CCL11 (Eotaxin) Promotes the Advancement of Aging-Related Cardiovascular Diseases.

Cellular senescence is a cell state that limits the proliferative lifespan of cells [18]. Two fundamental mechanisms that result in stable cell cycle arrest have been described: replicative and premature senescence. Replicative senescence is caused by a progressive shortening of telomeres upon each cell division. Additionally, replicative senescence represents physiological responses to prevent genomic instability and, therefore, the accumulation of DNA damage, which can be concluded as cell-intrinsic changes. Premature senescence, independent from telomere shortening, however, is considered a stress response induced by variable intrinsic and extrinsic insults, such as metabolic shock, oxidative and genotoxic stress, mitochondrial dysfunction, oncogenic activation, irradiation, or chemotherapeutic agents [18, 19, 20].

Unlike cell death, senescent cells still maintain metabolic activity for a period of time and show obvious changes, with typical cell cycle arrest, the senescence-associated secretory phenotype (SASP), macromolecular damage, and metabolic disorders [20]. Cellular senescence triggers profound phenotypic changes, expressing and secreting a plethora of soluble and insoluble factors, collectively termed the SASP [21]. The SASP constitutes a combination of bioactive secretions, and common phenotypes include the release of proinflammatory cytokines (interleukin-1 (IL-1), interleukin-6 (IL-6)), chemokines (C-X-C motif chemokine ligand (CXCL) 1, CXCL3), matrix metalloproteinases (MMPs), reactive oxygen species (ROS), growth factors (vascular endothelial growth factor (VEGF), angiogenin), and other signaling molecules secreted by senescent cells [19, 20]. Apart from being a consequence of cellular senescence, SASP also influences the microenvironment, mediating many pathophysiological effects [21].

Excessive oxidative stress and the inflammatory response are the two main causes of cell senescence (Fig. 1). ROS, including superoxide anion (O₂^–), hydroxyl (HO•), and hydroperoxyl (H₂O₂) radicals, are highly potent oxidants containing oxygen [22]. Mainly generated by mitochondria in cells through the electron transport chain, ROS could function as an important mediator in regulating cell growth, cell adhesion, differentiation, and cell death at a properly low level. However, higher ROS levels have been reported to severely damage DNA, lipids, and proteins, leading to cellular senescence [23].

A persistent chronic inflammatory response is another principle that can lead to aging. Based on observational studies of aged organisms, Claudio Franceschi proposed an important hypothesis: older organisms tend to develop a proinflammatory status characterized by high levels of proinflammatory markers in cells and tissues, first termed inflammageing in 2000 [24]. Several studies have also reported elevated serum and plasma levels of inflammatory factors in older individuals, such as IL-1, IL-6, C-reactive protein (CRP), and tumor necrosis factor α (TNF-α) [5, 25, 26, 27]. According to epidemiological studies, inflammageing has been demonstrated as a risk factor for aging-related diseases, such as CVDs, cancer, neurodegenerative diseases, chronic kidney disease, and dementia [28, 29, 30, 31, 32]. Therefore, the successful evaluation or prediction of the level of the chronic inflammatory response can be considered a measurement of our "real age". More importantly, it can predict or further intervene in various aging-related diseases. Based on the link between inflammation and aging, Buck Institute and Stanford University researchers created an inflammatory aging clock (iAGE) in 2021 to examine multiple immune system biomarkers in the blood and identify predictors of biological aging. These researchers successfully predicted the positive correlation between cardiovascular aging and immune markers, including serum CCL11 [6].

CCL11 could be produced by many secretory cells (Fig. 2), including smooth muscle cells (SMCs), endothelial cells (ECs), and immune cells (eosinophils, macrophages, T cells, and B cells) under the action of Th2-associated cytokines (IL-13, IL-10 and IL-4) [33, 34]. However, Th1-associated interferons (IL-17, bisphosphonates, 2 adrenergic receptor agonists, and fumaric acid) would suppress such a secretory process [13].

CCL11 has been reported to be involved in numerous cellular senescence. In asthma, the study revealed that eotaxin-1/CCL11 promoted ROS production and increased the activation of the DNA damage response (DDR), p-tumor protein p53 (TP53), and phosphorylated H2A histone family member X (γH2AX) in lung fibroblasts, accompanied by the promotion of cellular senescence and secretion of the SASP, including IL-6 and IL-8 [35]. In a study on human atopic dermatitis (AD), CCL11 was reported to be overexpressed in fibroblasts, causing the fibroblasts to destroy Ikkβ–NF-κB (inhibitor of nuclear factor kappa B kinase β subunit-nuclear factor-kappa B) under homeostasis conditions to abnormally induce skin inflammation, which is characterized by eosinophilic infiltration and a subsequent Th2 immune response [36]. For the epithelial cells, research has shown that airway epithelial lung cells overexpressing CCL11 were accompanied by increasing aging markers such as CDKN2A (p16INK4a), p21, p53, and serpin family E member 1 (SERPINE1) in atopic asthmatic patients [35]. For human coronary artery endothelial cells, CCL11 could increase oxidative stress and activation of mitogen-activated protein kinase (MAPK) p38, signal transducer and activator of transcription 3 (Stat3), and NF-κB, contributing to endothelial dysfunction during vascular lesion formation [37]. Moreover, the CCL11–CCR3 interaction mainly activated the phosphatidylinositol-3-kinase/protein kinase B (PI3K/Akt) signaling pathway in human umbilical vein endothelial cells (HUVECs), promoting endothelial cell migration and inducing weak proliferation [38]. CCL11 is also regarded as a potent chemotactic factor for SMCs, which could induce CCR3-dependent SMC migration [39]. Moreover, CCL11, together with stromal cell-derived factor (SDF), was verified to activate the growth factor receptor (EGFR) to induce matrix metalloproteinases-2 (MMP-2) through different mechanisms: both activated the PI3 kinase, extracellular regulated kinase (ERK) 1/2, and Akt signaling pathways, leading to SMC migration [40].

The regulatory effect of CCL11 in neurodegenerative diseases has been widely discussed. Growing evidence has been reviewed regarding the association between CCL11 and multiple neurodegenerative diseases, including Alzheimer's disease (AD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS),multiple sclerosis (MS), secondary progressive multiple sclerosis (SPMS), and chronic traumatic encephalopathy (CTE).

The current increasing incidence of neurodegenerative diseases has caused enormous losses to health and the global economy. Particularly, Alzheimer's disease, which is the most common neurodegenerative disorder, affects approximately 28 to 38 million people worldwide, followed by HD, ALS, and SPMS [41]. However, developing therapies for these diseases remains limited. Hence, a clear clarification of the neurodegenerative mechanisms involved is required. Recently, several lines have demonstrated that CCL11 and related molecules might contribute to degenerative processes in the central nervous system (CNS) [13].

CCL11 has been demonstrated to promote neurodegeneration by enhancing oxidative stress, reducing neurogenesis, and promoting neuroinflammation (Fig. 3). Research has indicated that CCL11 was mainly synthesized by microglia and then transported to the brain through the blood–brain barrier (BBB) [42]. Under the pressure of aging and neuroinflammation, choroid plexus epithelial cells could be provoked to secrete more CCL11 in vivo in old mice, such as astrocytes, pericytes, and microglia [13, 42, 43]. Furthermore, CCR3, the cognate receptor for CCL11, was also highly expressed within the brain by multiple varieties of neurocytes, including microglia, oligodendrocytes, astrocytes, and neurons [13]. Bijay Parajuli investigated that activated astrocytes could mainly secrete CCL11 and then promote microglial migration and production of ROS via nicotinamide adenine dinucleotide phosphate-oxidase 1 (NOX1) to induce excitotoxic neuronal cell death, which would potentiate the pathogenesis of various neurological disorders [43]. Moreover, the intensification of neuronal degeneration at the animal level further supported the positive correlation between CCL11 and neuronal senescence. Both young mice intraperitoneally injected with CCL11 and aging mice with naturally increasing CCL11 were detected with a significant decrease in neurogenesis [42]. Cognitive functions were consequently impaired, characterized by cellular changes consistent with markedly decreased adult neurogenesis and increased neuroinflammation, loss in synaptic plasticity, and behavioral deficits in contextual fear conditioning and radial arm water maze paradigms [42]. This evidence suggested a positive association between CCL11 and neurodegeneration.

Recent studies focusing on specific neurodegenerative diseases, such as Alzheimer's disease, HD, ALS, SPMS, and CTE, also presented similar tendencies, implying an essential role of CCL11 as a potential neuroinflammation biomarker for distinguishing between different neuroinflammation conditions and therapeutic targets in these disorders [44, 45, 46, 47, 48, 49, 50, 51, 52, 53].

CCL11, known for its role in neurodegenerative diseases by promoting neuroinflammation and oxidative damage, is also critical in vascular inflammation and atherosclerosis [40, 54, 55, 56]. These shared pathways suggest that CCL11 contributes to cardiovascular and neurodegenerative diseases through chronic inflammation, highlighting the interconnectedness of systemic aging processes.

Studies have also demonstrated that risk factors for cardiovascular diseases, including atherosclerosis and hypertension, are strongly associated with an increased risk of neurodegenerative diseases such as Alzheimer's [57, 58, 59]. The role of CCL11 in promoting vascular inflammation may be a key factor linking cardiovascular and neurodegenerative pathologies.

In addition to neurodegeneration, CCL11 has also been investigated in broader systemic aging mechanisms, including immune system decline and inflammation. CD4+ regulatory T (Treg) cells play an important role in immune tolerance and antitumor immunosuppression. CCL11 was shown to increase the CD4+CD25+Foxp3+ Treg cells proportion, CCR3 and Foxp3 expression, and the release of IL-2 and transforming growth factor (TGF) β1 in non-tumor-associated CD4+ T cells through the STAT5 signaling pathway [60]. Therapeutic interventions for cancer treatment would cause thymus damage and limit the recovery of protective immunity. These damages could be alleviated through CCR3-dependent colonization, which re-establishes the epithelial microenvironments that control thymopoiesis. Meanwhile, the expression of CCL11 would be triggered by natural killer T (NKT) cells through IL-4 receptor signaling, helping restore thymus function during the re-establishment of the adaptive immune system [61]. In research on the immunomodulatory compounds in cardiovascular disease, CCL11 was selected as a crucial inflammatory immunophenotype [62].

Regarding the effects of CCL11 on chronic inflammation, CCL11 is recognized for its contribution to chronic low-grade inflammation, which accelerates aging across various organ systems. Increased serum levels of CCL11 have been widely detected in multiple diseases with chronic inflammation of the airways, such as asthma and chronic obstructive pulmonary disease (COPD) [34, 35, 63, 64]. Additionally, growing evidence suggests an association between elevated CCL11 and gastrointestinal inflammation [65]. In a study on chronic inflammatory disease with disturbed bone remodeling, the CCL11/CCR3 pathway was also investigated to drive the expression of CCL11 in bone tissue and its novel role in osteoclast migration and resorption, which could be a new target for the treatment of inflammatory bone resorption [66].

Above all, various research studies show the involvement of CCL11 in neurodegeneration and broader systemic aging mechanisms, further suggesting a potential role for this chemokine in the pathogenesis of CVDs.

Despite significant decreases in prevalence over the last three decades, cardiovascular diseases remain the leading causes of morbidity and mortality in developed nations [67]. Further, aging is by far the strongest independent cardiovascular risk factor that dwarfs the impact of traditional risk factors, with 90% of all cardiovascular diseases occurring in adults aged 40 and older [67, 68].

Cardiovascular aging is a structural and fundamental degenerative alteration, with blood vessels gradually losing their original function with age. As a special type of organ senescence, cardiovascular aging is characterized by structural and functional vascular impairments, such as increased arterial stiffness and decreased compliance, which has deleterious effects on tissue oxygenation, nutrient delivery, and waste removal and, thus, negatively affects multiple organ functions [69]. At the cellular level, cardiovascular aging is mainly manifested by the morphological changes occurring on ECs in the intima and vascular SMCs in the mesa, as ECs and SMCs are critical building blocks of blood vessels and play dominant roles in age-induced cardiovascular dysfunction [2].

Apart from the intrinsic factors such as the phenotypic switch of ECs and SMCs, various extrinsic alterations caused by diseases and changes in cell–cell and cell–matrix interactions also promote vascular aging. Common extrinsic factors include atherosclerosis, hypertension, chronic inflammation, vascular wall stiffness, and vascular cell communication [2].

As an important group of aging-related diseases, cardiovascular diseases are also closely related to chronic inflammation. Epidemiological studies have found that inflammageing is a risk factor for cardiovascular diseases [5]. The characteristic targeting of relevant processes in experimental models has been shown to attenuate CVDs such as myocardial infarction and to predict cardiovascular events in various studies [28, 70].

Except for the neurodegenerative diseases mentioned above, the role of CCL11 in the pathogenesis of cardiovascular diseases remains a research priority (Fig. 4). The relevance of CCL11 to cardiovascular disease risk has been well established.

Regarding the mechanism through which abnormally elevated CCL11 affects vascular aging, studies showed that CCL11 contributes to chronic inflammation and cellular senescence in cardiovascular aging [37, 40, 71, 72]. CCL11 significantly increases vascular permeability through the downregulation of tight junction proteins, an increase in oxidative stress, and activation of MAPK p38, Stat3, and NF-κB in coronary artery endothelial cells, exerting a crucial impact on endothelial dysfunction during vascular lesion formation [37]. Research showed that MMP-2 is a member of the MMP family critical to SMC migration. As the regulator of proMMP-2 expression and by engaging in receptor cross-talk, CCL11 could induce proMMP-2 activation of the EGFR together with SDF, indicating its critical role in atherosclerosis, restenosis, and plaque rupturing [40]. Thus, CCL11 may directly contribute to angiogenesis [71]. Moreover, CCL11 has been selected as a typical proinflammatory phenotype in human vascular smooth muscle cells [72]. These lines have demonstrated that elevated CCL11 correlates with impaired angiogenesis and endothelial dysfunction, which are critical to atherosclerosis and other cardiovascular diseases.