Role of peroxisome proliferator-activated receptor gamma and its ligands in the control of immune responses

To ensure that efficient immune responses against dangerous antigens are raised while tolerance to self molecules is preserved, the immune system tightly regulates activation and survival of its cellular compartments through mechanisms only partially characterized. In this context, recent evidence indicates a role in immunity of the nuclear receptor PPAR-gamma, which is upregulated in activated lymphocytes and in dendritic cells. Preliminary in vitro studies indicate that PPAR-gamma activation profoundly alters the immune properties of these cells, usually leading to the inhibition of immune responses. Naturally occurring PPAR-gamma ligands include the cyclopentenone prostaglandins of the J series, which are present in bone marrow, thymus, and secondary lymphatic tissues. The levels of these metabolites are increased in inflamed tissues, where they exert strong anti-inflammatory effects leading to resolution of inflammation and wound healing. Cyclopentenone prostaglandins activate both PPAR-gamma-dependent and PPAR-gamma-independent pathways, possess intrinsic proapoptotic potential and are direct inhibitors of NF-kappaB signaling. The relevance of these effects in vivo still awaits proper evaluation in humans. Some of the newly described regulatory pathways might eventually be exploited in the treatment of immune diseases by means of PPAR-gamma ligands, such as thiazolidinediones or prostaglandins.     

Human multiple myeloma cells express peroxisome proliferator-activated receptor gamma and undergo apoptosis upon exposure to PPARgamma ligands

Multiple myeloma is essentially an incurable malignancy and it is therefore of great interest to develop new therapeutic approaches. We previously reported that human B cell-lymphomas express the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) and are killed by PPARgamma ligands. Herein, we investigate the therapeutic potential of PPARgamma ligands for multiple myeloma. The human multiple myeloma cell lines ANBL6 and 8226 express PPARgamma mRNA and protein. The PPARgamma ligands, 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) and ciglitazone, induced multiple myeloma cell apoptosis as determined by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) assay, loss of mitochondrial membrane potential, and caspase activation. Importantly, the ability of PPARgamma ligands to kill both multiple myeloma cell lines was not abrogated by Interleukin-6 (IL-6), a multiple myeloma growth survival factor. Finally, the RXR ligand 9-cis retinoic acid (9-cis RA) in combination with PPARgamma ligands greatly enhanced multiple myeloma cell killing. These new findings support that PPARgamma ligands may represent a novel therapy for multiple myeloma.     

过氧化物酶体增殖物激活受体-γ在增生性瘢痕中的表达及其意义

目的 探讨核转录因子过氧化物酶体增殖物激活受体-γ(PPAR-γ)在增生性瘢痕中的表达状况及其意义. 方法 术中切取16 例增生性瘢痕组织及每位患者手术时取皮修剪后的残余皮片.应用免疫组织化学检测增生性瘢痕和正常皮肤中PPAR-γ蛋白的表达并分析其与瘢痕增生程度的关系. 结果 免疫组织化学显示PPAR-γ蛋白在增生性瘢痕和正常皮肤中均有表达,但是在增生性瘢痕(32.18±17.84)中表达较正常皮肤(50.80±22.40)低. 结论 核内受体PPAR-γ在增生性瘢痕中呈下调表达,提示PPAR-γ的激活可能是增生性瘢痕治疗的一种新方法.     

Activation of peroxisome proliferator-activated receptor-gamma reverses squamous metaplasia and induces transitional differentiation in normal human urothelial cells

We observed that in urothelium, both cornifying and noncornifying forms of squamous metaplasia are accompanied by changes in the localization of the nuclear hormone receptors, peroxisome proliferator activated receptor gamma (PPAR-gamma) and retinoid X receptor (RXR-alpha). To obtain objective evidence for a role for PPAR-gamma-mediated signaling in urothelial differentiation, we examined expression of the cytokeratin isotypes CK13, CK20, and CK14 as indicators of transitional, terminal transitional, and squamous differentiation, respectively, in cultures of normal human urothelial cells. In control culture conditions, normal human urothelial cells showed evidence of squamous differentiation (CK14+, CK13-, CK20-). Treatment with the high-affinity PPAR-gamma agonist, troglitazone (TZ), resulted in gain of CK13 and loss of CK14 protein expression. The effect of TZ was significantly augmented when the autocrine-stimulated epidermal growth factor receptor pathway was inhibited and this resulted in induction of CK20 expression. The RXR-specific inhibitors PA452, HX531, and HX603 inhibited the TZ-induced CK13 expression, supporting a role for RXR in the induction of CK13 expression. Thus, signaling through PPAR-gamma can mediate transitional differentiation of urothelial cells and this is modulated by growth regulatory programs.     

Toll-like receptor 4 mediates cross-talk between peroxisome proliferator-activated receptor gamma and nuclear factor-kappaB in macrophages

The peroxisome proliferator-activated receptor gamma (PPARgamma) is expressed in macrophages and plays an important role in suppressing the inflammatory response. Lipopolysaccharides (LPS), which activate Toll-like receptor 4 (TLR4), reduced PPARgamma expression and function in peritoneal macrophages and macrophage cell lines. Moreover, pretreatment with the synthetic PPARgamma ligand, rosiglitazone did not prevent LPS-mediated downregulation of PPARgamma. Inhibition of PPARgamma expression was not blocked by cycloheximide, indicating that de novo protein synthesis is not required for LPS-mediated suppression of PPARgamma. Destabilization of PPARgamma messenger RNA (mRNA) was not observed in LPS-stimulated macrophages, suggesting that LPS regulates the synthesis of PPARgamma mRNA. LPS had no effect on PPARgamma expression in macrophages from TLR4 knockout mice, whereas LPS inhibited PPARgamma expression in cells that had been reconstituted to express functional TLR4. Targeting the TLR4 pathway with inhibitors of MEK1/2, p38, JNK and AP-1 had no effect on PPARgamma downregulation by LPS. However, inhibitors that target NEMO, IkappaB and NF-kappaB abolished LPS-mediated downregulation of PPARgamma in LPS-stimulated macrophages. Our data indicate that activation of TLR4 inhibits PPARgamma mRNA synthesis by an NF-kappaB-dependent mechanism. Low-density genomic profiling of macrophage-specific PPARgamma knockout cells indicated that PPARgamma suppresses inflammation under basal conditions, and that loss of PPARgamma expression is sufficient to induce a proinflammatory state. Our data reveal a regulatory feedback loop in which PPARgamma represses NF-kappaB-mediated inflammatory signalling in unstimulated macrophages; however, upon activation of TLR4, NF-kappaB drives down PPARgamma expression and thereby obviates any potential anti-inflammatory effects of PPARgamma in LPS-stimulated macrophages.     

Anti-inflammatory actions of peroxisome proliferator-activated receptor gamma agonists in Alzheimer’s disease

The role of inflammatory processes in the brains of Alzheimer’s Disease (AD) patients has recently attracted considerable interest. Indeed, the only demonstrated effective therapy for AD patients is long-term treatment with non-steroidal anti-inflammatory drugs (NSAIDs). The mechanistic basis of the efficacy of NSAIDs in AD remains unclear. However, the recent recognition that NSAIDs can bind to and activate the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ), has offered an explanation for the action of these drugs in AD. PPARγ activation leads to the inhibition of microglial activation and the expression of a broad range of proinflammatory molecules. The newly appreciated anti-inflammatory actions of PPARγ agonists may allow novel therapies for AD and other CNS indications with an inflammatory component.     

Activation of peroxisome proliferator-activated receptor gamma suppresses mast cell maturation involved in allergic diseases

Background:  Mast cells play a central role in allergic and inflammatory diseases. Several reports indicated role of peroxisome proliferator-activated receptor gamma (PPARγ) on mast cell function. However, there is no report about the role of PPARγ on differentiation of mast cells from the progenitors. In this study, we investigated the role of PPARγ in regulating bone marrow-derived mast cell maturation and the therapeutic implications for mast cell-related diseases such as atopic or contact dermatitis.
Methods:  We used in vitro cell culture system for mast cell differentiation from bone marrow-progenitors using specific ligands and lentiviral-mediated short hairpin RNA of PPARγ, and in vivo murine dermatitis models.
Results:  Activation of PPARγ inhibited the maturation of bone marrow progenitors into connective tissue-type mast cells (CTMCs) through up-regulation of GATA-4 and GATA-6 resulting in a decrease in expression of histidine decarboxylase and mast cell histamine content. In comparison, the differentiation of bone marrow progenitors into CTMCs was significantly accelerated by the knockdown of PPARγ expression by lentiviral-mediated short hairpin RNA. Peroxisome proliferator-activated receptor gamma ligand administration to mice inhibited the maturation of mast cells resulting in attenuation of atopic and contact dermatitis via diminishment of the number of mature mast cells.
Conclusion:  Our results indicate that PPARγ is one of master regulators on mast cell maturation and potentially useful for the therapy in various disorders involving mast cell activation.     

Peroxisome proliferator-activated receptor gamma activators affect the maturation of human monocyte-derived dendritic cells

Peroxisome proliferator-activated receptor gamma (PPARgamma ), a member of the nuclear receptor superfamily, has recently been described as a modulator of macrophage functions and as an inhibitor of T cell proliferation. Here, we investigated the role of PPARgamma in dendritic cells (DC), the most potent antigen-presenting cells. We showed that PPARgamma is highly expressed in immature human monocyte-derived DC (MDDC) and that it may affect the immunostimulatory function of MDDC stimulated with lipopolysaccharide (LPS) or via CD40 ligand (CD40L). We found that the synthetic PPARgamma agonist rosiglitazone (as well as pioglitazone and troglitazone) significantly increases on LPS- and CD40L-activated MDDC, the surface expression of CD36 (by 184% and 104%, respectively) and CD86 (by 54% and 48%), whereas it reduces the synthesis of CD80 (by 42% and 42%). Moreover, activation of PPARgamma resulted in a dramatic decreased secretion of the Th1-promoting factor IL-12 in LPS- and CD40L-stimulated cells (by 47% and 62%), while the production of IL-1beta, TNF-alpha, IL-6 and IL-10 was unaffected. Finally, PPARgamma ligands down-modulate the synthesis of IFN-gamma -inducible protein-10 (recently termed as CXCL10) and RANTES (CCL5), both chemokines involved in the recruitment of Th1 lymphocytes (by 49% and 30%), but not the levels of the Th2 cell-attracting chemokines,macrophage-derived chemokine (CCL22) and thymus and activation regulated chemokine (CCL17), in mature MDDC. Taken together, our data suggest that activation of PPARgamma in human DC may have an impact in the orientation of primary and secondary immune responses by favoring type 2 responses.     

Gastrin-releasing peptide receptors in non-neoplastic and neoplastic human breast

The regulatory peptide gastrin-releasing peptide (GRP) may play a role in human cancer as a stimulatory growth factor. To understand the potential role of GRP in human breast cancer, we have evaluated GRP receptor expression in human non-neoplastic and neoplastic breast tissues and in axillary lymph node metastases, using in vitro receptor autoradiography on tissue sections with [(125)I]Tyr(4)-bombesin and with [(125)I]D-Tyr(6), beta Ala(11), Phe(13), Nle(14)-bombesin(6-14) as radioligands. GRP receptors were detected, often in high density, in neoplastic epithelial mammary cells in 29 of 46 invasive ductal carcinomas, in 11 of 17 ductal carcinomas in situ, in 1 of 4 invasive lobular carcinomas, in 1 of 2 lobular carcinomas in situ, and in 1 mucinous and 1 tubular carcinoma. A heterogeneous GRP receptor distribution was found in the neoplastic tissue samples in 32 of 52 cases with invasive carcinoma and 12 of 19 cases with carcinoma in situ. The lymph node metastases (n = 33) from those primary carcinomas expressing GRP receptors were all positive, whereas surrounding lymphoreticular tissue was negative. GRP receptors were also present in high density but with heterogeneous distribution in ducts and lobules from all available breast tissue samples (n = 23). All of the receptors corresponded to the GRP receptor subtype of bombesin receptors, having high affinity for GRP and bombesin and lower affinity for neuromedin B. All tissues expressing GRP receptors were identified similarly with both radioligands. These data describe not only a high percentage of GRP receptor-positive neoplastic breast tissues but also for the first time a ubiquitous GRP receptor expression in nonneoplastic human breast tissue. Apart from suggesting a role of GRP in breast physiology, these data represent the molecular basis for potential clinical applications of GRP analogs such as GRP receptor scintigraphy, radiotherapy, or chemotherapy.     

Role of prostacyclin versus peroxisome proliferator-activated receptor beta receptors in prostacyclin sensing by lung fibroblasts

Prostacyclin and its mimetics are used therapeutically for the treatment of pulmonary hypertension. These drugs act via cell surface prostacyclin receptors (IP receptors); however, some of them can also activate the nuclear receptor peroxisome proliferator-activated receptor beta (PPARbeta). We examined the possibility that PPARbeta is a therapeutic target for the treatment of pulmonary hypertension. Using the newly approved (for pulmonary hypertension) prostacyclin mimetic treprostinil sodium, reporter gene assays for PPARbeta activation and measurement of lung fibroblast proliferation were analyzed. Treprostinil sodium was found to activate PPARbeta in reporter gene assays and to inhibit proliferation of human lung fibroblasts at concentrations consistent with an effect on PPARs but not on IP receptors. The effects of treprostinil sodium on human lung cell proliferation are mimicked by those of the highly selective PPARbeta ligand GW0742. There are no receptor antagonists for PPARbeta or for IP receptors, but by using lung fibroblasts cultured from mice lacking PPARbeta (PPARbeta-/-) or IP (IP-/-), we demonstrate that the antiproliferative effects of treprostinil sodium are mediated by PPARbeta and not IP in lung fibroblasts. These observations suggest that some of the local, longer-term benefits of treprostinil sodium on reducing the remodeling associated with pulmonary hypertension may be mediated by PPARbeta. This study is the first to identify PPARbeta as a potential therapeutic target for the treatment of pulmonary hypertension, which is important because orally active PPARbeta ligands have been developed for the treatment of dyslipidemia.     

Expression of peroxisome proliferator-activated receptor gamma in salivary duct carcinoma: immunohistochemical analysis of 15 cases.

Salivary duct carcinoma is a rare but highly aggressive tumor of the salivary glands that has poor prognosis. There is no effective cure for this tumor. Peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear receptor family with diverse biological functions that include mediation of adipocyte differentiation, regulation of the monocyte-macrophage anti-inflammatory activity, and inhibition of tumor cell proliferation. Natural (prostaglandin J2, PG-J2) and synthetic (thiazolinediones) PPARgamma ligands with anti-proliferative agonist activity have been identified. The expression of PPARgamma has been demonstrated in human colorectal, pancreas, breast, and prostate cancers but has never been explored in salivary duct carcinoma. The aim of our study was to investigate the expression patterns of PPARgamma in salivary duct carcinoma, a finding that may provide a mechanism for treating patients with this highly aggressive tumor. Archival formalin-fixed tissues from 15 salivary duct carcinoma cases were analyzed for PPARgamma expression by an immunohistochemical staining method using a monoclonal antibody against the PPARgamma. The tissue sections were subjected to antigen retrieval by a steam heat method. All the cases of salivary duct carcinoma originated from the parotid gland. Immunohistochemistry analyses showed positive expression of PPARgamma in 12 (80%) cases, whereas 3 (20%) were negative. Of the positive cases, 9 (75%), 2 (17%) and 1 (8%) showed strong, moderate, and weak staining, respectively. All staining was cytoplasmic. Nuclear staining was not observed. We conclude that PPARgamma is frequently (80%) expressed in salivary duct carcinoma, often at high levels, and is topographically located in the cytoplasm. The high-level expression of PPARgamma may provide a potential molecular target for the treatment of salivary duct carcinoma using agonist ligands.     

Anti-inflammatory actions of green tea catechins and ligands of peroxisome proliferator-activated receptors

Introduction Green tea catechins and peroxisome proliferator‐activated receptor (PPAR) agonists have been shown to reduce inflammation associated with collagen‐induced arthritis in mice ( Haqqi et al. 1999 ; Cuzzocrea et al. 2003 ) and to inhibit other markers of inflammation ( Vankemmelbeke et al. 2003 ; Sabatini et al. 2002 ; Singh et al. 2003 ; Jiang et al. 1998 ). We are investigating the mechanisms by which catechins and PPAR agonists interfere with the signalling pathways of the pro‐inflammatory cytokines, IL‐1 and TNF. Materials and methods Epigallocatechin gallate (EGCG), epicatechin gallate (ECG), epigallocatechin (EGC), epicatechin (EC) and fenofibrate were from Sigma Aldrich, Poole, UK. Rosiglitazone was from Cayman Chemical, Ann Arbor, MI, USA. Cytokines and IL‐8 ELISA were from NIBSC. IL‐6 ELISA from eBiosciences (San Diego, CA, USA). Cells used were primary fibroblasts from inflamed periodontal tissue or osteoarthritic chondrocytes from discarded tissue following surgery. Results We show the significant suppression of cytokine‐induced IL‐6 and IL‐8 release by EGCG, which is dose‐ and cell‐type dependent. The other three catechins have a much lesser effect. Preliminary results show that PPAR agonists also interfere with TNF‐induced IL‐8 production. Discussion Green tea has a long history of human consumption, and epidemiological studies have shown that it can reduce inflammatory diseases ( Greenwald et al. 2002 ). Our results confirm that EGCG has the strongest anti‐inflammatory effect, compared with the other three catechins. They also indicate that the mechanism of action of EGCG is cell‐type dependent. This could be because the compound targets components of pro‐inflammatory signalling pathways whose expression is restricted to certain differentiated cell types. Alternatively, the different sensitivities of cells to EGCG could reflect differences in their ability to metabolize the compound. Further experiments are needed to discriminate between these hypotheses.     

Interaction between a peroxisome proliferator-activated receptor gamma gene polymorphism and dietary fat intake in relation to body mass

The peroxisome proliferator-activated receptor gamma (PPAR gamma) is a critical regulator of adipogenesis. PPAR gamma+/- mice are resistant to high-fat diet-induced obesity and thus PPAR gamma may mediate physiological responses to dietary fat in other mammals. The aim of this study was to determine whether the human PPAR gamma proline to alanine substitution polymorphism (Pro12Ala) modifies the association between dietary fat and adiposity and plasma lipids. Subjects (n=2141) were controls selected for three case-control studies nested within the Nurses' Health Study, a large ongoing prospective cohort study. Associations between intake of total fat, fat subtypes and BMI were different in PPAR gamma 12Ala variant allele-carriers compared with non-carriers. Among homozygous wild-type Pro/Pro individuals, those in the highest quintile of total fat intake, had significantly higher mean body mass index (BMI) compared with those in the lowest quintile (27.3 versus 25.4 kg/m2, respectively; P-trend<0.0001) whereas among 12Ala variant allele-carriers there was no significant trend observed between dietary fat intake and BMI (P-trend=0.99; P-interaction=0.003). In contrast, intake of monounsaturated fat was not associated with BMI among homozygous wild-type women but was inversely associated with BMI among 12Ala variant allele-carriers (mean in lowest quintile=27.6 versus mean in highest quintile=25.5 kg/m2; P-trend=0.006; P-interaction=0.003). The relationship between dietary fat intake and plasma lipid concentrations also differed according to PPAR gamma genotype. These data suggest that PPAR gamma genotype is an important factor in physiological responses to dietary fat in humans.     

Activation of peroxisome proliferator-activated receptor beta/delta induces lung cancer growth via peroxisome proliferator-activated receptor coactivator gamma-1alpha

We previously demonstrated that a selective agonist of peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta), GW501516, stimulated human non-small cell lung carcinoma (NSCLC) growth, partly through inhibition of phosphatase and tensin homolog deleted on chromosome 10 expression. Here, we show that GW501516 also decreases the phosphorylation of AMP-activated protein kinase alpha (AMPKalpha), a major regulator of energy metabolism. This was mediated through specific activation of PPARbeta/delta, as a PPARbeta/delta small interfering RNA inhibited the effect. However, AMPKalpha did not mediate the growth-promoting effects of GW501516, as silencing of AMPKalpha did not inhibit GW501516-induced cell proliferation. Instead, we found that GW501516 stimulated peroxisome proliferator-activated receptor coactivator gamma (PGC)-1alpha, which activated the phosphatidylinositol 3 kinase (PI3-K)/Akt mitogenic pathway. An inhibitor of PI3-K, LY294002, had no effect on PGC-1alpha, consistent with PGC-1alpha being upstream of PI3-K/Akt. Of note, an activator of AMPKalpha, 5-amino-4-imidazole carboxamide riboside, inhibited the growth-promoting effects of GW501516, suggesting that although AMPKalpha is not responsible for the mitogenic effects of GW501516, its activation can oppose these events. This study unveils a novel mechanism by which GW501516 and activation of PPARbeta/delta stimulate human lung carcinoma cell proliferation, and suggests that activation of AMPKalpha may oppose this effect.     

An inverse relationship between peroxisome proliferator-activated receptor gamma and allergic airway inflammation in an allergen challenge model.

BACKGROUND: Peroxisome proliferator-activated receptor gamma (PPAR-gamma) expression has not been evaluated in bronchoalveolar lavage (BAL) cells from allergic asthmatic patients. OBJECTIVE: To determine whether inappropriate down-regulation of PPAR-gamma in alveolar macrophages may contribute to persistent airway inflammation in allergic asthma. METHODS: We used segmental allergen challenge as a model of in vivo experimental allergic asthmatic exacerbation and airway inflammation. PPAR-y gene expression was evaluated at baseline and 24 hours later in asthmatic patients and controls using real-time polymerase chain reaction. Immunofluorescence was used to determine cellular location of the PPAR-gamma protein. RESULTS: We demonstrate for the first time to our knowledge that PPAR-gamma messenger RNA and protein, which are highly expressed in alveolar macrophages of healthy individuals, are significantly reduced in asthmatic patients after segmental allergen challenge. In allergic asthmatic patients (n=9), PPAR-gamma gene expression decreased significantly from baseline to postchallenge BAL (median decrease, 45%; P = .008). Furthermore, immunofluorescence staining demonstrated that PPAR-gamma protein was associated with alveolar macrophages and not with inflammatory eosinophils and neutrophils. CONCLUSION: Results implicate down-regulation of PPAR-gamma in BAL cells as a potential factor in dysregulation of lung homeostasis in asthmatic patients. The present findings suggest that PPAR-gamma agonists could have a future role in asthma therapy and warrant further study.     

Overexpression of peroxisome proliferator-activated receptor gamma co-activator-1alpha leads to muscle atrophy with depletion of ATP

Peroxisome proliferator-activated receptor-gamma co-activator-1alpha (PGC-1alpha) is a key nuclear receptor co-activator for mitochondrial biogenesis. Here we report that overexpression of PGC-1alpha in skeletal muscles increased mitochondrial number and caused atrophy of skeletal muscle, especially type 2B fiber-rich muscles (gastrocnemius, quadriceps, and plantaris). Muscle atrophy became evident at 25 weeks of age, and a portion of the muscle was replaced by adipocytes. Mice showed increased energy expenditure and reduced body weight; thyroid hormone levels were normal. Mitochondria exhibited normal respiratory chain activity per mitochondrion; however, mitochondrial respiration was not inhibited by an ATP synthase inhibitor, oligomycin, clearly indicating that oxidative phosphorylation was uncoupled. Accordingly, ATP content in gastrocnemius was markedly reduced. A similar phenotype is observed in Luft's disease, a mitochondrial disorder that involves increased uncoupling of respiration and muscle atrophy. Our results indicate that overexpression of PGC-1alpha in skeletal muscle increases not only mitochondrial biogenesis but also uncoupling of respiration, resulting in muscle atrophy.