Essential amino acid tryptophan inhibits induction of inducible nitric oxide synthase gene expression in interleukin-1β stimulated hepatocytes

Background/objective: Tryptophan exerts protective effects against a variety of organ inflammation and injury, including liver. However, there are few scientific reports about the mechanisms involved in the action. Pro-inflammatory cytokine interleukin (IL)-1β stimulates the induction of inducible nitric oxide synthase (iNOS) expression and NO production in cultured hepatocytes (“in vitro liver injury model”), and the prevention of iNOS expression and NO production is considered to be an indicator of liver protection. This study aimed to examine whether tryptophan influences the induction of iNOS gene expression and the mechanisms.Methods: Tryptophan was added into primary cultures of rat hepatocytes stimulated by IL-1β. The iNOS induction, NO production and its signaling pathway were analyzed.Results: IL-1β induced iNOS gene expression, which was followed by iNOS expression and NO production. Tryptophan inhibited the expression of iNOS mRNA and protein, and decreased the production of NO. Transfection experiments with iNOS promoter-luciferase constructs revealed that tryptophan reduced the activities of iNOS mRNA synthesis and its stability. Tryptophan blocked two essential signaling pathways, the activation of nuclear factor (NF)-κB and upregulation of type I IL-1receptor (IL-1RI).Conclusions: Results indicate that tryptophan can prevent the NO production by the inhibition of iNOS gene expression, in part through NF-κB activation and IL-1RI upregulation, in inflamed hepatocytes. Tryptophan may be a potential therapeutic treatment in injured organs, including liver.Key words: tryptophan, inducible nitric oxide synthase, nitric oxide, cultured hepatocytes, nuclear factor-κB, type I interleukin-1 receptor