Ionizing radiation activates interferon-inducible Stat1-dependent pathway and leads to the formation of cross-resistance to irradiation and interferon.

2172

Interferons can interact with ionizing radiation (IR) by sensitizing tumor cells to cytotoxic effects of irradiation. However, molecular mechanisms of this interaction remain unclear. We previously showed that in vivo selection of radiosensitive SCC61 xenografts by repetitive cycles of ionizing radiation leads to the formation of radioresistant tumor (nu61). Using expressional profiling we found that nu61 over-express specific patterns of genes which is over represented by interferon-stimulated genes (ISGs) and include Stat1-major trans-activator of ISGs (N.N.Khodarev et. al., PNAS USA, 2004, 101:1714).The objectives of this study were to characterize effects of ionizing irradiation (IR) on induction/activation of Stat1 and other factors involved in IFN signaling and consequences of constitutive activation of IFN/Stat1 pathway. We reporthere that IR led to up-regulation of Stat1 and IRF-7 in SCC-61 and nu61. However, IRF-1 was expressed and up-regulated only in SCC61. 24 hours after irradiation synthesis of IFN alpha and up-regulation of Stat1-dependent ISGs were detected. This demonstratestheability of ionizing radiation to activate anIFN-inducible, Stat1-dependent pathway. Interaction of ionizing radiation and IFN-inducible signaling on the level of Stat1 was confirmed by findings that IR can stabilize Tyr701 phosphorylation of Stat1, increasing combined cytotoxicity. Stat1-dependent cross-talk between IR-and IFN-induced pathways was also confirmed by in vitro selection of SCC-61 against IFN gamma and IFN alpha. Selected IFN resistant clones demonstratedover-expression of Stat1 and were radioresistant compared with parental SCC-61. Using B16 mouse melanoma sub-lines which were passed (B16F10) or not passed (B16F1) through immunocompetent animals we showed that adaptation of tumor clones to the host microenvironment is accompanied by formation of cross-resistance to IR and IFN. Taken together, these data show that Stat1-dependent pathways mediate response of tumors to IR. These data also suggest that host interferons may provide negative selection leading to formation of tumor clones with aggressive properties and cross-resistance to IFNs and genotoxic stress.