ASSOCIATION OF MITOGEN-ACTIVATED PROTEIN-KINASES WITH MICROTUBULES INMOUSE MACROPHAGES

Taxol, a microtubule-binding diterpene, mimics many effects of lipopol ysaccharide (LPS) on mouse macrophages. The LPS-mimetic effects of tax ol appear to be under the same genetic control as responses to LPS its elf Thus we have postulated a role for microtubule-associated proteins (MAP) in the response of macrophages to LPS. Stimulation of macrophag es by LPS quickly induces the activation of mitogen-activated protein kinases (MAPK). MAPK are generally considered cytosolic enzymes. Herei n we report that much of the LPS-activatable pool of MAPK in primary m ouse peritoneal macrophages is microtubule associated. By immunofluore scence, MAPK were localized to colchicine- and nocodazole-disruptible filaments. From both mouse brain and RAW 264.7 macrophages, MAPK could be coisolated with polymerized tubulin. Fractionation of primary macr ophages into cytosol-, microfilament-, microtubule-, and intermediate filament-rich extracts revealed that similar to 10% of MAPK but none o f MAPK kinase (MEK1 and MEK2) was microtubule found. Exposure of macro phages to LPS did not change the proportion of MAPK bound to microtubu les, but preferentially activated the microtubule-associated pool. The se findings confirm the prediction that LPS activates a kinase bound t o microtubules. Together with LPS-mimetic actions of taxol and the sha red genetic control of responses to LPS and taxol, these results suppo rt the hypothesis that a major LPS-signaling pathway in mouse macropha ges may involve activation of one or more microtubule-associated kinas es.