During the ongoing coronavirus disease 2019 (COVID-19) pandemic, it has become clear that hyperactive inflammation is sometimes induced by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) via its spike antigen. This is mediated by cytokines released by inflammatory cells, such as monocytes.
An interesting new bioRxiv* preprint reports the suppression of monocyte glycolysis and cytokine production when exposed to the virus, following pretreatment with the commonly used drug, metformin.
Study: Metformin Suppresses Monocyte Immunometabolic Activation by SARS-CoV-2 and Spike Protein Subunit. Image Credit: Numstocker / Shutterstock.com
The hyper-inflammatory reaction induced by SARS-CoV-2 is believed to be largely responsible for much of the organ damage and respiratory distress associated with COVID-19. Various anti-inflammatory drugs have been used to treat this disease with inconsistent success.
The drugs’ lack of success is likely due to the limited understanding of the mechanisms that are responsible for the intense inflammation seen in COVID-19.
To this end, no common pattern of cytokine release has been identified among patients with severe or critical COVID-19. This has led to a diverse nomenclature for this syndrome, including cytokine storm, multisystem inflammatory syndrome, or macrophage activation syndrome.
Monocytes are innate immune cells that are classified as a subtype of mononuclear phagocytes. In COVID-19, monocytes reflect changes such as low human leukocyte antigen DR (HLA-DR) expression, high levels of CD16 expression, and cytokine production, all of which indicate a hyperinflammatory state.
Monocytes give rise to macrophages, which engulf pathogens and cellular debris, including SARS-CoV-2. However, this virus does not actually infect monocytes.
Immune cells in inflammation
In conditions that favor inflammation, as is the case in the SARS-CoV-2 infection, immune cells become metabolically active to promote inflammation and respond to the pathogenic stimulus.
To sustain such high rates of metabolism, immune cells will typically switch to aerobic glucose pathways that yield abundant adenosine triphosphate (ATP) molecules. As a result, these cells increase their production of pro-inflammatory mediators.
SARS-CoV-2 also reprograms lipid metabolism in the monocyte, which leads to the formation of lipid droplets that precede the production of inflammatory cytokines.
HIF-1α-mediated switch to glycolysis
The current study describes the phenomenon that occurs when monocytes bind with the SARS-CoV-2 spike S1 protein. This binding resets the metabolism of monocytes back to anaerobic glycolysis in a dose-dependent manner.
This increase in glycolysis is associated with an increased release of pro-inflammatory cytokines, notably interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α). These cytokines are closely linked to the COVID-19-related cytokine storm.
This switch to anaerobic metabolism is due to the presence of hypoxia-inducible factor-1α (HIF-1α). If this reprogramming could be inhibited, the onset of inflammation could likely be prevented.
Indeed, the HIF-1α inhibitor chetomin, when used to pretreat the macrophage before exposure to S1, led to the suppression of glycolysis and cytokine release.
“HIF-1α appears to be a master regulator of both glycolytic reprogramming and inflammatory activation of monocytes under S1 stimulation.”
Glycolysis suppression linked to cytokine reduction
Pre-treatment of monocytes with the glycolysis inhibitor 2-deoxyglucose (2-DG) also led to a drastic fall in cytokine levels. However, this molecule is also capable of broadly inhibiting mitochondrial metabolism.
Therefore, a control experiment was performed in which monocytes were deprived of glucose. When exposed to S1, the glucose-deprived monocytes experienced an increase in oxidative phosphorylation using fatty acid oxidation, which led to a rise in cytokines despite the inhibition of glycolysis. This was not seen with the use of 2-DG, which suppressed both glycolysis and mitochondrial metabolic pathways.
Though both chetomin and 2-DG inhibited glycolysis and cytokine release from monocytes following exposure to the spike antigen, they are not suitable for clinical administration. Conversely, metformin, which decreases glucose production, is already approved and widely used for the treatment of diabetes as well as for geriatric patients.
Pre-treatment of the monocytes with metformin before exposure to the spike antigen led to the suppression of glycolysis and mitochondrial respiration within the cells. Simultaneously, metformin pre-treatment was also found to prevent cytokine production in response to S1 exposure.
Further, pre-treatment with metformin led to the inhibition of cytokine release from monocytes following their exposure to wild-type SARS-CoV-2. This indicates that metformin is also able to block the effect of other pro-inflammatory proteins besides the spike protein.
What are the implications?
The current study identifies a possible mechanism responsible for hyperactive cytokine release during the early phase of the innate immune response to the virus. Since active infection with the virus does not occur within monocytes, this inflammation appears to be activated by either the viral spike protein, other structural viral proteins, or viral genomic material.
The exposure of the monocytes to viral proteins may be due to viral antigenemia and/or a result of the direct binding of the virus to macrophages. This S1-monocyte/macrophage recognition may also underlie the local inflammation of muscle tissue that is widely reported following COVID-19 vaccination.
Monocytes have low angiotensin-converting enzyme 2 (ACE2) levels, which is the best-known receptor for the SARS-CoV-2. However, the spike protein also binds to C-type lectins, which are signaling molecules on myeloid cells, as well as to the CD147 receptor that is present at high levels on monocytes and macrophages.
The finding that metformin regulates immunometabolic pathways triggered by SARS-CoV-2 is significant, as it offers a potential therapeutic approach to suppress inflammation in COVID-19. As an inexpensive, safe, and widely available drug, metformin offers an excellent avenue for further investigation.
“In summary, the SARS-CoV-2 spike protein induces a pro-inflammatory immunometabolic response in monocytes that can be suppressed by metformin, and metformin likewise suppresses inflammatory responses to live SARS-CoV-2. This has potential implications for the treatment of hyperinflammation during COVID-19.”
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
Cory, T. J., Emmons, R. S., Yarbro, J. R., et al. (2021). Metformin Suppresses Monocyte Immunometabolic Activation by SARS-CoV-2 and Spike Protein Subunit 1. bioRxiv preprint. doi:10.1101/2021.05.27.445991. https://www.biorxiv.org/content/10.1101/2021.05.27.445991v1