Data from MSK1-Mediated β-Catenin Phosphorylation Confers Resistance to PI3K/mTOR Inhibitors in Glioblastoma

Glioblastoma (GBM) represents a compelling disease for kinase inhibitor therapy because most of these tumors harbor genetic alterations that result in aberrant activation of growth factor–signaling pathways. The PI3K/mammalian target of the rapamycin (mTOR) pathway is dysregulated in over 50% of human GBM but remains a challenging clinical target. Inhibitors against PI3K/mTOR mediators have limited clinical efficacy as single agents. We investigated potential bypass mechanisms to PI3K/mTOR inhibition using gene expression profiling before and after PI3K inhibitor treatment by Affymetrix microarrays. Mitogen- and stress-activated protein kinase 1 (MSK1) was markedly induced after PI3K/mTOR inhibitor treatment and disruption of MSK1 by specific shRNAs attenuated resistance to PI3K/mTOR inhibitors in glioma-initiating cells (GIC). Further investigation showed that MSK1 phosphorylates β-catenin and regulates its nuclear translocation and transcriptional activity. The depletion of β-catenin potentiated PI3K/mTOR inhibitor-induced cytotoxicity and the inhibition of MSK1 synergized with PI3K/mTOR inhibitors to extend survival in an intracranial animal model and decreased phosphorylation of β-catenin at Ser⁵⁵². These observations suggest that MSK1/β-catenin signaling serves as an escape survival signal upon PI3K/mTOR inhibition and provides a strong rationale for the combined use of PI3K/mTOR and MSK1/β-catenin inhibition to induce lethal growth inhibition in human GBM. Mol Cancer Ther; 15(7); 1656–68. ©2016 AACR.