Mechanism of inhibitory effect of sodium orthovanadate on transcription-independent p53-induced apoptosis

Abstract

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 more potent antitapoptotic activity than other three chemical p53 inhibitors including pifithrin-alfa (PFT), a well-known inhibitor of p53. Although others inhibited the p53 transcriptional activity, they were not able to suppress p53-dependent apoptosis in irradiated MOLT-4 cells. To pursue the difference between the vanadate's effect and others, we chose PFT as a reference, and determined the effect of each inhibitor on p53-mediated apoptosis, especially focused on transcription-independent pathway. The following experiments revealed that vanadate showed more potent suppressive effect on p53-associated mitochondrial apoptotic events, such as the loss of mitochondrial membrane potential, the conformational change of Bax and Bak, the ubiquitilation and mitochondrial translocation of p53, and the interaction of p53 with Bcl-2. In addition, vanadate was capable of suppressing the apoptosis-inducing activity of mitochondrially targeted p53 in temperature-sensitive SaOS-2 stable transfectants. Our data demonstrate that vanadate can also suppress the transcription-independent pathway, and suggest the possibility that inhibition of both the transcription-dependent and transcription-independent pathways is needed to sufficient suppression of p53-mediated apoptosis.