p16INK4a reporter mice reveal age-promoting effects of environmental toxicants

Towards Healthy Longevity: Comprehensive Insights from Molecular Targets and Biomarkers to Biological Clocks

Aging is a complex and time-dependent decline in physiological function that affects most organisms, leading to increased risk of age-related diseases. Investigating the molecular underpinnings of aging is crucial to identify geroprotectors, precisely quantify biological age, and propose healthy longevity approaches. This review explores pathways that are currently being investigated as intervention targets and aging biomarkers spanning molecular, cellular, and systemic dimensions. Interventions that target these hallmarks may ameliorate the aging process, with some progressing to clinical trials. Biomarkers of these hallmarks are used to estimate biological aging and risk of aging-associated disease. Utilizing aging biomarkers, biological aging clocks can be constructed that predict a state of abnormal aging, age-related diseases, and increased mortality. Biological age estimation can therefore provide the basis for a fine-grained risk stratification by predicting all-cause mortality well ahead of the onset of specific diseases, thus offering a window for intervention. Yet, despite technological advancements, challenges persist due to individual variability and the dynamic nature of these biomarkers. Addressing this requires longitudinal studies for robust biomarker identification. Overall, utilizing the hallmarks of aging to discover new drug targets and develop new biomarkers opens new frontiers in medicine. Prospects involve multi-omics integration, machine learning, and personalized approaches for targeted interventions, promising a healthier aging population.

Resveratrol Alleviates Arsenic Exposure-Induced Liver Fibrosis in Rats by Inhibiting Hepatocyte Senescence

Arsenic is an environmental pollutant that has garnered considerable attention from the World Health Organization. Liver fibrosis is an advanced pathological stage of liver injury that can be caused by chronic arsenic exposure and has the potential to be reversed to prevent cirrhosis and hepatic malignancies. However, effective treatment options are currently limited. Given the profibrogenic effect of hepatocyte senescence, we established a rat model of sub-chronic sodium arsenite exposure and investigated the ability of resveratrol (RSV), a potential anti-senescence agent, to ameliorate arsenic-induced liver fibrosis and elucidate the underlying mechanism from the perspective of hepatocyte senescence. The results demonstrated that RSV was capable of mitigating fibrosis phenotypes in rat livers, including the activation of hepatic stellate cell (HSC), the generation of extracellular matrix, and the deposition of collagen fibers in the liver vascular zone, which are all induced by arsenic exposure. Furthermore, as an activator of the longevity factor SIRT1, RSV antagonized the arsenic-induced inhibition of SIRT1 expression, thereby restoring the suppression of the senescence protein p16 by SIRT1. This prevented arsenic-induced hepatocyte senescence, manifesting as a decrease in telomere shortening and a reduction in the release of senescence-associated secretory phenotype (SASP)-related proteins. In conclusion, this study demonstrated that RSV counteracts arsenic-induced hepatocyte senescence and the release of SASP-related proteins by restoring the inhibitory effect of SIRT1 on p16, thereby suppressing the activation of fibrotic phenotypes and mitigating liver fibrosis. These findings provide new insights for understanding the mechanism of arsenic-induced liver fibrosis, and more importantly, they reveal novel potential interventional approaches.

miR-21-Mediated Endothelial Senescence and Dysfunction Are Involved in Cigarette Smoke-Induced Pulmonary Hypertension through Activation of PI3K/AKT/mTOR Signaling

Smoking is a pathogenic factor for pulmonary hypertension (PH). Our previous study showed that serum miR-21 levels are elevated in smokers. miR-21 is considered as engaged in the PH process; however, its mechanisms remain unclear. In this investigation, we found that in the lung tissue of smoking-induced PH patients, the levels of miR-21 and aging markers (p21 and p16) were upregulated, and the function of pulmonary vascular endothelial cells was also impaired. Exposure of mice to cigarette smoke (CS) for four months caused similar changes in lung tissues and increased pulmonary arterial pressure, which were attenuated by knockout of miR-21. Further, human umbilical vein endothelial cells (HUVECs) exposed to cigarette smoke extract (CSE) revealed upregulation of miR-21 levels, depression of PTEN, activation of PI3K/AKT/mTOR signaling, an increase in senescence indexes, and enhanced dysfunction. Inhibiting miR-21 overexpression reversed the PTEN-mTOR signaling pathway and prevented senescence and dysfunction of HUVECs. In sum, our data indicate that miR-21-mediated endothelial senescence and dysfunction are involved in CS-induced PH through the activation of PI3K/AKT/mTOR signaling, which suggests that selective miR-21 inhibition offers the potential to attenuate PH.

Senescence and fibrosis in salivary gland aging and disease

Salivary gland hypofunction is highly prevalent in aged and diseased individuals leading to significant discomfort and morbidity. One factor that contributes to salivary gland hypofunction is cellular aging, or senescence. Senescent cells can impair gland function by secreting paracrine-acting growth factors and cytokines, known as senescence-associated secretory phenotype (SASP) factors. These SASP factors stimulate inflammation, propagate the senescent phenotype through the bystander effect, and stimulate fibrosis. As senotherapeutics that target senescent cells have shown effectiveness in limiting disease manifestations in other conditions, there is interest in the use of these drugs to treat salivary gland hypofunction. In this review, we highlight the contribution of senescence and fibrosis to salivary gland pathologies. We also discuss therapeutic approaches to eliminate or modulate the senescent SASP phenotype for treating age-related salivary gland diseases and extending health span.

Role of mitochondrial fusion proteins MFN2 and OPA1 on lung cellular senescence in chronic obstructive pulmonary disease

BACKGROUND

Mitochondrial dysfunction and lung cellular senescence are significant features involved in the pathogenesis of chronic obstructive pulmonary disease (COPD). Cigarette smoke (CS) stands as the primary contributing factor to COPD. This study examined mitochondrial dynamics, mitophagy and lung cellular senescence in COPD patients and investigated the effects of modulation of mitochondrial fusion [mitofusin2 (MFN2) and Optic atrophy 1 (OPA1)] on CS extract (CSE)-induced lung cellular senescence.

METHODS

Senescence-associated secretory phenotype (SASP) component mRNAs (IL-1β, IL-6, CXCL1 and CXCL8), mitochondrial morphology, mitophagy and mitochondria-related proteins (including phosphorylated-DRP1(p-DRP1), DRP1, MFF, MNF2, OPA1, PINK1, PARK2, SQSTM1/p62 and LC3b) and senescence-related proteins (including P16, H2A.X and Klotho) were measured in lung tissues or primary alveolar type II (ATII) cells of non-smokers, smokers and COPD patients. Alveolar epithelial (A549) cells were exposed to CSE with either pharmacologic inducer (leflunomide and BGP15) or genetic induction of MFN2 and OPA1 respectively.

RESULTS

There were increases in mitochondrial number, and decreases in mitochondrial size and activity in lung tissues from COPD patients. SASP-related mRNAs, DRP1 phosphorylation, DRP1, MFF, PARK2, SQSTM1/p62, LC3B II/LC3B I, P16 and H2A.X protein levels were increased, while MFN2, OPA1, PINK1 and Klotho protein levels were decreased in lung tissues from COPD patients. Some similar results were identified in primary ATII cells of COPD patients. CSE induced increases in oxidative stress, SASP-related mRNAs, mitochondrial damage and dysfunction, mitophagy and cellular senescence in A549 cells, which were ameliorated by both pharmacological inducers and genetic overexpression of MFN2 and OPA1.

CONCLUSIONS

Impaired mitochondrial fusion, enhanced mitophagy and lung cellular senescence are observed in the lung of COPD patients. Up-regulation of MFN2 and OPA1 attenuates oxidative stress, mitophagy and lung cellular senescence, offering potential innovative therapeutic targets for COPD therapy.

Benidipine calcium channel blocker promotes the death of cigarette smoke-induced senescent cells and improves lung emphysema

Smoking is the main risk factor for many lung diseases including chronic obstructive pulmonary disease. Cigarette smoke (CS) contains carcinogenic and reactive oxygen species that favor DNA mutations and perturb the homeostasis and environment of cells. CS induces lung cell senescence resulting in a stable proliferation arrest and a senescence-associated secretory phenotype. It was recently reported that senescent cell accumulation promotes several lung diseases. In this study, we performed a chemical screen, using an FDA-approved drug library, to identify compounds selectively promoting the death of CS-induced senescent lung cells. Aside from the well-known senolytic, ABT-263, we identified other potentially new senescence-eliminating compounds, including a new class of molecules, the dihydropyridine family of calcium voltage-gated channel (CaV) blockers. Among these blockers, Benidipine, decreased senescent lung cells and ameliorates lung emphysema in a mouse model. The dihydropyridine family of CaV blockers thus constitutes a new class of senolytics that could improve lung diseases. Hence, our work paves the way for further studies on the senolytic activity of CaV blockers in different senescence contexts and age-related diseases.

Assessing the potential molecular mechanism of arsenite-induced skin cell senescence

Arsenic exposure is a public health concern worldwide. Skin damage, as a typical lesion of arsenic exposure, the mechanism is still unknown. Studies have found that cellular senescence plays a key role in arsenic-induced skin damage, and the previous research found that the ERK/CEBPB signaling pathway may be an important molecular event of arsenic-induced skin cell senescence, but its specific mechanism is unknown. In this study, genetic engineering technology was used to construct stable HaCaT cell lines, and the role and mechanism of ERK/CEBPB signaling pathway in arsenic-induced HaCaT cell senescence were verified by knockdown and overexpression of ERK and CEBPB in both forward and backward. It was found that knockdown of CEBPB or ERK can downregulate the ERK/CEBPB signaling pathway and reduce arsenic-induced skin cell senescence. In contrast to knockdown, overexpression of CEBPB or ERK can upregulate the ERK/CEBPB signaling pathway and aggravate the senescence of skin cells caused by arsenic. These findings suggest that sodium arsenite can further promote SASP secretion and the expression of p53, p21 and p16 INK4a by activating the ERK/CEBPB signaling pathway, induce cell cycle arrest and trigger cellular senescence.

Understanding cellular senescence: pathways involved, therapeutics and longevity aiding

A normal somatic cell undergoes cycles of finite cellular divisions. The presence of surveillance checkpoints arrests cell division in response to stress inducers: oxidative stress from excess free radicals, oncogene-induced abnormalities, genotoxic stress, and telomere attrition. When facing such stress when undergoing these damages, there is a brief pause in the cell cycle to enable repair mechanisms. Also, the nature of stress determines whether the cell goes for repair or permanent arrest. As the cells experience transient or permanent stress, they subsequently choose the quiescence or senescence stage, respectively. Quiescence is an essential stage that allows the arrested/damaged cells to go through appropriate repair mechanisms and then revert to the mainstream cell cycle. However, senescent cells are irreversible and accumulate with age, resulting in inflammation and various age-related disorders. In this review, we focus on senescence-associated pathways and therapeutics understanding cellular senescence as a cascade that leads to aging, while discussing the recent details on the molecular pathways involved in regulating senescence and the benefits of therapeutic strategies against accumulated senescent cells and their secretions.

Regulatory mechanisms of the edible alga Ulva lactuca polysaccharide via modulation of gut microbiota in diabetic mice

In this study, Ulva lactuca polysaccharide (ULP) antihyperglycemic effect was assessed by monitoring changes in the gut microbiota of aging diabetic mice. The results showed that ULP alleviated type 2 diabetes by improving insulin tolerance, increasing SOD and CAT activities, and thus lowering blood glucose level. Moreover, ULP regulated the expressions of INSR and AMPK concurrent with inhibition the expression of JNK, JAK, STAT3, p16 and p38 to improve glucose metabolism dysfunction. Interestingly, the abundance of Alloprevotella and Pediococcus change might the key factor for ULP antihyperglycemic effectiveness in aging-related diabetes. These results suggest that ULP can exert a mechanism of blood glucose regulation by improving intestinal diversity composition asides from direct insulin mimetic actions.

PDK1 inhibition reduces autophagy and cell senescence through the PI3K/AKT signalling pathway in a cigarette smoke mouse emphysema model

A number of previous studies have demonstrated the pivotal role of PI3K/AKT signalling in cigarette smoke (CS)-induced emphysema, where phosphoinositide dependent protein kinase 1 (PDK1) is a critical component of this pathway. Therefore, the present study aimed to investigate the effects of a PDK1 inhibitor (GSK-2334470) on the expression levels of PI3K, AKT, cyclin-dependent kinase inhibitor 2A (p16) and LC3B in a CS + CS extract (CSE)-induced mouse emphysema model. CS exposure and intraperitoneal injections of CSE were combined for 4 weeks to establish an emphysema model. Mice (n=35) were randomly divided into the normal control, emphysema (CS), PI3K inhibitor (CS3) and PDK1 inhibitor (CS1) groups. Immunohistochemistry staining of lung tissues was used to measure the expression of the PI3K, PDK1 and AKT proteins in airway epithelial tissues. Immunofluorescence staining was also used to measure the levels of p16 and LC3BII protein expression in the airway epithelial tissues. In addition, PI3K, PDK1, AKT, p16 and LC3B protein expression was semi-quantified using western blotting. The expression of PDK1, PI3K and AKT proteins in the airway epithelial tissues was significantly increased in the CS + CSE group compared with that in the control group. The expression levels of p16 and LC3B were also increased as well in the CS + CSE group compared with those in the control group. The expression levels of PI3K, PDK1, AKT, LC3B and p16 in the airway epithelial tissues of the CS3 group were lower compared with those in the CS + CSE group. A decrease in the expression levels of PDK1, AKT, p16 and LC3B in the airway epithelial tissues of the CS1 group compared with those in the CS + CSE group was also observed. However, there were no significant differences in the expression levels of PI3K between the CS1 and the CS groups. The present study concluded that the inhibition of PDK1 can potentially reduce autophagy and cell senescence by downregulating the expression of PI3K/AKT pathway related proteins in airway epithelial cells, thereby protecting against CS + CSE-induced emphysema in mice.

Among Gerontogens, Heavy Metals Are a Class of Their Own: A Review of the Evidence for Cellular Senescence

Advancements in modern medicine have improved the quality of life across the globe and increased the average lifespan of our population by multiple decades. Current estimates predict by 2030, 12% of the global population will reach a geriatric age and live another 3-4 decades. This swelling geriatric population will place critical stress on healthcare infrastructures due to accompanying increases in age-related diseases and comorbidities. While much research focused on long-lived individuals seeks to answer questions regarding how to age healthier, there is a deficit in research investigating what aspects of our lives accelerate or exacerbate aging. In particular, heavy metals are recognized as a significant threat to human health with links to a plethora of age-related diseases, and have widespread human exposures from occupational, medical, or environmental settings. We believe heavy metals ought to be classified as a class of gerontogens (i.e., chemicals that accelerate biological aging in cells and tissues). Gerontogens may be best studied through their effects on the "Hallmarks of Aging", nine physiological hallmarks demonstrated to occur in aged cells, tissues, and bodies. Evidence suggests that cellular senescence-a permanent growth arrest in cells-is one of the most pertinent hallmarks of aging and is a useful indicator of aging in tissues. Here, we discuss the roles of heavy metals in brain aging. We briefly discuss brain aging in general, then expand upon observations for heavy metals contributing to age-related neurodegenerative disorders. We particularly emphasize the roles and observations of cellular senescence in neurodegenerative diseases. Finally, we discuss the observations for heavy metals inducing cellular senescence. The glaring lack of knowledge about gerontogens and gerontogenic mechanisms necessitates greater research in the field, especially in the context of the global aging crisis.

Senescent stromal cells: roles in the tumor microenvironment

Cellular senescence forms a barrier to tumorigenesis, by inducing cell cycle arrest in damaged and mutated cells. However, once formed, senescent cells often emit paracrine signals that can either promote or suppress tumorigenesis. There is evidence that, in addition to cancer cells, subsets of tumor stromal cells, including fibroblasts, endothelial cells, and immune cells, undergo senescence. Such senescent stromal cells can influence cancer development and progression and represent potential targets for therapy. However, understanding of their characteristics and roles is limited and few studies have dissected their functions in vivo. Here, we discuss current knowledge and pertinent questions regarding the presence of senescent stromal cells in cancers, the triggers that elicit their formation, and their potential roles within the tumor microenvironment.

Advanced age is associated with changes in alveolar macrophages and their responses to the stress of traumatic injury

Alveolar macrophages (AMs) are tissue-resident cells of the lower airways that perform many homeostatic functions critical for pulmonary health and protection against pathogens. However, little is known about the factors that shape AMs during healthy aging. In these studies, we sought to characterize age-related changes in AM phenotype, function, and responses to a physiologic stressor, that is, distal injury. Age was associated with a wide range of changes in cell surface receptor and gene expression by AMs, reflecting a unique alternatively activated phenotype. AMs from aged mice also exhibited markers of cellular senescence along with down-regulation of genes involved in growth and cell cycle pathways relative to young controls. Furthermore, AMs from aged mice showed a stunted transcriptional response to distal injury compared with AMs from young mice. Many changes were found to involve glucocorticoid-regulated genes, and corticosteroid treatment of primary AMs ex vivo revealed diminished transcriptional responses in cells from aged animals. These results demonstrate that there is a complex age-dependent AM phenotype associated with dysregulated stress hormone signaling that may interfere with AM responses to physiologic stressors and could contribute to AM dysfunction and the decline of pulmonary immunity during healthy aging.

Cellular senescence: a key therapeutic target in aging and diseases

Cellular senescence is a hallmark of aging defined by stable exit from the cell cycle in response to cellular damage and stress. Senescent cells (SnCs) can develop a characteristic pathogenic senescence-associated secretory phenotype (SASP) that drives secondary senescence and disrupts tissue homeostasis, resulting in loss of tissue repair and regeneration. The use of transgenic mouse models in which SnCs can be genetically ablated has established a key role for SnCs in driving aging and age-related disease. Importantly, senotherapeutics have been developed to pharmacologically eliminate SnCs, termed senolytics, or suppress the SASP and other markers of senescence, termed senomorphics. Based on extensive preclinical studies as well as small clinical trials demonstrating the benefits of senotherapeutics, multiple clinical trials are under way. This Review discusses the role of SnCs in aging and age-related diseases, strategies to target SnCs, approaches to discover and develop senotherapeutics, and preclinical and clinical advances of senolytics.

Cellular Senescence: Molecular Targets, Biomarkers, and Senolytic Drugs

Cellular senescence is defined as irreversible cell cycle arrest caused by various processes that render viable cells non-functional, hampering normal tissue homeostasis. It has many endogenous and exogenous inducers, and is closely connected with age, age-related pathologies, DNA damage, degenerative disorders, tumor suppression and activation, wound healing, and tissue repair. However, the literature is replete with contradictory findings concerning its triggering mechanisms, specific biomarkers, and detection protocols. This may be partly due to the wide range of cellular and in vivo animal or human models of accelerated aging that have been used to study senescence and test senolytic drugs. This review summarizes recent findings concerning senescence, presents some widely used cellular and animal senescence models, and briefly describes the best-known senolytic agents.

Assessing the Potential Value and Mechanism of Kaji-Ichigoside F1 on Arsenite-Induced Skin Cell Senescence

Chronic exposure to inorganic arsenic is a major environmental public health issue worldwide affecting more than 220 million of people. Previous studies have shown the correlation between arsenic poisoning and cellular senescence; however, knowledge regarding the mechanism and effective prevention measures has not been fully studied. First, the associations among the ERK/CEBPB signaling pathway, oxidative stress, and arsenic-induced skin cell senescence were confirmed using the HaCaT cell model. In the arsenic-exposed group, the relative mRNA and protein expressions of ERK/CEBPB signaling pathway indicators (ERK1, ERK2, and CEBPB), cell cycle-related genes (p21, p16^INK4a), and the secretion of SASP (IL-1α, IL-6, IL-8, TGF-β1, MMP-1, MMP-3, EGF, and VEGF) and the lipid peroxidation product (MDA) were significantly increased in cells (P < 0.05), while the activity of antioxidant enzyme (SOD, GSH-Px, and CAT) was significantly decreased (P < 0.05), and an increased number of cells accumulated in the G1 phase (P < 0.05). Further Kaji-ichigoside F1 intervention experiments showed that compared to that in the arsenic-exposed group, the expression level of the activity of antioxidant enzyme was significantly increased in the Kaji-ichigoside F1 intervention group (P < 0.05), but the indicators of ERK/CEBPB signaling pathway, cell cycle-related genes, and SASP were significantly decreased (P < 0.05), and the cell cycle arrest relieved to a certain extent (P < 0.05). Our study provides some limited evidence that the ERK/CEBPB signaling pathway is involved in low-dose arsenic-induced skin cell senescence, through regulating oxidative stress. The second major finding was that Kaji-ichigoside F1 can downregulate the ERK/CEBPB signaling pathway and regulate the balance between oxidation and antioxidation, alleviating arsenic-induced skin cell senescence. This study provides experimental evidence for further understanding of Kaji-ichigoside F1, a natural medicinal plant that may be more effective in preventing and controlling arsenic poisoning.

Novel Antioxidant Peptides from Crassostrea Hongkongensis Improve Photo-Oxidation in UV-Induced HaCaT Cells

Enzymatic hydrolysates from Oysters (OAH) display multiple biological activities. Previously, a 3~5 KDa oyster ultrafiltration component (OUP) showed a high property of preventing skin oxidation. Subsequently, we identified specific peptides with such activity. OUP was fractionated stepwise by Sephadex-G25 and RP-HPLC, and active fractions were screened using UV-irradiated HaCaT cells. The most active fractions (OP5-3) were analyzed by LC-MS/MS and a total of 17 peptides were identified. Results from mass spectrometry showed that OP5-3 consisted of peptides with a molecular weight range of 841.51–1786.92 Da. Six of these peptides were synthesized for validating the activity of resisting skin oxidation in the same cell model. All six peptides showed varying degrees of antioxidant activity, while pretreatment of HaCaT cells with AIVAEVNEAAK alleviated UV cytotoxicity, inhibited metalloproteinase 1 (MMP-1) expression, and showed the highest activity to resist UV-induced skin photo-oxidation among these peptides. In addition, results from molecular docking analysis of MMP-1 with AIVAEVNEAAK showed that AIVAEVNEAAK suppresses its enzymatic activity by directly interacting with MMP-1 and thus exhibit anti-photoaging activity.

How good is the evidence that cellular senescence causes skin ageing?

Skin is the largest organ of the body with important protective functions, which become compromised with time due to both intrinsic and extrinsic ageing processes. Cellular senescence is the primary ageing process at cell level, associated with loss of proliferative capacity, mitochondrial dysfunction and significantly altered patterns of expression and secretion of bioactive molecules. Intervention experiments have proven cell senescence as a relevant cause of ageing in many organs. In case of skin, accumulation of senescence in all major compartments with ageing is well documented and might be responsible for most, if not all, the molecular changes observed during ageing. Incorporation of senescent cells into in-vitro skin models (specifically 3D full thickness models) recapitulates changes typically associated with skin ageing. However, crucial evidence is still missing. A beneficial effect of senescent cell ablation on skin ageing has so far only been shown following rather unspecific interventions or in transgenic mouse models. We conclude that evidence for cellular senescence as a relevant cause of intrinsic skin ageing is highly suggestive but not yet completely conclusive.

Adipose tissue senescence is mediated by increased ATP content after a short-term high-fat diet exposure

In the context of obesity, senescent cells accumulate in white adipose tissue (WAT). The cellular underpinnings of WAT senescence leading to insulin resistance are not fully elucidated. The objective of the current study was to evaluate the presence of WAT senescence early after initiation of high‐fat diet (HFD, 1–10 weeks) in 5‐month‐old male C57BL/6J mice and the potential role of energy metabolism. We first showed that WAT senescence occurred 2 weeks after HFD as evidenced in whole WAT by increased senescence‐associated ß‐galactosidase activity and cyclin‐dependent kinase inhibitor 1A and 2A expression. WAT senescence affected various WAT cell populations, including preadipocytes, adipose tissue progenitors, and immune cells, together with adipocytes. WAT senescence was associated with higher glycolytic and mitochondrial activity leading to enhanced ATP content in HFD‐derived preadipocytes, as compared with chow diet‐derived preadipocytes. One‐month daily exercise, introduced 5 weeks after HFD, was an effective senostatic strategy, since it reversed WAT cellular senescence, while reducing glycolysis and production of ATP. Interestingly, the beneficial effect of exercise was independent of body weight and fat mass loss. We demonstrated that WAT cellular senescence is one of the earliest events occurring after HFD initiation and is intimately linked to the metabolic state of the cells. Our data uncover a critical role for HFD‐induced elevated ATP as a local danger signal inducing WAT senescence. Exercise exerts beneficial effects on adipose tissue bioenergetics in obesity, reversing cellular senescence, and metabolic abnormalities.

Vascular Endothelial Senescence: Pathobiological Insights, Emerging Long Noncoding RNA Targets, Challenges and Therapeutic Opportunities

Cellular senescence is a stable form of cell cycle arrest in response to various stressors. While it serves as an endogenous pro-resolving mechanism, detrimental effects ensue when it is dysregulated. In this review, we introduce recent advances for cellular senescence and inflammaging, the underlying mechanisms for the reduction of nicotinamide adenine dinucleotide in tissues during aging, new knowledge learned from p16 reporter mice, and the development of machine learning algorithms in cellular senescence. We focus on pathobiological insights underlying cellular senescence of the vascular endothelium, a critical interface between blood and all tissues. Common causes and hallmarks of endothelial senescence are highlighted as well as recent advances in endothelial senescence. The regulation of cellular senescence involves multiple mechanistic layers involving chromatin, DNA, RNA, and protein levels. New targets are discussed including the roles of long noncoding RNAs in regulating endothelial cellular senescence. Emerging small molecules are highlighted that have anti-aging or anti-senescence effects in age-related diseases and impact homeostatic control of the vascular endothelium. Lastly, challenges and future directions are discussed including heterogeneity of endothelial cells and endothelial senescence, senescent markers and detection of senescent endothelial cells, evolutionary differences for immune surveillance in mice and humans, and long noncoding RNAs as therapeutic targets in attenuating cellular senescence. Accumulating studies indicate that cellular senescence is reversible. A better understanding of endothelial cellular senescence through lifestyle and pharmacological interventions holds promise to foster a new frontier in the management of cardiovascular disease risk.

Cdkn1a transcript variant 2 is a marker of aging and cellular senescence

Cellular senescence is a cell fate response characterized by a permanent cell cycle arrest driven primarily the by cell cycle inhibitor and tumor suppressor proteins p16Ink4a and p21Cip1/Waf1. In mice, the p21Cip1/Waf1 encoding locus, Cdkn1a, is known to generate two transcripts that produce identical proteins, but one of these transcript variants is poorly characterized. We show that the Cdkn1a transcript variant 2, but not the better-studied variant 1, is selectively elevated during natural aging across multiple mouse tissues. Importantly, mouse cells induced to senescence in culture by genotoxic stress (ionizing radiation or doxorubicin) upregulated both transcripts, but with different temporal dynamics: variant 1 responded nearly immediately to genotoxic stress, whereas variant 2 increased much more slowly as cells acquired senescent characteristics. Upon treating mice systemically with doxorubicin, which induces widespread cellular senescence in vivo, variant 2 increased to a larger extent than variant 1. Variant 2 levels were also more sensitive to the senolytic drug ABT-263 in naturally aged mice. Thus, variant 2 is a novel and more sensitive marker than variant 1 or total p21Cip1/Waf1 protein for assessing the senescent cell burden and clearance in mice.

Slow-cycling (dormant) cancer cells in therapy resistance, cancer relapse and metastasis

It is increasingly appreciated that cancer cell heterogeneity and plasticity constitute major barriers to effective clinical treatments and long-term therapeutic efficacy. Research in the past two decades suggest that virtually all treatment-naive human cancers harbor subsets of cancer cells that possess many of the cardinal features of normal stem cells. Such stem-like cancer cells, operationally defined as cancer stem cells (CSCs), are frequently quiescent and dynamically change and evolve during tumor progression and therapeutic interventions. Intrinsic tumor cell heterogeneity is reflected in a different aspect in that tumors also harbor a population of slow-cycling cells (SCCs) that are not in the proliferative cell cycle and thus are intrinsically refractory to anti-mitotic drugs. In this Perspective, we focus our discussions on SCCs in cancer and on various methodologies that can be employed to enrich and purify SCCs, compare the similarities and differences between SCCs, CSCs and cancer cells undergoing EMT, and present evidence for the involvement of SCCs in surviving anti-neoplastic treatments, mediating tumor relapse, maintaining tumor dormancy and mediating metastatic dissemination. Our discussions make it clear that an in-depth understanding of the biological properties of SCCs in cancer will be instrumental to developing new therapeutic strategies to prevent tumor relapse and distant metastasis.

p16-3MR: A Novel Model to Study Cellular Senescence in Cigarette Smoke-Induced Lung Injuries

Cellular senescence and lung aging are associated with the pathogenesis of chronic obstructive pulmonary disease (COPD). COPD progresses with aging, and chronic smoking is the key susceptibility factor in lung pathological changes concurrent with mitochondrial dysfunction and biological aging. However, these processes involving cigarette smoke (CS)-mediated lung cellular senescence are difficult to distinguish. One of the impediments to studying cellular senescence in relation to age-related lung pathologies is the lack of a suitable in vivo model. In view of this, we provide evidence that supports the suitability of p16-3MR mice to studying cellular senescence in CS-mediated and age-related lung pathologies. p16-3MR mice have a trimodal reporter fused to the promoter of the p16^INK4a gene that enables detection, isolation, and selective elimination of senescent cells, thus making them a suitable model to study cellular senescence. To determine their suitability in CS-mediated lung pathologies, we exposed young (12–14 months) and old (17–20 months) p16-3MR mice to 30 day CS exposure and studied the expression of senescent genes (p16, p21, and p53) and SASP-associated markers (MMP9, MMP12, PAI-1, and FN-1) in air- and CS-exposed mouse lungs. Our results showed that this model could detect cellular senescence using luminescence and isolate cells undergoing senescence with the help of tissue fluorescence in CS-exposed young and old mice. Our results from the expression of senescence markers and SASP-associated genes in CS-exposed young and old p16-3MR mice were comparable with increased lung cellular senescence and SASP in COPD. We further showed alteration in the; (i) tissue luminescence and fluorescence, (ii) mRNA and protein expressions of senescent markers and SASP genes, and (iii) SA-β-gal activity in CS-exposed young and old p16-3MR mice as compared to their air controls. Overall, we showed that p16-3MR is a competent model for studying the cellular senescence in CS-induced pathologies. Hence, the p16-3MR reporter mouse model may be used as a novel tool for understanding the pathobiology of cellular senescence and other underlying mechanisms involved in COPD and fibrosis.

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.

Stimulation of epidermal hyperplasia and tumorigenesis by resident p16INK4a-expressing cells

p16^INK4a (CDKN2A) is a central tumor-suppressor and activator of senescence. We recently found that prolonged expression of p16^INK4a in epidermal cells induces hyperplasia and dysplasia through Wnt-mediated stimulation of neighboring keratinocytes. The study suggests a pro-tumorigenic function of p16^INK4a in early epidermal lesions, which could potentially be targeted by senolytic therapy.

Chronic expression of p16INK4a in the epidermis induces Wnt-mediated hyperplasia and promotes tumor initiation

p16^INK4a (CDKN2A) is a central tumor suppressor, which induces cell-cycle arrest and senescence. Cells expressing p16^INK4a accumulate in aging tissues and appear in premalignant lesions, yet their physiologic effects are poorly understood. We found that prolonged expression of transgenic p16^INK4a in the mouse epidermis induces hyperplasia and dysplasia, involving high proliferation rates of keratinocytes not expressing the transgene. Continuous p16^INK4a expression increases the number of epidermal papillomas formed after carcinogen treatment. Wnt-pathway ligands and targets are activated upon prolonged p16^INK4a expression, and Wnt inhibition suppresses p16^INK4a-induced hyperplasia. Senolytic treatment reduces p16^INK4a-expressing cell numbers, and inhibits Wnt activation and hyperplasia. In human actinic keratosis, a precursor of squamous cell carcinoma, p16^INK4a-expressing cells are found adjacent to dividing cells, consistent with paracrine interaction. These findings reveal that chronic p16^INK4a expression is sufficient to induce hyperplasia through Wnt-mediated paracrine stimulation, and suggest that this tumor suppressor can promote early premalignant epidermal lesion formation.

It is unclear how resident p16-expressing senescent cells affect the propensity of tissues to develop cancer. Here, the authors show that chronic p16 expression in the mouse epidermis causes hyperplasia and dysplasia through Wnt-mediated paracrine stimulation of proliferating keratinocytes, and can contribute to tumour formation.

High-throughput and label-free isolation of senescent murine mesenchymal stem cells

Under internal or external insults such as aging and oxidative stresses, cells are induced into a senescent state and stop cellular division permanently. As senescent cells (SnCs) accumulate, the regeneration capacity of biological tissue would be compromised, which has been found to be associated with a plethora of age-related disorders. Therefore, isolating SnCs becomes necessary. To address the lack of effective surface markers for SnCs isolation, a label-free microfluidic device was proposed in this paper, in which a spiral microchannel was deployed to isolate SnCs based on their size differences. We adopted a well-received cellular senescence model by exerting excessive oxidative stress to murine mesenchymal stem cells. This model was then validated through a series of SnCs characterizations including size measurement, p16INK4a expression level, senescence-associated beta-galactosidase, and doubling time. The senescence chip demonstrated an efficiency of 75% and viability over 85% at a flow rate of 5 ml/min. The average cell size from the inner outlet was 5 μm larger than that from the outer outlet. The isolated cells had a sixfold higher p16INK4a expression level. Overall, the chip had an area under curve of 0.719 in the receiver operating characteristic analysis, showing decent performance in sorting SnCs. By having the ability to perform size-based sorting at a high flow rate, such a microfluidic device can provide high-throughput and label-free isolation of SnCs. To further improve the isolation performance, the device can be modified to introduce additional physical biomarkers of SnCs such as stiffness. This device poses a good potential in purification for cytotherapy or estimation of biological age.

Cigarette smoking induces human CCR6+Th17 lymphocytes senescence and VEGF-A secretion

Chronic exposure to environmental pollutants is often associated with systemic inflammation. As such, cigarette smoking contributes to inflammation and lung diseases by inducing senescence of pulmonary cells such as pneumocytes, fibroblasts, and endothelial cells. Yet, how smoking worsens evolution of chronic inflammatory disorders associated with Th17 lymphocytes, such as rheumatoid arthritis, psoriasis, Crohn’s disease, and multiple sclerosis, is largely unknown. Results from human studies show an increase in inflammatory CD4⁺ Th17 lymphocytes at blood- and pulmonary level in smokers. The aim of the study was to evaluate the sensitivity of CD4⁺ Th17 lymphocytes to cigarette smoke-induced senescence. Mucosa-homing CCR6⁺ Th17- were compared to CCR6^neg -and regulatory T peripheral lymphocytes after exposure to cigarette smoke extract (CSE). Senescence sensitivity of CSE-exposed cells was assessed by determination of various senescence biomarkers (β-galactosidase activity, p16^Ink4a- and p21 expression) and cytokines production. CCR6⁺ Th17 cells showed a higher sensitivity to CSE-induced senescence compared to controls, which is associated to oxidative stress and higher VEGFα secretion. Pharmacological targeting of ROS- and ERK1/2 signalling pathways prevented CSE-induced senescence of CCR6⁺Th17 lymphocytes as well as VEGFα secretion. Altogether, these results identify mechanisms by which pro-oxidant environmental pollutants contribute to pro-angiogenic and pathogenic CCR6⁺Th17 cells, therefore potential targets for therapeutic purposes.

A cis-element within the ARF locus mediates repression of p16INK4A expression via long-range chromatin interactions

Loss of function of CDKN2A/B, also known as INK4/ARF [encoding p16INK4A, p15INK4B, and p14ARF (mouse p19Arf)], confers susceptibility to cancers, whereas its up-regulation during organismal aging provokes cellular senescence and tissue degenerative disorders. To better understand the transcriptional regulation of p16INK4A, a CRISPR screen targeting open, noncoding chromatin regions adjacent to p16INK4A was performed in a human p16INK4A-P2A-mCherry reporter cell line. We identified a repressive element located in the 3' region adjacent to the ARF promoter that controls p16INK4A expression via long-distance chromatin interactions. Coinfection of lentiviral dCas9-KRAB with selected single-guide RNAs against the repressive element abrogated the ARF/p16INK4A chromatin contacts, thus reactivating p16INK4A expression. Genetic CRISPR screening identified candidate transcription factors inhibiting p16INK4A regulation, including ZNF217, which was confirmed to bind the ARF/p16INK4A interaction loop. In summary, direct physical interactions between p16INK4A and ARF genes provide mechanistic insights into their cross-regulation.

Arsenic induces human chondrocyte senescence and accelerates rat articular cartilage aging

Arsenic-contaminated drinking water is known to be a serious human health problem. A previous epidemiological study has indicated that arsenic levels in blood were higher in arthritis patients compared to age-matched control subjects. Bone is known as an important arsenic store compartment in the body. Arsenic exposure has been suggested to promote senescence in human mesenchymal stem cells that may affect the balance of adipogenic and osteogenic differentiation. The toxicological effect and mechanism of arsenic exposure on articular chondrocytes still remain unclear. Here, we investigated the arsenic-induced senescence in cultured human articular chondrocytes and long-term arsenic-exposed rat articular cartilage. Arsenic trioxide (As₂O₃; 1-5 μM) significantly induced senescence in human articular chondrocytes by increasing senescence-associated β-galactosidase (SA-β-Gal) activity and protein expression of p16, p53, and p21. Arsenic induced the phosphorylation of p38 and c-Jun N-terminal kinase (JNK) proteins. The inhibitors of p38 and JNK significantly reversed the arsenic-induced chondrocyte senescence. Arsenic could also trigger the induction of GATA4-NF-κB signaling and senescence-associated secretory phenotype (SASP) by increasing IL-1α, IL-1β, TGF-β, TNF-α, CCL2, PAI-1, and MMP13 mRNA expression. The increased cartilage senescence and abrasion were also observed in a rat model long-term treatment with arsenic (0.05 and 0.5 ppm) in drinking water for 36 weeks as compared to age-matched control rats. The phosphorylation of p38 and JNK and the induction of GATA4-NF-κB signaling and SASP were enhanced in the rat cartilages. Taken together, these findings suggest that arsenic exposure is capable of inducing chondrocyte senescence and accelerating rat articular cartilage aging and abrasion.

Do senescence markers correlate in vitro and in situ within individual human donors?

Little is known on how well senescence markers in vitro and in situ correlate within individual donors. We studied correlations between the same and different in vitro markers. Furthermore, we tested correlations between in vitro markers with in situ p16INK4a positivity.

From 100 donors (20-91 years), cultured dermal fibroblasts were assessed for reactive oxygen species (ROS), telomere-associated foci (TAF), p16INK4a and senescence-associated β-gal (SAβ-gal), with/ without 0.6 µM rotenone for 3 days (short-term). In fibroblasts from 40 donors, telomere shortening, ROS and SAβ-gal were additionally assessed, with/ without 20 nM rotenone for 7 weeks (long-term). In skin from 52 donors, the number of p16INK4a positive dermal cells was assessed in situ.

More than half of the correlations of the same senescence markers in vitro between duplicate experiments and between short-term versus long-term experiments were significant. Half of the different senescence marker correlations were significant within the short-term and within the long-term experiments. The different senescence markers in vitro were not significantly correlated intra-individually with in situ p16INK4a positivity.
In conclusion, the same and different senescence markers are frequently correlated significantly within and between in vitro experiments, but in vitro senescence markers are not correlated with p16INK4a positivity in situ.

Mouse Models of Accelerated Cellular Senescence

Senescent cells accumulate in multiple tissues as virtually all vertebrate organisms age. Senescence is a highly conserved response to many forms of cellular stress intended to block the propagation of damaged cells. Senescent cells have been demonstrated to play a causal role in aging via their senescence-associated secretory phenotype and by impeding tissue regeneration. Depletion of senescent cells either through genetic or pharmacologic methods has been demonstrated to extend murine lifespan and delay the onset of age-related diseases. Measuring the burden and location of senescent cells in vivo remains challenging, as there is no marker unique to senescent cells. Here, we describe multiple methods to detect the presence and extent of cellular senescence in preclinical models, with a special emphasis on murine models of accelerated aging that exhibit a more rapid onset of cellular senescence.

Transgenic Animal Models to Visualize Cancer-Related Cellular Processes by Bioluminescence Imaging

Preclinical animal models are valuable tools to improve treatments of malignant diseases, being an intermediate step of experimentation between cell culture and human clinical trials. Among different animal models frequently used in cancer research are mouse and, more recently, zebrafish models. Indeed, most of the cellular pathways are highly conserved between human, mouse and zebrafish, thus rendering these models very attractive. Recently, several transgenic reporter mice and zebrafishes have been generated in which the luciferase reporter gene are placed under the control of a promoter whose activity is strictly related to specific cancer cellular processes. Other mouse models have been generated by the cDNA luciferase knockin in the locus of a gene whose expression/activity has increased in cancer. Using BioLuminescence Imaging (BLI), we have now the opportunity to spatiotemporal visualize cell behaviors, among which proliferation, apoptosis, migration and immune responses, in any body district in living animal in a time frame process. We provide here a review of the available models to visualized cancer and cancer-associated events in living animals by BLI and as they have been successful in identifying new stages of early tumor progression, new interactions between different tissues and new therapeutic responsiveness.

Pulmonary Diseases and Ageing

Chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis are regarded as a diseases of accelerated lung ageing and show all of the hallmarks of ageing, including telomere shortening, cellular senescence, activation of PI3 kinase-mTOR signaling, impaired autophagy, mitochondrial dysfunction, stem cell exhaustion, epigenetic changes, abnormal microRNA profiles, immunosenescence and a low grade chronic inflammation due to senescence-associated secretory phenotype (SASP). Many of these ageing mechanisms are driven by exogenous and endogenous oxidative stress. There is also a reduction in anti-ageing molecules, such as sirtuins and Klotho, which further accelerate the ageing process. Understanding these molecular mechanisms has identified several novel therapeutic targets and several drugs and dietary interventions are now in development to treat chronic lung disease.

Comparison of several commonly used detection indicators of cell senescence

Cell senescence is the state of irreversible growth arrest that can be triggered by a variety of different cellular stresses. Currently, the commonly used detection indicators involved in the study of cell senescence include senescence-associated β-galactosidase, Clusterin, Telomeres/Telomerase, senescence-associated heterochromatin foci, senescence-associated secretory phenotype, senescence marker protein-30, tumor suppressor genes p53 and p16, and other indicators such as Ki67 and decoy receptor 2. These indicators are widely used in the study of cell senescence, each with its own characteristics, advantages, and disadvantages. This review summarizes several commonly used cell senescence indicators and compares their accuracy, credibility, specificity, and the scope of their potential application.

Genetic Ablation of p16INK4a Does Not Protect against Cellular Senescence in Mouse Models of Chronic Obstructive Pulmonary Disease/Emphysema

Cigarette smoke (CS) affects DNA damage and cellular senescence signaling pathways in the pathogenesis of chronic obstructive pulmonary disease (COPD). p16INK4a (p16: a cyclin-dependent kinase inhibitor) is a key marker of cellular senescence, which is induced by CS in lung cells. It is thought that removal of p16 attenuates premature aging by removing senesced cells. However, the role of p16 in CS-induced stress-induced premature senescence (SIPS) and senescence-associated secretory phenotype (SASP) during the development of COPD/emphysema is not known. We hypothesize that p16 regulates cellular senescence and DNA damage/repair molecular signaling targets during chronic CS-induced inflammation and airspace enlargement in mouse models of COPD. We used p16 global knockout (KO) and p16 lung epithelial cell-specific KO (p16CreCC10) mice to determine whether p16 removal in lung epithelium augments or protects against cellular senescence (SIPS and SASP) in chronic CS- and elastase-induced development of COPD/emphysema in mice. p16 KO mice exposed to chronic CS and p16 lung epithelial cell-specific KO mice exposed to elastase did not show attenuation of lung inflammation, altered lung function, or airspace enlargement. p16 KO and p16CreCC10 exposed to CS and elastase showed increases in lung senescence-associated β-galactosidase activity. Thus, removal of p16-positive cells did not protect against airspace enlargement and decline in lung function induced in COPD mouse models. Our findings suggest that p16 is not the only key player associated with CS-induced cellular senescence phenotypes (SIPS and SASP), decline in lung function, and airspace enlargement in COPD/emphysema.

p16(Ink4a) and senescence-associated β-galactosidase can be induced in macrophages as part of a reversible response to physiological stimuli

Constitutive p16^Ink4a expression, along with senescence-associated β-galactosidase (SAβG), are commonly accepted biomarkers of senescent cells (SCs). Recent reports attributed improvement of the healthspan of aged mice following p16^Ink4a-positive cell killing to the eradication of accumulated SCs. However, detection of p16^Ink4a/SAβG-positive macrophages in the adipose tissue of old mice and in the peritoneal cavity of young animals following injection of alginate-encapsulated SCs has raised concerns about the exclusivity of these markers for SCs. Here we report that expression of p16^Ink4a and SAβG in macrophages is acquired as part of a physiological response to immune stimuli rather than through senescence, consistent with reports that p16^Ink4a plays a role in macrophage polarization and response. Unlike SCs, p16^Ink4a/SAβG-positive macrophages can be induced in p53-null mice. Macrophages, but not mesenchymal SCs, lose both markers in response to M1- [LPS, IFN-α, Poly(I:C)] and increase their expression in response to M2-inducing stimuli (IL-4, IL-13). Moreover, interferon-inducing agent Poly(I:C) dramatically reduced p16^Ink4a expression in vivo in our alginate bead model and in the adipose tissue of aged mice. These observations suggest that the antiaging effects following eradication of p16^Ink4a-positive cells may not be solely attributed to SCs but also to non-senescent p16^Ink4a/SAβG-positive macrophages.

TERT regulates telomere-related senescence and apoptosis through DNA damage response in male germ cells exposed to BPDE in vitro and to B[a]P in vivo

Increasing evidence shows that impaired telomere function is associated with male infertility, and various environmental factors are believed to play a pivotal role in telomerase deficiency and telomere shortening. Benzo[a]pyrene (B[a]P), a ubiquitous pollutant of polycyclic aromatic hydrocarbons (PAHs), can act as a reproductive toxicant; however, the adverse effect of B[a]P on telomeres in male reproductive cells has never been studied, and the related mechanisms remain unclear. In this study, we explored the effects of benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE), the active metabolite of B[a]P, on telomere dysfunction in mouse spermatocyte-derived cells (GC-2) and also the potential role of telomerase in BPDE-induced spermatogenic cell damage. The results showed that BPDE induced cell viability inhibition, senescence, and apoptosis in GC-2 cells in a dose-dependent manner. Shortened telomeres, telomere-associated DNA damage, reduced telomerase activity, and TERT expression were also observed in BPDE-treated cells, accompanied with the activation of DNA damage response pathway (ATM/Chk1/p53/p21). Moreover, by establishing the TERT knockdown and re-expression cell models, we found that TERT regulated telomere length and the expression of DNA damage response-related proteins to influence senescence and apoptosis in GC-2 cells. These in vitro findings were further confirmed in vivo in the testicular cells of rats orally administrated with B[a]P for 7 days. B[a]P treatment resulted in histological lesions, apoptosis, and senescence in the testes of rats, which were accompanied by shortened telomeres, reduced levels of TERT protein, and increased expression of DNA damage response-related proteins. In conclusion, it can be concluded that TERT-mediated telomere dysfunction contributes to B[a]P- and BPDE-induced senescence and apoptosis through DNA damage response in male reproductive cells.

Cellular Senescence and Lung Function during Aging. Yin and Yang

Cellular senescence is a cell fate decision and stress response that entails a permanent arrest of cell proliferation coupled to a complex secretory phenotype. Senescent cells increase in number with age in most, if not all, mammalian tissues, including the airways and lungs. They also increase at greater than expected numbers, compared with age-matched controls, at sites of age-related pathologies such as chronic obstructive pulmonary disorder and emphysema. The senescence response is a double-edged sword. The proliferative arrest suppresses the development of cancer by preventing the propagation of stressed or damaged cells that are at risk for neoplastic transformation. However, this arrest can also curtail the proliferation of stem or progenitor cells and thus hamper tissue repair and regeneration. Similarly, the secretory phenotype can promote wound healing by transiently providing growth factors and the initial inflammatory stimulus that is required for tissue repair. However, when chronically present, the secretory phenotype of senescent cells can drive pathological inflammation, which contributes to a host of age-related pathologies, including cancer. There are now transgenes and prototype small molecules that can clear senescent cells, at least in mouse models, and thus improve health span and median life span. The next challenge will be to develop interventions and supplements that can abrogate the deleterious effects of senescent cells while preserving their beneficial effects.

Senescence in Health and Disease

Many cellular stresses activate senescence, a persistent hyporeplicative state characterized in part by expression of the p16^INK4a cell-cycle inhibitor. Senescent cell production occurs throughout life and plays beneficial roles in a variety of physiological and pathological processes including embryogenesis, wound healing, host immunity, and tumor suppression. Meanwhile, the steady accumulation of senescent cells with age also has adverse consequences. These non-proliferating cells occupy key cellular niches and elaborate pro-inflammatory cytokines, contributing to aging-related diseases and morbidity. This model suggests that the abundance of senescent cells in vivo predicts "molecular," as opposed to chronologic, age and that senescent cell clearance may mitigate aging-associated pathology.

Senotherapy: growing old and staying young?

Cellular senescence, which has been linked to age-related diseases, occurs during normal aging or as a result of pathological cell stress. Due to their incapacity to proliferate, senescent cells cannot contribute to normal tissue maintenance and tissue repair. Instead, senescent cells disturb the microenvironment by secreting a plethora of bioactive factors that may lead to inflammation, regenerative dysfunction and tumor progression. Recent understanding of stimuli and pathways that induce and maintain cellular senescence offers the possibility to selectively eliminate senescent cells. This novel strategy, which so far has not been tested in humans, has been coined senotherapy or senolysis. In mice, senotherapy proofed to be effective in models of accelerated aging and also during normal chronological aging. Senotherapy prolonged lifespan, rejuvenated the function of bone marrow, muscle and skin progenitor cells, improved vasomotor function and slowed down atherosclerosis progression. While initial studies used genetic approaches for the killing of senescent cells, recent approaches showed similar effects with senolytic drugs. These observations open up exciting possibilities with a great potential for clinical development. However, before the integration of senotherapy into patient care can be considered, we need further research to improve our insight into the safety and efficacy of this strategy during short- and long-term use.

Senescence in COPD and Its Comorbidities

Chronic obstructive pulmonary disease (COPD) is regarded as a disease of accelerated lung aging. This affliction shows all of the hallmarks of aging, including telomere shortening, cellular senescence, activation of PI3 kinase-mTOR signaling, impaired autophagy, mitochondrial dysfunction, stem cell exhaustion, epigenetic changes, abnormal microRNA profiles, immunosenescence, and a low-grade chronic inflammation (inflammaging). Many of these pathways are driven by chronic exogenous and endogenous oxidative stress. There is also a reduction in antiaging molecules, such as sirtuins and Klotho, which further accelerate the aging process. COPD is associated with several comorbidities (multimorbidity), such as cardiovascular and metabolic diseases, that share the same pathways of accelerated aging. Understanding these mechanisms has helped identify several novel therapeutic targets, and several drugs and dietary interventions are now in development to treat multimorbidity.

The impact of autophagy on the development of senescence in primary tubular epithelial cells

Autophagy and senescence are 2 distinct pathways that are importantly involved in acute kidney injury and renal repair. Recent data indicate that the 2 processes might be interrelated. To investigate the potential link between autophagy and senescence in the kidney we isolated primary tubular epithelial cells (PTEC) from wild-type mice and monitored the occurrence of cellular senescence during autophagy activation and inhibition. We found that the process of cell isolation and transfer into culture was associated with a strong basal autophagic activation in PTEC. Specific inhibition of autophagy by silencing autophagy-related 5 (Atg5) counteracted the occurrence of senescence hallmarks under baseline conditions. Reduced senescent features were also observed in Atg5 silenced PTEC after γ-irradiation and during H-Ras induced oncogenic senescence, but the response was less uniform in these stress models. Senescence inhibition was paralleled by better preservation of a mature epithelial phenotype in PTEC. Interestingly, treatment with rapamycin, which acts as an activator of autophagy, also counteracted the occurrence of senescence features in PTEC. While we interpret the anti-senescent effect of rapamycin as an autophagy-independent effect of mTOR-inhibition, the more specific approach of Atg5 silencing indicates that overactivated autophagy can have pro-senescent effects in PTEC. These results highlight the complex interaction between cell culture dependent stress mechanisms, autophagy and senescence.

Emerging links among Chromosome Instability (CIN), cancer, and aging

Aneuploidy was predicted to cause cancer. To test the prediction, various Chromosome Instability (CIN) mice models that carry transgenic mutations in mitotic regulators have been created. The availability of these mice has aided researchers in discovering connections between CIN, cancer, and aging. This review will focus on recent interdisciplinary findings regarding how CIN and aneuploidy affect carcinogenesis, immune dysfunction, and aging. High CIN can be generated in vivo by various intrinsic alterations (e.g., gene mutation, epigenetic modification) and extrinsic/environmental challenges (e.g., biological, chemical, biophysical), while immune surveillance, cell death, and natural turnover can remove cells with CIN. CIN itself is mutagenic and may cause further cellular mutations, which can be carcinogenic. Mitotically damaged cells can activate senescence-related tumor suppressors (e.g., p21^WAF1 , p27^KIP1 , p16^INK4A ), which may lead to tissue-level senescence/aging through inflammatory paracrine mechanisms called Senescence-Associated Secretory Phenotype (SASP) and Senescence Inflammatory Response (SIR). Organs with high CIN show altered gene expressions in both organ-specific and non-specific manners. Organ-specific gene expression signatures include activation of oncogenic pathways. Non-organ-specific gene expression signatures include metabolic changes and downregulations in immune functions. Immune surveillance normally targets senescent cells and tetraploid cells, a form of aneuploidy, for elimination. However, with partial immune dysfunction, immune surveillance is weakened with systemic CIN. In this case, more senescent cells and aneuploid cells survive, which further leads to an inflammatory, pro-tumorigenic, and senescent/aging microenvironment. We also discuss how we may intervene in this sequence of events to prevent CIN- or age-related carcinogenesis and/or some aspects of tissue aging. © 2016 Wiley Periodicals, Inc.

Sustained p16INK4a expression is required to prevent IR-induced tumorigenesis in mice

Exposure of murine and human tissues to ionizing radiation (IR) induces the expression of p16^INK4a, a tumor suppressor gene and senescence/aging biomarker. Increased p16^INK4a expression is often delayed several weeks post exposure to IR. In this context, it remains unclear if it occurs to suppress aberrant cellular growth of potentially transformed cells or is simply a result of IR-induced loss of tissue homeostasis. To address this question, we used a conditional p16^INK4a null mouse model and determined the impact of p16^INK4a inactivation long-term post exposure to IR. We found that, in vitro, bone marrow stromal cells exposed to IR enter DNA replication following p16^INK4a inactivation. However, these cells did not resume growth; instead, they mostly underwent cell cycle arrest in G2. Similarly, delayed inactivation of p16^INK4a in mice several weeks post exposure to IR resulted in increased BrdU incorporation and cancer incidence. In fact, we found that the onset of tumorigenesis was similar whether p16^INK4a was inactivated before or after exposure to IR. Overall, our results suggest that IR-induced p16^INK4a dependent growth arrest is reversible in mice and that sustained p16^INK4a expression is necessary to protect against tumorigenesis.

Exercise Prevents Diet-Induced Cellular Senescence in Adipose Tissue

Considerable evidence implicates cellular senescence in the biology of aging and chronic disease. Diet and exercise are determinants of healthy aging; however, the extent to which they affect the behavior and accretion of senescent cells within distinct tissues is not clear. Here we tested the hypothesis that exercise prevents premature senescent cell accumulation and systemic metabolic dysfunction induced by a fast-food diet (FFD). Using transgenic mice that express EGFP in response to activation of the senescence-associated p16(INK4a) promoter, we demonstrate that FFD consumption causes deleterious changes in body weight and composition as well as in measures of physical, cardiac, and metabolic health. The harmful effects of the FFD were associated with dramatic increases in several markers of senescence, including p16, EGFP, senescence-associated β-galactosidase, and the senescence-associated secretory phenotype (SASP) specifically in visceral adipose tissue. We show that exercise prevents the accumulation of senescent cells and the expression of the SASP while nullifying the damaging effects of the FFD on parameters of health. We also demonstrate that exercise initiated after long-term FFD feeding reduces senescent phenotype markers in visceral adipose tissue while attenuating physical impairments, suggesting that exercise may provide restorative benefit by mitigating accrued senescent burden. These findings highlight a novel mechanism by which exercise mediates its beneficial effects and reinforces the effect of modifiable lifestyle choices on health span.

SNEV(Prp19/PSO4) deficiency increases PUVA-induced senescence in mouse skin

Senescent cells accumulate during ageing in various tissues and contribute to organismal ageing. However, factors that are involved in the induction of senescence in vivo are still not well understood. SNEV(P) (rp19/) (PSO) (4) is a multifaceted protein, known to be involved in DNA damage repair and senescence, albeit only in vitro. In this study, we used heterozygous SNEV(+/-) mice (SNEV-knockout results in early embryonic lethality) and wild-type littermate controls as a model to elucidate the role of SNEV(P) (rp19/) (PSO) (4) in DNA damage repair and senescence in vivo. We performed PUVA treatment as model system for potently inducing cellular senescence, consisting of 8-methoxypsoralen in combination with UVA on mouse skin to induce DNA damage and premature skin ageing. We show that SNEV(P) (rp19/) (PSO) (4) expression decreases during organismal ageing, while p16, a marker of ageing in vivo, increases. In response to PUVA treatment, we observed in the skin of both SNEV(P) (rp19/) (PSO) (4) and wild-type mice an increase in γ-H2AX levels, a DNA damage marker. In old SNEV(P) (rp19/) (PSO) (4) mice, this increase is accompanied by reduced epidermis thickening and increase in p16 and collagenase levels. Thus, the DNA damage response occurring in the mouse skin upon PUVA treatment is dependent on SNEV(P) (rp19/) (PSO) (4) expression and lower levels of SNEV(P) (rp19/) (PSO) (4) , as in old SNEV(+/-) mice, result in increase in cellular senescence and acceleration of premature skin ageing.

Absence of AMPKα2 accelerates cellular senescence via p16 induction in mouse embryonic fibroblasts

Emerging evidence suggests that activation of adenosine monophosphate-activated protein kinase (AMPK), an energy gauge and redox sensor, delays aging process. However, the molecular mechanisms by which AMPKα isoform regulates cellular senescence remain largely unknown. The aim of this study was to determine if AMPKα deletion contributes to the accelerated cell senescence by inducing p16(INK4A) (p16) expression thereby arresting cell cycle. The markers of cellular senescence, cell cycle proteins, and reactive oxygen species (ROS) were monitored in cultured mouse embryonic fibroblasts (MEFs) isolated from wild type (WT, C57BL/6J), AMPKα1, or AMPKα2 homozygous deficient (AMPKα1(-/-), AMPKα2(-/-)) mice by Western blot and cellular immunofluorescence staining, as well as immunohistochemistry (IHC) in skin tissue of young and aged mice. Deletion of AMPKα2, the minor isoform of AMPKα, but not AMPKα1 in high-passaged MEFs led to spontaneous cell senescence demonstrated by accumulation of senescence-associated-β-galactosidase (SA-β-gal) staining and foci formation of heterochromatin protein 1 homolog gamma (HP1γ). It was shown here that AMPKα2 deletion upregulates cyclin-dependent kinase (CDK) inhibitor, p16, which arrests cell cycle. Furthermore, AMPKα2 null cells exhibited elevated ROS production. Interestingly, knockdown of HMG box-containing protein 1 (HBP1) partially blocked the cellular senescence of AMPKα2-deleted MEFs via the reduction of p16. Finally, dermal cells senescence, including fibroblasts senescence evidenced by the staining of p16, HBP1, and Ki-67, in the skin of aged AMPKα2(-/-) mice was enhanced when compared with that in wild type mice. Taken together, our results suggest that AMPKα2 isoform plays a fundamental role in anti-oxidant stress and anti-senescence.

Clearance of senescent cells by ABT263 rejuvenates aged hematopoietic stem cells in mice

Senescent cells (SCs) accumulate with age and after genotoxic stress, such as total-body irradiation (TBI). Clearance of SCs in a progeroid mouse model using a transgenic approach delays several age-associated disorders, suggesting that SCs play a causative role in certain age-related pathologies. Thus, a 'senolytic' pharmacological agent that can selectively kill SCs holds promise for rejuvenating tissue stem cells and extending health span. To test this idea, we screened a collection of compounds and identified ABT263 (a specific inhibitor of the anti-apoptotic proteins BCL-2 and BCL-xL) as a potent senolytic drug. We show that ABT263 selectively kills SCs in culture in a cell type- and species-independent manner by inducing apoptosis. Oral administration of ABT263 to either sublethally irradiated or normally aged mice effectively depleted SCs, including senescent bone marrow hematopoietic stem cells (HSCs) and senescent muscle stem cells (MuSCs). Notably, this depletion mitigated TBI-induced premature aging of the hematopoietic system and rejuvenated the aged HSCs and MuSCs in normally aged mice. Our results demonstrate that selective clearance of SCs by a pharmacological agent is beneficial in part through its rejuvenation of aged tissue stem cells. Thus, senolytic drugs may represent a new class of radiation mitigators and anti-aging agents.