Prostanoid DP1 receptor agonist inhibits the pruritic activity in NC/Nga mice with atopic dermatitis

Molecular and cellular pruritus mechanisms in the host skin

Pruritus, also known as itching, is a complex sensation that involves the activation of specific physiological and cellular receptors. The skin is innervated with sensory nerves as well as some receptors for various sensations, and its immune system has prominent neurological connections. Sensory neurons have a considerable impact on the sensation of itching. However, immune cells also play a role in this process, as they release pruritogens. Disruption of the dermal barrier activates an immune response, initiating a series of chemical, physical, and cellular reactions. These reactions involve various cell types, including keratinocytes, as well as immune cells involved in innate and adaptive immunity. Collective activation of these immune responses confers protection against potential pathogens. Thus, understanding the molecular and cellular mechanisms that contribute to pruritus in host skin is crucial for the advancement of effective treatment approaches. This review provides a comprehensive analysis of the present knowledge concerning the molecular and cellular mechanisms underlying itching signaling in the skin. Additionally, this review explored the integration of these mechanisms with the broader context of itch mediators and the expression of their receptors in the skin.

Analyzing the Antinociceptive Effect of Interleukin-31 in Mice

The theory that an itch inhibits pain has been refuted; however, previous research did not investigate this theory for an interleukin-31 (IL-31)-induced itch. Previously, we have found that morphine-induced antinociception was partially reduced in IL-31 receptor A (IL-31RA)-deficient (IL-31RAKI) mice, indicating that IL-31RA may play an important role in morphine-induced peripheral antinociception. In the present study, we evaluated the effect of IL-31-induced analgesia on a 2,4,6-trinitrochlorobenzene (TNCB)-sensitized mice using a hot-plate test. This test evaluated the antinociceptive activity of morphine and non-steroidal anti-inflammatory drugs (NSAIDs). Repeated pretreatment with IL-31 showed significant antinociceptive action. Furthermore, its combination with morphine, but not with NSAIDs, increased the analgesic action. In contrast, treatment with TNCB and capsaicin decreased antinociception. Moreover, TNCB increased IL-31RA expression in the dorsal root ganglia at 24 h, whereas capsaicin inhibited it. The comparative action of several analgesics on TNCB or capsaicin was evaluated using a hot-plate test, which revealed that the antinociceptive activity was decreased or disappeared in response to capsaicin-induced pain in IL-31RAKI mice. These results indicate that the analgesic action of IL-31 involves the peripheral nervous system, which affects sensory nerves. These results provide a basis for developing novel analgesics using this mechanism.

Capsaicin suppresses interleukin-31-induced itching partially involved in inhibiting the expression of dorsal root ganglion interleukin-31 receptor A in male mice

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Topically applied capsaicin suppressed scratching behavior in NC/Nga mice, an animal model of atopic dermatitis, sustained for more than 72 h after application.

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Topically applied capsaicin suppressed IL-31-induced scratching behavior in BALB/c mice, sustained for more than 72 h after application.

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Topically applied capsaicin suppressed IL-31receptor A mRNA expression in the DRG, sustained for more than 72 h after application.

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This is the first report that an inhibitor of IL-31receptorA expression suggests a possible mechanism for atopic dermatitis treatment.

To elucidate the mechanisms underlying the antipruritic effect of capsaicin, we investigated how topical application of capsaicin (0.01, 0.1 and 1.0% w/v) affects spontaneous scratching in NC/Nga mice, inerleukin-31 (IL-31) induced in BALB/c mice, and IL-31 receptor A (IL-31RA) and transient receptor potential vanilloid member 1 (TRPV1) mRNA expression in dorsal root ganglia (DRG). Capsaicin concentration-dependently suppressed long-lasting scratching (over 1.0 s, itch-associated scratching) and short-lasting scratching (0.3–1.0 s, locomotor activity) immediately after the application. Total long-lasting scratching and short-lasting scratching counts for 24 h and IL-31RA mRNA expression in the DRG significantly decreased with increasing concentration of capsaicin. Furthermore, 1.0% capsaicin suppressed long-lasting scratching and short-lasting scratching for more than 72 h. At this point, DRG IL-31RAmRNA was significantly decreased, but there was no change in cutaneous IL-31RA and TRPV1 mRNA. Thus capsaicin suppresses long-lasting scratching by inhibiting IL-31RA mRNA expression in the DRG. Next, we examined the effect of capsaicin on IL-31-induced long-lasting scratching in BALB/c mice. Repeated administration of IL-31 (50 μg/kg, subcutaneous) every 12 h for 3 days apparently increased long-lasting scratching counts and IL-31RA mRNA in the DRG. These increases were significantly suppressed by pretreatment with 1.0% capsaicin. TRPV1 mRNA in the DRG was also decreased within 1–24 h after capsaicin application. These results suggest that the strong and prolonged antipruritic action for IL-31-induced itching of capsaicin was caused by desensitization of C-fibers, and, in addition, the long-lasting inhibition of IL-31RA mRNA expression in the DRG.

Intractable Itch in Atopic Dermatitis: Causes and Treatments

Itch or pruritus is the hallmark of atopic dermatitis and is defined as an unpleasant sensation that evokes the desire to scratch. It is also believed that itch is a signal of danger from various environmental factors or physiological abnormalities. Because histamine is a well-known substance inducing itch, H₁-antihistamines are the most frequently used drugs to treat pruritus. However, H₁-antihistamines are not fully effective against intractable itch in patients with atopic dermatitis. Given that intractable itch is a clinical problem that markedly decreases quality of life, its treatment in atopic dermatitis is of high importance. Histamine-independent itch may be elicited by various pruritogens, including proteases, cytokines, neuropeptides, lipids, and opioids, and their cognate receptors, such as protease-activated receptors, cytokine receptors, Mas-related G protein-coupled receptors, opioid receptors, and transient receptor potential channels. In addition, cutaneous hyperinnervation is partly involved in itch sensitization in the periphery. It is believed that dry skin is a key feature of intractable itch in atopic dermatitis. Treatment of the underlying conditions that cause itch is necessary to improve the quality of life of patients with atopic dermatitis. This review describes current insights into the pathophysiology of itch and its treatment in atopic dermatitis.

Eicosanoids in Skin Wound Healing

Wound healing is an important process in the human body to protect against external threats. A dysregulation at any stage of the wound healing process may result in the development of various intractable ulcers or excessive scar formation. Numerous factors such as growth factors, cytokines, and chemokines are involved in this process and play vital roles in tissue repair. Moreover, recent studies have demonstrated that lipid mediators derived from membrane fatty acids are also involved in the process of wound healing. Among these lipid mediators, we focus on eicosanoids such as prostaglandins, thromboxane, leukotrienes, and specialized pro-resolving mediators, which are produced during wound healing processes and play versatile roles in the process. This review article highlights the roles of eicosanoids on skin wound healing, especially focusing on the biosynthetic pathways and biological functions, i.e., inflammation, proliferation, migration, angiogenesis, remodeling, and scarring.

Molecular and Cellular Mechanisms of Itch in Psoriasis

Itch (or pruritus) was not previously recognized as a serious symptom of psoriasis. However, approximately 60-90% of psoriatic patients with pruritus have stated that it deteriorates their quality of life. Since conventional antipruritic therapies, such as antihistamines, only exert limited effects, the establishment of a treatment option for itch in psoriasis is urgently needed. Although a definitive drug is not currently available, various itch mediators are known to be involved in pruritus in psoriasis. In this review, we describe the clinical features of pruritus in psoriasis, classify a wide range of itch mediators into categories, such as the nervous, immune, endocrine, and vascular systems, and discuss the mechanisms by which these mediators induce or aggravate itch in the pathophysiology of psoriasis.

Prostanoids and leukotrienes in the pathophysiology of atopic dermatitis and psoriasis

Lipid mediators, such as prostanoids and leukotrienes (LTs), exert a range of actions through their own receptors on cell surfaces in various pathophysiological conditions. It has been reported that the production of prostanoids and LTs is significantly elevated in the skin lesions of some chronic inflammatory skin diseases, such as atopic dermatitis (AD) and psoriasis, showing the possible involvement of these lipid mediators in the development of those diseases. Although the actual significance of these lipid mediators in humans is still unclear, the findings from studies in mice suggest diverse roles of the lipid mediators in the progression or regulation of these diseases. For example, in a mouse AD model, prostaglandin D2 inhibits the induction of Th2 cells through DP receptor on Langerhans cells, while it promotes infiltration of Th2 cells through chemoattractant receptor-homologous molecule expressed on Th2 cells. In a psoriasis model, thromboxane A2-TP signaling promotes psoriatic dermatitis by facilitating IL-17 production from γδ T cells. In this short review, we summarize the current findings on the roles of prostanoids and LTs in AD and psoriasis as revealed by studies in mice, and discuss the potential of these lipid mediators as therapeutic targets in humans.

The Return of the Mast Cell: New Roles in Neuroimmune Itch Biology

The mast cell-nerve unit classically has represented a fundamental neuroimmune axis in the development of itch because of the traditional prominence of histamine as a pruritogen. However, it is appreciated increasingly that most chronic itch disorders are likely nonhistaminergic in nature, provoking the hypothesis that other novel effector itch mechanisms derived from mast cells are important. In this review, we present an overview of classical mast cell biology and put these concepts into the context of recent advances in our understanding of the regulation and function of the mast cell-nerve unit in itch biology.

RNA-seq profiling of skin in temperate and tropical cattle

Skin is a major thermoregulatory organ in the body controlling homeothermy, a critical function for climate adaptation. We compared genes expressed between tropical- and temperate-adapted cattle to better understand genes involved in climate adaptation and hence thermoregulation. We profiled the skin of representative tropical and temperate cattle using RNA-seq. A total of 214,754,759 reads were generated and assembled into 72,993,478 reads and were mapped to unique regions in the bovine genome. Gene coverage of unique regions of the reference genome showed that of 24,616 genes, only 13,130 genes (53.34%) displayed more than one count per million reads for at least two libraries and were considered suitable for downstream analyses. Our results revealed that of 255 genes expressed differentially, 98 genes were upregulated in tropically-adapted White Fulani (WF; Bos indicus) and 157 genes were down regulated in WF compared to Angus, AG (Bos taurus). Fifteen pathways were identified from the differential gene sets through gene ontology and pathway analyses. These include the significantly enriched melanin metabolic process, proteinaceous extracellular matrix, inflammatory response, defense response, calcium ion binding and response to wounding. Quantitative PCR was used to validate six representative genes which are associated with skin thermoregulation and epithelia dysfunction (mean correlation 0.92; p < 0.001). Our results contribute to identifying genes and understanding molecular mechanisms of skin thermoregulation that may influence strategic genomic selection in cattle to withstand climate adaptation, microbial invasion and mechanical damage.

[Topical Treatment of Pruritic Skin Disease and the Role of Community Pharmacists]

Itching, or pruritus, can be defined as an unpleasant sensation that evokes the desire to scratch. Pruritus is most commonly associated with a primary skin disorder such as atopic dermatitis (AD), psoriasis, etc., and can have a major impact on the quality of life of those patients. Itch-induced scratching can further damage the skin barrier, leading to a worsening of symptoms. For that reason, it is important to manage pruritus. Topical glucocorticoids are commonly the first-line therapy in the management of AD and psoriasis patients. We found that topical glucocorticoids induce pruritus in mice under certain conditions. Topical glucocorticoids may induce pruritus in a mouse model of allergic contact dermatitis via inhibition of prostaglandin (PG)D₂ production in antigen-mediated activated mast cells in the skin. Additionally, topical glucocorticoids do not induce pruritus in healthy skin. These results indicate the importance of controlling skin inflammation to a healthy level by applying sufficient quantities of glucocorticoids to avoid glucocorticoid-induced pruritus. However, topical "steroid phobia" is common in Japan, and most patients apply inadequate amounts of topical glucocorticoids for this reason. This may cause glucocorticoid-induced pruritus in patients by prolonging the skin inflammation. We conducted a survey regarding community pharmacists' instructions on the application quantity of topical glucocorticoids and found that most community pharmacists have experienced inappropriate instructions concerning this point.

G Protein-Coupled Receptors in Asthma Therapy: Pharmacology and Drug Action

Asthma is a heterogeneous inflammatory disease of the airways that is associated with airway hyperresponsiveness and airflow limitation. Although asthma was once simply categorized as atopic or nonatopic, emerging analyses over the last few decades have revealed a variety of asthma endotypes that are attributed to numerous pathophysiological mechanisms. The classification of asthma by endotype is primarily routed in different profiles of airway inflammation that contribute to bronchoconstriction. Many asthma therapeutics target G protein-coupled receptors (GPCRs), which either enhance bronchodilation or prevent bronchoconstriction. Short-acting and long-acting β 2-agonists are widely used bronchodilators that signal through the activation of the β 2-adrenergic receptor. Short-acting and long-acting antagonists of muscarinic acetylcholine receptors are used to reduce bronchoconstriction by blocking the action of acetylcholine. Leukotriene antagonists that block the signaling of cysteinyl leukotriene receptor 1 are used as an add-on therapy to reduce bronchoconstriction and inflammation induced by cysteinyl leukotrienes. A number of GPCR-targeting asthma drug candidates are also in different stages of development. Among them, antagonists of prostaglandin D2 receptor 2 have advanced into phase III clinical trials. Others, including antagonists of the adenosine A2B receptor and the histamine H4 receptor, are in early stages of clinical investigation. In the past decade, significant research advancements in pharmacology, cell biology, structural biology, and molecular physiology have greatly deepened our understanding of the therapeutic roles of GPCRs in asthma and drug action on these GPCRs. This review summarizes our current understanding of GPCR signaling and pharmacology in the context of asthma treatment. SIGNIFICANCE STATEMENT: Although current treatment methods for asthma are effective for a majority of asthma patients, there are still a large number of patients with poorly controlled asthma who may experience asthma exacerbations. This review summarizes current asthma treatment methods and our understanding of signaling and pharmacology of G protein-coupled receptors (GPCRs) in asthma therapy, and discusses controversies regarding the use of GPCR drugs and new opportunities in developing GPCR-targeting therapeutics for the treatment of asthma.

The 15d‑PGJ2 hydrogel ameliorates atopic dermatitis through suppression of the immune response

The present study examined the efficacy of the topical 15d-PGJ₂-poloxamer 407 hydrogel in an atopic dermatitis (AD) animal model. The 15d-PGJ₂ hydrogel was prepared and characterized. The examined rats possessed AD-Like cutaneous lesions, which were induced using 2,4-dinitrochlorobenzene, the rats were then treated with a hydrogel vehicle, 15d-PGJ₂ hydrogel or tacrolimus for 14 days. The rats were sacrificed and blood samples were collected to quantify the IgE levels. Subsequently, skin biopsies were stained with toluidine blue to identify mast cells and immunohistochemistry was performed for ROR-γt and TNF-α. Histological analyses demonstrated that 15d-PGJ₂ hydrogel significantly decreased mast cell infiltration (P<0.05) when compared with the AD-group. Tacrolimus at 0.1% exhibited decreased mast cell infiltration; however, this difference was not statistically significant from the AD-group. Topical 15d-PGJ₂ hydrogel and Tacrolimus 0.1% significantly reduced the serum levels of IgE (P<0.05) compared with the AD-group. Immunohistochemistry revealed a significant decrease in ROR-γt and TNF-α positive cell expression (P<0.05) in the 15d-PGJ₂ hydrogel group compared with the AD-group. In summary, topical administration of 15d-PGJ₂ hydrogel had a beneficial effect on AD symptoms, suggesting that this formulation may be a useful strategy for the treatment of AD.

Dihomo-γ-linolenic acid prevents the development of atopic dermatitis through prostaglandin D1 production in NC/Tnd mice

BACKGROUND

Atopic dermatitis (AD) is a chronic and relapsing skin disorder with pruritic skin symptoms. We previously reported that dihomo-γ-linolenic acid (DGLA) prevented the development of AD in NC/Tnd mice, though the mechanism remained unclear.

OBJECTIVE

We attempted to investigate the mechanism of preventive effect of DGLA on AD development in NC/Tnd mice.

METHODS

The clinical outcomes of NC/Tnd mice that were given diets containing DGLA, arachidonic acid, or eicosapentaenoic acid were compared. Lipid mediator contents in the skin in each group were also quantified. In addition, release of lipid mediators from RBL-2H3 mast cells treated with either DGLA or prostaglandin D1 (PGD1) was measured. Furthermore, effect of PGD1 on gene expression of thymic stromal lymphopoietin (TSLP) in PAM212 keratinocyte cells was determined.

RESULTS

Only DGLA containing diet suppressed the development of dermatitis in vivo. By quantifying the 20-carbon fatty acid-derived eicosanoids in the skin, the application of DGLA was found to upregulate PGD1, which correlated with a better outcome in NC/Tnd mice. Moreover, we confirmed that mast cells produced PGD1 after DGLA exposure, thereby exerting a suppressive effect on immunoglobulin E-mediated degranulation. PGD1 also suppressed gene expression of TSLP in keratinocytes.

CONCLUSION

These results suggest that oral administration of DGLA causes preventive effects on AD development in NC/Tnd mice by regulating the PGD1 supply.

Prostanoids in allergy

Prostanoids, which include prostaglandin and thromboxane, are metabolites of arachidonic acid released in various pathophysiological conditions. They induce a range of actions mediated through their respective receptors expressed on target cells. It has been demonstrated that each prostanoid receptor has multiple functions and that the effect of receptor stimulation can vary depending on context; this sometimes results in opposing effects, such as simultaneous excitatory and inhibitory outcomes. The balance between the production of each prostanoid and the expression of its receptors has been shown to be important for maintaining homeostasis but also involved in the development of various pathological conditions such as allergy. Here, we review the recent findings on the roles of prostanoids in allergy, especially focusing on atopic dermatitis and asthma.

Repeated application of glucocorticoids exacerbate pruritus via inhibition of prostaglandin D2 production of mast cells in a murine model of allergic contact dermatitis

Rebound is known to occur most typically when topical glucocorticoids are abruptly discontinued; however, its frequency and severity are poorly characterized. We previously created a novel murine model of topical glucocorticoid-induced pruritus; however, the mechanism underlying pruritus in this model has not been elucidated. Using this murine model, we aimed to determine the cause of augmentation of pruritus with a focus on the production of prostaglandin (PG) D(2). BALB/c mice with chronic allergic contact dermatitis induced by 5 weeks of repeated application of 2,4,6-trinitro-1-chlorobenzene (TNCB) were treated topically with dexamethasone for 5 weeks immediately after the elicitation of dermatitis and after ear-swelling and scratching behavior were measured. RBL-2H3 mast cells were used to investigate the effect of dexamethasone on degranulation or PGD(2) production in IgE/antigen-stimulated mast cells. The scratching behavior induced by TNCB was augmented by topical application of dexamethasone, but dexamethasone did not have any effect on scratching bouts in mice that had not been treated with TNCB. Topical dexamethasone reduced the PGD(2) level, which increase in TNCB-treated mice, to the baseline level. Moreover, dexamethasone significantly decreased the PGD(2) production in IgE/antigen-stimulated RBL-2H3 mast cells; however, the same concentration of dexamethasone did not have any effect on the degranulation of stimulated mast cells. Topical glucocorticoids may exacerbate pruritus in a mouse model of allergic contact dermatitis via inhibition of PGD(2) production in antigen-mediated activated mast cells in the skin.

Inverse agonist and pharmacochaperone properties of MK-0524 on the prostanoid DP1 receptor

Prostaglandin D₂ (PGD₂) acts through two G protein-coupled receptors (GPCRs), the prostanoid DP receptor and CRTH2 also known as DP1 and DP2, respectively. Several previously characterized GPCR antagonists are now classified as inverse agonists and a number of GPCR ligands are known to display pharmacochaperone activity towards a given receptor. Here, we demonstrate that a DP1 specific antagonist, MK-0524 (also known as laropiprant), decreased basal levels of intracellular cAMP produced by DP1, a Gα(s)-coupled receptor, in HEK293 cells. This reduction in cAMP levels was not altered by pertussis toxin treatment, indicating that MK-0524 did not induce coupling of DP1 to Gα(i/o) proteins and that this ligand is a DP1 inverse agonist. Basal ERK1/2 activation by DP1 was not modulated by MK-0524. Interestingly, treatment of HEK293 cells expressing Flag-tagged DP1 with MK-0524 promoted DP1 cell surface expression time-dependently to reach a maximum increase of 50% compared to control after 24 h. In contrast, PGD₂ induced the internalization of 75% of cell surface DP1 after the same time of stimulation. The increase in DP1 cell surface targeting by MK-0524 was inhibited by Brefeldin A, an inhibitor of transport from the endoplasmic reticulum-Golgi to the plasma membrane. Confocal microscopy confirmed that a large population of DP1 remained trapped intracellularly and co-localized with calnexin, an endoplasmic reticulum marker. Redistribution of DP1 from intracellular compartments to the plasma membrane was observed following treatment with MK-0524 for 24 h. Furthermore, MK-0524 promoted the interaction between DP1 and the ANKRD13C protein, which we showed previously to display chaperone-like effects towards the receptor. We thus report that MK-0524 is an inverse agonist and a pharmacochaperone of DP1. Our findings may have important implications during therapeutic treatments with MK-0524 and for the development of new molecules targeting DP1.

CRTH2 Antagonists

Prostaglandin D2 (PGD2) plays a key role in many of the physiological markings of allergic inflammation including vasodilation, bronchoconstriction, vascular permeability and lymphocyte recruitment. The action of this molecule is elicited through its two primary receptors, DP and CRTH2. Activation of CRTH2 leads to lymphocyte chemotaxis, potentiation of histamine release from basophils, production of inflammatory cytokines (IL-4, IL-5 and IL-13) by Th2 cells, eosinophil degranulation and prevention of Th2 cell apoptosis. As such, antagonism of CRTH2 has been reported to ameliorate the symptoms associated with various allergen challenge animal models including murine antigen induced lung inflammation, murine cigarette smoke induced lung inflammation, murine allergic rhinitis, guinea pig PGD2-induced airflow obstruction, guinea pig airway hyper-responsiveness, sheep airway hyper-responsiveness and murine contact hypersensitivity. CRTH2 antagonists fall into four broad categories: tricyclic ramatroban analogues, indole acetic acids, phenyl/phenoxy acetic acids and non-acid-containing tetrahydroquinolines. Numerous CRTH2 antagonists have been advanced into the clinic and early reports from two Phase II trials suggest promising activity in the alleviation of atopic symptoms.

Signaling by IL-31 and functional consequences

Cytokines are key to control cellular communication. Interleukin-31 (IL-31) was recently discovered as a new member of the IL-6 family of cytokines. IL-31 signals through a heterodimeric receptor composed of OSMR and IL-31RA, a complex that stimulates the JAK-STAT, the RAS/ERK and the PI3K/AKT signal transduction pathways. The available data suggests that IL-31 is important for both innate and adaptive immunity in tissues that are in close contact with the environment, i.e. the skin, the airways and the lung, and the lining of the intestine. Enhanced expression of IL-31 is associated with a number of diseases, including pruritic diseases such as atopic dermatitis, but also in allergy and inflammatory bowel disease. In these tissues IL-31 coordinates the interaction of different immune cells, including T-cells, mast cells, and eosinophils, with epithelial cells. In this review we have summarized the available data on IL-31 and its receptor, their expression pattern and how they are regulated. We describe the current state of knowledge of the involvement of IL-31 in diseases, both in humans and in mouse models. From these studies it is becoming clear that IL-31 plays an important role in the proper functioning of the skin and of airway and intestinal epithelia. The findings available suggest that IL-31 might be an interesting target for directed drug therapy.

D-type prostanoid receptor enhances the signaling of chemoattractant receptor-homologous molecule expressed on T(H)2 cells

BACKGROUND

Prostaglandin (PG) D(2) is substantially involved in allergic responses and signals through the 7 transmembrane-spanning/G protein-coupled receptors, chemoattractant receptor-homologous molecule expressed on T(H)2 cells (CRTH2), and D-type prostanoid (DP) receptor.

OBJECTIVE

Although the proinflammatory function of CRTH2 is well recognized and CRTH2 is hence considered an important emerging pharmacotherapeutic target, the role of the DP receptor in mediating the biological effects of PGD(2) in patients with allergic inflammation has remained unclear.

METHODS

The cross-talk of CRTH2 and DP receptors was investigated by using both a recombinant HEK293 cell model and human eosinophils in Ca(2+) mobilization assays, coimmunoprecipitation, Western blotting, radioligand binding, and immunofluorescence.

RESULTS

We show that CRTH2 and DP receptors modulate one another's signaling properties and form CRTH2/DP heteromers without altering their ligand-binding capacities. We find that the DP receptor amplifies the CRTH2-induced Ca(2+) release from intracellular stores and coincidentally forfeits its own signaling potency. Moreover, desensitization or pharmacologic blockade of the DP receptor hinders CRTH2-mediated signal transduction. However, CRTH2 internalization occurs independently of the DP receptor. In cells that express both receptors, pharmacologic blockade of Gα(q/11) proteins abolishes the Ca(2+) response to both CRTH2 and DP agonists, whereas inhibition of Gα(i) proteins selectively attenuates the CRTH2-mediated response but not the DP signal.

CONCLUSION

Our data demonstrate the capacity of DP receptors to amplify the biological response to CRTH2 activation. Therefore the CRTH2/DP heteromer might not only represent a functional signaling unit for PGD(2) but also a potential target for the development of heteromer-directed therapies to treat allergic diseases.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

Therapy of pruritus

IMPORTANCE OF THE FIELD

Pruritus is the predominant symptom of skin disease. Owing to the poorly understood pathophysiology, the development of effective treatment modalities for pruritus has proven to be particularly difficult. At present, there is no universally accepted therapy for itch. The purpose of this review is to provide an update on the treatment of pruritus.

AREAS COVERED IN THIS REVIEW

An overview of current, emerging and possible future therapies for pruritus is provided.

WHAT THE READER WILL GAIN

Insights into possible treatment regimes for pruritus in different clinical scenarios.

TAKE HOME MESSAGE

The therapy of pruritus is challenging and at present takes on an individualistic approach. Recent advancements in the mechanisms that underlie this distressing symptom have identified new targets for future therapy.

Prostaglandin D2 induces the production of human beta-defensin-3 in human keratinocytes

The antimicrobial peptide human beta-defensin-3 (hBD-3) is produced by epidermal keratinocytes and protects the skin from infections. This peptide induces the release of a lipid mediator, prostaglandin D(2) from dermal mast cells. Prostaglandin D(2) binds to cell-surface G protein-coupled receptors, D prostanoid receptor, and chemoattractant receptor-homologous molecule expressed on T helper cell type 2 (CRTH2). Both receptors are detected on epidermal keratinocytes. It is reported that prostaglandin D(2) is involved in cutaneous allergy, however, its role in antimicrobial defense is unknown. We examined the in vitro effects of prostaglandin D(2) on hBD-3 production in normal human keratinocytes. Prostaglandin D(2) enhanced hBD-3 secretion and mRNA expression in human keratinocytes. Prostaglandin D(2)-induced hBD-3 production was suppressed by the CRTH2 antagonist ramatroban and by antisense oligonucleotides against c-Jun and c-Fos, components of a transcription factor, activator protein-1 (AP-1). Prostaglandin D(2) enhanced the transcriptional activity and DNA binding of AP-1, expression, phosphorylation, and DNA binding of c-Fos proteins in keratinocytes. Prostaglandin D(2)-induced hBD-3 production, AP-1 activity, and c-Fos expression and phosphorylation were suppressed by U0126, PP2, and pertussis toxin, which are inhibitors of mitogen-activated protein kinase kinase (MEK), src, and G(i) proteins, respectively. The phosphorylation of extracellular signal-regulated kinase (ERK), downstream kinase of MEK, was induced by prostaglandin D(2), and suppressed by ramatroban, pertussis toxin, PP2, and U0126. These results suggest that prostaglandin D(2) induces hBD-3 production in human keratinocytes by activating AP-1 through the expression and phosphorylation of c-Fos via the CRTH2/G(i)/src/MEK/ERK pathway. Prostaglandin D(2) may promote cutaneous antimicrobial activity via hBD-3.

Induction of leukotriene B(4) and prostaglandin E(2) release from keratinocytes by protease-activated receptor-2-activating peptide in ICR mice

Protease-activated receptor-2 (PAR2) has been shown to play a key role in the pathophysiology of itch. However, the precise mechanism of PAR2-mediated itch remains largely unknown. In the present study, we investigated the effects of several agents on the scratching behavior induced by PAR2-activating peptide (SLIGRL-NH2). Pretreatment of experimental animals with tacrolimus or the 5-lipoxygenase inhibitor zileuton significantly reduced SLIGRL-NH2-induced scratching behavior, whereas histamine H(1) receptor antagonist cetirizine or the cyclooxygenase inhibitor indomethacin had little effect. Furthermore, intradermal injection of SLIGRL-NH2 increased cutaneous levels of LTB(4) and PGE(2). In vitro, SLIGRL-NH2 treatment enhanced LTB(4) and PGE(2) release from primary keratinocytes in a concentration-dependent manner. Preincubation of keratinocytes with zileuton resulted in a significant decrease of LTB(4) release and treatment of indomethacin led to a significant decrease of PGE(2) in response to SLIGRL-NH2 stimulation. In addition, SLIGRL-NH2-induced secretion of LTB(4) and PGE(2) was significantly inhibited by tacrolimus, whereas cetirizine had no effect. These results indicate that SLIGRL-NH2 stimulates LTB(4) and PGE(2) release from mouse keratinocytes and that enhancement of LTB(4) and PGE(2) secretion contributes to SLIGRL-NH2-induced scratching behavior in ICR mice.

Disruption of FADS2 gene in mice impairs male reproduction and causes dermal and intestinal ulceration

Delta-6 desaturase (D6D) catalyzes the first step in the synthesis of highly unsaturated fatty acids (HUFA) such as arachidonic (AA), docosapentaenoic (DPAn-6), and docosahexaenoic (DHA) acids, as well as the last desaturation of DPAn-6 and DHA. We created D6D-null mice (-/-), which enabled us to study HUFA deficiency without depleting their precursors. In -/-, no in vivo AA synthesis was detected after administration of [U-(13)C]linoleic acid (LA), indicating absence of D6D isozyme. Unexpectedly, all of the -/- developed ulcerative dermatitis when fed a purified diet lacking D6D products but containing ample LA. The -/- also exhibited splenomegaly and ulceration in duodenum and ileocecal junction. Male -/- lacked normal spermatozoa with a severe impairment of spermiogenesis. Tissue HUFAs in -/- declined differentially: liver AA and DHA by 95%, and a smaller decrease in brain and testes. Dietary AA completely prevented dermatitis and intestinal ulcers in -/-. DPAn-6 was absent in -/- brain under AA supplementation, indicating absence of D6D isozyme for DPAn-6 synthesis from AA. This study demonstrated a distinct advantage of the D6D-null mice (-/-) to elucidate (1) AA function without complication of LA deprivation and (2) DHA function in the nervous system without AA depletion or DPAn-6 replacement seen in traditional models.

Pivotal role of mast cells in pruritogenesis in patients with myeloproliferative disorders

Pruritus is a common symptom in patients with Philadelphia chromosome-negative myeloproliferative disorders (MPDs). The pathophysiology of MPD-associated pruritus is unclear. We have demonstrated that MPD mast cells (MCs) are involved by the malignant process. In the present study, we explored the hypothesis that MCs play an important role in the development of pruritogenesis in MPDs. We found that MPD MCs released significantly greater amounts of pruritogenic factors, including histamine, leukotrienes, and interleukin-31 (IL-31) than normal MCs. Elevated levels of IL-31 were also observed in MPD CD3(+) cell-conditioned media. MPD MCs exhibited increased migratory behavior in response to stem cell factor or interleukin-8, which was associated with increased filamentous-actin content. Furthermore, the presence of pruritus in MPDs was statistically correlated with a greater number of MCs being generated by CD34(+) cells, a greater number of MC colonies being formed by CD34(+) cells, decreased apoptosis and prostaglandin D(2) release by cultured MCs, and higher plasma levels of IL-31. These data demonstrate that functional abnormalities of MPD MCs probably lead to pruritogenesis in patients with MPDs. These studies provide cellular and molecular targets for the development of antipruritus drugs for patients with MPDs.

Clinical correlations of recent developments in the pathogenesis of atopic dermatitis

Atopic dermatitis is a chronic inflammatory skin disease with a steadily increasing prevalence affecting 10-20 of infants and 1-3 of adults globally. It is often the first clinical manifestation of atopic disease preceding asthma and allergic rhinitis. Probably half of the children with atopic dermatitis develop some other form of atopic disease later in life. The pathogenesis involves a complex interplay of factors including genetic predisposition due to altered immune or skin barrier function, interactions with the environment such as food and allergen exposures, and infectious triggers of inflammation. In this review, we summarize the recent advances in understanding the contribution of different factors in the pathophysiology of atopic dermatitis and how insights provide new therapeutic potential for its treatment.

Role of prostaglandins on mechanical scratching-induced cutaneous barrier disruption in mice

The role of prostaglandins (PGs) on mechanical scratching-induced cutaneous barrier disruption in mice was investigated by comparing the observed effects of arachidonic acid (AA) application. Scratching of the mouse skin with a stainless-steel wire brush (mechanical scratching) was associated with significant, scratch-count-dependent elevation of the transepidermal water loss (TEWL) and skin PG levels (especially PGD(2) and PGE(2)). Histological evidence of inflammation (crusta, acanthosis and neutrophilic infiltration) in the skin also became evident 24 h after mechanical scratching. On the other hand, while topical application of 0.1% AA to the mouse skin also increased the skin PG levels, but did not produce any increase of TEWL or histological evidence of inflammation in the skin. Topical application of cyclooxygenase inhibitors (indomethacin, piroxicam, aspirin, diclofenac and ketoprofen) decreased the spontaneous recovery rates from cutaneous barrier disruption. These results suggest that the elevation of cutaneous PG production induced by mechanical scratching is involved in the repair of the skin damage caused by the scratching.

Cyclooxygenase-1 inhibition delays recovery of the cutaneous barrier disruption caused by mechanical scratching in mice

BACKGROUND

Atopic dermatitis is a chronic inflammatory disease characterized by severe pruritus, and cutaneous barrier disruption by scratching contributes to further aggravation of the condition. We have previously shown that indomethacin delayed recovery from the effects of cutaneous barrier disruption caused by mechanical scratching in mice.

OBJECTIVES

This study was designed to assess the role of cyclooxygenase (COX)-1 and COX-2 inhibitors on recovery from the effects of cutaneous barrier disruption induced by mechanical scratching in mice.

METHODS

We examined the effects of SC-560 (a COX-1-selective inhibitor) or NS-398 (a COX-2-selective inhibitor) on recovery from the effects of cutaneous barrier disruption in mice induced by a wire brush, in terms of the skin prostaglandin (PG) levels.

RESULTS

While SC-560 significantly delayed recovery from the effects of cutaneous barrier disruption, NS-398 had no such effect. SC-560 was significantly more effective than NS-398 in reducing skin PG levels at 6 and 24 h after cutaneous barrier disruption. SC-560 strongly inhibited biosynthesis of cutaneous PGD(2) to a greater extent than that of other PGs.

CONCLUSIONS

COX-1-coupled PGD(2) biosynthesis may be an important factor in the recovery of cutaneous barrier disruption.

Role of scratch-induced cutaneous prostaglandin D2production on atopic-like scratching behaviour in mice

NC/Nga mice are known to develop scratching dermatitis akin to atopic dermatitis, under conventional (Conv), but not under the specific-pathogen-free (SPF) condition. In this study, we examined the effects of mechanical-scratching on the spontaneous scratching counts (sign of itching), in relation to the cutaneous prostaglandin D2 (PGD2) levels in NC/Nga or BALB/c mice. Mechanical-scratching increased the cutaneous barrier damage and PGD2 levels in both strain mice under the SPF condition. By 4 weeks of cohabitation with the skin-lesioned NC/Nga mice, both the increase in the spontaneous scratching and development of dermatitis score were higher in the Conv-NC/Nga than in the Conv-BALB/c mice. At this time-point, the cutaneous PGD2 level induced by mechanical-scratching was significantly lower in the Conv-NC/Nga when compared with that in the SPF-NC/Nga mice, and that in the Conv-BALB/c was almost equal to that in the SPF-BALB/c mice. With mechanical scratches, the cohabitation-induced scratching was suppressed in the Conv-BALB/c, but not in the Conv-NC/Nga mice. These results suggest that the scratch-induced cutaneous PGD2 inhibits scratching and the subsequent development of dermatitis in BALB/c, while the impaired scratch-induced cutaneous PGD2 production in the NC/Nga mice resulted in no suppression of scratching, and aggravated the dermatitis.

The anti-pruritic efficacy of TS-022, a prostanoid DP1 receptor agonist, is dependent on the endogenous prostaglandin D2 level in the skin of NC/Nga mice

TS-022 is a prostanoid DP(1) receptor agonist, originally developed as a novel anti-pruritic drug for atopic dermatitis. The drug has been shown to suppress scratching and improve the skin inflammation in the NC/Nga (NC) mouse, a model of atopic dermatitis. Corticosteroids are commonly used as effective agents for the treatment of atopic dermatitis. We examined the anti-pruritic efficacy of TS-022 in NC mice cohabited with skin-lesioned NC mice, which showed spontaneous scratching without skin lesions in the early phase and chronic itching with severe dermatitis in the late phase, in comparison with that of dexamethasone. We have previously reported that prostaglandin D(2) might have a physiological role in the inhibition of pruritus. While after 2 weeks of cohabitation with skin-lesioned NC mice (early phase of dermatitis, characterized by the appearance of spontaneous scratching), topically applied TS-022 exhibited a weak anti-pruritic effect in the NC mice, after 6 weeks of cohabitation (late phase, characterized by both chronic scratching and dermatitis), the drug exerted potent anti-pruritic activity. In contrast, dexamethasone exerted potent anti-pruritic effect in both the early and late phases. Indomethacin aggravated the scratching in the early phase, but had no effect in the late phase. The skin prostaglandin D(2) level was significantly increased in the early phase, to subsequently declined and return to the basal level in the late phase. The cutaneous ability for prostaglandin D(2) production following topical application of arachidonic acid or mechanical scratching was decreased in the late phase. Moreover, the expression level of the prostanoid DP(1) receptor in the skin was increased in the late phase. These findings suggest that the potent anti-pruritic activity of TS-022 in the late phase might be attributable to the decrease of endogenous prostaglandin D(2) production and increase of prostanoid DP(1) receptor expression.

Effects of TS-022, a newly developed prostanoid DP1 receptor agonist, on experimental pruritus, cutaneous barrier disruptions and atopic dermatitis in mice

TS-022, {4-[(1R, 2S, 3R, 5R)-5-Chloro-2-((S)-3-cyclohexyl-3-hydroxyprop-1-ynyl)-3-hydroxycyclopentyl] butylthio} acetic acid monohydrate, inhibits ADP-induced platelet aggregation, an effect significantly antagonized, as in the case of prostaglandin D(2) by the prostanoid DP(1) receptor antagonist (BW A868C). TS-022 is a prostanoid DP(1) receptor agonist, originally developed as a novel anti-pruritic drug for patients with atopic dermatitis. We examined the effects of TS-022 on experimental pruritus, cutaneous barrier disruption, and atopic dermatitis and in in vitro immune function tests. Topically applied TS-022 significantly suppressed scratching in skin-lesioned NC/Nga mice from a concentration of 2.5 nM, and this scratch-suppressive activity was significantly antagonized by BW A868C. Tacrolimus (FK-506) and dexamethasone, used as reference drugs for atopic dermatitis, also exhibited suppressive effects against scratching, but only at concentrations of 125 and 25,000 microM. TS-022 applied topically, once a day for 2 days, significantly accelerated repair of the cutaneous barrier disruption caused by mechanical scratching, from concentrations of 2.5 nM. This acceleration of repair of the disrupted cutaneous barrier by this drug was also significantly antagonized by BW A868C. FK-506 and dexamethasone showed no beneficial effects on the repair of the disrupted cutaneous barrier. Repeated topical application of 2.5 microM of TS-022 and 12.5 microM of FK-506 once a day for 6 weeks significantly improved the skin inflammation scores in the NC/Nga mice. In regard to the effects of TS-022 in vitro, the inhibitory activity of TS-022 against concanavalin A-induced cytokine production by splenocytes was marginal as compared with that of FK-506 or dexamethasone. These results suggest that the beneficial therapeutic effects of TS-022 in NC/Nga mice with atopic dermatitis are mediated by its suppressive effect on scratching and its effect of accelerating repair of the disrupted cutaneous barrier, both effects being attributable to its prostanoid DP(1) receptor agonistic activity.

Putative mechanism of the itch-scratch circle: repeated scratching decreases the cutaneous level of prostaglandin D2, a mediator that inhibits itching

In atopic dermatitis, scratching of the skin as a reaction to itching causes injury to the skin, which, in turn, further increases the itching resulting in the establishment of the so-called itch-scratch circle. We have shown that prostaglandin (PG) D2 plays an inhibitory role against pruritus in mice with atopic-like dermatitis; therefore, we examined the relationship between scratching and the cutaneous PGD2 level using an artificial scratching model with a wire brush. Mechanical scratching induced a temporary increase of the skin PGs levels (PGE2, PGD2, 6-ketoPGF1alpha, PGF2alpha). The skin PGD2 level and the ability of PGD2 production decreased at 48 h after repeated scratch, compared to that of normal skin, not so after single scratch. Immunohistochemical analysis and Western blotting revealed a decrease in the levels of cyclooxygenase-1 (COX-1) and hematopoietic PGD synthase in mechanically scratched skin. The reduced ability of the skin for PGD2 production following mechanical scratching could be caused by this decrease in the expression levels of COX-1 and PGD2 synthase. The results suggest that repeated scratching in mice decreases the ability of the skin to produce PGD2, which is an endogenous mediator that inhibits pruritus, resulting in the establishment of the itch-scratch circle.

Modulation of scratching behavior by silencing an endogenous cyclooxygenase-1 gene in the skin through the administration of siRNA

BACKGROUND

RNA interference (RNAi) is rapidly becoming a major tool that is revolutionizing research in the bioscience and biomedical fields. To apply the RNAi technique in vivo, it is crucial to develop appropriate methods of guiding the short interfering RNA (siRNA) molecules to the right tissues and cells. Here, we demonstrate an efficient method for performing gene knockdown in the body skin using the in vivo electro-transduction of siRNA. Using this method, we examined whether the targeted silencing of the cyclooxygenase (COX) gene in the skin could modulate the scratching behavior of an atopic dermatitis mouse model.

METHODS

NC/Nga mice were used as the atopic dermatitis model. Using our optimized in vivo electroporation conditions, siRNAs were introduced into the skin; the silencing efficiency was then analyzed by Western blotting, measuring the levels of prostaglandins, and immunohistochemistry. The scratching behaviors of the mice were measured using an automatic system.

RESULTS

Targeted silencing of the COX-1 gene using our in vivo siRNA technique significantly accelerated the scratching behavior of NC/Nga mice, whereas the COX-2 siRNA showed no effect. In addition, the effect of COX-1 siRNA was mimicked by treatment with a COX-1-selective inhibitor (SC-560).

CONCLUSIONS

We have demonstrated the successful silencing of endogenous gene expression in the skin using the intradermal transfection of unmodified siRNA via electroporation. Using this method, we revealed that COX- 1-mediated prostaglandins may act as endogenous inhibitors of scratching behavior.

Time course changes of scratching counts, dermatitis symptoms, and levels of cutaneous prostaglandins in NC/Nga mice

NC/Nga (NC) mice are known to develop dermatitis resembling atopic dermatitis (AD) in conventional (Conv) conditions, but not in specific pathogen-free (SPF) conditions. We reported that the ability of skin prostaglandin D(2) (PGD(2)) production, which might be the endogenous inhibitor of itching, was attenuated in skin-lesioned Conv-NC mice. We examined the age-related change in scratching, dermatitis symptoms, and skin PGs of SPF- and Conv-NC mice. In Conv-NC, PGD(2) increased at 7 weeks, at which scratching counts increased, but dermatitis did not develop. PGE(2), PGI(2) and PGF(2alpha) increased at 10 and 13 weeks, at which dermatitis developed. The ability to produce skin PGs was examined by measuring PGs after application of arachidonic acid or after mechanical scratching using a wire brush. In Conv-NC, PGD(2) production at 13 weeks was lower than at 7 weeks. In Conv-NC, hematopoietic PGD synthase (hPGDS) expression in the skin at 13 weeks was lower than at 7 weeks by Western blotting and immunohistochemical analysis. The increase of skin PGD(2) level in the early phase of the development of dermatitis is due to the stress of extensive scratching, but did not increase in spite of the stress of extensive scratching in the late phase, due to decreasing capacity of PGD(2) production attributable to decreasing hPGDS expression in Conv-NC mice. These results suggest that a decreased ability to produce skin PGD(2) production could enhance scratching and aggravate dermatitis in Conv-NC mice.

Prostaglandin D2 inhibits TGF-beta1-induced epithelial-to-mesenchymal transition in MDCK cells

In a separate study, we identified PGE2 as a potent inhibitor of TGF-beta1induced epithelial-mesenchymal transition (EMT) in cultured Madin-Darby canine kidney (MDCK) cells (Zhang A, Wang M-H, Dong Z, and Yang T. Am J Physiol Renal Physiol 291: F1323-F1331, 2006). This finding prompted us to examine the roles of other prostanoids: PGD2, PGF(2alpha), PGI2, and thromboxane A2 (TXA2). Treatment with 10 ng/ml TGF-beta1 for 3 days induced EMT as reflected by conversion to the spindle-like morphology, loss of E-cadherin, and activation of alpha-smooth muscle actin (alpha-SMA). Treatment with PGD2 remarkably preserved the epithelial-like morphology, restored the expression of E-cadherin, and abolished the activation of alpha-SMA. In contrast, PGF(2alpha), carbocyclic thromboxane A2, PGI2 and its stable analog beraprost were without an effect. MDCK cells expressed DP1 and DP2 receptors; however, the effect of PGD2 was neither prevented by DP1 antagonist BW-A868C or DP2 antagonist BAY-u3405 nor was mimicked by DP1 agonist BW-245C. cAMP-elevating agents forskolin and 8-Br-cAMP blocked EMT. However, cAMP blockers H89 and Rp-cAMP failed to block the effect of PGD2. PGD2 did not seem to act via its metabolites as 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2) levels in the medium following incubation with 3 microM PGD2 were well below the values predicted from the cross activity of the assay. Exposure to TGF-beta1 induced a threefold increase in reactive oxygen species production that was completely abolished by PGD2. We conclude that 1) PGD2, but not PGI2, PGF(2alpha), and TXA2 inhibit EMT, 2) PGD2 inhibits EMT independently of DP1 and DP2 receptors, and 3) PGD2 exhibits antioxidant property which may, in part, account for the antifibrotic action of this PG.

COX-1 inhibition enhances scratching behaviour in NC/Nga mice with atopic dermatitis

NC/Nga (NC) mice, spontaneously develop an eczematous atopic dermatitis (AD)-like skin lesion when kept under conventional condition (Conv), but not under specific pathogen-free (SPF) conditions, have been thought to be an animal model of AD. We have previously shown that PGD(2) and arachidonic acid inhibited the scratching behaviour of NC mice, while indomethacin enhanced it. This study was designed to assess the role of cyclooxygenase (COX)-1 and COX-2 in the itch-related scratching behaviour of NC mice. We examined the expression of COX in the skin using real-time PCR and Western blotting and the effects of SC-560 (a COX-1 selective inhibitor) or NS-398 (a COX-2 selective inhibitor) on scratching behaviour in relation to skin prostaglandin (PG) levels in NC mice. COX-1 mRNA expression was unchanged and protein expression decreased in Conv NC mice compared with that of SPF mice. By contrast, COX-2 mRNA and protein expression increased in Conv NC mice. SC-560 increased scratching behaviour and significantly reduced skin PGD(2), PGE(2) and PGF(2alpha) levels, but NS-398 did not have effects on scratching and skin PG level. Moreover, the topical application of PGD(2), which might be the endogenous inhibitor of itching, suppressed the SC-560-induced enhancement of scratching behaviour by NC mice. These results suggest COX-1-coupled skin PGD(2) biosynthesis plays a physiological role in inhibiting regulation of pruritus in NC mice with AD.

Role of COX-1 and COX-2 on skin PGs biosynthesis by mechanical scratching in mice

We examined the involvement of cyclooxygenase (COX)-1 and COX-2 on mechanical scratching-induced prostaglandins (PGs) production in the skin of mice. The dorsal regions of mice were scratched using a stainless brush. COXs expressions in the skin were analyzed using real-time PCR and Western blotting. The effect of acetylsalicylic acid (ASA) on the ability of PGs production were determined based on skin PGs level induced by arachidonic acid (AA) application. Mechanical scratching increased PGD2, PGE2, PGI2 and PGF(2 alpha). COX-1 was constitutively expressed and COX-2 expression was enhanced by scratching. Intravenous administration of ASA inhibited PGs biosynthesis in the normal skin. PGs levels of the skin 6h after ASA administration (ASA 6 h) were almost equal to those of the skin 10 min after ASA administration (ASA 10 min). In the scratched skin, AA-induced PGE2 and PGI2 of ASA 6 h were significantly higher than those of ASA 10 min. The skin PGD2 and PGF(2 alpha) of ASA 10 min were almost same to those of ASA 6 h. In the normal skin of COX-1-deficient mice, skin PGD2 level was lower than that of wild-type mice, although PGE2, PGI2 and PGF(2 alpha) levels were almost equal to those of wild type. In the scratched skin of COX-1-deficient mice, PGD2, PGE2, PGI2 and PGF(2 alpha) levels were lower than those of wild-type mice. These results suggested that cutaneous PGD2 could be mainly produced by COX-1, and PGE2 and PGI2 could be produced by COX-1 and COX-2, respectively, in mice.

Emerging roles of DP and CRTH2 in allergic inflammation

The lipid mediator prostaglandin D(2) (PGD(2)) has long been implicated in various inflammatory diseases including asthma. PGD(2) elicits biological responses by activating two seven-transmembrane (7TM) G-protein-coupled receptors, the D-prostanoid receptor DP and the chemoattractant receptor homologous-molecule expressed on T-helper-type-2 cells (CRTH2), which are linked to different signaling pathways. Understanding how immune cells integrate and coordinate signals that are triggered by the same ligand is crucial for the development of novel anti-inflammatory therapies. Here, we examine the roles of DP and CRTH2 in the orchestration of complex inflammatory processes, and discuss their importance as emerging targets for the treatment of asthma and inflammatory diseases.

Increased scratching counts depend on a decrease in ability of cutaneous prostaglandin D2 biosynthesis in NC/Nga mice with atopic dermatitis

Spontaneous and 2,4,6-trinitrochlorobenzene (TNCB)-induced dermatitis models using NC/Nga mice have been recognized as animal models of atopic dermatitis. We reported that scratching behavior leads to dermatitis in a spontaneous dermatitis but not in a TNCB-induced dermatitis. Prostaglandin D2 (PGD2) suppressed the scratching behavior of NC/Nga mice, suggesting that PGD2 plays a physiological role on inhibiting pruritus. We studied whether there was a difference in skin PG contents between spontaneous and TNCB-induced dermatitis. Spontaneous dermatitis was induced by cohabitation with NC/Nga mice having severe skin lesions. TNCB-induced dermatitis was caused by applications of TNCB. PGD2, PGE2, 6keto-PGF1alpha, and PGF2alpha contents in the skin were examined using enzyme-immunoassay kits. For studying ability to produce skin PGs, PG contents were evaluated after topical treatment of arachidonic acid (AA) or mechanical scratching. In spontaneous dermatitis, PGE2, 6keto-PGF1alpha, and PGF2alpha contents increased with dermatitis, but only PGD2 did not do so. In TNCB-induced dermatitis, PGD2, PGE2, 6keto-PGF1alpha, and PGF2alpha increased. Determination of skin PG contents after AA treatment or mechanical scratching revealed that skin PGD2 production of conventional group of spontaneous dermatitis was lower than the specific pathogen-free group. It seemed that ability of skin PGD2 production was attenuated in spontaneous dermatitis. These results suggest that enhancement of scratching behavior in spontaneous dermatitis was caused by the defect of ability to produce PGD2, which plays a physiological role in inhibiting pruritus, resulting in development of dermatitis.

Prostaglandin D2 and prostaglandin E2 accelerate the recovery of cutaneous barrier disruption induced by mechanical scratching in mice

The role of prostaglandins in mechanical scratching-induced cutaneous barrier disruption in mice was investigated. Skin prostaglandins contents were measured after cutaneous barrier function was disrupted by scratching using a stainless-steal wire brush (mechanical scratching), then effects of prostanoids on recovery of cutaneous barrier functions were examined. This mechanical scratching increased transepidermal water loss and skin prostaglandins (prostaglandin D2, prostaglandin E2, 6-keto-prostaglandin F1alpha and prostaglandin F2alpha) contents, count-dependently. Topical application of indomethacin immediately after cutaneous barrier disruption delayed the recovery period of cutaneous barrier disruption. We examined effects of several prostanoids (prostaglandin D2, prostaglandin E2, prostaglandin F2alpha, prostaglandin I2 and U46619) on delay of the recovery process of mechanical scratching-induced cutaneous barrier disruption with treatment of indomethacin. Topically applied prostaglandin D2 and prostaglandin E2 accelerated the recovery of cutaneous barrier disruption and topical application of prostaglandin J2, limaprost, sulprostone and ONO-4819, but not 13,14-dihydro-15-keto-prostaglandin D2, 15-deoxy-Delta(12,14)-prostaglandin J2, 17-phenyl-trinor-prostaglandin E2 or butaprost had effects on recovery of the cutaneous barrier. These results suggest that prostaglandin D2 and prostaglandin E2 accelerate the recovery process of cutaneous barrier disruption caused by mechanical scratching, via specific prostanoid DP1, EP3 and EP4 receptors.

Prostaglandin D2 Inhibits IgE-Mediated Scratching by Suppressing Histamine Release From Mast Cells

Effects of prostaglandin (PG) D(2), PGE(2), and PGI(2) on itch-associated scratching responses of mice and histamine release from the rat basophilic leukemia cell line RBL-2H3 were examined. PGD(2) and ketotifen but not PGE(2) and PGI(2) suppressed the scratching caused by ovalbumin injected into ovalbumin-sensitized mice. Ketotifen also suppressed compound 48/80-induced scratching but not PGD(2), PGE(2), and PGI(2). In vitro, PGD(2) suppressed the antigen-induced histamine release from RBL-2H3 cells, but PGE(2) and PGI(2) did not. These findings suggest that PGD(2) specifically suppressed IgE-mediated scratching by inhibiting IgE-mediated histamine release from mast cells.