System xc⁻ cystine/glutamate antiporter: an update on molecular pharmacology and roles within the CNS

Abstract

System x(c)(-) is an amino acid antiporter that typically mediates the exchange of extracellular l-cystine and intracellular L-glutamate across the cellular plasma membrane. Studied in a variety of cell types, the import of L-cystine through this transporter is critical to glutathione production and oxidative protection. The exchange-mediated export of L-glutamate takes on added significance within the CNS, as it represents a non-vesicular route of release through which this excitatory neurotransmitter can participate in either neuronal signalling or excitotoxic pathology. When both the import of L-cystine and the export of L-glutamate are taken into consideration, system x(c)(-) has now been linked to a wide range of CNS functions, including oxidative protection, the operation of the blood-brain barrier, neurotransmitter release, synaptic organization, viral pathology, drug addiction, chemosensitivity and chemoresistance, and brain tumour growth. The ability to selectively manipulate system x(c)(-), delineate its function, probe its structure and evaluate it as a therapeutic target is closely linked to understanding its pharmacology and the subsequent development of selective inhibitors and substrates. Towards that goal, this review will examine the current status of our understanding of system x(c)(-) pharmacology and the structure-activity relationships that have guided the development of an initial pharmacophore model, including the presence of lipophilic domains adjacent to the substrate binding site. A special emphasis is placed on the roles of system x(c)(-) within the CNS, as it is these actions that are among the most exciting as potential long-range therapeutic targets.

Figure 1

An xCT homology model was constructed by threading the hxCT sequence over the crystal structure of ApcT (RCSB: pdb3GIA) from M. jannaschii (GI:1591319) as reported by Goaux and co-workers (Shaffer et al., 2009) using FASTA (Pearson and Lipman, 1988) followed by multiple sequence alignments with ClustalW (Larkin et al., 2007). The structure of ApcT is shown as a white thread overlayed with the helical ribbons of the 12 TMDs of xCT: TMD 1A/B in light pink; TMD2 in dark pink; IL1 in dark blue; TMD3 in bright blue; TMD4 in light purple; TMD5 in teal, TMD6A/B in dark green; TMD7 in olive green; EL4A/B in silver; TMD8 in light green; TMD9 in yellow; TMD10 in gold; TMD11 in orange and TMD12 in red. Truncated intracellular N (light pink) and C (red) termini are shown with spheres. The cysteine-158 that participates in the disulphide bond with 4F2hc is highlighted with yellow spheres. The protein is shown in its inwardly facing Apo-form. A hypothetical substrate is depicted by the centrally located white surface as predicted by Schaffer et al., based upon the binding site of LeuT.

Figure 2

Acyclic amino acid substrates and inhibitors of the system x_c^- transporter.

Figure 3

Isoxazole and related heterocyclic substrate and inhibitors of the system x_c^- transporter.

Figure 4

Benzoic acid and aromatic sulphonic acid inhibitors of the system x_c^- transporter.

Figure 5

A ligand-based, superposition, 3D pharmacophore model for the substrate binding on Sx_c^-. A–E. l-Glu in green; l-Cys₂ in yellow; QA in teal; 4-S-CPG in purple; NACPA in red; TFMIH in orange; NEIH in blue and SSZ in bronze. F. Pharmacophore binding template.

Figure 6

Nrf-2/ARE activation pathway. Under basal conditions Nrf-2 is dimerized with KEAP1 and continuously targeted for degradation due to ubiquitination by Cul3. Electrophilies and oxidants can disrupt the dimerization of Nrf2 and KEAP directly by modifiying cysteine residues in KEAP1 or through phosphorylation of Nrf2 at Ser40 by protein kinases. Nrf2 is then free to translocate into the nucleus, bind with an adaptor protein (e.g. Mafs and ATF4) and increase ARE-driven transcription.

Figure 7

Functional roles of the system x_c^- transporter in the CNS.