Tannins protect against skin tumor promotion induced by ultraviolet-B radiation in hairless mice.

We recently showed that Tarapod tannic acid (TA), a hydrolyzable tannin extracted from the pods of the Tara tree (Caesalpinia spinosa), was more effective than other tannins tested at inhibiting ultraviolet-B (UV-B)-stimulated hydrogen peroxide activity (an indirect measure of free radicals) in the skin of hairless mice. We also found that Tarapod TA inhibited UV-B-induced ornithine decarboxylase activity and UV-B-stimulated DNA synthesis, two biochemical markers linked to the skin tumor-promoting ability of this physical carcinogen. For this reason, we examined the effect of topical application, force feeding (gavage), and intraperitoneal injections of Tarapod TA on mouse skin chronically treated with UV-B light. Mice were initiated by a single topical application of 7,12-dimethylbenz[a]anthracene (50 nmol) and promoted by two weekly treatments with UV-B light (250 mJ/cm2) for 25 weeks. Topical application of Tarapod TA, 20 minutes before irradiation, resulted in a dose-dependent inhibition of tumor incidence (number of mice with tumors) and tumor yield (number of tumors/mouse), with 8 mg of TA inhibiting tumor yield by 70% at Week 25. Intraperitoneal injections of low doses (10 mg/kg mouse body wt), but not of high doses (25 mg/kg body wt), of TA afforded protection against UV-B-induced papillomas. However, the protection by intraperitoneal injection was lower than that observed by topical application: 10 mg/kg body wt of TA reduced tumor yield by 55%. The force feeding of 10 mg of Tarapod TA before irradiation failed to significantly inhibit the yield of tumors at the end of the experiment but delayed tumor appearance by six weeks. These results suggest that plant tannins administered topically or injected intraperitoneally reduce the tumor-promoting effects of UV-B radiation and thus could be useful photoprotectants.