| The p53 tumour suppressor promotes cell-cycle arrest or apoptosis in response to cellular stress such as DNA damage and oncogenesis. It is not clear why p53 induces only G1 arrest in some cells but apoptosis in others. This thesis aims at understanding the mechanisms that govern cell-fate determination after p53 activation.; Murine erythroleukemia cells that lack endogenous p53 expression were transfected with a temperature-sensitive (ts) p53 allele. The erythroleukemia cells responded to p53 activation by undergoing G1 arrest, apoptosis and differentiation. Addition of a cytokine such as erythropoietin, c-kit ligand or interleukin-3 to the culture medium blocked p53-mediated apoptosis and differentiation but not p53-mediated G1 arrest. These observations indicate that apoptosis and G1 arrest can be effectively uncoupled through the action of cytokines acting as survival factors and are consistent with the idea that apoptosis and G1 arrest represent separate functions of p53. These experiments also demonstrate that the cellular environment plays an important role in the cell-fate determination after p53 activation. Further analysis of the EPO signalling pathways indicated that the EPO-mediated cell survival depends on JAK2 but independent of STAT5 and PI3′K.; The function of p53 as a tumour suppressor depends, at least in part, on its ability to bind to specific DNA sequences and activate transcription of target genes. p21WAF1 is the primary effector in p53-dependent G1 arrest. The pathways through which p53 promotes apoptosis, however, are not fully understood. We identified a number of p53 responsive genes by using differential display to analyze global gene expression in the murine erythroleukemia cells before and after the is p53 was activated. One of the new genes, which we named PIDD, encodes a predicted protein with a death domain. Pidd mRNA was induced by ionizing radiation in a p53-dependent manner and the basal level of Pidd RNA was dependent on p53 status. Overexpression of Pidd inhibited cell growth in a p53-like manner by inducing apoptosis. Antisense inhibition of Pidd expression attenuated p53-mediated apoptosis. Our data suggest that Pidd is an effector of p53-dependent apoptosis. |
