Abstract
BackgroundDCQ (2-benzoyl-3-phenyl-6,7-dichloroquinoxaline 1,4-dioxide), a synthetic quinoxaline 1,4-dioxide, enhances the cytotoxic effect of ionizing radiation (IR) in vivo and in vitro. We sought to clarify whether increased radiation-induced DNA damage, decreased rate of damage repair, and the generation of reactive oxygen species (ROS) contribute to DCQ enhancement of IR.MethodsMurine mammary adenocarcinoma EMT-6 cells were treated with DCQ for 4 h before exposure to 10 Gy IR. Treated cells were monitored for modulations in cell cycle, induction of DNA damage, and generation of ROS.ResultsCombined DCQ and IR treatments (DCQ+IR) induced rapid cell-cycle arrests in EMT-6 cells, particularly in S and G2/M phases. Alkaline comet assays revealed high levels of DNA damage in cells after exposure to DCQ+IR, consistent with damage-induced arrest. Unlike IR-only and DCQ-only treated cells, the damage induced by combined DCQ+IR was repaired at a slower rate. Combined treatment, compared to separate DCQ and IR treatments, activated DNA-protein kinase and induced more p-ATM, supporting a role for double strand breaks (DSBs), which are more toxic and difficult to repair than single strand breaks (SSBs). Contributing factors to DCQ radiosensitization appear to be the induction of ROS and DSBs.ConclusionCollectively, our findings indicate that radiosensitization by DCQ is mediated by DNA damage and decreased repair and that ROS are at least partially responsible.
Highlights
DCQ (2-benzoyl-3-phenyl-6,7-dichloroquinoxaline 1,4-dioxide), a synthetic quinoxaline 1,4-dioxide, enhances the cytotoxic effect of ionizing radiation (IR) in vivo and in vitro
DCQ Induces S Phase and G2/M Arrest in EMT-6 Cells Previous work has shown that DCQ, in combination with IR, induces apoptosis in EMT-6 cells 24 h post-treatment, and decreases their clonogenic survival [13]
To determine the direct effects of DCQ ± IR on cell cycle progression of EMT-6, cells were treated with 10 μM DCQ for 4 h followed by irradiation with 10 Gy IR, or separately treated
Summary
DCQ (2-benzoyl-3-phenyl-6,7-dichloroquinoxaline 1,4-dioxide), a synthetic quinoxaline 1,4-dioxide, enhances the cytotoxic effect of ionizing radiation (IR) in vivo and in vitro. We sought to clarify whether increased radiation-induced DNA damage, decreased rate of damage repair, and the generation of reactive oxygen species (ROS) contribute to DCQ enhancement of IR. Many malignant cells have incompetent cell-cycle controls, and DNA synthesis and replication may proceed despite the presence of unrepaired DNA damage, leading eventually to unviable daughter cells [1,2]. Such malignant cells are sensitive to therapies that induce DNA damage [3]. The hypoxic cytotoxin 7chloro-3-[(N, N-dimethylamino) propyl]amino]-2-quinoxalinecarbonitrile 1,4-dioxide hydrochloride (Q-85 HCl) has been shown to induce DNA damage under hypoxic conditions in CaCo-2 cells by producing reactive oxygen species (ROS) [4,5]. Studies on quinoxaline 1,4-dioxide has shown that it is reduced enzymatically into an active, oxygen-sensitive radical responsible for DNA cleavage [6]
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