Intrinsic radiation sensitivity: cellular signaling is the key.

The concept that the balance between DNA damage and repair determines intrinsic radiation sensitivity has dominated radiobiology for several decades. There is undeniably a cause- effect relationship between radiation-induced molecular alterations in the genomic DNA and cellular consequences. In the last decade, however, it has become obvious that the chromatin context affects the fate of damaged DNA and that cellular signaling is an important factor in defining intrinsic radiation sensitivity. Damaged DNA is the site of signal generation; however, alternative signaling at the plasma membrane is triggered: Reactive oxygen species (ROS) inactivate phosphatases and consequently cause activation of kinases localized at the plasma membrane; this includes ligand-independent activation of receptor kinases. Cells with an apparently functional DNA repair system may show increased radiation sensitivity due to deficiencies in specific kinases essential for repair activation and checkpoint control. Other signals that determine intrinsic radiosensitivity may affect proneness to apoptosis, the balance between DNA damage fixation and repair, and the translocation of proteins participating in the response to ionizing radiation. Interplay between the various signals decides the extent to which the repair of radiation-inflicted damage is supported or limited; in some cell types, this includes DNA-damage-independent processes guided by plasma membrane-generated signaling. Cellular signaling in the context of specific subcellular structures is the key to understanding how the molecular effects of radiation are expressed as biological consequences in various cell types. A systems approach should bring us closer to this end.