Cross-Desensitization of Chemoattractant Receptors Occurs at Multiple Levels: Evidence for a Role for Inhibition of Phospholipase C Activity

To define the molecular mechanisms of cross-regulation among chemoattractant receptors, we stably coexpressed, in a rat basophilic leukemia (RBL-2H3) cell line, epitope-tagged receptors for the chemoattractants formylmethionylleucylphenylalanine (fMLP), a peptide of the fifth component of the complement system (C5a), and interleukin-8 (IL-8). All the expressed receptors underwent homologous phosphorylation and desensitization upon agonist stimulation. When co-expressed, epitope-tagged C5a receptor (ET-C5aR) and epitope-tagged IL-8 receptor (ET-IL-8RA) were cross-phosphorylated by activation of the other. Activation of epitope- tagged fMLP receptor (ET-FR) also cross-phosphorylated ET-C5aR and ET-IL- 8RA, but ET-FR was totally resistant to cross-phosphorylation. Similarly, C5a and IL-8 stimulation of [35S]guanosine 5'-3-P-(thio) triphosphate (GTPγS) binding and Ca2+ mobilization were cross-desensitized by each other and by fMLP. Stimulation of [35S]GTPγS binding by fMLP was also not cross- desensitized by C5a or IL-8, however, Ca2+ mobilization was, suggesting a site of inhibition distal to G protein activation. Consistent with this desensitization of Ca2+ mobilization, inositol 1,4,5-trisphosphate release in RBL-2H3 cells expressing both ET-C5aR and ET-FR revealed that fMLP and C5a cross-desensitized each other's ability to stimulate phosphoinositide hydrolysis. Taken together, these results indicate that receptor cross- phosphorylation correlates directly with desensitization at the level of G protein activation. The ET-FR was resistant to this process. Of note, cross- desensitization of ET-FR at the level of phosphoinositide hydrolysis and Ca2+ mobilization was demonstrated in the absence of receptor phosphorylation. This suggests a new form of chemoattractant cross-regulation at a site distal to receptor/G protein coupling, involving the activity of phospholipase C.