Inhibitory mechanism of five natural flavonoids against murine norovirus

Background: Human noroviruses (HuNoV), which are responsible for acute gastroenteritis, are becoming a serious public health concern worldwide. Since no effective antiviral drug or vaccine for HuNoV has been developed yet, some natural extracts and their active components have been investigated for their ability to inhibit noroviruses. However, their exact antiviral mechanisms have not been investigated. Purpose: This study was performed to investigate the expression of interferon (IFN)-alpha, IFN-lambda, tumor necrosis factor-alpha (TNF-alpha), Mx, and zinc finger CCCH type antiviral protein 1 (ZAP), 2'-5' oligo (A) synthetase (OAS), and inducible nitric oxide synthase (iNOS) in RAW 264.7 cells pre-treated with fisetin, daidzein, quercetin, epigallocatechin gallate (EGCG), and epicatechin gallate (ECG) that have anti-noroviral activity. Study design: Based on the antiviral activity of the five flavonoids, recently reported by our group, the expression of antiviral factors such as IFN-alpha, IFN-lambda, TNF-alpha, IL-1 beta, IL-6, Mx, ZAP, OAS, and iNOS was investigated in RAW 264.7 cells pre-treated with these flavonoids. Methods: Anti-noroviral effect was determined by performing a plaque assay on cells treated with the flavonoid. RAW 264.7 cells were treated with fisetin, daidzein, quercetin, EGCG, and ECG. Then, mRNA of IFN-alpha, IFN-lambda, TNF-alpha, IL-1 beta, IL-6, Mx, ZAP, OAS, and iNOS were measured by real-time RT-PCR. IFN-alpha, TNF-alpha, IL-1 beta, and IL-6 proteins were measured by ELISA. Results: Pre-treatment with fisetin (50 mu M), fisetin (100 mu M), EGCG (100 mu M), quercetin (100 mu M), daidzein (50 mu M), and ECG (150 mu M) significantly reduced MNoV by 50.00 +/- 7.14 to 60.67 +/- 9.26%. The mRNA levels of IFN-alpha, IFN-lambda, TNF-alpha, Mx, and ZAP were upregulated in RAW 264.7 cells pre-treated with fisetin, quercetin, and daidzein, but not in those pre-treated with EGCG or ECG. Regarding protein levels, IFN-alpha was significantly induced in cells pre-treated with fisetin, quercetin, and daidzein, whereas TNF-alpha was significantly induced only in cells pre-treated with daidzein. Conclusion: Pre-treatment of RAW 264.7 cells with the five flavonoids inhibited MNoV by upregulating the expression of antiviral cytokines (IFN-alpha, IFN-lambda, and TNF-alpha) and interferon-stimulating genes (Mx and ZAP).