key: cord-0750836-jj7ut09g
authors: Chernyak, B. V.; Popova, E. N.; Prikhodko, A. S.; Grebenchikov, O. A.; Zinovkina, L. A.; Zinovkin, R. A.
title: COVID-19 and Oxidative Stress
date: 2020-12-28
journal: Biochemistry (Mosc)
DOI: 10.1134/s0006297920120068
sha: 73e2ccde7923243be24b2cff36bb5e9a3b721aea
doc_id: 750836
cord_uid: jj7ut09g

Pathogenesis of the novel coronavirus infection COVID-19 is the subject of active research around the world. COVID-19 caused by the SARS-CoV-2 is a complex disease in which interaction of the virus with target cells, action of the immune system and the body’s systemic response to these events are closely intertwined. Many respiratory viral infections, including COVID-19, cause death of the infected cells, activation of innate immune response, and secretion of inflammatory cytokines. All these processes are associated with the development of oxidative stress, which makes an important contribution to pathogenesis of the viral infections. This review analyzes information on the oxidative stress associated with the infections caused by SARS-CoV-2 and other respiratory viruses. The review also focuses on involvement of the vascular endothelium in the COVID-19 pathogenesis.

In the majority of cases, the novel coronavirus SARS CoV 2 causes respiratory disease requiring no spe cial medical intervention, but up to 20% of COVID 19 patients require hospitalization [1] . Severe COVID 19 infection triggers imbalanced and uncontrolled cytokine response (called cytokine storm), exuberant endothelial inflammatory reactions, and vascular thrombosis. These and probably other, yet unknown factors may lead to the development of acute respiratory distress syndrome (ARDS), a major cause of death of the COVID 19 patients [2, 3] . The pathological changes in organs and tissues are probably triggered by an imbalanced host reac tion to the infection, e.g., excessive activation of immune and endothelial cells, and platelets [4] . Most likely, oxida tive stress accompanying cell activation may profoundly impact COVID 19 pathogenesis.

Different viruses employ diverse mechanisms to induce oxidative stress that was originally described in 1979 for the Sendai virus [5] . Since then, numerous data confirmed the development of oxidative stress in various viral infections including respiratory diseases [6] . Following influenza infection, an excessive amount of reactive oxygen species (ROS) is produced in various tis sues [7] , including alveolar epithelium [8] and endotheli um [9] . Oxidative stress is typical for infection of human respiratory syncytial virus [10] , rhinoviruses [11] , and many other viruses. Patients infected with the influenza virus have a high level of oxidized biomolecules such as DNA, lipids, and proteins [12 14] . Moreover, elevated ROS production, upregulated NO synthase 2 (NOS2) expression, and high level of nitrated proteins indicating developed oxidative and nitrosative stress were observed in the lung tissue samples from the deceased influenza patients [15] . Overall, virtually all patients with viral infections are affected by chronic oxidative stress [16] impacting the disease pathogenesis including impaired BIOCHEMISTRY (Moscow) Vol. 85 Nos. 12 13 2020 immune functions, apoptosis, inflammatory response, as well as organ and tissue dysfunction [16] .

At the same time, ROS induced by viral infection should not be considered solely as harmful agents, because ROS are necessary for eradicating viruses phagocytosed by immune cells and also take part in signal transduction between various immune cells [17, 18] . Pulmonary alveo lar macrophages usually produce ROS at relatively low levels to be used primarily for intracellular signaling.

Thus, the adequate response to viral infection must involve strictly maintained redox homeostasis. Its shift towards excessive ROS production results in the develop ment of oxidative stress followed by cell and tissue damage.

Multiple isoforms of NADPH oxidase (NOX) and xanthine oxidase represent the main sources of ROS in immune cells. These enzymes catalyze the synthesis of superoxide anion via one electron reduction of oxygen. NOX2 oxidase is active in immune cell phagosomes and is predominantly involved in the ROS production during infection caused by human influenza virus [19] , rhi noviruses [20] , as well as respiratory syncytial virus and Sendai virus [21] . Activity of the NOX isoforms is neces sary for pathogen eradication, but excessive ROS produc tion worsens the course of the disease. Inhibition of NOX2 activity reduces destructive inflammatory reactions in the lung tissue during influenza [22, 23] as well as decreases virus titer and inflammatory cell infiltration [24] .

Mitochondria represent one of the crucial ROS sources in non immune cells, particularly endothelial cells [25] . It was shown that neutrophils and macrophages contain relatively few mitochondria, but formation of the mitochondrial ROS (mtROS) is necessary for intracellu lar signaling upon inflammatory reactions in these cells [26 28 ].

There are several common traits between oxidative stress and the risk of severe COVID 19 infection. A num ber of major risk factors related to COVID 19 severity and mortality have been identified: older age, Black and South Asian ethnicity, being male, low socioeconomic status, hyperglycemia, and obesity [29] . All these factors are asso ciated with enhanced oxidative stress [30 34 ]. However, the correlation does not signify a cause effect relationship between the oxidative stress level and COVID 19 severity. It may be assumed that the elevated level of oxidative stress aggravates the severity of COVID 19, whereas antioxidant supplementation could reduce its severity [35] .

Viral pneumonia caused by SARS CoV 2 induces overactivation of immune response in the lung tissues affected by virus replication. This pathological process is nearly always accompanied by oxidative stress. SARS CoV 2 is capable of causing severe pneumonia by infecting type II pneumocytes. These cells contain a large number of mitochondria [36] synthesizing acetyl CoA to be used for production of fatty acids and phospholipids, which consti tute pulmonary surfactants on the surface of epithelial cells [37] . Until now, however, it remains unclear whether COVID 19 triggers oxidative stress in the airway epitheli um. Patients with moderate and severe COVID 19 often develop respiratory distress compensated by oxygen thera py that could cause oxidative stress and ARDS [38, 39] . It was shown that hyperoxia induces ROS generation in mitochondria [25] inhibiting oxidative phosphorylation and lowering ATP level [40] . Thus, targeted protection of the pulmonary cell mitochondria represents a promising approach to prevent hyperoxia related lung tissue damage.

It is believed that monocytes and macrophages play a crucial role in the inflammatory reactions accompanying severe COVID 19 infection [41] . These immune cells release large amounts of pro inflammatory cytokines (IL 1β, IL 6, TNF, IL 8), which is typical for critically ill COVID 19 patients [ 42 44] .

Until now, the exact viral factors that initiate strong inflammatory reactions in macrophages during COVID 19 infection remain unknown. At present, there are three non contradicting hypotheses concerning this phenome non.

Firstly, these reactions could be triggered by non structured viral proteins, such as coronavirus 3a protein. SARS CoV 1 3a protein activates NLRP3 inflamma some in macrophages, which is accompanied by IL 1β activation and increase in mtROS level [45] . It is highly likely that the SARS CoV 2 3a protein displaying 72% similarity with SARS CoV 1 homologue may act in the same way [46] . It should be mentioned that mtROS are required for activation of the NLRP3 inflammasome as demonstrated in numerous studies [28] .

Secondly, there is experimental evidence that the complex of IgG antibody with SARS CoV 2 S protein from COVID 19 patients induces hyper inflammatory response in macrophages [47] . The excessive pro inflam matory activity of this immune complex is associated with the altered glycosylation in the IgG Fc tail [47] .

Finally, it has been demonstrated recently that SARS CoV 2 can productively infect monocytes and stimulate production of the pro inflammatory cytokines IL 1β, IL 6, and TNF [48] . The authors suggest that the elevated mtROS production promotes both viral replica tion and monocyte activation. However, it remains unknown as to what extent oxidative stress is prominent in the macrophages derived from the COVID 19 patients.

Endothelium is a target of both SARS CoV 2 coro navirus itself and pro inflammatory cytokines released during the COVID 19 infection [3] . Adhesion molecules (ICAM1, VCAM1, E selectin) are expressed on the sur face of endothelial cells in response to cytokines, which contribute to leukocyte adhesion and penetration across the vascular wall into the body tissues followed by their damage during infection. The activated endothelial cells release pro inflammatory cytokines and chemokines that recruit immune cells into the site of inflammation. It was shown in the model of influenza infection that activation of the pulmonary capillary endothelium created a vicious cycle of amplified immune response, which determined development of the cytokine storm [49] . The authors demonstrated that this cytokine amplification loop could be suppressed by activating sphingosine 1 phosphate (S1P) signaling thus preventing the cytokine storm. However, it remains unexplored whether such a mecha nism operates in COVID 19 infection and whether S1P may serve as a therapeutic target.

Endothelial activation by pro inflammatory cytokines is accompanied by the enhanced endothelial permeability for macromolecules that may induce lung edema. Insulating properties of the microvascular endothelium are determined mainly by the intercellular VE cadherin based adherens junctions. Cytokine induced disassembly of these junctions has been thorough ly studied using TNF as a prototypic pro inflammatory cytokine. TNF downregulates VE cadherin expression [50] and induces proteolytic shedding of its extracellular domains responsible for adherens junction [51] . TNF can also stimulate phosphorylation of tyrosine in VE cadherin disrupting its contact with beta catenin and junction related functions [52] . Moreover, TNF triggers actin cytoskeleton remodeling that affects contacts between VE cadherin cytosolic domains and beta catenin [53] . Finally, TNF causes caspase activation that might be involved in the cell-cell junction disassembly. In particular, it was demonstrated that TNF triggers activation of caspase 3 in the rat pulmonary microvascular endothelium cells, actin filament remodeling, release of beta catenin from the junctional complexes, and increase permeability of the cell monolayer [54] . Moreover, ICAM1 and other adhesion molecules play an active role in the disruption of adhesive contacts [55] . Enhancing pulmonary endothelial perme ability depends on the destruction of the surface glycoca lyx layer, which could occur owing to the TNF dependent hyaluronidase activation [56] . Moreover, TNF also acti vates transcription factor NF κB that controls expression of matrix metalloproteinases (MMPs) able to cleave and activate diverse pro inflammatory cytokines (IL 1β, TNF, and TGFβ1), adhesion molecules, and glycocalyx [57] . Disruption of the glycocalyx not only results in enhanced endothelial permeability but also increases availability of adhesion molecules, thereby stimulating leukocyte and platelet attachment.

Increased ROS production plays an important role in the TNF dependent upregulated expression of the adhesion molecules and enhanced endothelial perme ability. The mechanism underlying TNF induced ROS generation remains obscure, but some studies with mito chondria targeted antioxidant SkQ1 revealed that the mtROS production largely accounts for endothelial TNF effects [27, 58 60] . In particular, it was found that SkQ1 at nanomolar concentrations prevented upregulated expression of adhesion molecules, VE cadherin proteol ysis by matrix metalloprotease 9 (MMP9), actin cytoskeleton remodeling, as well as apoptosis. We suggest that the suppression of excessive endothelial activation per se by SkQ1 protected mice from death after intra venous administration of TNF [60] . Similar role of mtROS was established in the study examining TNF dependent endothelial expression of the Receptor for Advanced Glycation End Products (RAGE) [61] . Both endothelial pro inflammatory activation as well as RAGE expression are driven by the transcription factor NF κB implicating that this signaling pathway is targeted by mtROS. This conclusion was additionally confirmed by experiments where SkQ1 inhibited TNF dependent NF κB activation [27, 60] . Moreover, NADPH oxidase activity also mediated TNF effects in endothelial cells along with mtROS [61, 62] . It should be noted that not all ROS dependent TNF effects are related to NF κB activation. For instance, it was shown that similar to other inflammatory mediators, TNF was able to enhance endothelial permeability by activating small guanosine triphosphatases (GTPases) Rho and Rac1 [63] . Moreover, it was found that ROS in endothelial cells modulated cell to cell adhesion triggered by permanent ly active Rac (Tat RacV12) GTPase [64] . Mitochondria are main ROS targets in apoptotic endothelial cell induced by high TNF concentrations, and also in the sit uation of subthreshold activation of caspases cleaving intracellular adhesive contact proteins caused by sub lethal TNF exposure [58, 65] .

Expression of interleukin 6 (IL 6), one of the major pro inflammatory cytokines, is controlled by NF κB and enhanced upon exposure to IL 1β and TNF. In some cases, TNF induced endothelial inflammatory activation is also coupled to IL 6 production [66] . Both cytokines cooperatively act in acute inflammation phase. TNF induced IL 6 expression depends on mtROS and can be suppressed by SkQ1 [60] that is likely due to prevention of NF κB activation. IL 6 expression is also upregulated by mtROS under hypoxic conditions [67] as well as after IL 4 exposure [68] .

Signaling cascades triggered by IL 6 rely on tyrosine phosphorylation within the common receptor subunit gp130 [69] and involve STAT3 activation by Janus kinase (Jak), Notch signaling, and MAP kinase cascade. The level of IL 6 receptor in endothelial cells is very low, and IL 6 activity is exerted via trans signaling involving solu ble IL 6 receptor that activates gp130 [70] . It was shown that IL 6 induced trans signaling stimulates endothelial BIOCHEMISTRY (Moscow) Vol. 85 Nos. 12 13 2020 mtROS production [71] . Moreover, IL 6 induces oxida tive stress via activation of NADPH oxidase [72, 73] . IL 6 stimulates endothelial ICAM 1 expression, but contrary to TNF this signaling pathway is mediated by tran scriptional activity of the phosphorylated STAT3 rather than NF κB [74] . IL 6 induced STAT3 activation also stimu lates MCP 1 expression that could contribute to transition from neutrophil to monocyte inflammation stage [75] . IL 6 stimulates enhanced endothelial monolayer permeability in vitro [76, 77] . All these IL 6 induced effects are mediat ed by oxidative stress. MAP kinase activation triggered only transient increase in cell permeability, whereas long term effects were related to Jak dependent STAT3 activation [77] . It was demonstrated in the murine model that IL 6 stimulated increased lung endothelium vascular permeabil ity caused by mechanical ventilation [78] . Such observation is of special importance with respect to the widely used arti ficial lung ventilation in treatment of COVID 19 patients.

Along with pro inflammatory effects, IL 6 also low ers endothelial NO production via STAT3 dependent downregulation of the endothelial NO synthase expres sion (eNOS) [79] and its inactivation [80] . Moreover, IL 6 triggered oxidative stress restricts NO bioavailability enhancing vasomotor endothelial dysfunction.

Vascular endothelial growth factor (VEGF) could play an important role in disruption of the pulmonary endothelium function. Endothelial VEGF expression is upregulated upon inflammation under hypoxic condi tions. In particular, VEGF is expressed at higher level fol lowing IL 6 exposure [81] . Moreover, VEGF secretion by vascular smooth muscle cells wall is also stimulated by angiotensin II [82] . Upregulated VEGF level could be a major cause of enhanced angiogenesis in the pulmonary lesions during COVID 19. It is known that VEGF could not only enhance angiogenesis, but could also increase vascular permeability [83] as well as platelet [84] , leuko cyte, and T cell [85] adhesion.

It is worth mentioning that there are some intrinsic cellular mechanisms countering inflammatory activation, elevated endothelial permeability, and thrombogenesis upon inflammation [86] . Some prostaglandins such as PGE2, PGI2, and PGA2 [87] as well as anti inflammato ry cytokines IL 10 and growth factor TGFβ [88] exhibit protective effects. Activated C protein and thrombomod ulin protect endothelial cells from platelet triggered damage [89] .

Thus, proinflammatory cytokines IL 6 and TNF as well as VEGF may cooperatively induce endothelial dam age upon COVID 19. It can be suggested that along with the anti cytokine therapy widely used in COVID 19 treatment, the agents blocking VEGF and relevant signal ing cascades may be of clinical interest. Taking into con sideration that signaling pathways initiated by IL 6, TNF, and ATII depend on ROS and mtROS production, it seems promising to develop and test antioxidants includ ing those targeting mitochondria.

The first brief report on endothelium damage during COVID 19 was based on the data provided by anatomical pathology examination of three deceased patients [3] . In particular, signs of the SARS CoV 2 endothelial cell infection were found not only in the pulmonary vessels, but also in heart and other organs. Moreover, inflamma tory endothelial lesions presumably related to cytokine storm were observed, which were confirmed in subse quent investigations [90] .

Endothelial damage upon COVID 19 infection included disrupted cell-cell junctions, swelling, and lost contacts with the basal membrane [90] . In many cases they were found together with signs of thrombosis. Obvi ous disruption of the blood clotting system pointed at high probability of developing pulmonary thromboem bolism and deep vein thrombosis. At the same time, it was observed that vascular endothelium had markedly elevat ed levels of angiotensin converting enzyme 2 (ACE2) that acted as an entry receptor for SARS CoV 2 penetration into the cell. It was accompanied by the substantial increase in angiogenesis. In particular, Ackermann et al. [90] demonstrated that pulmonary angiogenesis during COVID 19 was increased by 2.7 fold in comparison with the one in patients died from consequences of the influenza virus A infection. It was suggested that the mechanism behind this phenomenon was based on acti vated intussusceptive angiogenesis typical to normal development, wound healing, and diverse pathologies [91] . During COVID 19, angiogenesis seems to result from the endothelium damage and hypoxia within the lung injury foci. Elevated ACE2 levels may be related to enhanced angiogenesis or its compensatory expression occurring after virus mediated blockade of its enzymatic activity, which awaits further investigation (hereinafter the data available in open access on July 20, 2020 are pre sented). It should be noted that the direct virus related endothelium damage is a crucial component in pathogen esis of the influenza virus infection as well [92] , but inten sity of this damage is profoundly lower and it rarely caused severe consequences in comparison with the infection caused by SARS CoV 2.

Interaction of the lung capillary endothelium with SARS CoV 2 could occur at early infection stage pre suming that potentially it might exit into the bloodstream without disrupting alveolar epithelial cells in close prox imity to the capillary endothelium. In the later stages of infection, massive release of the viruses to the blood flow from the large number of destroyed alveolar cells could cause infection of endothelial cells in other vessels. Even without invading sensitive cells, virus may induce some endothelial response by binding to ACE2 and suppressing its proteolytic activity. ACE2 (zinc metalloprotease) cleaves peptide hormone angiotensin II (ATII), which exerts multiple functions including inducing vasocon striction causing high blood pressure. Peptides formed due to ATII cleavage may stimulate signaling counteract ing ATII. While ACE2 activity declines after binding to viral S protein, ATII level may be markedly elevated in pulmonary capillaries. Local accumulation of ATII in the lungs was experimentally demonstrated in the bleomycin induced pulmonary fibrosis model [93] . In this model alveolar epithelial cells and pulmonary myofibroblasts were the main sources of ATII. Systemically elevated ATII in the blood flow during COVID19 seems highly unlikely because high viremia has not been observed even in the severe disease forms, whereas ACE2 is an ubiqui tous protein. Up to now, the elevated serum ATII level during COVID 19 infection was reported only in a single study [94] .

AT1R serves as a major ATII receptor that activates multilayered signaling in endothelial cells including MAP kinase axis, protein kinase C, as well as transcrip tion factor NF κB resulting in activation of NOX2, expression of cytokines, adhesion molecules, and cyclooxygenase 2 (COX2) [95] . Endothelial NOX2 acts as a major source of ROS production, which turned out to be required for AT1R downstream signaling [96] . It was found by Dikalov et al. [97, 98] that stimulation of ATII resulted in the elevated mtROS production that further enhanced NOX2 activity. Experiments with mitochondrial inhibitors and mitochondria targeted antioxidants revealed that the reduced mtROS produc tion suppressed AT1R signaling. The question regarding the cause of the increased mtROS production still remains open. Dikalov et al. demonstrated that the ATII induced signaling was lower in the cyclophilin D (CypD) knockout mice [99] . This mitochondrial pro tein serves as a regulatory component of the mitochon drial permeability transition pore (mPTP), opening of which may result in the increased mtROS level [100] . ROS produced by NOX2 may be one of the causes for pore opening. Thus, ATII initiates positive feedback loop leading to oxidative stress and endothelial dysfunc tion ( figure) .

ATII induced endothelial activation may occur cooperatively with pro inflammatory cytokines. In par ticular, IL 6 stimulates AT1R expression and ATII dependent signaling that result in the further enhance ment of oxidative stress and endothelial dysfunction Angiotensin II (ATII) interacts with AT1R receptor and induces ROS production via NADPH oxidase (NOX) in endothelial cells triggering mitochondrial oxidative stress and endothelial dysfunction. It is believed that SARS CoV 2 S protein binds to ACE2 causing its subsequent local or systemic depletion of this enzyme that cleaves ATII, which, in turn, increased ATII level. BIOCHEMISTRY (Moscow) Vol. 85 Nos. 12 13 2020 [101] . IL6 knockout mice exhibit lower endothelial dys function caused by administration of ATII [73] .

Furthermore, in the acute lung failure model caused by acid aspiration or with bacterial wall lipopolysaccha ride ACE2 knockout mice exhibited significantly more substantial tissue damage, whereas recombinant ACE2 or AT1R inhibitor protected from lung injury [102] . Upregulated expression of adhesion molecules, increased secretion of pro inflammatory cytokines and chemo kines, as well as elevated permeability result in acute inflammatory reaction in endothelium in the case of increase of the normal ATII level. Moreover, it is likely to enhance platelet deposition and release of the von Willebrand blood clotting factor that may account for one of causes of developing thrombosis [103] . Under normal conditions, impact of ATII on thrombogenesis seems to be insignificant [104] , but during COVID 19 such effect could be likely. In particular, the pro inflammatory cytokines may activate platelets in COVID 19 patients that presumably promotes thrombogenesis [105] .

Along with endothelial activation, ATII causes secretion of the pro inflammatory cytokines in alveolar epithelium [106] and changes in the alveolar fluid clear ance associated with inactivation of Na + channels [107] . Moreover, ATII induces alveolar epithelial mesenchymal transition that causes enhanced epithelial permeability and pulmonary edema [108] . Finally, high ATII levels trigger epithelial cell apoptosis [109] .

It should be noted that the hypothesized role of ATII in COVID 19 pathogenesis has been proposed repeatedly (e.g., see [110] ), but, however, has not been confirmed experimentally. Drugs blocking ATII production or AT1R signaling are commonly used in treating blood hyperten sion. Large scale study conducted by the New York University (NYU) Grossman School of Medicine [111] revealed that taking these drugs does not affect likelihood of infection or risk of severe COVID 19. Using recombi nant soluble ACE2 in vitro substantially lowered infection with SARS CoV 2 owing to competition with the natural ACE2 for virus binding [112] . It can be suggested that such protein would lower ATII level and prevent develop ing COVID 19 infection.

At present, no experimental data allowing to assess effects of infection of endothelial cells with SARS CoV 2 have been obtained. Studies examining interactions between the SARS CoV 1 and epithelial cells started in 2004 [113] were put on hold. No this type of studies were conducted for the MERS coronavirus. Only few studies aimed at investigating impaired endothelial function after infection with influenza A virus have been reported. In particular, it was shown that this virus in murine model lowered the level of endothelial Krüppel like Factor 2 (KLF2) [114] . This factor restricts inflammatory activa tion of endothelium, prevents disruption of permeability and development of atherosclerosis [115] . Interestingly, that Nrf2 is one of the targets of KLF2 in endothelium [116] , therefore it is likely that its activation explains pro tective effects of KLF2. THERAPY IN COVID 19 Currently, there are multiple trials underway that test antioxidants as therapeutic agents in COVID 19 (https:// clinicaltrials.gov/), but no results were available at the time of preparing current review. Nonetheless, antioxidants such NAC have been already included into the clinical pro tocols for treating moderate and severe COVID 19 [117] .

Most notably, use of antioxidants seems reasonable at the stage requiring inhibition of inflammatory reac tions during COVID 19. It is expected that such therapy may prevent organ and tissue damage due to cytokine storm and oxidative stress [118, 119] .

Furthermore, lowering oxidative stress by antioxi dants may result in the decreased viral load. Recent study with peripheral blood monocytes purified from the healthy volunteers demonstrated that SARS CoV 2 replication was suppressed by NAC and mitochondria targeted antioxidant MitoQ [48] allowing to assume that the decreased ROS levels prevent HIF1 α activation and subsequent metabolic switch to glycolysis necessary for coronavirus replication. However, this hypothesis requires further investigation.

Furthermore, To et al. [120] proposed another way of using mitochondria targeted antioxidant MitoTEMPO in the their study investigating its preventive and thera peutic action in murine model of H3N2 influenza virus infection. In particular, they found that intranasally administered MitoTEMPO decreased mouse mortality, virus titer, as well as lowered airway tract inflammation and decreased neutrophil infiltration. Precise antiviral mechanisms exerted by MitoTEMPO remain obscure; it is likely that the decrease of mtROS results in downregu lated expression of the cell cell adherens junctions, which explains decreased immune cell infiltration as well as reduced activity of NLRP3 inflammasome that produces IL 1β. The decline in virus titer could be explained by the elevated amount of antiviral interferon IFN 1β, which, however, was assessed solely at mRNA rather than protein level. It must be mentioned that the use of MitoTEMPO did not compromise adaptive immune response induced by pulmonary dendritic cells and did not affect popula tion of the lung B and T cell involved in humoral and cel lular immunity [120] .

Potentially, antioxidants may influence thrombogen esis, which is a common and dangerous complication during COVID 19. Cytokine storm may result in the ROS dependent apoptosis in endothelial cells [121] , whereas SkQ1 mediated mtROS decline prevents TNF induced apoptosis in vitro [65] . Lowering endothelial cell death could also prevent activation of thrombogenesis. BIOCHEMISTRY (Moscow) Vol. 85 Nos. 12 13 2020 Another approach in the fight against oxidative stress during COVID 19 involves induction of endogenous antioxidant systems. For instance, the transcription fac tor Nrf2 controls expression of antioxidant and other cell defense systems. Experiments with mice treated with ATII for 14 days demonstrated that Nrf2 activation with the help of tert butylhydroquinone lowered ROS level as well as decreased microvascular endothelial dysfunction and hypertension [122] . Similar data were also obtained in vitro in small artery derived endothelial cells by acti vating Nrf2 with sulforaphane [123] . The use of Nrf2 acti vators for COVID 19 therapy is discussed in more detail elsewhere [124] .

Thus, the use of mitochondria targeted antioxidants looks as a promising approach to lower oxidative stress and accompanying complications in viral infections. Further experiments with animal models and clinical tri als are necessary to reveal therapeutic potential for such approach.

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WNT/β catenin signaling induces IL 1β expression by alveolar epithelial cells in pulmonary fibrosis

The effect of endogenous angiotensin II on alveolar fluid clearance in rats with acute lung injury

Differential susceptibility to epithelial mesenchymal tran sition (EMT) of alveolar, bronchial and intestinal epithe lial cells in vitro and the effect of angiotensin II receptor inhibition

Angiotensin II induces apoptosis in human and rat alveolar epithelial cells

A hypothesis for patho biology and treatment of COVID 19: the centrality of ACE1/ACE2 imbalance

Renin angiotensin aldosterone system inhibitors and Rrsk of Covid 19

Inhibition of SARS CoV 2 infections in engineered human tissues using clinical grade soluble human ACE2

Tissue distribution of ACE2 protein, the functional receptor for SARS coron avirus. A first step in understanding SARS pathogenesis

Experimental lung injury reduces Krüppel like factor 2 to increase endothelial permeability via regulation of RAPGEF3 Rac1 signaling

KLF2 in regulation of NF κB mediated immune cell function and inflammation

KLF2 primes the antioxi dant transcription factor Nrf2 for activation in endothelial cells

Therapeutic blockade of inflammation in severe COVID 19 infection with intravenous n acetylcysteine

N acetyl cysteine may prevent COVID 19 associated cytokine storm and acute respiratory distress syndrome

Acetylcysteine: A poten tial therapeutic agent for SARS CoV 2

Mitochondrial reactive oxygen species con tribute to pathological inflammation during influenza A virus infection in mice

The role of endothelial cell apoptosis in inflammatory and immune diseases

NRF2 prevents hypertension, increased ADMA, microvascular oxidative stress, and dysfunction in mice with two weeks of ANG II infusion

Downregulation of nuclear factor erythroid 2 related factor and associated antioxi dant genes contributes to redox sensitive vascular dysfunc tion in hypertension

Trans cription factor Nrf2 as a potential therapeutic target for prevention of cytokine storm in COVID 19 patients