Pheochromocytoma: The First Metabolic Endocrine Cancer

Abstract

Dysregulated metabolism is one of the key characteristics of cancer cells. The most prominent alterations are present during regulation of cell respiration, which leads to a switch from oxidative phosphorylation to aerobic glycolysis. This metabolic shift results in activation of numerous signaling and metabolic pathways supporting cell proliferation and survival. Recent progress in genetics and metabolomics has allowed us to take a closer look at the metabolic changes present in pheochromocytomas (PHEO) and paragangliomas (PGL). These neuroendocrine tumors often exhibit dysregulation of mitochondrial metabolism, which is driven by mutations in genes encoding Krebs cycle enzymes or by activation of hypoxia signaling. Present metabolic changes are involved in processes associated with tumorigenesis, invasiveness, metastasis, and resistance to various cancer therapies. In this review, we discuss the metabolic nature of PHEOs/PGLs and how unveiling the metabolic disturbances present in tumors could lead to identification of new biomarkers and personalized cancer therapies. Clin Cancer Res; 22(20); 5001-11. ©2016 AACR SEE ALL ARTICLES IN THIS CCR FOCUS SECTION, "ENDOCRINE CANCERS REVISING PARADIGMS".

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 Krebs cycle. After entering the Krebs cycle, acetyl-CoA condensates with oxaloacetate to produce citrate, 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 α-ketoglutarate by IDH. α-ketoglutarate is then either converted to succinyl-CoA by α-KGDH complex or exits the mitochondria and serves as a precursor for amino acid biosynthesis. Succinyl-CoA is either transformed to succinate in the reaction catalyzed by SUCLG or can be utilized for porphyrin biosynthesis. Succinate is then oxidized to fumarate by SDH, which also represents complex II of the ETC (dotted circle/ellipse). Fumarate is hydrated to malate by FH and, finally, malate is oxidized by MDH to restore oxaloacetate. In the Krebs cycle, hydrogen atoms reduce NAD⁺ and FAD to NADH + 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 synthetize nucleotides. The triose phosphates can be used for lipids and phospholipids. In normal cells, amino acids follow the physiological turnover of the proteins and little part is used to synthetize the nucleotide bases. After deamination, the remainder of amino acids are used for energy production. When Krebs cycle ketoacids are consumed or removed, they need to be replaced to permit the Krebs cycle sustained function. This process is called anaplerosis and is tightly coupled with cataplerosis (100). The anaplerotic reactions of Krebs cycle include the catabolism of essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) as well as odd chain fatty acids. Anaplerotic reactions provide the Krebs cycle with fumarate, oxaloacetate, α-ketoglutarate, malate, and succinyl-CoA. Oxaloacetate is formed via carboxylation of pyruvate by PC, from malate through oxidation by malate dehydrogenase, or by transamination of aspartate. Pyruvate can also be decarboxylated to malate. Glutaminolysis (C) serves as the source of the Krebs cycle intermediate α-ketoglutarate and oxidation of odd chain fatty acids or metabolism of methionine and isoleucine provide succinyl-CoA. Acetyl-CoA can be replenished from β-oxidation of fatty acids (D). Abbreviations: α-KGDH, alpha-ketoglutarate dehydrogenase; ACLY, ATP-citrate lyase; ACO, aconitase; ADP, adnesoine diphosphate; Aldo, aldolase; ATP, adenosine triphosphate; CO₂, carbon dioxide; CoA, coenzyme A; CS, citrate synthase; DHAP, dihydroxyacetone phosphate; Eno, enolase; ETC, electron transport chain; FAD, flavin adenine dinucleotide; FADH₂, reduced FAD; FH, fumarate hydratase; G3P, glycerol-3-phosphate; GA3PD, glyceraldehyde-3-phosphate dehydrogenase; GABA, gamma-aminobutyric acid; GDP, guanosine diphosphate; GLDH, glutamate dehydrogenase; GTP, guanosine triphosphate; H₂O, water; HK, hexokinase; HS-CoA, Coenzyme A; IDH, isocitrate dehydrogenase; LDH, lactate dehydrogenase; MDH, malate dehydrogenase; NAD⁺, nicotinamide adenine dinucleotide, oxidized; NADH, reduced form of NAD; PC, pyruvate carboxylase; PDH, pyruvate dehydrogenase; PFK, phosphofructokinase; PGI, glucose-6-phosphate isomerase; PGK, phosphoglycerate kinase; PGM, phosphoglycerate mutase; P_i, inorganic phosphate; PK, pyruvate kinase; SDH, succinate dehydrogenase; SUCLG, succinyl-CoA synthetase; TCA, tricarboxylic acid; TPI, triosephosphate isomerase

Figure 2. Metabolic changes in PHEO/PGL

Schematic representation of mitochondrial genes as well as others involved in PHEO/PGL development with emphasis to the Krebs cycle enzymes, as explained in the text. Dotted arrows represent changes resulting from mutations in certain proteins. Actual treatment targets are explained in Table 2 and Supplementary Table S1. Abbreviations: α-KGDH, alpha-ketoglutarate dehydrogenase; ACO, aconitase; Akt, RAC-alpha serine/threonine-protein kinase; CoA, coenzyme A; CS, citrate synthase; c-Myc, Myc proto oncogene; eIF-4E, eukaryotic translation initiation factor 4E; ERK, mitogen-activated protein kinase 2; ETC, electron transport chain; FH, fumarate hydratase; HIF-α, hypoxia-inducible factor alpha; HK, hexokinase; HS-CoA, Coenzyme A; IDH, isocitrate dehydrogenase; LDH, lactate dehydrogenase; MAX, myc-associated factor X; mTORC1, mammalian target of rapamycin complex 1; mTORC2, mammalian target of rapamycin complex 2; MDH2, malate dehydrogenase 2; NF1, neurofibromin 1; PDH, pyruvate dehydrogenase; PHD, prolyl hydroxylase domain protein; PI3K, phosphoinositide 3-kinase; pVHL, von Hippel-Lindau protein; Raptor, regulatory associated protein of mTOR; RAS, rat sarcoma oncogene; RET, rearranged during transfection proto-oncogene; Rheb, RAS homolog enriched in brain; ROS, reactive oxygen species; S6K, S6 kinase; SDH, succinate dehydrogenase; SUCLG, succinyl-CoA synthetase; TMEM127, transmembrane protein 127; TSC1/2, tuberous sclerosis complex 1/2

Figure 3. Inhibition of α-ketoglutarate-dependent dioxygenases by Krebs cycle intermediates and 2-HG

Mutations in SDHx and FH genes lead to an accumulation of succinate and fumarate, mutated IDH1/2 exhibit neomorphic activity that results in conversion of α-ketoglutarate to oncometabolite, D2HG. Under hypoxic conditions, L2HD accumulation occurs, as described in the text. Succinate, fumarate, D2HG, and L2HG function as a competitive inhibitors of α-ketoglutarate-dependent dioxygenases. Reactions of α-ketoglutarate-dependent dioxygenases potentially inhibited by succinate, fumarate, D2HG, and L2HG are depicted in the lower part of the scheme. All four reactions convert α-ketoglutarate to succinate and CO₂, incorporate O₂, and require iron and ascorbate as cofactors. Inhibition of these reactions results in DNA and histone hypermethylation, activation of hypoxic responses, and inhibition of collagen maturation and folding. Abbreviations: Δ, mutant; 2-HG, 2-hydroxyglutarate; α-KGDH, alpha-ketoglutarate dehydrogenase; ACO, aconitase; CO₂, carbon dioxide; CS, citrate synthase; D2HG, D-2-hydroxyglutarate; FH, fumarate hydratase; HIF, hypoxia-inducible factor, IDH, isocitrate dehydrogenase; JMJD3, Jumoni C domain-containing histone lysine demethylases; L2HG, L-2-hydroxyglutarate; LDHA, lactate dehydrogenase A, LHD, lysyl hydroxylase, MDH, malate dehydrogenase; O₂, oxygen; PHD, prolyl hydroxylases, SDH, succinate dehydrogenase; SUCLG, succinyl-CoA synthetase; TET, ten-eleven translocation family of 5-methylcytosine (5mC) hydroxylases