Radiation-induced autophagy in normal and cancer cells: towards novel cytoprotection and radio-sensitization policies?

Autophagy or Type II programmed cell death (PCD) is a major intracellular pathway for the degradation and recycling of proteins, ribosomes and entire organelles. The role of this pathway in the antitumor effect of radiotherapy and in radiation toxicity is obscure. A complicated machinery of genes and proteins is involved in the regulation of autophagy as a response to a variety of stress factors including hypoxia, nutrient deprivation, cytotoxic agents and radiotherapy. Continuously accumulating data suggest that autophagic response of cancer cells to radiotherapy is a major pathway which, in contrast to apoptosis that leads to death, may lead to either death or cellular survival. A variety of agents have been recognized that induce or block autophagy, directly interfering with the cytotoxic effect of radiotherapy. Simultaneous targeting of autophagy and apoptosis during radiotherapy seems to further augment the antitumor effect. Radiobiology research should focus on the differential effect of fractionation on the induction of autophagy in different tumors and on the manipulation of this with autophagy triggering agents. Whether manipulation of this pathway in normal tissues may be used to confer cytoprotection also deserves thorough investigation. Moreover, the role of pretreatment autophagic indices in tumor cells in predicting radiotherapy and chemotherapy outcome should be examined in translational studies.