SOD2-mediated adaptive responses induced by low-dose ionizing radiation via TNF signaling and amifostine

Manganese superoxide dismutase (SOD2)-mediated adaptive processes that protect against radiation-induced micronucleus formation can be induced in cells after a 2-Gy exposure by previously exposing them to either low-dose ionizing radiation (10 cGy) or WR1065 (40 mu M), the active thiol form of amifostine. Although both adaptive processes culminate in elevated levels of SOD2 enzymatic activity, the underlying pathways differ in complexity, with the tumor necrosis factor alpha (TNF alpha) signaling pathway implicated in the low-dose radiation-induced response, but not in the thiol-induced pathway. The goal of this study was the characterization of the effects of TNFa receptors 1 and 2 (TNFR1, TNFR2) on the adaptive responses induced by low-dose irradiation or thiol exposure using micronucleus formation as an endpoint. BFS-1 wild-type cells with functional TNFR1 and 2 were exposed 24 h before a 2-Gy dose of ionizing radiation to either 10 cGy or a 40 mu M dose of WR1065. BFS2C-SHO2 cells, defective in TNFR1, and BFS2C-SH22 cells, defective in both TNFR1 and TNFR2 and generated from BFS2C-SHO2 cells by transfection with a murine TNFR2-targeting vector and confirmed to be TNFR2 defective by quantitative PCR, were also exposed under similar conditions for comparison. A 10-cGy dose of radiation induced a significant elevation in SOD2 activity in BFS-1 (P<0.001) and BFS2C-SHO2 (P=0.005) but not BFS2C-SH22 cells (P= 0.433), compared to their respective untreated controls. In contrast, WR1065 significantly induced elevations in SOD2 activity in all three cell lines (P= 0.001, P= 0.007, P= 0.020, respectively). A significant reduction in the frequency of radiation-induced micronuclei was observed in each cell line when exposure to a 2-Gy challenge dose of radiation occurred during the period of maximal elevation in SOD2 activity. However, this adaptive effect was completely inhibited if the cells were transfected 24 h before low-dose radiation or thiol exposure with SOD2 siRNA. Under the conditions tested, TNFR1 and 2 inhibition negatively affected the low-dose radiation-induced but not the thiol-induced adaptive responses observed to be mediated by elevations in SOD2 activity. (C) 2011 Elsevier Inc. All rights reserved.