Inhibitory Effect of Curcumin on TPA (12-O-Tetradecanoyl Phorbol-13-acetate)-Induced Activation of Protein Kinase C Isoenzyme-Epsilon and c-fos Protein Level in Human Keratinocytes

Pornngarm Limtrakul*, Wanida Chearwae and Sasisopin Luekumharn

ABSTRACT: Protein Kinase C (PKC) is a family of isoenzymes that are normally expressed in human keratinocytes. The expression pattern of PKC under induction of 12-O-tetradecanoyl-phorbol-13-acetate (TPA), a potent tumor promoter, is herein reported. Curcumin was evaluated for its effect on TPA-activated PKC-ε because PKC-ε has been found to have unique oncogenic potential and substrate specificity. Curcumin was shown previously to inhibit TPA-induced AP-1 activity. c-fos protein is inducible by TPA and thus is associated with c-jun to result in an increased AP-1 activity. In the present study the effect of curcumin on TPA-induced c-fos protein level was determined. Human keratinocytes were treated with 160 nM TPA for 1, 2 and 18 h. Upon stimulation by TPA for 1 or 2 h, PKC-α and -ε isoenzymes were translocated from cytosol to membrane by 70 or 85 % and they were completely down regulated after treatment for 18 h. However, effects were not found in PKC-δ isoenzyme. Treatment with 20, 40 and 50 µM curcumin inhibited TPA-induced translocation of PKC-ε isoenzyme from the cytosol to the membrane in a dose-dependent manner. Curcumin alone did not affect the translocation of PKC-ε isoenzyme. The expression level of c-fos protein in human keratinocytes was induced by 160 nM TPA with the maximum result at 2 h, as measured by enhanced chemiluminesence Western blotting detection system. Curcumin pretreated cells decreased TPA-induced c-fos expression in a dose-dependent manner. In conclusion, the induction of c-fos expression and PKC-ε activation by TPA occurred in human keratinocytes and this signalling was inhibited by curcumin.

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Department of Biochemistry, Faculty of Medicine, Chiang Mai University, Chaing Mai 50200, Thailand.
* Corresponding author, E-mail: plimtrak@mail.med.cmu.ac.th

Received 17 Sep 2004, Accepted 28 Dec 2004