Targeting mitochondrial metabolism for precision medicine in cancer

Abstract

During decades, the research field of cancer metabolism was based on the Warburg effect, described almost one century ago. Lately, the key role of mitochondria in cancer development has been demonstrated. Many mitochondrial pathways including oxidative phosphorylation, fatty acid, glutamine, and one carbon metabolism are altered in tumors, due to mutations in oncogenes and tumor suppressor genes, as well as in metabolic enzymes. This results in metabolic reprogramming that sustains rapid cell proliferation and can lead to an increase in reactive oxygen species used by cancer cells to maintain pro-tumorigenic signaling pathways while avoiding cellular death. The knowledge acquired on the importance of mitochondrial cancer metabolism is now being translated into clinical practice. Detailed genomic, transcriptomic, and metabolomic analysis of tumors are necessary to develop more precise treatments. The successful use of drugs targeting metabolic mitochondrial enzymes has highlighted the potential for their use in precision medicine and many therapeutic candidates are in clinical trials. However, development of efficient personalized drugs has proved challenging and the combination with other strategies such as chemocytotoxic drugs, immunotherapy, and ketogenic or calorie restriction diets is likely necessary to boost their potential. In this review, we summarize the main mitochondrial features, metabolic pathways, and their alterations in different cancer types. We also present an overview of current inhibitors, highlight enzymes that are attractive targets, and discuss challenges with translation of these approaches into clinical practice. The role of mitochondria in cancer is indisputable and presents several attractive targets for both tailored and personalized cancer therapy.

Fig. 1. Tricarboxylic acid (TCA) cycle fuels.

Carbons derived from glucose, fatty acids, and glutamine enter the TCA cycle, which produces NADH and FADH₂, necessary for the formation of ATP through the electron transport chain (ETC). Glucose is processed into pyruvate via glycolysis, resulting in lactate by lactate dehydrogenase (LDH) or enters the mitochondria through the mitochondrial pyruvate carrier (MPC). Once in the mitochondria, pyruvate will be converted into acetyl-CoA by pyruvate dehydrogenase (PDH). Fatty acids are transferred to the mitochondria by carnitine palmitoyltransferases (CPT1/2) and provide acetyl-CoA to the TCA cycle through β-oxidation. Glutamine enters mitochondria through the solute carrier family 1 member 5 (SCL1A5) transporter and is converted into glutamate by glutaminases (GLS) and into α-ketoglutarate (α-KG) by glutamate dehydrogenase (GDH).

Fig. 2. Targets of the tricarboxylic acid cycle (TCA) cycle and the electron transport chain (ETC).

Glucose-derived pyruvate enters mitochondria via MPC1/2 transporters and is converted to acetyl-CoA via PDH. Inhibition of mitochondria pyruvate transporter using UK5099 attenuates OXPHOS. Alterations in enzymes of the TCA cycle results in production of oncometabolites. These include 2-HG, fumarate, and succinate (dark red). Inhibitors of these enzymes include AGI-5198, AG-221, and AG-881 for IDH, and CPI-613 for KGDHC. Both NADH and FADH₂ produced in the TCA cycle transfer electrons to Complex I and II. Electrons then pass through a series of redox reactions producing energy for transportation of protons to the IMS, generating enough energy to produce ATP. Abbreviations: TCA tricarboxylic acid, ETC electron transport chain, MPC1/2 mitochondrial pyruvate carrier 1/2, PDH pyruvate dehydrogenase, CS citrate synthase, ACO2 aconitase 2, IDH isocitrate dehydrogenase, KGDHC α-ketoglutarate dehydrogenase complex, SCS succinyl-CoA synthetase, SDH succinate dehydrogenase, FH fumarate hydratase, MDH malate dehydrogenase, 2-HG 2-hydroxyglutarate, IMS intermembrane mitochondrial space, IMM inner mitochondrial membrane, Mt matrix mitochondrial matrix, FADH₂ flavin adenine dinucleotide.

Fig. 3. One carbon metabolism.

Cytosolic and mitochondrial one carbon metabolism pathway indicating the available inhibitors for different enzymes. Folate is converted to DHF and further to THF via DHFR, a process that consumes NADPH. Serine is converted by SHMT1 (cytosolic) or SHMT2 (mitochondrial) to glycine. The one carbon unit resulting from the reaction is transferred to THF forming 5,10-methylene-THF, which is oxidized by cytosolic MTHFD1 or mitochondrial MTHFD2/MTHFD2L to 10-formyl-THF via the intermediate 5,10-methenyl-THF, generating NAD(P)H. The one carbon unit in 10-formyl-THF can be converted either to formate, generating ATP from ADP by MTHFD1 or MTHFD1L, or metabolized to THF and released as a CO₂ via ALDH1L2. Formate is present in mitochondria and is used as substrate for the bidirectional enzyme MTHFD1 to form 10-formyl-THF, for de novo purine synthesis, and 5,10-methylene-THF for thymidylate synthesis as well as for the methionine cycle. Several inhibitors have been identified that target one carbon metabolism, including methotrexate and aminopterin for DHFR, SHIN1/2 for SHMT1/2, and LY345899 and DS18561882 for MTHFD1/2. Abbreviations: DHF dihydrofolate, THF tetrahydrofolate, DHFR dihydrofolate reductase, SHMT1/2 serine hydroxymethyltransferase 1/2, THF tetrahydrofolate, MTHFD2 methylenetetrahydrofolate dehydrogenase 2, MTHFD1/2L methylenetetrahydrofolate dehydrogenase 1/2 like, ALDH1L2 aldehyde dehydrogenase 1 family member L2 also known as 10-formyl-THF dehydrogenase, TYMS thymidylate synthase.

Fig. 4. Inhibitors of glutamine metabolism and β-oxidation (FAO).

Glutaminase (GLS) converts glutamine to glutamate, which is further metabolized to α-KG by GDH. In turn, α-KG can be converted to citrate. Both α-KG and citrate can be exported to the cytosol for use as precursors for de novo fatty acid synthesis. Glutamate can also be metabolized by GPT2/GOT2 aminotransferases, producing alanine/aspartate and α-KG. Several inhibitors of glutamine metabolism have been described, including BPTES, CB-830, CB-893, and compounds 968 for GLS, EGCG and R162 for GDH and AOA, and cycloserine for GOT2 and GPT2, respectively. For β-oxidation (FAO), fatty acids need to be transferred to mitochondria by the action of CPT1/2. The most used inhibitors of FAO are etomoxir and teglicar (ST1326), both targeting CPT1. Moreover, perhexiline acts as a dual CPT1/2 inhibitor. Abbreviations: α-KG, α-ketoglutarate; GLS, glutaminase; GDH, glutamate dehydrogenase; GPT2, mitochondrial glutamate-pyruvate transaminase 2; GOT2, aspartate aminotransferase; EGCG, epigallocatechin-3-gallate; AOA, amino-oxyacetic acid; IMS, intermembrane mitochondrial space.

Fig. 5. Mitochondrial ROS and antioxidant systems.

Electrons can leak from the ETC, especially from Complex I and III (marked with yellow stars), from the reverse electron transport chain (RET) or mitochondrial enzymes as proline dehydrogenase (PRODH), glycerophosphate dehydrogenase (GPDH/GPD2), mitochondrial dehydroorotate dehydrogenase (DHODH), mono-amino oxidase (MAO), 2-oxoglutarate dehydrogenase (OGDH), and pyruvate dehydrogenase (PDH). This electron leakage can result in the formation of superoxide radical (O₂^•–), which superoxide dismutases (SOD) convert it into hydrogen peroxide (H₂O₂). H₂O₂ will be processed into water (H₂O) by the antioxidant systems. The glutathione system includes the glutathione (GSH) peroxidase (GPX1) and the glutathione disulfide (GSSG) reductase (GR). The peroxiredoxin-thioredoxin system alternates between reduced to oxidized peroxiredoxin (PRDX) and thioredoxin (TRX), and thioredoxin reductase (TrxR), which catalyzes the NADPH-dependent reduction of TRX.