P53 LEVELS, FUNCTIONAL DOMAINS, AND DNA-DAMAGE DETERMINE THE EXTENT OF THE APOPTOTIC RESPONSE OF TUMOR-CELLS

It is well established that induction of the p53 tumor suppressor prot ein in cells can lead to either cell cycle arrest or apoptosis. To fur ther understand features of p53 that contribute to these cell response s several p53-null Saos2 and H1299 cell lines were generated that expr ess wild-type or mutant forms of p53, or the cyclin-dependent kinase i nhibitor p21/WAF1, under a tetracycline-regulated promoter. Our result s show that the cellular level of p53 can dictate the response of the cell such that lower levels of p53 result in arrest whereas higher lev els result in apoptosis; nevertheless, DNA damage can heighten the apo ptotic response to p53 without altering the protein level of p53 in ce lls. We also demonstrate that arrest and apoptosis are two genetically separable functions of p53 because a transcriptionally incompetent p5 3 can induce apoptosis but not arrest, whereas induction of p21/WAF1, which is a major transcriptional target of p53, can induce arrest but not apoptosis. Finally, we show that a full apoptotic response to p53 requires bath its amino and carboxyl terminus, and our data suggest th at there is synergism between transcription-dependent and -independent functions of p53 in apoptosis. Thus, there are multiple independent c ellular responses to p53 that together may account for the extraordina rily high frequency of p53 mutations in diverse types of human tumors. The implications of these results are discussed and a model is propos ed.