Effect of ionizing radiation on wild-type and mutant mouse leukemia L1210 cells

Wild-type (WT) mouse leukemia L1210 cells express steady-state levels of p53 mRNA and protein. However, the p53 expressed by the wild-type L1210 cells was found to be a mutant form of p53 (relative to normal mouse fibroblast p53 sequence) having a point mutation in the DNA binding domain of p53. A deoxyadenosine-resistant L1210 cell line (Y8) derived from the parental WT cells had previously been shown to lack the expression of p53 but to respond to cycloheximide (CHX) treatment by superinduction of p53 mRNA. The mRNA for p53 induced by CHX had the same sequence as the p53 from normal mouse fibroblasts. Although the Y8 cells had no constitutive levels of p53 mRNA or protein, the Y8 cells expressed constitutive levels of WAF1 mRNA and protein. Gadd45 mRNA was also present in the Y8 cells. Subjecting the WT or Y8 cells to ionizing radiation did not result in a G0/G1 cell cycle block; the cells blocked in G2/M. The Y8 cells were much more sensitive to the irradiation treatment than the WT cells, resulting in marked increases in apoptosis in the Y8 cells. Although radiation treatment induced p53 mRNA, but no p53 protein, in the Y8 cells, WAF1 mRNA was induced in the Y8 cells. These data indicate that there are p53-independent pathway(s) that may still involve WAF1 and Gadd45 with respect to cell cycle control and apoptosis.