Urocanic acid isomers: relation to body site, pigmentation, stratum corneum thickness and photosensitivity

Low-concentration imiquimod treatment promotes enhanced skin barrier functions through epidermal melanization reaction regulation

The primary function of the skin is to form a mechanical, permeability, antimicrobial, and ultraviolet radiation barrier, which is essential for maintaining physiological homeostasis. Our previous studies demonstrated that cutaneous pigmentation could promote skin barrier function in addition to providing anti-ultraviolet irradiation defense. The present study aimed to develop a new regimen that enhances skin barrier function by regulating skin pigmentation using low-concentration imiquimod. Results showed that topical application of low-concentration imiquimod effectively induced skin hyperpigmentation in the dorsal skin and external ear of mice without inducing inflammatory cell infiltration. An in vitro study also revealed that low-concentration imiquimod did not induce any cytotoxic effects on melanoma cells but triggered excessive melanin synthesis. In coculture systems, low-concentration imiquimod was noted to increase tyrosinase activity in a broader cellular context, revealing the potential role of neighboring cells in melanin production. The next-generation sequencing result indicated that PKCη and Dnm3 might regulate melanin synthesis and release during imiquimod treatment. Overall, our study presents new insights into the regulation of melanin production by low-concentration imiquimod, both in a mice model and cultured cells. Furthermore, our study highlights the potential benefits of imiquimod in promoting melanin synthesis without causing skin disruptions or inducing inflammation, validating its potential to serve as a method for enhancing skin barrier functions by regulating the epidermal melanization reaction.

The evolution of human skin pigmentation involved the interactions of genetic, environmental, and cultural variables

The primary biological role of human skin pigmentation is as a mediator of penetration of ultraviolet radiation (UVR) into the deep layers of skin and the cutaneous circulation. Since the origin of Homo sapiens, dark, protective constitutive pigmentation and strong tanning abilities have been favored under conditions of high UVR and represent the baseline condition for modern humans. The evolution of partly depigmented skin and variable tanning abilities has occurred multiple times in prehistory, as populations have dispersed into environments with lower and more seasonal UVR regimes, with unique complements of genes and cultural practices. The evolution of extremes of dark pigmentation and depigmentation has been rare and occurred only under conditions of extremely high or low environmental UVR, promoted by positive selection on variant pigmentation genes followed by limited gene flow. Over time, the evolution of human skin pigmentation has been influenced by the nature and course of human dispersals and modifications of cultural practices, which have modified the nature and actions of skin pigmentation genes. Throughout most of prehistory and history, the evolution of human skin pigmentation has been a contingent and non-deterministic process.

Analytical tools for urocanic acid determination in human samples: A review

Urocanic acid is a chromophore found in the skin that has been identified as an important immunosuppressant and carcinogenesis mediator through its photoisomerization from trans to cis form induced by ultraviolet radiation. Research on analytical methods that explore urocanic acid isomerization is indispensable to fully understand the deleterious effects mediated by this biomarker. In this context, the current relevant analytical methods for determination of these isomers in human samples are summarized in this review. The methods presented here are applicable to human samples collected by noninvasive methods (or minimally invasive), encompassing an array of analytical techniques, including high-performance capillary electrophoresis, confocal Raman spectroscopy, gas chromatography, high-performance liquid chromatography, and mass spectrometry, among others. Developed high-performance liquid chromatography methods have proven to be advantageous, allowing noninvasive collections for in vivo analysis and the confocal Raman, specially, for real-time analysis. Among all these methods, high-performance liquid chromatography is the most investigated one with mass spectrometry or ultraviolet detector, and the mass spectrometry detector being the most studied in the last years, demonstrating high sensitivity, very low detection limits, and accurate identification, especially for clinical investigations.

Genome-wide association study identifies 143 loci associated with 25 hydroxyvitamin D concentration

Vitamin D deficiency is a candidate risk factor for a range of adverse health outcomes. In a genome-wide association study of 25 hydroxyvitamin D (25OHD) concentration in 417,580 Europeans we identify 143 independent loci in 112 1-Mb regions, providing insights into the physiology of vitamin D and implicating genes involved in lipid and lipoprotein metabolism, dermal tissue properties, and the sulphonation and glucuronidation of 25OHD. Mendelian randomization models find no robust evidence that 25OHD concentration has causal effects on candidate phenotypes (e.g. BMI, psychiatric disorders), but many phenotypes have (direct or indirect) causal effects on 25OHD concentration, clarifying the epidemiological relationship between 25OHD status and the health outcomes examined in this study.

Reference values for skin microanatomy: A systematic review and meta-analysis of ex vivo studies

BACKGROUND

Few studies have characterized reference values of normal human skin microanatomy parameters.

OBJECTIVE

To quantify histologic measurements of epidermal thickness, melanocyte density, hair follicle density, and eccrine gland density as a function of age and anatomic site.

METHOD

We searched the PubMed, Embase, Web of Science, and Cochrane databases for articles published through May 25, 2017. Two reviewers independently screened 2016 articles; 327 relevant articles and 151 additional articles found via forward or reference citations underwent full-text review by 1 of 4 reviewers for relevance, data extraction, and critical appraisal. Weighted averages, meta-analysis, and meta-regression were used in statistical analysis.

RESULTS

A total of 56 articles were included; when all anatomic locations were used, the overall estimates for epidermal thickness, melanocyte density, hair follicle density, and eccrine gland density were 99.75 μm (95% confidence interval [CI], 83.25-116.25), 955.05 cells/mm² (95% CI. 880.89-1029.21), 1.40 hairs/mm² (95% CI. 0.91-1.89), and 1.28 glands/mm² (95% CI. 0.91-1.64), respectively.

LIMITATIONS

There was significant data heterogeneity across studies, possibly because of differences in histological techniques and absence of standardized microanatomy definitions.

CONCLUSIONS

We established summary estimates for normal human skin microanatomy parameters.

The impact of skin colour on human photobiological responses

Terrestrial solar ultraviolet radiation (UVR) exerts both beneficial and adverse effects on human skin. Epidemiological studies show a lower incidence of skin cancer in people with pigmented skins compared to fair skins. This is attributed to photoprotection by epidermal melanin, as is the poorer vitamin D status of those with darker skins. We summarize a wide range of photobiological responses across different skin colours including DNA damage and immunosuppression. Some studies show the generally modest photoprotective properties of melanin, but others show little or no effect. DNA photodamage initiates non‐melanoma skin cancer and is reduced by a factor of about 3 in pigmented skin compared with white skin. This suggests that if such a modest reduction in DNA damage can result in the significantly lower skin cancer incidence in black skin, the use of sunscreen protection might be extremely beneficial for susceptible population. Many contradictory results may be explained by protocol differences, including differences in UVR spectra and exposure protocols. We recommend that skin type comparisons be done with solar‐simulated radiation and standard erythema doses or physical doses (J/m²) rather than those based solely on clinical endpoints such as minimal erythema dose (MED).

The symbiosis of phototherapy and photoimmunology

The health benefits of natural sunlight have been noted since the rise of civilization, even without the knowledge of its mechanisms of action. Currently, phototherapy remains an effective and widely used treatment for a variety of skin diseases. Ultraviolet radiation, from either the sun or artificial light sources, has a profound immunomodulatory effect that is responsible for its beneficial clinical outcomes. Ultraviolet radiation mostly induces the innate while suppressing the adaptive immune system, leading to both local and systemic effects. It is antigen specific, acts on both effector and regulatory T cells, alters antigen-presenting cell function, and induces the secretion of cytokines and soluble mediators. This review provides an overview of the immunologic mechanisms by which ultraviolet radiation is responsible for the therapeutic effects of phototherapy.

Microwave discharge electrodeless lamps (MDEL). Part VII. Photo-isomerization of trans-urocanic acid in aqueous media driven by UV light from a novel Hg-free Dewar-like microwave discharge thermally-insulated electrodeless lamp (MDTIEL). Performance evaluation

A novel mercury-free Dewar-like (double-walled structure) microwave discharge thermally-insulated electrodeless lamp (MDTIEL) was fabricated and its performance evaluated using the photo-isomerization of trans-urocanic acid (trans-UA) in aqueous media as a test process driven by the emitted UV light when ignited with microwave radiation. The photo-isomerization processes trans-UA → cis-UA and cis-UA → trans-UA were re-visited using light emitted from a conventional high-pressure Hg light source and examined for the influence of UV light irradiance and solution temperature; the temperature dependence of the trans → cis process displayed a negative activation energy, E(a) = -1.3 cal mol(-1). To control the photo-isomerization of urocanic acid from the heat usually dissipated by a microwave discharge electrodeless lamp (single-walled MDEL), it was necessary to suppress the microwave-initiated heat. For comparison, the gas-fill in the MDEL lamp, which typically consists of a mixture of Hg and Ar, was changed to the more eco-friendly N(2) gas in the novel MDTIEL device. The dynamics of the photo-isomerization of urocanic acid driven by the UV wavelengths of the N(2)-MDTIEL light source were compared to those from the more conventional single-walled N(2)-MDEL and Hg/Ar-MDEL light sources, and with those from the Hg lamp used to irradiate, via a fiber optic, the photoreactor located in the wave-guide of the microwave apparatus. The heating efficiency of a solution with the double-walled N(2)-MDTIEL was compared to the efficiency from the single-walled N(2)-MDEL device. Advantages of N(2)-MDTIEL are described from a comparison of the dynamics of the trans-UA → cis-UA process on the basis of unit surface area of the lamp and unit power consumption. The considerably lower temperature on the external surface of the N(2)-MDTIEL light source should make it attractive in carrying out photochemical reactions that may be heat-sensitive such as the photothermochromic urocanic acid system.

cis-Urocanic acid enhances prostaglandin E2 release and apoptotic cell death via reactive oxygen species in human keratinocytes

Urocanic acid (UCA) is a major UVR-absorbing skin molecule that undergoes trans to cis photoisomerization in the epidermis following UVR exposure. Murine studies have established that cis-UCA is an important mediator of UVR-induced immune suppression, but little is known about its signaling pathway. We have previously demonstrated that treatment of normal human epidermal keratinocytes with cis-UCA resulted in increased synthesis of prostaglandin E(2) (PGE(2)) and cell death. Here, using immortalized human keratinocytes, we report that cis-UCA but not trans-UCA generates reactive oxygen species (ROS) in a dose-dependent manner and that the natural antioxidant α-tocopherol can reduce this ROS generation, subsequent PGE(2) release, and apoptotic cell death. Western blot analysis revealed that cis-UCA leads to a transient phosphorylation of EGFR as well as downstream mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase (ERK) and p38. Pharmacological inhibition of their activity attenuated PGE(2) release induced by cis-UCA. After transient activation, cis-UCA downregulated EGFR protein expression that corresponded to activation of caspase-3. In addition, pretreatment with α-tocopherol inhibited EGFR downregulation and caspase-3 activation induced by cis-UCA. These results suggest that cis-UCA exerts its effects on human keratinocytes via intracellular ROS generation that modulates EGFR signaling and subsequently induces PGE(2) synthesis and apoptotic cell death.

Urocanic acid in the skin: a mixed blessing?

Located in the stratum corneum, urocanic acid is a major epidermal chromophore for UVR. This simple molecule has attracted a great deal of research interest over the past half century, initially as a putative "natural sunscreen" and later as a mediator of photoimmunosuppression with a consequent role in photocarcinogenesis. For the first time, Barresi and colleagues provide robust evidence for the photoprotective role of endogenous urocanic acid and reopen the debate on the relative "beneficial" and "detrimental" properties of this molecule.

Absorption spectra of human skin in vivo in the ultraviolet wavelength range measured by optoacoustics

Knowledge of the optical properties of human skin in the ultraviolet range is fundamental for photobiologic research. However, optical properties of human skin in the ultraviolet spectral range have so far mainly been measured ex vivo. We have determined the absorption spectra of human skin in vivo in the wavelength range from 290 to 341 nm in 3 nm steps using laser optoacoustics. In this technique, optical properties are derived from the pressure profile generated by absorbed light energy in the sample. In a study on 20 subjects belonging to phototypes I-IV, we studied the optical properties at the volar and dorsal aspect of the forearm as well as on the thenar. Analysis of the measured absorption spectra shows that comparable skin areas-like different sides of the forearm-have qualitatively similar optical characteristics. Still, the optical properties may vary substantially within the same area, probably due to the skin structure and inhomogeneities. Comparison of the spectra from different skin sites indicates that the spectral characteristics of the stratum corneum and its chromophores play an important role for the optical properties of human skin in vivo in the ultraviolet B range.

Wavelength-dependent penetration depths of ultraviolet radiation in human skin

The wavelength-dependent penetration depth of ultraviolet radiation in human skin is a fundamental parameter for the estimation of the possible photobiological impact of ultraviolet (UV) radiation. We have determined the absorption spectra of human skin in vivo in the wavelength range from 290 to 341 nm in 3-nm steps using laser optoacoustics and calculated the respective penetration depths. Data were analyzed with respect to different skin regions and skin phototype of the 20 subjects in the study (phototype I: n=3; II: n=7; III: n=5; IV: n=5), revealing large variability between individuals. The penetration depth of UV radiation in human skin is highly dependent on wavelength and skin area, but no significant dependence on skin phototype could be found.

Recent advances in urocanic acid photochemistry, photobiology and photoimmunology

Urocanic acid (UCA), produced in the upper layers of mammalian skin, is a major absorber of ultraviolet radiation (UVR). Originally thought to be a 'natural sunscreen', studies conducted a quarter of a century ago proposed that UCA may be a chromophore for the immunosuppression that follows exposure to UVR. With its intriguing photochemistry, its role in immunosuppression and skin cancer development, and skin barrier function, UCA continues to be the subject of intense research effort. This review summarises the photochemical, photobiological and photoimmunological findings regarding UCA, published since 1998.

Studies to determine the immunomodulating effects of cis-urocanic acid

Exposure to ultraviolet (UV) radiation, particularly the UVB wavelengths, leads not only to DNA damage but also to suppression of cell-mediated immunity to antigens encountered shortly after the irradiation. One initiator of this complex process is cis-urocanic acid (cis-UCA), which is formed from the naturally occurring trans isomer in the epidermis on absorption of UV. cis-UCA has been shown to have immunomodulating properties in a variety of in vivo and in vitro experimental systems, although its mechanism of action is not yet clear. This article covers methods of preparing cis-UCA and of analyzing UCA isomers in various human and mouse tissues. Experiments that demonstrate that cis-UCA is immunosuppressive are described. The final section deals with the preparation and characterization of a monoclonal antibody with specificity for cis-UCA.

Repeated ultraviolet exposure affords the same protection against DNA photodamage and erythema in human skin types II and IV but is associated with faster DNA repair in skin type IV

We have investigated the photoprotective properties of induced pigmentation using erythema and epidermal DNA photodamage as endpoints. Previously unexposed buttock skin of 12 young, healthy adults (six skin type II and six skin type IV) was exposed daily (Monday to Friday) for 2 wk (days 1-12) with 0.65 minimal erythema dose of solar simulated radiation. Mean skin type IV minimal erythema dose was 1.8-fold greater than for skin type II. Compared to skin type II, solar simulated radiation treatments produced less erythema and more tanning in skin type IV. To assess DNA photodamage, biopsies were taken and prepared for paraffin sections that were stained with a monoclonal antibody for thymine dimers. Thymine dimers were quantified by image analysis. The single exposure data (0.65 and 2 minimal erythema dose) showed that DNA damage was related to physical dose (J per cm2) independent of skin type. Our data also showed that DNA photodamage accumulates in both skin types with repeated, suberythemal doses of solar simulated radiation. On day 12, there were more thymine dimers in skin type IV than skin type II, again indicating that physical rather than biologic dose determines the level of DNA damage. Comparisons on days 12 and 19, however, showed a much greater loss of thymine dimers in skin type IV, suggesting better thymine dimer repair. Protection factors for erythema and thymine dimers were calculated and shown to be about 2 in both skin types. This provides further indirect evidence that DNA is a chromophore for erythema, but also suggests that a tan may not be the major factor in natural photoprotection.

Effects of solar radiation on the human immune system

On UV irradiation of the skin, a complex cascade of immunological changes results, initiated by cutaneous chromophores and ending in suppression of some local and systemic immune responses. In this review, the stages in this process are outlined first, concentrating on the roles of DNA and urocanic acid as photoreceptors. Evidence indicating UV-induced immunomodulation of delayed hypersensitivity and resistance to infectious diseases in human subjects follows. Aspects of genetic susceptibility to the immunosuppressive effects of UV exposure and extrapolation of the data obtained in animal models to the human situation are included. Finally uncertain and unknown factors relating to the impact of UV on the human immune system are discussed.

Broad-spectrum sunscreens offer protection against urocanic acid photoisomerization by artificial ultraviolet radiation in human skin

Cis-urocanic acid (UCA) has been indicated as an important mediator of ultraviolet (UV)-induced immunosuppression. In this study we describe a rapid, noninvasive method for the determination of the protective capacity of various sunscreens against the UV-induced isomerization of trans-UCA into its cis form. For this purpose we applied sunscreens prior to in vivo exposure of human volunteers with single or repeated broadband UVB irradiations of 100 mJ per cm2. We found significant but different levels of protection against UCA photoisomerization by all sunscreens that correlated with the sun protection factor. A comparison of various sunscreens with a sun protection factor of 10, showed that the best protection was offered by the sunscreens (containing organic UV filters or TiO2) with broad absorption spectra. The ability to inhibit cis-UCA formation was not influenced by the penetration characteristics of sunscreens, as determined by application of the sunscreen on quartz glass that was placed on the skin, preventing penetration of sunscreen in the skin. In addition ex vivo UV exposure of human skin was employed to permit other tests of immunomodulation, in this case the mixed epidermal cell lymphocyte reaction. The advantage of this ex vivo method is that there is no need to take biopsies from volunteers. Ex vivo irradiation of human skin with a single dose of 200 mJ per cm2 resulted in similar protection by the sunscreens against cis-UCA formation as in the in vivo system. Furthermore, the mixed epidermal cell lymphocyte reaction data correlated with the cis-UCA findings. We conclude that UCA isomerization is an excellent method to determine sunscreen efficacy and that broad-spectrum sunscreens offer good immunoprotection.

The effects of ultraviolet radiation on the human immune system

The adverse outcome of increased ultraviolet (UV) irradiation on human health is currently of concern. While many experiments have been carried out in rodent models, fewer have been designed to test the effects of UV exposure in human subjects. This review concentrates on the modulations induced in the human immune system by UV, and outlines changes in antigen presentation by Langerhans cells and macrophages, in the activities of natural killer cells and T cells, and in cytokine regulation. Precautionary measures which might be taken to help protect people against the immunosuppressive action of UV irradiation are considered.

The degradation of L-histidine and trans- and cis-urocanic acid by bacteria from skin and the role of bacterial cis-urocanic acid isomerase

UV-B radiation suppresses cell-mediated immunity. Histidine forms trans-urocanic acid (trans-UCA) enzymatically in the stratum corneum. Photoisomerization of trans-UCA to cis-urocanic acid (cis-UCA) has been proposed for the initiation of an immunosuppressive process. Many microorganisms described in the literature metabolize histidine and/or trans-UCA. Our enrichment cultures of soil and sewage contain organisms that can degrade cis-UCA. We have tested microorganisms for degradation of cis-UCA, trans-UCA, or L-histidine when they are incorporated at 0.2% in nutrient broth. Six out of 10 selected genera isolated by our clinical microbiology laboratory degrade one or more of the imidazole substrates. We have cultured over 60 aerobic isolates from human skin. Of these, 33 degrade one or more of the three imidazole substrates and 12 degrade cis-UCA. Isolates from BALB/c mice are also active on cis-UCA. We have identified a cis-UCA-degrading bacterium as Micrococcus luteus. Four ATCC strains of M. luteus have been tested and three are active on histidine or trans-UCA; two are active on cis-UCA. Micrococci that degrade cis-UCA contain a new enzyme, cis-UCA isomerase, which converts the substrate to the trans-isomer. This enzyme provides access to the classical L-histidine degradation pathway. We hypothesize that an epidermal microflora that degrades L-histidine, trans-UCA, or cis-UCA influences the concentration of urocanic acids on the skin and, thus, affects immune suppression.

Urocanic acid isomers in patients with basal cell carcinoma and cutaneous malignant melanoma

Urocanic acid (UCA) is a major chromophore for ultraviolet (UV) radiation in the skin. On UV exposure, the naturally occurring trans-isomer converts to the cis-isomer in a dose-dependent manner. Accumulating evidence indicates that cis-UCA acts as an initiator of the UV-induced suppression of certain skin immune functions. This immunomodulation is recognized as an important factor in the development of skin cancer. In this study, pigmentation and UCA isomers were measured in 29 patients with previous basal cell carcinoma (BCC), 23 patients with previous cutaneous malignant melanoma (MM), and 32 healthy controls. Measurements were performed on UV-exposed (forehead, upper back) and UV non-exposed (buttock) skin. No significant differences in pigmentation percentage, total UCA concentration, relative (%) or absolute (nmol/cm2) cis-UCA concentration were observed between the groups in any of the body sites studied. The net production of cis-UCA after irradiation with a single test UV dose was evaluated. The relative production of cis-UCA following irradiation was significantly higher in both cancer groups when compared with the control group, while no significant difference was found between the BCC and the MM patients.