HYPER-RADIOSENSITIVITY AND INDUCED RADIORESISTANCE (HRS/IRR) : the effect of using different dose-rates for pre-exposure on the hyper-radiosensitivity in T-47D cells

Abstract

Irradiating T-47D cells with doses below 1 Gy revealed that cells of this line express hyper-radiosensitivity (HRS). In the present study HRS has been investigated when a challenge dose was given 6 or 24 hours after a priming dose delivered with varying dose-rates. In addition the effect of a challenge dose to T-47D cells that had been pre-irradiated for several months by incorporated tritium was investigated.

A 60Co-source at the Radium Hospital was used for all irradiations. It was chosen to give a fixed priming dose of 0.3 Gy, but dose-rates were varied between 0.045 Gy/h, 0.32 Gy/h, 0.9 Gy/h or high dose-rate (HDR) of 1 Gy/min. Immediately after the priming exposure, the cells were trypsinized and seeded in small flasks (25 cm2) and incubated for 6 or 24 hours before they received the HDR challenge doses.

DNA histograms revealed that the T-47D cells in the cell culture used in the experiments with priming exposures of 0.32 and 0.045 Gy/h and also in one of the experiments with HDR priming, had been mixed with cells of another stemline. The mixed cell culture (denoted T-47Dmix) was also used in the experiment with incorporation of tritium into T-47d cells, but in this case it was found from DNA histograms that only the control cells were mixed, but that the cultures exposed to protracted low dose-rate (LDR) irradiation consisted of only T-47d cells. Investigations were made that suggested that the cells mixed into the T-47D cells were of the line NHIK 3025.

The following was observed from the experiments:

T-47D cells clearly express both HRS/IRR and the adaptive response in the low-dose range (below 1 Gy).

NHIK 3025 cells do not express HRS/IRR.

T-47D cells that had been pre-irradiated by tritium-decay electrons did not express HRS/IRR.

T-47D cells with functional pRb adapted to the continuous irradiation with electrons from incorporated tritium, while the putative NHIK 3025 cells without functional pRb were eradicated by the same treatment.

When priming doses were delivered with the lowest dose-rates used (i.e. 0.32 Gy/h and 0.045 Gy/h) the priming effect as measured 6 hours following the termination of the priming exposure seemed to be larger than when the priming dose was delivered with higher dose-rates (i.e. HDR or 0.9 Gy/h).

24 hours after the priming doses delivered with 0.32 Gy/h and 0.045 Gy/h, HRS was still absent (perhaps even further reduced), while 24 hours after a HDR priming dose HRS was partially restored.

These results led to speculations concerning:

The influence of micro-environmental conditioning effects on the radiation response of NHIK 3025 cells that are growing in a mixed culture with T-47D cells under conditions that would normally be lethal for NHIK 3025 cells.

An alternative theory for HRS/IRR in which it is assumed that the repair processes of the cell are permanently induced. When radiation damages are so small, that the tissue as a whole would profit from cell suicide relative to repair with the danger of mis-repair, the repair processes are subdued by apoptosis.

The involvement of pRb in the suppression of apoptosis of the alternative theory for HRS/IRR and in the adaptation of T-47D cells to continuous irradiation by decay electrons from incorporated tritium.

The possibility that there are two different regulation pathways for induction of the cell-protective mechanisms that reduce HRS after pre-irradiation: One that is dose dependent, instantly induced by HDR irradiation but not by LDR irradiation, and which induces short-lasting mechanisms. Another pathway that works over time and is induced by LDR and probably also by HDR irradiation. The effect and duration of the mechanisms induced by this late-responding pathway depends on the duration of exposure.