Abstract 3774: Hyperthermia treatment overcomes temozolomide resistance in glioma cells by downregulating MGMT expression and increasing temozolomide uptake

Glioblastoma multiforme (GBM) is the most common and malignant brain tumor. Current standard treatment for GBM following maximum safe resection includes temozolomide (TMZ) with concurrent radiotherapy. Despite the improvement of survival by TMZ and radiation, the overall prognosis remains poor for GBM patients. This is partially attributed to drug resistance. Expression of DNA repair protein O6-methylguanine methyltransferase (MGMT) in gliomas confers TMZ resistance and a worse prognosis. Therefore, development of novel therapy to suppress MGMT expression and overcome TMZ resistance is highly desired. Hyperthermia has been shown to increase TMZ sensitivity in melanoma and this provides a good rationale to evaluate whether hyperthermia overcomes TMZ resistance in glioblastoma cells. In this study, we investigated the effect of hyperthermia treatment (43 °C) on TMZ sensitivity of four human glioblastoma cell lines using clonogenic assays. Our results showed that hyperthermia treatment increased TMZ-induced cell death in TMZ-resistant glioma cells, but had no additional beneficial effect in the TMZ-sensitive cells. Hyperthermia in combination with radiation further overcame TMZ resistance. To study the molecular targets, we found that hyperthermia downregulated MGMT expression through increasing proteasome-mediated MGMT degradation, which can be reversed by the proteasome inhibitor bortezomib. Hyperthermia had no effects on p53 expression and MGMT promoter methylation which was responsible for MGMT gene silencing. In addition, liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed an increase of TMZ and its downstream product 5-aminoimidazole-4-carboxamide (AIC) in hyperthermia-treated cells, indicating an increase of TMZ drug uptake and breakdown. In conclusion, these results suggest that hyperthermia overcomes MGMT-mediated resistance of glioblastoma cell lines to TMZ and increases TMZ drug uptake. This study suggests a potential benefit of using hyperthermia in the treatment of GBM, particularly those with more adverse prognosis. This work was supported by Debrule Research Fund. Citation Format: Chen-Ting Lee, Aaron Blackley, Chelsea Landon, Ivan Spasojevic, John P. Kirkpatrick, Mark W. Dewhirst. Hyperthermia treatment overcomes temozolomide resistance in glioma cells by downregulating MGMT expression and increasing temozolomide uptake. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3774. doi:10.1158/1538-7445.AM2014-3774