Immunosuppressant FK506 inhibits inducible nitric oxide synthase gene expression at a step of NF-κB activation in rat hepatocytes

Background/Aims: Recent evidence indicates that an increase in nitric oxide production after liver transplantation is associated with acute allograft rejection. Nitric oxide mediates cellular injury under various pathological conditions in the liver. Studies were performed to determine whether the immunosuppressants FK506 and cyclosporin A directly influence gene expression of inducible nitric oxide synthase by interleukin 1β in hepatocytes. Methods: Primary cultures of rat hepatocytes were treated with interleukin 1β in the presence and absence of FK506 or cyclosporin A. Release of nitrite (nitric oxide metabolite) into culture medium, levels of inducible nitric oxide synthase protein and mRNA, and activation of nuclear factor-κB were compared with the two drugs. Results: Interleukin 1β increased levels of inducible nitric oxide synthase protein and inducible nitric oxide synthase mRNA, as well as nitric oxide production, in the cultured hepatocytes. Nuclear factor-κB, an important transcription factor in inducible nitric oxide synthase gene expression in response to inflammation, also appeared in the nuclear fraction of hepatocytes after addition of interleukin 1β. FK506 markedly inhibited the nitric oxide formation, inducible nitric oxide synthase protein synthesis and inducible nitric oxide synthase mRNA expression induced by interleukin 1β, but cyclosporin A had no effects. Furthermore, FK506 inhibited nuclear factor-κB activation and decreased mRNA levels of the p50/p65 subunits of nuclear factor-κB. Conclusions: These results demonstrate that FK506, but not cyclosporin A, inhibits the induction of inducible nitric oxide synthase expression during nuclear factor-κB activation. FK506 may influence liver function during diseases by modulating the nitric oxide pathway, in addition to its immunosuppressive effect.