The cell cycle dependence of thermotolerance. II. CHO cells heated at 45.0 degrees C.

This report extends our investigations of the cell cycle dependence of the expression of thermotolerance to include tolerance expressed by Chinese hamster ovary (CHO) cells exposed to 45.0 degrees C hyperthermia. We examined the response of asynchronous cells following exposure at 45.0 degrees C. A maximum in thermotolerance under these conditions was reached approximately 12 hr after a 15-min exposure to 45.0 degrees C hyperthermia and progressively decreased thereafter. Cells were delayed in S and G2 phase for 24 hr, after which time cell growth resumed. We then characterized the response of CHO cell populations synchronized in G1 or early or late S phase. We observed that the expression of tolerance depended on the position of cells in the cell cycle and was modulated by changes in the sensitivity of cells as they progressed through the cell cycle subsequent to the tolerance induction dose. We measured the variation in the sensitivity of these cells to 45.0 degrees C hyperthermia throughout the cell cycle and found substantial changes as cells progressed through S phase. Cells in early S phase were the most sensitive to heat at this temperature, and as these cells progressed through S phase, they became progressively more resistant. In addition, G1 cells were delayed for approximately 15 to 18 hr by a 15-min, 45.0 degrees C heat pulse, whereas S-phase cells were delayed to a lesser extent. The data presented in this report suggest that the induction of thermotolerance is relatively non-cell-cycle specific, but the magnitude of expression of tolerance depends on the position of cells in the cell cycle at the time of the subsequent challenge heat dose.