The transport of cationic amino acids in human airway cells: expression of system y+L activity and transepithelial delivery of NOS inhibitors

The transport of arginine has been characterized in human airway Calu-3 cells. As assessed with RT-PCR, Calu-3 cells express the genes for several transporters, such as the system y+-related SLC7A1, SLC7A2, and SLC7A4; the system y+L-related SLC7A6, SLC7A7, and SLC3A2; and the system B0,+-related SLC6A14. In polarized Calu-3 cell monolayers, apical arginine influx has a leucine-sensitive, sodium-dependent component and a leucine- and lysine-resistant sodium-independent fraction. At the basolateral membrane, arginine transport was fully sodium-independent and partially inhibited by leucine provided that sodium was present in the extracellular medium. Moreover, extracellular leucine trans-stimulated arginine efflux from the basolateral membrane in the presence, but not in the absence, of sodium. The transepithelial, apical to basolateral, arginine transport strictly depended on the presence of sodium and was markedly inhibited by apical leucine, but significantly trans-stimulated by the neutral amino acid added at the basolateral side. When added at the apical side, the NOS-inhibitors NMMA and NIL, CAA analogs with a free carboxyl group, markedly inhibited the apical arginine influx and the transepithelial flux of the cationic amino acid. The same compounds trans-stimulated basolateral arginine efflux. None of these effects were observed in the presence of the methyl ester analog NAME. The basolateral medium of Calu-3 cell monolayers, obtained after incubation in the presence of the three inhibitors at the apical side, inhibited the production of NO by activated murine macrophages. The inhibitory effect of the Calu-3 cell conditioned medium was time-dependent and markedly higher with NMMA and NIL than with NAME. Moreover, the NOS-inhibitory effect of the medium was significantly enhanced if NMMA and NIL, at the apical side, and basolateral leucine were simultaneously present during the conditioning procedure. These results indicate that 1) human airway epithelial cells express a functional system y+L at the basolateral membrane; 2) in this model, transepithelial arginine transport involves apical influx through system B0,+ and basolateral efflux through system y+L, and 3) the same transporters also perform an efficient transepithelial transport of amino acid-like NOS inhibitors.