The thrombin receptor extracellular domain contains sites crucial for peptide ligand-induced activation

A thrombin receptor (TR) demonstrating a unique activation mechanism has recently been isolated from a megakaryocytic (Dami) cell line. To further study determinants of peptide ligand-mediated activation phenomenon, we have isolated, cloned, and stably expressed the identical receptor from a human umbilical vein endothelial cell (HUVEC) library. Chinese hamster ovary (CHO) cells expressing a functional TR (CHO-TR), platelets, and HUVECs were then used to specifically characterize α-thrombin- and peptide ligand-induced activation responses using two different antibodies: anti-TR^34-52 directed against a 20-amino acid peptide spanning the thrombin cleavage site, and anti-TR^1-160 generated against the NH₂-terminal 160 amino acids of the TR expressed as a chimeric protein in Escherichia coli. Activation-dependent responses to both α-thrombin (10 nM) and peptide ligand (20 μM) were studied using fura 2-loaded cells and microspectrofluorimetry. Whereas preincubation of CHO-TR with anti-TR^34-52 abolished only α-thrombin-induced [Ca²⁺]_i transients, preincubation with anti-TR^1-160 abrogated both α-thrombin- and peptide ligand-induced responses. This latter inhibitory effect was dose dependent and similar for both agonists, with an KC₅₀ of ∼ 90 μg/ml. Anti-TR^1-160 similarly abolished peptide ligand-induced [Ca²⁺]_i transients in platelets and HUVECs, whereas qualitatively different responses characterized by delayed but sustained elevations in [Ca²⁺]_i, transients were evident using α-thrombin. Platelet aggregation to low concentrations of both ligands was nearly abolished by anti-TR^1-160, although some shape change remained; anti-TR^34-52 only inhibited α-thrombin-induced aggregation. These data establish that a critical recognition sequence for peptide ligand-mediated receptor activation is contained on the NH2-terminal portion of the receptor, upstream from the first transmembrane domain. Furthermore, α-thrombin-induced activation of HUVECs and platelets may be partially mediated by an alternative mechanism(s) or receptor(s).