Extracellular Nicotinamide Phosphoribosyltransferase Is a Component of the Senescence-Associated Secretory Phenotype.

Cellular senescence is a stress or damage response by which a cell adopts of state of essentially permanent proliferative arrest, coupled to the secretion of a number of biologically active molecules. This senescence-associated secretory phenotype (SASP) underlies many of the degenerative and regenerative aspects of cellular senescence - including promoting wound healing and development, but also driving diabetes and multiple age-associated diseases. We find that nicotinamide phosphoribosyltransferase (NAMPT), which catalyzes the rate-limiting step in nicotinamide adenine dinucleotide (NAD) biosynthesis, is elevated in senescent cells without a commensurate increase in NAD levels. This elevation is distinct from the acute DNA damage response, in which NAD is depleted, and recovery of NAD by NAMPT elevation is AMPK-activated protein kinase (AMPK)-dependent. Instead, we find that senescent cells release extracellular NAMPT (eNAMPT) as part of the SASP. eNAMPT has been reported to be released as a catalytically active extracellular vesicle-contained dimer that promotes NAD increases in other cells and extends lifespan, and also as free monomer that acts as a damage-associated molecular pattern and promotes conditions such as diabetes and fibrosis. Senescent cells released eNAMPT as dimer, but surprisingly eNAMPT appeared in the soluble secretome while being depleted from exosomes. Finally, diabetic mice showed elevated levels of eNAMPT, and this was lowered by treatment with the senolytic drug, ABT-263. Together, these data reveal a new SASP factor with implications for NAD metabolism.

Coordinated post-transcriptional control of oncogene-induced senescence by UNR/CSDE1.

Oncogene-induced senescence (OIS) is a form of stable cell-cycle arrest arising in response to oncogenic stimulation. OIS must be bypassed for transformation, but the mechanisms of OIS establishment and bypass remain poorly understood, especially at the post-transcriptional level. Here, we show that the RNA-binding protein UNR/CSDE1 enables OIS in primary mouse keratinocytes. Depletion of CSDE1 leads to senescence bypass, cell immortalization, and tumor formation, indicating that CSDE1 behaves as a tumor suppressor. Unbiased high-throughput analyses uncovered that CSDE1 promotes OIS by two independent molecular mechanisms: enhancement of the stability of senescence-associated secretory phenotype (SASP) factor mRNAs and repression of Ybx1 mRNA translation. Importantly, depletion of YBX1 from immortal keratinocytes rescues senescence and uncouples proliferation arrest from the SASP, revealing multilayered mechanisms exerted by CSDE1 to coordinate senescence. Our data highlight the relevance of post-transcriptional control in the regulation of senescence.

Therapeutic Potential of Senolytics in Cardiovascular Disease.

Ageing is the biggest risk factor for impaired cardiovascular health, with cardiovascular disease being the leading cause of death in 40% of individuals over 65 years old. Ageing is associated with both an increased prevalence of cardiovascular disease including heart failure, coronary artery disease, and myocardial infarction. Furthermore, ageing is associated with a poorer prognosis to these diseases. Genetic models allowing the elimination of senescent cells revealed that an accumulation of senescence contributes to the pathophysiology of cardiovascular ageing and promotes the progression of cardiovascular disease through the expression of a proinflammatory and profibrotic senescence-associated secretory phenotype. These studies have resulted in an effort to identify pharmacological therapeutics that enable the specific elimination of senescent cells through apoptosis induction. These senescent cell apoptosis-inducing compounds are termed senolytics and their potential to ameliorate age-associated cardiovascular disease is the focus of this review.

α-1 Adrenergic receptor antagonist doxazosin reverses hepatic stellate cells activation via induction of senescence.

BACKGROUND: Activated hepatic stellate cells (aHSCs) are the main effector cells during liver fibrogenesis. α-1 adrenergic antagonist doxazosin (DX) was shown to be anti-fibrotic in an in vivo model of liver fibrosis (LF), but the mechanism remains to be elucidated. Recent studies suggest that reversion of LF can be achieved by inducing cellular senescence characterized by irreversible cell-cycle arrest and acquisition of the senescence-associated secretory phenotype (SASP). AIM: To elucidate the mechanism of the anti-fibrotic effect of DX and determine whether it induces senescence. METHODS: Primary culture-activated rat HSCs were used. mRNA and protein expression were measured by qPCR and Western blot, respectively. Cell proliferation was assessed by BrdU incorporation and xCelligence analysis. TGF-ß was used for maximal HSC activation. Norepinephrine (NE), PMA and m-3M3FBS were used to activate alpha-1 adrenergic signaling. RESULTS: Expression of Col1α1 was significantly decreased by DX (10 µmol/L) at mRNA (-30 %) and protein level (-50 %) in TGF-ß treated aHSCs. DX significantly reduced aHSCs proliferation and increased expression of senescence and SASP markers. PMA and m-3M3FBS reversed the effect of DX on senescence markers. CONCLUSION: Doxazosin reverses the fibrogenic phenotype of aHSCs and induces the senescence phenotype.

Senescence-associated hyper-activation to inflammatory stimuli in vitro.

Aging is associated with an increased susceptibility to adverse inflammatory conditions such as sepsis and cytokine storm. We hypothesized that senescent cells (SnCs) play a central role in this age-associated pathology in part due to their expression of the senescence-associated secretory phenotype (SASP), which may prime SnCs to inflammatory stimulation. To test this hypothesis, we examined the expression of various inflammatory cytokines and chemokines at the levels of gene transcription and protein production in various SnCs in vitro in response to lipopolysaccharide (LPS), interleukin-1ß (IL1ß), and tumor necrosis factor α (TNFα) stimulation. We found that SnCs not only expressed higher basal levels of various inflammatory cytokines and chemokines as a manifestation of the SASP, but more importantly exhibited hyper-activation of the induction of a variety of inflammatory mediators in response to LPS, IL1ß and TNFα stimulation as compared with non-SnCs. This senescence-associated hyper-activation is likely mediated in part via the p38MAPK (p38) and NFκB pathways because LPS stimulation elicited significantly higher levels of p38 phosphorylation and NFκB p65 nuclear translation in SnCs when compared to their non-senescent counterparts and inhibition of these pathways with losmapimod (a p38 specific inhibitor) and BMS-345541 (a selective NFκB inhibitor) attenuated LPS-induced expression of IL6, TNFα, CCL5, and IL1ß mRNA in SnCs. These findings suggest that SnCs may play an important role in the age-related increases in the susceptibility to developing an exacerbated inflammatory response and highlight the potential to use senotherapeutics to ameliorate the severity of various devastating inflammatory conditions in the elderly.

Impact of Senescent Cell Subtypes on Tissue Dysfunction and Repair: Importance and Research Questions.

Cellular senescence, first observed and defined through cell culture studies, is a cell fate associated with essentially permanent cell cycle arrest and that can be triggered by a variety of inducers. Emerging evidence suggests senescence is a dynamic process with diverse functional characteristics. Depending on the tissue, type of inducer, and time since induction, senescent cells can promote tissue repair and re-modeling, prevent tumor development, or contribute to age-related disorders and chronic diseases, including cancers. Senescent cell characteristics appear to depend on multiple factors and be influenced by the milieu and other senescent cells locally and at a distance. We review diverse phenotypes of senescent cells originating from different cell types, senescence inducers over time since induction of senescence, and across conditions and diseases. This background is essential to inform further understanding about senescent cell subtypes and will point towards rational senescence-modulating strategies for achieving therapeutic benefit.

Cellular senescence as a possible link between prostate diseases of the ageing male.

Senescent cells accumulate with age in all tissues. Although senescent cells undergo cell-cycle arrest, these cells remain metabolically active and their secretome - known as the senescence-associated secretory phenotype - is responsible for a systemic pro-inflammatory state, which contributes to an inflammatory microenvironment. Senescent cells can be found in the ageing prostate and the senescence-associated secretory phenotype and can be linked to BPH and prostate cancer. Indeed, a number of signalling pathways provide biological plausibility for the role of senescence in both BPH and prostate cancer, although proving causality is difficult. The theory of senescence as a mechanism for prostate disease has a number of clinical implications and could offer opportunities for targeting in the future.

Advanced maternal age induces fetal growth restriction through decreased placental inflammatory cytokine expression and immune cell accumulation in mice.

Advanced maternal age is a risk factor for female infertility, and placental dysfunction is considered one of the causes of pregnancy complications. We investigated the effects of advanced maternal aging on pregnancy outcomes and placental senescence. Female pregnant mice were separated into three groups: young (3 months old), middle (8-9 months old), and aged (11-13 months old). Although the body weights of young and middle dams gradually increased during pregnancy, the body weight of aged dams only increased slightly. The placental weight and resorption rate were significantly higher, and live fetal weights were reduced in a maternal age-dependent manner. Although mRNA expression of senescence regulatory factors (p16 and p21) increased in the spleen of aged dams, mRNA expression of p16 did not change and that of p21 was reduced in the placenta of aged dams. Using a cytokine array of proteins extracted from placental tissues, the expression of various types of senescence-associated secretory phenotype (SASP) factors was decreased in aged dams compared with young and middle dams. The aged maternal placenta showed reduced immune cell accumulation compared with the young placenta. Our present results suggest that models using pregnant mice older than 8 months are more suitable for verifying older human pregnancies. These findings suggest that general cellular senescence programs may not be included in the placenta and that placental functions, including SASP production and immune cell accumulation, gradually decrease in a maternal age-dependent manner, resulting in a higher rate of pregnancy complications.

Astrocyte senescence and SASP in neurodegeneration: tau joins the loop.

Tau accumulation is a core component of Alzheimer's disease and other neurodegenerative tauopathies. While tau's impact on neurons is a major area of research, the effect of extracellular tau on astrocytes is largely unknown. This article summarizes our recent studies showing that astrocyte senescence plays a critical role in neurodegenerative diseases and integrates extracellular tau into the regulatory loop of senescent astrocyte-mediated neurotoxicity. Human astrocytes in vitro undergoing senescence were shown to acquire the inflammatory senescence-associated secretory phenotype (SASP) and toxicity to neurons, which may recapitulate aging- and disease-associated neurodegeneration. Here, we show that human astrocytes exposed to extracellular tau in vitro also undergo cellular senescence and acquire a neurotoxic SASP (e.g. IL-6 secretion), with oxidative stress response (indicated by upregulated NRF2 target genes) and a possible activation of inflammasome (indicated by upregulated ASC and IL-1ß). These findings suggest that senescent astrocytes induced by various conditions and insults, including tau exposure, may represent a therapeutic target to inhibit or delay the progression of neurodegenerative diseases. We also discuss the pathological activity of extracellular tau in microglia and astrocytes, the disease relevance and diversity of tau forms, therapeutics targeting senescence in neurodegeneration, and the roles of p53 and its isoforms in astrocyte-mediated neurotoxicity and neuroprotection.

Cellular senescence: Silent operator and therapeutic target in cancer.

The process of carcinogenesis and its progression involves an intricate interplay between a number of signaling networks, metabolic pathways and the microenvironment. These include an alteration in the cellular redox metabolism and deregulation of cell cycle checkpoints. Similar to the dichotomy of redox signaling in cancer cell fate and state determination, a diverging effect of an irreversible cell cycle arrest or senescence on carcinogenesis has been demonstrated. In this regard, while overwhelming oxidative stress has a damaging effect on tissue architecture and organ function and promotes death execution, a mild "pro-oxidant" environment is conducive for cell proliferation, growth and survival. Similarly, cellular senescence has been shown to elicit both a tumor suppressor and an oncogenic effect in a context-dependent manner. Notably, there appears to be a crosstalk between these two critical regulators of cell fate and state, particularly from the standpoint of the divergent effects on processes that promote or abate carcinogenesis. This review aims to provide an overview of these overarching themes and attempts to highlight critical intersection nodes, which are emerging as potential diagnostic and/or therapeutic targets for novel anticancer strategies.