Quinones are reduced by 6-tetrahydrobiopterin in human keratinocytes, melanocytes, and melanoma cells

Assessing the Oxidative State of the Skin by Combining Classical Tape Stripping with ORAC Assay

The antioxidant barrier system of the skin acts as the main defence against environmental pro-oxidants. Impaired skin oxidative state is linked to unhealthy conditions such as skin autoimmune diseases and cancer. Thus, the evaluation of the overall oxidative state of the skin plays a key role in further understanding and prevention of these disorders. This study aims to present a novel ex vivo model to evaluate the skin oxidative state by the measurement of its antioxidant capacity (AOC). For this the ORAC assay was combined with classical tape stripping and infrared densitometry to evaluate the oxidative state of the stratum corneum (SC). Outcomes implied the suitability of the used model to determine the intrinsic antioxidant capacity (iAOC) of the skin. The average iAOC of untreated skin was determined as 140 ± 7.4 µM TE. Skin exposure to UV light for 1 h reduced the iAOC by about 17%, and exposure for 2 h decreased the iAOC by about 30%. Treatment with ascorbic acid (AA) increased the iAOC in a dose-dependent manner and reached an almost two-fold iAOC when 20% AA solution was applied on the skin. The application of coenzyme Q10 resulted in an increase in the iAOC at low doses but decreased the iAOC when doses > 1% were applied on the skin. The results show that the combination of classical tape stripping and ORAC assay is a cost-effective and versatile method to evaluate the skin oxidative state and the pro-oxidate and antioxidative effects of topical skin treatments on the iAOC of the skin. Therefore, the model can be considered to be a valuable tool in skin research.

Revisiting the role of melatonin in human melanocyte physiology: A skin context perspective

The evolutionarily ancient methoxyindoleamine, melatonin, has long perplexed investigators by its versatility of functions and mechanisms of action, which include the regulation of vertebrate pigmentation. Although first discovered through its potent skin-lightening effects in amphibians, melatonin's role in human skin and hair follicle pigmentation and its impact on melanocyte physiology remain unclear. Synthesizing our limited current understanding of this role, we specifically examine its impact on melanogenesis, oxidative biology, mitochondrial function, melanocyte senescence, and pigmentation-related clock gene activity, with emphasis on human skin, yet without ignoring instructive pointers from nonhuman species. Given the strict dependence of melanocyte functions on the epithelial microenvironment, we underscore that melanocyte responses to melatonin are best interrogated in a physiological tissue context. Current evidence suggests that melatonin and some of its metabolites inhibit both, melanogenesis (via reducing tyrosinase activity) and melanocyte proliferation by stimulating melatonin membrane receptors (MT1, MT2). We discuss whether putative melanogenesis-inhibitory effects of melatonin may occur via activation of Nrf2-mediated PI3K/AKT signaling, estrogen receptor-mediated and/or melanocortin-1 receptor- and cAMP-dependent signaling, and/or via melatonin-regulated changes in peripheral clock genes that regulate human melanogenesis, namely Bmal1 and Per1. Melatonin and its metabolites also accumulate in melanocytes where they exert net cyto- and senescence-protective as well as antioxidative effects by operating as free radical scavengers, stimulating the synthesis and activity of ROS scavenging enzymes and other antioxidants, promoting DNA repair, and enhancing mitochondrial function. We argue that it is clinically and biologically important to definitively clarify whether melanocyte cell culture-based observations translate into melatonin-induced pigmentary changes in a physiological tissue context, that is, in human epidermis and hair follicles ex vivo, and are confirmed by clinical trial results. After defining major open questions in this field, we close by suggesting how to begin answering them in clinically relevant, currently available preclinical in situ research models.

Advances in the Application and Impact of MicroRNAs as Therapies for Skin Disease

The advent of RNA interference (RNAi) technology has profoundly impacted molecular biology research and medicine but has also advanced the field of skin care. Both effector molecules of RNAi, short-interfering RNA molecules and microRNAs (miRNAs), have been explored for their relative impact and utility for treating a variety of skin conditions. These post-transcriptional RNA regulatory molecules down-modulate protein expression through targeting of the 3' untranslated regions of messenger RNAs, leading to their degradation or repression through sequestration. As researchers hunt for genetic linkages to skin diseases, miRNA regulators have emerged as key players in the biology of keratinocytes, fibroblasts, melanocytes, and other cells of the skin. Herein, we attempt to coalesce the current efforts to combat various skin disorders and diseases through the development of miRNA-based technologies.

In vitro Dermal Absorption of Hydroquinone: Protocol Validation and Applicability on Illegal Skin-Whitening Cosmetics

In Europe, hydroquinone is a forbidden cosmetic ingredient. It is, however, still abundantly used because of its effective skin-whitening properties. The question arises as to whether the quantities of hydroquinone used become systemically available and may cause damage to human health. Dermal absorption studies can provide this information. In the EU, dermal absorption has to be assessed in vitro since the Cosmetic Regulation 1223/2009/EC forbids the use of animals. To obtain human-relevant data, a Franz diffusion cell protocol was validated using human skin. The results obtained were comparable to those from a multicentre validation study. The protocol was applied to hydroquinone and the dermal absorption ranged between 31 and 44%, which is within the range of published in vivo human values. This shows that a well-validated in vitro dermal absorption study using human skin provides relevant human data. The validated protocol was used to determine the dermal absorption of illegal skin-whitening cosmetics containing hydroquinone. All samples gave high dermal absorption values, rendering them all unsafe for human health. These results add to our knowledge of illegal cosmetics on the EU market, namely that they exhibit a negative toxicological profile and are likely to induce health problems.

The melanogenesis and mechanisms of skin-lightening agents--existing and new approaches

Skin-lightening products are commercially available for cosmetic purposes to obtain lighter skin complexion. Clinically, they are also used for treatment of hyperpigmentary disorders such as melasma, café au lait spot and solar lentigo. All of these target naturally melanin production, and many of the commonly used agents are known as competitive inhibitors of tyrosinase, one of the key enzymes in melanogenesis. In this review, we present an overview of commonly used skin-whitening ingredients that are commercialized, but we also hypothesize on other mechanisms that could be important targets to control skin pigmentation such as for example regulation of the adrenergic and glutaminergic signalling and also control of tetrahydrobiopterins in the human skin.

In vivo and in vitro evidence for epidermal H2O2-mediated oxidative stress in piebaldism

Piebaldism is characterised by the absence of pigment in patches on the skin, usually present at birth. Mutations in the kit gene are documented. Clinically this disorder can mimic vitiligo. Here, we show for the first time the presence of oxidised pteridine-induced fluorescence in association with H2O2-mediated stress in piebald patches employing Wood's light and in vivo FT-Raman spectroscopy. In situ immunofluorescence data revealed low catalase and methionine sulphoxide reductase A (MSRA) levels whereas thioredoxin reductase and methionine sulphoxide reductase B (MSRB) are not affected. We also show low superoxide dismutase levels in these patients. The presence of thioredoxin reductase provides capacity to reduce H2O2, a mechanism which is absent in vitiligo. Importantly, this enzyme reduces biopterin back to the functioning cofactor 6-tetrahydrobiopterin. The absence of MSRA indicates deficient methionine sulphoxide repair in the cytosol, meanwhile the presence of MSRB is helpful to protect the nucleus. Taken together, we have identified H2O2-mediated stress in piebald skin with distinct differences to vitiligo.

Effects of tetrahydropterines on the generation of quinones catalyzed by tyrosinase

Tetrahydrobiopterine (6BH(4)) can diminish the oxidative stress undergone by keratinocytes and melanocytes by reducing the o-quinones generated by the oxidation of the corresponding o-diphenols. We found that 6BH(4) and their analogs reduced all the o-quinones studied. The formal potentials of different quinone/diphenol pairs indicate that the o-quinones with withdrawing groups are more potent oxidants than those with donating groups.

A plaidoyer for cutaneous enzymology: our view of some important unanswered questions on the contributions of selected key enzymes to epidermal homeostasis

This review highlights the importance of enzymology, a field of great neglect in current cutaneous biology research. It was therefore the aim by using selected examples of epidermal enzymes and their action including some open questions to demonstrate the importance of this area. Clearly a thorough understanding of basic knowledge in this field is needed which in turn offers a plethora of innovative research projects for a curious mind. Moreover, in order to gain the closest understanding to the truth instead of generating esoteric results, emphasis is put forward on a holistic view utilizing a combination of modern and sometimes old methods to get the answer. Last but not least the bench work is only useful for the welfare of our patients if we can apply our basic knowledge.

The Ca2+-binding capacity of epidermal furin is disrupted by H2O2-mediated oxidation in vitiligo

The Ca(2+)-dependent precursor convertase furin is abundantly expressed in epidermal keratinocytes and melanocytes. In this context, it is noteworthy that proopiomelanocortin (POMC) cleavage is also processed by furin, leading to ACTH, beta-lipotropin, and beta-endorphin. All prohormone convertases including furin are regulated by Ca(2+). Because numerous epidermal peptides and enzymes are affected by H(2)O(2)-mediated oxidation, including the POMC-derived peptides alpha-MSH and beta-endorphin as shown in the epidermis of patients with vitiligo, we here asked the question of whether furin could also be a possible target for this oxidation mechanism by using immunofluorescence, RT-PCR, Western blotting, Ca(2+)-binding studies, and computer modeling. Our results demonstrate significantly decreased in situ immunoreactivity of furin in the epidermis of patients with progressive vitiligo (n = 10), suggesting H(2)O(2)-mediated oxidation. This was confirmed by (45)Ca(2+)-binding studies with human recombinant furin identifying the loss of one Ca(2+)-binding site from the enzyme after oxidation with H(2)O(2). Computer simulation supported alteration of one of the two Ca(2+)-binding sites on furin. Taken together, our results implicate that the Ca(2+)-dependent proteolytic activity of this convertase is targeted by H(2)O(2), which in turn could contribute to the reduced epidermal expression of the POMC-derived peptides alpha-MSH and beta-endorphin as documented earlier in patients with vitiligo.