Atherosclerosis is a chronic inflammatory disease. A healthy endothelium actively reduces thrombosis, vascular inflammation, and hypertrophy in addition to mediating endothelium-dependent vasodilation. Endothelial dysfunction and inflammation are responsible for various pathologies involved in atherosclerosis.

Recent clinical data indicate that plaque disruption rather than plaque stenosis is related to cardiovascular events and mortality in patients [91]. Unstable plaques feature a large necrotic core, high macrophage content, and thin fibrous cap [92]. Evidence indicates plaque destabilization is caused by inflammation due to immune cell infiltration in atherosclerotic lesions. Matrix degradation and apoptosis, two crucial factors in plaque stability, are controlled by inflammatory mediators from macrophages and T cells. Owen’s group recently revealed that ACTA2+ myofibroblast-like cells originate not only from smooth muscle cells but also from endothelial cells and macrophages via EndMT and macrophage-to-mesenchymal transition (MMT), respectively [93]. Another study supports that control of EndMT could be a valid strategy for plaque stabilization. Kovacic et al. examined histone post-translational modification during EndMT and found that histone deacetylation by HDAC9 is increased. Inhibition of class IIa HDAC family members, blocked-EndMT-mediated gene induction, and endothelial HDAC9 knockout reduced atherosclerosis and enhanced plaque stability [94]. OCT4, a Yamanaka factor, was recently found to be activated in ECs during atherogenesis and plays an important role in plaque stability. Endothelial deletion of OCT4 exacerbated atherosclerosis in ApoE-null mice plaque stability by regulating endothelial ABCG2 induction to control excessive hemes and ROS [95]. It has been observed that oscillatory shear stress enhances the activity of metalloproteinase (MMP), drives collagen degradation, and weakens fibrous caps, in addition to its role in vascular remodeling with matrix degradation [96,97]. A recent in vivo investigation on coronary arterial plaque clearly demonstrated that the low shear stress area was highly correlated with macrophages, cholesterol crystals, 18F-NaF activity, active microcalcification, and thin-cap fiberoatheroma (TCFA) thinning [98]. Proinflammatory signaling and anerobic metabolism are tightly correlated with atherosclerotic macrophages through the involvement of HIF-1 and PFKFB3. High PFKFB3 expression in mice is linked to an unstable plaque, whereas suppression of PFKFB3 activity stabilizes the plaque [99,100,101,102]. Many recent studies have proven that fluid dynamics and endothelial shear stress are the main pathological factors leading to vulnerable plaques [103].

Intimal calcification is associated with atherosclerosis. Unstable plaques are at risk of rupture due to the involvement of inflammation and microcalcification, which are controlled by wall shear stress [104,105,106,107]. Early proinflammatory and osteogenic cytokines from macrophages including TNF-𝛼, IGF-1, TGF-β, IL-1β, IL-6, and IL-8 induce differentiation of the vascular smooth muscle cells into osteoblast-like cells, initiating microcalcification [108]. TNF-𝛼 induces a reduction in BMPR2, enhancing BMP-9 for osteogenic differentiation in ECs, and induces calcification, thin fibrous cap formation, and plaque rupture [96,97,109,110,111]. Vascular calcification is inhibited by KLF2-mediated suppression of endothelial BMP/SMAD1/5 signaling resulting from laminar flow [112]. The metabolic change from OXPHOS to aerobic glycolysis and downregulation of PPAR-𝜰 resulting from enhanced WNT/β-catenin pathway activation in atherosclerotic lesions increases vascular calcification via lactate secretion. Inhibition of glycolysis by 3-PO, the PFKFB3 inhibitor, reduces EC differentiation and VSMC calcification and enhances cell survival [113,114,115,116]. Recent studies demonstrated that FGF21 reduces vascular calcification and improves vascular function by increasing antioxidant SOD and reducing oxidative stress [117,118].

The hallmarks of the devastating condition known as pulmonary arterial hypertension (PAH) are neointimal lesions, small vessel constriction, and large vessel stiffness. Pulmonary arterial hypertension occurs when most of the small arteries throughout the lungs narrow in diameter, which leads to increased pulmonary vascular resistance, subsequent right heart failure, and premature death [119]. Endothelial dysfunction is a key factor in the onset and development of artery remodeling and disease progression in pulmonary arterial hypertension (PAH) [120,121,122]. EC dysfunction can be caused by several factors, including inflammatory cytokines, hypoxia, toxins, and external stimuli such as shear stress. Among them, oscillatory shear stress, which leads to disturbed flow, fluid dynamics, and ongoing pathogenic mechanisms, has a substantial impact on PAH patients with neointimal lesion formation, leading to vessel remodeling, which increases pressure in the blood flow [119,123]. PAH begins with a malfunctioning EC, progresses with increased cell proliferation and decreased apoptosis, and finally matures with senescence, which makes PAH difficult to reverse. BMPR2 mutation is frequently found in familial PAH and idiopathic PAH patients. Endothelial BMPR2 controls TGF-β-dependent EndMT, and BMPR2 ligand BMP9 administration has been used to reverse PAH in a rat model. Pulmonary inflammation is believed to put patients with BMPR2 mutations at risk of PAH development [122]. Single-cell RNA sequencing of endothelial cells from a PAH mouse model showed that ECs upregulate genes in the MHC class II pathway, supporting the role of ECs in the inflammatory responses in PAH [124]. A recent report by Chan et al. indicated endothelial senescence is another factor driving PAH [125]. Induction of endothelial senescence was induced by blocking fraxacin (FXN) and mitochondria iron–sulfur (Fe–S) cluster assembly protein, and PAH patients showed reduced fraxacin expression and EC senescence. In addition, senolytic treatment prevented FXN-dependent PAH development in mice. Shyy et al. reported that the transcriptional coactivator MED1 is downregulated in PAH patients and animal models. MDE1 is associated with KLF4, an important transcriptional regulator in endothelial homeostasis, and mediates the expression of BMPR2, ETF, and TGFBR2 [126]. Several studies have demonstrated that disturbed blood flow initiates PAH with EC inflammation, barrier dysfunction, and smooth muscle cell migration through downregulation of eNOS / AKT pathway [127]. Metabolic alterations such as aerobic glycolysis are involved in the development of PAH. Inhibition of the glycolytic enzyme PFKFB3 inhibited PAH progression by reducing vascular remodeling, endothelial inflammation, and leukocyte recruitment [128,129,130]. BMPR2 mediated NOTCH1 activation [131] and increased the nuclear localization of HDAC4 and HDAC5, which are involved in the promotion of EC proliferation and a reduction in apoptosis in PAH ECs [132].

Sepsis is caused by uncontrolled immune responses to bacterial or viral infections, damaging host tissues. Endothelial proinflammatory phenotype changes are responsible for many sepsis-induced pathological events, including vascular permeability, increased coagulation, and hypoperfusion [133].

Sepsis-induced vascular leakage leads to fluid accumulation and multiorgan failure. A recent study on the ProCESS Trial showed that endothelial permeability markers (sFLT-1, Ang-2, and VEGF) were significantly lower in number in sepsis survivors [134]. Endotoxins or viral pathogens cause vascular leakage via EC junction disassembly or cell death. VE-cadherin is a key adherens junction protein, and its disengagement leads to paracellular permeability. Tyrosine phosphorylation of VE-cadherin induces its intracellular relocalization and loss of cell–cell adhesion. Inflammatory ligands activate Rho-dependent actin stress fiber formation and contractility and paracellular pore formation. Evidence shows that the blockade of endothelial permeability can be a valid strategy for decreasing sepsis mortality [135,136]. Recently, Okada et al. revealed a signaling pathway in ROBO4-dependent endothelial barrier stabilization using small-molecule screening [137]. Endothelial TGF-β-ALK5-Smad2/3 signaling led to ROBO4 expression, whereas BMP9-ALK1-Smad1/5 signaling blocked it, and the ALK1 inhibitor suppressed vascular hyperpermeability and mortality in COVID-19 mouse models. Lactate is known to be a sepsis biomarker, and its newly identified effect on the endothelial barrier has been reported. According to a report by Li et al., lactate disorganizes the VE-cadherin complex via VE-cadherin cleavage and internalization via calpain and Erk2 activation [138].

Sepsis is often associated with subclinical hypercoagulability or acute disseminated intravascular coagulation (DIC) accompanied by widespread microvascular thrombosis and consumption of platelets and coagulation proteins, leading to bleeding [139]. Sepsis induces a shift in the EC phenotype toward a procoagulant and anti-fibrinolytic status. Activated ECs, in the presence of proinflammatory molecules, express tissue factor (TF), which binds with circulating coagulation factor VII in the extrinsic pathway of the blood coagulation system. Activated ECs promote platelet adhesion and aggregation via the upregulation of various cell surface adhesion molecules [140]. During sepsis, PAI-1(plasminogen activator inhibitor-1) is released by ECs, leading to the inhibition of fibrinolysis and hypercoagulation.

Sepsis leads to high cardiac output and decreased peripheral resistance due to vessel dilation and hypotension. Inflammatory ligands cause increased NO synthesis from ECs and smooth muscle cell relaxation [141]. NO also reacts with ROS to form nitrate and nitrate-inducing cytotoxicities [142].

Proinflammatory stimuli such as diabetes, hypercholesterolemia, disturbed flow, ECM remodeling, and inflammatory cytokines induce activation of ECs and modification of phenotypes via the upregulation of inflammatory proteins and activation of several signaling pathways. This proinflammatory phenotype causes EC dysfunction, leading to weakening of the barrier, an increase in permeability, an increase in inflammation, EndoMT, senescence, etc, which is shown in Figure 2. These factors are the main causes of the development of most cardiovascular diseases.