Dual coupling of the alpha-thrombin receptor to signal-transduction pathways involving phosphatidylinositol and phosphatidylcholine metabolism.

Addition of alpha-thrombin to quiescent IIC9 cells results in the activation of lipid-metabolizing enzymes associated with signal-transduction cascades. These enzymes include phosphatidylinositol (PI)-specific phospholipase C (PI-PLC), phosphatidylcholine (PC)-specific phospholipases C and D and phospholipase A2 (PLA2). Whereas the alpha-thrombin receptor has been shown to couple with PI-PLCs, it is not clear whether this receptor, or a putative second receptor, couples to one or more of the other phospholipases. In this report we determine whether the cloned receptor couples to all or a subset of these enzymes. We show that (i) an alpha-thrombin-receptor-activating peptide also elicits the above responses and (ii) addition of enterokinase to IIC9 cells, stably transfected with an alpha-thrombin receptor (enterokinase- responsive alpha-thrombin receptor, EKTR) containing an enterokinase cleavage site in place of an alpha-thrombin cleavage site, stimulates both PI and PC hydrolysis, including PLA2. Enterokinase also induces mitogenesis in the IIC9s transfected with EKTR. These results indicate that, in addition to initiating a mitogenic signalling cascade, the cloned alpha-thrombin receptor couples to enzymes involved in generating PC-derived, as well as PI-derived, second-messenger molecules in IIC9s. Additionally, using the cells transfected with EKTR, we further demonstrate that only activated, i. e. cleaved, receptors are desensitized.