The role of senescence,telomere dysfunction and shelterin in vascular aging

In the United States and other westernized nations, CVDs are the leading cause of death in adults over 65 years of age. Large artery stiffness and endothelial dysfunction are increased with age and age‐associated arterial dysfunction is an important antecedent of CVDs. One age‐associated change that may contribute to vascular dysfunction and CVD risk is an increase in the number of resident senescent cells in the vasculature. Senescent cells display a pro‐oxidant, pro‐inflammatory phenotype known as the SASP. However, the mechanisms that drive the SASP and the vascular aging phenotype remain elusive. A putative mechanism is the involvement of oxidative stress and inflammation in telomere function. Telomeres are the end caps of chromosomes which are maintained by a six‐protein complex known as shelterin. Disruption of shelterin can uncap telomeres and induce cellular senescence. Accordingly, in this review, we propose that oxidative stress and inflammation disrupt shelterin in vascular cells, driving telomere dysfunction and that this mechanism may be responsible for the induction of SASP. The proposed mechanisms may represent some of the initial changes that lead to vascular dysfunction in advanced age.     

Liver-specific knockdown of IGF-1 decreases vascular oxidative stress resistance by impairing the Nrf2-dependent antioxidant response: a novel model of vascular aging

Recent studies demonstrate that age-related dysfunction of NF-E2-related factor-2 (Nrf2)-driven pathways impairs cellular redox homeostasis, exacerbating age-related cellular oxidative stress and increasing sensitivity of aged vessels to oxidative stress-induced cellular damage. Circulating levels of insulin-like growth factor (IGF)-1 decline during aging, which significantly increases the risk for cardiovascular diseases in humans. To test the hypothesis that adult-onset IGF-1 deficiency impairs Nrf2-driven pathways in the vasculature, we utilized a novel mouse model with a liver-specific adeno-associated viral knockdown of the Igf1 gene using Cre-lox technology (Igf1(f/f) + MUP-iCre-AAV8), which exhibits a significant decrease in circulating IGF-1 levels (~50%). In the aortas of IGF-1-deficient mice, there was a trend for decreased expression of Nrf2 and the Nrf2 target genes GCLC, NQO1 and HMOX1. In cultured aorta segments of IGF-1-deficient mice treated with oxidative stressors (high glucose, oxidized low-density lipoprotein, and H(2)O(2)), induction of Nrf2-driven genes was significantly attenuated as compared with control vessels, which was associated with an exacerbation of endothelial dysfunction, increased oxidative stress, and apoptosis, mimicking the aging phenotype. In conclusion, endocrine IGF-1 deficiency is associated with dysregulation of Nrf2-dependent antioxidant responses in the vasculature, which likely promotes an adverse vascular phenotype under pathophysiological conditions associated with oxidative stress (eg, diabetes mellitus, hypertension) and results in accelerated vascular impairments in aging.     

Antisenescence as a novel therapeutic strategy for vascular aging

Vascular cells have a finite lifespan when cultured in vitro and eventually enter an irreversible growth arrest state called "cellular senescence." It has been reported that many of the changes in senescent vascular cell behavior are consistent with the changes seen in age-related vascular diseases. Recently, senescent vascular cells have been demonstrated in human atherosclerotic lesions but not non-atherosclerotic lesions. Moreover, these cells express increased levels of proinflammatory molecules and decreased levels of endothelial nitric oxide synthase, suggesting that cellular senescence in vivo contributes to the pathogenesis of human atherosclerosis. One widely discussed hypothesis of senescence is the telomere hypothesis. An increasing body of evidence has established the critical role of the telomere in vascular cell senescence. More recent evidence suggests that telomere-independent mechanisms are implicated in vascular cell senescence. Activation of Ras, an important signaling molecule involved in atherogenic stimuli, induces vascular cell senescence and thereby promotes vascular inflammation in vitro and in vivo. Constitutive activation of Akt also induces vascular cell senescence. This novel role of Akt in regulating the cellular lifespan may contribute to various human diseases including atherosclerosis and diabetes mellitus. Although a causal link between vascular aging and vascular cell senescence remains elusive, a large body of data is consistent with cellular senescence contributing to age-associated vascular disorders. This review considers the clinical relevance of vascular cell senescence in vivo and discusses the potential of antisenescence therapy for human atherosclerosis.     

Vascular cell senescence and vascular aging

Vascular cells have a finite lifespan when cultured in vitro and eventually enter an irreversible growth arrest called "cellular senescence". A number of genetic animal models carrying targeted disruption of the genes that confer the protection against senescence in vitro have been reported to exhibit the phenotypes of premature aging. Similar mutations have been found in the patients with premature aging syndromes. Many of the changes in senescent vascular cell behavior are consistent with the changes seen in age-related vascular diseases. We have demonstrated the presence of senescent vascular cells in human atherosclerotic lesions but not in non-atherosclerotic lesions. Moreover, these cells express increased levels of pro-inflammatory molecules and decreased levels of endothelial nitric oxide synthase, suggesting that cellular senescence in vivo contributes to the pathogenesis of human atherosclerosis. One widely discussed hypothesis of senescence is the telomere hypothesis. An increasing body of evidence has established the critical role of the telomere in vascular cell senescence. Another line of evidence suggests that telomere-independent mechanisms are also involved in vascular cell senescence. Activation of Ras, an important signaling molecule involved in atherogenic stimuli, induces vascular cell senescence and thereby promotes vascular inflammation in vitro and in vivo. It is possible that mitogenic-signaling pathways induce telomere-dependent and telomere-independent senescence, which results in vascular dysfunction. Further understanding of the mechanism underlying cellular senescence will provide insights into the potential of antisenescence therapy for vascular aging.     

Mechanisms of Vascular Aging,A Geroscience Perspective: JACC Focus Seminar

Age-related pathological alterations of the vasculature have a critical role in morbidity and mortality of older adults. In epidemiological studies, age is the single most important cardiovascular risk factor that dwarfs the impact of traditional risk factors. To develop novel therapeutic interventions for prevention of age-related vascular pathologies, it is crucial to understand the cellular and molecular mechanisms of vascular aging. In this review, shared molecular mechanisms of aging are considered in terms of their contribution to the pathogenesis of macrovascular and microvascular diseases associated with old age. The role of cellular senescence in development of vascular aging phenotypes is highlighted, and potential interventions to prevent senescence and to eliminate senescent cells for prevention of vascular pathologies are presented. The evidence supporting a role for interorgan communication and circulating progeronic and antigeronic factors in vascular aging is discussed.     

Rejuvenation of senescent cells—The road to postponing human aging and age-related disease?

Cellular senescence is the state of permanent inhibition of cell proliferation. Replicative senescence occurs due to the end replication problem and shortening telomeres with each cell division leading to DNA damage response (DDR). The number of short telomeres increases with age and age-related pathologies. Stress induced senescence, although not accompanied by attrition of telomeres, is also attributed to the DDR induced by irreparable DNA lesions in telomeric DNA. Senescent cells characterized by the presence of γH2AX, the common marker of double DNA strand breaks, and other senescence markers including activity of SA-β-gal, accumulate in tissues of aged animals and humans as well as at sites of pathology. It is believed that cellular senescence evolved as a cancer barrier since non-proliferating senescent cells cannot be transformed to neoplastic cells. On the other hand senescent cells favor cancer development, just like other age-related pathologies, by creating a low grade inflammatory state due to senescence associated secretory phenotype (SASP). Reversal/inhibition of cellular senescence could prolong healthy life span, thus many attempts have been undertaken to influence cellular senescence. The two main approaches are genetic and pharmacological/nutritional modifications of cell fate. The first one concerns cell reprogramming by induced pluripotent stem cells (iPSCs), which in vitro is effective even in cells undergoing senescence, or derived from very old or progeroid patients. The second approach concerns modification of senescence signaling pathways just like TOR-induced by pharmacological or with natural agents. However, knowing that aging is unavoidable we cannot expect its elimination, but prolonging healthy life span is a goal worth serious consideration.     

Involvement of IGF binding protein 5 in prostaglandin E<Subscript>2</Subscript>-induced cellular senescence in human fibroblasts

Inflammation is an underlying basis for the molecular alterations that link aging and age-related pathological processes. In a previous study, we found that secretory phospholipase A₂ (sPLA₂) induced cellular senescence in human dermal fibroblasts (HDFs). To further investigate the association of inflammation with cellular senescence, the effects of PGE₂ on cellular senescence in HDFs were investigated, since PGE₂ is the most abundant prostanoid. PGE₂ treatment induces cellular senescence, as determined by a decrease in cell proliferation and an increase in senescence-associated β-galactosidase staining. Notably, PGE₂ treatment increased the IGFBP5 protein level. While treatment with PGE₂ antagonists repressed PGE₂-induced cellular senescence, increasing intracellular cAMP accelerated cellular senescence. Down-regulation of IGFBP5 inhibited PGE₂-induced cellular senescence. Taken together, these results suggest that PGE₂ may play an important role in controlling cellular senescence of HDFs through the regulation of IGFBP5 and therefore may contribute to inflammatory disorders associated with aging.     

Endothelial cell senescence in human atherosclerosis: role of telomeres in endothelial dysfunction

BACKGROUND: The functional changes associated with cellular senescence may be involved in human aging and age-related vascular disorders. We have shown the important role of telomeres and telomerase in vascular cell senescence in vitro. Progressive telomere shortening in vivo has been observed in the regions susceptible to atherosclerosis, implicating its contributions to atherogenesis. However, whether senescent vascular cells are present in the vascularture and contribute to the pathogenesis of atherosclerosis remains unclear. METHODS AND RESULTS: Senescence-associated beta-galactosidase (beta-gal) activity was examined in the coronary arteries and the internal mammary arteries retrieved from autopsied individuals who had ischemic heart diseases. Strong beta-gal staining was observed in atherosclerotic lesions of the coronary arteries but not in the internal mammary arteries. An immunohistochemical analysis using anti-factor VIII antibody demonstrated that beta-gal stained cells are vascular endothelial cells. To determine whether endothelial cell senescence causes endothelial dysfunction, we induced senescence in human aortic endothelial cells (HAECs) by inhibiting telomere function and examined the expression of intercellular adhesion molecule (ICAM)-1 and endothelial nitric oxide synthase (NOS) activity. Senescent HAECs exhibited increased ICAM-1 expression and decreased eNOS activity, both of which are alterations implicated in atherogenesis. In contrast, introduction of telomerase catalytic component significantly extended the life span and inhibited the functional alterations associated with senescence in HAECs. CONCLUSIONS: Vascular endothelial cells with senescence-associated phenotypes are present in human atherosclerotic lesions, and endothelial cell senescence induced by telomere shortening may contribute to atherogenesis.     

Curcumin induces senescence of primary human cells building the vasculature in a DNA damage and ATM-independent manner

Curcumin is considered not only as a supplement of the diet but also as a drug in many types of diseases and even as a potential anti-aging compound. It can reduce inflammation that increases with age and accompanies almost all age-related diseases. It has been suggested that curcumin can play a beneficial role in the cardiovascular system. However, there are also data showing that curcumin can induce senescence in cancer cells, which is a beneficial effect in cancer therapy but an undesirable one in the case of normal cells. It is believed that cellular senescence accompanies age-related changes in the cardiovascular system. The aim of this study was to check if curcumin, in a certain range of concentrations, can induce senescence in cells building the vasculature. We have found that human vascular smooth muscle and endothelial cells derived from aorta are very sensitive to curcumin treatment and can senesce upon treatment with cytostatic doses. We observed characteristic senescence markers but the number of DNA damage foci decreased. Surprisingly, in vascular smooth muscle cell (VSMC) activation of DNA damage response pathway downstream of ataxia-telangiectasia mutated (ATM) was observed. ATM silencing and the supplementation of antioxidants, N-acetyl-L-cysteine (NAC) or trolox, did not reduce the number of senescent cells. Thus, we have shown that curcumin can induce senescence of cells building the vasculature, which is DNA damage and ATM independent and is not induced by increased reactive oxygen species (ROS) level. We postulate that an increase in the bioavailability of curcumin should be introduced very carefully considering senescence induction as a side effect.

Electronic supplementary material

The online version of this article (doi:10.1007/s11357-014-9744-y) contains supplementary material, which is available to authorized users.     

Aging-induced adaptations of microvascular reactivity

Control of blood flow to skeletal muscle depends on the vasomotor tone present in the resistance vasculature. Although muscle blood flow has been shown to decline with advancing age, our knowledge of how alterations of reactivity of the resistance vasculature contribute to reduced delivery or altered distribution of blood in the aged is limited. Recent work has demonstrated that age alters the reactivity of resistance arteries and arterioles from skeletal muscle, and that impairment of both vasodilator and vasoconstrictor responses occurs with advancing age. The alterations in cellular mechanisms that contribute to age-related impairment of vasoreactive responses encompass both the vascular endothelium and smooth muscle, and differ in muscles of varying function and fiber type. Current research suggests that some degree of age-induced endothelial dysfunction occurs in resistance arteries and arterioles from most skeletal muscle; however, the severity of endothelial impairment appears greater in resistance arteries and arterioles from highly oxidative locomotory muscles. Age-related impairment of vasoconstrictor responses to metabolites and endogenous constrictor agents has also been documented. These age-related reductions in vasoreactivity that occur in the skeletal muscle resistance vasculature may contribute to inadequate delivery or distribution of blood flow during exercise and ultimately be a factor in loss of exercise capacity that occurs with advancing age.     

Senescence research from historical theory to future clinical application

Historically, the findings from cellular lifespan studies have greatly affected aging research. The discovery of replicative senescence by Hayflick developed into research on telomeres and telomerase, while stress-induced senescence became known as a telomere-independent event. Senescence-inducing signals comprise several tumor suppressors or cell cycle inhibitors, e.g., p53, cyclin-dependent kinase inhibitor p16 Ink4a and others. Stress-induced senescence serves as a physiological barrier to oncogenesis in vivo, while it activates senescence-associated secretary phenotype, inducing chronic inflammation. Thus, beside telomere length, p16, p53 and inflammatory cytokines have been utilized as biomarkers for cellular senescence. Telomere lengths in human leukocytes correlate well with events of aging-related lifestyle diseases, indicating the importance of cellular senescence in organismal aging. As such, the development of senescence research will have significant future clinical applications, e.g., senolysis. Geriatr Gerontol Int 2021; 21: 125–130 .     

Senescence associated secretory phenotype profile from primary lung mice fibroblasts depends on the senescence induction stimuli

Cellular senescence is a multifactorial phenomenon of growth arrest and distorted function, which has been recognized as an important feature during tumor suppression mechanisms and a contributor to aging. Senescent cells have an altered secretion pattern called Senescence-Associated Secretory Phenotype (SASP) that comprises a complex mix of factors including cytokines, growth factors, chemokines, and matrix metalloproteinases. SASP has been related with local inflammation that leads to cellular transformation and neurodegenerative diseases. Various pathways for senescence induction have been proposed; the most studied is replicative senescence due to telomere attrition called replicative senescence (RS). However, senescence can be prematurely achieved when cells are exposed to diverse stimuli such as oxidative stress (stress-induced premature senescence, SIPS) or proteasome inhibition (proteasome inhibition-induced premature senescence, PIIPS). SASP has been characterized in RS and SIPS but not in PIIPS. Hence, our aim was to determine SASP components in primary lung fibroblasts obtained from CD-1 mice induced to senescence by PIIPS and compare them to RS and SIPS. Our results showed important variations in the 62 cytokines analyzed, while SIPS and RS showed an increase in the secretion of most cytokines, and in PIIPS only 13 were incremented. Variations in glutathione-redox balance were also observed in SIPS and RS, and not in PIIPS. All senescence types SASP displayed a pro-inflammatory profile and increased proliferation in L929 mice fibroblasts exposed to SASP. However, the behavior observed was not exactly the same, suggesting that the senescence induction pathway might encompass dissimilar responses in adjacent cells and promote different outcomes.     

Aging and atherosclerosis: mechanisms, functional consequences, and potential therapeutics for cellular senescence

Atherosclerosis is classed as a disease of aging, such that increasing age is an independent risk factor for the development of atherosclerosis. Atherosclerosis is also associated with premature biological aging, as atherosclerotic plaques show evidence of cellular senescence characterized by reduced cell proliferation, irreversible growth arrest and apoptosis, elevated DNA damage, epigenetic modifications, and telomere shortening and dysfunction. Not only is cellular senescence associated with atherosclerosis, there is growing evidence that cellular senescence promotes atherosclerosis. This review examines the pathology of normal vascular aging, the evidence for cellular senescence in atherosclerosis, the mechanisms underlying cellular senescence including reactive oxygen species, replication exhaustion and DNA damage, the functional consequences of vascular cell senescence, and the possibility that preventing accelerated cellular senescence is a therapeutic target in atherosclerosis.