Evidence for radiosensitizing by gliotoxin in HL-60 cells: implications for a role of NF-kappaB 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-kappaB). NF-kappaB activation in turn can lead to an inhibition of radiation-induced apoptotic cell death. Thus, inhibition of NF-kappaB 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-kappaB 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-kappaB activation since radiation of HL-60 cells with clinically relevant doses of radiation induced only a marginal increase in NF-kappaB activity, and selective inhibition of NF-kappaB 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-kappaB 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.
Baust
H
H
Department of Radiation Oncology, University of Ulm, D-89081 Ulm, Germany. heinrich.baust@medizin.uni-ulm.de
Schoke
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A
Brey
A
A
Gern
U
U
Los
M
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Schmid
R M
RM
Röttinger
E M
EM
Seufferlein
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T
D017209:Apoptosis / Q000187:drug effects / Q000528:radiation effects; D020169:Caspases / Q000502:physiology; D003513:Cycloheximide / Q000494:pharmacology; D004247:DNA / Q000378:metabolism; D016195:G2 Phase; D005912:Gliotoxin / Q000494:pharmacology*; D018922:HL-60 Cells; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases; D007783:Lactones / Q000494:pharmacology; D020928:Mitogen-Activated Protein Kinases / Q000502:physiology; D008938:Mitosis; D016328:NF-kappa B / Q000502:physiology*; D011506:Proteins / Q000378:metabolism; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D051636:X-Linked Inhibitor of Apoptosis Protein
D007783:Lactones; D016328:NF-kappa B; D011506:Proteins; D011838:Radiation-Sensitizing Agents; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; C098869:clasto-lactacystin beta-lactone; D005912:Gliotoxin; D004247:DNA; D003513:Cycloheximide; D048031:JNK Mitogen-Activated Protein Kinases; D020928:Mitogen-Activated Protein Kinases; D020169:Caspases