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Vitamin D and A derivatives are well-known endogenous substances responsible for skin homeostasis. In this study we topically treated shaved mouse skin with a vitamin D agonist (MC903) or vitamin D antagonist/partial agonist (ZK159222) and compared the changes with acetone (control treatment) treatment for 14 days. Topical treatment with ZK159222 resulted in increased expression of genes involved in retinoic acid synthesis, increased retinoic acid concentrations and increased expression of retinoid target genes. Clustering the altered genes revealed that heparin-binding epidermal growth factor-like growth factor, the main driver of epidermal hyperproliferation, was increased via RARγ-mediated pathways, while other clusters of genes were mainly decreased which were comparable to the changes seen upon activation of the RARα-mediated pathways. In summary, we conclude that epidermal hyperproliferation of mouse skin in response to a topically administered vitamin D receptor antagonist/partial agonist (ZK159222) is induced via increased retinoic acid synthesis, retinoic acid levels and increased RARγ-mediated pathways.

Carotenoids and retinoids are known to alter the allergic response with important physiological roles in the skin and the immune system. In the human organism various carotenoids are present, some of which are retinoid precursors. The bioactive derivatives of these retinoids are the retinoic acids, which can potently activate nuclear hormone receptors such as the retinoic acid receptor and the retinoid X receptor. In this study, we aimed to assess how plasma carotenoid and retinoid concentrations along with the ratio of their isomers are altered in atopic dermatitis (AD) patients (n = 20) compared to healthy volunteers (HV, n = 20). The study indicated that plasma levels of the carotenoids lutein (HV 198 &plusmn, 14 ng/mL, AD 158 &plusmn, 12 ng/mL, p = 0.02, all values in mean &plusmn, SEM), zeaxanthin (HV 349 &plusmn, 30 ng/mL, AD 236 &plusmn, 18 ng/mL, p &le, 0.01), as well as the retinoids retinol (HV 216 &plusmn, 20 ng/mL, AD 167 &plusmn, 17 ng/mL, p = 0.04) and all-trans-retinoic acid (HV 1.1 &plusmn, 0.1 ng/mL, AD 0.7 &plusmn, 0.1 ng/mL, p = 0.04) were significantly lower in the AD-patients, while lycopene isomers, &alpha, carotene, and &beta, carotene levels were comparable to that determined in the healthy volunteers. In addition, the ratios of 13-cis- vs. all-trans-lycopene (HV 0.31 &plusmn, 0.01, AD 0.45 &plusmn, 0.07, p = 0.03) as well as 13-cis- vs. all-trans-retinoic acid (HV 1.4 &plusmn, 0.2, AD 2.6 &plusmn, 0.6, p = 0.03) were increased in the plasma of AD-patients indicating an AD-specific 13-cis-isomerisation. A positive correlation with SCORAD was calculated with 13-cis- vs. all-trans-lycopene ratio (r = 0.40, p = 0.01), while a negative correlation was observed with zeaxanthin plasma levels (r = &minus, 0.42, p = 0.01). Based on our results, we conclude that in the plasma of AD-patients various carotenoids and retinoids are present at lower concentrations, while the ratio of selected lycopene isomers also differed in the AD-patient group. An increase in plasma isomers of both lycopene and retinoic acid may cause an altered activation of nuclear hormone receptor signaling pathways and thus may be partly responsible for the AD-phenotype.

Various health benefits of carotenoids have been described. However, while human observational studies generally suggest positive health effects, supplementation with relatively high doses of individual carotenoids (supplements) have partly produced adverse effects. In the present study, we investigated the effect of several carotenoids on the proteomic response of male Mongolian gerbils (aged 6 weeks). Five groups of gerbils (n = 6 per group) received either retinol (vitamin A/53 mg per kg bw), all-trans β-carotene (pro-vitamin A/100 mg kg-1), the non-pro vitamin A carotenoid lutein (100 mg kg-1), the acyclic carotenoid lycopene (100 mg kg-1) or vehicle (Cremophor EL), via oral single gavage. Gerbils were 12 h post-prandially sacrificed and blood plasma, liver, and white adipose tissue were collected. For liver and adipose tissue, a 2D-DIGE (difference gel electrophoresis) approach was conducted; for plasma, proteomic analyses were achieved by liquid chromatography-mass spectrometry. Compared to controls (vehicle), various proteins were showing significant abundance variations in plasma (66), liver (29) and adipose tissue (19), especially regarding structure (22), protein metabolism (15) and immune system/inflammation (19) functions, while proteins related to antioxidant effects were generally less abundant, suggesting no in vivo relevance. Surprisingly, a large overlap in protein regulation was found between lycopene and retinol exposure, while other carotenoids, including all-trans β-carotene, did not show this overlap. Mainly retinoid acid receptor co-regulated proteins may mechanistically explain this overlapping regulation. This overlapping regulation may be related to common nuclear hormone receptor mediated signalling, though further studies using synthetic ligands of retinoid receptors targeting protein regulation are needed for confirmation.

Lipoxygenases (LOX) and cyclooxygenase (COX) are the main enzymes for PUFA metabolism to highly bio-active prostaglandins, leukotrienes, thromboxanes, lipoxins, resolvins and protectins. LOX and COX pathways are important for the regulation of pro-inflammatory or pro-resolving metabolite synthesis and metabolism for various inflammatory diseases such as atopic dermatitis (AD). In this study, we determined PUFAs and PUFA metabolites in serum as well as affected and non-affected skin samples from AD patients and the dermal expression of various enzymes, binding proteins and receptors involved in these LOX and COX pathways. Decreased EPA and DHA levels in serum and reduced EPA level in affected and non-affected skin were found; in addition, n3/n6-PUFA ratios were lower in affected and non-affected skin and serum. Mono-hydroxylated PUFA metabolites of AA, EPA, DHA and the sum of AA, EPA and DHA metabolites were increased in affected and non-affected skin. COX1 and ALOX12B expression, COX and 12/15-LOX metabolites as well as various lipids, which are known to induce itch (12-HETE, LTB4, TXB2, PGE2 and PGF2) and the ratio of pro-inflammatory vs pro-resolving lipid mediators in non-affected and affected skin as well as in the serum of AD patients were increased, while n3/n6-PUFAs and metabolite ratios were lower in non-affected and affected AD skin. Expression of COX1 and COX-metabolites was even higher in non-affected AD skin. To conclude, 12/15-LOX and COX pathways were mainly upregulated, while n3/n6-PUFA and metabolite ratios were lower in AD patients skin. All these parameters are a hallmark of a pro-inflammatory and non-resolving environment in affected and partly in non-affected skin of AD patients.

Atopic dermatitis (AD) is a chronically relapsing inflammatory skin disease, which is characterized by a disrupted epidermal barrier function present both in affected skin and in non-affected skin. Mainly glucocorticosteroids were used in topical and systemic atopy treatments because of their potent anti-inflammatory effects, unfortunately with strong side effects. In this study we determined that 2 out of 16 steroids were significantly different in healthy volunteers vs AD-patients. Cortisone, which is higher in AD-patients plasma, is a direct precursor of the bioactive corticosteroid cortisol, which just displays a higher tendency and is known for its potent anti-inflammatory effects. In addition a tendency of reduced levels of the anti-inflammatory ER ligand estrone was found in AD-patients. DHEA is a precursor of testosterone, its levels just display a lower tendency in male AD-patients, while its sulfonation metabolite DHEAS is lower in male and female AD-patients. We found and conclude that altered steroid levels in the plasma of AD-patients indicate altered vitamin D signaling (based on reduced DHEA sulfonation) and increased feedback for anti-inflammatory signaling (increased levels of cortisone) present in AD-patients. This article is protected by copyright. All rights reserved.

Lipoxygenases (LOX) and cyclooxygenases (COX) are the main enzymes for poly-unsaturated fatty acid (PUFA) metabolism to highly bioactive prostaglandins, leukotrienes, thromboxanes and protectins. LOX and COX pathways are highly important for the regulation of pro- and anti-inflammatory active metabolite synthesis and metabolism in various inflammatory diseases like atopic diseases (AD). In this study using QRT-PCR, we found that in PBMCs the expression of 5-LOX, 12-LOX, 15-LOX and COX pathways and further enzymatic pathways like various leukotriene-hydoxylases, leukotriene-, prostaglandin-, and thromboxane-synthases as well as various of their membrane based receptors are mainly significantly down-regulated in AD-patients vs. healthy volunteers. In addition, using HPLC MS-MS we determined up to 19 different metabolites originating from eicosapentaenoic acid (EPA), docosapentaenoic acid (DHA) and arachidonic acid (AA) ranging from hydroxylated-PUFA derivatives and further bioactive derivatives like thromboxanes, leukotrienes, prostaglandins and protectins originating from LOX and COX metabolism. In PBMCs from AD-patients LOX and COX pathways were down-regulated. We conclude from this study, that in PBMCs from AD-patients in comparison to healthy volunteers, a systemic down-regulation of LOX- and COX-responses occurs to generally reduce eicosanoid/docosanoid synthesis during the current allergic inflammatory status.

cope The aim of this study was to compare if lycopene also possesses pro-vitamin A (VA) activity comparable to known VA derivatives. Materials and methods We used a transgenic retinoic acid response element reporter mouse model (n = 8, per group) for this study, and after the initial wash out of VA using a vitamin A deficient diet (VAD) for 18 weeks, the animals were supplemented further with (a) VAD-fed mice, (b) VAD-fed mice plus retinol (20 mg/kg bw), (c) VAD-fed mice plus β-carotene (40 mg/kg bw), and (d) VAD-fed mice plus lycopene (40 mg/kg bw). Using ex vivo scanning and gene expression analysis of retinoid target and VA marker gene analysis in various organs of these supplemented mice (b, c, d), we found increased luciferase activity and normalized marker and target gene analysis compared to group a. Conclusions Lycopene can restore VA deficiency and compensate VA for retinoic acid receptor (RAR)-mediated signaling as the major function of VA in the mammalian organism. Lycopene administration can initiate upregulation of RAR-mediated signaling in various organs in VAD-fed animals via potential novel bioactive lycopene metabolites. This indicates that lycopene possesses partial pro-VA activity in mice transmitted via RAR-mediated signaling.

TSLP is an important trigger and initiator for various atopic diseases mainly atopic dermatitis (AD). Activators of nuclear hormone receptors like bioactive vitamin A and D derivatives are known to induce TSLP up-regulation in the skin. In this study, various combinations of synthetic specific agonists and antagonists of the retinoic acid receptors (RARs), retinoid X receptors (RXRs) and vitamin D receptor (VDR) were topically administered to mice. The aim of the study was to elucidate via which nuclear hormone receptor pathways TSLP is regulated and how this regulation is connected to the development and phenotype of atopic dermatitis. TSLP expression was monitored using QRT-PCR and serum TSLP levels using ELISA. Synthetic agonists of the VDR and RARγ as well as the natural agonist all-trans retinoic acid (ATRA) increased TSLP expression in the skin, while an RXR agonist was not active. Treatments with antagonists of RXRs and RARs in addition to RARα-agonists reduced skin TSLP expression. Strong activation was found after a combination of a VDR and an RXR agonist (ca. 5 times induction) and even stronger by an RARγ and an RXR agonist treatment (ca. 48 times induction). We conclude that besides VDR-mediated signaling mainly RARγ-RXR mediated pathways in the skin are important patho-physiological triggers for increased skin TSLP expression. We conclude that topical synthesized retinoids stimulated by internal or external triggers or topically applied induce TSLP production and are thereby important triggers for atopic dermatitis prevalence.

Atopic dermatitis (AD) is one of the most common skin diseases. Various features present in AD like inflammation, reduced apoptosis, altered epidermal differentiation and hyperproliferation as well as permeability dysfunction are also regulated by retinoids. The aim of our study is to identify the retinoid signalling pathways and retinoid concentration profiles in AD skin. Human skin biopsies were obtained from healthy volunteers (HS) (n=6) and patients with AD (n=6), with both affected (AS) and non-affected (NAS) skin. The gene expression of retinoid receptors, retinoid-binding proteins and retinoid-metabolizing enzymes was investigated by QRT-PCR. Retinoid concentrations in serum and skin were measured via high performance liquid chromatography mass spectrometry-mass spectrometry. Our results show that the target gene expression of retinoid receptor regulated pathways is significantly decreased in AS and NAS of patients with AD. CYP26A1, transglutaminase 2 and retinoic acid receptor responder 1 decreased in NAS and AS in comparison with HS. The main retinoic acid synthesizing enzyme, retinal dehydrogenase 1, was significantly lower expressed in NAS (0.1%) and AS (1%) in patients with AD. Analysis of retinoid concentration in serum and skin showed comparable all-trans retinoic acid (ATRA) and retinol (ROL) concentrations from AD and healthy serum, but strongly reduced ATRA and ROL concentrations in affected and non-affected skin in comparison with healthy skin. Our data indicate that retinoid transport, synthesis, concentrations and signalling are strongly decreased in the affected but also in non-affected skin of patients with AD suggesting a general intrinsic influence on skin retinoid signalling pathway in patients with AD.

It is common wisdom in pharmacology that fluorination can significantly increase the efficacy of the active components in pharmaceuticals – actually, ca. 30% of the best-selling drugs worldwide contain fluorinated compounds. The aim of the current study was to assess the potential anti-inflammatory effects of cannabidiol (CBD), the major non-psychoactive component of the pant Cannabis sativa, and its fluorinated derivative (HUF-101) in various experimental systems modeling atopic dermatitis (AD). For the in vitro AD model, human epidermal keratinocytes were challenged with the combination of Staphylococcus aureus enterotoxin B (SEB) and thymic stromal lymphopoietin (TSLP), and expressions of certain marker molecules were assessed by RT-qPCR and ELISA. For the in vivo model, mice were sensitized with 2% oxazolone (OXA) before elicitation. Test compounds were applied topically (1 and 10 μM) after inducing skin inflammation and edema formation (in the ears) was measured with an engineer’s micrometer. In the in vitro model, expressions of certain pro-inflammatory cytokines (e.g. interleukin [IL]-1α, IL-1β, IL-6 and IL-8) were significantly down-regulated upon the administration of CBD and HUF-101. Of great importance, however, HUF-101 exhibited significantly higher potency in comparison to CBD. In the in vivo model, topical application of 1 μM CBD significantly reduced the OXA-induced ear edema; however, 10 μM CBD exerted insignificant effect. In contrast, HUF-101 attenuated OXA-induced edema formation at both concentrations. Intriguingly, similar to the in vitro conditions, the anti-inflammatory potency of HUF-101 was significantly greater than that of CBD. Our study provides the first evidence that CBD and its fluorinated derivative exert significant anti-inflammatory actions in models of AD. These intriguing data invite further pre-clinical and clinical studies to exploit the therapeutic potential of certain CBD derivatives in cutaneous inflammatory conditions.

Little is known about the altered lipid metabolism-related transcriptional events occuring in sebaceous glands of patients with acne vulgaris. Peroxisome proliferator-activated receptor (PPAR)γ, a lipid-activated transcription factor, is implicated in differentiation and lipid metabolism of sebocytes. We have observed that PPARγ and its target genes, ADRP (adipose differentiation related protein) and PGAR (PPARγ angioprotein related protein) are expressed at lower levels in sebocytes from patients with acne than in those from healthy controls (HCs) Furthermore, endogenous PPARγ activator lipids such as arachidonic acid-derived keto-metabolites (e.g. 5KETE, 12KETE) are increased in acne-involved and nonacne-involved skin of patients with acne, compared with skin from healthy individuals. Our findings highlight the possible anti-inflammatory role of endogenous ligand-activated PPARγ signaling in human sebocyte biology, and suggest that modulating PPARγ- expression and thereby signaling might be a promising strategy for the clinical management of acne vulgaris.

Fatty acid concentrations, in particular n-3 and n-6 polyunsaturated fatty acids (PUFAs), have been described to be dysregulated in atopic dermatitis (AD) patients. The role of genetic polymorphisms of fatty acid enzymes in AD is controversial. We determined in a Hungarian cohort of healthy volunteers (n = 20) and AD patients (n = 20) triglyceride-, sterol- and phospholipid-bound fatty acids in the plasma, mRNA expression of fatty acid desaturase 2 (FADS2) and stearoyl-coenzyme A desaturase 1 in peripheral blood mononuclear cells (PBMCs) and FADS2 concentrations in plasma. We observed higher levels of monounsaturated fatty acids, 16:1 versus 16:0 ratios in phospholipids, triglycerides and sterol esters in patients compared to healthy subjects. In addition higher levels of the FADS2-derived n-6 PUFAs γ-linolenic acid and dihomo-γ-linolenic acid were observed in PBMCs of patients as well as lower levels of n-3 PUFAs. We conclude that the increased expression of FADS2 in PBMCs, as a representative tissue accessible from human blood of AD patients, might be responsible for higher levels of FADS2-derived n-6 PUFAs and lower n-3 PUFA levels in patients.

Dietary fat and vitamin A provide important precursors for potent bioactive ligands of nuclear hormone receptors, which regulate various enzymes involved in lipid homeostasis, metabolism and inflammation. We determined the effects of dietary fat and dietary vitamin A on hepatic expression of two fatty acid metabolizing enzymes, elongase 6 (ELOVL6) and stearoyl-coenzyme A desaturase 1 (SCD1) and the concentration of saturated fatty acids (SAFA) and monounsaturated fatty acid (MUFA) of phospholipids in serum and liver. Mice (n = 6) were fed 4 weeks with diets containing 2, 5 and 25 % of fat or vitamin A (0, 2,500 and 326,500 RE/kg as retinyl palmitate). MUFAs and SAFAs were measured using GC and ESI–MS/MS. Hepatic expression of metabolizing enzymes was determined using QRT-PCR. ELOVL6 was significantly down-regulated in response to a high-fat diet (p

Endogenous retinoids like all-trans retinoic acid (ATRA) play important roles in skin homeostasis and skin-based immune responses. Moreover, retinoid signaling was found to be dysregulated in various skin diseases. The present study used topical application of selective agonists and antagonists for retinoic acid receptors (RARs) α and γ and retinoid-X receptors (RXRs) for two weeks on mouse skin in order to determine the role of retinoid receptor subtypes in the gene regulation in skin. We observed pronounced epidermal hyperproliferation upon application of ATRA and synthetic agonists for RARγ and RXR. ATRA and the RARγ agonist further increased retinoid target gene expression (Rbp1, Crabp2, Krt4, Cyp26a1, Cyp26b1) and the chemokines Ccl17 and Ccl22. In contrast, a RARα agonist strongly decreased the expression of ATRA-synthesis enzymes, of retinoid target genes, markers of skin homeostasis, and various cytokines in the skin, thereby markedly resembling the expression profile induced by RXR and RAR antagonists. Our results indicate that RARα and RARγ subtypes possess different roles in the skin and may be of relevance for the auto-regulation of endogenous retinoid signaling in skin. We suggest that dysregulated retinoid signaling in the skin mediated by RXR, RARα and/or RARγ may promote skin-based inflammation and dysregulation of skin barrier properties.

Retinoid-X receptor (RXR)- and retinoic acid receptor (RAR)-mediated signaling is induced by retinoic acids (RA), which are involved in the regulation of skin permeability, differentiation and immune response. Dysregulation of retinoid signaling is present in various skin disorders. Topically and systemically administered synthetic RAR or RXR agonists might influence retinoid-mediated signaling in the skin of RARE reporter animals and gene expression analysis for retinoid, skin homeostasis and skin inflammation marker genes and local retinoid concentrations. Mice were treated orally and topically with synthetic ligands and bioimaging, QRT-PCR and retinoid analysis were performed. Topical application of the synthetic RAR ligand AM580 significantly enhanced retinoid signaling in skin while topical application of the RXR ligand LG268 did not influence retinoic acid receptor re-sponse elements (RARE)-mediated signaling. Systemic treatments with LG268 decreased the expression of genes involved in skin homeostasis, RA synthesis and skin RA concentrations, while it increased various markers for skin inflammation and RA degradation, which corresponds to decreased skin RARE signaling. We conclude from these observations that increased systemic concentrations of an RXR -ligand may be one reason for reduced retinoid signaling, -reduced all-trans RA levels in the skin, reduced epidermal homeostasis and increased skin inflammation marker expression with potential relevance for various skin disorders, like atopic dermatitis.

We hypothesized that higher provitamin A carotenoid serum levels may be associated with higher concentrations of all-trans retinoic acid (ATRA) and atopy. Concentration of ATRA was measured by high-performance liquid chromatography in sera from German domestic and Turkish migrants' children. ATRA serum levels were significantly higher in German children if compared with Turkish children and correlated with those of β-carotene (rs = 0.692) and other provitamin A carotenoids. They did not differ significantly between atopic and nonatopic individuals. Serum levels of ATRA are related to those of provitamin A carotenoids but are not directly related to atopy in the present study.

Nuclear hormone receptors are transcription factors that can be activated by nutrition-derived ligands and alter the expression of various specific target genes. Stearoyl-Coenzyme A desaturase (SCD1) converts palmitic acid (16:0) to palmitoleic acid (16:1n-7) as well as stearic acid (18:0) to oleic acid (18:1n-9). At the same time, elongase 6 (ELOVL6) elongates 16:1n-7 and 18:1n-9 to vaccenic acid (18:1n-7) and eicosenoic acid (20:1n-9). We examined how synthetic selective ligands of nuclear hormone receptors alter the gene expression of hepatic enzymes in mice. In addition, we examined how the regulation of these two enzymes influences fatty acid composition of phospholipids in liver and plasma. Mice were gavaged daily for 1 week with synthetic ligands of peroxisome proliferator-activated receptor (PPAR) α, β/δ, γ, liver X receptor (LXR), retinoic acid receptor (RAR) and retinoid-X receptor (RXR) for 1 week. Phospholipids from liver and plasma were analysed using ESI-MS/MS and GC after saponification. Hepatic gene expression of SCD1 and ELOVL6 was measured using QRT-PCR. SCD1 and ELOVL6 expression increased after the gavage of LXR and RXR ligands. The analysis of fatty acid composition of total phospholipids in plasma and liver showed increased percentage contributions of the SCD1 and ELOVL6 products 18:1n-9, 18:1n-7 and 20:1n-9 after LXR and RXR ligand application. Analysis of total phospholipids from plasma and liver revealed a significant increase in monounsaturated fatty acids bound in phosphatidylcholine (PtdCho) and lysophosphatidylcholine (PtdEtn) after LXR and RXR ligand administration. Increased hepatic gene expression of SCD1 and ELOVL6 after gavage of selective RXR or LXR ligands to mice resulted in increased concentrations of their metabolic products in phospholipids of liver and plasma.

Dietary fatty acids have been shown to influence allergic sensitisation. Bothn-3 andn-6 PUFA are involved in targeted mediation of inflammatory responses during allergic sensitisation and manifestation of atopic diseases. In the present experiments we investigated whether supplementation of DHA-enriched fish oil partly substituting dietary sunflower-seed oil, in comparison with sunflower-seed oil, supplemented to mice influences fatty acid composition of serum lipid classes. The effects of the two different diets were also investigated depending on allergic sensitisation. Supplementation of DHA and EPA in doses of 2 and 0·12 % (w/w) to non-sensitised and sensitised mice resulted in significantly increased percentile contributions of DHA to all lipid classes. In contrast, serum values of then-6 PUFA arachidonic acid (AA) were significantly lower, both in non-sensitised and sensitised mice fed the DHA-enriched diet. The fatty acid composition of serum lipids also reflected allergic sensitisation: the EPA:AA ratio in TAG, cholesteryl esters and phospholipids in non-supplemented animals fell to 23, 29 and 29 % respectively of the original value after allergic sensitisation, whereas it decreased to 70, 80 and 76 % respectively only in the animals supplemented with DHA. In summary, allergic sensitisation alone decreased significantly the EPA:AA ratios in serum TAG, while concomitant supplementation of DHA-enriched fish oil ameliorated this decrease. We postulate from the present results that the amelioration of the severity of allergic sensitisation after DHA supplementation may be linked to altered ratios of the eicosanoid precursors EPA and AA as well as DHA needed for further metabolic activation to pro- or anti-inflammatory bioactive lipids.

Nuclear receptor-mediated signaling via RARs and PPARδ is involved in the regulation of skin homeostasis. Moreover, activation of both RAR and PPARδ was shown to alter skin inflammation. Endogenous all-trans retinoic acid (ATRA) can activate both receptors depending on specific transport proteins: Fabp5 initiates PPARδ signaling whereas Crabp2 promotes RAR signaling. Repetitive topical applications of ovalbumin (OVA) in combination with intraperitoneal injections of OVA or only intraperitoneal OVA applications were used to induce allergic dermatitis. In our mouse model, expression of IL-4, and Hbegf increased whereas expression of involucrin, Abca12 and Spink5 decreased in inflamed skin, demonstrating altered immune response and epidermal barrier homeostasis. Comprehensive gene expression analysis showed alterations of the cutaneous retinoid metabolism and retinoid-mediated signaling in allergic skin immune response. Notably, ATRA synthesis was increased as indicated by the elevated expression of retinaldehyde dehydrogenases and increased levels of ATRA. Consequently, the expression pattern of genes downstream to RAR was altered. Furthermore, the increased ratio of Fabp5 vs. Crabp2 may indicate an up-regulation of the PPARδ pathway in allergen-induced dermatitis in addition to the altered RAR signaling. Thus, our findings suggest that ATRA levels, RAR-mediated signaling and signaling involved in PPARδ pathways are mainly increased in allergen-induced dermatitis and may contribute to the development and/or maintenance of allergic skin diseases.