Inhibitory Effects of Plant Tannins on Ultraviolet Light‐Induced Epidermal DNA Synthesis in Hairless Mice

Naturally occurring hydrolyzable (HT) and condensed (CT) tannins and their monomeric units were tested for their ability to inhibit the stimulation of DNA synthesis by UVB radiation. Hairless mice were irradiated with either single (200 mJ/cm2) or multiple (150 mJ/cm2) doses of UVB applied at 24 h intervals and epidermal DNA synthesis was measured at different times after the last of these treatments. The peak of DNA synthesis that is observed 48-56 h after a single UVB irradiation shifts to an earlier time of 16-24 h after multiple UVB treatments. Interestingly, the early inhibitory period of DNA synthesis observed 8 h after a single UVB treatment is not detected following multiple UVB treatments. Rather, DNA synthesis is stimulated six-fold 24 h after multiple UVB treatment, a response that is higher than the peak occurring 48-56 h after a single UVB irradiation. The disappearance of the early period of inhibition when the peak of DNA synthesis shifts to an earlier time may be linked to reactive oxygen species brought to the epidermis by infiltrating leukocytes, which, in turn, act as second messengers to stimulate growth signals in cells. Topical applications of HT or CT remarkably inhibit the DNA responses to single and multiple UVB treatments, an effect that is dependent on the dose and time of administration. Indeed, the peak stimulation of DNA synthesis is maximally inhibited when 17 mg of Tarapod tannic acid (TA), an HT, are applied topically 20 min before a single UVB treatment. The polymeric tannins inhibited DNA synthesis to a greater degree than equal doses of their monomeric units, gallic acid and catechin. These results suggest that various oligomeric HT and CT may be useful against tumor-promoting responses associated with the exposure of skin to physical carcinogens.