SV40 T/t-antigens sensitize mammary gland epithelial cells to oxidative stress and apoptosis

As shown recently, the SV40 T/t-antigens (T/t-ag) exert a strong apoptotic activity in mouse mammary gland epithelial cells (ME-cells) leading to premature gland involution at late pregnancy. This high spontaneous cell death rate (20%) is also maintained in T/t-ag positive ME-tissue culture cell lines (e.g., 8/61-A), but not in those ME-cells that have switched off the SV40 T/t-transgene expression. In this study, we demonstrate for the first time that the T/t-ag sensitize ME-cells to oxidative stress leading to apoptosis. Treatment of the 8/61-A ME-cells with catalase, a scavenger of H2O2, completely blocked spontaneous cell death, which was linked to downregulation of caspase-3 activity. Furthermore, exposure of the cells to low concentrations of H2O2 highly increased the apoptosis rate. These findings suggest that the T/t-ag positive ME-cells contain either elevated levels of reactive oxygen species or reduced antioxidant activities. During spontaneous and H2O2-induced apoptosis, the activity of caspase-3 is significantly increased. In addition, the 8/61-A cells accumulated p21 and Bax proteins while the level of the anti-apoptotic protein Bcl-2 decreased implying a posttranscriptional regulation of apoptosis.