Activation of cyclin A-dependent kinases associated with WAF1 degradation during radiation-induced apoptosis

We describe here the further characterisation of the radiation response of a pair of isogenic Burkitt's lymphoma cell lines which differ significantly in their susceptibility to radiation-induced apoptosis. In both cases a marked inhibition of cyclin A-dependent kinase activity was observed at 4 h post-irradiation which recovered to normal levels in the susceptible line by 12 h but remained inhibited in the resistant cell line. Under these conditions the cellular abundance of p58cyclinA and p33cdk2 did not significantly change in the two cell types and there was no evidence for phosphorylation changes in p33cdk2 which might account for the activity differences. In parallel with the changes in activity, p21WAF1 increased initially in both cell lines, declined in the sensitive cell line as the activity recovered but remained high in the resistant cell line. This appears to be explained by a more rapid turn-over of p21WAF1 in the sensitive cell line and an increased association of p21WAF1 with cyclin kinase as determined by immunoprecipitation. These results implicate p21WAF1 in the regulation of cyclin-dependent kinases during radiation-induced apoptosis, with persistence of induced p21WAF1 being associated with a more resistant phenotype.