CCR FOCUS CCR Focus Pheochromocytoma: The First Metabolic Endocrine Cancer Ivana Jochmanova1,2 and Karel Pacak1 Abstract Dysregulated metabolism is one of the key characteristics of mutations in genes encoding Krebs cycle enzymes or by activation cancer cells. The most prominent alterations are present during of hypoxia signaling. Present metabolic changes are involved in regulation of cell respiration, which leads to a switch from oxidative processes associated with tumorigenesis, invasiveness, metastasis, phosphorylation to aerobic glycolysis. This metabolic shift results and resistance to various cancer therapies. In this review, we discuss in activation of numerous signaling and metabolic pathways the metabolic nature of PHEOs/PGLs and how unveiling the supporting cell proliferation and survival. Recent progress in genet- metabolic disturbances present in tumors could lead to identifica- ics and metabolomics has allowed us to take a closer look at the tion of new biomarkers and personalized cancer therapies. Clin metabolic changes present in pheochromocytomas (PHEO) and Cancer Res; 22(20); 5001–11. Ó2016 AACR. paragangliomas (PGL). These neuroendocrine tumors often exhibit See all articles in this CCR Focus section, "Endocrine Cancers: dysregulation of mitochondrial metabolism, which is driven by Revising Paradigms." Introduction shift in cell metabolism causes cancer cells to present with increased bioenergetics and altered anaplerotic (intermediate Recently, substantial progress has occurred in the understanding replenishing) processes driven by activation of mechanisms sup- of the pathophysiologic mechanisms involved in various cancers. porting cell survival (6). However, the Warburg effect itself is not Advancements in cancer research and molecular biology (including sufficient to sustain cell proliferation (7). First, a cancer cell has to genomics) have helped to identify genes encoding metabolic increase its uptake of nutrients from the environment, especially enzymes and alterations in multiple signaling pathways that are glucose and glutamine, which are the major nutrients needed for involved in tumorigenesis. Several lines of evidence suggest that cancer cell survival and proliferation. They provide the cancer cell, activation of oncogenic signaling pathways leads to reprogram- through catabolism, with sufficient pools of carbon intermediates ming of cell metabolism to fuel extensive cell proliferation and used for synthesis of various macromolecules and for ATP pro- support cell survival (1, 2). Moreover, some of these metabolic duction. Second, to satisfy energy needs and ensure accelerated alterations seem to be required for malignant transformation, and growth and proliferation, cancer cell metabolic reprogramming this makes metabolic alterations in the cell one of the key hallmarks also includes an increase in protein, lipid, and nucleic acid of cancer (1, 3). Thus, cancer metabolism is becoming paramount biosynthesis (1). For essential biosynthetic processes, cancer cells in understanding cancer pathophysiology and, therefore, tumor use precursors derived from intermediates of the Krebs (tricar- development, progression, senescence, and metastasis. boxylic acid) cycle, which serves as a hub for these processes (8). Decades ago, during the early period of cancer research, the link Because of this, the Krebs cycle is considered one of the key between carcinogenesis and cell metabolism alterations was pro- metabolic pathways, and if dysregulated, its dysfunction may posed. In 1924, the German biochemist Otto Warburg hypoth- result in tumorigenesis of certain tumors, including pheochro- esized that cancer is a result of damage to the mitochondrial mocytomas (PHEO) and paragangliomas (PGL). respiratory function and, therefore, the replacement of oxidative PHEOs and PGLs are rare neuroendocrine tumors arising from phosphorylation (OXPHOS) by aerobic glycolysis for adenosine chromaffin cells in the adrenal medulla or from extra-adrenal triphosphate (ATP) production. This became known as the War- sympathetic and parasympathetic paraganglia, respectively (9, burg effect (4, 5). Compared with normal, healthy cells, such a 10). These tumors, especially those arising from the sympathetic nervous system, are usually characterized by catecholamine pro- 1Section on Medical Neuroendocrinology, Eunice Kennedy Shriver duction, which is responsible for clinical symptoms associated National Institute of Child Health and Human Development, NIH, with PHEO/PGL. On the other hand, parasympathetic PGLs 2 Bethesda, Maryland. First Department of Internal Medicine, Medical (head and neck PGLs) are mostly nonfunctional (11, 12). The Faculty of P.J. Safarik University in Kosice, Kosice, Slovakia. majority of PHEOs/PGLs present as benign tumors; however, Note: Supplementary data for this article are available at Clinical Cancer metastasis can occur, notably, in patients with a specific genetic Research Online (http://clincancerres.aacrjournals.org/). background (13–16). Corresponding Author: Karel Pacak, Eunice Kennedy Shriver National Institute Previous and recent genetic discoveries in PHEO/PGL of Child Health and Human Development, National Institutes of Health, Building research have led to the identification of PHEO/PGL-related 10, CRC, 1E-3140, 10 Center Drive, MSC-1109, Bethesda, MD 20892-1109. Phone: unique metabolic abnormalities or pathways involved in 301-402-4594; Fax: 301-402-0884; E-mail: [email protected] oxygen sensing, hypermethylation, DNA repair, upregulation doi: 10.1158/1078-0432.CCR-16-0606 of specific transporters and/or receptors, and particularly, Ó2016 American Association for Cancer Research. Krebs cycle enzymes (17–20). These changes are tightly linked www.aacrjournals.org 5001 Downloaded from clincancerres.aacrjournals.org on September 29, 2021. © 2016 American Association for Cancer Research. CCR FOCUS GA3PD PGK PGM Amino acids, G3P 1,3-bisphosphoglycerate 3-phosphoglycerate 2-phosphoglycerate pyrimidines (nucleotides) NAD+ NADH + H+ ADP ATP Eno TPI H2O DHAP G3P Phosphoenol pyruvate Aldo ADP PK Fructose-1,6-bisphosphate ATP LDH D ADP Pyruvate L-lactate PFK β ATP -oxidation of fatty acids + Fructose-6-bisphosphate Nucleotides NAD + HS-CoA CO + NADH + H+ Fatty acids, B 2 PGI Pyruvate Acetyl-CoA lipids, steroids Amino sugars, - PDH HCO3 + ATP Glucose-6-phosphate glycolipids, PC glycoproteins ADP + P HS-CoA Malonyl-CoA ADP i HK + Oxaloacetate CS ATP NADH + H ACLY Glucose + Citrate Citrate Acetyl-CoA α-ketoglutarate Glutamate NAD MDH2 ACO H2O Oxaloacetate Aspartate Oxaloacetate L-malate cis-Aconitate Malate Acetate H O FH 2 Krebs cycle A Asparagine ACO H2O Phenyalanine Fumarate Tyrosine NADH + H+ FADH 2 Purines Isocitrate SDH + (nucleotides) O FAD NAD 2 IDH ETC Succinate NADH + H+ + CO Histidine 2 GLDH Proline α-ketoglutarate Glutamate SUCLG α Arginine -KGDH + H O HS-CoA + GTP NAD + HS-CoA Glutamine 2 NH3 Succinyl-CoA + Alanine Pyruvate GTP + Pi NADH + H + CO ATP + H O 2 ADP 2 GABA C Glucose ATP transporter Valine Isoleucine Succinate Methionine Threonine Odd-chain fatty acids Porphyrins, Heme Glucose Glutamine © 2016 American Association for Cancer Research Figure 1. The Krebs (TCA) cycle and anaplerotic/cataplerotic pathways. After entering the cell, glucose is phosphorylated by HK1, and then most of it is degraded via glycolysis (A) to pyruvate. Pyruvate enters the mitochondria, where it is decarboxylated and oxidized by PDH enzyme complex to acetyl-CoA, the main source of energy for the Krebs cycle. After entering the Krebs cycle, acetyl-CoA condensates with oxaloacetate to produce citrate, which is catalyzed by CS. Citrate either stays in the mitochondria and is converted to isocitrate by ACO or is exported to the cytoplasm to be used as a precursor for lipid biosynthesis (via conversion by ACLY). Isocitrate is subsequently decarboxylated to a-ketoglutarate by IDH; a-ketoglutarate is then either converted to succinyl-CoA by a-KGDH complex or exits the mitochondria and serves as a precursor for amino acid biosynthesis. Succinyl-CoA is either transformed in to succinate in the reaction catalyzed by SUCLG or can be used for porphyrin biosynthesis. Succinate is then oxidized to fumarate by SDH, which also represents complex II of the ETC (red dotted circle). Fumarate is hydrated to malate by FH, and, finally, malate is oxidized by MDH to restore oxaloacetate. In the Krebs cycle, hydrogen atoms reduce NADþ and FAD to þ NADH þ H and FADH2 respectively, which feed the ETC to produce ATP. The Krebs cycle as a biosynthetic pathway produces intermediates that leave the cycle (cataplerosis) to be converted primarily to glutamate, GABA, glutamine, and aspartate and also to glucose derivatives and fatty acids. A minor part of glycolytic glucose-6-phosphate is redirected to the pentose phosphate pathway (B) to produce ribose-5-phosphate and NADPH, which will be used to synthesize nucleotides. The triose phosphates can be used for lipids and phospholipids. In normal cells, amino acids follow the physiologic turnover of the proteins, and little part is used to synthesize the nucleotide bases. After deamination, the remaining amino acids are used for energy production. When Krebs cycle ketoacids are consumed or removed, they need to be replaced to permit the sustained function of the Krebs cycle. This process is called "anaplerosis" and is tightly coupled with cataplerosis (100). The anaplerotic reactions of the Krebs cycle include the catabolism of essential amino acids (histidine, isoleucine, leucine, lysine, methionine,