Absence of a radiation-induced first-cycle G₁-S arrest in p53⁺ human tumor cells synchronized by mitotic selection

It is well known that normal human diploid fibroblasts undergo a significant, p53-dependent arrest in the G₁ phase of the cell cycle after exposure to ionizing radiation. The presence and magnitude of a G₁ arrest in human tumor cell lines, however, has been controversial, particularly in cells derived from solid tumors and irradiated during exponential growth. To examine this question more precisely, we synchronized cells by mitotic selection and irradiated them in very early G₁ prior to any of the described G₁ checkpoints. Progression of cells from G₁ into the S phase was monitored by autoradiographic measurement of cumulative labeling indices and by flow cytometric analysis. Three different human tumor cell lines confirmed as expressing normal p53 function were examined, i.e., lines derived from an adenocarcinoma of the colon (RKO), a breast cancer (MCF-7), and a squamous cell carcinoma (SCC61). Following irradiation with 4-8 Gy, there was a transient delay in progression from G₁ into S phase, lasting approximately 2 h, and in two of the three cell lines (RKO and MCF-7), a small fraction of cells (5-8%) never entered the first S phase. Although there was no evidence for a prolonged G₁ arrest, the expected G₂ delay was observed in all three cell lines. When irradiated RKO cells were resynchronized at the next mitosis, approximately 30% of the cells did not enter the second S phase. This latter finding is consistent with earlier reports on the kinetics of radiation-induced reproductive failure in mammalian cells. These results indicate that cells derived from human solid tumors that express normal p53 may respond to irradiation quite differently than do normal cells in terms of G₁ checkpoint control.