Protein phosphatase inhibitors and bone resorption: inhibition by okadaic acid and biphasic actions of calyculin-A.

PTH receptors on osteoblasts and calcitonin receptors on osteoclasts are coupled to adenylate cyclase. Despite similar transduction mechanisms, these hormones have opposing physiological actions. We investigated the consequences of persistent protein phosphorylation on bone resorption in neonatal mouse calvariae using okadaic acid (OA) and calyculin-A, two inhibitors of protein phosphatase-1 and -2A. These two inhibitors caused different responses in bone at picomolar and low nanomolar concentrations. OA inhibited, in a dose-dependent manner, bone resorption stimulated by PTH, 1,25-Dihydroxyvitamin D3, phorbol ester, and prostaglandin E2 (PGE2). OA did not inhibit the generation of the second messengers cAMP or PGs and did not have nonspecific toxic effects, as measured by protein and RNA synthesis. Thus, OA appeared to mimic the global inhibitory action of calcitonin on bone resorption. Unlike OA, calyculin-A elicited a biphasic dose response. At concentrations of 3.3 nM and greater, calyculin-A inhibited, in a dose-dependent manner, stimulated bone resorption. However, calyculin-A alone, at 0.625 and 2.5 nM, stimulated bone resorption via a PG-independent pathway. In calvariae, OA and calyculin-A increased phosphorylation of a 58- to 60-kilodalton protein. A protein of similar molecular mass was hyperphosphorylated in OA-treated ROS 17/2.8 osteoblast-like cells. We conclude that in addition to hormonal regulation of protein kinase activity, protein dephosphorylation plays a functionally important role in the modulation of bone resorption.