title: Chemosensitization of cancer in vitro and in vivo by nitric oxide signaling.
abstract: OBJECTIVE
  Hypoxia contributes to drug resistance in solid cancers, and studies have revealed that low concentrations of nitric oxide (NO) mimetics attenuate hypoxia-induced drug resistance in tumor cells in vitro. Classic NO signaling involves activation of soluble guanylyl cyclase, generation of cyclic GMP (cGMP), and activation of cGMP-dependent protein kinase. Here, we determined whether chemosensitization by NO mimetics requires cGMP-dependent signaling and whether low concentrations of NO mimetics can chemosensitize tumors in vivo.
  
  
  METHODS
  Survival of human prostate and breast cancer cells was assessed by clonogenic assays following exposure to chemotherapeutic agents. The effect of NO mimetics on tumor chemosensitivity in vivo was determined using a mouse xenograft model of human prostate cancer. Drug efflux in vitro was assessed by measuring intracellular doxorubicin-associated fluorescence.
  
  
  RESULTS
  Low concentrations of the NO mimetics glyceryl trinitrate (GTN) and isosorbide dinitrate attenuated hypoxia-induced resistance to doxorubicin and paclitaxel. Similar to hypoxia-induced drug resistance, inhibition of various components of the NO signaling pathway increased resistance to doxorubicin, whereas activation of the pathway with 8-bromo-cGMP attenuated hypoxia-induced resistance. Drug efflux was unaffected by hypoxia and inhibitors of drug efflux did not significantly attenuate hypoxia-induced chemoresistance. Compared with mice treated with doxorubicin alone, tumor growth was decreased in mice treated with doxorubicin and a transdermal GTN patch. The presence of GTN and GTN metabolites in plasma samples was confirmed by gas chromatography.
  
  
  CONCLUSIONS
  Tumor hypoxia induces resistance to anticancer drugs by interfering with endogenous NO signaling and reactivation of NO signaling represents a novel approach to enhance chemotherapy.
authors.affiliation: Department of Anatomy, Queen's University, Kingston, Ontario, Canada.
authors.forename: Lisa J
authors.initials: LJ
authors.lastname: Frederiksen
authors.forename: Richard
authors.initials: R
authors.lastname: Sullivan
authors.forename: Lori R
authors.initials: LR
authors.lastname: Maxwell
authors.forename: Shannyn K
authors.initials: SK
authors.lastname: Macdonald-Goodfellow
authors.forename: Michael A
authors.initials: MA
authors.lastname: Adams
authors.forename: Brian M
authors.initials: BM
authors.lastname: Bennett
authors.forename: D Robert
authors.initials: DR
authors.lastname: Siemens
authors.forename: Charles H
authors.initials: CH
authors.lastname: Graham
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D001943:Breast Neoplasms / Q000378:metabolism; D015687:Cell Hypoxia / Q000187:drug effects / Q000502:physiology; D045744:Cell Line, Tumor; D006152:Cyclic GMP / Q000378:metabolism; D019008:Drug Resistance, Neoplasm / Q000187:drug effects*; D005260:Female; D006801:Humans; D007548:Isosorbide Dinitrate / Q000378:metabolism / Q000494:pharmacology; D008297:Male; D051379:Mice; D009374:Neoplasms, Experimental / Q000188:drug therapy / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism*; D020030:Nitric Oxide Donors / Q000097:blood / Q000378:metabolism / Q000494:pharmacology; D005996:Nitroglycerin / Q000097:blood / Q000378:metabolism / Q000494:pharmacology; D011471:Prostatic Neoplasms / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*
chemical_list: D000970:Antineoplastic Agents; D020030:Nitric Oxide Donors; D009569:Nitric Oxide; D005996:Nitroglycerin; D006152:Cyclic GMP; D007548:Isosorbide Dinitrate