SARS-Cov2, is a newly emerged strain of coronavirus responsible for
causing the coronavirus disease-2019 pandemic (COVID-19), wreaking havoc
worldwide. This virus belongs to the genus Betacoronaviruswhose other
members-severe acute respiratory syndrome coronavirus (SARS-CoV) and
Middle East respiratory syndrome coronavirus (MERS-CoV) are known for
two previous outbreaks. Although, SARS and MERS caused deadly pneumonia
with mortality rates of 10% and 36% respectively1,
much higher as compared to 2.3% for SARS-CoV2
infections2, the overall number of deaths from
SARS-CoV2 far outweighs the previous outbreaks. At genome level,
SARS-CoV2 is more similar to a bat SARSr-CoV showing 96% genome
identity than that of SARS and MERS sharing about 79% and 50% of
genome identity respectively3,4. Two of the
significant predictors of severity and death in patients affected by
SARS, MERS and pandemic influenza are diabetes and uncontrolled
glycemia5-7. Interestingly, majority of deaths in the
SARS-CoV2 infection have occurred in patients suffering from metabolic
co-morbidities and indeed metabolic co-morbidities have been reported to
portend worse disease outcomes in multiple studies carried out in
different geographical locations, with completely different ethnic
populations.8-10 But, the reason for this often-fatal
role of metabolic co-morbidities, more apparent in the case of SARS-CoV2
infection, is not yet mechanistically clear. We aimed at understanding
the pathogenetic mechanisms underlying this prognostic implication of
metabolic disorders in COVID-19 that may differentiate SARS-CoV2
infection from few other respiratory viruses that caused more limited
pandemics in the recent past.
In order to do so, we undertook a meta-analysis of three publicly
available gene expression studies (RNA-sequencing as well as microarray
data) done on human lung epithelial cells (HLECs) (either A549 or Calu-3
cells) infected with different respiratory viruses, viz. SARS-CoV2,
Respiratory Syncytial Virus or RSV, H1N1 Influenza, SARS and H3N2
Influenza11-13. The HLEC transcriptome on infection
with these viruses revealed transcriptional signatures shared between
the different viruses as well as transcriptional regulations exclusive
to each of the viruses, as expected (Figure 1A). SARS-CoV2 was found to
drive differential regulation of maximum number of genes among the
datasets analyzed. Further analysis of the significantly regulated gene
expressions revealed the key enriched pathways that are differentially
regulated among these viruses (Figure 1B, C; Supplemental information).
In Figure 1B and C, we represented pathways, among those enriched, which
have role in cellular metabolic regulation and for which the nature of
regulation of the pathway could be ascertained and excluded pathways
that are related to diseases of no apparent relevance. Amongst these,
modulation of a few key metabolic pathways (viz. AMPK signaling pathway,
HIF1αsignaling pathway, mitophagy, AGE-RAGE signaling pathway, pentose
phosphate pathway etc.) as well as the cellular senescence pathway and
lysosome mediated degradation pathway were found to be exclusive to
SARS-CoV2 infection (Figure 1B,C). The AGE-RAGE signaling pathway and
cellular senescence pathway were upregulated (Figure 1B), whereas the
AMPK signaling pathway, HIF1α signaling pathway, mitophagy, pentose
phosphate pathway and the lysosome pathway were all downregulated
(Figure 1C). Interestingly, dysregulation of most of these metabolic
pathways (upregulation of AGE-RAGE signaling and downregulation of AMPK
signaling pathway, HIF1α signaling pathway, mitophagy as well as the
pentose phosphate pathway) have all been previously implicated in
systemic disease pathogenesis observed in insulin resistance, Type 2
diabetes and its associated complications14-18.
Type 2 diabetes mellitus (T2DM) and obesity are two of the major
co-morbidities portending worse prognostic outcomes in Covid-19
patients8-10. Interestingly,previous reports have
established the increased association of asthma and chronic obstructive
pulmonary disease(COPD) with these metabolic
disorders19. Infact, these diseases associated with
increased systemic metabolic disorder, are known to cause metabolic
dysregulations in HLECs, which possibly underlie the predisposition of
diabetic individuals towards asthma and COPD20,21. In
order to compare the transcriptomic signature of SARS-Cov2 infected lung
and diabetic lung, we performed an integrated analysis of the HLEC
transcriptome in response to SARS-CoV2 infection as well as a COVID-19
bronchoalveolar lavage22and autopsied lung
transcriptome23 with lung transcriptome from two
different datasets of preclinical rodent models of diet-induced
obesity24,25 (Figure 2A). Interestingly, we found
significant overlaps in how some of these key pathways are
transcriptionally regulated in lung in these two distinct clinical
contexts of SARS-CoV2 infection and metabolic disorder (Figure 2B). In
Figure 1D, selection of pathways were based on whether 1) pathway
enrichment and the nature of regulation are shared between at least two
datasets, 2) are not enriched in any of the other 4 virus datasets (RSV,
SARS, H1N1 and H3N2), 3) are related to cellular metabolic regulation,
4) are represented in either Figure 1B or 1C and 5) are not primarily
linked to diseases of no apparent relevance. Thus, the similarity in the
dysregulation of pathways in HLECs/lung upon SARS-CoV2 infection and
metabolic disorders was apparent.
Our meta-analysis led us to hypothesize that with underlying metabolic
co-morbidities, e.g. T2DM, this already present metabolic dysregulation
of the HLECs is further aggravated upon SARS-CoV2 infection leading to
faster and more widespread disruption of epithelial integrity and
pulmonary damage, leading to worse disease outcomes. This provides a
possible explanation for the distinction observed in the characteristics
of the population subgroups worst affected by SARS-CoV2 and the other
above-mentioned viruses including viruses belonging to the same family,
such as SARS.
We envisage that any therapeutic agent that can target these key
featured pathways, as shown in Figure 2B, should be of interest in
SARS-CoV2 infection. This also points to a hitherto untapped potential
for use of the widely used anti-diabetic drug metformin in COVID-19,
which is an established modulator of most of these dysregulated
pathways26-30. Indeed, metformin has been reported to
inhibit AGEs, thus downregulating the AGE-RAGE signaling pathway
associated with diabetes complications26. Metformin
also enhances mitophagy in patients with Type 2
diabetes27. This anti-diabetic drug has also been
reported to maintain cellular integrity by hindering cellular
senescence28. The role of metformin in upregulation of
the AMPK signaling pathway is one of its most well-established
mechanisms of action involved in treating diabetes, reported in multiple
studies29. Metformin has also been reported to
modulate fatty acid metabolism30.
In addition to its projected effect on maintenance of lung epithelial
integrity through these metabolic pathways in the epithelial cells,
metformin has also been shown to boost CD8+ T cell
memory though metabolic reprogramming, an effect which will also add to
its potential therapeutic action in COVID-1931,32. In
a preclinical model on pulmonary effects of air pollution it was also
shown that metformin can attenuate the production of the proinflammatory
cytokine IL-6 from alveolar macrophages33, which is
also a critical component of the hyperimmune response in
COVID-1934. Moreover, vascular thromboembolism has
been shown to be a unique feature in a number of COVID-19 patients in
different studies35,36 and metformin has been shown to
have an antithrombotic effect through inhibition of platelet activation
and maintenance of endothelial integrity37,38. Thus,
metformin, due to its multifaceted pharmacodynamics, is well poised for
acting as a therapeutic agent in SARS-CoV2 infections. But for
ascertaining the therapeutic potential of metformin, mechanistic studies
in the context of SARS-CoV2 infection, as well as comprehensive clinical
trials in COVID-19 patients with or without metabolic co-morbidities,
are warranted.