Environmental and Molecular Mutagenesis 16:37-43 (1990)
Melanin Photosensitizes Ultraviolet Light (UVC) DNA Damage in Pigmented Cells Christine A. Huselton and Helene Z. Hill Section of Cancer Biology, Department of Radiology, New Jersey Medical School, Newark, New Jersey Melanins, pigments of photoprotection and camouflage, are very photoreactive and can both absorb and emit active oxygen species. Nevertheless, black skinned individuals rarely develop skin cancer and melanin is assumed to act as a solar screen. Since D N A is the target for solar carcinogenesis, the effect of melanin on Ultraviolet (UV)-induced thyrnine lesions was examined in mouse melanoma and carcinoma cells that varied in melanin content. Cells prelabeled with 14C-dThd were irradiated with UVC; D N A was isolated, purified, degraded to bases by acid hydrolysis and analyzed by HPLC. Thymine dimers were detected in all of the extracts of irradiated cells. Melanotic and hypomelanotic but not mammary carcinoma cell D N A from ir-
radiated cells contained hydrophilic thymine derivatives. The quantity of these damaged bases was a function of both the UVC dose and the cellular melanin content. One such derivative was identified by gas chromatographymass spectroscopy as thymine glycol. The other appears to be derived from thymine glycol by further oxidation during acid hydrolysis of the DNA. The finding of oxidative DNA damage in melanin-containing cells suggests that melanin may be implicated in the etiology of Caucasian skin cancer, particularly melanoma. Furthermore, the projected decrease in stratospheric ozone could impact in an unanticipated deleterious manner on dark-skinned individuals.
Key words: solar carcinogenesis, ozone depletion, melanoma
INTRODUCTION While degree of skin pigmentation correlates with skin cancer incidence-the darker the skin, the lower the riskthe role of tanning in skin protection is not clear. Some tans, such as those induced by ultraviolet light A (UVA), provide little protection against sunburn reaction, an index for carcinogenic risk [Kaidbey and Kligman, 1978; Gange et al., 19851. Furthermore, there is a high correlation between the incidence of non-melanoma skin cancer, particularly squamous cell cancer and lifetime sun exposure. This is true for all populations. regardless of intrinsic tanning ability [Webb, 19771. Not all melanomas can be attributed to sun exposure, but their development on sun-exposed areas is correlated with episodes of intense exposure at an early age [D’Arcy et al., 1984, 1986; Elwood et al., 1985; Osterlind et al.. 19881. The melanoma rate is doubling every decade worldwide [Lee and Carter, 1970; Elwood and Lee. 1975; Magnus, 1977; Ohsumi and Seiji, 19771. At this rate, by 2010, it will cause 10%of U.S. cancer deaths. Understanding melanin’s role in skin cancer etiology becomes increasingly important as the protective stratospheric ozone layer declines. The projected increase in UVB is expected to exacerbate the already significant increase in skin cancer rate. Melanins absorb light over a wide range of wavelengths @I
1990 Wiley-Liss, Inc.
with no characteristic absorption bands in the ultraviolet, visible, or near-infrared regions, but with a rate that increases with decreasing wavelength [Menon et al., 1983; Sarna and Sealy, 1984a,b; Kollias and Baqer, 19871. The primary function of epidermal melanins is believed to be passive screening of harmful UV radiation. However, their chemical structure, physical nature, and anatomic distribution suggest a more complex role [Sealy, 19841. Melanins are naturally occurring free radicals. They can scavenge .OH [Sarna et al., 19861, O,-. [Korytowski et a]., 19861, and ‘0, [Sarna et al., 1985; Sealy et al., 1984). On illumination, the free radical content of melanins increases [Sarna and Sealy, 1984a.b], O2 is consumed [Sarna and Sealy, 1984b1, and 0,- and H,O, are produced [Korytowski et al., 19871. -OH forms by a Fenton-type reaction [Korytowski et a]., 19871. The quantum yields of these reactions increase markedly as the wavelength decreases
Received June 12. 1989; revised and accepted February 27, 1990. Address reprint requests to Helene 2. Hill. Section of Cancer Biology. Department of Radiology, New Jersey Medical School, MSB-E586, 185 South Orange Avenue, Newark, NJ 07103-2757.
38
Huselton and Hill
TABLE I. Melanin Content of the Three Cell Lines Cell line
Melanin content (odcell)
EMT6 S9liamel S91113
Melanin Photosensitizes Ultraviolet Light (UVC) DNA Damage in Pigmented Cells Christine A. Huselton and Helene Z. Hill Section of Cancer Biology, Department of Radiology, New Jersey Medical School, Newark, New Jersey Melanins, pigments of photoprotection and camouflage, are very photoreactive and can both absorb and emit active oxygen species. Nevertheless, black skinned individuals rarely develop skin cancer and melanin is assumed to act as a solar screen. Since D N A is the target for solar carcinogenesis, the effect of melanin on Ultraviolet (UV)-induced thyrnine lesions was examined in mouse melanoma and carcinoma cells that varied in melanin content. Cells prelabeled with 14C-dThd were irradiated with UVC; D N A was isolated, purified, degraded to bases by acid hydrolysis and analyzed by HPLC. Thymine dimers were detected in all of the extracts of irradiated cells. Melanotic and hypomelanotic but not mammary carcinoma cell D N A from ir-
radiated cells contained hydrophilic thymine derivatives. The quantity of these damaged bases was a function of both the UVC dose and the cellular melanin content. One such derivative was identified by gas chromatographymass spectroscopy as thymine glycol. The other appears to be derived from thymine glycol by further oxidation during acid hydrolysis of the DNA. The finding of oxidative DNA damage in melanin-containing cells suggests that melanin may be implicated in the etiology of Caucasian skin cancer, particularly melanoma. Furthermore, the projected decrease in stratospheric ozone could impact in an unanticipated deleterious manner on dark-skinned individuals.
Key words: solar carcinogenesis, ozone depletion, melanoma
INTRODUCTION While degree of skin pigmentation correlates with skin cancer incidence-the darker the skin, the lower the riskthe role of tanning in skin protection is not clear. Some tans, such as those induced by ultraviolet light A (UVA), provide little protection against sunburn reaction, an index for carcinogenic risk [Kaidbey and Kligman, 1978; Gange et al., 19851. Furthermore, there is a high correlation between the incidence of non-melanoma skin cancer, particularly squamous cell cancer and lifetime sun exposure. This is true for all populations. regardless of intrinsic tanning ability [Webb, 19771. Not all melanomas can be attributed to sun exposure, but their development on sun-exposed areas is correlated with episodes of intense exposure at an early age [D’Arcy et al., 1984, 1986; Elwood et al., 1985; Osterlind et al.. 19881. The melanoma rate is doubling every decade worldwide [Lee and Carter, 1970; Elwood and Lee. 1975; Magnus, 1977; Ohsumi and Seiji, 19771. At this rate, by 2010, it will cause 10%of U.S. cancer deaths. Understanding melanin’s role in skin cancer etiology becomes increasingly important as the protective stratospheric ozone layer declines. The projected increase in UVB is expected to exacerbate the already significant increase in skin cancer rate. Melanins absorb light over a wide range of wavelengths @I
1990 Wiley-Liss, Inc.
with no characteristic absorption bands in the ultraviolet, visible, or near-infrared regions, but with a rate that increases with decreasing wavelength [Menon et al., 1983; Sarna and Sealy, 1984a,b; Kollias and Baqer, 19871. The primary function of epidermal melanins is believed to be passive screening of harmful UV radiation. However, their chemical structure, physical nature, and anatomic distribution suggest a more complex role [Sealy, 19841. Melanins are naturally occurring free radicals. They can scavenge .OH [Sarna et al., 19861, O,-. [Korytowski et a]., 19861, and ‘0, [Sarna et al., 1985; Sealy et al., 1984). On illumination, the free radical content of melanins increases [Sarna and Sealy, 1984a.b], O2 is consumed [Sarna and Sealy, 1984b1, and 0,- and H,O, are produced [Korytowski et al., 19871. -OH forms by a Fenton-type reaction [Korytowski et a]., 19871. The quantum yields of these reactions increase markedly as the wavelength decreases
Received June 12. 1989; revised and accepted February 27, 1990. Address reprint requests to Helene 2. Hill. Section of Cancer Biology. Department of Radiology, New Jersey Medical School, MSB-E586, 185 South Orange Avenue, Newark, NJ 07103-2757.
38
Huselton and Hill
TABLE I. Melanin Content of the Three Cell Lines Cell line
Melanin content (odcell)
EMT6 S9liamel S91113
