Pharmacological CDK4/6 inhibition reveals a p53-dependent senescent state with restricted toxicity

Cellular senescence is a state of stable growth arrest and a desired outcome of tumor suppressive interventions. Treatment with many anti‐cancer drugs can cause premature senescence of non‐malignant cells. These therapy‐induced senescent cells can have pro‐tumorigenic and pro‐disease functions via activation of an inflammatory secretory phenotype (SASP). Inhibitors of cyclin‐dependent kinases 4/6 (CDK4/6i) have recently proven to restrain tumor growth by activating a senescence‐like program in cancer cells. However, the physiological consequence of exposing the whole organism to pharmacological CDK4/6i remains poorly characterized. Here, we show that exposure to CDK4/6i induces non‐malignant cells to enter a premature state of senescence dependent on p53. We observe in mice and breast cancer patients that the CDK4/6i‐induced senescent program activates only a partial SASP enriched in p53 targets but lacking pro‐inflammatory and NF‐κB‐driven components. We find that CDK4/6i‐induced senescent cells do not acquire pro‐tumorigenic and detrimental properties but retain the ability to promote paracrine senescence and undergo clearance. Our results demonstrate that SASP composition is exquisitely stress‐dependent and a predictor for the biological functions of different senescence subsets.

The Rise and the Death of Senescent Cells: From Mechanisms to Interventions

Aging is at the root of age-related diseases and therapies targeting basic age-associated mechanisms have the potential to extend healthy lifespan. A common feature of older organisms is the accumulation of senescent cells – cells that have irreversibly lost the capacity to undergo replication. Senescent cells are characterized by an irreversible cell cycle arrest and by the Senescence-Associated Secretory Phenotype (SASP), which include many tissue remodeling and pro-inflammatory factors. Senescent cells are intermittently present during embryogenesis and in young organisms. On the contrary senescent cells accumulate and persist in aging tissues. Significantly, these persistent senescent cells can drive low-grade chronic inflammation, and their genetic or pharmacological elimination is sufficient to delay a number of diseases and to improve health span. Here, I will discuss the mechanisms by which senescent cells can promote tissue aging and dysfunction and the potential of targeting senescent cells to delay human aging.

The Quest to Define and Target Cellular Senescence in Cancer

Cellular senescence represents a double-edged sword in cancer and its therapy. On one side, senescence-associated growth arrest and immunomodulatory properties exert potent antimalignant functions. On the other side, senescence bypass and secretory phenotype are associated with tumor progression and relapse. Recent studies have demonstrated the enormous potential to combine pro- to antisenescence interventions as a new anticancer approach. However, the heterogeneity of senescence-associated features makes definition and targeting of therapy-induced senescent cells a challenging task. Here, we describe these challenges and discuss how to exploit senescence-associated features to improve treatment efficacy and tolerability.

A recurrent chromosomal inversion suffices for driving escape from oncogene-induced senescence via subTAD reorganization

Oncogene-induced senescence (OIS) is an inherent and important tumor suppressor mechanism. However, if not removed timely via immune surveillance, senescent cells also have detrimental effects. Although this has mostly been attributed to the senescence-associated secretory phenotype (SASP) of these cells, we recently proposed that "escape" from the senescent state is another unfavorable outcome. The mechanism underlying this phenomenon remains elusive. Here, we exploit genomic and functional data from a prototypical human epithelial cell model carrying an inducible CDC6 oncogene to identify an early-acquired recurrent chromosomal inversion that harbors a locus encoding the circadian transcription factor BHLHE40. This inversion alone suffices for BHLHE40 activation upon CDC6 induction and driving cell cycle re-entry of senescent cells, and malignant transformation. Ectopic overexpression of BHLHE40 prevented induction of CDC6-triggered senescence. We provide strong evidence in support of replication stress-induced genomic instability being a causative factor underlying "escape" from oncogene-induced senescence.

Identification of distinct and age-dependent p16High microglia subtypes

Cells expressing high levels of the cyclin‐dependent kinase (CDK)4/6 inhibitor p16 (p16^High) accumulate in aging tissues and promote multiple age‐related pathologies, including neurodegeneration. Here, we show that the number of p16^High cells is significantly increased in the central nervous system (CNS) of 2‐year‐old mice. Bulk RNAseq indicated that genes expressed by p16^High cells were associated with inflammation and phagocytosis. Single‐cell RNAseq of brain cells indicated p16^High cells were primarily microglia, and their accumulation was confirmed in brains of aged humans. Interestingly, we identified two distinct subpopulations of p16^High microglia in the mouse brain, with one being age‐associated and one present in young animals. Both p16^High clusters significantly differed from previously described disease‐associated microglia and expressed only a partial senescence signature. Taken together, our study provides evidence for the existence of two p16‐expressing microglia populations, one accumulating with age and another already present in youth that could positively and negatively contribute to brain homeostasis, function, and disease.

Hepatic stellate cell senescence in liver fibrosis: Characteristics, mechanisms and perspectives

Myofibroblasts play an important role in fibrogenesis. Hepatic stellate cells are the main precursors of myofibroblasts. Cellular senescence is the terminal cell fate in which proliferating cells undergo irreversible cell cycle arrest. Senescent hepatic stellate cells were identified in liver fibrosis. Senescent hepatic stellate cells display decreased collagen production and proliferation. Therefore, induction of senescence could be a protective mechanism against progression of liver fibrosis and the concept of therapy-induced senescence has been proposed to treat liver fibrosis. In this review, characteristics of senescent hepatic stellate cells and the essential signaling pathways involved in senescence are reviewed. Furthermore, the potential impact of senescent hepatic stellate cells on other liver cell types are discussed. Senescent cells are cleared by the immune system. The persistence of senescent cells can remodel the microenvironment and interact with inflammatory cells to induce aging-related dysfunction. Therefore, senolytics, a class of compounds that selectively induce death of senescent cells, were introduced as treatment to remove senescent cells and consequently decrease the disadvantageous effects of persisting senescent cells. The effects of senescent hepatic stellate cells in liver fibrosis need further investigation.

Cellular senescence impairs the reversibility of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) in congenital cardiac shunts can be reversed by hemodynamic unloading (HU) through shunt closure. However, this reversibility potential is lost beyond a certain point in time. The reason why PAH becomes irreversible is unknown. In this study, we used MCT+shunt-induced PAH in rats to identify a dichotomous reversibility response to HU, similar to the human situation. We compared vascular profiles of reversible and irreversible PAH using RNA sequencing. Cumulatively, we report that loss of reversibility is associated with a switch from a proliferative to a senescent vascular phenotype and confirmed markers of senescence in human PAH-CHD tissue. In vitro, we showed that human pulmonary endothelial cells of patients with PAH are more vulnerable to senescence than controls in response to shear stress and confirmed that the senolytic ABT263 induces apoptosis in senescent, but not in normal, endothelial cells. To support the concept that vascular cell senescence is causal to the irreversible nature of end-stage PAH, we targeted senescence using ABT263 and induced reversal of the hemodynamic and structural changes associated with severe PAH refractory to HU. The factors that drive the transition from a reversible to irreversible pulmonary vascular phenotype could also explain the irreversible nature of other PAH etiologies and provide new leads for pharmacological reversal of end-stage PAH.

Physiological hypoxia restrains the senescence-associated secretory phenotype via AMPK-mediated mTOR suppression

Cellular senescence is a state of stable proliferative arrest triggered by damaging signals. Senescent cells persist during aging and promote age-related pathologies via the pro-inflammatory senescence-associated secretory phenotype (SASP), whose regulation depends on environmental factors. In vivo, a major environmental variable is oxygenation, which varies among and within tissues. Here, we demonstrate that senescent cells express lower levels of detrimental pro-inflammatory SASP factors in physiologically hypoxic environments, as measured in culture and in tissues. Mechanistically, exposure of senescent cells to low-oxygen conditions leads to AMPK activation and AMPK-mediated suppression of the mTOR-NF-κB signaling loop. Finally, we demonstrate that treatment with hypoxia-mimetic compounds reduces SASP in cells and tissues and improves strength in chemotherapy-treated and aged mice. Our findings highlight the importance of oxygen as a determinant for pro-inflammatory SASP expression and offer a potential new strategy to reduce detrimental paracrine effects of senescent cells.

To breathe or not to breathe: Understanding how oxygen sensing contributes to age-related phenotypes

Aging is characterized by a progressive loss of tissue integrity and functionality due to disrupted homeostasis. Molecular oxygen is pivotal to maintain tissue functions, and aerobic species have evolved a sophisticated sensing system to ensure proper oxygen supply and demand. It is not surprising that aberrations in oxygen and oxygen-associated pathways subvert health and promote different aspects of aging. In this review, we discuss emerging findings on how oxygen-sensing mechanisms regulate different cellular and molecular processes during normal physiology, and how dysregulation of oxygen availability lead to disease and aging. We describe various clinical manifestations associated with deregulation of oxygen balance, and how oxygen-modulating therapies and natural oxygen oscillations influence longevity. We conclude by discussing how a better understanding of oxygen-related mechanisms that orchestrate aging processes may lead to the development of new therapeutic strategies to extend healthy aging.

Algorithmic assessment of cellular senescence in experimental and clinical specimens

The development of genetic tools allowed for the validation of the pro-aging and pro-disease functions of senescent cells in vivo. These discoveries prompted the development of senotherapies-pharmaceutical interventions aimed at interfering with the detrimental effect of senescent cells-that are now entering the clinical stage. However, unequivocal identification and examination of cellular senescence remains highly difficult because of the lack of universal and specific markers. Here, to overcome the limitation of measuring individual markers, we describe a detailed two-phase algorithmic assessment to quantify various senescence-associated parameters in the same specimen. In the first phase, we combine the measurement of lysosomal and proliferative features with the expression of general senescence-associated genes to validate the presence of senescent cells. In the second phase we measure the levels of pro-inflammatory markers for specification of the type of senescence. The protocol can help graduate-level basic scientists to improve the characterization of senescence-associated phenotypes and the identification of specific senescent subtypes. Moreover, it can serve as an important tool for the clinical validation of the role of senescent cells and the effectiveness of anti-senescence therapies.

The struggle of a good friend getting old: cellular senescence in viral responses and therapy

Cellular senescence is a state of stable cell cycle arrest associated with macromolecular alterations and secretion of pro-inflammatory cytokines and molecules. Senescence-associated phenotypes restrict damage propagation and activate immune responses, two essential processes involved in response to viral infections. However, excessive accumulation and persistence of senescent cells can become detrimental and promote pathology and dysfunctions. Various pharmacological interventions, including antiviral therapies, lead to aberrant and premature senescence. Here, we review the molecular mechanisms by which viral infections and antiviral therapy induce senescence. We highlight the importance of these processes in attenuating viral dissemination and damage propagation, but also how prematurely induced senescent cells can promote detrimental adverse effects in humans. We describe which sequelae due to viral infections and treatment can be partly due to excessive and aberrant senescence. Finally, we propose that pharmacological strategies which eliminate senescent cells or suppress their secretory phenotype could mitigate side effects and alleviate the onset of additional morbidities. These strategies can become extremely beneficial in patients recovering from viral infections or undergoing antiviral therapy.

Biological functions of therapy-induced senescence in cancer

Therapy-induced cellular senescence is a state of stable growth arrest induced by common cancer treatments such as chemotherapy and radiation. In an oncogenic context, therapy-induced senescence can have different consequences. By blocking cellular proliferation and by facilitating immune cell infiltration, it functions as tumor suppressive mechanism. By fueling the proliferation of bystander cells and facilitating metastasis, it acts as a tumor promoting factor. This dual role is mainly attributed to the differential expression and secretion of a set of pro-inflammatory cytokines and tissue remodeling factors, collectively known as the Senescence-Associated Secretory Phenotype (SASP). Here, we describe cell-autonomous and non-cell-autonomous mechanisms that senescent cells activate in response to chemotherapy and radiation leading to tumor suppression and tumor promotion. We present the current state of knowledge on the stimuli that affect the activation of these opposing mechanisms and the effect of senescent cells on their micro-environment eg. by regulating the functions of immune cells in tumor clearance as well as strategies to eliminate senescent tumor cells before exerting their deleterious side-effects.

Unravelling Heterogeneity of Amplified Human Amniotic Fluid Stem Cells Sub-Populations

Human amniotic fluid stem cells (hAFSCs) are broadly multipotent immature progenitor cells with high self-renewal and no tumorigenic properties. These cells, even amplified, present very variable morphology, density, intracellular composition and stemness potential, and this heterogeneity can hinder their characterization and potential use in regenerative medicine. Celector^® (Stem Sel ltd.) is a new technology that exploits the Non-Equilibrium Earth Gravity Assisted Field Flow Fractionation principles to characterize and label-free sort stem cells based on their solely physical characteristics without any manipulation. Viable cells are collected and used for further studies or direct applications. In order to understand the intrapopulation heterogeneity, various fractions of hAFSCs were isolated using the Celector^® profile and live imaging feature. The gene expression profile of each fraction was analysed using whole-transcriptome sequencing (RNAseq). Gene Set Enrichment Analysis identified significant differential expression in pathways related to Stemness, DNA repair, E2F targets, G2M checkpoint, hypoxia, EM transition, mTORC1 signalling, Unfold Protein Response and p53 signalling. These differences were validated by RT-PCR, immunofluorescence and differentiation assays. Interestingly, the different fractions showed distinct and unique stemness properties. These results suggest the existence of deep intra-population differences that can influence the stemness profile of hAFSCs. This study represents a proof-of-concept of the importance of selecting certain cellular fractions with the highest potential to use in regenerative medicine.

ARDD 2020: from aging mechanisms to interventions

Aging is emerging as a druggable target with growing interest from academia, industry and investors. New technologies such as artificial intelligence and advanced screening techniques, as well as a strong influence from the industry sector may lead to novel discoveries to treat age-related diseases. The present review summarizes presentations from the 7th Annual Aging Research and Drug Discovery (ARDD) meeting, held online on the 1st to 4th of September 2020. The meeting covered topics related to new methodologies to study aging, knowledge about basic mechanisms of longevity, latest interventional strategies to target the aging process as well as discussions about the impact of aging research on society and economy. More than 2000 participants and 65 speakers joined the meeting and we already look forward to an even larger meeting next year. Please mark your calendars for the 8th ARDD meeting that is scheduled for the 31st of August to 3rd of September, 2021, at Columbia University, USA.

Cellular Senescence and the Senescence-Associated Secretory Phenotype as Drivers of Skin Photoaging

Chronic exposure to UVR is known to disrupt tissue homeostasis, accelerate the onset of age-related phenotypes, and increase the risk for skin cancer-a phenomenon defined as photoaging. In this paper, we review the current knowledge on how UV exposure causes cells to prematurely enter cellular senescence. We describe the mechanisms contributing to the accumulation of senescent cells in the skin and how the persistence of cellular senescence can promote impaired regenerative capacity, chronic inflammation, and tumorigenesis associated with photoaging. We conclude by highlighting the potential of senolytic drugs in delaying the onset and progression of age-associated phenotypes in the skin.

Cellular senescence as a potential mediator of COVID-19 severity in the elderly

SARS-CoV-2 is a novel betacoronavirus which infects the lower respiratory tract and can cause coronavirus disease 2019 (COVID-19), a complex respiratory distress syndrome. Epidemiological data show that COVID-19 has a rising mortality particularly in individuals with advanced age. Identifying a functional association between SARS-CoV-2 infection and the process of biological aging may provide a tractable avenue for therapy to prevent acute and long-term disease. Here, we discuss how cellular senescence-a state of stable growth arrest characterized by pro-inflammatory and pro-disease functions-can hypothetically be a contributor to COVID-19 pathogenesis, and a potential pharmaceutical target to alleviate disease severity. First, we define why older COVID-19 patients are more likely to accumulate high levels of cellular senescence. Second, we describe how senescent cells can contribute to an uncontrolled SARS-CoV-2-mediated cytokine storm and an excessive inflammatory reaction during the early phase of the disease. Third, we discuss the various mechanisms by which senescent cells promote tissue damage leading to lung failure and multi-tissue dysfunctions. Fourth, we argue that a high senescence burst might negatively impact on vaccine efficacy. Measuring the burst of cellular senescence could hypothetically serve as a predictor of COVID-19 severity, and targeting senescence-associated mechanisms prior and after SARS-CoV-2 infection might have the potential to limit a number of severe damages and to improve the efficacy of vaccinations.

A Senescence-Centric View of Aging: Implications for Longevity and Disease

Cellular senescence is a state of stable cell cycle arrest associated with macromolecular alterations and secretion of proinflammatory cytokines and molecules. From their initial discovery in the 1960s, senescent cells have been hypothesized as potential contributors to the age-associated loss of regenerative potential. Here, we discuss recent evidence that implicates cellular senescence as a central regulatory mechanism of the aging process. We provide a comprehensive overview of age-associated pathologies in which cellular senescence has been implicated. We describe mechanisms by which senescent cells drive aging and diseases, and we discuss updates on exploiting these mechanisms as therapeutic targets. Finally, we critically analyze the use of senotherapeutics and their translation to the clinic, highlighting limitations and suggesting ideas for future applications and developments.

Cellular Senescence Promotes Skin Carcinogenesis through p38MAPK and p44/42MAPK Signaling

Cellular senescence entails an irreversible growth arrest that evolved in part to prevent cancer. Paradoxically, senescent cells secrete proinflammatory and growth-stimulatory molecules, termed the senescence-associated secretory phenotype (SASP), which is correlated with cancer cell proliferation in culture and xenograft models. However, at what tumor stage and how senescence and the SASP act on endogenous tumor growth in vivo is unknown. To understand the role of senescence in cancer etiology, we subjected p16-3MR transgenic mice, which permit the identification and selective elimination of senescent cells in vivo, to the well-established two-step protocol of squamous cell skin carcinoma, in which tumorigenesis is initiated by a carcinogen 7,12-dimethylbenz[α]anthracene, and then promoted by 12-O-tetradecanoyl-phorbol-13-acetate (TPA). We show that TPA promotes skin carcinogenesis by inducing senescence and a SASP. Systemic induction of senescence in nontumor-bearing p16-3MR mice using a chemotherapy followed by the two-step carcinogenesis protocol potentiated the conversion of benign papillomas to carcinomas by elevating p38MAPK and MAPK/ERK signaling. Ablation of senescent cells reduced p38MAPK and MAPK/ERK signaling, thereby preventing the progression of benign papillomas to carcinomas. Thus, we show for the first time that senescent cells are tumor promoters, not tumor initiators, and that they stimulate skin carcinogenesis by elevating p38MAPK and MAPK/ERK signaling. These findings pave the way for developing novel therapeutics against senescence-fueled cancers. SIGNIFICANCE: These findings identify chemotherapy-induced senescence as a culprit behind tumor promotion, suggesting that elimination of senescent cells after chemotherapy may reduce occurrence of second cancers decades later. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/17/3606/F1.large.jpg.

Senescent Cells in Cancer Therapy: Friends or Foes?

Several cancer interventions induce DNA damage and promote senescence in cancer and nonmalignant cells. Senescent cells secrete a collection of proinflammatory factors collectively termed the senescence-associated secretory phenotype (SASP). SASP factors are able to potentiate various aspects of tumorigenesis, including proliferation, metastasis, and immunosuppression. Moreover, the accumulation and persistence of therapy-induced senescent cells can promote tissue dysfunction and the early onset of various age-related symptoms in treated cancer patients. Here, we review in detail the mechanisms by which cellular senescence contributes to cancer development and the side effects of cancer therapies. We also review how pharmacological interventions to eliminate senescent cells or inhibit SASP production can mitigate these negative effects and propose therapeutic strategies based on the age of the patient.

Link between increased cellular senescence and extracellular matrix changes in COPD

Chronic obstructive pulmonary disease (COPD) is associated with features of accelerated aging, including cellular senescence, DNA damage, oxidative stress, and extracellular matrix (ECM) changes. We propose that these features are particularly apparent in patients with severe, early-onset (SEO)-COPD. Whether fibroblasts from COPD patients display features of accelerated aging and whether this is also present in relatively young SEO-COPD patients is unknown. Therefore, we aimed to determine markers of aging in (SEO)-COPD-derived lung fibroblasts and investigate the impact on ECM. Aging hallmarks and ECM markers were analyzed in lung fibroblasts from SEO-COPD and older COPD patients and compared with fibroblasts from matched non-COPD groups (n = 9-11 per group), both at normal culture conditions and upon Paraquat-induced senescence. COPD-related differences in senescence and ECM expression were validated in lung tissue. Higher levels of cellular senescence, including senescence-associated β-galactosidase (SA-β-gal)-positive cells (19% for COPD vs. 13% for control) and p16 expression, DNA damage (γ-H2A.X-positive nuclei), and oxidative stress (MGST1) were detected in COPD compared with control-derived fibroblasts. Most effects were also different in SEO-COPD, with SA-β-gal-positive cells only being significant in SEO-COPD vs. matched controls. Lower decorin expression in COPD-derived fibroblasts correlated with higher p16 expression, and this association was confirmed in lung tissue. Paraquat treatment induced cellular senescence along with clear changes in ECM expression, including decorin. Fibroblasts from COPD patients, including SEO-COPD, display higher levels of cellular senescence, DNA damage, and oxidative stress. The association between cellular senescence and ECM expression changes may suggest a link between accelerated aging and ECM dysregulation in COPD.

Early ageing after cytotoxic treatment for testicular cancer and cellular senescence: Time to act

Treatment of testicular cancer (TC) has an exceptionally high success rate compared to other cancer types; even in case of metastasized disease, 80-90 % of TC patients can be cured. Consequently, attention has been drawn to a potential downside of this treatment success: late adverse treatment effects such as the accelerated development of otherwise age-associated features like cardiovascular disease and second malignancies. Underlying mechanisms are poorly understood. Emerging data suggest that cytotoxic treatment induces cellular senescence, resulting in secretion of inflammatory factors contributing to this early ageing phenotype. Molecular and cellular characterization of this early ageing will enhance understanding the pathogenesis of TC treatment-induced morbidity and contribute to better recognition and prevention of late effects. In this review, we describe clinical manifestations of the early ageing phenotype among TC survivors, and subsequently focus on potential underlying mechanisms. We discuss the clinical implications and describe perspectives for future research and intervention strategies.

Cellular Senescence: Defining a Path Forward

Cellular senescence is a cell state implicated in various physiological processes and a wide spectrum of age-related diseases. Recently, interest in therapeutically targeting senescence to improve healthy aging and age-related disease, otherwise known as senotherapy, has been growing rapidly. Thus, the accurate detection of senescent cells, especially in vivo, is essential. Here, we present a consensus from the International Cell Senescence Association (ICSA), defining and discussing key cellular and molecular features of senescence and offering recommendations on how to use them as biomarkers. We also present a resource tool to facilitate the identification of genes linked with senescence, SeneQuest (available at http://Senequest.net). Lastly, we propose an algorithm to accurately assess and quantify senescence, both in cultured cells and in vivo.

SILAC Analysis Reveals Increased Secretion of Hemostasis-Related Factors by Senescent Cells

Cellular senescence irreversibly arrests cell proliferation, accompanied by a multi-component senescence-associated secretory phenotype (SASP) that participates in several age-related diseases. Using stable isotope labeling with amino acids (SILACs) and cultured cells, we identify 343 SASP proteins that senescent human fibroblasts secrete at 2-fold or higher levels compared with quiescent cell counterparts. Bioinformatic analysis reveals that 44 of these proteins participate in hemostasis, a process not previously linked with cellular senescence. We validated the expression of some of these SASP factors in cultured cells and in vivo. Mice treated with the chemotherapeutic agent doxorubicin, which induces widespread cellular senescence in vivo, show increased blood clotting. Conversely, selective removal of senescent cells using transgenic p16-3MR mice showed that clearing senescent cells attenuates the increased clotting caused by doxorubicin. Our study provides an in-depth, unbiased analysis of the SASP and unveils a function for cellular senescence in hemostasis.