title: Diminished capacity for p53 in mediating a radiation-induced G1 arrest in established human tumor cell lines.
abstract: It has been reported that the p53 gene mediates an ionizing radiation-induced G1 arrest in mammalian cells. To further characterize this important phenomenon, a panel of seven human diploid fibroblast cell strains and 14 human tumor cell lines from a variety of sources with both wild-type and mutant p53 status were assayed for their susceptibility to G1 arrest after gamma-ray irradiation by a continuous labeling [3H]thymidine incorporation technique. An irreversible G1-block involving 20-70% of the cell population was observed in diploid fibroblasts irradiated with 4 Gy. The block was abolished by transfection with the Human Papilloma Virus E6 gene and in an ataxia telangiectasia (AT) cell line, indicating a role for the AT and p53 genes respectively in this process. In contrast to wild-type normal fibroblast cell strains, the G1-block in all tumor cell lines was significantly reduced, irrespective of their p53 status. None of the nine human tumor cell lines with mutant p53 genes showed a significant G1-block following irradiation with 4 Gy. Among the five tumor cell lines expressing wild-type p53, two showed no apparent G1-block. The remaining three showed a G1-block involving only 8-15% of the cell population, a block much smaller in magnitude than that seen in diploid fibroblasts. Finally, a diploid fibroblast cell strain and a tumor cell line, both showing a normal p53 and p21/WAF1 expression pattern, were examined for pRb phosphorylation before and after irradiation. The diploid fibroblast cell strain showed a significant G1-arrest and a clear inhibition of pRb phosphorylation by irradiation whereas the tumor cells showed no G1-arrest and no inhibition of pRb phosphorylation. These results suggest that (1) multiple genetic factors may modulate the occurrence and magnitude of the G1-arrest induced by exposure to ionizing radiation, (2) the capacity for p53 to mediate a radiation-induced G1 arrest is significantly reduced in tumor cells, (3) the disruption of G1-block modulating factor(s) other than p53 may be an important step in carcinogenesis.
mesh_terms: D001260:Ataxia Telangiectasia; D001483:Base Sequence; D015153:Blotting, Western; D002453:Cell Cycle / Q000235:genetics*; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000096:biosynthesis; D017931:DNA Primers; D004261:DNA Replication / Q000528:radiation effects; D004791:Enzyme Inhibitors / Q000378:metabolism; D005347:Fibroblasts; D016193:G1 Phase; D005720:Gamma Rays; D015870:Gene Expression / Q000528:radiation effects*; D016158:Genes, p53; D006801:Humans; D008969:Molecular Sequence Data; D009154:Mutation; D010766:Phosphorylation; D016133:Polymerase Chain Reaction; D012016:Reference Values; D016160:Retinoblastoma Protein / Q000378:metabolism; D013936:Thymidine / Q000378:metabolism; D014407:Tumor Cells, Cultured; D016159:Tumor Suppressor Protein p53 / Q000096:biosynthesis
chemical_list: C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D017931:DNA Primers; D004791:Enzyme Inhibitors; D016160:Retinoblastoma Protein; D016159:Tumor Suppressor Protein p53; D013936:Thymidine