Seasonal variation in urocanic acid isomers in human skin

A deep dive into UV-based phototherapy: Mechanisms of action and emerging molecular targets in inflammation and cancer

UV-based phototherapy (including psoralen plus UVA (PUVA), UVB and UVA1) has a long, successful history in the management of numerous cutaneous disorders. Photoresponsive diseases are etiologically diverse, but most involve disturbances in local (and occasionally systemic) inflammatory cells and/or abnormalities in keratinocytes that trigger inflammation. UV-based phototherapy works by regulating the inflammatory component and inducing apoptosis of pathogenic cells. This results in a fascinating and complex network of simultaneous events-immediate transcriptional changes in keratinocytes, immune cells, and pigment cells; the emergence of apoptotic bodies; and the trafficking of antigen-presenting cells in skin-that quickly transform the microenvironment of UV-exposed skin. Molecular elements in this system of UV recognition and response include chromophores, metabolic byproducts, innate immune receptors, neurotransmitters and mediators such as chemokines and cytokines, antimicrobial peptides, and platelet activating factor (PAF) and PAF-like molecules that simultaneously shape the immunomodulatory effects of UV and their interplay with the microbiota of the skin and beyond. Phototherapy's key effects-proapoptotic, immunomodulatory, antipruritic, antifibrotic, propigmentary, and pro-prebiotic-promote clinical improvement in various skin diseases such as psoriasis, atopic dermatitis (AD), graft-versus-host disease (GvHD), vitiligo, scleroderma, and cutaneous T-cell lymphoma (CTCL) as well as prevention of polymorphic light eruption (PLE). As understanding of phototherapy improves, new therapies (UV- and non-UV-based) are being developed that will modify regulatory T-cells (Treg), interact with (resident) memory T-cells and /or utilize agonists and antagonists as well as antibodies targeting soluble molecules such as cytokines and chemokines, transcription factors, and a variety of membrane-associated receptors.

Seasonal variations in the skin parameters of Caucasian women from Central Europe

BACKGROUND

The human skin is greatly affected by external factors such as UV radiation (UVR), ambient temperature (T), and air humidity. These factors oscillate during the year giving rise to the seasonal variations in the skin properties. The aim of this study was to evaluate the effect of seasons, environmental T, relative and absolute humidity on the skin parameters of Caucasian women, perform a literature review and discuss the possible factors lying behind the found changes.

MATERIALS AND METHODS

We measured stratum corneum (SC) hydration, transepidermal water loss (TEWL), sebum level, erythema index, and elasticity parameters R2 and R7 on the forehead and the cheek of Caucasian women from the Czech Republic throughout the year. We also performed a non-systematic literature review focused on the seasonal variations in these skin parameters.

RESULTS

We confirmed a well-documented low SC hydration and sebum production in winter. In spring, we found the lowest TEWL (on the forehead) and the highest SC hydration but also the highest erythema index and the lowest elasticity presumably indicating skin photodamage. For most of the skin parameters, the seasonal variations probably arise due to a complex action of different factors as we extensively discussed.

CONCLUSION

The data about the seasonal variations in the skin parameters are still highly inconsistent and further studies are needed for better understanding of the normal skin changes throughout the year.

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.

A review of studies on the effects of ultraviolet irradiation on the resistance to infections: evidence from rodent infection models and verification by experimental and observational human studies

Recent studies on the immunosuppressive effects of ultraviolet radiation (UVR) and the related resistance to infections in rodents and humans are presented. The waveband dependency of trans-to-cis isomerisation of urocanic acid in the stratum corneum and the role of DNA damage in UVR-induced erythema and immunosuppression were investigated to further elucidate the underlying mechanisms. Furthermore, human experimental studies on UVR-induced immunomodulation were performed. It appeared that the doses needed to suppress various immune parameters in humans (e.g. NK activity, contact hypersensitivity) were higher than those needed in experiments in rodents. Still, extrapolation of experimental animal data to the human situation showed that UVR may impair the resistance to different systemic infections at relevant outdoor doses. In observational human studies we aimed to substantiate the relevance of UVR for infections in humans. It was shown that sunny season was associated with a slightly retarded but clinically non-relevant antibody response to hepatitis B vaccination. Furthermore, sunny season appeared to be associated with a small decline in the number of CD4+ T-helper cells in a cohort of HIV-infected persons and a higher recurrence of herpes simplex and herpes zoster in a cohort of renal transplant recipients. However, in a study among young children a higher exposure to solar UVR was associated with a lower occurrence of upper respiratory tract symptoms. As disentangling the effects of UVR from other relevant factors is often impossible in observational studies, concise quantitative risk estimations for the human situation cannot be given at present.

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.

Urocanic acid isomers in patients with non-melanoma skin cancer

BACKGROUND

Cis-urocanic acid (cis-UCA), formed from the naturally occurring trans-isomer in the epidermis on ultraviolet (UV) radiation, initiates some of the changes leading to UV-induced immunosuppression, but its role in cutaneous carcinogenesis has not been fully investigated.

OBJECTIVES

To measure the concentration of UCA isomers in the photoexposed and non-photoexposed skin of patients with multiple non-melanoma skin cancer (NMSC), enrolled in different periods of the year, in comparison with control subjects.

PATIENTS/METHODS

UCA isomers were determined by high-performance liquid chromatography analysis in samples from the outer arm (photoexposed site) and buttock (non-photoexposed site) obtained from 20 patients and 19 controls during the winter period (October to April), and from five patients and 11 controls during the summer period (June to September).

RESULTS

In the winter months, no difference was found between patients and controls in the concentration of UCA or the percentage of cis-UCA in either site. In the summer months, the percentage of cis-UCA in the buttock of patients and controls was similar but it was significantly higher in the arm of the controls (42%) than in the patients (17%).

CONCLUSIONS

We conclude that different behaviour regarding sun exposure is the most likely explanation for these results, and that the concentration of UCA and its isomers does not reflect a tendency for individuals to develop NMSC.

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 and photosensitivity in healthy children

Episodes of intense sun exposure, particularly in childhood, seem to carry a risk for the development of malignant melanoma in later life. However, little is known about photosensitivity and natural photoprotection in children. In adult subjects, photoprotection is provided mainly by the epidermal content of melanin and the thickness of the stratum corneum, while the amount of urocanic acid (UCA), a major ultraviolet-absorbing component of the stratum corneum, is not thought to contribute significantly to photoprotection. The minimal erythema dose (MED) was determined in 22 healthy children aged 6-13 years and in 36 healthy adults (mean age 28.1 years). Pigmentation was measured at six body sites by use of reflectance spectroscopy and the concentration of UCA isomers was measured in a sun-exposed area (upper back) and in unexposed buttock skin. No significant differences between children and adults were found, either in pigmentation at exposed and unexposed body sites, or in MED. The concentration of total UCA was significantly higher in the children than in the adults on the buttock (median 22.2 vs. 13.6 nmol/cm2), but not on the back. On exposed back skin, the children had a significantly higher percentage of cis-UCA than the adults (median 60.1 vs. 28.3%), while no difference was found on the buttock. In both groups, a significant correlation was found between pigmentation and MED (children: Spearman correlation coefficient 0. 58, P = 0.006; adults: Spearman correlation coefficient 0.69, P < 0. 0001), indicating that pigmentation is of major importance in determining photosensitivity in children as well as in adults. The concentration of total UCA did not correlate with the MED in either group.