βgal; Immunostaining
6.  Cellular Senescence - its role in cancer and the response to ionizing radiation 
Genome Integrity  2011;2:7.
Cellular senescence is a normal biological process that is initiated in response to a range of intrinsic and extrinsic factors that functions to remove irreparable damage and therefore potentially harmful cells, from the proliferative pool. Senescence can therefore be thought of in beneficial terms as a tumour suppressor. In contrast to this, there is a growing body of evidence suggesting that senescence is also associated with the disruption of the tissue microenvironment and development of a pro-oncogenic environment, principally via the secretion of senescence-associated pro-inflammatory factors. The fraction of cells in a senescent state is known to increase with cellular age and from exposure to various stressors including ionising radiation therefore, the implications of the detrimental effects of the senescent phenotype are important to understand within the context of the increasing human exposure to ionising radiation. This review will discuss what is currently understood about senescence, highlighting possible associations between senescence and cancer and, how exposure to ionising radiation may modify this.
PMCID: PMC3169443  PMID: 21834983
Ionising radiation; premature senescence; SASP; inflammation; age-related pathologies
7.  Scoring of senescence signalling in multiple human tumour gene expression datasets, identification of a correlation between senescence score and drug toxicity in the NCI60 panel and a pro-inflammatory signature correlating with survival advantage in peritoneal mesothelioma 
BMC Genomics  2010;11:532.
Cellular senescence is a major barrier to tumour progression, though its role in pathogenesis of cancer and other diseases is poorly understood in vivo. Improved understanding of the degree to which latent senescence signalling persists in tumours might identify intervention strategies to provoke "accelerated senescence" responses as a therapeutic outcome. Senescence involves convergence of multiple pathways and requires ongoing dynamic signalling throughout its establishment and maintenance. Recent discovery of several new markers allows for an expression profiling approach to study specific senescence phenotypes in relevant tissue samples. We adopted a "senescence scoring" methodology based on expression profiles of multiple senescence markers to examine the degree to which signals of damage-associated or secretory senescence persist in various human tumours.
We first show that scoring captures differential induction of damage or inflammatory pathways in a series of public datasets involving radiotherapy of colon adenocarcinoma, chemotherapy of breast cancer cells, replicative senescence of mesenchymal stem cells, and progression of melanoma. We extended these results to investigate correlations between senescence score and growth inhibition in response to ~1500 compounds in the NCI60 panel. Scoring of our own mesenchymal tumour dataset highlighted differential expression of secretory signalling pathways between distinct subgroups of MPNST, liposarcomas and peritoneal mesothelioma. Furthermore, a pro-inflammatory signature yielded by hierarchical clustering of secretory markers showed prognostic significance in mesothelioma.
We find that "senescence scoring" accurately reports senescence signalling in a variety of situations where senescence would be expected to occur and highlights differential expression of damage associated and secretory senescence pathways in a context-dependent manner.
PMCID: PMC3091681  PMID: 20920304
8.  Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders 
Nature  2011;479(7372):232-236.
Advanced age is the main risk factor for most chronic diseases and functional deficits in humans, but the fundamental mechanisms that drive ageing remain largely unknown, impeding the development of interventions that might delay or prevent age-related disorders and maximize healthy lifespan. Cellular senescence, which halts the proliferation of damaged or dysfunctional cells, is an important mechanism to constrain the malignant progression of tumour cells1,2. Senescent cells accumulate in various tissues and organs with ageing3 and have been hypothesized to disrupt tissue structure and function because of the components they secrete4,5. However, whether senescent cells are causally implicated in age-related dysfunction and whether their removal is beneficial has remained unknown. To address these fundamental questions, we made use of a biomarker for senescence, p16Ink4a, to design a novel transgene, INK-ATTAC, for inducible elimination of p16Ink4a-positive senescent cells upon administration of a drug. Here we show that in the BubR1 progeroid mouse background, INK-ATTAC removes p16Ink4a-positive senescent cells upon drug treatment. In tissues—such as adipose tissue, skeletal muscle and eye—in which p16Ink4a contributes to the acquisition of age-related pathologies, life-long removal of p16Ink4a-expressing cells delayed onset of these phenotypes. Furthermore, late-life clearance attenuated progression of already established age-related disorders. These data indicate that cellular senescence is causally implicated in generating age-related phenotypes and that removal of senescent cells can prevent or delay tissue dysfunction and extend healthspan.
PMCID: PMC3468323  PMID: 22048312
9.  Lamin B1 loss is a senescence-associated biomarker 
Molecular Biology of the Cell  2012;23(11):2066-2075.
This study rigorously defines lamin B1 loss as a marker of senescence in response to all classic signals of senescence, including DNA damage, oncogene activation, and replicative exhaustion. This decline is induced by activation of either the p53 or the pRb pathway and occurs in vivo in response to a senescence-inducing dose of radiation.
Cellular senescence is a potent tumor-suppressive mechanism that arrests cell proliferation and has been linked to aging. However, studies of senescence have been impeded by the lack of simple, exclusive biomarkers of the senescent state. Senescent cells develop characteristic morphological changes, which include enlarged and often irregular nuclei and chromatin reorganization. Because alterations to the nuclear lamina can affect both nuclear morphology and gene expression, we examined the nuclear lamina of senescent cells. We show here than lamin B1 is lost from primary human and murine cell strains when they are induced to senesce by DNA damage, replicative exhaustion, or oncogene expression. Lamin B1 loss did not depend on the p38 mitogen-activated protein kinase, nuclear factor-κB, ataxia telangiectasia–mutated kinase, or reactive oxygen species signaling pathways, which are positive regulators of senescent phenotypes. However, activation of either the p53 or pRB tumor suppressor pathway was sufficient to induce lamin B1 loss. Lamin B1 declined at the mRNA level via a decrease in mRNA stability rather than by the caspase-mediated degradation seen during apoptosis. Last, lamin B1 protein and mRNA declined in mouse tissue after senescence was induced by irradiation. Our findings suggest that lamin B1 loss can serve as biomarker of senescence both in culture and in vivo.
PMCID: PMC3364172  PMID: 22496421
10.  Novel ARF/p53-independent senescence pathways in cancer repression 
Cellular senescence, which can be induced by various stimuli, is a stress response that manifests as irreversible cell cycle arrest. Recent studies have revealed that cellular senescence can serve as a critical barrier for cancer development. Induction of cellular senescence by oncogenic insults, such as Ras over-expression or by inactivation of PTEN tumor suppressor, triggers an ARF/p53-dependent tumor-suppressive effect which can significantly restrict cancer progression. Given the important role of the ARF/p53 pathway in cellular senescence and tumor suppression, drugs that stabilize p53 expression have been developed and tested in clinical trials. However, a major hurdle for p53 targeting in cancer treatment arises from the frequent deficiency or mutation of ARF or p53 in human cancers, which, in turn, profoundly compromises their tumor-suppressive ability. Recent discoveries of novel regulators involved in ARF/p53-independent cellular senescence not only reveal novel paradigms for cellular senescence but also provide alternative approaches for cancer therapy.
PMCID: PMC3296232  PMID: 21594579
Skp2; p53; Cellular senescence; Cancer therapy
11.  Cellular senescence and organismal aging 
Mechanisms of Ageing and Development  2008;129(7-8):467-474.
Cellular senescence, first observed and defined using in vitro cell culture studies, is an irreversible cell cycle arrest which can be triggered by a variety of factors. Emerging evidence suggests that cellular senescence acts as an in vivo tumor suppression mechanism by limiting aberrant proliferation. It has also been postulated that cellular senescence can occur independently of cancer and contribute to the physiological processes of normal organismal aging. Recent data have demonstrated the in vivo accumulation of senescent cells with advancing age. Some characteristics of senescent cells, such as the ability to modify their extracellular environment, could play a role in aging and age related pathology. In this review, we examine current evidence that links cellular senescence and organismal aging.
PMCID: PMC3297662  PMID: 18502472
Cellular senescence; Aging; Telomeres
12.  How Does Proliferative Homeostasis Change With Age? What Causes It and How Does It Contribute to Aging? 
The notion that there might be a cellular basis for aging stems from research that began several decades ago and was proposed to explain the loss of proliferative homeostasis, which is a hallmark of complex animals. Recent years have seen growing support for the idea that two cell fates—apoptosis and cellular senescence, both now well-established tumor suppressor mechanisms—may be important drivers of aging phenotypes and age-related disease. However, there remain many unanswered questions, some quite basic, about how these processes change with age and how they might contribute to aging. It is now clear that failures in apoptosis or senescence can result in hyperproliferative diseases such as cancer. Less is known about whether and how increased apoptosis or senescence can cause tissue degeneration and aging. In addition, there is now a growing recognition that cellular senescence can have cell-nonautonomous effects within tissues. New molecular tools and model organisms, some already on the horizon, will need to be developed to better understand the roles of apoptosis and cellular senescence in age-associated changes in proliferative homeostasis.
PMCID: PMC2655008  PMID: 19228778
Proliferative homeostasis; Aging
13.  Senescent cells as a source of inflammatory factors for tumor progression 
Cancer Metastasis Reviews  2010;29(2):273-283.
Cellular senescence, which is associated with aging, is a process by which cells enter a state of permanent cell cycle arrest, therefore constituting a potent tumor suppressive mechanism. Recent studies show that, despite the beneficial effects of cellular senescence, senescent cells can also exert harmful effects on the tissue microenvironment. The most significant of these effects is the acquisition of a senescent-associated secretory phenotype (SASP), which entails a striking increase in the secretion of pro-inflammatory cytokines. Here, we summarize our knowledge of the SASP and the impact it has on tissue microenvironments and ability to stimulate tumor progression.
PMCID: PMC2865636  PMID: 20390322
Aging; Senescence; Cancer; Inflammation; Cytokines; Interleukins; Proliferation; Invasion; Migration
14.  Oxidative stress in secondary osteoarthritis: from cartilage destruction to clinical presentation? 
Orthopedic Reviews  2010;2(2):e23.
Due to an increasing life expectance, osteoarthritis (OA) is one of the most common chronic diseases. Although strong efforts have been made to regenerate degenerated joint cartilage, OA is a progressive and irreversible disease up to date. Among other factors the dysbalance between free radical burden and cellular scavenging mechanisms defined as oxidative stress is a relevant part of OA pathogenesis. Here, only little data are available about the mediation and interaction between different joint compartments. The article provides a review of the current literature regarding the influence of oxidative stress on cellular aging, senescence and apoptosis in different joint compartments (cartilage, synovial tissue and subchondral bone). Free radical exposure is known to promote cellular senescence and apoptosis. Radical oxygen species (ROS) involvement in inflammation, fibrosis control and pain nociception has been proven. The data from literature indicates a link between free radical burden and OA pathogenesis mediating local tissue reactions between the joint compartments. Hence, oxidative stress is likely not only to promote cartilage destruction but also to be involved in inflammative transformation, promoting the transition from clinically silent cartilage destruction to apparent OA. ROS induced by exogenous factors such as overload, trauma, local intraarticular lesion and consecutive synovial inflammation cause cartilage degradation. In the affected joint, free radicals mediate disease progression. The interrelationship between oxidative stress and OA etiology might provide a novel approach to the comprehension and therefore modification of disease progression and symptom control.
PMCID: PMC3143971  PMID: 21808712
osteoarthritis; free radicals; oxidative damage.
15.  Melanomagenesis: Overcoming the Barrier of Melanocyte Senescence 
Cell cycle (Georgetown, Tex.)  2008;7(13):1944-1948.
Although melanoma ultimately progresses to a highly aggressive and metastatic disease that is typically resistant to currently available therapy, it often begins as a benign nevus consisting of a clonal population of hyperplastic melanocytes that cannot progress because they are locked in a state of cellular senescence. Once senescence is overcome, the nevus can exhibit dysplastic features and readily progress to more lethal stages. Recent advances have convincingly demonstrated that senescence represents a true barrier to the progression of many types of cancer, including melanoma. Thus, understanding the mechanism(s) by which melanoma evades senescence has become a priority in the melanoma research community. Senescence in most cells is regulated through some combination of activities within the RB and p53 pathways. However, differences discovered among various tumor types, some subtle and others quite profound, have revealed that senescence frequently operates in a context-dependent manner. Here we review what is known about melanocyte senescence, and how such knowledge may provide a much-needed edge in our struggles to contain or perhaps vanquish this often-fatal malignancy.
PMCID: PMC2678050  PMID: 18604170
16.  Lessons from senescence: chromatin maintenance in non-proliferating cells 
Biochimica et biophysica acta  2011;1819(0):322-331.
Cellular senescence is an irreversible proliferation arrest, thought to contribute to tumour suppression, proper wound healing and, perhaps, tissue and organismal aging. Two classical tumor suppressors, p53 and pRB, control cell cycle arrest associated with senescence. Profound molecular changes occur in cells undergoing senescence. At the level of chromatin, for example, senescence associated heterochromatic foci (SAHF) form in some cell types. Chromatin is inherently dynamic and likely needs to be actively maintained to achieve a stable cell phenotype. In proliferating cells chromatin is maintained in conjunction with DNA replication, but how non-proliferating cells maintain chromatin structure is poorly understood. Some histone variants, such as H3.3 and macroH2A increase as cells undergo senescence, suggesting histone variants and their associated chaperones could be important in chromatin structure maintenance in senescent cells. Here, we discuss options available for senescent cells to maintain chromatin structure and the relative contribution of histone variants and chaperones in this process.
PMCID: PMC3895594  PMID: 21839870
17.  Implications of Cellular Senescence in Tissue Damage Response, Tumor Suppression, and Stem Cell Biology 
Cellular senescence is characterized by an irreversible cell cycle arrest that, when bypassed by mutation, contributes to cellular immortalization. Activated oncogenes induce a hyperproliferative response, which might be one of the senescence cues. We have found that expression of such an oncogene, Akt, causes senescence in primary mouse hepatoblasts in vitro. Additionally, AKT-driven tumors undergo senescence in vivo following p53 reactivation and show signs of differentiation. In another in vivo system, i.e., liver fibrosis, hyperproliferative signaling through AKT might be a driving force of the senescence in activated hepatic stellate cells. Senescent cells up-regulate and secrete molecules that, on the one hand, can reinforce the arrest and, on the other hand, can signal to an innate immune system to clear the senescent cells. The mechanisms governing senescence and immortalization are overlapping with those regulating self-renewal and differentiation. These respective control mechanisms, or their disregulation, are involved in multiple pathological conditions including fibrosis, wound healing, and cancer. Understanding extracellular cues that regulate these processes may enable new therapies for these conditions.
PMCID: PMC3285266  PMID: 19150958
18.  Cellular senescence in the glaucomatous outflow pathway 
Experimental gerontology  2005;40(8-9):745-748.
The mechanisms responsible for the progressive malfunction of the trabecular meshwork (TM)–Schlemm’s canal (SC) conventional outflow pathway tissue in primary open angle glaucoma (POAG) are still not fully understood. To determine whether POAG is characterized by an accumulation of senescent cells, similar to what has been described in other diseases, we have compared the levels of the senescence marker senescence-associated-β-galactosidase (SA-β-gal) in the outflow pathway cells of POAG and age-matched control donors. POAG donors demonstrated a statistically significant fourfold increase in the percentage of SA-β-gal positive cells. These results suggest a potential role for cellular senescence in the pathophysiology of the outflow pathway.
PMCID: PMC3152456  PMID: 16051457
Trabecular meshwork; Senescence; POAG; Glaucoma; Senescence-associated-β-galactosidase
19.  p41-Arc, a regulatory subunit of Arp2/3 complex, can induce premature senescence in the absence of p53 and Rb 
Experimental & Molecular Medicine  2011;43(7):389-392.
Cellular senescence is a tumor-suppressive process instigated by proliferation in the absence of telomere replication, by cellular stresses such as oncogene activation, or by activation of the tumor suppressor proteins, such as Rb or p53. This process is characterized by an irreversible cell cycle exit, a unique morphology, and expression of senescence-associated-β-galactosidase (SA-β-gal). Despite the potential biological importance of cellular senescence, little is known of the mechanisms leading to the senescent phenotype. p41-Arc has been known to be a putative regulatory component of the mammalian Arp2/3 complex, which is required for the formation of branched networks of actin filaments at the cell cortex. In this study, we demonstrate that p41-Arc can induce senescent phenotypes when it is overexpressed in human tumor cell line, SaOs-2, which is deficient in p53 and Rb tumor suppressor genes, implying that p41 can induce senescence in a p53-independent way. p41-Arc overexpression causes a change in actin filaments, accumulating actin filaments in nuclei. Therefore, these results imply that a change in actin filament can trigger an intrinsic senescence program in the absence of p53 and Rb tumor suppressor genes.
PMCID: PMC3158497  PMID: 21628992
p41-Arc; Arp2/3; p53; senescence
20.  Pituitary Senescence: The Evolving Role of Pttg 
Despite the high prevalence of pituitary adenomas they are invariably benign, indicative of unique intrinsic mechanisms controlling pituitary cell proliferation. Cellular senescence is characterized by a largely irreversible cell cycle arrest and constitutes a strong anti-proliferative response, which can be triggered by DNA damage, chromosomal instability and aneuploidy, loss of tumor suppressive signaling or oncogene activation. In vivo senescence is an important protective mechanisms against cancer. Here we discuss prospective mechanisms underlying senescence-associated molecular pathways activated in benign pituitary adenomas. Both deletion and overexpression of pituitary tumor transforming gene (Pttg) promote chromosomal instability and aneuploidy. Pttg deletion abrogates tumor development by activating p53/p21-dependent senescence pathways. Abundant PTTG in GH-secreting pituitary adenomas also triggers p21-dependent senescence. Pituitary p21 may therefore safeguard against further chromosomal instability by constraining pituitary tumor growth. These observations point to senescence as a target for effective therapy for both tumor silencing and growth restraint towards development of pituitary malignancy
PMCID: PMC2906651  PMID: 20153804
21.  Protein Oxidative Damage at the Crossroads of Cellular Senescence, Aging, and Age-Related Diseases 
Protein damage mediated by oxidation, protein adducts formation with advanced glycated end products and with products of lipid peroxidation, has been implicated during aging and age-related diseases, such as neurodegenerative diseases. Increased protein modification has also been described upon replicative senescence of human fibroblasts, a valid model for studying aging in vitro. However, the mechanisms by which these modified proteins could impact on the development of the senescent phenotype and the pathogenesis of age-related diseases remain elusive. In this study, we performed in silico approaches to evidence molecular actors and cellular pathways affected by these damaged proteins. A database of proteins modified by carbonylation, glycation, and lipid peroxidation products during aging and age-related diseases was built and compared to those proteins identified during cellular replicative senescence in vitro. Common cellular pathways evidenced by enzymes involved in intermediate metabolism were found to be targeted by these modifications, although different tissues have been examined. These results underscore the potential effect of protein modification in the impairment of cellular metabolism during aging and age-related diseases.
PMCID: PMC3483731  PMID: 23125894
22.  Quantitative Model of Cell Cycle Arrest and Cellular Senescence in Primary Human Fibroblasts 
PLoS ONE  2012;7(8):e42150.
Primary human fibroblasts in tissue culture undergo a limited number of cell divisions before entering a non-replicative “senescent” state. At early population doublings (PD), fibroblasts are proliferation-competent displaying exponential growth. During further cell passaging, an increasing number of cells become cell cycle arrested and finally senescent. This transition from proliferating to senescent cells is driven by a number of endogenous and exogenous stress factors. Here, we have developed a new quantitative model for the stepwise transition from proliferating human fibroblasts (P) via reversibly cell cycle arrested (C) to irreversibly arrested senescent cells (S). In this model, the transition from P to C and to S is driven by a stress function γ and a cellular stress response function F which describes the time-delayed cellular response to experimentally induced irradiation stress. The application of this model based on senescence marker quantification at the single-cell level allowed to discriminate between the cellular states P, C, and S and delivers the transition rates between the P, C and S states for different human fibroblast cell types. Model-derived quantification unexpectedly revealed significant differences in the stress response of different fibroblast cell lines. Evaluating marker specificity, we found that SA-β-Gal is a good quantitative marker for cellular senescence in WI-38 and BJ cells, however much less so in MRC-5 cells. Furthermore we found that WI-38 cells are more sensitive to stress than BJ and MRC-5 cells. Thus, the explicit separation of stress induction from the cellular stress response, and the differentiation between three cellular states P, C and S allows for the first time to quantitatively assess the response of primary human fibroblasts towards endogenous and exogenous stress during cellular ageing.
PMCID: PMC3413708  PMID: 22879912
23.  Senescence induction; a possible cancer therapy 
Molecular Cancer  2009;8:3.
Cellular immortalization is a crucial step during the development of human cancer. Primary mammalian cells reach replicative exhaustion after several passages in vitro, a process called replicative senescence. During such a state of permanent growth arrest, senescent cells are refractory to physiological proliferation stimuli: they have altered cell morphology and gene expression patterns, although they remain viable with preserved metabolic activity. Interestingly, senescent cells have also been detected in vivo in human tumors, particularly in benign lesions. Senescence is a mechanism that limits cellular lifespan and constitutes a barrier against cellular immortalization. During immortalization, cells acquire genetic alterations that override senescence. Tumor suppressor genes and oncogenes are closely involved in senescence, as their knockdown and ectopic expression confer immortality and senescence induction, respectively. By using high throughput genetic screening to search for genes involved in senescence, several candidate oncogenes and putative tumor suppressor genes have been recently isolated, including subtypes of micro-RNAs. These findings offer new perspectives in the modulation of senescence and open new approaches for cancer therapy.
PMCID: PMC2631463  PMID: 19133111
24.  POZ/BTB and AT-hook-containing zinc finger protein 1 (PATZ1) inhibits endothelial cell senescence through a p53 dependent pathway 
Cell Death and Differentiation  2011;19(4):703-712.
Vascular cell senescence, induced by the DNA damage response or inflammatory stress, contributes to age-associated vascular disease. Using complementary DNA microarray technology, we found that the level of POZ/BTB and AT-hook-containing zinc finger protein 1 (PATZ1) is downregulated during endothelial cell (EC) senescence. PATZ1 may have an important role as a transcriptional repressor in chromatin remodeling and transcription regulation; however, the role of PATZ1 in EC senescence and vascular aging remains unidentified. Knockdown of PATZ1 in young cells accelerated premature EC senescence, which was confirmed by growth arrest, increased p53 protein level and senescence-associated β-galactosidase (SA-β-gal) activity, and repression of EC tube formation. In contrast, overexpression of PATZ1 in senescent cells reversed senescent phenotypes. Cellular senescence induced by PATZ1 knockdown in young cells was rescued by knockdown of p53, but not by knockdown of p16INK4a. PATZ1 knockdown increased ROS levels, and pretreatment with N-acetylcysteine abolished EC senescence induced by PATZ1 knockdown. Notably, PATZ1 immunoreactivity was lower in ECs of atherosclerotic tissues than those of normal arteries in LDLR−/− mice, and immunoreactivity also decreased in ECs of old human arteries. These results suggest that PATZ1 may have an important role in the regulation of EC senescence through an ROS-mediated p53-dependent pathway and contribute to vascular diseases associated with aging.
PMCID: PMC3307983  PMID: 22052190
cell aging; PATZ1; P53; ROS; vascular aging; atherosclerosis
25.  A novel type of cellular senescence that can be enhanced in mouse models and human tumor xenografts to suppress prostate tumorigenesis 
Irreversible cell growth arrest, a process termed cellular senescence, is emerging as an intrinsic tumor suppressive mechanism. Oncogene-induced senescence is thought to be invariably preceded by hyperproliferation, aberrant replication, and activation of a DNA damage checkpoint response (DDR), rendering therapeutic enhancement of this process unsuitable for cancer treatment. We previously demonstrated in a mouse model of prostate cancer that inactivation of the tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (Pten) elicits a senescence response that opposes tumorigenesis. Here, we show that Pten-loss–induced cellular senescence (PICS) represents a senescence response that is distinct from oncogene-induced senescence and can be targeted for cancer therapy. Using mouse embryonic fibroblasts, we determined that PICS occurs rapidly after Pten inactivation, in the absence of cellular proliferation and DDR. Further, we found that PICS is associated with enhanced p53 translation. Consistent with these data, we showed that in mice p53-stabilizing drugs potentiated PICS and its tumor suppressive potential. Importantly, we demonstrated that pharmacological inhibition of PTEN drives senescence and inhibits tumorigenesis in vivo in a human xenograft model of prostate cancer. Taken together, our data identify a type of cellular senescence that can be triggered in nonproliferating cells in the absence of DNA damage, which we believe will be useful for developing a “pro-senescence” approach for cancer prevention and therapy.
PMCID: PMC2827955  PMID: 20197621

Results 1-25 (1064639)