NRF2 activators restrict coronaviruses by targeting a network involving ACE2, TMPRSS2, and XPO1
Nuclear factor erythroid 2-related factor 2 (NRF2) is a master regulator of anti-oxidative and detoxifying cell responses. In addition, it plays important roles in host cell defenses against pathogenic viruses, and small molecules that activate NRF2 signaling can exert potent antiviral effects. We recently found that the NRF2 activators 4-octyl itaconate (4OI), bardoxolone (BARD), and sulforaphane (SFN) interfere with influenza A virus replication by blocking the nuclear export factor exportin 1 (XPO1), which did not require NRF2 signaling. Here, we have assessed their potential to inhibit highly pathogenic (SARS-CoV-2) and seasonal (hCoV-229E) coronaviruses and begun to elucidate the involved mechanisms of action. Using human cell lines and iPSC-derived vascular endothelial cells, we find that NRF2 knock-out or knock-down enhances infection by both viruses, indicating that physiologic NRF2 signaling restricts human coronavirus infection. 4OI, BARD, SFN, as well as the XPO1 blocker Selinexor (SEL), greatly limit infection by both viruses, but in an NRF2-independent manner. Strikingly, the compounds (particularly 4OI) downregulate ACE2 and TMPRSS2 mRNA and protein in Calu3 cells, leading to a >10-fold reduction in viral cell entry by 4OI and SEL, as assessed using SARS-CoV-1 and -2 spike protein VSV pseudotypes. A cycloheximide chase experiment revealed that 4OI dramatically reduces ACE2 half-life, which requires the E3 ligases NEDD4L and MCM1, suggesting that 4OI targets ACE2 for destruction by the proteasome. Moreover, 4OI and SEL reduce XPO1 protein levels, and all compounds reduce XPO1 mRNA levels. Co-incubation experiments of 4OI and the transcription blocker actinomycin D in A549 cells suggest that 4OI acts primarily by interfering with transcription of the XPO1 gene. XPO1 knock-down markedly reduces 229E replication. All four compounds interfere with 229E infection, but do not alter expression of ANPEP, the cellular receptor for this virus. Their anti-229E efficacy depends on expression of XPO1 in host cells in the order of SEL (most dependent) >4OI >SFN >BARD (least dependent), suggesting that especially BARD interferes with 229E infectivity via yet another, unknown, target. Taken together, these results suggest that "NRF2 activators" act as potent antivirals against human coronaviruses by targeting diverse host factors which are critical for viral infectivity. Author summaryHost-directed antiviral compounds act by a variety of mechanisms. For instance, they stimulate cellular antiviral immune responses and target host cell factors which are required for the viral life cycle. Pharmacologic activation of the NRF2 signaling pathway is a particularly attractive antiviral strategy, as this pathway restricts replication of a variety of viruses and also protects cells from excessive inflammation and oxidative stress resulting from accumulation of reactive oxygen species. In our previous study of the NRF2 activators bardoxolone, sulforaphane, and 4-octyl itaconate as host-directed treatments for influenza A virus infection, we found that these compounds interfered with replication of the virus. Unexpectedly, this antiviral activity was completely independent of NRF2 signaling, but resulted from blocking the nuclear export factor XPO1. In the present study, we find that these compounds limit infection by SARS-CoV-2 and hCoV-229E and that, again, this antiviral effect is NRF-independent. Instead, it depends to a large extent on downregulating ACE2 and TMPRSS2 (the major host cell receptors for SARS-CoV-1 and 2) and blocking/downregulating XPO1. Our results underscore the potential of "NRF2 activators" as adjunct treatments for viral infections, as they protect the host by anti-oxidative, anti-inflammatory, and cytoprotective mechanisms and also interfere with diverse host factors required for the viral life cycle.