title: Riluzole enhances ionizing radiation-induced cytotoxicity in human melanoma cells that ectopically express metabotropic glutamate receptor 1 in vitro and in vivo.
abstract: OBJECTIVE
  Brain metastases are a common preterminal event in patients with metastatic melanoma and require radiation therapy. Our group has previously shown that human GRM1 (hGRM1) expressing melanoma cells release excess extracellular glutamate and are growth inhibited by riluzole, an inhibitor of glutamate release. Riluzole-treated cells accumulate in G(2)/M phase of the cell cycle at 24 hours, and then undergo apoptotic cell death. We evaluated whether riluzole enhanced radiosensitivity in melanoma cells.
  
  
  METHODS
  Clonogenic assays were performed to evaluate clonogenic survival after treatment in hGRM1 expressing and nonexpressing melanoma cells. Western immunoblots were performed to confirm apoptotic cell death. A xenograft mouse model was used to validate the in vitro experiments. Tumors harvested from the xenografts were fixed and stained for apoptosis and DNA damage markers.
  
  
  RESULTS
  In the hGRM1-positive cell lines C8161 and UACC903, riluzole enhanced the lethal effects of ionizing radiation; no difference was seen in the hGRM1-negative UACC930 cell line. C8161 cells treated with riluzole plus irradiation also showed the highest levels of the cleaved forms of PARP and caspase-3; excised C8161 xenografts showed the greatest number of apoptotic cells by immunohistochemistry (P < 0.001). On cell cycle analysis, a sequence-dependent enrichment in the G(2)/M phase was shown with the combination of riluzole and irradiation. Xenografts treated with riluzole and weekly radiation fractions showed significant growth inhibition and revealed markedly increased DNA damage.
  
  
  CONCLUSIONS
  We have shown, in vitro and in vivo, that the combination of riluzole and ionizing radiation leads to greater cytotoxicity. These results have clinical implications for patients with brain metastases receiving whole brain radiation therapy.
authors.affiliation: Department of Radiation Oncology, UMDNJ-Robert Wood Johnson Medical School, Piscataway, New Jersey, USA.
authors.forename: Atif J
authors.initials: AJ
authors.lastname: Khan
authors.forename: Brian
authors.initials: B
authors.lastname: Wall
authors.forename: Stuti
authors.initials: S
authors.lastname: Ahlawat
authors.forename: Camille
authors.initials: C
authors.lastname: Green
authors.forename: Devora
authors.initials: D
authors.lastname: Schiff
authors.forename: Janice M
authors.initials: JM
authors.lastname: Mehnert
authors.forename: James S
authors.initials: JS
authors.lastname: Goydos
authors.forename: Suzie
authors.initials: S
authors.lastname: Chen
authors.forename: Bruce G
authors.initials: BG
authors.lastname: Haffty
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000528:radiation effects; D001932:Brain Neoplasms / Q000188:drug therapy / Q000532:radiotherapy / Q000556:secondary*; D053148:Caspase 3 / Q000378:metabolism; D002453:Cell Cycle / Q000187:drug effects / Q000528:radiation effects; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects / Q000528:radiation effects; D003131:Combined Modality Therapy; D018683:Excitatory Amino Acid Agents / Q000494:pharmacology*; D006657:Histones / Q000378:metabolism; D006801:Humans; D008545:Melanoma / Q000188:drug therapy / Q000473:pathology* / Q000532:radiotherapy; D051379:Mice; D008819:Mice, Nude; D011836:Radiation Tolerance / Q000187:drug effects; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D018094:Receptors, Metabotropic Glutamate / Q000378:metabolism*; D019782:Riluzole / Q000494:pharmacology*; D047368:Tumor Burden / Q000187:drug effects / Q000528:radiation effects; D023041:Xenograft Model Antitumor Assays
chemical_list: D018683:Excitatory Amino Acid Agents; C468783:H2AX protein, human; D006657:Histones; D011838:Radiation-Sensitizing Agents; D018094:Receptors, Metabotropic Glutamate; C104077:metabotropic glutamate receptor type 1; D019782:Riluzole; D053148:Caspase 3