© 2018. Published by The Company of Biologists Ltd | Disease Models & Mechanisms (2018) 11, dmm033365. doi:10.1242/dmm.033365 REVIEW SPECIAL COLLECTION: CANCER METABOLISM Hidden features: exploring the non-canonical functions of metabolic enzymes Peiwei Huangyang1,2 and M. Celeste Simon1,3,* ABSTRACT A key finding from studies of metabolic enzymes is the existence The study of cellular metabolism has been rigorously revisited over the of mechanistic links between their nuclear localization and the past decade, especially in the field of cancer research, revealing new regulation of transcription. By modulating gene expression, insights that expand our understanding of malignancy. Among these metabolic enzymes themselves facilitate adaptation to rapidly insights isthe discovery that various metabolic enzymes have surprising changing environments. Furthermore, they can directly shape a ’ activities outside of their established metabolic roles, including in cell s epigenetic landscape (Kaelin and McKnight, 2013). the regulation of gene expression, DNA damage repair, cell cycle Strikingly, several metabolic enzymes exert completely distinct progression and apoptosis. Many of these newly identified functions are functions in different cellular compartments. Nuclear fructose activated in response to growth factor signaling, nutrient and oxygen bisphosphate aldolase, for example, directly interacts with RNA ́ availability, and external stress. As such, multifaceted enzymes directly polymerase III to control transcription (Ciesla et al., 2014), whereas, link metabolism to gene transcription and diverse physiological and in the cytosol, it mediates signal transduction, vesicle trafficking pathological processes to maintain cell homeostasis. In this Review, we and cell motility (Lincet and Icard, 2015). Indeed, a growing list of summarize the current understanding of non-canonical functions of multifaceted enzymes supports the possibility that cells employ multifaceted metabolic enzymes in disease settings, especially cancer, existing proteins in different and efficient ways, without the need to and discuss specific circumstances in which they are employed. We replicate or transcribe additional genes. also highlight the important role of subcellular localization in activating The aim of this Review is to provide an overview of metabolic these novel functions. Understanding their non-canonical properties enzymes for which non-canonical functions have been identified should enhance the development of new therapeutic strategies for (Table 1) and to consider their implications in cancer. We begin by cancer treatment. discussing how some metabolic enzymes translocate to the nucleus and regulate gene expression in different contexts, explaining how KEY WORDS: Cancer metabolism, Metabolic enzymes, environmental cues are quickly relayed to regulate gene expression. Non-canonical functions We then describe the unexpected subcellular localization of several metabolic enzymes and their surprising roles in regulating major Introduction cell functions, such as cell cycle progression, DNA damage repair Altered metabolism has long been observed in cancer cells (Warburg, and apoptosis. We also review several unresolved questions about 1956a,b). With the development of new experimental techniques, multifunctional enzymes and discuss their potential therapeutic advances in cancer metabolism research have greatly enhanced our implications in cancer. understanding of how cancer cells benefit from altered metabolism to support their growth. For example, subcellular fractionation revealed Regulation of gene transcription that the majority of key glycolytic enzymes are actually present in the A growing number of metabolic enzymes have been observed in the nucleus (Kim and Dang, 2005). It is intriguing to speculate that these nucleus. It is reasonable to speculate that these enzymes are required enzymes have unexpected nuclear functions, such as activating gene for special nuclear functions, such as fundamental regulation of expression, which impacts specific cell decisions in response to fuel transcription and of the epigenome. Metabolic enzymes are highly supply and demand. An emerging paradigm proposes that metabolic sensitive to nutrient supply and demand, representing an efficient way enzymes, rather than simply being components of biochemical to quickly establish adaptive responses by sensing metabolic stress and pathways, are multi-functional proteins. They can act as mediators simultaneously modulating transcription. These enzymes participate in between growth stimuli, signaling pathways and downstream transcriptional control primarily by: (1) directly binding to target genes effectors, over and above the changes in metabolism, contributing to as transcription factors; (2) providing essential substrates for post- many other biological functions, such as gene transcription, apoptosis translational modifications; and (3) forming a transcription complex and cell cycle progression. with other proteins. It is intriguing to propose that, in order to adjust to external cues, especially those driven by nutrient availability, cells coordinate their metabolic state and gene transcription through nuclear 1Abramson Family Cancer Research Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA. 2Departments of Cancer metabolic enzymes via complex mechanisms. Biology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA. 3Cell and Developmental Biology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA. Direct regulation Many glycolytic (see Box 1 for a glossary of terms) enzymes are *Author for correspondence ([email protected]) involved in transcriptional regulation by acting as transcription M.C.S., 0000-0002-7885-3359 cofactors or by directly binding to DNA (reviewed in Boukouris et al., 2016; Yu and Li, 2017). The ability of metabolic enzymes to This is an Open Access article distributed under the terms of the Creative Commons Attribution directly regulate gene transcription was first discovered in yeast, License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed. where glucose inhibits the expression of genes involved in its Disease Models & Mechanisms 1 REVIEW Disease Models & Mechanisms (2018) 11, dmm033365. doi:10.1242/dmm.033365 Table 1. Summary of enzymes with non-canonical functions Canonical functions Non-canonical functions Subcellular Subcellular Enzymes Function localization Function localization References Hexokinase 2 (HK2) Convert phosphorylate glucose Cytosol Repress the expression of Nucleus Ahuatzi et al., 2004; to glucose 6-phosphate genes involved in glucose Cheung et al., 2012; (G6P) repression Chiara et al., 2008; Protect cells from apoptosis Mitochondria Gottlob et al., 2001; by regulating the Majewski et al., mitochondrial permeability 2004; Pastorino transition pore and by et al., 2002 limiting the production of reactive oxygen species Phosphoglucose isomerase Interconvert G6P to fructose Cytosol Act as an autocrine factor Extracellular Ahmad et al., 2011; Fu (PGI) 6-phosphate (F6P) extracellularly to elicit cell localization et al., 2011; migration and proliferation Funasaka et al., 2007a,b; Sun et al., 1999; Watanabe et al., 1996 Phosphofructokinase (PFK) Phosphorylate F6P to fructose Cytosol Bind to transcription factor Nucleus Enzo et al., 2015 1,6-bisphosphate (F1,6BP) TEAD and stabilize its interaction with YAP/TAZ, and promote gene expression 6-phosphofructose-2-kinase/ Interconvert F6P to fructose Cytosol Promote cell cycle Nucleus Yalcin et al., 2014; fructose-2,6- 2,6-bisphosphate (F2,6BP) progression by Yalcin et al., 2009 bisphosphatase 3 upregulating cell cycle (PFKFB3) protein expression and downregulating cell cycle inhibitor p27 Fructose 1,6-bisphosphatase Hydrolyze F1,6BP to F6P Cytosol Bind to hypoxia-inducible Nucleus Li et al., 2014 1 (FBP1) factors (HIFs) and inhibit their transcriptional activation of glycolytic gene expression Aldolase A Split F1,6BP to Cytosol Participate in cytokinesis Cytoskeleton Buscaglia et al., 2006; dihydroxyacetone phosphate through its interaction with Hu et al., 2016; Kao (DHAP) and glyceraldehyde F-actin and Wiskott-Aldrich et al., 1999; 3-phosphate (G3P) syndrome protein (WASP) Ritterson Lew and family proteins Tolan, 2012 Glyceraldehyde 3-phosphate Interconvert G3P to Cytosol Bind to colony-stimulating Nucleus Grosse et al., 1986; dehydrogenase (GAPDH) 1,3-bisphophoglycerate factor-1 (CSF-1) mRNA Hara et al., 2005; (1,3BPG) and stabilize its transcripts Sen et al., 2008; Function as a key component Nucleus Sundararaj et al., in the OCA-S complex, 2004; Zheng et al., mediate its activation of 2003; Zhou et al., H2B during S phase and 2008 promote cell cycle progression Protects telomeres against Nucleus rapid shortening Increases DNA synthesis Nucleus during S phase via direct binding to single-stranded DNA (ssDNA) and stimulating the DNA- polymerase-α–primase complex GAPDH S-nitrosylation Nucleus promotes its nuclear translocation and triggers apoptosis Continued Disease Models & Mechanisms 2 REVIEW Disease Models & Mechanisms (2018) 11, dmm033365. doi:10.1242/dmm.033365 Table 1. Continued Canonical functions Non-canonical functions Subcellular Subcellular Enzymes Function localization Function localization References Facilitate apoptosis