Characterization of the receptor responsible for thrombin-induced intracellular calcium responses in osteoblast-like cells.

The receptor responsible for the increase in intracellular calcium concentration ([Ca2+]i) after the addition of thrombin to the human osteoblast-like cell line Saos-2 has been characterized. Thrombin caused a dose-dependent increase in [Ca2+]i; a half-maximal stimulation was observed with 3.2 +/- 1.1 nM thrombin. The human platelet thrombin receptor is activated by thrombin cleavage to create a new NH2 terminus that acts as a tethered ligand, and peptides based on the tethered ligand can activate the receptor independently of thrombin. Northern analysis indicated the presence of mRNA encoding the platelet receptor in Saos-2 cells, and surface expression of the receptor was demonstrated by immunocytochemistry. A tethered ligand peptide (SFLLRNPNDKYEPF, single-letter amino acid code) was found to increase [Ca2+]i. The maximal response to the peptide was similar to that observed with thrombin, and a half-maximal response was observed with 22 +/- 6 microM peptide. The time course of the increase in [Ca2+]i with the peptide was different than that observed with thrombin; a pronounced shoulder was observed after an initial sharp rise. The phenylalanine in the second position of the agonist peptide and the arginine in the fifth position were shown to be essential for its activity. The requirement for proteolysis of the receptor for the thrombin-dependent increase in [Ca2+]i was demonstrated by two methods. Antibodies that reacted with the cleavage site of the receptor abolished the effect of thrombin on [Ca2+]i. In addition, a mutant of thrombin without catalytic activity as well as chemically inactivated thrombin failed to cause an increase in [Ca2+]i. Similar results were obtained with the rat osteoblast-like cell line UMR-106; a tethered ligand peptide based on the rat sequence induced an increase in [Ca2+]i, and antibodies to the cleavage site of the rat receptor inhibited the effect of thrombin.