Nrf2 Activator PB125® as a Potential Therapeutic Agent against COVID-19

Abstract

Nrf2 is a transcription factor that regulates cellular redox balance and the expression of a wide array of genes involved in immunity and inflammation, including antiviral actions. Nrf2 activity declines with age, making the elderly more susceptible to oxidative stress-mediated diseases, which include type 2 diabetes, chronic inflammation, and viral infections. Published evidence suggests that Nrf2 activity may regulate important mechanisms affecting viral susceptibility and replication. We examined gene expression levels by GeneChip microarray and by RNA-seq assays. We found that the potent Nrf2-activating composition PB125^® downregulates ACE2 and TMPRSS2 mRNA expression in human liver-derived HepG2 cells. ACE2 is a surface receptor and TMPRSS2 activates the spike protein for SARS-CoV-2 entry into host cells. Furthermore, in endotoxin-stimulated primary human pulmonary artery endothelial cells, we report the marked downregulation by PB125 of 36 genes encoding cytokines. These include IL-1-beta, IL-6, TNF-α, the cell adhesion molecules ICAM-1, VCAM-1, and E-selectin, and a group of IFN-γ-induced genes. Many of these cytokines have been specifically identified in the "cytokine storm" observed in fatal cases of COVID-19, suggesting that Nrf2 activation may significantly decrease the intensity of the storm.

Keywords: ACE2; ALI; COVID-19; CXCL10; HDAC5; LIF; NFE2L2; Nrf2; SARS-CoV-2; TMPRSS2; coronavirus; cytokine storm.

Figure 1

IL-6 protein release is attenuated by Nrf2 activation. HPAECs pretreated with 5 μg/mL PB125 and then stimulated for 5 h with 20 ng/mL LPS had significantly lower levels of IL-6 released into the culture media than vehicle-pretreated HPAECs stimulated with LPS (n = 3 in each group).

Figure 2

Regulation of pro- and antiviral genes by PB125. HepG2 cells were cultured overnight in 24-well plates with control vs. 16 µg/mL PB125 and gene expressions were determined using RNA-seq analysis on four biological replicates. All six genes differed from control by p < 0.04.

Figure 3

The expression of 36 LPS-induced cytokines in cultured HPAECs was strongly inhibited by PB125 at 5 μg/mL in culture medium. Control expressions were normalized to 100% expression (0% suppression).

Figure 4

The replication cycle of SARS-CoV-2. Binding of virus to the cell membrane (1) occurs via ACE2 receptors. The spike protein must then be cleaved (indicated as scissors representing serine protease TMPRSS2) to allow entry into the cell (2). The activated spike protein penetrates the cell membrane (3), allowing entry of the viral genome (4), which is replicated, translated, and assembled into mature virus particles (5). On the left, TMPRSS2 inhibition is shown by the antiviral drugs Camostat and Nafamostat, as well as by plasminogen activator inhibitor, PAI-1, encoded by the SERPINE1 gene. PB125 upregulates PAI-2 and downregulates both TMPRSS2 and ACE2 in HepG2 cells.

Figure 5

Development and resolution of an acute inflammatory event. (1) Initiation occurs with bacterial or viral infection, which triggers (2) local production of cytokines by endothelial cells to call in inflammatory cells to neutralize the invasion. (3) An attack ensues in which superoxide and secondary oxidants are produced, phagocytosis occurs, and more cytokines are released by the first responders, calling in subsequent waves of activated inflammatory cells. (4) In young healthy cells, the oxidative stress generated by the battle activates Nrf2 and, within hours, the nearby tissues are inhibiting further tissue cytokine production, rescuing host cells from further damage and permitting (5) repair, clean up, and recovery. Alternatively, in (4a), older cells deficient in Nrf2 may be unable to mount Nrf2 sufficient activation to break the self-sustaining chain reaction, resulting in an uncontrolled cytokine storm that ultimately destroys the host tissue and leads to death. A more robust activation of the limited Nrf2 available in older cells may be provided by pharmacological or phytochemical Nrf2 activators.