Unmasking Transcriptional Heterogeneity in Senescent Cells

Senescence and cancer - role and therapeutic opportunities

Cellular senescence is a state of stable, terminal cell cycle arrest associated with various macromolecular changes and a hypersecretory, pro-inflammatory phenotype. Entry of cells into senescence can act as a barrier to tumorigenesis and, thus, could in principle constitute a desired outcome for any anticancer therapy. Paradoxically, studies published in the past decade have demonstrated that, in certain conditions and contexts, malignant and non-malignant cells with lastingly persistent senescence can acquire pro-tumorigenic properties. In this Review, we first discuss the major mechanisms involved in the antitumorigenic functions of senescent cells and then consider the cell-intrinsic and cell-extrinsic factors that participate in their switch towards a tumour-promoting role, providing an overview of major translational and emerging clinical findings. Finally, we comprehensively describe various senolytic and senomorphic therapies and their potential to benefit patients with cancer.

The entry of cells into senescence can act as a barrier to tumorigenesis; however, in certain contexts senescent malignant and non-malignant cells can acquire pro-tumorigenic properties. The authors of this Review discuss the cell-intrinsic and cell-extrinsic mechanisms involved in both the antitumorigenic and tumour-promoting roles of senescent cells, and describe the potential of various senolytic and senomorphic therapeutic approaches in oncology.

Cellular senescence is a natural barrier to tumorigenesis; senescent cells are widely detected in premalignant lesions from patients with cancer.

Cellular senescence is induced by anticancer therapy and can contribute to some treatment-related adverse events (TRAEs).

Senescent cells exert both protumorigenic and antitumorigenic effects via cell-autonomous and paracrine mechanisms.

Pharmacological modulation of senescence-associated phenotypes has the potential to improve therapy efficacy and reduce the incidence of TRAEs.

Senescence Markers in Peripheral Blood Mononuclear Cells in Amnestic Mild Cognitive Impairment and Alzheimer's Disease

Recent studies suggest that cellular senescence plays a role in Alzheimer’s Disease (AD) pathogenesis. We hypothesize that cellular senescence markers might be tracked in the peripheral tissues of AD patients. Senescence hallmarks, including altered metabolism, cell-cycle arrest, DNA damage response (DDR) and senescence secretory associated phenotype (SASP), were measured in peripheral blood mononuclear cells (PBMCs) of healthy controls (HC), amnestic mild cognitive impairment (aMCI) and AD patients. Senescence-associated βeta-galactosidase (SA-β-Gal) activity, G0-G1 phase cell-cycle arrest, p16 and p53 were analyzed by flow cytometry, while IL-6 and IL-8 mRNA were analyzed by qPCR, and phosphorylated H2A histone family member X (γH2AX) was analyzed by immunofluorescence. Senescent cells in the brain tissue were determined with lipofuscin staining. An increase in the number of senescent cells was observed in the frontal cortex and hippocampus of advanced AD patients. PBMCs of aMCI patients, but not in AD, showed increased SA-β-Gal compared with HCs. aMCI PBMCs also had increased IL-6 and IL8 mRNA expression and number of cells arrested at G0-G1, which were absent in AD. Instead, AD PBMCs had significantly increased p16 and p53 expression and decreased γH2Ax activity compared with HC. This study reports that several markers of cellular senescence can be measured in PBMCs of aMCI and AD patients.

A new gene set identifies senescent cells and predicts senescence-associated pathways across tissues

Although cellular senescence drives multiple age-related co-morbidities through the senescence-associated secretory phenotype, in vivo senescent cell identification remains challenging. Here, we generate a gene set (SenMayo) and validate its enrichment in bone biopsies from two aged human cohorts. We further demonstrate reductions in SenMayo in bone following genetic clearance of senescent cells in mice and in adipose tissue from humans following pharmacological senescent cell clearance. We next use SenMayo to identify senescent hematopoietic or mesenchymal cells at the single cell level from human and murine bone marrow/bone scRNA-seq data. Thus, SenMayo identifies senescent cells across tissues and species with high fidelity. Using this senescence panel, we are able to characterize senescent cells at the single cell level and identify key intercellular signaling pathways. SenMayo also represents a potentially clinically applicable panel for monitoring senescent cell burden with aging and other conditions as well as in studies of senolytic drugs.

Skin-Aging Pigmentation: Who Is the Real Enemy?

Skin aging is induced and sustained by chronological aging and photoaging. Aging skin pigmentation such as mottled pigmentation (senile lentigo) and melasma are typical signs of photoaging. The skin, like other human organs, undergoes cellular senescence, and senescent cells in the skin increase with age. The crosstalk between melanocytes as pigmentary cells and other adjacent types of aged skin cells such as senescent fibroblasts play a role in skin-aging pigmentation. In this review, we provide an overview of cellular senescence during the skin-aging process. The discussion also includes cellular senescence related to skin-aging pigmentation and the therapeutic potential of regulating the senescence process.

Simple Detection of Unstained Live Senescent Cells with Imaging Flow Cytometry

Cellular senescence is a hallmark of aging and a promising target for therapeutic approaches. The identification of senescent cells requires multiple biomarkers and complex experimental procedures, resulting in increased variability and reduced sensitivity. Here, we propose a simple and broadly applicable imaging flow cytometry (IFC) method. This method is based on measuring autofluorescence and morphological parameters and on applying recent artificial intelligence (AI) and machine learning (ML) tools. We show that the results of this method are superior to those obtained measuring the classical senescence marker, senescence-associated beta-galactosidase (SA-β-Gal). We provide evidence that this method has the potential for diagnostic or prognostic applications as it was able to detect senescence in cardiac pericytes isolated from the hearts of patients affected by end-stage heart failure. We additionally demonstrate that it can be used to quantify senescence “in vivo” and can be used to evaluate the effects of senolytic compounds. We conclude that this method can be used as a simple and fast senescence assay independently of the origin of the cells and the procedure to induce senescence.

Cellular senescence and senolytics: the path to the clinic

Interlinked and fundamental aging processes appear to be a root-cause contributor to many disorders and diseases. One such process is cellular senescence, which entails a state of cell cycle arrest in response to damaging stimuli. Senescent cells can arise throughout the lifespan and, if persistent, can have deleterious effects on tissue function due to the many proteins they secrete. In preclinical models, interventions targeting those senescent cells that are persistent and cause tissue damage have been shown to delay, prevent or alleviate multiple disorders. In line with this, the discovery of small-molecule senolytic drugs that selectively clear senescent cells has led to promising strategies for preventing or treating multiple diseases and age-related conditions in humans. In this Review, we outline the rationale for senescent cells as a therapeutic target for disorders across the lifespan and discuss the most promising strategies-including recent and ongoing clinical trials-for translating small-molecule senolytics and other senescence-targeting interventions into clinical use.

Targeting senescent cells for a healthier longevity: the roadmap for an era of global aging

Aging is a natural but relentless process of physiological decline, leading to physical frailty, reduced ability to respond to physical stresses (resilience) and, ultimately, organismal death. Cellular senescence, a self-defensive mechanism activated in response to intrinsic stimuli and/or exogenous stress, is one of the central hallmarks of aging. Senescent cells cease to proliferate, while remaining metabolically active and secreting numerous extracellular factors, a feature known as the senescence-associated secretory phenotype. Senescence is physiologically important for embryonic development, tissue repair, and wound healing, and prevents carcinogenesis. However, chronic accumulation of persisting senescent cells contributes to a host of pathologies including age-related morbidities. By paracrine and endocrine mechanisms, senescent cells can induce inflammation locally and systemically, thereby causing tissue dysfunction, and organ degeneration. Agents including those targeting damaging components of the senescence-associated secretory phenotype or inducing apoptosis of senescent cells exhibit remarkable benefits in both preclinical models and early clinical trials for geriatric conditions. Here we summarize features of senescent cells and outline strategies holding the potential to be developed as clinical interventions. In the long run, there is an increasing demand for safe, effective, and clinically translatable senotherapeutics to address healthcare needs in current settings of global aging.

Gonadal sex patterns p21-induced cellular senescence in mouse and human glioblastoma

Males exhibit higher incidence and worse prognosis for the majority of cancers, including glioblastoma (GBM). Disparate survival may be related to sex-biased responses to treatment, including radiation. Using a mouse model of GBM, we show that female cells are more sensitive to radiation, and that senescence represents a major component of the radiation therapeutic response in both sexes. Correlation analyses revealed that the CDK inhibitor p21 and irradiation induced senescence were differentially regulated between male and female cells. Indeed, female cellular senescence was more sensitive to changes in p21 levels, a finding that was observed in wildtype and transformed murine astrocytes, as well as patient-derived GBM cell lines. Using a novel Four Core Genotypes model of GBM, we further show that sex differences in p21-induced senescence are patterned during early development by gonadal sex. These data provide a rationale for the further study of sex differences in radiation response and how senescence might be enhanced for radiation sensitization. The determination that p21 and gonadal sex are required for sex differences in radiation response will serve as a foundation for these future mechanistic studies.

Multiple time-series expression trajectories imply dynamic functional changes during cellular senescence

Cellular senescence is a dynamic process driven by epigenetic and genetic changes. Although some transcriptomic signatures of senescent cells have been discovered, how these senescence-related signals change over time remains largely unclear. Here, we profiled the transcriptome dynamics of human dermal fibroblast (HDF) cells in successive stages of growth from proliferation to senescence. Based on time-series expression profile analysis, we discovered four trajectories (C1, C2, C3, C4) that are dynamically expressed as senescence progresses. While some genes were continuously up-regulated (C4) or down-regulated (C2) with aging, other genes did not change linearly with cell proliferation, but remained stable until entering the senescent state (C1, C3). Further analysis revealed that the four modes were enriched in different biological pathways, including regulation of cellular senescence. These findings provide a new perspective on understanding the dynamic regulatory mechanism of cellular senescence.

DNA damage to β cells in culture recapitulates features of senescent β cells that accumulate in type 1 diabetes

OBJECTIVE

Type 1 Diabetes (T1D) is characterized by progressive loss of insulin-producing pancreatic β cells as a result of autoimmune destruction. In addition to β cell death, recent work has shown that subpopulations of β cells acquire dysfunction during T1D. We previously reported that β cells undergoing a DNA damage response (DDR) and senescence accumulate during the pathogenesis of T1D. However, the question of how senescence develops in β cells has not been investigated.

METHODS

Here, we tested the hypothesis that unrepaired DNA damage in the context of genetic susceptibility triggers β cell senescence using culture models including the mouse NIT1 β cell line derived from the T1D-susceptible nonobese diabetic (NOD) strain, human donor islets and EndoC β cells. DNA damage was chemically induced using etoposide or bleomycin and cells or islets were analyzed by a combination of molecular assays for senescence phenotypes including Western blotting, qRT-PCR, Luminex assays, flow cytometry and histochemical staining. RNA-seq was carried out to profile global transcriptomic changes in human islets undergoing DDR and senescence. Insulin ELISAs were used to quantify glucose-stimulated insulin secretion from chemically-induced senescent human islets, EndoC β cells and mouse β cell lines in culture.

RESULTS

Sub-lethal DNA damage in NIT1 cells led to several classical hallmarks of senescence including sustained DDR activation, growth arrest, enlarged flattened morphology and a senescence-associated secretory phenotype (SASP) resembling what occurs in primary β cells during T1D in NOD mice. These phenotypes differed between NIT1 cells and the MIN6 β cell line derived from a non-T1D susceptible mouse strain. RNA-seq analysis of DNA damage-induced senescence in human islets from two different donors revealed a p53 transcriptional program and upregulation of prosurvival and SASP genes, with inter-donor variability in this response. Inter-donor variability in human islets was also apparent in the extent of persistent DDR activation and SASP at the protein level. Notably, chemically induced DNA damage also led to DDR activation and senescent phenotypes in EndoC-βH5 human β cells, confirming that this response can occur directly in a human β cell line. Finally, DNA damage led to different effects on glucose-stimulated insulin secretion in mouse β cell lines as compared with human islets and EndoC β cells.

CONCLUSIONS

Taken together, these findings suggest that some of the phenotypes of senescent β cells that accumulate during the development of T1D in the NOD mouse and humans can be modeled by chemically induced DNA damage to mouse β cell lines, human islets and EndoC β cells in culture. The differences between β cells from different mouse strains and different human islet donors and EndoC β cells highlights species differences and the role for genetic background in modifying the β cell response to DNA damage and its effects on insulin secretion. These culture models will be useful tools to understand some of the mechanisms of β cell senescence in T1D.

Acid-sensitive ion channel 1a mediates osteoarthritis chondrocyte senescence by promoting Lamin B1 degradation

Osteoarthritis (OA) is a common and debilitating chronic joint disease, which is characterized by degeneration of articular cartilage and the aging of chondrocytes. Acid-sensitive ion channel 1a (ASIC1a) is a proton-activated cationic channel abundant in chondrocytes, which senses and regulates joint cavity pH. Our previous study demonstrated that ASIC1a was involved in acid-induced rat articular chondrocyte senescence, but the mechanistic basis remained unclear. In this study, we explored the mechanism of ASIC1a in chondrocyte senescence and OA. The results showed that senescence-related-β-galactosidase, senescence-related markers (p53 and p21) and the autophagy-related protein Beclin-1 were found to be increased, but Lamin B1 was found to be reduced with acid (pH 6.0) treatment. These effects were inhibited by ASIC1a-specific blocker psalmotoxin-1 or ASIC1a-short hairpin RNA respectively in chondrocytes. Moreover, Silencing of Lamin B1 enhanced ASIC1a-mediated chondrocyte senescence, this effect was reversed by overexpression of Lamin B1, indicating that Lamin B1 was involved in ASIC1a-mediated chondrocyte senescence. Further, blockade of ASIC1a inhibits acid-induced autophagosomes and Beclin-1 protein expression, suggesting that ASIC1a is involved in acid-induced chondrocyte autophagy. Blocking autophagy with chloroquine inhibited Beclin-1 and increased Lamin B1 in acid-induced chondrocyte senescence. We further demonstrated that ASIC1a-mediated reduction of Lamin B1 expression was caused by autophagy pathway-dependent protein degradation. Finally, blocking ASIC1a protected cartilage tissue, restored Lamin B1 levels and inhibited chondrocyte senescence in a rat OA model. In summary, these findings suggest that ASIC1a may promote Lamin B1 degradation to mediate osteoarthritis chondrocyte senescence through the autophagy pathway.

Systemic induction of senescence in young mice after single heterochronic blood exchange

ABSTACT

Ageing is the largest risk factor for many chronic diseases. Studies of heterochronic parabiosis, substantiated by blood exchange and old plasma dilution, show that old-age-related factors are systemically propagated and have pro-geronic effects in young mice. However, the underlying mechanisms how bloodborne factors promote ageing remain largely unknown. Here, using heterochronic blood exchange in male mice, we show that aged mouse blood induces cell and tissue senescence in young animals after one single exchange. This induction of senescence is abrogated if old animals are treated with senolytic drugs before blood exchange, therefore attenuating the pro-geronic influence of old blood on young mice. Hence, cellular senescence is neither simply a response to stress and damage that increases with age, nor a chronological cell-intrinsic phenomenon. Instead, senescence quickly and robustly spreads to young mice from old blood. Clearing senescence cells that accumulate with age rejuvenates old circulating blood and improves the health of multiple tissues.

Human Umbilical Cord-Derived Mesenchymal Stem Cells Ameliorate Skin Aging of Nude Mice Through Autophagy-Mediated Anti-Senescent Mechanism

Skin aging is a currently irreversible process, affected by increased oxidative stress, activated cellular senescence, and lacked regeneration of the dermal layer. Mesenchymal stem cells (MSCs), such as human umbilical cord-derived MSCs (hucMSCs), have pro-regeneration and anti-aging potencies. To explore whether hucMSCs can be used to treat skin aging, this study employed skin-aging model of nude mice to conduct in vivo assays, including biochemical analysis of superoxide dismutase (SOD) and malondialdehyde (MDA), gross observation, histopathological observation, and immunohistochemical analysis. To clarify how hucMSCs work on skin aging, this study employed skin-aging model of human dermal fibroblasts (HDFs) to conduct in vitro assays by applying conditional medium of hucMSCs (CMM), including wound healing assay, senescence staining, flow cytometric oxidative detection, real time PCR, and western blot analysis. The in vivo data demonstrated that hucMSCs dose-dependently removed wrinkles, smoothed skin texture, and increased dermal thickness and collagen production of aged skin by reversing SOD and MDA levels and up-regulating Col-1 and VEGF expressions, indicating anti-oxidative and pro-regenerative effects against skin aging. The in vitro data revealed that hucMSCs significantly reversed the senescence of HDFs by promoting cell migration, inhibiting ROS production, and restoring the overexpressions of oxidative and senescent markers through paracrine mode of action, and the paracrine mechanism was mediated by the inhibition of autophagy. This study provided novel knowledge regarding the anti-aging efficacy and paracrine mechanism of hucMSCs on skin, making hucMSCs-based therapy a promising regime for skin aging treatment.

Connecting aging biology and inflammation in the omics era

Aging is characterized by the accumulation of damage to macromolecules and cell architecture that triggers a proinflammatory state in blood and solid tissues, termed inflammaging. Inflammaging has been implicated in the pathogenesis of many age-associated chronic diseases as well as loss of physical and cognitive function. The search for mechanisms that underlie inflammaging focused initially on the hallmarks of aging, but it is rapidly expanding in multiple directions. Here, we discuss the threads connecting cellular senescence and mitochondrial dysfunction to impaired mitophagy and DNA damage, which may act as a hub for inflammaging. We explore the emerging multi-omics efforts that aspire to define the complexity of inflammaging - and identify molecular signatures and novel targets for interventions aimed at counteracting excessive inflammation and its deleterious consequences while preserving the physiological immune response. Finally, we review the emerging evidence that inflammation is involved in brain aging and neurodegenerative diseases. Our goal is to broaden the research agenda for inflammaging with an eye on new therapeutic opportunities.

Cellular Senescence and Ageing: Mechanisms and Interventions

The influence of the activation of a cellular phenotype termed senescence and it’s importance in ageing and age-related diseases is becoming more and more evident. In fact, there is a huge effort to tackle these diseases via therapeutic drugs targeting senescent cells named senolytics. However, a clearer understanding of how senescence is activated and the influence it has on specific cellular types and tissues is needed. Here, we describe general triggers and characteristics of senescence. In addition, we describe the influence of senescent cells in ageing and different age-related diseases.

Amphiregulin mediates non-cell-autonomous effect of senescence on reprogramming

Cellular senescence is an irreversible growth arrest with a dynamic secretome, termed the senescence-associated secretory phenotype (SASP). Senescence is a cell-intrinsic barrier for reprogramming, whereas the SASP facilitates cell fate conversion in non-senescent cells. However, the mechanisms by which reprogramming-induced senescence regulates cell plasticity are not well understood. Here, we investigate how the heterogeneity of paracrine senescence impacts reprogramming. We show that senescence promotes in vitro reprogramming in a stress-dependent manner. Unbiased proteomics identifies a catalog of SASP factors involved in the cell fate conversion. Amphiregulin (AREG), frequently secreted by senescent cells, promotes in vitro reprogramming by accelerating proliferation and the mesenchymal-epithelial transition via EGFR signaling. AREG treatment diminishes the negative effect of donor age on reprogramming. Finally, AREG enhances in vivo reprogramming in skeletal muscle. Hence, various SASP factors can facilitate cellular plasticity to promote reprogramming and tissue repair.

Induction of senescence upon loss of the Ash2l core subunit of H3K4 methyltransferase complexes

Gene expression is controlled in part by post-translational modifications of core histones. Methylation of lysine 4 of histone H3 (H3K4), associated with open chromatin and gene transcription, is catalyzed by type 2 lysine methyltransferase complexes that require WDR5, RBBP5, ASH2L and DPY30 as core subunits. Ash2l is essential during embryogenesis and for maintaining adult tissues. To expand on the mechanistic understanding of Ash2l, we generated mouse embryo fibroblasts (MEFs) with conditional Ash2l alleles. Upon loss of Ash2l, methylation of H3K4 and gene expression were downregulated, which correlated with inhibition of proliferation and cell cycle progression. Moreover, we observed induction of senescence concomitant with a set of downregulated signature genes but independent of SASP. Many of the signature genes are FoxM1 responsive. Indeed, exogenous FOXM1 was sufficient to delay senescence. Thus, although the loss of Ash2l in MEFs has broad and complex consequences, a distinct set of downregulated genes promotes senescence.

Inhibiting the IRE1α Axis of the Unfolded Protein Response Enhances the Antitumor Effect of AZD1775 in TP53 Mutant Ovarian Cancer

Targeting the G2/M checkpoint mediator WEE1 has been explored as a novel treatment strategy in ovarian cancer, but mechanisms underlying its efficacy and resistance remains to be understood. Here, it is demonstrated that the WEE1 inhibitor AZD1775 induces endoplasmic reticulum stress and activates the protein kinase RNA-like ER kinase (PERK) and inositol-required enzyme 1α (IRE1α) branches of the unfolded protein response (UPR) in TP53 mutant (mtTP53) ovarian cancer models. This is facilitated through NF-κB mediated senescence-associated secretory phenotype. Upon AZD1775 treatment, activated PERK promotes apoptotic signaling via C/EBP-homologous protein (CHOP), while IRE1α-induced splicing of XBP1 (XBP1s) maintains cell survival by repressing apoptosis. This leads to an encouraging synergistic antitumor effect of combining AZD1775 and an IRE1α inhibitor MKC8866 in multiple cell lines and preclinical models of ovarian cancers. Taken together, the data reveal an important dual role of the UPR signaling network in mtTP53 ovarian cancer models in response to AZD1775 and suggest that inhibition of the IRE1α-XBP1s pathway may enhance the efficacy of AZD1775 in the clinics.

Characterization of cellular senescence in aging skeletal muscle

Senescence is a cell fate that contributes to multiple aging-related pathologies. Despite profound age-associated changes in skeletal muscle (SkM), whether its constituent cells are prone to senesce has not been methodically examined. Herein, using single cell and bulk RNA-sequencing and complementary imaging methods on SkM of young and old mice, we demonstrate that a subpopulation of old fibroadipogenic progenitors highly expresses p16 Ink4a together with multiple senescence-related genes and, concomitantly, exhibits DNA damage and chromatin reorganization. Through analysis of isolated myofibers, we also detail a senescence phenotype within a subset of old cells, governed instead by p2 Cip1 . Administration of a senotherapeutic intervention to old mice countered age-related molecular and morphological changes and improved SkM strength. Finally, we found that the senescence phenotype is conserved in SkM from older humans. Collectively, our data provide compelling evidence for cellular senescence as a hallmark and potentially tractable mediator of SkM aging.

Exerkines and long-term synaptic potentiation: Mechanisms of exercise-induced neuroplasticity

Physical exercise may improve cognitive function by modulating molecular and cellular mechanisms within the brain. We propose that the facilitation of long-term synaptic potentiation (LTP)-related pathways, by products induced by physical exercise (i.e., exerkines), is a crucial aspect of the exercise-effect on the brain. This review summarizes synaptic pathways that are activated by exerkines and may potentiate LTP. For a total of 16 exerkines, we indicated how blood and brain exerkine levels are altered depending on the type of physical exercise (i.e., cardiovascular or resistance exercise) and how they respond to a single bout (i.e., acute exercise) or multiple bouts of physical exercise (i.e., chronic exercise). This information may be used for designing individualized physical exercise programs. Finally, this review may serve to direct future research towards fundamental gaps in our current knowledge regarding the biophysical interactions between muscle activity and the brain at both cellular and system levels.

Potential Methods of Targeting Cellular Aging Hallmarks to Reverse Osteoarthritic Phenotype of Chondrocytes

Osteoarthritis (OA) is a chronic degenerative joint disease that causes pain, physical disability, and life quality impairment. The pathophysiology of OA remains largely unclear, and currently no FDA-approved disease-modifying OA drugs (DMOADs) are available. As has been acknowledged, aging is the primary independent risk factor for OA, but the mechanisms underlying such a connection are not fully understood. In this review, we first revisit the changes in OA chondrocytes from the perspective of cellular hallmarks of aging. It is concluded that OA chondrocytes share many alterations similar to cellular aging. Next, based on the findings from studies on other cell types and diseases, we propose methods that can potentially reverse osteoarthritic phenotype of chondrocytes back to a healthier state. Lastly, current challenges and future perspectives are summarized.

Senescent cells limit p53 activity via multiple mechanisms to remain viable

Super-enhancers regulate genes with important functions in processes that are cell type-specific or define cell identity. Mouse embryonic fibroblasts establish 40 senescence-associated super-enhancers regardless of how they become senescent, with 50 activated genes located in the vicinity of these enhancers. Here we show, through gene knockdown and analysis of three core biological properties of senescent cells that a relatively large number of senescence-associated super-enhancer-regulated genes promote survival of senescent mouse embryonic fibroblasts. Of these, Mdm2, Rnase4, and Ang act by suppressing p53-mediated apoptosis through various mechanisms that are also engaged in response to DNA damage. MDM2 and RNASE4 transcription is also elevated in human senescent fibroblasts to restrain p53 and promote survival. These insights identify key survival mechanisms of senescent cells and provide molecular entry points for the development of targeted therapeutics that eliminate senescent cells at sites of pathology.

Doxorubicin-induced senescence in normal fibroblasts promotes in vitro tumour cell growth and invasiveness: the role of Quercetin in modulating these processes

Ageing is a complex biological phenomenon representing the major risk factor for developing age-related diseases, such as cardiovascular pathologies, neurodegenerative diseases, and cancer. Geroscience, the new vision of gerontology, identifies cellular senescence as an interconnected biological process that characterises ageing and age-related diseases. Therefore, many strategies have been employed in the last years to reduce the harmful effects of senescence, and among these, the most intriguing ones use nutraceutical compounds. Here we show that a pre-treatment with Quercetin, a bioactive flavonoid present in many fruits and vegetables, increasing cellular antioxidant defence, can alleviate Doxorubicin (Doxo)-induced cellular senescence in human normal WI-38 fibroblasts. Furthermore, our work demonstrates that Quercetin pre-treatment, reducing the number of senescent cells and the production of the senescence-associated secretory phenotype (SASP) factors, can decrease the pro-tumour effects of conditioned medium from Doxo-induced senescent fibroblasts on osteosarcoma cells. Overall, our findings are consistent with the hypothesis that targeting senescent cells can be an emerging strategy for cancer treatment, especially in elderly patients, in which senescent cells are already abundant in several tissues and organs.

Albumin-induced premature senescence in human renal proximal tubular cells and its relationship with intercellular fibrosis

The presence of senescent cells is associated with renal fibrosis. This study aims to investigate the effect of albumin-induced premature senescence on tubulointerstitial fibrosis and its possible mechanism in vitro. Different concentrations of bovine serum albumim (BSA) with or without si-p21 are used to stimulate HK-2 cells for 72 h, and SA-β-gal activity, senescence-associated secretory phenotypes (SASPs), LaminB1 are used as markers of senescence. Immunofluorescence staining is performed to characterize the G2/M phase arrest between the control and BSA groups. Alterations in the DNA damage marker γ-H2AX, fibrogenesis, and associated proteins at the G2/M phase, such as p21, p-CDC25C and p-CDK1, are evaluated. Compared with those in the control group, the SA-β-gal activity, SASP, and γ-H2AX levels are increased in the BSA group, while the level of LaminB1 is decreased. Meanwhile, HK-2 cells blocked at the G2/M phase are significantly increased under the stimulation of BSA, and the levels of p21, p-CDC25C and p-CDK1, as well as fibrogenesis are also increased. When p21 expression is inhibited, the levels of p-CDC25C and p-CDK1 are decreased and the G2/M phase arrest is improved, which decreases the production of fibrogenesis. In conclusion, BSA induces renal tubular epithelial cell premature senescence, which regulates the G2/M phase through the CDC25C/CDK1 pathway, leading to tubulointerstitial fibrosis.

Cellular senescence in neuroblastoma

Neuroblastoma is a tumour that arises from the sympathoadrenal lineage occurring predominantly in children younger than five years. About half of the patients are diagnosed with high-risk tumours and undergo intensive multi-modal therapy. The success rate of current treatments for high-risk neuroblastoma is disappointingly low and survivors suffer from multiple therapy-related long-term side effects. Most chemotherapeutics drive cancer cells towards cell death or senescence. Senescence has long been considered to represent a terminal non-proliferative state and therefore an effective barrier against tumorigenesis. This dogma, however, has been challenged by recent observations that infer a much more dynamic and reversible nature for this process, which may have implications for the efficacy of therapy-induced senescence-oriented treatment strategies. Neuroblastoma cells in a dormant, senescent-like state may escape therapy, whilst their senescence-associated secretome may promote inflammation and invasiveness, potentially fostering relapse. Conversely, due to its distinct molecular identity, senescence may also represent an opportunity for the development of novel (combination) therapies. However, the limited knowledge on the molecular dynamics and diversity of senescence signatures demands appropriate models to study this process in detail. This review summarises the molecular knowledge about cellular senescence in neuroblastoma and investigates current and future options towards therapeutic exploration.

Targeting tumor cell senescence and polyploidy as potential therapeutic strategies

Senescence is a unique state of growth arrest that develops in response to a plethora of cellular stresses, including replicative exhaustion, oxidative injury, and genotoxic insults. Senescence has been implicated in the pathogenesis of multiple aging-related pathologies, including cancer. In cancer, senescence plays a dual role, initially acting as a barrier against tumor progression by enforcing a durable growth arrest in premalignant cells, but potentially promoting malignant transformation in neighboring cells through the secretion of pro-tumorigenic drivers. Moreover, senescence is induced in tumor cells upon exposure to a wide variety of conventional and targeted anticancer drugs (termed Therapy-Induced Senescence-TIS), representing a critical contributing factor to therapeutic outcomes. As with replicative or oxidative senescence, TIS manifests as a complex phenotype of macromolecular damage, energetic dysregulation, and altered gene expression. Senescent cells are also frequently polyploid. In vitro studies have suggested that polyploidy may confer upon senescent tumor cells the ability to escape from growth arrest, thereby providing an additional avenue whereby tumor cells escape the lethality of anticancer treatment. Polyploidy in tumor cells is also associated with persistent energy production, chromatin remodeling, self-renewal, stemness and drug resistance - features that are also associated with escape from senescence and conversion to a more malignant phenotype. However, senescent cells are highly heterogenous and can present with variable phenotypes, where polyploidy is one component of a complex reversion process. Lastly, emerging efforts to pharmacologically target polyploid tumor cells might pave the way towards the identification of novel targets for the elimination of senescent tumor cells by the incorporation of senolytic agents into cancer therapeutic strategies.

Exploiting senescence for the treatment of cancer

Senescence is a cellular response to a variety of stress signals that is characterized by a stable withdrawal from the cell cycle and major changes in cell morphology and physiology. While most research on senescence has been performed on non-cancer cells, it is evident that cancer cells can also mount a senescence response. In this Review, we discuss how senescence can be induced in cancer cells. We describe the distinctive features of senescent cancer cells and how these changes in cellular physiology might be exploited for the selective eradication of these cells (senolysis). We discuss activation of the host immune system as a particularly attractive way to clear senescent cancer cells. Finally, we consider the challenges and opportunities provided by a 'one-two punch' sequential treatment of cancer with pro-senescence therapy followed by senolytic therapy.

The origins of cancer cell dormancy

Cancer cell dormancy has emerged as an important nongenetic driver of drug resistance. Dormant cells are characterized by a reversible cell cycle exit. They represent a reservoir for eventual cancer relapse, and upon reactivation, can fuel metastatic disease. Although dormant cells were originally believed to emerge from a drug-resistant pre-existing cancer subpopulation, this notion has been recently challenged. Here, we review recent evidence indicating that dormancy represents an adaptive strategy employed by cancer cells to avoid the cytotoxic effects of antitumor therapy. Furthermore, we outline the molecular pathways engaged by cancer cells to enter dormancy upon drug exposure, with a focus on cellular senescence as a driver of dormancy.

Cellular Senescence in Aging Lungs and Diseases

Cellular senescence represents a state of irreversible cell cycle arrest occurring naturally or in response to exogenous stressors. Following the initial arrest, progressive phenotypic changes define conditions of cellular senescence. Understanding molecular mechanisms that drive senescence can help to recognize the importance of such pathways in lung health and disease. There is increasing interest in the role of cellular senescence in conditions such as chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) in the context of understanding pathophysiology and identification of novel therapies. Herein, we discuss the current knowledge of molecular mechanisms and mitochondrial dysfunction regulating different aspects of cellular senescence-related to chronic lung diseases to develop rational strategies for modulating the senescent cell phenotype in the lung for therapeutic benefit.

Cellular senescence

Cellular senescence defines a state of stable and generally irreversible proliferative arrest associated with various morphological, structural and functional changes (Figure 1), including enhanced expression and secretion of pro-inflammatory and tissue-remodelling mediators. This state is crucial in tissue physiology and pathology and arises as a response to potentially damaging stress signals. Whether the activation of a senescence state provides benefits or detriments for tissue function and homeostasis is strictly dependent on the context. Cell senescence acts as a potent tumour-suppressive mechanism limiting the proliferation of cells at risk of malignant transformation and supports the repair of acute tissue damage, but also represents a key driver of ageing and age-related diseases.

Potential regulators of the senescence-associated secretory phenotype during senescence and ageing

Senescent cells express and secrete a variety of extracellular modulators that include cytokines, chemokines, proteases, growth factors and some enzymes associated with ECM remodeling, defined as the senescence-associated secretory phenotype (SASP). SASP reinforces senescent cell cycle arrest, stimulates and recruits immune cells for immune-mediated clearance of potentially tumorigenic cells, limits or induces fibrosis and promotes wound healing and tissue regeneration. On the other hand, SASP mediates chronic inflammation leading to destruction of tissue structure and function and stimulating the growth and survival of tumour cells. SASP is highly heterogeneous and the role of SASP depends on the context. The regulation of SASP occurs at multiple levels including chromatin remodelling, transcription, mRNA translation, intracellular trafficking and secretion. Several SASP modulators have already been identified setting the stage for future research on their clinical applications. In this review, we summarize in detail the potential signalling pathways that trigger and regulate SASP production during ageing and senescence.

Exploring the role of senescence inducers and senotherapeutics as targets for anticancer natural products

During the last few decades, cancer has remained one of the deadliest diseases that endanger human health, emphasizing urgent drug discovery. Cellular senescence has gained a great deal of attention in recent years because of its link to the development of cancer therapy. Senescent cells are incapable of proliferating due to irreversibly inhibited the initiation of the cell cycle pathways. However, senescent cells aggregate in tissues and produce a pro-inflammatory secretome called senescence-associated secretory phenotype (SASP) that can cause serious harmful effects if not managed properly. There is mounting evidence that senescent cells lead to various phases of tumorigenesis in various anatomical sites, owing mostly to the paracrine activities of the SASP. Therefore, a new treatment field called senotherapeutics has been established. Senotherapeutics are newly developed anticancer agents that have been demonstrated to inhibit cancer effectively. In light of recent findings, several promising natural products have been identified as senescence inducers and senotherapeutics, including, miliusanes, epigallocatechin gallate, phloretin, silybin, resveratrol, genistein, sulforaphane, quercetin, allicin, fisetin, piperlongumine, berberine, triptolide, tocotrienols and curcumin analogs. Several of them have already been validated through preclinical trials and exert an enormous potential for clinical trials. This review article focuses on and summarises the latest advances on cellular senescence and its potential as a target for cancer treatment and highlights the well-known natural products as senotherapeutics for cancer treatment.

Transcription Factor 4 loss-of-function is associated with deficits in progenitor proliferation and cortical neuron content

Transcription Factor 4 (TCF4) has been associated with autism, schizophrenia, and other neuropsychiatric disorders. However, how pathological TCF4 mutations affect the human neural tissue is poorly understood. Here, we derive neural progenitor cells, neurons, and brain organoids from skin fibroblasts obtained from children with Pitt-Hopkins Syndrome carrying clinically relevant mutations in TCF4. We show that neural progenitors bearing these mutations have reduced proliferation and impaired capacity to differentiate into neurons. We identify a mechanism through which TCF4 loss-of-function leads to decreased Wnt signaling and then to diminished expression of SOX genes, culminating in reduced progenitor proliferation in vitro. Moreover, we show reduced cortical neuron content and impaired electrical activity in the patient-derived organoids, phenotypes that were rescued after correction of TCF4 expression or by pharmacological modulation of Wnt signaling. This work delineates pathological mechanisms in neural cells harboring TCF4 mutations and provides a potential target for therapeutic strategies for genetic disorders associated with this gene.

In vivo cyclic induction of the FOXM1 transcription factor delays natural and progeroid aging phenotypes and extends healthspan

The FOXM1 transcription factor exhibits pleiotropic C-terminal transcriptional and N-terminal non-transcriptional functions in various biological processes critical for cellular homeostasis. We previously found that FOXM1 repression during cellular aging underlies the senescence phenotypes, which were vastly restored by overexpressing transcriptionally active FOXM1. Yet, it remains unknown whether increased expression of FOXM1 can delay organismal aging. Here, we show that in vivo cyclic induction of an N-terminal truncated FOXM1 transgene on progeroid and naturally aged mice offsets aging-associated repression of full-length endogenous Foxm1, reinstating both transcriptional and non-transcriptional functions. This translated into mitigation of several cellular aging hallmarks, as well as molecular and histopathological progeroid features of the short-lived Hutchison-Gilford progeria mouse model, significantly extending its lifespan. FOXM1 transgene induction also reinstated endogenous Foxm1 levels in naturally aged mice, delaying aging phenotypes while extending their lifespan. Thus, we disclose that FOXM1 genetic rewiring can delay senescence-associated progeroid and natural aging pathologies.

Detection of Cellular Senescence in Human Primary Melanocytes and Malignant Melanoma Cells In Vitro

Detection and quantification of senescent cells remain difficult due to variable phenotypes and the absence of highly specific and reliable biomarkers. It is therefore widely accepted to use a combination of multiple markers and cellular characteristics to define senescent cells in vitro. The exact choice of these markers is a subject of ongoing discussion and usually depends on objective reasons such as cell type and treatment conditions, as well as subjective considerations including feasibility and personal experience. This study aims to provide a comprehensive comparison of biomarkers and cellular characteristics used to detect senescence in melanocytic systems. Each marker was assessed in primary human melanocytes that overexpress mutant BRAFV600E, as it is commonly found in melanocytic nevi, and melanoma cells after treatment with the chemotherapeutic agent etoposide. The combined use of these two experimental settings is thought to allow profound conclusions on the choice of senescence biomarkers when working with melanocytic systems. Further, this study supports the development of standardized senescence detection and quantification by providing a comparative analysis that might also be helpful for other cell types and experimental conditions.

Biology of aging: Oxidative stress and RNA oxidation

The prevalence of aged people has increased rapidly in recent years and brings profound demographic changes worldwide. The multi-level progression of aging occurs at diverse stages of complexity, from cell to organ systems and eventually to the human as a whole. The cellular and molecular damages are usually regulated by the cells; repair or degrade mechanisms. However, these mechanisms are not entirely functional; their effectiveness decreases with age due to influence from endogenous sources like oxidative stress, which all contribute to the aging process. The hunt for novel strategies to increase the man's longevity since ancient times needs better understandings of the biology of aging, oxidative stress, and their roles in RNA oxidation. The critical goal in developing new strategies to increase the man's longevity is to compile the novel developed knowledge on human aging into a single picture, preferably able to understand the biology of aging and the contributing factors. This review discusses the biology of aging, oxidative stress, and their roles in RNA oxidation, leading to aging in humans.

Single-cell transcriptomics identifies Mcl-1 as a target for senolytic therapy in cancer

Cells subjected to treatment with anti-cancer therapies can evade apoptosis through cellular senescence. Persistent senescent tumor cells remain metabolically active, possess a secretory phenotype, and can promote tumor proliferation and metastatic dissemination. Removal of senescent tumor cells (senolytic therapy) has therefore emerged as a promising therapeutic strategy. Here, using single-cell RNA-sequencing, we find that senescent tumor cells rely on the anti-apoptotic gene Mcl-1 for their survival. Mcl-1 is upregulated in senescent tumor cells, including cells expressing low levels of Bcl-2, an established target for senolytic therapy. While treatment with the Bcl-2 inhibitor Navitoclax results in the reduction of metastases in tumor bearing mice, treatment with the Mcl-1 inhibitor S63845 leads to complete elimination of senescent tumor cells and metastases. These findings provide insights on the mechanism by which senescent tumor cells survive and reveal a vulnerability that can be exploited for cancer therapy.

Cell senescence remains a barrier to tumor elimination in many cancers. Here, the authors use single cell RNA-seq to identify a role for Mcl-1 in senescent cell survival, and show that Mcl-1 inhibition may be an effective therapeutic strategy.

Targeting cellular senescence as a novel treatment for osteoarthritis

Cellular senescence is associated with normal development and wound healing, but has also been implicated in the pathogenesis of numerous aging-related diseases including osteoarthritis (OA). Treatment strategies for OA are being developed that target senescent cells and the paracrine and autocrine secretions of the senescence-associated secretory phenotype (SASP). The field of potential therapies continues to expand as new mechanistic targets of cell senescence and the SASP are identified. Ongoing pre-clinical and clinical studies of drugs targeting cellular senescence yield significant promise, but have yet to demonstrate long-term efficacy. Therapeutic targeting of senescence is challenged by the diverse phenotypes of senescent cells, which can vary depending on age, species, tissue source, and type of physiologic stressor. Accordingly, there remains considerable demand for more studies to further develop and assess senotherapeutics as disease-modifying treatments for OA.

Short-term senolytic treatment: a paradigm to promote fracture repair during aging

Increased age is blamed for a wide range of bone physiological changes, and although the underlying mechanisms affecting the decreased capacity for fracture healing are not fully understood, they are clearly linked to changes at the cellular level. Recent evidence suggests potential roles of senescent cells in response to most tissue injuries, including bone fractures. In this issue of the JCI, Liu, Zhang, and co-authors showed that a senolytic drug cocktail cleared senescent cells from the callus and improved bone fracture repair in aged mice. Understanding how senescent cells emerge at fracture sites and how their timely removal improves fracture healing should provide insights for effective therapeutic approaches in old age.

Lamin B1 deletion in myeloid neoplasms causes nuclear anomaly and altered hematopoietic stem cell function

Abnormal nuclear morphology is a hallmark of malignant cells widely used in cancer diagnosis. Pelger-Huët anomaly (PHA) is a common abnormality of neutrophil nuclear morphology of unknown molecular etiology in myeloid neoplasms (MNs). We show that loss of nuclear lamin B1 (LMNB1) encoded on chromosome 5q, which is frequently deleted in MNs, induces defects in nuclear morphology and human hematopoietic stem cell (HSC) function associated with malignancy. LMNB1 deficiency alters genome organization inducing in vitro and in vivo expansion of HSCs, myeloid-biased differentiation with impaired lymphoid commitment, and genome instability due to defective DNA damage repair. Nuclear dysmorphology of neutrophils in patients with MNs is associated with 5q deletions spanning the LMNB1 locus, and lamin B1 loss is both necessary and sufficient to cause PHA in normal and 5q-deleted neutrophils. LMNB1 loss thus causes acquired PHA and links abnormal nuclear morphology with HSCs and progenitor cell fate determination via genome organization.

Measurement of Protein Turnover Rates in Senescent and Non-Dividing Cultured Cells with Metabolic Labeling and Mass Spectrometry

Mounting evidence has shown that the accumulation of senescent cells in the central nervous system contributes to neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Cellular senescence is a state of permanent cell cycle arrest that typically occurs in response to exposure to sub-lethal stresses. However, like other non-dividing cells, senescent cells remain metabolically active and carry out many functions that require unique transcriptional and translational demands and widespread changes in the intracellular and secreted proteomes. Understanding how protein synthesis and decay rates change during senescence can illuminate the underlying mechanisms of cellular senescence and find potential therapeutic avenues for diseases exacerbated by senescent cells. This paper describes a method for proteome-scale evaluation of protein half-lives in non-dividing cells using pulsed stable isotope labeling by amino acids in cell culture (pSILAC) in combination with mass spectrometry. pSILAC involves metabolic labeling of cells with stable heavy isotope-containing versions of amino acids. Coupled with modern mass spectrometry approaches, pSILAC enables the measurement of protein turnover of hundreds or thousands of proteins in complex mixtures. After metabolic labeling, the turnover dynamics of proteins can be determined based on the relative enrichment of heavy isotopes in peptides detected by mass spectrometry. In this protocol, a workflow is described for the generation of senescent fibroblast cultures and similarly arrested quiescent fibroblasts, as well as a simplified, single-time point pSILAC labeling time-course that maximizes coverage of anticipated protein turnover rates. Further, a pipeline is presented for the analysis of pSILAC mass spectrometry data and user-friendly calculation of protein degradation rates using spreadsheets. The application of this protocol can be extended beyond senescent cells to any non-dividing cultured cells such as neurons.

Spatial transcriptomic analysis reveals inflammatory foci defined by senescent cells in the white matter, hippocampi and cortical grey matter in the aged mouse brain

There is strong evidence that aging is associated with an increased presence of senescent cells in the brain. The finding that treatment with senolytic drugs improves cognitive performance of aged laboratory mice suggests that increased cellular senescence is causally linked to age-related cognitive decline. The relationship between senescent cells and their relative locations within the brain is critical to understanding the pathology of age-related cognitive decline and dementia. To assess spatial distribution of cellular senescence in the aged mouse brain, spatially resolved whole transcriptome mRNA expression was analyzed in sections of brains derived from young (3 months old) and aged (28 months old) C57BL/6 mice while capturing histological information in the same tissue section. Using this spatial transcriptomics (ST)-based method, microdomains containing senescent cells were identified on the basis of their senescence-related gene expression profiles (i.e., expression of the senescence marker cyclin-dependent kinase inhibitor p16INK4A encoded by the Cdkn2a gene) and were mapped to different anatomical brain regions. We confirmed that brain aging is associated with increased cellular senescence in the white matter, the hippocampi and the cortical grey matter. Transcriptional analysis of the senescent cell-containing ST spots shows that presence of senescent cells is associated with a gene expression signature suggestive of neuroinflammation. GO enrichment analysis of differentially expressed genes in the outer region of senescent cell-containing ST spots ("neighboring ST spots") also identified functions related to microglia activation and neuroinflammation. In conclusion, senescent cells accumulate with age in the white matter, the hippocampi and cortical grey matter and likely contribute to the genesis of inflammatory foci in a paracrine manner.

Spatially Confined Intervention of Cellular Senescence by a Lysosomal Metabolism Targeting Molecular Prodrug for Broad-Spectrum Senotherapy

Specific intervention of senescent cells (SnCs) is emerging as a powerful means to counteract aging and age-related diseases. Canonical methods are generally designed to target SnC-associated signaling pathways, which are however dynamically changing and highly heterogeneous in SnCs, significantly limiting the effectiveness. Here, we present a tailor-made molecular prodrug targeting lysosome dysfunction, a unique feature shared by virtually all types of SnCs. The prodrug comprises three modules all targeting the altered lysosomal programs in SnCs, namely, a recognizing unit towards the elevated lysosome content, a linker cleavable by the activated lysosomal enzyme, and a lysosomotropic agent targeting the increased lysosomal membrane sensitivity. This spatially confined design enables killing broad-spectrum SnCs, with high specificity over non-SnCs. Along with in vivo benefits, this work offers a way to significantly expand the applicability of senotherapy in a wide range of diseases.

The flavonoid corylin exhibits lifespan extension properties in mouse

In the long history of traditional Chinese medicine, single herbs and complex formulas have been suggested to increase lifespan. However, the identification of single molecules responsible for lifespan extension has been challenging. Here, we collected a list of traditional Chinese medicines with potential longevity properties from pharmacopeias. By utilizing the mother enrichment program, we systematically screened these traditional Chinese medicines and identified a single herb, Psoralea corylifolia, that increases lifespan in Saccharomyces cerevisiae. Next, twenty-two pure compounds were isolated from Psoralea corylifolia. One of the compounds, corylin, was found to extend the replicative lifespan in yeast by targeting the Gtr1 protein. In human umbilical vein endothelial cells, RNA sequencing data showed that corylin ameliorates cellular senescence. We also examined an in vivo mammalian model, and found that corylin extends lifespan in mice fed a high-fat diet. Taken together, these findings suggest that corylin may promote longevity.

Why Senescent Cells Are Resistant to Apoptosis: An Insight for Senolytic Development

Cellular senescence is a process that leads to a state of irreversible cell growth arrest induced by a variety of intrinsic and extrinsic stresses. Senescent cells (SnCs) accumulate with age and have been implicated in various age-related diseases in part via expressing the senescence-associated secretory phenotype. Elimination of SnCs has the potential to delay aging, treat age-related diseases and extend healthspan. However, once cells becoming senescent, they are more resistant to apoptotic stimuli. Senolytics can selectively eliminate SnCs by targeting the SnC anti-apoptotic pathways (SCAPs). They have been developed as a novel pharmacological strategy to treat various age-related diseases. However, the heterogeneity of the SnCs indicates that SnCs depend on different proteins or pathways for their survival. Thus, a better understanding of the underlying mechanisms for apoptotic resistance of SnCs will provide new molecular targets for the development of cell-specific or broad-spectrum therapeutics to clear SnCs. In this review, we discussed the latest research progresses and challenge in senolytic development, described the significance of regulation of senescence and apoptosis in aging, and systematically summarized the SCAPs involved in the apoptotic resistance in SnCs.

Senescent macrophages in the human adipose tissue as a source of inflammaging

Obesity is a major risk factor for type 2 diabetes and a trigger of chronic and systemic inflammation. Recent evidence suggests that an increased burden of senescent cells (SCs) in the adipose tissue of obese/diabetic animal models might underlie such pro-inflammatory phenotype. However, the role of macrophages as candidate SCs, their phenotype, the distribution of SCs among fat depots, and clinical relevance are debated. The senescence marker β-galactosidase and the macrophage marker CD68 were scored in visceral (vWAT) and subcutaneous (scWAT) adipose tissue from obese patients (n=17) undergoing bariatric surgery and control patients (n=4) subjected to cholecystectomy. A correlation was made between the number of SCs and BMI, serum insulin, and the insulin resistance (IR) index HOMA. The monocyte cell line (THP-1) was cultured in vitro in high glucose milieu (60 mM D-glucose) and subsequently co-cultured with human adipocytes (hMADS) to investigate the reciprocal inflammatory activation. In obese patients, a significantly higher number of SCs was observed in vWAT compared to scWAT; about 70% of these cells expressed the macrophage marker CD68; and the number of SCs in vWAT, but not in scWAT, positively correlated with BMI, HOMA-IR, and insulin. THP-1 cultured in vitro in high glucose milieu acquired a senescent-like phenotype (HgSMs), characterized by a polarization toward a mixed M1/M2-like secretory phenotype. Co-culturing HgSMs with hMADS elicited pro-inflammatory cytokine expression in both cell types, and defective insulin signaling in hMADS. In morbid obesity, expansion of visceral adipose depots involves an increased burden of macrophages with senescent-like phenotype that may promote a pro-inflammatory profile and impair insulin signaling in adipocytes, supporting a framework where senescent macrophages fuel obesity-induced systemic inflammation and possibly contribute to the development of IR.

Lipids as Regulators of Cellular Senescence

Lipids are key macromolecules that perform a multitude of biological functions ranging from maintaining structural integrity of membranes, energy storage, to signaling molecules. Unsurprisingly, variations in lipid composition and its levels can influence the functional and physiological state of the cell and its milieu. Cellular senescence is a permanent state of cell cycle arrest and is a hallmark of the aging process, as well as several age-related pathologies. Senescent cells are often characterized by alterations in morphology, metabolism, chromatin remodeling and exhibit a complex pro-inflammatory secretome (SASP). Recent studies have shown that the regulation of specific lipid species play a critical role in senescence. Indeed, some lipid species even contribute to the low-grade inflammation associated with SASP. Many protein regulators of senescence have been well characterized and are associated with lipid metabolism. However, the link between critical regulators of cellular senescence and senescence-associated lipid changes is yet to be elucidated. Here we systematically review the current knowledge on lipid metabolism and dynamics of cellular lipid content during senescence. We focus on the roles of major players of senescence in regulating lipid metabolism. Finally, we explore the future prospects of lipid research in senescence and its potential to be targeted as senotherapeutics.