On the role of cyclic AMP as a mediator of bone resorption: γ-Interferon completely inhibits cholera toxin- and forskolin-induced but only partially inhibits parathyroid hormone-stimulated 45Ca release from mouse calvarial bones

The effects of γ-interferon (γ-IFN) on bone resorption and cyclic AMP formation stimulated by parathyroid hormone (PTH), forskolin, and cholera toxin have been studied in cultured neonatal mouse calvarial bones. Bone resorption was assessed by the release of 45Ca from prelabeled mouse calvarial bone fragments. Cyclic AMP formation was quantified by analyzing the amount of the nucleotide in calvarial bone tissue. γ-IFN completely blocked the 45Ca release response to forskolin and cholera toxin in 96 h cultures. In contrast, the 45Ca release response to PTH was only partially inhibited, an effect that was seen over a wide range of PTH concentrations. The inhibitory effect of γ-IFN was dose dependent, with a threshold for action at 10 U/ml. Forskolin-stimulated 45Ca release could only be inhibited when γ-IFN was added simultaneously with forskolin; γ-IFN added to bones prestimulated with forskolin had no effect. The inhibitory effect of γ-IFN on PTH-stimulated 45Ca release was seen first after a time lag of 48 h. In contrast, calcitonin caused an inhibition after only 3 h. PTH and cholera toxin stimulation of radioactive calcium release was also inhibited by γ-IFN in bones treated with indomethacin. γ-IFN inhibited forskolin-induced 45Ca release in bones treated with the mitotic inhibitor hydroxyurea. No effect of γ-IFN on cyclic AMP formation induced by PTH, cholera toxin, or forskolin could be seen. These data show that γ-IFN inhibits forskolin- and cholera toxin-induced bone resorption by a mechanism unrelated to prostaglandin production or mitotic activity. We suggest that γ-IFN causes a delayed inhibition of bone resorption in mouse calvariae by inhibiting the recruitment of new osteoclasts and thereby totally blocks forskolin- and cholera toxin-induced bone resorption. The bone-resorptive effect of PTH is only partially blocked since the initial stages in the mechanism of action of PTH involve rapid activation of existing osteoclasts, insensitive to inhibition by γ-IFN. These data lend further support to the view that PTH stimulation of bone resorption is not solely mediated by cyclic AMP.