Endothelial dysfunction is “an imbalance between vasodilatating and vasoconstricting substances produced by (or acting on) endothelial cells”[10]. Inflammation leads to functional and structural changes in vascular endothelium and its activation (Figure 3). These changes initially include increased leukocyte adhesiveness, leukocyte diapedesis, and vascular smooth muscle tone and procoagulant activity[5,11]. Activation of endothelial cells is promoted by the interactions between integrins and chemokine receptors with endothelial and mucosal ligands, the CAM superfamily, and chemokines[12,13]. Activated endothelium expresses CAMs and chemokines, which promote the recruitment of leukocytes[14,15]. Adhesion is the most important process determining which leukocytes migrate to tissues[16]. The three main classes of CAMs are selectins, integrins, and the immunoglobulin CAM superfamily, including intercellular adhesion molecule-1 (ICAM-1), ICAM-2, and vascular cell adhesion molecule-1 (VCAM-1))[17]. This process is mainly mediated by leukocyte CD11a/CD18 binding to ICAM-1 in the gut or by α4-β1 or α4-β7 binding to VCAM-1 and mucosal addressin cell adhesion molecule-1 (MAdCAM-1)[12].

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Figure 3 Inflammatory activation of endothelium. TNF: Tumour necrosis factor; IL: Interleukin.

Cultures of human intestinal microvascular endothelial cells (HIMECs) have revealed unique leukocyte adhesion and growth patterns[18]. Significant changes, particularly an enhanced capacity to adhere to leukocytes, occur in chronically inflamed microvessels of patients with IBD compared to vessels from healthy controls and patients without IBD[5]. Microvascular expression of ICAM-1 and VCAM-1 is upregulated in patients with IBD[14,16]. Increased MAdCAM-1 expression is also observed, which increases recruitment of α4 integrin-expressing leukocytes[19]. Vowinkel et al[20] demonstrated a significant increase in endothelial expression of CD40, resulting in increased recruitment of leukocytes.

The risk for hyperhomocysteinemia is significantly higher in patients with IBD than in those without the disease[21,22]. HIMECs are activated by homocysteine alone or homocysteine combined with tumor necrosis factor-α (TNF-α), leading to enhanced VCAM-1 expression, increased production of monocyte chemotactic protein-1, and phosphorylation of p38 mitogen-activated protein kinase[16,23].

Decreased NO production occurs in patients with IBD, which may be caused by increased arginase activity, an enzyme that competes with NOS, and a lack of inducible NOS mRNA expression[16,24,25]. Vascular endothelium is also a TNF-α target[11]. Binding of TNF-α to the TNF receptor leads to diminished eNOS protein expression and suppresses eNOS activity, which prevents degradation of asymmetric dimethylarginine (ADMA), an endogenous eNOS inhibitor[11,26]. Reactive oxygen species are produced in the inflamed area, which generate oxidative stress in vWF molecules. This process inhibits cleavage of vWF molecules by disintegrin and metalloproteinase with thrombospondin motif-13 and contributes to the creation of ultra-large vWF multimers leading to microvascular thrombosis in patients with IBD[13,17,27].

Structural changes in vascular endothelium include capillary and venule remodeling and proliferation of endothelial cells[28]. Angiogenesis is a crucial process that sustains chronic inflammation in the gastrointestinal tract. Numerous inflammatory cell types, including macrophages, lymphocytes, mast cells, and fibroblasts, produce angiogenic factors and promote pathological angiogenesis in inflammatory tissues[16,29]. Hypoxia in the inflamed area stimulates angiogenesis by upregulating vascular endothelial growth factor (VEGF), fibroblast growth factor, and TNF-α[29]. Rutella et al[30] suggested that inflamed HIMECs recruit platelets to produce higher levels of pro-angiogenic VEGF-A and CD40 ligand (CD40L) in patients with active ulcerative colitis (UC) and Crohn’s disease (CD)[30].

The CD40-CD40L pathway has a dual role, directly stimulating mucosal angiogenesis and activating CD40L-expressing T cells, which in turn stimulates intestinal fibroblasts to release angiogenic cytokines[31]. CD40 is upregulated by several cell types in patients with IBD, including antigen-presenting cells, monocytes, and endothelial cells. CD40L is overexpressed by T cells and platelets, and plasma CD40L levels are particularly high in patients with active IBD[31,32]. The CD40-CD40L pathway promotes mucosal inflammation and triggers increased production of various cytokines and cell adhesion molecules[31]. CD40 binding stimulates the production of TNF-α, and TNF-α increases CD40 expression[32].

Intestinal endothelium is also involved in innate immunity by expressing toll-like receptors (TLRs), which increase the expression of proinflammatory genes and promote leukocyte chemotaxis, phagocytosis, and cytotoxicity as a second-line defense against bacterial entry[33]. Expression of TLRs on vascular endothelial cells is upregulated by vascular inflammation, as well as by lipopolysaccharide. Endothelial cells detect both extracellular and intracellular microbial invaders through TLRs and activate the nucleotide oligomerization domain family of proteins[34]. Activated TLRs upregulate endothelial leukocyte adhesion molecules, increasing leukocyte transmigration[35].

Tolstanova et al[36] demonstrated that damage to endothelium and increased vascular permeability lead to hypoxia in four UC animal models, which increases the levels of oxygen-derived toxic free radicals and induces epithelial cell injury followed by the development of erosions und ulcerations. Genetic predisposition, activation of the VEGF gene, and activation of TLR2 by oxidation stress are responsible for increased endothelial microvascular permeability in patients with UC[36].

The lymphatic vessels are responsible for intestinal lymphatic transport and elimination of accumulated fluid, inflammatory cells, and mediators. However, inflammatory mediators released alter lymphatic vessel function and impair lymph flow in patients with IBD, which exacerbates tissue edema and the accumulation of dead cells and bacteria. Lymphangiogenesis is mediated by binding of the lymphatic vascular endothelial selective growth factors VEGF-C and VEGF-D/VEGF receptor-3[37].

Lymphatic obstruction, submucosal edema, and extended lymphangiogenesis are observed in patients with IBD[13,16]. Rahier et al[38] reported that lymphatic density increases significantly in patients with CD and UC in both inflamed and non-inflamed intestinal mucosa. Tonelli et al[39] demonstrated a correlation between parietal wall thickness and the severity of changes in lymphatic drainage in patients with IBD and stenosis, and reported that these abnormalities disappeared after strictureplasty. It remains unclear whether the impaired lymphatic function in patients with IBD is a consequence of inflammation or a preceding lymphangitis-like event[40].

Chronic systemic inflammatory diseases are associated with accelerated atherosclerosis, a higher rate of cardiovascular disease (CVD), and increased morbidity and mortality compared to the general population[11,61-63]. Inflammatory mediators, such as CRP, TNF-α, interleukin (IL)-6, IL-18, and CD40L, are involved in the pathogenesis of inflammation and atherosclerosis[64,65]. Endothelial dysfunction is considered the initial step in the pathogenesis of atherosclerosis in the general population and may be a marker for future risk of cardiovascular events[66].

The risk for CVD in patients with IBD is controversial. Some studies have demonstrated an increased risk for CVD in these patients[66-68], whereas others have provided no evidence for an increased risk of mortality due to CVD[69-71]. Ruisi et al[72] found no increased risk for CVD in patients with IBD without traditional CVD risk factors after a 2-year follow-up. A meta-analysis performed by Fumery et al[73] and composed of 33 observational studies showed a higher risk for ischemic heart disease and arterial thromboembolism in patients with IBD but no increased risk for cardiovascular mortality. In addition, Singh et al[74] reported a 19% higher risk of CVD in patients with IBD and a modest increased risk for cardiovascular morbidity (from cerebrovascular accidents and ischemic heart disease), mostly in female patients[74]. Kristensen et al[75] performed a population-based study and demonstrated an increased risk for myocardial infarction in patients with IBD during flare-ups and during persistent disease activity, but not in patients in remission[76].

Coronary flow reserve is impaired in patients with IBD, suggesting dysfunction in coronary microcirculation[76].

Fan et al[77] concluded that traditional risk factors, such as age, systolic blood pressure, body mass index, diabetes, and triglycerides, rather than inflammatory markers, are major predictors of arterial stiffness in patients with an inflammatory disease. In contrast, Ozturk et al[49] suggested that patients with IBD without traditional cardiovascular risk factors have higher risks for endothelial dysfunction and atherosclerosis.

Some studies have suggested that patients with IBD, particularly those with CD, exhibit low levels of total cholesterol and low-density lipoprotein (LDL)[78]. However, Sappati Biyyani et al[79] reported lower total cholesterol but higher LDL and lower high-density lipoprotein levels in patients with IBD in a large retrospective study. Changes in the lipid profile do not depend on disease activity[77]. Despite a lower incidence of traditional CVD risk factors in patients with IBD, these patients are diagnosed with coronary artery disease at a younger age[67,80]. It is important to emphasize that some patients with CD are tobacco smokers, which could also contribute to enhanced endothelial damage[81]. These findings indicate that large prospective studies are needed to determine the actual risks for CVD in patients with IBD.

Pharmacological agents directed against TNF-α-mediated inflammation may decrease the risk for endothelial dysfunction and CVD in patients with IBD. Some studies that have addressed rheumatoid arthritis have demonstrated decreased levels of endothelial dysfunction biomarkers in plasma after anti-TNF-α therapy[11]. Other studies have shown improved endothelium-dependent vasodilation in patients with inflammatory diseases, including rheumatoid arthritis, spondyloarthritis, psoriasis, and IBD after initiating anti-TNF-α therapy[11,48,82]. However, whether this effect is permanent or transient remains controversial[11]. Arijs et al[83] demonstrated that inflammatory cells from the intestinal lamina propria disappear after an infliximab infusion and that previously upregulated cell adhesion molecules are restored. Some authors have suggested that anti-TNF-α antibodies inhibit angiogenesis in patients with IBD[84,85]. Danese et al[32] showed that anti-TNF-α downregulates the CD40/CD40L pathway.

The effects of TNF-α on the cardiovascular system could be both beneficial and harmful due to concentration-related differences in activation of different receptors[63,86].

Data regarding corticosteroids are controversial as well. However, a systematic review of studies that assessed steroid administration in patients with rheumatoid arthritis did not report any effects of steroid use on endothelial function[87].

At the same time, it is probable that methotrexate improves endothelial-dependent vasodilation in patients with rheumatoid arthritis[62]. A weekly low dose of methotrexate reduces disease-associated cardiovascular mortality. Thornton et al[88] suggested that methotrexate induces cytoprotective genes by activating adenosine monophosphate protein kinase-cyclic AMP response element binding protein signaling, which prevents CVD.

In vitro studies have indicated that cyclosporine induces increased activation of endothelial cells[89]. However, some evidence suggests that cyclosporine A inhibits angiogenesis targeting VEGF-A[84,90]. At the same time, azathioprine and its metabolite, 6-mercaptopurine, may play a protective role against the activation of endothelial cells[91].

New therapeutic options for patients with IBD include anti-adhesion molecules, such as vedolizumab, etrolizumab, and PF-00547659[92]. Vedolizumab targets α4-β7 integrins and blocks interactions between α4-β7 integrins and MAdCAM-1. Etrolizumab targets the β7 subunit, and PF-00547659 is a monoclonal antibody against MAdCAM-1[92]. Anti-chemokine receptor CCR 9 interferes with the activation of inflammatory cells in the intestine. Other medications, such as laquinimod and tofacitinib, reduce the synthesis of pro-inflammatory cytokines[92].

As the lymphatic endothelium seems to play an adaptive role in the inflammation that occurs in patients with IBD, the inflammatory process may also be affected by stimulating lymphatic vessel functions, such as drainage and clearance of bacterial antigens, together with adaptive immunity[37]. D’Alessio et al[37] demonstrated that adenoviral induction of the prolymphangiogenic factor VEGF-C in two different animal models significantly prevents the development of acute and chronic colitis, which supports the potential use of lymphangiogenic growth factors as a novel therapeutic approach[37].

Improved endothelial function may also be achieved by increasing NO availability, as antioxidants affect the decrease in NO breakdown. Dietary components, such as vitamins C and E, have potential benefits on endothelial function and could prevent CVD[93]. Recent studies provide no evidence for a supportive role of vitamin D in endothelial function[94]. Ibrahim et al[95] showed that treatment with long-chain n-3 polyunsaturated fatty acid decreases the expression of adhesion molecules in endothelial cells and has a potential anti-angiogenic role in animal models and in vitro.