Cycling hypoxia increases U87 glioma cell radioresistance via ROS induced higher and long-term HIF-1 signal transduction activity.
Glioblastoma multiforme (GBM) tumors are the most common type of brain tumors and resistance to radiotherapy. This study aimed to investigate the differential effect and mechanism of tumor microenvironments, cycling hypoxia and non-interrupted hypoxia, on tumor cell radiosensitivity in the human U87 glioblastoma tumor model. We exposed U87 cells and mice bearing U87 glioma to experimentally imposed cycling or non-interrupted hypoxic stress in vitro and in vivo prior to treatment with ionizing irradiation. Clonogenic survival assay and tumor growth measurements were performed to determine tumor radiosensitivity. The differential regulation of non-interrupted vs. cycling hypoxia by hypoxia-inducible factor-1 (HIF-1) and the impact of HIF-1α on hypoxia-induced radioresistance were assessed by molecular assay and RNAi-knockdown technology. Our results demonstrated that cycling hypoxia induced higher and longer term HIF-1 signal transduction activity via reactive oxygen species (ROS) in U87 cells compared with non-interrupted hypoxia. Cycling hypoxia-induced HIF-1α activation reflected ROS mediated HIF-1α synthesis and stabilization, whereas non-interrupted hypoxia-induced HIF-1α activation was due to decreased HIF-1α degradation resulting from decreased prolyl hydroxylation. With regard to tumor radiosensitivity, cycling hypoxia induced more tumor cell radioresistance and a decreased response to radiotherapy in U87 cells compared with non-interrupted hypoxia. HIF-1 knockdown during in vitro and in vivo hypoxic stresses combined with radiotherapy suppressed cycling and non-interrupted hypoxia-induced radioresistance while increasing overall tumor radiosensitivity. Our results suggest that cycling hypoxia induces more radioresistance than non-interrupted hypoxia in U87 gliomas, and ROS mediated HIF-1α activation is a crucial mechanism involved in hypoxia-induced differential radioresistant in U87 gliomas.
Graduate Institute of Basic Medical Science, Center for Neuropsychiatry, and Graduate Institute of Immunology, China Medical University and Hospital, Taichung, Taiwan, ROC. chhsiehcmu@mail.cmu.edu.tw
Chia-Hung
CH
Hsieh
Cheng-Hung
CH
Lee
Ji-An
JA
Liang
Chun-Yen
CY
Yu
Woei-Cherng
WC
Shyu
D000818:Animals; D001932:Brain Neoplasms / Q000235:genetics* / Q000378:metabolism / Q000473:pathology*; D015687:Cell Hypoxia / Q000235:genetics / Q000502:physiology; D045744:Cell Line, Tumor; D005910:Glioma / Q000235:genetics* / Q000378:metabolism / Q000473:pathology*; D006801:Humans; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D010101:Oxygen Consumption / Q000235:genetics / Q000502:physiology; D010507:Periodicity; D034741:RNA, Small Interfering / Q000494:pharmacology; D011836:Radiation Tolerance / Q000187:drug effects / Q000235:genetics*; D017382:Reactive Oxygen Species / Q000378:metabolism / Q000494:pharmacology*; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology; D013997:Time Factors; D023041:Xenograft Model Antitumor Assays
C497442:HIF1A protein, human; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D034741:RNA, Small Interfering; D017382:Reactive Oxygen Species