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

Peroxisome Proliferator‐activated Receptors and their Relevance to Dermatology

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily and are expressed in a variety of tissues including skin and cells of the immune system. They act as ligand-dependent transcription factors which heterodimerize with retinoid X receptors to allow binding to and activation of PPAR responsive genes. Through this mechanism, PPAR ligands can control a wide range of physiological processes. Based on their effects in vitro and in vivo PPAR agonists and antagonists have the potential to become important therapeutic agents for the treatment of various skin diseases. PPARs can also be activated directly by phosphorylation to have ligand-independent effects. This review will discuss the physiology of PPARs relating this to skin pathology and their role as a target for novel therapies.

Inhibition of IL-10-induced STAT3 activation by 15-deoxy-Δ12,14-prostaglandin J2

OBJECTIVES

15-Deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) is a natural ligand that activates the peroxisome proliferator-activated receptor (PPAR)-gamma, a member of the nuclear receptor family implicated in the regulation of lipid metabolism and adipocyte differentiation. Recent data have shown that 15d-PGJ2 exerts anti-inflammatory action via inhibition of the interferon gamma (IFN-gamma)-induced Jak-STAT signalling pathway. The anti-inflammatory effect of IL-10 is mediated via activated STAT3 (signal transducer and activator of transcription 3). In this study, we investigated whether 15d-PGJ2 inhibit IL-10-induced STAT activation.

METHODS

We used western blotting, flow cytometric analysis and a real-time polymerase chain reaction.

RESULTS

15d-PGJ2 blocked IL-10-induced STAT1 and STAT3 activation in primary human monocytes, macrophages and THP-1 cells. Inhibition was not specific for IL-10, as induction of STAT activation by IFN-gamma and IL-6 was also inhibited by 15d-PGJ2. Inhibition of IL-10 signalling was induced within 1 h after pretreatment of 15d-PGJ2. Other PPARgamma agonists, such as troglitazone, did not inhibit IL-10 signalling. Treatment with GW9662, a specific PPARgamma antagonist, had no effect on 15d-PGJ2-mediated inhibition of IL-10 signalling even at higher concentrations (50 microM), indicating that 15d-PGJ2 affects the IL-10-induced Jak-STAT signalling pathway via an PPARgamma-independent mechanism. Actinomycin D had no effect on 15d-PGJ2-mediated inhibition of IL-10 signalling, indicating that inhibition of IL-10 signalling occurs independently of de novo gene expression. Also, inhibitors of extracellular signal-regulated kinase (ERKs) (PD98059), p38 MAPK (mitogen-activated protein kinase) (SB203580) and protein kinase C (PKC) (GF109203X, calphostin C) had no effect on 15d-PGJ2-mediated inhibition of IL-10 signalling. These results show that MAPKs and PKC are not involved in the inhibition of IL-10 signalling.

CONCLUSIONS

We showed that 15d-PGJ2 non-specifically inhibits STAT signalling of the anti-inflammatory cytokine IL-10 as well as the proinflammatory cytokine IFN-gamma. These findings indicate the possibility that 15d-PGJ2 can have adverse effects in the management of diseases in which IL-10 plays a critical role in the suppression of inflammation.

Peroxisome proliferator-activated receptor gamma activation decreases neuroinflammation in brain after stress in rats

BACKGROUND

A growing body of evidence has demonstrated that peroxisome proliferator-activated receptor gamma (PPARgamma) play a role in brain inflammatory conditions because various PPARgamma ligands inhibit proinflammatory mediators, such as cytokines (tumor necrosis factor alpha [TNFalpha]) and inducible nitric oxide synthase (NOS-2). As has been previously shown, immobilization stress and stress-related neuropsychologic conditions are followed by accumulation of oxidative/nitrosative mediators in brain after the release of cytokines, nuclear factor kappaB activation, and NOS-2 and cyclooxygenase 2 (COX-2) expression in the brain.

METHODS

To assess whether PPARgamma activation can modify the accumulation of oxidative/nitrosative species seen in brain after stress, and to study the mechanisms by which this effect is achieved, young-adult male Wistar rats (control and immobilized during 6 hours) were injected (IP) with the high-affinity ligand rosiglitazone (RS) at the onset of stress.

RESULTS

Stress increased PPARgamma expression in cortical neurons and glia as assessed by Western blot and immunohistochemistry. In stressed animals, RS (1-3 mg/kg) decreased stress-induced increases in NOS-2 activity. On the other hand, the PPARgamma ligand decreased stress-induced malondialdehyde (an indicator of lipid peroxidation) accumulation in cortex and prevented oxidation of the main antioxidant glutathione. The mechanisms involved in the antioxidative properties of RS in stress involve nuclear factor KB blockade (by preventing stress-induced IkappaBalpha decrease) and inhibition of TNFalpha release in stressed animals. At the doses tested, RS did not decrease COX-2 expression and prostaglandin E2 release during stress. Finally, RS also decreased chronic (repeated immobilization for 21 days) stress-induced accumulation of oxidative/nitrosative mediators.

CONCLUSIONS

Taken together, these findings suggest a role for this antiinflammatory pathway in the brain response to stress and the possibility of pharmacologic modulation for preventing accumulation of oxidative/nitrosative species and subsequent brain damage in stress-related neuropsychologic conditions.

Nanomolar and micromolar effects of 15-deoxy-delta 12,14-prostaglandin J2 on amnion-derived WISH epithelial cells: differential roles of peroxisome proliferator-activated receptors gamma and delta and nuclear factor kappa B

15-Deoxy delta(12,14)-prostaglandin J(2) (15d-PGJ(2)), an activator of peroxisome proliferator-activated receptor (PPAR)-gamma and -delta, is a prostanoid metabolite with anti-inflammatory actions. In intrauterine tissues, proinflammatory cytokines and prostaglandins have been identified as playing key roles in the maintenance of pregnancy and the onset of labor. We investigated and compared the early (<3 h) effects of 15d-PGJ(2) with rosiglitazone (PPAR-gamma ligand) and 2-methyl-4-((4-methyl-2-(4-trifluoromethylphenyl)-1,3-thiazol-5-yl)-methylsulfanyl)phenoxy-acetic acid (GW501516) (PPAR-delta ligand) on interleukin (IL)-1beta-induced prostaglandin and cytokine production by amnion-derived WISH cells. We show that 15d-PGJ(2) exerts differential effects depending on concentration. At low concentrations (<0.1 microM), 15d-PGJ(2) inhibited IL-1beta-stimulated prostaglandin E(2) (PGE(2)) but not cytokine (IL-6/IL-8) production or cyclooxygenase-2 (COX-2) expression. This effect was attenuated by a PPAR-gamma inhibitor [2-chloro-5-nitro-N-phenyl-benzamide (GW9662)], by transfection with a dominant-negative PPAR construct, and was reproduced by the PPAR-gamma ligand rosiglitazone. At higher concentrations (1-10 microM), 15d-PGJ(2) inhibited IL-1beta-stimulated PGE(2) and cytokine production and COX-2 expression, and this effect was not blocked by GW9662. Rosiglitazone at high concentrations (1-10 microM) stimulated PGE(2) production in the absence or presence of the dominant-negative PPAR. The PPAR-delta ligand GW501516 also inhibited IL-1beta-stimulated PGE(2) production but only at high concentrations (1 microM). IL-1beta-induced nuclear factor-kappaB (NF-kappaB) DNA binding activity was significantly inhibited by 15d-PGJ(2) (10 microM) and GW501516 (1 microM) but increased with 10 microM rosiglitazone. We conclude that 1) at low concentrations, 15d-PGJ(2) acts through a PPAR-gamma signaling pathway; b) at higher concentrations, its actions are mediated most likely through other pathways such as activation of PPAR-delta and/or inhibition of NF-kappaB; and 3) rosiglitazone exerts PPAR-independent effects at high concentrations (>1 microM).

15-Deoxy-Δ12,14-Prostaglandin J2(15d-PGJ2) Signals Through Retinoic Acid Receptor–Related Orphan Receptor-α but Not Peroxisome Proliferator–Activated Receptor-γ in Human Vascular Endothelial Cells

OBJECTIVE

15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2), a natural ligand of the peroxisome proliferator-activated receptor-gamma (PPARgamma), has been shown to inhibit proinflammatory gene expression, but the signaling mechanisms involved remain unclear. Because retinoic acid receptor-related orphan receptor-alpha (RORalpha) has been reported to suppress tumor necrosis factor-alpha (TNF-alpha)-induced expression of proinflammatory genes, we hypothesized that 15d-PGJ2 may induce RORalpha expression resulting in inhibition of proinflammatory gene expression.

METHODS AND RESULTS

We demonstrate that 15d-PGJ2 induced RORalpha1 and RORalpha4 expression and inhibited TNF-alpha-induced vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) expression in human umbilical vein endothelial cells (HUVECs). In contrast, the synthetic PPARgamma ligand pioglitazone weakly induced RORalpha4 expression but did not affect RORalpha1 expression or TNF-alpha-induced gene expression. Biphenol A diglycidyl ether, a PPARgamma antagonist, did not block the effect of 15d-PGJ2 on RORalpha expression. Adenovirus-mediated overexpression of RORalpha1 inhibited TNF-alpha-induced VCAM-1 and ICAM-1 expression, and overexpression of a mutant form of RORalpha1 (RORalpha1Delta), which inhibited transcriptional activity of RORalpha1 and RORalpha4, attenuated its inhibition. Furthermore, we found that RORalpha1Delta attenuated the inhibitory actions of 15d-PGJ2 on TNF-alpha-induced VCAM-1 and ICAM-1 expression.

CONCLUSIONS

These results suggest that 15d-PGJ2 inhibits TNF-alpha-induced expression of proinflammatory genes mediated in part via induction of RORalpha in HUVECs. This mechanism provides a novel insight into PPARgamma-independent actions of 15d-PGJ2.

S-nitrosation of the insulin receptor, insulin receptor substrate 1, and protein kinase B/Akt: a novel mechanism of insulin resistance

Evidence demonstrates that exogenous nitric oxide (NO) and the NO produced by inducible nitric oxide synthase (iNOS) can induce insulin resistance in muscle. Here, we investigated whether this insulin resistance could be mediated by S-nitrosation of proteins involved in early steps of the insulin signal transduction pathway. Exogenous NO donated by S-nitrosoglutathione (GSNO) induced in vitro and in vivo S-nitrosation of the insulin receptor beta subunit (IRbeta) and protein kinase B/Akt (Akt) and reduced their kinase activity in muscle. Insulin receptor substrate (IRS)-1 was also rapidly S-nitrosated, and its expression was reduced after chronic GSNO treatment. In two distinct models of insulin resistance associated with enhanced iNOS expression-diet-induced obesity and the ob/ob diabetic mice-we observed enhanced S-nitrosation of IRbeta/IRS-1 and Akt in muscle. Reversal of S-nitrosation of these proteins by reducing iNOS expression yielded an improvement in insulin action in both animal models. Thus, S-nitrosation of proteins involved in insulin signal transduction is a novel molecular mechanism of iNOS-induced insulin resistance.

Peroxisome proliferator-activated receptor delta suppresses 11beta-hydroxysteroid dehydrogenase type 2 gene expression in human placental trophoblast cells

Accumulating evidence suggests that the human placental enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) plays a key role in fetal development by controlling fetal exposure to maternal glucocorticoids. Recently, the nuclear peroxisome proliferator-activated receptor delta (PPAR delta) has been found to be the most abundantly expressed PPAR subtype in the human placenta, but its function in this organ is unknown. Given that PPAR delta-null mice exhibited placental defects and consequent intrauterine growth restriction, the present study was undertaken to examine the hypothesis that PPAR delta regulates human placental function in part by targeting 11beta-HSD2. Using cultured human trophoblast cells as a model system, we demonstrated that 1) the putative PPAR delta agonist carbaprostacyclin (cPGI2) reduced 11beta-HSD2 activity as well as 11beta-HSD2 expression at both protein and mRNA levels; 2) GW610742 (a selective PPAR delta agonist) mimicked the effect of cPGI2, whereas indomethacin (a known ligand for PPARalpha and PPAR gamma) had no effect; 3) the cPGI2-induced down-regulation of 11beta-HSD2 mRNA did not require de novo protein synthesis; 4) cPGI2 suppressed HSD11B2 promoter activity, but did not alter the half-life of 11beta-HSD2 mRNA; and 5) the inhibitory effect of cPGI2 on HSD11B2 promoter activity was abrogated in trophoblast cells cotransfected with a dominant negative PPAR delta mutant. Taken together, these findings suggest that activation of PPAR delta down-regulates HSD11B2 gene expression in human trophoblast cells, and that this effect is mediated primarily at the transcriptional level. Thus, the present study reveals 11beta-HSD2 as an additional target for PPAR delta and identifies a molecular mechanism by which this nuclear receptor may regulate human placental function.

Mechanism of the anti-inflammatory effect of thiazolidinediones: relationship with the glucocorticoid pathway

The glucocorticoid receptor (GR) and peroxisome proliferator-activated receptors (PPARs) play important roles in both physiological and pathological conditions such as cell differentiation, lipolysis, control of glucose metabolism, immunity, and inflammation. In fact, recent studies suggest that the thiazolidinedione (TZD) class of PPAR-gamma ligands, like glucocorticoids, may also be clinically beneficial in several inflammatory diseases, even if the molecular mechanisms responsible for these activities have not yet been clarified. In this study, by using a murine model of inflammation, the carrageenin-induced paw edema in mouse, we show that the anti-inflammatory activity exhibited by the PPAR-gamma agonists rosiglitazone and ciglitazone is reversed by the GR antagonist RU486 (17 beta-hydroxy-11 beta-[4-dimethylamino phenyl]-17 alpha-[1-propynyl]estra-4,9-dien-3-one). Moreover, by using a conditional GR null cell line, we demonstrate, for the first time to our knowledge, that one of the possible mechanisms explaining the anti-inflammatory activity of TZDs is their ability to activate GR nuclear translocation. In addition, by using J774 cell line lacking PPAR-gamma, we demonstrate that PPAR-gamma expression could not be essential for TZD-mediated GR nuclear translocation, thus explaining, at least in part, the molecular mechanism underlying their anti-inflammatory activity.

The fatty acid-binding protein, aP2, coordinates macrophage cholesterol trafficking and inflammatory activity. Macrophage expression of aP2 impacts peroxisome proliferator-activated receptor gamma and IkappaB kinase activities

Fatty acid-binding proteins are cytosolic fatty acid chaperones, and the adipocyte isoform, aP2, plays an important role in obesity and glucose metabolism. Recently, this protein has been detected in macrophages where it strongly contributes to the development of atherosclerosis. Here, we investigated the role of aP2 in macrophage biology and the molecular mechanisms underlying its actions. We demonstrate that aP2-deficient macrophages display defects in cholesterol accumulation and alterations in pro-inflammatory responsiveness. Deficiency of aP2 alters the lipid composition in macrophages and enhances peroxisome proliferator-activated receptor gamma activity, leading to elevated CD36 expression and enhanced uptake of modified low density lipoprotein. The increased peroxisome proliferator-activated receptor gamma activity in aP2-deficient macrophages is also accompanied by a significant stimulation of the liver X receptor alpha-ATP-binding cassette transporter A1-mediated cholesterol efflux pathway. In parallel, aP2-deficient macrophages display reduced IkappaB kinase and NF-kappaB activity, resulting in suppression of inflammatory function including reduced cyclooxygenase-2 and inducible nitric-oxide synthase expression and impaired production of inflammatory cytokines. Our results demonstrate that aP2 regulates two central molecular pathways to coordinate macrophage cholesterol trafficking and inflammatory activity.

Peroxisome proliferator-activated receptor-γ ligands reduce inflammation and infarction size in transient focal ischemia

Newly developed insulin-sensitizing agents, which target the nuclear receptor peroxisome proliferator-activated receptor-gamma have recently been appreciated to exhibit potent anti-inflammatory actions. Since stroke is associated with an intense inflammatory response, we reasoned that these agents may ameliorate injury from stroke. We report that administration of troglitazone or pioglitazone 24 h before and at the time of cerebral infarction dramatically reduced infarction volume and improved neurological function following middle cerebral artery occlusion in rats. Furthermore, we find that delayed therapy also significantly reduced infarct volume. The brains of the drug-treated animals displayed reduced inflammation as evidenced by decreased immunoreactivity for microglial/macrophage markers and reduced protein and mRNA for interleukin-1beta, cyclooxygenase-2 and inducible nitric oxide synthase. We argue that the beneficial effects of these drugs are likely due to reduced expression of these inflammatory mediators, which are known to exacerbate ischemic injury following stroke. These results are of particular relevance to diabetic patients chronically treated with these agents who may benefit from the neuroprotective actions of these drugs.

Inhibition of interleukin-1?-induced cyclooxygenase 2 expression in human synovial fibroblasts by 15-deoxy-?12,14-prostaglandin J2 through a histone deacetylase-independent mechanism

OBJECTIVE

The cyclooxygenase (COX) metabolite, 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)), has been reported to inhibit the expression of a number of genes involved in the pathogenesis of arthritis. However, its effects on COX-2 remain controversial. We undertook this study to investigate the effects of 15d-PGJ(2) on interleukin-1beta (IL-1beta)-induced COX-2 expression in human synovial fibroblasts (HSFs).

METHODS

HSFs were cultured with IL-1beta in the absence or presence of 15d-PGJ(2), and the levels of COX-2 protein and messenger RNA (mRNA) expression were evaluated using Western blotting and real-time reverse transcriptase-polymerase chain reaction, respectively. COX-2 promoter activity was analyzed in transient transfection experiments. Chromatin immunoprecipitation assays were performed to evaluate the level of histone acetylation and the recruitment of histone deacetylases (HDACs) 1, 2, and 3 and histone acetylase (HAT) p300 to the COX-2 promoter.

RESULTS

IL-1beta-induced COX-2 protein and mRNA expression, as well as COX-2 promoter activation, were inhibited by 15d-PGJ(2). Troglitazone, a selective peroxisome proliferator-activated receptor gamma (PPARgamma) ligand, enhanced COX-2 expression, while GW9662, a specific PPARgamma antagonist, relieved the suppressive effect of 15d-PGJ(2). IL-1beta-induced histone H3 acetylation was selectively blocked by 15d-PGJ(2). The reduction of histone H3 acetylation did not correlate with the recruitment of HDACs to the COX-2 promoter. Also, treatment with the specific HDAC inhibitor, trichostatin A, did not relieve the suppressive effect of 15d-PGJ(2), indicating that HDACs are not involved in the inhibitory effect of 15d-PGJ(2). Furthermore, 15d-PGJ(2) blocked IL-1beta-induced recruitment of p300 to the COX-2 promoter, which may be the mechanism for decreased histone H3 acetylation and COX-2 expression. In accordance with this, overexpression of p300, but not of a mutant p300 lacking HAT activity, relieved the inhibitory effect of 15d-PGJ(2) on COX-2 promoter activation.

CONCLUSION

These data suggest that 15d-PGJ(2) can inhibit IL-1beta-induced COX-2 expression by an HDAC-independent mechanism, probably by interfering with HAT p300.

Peroxisome proliferator-activated receptor agonists inhibit interleukin-1beta-mediated nitric oxide production in cultured lacrimal gland acinar cells

Development of dry eye disease often occurs in individuals with autoimmune disorders such as Sjögren's syndrome. The cause of dry eye in these patients is thought to be due, at least in part, to lymphocytic infiltration of the lacrimal glands, with subsequent loss of secretion of the aqueous component of tear film. How this lymphocytic infiltration leads to loss of secretion is not fully understood. We have previously shown that the proinflammatory cytokine, interleukin-1beta (IL-1beta), can stimulate the production of nitric oxide (NO) in cultured lacrimal gland acinar cells. It is possible that IL-1beta, produced by the infiltrating macrophages, stimulates production of inducible nitric oxide synthase (iNOS), and subsequently excessive production of NO. Peroxynitrate and other radical byproducts associated with excessive synthesis of NO may be detrimental to normal function of the lacrimal gland. Here we show that the peroxisome proliferator-activated receptor (PPAR)alpha and gamma agonists can inhibit NO production in cultured lacrimal gland acinar cells. Further, this is accomplished without loss of iNOS expression or tetrahydrobiopterin. These data suggest that the use of ointments or eye drops containing these PPAR agonists may provide an effective therapeutic intervention for the prevention of dry eye in Sjögren's syndrome patients.

Endogenous Inhibitors of Nuclear Factor-κB, An Opportunity for Cancer Control: Fig. 1

Excessive and prolonged activation of nuclear factor-kappaB (NF-kappaB) has been linked to numerous human diseases, especially cancer, because of the elevated expression of genes encoding antiapoptotic proteins, cytokines, chemokines, cell adhesion molecules, and so on. Eukaryotic cells have developed multiple mechanisms to keep this ubiquitous transcription factor in check. In addition to the inhibitor of kappaB family proteins, a number of endogenous molecules that negatively regulate the activation or activity of NF-kappaB have been identified. These molecules include A20, CYLD, cyPG15-deoxy-Delta(12,14)-prostaglandin J(2), Foxj1, Twist proteins, and beta-arrestins. The extended list of these endogenous inhibitors of NF-kappaB may provide new opportunities for the development of novel strategies for the intervention of malignant transformation. The question to be asked is how NF-kappaB is sustained activated in a number of cancers in which so many antagonists are surrounded.

Peroxisome proliferator-activated receptor-γ and its ligands attenuate biologic functions of human natural killer cells

Interferon-γ (IFN-γ) production and cytolytic activity are 2 major biologic functions of natural killer (NK) cells that are important for innate immunity. We demonstrate here that these functions are compromised in human NK cells treated with peroxisome proliferator-activated-γ (PPAR-γ) ligands via both PPAR-γ-dependent and -independent pathways due to variation in PPAR-γ expression. In PPAR-γ-null NK cells, 15-deoxy-Δ12,14 prostaglandin J2 (15d-PGJ2), a natural PPAR-γ ligand, reduces IFN-γ production that can be reversed by MG132 and/or chloroquine, and it inhibits cytolytic activity of NK cells through reduction of both conjugate formation and CD69 expression. In PPARγ-positive NK cells, PPAR-γ activation by 15d-PGJ2 and ciglitazone (a synthetic ligand) leads to reduction in both mRNA and protein levels of IFN-γ. Overexpression of PPAR-γ in PPAR-γ-null NK cells reduces IFN-γ gene expression. However, PPAR-γ expression and activation has no effect on NK cell cytolytic activity. In addition, 15d-PGJ2 but not ciglitazone reduces expression of CD69 in human NK cells, whereas CD44 expression is not affected. These results reveal novel pathways regulating NK cell biologic functions and provide a basis for the design of therapeutic agents that can regulate the function of NK cells within the innate immune response. (Blood. 2004;104:3276-3284)

Repression of IFN-gamma expression by peroxisome proliferator-activated receptor gamma

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors expressed in a wide variety of cells. Our studies and others have demonstrated that both human and murine T cells express PPARgamma and that expression can be augmented over time in mitogen-activated splenocytes. PPARgamma ligands decrease proliferation and IL-2 production, and induce apoptosis in both B and T cells. PPARgamma ligands have also been shown to be anti-inflammatory in multiple models of inflammatory disease. In the following study, we demonstrate for the first time that PPARgamma is expressed in both murine CD4 and CD8 cells and that PPARgamma ligands directly decrease IFN-gamma expression by murine and transformed T cell lines. Unexpectedly, GW9662, a PPARgamma antagonist, increases lymphocyte IFN-gamma expression. Transient transfection studies reveal that PPARgamma ligands, in a PPARgamma-dependent manner, potently repress an IFN-gamma promoter construct. Repression localizes to the distal conserved sequence of the IFN-gamma promoter. Our studies also demonstrate that PPARgamma acts on the IFN-gamma promoter by interfering with c-Jun activation. These studies suggest that many of the observed anti-inflammatory effects of PPARgamma ligands may be related to direct inhibition of IFN-gamma by PPARgamma.

Peroxisome Proliferation-Activated Receptor (PPAR)γ Is Not Necessary for Synthetic PPARγ Agonist Inhibition of Inducible Nitric-Oxide Synthase and Nitric Oxide

Peroxisome proliferation-activated receptor (PPAR)gamma agonists inhibit inducible nitric-oxide synthase (iNOS), tumor necrosis factor-alpha, and interleukin-6. Because of these effects, synthetic PPARgamma agonists, including thiazolidinediones, are being studied for their impact on inflammatory disease. The anti-inflammatory concentrations of synthetic PPARgamma agonists range from 10 to 50 microM, whereas their binding affinity for PPARgamma is in the nanomolar range. The specificity of synthetic PPARgamma agonists for PPARgamma at the concentrations necessary for anti-inflammatory effects is thus in question. We report that PPARgamma is not necessary for the inhibition of iNOS by synthetic PPARgamma agonists. RAW 264.7 macrophages possess little PPARgamma, yet lipopolysaccharide (LPS)/interferon (IFN)gamma-induced iNOS was inhibited by synthetic PPARgamma agonists at 20 microM. Endogenous PPARgamma was inhibited by the transfection of a dominant-negative PPARgamma construct into murine mesangial cells. In the transfected cells, synthetic PPARgamma agonists inhibited iNOS production at 10 microM, similar to nontransfected cells. Using cells from PPARgamma Cre/lox conditional knockout mice, baseline and LPS/IFNgamma-induced nitric oxide levels were higher in macrophages lacking PPARgamma versus controls. However, synthetic PPARgamma agonists inhibited iNOS at 10 microM in the PPARgamma-deficient cells, similar to macrophages from wild-type mice. These results indicate that PPARgamma is not necessary for inhibition of iNOS expression by synthetic PPARgamma agonists at concentrations over 10 microM. Intrinsic PPARgamma function, in the absence of synthetic agonists, however, may play a role in inflammatory modulation.

Antineoplastic effects of peroxisome proliferator-activated receptor gamma agonists

Peroxisome proliferator-activated receptors (PPAR) are members of a superfamily of nuclear hormone receptors. Activation of PPAR isoforms elicits both antineoplastic and anti-inflammatory effects in several types of mammalian cells. PPARs are ligand-activated transcription factors and have a subfamily of three different isoforms: PPAR alpha, PPAR gamma, and PPAR beta/delta. All isoforms heterodimerise with the 9-cis-retinoic acid receptor RXR, and play an important part in the regulation of several metabolic pathways, including lipid biosynthesis and glucose metabolism. Endogenous ligands of PPAR gamma include long-chain polyunsaturated fatty acids, eicosanoid derivates, and oxidised lipids. Newly developed synthetic ligands include thiazolidinediones-a group of potent PPAR gamma agonists and antidiabetic agents. Here, we review PPAR gamma-induced antineoplastic signalling pathways, and summarise the antineoplastic effects of PPAR gamma agonists in different cancer cell lines, animal models, and clinical trials.

Diverse regulation of NF-kappaB and peroxisome proliferator-activated receptors in murine nonalcoholic fatty liver

Fatty liver is highly sensitive to inflammatory activation. Peroxisome proliferator-activated receptors (PPAR) have anti-inflammatory effects and regulate lipid metabolism in the fatty liver. We hypothesized that fatty liver leads to endotoxin sensitivity through an imbalance between pro- and anti-inflammatory signals. Leptin-deficient, ob/ob mice and their lean littermates were challenged with single or double insults and pro- and anti-inflammatory pathways were tested on cytokine production and activation of nuclear regulatory factors NF-kappaB and peroxisome proliferator receptor element (PPRE). Ob/ob mice produced significantly higher serum tumor necrosis factor alpha (TNF-alpha) and interleukin (IL) 6 and showed increased hepatic NF-kappaB activation compared to lean littermates after stimulation with a single dose of lipopolysaccharide (LPS) or alcohol. In ob/ob mice, double insults with alcohol and LPS augmented proinflammatory responses mediated by increased degradation of inhibitory kappaB (IkappaB)-alpha and IkappaB-beta and preferential induction of the p65/p50 NF-kappaB heterodimer. In lean mice, in contrast, acute alcohol attenuated LPS-induced TNF-alpha, IL-6 production, and NF-kappaB activation through reduced IkappaB-alpha degradation and induction of p50/p50 homodimers. PPRE binding was increased in fatty but not in lean livers after alcohol or LPS stimulation. However, cotreatment with alcohol and LPS reduced both PPRE binding and nuclear levels of PPAR-alpha in fatty livers but increased those in lean livers. In conclusion, our results show opposite PPRE and NF-kappaB activation in fatty and lean livers. PPAR activation may represent an anti-inflammatory mechanism that fails in the fatty liver on increased proinflammatory pressure. Thus, an imbalance between PPAR-mediated anti-inflammatory and NF-kappaB-mediated proinflammatory signals may contribute to increased inflammation in the fatty liver.

15-Deoxy-Δ12,14-Prostaglandin J2 Inhibits Glucocorticoid Binding and Signaling in Macrophages through a Peroxisome Proliferator-Activated Receptor γ-Independent Process

15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)) is involved in the control of inflammatory reaction. We tested the hypothesis that 15d-PGJ(2) would exert this control in part by modulating the sensitivity of inflammatory cells to glucocorticoids. Human U937cells and mouse RAW 264.7 cells were exposed to 15d-PGJ(2), and binding experiments were performed with [(3)H]dexamethasone as a glucocorticoid receptor (GR) ligand. 15d-PGJ(2) caused a transient and concentration-dependent decrease in [(3)H]dexamethasone-specific binding to either cells through a decrease in the number of GR per cell without significant modification of the K(d) value. These changes were related to functional alteration of the GR rather than to a decrease in GR protein. They did not require the engagement of peroxisome proliferator-activated receptor gamma (PPARgamma), because the response to 15d-PGJ(2) was neither mimicked by the PPARgamma agonist ciglitazone nor prevented by the PPARgamma antagonist bisphenol A diglycidyl ether. 15d-PGJ(2) altered GR possibly through the interaction of its cyclopentenone ring with GR cysteine residues because the cyclopentenone ring per se could mimic the effect of 15d-PGJ(2), and modification of GR cysteine residues with methyl methanethiosulfonate suppressed the response to 15d-PGJ(2). Finally, 15d-PGJ(2)-induced decreases in glucocorticoid binding to GR resulted in parallel decreases in the ability of GR to activate the transcription of a glucocorticoid-inducible reporter gene and to reduce the expression of monocyte chemoattractant protein-1. Together these data suggest that 15d-PGJ(2) limits glucocorticoid binding and signaling in monocytes/macrophages through a PPARgamma-independent and cyclopentenone-dependent mechanism. It provides a way in which 15d-PGJ(2) would exert proinflammatory activities in addition to its known anti-inflammatory activities.

Peroxisome proliferator-activated receptor-gamma: therapeutic target for diseases beyond diabetes: quo vadis?

The discovery that the insulin-sensitising thiazolidinediones (TZDs), specific peroxisome proliferator-activated receptor-gamma (PPARgamma) agonists, have antiproliferative, anti-inflammatory and immunomodulatory effects has led to the evaluation of their potential use in the treatment of diabetic complications and inflammatory, proliferative diseases in non-insulin-resistant, euglycaemic individuals. Apart from improving insulin resistance, plasma lipids and systemic inflammatory markers, ameliorating atherosclerosis and preventing coronary artery restenosis in diabetic subjects, currently approved TZDs have been shown to improve psoriasis and ulcerative colitis in euglycaemic human subjects. These data imply that the activation of PPAR-gamma may improve cardiovascular risk factors and cardiovascular outcomes in both insulin-resistant diabetic and non-diabetic individuals. Through their immunomodulatory and anti-inflammatory actions, TZDs and other PPAR-gamma agonists may prove to be effective in treating diseases unrelated to insulin resistance, such as autoimmune (e.g., multiple sclerosis), atopic (e.g., asthma, atopic dermatitis) and other inflammatory diseases (e.g., psoriasis, ulcerative colitis). Newer and safer selective PPAR-gamma agonists are presently under development. Furthermore, of considerable interest is the recent discovery that a unique subset of currently prescribed antihypertensive angiotensin II Type 1 receptor antagonists has selective PPAR-gamma-modulating activity. These discoveries pave the way for the development of drugs for treating chronic multigenic cardiovascular and metabolic diseases, for which therapy is presently insufficient or non-existent. The potential utility of the currently available TZDs rosiglitazone and pioglitazone and PPAR-gamma-modulating angiotensin II Type 1 receptor antagonists in treating cardiovascular, metabolic and inflammatory diseases in insulin resistant and euglycaemic states is of immense clinical potential and should be investigated.

Peroxisome proliferator-activated receptor γ: Its role in metabolic syndrome

Here we review PPARgamma function in relation to human adipogenesis, insulin sensitization, lipid metabolism, blood pressure regulation and prothrombotic state to perhaps provide justification for this nuclear receptor remaining a key therapeutic target for the continuing development of agents to treat human metabolic syndrome.

Mitochondrial nitric oxide synthase

Nitric oxide (NO*) can bind to and inhibit the terminal enzyme of the mitochondrial respiratory chain, cytochrome c oxidase (complex IV). In vivo, NO* is made by the NO* synthase (NOS) family of enzymes, and considerable debate has recently arisen regarding a NOS inside mitochondria (termed 'mtNOS'). Such an enzyme is an intriguing proposition, since it affords unique organelle-based regulatory mechanisms for NO* synthesis, and has considerable implications for mitochondrial function. This review serves to discuss some of the current issues regarding mtNOS, such as its isoform identity, the availability of co-factors and substrates within the organelle, and potential physiological vs. pathological roles for the enzyme, all within the broader context of mitochondrial regulation by NO*.

Inhibition of inducible nitric-oxide synthase by activators of AMP-activated protein kinase: a new mechanism of action of insulin-sensitizing drugs

AMP-activated protein kinase (AMPK), an energy-sensing enzyme that is activated in response to cellular stress, is a critical signaling molecule for the regulation of multiple metabolic processes. AMPK has recently emerged as an attractive novel target for the treatment of obesity and type 2 diabetes because its activation increases fatty acid oxidation and improves glucose homeostasis. Here we show that pharmacological activation of AMPK by insulin-sensitizing drugs markedly inhibits inducible nitric-oxide synthase (iNOS), a proinflammatory mediator in endotoxic shock and in chronic inflammatory states including obesity-linked diabetes. AMPK-mediated iNOS inhibition was observed in several cell types (myocytes, adipocytes, macrophages) and primarily resulted from post-transcriptional regulation of the iNOS protein. AMPK activation in vivo also blunted iNOS induction in muscle and adipose tissues of endotoxin-challenged rats. Reduction of AMPK expression by small interfering RNA reversed the inhibitory effects of AMPK activators on iNOS expression and nitric oxide production in myocytes. These results indicate that AMPK is a novel anti-inflammatory signaling pathway and thus represents a promising therapeutic target for immune-inflammatory disorders.

Protection by pioglitazone in the MPTP model of Parkinson's disease correlates with I kappa B alpha induction and block of NF kappa B and iNOS activation

Inflammation has been implicated in the pathogenesis of Parkinson's disease (PD). In the chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD, inducible NO synthase (iNOS) derived nitric oxide (NO) is an important mediator of dopaminergic cell death. Ligands of the peroxisome proliferator-activated receptor (PPAR) exert anti-inflammatory effects. We here investigated whether pioglitazone, a PPARgamma agonist, protected mice from MPTP-induced dopaminergic cell loss, glial activation, and loss of catecholamines in the striatum. As shown by western blot, PPARgamma was expressed in the striatum and the substantia nigra of vehicle- and MPTP-treated mice. Oral administration of 20 mg/(kg day) of pioglitazone protected tyrosine hydroxylase (TH)-positive substantia nigra neurons from death induced by 5 x 30 mg/kg MPTP. However, the decrease of dopamine in the striatum was only partially prevented. In mice treated with pioglitazone, there were a reduced activation of microglia, reduced induction of iNOS-positive cells and less glial fibrillary acidic protein positive cells in both striatum and substantia nigra pars compacta. In addition, treatment with pioglitazone almost completely blocked staining of TH-positive neurons for nitrotyrosine, a marker of NO-mediated cell damage. Because an increase in inhibitory protein-kappa-Balpha (IkappaBalpha) expression and inhibition of translocation of the nuclear factor kappaB (NFkappaB) subunit p65 to the nucleus in dopaminergic neurons, glial cells and astrocytes correlated with the protective effects of pioglitazone, our results suggest that pioglitazone sequentially acts through PPARgamma activation, IkappaBalpha induction, block of NFkappaB activation, iNOS induction and NO-mediated toxicity. In conclusion, treatment with pioglitazone may offer a treatment opportunity in PD to slow the progression of disease that is mediated by inflammation.

Nuclear receptor signaling in macrophages

Macrophages play diverse roles in host defense and in maintenance of homeostasis. Based on their ability to promote inflammatory responses, inappropriate macrophage function also contributes to numerous pathological processes, including atherosclerosis, rheumatoid arthritis and inflammatory bowel disease. Members of the nuclear receptor superfamily of ligand-dependent transcriptions factors have emerged as key regulators of inflammation and lipid homeostasis in macrophages. These include the glucocorticoid receptor (GR), which inhibits inflammatory programs of gene expression in response to natural corticosteroids and synthetic anti-inflammatory ligands such as dexamethasone. Also, in response to endogenous eicosanoids and oxysterols, respectively, peroxisome proliferator-activated receptors (PPARs) and liver X receptors (LXRs) regulate transcriptional programs involved in inflammatory responses and lipid homeostasis. Identification of their mechanisms of action should help guide the development of new therapeutic agents useful in the treatment of diseases in which macrophages play critical pathogenic roles.

NF-kappa B-binding activity in an animal diet-induced overweightness model and the impact of subsequent energy restriction

An impaired immune function linked to obesity has been shown in both human and animal studies. The purpose of this work was to analyse the hypothesis that PPAR gamma 1 participates in the inhibition of the immune response by affecting the DNA-binding ability of the NF-kappa B complex and whether the SREBP-1 expression can regulate PPAR gamma 1 expression in spleen. Diet-induced overweight rats showed higher PPAR gamma 1 (p<0.05) and lower SREBP-1 (p<0.01) mRNA expression levels with an inhibition of the DNA-binding ability of NF-kappa B compared to control rats as determined by gel-shift analysis. On the other hand, energy restriction decreased SREBP-1 (p<0.01) mRNA expression with no differences in PPAR gamma 1 mRNA expression compared to non-restricted rats, which was accompanied by a restoration in the DNA-binding ability of NF-kappa B as shown by gel-shift analysis. These results suggest that PPAR gamma 1 may be involved in the altered immune response through changes in the activity of NF-kappa B.

Expression and localization of peroxisome proliferator-activated receptors and nuclear factor kappaB in normal and lesional psoriatic skin

Abnormal epidermal proliferation and differentiation characterize the inflammatory skin disease psoriasis. Here we demonstrate that expression of PPARdelta mRNA and protein is markedly upregulated in psoriatic lesions and that lipoxygenase products accumulating in psoriatic lesions are potent activators of PPARdelta. The expression levels of NF-kappaB p50 and p65 were not significantly altered in lesional compared with nonlesional psoriatic skin. In the basal layer of normal epidermis both p50 and p65 were sequestered in the cytoplasm, whereas p50, but not p65, localized to nuclei in the suprabasal layers, and this distribution was maintained in lesional psoriatic skin. In normal human keratinocytes PPAR agonists neither impaired IL-1beta-induced translocation of p65 nor IL-1beta-induced NF-kappaB DNA binding. We show that PPARdelta physically interacts with the N-terminal Rel homology domain of p65. Irrespective of the presence of agonists none of the PPAR subtypes decreased p65-mediated transactivation in keratinocytes. In contrast p65, but not p50, was a potent repressor of PPAR-mediated transactivation. The p65-dependent repression of PPARdelta- but not PPARalpha- or PPARgamma-mediated transactivation was partially relieved by forced expression of the coactivators p300 or CBP. We suggest that deficient NF-kappaB activation in chronic psoriatic plaques permitting unabated PPARdelta-mediated transactivation contributes to the pathologic phenotype of psoriasis.

PPARalpha activators inhibit vascular endothelial growth factor receptor-2 expression by repressing Sp1-dependent DNA binding and transactivation

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors, originally implicated in the regulation of lipid and glucose homeostasis. In addition, natural and synthetic PPAR activators may control inflammatory processes by inhibition of distinct proinflammatory genes. As signaling via the vascular endothelial growth factor receptor-2 (VEGFR2) pathway is critical for angiogenic responses during chronic inflammation, we explored whether known antiinflammatory effects of PPAR ligands are mediated in part through diminished VEGFR2 expression. In this study, PPARalpha agonists are found to inhibit endothelial VEGFR2 expression, whereas predominant PPARgamma ligands remained without discernible effects. Time- and concentration-dependent inhibition is demonstrated both at the level of protein and mRNA VEGFR2 expression. Inhibitory effects of PPARalpha agonists on transcriptional activity of the VEGFR2 promoter are conveyed by an element located between base pairs -60 and -37 that contains two adjacent consensus Sp1 transcription factor binding sites. Constitutive Sp1-containing complex formation to this sequence is decreased by PPARalpha treatment, indicating that VEGFR2 gene expression is inhibited by repressing Sp1 site-dependent DNA binding and transactivation. Our coimmunoprecipitation experiments revealed enhanced protein interactions between PPARalpha and Sp1 on PPARalpha activation, thus constituting a probable mechanism by which PPARalpha activators decrease Sp-dependent binding activity to the VEGFR2 promoter. Hence, molecular mechanisms by which PPARs modulate the rate of gene transcription may include direct interactions between specific transcription factors and PPARs that ultimately result in reduced DNA binding to their respective response elements.

Peroxisome proliferator-activated receptor gamma ligands regulate myeloperoxidase expression in macrophages by an estrogen-dependent mechanism involving the -463GA promoter polymorphism

A functional myeloperoxidase (MPO) promoter polymorphism, -463GA, has been associated with incidence or severity of inflammatory diseases, including atherosclerosis and Alzheimer's disease, and some cancers. The polymorphism is within an Alu element encoding four hexamer repeats recognized by nuclear receptors (AluRRE). Here we show that peroxisome proliferator-activated receptor gamma (PPARgamma) agonists strongly regulate MPO gene expression through the AluRRE. Opposite effects were observed in granulocyte/macrophage colony-stimulating factor (GMCSF)- versus macrophage colony-stimulating factor (MCSF)-derived macrophages (Mphi): Expression was markedly up-regulated (mean 26-fold) in MCSF-Mphi and down-regulated (34-fold) in GMCSF-Mphi. This was observed with rosiglitazone and three other PPARgamma ligands of the thiazolidinedione class, as well as the natural prostaglandin metabolite 15-deoxy-Delta(12,14) prostaglandin J(2). The selective PPARgamma antagonist, GW9662, blocked both the positive and negative effects on MPO expression. Gel retardation assays showed PPARgamma bound hexamers 3/4, and estrogen receptor-alpha bound hexamers 1/2, with -463A in hexamer 1 enhancing binding. Estrogen blocked PPARgamma effects on MPO expression, especially for the A allele. Charcoal filtration of fetal calf serum eliminated the block of PPARgamma, whereas replenishing the medium with 17beta-estradiol reinstated the block. These findings suggest a model in which estrogen receptor binds the AluRRE, preventing PPARgamma binding to the adjacent site. The positive and negative regulation by PPARgamma ligands, and the block by estrogen, was also observed in transgenic mice expressing the G and A alleles. The mouse MPO gene, which lacks the primate-specific AluRRE, was unresponsive to PPARgamma ligands, suggesting the human MPO transgenes will enhance the utility of mouse models for diseases involving MPO, such as atherosclerosis and Alzheimer's.

Peroxisome Proliferator Activator Receptor-γ Ligands, 15-Deoxy-Δ12,14-Prostaglandin J2 and Ciglitazone, Reduce Systemic Inflammation in Polymicrobial Sepsis by Modulation of Signal Transduction Pathways

Peroxisome proliferator activator receptor-gamma (PPARgamma) is a nuclear receptor that controls the expression of several genes involved in metabolic homeostasis. We investigated the role of PPARgamma during the inflammatory response in sepsis by the use of the PPARgamma ligands, 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)) and ciglitazone. Polymicrobial sepsis was induced by cecal ligation and puncture in rats and was associated with hypotension, multiple organ failure, and 50% mortality. PPARgamma expression was markedly reduced in lung and thoracic aorta after sepsis. Immunohistochemistry showed positive staining for nitrotyrosine and poly(ADP-ribose) synthetase in thoracic aortas. Plasma levels of TNF-alpha, IL-6, and IL-10 were increased. Elevated activity of myeloperoxidase was found in lung, colon, and liver, indicating a massive infiltration of neutrophils. These events were preceded by degradation of inhibitor kappaBalpha (IkappaBalpha), activation of IkappaB kinase complex, and c-Jun NH(2)-terminal kinase and, subsequently, activation of NF-kappaB and AP-1 in the lung. In vivo treatment with ciglitazone or 15d-PGJ(2) ameliorated hypotension and survival, blunted cytokine production, and reduced neutrophil infiltration in lung, colon, and liver. These beneficial effects of the PPARgamma ligands were associated with the reduction of IkappaB kinase complex and c-Jun NH(2)-terminal kinase activation and the reduction of NF-kappaB and AP-1 DNA binding in the lung. Furthermore, treatment with ciglitazone or 15d-PGJ(2) up-regulated the expression of PPARgamma in lung and thoracic aorta and abolished nitrotyrosine formation and poly(ADP-ribose) expression in aorta. Our data suggest that PPARgamma ligands attenuate the inflammatory response in sepsis through regulation of the NF-kappaB and AP-1 pathways.

Molecular Basis for the Direct Inhibition of AP-1 DNA Binding by 15-Deoxy-Δ12,14-prostaglandin J2

Cyclopentenone prostaglandins may interfere with cellular functions by multiple mechanisms. The cyclopentenone 15-deoxy-Delta 12,14-prostaglandin J2 (15d-PGJ2) has been reported to inhibit the activity of the transcription factor AP-1 in several experimental settings. We have explored the possibility of a direct interaction of 15d-PGJ2 with AP-1 proteins. Here we show that 15d-PGJ2 covalently modifies c-Jun and directly inhibits the DNA binding activity of AP-1. The modification of c-Jun occurs both in vitro and in intact cells as detected by labeling with biotinylated 15d-PGJ2 and mass spectrometry analysis. Attachment of the cyclopentenone prostaglandin occurs at cysteine 269, which is located in the c-Jun DNA binding domain. In addition, 15d-PGJ2 can promote the oligomerization of a fraction of c-Jun through the formation of intermolecular disulfide bonds or 15d-PGJ2-bonded dimers. Our results identify a novel site of interaction of 15d-PGJ2 with the AP-1 activation pathway that may contribute to the complex effects of cyclopentenone prostaglandins on the cellular response to pro-inflammatory agents. They also show the first evidence for the induction of protein cross-linking by 15d-PGJ2.

Antioxidative, antinitrative, and vasculoprotective effects of a peroxisome proliferator-activated receptor-gamma agonist in hypercholesterolemia

BACKGROUND

Peroxisome proliferator-activated receptor (PPAR) signaling pathways have been reported to exert anti-inflammatory effects and attenuate atherosclerosis formation. However, the mechanisms responsible for their anti-inflammatory and antiatherosclerotic effects remain largely unknown. The present study tested the hypothesis that a PPARgamma agonist may exert significant endothelial protection by antioxidative and antinitrative effects.

METHODS AND RESULTS

Male New Zealand White rabbits were randomized to receive a normal (control) or a high-cholesterol diet and treated with vehicle or rosiglitazone (a PPARgamma agonist) 3 mg x kg(-1) x d(-1) for 5 weeks beginning 3 weeks after the high-cholesterol diet. At the end of 8 weeks of a high-cholesterol diet, the rabbits were killed, and the carotid arteries were isolated. Bioactive nitric oxide was determined functionally (endothelium-dependent vasodilatation) and biochemically (the phosphorylation of vasodilator-stimulated phosphoprotein, or P-VASP). Vascular superoxide production, PPARgamma, gp91phox, and inducible nitric oxide synthase (iNOS) expression, and vascular ONOO- formation were determined. Hypercholesterolemia caused severe endothelial dysfunction and reduced P-VASP, despite a marked increase in iNOS expression and total NOx production. Treatment with rosiglitazone enhanced PPARgamma expression, improved endothelium-dependent vasodilatation, preserved P-VASP, suppressed gp91phox and iNOS expression, reduced superoxide and total NOx production, and inhibited nitrotyrosine formation.

CONCLUSIONS

The PPARgamma agonist rosiglitazone exerted a significant vascular protective effect in hypercholesterolemic rabbits, most likely by attenuation of oxidative and nitrative stresses. The endothelial protective effects of PPARgamma agonists may reduce leukocyte accumulation in vascular walls and contribute to their antiatherosclerotic effect.

Antiinflammatory roles of peroxisome proliferator-activated receptor gamma in human alveolar macrophages

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcriptional factor belonging to the nuclear receptor superfamily. PPARgamma, which is predominantly expressed in adipose tissue, plays a major regulatory role in glucose metabolism and adipogenesis. Interestingly, recent studies have demonstrated PPARgamma expression in monocytes/macrophages and its antiinflammatory activities. However, it is unclear whether alveolar macrophages (AMs) express functional PPARgamma. The present study was conducted to investigate the expression of PPARgamma by AMs and to elucidate its functional role. Using reverse transcription-polymerase chain reaction and Western blotting, we demonstrated the strong expression of PPARs messenger RNA and protein in freshly isolated human AMs. Ligands of PPARgamma, 15-deoxy-delta(12,14)prostaglandin J2, and troglitazone significantly decreased LPS-induced tumor necrosis factor-alpha production by AMs. These ligands markedly upregulated the expression of CD36, a scavenger receptor that mediates the phagocytosis of apoptotic neutrophils. Indeed, ligand-treated AMs ingested a significantly higher number of apoptotic neutrophils than untreated AMs. These data indicate that PPARgamma expressed by AMs play an antiinflammatory role through inhibiting cytokine production and increasing their CD36 expression together with the enhanced phagocytosis of apoptotic neutrophils, which is an essential process for the resolution of inflammation. This suggests the potential therapeutic application of PPARgamma ligands in inflammatory disorders of the lung.

Repression of p65 transcriptional activation by the glucocorticoid receptor in the absence of receptor-coactivator interactions

Glucocorticoids are among the most potent antiinflammatory agents, acting through the glucocorticoid receptor (GR) to suppress gene expression of a variety of cytokines. This appears to be via transcriptional interference (or transrepression) of key regulatory factors such as nuclear factor-kappaB and activator protein 1. Ligand-bound GR can also activate gene transcription (transactivation) via direct binding to glucocorticoid response elements. Transactivation by GR is potentiated by accessory coactivators such as steroid receptor coactivator 1 and peroxisome proliferator-activated receptor gamma coactivator 1, whereas the role of these proteins in transrepression is unclear.Here, we show that GR can recruit several coactivator receptor interacting domains in a ligand-dependent manner. All interactions require the charge clamp defined by K579/E755, while a subset also requires a second charge clamp defined by R585/D590, within the GR ligand-binding domain. A point mutation, E755A, abolished all GR-receptor interacting domain interactions and led to a decrease in GR-mediated transactivation, but did not significantly affect GR-mediated transrepression of Gal4-p65 activity. Overexpression of a GR-interacting coactivator peptide blocked transactivation but did not affect transrepression of p65 or TNFalpha-induced IL-6 promoter activity. Finally, the GR antagonist RU486 did not recruit coactivators to GR but maintained the ability to transrepress p65 activity. Our data suggest that different coactivators utilize distinct contact points to interact with GR. Although GR interactions with specific coactivators are critical for transactivation, they appear to be dispensable for at least certain aspects of GR-mediated transrepression of nuclear factor-kappaB. This is consistent with the notion that all GR- mediated repression is not intrinsically linked to activation and can be separated mechanistically.

Peroxisome proliferator-activated receptor gamma activity is deficient in alveolar macrophages in pulmonary sarcoidosis

The ligand-activated transcription factor, peroxisome proliferator-activated receptor gamma (PPAR gamma), has pleiotropic effects on lipid and glucose metabolism as well as modulating immune activity. In Th1-predominant models of inflammatory bowel disease and arthritis, PPAR gamma ligands can ameliorate clinical disease severity, partly by downregulating a range of inflammatory cytokines. However, PPAR gamma has not been evaluated in chronic sarcoidosis, a disease characterized by persistent activation of Th1 immune responses in alveolar macrophages. We hypothesized that a deficiency of PPAR gamma activity contributes to ongoing inflammation in pulmonary sarcoidosis via failure to repress proinflammatory transcription factors. To address this, we studied eight patients with active sarcoidosis and nine healthy control subjects by bronchoscopy. Bronchoalveolar lavage specimens from patients revealed a striking reduction of PPAR gamma activity by electrophoretic mobility shift assay in alveolar macrophages compared with healthy control subjects, with a concomitant upregulation of nuclear factor (NF)-kappa B activity. Immunostaining and real-time polymerase chain reaction demonstrated reductions of PPAR gamma nuclear protein and gene expression. The data show for the first time that alveolar macrophages from patients with active sarcoidosis exhibit activation of NF-kappa B and deficiency of PPAR gamma. Although these results do not demonstrate a direct causal effect, they are consistent with the hypothesis that insufficient PPAR gamma activity contributes to ongoing dysregulated inflammation in pulmonary sarcoidosis by failing to suppress NF-kappa B.

Peroxisome proliferator-activated receptor gamma and ligands inhibit surfactant protein B gene expression in the lung

Pulmonary nonciliated bronchiolar epithelial cells (Clara cells) and alveolar type II (AT II) epithelial cells are responsible for surfactant synthesis and secretion. These cells are highly lipogenic with a high lipid turnover rate. Although only 10% of surfactant lipids are neutral lipids, they play very important roles in maintaining pulmonary surfactant homeostasis. Many metabolic intermediate products of neutral lipids serve as ligands for various nuclear receptors that bind to target genes to influence gene transcription. In this report, the functional role of the neutral lipid metabolites, 15-deoxy-Delta12,14-prostaglandin J2 and 9-hydroxyoctadecanoic acids, and peroxisome proliferator-activated receptor gamma was evaluated in surfactant protein B gene regulation. These reagents down-regulated surfactant protein B gene expression in respiratory epithelial cells at the transcriptional level in both cell line and whole lung explant systems. The studies support the concept that surfactant protein B homeostasis is influenced by neutral lipid metabolites in the lung.

The central role of fat and effect of peroxisome proliferator-activated receptor-gamma on progression of insulin resistance and cardiovascular disease

Recent evidence suggests that progression of insulin resistance parallels progression of atherosclerosis. Fat plays an integral role in the development of type 2 diabetes and vascular injury. The balance of adipose-derived substances, including free fatty acids, tumor necrosis factor-alpha, leptin, adiponectin, and plasminogen activator inhibitor-1, determine both insulin action and the state of vascular inflammation. Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) ligands promote the balance of these substances to enhance insulin-mediated glucose uptake and decrease inflammation. PPAR-gamma ligands reverse the major defect of the insulin resistance syndrome and have important effects that inhibit atherosclerosis, improve endothelial cell function, and attenuate inflammation. Although more research is needed, data suggest that PPAR-gamma ligands may prevent the progression of insulin resistance to diabetes and endothelial dysfunction to atherosclerosis.

Inhibition of IFN-gamma-mediated inducible nitric oxide synthase induction by the peroxisome proliferator-activated receptor gamma agonist, 15-deoxy-delta 12,14-prostaglandin J2, involves inhibition of the upstream Janus kinase/STAT1 signaling pathway

Peroxisome proliferator-activated receptor gamma (PPARgamma) ligands have been reported to exert anti-inflammatory activities in macrophages by competition for transcriptional coactivators with some transcriptional factors, including NF-kappaB. In the present study the influence of PPARgamma activators on IFN-gamma-elicited macrophage stimulation and signaling cascades was investigated. The results show that IFN-gamma-induced inducible NO synthase (iNOS) gene transcription, iNOS protein induction, and NO production are more sensitive to inhibition by 15-deoxy-Delta(12,14)-prostaglandin J(2) (15dPGJ(2)) than by the other two PPARgamma agonists, GW1929 and ciglitazone. Delayed addition of 15dPGJ(2) for 2 h resulted in reduced inhibition, suggesting action by 15dPGJ(2) on the upstream signaling cascades. Immunoblotting, DNA binding, and reporter gene assays consistently revealed the inhibitory ability of 15dPGJ(2), but not GW1929 or ciglitazone, on IFN-gamma-elicited signaling cascades, including tyrosine phosphorylation of Janus tyrosine protein kinase 2 and STAT1, DNA binding, and IFN regulatory factor-1 trans-activation of STAT1. These effects of 15dPGJ(2) were not abrogated by the PPARgamma antagonist, bisphenol A diglycidyl ether, indicating the PPARgamma-independent actions. 15dPGJ(2) also attenuated IL-6-induced tyrosine phosphorylation of STAT1 and STAT3 in Hep3B hepatoma cells. Consistent with the inhibitory effect of reactive oxygen species on STAT1 signaling, STAT1 inhibition by 15dPGJ(2) was abrogated by N-acetylcysteine, glutathione, superoxide dismutase, and catalase. Furthermore, 15dPGJ(2)-induced inhibition of STAT1 phosphorylation and NO production still occurred in the presence of peroxovanadate, ruling out the action mechanism of 15dPGJ(2) on tyrosine phosphatase. Taken together, for the first time in this study we demonstrate that 15dPGJ(2) can inhibit cytokine-stimulated Janus kinase 2-STAT signaling through a PPARgamma-independent, reactive oxygen species-dependent mechanism. These data provide a novel molecular mechanism of iNOS inhibition by 15dPGJ(2) and confirm its physiological role in anti-inflammation.

Peroxisome proliferator-activated receptor alpha negatively regulates T-bet transcription through suppression of p38 mitogen-activated protein kinase activation

Expression of the nuclear hormone receptor peroxisome proliferator-activated receptor alpha (PPARalpha) in resting lymphocytes was recently established, although the physiologic role(s) played by this nuclear hormone receptor in these cell types remains unresolved. In this study, we used CD4(+) T cells isolated from PPARalpha(-/-) and wild-type mice, as well as cell lines that constitutively express PPARalpha, in experiments designed to evaluate the role of this hormone receptor in the regulation of T cell function. We report that activated CD4(+) T cells lacking PPARalpha produce increased levels of IFN-gamma, but significantly lower levels of IL-2 when compared with activated wild-type CD4(+) T cells. Furthermore, we demonstrate that PPARalpha regulates the expression of these cytokines by CD4(+) T cells in part, through its ability to negatively regulate the transcription of T-bet. The induction of T-bet expression in CD4(+) T cells was determined to be positively influenced by p38 mitogen-activated protein (MAP) kinase activation, and the presence of unliganded PPARalpha effectively suppressed the phosphorylation of p38 MAP kinase. The activation of PPARalpha with highly specific ligands relaxed its capacity to suppress p38 MAP kinase phosphorylation and promoted T-bet expression. These results demonstrate a novel DNA-binding independent and agonist-controlled regulatory influence by the nuclear hormone receptor PPARalpha.

Inducible nitric oxide synthase expression inhibition by adenovirus E1A

Nitric oxide (NO) is an antiviral effector of the innate immune system. Viruses that can interfere with NO synthesis may be able to replicate more rapidly than viruses that cannot limit NO synthesis. We show that the adenovirus E1A protein inhibits NO production by decreasing expression of the inducible NO synthase (NOS2). The amino-terminal portion of E1A decreases transactivation of the NOS2 5'-flanking region, limiting the DNA binding activity of NF-kappaB and inhibiting NOS2 expression. E1A is thus able to deactivate a critical component of the host defense against viral infection. Viral inhibition of NO production is a mechanism that may enable certain viruses to evade the host innate immune system.

Cardiac hypertrophy: the good, the bad, and the ugly

Cardiac hypertrophy is the heart's response to a variety of extrinsic and intrinsic stimuli that impose increased biomechanical stress. While hypertrophy can eventually normalize wall tension, it is associated with an unfavorable outcome and threatens affected patients with sudden death or progression to overt heart failure. Accumulating evidence from studies in human patients and animal models suggests that in most instances hypertrophy is not a compensatory response to the change in mechanical load, but rather is a maladaptive process. Accordingly, modulation of myocardial growth without adversely affecting contractile function is increasingly recognized as a potentially auspicious approach in the prevention and treatment of heart failure. In this review, we summarize recent insights into hypertrophic signaling and consider several novel antihypertrophic strategies.

Peroxisome proliferator-activated receptors: regulation of transcriptional activities and roles in inflammation

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear receptor superfamily. Three PPARs isoforms have been characterized: PPARalpha, beta/delta and gamma. As other nuclear receptors, the PPARs are organized in distinct functional domains: A/B, C or DNA binding domain (DBD), D, E or ligand binding domain (LBD) and F. The A/B domain contains the activation function 1 (AF-1) which is transcriptionally active in absence of ligands. The DBD and the LBD of the PPARs determine the specificity of promoter DNA sequence recognition and ligand recognition, respectively. An activation function 2 (AF-2) is contained in the E domain, which mediates the ligand-dependent activation of the receptor. The transcriptional activity of the PPARs is regulated by post-translational modifications, such as phosphorylation and ubiquitination. Phosphorylation of PPARs is controlled by environmental factors activating different kinase pathways leading to the modulation of their activities. PPARs degradation by the ubiquitin-proteasome system modulates the intensity of the ligand response by controlling the level of PPAR proteins in the cells. PPARs also control the expression of genes implicated in the inflammatory response via negative interference with different inflammatory pathways, such as NFkappaB, AP-1, C/EBP beta, STAT-1 and NFAT. As such, PPARs influence inflammatory cytokine production and cell recruitment to the inflammatory sites. A better understanding of the mechanism of action of PPARs could improve the design of more specific and more efficient novel drugs. Molecules with dissociated effects could be useful for the treatment of lipid disorders or inflammation.

Differential expression of inducible nitric oxide synthase and peroxisome proliferator-activated receptor gamma in non-small cell lung carcinoma

Both inducible nitric oxide synthase (iNOS) and peroxisome proliferator-activated receptor gamma (PPARgamma) are closely associated with the development of human cancer. Although the expression of iNOS has been studied in non-small cell lung carcinoma (NSCLC), the level of PPARgamma has not been examined in tumorous and non-tumorous tissues from NSCLC. The present study analysed the levels of both iNOS and PPARgamma in NSCLC tissues and in lung cell lines. The possible role of these two molecules in the carcinogenesis of lung cancer was investigated. The expression of iNOS was significantly higher in the tumorous tissues than in the non-tumorous ones. In contrast to this pattern of iNOS protein expression, the level of PPARgamma was much lower in the tumorous tissues than in the non-tumorous samples. A similar result was also obtained in vitro using human lung cancer cell lines and normal lung cells. Immunohistochemical examination revealed that PPARgamma expression in the non-tumorous tissues was more likely to be located in the nucleus whereas it was present in both the nucleus and cytoplasm of the tumorous tissues. The intensity of iNOS expression was stronger in the nucleus than in the cytoplasm of the tumorous tissues. More than 50% of the cases tested did not express iNOS protein in the non-tumorous tissues. Statistical analysis indicated a negative correlation between iNOS and PPARgamma levels in the NSCLC tissues. In conclusion, this study demonstrated differing expressions for iNOS and PPARgamma in NSCLC tissues. Since activated PPARgamma is able to inhibit the expression of iNOS and the generation of iNOS is particularly associated with the inflammatory and environmental factors of lung cancer risk, this discrepant expression pattern may be associated with the pathogenesis of NSCLC.

Peroxisome proliferator-activated receptor gamma inhibits expression of minichromosome maintenance proteins in vascular smooth muscle cells

Using a cDNA array consisting only of cell cycle genes, we found that a novel nonthiazolidinedione partial peroxisome proliferator-activated receptor gamma (PPARgamma) agonist (nTZDpa) inhibited expression of minichromosome maintenance (MCM) proteins 6 and 7 in vascular smooth muscle cells. MCM proteins are required for the initiation and elongation stages of DNA replication and are regulated by the transcription factor E2F. Mitogen-induced MCM6 and MCM7 mRNA expression was potently inhibited by nTZDpa and to a lesser degree by the full PPARgamma agonist, rosiglitazone. Inhibition of MCM6 and MCM7 expression by nTZDpa and rosiglitazone paralleled their effect to inhibit phosphorylation of the retinoblastoma protein and cell proliferation. Transient transfection experiments revealed that the nTZDpa inhibited mitogen-induced MCM6 and MCM7 promoter activity, implicating a transcriptional mechanism. Adenoviral-mediated E2F overexpression reversed the suppressive effect of nTZDpa on MCM6 and MCM7 expression. Furthermore, activity of a luciferase reporter plasmid driven by multiple E2F elements was inhibited by nTZDpa, indicating that their down-regulation by nTZDpa involves an E2F-dependent mechanism. Overexpression of dominant-negative PPARgamma or addition of a PPARgamma antagonist, GW 9662, blocked nTZDpa inhibition of MCM7 transcription. Adenovirus-mediated overexpression of constitutively active PPARgamma inhibited MCM7 expression in a similar manner as the nTZDpa. These findings provide strong evidence that activation of PPARgamma attenuates MCM7 transcription and support the important role of this nuclear receptor in regulating vascular smooth muscle cell proliferation.

PPARgamma and PPARdelta negatively regulate specific subsets of lipopolysaccharide and IFN-gamma target genes in macrophages

Natural and synthetic agonists of the peroxisome proliferator-activated receptor gamma (PPARgamma) regulate adipocyte differentiation, glucose homeostasis, and inflammatory responses. Although effects on adipogenesis and glucose metabolism are genetically linked to PPARgamma, the PPARgamma dependence of antiinflammatory responses of these substances is less clear. Here, we have used a combination of mRNA expression profiling and conditional disruption of the PPARgamma gene in mice to characterize programs of transcriptional activation and repression by PPARgamma agonists in elicited peritoneal macrophages. Natural and synthetic PPARgamma agonists, including the thiazolidinedione rosiglitazone (Ro), modestly induced the expression of a surprisingly small number of genes, several of which were also induced by a specific PPARdelta agonist. The majority of these genes encode proteins involved in lipid homeostasis. In contrast, Ro inhibited induction of broad subsets of lipopolysaccharide and IFN-gamma target genes in a gene-specific and PPARgamma-dependent manner. At high concentrations, Ro inhibited induction of lipopolysaccharide target genes in PPARgamma-deficient macrophages, at least in part by activating PPARdelta. These studies establish overlapping transactivation and transrepression functions of PPARgamma and PPARdelta in macrophages and suggest that a major transcriptional role of PPARgamma is negative regulation of specific subsets of genes that are activated by T helper 1 cytokines and pathogenic molecules that signal through pattern recognition receptors. These findings support a physiological role of PPARgamma in regulating both native and acquired immune responses.

Induction of heme oxygenase-1 expression in murine macrophages is essential for the anti-inflammatory effect of low dose 15-deoxy-Delta 12,14-prostaglandin J2

15-Deoxy-Delta 12,14-prostaglandin J2 (15d-PGJ2), a cyclopentenone prostaglandin, displays a potent anti-inflammatory effect at micromolar concentrations (>2 microM) through direct inhibition of nuclear factor (NF)-kappa B activation. Here we show that at submicromolar concentrations (0.1-0.5 microM) 15d-PGJ2 retains the ability to suppress the production of tumor necrosis factor-alpha (TNF-alpha) and nitric oxide (NO) in lipopolysaccharide (LPS)-activated murine J774 macrophages under the conditions of a prolonged incubation (>12 h). Western blot analysis revealed that the expression of the cytoprotective enzyme, heme oxygenase-1 (HO-1), was induced and coincident with the anti-inflammatory action of 15d-PGJ2. Inhibition of HO-1 activity or scavenging carbon monoxide (CO), a byproduct derived from heme degradation, significantly attenuated the suppressive activity of 15d-PGJ2. Furthermore, LPS-induced NF-kappa B activation assessed by the inhibitory protein of NF-kappa B(I kappa B) degradation and p50 nuclear translocation was diminished in cells subjected to prolonged treatment with the low concentration of 15d-PGJ2. Treatment of cells with the protein synthesis inhibitor, cycloheximide, or the specific p38 MAP kinase inhibitor, SB203580, blocked the induction of HO-1 and suppression of LPS-induced I kappa B degradation mediated by 15d-PGJ2. Likewise, HO inhibitor and CO scavenger were effective in abolishing the inhibitory effects of 15d-PGJ2 on NF-kappa B activation induced by LPS. The functional role of CO was further demonstrated by the use of a CO releasing molecule, tricarbonyldichlororuthenium(II) dimer, which significantly suppressed LPS-induced nuclear translocation of p50 as assessed by confocal immunofluorescence. Collectively, these data suggest that even at submicromolar concentrations 15d-PGJ2 can exert an anti-inflammatory effect in macrophages through a mechanism that involves the action of HO/CO.

Activation-induced PPARgamma expression sensitizes primary human T cells toward apoptosis

Phytohemagglutinin (PHA) elicited expression of peroxisome proliferator-activated receptor gamma (PPARgamma) in primary human T cells via the PPARgamma3 promoter, as shown by reverse transcription-polymerase chain reaction. Electrophoretic mobility shift assay demonstrated no correlation between PPARgamma expression and its activation. However, addition of specific PPARgamma agonists such as ciglitazone or 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) for 1 h following PHA pretreatment provoked PPARgamma activation verified by supershift analysis. Taking the proapoptotic properties of PPARgamma into consideration, we analyzed induction of apoptosis in activated T cells in response to PPARgamma agonists. Cells exposed to PPARgamma agonists alone revealed minor cell death compared with controls, whereas treatment with 15d-PGJ(2) or ciglitazone for 4 h subsequent to PHA stimulation significantly increased cell demise, which was attenuated by the pan-caspase inhibitor zVAD, pointing to apoptosis as the underlying mechanism. These data may be relevant for pathophysiological conditions accompanied with lymphopenia of T cells under conditions such as sepsis.

New insights on oxidative stress and diabetic complications may lead to a "causal" antioxidant therapy

Evidence implicates hyperglycemia-derived oxygen free radicals as mediators of diabetic complications. However, intervention studies with classic antioxidants, such as vitamin E, failed to demonstrate any beneficial effect. Recent studies demonstrate that a single hyperglycemia-induced process of overproduction of superoxide by the mitochondrial electron-transport chain seems to be the first and key event in the activation of all other pathways involved in the pathogenesis of diabetic complications. These include increased polyol pathway flux, increased advanced glycosylation end product formation, activation of protein kinase C, and increased hexosamine pathway flux. Superoxide overproduction is accompanied by increased nitric oxide generation, due to an endothelial NOS and inducible NOS uncoupled state, a phenomenon favoring the formation of the strong oxidant peroxynitrite, which in turn damages DNA. DNA damage is an obligatory stimulus for the activation of the nuclear enzyme poly(ADP-ribose) polymerase. Poly(ADP-ribose) polymerase activation in turn depletes the intracellular concentration of its substrate NAD(+), slowing the rate of glycolysis, electron transport, and ATP formation, and produces an ADP-ribosylation of the GAPDH. These processes result in acute endothelial dysfunction in diabetic blood vessels that, convincingly, also contributes to the development of diabetic complications. These new findings may explain why classic antioxidants, such as vitamin E, which work by scavenging already-formed toxic oxidation products, have failed to show beneficial effects on diabetic complications and may suggest new and attractive "causal" antioxidant therapy. New low-molecular mass compounds that act as SOD or catalase mimetics or L-propionyl-carnitine and lipoic acid, which work as intracellular superoxide scavengers, improving mitochondrial function and reducing DNA damage, may be good candidates for such a strategy, and preliminary studies support this hypothesis. This "causal" therapy would also be associated with other promising tools such as LY 333531, PJ34, and FP15, which block the protein kinase beta isoform, poly(ADP-ribose) polymerase, and peroxynitrite, respectively. While waiting for these focused tools, we may have other options: thiazolinediones, statins, ACE inhibitors, and angiotensin 1 inhibitors can reduce intracellular oxidative stress generation, and it has been suggested that many of their beneficial effects, even in diabetic patients, are due to this property.

Peroxisome proliferator-activated receptors: a critical review on endogenous pathways for ligand generation

Lipid mediators can exert their effects by interactions with well-characterised cell surface G-protein-linked receptors. Recently, a group of intracellular receptors have been identified that are activated by a large variety of lipid-derived mediators. Amongst these novel targets, the peroxisome proliferator-activated receptors (PPARs), a family of three (PPARalpha, beta/delta and gamma) nuclear receptor/transcription factors have become a major area for investigation. PPARs are found throughout the body, where they have diverse roles regulating lipid homeostasis, cellular differentiation, proliferation and the immune response. There is a great interest, therefore, in the roles of PPARs in a variety of pathological conditions, including diabetes, atherosclerosis, cancer and chronic inflammation. Although, a number of naturally occurring compounds can activate PPARs, it has been difficult, as yet, to characterise any of these mediators as truly endogenous ligands. These findings have lead to the suggestion that PPARs may act just as general lipid sensors. Acting as lipid sensors, PPARs may take changes in lipid/fatty acid balance in the diet or local metabolism and translate them to tissue-specific ligands, exerting tissue-specific effects. Using classical pharmacological criteria for endogenous mediator classification we will critically discuss the variety of pathways for putative ligand generation.

Proteomic analysis of S-nitrosylated proteins in mesangial cells

NO participates in numerous biological events in a variety of cell types including activated glomerular mesangial cells. Many of these events appear to be independent of the known effects of NO on soluble guanylyl cyclase. NO derived from all major isoforms of NO synthase can S-nitrosylate cysteine residues in target proteins, potentially altering their functional activities. Recent evidence suggests that S-nitrosylation is specific, is regulated, and may play an important regulatory role akin to phosphorylation. In the present study, the "biotin-switch" method of isolating S-nitrosylated proteins was coupled with two-dimensional PAGE protein separation followed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and peptide mass fingerprinting to identify target proteins for S-nitrosylation in murine mesangial cells treated with NO donors or appropriate controls. This approach resolved 790 protein spots. We analyzed the most abundant spots and identified 34 known proteins. Of these, 31 are unique S-nitrosylated proteins not previously identified, including signaling proteins, receptors and membrane proteins, cytoskeletal or cell matrix proteins, and cytoplasmic proteins. Prominent among these were peroxisome proliferator activated receptor gamma, uroguanylin, GTP-binding protein alpha, protein 14-3-3, NADPH-cytochrome P450 oxidoreductase, transcription factor IIA, melusin, mitosin, phospholipase A2-activating protein, and protein-tyrosine phosphatase. The in vivo induction of S-nitrosylation was assayed by treating mesangial cells with interleukin-1beta followed by the biotin-switch and Western blot of selected targets. These results broaden our knowledge of potential signal transduction pathways and other cell functions mediated by NO S-nitrosylation.