Characterization of the signal transduction pathway mediating gamma ray-induced inhibition of DNA synthesis in human cells: indirect evidence for involvement of calmodulin but not protein kinase C nor p53
Cultured cells from patients inheriting the rare cancer-prone and radiotherapy-sensitive disorder ataxia-telangiectasia (A-T) exhibit anomalies in cell cycle control and protein kinase C (PKC)-mediated upregulation of p53 protein following exposure to ionizing radiation. It remains unclear, however, as to whether this irregularity in a p53-dependent signal transduction pathway controlling the G1/S checkpoint is causally linked to the most consistent molecular hallmark of A-T-namely, marked attenuation in the inhibition of replicative DNA synthesis at early times (< or = 2 h) after irradiation [radioresistant DNA synthesis (RDS)]. We report here that treatment of normal human fibroblast strains with inhibitors of calmodulin (CaM) (i.e. W7 and W13) and CaM-dependent protein kinases II and IV (i.e. KN62) prior to radiation exposure elicits an 'A-T-like' RDS phenotype, whereas treatment with PKC inhibitors (e.g. staurosporine) does not produce this response. Moreover, at 1 h post-gamma irradiation A-T fibroblasts undergo normal induction of p53 protein while exhibiting the RDS trait. At later times (e.g. 4 h) following irradiation, however, these A-T cells contain abnormally low levels of p53 protein, as do their lymphoblastoid cell line counterparts during the entire post-gamma ray incubation period. On the other hand, human cells which either lack the p53 gene completely (i.e. HL60 leukemia cells) or harbor a germline mutation in the gene (i.e. Li-Fraumeni syndrome cells) shut down their DNA replication machinery normally upon sustaining radiation damage. We thus conclude that the transitory delay in DNA synthesis routinely experienced by human cells in the face of radiation injury is mediated through a CaM-dependent regulatory cascade which involves neither PKC nor p53 protein. Accordingly, A-T cells appear to be malfunctional in at least two distinct radiation-responsive signalling pathways, one regulating the G1/S checkpoint and governed by p53 and PKC and another controlling passage through S phase and requiring CaM.