Thioredoxin reductase （TrxR） is an important enzyme responsible for the removal of excess reactive oxygen species （ROS） in mammalian cells. This study aimed to investigate the roles of ROS in the anti-tumor effects of ethaselen, a TrxR inhibitor. In a human lung cancer cell line A549, we observed rapidly elevated ROS levels, loss of mitochondrial potential, and cell death with the treatment of ethaselen. Such changes were prevented when cells were pre-incubated with of N-acetyl-cysteine （NAC）, a ROS scavenger. The apoptosis study by Annexin V-PI assay and protein assay by Western blot suggested that mitochondrial pathway was involved in the initiation of apoptosis. In ethaselen-treated cells, cyclosporin A （CsA）, an inhibitor of mitochondrial permeability transition pores （PTPs）, inhibited the release of cytochrome c and reduced cell death in ethaselen-treated cells, while the protein levels of some Bcl-2 family proteins remained unchanged. These results suggest that ROS mediated the ethaselen-induced rapid apoptosis, at least, partially through the enhancement of mitochondrial membrane permeability, which may ultimately lead to the release of pro-apoptotic proteins. Our study, for the first time, demonstrated that the anti-tumor effects of ethaselen, including the early phase of cellular response, are associated with ROS. More studies are needed to confirm our conclusion, as well as to further explore the underlying mechanisms in the anti-tumor effects of ethaselen.
