Distinct Roles of Receptor Phosphorylation, G Protein Usage, and Mitogen-Activated Protein Kinase Activation on Platelet Activating Factor-Induced Leukotriene C4 Generation and Chemokine Production
Platelet activating factor (PAF) interacts with cell surface G protein-coupled receptors on leukocytes to induce degranulation, leukotriene C4 (LTC4) generation, and chemokine CCL2 production. Using a basophilic leukemia RBL-2H3 cell line expressing wild-type PAF receptor (PAFR) and a phosphorylation-deficient mutant (mPAFR), we have previously demonstrated that receptor phosphorylation mediates desensitization of PAF-induced degranulation. Here, we sought to determine the role of receptor phosphorylation on PAF-induced LTC4 generation and CCL2 production. We found that PAF caused a significantly enhanced LTC4 generation in cells expressing mPAFR when compared with PAFR cells. In contrast, PAF-induced CCL2 production was greatly reduced in mPAFR cells. Pertussis toxin and U0126, which inhibit Gi and p44/42 mitogen-activated protein kinase (ERK) activation, respectively, caused very little inhibition of PAF-induced CCL2 production (∼20% inhibition). In contrast, these inhibitors almost completely blocked both PAF-induced ERK phosphorylation and LTC4 generation in PAFR cells. However, in mPAFR cells pertussis toxin only partially inhibited PAF-induced ERK phosphorylation. A Ca2+/calmodulin inhibitor had no effect on PAF-induced ERK phosphorylation in PAFR cells but completely blocked the response in mPAFR cells. These data demonstrate that receptor phosphorylation, which serves to desensitize PAF-induced LTC4 generation, is required for chemokine CCL2 production. They also indicate a previously unrecognized selectivity in G protein usage and ERK activation for PAF-induced responses. Whereas PAF-induced CCL2 production is, in large part, mediated independently of Gi activation or ERK phosphorylation, LTC4 generation requires ERK phosphorylation, which is mediated by different G proteins depending on the phosphorylation status of the receptor.