Prevention of DNA photodamage by vitamin E compounds and sunscreens: Rolesof ultraviolet absorbance and cellular uptake

Topical application of alpha-tocopherol (alpha TH), the most prominent natu rally occurring form of vitamin E, inhibits ultraviolet (UV) B-induced phot ocarcinogenesis and DNA photodamage in C3H mice in vivo. In this study, we compared alpha TH with other vitamin E compounds and with three commercial sunscreen compounds for their ability to inhibit DNA photodamage in C3H mou se skin in vivo. When applied in a 5% dispersion in a neutral cream vehicle , alpha-tocopherol (alpha TH), gamma-tocopherol (gamma TH), and delta-tocop herol (delta TH) each produced a statistically significant inhibition of th ymine dimer formation, whereas alpha-tocopherol acetate (alpha TAc) and alp ha-tocopherol methyl ether (alpha TOMe) did not. Application of 5% dispersi ons of the commercial sunscreen agent octylmethoxycinnamate also inhibited dimer formation, whereas ethylhexyl salicylate and oxybenzone did not, desp ite their considerably greater UVB absorbances than alpha TH. To test the h ypothesis that cellular uptake and distribution are necessary for optimal p hotoprotection by tocopherols, photoprotection was studied in mouse 308 ker atinocyte cells in vitro. Preincubation of 308 cells with 1 mu M alpha TH f or at least 2 h before exposure to 2.5 J/m(2)/s UVB for 10 min significantl y (P < 0.05) attenuated thymine dimer formation. Pre-incubation with 1 mu M gamma TH, delta TH, alpha TAc, or alpha TOMe for 2 h did not inhibit thymi ne dimer formation significantly. Uptake of alpha TH was measured after inc ubation with 1 mu M [H-2(3)]alpha TH (d(3)-alpha TH) and resulted in a time -dependent increase in alpha TH levels. Use of d(3)-alpha TH allowed separa te, simultaneous measurement of added alpha TH and unlabeled endogenous alp ha TH by gas chromatography-mass spectrometry. Accumulation of 167 +/- 62 p mol alpha TH/mg protein was measured within 1 h in whole-cell fractions, d( 3)-alpha TH in the nuclear fraction reached levels of 15 +/- 4 pmol d(3)-al pha TH/mg protein at 2 h. Accumulation of alpha TH in the whole cell and nu clei corresponded temporally with significant protection against DNA photod amage. The kinetics of accumulation of the three tocopherols in whole cells and in nuclei were similar. Although only alpha TH conferred significant p rotection compared with irradiated controls at 2 h, the differences between individual tocopherols were not statistically significant. This work sugge sts that incorporation of tocopherol compounds into sunscreen products conf ers protection against procarcinogen ic DNA photodamage and that cellular u ptake and distribution of tocopherol compounds is necessary for their optim al photoprotection. Mol. Carcinog. 24:169-176, 1999. (C) 1999 Wiley-Liss, I nc.