Nitric oxide synthase 2 through an autocrine loop via respiratory epithelial cell-derived mediator

Respiratory epithelium expresses nitric oxide synthase 2 (NOS2) continuousl y in vivo; however, mechanisms responsible for its expression are only part ially understood. We definitively identify an autocrine mechanism of induct ion and maintenance of NOS2 in human airway epithelial cells through the sy nthesis and secretion of a soluble mediator. Short exposure of human airway cells to interferon (IFN)-gamma leads to prolonged NOS2 expression. Transf er of the overlying culture medium (conditioned medium) induces NOS2 expres sion in other airway epithelial cells, suggesting the presence of an interm ediary substance regulating NOS2 expression in an autocrine loop. Character ization of the soluble mediator reveals that it is stable and transferable in conditioned medium for up to 7 days. However, soluble mediator does not induce NOS2 mRNA in human alveolar macrophages, indicating that the respons e to soluble mediator is unique to human respiratory epithelium. Soluble me diator is heat labile but is not inactivated by acid treatment, unlike IFN- gamma itself. Importantly, IFN regulatory factor-1, which is critical for m urine NOS2 expression, is expressed and activated by soluble mediator throu gh the signal transducer and activator of transcription-1-dependent pathway . Based on these findings, we propose novel regulatory mechanisms for NOS2 expression in human airway epithelium.