Systemic inflammation occurring in COPD patients may contribute to the development of both pulmonary and systemic endothelial dysfunction. Activation and alteration of adhesion molecules in ECs promote the inflammatory reactions occurring in pulmonary vessels. The surface activation of cellular adhesion molecules represents a common endothelial response to a variety of toxic and inflammatory stimuli and their shedding into the systemic circulation provides evidence of endothelial dysfunction.

As described above, the endothelium produces and activates molecules and inflammatory factors, which exit the bloodstream and reach the target tissue through the endothelium. Trafficking of leukocytes out of the circulation into tissues is a highly regulated, complex, multistep process. Physiologically, inflammation is initiated by a relatively loose adhesion of leukocytes to ECs followed by a firmer adhesion and transmigration of leukocytes across pulmonary ECs [45]. Specifically, neutrophils are provided by two different mechanisms: The first one is called paracellular transmigration and consists of a connection with the apical domain of ECs via cell surface proteins (e.g., pseudopods) over one or between two ECs. The second mechanism is called TEM by which neutrophils can extravasate at junction regions between two ECs. This process consists of multiple steps of interaction between neutrophils and ECs and has a relevant role in the inflammatory response [46]. TEM is a mechanism by which endothelium may play a role in COPD and therefore may be important in the development of inflammation and in the pathogenesis of COPD. Patients with COPD have high levels of TEM and during this process, the MAC-1, an important human cell surface receptor found on B and T lymphocytes, is upregulated [47]. MAC-1 binds to ICAM-1 on the surface of ECs resulting in increased ICAM-1. This mechanism could be clinically relevant, as when ICAM-1 serum levels are high, lung function is reduced. Conversely, when ICAM-1 is turned off, pulmonary inflammation is reduced, supporting the hypothesis that ICAM-1 upregulation could relate to the increased inflammation seen in COPD patients [48]. Furthermore, another study by Johnston et al. [49] suggested that the upregulation of ICAM-1 could be reduced by an anti-ICAM-1 molecule, administered at the initial stage of the disease, thus preventing COPD exacerbations and its adverse outcomes [49, 50]. Another relevant adhesion molecule involved in TEM is ELAM-1. This has also been found upregulated in COPD patients’ serum and its levels are particularly high in patients with chronic bronchitis. Altogether, this evidence further supports the involvement of adhesion molecules in lung inflammation and COPD pathogenesis [51].

In the last years, scientists improved the research about a variety of COPD inflammatory biomarkers [52]. This research field is interesting for the study of disease progression, since inflammatory biomarkers are diagnostic and prognostic factors of COPD patients with acute exacerbation of COPD (AECOPD). For example, are studied as markers for degradation and repair processes factors like neutrophil elastase (NE) or matrix metalloproteases (MMP), which are detectable in bronchoalveolar lavage fluid (BALF) or sputum [53]. Among the most significative systemic markers of COPD there are fibrinogen and acute C-reactive protein (CRP), which is one of the predictive factors. This project, called “Evaluation of COPD Longitudinally to Identify Predictive Surrogate End-points (ECLIPSE)”, is a three-year longitudinal study whose main objective was to identify factors that predict disease progression in individuals with different endotype of COPD [54]. Literature studies show that serum CRP is associated with mortality, morbidity, number of exacerbations and is inversely related to lung function. Increased serum concentrations of IL-6, tumor necrosis factor α (TNFα), and monocyte chemoattractant protein-1 (MCP-1) are also related to systemic inflammation in COPD patients [55].

As described in the sections above, the inflammatory endotype in COPD can be mediated by neutrophils [T helper 1 (Th1)] and by eosinophils (Th2) [56]. Eosinophil recruitment to inflammation sites is mediated by eotaxins [eotaxin-1 (CCL11), eotaxin-2 (CCL24), eotaxin-3 (CCL26)]. These chemokines are considered as candidate biomarkers to monitor COPD development and progression since they are upregulated in COPD patients [57]. The main source of eotaxins are epithelial cells, motile cells present in BALF (macrophages, monocytes, basophils, eosinophils, lymphocytes), and structural cells like fibroblasts or smooth muscle cells [58]. Eotaxins can also be released by ECs but there are few studies supporting this statement. One of these showed that CCL11 can be released by both epithelial and ECs to favor angiogenesis. Unfortunately, none of these studies describe the release of eotaxins by ECs in the context of COPD [59]. Similar considerations can be made for neutrophils. Indeed, there is evidence about neutrophil chemoattractant cytokines [C-X-C motif chemokine ligand 1 (CXCL1) and CXCL5)] release by ECs but not in the context of COPD even if they are dysregulated in COPD patients [60, 61]. To study Th2 response is particularly appealing since it defines a specific COPD patient population as shown by the ECLIPSE study reporting that eosinophilic inflammation was present in 37.4% of patients [62]. In these patients, eosinophil count might be relevant to predict their disease progression and to choose the most appropriate pharmacological therapy. Indeed, peripheral eosinophil level from a complete blood count has been evaluated as a surrogate marker for corticosteroid responsiveness and eosinophilic bronchitis [63]. A higher blood eosinophil count appears to indicate a subgroup of COPD patients in which the use of inhaled corticosteroids (ICS) results in reduced exacerbation frequency and when ICS [64] is withdrawn significant increase in peripheral eosinophil count has been associated with a higher exacerbation rate [65]. To deepen these issues could help to improve the discovery of therapeutic targets and to identify patients who are likely to respond to classic or novel treatments.

Neurogenic inflammation is confined to the central and peripheral nervous system and is caused by nerve activation inducing the release of neuropeptides and rapid extravasation. During airway inflammation, neuropeptides are secreted from the airways in response to a multitude of inflammatory mediators, such as exogenous irritants like CS or gas. According to this, neurogenic inflammation may participate in the development and progression of chronic inflammatory airway diseases such as allergic asthma or COPD. Among the large variety of neuropeptides stored in and secreted from airway sensory nerves the most studied are the tachykinins like substance P (SP), neurokinin A (NKA), and the calcitonin gene-related peptide (CGRP) [66]. Tachykinins can bind to three different receptors, called neurokinin receptors (NK). Specifically, SP acts via NK1, NKA acts via NK2 and neurokinin B acts via NK3. The first one is predominantly localized to airway epithelium, submucosal glands, and vessels, the second one is mainly expressed by airway smooth muscle, while NK3 is expressed by airway parasympathetic ganglia. Tachykinins exert various actions on airway functions. They have a direct effect on coronary arteries to decrease or increase tone. Stimulation of NK1 receptors on the endothelium causes vasodilatation mediated by NO. They can vasodilate or vasoconstrict the state of the endothelium depending on the situation; this state appears to be of considerable importance [67]. Tachykinins also enhance eosinophils chemotaxis through the activation of alveolar macrophages and monocytes that release inflammatory cytokines such as IL-6. CGRP is a 37 amino acid peptide that is abundantly expressed in the respiratory tract of several species and is co-stored and co-localized with tachykinins in sensory nerve fibres innervating both upper and lower airways. CGRP is a powerful vasodilator, with a long-lasting effect on bronchial vessels and this is evident in vitro and in vivo. Mapping of the receptors showed that CGRP receptors are predominantly situated in the bronchial vessels, in the epithelium of the human airways, and in the smooth muscles on which it has effects on tone by acting indirectly through the release of other constrictors, such as endothelin. Like tachykinins, CGRP is chemotactic for eosinophils [68]. The molecular mechanisms underlying the different events of neurogenic inflammation in the airways of mammals have been identified thanks to animal models. Based on data obtained from these studies, the existence of a neurogenic inflammatory component in human airway diseases such as asthma and COPD seems very likely. Nevertheless, further studies are required to assess the clinical relevance of this inflammatory process in the onset and progression of COPD [69].

Oxidative stress is another crucial mechanism underlying COPD development and may be a causal link in COPD comorbidities such as CVD [70]. Oxidative stress is caused by an alteration of the prooxidant/antioxidant balance that results in the production and accumulation of ROS. ROS refer to a variety of molecules, either containing unpaired electrons (such as superoxide, O₂⁻) or having a strong ability to oxidize other molecules (such as hydrogen peroxide, H₂O₂) [71], and are produced by both endogenous and exogenous factors. Inflammatory cells, infiltrated in pulmonary arteries, may be one of the major sources of endogenous ROS generated mainly by xanthine oxidase, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, and mitochondria. On the other hand, genetic factors, CS, and air pollutants are the main producers of exogenous ROS [72]. More in detail, CS favors the activation of inflammatory cells through the recruitment of transcription factors nuclear factor-kappa B (NF-κB), p38, mitogen-activated protein kinase (MAPK), and post-translational modification of histone deacetylase (HDAC) in macrophages [73]. These processes stimulate the cells to release pro-inflammatory cytokines and activate the immune system and inflammation [74]. Experimental studies performed with cultured rat pulmonary microvascular ECs demonstrated that exposure to tobacco smoke condensate causes increased transcription and activity of the ROS generating enzyme xanthine dehydrogenase/oxidase [75]. Among the causes of oxidative stress, there is also a decrease in antioxidants. Antioxidants, such as superoxide dismutase (SOD), glutathione (GSH), and nuclear factor erythroid 2-related factor 2 (Nrf2) are thought to play very important roles in the lungs since they are continuously exposed to oxidant endogenous and exogenous risk factors. NO released from the vascular endothelium is also an antioxidative factor that neutralizes O₂⁻ [76] and impairment of NO synthesis marks the onset of endothelial dysfunction that is mainly mediated by the endothelial NO synthase (eNOS) uncoupling mechanism [77].

Increased oxidative stress and ROS overproduction may cause not only endothelial dysfunction but also programmed ECs death such as ferroptosis, pyroptosis, or necroptosis that are involved in the pathophysiology of many diseases, like COPD [78]. Ferroptosis is an iron-dependent process and is characterized by lipid peroxidation, which is the reaction by which OH^– attacks the carbon-carbon double bonds of lipids. It is directed by the glutathione antioxidant system, which is formed by GSH, glutathione peroxidase (GPX), and glutaredoxin (GRX) but most importantly it can effectively prevent ROS overproduction [79]. In recent years, studies have shown that ferroptosis is closely related to ECs death. For example, Qin et al. [80] found that zinc oxide nanoparticles could induce iron and lipid peroxidation in ECs depending on the dose and time. Luo et al. [81] showed that ferroptosis is related to endothelial dysfunction, meanwhile, Sheng et al. [82] showed that lysophosphatidylcholine (LPC) can induce an increase in the amount of intracellular iron and lipid peroxide levels and mitochondrial atrophy in ECs. In a study conducted by Wang et al. [83] they demonstrated, with in vitro and in vivo experiments, that pulmonary microvascular endothelial barrier dysfunction plays an important role in the mechanism by which COPD affects the progression of atherosclerosis and that oxidative stress and ferroptosis are involved in this effect. Another mechanism of programmed cell death is pyroptosis. The molecular features of pyroptosis include the activation of inflammasome, membrane pore formation, and pro-inflammatory cytokine maturation and release. Pyroptosis may require activation of caspase-1, depending on whether it is activated or not, it may be involved in classical or non-classical inflammatory pathways. Among the classical inflammasome pathway, Nod-like receptor pyrin domain-containing protein 3 (NLRP3) inflammasome-mediated pyroptosis is the most extensively studied phenomenon and can be activated by different stimuli, first of all, ROS. The NLRP3 inflammasome in ECs can be caused by stimuli such as low-density lipoprotein oxidized, hyperglycemia, and nicotine, thus leading to the death of ECs [84, 85]. For example, Wu et al. [86] found that oxidized low-density lipoprotein (ox-LDL) induced, in a dose-dependent manner, the upregulation of NLRP3, caspase-1, and IL-1β in ECs. Hang et al. [87] found that ox-LDL stimulated in ECs NLRP3 inflammasome activation, increased IL-1β and IL-18 maturation and secretion, increased intracellular ROS and increased lactate dehydrogenase (LDH) release in ECs. Zhang et al. [88] found that nicotine could activate the NLRP3 inflammasome. The team also found that due to the activation of NLRP3 inflammation can promote the destruction of the proteins of the narrow junction between ECs, resulting in increased vascular permeability. Cau et al. [89] found that angiotensin II (Ang-II) induces endothelial dysfunction, vascular remodeling, and hypertension through NLRP3 inflammasome activation. The last cell death program we here describe is necroptosis, a genetically encoded mechanism of necrotic regulated cell death defined by rupture of the plasma membrane. Necroptosis causes the release of damage-associated molecular patterns (DAMPs) into the extracellular environment and for this reason is highly proinflammatory. Indeed, Pouwels et al. [90] discovered that CS-induced necroptosis and the release of DAMPs trigger neutrophilic airway inflammation in mice BAL and this effect was reduced by treatment with the necroptosis inhibitor necrostatin-1. Wang et al. [91] reported a novel regulatory mechanism of necroptosis-mediated inflammation. Endoplasmic reticulum chaperone glucose-regulating protein 78 (GRP78) promoted a CS extract (CSE)-induced inflammatory response and mucus hyperproduction in airway epithelial cells, likely through the upregulation of necroptosis and subsequent activation of the NF-κB and activator protein-1 pathways [91, 92]. Based on data present in literature, necroptosis is increased in the lungs of COPD patients and its inhibition attenuates CS-induced emphysema, airway inflammation, and remodeling making this process an attractive candidate for COPD patients. Nevertheless, there is not enough data about the correlation between necroptosis and ECs, this could be a starting point for new research.

Cellular senescence is now considered an important driving mechanism for chronic lung diseases contributing to the pathophysiology of COPD and CVD.

Cellular senescence is due to telomere shortening (replicative senescence) and oxidative stress (stress-related senescence) with activation of p53 and p16 that lead to activation of p21 and cell cycle arrest, respectively [93, 94]. Patients with COPD showed, accumulated in the lungs, senescent cells, including alveolar epithelial and ECs [95–97]. Exposure to CS is among the most relevant factors that cause oxidative stress and is also an important inducer of senescence in COPD, increases markers of senescence in the epithelial cells of the airways, resulting in endothelial damage. Chronic CS exposure causes pulmonary endothelial dysfunction in animal models, healthy young smokers, and chronic smokers with COPD [98, 99]. Accordingly in a study performed by Paschalaki et al. [100] using circulating endothelial progenitors named endothelial colony-forming cells, accelerated endothelial senescence in smokers and patients with COPD was shown compared with healthy controls. They also demonstrated that patients with COPD on ICS exhibit significantly reduced endothelial senescence [100]. Another very recent study by Zeng et al. [101], documented that CSE averages the ubiquitin-specific protease (USP7)/p300 dependent high expression of p53 and activates the expression of target p53 genes, especially p21. They also showed that the activation of p53, via p21, inhibits cell activity and leads to cell cycle arrest with premature senescence of ECs progenitors [101]. Finally, studies reported that senescent pulmonary ECs release more inflammatory markers and this boosts the inflammatory state in COPD patients [102].

Another mechanism that plays an important role in the pathogenesis of COPD is apoptosis and interestingly, increased apoptotic events correlate with ECs damage [103]. Apoptosis is not a marginal event in COPD. Indeed, recent studies suggest that apoptosis of pulmonary vascular ECs plays a role in initiating and participating in the pathogenesis of COPD [104]. Studies show that the number of dead ECs in the lungs of patients with COPD is increased as assessed by specific nuclei fragmentation, which is a clear feature of apoptotic cells [105]. The importance of apoptosis in COPD, especially in the pathogenesis of emphysema, originates from in vitro, in vivo, and in human studies.

In an in vitro study performed by Farid et al. [106], the authors studied the effect of CSE exposure on VEGF release by human lung fibroblasts demonstrating that the TGF-β receptor/mothers against decapentaplegic homolog 3 (Smad3) pathway mediates CSE-induced VEGF production. VEGF plays important roles by mediating angiogenesis and vascular permeability, inducing cell proliferation, and preventing ECs death. Following CS administration VEGF levels decrease and in parallel emphysematous degeneration increases [106]. In line with this observation, Taraseviciene-Stewart et al. [107] by inducing apoptosis of rat pulmonary vascular ECs, with an intraperitoneal injection of CSE, successfully established a rat emphysema model. To explain this result, they suggested that chronic blockade of VEGF induces alveolar cells apoptosis and emphysema. Along this line, another preclinical study suggests that it is possible to induce emphysema in rodents by deliberately causing endothelial apoptosis through VEGF blockade [108]. The authors treated rodents with a VEGF receptor inhibitor which resulted in EC apoptosis and emphysema. Conversely, when rodents were treated with a caspase inhibitor, apoptosis, and emphysema were reverted, suggesting that apoptosis of ECs may be key in emphysema development [109]. Evidence of increased apoptosis of ECs in humans has also been published. Segura-Valdez et al. [110] were the first to show increased apoptosis in the lungs of COPD patients. They showed that ECs and arterioles exhibited prominent intranuclear staining by in situ-end labeling of fragmented DNA [110]. Moreover, Kasahara et al. [109] described an increased apoptosis of alveolar epithelial and ECs in lung tissue from patients with emphysema. Different and contrasting studies suggest that VEGF and HIF-1α levels change with the severity of pathology. For example, in the study by Yasuo et al. [111], patients with emphysema displayed reduced levels of HIF-1α in lung ECs. On the other hand, a study by Lee et al. [112] showed increased levels of VEGF and HIF-1α in patients with chronic bronchitis. These contrasting findings suggest that the endothelium is variably involved and VEGF is not uniquely implicated in the mechanism of endothelial apoptosis in COPD patients. In this regard, other players could be the cystic fibrosis transmembrane regulator (CFTR) and AAT. CFTR, who participates in the apoptotic process, is necessary for stress-induced apoptosis of human pulmonary ECs as confirmed by the evidence its inhibition, by hydrogen peroxide, attenuates EC apoptosis [113]. AAT has an important role in preventing caspase-3 activation and therefore apoptosis in pulmonary ECs [114]. Overall, endothelial apoptosis may contribute to the destruction of alveoli and consequently, to the onset and progression of emphysema in COPD patients.

Therefore, understanding the mechanisms by which apoptosis is induced in the pulmonary endothelium is important for the development of new therapies to prevent or treat lung diseases characterized by endothelial dysfunction.