Temozolomide induces activation of Wnt/β-catenin signaling in glioma cells via PI3K/Akt pathway: implications in glioma therapy

Glioblastoma (GBM) is the most aggressive type of glioma. Temozolomide (TMZ) is currently the drug of choice used for post-operative chemotherapy of GBM. However, the presence of intrinsic and acquired resistance hinders the success of chemotherapy. To understand the TMZ resistant mechanisms in glioma, we investigated the alterations in cellular signaling pathways by performing transcriptome analysis of TMZ treated glioma cells. Gene Set Enrichment Analysis (GSEA) indicated a significant enrichment of Wnt/β-catenin signaling besides many other pathways in TMZ treated cells. Further, we demonstrate that TMZ treatment increased the activity from TOPflash reporter, (a Wnt responsive reporter), enhanced the levels of pGSK-3β (S9) and reduced the levels of p-β-catenin (S33/37/T41) with a concomitant increase in transcript and protein levels of Wnt targets in a concentration and time-dependent manner. While TMZ treated cells did not show alteration in any of the Wnt ligands, PI3K inhibitor (LY294002) treatment repressed Akt activation and abolished the TMZ-mediated induction of Wnt/β-catenin pathway. In addition, we show that Wnt/β-catenin signaling activation by TMZ is independent of ATM/Chk2 pathway. Further, we also demonstrate the activation of mTOR pathway after TMZ treatment. Thus, our results demonstrate that activation of Wnt/β-catenin pathway involves an ATM/Chk2- independent PI3K/Akt/GSK-3 cascade in TMZ treated cells and further provides mechanistic basis for the chemoresistance of glioma to TMZ.