The role of MyD88 and TLR4 in the LPS‐mimetic activity of Taxol

Taxol can mimic bacterial lipopolysaccharide (LPS) by activating mouse macrophages in a cell cycle‐independent, LPS antagonist‐inhibitable manner. Macrophages from C3H/HeJ mice, which have a spontaneous mutation in Toll‐like receptor 4 (TLR4), are hyporesponsive to both LPS and Taxol, suggesting that LPS and Taxol may share a signaling pathway involving TLR4. To determine whether TLR4 and its interacting adaptor molecule MyD88 are necessary for Taxol's LPS mimetic actions, we examined Taxol responses of primary macrophages from genetically defective mice lacking either TLR4 (C57BL/10ScNCr) or MyD88 (MyD88 knockout). When stimulated with Taxol, macrophages from wild‐type mice responded robustly by secreting both TNF and NO, while macrophages from either TLR4‐deficient C57BL/10ScNCr mice or MyD88 knockout mice produced only minimal amounts of TNF and NO. Taxol‐induced NF‐κB‐driven luciferase activity was reduced after transfection of RAW 264.7 macrophages with a dominant negative version of mouse MyD88. Taxol‐induced microtubule‐associated protein kinase (MAPK) activation and NF‐κB nuclear translocation were absent from TLR4‐null macrophages, but were preserved in MyD88 knockout macrophages with a slight delay in kinetics. Neither Taxol‐induced NF‐κB activation, nor IκB degradation was affected by the presence of phosphatidylinositol 3‐kinaseinhibitors. These results suggest that Taxol and LPS not only share a TLR4/MyD88‐dependent pathway in generating inflammatory mediators, but also share a TLR4‐dependent/MyD88‐independent pathway leading to activation of MAPK and NF‐κB.