Proteasome inhibitor has been proven to be an effective anticancer compound in many tumor models, including Glioblastoma multiforme (GBM). In this study, we found that the proteasome inhibitor Velcade (PS-341/bortezomib) activated the PI3K/Akt pathway although causing GBM cells death. Therefore, we sought to investigate if the PI3K inhibitor, ZSTK474, would enhance the effectiveness of PS-341 in anticancer therapy. Three GBM cells lines (U87, U251, U138) were exposed to Velcade (10-7 M, 24 hr). We found that phosphorylation of Akt, 4E binding protein-1 (4EBP1), and mammalian target of rapamycin (mTOR) was induced by Valcade in these GBM cells compared to the controls. To study the role of the PI3K/Akt signal pathway in the Velcade induced growth arrest in GBM cells, this pathway was blocked using the PI3K inhibitor ZSTK474. These three cell lines were treated with serial dilutions of either Velcade or ZSTK474 or both drugs together, and MTT [3-(4,5-dimethylthiazol-2-yl)- 2,5-diphenyl tetrazolium bromide] assay results showed that combination of ZSTK474 and Velcade synergistically inhibited proliferation of GBM cell lines. In the presence of ZSTK474, the phosphorylation of Akt, 4EBP1, and mTOR was markedly decreased in all cell lines. Cells apoptosis was increased when exposed to Velcade and ZSTK474 in combination as shown by Annexin V analysis. To confirm the decrease of p-4EBP1 in the presence of both drugs, the unphosphorylated binding protein 4EBP1 was also detected. In all cell lines in the presence of Velcade and ZSTK474, 4EBP1 becoming unphosphorylated. Cyclin D1, another protein promoting proliferation in the PI3K/Akt pathway, was up-regulated in the presence of Velcade alone, but down-regulated in the presence of both Valcade and ZSTK474 together. In summary, the anticancer ability of Valcade is increased by combination with PI3K pathway inhibitor ZSTK474 in glioblastoma multiforme.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2801. doi:1538-7445.AM2012-2801