Abstract 2662: Radiosensitizing potential of the PI3K inhibitor, BKM-120 and the HSP 90 inhibitor, HSP990, is dependent on PTEN status of glioblastoma cells

Abstract

Abstract Introduction: Radiation therapy plays an essential role in the treatment of Glioblastoma Multiforme (GBM), the most common primary adult brain tumor. GBMs frequently contain mutations in the tumor suppressor gene, PTEN, a down regulator of the pro-survival phosphatidylinositide-3-kinase (PI3K) pathway. PTEN loss causes overactivation of the pathway inducing radioresistance. Heat-shock protein (HSP) 90 is overexpressed in GBM; it is a chaperone for many oncogenic proteins and plays a key role in assisting tumor cell survival, proliferation, invasion, metastasis and tumor angiogenesis. The purpose of this study was to investigate the radiosensitizing potential of an HSP 90 inhibitor, (HSP990) and a PI3K inhibitor, (BKM-120) in U87 GBM cells expressing PTEN or null for PTEN. Methods: Two isogenic U87 cell lines, stably transfected with wildtype PTEN (U87 wtPTEN) or lacking PTEN (U87PTEN null) were used. Radiosensitivity was determined by clonogenic and apoptotic assays (Annexin V kit). DNA damage and repair was assessed by quantifying gamma Histone 2AX (γH2AX) foci. Immunoblots were performed to assess pAkt levels. HSP990 and BKM -120 were provided by Novartis Pharmaceuticals. Results: Neither HSP990 or BKM -120, alone or in combination, potentiated radiation-induced DNA damage in either cell line, as assessed by γH2AX foci. Radiation alone (6 Gy) activated AKT phosphorylation, with maximum activity occurring one hour post-radiation in both cell lines. Radiation-induced Akt phosphorylation was strongly inhibited by BKM -120 alone or in combination with HSP990 but not by HSP990 alone in both cell lines. In clonogenic assays, both drugs (alone or in combination), sensitized PTEN positive and PTEN null cells to radiation. Whereas the two drugs had similar efficacy in PTEN null cells, HSP990 was a better radiosensitizer than BKM -120 in U87 wtPTEN cells. Apoptosis assays revealed very little cell death 1 hr or 24 hr following drug and RT treatments. After 5 days, the percentage of apoptotic/necrotic cells increased >30% over control following treatment with BKM -120 alone or in combination with HSP 990 in PTEN null cells. In contrast, PTEN positive cells were more resistant to cell death following BKM -120. Conclusions: The results suggest that PI3K inhibition with BKM -120 will enhance radiotherapy for GBM lacking PTEN activity. HSP 90 inhibition by HSP990 may be of benefit in enhancing radiotherapy for GBM expressing functional PTEN. Thus, it appears that the optimal choice of radiosensitizer depends upon PTEN status. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2662. doi:10.1158/1538-7445.AM2011-2662