title: Protein phosphatase 2A mediates dormancy of glioblastoma multiforme-derived tumor stem-like cells during hypoxia.
abstract: OBJECTIVE
  The hypoxic microenvironment of glioblastoma multiforme (GBM) is thought to increase resistance to cancer therapies. Recent evidence suggests that hypoxia induces protein phosphatase 2A (PP2A), a regulator of cell cycle and cell death. The effects of PP2A on GBM tumor cell proliferation and survival during hypoxic conditions have not been studied.
  
  
  METHODS
  Expression of PP2A subunits and HIF-α proteins was measured in 65 high-grade astrocytoma and 18 non-neoplastic surgical brain specimens by western blotting. PP2A activity was measured by an immunoprecipitation assay. For in vitro experiments, GBM-derived tumor stem cell-like cells (TSCs) were exposed to severe hypoxia produced by either CoCl₂ or 1% O₂. PP2A activity was inhibited either by okadaic acid or by shRNA depletion of the PP2A C subunit. Effects of PP2A activity on cell cycle progression and cell survival during hypoxic conditions were assessed using flow cytometry.
  
  
  RESULTS
  In our patient cohort, PP2A activity was positively correlated with HIF-1∝ protein expression (P = 0.002). Patients with PP2A activity levels above 160 pMP had significantly worse survival compared to patients with levels below this threshold (P = 0.002). PP2A activity was an independent predictor of survival on multivariable analysis (P = 0.009). In our in vitro experiments, we confirmed that severe hypoxia induces PP2A activity in TSCs 6 hours after onset of exposure. PP2A activity mediated G1/S phase growth inhibition and reduced cellular ATP consumption in hypoxic TSCs. Conversely, inhibition of PP2A activity led to increased cell proliferation, exhaustion of intracellular ATP, and accelerated P53-independent cell death of hypoxic TSCs.
  
  
  CONCLUSIONS
  Our results suggest that PP2A activity predicts poor survival in GBM. PP2A appears to reduce the metabolic demand of hypoxic TSCs and enhances tumor cell survival. Modulation of PP2A may be a potential target for cancer therapy.
authors.affiliation: Department of Neurological Surgery, Weill Cornell Brain Tumor Center, Weill Cornell Medical College, New York Presbyterian Hospital, New York, New York, United States of America. chh9045@nyp.org
authors.forename: Christoph P
authors.initials: CP
authors.lastname: Hofstetter
authors.forename: Jan-Karl
authors.initials: JK
authors.lastname: Burkhardt
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Shin
authors.forename: Demirkan B
authors.initials: DB
authors.lastname: Gürsel
authors.forename: Lynn
authors.initials: L
authors.lastname: Mubita
authors.forename: Ramana
authors.initials: R
authors.lastname: Gorrepati
authors.forename: Cameron
authors.initials: C
authors.lastname: Brennan
authors.forename: Eric C
authors.initials: EC
authors.lastname: Holland
authors.forename: John A
authors.initials: JA
authors.lastname: Boockvar
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D001932:Brain Neoplasms / Q000201:enzymology* / Q000473:pathology* / Q000601:surgery; D016923:Cell Death; D015687:Cell Hypoxia; D049109:Cell Proliferation; D002470:Cell Survival; D056767:Cyclin G2 / Q000378:metabolism; D005260:Female; D016193:G1 Phase; D005909:Glioblastoma / Q000201:enzymology* / Q000473:pathology* / Q000601:surgery; D006801:Humans; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D014411:Neoplastic Stem Cells / Q000201:enzymology* / Q000473:pathology*; D011379:Prognosis; D016016:Proportional Hazards Models; D054648:Protein Phosphatase 2 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D021122:Protein Subunits / Q000037:antagonists & inhibitors / Q000378:metabolism; D016196:S Phase
chemical_list: D056767:Cyclin G2; D021122:Protein Subunits; D000255:Adenosine Triphosphate; D054648:Protein Phosphatase 2