Ionizing radiation induces blockade of c-Jun N-terminal kinase-dependent cell death pathway in a manner correlated with p21^Cip/WAF1 induction in primary cultured normal human fibroblasts

During radiotherapy of cancer, neighboring normal cells may receive sub-lethal doses of radiation. To investigate whether such low levels of radiation modulate normal cell responses to death stimuli, primary cultured human fibroblasts were exposed to various doses of γ-rays. Analysis of cell viability using an exclusion dye propidium iodide revealed that the irradiation up to 10 Gy killed the fibroblasts only to a minimal extent. In contrast, the cells efficiently lost their viability when exposed to 0.5-0.65 mM H₂O₂. This type of cell death was accompanied by JNK activation, and was reversed by the use of a JNK-specific inhibitor SP600125. Interestingly, H₂O₂ failed to kill the fibroblasts when these cells were pre-irradiated, 24 h before H₂O₂ treatment, with 0.25-0.5 Gy of γ-rays. These cytoprotective doses of γ-rays did not enhance cellular capacity to degrade H₂O ₂, but elevated cellular levels of p21^Cip/WAF1, a p53 target that can suppress H₂O₂-induced cell death by blocking JNK activation. Consistently, H₂O₂-induced JNK activation was dramatically suppressed in the pre-irradiated cells. The overall data suggests that ionizing radiation can impart normal fibroblasts with a survival advantage against oxidative stress by blocking the process leading to JNK activation.