Skin-targeted inhibition of PPAR β/δ by selective antagonists to treat PPAR β/δ-mediated psoriasis-like skin disease in vivo

Association between genetically proxied PPARG activation and psoriasis vulgaris: a Mendelian randomization study

BACKGROUND

Psoriasis is a common autoimmune disease in clinical practice, and previous observational studies have suggested that PPARG agonists such as Pioglitazone may be potential therapeutic agents. However, due to interference from various confounding factors, different observational studies have not reached a unified conclusion. We aim to evaluate the potential use of PPARG agonists for treating psoriasis from a new perspective through drug-targeted Mendelian randomization (MR) analysis.

MATERIALS AND METHODS

This study includes data on 8,876 individuals for acute myocardial infarction from GWAS, and LDL cholesterol data from 343,621 Europeans. FinnGen contributed psoriasis vulgaris data for 403,972 individuals. The DrugBank10 databases function to identify genes encoding protein products targeted by active constituents of lipid-modifying targets. A two-sample MR analysis and summary-data-based MR (SMR) analysis estimated the associations between expressions of drug target genes and symptoms of psoriasis vulgaris. A multivariable MR study was further conducted to examine if the observed association was direct association.

RESULTS

SMR analysis revealed that enhanced PPARG gene expression in the blood (equivalent to a one standard deviation increase) was a protective factor for psoriasis vulgaris (beta = -0.2017, se = 0.0723, p = 0.0053). Besides, there exists an MR association between LDL mediated by PPARG and psoriasis vulgaris outcomes (beta = -3.9169, se = 0.5676, p = 5.17E-12). These results indicate that PPARG is a therapeutic target for psoriasis, suggesting that psoriasis may be a potential indication for PPARG agonists.

CONCLUSION

This study confirms that therapeutic activation of PPARG helps suppress the development of psoriasis. Psoriasis may be a new indication for PPARG agonists, such as Pioglitazone. In the future, new anti-psoriatic drugs could be developed targeting PPARG.

PPARδ Agonist GW501516 Suppresses the TGF-β-Induced Profibrotic Response of Human Bronchial Fibroblasts from Asthmatic Patients

The airway wall remodeling observed in asthma is associated with subepithelial fibrosis and enhanced activation of human bronchial fibroblasts (HBFs) in the fibroblast to myofibroblast transition (FMT), induced mainly by transforming growth factor-β (TGF-β). The relationships between asthma severity, obesity, and hyperlipidemia suggest the involvement of peroxisome proliferator-activated receptors (PPARs) in the remodeling of asthmatic bronchi. In this study, we investigated the effect of PPARδ ligands (GW501516 as an agonist, and GSK0660 as an antagonist) on the FMT potential of HBFs derived from asthmatic patients cultured in vitro. This report shows, for the first time, the inhibitory effect of a PPARδ agonist on the number of myofibroblasts and the expression of myofibroblast-related markers-α-smooth muscle actin, collagen 1, tenascin C, and connexin 43-in asthma-related TGF-β-treated HBF populations. We suggest that actin cytoskeleton reorganization and Smad2 transcriptional activity altered by GW501516 lead to the attenuation of the FMT in HBF populations derived from asthmatics. In conclusion, our data demonstrate that a PPARδ agonist stimulates antifibrotic effects in an in vitro model of bronchial subepithelial fibrosis. This suggests its potential role in the development of a possible novel therapeutic approach for the treatment of subepithelial fibrosis during asthma.

Mead acid inhibits retinol-induced irritant contact dermatitis via peroxisome proliferator-activated receptor alpha

Retinol is widely used in topical skincare products to ameliorate skin aging and treat acne and wrinkles; however, retinol and its derivatives occasionally have adverse side effects, including the induction of irritant contact dermatitis. Previously, we reported that mead acid (5,8,11-eicosatrienoic acid), an oleic acid metabolite, ameliorated skin inflammation in dinitrofluorobenzene-induced allergic contact hypersensitivity by inhibiting neutrophil infiltration and leukotriene B₄ production by neutrophils. Here, we showed that mead acid also suppresses retinol-induced irritant contact dermatitis. In a murine model, we revealed that mead acid inhibited keratinocyte abnormalities such as keratinocyte hyperproliferation. Consistently, mead acid inhibited p38 MAPK (mitogen-activated protein kinase) phosphorylation, which is an essential signaling pathway in the keratinocyte hyperplasia induced by retinol. These inhibitory effects of mead acid were associated with the prevention of both keratinocyte hyperproliferation and the gene expression of neutrophil chemoattractants, including Cxcl1 and Cxcl2, and they were mediated by a PPAR (peroxisome proliferator-activated receptor)-α pathway. Our findings identified the anti-inflammatory effects of mead acid, the use of which can be expected to minimize the risk of adverse side effects associated with topical retinoid application.

Parsing the Role of PPARs in Macrophage Processes

Cells are richly equipped with nuclear receptors, which act as ligand-regulated transcription factors. Peroxisome proliferator activated receptors (PPARs), members of the nuclear receptor family, have been extensively studied for their roles in development, differentiation, and homeostatic processes. In the recent past, there has been substantial interest in understanding and defining the functions of PPARs and their agonists in regulating innate and adaptive immune responses as well as their pharmacologic potential in combating acute and chronic inflammatory disease. In this review, we focus on emerging evidence of the potential roles of the PPAR subtypes in macrophage biology. We also discuss the roles of dual and pan PPAR agonists as modulators of immune cell function, microbial infection, and inflammatory diseases.

Keratinocyte-derived IL-1β induces PPARG downregulation and PPARD upregulation in human reconstructed epidermis following barrier impairment

Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear hormone receptors. In skin, PPARs modulate inflammation, lipid synthesis, keratinocyte differentiation and proliferation and thus are important for skin barrier homeostasis. Accordingly, PPAR expression is altered in various skin conditions that entail epidermal barrier impairment, that is atopic dermatitis (AD) and psoriasis. Using human epidermal equivalents (HEEs), we established models of acute epidermal barrier impairment devoid of immune cells. We assessed PPAR and cytokine expression after barrier perturbation and examined effects of keratinocyte-derived cytokines on PPAR expression. We show that acetone or SDS treatment causes graded impairment of epidermal barrier function. Furthermore, we demonstrate that besides IL-1β and TNFα, IL-33 and TSLP are highly relevant markers for acute epidermal barrier impairment. Both SDS- and acetone-mediated epidermal barrier impairment reduce PPARG expression levels, whereas only SDS enhances PPARD expression. In line with findings in IL-1β and TNFα-treated HEEs, abrogation of IL-1 signalling restores PPARG expression and limits the increase of PPARD expression in SDS-induced epidermal barrier impairment. Thus, following epidermal barrier perturbation, keratinocyte-derived IL-1β and partly TNFα modulate PPARG and PPARD expression. These results emphasize a role for PPARγ and PPARβ/δ in acute epidermal barrier impairment with possible implications for diseases such as AD and psoriasis.

PPARdelta in Affected Atopic Dermatitis and Psoriasis: A Possible Role in Metabolic Reprograming

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors expressed in the skin. Three PPAR isotypes, α (NRC1C1), β or δ (NRC1C2) and γ (NRC1C3), have been identified. After activation through ligand binding, PPARs heterodimerize with the 9-cis-retinoic acid receptor (RXR), another nuclear hormone receptor, to bind to specific PPAR-responsive elements in regulatory regions of target genes mainly involved in organogenesis, cell proliferation, cell differentiation, inflammation and metabolism of lipids or carbohydrates. Endogenous PPAR ligands are fatty acids and fatty acid metabolites. In past years, much emphasis has been given to PPARα and γ in skin diseases. PPARβ/δ is the least studied PPAR family member in the skin despite its key role in several important pathways regulating inflammation, keratinocyte proliferation and differentiation, metabolism and the oxidative stress response. This review focuses on the role of PPARβ/δ in keratinocytes and its involvement in psoriasis and atopic dermatitis. Moreover, the relevance of targeting PPARβ/δ to alleviate skin inflammation is discussed.

Discovery and Structure-Activity Relationships of Novel Template, Truncated 1'-Homologated Adenosine Derivatives as Pure Dual PPARγ/δ Modulators

Following our report that A₃ adenosine receptor (AR) antagonist 1 exhibited a polypharmacological profile as a dual modulator of peroxisome proliferator-activated receptor (PPAR)γ/δ, we discovered a new template, 1'-homologated adenosine analogues 4a-4t, as dual PPARγ/δ modulators without AR binding. Removal of binding affinity to A₃AR was achieved by 1'-homologation, and PPARγ/δ dual modulation was derived from the structural similarity between the target nucleosides and PPAR modulator drug, rosiglitazone. All the final nucleosides were devoid of AR-binding affinity and exhibited high binding affinities to PPARγ/δ but lacked PPARα binding. 2-Cl derivatives exhibited dual receptor-binding affinity to PPARγ/δ, which was absent for the corresponding 2-H derivatives. 2-Propynyl substitution prevented PPARδ-binding affinity but preserved PPARγ affinity, indicating that the C2 position defines a pharmacophore for selective PPARγ ligand designs. PPARγ/δ dual modulators functioning as both PPARγ partial agonists and PPARδ antagonists promoted adiponectin production, suggesting their therapeutic potential against hypoadiponectinemia-associated cancer and metabolic diseases.

Recent Insights on the Role of PPAR-β/δ in Neuroinflammation and Neurodegeneration, and Its Potential Target for Therapy

Peroxisome proliferator-activated receptor (PPAR) β/δ belongs to the family of hormone and lipid-activated nuclear receptors, which are involved in metabolism of long-chain fatty acids, cholesterol, and sphingolipids. Similar to PPAR-α and PPAR-γ, PPAR-β/δ also acts as a transcription factor activated by dietary lipids and endogenous ligands, such as long-chain saturated and polyunsaturated fatty acids, and selected lipid metabolic products, such as eicosanoids, leukotrienes, lipoxins, and hydroxyeicosatetraenoic acids. Together with other PPARs, PPAR-β/δ displays transcriptional activity through interaction with retinoid X receptor (RXR). In general, PPARs have been shown to regulate cell differentiation, proliferation, and development and significantly modulate glucose, lipid metabolism, mitochondrial function, and biogenesis. PPAR-β/δ appears to play a special role in inflammatory processes and due to its proangiogenic and anti-/pro-carcinogenic properties, this receptor has been considered as a therapeutic target for treating metabolic syndrome, dyslipidemia, carcinogenesis, and diabetes. Until now, most studies were carried out in the peripheral organs, and despite of its presence in brain cells and in different brain regions, its role in neurodegeneration and neuroinflammation remains poorly understood. This review is intended to describe recent insights on the impact of PPAR-β/δ and its novel agonists on neuroinflammation and neurodegenerative disorders, including Alzheimer’s and Parkinson’s, Huntington’s diseases, multiple sclerosis, stroke, and traumatic injury. An important goal is to obtain new insights to better understand the dietary and pharmacological regulations of PPAR-β/δ and to find promising therapeutic strategies that could mitigate these neurological disorders.

The peroxisome proliferator-activated receptor-β/δ antagonist GSK0660 mitigates retinal cell inflammation and leukostasis

Diabetic retinopathy (DR) is triggered by retinal cell damage stimulated by the diabetic milieu, including increased levels of intraocular free fatty acids. Free fatty acids may serve as an initiator of inflammatory cytokine release from Müller cells, and the resulting cytokines are potent stimulators of retinal endothelial pathology, such as leukostasis, vascular permeability, and basement membrane thickening. Our previous studies have elucidated a role for peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in promoting several steps in the pathologic cascade in DR, including angiogenesis and expression of inflammatory mediators. Furthermore, PPARβ/δ is a known target of lipid signaling, suggesting a potential role for this transcription factor in fatty acid-induced retinal inflammation. Therefore, we hypothesized that PPARβ/δ stimulates both the induction of inflammatory mediators by Müller cells as well the paracrine induction of leukostasis in endothelial cells (EC) by Müller cell inflammatory products. To test this, we used the PPARβ/δ inhibitor, GSK0660, in primary human Müller cells (HMC), human retinal microvascular endothelial cells (HRMEC) and mouse retina. We found that palmitic acid (PA) activation of PPARβ/δ in HMC leads to the production of pro-angiogenic and/or inflammatory cytokines that may constitute DR-relevant upstream paracrine inflammatory signals to EC and other retinal cells. Downstream, EC transduce these signals and increase their synthesis and release of chemokines such as CCL8 and CXCL10 that regulate leukostasis and other cellular events related to vascular inflammation in DR. Our results indicate that PPARβ/δ inhibition mitigates these upstream (MC) as well as downstream (EC) inflammatory signaling events elicited by metabolic stimuli and inflammatory cytokines. Therefore, our data suggest that PPARβ/δ inhibition is a potential therapeutic strategy against early DR pathology.

Peroxisomes and Oxidative Stress: Their Implications in the Modulation of Cellular Immunity During Mycobacterial Infection

Host redox dependent physiological responses play crucial roles in the determination of mycobacterial infection process. Mtb explores oxygen rich lung microenvironments to initiate infection process, however, later on the bacilli adapt to oxygen depleted conditions and become non-replicative and unresponsive toward anti-TB drugs to enter in the latency stage. Mtb is equipped with various sensory mechanisms and a battery of pro- and anti-oxidant enzymes to protect themselves from the host oxidative stress mechanisms. After host cell invasion, mycobacteria induces the expression of NADPH oxidase 2 (NOX2) to generate superoxide radicals (O 2 - ), which are then converted to more toxic hydrogen peroxide (H2O2) by superoxide dismutase (SOD) and subsequently reduced to water by catalase. However, the metabolic cascades and their key regulators associated with cellular redox homeostasis are poorly understood. Phagocytosed mycobacteria en route through different subcellular organelles, where the local environment generated during infection determines the outcome of disease. For a long time, mitochondria were considered as the key player in the redox regulation, however, accumulating evidences report vital role for peroxisomes in the maintenance of cellular redox equilibrium in eukaryotic cells. Deletion of peroxisome-associated peroxin genes impaired detoxification of reactive oxygen species and peroxisome turnover post-infection, thereby leading to altered synthesis of transcription factors, various cell-signaling cascades in favor of the bacilli. This review focuses on how mycobacteria would utilize host peroxisomes to alter redox balance and metabolic regulatory mechanisms to support infection process. Here, we discuss implications of peroxisome biogenesis in the modulation of host responses against mycobacterial infection.

Genomics of lipid-laden human hepatocyte cultures enables drug target screening for the treatment of non-alcoholic fatty liver disease

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is a major health burden in need for new medication. To identify potential drug targets a genomic study was performed in lipid-laden primary human hepatocyte (PHH) and human hepatoma cell cultures.

METHODS

PHH, HuH7 and HepG2 hepatoma cell cultures were treated with lipids and/or TNFα. Intracellular lipid load was quantified with the ORO assay. The Affymetrix HG-U133+ array system was employed to perform transcriptome analysis. The lipid droplet (LD) growth and fusion was determined by fluorescence microscopy. LD associated proteins were imaged by confocal immunofluorescence microscopy and confirmed by Western immunoblotting. Bioinformatics defined perturbed metabolic pathways.

RESULTS

Whole genome expression profiling identified 227, 1031 and 571 significant regulated genes. Likewise, the combined lipid and TNFα treatment of PHH, HuH7 and HepG2 cell cultures revealed 154, 1238 and 278 differentially expressed genes. Although genomic responses differed among in-vitro systems, commonalities were ascertained by filtering the data for LD associated gene regulations. Among others the LD-growth and fusion associated cell death inducing DFFA like effector C (CIDEC), perilipins (PLIN2, PLIN3), the synaptosome-associated-protein 23 and the vesicle associated membrane protein 3 were strongly up-regulated. Likewise, the PPAR targets pyruvate-dehydrogenase-kinase-4 and angiopoietin-like-4 were up-regulated as was hypoxia-inducible lipid droplet-associated (HILPDA), flotilin and FGF21. Their inhibition ameliorates triglyceride and cholesterol accumulation. TNFα treatment elicited strong induction of the chemokine CXCL8, the kinases MAP3K8, MAP4K4 and negative regulators of cytokine signaling, i.e. SOCS2&SOCS3. Live cell imaging of DsRED calreticulin plasmid transfected HuH7 cells permitted an assessment of LD growth and fusion and confocal immunofluorescence microscopy evidenced induced LD-associated PLIN2, CIDEC, HIF1α, HILPDA, JAK1, PDK4 and ROCK2 expression. Notwithstanding, CPT1A protein was repressed to protect mitochondria from lipid overload. Pharmacological inhibition of the GTPase-dynamin and the fatty acid transporter-2 reduced lipid uptake by 28.5 and 35%, respectively. Finally, a comparisons of in-vitro/NAFLD patient biopsy findings confirmed common gene regulations thus demonstrating clinical relevance.

CONCLUSION

The genomics of fat-laden hepatocytes revealed LD-associated gene regulations and perturbed metabolic pathways. Immunofluorescence microscopy confirmed expression of coded proteins to provide a rationale for therapeutic intervention strategies. Collectively, the in-vitro system permits testing of drug candidates.

Selective deletion of PPARβ/δ in fibroblasts causes dermal fibrosis by attenuated LRG1 expression

Connective tissue diseases of the skin are characterized by excessive collagen deposition in the skin and internal organs. Fibroblasts play a pivotal role in the clinical presentation of these conditions. Nuclear receptor peroxisome-proliferator activated receptors (PPARs) are therapeutic targets for dermal fibrosis, but the contribution of the different PPAR subtypes are poorly understood. Particularly, the role of fibroblast PPARβ/δ in dermal fibrosis has not been elucidated. Thus, we generated a mouse strain with selective deletion of PPARβ/δ in the fibroblast (FSPCre-Pparb/d-/-) and interrogated its epidermal and dermal transcriptome profiles. We uncovered a downregulated gene, leucine-rich alpha-2-glycoprotein-1 (Lrg1), of previously unknown function in skin development and architecture. Our findings suggest that the regulation of Lrg1 by PPARβ/δ in fibroblasts is an important signaling conduit integrating PPARβ/δ and TGFβ1-signaling networks in skin health and disease. Thus, the FSPCre-Pparb/d-/- mouse model could serve as a novel tool in the current gunnery of animal models to better understand dermal fibrosis.

Integrative analyses reveal biological pathways and key genes in psoriasis

BACKGROUND

Psoriasis is a complex disease influenced by both genetic and environmental factors with abnormal gene expression in lesional skin. However, no studies are available on genome-scale gene expression of psoriatic lesions in the Chinese population. In addition, systematic studies on the biological pathways, pathogenicity and interaction networks of psoriasis-related genes with abnormal expression profiles require further investigation.

OBJECTIVES

To further explore the associated pathways in psoriasis by functional analysis and to identify the key genes by gene pathogenicity analysis.

METHODS

We performed RNA sequencing on 60 skin biopsy samples from patients with psoriasis and healthy controls to identify the primary differentially expressed genes in psoriatic lesional skin. We retrieved all reported psoriasis-associated genes and performed integrative analyses covering gene expression profiling, pathway analysis, gene pathogenicities and protein-protein interaction networks.

RESULTS

We found that internal and external stimuli may activate immunoinflammatory responses to promote the development of psoriasis. Pathways associated with infectious diseases and cancers were identified by functional and pathway analyses. The gene pathogenicity analysis revealed five key genes in psoriasis: PPARD, GATA3, TIMP3, WNT5A and PTTG1.

CONCLUSIONS

Our analyses showed that genes contributed to the pathogenesis of psoriasis by activating risk pathways with components abnormality in expression. We identified five potentially pathogenic genes for psoriasis that may serve as important biomarkers for the diagnosis and treatment.

Polypharmacology of N6-(3-Iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA) and Related A3 Adenosine Receptor Ligands: Peroxisome Proliferator Activated Receptor (PPAR) γ Partial Agonist and PPARδ Antagonist Activity Suggests Their Antidiabetic Potential

A3 adenosine receptor (AR) ligands including A3 AR agonist, N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (1a, IB-MECA) were examined for adiponectin production in human bone marrow mesenchymal stem cells (hBM-MSCs). In this model, 1a significantly increased adiponectin production, which is associated with improved insulin sensitivity. However, A3 AR antagonists also promoted adiponectin production in hBM-MSCs, indicating that the A3 AR pathway may not be directly involved in the adiponectin promoting activity. In a target deconvolution study, their adiponectin-promoting activity was significantly correlated to their binding activity to both peroxisome proliferator activated receptor (PPAR) γ and PPARδ. They functioned as both PPARγ partial agonists and PPARδ antagonists. In the diabetic mouse model, 1a and its structural analogues A3 AR antagonists significantly decreased the serum levels of glucose and triglyceride, supporting their antidiabetic potential. These findings indicate that the polypharmacophore of these compounds may provide therapeutic insight into their multipotent efficacy against various human diseases.

Nuclear receptor function in skin health and disease: therapeutic opportunities in the orphan and adopted receptor classes

The skin forms a vital barrier between an organism's external environment, providing protection from pathogens and numerous physical and chemical threats. Moreover, the intact barrier is essential to prevent water and electrolyte loss without which terrestrial life could not be maintained. Accordingly, acute disruption of the skin through physical or chemical trauma needs to be repaired timely and efficiently as sustained skin pathologies ranging from mild irritations and inflammation through to malignancy impact considerably on morbidity and mortality. The Nuclear Hormone Receptor Family of transcriptional regulators has proven to be highly valuable targets for addressing a range of pathologies, including metabolic syndrome and cancer. Indeed members of the classic endocrine sub-group, such as the glucocorticoid, retinoid, and Vitamin D receptors, represent mainstay treatment strategies for numerous inflammatory skin disorders, though side effects from prolonged use are common. Emerging evidence has now highlighted important functional roles for nuclear receptors belonging to the adopted and orphan subgroups in skin physiology and patho-physiology. This review will focus on these subgroups and explore the current evidence that suggests these nuclear receptor hold great promise as future stand-alone or complementary drug targets in treating common skin diseases and maintaining skin homeostasis.

The role of PPARγ-mediated signalling in skin biology and pathology: new targets and opportunities for clinical dermatology

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that modulate the expression of multiple different genes involved in the regulation of lipid, glucose and amino acid metabolism. PPARs and cognate ligands also regulate important cellular functions, including cell proliferation and differentiation, as well as inflammatory responses. This includes a role in mediating skin and pilosebaceous unit homoeostasis: PPARs appear to be essential for maintaining skin barrier permeability, inhibit keratinocyte cell growth, promote keratinocyte terminal differentiation and regulate skin inflammation. They also may have protective effects on human hair follicle (HFs) epithelial stem cells, while defects in PPARγ-mediated signalling may promote the death of these stem cells and thus facilitate the development of cicatricial alopecia (lichen planopilaris). Overall, however, selected PPARγ modulators appear to act as hair growth inhibitors that reduce the proliferation and promote apoptosis of hair matrix keratinocytes. The fact that commonly prescribed PPARγ-modulatory drugs of the thiazolidine-2,4-dione class can exhibit a battery of adverse cutaneous effects underscores the importance of distinguishing beneficial from clinically undesired cutaneous activities of PPARγ ligands and to better understand on the molecular level how PPARγ-regulated cutaneous lipid metabolism and PPARγ-mediated signalling impact on human skin physiology and pathology. Surely, the therapeutic potential that endogenous and exogenous PPARγ modulators may possess in selected skin diseases, ranging from chronic inflammatory hyperproliferative dermatoses like psoriasis and atopic dermatitis, via scarring alopecia and acne can only be harnessed if the complexities of PPARγ signalling in human skin and its appendages are systematically dissected.

Keratinocyte differentiation and upregulation of ceramide synthesis induced by an oat lipid extract via the activation of PPAR pathways

Activation of peroxisome proliferator-activated receptors (PPARs) has been shown to have an important role in skin barrier function by regulating differentiation and lipid synthesis in keratinocytes. Oat (Avena sativa) has long been used as a soothing agent to relieve skin irritations, and the clinical benefits of topical oat formulations have been proven; however, the mechanistic understanding of oat's mode of action remains unknown. We investigated whether an oat lipid extract could activate PPARs and subsequently increase epidermal lipid synthesis and differentiation markers. Primary human epidermal keratinocytes and transformed cell lines were treated with PPAR agonists and oat lipid extracts to investigate the PPAR agonism. PPAR target genes and epidermal differentiation markers were analysed using quantitative real-time PCR and HPTLC analysis. Oat lipid extract demonstrated robust dual agonism for PPARα and PPARβ/δ, and increased direct PPAR target gene induction in primary human keratinocytes. In addition, oat oil treatment increased both receptor expression and, consistent with the literature on PPARs, oat oil treatment resulted in a significant upregulation of differentiation genes (involucrin, SPRRs and transglutaminase 1) and ceramide processing genes (β-glucocerebrosidase, sphingomyelinases 3 and ABCA12). Further, oat oil treatment in keratinocytes significantly increased ceramide levels (70%), suggesting a functional translation of PPAR activation by oat oil in keratinocytes. Taken together, these results demonstrate that oat lipids possess robust dual agonistic activities for PPARα and PPARβ/δ, increase their gene expression and induce differentiation and ceramide synthesis in keratinocytes, which can collectively improve skin barrier function.

Synthesis of 5-trifluoromethyl-2-sulfonylpyridine PPARβ/δ antagonists: Effects on the affinity and selectivity towards PPARβ/δ

The covalent modification of peroxisome-proliferator activated receptor β/δ (PPARβ/δ) is part of the mode of action of 5-trifluoromethyl-2-sulfonylpyridine PPARβ/δ antagonists such as GSK3787 and CC618. Herein, the synthesis and in vitro biological evaluation of a range of structural analogues of the two antagonists are reported. The new ligands demonstrate that an improvement in the selectivity of 5-trifluoromethyl-2-sulfonylpyridine antagonists towards PPARβ/δ is achievable at the expense of their immediate affinity for PPARβ/δ. However, their putatively covalent and irreversible mode of action may ensure their efficacy over time, as observed in time-resolved fluorescence resonance energy transfer (TR-FRET)-based ligand displacement assays.

Peroxisome proliferator-activated receptors (PPARs) and PPAR agonists: the 'future' in dermatology therapeutics?

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors and comprise three different isoforms namely PPARα, PPARγ, and PPARβ/δ with PPARβ/δ being the predominant subtype in human keratinocytes. After binding with specific ligands, PPARs regulate gene expression, cell growth and differentiation, apoptosis, inflammatory responses, and tumorogenesis. PPARs also modulate a wide variety of skin functions including keratinocyte proliferation, epidermal barrier formation, wound healing, melanocyte proliferation, and sebum production. Recent studies have shown the importance of PPARs in the pathogenesis of many dermatological disorders. Clinical trials have suggested possible role of PPAR agonists in the management of various dermatoses ranging from acne vulgaris, psoriasis, hirsutism, and lipodystrophy to cutaneous malignancies including melanoma. This article is intended to be a primer for dermatologists in their understanding of clinical relevance of PPARs and PPAR agonists in dermatology therapeutics.

The role of nuclear receptors in regulation of Th17/Treg biology and its implications for diseases

Nuclear receptors in the cell play essential roles in environmental sensing, differentiation, development, homeostasis,and metabolism and are thus highly conserved across multiple species. The anti-inflammatory role of nuclear receptors in immune cells has recently gained recognition. Nuclear receptors play critical roles in both myeloid and lymphoid cells, particularly in helper CD41 T-cell type 17 (Th17) and regulatory T cells (Treg). Th17 and Treg are closely related cell fates that are determined by orchestrated cytokine signaling. Recent studies have emphasized the interactions between nuclear receptors and the known cytokine signals and how such interaction affects Th17/Treg development and function.This review will focus on the most recent discoveries concerning the roles of nuclear receptors in the context of therapeutic applications in autoimmune diseases.

RNA-seq identifies a role for the PPARβ/δ inverse agonist GSK0660 in the regulation of TNFα-induced cytokine signaling in retinal endothelial cells

PURPOSE

The peroxisome proliferator-activated receptor beta/delta (PPARβ/δ) is a transcription factor with roles in metabolism, angiogenesis, and inflammation. It has yet undefined roles in retinal inflammation and diabetic retinopathy (DR). We used RNA-seq to better understand the role of the antagonist and inverse agonist of PPARβ/δ, GSK0660, in TNFα-induced inflammation. Understanding the underlying mechanisms of vascular inflammation could lead to new treatments for DR.

METHODS

RNA was isolated from human retinal microvascular endothelial cells treated with a vehicle, TNFα, or TNFα plus GSK0660. RNA-seq was performed with a 50 bp single read protocol. The differential expression was determined using edgeR and gene ontology, and a pathway analysis was performed using DAVID. RNA-seq validation was performed using qRT-PCR using the primers for ANGPTL4, CCL8, NOV, CXCL10, and PDPK1.

RESULTS

TNFα differentially regulated 1,830 transcripts, many of which are involved in the cytokine-cytokine receptor interaction, chemokine signaling, and inflammatory response. Additionally, TNFα highly upregulated genes involved in leukocyte recruitment, including CCL5, CX3CL1, and CXCL10. GSK0660 differentially regulated 273 transcripts in TNFα-treated cells compared to TNFα alone. A pathway analysis revealed the enrichment of cytokine-cytokine receptor signaling. In particular, GSK0660 blocks the TNFα-induced upregulation of CCL8, a chemokine involved in leukocyte recruitment.

CONCLUSIONS

TNFα regulates several genes related to retinal leukostasis in retinal endothelial cells. GSK0660 blocks the effect of TNFα on the expressions of cytokines involved in leukocyte recruitment, including CCL8, CCL17, and CXCL10 and it may therefore block TNFα-induced retinal leukostasis.

The transcriptional PPARβ/δ network in human macrophages defines a unique agonist-induced activation state

Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is a lipid ligand-inducible transcription factor with established metabolic functions, whereas its anti-inflammatory function is poorly understood. To address this issue, we determined the global PPARβ/δ-regulated signaling network in human monocyte-derived macrophages. Besides cell type-independent, canonical target genes with metabolic and immune regulatory functions we identified a large number of inflammation-associated NFκB and STAT1 target genes that are repressed by agonists. Accordingly, PPARβ/δ agonists inhibited the expression of multiple pro-inflammatory mediators and induced an anti-inflammatory, IL-4-like morphological phenotype. Surprisingly, bioinformatic analyses also identified immune stimulatory effects. Consistent with this prediction, PPARβ/δ agonists enhanced macrophage survival under hypoxic stress and stimulated CD8⁺ T cell activation, concomitantly with the repression of immune suppressive target genes and their encoded products CD274 (PD-1 ligand), CD32B (inhibitory Fcγ receptor IIB) and indoleamine 2,3-dioxygenase 1 (IDO-1), as well as a diminished release of the immune suppressive IDO-1 metabolite kynurenine. Comparison with published data revealed a significant overlap of the PPARβ/δ transcriptome with coexpression modules characteristic of both anti-inflammatory and pro-inflammatory cytokines. Our findings indicate that PPARβ/δ agonists induce a unique macrophage activation state with strong anti-inflammatory but also specific immune stimulatory components, pointing to a context-dependent function of PPARβ/δ in immune regulation.

Involvement of covalent interactions in the mode of action of PPARβ/δ antagonists

Investigations on the mode of action of several different chemical modulators of the peroxisome proliferator-activated receptor β/δ (PPARβ/δ) have been reported using MS and NMR experiments.

Fetal inhibition of inflammation improves disease phenotypes in harlequin ichthyosis

Harlequin ichthyosis (HI) is a severe skin disease which leads to neonatal death in ∼50% of cases. It is the result of mutations in ABCA12, a protein that transports lipids required to establish the protective skin barrier needed after birth. To better understand the life-threatening newborn HI phenotype, we analysed the developing epidermis for consequences of lipid dysregulation in mouse models. We observed a pro-inflammatory signature which was characterized by chemokine upregulation in embryonic skin which is distinct from that seen in other types of ichthyosis. Inflammation also persisted in grafted HI skin. To examine the contribution of inflammation to disease development, we overexpressed interleukin-37b to globally suppress fetal inflammation, observing considerable improvements in keratinocyte differentiation. These studies highlight inflammation as an unexpected contributor to HI disease development in utero, and suggest that inhibiting inflammation may reduce disease severity.

Physiological functions of peroxisome proliferator-activated receptor β

The peroxisome proliferator-activated receptors, PPARα, PPARβ, and PPARγ, are a family of transcription factors activated by a diversity of molecules including fatty acids and fatty acid metabolites. PPARs regulate the transcription of a large variety of genes implicated in metabolism, inflammation, proliferation, and differentiation in different cell types. These transcriptional regulations involve both direct transactivation and interaction with other transcriptional regulatory pathways. The functions of PPARα and PPARγ have been extensively documented mainly because these isoforms are activated by molecules clinically used as hypolipidemic and antidiabetic compounds. The physiological functions of PPARβ remained for a while less investigated, but the finding that specific synthetic agonists exert beneficial actions in obese subjects uplifted the studies aimed to elucidate the roles of this PPAR isoform. Intensive work based on pharmacological and genetic approaches and on the use of both in vitro and in vivo models has considerably improved our knowledge on the physiological roles of PPARβ in various cell types. This review will summarize the accumulated evidence for the implication of PPARβ in the regulation of development, metabolism, and inflammation in several tissues, including skeletal muscle, heart, skin, and intestine. Some of these findings indicate that pharmacological activation of PPARβ could be envisioned as a therapeutic option for the correction of metabolic disorders and a variety of inflammatory conditions. However, other experimental data suggesting that activation of PPARβ could result in serious adverse effects, such as carcinogenesis and psoriasis, raise concerns about the clinical use of potent PPARβ agonists.

Role of PPAR, LXR, and PXR in epidermal homeostasis and inflammation

Epidermal lipid synthesis and metabolism are regulated by nuclear hormone receptors (NHR) and in turn epidermal lipid metabolites can serve as ligands to NHR. NHR form a large superfamily of receptors modulating gene transcription through DNA binding. A subgroup of these receptors is ligand-activated and heterodimerizes with the retinoid X receptor including peroxisome proliferator-activated receptor (PPAR), liver X receptor (LXR) and pregnane X receptor (PXR). Several isotypes of these receptors exist, all of which are expressed in skin. In keratinocytes, ligand activation of PPARs and LXRs stimulates differentiation, induces lipid accumulation, and accelerates epidermal barrier regeneration. In the cutaneous immune system, ligand activation of all three receptors, PPAR, LXR, and PXR, has inhibitory properties, partially mediated by downregulation of the NF-kappaB pathway. PXR also has antifibrotic effects in the skin correlating with TGF-beta inhibition. In summary, ligands of PPAR, LXR and PXR exert beneficial therapeutic effects in skin disease and represent promising targets for future therapeutic approaches in dermatology. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Peroxisome proliferator-activated receptor delta and cardiovascular disease

Recent reports have shown that peroxisome proliferator-activated receptor delta (PPARD) plays an important role in different vascular processes suggesting that PPARD is a significant modulator of cardiovascular disease. This review will focus on PPARD in relation to cardiovascular risk factors based on cell, animal and human data. Mouse studies suggest that Ppard is an important metabolic modulator that may have implications for cardiovascular disease (CVD). Specific human PPARD gene variants show no clear association with CVD but interactions between variants and lifestyle factors might influence disease risk. During recent years, development of specific and potent PPARD agonists has also made it possible to study the effects of PPARD activation in humans. PPARD agonists seem to exert beneficial effects on dyslipidemia and insulin-resistant syndromes but safety issues have been raised due to the role that PPARD plays in cell proliferation. Thus, large long term outcome as well as detailed safety and tolerability studies are needed to evaluate whether PPARD agonists could be used to treat CVD in humans.

Nuclear hormone receptor functions in keratinocyte and melanocyte homeostasis, epidermal carcinogenesis and melanomagenesis

Skin homeostasis is maintained, in part, through regulation of gene expression orchestrated by type II nuclear hormone receptors in a cell and context specific manner. This group of transcriptional regulators is implicated in various cellular processes including epidermal proliferation, differentiation, permeability barrier formation, follicular cycling and inflammatory responses. Endogenous ligands for the receptors regulate actions during skin development and maintenance of tissue homeostasis. Type II nuclear receptor signaling is also important for cellular crosstalk between multiple cell types in the skin. Overall, these nuclear receptors are critical players in keratinocyte and melanocyte biology and present targets for cutaneous disease management.