Abstract 5683: Growth hormone as a SASP component

Senescence represents a fundamental response to cancer therapy. Accumulating senescent cells contribute to the deleterious outcomes of cancer therapy including cancer relapse, effects that may be largely mediated by the Senescence-Associated Secretory Phenotype (SASP). In this work, we show that tumor cells induced into senescence by etoposide retain proliferative capacity based on their capacity to generate proliferating colonies in culture as well as giving rise to viable tumors in vivo. Using a flow cytometry-based enrichment approach based on enlarged size and expression of Senescence-Associated β-galactosidase (SA-β-gal), we were able to utilize real time imaging to establish the re-emergence of non-small cell lung cancer cells from senescence-based arrest and the generation or proliferating daughter cells (i.e. self-renewal). Moreover, we implemented High-Speed Live-Cell Interferometry (HSLCI) to provide a single-cell lineage tracking of dividing senescent cells. The recovery from senescence was accompanied by resolution of several senescence-associated hallmarks, specifically SA-β-gal activity, p21Waf1/Cip1 and several components of the SASP (IL-1β, IL-6 and CXCL1). Our data suggests that Therapy-Induced Senescence (TIS) may ultimately be a transient process in that at least a subpopulation of tumor cells can recover proliferative capacity. We further demonstrate that the senolytic agent, ABT263, which has been shown to eliminate senescent cells from aging-related animal models can also eliminate senescent tumor cells that persistent after exposure to chemotherapy by shifting the response towards apoptotic cell death. Furthermore, sequential administration of ABT263 interferes with the ability of tumor cells induced into senescence by chemotherapy to recover growth potential. These studies suggest that senescent tumor cells can potentially contribute to cancer relapse by acquiring proliferative properties. The use of senolytic agents after induction of senescence by conventional or targeted therapies allows for the clearance of residual (possibly dormant) senescent tumor cells, which could serve to suppress disease recurrence and cancer mortality. Citation Format: Tareq Saleh, Liliya Tyutyunyk-Massey, Graeme F. Murray, Moureq R. Alotaibi, Ajinkya S. Kawale, Zeinab Elsayed, Scott C. Henderson, Vasily Yakovlev, Lynne W. Elmore, Amir Toor, Hisashi Harada, Jason Reed, Joseph W. Landry, David A. Gewirtz. Elimination of senescent tumor cells by ABT263 interferes with proliferative recovery and provides a two-hit therapeutic approach [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 901.

Cellular senescence is a state of stable growth arrest in response to stress, which is a fundamental process of biological aging. They secrete products, the Senescence-Associated Secretory Phenotype (SASP), which consists of inflammatory cytokines, chemokines, growth factors and matrix remodeling proteins. Senescent cells accumulate with advancing age and partial elimination of senescent cells can reverse age-related dysfunction and increase mean lifespan in mice. However, it is not clear whether components of the SASP can be measured in human plasma and serve as aging biomarkers. Here we generated a candidate panel of senescence markers based on a multiplexed bead-based assay of proteins secreted by senescent preadipocytes, endothelial cells, fibroblasts, myoblasts, preadipocytes, and epithelial cells compared to non-senescent controls. The SASP undoubtedly varies by cell type; however, we observed that multiple components of the SASP are conserved. We then assessed circulating SASP components in human plasma samples from Mayo Clinic Biobank participants (n=280, 20 male and 20 female per decade, age 20-90) using the same method. We confirmed that components of the SASP can be quantified in human plasma with the multiplexed bead-based assay and observed several SASP components robustly increase with chronological age in humans. Our study illustrates that senescence-associated secretome components are detectable in human plasma and could potentially serve as biomarkers of systemic aging and senescent cell burden.

Cellular senescence is characterized by stable cell cycle arrest and a secretory program that modulates the tissue microenvironment. The aberrant accumulation of senescent cells in aged and cancerous tissue triggers inflammatory signaling through a senescence-associated secretory phenotype (SASP), promoting aging and tumor progression. Pharmacologically, clearing senescent cells has been shown to have promising effects against cancer and age-related pathologies in preclinical models as well as human pilot clinical trials. However, current senescent cell elimination strategies are focused on the rare senescence targets that are under-represented on normal cells. To date, there are no universal senescence cell specific markers. Therefore, the use of senolytics in the clinic is still limited due to their cytotoxicity to normal cells. A more effective senescence cell targeted approach is needed to reduce side effects and increase the efficiency of senescence cell removal. Conditionally Active Biologics (CAB) technology is a proprietary platform that is unique in its ability to selectively activate in the context of diseased tissues, but not normal tissues, taking advantage of the acidic pH conditions in the inflammatory tumor/aged tissue microenvironment. Senescent cells, like tumor cells, are dependent on glycolysis for energy metabolism, and thus generate a glycolytic microenvironment (GME) that resembles that of the tumor microenvironment. Since our CAB technology is already being deployed in cancer therapy by targeting cell surface markers such as AXL, ROR2, and CTLA4 in the clinical stage, we explored whether CAB technology will allow selective removal of senescent cells in SASP-associated microenvironments such as the GME. To identify senescence-specific surface antigens that are broadly and specifically upregulated in senescent cells, we screened our CAB cell surface antigen library in different senescence models including CDK4/6 inhibitor-induced senescent human breast cancer cells, human lung fibroblast cells, as well as in vivo aging mouse tissues and chronic kidney disease mouse model. We have identified several novel senescence markers which show significantly elevated expression in senescent cells. Furthermore, in vitro binding assays demonstrated that CAB antibodies targeting these senescence markers have no or very low binding to the target antigen on senescence cells in alkaline physiological conditions but have strong binding in glycolytic/acidic SASP conditions. Moreover, we performed antibody-dependent cell-mediated cytotoxicity assays for evaluating the functional activities of CAB antibodies against the senescence targets. Our data demonstrated that CAB antibodies were more potent against senescent cells compared to proliferating cells and displayed high selectivity for the glycolytic/acidic microenvironment condition. In conclusion, CAB technology should provide a new generation of biologics with an increased safety margin and therapeutic index in the clinic targeting senescence cells in cancer and age-related diseases. Citation Format: Jian Chen, Jing Wang, Matt Lucas, Haizhen Liu, Christina Wheeler, Kyrie Johnson, Kathryn Woodard, Cathy Chang, Gerhard Frey, William J. Boyle, Jay M. Short. Targeting senescence cells in cancer and aging by conditionally active biologic therapeutics [abstract]. In: Proceedings of the AACR Special Conference: Aging and Cancer; 2022 Nov 17-20; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2022;83(2 Suppl_1):Abstract nr B022.

Non-resolving inflammation is the underpinning of several prevalent diseases including atherosclerosis. Understanding new mechanisms to promote the resolution of inflammation in atherosclerosis are of interest. Resolution is mediated by resolvins, including Resolvin D1 (RvD1). We recently showed that RvD1 prevents lesional necrosis in Ldlr -/- mice. However, the mechanisms underlying RvD1’s protective actions remain unknown. In this regard, the accumulation of senescent cells (SCs) was recently shown to promote necrotic core formation in Ldlr -/- mice. SCs are harmful because they possess a pro-inflammatory and proteolytic phenotype called the senescence associated secretory phenotype (SASP). Because RvD1 decreased lesional necrosis, we questioned whether RvD1’s actions were through limiting SCs in plaques. First, we found that human symptomatic plaques had significantly less RvD1 and significantly more SCs (quantified by positive p16 INK4A immunofluorescence staining) compared with asymptomatic plaques. To prove causation, we administered RvD1 to Ldlr -/- mice during advanced atherosclerosis and observed a significant decrease in lesional SCs and necrosis compared with vehicle controls. Mechanistically, we found that RvD1 significantly decreased the SASP as well as beta-galactosidase and p16 INK4A in WI-38 senescent cells. Importantly, a major component of the SASP is ADAM17, which is an enzyme that cleaves (and thus inactivates) a critical efferocytosis receptor on macrophages called MerTK. Because SCs are not cleared in plaques we questioned whether the SASP deranges efferocytosis mechanisms via their ability to cleave MerTK on macrophages. To investigate whether MerTK cleavage deranges SC clearance in plaques, we transplanted bone marrow from wild type (WT) or MerTK cleavage resistant (Mertk CR ) mice into Ldlr -/- mice, and quantified the levels of lesional p16 INK4A after 12 weeks of Western Diet feeding. Mertk CR mice had significantly less p16 INK4A levels and smaller necrotic cores compared with WT controls. Overall, we identified new functions for RvD1 and uncovered, for the first time, an endogenous mechanism for SC clearance. Our findings may provide new therapeutic strategies to treat atherosclerosis.

Skin diseases affect nearly one quarter of the global population, with prevalence rising sharply among older adults. By 2050, the number of individuals over 60 years will double, making age-related dermatological conditions an increasing public health concern. A central process underlying aging is cellular senescence, a stable form of growth arrest induced by diverse stressors, including DNA damage, telomere attrition, and oncogenic signaling. Senescent keratinocytes, fibroblasts, melanocytes, and immune cells accumulate in the skin with age and secrete a pro-inflammatory senescence-associated secretory phenotype (SASP) that actively shapes the cutaneous immune landscape. Although senescence can promote tissue repair and tumour suppression, the persistence of senescent cells drives inflammation, impairs immunity function, and contributes to pathology, a concept now termed senopathy. In this review, we first examine the crosstalk between senescent stromal and immune cells in human skin. Then we discuss how SASP from senescent fibroblasts inhibits the function of resident T cell, while senescent T cells adopt a paradoxical state, hyperfunctional yet non-proliferative, that can accelerate tissue damage. We further highlight the immune evasion strategies which enable senescent cells to persist and drive inflammaging. Insights from patients with Familial Melanoma Syndrome (germline CDKN2A mutations) illustrate how defective senescence pathways across multiple cell types impair cutaneous immunosurveillance and increase melanoma risk. Finally, we explore evidence for stromal- and immune-mediated cutaneous senopathies, including psoriasis, lupus, vitiligo, and leishmaniasis, where senescent cells actively drive the diseases’ progression. Based on the analysis, we propose that the skin represents a powerful and accessible model for investigating the interplay between senescent immune and non-immune cells across the lifespan, with therapeutic implications for aging and age-related pathologies.

Oridonin inhibits SASP by blocking p38 and NF-κB pathways in senescent cells

Introduction: Senescent cells have emerged as bona fide drivers of ageing and age-related cardiovascular disease, with senescent cells accumulating in the aged heart and following damage/injury. We have shown that the removal of senescent cells using senolytics can rejuvenate the regenerative capacity of the aged heart. Aim: To investigate the effects of cell senescence and the action of the senolytics, Dasatinib (D) and Quercetin (Q) on human iPSC-derived cardiomyocyte survival and cell cycle, and endothelial cell survival, cell cycle, migration and tube formation in vitro. Methods and Results: We developed a transwell insert co-culture stress-induced premature senescence human cell model system to test the effects of senolytics D+Q in vitro. Co-culture of iPSC-derived cardiomyocytes (iPSC-CMs) with senescent cardiac stromal progenitor cells (senCPCs) led to decreased number and DNA-synthesising activity of iPSC-CMs. Treatment with senolytics D+Q led to the elimination of senCPCs in the co-culture and the rescue of iPSC-CM number and DNA synthesis. Treatment of HUVECs with senCPC conditioned media decreased HUVEC number, cell cycle activity, migration, and tube formation. Treatment of HUVECs with D+Q conditioned media rescued HUVEC number, migration and tube formation. Next, we investigated the effects of co-culture of senescent HUVECs (senHUVECs) with HUVECs and showed decreased HUVEC number and DNA synthesis. Treatment with senolytics D+Q led to the elimination of senHUVECs in the co-culture and ameliorated HUVEC number, but not DNA synthesis. Treatment of HUVECs with conditioned media from senHUVECs led to decreased HUVEC migration and tube formation. Treatment of HUVECs with D+Q conditioned media improved HUVEC tube formation but not migration. Luminex analysis of the conditioned media from iPSC-CM and HUVEC co-cultures revealed upregulation of senescence-associated secretory phenotype (SASP) factors, but the level of SASP factors was reduced with the application of D+Q. Conclusion: Senescent cell removal by senolytics D+Q shows therapeutic potential in rejuvenating the reparative activity of human cardiomyocytes and endothelial cells. These results open the path to further studies on using senolytic therapy in age-related cardiac deterioration and rejuvenation. Potential impact of the findings: Senescent cells and their SASP present a promising therapeutic target to rejuvenate the heart’s reparative potential. Clinical trials using senolytics D+Q are already underway and thus far have shown promising results. Further pre-clinical studies are warranted for evidence-based clinical trials using senolytics in age-related cardiovascular diseases.

Cellular senescence is the process of the permanent proliferative arrest of cells in response to various inducers. It is accompanied by typical morphological changes, in addition to the secretion of bioactive molecules, including proinflammatory cytokines and chemokines [known as the senescence-associated secretory phenotype (SASP)]. Thus, senescent cells may affect their local environment and induce a so-called ‘bystander’ senescence through the state of SASP. The phenotypes of senescent cells are determined by the type of agent inducing cellular stress and the cell lineages. To characterise the phenotypes of senescent cancer cells, two murine cell lines were employed in the present study: TC-1 and B16F10 (B16) cells. Two distinct senescence inductors were used: Chemotherapeutic agent docetaxel (DTX) and a combination of immunomodulatory cytokines, including interferon γ (IFNγ) and tumour necrosis factor α (TNFα). It was demonstrated that DTX induced senescence in TC-1 and B16 tumour cell lines, which was demonstrated by growth arrest, positive β-galactosidase staining, increased p21Waf1 (p21) expression and the typical SASP capable of inducing a ‘bystander’ senescence. By contrast, treatment with a combination of T helper cell 1 cytokines, IFNγ and TNFα, induced proliferation arrest only in B16 cells. Despite the presence of certain characteristic features resembling senescent cells (proliferation arrest, morphological changes and increased p21 expression), these cells were able to form tumours in vivo and started to proliferate upon cytokine withdrawal. In addition, B16 cells were not able to induce a ‘bystander’ senescence. In summary, the present study described cell line- and treatment- associated differences in the phenotypes of senescent cells that may be relevant in optimization of cancer chemo- and immunotherapy.

We tested the hypothesis that 1,25‐dihydroxyvitamin D3[1α,25(OH)2D3] has antiaging effects via upregulating nuclear factor (erythroid‐derived 2)‐like 2 (Nrf2), reducing reactive oxygen species (ROS), decreasing DNA damage, reducing p16/Rb and p53/p21 signaling, increasing cell proliferation, and reducing cellular senescence and the senescence‐associated secretory phenotype (SASP). We demonstrated that 1,25(OH)2D3‐deficient [1α(OH)ase−/−] mice survived on average for only 3 months. Increased tissue oxidative stress and DNA damage, downregulated Bmi1 and upregulated p16, p53 and p21 expression levels, reduced cell proliferation, and induced cell senescence and the senescence‐associated secretory phenotype (SASP) were observed. Supplementation of 1α(OH)ase−/− mice with dietary calcium and phosphate, which normalized serum calcium and phosphorus, prolonged their average lifespan to more than 8 months with reduced oxidative stress and cellular senescence and SASP. However, supplementation with exogenous 1,25(OH)2D3 or with combined calcium/phosphate and the antioxidant N‐acetyl‐l‐cysteine prolonged their average lifespan to more than 16 months and nearly 14 months, respectively, largely rescuing the aging phenotypes. We demonstrated that 1,25(OH)2D3exerted an antioxidant role by transcriptional regulation of Nrf2 via the vitamin D receptor (VDR). Homozygous ablation of p16 or heterozygous ablation of p53 prolonged the average lifespan of 1α(OH)ase−/− mice on the normal diet from 3 to 6 months by enhancing cell proliferative ability and reducing cell senescence or apoptosis. This study suggests that 1,25(OH)2D3 plays a role in delaying aging by upregulating Nrf2, inhibiting oxidative stress and DNA damage，inactivating p53‐p21 and p16‐Rb signaling pathways, and inhibiting cell senescence and SASP.

Glioblastomas (GBM) are treated with high doses of ionizing radiation (IR), yet these tumors inevitably recur, and the recurrent tumors are highly therapy resistant. An understanding of mechanisms driving GBM recurrence is critical for improving therapy. During GBM therapy, both the tumor and the normal brain tissue surrounding the tumor are treated with up to 60 Gy of IR. IR is a potent inducer of senescence, and senescent stromal cells can promote the growth of neighboring tumor cells by secreting proteases, cytokines and growth factors that create a senescence-associated secretory phenotype (SASP). We have shown previously that IR-induced senescence of astrocytes in the brain leads to the secretion of SASP factors that promote the growth and invasiveness of tumor cells in mouse models of GBM (Fletcher-Sananikone, et al, Cancer Research, 2021). Following up on this study, we irradiated a panel of GBM cell lines and found that a significant fraction of these cells senesced rapidly in a p21-dependent manner. Senescent glioma cells upregulated SASP genes in a NF-kB-dependent manner, prominently, the interleukin IL6, which is an activator of the JAK-STAT pathway. Conditioned media from senescent GBM cells activated the JAK-STAT pathway in non-senescent GBM cells and promoted tumor cell proliferation and radiation resistance. Interestingly, conditioned media from senescent GBM cells could also activate the NF-kB pathway and induce SASP in non-senescent cells. These results indicate that senescent GBM cells can activate pro-tumorigenic pathways in their non-senescent counterparts and SASP can spread from senescent to non-senescent cells. Bioinformatic analyses of the transcriptomic profiles of irradiated GBM cells and the TCGA database revealed that the inhibitor of apoptosis protein cIAP2 is a critical survival factor for senescent glioma cells. We find that targeting cIAP2 using Smac mimetics triggers apoptosis of senescent GBM cells with minimal toxicity towards normal brain cells like astrocytes. Upregulation of cIAP2 in irradiated tumor cells was also seen in multiple PDX models of GBM. Using these PDX lines, we demonstrate that adjuvant senolytic treatment can delay or even prevent recurrence in pre-clinical mouse models of GBM. In sum, our findings illustrate how senescence of both stromal and tumor cells promote GBM recurrence via different mechanisms and underscore the potential utility of adjuvant senolytic therapy for blunting GBM recurrence after radiotherapy. Citation Format: Sandeep Burma, Nozomi Tomimatsu, Luis Cristofaro, Suman Kanji, Benjamin Jordan, Bipasha Mukherjee. DNA damage-induced senescence as a driver of glioblastoma recurrence [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr A009.

Cellular senescence is a double-edged sword in cancer biology, initially acting as a tumor-suppressive mechanism but later contributing to cancer progression and therapy resistance. Senescent cells, characterized by stable cell cycle arrest, secrete a complex array of bioactive molecules known as the senescence-associated secretory phenotype (SASP), which fosters chronic inflammation, disrupts tissue architecture, and promotes tumorigenesis through paracrine signaling. Accumulation of these cells in the tumor microenvironment can enhance malignancy, drive metastasis, and impair treatment outcomes. Senotherapeutics, have emerged as promising strategies for targeting senescent cells in cancer therapy. These agents selectively induce apoptosis in senescent cells while preserving normal tissues, representing a paradigm shift in oncology. Senotherapeutics can function as standalone treatments by clearing senescent tumor cells or as adjuvants to chemotherapy and radiotherapy, effectively eliminating residual therapy-induced senescent cells that may contribute to relapse. This dual approach allows for reduced treatment toxicity, improved therapeutic efficacy, and decreased tumor recurrence. Furthermore, targeting non-cancerous senescent cells may help suppress inflammation-driven tumorigenesis, slow disease progression, and enhance patient outcomes. Despite their promise, challenges remain in optimizing senotherapeutic strategies, identifying precise biomarkers, and minimizing off-target effects. This review explores the mechanisms of cellular senescence, its role in tumor dynamics, and the potential of senotherapeutics as a novel adjunct in cancer treatment. By integrating senotherapeutics with existing modalities, the field moves closer to more effective, personalized cancer interventions, warranting further preclinical and clinical investigation.

The aging process is associated with a low-grade chronic inflammation and the accumulation of senescent cells into tissues. Diverse stresses can trigger cellular senescence, a cell fate characterized by cell-cycle arrest and flat morphology. Oncogenic signaling can also induce cellular senescence which has been termed oncogene-induced senescence (OIS). Senescent cells display a pro-inflammatory phenotype which has been called the senescence-associated secretory phenotype (SASP). The secretomes associated with SASP contain colony-stimulating factors and chemokines which stimulate the generation of myeloid-derived suppressor cells (MDSC) by enhancing myelopoiesis in bone marrow and spleen. Enhanced myelopoiesis and increased level of MDSCs have been observed in bone marrow, spleen, and blood in both tumor-bearing and aged mice. Immunosuppressive MDSCs are recruited via chemotaxis into inflamed tissues where they proliferate and consequently suppress acute inflammatory reactions by inhibiting the functions of distinct components of innate and adaptive immunity. For instance, MDSCs stimulate the activity of immunosuppressive regulatory T-cells (Tregs). They also increase the expression of amino acid catabolizing enzymes and the secretion of anti-inflammatory cytokines, e.g. IL-10 and TGF-β, and reactive oxygen species (ROS). On the other hand, the accumulation of MDSCs into tissues exerts harmful effects in chronic pathological disorders, e.g. tumors and many age-related diseases, since the immunosuppression induced by MDSCs impairs the clearance of senescent and cancer cells and also disturbs the maintenance of energy metabolism and tissue proteostasis. The co-operation between senescent cells and immunosuppressive MDSCs regulates not only tumorigenesis and chronic inflammatory disorders but it also might promote inflammaging during the aging process.

<b>Introduction:</b> Chronic obstructive pulmonary disease (COPD) is associated with accelerated lung aging. In the COPD lung, there is an accumulation of senescent cells that may, in part, be due to a lack of clearance. To investigate this, a robust source of well-characterised senescent cells is required. Doxorubicin (DOXO) and etoposide (Etop) at sub-cytotoxic concentrations activate the DNA-damage response (DDR) pathway, leading to the induction of cellular senescence and could be used to generate a senescent cell population for study. <b>Aim:</b> To induce senescence in BEAS-2B cells using DOXO and Etop. <b>Methods:</b> Senescent BEAS-2B cells were generated by repeated exposure to 50nM DOXO or 1mM Etop for a total of 6 days. Cell cycle checkpoint inhibitors, anti-aging molecule sirtuin-1 and senescence associated secretory phenotype (SASP) marker expression were measured using western blotting and ELISA. Senescent cells were identified by senescence-associated-b-galactosidase (SA-β-gal) expression. <b>Results:</b> In BEAS-2B cells, 50nM DOXO treatment induced senescence in ~60% of cells (n=7, p&lt;0.01), p21 protein expression (4-fold, p&lt;0.01), reduced sirtuin-1 protein expression (~50%, p&lt;0.05) and induced secretion of IL-6 (10-fold, p&lt;0.05), CXCL-8 (6-fold, p&lt;0.05) and PAI-1 (3-fold, p&lt;0.01). 1mM etoposide treatment induced ~60% SA-b-gal (n=5, p&lt;0.001), p21 protein expression (8-fold, p=0.1), secretion of IL-6 (19-fold, p=0.09), CXCL-8 (11-fold, p&lt;0.05) and PAI-1 (3-fold, p&lt;0.01). <b>Conclusion:</b> Sub-cytotoxic double-hit DOXO and Etop treatments are required to induce senescence in BEAS-2B cells via p21 induction of cell cycle arrest and may provide a model that can be used in future experiments.

Cellular senescence prevents the proliferation of cells at risk for neoplastic transformation. Nonetheless, the senescence response is thought to be antagonistically pleiotropic and thus contribute to aging phenotypes, including, ironically, late life cancers. The cancer-promoting activity of senescent cells is likely due to secreted molecules, the identity of which remains largely unknown. Here, we have shown that senescent fibroblasts, much more than presenescent fibroblasts, stimulate tumor vascularization in mice. Weakly malignant epithelial cells co-injected with senescent fibroblasts had larger and greater numbers of blood vessels compared with controls. Accordingly, increased vascular endothelial growth factor (VEGF) expression was a frequent characteristic of senescent human and mouse fibroblasts in culture. Importantly, conditioned medium from senescent fibroblasts, more than medium from presenescent cells, stimulates cultured human umbilical vein endothelial cells to invade a basement membrane, a hallmark of angiogenesis. Increased VEGF expression was specific to the senescent phenotype and increased whether senescence was induced by replicative exhaustion, overexpression of p16(Ink4a), or overexpression of oncogenic RAS. The senescence-dependent increase in VEGF production was accompanied by very little increase in hypoxic-inducible (transcription) factor 1 alpha protein levels, and hypoxia further induced VEGF in senescent cells. This result suggests the rise in VEGF expression at senescence is not a hypoxic response. Our findings may in part explain why senescent cells stimulate tumorigenesis in vivo and support the idea that senescent cells may facilitate age-associated cancer development by secreting factors that promote malignant progression.

Cellular senescence is a state of stable cell cycle arrest triggered by internal and external stressors. A major hallmark of senescent cells is the senescent associated secretory phenotype (SASP), an inflammatory secretome consisting of cytokines, chemokines, growth factors and proteases. When transient in nature, senescent cells are beneficial, aiding in such physiological processes as tissue repair and regeneration. However, when senescent cells persist, SASP-driven inflammation promotes tissue and organ dysfunction. Through direct cytotoxicity, the immune system regulates an individuals senescent cell load. Compared to younger subjects, older adults exhibit an increased senescent cell burden that has been suggested to drive the development of age-related pathologies. The accumulation of senescent cells that occurs with age is accompanied by remodelling of the immune system. Termed immunesenescence, ageing results in marked alterations in the innate and adaptive arms of the immune system. Here, we discuss immunosurveillance of senescent cells in the context of an ageing immune system, suggesting that immunesenescence underpins the increased senescent cell load of older adults. Moreover, we propose the idea of a vicious cycle, whereby, via the SASP, senescent cells exacerbate immunesenescence by promoting dysregulation in aspects of the immune response that are otherwise unaffected by the ageing process.KeywordsAgeingCellular senescenceImmunesenescenceSenescence associated secretory phenotypeSenescent cells