Contributed by guest blogger: Brian Lu ’13
Zinc ions function in many different cellular processes and have been shown to play important roles in the proper folding and activity of various cellular enzymes and transcription factors, but zinc ion concentrations are kept relatively low in the cell by metallothioneins. High concentration of zinc ions and compounds that stimulate cellular import of zinc ions have been shown to inhibit the replication of various RNA viruses, including influenza virus, respiratory syncytial virus, and several picornaviruses, but details of the effects of zinc ions on nidoviruses are not well understood. Nidoviruses include major pathogens of both human and livestock, including severe acute respiratory syndrome coronavirus (SARS-CoV), the arterivirus equine arteritis virus (EAV), and porcine reproductive and respiratory syndrome virus (PRRSV). A recent study suggests that zinc ions also inhibit nidovirus replication by blocking RNA synthesis.
As high concentrations of zinc is known to inhibit cellular translation, the researchers tested if high concentrations of zinc would also inhibit viral translation. After determining the concentration of pyrithione (PT) cells will tolerate without negative effects, cells were incubated with non-toxic concentrations of PT and zinc ions. PT, functioning as an ionophore, stimulated the cellular import of zinc ions and increased the cellular concentration of zinc. The results showed a dose-dependent inhibition of viral gene expression of both SARS-CoV and EAV by the addition of PT. The inhibition of viral gene expression appears to be the result of direct inhibition of RNA-dependent RNA polymerase (RdRp) activity. The researchers also observed a dose-dependent decrease in RNA synthesis for SARS-CoV and EAV by testing the effect of zinc on the virus’s replication/transcription complex (RTC). RNA synthesis, separate from mRNA synthesis for gene expression, is an integral part of viral replication, and a decrease in RNA synthesis would imply a decrease in viral replication as well. Interestingly, the zinc ion’s effects on RNA synthesis are reversible. The addition of magnesium-saturated ethylenediaminetetraacetic acid (MgEDTA) restored RTC activity in both EAV and SARS-CoV. MgEDTA ionizes to magnesium ions and EDTA in solution, which binds to the zinc ions and prevents them from interacting with viral RTC. Adding zinc ions at different stages of RNA synthesis showed that zinc inhibits synthesis at the initiation stage for EAV but inhibits synthesis at the initiation and elongation stages for SARS-CoV.
The use of zinc ions and PT as inhibitors of nidovirus replication in cell culture can be further investigated for use as antiviral compounds, and a better understanding of the inhibition mechanism may yield future antiviral drugs against SARS and other nidovirus-related diseases. But before zinc can be used as an antiviral compound, several questions need to be answered. What is the exact mechanism for RdRp inhibition? What are the systemic effects of PT? What levels of zinc and PT would be safe for an organism? The U.S. Food and Drug Administration has approved pyrithione zinc less than 2 percent in concentration for topical use in treating dandruff, but there are no guidelines for internal uses of pyrithione zinc. It is known, however, that industrial concentrations of PT zinc is highly toxic. Additionally, in depth structural analysis and mutational studies of nidovirus RdRps is needed to determine a structural mechanism for zinc-induced inhibition of RdRp activity. Unfortunately, zinc ion binding is very fleeting and not detectable with currently available methods. As such, more sensitive methods of detecting zinc binding may be needed before the mechanism for zinc-induced inhibition of RdRp activity can be determined. Water-soluble zinc-ionophore may be better suited as the compound appears to be non-toxic even at concentrations that were effective against tumors in a mouse model. The reversible property of zinc-induced inhibition can be used in future research to gain a better understanding of nidoviral RNA synthesis. If pyrithione zinc is shown to be safe and effective in animal models, it still has to go through clinical trials before it can be used as an antiviral treatment.
Brian Lu is a junior at Vassar College, majoring in biochemistry.