Melanocytes are deficient in repair of oxidative DNA damage and UV-induced photoproducts

Abstract

Melanomas occur mainly in sunlight-exposed skin. Xeroderma pigmentosum (XP) patients have 1,000-fold higher incidence of melanoma, suggesting that sunlight-induced "bulky" photoproducts are responsible for melanomagenesis. Sunlight induces a high level of reactive oxygen species in melanocytes (MCs); oxidative DNA damage (ODD) may thus also contribute to melanomagenesis, and XP gene products may participate in the repair of ODD. We examined the effects of melanin on UVA (320-400 nm) irradiationinduced ODD and UV photoproducts and the repair capacity in MC and XP cells for ODD and UV-induced photoproducts. Our findings indicate that UVA irradiation induces a significantly higher amount of formamidopyrimidine glycosylase-sensitive ODD in MCs than in normal human skin fibroblasts (NHSFs). In contrast, UVA irradiation induces an insignificant amount of UvrABC-sensitive sites in either of these two types of cells. We also found that, compared to NHSFs, MCs have a reduced repair capacity for ODD and photoproducts; H 2 O 2 modified-and UVC-irradiated DNAs induce a higher mutation frequency in MCs than in NHSFs; and, XP complementation group A (XPA), XP complementation group C, and XP complementation group G cells are deficient in ODD repair and ODD induces a higher mutation frequency in XPA cells than in NHSFs. These results suggest that: (i) melanin sensitizes UVA in the induction of ODD but not bulky UV photoproducts; (ii) the high susceptibility to UVA-induced ODD and the reduced DNA repair capacity in MCs contribute to carcinogenesis; and (iii) the reduced repair capacity for ODD contributes to the high melanoma incidence in XP patients.DNA repair and mutagenesis | UV photoproducts | ultraviolet light | melanoma | xeroderma pigmentosum I t has long been recognized that sunlight exposure is the major cause of skin cancers, including melanoma (1-3). Although evidence from both epidemiological and molecular studies show that photoproducts induced by UVB (290-320 nm) irradiation are the major cause for nonmelanoma skin cancer (4), epidemiological studies strongly implicate UVA (320-400 nm) irradiation in sunlight exposure-related cutaneous melanomagenesis (5, 6). In animal models, it has been found that both UVA and UVB irradiation can trigger melanocytic hyperplasia and melanomagenesis (7-9). Although UVB radiation can induce both cyclobutane pyrimidine dimers (CPDs) and pyrimidine < 6-4 > pyrimidone photoproducts (<6-4 > PPs), which can trigger mutagenesis and carcinogenesis (10, 11), the photoproduct yield of these types of DNA damage by UVA irradiation is two to three orders of magnitude lower than by UVB radiation (12). Thus, exactly how UVA acts to promote melanomagenesis remains controversial. It has been found that XP patients have 1,000-fold higher incidence of melanoma than normal individuals (13). Xeroderma pigmentosum cells are sensitive to UV radiation but resistant to ionizing radiation, and XP gene products are also crucial for CPD and <6-4 > PP repair (14). Therefore, it is possible that UVA irradiation...

Role of the pro-survival molecule Bfl-1 in melanoma

“…In melanoma, Bim downregulation has been reported in TRAIL-resistant cells, possibly as a result of the associated high levels of ERK1/2 activation 305 . Bad is also downregulated in melanoma cells which display hyperactivated MAPK pathways and the dephosphorylation of this protein is the key step in MEK inhibitor induced apoptosis in melanoma cells 306 , as is the translocation of Bmf 307 . However there is no evidence suggesting Bad inactivation, mutation or dysregulation contributes to the development of melanoma.…”

“…Melanocytes may not be more susceptible to gaining mutations than other cells in the skin, but once a DNA mutation has occurred, melanocytes appear to have a reduced ability, compared with other human fibroblasts, to repair the DNA damage 307 . In addition, melanocytes constitutively express high levels of the pro-survival protein Bcl-2 164 , which allows them to escape cell suicide.…”

The effects of phototherapy and melanocytes on keratinocytes

“…by various factors, including UV light, oxidation and keratinocytes (31,32). The association between melanocytes and keratinocytes is essential during the pathogenesis of vitiligo; however, few studies have focused on the effects of melanocytes on healthy keratinocytes.…”

Melanocytes and Oxidative Stress

“…While melanin can exhibit a photoprotective effect, it's pro-oxidant and antioxidant activity appears to depend on the redox state of the melanocytes [20], on the relative eumelanin/phaeomelanin contents, the levels of melanin intermediates [21], and the composition and concentration of reactive metal ions in the melanosome microenvironment [22]. Generation of H 2 O 2 in response to UV correlates inversely with melanin content of the skin, suggesting an antioxidant property of the latter [23] while cultures of human melanocytes with high melanin content were reported to be more vulnerable to UVA-induced oxidative DNA damage than melanocytes with lower melanin content [24]. Still in cultured human melanocytes, UVA-stimulation appears to increase DNA damage most likely through phaeomelanin and/or melanin intermediates [25].…”