Intracellular Signaling Mechanism of Bradykinin in Osteoblast-Like Cells

Comparison with Prostaglandin E₂

Abstract

Bradykinin is recognized to be involved in the process of bone resorption in chronic inflammatory diseases. We previously reported that prostaglandin E₂ (PGE₂), known as a potent bone resorbing agent, induces phosphoinositide hydrolysis, cAMP production and Ca²⁺ influx in osteoblast-like MC3T3-E1 cells, and these dose-dependencies are different to one another. To clarify the signaling mechanism of bradykinin, we compared the intracellular signaling system of bradykinin with that of PGE₂ in these cells. Bradykinin stimulated Ca²⁺ influx dose-dependently in the range between 0.1nM and 0.1μM even in the presence of nifedipine, a Ca²⁺ antagonist that inhibits the voltage-dependent L-type Ca²⁺ channel. The maximum effect of bradykinin (0.1μM) on Ca²⁺ influx was almost as great as that of PGE₂ (0.5μM). Bradykinin had little effect on cAMP accumulation, while PGE₂ significantly stimulated it. Bradykinin stimulated the formation of inositol phosphates much less strongly than PGE₂. Bradykinin stimulated inositol 1, 4, 5-trisphosphate [Ins(1, 4, 5)P₃] formation dose-dependently between 0.1nM and 0.1μM, and the dose-dependent curves of bradykinin-induced Ca²⁺ influx and Ins(1, 4, 5)P₃ were similar. However, the maximum effect of PGE₂ (10μM) on Ins (1, 4, 5) P₃ formation was about 2-fold higher than that of bradykinin (0.1μM). These results suggest that bradykinin induces Ca²⁺ influx independent of the voltage-dependent L-type Ca²⁺ channel and phosphoinositide hydrolysis in a similar dose-dependent manner in osteoblast-like cells.