The cloned platelet thrombin receptor couples to at least two distinct effectors to stimulate phosphoinositide hydrolysis and inhibit adenylyl cyclase.

Abstract

Thrombin both stimulates phosphoinositide hydrolysis and inhibits adenylyl cyclase in a variety of cell types. Whether the cloned human platelet thrombin receptor accounts for both of these signaling events is unknown. We report that thrombin receptor agonist peptide causes both phosphoinositide hydrolysis and inhibition of adenylyl cyclase in naturally thrombin-responsive CCL-39 cells. To exclude the possibility that the agonist peptide or thrombin itself may activate these pathways via distinct receptors and to circumvent a lack of suitable thrombin receptor-null cells, we utilized a designed "enterokinase receptor," a thrombin receptor with its thrombin cleavage recognition sequence LDPR replaced by DDDDK, the enterokinase cleavage recognition sequence. Transfection of enterokinase-unresponsive cells with this construct conferred both enterokinase-sensitive phosphoinositide hydrolysis and inhibition of adenylyl cyclase. The phosphoinositide hydrolysis response was largely insensitive to pertussis toxin, whereas the adenylyl cyclase response was completely blocked by pertussis toxin. These data show that the cloned thrombin receptor can effect both phosphoinositide hydrolysis and inhibition of adenylyl cyclase via at least two distinct effectors, most likely Gq-like and Gi-like G-proteins.