Molecular Cloning of the Platelet P2TAC ADP Receptor: Pharmacological Comparison with Another ADP Receptor, the P2Y1 Receptor

Platelet activation plays an essential role in thrombosis. ADP-induced platelet aggregation is mediated by two distinct G protein-coupled ADP receptors, Gq-linked P2Y(1), and Gi-linked P2T(AC), which has not been cloned. The cDNA encoding a novel G protein-coupled receptor, termed HORK3, was isolated. The HORK3 gene and P2Y(1) gene were mapped to chromosome 3q21-q25. HORK3, when transfected in the rat glioma cell subline (C6-15), responded to 2-methylthio-ADP (2MeSADP) (EC(50) = 0.08 nM) and ADP (EC(50) = 42 nM) with inhibition of forskolin-stimulated cAMP accumulation. 2MeSADP (EC(50) = 1.3 nM) and ADP (EC(50) = 18 nM) also induced intracellular calcium mobilization in P2Y(1)-expressing cells. These results show that HORK3 is a Gi/o-coupled receptor and that its natural ligand is ADP. AR-C69931 MX and 2MeSAMP, P2T(AC) antagonists, selectively inhibited 2MeSADP-induced adenylyl cyclase inhibition in HORK3-expressing cells. On the other hand, A3P5PS, a P2Y(1) antagonist, blocked only 2MeSADP-induced calcium mobilization in P2Y(1)-expressing cells. HORK3 mRNA was detected in human platelets and the expression level of HORK3 was equivalent to that of P2Y(1). These observations indicate that HORK3 has the characteristics of the proposed P2T(AC) receptor. We have also determined that [(3)H]2MeSADP binds to cloned HORK3 and P2Y(1). Competition binding experiments revealed a similarity in the rank orders of potency of agonists and the selectivity of antagonists as obtained in the functional assay. These results support the view that P2Y(1) functions as a high-affinity ADP receptor and P2T(AC) as a low-affinity ADP receptor in platelets.