4 other hand, the same drugs activate Nrf2 as evident from the upregulation of Nrf2 targets such as glutathione reductase, a glutathione peroxidase, and GST [69]. The thereby also upregulated TrxR1 might interfere with the inhibition of enzyme activity and facilitate cancer cell growth. This should be considered when selecting drugs to inhibit TrxR activity. However, TrxR1 is not the only enzyme required for pro- liferation. Cells from mice with a diﬀerentiated hepatocyte- speciﬁc KO of TrxR1 were able to proliferate [70]. Prolif- eration of hepatocytes lacking TrxR1 was observed by in vivo staining [71] and MEFs from conditioned TrxR1 KO mouse embryos did not show impaired proliferation [72]. Thus, TrxR1 deletion appears to be compensated by another system. This most probably is the glutathione (GSH) system as demonstrated by a severely reduced replicative index if also GSH is depleted [71], and by an upregulation of GSH metabolizing enzymes in MEFs from conditioned TrxR1 KO mice [72]. The latter obviously is achieved in TrxR1- depleted cells by an activation of Nrf2 [73]. Deletion of TrxR1 in all parenchymal hepatocytes of mice resulted in a compensatory upregulation of Nrf2 targets including GSTs, GPx2, and sulﬁredoxin [73]. It was, thus, concluded that ablation of txnrd1 encoding TrxR1 would mimic oxidative challenge and switch on a constitutively active Nrf2 pathway [73]. What is activating Nrf2 when TrxR1 is absent is unclear, but it ﬁts with the observations made with TrxR1 inhibitors (see above) and the upregulation of Nrf2 targets in moderate selenium deﬁciency in mice [74, 75]. A challenging hypothesis would be that the thioredoxin system maintains critical thiol groups in Keap1 in the reduced state and, thereby, prevents Nrf2 release. That Trx is able to reduce Cys151 in Keap1 has recently indeed been shown [76]. This way TrxR1 would serve as turn-oﬀ signal for the Nrf2 system. In sum, a critical balance between Nrf2 and TrxR1 activities might exist which appears worth to be further investigated. 4. GPx2 The gastrointestinal glutathione peroxidase (GPx2) was ﬁrst detected in the gastrointestinal system [77]. There it obvi- ously plays a role in proliferating cells since its concentration is highest at crypt bases where proliferation takes place [78]. During human colon carcinogenesis, GPx2 is transiently increased with the highest expression in early adenoma and decreasing amounts in late stages of malignancy [78, 79]. It is also upregulated during the neoplastic transformation of squamous epithelial cells [80] and in lung adenocarcinomas of smokers [81] indicating that its expression is not restricted to the gastrointestinal system, but rather characteristic for rapidly dividing epithelial cells in general [82]. Evidences for a protective role are provided by genetically modiﬁed animals. A GPx2 KO rendered mice more susceptible to skin cancer development upon γ-irradiation [83]. Mice in which both GPx1 and GPx2 had been knocked out developed ileocolitis [84] and later intestinal cancer [85]. The lack of GPx2 was more detrimental, since one intact allele of GPx2 (but not of GPx1) was suﬃcient to prevent intestinal International Journal of Cell Biology inﬂammation [82]. Mechanistic studies with HT29 cells with a stable downregulation of GPx2 by siRNA revealed that GPx2 suppresses COX2 expression and PGE2 production [86]. Furthermore, the siGPx2 cells exhibited an enhanced invasive potential and migrated faster in a wound healing assay [87]. Both eﬀects obviously required the upregulated COX2 activity since celecoxib, a speciﬁc COX2 inhibitor, rescued the eﬀects to the level observed in control cells. A protective role of GPx2 can also be inferred from its induction by Nrf2 [52]. Thus, the majority of the ﬁndings described so far characterize GPx2 as an anti-inﬂammatory enzyme. The function of GPx2 as an anticarcinogenic enzyme is less clear, and evidence for an additional procarcinogenic role is increasing. Apoptosis at colonic crypt bases is drastically increased in GPx2 KO mice [88]. Inhibition of apoptosis may reﬂect the physiological function of GPx2 in crypt bases where it appears to support cell proliferation in the self-renewal of the intestinal mucosa. Cancer cells, however, will proﬁt from not being eliminated by apoptosis which might well be the reason why the siGPx2 cells were not able to grow anchorage-independently and developed into much smaller tumors than WT cells when injected into nude mice [87]. Indeed, GPx2 expression is higher in proliferating cancer stem cells compared to their diﬀerentiated progeny [89]. Transcriptional regulation further points into a pro- carcinogenic direction. GPx2 is induced by ΔNp63 [90], a transcription factor necessary for cell proliferation, and its overexpression inhibited oxidant-mediated apoptosis [90]. Activation of the GPx2 promoter by β-catenin [91], which is the key mediator in the Wnt pathway and constitutively active in most of intestinal cancers, can again be interpreted controversially, either as an attempt to counteract carcino- genesis or to sustain cancer cell growth. Some tentative answers may be derived from a study using an inﬂammation triggered model of colon carcino- genesis, the azoxymethane (AOM)/dextransulfate sodium (DSS) mouse model. In this study AOM/DSS treatment was combined with feeding WT and GPx2 KO mice a moderately Se-deﬁcient, Se-adequate, and Se-supranutritional diet [92]. All AOM/DSS-treated mice developed colitis which was generally more severe in GPx2 KO mice than in WT mice under all Se states and especially high in moderate Se- deﬁciency. Inﬂammation and accordingly tumor formation were decreased under the Se supranutritional diet. Tumor numbers per animal tended to be higher in GPx2 KO mice at all selenium diets and were decreased by supranutritional selenium. In contrast, tumor size was smaller in GPx2 KO mice at the moderate selenium deﬁciency and in the supranutritional status, which correlates with the smaller tumors in nude mice developing from HT29 cells in which GPx2 was knocked down (see above). In the same experiment the eﬀect of sulforaphane (SFN), a well-known Nrf2 activator, was tested. Surprisingly, it enhanced colitis in Se-poor WT and GPx2 KO mice but decreased it in Se-adequate mice to an identical score in both genotypes. The same dependency on selenium was observed for the reduction in the number of tumors and apoptotic cells by SFN in both GPx2 KO and WT mice. This indicates