Leukotriene B4Increases Intracellular Calcium Concentration and Phosphoinositide Metabolism in Mouse Osteoblasts via Cyclic Adenosine 3′, 5′-Monophosphate-Independent Pathways*

Abstract

Leukotriene B4 is one of a number of agents which stimulate bone resorption by acting on osteoblasts. Some agonists, such as PTH or prostaglandins, are known to activate adenylate cyclase in osteoblasts, whereas others, such as vitamin D3, have no effect on adenylate cyclase. Recent evidence suggests that both classes of agonist may raise the intracellular calcium concentration, although the relative importance for bone resorption of calcium mobilization and adenylate cyclase activity in the osteoblast is not clear. Here it is shown 1) that leukotriene B4 does not activate but may be inhibitory toward adenylate cyclase in intact osteoblasts or membrane preparations, 2) that leukotriene B4 causes an elevation of intracellular calcium levels in osteoblast monolayers, 3) leukotriene B4 rapidly activates phosphatidylinositol bisphosphate breakdown in osteoblast membranes and intact osteoblasts, and 4) that leukotriene B4 stimulates phosphatidylinositol kinase activity concurrently with phosphoinositidase C in intact osteoblasts over a similar timescale. These results suggest that leukotriene B4 may increase the concentration of intracellular calcium in osteoblasts by stimulating phosphoinositide turnover, and support the proposal that calcium signaling rather than activation of adenylate cyclase in osteoblasts may be of overriding importance in the regulation of bone resorption.