Structure, function, and regulation of human cystine/glutamate transporter in retinal pigment epithelial cells

PURPOSE. The purpose of this investigation was to provide evidence for the expression of the cystine/glutamate transporter (x_c^-) in the human retinal pigment epithelial cell line ARPE-19, clone the light chain of the transporter from an ARPE-19 cell cDNA library and study its function, and investigate the regulation of this transporter by nitric oxide (NO) in ARPE-19 cells. METHODS. Uptake of radiolabeled cystine and glutamate was measured in ARPE-19 cells. The functional identity of x_c^- in these cells was established by substrate specificity and Na⁺-independence of the uptake process. The human x_c^- light chain (human xCT) was cloned from an ARPE-19 cell cDNA library. The functional identity of the cloned human xCT was investigated by heterologous coexpression of the light chain with the heavy chain (human 4F2hc) in HeLa cells. ARPE-19 cells were treated with or without the NO donor 3-nitroso-N-acetylpenicillamine (SNAP) and the expression of x_c^- was studied at the functional and molecular levels. RESULTS. ARPE-19 cells take up cystine as well as glutamate in the absence of Na⁺. Substrate specificity studies indicate that although the uptake of cystine in the absence of Na⁺ is mediated by multiple amino acid transport systems including x_c^-, the uptake of glutamate in the absence of Na⁺ occurs exclusively via x_c^-. The human xCT cloned from ARPE-19 cells is a protein of 501 amino acids. These cells express the heavy chain 4F2hc as evidenced from RT-PCR analysis. Coexpression of human xCT with 4F2hc in HeLa cells leads to the induction of cystine and glutamate uptake with characteristics similar to that of x_c^-. The activity of x_c^- in ARPE-19 cells is upregulated by SNAP, and the process is associated with an increase in the expression of xCT with no detectable change in the expression of 4F2hc. Conclusions. ARPE-19 cells express the cystine/glutamate transporter x_c^- (the light chain xCT and the heavy chain 4F2hc) as is evident from functional and molecular studies. NO upregulates this transport system and the process is associated with an increase in xCT mRNA but with no change in 4F2hc mRNA.