Effect of oxidative stress on cystine transportation by xC‾ antiporter

We performed computer simulations to investigate the effect of oxidation on the extracellular cystine (CYC) uptake by the xC^- antiporter. The latter is important for killing of cancer cells. Specifically, applying molecular dynamics (MD) simulations we studied the transport of CYC across xCT, i.e., the light subunit of the xC^- antiporter, in charge of bidirectional transport of CYC and glutamate. We considered the outward facing (OF) configuration of xCT, and to study the effect of oxidation, we modified the Cys₃₂₇ residue, located in the vicinity of the extracellular milieu, to cysteic acid (CYO₃₂₇). Our computational results showed that oxidation of Cys₃₂₇ results in a free energy barrier for CYC translocation, thereby blocking the access of CYC to the substrate binding site of the OF system. The formation of the energy barrier was found to be due to the conformational changes in the channel. Analysis of the MD trajectories revealed that the reorganization of the side chains of the Tyr₂₄₄ and CYO₃₂₇ residues play a critical role in the OF channel blocking. Indeed, the calculated distance between Tyr₂₄₄ and either Cys₃₂₇ or CYO₃₂₇ showed a narrowing of the channel after oxidation. The obtained free energy barrier for CYC translocation was found to be 33.9kJmol^-1, indicating that oxidation of Cys₃₂₇, by e.g., cold atmospheric plasma, is more effective in inhibiting the xC^- antiporter than in the mutation of this amino acid to Ala (yielding a barrier of 32.4kJmol^-1). The inhibition of the xC^- antiporter may lead to Cys starvation in some cancer cells, eventually resulting in cancer cell death.