Tamoxifen inhibits phorbol ester stimulated osteoclastic bone resorption: an effect mediated by calmodulin.
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.
Williams
J P
JP
Department of Internal Medicine, University of Kentucky, Lexington 40536, USA. jpwill2@pop.uky.edu
McKenna
M A
MA
Thames
A M
AM
McDonald
J M
JM
D000818:Animals; D001862:Bone Resorption / Q000503:physiopathology*; D002147:Calmodulin / Q000378:metabolism*; D002462:Cell Membrane / Q000201:enzymology; D002478:Cells, Cultured; D002645:Chickens; D003600:Cytosol / Q000201:enzymology; D015536:Down-Regulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D007527:Isoenzymes / Q000378:metabolism; D010010:Osteoclasts / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism; D013629:Tamoxifen / Q000494:pharmacology*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D014268:Trifluoperazine / Q000494:pharmacology
D002147:Calmodulin; D004791:Enzyme Inhibitors; D007527:Isoenzymes; D013629:Tamoxifen; D014268:Trifluoperazine; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate