Characterization of cationic amino acid transporter and its gene expression in rat hepatic stellate cells in relation to nitric oxide production

Background/Aims: Nitric oxide is a potent mediator of hepatic sinusoidal he modynamics and affects hepatic stellate cells (Ito cells, fat-storing cells ). Although nitric oxide production may depend on the induction of inducibl e nitric oxide synthase and on transport of extracellular L-arginine, the p recise mechanisms controlling nitric oxide production in stellate cells hav e not been well characterized. Methods: Using stellate cells prepared from the male Wistar rat, kinetic an alysis of L-arginine transport and reverse transcription-polymerase chain r eaction for cationic amino acid transporter were carried out. The effect of tumor necrosis factor-alpha and interferon-gamma on L-arginine transport, mRNA expression of cationic amino acid transporter and inducible nitric oxi de synthase, and nitric oxide production of stellate cells was assessed. Results: The L-arginine transport system functioning in the transformed hep atic stellate cells was system y(+), possibly mediated by cationic amino ac id transporter-1 and cationic amino acid transporter-2B (Km similar to 50 m u M). Tumor necrosis factor-alpha enhanced cationic amino acid transporter- 2B mRNA expression and L-arginine transport, whereas cationic amino acid tr ansporter-1 mRNA expression remained unchanged. Interferon-gamma induced th e expression of inducible nitric oxide synthase mRNA without obvious change s in L-arginine transport. Interferon-gamma in combination with tumor necro sis factor-alpha induced nitric oxide production with an enhancement in cat ionic amino acid transporter-2B mRNA expression, inducible nitric oxide syn thase mRNA expression, and L-arginine transport, while extracellular L-lysi ne competitively inhibited this nitric oxide production. Conclusions: In transformed hepatic stellate cells, tumor necrosis factor-a lpha and interferon-gamma have a crucial role in nitric oxide production, a nd extracellular L-arginine transport and inducible nitric oxide synthase e xpression are regulated in a differential cytokine-specific manner. As the estimated Km of L-arginine transporter in transformed hepatic stellate cell s is very similar to the physiological L-arginine concentration in portal v ein, we assume that increased portal L-arginine concentration may easily af fect sinusoidal blood flow through enhancement of autocrine nitric oxide pr oduction in transformed hepatic stellate cells of diseased liver.