Liver macrophages (Kupffer cells) respond to many stimulations with the production of bioactive substances including cytokines, eicosanoids, and inorganic radicals. In this study the activation of transcription factors by substances inducing cytokine gene expression or superoxide formation in rat Kupffer cells was examined. Using primary cultures of rat Kupffer cells the role of NF-KB and activator protein 1 (AP-1) in the expression of the tumor necrosis factor-alpha (TNF-a) gene by lipopolysaccharide (LPS) was investigated. Both transcription factors were strongly activated but with different kinetics. Maximal DNA-binding activity was induced with 50 ng of L P S / d of medium and persisted for at least 24 hours. At that time, NF-KBas well as AP-1-DNA complexes decreased their mobilities in native gels. Among the cytokines tested only TNF-a and macrophage colony-stimulating factor (M-CSF) were able to activate NF-KB in Kupffer cells. Phorbol ester and zymosan activated AP-1 but not NF-KB; the treatment of zymosan yielding a modified form of AP-1. Of all substances found to interfere with TNF-a production by Kupffer cells (pyrrolidine dithiocarbamate, dexamethasone, prostaglandin EO, interleukin [IL]-4, IL-10, and transforming growth factor-beta [TGF-P]) only pyrrolidine dithiocarbamate was able to completely inhibit the activation of NF-KB by LPS. Although not abrogating the LPS activation of NF-KB, dexamethasone inhibited that of AP-1. The results indicate a direct participation of NF-KB in the regulation of TNF-cy synthesis and a differential ef-