Introduction: The hypoxic microenvironment of glioblastoma multiforme (GBM) is thought to increase resistance to cancer therapies. Recent evidence suggests that hypoxia induces activity of protein phosphatase 2A (PP2A), a regulator of cell cycle and apoptosis. The effects of PP2A activity on tumor cell proliferation and survival in GBM have not been studied.
Methods: 65 high-grade astrocytomas and 18 control brain specimens were evaluated for PP2A, HIF-1a, Cyclin G2, AKT and PLK expression as well as for PP2A activity. PP2A activity was modulated by okadaic acid or by shRNA in hypoxic GBM-derived tumor stem-like cells (TSCs). Effects of PP2A activity on cell cycle and cell survival were assessed using flow cytometry.
Results: High PP2A activity was detected in hypoxic GBM specimens and was associated with poor clinical prognosis. Moderate hypoxia induced PP2A activity in TSCs in vitro. PP2A activity was mirrored by increased expression of cyclin G2, an unconventional cyclin that mediates G1/S phase cell cycle arrest. Cyclin G2 was detected as a binding partner of PP2A in TSCs. Inhibition of PP2A activity by okadaic acid or shRNA partially reversed cell cycle arrest typically seen in hypoxic TSCs. Inhibition of PP2A in hypoxic cells increased PLK expression, AKT phosphorylation and was associated with increased cell death.
Conclusions: Our results demonstrate that PP2A activity mediates dormancy of hypoxic GBM-derived TSCs. Pharmacological inhibition of PP2A decreases TSC survival in hypoxic conditions and may be a possible target for cancer therapy.
Patient Care: In the present study, we propose protein phosphatase 2A (PP2A) as a potential mediator between hypoxia and increased tumor resistance. In vitro experiments demonstrate that inhibition of hypoxia-induced PP2A activity allows for apoptotic cell death of tumor stem-like cells (TSCs) during hypoxia, while no measurable effect is seen on TSCs in normoxia. By targeting hypoxic tumor cells, pharmacological inhibition of PP2A may be a valuable strategy in the treatment of the most aggressive GBM.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the role of protein phosphatases in tumor cell survival, 2) Identify the impact of hypoxia on the behavior of GBM-derived stem cells, 3) Discuss strategies to target hypoxic tumor cells with cytotoxic agents.