Ionizing radiation-induced, mitochondria-dependent generation of reactive oxygen/nitrogen

Transient generation of reactive oxygen or nitrogen (ROS/RNS), detected wit h dihydrodichlorofluoroscein by fluorescence microscopy, occurs within minu tes of exposing cells to ionizing radiation. In the 1-10 Gy dose range, the amount of ROS/RNS produced/cell is constant, but the percentage of produci ng cells increases with dose (20 to 80%). Reversible depolarization of the mitochondrial membrane potential (Delta Psi) and decrease in fluorescence o f a mitochondria-entrapped dye, calcein, are observed coincidentally. Radia tion-induced ROS/RNS, Delta Psi depolarization, and calcein fluorescence de crease are inhibited by the mitochondrial permeability transition inhibitor , cyclosporin A, but not the structural analogue, cyclosporin H. Radiation- stimulated ROS/RNS is also inhibited by overexpressing the Ca2+-binding pro tein, calbindin 28K, or treating cells with an intracellular Ca2+ chelator, Radiation-induced ROS/RNS is observed in several cell types with the excep tion of rho (o) cells deficient in mitochondrial electron transport, rho (o ) cells show neither radiation-induced ROS/RNS production nor Delta Psi dep olarization, We propose that radiation damage in a few mitochondria is tran smitted via a reversible, Ca2+-dependent mitochondrial permeability transit ion to adjacent mitochondria with resulting enhanced ROS/RNS generation. Me asurements of radiation-induced mitogen-activated protein kinase activity i ndicate that this sensing/amplification mechanism is necessary for activati on of some cytoplasmic signaling pathways by low doses of radiation.