Use of minoxidil to demonstrate that prostacyclin is not the mediator of bone resorption stimulated by growth factors in mouse calvariae.

Growth factors, such as human transforming growth factor-alpha (hTGF alpha) and epidermal growth factor, as well as human tumor necrosis factor (hTNF) stimulate the resorption of bone in neonatal mouse calvariae in organ culture via a prostaglandin (PG)-mediated pathway. In response to such factors mouse calvariae produce substantial quantities of prostaglandin E2 (PGE2) and prostacyclin (PGI2). We have selectively inhibited the production of PGI2, but not PGE2, using the drug minoxidil and have measured the effects on stimulated bone resportion and arachidonic acid metabolism. The increased production of 6-keto-PGF1 alpha (6k-PGF1 alpha), the hydrolytic product of PGI2, stimulated by recombinant hTGF alpha and hTNF as well as murine epidermal growth factor was inhibited by minoxidil. There was no inhibition by minoxidil of PGE2 production. Despite essentially complete inhibition of stimulated 6k-PGF1 alpha formation, there was no inhibition of bone resorption. The possibility was investigated that growth factors and TNF enhanced enzymatic conversion of PGI2 to 6k-PGE1, which stimulates bone resorption in mouse calvariae with a potency about one fourth that of PGE2. Enzymatic conversion of PGI2 to 6k-PGE1 is inhibited by rutin. Rutin did not inhibit bone resorption stimulated by hTGF alpha or hTNF. We conclude, on the basis of these new results and previously published data, that the cyclooxygenase product that acts as the mediator of bone resorption enhanced by growth factors and TNF in mouse calvariae is probably PGE2.