Parathyroid hormone-induced calcium efflux from cultured bone is mediated by protein kinase C translocation

Activa-tion of adenylate cyclase is believed to be the major intracellular mediator of bone rsorption induced by parathyroid hormone (PTH), prostaglandins, and various bone resorbing cytokines. Studies have demonstrated a dissociation between PTH-induced bone reorption and adenosine 3′,5′-cyclic monophosphate (cAMP) formation, as well as suggested a role of protein kinase C (PKC) in mediating in part the actions of PTH. We therefore investigated the relative contribution of the adenylate cyclase or PKC signal transduction pathways in mediating the PTH-induced net calcium release from cultured neonatal calvariae, an in vitro model of bone résorption. PTH (10^-11 to 10^-7 M) caused a dose-dependent increase in calcium efflux from cultured bone and activated both cAMP and PKC. To determine the role of each of these second messengers in mediating PTH-induced calcium release from bone, calvariae were preincubated with either the adenylate cyclase inhibitor SQ-22536 (10^-5 to 10^-4 M) or the PKC inhibitor staurosporine (10^-7 M) before coincubation with PTH. Compared with control, PTH caused a significant calcium efflux, whereas preincubation with SQ-22536 had no effect on basal calcium efflux and partially inhibited the calcium efflux caused by PTH. In contrast preincubation with staurosporine completely obliterated the PTH-induced calcium efflux. PTH is a potent stimulator of calcium release and activates both the cAMP and PKC signal transduction pathways in cultured bone. Inhibition of PTH-stimulated PKC activity completely abolished the PTH-induced calcium efflux from calvariae, whereas PTH-induced calcium efflux persisted despite adenylate cyclase inhibition. Thus the bone resorbing effect of PTH appears to be dependent predominantly on activation of PKC.