Tamoxifen inhibits bone resorption by disrupting calmodulin-dependent processes. 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 calmodulin levels four-fold (k0.5 = 0.1-0.3 microM). In contrast, tamoxifen inhibited osteoclast activity approximately 60% with an IC50 of 1.5 microM, had no apparent effect on protein kinase C activity in whole-cell lysates, and reduced protein kinase C alpha recovered by immunoprecipitation 75%. Phorbol esters stimulated resorption in a time-dependent manner that was closely correlated with a similar-fold increase in calmodulin. Protein kinase C alpha, beta, delta, epsilon, and zeta were all down-regulated in response to phorbol ester treatment. Tamoxifen and trifluoperazine inhibited PMA-dependent increases in bone resorption and calmodulin by 85 +/- 10%. Down-regulation of protein kinase C isoforms by phorbol esters suggests that the observed increases in bone resorption and calmodulin levels are most likely due to a mechanism independent of protein kinase C and dependent on calmodulin. In conclusion, the data suggest that protein kinase C negatively regulates calmodulin expression and support the hypothesis that the effects of both phorbol esters and tamoxifen on osteoclast activity is mediated by calmodulin.