Protein kinase C-dependent phosphorylation regulates osteoclast calcium-sensing

Osteoclasts display a membrane Ca(2+)-sensing mechanism capable of detecting the extracellular calcium concentration ([Ca2+]o), and to induce increase of [Ca2+]i and inhibition of bone resorption. The ultimate result of the stimulation of such sensing is probably the activation of protein kinase C (PKC). To demonstrate whether PKC plays a role in the control of the osteoclast activity, we treated rabbit single osteoclasts with agents known to activate or to inhibit the enzyme. We measured [Ca2+]i in single fura 2-loaded single cells and found that activation of PKC by phorbol esters doubled the [Ca2+]o-induced [Ca2+]i elevation, whereas inhibition of the enzyme by H7, staurosporine or sphingosine, completely blocked the ability of the cell to respond to elevated [Ca2+]i. By contrast, a control inactive agent, 4Aphorbol, failed to modify the cellular response to elevated [Ca2+]o. We conclude that PKC plays a synergistic role in the regulation of osteoclast Ca(2+)-sensing. Since we have previously demonstrated that activation of PKA up-regulates the Ca(2+)-sensing as well, we hypothesize that such mechanism is positively fed-back by both PKA and PKC-dependent threonine/serine phosphorylations.