Rifampin (rifampicin), an important antibiotic agent and a major drug used for the treatment of tubercu-losis, exerts immunomodulatory effects. Previous studies have found that rifampin increases inducible nitric oxide (NO) synthase (iNOS) expression and NO production. The present study investigated the potential mechanism(s) underlying these actions. The incubation of human lung epithelial A549 cells with a cytokine mix (interleukin-lß, tumor necrosis factor alpha, and gamma interferon) induced the expression of iNOS mRNA. The addition of rifampin increased the INOS level by 1.9 ± 0.3-fold at a dose of 10 f¿g/ml (P < 0.01) and by 4.0 ± 0.3-fold at a dose of 50 |xg/ml (P < 0.001). Rifampin treatment also affected the transcription factors that regulate iNOS mRNA: there was an increased and prolonged degradation of the inhibitory subunit of NF-KB, a corresponding increase in the level of cytokine-induced DNA binding of NF-KB (2.1 ± 0.2-fold), and a decrease in the level of expression of peroxisome proliferator-activated receptor gamma (PPARY). Specifically, the level of PPARY expression dropped by 15% in response to cytokine stimulation and by an additional 40% when rifampin was added (P < 0.001). Rifampin had no effect on the activation of mitogen-activated protein kinases or the signal transducer and transcription activator (STAT-I). In conclusion, rifampin augments NO production by upregulating iNOS mRNA. It also increases the level of NF-KB activation and decreases the level of PPARY expression. The increases in the levels of NF-KB activation and NO production probably contribute to the therapeutic effects of rifampin. However, given the role of NF-KB in upregulating many inflammatory genes and the roles of PPARY in downregulating inflammatory genes and in lipid and glucose metabolism,these findings have implications for potential adverse effects of rifampin in patients with chronic inflammatory diseases and glucose or lipid disorders.