The current status of the adaptive response to ionizing radiation in mammalian cells

The results of numerous studies indicate that cells can become refractory to the detrimental effect of ionizing radiation when previously exposed to a low, “adapting dose”;. This phenomenon has been termed an “adaptive response”; to ionizing radiation. It has been postulated that the induced radioresistance is due to the induction of DNA repair systems which efficiently protect the adapted cells from the effects of a subsequent, high “challenging dose”;. However, a direct proof of this hypothesis is still lacking. The analyzed endpoints include chromosomal aberrations, survival, mutations, genetic instability and DNA damage repair measured by the comet assay. Frequently contradictory results were published by different authors. For example some authors observed a reduced frequency of apoptosis in adapted cells, whereas others reported the opposite. The source of variablity of the adaptive response in human lymphocytes remains unresolved. While there is no doubt that an adapting dose can trigger some protecting mechanisms within the cell it appears that there is no single, universal mechanism of the adaptive response that is valid for all cell types and irradiation conditions.