p53 mediated apoptosis in HeLa cells: transcription dependent and independent mechanisms

The most frequent target for genetic alterations in human cancers is the p53 tumor suppressor gene. Mutations in p53 abrogate its ability to inhibit cell growth and to suppress tumor progression. The anti-proliferative activity of p53 can be mediated by the induction of growth arrest and/or programmed cell death (apoptosis). Recent in vivo studies support the involvement of apoptosis in tumor suppression by p53. To gain further insight into the mechanisms by which p53 induces apoptosis, the activity of p53 was studied in HeLa cells using a transient transfection assay. To define the functional domains of p53 required for apoptosis a C-terminal deletion mutant of p53 was used. This mutant, p53d1214, lacks the oligomerization domain, the nuclear localization signal and a large part of the core DNA binding domain. This mutant was shown to be deficient in sequence specific transactivation activity. Overexpression of wt p53 induced an efficient apoptosis in transiently transfected HeLa cells. Surprisingly p53d1214, containing only the first 214 N-terminal residues induced extensive apoptosis. The induction of apoptosis by p53d1214 is slower than that induced by wt p53. Furthermore, p53d1214 suppressed the transformation of rat embryo fibroblasts by several oncogene combinations, such as myc plus ras. In view of the fact that p53d1214 lacks measurable transactivation potential, our findings suggest the existence of two p53 dependent-apoptotic pathways--one involves activation of specific target genes, and the other is independent of it. Transactivation independent apoptosis may play a central role in tumor suppression by p53.