Dual functions of prostaglandin D2 in murine contact hypersensitivity via DP and CRTH2

Modulation of immunity by the secreted phospholipase A2 family

Among the phospholipase A2 (PLA2 ) superfamily, which typically catalyzes the sn-2 hydrolysis of phospholipids to yield fatty acids and lysophospholipids, the secreted PLA2 (sPLA2 ) family contains 11 isoforms in mammals. Individual sPLA2 s have unique enzymatic specificity toward fatty acids and polar heads of phospholipid substrates and display distinct tissue/cellular distributions, suggesting their distinct physiological functions. Recent studies using knockout and/or transgenic mice for a full set of sPLA2 s have revealed their roles in modulation of immunity and related disorders. Application of mass spectrometric lipidomics to these mice has enabled to identify target substrates and products of individual sPLA2 s in given tissue microenvironments. sPLA2 s hydrolyze not only phospholipids in the plasma membrane of activated, damaged or dying mammalian cells, but also extracellular phospholipids such as those in extracellular vesicles, microbe membranes, lipoproteins, surfactants, and dietary phospholipids, thereby exacerbating or ameliorating various diseases. The actions of sPLA2 s are dependent on, or independent of, the generation of fatty acid- or lysophospholipid-derived lipid mediators according to the pathophysiological contexts. In this review, we make an overview of our current understanding of the roles of individual sPLA2 s in various immune responses and associated diseases.

Regulatory Roles of Phospholipase A2 Enzymes and Bioactive Lipids in Mast Cell Biology

Lipids play fundamental roles in life as an essential component of cell membranes, as a major source of energy, as a body surface barrier, and as signaling molecules that transmit intracellular and intercellular signals. Lipid mediators, a group of bioactive lipids that mediates intercellular signals, are produced via specific biosynthetic enzymes and transmit signals via specific receptors. Mast cells, a tissue-resident immune cell population, produce several lipid mediators that contribute to exacerbation or amelioration of allergic responses and also non-allergic inflammation, host defense, cancer and fibrosis by controlling the functions of microenvironmental cells as well as mast cell themselves in paracrine and autocrine fashions. Additionally, several bioactive lipids produced by stromal cells regulate the differentiation, maturation and activation of neighboring mast cells. Many of the bioactive lipids are stored in membrane phospholipids as precursor forms and released spatiotemporally by phospholipase A₂ (PLA₂) enzymes. Through a series of studies employing gene targeting and lipidomics, several enzymes belonging to the PLA₂ superfamily have been demonstrated to participate in mast cell-related diseases by mobilizing unique bioactive lipids in multiple ways. In this review, we provide an overview of our current understanding of the regulatory roles of several PLA₂-driven lipid pathways in mast cell biology.

PGD2 and CRTH2 counteract Type 2 cytokine-elicited intestinal epithelial responses during helminth infection

Type 2 inflammation is associated with epithelial cell responses, including goblet cell hyperplasia, that promote worm expulsion during intestinal helminth infection. How these epithelial responses are regulated remains incompletely understood. Here, we show that mice deficient in the prostaglandin D2 (PGD2) receptor CRTH2 and mice with CRTH2 deficiency only in nonhematopoietic cells exhibited enhanced worm clearance and intestinal goblet cell hyperplasia following infection with the helminth Nippostrongylus brasiliensis. Small intestinal stem, goblet, and tuft cells expressed CRTH2. CRTH2-deficient small intestinal organoids showed enhanced budding and terminal differentiation to the goblet cell lineage. During helminth infection or in organoids, PGD2 and CRTH2 down-regulated intestinal epithelial Il13ra1 expression and reversed Type 2 cytokine-mediated suppression of epithelial cell proliferation and promotion of goblet cell accumulation. These data show that the PGD2-CRTH2 pathway negatively regulates the Type 2 cytokine-driven epithelial program, revealing a mechanism that can temper the highly inflammatory effects of the anti-helminth response.

Mast Cell-Specific Deletion of Group III Secreted Phospholipase A2 Impairs Mast Cell Maturation and Functions

Tissue-resident mast cells (MCs) have important roles in IgE-associated and -independent allergic reactions. Although microenvironmental alterations in MC phenotypes affect the susceptibility to allergy, understanding of the regulation of MC maturation is still incomplete. We previously reported that group III secreted phospholipase A2 (sPLA2-III) released from immature MCs is functionally coupled with lipocalin-type prostaglandin D2 (PGD2) synthase in neighboring fibroblasts to supply a microenvironmental pool of PGD2, which in turn acts on the PGD2 receptor DP1 on MCs to promote their proper maturation. In the present study, we reevaluated the role of sPLA2-III in MCs using a newly generated MC-specific Pla2g3-deficient mouse strain. Mice lacking sPLA2-III specifically in MCs, like those lacking the enzyme in all tissues, had immature MCs and displayed reduced local and systemic anaphylactic responses. Furthermore, MC-specific Pla2g3-deficient mice, as well as MC-deficient KitW-sh mice reconstituted with MCs prepared from global Pla2g3-null mice, displayed a significant reduction in irritant contact dermatitis (ICD) and an aggravation of contact hypersensitivity (CHS). The increased CHS response by Pla2g3 deficiency depended at least partly on the reduced expression of hematopoietic PGD2 synthase and thereby reduced production of PGD2 due to immaturity of MCs. Overall, our present study has confirmed that MC-secreted sPLA2-III promotes MC maturation, thereby facilitating acute anaphylactic and ICD reactions and limiting delayed CHS response.

Positive and negative roles of lipids in mast cells and allergic responses

Mast cells are a central immune cell population that are crucial in allergic responses. They secrete granule contents and cytokines and produce a panel of lipid mediators in response to FcεRI-dependent or independent stimuli. Leukotrienes and prostaglandins derived from ω6 arachidonic acid, or specialized pro-resolving lipid mediators derived from ω3 eicosapentaenoic and docosahexaenoic acids, exert pleiotropic effects on various cells in the tissue microenvironment, thereby positively or negatively regulating allergic responses. Mast cells also express the inhibitory receptors CD300a and CD300f, which recognize structural lipids. CD300a or CD300f binding to externalized phosphatidylserine or extracellular ceramides, respectively, inhibits FcεRI-mediated mast cell activation. The inhibitory CD300-lipid axis downregulates IgE-driven, mast cell-dependent type I hypersensitivity through different mechanisms. Herein, we provide an overview of our current understanding of the biological roles of lipids in mast cell-dependent allergic responses.

Deficiency of CRTH2, a Prostaglandin D2 Receptor, Aggravates Bleomycin-induced Pulmonary Inflammation and Fibrosis

Chemoattractant receptor homologous with T-helper cell type 2 cells (CRTH2), a receptor for prostaglandin D₂, is preferentially expressed on T-helper cell type 2 lymphocytes, group 2 innate lymphoid cells, eosinophils, and basophils, and elicits the production of type 2 cytokines, including profibrotic IL-13. We hypothesized that lack of CRTH2 might protect against fibrotic lung disease, and we tested this hypothesis using a bleomycin-induced lung inflammation and fibrosis model in CRTH2-deficient (CRTH2^-/-) or wild-type BALB/c mice. Compared with wild-type mice, CRTH2^-/- mice treated with bleomycin exhibited significantly higher mortality, enhanced accumulation of inflammatory cells 14-21 days after bleomycin injection, reduced pulmonary compliance, and increased levels of collagen and total protein in the lungs. These phenotypes were associated with decreased levels of IFN-γ, IL-6, IL-10, and IL-17A in BAL fluid. Adoptive transfer of splenocytes from wild-type, but not CRTH2^-/-, mice 2 days before injection of bleomycin resolved the sustained inflammation as well as the increased collagen and protein accumulation in the lungs of CRTH2^-/- mice. We consider that the disease model is driven by γδT cells that express CRTH2; thus, the adoptive transfer of γδT cells could ameliorate bleomycin-induced alveolar inflammation and fibrosis.

Prostaglandin regulation of T cell biology

Prostaglandins (PG) are pleiotropic bioactive lipids involved in the control of many physiological processes, including key roles in regulating inflammation. This links PG to the modulation of the quality and magnitude of immune responses. T cells, as a core part of the immune system, respond readily to inflammatory cues from their environment, and express a diverse array of PG receptors that contribute to their function and phenotype. Here we put in context our knowledge about how PG affect T cell biology, and review advances that bring light into how specific T cell functions that have been newly discovered are modulated through PG. We will also comment on drugs that target PG metabolism and sensing, their effect on T cell function during disease, and we will finally discuss how we can design new approaches that modulate PG in order to maximize desired therapeutic T cell effects.

Effects of Supplementary Seleno-L-methionine on Atopic Dermatitis-Like Skin Lesions in Mice

Effects of selenium supplementation on atopic dermatitis (AD) were investigated by administering seleno-L-methionine (SeMet) using a mouse model of AD caused by repeated application of 2,4,6-trinitrochlorobenzene (TNCB). BALB/c mice were sensitized with TNCB to the abdomen on day -7; then, TNCB was applied repeatedly to each ear three times a week from days 0 to 23. SeMet was orally administered to the mice from days 0 to 23. The efficacy of SeMet on AD was assessed by measuring ear thickness, histologic evaluation, serum total immunoglobulin (Ig) E levels, and expression of interleukin (IL)-4 in the ear and superficial parotid lymph node. Ear thickness was remarkably increased by repeated application of TNCB, and SeMet significantly suppressed ear thickness in BALB/c mice. SeMet inhibited epidermal hyperplasia and dense infiltration of inflammatory cells. The number of TNCB-induced mast cells was significantly decreased by SeMet. Serum total IgE levels that increased by the repeated application of TNCB were significantly suppressed by SeMet. Repeated application of TNCB induced expression of IL-4, a T-helper (Th) 2 cytokine, in the ear and superficial parotid lymph node of BALB/c mice and its expression was significantly inhibited by SeMet. These results demonstrated that SeMet supplementation suppresses AD-like skin lesions in BALB/c mice and inhibits the expression of total IgE and IL-4.

Host Lipid Mediators in Leprosy: The Hypothesized Contributions to Pathogenesis

The spectrum of clinical forms observed in leprosy and its pathogenesis are dictated by the host's immune response against Mycobacterium leprae, the etiological agent of leprosy. Previous results, based on metabolomics studies, demonstrated a strong relationship between clinical manifestations of leprosy and alterations in the metabolism of ω3 and ω6 polyunsaturated fatty acids (PUFAs), and the diverse set of lipid mediators derived from PUFAs. PUFA-derived lipid mediators provide multiple functions during acute inflammation, and some lipid mediators are able to induce both pro- and anti-inflammatory responses as determined by the cell surface receptors being expressed, as well as the cell type expressing the receptors. However, little is known about how these compounds influence cellular immune activities during chronic granulomatous infectious diseases, such as leprosy. Current evidence suggests that specialized pro-resolving lipid mediators (SPMs) are involved in the down-modulation of the innate and adaptive immune response against M. leprae and that alteration in the homeostasis of pro-inflammatory lipid mediators versus SPMs is associated with dramatic shifts in the pathogenesis of leprosy. In this review, we discuss the possible consequences and present new hypotheses for the involvement of ω3 and ω6 PUFA metabolism in the pathogenesis of leprosy. A specific emphasis is placed on developing models of lipid mediator interactions with the innate and adaptive immune responses and the influence of these interactions on the outcome of leprosy.

Exacerbation of Aging-Associated and Instability-Induced Murine Osteoarthritis With Deletion of D Prostanoid Receptor 1, a Prostaglandin D2 Receptor

OBJECTIVE

D prostanoid receptor 1 (DP1), a receptor for prostaglandin D₂ , plays important roles in inflammation and cartilage metabolism. However, its role in the pathogenesis of osteoarthritis (OA) remains unknown. This study was undertaken to explore the roles of DP1 in the development of OA in murine models and to evaluate the efficacy of a DP1 selective agonist in the treatment of OA.

METHODS

The development of aging-associated OA and destabilization of the medial meniscus (DMM)-induced OA was compared between DP1-deficient (DP1^-/- ) and wild-type (WT) mice. The progression of OA was assessed by histology, immunohistochemistry, and micro-computed tomography. Cartilage explants from DP1^-/- and WT mice were treated with interleukin-1α (IL-1α) ex vivo, to evaluate proteoglycan degradation. The effect of intraperitoneal administration of the DP1 selective agonist BW245C on OA progression was evaluated in WT mice.

RESULTS

Compared to WT mice, DP1^-/- mice had exacerbated cartilage degradation in both models of OA, and this was associated with increased expression of matrix metalloproteinase 13 and ADAMTS-5. In addition, DP1^-/- mice demonstrated enhanced subchondral bone changes. Cartilage explants from DP1^-/- mice showed enhanced proteoglycan degradation following treatment with IL-1α. Intraperitoneal injection of BW245C attenuated the severity of DMM-induced cartilage degradation and bony changes in WT mice.

CONCLUSION

These findings indicate a critical role for DP1 signaling in OA pathogenesis. Modulation of the functions of DP1 may constitute a potential therapeutic target for the development of novel OA treatments.

Dual Roles of Group IID Phospholipase A2 in Inflammation and Cancer

Phospholipase A2 enzymes have long been implicated in the promotion of inflammation by mobilizing pro-inflammatory lipid mediators, yet recent evidence suggests that they also contribute to anti-inflammatory or pro-resolving programs. Group IID-secreted phospholipase A2 (sPLA2-IID) is abundantly expressed in dendritic cells in lymphoid tissues and resolves the Th1 immune response by controlling the steady-state levels of anti-inflammatory lipids such as docosahexaenoic acid and its metabolites. Here, we show that psoriasis and contact dermatitis were exacerbated in Pla2g2d-null mice, whereas they were ameliorated in Pla2g2d-overexpressing transgenic mice, relative to littermate wild-type mice. These phenotypes were associated with concomitant alterations in the tissue levels of ω3 polyunsaturated fatty acid (PUFA) metabolites, which had the capacity to reduce the expression of pro-inflammatory and Th1/Th17-type cytokines in dendritic cells or lymph node cells. In the context of cancer, however, Pla2g2d deficiency resulted in marked attenuation of skin carcinogenesis, likely because of the augmented anti-tumor immunity. Altogether, these results underscore a general role of sPLA2-IID as an immunosuppressive sPLA2 that allows the microenvironmental lipid balance toward an anti-inflammatory state, exerting beneficial or detrimental impact depending upon distinct pathophysiological contexts in inflammation and cancer.

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.

Secreted phospholipase A2 and mast cells

Phospholipase A2s (PLA2s) are a group of enzymes that hydrolyze the sn-2 position of phospholipids to release (typically unsaturated) fatty acids and lysophospholipids, which serve as precursors for a variety of bioactive lipid mediators. Among the PLA2 superfamily, secreted PLA2 (sPLA2) enzymes comprise the largest subfamily that includes 11 isoforms with a conserved His-Asp catalytic dyad. Individual sPLA2 enzymes exhibit unique tissue and cellular localizations and specific enzymatic properties, suggesting their distinct biological roles. Recent studies using transgenic and knockout mice for individual sPLA2 isofoms have revealed their involvement in various pathophysiological events. Here, we overview the current state of knowledge about sPLA2s, specifically their roles in mast cells (MCs) in the context of allergology. In particular, we highlight group III sPLA2 (PLA2G3) as an "anaphylactic sPLA2" that promotes MC maturation and thereby anaphylaxis through a previously unrecognized lipid-orchestrated circuit.

Mast cell-derived prostaglandin D2 inhibits colitis and colitis-associated colon cancer in mice

Compared with prostaglandin E2, which has an established role in cancer, the role of the COX metabolite prostaglandin D2 (PGD2) in chronic inflammation leading to tumorigenesis is uncertain. In this study, we investigated the role of PGD2 in colitis and colitis-associated colon cancer (CAC) using genetically modified mice and an established model of inflammatory colon carcinogenesis. Systemic genetic deficiency in hematopoietic PGD synthase (H-PGDS) aggravated colitis and accelerated tumor formation in a manner associated with increased TNFα expression. Treatment with a TNFα receptor antagonist attenuated colitis regardless of genotype. Histologic analysis revealed that infiltrated mast cells strongly expressed H-PGDS in inflamed colons. Mast cell-specific H-PGDS deficiency also aggravated colitis and accelerated CAC. In contrast, treatment with a PGD2 receptor agonist inhibited colitis and CAC. Together, our results identified mast cell-derived PGD2 as an inhibitor of colitis and CAC, with implications for its potential use in preventing or treating colon cancer.

Prostaglandin D2 receptor D-type prostanoid receptor 2 mediates eosinophil trafficking into the esophagus

Eosinophilic esophagitis is characterized by eosinophil-predominant inflammation in the esophagus. How eosinophils migrate and infiltrate into the esophagus, however, is less clear. Our previous study demonstrated that mast cell activation led to eosinophil infiltration in the esophagus. Prostaglandin D2 (PGD2) is an important mediator released from activated mast cells. The present study aims to determine whether PGD2 induces eosinophil infiltration into the esophagus via a d-type prostanoid receptor 2 (DP2) receptor-dependent mechanism. Using an in vivo guinea pig model, PGD2, d-type prostanoid receptor 1 (DP1) agonist, or DP2 agonist were injected into the esophagus. Esophageal tissues were removed 2 hours after injections and proceeded to either hematoxylin-eosin (HE) staining or immunofluorescent staining of eosinophil major basic protein (MBP) to compare each treatment-induced eosinophil infiltration in the esophagus. In a separate study, ovalbumin (OVA)-sensitized guinea pigs were pretreated with either DP2 or DP1 antagonists, followed by inhalation of OVA to induce mast cell activation. Esophageal tissues were then processed for immunofluorescent staining of MBP. PGD2 injection in the esophagus led to an increase of eosinophil infiltration in esophageal epithelium at the injection site as revealed by HE staining. Increased infiltration of eosinophils was further confirmed by the increased presence of MBP-labeled immunopositive (MBP-LI) cells in esophageal epithelium. Injection with DP2 agonist 15(R)-PGD2, but not DP1 agonist BW 245C, mimicked the PGD2-induced response. In OVA-sensitized animals, antigen inhalation increased MBP-LI cells in esophageal epithelium. Pretreatment with DP2 antagonist BAY-u3405, but not DP1 antagonist BW 868C, inhibited the antigen inhalation-induced increase of MBP-LI cells in esophageal epithelium. These data support the hypothesis that PGD2 induces eosinophil trafficking into the esophageal epithelium via a DP2-mediated pathway, suggesting a role of DP2 antagonist in the prevention of eosinophilic esophagitis.

Polyunsaturated Fatty Acid-derived lipid mediators and T cell function

Fatty acids are involved in T cell biology both as nutrients important for energy production as well as signaling molecules. In particular, polyunsaturated fatty acids are known to exhibit a range of immunomodulatory properties that progress through T cell mediated events, although the molecular mechanisms of these actions have not yet been fully elucidated. Some of these immune activities are linked to polyunsaturated fatty acid-induced alteration of the composition of cellular membranes and the consequent changes in signaling pathways linked to membrane raft-associated proteins. However, significant aspects of the polyunsaturated fatty acid bioactivities are mediated through their transformation to specific lipid mediators, products of cyclooxygenase, lipoxygenase, or cytochrome P450 enzymatic reactions. Resulting bioactive metabolites including prostaglandins, leukotrienes, and endocannabinoids are produced by and/or act upon T leukocytes through cell surface receptors and have been shown to alter T cell activation and differentiation, proliferation, cytokine production, motility, and homing events. Detailed appreciation of the mode of action of these lipids presents opportunities for the design and development of therapeutic strategies aimed at regulating T cell function.

Indomethacin inhibits eosinophil migration to prostaglandin D2 : therapeutic potential of CRTH2 desensitization for eosinophilic pustular folliculitis

Indomethacin is a cyclo-oxygenase inhibitor, and shows therapeutic potential for various eosinophilic skin diseases, particularly eosinophilic pustular folliculitis. One of the unique characteristics of indomethacin is that, unlike other non-steroidal anti-inflammatory drugs, it is a potent agonist of chemoattractant receptor-homologous molecule expressed on T helper type 2 cells (CRTH2), a receptor for prostaglandin D2 (PGD2 ). This study investigated the pharmacological actions of indomethacin on eosinophil migration to clarify the actual mechanisms underlying the therapeutic effects of indomethacin on eosinophilic pustular folliculitis. Eosinophils exhibited chemokinetic and chemotactic responses to both PGD2 and indomethacin through CRTH2 receptors. Pre-treatment of eosinophils with indomethacin greatly inhibited eosinophil migration to PGD2 and, to a much lesser extent, to eotaxin (CCL11); these effects could be mediated by homologous and heterologous desensitization of eosinophil CRTH2 and CCR3, respectively, by agonistic effects of indomethacin on CRTH2. Indomethacin also cancelled a priming effect of Δ(12) -PGJ2 , a plasma metabolite of PGD2 , on eosinophil chemotaxis to eotaxin. Indomethacin down-modulated cell surface expression of both CRTH2 and CCR3. Hair follicle epithelium and epidermal keratinocytes around eosinophilic pustules together with the eccrine apparatus of palmoplantar lesions of eosinophilic pustular folliculitis were immunohistochemically positive for lipocalin-type PGD synthase. Indomethacin may exert therapeutic effects against eosinophilic skin diseases in which PGD2 -CRTH2 signals play major roles by reducing eosinophil responses to PGD2 .

The CRTH2 agonist Pyl A prevents lipopolysaccharide-induced fetal death but induces preterm labour

We have previously demonstrated that the anti-inflammatory prostaglandin 15-deoxy-Δ 12,14-prostaglandin J(2) (15dPGJ(2)) delays inflammation-induced preterm labour in the mouse and improves pup survival through the inhibition of nuclear factor-κB (NF-κB) by a mechanism yet to be elucidated. 15dPGJ(2) is an agonist of the second prostaglandin D(2) receptor, chemoattractant receptor homologous to the T helper 2 cell (CRTH2). In human T helper cells CRTH2 agonists induce the production of the anti-inflammatory interleukins IL-10 and IL-4. We hypothesized that CRTH2 is involved in the protective effect of 15dPGJ(2) in inflammation-induced preterm labour in the murine model. We therefore studied the effects of a specific small molecule CRTH2 agonist on preterm labour and pup survival. An intrauterine injection of lipopolysaccharide (LPS) was administered to CD1 mice at embryonic day 16, ± CRTH2 agonist/vehicle controls. Mice were killed at 4.5 hr to assess fetal wellbeing and to harvest myometrium and pup brain for analysis of NF-κB, and T helper type 1/2 interleukins. To examine the effects of the CRTH2 agonist on LPS-induced preterm labour, mice were allowed to labour spontaneously. Direct effects of the CRTH2 agonist on uterine contractility were examined ex vivo on contracting myometrial strips. The CRTH2 agonist increased fetal survival from 20 to 100% in LPS-treated mice, and inhibited circular muscle contractility ex vivo. However, it augmented LPS-induced labour and significantly increased myometrial NF-κB, IL-1β, KC-GRO, interferon-γ and tumour necrosis factor-α. This suggests that the action of 15dPGJ(2) is not via CRTH2 and therefore small molecule CRTH2 agonists are not likely to be beneficial for the prevention of inflammation-induced preterm labour.

Prostaglandin E2-EP3 axis in fine-tuning excessive skin inflammation by restricting dendritic cell functions

Prostaglandin E₂ (PGE₂) is produced in the skin and is suggested to play a role in the regulation of cutaneous immune homeostasis and responses. However, the multifaceted functions of PGE₂ continue to elude our understanding, especially because of the multiplicity of PGE₂ receptors—EP1, EP2, EP3, and EP4. While cAMP-elevating EP4 is known to activate the functions of cutaneous dendritic cells (DCs), including Langerhans cells (LCs) and dermal DCs, the role of cAMP-suppressing EP3 in this process remains unknown. Here we demonstrated that an EP3 receptor selective agonist, ONO-AE-248, inhibited chemotaxis and co-stimulatory molecule expressions of DCs in vitro. A suboptimal dose of antigen was sufficient to induce contact hypersensitivity in EP3-deficient mice. Intriguingly, EP3 deficiency did not impair skin inflammation at all when the antigen dose was sufficiently high. EP3 limited the functions of cutaneous DCs only when the antigen dose was low. In contrast to EP4, the observed unappreciated function of EP3 may stabilize the cutaneous DCs to halt the impetuous response to a suboptimal dose of antigen. Taken together, PGE₂-EP3 signaling is essential for fine-tuning excessive skin inflammation by restricting DC functions.

Prostaglandin D2 inhibits wound-induced hair follicle neogenesis through the receptor, Gpr44

Prostaglandins (PGs) are key inflammatory mediators involved in wound healing and regulating hair growth; however, their role in skin regeneration after injury is unknown. Using wound-induced hair follicle neogenesis (WIHN) as a marker of skin regeneration, we hypothesized that PGD₂ decreases follicle neogenesis. PGE₂ and PGD₂ were elevated early and late respectively during wound healing. The levels of WIHN, lipocalin-type prostaglandin D₂ synthase (Ptgds) and its product PGD₂ each varied significantly among background strains of mice after wounding and all correlated such that the highest Ptgds and PGD₂ levels were associated with the lowest amount of regeneration. Additionally, an alternatively spliced transcript variant of Ptgds missing exon 3 correlated with high regeneration in mice. Exogenous application of PGD₂ decreased WIHN in wild type mice and PGD₂ receptor Gpr44 null mice showed increased WIHN compared to strain-matched control mice. Furthermore, Gpr44 null mice were resistant to PGD₂-induced inhibition of follicle neogenesis. In all, these findings demonstrate that PGD₂ inhibits hair follicle regeneration through the Gpr44 receptor and imply that inhibition of PGD₂ production or Gpr44 signaling will promote skin regeneration.

[Functions of prostaglandin receptors in contact dermatitis and application to drug discovery]

Contact dermatitis is an inflammatory skin disease caused by toxic factors that activate the skin innate immunity (irritant contact dermatitis) or by a T cell-mediated hypersensitivity reaction (allergic contact dermatitis). These inflammatory skin diseases are sometimes still not easy to control. Therefore, the development of new effective drugs with fewer side effects is anticipated. In the skin under pathophysiological conditions, multiple prostaglandins are produced and their receptors are expressed in time- and/or cell-dependent manners. However, the precise role of prostaglandins and their receptors in contact dermatitis has not been fully understood. Recently, studies using mice with a disruption of each prostaglandin receptor gene, as well as receptor-selective compounds revealed that prostaglandin receptors have manifold functions, sometimes resulting in opposite outcomes. Here, we review new advances in the roles of prostaglandin receptors in contact hypersensitivity as a cutaneous immune response model, and also discuss the clinical potentials of receptor-selective drugs.

Mast cell chemotaxis - chemoattractants and signaling pathways

Migration of mast cells is essential for their recruitment within target tissues where they play an important role in innate and adaptive immune responses. These processes rely on the ability of mast cells to recognize appropriate chemotactic stimuli and react to them by a chemotactic response. Another level of intercellular communication is attained by production of chemoattractants by activated mast cells, which results in accumulation of mast cells and other hematopoietic cells at the sites of inflammation. Mast cells express numerous surface receptors for various ligands with properties of potent chemoattractants. They include the stem cell factor (SCF) recognized by c-Kit, antigen, which binds to immunoglobulin E (IgE) anchored to the high affinity IgE receptor (FcεRI), highly cytokinergic (HC) IgE recognized by FcεRI, lipid mediator sphingosine-1-phosphate (S1P), which binds to G protein-coupled receptors (GPCRs). Other large groups of chemoattractants are eicosanoids [prostaglandin E₂ and D₂, leukotriene (LT) B₄, LTD₄, and LTC₄, and others] and chemokines (CC, CXC, C, and CX3C), which also bind to various GPCRs. Further noteworthy chemoattractants are isoforms of transforming growth factor (TGF) β1–3, which are sensitively recognized by TGF-β serine/threonine type I and II β receptors, adenosine, C1q, C3a, and C5a components of the complement, 5-hydroxytryptamine, neuroendocrine peptide catestatin, tumor necrosis factor-α, and others. Here we discuss the major types of chemoattractants recognized by mast cells, their target receptors, as well as signaling pathways they utilize. We also briefly deal with methods used for studies of mast cell chemotaxis and with ways of how these studies profited from the results obtained in other cellular systems.