Role of phospholipase C beta 3 phosphorylation in the desensitization of cellular responses to platelet-activating factor

Platelet-activating factor (PAF) stimulates a diverse array of cellular responses through receptors coupled to G proteins that activate phospholipase C (PLC). Truncation of the cytoplasmic tail of the receptor to remove phosphorylation sites (mutant PAF receptor, mPAFR) results in enhancement of PAF-stimulated responses. Here we demonstrate that PAF or phorbol 12-myristate 13-acetate (PMA) pretreatment inhibited wild type PAFR-induced PLC-mediated responses by similar to 90%, whereas these responses to the phosphorylation deficient mPAFR mere inhibited by similar to 50%, despite normal G protein coupling, suggesting a distal inhibitory locus. PAF and PR as well as a membrane permeable cyclic AMP analog, stimulated phosphorylation of PLC beta 3. A protein kinase C (PKC) inhibitor blocked phosphorylation of PLC beta 3 stimulated by PAF and PMA but not by cAMP, Activation of protein kinase A (PKA) by cAMP did not result in inhibition of Ca2+ mobilization stimulated by PAF. In contrast, cAMP did inhibit the response to formylpeptide chemoattractant receptor. These data suggest that homologous desensitization of PAF-mediated responses is regulated via phosphorylation at two levels in the signaling pathway, one at the receptor and the other at PLC beta 3 mediated by PKC but not by PKA, Phosphorylation of PLC beta 3 by PKA could explain the inhibition of formylpeptide chemoattractant receptor signaling by cAMP. As PAF and formylpeptide chemoattractant receptors activate PLC via different G proteins, phosphorylation of PLC beta 3 by PKC and PKA could provide distinct regulatory control for classes of G protein-coupled receptors.