Regulation of human skin pigmentation and responses to ultraviolet radiation

Melanocyte pigmentation stiffens murine cardiac tricuspid valve leaflet

Pigmentation of murine cardiac tricuspid valve leaflet is associated with melanocyte concentration, which affects its stiffness. Owing to its biological and viscoelastic nature, estimation of the in situ stiffness measurement becomes a challenging task. Therefore, quasi-static and nanodynamic mechanical analysis of the leaflets of the mouse tricuspid valve is performed in the current work. The mechanical properties along the leaflet vary with the degree of pigmentation. Pigmented regions of the valve leaflet that contain melanocytes displayed higher storage modulus (7-10 GPa) than non-pigmented areas (2.5-4 GPa). These results suggest that the presence of melanocytes affects the viscoelastic properties of the mouse atrioventricular valves and are important for their proper functioning in the organism.

Correlation between melanogenic and catalase activity in in vitro human melanocytes: a synergic strategy against oxidative stress

UV-induced DNA damage can lead to melanoma, the most dangerous form of skin cancer. Understanding the mechanisms employed by melanocytes to protect against UV is therefore a key issue. In melanocytes, catalase is the main enzyme responsible for degrading hydrogen peroxide and we have previously shown that that low basal levels of catalase activity are associated with the light phototype in in vitro and ex vivo models. Here we investigate the possible correlation between its activity and melanogenesis in primary cultures of human melanocytes. We show that while the total melanin concentration is directly correlated to the level of pigmentation, the more the degree of pigmentation increased, the lower the proportion of pheomelanin present. Moreover, in human melanocytes in vitro, catalase-specific mRNA, protein and enzymatic activity were all directly correlated with total cellular melanin content. We also observed that immediately after a peroxidative treatment, the increase in reactive oxygen species was inversely associated with pigmentation level. Darkly pigmented melanocytes therefore possess two protective strategies represented by melanins and catalase activity that are likely to act synergistically to counteract the deleterious effects of UV radiation. By contrast, lightly pigmented melanocytes possess lower levels of melanogenic and catalase activity and are therefore more susceptible to accumulate damage after UV exposition.

Long-lasting molecular changes in human skin after repetitive in situ UV irradiation

It is known that UV modulates the expression of paracrine factors that regulate melanocyte function in the skin. We investigated the consequences of repetitive UV exposure of human skin in biopsies of 10 subjects with phototypes 2-3.5 taken 1-4 years later. The expression of melanogenic factors (TYR, MART1, MITF), growth factors/receptors (SCF/KIT, bFGF/FGFR1, ET1/EDNRB, HGF, GM-CSF), adhesion molecules (beta-catenin, E-cadherin, N-cadherin), cell cycle proteins (PCNA, cyclins D1, E2) as well as Bcl-2, DKK1, and DKK3, were analyzed by immunohistochemistry. Most of those markers showed no detectable changes at > or = 1 year after the repetitive UV irradiation. Although increased expression of EDNRB protein was detected in 3 of 10 UV-irradiated subjects, there was no detectable change in the expression of ET1 protein or in EDNRB mRNA levels. In summary, only the expression of TYR, MART1, and/or EDNRB, and only in some subjects, was elevated at > or = 1 year after UV irradiation. Thus the long-term effects of repetitive UV irradiation on human skin did not lead to significant changes in skin morphology and there is considerable subject-to-subject variation in responses. The possibility that changes in the expression and function of EDNRB triggers downstream activation of abnormal melanocyte proliferation and differentiation deserves further investigation.

Looking older: fibroblast collapse and therapeutic implications

Skin appearance is a primary indicator of age. During the last decade, substantial progress has been made toward understanding underlying mechanisms of human skin aging. This understanding provides the basis for current use and new development of antiaging treatments. Our objective is to review present state-of-the-art knowledge pertaining to mechanisms involved in skin aging, with specific focus on the dermal collagen matrix. A major feature of aged skin is fragmentation of the dermal collagen matrix. Fragmentation results from actions of specific enzymes (matrix metalloproteinases) and impairs the structural integrity of the dermis. Fibroblasts that produce and organize the collagen matrix cannot attach to fragmented collagen. Loss of attachment prevents fibroblasts from receiving mechanical information from their support, and they collapse. Stretch is critical for normal balanced production of collagen and collagen-degrading enzymes. In aged skin, collapsed fibroblasts produce low levels of collagen and high levels of collagen-degrading enzymes. This imbalance advances the aging process in a self-perpetuating, never-ending deleterious cycle. Clinically proven antiaging treatments such as topical retinoic acid, carbon dioxide laser resurfacing, and intradermal injection of cross-linked hyaluronic acid stimulate production of new, undamaged collagen. Attachment of fibroblasts to this new collagen allows stretch, which in turn balances collagen production and degradation and thereby slows the aging process. Collagen fragmentation is responsible for loss of structural integrity and impairment of fibroblast function in aged human skin. Treatments that stimulate production of new, nonfragmented collagen should provide substantial improvement to the appearance and health of aged skin.

Pigmentation effects of solar-simulated radiation as compared with UVA and UVB radiation

Different wavelengths of ultraviolet (UV) radiation elicit different responses in the skin. UVA induces immediate tanning and persistent pigment darkening through oxidation of pre-existing melanin or melanogenic precursors, while UVB induces delayed tanning which takes several days or longer to develop and requires activation of melanocytes. We compared the effects of a 2-week repetitive exposure of human skin to solar-simulated radiation (SSR), UVA or UVB at doses eliciting comparable levels of visible tanning and measured levels of melanins and melanin-related metabolites. Levels of eumelanin and pheomelanin were significantly higher in the order of SSR, UVB, UVA or unexposed control skin. Levels of free 5-S-cysteinyldopa (5SCD) were elevated about 4-fold in SSR- or UVB-exposed skin compared with UVA-exposed or control skin. Levels of protein-bound form of 5SCD tended to be higher in SSR- or UVB-exposed skin than in UVA-exposed or control skin. Total levels of 5-hydroxy-6-methoxyindole-2-carboxylic acid (5H6MI2C) and 6H5MI2C were higher in SSR- than in UVB-exposed or control skin. These results show that SSR is more effective in promoting delayed tanning than UVB radiation alone, suggesting a synergistic effect of UVA radiation. Furthermore, free 5SCD may serve as a good marker of the effect of SSR and UVB.

Timeline and distribution of melanocyte precursors in the mouse heart

Apart from the well-studied melanocytes of the skin, eye and inner ear, another population has recently been described in the heart. In this study, we tracked cardiac melanoblasts using in situ hybridization with a dopachrome tautomerase (Dct) probe and Dct-LacZ transgenic mice. Large numbers of melanoblasts were found in the atrioventricular (AV) endocardial cushions at embryonic day (E) 14.5 and persisted in the AV valves into adulthood. The earliest time Dct-LacZ-positive cells were observed in the AV endocardial cushions was E12.5. Prior to that, between E10.5 and E11.5, small numbers of melanoblasts traveled between the post-otic area and third somite along the anterior and common cardinal veins and branchial arch arteries with other neural crest cells expressing CRABPI. Cardiac melanocytes were not found in the spotting mutants Ednrb s-l/s-l and Kit w-v/w-v, while large numbers were observed in transgenic mice that overexpress endothelin 3. These results indicate that cardiac melanocytes depend on the same signaling molecules known to be required for proper skin melanocyte development and may originate from the same precursor population. Cardiac melanocytes were not found in zebrafish or frog but were present in quail suggesting an association between cardiac melanocytes and four-chambered hearts.

Cyclobutane pyrimidine dimer formation and p53 production in human skin after repeated UV irradiation

Substantial differences in DNA damage caused by a single UV irradiation were found in our previous study on skin with different levels of constitutive pigmentation. In this study, we assessed whether facultative pigmentation induced by repeated UV irradiation is photoprotective. Three sites on the backs of 21 healthy subjects with type II-III skin were irradiated at 100-600 J/m(2) every 2-7 days over a 4- to 5-week period. The three sites received different cumulative doses of UV (1900, 2900 or 4200 J/m(2)) and were biopsied 1 day after the last irradiation. Biomarkers examined included pigment content assessed by Fontana-Masson staining, melanocyte function by expression of melanocyte-specific markers, DNA damage as cyclobutane pyrimidine dimers (CPD), nuclear accumulation of p53, apoptosis determined by TUNEL assay, and levels of p21 and Ser46-phosphorylated p53. Increases in melanocyte function and density, and in levels of apoptosis were similar among the 3 study sites irradiated with different cumulative UV doses. Levels of CPD decreased while the number of p53-positive cells increased as the cumulative dose of UV increased. These results suggest that pigmentation induced in skin by repeated UV irradiation protects against subsequent UV-induced DNA damage but not as effectively as constitutive pigmentation.

Human melanocytes expressing MC1R variant alleles show impaired activation of multiple signaling pathways

Variant alleles of the human MC1R gene are strongly associated with red hair color, fair skin and poor tanning ability (RHC-trait). Recently, we demonstrated that melanocytes harboring RHC-associated alleles have markedly reduced surface expression and/or impaired G-protein coupling of the corresponding receptor protein. The consequences of such a deficit on MC1R-mediated signaling pathways have now been quantitatively evaluated utilizing strains of human primary melanocytes homozygous for RHC-associated variant alleles and comparing responses to wild-type strains. The ability of melanocortin peptides to increase transcription of cAMP-dependent pigmentation genes, including MITF and SLC45A2, was abrogated in melanocytes with RHC-associated variant alleles, an effect that may contribute to the RHC phenotype. Activation of the c-Fos transcription factor gene was also severely compromised, a finding of potential relevance for non-pigmentary roles of MC1R. We also confirmed p38 signaling as an MC1R-regulated pathway and identified a large synergistic interaction between UV irradiation and MC1R stimulation for the activation of p38. This synergism was impaired in melanocytes expressing RHC variants of MC1R which may be relevant for the poor tanning ability associated with individuals possessing these alleles.

BRAF(E600) in benign and malignant human tumours

Of the RAF family of protein kinases, BRAF is the only member to be frequently activated by mutation in cancer. A single amino acid substitution (V600E) accounts for the vast majority and results in constitutive activation of BRAF kinase function. Its expression is required to maintain the proliferative and oncogenic characteristics of BRAF(E600)-expressing human tumour cells. Although BRAF(E600) acts as an oncogene in the context of additional genetic lesions, in primary cells it appears to be associated rather with transient stimulation of proliferation. Eventually, BRAF(E600) signalling triggers cell cycle arrest with the hallmarks of cellular senescence, as is illustrated by several recent studies in cultured cells, animal models and benign human lesions. In this review, we will discuss recent advances in our understanding of the role of BRAF(E600) in benign and malignant human tumours and the implications for therapeutic intervention.