Mechanisms of Desensitization and Resensitization of Proteinase-activated Receptor-2

Proteinase-activated receptor-2 (PAR-2) is a G-protein-coupled receptor that is expressed by intestinal epithelial cells, which are episodically exposed to pancreatic trypsin in the intestinal lumen. Trypsin cleaves PAR-2 to expose a tethered ligand, which irreversibly activates the receptor. Thus, PAR-2 may desensitize and resensitize by novel mechanisms. We examined these mechanisms in kidney epithelial cells, stably expressing human PAR-2, and intestinal epithelial cells, which naturally express PAR-2. Trypsin stimulated a prompt increase in [Ca 2+ ] i , due to mobilization of intracellular Ca 2+ , followed by a sustained plateau, due to influx of extracellular Ca 2+ . Repeated application of trypsin caused marked desensitization of this response, which is due in part to ( a ) irreversible cleavage of the receptor by trypsin and ( b ) protein kinase C-mediated termination of signaling. Trypsin exposure resulted in internalization of PAR-2 into early endosomes and then lysosomes; but endocytosis was not the mechanism of rapid desensitization. Thus, activated PAR-2 is endocytosed and degraded. The Ca 2+ response to trypsin resensitized by 60-90 min. Brefeldin A, which disrupted Golgi stores of PAR-2, and cycloheximide, which inhibited protein synthesis, markedly attenuated resensitization. Thus, PAR-2-mediated Ca 2+ mobilization desensitizes by irreversible receptor cleavage, protein kinase C-mediated termination of signaling, and PAR-2 targeting to lysosomes. It resensitizes by mobilization of large Golgi stores and synthesis of new receptors.