Activation of the NRF2 pathway and its impact on the prognosis of anaplastic glioma patients

Anaplastic gliomas and glioblastomas are aggressive brain tumors with median survival times ranging from 72.1 to 82.6 months in anaplastic gliomas and 14.6 months in glioblastomas.1,2 One of the main causes of the poor prognosis is chemo- and radioresistance. Thus, elucidation of mechanisms underlying this resistance is urgent to improve the treatment outcomes of malignant gliomas. The system involving Kelch-like ECH (erythroid cell-derived protein with cap'n'collar homology)-associated protein 1 (KEAP1) and nuclear factor erythroid 2–related factor 2 (NRF2) plays pivotal roles in protecting normal and neoplastic cells from oxidative and electrophilic insults by activating cytoprotective genes. These gene products are involved in glutathione synthesis (eg, glutamate cysteine ligase of catalytic [GCLC] and of modifier subunit [GCLM] genes), reactive oxygen species (ROS) elimination, xenobiotic metabolism (eg, NAD(P)H:quinone oxidoreductase gene [NQO1]), and crossmembrane transport, often resulting in multidrug resistance.3 Under normal conditions, KEAP1 ubiquitinates NRF2, and NRF2 is degraded by the proteosome. In the presence of electrophiles or ROS, KEAP1 is inactivated, and NRF2 is stabilized, binds to antioxidant response elements (AREs), and induces the cytoprotective genes. Recently, constitutive activation of NRF2 was found in various cancers. Multiple causes of the constitutive activation of NRF2 have been described, including somatic mutations of KEAP1 or NRF2, reduced expression of KEAP1, and accumulation of p62.4 Increased NRF2 activity confers cell growth advantage and resistance to chemo- and radiotherapy on lung, prostate, gallbladder, and ovarian cancers5–9 and leads to poor prognosis in lung, esophagus, gallbladder, and breast cancers.10–14 Although similar impacts of NRF2 activation on glioblastoma cell lines and a small cohort of glioblastoma patients have been reported,15,16 the prognostic significance of NRF2 activation in malignant gliomas remains to be elucidated. Somatic mutations in the isocitrate dehydrogenase 1 (IDH1) gene or IDH2 gene have been identified in adult gliomas.17–19 These mutations occur at a very early stage of gliomatogenesis and confer genetic and prognostic differences on anaplastic gliomas and glioblastomas.17–19 Malignant gliomas with IDH1/2 mutations had better prognoses than those with wild-type IDH1/2.17–19 Although the hypermethylated phenotype is frequently found in World Health Organization grades II and III gliomas with IDH1/2 mutations, underlying mechanisms for better prognosis remain unclear. Recently, IDH1 mutations have been shown to sensitize glioma cells to bis-chloroethylnitrosourea (BCNU)–induced oxidative stress by decreasing the amount of the reduced form of glutathione.20 Because the synthesis and reduction of glutathione is often heavily dependent on NRF2 activity,4 we hypothesized that the NRF2 pathway was suppressed in the presence of mutated IDH1, leading to the better prognosis of malignant gliomas with IDH1/2 mutations. To test this hypothesis, we characterized the prevalence, possible causes, and prognostic value of the NRF2 activation in anaplastic gliomas with mutated or wild-type IDH1/2 and glioblastomas. We also evaluated the association between IDH1/2 mutations and NRF2 activity by analyzing the expression of NRF2 and its target genes in surgical specimens, and confirmed the effect of IDH1 mutation on NRF2 activity, using IDH1 mutant-expressing cells.