Chapter 18 - UV-induced pigmentation in human skin

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Abstract

Pigmentation, whether constitutive (i.e. base skin colour) or facultative (induced by UVR), is considered to have photoprotective properties in human skin. Much of the evidence for this is epidemiological and is based on the inverse relationship between skin cancer risk and skin pigmentation. Some studies have evaluated the photoprotective role of facultative pigmentation using erythema and epidermal DNA photodamage caused by a single UVR exposure. In general, pigmentation-related protection factors against these endpoints have been shown to be about 2–3 in different white skin types and were independent of the level of pigmentation. In vitro studies suggest that epidermal pigmentation may be the mammalian equivalent of a bacterial SOS response. We have shown no difference in human epidermal DNA repair capacity in different skin types after a single exposure to solar simulating radiation (SSR). However, repeat SSR exposure studies suggest that DNA repair may be induced in skin types IV that tan well, but not in skin types II that tan poorly. One interpretation of these studies is that pigmentation ability is an indicator of inducible DNA repair capacity and it is this, rather than the photoprotective properties of pigment per se, that is important in the prevention of skin cancer.

Introduction

Human skin may be classified according to its constitutive colour and its ability to undergo melanogenesis (tanning) in response to UVR exposure. These properties are often described as constitutive and facultative pigmentation respectively and, along with sensitivity to sunburn, are the basis of skin phototype as summarised in Table 1. The adverse acute and long-term effects of solar exposure are well established and, in general, are inversely related to skin type, with skin type I being the most sensitive and skin type VI being the most resistant. It is widely assumed that sensitivity to UVR is directly related to pigmentation or tanning ability and this assumption is primarily based on epidemiological evidence that shows that skin cancers are much less common in people who tan well (e.g. skin types III and IV) or who have high levels of constitutive pigmentation (e.g. skin types V and VI) [1, 2]. Furthermore, studies comparing dark-skinned peoples with related albinos show the latter to have a higher incidence of photodamage [3] and non-melanoma skin cancer [4]. However, comparisons based on pigmentation alone may fail to take into account other factors that may be important in UVR sensitivity, such as DNA repair capacity and sensitivity to UVR-induced immunosuppression that is widely believed to be important in human skin cancer [5]. For example, studies by Kelly et al. [6] have shown that on previously unexposed white buttock skin, cell-mediated immunity is more readily suppressed in skin types I/II when compared with skin types III/IV. These differences were highly significant and the observations held good whether solar simulated radiation (SSR) doses were expressed in terms of individual MED or in J/cm2. Such data strongly suggest inherent differences in UVR sensitivity that are not obviously related to erythema or pigmentation.

This chapter will review the evidence for the photoprotective properties of UVR-induced pigmentation against the acute effects of UVR and will focus primarily on human studies in vivo that have been done with environmentally relevant sources of UVR.

Section snippets

Melanins

The singular term melanin is widely used to describe the skin's red-brown-black pigment which resides in the epidermis. However, this description belies the range of epidermal pigments and the plural term melanins is more apt. The natural structures of melanins are still poorly defined but are believed to be polymers with multiple-monomer units linked by non-hydrolysable bonds. Consequently, degradation for chemical analysis is difficult and the overall chemical structures of melanins remain

Evolutionary significance of melanins

Skin reflectance of visible radiation, as a marker of melanin, in indigenous populations is inversely correlated with absolute latitude and UVR levels [25] suggesting that skin type differences have evolved in response to ambient UVR. It is generally thought that melanogenesis is associated with photoprotection and it has been suggested that skin colour represents a compromise between photoprotection from UVB-induced damage and the need to synthesise vitamin D, also a UVB-dependent process.

Conclusions

There is no doubt that constitutive pigmentation or the ability to tan reduces the adverse acute and long-term effects of solar UVR exposure. It is has been widely assumed that this is mainly due to the optical screening properties of melanin per se. However, there is increasing evidence that skin type-related UVR responsivity, that is believed to be associated with melanogenesis, may be more complex than previously thought. In particular, it seems possible that the ability to tan is a

Acknowledgements

We thank Mr. Graham Harrison and Dr. Christian Kipp for preparing the figures.

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