Trp-Lys-Tyr-Met-Val-Met (WKYMVM) is a novel potent peptide which can stimulate phosphoinositide hydrolysis in U937 as well as U266 and HL-60 cells (Baek et al., J. Biol. Chem. 271, 8170 (1996)). The peptide also induces superoxide generation in human neutrophils (Seo et al., J. Immunol. 158, 1896 (1997)). However, the signaling pathway down-stream of PLC set in motion by the peptide is not yet completely understood. We studied the signaling pathway of the peptide with the goal of elucidating the mechanism of the peptide's action. WKYMVM induced a rapid and transient activation of the ERKs in human histiocytic lymphoma cells, U937. The ERK1 activation peaked at 5 min and returned to the basal level after 30 min. The ERK1 stimulation by the peptide was partially inhibited by pretreatment of the cells with pertussis toxin (PTX), implicating G-protein involvement in the peptide's action. Pretreatment of staurosporine, protein kinase C (PKC) inhibitor, or PKC down-regulating PMA had no impact on the ERK1 activation by the peptide, indicating that the signaling pathway is independent of PKC activation. Pretreatment of the cells with neomycin and intracellular Ca2+ mobilizing reagents had also no effect on the ERK1 activation by the peptide. However, pretreatment with wortmannin or LY294002, the inhibitor of phosphatidylinositol 3 kinase (PI-3K), strongly inhibited peptide-stimulated ERK1 activation. Our results suggest that PI-3K may be an important participant in the ERK cascade induced by the peptide. Furthermore, the treatment of U937 cells with the peptide activated p74Raf-1, an upstream kinase of ERK. Taken together, our results suggest that the peptide activate ERK via a G-protein/PI-3K/Ras/Raf-1 mediated signaling pathway in U937 cells.