Sodium orthovanadate is a bifunctional inhibitor of transcription-dependent and -independent p53-mediated apoptosis

We recently reported a novel suppressive effect of sodium orthovanadate (vanadate) on the DNA-binding activity of p53. In this study, we initially showed that vanadate had a more potent antiapoptotic activity than three other chemical p53 inhibitors, including pifithrin-α (PFTα), a well-known inhibitor of p53. Although the other agents inhibited the p53 transcriptional activity, they did not suppress p53-dependent apoptosis in irradiated MOLT-4 cells. Further investigations using cells with defective or impaired p53 function indicated vanadate’s specificity for p53 in suppressing DNA damage-induced apoptosis. To investigate the cause for the different effects of vanadate and the other inhibitors, we chose PFTα and PFTµ (an inhibitor of p53-mediated transcription-independent apoptotic pathway), as references, and determined the effect of them and vanadate on p53-mediated apoptosis, with a particular focus on the transcription-independent pathway. We found that vanadate suppressed the p53-associated apoptotic events at the mitochondria, such as the loss of the mitochondrial membrane potential, the conformational change of Bax and Bak, the mitochondrial translocation of p53, and its interaction with Bcl-2. Vanadate also suppressed the apoptosis-inducing activity of mitochondrially-targeted temperature-sensitive p53 in stable transfectants of the SaOS-2 cell line. Finally, we tested vanadate’s potential use as a radioprotector. Vanadate completely protected mice from a sublethal dose of 8 Gy and partially from a lethal dose of 12 Gy. Our data demonstrate that vanadate can suppress both the transcription-dependent and the transcription-independent pathways, and suggest that both pathways must be inhibited to completely block p53-mediated apoptosis.