Tamoxifen inhibits phorbol ester stimulated osteoclastic bone resorption: An effect mediated by calmodulin

Tamoxifen inhibits bone resorption by disrupting calmodulin-dependent proce sses. Since tamoxifen inhibits protein kinase C in other cells, we compared the effects of tamoxifen and the phorbol ester, phorbol myristate acetate, on osteoclast activity. Phorbol esters stimulate bone resorption and calmo dulin levels four-fold (k(0.5) = 0.1-0.3 muM). In contrast, tamoxifen inhib ited osteoclast activity similar to 60% with an IC50 of 1.5 muM, had no app arent effect on protein kinase C activity in whole-cell lysates, and reduce d protein kinase Calpha recovered by immunoprecipitation 75%. Phorbol ester s stimulated resorption in a time-dependent manner that was closely correla ted with a similar-fold increase in calmodulin. Protein kinase C alpha, bet a, delta, epsilon, and zeta were all down-regulated in response to phorbol ester treatment. Tamoxifen and trifluoperazine inhibited PMA-dependent incr eases in bone resorption and calmodulin by 85 +/- 10%. Down-regulation of p rotein kinase C isoforms by phorbol esters suggests that the observed incre ases in bone resorption and calmodulin levels are most likely due to a mech anism independent of protein kinase C and dependent on calmodulin. In concl usion, the data suggest that protein kinase C negatively regulates calmodul in expression and support the hypothesis that the effects of both phorbol e sters and tamoxifen on osteoclast activity is mediated by calmodulin.