Kathrin Schlenz

The deceptive nature of UVA tanning versus the modest protective effects of UVB tanning on human skin.

The relationship between human skin pigmentation and protection from ultraviolet (UV) radiation is an important element underlying differences in skin carcinogenesis rates. The association between UV damage and the risk of skin cancer is clear, yet a strategic balance in exposure to UV needs to be met. Dark skin is protected from UV‐induced DNA damage significantly more than light skin owing to the constitutively higher pigmentation, but an as yet unresolved and important question is what photoprotective benefit, if any, is afforded by facultative pigmentation (i.e. a tan induced by UV exposure). To address that and to compare the effects of various wavelengths of UV, we repetitively exposed human skin to suberythemal doses of UVA and/or UVB over 2 weeks after which a challenge dose of UVA and UVB was given. Although visual skin pigmentation (tanning) elicited by different UV exposure protocols was similar, the melanin content and UV‐protective effects against DNA damage in UVB‐tanned skin (but not in UVA‐tanned skin) were significantly higher. UVA‐induced tans seem to result from the photooxidation of existing melanin and its precursors with some redistribution of pigment granules, while UVB stimulates melanocytes to up‐regulate melanin synthesis and increases pigmentation coverage, effects that are synergistically stimulated in UVA and UVB‐exposed skin. Thus, UVA tanning contributes essentially no photoprotection, although all types of UV‐induced tanning result in DNA and cellular damage, which can eventually lead to photocarcinogenesis.

Pigmentation effects of solar simulated radiation as compared with UVA and UVB radiation

Different wavelengths of ultraviolet (UV) radiation elicit different responses in the skin. UVA induces immediate tanning and persistent pigment darkening through oxidation of pre‐existing melanin or melanogenic precursors, while UVB induces delayed tanning which takes several days or longer to develop and requires activation of melanocytes. We compared the effects of a 2‐week repetitive exposure of human skin to solar‐simulated radiation (SSR), UVA or UVB at doses eliciting comparable levels of visible tanning and measured levels of melanins and melanin‐related metabolites. Levels of eumelanin and pheomelanin were significantly higher in the order of SSR, UVB, UVA or unexposed control skin. Levels of free 5‐S‐cysteinyldopa (5SCD) were elevated about 4‐fold in SSR‐ or UVB‐exposed skin compared with UVA‐exposed or control skin. Levels of protein‐bound form of 5SCD tended to be higher in SSR‐ or UVB‐exposed skin than in UVA‐exposed or control skin. Total levels of 5‐hydroxy‐6‐methoxyindole‐2‐carboxylic acid (5H6MI2C) and 6H5MI2C were higher in SSR‐ than in UVB‐exposed or control skin. These results show that SSR is more effective in promoting delayed tanning than UVB radiation alone, suggesting a synergistic effect of UVA radiation. Furthermore, free 5SCD may serve as a good marker of the effect of SSR and UVB.