Metabolic Signaling to the Nucleus in Cancer

Molecular Cell

Review Metabolic Signaling to the Nucleus in Cancer Sydney L. Campbell1,2 and Kathryn E. Wellen1,2,3,* 1Department

of Cancer Biology Family Cancer Research Institute Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA 3Lead Contact *Correspondence: [email protected] https://doi.org/10.1016/j.molcel.2018.07.015 2Abramson

Nutrient-sensing mechanisms ensure that cellular activities are coordinated with nutrient availability. Recent work has established links between metabolite pools and protein post-translational modifications, as metabolites are substrates of enzymes that add or remove modifications such as acetylation, methylation, and glycosylation. Cancer cells undergo metabolic reprogramming and exhibit metabolic plasticity that allows them to survive and proliferate within the tumor microenvironment. In this article we review the evidence that, in cancer cells, nutrient availability and oncogenic metabolic reprogramming impact the abundance of key metabolites that regulate signaling and epigenetics. We propose models to explain how these metabolites may control locus-specific chromatin modification and gene expression. Finally, we discuss emerging roles of metabolites in regulating malignant phenotypes and tumorigenesis via transcriptional control. An improved understanding of how metabolic alterations in cancer affect nuclear gene regulation could uncover new vulnerabilities to target therapeutically. Introduction The capacity of cells to sense nutrients and modulate their activities accordingly is central to life and to normal cellular function. Cancer cells face diverse metabolic challenges, including nutrient-limited microenvironments, changing environmental conditions during tumor progression and metastasis, and exposure to cancer treatments that directly or indirectly impact metabolic pathways. Understanding how cancer cells assess their metabolic resources to adapt to these challenges could open doors to improved cancer therapeutic strategies. Cells monitor nutrient availability through the core nutrient sensing AMP-activated protein kinase (AMPK) and mechanistic target of Rapamycin (mTOR) pathways (Ben-Sahra and Manning, 2017; Garcia and Shaw, 2017; Hardie et al., 2016; Saxton and Sabatini, 2017). Additionally, transcription factors such as sterol response element-binding protein (SREBP) and carbohydrate response element-binding protein (ChREBP) have well-described roles in cholesterol- and glucose-dependent gene regulation, respectively (Abdul-Wahed et al., 2017; DeBose-Boyd and Ye, 2018). Extensive study of these pathways has led to fundamental insights into physiology and disease. Recent reviews cited above have covered each of these pathways in depth. In addition to these major nutrient-sensing pathways, metabolite abundance can be sensed by the cell via post-translational modifications, since many of the enzymes that establish these modifications utilize metabolites as substrates. Such modifications include acetylation (using acetyl-CoA as the acetyl donor), methylation (requiring S-adenosylmethionine [SAM]), and O-GlcNAcylation (employing uridine diphosphate N-acetylglucosamine [UDPGlcNAc]) (Figure 1). Similarly, a-ketoglutarate (a-KG)-dependent ten-eleven translocation (TET) DNA hydroxylases and JmjC domain-containing (JMJD) histone demethylases are competitively inhibited by metabolites that are structurally similar to a-KG, such as succinate, fumarate, (D)-2-hydroxyglutarate 398 Molecular Cell 71, August 2, 2018 ª 2018 Elsevier Inc.

(2-HG), and (L)-2-HG. Importantly, many of these metabolite substrates appear to be present at levels in cells that can limit enzyme reaction rates (Fan et al., 2015; Reid et al., 2017). Nutrient-sensitive chromatin or transcription factor modifications thus have the potential to modulate gene expression in accordance with metabolic resources (Figure 1). In order to support growth and proliferation, cancer cells remodel metabolism to enable synthesis of lipids, proteins, and nucleic acids. Moreover, flexibility in nutrient acquisition and utilization enables tumor cells to adapt and survive when faced with nutrient scarcity. These metabolic alterations have the potential to impact chromatin modification and gene expression and thereby to modulate cellular functions (Pavlova and Thompson, 2016). Isocitrate dehydrogenase 1 and 2 (IDH1 and IDH2) mutations in cancer are clear examples in which metabolic alterations drive malignant phenotypes at least in part through changes in chromatin modification. Gain-offunction mutations in IDH1 and IDH2 lead to production of (D)-2-HG, which inhibits a-ketoglutarate-dependent dioxygenases, including JMJD histone demethylases and TET enzymes (Ye et al., 2018). Intense efforts to target these enzymes are underway, and recent clinical trials in acute myeloid leukemia patients treated with inhibitors targeting mutant IDH1 or mutant IDH2 show some patients responding for a period of several months (DiNardo et al., 2018; Stein et al., 2017). In addition to IDH mutation, metabolic alterations driven by oncogenic metabolic reprogramming or by nutrient availability have the potential to alter signaling and epigenetics in cancer cells to impact tumor formation or growth. In this article, we will explore the evidence that altered metabolite levels in cancer cells can impact malignant phenotypes through the regulation of gene expression. We will discuss how availability of specific metabolites can signal to the nucleus through regulation of chromatin modifications or transcription

Molecular Cell

Review Figure 1. Metabolites as Signaling Molecules In addition to their roles as metabolic reaction intermediates, metabolites are substrates for posttranslational modifications. Acetyl-CoA (in blue), generated from citrate or acetate, provides acetyl groups for acetylation. SAM (in red), generated in the methionine cycle as part of one-carbon metabolism, supplies carbons for methylation. UDP-GlcNAc (in orange) is used for glycosylation reactions. TCA cycle intermediate a-ketoglutarate (in green) is a required substrate for TET DNA hydroxylases and JMJD histone demethylases, while fumarate and succinate (in green) have inhibitory effects.

factor activities, focusing on acetyl-CoA, SAM, UDP-GlcNAc, and the TCA cycle intermediates a-KG, succinate, and fumarate. For each of these metabolites, we will examine how metabolic perturbations in cancer alter these signals to contribute to tumorigenesis and cancer progression. Finally, we will consider some of the central questions that should be addressed to advance our understanding of the roles of metabolites as signaling molecules in cancer. Acetyl-CoA Acetyl-CoA is both a central metabolic intermediate and the acetyl donor for acetylation reactions, positioning it at the crossroads of metabolism and signaling (Sivanand et al., 2018). In mitochondria, acetyl-CoA is generated from several sources, including pyruvate, acetate, fatty acid b-oxidation, and amino acid catabolism. Mitochondrial acetyl-CoA is required for citrate synthesis in the TCA cycle. Since acetyl-CoA cannot pass through mitochondrial membranes, it must be synthesized sepa-

rately in other subcellular compartments. It is produced in the cytosol and nucleus from mitochondria-exported citrate by ATP-citrate lyase (ACLY), as well as from acetate by acyl-CoA synthetase short-chain family member 2 (ACSS2) (Sivanand et al., 2018). The pyruvate dehydrogenase complex (PDC), which converts pyruvate to acetyl-CoA, also translocates from the mitochondria to the nucleus under certain conditions, such as in response to mitochondrial stress or during specific developmental phases (Nagaraj et al., 2017; Sutendra et al., 2014). Acetyl-CoA is needed in the cytosol for biosynthetic processes, including de novo fatty acid and cholesterol synthesis, and is also used for post-translational lysine (Nε) acetylation throughout the cell. Acetyl-CoA thus serves critical functions both in metabolism and in cell signaling. Acetyl-CoA levels rise and fall with nutrient abundance, and thus it has been proposed that acetyl-CoA concentration may serve as a signal to the cell of nutrient abundance (Shi and Tu, 2015; Sivanand et al., 2018). Acetyl-CoA concentrations are approximately 3–20 mM in mammalian cells (Chen et al., 2016; Lee et al., 2014), which could potentially limit the activity of some lysine acetyltransferases (KATs) such as p300, which has a reported Km for acetyl-CoA within this range (6.7 mM) (Fan et al., 2015; Liu et al., 2008). Moreover, many KATs are inhibited by their product CoA, and the acetyl-CoA:CoA ratio may also be important in regulating acetylation in response to metabolic changes (Fan et al., 2015; Lee et al., 2014; Montgomery et al., 2015). Nutrient limitation or glycolytic inhibition leads to decreased acetyl-CoA levels and in turn reduced histone and non-histone protein acetylation (Cluntun et al., 2015; Lee et al., 2014; Marin˜o et al., 2014; Wellen et al., 2009). Consistent with the proposed role of acetyl-CoA in nutrient sensing, genes related to cell growth are prominently acetyl-CoA responsive in yeast (Cai et al., 2011), and in both yeast and mammalian cells, cell cycle gene expression is linked to acetyl-CoA abundance

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Figure 2. Metabolic Signaling Mechanisms Mediate Feedforward Loops in Control of Nutrient Uptake (A) In adipocytes, acetyl-CoA generated from glucose both supports fatty acid synthesis and provides acetyl groups for histone acetylation at genes that promote glucose metabolism, including GLUT4 (Wellen et al., 2009). (B) Lung cancer cells with high expression of LAT1 exhibit increased SAM and H3K27Me3 levels. LAT1 expression is increased by activity of the methyltransferase EZH2, allowing for sustained methionine uptake and SAM production for histone methylation (Dann et al., 2015). (C) In breast cancer cells, high levels of O-GlcNAcylation supported by glucose use in the HBP result in suppressed a-KG levels, which reduces prolyl hydroxylase activity and promotes HIF-1 stabilization. HIF-1 targets include GLUT1, expression of which promotes glucose uptake, allowing sustained HBP activity (Ferrer et al., 2014).

(Lee et al., 2014; Shi and Tu, 2013). Acetyl-CoA has been associated with promoting metabolic gene expression for nutrient uptake and macromolecular biosynthesis and with suppressing catabolic metabolism. For example, in adipocytes, expression of Slc2a4, encoding the glucose transporter GLUT4 as well as other glycolytic genes, is acetyl-CoA responsive (Wellen et al., 2009; Zhao et al., 2016), suggesting that glucose-dependent acetyl-CoA production may mediate a positive feedback loop supporting glucose uptake and metabolism in this cell type (Figure 2A). Similarly, in hypoxic liver cancer cells, acetyl-CoA production from acetate is linked to epigenetic control of lipid biosynthesis gene expression (Gao et al., 2016). Conversely, acetyl-CoA abundance suppresses autophagy, a major catabolic process in the cell, in a p300-dependent manner (Marin˜o et al., 2014). Thus, signaling roles of acetyl-CoA appear to be broadly involved in cellular responses to high nutrient abundance. In addition to nutrient availability, oncogenic signaling and microenvironmental conditions such as hypoxia regulate acetyl-CoA production and utilization, impacting its use in biosynthetic processes and its roles in signaling. For example, MYC expression increases glucose-dependent fatty acid synthesis and histone acetylation (Morrish et al., 2009). AKT phosphorylates ACLY at serine 455, enhancing its activity (Berwick et al., 2002; Potapova et al., 2000), and expression of constitutively active AKT promotes acetyl-CoA production and histone H3 and H4 acetylation in cancer cells (Lee et al., 2014). In human gliomas and prostate tumors, pAKT-S473 and global histone acetylation levels positively correlate (Lee et al., 2014). Thus, oncogenic signaling generally promotes ACLY-dependent acetyl-CoA production and utilization. Hypoxia conversely sup-

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presses glucose-dependent fatty acid synthesis and histone acetylation but stimulates ACSS2 upregulation and acetate use in these processes (Gao et al., 2016; Schug et al., 2015). Both ACLY and ACSS2 expression are upregulated in multiple cancer types, and genetic or pharmacological targeting of these enzymes produces anti-cancer effects in preclinical models (Comerford et al., 2014; Hatzivassiliou et al., 2005; Migita et al., 2008; Schug et al., 2015). A key question arising from these studies is how the availability of acetyl-CoA can regulate histone acetylation at specific loci and expression of distinct sets of genes. Two interrelated themes that may explain this specificity in gene regulation are beginning to emerge: (1) spatiotemporal control of acetyl-CoA production within the nucleus (Figures 3A and 3B) and (2) transcription factor regulation in response to acetyl-CoA (Figure 3C). These models are expanded upon below. Spatiotemporal Control of Acetyl-CoA Production within the Nucleus ACLY, ACSS2, and PDC have each been found within nuclei of cancer cells (Comerford et al., 2014; Sutendra et al., 2014; Wellen et al., 2009), suggesting that they may have distinct roles there. Findings from recent studies suggest that dynamic posttranslational modification of these enzymes within the nucleus or their association with KATs and transcription factors may account for their roles in chromatin regulation. For example, ACLY is phosphorylated within the nucleus in response to DNA damage signaling, where it promotes histone H4 acetylation near sites of DNA double-strand breaks, thus enabling BRCA1 recruitment and DNA repair by homologous recombination (Sivanand et al., 2017). Dynamic ACLY phosphorylation may allow a rapid increase in the capture and cleavage of citrate in the

Molecular Cell

Review

Figure 3. Models to Explain How Acetyl-CoA Abundance or Production Can Regulate Expression of Specific Genes and Histone Acetylation at Distinct Loci (A) Post-translational modification of nuclear acetyl-CoA producers. AcetylCoA-producing enzymes, such as ACLY, are localized to the nucleus as well as to the cytosol. Stimulus-induced post-translational modification of acetyl-CoA producers could allow the temporal control of acetyl-CoA production within the nucleus. For example, nuclear ACLY is phosphorylated in response to DNA damage signaling and regulates H4 acetylation at sites of DNA double-strand breaks to promote DNA repair by homologous recombination (Sivanand et al., 2017). ACLY utilizes mitochondria-derived citrate as its substrate, and its phosphorylation may allow increased nuclear capture of citrate for acetyl-CoA production when demand for acetylation increases. (B) Chromatin recruitment of acetyl-CoA producers. ACSS2 can be recruited to chromatin and associate with KATs and transcription factors, promoting

nucleus when demand for acetyl-CoA rises in response to a stimulus such as DNA damage (Figure 3A). ACSS2, on the other hand, has been shown to be recruited to specific genomic loci to supply acetyl-CoA for site-specific histone acetylation. For example, ACSS2 nuclear localization and chromatin recruitment participates in neuronal gene regulation, in which ACSS2 associates with the KAT p300 and facilitates expression of neuronal identity genes (Mews et al., 2017). Similarly, in glioblastoma cell lines, ACSS2 was found to translocate to the nucleus under low-glucose conditions upon phosphorylation by AMPK. In the nucleus, ACSS2 binds transcription factor EB (TFEB) to facilitate adaptation to low glucose by promoting expression of lysosomal biogenesis and autophagy-related genes (Li et al., 2017). Since cellular acetyl-CoA levels fall when glucose is limited (Lee et al., 2014), it is possible that a localized source of acetyl-CoA generated by ACSS2 is particularly important in this context to ensure adequate substrate availability to KATs for histone acetylation. Moreover, given the role of ACSS2 in recapturing acetate produced by HDAC reactions (Bulusu et al., 2017), chromatin localization of this enzyme may allow for the efficient redistribution of acetylation marks for gene regulation (Figure 3B). Nuclear functions of PDC have also recently been implicated in gene regulation. In prostate cancer, nuclear PDC facilitates H3K9 acetylation at promoter regions of lipogenic genes, including ACLY and SQLE, promoting lipogenesis and supporting cell proliferation (Chen et al., 2018). Nuclear PDC has also been found in association with STAT5A in adipocytes (Richard et al., 2017), as well as with the arylhydrocarbon receptor in HeLa cells (Matsuda et al., 2016). Thus, accumulating evidence suggests that compartmentalized acetyl-CoA metabolism facilitates histone acetylation for gene regulation and DNA repair. A more in-depth discussion of the roles of spatial and temporal control of acetyl-CoA metabolism can be found in Sivanand et al., 2018. Transcription Factor Regulation in Response to Acetyl-CoA Emerging evidence suggests that acetyl-CoA abundance can influence gene expression through regulation of transcription factors (Figure 3C). Lysine acetylation is a highly prevalent posttranslational modification found on thousands of sites across cellular proteins, including many transcription factors and coregulators as well as chromatin-modifying and remodeling enzymes (Choudhary et al., 2014). Acetylation can impact the function of a transcription factor by altering its stability, its subcellular localization, or its ability to bind to DNA (Choudhary et al., 2014). In breast cancer, acetyl-CoA abundance promotes acetylation and nuclear translocation of SMAD2, upon which SMAD2 activates expression of transcription factors SNAIL and SLUG, histone acetylation at specific loci and gene regulation (Li et al., 2017; Mews et al., 2017). ACSS2 is crucial for recapturing acetate produced by HDAC reactions to regenerate acetyl-CoA used by KATs (Bulusu et al., 2017), highlighting the logic behind its emerging role at chromatin. (C) Acetyl-CoA-dependent control of transcription factor localization or activity. Nuclear translocation or activity of specific transcription factors is responsive to acetyl-CoA abundance, through direct acetylation or through indirect mechanisms, allowing acetyl-CoA-dependent gene regulation (Galdieri et al., 2016; Lee et al., 2018; Rios Garcia et al., 2017). Through transcription factor association with KATs, histone acetylation at target genes is regulated.

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Molecular Cell

Review In addition to direct regulation by acetylation, transcription factor activity can be influenced by acetyl-CoA concentration through indirect mechanisms. In glioblastoma cells, acetyl-CoA availability promotes cell migration and adhesion to the extracellular matrix in a manner dependent on the Ca2+-responsive transcription factor NFAT1. Acetyl-CoA abundance corresponds with increased intracellular Ca2+ levels, promoting Ca2+-dependent NFAT1 nuclear translocation when acetyl-CoA is abundant (Lee et al., 2018). NFAT transcription factors operate in conjunction with the KATs p300/CBP (Garcı´a-Rodrı´guez and Rao, 1998), and ChIP-seq analysis demonstrated potent acetyl-CoA-dependent regulation of H3K27 acetylation at the promoters of cell adhesion genes (Lee et al., 2018). Thus, acetyl-CoA-dependent coordination of transcription factor activity is one mechanism through which specific sets of genes are regulated in response to nutrient abundance or metabolic alterations.

Figure 4. Diagrams of One-Carbon Metabolism and the Hexosamine Biosynthesis Pathway (A) SAM, the substrate for methylation reactions, is generated through the methionine cycle. Serine also supports SAM production through one-carbon contribution to the folate cycle to allow regeneration of methionine. (B) The HBP branches off of glycolysis at fructose-6-phosphate. In addition to glucose, the HBP takes inputs from glutamine, acetyl-CoA, and uridine triphosphate as well as energy input from ATP. The product, UDP-GlcNAc, provides substrate for O-GlcNAcylation and N-glycosylation and also is converted to other nucleotide sugars used in glycosylation, including galactose and CMP-sialic acid.

increasing cell invasion capabilities. Inhibition of acetyl-CoA carboxylase (ACC) 1, which synthesizes malonyl-CoA from acetyl-CoA to initiate fatty acid synthesis, results in increased acetyl-CoA levels and, in turn, SMAD2 acetylation and increased metastasis (Rios Garcia et al., 2017). Similarly, metformin treatment of cancer cells results in AMPK-dependent ACC inhibition, corresponding with increased H3 and H4 histone acetylation as well as increased acetylation of the p65 subunit of the transcription factor NF-kB and expression of NF-kB target genes (Galdieri et al., 2016). Transcription factor acetylation may also be tied directly to specific acetyl-CoA producers in some contexts; for example, CBP-mediated hypoxia-inducible factor (HIF)-2a acetylation in hypoxia is dependent on ACSS2 (Xu et al., 2014).

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One-Carbon Metabolism: S-Adenosylmethionine The methylation of histones and DNA requires the one-carbon donor SAM, which is produced in the methionine cycle. SAM is synthesized from methionine and ATP by methionine adenosyltransferase (MAT). After transfer of the methyl group from SAM to the substrate, S-adenosyl homocysteine (SAH) is produced along with the methylated substrate. SAH is then converted to homocysteine, and finally methionine is regenerated after donation of a methyl group from 5-methyl-tetrahydrofolate (THF) (Locasale, 2013) (Figure 4A). Methylation of histones and DNA is sensitive to the abundance of SAM or the [SAM]:[SAH] ratio, since SAH can act as an inhibitor of methyltransferases and the Km and Ki for SAM and SAH, respectively, of many methyltransferases falls within physiological range (Reid et al., 2017). Restricting methionine in cultured cells markedly suppresses levels of SAM and SAH. This coincides with decreased global methylation of several histone lysines, with H3K4me3, a chromatin mark generally associated with open chromatin and active transcription, the most sensitive (Mentch et al., 2015). Interestingly, the genome-wide locations of H3K4me3 peaks remain consistent upon methionine restriction, while peak width at specific genes is sensitive to methionine availability in cancer cells (Dai et al., 2018). Peak width correlates closely with gene expression, and methionine-responsive genes in colon cancer cells include those involved in cell cycle and cancer-related pathways (Dai et al., 2018). Understanding the impact of such regulation on tumorigenesis awaits further study. Notably, however, metabolic regulation of histone methylation is observed in vivo; upon consumption of a methionine-restricted diet, SAM and global H3K4me3 levels decrease in the livers of mice (Mentch et al., 2015). Thus, methionine availability impacts SAM levels and histone methylation in vitro and in vivo. Intracellular methionine concentration is also impacted by the controlled expression of cell surface transporters. Methionine can be transported into cells through the amino acid transporter LAT1 (SLC7A5), which forms a heterodimer with SLC3A2 (Napolitano et al., 2015). Expression of LAT1 and SLC3A2 are regulated by MYC (Gao et al., 2009; Hayashi et al., 2012; Wang et al., 2011), and the cell surface localization of SLC3A2 is responsive to AKT-mTOR signaling (Edinger and Thompson, 2002), suggesting that many cancer cells may upregulate the

Molecular Cell

Review import of methionine and other amino acids. In a lung cancer cell line, those cells with high expression of LAT1 exhibit increased abundance of SAM, elevated activity of the histone methyltransferase EZH2 and higher H3K27me3 levels, and enhanced subcutaneous tumor growth compared to those cells with low LAT1 expression. Silencing of LAT1 or MAT2A suppresses H3K27me3 and tumor growth (Dann et al., 2015). Interestingly, expression of LAT1 itself is enhanced by EZH2 activity, suggesting a positive feedback loop that further drives methionine uptake to sustain EZH2 activity (Figure 2B). These data indicate that controlling the uptake of methionine into cells can regulate SAM and histone methylation levels. Serine metabolism is also an important determinant of SAM levels. Serine is a one-carbon donor to the folate cycle and allows the regeneration of methionine for SAM synthesis. Serine conversion to glycine by serine hydroxymethyltransferase (SHMT) generates 5,10-methylene-THF, which is subsequently reduced by methylene tetrahydrofolate reductase (MTHFR) to 5-methyl-THF within the folate cycle (Locasale, 2013). As mentioned earlier, methionine is regenerated by 5-methyl-THF donating a methyl group to homocysteine (Figure 4A). Along with allowing regeneration of methionine, serine can also impact the SAM cycle by supporting synthesis of ATP, which is required for generation of SAM from methionine (Maddocks et al., 2016). Consistent with this demand for serine, a diet restricted in serine and glycine can limit growth of certain cancers, such as MYCdriven lymphoma or intestinal cancer driven by APC inactivation (Maddocks et al., 2017). In addition to serine uptake, serine levels can also be increased via regulation of its synthesis from the glycolytic intermediate 3-phosphoglycerate. Amplification of the gene encoding phosphoglycerate dehydrogenase (PHGDH), the first enzyme in the de novo serine biosynthesis pathway, occurs in several cancers including breast cancer and melanoma (Locasale et al., 2011; Possemato et al., 2011). Increased serine biosynthesis is also driven by histone methyltransferase EHMT2 (G9A) overexpression in osteosarcoma. Knockdown of EHMT2 in U2OS cells results in decreased H3K9me1 at the promoter regions of the genes encoding PHGDH and phosphoserine aminotransferase (PSAT), the second enzyme in the serine biosynthesis pathway, reducing their expression and downregulating de novo serine synthesis. Overexpression of EHMT2 increases growth of U2OS cell subcutaneous tumors in a manner dependent on increased serine biosynthesis (Ding et al., 2013). These data suggest that, similar to EZH2 and regulation of LAT1, EHMT2 expression may engage a positive feedback loop to increase serine synthesis to provide SAM as a substrate for EHMT2. Finally, enhanced serine synthesis has been observed in the context of Kras and Lkb1 mutations in pancreatic cancer, in which serine biosynthesis genes, including Psat, are upregulated. PSAT silencing in pancreatic cancer cells decreases SAM abundance and in turn DNA methylation at long interspersed nuclear (LINE) elements, increasing their expression. These findings uncovered a therapeutic vulnerability: subcutaneous tumors generated from cells with Kras and Lkb1 mutations exhibit enhanced sensitivity to DNA methyltransferase inhibition (Kottakis et al., 2016). Thus, serine biosynthesis is upregulated by several distinct mechanisms in cancer cells and

may impact the regulation of histone or DNA methylation, pointing to potential metabolic and epigenetic vulnerabilities in these tumors. Analogous to the growing evidence for coordination of local acetyl-CoA production within the nucleus (Figures 3A and 3B), recent evidence argues that nuclear SAM production may also be important for regulation of specific genes. In yeast, SAM synthesis enzymes were identified within a serine-responsive, chromatin-associated complex termed SESAME that regulates locus-specific histone methylation (Li et al., 2015). Moreover, in liver cancer cells, nuclear MATII associates with the DNA-binding protein MafK, where it provides substrate for histone methyltransferases to regulate MafK target genes (Katoh et al., 2011). Further study is required to determine the exact roles of these MAT-chromatin modifier interactions in cancer. The Hexosamine Biosynthetic Pathway: UDP N-Acetylglucosamine The hexosamine biosynthetic pathway (HBP) produces nucleotide sugars that are substrates for glycosylation (Figure 4B). The HBP branches off of glycolysis from fructose-6-phosphate, which is converted to glucosamine-6-phosphate using glutamine as the nitrogen donor. Glucosamine-6-phosphate is acetylated, generating N-acetylglucosamine-6-phosphate (GlcNAc6-phosphate), which is then converted to GlcNAc-1-phosphate and finally attached to the nucleotide UTP, generating UDP-Nacetylglucosamine (UDP-GlcNAc) (Denzel and Antebi, 2015; Ferrer et al., 2016). UDP-GlcNAc is a key substrate for both Nand O-linked glycosylation, as will be discussed below, and it is also used for synthesis of UDP-N-acetylgalactosamine (UDP-GalNAc) and CMP-sialic acid, other important substrates for production of glycoproteins (Denzel and Antebi, 2015). As the HBP takes inputs from several major metabolic pathways in the cell, and because flux into the pathway is impacted by both glucose and glutamine metabolism (Araujo et al., 2017; Wellen et al., 2010), the HBP has been proposed as a nutrientsensing pathway (Ferrer et al., 2016; Hardiville´ and Hart, 2014). Moreover, metabolic reprogramming by c-MYC, mutant KRAS, or hypoxic conditions increases flux into the HBP (Guillaumond et al., 2013; Morrish et al., 2009; Ying et al., 2012), and genetic or pharmacological targeting of HBP enzymes has been found to suppress proliferation, cell migration, and tumor growth (Ricciardiello et al., 2018; Ying et al., 2012). Thus, the HBP supports malignant phenotypes, and both nutrient availability and metabolic reprogramming can impact HBP activity. The O-GlcNAc modification is a single O-linked GlcNAc posttranslational modification to serine and threonine residues. O-GlcNAc modifications are added by a single enzyme, O-GlcNAc transferase (OGT), and removed by a single enzyme, O-GlcNAcase (OGA). O-GlcNAcylation is upregulated in many cancers, including breast, prostate, and pancreatic, and silencing of OGT suppresses proliferation and tumor growth (Caldwell et al., 2010; Lynch et al., 2012; Ma et al., 2013; Sodi et al., 2018). Glucose-dependent O-GlcNAcylation is sufficient to enhance oncogenic signaling and drive malignant phenotypes, such as loss of polarity in breast epithelial cells (Onodera et al., 2014). High cellular O-GlcNAcylation in cancer cells also results in diminished a-KG levels, thus reducing HIF-1a prolyl Molecular Cell 71, August 2, 2018 403

Molecular Cell

Review hydroxylation (an a-KG-dependent reaction) and enhancing HIF stability, promoting expression of target genes such as GLUT1. This promotes sustained high rates of glycolysis, suggesting that cancer cells engage a feedforward metabolic signaling loop via the HBP to sustain oncogenic metabolic features (Ferrer et al., 2014) (Figure 2C). O-GlcNAcylation is also implicated in other processes that support tumor growth, including augmenting lipid biosynthesis by regulating SREBP in an AMPK-dependent manner (Sodi et al., 2018) and promoting expression and stabilization of HSP70 and HSP90 to combat proteotoxic stress (Zachara et al., 2004). These data indicate that tumors may maintain high O-GlcNAcylation to support various processes that enable tumor growth. Although in many cases the specific O-GlcNAcylated proteins that mediate particular oncogenic phenotypes are not known, recent studies have delineated roles for distinct O-GlcNAcylation events. For example, in pancreatic cancer cells, the transcription factor YAP is O-GlcNAcylated in a glucose-dependent manner, promoting its nuclear localization, association with its partner TEAD, and transcription of its target genes. Expression of a mutant form of YAP that cannot be O-GlcNAcylated in pancreatic cancer cells impairs subcutaneous tumor growth (Peng et al., 2017). Similarly, the KEAP1/NRF2 pathway, which is involved in antioxidant gene regulation, is regulated by O-GlcNAcylation in a glucose-dependent manner. The adaptor protein KEAP1 is O-GlcNAcylated, which promotes its association with the E3 ligase CUL3 and subsequent ubiquitination and degradation of NRF2. Thus, under nutrient-limited conditions, reduced KEAP1 O-GlcNAcylation promotes NRF2 transcriptional activity and resistance to oxidative stress (Chen et al., 2017). Through mechanisms such as these, O-GlcNAcylation links glucose availability to transcriptional regulation. Nutrient-sensitive changes in N-linked glycosylation can also impact nuclear gene expression. The availability of UDP-GlcNAc impacts N-glycan branching and surface expression of growth factor receptors, thus sustaining high levels of signaling (Lau et al., 2007; Wellen et al., 2010). In addition, UDP-GlcNAcdependent N-glycosylation of SREBP cleavage-activating protein (SCAP) is critical for activation of the transcription factor SREBP-1. In normal conditions, SREBP-1 is retained in the endoplasmic reticulum by association with its binding partner, SCAP, which interacts with resident ER protein INSIG. The absence of sterols promotes the dissociation of INSIG from SCAP and allows for SCAP and SREBP-1 to translocate to the Golgi, where SREBP-1 is cleaved to allow its translocation to the nucleus. However, N-glycosylation of SCAP is required for this process; when glucose is absent and UDP-GlcNAc levels are in turn reduced, SCAP is insufficiently N-glycosylated and SREBP-1 is not activated (Cheng et al., 2015). In this manner, transcriptional control of lipid synthesis depends on glucose availability and the HBP. Further, since glucose also supplies acetyl-CoA, the building block for lipid synthesis, this mechanism may prevent activation of lipid synthesis when insufficient precursor is available. Orthotopic tumor growth is significantly impaired from glioblastoma cells expressing a mutant form of SCAP that cannot be N-glycosylated (Cheng et al., 2015). Taken together, glucose availability and in turn UDP-GlcNAc availability can impact how a cell interprets its nutritional context through

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both intracellular O-GlcNAcylation and regulation of membrane protein N-glycosylation. TCA Cycle Intermediates: a-Ketoglutarate, Succinate, and Fumarate TCA cycle intermediates are also critical signaling molecules. For example, a-KG serves as a substrate for dioxygenase enzymes, including the JMJD histone demethylases; TET DNA hydroxylases, which facilitate a multi-step DNA demethylation (Wu and Zhang, 2017); and prolyl hydroxylases that regulate HIFs (Nowicki and Gottlieb, 2015). Succinate and fumarate competitively inhibit these enzymes (Nowicki and Gottlieb, 2015). The relative concentrations of these metabolites are important in regulating histone and DNA methylation as well as HIF-1a stabilization. Aberrant activation of HIF-1a is one mechanism contributing to tumorigenesis when succinate and fumarate accumulate. Loss-of-function mutations resulting in succinate dehydrogenase (SDH) (mutations in SDHA, SDHB, SDHC, or SDHD) or fumarate hydratase (FH) deficiency result in excess succinate or fumarate, respectively, and thus inhibition of HIF prolyl hydroxylases. As a consequence of this, SDH or FH deficiency promotes the stabilization of HIF-1a in normoxic conditions, allowing expression of HIF target genes such as VEGF and GLUT1 that could promote angiogenesis and glucose metabolism (Isaacs et al., 2005; Selak et al., 2005). HIF-1a stabilization in either FH-inhibitor-treated or SDH-deficient cells can be overcome by adding increasing concentrations of a cell-permeable derivative of a-KG (Isaacs et al., 2005; MacKenzie et al., 2007), highlighting that the balance between succinate, fumarate, and a-KG levels determines the activity of HIF prolyl hydroxylases. The relative concentrations of succinate, fumarate, and a-KG also have broad effects on histone and DNA methylation in the nucleus and thus gene expression. In addition to increasing HIF-1a levels, silencing of FH or SDH in cultured cells or in vivo in the liver inhibits demethylation of histones and DNA (Xiao et al., 2012). Similarly, SDH mutation in paraganglioma corresponds with increased DNA and histone methylation and repression of neuroendocrine differentiation genes. In this analysis, patients with the most hypermethylated tumors, mostly with SDH mutations, had poorer overall and metastasis-free survival as compared to patients with less methylated tumors. Interestingly, the only hypermethylated tumor in this study that did not have an SDH mutation had an inactivating mutation in FH (Letouze´ et al., 2013). SDH-deficient gastrointestinal stromal tumors also show high global DNA methylation levels, comparable to that observed in IDH mutant glioblastoma (Killian et al., 2013). Recent work has produced mechanistic insights into roles of fumarate in promoting tumorigenesis via epigenetic control. FH deficiency in renal cancer corresponds with expression of epithelial-to-mesenchymal transition (EMT)-associated genes. This is mediated through epigenetic silencing of miR-200ba429, a microRNA cluster that normally functions to restrict expression of genes such as Snai2, Zeb1, and Zeb2 that coordinate EMT. Fumarate accumulation increases methylation at a regulatory CpG island, resulting in suppression of miR-200 family microRNAs (Sciacovelli et al., 2016). Thus, accumulation of metabolites such as succinate and fumarate can drive epigenetic alterations that promote malignant phenotypes.

Molecular Cell

Review Recent evidence indicates that production of (L)-2HG from a-KG catalyzed by enzymes such as lactate dehydrogenase and malate dehydrogenase may also be a relevant mechanism of gene regulation in cancer cells (Ye et al., 2018). Acidic pH promotes these promiscuous reactions, leading to HIF-1a stabilization and increased histone methylation (Intlekofer et al., 2015, 2017; Oldham et al., 2015). Thus, in addition to mutations in genes encoding TCA cycle enzymes, microenvironmental conditions can impact metabolite availability and the activity of a-KGdependent dioxygenase enzymes. Concluding Remarks Accumulating evidence supports the concept that metabolites serve as critical signaling molecules that can impact cancer development and progression. Here we have discussed the roles of several key metabolites involved in the regulation of gene expression and DNA repair and their ties to tumorigenesis. Of course, the pathways and metabolites highlighted in this article are far from the only relevant mechanisms through which metabolites signal to the nucleus. NAD+, for example, is also a crucial signaling metabolite. In addition to its role as a cofactor in metabolic redox reactions, NAD+ is a substrate of sirtuin deacetylases and PARP enzymes, which regulate diverse cellular processes (Canto´ et al., 2015; Gupte et al., 2017). NAD+ levels are impacted by numerous stimuli and conditions, including caloric restriction, aging, exercise, and circadian oscillations (Rajman et al., 2018), and recent evidence suggests that compartment-specific NAD+ levels fall within a range that can regulate the reaction rates of these enzymes (Ryu et al., 2018). Evidence exists for deregulation of NAD+ metabolism in cancer. For example, reduced NAD+ synthesis promotes DNA damage in early stages of liver cancer development, and supplementation of an NAD+ precursor suppresses tumorigenesis (Tummala et al., 2014). Lysine acylation, such as succinylation, malonylation, and crotonylation, is also emerging as a link between metabolism and gene regulation, though the functions of these modifications are not well understood (Sabari et al., 2017). Abundance of these modifications on histones closely correlates with the levels of the corresponding acyl-CoAs (Simithy et al., 2017). How NAD+ and acylCoA availability are altered in cancer and how each impacts protein modifications, cancer cell phenotypes, and in turn tumorigenesis are exciting areas for future investigation. In order to mechanistically understand how metabolites modulate cellular phenotypes and activities, it will be crucial to further elucidate how specific genes are regulated in response to changes in the availability of a metabolite. As we have discussed, two ways that specificity may be achieved are through the coordinated spatial and temporal control of metabolite production and through metabolite-dependent regulation of transcription factors (Figure 3). Metabolic state could also potentially modify cancer phenotypes that arise as a consequence of mutations in or altered expression of epigenetic enzymes with differential sensitivities to the availability of their metabolite substrates. Defining compartment-specific interacting partners, mechanisms of nuclear trafficking, and roles for dynamic posttranslational modification of relevant metabolic enzymes will aid in further defining the functions of metabolites in gene regulation and DNA repair.

From a therapeutic standpoint, metabolic regulation of chromatin modification or transcriptional programs has the potential to expose novel vulnerabilities. Might metabolic alterations in cancer cells enhance sensitivity to epigenetic therapies? Could targeting metabolic pathways promote differentiation or suppress EMT and metastasis? Further, given that acetylation, methylation, and glycosylation can be influenced by nutrient availability, might a patient’s diet also modulate tumor formation and growth at least in part via metabolic signaling mechanisms? Could targeting the tumor microenvironment influence metabolite availability to cancer cells and thereby metabolic signaling? As discussed, initial evidence in support of each of these possibilities has emerged, but knowledge of how metabolitedependent signaling mechanisms might be exploited for therapeutic benefit is still quite limited and is an important area for investigation. In sum, it is now clear that metabolites play key roles in regulating nuclear processes. Future work will continue to elucidate the mechanisms linking metabolite availability to gene regulation, with the hope that understanding these mechanisms will aid in developing new cancer therapies or in identifying cancer prevention strategies. ACKNOWLEDGMENTS We acknowledge National Cancer Institute grants R01 CA174761 and R01 CA228339 to K.E.W. and predoctoral fellowship F31 CA217070 to S.L.C. We thank Martin Carroll, Donita Brady, Brian Altman, Rebekah Brooks, and members of the laboratory of K.E.W. for helpful discussion and valuable comments on the manuscript. REFERENCES Abdul-Wahed, A., Guilmeau, S., and Postic, C. (2017). Sweet Sixteenth for ChREBP: Established Roles and Future Goals. Cell Metab. 26, 324–341. Araujo, L., Khim, P., Mkhikian, H., Mortales, C.L., and Demetriou, M. (2017). Glycolysis and glutaminolysis cooperatively control T cell function by limiting metabolite supply to N-glycosylation. eLife 6, 1–16. Ben-Sahra, I., and Manning, B.D. (2017). mTORC1 signaling and the metabolic control of cell growth. Curr. Opin. Cell Biol. 45, 72–82. Berwick, D.C., Hers, I., Heesom, K.J., Moule, S.K., and Tavare´, J.M. (2002). The identification of ATP-citrate lyase as a protein kinase B (Akt) substrate in primary adipocytes. J. Biol. Chem. 277, 33895–33900. Bulusu, V., Tumanov, S., Michalopoulou, E., van den Broek, N.J., MacKay, G., Nixon, C., Dhayade, S., Schug, Z.T., Vande Voorde, J., Blyth, K., et al. (2017). Acetate Recapturing by Nuclear Acetyl-CoA Synthetase 2 Prevents Loss of Histone Acetylation during Oxygen and Serum Limitation. Cell Rep. 18, 647–658. Cai, L., Sutter, B.M., Li, B., and Tu, B.P. (2011). Acetyl-CoA induces cell growth and proliferation by promoting the acetylation of histones at growth genes. Mol. Cell 42, 426–437. Caldwell, S.A., Jackson, S.R., Shahriari, K.S., Lynch, T.P., Sethi, G., Walker, S., Vosseller, K., and Reginato, M.J. (2010). Nutrient sensor O-GlcNAc transferase regulates breast cancer tumorigenesis through targeting of the oncogenic transcription factor FoxM1. Oncogene 29, 2831–2842. Canto´, C., Menzies, K.J., and Auwerx, J. (2015). NAD(+) Metabolism and the Control of Energy Homeostasis: A Balancing Act between Mitochondria and the Nucleus. Cell Metab. 22, 31–53. Chen, W.W., Freinkman, E., Wang, T., Birsoy, K., and Sabatini, D.M. (2016). Absolute quantification of matrix metabolites reveals the dynamics of mitochondrial metabolism. Cell 166, 1324–1337.e11.

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