Fcγ Receptor-mediated Mitogen-activated Protein Kinase Activation in Monocytes Is Independent of Ras

Abstract

Receptors for the Fc portion of immunoglobulin molecules (FcR) present on leukocyte cell membranes mediate a large number of cellular responses that are very important in host defense, including phagocytosis, cell cytotoxicity, production and secretion of inflammatory mediators, and modulation of the immune response. Cross-linking of FcR with immune complexes leads, first to activation of protein-tyrosine kinases. The molecular events that follow and that transduce signals from these receptors to the nucleus are still poorly defined. We have investigated the signal transduction pathway from Fc receptors that leads to gene activation and production of cytokines in monocytes. Cross-linking of FcR, on the THP-1 monocytic cell line, by immune complexes resulted in both activation of the transcription factor NF-kappaB and interleukin 1 production. These responses were completely blocked by tyrosine kinase inhibitors. In contrast, expression of dominant negative mutants of Ras and Raf-1, in these cells, did not have any effect on FcR-mediated nuclear factor activation, suggesting that the mitogen-activated protein kinase (MAPK) signaling pathway was not used by these receptors. However, MAPK activation was easily detected by in vitro kinase assays, after FcR cross-linking with immune complexes. Using the specific MAPK/extracellular signal-regulated kinase kinase (MAPK kinase) inhibitor PD98059, we found that MAPK activation is necessary for FcR-dependent activation of the nuclear factor NF-kappaB. These results strongly suggest that the signaling pathway from Fc receptors leading to expression of different genes important to leukocyte biology, initiates with tyrosine kinases and requires MAPK activation; but in contrast to other tyrosine kinase receptors, FcR-mediated MAPK activation does not involve Ras and Raf.