Activation and regulation of platelet-activating factor receptor: role of G(i) and G(q) in receptor-mediated chemotactic, cytotoxic, and cross-regulatory signals.

Platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycerolphosphocholine; PAF) induces leukocyte accumulation and activation at sites of inflammation via the activation of a specific cell surface receptor (PAFR). PAFR couples to both pertussis toxin-sensitive and pertussis toxin-insensitive G proteins to activate leukocytes. To define the role(s) of G(i) and G(q) in PAF-induced leukocyte responses, two G-protein-linked receptors were generated by fusing G alpha(i3) (PAFR-G alpha(i3)) or G alpha(q) (PAFR-G alpha(q)) at the C terminus of PAFR. Rat basophilic leukemia cell line (RBL-2H3) stably expressing wild-type PAFR, PAFR-G alpha(i3), or PAFR-G alpha(q) was generated and characterized. All receptor variants bound PAF with similar affinities to mediate G-protein activation, intracellular Ca2+ mobilization, phosphoinositide (PI) hydrolysis, and secretion of beta-hexosaminidase. PAFR-G alpha(i3) and PAFR-G alpha(q) mediated greater GTPase activity in isolated membranes than PAFR but lower PI hydrolysis and secretion in whole cells. PAFR and PAFR-G alpha(i3), but not PAFR-G alpha(q), mediated chemotaxis to PAF. All three receptors underwent phosphorylation and desensitization upon exposure to PAF but only PAFR translocated beta arrestin to the cell membrane and internalized. In RBL-2H3 cells coexpressing the PAFRs along with CXCR1, IL-8 (CXCL8) cross-desensitized Ca2+ mobilization to PAF by all the receptors but only PAFR-G alpha(i3) activation cross-inhibited the response of CXCR1 to CXCL8. Altogether, the data indicate that G(i) exclusively mediates chemotactic and cross-regulatory signals of the PAFR, but both G(i) and G(q) activate PI hydrolysis and exocytosis by this receptor. Because chemotaxis and cross-desensitization are exclusively mediated by G(i), the data suggest that differential activation of both G(i) and G(q) by PAFR likely mediate specific as well as redundant signaling pathways.