Antiviral and Immunomodulatory Effects of Phytochemicals from Honey against COVID-19: Potential Mechanisms of Action and Future Directions

Abstract

The new coronavirus disease (COVID-19), caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), has recently put the world under stress, resulting in a global pandemic. Currently, there are no approved treatments or vaccines, and this severe respiratory illness has cost many lives. Despite the established antimicrobial and immune-boosting potency described for honey, to date there is still a lack of evidence about its potential role amid COVID-19 outbreak. Based on the previously explored antiviral effects and phytochemical components of honey, we review here evidence for its role as a potentially effective natural product against COVID-19. Although some bioactive compounds in honey have shown potential antiviral effects (i.e., methylglyoxal, chrysin, caffeic acid, galangin and hesperidinin) or enhancing antiviral immune responses (i.e., levan and ascorbic acid), the mechanisms of action for these compounds are still ambiguous. To the best of our knowledge, this is the first work exclusively summarizing all these bioactive compounds with their probable mechanisms of action as antiviral agents, specifically against SARS-CoV-2.

Keywords: 2019-nCoV; SARS-CoV-2; antiviral activity; antiviral agent; antiviral immunity.

Figure 1

The schematic mechanism of replication of SARS-CoV-2 into host cell. Spike (S) protein on the surface of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) recognizes the ACE2 receptor on the cellular membrane of host cell. After receptor binding, the virus enters host cell cytosol via cleavage of S protein by transmembrane protease/serine subfamily member 2 (TMPRSS2), followed by fusion of the viral and cellular membranes. The conformational changes at the S1 and S2 subunits facilitate the virus–cell fusion via the endosomal pathway. The viral genome is released into the cytoplasm and translated through the ribosomal frame, shifting to generate replicas polyproteins pp1a and pp1b. Negative-sense RNA intermediates are generated to serve as the templates for the synthesis of positive-sense genomic RNA (gRNA) and sub-genomic RNAs (sgRNAs). The gRNA is packaged by the structural proteins to assemble progeny virions. Shorter sgRNAs encode conserved structural and accessory proteins. Following gRNA and sgRNA synthesis, the viral proteins and genome RNA are inserted into virions and assembled in the ER-Golgi intermediate compartment (ERGIC) and then transported in the vesicle to the plasma membrane before releasing out via exocytosis pathway [12,13,14,15,16].

Figure 2

Classification of therapeutic substances being assessed in clinical trials, 305 trials in different stages, against COVID-19, according to ‘Milken institute COVID-19 treatment and vaccine tracker’ on 8 September 2020 [23].

Figure 3

The principal of oxidative damage and the role of antioxidants in scavenging free radicals. (A) The free radicals generated from endogenous sources, at limited concentrations, are considered important for regulation of cell maturation, in addition to their role in immune defense. Excessive concentrations of these unstable molecules can result from illness conditions and exogenous sources, and thus lead to oxidative damage (imbalance between free radical and antioxidant concentrations). This status results in cell injury/death based on the extremely high reactivity of free radicals with vital cellular molecules including lipids, amino acids, proteins, and DNA. (B) Antioxidants are necessary to stop oxidative damage by neutralizing free radicals. They own this unique role due to their capability to give an electron to free radicals that have unpaired electrons to make them stable and unharmful. ROS, reactive oxygen species; RNS, reactive nitrogen species (adapted from Al-Hatamleh et al., 2020 [36]).

Figure 4

Potential mechanisms of action of honey as an immunomodulatory agent. These mechanisms relied on the antioxidant activity of honey. This activity inhibits oxidative stress and results in stopping harm to the vital cellular components (lipids, amino acids/proteins, and DNA), which promotes lymphocytes proliferation and activation. On the other hand, inhibition of the mitogen-activated protein kinases (MAPK) and the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) pathways results in complicated cellular mechanisms finished with suppression of pro-inflammatory genes, and thus blocks the expression of pro-inflammatory cytokines. In addition, the antioxidant activity also reduces the release of arachidonic acid, which results in the oxidation of membrane phospholipids, and thus reduces its metabolites (leukotrienes and prostaglandins) that are considered as important inflammatory mediators [34,49].

Figure 5

The signaling pathways of MD-2/TLR4 axis that lead to stimulating the immune responses. Through binding to MD-2 hydrophobic pockets, viral proteins activate the MD-2/TLR4 signaling axis. This interaction enables two cytoplasmic signaling domains; MyD88 via Toll–IL-1 receptor (TIR) domain-containing adaptor protein (TIRAP)/MyD88 adapter-like (Mal), and TIR domain-containing adaptor-inducing interferon-β (TRIF) via TRIF-related adaptor molecule (TRAM). The MyD88/TIRAP pathway uses the members of IL-1 receptor-associated kinases 1 and 6 (IRAK1/4), TNF receptor-associated factor 6 (TRAF6), and transforming growth factor beta-activated kinase 1 (TAK1) complex to activate two transcription factors; nuclear factor kappa B (NF-kB), through the I kappa B kinase (IKK) complex, and activator protein-1 (AP-1), through the mitogen-activated protein kinases (MAPK). Both NF-kB and AP-1 regulates gene expression in response to pathogen infections and controls cytokines expression. On the other side, the TRIF/TRAM pathway activates the transcription factor interferon regulatory factor 3 (IRF3) and IRF7, through TANK binding kinase 1 (TBK1), that is involved in the regulation of innate immune responses [80,81,82]. JNK, c-jun n-terminal kinase; ERK, extracellular signal-regulated kinase.

Figure 6

The hypothesized mechanisms of action for NO antiviral effect through the MD-2/TLR4 signaling axis, again implicated in the viral infection at the intracellular level. The MD-2/TLR4 signals activate the transcription of the nitric oxide synthase 2 (NOS2) gene through activation of MAPK and NF-kB. This activation results in expression of NOS2 mRNA to produce NOS enzymes that are responsible for producing NO by the conversion of l-arginine (Arg) into l-citrulline (Cit), with existing nicotinamide adenine dinucleotide phosphate (NADPH) and oxygen (O₂). There are three isoforms from NOS enzymes that produce NO; neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS); each of them is expressed in different cell types. In the cell infected by a virus, NO might inhibit viral protease enzymes by blocking their cleavage ability of viral polyproteins. This process would inhibit the synthesis of viral RNA, and thus inhibit viral replication.

Figure 7

The potential antiviral mechanisms of action for honey contain caffeic acid, CAPE, galangin, and chrysin against SARS-CoV-2. Through an endosomal pathway, SARS-CoV-2 enters the host cell upon binding its S protein to the cellular receptor ACE2. (A) The viral RNA is unveiled in the cytoplasm and the pp1a and pp1ab polyproteins cleaved by the proteases (3CL^pro and PL^pro) to form nonstructural proteins (nsps) as helicase (for viral RNA synthesis) and the RNA replicase–transcriptase complex (RTC), which includes RNA-dependent RNA polymerase (RdRp) (for virion assembly). RdRp is responsible for the replication of structural protein RNA. The nucleocapsids residing in the cytoplasm are assembled from genomic RNA, whereas the structural proteins S1, S2, E, and M are translated by ribosomes in the endoplasmic reticulum (ER), and then released for preparation of virion assembly. The structural proteins then fuse with virion assembly to virus assembling, which is then transported through the Golgi apparatus to be released via exocytosis. (B) When honey containing caffeic acid (Caf), CAPE, galangin (Gal), and chrysin (ChR) is being consumed, those compounds could enter the infected cells and inhibit 3CL^pro. This inhibition is based on the chemical interactions of these compounds with 3CL^pro amino acid residues; (1) Caf with GLN-189, HIE-164 through hydrogen (H) bonding, and with HIE-41 through π–π stacking interaction. (2) CAPE with THR-24 and THR-26 through H-bonding, and with HIE-41 through π–π interaction. (3) Chr with SER-46, THR-24 and THR-26 through H-bonding, and with HIE41 through π–π interaction. (4) Gal with SER-46 and THR-24 through H-bonding, and with HIE-41 through π–π interaction. As a result, the process of pp1a and pp1ab polyprotein cleavage will fail to form nsps and RdRp, which means that protein replication cannot be completed and finally, SARS-CoV-2 replication will be stopped.