ABSENCE OF A RADIATION-INDUCED FIRST-CYCLE G(1)-S ARREST IN P53(-CELLS SYNCHRONIZED BY MITOTIC SELECTION() HUMAN TUMOR)

It is well known that normal human diploid fibroblasts undergo a signi ficant, p53-dependent arrest in the G(1) phase of the cell cycle after exposure to ionizing radiation. The presence and magnitude of a G(1) arrest in human tumor cell lines, however, has been controversial, par ticularly in cells derived from solid tumors and irradiated during exp onential growth. To examine this question more precisely, we synchroni zed cells by mitotic selection and irradiated them in very early G(1) prior to any of the described G(1) checkpoints. Progression of cells f rom G(1) into the S phase was monitored by autoradiographic measuremen t of cumulative labeling indices and by flow cytometric analysis. Thre e 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 carcinom a (SCC61). Following irradiation with 4-8 Gy, there was a transient de lay in progression from G(1) into S phase, lasting approximately 2 h, and in two of the three cell lines (RKO and MCF-7), a small fraction o f cells (5-8%) never entered the first S phase. Although there was no evidence for a prolonged G(1) arrest, the expected G(2) delay was obse rved in all three cell Lines. When irradiated RKO cells were resynchro nized at the next mitosis, approximately 30% of the cells did not ente r the second S phase. This latter finding is consistent with earlier r eports on the kinetics of radiation-induced reproductive failure in ma mmalian cells. These results indicate that cells derived from human so lid tumors that express normal p53 may respond to irradiation quite di fferently than do normal cells in terms of G(1) checkpoint control.