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Cysteine plays critical roles in cellular biosynthesis, enzyme catalysis, and redox metabolism. The intracellular cysteine pool can be sustained by cystine uptake or de novo synthesis from serine and homocysteine. Demand for cysteine is increased during tumorigenesis for generating glutathione to deal with oxidative stress. While cultured cells have been shown to be highly dependent on exogenous cystine for proliferation and survival, how diverse tissues obtain and use cysteine in vivo has not been characterized. We comprehensively interrogated cysteine metabolism in normal murine tissues and cancers that arise from them using stable isotope 13C1-serine and 13C6-cystine tracing. De novo cysteine synthesis was highest in normal liver and pancreas and absent in lung tissue, while cysteine synthesis was either inactive or downregulated during tumorigenesis. In contrast, cystine uptake and metabolism to downstream metabolites was a universal feature of normal tissues and tumors. However, differences in glutathione labeling from cysteine were evident across tumor types. Thus, cystine is a major contributor to the cysteine pool in tumors, and glutathione metabolism is differentially active across tumor types. Significance: Stable isotope 13C1-serine and 13C6-cystine tracing characterizes cysteine metabolism in normal murine tissues and its rewiring in tumors using genetically engineered mouse models of liver, pancreas, and lung cancers.

Cysteine plays critical roles in cellular biosynthesis, enzyme catalysis, and redox metabolism. The intracellular cysteine pool can be sustained by cystine uptake or de novo synthesis from serine and homocysteine. Demand for cysteine is increased during tumorigenesis for generating glutathione to deal with oxidative stress. While cultured cells have been shown to be highly dependent on exogenous cystine for proliferation and survival, how diverse tissues obtain and use cysteine in vivo has not been characterized. We comprehensively interrogated cysteine metabolism in normal murine tissues and cancers that arise from them using stable isotope 13C1-serine and 13C6-cystine tracing. De novo cysteine synthesis was highest in normal liver and pancreas and absent in lung tissue, while cysteine synthesis was either inactive or downregulated during tumorigenesis. In contrast, cystine uptake and metabolism to downstream metabolites was a universal feature of normal tissues and tumors. However, differences in glutathione labeling from cysteine were evident across tumor types. Thus, cystine is a major contributor to the cysteine pool in tumors, and glutathione metabolism is differentially active across tumor types.Significance:Stable isotope 13C1-serine and 13C6-cystine tracing characterizes cysteine metabolism in normal murine tissues and its rewiring in tumors using genetically engineered mouse models of liver, pancreas, and lung cancers.

Figure S6 contains total metabolite levels in lung cancer models related to Figure 5

Mutations in the KEAP1-NRF2 pathway occur in up to a third of non-small cell lung cancer (NSCLC) cases and often confer resistance to therapy and poor outcomes. Here, we developed murine alleles of the KEAP1 and NRF2 mutations found in human NSCLC and comprehensively interrogated their impact on tumor initiation and progression. Chronic Nrf2 stabilization by Keap1 or Nrf2 mutation was not sufficient to induce tumorigenesis, even in the absence of tumor suppressors p53 or Lkb1. When combined with KrasG12D/+, constitutive Nrf2 activation promoted lung tumor initiation and early progression of hyperplasia to low-grade tumors but impaired their progression to advanced-grade tumors, which was reversed by Nrf2 deletion. Finally, NRF2 overexpression in KEAP1 mutant NSCLC cell lines was detrimental to cell proliferation, viability, and anchorage-independent colony formation. Collectively, our results establish the context-dependence and activity threshold for NRF2 during the lung tumorigenic process.SignificanceThis study reports murine lung cancer models harboring mutations in the Keap1/Nrf2 pathway and highlights the context-dependent and diverse roles of Nrf2 during lung tumor initiation and progression.

SUMMARYCysteine plays critical roles in cellular biosynthesis, enzyme catalysis, and is an essential contributor to redox metabolism. While cultured cells are highly dependent on exogenous cystine for proliferation and survival, how diverse tissues obtain and use cysteine in vivo has not been characterized. We comprehensively interrogated cysteine metabolism in normal murine tissues and the cancer that arise from them using stable isotope 13C-serine and 13C-cystine tracing. We found that de novo cysteine synthesis was highest in normal liver and pancreas and absent in lung tissue. In tumors, cysteine synthesis was either inactive or downregulated during tumorigenesis. By contrast, cystine uptake and metabolism to downstream metabolites was a universal feature of normal tissues and tumors. Differences in cysteine catabolism were evident across tumor types, including glutathione synthesis. Thus, cystine is a major contributor to the cysteine pool in tumors and cysteine catabolic pathways are differentially active across tumor types.

Lung cancer is the second most diagnosed type of cancer and the leading cause of cancer-related mortality. 85% of all lung cancer cases are non-small cell lung cancer (NSCLC), a large proportion of which are only diagnosed at advanced stages (II-IV) due to lack of early diagnostic techniques. Standard first-line treatment for late-stage inoperable NSCLC still is chemotherapy regiments. However, most NSCLC patients fail to respond to these regimens or relapse upon treatment. Thus, there is a pressing need to identify novel therapeutic targets that can more effectively treat NSCLC. Quinolinic acid phosphoribosyl transferase (QPRT) is a rate-limiting enzyme in the tryptophan catabolic pathway which fuels de novo NAD+ production, whose expression as previously been shown to have a prognostic value in certain cancers. Here, we demonstrate that high QPRT levels is a feature of a large proportion of NSCLC cell lines and that QPRT induction occurs in genetically engineered models of NSCLC (KP and KL) when compared to the normal lung. Considering the critical role of NAD+ levels to enable high rates of proliferation, we hypothesized that QPRT induction enables lung cancer cells to thrive. Strikingly, QPRT knockdown in a panoply of NSCLC cell lines results in pronounced suppression of tumor cells both in 2D and 3D conditions. Interestingly, we observe that QPRT suppression does not affect proliferation or cell cycle progression but rather due to pronounced induction of cell death. Surprisingly, while we observed DNA damage upon QPRT suppression, NAD+ levels were not affected by QPRT suppression indicating that QPRT’s effects in NSCLC are independent of its contribution to the NAD+ pools. Together, our results point for QPRT as a novel and effective therapeutic target for NSCLC. Citation Format: Hossein Kashfi, Nathan Ward, Didem Ilter, Stanislav Drapela, Aimee Falzone, Stephen Gardell, Gina M. DeNicola, Ana P. Gomes. Quinolinic acid phosphoribosyl transferase (QPRT) is an essential liability of non-small cell lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2161.

Background d-3-phosphoglycerate dehydrogenase (PHGDH), which encodes the first enzyme in serine biosynthesis, is overexpressed in human cancers and has been proposed as a drug target. However, whether PHGDH is critical for the proliferation or homeostasis of tissues following the postnatal period is unknown. Methods To study PHGDH inhibition in adult animals, we developed a knock-in mouse model harboring a PHGDH shRNA under the control of a doxycycline-inducible promoter. With this model, PHGDH depletion can be globally induced in adult animals, while sparing the brain due to poor doxycycline delivery. Results We found that PHGDH depletion is well tolerated, and no overt phenotypes were observed in multiple highly proliferative cell compartments. Further, despite detectable knockdown and impaired serine synthesis, liver and pancreatic functions were normal. Interestingly, diminished PHGDH expression reduced liver serine and ceramide levels without increasing the levels of deoxysphingolipids. Further, liver triacylglycerol profiles were altered, with an accumulation of longer chain, polyunsaturated tails upon PHGDH knockdown. Conclusions These results suggest that dietary serine is adequate to support the function of healthy, adult murine tissues, but PHGDH-derived serine supports liver ceramide synthesis and sustains general lipid homeostasis.

Circular RNAs (circRNAs) are a class of non-coding RNAs that feature a covalently closed ring structure formed through backsplicing. circRNAs are broadly expressed and contribute to biological processes through a variety of functions. Standard gain-of-function and loss-of-function approaches to study gene functions have significant limitations when studying circRNAs. Overexpression studies in particular suffer from the lack of efficient genetic tools. While mammalian expression plasmids enable transient overexpression of circRNAs in cultured cells, most cell biological studies require long-term ectopic expression. Here we report the development and characterization of genetic tools enabling stable circRNA overexpressionin vitroandin vivo. We demonstrated that circRNA expression constructs can be delivered to cultured cells via transposons, whereas lentiviral vectors have limited utility for the delivery of circRNA constructs. We further showed that circRNA transposons can be supplied to mouse livers via hydrodynamic tail vein injection, resulting in ectopic circRNA expression in a hepatocellular carcinoma mouse model. Furthermore, we generated genetically engineered mice harboring circRNA expression constructs. We demonstrate that this approach enables constitutive, global circRNA overexpression as well as inducible circRNA expression directed specifically to melanocytes in a melanoma mouse model. Overall, these tools expand the genetic toolkit available for the functional characterization of circRNAs of interest.

Nucleotide nicotinamide transhydrogenase contributes to non-small cell lung carcinoma., In this study, Ward et al. (https://doi.org/10.1084/jem.20191689) provide exciting evidence that nucleotide nicotinamide transhydrogenase (NNT), a mitochondrial matrix–located enzyme harnessing the proton gradient to generate NADPH using NADH, markedly contributes to non-small cell lung carcinoma (NSCLC), which is abrogated in the murine C57BL/6J background, a strain known to be deficient in NNT.

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., Graphical abstract Image 1, Highlights • Aberrant activation of NRF2 in non-small cell lung cancer promotes resistance to β-lapachone via the antioxidant defense. • Inhibition of the thioredoxin-dependent system and superoxide dismutase 1 increase sensitivity to β-lapachone treatment. • Mutations in the NRF2/KEAP1 pathway might serve as predictive biomarker for response to β-lapachone in patients.

Lung cancer is responsible for the most cancer-related deaths worldwide. Within the most prominent histological subtype, non-small cell lung cancer (NSCLC), there is an unmet clinical need: lung adenocarcinomas (ADCs) driven by mutant KRAS. Within this subset of tumors, KRAS mutations co-occur with mutations in tumor suppressor genes including TP53 and the redox regulator KEAP1. KEAP1 is the negative regulator of transcription factor NRF2, which directs the antioxidant response and multiple facets of metabolism. In NSCLC, alterations in the KEAP1-NRF2 circuit result in constitutive NRF2 activation and are often associated with resistance to therapy and poor outcomes in patients. While NRF2 hyperactivation has been associated with tumor progression, our lab's recent findings suggest that this may be context-dependent, and that too much NRF2 activation may be detrimental. To study the role of NRF2 hyperactivation on tumor progression, we have utilized KRAS mutant genetically engineered mouse models of NSCLC harboring TP53 deletion. These studies are based on our lab's finding that the homozygous KEAP1R554Q loss-of-function mutation decreases tumor size in a Kras mutant, Trp53-deficient (KP) lung ADC model (Kang et al. 2019 eLife). In parallel to these studies, we have also developed a conditional murine allele of the NRF2D29H mutation found in human NSCLC to serve as a secondary model of NRF2 hyperactivation in the KP mouse (KPN). Consistent with our homozygous KEAP1 mutant model (KPKK), we found that KPN mice demonstrated constitutive NRF2 activation, as observed by increased immunohistochemical staining of canonical NRF2 target, NQO1. This degree of NRF2 activation in KPN mice was slightly lower than that of KPKK mice, suggesting that the KPN mouse is an intermediate model of NRF2 activation. Supportingly, we also found that KPN mice had decreased tumor burden, although not to the same extent as KPKK mice. Interestingly, our heterozygous KEAP1 mutant model (KPK) demonstrates only modest NRF2 activation but did not exhibit decreased tumor burden. Importantly, analyses of tumor number suggested that KPKK and KPN tumors are impaired in tumor progression, rather than initiation. KPKK and KPN tumors also exhibited lower proliferative indices when compared to KP mice, in correspondence with their reduced tumor burden. Collectively, these results suggest that there may be a threshold for NRF2 activation to block tumor progression in the KP model. Current studies are focused on determining whether this impediment to tumor burden is NRF2-dependent, and what NRF2-dependent mechanisms may impair tumor progression. Importantly, these studies may help identify whether a threshold for NRF2 hyperactivation to promote or block tumor progression exists, and if this can be therapeutically exploited in patients with KRAS mutant, TP53-deficient lung tumors. Citation Format: Janine M. DeBlasi, Aimee Falzone, Gina M. DeNicola. Does a threshold exist for NRF2 hyperactivation to block tumor progression in KRAS mutant, TP53-deficient NSCLC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2507.

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.HighlightsAberrant activation of NRF2 in non-small cell lung cancer promotes resistance to β-lapachone via the antioxidant defense.Inhibition of the thioredoxin-dependent system and superoxide dismutase 1 increase sensitivity to β-lapachone treatment.Mutations in the NRF2/KEAP1 pathway might serve as predictive biomarker for response to β-lapachone in patients.

BackgroundD-3-phosphoglycerate dehydrogenase (PHGDH), which encodes the first enzyme in serine biosynthesis, is overexpressed in human cancers and has been proposed as a drug target. However, whether PHGDH is critical for the proliferation or homeostasis of tissues following the postnatal period is unknown.MethodsTo study PHGDH inhibition in adult animals, we developed a knock-in mouse model harboring a PHGDH shRNA under the control of a doxycycline-inducible promoter. With this model, PHGDH depletion can be globally induced in adult animals, while sparing the brain due to poor doxycycline delivery.ResultsWe found that PHGDH depletion is well tolerated and no overt phenotypes were observed in multiple highly proliferative cell compartments. Further, despite detectable knockdown, liver and pancreatic function were normal. Interestingly, diminished PHGDH expression in the liver reduced serine and ceramide levels without increasing the levels of deoxysphingolipids. Further, liver triacylglycerol profiles were altered, with an accumulation of longer chain, polyunsaturated tails upon PHGDH knockdown.ConclusionsThese results suggest that dietary serine is adequate to support the function of healthy, adult murine tissues, but PHGDH-derived serine supports liver ceramide synthesis and sustains general lipid homeostasis.

NRF2 is emerging as a major regulator of cellular metabolism. However, most studies have been performed in cancer cells, where co-occurring mutations and tumor selective pressures complicate the influence of NRF2 on metabolism. Here we use genetically engineered, non-transformed primary murine cells to isolate the most immediate effects of NRF2 on cellular metabolism. We find that NRF2 promotes the accumulation of intracellular cysteine and engages the cysteine homeostatic control mechanism mediated by cysteine dioxygenase 1 (CDO1), which catalyzes the irreversible metabolism of cysteine to cysteine sulfinic acid (CSA). Notably

Human lung tumors exhibit robust and complex mitochondrial metabolism, likely precipitated by the highly oxygenated nature of pulmonary tissue. As ROS generation is a byproduct of this metabolism, reducing power in the form of nicotinamide adenine dinucleotide phosphate (NADPH) is required to mitigate oxidative stress in response to this heightened mitochondrial activity. Nicotinamide nucleotide transhydrogenase (NNT) is known to sustain mitochondrial antioxidant capacity through the generation of NADPH, however its function in non-small cell lung cancer (NSCLC) has not been established. We found that NNT expression significantly enhances tumor formation and aggressiveness in mouse models of lung tumor initiation and progression. We further show that NNT loss elicits mitochondrial dysfunction independent of substantial increases in oxidative stress, but rather marked by the diminished activities of proteins dependent on resident iron-sulfur clusters. These defects were associated with both NADPH availability and ROS accumulation, suggesting that NNT serves a specific role in mitigating the oxidation of these critical protein cofactors.

Cysteine is required for maintaining cellular redox homeostasis in both normal and transformed cells. Deprivation of cysteine induces the iron-dependent form of cell death known as ferroptosis; however, the metabolic consequences of cysteine starvation beyond impairment of glutathione synthesis are poorly characterized. Here, we find that cystine starvation of non-small cell lung cancer cell lines induces an unexpected accumulation of γ-glutamyl-peptides, which are produced due to a non-canonical activity of glutamate-cysteine ligase catalytic subunit (GCLC). This activity is enriched in cell lines with high levels of NRF2, a key transcriptional regulator of GCLC, but is also inducible in healthy murine tissues following cysteine limitation. γ-glutamyl-peptide synthesis limits the accumulation of glutamate, thereby protecting against ferroptosis. These results indicate that GCLC has a glutathione-independent, non-canonical role in the protection against ferroptosis by maintaining glutamate homeostasis under cystine starvation.

NRF2 is emerging as a major regulator of cellular metabolism. However, most studies have been performed in cancer cells, where co-occurring mutations and tumor selective pressures complicate the influence of NRF2 on metabolism. Here we use genetically engineered, non-transformed primary murine cells to isolate the most immediate effects of NRF2 on cellular metabolism. We find that NRF2 promotes the accumulation of intracellular cysteine and engages the cysteine homeostatic control mechanism mediated by cysteine dioxygenase 1 (CDO1), which catalyzes the irreversible metabolism of cysteine to cysteine sulfinic acid (CSA). Notably, CDO1 is preferentially silenced by promoter methylation in human non-small cell lung cancers (NSCLC) harboring mutations in KEAP1, the negative regulator of NRF2. CDO1 silencing promotes proliferation of NSCLC by limiting the futile metabolism of cysteine to the wasteful and toxic byproducts CSA and sulfite (SO32-), and depletion of cellular NADPH. Thus, CDO1 is a metabolic liability for NSCLC cells with high intracellular cysteine, particularly NRF2/KEAP1 mutant cells., eLife digest Cancers form in humans and other animals when cells of the body develop mutations that allow them to grow and divide uncontrollably. The set of chemical reactions happening inside cancer cells, referred to as “metabolism”, can be very different to metabolism in the healthy cells they originate from. Some of these differences are directly caused by mutations, while others are a result of the environment surrounding the cancer cells as they develop into a tumor. A protein called NRF2 is often overactive in human tumors due to mutations in its inhibitor protein KEAP1. Previous studies have shown that NRF2 changes the metabolism of cancer cells by switching specific genes on or off. However, since cancer cells also have other mutations that could mask or amplify some of the effects of NRF2, the precise role of this protein in metabolism remains unclear. To address this question, Kang et al. generated mice that could switch between producing the normal KEAP1 protein or a mutant version that is unable to inhibit NRF2. The mouse model was then used to examine the immediate effects of activating the NRF2 protein. This revealed that NRF2 altered how mouse cells used a molecule called cysteine, which is required to make proteins and other cell components. When NRF2 was active, some of the cysteine molecules were converted into two wasteful and toxic particles by an enzyme called CDO1. Kang et al. found that inactivating CDO1 in human lung cancer cells prevented these wasteful particles from being produced. This allows cancer cells to grow more rapidly, and may explain why human tumors generally evolve to shut down CDO1. The findings of Kang et al. show that not all of the changes in metabolism caused by individual mutations in cancer cells help tumors to grow. As a tumor develops it may need to acquire further mutations to override the negative effects of these changes in metabolism. In the future these findings may help researchers develop new therapies that reactivate or mimic CDO1 to limit the growth of tumors.