Peroxisome proliferator-activated receptor gamma-dependent repression of the inducible nitric oxide synthase gene

Reciprocal regulation of inflammation and lipid metabolism by liver X receptors

Macrophages have important roles in both lipid metabolism and inflammation and are central to the pathogenesis of atherosclerosis. The liver X receptors (LXRs) are established mediators of lipid-inducible gene expression, but their role in inflammation and immunity is unknown. We demonstrate here that LXRs and their ligands are negative regulators of macrophage inflammatory gene expression. Transcriptional profiling of lipopolysaccharide (LPS)-induced macrophages reveals reciprocal LXR-dependent regulation of genes involved in lipid metabolism and the innate immune response. In vitro, LXR ligands inhibit the expression of inflammatory mediators such as inducible nitric oxide synthase, cyclooxygenase (COX)-2 and interleukin-6 (IL-6) in response to bacterial infection or LPS stimulation. In vivo, LXR agonists reduce inflammation in a model of contact dermatitis and inhibit inflammatory gene expression in the aortas of atherosclerotic mice. These findings identify LXRs as lipid-dependent regulators of inflammatory gene expression that may serve to link lipid metabolism and immune functions in macrophages.

Dimer-specific potentiation of NGFI-B (Nur77) transcriptional activity by the protein kinase A pathway and AF-1-dependent coactivator recruitment

The NGFI-B (Nur77) subfamily of orphan nuclear receptors (NRs), which also includes Nurr1 and NOR1, bind the NurRE regulatory element as either homo- or heterodimers formed between subfamily members. These NRs mediate the activation of pituitary proopiomelanocortin (POMC) gene transcription by the hypothalamic hormone corticotropin-releasing hormone (CRH), an important link between neuronal and endocrine components of the hypothalamo-pituitary-adrenal axis. CRH effects on POMC transcription do not require de novo protein synthesis. We now show that CRH signals activate Nur factors through the cyclic AMP/protein kinase A (PKA) pathway. CRH and PKA rapidly increase nuclear DNA binding activity of NGFI-B dimers but not monomers. Accordingly, CRH- or PKA-activated Nur factors enhance dimer (but not monomer) target response elements. We also show that p160/SRC coactivators are recruited to Nur dimers (but not to monomers) and that coactivator recruitment to the NurRE is enhanced in response to CRH. Moreover, PKA- and coactivator-induced potentiation of NGFI-B activity are primarily exerted through the N-terminal AF-1 domain of NGFI-B. The TIF2 (SRC-2) glutamine-rich domain is required for this activity. Taken together, these results indicate that Nur factors behave as endpoint effectors of the PKA signaling pathway acting through dimers and AF-1-dependent recruitment of coactivators.

Regulation of the Expression of Inducible Nitric Oxide Synthase

Nitric oxide (NO), generated by the inducible isoform of nitric oxide synthase (iNOS), has been described to have beneficial microbicidal, antiviral, antiparasital, immunomodulatory, and antitumoral effects. However, aberrant iNOS induction at the wrong place or at the wrong time has detrimental consequences and seems to be involved in the pathophysiology of several human diseases. iNOS is primarily regulated at the expression level by transcriptional and post-transcriptional mechanisms. iNOS expression can be induced in many cell types with suitable agents such as bacterial lipopolysaccharides (LPS), cytokines, and other compounds. Pathways resulting in the induction of iNOS expression may vary in different cells or different species. Activation of the transcription factors NF-kappaB and STAT-1alpha, and thereby activation of the iNOS promoter, seems to be an essential step for iNOS induction in most cells. However, at least in the human system, also post-transcriptional mechanism are critically involved in the regulation of iNOS expression. The induction of iNOS can be inhibited by a wide variety of immunomodulatory compounds acting at the transcriptional levels and/or post-transcriptionally.

Nitric Oxide Signalling with a Special Focus on Lipid-Derived Mediators

The ways in which cells communicate among each other concerns all aspects of biology, from developmental processes to diseases. Nitric oxide (NO) is one of the most remarkable and unusual regulatory molecules. It is a labile free radical gas that is not stored but generated on demand, and has been implicated in an extraordinarily diverse range of physiological and pathophysiological functions. The modulation of cell signalling by free radicals is an emerging area of research that provides insight into the orchestration of cell adaptation to a changing microenvironment. In a multicellular organism this serves to coordinate complex physiological responses, such as inflammation. Cell signalling is also accompanied by rapid remodelling of membrane lipids by activated lipases. The discovery that NO, which does not reversibly interact with membrane receptors like conventional hormones and growth factors, targets enzymes such as phospholipase A2, sphingomyelinases or ceramidases, has stimulated growing interest in the crosstalk between redox and lipid signalling.

Novel pituitary ligands: peroxisome proliferator activating receptor-gamma

Pituitary tumors cause considerable morbidity due to local invasion, hypopituitarism, or hormone hypersecretion. In many cases, no suitable drug therapies are available, and surgical excision is currently the only effective treatment. We have recently demonstrated abundant expression of nuclear hormone receptor PPAR-gamma in human pituitary tumors of different subtypes. PPAR-gamma activators (thiazolidinediones) induced G0-G1 cell-cycle arrest and apoptosis in human, and murine corticotroph, somatolactotroph, and gonadotroph pituitary tumor cells, and suppressed in vitro hormone secretion. In vivo development and growth of murine corticotroph, somatolactotroph and gonadotroph tumors, generated by subcutaneous injection of ACTH-secreting AtT20, PRL- and GH-secreting GH3, and LH-secreting LbetaT2, and alpha-T3 cells, was markedly suppressed in rosiglitazone treated mice, and plasma ACTH, and serum corticosterone, GH, PRL and LH levels were attenuated in all treated animals. PPAR-gamma is an important novel molecular target in pituitary adenoma cells and as PPAR-gamma ligands inhibit tumor cell growth and ACTH, GH, PRL and LH secretion in vitro and in vivo, thiazolidinediones are proposed as a novel oral medical management for pituitary tumors.

Identification of a functional peroxisome proliferator-activated receptor response element in the rat catalase promoter

Peroxisomal proliferator-activated receptor (PPAR)gamma has been shown to decrease the inflammatory response via transrepression of proinflammatory transcription factors. However, the identity of PPARgamma responsive genes that decrease the inflammatory response has remained elusive. Because generation of the reactive oxygen species hydrogen peroxide (H(2)O(2)) plays a role in the inflammatory process and activation of proinflammatory transcription factors, we wanted to determine whether the antioxidant enzyme catalase might be a PPARgamma target gene. We identified a putative PPAR response element (PPRE) containing the canonical direct repeat 1 motif, AGGTGA-A-AGTTGA, in the rat catalase promoter. In vitro translated PPARgamma and retinoic X receptor-alpha proteins were able to bind to the catalase PPRE. Promoter deletion analysis revealed that the PPRE was functional, and a heterologous promoter construct containing a multimerized catalase PPRE demonstrated that the PPRE was necessary and sufficient for PPARgamma-mediated activation. Treatment of microvascular endothelial cells with PPARgamma ligands led to increases in catalase mRNA and activity. These results demonstrate that PPARgamma can alter catalase expression; this occurs via a PPRE in the rat catalase promoter. Thus, in addition to transrepression of proinflammatory transcription factors, PPARgamma may also be modulating catalase expression, and hence down-regulating the inflammatory response via scavenging of reactive oxygen species.

PPARalpha inhibits TGF-beta-induced beta5 integrin transcription in vascular smooth muscle cells by interacting with Smad4

Integrins play an important role in vascular smooth muscle cell (VSMC) migration, a crucial event in the development of restenosis and atherosclerosis. Transforming growth factor-beta (TGF-beta) is highly expressed in restenotic and atherosclerotic lesions, and known to induce integrin expression. Peroxisome proliferator-activated receptor alpha (PPARalpha), a member of the nuclear receptor superfamily, regulates gene expression in a variety of vascular cells. We investigated the effects of PPARalpha ligands on TGF-beta-induced beta3 and beta5 integrin expression and potential interaction between PPARalpha and TGF-beta signaling. PPARalpha ligands WY-14643 (100 micromol/L) and 5,8,11,14-eicosatetranoic acid (ETYA, 50 micromol/L) inhibited TGF-beta-induced beta5 integrin protein expression by 72+/-6.8% and 73+/-7.1%, respectively (both P<0.05). TGF-beta-stimulated beta3 integrin expression was not affected by PPARalpha ligands. Both PPARalpha ligands also suppressed TGF-beta-induced beta5 integrin mRNA levels. PPARalpha ligands inhibited TGF-beta-inducible transcription of beta5 integrin by an interaction with a TGF-beta response element between nucleotides -63 and -44, which contains a Sp1/Sp3 transcription factor binding site. Nuclear complexes binding to the TGF-beta response region contained Sp1/Sp3 and TGF-beta-regulated Smad 2, 3, and 4 transcription factors. TGF-beta-stimulated Sp1/Smad4 nuclear complex formation was inhibited by WY-14643 and ETYA with a parallel induction of PPARalpha/Smad4 interactions. However, in vitro pull-down experiments failed to demonstrate direct binding between PPARalpha/Smad4. Both PPARalpha ligands blocked PDGF-directed migration of TGF-beta-pretreated VSMCs, a process mediated, in part, by beta5 integrins. The present study demonstrates that PPARalpha activators inhibit TGF-beta-induced beta5 integrin transcription in VSMCs through a novel indirect interaction between ligand-activated PPARalpha and the TGF-beta-regulated Smad4 transcription factors. The full text of this article is available at http://www.circresaha.org.

UVB light suppresses nitric oxide production by murine keratinocytes and macrophages

Nitric oxide is an important mediator of excessive cell growth and inflammation associated with many epidermal proliferative disorders. It is a highly reactive oxidant generated in keratinocytes and macrophages via the inducible form of the enzyme nitric oxide synthase (NOS2). In the present studies, we examined the effects of ultraviolet light (UVB, 2.5-25mJ/cm(2)) on interferon-gamma (IFN-gamma)-induced expression of NOS2 in these cells. Transient transfection assays using wild-type and mutant NOS2 promoter/luciferase reporter constructs showed that DNA binding of the transcription factors Stat1 and NF-kappaB was essential for optimal expression of the NOS2 gene. Whereas NF-kappaB was constitutively expressed in both cell types, Stat1 phosphorylation and nuclear binding activity were dependent upon IFN-gamma. UVB light, which is used therapeutically to treat inflammatory dermatosis, was found to suppress IFN-gamma-induced expression of NOS2 mRNA and protein, and nitric oxide production in both keratinocytes and macrophages. In macrophages, this was associated with complete inhibition of NF-kappaB nuclear binding activity and partial (approximately 20-25%) reduction of Stat1 activity. In keratinocytes, both responses were partially reduced at the highest doses of UVB light (15-25mJ/cm(2)). Whereas in macrophages UVB light suppressed NOS2 wild-type promoter-luciferase reporter activity, this activity was stimulated in keratinocytes. These data suggest that UVB light functions to suppress NOS2 gene expression in macrophages by inhibiting the activity of key regulatory transcription factors. In contrast, in keratinocytes, inhibition occurs downstream of NOS2 promoter activity.

The role of transcription factors in allergic inflammation

The induction of allergic inflammation and the expression of allergic disorders are dependent on the coordinated regulation of numerous genes. The products of these genes determine lymphocyte phenotype, immunologic responsiveness, eosinophil and mast cell development, activation, migration and life span, adhesion molecule expression, cytokine synthesis, cell-surface receptor display, and processes governing fibrosis and tissue repair. Although the expression of gene products involved in these processes is regulated at multiple levels (eg, transcription, mRNA processing, translation, phosphorylation, and degradation), transcription represents an essential and often the most important determinant of their contribution to cellular function. Signal-dependent and cell type-specific regulation of gene expression is generally achieved by means of combinatorial interactions between sequence-specific transcription factors that recruit chromatin remodeling machinery and general transcription factors to promoter and enhancer regions of RNA polymerase II-dependent genes. As targets of signal-transduction pathways, transcription factors integrate the response of the cell to the myriad of inputs it receives. This integration can be accomplished by the effect of signaling cascades on the activation status or subcellular locus of transcription factors or by transcription factor dimerization induced by means of ligand binding. This review will identify the major families of transcription factors important in allergic mechanisms and discuss their interactions, their mechanisms of action, and their interrelated and competitive actions, as well as implications for therapy of allergic disorders.

Signal transducers and activators of transcription 3 (STAT3) inhibits transcription of the inducible nitric oxide synthase gene by interacting with nuclear factor kappaB

Prolific generation of NO by inducible nitric oxide synthase (iNOS) can cause unintended injury to host cells during glomerulonephritis and other inflammatory diseases. While much is known about the mechanisms of iNOS induction, few transcriptional repressors have been found. We explored the role of signal transducers and activators of transcription 3 (STAT3) proteins in interleukin (IL)-1beta- and lipopolysaccharide (LPS)+interferon (IFN)-gamma-mediated iNOS induction in murine mesangial cells. Both stimuli induced rapid phosphorylation of STAT3 and sequence-specific STAT3 DNA-binding activity. Supershift assays with a STAT3 element probe demonstrated that nuclear factor kappaB (NF-kappaB) p65 and p50 complexed with STAT3 in the DNA-protein complex. The direct interaction of STAT3 and NF-kappaB p65 was verified in vivo by co-immunoprecipitation and in vitro by pull-down assays with glutathione S-transferase-NF-kappaB p65 fusion protein and in vitro -translated STAT3alpha. Overexpression of STAT3 dramatically inhibited IL-1beta- or LPS+IFN-gamma-mediated induction of iNOS promoter-luciferase constructs that contained the wild-type iNOS promoter or ones harbouring mutated STAT-binding elements. In tests of indirect inhibitory effects of STAT3, overexpression of STAT3 dramatically inhibited the activity of an NF-kappaB-dependent promoter devoid of STAT-binding elements without affecting NF-kappaB DNA-binding activity. Thus STAT3, via direct interactions with NF-kappaB p65, serves as a dominant-negative inhibitor of NF-kappaB activity to suppress indirectly cytokine induction of the iNOS promoter in mesangial cells. These results provide a new model for the termination of NO production by activated iNOS following exposure to pro-inflammatory stimuli.

Peroxisome proliferator-activated receptor-gamma in macrophage lipid homeostasis

Peroxisome proliferator-activated receptor-gamma (PPARgamma), a fatty acid receptor, has received particular attention as the molecular target of insulin-sensitizing drugs, and as a regulator of lipid accumulation by the coronary artery macrophages known as foam cells. Controversial results have been reported regarding the consequences of PPARgamma activation in the inflammatory response, the progression or improvement of the atherosclerotic lesion, and the identity of target tissues (muscle or fat) for PPARgamma-specific antidiabetic drugs. A clear understanding of how PPARgamma functions in each of these processes is therefore necessary to advance its utility as a therapeutic target. Receptor-dependent and -independent actions of PPARgamma agonists have been carefully examined with a combination of Pparg-knockout mice, PPARgamma-null embryonic stem cells, PPARgamma-specific drugs, and mouse models of atherosclerosis. Through those combined studies, a physiological and therapeutic role for PPARgamma in lipid management by the macrophage has emerged.

Liver X receptors: Xcreting Xol to combat atherosclerosis

Liver X receptors (LXRs) are nuclear receptors that act as metabolic sensors for cellular cholesterol (Xol) and oxysterol content. Increased oxysterol levels activate LXRs, which then induce: the removal of cholesterol out of peripheral cells; transport of this cholesterol to the liver; excretion of cholesterol through production of bile acids; and inhibition of intestinal cholesterol absorption. Recent evidence indicates that LXRs are not only master regulators of cholesterol homeostasis, but also decrease the development of atherosclerosis - a disease intimately linked with abnormal cholesterol homeostasis. This evidence shows that LXRs are promising drug development targets for atherosclerosis.

Emerging roles of PPARs in inflammation and immunity

Lipids and lipid metabolism have well-documented regulatory effects on inflammatory processes. Recent work has highlighted the role of the peroxisome proliferator-activated receptors (PPARs)--a subset of the nuclear-hormone-receptor superfamily that are activated by various lipid species--in regulating inflammatory responses. Here, we describe how the PPARs, through their interactions with transcription factors and other cell-signalling systems, have important regulatory roles in innate and adaptive immunity.

PPAR agonists amplify iNOS expression while inhibiting NF-kappaB: implications for mesangial cell activation by cytokines

In acute inflammation, the transcription factor NF-kappaB is activated and increases the expression of multiple pro-inflammatory genes. Agonists of peroxisome proliferator activated receptors (PPAR) have been reported to exert antiinflammatory effects in various systems. In keeping with such an antiinflammatory role, it was found that several PPAR agonists, including Wy14,643, clofibrate, carbaprostacyclin, and ciglitazone inhibited NF-kappaB activity and increased IkappaBalpha levels in cytokine-stimulated mesangial cells (MC). Activation of NF-kappaB has been found to be crucial to the cytokine-elicited expression of inducible nitric oxide synthase (iNOS). Despite the inhibitory effect of PPAR agonists on NF-kappaB activity, this study provides experimental data demonstrating that these agonists amplify cytokine-elicited NO generation in MC, potentiating iNOS protein expression approximately threefold. The upregulation of iNOS expression occurred at the mRNA level and apparently did not result from iNOS mRNA stabilization. Clofibrate and ciglitazone amplified the cytokine-elicited stimulation of a 16-Kb human iNOS promoter construct in stably transfected MC, suggesting that PPAR agonists potentiate iNOS induction through transcriptional mechanisms. MC express all three PPAR proteins. However, iNOS potentiation did not correlate with increased PPAR activity. In addition, Wy14,643-induced amplification of cytokine-elicited iNOS levels also occurred in RAW264.7 macrophages and in human epithelial Caco-2 and HT-29 cells. The observation that these epithelial cell lines express an inactive, truncated PPARalpha variant suggests that a classical PPARalpha agonist, such as Wy14,643, may act through PPARalpha-independent mechanisms. In conclusion, these results show that, despite reducing NF-kappaB activity, PPAR agonists may amplify the expression of certain NF-kappaB-dependent genes that are relevant to the inflammatory process, like iNOS.

Activation of peroxisome proliferator-activated receptor gamma by nitric oxide in monocytes/macrophages down-regulates p47phox and attenuates the respiratory burst

NO appears as an important determinant in auto and paracrine macrophage function. We hypothesized that NO switches monocyte/macrophage function from a pro- to an anti-inflammatory phenotype by activating anti-inflammatory properties of the peroxisome proliferator-activated receptor (PPAR)gamma. NO-releasing compounds (100 micro M S-nitrosoglutathione or 50 micro M spermine-NONOate) as well as inducible NO synthase induction provoked activation of PPARgamma. This was proven by EMSAs, with the notion that supershift analysis pointed to the involvement of PPARgamma. PCR analysis ruled out induction of PPARgamma mRNA as a result of NO supplementation. Reporter assays, with a construct containing a triple PPAR response element in front of a thymidine kinase minimal promoter driving the luciferase gene, were positive in response to NO delivery. DNA binding capacity as well as the transactivating capability of PPARgamma were attenuated by addition of the antioxidant N-acetyl-cysteine or in the presence of the NO scavenger 2-phenyl-4,4,5,6-tetramethyl-imidazoline-1-oxyl 3-oxide. Having established that NO but not lipophilic cyclic GMP analogs activated PPARgamma, we verified potential anti-inflammatory consequences. The oxidative burst of macrophages, evoked by phorbol ester, was attenuated in association with NO-elicited PPARgamma activation. A cause-effect relationship was demonstrated when PPAR response element decoy oligonucleotides, supplied in front of NO delivery, allowed to regain an oxidative response. PPARgamma-mediated down-regulation of p47 phagocyte oxidase, a component of the NAD(P)H oxidase system, was identified as one molecular mechanism causing inhibition of superoxide radical formation. We conclude that NO participates in controlling the pro- vs anti-inflammatory phenotype of macrophages by modulating PPARgamma.

Protective action of the peroxisome proliferator-activated receptor-gamma agonist pioglitazone in a mouse model of Parkinson's disease

We examined the effect of pioglitazone, a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist of the thiazolidinedione class, on dopaminergic nerve cell death and glial activation in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. The acute intoxication of C57BL/6 mice with MPTP led to nigrostriatal injury, as determined by tyrosine hydroxylase (TH) immunocytochemistry, and HPLC detection of striatal dopamine and metabolites. Damage to the nigrostriatal dopamine system was accompanied by a transient activation of microglia, as determined by macrophage antigen-1 (Mac-1) and inducible nitric oxide synthase (iNOS) immunoreactivity, and a prolonged astrocytic response. Orally administered pioglitazone (approximately 20 mg/kg/day) attenuated the MPTP-induced glial activation and prevented the dopaminergic cell loss in the substantia nigra pars compacta (SNpc). In contrast, there was little reduction of MPTP-induced dopamine depletion, with no detectable effect on loss of TH immunoreactivity and glial response in the striatum of pioglitazone-treated animals. Low levels of PPARgamma expression were detected in the ventral mesencephalon and striatum, and were unaffected by MPTP or pioglitazone treatment. Since pioglitazone affects primarily the SNpc in our model, different PPARgamma-independent mechanisms may regulate glial activation in the dopaminergic terminals compared with the dopaminergic cell bodies after acute MPTP intoxication.

Effects of peroxisome proliferator-activated receptor agonists on LPS-induced neuronal death in mixed cortical neurons: associated with iNOS and COX-2

In neurodegenerative disease, the use of non-steroidal anti-inflammatory drugs (NSAIDs) has been regarded as beneficial. The NSAID, an inhibitor of cyclooxygenase (COX), has been also suggested as a ligand of the peroxisome proliferator-activated receptor (PPAR). In cortical neuron-glial co-cultures, we examined the effect of PPAR agonists on lipopolysaccharide(LPS)-induced neuronal death, which has been known to be NO-dependent. LPS induced iNOS expression and the release of nitric oxide in microglia, and COX-2 expression in neurons. PPAR-gamma agonists such as 15d-PGJ(2), ciglitazone and troglitazone prevented LPS-induced neuronal death and abolished LPS-induced NO and PGE(2) release, however PPAR-alpha agonists such as clofibrate and WY14,643 did not produce the same results. PPAR-gamma agonists also reduced LPS-induced iNOS and COX-2 expression, which suggested by interfering with the NF-kappaB signal pathway.

Roles of peroxisome proliferator-activated receptor gamma in lipid homeostasis and inflammatory responses of macrophages

Monocytes play a critical role in atherogenesis by their inflammatory signals and differentiation into macrophage foam cells through cholesterol accumulation. The seminal finding of high levels of the peroxisome proliferator-activated receptor gamma in macrophage foam cells has opened up the prospect that its ligands, most importantly the thiazolidinedione class of drugs, might directly influence the development of atheromatous lesions. The present review weighs the growing evidence on regulation of both inflammatory responses and cholesterol homeostasis in macrophages by peroxisome proliferator-activated receptor gamma ligands with regard to their overall impact as antiatherogenic agents.

Mechanisms regulating adipocyte expression of resistin

Resistin, also known as Adipocyte Secreted Factor (ADSF) and Found in Inflammatory Zone 3 (FIZZ3), is a mouse protein with potential roles in insulin resistance and adipocyte differentiation. The resistin gene is expressed almost exclusively in adipocytes. Here we show that a proximal 264-base pair fragment of the mouse resistin promoter is sufficient for expression in adipocytes. Ectopic expression of the adipogenic transcription factor CCAAT/enhancer-binding protein (C/EBPalpha) was sufficient for expression in non-adipogenic cells. C/EBPalpha binds specifically to a site that is essential for expression of the resistin promoter. Chromatin immunoprecipitation studies of the endogenous gene demonstrated adipocyte-specific association of C/EBPalpha with the proximal resistin promoter in adipocytes but not preadipocytes. C/EBPalpha binding was associated with the recruitment of coactivators p300 and CREB-binding protein and a dramatic increase in histone acetylation in the vicinity of the resistin promoter. The antidiabetic thiazolidinedione (TZD) drug rosiglitazone reduced resistin expression with an ED(50) similar to its K(d) for binding to peroxisome proliferator activated receptor gamma (PPARgamma). Other TZD- and non-TZD PPARgamma ligands also down-regulated resistin expression. However, no functional PPARgamma binding site was found within 6.2 kb of the transcriptional start site, suggesting that if PPARgamma is involved, it is either acting at a long distance from the start site, in an intron, or indirectly. Nevertheless, rosiglitazone treatment selectively decreased histone acetylation at the resistin promoter without a change in occupation by C/EBPalpha, CREB-binding protein, or p300. Thus, adipocyte specificity of resistin gene expression is because of C/EBPalpha binding, leading to the recruitment of transcriptional coactivators and histone acetylation that is characteristic of an active chromatin environment. TZD reduces resistin gene expression at least in part by reducing histone acetylation associated with the binding of C/EBPalpha in mature adipocytes.

Differential gene regulation by PPARgamma agonist and constitutively active PPARgamma2

The PPARgamma is a key adipogenic determination factor. Ligands for PPARgamma such as antidiabetic thiazolidinedione (TZD) compounds are adipogenic, and many adipocyte genes that are activated by TZDs contain binding sites for PPARgamma. Like ligands for other nuclear receptors, TZDs can regulate genes positively or negatively. Here, we sought to understand the importance of positive regulation of gene expression by PPARgamma in adipogenesis. Fusion of the potent viral transcriptional activator VP16 to PPARgamma2 (VP16-PPARgamma) created a transcription factor that constitutively and dramatically activated transcription of PPARgamma-responsive genes in the absence of ligand. Forced expression of VP16-PPARgamma in 3T3-L1 preadipocytes using retroviral vectors led to adipogenesis in the absence of standard differentiating medium or any exogenous PPARgamma ligand. Gene microarray analysis revealed that VP16-PPARgamma induced many of the genes associated with adipogenesis and adipocyte function. Thus, direct up-regulation of gene expression by PPARgamma is sufficient for adipogenesis. TZD-induced adipogenesis up-regulated many of the same genes, although some were divergently regulated, including resistin, whose gene expression was reduced inVP16-PPARgamma adipocytes treated with TZDs. These results show that, although activation of PPARgamma by a heterologous activation domain is sufficient for adipogenesis, it is not equivalent to TZD treatment. This conclusion has important implications for understanding biological effects of the TZDs on adipogenesis and insulin sensitization.

Conjugated linoleic acid decreases production of pro-inflammatory products in macrophages: evidence for a PPAR gamma-dependent mechanism

Conjugated linoleic acid (CLA) is a dietary fatty acid that has received considerable attention due to its unique properties in rodent models including anti-cancer, anti-atherogenic and anti-diabetic effects. The effects of CLA are similar to those seen with ligands for peroxisome proliferator-activated receptor (PPARs), most notably of the PPAR gamma subtype. With the recent observation of a role for PPAR gamma in regulation of immune responses, we suspected that CLA could affect immune function, in particular macrophage activity. The goal of our study was to examine whether this dietary fatty acid has anti-inflammatory properties similar to those reported for PPAR gamma activators such as 15-deoxy prostaglandin J(2) (PGJ(2)). In reporter assays, various CLA isomers activated PPAR gamma in RAW264.7 mouse macrophage (RAW) cells. CLA decreased the interferon-gamma (IFN gamma)-induced mRNA expression of mediators of inflammation including cyclooxygenase 2 (COX2), inducible NOS (iNOS), and tumor necrosis factor alpha (TNFalpha). Reporter assays also demonstrated reduced IFN gamma-stimulated transcriptional activity of the iNOS and COX2 promoters by CLA. Consequently, CLA decreased the production of PGE(2), TNFalpha and the inflammatory agent nitric oxide (NO) in RAW cells treated with IFN gamma. Other pro-inflammatory cytokines such as IL-1 beta and IL-6 were similarly decreased by CLA treatment of RAW cells. In addition, various CLA isomers induced HL60 cell differentiation along the monocytic lineage as assessed by measuring expression of the cell surface marker CD14. This differentiation process, as well as the regulation of iNOS and COX2 by 15dPGJ(2), is believed to involve PPAR gamma. Mutations of Leu(468) and Glu(471) to alanine in helix 12 of the ligand-binding domain of PPAR gamma resulted in a protein with strong dominant-negative activity (dnPPAR gamma). Transfecting dnPPAR gamma into RAW cells eliminated the ability of various CLA isomers to regulate the iNOS reporter construct. Taken together, these results suggest that CLA has anti-inflammatory properties that are mediated, at least in part, by the nuclear hormone receptor PPAR gamma.

Transcription suppression of thromboxane receptor gene by peroxisome proliferator-activated receptor-gamma via an interaction with Sp1 in vascular smooth muscle cells

Thromboxane (TX) A(2) exerts contraction and proliferation of vascular smooth muscle cells (VSMCs) via its specific membrane TX receptor (TXR), possibly leading to the progression of atherosclerosis. A nuclear hormone receptor, peroxisome proliferator-activated receptor (PPAR)-gamma, has recently been reported to be expressed in VSMCs. Here we examined a role of PPAR-gamma in TXR gene expression in VSMCs. PPAR-gamma ligands 15-deoxy-Delta(12,14)-prostaglandin J(2) and troglitazone reduced TXR mRNA expression levels as well as cell growth as assessed by [(3)H]thymidine incorporation. Transcriptional activity of the TXR gene promoter was suppressed with PPAR-gamma ligands, and the suppression was augmented further by PPAR-gamma overexpression. By deletion and mutation analyses, the transcription suppression was shown to be the result of a -22/-7 GC box-related sequence (upstream of transcription start site). Electrophoretic mobility shift assays also showed that the sequence was bound by Sp1 but not by PPAR-gamma, and the formation of a Sp1 small middle dotDNA complex was inhibited either by coincubation with PPAR-gamma or PPAR-gamma ligand treatment of VSMCs. Moreover, glutathione S-transferase pull-down assays demonstrated a direct interaction between PPAR-gamma and Sp1. In conclusion, PPAR-gamma suppresses TXR gene transcription via an interaction with Sp1. PPAR-gamma may possibly have an antiatherosclerotic action by inhibiting TXR gene expression in VSMCs.

Nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha) is expressed in resting murine lymphocytes. The PPARalpha in T and B lymphocytes is both transactivation and transrepression competent

Peroxisome proliferator-activated receptors (PPARs) are transcription factors that belong to the nuclear hormone receptor superfamily. PPARalpha and PPARgamma ligands have been demonstrated to exert anti-inflammatory activities in macrophages by repressing the activities of several transcription factors. PPARgamma is expressed in T lymphocytes and may play a role in cytokine production, cellular proliferation, and susceptibility to apoptosis. Herein, we demonstrate that T and B lymphocytes constitutively express PPARalpha. PPARalpha represents the predominant isoform expressed in lymphocytes, whereas PPARgamma dominates in all cell types of the myeloid lineage. PPARalpha expression was down-regulated following T-cell activation while PPARgamma expression increased under the same activating conditions. PPARalpha expression in T cells may be regulated by microenvironmental factors, because Peyer's patch T cells expressed far greater levels of PPARalpha than T cells isolated from peripheral lymphoid organs. Exposure to specific ligand determined that PPARalpha in lymphocytes can effectively transactivate a peroxisome proliferator response element reporter construct. PPARalpha's ability to regulate endogenous genes, however, required treatment with histone deacetylase inhibitors. Finally, ligand activation of lymphocyte PPARalpha antagonized NF-kappaB. Our observation that a functional PPARalpha exists within T cells and B lymphocytes suggests an expanding role for this nuclear receptor in cells of the immune system.

Alpha1-antichymotrypsin/Alzheimer's peptide Abeta(1-42) complex perturbs lipid metabolism and activates transcription factors PPARgamma and NFkappaB in human neuroblastoma (Kelly) cells

Amyloid-beta peptide (Abeta) and the serpin proteinase inhibitor alpha1-antichymotrypsin (ACT) are components of the amyloid plaques associated with Alzheimer's disease (AD). Abeta exists in soluble monomeric and oligomeric forms and in an insoluble polymerised fibrillar form, but it is not clear which of these plays the most important role in the etiology of AD. In vitro, Abeta(1-42) interacts with ACT, and as a result of this, ACT loses its proteinase inhibitor activity and polymerisation of Abeta(1-42) is promoted. Here we provide evidence that new molecular forms resulting from incubation of ACT with Abeta(1-42) have multiple cellular level effects on neuronal cells. The mixture of soluble Abeta and an ACT/Abeta complex formed by 2 hr incubation at a 10:1 molar ratio of Abeta:ACT strongly induce cellular proliferation and expression of transcription factors peroxisome proliferator-activated receptor-gamma (PPARgamma) and NFkappaB, and also increase uptake and depress degradation of native and oxidised low-density lipoprotein (LDL) by cells. Similar but less pronounced effects are seen when cells are exposed to the Abeta peptide alone preincubated for 2 hr. Abeta(1-42) and to a lesser extent ACT/Abeta(1-42) complex mixture prepared by 2 hr incubation both inhibit association of native LDL with cells. Neither ACT alone nor the Abeta(1-42) and ACT/Abeta(1-42) forms prepared by 24-hr incubation show any significant effects in these assays. We propose that specific molecular forms of Abeta(1-42) and ACT/Abeta(1-42) complex mixture, both dependent on the abundances of Abeta(1-42) and ACT/Abeta(1-42) in vivo and on their time of exposure to each other, have cellular effects which are important for the initiation and progression of the pathologies associated with AD.

Repression of glucagon gene transcription by peroxisome proliferator-activated receptor gamma through inhibition of Pax6 transcriptional activity

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is involved in glucose homeostasis and synthetic PPARgamma ligands, the thiazolidinediones, a new class of antidiabetic agents that reduce insulin resistance and, as a secondary effect, reduce hepatic glucose output. PPARgamma is highly expressed in normal human pancreatic islet alpha-cells that produce glucagon. This peptide hormone is a functional antagonist of insulin stimulating hepatic glucose output. Therefore, the effect of PPARgamma and thiazolidinediones on glucagon gene transcription was investigated. After transient transfection of a glucagon-reporter fusion gene into a glucagon-producing pancreatic islet cell line, thiazolidinediones inhibited glucagon gene transcription when PPARgamma was coexpressed. They also reduced glucagon secretion and glucagon tissue levels in primary pancreatic islets. A 5'/3'-deletion and internal mutation analysis indicated that a pancreatic islet cell-specific enhancer sequence (PISCES) motif within the proximal glucagon promoter element G1 was required for PPARgamma responsiveness. This sequence motif binds the paired domain transcription factor Pax6. When the PISCES motif within G1 was mutated into a GAL4 binding site, the expression of GAL4-Pax6 restored glucagon promoter activity and PPARgamma responsiveness. GAL4-Pax6 transcriptional activity was inhibited by PPARgamma in response to thiazolidinedione treatment also at a minimal viral promoter. These results suggest that PPARgamma in a ligand-dependent but DNA binding-independent manner inhibits Pax6 transcriptional activity, resulting in inhibition of glucagon gene transcription. These data thereby define Pax6 as a novel functional target of PPARgamma and suggest that inhibition of glucagon gene expression may be among the multiple mechanisms through which thiazolidinediones improve glycemic control in diabetic subjects.

Peroxisome proliferator-activated receptor-gamma ligands inhibit nuclear but not cytosolic extracellular signal-regulated kinase/mitogen-activated protein kinase-regulated steps in vascular smooth muscle cell migration

Vascular smooth muscle cell (VSMC) migration involves adhesion, locomotion, and invasion regulated by various signaling molecules, among which the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinases (MAPK) play a critical role. We have shown that the peroxisome proliferator-activated receptor-gamma (PPAR-gamma) ligands troglitazone and rosiglitazone inhibit VSMC migration downstream of ERK MAPK. The purpose of the current study was to more specifically determine which step(s) in VSMC migration are targeted by inhibition of the ERK MAPK pathway or activation of PPAR-gamma. VSMC adhesion was not affected by the ERK MAPK pathway inhibitor PD98059 or PPAR-gamma ligands. Phosphorylation and activation of myosin light chain kinase (MLCK) play important roles in cell locomotion. Platelet-derived growth factor (PDGF)-induced MLCK phosphorylation (1.7-fold) was completely blocked by PD98059 at 30 microM (p < 0.05), but not by troglitazone or rosiglitazone. PDGF-directed migration (5.8-fold) was inhibited by PD98059 (-88% at 30 microM) and the MLCK inhibitor ML9 (0.1-1 microM, -84% at 1 microM) (all p < 0.05). The transcription factor Ets-1 mediates matrix metalloproteinase induction required for tissue invasion by VSMC. PDGF (20 ng/ml) stimulated an Ets-1 protein expression (14-fold at 60 min) in VSMC, which was inhibited by PD98059 (-72% at 30 microM), troglitazone (-69% at 20 microM), and rosiglitazone (-54% at 10 microM) (all p < 0.05). Immunohistochemistry of rat aortae 2 h after balloon injury showed a dramatic upregulation of Ets-1, which was markedly inhibited in animals that had received troglitazone treatment. In contrast, phosphorylated ERK MAPK was not affected by troglitazone. These data are consistent with PPAR-gamma ligands exerting their anti-migratory effects downstream of ERK MAPK activation by blocking nuclear events, such as Ets-1 expression, required for cell invasion in response to arterial injury.

Peroxisome proliferator-activated receptor gamma ligands differentially modulate muscle cell differentiation and MyoD gene expression via peroxisome proliferator-activated receptor gamma -dependent and -independent pathways

The effects of distinct classes of peroxisome proliferator-activated receptor gamma (PPARgamma) ligands on myogenesis and MyoD gene expression were examined in mouse skeletal muscle C2C12 myoblasts. Treatment of C2C12 cells with the PPARgamma ligand, 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2), repressed morphologically defined myogenesis and reduced endogenous mRNA levels of the myogenic differentiation markers MyoD, myogenin, and alpha-actin. In contrast, two synthetic PPARgamma ligands, L-805645 and ciglitazone, exhibited no effects. In transient transfection assays, 15d-PGJ2 specifically inhibited the expression of a MyoD promoter-luciferase reporter gene (MyoDLuc) in a cell type- and promoter-specific manner, indicating that 15d-PGJ2 functions in part by repressing MyoD gene transcription. The inhibition of MyoD gene expression by 15d-PGJ2 is mediated by the distal region of the MyoD gene promoter. PPARgamma on its own also inhibited MyoDLuc expression and further augmented the 15d-PGJ2 response. In contrast, L-805645 and ciglitazone did not inhibit MyoDLuc expression on their own but did so in the presence of ectopically expressed PPARgamma. Interestingly, a transdominant inhibitor of PPARgamma (hPPARgamma2Delta500) had no effect on the 15d-PGJ2-dependent repression of MyoDLuc expression but overcame L-805645/PPARgamma-dependent repression. Finally, saturating concentrations of L-805645, which did not affect myogenesis, failed to ablate 15d-PGJ2-mediated repression of the myogenic program. Thus, distinct PPARgamma ligands may repress MyoD gene expression through PPARgamma-dependent and -independent pathways, and 15d-PGJ2 can inhibit the myogenic program independent of its cognate receptor, PPARgamma.

Peroxisome proliferator-activated receptors in macrophage biology: friend or foe?

Peroxisome proliferator-activated receptor (PPAR)-gamma is a nuclear hormone receptor, with a well-established role in adipogenesis and glucose metabolism. Over the past 3 years several laboratories have reported that this protein can influence macrophage responses to a variety of inflammatory stimuli. The effect of PPAR-gamma activation on macrophage lipid uptake, cholesterol efflux, and cytokine production have all recently been examined in several in-vitro culture systems. In addition, PPAR-gamma ligands have been shown to influence atherosclerotic lesion formation in murine models of that disease. This review attempts to summarize and critically evaluate that work and its implications for the use of PPAR-gamma activators in understanding and treating the pathogenetic processes that contribute to atherosclerotic plaque formation.

Targeted disruption of inducible nitric oxide synthase protects against obesity-linked insulin resistance in muscle

Inducible nitric oxide synthase (iNOS) is induced by inflammatory cytokines in skeletal muscle and fat. It has been proposed that chronic iNOS induction may cause muscle insulin resistance. Here we show that iNOS expression is increased in muscle and fat of genetic and dietary models of obesity. Moreover, mice in which the gene encoding iNOS was disrupted (Nos2-/- mice) are protected from high-fat-induced insulin resistance. Whereas both wild-type and Nos2-/- mice developed obesity on the high-fat diet, obese Nos2-/- mice exhibited improved glucose tolerance, normal insulin sensitivity in vivo and normal insulin-stimulated glucose uptake in muscles. iNOS induction in obese wild-type mice was associated with impairments in phosphatidylinositol 3-kinase and Akt activation by insulin in muscle. These defects were fully prevented in obese Nos2-/- mice. These findings provide genetic evidence that iNOS is involved in the development of muscle insulin resistance in diet-induced obesity.

15-Deoxy-Delta 12,14-prostaglandin J2 inhibition of NF-kappaB-DNA binding through covalent modification of the p50 subunit

Cyclopentenone prostaglandins display anti-inflammatory activities and interfere with the signaling pathway that leads to activation of transcription factor NF-kappaB. Here we explore the possibility that the NF-kappaB subunit p50 may be a target for the cyclopentenone 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)). This prostaglandin inhibited the DNA binding ability of recombinant p50 in a dose-dependent manner. The inhibition required the cyclopentenone moiety and could be prevented but not reverted by glutathione and dithiothreitol. Moreover, a p50 mutant with a C62S mutation was resistant to inhibition, indicating that the effect of 15d-PGJ(2) was probably due to its interaction with cysteine 62 in p50. The covalent modification of p50 by 15d-PGJ(2) was demonstrated by reverse-phase high pressure liquid chromatography and mass spectrometry analysis that showed an increase in retention time and in the molecular mass of 15d-PGJ(2)-treated p50, respectively. The interaction between p50 and 15d-PGJ(2) was relevant in intact cells. 15d-PGJ(2) effectively inhibited cytokine-elicited NF-kappaB activity in HeLa without reducing IkappaBalpha degradation or nuclear translocation of NF-kappaB subunits. 15d-PGJ(2) reduced NF-kappaB DNA binding activity in isolated nuclear extracts, suggesting a direct effect on NF-kappaB proteins. Finally, treatment of HeLa with biotinylated-15d-PGJ(2) resulted in the formation of a 15d-PGJ(2)-p50 adduct as demonstrated by neutravidin binding and immunoprecipitation. These results clearly show that p50 is a target for covalent modification by 15d-PGJ(2) that results in inhibition of DNA binding.

Peroxisome proliferator-activated receptor-gamma-independent inhibition of macrophage activation by the non-thiazolidinedione agonist L-796,449. Comparison with the effects of 15-deoxy-delta(12,14)-prostaglandin J(2)

The effects of L-796,449 (3-chloro-4-(3-(3-phenyl-7-propylbenzofuran-6-yloxy)propylthio)phenylacetic acid; referred to henceforth as compound G), a thiazolidinedione-unrelated peroxisome proliferator activated-receptor-gamma (PPAR-gamma) agonist, on early signaling in lipopolysaccharide-activated RAW 264.7 macrophages were analyzed and compared with those elicited by 15-deoxy-Delta(12,14)-prostaglandin J(2) and the thiazolidinedione rosiglitazone. Compound G inhibited the activation of nuclear factor kappa B through the impairment of the targeting and degradation of I kappa B proteins and promoted a redistribution of I kappa B alpha and I kappa B beta in the nucleus of activated cells. Compound G inhibited I kappa B kinase (IKK) activity both in vivo and in vitro, suggesting a direct mechanism of interaction between this molecule and the IKK complex. The effect of compound G on IKK activity was independent of PPAR-gamma engagement because RAW 264.7 cells expressed negligible levels of this nuclear receptor, and rosiglitazone failed to mimic these actions. Moreover, treatment of activated macrophages with compound G enhanced the synthesis of superoxide anion, which, in combination with the NO produced under activation conditions, triggered apoptosis through the intracellular synthesis of peroxynitrite. These results suggest that compound G might contribute to the resolution of inflammation by favoring the induction of apoptosis through mechanisms independent of PPAR-gamma engagement.

Target genes of peroxisome proliferator-activated receptor gamma in colorectal cancer cells

Activation of the nuclear hormone peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits cell growth and promotes differentiation in a broad spectrum of epithelial derived tumor cell lines. Here we utilized microarray technology to identify PPARgamma gene targets in intestinal epithelial cells. For each gene, the induction or repression was seen with two structurally distinct PPARgamma agonists, and the change in expression could be blocked by co-treatment with a specific PPARgamma antagonist. A majority of the genes could be regulated independently by a retinoid X receptor specific agonist. Genes implicated in lipid transport or storage (adipophilin and liver fatty acid-binding protein) were also activated by agonists of PPAR subtypes alpha and/or delta. In contrast, PPARgamma-selective targets included genes linked to growth regulatory pathways (regenerating gene IA), colon epithelial cell maturation (GOB-4 and keratin 20), and immune modulation (neutrophil-gelatinase-associated lipocalin). Additionally, three different genes of the carcinoembryonic antigen family were induced by PPARgamma. Cultured cells treated with PPARgamma ligands demonstrated an increase in Ca(2+)-independent, carcinoembryonic antigen-dependent homotypic aggregation, suggesting a potential role for PPARgamma in regulating intercellular adhesion. Collectively, these results will help define the mechanisms by which PPARgamma regulates intestinal epithelial cell biology.

Maturity-onset diabetes of the young Type 1 (MODY1)-associated mutations R154X and E276Q in hepatocyte nuclear factor 4alpha (HNF4alpha) gene impair recruitment of p300, a key transcriptional co-activator

Hepatocyte nuclear factor 4alpha (HNF4alpha) is a nuclear receptor involved in glucose homeostasis and is required for normal beta-cell function. Mutations in the HNF4alpha gene are associated with maturity-onset diabetes of the young type 1. E276Q and R154X mutations were previously shown to impair intrinsic transcriptional activity (without exogenously supplied co-activators) of HNF4alpha. Given that transcriptional partners of HNF4alpha modulate its intrinsic transcriptional activity and play crucial roles in HNF4alpha function, we investigated the effects of these mutations on potentiation of HNF4alpha activity by p300, a key co-activator for HNF4alpha. We show here that loss of HNF4alpha function by both mutations is increased through impaired physical interaction and functional cooperation between HNF4alpha and p300. Impairment of p300-mediated potentiation of HNF4alpha transcriptional activity is of particular importance for the E276Q mutant since its intrinsic transcriptional activity is moderately affected. Together with previous results obtained with chicken ovalbumin upstream promoter-transcription factor II, our results highlight that impairment of recruitment of transcriptional partners represents an important mechanism leading to abnormal HNF4alpha function resulting from the MODY1 E276Q mutation. The impaired potentiations of HNF4alpha activity were observed on the promoter of HNF1alpha, a transcription factor involved in a transcriptional network and required for beta-cell function. Given its involvement in a regulatory signaling cascade, loss of HNF4alpha function may cause reduced beta-cell function secondary to defective HNF1alpha expression. Our results also shed light on a better structure-function relationship of HNF4alpha and on p300 sequences involved in the interaction with HNF4alpha.

Differential recruitment of the mammalian mediator subunit TRAP220 by estrogen receptors ERalpha and ERbeta

Estrogen receptors (ERs) associate with distinct transcriptional coactivators to mediate activation of target genes in response to estrogens. Previous work has provided multiple evidence for a critical role of p160 coactivators and associated histone acetyltransferases in estrogen signaling. In contrast, the involvement of the mammalian mediator complex remains to be established. Further, although the two subtypes ERalpha and ERbeta appear to be similar in regard to principles of LXXLL-mediated coactivator binding to the AF-2 activation domain, there are indications that the context-dependent transcriptional activation profiles of the two ERs can be quite distinct. Potentially, this could be attributed to differences with regard to coregulator recruitment. We have here studied the interactions of the nuclear receptor-binding subunit of the mammalian mediator complex, referred to as TRAP220, with ERalpha and ERbeta. In comparison to the p160 coactivator TIF2, we find that TRAP220 displays ERbeta preference. Here, we show that this is a feature of the binding specificity of the TRAP220 LXXLL motifs and demonstrate that the ER subtype-specific F-domain influences TRAP220 interaction. Such differences with regard to coactivator recruitment indicate that the relative importance of individual coregulators in estrogen signaling could depend on the dominant ER subtype.

Differential regulation of chemokine gene expression by 15-deoxy-delta 12,14 prostaglandin J2

Ligands for peroxisome proliferator-activated receptor gamma (PPARgamma), such as 15-deoxy-Delta(12,14)PGJ2 (15d-PGJ2) have been proposed as a new class of antiinflammatory compounds with possible clinical applications. As there is some controversy over the inhibitory effects of 15d-PGJ2 on chemokine gene expression, we investigated whether 15d-PGJ2 itself affected chemokine gene expression in human monocytes/macrophages and two monocytic cell lines. Here we demonstrate that the 15d-PGJ2 can induce IL-8 gene expression. In contrast, monocyte chemoattractant protein-1 gene expression was suppressed by 15d-PGJ2, while the expression of RANTES was unaltered. Furthermore, concomitant treatment of monocytes/macrophages with 15d-PGJ2 (2.5 x 10(-6) M) potentiated LPS-induced gene expression of IL-8 mRNA, but suppressed PMA-induction of IL-8 mRNA. In addition, treatment of U937 and THP-1 cells with 15d-PGJ2 also resulted in induction of IL-8 gene expression. Further studies demonstrated that 15d-PGJ2 regulated IL-8 gene expression via a ligand-specific and PPARgamma-dependent pathway. Our observations revealed a previous unappreciated function and mechanism of 15d-PGJ2-mediated regulation of cytokine gene expression in monocytes/macrophages.

Peroxisome proliferator-activated receptor-gamma regulates airway epithelial cell activation

The peroxisome proliferator-activated receptors (PPARs) are nuclear hormone transcription factors that regulate genes associated with lipid and glucose metabolism. Recent evidence suggests that PPAR-gamma may also act as a negative immunomodulator. To investigate the potential role of PPAR-gamma in regulating airway inflammation, we characterized the expression and function of PPAR-gamma in airway epithelial cells. Airway epithelial cells constitutively express PPAR-gamma-specific messenger RNA and protein. Further, airway epithelial PPAR-gamma is inducible by interleukin (IL)-4 in NIH-A549 cells. Two PPAR-gamma agonists, the prostaglandin D2 metabolite 15-deoxy-(Delta)(12,14) prostaglandin J2 (15d-PGJ2) and a thiazolidinedione, ciglitazone, were used to study the effects of PPAR-gamma activation on airway epithelial cytokine expression. Activation of PPAR-gamma stimulated a PPAR-responsive reporter gene in a ligand-specific manner. In NIH-A549 cells, both ligands also blocked the cytokine-induced expression of the inducible form of nitric oxide synthase in a dose-dependent manner. In contrast, ciglitazone alone had a slight effect on cytokine-induced IL-8 secretion, but markedly inhibited IL-8 secretion from cells pretreated with IL-4. The demonstration of PPAR-gamma expression and function in airway epithelial cells expands the immunoregulatory role of PPARs and suggests a critical role for PPAR-gamma in antagonizing proinflammatory pathways in the airways.

Cyclopentenone prostaglandins: new insights on biological activities and cellular targets

The cyclopentenone prostaglandins PGA2, PGA1, and PGJ2 are formed by dehydration within the cyclopentane ring of PGE2, PGE1, and PGD2. PGJ2 is metabolized further to yield Delta(12)-PGJ(2) and 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)). Various compounds within the cyclopentenone prostaglandin family possess potent anti-inflammatory, anti-neoplastic, and anti-viral activity. Most actions of the cyclopentenone prostaglandins do not appear to be mediated by binding to G-protein coupled prostanoid receptors. Rather, the bioactivity of these compounds results from their interaction with other cellular target proteins. 15-deoxy-Delta(12,14)-PGJ(2) is a high affinity ligand for the nuclear receptor PPARgamma and modulates gene transcription by binding to this receptor. Other activities of the cyclopentenone prostaglandins are mediated by the reactive alpha,beta-unsaturated carbonyl group located in the cyclopentenone ring. The transcription factor NF-kappaB and its activating kinase are key targets for the anti-inflammatory activity of 15d-PGJ2, which inhibits NF-kappaB-mediated transcriptional activation by PPARgamma-dependent and independent molecular mechanisms. Other cyclopentenone prostaglandins, such as Delta(7)-PGA1 and Delta(12)-PGJ2, have strong anti-tumor activity. These compounds induce cell cycle arrest or apoptosis of tumor cells depending on the cell type and treatment conditions. We review here recent progress in understanding the mechanisms of action of the cyclopentenone prostaglandins and their possible use as therapeutic agents.

Inhibition of cellular proliferation through IkappaB kinase-independent and peroxisome proliferator-activated receptor gamma-dependent repression of cyclin D1

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-regulated nuclear receptor superfamily member. Liganded PPARgamma exerts diverse biological effects, promoting adipocyte differentiation, inhibiting tumor cellular proliferation, and regulating monocyte/macrophage and anti-inflammatory activities in vitro. In vivo studies with PPARgamma ligands showed enhancement of tumor growth, raising the possibility that reduced immune function and tumor surveillance may outweigh the direct inhibitory effects of PPARgamma ligands on cellular proliferation. Recent findings that PPARgamma ligands convey PPARgamma-independent activities through IkappaB kinase (IKK) raises important questions about the specific mechanisms through which PPARgamma ligands inhibit cellular proliferation. We investigated the mechanisms regulating the antiproliferative effect of PPARgamma. Herein PPARgamma, liganded by either natural (15d-PGJ(2) and PGD(2)) or synthetic ligands (BRL49653 and troglitazone), selectively inhibited expression of the cyclin D1 gene. The inhibition of S-phase entry and activity of the cyclin D1-dependent serine-threonine kinase (Cdk) by 15d-PGJ(2) was not observed in PPARgamma-deficient cells. Cyclin D1 overexpression reversed the S-phase inhibition by 15d-PGJ(2). Cyclin D1 repression was independent of IKK, as prostaglandins (PGs) which bound PPARgamma but lacked the IKK interactive cyclopentone ring carbonyl group repressed cyclin D1. Cyclin D1 repression by PPARgamma involved competition for limiting abundance of p300, directed through a c-Fos binding site of the cyclin D1 promoter. 15d-PGJ(2) enhanced recruitment of p300 to PPARgamma but reduced binding to c-Fos. The identification of distinct pathways through which eicosanoids regulate anti-inflammatory and antiproliferative effects may improve the utility of COX2 inhibitors.

Peroxisome proliferator-activated receptor gamma ligands suppress the transcriptional activation of cyclooxygenase-2. Evidence for involvement of activator protein-1 and CREB-binding protein/p300

We investigated whether peroxisome proliferator-activated receptor gamma (PPARgamma) ligands (ciglitazone, troglitazone, and 15-deoxy-Delta(12,14) prostaglandin J(2)) inhibited cyclooxygenase-2 (COX-2) induction in human epithelial cells. Ligands of PPARgamma inhibited phorbol ester (phorbol 12-myristate 13-acetate, PMA)-mediated induction of COX-2 and prostaglandin E(2) synthesis. Nuclear run-offs revealed increased rates of COX-2 transcription after treatment with PMA, an effect that was inhibited by PPARgamma ligands. PMA-mediated induction of COX-2 promoter activity was inhibited by PPARgamma ligands; this suppressive effect was prevented by overexpressing a dominant negative form of PPARgamma or a PPAR response element decoy oligonucleotide. The stimulatory effects of PMA were mediated by a cyclic AMP response element in the COX-2 promoter. Treatment with PMA increased activator protein-1 (AP-1) activity and the binding of c-Jun, c-Fos, and ATF-2 to the cyclic AMP response element, effects that were blocked by PPARgamma ligands. These findings raised questions about the mechanism underlying the anti-AP-1 effect of PPARgamma ligands. The induction of c-Jun by PMA was blocked by PPARgamma ligands. Overexpression of either c-Jun or CREB-binding protein/p300 partially relieved the suppressive effect of PPARgamma ligands. When CREB-binding protein and c-Jun were overexpressed together, the ability of PPARgamma ligands to suppress PMA-mediated induction of COX-2 promoter activity was essentially abrogated. Bisphenol A diglycidyl ether, a compound that binds to PPARgamma but lacks the ability to activate transcription, also inhibited PMA-mediated induction of AP-1 activity and COX-2. Taken together, these findings are likely to be important for understanding the anti-inflammatory and anti-cancer properties of PPARgamma ligands.

Regulation of cyclooxygenase-2 expression by nitric oxide in cells

Two different cyclooxygenases (COXs) are functional in mammals: COX-1 and COX-2. COX-2 is mainly an inducible isoform that shares significant features with inducible nitric oxide synthase (iNOS) in terms of its tissue distribution and participation in pathophysiological phenomena. Furthermore, the product of iNOS catalysis, nitric oxide (NO), is an important regulator of COX-2 activity and expression, and the products of COX-1 and COX-2 (diverse prostanoids) may also influence iNOS expression. Both positive and negative effects of NO on COX-2 expression have been encountered in experimental systems, showing that the outcome of the NO-COX-2 interaction is exquisitely dependent upon the temporal frame and the cell type studied. The pathophysiological significance of NO-COX cross-talk also arises from in vivo studies, in which most evidence points to a positive effect of NO on COX-2 activity and/or expression. This emphasizes the need to understand the underlying mechanisms. Among these, the capacity of NO and its effector cyclic GMP to modulate the function of several target proteins, including transcription factors such as nuclear factor-kappaB and activator protein-1, appears as the key pathway by which NO may regulate COX-2 expression. Given the capacity of some prostanoids to modulate the inflammatory response, the interplay between NO synthase and COX pathways stands at the center of the pathophysiological basis of inflammatory diseases.