Paclitaxel (Taxol), a microtubule stabilizer with antitumor activity, mimics certain effects of lipopolysaccharide (LPS) in murine macrophages. We examined the mechanism by which Taxol regulates the expression of inducible nitric oxide synthase (iNOS) in a murine macrophage cell line. Taxol alone induced iNOS mRNA and promoter activity, but no iNOS protein or NO production. The stability of the iNOS mRNA formed in response to Taxol was lower than that formed in response to IFN-gamma or LPS, and this may have been responsible for the lack of induction of iNOS protein and NO. However, IFN-gamma synergized with Taxol by increasing iNOS mRNA stability, and upregulating levels of iNOS mRNA and protein, promoter activity, and NO production. Transfection experiments with 5'-serial deletions and site-directed mutants of the iNOS promoter revealed that the pair of upstream and downstream NF-kappaB sites was crucial for promoter activity in response to Taxol, as in the case of LPS. Electrophoretic mobility shift assays showed that both Taxol and LPS rapidly activated identical NF-kappaB complexes that Could bind to the iNOS promoter. These results suggest that Taxol shares a signaling pathway for transcriptional activation of iNOS with LPS, but that the stability of the iNOS mRNA induced by Taxol is different from that induced by LPS. (C) 2004 Elsevier Inc. All rights reserved.