Role of Phospholipases, Protein Kinases and Calcium in FcεRI-Induced Secretion

Disruption of lipid order by short-chain ceramides correlates with inhibition of phospholipase D and downstream signaling by FcepsilonRI

Specialized plasma membrane domains known as lipid rafts participate in signal transduction and other cellular processes, and their liquid-ordered properties appear to be important for their function. We investigated the possibility of using amphiphiles to disrupt lipid rafts and thereby inhibit IgE-FcepsilonRI signaling. We find that short-chain ceramides - C2-ceramide and C6-ceramide - decrease plasma membrane lipid order and reduce the extent of fluorescence resonance energy transfer between lipid-raft-associated molecules on intact cells; by contrast, biologically inactive C2-dihydroceramide does neither. Structural perturbations by these ceramides parallel their inhibitory effects on antigen-stimulated Ca2+ mobilization in RBL mast cells in the presence and absence of extracellular Ca2+. Similar inhibition of Ca2+ mobilization is caused by n-butanol, which prevents phosphatidic acid production by phospholipase D, but not by t-butanol, which does not prevent phosphatidic acid production. These results and previously reported effects of short-chain ceramides on phospholipase D activity prompted us to compare the effects of C2-ceramide, C2-dihydroceramide and C16-ceramide on phospholipase D1 and phospholipase D2 activities in vitro. We find that the effects of these ceramides on phospholipase D1 activity strongly correlate with their effects on antigen-stimulated Ca2+ mobilization and with their disruption of lipid order. Our results indicate that phospholipase D activity is upstream of antigen-stimulated Ca2+ mobilization in these cells, and they demonstrate that ceramides can serve as useful probes for investigating roles of plasma membrane structure and phospholipase D activity in cellular signaling.

Synthesis and secretion of monocyte chemotactic protein-1 stimulated by the high affinity receptor for IgE

In prior studies aggregation of the high affinity receptors for IgE, Fc epsilon RI, on a rat mast cell line, RBL-2H3, stimulated transcription of the gene for monocyte chemotactic protein-1 (MCP-1) and secretion of the protein. Unexpectedly, those delayed events appeared much less constrained by kinetic proofreading than had been documented for other receptor-initiated responses. The results of the present experiments are consistent with the proposal that the biosynthesis and secretion of MCP-1 result from a soluble messenger formed in the reaction cascades initiated by the receptor, and that Ca(2+) could serve as that messenger. Interestingly, whereas receptor-mediated signals were required for transcription of the gene for MCP-1 and secretion of the chemokine, such signals were not required for the intervening step of translation of its mRNA.