title: Cycling hypoxia induces chemoresistance through the activation of reactive oxygen species-mediated B-cell lymphoma extra-long pathway in glioblastoma multiforme.
abstract: BACKGROUND
  Cycling hypoxia is a well-recognized phenomenon within animal and human solid tumors. It contributes to the resistance to cytotoxic therapies through anti-apoptotic effects. However, the mechanism underlying cycling hypoxia-mediated anti-apoptosis remains unclear.
  
  
  METHODS
  Reactive oxygen species (ROS) production, activation of the hypoxia-inducible factor-1 alpha (HIF-1α) and nuclear factor-κB (NF-κB) signaling pathways, B-cell lymphoma extra-long (Bcl-xL) expression, caspase activation, and apoptosis in in vitro hypoxic stress-treated glioblastoma cells or tumor hypoxic cells derived from human glioblastoma xenografts were determined by in vitro ROS analysis, reporter assay, western blotting analysis, quantitative real-time PCR, caspase-3 activity assay, and annexin V staining assay, respectively. Tempol, a membrane-permeable radical scavenger, Bcl-xL knockdown, and specific inhibitors of HIF-1α and NF-κB were utilized to explore the mechanisms of cycling hypoxia-mediated resistance to temozolomide (TMZ) in vitro and in vivo and to identify potential therapeutic targets.
  
  
  RESULTS
  Bcl-xL expression and anti-apoptotic effects were upregulated under cycling hypoxia in glioblastoma cells concomitantly with decreased responses to TMZ through ROS-mediated HIF-1α and NF-κB activation. Tempol, YC-1 (HIF-1 inhibitor), and Bay 11-7082 (NF-κB inhibitor) suppressed the cycling hypoxia-mediated Bcl-xL induction in vitro and in vivo. Bcl-xL knockdown and Tempol treatment inhibited cycling hypoxia-induced chemoresistance. Moreover, Tempol treatment of intracerebral glioblastoma-bearing mice combined with TMZ chemotherapy synergistically suppressed tumor growth and increased survival rate.
  
  
  CONCLUSIONS
  Cycling hypoxia-induced Bcl-xL expression via ROS-mediated HIF-1α and NF-κB activation plays an important role in the tumor microenvironment-promoted anti-apoptosis and chemoresistance in glioblastoma. Thus, ROS blockage may be an attractive therapeutic strategy for tumor microenvironment-induced chemoresistance.
authors.affiliation: Aging Medicine Program, China Medical University, Taichung, Taiwan. weiling325@gmail.com.|Department of Psychiatry, Taichung Veterans General Hospital, Taichung, Taiwan. weiling325@gmail.com.
authors.forename: Wei-Ling
authors.initials: WL
authors.lastname: Chen
authors.affiliation: Graduate Institute of Basic Medicine, Fu Jen Catholic University, Taipei, Taiwan. 075006@mail.fju.edu.tw.
authors.forename: Chi-Chung
authors.initials: CC
authors.lastname: Wang
authors.affiliation: Graduate Institute of Basic Medical Science, China Medical University, Taichung, Taiwan. polo24862001@yahoo.com.tw.
authors.forename: Yu-Jung
authors.initials: YJ
authors.lastname: Lin
authors.affiliation: Department of Physiology and Pharmacology, Chang Gung University, Tao-Yuan, Taiwan. emailpaulwu@yahoo.com.
authors.forename: Chung-Pu
authors.initials: CP
authors.lastname: Wu
authors.affiliation: Aging Medicine Program, China Medical University, Taichung, Taiwan. chhsiehcmu@gmail.com.|Graduate Institute of Basic Medical Science, China Medical University, Taichung, Taiwan. chhsiehcmu@gmail.com.|Department of Medical Research, China Medical University Hospital, Taichung, Taiwan. chhsiehcmu@gmail.com.|Department of Biomedical Informatics, Asia University, Taichung, Taiwan. chhsiehcmu@gmail.com.
authors.forename: Chia-Hung
authors.initials: CH
authors.lastname: Hsieh
mesh_terms: D000818:Animals; D001932:Brain Neoplasms / Q000378:metabolism*; D005909:Glioblastoma / Q000378:metabolism*; D006801:Humans; D000860:Hypoxia; D016393:Lymphoma, B-Cell / Q000378:metabolism*; D008297:Male; D051379:Mice; D008819:Mice, Nude; D017382:Reactive Oxygen Species / Q000378:metabolism*
chemical_list: D017382:Reactive Oxygen Species