Cysteine metabolic circuitries: druggable targets in cancer

Abstract

To enable survival in adverse conditions, cancer cells undergo global metabolic adaptations. The amino acid cysteine actively contributes to cancer metabolic remodelling on three different levels: first, in its free form, in redox control, as a component of the antioxidant glutathione or its involvement in protein s-cysteinylation, a reversible post-translational modification; second, as a substrate for the production of hydrogen sulphide (H₂S), which feeds the mitochondrial electron transfer chain and mediates per-sulphidation of ATPase and glycolytic enzymes, thereby stimulating cellular bioenergetics; and, finally, as a carbon source for epigenetic regulation, biomass production and energy production. This review will provide a systematic portrayal of the role of cysteine in cancer biology as a source of carbon and sulphur atoms, the pivotal role of cysteine in different metabolic pathways and the importance of H₂S as an energetic substrate and signalling molecule. The different pools of cysteine in the cell and within the body, and their putative use as prognostic cancer markers will be also addressed. Finally, we will discuss the pharmacological means and potential of targeting cysteine metabolism for the treatment of cancer.

Fig. 1. Cysteine metabolic fate.

Cysteine has different fates, including the synthesis of glutathione or proteins, oxidative or non-oxidative catabolism and reversible post-translational protein modification (protein cysteinylation, production of reactive sulphide species and oxidation to cysteine disulphides).

Fig. 2. Cysteine: an intermediate and a supplier of several metabolic pathways.

a Cysteine can be taken up in the form of cystine, through the cystine–glutamate antiporter transport system (xCT), or as cysteine through the excitatory amino acid transporter 3 (EAAT3) or the alanine-serine-cysteine-transporter 2 (ASCT2). Cysteine metabolism is tightly linked to that of glutamine, forming a network of amino acids capable of supplying the core metabolic pathways that underlie pivotal processes in cancer: reactive oxygen species (ROS) scavenging and chemoresistance dependent on glutathione synthesis; carbon and energy metabolism through fatty acid synthesis and the tricarboxylic acid (TCA) cycle, one-carbon metabolism and the production of ATP by the mitochondrial electron transfer chain (mETC), and sulphur and energy production as a generator of hydrogen sulphide (H₂S), an electron (e⁻) donor for the mETC. b Cysteine (Cys) is a precursor of other organic compounds, such as homocysteine (Hcy), 3-mercaptopyruvate (3-MP), cystathionine, cystine, cysteine sulphinic acids (CSA), 3-sulphopyruvate (3-SP), hypotaurine (hT), taurine, glutathione (reduced, GSH), γ-glutamyl-cysteine (Glu-Cys) and cysteinylglycine (Cys-Gly).

Fig. 3. Metabolic pathways involved in cysteine catabolism.

Cysteine can be a substrate for hydrogen sulphide (H₂S) synthesis, be oxidatively catabolised to taurine or be a substrate for glutathione production. a The trans-sulphuration pathway. The enzymes cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) catalyse the conversion of homocysteine into cysteine, generate hydrogen sulphide through several alternative reactions or cysteine per/polysulphide (CysSS_(n)H) with cystine as substrate. Cysteine is converted by cysteine aminotransferase (CAT) into 3-mercaptopyruvate (3-MP), which is a substrate of 3-MP sulphurtransferase (3-MST), of which there is a cytosolic and a mitochondrial isoform. 3-MST can also generate CysS_(n)SH and glutathione per-/polysulphide (GS_(n)SH), respectively, using Cys and GSH as sulphur acceptors. H₂S is catabolised by a mitochondrial sulphide oxidation pathway. H₂S is a substrate of sulphide:quinone oxidoreductase (SQR), which preferentially uses glutathione as a sulphur acceptor, to generate oxidised glutathione (GSSH) that is converted back into GSH by persulphide dioxygenase (PDO) using oxygen as co-substrate, yielding sulphite (SO₃²⁻) as co-product. Oxidised glutathione is also a substrate of rhodanese (Rhod), which uses SO₃²⁻ as co-substrate to yield thiosulphate (S₂O₃²⁻) and glutathione. SO₃²⁻ oxidation by sulphite oxidase yields sulphate (SO₄²⁻). b Cysteine oxidation. Cysteine can be oxidised by cysteine dioxygenase (CDO) to cysteinesulphinate (CSA), which is converted by cysteinesulphinate decarboxylase (CSD) into hypotaurine (hT), which is further oxidised to taurine. Alternatively, cysteinesulphinate can be transaminated by aspartate aminotransferase (AAT) to 3-sulphinopyruvate (3-SP), which decomposes to form pyruvate and sulphite/sulphate. c Cysteine is a substrate for glutathione generation. Cysteine is converted into γ-glutamyl-cysteine (Glu-Cys) by glutamate-cysteine ligase (GCL) and subsequently to glutathione by glutathione synthase (GS). Conversely, GSH is converted into cysteinylglycine (Cys-Gly) by γ-glutamyl transpeptidase (γGT) and finally back to cysteine by dipeptidase (dPP).

Fig. 4. Protein s-cysteinylation.

The oxidation of a cysteine residue within a protein can result in the formation of a cysteinyl radical. l-Cystine is reduced to l-cysteine under the action of l-cystine reductase. Reaction between protein cysteinyl residues and low molecular weight thiols such as free cysteine can yield s-cysteinylated proteins.

Fig. 5. Targeting cysteine metabolism.

a Drugs targeting cysteine metabolism. b Key inhibition targets and interplay between the trans-sulphuration pathway, the xCT antiporter and ferroptosis. c Inhibition of glutathione- and thioredoxin-dependent antioxidant pathways by sulphasalazine or auranofin combined with L-BSO. d Novel selenium-chrysin nanoformulations acting as inhibitors of glutathione bioavailability/synthesis and CBS activity. CBS cystathionine β-synthase, CSE cystathionine γ-lyase, GCS γ-glutamyl cysteine synthetase, GSS glutathione synthetase, Pyr pyruvate, α-KB α-ketobutyrate, GSH glutathione reduced form, GSSG glutathione oxidised form, ROS reactive oxygen species, FIN ferroptosis-inducing compounds.