The role of intercellular communication and oxidative metabolism in the propagation of ionizing radiation-induced biological effects
Coordinated interactions of specific molecular and biochemical processes are likely involved in the cellular responses to stresses induced by different ionizing radiations with distinctive linear energy transfer (LET) properties. Here, we investigated the roles and mechanisms of gap junction intercellular communication and oxidative metabolism in modulating cell killing and repair of potentially lethal damage (PLDR) in confluent AG1522 human fibroblasts exposed to 1 GeV protons (LET~0.2 keV/[mu]m), [superscript 137]Cs [gamma] rays (LET~0.9 keV/[mu]m), [superscript 241]Am [alpha] particles (LET~122 keV/[mu]m) or 1 GeV/u iron ions (LET~151 keV/[mu]m) at doses by which all cells in the exposed cultures are irradiated. As expected, [alpha]-particles and iron ions were more effective than protons and [gamma] rays at inducing cell killing. Holding [gamma]- or proton-irradiated cells in the confluent state for several hours after irradiation promoted increased survival and decreased chromosomal damage. However, maintaining [alpha]-particle or iron ion-irradiated cells in the confluent state for various times prior to subculture resulted in increased rather than decreased lethality, and was associated with persistent DNA damage and increased protein oxidation and lipid peroxidation. Inhibiting gap junction communication with 18-[alpha]-glycyrrhetinic acid or by knockdown of connexin43, a constitutive protein of junctional channels in these cells, protected against the toxic effects expressed in these cells during confluent holding. Up-regulation of antioxidant defense by ectopic over-expression of glutathione peroxidase, protected against cell killing by [alpha]-particles when cells were analyzed shortly after exposure. However, it did not attenuate the decrease in survival during confluent holding. Together, these findings indicate that the damaging effect of [alpha] particles results in oxidative stress, and the toxic effects in the hours following irradiation are amplified by intercellular communication, but the communicated molecule(s) is unlikely to be a substrate of glutathione peroxidase. To further understand the role of GJIC, we tested the effect of specific connexin channel permeabilities on radiation-induced cell killing and induction of DNA damage. We used human adenocarcinoma (HeLa) cells in which specific connexins can be expressed in the absence of endogenous connexins. When exposed to protons, [gamma] rays, [alpha] particles, or iron ions, connexin26 and connexin43 channels mediated the propagation of toxic effects among irradiated cells; in contrast, connexin32 channels conferred protective effects. Collectively, these studies provide a novel mechanistic understanding of the molecular events that mediate the fate of cell populations exposed to different types of ionizing radiation. They show that the LET of the radiation significantly impacts these events. The enhancement of cell killing in the hours after exposure of tumor cells to high charge and high energy particles and or [alpha] particles support the use of these particles in cancer radiotherapy. Characterization of the molecules that are communicated through junctional channels from tumor to normal cells would help formulate countermeasures to protect normal tissues during radiotherapy. Future in vivo research would contribute to validating these concepts.