Induction of quiescent S-phase cells by irradiation and/or hyperthermia. I. Time and dose dependence

DNA synthetic activity and DNA content of individual cells can be determined simultaneously by means of two-parameter flow cytometry. We used this method to study the effects of irradiation and/or hyperthermia on the proliferation of human melanoma cells in vitro. In untreated cultures, most of the cells with an S-phase DNA content showed incorporation of BrdU, but a small percentage did not. The fraction of these quiescent S-phase cells increased after irradiation (up to 8 Gy X-rays) and/or hyperthermia (up to 6 h at 42 degrees C or up to 2 h at 43 degrees C). Four days after the treatment up to 50% of the S-phase cells did not incorporate BrdU. There was a clear dose dependence for irradiation and hyperthermia alone or in combination. Generally, the combined effects seemed to be additive. Possible pitfalls of the technique used were taken into consideration. Our main practical conclusion is that single-parameter measurements of DNA content are insufficient to characterize the proliferative status of cell populations, especially after irradiation and/or hyperthermia, because a large part of those cells identified as being in S-phase may be quiescent.