17β-estradiol induces temozolomide resistance through NRF2-mediated redox homeostasis in glioblastoma

Glioblastoma multiforme (GBM) is the most fatal cancer among brain tumors, and the standard treatment of GBM patients is surgical tumor resection followed by radiotherapy and temozolomide (TMZ) chemotherapy. However, tumors always recur due to the developing drug resistance. It has been shown that neurosteroids, including dehydroepiandrosterone and 17 beta-estradiol, are synthesized in TMZ-resistant GBM tumors. Therefore, we sought to explore the possible role of 17 beta-estradiol in the development of drug resistance in GBM. Bioinformatics analysis revealed that aromatase/cytochrome P450 19A1 expression was gradually increased in the development from normal, astrocytoma to GBM. The level of 17 beta-estradiol was significantly increased in TMZ-resistant cells characterized by ultra performance liquid chromatography-tandem mass spectrometry. Furthermore, 17 beta-estradiol attenuated TMZ-induced cell death and reduced reactive oxygen species production by mitochondria. In addition, 17 beta-estradiol attenuated oxidative stress by increasing the expression of superoxide dismutase 1/2, catalase, and nuclear factor erythroid 2-related factor (NRF) 2. We found that NRF2 expression was essential for the induction of drug resistance by 17 beta-estradiol through the reduction of oxidative stress in GBM.