Multiple signal transduction pathways regulate TNF-induced actin reorganization in macrophages: Inhibition of Cdc42-mediated filopodium formation by TNF

TNF is known to regulate macrophage (M phi) migration, but the signaling pa thways mediating this response have not been established. Here we report th at stimulation of the 55-kDa TNF receptor (TNFR-1) induced an overall decre ase in filamentous actin (F-actin), inhibited CSF-1- and Cdc42-dependent fi lopodium formation, and stimulated macropinocytosis. Using a panel of TNFR- 1 mutants, the regions of the receptor required for each of these responses were mapped. The decrease in F-actin required both the death domain and th e membrane proximal part of the receptor, whereas inhibition of filopodium formation and increased pinocytosis were only dependent upon a functional d eath domain. When the TNF-induced decrease in F-actin was inhibited using e ither receptor mutants or the compound D609, TNF-stimulated actin reorganiz ation at the cell cortex became apparent. This activity was dependent upon the FAN-binding region of TNFR-1, We conclude that different domains of TNF R-1 mediate distinct changes in the M phi cytoskeleton, and that the abilit y of TNF to inhibit M phi chemotaxis may be due to decreased filopodium for mation downstream of Cdc42.