Signaling and Cross-talk by C5a and UDP in Macrophages Selectively Use PLCβ3 to Regulate Intracellular Free Calcium

Studies in fibroblasts, neurons, and platelets have demonstrated the integration of signals from different G-protein coupled receptors (GPCRs) in raising intracellular free Ca2+. To study signal integration in macrophages, we screened RAW264.7 cells and bone marrow-derived macrophages (BMDM) for their Ca2+ response to GPCR ligands. We found a synergistic response to complement component 5a (C5a) in combination with uridine 5’-diphosphate (UDP), platelet activating factor (PAF) or lysophosphatidic acid (LPA). The C5a response was Gai-dependent, while the UDP, PAF, and LPA responses were Gaqdependent. Synergy between C5a and UDP, mediated by the C5a and P2Y6 receptors, required dual receptor occupancy, and affected the initial release of Ca2+ from intracellular stores as well as sustained Ca2+ levels. C5a and UDP synergized in generating inositol-1,4,5-trisphosphate, suggesting synergy in activating phospholipase C (PLC) s. Macrophages expressed transcripts for three PLCs isoforms (PLCs2, PLCs3, and PLCs4), but GPCR ligands selectively used these isoforms in Ca2+ signaling. C5a predominantly used PLCs3, while UDP used PLCs3 but also PLCs4. Neither ligand required PLCs2. Synergy between C5a and UDP likewise depended primarily on PLCs3. Importantly, the Ca2+ signaling deficiency observed in PLCs3-deficient BMDM was reversed by reconstitution with PLCs3. Neither PI-3 kinase nor PKC was required for synergy. In contrast to Ca2+, PI3-kinase activation by C5a was inhibited by UDP, as was macropinocytosis, which depends on PI3- kinase. PLCs3 may thus provide a selective target for inhibiting Ca2+ responses to mediators of inflammation, including C5a, UDP, PAF, and LPA.