Endothelial function. The healthy endothelium serves diverse biologic functions, including 1) serving as a semipermeable barrier and regulating substance exchange/transport and innate immunity; 2) regulation of vascular tone by balancing the production of vasodilators [such as NO, PGI₂, H₂S, and endothelium-derived hyperpolarizing factor (EDHF)] and vasoconstrictors (such as ET-1 and Ang-II); 3) mediating mechanotransduction via mechanosensors/mechanosensitive complexes and gene expression via transcription factors KLF2/KLF4/Nrf2, leading to increased NO and H₂S production; 4) repairing vascular injury by accelerating re-endothelialization and with the assistance of VEGF, NO, and other growth factors (GFs); 5) regulating hemostasis by secreting antiplatelet and anticoagulant molecules such as NO, tissue plasminogen activator (t-PA), urokinase-type plasminogen activator (u-PA), tissue factor pathway inhibitor (TFPI), and thrombomodulin (Thbd) while reducing TF and PAI-1; 6) regulating angiogenesis via producing factors, including, VEGF, NO, MMPs/tissue inhibitor metalloproteinase (TIMPs), and triggering endothelial proliferation, migration, and tube formation; and 7) serving as an important cell type for the metabolism of glucose, lipids, amino acids, fatty acids, ATP, lactate, and acetyl-CoA and maintaining oxygen and nutrient supply to all tissues in the body. The glycocalyx microstructure is involved in regulating multiple aspects of endothelial function as depicted.

Multiple layers of endothelial dysfunction. Endothelial dysfunction is characterized by a shift from executing physiologic functions of the vascular endothelium to inflammation, hyperpermeability, leukocyte adhesion, eNOS uncoupling, altered endothelial cell metabolism, oxidative stress, vasoconstriction, injury and cell death, senescence, and EndoMT. Endothelial dysfunction is more of a composite of these dysfunctional aspects, with impaired vasodilatation being one important layer of endothelial dysfunction.

Triggers of endothelial dysfunction. There are different factors that can trigger endothelial dysfunction: 1) lipid [via modified LDL, dysfunctional high-density lipoprotein (HDL), lysophosphatidylcholine (LPC), oxidized phosphatidylcholine (OxPC), and cholesterol crystal]; 2) inflammation [by proinflammatory cytokines IL-1β, IL-6, TNF-α, and high-sensitivity c-reactive protein, LPS or virus infection, high glucose (HG)/methylglyoxal (MGO)/AGE, Hcy, and disturbed flow]; 3) oxidative stress including H₂O₂, superoxide, mitochondrial ROS, hyperglycemia-associated stimuli, and disturbed flow; 4) different drugs such as anti–human immunodeficiency virus (HIV) drugs, anticancer agent doxorubicin, and vascular endothelial growth factor (VEGF) inhibitors may also trigger endothelial dysfunction; 5) other risk factors associated with endothelial dysfunction include aging, smoking, physical inactivity, unhealthy diet, irradiation, air pollution (i.e., PM2.5), and psychologic stress etc.

Endothelial dysfunction in CVD and beyond. Endothelial dysfunction is generally associated with many disease conditions due to imbalanced production of vasodilators and vasoconstrictors, or many other factors. Major disease conditions associated with endothelial dysfunction include atherosclerosis, hypertension and stroke, diabetes, peripheral arterial disease, metabolic syndrome (obesity, insulin resistance), chronic kidney disease, pre-eclampsia, recently reported COVID-19, and many others (not depicted here).

Role of endothelial dysfunction in atherosclerosis. Oxidative stress, inflammation, and other risk factors play an important role in endothelial cell activation. The resulting endothelial cell injury/multiple modes of cell death, hyperpermeability, and increased expression of various adhesion molecules (such as ICAM-1, VCAM-1, MCP-1, E-selectin) and KLF2 downregulation attract leukocyte adherence to the activated endothelium and penetrate the arterial wall to initiate atherosclerosis. A reduction in endothelium-derived NO production or increased eNOS uncoupling [increased ROS, reactive nitrogen species (RNS), and ADMA and decreased DHFR, BH4/dihydrobiopterin (BH2), and GCH1] occurs as one major cause of endothelial phenotypical alterations presented as endothelium-dependent vasoconstriction [increased levels of ET-1, Ang-II, and TxA₂ and decreased levels of NO, PGI₂, and endothelium-derived hyperpolarizing factor (EDHF)] and leads to the progression of atherosclerosis. Atherorelevant stimuli also cause endothelial cell senescence (evidenced by increased senescence-associated β-galactosidase (β-gal) staining and PAI-1 secretion and decreased activity of sirtuins) and EndoMT [evidenced by increased markers of mesenchymal cells (α-SMA, vimentin, FSP1, and Snail, etc.] and decreased endothelial markers (CD31 and VE-cadherin). In addition, the exposure of endothelial cells to persistent proatherogenic stimuli further causes impaired inflammation resolution, atherothrombosis, and intraplaque hemorrhage, which lead to plaque rupture. Genes in yellow denotes that the expression of these genes was downregulated during atheroprogression. SA-β-gal, senescence-associated beta-galactosidase; TFPI, tissue factor pathway inhibitor; TM, thrombomodulin; tPA, tissue plasminogen activator; uPA, urokinase-type plasminogen activator.

Role of endothelial dysfunction in hypertension. The vascular endothelium dysfunction resulting from ROS generation, decreased NO bioavailability, inflammation, senescence, and EndoMT and may cause hypertension. Endothelial dysfunction contributes to hypertension via different mechanisms, including altered vascular tone, sympathetic tone, and Na⁺ retention, which is followed by increased aortic stiffness, vascular remodeling, and vessel resistance; these events collectively lead to hypertension. Measurement of pulse wave velocity, pulse pressure, or volume expansion are frequently used as clinical assessments of the impact of chronic hypertension.

Endothelial dysfunction caused by SARS-CoV-2 infection. SARS-CoV-2 enters host cells by binding to ACE2 on the cell membrane (left), which is subsequently endocytosed, potentially causing endothelitis and endothelial dysfunction. Virus infection leads to immune activation and increases the production of proinflammatory cytokines (accompanying cytokine storm), in particular IL-6, TNF-α, IL-1β, MCP-1, and PAI-1, that ultimately damage endothelial cells. Cytokine release and immune cell activation after SARS-CoV-2 infection lead to endothelial activation at multiple layers, including hyperpermeability, oxidative stress, inflammation, leukocyte infiltration, coagulation and thrombosis, vasoconstriction, NET production, apoptosis/pyroptosis, and abnormal angiogenesis. Moreover, because of the crosstalk among different layers of endothelial dysfunction, it is plausible that abnormal endothelial cell metabolism (glycolysis, fatty acid oxidation, oxidative phosphorylation, etc.), EndoMT, and inflammasome activation may also be related to endothelial dysfunction in COVID-19. Several known risk factors associated with endothelial dysfunction in COVID-19 include aging, male gender, other metabolic disorders, and comorbidities like diabetes, hypertension, and CVD. EC, endothelial cells; ED, endothelial dysfunction.

Potential therapies for COVID-19–induced endothelial dysfunction. SARS-CoV-2 infection with endothelial cells causes a myriad of events, including apoptosis, coagulation/thrombosis, inflammation and vascular leakage. Several drugs that exhibit protective effects against endothelial dysfunction in COVID-19 include statins, vitamin C (Vit C), anticoagulants, tocilizumab, dexamethasone, and SGLT2i (function mechanistically by inhibiting ROS, apoptosis, cytokine storm, inflammation, and thrombosis). Adrecizumab may exert its effects via inhibition of vascular and capillary leakage during COVID-19.