Urocanic acid and immunosuppression

Keratinocyte Carcinoma and Photoprevention: The Protective Actions of Repurposed Pharmaceuticals, Phytochemicals and Vitamins

Simple Summary

Keratinocyte carcinoma is the most common type of cancer. Sun exposure and ultraviolet radiation are significant contributors to the development of carcinogenesis, mediated by DNA damage, increased oxidative stress, inflammation, immunosuppression and dysregulated signal transduction. Photoprevention involves using different compounds to delay or prevent ultraviolet radiation-induced skin cancer. In this review, we look at new avenues for systemic photoprevention that are based on pharmaceuticals, plant-derived phytochemicals and vitamins. We also investigate the mechanisms underlying these strategies for preventing the onset of carcinogenesis.

Abstract

Ultraviolet radiation (UVR) arising from sun exposure represents a major risk factor in the development of keratinocyte carcinomas (KCs). UVR exposure induces dysregulated signal transduction, oxidative stress, inflammation, immunosuppression and DNA damage, all of which promote the induction and development of photocarcinogenesis. Because the incidence of KCs is increasing, better prevention strategies are necessary. In the concept of photoprevention, protective compounds are administered either topically or systemically to prevent the effects of UVR and the development of skin cancer. In this review, we provide descriptions of the pathways underlying photocarcinogenesis and an overview of selected photoprotective compounds, such as repurposed pharmaceuticals, plant-derived phytochemicals and vitamins. We discuss the protective potential of these compounds and their effects in pre-clinical and human trials, summarising the mechanisms of action involved in preventing photocarcinogenesis.

Non-Melanoma Skin Cancers: Biological and Clinical Features

Non-melanoma skin cancers (NMSCs) include basal cell carcinoma (BCC), squamous cell carcinoma (SCC) and Merkel cell carcinoma (MCC). These neoplasms are highly diverse in their clinical presentation, as well as in their biological evolution. While the deregulation of the Hedgehog pathway is commonly observed in BCC, SCC and MCC are characterized by a strikingly elevated mutational and neoantigen burden. As result of our improved understanding of the biology of non-melanoma skin cancers, innovative treatment options including inhibitors of the Hedgehog pathway and immunotherapeutic agents have been recently investigated against these malignancies, leading to their approval by regulatory authorities. Herein, we review the most relevant biological and clinical features of NMSC, focusing on innovative treatment approaches.

Conformational isomers of trans-urocanic acid observed by rotational spectroscopy

Rotational spectra have been measured and assigned for four conformers of trans-urocanic acid. The acid was transferred into the gas phase through laser vaporisation of a solid sample, mixed with a neon buffer gas and then cooled through supersonic expansion. Molecules and complexes in the expanding gas jet were probed through chirped-pulse, Fourier transform microwave spectroscopy between 2.0 and 18.5 GHz. Rotational constants, A0, B0 and C0; centrifugal distortion constants, ΔJ and ΔJK; and nuclear quadrupole coupling constants of the nitrogen atoms, χaa(N) and χbb(N)-χcc(N), were determined for the various conformers. Data were obtained for ten isotopologues of the conformer that was observed to yield the spectrum of highest intensity. Substitution (rs) coordinates were determined for all carbon atoms and two hydrogen atoms of this conformer. Other observed spectra were assigned to conformers on the basis of excellent agreement between calculated and experimentally-determined rotational constants, and empirical observations of the relative intensities of a- and b-type transitions. The results of DFT calculations imply high barriers to the interconversion of assigned conformers.

Final Report on the Safety Assessment of Urocanic Acid

Urocanic Acid is a naturally occurring metabolite of histidine. The trans-Urocanic Acid isomer is found as a normal constituent of the epidermis, where it accumulates because there are only very low levels of the enzyme urocanase available to break it down; the accumulation causes trans-Urocanic Acid excretion in sweat. On exposure to UV radiation present in sunlight, the trans-Urocanic Acid isomer converts to the cis-Urocanic Acid isomer. In cosmetic formulations, Urocanic Acid is used as a skin-conditioning agent and as a sunscreen. Several questions were specifically considered in this safety assessment, including the extent to which applied Urocanic Acid is absorbed by the skin and, if absorbed, what the effect is on endogenous levels. Recognizing that photoisomerization is likely to occur in the skin, what is the resultant ability of cis-Urocanic Acid to act as an immunosuppressant? If the ingredient does cause immunosuppression, is there concomitant enhancement of photo-carcinogenesis? The available data indicate that Urocanic Acid is absorbed in mouse and human skin, although at a faster rate in mouse skin. Limited human data suggest that there is no increase in the total level (endogenous + applied) of Urocanic Acid in the skin over a 16-week period. Extensive animal data indicate that cis-Urocanic Acid is an immunosuppressant, but the clinical data are inconclusive as to the immunosuppressant effect of Urocanic Acid in humans (it may be problematic that Urocanic Acid was not exposed to UV radiation in the clinical tests). To directly assess the question of enhanced photocarcinogenesis, the results of two studies were considered. In one study of hairless mice, no neoplasms were found in the group exposed only to trans-Urocanic Acid, carcinomas were found in the group that received UV exposure and no trans-Urocanic Acid, and a significantly greater number of carcinomas was found in the group exposed to trans-Urocanic Acid followed by UV exposure. In a second study, using three similar groups of hairless mice (Urocanic Acid alone, UV alone, and UV plus varying concentrations of Urocanic Acid), all groups showed comparable numbers of carcinomas, papillomas, and other tumors. While there was concern about the influence of the methodologies on the interpretation of results in these two studies, the results from neither study could be discounted. Only further study, therefore, can resolve the questions of the immunosuppressive effect of Urocanic Acid in humans and whether the immunosuppressive effect in animals is linked to the incidence of cancer in those animals. The additional information needed includes human photoimmunosuppression data, data on the modulation of photocarcinogenicity using specified procedures, and a DNA adduct study in vivo and in vitro. Until these data are available, it cannot be concluded that Urocanic Acid is safe for use in cosmetic formulations.

Mechanisms of Photoaging and Cutaneous Photocarcinogenesis, and Photoprotective Strategies with Phytochemicals

Photoaging and photocarcinogenesis are primarily due to solar ultraviolet (UV) radiation, which alters DNA, cellular antioxidant balance, signal transduction pathways, immunology, and the extracellular matrix (ECM). The DNA alterations include UV radiation induced thymine-thymine dimers and loss of tumor suppressor gene p53. UV radiation reduces cellular antioxidant status by generating reactive oxygen species (ROS), and the resultant oxidative stress alters signal transduction pathways such as the mitogen-activated protein kinase (MAPK), the nuclear factor-kappa beta (NF-κB)/p65, the janus kinase (JAK), signal transduction and activation of transcription (STAT) and the nuclear factor erythroid 2-related factor 2 (Nrf2). UV radiation induces pro-inflammatory genes and causes immunosuppression by depleting the number and activity of the epidermal Langerhans cells. Further, UV radiation remodels the ECM by increasing matrixmetalloproteinases (MMP) and reducing structural collagen and elastin. The photoprotective strategies to prevent/treat photoaging and photocarcinogenesis include oral or topical agents that act as sunscreens or counteract the effects of UV radiation on DNA, cellular antioxidant balance, signal transduction pathways, immunology and the ECM. Many of these agents are phytochemical derivatives and include polyphenols and non-polyphenols. The flavonoids are polyphenols and include catechins, isoflavones, proanthocyanidins, and anthocyanins, whereas the non-flavonoids comprise mono phenolic acids and stilbenes. The natural sources of polyphenols include tea, cocoa, grape/wine, soy, pomegranate, and Polypodium leucotomos. The non-phenolic phytochemicals include carotenoids, caffeine and sulphoraphance (SFN). In addition, there are other phytochemical derivatives or whole extracts such as baicalin, flavangenol, raspberry extract, and Photomorphe umbellata with photoprotective activity against UVB radiation, and thereby carcinogenesis.

Fernblock, a nutriceutical with photoprotective properties and potential preventive agent for skin photoaging and photoinduced skin cancers

Many phytochemicals are endowed with photoprotective properties, i.e., the capability to prevent the harmful effects of excessive exposure to ultraviolet (UV) light. These effects include photoaging and skin cancer, and immunosuppression. Photoprotection is endowed through two major modes of action: UV absorption or reflection/scattering; and tissue repair post-exposure. We and others have uncovered the photoprotective properties of an extract of the fern Polypodium leucotomos (commercial name Fernblock). Fernblock is an all-natural antioxidant extract, administered both topically (on the skin) or orally. It inhibits generation of reactive oxygen species (ROS) production induced by UV including superoxide anion. It also prevents damage to the DNA, inhibits UV-induced AP1 and NF-κB, and protects endogenous skin natural antioxidant systems, i.e., CAT, GSH, and GSSR. Its photoprotective effects at a cellular level include a marked decrease of UV-mediated cellular apoptosis and necrosis and a profound inhibition of extracellular matrix remodeling. These molecular and cellular effects translate into long-term inhibition of photoaging and carcinogenesis that, together with its lack of toxicity, postulate its use as a novel-generation photoprotective nutriceutical of phytochemical origin.

Polypodium leucotomos extract inhibits trans-urocanic acid photoisomerization and photodecomposition

In this report, we demonstrate a possible molecular mechanism by which a hydrophilic extract of the leaves of the fern Polypodium leucotomos (Fernblock, PL) blocks ultraviolet (UV)-induced skin photodamage. The extract inhibits UVA and UVB light induced photoisomerization of trans-urocanic acid (t-UCA), a common photoreceptor located in the stratum corneum, and also blocks its photodecomposition in the presence of oxidizing reagents such as H2O2, and titanium dioxide (TiO2). PL protects in vitro human fibroblasts from UV-induced death as well. These results suggest the potential of employing the PL extract as a component of sunscreen moistures in order to prevent photodecomposition of t-UCA, to inhibit UV-induced deleterious effects of TiO2 and to protect skin cells and endogenous molecules directly involved in skin immunosurveillance.

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.

Some photophysical studies of cis- and trans-urocanic acid

Urocanic acid, an important human skin chromophore, undergoes a variety of photochemical transformations when exposed to the near-UV portion of sunlight and natural daylight, the principal reaction being the transformation from the stable trans- or (E)-form of the chromophore (trans-UA) to the biologically active cis- or (Z)-form (cis-UA), which is claimed to induce immunosuppression linked to the onset of skin cancer. This study is concerned with the comparative photophysical behaviour of the two urocanic acid isomers in aqueous solution using both continuous irradiation and pulsed irradiation techniques. The UV absorption maximum for both isomers occurs in the region of 270 nm with the absorption shape varying characteristically with pH, the cis-isomer showing a lower overall molar absorptivity. Both isomers exhibit weak fluorescence (quantum yields estimated to be less than 10(-4)) with each isomer showing small differences in the way in which pH and excitation wavelength influence the fluorescence emission characteristics. Pulsed nanosecond laser irradiation at 266 nm of aqueous solutions at pH 7 shows that both isomers undergo photo-ionisation with a quantum yield of 0.02 for the hydrated electron production, a quantum yield value comparable with that for photoisomerisation at this wavelength. Laser flash studies also show that the photo-ionised species reacts efficiently with oxygen (quenching rate kQ = 1.3 x 10(9) M(-1) s(-1)), while some preliminary experiments indicate that both cis- and trans-urocanic acids react with the semiquinone radical of L-3,4-dihydroxyphenylalanine (L-DOPA) with a fast reaction rate constant of approximately 5 x 10(7) M(-1) s(-1). The photophysical characteristics of trans-UA and cis-UA reported here are discussed in the context of other recent pulsed irradiation studies on urocanic acid over nanosecond and picosecond time scales, in an attempt to clarify the complex photo-behaviour of this interesting biomolecule.

Interaction of UVB-absorbing sunscreen ingredients with cutaneous molecules may alter photoimmune protection

Studies of the photoimmunoprotective properties of sunscreens have produced disparate results. In this study in hairless mice, we compared two UVB absorbers, 2-ethylhexyl-p-methoxycinnamate (2-EHMC) and octyl-N-dimethyl-p-aminobenzoate (o-PABA), individually formulated in a common base lotion with a sunburn protection factor of 6. We measured their capacity to protect against suppression of the contact hypersensitivity (CHS) induced by three daily exposures of the dorsum to 6x the minimal erythemal/edematous dose (MED) of solar-simulated UV radiation (SSUV), in comparison with base lotion-treated mice exposed to 3 x 1 MED of SSUV. All treatments produced a similar minimal erythema. CHS was equally suppressed in mice irradiated through o-PABA and base lotion, but the suppression was significantly reduced in mice irradiated through 2-EHMC. Neither UVB absorber inhibited the epidermal photoisomerization to the immunosuppressive mediator, cis-urocanic acid. However, when mice were treated with exogenous cis-urocanic acid topically on the dorsum, but not when injected subcutaneously on the abdomen, suppression of CHS was observed in o-PABA- and base lotion-treated mice, but not in 2-EHMC-treated mice. Thus, the enhanced immunoprotection in mice irradiated through 2-EHMC apparently resulted from the direct inactivation of epidermal cis-urocanic acid by 2-EHMC. We conclude that comparative assessment of photoimmunoprotection by UV absorbers requires SSUV, erythemally matched exposures and consideration of potential interactions with cutaneous molecules.

Isoflavonoid compounds from red clover (Trifolium pratense) protect from inflammation and immune suppression induced by UV radiation

Isoflavones derived from many edible plants have been reported to possess significant antioxidant, estrogenic and tyrosine kinase inhibitory activity. Genistein has been found previously to provide protection from oxidative damage induced by UV radiation both in vitro and following dietary administration. We have therefore examined the potential of a number of isoflavones from red clover (Trifolium pratense) and some metabolically related compounds to offer protection from UV irradiation in hairless mice by topical application after UV exposure. We show that whereas the primary isoflavones, daidzein, biochanin A and formononetin, were inactive, 20 microM lotions of genistein and the metabolites equol, isoequol and the related derivative dehydroequol had powerful potential to reduce the inflammatory edema reaction and the suppression of contact hypersensitivity induced by moderate doses of solar-simulated UV radiation. For equol the protection was concentration dependent and 5 microM equol markedly reduced the UV-induced inflammation but abrogated the UV-induced immunosuppression. Equol protected similarly from immunosuppression induced by the putative epidermal mediator, cis-urocanic acid (UCA), indicating a potential mechanism of action involving inactivation of this UV-photoproduct. Since immunosuppression induced by both UV radiation and by cis-UCA appears to be an oxidant-dependent response our observations support the actions of these topically applied isoflavones and their metabolites as antioxidants. They also indicate that lotions containing equol, unlike topical UV sunscreens, more readily protect the immune system from photosuppression than from the inflammation of the sunburn reaction, even when applied after exposure, and thus such compounds may have a future role as sun-protective cosmetic ingredients.

cis-Urocanic acid does not induce the expression of immunosuppressive cytokines in murine keratinocytes

trans-Urocanic acid (UCA) acts as a chromophore for UV radiation in the epidermis and isomerizes to cis-UCA which then initiates some of the changes leading to UV-induced immunosuppression. The mechanism of the immunomodulation by cis-UCA is unknown at present, but one possibility is that the interaction between cis-UCA and keratinocytes causes the release of immunosuppressive cytokines locally. To test this hypothesis, PAM-212 cells, a murine keratinocyte cell line, were incubated with 0.10-100 micrograms/mL trans- and cis-UCA for 6 or 24 h, respectively. The expression of interleukin (IL)-10, transforming growth factor (TGF)-beta and tumor necrosis factor (TNF)-alpha messenger RNA (mRNA) was then measured by reverse transcription-polymerase chain reaction in comparison with the mRNA for the house-keeping gene, beta-actin. No change or significant induction of any of the cytokine messages occurred. However, the expression of IL-10 messenger RNA (mRNA) was induced 24 h after UVB irradiation (300 J/m2) and that of TNF-alpha mRNA occurred 6 h after treatment with phorbol myristate acetate. The expression of IL-10 protein was also examined by immunostaining in both PAM-212 cells and B16-F10 murine melanoma cells between 3 and 48 h after incubation with 10 and 100 micrograms/mL cis- and trans-UCA. No alteration was seen with either isomer at either concentration. In contrast, UVB irradiation of both cell lines resulted in a marked increase in intracellular IL-10 protein at 24 and 48 h. Therefore the upregulation of the immunosuppressive cytokines, IL-10, TNF-alpha and TGF-beta, in keratinocytes is unlikely to be the mechanism by which cis-UCA induces immunosuppression in mice.

Spectroscopic and dynamic studies of the epidermal chromophores trans-urocanic acid and eumelanin

Photoaging and skin cancer can result from the exposure of skin to ultraviolet A (UV-A, 320-400 nm) radiation. The detailed chemical mechanisms by which these processes occur are not known, but they must begin with the absorption of a UV-A photon by one or more photoreceptor(s) within the skin. The situation is complicated by the lack of understanding of the photoreactions of many of the UV-A-absorbing molecules in skin. In this Account, we describe recent research efforts directed at elucidating the UV-A-induced photoreactivity of two light-absorbing epidermal photoreceptors: trans-urocanic acid and eumelanin.

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 role of urocanic acid in UVB-induced suppression of immunity to Trichinella spiralis infection in the rat

The naturally occurring trans-isomer of urocanic acid (trans-UCA), found in the stratum corneum, absorbs ultraviolet light (UV) and isomerizes to the cis-form. Cis-UCA has been shown to impair some cellular immune responses, and has been proposed as an initiator of the suppression that follows UV irradiation. UVB exposure leads to an increase in cis-UCA in the skin of rats from about 10% to 40% of the total UCA. Previously it has been demonstrated that UVB lowers immune responses to Trichinella spiralis after oral infection of rats with the parasitic worm. In the present study we investigated the role of cis-UCA in the control of this parasitic infection. Rats were infected orally with T. spiralis and injected with different doses of cis- or trans-UCA subcutaneously. Mitogenic responses and the mixed lymphocyte reaction were not affected by either isomer. In contrast, the number of T. spiralis larvae in muscle tissue of infected rats was increased significantly in the cis-UCA-treated animals compared with the trans-UCA-treated animals. In addition, delayed-type hypersensitivity (DTH) to T. antigen in infected rats was significantly impaired by cis-UCA but not by trans-UCA. If rats were injected with a monoclonal antibody with specificity for cis-UCA 2 hr prior to UVB exposure, the UVB-induced suppression in DTH to T. spiralis and the increase in larvae counts were significantly inhibited compared with rats that were similarly injected with a control antibody. Thus cis-UCA can inhibit the specific resistance to parasitic infections and acts as an important mediator of UVB-induced suppression of immunity to T. spiralis in the rat.

Histamine involvement in UVB- and cis-urocanic acid-induced systemic suppression of contact hypersensitivity responses

Studies in experimental models have implicated histamine and prostanoids in ultra-violet B (UVB)- and cis-urocanic acid (UCA)-induced systemic immunosuppression. This study examined the hypothesis that UVB irradiation and cis-UCA suppressed contact hypersensitivity responses to hapten by induction of histamine, which in turn evoked a prostanoid-dependent component of immunosuppression. BALB/c mice were administered with a cis-UCA monoclonal antibody, a combination of histamine types 1 and 2 receptor antagonists, or indomethacin. Mice were sensitized to 2,4,6-trinitrochlorobenzene (TNCB) on their ventral surface 5 days after UVB irradiation, or cis-UCA or histamine administration. Ears were challenged with TNCB 5 days later. Cis-UCA antibody inhibited the suppressive effects of UVB by approximately 60% and confirmed that suppression of contact hypersensitivity responses by UVB was due, at least in part, to mechanisms involving cis-UCA. Histamine suppressed contact hypersensitivity responses and the effects of cis-UCA and histamine were not cumulative, suggesting that cis-UCA and histamine signal largely through the same pathway. The immunosuppressive effects of histamine were not affected by the cis-UCA antibody, consistent with the model that histamine acts downstream of cis-UCA. Administration of histamine receptor antagonists and indomethacin each approximately halved the UVB- and cis-UCA-induced systemic suppression of contact hypersensitivity responses. The effects of the reagents that inhibited the action of histamine and prevented prostanoid production were not cumulative, and suggested involvement in the same pathway. These results support the involvement of cis-UCA, histamine and prostanoids, in a sequence, in UVB-induced systemic suppression of contact hypersensitivity responses.

Ultraviolet B-exposed and soluble factor-pre-incubated epidermal Langerhans cells fail to induce contact hypersensitivity and promote DNP-specific tolerance

Acute low-dose ultraviolet B (UVB) radiation impairs the induction of contact hypersensitivity (CH) and induces tolerance in UVB-susceptible strains of mice when dinitrofluorobenzene (DNFB) is applied to an irradiated skin surface. We are interested in learning the cellular and molecular bases for the existence of UVB susceptibility in certain strains of mice. CH was induced by subcutaneous injections into naive syngeneic C57BL/6 and BALB/c mice of dinitrophenyl (DNP)-derivatized Thy-1(+)-depleted epidermal cells enriched for Ia+ cells (LC/DNP, 2 x 10(4) cells per mouse). Tolerance was detected by applying 185 microg of DNFB epicutaneously to mice treated 2 wk earlier with a putative tolerating regimen and testing CH expression. We found that LC/DNP obtained from C57BL/6 skin 2 h after UVB irradiation (400 J per m2) failed to induce CH and induced DNP-specific tolerance instead; by contrast, similar cells obtained from same or even higher dose (400 J per m2 and 1200 J per m2) UVB-exposed BALB/c skin induced vigorous CH, and no tolerance was detected. In both C57BL/6 and BALB/c mice, Ia+-depleted EC/DNP neither sensitized naive syngeneic mice nor induced tolerance. LC/DNP prepared from unirradiated trunk skin of either C57BL/6 or BALB/c mice and pre-incubated in vitro for 2 h with cis-UCA, TNF-alpha, or IL-10 failed to induce intense CH; instead, all induced DNP-specific tolerance. Pre-incubation of similar LCs with alpha-MSH in vitro for 2 h also failed to induce CH but did not cause tolerance. Thus, single low-dose UVB irradiation alters the immunogenic and tolerogenic potentials of LCs only in UVB-susceptible mice; by contrast, pre-treatment of LCs with UVB-dependent soluble factors can achieve effects similar to UVB irradiation in both UVB-susceptible and -resistant strains of mice. These findings demonstrate that UVB susceptibility in mice may be determined by the production of UVB-dependent soluble factors within UVB-irradiated skin.

Differential suppression of the human mixed epidermal cell lymphocyte reaction (MECLR) and mixed lymphocyte reaction (MLR) by cis-urocanic acid

Cis-urocanic acid (UCA), formed in the stratum corneum by UV irradiation of trans-UCA has been proposed as a mediator of UV-induced immunosuppression in the skin. In this study, we examined the in vitro effect of cis-UCA (6-100 micrograms/mL) on the human mixed lymphocyte reaction (MLR) and the mixed epidermal cell lymphocyte reaction (MECLR). Addition of cis-UCA (purified or in a mixture with trans-UCA) did not affect the MLR but was able to induce a 20% suppression of the MECLR responses. Because this effect of cis-UCA on the MECLR was not as strong as could be expected from previous in vivo results, we designed a set of experiments in order to enhance the in vitro immunosuppressive capacity of cis-UCA. Firstly, we preincubated epidermal cells with UCA (50 micrograms/mL). for 3 or 6 days before culture in the MECLR because in vivo repeated UV exposure can lead to a photostationary state, where cis-UCA may be present for several weeks. This pretreatment with cis-UCA resulted in a maximal decrease of the MECLR response of 27%, whereas trans-UCA had no effect. Secondly, we investigated whether UVB irradiation of epidermal cells could make cells more sensitive to cis-UCA. However, addition of trans- or cis-UCA did not potentiate the reduced alloactivating capacity of UVB-irradiated cells. Finally, we examined the possibility of a synergistic effect of cis-UCA with histamine. Addition of histamine suppressed the MLR and MECLR responses, but neither cis- nor trans-UCA were able to modulate this decrease. We conclude that cis-UCA can partly downregulate the human MECLR but not the MLR. The mechanism involved in this differential downregulation is not known. In this respect it is striking that cis-UCA dose not potentiate the UVB- or histamine-induced suppression of the MECLR.

Quantitative analysis of histidine and cis and trans isomers of urocanic acid by high-performance liquid chromatography: a new assay method and its application

To elucidate the factors involved in dry skin and the skin damage caused by UV light, it is necessary to analyze small amounts of stratum corneum to determine amino acid contents. A new assay method for this purpose is described. Dabsylated amino acids including histidine and the cis and trans isomers of urocanic acid were analyzed quantitatively by high-performance liquid chromatography (HPLC), using a reversed-phase column. Histidine and the isomers of urocanic acid were separated from 36 other amino acids thought to be present in the extract of stratum corneum. In the presence of the 36 amino acids, standard calibration curves were obtained from 0.25 to 2.5 pmol/microliter, for histidine and for both isomers of urocanic acid. The coefficients of variation for the reproducibility of the analysis at 1.0 pmol/microliter were 3.8%, 2.9% and 2.5% for the cis and trans isomers of urocanic acid and for histidine, respectively. Amounts of 2 to 50 pmol of cis and trans isomers of urocanic acid and histidine in the stratum corneum were detected. The ratio of the cis to the trans isomer of urocanic acid in sunburned stratum corneum was more than three times that in normal stratum corneum. This method appears to be useful for the determination of small amounts of histidine and of the cis and trans isomers of urocanic acid in the stratum corneum.

Molecular aspects of UVB-induced immunosuppression

Ultraviolet light can affect the immune system locally as well as systemically leading to an impaired resistance to neoplastic cells and/or infections. Prior to the biological effect, UVB must be absorbed by a chromophore in the skin where it will give a signal that can lead to an altered immune response in the skin or elsewhere. These altered immune responses may be constituted by alteration in among others: cytokine profile, growth factors and costimulatory signals. Several hypotheses about the identity of the photoreceptor have been put forward. One photoreceptor in the skin is urocanic acid (UCA), that can isomerize from the trans- to the cis-isomer. The cis-isomer has immunosuppressive properties. Another photoreceptor is DNA that also efficiently absorbs UV wavelengths. After absorption the structure of the DNA molecule is altered. This alteration might lead to gene activation responsible for the immunotoxic outcome (altered gene expression). It has been demonstrated that the formation of DNA photoproducts by UV light is associated with the activation of many genes. Several studies indicate that UV-induced DNA damage, in the form of cyclobutyl pyrimidine dimers plays a role in UV-induced suppression of the immune system locally as well as systemically. In mice that were injected with liposomes containing the excision repair enzyme T4 endonuclease UVB-induced dimers were removed more efficiently as compared to control mice. In these mice UV-induced immunosuppression was prevented. Pilot studies by Kripke et al. indicated that the release of IL-IO and TNF alpha that are both induced by DNA damage might be involved. In preliminary studies with mice that were deficient with respect to DNA repair lower doses of UV were needed for the induction of immunosuppression as compared to their normal littermates. These studies indicate that altered gene expression plays a pivotal role in UVB-induced immunosuppression. In addition to a role for UCA and DNA in UV-induced immunosuppression it is postulated recently that signal transduction (EGF-receptor mediated upregulation of phospholipase A2) and transcription factors (NF kappa B, p91) are involved in UV-induced immunomodulation.

Mexoryl SX protects against solar-simulated UVR-induced photocarcinogenesis in mice

The aim of this study was to determine, for regulatory purposes, the potential of Mexoryl SX, a broad UVA absorber that also absorbs to some extent in the UVB, to modify the UV radiation (UVR)-induced murine skin tumor development and growth. Skh-hr1 mice were exposed to solar-simulated UVR 5 days per week for 40 weeks. Two control groups were irradiated without topical application, three groups received a sunscreen preparation containing either the UVA absorber, Mexoryl SX at 5 or 10% concentration, or a filter that absorbs principally in the UVB, 2-ethylhexyl-p-methoxycinnamate (2-EHMC) at 5% concentration, introduced as a comparator test article. Sunscreen application was performed before UVR exposure 3 days per week and after UVR exposure on the other 2 days (consistent with the design of a standard photocarcinogenesis safety test). Two different weekly UVR doses were administrated: the lower dose was given to one group of unprotected animals, whereas the higher dose was administrated to the other unprotected group and to the three sunscreen-treated groups. The two UVR control groups demonstrated a UVR-dependent response for cumulative tumor prevalence, tumor yield and median latent period. Neither concentration of Mexoryl SX increased the probability of tumor development; consistent with the principles for safety testing, this provides evidence in that it is safe for use in sunlight. Although this study was explicitly designed as a safety test, the results also provide some clues about the efficacy of Mexoryl SX in decreasing the probability of tumor development. Topical administration of Mexoryl SX, at both concentrations, resulted in a 6 week delay in the median latent period compared to high UVR controls, whereas 5% 2-EHMC delayed the median latent periods only by 2 weeks. Tumor prevalence and yield show the same efficacy differences between the two sunscreen ingredients. Tumor protection factors were calculated from these results and found to be equal to 2.4 for the two preparations containing Mexoryl SX and to 1.3 for the 5% 2-EHMC preparation. These findings illustrate the efficacy of Mexoryl SX in preventing UVR-induced carcinogenesis.

Investigating the red shift between in vitro and in vivo urocanic acid photoisomerization action spectra

Trans-urocanic acid (UCA) is found in the upper layer of the skin and UV irradiation induces its photoisomerization to cis-UCA. Cis-UCA mimics some of the immunosuppressive properties of UV exposure. The wavelength dependence for in vitro photoisomerization of trans-UCA (15 microM) over the spectral range 250 nm-340 nm (10 nm intervals) was determined. The action spectrum revealed that maximal cis-UCA production occurred at 280 nm, which is red-shifted by 10-12 nm from its absorption peak at 268 nm and differs markedly from the reported action spectra for cis-UCA production in mouse skin in vivo, which peaks at 300-310 nm. The reasons for the red shift between the in vitro and in vivo action spectra are not clear. There is limited evidence suggesting that the UV absorption maximum of trans-UCA red shifts from 268 nm in vitro to 310 nm on interaction with stratum corneum proteins in vivo. This phenomenon was investigated by applying trans-UCA (2.5 mg/cm2) in an oil emulsion to isolated human stratum corneum. After incubation at 37 degrees C for 1 h, the absorption spectra of stratum corneum with UCA and with oil only were compared using a Xe arc source and a spectroradiometer. A moderate red shift in trans-UCA absorption from approximately 268 nm to 280 nm was observed. In summary, we suggest that the 10-12 nm red shift between the UCA absorption spectrum peak and the action spectrum peak in vitro may be accounted for by the wavelength dependence of quantum yields reported over the 254-313 nm range. The red shift between the in vitro and in vivo photoisomerization action spectra may result from the 10 to 12 nm red shift in the absorption of UCA in association with stratum corneum proteins, combined with increasing quantum yields over the 254-313 nm range.

The role of cis-urocanic acid in UVB-induced suppression of contact hypersensitivity

Ultraviolet light B (UVB) is well recognized to suppress the contact hypersensitivity (CHS) response and it has been postulated that cis-urocanic acid (UCA) is a mediator of the immunosuppression. This study was designed to examine the effect of UCA on CHS and to clarify its role in UVB-induced immunosuppression in C57BL/6 mice. Intradermal injection of 0.5-50 micrograms cis-UCA into the ear 2 h before DNFB sensitization resulted in a 60-70% reduction of CHS assessed by ear swelling, whereas trans-UCA did not have a significant effect on CHS except at a high dose (50 micrograms) which showed a 20-40% suppression. Intraperitoneal injection of anti-cis-UCA antibody before administration of cis-UCA abrogated the suppression. To examine the effect of UCA on UVB-induced immunosuppression, some mice were pre-treated with anti-cis-UCA antibody and then exposed to UVB (960 J/m2). After 3 days the mice were sensitized either on the irradiated abdominal skin or on the unirradiated dorsal surface of the right ear followed by the challenge on the left ear. The CHS response was significantly suppressed in UVB-irradiated mice both locally (abdominal sensitization, suppression was 45%, P < 0.001) and systemically (ear sensitization, suppression was 53%, P < 0.0025). The CHS response was partially restored in both abdominal sensitized mice and ear sensitized mice by pre-treatment with anti-cis-UCA antibody. These results confirmed the immunosuppressive effects of cis-UCA on CHS and suggest that cis-UCA plays a role in UVB-induced local and systemic immunosuppression.

The role of urocanic acid in UV-induced immunosuppression: recent advances (1992-1994)

Cis-urocanic acid (UCA), formed in the epidermis by UV irradiation of trans-UCA, has been implicated as a mediator of the immunosuppression induced by UV exposure of the skin. This review covers recent work in which the wavelength dependence of cis-UCA formation, the interaction of UCA isomers with DNA, the effects of UCA isomers on the immune system and their interaction with histamine are examined. Results are frequently conflicting, particularly when considering the possible mode of action of cis-UCA but, overall, a multifaceted role for UCA in immunomodulation by UV radiation is substantiated.

The effect of chronic low-dose UVB radiation on Langerhans cells, sunburn cells, urocanic acid isomers, contact hypersensitivity and serum immunoglobulins in mice

C3H mice were irradiated three times a week for up to 6 weeks with either 500 J/m2 or 1000 J/m2 broadband UVB (270-350 nm) or 3000 J/m2 narrowband UVB (311-312 nm; TL01 source). Each dose was suberythemal to the mouse strain used. The number of Langerhans cells (LC) in the epidermis was reduced by over 50% after 2 weeks of irradiation with the UVB source and by 20% following TL01 irradiation. Continued irradiation for up to 6 weeks resulted in no further decrease in LC numbers in the case of the UVB source but a steady decline to 40% in the case of the TL01 source. Sunburn cells were detected following irradiation with both sources but the numbers were very low in comparison with acute exposure. Ultraviolet-B exposure resulted in doubling of the thickness of the epidermis throughout the 6 weeks of irradiation while TL01 exposure did not alter epidermal thickness. Conversion of trans- to cis-urocanic acid (UCA) was observed with both UVB and TL01 sources. The percentage of cis-UCA started to return to normal after 4 weeks of TL01 exposure despite continued irradiation. As observed following a single exposure, the contact hypersensitivity (CH) response was significantly reduced following 6 weeks of UVB irradiation but was unaffected by TL01 exposure, indicating no correlation between cis-UCA levels and CH response. Total serum immunoglobulin levels remained unchanged throughout the 6 weeks of UVB or TL01 irradiation but IgE titers significantly increased in all cases in the first 2 weeks of irradiation, indicating a possible shift to a TH2 cytokine profile.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunomodulation at the initiation of phototherapy and photochemotherapy

The numbers and function of circulating lymphocyte subsets are within normal ranges in patients with psoriasis and are not affected by 4 weeks of ultraviolet (UV) therapy, except for a suppression in natural killer (NK) cell activity. However, it is possible that immunomodulation might occur at the initiation of phototherapy with a return to control values on more prolonged UV exposure. Thus, in this study the responses of 15 patients with chronic plaque psoriasis undergoing broad-band UVB therapy, 10 narrow-band (311-313 nm) UVB therapy and 10 PUVA therapy were compared. In each case, samples were taken immediately before starting treatment and 1 week later. Broad-band UVB and PUVA therapy had no effect on NK activity, but a significant reduction was found in the group receiving narrow-band UVB. In vitro lymphoproliferative responses to mitogens and to herpes simplex virus antigens did not alter with therapy, except there was a significant increase in mitogen responses (at optimal mitogen concentrations only) in the narrow-band UVB group. Generally no alterations in overall percentages of circulating mononuclear cells were found in any group. Samples were taken from the epidermis of the forearm and back of the patients receiving narrow-band UVB for the quantification of urocanic acid (UCA) isomers. The total UCA concentration remained unchanged after 1 week of therapy, while the percentage of cis-UCA increased significantly at both sites in the majority of patients. However, this rise did not correlate with the decrease in NK cell activity and the two parameters may not be related causally.

Treatment of contact hypersensitivity with urocanic acid

In order to investigate the effect of cis-urocanic acid (UCA) on a delayed-type hypersensitivity response in humans, a contact hypersensitivity reaction was induced on four test sites on the back of 33 volunteer subjects. The first test site was pretreated with cis-UCA immediately before application of the allergen. The second and third test sites were posttreated on the second and third days of the hypersensitivity response with cis-UCA and a class III corticosteroid, respectively. The fourth test site was used as a positive control. The cutaneous blood flow of the test sites was measured using laser Doppler flowmetry. Pretreatment with cis-UCA reduced the hypersensitivity response significantly. It is possible that cis-UCA could be used in the preventive treatment of contact hypersensitivity responses.