Transmembrane Signaling : An Model for Signal Transduction Ion-flux-independent by Complexed Fc Receptors

Fluxes of Na+/K + that precede effector functions in stimulated phagocytes are thought to play a role in signal transduction. To examine this hypothesis, phagocytosis, phagosomal acidification, and superoxide anion generation (02-) were stimulated in media in which the Na + was replaced with K + or choline +. Counts of particles internalized and assessment of acidification of the phagosomes by acridine orange staining indicated that Na+/ K + fluxes were not necessary for phagocytosis or phagosomal acidification in J774.2 macrophages. Phagocytosis mediated by the ionophoretic Fc receptor~2b/~l of J774.2 macrophages was equally independent of a Na + gradient. Na+/K + fluxes did not dictate the rate of 02generation in human monocytes. Therefore, in at least these three effector functions, Na+/K + fluxes stimulated by Fcand non-specific receptor binding play neither a signaling nor an enhancing role. An ion-flux-independent model for transmembrane signaling by the Fc receptor is proposed. Others have shown that there is an apparent dependence on the external Na + concentration for 02generation and lysosomal secretion by neutrophils. These neutrophils had been pretreated with NH4 + during a routine purification step. 02generation stimulated by opsonized zymosan or phorbol myristate acetate, by monocytes or monocyte-derived macrophages, and phagocytosis of opsonized zymosan by J774.2 macrophages, showed dependence on external Na + only if these cells had been pre-treated with NH4 +. Brief NH4 + pre-treatment would be expected to acidify the cytoplasm of the cells. The reversal of this acidification is known to require Na + for H + extrusion through the Na+/H + antiport, thus explaining the apparent Na ÷ dependence. Mononuclear (monocytes, macrophages) and polymorphonuclear phagocytes (neutrophils) share certain functions. The binding of particulate or soluble ligands to surface receptors induces the production of microbicidal oxygen radicals, phagocytosis of particulate ligands, and lysosomal secretion, first to the outside of the cell and then into the phagosomal vacuole. Some of the surface receptors that are involved in initiating these pleiotypic responses, collectively called phagocyte effector functions, have been studied to determine how receptors signal these functions. It is known that internalization of ligand is not necessary for signaling. Immobilization of ligands on a non-phagocytosable surface (1) or treatment of phagocytes with cytochalasin B, an inhibitor ofendocytosis, does not reduce inflammatory responses after receptor stimulation (2). Thus, stimulus-response coupling between a ligand-bound receptor and cytoplasmic effector functions is apparently a surface phenomenon. Evidence, much of it indirect, suggests that the signal for phagocyte effector function is Na + influx. Stimulatory ligandreceptor interaction of phagocytic cells increases the permeability of their plasma membranes to Na + (3). This increased ion permeability results in a change in membrane potential (4) that is dependent on Na+/K + fluxes (4-6). The membrane potential change always precedes ligand-stimulated inflammatory responses (2, 7-12). Membrane permeability to other ions, such as Ca 2+ (3, 13), also increases, but Ca 2+ influx does not account for the membrane potential changes (2), nor is it necessary for the stimulation of inflammatory responses (14). Membrane potential changes that are dependent on Na+/K + fluxes have been recorded by microelectrode (5, 6, 12) and estimated by the distribution of the lipophilic cations (2, 4, 7, 9, 11) across the membrane in several cell types after the addition of stimulatory ligand. In a manner resembling the response of the postsynaptic membrane of muscle to Na + THE JOURNAL OF CELL BIOLOGY • VOLUME 99 DECEMBER 1984 2231-2240 © The Rockefeller University Press • 0021-9525/84/12/2231/10 $1.00 2 2 31 on F ebuary 1, 2013 jcb.rress. org D ow nladed fom Published December 1, 1984