MGST1, a new soldier of NRF2 in the battle against ferroptotic death

In this issue of Cell Chemical Biology, Kuang et al., 2021Kuang F. Liu J. Xie Y. Tang D. Kang R. MGST1 is a redox-sensitive repressor of ferroptosis in pancreatic cancer cells.Cell Chem. Biol. 2021; 28 (this issue): 765-775.e5Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar identify microsomal glutathione-S-transferase 1 (MGST1) as an NRF2 target gene that suppresses ferroptosis in pancreatic cancer cells. Mechanistically, MGST1 binds ALOX5 during ferroptosis induction, inhibiting lipid peroxide production. Thus, MGST1 could represent a viable therapeutic target for treating pancreatic cancer. In this issue of Cell Chemical Biology, Kuang et al., 2021Kuang F. Liu J. Xie Y. Tang D. Kang R. MGST1 is a redox-sensitive repressor of ferroptosis in pancreatic cancer cells.Cell Chem. Biol. 2021; 28 (this issue): 765-775.e5Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar identify microsomal glutathione-S-transferase 1 (MGST1) as an NRF2 target gene that suppresses ferroptosis in pancreatic cancer cells. Mechanistically, MGST1 binds ALOX5 during ferroptosis induction, inhibiting lipid peroxide production. Thus, MGST1 could represent a viable therapeutic target for treating pancreatic cancer. Cancer drug resistance continues to represent a significant obstacle in the successful treatment of most cancers. Along these lines, a great deal of scientific effort remains focused on understanding the mechanisms that drive chemoresistance, with the continued goal of generating more effective therapies to improve patient outcomes. As one might expect, many of the same critical pro-survival features that promote tumor progression, collectively known as the “hallmarks of cancer” (Hanahan and Weinberg, 2011Hanahan D. Weinberg R.A. Hallmarks of cancer: the next generation.Cell. 2011; 144: 646-674Abstract Full Text Full Text PDF PubMed Scopus (40071) Google Scholar), also facilitate the ability of cancer cells to survive harsh chemotherapeutic regimens. While all of the identified hallmarks play an important role in mediating tumor survival, the ability of cancer cells to evade the activation of pro-death cascades, such as apoptosis, is an important indicator of the difficult nature of successfully triggering cancer cell death. This inherent resilience is a key contributor to both intrinsic and acquired chemotherapeutic resistance in cancer cells and has led to the continual search for better cancer treatment strategies. Interestingly, the quest to find and develop therapeutics to treat resistant cancer cells led to the discovery of a previously unidentified mode of iron- and lipid peroxide-dependent cell death termed ferroptosis (Dixon et al., 2012Dixon S.J. Lemberg K.M. Lamprecht M.R. Skouta R. Zaitsev E.M. Gleason C.E. Patel D.N. Bauer A.J. Cantley A.M. Yang W.S. et al.Ferroptosis: an iron-dependent form of nonapoptotic cell death.Cell. 2012; 149: 1060-1072Abstract Full Text Full Text PDF PubMed Scopus (4286) Google Scholar; Stockwell et al., 2017Stockwell B.R. Friedmann Angeli J.P. Bayir H. Bush A.I. Conrad M. Dixon S.J. Fulda S. Gascón S. Hatzios S.K. Kagan V.E. et al.Ferroptosis: a regulated cell death nexus linking metabolism, redox biology, and disease.Cell. 2017; 171: 273-285Abstract Full Text Full Text PDF PubMed Scopus (2102) Google Scholar). Since its emergence as a mode of cell death, ferroptosis has become an increasingly popular means to kill apoptosis-resistant cancer cell types across a variety of cancer contexts (Chen et al., 2021bChen X. Kang R. Kroemer G. Tang D. Broadening horizons: the role of ferroptosis in cancer.Nat. Rev. Clin. Oncol. 2021; 18: 280-296Crossref PubMed Scopus (309) Google Scholar). In particular, the discovery and development of ferroptosis inducers as stand-alone or adjuvant chemotherapeutic agents represents an actively growing area of interest. Importantly, even though a number of critical mediators of this cascade have been identified (i.e., xCT and GPX4), which serve as key targets of already established ferroptosis-inducing compounds, the search for other relevant pro- and anti-ferroptotic effectors continues. One important upstream regulator of numerous mediators of the ferroptosis cascade is the transcription factor nuclear factor erythroid 2 (NF-E2)-related factor 2 (NRF2). Specifically, many of the genes encoding key components of iron storage and transport (FTH/FTL1 and SLC40A1/FPN), reduction of lipid peroxides (GPX4), and glutathione metabolism (GCLC/GCLM and SLC7A11/xCT) are all known NRF2 target genes (Anandhan et al., 2020Anandhan A. Dodson M. Schmidlin C.J. Liu P. Zhang D.D. Breakdown of an ironclad defense system: the critical role of NRF2 in mediating ferroptosis.Cell Chem. Biol. 2020; 27: 436-447Abstract Full Text Full Text PDF PubMed Scopus (72) Google Scholar) (Figure 1). Importantly, NRF2 has also been shown to play an important role in all of the hallmarks of cancer, including evading cell death (Rojo de la Vega et al., 2018Rojo de la Vega M. Chapman E. Zhang D.D. NRF2 and the hallmarks of cancer.Cancer Cell. 2018; 34: 21-43Abstract Full Text Full Text PDF PubMed Scopus (577) Google Scholar). As such, targeting NRF2 or its downstream effectors represents a viable strategy to kill resistant cancer cells via the induction of ferroptosis. In this issue of Cell Chemical Biology, Kuang et al. identify microsomal glutathione-S-transferase (MGST1) as an NRF2-dependent suppressor of ferroptosis in a pancreatic cancer setting (Kuang et al., 2021Kuang F. Liu J. Xie Y. Tang D. Kang R. MGST1 is a redox-sensitive repressor of ferroptosis in pancreatic cancer cells.Cell Chem. Biol. 2021; 28 (this issue): 765-775.e5Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar) (Figure 1). Using Erastin and RSL3, inhibitors of xCT and GPX4, respectively, the authors show that MGST1 levels are increased during ferroptosis induction in CFPAC and PANC2.03 pancreatic cancer cells. Furthermore, they demonstrate that MGST1 upregulation by ferroptosis inducers is NRF2 dependent, and that knockdown of either NRF2 or MGST1 can increase pancreatic cancer cell sensitivity to ferroptosis inducers. An important point discussed by the authors is that pancreatic ductile adenocarcinoma (PDAC) represents a particularly aggressive cancer type with a poor prognosis. Compounding this problem is the fact that most PDAC cases are typically identified too late for surgical removal and have a strong propensity for developing chemoresistance over time. Supporting the notion that MGST1 could represent a key mechanism of PDAC resistance, the authors show that high MGST1 levels correlate with decreased patient survival. Thus, identifying NRF2-dependent effectors of PDAC drug resistance, such as MGST1, and developing therapeutics to overcome NRF2 and its downstream mediators of resistance is of critical importance. The main damage-promoting effectors of ferroptotic cell death are lipid peroxides, which are thought to cause the membrane disruption and structural instability that in turn lead to the main morphological features of ferroptosis, including cell swelling, moderate chromatin condensation, and eventual loss of plasma membrane integrity (Tang et al., 2021Tang D. Chen X. Kang R. Kroemer G. Ferroptosis: molecular mechanisms and health implications.Cell Res. 2021; 31: 107-125Crossref PubMed Scopus (313) Google Scholar). Importantly, lipid peroxidation can result from both non-enzymatic (Fenton chemistry) and enzymatic (lipoxygenases and cyclooxygenases) oxidation of polyunsaturated fatty acids (PUFAs). Thus, an inability to properly metabolize lipid peroxides can lead to excessive lipid peroxidation and ferroptosis. Contrastingly, increased activation of key lipid detoxifying enzymes, or as was the case in the Kuang study, inhibition of the enzymatic production of lipid peroxides, both represent important aspects of surviving oxidative stress-inducing chemotherapeutic agents, including ferroptosis inducers that might be used in this capacity. Accordingly, increased expression of PUFA metabolizing enzymes, including ACSL4, PTGS2/COX-2, ALOX5, ALOX12, and ALOX15, are all considered biomarkers of ferroptotic death (Chen et al., 2021aChen X. Comish P.B. Tang D. Kang R. Characteristics and biomarkers of ferroptosis.Front. Cell Dev. Biol. 2021; 9: 637162Crossref PubMed Scopus (63) Google Scholar). This implies that enzymatic production of lipid peroxides is an important consideration when designing chemotherapy strategies targeting ferroptosis. In the case of the Kuang et al. study, one of the most interesting aspects of MGST1 prevention of ferroptotic cell death is its mechanism of suppression, which involves binding and subsequent inactivation of the lipid peroxide generating enzyme arachidonate 5-lipoxygenase (ALOX5) without changing its expression level. Intriguingly, MGST1 inhibition of lipoxygenases is limited, at least in pancreatic cancer cells, to ALOX5, as neither ALOX12 nor ALOX15 interact with MGST1 following ferroptosis induction. The authors also clearly show that MGST1-dependent inhibition of ALOX5 is critical in suppressing lipid peroxide production and ferroptotic death, as knockdown of MGST1 further enhanced the production of oxidized phosphatidylethanolamine-polyunsaturated fatty acids (PE-PUFAs), and knockdown of either ALOX5 or MGST1 reduced erastin and RSL3-induced lipid peroxidation and ferroptotic cell death. The possible translational relevance of the NRF2-MGST1 axis was further validated using an in vivo xenograft model, where the authors use imidazole-ketone erastin (IKE), an erastin derivative recently developed by the Stockwell group (Zhang et al., 2019Zhang Y. Tan H. Daniels J.D. Zandkarimi F. Liu H. Brown L.M. Uchida K. O’Connor O.A. Stockwell B.R. Imidazole ketone erastin induces ferroptosis and slows tumor growth in a mouse lymphoma model.Cell Chem. Biol. 2019; 26: 623-633.e9Abstract Full Text Full Text PDF PubMed Scopus (194) Google Scholar), to show that loss of NRF2 or MGST1 can sensitize pancreatic cancer cell xenografts to IKE-induced ferroptosis. Overall, the study by Kuang and colleagues uncovers an axis by which NRF2 suppresses the efficacy of pro-ferroptotic compounds via upregulation of a cytoprotective target gene, in this case MGST1. This work highlights the continued importance of not only determining both pan-cancer and cancer-specific mechanisms of chemoresistance, but also the development of therapeutics capable of successfully initiating the ferroptosis cascade. The possible utilization of ferroptosis inducers as chemotherapeutic agents is promising; however, as this study clearly indicates, many of the challenges facing pro-apoptotic chemotherapeutic drugs (i.e., NRF2 upregulation) may also be faced in the context of targeting ferroptosis. Along these lines, identification of what pathways might suppress the full chemotherapeutic efficacy of ferroptosis-inducing compounds will not only facilitate the improvement of current drugs, but also ensure the proper tailoring of adjuvant/multidrug chemotherapeutic approaches to better treat specific cancer types or stages. This has already been shown to an extent in the case of IKE, with this study indicating that IKE alone already has some efficacy in preventing xenograft tumor growth; however, the tumor-killing capability of IKE was greatly enhanced by additional inhibition of MGST1 or NRF2. Thus, as our understanding of the pro- and anti-ferroptotic mechanisms relevant to specific cancer types continues to grow, so too will the development of more efficacious and targeted therapies capable of harnessing this cell death cascade to overcome chemoresistance. The authors declare no competing interests. MGST1 is a redox-sensitive repressor of ferroptosis in pancreatic cancer cellsKuang et al.Cell Chemical BiologyFebruary 3, 2021In BriefKuang et al. demonstrate that MGST1 plays a role in preventing ferroptosis by inhibiting ALOX5 activity and subsequent lipid peroxidation in human pancreatic cancer cells. These findings indicate that targeting the MGST1 pathway has the potential to modulate ferroptosis sensitivity in cancer therapy. Full-Text PDF Open Archive