NRF2 Rewires Cellular Metabolism to Support the Antioxidant Response

The transcription factor (nuclear factor-erythroid 2 p45-related factor 2, NRF2) is a master regulator of the cellular response to oxidative insults. While antioxidant response enzymes are well-characterized transcriptional targets of NRF2, it is recently becoming clear that NRF2 also supports cellular detoxification through metabolic rewiring to support the antioxidant systems. In this chapter, we discuss the regulation of NRF2 and how NRF2 activation promotes the antioxidant defense of cells. Furthermore, we discuss how reactive oxygen species influence cellular metabolism and how this affects antioxidant function. We also discuss how NRF2 reprograms cellular metabolism to support the antioxidant response and how this functions to funnel metabolic intermediates into antioxidant pathways. This chapter concludes by exploring how these factors may contribute to both normal physiology and disease

Inhibition of TXNRD or SOD1 overcomes NRF2-mediated resistance to β-lapachone

Alterations in the NRF2/KEAP1 pathway result in the constitutive activation of NRF2, leading to the aberrant induction of antioxidant and detoxification enzymes, including NQO1. The NQO1 bioactivatable agent β-lapachone can target cells with high NQO1 expression but relies in the generation of reactive oxygen species (ROS), which are actively scavenged in cells with NRF2/KEAP1 mutations. However, whether NRF2/KEAP1 mutations influence the response to β-lapachone treatment remains unknown. To address this question, we assessed the cytotoxicity of β-lapachone in a panel of NSCLC cell lines bearing either wild-type or mutant KEAP1. We found that, despite overexpression of NQO1, KEAP1 mutant cells were resistant to β-lapachone due to enhanced detoxification of ROS, which prevented DNA damage and cell death. To evaluate whether specific inhibition of the NRF2-regulated antioxidant enzymes could abrogate resistance to β-lapachone, we systematically inhibited the four major antioxidant cellular systems using genetic and/or pharmacologic approaches. We demonstrated that inhibition of the thioredoxin-dependent system or copper-zinc superoxide dismutase (SOD1) could abrogate NRF2-mediated resistance to β-lapachone, while depletion of catalase or glutathione was ineffective. Interestingly, inhibition of SOD1 selectively sensitized KEAP1 mutant cells to β-lapachone exposure. Our results suggest that NRF2/KEAP1 mutational status might serve as a predictive biomarker for response to NQO1-bioactivatable quinones in patients. Further, our results suggest SOD1 inhibition may have potential utility in combination with other ROS inducers in patients with KEAP1/NRF2 mutations

K-Ras oncogene-induced ROS detoxification promotes tumorigenesis

The contrasting outcomes of tumor promotion or cellular senescence have both been ascribed to increased levels of reactive oxygen species (ROS). Instead, I demonstrate that endogenous expression of K-RasG12D in primary murine cells and tissues unexpectedly triggers a reduction of ROS due to the activation of an antioxidant program by the Nrf2 transcription factor. Persistent Nrf2 activation maintains the decreased intracellular redox state and promotes hyperproliferation of K-RasG12D expressing cells, and this occurs through engagement of the mitogen activated protein kinase (MAPK) Ras effector pathway. These findings are not shared by cells ectopically overexpressing oncogenic Ras, where transient induction of Nrf2, ROS production and cellular senescence are instead observed. Nrf2 is activated in human and mouse pancreatic cancer, and Nrf2 deficiency or glutathione depletion both impair K-RasG12D – dependent cellular proliferation and carcinogenesis in vivo. Therefore, decreased ROS can promote carcinogenesis, presenting new opportunities to treat and prevent K-Ras mutant cancers

K-Ras oncogene-induced ROS detoxification promotes tumorigenesis

The contrasting outcomes of tumor promotion or cellular senescence have both been ascribed to increased levels of reactive oxygen species (ROS). Instead, I demonstrate that endogenous expression of K-RasG12D in primary murine cells and tissues unexpectedly triggers a reduction of ROS due to the activation of an antioxidant program by the Nrf2 transcription factor. Persistent Nrf2 activation maintains the decreased intracellular redox state and promotes hyperproliferation of K-RasG12D expressing cells, and this occurs through engagement of the mitogen activated protein kinase (MAPK) Ras effector pathway. These findings are not shared by cells ectopically overexpressing oncogenic Ras, where transient induction of Nrf2, ROS production and cellular senescence are instead observed. Nrf2 is activated in human and mouse pancreatic cancer, and Nrf2 deficiency or glutathione depletion both impair K-RasG12D – dependent cellular proliferation and carcinogenesis in vivo. Therefore, decreased ROS can promote carcinogenesis, presenting new opportunities to treat and prevent K-Ras mutant cancers

The Non-Essential Amino Acid Cysteine Becomes Essential for Tumor Proliferation and Survival

The non-essential amino acid cysteine is used within cells for multiple processes that rely on the chemistry of its thiol group. Under physiological conditions, many non-transformed tissues rely on glutathione, circulating cysteine, and the de novo cysteine synthesis (transsulfuration) pathway as sources of intracellular cysteine to support cellular processes. In contrast, many cancers require exogeneous cystine for proliferation and viability. Herein, we review how the cystine transporter, xCT, and exogenous cystine fuel cancer cell proliferation and the mechanisms that regulate xCT expression and activity. Further, we discuss the potential contribution of additional sources of cysteine to the cysteine pool and what is known about the essentiality of these processes in cancer cells. Finally, we discuss whether cyst(e)ine dependency and associated metabolic alterations represent therapeutically targetable metabolic vulnerabilities

mTORC1-Induced HK1-Dependent Glycolysis Regulates NLRP3 Inflammasome Activation

The mammalian target of rapamycin complex 1 (mTORC1) regulates activation of immune cells and cellular energy metabolism. Although glycolysis has been linked to immune functions, the mechanisms by which glycolysis regulates NLRP3 inflammasome activation remain unclear. Here, we demonstrate that mTORC1-induced glycolysis provides an essential mechanism for NLRP3 inflammasome activation. Moreover, we demonstrate that hexokinase 1 (HK1)-dependent glycolysis, under the regulation of mTORC1, represents a critical metabolic pathway for NLRP3 inflammasome activation. Downregulation of glycolysis by inhibition of Raptor/mTORC1 or HK1 suppressed both pro-IL-1β maturation and caspase-1 activation in macrophages in response to LPS and ATP. These results suggest that upregulation of HK1-dependent glycolysis by mTORC1 regulates NLRP3 inflammasome activation