title: Acquired resistance to cetuximab is associated with the overexpression of Ras family members and the loss of radiosensitization in head and neck cancer cells.
abstract: OBJECTIVE
  Cetuximab in combination with radiation therapy is used to treat patients with head and neck squamous cell carcinoma (HNSCC). In the present study, the mechanism of acquired resistance to cetuximab in HNSCC cells was investigated in vitro.
  
  
  METHODS
  The HNSCC cell lines UT5 and SAS and UT5 cells with acquired resistance to cetuximab (UT5R9) were used. The radiotoxicity potentials of cetuximab and inhibitors of PI3K, MAPK and farnesylation were tested using a clonogenic survival assay. Western blotting was used to evaluate protein expression. The levels of EGFR ligands were detected by ELISA.
  
  
  RESULTS
  Cetuximab inhibited proliferation and induced radiosensitization in UT5 cells but not in SAS cells. In comparison with UT5 cells, cetuximab-resistant SAS cells markedly overexpressed the K-Ras, H-Ras and N-Ras proteins, as detected by Western blotting. Resistance in UT5R9 cells was associated with the overexpression of the K-Ras, H-Ras and N-Ras proteins as well as an increase in the autocrine production of the EGFR ligands amphiregulin and transforming growth factor α (TGFα). UT5R9 cells were significantly more radioresistant than UT5 cells. Radioresistant UT5R9 cells were not radiosensitized by cetuximab, but knocking down H-RAS and N-RAS with siRNA and targeting Ras farnesylation using the farnesyltransferase inhibitor lonafarnib induced radiosensitization in these cells. Targeting PI3K and MEK revealed that the activation of the PI3K/Akt pathway but not the MAPK/ERK pathway is associated with radioresistance in UT5R9 cells.
  
  
  CONCLUSIONS
  Targeting Ras and PI3K activity improves the outcome of irradiation in cetuximab-resistant HNSCC cell lines in vitro.
authors.affiliation: Division of Radiobiology & Molecular Environmental Research, Department of Radiation Oncology, University of Tuebingen, Germany; Translational Radiooncology Laboratory, Department of Radiooncology and Radiotherapy, Charité Campus Mitte, Charité Universitätsmedizin, Berlin, Germany.
authors.forename: Mohammad
authors.initials: M
authors.lastname: Saki
authors.forename: Mahmoud
authors.initials: M
authors.lastname: Toulany
authors.forename: H Peter
authors.initials: HP
authors.lastname: Rodemann
mesh_terms: D061067:Antibodies, Monoclonal, Humanized / Q000494:pharmacology*; D000970:Antineoplastic Agents / Q000494:pharmacology*; D045744:Cell Line, Tumor; D000068818:Cetuximab; D019008:Drug Resistance, Neoplasm; D066246:ErbB Receptors / Q000502:physiology; D006258:Head and Neck Neoplasms / Q000188:drug therapy / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology; D011836:Radiation Tolerance; D018631:ras Proteins / Q000502:physiology*
chemical_list: D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents; D019869:Phosphatidylinositol 3-Kinases; D066246:ErbB Receptors; D018631:ras Proteins; D000068818:Cetuximab
keywords: Cetuximab; EGFR; HNSCC; PI3K; RAS