High apoptotic threshold mediates p53 dependent decision between arrest and apoptosis

In response to stress, the p53 tumor suppressor induces arrest or apoptosis by transcriptionally regulating genes that mediate these processes. It has been proposed that the levels of p53 can influence the choice between these different outcomes, but the mechanisms involved are not clear. To gain mechanistic understanding of this p53-dependent cell fate decision, we generated a p53 inducible system that allowed tight regulation of p53 expression in human mammary epithelial cells. We used microarrays to detail the global programme of gene expression underlying cellularisation and identified distinct classes of up-regulated genes during this process. Using microarray and chromatin immunoprecipitation analysis, we showed that low and high levels of p53 bind to and activate the same set of pro arrest and pro apoptotic target genes, induced to lower and higher levels, respectively. We propose that the cell fate decision between arrest and apoptosis in these cells is determined by a higher threshold required for p53 dependent apoptosis. We suggest that high level p53 activation is crucial in order to achieve maximum efficacy of p53 targeted cancer therapies.