The synthetic PPARgamma agonist troglitazone inhibits IL-5-induced CD69 upregulation and eosinophil-derived neurotoxin release from eosinophils

The Role of PPAR-γ in Allergic Disease

PURPOSE OF REVIEW

The incidence of allergic diseases such as asthma, rhinitis and atopic dermatitis has risen at an alarming rate over the last century. Thus, there is a clear need to understand the critical factors that drive such pathologic immune responses. Peroxisome proliferator-activated receptor-γ (PPAR-γ) is a nuclear receptor that has emerged as an important regulator of multiple cell types involved in the inflammatory response to allergens; from airway epithelial cells to T Helper (TH) cells.

RECENT FINDINGS

Initial studies suggested that agonists of PPAR-γ could be employed to temper allergic inflammation, suppressing pro-inflammatory gene expression programs in epithelial cells. Several lines of work now suggest that PPAR-γ plays an essential in promoting 'type 2' immune responses that are typically associated with allergic disease. PPAR-γ has been found to promote the functions of TH2 cells, type 2 innate lymphoid cells, M2 macrophages and dendritic cells, regulating lipid metabolism and directly inducing effector gene expression. Moreover, preclinical models of allergy in gene-targeted mice have increasingly implicated PPAR-γ in driving allergic inflammation. Herein, we highlight the contrasting roles of PPAR-γ in allergic inflammation and hypothesize that the availability of environmental ligands for PPAR-γ may be at the heart of the rise in allergic diseases worldwide.

FABP4 regulates eosinophil recruitment and activation in allergic airway inflammation

Fatty acid binding protein 4 (FABP4), a member of a family of lipid-binding proteins, is known to play a role in inflammation by virtue of its ability to regulate intracellular events such as lipid fluxes and signaling. Studies have indicated a proinflammatory role for FABP4 in allergic asthma although its expression and function in eosinophils, the predominant inflammatory cells recruited to allergic airways, were not investigated. We examined expression of FABP4 in murine eosinophils and its role in regulating cell recruitment in vitro as well as in cockroach antigen (CRA)-induced allergic airway inflammation. CRA exposure led to airway recruitment of FABP4-expressing inflammatory cells, specifically eosinophils, in wild-type (WT) mice. FABP4 expression in eosinophils was induced by TNF-α as well as IL-4 and IL-13. FABP4-deficient eosinophils exhibited markedly decreased cell spreading/formation of leading edges on vascular cell adhesion molecule-1 and significantly decreased adhesion to intercellular adhesion molecule-1 associated with reduced β2-integrin expression relative to WT cells. Furthermore, FABP4-deficient eosinophils exhibited decreased migration, F-actin polymerization, calcium flux, and ERK(1/2) phosphorylation in response to eotaxin-1. In vivo, CRA-challenged FABP4-deficient mice exhibited attenuated eosinophilia and significantly reduced airway inflammation (improved airway reactivity, lower IL-5, IL-13, TNF-α, and cysteinyl leukotriene C4 levels, decreased airway structural changes) compared with WT mice. In conclusion, expression of FABP4 in eosinophils is induced during conditions of inflammation and plays a proinflammatory role in the development of allergic asthma by promoting eosinophil adhesion and migration and contributing to the development of various aspects of airway inflammation.

Functional analysis of free fatty acid receptor GPR120 in human eosinophils: implications in metabolic homeostasis

Recent evidence has shown that eosinophils play an important role in metabolic homeostasis through Th2 cytokine production. GPR120 (FFA4) is a G protein-coupled receptor (GPCR) for long-chain fatty acids that functions as a regulator of physiological energy metabolism. In the present study, we aimed to investigate whether human eosinophils express GPR120 and, if present, whether it possesses a functional capacity on eosinophils. Eosinophils isolated from peripheral venous blood expressed GPR120 at both the mRNA and protein levels. Stimulation with a synthetic GPR120 agonist, GW9508, induced rapid down-regulation of cell surface expression of GPR120, suggesting ligand-dependent receptor internalization. Although GPR120 activation did not induce eosinophil chemotactic response and degranulation, we found that GW9508 inhibited eosinophil spontaneous apoptosis and Fas receptor expression. The anti-apoptotic effect was attenuated by phosphoinositide 3-kinase (PI3K) inhibitors and was associated with inhibition of caspase-3 activity. Eosinophil response investigated using ELISpot assay indicated that stimulation with a GPR120 agonist induced IL-4 secretion. These findings demonstrate the novel functional properties of fatty acid sensor GPR120 on human eosinophils and indicate the previously unrecognized link between nutrient metabolism and the immune system.

The peroxisome proliferator-activated receptor gamma agonist rosiglitazone ameliorates murine lupus by induction of adiponectin

Systemic lupus erythematosus (SLE) is an inflammatory autoimmune disease for which current therapy is suboptimal. SLE is characterized by autoantibody production, with renal disease and premature atherosclerosis being common and severe manifestations causing appreciable morbidity and mortality. Peroxisome proliferator-activated receptor gamma (PPARgamma) agonists are widely used in the treatment of diabetes mellitus for their insulin-sensitizing properties, but also have immunomodulatory effects. In this report, we show that the PPARgamma agonist rosiglitazone reduces autoantibody production, renal disease, and atherosclerosis in mouse models of SLE. The beneficial effect of rosiglitazone on SLE manifestations depends on the induction of adiponectin, because rosiglitazone has no effect on autoantibody production or renal disease in lupus mice that lack adiponectin. In addition, lupus mice that lack adiponectin develop more severe disease than adiponectin-sufficient lupus mice, indicating that endogenous adiponectin is involved in regulating disease activity. Furthermore, administration of exogenous adiponectin ameliorates disease. These experiments suggest that PPARgamma agonists may be useful agents for the treatment of SLE. They also demonstrate that induction of adiponectin is a major mechanism underlying the immunomodulatory effects of PPARgamma agonists.

The synthetic PPARgamma agonist troglitazone inhibits eotaxin-enhanced eosinophil adhesion to ICAM-1-coated plates

Accumulation and activation of eosinophils in tissue are critical events in the allergic inflammatory response and adhesion molecules play important roles in this process. We previously demonstrated that human eosinophils expressed a nuclear receptor, peroxisome proliferator-activated receptor gamma (PPARgamma), and that stimulation with a PPARgamma agonist attenuated cytokine/chemokine-induced eosinophil activation, such as survival, chemotaxis and degranulation. In the present study, we investigated the effect of troglitazone, a synthetic PPARgamma agonist, on adherence to intercellular adhesion molecule-1 (ICAM-1). Eosinophils were purified from human peripheral blood, and the functional adherence to recombinant soluble ICAM-1-coated plates was examined. We found that in the presence of eotaxin, troglitazone inhibited eosinophil adherence in a concentration-dependent manner. This novel activity appears to be associated with modulation of qualitative change of integrins in response to eotaxin, because quantitative reduction of CD11a, CD11b and CD18 expression by troglitazone was not observed using flow cytometry. The PPARgamma agonist troglitazone has a potent inhibitory effect on eosinophil adhesion to ICAM-1, and this may be a therapeutic modality for the treatment of eosinophil-related diseases including bronchial asthma.

Anti- and proinflammatory effects of 15-deoxy-delta-prostaglandin J2(15d-PGJ2) on human eosinophil functions

The cyclopentenone prostaglandin 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) is recognized as a potent lipid mediator that is derived from PGD(2), which is produced abundantly in allergic inflammatory sites. It is now established that 15d-PGJ(2) negatively regulates cellular functions through its intracellular targets such as peroxisome proliferator-activated receptor-gamma (PPARgamma). However, recent studies revealed that 15d-PGJ(2) appears to possess not only anti-inflammatory activities but also a proinflammatory potential depending on its concentration and the activation state of the target cell. For instance, at low concentrations, 15d-PGJ(2) enhances eotaxin-induced chemotaxis, shape change, and actin reorganization in eosinophils through its ligation with PPARgamma. Moreover, 15d-PGJ(2) itself is a potent chemoattractant, and it induces calcium mobilization, and up-regulates CD11b expression through its membrane receptor--chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2). Conversely, at high concentrations, 15d-PGJ(2) inhibits eosinophil survival by inducing apoptosis in a PPARgamma-independent manner. Here, we discuss the pathophysiological roles of 15d-PGJ(2) that could act as a paracrine, autocrine, and intracrine substance to regulate eosinophil functions.

Procaterol upregulates peroxisome proliferator-activated receptor-gamma expression in human eosinophils

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear receptor that regulates immune reaction. We have previously demonstrated that human eosinophils express PPARgamma and that stimulation with a synthetic agonist for PPARgamma attenuated the factor-induced eosinophil activations. However, the modulator of PPARgamma expression in eosinophils has not yet been studied. In this study, we investigated the effect of procaterol, the synthetic beta2-adrenoceptor agonist widely used as bronchodilators in asthma, on the PPARgamma expression in eosinophils. Purified human peripheral blood eosinophil and the eosinophilic cell line EoL-1 were cultured with procaterol. This was followed by PPARgamma measurement using flow cytometer and quantitative real-time RT-PCR. We observed that PPARgamma was constitutively expressed by EoL-1 and the purified eosinophils and that the therapeutic concentration (10(-9)M) of procaterol markedly enhanced PPARgamma protein expression, which was reversed by the selective beta2-adrenoceptor antagonist ICI-118551. The PPARgamma mRNA expression in EoL-1 and eosinophils was also induced by procaterol. These findings suggest that procaterol could modulate the eosinophil function by increasing the expression of PPARgamma.

Theophylline and dexamethasone induce peroxisome proliferator-activated receptor-gamma expression in human eosinophils

Eosinophils are major effector cells in allergic diseases including asthma. Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear receptor that regulates immune reaction. We have previously demonstrated that human eosinophils express PPARgamma and that stimulation with a synthetic agonist for PPARgamma attenuated the factor-induced eosinophil survival and chemotaxis. However, the modulator of the eosinophil PPARgamma expression has not yet been studied. In this study, we investigated the effect of theophylline and dexamethasone (widely used drugs in the treatment of asthma) on PPARgamma expression in eosinophils. Purified human peripheral blood eosinophils were cultured, and therapeutic concentrations of theophylline and dexamethasone were added. Subsequently, PPARgamma was measured using quantitative real-time RT-PCR and flow cytometry. Theophylline and dexamethasone markedly enhanced both mRNA and protein levels of PPARgamma. These findings suggest that the increase in PPARgamma expression on eosinophils may play a role in the anti-inflammatory effects of theophylline and dexamethasone.

Cis-9,Trans-11-CLA exerts anti-inflammatory effects in human bronchial epithelial cells and eosinophils: Comparison to Trans-10,Cis-12-CLA and to linoleic acid

Interaction of eosinophils and bronchial epithelial cells plays a pivotal role in maintaining inflammatory airway disease. Since conjugated linoleic acids (CLA) are suggested to exert anti-inflammatory effects, one purpose of this study was to compare cis-9,trans-11-CLA and trans-10,cis-12-CLA with regard to their influence on the stimulus-induced activation of eosinophils. ECP (eosinophil cationic protein) released in co-culture of stimulated and CLA-treated eosinophils with stimulated bronchial epithelial cells (BEAS-2B) was measured and cis-9,trans-11-CLA was found to be most potent in inhibiting ECP formation. Further, expression of the activation markers CD69 and CD13 induced by various stimuli (TNF-alpha, IL-5, IL-3) was significantly reduced in the presence of cis-9,trans-11-CLA. Subsequently, various concentrations of cis-9,trans-11-CLA vs. linoleic acid (LA, cis-9,cis-12-octadecadienoic acid) were tested for the effect on proliferative response and release of the pro-inflammatory cytokine IL-8 in stimulated BEAS-2B. Addition of cis-9,trans-11-CLA attenuated cell growth and significantly reduced IL-8 production at mRNA and protein levels. In contrast, LA had a slight stimulating effect on proliferation and was less effective in reducing the cytokine release. It was demonstrated that the inhibitory effect of cis-9,trans-11-CLA on IL-8 production is mediated through activation of the nuclear receptor PPARgamma, since blocking the receptor with a selective antagonist (GW9662) restored the stimulus-induced enhancement in IL-8 mRNA expression and protein secretion. PPARgamma has previously been shown to be closely involved in the downregulation of inflammation during hyperresponsiveness related to pulmonary immune responses. Thus, targeting PPARgamma, cis-9,trans-11-CLA might be of therapeutic value in the focus of airway disease while ameliorating inflammatory processes by affecting epithelial and eosinophil functions.

Physiological levels of 15-deoxy-Delta12,14-prostaglandin J2 prime eotaxin-induced chemotaxis on human eosinophils through peroxisome proliferator-activated receptor-gamma ligation

15-Deoxy-Delta12,14-PGJ2 (15d-PGJ2), mainly produced by mast cells, is known as a potent lipid mediator derived from PGD2 in vivo. 15d-PGJ2 was thought to exert its effects on cells exclusively through peroxisome proliferator-activated receptor-gamma (PPARgamma) and chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2), which are both expressed on human eosinophils. However, the physiological role of 15d-PGJ2 remains unclear, because the concentration generated in vivo is generally much lower than that required for its biological functions. In the present study we found that low concentrations (picomolar to low nanomolar) of 15d-PGJ2 and a synthetic PPARgamma agonist markedly enhanced the eosinophil chemotaxis toward eotaxin, and the effect was decreased in a dose-dependent manner. Moreover, at a low concentration (10(-10) M), 15d-PGJ2 and troglitazone primed eotaxin-induced shape change and actin polymerization. These priming effects were completely reversed by a specific PPARgamma antagonist, but were not mimicked by CRTH2 agonist 13,14-dihydro-15-keto-PGD2, suggesting that the effects were mediated through PPARgamma ligation. The effect exerted by 15d-PGJ2 parallels the enhancement of Ca2+ influx, but is not associated with the ERK, p38 MAPK, and NF-kappaB pathways. Furthermore, the time course and treatment of eosinophils with actinomycin D, an inhibitor of gene transcription, indicated that the transcription-independent pathway had a role in this process. PPARgamma might interact with an eotaxin-induced cytosolic signaling pathway, because PPARgamma is located in the eosinophil cytosol. Taken together with current findings, these results suggest that under physiological conditions, 15d-PGJ2 contributes to allergic inflammation through PPARgamma, which plays a role as a biphasic regulator of immune response.