The new aryl hydrocarbon receptor antagonist E/Z-2-benzylindene-5,6-dimethoxy-3,3-dimethylindan-1-one protects against UVB-induced signal transduction

Monoterpenoid aryl hydrocarbon receptor allosteric antagonists protect against ultraviolet skin damage in female mice

The human aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is a pivotal regulator of human physiology and pathophysiology. Allosteric inhibition of AhR was previously thought to be untenable. Here, we identify carvones as noncompetitive, insurmountable antagonists of AhR and characterize the structural and functional consequences of their binding. Carvones do not displace radiolabeled ligands from binding to AhR but instead bind allosterically within the bHLH/PAS-A region of AhR. Carvones do not influence the translocation of ligand-activated AhR into the nucleus but inhibit the heterodimerization of AhR with its canonical partner ARNT and subsequent binding of AhR to the promoter of CYP1A1. As a proof of concept, we demonstrate physiologically relevant Ahr-antagonism by carvones in vivo in female mice. These substances establish the molecular basis for selective targeting of AhR regardless of the type of ligand(s) present and provide opportunities for the treatment of disease processes modified by AhR.

Preventive Effect of Pharmaceutical Phytochemicals Targeting the Src Family of Protein Tyrosine Kinases and Aryl Hydrocarbon Receptor on Environmental Stress-Induced Skin Disease

The skin protects our body; however, it is directly exposed to the environment and is stimulated by various external factors. Among the various environmental factors that can threaten skin health, the effects of ultraviolet (UV) and particulate matter (PM) are considered the most notable. Repetitive exposure to ultraviolet and particulate matter can cause chronic skin diseases such as skin inflammation, photoaging, and skin cancer. The abnormal activation of the Src family of protein tyrosine kinases (SFKs) and the aryl hydrocarbon receptor (AhR) in response to UV and/or PM exposure are involved in the development and aggravation of skin diseases. Phytochemicals, chemical compounds of natural plants, exert preventive effects on skin diseases through the regulation of various signaling pathways. Therefore, this review aims to highlight the efficacy of phytochemicals as potential nutraceuticals and pharmaceutical materials for the treatment of skin diseases, primarily by targeting SFK and AhR, and to explore the underlying mechanisms of action. Future studies are essential to validate the clinical potential for the prevention and treatment of skin diseases.

Functions of the aryl hydrocarbon receptor (AHR) beyond the canonical AHR/ARNT signaling pathway

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor regulating adaptive and maladaptive responses toward exogenous and endogenous signals. Research from various biomedical disciplines has provided compelling evidence that the AHR is critically involved in the pathogenesis of a variety of diseases and disorders, including autoimmunity, inflammatory diseases, endocrine disruption, premature aging and cancer. Accordingly, AHR is considered an attractive target for the development of novel preventive and therapeutic measures. However, the ligand-based targeting of AHR is considerably complicated by the fact that the receptor does not always follow the beaten track, i.e. the canonical AHR/ARNT signaling pathway. Instead, AHR might team up with other transcription factors and signaling molecules to shape gene expression patterns and associated physiological or pathophysiological functions in a ligand-, cell- and micromilieu-dependent manner. Herein, we provide an overview about some of the most important non-canonical functions of AHR, including crosstalk with major signaling pathways involved in controlling cell fate and function, immune responses, adaptation to low oxygen levels and oxidative stress, ubiquitination and proteasomal degradation. Further research on these diverse and exciting yet often ambivalent facets of AHR biology is urgently needed in order to exploit the full potential of AHR modulation for disease prevention and treatment.

Effects of Air Pollution on Cellular Senescence and Skin Aging

The human skin is exposed daily to different environmental factors such as air pollutants and ultraviolet (UV) light. Air pollution is considered a harmful environmental risk to human skin and is known to promote aging and inflammation of this tissue, leading to the onset of skin disorders and to the appearance of wrinkles and pigmentation issues. Besides this, components of air pollution can interact synergistically with ultraviolet light and increase the impact of damage to the skin. However, little is known about the modulation of air pollution on cellular senescence in skin cells and how this can contribute to skin aging. In this review, we are summarizing the current state of knowledge about air pollution components, their involvement in the processes of cellular senescence and skin aging, as well as the current therapeutic and cosmetic interventions proposed to prevent or mitigate the effects of air pollution in the skin.

Rosmarinus officinalis L. Leaf Extracts and Their Metabolites Inhibit the Aryl Hydrocarbon Receptor (AhR) Activation In Vitro and in Human Keratinocytes: Potential Impact on Inflammatory Skin Diseases and Skin Cancer

Aryl hydrocarbon receptor (AhR) activation by environmental agents and microbial metabolites is potentially implicated in a series of skin diseases. Hence, it would be very important to identify natural compounds that could inhibit the AhR activation by ligands of microbial origin as 6-formylindolo[3,2-b]carbazole (FICZ), indirubin (IND) and pityriazepin (PZ) or the prototype ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Five different dry Rosmarinus officinalis L. extracts (ROEs) were assayed for their activities as antagonists of AhR ligand binding with guinea pig cytosol in the presence of [³H]TCDD. The methanolic ROE was further assayed towards CYP1A1 mRNA induction using RT-PCR in human keratinocytes against TCDD, FICZ, PZ, and IND. The isolated metabolites, carnosic acid, carnosol, 7-O-methyl-epi-rosmanol, 4′,7-O-dimethylapigenin, and betulinic acid, were assayed for their agonist and antagonist activity in the presence and absence of TCDD using the gel retardation assay (GRA). All assayed ROE extracts showed similar dose-dependent activities with almost complete inhibition of AhR activation by TCDD at 100 ppm. The methanol ROE at 10 ppm showed 99%, 50%, 90%, and 85% inhibition against TCDD, FICZ, IND, and PZ, respectively, in human keratinocytes. Most assayed metabolites exhibited dose-dependent antagonist activity. ROEs inhibit AhR activation by TCDD and by the Malassezia metabolites FICZ, PZ, and IND. Hence, ROE could be useful for the prevention or treatment of skin diseases mediated by activation of AhR.

The Aryl Hydrocarbon Receptor in the Pathogenesis of Environmentally-Induced Squamous Cell Carcinomas of the Skin

Cutaneous squamous cell carcinoma (SCC) is one of the most frequent malignancies in humans and academia as well as public authorities expect a further increase of its incidence in the next years. The major risk factor for the development of SCC of the general population is the repeated and unprotected exposure to ultraviolet (UV) radiation. Another important risk factor, in particular with regards to occupational settings, is the chronic exposure to polycyclic aromatic hydrocarbons (PAH) which are formed during incomplete combustion of organic material and thus can be found in coal tar, creosote, bitumen and related working materials. Importantly, both exposomal factors unleash their carcinogenic potential, at least to some extent, by activating the aryl hydrocarbon receptor (AHR). The AHR is a ligand-dependent transcription factor and key regulator in xenobiotic metabolism and immunity. The AHR is expressed in all cutaneous cell-types investigated so far and maintains skin integrity. We and others have reported that in response to a chronic exposure to environmental stressors, in particular UV radiation and PAHs, an activation of AHR and downstream signaling pathways critically contributes to the development of SCC. Here, we summarize the current knowledge about AHR's role in skin carcinogenesis and focus on its impact on defense mechanisms, such as DNA repair, apoptosis and anti-tumor immune responses. In addition, we discuss the possible consequences of a simultaneous exposure to different AHR-stimulating environmental factors for the development of cutaneous SCC.

Targeting Tryptophan Catabolism in Cancer Immunotherapy Era: Challenges and Perspectives

Metabolism of tryptophan (Trp), an essential amino acid, represent a major metabolic pathway that both promotes tumor cell intrinsic malignant properties as well as restricts antitumour immunity, thus emerging as a drug development target for cancer immunotherapy. Three cytosolic enzymes, namely indoleamine 2,3-dioxygenase 1 (IDO1), IDO2 and tryptophan 2,3-dioxygenase (TDO2), catalyzes the first-rate limiting step of the degradation of Trp to kynurenine (Kyn) and modulates immunity toward immunosuppression mainly through the aryl hydrocarbon receptor (AhR) activation in numerous types of cancer. By restoring antitumor immune responses and synergizing with other immunotherapies, the encouraging preclinical data of IDO1 inhibitors has dramatically failed to translate into clinical success when combined with immune checkpoints inhibitors, reigniting the debate of combinatorial approach. In this review, we i) provide comprehensive evidences on immunomodulatory role of the Trp catabolism metabolites that highlight this pathway as relevant target in immuno-oncology, ii)ii) discuss underwhelming results from clinical trials investigating efficacy of IDO1 inhibitors and underlying mechanisms that might have contributed to this failure, and finally, iii) discuss the current state-of-art surrounding alternative approaches of innovative antitumor immunotherapies that target molecules of Trp catabolism as well as challenges and perspectives in the era of immunotherapy.

Inhibition of 6-formylindolo[3,2-b]carbazole metabolism sensitizes keratinocytes to UVA-induced apoptosis: Implications for vemurafenib-induced phototoxicity

Ultraviolet (UV) B irradiation of keratinocytes results in the formation of the tryptophan photoproduct 6-formylindolo[3,2-b]carbazole (FICZ) which is a high-affinity ligand for the aryl hydrocarbon receptor (AHR). The resulting activation of AHR signaling induces the expression of cytochrome P450 (CYP) 1A1 which subsequently metabolizes FICZ. Importantly, FICZ is also a nanomolar photosensitizer for UVA radiation. Here, we assess whether a manipulation of the AHR-CYP1A1 axis in human epidermal keratinocytes affects FICZ/UVA-induced phototoxic effects and whether this interaction might be mechanistically relevant for the phototoxicity of the BRAF inhibitor vemurafenib. Treatment of keratinocytes with an AHR agonist enhanced the CYP1A1-catalyzed metabolism of FICZ and thus prevented UVA photosensitization, whereas an inhibition of either AHR signaling or CYP1A1 enzyme activity resulted in an accumulation of FICZ and a sensitization to UVA-induced oxidative stress and apoptosis. Exposure of keratinocytes to vemurafenib resulted in the same outcome. Specifically, CYP phenotyping revealed that vemurafenib is primarily metabolized by CYP1A1 and to a lesser degree by CYP2J2 and CYP3A4. Hence, vemurafenib sensitized keratinocytes to UVA-induced apoptosis by interfering with the CYP1A1-mediated oxidative metabolism of FICZ. In contrast to this pro-apoptotic effect, a treatment of UVB-damaged keratinocytes with vemurafenib suppressed apoptosis, a process which might contribute to the skin carcinogenicity of the drug. Our results provide insight into the mechanisms responsible for the photosensitizing properties of vemurafenib and deliver novel information about its metabolism which might be relevant regarding potential drug-drug interactions. The data emphasize that the AHR-CYP1A1 axis contributes to the pathogenesis of cutaneous adverse drug reactions.

Symurban Nanocrystals for Advanced Anti-Pollution Skincare

Several of most common dermatoses worldwide, e.g., psoriasis and atopic dermatitis, are worsened in their clinical picture when the skin is regularly exposed to an increased air pollution level, e.g., particulate matter. This is explainable by the activation of the aryl hydrocarbon receptor (AhR) in the skin, which results in an increased release of proinflammatory cytokines and matrix metalloproteinases. Symurban is a competitive AhR antagonist and thus allows the effective protection of skin. In order to improve its dermal bioavailability as a poorly soluble active agent (0.25 µg/mL), nanocrystals were prepared and evaluated. Nanocrystals are pure active crystals reduced in particle size to the submicron range of 100 to 1000 nm. They feature the properties of nanocrystals, such as increased saturation solubility and dissolution velocity, without having to be declared as nanomaterial. Production methods and parameters were systematically investigated. Wet bead milling at 2000 rpm for 30 min yielded the best results. A z-average of 280 nm was achieved for a 10% Symurban suspension with a polydispersity index of 0.20, indicating a narrow size distribution. For the long-term stabilization of the nanocrystal suspension, the performance of 15 surfactants of different categories and HLB values were investigated and evaluated. It was found that non-ionic surfactants in general were better able to stabilize the system than anionic or amphoteric surfactants. Highest stability of over 12 months at 25 °C was achieved with 2% Plantacare 810 UP, an ECOCERT surfactant with high skin tolerance. The suspension was also chemically long-term stable with >97% of remaining Symurban over 12 months. The saturation solubility of Symurban as nanocrystals was significantly increased from 0.25 to 2.9 μg/mL, which corresponds to a factor of >11. In a case study of one male volunteer with healthy skin conditions, penetration profiles of Symurban nanocrystal hydrogel and commercial anti-pollution serum containing an identical amount of Symurban were determined and compared. After 20 min of exposure, the relative Symurban concentration in the deeper skin layers (tape 19-30) was more than two times higher for nanocrystal hydrogel (16%) than the commercial serum (7%). These results suggest that nanocrystals are a promising delivery system for the poorly soluble anti-pollution agent Symurban.

Role of the Aryl Hydrocarbon Receptor in Environmentally Induced Skin Aging and Skin Carcinogenesis

The skin is constantly exposed to a variety of environmental threats, including solar electromagnetic radiation, microbes, airborne particulate matter, and chemicals. Acute exposure to these environmental factors results in the activation of different signaling pathways that orchestrate adaptive stress responses to maintain cell and tissue homeostasis. Chronic exposure of skin to these factors, however, may lead to the accumulation of damaged macromolecules and loss of cell and tissue integrity, which, over time, may facilitate aging processes and the development of aging-related malignancies. One transcription factor that is expressed in all cutaneous cells and activated by various environmental stressors, including dioxins, polycyclic aromatic hydrocarbons, and ultraviolet radiation, is the aryl hydrocarbon receptor (AHR). By regulating keratinocyte proliferation and differentiation, epidermal barrier function, melanogenesis, and immunity, a certain degree of AHR activity is critical to maintain skin integrity and to adapt to acute stress situations. In contrast, a chronic activation of cutaneous AHR signaling critically contributes to premature aging and the development of neoplasms by affecting metabolism, extracellular matrix remodeling, inflammation, pigmentation, DNA repair, and apoptosis. This article provides an overview of the detrimental effects associated with sustained AHR activity in chronically stressed skin and pinpoints AHR as a promising target for chemoprevention.

Assessing customers' perceived value of the anti-haze cosmetics under haze pollution

Air pollution caused by haze may damage residents' skin, which affects their willingness to buy anti-haze cosmetics. Thus, this paper pays attention to residents' willingness to buy anti-haze cosmetics through customer perceived value theory. In this study, it is assumed that all consumers are rational economic people and that anti-haze cosmetics on the market can effectively cleanse, isolate and repair haze damage. Based on customer perceived value theory and the characteristic of anti-haze cosmetics, four main drivers and had been selected out, that is, monetary value, social value, brand value and perceived haze awareness. Then, based on 314 valid questionnaires, the structural equation analysis method had been used to analyze the impact of the four dimensions of customer perceived value theory on perceived value and consumers' purchase intention of anti-haze cosmetics. The empirical results showed that monetary value, social value, and perceived haze awareness had positive impact on perceived value and purchase intention of anti-haze cosmetics, while brand value only affects perceived value. Perceived value was a partial mediator between the relations of monetary value, social value, perceived haze awareness and purchase intention, while played a completely mediating role between brand value and purchase intention. Then, we also found that price sensitivity played a significant down-modulatory role in perceived value and purchase intention. Furthermore, the perceived haze awareness is the term that we first proposed in this realm, which extends and optimizes Customer Perceived Value. Finally, according to the survey results, consumers would be willing to pay an additional 56.1 yuan per month for the anti-haze effect of cosmetics.

The Toll-like receptor agonist imiquimod is metabolized by aryl hydrocarbon receptor-regulated cytochrome P450 enzymes in human keratinocytes and mouse liver

The Toll-like receptor 7 agonist imiquimod (IMQ) is an approved drug for the topical treatment of various skin diseases that, in addition, is currently tested in multiple clinical trials for the immunotherapy of various types of cancers. As all of these trials include application of IMQ to the skin and evidence exists that exposure to environmental pollutants, i.e., tobacco smoke, affects its therapeutic efficacy, the current study aims to elucidate the cutaneous metabolism of the drug. Treatment of human keratinocytes with 2.5 µM benzo[a]pyrene (BaP), a tobacco smoke constituent and aryl hydrocarbon receptor (AHR) agonist, for 24 h induced cytochrome P450 (CYP) 1A enzyme activity. The addition of IMQ 30 min prior measurement resulted in a dose-dependent inhibition of CYP1A activity, indicating that IMQ is either a substrate or inhibitor of CYP1A isoforms. Incubation of 21 recombinant human CYP enzymes with 0.5 µM IMQ and subsequent LC-MS analyses, in fact, identified CYP1A1 and CYP1A2 as being predominantly responsible for IMQ metabolism. Accordingly, treatment of keratinocytes with BaP accelerated IMQ clearance and the associated formation of monohydroxylated IMQ metabolites. A co-incubation with 5 µM 7-hydroxyflavone, a potent inhibitor of human CYP1A isoforms, abolished basal as well as BaP-induced IMQ metabolism. Further studies with hepatic microsomes from CD-1 as well as solvent- and β-naphthoflavone-treated CYP1A1/CYP1A2 double knock-out and respective control mice confirmed the critical contribution of CYP1A isoforms to IMQ metabolism. Hence, an exposure to life style-related, dietary, and environmental AHR ligands may affect the pharmacokinetics and, thus, treatment efficacy of IMQ.

Photoimmunology: how ultraviolet radiation affects the immune system

Ultraviolet (UV) radiation is a ubiquitous component of the environment that has important effects on a wide range of cell functions. Short-wavelength UVB radiation induces sunburn and is a potent immunomodulator, yet longer-wavelength, lower-energy UVA radiation also has effects on mammalian immunity. This Review discusses current knowledge regarding the mechanisms by which UV radiation can modify innate and adaptive immune responses and how this immunomodulatory capacity can be both beneficial in the case of inflammatory and autoimmune diseases, and detrimental in the case of skin cancer and the response to several infectious agents.

Extract of Deschampsia antarctica (EDA) Prevents Dermal Cell Damage Induced by UV Radiation and 2,3,7,8-Tetrachlorodibenzo-p-dioxin

Exposure to natural and artificial light and environmental pollutants are the main factors that challenge skin homeostasis, promoting aging or even different forms of skin cancer through a variety of mechanisms that include accumulation of reactive oxygen species (ROS), engagement of DNA damage responses, and extracellular matrix (ECM) remodeling upon release of metalloproteases (MMPs). Ultraviolet A radiation is the predominant component of sunlight causative of photoaging, while ultraviolet B light is considered a potentiator of photoaging. In addition, different chemicals contribute to skin aging upon penetration through skin barrier disruption or hair follicles, aryl hydrocarbon receptors (AhR) being a major effector mechanism through which toxicity is exerted. Deschampsia antarctica is a polyextremophile Gramineae capable of thriving under extreme environmental conditions. Its aqueous extract (EDA) exhibits anti- photoaging in human skin cells, such as inhibition of MMPs, directly associated with extrinsic aging. EDA prevents cellular damage, attenuating stress responses such as autophagy and reducing cellular death induced by UV. We demonstrate that EDA also protects from dioxin-induced nuclear translocation of AhR and increases the production of loricrin, a marker of homeostasis in differentiated keratinocytes. Thus, our observations suggest a potential use exploiting EDA's protective properties in skin health supplements.

Implications of tryptophan photoproduct FICZ in oxidative stress and terminal differentiation of keratinocytes

Ultraviolet B (UVB) irradiation activates aryl hydrocarbon receptor (AHR), generates reactive oxygen species (ROS) and mediates photocarcinogenesis and photoaging. 6-Formylindolo[3,2-b]carbazole (FICZ) is a tryptophan photoproduct generated by UVB exposure. FICZ exhibits similar biological effects to UVB, including AHR ligation and ROS production. FICZ also acts as a potent photosensitizer for UVA and the production of ROS is synergistically augmented in the simultaneous presence of FICZ and UVA. In contrast, FICZ upregulates the expression of terminal differentiation molecules such as filaggrin and loricrin via AHR. In parallel with this, the administration of FICZ inhibits skin inflammation in a murine psoriasis and dermatitis model. In this article, we summarize the harmful and beneficial aspects of FICZ in skin pathology.

The AHR represses nucleotide excision repair and apoptosis and contributes to UV-induced skin carcinogenesis

Ultraviolet B (UVB) radiation induces mutagenic DNA photoproducts, in particular cyclobutane pyrimidine dimers (CPDs), in epidermal keratinocytes (KC). To prevent skin carcinogenesis, these DNA photoproducts must be removed by nucleotide excision repair (NER) or apoptosis. Here we report that the UVB-sensitive transcription factor aryl hydrocarbon receptor (AHR) attenuates the clearance of UVB-induced CPDs in human HaCaT KC and skin from SKH-1 hairless mice. Subsequent RNA interference and inhibitor studies in KC revealed that AHR specifically suppresses global genome but not transcription-coupled NER. In further experiments, we found that the accelerated repair of CPDs in AHR-compromised KC depended on a modulation of the p27 tumor suppressor protein. Accordingly, p27 protein levels were increased in AHR-silenced KC and skin biopsies from AHR^−/− mice, and critical for the improvement of NER. Besides increasing NER activity, AHR inhibition was accompanied by an enhanced occurrence of DNA double-strand breaks triggering KC apoptosis at later time points after irradiation. The UVB-activated AHR thus acts as a negative regulator of both early defense systems against carcinogenesis, NER and apoptosis, implying that it exhibits tumorigenic functions in UVB-exposed skin. In fact, AHR^−/− mice developed 50% less UVB-induced cutaneous squamous cell carcinomas in a chronic photocarcinogenesis study than their AHR^+/+ littermates. Taken together, our data reveal that AHR influences DNA damage-dependent responses in UVB-irradiated KC and critically contributes to skin photocarcinogenesis in mice.

Aryl hydrocarbon receptor (AhR) a possible target for the treatment of skin disease

Aryl hydrocarbon receptor (AhR) is a transcription factor expressed in all skin cells type. It responds to exogenous and endogenous chemicals by inducing/repressing the expression of several genes with toxic or protective effects in a wide range of species and tissues. In healthy skin, AhR signalling contributes to keratinocytes differentiation, skin barrier function, skin pigmentation, and mediates oxidative stress. In the last years, some studies have shown that AhR seems to be involved in the pathogenesis of some skin diseases, even if the currently available data are contradictory. Indeed, while the blocking the AhR signalling activity could prevent or treat skin cancer, the AhR activation seems to be advantageous for the treatment of inflammatory skin diseases. Therefore, for its multifaceted role in skin diseases, AhR seems to be an attractive therapeutic target. Indeed, recently some molecules have been identified for the prevention of skin cancer and the treatment of inflammatory skin diseases.

Mechanisms of aging and development-A new understanding of environmental damage to the skin and prevention with topical antioxidants

Recent research has given us new insights into the molecular biology of extrinsic aging of the skin. Not only does UV irradiation directly cause photoaging of the skin, but also environmental pollutants significantly damage exposed skin by several mechanisms. Exposure to the noxious gases of air pollution with simultaneous exposure to UVA can act synergistically to initiate skin cancer. Also ozone generated from pollutants reacting with UV induces oxidative stress of the skin's surface via formation of lipid peroxidation products, with cascading consequences to deeper layers. Furthermore, new studies have demonstrated that particulate matter (PM) pollutants can penetrate the skin transepidermally and through hair follicles to induce skin aging via the aryl hydrocarbon receptor (AHR), a recently discovered ligand-activated transcription factor that regulates and protects keratinocytes, melanocytes, and fibroblasts. With this understanding that extrinsic aging of the skin is not only due to photoaging, we realize the necessity of protection beyond sunscreen. Fortunately, correctly formulated topical antioxidants can prevent damage inflicted by both UV and environmental pollution.

Allelic variants of the aryl hydrocarbon receptor differentially influence UVB-mediated skin inflammatory responses in SKH1 mice

The mouse strain SKH1 is widely used in skin research due to its hairless phenotype and intact immune system. Due to the complex nature of aryl hydrocarbon receptor (AHR) function in the skin, the development of additional in vivo models is necessary to study its role in cutaneous homeostasis and pathology. Variants of the Ah allele, exist among different mouse strains. The Ah^b-1 and Ah^d alleles express high and low affinity ligand binding forms of the AHR, respectively. The outbred SKH1 mice express the Ah^b-2 and/or Ah^d alleles. SKH1 mice were crossed with C57BL/6J mice, which harbor the Ah^b-1 allele, to create useful models for studying endogenous AHR function. SKH1 mice were bred to be homozygous for either the Ah^b-1 or Ah^d allele to establish strains for use in comparative studies of the effects of differential ligand-mediated activation through gene expression changes upon UVB exposure. Ah^b-1 or Ah^d allelic status was confirmed by DNA sequence analysis. We tested the hypothesis that SKH1-Ah^b-1 mice would display enhanced inflammatory signaling upon UVB exposure compared to SKH1-Ah^d mice. Differential basal AHR activation between the strains was determined by assessing Cyp1a1 expression levels in the small intestine, liver, and skin of the SKH1-Ah^b-1 mice compared to SKH1-Ah^d mice. To determine whether SKH1-Ah^b-1 mice are more prone to a pro-inflammatory phenotype in response to UVB, gene expression of inflammatory mediators was analyzed. SKH1-Ah^b-1 mice expressed enhanced gene expression of the chemotactic factors Cxcl5, Cxcl1, and Ccl20, as well as the inflammatory signaling factors S100a9 and Ptgs2, compared to SKH1-Ah^d mice in skin. These data supports a role for AHR activation and enhanced inflammatory signaling in skin.

Editor's Highlight: Ah Receptor Activation Potentiates Neutrophil Chemoattractant (C-X-C Motif) Ligand 5 Expression in Keratinocytes and Skin

Chemokines are components of the skin microenvironment, which enable immune cell chemotaxis. Traditionally, transcription factors involved in inflammatory signaling (eg, NFκB) are important mediators of chemokine expression. To what extent xenobiotics and their associated receptors control chemokine expression is poorly understood. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor known to mediate physiological responses in the skin through the regulation of genes involved in xenobiotic metabolism, epidermal differentiation, and immunity. Here, we demonstrate that AHR activation within primary mouse keratinocytes regulates the expression of a neutrophil directing chemokine (C-X-C motif) ligand 5 (Cxcl5). AHR-mediated regulation of Cxcl5 is because of direct transcriptional activity upon treatment with AHR agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Additionally, AHR mediates enhanced induction of Cxcl5 upon exposure to an agonist and the inflammatory cytokine interleukin 1 beta. This synergy is confined primarily to keratinocytes, as dermal fibroblasts did not achieve the same level of combinatorial induction. AHR-specific antagonists were able to reduce basal and induced levels of Cxcl5, demonstrating the potential for pharmacological intervention. Exposure of C57BL/6 J mice to ultraviolet (UV) light followed by topical treatment with the AHR agonist formylindolo(3,2-b)carbazole (FICZ) significantly induced Cxcl5 expression in skin compared with UV alone, and this response was absent in Ahr-/- mice. These results establish AHR as an important mediator of Cxcl5, with implications for the treatment of inflammatory skin diseases.

An endogenous tryptophan photo-product, FICZ, is potentially involved in photo-aging by reducing TGF-β-regulated collagen homeostasis

BACKGROUND

Persistent ultraviolet (UV) radiation in the form of sunlight causes photo-aging of the skin by reducing the production of type I collagen, the major constituent of the extracellular matrix of the dermis. Transforming growth factor (TGF)-β transforms dermal fibroblasts into α2-smooth muscle actin (ACTA2)-expressing myofibroblasts. Myofibroblasts produce a precursor form of type I collagen, type I procollagen (collagen I), consisting of pro-alpha1 (produced by the COL1A1 gene) and pro-alpha2 chains (produced by the COL1A2 gene). Smad2/3 is a key downstream molecule of TGF-β signaling. The mechanisms through which UV inhibits collagen I synthesis are not fully understood. 6-Formylindolo[3,2-b]carbazole (FICZ) is an endogenous tryptophan photo-metabolite generated by UV irradiation. FICZ is well known as a high-affinity ligand for aryl hydrocarbon receptor (AHR). However, the physiological roles of FICZ in photo-aging have yet to be addressed.

OBJECTIVE

To evaluate the effects of FICZ on the TGF-β-mediated ACTA2 and collagen I expression in normal human dermal fibroblasts (NHDFs).

METHODS

Quantitative real-time polymerase chain reaction and western blot analysis were performed to determine the expression of ACTA2, COL1A1, and COL1A2 in NHDFs with or without FICZ and TGF-β. The phosphorylated Smad2/3 (pSmad2/3) protein levels in cytoplasmic or nuclear portions were investigated by western blot analysis. Immunofluorescence staining was conducted to evaluate pSmad2/3 localization, and F-actin staining with phalloidin was performed to visualize actin polymerization in myofibroblasts. The actions of FICZ on the TGF-β-mediated collagen I expression and nuclear translocation of pSmad2/3 were analyzed in the presence of selective AHR antagonists or in AHR-knockdown NHDFs.

RESULTS

We found that FICZ significantly inhibited the TGF-β-induced upregulation of mRNA and protein levels of ACTA2 and collagen I and actin polymerization in myofibroblasts. FICZ did not disturb the phosphorylation of Smad2/3. Notably, FICZ reduced the expression of pSmad2/3 in the nucleus, while it increased that in the cytoplasm, suggesting that it inhibits the nuclear translocation of pSmad2/3 induced by TGF-β. The inhibitory actions of FICZ on the TGF-β-mediated collagen I expression and nuclear translocation of pSmad2/3 were independent of AHR signaling. Another endogenous AHR agonist, kynurenine, also inhibited the TGF-β-mediated ACTA2 and collagen I upregulation in NHDFs in an AHR-independent manner; however, its effects were insignificant in comparison with those of FICZ.

CONCLUSIONS

These findings suggest that the endogenous photo-product FICZ may be a key chromophore that involves in photo-aging. Downregulation of FICZ signaling is thus a potential strategy to protect against photo-aging.

The aryl hydrocarbon receptor repressor - More than a simple feedback inhibitor of AhR signaling: Clues for its role in inflammation and cancer

The aryl hydrocarbon receptor repressor (AhRR) was first described as a specific competitive repressor of aryl hydrocarbon receptor (AhR) activity based on its ability to dimerize with the AhR nuclear translocator (ARNT) and through direct competition of AhR/ARNT and AhRR/ARNT complexes for binding to dioxin-responsive elements (DREs). Like AhR, AhRR belongs to the basic Helix-Loop-Helix/Per-ARNT-Sim (bHLH/PAS) protein family but lacks functional ligand-binding and transactivation domains. Transient transfection experiments with ARNT and AhRR mutants examining the inhibitory mechanism of AhRR suggested a more complex mechanism than the simple mechanism of negative feedback through sequestration of ARNT to regulate AhR signaling. Recently, AhRR has been shown to act as a tumor suppressor gene in several types of cancer cells. Furthermore, epidemiological studies have found epigenetic changes and silencing of AhRR associated with exposure to cigarette smoke and cancer development. Additional studies from our laboratories have demonstrated that AhRR represses other signaling pathways including NF-κB and is capable of regulating inflammatory responses. A better understanding of the regulatory mechanisms of AhRR in AhR signaling and adverse outcome pathways leading to deregulated inflammatory responses contributing to tumor promotion and other adverse health effects is expected from future studies. This review article summarizes the characteristics of AhRR as an inhibitor of AhR activity and highlights more recent findings pointing out the role of AhRR in inflammation and tumorigenesis.

Arylidene indanone scaffold: medicinal chemistry and structure–activity relationship view

Arylidene indanone (AI) scaffolds are considered as the rigid cousins of chalcones, incorporating the α,β-unsaturated ketone system of chalcones forming a cyclic 5 membered ring.

[Environmentally induced (extrinsic) skin aging]

Chronic exposure to ultraviolet light, particularly as a component of natural sunlight, is a major cause of environmentally induced aging of the skin. In addition, other environmental factors for premature skin aging include longer wavelength radiation in the visible light region and in particular in the shortwave infrared radiation region. Furthermore, particulate and gaseous components of air pollution significantly contribute to the aging process.

The Aryl Hydrocarbon Receptor in Immunity: Tools and Potential

The signaling pathway of the evolutionary old transcription factor AhR is inducible by a number of small molecular weight chemicals, including toxicants such as polycyclic aromatic hydrocarbons, bacterial toxic pigments, and physiological compounds such as tryptophan derivatives or dietary indoles. AhR activation is of immunological importance, but at the same time mediates toxicity of environmental pollutants, such as immunosuppression by dioxins. Measuring AhR activity and identification of ligands is thus of great interest for a variety of research fields. In this chapter, I briefly introduce the AhR signaling pathway, its role in immunology, and the tools and assays needed to analyze AhR signaling. Both are also needed when therapeutic applications are envisioned.

Urban particulate matter down-regulates filaggrin via COX2 expression/PGE2 production leading to skin barrier dysfunction

We explored the regulation of filaggrin, cyclooxygenase 2 (COX2) and prostaglandin E2 (PGE2) expression induced by urban particulate matter (PM) in human keratinocytes. In addition, we investigated the signaling pathways involved in PM-induced effects on COX2/PGE2 and filaggrin. PMs induced increases in COX2 expression and PGE2 production, and decreased filaggrin expression. These effects were attenuated by pretreatment with COX2 inhibitor and PGE2 receptor antagonist, or after transfection with siRNAs of the aryl hydrocarbon receptor (AhR), gp91phox and p47phox. Furthermore, PM-induced generation of reactive oxygen species (ROS) and NADPH oxidase activity was attenuated by pretreatment with an AhR antagonist (AhRI) or antioxidants. Moreover, Nox-dependent ROS generation led to phosphorylation of ERK1/2, p38, and JNK, which then activated the downstream molecules NF-κB and AP-1, respectively. In vivo studies in PMs-treated mice showed that AhRI and apocynin (a Nox2 inhibitor) had anti-inflammatory effects by decreasing COX2 and increasing filaggrin expression. Our results reveal for the first time that PMs-induced ROS generation is mediated through the AhR/p47 phox/NADPH oxidase pathway, which in turn activates ERK1/2, p38/NF-κB and JNK/AP-1, and which ultimately induces COX2 expression and filaggrin downregulation. Up-regulated expression of COX2 and production of PGE2 may lead to impairment of skin barrier function.

Derinat Protects Skin against Ultraviolet-B (UVB)-Induced Cellular Damage

Ultraviolet-B (UVB) is one of the most cytotoxic and mutagenic stresses that contribute to skin damage and aging through increasing intracellular Ca²⁺ and reactive oxygen species (ROS). Derinat (sodium deoxyribonucleate) has been utilized as an immunomodulator for the treatment of ROS-associated diseases in clinics. However, the molecular mechanism by which Derinat protects skin cells from UVB-induced damage is poorly understood. Here, we show that Derinat significantly attenuated UVB-induced intracellular ROS production and decreased DNA damage in primary skin cells. Furthermore, Derinat reduced intracellular ROS, cyclooxygenase-2 (COX-2) expression and DNA damage in the skin of the BALB/c-nu mice exposed to UVB for seven days in vivo. Importantly, Derinat blocked the transient receptor potential canonical (TRPC) channels (TRPCs), as demonstrated by calcium imaging. Together, our results indicate that Derinat acts as a TRPCs blocker to reduce intracellular ROS production and DNA damage upon UVB irradiation. This mechanism provides a potential new application of Derinat for the protection against UVB-induced skin damage and aging.

The aryl hydrocarbon receptor in barrier organ physiology, immunology, and toxicology

The aryl hydrocarbon receptor (AhR) is an evolutionarily old transcription factor belonging to the Per-ARNT-Sim-basic helix-loop-helix protein family. AhR translocates into the nucleus upon binding of various small molecules into the pocket of its single-ligand binding domain. AhR binding to both xenobiotic and endogenous ligands results in highly cell-specific transcriptome changes and in changes in cellular functions. We discuss here the role of AhR for immune cells of the barrier organs: skin, gut, and lung. Both adaptive and innate immune cells require AhR signaling at critical checkpoints. We also discuss the current two prevailing views-namely, 1) AhR as a promiscuous sensor for small chemicals and 2) a role for AhR as a balancing factor for cell differentiation and function, which is controlled by levels of endogenous high-affinity ligands. AhR signaling is considered a promising drug and preventive target, particularly for cancer, inflammatory, and autoimmune diseases. Therefore, understanding its biology is of great importance.

The Janus-Faced Role of Aryl Hydrocarbon Receptor Signaling in the Skin: Consequences for Prevention and Treatment of Skin Disorders

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor expressed in all skin cell types, which is critically involved in the pathogenesis of a variety of skin diseases and thus represents a potential therapeutic target. Recent studies indicate that blocking AHR activation is desirable in some skin conditions, whereas the opposite, i.e., stimulation of AHR activation, is beneficial in another group of skin disorders. We here propose a model based on qualitative differences in canonical versus non-canonical AHR signaling to reconcile these seemingly contradictory observations.

Gene regulation of filaggrin and other skin barrier proteins via aryl hydrocarbon receptor

Aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor that binds to structurally diverse chemicals including dioxins, coal tar, flavonoids and tryptophan photoproducts. Upon ligation, cytoplasmic AHR translocates to the nucleus, heterodimerizes with aryl hydrocarbon receptor nuclear translocator and mediates numerous biological effects by inducing the transcription of various AHR-responsive genes such as epidermal barrier proteins. The activation of AHR usually generates oxidative stress. However, AHR also mediates antioxidant signaling by a plethora of ligands via nuclear factor-erythroid 2-related factor-2. Both oxidative and antioxidant ligands upregulate the expression of the filaggrin gene. We review the role of AHR signaling in the gene regulation of epidermal barrier proteins.

Environment-induced lentigines: formation of solar lentigines beyond ultraviolet radiation

There is no doubt that ultraviolet radiation (UVR) contributes to the generation of acquired lentigines in human skin, as indicated by the term solar lentigo. A growing number of recent epidemiological and mechanistic studies, however, strongly suggest that in addition to UVR, other environmental factors contribute to lentigines' formation as well. We therefore here introduce the term 'environment-induced lentigo' (EIL) to refer to acquired pigment spots of human skin. In this view point, we (i) summarize the existing evidence to support a role of environmental toxicants other than UVR in the pathogenesis of EILs, (ii) we argue that activation of aryl hydrocarbon receptor (AHR) signalling by UVR and environmental toxicants is critically involved in triggering and sustaining a crosstalk between melanocytes, keratinocytes and fibroblasts, which then causes the development and persistence of EILs in human skin, and (iii) we discuss clinical implications for the prevention and treatment of EILs resulting from this concept.

The Tryptophan-Derived Endogenous Aryl Hydrocarbon Receptor Ligand 6-Formylindolo[3,2-b]Carbazole Is a Nanomolar UVA Photosensitizer in Epidermal Keratinocytes

Endogenous UVA-chromophores may act as sensitizers of oxidative stress underlying cutaneous photoaging and photocarcinogenesis, but the molecular identity of non-DNA key chromophores displaying UVA-driven photodyamic activity in human skin remains largely undefined. Here we report that 6-formylindolo[3,2-b]carbazole (FICZ), a tryptophan photoproduct and endogenous high affinity aryl hydrocarbon receptor (AhR) agonist, acts as a nanomolar photosensitizer potentiating UVA-induced oxidative stress irrespective of AhR ligand activity. In human HaCaT and primary epidermal keratinocytes, photodynamic induction of apoptosis was elicited by the combined action of solar simulated UVA and FICZ, whereas exposure to the isolated action of UVA or FICZ did not impair viability. In a human epidermal tissue reconstruct, FICZ/UVA-cotreatment caused pronounced phototoxicity inducing keratinocyte cell death, and FICZ photodynamic activity was also substantiated in a murine skin exposure model. Array analysis revealed pronounced potentiation of cellular heat shock, ER stress, and oxidative stress response gene expression observed only upon FICZ/UVA-cotreatment. FICZ photosensitization caused intracellular oxidative stress, and comet analysis revealed introduction of formamidopyrimidine-DNA glycosylase (FPG)-sensitive oxidative DNA lesions suppressible by antioxidant cotreatment. Taken together, our data demonstrate that the endogenous AhR ligand FICZ displays nanomolar photodynamic activity representing a molecular mechanism of UVA-induced photooxidative stress potentially operative in human skin.

Role of AhR/ARNT system in skin homeostasis

Aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that binds to structurally diverse synthetic and naturally occurring chemicals including dioxins, flavonoids, tryptophan photoproducts, and Malassezia metabolites. Upon binding to its ligands, cytoplasmic AhR translocates to the nucleus, heterodimerizes with aryl hydrocarbon receptor nuclear translocator (ARNT), and mediates numerous biological and toxicological effects by inducing the transcription of various AhR-responsive genes. AhR ligation controls oxidation/antioxidation, epidermal barrier function, photo-induced response, melanogenesis, and innate immunity. This review summarizes recent advances in the understanding of the regulatory mechanisms of skin homeostasis mediated by the AhR/ARNT system.