Cat2 L-arginine transporter-deficient fibroblasts can sustain nitric oxide production

High-output nitric oxide (NO) production by nitric oxide synthase 2 (NOS2) contributes to normal cellular processes and pathophysiological conditions. The transport of L-arginine, the substrate for NOS2, is required for sustained NO production by NOS2. L-Arginine can be transported by several kinetically defined transport systems, although the majority of arginine uptake is mediated by transport system y(+), encoded by the Cat1-3 gene family. Using macrophages from Cat2-deficient mice, we previously determined that arginine uptake via CAT2 is absolutely required for sustained NO production. Because NO production by fibroblasts is important in wound healing, we sought to determine whether CAT2 is required for NO production in cytokine-stimulated Cat2-deficient and wild-type embryonic fibroblasts. Although macrophages and fibroblasts both required extracellular L-arginine for NO production, NO synthesis by activated Cat2(-/-) fibroblasts was reduced only 19%, whereas Cat2(-/-) macrophages were virtually unable to produce NO. As expected, activated Cat2(-/-) fibroblasts had reduced system y(+)-mediated arginine uptake. However, their reduced NO output was not the result of a significant difference in intracellular L-arginine levels following cytokine stimulation. Uptake experiments revealed that the L-arginine transport system y(+)L was the major cationic amino acid carrier in fibroblasts of both genotypes. We conclude that NO production in embryonic fibroblasts is only partially dependent on CAT2 and that other compensating transporters provide arginine for NOS2-mediated NO synthesis. The data demonstrate that fibroblasts and macrophages have differential dependence on CAT2-mediated L-arginine transport for NO synthesis. The important physiological implication of this finding is discussed.