Kinetic analysis of cystine uptake and inhibition pattern of sulfasalazine in A549 cells

Abstract

Cystine/glutamate transporter (xCT) is an antiporter involved in cystine uptake and glutamate efflux. However, there are very few reports regarding the kinetic analysis of xCT for cystine uptake using cancer cell lines, as well as the inhibition pattern of sulfasalazine, an inhibitor of xCT, for cystine uptake. Therefore, the purpose of this study was to clarify the kinetics of xCT in A549 cells, human lung cancer cells, and to reveal the inhibition pattern of sulfasalazine. Cystine uptake occurred in a time-dependent manner, with linear cystine uptake observed for 5 min. Additionally, sulfasalazine inhibited cystine uptake in a concentration-dependent manner, presenting an IC₅₀ value of 24.7 ± 5.6 μM. Cystine uptake was saturated with increasing concentration, demonstrating K_m and V_max values of 179.4 ± 26.7 μM and 30.4 ± 2.3 nmol/min/mg protein, respectively. Moreover, during cystine uptake with sulfasalazine, K_m and V_max were >300 μM and 8.0 ± 1.5 nmol/min/mg protein, respectively, suggesting that sulfasalazine might demonstrate a mixed inhibition pattern. Furthermore, xCT siRNA decreased the xCT mRNA level and reduced cystine uptake. In conclusion, xCT was involved in the cystine uptake in A549 cells and sulfasalazine showed a mixed inhibition pattern to xCT.