Thrombin, but not bradykinin, stimulates proliferation in isolated human osteoblasts, via a mechanism not dependent on endogenous prostaglandin formation.

Osteolysis or osteosclerosis often occurs in bone tissue adjacent to chronic inflammatory processes. Numerous cytokines and inflammatory mediators have been implicated as osteoclast-activating agents, explaining inflammation-induced bone resorption. In many cases, the cause of the sclerosis seen in these lesions is less thoroughly investigated. We have studied the effects of thrombin and bradykinin, 2 inflammatory mediators, on the rate of proliferation in isolated human osteoblasts (hOBs). Thrombin, at and above 1 U/mL, stimulated the rate of thymidine incorporation into hOBs. The absolute cell number also increased, as measured by an assay based on the detection of cell metabolism. A synthetic peptide ligand for the thrombin receptor enhanced the rate of [3H]thymidine incorporation in hOBs, indicating that thrombin-induced proliferation is mediated via the tetheric thrombin receptor. The thrombin-induced proliferation was not affected by indomethacin, excluding prostanoids as mediators of this effect. Bradykinin did not affect either the rate of thymidine incorporation, or number of cells in long-term cultures of hOBs. In conclusion, the inflammatory mediator, thrombin, stimulates proliferation in isolated human osteoblasts probably via the recently described G-protein-coupled tetheric thrombin receptor. Thrombin may therefore be involved as a mediator of inflammation-induced sclerosis and bone formation.