INHIBITORY EFFECTS OF PLANT TANNINS ON ULTRAVIOLET LIGHT-INDUCED EPIDERMAL DNA-SYNTHESIS IN HAIRLESS MICE

Naturally occurring hydrolyzable (HT) and condensed (CT) tannins and t heir monomeric units were tested for their ability to inhibit the stim ulation of DNA synthesis by UVB radiation, Hairless mice were irradiat ed with either single (200 mJ/cm(2)) or multiple (150 mJ/cm(2)) doses of UVB applied at 24 h intervals and epidermal DNA synthesis was measu red 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, I nterestingly, 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 mu ltiple UVB treatment, a response that is higher than the peak occurrin g 48-56 h after a single UVB irradiation. The disappearance of the ear ly period of inhibition when the peak of DNA synthesis shifts to an ea rlier time may be linked to reactive oxygen species brought to the epi dermis by infiltrating leukocytes, which, in turn, act as second messe ngers 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 admin istration. Indeed, the peak stimulation of DNA synthesis is maximally inhibited when 17 mg of Tarapod tannic acid (TA), an HT, are applied t opically 20 min before a single UVB treatment. The polymeric tannins i nhibited DNA synthesis to a greater degree than equal doses of their m onomeric units, gallic acid and catechin, These results suggest that v arious oligomeric MT and CT may be useful against tumor-promoting resp onses associated with the exposure of skin to physical carcinogens.