The controversial effect of smoking and nicotine in SARS-CoV-2 infection

In the first step, articles containing the MeSH terms of “smoke” and “COVID-19” and “meta-analysis” were investigated only in PubMed (16 Jul 2021) (Fig. 1-a). Meta-analysis studies with more than 10 studies were summarized in Tables 1 and 2.

Also, articles containing the MeSH terms of “nicotine” and “COVID-19” were systematically searched in the online databases of PubMed, Google Scholar (Fig. 1-b). Then, all the articles were initially evaluated, and the preliminary design for the signaling and metabolic pathway involved in coronavirus infection and smoking/nicotine responses was performed. We performed a systematic search on (SARS-CoV-2 OR COVID-19) in PubMed and manually reviewed all the publications. Subsequently, all related articles were reviewed and included in this review (2021, June). Since the articles related to the study of the effect of nicotine in COVID-19 were very sparse, two other searching strategies regarding the effect of pure nicotine on different (A) cells, including epithelial, fibroblast, endothelial, and dendritic, macrophages, T, and B cells, and (2) organs, including respiratory, nervous, metabolic, cardiovascular, and urogenital systems. Generally, 12 diseases were candidate according to their similarity with the pulmonary and extrapulmonary complications of COVID-19. Also, Finally, the effect of smoking/nicotine on other common coronaviruses (SARS and MERS) was also evaluated and added to the study.

Tobacco constitutes a varied mixture of > 8000 chemicals, including nicotine, carbon monoxide, nitrogen oxides, pro-oxidants (e.g. free radicals), aromatic amines, catechols, inorganics (e.g. nickel, chromium, and cadmium) [60, 61]. Although the exact mechanisms by which smoking causes diseases are unclear, chronic inflammation is the main factor in the development of diseases relevant to smoking such as lung cancer, heart disease, COPD, and asthma [61, 62]. Nicotine (C₁₀H₁₄N₂), a potent parasympathomimetic alkaloid, is the primary addictive tobacco smoke component. It was named after Jean Nicot, the French ambassador to Portugal, who introduced tobacco seed in Paris in 1550 [63]. A single cigarette tar contains about 10 to 12 mg nicotine, and when a cigarette is smoked, between 1 and 2 mg is inhaled into the lungs [64]. Nicotine interferes with the natural functioning of acetylcholine and over-stimulates acetylcholine receptors. In addition to the activation of nicotinic receptors, nicotine can be absorbed from cellular membranes, especially through the epithelium of the lower respiratory system and alveolus [65]. After absorption through the lungs, nicotine is metabolized in the liver by CYP2A6, UDP glucuronosyltransferase (UGT), and flavin-containing monooxygenase (FMO) enzymes. Cotinine is the principal (70–80%) product of nicotine metabolization and is used as a biomarker in smokers, second-hand smokers, and children exposed to smoke (detectable in blood, urine, saliva, hair, and nail) [66, 67].

Nicotine is only one of 8,000 tobacco components in cigarettes; therefore the consequences of smoking cannot be attributed only to nicotine [68]. In one study, the cytotoxicity of cigarette smoke extract (CSE) containing nicotine (CSE-N) and nicotine-free portion (CSE-O) on human bronchial epithelial cells was studied (BEAS-2B). CSE and CSE-O were shown to be hazardous to BEAS-2B cells, but CSE-N exhibited substantially lower cytotoxicity. CSE-O, but not CSE or CSE-N, enhanced apoptosis in cells stably expressing CYP2A13 (B-2A13). Cytochrome P450 2A13 (CYP2A13), an extrahepatic enzyme mostly expressed in the human respiratory system, has been discovered to mediate cigarette smoke metabolism and toxicity. These findings show that the nicotine component reduces the metabolic activation of CYP2A13 to CSE. As a result, it may be useful in lowering the toxicity produced by other tobacco components [69]. In fact, much more research is required to establish whether pure nicotine has detrimental or good effects, although it can be stated that many of the consequences induced by cigarette smoke, particularly in respiratory problems, are caused by other components in cigarette smoke.

Epithelial cells form a mechanical barrier that prevents pathogenic substances and secrete mucus and antimicrobial peptides in host defense. Nicotine changes the viability, morphology, and motility of epithelium in various organs, including the respiratory system and oral mucosa [87‐89]. Nicotine increases the expression of kidney injury molecule-1, the classic epithelial-mesenchymal transition (EMT) markers, vimentin, fibronectin, and production of α-smooth muscle actin (α-SMA), TGF-β, MCP-1, and ROS in the renal epithelium. Nicotine contributes to renal injury via STAT3, JNK, and AP-1 [90, 91]. In addition to the increased frequency of eosinophils in lamina propria (a thin layer of connective tissue), nicotine enhances the expression of ICAM1 on the submucosal connective tissue of the intestine and trachea [92]. Upon exposure to nicotine, the release of granulocyte-macrophage colony-stimulating factor (GM-CSF) and the activation of Akt are also stimulated in airway epithelial cells (AECs) [93, 94]. In response to chronic nicotine exposure, the upregulation of nAChRs occurs in bronchial epithelial cells (BECs), leading to the activation of PKC and MAPK p38. Similarly, the p42/44 signaling pathway is activated in airway fibroblasts [71]. The presence of nicotine also suppresses inflammatory factors such as MUC5AC in airway epithelial cells [95]. The bronchial epithelial cell showed apoptosis and senescence via ROS mediated autophagy-impairment mediated by nicotine [96]. Same to macrophages derived from human monocytes (MDMs) and type II pneumocytes, nicotine decreases the expression of the TLR-2, TLR-4, and NOD-2 on epithelial cells, which happens without alteration of the viability of the cells. Nicotine also reduces the expression of the surfactant protein (SP)-D in type II pneumocytes. During Mycobacterium tuberculosis infection, nicotine decreases IL6 and CCL5 in epithelial cells (EpCs), whereas in MDMs, there is a decrease in IL6, IL8, IL10, and TNF-α, and CCL2, CXCL9, and CXCL10. The authors pointed that nicotine probably modulates the expression of these molecules by an independent nAChRs pathway [97]. IL22 cytokine is a vital link between immunity and barrier function. By increasing transepithelial resistance and epithelial cell regeneration and repair, IL22 pathway maintenances epithelial cell health and the lung function [98]. Nicotine also downregulates the expression of IL-22Rα1 which is followed by suppression of AECs response to IL22 [99].

As the primary source of the extracellular matrix (ECM), fibroblasts provide the structural framework for tissues and play a critical role in wound healing [100]. Over the past decades, many studies have provided evidence for nicotine involvement in the modulation of fibroblast activation and function in various systems, such as the heart, lung, gingiva, prostate, and joints. However, gene expression changes may not be identical. Also, nicotine interrupts the airway epithelium’s barrier function, leading to the release of inflammatory proteins into the systemic circulation and causes multi-organ fibrosis [101]. TGF-β is described as the key molecule responsible for nicotine-induced fibrosis in various organs [102‐104].

Further, nicotine administration results in increased fibronectin (FN1) gene expression in rat hearts [105]. Nicotine also alters the deposition and phagocytosis of collagen in fibroblast cells [106]. In a dose-dependent manner, nicotine raises nerve growth factor (NGF) secretion by lung fibroblasts and stimulated NF-κB nuclear translocation and transcriptional activity [107]. Both fibroblast proliferation via MEK-1/ERK and their collagen type I expression in the lung via α7nAChRs are also induced by nicotine [108]. Also, nicotine tempers fibroblast proliferation via modulation of growth factors and microRNA, particularly by targeting miR-24 [109].

The endothelium is a thin layer of cells between the interior surface of blood vessels and lymphatic vessels. They have various tasks like; permeability, leukocyte trafficking, vascular tone regulation, angiogenesis, and immunity. Hence, the impaired endothelial function leads to inflammatory diseases, remarkably vascular disorders (i.e., atherosclerosis) [110]. Nicotine is one of the main components of cigarettes that can cause endothelial dysfunction. Nicotine exposure gives rise to differential gene expression of endothelial cells, growth of atherosclerotic plaques, and angiogenesis [111, 112]. Human coronary endothelial cells treated with nicotine showed increased expression of ACEI, nitric oxide synthase, and vascular cell adhesion molecule-1 (VCAM1) [113]. Angiotensin II Type I Receptor Antagonism (AT1) was found to diminish the adverse effects of nicotine on cardiac structure and function [105]. nAChRs are also expressed on the upper respiratory endothelial cells, neighboring to ACE-2 receptors [114].

Since DCs facilitate the initiation of cell-mediated adaptive immune responses, they are pivotal cells of the innate immunity system. Dendritic cells are well-known as the most professional APCs with high expression of major histocompatibility complex (MHC) class II molecules (MHC-II), and they can uptake, process, and present antigens to naïve T cells and promote their polarization into different T cell subsets [115]. Dendritic cells can uptake antigens via several mechanisms such as receptor-mediated endocytosis (using mannose receptor, FcγRI, and FcγRII), macropinocytosis, and phagocytosis (via CD36 and αvβ3 or αvβ5 integrins) [116]. Although rare in human blood, DCs are heterogeneous cell populations with different subsets, including plasmacytoid DC (pDC) and classical DC or myeloid DC (cDC). The latter can further be divided into two subpopulations of cDC1 and cDC2. The DCs subsets have mouse counterparts as well [117].

Aicher et al. showed that nicotine could enhance HLA-DR expression, costimulatory molecules (CD86 and CD40), and adhesion molecules (LFA-1 and its ligand, CD54) in human DCs. Therefore, nicotine increases the production of IL12 in DCs and stimulates the activation of T cells, which is followed by augmented IL2 production and increased CD40L expression. They proposed that ERK1/2, p38 MAPK, and Akt signaling pathways may mediate the effects of nicotine on DCs [118]. For the first time, Nouri-Shirazi et al. provide evidence of the immunosuppressive effect of nicotine on human DCs. They could demonstrate that nicotine exposure alters the function of monocyte-derived DCs, including (1) antigen uptake: no significant change was observed in macropinocytosis, but endocytic and phagocytic properties of immature DCs were reduced. In this context, they observed downregulation of mannose receptor without a change in CD36 expression; (2) maturation and cytokine production: DCs matured in response to lipopolysaccharide; however, they produced a lower level of pro-inflammatory cytokines, including IL1β, IL10, IL12, and TNF-α; and (3) T cell responses: in the presence of nicotine, DCs showed an anti-proliferative effect on T cells which finally led to the inhibition of Th1 polarization and IFN-γ production. The latter was linked to the downregulation of IL12 as well as CD86 (B7.1) and CD80 (B7.2) in human DCs [119, 120]. Correspondingly, Nouri-Shirazi et al. provided similar evidence for nicotine’s effect on mouse DCs [121]. Later, the association of nicotinic environment and dose-dependent functioning of DCs was described; The pro-apoptotic activity in DCs in high doses of nicotine and anti-apoptotic effect in the lower doses [122]. In a recent study, nicotine-treated DCs showed increased expression of CD40 and CD197 and decreased expression of CD86, MHC-II, and CCP3 (an apoptotic molecule). Interestingly, nicotine did not alter CD80 levels in DCs. While the phagocytosis property of DCs was attenuated in the presence of nicotine, no change was seen in its endocytosis function [123].

Macrophages, as mononuclear phagocytes, are highly heterogeneous cells that differentiated from monocytes. They have multiple functions, such as phagocytosis, antigen presentation, and the production of different types of cytokines [124]. Based on their role, several distinct subsets of macrophages have been defined: (1) classically activated macrophages (M1) which mediate antimicrobial defense via secretion of pro-inflammatory cytokines, (2) alternatively activated macrophages (M2) with anti-inflammatory function, (3) regulatory macrophages (Mregs) which secrete large amounts of IL10, (4) tumor-associated macrophages (TAMs) which suppress anti-tumor immunity, and finally (5) myeloid-derived suppressor cells (MDSCs) as a group of immature cells linked to TAMs [125].

It has been reported that exposure of human macrophages to nicotine increases the intracellular ROS through the activation of Src/PI3K/Akt cascade and NADPH oxidase, which activates AMPK/MAPKs and NF-κB signaling. Flowingly, the activation of these signaling pathways upregulates the expression of IL8, a pro-inflammatory chemokine [126]. While nicotine upregulates the expression of IL8, the expression of IL1β, IL6, TNFα, MIP-1α, MIP-1β, and MCP-1 are downregulated. It also attenuates the phagocytosis of macrophages by affecting phagocytic recognition molecules, SR-A1 and TLR-2. [127]. However, macrophages treated with nicotine exhibited upregulated expression of CD36, possibly via the ROS/PKCδ/PPARγ signaling pathway [128]. The exposure of mice to nicotine also showed a rise in macrophages’ infiltration into the bronchoalveolar lavage fluid [129]. Besides the induction of inflammatory macrophages, nicotine treatments promote the M2 polarization, reduces the secretion of several proinflammatory cytokines in macrophages, and suppresses the alveolar macrophages to present the inhaled antigen to lymphocytes [130, 131].

In human monocytes, activation of α7nAChR inhibits IκB phosphorylation, followed by inhibition of nuclear localization and transcriptional activation of NF-κB [132]. Likewise, in murine macrophages, the activated α7nAChR can mediate Jak2-STAT3 activation and then can induce the phosphorylation of STAT3, which subsequently downregulates pro-inflammatory cytokines (e.g., IL6, IL12, and TNF-α) [133]. Similarly, overexpression of IRAK-M (a negative regulator of TLR-mediated immune responses) in human macrophages through JAK2/STAT3/PI3K not only mediates the anti-inflammatory effect of nicotine via α7nAChRs but also triggers the hyperresponsivity [134]. Altogether, these data suggest that nicotine exerts a dual role in macrophages. In a recent study, it has been shown that the effect of nicotine on infected macrophages is contradictory to uninfected macrophages; (1) uninfected macrophages express anti-inflammatory responses, including polarization of CD206⁺ M2 macrophages and increased expression of IL10, whereas (2) infected macrophages show inflammatory responses, including differentiation to M1 macrophages and increased expression of iNOS, TNF-α, and IL6. Additionally, they have identified a dose-dependent effect of nicotine on macrophages through α7nAChRs [135].

Once developed in the thymus, T cells circulate throughout the body to participate in the cell-mediated adaptive immune response. Overall, there are two specific T cells, including CD4⁺ T cells (helper T cells; Th) and CD8⁺ T cells (cytotoxic T cells; Tc). The CD4⁺ Th subset includes Th1, Th2, Th9, Th17, Th22, Treg, and Tfh categorized according to their surface molecules, transcription factors, and functions. In parallel, there are such subsets for CD8⁺ T cells [136].

Many studies have been performed to show how nicotine alters T cell-mediated immune responses. With a dose-dependent response, the serum level of cotinine, the primary metabolite of nicotine catabolism, was correlated with increased naive (CD45RA⁺) and decreased memory (CD45RO⁺) CD3⁺ CD4⁺ T cells. However, the CD8⁺ T cell population showed no significant difference in passive smokers than controls [137]. Besides α7nAChRs, nicotine appeared to increment Th2 cells via the activation of α4nAChRs induced through the Gprin1/CDC42 signaling pathway. The α4nAChRs-expressing Th lymphocytes were found in the circulation system, spleen, bone marrow, and thymus [138]. Additionally, nicotine-treated mice showed a higher frequency of PD-1^- IL7R⁺ CD8⁺ T cells (non-exhausted phenotype associated with loss of tolerance) in bone marrow and spleen. These cells were promoted to produce survivin. In parallel with higher expression and activation of α4/α7nAChRs, the transcription factors of T-bet and Blimp-1 were downregulated in CD8 + T cells [139].

In the context of nicotine’s immunosuppressive properties, a pivotal role has been attributed to the expression and activation of α7 nAChRs on CD4⁺ T cells. Nicotine skewed polarization from Th1 and Th17 cells to Th2 cells. Although both Th1 (IFN-γ and TNF-α) and Th17 (IL17, IL21, and IL22) cytokines were downregulated, the Th2 cytokine (IL4) was upregulated. This effect was further accompanied by reducing T-bet and augmentation of GATA-3 and lower NF-κB-mediated transcription of I-κB and IL-2 [140]. According to a recent study, the co-culture of CD4⁺ T cells and DCs in a nicotinic environment reduced the differentiation to Th1 (IFN-γ), increased the differentiation to Treg (FOXP3) and Th2 (IL6, IL10, and IL13) cells [123]. Nicotine stimulation also reduced the proportion of IL-22-producing PBMCs, particularly CD4⁺ T cells. By targeting IL-22Rα1 expression, the nicotine impaired IL-22/IL-22R signaling axis [99].

B cells are well-known for their ability to support the antibody-mediated immune responses along with their immunoregulatory functions. Typically, B cells are characterized as CD19⁺ and B220/CD45R⁺ cells. There are various types of B cell populations, including (1) B1 cells (CD19⁺ and B220^low/-), which can be detected in most tissues, (2) follicular B cells (FO or B2), (3) marginal zone B cells (MZ), (4) transitional B cells, and (5) regulatory B cells (Bregs); Breg-derived IL10 has a pivotal role in tolerance and differentiation of Treg cells [141‐143].

Based on a preliminary study, the lack of nicotine-binding sites was shown on B lymphocytes [144]. Later, as the first demonstration, both α4- and α7nAChRs were identified on nicotine treated B lymphocyte-derived cell lines. The authors concluded that long-term exposure with nicotine led to the upregulation of nicotinic receptors coupled with increased cell proliferation and suppressed antibody production. The nAChRs regulated B lymphocyte activation and immune response through CD40 signaling pathway [145, 146]. Skok et al. further confirmed a higher count of B cells (lymphopoiesis) in nicotine-treated mice. Nicotine-treated B cells develop from their early precursor, pre-B cells (B220⁺ IgM^- CD43^-) [147]. They have recently reported that not only CD5⁺ and Foxp3⁺ B lymphocytes are enriched with a high amount of α7nAChR, even it is necessary for the formation, induction, and functioning of regulatory B lymphocytes [148].

Overall, nicotine influences on immune cell responses seem to be profoundly dependent on experimental design, nicotine concentration, and exposure to nicotine. As discussed below, it is plausible that environmental signals, including autoimmune and infectious disease models, also implicate in the fate of nicotine treated cells. More studies are required to assess both inflammatory and anti-inflammatory effects of nicotine on cells of the immune system.

Neutrophils are the most abundant type of leukocytes in blood circulation in humans. They are known as a main part of the innate immune system and are the first cells that migrate toward inflamed or infected sites [149]. There are few studies on the effect of nicotine on neutrophils. In the study of Iho et al. the blood level of IL-8 elevated in smokers compared with non-smokers. Increased production of IL-8 was observed in nicotine-stimulated neutrophils in a time- and concentration-dependent manners. Nicotine-induced IL-8 formation is mediated via nAChR, depends on peroxynitrite generation and following NF-κB activation [150]. Also, nicotine stimulates neutrophils to release neutrophil extracellular traps (NETs) in a dose-dependent manner. nAChRs through activation of Akt and PAD4 and without activation of Nox2 produce nicotine-induced NET. These results show that nicotine can be involved in smoking-related diseases [151].

Mast cells play a key role in inflammation caused by allergic reactions [152]. They express receptors for IgE (FcεRI) which bind to allergens in vivo and induce the release of Th2 cytokines and cysteinyl leukotrienes (cysLTs) [152‐156]. Activated mast cells are involved in pathologies of some pulmonary diseases such as COPD, emphysema, asthma, and COVID-19 [27, 157‐159].

Several studies investigated the mechanism by which nicotine may affect mast cells. An in vitro study showed nicotine could suppress the delayed phase of activated mast cells via α7/α9/α10 nAChRs and could inhibit the cytosolic phospholipase A2/MAP kinase pathway [160]. Another in vitro study demonstrated cigarette smoke medium (CSM) suppressed c-kit and FcεRI expression in bone-marrow-derived mast cells (BMMCs) and inhibited the development of mast cells in a toll-like receptor 4 (TLR4)-independent manner [161]. TLR4 a protein that play the role in the maturation of DC and B cells [162, 163]. Exposure of RBL-2H3, as a mast cell model, to cigarette smoke condensate (CSC) stimulated MCPTs, especially tryptases, secretion. In line with this finding, several in vivo investigations showed there is an adverse correlation between MCPTs levels and airway function, suggesting the contribution of MCPTs in airway remodeling in smokers [164]. Increases in mucosal mast cells (MMCs) were shown in the colonic mucosa from mice with food allergy [165]. nicotine inhibited the activation of MMCs via a7 nAChRs and induced the expression of cytokines (Th1 and Th2 types), subsequently ameliorated food allergy mice [166].

According to a WHO report based on the monitoring of patients with COVID-19 after recovery in 2022, these people, particularly those with co-morbidities, displayed many symptoms for months after recovery [178]. Post-acute COVID-19 syndrome, also known as ‘post-COVID syndrome,‘ ‘long COVID,‘ ‘persistent long COVID,‘ or ‘post-acute COVID sequelae (PACS),‘ is a pathological disease characterized by persistent physical, medical, and cognitive sequelae after acute COVID-19 [179]. Post-acute COVID-19 syndrome, like COVID-19, can impact various systems and organs, including the respiratory, cardiovascular, neurological, gastrointestinal, and musculoskeletal systems [180]. There is significant evidence that post-acute COVID-19 syndrome can impact the whole COVID-19 patient population, and that around 20% and 10% of SARS-CoV-2 positive patients have symptoms that last at least 5 or 12 weeks, respectively [181].

The number of research on the influence of smoking on Post-COVID-19 symptoms is limited, and we will discuss the scant information that exists in this area in this section. According to the results of a 2022 study done in France, smoking can contribute to a rise in Post-Covid-19 symptoms such as cutaneous disorders, tachycardia and/or hypertension. In fact, this study discovered a link between smoking in women and the occurrence of these symptoms following recovery [178]. Another study found that the majority of COVID-19 recovered patients have varying degrees of functional impairments ranging from insignificant to severe based on Post-COVID-19 Functional Status (PCFS). Age, gender, periodic influenza vaccination, smoking, length since symptoms began, requirement for oxygen or ICU admission, and lastly the existence of concomitant comorbidities all had an impact on these limits [182]. Based on the findings of a 2022 study, smoking and vaping are not only risk factors for a more severe clinical form and slower progression of COVID-19, but they are also risk factors for the development of lengthy and persistent post-COVID-19 symptoms [183]. Finally, it is crucial to note that all of the above cases are associated to smoking and post-COVID-19 symptoms, and these cases cannot be attributed to nicotine alone; additional research in this area is required.

While neurodegenerative diseases appeared to be more prevalent in smokers than non-smokers, epidemiological studies during the early 1960s provide the first evidence of more smoking is used, less incidence of Parkinson’s disease is observed [191, 192]. Since then, several studies have proven the critical role of nAChRs activation in modulating neuro-immune pathways, and nicotine has been investigated as an effective therapeutic in neurodegeneration and neuroinflammatory diseases [193, 194]. Although nAChRs are widely distributed in different nervous system regions, nicotine neuroprotection in the brain and spinal cord is mainly mediated via α7nAChRs and α4β2 [195]. Below we will review the impact of nicotine in Alzheimer’s disease (AD), Parkinson’s disease (PD), and multiple sclerosis (MS), which are among the major cause of neurological impairment in middle and late life [196].

Cardiovascular diseases (CVDs) are the first cause of death worldwide. As a coronary artery disease, atherosclerosis is a major cause of heart attack, stroke, and peripheral arteries. Generally, chronic nicotine exposure enhances atherosclerosis in animal and human studies. After nicotine administration, impaired and diminished clearance of LDL and accelerated lipid transfer from HDL increases their deposition in the arterial wall, which result in the atherogenic lipid profile (increased LDL, decreased HDL/total cholesterol ratio) [276, 277]. Nicotine also raises heart rate and blood pressure, and platelet aggregation. The latter contributes to plaque growth and thrombosis [278]. Also, nicotine significantly induces vascular smooth muscle cells’ proliferation via the upregulation of bFGF mitogen and several MMPs, which leads to an augmentation of lesions and development of intimal hyperplasia, atherosclerosis, and aneurysm. Chronic nicotine consumption profoundly activates macrophages via nAChRs to express pro-inflammatory cytokines, aggravating atherosclerosis and aortic lesions [279]. Distribution of α7nAChRs on endothelial cells pointedly indicates nicotine-induced angiogenesis in the endothelium [278]. Nicotine induces proliferation, migration, and tube formation in endothelial cells. There is a close association between neovascularization and atherosclerotic plaque progression, which directly increased by nicotine [280]. However, chronic exposure of endothelial cells to nicotine impairs angiogenesis and decreased cell migration and tabular structure formation [281, 282]. In addition, nicotine stimulates the migration of vascular smooth muscle cell and structural changes in vascular endothelium via p38 and p44/42 signaling axis [283]. Treatment with nicotine also activates NF-κB signaling and increases proliferation of endothelial and smooth muscle cells [284].

Additionally, nicotine exposed mice show upregulated expression of VCAM-1, COX-2, platelet-derived growth factor β (PDGFβ), and molecules involved in downstream of NF-κB signaling pathway in aorta lesions [279]. The upregulation of PDGF promotes a phenotypic switching from vascular smooth muscle cells into myofibroblasts and osteoblast-like cells that secrete and modify the endothelium and cardia ECM by collagen and osteopontin [285]. Inhibition of α1nAChR reduced myofibroblasts in the aortic wall by 80% resulting in attenuated calcification and reduced immune cells in the lesions [286]. Treatment of endothelial cells by nicotine induces calcium internalization and accumulation via α7nAChR activation and induction of angiogenesis [287]. Furthermore, via the upregulation of VEGF and FGF, nicotine activates other angiogenic signaling pathways in endothelial cells [288‐290]. Therefore, nicotine induces the proliferation of smooth muscle cells, endothelial cells, and accumulation of macrophages in plaques and lesions, promoting atherosclerosis and vessel hardening via calcification.

Kidneys are essential organ for filtering waste and toxic materials out of the blood. Diseases related to impaired kidney function is increasing with global prevalence of 9·1% (2019) [291]. Nicotine mainly affects kidneys via α7-nAChR signaling and worsens renal injury in chronic kidney disease (CKD), acute nephritis, and subtotal nephrectomy [292]. Different subtypes of nAChRs are distributed in renal cells, especially mesangial cells. Nicotine stimulates differentiation and specialization of cells in kidneys, the proliferation of mesangial cells, and fibronectin production, mediated through PKC activation, ERK1/2 phosphorylation, and NADPH oxidase and ROS generation [252, 292‐294]. Also, COX-2 and prostaglandin generation are involved in nicotine-mediated renal injury [295]. Besides, nicotine mediates inflammation and fibrosis by TGF-β production (by activation of STAT3) and upregulation of vimentin, α-SMA, and fibronectin [296].

Several studies have also reported anti-inflammatory responses and attenuation of proteinuric renal injury due to nicotine treatment [297, 298]. Nicotine has renoprotective effects in kidney diseases. Oral nicotine delivery in rats with proteinuria-induced renal inflammation showed dose-dependent reduces proteinuria, glomerular desmin deposition, decreased glomerular podocin, decreased focal glomerulosclerosis (FGS) score, and reduced infiltration of macrophages and myofibroblasts. Renal inflammatory cytokines were also downregulated in monocytes of nicotine treated rats [299]. Furthermore, long-term nicotine delivery has been shown to slow-down proteinuria by reducing glomerulosclerosis and interstitial fibrosis [297, 300‐302].

SARS, MERS, and CODIV-19 show similar aggressive pathology. They extensively affect multiple tissues and organs accompanied by uncontrolled cytokine storm and delayed interferon-gamma response, multi-organ fibrosis, impaired tissue remodeling, and organ failure [303, 304]. As mentioned in the former section, nAChRs are expressed in different lung cells, including epithelial cells, fibroblast cells, and RAS components [16]. The effect of nicotine in the renin-angiotensin system is also well studied. Overall, tobacco smoke and nicotine play complicated dynamic functions against coronavirus infections, which are mediated through different mechanisms such as overexpression of ACE2 and inhibition/dysregulation of inflammatory processes [305, 306].

Limited data are indicating the involvement of smoking at an increased risk of MERS infection. Infected smokers during the MERS-CoV epidemic of 2003 showed more mortality rate than infected non-smokers [307, 308]. Alraddadi et al. investigated the smoking history in a population of MERS infected cases and asserted that active smokers are more likely among patients than controls [309]. After this report, Seys et al. reported the upregulation (mRNA and protein level) of dipeptidyl peptidase-4 (DPP4/CD26), the receptor involved in MERS infection lung tissue of smokers and COPD patients. DPP4 is mostly expressed in various alveolar cells, including epithelial, endothelial, macrophages, and other immune cells in the submucosal region of airway epithelium and lymphoid aggregates, but not in bronchial epithelial cells [310]. Also, binding of the spike protein of SARS–CoV-2 to DPP4 is a critical factor for infection [311]. Interestingly, a positive correlation between ACE2 and DPP4 is reported recently in several normal tissues [312]. DPP4 is a natural surface marker for T cells and is involved in the immune regulation of several biological processes. DPP4, the same as ACE2, is much expressed in lower airways, lung parenchyma, interstitium, and pleural mesothelia, which are the most infected areas in infected patients [313‐315]. Like DPP4, the upregulated expression of ACE2 has been shown in smokers and COPD patients [316‐318]. However, it is downregulated after infection, but ACE remains unchanged, which results in severe acute respiratory failure [319‐321]. Imai et al. demonstrated that loss of ACE2 expression in ace2 knockout mice causes very severe ARDS complications, including increased immune cell infiltration, lung edema, and vascular permeability [319]. Therefore, SARS infection downregulates the expression of ACE2 as a post-infection regulatory mechanism and ARDS [321]. Therefore, the upregulation of ACE2 via nicotine may play a protective role after SARS infections. We could not find any research regarding the direct effect of nicotine in MERS and SARS infections.

Nicotine has a dual effect on patients with COVID-19. On the one hand, nicotine has anti-inflammatory properties, and on the other hand, it would allow more viral entry. Since cytokine storm is the hallmark of SARS-CoV-2 infections, nicotine could diminish it through alpha7-contained nAChRs [322]. Furthermore, agonists of alpha7-nAChRs, nicotine or GTS-21, reduce cytokine storm mediators such as high mobility group box-1 (HMGB1) [323]. HMGB1 is a nuclear protein that is released into the extracellular spaces, including the airways and the blood circulation to promote inflammation [323]. High levels of HMGB1 have been shown in acute lung injury caused by infectious and severe COVID-19 patients, therefore, it could be a therapeutic target for patients with severe COVID-19 [324]. There are controversial reports regarding viral entry by smoking and nicotine use. Some studies showed that ACE2 increased by using nicotine and/or smoking inhabits [325‐328]. While Caruso et al. demonstrated that the protein expression of ACE2 diminished in bronchial epithelial cells after smoking exposure [329]. another study reported ACE2 mRNA levels in bronchial epithelial cells from current smokers were similar to never smokers [330]. As mentioned above, in addition to receptors, proteases are needed for virus entrance into host cells. The effect of nicotine/smoking on these proteases is different. The cellular furin levels were reduced by smoking [331], whilst TMPRSS4 and TMPRSS2 increased and unchanged in bronchial epithelial cells from current smokers in comparison with never smokers [330].

Additionally, data mining and in silico studies for evaluating the effects of smoking/nicotine on COVID-19 patients were performed. For instance, to determine the relationship between vaping/smoking and the expression of inflammasomes and inflammatory cytokines, Lee et al. used transcriptome datasets (GSE138326 and GSE112073). They found upregulated pro-inflammatory markers and inflammasome genes in smoking and nicotine use as well as e-cigarettes containing flavor. The upregulation of CCL5 and CCR1 in cigarette smoking and CCL5 and CCR1 in vaping e-cigarettes containing flavor/nicotine have been reported. Also, the upregulation inflammasome-related genes (CXCL1, CXCL2, NOD2, and ASC) in smoking and vaping have been observed, revealing the negative effect of vaping/smoking on the inflammation and susceptibility to SARS-CoV-2 infection [328]. Furthermore, in silico studies showed interaction of α7 nAChRs with RBDs of SARS-CoV-2 spike glycoproteins, indicating dysregulation of the nicotinic cholinergic system and leading to cytokine storm in COVID-19 patients [332, 333]. Mohammadi et al. demonstrated the interaction between ACE2 human receptor and nicotine by molecular dynamic simulations. They also revealed the combination of favipiravir (antiviral therapeutic for SARS-CoV-2) with nicotine could successfully block 6LZG, a main active site of ACE2-S protein, suggesting the potential influence of this combination for blocking ACE2 versus SARS-CoV-2 [334].

Several systematic reviews and meta-analyses evaluated the association between smoking and COVID-19 hospitalization. Some of these studies showed low smoking prevalence among hospitalized patients with COVID-19, while others demonstrated the inverse trend [335‐346]. The latest meta-analysis that included 109 studies with 517,020 patients showed that smoking was related to COVID-19 severity. Also smoking elevated the risk of ICU admission and death in patients with COVID-19, but was not relevant to mechanical ventilation. This study reported former smokers had a risk of progressing COVID-19 severity compared with current smokers. Current smokers were significantly associated with the severity of COVID-19 compared with non-smokers [344]. According to this meta-analysis that has been shown a positive correlation between smoking and COVID-19 progression, it is not clear whether nicotine would prevent negative outcomes among hospitalized patients with COVID-19.

Unfortunately, we were unable to find a publication that looked into the influence of nicotine dosage, age, and COVID disease. However, the findings of other research demonstrate that increasing age is a risk factor for mortality caused by COVID. COPD patients, smokers, quitters, and persons, who have never smoked, on the other hand, have various reactions to this virus [347].

To the best of our knowledge, there is no clinical trial to demonstrate the effects of smoking/nicotine on outcomes in COVID-19 patients. To clarify the effect of nicotine well-designed clinical trials are required for the assessment of its therapeutic agents.