Taxol, an antitumor agent derived from a plant, mimics the action of lipopolysaccharide (LPS) in mice but not in humans. Although Taxol is structurally unrelated to LPS, Taxol and LPS are presumed to share a receptor or signaling molecule. The LPS-mimetic activity of Taxol is not observed in LPS-hyporesponsive C3H/HeJ mice, which possess a point mutation in Toll-like receptor 4 (TLR4); therefore, TLR4 appears to be involved in both Taxol and LPS signaling. In addition, TLR4 was recently shown to physically associate with MD-2, a molecule that confers LPS responsiveness on TLR4. To determine whether TLR4⅐MD-2 complex mediates a Taxolinduced signal, we constructed transformants of the mouse pro-B cell line, Ba/F3, expressing mouse TLR4 alone, both mouse TLR4 and mouse MD-2, and both mouse MD-2 and mouse TLR4 lacking the cytoplasmic portion, and then examined whether Taxol induced NFB activation in these transfectants. Noticeable NFB activation by Taxol was detected in Ba/F3 expressing mouse TLR4 and mouse MD-2 but not in the other transfectants. Coexpression of human TLR4 and human MD-2 did not confer Taxol responsiveness on Ba/F3 cells, suggesting that the TLR4⅐MD-2 complex is responsible for the species specificity with respect to Taxol responsiveness. Furthermore, Taxol-induced NFB activation via TLR4⅐MD-2 was blocked by an LPS antagonist that blocks LPS-induced NFB activation via TLR4⅐MD-2. These results demonstrated that coexpression of mouse TLR4 and mouse MD-2 is required for Taxol responsiveness and that the TLR4⅐MD-2 complex is the shared molecule in Taxol and LPS signal transduction in mice.Taxol, a diterpene purified from the bark of the Western yew (Taxus brevifolia) (1), is an antitumor agent that blocks mitosis by binding and stabilizing microtubules (2, 3). Ding et al. (4) found that Taxol induces the secretion of tumor necrosis factor and down-regulation of tumor necrosis factor receptors in murine macrophages. Although the structure of Taxol is quite different from that of lipopolysaccharide (LPS), 1 Taxol has been shown to possess many LPS-like activities, such as tyrosine phosphorylation of microtubule-associated protein kinases (5), induction of LPS-inducible gene expression (6), and activation of NFB (7). Interestingly, Taxol mimics the actions of LPS on murine macrophages but not on human LPS-responsive cells including macrophages (8,9).LPS contains polysaccharide and lipid A portion, the latter of which mediates many LPS responses (10). Several synthetic and natural lipid A analogs, which lack LPS-like activities, have been shown to retain the ability to block LPS-induced cellular responses (11-13). Taxol-induced signaling events in murine macrophages are blocked by some of these LPS antagonists, suggesting that LPS and Taxol share a receptor or signaling molecule (14). Although the target of the antagonists was not defined well, membrane-bound CD14 (mCD14), which has been demonstrated to be involved in LPS-induced signaling events on macrophages (15), might not be the target, because the ...