Edouard Mullarky

Glutamine supports pancreatic cancer growth through a Kras-regulated metabolic pathway

Cancer cells have metabolic dependencies that distinguish them from their normal counterparts. Among these dependencies is an increased use of the amino acid glutamine to fuel anabolic processes. Indeed, the spectrum of glutamine-dependent tumours and the mechanisms whereby glutamine supports cancer metabolism remain areas of active investigation. Here we report the identification of a non-canonical pathway of glutamine use in human pancreatic ductal adenocarcinoma (PDAC) cells that is required for tumour growth. Whereas most cells use glutamate dehydrogenase (GLUD1) to convert glutamine-derived glutamate into α-ketoglutarate in the mitochondria to fuel the tricarboxylic acid cycle, PDAC relies on a distinct pathway in which glutamine-derived aspartate is transported into the cytoplasm where it can be converted into oxaloacetate by aspartate transaminase (GOT1). Subsequently, this oxaloacetate is converted into malate and then pyruvate, ostensibly increasing the NADPH/NADP+ ratio which can potentially maintain the cellular redox state. Importantly, PDAC cells are strongly dependent on this series of reactions, as glutamine deprivation or genetic inhibition of any enzyme in this pathway leads to an increase in reactive oxygen species and a reduction in reduced glutathione. Moreover, knockdown of any component enzyme in this series of reactions also results in a pronounced suppression of PDAC growth in vitro and in vivo. Furthermore, we establish that the reprogramming of glutamine metabolism is mediated by oncogenic KRAS, the signature genetic alteration in PDAC, through the transcriptional upregulation and repression of key metabolic enzymes in this pathway. The essentiality of this pathway in PDAC and the fact that it is dispensable in normal cells may provide novel therapeutic approaches to treat these refractory tumours.

Phosphoglycerate dehydrogenase diverts glycolytic flux and contributes to oncogenesis

Most tumors exhibit increased glucose metabolism to lactate, however, the extent to which glucose-derived metabolic fluxes are used for alternative processes is poorly understood. Using a metabolomics approach with isotope labeling, we found that in some cancer cells a relatively large amount of glycolytic carbon is diverted into serine and glycine metabolism through phosphoglycerate dehydrogenase (PHGDH). An analysis of human cancers showed that PHGDH is recurrently amplified in a genomic region of focal copy number gain most commonly found in melanoma. Decreasing PHGDH expression impaired proliferation in amplified cell lines. Increased expression was also associated with breast cancer subtypes, and ectopic expression of PHGDH in mammary epithelial cells disrupted acinar morphogenesis and induced other phenotypic alterations that may predispose cells to transformation. Our findings show that the diversion of glycolytic flux into a specific alternate pathway can be selected during tumor development and may contribute to the pathogenesis of human cancer.

Vitamin C selectively kills KRAS and BRAF mutant colorectal cancer cells by targeting GAPDH

Getting all stressed out by vitamin C Few experimental cancer therapies have incited as much debate as vitamin C. Yet the mechanistic effect of vitamin C on cancer cells is still poorly understood. Yun et al. studied human colorectal cancer cells with KRAS or BRAF mutations and found that they “handle” vitamin C in a different way than other cells, ultimately to their detriment (see the Perspective by Reczek and Chandel). Because a certain receptor is up-regulated in the mutant cells, they take up the oxidized form of vitamin C (dehydroascorbate). This leads to oxidative stress, inactivation of a glycolytic enzyme required by the mutant cells for growth, and finally cell death. Whether the selective toxicity of vitamin C to these mutant cells can be exploited therapeutically remains unclear. Science, this issue p. 1391; see also p. 1317 Cancer cells with certain mutations take up the oxidized form of vitamin C, which fatally disrupts their metabolism. [Also see Perspective by Reczek and Chandel] More than half of human colorectal cancers (CRCs) carry either KRAS or BRAF mutations and are often refractory to approved targeted therapies. We found that cultured human CRC cells harboring KRAS or BRAF mutations are selectively killed when exposed to high levels of vitamin C. This effect is due to increased uptake of the oxidized form of vitamin C, dehydroascorbate (DHA), via the GLUT1 glucose transporter. Increased DHA uptake causes oxidative stress as intracellular DHA is reduced to vitamin C, depleting glutathione. Thus, reactive oxygen species accumulate and inactivate glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Inhibition of GAPDH in highly glycolytic KRAS or BRAF mutant cells leads to an energetic crisis and cell death not seen in KRAS and BRAF wild-type cells. High-dose vitamin C impairs tumor growth in Apc/KrasG12D mutant mice. These results provide a mechanistic rationale for exploring the therapeutic use of vitamin C for CRCs with KRAS or BRAF mutations.

NRF2 regulates serine biosynthesis in non–small cell lung cancer

Tumors have high energetic and anabolic needs for rapid cell growth and proliferation, and the serine biosynthetic pathway was recently identified as an important source of metabolic intermediates for these processes. We integrated metabolic tracing and transcriptional profiling of a large panel of non–small cell lung cancer (NSCLC) cell lines to characterize the activity and regulation of the serine/glycine biosynthetic pathway in NSCLC. Here we show that the activity of this pathway is highly heterogeneous and is regulated by NRF2, a transcription factor frequently deregulated in NSCLC. We found that NRF2 controls the expression of the key serine/glycine biosynthesis enzyme genes PHGDH, PSAT1 and SHMT2 via ATF4 to support glutathione and nucleotide production. Moreover, we show that expression of these genes confers poor prognosis in human NSCLC. Thus, a substantial fraction of human NSCLCs activates an NRF2-dependent transcriptional program that regulates serine and glycine metabolism and is linked to clinical aggressiveness.

PHGDH amplification and altered glucose metabolism in human melanoma.

The metabolic requirements of cancer cells differ from that of their normal counterparts. To support their proliferation, cancer cells switch to a fermentative metabolism that is thought to support biomass production. Instances where metabolic enzymes promote tumorigenesis remain rare. However, an enzyme involved in the de novo synthesis of serine, 3‐phosphoglycerate dehydrogenase (PHGDH), was recently identified as a putative oncogene. The potential mechanisms by which PHGDH promotes cancer are discussed.

Identification of a small molecule inhibitor of 3-phosphoglycerate dehydrogenase to target serine biosynthesis in cancers

Significance Serine supports a number of anabolic processes, including protein, lipid, and nucleic acid synthesis. Cells can either import serine or synthesize it de novo. Recently, overexpression of 3-phosphoglycerate dehydrogenase (PHGDH), the gene encoding the first committed step of serine synthesis, via focal amplification and other mechanisms, has been identified in human cancers. Cancer cell lines that overexpress PHGDH are uniquely sensitive to PHGDH knockdown whereas lines that express little PHGDH are insensitive, suggesting that PHGDH may be a clinically interesting target. Here, we report the discovery of a specific small molecule inhibitor of PHGDH, which enables preclinical evaluation of PHGDH as a target in cancer and provides a tool to study the biology of de novo serine synthesis. Cancer cells reprogram their metabolism to promote growth and proliferation. The genetic evidence pointing to the importance of the amino acid serine in tumorigenesis is striking. The gene encoding the enzyme 3-phosphoglycerate dehydrogenase (PHGDH), which catalyzes the first committed step of serine biosynthesis, is overexpressed in tumors and cancer cell lines via focal amplification and nuclear factor erythroid-2-related factor 2 (NRF2)-mediated up-regulation. PHGDH-overexpressing cells are exquisitely sensitive to genetic ablation of the pathway. Here, we report the discovery of a selective small molecule inhibitor of PHGDH, CBR-5884, identified by screening a library of 800,000 drug-like compounds. CBR-5884 inhibited de novo serine synthesis in cancer cells and was selectively toxic to cancer cell lines with high serine biosynthetic activity. Biochemical characterization of the inhibitor revealed that it was a noncompetitive inhibitor that showed a time-dependent onset of inhibition and disrupted the oligomerization state of PHGDH. The identification of a small molecule inhibitor of PHGDH not only enables thorough preclinical evaluation of PHGDH as a target in cancers, but also provides a tool with which to study serine metabolism.

α-Ketothioamide Derivatives: A Promising Tool to Interrogate Phosphoglycerate Dehydrogenase (PHGDH)

Given the putative role of PHGDH in cancer, development of inhibitors is required to explore its function. In this context, we established and validated a straightforward enzymatic assay suitable for high-throughput screening and we identified inhibitors with similar chemical scaffolds. Through a convergent pharmacophore approach, we synthesized α-ketothioamides that exhibit interesting in vitro PHGDH inhibition and encouraging cellular results. These novel probes may be used to understand the emerging biology of this metabolic target.

Erratum: NRF2 regulates serine biosynthesis in non–small cell lung cancer

Nat. Genet. 47, 1475–1481 (2015); published online 19 October 2015; corrected after print 15 February 2016 In the version of this article initially published, the colors of the lines in the key in the top right corner of Figure 5h were incorrect. The line labeled “High” should be red and the line labeled “Low” should be blue.

Diverting Glycolysis to Combat Oxidative Stress

Reactive oxygen species (ROS) are an intricate part of normal cellular physiology. In excess, however, ROS can damage all three major classes of macromolecules and compromise cell viability. We briefly discuss the physiology of ROS but focus on the mechanisms cells use to preserve redox homeostasis upon oxidative stress, with particular emphasis on glycolysis. ROS inhibits multiple glycolytic enzymes, including glyceraldehyde 3-phosphate dehydrogenase, pyruvate kinase M2, and phosphofructokinase-1. Consistently, glycolytic inhibition promotes flux into the oxidative arm of the pentose phosphate pathway to generate NADPH. NADPH is critically important, as it provides the reducing power that fuels the protein-based antioxidant systems and recycles oxidized glutathione. The unique ability of pyruvate kinase M2 inhibition to promote serine synthesis in the context of oxidative stress is also discussed.

Erratum: Identification of a small molecule inhibitor of 3-phosphoglycerate dehydrogenase to target serine biosynthesis in cancers (Proceedings of the National Academy of Sciences of the United States of America (2016) 113 (1778-1783) DOI:10.1073/pnas.1521548113)

BIOCHEMISTRY Correction for “Identification of a small molecule inhibitor of 3-phosphoglycerate dehydrogenase to target serine biosynthesis in cancers,” by Edouard Mullarky, Natasha C. Lucki, Reza Beheshti Zavareh, Justin L. Anglin, Ana P. Gomes, Brandon N. Nicolay, Jenny C. Y. Wong, Stefan Christen, Hidenori Takahashi, Pradeep K. Singh, John Blenis, J. David Warren, Sarah-Maria Fendt, John M. Asara, Gina M. DeNicola, Costas A. Lyssiotis, Luke L. Lairson, and Lewis C. Cantley, which appeared in issue 7, February 16, 2016, of Proc Natl Acad Sci USA (113:1778–1783; first published February 1, 2016; 10.1073/pnas.1521548113). The editors note that the date on which this manuscript was sent for review was originally published incorrectly as December 7, 2015. The date should instead appear as November 2, 2015.