Potential Therapeutic Effect of Glycogen Synthase Kinase 3β Inhibition against Human Glioblastoma

Purpose: Glioblastoma represents the malignant brain tumor that is most refractory to treatment and in which the identification of molecular target(s) is urgently required. We investigated the expression, activity, and putative pathologic role of glycogen synthase kinase 3 beta (GSK3 beta), an emerging therapeutic target for neurodegenerative diseases, in human glioblastoma. Experimental Design: The active fraction of GSK3 beta that is phosphorylated at the tyrosine 216 residue (pGSK3 beta(Y216)) was identified in glioblastoma cell lines. GSK3 beta activity for phosphorylating its substrate was detected in these cells by nonradioisotopic in vitro kinase assay. Results: Higher expression levels of GSK3 beta and pGSK3 beta(Y216) were frequently detected in glioblastomas compared with nonneoplastic brain tissues. Inhibition of GSK3 beta activity by escalating doses of a small-molecule inhibitor (AR-A014418) or inhibition of its expression by RNA interference induced the apoptosis and attenuated the survival and proliferation of glioblastoma cells in vitro. Inhibition of GSK3 beta was associated with increased expression of p53 and p21 in glioblastoma cells with wild-type p53 and with decreased Rb phosphorylation and expression of cyclin-dependent kinase 6 in all glioblastoma cell lines. Administration of AR-A014418 at a low dose significantly sensitized glioblastoma cells to temozolomide and 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea, chemotherapeutic agents used in the clinical setting, as well as to ionizing radiation. Conclusion: These results indicate that GSK3 beta exerts a pathologic role by promoting the survival and proliferation of glioblastoma cells and by protecting them from apoptosis via the inactivation of p53- and/or Rb-mediated pathways. Consequently, we propose that GSK3 beta provides a potential therapeutic target in glioblastoma.