International Journal of Cell Biology Nrf2 Nrf2 Nrf2 Nrf2 GSH system GPx2 TrxR1 GPx2 TrxR1 2 true for healthy cells, solely beneﬁcial functions of Nrf2 have been questioned especially in cancer. Recent data revealed a “dark” side of Nrf2. Its upregulation in cancer cells provides an advantage for these cells to grow and, in addition, makes them resistant against chemotherapy (reviewed in [7, 8]). Thus, also a beneﬁt of Nrf2 activation might depend on the cancer stage. Evidence to support this idea is summarized in view of the mutual regulation of selenium/selenoproteins and Nrf2 (see Figure 1). 2. Nrf2 Nrf2 is a transcription factor which is kept in the cytosol by Keap1. Keap1 acts as substrate adaptor for the Cul3- Rbx1 E3 ligase which ubiquitylates Nrf2 for proteasomal degradation. Dissociation of this complex is achieved by thiol modiﬁcation of Keap1 preventing degradation and allowing newly synthesized Nrf2 to translocate into the nucleus. There it binds to the antioxidant/electrophile responsive element (ARE/EpRE) in the promoter region of its target genes. The mechanism of activation is complex but has become relatively clear in the very recent years and is described in multiple reviews [9–18]. Nrf2 regulates the expression of proteins that collectively favour cell survival. These comprise enzymes that directly or indirectly have antioxidant functions, are molecular chaperones and proteins that enhance glutathione syn- thesis and regeneration, belong to enzymes of the phase 2 detoxiﬁcation drug metabolism systems, and recognize, repair, and remove damaged proteins and DNA. Also proteins that regulate the expression of other transcription factors, growth factors, and receptors and inhibit cytokine- mediated inﬂammation and autophagy are targets of Nrf2 (reviewed in [9, 19]). The realm of Nrf2 activators com- prises endogenous signaling molecules produced during normal oxygen metabolism and under inﬂammation or other stress situations. Among activators are H2O2, ROOH, ONOO−, oxoaldehydes, and ketones, or cyclopentenones, like 15-deoxy-Δ12,14-prostaglandin J2. Exogenous activators comprise dietary isothiocyanates, thiocarbamates, trivalent arsenicals, quinones, dithiolethiones, vicinal dimercaptanes, certain statins, and heavy metals [9, 12, 19]. Activation by the later group allows the conclusion that these compounds may act via initiation of a moderate oxidative/electrophilic stress. The protective role of Nrf2 is also demonstrated by genetic ablation. Nrf2-deﬁcient mice are more susceptible to carcinogen-induced cancers [20] and develop more severe intestinal inﬂammation and a higher number of aberrant crypts than controls upon dextran sulfate sodium treatment [21–23]. These ﬁndings indicate a role of Nrf2 in the prevention of carcinogenesis, especially if inﬂammation triggered. In contrast, without challenge, Nrf2-deﬁcient mice did not show an obvious phenotype [24]. The Nrf2 system, thus, appears to be an emergency device that comes into play if a stress is severe enough that it can no longer be handled by constitutively active systems. An enhanced Nrf2 activity by moderate stress makes cells resistant to a subsequent more severe oxidative and electrophilic stress and, thus, works like Nrf2 +Se (a) −Se (b) Figure 1: Interplay between Nrf2 and the selenoproteins thiore- doxin reductase-1 (TrxR1) and glutathione peroxidase-2 (GPx2). (a) In the presence of selenium, the activation of Nrf2 leads to increased mRNA of both enzymes which can be translated into respective proteins. (b) Under selenium deﬁciency, Nrf2 is activated which in principle can lead to an induction of TrxR1 and GPx2 mRNA. Due to lack of selenium, the proteins cannot be synthesized. Decrease in TrxR1 further activates Nrf2, which subsequently upregulates enzymes of the glutathione (GSH) system. These, in part at least, can compensate the reduced TrxR1 activity. For details see text. a vaccination. This way, by upregulation of defense systems, Nrf2 can prevent cancer initiation by elimination of reactive oxygen species and detoxiﬁcation of carcinogens. However, also this coin has two sites, since Nrf2 acti- vation may not be beneﬁcial under all circumstances. Not only normal but also tumor cells may beneﬁt from the protective function of Nrf2 as evidenced by an increase of Nrf2 and its targets in many cancer cell lines (reviewed in [7, 17]). The physiological result of the upregulation, that is, inhibition of apoptosis and autophagy, and increase of proteasomal degradation of damaged proteins, provides a superior survival chance also for tumors. Accordingly, Nrf2 was expressed in a signiﬁcantly higher proportion of endometrial serous carcinoma, the most aggressive subtype of endometrial cancer [25]. Nrf2 knockdown inhibited tumor growth from human cervical carcinoma cells in xenograft studies [26] and increased eﬃciency of chemother- apy in mice bearing subcutaneous tumors of these cells. In an urethane-induced lung cancer mouse model, Nrf2 deletion decreased tumorigenesis and facilitated death of early initiated cells by apoptosis [27]. These ﬁndings support cell survival properties of Nrf2 also in cancer cells. A constitutive activity of Nrf2 is often reached by muta- tions in Keap1 and Nrf2 genes themselves [28]. Somatic mu- tations and loss of heterozygosity of Keap1 were ﬁrst found in small cell lung carcinoma cell lines [29] and in non-small- cell lung cancers [30] associated with an upregulation of Nrf2 and respective target genes. RNAi-mediated downregulation of Nrf2 in these cells suppressed tumor growth in xenograft experiments and increased sensitivity to chemotherapy [31]. Somatic Keap1 mutations have also been found in cancers of gallbladder and hepatic bile duct [32] and prostate [33]. Nrf2 missense mutations were identiﬁed in lung cancers [34] and in squamous cell carcinomas of oesophagus and skin [35]. The mutations lie in the Nrf2-Keap1 interaction area, which