Targeting amino acid metabolism for cancer therapy

Abstract

To support sustained biomass accumulation, tumor cells undergo metabolic reprogramming. Nutrient transporters and metabolic enzymes are regulated by the same oncogenic signals that drive cell-cycle progression. Some of the earliest cancer therapies used antimetabolites to disrupt tumor metabolism, and there is now renewed interest in developing drugs that target metabolic dependencies. Many cancers exhibit increased demand for specific amino acids, and become dependent on either an exogenous supply or upregulated de novo synthesis. Strategies to exploit such 'metabolic addictions' include depleting amino acids in blood serum, blocking uptake by transporters and inhibiting biosynthetic or catabolic enzymes. Recent findings highlight the importance of using appropriate model systems and identifying target patient groups as potential therapies advance into the clinic.

Figure 1

Plasma arginine depletion for cancer therapy. Clinical trials are currently evaluating PEGylated bacterial arginine deiminase (ADI-PEG20) and recombinant human arginase 1 (rhArg1-PEG, or BCT-100) for treatment of a variety of cancers. Certain tumors lose expression of argininosuccinate synthetase (ASS1), the enzyme that catalyzes formation of argininosuccinate from aspartate and citrulline. Consequently, these tumors are unable to synthesize arginine de novo (indicated by dotted lines), and rely on extracellular arginine supply and uptake through members of the SLC7 family of transporters. Loss of ASS1 benefits tumor cells by increasing aspartate supply for pyrimidine synthesis. By depleting plasma arginine, ADI-PEG20 and rhArg1-PEG/BCT-100 effectively starve these tumor cells of arginine. A potential mechanism for resistance to this treatment strategy is re-expression of ASS1, leading to restored de novo arginine synthesis. Thick blue arrows indicate multistep pathways and blue text indicates their products. Abbreviations: ARG1, arginase 1; ASL, argininosuccinate lyase; ASS1, argininosuccinate synthetase; GLS, glutaminase; GOT, aspartate aminotransferase; OTC, ornithine transcarbamylase.

Figure 2

Glutamine/glutamate utilization in proliferating cells. Glutamine (Gln) and glutamate (Glu), shown in dark green text, donate carbon and/or nitrogen to diverse biosynthetic processes in proliferating cells. Glutamine is taken up from the extracellular environment by a number of SLC superfamily transporters, or synthesized de novo from glutamate and ammonia by glutamine synthetase (GLUL). A heterodimer of SLC7A5 (LAT1) and SLC3A2 couples glutamine efflux to uptake of aromatic and branched-chain amino acids, and SLC7A11 (xCT) couples glutamate efflux to cystine uptake. In the cytosol, glutamine donates its amide nitrogen (yielding glutamate) to a number of biosynthetic reactions catalyzed by glutamine amidotransferases, including reactions in the purine, pyrimidine and hexosamine biosynthetic pathways and also the asparagine synthetase reaction. In the mitochondrion, glutamine is converted to glutamate by glutaminases (GLS or GLS2), with the amide nitrogen released as ammonia. Cytosolic and mitochondrial glutamate can exchange via the aspartate/glutamate carrier SLC25A13. In the cytosol, glutamate together with glycine and cysteine form the antioxidant tripeptide glutathione. Reversible transaminases, some of which have cytosolic and mitochondrial isoforms, interchange glutamate and α-ketoglutarate (α-KG), coupled to the interchange of α-ketoacids and amino acids including serine, alanine and aspartate. In the mitochondrion, glutamate can also be deaminated by glutamate dehydrogenase (GLUD1/2) to yield α-KG and ammonia. Glutamate can also be directly incorporated in the proline synthesis pathway. Thick blue arrows indicate multistep pathways and blue text indicates their products. Abbreviations: AA, amino acids;α-KG, α-ketoglutarate; BCAT, branched-chain amino transaminase; GLUL, glutamine synthetase; GLS/GLS2, glutaminase/glutaminase 2; GOT, aspartate aminotransferase; GPT, alanine transaminase; MDH, malate dehydrogenase; ME, malic enzyme; MPC, mitochondrial pyruvate carrier; OAA, oxaloacetate, PDC, pyruvate dehydrogenase complex; 3-PHP, 3-phosphohydroxypyruvate; 3-PS, 3-phosphoserine; Succ-CoA, succinyl-Coenzyme A. Amino acids are denoted by their standard three-letter abbreviations.

Figure 3

Serine/glycine metabolism in proliferating cells. (a) The serine synthesis pathway (SSP). Serine can be imported from the extracellular environment by several members of the SLC superfamily of transporters, and it can also be synthesized de novo in the cytosol via the SSP. The SSP branches from glycolysis at the point of 3-phosphoglycerate (3-PG) and involves three sequential reactions catalyzed by 3-phosphoglycerate dehydrogenase (PHGDH), phosphoserine aminotransferase (PSAT1) and phosphoserine phosphatase (PSPH). Up to 10% of glycolytic carbon is shunted into the SSP in cancer cells. The gene encoding PHGDH is recurrently amplified in a subset of human cancers, and is frequently overexpressed even in the absence of amplification. The endproduct of the SSP, serine, allosterically activates pyruvate kinase M2 (PKM2), allowing glycolytic flux to increase. Thus, when serine is depleted PKM2 activity becomes suppressed and glycolytic intermediates including 3-PG accumulate to supply the SSP. In addition, the glycolytic intermediate 2-phosphoglycerate (2-PG) activates PHGDH. Thick blue arrows indicate multistep pathways and blue text indicates their products. Dotted green line and ‘+’ symbol indicate enzymatic activation mechanisms. (b) Contribution of serine and glycine to one-carbon metabolism. In proliferating mammalian cells, the mitochondrial isoform serine hydroxymethyltransferase 2 (SHMT2), and not cytosolic SHMT1, is the primary catalyst of serine to glycine conversion (the SHMT1 reaction is indicated by a thin dotted line, and the SHMT2 reaction by a thick solid line, to illustrate the primary flow of carbon). The SHMT reaction transfers a one-carbon unit from serine to tetrahydrofolate (THF), generating glycine and 5,10-methylenetetrahydrofolate (5,10-CH₂-THF). The mitochondrial glycine cleavage system (GCS) transfers a one-carbon unit from glycine to THF, also generating 5,10-CH₂-THF. This intermediate is oxidized in the mitochondrion by methylenetetrahydrofolate dehydrogenase 2 (MTHFD2) or MTHFD2-like (MTHFD2L) to yield 10-formyl-THF. THF can be regenerated from 10-formyl-THF, with the release of formate, in a reaction catalyzed by MTHFD1L. After transport into the cytosol, formate is consumed by the trifunctional MTHFD1 along with cytosolic THF, generating cytosolic 10-formyl-THF, which is required for purine synthesis, and cytosolic 5,10-CH₂-THF. The latter intermediate donates the one-carbon unit to convert deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), a reaction catalyzed by thymidylate synthase (TYMS). The other product of the TYMS reaction, dihydrofolate, is converted back to tetrahydrofolate by the enzyme dihydrofolate reductase (DHFR). TYMS is a target of the commonly used chemotherapy drugs pemetrexed and 5-fluorouracil (5-FU), and DHFR is a target of pemetrexed and methotrexate. Cytosolic 5,10-CH₂-THF also donates a one-carbon unit for homocysteine methylation in the methionine cycle. Thick blue arrows indicate multistep pathways and blue text indicates their products. Reactions involving transfer of one-carbon units are highlighted with the text ‘1C’ in blue. Abbreviations:α-KG, α-ketoglutarate; ENO, enolase; LDH, lactate dehydrogenase; PGAM, phosphoglycerate mutase; Pi, inorganic phosphate; 3-PG, 3-phosphoglycerate; SSP, serine synthesis pathway. Amino acids are denoted by their standard three-letter abbreviations.