Thoracic aortic aneurysms and AAAs are life-threatening aortic diseases, the risk of which is known to increase with age. Endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) from AAA patients show telomere attrition and oxidative DNA damage.25 Human VSMCs from AAA patients exhibited high levels of phosphorylated histone H2AX (γH2AX), but no correlation was found with patients' chronological age.33 Sirtuins are protein deacetylases that are known to inhibit aging and contribute to longevity.34 Suppression of vascular cell senescence by the activation of sirtuin 1 (SIRT1) prevented AAA formation, but AAA formation was augmented by promoting cell senescence in these cells.35 It is well known that calorie restriction (CR) can extend the lifespan in organisms ranging from Caenorhabditis elegans to mammals.3637 CR can reduce the incidence of AAA formation induced by the peptide hormone angiotensin II (AngII) in mice.38 Systemic genetic depletion of p53 also contributed to the suppression of AAA formation.39 Patients with thoracic aortic aneurysm/dissection exhibited elevated numbers of p53-, p21-, and p19-positive cells in the vessel wall, along with increased SA-β-gal activity.40 These results indicate that cellular senescence plays a pathogenic role in aortic aneurysm.383940

Studies have suggested a close connection between cellular senescence and coronary artery disease.741 In patients with ischemic heart disease, the number of senescent cells was higher in the coronary arteries than in the internal mammary arteries.7 A large amount of evidence now indicates that telomere length and telomerase activity in peripheral leukocytes and circulating progenitor cells are involved in human cardiovascular diseases.4243 It has been reported that endothelial progenitor cells from coronary heart disease patients exhibited telomere attrition and reduced telomerase activity.41 In addition, leukocyte telomere length showed an inverse association with the risk of coronary heart disease independent of conventional vascular risk factors.44 MicroRNA (miRNA) is a type of small non-coding RNA, and studies have indicated that some subsets of miRNA play a role in either maintaining or disturbing systemic homeostasis.4546 Expression of miR-23a has been shown to be higher in peripheral blood mononuclear cells in patients with coronary artery disease than in controls.47 Telomeric repeat binding factor-2 (TRF-2) plays a critical role in telomere maintenance, and miR-23a has been shown to reduce the levels of TRF-2.47 The level of miR-23a was higher in coronary artery disease patients than in controls, suggesting that it affects the progression of coronary atherosclerosis via downregulation of TRF-2.47 Nutrients and oxygen are delivered via capillaries, and the formation of the capillary network is critical for maintaining organ function.1148 Vascular endothelial growth factor-A (VEGF-A), an angiogenic molecule, has been extensively studied, and suppression of VEGF-A has been reported to result in a lack of tissue remodeling.1148 Several splice variants or isoforms of VEGF-A exist, all of which are generally recognized to elicit a pro-angiogenic response; however, some isoforms have been reported to become anti-angiogenic molecules.4950 For example, under metabolic stress, it has been shown that levels of VEGFA₁₆₅b increased and became pathogenic by reducing angiogenesis in a murine ischemic limb model.50 Senescent human aortic ECs (HAECs) express a high level of VEGFA₁₆₅b, and interestingly, the transcript Vegfa₁₆₅b is present at higher levels in patients with coronary heart disease.51 These results indicate that dysregulation of the splicing of VEGF-A into its various isoforms may be a key feature of EC senescence involved in the pathogenesis of coronary heart disease.51

The ankle-brachial index (ABI) is defined as the ratio of ankle systolic blood pressure to that of the arm. A decline in the ABI is known to be indicative of PAD.52 As the ABI decreases with age, this index has traditionally been considered to be independent of traditional risk factors for PAD.53 The prevalence of PAD, and in particular pathological PAD, increases with age.54 It was reported that aging caused a drop in perfusion in a hind limb ischemia model.55 Furthermore, aging was shown to exacerbate injuries mediated by ischemia-reperfusion in skeletal muscle.56 In one study, hind limb ischemia was generated in young, old, and very old mice administered a telomerase activator.57 Blood flow recovery was suppressed in the ischemic limbs of old mice compared to young mice, and recovery in old mice improved in response to administration of a telomerase activator, TA-65.57 The expression of hypoxia inducible factor 1α, VEGF-A, and peroxisome proliferator-activated receptor gamma coactivator 1-alpha was reduced in old mice, but increased following TA-65 administration.57 Expression of senescence markers—including p53, p21, and p16—increased in ischemic limbs on day 3 in old mice, although the expression of these senescence markers was ameliorated in old mice given TA-65.57 This indicates that telomerase activation is a potential therapeutic agent for ischemic tissues in aged individuals.57 Serum from PAD patients showed acceleration in the aging phenotype of HAECs.58 This was suppressed by administering sulodexide (a mixture of glycosaminoglycans).58 The results of that study indicate that serum from PAD patients contains pro-senescent molecules that accelerate arteriosclerosis.58 However, to date, few studies have investigated the role of cellular senescence in PAD or the role of senescence processes in VSMCs specifically.57

With aging, VSMCs undergo transformation from a “contractile” to a “synthetic” phenotype, show enhanced inducible nitric oxide synthase activity, and exhibit high expression of intercellular adhesion molecule-1 (ICAM-1) and angiotensinogen in response to stress.362 Senescent VSMCs have been reported to accumulate in atherosclerotic plaques of patients with ischemic heart disease, AAA, and PAD.725 Approximately 20% of the VSMCs in a carotid artery plaque stained positive for p16, p21, and SA-β-gal.61

In cell culture conditions, VSMCs from the fibrous cap of an atheroma exhibited telomere attrition compared to those from normal media.61 Furthermore, VSMCs with short telomeres were found to be positive for SA-β-gal activity, in addition to their increased levels of p16 and p21.61 Oxidative stress induced DNA damage in VSMCs, which was associated with a reduction in telomerase activity, telomere attrition, and cellular senescence.61 In atherosclerotic plaques, VSMCs become senescent, and the levels of TRF-2 are reduced in these cells.63 TRF-2 is known to be localized in telomeres, where it mediates their protective activity.64 In vitro studies have demonstrated that increased expression of TRF-2 reduces DNA damage, accelerates DNA repair, and suppresses the process of senescence.65 Inhibition of TRF-2 resulted in the opposite phenotype.65 Finally, VSMC-specific TRF-2 suppression led to the progression of atherosclerosis in vivo in mice, which was ameliorated in VSMC-specific TRF-2 overexpression in vivo in mice.65 The peptide hormone AngII is involved in vasoconstriction, ultimately resulting in increased blood pressure.66 It was previously reported that AngII induced senescence in VSMCs in Apoe^−/− mice.67 Smooth muscle 22α, an actin-binding protein, was recently shown to promote AngII-induced senescence by the suppression of E3 ubiquitin ligase (Mdm2)-induced p53 degradation in mice.68 Senescent VSMCs in carotid plaques stained positive for interleukin-6, suggesting that senescent VSMCs contribute to vascular remodeling by acquiring the senescence-associated secretory phenotype.69 Dysregulation of the production of growth factors and modifiers of the extracellular matrix results in senescent VSMCs, further accelerating the remodeling process in vessels.69 Suppression of SIRT1 in VSMCs increased vascular cell senescence, upregulated p21 expression, enhanced vascular inflammation, and ultimately promoted pathologies associated with AAA.35 In contrast, specific SIRT1 overexpression inhibited AngII-induced AAA formation and progression in Apoe^−/− mice.35

It is well known that CR can prolong the lifespan.70 Interestingly, CR has also been shown to contribute to vascular health, as CR inhibited AAA formation in AngII-infused Apoe^−/− mice.38 Furthermore, in an AAA mice model, CR led to an increase in SIRT1 expression in VSMCs, and genetic depletion of Sirt1 in VSMCs abolished the protective role of CR.38 Nicotinamide phosphoribosyltransferase (NAMPT) is a rate-limiting enzyme that initiates nicotinamide adenine dinucleotide (NAD⁺) production.71 Reduced NAD⁺ levels are associated with aging and age-associated diseases and contribute to the progression of pathologies in these disorders.71 In ascending dilated aortas from patients with aortopathy, an inverse relationship was found between NAMPT in VSMCs and aortic diameter.72 Depletion of Nampt in VSMCs led to dilatation of the aorta in association with a 43% reduction in NAD⁺ in the media.72 Infusion of AngII in a VSMC-Nampt knockout mouse model led to aortic medial hemorrhage and dissection or tearing.72 In addition, VSMCs showed high levels of p16 and SA-β-gal activity, indicating that they were senescent, not apoptotic.72 These results suggest that suppression of VSMC senescence contributes to the inhibition of pathologies associated with atherosclerotic diseases.387172

It is well known that ECs play a role in the maintenance of vascular homeostasis, and that EC dysfunction is associated with a pro-inflammatory phenotype, an altered angiogenic response, and pro-oxidant and pro-thrombotic states.151673 The proliferation and migration capacity of aged ECs is diminished, which is associated with reduced nitric oxide production.74 Studies have suggested that EC dysfunction plays a causal role in alterations of blood pressure, systemic metabolism, and the coagulation system, and may play a pivotal role in cellular senescence.75 In a murine left ventricular pressure overload model, the expression of p53 was increased, leading to overexpression of ICAM-1 in these cells, which promoted cardiac inflammation.16 Levels of p53 in ECs also increased under metabolic stress, ultimately resulting in diminished endothelium-dependent vasodilatation and ischemia-induced angiogenesis.15 In addition, elevated levels of p53 in ECs induced systemic metabolic dysfunction.75 Senescent ECs have been shown to exist in atherosclerotic plaques in coronary arteries.7 In contrast, the expression of these senescent cells was low in the internal mammary arteries.7 In the coronary arteries, SA-β-gal activity was high in cells located on the luminal surface, indicating that ECs in the coronary arteries contain SA-β-gal.7 Disturbance of flow in the ascending aorta and aortic arch was shown to promote EC senescence in atherosclerotic mice.76 Oxidized low density lipoprotein (LDL) is a well-established atherogenic lipoprotein.77 The most electronegative subfraction of LDL is L5, which was recently demonstrated to promote EC senescence.63 Furthermore, L5 administration increased levels of γH2AX and SA-β-gal activity in the aortic endothelium of mice.63 A high-caloric diet increased L5 levels in circulation, which also promoted a senescent-like phenotype in the aortic endothelium of Syrian hamsters.63 Regarding cellular pathways, an in vitro study showed that L5 induced EC senescence via activation of the ATM/Chk2/p53 pathway.63 Long non-coding RNA H19 (lncRNA H19), has been shown to be downregulated in the endothelium of aged mice.78 The loss of lncRNA H19 led to increased p16 and p21 levels in vitro.78 In addition, lncRNA H19 depletion increased expression of signal transducer and activator of transcription 3 (STAT3), while suppression of lncRNA H19 reduced STAT3.78 Additional studies have reported that increased H19 levels contributed to an inhibition of STAT3-mediated cell senescence.78 Several molecules and methods have been reported to inhibit senescence in ECs.7980 Polyphenol suppressed reactive oxygen species (ROS) production, reduced endothelial p53 levels, and increased nitric oxide levels in the aorta, contributing to an inhibition of endothelial dysfunction.79 Both L-citrulline and L-arginine supplementation reduced SA-β-gal activity and p16 levels, and increased telomerase activity in human umbilical vein ECs maintained in a high-glucose medium.80 Administration of these amino acids also led to a reduction of SA-β-gal activity in the abdominal aorta of Zucker diabetic fatty rats with diabetes.80 Omega-3 fatty acids (eicosapentaenoic acid and docosahexaenoic acid) suppressed a hydrogen peroxide-induced increase in SA-β-gal activity in HAECs.81 Aging increased levels of the senescence marker p19 in arteries, and this was suppressed by dietary rapamycin administration, resulting in enhanced endothelium-dependent dilatation in carotid arteries.82 In humans and rodents, it is well known that physical activity improves vascular structure and function.83 The bioavailability of nitric oxide decreases with age, which is associated with high ROS levels and reduced physical activity.84858687 Compared to sedentary older individuals, older individuals who regularly exercised had lower levels of p53, p21, and p16 in ECs from the brachial artery and antecubital vein.88 This indicates that physical activity inhibits cellular senescence in human vessels.88

Atherosclerotic lesions develop and progress through excessive levels of lipids and chronic inflammatory responses.60 Evidence indicates that cellular senescence in immune cells plays a causal role in the progression of atherosclerosis.23899091 Links between immune cell senescence and atherosclerosis have been reported.23899091Telomere attrition in leukocytes in aged individuals has been linked with a high mortality rate, of which a moderate number of cases are attributable to cardiovascular deaths.23899091 Monocytes from atherosclerotic patients showed high levels of ROS and pro-inflammatory cytokine production.89 Senescent intimal foam cells have been shown to accumulate in atherosclerotic lesions and become drivers of atheroma formation.23 Another report showed that p16^INK4a can induce cellular senescence in macrophages, causing these cells to become pro-inflammatory.91 Evidence indicates that leukocyte senescence can accelerate plaque formation in atherosclerosis.2391 Inhibition of the senescence process in immune cells is another important concept to be considered in the treatment of atherosclerotic vascular disorders.238991