Endothelial cells are constantly exposed to circulating blood and pathogenic stimuli and flow/shear stress that predispose them to damage and induce senescence. Consequently, senescent cell burden is high in well-vascularized tissues with many endothelial cells ^[38][84]. Endothelial cells are also one of the first cell types to manifest increasing effects of biological age ^[39][85]. PMEF as an index of vascular aging correlates better with advanced vascular aging as estimated by artificial intelligence (AI) analyses of ECGs than with actual age. This suggests that vascular aging as manifested by endothelial function is associated with accelerated physiological aging ^[40][86]. The hemodynamic environment in blood vessels is an important factor, evidenced by the fact that endothelial cells exposed to high shear stress have high cellular turnover that predisposes to replicative senescence ^[41][42][87,88]. Lifestyle-related metabolic conditions, such as dyslipidemia, are risk factors for cardiovascular diseases and promote endothelial senescence. Dyslipidemia propagates endothelial cell senescence due to increased oxidation of low-density-lipoproteins ^[43][45]. Type 2 diabetes may also cause endothelial cell senescence due to increases in circulating glucose and excessive insulin signaling [44]. Oxidized low-density-lipoproteins and insulin activate the phospho- inositol 3-kinase (PI3K)/RACα serine/threonine protein kinase (AKT1) signaling pathway, inhibiting forkhead box protein O(FOXO) 3A, resulting in reduced manganese superoxide dismutase (SOD) activity and a subsequent increase in reactive oxygen species (ROS). In vivo and in vitro studies have demonstrated that high glucose levels promote endothelial cell senescence by NADPH oxidase-, SOD-, and sirtuin- (SIRT) mediated ROS accumulation and increased pro-inflammatory SASP factor production ^[45][46][89,90]. ROS trigger the p53/p21^CIP1/WAF1 DNA damage response pathway, inducing proliferative arrest and cellular senescence. Chemotherapeutic drugs such as doxorubicin are also known to increase mitochondrial ROS ^[47][91] and production of arterial SASP factors ^[48][92].

Senescent ECs exhibit functional abnormalities such as decreased expression of vasoprotective factors and increases in inflammatory cytokines and adhesion molecules. Senescent ECs also dysregulate blood flow and cause barrier dysfunction. They do not proliferate, impeding capacity to repair the blood vessel lumen. ECs produce the potent vasodilatory molecule nitric oxide (NO), which in addition to its vasodilatory effect mediated by VSMC relaxation, also reduces endothelial expression of adhesion molecules, improves barrier function, and prevents coagulation ^[49][93]. Furthermore, NO may have a protective effect against cellular senescence ^[50][94]. Consequences of senescence-driven EC dysfunction include reduced vascular dilation, partly due to increased p53 activity leading to decreased active endothelial nitric oxide synthase (eNOS) levels ^[51][95]. Inhibiting p53 activation in endothelium has been demonstrated to increase eNOS production ^[52][96]. Evidence exists that high glucose levels reduce telomerase activity, length, and phosphorylation of eNOS, thereby reducing NO production ^[53][97]. Persistent senescent endothelial cells can induce increases in Ang II ^[54][98] and endothelin 1 (ET1) levels ^[55][99], contributing to elevated blood pressure. Reduced NO synthesis and increased Ang II levels, along with studies of senescence markers, connect EC senescence to a common cardiovascular risk factor, hypertension. As the endothelium effectively is an endocrine tissue, senescent ECs may cause wide-ranging systemic consequences ^[56][100]. Reduced vasodilation due to senescent endothelial cell accumulation has also been associated with heart failure with preserved ejection fraction (HFpEF) ^[57][101]. Senescent endothelial cells exhibit increased production of ROS and certain SASP markers, such as intercellular adhesion molecule-1 (ICAM-1) ^[58][59][102,103], IL-1 ^[60][104], IL-6 ^[61][105], IL-8 ^[62][106], and MCP-1 ^[62][106], facilitating immune cell infiltration of tissues. Senescent ECs cause increased thrombosis risk and susceptibility to atherosclerosis due to increased plasminogen activator-1 (PAI-1) expression as well as reduced endothelial nitric oxide synthase (eNOS) ^[63][107]. Cyclo-oxygenase activity is altered in senescent ECs, as demonstrated by decreased prostaglandin I2 levels and increased thromboxane A2 production. The SASP factors considered above, including inflammatory proteins and growth factors, are recognized risk factors for atherosclerosis. Unsurprisingly, senescent ECs are found in high numbers in human atherosclerotic plaques [5]. Recent studies have identified endothelial cell senescence as a contributor to pulmonary hypertension. EC senescence seems to drive dysfunctional phenotypes through upregulation of several canonical SASP factors, such as IL-6, TNF-α, and PAI-1, leading to dysfunction in other vascular cells ^[64][65][108,109]. Angiogenesis is another aspect of endothelial function, protecting tissues against ischemia in ischemic heart disease and peripheral arterial diseases, primarily through vascular endothelial growth factor (VEGF) ^[66][110], fibroblast growth factor (FGF) ^[67][111], and hypoxia-inducible factor (HIF) 1α ^[68][112]. Senescent ECs decrease expression of these factors in vessels due to signaling through the p53-p21 senescence pathways, as evidenced by increased angiogenesis after this pathway is inhibited ^[52][96]. Angiogenesis may be associated with the development and vulnerability to atherosclerotic plaques. Further studies investigating this possibility are needed.

Vascular smooth muscle cells (VSMCs) are the main drivers of atherosclerosis, but senescent VSMCs appear not to be involved in initial plaque formation; rather, they may primarily lead to increased plaque size ^[69][113]. Accumulation of senescent VSMCs in atherosclerotic plaques is evidenced by reduced proliferation ^[70][114], larger and flatter cell morphology ^[71][115], increased expression of p16 ^INK4a and p21 ^{CIP1/WAF1 [72]}[116], and shortened or dysfunctional telomeres ^[72][116]. VSMC senescence can be caused by many factors, such as chronic inflammation, dysregulated local calcium metabolism, and increased oxidative stress. Conditions considered to be significant risk factors for atherosclerotic cardiovascular disease such as diabetes, hypertension, dyslipidemia, and smoking increase reactive oxygen species (ROS) in the vessel wall ^[73][117]. One significant source of ROS is nicotinamide dinucleotide phosphate (NADPH) oxidases (NOXs) ^[74][118]. Accelerated senescence of aortic VSMCSs, increased DNA damage, and a proinflammatory secretory profile were found in young mice with NOX4 overexpression, with the suggested mechanism being increased superoxide and hydrogen peroxide production ^[74][118]. ROS have also been tied to accelerated telomere depletion, which potentially contributes to the finding that VSMCs covering atherosclerotic plaques exhibit telomere shortening ^[72][116]. VSMCs are influenced by neural and endocrine signaling, and crucially, by paracrine mediators from ECs. The renin-angiotensin-aldosterone system (RAAS) is a regulator of VSMC activity, and high levels of Ang II are connected to VSMC senescence ^[75][43]. Inhibition of RAAS signaling may attenuate premature senescence and proinflammatory cytokine production caused by Ang II ^[75][43]. Other signals driving VSMC senescence are chronic exposure to high levels of the coagulation factor Xa ^[76][77][119,120], leading to chronic inflammation and stimulating p53 and insulin-like growth factor binding protein 5 (IGFBP-5) expression ^[78][121]. The monocyte chemotactic protein-1 and transforming growth factor-ß signaling pathways are additional players in VSMC senescence ^[79][122]. There are proteins that may delay cellular senescence, such as sirtuins ^[80][123]. In mice, sirtuin protein 6 (SIRT6) may counteract onset of senescence in VSMCs. Other beneficial effects of sirtuin activation may be increased atherosclerotic plaque stability, protection against telomeric damage, and reduced inflammatory cytokine production ^[81][124].

As VSMCs senesce, they can lead to increased production of proinflammatory proteins such as IL-1α, IL-1β, IL-6, IL-8, IL-18, and TNF-α ^[82][125]. These SASP factors and reduced anti-inflammatory protein expression drive chemotaxis of immune cells and increase endothelial cell adhesion molecule expression. Collagen production by senescent VSMCs and surrounding cells is consequently reduced, making atherosclerotic plaques more vulnerable to rupture ^[83][84][85][126,127,128]. Moreover, senescent VSMCs have increased production of elastase and other matrix-degrading proteases and add less collagen to the surrounding extracellular matrix (ECM) ^[86][129]. The contractile capacity of aged VSMCs is reduced ^[87][130] due to reduced expression of proteins involved in muscle contraction, such as α-smooth muscle actin (α-SMA), smooth muscle myosin heavy chain (SM-MHC), and calponin ^[88][89][131,132]. Ion channel types on the cellular membrane change: their numbers decrease and responses to mediators secreted by endothelial cells are diminished ^[88][89][131,132]. These changes may contribute to hypertension due to blood flow dysregulation and arterial stiffness. Senescent VSMCs may further contribute to this stiffness due to increased production of osteogenic mediators such as osteopontin (OPN), osteoprotegerin (OPG), runt-associated transcription factor 2 (Runx-2), bone morphogenetic protein 2 (BMP-2), and alkaline phosphatase (ALP) as a response to oxidative stress and inflammation ^[82][90][125,133]. Attenuators of this calcification include myostatin, an activator of the mTOR pathway that reduces osteogenetic factor expression ^[91][134]. Interestingly, no correlation was found between p21-expressing VSMCs and pulmonary hypertension, suggesting that senescent VSMCs may not be a primary driver of this disease ^[92][135].

As cardiomyocytes are terminally differentiated, post-mitotic cells in adult humans (with a yearly renewal capacity of less than 1% ^[93][136]), senescence in cardiomyocytes is challenging to define precisely compared to proliferative cells. Evidence from animals and humans has indicated that post-mitotic cardiomyocyte senescence is mediated by length-independent telomeric damage ^[94][95][54,137]. In mice, it has been reported that cardiomyocyte size, ROS production, and p53 or p16^Ink4a expression increases with chronological age together with telomere attrition ^[94][96][54,138]. These findings are signs of cellular senescence also encountered in senescent normally proliferating cell types. In addition to biological aging, senescent cardiomyocytes may be implicated in multiple pathologies, including dysfunctional remodeling after myocardial infarction ^[97][139] and hypertrophic cardiomyopathy. In heart failure due to sustained pressure overload, p53-expressing cells were present in increased numbers. Senescent cell accumulation was shown to inhibit Hypoxia-Inducible Factor (HIF)-1 activity and impair cardiac angiogenesis and systolic function ^[68][112]. Cardiomyocyte remodeling is initiated by p53-independent mitochondrial activation and is characterized by hypertrophy, but continuous stimuli from volume overload led to p53-dependent mitochondrial inhibition, morphological elongation, and heart failure ^[98][140]. Cardiotoxic drugs such as doxorubicin can induce premature senescence in cardiomyocytes as evidenced by increases in p16^Ink4a and p53/p21^Cip1/Waf1 expression, increased SA-ßgal signaling, decreased cardiac troponin I phosphorylation, and decreased telomerase activity ^[99][100][141,142], potentially contributing to doxorubicin-induced cardiomyopathy. Paracrine mediators from ECs, fibroblasts, and immune cells are possibly also drivers of cardiomyocyte senescence. ECs promote cardiomyocyte maturation during infancy through growth factors such as platelet-derived growth factor (PDGF). With increasing age, senescent ECs instead drive inflammation and senescence in cardiomyocytes through release of factors such as tumor growth factor-β (TGFß) and IL-6. Decreased NO production by senescent ECs directly exacerbated cardiomyocyte contractility by disrupting the fine regulation of excitation–contraction coupling, dysregulating autonomic signaling, and impacting mitochondrial respiration in addition to vascular-dependent aspects such as increasing vascular stiffness, inflammation, thrombosis, and impairment of angiogenesis ^[101][143]. Consistent with this, endothelial senescence has been demonstrated to play a key role in heart failure. Senescent fibroblasts also secrete these markers, along with TNF-α and IGF-1. Immune cells such as macrophages may drive senescence by producing factors such as IL-1ß. SASP factors from adipose tissue are also involved in cardiomyocyte aging and heart failure ^[102][144].

Senescent cardiomyocytes produce SASP factors, including proinflammatory cytokines and chemokines, growth modulators, angiogenetic factors, and matrix metalloproteinases (MMPs) ^[103][145]. Examples include CCN family member 1 (CCN1), interleukins (IL1α, IL1β, and IL6), TNF-α, MCP1, endothelin 3 (Edn3), TGFβ, and growth and differentiation factor 15 (GDF15) ^[94][104][54,146]. These factors can contribute to cardiac remodeling and dysfunction. As cardiomyocytes consume more energy than most other types of cells, well-functioning cellular metabolism is essential for homeostasis. Dysfunctional metabolism may contribute to cardiomyocyte senescence during aging and the development of various diseases. When cardiomyocytes become senescent, p53 upregulation shifts the cell toward glucose metabolism. This increases activation of the insulin growth factor receptor, promoting senescence and increasing the release of SASP factors. Age-related declines in mitochondrial enzyme levels may also cause further toxicity due to metabolite buildup. For example, carnitine palmitoyltransferase 1 (CPT1), one of the rate-limiting enzymes in the fatty acid oxidation and glucose oxidation pathways, has been shown to be decreased in cardiomyocytes of aging rats ^[105][147]. In addition, expression of other regulators of fat metabolism, such as peroxisome proliferator-activated receptor α (PPARα) and PGC-1α, is also reduced with age ^[106][148]. As these enzymes decrease with age, consequent intracellular lipid buildup may induce senescence. Core metabolism-regulating enzymes such as AMP-activated protein kinase (AMPK), NAD⁺-dependent sirtuins, FOXOs, and mammalian target of rapamycin (mTOR) play prominent roles in driving or inhibiting cardiomyocyte senescence. AMPK activation is reduced in aged myocardial tissues, and activation of AMPK improves mitochondrial dynamics, reduces ER stress, improves the function of cardiomyocytes, and represses cardiomyocyte senescence ^{[107][108][109]}[149,150,151]. Sirtuins are NAD+-dependent cell metabolism and senescence regulators, and contractile dysfunction is linked to NAD+ depletion ^[110][152]. Increased mTOR activity, on the other hand, has been linked to pathologic cardiomyocyte hypertrophy.

Immune cells play a prominent role in CVDs, as demonstrated by a report showing that atherosclerotic plaques with high macrophage content were more vulnerable to rupture ^{[111][112][113]}[153,154,155]. The detrimental effects of immune cells may partly be due to SASP secretion that may increase inflammatory cell migration and cause local damage. For example, the SASP directly drives inflammation through IL-1α translocation to the cell surface, which activates neighboring VSMCs, ECs, and macrophages, causing the spread of inflammation, and promotes atherosclerosis through secondary proinflammatory cytokines ^[79][122].

Myocardial cellular composition includes resident immune cells such as subsets of leukocytes encountered in the healthy heart. In a recent study, flow cytometry of cardiac tissue showed approximately 10³ leukocytes per milligram of tissue in the steady state and 3380 ± 1279 CD45⁺ cells per cubic millimeter of tissue. Three-dimensional reconstructions of immunohistochemistry images detected CD45⁺ cells even within the healthy myocardium. Interestingly, cardiac muscle contained 12 times more leukocytes per milligram of tissue compared to skeletal muscle. Of all the leukocytes found in the healthy heart, only ∼13% were in direct contact with the bloodstream. Fundamental changes in cardiac leukocyte composition occur over time, affecting macrophages and T cells. The resident cardiac macrophage population (primarily CD206⁺ cells) significantly diminishes with aging, being replaced by granulocytes. Despite the reduction in macrophage numbers, aging does not affect the ratio of the F4/80⁺CD206⁺ and F4/80⁺CD206⁻ subsets. Still, a slight increase in C-C motif chemokine receptor 2-(CCR2)-expressing macrophages was observed in the hearts of 12- to 15-month-old animals, suggesting possible macrophage replenishment ^[114][156]. Cardiac macrophages have been reported to improve electrical conduction in the atrioventricular node, evidenced by cells being fused together with elongated macrophages expressing connexin-43 in the adult heart. Using the Cd11bDTR mouse model, macrophage ablation was shown to cause progressive atrioventricular blockage. These findings elucidate the role of macrophages in normal and aberrant cardiac conduction ^[115][157]. Macrophages lodge within the healthy myocardium phagocytose cell components, including mitochondria, of damaged cardiomyocytes. Immune system aging may increase formally senescent immune cell burden, potentially contributing to morbidity and mortality. After myocardial infarction, cellular debris from dead cardiac cells is cleared by neutrophil and macrophage phagocytosis. Depletion of cardiac macrophages results in defective elimination of mitochondria from myocardial tissue, inflammasome activation, impaired autophagy, abnormal mitochondrial accumulation in cardiomyocytes, metabolic alterations, and ventricular dysfunction ^[116][158]. With increasing age, cardiac macrophages appear to become senescent-like, starting to secrete damaging matrix metalloproteinases (MMPs) and CCL2, both shown to drive cardiomyocyte hypertrophy. The pathological effects of immunosenescence on CVDs can be further exacerbated by increased levels of osteopontin (OPN) and/or visceral obesity.

A recent study demonstrated that macrophages are the most common p16^Ink4a/SA-βgal-positive cells accumulating in aging mice ^[117][159]. In the CVD setting, leukocytes with short telomeres, a sign of senescence, have been found in atherosclerotic coronary arteries. Senescent-like macrophages appear to have increased SA-β-Gal activity and p53 and p16 ^INK4a expression, display impaired cholesterol efflux, and exacerbate atherosclerosis ^[118][119][160,161]. Foamed macrophages exhibiting senescence markers may possibly secrete inflammatory cytokines, chemokines, and metalloproteinases in atherosclerotic plaques ^[120][162]. There are three distinct immune cell types in atherosclerotic plaques, each with a different morphology when analyzed by transmission electron microscopy (TEM). These three are the elongated, vacuolated cells located in the endothelial layer, spindly foam cells with histological properties of VSMCs, and large foamy cells resembling lipid-loaded macrophages producing X-galactosidase (X-Gal) crystals ^[120][162]. Oxidized low-density lipoprotein (LDL) inhibits macrophage proliferation and migration, induces cellular senescence, and promotes the secretion of inflammatory factors such as TNF-α, monocyte chemoattractant protein-1(MCP-1), and IL-1β, possibly establishing a positive feedback loop ^[121][163].

Besides macrophages, senescent-like T cells may also be involved in the pathogenesis of chronic inflammatory diseases, including vascular diseases ^[122][82]. In T cells, oxidative stress reduces telomerase activity, causing a T cell senescent-like state and creating proinflammatory phenotypes within plaques ^{[123][124][125]}[164,165,166]. Senescent-like T cells with the CD8⁺CD57⁺CD27⁻CD28^null phenotype produce large amounts of IFN-γ and TNF-α and may promote inflammation in atherosclerotic disease ^[126][167]. Furthermore, telomere shortening in T cells has been observed in patients with atherosclerosis. Terminal restriction fragment (TRF) analysis has indicated that the mean length of the TRF in leukocytes of coronary artery disease (CAD) patients is shorter than in controls with no family history of CAD ^[127][168]. It is still unclear whether senescent-like T cell accumulation is the cause or the result of atherosclerosis; however, senescent T cells have been implicated in damaging VSMCs and ECs by producing perforin and granzymes, which may drive atherosclerosis ^[83][126]. Senescent-like T cells also secrete IFN-γ, which activates macrophages and increases metalloproteinase production ^[128][169]. The resulting ECM destruction may again play a part in atherosclerosis ^[129][46]. During aging, T cells accumulate potentially inflammatory cholesterol. The cholesterol efflux pathways behind this accumulation suppress T cell apoptosis and cause a senescent-like state, contributing to atherosclerosis in middle-aged mice ^[130][170]. Senescent T cells have been reported to drive hypertension. A higher frequency of CD57⁺CD28⁻CD8⁺ T cells and increased expression of CXCL11 has been noted in patients with hypertension compared to healthy controls, suggesting that immunosenescent cytotoxic CD8⁺ T cells are linked to hypertension ^[131][171]. Senescent CD4⁺ T cells producing interferon-gamma (IFN- γ) are found in high numbers in patients with acute coronary syndromes and may contribute to decline in myocardial function. Senescent CD8⁺ T cells have been connected to increased mortality six months after suffering a myocardial infarction ^[132][172]. In HIV-positive women, they were associated with carotid artery disease, and in CMV-seropositive patients, their numbers correlated with worsening left ventricular function ^[133][173]. IFN-γ-producing CD28^null CD4⁺ T cells were shown to accumulate in lymph nodes draining the heart of aged mice, and implanting these cells to young mice caused inflammation ^[114][134][156,174]. It has also been speculated that senescent CD4⁺ T cells might infiltrate the heart and promote inflammation and an increased stress response, causing heart failure ^[135][175]. Senescent CD57⁺CD8⁺ T cells have been observed in patients with acute myocardial infarction (MI) in higher concentrations than controls, and their numbers correlate with post-MI cardiovascular mortality ^[132][172]. Perhaps senescent cell IFNγ-driven IL-17 secretion may alter IL-23 levels and impact T lymphocyte subsets and contribute to post-MI dysfunction.

Endothelial progenitor cells (EPCs) from the bone marrow can participate in postnatal neovascularization and vascular repair ^{[136][137][138]}[176,177,178]. Declines in EPC function, proliferation, and telomere length are potentially detrimental to vascular EC function and contribute to reduced neovascularization and atherosclerosis ^[139][140][179,180]. In vitro, senescent EPCs form fewer capillaries and grow more slowly than non-senescent controls ^[141][181]. Senescent EPCs may impede vascular healing and worsen age-related vascular diseases.

Cardiac progenitor cells (CPC’s) can differentiate into cardiomyocytes, VSMCs, and ECs, and their myogenic potential is especially important since differentiated cardiomyocytes have low proliferative capacity. One characteristic of CPCs is expression of the protein c-kit; in the adult heart, only approximately 2% of the cells express this protein. Senescent CPCs accumulate in the heart right atrial appendage [7] and most CPCs in human hearts become senescent in old age [7]. Patients with cardiac diseases, particularly ischemic heart diseases, have damaged cardiomyocytes and may require myocardial regeneration from progenitor cells. The increasingly senescent CPCs may not be able to maintain homeostasis, repair damage, or regenerate after injury ^{[142][143][144]}[182,183,184]. In chronic heart failure, senescent CPCs may hinder myocardial ability to regenerate and cause further fibrotic remodeling.

Senescent CPCs can produce proinflammatory and profibrotic SASP components such as IL-1ß, IL-6, IL-8, PAI-1, and MMP-3 ^[145][51]. These paracrine mediators spread senescence, as shown by conditioned media from senescent CPCs causing an increase in senescence markers in a non-senescent CPC population [7].

Cardiac tissue obtained from nonaged (50- to 64-year-old) patients with type 2 diabetes mellitus (T2DM) and without DM (NDM) and post-infarct cardiomyopathy had higher ROS production in T2DM, which was associated with an increased number of senescent/dysfunctional T2DM-human CPCs with reduced proliferation, clonogenesis/spherogenesis, and myogenic differentiation versus NDM-CPCs in vitro. Moreover, T2DM-CPCs expressed a pathologic SASP ^[146][185].

Further studies are needed to more fully elucidate the characteristics of senescent EPCs and CPCs and their role in CVD development. As of today, consensus about methods for identifying these cells and their function has not been reached. As discussed above, several groups have independently reported that senolytic administration improves CPC function and have suggested that progenitor cell senescence may become a target for future CVD interventions.

Cellular senescence was originally discovered by L. Hayflick et al. using fibroblasts ^[147][38]. In the cardiovascular system, fibroblasts may be the most abundant cell type. Increased biological age has been linked to fibroblast senescence, evidenced by fibroblasts containing X-Gal crystals in the pericardium. Recent studies have reported that senescent biomarkers, including p16^Ink4a and p21^Cip1/Waf1, were increased in post-myocardial infarction mouse hearts, and costaining of α-SMA with p53 or p16 supports the possibility that senescent myofibroblast numbers are increased in infarct-border regions ^[148][149][186,187]. Senescent cardiac fibroblast accumulation was also noted in murine models of overload-induced cardiac hypertrophy. Senescence markers such as p16^Ink4a, p21^Cip1/Waf1, and SA-ßgal were higher in cardiac fibroblasts (CF), up to 20 times compared to sham models. These senescent cells accumulated within fibrotic areas ^[150][188]. Induced cardiac hypertrophy in transgenic mice with high ß1-adenoreceptor expression also increased the number of CFs expressing these three senescence markers. In heart biopsies from patients with idiopathic cardiomyopathy, there were significant increases in p16^INK4a, p21^CIP1/WAF1, and SA-ßgal positive cell populations. Another group found that heart tissue from patients undergoing ablation for atrial fibrillation (AF) exhibited increases in senescence markers co-localized with vimentin and α-SMA. Senescent fibroblasts were found to accumulate in the adventitial layer of blood vessels in the lungs of a pulmonary hypertension mouse model ^[151][152][189,190]. Another recently discovered marker of CF senescence is osteopontin (OPN) from peripheral adipose tissue, which potentially contributes to cardiac aging ^[153][191].

When activated by injury, resident cardiac fibroblasts (CFs) may transition into being myofibroblasts with α-smooth muscle actin expression, possibly helping to attenuate injury due to increased production of extracellular matrix (ECM) components. During and after repair, these myofibroblasts may mature into matrifibroblasts or return to their initial state. With continuous stress, CFs can undergo apoptosis or develop a senescent-like state with increased cell cycle-arresting proteins and SASP expression. Senescent CFs and myofibroblasts have been suggested to drive pathologic fibrosis and can be identified through their increases in platelet-derived growth factor, vimentin, and α-smooth muscle actin co-localized with senescence markers. Senescent CFs may contribute to cardiomyocyte senescence through paracrine signals and extracellular matrix modulation (ECM) ^[154][192]. Senescent fibroblasts also secrete IL-33, which attenuates cardiomyocyte senescence after hypoxic injury ^[155][193]. Fibroblasts normally express integrins, crucial for immune cell adhesion and surveillance and, through paracrine signaling to the ECM and the actin cytoskeleton, may contribute to ECM homeostasis ^[156][194]. Senescent fibroblasts and myofibroblasts are abundant within fibrotic areas and are involved in fibrotic myocardial pathologies. Interestingly, inducing senescence prematurely by CCN-1 decreases fibrosis in murine models ^[150][188]. This is in line with another finding demonstrating that a transient rise in the number of senescent fibroblasts reduces the fibrotic response after cardiac injury ^[157][195]. This may be due to senescent cells suppressing non-senescent fibroblast proliferation. Another explanation may be that senescent fibroblasts cannot proliferate and therefore cannot enlarge fibrotic areas. Oxidative stress may also be a contributor to AF development ^[158][196] and SASP factor release from senescent fibroblasts may worsen this condition.