Tolerance of high levels of wild-type p53 in transformed epithelial cells dependent on auto-regulation by mdm-2

A significant proportion of human cancers express high levels of p53 protein in the absence of an underlying mutation in the gene. Using transformed (Vh1) and non-transformed (FRTL-5) rat thyroid epithelial cell lines as a model, we have examined the mechanisms by which high levels of wild-type p53 may be tolerated. Stable transfection with p53-dependent reporter constructs demonstrated that the 'excess' wild-type p53 in Vh1 cells is not associated with a comparable increase in p53-dependent transcription (though the response to u.v. irradiation is retained). Mdm-2, which binds p53 and inhibits its transactivation activity, is overexpressed in Vh1 cells in the absence of gene amplification and in a p53-dependent manner. Furthermore disruption of p53-mdm-2 complex formation in Vh1 cells by microinjection of an antibody to the p53-binding domain of mdm-2 resulted in a dramatic increase in p53-dependent transcription. Since only a small proportion of the p53 in Vh1 cells was found to be in complex with mdm-2 (the majority of unbound protein being in a latent form), this suggests that mdm-2 selectively binds a pool of p53 that would otherwise be active as a sequence-specific activator of transcription. We suggest that, in some types of tumour, the 'sensitivity' of the p53-driven mdm-2 feedback loop may be sufficient to prevent free, active p53 reaching the level required for growth arrest or apoptosis, making them an ideal target for therapies designed to disrupt p53-mdm-2 interactions.