CROSS-LINKING OF SURFACE IGM STIMULATES THE RAS RAF-1/MEK/MAPK CASCADE IN HUMAN B-LYMPHOCYTES/

The mechanism by which mitogen-activated protein kinase (MAPK) is acti vated in human B cells following cross-linking of the antigen receptor was investigated. Following anti-IgM antibody and phorbol 12-myristat e 13-acetate (PMA) stimulation, we demonstrate the activation of Res, Raf-1, and MAPK/ERK kinase (MEK), all of which are thought to particip ate in an important signaling cascade that leads to MAPK activation. W e detected the kinase activities of Raf-1 and MEK toward purified reco mbinant substrates for each in this pathway (MEK for Raf-1 and MAPK fo r MEK). Following stimulation with either anti-IgM or PMA, Ras activat ion was observed, and the ability of Raf-1 to phosphorylate recombinan t kinase-inactive MEK was increased by approximately 10-fold. Similarl y, MEK activity toward kinase-active or -inactive recombinant MAPK als o increased upon anti-IgM or PMA treatment. Furthermore, the activatio n of both MAPK and p(90rsk) was demonstrated under identical condition s in the B cells. We conclude that activation of B lymphocytes through the antigen receptor stimulates distinct members of the Ras/Raf-1/MEK cascade and this mechanism is likely to be responsible for MAPK and p (90rsk) activation in these cells.