PurposeThe 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.Experimental DesignExpression 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.ResultsIn 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.ConclusionsOur 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.

Purpose: Recurrent malignant glioma carries a dismal prognosis, and novel therapies are needed. We examined the feasibility and safety of vaccination with irradiated autologous glioma cells mixed with irradiated GM-K562 cells in patients undergoing craniotomy for recurrent malignant glioma. Experimental Design: We initiated a phase I study examining the safety of 2 doses of GM-K562 cells mixed with autologous cells. Primary endpoints were feasibility and safety. Feasibility was defined as the ability for 60% of enrolled subjects to initiate vaccination. Dose-limiting toxicity was assessed via a 3+3 dose-escalation format, examining irradiated tumor cells mixed with 5 × 106 GM-K562 cells or 1 × 107 GM-K562 cells. Eligibility required a priori indication for resection of a recurrent high-grade glioma. We measured biological activity by measuring delayed type hypersensitivity (DTH) responses, humoral immunity against tumor-associated antigens, and T-lymphocyte activation. Results: Eleven patients were enrolled. Sufficient numbers of autologous tumor cells were harvested in 10 patients, all of whom went on to receive vaccine. There were no dose-limiting toxicities. Vaccination strengthened DTH responses to irradiated autologous tumor cells in most patients, and vigorous humoral responses to tumor-associated angiogenic cytokines were seen as well. T-lymphocyte activation was seen with significantly increased expression of CTLA-4, PD-1, 4-1BB, and OX40 by CD4+ cells and PD-1 and 4-1BB by CD8+ cells. Activation was coupled with vaccine-associated increase in the frequency of regulatory CD4+ T lymphocytes. Conclusions: Vaccination with irradiated autologous tumor cells mixed with GM-K562 cells is feasible, well tolerated, and active in patients with recurrent malignant glioma. Clin Cancer Res; 22(12); 2885–96. ©2016 AACR.