Cell cycle progression and radiation survival following prolonged hypoxia and re-oxygenation

PURPOSE: To investigate cell cycle progression and radiation survival following prolonged hypoxia and re-oxygenation. MATERIALS AND METHODS: NHIK 3025 human cervical carcinoma cells were exposed to extremely hypoxic conditions (<4ppm O2) for 20 h and then re-oxygenated. The subsequent cell cycle progression was monitored by analysing cell cycle distribution at different time-points after re-oxygenation using two-dimensional flowcytometry. The clonogenic survival after a 3.6 Gy X-ray dose was also measured at each of these time-points. The measured radiation survival was compared with theoretical predictions based on cell cycle distribution and the radiation age response of the cells. RESULTS: Following re-oxygenation the cells resumed cell cycle progression, completed S-phase, and then accumulated in G2. Non-clonogenic cells remained permanently arrested in G2, while the remainder of the cells completed mitosis after a few hours delay. The radiation survival of the hypoxia-pretreated cell population remained lower than for an exponentially growing control population for the investigated 50h of re-oxygenation. However, following 7 h of re-oxygenation, the radiation survival of the hypoxia-treated cell population correlated well with theoretically predicted values based on cell cycle distribution and radiation age response. CONCLUSIONS: The work demonstrates that prolonged hypoxia followed by re-oxygenation results in a G2 delay similar to that observed after DNA damage. Furthermore, chronic hypoxia results in decreased radiation survival for at least 50h following the reintroduction of oxygen. The hypoxia-induced radiosensitization following 7 h of re-oxygenation could in large part be explained by the synchronous cell cycle progression that occurred.