Modulation of protein kinase C by curcumin; inhibition and activation switched by calcium ions.

Background And Purpose: 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.

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

Key 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 And Implications: 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.