title: Deubiquitylating enzyme USP9x regulates radiosensitivity in glioblastoma cells by Mcl-1-dependent and -independent mechanisms.
abstract: Glioblastoma is a very aggressive form of brain tumor with limited therapeutic options. Usually, glioblastoma is treated with ionizing radiation (IR) and chemotherapy after surgical removal. However, radiotherapy is frequently unsuccessful, among others owing to resistance mechanisms the tumor cells have developed. Antiapoptotic B-cell leukemia (Bcl)-2 family members can contribute to radioresistance by interfering with apoptosis induction in response to IR. Bcl-2 and the closely related Bcl-xL and Mcl-1 are often overexpressed in glioblastoma cells. In contrast to Bcl-2 and Bcl-xL, Mcl-1 is a short-lived protein whose stability is closely regulated by ubiquitylation-dependent proteasomal degradation. Although ubiquitin ligases facilitate degradation, the deubiquitylating enzyme ubiquitin-specific protease 9x (USP9x) interferes with degradation by removing polyubiquitin chains from Mcl-1, thereby stabilizing this protein. Thus, an inability to downregulate Mcl-1 by enhanced USP9x activity might contribute to radioresistance. Here we analyzed the impact of USP9x on Mcl-1 levels and radiosensitivity in glioblastoma cells. Correlating Mcl-1 and USP9x expressions were significantly higher in human glioblastoma than in astrocytoma. Downregulation of Mcl-1 correlated with apoptosis induction in established glioblastoma cell lines. Although Mcl-1 knockdown by siRNA increased apoptosis induction after irradiation in all glioblastoma cell lines, USP9x knockdown significantly improved radiation-induced apoptosis in one of four cell lines and slightly increased apoptosis in another cell line. In the latter two cell lines, USP9x knockdown also increased radiation-induced clonogenic death. The massive downregulation of Mcl-1 and apoptosis induction in A172 cells transfected with USP9x siRNA shows that the deubiquitinase regulates cell survival by regulating Mcl-1 levels. In contrast, USP9x regulated radiosensitivity in Ln229 cells without affecting Mcl-1 levels. We conclude that USP9x can control survival and radiosensitivity in glioblastoma cells by Mcl-1-dependent and Mcl-1-independent mechanisms.
authors.affiliation: Department of Radiation Oncology, University of Tuebingen, Tuebingen, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Wolfsperger
authors.affiliation: Department of Radiation Oncology, University of Tuebingen, Tuebingen, Germany.
authors.forename: S A
authors.initials: SA
authors.lastname: Hogh-Binder
authors.affiliation: Department of Neuropathology, Institute of Pathology and Neuropathology, University of Tuebingen, Tuebingen, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Schittenhelm
authors.affiliation: Department of Neurosurgery, University of Tuebingen, Tuebingen, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Psaras
authors.affiliation: Institute for Cell Biology, University Hospital Essen, Essen, Germany.
authors.forename: V
authors.initials: V
authors.lastname: Ritter
authors.affiliation: Institute for Cell Biology, University Hospital Essen, Essen, Germany.
authors.forename: L
authors.initials: L
authors.lastname: Bornes
authors.affiliation: Department of Radiation Oncology, University of Tuebingen, Tuebingen, Germany.
authors.forename: S M
authors.initials: SM
authors.lastname: Huber
authors.affiliation: Institute for Cell Biology, University Hospital Essen, Essen, Germany.
authors.forename: V
authors.identifier: http://orcid.org/0000-0003-1058-2107
authors.initials: V
authors.lastname: Jendrossek
authors.affiliation: Institute for Cell Biology, University Hospital Essen, Essen, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Rudner
mesh_terms: D001932:Brain Neoplasms / Q000235:genetics* / Q000473:pathology; D045744:Cell Line, Tumor; D005260:Female; D005909:Glioblastoma / Q000235:genetics* / Q000473:pathology; D006801:Humans; D008297:Male; D064549:Myeloid Cell Leukemia Sequence 1 Protein / Q000235:genetics* / Q000378:metabolism; D011836:Radiation Tolerance; D014162:Transfection; D043222:Ubiquitin Thiolesterase / Q000235:genetics* / Q000378:metabolism*
chemical_list: D064549:Myeloid Cell Leukemia Sequence 1 Protein; C506234:USP9X protein, human; D043222:Ubiquitin Thiolesterase