Elucidation and Exploitation of GSK3 as a Novel Glioma Therapeutic Target

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We have demonstrated that multiple small molecular inhibitors of GSK3 activity and genetic downregulation of GSK3alpha/beta significantly inhibit glioma cell survival. Among the small molecules used, LY317615 was developed by Eli Lilly Pharmaceuticals as an ATP-competitive inhibitor of PKC-beta (PKC-b) to inhibit VEGF-stimulated endothelial proliferation and applied in preclinical tumor models where it demonstrated significant anti-angiogenic activity. Given that other PKC isoenzymes have been shown to contribute to tumor cell survival and proliferation, we sought to investigate whether Enzastaurin could exert anti-proliferation activity on glioma cells directly by inhibiting PKC-b activity. We found that LY317615 exerts potent anti-proliferation activity on glioma cell lines at pharmacologically achievable concentrations (IC50 of 10mM). We sought to determine the anti-proliferative mechanism of LY317615 on glioma cells. Cell Cycle analysis preformed by BrdU/PI staining of LY317615-treated U251 revealed a drug-induced G2/M arrest and apoptosis as early as 24hr after treatment. To elucidate mechanisms responsible for the antiglioma effects of LY317615, we performed gene expression profiling in hopes of identifying potential downstream effectors of PKC-b inhibition. Striking, were alterations among components of the Wnt pathway within the nearly 1400 mRNA transcripts significantly altered following glioma cell exposure to LY317615. The strongest up-regulated gene (more than 40-fold) was axin 2 mRNA, a known component of the Wnt negative feedback loop. In addition to axin 2, we found highly significant changes in expression of at least 20 genes, such as CyclinD1, that are known to be the targets of b-catenin, the down-stream effector of the Wnt pathway. Further investigation of this pathway by both pharmacological and genetic means have suggested that activation of Wnt pathway in glioma cell lines leads to cell death. Specifically, we have demonstrated that the potency of the cytotoxic effects is directly correlated with decreased enzyme activity-activating phosphorylation of GSK3 alpha/beta Y276/Y216 and with increased enzyme activity-inhibitory phosphorylation of GSK3alpha S21. Inhibition of GSK3 activity results in a cytotoxicity-dependent increase in c-MYC activity thereby inducing expression of Bim, bax and DR4/DR5. Down-regulation of GSK3 activity also leads to a drop in FLIP protein and up-regulation of TRAIL. In addition to up-regulation of components of the TRAIL-associated extrinsic apoptotic pathway, downregulation of GSK3 activity results in alteration of intracellular glucose metabolism resulting in dissociation of hexokinase (HK) II from outer mitochondrial membrane with subsequent mitochondrial destabilization. Finally, inhibition of GSK3 activity causes a dramatic decrease in intracellular nuclear factor-kappa B (NF-kappaB) activity. Thus, inhibition of GSK3 activity results in c-MYC dependent glioma cell death through multiple mechanisms all of which converge on the apoptotic pathways. These data support the hypothesis that GSK3 may be important therapeutic target for gliomas. Based on the promising preclinical data, we initiated a clinical trial of LY317615 in patients with recurrent high-grade gliomas