Use of a photoactivatable taxol analogue to identify unique cellular targets in murine macrophages: Identification of murine CD18 as a major taxol-binding protein and a role for mac-1 in taxol-induced gene expression

Taxol, a potent antitumor agent that binds beta-tubulin and promotes microt ubule assembly, results in mitotic arrest at the G(2)/m phase of the cell c ycle. More recently, Taxol was shown to be a potent LPS mimetic in murine, but not in human macrophages, stimulating signaling pathways and gene expre ssion indistinguishably from LPS, Although structurally unrelated to LPS, T axol's LPS-mimetic activities are blocked by inactive structural analogues of LPS, indicating that despite the species-restricted effects of Taxol, LP S and Taxol share a common receptor/signaling complex that might be importa nt in LPS-induced human diseases. To identify components of the putatively shared Taxol/LPS receptor, a novel, photoactivatable Taxol analogue was emp loyed to identify unique Taxol-binding proteins in murine macrophage membra nes. Seven major Taxol-binding proteins, ranging from similar to 50 to 200 kDa, were detected. Although photoactivatable Taxol analogue failed to bind to CD14, the prominent Taxol-binding protein was identified as CD18, the s imilar to 96-kDa common component of the beta(2) integrin family. This find ing aas supported by the concomitant failure of macrophage membranes from M ac-1 knockout mice to express immunoreactive CD18 and the major Taxol-bindi ng protein. In addition, Taxol-induced IL-12 p40 mRNA was markedly reduced in Mac-1 knockout macrophages and anti-Mac-1 Ab blocked secretion of IL-12 p70 in Taxol- and LPS-stimulated macrophages. Since CD18 has been described as a participant in LPS-induced binding and signal transduction, these dat a support the hypothesis that the interaction of murine CD18 with Taxol is involved in its proinflammatory activity.