Author:

Keywords:

G0B1719N#54971383

Abstract:

Glioblastoma (GBM) is the most common primary brain tumor in adults. Current standard therapy is surgery followed by radiotherapy, with concurrent and adjuvant temozolomide (TMZ) chemotherapy. GBM is characterized by almost uniformly fatal outcome, highlighting the unmet clinical need for more efficient, biomarker-guided treatments. Protein phosphatase methylesterase-1 (PME-1), a regulator of the tumor suppressive phosphatase PP2A, promotes PP2A demethylation and inactivation, and is overexpressed in 44% of GBM, associated with increased tumor grade and cellular proliferation. In the current study, we first aimed to investigate how reactive oxygen species (ROS), a frequent by-product of radiotherapy and temozolomide chemotherapy, regulate PP2A function via its methylesterase PME-1, and how PME-1 overexpression impacts the response of GBM cells to oxidative stress. We found that in two glioblastoma cell lines, U87MG and U251MG, expression of PME-1 is positively correlated with the sensitivity of the cells to H2O2 or t-BHP-induced oxidative stress. Experiments using the irreversible pharmacologic PME-1 inhibitor, AMZ30, and different PME-1 mutants, revealed that the methylesterase function, the PP2A binding capacity and the nuclear localization of PME-1 are all important for the sensitizing effect of PME-1 expression. Furthermore, we identified increased nuclear localization of the PP2A B55α subunit, increased binding of PP2A-B55α to PME-1 and increased B55α-bound PP2A-C demethylation upon oxidative stress. Lastly, we uncovered increased stress-induced phosphorylation and activity of MAPKAPK2 and RIPK1 in PME-1 overexpressing U87MG cells, which caused the observed sensitization to t-BHP treatment. Our data reveal a novel role for PME-1 in oxidative stress-induced GBM cell death, regulating nuclear PP2A-B55α activity and MAPKAPK2-RIPK1 signaling. Patients with GBM tumors overexpressing PME-1, although having a worse prognosis due to increased cellular proliferation of the tumor, could actually be more responsive to oxidative stress-inducing therapies. In addition, our study aimed to explore if PME-1 expression impacts the growth of the U87MG and U251MG GBM cell lines. Our findings unveiled a cell-type dependent effect of PME-1 modulation on cell growth, with only the U87MG cell line being affected by PME-1 modulation. Nonetheless, the growth of both GBM cell lines was inhibited by the PME-1 inhibitor AMZ30, revealing a potential therapeutic opportunity for targeting PME-1 in GBM tumors. Lastly, we assessed the TMZ sensitivity of the different PME-1 modulated cell lines, once again revealing a cell-type dependent response. For full understanding of the cell-type dependent effects of PME-1 modulation on cell growth and TMZ response, additional experiments should be performed.