Mechanism of measles virus inhibition by retinoids: The role of signaling by retinoid nuclear receptors and type I interferon

Measles-associated morbidity and mortality can be decreased in response to treatment with vitamin A. However, the mechanism underlying this beneficial effect is unknown. In this work, we investigate the molecular interaction between measles virus (MeV) and retinoids in vitro. Our first goal was to assess whether retinoids influence the in vitro replication of MeV. We found all-trans retinoic acid (ATRA), as well as other natural and synthetic retinoids, inhibited MeV replication in primary cells and a range of cell lines. This inhibitory effect was mediated through retinoid nuclear receptor signaling, and retinoic acid receptor alpha (RARalpha) in particular. Next, we sought to determine the role of type I interferon (IFN), since both MeV and retinoids can modulate IFN signaling. Type I IFN blocking antibodies abrogated the inhibitory effects of ATRA on MeV replication. IFN-stimulated genes (ISG) were upregulated by ATRA in MeV-infected cell cultures when compared to either treatment or infection alone. Using a transwell system, we determined that this increased gene expression occurred in initially uninfected bystander cells. The ATRA-treated bystander cells did not support substantial viral replication when subsequently challenged with MeV. Finally, we identified retinoid-induced gene I (RIG-I) as a link between the RARalpha- and IFN-dependent mechanism of MeV inhibition by retinoids. We found that RIG-I can be transcriptionally regulated by ATRA. Furthermore, RARalpha function was necessary for the induction of RIG-I by MeV and ATRA, and could bind to the RIG-I promoter. A retinoic acid response element (RARE) in the RIG-I promoter region was identified, and RIG-I promoter constructs were shown to produce luciferase in response to ATRA. Finally, we showed that functional RIG-I was necessary for ATRA to inhibit MeV replication. In summary, we have demonstrated that transcriptional regulation of RIG-I by ATRA through RARalpha is required for inhibition of MeV. Furthermore, initially uninfected bystander cells upregulate their expression of ISGs, making them refractory to productive infection during subsequent rounds of viral replication. Therefore, we conclude that ATRA inhibits MeV replication through the RARalpha-dependent regulation of RIG-I and IFN signaling.