SKIN DELIVERY OF VITAMIN E 259 3 2 1 0 Dermis Epidermis [] Control [] UV+ Vit E Figure 8. Lipid hydroperoxide levels in epidermis and dermis from hairless mice after topical vitamin E application and ultraviolet irradiation. Data are mean _+ SE. n = 5. P 0.05 compared with non-irradiated group. Adapted from reference 48. Ascorbate levels were increased both in epidermis and dermis, and total glutathione and GSH levels increased in the epidermis. Thus the skin's antioxidant system could be modulated by changing a single network parameter, demonstrating the interdependence of the various antioxidant species involved. Topical ot-tocopherol treatment is capable of enhancing the overall antioxidant capacity of murine skin. Although the underlying mechanism is unclear, it appears that strongly increased ot-tocopherol concentrations modulate gene regulation of enzymatic antioxidants, either directly or indirectly by lower cellular levels of reactive oxygen species. However, as ascorbate cannot be syn- thesized by murine skin cells, other mechanisms such as a decreased demand for its activity as a free radical quencher may apply for this vitamin. Protection against UV- induced lipid peroxidation may in part be due to increased levels and improved anti- oxidant capacity of these co-oxidants. This study demonstrated that topical ot-tocopherol administration modulates enzymatic and non-enzymatic antioxidant levels in the skin and significantly inhibits lipid per- oxidation induced by UV irradiation. UV irradiation can affect the antioxidant concen- trations in three ways: (a) direct absorbance of light (b) reaction with reactive oxygen species generated by interaction between photosensitizers and UV light and (c) anti- oxidant recycling mechanisms, where one antioxidant is spared at the expense of another. Several groups have studied the potential protective effects of various antioxidants, including vitamin E, against UV exposure. Khettab and colleagues (49) reported that topical application of the vitamin diminished the production of lipid peroxides in the epidermis. There was a 60% reduction in the production ofmalondialdehyde in the skin. Malondialdehyde is the end product of peroxidation of unsaturated fatty acid by oxy-

260 JOURNAL OF COSMETIC SCIENCE genated free radicals. Kaidby and Kligman (50) found the levels of lipid peroxides in the skin of vitamin E-deficient rats to be greater than those of control animals, although there was only a modest increase after UV exposure. Record eta/. (45) compared oral and topical vitamin E application for three weeks before a single exposure of UV light. Lipid peroxidation levels measured as the malondialdehyde content was used to estimate the degree of damage caused by the radiation. They found the topical form of the vitamin E to be far more effective at preventing the increase in lipid peroxidation than the dietary supplement, probably because of the higher skin levels attained. They concluded that after topical application, vitamin E is retained in the epidermis for at least 24 h while still affording protection, suggesting that vitamin E is not merely acting as a sunscreen but that it reduces free radical-driven damage to the cell membrane. Simulation modeling allows calculation of TBARS, which are the thiobarbituric reacting substances, as a marker for the peroxidation process: PUFA x Perox TBARS = (Eq. 1) Tocopherol + Antioxidant PUFA x Perox TBARS = (Eq. 2) Log (tocopherol) PUFA are the amounts of polyunsaturated fatty acids with three to six double bonds. Perox is the peroxidizability of PUFA. Tocopherol is the amount of total tocopherol, and antioxidant is the amount of antioxidant in addition to total tocopherol. Topical appli- cation of a 0.25% (x-tocopherol solution in repeated doses before and after UV irradia- tion significantly reduces the formation of TBARS in mouse skin (49). Topical 5% c•-tocopherol sorbate (0.1 ml applied for three weeks, three times per week) significantly reduced UVB-induced formation of lipid-derived alkoxyl radicals in skin of hairless mice (51). Topical application as a 5% c•-tocopheryl acetate solution or a 5% [•-carotene solution 15 min prior to UVB/A exposure on mouse skin reduced the formation of UV-induced TBARS (52). Girard et aL (53) describe a cutaneous peroxide measurement method using a fluorogenic probe to determine free-radical activity on the stratum corneum and provide a quantitative evaluation of the scavenging effect of cosmetic and dermopharmaceutical products. PHOTOCARCINOGENESIS Skin cancer is the most prevalent type of human malignancy. Epidemiological and experimental evidence indicates that UV radiation in sunlight induces non-melanoma skin cancer (54,55). One of the contributing factors is speculated to be the UVB-induced suppression of cell-mediated immunity, predisposing to the development of skin cancer and infection. UVB causes local immunosuppression even at suberythemal UVB doses (56). UV irradiation is known to induce free radicals and lipid peroxidation in the skin, which may lead to DNA damage and cancer (57,58). The finding that a number of antioxidants can reduce photocarcinognesis in mice (59,60) suggests that oxidative reactions are important in photocarcinogenesis and can serve as targets for skin cancer prevention. Topical vitamin E treatment has been found to inhibit immunosuppression and tumorigenesis induced by UV irradiation. UVB irradiation from sunlight can directly modify nucleic acids by forming cyclobutane