Critical roles of PPAR beta/delta in keratinocyte response to inflammation

Non-classical Transcriptional Activity of Retinoic Acid

It has long been established that the transcriptional activity of retinoic acid (RA) is mediated by members of the nuclear receptor family of ligand-activated transcription factors termed RA receptors (RARs). More recent observations have established that RA also activates an additional nuclear receptor, PPARβ/δ. Partitioning RA between RARs and PPARβ/δ is governed by different intracellular lipid-binding proteins: cellular RA binding protein 2 (CRABP2) delivers RA to nuclear RARs and a fatty acid binding protein (FABP5) delivers the hormone from the cytosol to nuclear PPARβ/δ. Consequently, RA signals through RARs in CRABP2-expressing cells, but activates PPARβ/δ in cells that express a high level of FABP5. RA elicits different and sometimes opposing responses in cells that express different FABP5/CRABP2 ratios because PPARβ/δ and RARs regulate the expression of distinct sets of genes. An overview of the observations that led to the discovery of this non-classical activity of RA are presented here, along with a discussion of evidence demonstrating the involvement of the dual transcriptional activities of RA in regulating energy homeostasis, insulin responses, and adipocyte and neuron differentiation.

Zebrafish as a Model to Study the Role of Peroxisome Proliferating-Activated Receptors in Adipogenesis and Obesity

The Peroxisome Proliferator-Activated Receptors (PPARs) PPARA and PPARD are regulators of lipid metabolism with important roles in energy release through lipid breakdown, while PPARG plays a key role in lipid storage and adipogenesis. The aim of this review is to describe the role of PPARs in lipid metabolism, adipogenesis, and obesity and evaluate the zebrafish as an emerging vertebrate model to study the function of PPARs. Zebrafish are an appropriate model to study human diseases, including obesity and related metabolic diseases, as pathways important for adipogenesis and lipid metabolism which are conserved between mammals and fish. This review synthesizes knowledge on the role of PPARs in zebrafish and focuses on the putative function of PPARs in zebrafish adipogenesis. Using in silico analysis, we confirm the presence of five PPARs (pparaa, pparab, pparda, ppardb, and pparg) in the zebrafish genome with 67–74% identity to human and mouse PPARs. During development, pparda/b paralogs and pparg show mRNA expression around the swim bladder and pancreas, the region where adipocytes first develop, whereas pparg is detectable in adipocytes at 15 days post fertilization (dpf). This review indicates that the zebrafish is a promising model to investigate the specific functions of PPARs in adipogenesis and obesity.

Kruppel-like factor 2 suppresses mammary carcinoma growth by regulating retinoic acid signaling

The transcription factor Kruppel-like factor 2 (KLF2) displays anticarcinogenic activities but the mechanism that underlies this activity is unknown. We show here that KLF2 is markedly downregulated in human breast cancers and that its expression positively correlates with breast cancer patient survival. We show further that KLF2 suppresses tumor development by controlling the transcriptional activity of the vitamin A metabolite retinoic acid (RA). RA regulates gene transcription by activating two types of nuclear receptors: RA receptors (RARs), which inhibit tumor development, and peroxisome proliferator-activated receptor β/δ (PPARβ/δ), which promotes tumorigenesis. The partitioning of RA between these receptors is regulated by two carrier proteins: cellular retinoic acid-binding protein 2 (CRABP2), which delivers RA to RARs, and fatty acid-binding protein 5 (FABP5), which shuttles ligands to PPARβ/δ. We show that KLF2 induces the expression of CRABP2 and RARγ and inhibits the expression FABP5 and PPARβ/δ thereby shifting RA signaling from the pro-carcinogenic FABP5/PPARβ/δ to the growth-suppressing CRABP2/RAR path. The data thus reveal that KLF2 suppresses tumor growth by controlling the transcriptional activities of RA.

Expression of Prostacyclin-Synthase in Human Breast Cancer: Negative Prognostic Factor and Protection against Cell Death In Vitro

Endogenously formed prostacyclin (PGI2) and synthetic PGI2 analogues have recently been shown to regulate cell survival in various cell lines. To elucidate the significance of PGI2 in human breast cancer, we performed immunohistochemistry to analyze expression of prostacyclin-synthase (PGIS) in 248 human breast cancer specimens obtained from surgical pathology files. We examined patients' 10-year survival retrospectively by sending a questionnaire to their general practitioners and performed univariate analysis to determine whether PGIS expression correlated with patient survival. Lastly, the effects of PGI2 and its analogues on cell death were examined in a human breast cancer cell line (MCF-7) and a human T-cell leukemia cell line (CCRF-CEM). PGIS expression was observed in tumor cells in 48.7% of samples and was associated with a statistically significant reduction in 10-year survival (P = 0.038; n = 193). Transient transfection of PGIS into MCF-7 cells exposed to sulindac increased cell viability by 50% and exposure to carbaprostacyclin protected against sulindac sulfone induced apoptosis in CCRF-CEM cells. Expression of PGIS is correlated with a reduced patient survival and protects against cell death in vitro, suggesting that PGIS is a potential therapeutic target in breast cancer.

Cross Talk between Proliferative, Angiogenic, and Cellular Mechanisms Orchestred by HIF-1α in Psoriasis

Psoriasis is a chronic inflammatory skin disease where the altered regulation in angiogenesis, inflammation, and proliferation of keratinocytes are the possible causes of the disease, and the transcription factor “hypoxia-inducible factor 1-alpha” (HIF-1α) is involved in the homeostasis of these three biological phenomena. In this review, the role of HIF-1α in the cross talk between the cytokines and cells of the immunological system involved in the pathogenesis of psoriasis is discussed.

Integrative and systemic approaches for evaluating PPARβ/δ (PPARD) function

The peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptors that function as transcription factors regulating the expression of genes involved in cellular differentiation, development, metabolism and also tumorigenesis. Three PPAR isotypes (α, β/δ and γ) have been identified, among which PPARβ/δ is the most difficult to functionally examine due to its tissue-specific diversity in cell fate determination, energy metabolism and housekeeping activities. PPARβ/δ acts both in a ligand-dependent and -independent manner. The specific type of regulation, activation or repression, is determined by many factors, among which the type of ligand, the presence/absence of PPARβ/δ-interacting corepressor or coactivator complexes and PPARβ/δ protein post-translational modifications play major roles. Recently, new global approaches to the study of nuclear receptors have made it possible to evaluate their molecular activity in a more systemic fashion, rather than deeply digging into a single pathway/function. This systemic approach is ideally suited for studying PPARβ/δ, due to its ubiquitous expression in various organs and its overlapping and tissue-specific transcriptomic signatures. The aim of the present review is to present in detail the diversity of PPARβ/δ function, focusing on the different information gained at the systemic level, and describing the global and unbiased approaches that combine a systems view with molecular understanding.

Targeting Peroxisome Proliferator-Activated Receptor-β/δ (PPARβ/δ) for Cancer Chemoprevention

The role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in cancer remains contentious due in large part to divergent publications indicating opposing effects in different rodent and human cell culture models. During the past 10 years, some facts regarding PPARβ/δ in cancer have become clearer, while others remain uncertain. For example, it is now well accepted that (1) expression of PPARβ/δ is relatively lower in most human tumors as compared to the corresponding non-transformed tissue, (2) PPARβ/δ promotes terminal differentiation, and (3) PPARβ/δ inhibits pro-inflammatory signaling in multiple in vivo models. However, whether PPARβ/δ is suitable to target with natural and/or synthetic agonists or antagonists for cancer chemoprevention is hindered because of the uncertainty in the mechanism of action and role in carcinogenesis. Recent findings that shed new insight into the possibility of targeting this nuclear receptor to improve human health will be discussed.

MiR-138/peroxisome proliferator-activated receptor β signaling regulates human hypertrophic scar fibroblast proliferation and movement in vitro

Excessive scars affect a patient's quality of life, both physically and psychologically, by causing pruritus, pain and contractures. Because there is a poor understanding of the complex mechanisms underlying the processes of hypertrophic scar formation, most therapeutic approaches remain clinically unsatisfactory. In this study, we found that miR-138 was downregulated and peroxisome proliferator-activated receptor (PPARβ) was inversely upregulated in hypertrophic scar tissues compared to in paired normal skin tissues. Using a dual-luciferase assay, we validated that miR138 directly targets PPARβ and regulates its expression at the transcriptional and translational levels. In gain-and-loss experiments, we found that miR-138/PPARβ signaling regulated human hypertrophic scar fibroblast proliferation and movement, and affected scarring-related protein expression, which suggests that miR-138/PPARβ signaling is important for hypertrophic scarring. Thus, our study provides evidence to help determine whether miR-138/PPARβ signaling may be a potential target for hypertrophic scarring management.

Hyaluronidase modulates inflammatory response and accelerates the cutaneous wound healing

Hyaluronidases are enzymes that degrade hyaluronan an important constituent of the extracellular matrix. They have been used as a spreading agent, improving the absorption of drugs and facilitating the subcutaneous infusion of fluids. Here, we investigated the influence of bovine testes hyaluronidase (HYAL) during cutaneous wound healing in in vitro and in vivo assays. We demonstrated in the wound scratch assay that HYAL increased the migration and proliferation of fibroblasts in vitro at low concentration, e.g. 0.1 U HYAL enhanced the cell number by 20%. HYAL presented faster and higher reepithelialization in in vivo full-thickness excisional wounds generated on adult Wistar rats back skin already in the early phase at 2^nd day post operatory compared to vehicle-control group. Wound closured area observed in the 16 U and 32 U HYAL treated rats reached 38% and 46% compared to 19% in the controls, respectively. Histological and biochemical analyses supported the clinical observations and showed that HYAL treated wounds exhibited increased granulation tissue, diminished edema formation and regulated the inflammatory response by modulating the release of pro and anti-inflammatory cytokines, growth factor and eicosanoids mediators. Moreover, HYAL increased gene expression of peroxisome proliferator-activated receptors (PPAR) γ and PPAR β/δ, the collagen content in the early stages of healing processes as well as angiogenesis. Altogether these data revealed that HYAL accelerates wound healing processes and might be beneficial for treating wound disorders.

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.

Peroxisome proliferator-activated receptor δ modulates MMP-2 secretion and elastin expression in human dermal fibroblasts exposed to ultraviolet B radiation

BACKGROUND

Changes in skin connective tissues mediated by ultraviolet (UV) radiation have been suggested to cause the skin wrinkling normally associated with premature aging of the skin. Recent investigations have shown that peroxisome proliferator-activated receptor (PPAR) δ plays multiple biological roles in skin homeostasis.

OBJECTIVE

We attempted to investigate whether PPARδ modulates elastin protein levels and secretion of matrix metalloproteinase (MMP)-2 in UVB-irradiated human dermal fibroblasts (HDFs) and mouse skin.

METHODS

These studies were undertaken in primary HDFs or HR-1 hairless mice using Western blot analyses, small interfering (si)RNA-mediated gene silencing, and Fluorescence microscopy.

RESULTS

In HDFs, UVB irradiation induced increased secretion of MMP-2 and reduced levels of elastin. Activation of PPARδ by GW501516, a ligand specific for PPARδ, markedly attenuated UVB-induced MMP-2 secretion with a concomitant increase in the level of elastin. These effects were reduced by the presence of siRNAs against PPARδ or treatment with GSK0660, a specific inhibitor of PPARδ. Furthermore, GW501516 elicited a dose- and time-dependent increase in the expression of elastin. Modulation of MMP-2 secretion and elastin levels by GW501516 was associated with a reduction in reactive oxygen species (ROS) production in HDFs exposed to UVB. Finally, in HR-1 hairless mice, administration of GW501516 significantly reduced UVB-induced MMP-2 expression with a concomitant increase in elastin levels, and these effects were significantly reduced by the presence of GSK0660.

CONCLUSION

Our results suggest that PPARδ-mediated modulation of MMP-2 secretion and elastin expression may contribute to the maintenance of skin integrity by inhibiting ROS generation.

Src is activated by the nuclear receptor peroxisome proliferator-activated receptor β/δ in ultraviolet radiation-induced skin cancer

Although non-melanoma skin cancer (NMSC) is the most common human cancer and its incidence continues to rise worldwide, the mechanisms underlying its development remain incompletely understood. Here, we unveil a cascade of events involving peroxisome proliferator-activated receptor (PPAR) β/δ and the oncogene Src, which promotes the development of ultraviolet (UV)-induced skin cancer in mice. UV-induced PPARβ/δ activity, which directly stimulated Src expression, increased Src kinase activity and enhanced the EGFR/Erk1/2 signalling pathway, resulting in increased epithelial-to-mesenchymal transition (EMT) marker expression. Consistent with these observations, PPARβ/δ-null mice developed fewer and smaller skin tumours, and a PPARβ/δ antagonist prevented UV-dependent Src stimulation. Furthermore, the expression of PPARβ/δ positively correlated with the expression of SRC and EMT markers in human skin squamous cell carcinoma (SCC), and critically, linear models applied to several human epithelial cancers revealed an interaction between PPARβ/δ and SRC and TGFβ1 transcriptional levels. Taken together, these observations motivate the future evaluation of PPARβ/δ modulators to attenuate the development of several epithelial cancers.

Role of Peroxisome Proliferator-Activated Receptor (PPAR)δ in Embryonic Stem Cell Proliferation

The peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that belong to the nuclear receptor family. It is well known that PPARs function as regulators of lipid and lipoprotein metabolism and glucose homeostasis, as well as influence cellular proliferation, differentiation and apoptosis. However, the role of the PPARs with regard to embryonic stem (ES) cells remains unknown. We will review the function of the PPARδ, one of the three PPAR isoforms, α, δ (also called β/δ), and γ, in ES cells and its role in embryo development. In addition, pluripotent mouse ES cells can be expanded in large numbers in vitro due to the process of symmetrical self-renewal. Here we describe how PPARδ sustains ES cell proliferation.

Structural basis for ligand regulation of the fatty acid-binding protein 5, peroxisome proliferator-activated receptor β/δ (FABP5-PPARβ/δ) signaling pathway

Fatty acid-binding proteins (FABPs) are a widely expressed group of calycins that play a well established role in solubilizing cellular fatty acids. Recent studies, however, have recast FABPs as active participants in vital lipid-signaling pathways. FABP5, like its family members, displays a promiscuous ligand binding profile, capable of interacting with numerous long chain fatty acids of varying degrees of saturation. Certain "activating" fatty acids induce the protein's cytoplasmic to nuclear translocation, stimulating PPARβ/δ transactivation; however, the rules that govern this process remain unknown. Using a range of structural and biochemical techniques, we show that both linoleic and arachidonic acid elicit FABP5's translocation by permitting allosteric communication between the ligand-sensing β2 loop and a tertiary nuclear localization signal within the α-helical cap of the protein. Furthermore, we show that more saturated, nonactivating fatty acids inhibit nuclear localization signal formation by destabilizing this activation loop, thus implicating FABP5 specifically in cis-bonded, polyunsaturated fatty acid signaling.

Acanthosis Nigricans and insulin levels in a south Indian population-(ADEPS paper 2)

SUMMARY

OBJECTIVE

Aim of the study was to determine the prevalence of Acanthosis Nigricans (AN) in a central Kerala south Indian population and to evaluate its correlations with diabetes, obesity, insulin levels and other factors.

METHODS

A cross-sectional community survey including physical examination and biochemical evaluation was conducted among adults above the age of 18 years in central Kerala In the first phase of the study 3069 participants were surveyed using questionnaires regarding socioeconomic status and medical details. Among them 986 subjects were evaluated in the second phase of the study which included anthropometric measurements, examination for blood pressure and Acanthosis Nigricans, blood tests such as blood glucose, fasting lipids. Fasting serum insulin level was measured for all non-diabetic subjects. Statistical analysis was done using SPSS 11.0 version software.

RESULTS

RESULTS of the study showed that 16.1% of population had AN and it was significantly higher among females (19.6%) than males (11.4%). Prevalence of AN was highest in 30-40 year age group and it decreased with the age. Prevalence of AN correlated positively with female gender, obesity, high triglyceride levels and presence of diabetes. Presence of AN was significantly associated with higher fasting insulin levels. Males with AN had significantly higher insulin values than females with AN.

CONCLUSION

AN has stronger clinical relevance among males than females and it can be used as a marker of insulin resistance in south Indian population especially if obesity and family history of diabetes are also present.

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.

Molecular mechanisms of mouse skin tumor promotion

Multiple molecular mechanisms are involved in the promotion of skin carcinogenesis. Induction of sustained proliferation and epidermal hyperplasia by direct activation of mitotic signaling pathways or indirectly in response to chronic wounding and/or inflammation, or due to a block in terminal differentiation or resistance to apoptosis is necessary to allow clonal expansion of initiated cells with DNA mutations to form skin tumors. The mitotic pathways include activation of epidermal growth factor receptor and Ras/Raf/mitogen-activated protein kinase signaling. Chronic inflammation results in inflammatory cell secretion of growth factors and cytokines such as tumor necrosis factor-α and interleukins, as well as production of reactive oxygen species, all of which can stimulate proliferation. Persistent activation of these pathways leads to tumor promotion.

Effects of 4-nonylphenol on hepatic gene expression of peroxisome proliferator-activated receptors and cytochrome P450 isoforms (CYP1A1 and CYP3A4) in juvenile sole (Solea solea)

The objective of the present study was to investigate the modulatory effects of the xenoestrogen 4-nonylphenol (4-NP) on hepatic peroxisome proliferator-activated receptor (PPAR) α and β gene expression patterns in relation to the detoxification pathways mediated by cytochrome P450 isoforms (CYP1A1 and CYP3A4). Waterborne 4-NP-induced effects were compared with those of 10(-8)M 17β-estradiol (E2) by using in vivo dose-response experiments carried out with juvenile sole (Solea solea). Compared to the controls, significantly higher levels of PPARα mRNAs were found in fish treated with E2 or 4-NP (10(-6)M) 3 d after exposure; the highest dose of 4-NP also caused up-regulation of retinoid X receptor α (RXRα) transcript levels. On the contrary, PPARβ gene expression was not modulated by E2 or 4-NP. Our data show that 4-NP-induced PPARα mRNA levels coincide with suppression of CYP1A1 and CYP3A4 expression similarly to E2. The results from these in vivo studies suggest the presence of cross-talk between nuclear receptor-mediated signaling pathways and PPARα that may result in modulation of CYP450 isoforms expression following 4-NP treatment in sole liver.

Ceramides stimulate caspase-14 expression in human keratinocytes

Caspase-14 is an enzyme that is expressed predominantly in cornifying epithelia and catalyses the degradation of profilaggrin. Additionally, caspase-14 plays an important role in the terminal differentiation of keratinocytes. However, how caspase-14 expression is regulated remains largely unknown. Here we demonstrate that ceramides (C(2) -Cer and C(6) -Cer), but not other sphingolipids (C(8) -glucosylceramides, sphinganine, sphingosine-1-phosphate or ceramide-1-phosphate), increase caspase-14 expression (mRNA and protein) in cultured human keratinocytes in a dose- and time-dependent manner. Inhibitors of glucosylceramide synthase and ceramidase increase endogenous ceramide levels and also increase caspase-14 expression, indicating an important regulatory role for ceramides and suggesting that the conversion of ceramides to other metabolites is not required. The increase in caspase-14 expression induced by ceramides is first seen at 16 h and requires new protein synthesis, suggesting that the ceramide-induced increase is likely an indirect effect. Furthermore, ceramides increase caspase-14 gene expression primarily by increasing transcription. Blocking de novo synthesis of ceramides does not affect caspase-14 expression, suggesting that basal expression is not dependent on ceramide levels. These studies show that ceramides, an important structural lipid, stimulate caspase-14 expression providing a mechanism for coordinately regulating the formation of lipid lamellar membranes with the formation of corneocytes.

PPARδ induces estrogen receptor-positive mammary neoplasia through an inflammatory and metabolic phenotype linked to mTOR activation

The peroxisome proliferator-activated receptor-δ (PPARδ) regulates a multitude of physiological processes associated with glucose and lipid metabolism, inflammation, and proliferation. One or more of these processes are potential risk factors for the ability of PPARδ agonists to promote tumorigenesis in the mammary gland. In this study, we describe a new transgenic mouse model in which activation of PPARδ in the mammary epithelium by endogenous or synthetic ligands resulted in progressive histopathologic changes that culminated in the appearance of estrogen receptor- and progesterone receptor-positive and ErbB2-negative infiltrating ductal carcinomas. Multiparous mice presented with mammary carcinomas after a latency of 12 months, and administration of the PPARδ ligand GW501516 reduced tumor latency to 5 months. Histopathologic changes occurred concurrently with an increase in an inflammatory, invasive, metabolic, and proliferative gene signature, including expression of the trophoblast gene, Plac1, beginning 1 week after GW501516 treatment, and remained elevated throughout tumorigenesis. The appearance of malignant changes correlated with a pronounced increase in phosphatidylcholine and lysophosphatidic acid metabolites, which coincided with activation of Akt and mTOR signaling that were attenuated by treatment with the mTOR inhibitor everolimus. Our findings are the first to show a direct role of PPARδ in the pathogenesis of mammary tumorigenesis, and suggest a rationale for therapeutic approaches to prevent and treat this disease.

Peroxisome proliferator-activated receptor-β/δ regulates angiogenic cell behaviors and oxygen-induced retinopathy

PURPOSE

To develop new therapies against ocular neovascularization (NV), we tested the effect of peroxisome proliferator-activated receptor-β/δ (PPAR-β/δ) agonism and antagonism on angiogenic behaviors and in human retinal microvascular endothelial cells (HRMEC) and on preretinal NV in rat oxygen-induced retinopathy (OIR).

METHODS

HRMECs were treated with the PPAR-β/δ agonist GW0742 and the antagonist GSK0660. Messenger RNA levels of a PPAR-β/δ target gene, angiopoietin-like-4 (angptl4) were assayed by qRT-PCR. HRMEC proliferation and tube formation were assayed according to standard protocols. OIR was induced in newborn rats by exposing them to alternating 24-hour episodes of 50% and 10% oxygen for 14 days. OIR rats were treated with GW0742 or GSK0660. Angptl4 protein levels were assessed by ELISA and preretinal NV was quantified by adenosine diphosphatase staining.

RESULTS

GW0742 significantly increased angptl4 mRNA, and GSK0660 significantly decreased angptl4 mRNA. GW0742 had no effect on HRMEC proliferation, but caused a significant and dose-responsive increase in tube formation. GSK0660 significantly reduced serum-induced HRMEC proliferation and tube formation in a dose-dependent manner. Intravitreal injection of GW0742 significantly increased total retinal Angptl4 protein, but intravitreal injection of GSK0660 had no effect. Intravitreal injection of GW0742 significantly increased retinal NV, as did GW0742 administered by oral gavage. Conversely, both intravitreal injection and intraperitoneal injection of GSK0660 significantly reduced retinal NV.

CONCLUSIONS

PPAR-β/δ activation exacerbates, and its inhibition reduces, preretinal NV. PPAR-β/δ may regulate preretinal NV through a prodifferentiation/maturation mechanism that depends on Angptl4. Pharmacologic inhibition of PPAR-β/δ may provide a rational basis for therapeutic targeting of ocular NV.

Effects of peroxisome proliferator-activated receptor-β activation in endothelin-dependent hypertension

AIMS

We analysed the chronic effects of the peroxisome proliferator-activated receptor β/δ (PPAR-β) agonist GW0742 on the renin-independent hypertension induced by deoxycorticosterone acetate (DOCA)-salt.

METHODS AND RESULTS

Rats were treated for 5 weeks with: control-vehicle, control-GW0742 (5 or 20 mg kg(-1) day(-1)), DOCA-vehicle, DOCA-GW0742 (5 or 20 mg kg(-1) day(-1)), DOCA-GSK0660 (1 mg kg(-1) day(-1)), and DOCA-GSK0660-GW0742. Rats receiving DOCA-vehicle showed increased systolic blood pressure, left ventricular and kidney weight indices, endothelin-1 (ET-1), and malondialdehyde plasma levels, urinary iso-PGF2α excretion, impaired endothelium-dependent relaxation to acetylcholine, and contraction to ET-1 when compared with controls. Aortic reactive oxygen species content, NADPH oxidase activity, and p47(phox), p22(phox), NOX-4, glutathione peroxidase 1, hemeoxygenase-1, and preproET-1 expression were increased, whereas catalase and regulators of G protein-coupled signalling proteins (RGS)5 expression were decreased in the DOCA-vehicle group. GW0742 prevented the development of hypertension in a dose-dependent manner but the reduction of renal and cardiac hypertrophy, systemic and vascular oxidative stress markers, and improvement of endothelial dysfunction were only observed after the higher dose. GW0742, at 20 mg kg(-1) day(-1), attenuated ET-1 contraction by increasing RGS5 expression and restored the intracellular redox balance by reducing NADPH-oxidase activity, and by increasing the antioxidant genes expression. The PPAR-β antagonist GSK0660 prevented all vascular changes induced by GW0742 but not its antihypertensive effects.

CONCLUSION

Vascular protective effects of GW0742 operate via PPAR-β by interference with the ET-1 signalling as a result of increased expression of RGS5 and up-regulation of antioxidant genes and via PPAR-β-independent mechanisms to decrease blood pressure.

Genetic ablation of the fatty acid-binding protein FABP5 suppresses HER2-induced mammary tumorigenesis

The fatty acid-binding protein FABP5 shuttles ligands from the cytosol to the nuclear receptor PPARβ/δ (encoded for by Pparδ), thereby enhancing the transcriptional activity of the receptor. This FABP5/PPARδ pathway is critical for induction of proliferation of breast carcinoma cells by activated epidermal growth factor receptor (EGFR). In this study, we show that FABP5 is highly upregulated in human breast cancers and we provide genetic evidence of the pathophysiologic significance of FABP5 in mammary tumorigenesis. Ectopic expression of FABP5 was found to be oncogenic in 3T3 fibroblasts where it augmented the ability of PPARδ to enhance cell proliferation, migration, and invasion. To determine whether FABP5 is essential for EGFR-induced mammary tumor growth, we interbred FABP5-null mice with MMTV-ErbB2/HER2 oncomice, which spontaneously develop mammary tumors. FABP5 ablation relieved activation of EGFR downstream effector signals, decreased expression of PPARδ target genes that drive cell proliferation, and suppressed mammary tumor development. Our findings establish that FABP5 is critical for mammary tumor development, rationalizing the development of FABP5 inhibitors as novel anticarcinogenic drugs.

PPARδ inhibits UVB-induced secretion of MMP-1 through MKP-7-mediated suppression of JNK signaling

In the present study, we investigated the role of peroxisome proliferator-activated receptor (PPAR) δ in modulating matrix-degrading metalloproteinases and other mechanisms underlying photoaging processes in the skin. In human dermal fibroblasts (HDFs), activation of PPARδ by its specific ligand GW501516 markedly attenuated UVB-induced secretion of matrix metalloproteinase (MMP)-1, concomitant with decreased generation of reactive oxygen species. These effects were significantly reduced in the presence of PPARδ small interfering RNA and GSK0660. Furthermore, c-Jun N-terminal kinase (JNK), but not p38 or extracellular signal-regulated kinase, mediated PPARδ-dependent inhibition of MMP-1 secretion in HDFs exposed to UVB. PPARδ-mediated messenger RNA stabilization of mitogen-activated protein kinase phosphatase (MKP)-7 was responsible for the GW501516-mediated inhibition of JNK signaling. Inhibition of UVB-induced secretion of MMP-1 by PPARδ was associated with the restoration of types I and III collagen to levels approaching those in cells not exposed to UVB. Finally, in HR-1 hairless mice exposed to UVB, administration of GW501516 significantly reduced wrinkle formation and skin thickness, downregulated MMP-1 and JNK phosphorylation, and restored the levels of MKP-7, types I and III collagen. These results suggest that PPARδ-mediated inhibition of MMP-1 secretion prevents some effects of photoaging and maintains the integrity of skin by inhibiting the degradation of the collagenous extracellular matrix.

Regulation of retinoid-mediated signaling involved in skin homeostasis by RAR and RXR agonists/antagonists in mouse skin

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

Effect of genetic polymorphism +294T/C in peroxisome proliferator-activated receptor delta on the risk of ischemic stroke in a Tunisian population

PPARδ +294T/C polymorphism was investigated in diabetics, in normolipidemic healthy controls, in dyslipidemic and nondyslipidemic coronary artery disease patients but never in ischemic stroke patients. The aim of this study was to explore, for the first time, the relationship between the genetic polymorphism of PPARδ and the risk of ischemic stroke among patients with diabetes. The study group consisted of 196 patients with ischemic stroke and 192 controls. Plasma concentrations of total cholesterol, triglycerides, low-, and high-density lipoprotein did not differ significantly between subjects carrying the TT genotype and those carrying the CC/TC genotype in both ischemic stroke patients (with or without diabetes) and control groups. The +294C allele (CC + CT genotypes) as compared with TT genotypes was found to be higher in total ischemic stroke patients than in controls. On the other hand, no interaction between diabetes and PPAR +294T/C polymorphism on the risk of ischemic stroke was found (p = 0.089). The PPARδ +294T/C polymorphism was associated with the risk of ischemic stroke in Tunisian subjects. This polymorphism has no influence on plasma lipoprotein concentrations and body mass index either in healthy subjects or in ischemic stroke patients with or without diabetes both in males and females.

MicroRNA-9 regulates the expression of peroxisome proliferator-activated receptor δ in human monocytes during the inflammatory response

PPARδ is involved in the inflammatory response and its expression is induced by cytokines, however, limited knowledge has been produced regarding its regulation. Since recent findings have shown that microRNAs, which are small non-coding RNAs that regulate gene expression, are involved in the immune response, we set out to investigate whether PPARδ can be regulated by microRNAs expressed in monocytes. Bioinformatic analysis identified a putative miR-9 target site within the 3′-UTR of PPARδ that was subsequently verified to be functional using reporter constructs. Primary human monocytes stimulated with LPS showed a downregulation of PPARδ and its target genes after 4 h while the expression of miR-9 was induced. Analysis of pro-inflammatory (M1) and anti-inflammatory (M2) macrophages showed that human PPARδ mRNA as well as miR-9 expression was higher in M1 compared to M2 macrophages. Furthermore, treatment with the PPARδ agonist, GW501516, induced the expression of PPARδ target genes in the pro-inflammatory M1 macrophages while no change was observed in the anti-inflammatory M2 macrophages. Taken together, these data suggest that PPARδ is regulated by miR-9 in monocytes and that activation of PPARδ may be of importance in M1 pro-inflammatory but not in M2 anti-inflammatory macrophages 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.

Nuclear control of the inflammatory response in mammals by peroxisome proliferator-activated receptors

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that play pivotal roles in the regulation of a very large number of biological processes including inflammation. Using specific examples, this paper focuses on the interplay between PPARs and innate immunity/inflammation and, when possible, compares it among species. We focus on recent discoveries establishing how inflammation and PPARs interact in the context of obesity-induced inflammation and type 2 diabetes, mostly in mouse and humans. We illustrate that PPAR γ ability to alleviate obesity-associated inflammation raises an interesting pharmacologic potential. In the light of recent findings, the protective role of PPAR α and PPAR β / δ against the hepatic inflammatory response is also addressed. While PPARs agonists are well-established agents that can treat numerous inflammatory issues in rodents and humans, surprisingly very little has been described in other species. We therefore also review the implication of PPARs in inflammatory bowel disease; acute-phase response; and central, cardiac, and endothelial inflammation and compare it along different species (mainly mouse, rat, human, and pig). In the light of the data available in the literature, there is no doubt that more studies concerning the impact of PPAR ligands in livestock should be undertaken because it may finally raise unconsidered health and sanitary benefits.

Liver X receptor and peroxisome proliferator-activated receptor as integrators of lipid homeostasis and immunity

Lipid metabolism has emerged as an important modulator of innate and adaptive immune cell fate and function. The lipid-activated transcription factors peroxisome proliferator-activated receptor (PPAR) α, β/δ, γ and liver X receptor (LXR) are members of the nuclear receptor superfamily that have a well-defined role in regulating lipid homeostasis and metabolic diseases. Accumulated evidence over the last decade indicates that PPAR and LXR signaling also influence multiple facets of inflammation and immunity, thereby providing important crosstalk between metabolism and immune system. Herein, we provide a brief introduction to LXR and PPAR biology and review recent discoveries highlighting the importance of PPAR and LXR signaling in the modulation of normal and pathologic states of immunity. We also examine advances in our mechanistic understanding of how nuclear receptors impact immune system function and homeostasis. Finally, we discuss whether LXRs and PPARs could be pharmacologically manipulated to provide novel therapeutic approaches for modulation of the immune system under pathologic inflammation or in the context of allergic and autoimmune disease.

Targeting peroxisome proliferator-activated receptor-β/δ in colon cancer: how to aim?

Peroxisome proliferator-activated receptor-β/δ (PPARδ) is a ubiquitously expressed, ligand-activated transcriptional factor that performs diverse critical functions in normal cells (e.g., fatty acid metabolism, obesity, apoptosis, and inflammation). Various studies in humans have found that PPARδ is upregulated in primary colorectal cancers; however, these findings have been challenged by those of other reports. Similarly, various in vitro and in vivo mechanistic pre-clinical models have yielded data demonstrating that PPARδ promotes colonic tumorigenesis, but other models have yielded data that contradicts this notion. Definitive studies are therefore needed to establish the exact role of PPARδ in human colorectal tumorigenesis and to provide a theoretical basis for PPARδ therapeutic targeting.

Upregulation of ski in fibroblast is implicated in the peroxisome proliferator--activated receptor δ-mediated wound healing

BACKGROUND/AIM

Both peroxisome proliferator-activated receptor (PPAR) δ and Ski are investigate the interaction of PPARδ and Ski and this interaction-associated effect in wound healing.

METHODS

Effect of PPARδ activation on Ski expression was detected in rat skin fibroblasts by real-time PCR and western blot. Luciferase assay, electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assay were performed to identify the binding site of PPARδ in the promoter region of rat Ski gene. And the functional activity of PPARδ regulation to Ski was detected in fibroblast proliferation and rat skin wound healing model.

RESULTS

PPARδ agonist GW501516 upregulated Ski expression in a dose-dependent manner. Direct repeat-1 (DR1) response element locating at -865∼-853 in Ski promoter region was identified to mediate PPARδ binding to Ski and associated induction of Ski. Furthermore, PPARδ upregulated Ski to promote fibroblasts proliferation and rat skin wound repair, which could be largely blocked by pre-treated with Ski RNA interference.

CONCLUSION

This study demonstrates that Ski is a novel target gene for PPARδ and upregulation of Ski to promote fibroblast proliferation is implicated in the PPARδ-mediated wound healing.

Ligand-activated PPARδ inhibits UVB-induced senescence of human keratinocytes via PTEN-mediated inhibition of superoxide production

UV radiation-mediated photodamage to the skin has been implicated in premature aging and photoaging-related skin cancer and melanoma. Little is known about the cellular events that underlie premature senescence, or how to impede these events. In the present study we demonstrate that PPARδ (peroxisome-proliferator-activated receptor δ) regulates UVB-induced premature senescence of normal keratinocytes. Activation of PPARδ by GW501516, a specific ligand of PPARδ, significantly attenuated UVB-mediated generation of ROS (reactive oxygen species) and suppressed senescence of human keratinocytes. Ligand-activated PPARδ up-regulated the expression of PTEN (phosphatase and tensin homologue deleted on chromosome 10) and suppressed the PI3K (phosphatidylinositol 3-kinase)/Akt pathway. Concomitantly, translocation of Rac1 to the plasma membrane, which leads to the activation of NADPH oxidases and generation of ROS, was significantly attenuated. siRNA (small interfering RNA)-mediated knockdown of PTEN abrogated the effects of PPARδ on cellular senescence, on PI3K/Akt/Rac1 signalling and on generation of ROS in keratinocytes exposed to UVB. Finally, when HR-1 hairless mice were treated with GW501516 before exposure to UVB, the number of senescent cells in the skin was significantly reduced. Thus ligand-activated PPARδ confers resistance to UVB-induced cellular senescence by up-regulating PTEN and thereby modulating PI3K/Akt/Rac1 signalling to reduce ROS generation in keratinocytes.

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

We have previously shown that peroxisome proliferator activating receptor ß/δ (PPAR β/δ is overexpressed in psoriasis. PPAR β/δ is not present in adult epidermis of mice. Targeted expression of PPAR β/δ and activation by a selective synthetic agonist is sufficient to induce an inflammatory skin disease resembling psoriasis. Several signalling pathways dysregulated in psoriasis are replicated in this model, suggesting that PPAR β/δ activation contributes to psoriasis pathogenesis. Thus, inhibition of PPAR β/δ might harbour therapeutical potential. Since PPAR β/δ has pleiotropic functions in metabolism, skin-targeted inhibition offer the potential of reducing systemic adverse effects. Here, we report that three selective PPAR β/δ antagonists, GSK0660, compound 3 h, and GSK3787 can be formulated for topical application to the skin and that their skin concentration can be accurately quantified using ultra-high performance liquid chromatography (UPLC)/mass spectrometry. These antagonists show efficacy in our transgenic mouse model in reducing psoriasis-like changes triggered by activation of PPAR β/δ. PPAR β/δ antagonists GSK0660 and compound 3 do not exhibit systemic drug accumulation after prolonged application to the skin, nor do they induce inflammatory or irritant changes. Significantly, the irreversible PPAR β/δ antagonist (GSK3787) retains efficacy when applied topically only three times per week which could be of practical clinical usefulness. Our data suggest that topical inhibition of PPAR β/δ to treat psoriasis may warrant further exploration.

Discovery, design and synthesis of Y-shaped peroxisome proliferator-activated receptor δ agonists as potent anti-obesity agents in vivo

We have discovered and demonstrated the in vitro and in vivo PPARδ-selective activity of novel Y-shaped agonists. These compounds activated hPPARδ with EC(50) values between 1 and 523 nM. Surprisingly, compounds 10a, 11d, 11e and 11f were the most potent and most selective hPPARδ agonists with 10(4)-fold selectivity over the other two subtypes, namely, hPPARα and hPPARγ. The PPARδ ligands 10a, 11e and 11f showed good bioavailability and in vivo efficacy.

Dissecting the role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in colon, breast, and lung carcinogenesis

Peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) is a promising drug target since its agonists increase serum high-density lipoprotein; decrease low-density lipoprotein, triglycerides, and insulin associated with metabolic syndrome; improve insulin sensitivity; and decrease high fat diet-induced obesity. PPARβ/δ agonists also promote terminal differentiation and elicit anti-inflammatory activities in many cell types. However, it remains to be determined whether PPARβ/δ agonists can be developed as therapeutics because there are reports showing either pro- or anti-carcinogenic effects of PPARβ/δ in cancer models. This review examines studies reporting the role of PPARβ/δ in colon, breast, and lung cancers. The prevailing evidence would suggest that targeting PPARβ/δ is not only safe but could have anti-carcinogenic protective effects.

The nuclear envelope protein Nesprin-2 has roles in cell proliferation and differentiation during wound healing

Nesprin-2, a type II transmembrane protein of the nuclear envelope, is a component of the LINC complex that connects the nuclear lamina with the actin cytoskeleton. To elucidate its physiological role we studied wound healing in Nesprin-2 Giant deficient mice and found that a loss of the protein affected wound healing particularly at later stages during fibroblast differentiation and keratinocyte proliferation leading to delayed wound closure. We identified altered expression and localization of transcription factors as one of the underlying mechanisms. Furthermore, the actin cytoskeleton which surrounds the nucleus was altered and keratinocyte migration was slowed down and focal adhesion formation enhanced. We also uncovered a new activity of Nesprin-2. When we probed for an interaction of Nesprin-2 Giant with chromatin we observed in ChIP Seq experiments an association of the protein with heterochromatic and centromeric DNA. Through this activity Nesprin-2 can affect the nuclear landscape and gene regulation. Our findings suggest functions for Nesprin-2 at the nuclear envelope (NE) in gene regulation and in regulation of the actin cytoskeleton which impact on wound healing.

New insights into the role of peroxisome proliferator-activated receptors in regulating the inflammatory response after tissue injury

Major trauma results in a strong inflammatory response in injured tissue. This posttraumatic hyperinflammation has been implied in the adverse events leading to a breakdown of host defense mechanisms and ultimately to delayed organ failure. Ligands to peroxisome proliferator-activated receptors (PPARs) have recently been identified as potent modulators of inflammation in various acute and chronic inflammatory conditions. The main mechanism of action mediated by ligand binding to PPARs is the inhibition of the nuclear transcription factor NF-κB, leading to downregulation of downstream gene transcription, such as for genes encoding proinflammatory cytokines. Pharmacological PPAR agonists exert strong anti-inflammatory properties in various animal models of tissue injury, including central nervous system trauma, ischemia/reperfusion injury, sepsis, and shock. In addition, PPAR agonists have been shown to induce wound healing process after tissue trauma. The present review was designed to provide an up-to-date overview on the current understanding of the role of PPARs in the pathophysiology of the inflammatory response after major trauma. Therapeutic options for using recombinant PPAR agonists as pharmacological agents in the management of posttraumatic inflammation will be discussed.

The role of peroxisome proliferator-activated receptors in carcinogenesis and chemoprevention

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that are involved in regulating glucose and lipid homeostasis, inflammation, proliferation and differentiation. Although all of these functions might contribute to the influence of PPARs in carcinogenesis, there is a distinct need for a review of the literature and additional experimentation to determine the potential for targeting PPARs for cancer therapy and cancer chemoprevention. As PPAR agonists include drugs that are used for the treatment of metabolic diseases, a more complete understanding of the roles of PPARs in cancer will aid in determining any increased cancer risk for patients undergoing therapy with PPAR agonists.

Modulation of gastrointestinal inflammation and colorectal tumorigenesis by peroxisome proliferator-activated receptor-β/δ (PPARβ/δ)

Critical physiological roles of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) include the regulation glucose and lipid homeostasis, cellular differentiation, and modulation of inflammation. The potential for targeting PPARβ/δ for the prevention and treatment of metabolic diseases or cancer, is compromised because of major inconsistencies in the literature. This is due primarily to uncertainty regarding the effect of PPARβ/δ and its activation on cell proliferation, apoptosis and cell survival. This review summarizes both the confirmed and conflicting mechanisms that have been described for PPARβ/δ and the potential for targeting this nuclear receptor for the prevention and treatment of colon cancer.