Constitutive skin pigmentation dramatically affects the incidence of skin cancers, and in the US, rates of basal and squamous cell carcinomas and melanomas are dramatically higher in Whites than in Blacks. Cyclobutane pyrimidine dimers (CPD) and (6-4)-photoproducts (64PP), the two major types of DNA lesions resulting from UVB exposure, are potentially carcinogenic as is the oxidative damage resulting from UVA exposure. Melanin reduces DNA photoproducts in vivo and in vitro and can scavenge reactive oxygen species. Thus, melanin content of melanocytes plays a significant role in minimizing UV damage and in enhancing cell survival. However, much remains unknown about relationships between DNA damage/repair and different types, quantities, forms and distribution of melanin in skin of different racial/ethnic origin and different phototypes. Our collaborations with the FDA have been highly productive and have provided some insights into these basic questions. Several key observations have evolved from these studies: (1) DNA damage is greatest immediately following UV exposure in all subjects and is gradually repaired thereafter;(2) rates and efficiencies of removal of DNA lesions differs dramatically between subjects in all groups;(3) while DNA damage is most severe in light, UV-sensitive skin, even the darkest, UV-resistant skin incurs significant DNA damage after UV exposure at levels less than 1 MED;and (4) there is an inverse correlation between skin pigmentation and DNA damage. Thus, even very low UV exposures cause significant damage to DNA in all types of skin, underscoring the contention that there is no such thing as totally UV-resistant human skin. Our studies, combined with others on this topic, suggest that at least 5 factors contribute to UV-induced carcinogenesis and the various risks seen in human populations: (1) the amount of UV-induced DNA damage, (2) the identity and function of gene(s) damaged, (3) the nature of cells damaged, (4) the efficiency of post-exposure removal/repair of DNA damage, and (5) the removal of irreversibly UV-damaged cells by apoptosis or other mechanisms. The sum of those processes may explain the dramatically higher incidence of skin cancers in light-skinned subjects compared to dark-skinned subjects. We have completed our studies on the effects of acute or repetitive UV exposure on human skin of various phototypes. Such work has gained urgency since repeated UV exposure by sunlamps has been linked with squamous cell carcinoma and cutaneous melanoma. Our results on the effects of repetitive UV exposure further emphasize that the distribution of melanin is important to the appearance of visible color and to photoprotection of the skin;however DNA damage is not eliminated during UV-induced pigmentation. Important issues that we are now studying include defining: (1) whether production of eumelanin versus pheomelanin has any consequence on photoprotection, (2) whether facultative (induced) pigmentation of the skin provides added protection against UV damage, (3) the role of DNA repair in minimizing long-term damage to the skin and subsequent photocarcinogenesis, and (4) the identity and regulation of UV-induced factors that modulate responses to that environmental stress. Given the importance of skin pigmentation to reduce the risk of photocarcinogenesis, these studies are critical to understanding parameters involved in photocarcinogenesis, as well as to developing effective strategies to minimize such risk. We have now measured parameters that had been characterized in our single UV dose study to determine levels of DNA damage and melanogenic functions in the skin during tanning elicited by repetitive UV exposure. Increased expression of MITF occurs most quickly after repetitive UV exposure (within 1-2 d), increased expression of melanosomal proteins such as TYR, TYRP1, Pmel17 and DCT is slower (1 wk), while increases in melanin synthesis take longer (3 wk) and increases in melanocyte density take even longer (4-5 wk). The distribution of melanin in the skin is important to visible pigmentation and to its photoprotective capacity. It is clear that relatively small changes in melanin content and/or distribution can make relatively large changes in visible pigmentation. Those affect not only constitutive pigmentation that defines racial/ethnic differences, but also responses to UV exposure. Long-term follow-up studies have revealed a new type of UV-induced pigmentation, termed LLP (long-lasting pigmentation), which is defined as skin pigmentation that persists more than 9 months after the end of UV exposure, in some cases up to 3 or 4 years later. We are currently studying the mechanisms involved in LLP and whether that is associated (positively or negatively) with increased risk for skin photocarcinogenesis. The data from our studies suggest that the induction of DNA damage as a by-product of tanning is a significant source of delayed risks, including skin cancer. The induction/removal of DNA photoproducts in fair skin (phototypes 2 or 3) were measured for repeated UV exposures 1 day after the final exposure. Following repeated UV exposure, redistribution of melanin to upper layers of the skin is an immediate response followed by de novo melanin synthesis. Based on diffuse reflectance measurements and melanin content assessed by Fontana-Masson staining, repeated UV doses produce a moderate to dark brown tan which reaches a plateau after several exposures, indicating that the pigment system reaches saturation. Pigmentation remains elevated for >4 wk after the final exposure showing that once a tan develops, continued frequent UV exposure is not necessary to maintain it. The 2-3 fold increase in melanin content of each layer of the epidermis elicited by repetitive UV exposure is consistent with the increased density of melanocytes in the basal layer of the skin. We have also examined UV-irradiated specimens obtained 3 days after a 2 week course of repeated doses of SSR, UVA or UVB. It has been proposed that melanin protects against UVB more efficiently than against UVA, and that eumelanin absorbs UV more efficiently than pheomelanin, but those assumptions need to be tested. According to our UV reflectance analysis, different types of UV affect melanin content in human skin in different manners. We analyzed the relationship between CPD damage and melanin content using immunohistochemistry and Fontana Masson staining. Those results are consistent with our earlier studies and confirm that different subjects with apparently identical skin phototype have distinct responses to repetitive UVA and/or UVB radiation. UVB stimulates the melanogenic system, increasing factors involved in melanin production and eventually in melanin content, and provides a small measure of added photoprotection. In contrast, repetitive UVA exposure, while leading to comparable visible skin tanning, has no detectable effect on the melanogenic system and elicits no increase in melanin content, thus providing absolutely no photoprotective benefit. We have completed our DNA microarray analyses of human skin exposed to repetitive UVA and/or UVB which provides important clues as to the in situ responses of human skin to different types of UV, and also adds to our understanding of the mechanisms of skin tanning in response to UVA and/or UVB. We hypothesize that the visible tan elicited by UVA results from oxidation of existing melanin (or its precursors) and that UVA-induced tans (such as those promoted by the tanning industry) give tanners a false sense of protection against subsequent UV exposure, which may be worsening skin cancer incidence.
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