Photoprotective actions of topically applied vitamin E.

Topical application of vitamin E has been shown to decrease the incidence of ultraviolet (UV)-induced skin cancer in mice. Vitamin E provides protection against UV-induced skin photodamage through a combination of antioxidant and UV absorptive properties. Topical application of alpha-tocopherol on mouse skin inhibits the formation of cyclobutane pyrimidine photoproducts. However, topically applied alpha-tocopherol is rapidly depleted by UVB radiation in a dose-dependent manner. The photooxidative fate of the alpha-tocopherol depends on the local environment of the vitamin E. alpha-Tocopherol quinone and alpha-tocopherol quinone epoxides are principal photoproducts of vitamin E that has penetrated into the epidermal layer of the skin, whereas tocopherol dimers and trimers are formed from alpha-tocopherol in a bulk phase at the skin surface. Dimer and trimer products may participate in prevention of UV-induced photodamage.

Inhibitory effects of deferoxamine on UVB-induced AP-1 transactivation.

Production of reactive oxygen species (ROS) by iron can contribute directly to DNA and protein damage and may contribute to cell signaling and proliferation. We have examined the effects of the iron(III) chelator deferroxamine (DFO) and iron (FeCl(3)) on UVB (290-320 nm)-induced activator protein 1 (AP-1) signaling. The ability of DFO to inhibit UVB-induced AP-1 transactivation was tested in a human keratinocyte cell line stably transfected with a luciferase reporter driven by a single AP-1 element. DFO treatment 24 h prior to UVB irradiation reduced UVB-induced AP-1 transactivation by approximately 80%, with the effect of DFO diminishing as pre-treatment time was shortened. Treatment with FeCl(3) a minimum of 6 h prior to UVB potentiated the UVB induction of AP-1 transactivation by 2-3-fold. DFO was able to ablate both the UVB induction of AP-1 transactivation as well as the potentiation by FeCl(3). The antioxidants Trolox and N-acetyl cysteine were both able to inhibit UVB-induced AP-1 transactivation and Trolox was able to inhibit the potentiation of UVB-induced AP-1 by FeCl(3). These results indicate that UVB-induced AP-1 activation may be in part due to oxidant effects of UVB and intercellular iron.

UV-B-Induced photooxidation of vitamin E in mouse skin.

Topically applied alpha-tocopherol (alpha-TH, vitamin E) inhibits UV-B (290-320 nm) photocarcinogenesis, yet alpha-TH undergoes rapid photooxidation by UV-B in vitro. To examine the effect of UV-B on alpha-TH in vivo, we studied the fate of alpha-TH in UV-B-irradiated mouse skin. [14C]-alpha-TH was applied to mouse skin at various times prior to UV-B irradiation. UV-B irradiation for 1 h at a dose rate of 2.6-2.9 J m-2 s-1 resulted in consumption of 40-60% of the applied dose and formation of oxidation products. The major product fraction formed in UV-B-irradiated mice treated topically with alpha-TH contained an alpha-TH dihydroxy dimer and its two-electron oxidation product, a spirodimer. Products previously identified as being derived from photochemical or peroxyl radical scavenging reactions of alpha-TH were also observed, including alpha-tocopherolquinone (alpha-TQ), alpha-tocopherolquinone 2, 3-epoxide (alpha-TQE 1), alpha-tocopherolquinone 5,6-epoxide (alpha-TQE 2), and 8a-(hydroperoxy)epoxytocopherones. These results indicate that topically applied alpha-TH is extensively oxidized in skin and suggest that alpha-TH photoproducts may be involved in the observed effects of topically applied vitamin E in UV-B-irradiated skin.

Effect of UVB on hydrolysis of alpha-tocopherol acetate to alpha-tocopherol in mouse skin.

We have assessed the hydrolysis of alpha-tocopherol acetate (alpha-TAc) to the active antioxidant alpha-tocopherol (alpha-TH) in mouse epidermis and in supernatant from epidermal homogenates. Topically administered alpha-TH prevents UVB photocarcinogenesis in C3H mice, whereas alpha-TAc does not. Hydrolysis in skin was monitored in mice treated topically with deuterium labeled alpha-TAc (d3-alpha-TAc). Epidermal samples were isolated from mice and analyzed for endogenous (d0-alpha-TAc) and d3-alpha-TH by gas chromatography-mass spectrometry. Within 24 h, the levels of d3-alpha-TH increased up to 10-fold over endogenous d0-alpha-TH levels; however, in mice irradiated with UVB prior to the application of d3-alpha-TAc, levels of d3-alpha-TH increased up to 30-40-fold over endogenous d0-alpha-TH. This enhancement of alpha-TAc hydrolysis increased with increasing UVB dose. Prior UVB exposure may increase hydrolysis of alpha-TAc by increasing epidermal esterase activity. Nonspecific esterase activity was measured in the 2000 x g supernatant from epidermis of unirradiated and irradiated mice. Alpha-napthyl acetate, a nonspecific esterase substrate, was converted to alpha-napthol in supernatants from unirradiated mice. Hydrolysis to alpha-napthol increased approximately 3-fold in supernatants from irradiated mice. Hydrolysis of alpha-TAc to alpha-TH also occurred in supernatant from unirradiated mice, and this hydrolysis increased approximately 3-fold in supernatant from irradiated animals. These data indicate that nonspecific esterase activity was increased by UVB in the skin, that alpha-TAc is converted to alpha-TH in the homogenate fraction containing nonspecific esterase, and that UVB exposure modulates the metabolism of alpha-TAc to alpha-TH in vivo.