Melanin Offers Protection Against Induction of Cyclobutane Pyrimidine Dimers and 6-4 Photoproducts by UVB in Cultured Human Melanocytes¶

New insights from non-invasive imaging: from prospection of skin photodamages to training with mobile application

The incidence of non‐melanoma skin cancer is on the rise and melanoma is among the most common cancers in the United States. Establishing an early diagnosis is essential for improving the prognosis of patients with skin cancer. High‐resolution non‐invasive imaging techniques may represent key tools for helping to identify and monitor early signs of skin cancer in seemingly healthy skin. Cumulative lifetime sun exposure leads to photoaging and photocarcinogenenis and the reaction of the skin to this solar‐induced damage is balanced between the DNA repair and photoprotection defence mechanisms of melanocytes and keratinocytes. In the first part of this article we provide an overview of these defence mechanisms and of the photoaging process, and discuss how non‐invasive imaging can be used to evaluate these changes. We then propose a model in which skin aging manifestations can be classified according to subject‐specific sun‐damage reaction profiles observed by reflectance confocal microscopy (RCM) and optical coherence tomography (OCT). These photoaging profiles include an atrophic phenotype characterized by actinic keratosis, and a hypertrophic phenotype characterized by hyperplastic pigmented skin. According to our model, these phenotypes may be predictive of predispositions to different types of skin cancer: squamous cell carcinoma for the atrophic phenotype and lentigo maligna and freckles for the hypertrophic phenotype. In addition to RCM and OCT, dermoscopy is another non‐invasive technique that has improved the diagnosis of skin cancer. In the second part of this article, we describe how the YouDermoscopy™ application can improve skills and thus enhance the dermoscopic recognition of sun‐induced skin tumours, and then show how this training tool enables its users to collaborate with dermatologists worldwide to obtain second opinions for the diagnosis of ambiguous lesions. Altogether, RCM, OCT and dermoscopy are valuable tools that can contribute significantly to improving the early diagnosis of precancerous and cancerous lesions.

Ultraviolet B, melanin and mitochondrial DNA: Photo-damage in human epidermal keratinocytes and melanocytes modulated by alpha-melanocyte-stimulating hormone

Alpha-melanocyte-stimulating hormone (alpha-MSH) increases melanogenesis and protects from UV-induced DNA damage. However, its effect on mitochondrial DNA (mtDNA) damage is unknown. We have addressed this issue in a pilot study using human epidermal keratinocytes and melanocytes incubated with alpha-MSH and irradiated with UVB. Real-time touchdown PCR was used to quantify total and deleted mtDNA. The deletion detected encompassed the common deletion but was more sensitive to detection. There were 4.4 times more mtDNA copies in keratinocytes than in melanocytes. Irradiation alone did not affect copy numbers. Alpha-MSH slightly increased copy numbers in both cell types in the absence of UVB and caused a similar small decrease in copy number with dose in both cell types. Deleted copies were nearly twice as frequent in keratinocytes as in melanocytes. Alpha-MSH reduced the frequency of deleted copies by half in keratinocytes but not in melanocytes. UVB dose dependently led to an increase in the deleted copy number in alpha-MSH-treated melanocytes. UVB irradiation had little effect on deleted copy number in alpha-MSH-treated keratinocytes. In summary, alpha-MSH enhances mtDNA damage in melanocytes presumably by increased melanogenesis, while α-MSH is protective in keratinocytes, the more so in the absence of irradiation.

Increased melanogenesis is a risk factor for oxidative DNA damage--study on cultured melanocytes and atypical nevus cells

Melanin synthesis is an oxygen-dependent process that acts as a potential source of reactive oxygen species (ROS) inside pigment-forming cells. The synthesis of the lighter variant of melanin, pheomelanin, consumes cysteine and this may limit the capacity of the cellular antioxidative defense. We show that tyrosine-induced melanogenesis in cultured normal human melanocytes (NHM) is accompanied by increased production of ROS and decreased concentration of intracellular glutathione. Clinical atypical (dysplastic) nevi (DN) regularly contain more melanin than do normal melanocytes (MC). We also show that in these cultured DN cells three out of four exhibit elevated synthesis of pheomelanin and this is accompanied by their early senescence. By using various redox-sensitive molecular probes, we demonstrate that cultured DN cells produce significantly more ROS than do normal MC from the same donor. Our experiments employing single-cell gel electrophoresis (comet assay) usually reveal higher fragmentation of DNA in DN cells than in normal MC. Even if in some cases the normal alkaline comet assay shows no differences in DNA fragmentation between DN cells and normal MC, the use of the comet assay with formamidopyrimidine DNA glycosylase can disclose that the DNA of the cultured DN cells harbor more oxidative damage than the DNA of normal MC from the same person.

Photoprotection of bacterial-derived melanin against ultraviolet A-induced cell death and its potential application as an active sunscreen

BACKGROUND

The increase in the incidence of non-melanoma skin tumours, photoaging, and immunosuppression demand for more effective sunscreen on ultraviolet A (UVA) irradiation.

OBJECTIVES

The aim of the study is to evaluate the photoprotective effects of a bacterial-derived melanin against UVA-induced damages in vitro and in vivo. Methods Human fibroblasts were used to assess the role of the bacterial-derived melanin on cell viability against UVA. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and nuclear morphology were employed to evaluate the photoprotection at the cellular level. Fluorometric assays were performed to detect the formation of reactive oxygen species (ROS) in the cells. Evaluations of the bacterial-derived melanin as a sunscreen were measured by transmission test and persistent pigment darkening on human skin.

RESULTS

Bacterial-derived melanin efficiently scavenged ROS in the fibroblasts after UVA irradiation. The cell viability of xeroderma pigmentosum (XP) fibroblast treated with varied doses of melanin increased dramatically in comparison with untreated control and the treated XP fibroblasts became more resistant to UVA-induced apoptosis than normal fibroblasts. Although the relative transmission didn't change too much with different concentration of bacterial-derived melanin, this melanin could keep UVA-irradiated skin from pigment darkening and act as an active sunscreen on skin.

CONCLUSIONS

The bacterial-derived melanin provided significant protection to fibroblast cell and human skin against the UVA radiation. It has the potential to be developed as an active sunscreen for the patients with photosensitivity skin to sun exposure.

The phototoxicity of aged human retinal melanosomes

The purpose of this study was to determine whether an age-related increase in photoreactivity of human retinal melanosomes (MS) can cause phototoxicity to retinal pigment epithelium (RPE) cells. MS were isolated post mortem from young (20-30 years, young human melanosomes [YHMs]) and old (60-90 years, old human melanosomes [OHMs]) human eyes and from young bovine eyes (bovine melanosomes [BMs]). Confluent cultured ARPE-19 cells were fed equivalent numbers of OHMs or BMs and accumulated similar amounts of melanin as determined by electron paramagnetic resonance assay. Cells with and without MS were either maintained in the dark or exposed to blue light for up to 96 h and assessed for alterations in cell morphology, cell viability and lysosomal integrity. Incubation of cells in dark in the presence of internalized MS or irradiation of cells with blue light in the absence or presence of BMs did not significantly affect cell viability. However, exposures to blue light in the presence of OHMs resulted in abnormal cell morphology, up to approximately 75% decrease in mitochondrial activity, loss of lysosomal pH and cell death. OHMs contained significantly less melanin than YHMs, supporting the hypothesis that melanin undergoes degradation during RPE aging. Our results demonstrate that aged MS can be phototoxic to human RPE cells and support a contributing role of MS in RPE aging and in the pathogenesis of age-related macular degeneration.

Melanin content and MC1R function independently affect UVR-induced DNA damage in cultured human melanocytes

Malignant transformation of melanocytes leads to melanoma, the most fatal form of skin cancer. Ultraviolet radiation (UVR)-induced DNA photoproducts play an important role in melanomagenesis. Cutaneous melanin content represents a major photoprotective mechanism against UVR-induced DNA damage, and generally correlates inversely with the risk of skin cancer, including melanoma. Melanoma risk is also determined by susceptibility genes, one of which is the melanocortin 1 receptor (MC1R) gene. Certain MC1R alleles are strongly associated with melanoma. We hereby present experimental evidence for the role of two melanoma risk factors, constitutive pigmentation, as assessed by total melanin, eumelanin and pheomelanin contents, and MC1R genotype and function, in determining the induction and repair of DNA photoproducts in cultured human melanocytes after irradiation with increasing doses of UVR. We found that total melanin and eumelanin contents (MC and EC) correlated inversely with the extent of UVR-induced growth arrest, apoptosis and induction of cyclobutane pyrimidine dimers (CPD), but not with hydrogen peroxide release in melanocytes expressing functional MC1R. In comparison, melanocytes with loss-of-function MC1R, regardless of their MC or EC, sustained more UVR-induced apoptosis and CPD, and exhibited reduced CPD repair. Therefore, MC, mainly EC, and MC1R function are independent determinants of UVR-induced DNA damage in melanocytes.

T-oligos augment UV-induced protective responses in human skin

We have shown that DNA oligonucleotides substantially homologous to the telomere 3-prime overhang sequence (T-oligos) increase DNA repair capacity (DRC) in cultured human cells and decrease UV-induced mutation rate and photocarcinogenesis in mouse skin. To investigate the protective effects of T-oligos in intact human skin, paired skin explants obtained from adult donors were treated with T-oligos or diluent alone for 24 h, then UVB- or sham-irradiated, and processed after 6, 24, 48, 72, and 96 h for histological analysis. After UV irradiation apoptotic epidermal cells were comparable in diluent- and T-oligo-treated skin. Proliferating (Ki67+) cells were sparse in sham-irradiated skin and for 24 h after UV in both diluent- and T-oligo-treated specimens. However, compared to diluent controls, at 48 and 72 h T-oligos significantly inhibited UV-induced rebound hyperproliferation. Maximum and comparable cyclobutane pyrimidine dimers (CPDs) were detected immediately after UV irradiation in diluent- and T-oligo-treated skin, but CPDs were strikingly reduced in T-oligo- vs. diluent-treated skin at 24, 48, and 72 h. Total and activated p53 protein was increased in T-oligo- vs. diluent-pretreated skin at the time of irradiation, and up to 3-fold increases persisted for 24 h post-UV. Over 5 days, UV irradiation and T-oligo comparably increased expression of melanogenic proteins and each increased epidermal melanin content 3- to 5-fold, with distinct nuclear capping in many keratinocytes. In combination, these findings predict that T-oligo treatment will increase melanogenesis, prolong epidermal arrest, and increase DNA repair rate after UV irradiation, thus decreasing the severity of acute and chronic photodamage in human skin. Moreover, the data document an inducible SOS-like response consisting of increased melanogenesis and increased DNA repair capacity in human skin following UV-induced damage that is also produced by T-oligos in the absence of initial damage.