Decreased intracellular superoxide levels activate Sindbis virus-induced apoptosis

Infection of many cultured cell types with Sindbis virus (SV), an alphaviru s, triggers apoptosis through a commonly utilized caspase activation pathwa y. However, the upstream signals by which SV activates downstream apoptotic effecters, including caspases, remain unclear. Here we report that in AT-3 prostate carcinoma cells, SV infection decreases superoxide (O-radical ani on) levels within minutes of infection as monitored by an aconitase activit y assay. This SV-induced decrease in O-radical anion levels appears to acti vate or modulate cell death, as a recombinant SV expressing the O-radical a nion scavenging enzyme, copper/zinc superoxide dismutase (SOD), potentiates SV-induced apoptosis. A recombinant SV expressing a mutant form of SOD, wh ich has reduced SOD activity, has no effect. The potentiation of SV-induced apoptosis by wild type SOD is because of its ability to scavenge intracell ular OX rather than its ability to promote the generation of hydrogen perox ide. Pyruvate, a peroxide scavenger, does not affect the ability of wild ty pe SOD to potentiate cell death; and increasing the intracellular catalase activity via a recombinant SV vector has no effect on SV-induced apoptosis, Moreover, increasing intracellular O-radical anion by treatment of 3T3 cel ls with paraquat protects them from SV-induced death. Altogether, our resul ts suggest that SV may activate apoptosis by reducing intracellular superox ide levels and define a novel redox signaling pathway by which viruses can trigger cell death.