mTOR inhibition in COVID-19: A commentary and review of efficacy in RNA viruses

Abstract

In this commentary, we shed light on the role of the mammalian target of rapamycin (mTOR) pathway in viral infections. The mTOR pathway has been demonstrated to be modulated in numerous RNA viruses. Frequently, inhibiting mTOR results in suppression of virus growth and replication. Recent evidence points towards modulation of mTOR in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. We discuss the current literature on mTOR in SARS-CoV-2 and highlight evidence in support of a role for mTOR inhibitors in the treatment of coronavirus disease 2019.

1 |. MANUSCRIPT

The coronavirus disease 2019 (COVID-19) pandemic that emerged in December of 2019 has rapidly spread around the world infecting more than 50 million people and resulting in over 1.2 million deaths. Without an effective vaccine or readily available preventive medication, examining all facets of the disease process, from the beginning of the infection¹ to its sequelae,²is critical to exploring targeted interventions. In this review, we look through the lens of the viral life cycle and how critical host proteins that help promote viral replication are key to identifying potential cellular mechanisms that can become therapeutic targets.

It is well known that viruses cannot replicate on their own and require host transcription and translation machinery to reproduce their genome and associated proteins. Viruses co-opt and modulate normal cellular pathways to facilitate this process.³ Of particular interest in the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) life cycle is its modulation of the mammalian target of rapamycin (mTOR) molecule and its pathways.⁴mTOR is an evolutionarily conserved serine/threonine kinase, regulating two protein complexes mTORC1 and mTORC2, integral to cellular growth. Importantly, mTOR signaling is indispensable to viral translation (Figure 1). Previous research from our group used biophysical modeling of the SARS-CoV-2 life cycle to identify interruption of viral translation as an especially sensitive step to target for inhibition of viral replication.⁵Multiple FDA approved mTOR inhibitors currently exist including metformin (originally termed fluamine and developed as an anti-influenza drug), rapamycin, and everolimus.

FIGURE 1.

Inhibition of mTORC1 by mTOR inhibitors may promote autophagy of infected cells and inhibit translation of SARS-CoV-2 viral polymerase and structural proteins. mTOR, mammalian target of rapamycin; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2

It is well documented that both DNA (i.e., adenoviruses, cytomegalovirus, and herpesviruses) and RNA (i.e., Middle Eastern respiratory syndrome coronavirus [MERS-CoV], influenza, HIV, West Nile virus [WNV], and Zika virus [ZIKV]) viruses modulate the mTOR pathway by activation of phosphoinositide 3-kinase (PI3K), Akt, or mTOR itself.³There is ample evidence to suggest that mTOR inhibition suppresses viral protein synthesis in addition to interfering with virus-mediated transcription events.³

1.1 |. Mammalian target of rapamycin inhibition in RNA viruses

For example, WNV activates the mTOR pathway through PI3K, triggering downstream activators of protein synthesis. Pharmacologic inhibition of mTOR resulted in a significantly reduced growth of WNV in vitro. Further, inducible knockout of mTOR cofactors prevented WNV-induced mTOR activation.⁶

ZIKV and hepatitis C virus (HCV) have been shown to promote their own viral replication by modulating the autophagy response. ZIKV is thought to inhibit the Akt–mTOR pathway promoting autophagy and then subverting the autophagic machinery to promote its own replication. Treatment with metformin was found to inhibit ZIKV in vitro.⁷

HCV is also able to modulate mTOR along multiple points of the pathway and makes use of the autophagic process to establish an infection, and then promotes the growth of hepatocytes, facilitating a persistent infection. Multiple studies have found that metformin inhibits growth of HCV infected cells and suppresses HCV replication via mTOR.^8,9

The 1918 Influenza A virus, has also been shown to hijack the Akt/mTOR signaling pathways to promote viral replication, with inhibition of mTOR severely impairing its replication.¹⁰Treatment with buformin (an mTOR inhibitor) was found to be associated with improved survival in mouse models of influenza.¹¹In a study of 200 patients with H3N2 influenza, treatment with the biguanides phenformin and buformin was associated with reduced incidence of influenza (5.4%) as compared to the control group (24%, p < .001).¹² Futhermore, mTOR inhibition has also been shown to improve clinical outcomes in randomized controlled trials of patients with H1N1 influenza requiring mechanical ventilation.^13,14

mTOR inhibition with guanidine (a derivative of metformin) is also associated with cytopathic effects on poliovirus both in vitro and in primate models.^15,16Guanidine has also been shown to inhibit enterovirus in vitro.^17,18

1.2 |. mTOR blockage inhibits Middle Eastern respiratory syndrome in vitro

mTOR pathway modulation has also been demonstrated in viruses more closely related to SARS-CoV-2. Kinome analysis of MERS-CoV identified mTOR pathway modulation during infection. In vitro experiments of MERS-CoV have demonstrated that PI3K/AKT/mTOR signaling responses play a critical role in viral pathogenesis. Inhibition of the mTOR pathway pre-infection resulted in 60% inhibition of MERS-CoV infection in vitro.¹⁹

1.3 |. mTOR in severe acute respiratory syndrome coronavirus 2

A recent pre-print identified the mTOR–PI3K–AKT pathway as a key signaling pathway in SARS-CoV-2 infection. The authors evaluated three mTOR inhibitors in vitro and identified significant viral inhibition of SARS-CoV-2 with nanomolar drug concentrations of each drug.^{20 A} proteo-transcriptomics analysis in SARS-CoV-2 infected cells identified multiple pathways that exhibited in vitro modulation during the course of the infection. Specifically, all implicated pathways converged on mTOR signaling, however it is not clear how each contributes to the viral life cycle or whether it promotes viral replication and growth.²¹However, if the SARS-CoV-2 cycle and its modulation of cellular pathways is similar to MERS-CoV and other RNA viruses, this presents a potential target to explore for novel or repurposed treatments. Biophysical modeling of SARS-Cov2, predicts that targeting of viral transcription, translation, or both, represent high sensitivity targets for therapeutic inhibition and are likely to result in inhibition of viral replication.^{5 A} SARS-CoV-2 human protein-protein interaction map was recently performed to reveal potential drug targets. The mTOR inhibitors rapamycin and sapanisertib and mTORC1 protein complex modulator metformin were among the proposed drugs due to evidence of mTORC1 involvement. Interestingly, multiple recent retrospective studies have shown a significant mortality benefit for COVID-19 patients on metformin.^22–24

2 |. CONCLUSION

The literature surrounding potential therapeutic targets for COVID-19 is continuously evolving as understanding of the pathogenesis and viral cycle of SARS-CoV-2 improves. In this brief review we present the growing evidence for the role of the PI3K/Akt/mTOR pathway in the cellular response to SARS-CoV-2. Additional biochemical studies and clinical trials are urgently needed to further elucidate the exact role of mTOR inhibitors and modulators in the treatment of COVID-19.