Differential post-translational modification of the tumour suppressor proteins Rb and p53 modulate the rates of radiation-induced apoptosis in vivo

Abstract

Ionizing radiation induces p53-dependent apoptosis in the spleen, providing a model system to study p53 regulated events in a normal cell type. We have developed an in vivo model that identi®es genetic dierences in the regulation of p53-mediated apoptosis and addresses whether altered post-translational events in the p53 ± p21/Rb axis modulate the sensitivity of cells to radiation-induced cell death in vivo. Splenocytes from mice with distinct genetic backgrounds (DBA/2 and C57BL/6) exhibit dierences in the rate of apoptosis. Whilst no obvious strain dierences in protein levels of Bcl-2 or the cyclin-CDKs were observed, early posttranslational regulatory events in the p53 ± p21/Rb axis showed striking dierences in the two mouse strains. Cells from C57BL/6 animals undergo more rapid apoptosis after irradiation resulting from elevated levels and rapid induction of p53, pronounced Rb-cleavage, and the absence of a sustained induction of p21. In contrast, cells from DBA/2 animals have a reduced rate of apoptosis following irradiation with elevated levels of hyperphosphorylated Rb and a sustained induction of the p21 protein that is coincident with the C-terminal phosphorylation of p53. These data suggest that quantitative dierences in the level of p21 protein can aect the rate of apoptosis in vivo, consistent with the view that p21 is an anti-apoptotic eector of p53. However, striking dierences in the Rb protein ± caspase cleavage or hyperphosphorylation ± in the same cell type, but in dierent genetic backgrounds, demonstrates that p53-dependent apoptosis can be modulated in vivo by genetic factors that impinge upon the pro-or antiapoptotic potential of Rb. In addition, we show that Rb cleavage is p53-dependent and that its phosphorylation status can be uncoupled from p21 expression. This study highlights the possibility that genetic factors can be identi®ed that aect dierential sensitivity of cells to ionizing radiation in vivo Oncogene (2001) 20, 3597 ± 3608.

Radiation-induced genomic instability and bystander effects: inter-related nontargeted effects of exposure to ionizing radiation

“…Transcriptionally active p53 can represses or activate genes, and there is increasing evidence for differential activation of different p53 target genes in different cells and tissues (Gorospe et al, 1997;Yu et al, 1999;Bouvard et al, 2000;Burns et al, 2001;Fei et al, 2002). It is now clear that the differential induction of specific p53-target genes is a genetically modified process (Wallace et al, 2001;Coates et al, 2003). The data for haemopoietic cells are consistent with a greater p53-mediated transcriptional activation of the CDKN1A/ p21 gene following irradiation tending to reduce the apoptotic response (CBA/Ca and DBA/2) and a greater upregulation of Bax reinforcing the apoptotic programme (C57BL/6).…”

“…The genotype dependency of the expression of radiation-induced chromosomal instability (Ponnaiya pathway for the expression of radiation-induced genomic instability and bystander effects in haemopoietic cells are shown schematically as a differential sectoring of response in genetic strains, susceptible to radiation-induced chromosomal instability and expressing low levels of delayed death (CBA/Ca and DBA/2) and those relatively resistant to the expression of chromosomal instability expressing high levels of delayed death (C57BL/6) et al, 1997; Watson et al, 1997) and apoptotic response (Clutton et al, 1996b;Mothersill et al, 1999;Wright, 2002), together with the many observations of genetic factors influencing the response to ionizing radiation, prompted an investigation to study the potential for genetic modification of cell-type-specific p53 and apoptotic responses that might relate to genetically determined differences in the longer-term outcomes of radiation exposure Wallace et al, 2001;Coates et al, 2003). In these investigations, strain-dependent differences in the amount, timing, distribution and transcriptional activity of p53 in tissues taken from identically treated inbred strains of mice were demonstrated.…”

Heritability of DNA-damage-induced apoptosis and its relationship with age in lymphocytes from female twins

“…The reduced AR seen in some mouse strains such as DBA/2 has been correlated with an increased incidence of genetic defects in irradiated cells (Watson et al, 1997;Wallace et al, 2001). The demonstration of a strong level of heritability for AR in lymphocytes from human volunteers in our study, along with the confirmation of an age-related decrease in AR, are important findings that suggest a number of further avenues of research.…”

Apoptosis, ageing and cancer susceptibility

“…The general idea that apoptotic response to DNA damage and susceptibility to cancer are linked is supported by work in mice. Splenocytes from C57BL/6 mice show a large and rapid apoptotic response to 1 Gy radiation, in comparison with splenocytes from DBA/2 mice, in which the response is slower and reduced in extent (Wallace et al, 2001). Radiation-induced malignancy is greater in DBA/2 as compared with C57BL/6 mice.…”