Phytochemicals: Potential Therapeutic Interventions Against Coronavirus-Associated Lung Injury

Since the outbreak of coronavirus disease 2019 (COVID-19) in December 2019, millions of people have been infected and died worldwide. However, no drug has been approved for the treatment of this disease and its complications, which urges the need for finding novel therapeutic agents to combat. Among the complications due to COVID-19, lung injury has attained special attention. Besides, phytochemicals have shown prominent anti-inflammatory effects and thus possess significant effects in reducing lung injury caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Also, the prevailing evidence reveales the antiviral effects of those phytochemicals, including anti-SARS-CoV activity, which could pave the road in providing suitable lead compounds in the treatment of COVID-19. In the present study, candidate phytochemicals and related mechanisms of action have been shown in the treatment/protection of lung injuries induced by various methods. In terms of pharmacological mechanism, phytochemicals have shown potential inhibitory effects on inflammatory and oxidative pathways/mediators, involved in the pathogenesis of lung injury during COVID-19 infection. Also, a brief overview of phytochemicals with anti-SARS-CoV-2 compounds has been presented.


INTRODUCTION
The complex pathophysiological mechanisms behind viral diseases, along with the associated side effects of the present conventional drugs, urge the need for introducing alternative treatments. Among viral infections, severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome (MERS-CoV), and the newest human CoVs (HCoVs) associated with the outbreak of coronavirus disease 2019-SARS-CoV-2 (COVID-19) have caused acute respiratory distress syndrome . Based on the pathological findings, the inflammatory cytokines/signaling pathways lead to pulmonary edema and, ultimately, lung injury in COVID-19 patients (Merad and Martin, 2020). Considering their potential effects in targeting several dysregulated mediators, phytochemicals could be auspicious agents in the treatment/ management of various diseases (Mani et al., 2020). The medicinal plants and phytochemicals target multiple proinflammatory and oxidative mediators such as tumor necrosis factor-α (TNF-α), interleukin-(IL-) 1β, IL-6, IL-8, matrix metalloproteinases (MMPs), nuclear factor-kappa B (NF-κB), mitogen-activated protein kinase (MAPK), cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), and reactive oxygen species (ROS). Therefore, owing to the involvement of inflammation and oxidative stress in the pathogenesis of lung injury, phytochemicals have attracted particular attention to providing novel agents in combating coronaviruses and related complications (Bellik et al., 2012;Cornélio Favarin et al., 2013b). This article presents an overview of phytochemicals, including alkaloids, coumarins, polyphenols, especially flavonoids, quinones, and terpenes to show noticeable effects against lung injury. Therefore, they could be introduced as ameliorative agents against SARS-CoV-2-induced lung injury. Moreover, based on their simultaneous antiviral and preventive effects against lung injury, some phytochemicals such as matrine, cepharanthine, osthole, wogonin, myricetin, and triptolide have been also provided as promising candidates in the management of COVID-19. In general, this review article aims to introduce phytochemicals as potential therapeutic agents against coronavirus complications, focusing on lung injury.

CORONAVIRUSES AND PATHOGENESIS: FOCUSING ON LUNG INJURY
In striking contrast to the history of HCoVs, as relatively harmless respiratory pathogens, the outbreak of SARS and the emergence of MERS pose the CoVs as important pathogens in respiratory tract infections. SARS-CoV, MERS-CoV, and SARS-CoV-2 can cause clinical complications leading to severe diseases presented as acute respiratory distress syndrome (ARDS) (Yin and Wunderink, 2018). HCoVs contain single-stranded, polycistronic RNA genomes of positive polarity (∼30 kb). These viral genomes are translated into multiple nonstructural proteins (ORF1a and ORF1b), structural proteins (S, E, M, and N), and lineage-specific accessory proteins showing differences in these viruses. For instance, in the case of SARS-CoV, accessory proteins include ORF3a, ORF3b, ORF6, ORF7a, ORF7b, ORF8a, ORF8b, and ORF9b (Fung et al., 2020).
The most common clinical symptoms in SARS-CoV-2 include fever, cough, dyspnea, fatigue, headache, myalgia, and diarrhea. Some patients afterward suffer from shortness of breath and recurrent or ongoing fever. In nearly 13% of patients, intense care treatment (e.g., mechanical ventilation) should be applied Wang D. et al., 2020). The pathobiology of SARS-CoV-2 and related molecular mechanisms behind the coronavirus-associated lung injury are not yet completely understood; however, the role of some key molecular intermediates are not deniable (Marini and Gattinoni, 2020). Among those signaling mediators, TNF-α, IL-1, IL-6, IL-8, and IL-1β, NF-κB, MMPs, MAPK, and COX-2 seem to play critical roles in the pathogenesis of COVID-19 and associated lung injury (Fakhri et al., 2020b;Merad and Martin, 2020). In terms of ROS, iNOS, as well as nuclear factor erythroid 2-like 2 (Nrf2), autophagy-related molecules (LC-3II, Atg5, and Beclin1), and Janus kinase-signal transducers and activator of transcription (JAK/STATs) pathway have shown an important role (Seif et al., 2017). From the other point of view, the extracellular signal-regulated kinase (ERK) and protein kinase B (Akt) signaling pathways are of the other dysregulated mediators following lung injury (Mo et al., 2014;Tsai et al., 2015;Jin et al., 2018). In COVID-19 patients, angiotensin-converting enzyme 2 (ACE2) receptor, located on alveolar epithelial cells, has attracted growing attention, as a high-affinity receptor and cotransporter for SARS-CoV-2 entrance to the lung (South et al., 2020;Ziai et al., 2020). Dysregulation of ACE2/Ag (1-7)/Mas receptor and ACE1/Ag II/Ag II type 1 receptor pathways could enhance ACE2, thereby increasing the chances of the viral entry (Rico-Mesa et al., 2020;South et al., 2020). Besides, the dysregulation of ACE2 by SAR-CoV-2 infection inhibits the degradation of Ag II into angiotensin (Ag) (1-7), exacerbates inflammation, and leads to vascular permeability, as well as cardiovascular/lung complications South et al., 2020). Based on the pathological findings, an edematous lung with increased weight was also observed in this disease (Ding et al., 2003;Nicholls et al., 2003). Large multinucleated cells (macrophages and pneumocytes) and atypical enlarged pneumocytes comprise large nuclei, prominent nucleoli, and amphophilic granular cytoplasm, which have often been observed in the lungs of SARS patients. However, none of these signs can be considered as a unique feature of SARS-related pathology. The other pathological features usually observed in SARS include squamous metaplasia of bronchial and alveolar epithelial cells; cilia loss of bronchiolar epithelial cells; subpleural multiplication of fibrogranulative tissue in small airways and airspaces; vascular injury hemophagocytosis in residing mononuclear cells in pulmonary tissue; and apoptosis in epithelial cells, lymphocytes, monocytes/macrophages, and pneumocytes (Gu and Korteweg, 2007). Apart from a respiratory infection, gastrointestinal and central nervous system (CNS) infection was also reported in some patients suffering from SARS (Fung et al., 2020). Additionally, in most SARS autopsies, both extensive necrosis of the spleen and atrophy of the white pulp were reported. Reduction of CD4 + , CD8 + , and CD20 + lymphocytes, dendritic cells, macrophages, and natural killer cells residing in the spleen, as well as atrophy and decrement of the lymph nodes lymphocytes, were often observed. The presence of SARS-CoV was also confirmed in circulating monocytes and T lymphocytes and to some degree in B lymphocytes and natural killer cells (Chong et al., 2004;Gu et al., 2005). The liver is another organ that is affected during the course of this disease. For example, the increment of serum alanine aminotransferase level in SARS patients was associated with some adverse outcomes. Besides, hemophagocytosis or bone marrow hypoplasia, destruction of epithelial cells in the thyroid glands, myofiber necrosis and atrophy of skeletal muscle tissue, and necrosis and vacuities of the adrenal medulla can occur in some SARS patients (Gu and Korteweg, 2007).

PHYTOCHEMICALS AS POTENTIAL AGENTS FOR CORONAVIRUS-ASSOCIATED LUNG INJURY
Medicinal plants and isolated phytochemicals can cover multiple therapeutic targets at the same time and lie in the fact that they are widely used in the treatment of various diseases, including viral diseases and related complications. Since infection with any of the viruses of the Coronaviridae family, including SARS-CoV-2, can cause severe damage to the pulmonary system (Ding et al., 2003), the plant-derived secondary metabolites can play a significant role in reducing these pulmonary complications. The phytochemicals with different molecular targets and signaling mechanisms, including reducing proinflammatory and oxidant mediators such as TNF-α, IL-1, IL-6, IL-8, IL-1β, NF-κB, MMPs, iNOS, MAPK, COX-2, and ROS, minimize lung injury. Therefore, protective effects on lung injury, along with other effects, including antiviral (especially anti-CoVs) effects, have attracted the attention of many researchers on the use of phytochemicals as potential strategies for discovering new anti-CoV agents regarding controlling related complication (Bellik et al., 2012;Cornélio Favarin et al., 2013b).

Alkaloids
Alkaloids are one of the largest classes of natural products that are mainly found in several plant families such as Solanaceae, Ranunculaceae, Rubiaceae, Papaveraceae, Amaryllidaceae, and Fabaceae. The main feature of this group is the presence of the nitrogen atom in their structure (Yang and Stöckigt, 2010). Several studies showed that alkaloids have the potential of reducing lung injury induced by different methods. Sinomenine ( Figure 2) is an isoquinoline alkaloid that is isolated from the stem and rhizome of Sinomenium acutum (Thunb.) Rehder & E.H.Wilson (Menispermaceae). It reduced the lung injury induced by lipopolysaccharides (LPS) and Escherichia coli, via regulation of inflammatory signaling pathway, including the downregulation of IL-6, IL-1β, TNF-α, NF-κB, iNOS, and COX-2 and upregulation of the protective antiinflammatory adenosine A2A receptor. Sinomenine also inhibited oxidative stress markers, including the increase of the superoxide dismutase (SOD) and the decrease of the malondialdehyde (MDA) . Besides, sinomenine [100 mg/kg, intraperitoneally (i.p.)] upregulated the expression of Nrf2 and autophagy-related molecules (LC-3II, Atg5, and Beclin1), as critical mediators in increasing cell resistance against oxidative stress and inflammation, 1 h after inducing lung injury by LPS (8 mg/kg) in mice. Moreover, lung wet/dry (W/D) ratio, pulmonary edema, and the protein leakage into bronchoalveolar lavage fluid (BALF), as the pathological markers of lung injury, were decreased by sinomenine . In addition, six isosteroid alkaloids (imperialine, verticinone, verticine, imperialine-3-β-Dglucoside, delavine, and peimisine) and total alkaloid extraction isolated from bulbs of Fritillaria cirrhosa D.Don (Liliaceae) showed the protective effects on lung injury, induced by LPS and cigarette smoke, increase the expression of Nrf2 and heme oxygenase (HO-1), and reduce ROS production, IL-6, and TNF-α expression in vivo and in vitro Liu S. et al., 2020).
Toll-like receptor 4 (TLR4) is an inflammatory signaling pathway whose expression is increased in acute lung injury (Yang H.-Z. et al., 2012). Sophocarpine (25 and 50 mg/kg, i.p.), quinolizidine alkaloid isolated from the seeds of Sophora alopecuroides L. (Fabaceae), reduced LPS-induced lung injury in mice by the inhibition of TLR4 expression . Zhang et al. reported that tabersonine, as a monoterpenoid indole alkaloid isolated from the root of Catharanthus roseus (L.) G.Don (Apocynaceae), has shown a protective effect on lung injury induced by LPS in vivo (20 mg/kg, i.p.) and in vitro (mouse bone marrowderived macrophages, 10 µM). This study showed that tabersonine decreased the expression of TNF receptor-associated factor 6 (TRAF6) and thereby blocked p38MAPK-activated protein kinase 2 (MAPK/MK2) and NF-κB activities. The amelioration of the aforementioned signaling pathways/mediators leads to the inhibition of proinflammatory mediators and the reduction of pathological indices of lung injury such as total protein concentrations in BALF ameliorated lung injury .
Berberine, an isoquinoline alkaloid isolated from different species such as Berberis vulgaris L. (Berberidaceae) and Coptis chinensis Franch. (Ranunculaceae), has indicated protective effects on LPS-induced lung injury via activating Nrf2 and increasing the expression of HO-1 in C57BL/6 mice at 10 mg/kg (i.p., 24 and 2 h before injection of LPS, 2.5 mg/kg), as well as the in vitro manner on the human bronchial epithelial cell line at 5 and 10 μM concentrations. Berberine also reduced the pulmonary edema and the protein leakage into BALF of mice (Liang et al., 2019).

Coumarins
Coumarins are the heterocyclic phytochemicals with 2H-1benzopyran-2-one chemical structure. The Apiaceae is one of the greatest plant families that coumarins are isolated from its species (Ribeiro and Kaplan, 2002). Anti-inflammatory and antioxidant properties are two prominent effects of coumarins along with other pharmacological and biological activities such as cytotoxic and anticancer, antiviral, antiangiogenic, anticoagulant, edema-protective, and anxiolytic effect (Fylaktakidou et al., 2004;Venugopala et al., 2013;Srikrishna et al., 2018). The downregulation of inflammatory mediators, including NF-κB, TNF-α, iNOS, and MAPKs pathway, and inhibiting oxidative factors such as ROS and free radicals are critical mechanisms of coumarins. Therefore, coumarinic compounds are thought to exert anti-inflammatory effects on therapeutic applications against lung injury induced by LPS and other destructive inducers (Fylaktakidou et al., 2004;Bansal et al., 2013).
Another study showed that the praeruptorins D and E (80 mg/kg, gavage), as pyranocoumarins found in Kitagawia praeruptora (Dunn) Pimenov (syn. Peucedanum praeruptorum) roots, similar to daphnetin inhibited NF-κB and interconnected inflammatory cytokines (IL-6 and TNF-α) in male BALB/c mice with lung injury induced by intranasal administration of LPS (40 μg/ml) and hydrochloric acid (0.1 N). The total protein level, neutrophils, and cell infiltration in BALF were also reduced at 40 and 80 mg/kg of daphnetin .
Frontiers in Pharmacology | www.frontiersin.org Volume 11 | Article 588467 regulated by retinoic acid-related orphan receptor gamma t (RORγt). Esculetin (20 and 40 mg/kg, i.p.) as a hydroxycoumarin is widely found in Fraxinus spp., with the potential of reducing lung injury via inhibiting the RORγt and then the suppression of IL-17 in mice. At the same doses, esculetin also inhibited MAPKs and neutrophils/macrophages entry in mice lung . Besides, the protective effects of osthole, as prenylated coumarins purified first from the fruit of Cnidium monnieri (L.) Cusson (Apiaceae), were reported in several in vivo and in vitro studies (Table 1). Reducing the expression of cytokines (IL-1β, IL-6, and TNF-α) and blocking the NF-κB and ERK and Akt signaling pathway are of the critical protective mechanisms of osthole on lung injury (Mo et al., 2014;Tsai et al., 2015;Jin et al., 2018). Also, Shi and coworkers proposed that inhibition of ACE2 and Ag (1-7) depletion in lung tissues are other protective mechanisms of osthole (40 mg/kg, gavage) against the lung injury induced by LPS (Shi et al., 2013). Also, ACE2 has shown ameliorating effects on lung injury complications induced by acid, LPS, and viruses, including SARS coronavirus and influenza (Gu et al., 2016).
Isofraxidin is another hydroxycoumarin, isolated from Fraxinus spp., with prominent anti-inflammatory effects, especially pulmonary inflammations induced by influenza virus Majnooni et al., 2020). Also, isofraxidin (5, 10, and 15 mg/kg, i.p.) showed improving effects on LPSinduced lung injury via reducing the production of inflammatory cytokines (IL-6 and TNF-α) and prostaglandin E 2 (PGE2). Consequently, it blocked the secretion of PGE2 in mice serum and BALF, also reduced COX-2 gene expression, and led to further improvement of lung damage (Niu et al., 2015).
According to the prominent anti-inflammatory effects of natural coumarins, along with their other pharmacological effects, these compounds can be introduced as one of the new sources of drug discovery for the protection and treatment of lung injury.
In general, due to the anti-inflammatory and antioxidant effects of polyphenol compounds, as well as their antiviral effects (Table 1), this category of secondary metabolisms of plants has the potential to treat COVID-19 and its complications, including lung injuries. However, the pharmacokinetic parameters of these compounds should be considered .

Quinones
Quinones are another class of phytochemicals with an aromatic ring attached to two carbonyl groups in their structure, including anthraquinones, benzoquinones, naphthoquinones, phenanthrenequinones, and polycyclic quinones derivatives (Figure 3). Several investigations showed that the quinones derivatives have demonstrated protective effects on lung injury by various mechanisms. Chen and coworkers reported that a phenanthrenequinone isolated from Salvia miltiorrhiza Bunge (Lamiaceae), Tanshinone IIA, suppressed the nucleotide-binding oligomerization domain-like receptors pyrin domain-containing protein 3 (NLRP3), as an inflammatory signaling pathway, at 10 mg/kg i.v. in rats, thereby reducing the oleic acid-induced lung injury . Also, emodin, an anthraquinone found in different laxative plants such as Rheum rhabarbarum L. (Polygonaceae), showed protective effects on LPS-lung injury via activating autophagy pathways at 20 mg/kg i.p. in rats . Shikonin (a naphthoquinone) and thymoquinone (a benzoquinone) are other quinones with protective effects on lung injury (Kanter, 2011;Liang et al., 2013) (Table 1).
In general, due to the protective effects of the aforementioned phytochemicals on lung injuries, these compounds can be used as a protector and treatment in lung injuries leftover from coronavirus activity, including COVID-19. Given the antiviral effects (especially anticoronavirus) of some of the compounds listed in Table 1, this role could lead researchers to find much more effective multitarget compounds in the treatment of patients with COVID-19 and its complications.

CONCLUSION
Since the World Health Organization (WHO) announced the pandemic of COVID-19 disease (March 11, 2020), no effective treatment or vaccine has been introduced to treat this disease. Besides, to eliminate the SARS-CoV-2, conventional medications have either failed or been used taking them in doses higher than their therapeutic index leading to side effects (Ianevski et al., 2020;. On the other hand, due to their multitarget character, phytochemicals have always been of the options for discovering drug molecules to treat complicated diseases, including viral diseases and their complications. On the other hand, lung injury is the main COVID-19 complication that happens with inflammatory cascades by SARS-CoV-2 (Fakhri et al., 2020b;Merad and Martin, 2020). In the present review, we described the candidate phytochemicals with protective effects on lung injuries induced by various methods, as well as their pharmacological mechanisms (Figure 4). In addition, we showed some phytochemicals possessing protective effects against lung injury, with a focus on cepharanthine, epigoitrin, isofraxidin, osthole, resveratrol, apigenin, kaempferol, myricetin, quercetin, chlorogenic acid, chicoric acid, emodin, thymoquinone, betulinic acid, eucalyptol, oridonin, zerumbone, glycyrrhizin, and sulforaphane and their antiviral activities ( Table 1). On the other hand, despite the effectiveness of natural secondary metabolites in combating viral diseases, providing the novel drug delivery systems helps to drawback their pharmacokinetic limitations (Abbaszadeh et al., 2020;Fakhri et al., 2020a). Such reports could pave the way for discovering alternative drugs with anti-CoV effects and the potential in controlling the complication of COVID-19. Additional studies are needed to reveal the precise dysregulated pathways in COVID-19 and clarify the potential effects of phytochemicals on humans.

AUTHOR CONTRIBUTIONS
MM and MF contributed to conceptualization; MM, SF, and MF contributed to designing the structure of the paper; MM and SF contributed to software; MM, SF, YS, NK, KS, PM, MG, MF, and JE contributed to drafting the manuscript; and MM, SF, MF, and JE contributed to reviewing and editing the paper.