GA inhibits HSV-1 and ZIKV infection
We tested GA on HSV-1, a rapidly replicating and lytic DNA virus, and on ZIKV, an RNA virus which has neither glycoprotein conservation nor common receptors with HCMV or HSV-1. To test whether GA inhibits HSV-1, we designed an experiment to test the direct effect of GA on the virus. For this experiment, we used HEp-2 and 293T cells. 1 × 107 PFU HSV-1 strain F was treated for 1 hour with GA (50 µM) or with vehicle in serum free DMEM and then each of these were used to infect HEp2 or 293T cells at an MOI of 0.5 in 199V medium with a final concentration of 2.5 µM GA. As a control, the virus that was originally treated with the vehicle was then supplemented also with 2.5 µM GA. To test for successful viral infection of HEp2 and 293T cells, production of HSV-1 immediate early (ICP27), early (ICP8), and late (US11) proteins was analyzed by Western Blot (Fig. 2A,B). The results showed that in the treated HSV-1 F stock there was complete inhibition of viral replication, as indicated by lack of protein synthesis. This implies that viral entry may be a target of GA, because direct treatment of HSV-1 with GA blocks downstream HSV-1 protein production. To test the effect of GA on HSV-1 replication, HEp2 cell monolayers were grown to 90% confluency in a 6-well plate, treated with 10 µM GA C15:1 or vehicle for 1 hour in 199V medium, and then inoculated with HSV-1 strain F for protein analysis. HSV-1 ICP27, ICP8, and US11 proteins were detected by Western Blot (Fig. 2C). The results showed profound inhibition of immediate early, early, and late viral proteins. The inhibition of the full temporal range of HSV-1 proteins implies that inhibition of viral replication occurs by blocking the entry of virions into the target cell. To evaluate the effect of GA on progeny virions, the supernatant of GFP-HSV-1 strain 17+ infected Vero cells at an MOI of 1 was collected after 20 h and titered by plaque reduction assay (Fig. 2D). The results showed a significant decrease of approximately 2 log in the GA-treated cell titers.
Figure 2 GA inhibits HSV-1 infection. (A) 1 × 107 PFU HSV-1 strain F were treated for 1 hour with 50 µM GA (Lanes 1–3) or with vehicle (Lanes 4–6) in serum free DMEM and then were used to infect HEp2 or 293T cells at an MOI of 0.5 in 199V medium with final concentration of 2.5 µM GA. 6-well cultures of HEp-2 (A) or 293T (B) were then infected, and at 5, 10 and 29 hours post-inoculation, cells were collected and a fraction of the total cell lysate was subject to Western Blot analysis using antibodies directed against ICP8, ICP27, US11 and β-Actin. (C) HEp2 cell monolayers were treated with 10 µM GA C15:1 or vehicle for 1 hour in 199V medium and then infected with HSV-1 strain F at 4, 8, 12, 24 and 32 hours post-inoculation cells were collected and a fraction of the total cell lysate was subject to Western Blot analysis using antibodies directed against ICP8, ICP27, US11 and β-Actin. Normalized ratios of protein expression are in the bar graph. (D) Titration by plaque assay of Vero cells pretreated with 10 µM GA C15:1 or vehicle and then infected with GFP-HSV-1 strain 17+.
To test the effect of GA on ZIKV infection, NHA were grown to 90% confluency in a 24-well plate, treated with GA or DMSO for 3 hours without serum and then infected with ZIKV strain PRVABC59 at an MOI of 0.3. The next day, supernatant was replaced with fresh medium containing GA or DMSO and cells were incubated at 37 °C, 5% CO2. On day 7, viability was determined with the MTS Cell Proliferation Assay (Promega), and then these cells were harvested to extract total RNA for quantification of ZIKV RNA by Taqman based qRT-PCR. The results showed 70% to 80% viability at 5 µM to 20 µM GA, compared to less than 40% viability for the vehicle-treated cells. Furthermore, an 80–90% decrease in ZIKV RNA was observed at 5 µM to 20 µM GA (Fig. 3A,B). We concluded that GA inhibits entry of Zika virions and prevents NHA cell death. This suggests that the mechanism of inhibition that appears to be targeted in HSV and HCMV by GA is conserved among enveloped viruses that have no homologous glycoproteins and use different cell receptors for entry. To test the direct effect of GA on ZIKV, 5 × 105 PFU ZIKV were treated for 1 hour with GA (10 µM/ml) or with vehicle in 199V medium, and then were used to infect Vero cells at an MOI of 0.5. Supernatant was collected at 4, 24 and 48 hours post-inoculation for cell free viral RNA copy number determination (Fig. 3C). The results indicated that the GA-treated ZIKV was completely inhibited, as indicated by cell free ZIKV RNA copy number. This suggests that viral entry of ZIKV, may be a target of GA because direct treatment of ZIKV with GA blocks downstream ZIKV RNA production.
Figure 3 GA inhibits ZIKV infection. (A) Viability of NHA infected with ZIKV and treated with GA. NHA were grown in 24-well plates to >80% confluency and treated with GA (0–20 µM) or DMSO for 3 hours and infected with ZIKV strain PRVABC59 at an MOI of 0.3. At 12 hpi, supernatants were carefully removed and replaced with fresh AGM. (B) 7 days post infection, samples were analyzed for cell viability by the MTS assay and then live cells were harvested for RNA and processed with Taqman based real-time PCR to quantify ZIKV RNA. Experiments were performed in duplicate three independent times and the data were analyzed by t-test (* indicates p ≤ 0.05). (C) Cell free ZIKV in 199V medium was pretreated with 10 µM GA or with DMSO for 1 hour at 37 °C. The pretreated ZIKV was used to infect Vero cells grown in 6 well plates >80% confluency with 0.5 MOI for 1 hour. The infected cells were washed twice with warm DPBS and supplemented with DMEM supplemented with 5% FBS. Supernatant was collected at 4, 24 and 48 hours post-inoculation for cell free viral RNA copy number determination. Experiments were performed in triplicates.