Evidence for radiosensitizing by gliotoxin in HL-60 cells: implications for a role of NF-κB independent mechanisms

Radioresistance markedly impairs the efficacy of tumor radiotherapy and may involve antiapoptotic signal transduction pathways that prevent radiation-induced cell death. A common cellular response to genotoxic stress induced by radiation is the activation of the nuclear factor kappa B (NF- κ B). NF- κ B activation in turn can lead to an inhibition of radiation-induced apoptotic cell death. Thus, inhibition of NF- κ B activation is commonly regarded as an important strategy to abolish radioresistance. Among other compounds, the fungal metabolite gliotoxin (GT) has been reported to be a highly selective inhibitor of NF- κ B activation. Indeed, low doses of GT were sufficient to significantly enhance radiation-induced apoptosis in HL-60 cells. However, this effect turned out to be largely independent of NF- κ B activation since radiation of HL-60 cells with clinically relevant doses of radiation induced only a marginal increase in NF- κ B activity, and selective inhibition of NF- κ B by SN50 did not result in a marked enhancement of GT-induced apoptosis. GT induced activation of JNKs, cytochrome c release from the mitochondria and potently stimulated the caspase cascade inducing cleavage of caspases −9, −8, −7 and −3. Furthermore, cleavage of the antiapoptotic protein X-linked IAP and downregulation of the G2/M-specific IAP-family member survivin were observed during GT-induced apoptosis. Finally, the radiation-induced G2/M arrest was markedly reduced in GT-treated cells most likely due to the rapid induction of apoptosis. Our data demonstrate that various other pathways apart from the NF- κ B signaling complex can sensitize tumor cells to radiation and propose a novel mechanism for radiosensitization by GT, the interference with the G2/M checkpoint that is important for repair of radiation-induced DNA damage in p53-deficient tumor cells.