The Non-Essential Amino Acid Cysteine Becomes Essential for Tumor Proliferation and Survival

Abstract

The non-essential amino acid cysteine is used within cells for multiple processes that rely on the chemistry of its thiol group. Under physiological conditions, many non-transformed tissues rely on glutathione, circulating cysteine, and the de novo cysteine synthesis (transsulfuration) pathway as sources of intracellular cysteine to support cellular processes. In contrast, many cancers require exogeneous cystine for proliferation and viability. Herein, we review how the cystine transporter, xCT, and exogenous cystine fuel cancer cell proliferation and the mechanisms that regulate xCT expression and activity. Further, we discuss the potential contribution of additional sources of cysteine to the cysteine pool and what is known about the essentiality of these processes in cancer cells. Finally, we discuss whether cyst(e)ine dependency and associated metabolic alterations represent therapeutically targetable metabolic vulnerabilities.

Keywords: CBS; CSE; GGT; cystathionine; cysteine; cystine; glutathione; macropinocytosis; transsulfuration; xCT.

Figure 1

Acquisition of cysteine from extracellular cystine through xCT and de novo production of cysteine through the transsulfuration pathway. (Left) Extracellular cystine is transported into the cell through xCT while glutamate is exported. Thioredoxin or glutathione reduce cystine to cysteine, which is subsequently used for the synthesis of proteins, glutathione, and other sulfur-containing molecules. (Right) The de novo cysteine synthesis pathway (transsulfuration) requires the sulfur group from methionine. In the methionine cycle, methionine is adenosylated to produce SAM. SAM donates a methyl group to a methyl acceptor (protein, RNA, or DNA) to generate SAH. SAH hydrolase (not shown) generates Hcy from SAH. Hcy can regenerate methionine by accepting a methyl group from betaine or 5-MTHF from the folate cycle. Hcy can alternatively exit the methionine cycle via its condensation with serine by CBS in the first step of the transsulfuration pathway to produce Cth. Cth is subsequently hydrolyzed by CSE to ammonia (not shown), α-ketobutyrate (not shown), and cysteine. 5-MTHF: 5-tetramethylhydrofolate, 5-THF: 5-tetrahydrofolate, Cth: cystathionine, CBS: cystathionine β-synthase, CSE: cystathionine γ-lyase, CysSH: cysteine, CysSSCys: cystine, DMG: dimethylglycine, GSH: glutathione, Hcy: homocysteine, Met: methionine, SAH: S-adenosylhomocysteine, SAM: S-adenosylmethionine, TXN: thioredoxin.

Figure 2

Cells acquire cysteine from extracellular glutathione through the γ-glutamyl cycle, cysteine uptake, and lysosomal protein scavenging by macropinocytosis with cystine export. (Left) Extracellular cysteine is transported into the cell through unidentified cysteine transporters, possibly EAAT3. (Center) The γ-glutamyl bond in extracellular glutathione is cleaved by GGT to create 5-oxoproline and the dipeptide cysteinylglycine. Cysteinylglycine can be taken up by the cell through PEPT2 and cleaved intracellularly by non-specific dipeptidases to cysteine and glycine. Cysteinylglycine may also be cleaved to cysteine and glycine extracellularly by ApN, followed by the cellular uptake of cysteine by cysteine transporters or cystine via xCT (not shown). (Right) Soluble proteins in the extracellular fluid are engulfed by the cell through macropinocytosis. The macropinosome containing proteins travels to and fuses with the lysosome. The disulfide bridges of the proteins are broken, and the protein is linearized by interaction with cysteine imported into the lysosome. The protein is hydrolyzed into its constituent amino acids, including cystine. Lysosomal cystine is subsequently exported to the cytosol by the lysosomal cystine transporter, cystinosin, and reduced by thioredoxin or glutathione to cysteine. ApN: Aminopeptidase N, CysSH: cysteine, CysSSCys: cystine, EAAT3: excitatory amino acid transporter 3, GGT: γ-glutamyltransferase, GSH: glutathione, GSSG: oxidized glutathione, PEPT2: peptide transporter 2, TXN: thioredoxin.