Zölzer, F. and Streffer, C. G₂-Phase Delays after Irradiation and/or Heat Treatment as Assessed by Two-Parameter Flow Cytometry.

Similar to what has been observed after irradiation, the fraction of G₂-phase cells increases as a consequence of heat treatment. On the basis of cell cycle distributions alone, however, it is difficult to say whether the two results are related. In particular, comparison is complicated by the fact that the accompanying changes in the S-phase transition are different. These changes play a minor role after irradiation but constitute by far the most important cell cycle effect after heat treatment. Two-parameter flow cytometry was used here to study the proliferation of human melanoma cells in vitro. Cultures were pulse-labeled with BrdU after irradiation and/or heat treatment and were fixed either immediately or after a delay of up to 36 h. DNA-synthesizing cells were identified with the help of an FITC-conjugated antibody against BrdU; DNA was quantified after staining with propidium iodide. In this way, the cell cycle distribution could be determined and the progression through the cell cycle could be analyzed. From the movement of labeled cells through the cycle, in particular the appearance of labeled cells in the G₁ compartment (after they had gone through mitosis), the delay in G₂ phase could be determined. The duration of the G₂/M phase in control cells was about 6 h. This was increased to 12, 13 and 16 h after irradiation (4 Gy X rays), heat treatment (1 h at 43°C), and a combination of the two, respectively. In all these cases, the G₂-phase block was completely overcome within 48 h after treatment, whereas changes in the S phase were still observable at this time. As expected, the radiation-induced G₂-phase block was almost completely removed by incubating the cells with 5 or 10 mM caffeine. In the case of hyperthermia alone or in combination with radiation, however, caffeine was somewhat less effective. This does not mean, however, that the mechanisms involved are necessarily different. It can also be seen as a result of the differences in the time course of events. The long delay in S phase after heat treatment may lead to a loss of susceptibility to caffeine by the time the cells move into the G₂ phase.