4.2 Antiviral action of H2S and its underlying
mechanisms in relation to COVID-19 infection
Burgeoning evidence shows that H2S donors such as NaHS
and GYY4137 exhibit excellent effects against the family of enveloped
RNA viruses of which SARS-CoV-2 is a member (Li et al., 2015; Ivanciuc
et al., 2016; Bazhanov et al., 2017; Bazhanov et al., 2018) Also,
natural sources of exogenous H2S such as diallyl
sulfide, diallyl disulfide and diallyl trisulfide, which are derived
from garlic, have been reported to reduce viral load of cytomegalovirus
(another enveloped virus) in infected organs of humans and rodents (Fang
et al., 1999). Furthermore, sinigrin, a precursor of the
H2S donor allyl isothiocyanate, and obtained from the
root extract of Isatis indigotica plant for Chinese traditional
medicine (Martelli et al., 2020), inhibited the function of
3-chymotrypsin-like protease, the main protease of SARS-CoV, which
caused the 2002–2004 outbreak of severe acute respiratory syndrome (Lin
et al., 2005).
There are several mechanisms that underlie the antiviral action of
H2S. Firstly, the antiviral activity of
H2S has been suggested to be partly linked to its
antioxidant property - activating and increasing the levels of other
antioxidant enzymes including glutathione (GSH), the most abundant
naturally occurring antioxidant in the body, which inhibits
overproduction of reactive oxygen species (ROS; a destructive mediator
in tissue injury) and its consequent oxidative stress (Palamara et al.,
1995; Kim et al., 2020). Interestingly, ROS-induced oxidative stress has
been associated with viral infection in the kidney (Horoz et al., 2006),
impairing the kidney’s antioxidant defense system. Moreover, Kim et al.
(2020) recently predicted in their study involving high-throughput
artificial intelligence-based binding affinity that GSH interacts with
and possibly inhibits the action of ACE2 and TMPRSS2, the two proteins
that facilitate SARS-COV-2 entry into the kidney. Secondly, findings
from a recent study also suggested that H2S may also
exhibit its antiviral activity against SARS-CoV-2 by interfering with
ACE2 and TMPRSS2 and blocking the attachment of the virus to these host
proteins (Yang et al., 2019), and thereby inhibiting entry of the virus
into the host cell (Fig. 2). Besides, administration of
H2S via its donor molecule NaHS has been reported to
upregulate carotid ACE2 expression and reduced organ damages in a mouse
model of carotid artery ligation (Lin et al., 2017). A recent molecular
dynamics simulation study showed that reduction of disulfides in ACE2
and S protein of SARS-CoV-2 into sulfydryl groups impairs the binding of
the S protein of SARS-CoV-2 to ACE-2 (Hati et al., 2020). Interestingly,
administration of N-acetylcysteine (NAC; an antioxidant
H2S donor and a source of cysteine for endogenous GSH
production) disrupted the disulfides, leading to inhibition of
SARS-CoV-2 into the host cell (Manček-Keber et al., 2021). Moreover, NAC
is a known mucolytic agent that breaks disulfide bonds in mucus, making
it less viscous and easier to be expelled by other mucoactive agents
(expectorants and mucokinetics) together with the action of the ciliary
apparatus of the respiratory system. Thus, H2S
facilitates elimination of potentially harmful viruses such as
SARS-CoV-2, suggesting its antiviral action in COVID-19.
Thirdly, the antiviral action of H2S involves Toll-like
receptors (TLRs), a class of pattern recognition receptors (PRRs) that
initiate innate immune response for early immune recognition of a
pathogen. Following release of viral RNA (i.e. pathogen-associated
molecular pattern) into host cells, it is recognized by PRRs such as
TLRs in the host immune cells, which activates production and secretion
of large amounts of proinflammatory cytokines and chemokines responsible
for cytokine storm and organ damage as seen in the various kidney
conditions discussed in previous sections (Sallenave and Guillot, 2020).
Chen and colleagues (2021) recently reported that deficiency in
endogenous H2S level contributes to sepsis-induced
myocardial dysfunction (SIMD) in humans and mice via increased
expression of TLRs. However, administration of NaHS in SIMD mice
inhibited TLR pathway and prevented TLR-mediated inflammation. Although
this study was not in relation to viruses, it is likely that antiviral
action of H2S involves the same mechanism. Besides,
H2S has been reported to inhibit activation and nuclear
translocation of nuclear factor-kappaB (NF-κB; an inflammatory-related
transcription factor) and thereby suppressing the transcription of
pro-inflammatory genes, leading to inhibition of the secretion of
virus-induced chemokines and cytokines (Li et al., 2015). Fourthly,
post-mortem examination of transplanted kidney, lungs and heart of
COVID-19 deceased patients revealed endotheliitis and accumulation of
apoptotic bodies (Varga et al., 2020), suggesting that inflammation of
the endothelium (an important gatekeeper of cardiovascular health and
homeostasis) and apoptotic cell death contributed to dysfunction or
malfunction of these organs in the COVID-19 patients, which resulted in
death of these patients. As a potential therapy for COVID-19 patients,
there are studies showing the ameliorative effect of H2S
endothelial dysfunction in cardiovascular disorders such as
hypertension, atherosclerosis, hyperhomocysteinemia as well as in
diabetes (Citi et al., 2020; Sun et al., 2020). Besides, overactivation
of the sympathetic nervous system has recently been implicated in
COVID-19 patients with pre-existing chronic lung diseases, kidney
diseases, cardiovascular pathologies, obesity and diabetes mellitus
through factors including ACE2 imbalance, which contributes to organ
damage in these patients (Porzionato et al., 2020). Interestingly,
H2S donors such as NaHS are well-known to suppress
sympathetic activation (Kulkarni et al., 2009; Guo et al., 2011; Duan et
al., 2015; Salvi et al., 2016), and therefore inhibition of sympathetic
outflow could be a potential therapeutic mechanism by
H2S donors for COVID-19 patients.
Another mechanism underlying the antiviral action of H2S
in relation to COVID-19 involves interaction with endoplasmic reticulum
(ER) stress-related proteins. A recent preliminary virtual screening
study in patients with COVID-19 pneumonia revealed higher gene
expression and serum concentrations of glucose-regulated protein 78
(GRP78; an ER stress protein and the host cell surface protein to which
the Spike protein of SARS-CoV-2 binds as revealed by molecular docking)
compared to pneumonia patients without COVID-19 (Palmeira et al., 2020).
There are studies showing the inhibitory action of H2S
donors on GRP78 and other ER stress-related proteins in experimental
models of human diseases. Yi et al. (2018) reported that administration
of NaHS downregulated the expression of GRP78 and other ER
stress-related proteins in uranium-induced rat renal proximal tubular
epithelial cells and mitigated ER stress via activation of
Akt/GSK-3β/Fyn-Nrf2 pathway, a protective molecular pathway. Thisin vitro result supports a previous result by Wei et al. (2010)
who observed attenuation of hyperhomocysteinemia-induced cardiomyocyte
injury following H2S administration in rats.
Administration of NaHS also markedly inhibited cigarette smoke-induced
overexpression GRP78 and other markers of ER stress-mediated apoptosis
and prevented lung tissue damage (Lin et al., 2017). These pieces of
experimental evidence suggest that H2S donors could be
potential antiviral agents that serve to treat COVID-19 patients by
preventing entry of SARS-CoV-2 into host cells via inhibition or
downregulation of the expression of GRP78 and other ER stress-related
proteins, thereby preventing apoptosis and organ damage. In addition to
all these mechanisms, we also reported that H2S
decreases renal expression of kidney injury molecule (KIM-1; a biomarker
of human renal proximal tubular injury) (Dugbartey et al., 2015a;
Dugbartey et al., 2015b), which has recently been found to be associated
with COVID-19 nephropathy and potential receptor for SARS-CoV-2 entry
into renal and lung cells (Wan et al., 2021). Renal and lung epithelial
cells of humans and mice co-expressed KIM-1 and SARS-CoV-2 Spike protein
(Ichimura et al., 2020), suggesting that KIM-1 could directly bind to
SARS-CoV-2 Spike protein following its induction by AKI or other
pathological conditions involving the kidney, as this interaction was
inhibited by anti-KIM-1 antibodies and the KIM-1 inhibitor, TW-37
(Ichimura et al., 2020). Yang et al. (2021) also implicated KIM-1 and
ACE2 in a synergistic interaction which mediated the invasion of
SARS-CoV-2 in kidney cells and worsened COVID-19 infection in the
kidney. We recently showed that activation of endogenous
H2S production by dopamine administration increases
renal expression of H2S-producing enzymes (CBS, CSE and
3-MST) and serum H2S level and decreases renal KIM-1
expression, leading to increased kidney protection in a rat model of
deep hypothermia/rewarming-induced AKI (Dugbartey et al., 2015a). We
also observed decreased expression of KIM-1 in renal tubules and
preservation of renal structures following administration of
5’-adenosine monophosphate, which correlated with increased renal
H2S-producing enzymes and serum H2S
level in a hamster model of therapeutic hypothermia (Dugbartey et al.,
2015b). These observations together with other potential mechanisms that
decrease KIM-1 expression in kidney and lung tissues suggest that
H2S may offer a new therapy for COVID-19-associated
nephropathy and pneumopathy. Other mechanisms underlying the antiviral
action of H2S or H2S donors include
inhibition of gene transcription along with antiviral immunosuppressive
effect, as was reported in human cytomegalovirus (Zhen et al., 2006) and
alterations of the viral membrane, as XM-01 (an H2S
donor) inhibited the activities of enveloped viruses but had no effect
on non-enveloped viruses (Pacheco et al., 2017). The findings from all
these studies strongly suggest that H2S donors could
serve a therapeutic purpose in COVID-19 and its complications including
COVID-19-associated nephropathy (Fig. 2).