Characterization of structural p53 mutants which show selective defects in apoptosis but not cell cycle arrest

Suppression of tumor cell growth by p53 results from the activation of both apoptosis and cell cycle arrest, functions which have been shown to be separable activities of p53. We have characterized a series of p53 mutants with amino acid substitutions at residue 175 and show that these mutants fall into one of three classes: class I, which is essentially wild type for apoptotic and cell cycle arrest functions; class II, which retains cell cycle arrest activity but is impaired in the induction of apoptosis; and class III, which is defective in both activities. Several residue 175 mutants which retain cell cycle arrest function have been detected in cancers, and we show that these have lost apoptotic function. Furthermore, several class II mutants have been found to be temperature sensitive for apoptotic activity while showing constitutive cell cycle arrest function. Taken together, these mutants comprise an excellent system with which to investigate the biochemical nature of p53-mediated apoptosis, the function of principal importance in tumor suppression. All of the mutants that showed loss of apoptotic function also showed defects in the activation of promoters from the potential apoptotic targets Bax and the insulin-like growth factor-binding protein 3 gene (IGF-BP3), and a correlation between full apoptotic activity and activation of both of these promoters was also seen with the temperature-sensitive mutants. However, a role for additional apoptotic activities of p53 was suggested by the observation that some mutants retained significant apoptotic function despite being impaired in the activation of Bax- and IGF-BP3-derived promoters. In contrast to the case of transcriptional activation, a perfect correlation between transcriptional repression of the c-fos promoter and the ability to induce apoptosis was seen, although the observation that Bax expression induced a similar repression of transcription from this promoter suggests that this may be a consequence, rather than a cause, of apoptotic death.