title: Pharmacological inhibition of poly(ADP-ribose) polymerase-1 modulates resistance of human glioblastoma stem cells to temozolomide.
abstract: BACKGROUND
  Chemoresistance of glioblastoma multiforme (GBM) has been attributed to the presence within the tumor of cancer stem cells (GSCs). The standard therapy for GBM consists of surgery followed by radiotherapy and the chemotherapeutic agent temozolomide (TMZ). However, TMZ efficacy is limited by O6-methylguanine-DNA-methyltransferase (MGMT) and Mismatch Repair (MMR) functions. Strategies to counteract TMZ resistance include its combination with poly(ADP-ribose) polymerase inhibitors (PARPi), which hamper the repair of N-methylpurines. PARPi are also investigated as monotherapy for tumors with deficiency of homologous recombination (HR). We have investigated whether PARPi may restore GSC sensitivity to TMZ or may be effective as monotherapy.
  
  
  METHODS
  Ten human GSC lines were assayed for MMR proteins, MGMT and PARP-1 expression/activity, MGMT promoter methylation and sensitivity to TMZ or PARPi, alone and in combination. Since PTEN defects are frequently detected in GBM and may cause HR dysfunction, PTEN expression was also analyzed. The statistical analysis of the differences in drug sensitivity among the cell lines was performed using the ANOVA and Bonferroni's post-test or the non-parametric Kruskal-Wallis analysis and Dunn's post-test for multiple comparisons. Synergism between TMZ and PARPi was analyzed by the median-effect method of Chou and Talalay. Correlation analyses were done using the Spearman's rank test.
  
  
  RESULTS
  All GSCs were MMR-proficient and resistance to TMZ was mainly associated with high MGMT activity or low proliferation rate. MGMT promoter hypermethylation of GSCs correlated both with low MGMT activity/expression (Spearman's test, P = 0.004 and P = 0.01) and with longer overall survival of GBM patients (P = 0.02). Sensitivity of each GSC line to PARPi as single agent did not correlate with PARP-1 or PTEN expression. Notably, PARPi and TMZ combination exerted synergistic antitumor effects in eight out of ten GSC lines and the TMZ dose reduction achieved significantly correlated with the sensitivity of each cell line to PARPi as single agent (P = 0.01).
  
  
  CONCLUSIONS
  The combination of TMZ with PARPi may represent a valuable strategy to reverse GSC chemoresistance.
authors.affiliation: Department of System Medicine, University of Rome "Tor Vergata", Via Montpellier 1, 00133 Rome, Italy. tentori@uniroma2.it.
authors.forename: Lucio
authors.initials: L
authors.lastname: Tentori
authors.forename: Lucia
authors.initials: L
authors.lastname: Ricci-Vitiani
authors.forename: Alessia
authors.initials: A
authors.lastname: Muzi
authors.forename: Fabio
authors.initials: F
authors.lastname: Ciccarone
authors.forename: Federica
authors.initials: F
authors.lastname: Pelacchi
authors.forename: Roberta
authors.initials: R
authors.lastname: Calabrese
authors.forename: Daniele
authors.initials: D
authors.lastname: Runci
authors.forename: Roberto
authors.initials: R
authors.lastname: Pallini
authors.forename: Paola
authors.initials: P
authors.lastname: Caiafa
authors.forename: Grazia
authors.initials: G
authors.lastname: Graziani
mesh_terms: D018906:Antineoplastic Agents, Alkylating / Q000494:pharmacology*; D045744:Cell Line, Tumor; D018899:CpG Islands; D019175:DNA Methylation; D003606:Dacarbazine / Q000031:analogs & derivatives* / Q000494:pharmacology; D019008:Drug Resistance, Neoplasm; D005909:Glioblastoma / Q000235:genetics / Q000378:metabolism* / Q000401:mortality; D006801:Humans; D014411:Neoplastic Stem Cells / Q000187:drug effects* / Q000378:metabolism*; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000235:genetics / Q000378:metabolism; D000071137:Poly (ADP-Ribose) Polymerase-1; D000067856:Poly(ADP-ribose) Polymerase Inhibitors; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism; D011401:Promoter Regions, Genetic; D000077204:Temozolomide
chemical_list: D018906:Antineoplastic Agents, Alkylating; D000067856:Poly(ADP-ribose) Polymerase Inhibitors; D003606:Dacarbazine; D019853:O(6)-Methylguanine-DNA Methyltransferase; C499320:PARP1 protein, human; D000071137:Poly (ADP-Ribose) Polymerase-1; D011065:Poly(ADP-ribose) Polymerases; D000077204:Temozolomide