Ability of tannins extracted from the leaves of various trees and shrubs to inhibit the biomarkers of tumor promotion in mouse skin in vivo

Eight heterogeneous tannin samples (HTSs) extracted from various tree/shrub leaves of African and Himalayan origin were tested topically for their ability to inhibit the biomarkers of tumor promotion in mouse skin in vivo. HTS2 (from Dichostachys cinerea) and HTS6 (from Cassia sieberiana) consistently inhibit tumor promoter-stimulated ornithine decarboxylase activity, DNA synthesis, hydroperoxide production, and edema formation almost as much as loblolly pine bark condensed tannin (LPB-CT), which is known to inhibit skin tumor promotion. The other HTSs tested have lesser or only partial inhibitory effects. The ability of HTSs to inhibit the biomarkers of tumor promotion may be related to their reducing power but there is no apparent correlation between their inhibitory effects and their proanthocyanidin contents expressed as absorbance units, protein precipitation activities, and relative degrees of polymerization. HTS6 is effective against a wide spectrum of tumor-promoting agents unrelated to 12-O-tetradecanoyl-phorbol-13-acetate. The antioxidant effects of HTS6 and LPB-CT are similar but do not resemble that of tannic acid. HTS6 and LPB-CT both fail to alter the covalent binding of a tumor-initiating dose of 7,12-dimethylbenz[a] anthracene to DNA but inhibit the stimulation of DNA synthesis caused by a carcinogenic dose of this compound. Some foliage tannins, therefore, have potent antioxidant and anti-inflammatory activities and may inhibit hyperplasia and tumor promotion but their efficacy may vary considerably depending on their origin, chemical composition, and biological properties.