Tumor suppressor and aging biomarker p16(INK4a) induces cellular senescence without the associated inflammatory secretory phenotype

Effect of high-dose irradiation on human bone-marrow-derived mesenchymal stromal cells

Cell therapy using multipotent mesenchymal stromal cells (MSCs) is of high interest in various indications. As the pleiotropic effects mediated by MSCs rely mostly on their unique secretory profile, long-term persistence of ex-vivo-expanded cells in the recipient may not always be desirable. Irradiation is a routine procedure in transfusion medicine to prevent long-term persistence of nucleated cells and could therefore also be applied to MSCs. We have exposed human bone-marrow-derived MSCs to 30 or 60 Gy of γ-irradiation and assessed cell proliferation, clonogenicity, differentiation, cytokine levels in media supernatants, surface receptor profile, as well as expression of proto-oncogenes/cell cycle markers, self-renewal/stemness markers, and DNA damage/irradiation markers. Irradiated MSCs show a significant decrease in proliferation and colony-forming unit-fibroblasts. However, a subpopulation of surviving cells is able to differentiate, but is unable to form colonies after irradiation. Irradiated MSCs showed stable expression of CD73 and CD90 and absence of CD3, CD34, and CD45 during a 16-week follow-up period. We found increased vascular endothelial growth factor (VEGF) levels and a decrease of platelet-derived growth factor (PDGF)-AA and PDGF-AB/BB in culture media of nonirradiated cells. Irradiated MSCs showed an inverse pattern, that is, no increase of VEGF, and less consumption of PDGF-AA and PDGF-AB/BB. Interestingly, interleukin-6 (IL-6) levels increased during culture regardless of irradiation. Cells with lower sensitivity toward γ-irradiation showed positive β-galactosidase activity 10 days after irradiation. Gene expression of both irradiated and nonirradiated MSCs 13-16 weeks after irradiation with 60 Gy predominantly followed the same pattern; cell cycle regulators CDKN1A (p21) and CDKN2A (p16) were upregulated, indicating cell cycle arrest, whereas classical proto-oncogenes, respectively, and self-renewal/stemness markers MYC, TP53 (p53), and KLF4 were downregulated. In addition, DNA damage/irradiation markers ATM, ATR, BRCA1, CHEK1, CHEK2, MDC1, and TP53BP1 also mostly showed the same pattern of gene expression as high-dose γ-irradiation. In conclusion, we demonstrated the existence of an MSC subpopulation with remarkable resistance to high-dose γ-irradiation. Cells surviving irradiation retained their trilineage differentiation capacity and surface marker profile but changed their cytokine secretion profile and became prematurely senescent.

The role of senescent cells in ageing

Cellular senescence has historically been viewed as an irreversible cell-cycle arrest mechanism that acts to protect against cancer, but recent discoveries have extended its known role to complex biological processes such as development, tissue repair, ageing and age-related disorders. New insights indicate that, unlike a static endpoint, senescence represents a series of progressive and phenotypically diverse cellular states acquired after the initial growth arrest. A deeper understanding of the molecular mechanisms underlying the multi-step progression of senescence and the development and function of acute versus chronic senescent cells may lead to new therapeutic strategies for age-related pathologies and extend healthy lifespan.

Senescence and the pro-tumorigenic stroma

Hayflick and Moorhead first described senescence in the late 1960's as a permanent growth arrest that primary cells underwent after a defined number of cellular divisions in culture. This observation gave rise to the hypothesis that cells contained an internal counting mechanism that limited cellular division and that this limit was an important barrier to cellular transformation. What began as an in vitro observation has led to an immense body of work that reaches into all fields of biology and is of particular interest in the areas of aging, tissue regeneration, and tumorigenesis. The initially simplistic view that senescence limits cellular division and contributes to aging while stymying tumorigenesis has now evolved into an important and complex biological process that has numerous caveats and often opposing effects on tumorigenesis. In this review, we limit our discussion to the complex role senescence plays in tumorigenesis. Throughout the review we attempt to draw many parallels to other systems including the role senescent cells play in the tumor microenvironment and their significant molecular and phenotypic similarities to cancer associated fibroblasts (CAFs).

Coordinated control of senescence by lncRNA and a novel T-box3 co-repressor complex

Cellular senescence is a crucial tumor suppressor mechanism. We discovered a CAPERα/TBX3 repressor complex required to prevent senescence in primary cells and mouse embryos. Critical, previously unknown roles for CAPERα in controlling cell proliferation are manifest in an obligatory interaction with TBX3 to regulate chromatin structure and repress transcription of CDKN2A-p16^INK and the RB pathway. The IncRNA UCA1 is a direct target of CAPERα/TBX3 repression whose overexpression is sufficient to induce senescence. In proliferating cells, we found that hnRNPA1 binds and destabilizes CDKN2A-p16^INK mRNA whereas during senescence, UCA1 sequesters hnRNPA1 and thus stabilizes CDKN2A-p16^INK. Thus CAPERα/TBX3 and UCA1 constitute a coordinated, reinforcing mechanism to regulate both CDKN2A-p16^INK transcription and mRNA stability. Dissociation of the CAPERα/TBX3 co-repressor during oncogenic stress activates UCA1, revealing a novel mechanism for oncogene-induced senescence. Our elucidation of CAPERα and UCA1 functions in vivo provides new insights into senescence induction, and the oncogenic and developmental properties of TBX3.

DOI:http://dx.doi.org/10.7554/eLife.02805.001

Cell division and growth are essential for survival. But it is equally important that cells can stop dividing, because failing to do so can lead to the uncontrolled tumor growth seen in cancer. One such quality control mechanism is called senescence, which stops the growth and multiplication of cells that are old, damaged or behaving in ways that may harm the organism. All cells eventually stop dividing and undergo senescence, but a number of factors may trigger the process early, such as DNA damage, stress or the appearance of cancer-causing proteins.

Senescence can be harmful if it occurs too early in life and interferes with normal growth. Severe birth defects—including fatal heart problems and limb malformations—occur if senescence is inappropriately triggered early in development. Mutations in a gene encoding a protein called TBX3 have been linked to these severe birth defects.

Normally, TBX3 stops the production of other proteins that trigger senescence in early development, and helps to maintain stable conditions in adult cells. Understanding how it does so could help scientists understand normal cell function and aging, and also help to find ways to trigger senescence in cancerous cells.

Kumar et al. found that a protein called CAPERα—for short Coactivator of AP1 and Estrogen Receptor—forms a complex with TBX3 that stops cells dividing in living organisms in at least two different ways. One way is by altering how DNA is folded. The other way involves a non-coding strand of RNA from a gene called UCA1: this RNA prevents the degradation of proteins that stop cell division.

In normal proliferating cells, the CAPERα/TBX3 protein complex prevents the production of UCA1 RNA. In contrast, in cells that received a cancer causing stimulus, TBX3 and CAPERα physically separate: this activates production of UCA1 RNA and causes senescence. Further studies will be required to establish exactly how the CAPERα/TBX3 protein complex interacts with DNA and RNA to control senescence and prevent cancer.

DOI:http://dx.doi.org/10.7554/eLife.02805.002

Cell senescence: role in aging and age-related diseases

Cell senescence is one of the major paradigms of aging research. It started with the demonstration by L. Hayflick of the limited number of divisions by normal, nontransformed cells, not shown by transformed malignant cells, this processes being largely regulated by the telomere-telomerase system. A complete renewal of this discipline came from the demonstration that cells can enter senescence at any time by an anti-oncogene-triggered pathway, enabling them to escape malignancy. The senescent cell became a major actor of the aging process, among others, by the acquisition of the senescence-associated secretory phenotype. This chapter is devoted to the regulatory process involved in the acquisition of the senescent cell phenotype and its role in organismal aging.

Crosstalk between the Rb pathway and AKT signaling forms a quiescence-senescence switch

Cell-cycle arrest in quiescence and senescence is largely orchestrated by the retinoblastoma (Rb) tumor-suppressor pathway, but the mechanisms underlying the quiescence-senescence switch remain unclear. Here, we show that the crosstalk between the Rb-AKT-signaling pathways forms this switch by controlling the overlapping functions of FoxO3a and FoxM1 transcription factors in cultured fibroblasts. In the absence of mitogenic signals, although FoxM1 expression is repressed by the Rb pathway, FoxO3a prevents reactive oxygen species (ROS) production by maintaining SOD2 expression, leading to quiescence. However, if the Rb pathway is activated in the presence of mitogenic signals, FoxO3a is also inactivated by AKT, thus reducing SOD2 expression and consequently allowing ROS production. This situation elicits senescence through irreparable DNA damage. We demonstrate that this pathway operates in mouse liver, indicating that this machinery may contribute more broadly to tissue homeostasis in vivo.

p16INK4a and its regulator miR-24 link senescence and chondrocyte terminal differentiation-associated matrix remodeling in osteoarthritis

INTRODUCTION

Recent evidence suggests that tissue accumulation of senescent p16INK4a-positive cells during the life span would be deleterious for tissue functions and could be the consequence of inherent age-associated disorders. Osteoarthritis (OA) is characterized by the accumulation of chondrocytes expressing p16INK4a and markers of the senescence-associated secretory phenotype (SASP), including the matrix remodeling metalloproteases MMP1/MMP13 and pro-inflammatory cytokines interleukin-8 (IL-8) and IL-6. Here, we evaluated the role of p16INK4a in the OA-induced SASP and its regulation by microRNAs (miRs).

METHODS

We used IL-1-beta-treated primary OA chondrocytes cultured in three-dimensional setting or mesenchymal stem cells differentiated into chondrocyte to follow p16INK4a expression. By transient transfection experiments and the use of knockout mice, we validate p16INK4a function in chondrocytes and its regulation by one miR identified by means of a genome-wide miR-array analysis.

RESULTS

p16INK4a is induced upon IL-1-beta treatment and also during in vitro chondrogenesis. In the mouse model, Ink4a locus favors in vivo the proportion of terminally differentiated chondrocytes. When overexpressed in chondrocytes, p16INK4a is sufficient to induce the production of the two matrix remodeling enzymes, MMP1 and MMP13, thus linking senescence with OA pathogenesis and bone development. We identified miR-24 as a negative regulator of p16INK4a. Accordingly, p16INK4a expression increased while miR-24 level was repressed upon IL-1-beta addition, in OA cartilage and during in vitro terminal chondrogenesis.

CONCLUSIONS

We disclosed herein a new role of the senescence marker p16INK4a and its regulation by miR-24 during OA and terminal chondrogenesis.

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.

The senescence arrest program and the cell cycle

All living organisms are subject to progressive loss of function and damage to their tissues, a process known as aging. At the cellular level, the accumulation of damage to DNA, proteins, and organelles induces cellular senescence, a stress-response pathway that likely influences the aging process. Although the senescence arrest program was initially described in vitro, accumulating evidence suggests that this damage response program occurs in a variety of pathologic settings. This review discusses aspects of the senescence program, their interrelationships with damage arrest pathways, the cell cycle, and the impact of senescence in vivo.

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.

Construction of an integral formula of biological age for a healthy Chinese population using principle component analysis

BACKGROUND

Whereas chronological age (CA) cannot distinguish functional differences among individuals of the same age, the biological age (BA) may be used to reflect the functional state of the body. The purpose of this study was to construct an integral formula of the BA, by using principle component analysis (PCA).

METHODS

The vital organ function of 505 healthy individuals of Han origin (age 35-91 years) was examined. A total of 114 indicators of cardiovascular, pulmonary, and brain functions, and clinical, inflammatory, genetic, psychological, and life habit factors were assessed as candidate indicators of aging. Candidate indicators were submitted with CA to correlation and redundancy analyses. The PCA method was used to build an integral formula of the BA for the population.

RESULTS

Seven biomarkers were selected in accordance with a certain load standard. These biomarkers included the trail making test (TMT), pulse pressure (PP), mitral valve annulus ventricular septum of the peak velocity of early filling (MVES), minimum carotid artery intimal-medial thickness (IMTmin), maximum internal diameter of the carotid artery (Dmax), maximal midexpiratory flow rate 75/25 (MMEF75/25), and Cystatin C (CysC). The formula for the BA was: BA = 0.0685 (TMT) + 0.267 (PP) - 1.375 (MVES) + 22.443 (IMTmin) + 2.962 (Dmax) - 2.332 (MMEF75/25) + 16.104 (CysC) + 0.137 (CA) + 0.492.

CONCLUSION

Several genetic and lifestyle indicators were considered as candidate markers of aging. However, ultimately, only markers reflecting the function of the vital organs were included in the BA formula. This study represents a useful attempt to employ multiple indicators to build a comprehensive BA evaluation formula of aging populations.

Aging increases CCN1 expression leading to muscle senescence

Using microarray analysis, we found that aging sarcopenia is associated with a sharp increase in the mRNA of the matricellular protein CCN1 (Cyr61/CTGF/Nov). CCN1 mRNA was upregulated 113-fold in muscle of aged vs. young rats. CCN1 protein was increased in aging muscle in both rats (2.8-fold) and mice (3.8-fold). When muscle progenitor cells (MPCs) were treated with recombinant CCN1, cell proliferation was decreased but there was no change in the myogenic marker myoD. However, the CCN1-treated MPCs did express a senescence marker (SA-βgal). Interestingly, we found CCN1 increased p53, p16(Ink4A), and pRP (hypophosphorylated retinoblastoma protein) protein levels, all of which can arrest cell growth in MPCs. When MPCs were treated with aged rodent serum CCN1 mRNA increased by sevenfold and protein increased by threefold suggesting the presence of a circulating regulator. Therefore, we looked for a circulating regulator. Wnt-3a, a stimulator of CCN1 expression, was increased in serum from elderly humans (2.6-fold) and aged rodents (2.0-fold) compared with young controls. We transduced C2C12 myoblasts with wnt-3a and found that CCN1 protein was increased in a time- and dose-dependent manner. We conclude that in aging muscle, the circulating factor wnt-3a acts to increase CCN1 expression, prompting muscle senescence by activating cell arrest proteins.

Is HIV a model of accelerated or accentuated aging?

BACKGROUND

Antiretroviral therapy has reduced the incidence of adverse events and early mortality in HIV-infected persons. Despite these benefits, important comorbidities that increase with age (eg, diabetes, cardiovascular disease, cancer, liver disease, and neurocognitive impairment) are more prevalent in HIV-infected persons than in HIV-uninfected persons at every age, and geriatric syndromes such as falls and frailty occur earlier in HIV-infected persons. This raises a critical research question: Does HIV accelerate aging through pathways and mechanisms common to the aging process or is HIV simply an additional risk factor for a wide number of chronic conditions, thus accentuating aging?

METHODS

Extensive literature review.

RESULTS

The purpose of this review is to briefly outline the evidence that age-related clinical syndromes are exacerbated by HIV, examine the ways in which HIV is similar, and dissimilar from natural aging, and assess the validity of HIV as a model of premature aging. Specific biomarkers of aging are limited in HIV-infected hosts and impacted by antiretroviral therapy, and a high rate of modifiable life style confounders (eg, smoking, substance abuse, alcohol) and coinfections (eg, hepatitis) in HIV-infected participants.

CONCLUSIONS

There is a need for validated biomarkers of aging in the context of HIV. Despite these differences, welldesigned studies of HIV-infected participants are likely to provide new opportunities to better understand the mechanisms that lead to aging and age-related diseases.

Cardiac Hegemony of Senescence

Cardiac senescence and age-related disease development have gained general attention and recognition in the past decades due to increased accessibility and quality of health care. The advancement in global civilization is complementary to concerns regarding population aging and development of chronic degenerative diseases. Cardiac degeneration has been rigorously studied. The molecular mechanisms of cardiac senescence are on multiple cellular levels and hold a multilayer complexity level, thereby hampering development of unambiguous treatment protocols. In particular, the synergistic exchange of the senescence phenotype through a senescence secretome between myocytes and stem cells appears complicated and is of great future therapeutic value. The current review article will highlight hallmarks of senescence, cardiac myocyte and stem cell senescence, and the mutual exchange of senescent secretome. Future cardiac cell therapy approaches require a comprehensive understanding of myocardial senescence to improve therapeutic efficiency as well as efficacy.

Lifetime opiate exposure as an independent and interactive cardiovascular risk factor in males: a cross-sectional clinical study

Introduction

While several studies have identified an increased incidence of cardiovascular disorders in opiate dependence, neither opiates as a cardiovascular risk factor nor their effect on central arterial function has been considered.

Methods

Pulse wave analysis (SphygmoCor, AtCorMedical Pty Limited, Sydney, NSW, Australia) was undertaken on a cohort of controls and opiate dependent patients and the results compared to their lifetime opiate exposure.

Results

Controls (N = 401) were compared with 465 opiate dependent men. The mean (log) ages were different and were found to be 28.80 ± 0.49 years versus 35.02 ± 0.39 years (P < 0.0001), respectively. Of the opiate dependent group, 87.7% were treated with buprenorphine, 8.8% with methadone, and 3.4% with naltrexone. Multiple regression analysis was used to adjust for chronologic age (CA). At CA of 60 years, the modeled age in the controls was 66.40 years, and that in the addicted group was 73.11 years, an advancement of 6.71 years, or 10.10%. Exacerbations of age dependent changes in central arterial stiffness, central pressures, pulse rate, ejection duration, diastolic duration, and subendocardial perfusion ratio by opiate dependence were all noted (P < 0.05). Current heroin dose, heroin duration, and the dose duration interaction were all significantly related to the vascular (or “reference”) age (RA)/CA ratio (all P < 0.006). After multivariate adjustment, the opiate dose duration was independently predictive of RA (P < 0.02). Opiate dose and/or duration were included in a further 25 terms.

Conclusion

These data show that opiate use is not benign for the male cardiovascular system, but has a dose response relationship to central arterial stiffness and thus cardiovascular aging, acting independently and interactively with established cardiovascular risk factors. These findings imply accelerated organismal aging.

Whole brain radiation-induced vascular cognitive impairment: mechanisms and implications

Mild cognitive impairment is a well-documented consequence of whole brain radiation therapy (WBRT) that affects 40-50% of long-term brain tumor survivors. The exact mechanisms for the decline in cognitive function after WBRT remain elusive and no treatment or preventative measures are available for use in the clinic. Here, we review recent findings indicating how changes in the neurovascular unit may contribute to the impairments in learning and memory. In addition to affecting neuronal development, WBRT induces profound capillary rarefaction within the hippocampus - a region of the brain important for learning and memory. Therapeutic strategies such as hypoxia, which restore the capillary density, result in the rescue of cognitive function. In addition to decreasing vascular density, WBRT impairs vasculogenesis and/or angiogenesis, which may also contribute to radiation-induced cognitive decline. Further studies aimed at uncovering the specific mechanisms underlying these WBRT-induced changes in the cerebrovasculature are essential for developing therapies to mitigate the deleterious effects of WBRT on cognitive function.

Evaluating the physiological reserves of older patients with cancer: the value of potential biomarkers of aging?

Aging of an individual entails a progressive decline of functional reserves and loss of homeostasis that eventually lead to mortality. This process is highly individualized and is influenced by multiple genetic, epigenetic and environmental factors. This individualization and the diversity of factors influencing aging result in a significant heterogeneity among people with the same chronological age, representing a major challenge in daily oncology practice. Thus, many factors other than mere chronological age will contribute to treatment tolerance and outcome in the older patients with cancer. Clinical/comprehensive geriatric assessment can provide information on the general health status of individuals, but is far from perfect as a prognostic/predictive tool for individual patients. On the other hand, aging can also be assessed in terms of biological changes in certain tissues like the blood compartment which result from adaptive alterations due to past history of exposures, as well as intrinsic aging processes. There are major signs of 'aging' in lymphocytes (e.g. lymphocyte subset distribution, telomere length, p16INK4A expression), and also in (inflammatory) cytokine expression and gene expression patterns. These result from a combination of the above two processes, overlaying genetic predispositions which contribute significantly to the aging phenotype. These potential "aging biomarkers" might provide additional prognostic/predictive information supplementing clinical evaluation. The purpose of the current paper is to describe the most relevant potential "aging biomarkers" (markers that indicate the biological functional age of patients) which focus on the biological background, the (limited) available clinical data, and technical challenges. Despite their great potential interest, there is a need for much more (validated) clinical data before these biomarkers could be used in a routine clinical setting. This manuscript tries to provide a guideline on how these markers can be integrated in future research aimed at providing such data.

Mesenchymal stroma: primary determinant and therapeutic target for epithelial cancer

Multifocal and recurrent epithelial tumors, originating from either dormant or de novo cancer cells, are major causes of morbidity and mortality. The age-dependent increase of cancer incidence has long been assumed to result from the sequential accumulation of cancer-driving or -facilitating mutations with induction of cellular senescence as a protective mechanism. However, recent evidence suggests that the initiation and development of epithelial cancer results from a close interplay with its altered tissue microenvironment, with chronic inflammation, stromal senescence, autophagy, and the activation of cancer-associated fibroblasts (CAFs) playing possible primary roles. We will discuss recent progress in these areas, and highlight how this understanding may be used for devising novel preventive and therapeutic approaches to the epithelial cancer problem.

Senescent cardiac fibroblast is critical for cardiac fibrosis after myocardial infarction

Senescence is a recognized mechanism of cardiovascular diseases; however, its contribution to myocardial fibrosis and rupture after infarction and the underlying mechanisms remain unclear. Here we showed that senescent cardiac fibroblasts markedly accumulated in heart after myocardial infarction. The expression of key senescence regulators, especially p53, was significantly up-regulated in the infarcted heart or hypoxia-treated fibroblasts. Furthermore, knockdown of endogenous p53 by siRNA in fibroblasts markedly reduced hypoxia-induced cell senescence, cytokine expression but increased collagen expression, whereas increased expression of p53 protein by adenovirus infection had opposite effects. Consistent with in vitro results in cardiac fibroblasts, p53 deficiency in vivo significantly decreased the accumulation of senescent fibroblasts, the infiltration of macrophages and matrix metalloproteinases, but enhanced collagen deposition after myocardial infarction. In conclusion, these results suggest that the p53-mediated fibroblast senescence limits cardiac collagen production, and inhibition of p53 activity could represent a novel therapeutic target to increase reparative fibrosis and to prevent heart rupture after myocardial infarction.

mTORC1 and p53: clash of the gods?

A balance must be struck between cell growth and stress responses to ensure that cells proliferate without accumulating damaged DNA. This balance means that optimal cell proliferation requires the integration of pro-growth and stress-response pathways. mTOR (mechanistic target of rapamycin) is a pleiotropic kinase found in complex 1 (mTORC1).The mTORC1 pathway governs a response to mitogenic signals with high energy levels to promote protein synthesis and cell growth. In contrast, the p53DNA damage response pathway is the arbiter of cell proliferation, restraining mTORC1 under conditions of genotoxic stress. Recent studies suggest a complicated integration of these pathways to ensure successful cell growth and proliferation without compromising genome maintenance. Deciphering this integration could be key to understanding the potential clinical usefulness of mTORC1 inhibitors like rapamycin. Here we discuss how these p53-mTORC1 interactions might play a role in the suppression of cancer and perhaps the development of cellular senescence and organismal aging.

1,25(OH)2D deficiency induces temporomandibular joint osteoarthritis via secretion of senescence-associated inflammatory cytokines

1,25-Dihydroxyvitamin D [1,25(OH)(2)D] insufficiency appears to be associated with several age-related diseases. Insufficient levels of serum 25-hydroxyvitamin D has been shown to lead to the progression of osteoarthritis (OA) while underlying biological mechanisms remain largely unknown. In this study, we sought to determine whether 1,25(OH)(2)D deficiency has a direct effect on the process of murine temporomandibular joint (TMJ) OA in 25-hydroxyvitamin D 1α-hydroxylase knockout [1α(OH)ase(-/-)] mice that had been fed a rescue diet (high calcium, phosphate, and lactose) from weaning until 6 or 18 months of age. Our results showed that the bone mineral density and subchondral bone volume were reduced in mandibular condyles, articular surfaces were collapsed, the thickness of articular cartilage and cartilage matrix protein abundance were progressively decreased and eventually led to an erosion of articular cartilage of mandibular condyles. We also found that DNA damage, cellular senescence and the production of senescence-associated inflammatory cytokines were increased significantly in 1α(OH)ase(-/-) mice. This study demonstrates that 1,25(OH)(2)D deficiency causes an erosive TMJ OA phenotype by inducing DNA damage, cellular senescence and the production of senescence-associated inflammatory cytokines. Our results indicate that 1,25(OH)(2)D plays an important role in preventing the development and progression of OA.

Hypoxia-induced senescence contributes to the regulation of microenvironment in melanomas

Senescence, an irreversible state of cell cycle arrest, maintains metabolic activity. Although being a barrier against tumor development, senescence could also promote tumor progression by influencing the microenvironment. Necrosis is a common feature of various malignant tumors, which also has two opposing effects: pro-tumor by chronic inflammation and anti-tumor by effective cell clearance. However, the role of senescence in melanoma and whether it is associated with necrosis remain unclear. By detecting senescence-associated β-galactosidase activity and pimonidazole (hypoxia probe), we found that senescent cells (SA-β-gal positive) are mainly located around the necrotic/hypoxic areas of melanoma from C57BL/6J mice. Moreover, treatment of hypoxia induced irreversibly cellular senescence in vitro. In addition, the senescent cells may facilitate microenvironment modulation and promote the invasion of melanoma cells by secreting matrix metalloproteinase-2(MMP-2). Moreover, Kaplan-Meier analysis showed that the presence of necrosis in melanomas had an inverse correlation with patient survival and may serve as an independent prognostic marker. Therefore, hypoxic stress imposed on melanomas may lead to cellular senescence surrounding necrotic areas, and the adverse effects of necrosis in tumor may be attributed to the adjacent senescent cells with senescence-associated secretion phenotype (SASP), including secretion of MMP-2.

DPY30 regulates pathways in cellular senescence through ID protein expression

Cellular senescence is an intrinsic defense mechanism to various cellular stresses: while still metabolically active, senescent cells stop dividing and enter a proliferation arrest. Here, we identify DPY30, a member of all mammalian histone H3K4 histone methyltransferases (HMTases), as a key regulator of the proliferation potential of human primary cells. Following depletion of DPY30, cells show a severe proliferation defect and display a senescent phenotype, including a flattened and enlarged morphology, elevated level of reactive oxygen species (ROS), increased SA-β-galactosidase activity, and formation of senescence-associated heterochromatin foci (SAHFs). While DPY30 depletion leads to a reduced level of H3K4me3-marked active chromatin, we observed a concomitant activation of CDK inhibitors, including p16INK4a, independent of H3K4me3. ChIP experiments show that key regulators of cell-cycle progression, including ID proteins, are under direct control of DPY30. Because ID proteins are negative regulators of the transcription factors ETS1/2, depletion of DPY30 leads to the transcriptional activation of p16INK4a by ETS1/2 and thus to a senescent-like phenotype. Ectoptic re-introduction of ID protein expression can partially rescue the senescence-like phenotype induced by DPY30 depletion. Thus, our data indicate that DPY30 controls proliferation by regulating ID proteins expression, which in turn lead to senescence bypass.

A new strategy for treatment of liver fibrosis: letting MicroRNAs do the job

MicroRNAs (miRNAs) are short, endogenous, noncoding RNA molecules that regulate gene expression at a post-translational level. MiRNAs have been recognized in the regulation of physiological conditions. Moreover, awareness of the association between dysregulated miRNAs and human diseases is increasing, which consequently brings miRNAs to the frontline in the development of novel therapeutic strategies. We review the latest advances in our knowledge of the involvement of miRNAs in fibrosis with particular emphasis on hepatic fibrosis and the possibilities in the near future for miRNA-based therapy for targeted treatment of liver fibrosis. With recent advances in our understanding of the important role of senescence in the resolution of activated hepatic stellate cells (HSCs), we suggested the therapeutic potential of inducing activated HSCs into senescence by an miRNA-based strategy.

Obesity-induced gut microbial metabolite promotes liver cancer through senescence secretome

Obesity has become more prevalent in most developed countries over the past few decades, and is increasingly recognized as a major risk factor for several common types of cancer. As the worldwide obesity epidemic has shown no signs of abating, better understanding of the mechanisms underlying obesity-associated cancer is urgently needed. Although several events were proposed to be involved in obesity-associated cancer, the exact molecular mechanisms that integrate these events have remained largely unclear. Here we show that senescence-associated secretory phenotype (SASP) has crucial roles in promoting obesity-associated hepatocellular carcinoma (HCC) development in mice. Dietary or genetic obesity induces alterations of gut microbiota, thereby increasing the levels of deoxycholic acid (DCA), a gut bacterial metabolite known to cause DNA damage. The enterohepatic circulation of DCA provokes SASP phenotype in hepatic stellate cells (HSCs), which in turn secretes various inflammatory and tumour-promoting factors in the liver, thus facilitating HCC development in mice after exposure to chemical carcinogen. Notably, blocking DCA production or reducing gut bacteria efficiently prevents HCC development in obese mice. Similar results were also observed in mice lacking an SASP inducer or depleted of senescent HSCs, indicating that the DCA-SASP axis in HSCs has key roles in obesity-associated HCC development. Moreover, signs of SASP were also observed in the HSCs in the area of HCC arising in patients with non-alcoholic steatohepatitis, indicating that a similar pathway may contribute to at least certain aspects of obesity-associated HCC development in humans as well. These findings provide valuable new insights into the development of obesity-associated cancer and open up new possibilities for its control.

Curcumin triggers p16-dependent senescence in active breast cancer-associated fibroblasts and suppresses their paracrine procarcinogenic effects

Activated cancer-associated fibroblasts (CAFs) or myofibroblasts not only facilitate tumor growth and spread but also affect tumor response to therapeutic agents. Therefore, it became clear that efficient therapeutic regimens should also take into account the presence of these supportive cells and inhibit their paracrine effects. To this end, we tested the effect of low concentrations of curcumin, a pharmacologically safe natural product, on patient-derived primary breast CAF cells. We have shown that curcumin treatment upregulates p16(INK4A) and other tumor suppressor proteins while inactivates the JAK2/STAT3 pathway. This reduced the level of alpha-smooth muscle actin (α-SMA) and the migration/invasion abilities of these cells. Furthermore, curcumin suppressed the expression/secretion of stromal cell-derived factor-1 (SDF-1), interleukin-6 (IL-6), matrix metalloproteinase-2 (MMP-2), MMP-9, and transforming growth factor-β, which impeded their paracrine procarcinogenic potential. Intriguingly, these effects were sustained even after curcumin withdrawal and cell splitting. Therefore, using different markers of senescence [senescence-associated β-galactosidase (SA-β-gal) activity, Ki-67 and Lamin B1 levels, and bromodeoxyuridine incorporation], we have shown that curcumin markedly suppresses Lamin B1 and triggers DNA damage-independent senescence in proliferating but not quiescent breast stromal fibroblasts. Importantly, this curcumin-related senescence was p16(INK4A)-dependent and occurred with no associated inflammatory secretory phenotype. These results indicate the possible inactivation of cancer-associated myofibroblasts and present the first indication that curcumin can trigger DNA damage-independent and safe senescence in stromal fibroblasts.

Ionizing radiation promotes the acquisition of a senescence-associated secretory phenotype and impairs angiogenic capacity in cerebromicrovascular endothelial cells: role of increased DNA damage and decreased DNA repair capacity in microvascular radiosensitivity

Cerebromicrovascular rarefaction is believed to play a central role in cognitive impairment in patients receiving whole-brain irradiation therapy. To elucidate the mechanism underlying the deleterious effects of γ-irradiation on the cerebral microcirculation, rat primary cerebromicrovascular endothelial cells (CMVECs) were irradiated in vitro. We found that in CMVECs, γ-irradiation (2-8 Gy) elicited increased DNA damage, which was repaired less efficiently in CMVECs compared with neurons, microglia, and astrocytes. Increased genomic injury in CMVECs associated with increased apoptotic cell death. In the surviving cells, γ-irradiation promotes premature senescence (indicated by SA-β-galactosidase positivity and upregulation of p16 (INK4a) ), which was associated with impaired angiogenic capacity (decreased proliferation and tube-forming capacity). γ-Irradiated CMVECs acquired a senescence-associated secretory phenotype, characterized by upregulation of proinflammatory cytokines and chemokines (including IL-6, IL-1α, and MCP-1). Collectively, increased vulnerability of γ-irradiated CMVECs and their impaired angiogenic capacity likely contribute to cerebromicrovascular rarefaction and prevent regeneration of the microvasculature postirradiation. The acquisition of a senescence-associated secretory phenotype in irradiated CMVECs is biologically highly significant as changes in the cytokine microenvironment in the hippocampus may affect diverse biological processes relevant for normal neuronal function (including regulation of neurogenesis and the maintenance of the blood brain barrier).

The role of cellular senescence in the gastrointestinal mucosa

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.

p53-dependent release of Alarmin HMGB1 is a central mediator of senescent phenotypes

Cellular senescence irreversibly arrests proliferation in response to potentially oncogenic stress. Senescent cells also secrete inflammatory cytokines such as IL-6, which promote age-associated inflammation and pathology. HMGB1 (high mobility group box 1) modulates gene expression in the nucleus, but certain immune cells secrete HMGB1 as an extracellular Alarmin to signal tissue damage. We show that nuclear HMGB1 relocalized to the extracellular milieu in senescent human and mouse cells in culture and in vivo. In contrast to cytokine secretion, HMGB1 redistribution required the p53 tumor suppressor, but not its activator ATM. Moreover, altered HMGB1 expression induced a p53-dependent senescent growth arrest. Senescent fibroblasts secreted oxidized HMGB1, which stimulated cytokine secretion through TLR-4 signaling. HMGB1 depletion, HMGB1 blocking antibody, or TLR-4 inhibition attenuated senescence-associated IL-6 secretion, and exogenous HMGB1 stimulated NF-κB activity and restored IL-6 secretion to HMGB1-depleted cells. Our findings identify senescence as a novel biological setting in which HMGB1 functions and link HMGB1 redistribution to p53 activity and senescence-associated inflammation.

Clinical significance of cell cycle inhibitors in hepatocellular carcinoma

It is well accepted that cell cycle regulators are strongly implicated in the progression of cancer development. p16 and p27 are potent cyclin-dependent kinase (CDK) inhibitors involved in G1 phase progression, and are regarded as adverse prognostic biomarkers for various types of cancers. It has been reported that the main mechanism for p16 inactivation is aberrant DNA methylation, while p27 is exclusively inactivated by proteasome-mediated protein degradation. We have found that p27 is decreased in around half of hepatocellular carcinomas (HCCs), and in some cases p27 is inactivated by inappropriate interaction with cyclin D1/CDK4 complexes. In such cases, p16 is concomitantly inactivated through DNA methylation. Taking into consideration the complex interaction between p16 and p27, a comprehensive analysis including p16 and p27 would be useful for predicting the prognosis of HCC patients.

Cellular senescence and the senescent secretory phenotype: therapeutic opportunities

Aging is the largest risk factor for most chronic diseases, which account for the majority of morbidity and health care expenditures in developed nations. New findings suggest that aging is a modifiable risk factor, and it may be feasible to delay age-related diseases as a group by modulating fundamental aging mechanisms. One such mechanism is cellular senescence, which can cause chronic inflammation through the senescence-associated secretory phenotype (SASP). We review the mechanisms that induce senescence and the SASP, their associations with chronic disease and frailty, therapeutic opportunities based on targeting senescent cells and the SASP, and potential paths to developing clinical interventions.

Dynamics of senescent cell formation and retention revealed by p14ARF induction in the epidermis

Cellular senescence, a state of cell-cycle arrest accompanied by dramatic morphologic and metabolic changes, is a central means by which cells respond to physiologic stress and oncogene activity. Senescence is thought to play important roles in aging and in tumor suppression, yet the dynamics by which senescent cells are formed, their effects on tissue function and their eventual fate are poorly understood. To study cellular senescence within an adult tissue, we developed transgenic mice inducibly expressing p14(ARF) (human ortholog of murine p19(ARF)), a central activator of senescence. Induction of p14(ARF) in the epidermis rapidly led to widespread apoptosis and cell-cycle arrest, a stage that was transient, and was followed by p53-dependent cellular senescence. The endogenous Cdkn2a products p19(ARF) and p16(Ink4a) were activated by the transgenic p14(ARF) through p53, revealing a senescence-promoting feed-forward loop. Commitment of cells to senescence required continued p14(ARF) expression, indicating that entry into this state depends on a persistent signal. However, once formed, senescent cells were retained in the epidermis, often for weeks after transgene silencing, indicating an absence of an efficient rapidly acting mechanism for their removal. Stem cells in the hair follicle bulge were largely protected from apoptosis upon p14(ARF) induction, but irreversibly lost their ability to proliferate and initiate follicle growth. Interestingly, induction of epidermal hyperplasia prevented the appearance of senescent cells upon p14(ARF) induction. Our findings provide basic insights into the dynamics of cellular senescence, a central tumor- suppressive mechanism, and reveal the potential for prolonged retention of senescent cells within tissues.

Senescent cells and their secretory phenotype as targets for cancer therapy

Cancer is a devastating disease that increases exponentially with age. Cancer arises from cells that proliferate in an unregulated manner, an attribute that is countered by cellular senescence. Cellular senescence is a potent tumor-suppressive process that halts the proliferation, essentially irreversibly, of cells at risk for malignant transformation. A number of anti-cancer drugs have emerged that induce tumor cells to undergo cellular senescence. However, although a senescence response can halt the proliferation of cancer cells, the presence of senescent cells in tissues has been associated with age-related diseases, including, ironically, late-life cancer. Thus, anti-cancer therapies that can induce senescence might also drive aging phenotypes and age-related pathology. The deleterious effects of senescent cells most likely derive from their senescence-associated secretory phenotype or SASP. The SASP entails the secretion of numerous inflammatory cytokines, growth factors and proteases that can render the tissue microenvironment favorable for tumor growth. Here, we discuss the beneficial and detrimental effects of inducing cellular senescence, and propose strategies for targeting senescent cells as a means to fight cancer.

Detection of the senescence-associated secretory phenotype (SASP)

Cellular senescence suppresses cancer by eliminating potentially oncogenic cells, participates in tissue repair, contributes to cancer therapy, and promotes organismal aging. Numerous activities of senescent cells depend on the aptitude of these cells to secrete myriads of bioactive molecules, a behavior termed the senescence-associated secretory phenotype (SASP). The SASP supports cell-autonomous functions like the senescence-associated growth arrest, and mediates paracrine interactions between senescent cells and their surrounding microenvironment. The biological functions and the regulation of the SASP are beginning to emerge, and current SASP assessment techniques include the analysis of SASP factors at the mRNA level, the direct measurement of factors inside or outside the cell (i.e., secreted), and the detection of SASP-provoked cellular responses. Here, we focus on a simple approach to collect SASP-conditioned media in order to directly measure secreted SASP factors using sandwich enzyme-linked immunosorbent assay. As an example, we discuss the assessment of the major SASP factor interleukin-6 in senescent human fibroblasts. Supplemental notes are provided to easily adapt this procedure to other SASP factors, change cell types, or scale the techniques for different volumes or high-throughput measurements. These techniques should facilitate the discovery of novel functions and regulators of the SASP.

Dexamethasone reduces sensitivity to cisplatin by blunting p53-dependent cellular senescence in non-small cell lung cancer

Introduction

Dexamethasone (DEX) co-treatment has proved beneficial in NSCLC patients, improving clinical symptoms by the reduction of side effects after chemotherapy. However, recent studies have shown that DEX could render cancer cells more insensitive to cytotoxic drug therapy, but it is not known whether DEX co-treatment could influence therapy-induced senescence (TIS), and unknown whether it is in a p53-dependent or p53-independent manner.

Methods

We examined in different human NSCLC cell lines and detected cellular senescence after cisplatin (DDP) treatment in the presence or absence of DEX. The in vivo effect of the combination of DEX and DDP was assessed by tumor growth experiments using human lung cancer cell lines growing as xenograft tumors in nude mice.

Results

Co-treatment with DEX during chemotherapy in NSCLC resulted in increased tumor cell viability and inhibition of TIS compared with DDP treated group. DEX co-treatment cells exhibited the decrease of DNA damage signaling pathway proteins, the lower expression of p53 and p21^CIP1, the lower cellular secretory program and down-regulation of NF-κB and its signaling cascade. DEX also significantly reduced DDP sensitivity in vivo.

Conclusions

Our results underscore that DEX reduces chemotherapy sensitivity by blunting therapy induced cellular senescence after chemotherapy in NSCLC, which may, at least in part, in a p53-dependent manner. These data therefore raise concerns about the widespread combined use of gluocorticoids (GCs) with antineoplastic drugs in the clinical management of cancer patients.

Senescent cells: a novel therapeutic target for aging and age-related diseases

Aging is the main risk factor for most chronic diseases, disabilities, and declining health. It has been proposed that senescent cells--damaged cells that have lost the ability to divide--drive the deterioration that underlies aging and age-related diseases. However, definitive evidence for this relationship has been lacking. The use of a progeroid mouse model (which expresses low amounts of the mitotic checkpoint protein BubR1) has been instrumental in demonstrating that p16(Ink4a)-positive senescent cells drive age-related pathologies and that selective elimination of these cells can prevent or delay age-related deterioration. These studies identify senescent cells as potential therapeutic targets in the treatment of aging and age-related diseases. Here, we describe how senescent cells develop, the experimental evidence that causally implicates senescent cells in age-related dysfunction, the chronic diseases and disorders that are characterized by the accumulation of senescent cells at sites of pathology, and the therapeutic approaches that could specifically target senescent cells.

Postmitotic neurons develop a p21-dependent senescence-like phenotype driven by a DNA damage response

In senescent cells, a DNA damage response drives not only irreversible loss of replicative capacity but also production and secretion of reactive oxygen species (ROS) and bioactive peptides including pro-inflammatory cytokines. This makes senescent cells a potential cause of tissue functional decline in aging. To our knowledge, we show here for the first time evidence suggesting that DNA damage induces a senescence-like state in mature postmitotic neurons in vivo. About 40–80% of Purkinje neurons and 20–40% of cortical, hippocampal and peripheral neurons in the myenteric plexus from old C57Bl/6 mice showed severe DNA damage, activated p38MAPkinase, high ROS production and oxidative damage, interleukin IL-6 production, heterochromatinization and senescence-associated β-galactosidase activity. Frequencies of these senescence-like neurons increased with age. Short-term caloric restriction tended to decrease frequencies of positive cells. The phenotype was aggravated in brains of late-generation TERC−/− mice with dysfunctional telomeres. It was fully rescued by loss of p21(CDKN1A) function in late-generation TERC−/−CDKN1A−/− mice, indicating p21 as the necessary signal transducer between DNA damage response and senescence-like phenotype in neurons, as in senescing fibroblasts and other proliferation-competent cells. We conclude that a senescence-like phenotype is possibly not restricted to proliferation-competent cells. Rather, dysfunctional telomeres and/or accumulated DNA damage can induce a DNA damage response leading to a phenotype in postmitotic neurons that resembles cell senescence in multiple features. Senescence-like neurons might be a source of oxidative and inflammatory stress and a contributor to brain aging.

Cellular senescence induced by CD158d reprograms natural killer cells to promote vascular remodeling

Natural killer (NK) cells, which have an essential role in immune defense, also contribute to reproductive success. NK cells are abundant at the maternal-fetal interface, where soluble HLA-G is produced by fetal trophoblast cells during early pregnancy. Soluble HLA-G induces a proinflammatory response in primary, resting NK cells on endocytosis into early endosomes where its receptor, CD158d, resides. CD158d initiates signaling through DNA-PKcs, Akt, and NF-κB for a proinflammatory and proangiogenic response. The physiological relevance of this endosomal signaling pathway, and how activation of CD158d through soluble ligands regulates NK cell fate and function is unknown. We show here that CD158d agonists trigger a DNA damage response signaling pathway involving cyclin-dependent kinase inhibitor p21 expression and heterochromatin protein HP1-γ phosphorylation. Sustained activation through CD158d induced morphological changes in NK cell shape and size, and survival in the absence of cell-cycle entry, all hallmarks of senescence, and a transcriptional signature of a senescence-associated secretory phenotype (SASP). SASP is a program that can be induced by oncogenes or DNA damage, and promotes growth arrest and tissue repair. The secretome of CD158d-stimulated senescent NK cells promoted vascular remodeling and angiogenesis as assessed by functional readouts of vascular permeability and endothelial cell tube formation. Retrospective analysis of the decidual NK cell transcriptome revealed a strong senescence signature. We propose that a positive function of senescence in healthy tissue is to favor reproduction through the sustained activation of NK cells to remodel maternal vasculature in early pregnancy.

Aging, cellular senescence, and cancer

For most species, aging promotes a host of degenerative pathologies that are characterized by debilitating losses of tissue or cellular function. However, especially among vertebrates, aging also promotes hyperplastic pathologies, the most deadly of which is cancer. In contrast to the loss of function that characterizes degenerating cells and tissues, malignant (cancerous) cells must acquire new (albeit aberrant) functions that allow them to develop into a lethal tumor. This review discusses the idea that, despite seemingly opposite characteristics, the degenerative and hyperplastic pathologies of aging are at least partly linked by a common biological phenomenon: a cellular stress response known as cellular senescence. The senescence response is widely recognized as a potent tumor suppressive mechanism. However, recent evidence strengthens the idea that it also drives both degenerative and hyperplastic pathologies, most likely by promoting chronic inflammation. Thus, the senescence response may be the result of antagonistically pleiotropic gene action.

Astrocyte senescence as a component of Alzheimer's disease

Aging is the main risk factor for Alzheimer’s disease (AD); however, the aspects of the aging process that predispose the brain to the development of AD are largely unknown. Astrocytes perform a myriad of functions in the central nervous system to maintain homeostasis and support neuronal function. In vitro, human astrocytes are highly sensitive to oxidative stress and trigger a senescence program when faced with multiple types of stress. In order to determine whether senescent astrocytes appear in vivo, brain tissue from aged individuals and patients with AD was examined for the presence of senescent astrocytes using p16^INK4a and matrix metalloproteinase-1 (MMP-1) expression as markers of senescence. Compared with fetal tissue samples (n = 4), a significant increase in p16^INK4a-positive astrocytes was observed in subjects aged 35 to 50 years (n = 6; P = 0.02) and 78 to 90 years (n = 11; P<10⁻⁶). In addition, the frontal cortex of AD patients (n = 15) harbored a significantly greater burden of p16^INK4a-positive astrocytes compared with non-AD adult control subjects of similar ages (n = 25; P = 0.02) and fetal controls (n = 4; P<10⁻⁷). Consistent with the senescent nature of the p16^INK4a-positive astrocytes, increased metalloproteinase MMP-1 correlated with p16^INK4a. In vitro, beta-amyloid 1–42 (Aβ_1–42) triggered senescence, driving the expression of p16^INK4a and senescence-associated beta-galactosidase. In addition, we found that senescent astrocytes produce a number of inflammatory cytokines including interleukin-6 (IL-6), which seems to be regulated by p38MAPK. We propose that an accumulation of p16^INK4a-positive senescent astrocytes may link increased age and increased risk for sporadic AD.

The impact of mRNA turnover and translation on age-related muscle loss

The deterioration of skeletal muscle that develops slowly with age, termed sarcopenia, often leads to disability and mortality in the elderly population. As the proportion of elderly citizens continues to increase due to the dramatic rise in life expectancy, there are rising concerns about the healthcare cost and social burden of caring for geriatric patients. Thus, there is a growing need to understand the underlying mechanisms of sarcopenic muscle loss so that more efficacious therapies may be developed. Building evidence suggests that the onset of age-related muscle loss is linked to the age-related changes in gene expression that occur during sarcopenia. In recent work, the posttranscriptional regulation of gene expression by RNA-binding proteins (RBPs) and microRNA (miRNA) involved in the turnover and translation of mRNA were shown as key players believed to be involved in the induction of muscle wasting. Furthermore, posttranscriptional regulation may also be linked to the reduced ability of muscle satellite cells to contribute to muscle mass during ageing, a key contributing factor to sarcopenic progression. Here we highlight how the activation of pathways such as the p38 MAPK and the phosphoinositide 3-kinase (PI3K) pathways alter the ability of RBPs to regulate the expression of their target mRNAs encoding proteins involved in cell cycle (p21 and p16), as well as myogenesis (Pax7, myogenin and MyoD). Further investigation into the role of RBPs and miRNA during sarcopenia may provide new insights into the development and progression of this disorder, which may lead to the development of new treatment options for elderly patients suffering from sarcopenia.

CDK inhibitors (p16/p19/p21) induce senescence and autophagy in cancer-associated fibroblasts, "fueling" tumor growth via paracrine interactions, without an increase in neo-angiogenesis

Here, we investigated the compartment-specific role of cell cycle arrest and senescence in breast cancer tumor growth. For this purpose, we generated a number of hTERT-immortalized senescent fibroblast cell lines overexpressing CDK inhibitors, such as p16(INK4A), p19(ARF) or p21(WAF1/CIP1). Interestingly, all these senescent fibroblast cell lines showed evidence of increased susceptibility toward the induction of autophagy (either at baseline or after starvation), as well as significant mitochondrial dysfunction. Most importantly, these senescent fibroblasts also dramatically promoted tumor growth (up to ~2-fold), without any comparable increases in tumor angiogenesis. Conversely, we generated human breast cancer cells (MDA-MB-231 cells) overexpressing CDK inhibitors, namely p16(INK4A) or p21(WAF1/CIP1). Senescent MDA-MB-231 cells also showed increased expression of markers of cell cycle arrest and autophagy, including β-galactosidase, as predicted. Senescent MDA-MB-231 cells had retarded tumor growth, with up to a near 2-fold reduction in tumor volume. Thus, the effects of CDK inhibitors are compartment-specific and are related to their metabolic effects, which results in the induction of autophagy and mitochondrial dysfunction. Finally, induction of cell cycle arrest with specific inhibitors (PD0332991) or cellular stressors [hydrogen peroxide (H₂O₂) or starvation] indicated that the onset of autophagy and senescence are inextricably linked biological processes. The compartment-specific induction of senescence (and hence autophagy) may be a new therapeutic target that could be exploited for the successful treatment of human breast cancer patients.

Oncogene-induced senescence and melanoma: where do we stand?

In recent years, many groups have detected biomarkers of cellular senescence in a plethora of neoplastic lesions, in model systems, and humans. Indeed, we have come to realize that oncogene-induced senescence (OIS) acts as a potent barrier to oncogenic transformation, operating alongside cell death programs. We have begun to uncover some of its underlying principles, but many fundamental questions remain. In this perspective, some of the 'knowns' and 'unknowns' of OIS are discussed, with a focus on melanomagenesis.

Cancers co-opt cohabitants' catabolism: autophagy and senescence in the tumor stroma

Comment on: Capparelli C, et al. Cell Cycle 2012; 11:2272-84 and Capparelli C, et al. Cell Cycle 2012; 11:2285-302.

Autophagy and senescence in cancer-associated fibroblasts metabolically supports tumor growth and metastasis via glycolysis and ketone production

Senescent fibroblasts are known to promote tumor growth. However, the exact mechanism remains largely unknown. An important clue comes from recent studies linking autophagy with the onset of senescence. Thus, autophagy and senescence may be part of the same physiological process, known as the autophagy-senescence transition (AST). To test this hypothesis, human fibroblasts immortalized with telomerase (hTERT-BJ1) were stably transfected with autophagy genes (BNIP3, CTSB or ATG16L1). Their overexpression was sufficient to induce a constitutive autophagic phenotype, with features of mitophagy, mitochondrial dysfunction and a shift toward aerobic glycolysis, resulting in L-lactate and ketone body production. Autophagic fibroblasts also showed features of senescence, with increased p21(WAF1/CIP1), a CDK inhibitor, cellular hypertrophy and increased β-galactosidase activity. Thus, we genetically validated the existence of the autophagy-senescence transition. Importantly, autophagic-senescent fibroblasts promoted tumor growth and metastasis, when co-injected with human breast cancer cells, independently of angiogenesis. Autophagic-senescent fibroblasts stimulated mitochondrial metabolism in adjacent cancer cells, when the two cell types were co-cultured, as visualized by MitoTracker staining. In particular, autophagic ATG16L1 fibroblasts, which produced large amounts of ketone bodies (3-hydroxy-butyrate), had the strongest effects and promoted metastasis by up to 11-fold. Conversely, expression of ATG16L1 in epithelial cancer cells inhibited tumor growth, indicating that the effects of autophagy are compartment-specific. Thus, autophagic-senescent fibroblasts metabolically promote tumor growth and metastasis, by paracrine production of high-energy mitochondrial fuels. Our current studies provide genetic support for the importance of “two-compartment tumor metabolism” in driving tumor growth and metastasis via a simple energy transfer mechanism. Finally, β-galactosidase, a known lysosomal enzyme and biomarker of senescence, was localized to the tumor stroma in human breast cancer tissues, providing in vivo support for our hypothesis. Bioinformatic analysis of genome-wide transcriptional profiles from tumor stroma, isolated from human breast cancers, also validated the onset of an autophagy-senescence transition. Taken together, these studies establish a new functional link between host aging, autophagy, the tumor microenvironment and cancer metabolism.

Translational research in immune senescence: assessing the relevance of current models

Advancing age is accompanied by profound changes in immune function; some are induced by the loss of critical niches that support development of naïve cells (e.g. thymic involution), others by the intrinsic physiology of long-lived cells attempting to maintain homeostasis, still others by extrinsic effects such as oxidative stress or long-term exposure to antigen due to persistent viral infections. Once compensatory mechanisms can no longer maintain a youthful phenotype the end result is the immune senescent milieu - one characterized by chronic, low grade, systemic inflammation and impaired responses to immune challenge, particularly when encountering new antigens. This state is associated with progression of chronic illnesses like atherosclerosis and dementia, and an increased risk of acute illness, disability and death in older adults. The complex interaction between immune senescence and chronic illness provides an ideal landscape for translational research with the potential to greatly affect human health. However, current animal models and even human investigative strategies for immune senescence have marked limitations, and the reductionist paradigm itself may be poorly suited to meet these challenges. A new paradigm, one that embraces complexity as a core feature of research in older adults is required to address the critical health issues facing the burgeoning senior population, the group that consumes the majority of healthcare resources. In this review, we outline the major advantages and limitations of current models and offer suggestions for how to move forward.