Modulation of protein kinase C by curcumin; inhibition and activation switched by calcium ions.
OBJECTIVE
Previous studies have identified the natural polyphenol curcumin as a protein kinase C (PKC) inhibitor. In contrast, we found significant stimulation of PKC activity following curcumin treatment. Thus, the mechanism of curcumin interaction with PKC was investigated.


METHODS
We employed phosphorylation assays in the presence of soluble or membrane-bound PKC substrates, followed by SDS-PAGE, autoradiography and phosphorylation intensity measurements.


RESULTS
Curcumin inhibited PKC in the absence of membranes whereas stimulation was observed in the presence of membranes. Further analysis indicated that curcumin decreased PKC activity by competition with Ca(2+) stimulation of the kinase, resulting in inhibition of activity at lower Ca(2+) concentrations and stimulation at higher Ca(2+) concentrations. The role of the membrane is likely to be facilitation of Ca(2+)-binding to the kinase, thus relieving the curcumin inhibition observed at limited Ca(2+) concentrations. Curcumin was found to mildly stimulate the catalytic subunit of PKC, which does not require Ca(2+) for activation. In addition, studies on Ca(2+)-independent PKC isoforms as well as another curcumin target (the sarcoplasmic reticulum Ca(2+)-ATPase) confirmed a correlation between Ca(2+) concentration and the curcumin effects.


CONCLUSIONS
Curcumin competes with Ca(2+) for the regulatory domain of PKC, resulting in a Ca(2+)-dependent dual effect on the kinase. We propose that curcumin interacts with the Ca(2+)-binding domains in target proteins. To our knowledge, this is the first study that defines an interaction domain for curcumin, and provides a rationale for the broad specificity of this polyphenol as a chemopreventive drug.
Mahmmoud
Y A
YA
Institute of Physiology and Biophysics, Ole Worms Allé 1185, University of Aarhus, Aarhus C, Denmark. yam@biophys.au.dk
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001345:Autoradiography; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D020134:Catalytic Domain; D002413:Cations, Divalent; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D003474:Curcumin / Q000737:chemistry* / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000737:chemistry / Q000378:metabolism; D008081:Liposomes / Q000737:chemistry; D010766:Phosphorylation; D011493:Protein Kinase C / Q000737:chemistry* / Q000378:metabolism; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D013268:Stimulation, Chemical
D002413:Cations, Divalent; D007527:Isoenzymes; D008081:Liposomes; D021122:Protein Subunits; D000255:Adenosine Triphosphate; D011493:Protein Kinase C; D000252:Calcium-Transporting ATPases; D003474:Curcumin; D002118:Calcium