No abstract

Ml clone S6 myeloid leukemic cells do not express detectable p53 protein. When stable transfected with a temperature-sensitive mutant of p53, these cells undergo rapid cell death upon induction of wild-type (wt) p53 activity at the permissive temperature. This process has features of apoptosis. In a number of other cell systems, wt p53 activation has been shown to induce a growth arrest. Yet, wt 53 fails to induce a measurable growth arrest in Ml cells, and cell cycle progression proceeds while viability is being lost. There exists, however, a relationship between the cell cycle and p53-mediated death, and cells in G, appear to be preferentially susceptible to the death-inducing activity of wt p53. In addition, p53-mediated Ml cell death can be inhibited by interleukin-6. The effect of the cytokine is specific to p53-mediated death, since apoptosis elicited by serum deprivation is refractory to interleukin-6. Our data imply that p53-mediated cell death is not dependent on the induction of a growth arrest but rather may result from mutually incompatible growthregulatory signals.The p53 phosphoprotein is the product of a tumor suppressor gene, whose inactivation may play a role in the development and progression of many types of cancer (reviewed in references 5, 26, 35, and 43). In most cases,

In addition to the virus-coded large-T and small-t antigens, two new classes of proteins were immunoprecipitated by anti-simian virus 40 (SV40) tumor serum from extracts of various SV40-transformed cell lines. These were as follows: (i) proteins (termed "super-T proteins") with an Mr higher than that of large-T antigen (86,000), which were found in many SV40-transformed cell lines derived from mouse and rat cells (super-T proteins and large-T antigen appeared to have closely related structures as judged by the Chromobead elution patterns of their methionine-labeled tryptic peptides); (ii) proteins (termed "55K proteins") with an Mr ranging from 50,000 to 60,000, which were present in all SV40-transformed cell lines examined so far, including those obtained by chromosome-mediated gene transfer. The 55K proteins were not structurally related to large-T antigen, as judged by the Chromobead elution patterns of their methionine-labeled tryptic peptides. Our data are compatible with the assumption that the 55K proteins are largely or totally cell coded. with 25 to 50 ,uCi of L-[35S]methionine per ml (750 to 930 Ci/mmol; The Radiochemical Centre, Amersham, England) or with 100 JCi of 32PO4 per ml (C.E.A., France) in methionine (or phosphate)-free medium. SV40-infected BMK and SV40-infected CV1 cells were labeled 24 h and 72 h, respectively, after infection, with the same radioactive molecules, specific activities, length of time, and media as described for trans-472

The mechanisms by which the p53 tumour suppressor protein would, in vivo, co-ordinate the adaptive response to genotoxic stress is poorly understood. p53 has been shown to transactivate several genes that could be involved in two main cellular responses, growth arrest and apoptosis. To get further insight into the tissuespeci®c regulation of p53 transcriptional activity, we performed an extensive study looking at the expression of four well characterized p53-responsive genes, before and after g-irradiation in p53 wild-type (p53+/+) and p53-de®cient (p537/7) mice. The waf1, bax, fas and mdm2 genes were chosen for their dierent potential roles in the cellular response to stress. Our data demonstrate the strict p53-dependence of mRNA upregulation for bax, fas and mdm2 in irradiated tissues and con®rm such ®ndings for waf1. They further highlight complex levels of regulatory mechanisms that could lead, in vivo, to selective transcriptional activation of genes by p53. In addition, our results provide arguments for the involvement of p53 in the basal mRNA expression of the four genes in some organs. Finally, in situ expression of Bax and p21Waf-1 protein suggests, at least in lymphoid organs, a direct correlation between selective p53-target gene expression and a particular response of a cell to ionising radiation.