Mechanisms of Skin Tanning in Different Racial/Ethnic Groups in Response to Ultraviolet Radiation

Abstract

Ultraviolet radiation stimulates pigmentation in human skin, but the mechanism(s) whereby this increase in melanin production (commonly known as tanning) occurs is not well understood. Few studies have examined the molecular consequences of UV on human skin of various racial backgrounds in situ. We investigated the effects of UV on human skin of various races before and at different times after a single 1 minimal erythemal dose UV exposure. We measured the distribution of DNA damage that results, as well as the melanin content/distribution and the expression of various melanocyte-specific genes. The density of melanocytes at the epidermal:dermal junction in different types of human skin are remarkably similar and do not change significantly within 1 wk after UV exposure. The expression of melanocyte-specific proteins (including TYR (tyrosinase), TYRP1 (tyrosinase-related protein 1), DCT (tyrosinase-related protein 2), MART1 (melanoma antigens recognized by T-cells) gp100 (Pmel17/silver), and MITF (micropthalmia transcription factor)) increased from 0 to 7 d after UV exposure, but the melanin content of the skin increased only slightly. The most significant change, however, was a change in the distribution of melanin from the lower layer upwards to the middle layer of the skin, which was more dramatic in the darker skin. These results provide a basis for understanding the origin of different skin colors and responses to UV within different races.

The Regulation of Skin Pigmentation

“…Many factors regulate constitutive skin color, the most obvious of them being UV in what is commonly termed the tanning reaction (28,29). Recent studies have outlined the complex kinetics of responses of the skin to UV radiation, which result in tanning over the course of several weeks (30).…”

“…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).…”

“…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.…”

Modifying skin pigmentation – approaches through intrinsic biochemistry and exogenous agents

“…Although the mechanisms involved in pigmentation are similar in skin of different racial/ethnic groups, melanin transfer from the basal level of the epidermis upwards is more effective in dark skin than in fair skin resulting in a better protection of the lower epidermis in dark skin [40,41].…”

The fibroblast-derived paracrine factor neuregulin-1 has a novel role in regulating the constitutive color and melanocyte function in human skin

“…UV stimulates the expression and function of many melanocyte-specific proteins, such as tyrosinase (TYR), tyrosinase-related protein 1 (TYRP1), tyrosinase-related protein 2 (DCT), and melanoma antigen recognized by T-cells 1 (MART1), and those increases are typically more dramatic in darker skin types than in lighter skin types (Miyamura et al, 2007;Tadokoro et al, 2005). Despite that diversity, the density of melanocytes in all types of skin is virtually identical (Yamaguchi et al, 2006;Yamaguchi and Hearing, 2005) and the differences in visible skin color depend on the amount of melanin produced, the efficiency of melanine transfer from melanocytes to keratinocytes, the ratio of pheomelanin to eumelanin synthesized, etc.…”