Effects of tannic acid on 12-O-tetradecanoylphorbol-13-acetate-induced protein kinase C activation in NIH 3T3 cells.

Tannic acid (TA) is a naturally occurring phenol, which has been found to display an antipromotion effect on mouse skin carcinogenesis. In order to explore the molecular mechanism, we have examined the process of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced protein kinase C (PKC) activation, including phorbol ester binding, enzyme translocation, autophosphorylation and substrate phosphorylation, and finally the TPA-stimulated DNA synthesis. In an initial study, we found that TA slightly inhibited [3H]phorbol dibutyrate (PDBu) binding to intact cells, and only 30% reduction of phorbol ester binding was observed at the highest dose of TA (100 microM). Further analysis by Scatchard plot showed that TA reduced the Bmax of [3H]PDBu from 1.3 pmol/10(6) cells to 1.1 pmol/10(6) cells, but the Kd was increased from 24 to 30 nM. Analysis by western blot indicated that TA did not interfere with the TPA-induced PKC translocation, whereas TA effectively blocked the TPA-evoked phosphorylation of the membrane-bound PKC moiety and its 80 kDa substrate in a dose-dependent manner. We also found that pre- or post-treatment with TA both lead to a similar reduction of 80 kDa protein phosphorylation and that the TPA-stimulated DNA synthesis was also inhibited by TA in a dose-dependent manner, suggesting that the blockage of protein phosphorylation by TA was of biological significance. In conclusion, the work presented here demonstrated that the antitumor promoting effect of TA was not mediated by competing for the binding site with phorbol ester or interrupting the PKC translocation, but rather by effectively blocking phosphorylation by membrane-bound PKC, possibly through altering the biophysical properties of the membrane environment.