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

Purpose: To investigate tell cycle progression and radiation survival follo wing prolonged hypoxia and re-oxygenation. Materials and methods: NHIK 3025 human cervical carcinoma cells were expose d to extremely hypoxic conditions (< 4 ppm O-2) for 20 h and then re-oxygen ated. 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 do se was also measured at each of these time-points. The measured radiation s urvival was compared with theoretical predictions based on cell cycle distr ibution 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 G(2) Non-clonogenic cells remai ned permanently arrested in G(2), while the remainder of the cells complete d mitosis after a few hours delay. The radiation survival of the hypoxia-pr etreated cell population remained lower than for an exponentially growing c ontrol population for the investigated 50 h of re-oxygenation. However, fol lowing 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-ox ygenation results in a G(2) delay similar to that observed after DNA damage . Furthermore, chronic hypoxia results in decreased radiation survival for at least 50 h following the reintroduction of oxygen. The hypoxia-induced r adiosensitization following 7 h of re-oxygenation could in large part be ex plained by the synchronous cell cycle progression that occurred.