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European nations established only from Medieval times - whites are very new to Europe
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[QUOTE]Originally posted by meninarmer: [QB] Role of Melanin in Photoprotection of the Skin TOP INTRODUCTION Overview: Architecture of the... Biochemical Considerations Developmental Considerations Regulation of Constitutive Skin... Regulation of Facultative Skin... Role of Melanin in... Disrupted Regulation of Skin... Approaches to Regulating Skin... REFERENCES Lightly pigmented skin has a dramatically increased risk of skin cancers, including melanomas, much higher (15–70-fold) than in darker skin (38, 39). Because skin pigmentation is primarily regulated by the MC1R, its gene is considered a susceptibility gene for melanoma (40). UV is harmful to human skin because of its production of various types of cellular damage, most notably oxidative damage and two major types of DNA damage: cyclobutane pyrimidine dimers and 6,4-photoproducts (41). Such molecular lesions have significant long-term effects on tissue if not repaired quickly and correctly. There is increasing evidence that DNA damage/repair itself can induce skin pigmentation. Small DNA fragments, such as thymine dinucleotides, enhance pigmentation of melanocytic cells and can stimulate TYR mRNA levels and responses to MSH (42). p53, which regulates the cell cycle and the repair of DNA damage, as well as the induction of apoptosis (32), can also up-regulate POMC/MSH expression by keratinocytes in response to UV, thereby inducing pigmentation (35). The involvement of MC1R with UV induction of skin pigmentation is complex and is regulated at many levels (43). MC1R regulates melanocyte function primarily via MITF, which in turn regulates melanogenesis and dendricity. MITF expression is stimulated relatively quickly, and significant increases are seen within 1 day of UV exposure. The downstream targets of MITF, e.g. TYR, Pmel17, and DCT, respond more slowly and reach maxima from 1–3 weeks after UV exposure. It takes several weeks after UV exposure before significant increases in melanin synthesis or melanocyte density occur. In addition to its role in pigmentation, MC1R regulates many other properties of melanocytes, such as the activation of DNA repair and other anti-photocarcinogenic activities that are important for protection against the deleterious effects of UV (44). Although UV increases expression of melanogenic genes similarly in skin of different racial/ethnic groups (29), there are some significant differences including melanin redistribution, protection against DNA damage, and induction of apoptosis in melanin-containing keratinocytes (21, 45). UV stimulates the transfer of melanin from the lower epidermis upward and prevents DNA damage in the lower epidermis more significantly in dark skin than in fair skin (29, 45). UV induces significantly more apoptosis in dark skin than in fair skin, which suggests a more efficient removal of UV-damaged cells; this may play a role in the decreased photocarcinogenesis of darker skin. [IMG]http://i32.photobucket.com/albums/d25/GoldenMenes/UV_Pene.gif[/IMG] Note the different levels of UV penetration which explains why blacks absorb and convert UV->Vitamin D and why long term, irreparable cellular damage is highly likely in whites. Reference: UV-induced DNA damage and melanin content in human skin differing in racial/ethnic origin. Taketsugu Tadokoro, Nobuhiko Kobayashi, Barbara Z. Zmudzka, Shosuke Ito, Kazumasa Wakamatsu, Yuji Yamaguchi, Katalin S. Korossy, Sharon A. Miller, Janusz Z. Beer, and Vincent J. Hearing, National Institute Of Health, 2003 [/QB][/QUOTE]
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