| ABSTRACT: |
|
The anti-oxidant and the anti-tumor-promotion activities of several
hydrolyzable tannins (HTs), including a commercial tannic-acid (TA)
mixture, were examined in mouse skin treated with
12-O-tetradecanoylphorbol-13-acetate (TPA) in vivo. A single application
of TPA gradually increases the hydroperoxide (HPx)-producing activity of
the epidermis, which is maximally stimulated at 3 days and returns to
control levels at 9 days. Pre-treatments with TA and ellagic acid (EA)
strongly inhibit, in a dose-dependent manner, this HPx response to TPA.
Total inhibition by TA lasts for about 16 hr, beyond which it is
substantially reduced but not completely lost. TA can also reduce the
level of epidermal HPx when it is applied 36 hr after the tumor promoter.
EA is an antioxidant 10 times more potent than TA and n-propyl gallate
(PG), which are equally effective against TPA-induced HPx production.
Gallic acid is the least effective of the HTs in inhibiting HPx formation.
TA also inhibits the production of HPx induced by several structurally
different tumor promoters and the greater HPx responses produced by
repeated TPA treatments. When applied 20 min before each promotion
treatment, twice a week for 45 weeks, several HTs inhibit the incidence
and yield of papillomas and carcinomas promoted by TPA in initiated skin.
Overall, TA is more effective than EA and PG in inhibiting skin-tumor
promotion by TPA, suggesting that the anti-oxidant effects of HTs are
essential but not sufficient for their anti-tumor-promotion
activity. |