The PPARdelta agonist GW501516 suppresses interleukin-6-mediated hepatocyte acute phase reaction via STAT3 inhibition

Mitogen-Activated Protein Kinase and Exploratory Nuclear Receptor Crosstalk in Cancer Immunotherapy

The three major mitogen-activated protein kinase (MAPK) pathways (ERK1/2, p38, and JNK/SAPK) are upstream regulators of the nuclear receptor superfamily (NRSF). These ligand-activated transcription factors are divided into subclasses comprising receptors for endocrine hormones, metabolic compounds (e.g., vitamins, diet), xenobiotics, and mediators released from host immune reactions such as tissue injury and inflammation. These internal and external cues place the NRSF at the frontline as sensors and translators of information from the environment towards the genome. For most of the former "orphan" receptors, physiological and synthetic ligands have been identified, opening intriguing opportunities for combination therapies with existing cancer medications. Hitherto, only preclinical data are available, warranting further validation in clinical trials in patients. The current review summarized the existing literature covering the expression and function of NRSF subclasses in human solid tumors and hematopoietic malignancies and their modulatory effects on innate (e.g., macrophages, dendritic cells) and adaptive (i.e., T cell subsets) immune cells, encouraging mechanistic and pharmacological studies in combination with current clinically approved therapeutics against immune checkpoint molecules (e.g., PD1).

Strawberry notch homolog 2 regulates the response to interleukin-6 in the central nervous system

BACKGROUND

The cytokine interleukin-6 (IL-6) modulates a variety of inflammatory processes and, context depending, can mediate either pro- or anti-inflammatory effects. Excessive IL-6 signalling in the brain is associated with chronic inflammation resulting in neurodegeneration. Strawberry notch homolog 2 (Sbno2) is an IL-6-regulated gene whose function is largely unknown. Here we aimed to address this issue by investigating the impact of Sbno2 disruption in mice with IL-6-mediated neuroinflammation.

METHODS

Mice with germline disruption of Sbno2 (Sbno2-/-) were generated and crossed with transgenic mice with chronic astrocyte production of IL-6 (GFAP-IL6). Phenotypic, molecular and transcriptomic analyses were performed on tissues and primary cell cultures to clarify the role of SBNO2 in IL-6-mediated neuroinflammation.

RESULTS

We found Sbno2-/- mice to be viable and overtly normal. By contrast GFAP-IL6 × Sbno2-/- mice had more severe disease compared with GFAP-IL6 mice. This was evidenced by exacerbated neuroinflammation and neurodegeneration and enhanced IL-6-responsive gene expression. Cell culture experiments on primary astrocytes from Sbno2-/- mice further showed elevated and sustained transcript levels of a number of IL-6 stimulated genes. Notably, despite enhanced disease in vivo and gene expression both in vivo and in vitro, IL-6-stimulated gp130 pathway activation was reduced when Sbno2 is disrupted.

CONCLUSION

Based on these results, we propose a role for SBNO2 as a novel negative feedback regulator of IL-6 that restrains the excessive inflammatory actions of this cytokine in the brain.

Investigation of PPARβ/δ within Human Dental Pulp Cells: A Preliminary In Vitro Study

Controlling the inflammatory response to restore tissue homeostasis is a crucial step to maintain tooth vitality after pathogen removal from caries-affected dental tissues. The nuclear peroxisome proliferator-activated receptor beta/delta (PPARβ/δ) is a ligand-activated transcription factor with emerging anti-inflammatory roles in many cells and tissues. However, its expression and functions are poorly understood in human dental pulp cells (hDPCs). Thus, this study evaluated PPARβ/δ expression and assessed the anti-inflammatory effects evoked by activation of PPARβ/δ in lipopolysaccharide- (LPS-) induced hDPCs. Our results showed that hDPCs constitutively expressed PPARβ/δ mRNA/protein, and treatment with LPS increased PPARβ/δ mRNA expression. The selective PPARβ/δ agonist GW0742 significantly decreased inflammation-related mRNA expression in hDPCs (IL6, IL1β, TNFα, MMP1, and MMP2) and RAW264.7 cells (Il6 and Tnfα). Further, PPARβ/δ agonist attenuated MMP2/9 gelatinolytic activity in hDPCs. Previously LPS-conditioned hDPCs increased the migration of RAW264.7 cells through the membrane of a Transwell coculture system. Conversely, pretreatment with GW0742 markedly decreased macrophage recruitment. These findings provide among the first evidence that hDPCs express PPARβ/δ. In addition, they suggest that activation of PPARβ/δ by GW0742 can attenuate some cellular and molecular in vitro aspects related to the inflammatory process, pointing out to investigate its potential target role in dental pulp inflammation.

The role of peroxisome proliferator-activated receptors (PPAR) in immune responses

Peroxisome proliferator-activated receptors (PPARs) are fatty acid-activated transcription factors of nuclear hormone receptor superfamily that regulate energy metabolism. Currently, three PPAR subtypes have been identified: PPARα, PPARγ, and PPARβ/δ. PPARα and PPARδ are highly expressed in oxidative tissues and regulate genes involved in substrate delivery and oxidative phosphorylation (OXPHOS) and regulation of energy homeostasis. In contrast, PPARγ is more important in lipogenesis and lipid synthesis, with highest expression levels in white adipose tissue (WAT). In addition to tissues regulating whole body energy homeostasis, PPARs are expressed in immune cells and have an emerging critical role in immune cell differentiation and fate commitment. In this review, we discuss the actions of PPARs in the function of the innate and the adaptive immune system and their implications in immune-mediated inflammatory conditions.

Endoplasmic Reticulum Stress Activation in Alport Syndrome Varies Between Genotype and Cell Type

Alport syndrome is a hereditary progressive chronic kidney disease caused by mutations in type IV collagen genes COL4A3/4/5. X-linked Alport syndrome (XLAS) is caused by mutations in the COL4A5 gene and is the most common form of Alport syndrome. A strong correlation between the type of COL4A5 mutation and the age developing end-stage renal disease in male patients has been found. Mutation to the α (IV) chain causes retention of the protein to the endoplasmic reticulum lumen, which causes endoplasmic reticulum stress (ERS) and subsequent exertion of deleterious intracellular effects through the activation of ERS. The exact time point that mutant type IV collagen α chain exerts its deleterious effects remains elusive. In this study, we explored the relationship between the COL4A5 genotype and cell type in ERS activation. We obtained skin fibroblasts from Alport syndrome patients with different COL4A5 mutation categories [i.e., a missense mutation (c.4298G > T, p.Gly1433Val) in exon 47, a splicing mutation (c.1949–1G > A) in intron 25 and an insertion (c.573_c.574insG, p. Pro193Alafs*23) in exon 10], and then reprogrammed these fibroblasts into induced pluripotent stem cells (iPSCs). Interestingly, no significant dysregulation of ERS pathway markers was observed for the three COL4A5 mutant iPSCs; however, significant activation of ERS in COL4A5 mutant fibroblasts was observed. In addition, we found that the activation levels of some ERS markers in fibroblasts varied among the three COL4A5 mutation types. Mutant COL4A5 proteins were demonstrated to have different effects on cells at different stages of ontogenesis, providing a theoretical basis for choosing the timing of intervention. The observed differences in activation of ERS by the COL4A5 mutant fibroblasts may contribute to the intracellular molecular mechanisms that describe the correlation between genotype and clinical features in XLAS.

Improved Prediction of Survival by a Risk Factor-Integrating Inflammatory Score in Sorafenib-Treated Hepatocellular Carcinoma

BACKGROUND AND AIMS

Inflammation affects progression of hepatocellular carcinoma (HCC). We therefore postulate that systemic inflammatory markers could help to predict prognosis in HCC patients receiving sorafenib therapy.

METHODS

Overall survival (OS) of HCC patients receiving palliative sorafenib treatment was correlated with the neutrophil to lymphocyte ratio (NLR), platelet to lymphocyte ratio (PLR), C-reactive protein to albumin ratio (CAR), Glasgow prognostic score (GPS) and the modified GPS (mGPS) along with clinicopathological parameters. Predictors of OS were assessed by multivariable Cox regression and receiver operating characteristics and area under the curve (ROC-AUC) analyses.

RESULTS

Patients receiving sorafenib (n = 120) for advanced HCC (Barcelona Clinic Liver Cancer stage C) were explored by retrospective analysis. Findings were subsequently validated by a second HCC cohort (n = 113) receiving sorafenib at two independent treatment centers. Multivariable assessment across these HCC cohorts confirmed a stable association of CAR (p ≤ 0.001), GPS (p ≤ 0.01) and mGPS (p ≤ 0.004) with OS. This study also identified Eastern Cooperative Oncology Group (ECOG) performance score (p < 0.001) and portal thrombosis (p = 0.002) as prognostic factors and uncovered an inconsistent OS association of NLR and PLR in HCC patients. Additional combined analysis of ECOG, portal thrombosis and GPS within an extended score (GPS-EP) was associated with OS (p = 0.021), which was confirmed within the validation cohort (p = 0.001). In sorafenib-treated HCC, the ROC-AUC value for the prediction of 12-month survival was 0.761 (CAR >/≤0.37 cut-off, p < 0.001), 0.766 (GPS, p < 0.001) and 0.754 (mGPS, p < 0.001), respectively. In comparison to this, GPS-EP achieved a higher AUC of 0.826 (0.746-0.907) for the 12-month survival prediction, resulting in a 64.4% sensitivity and 83.3% specificity at a > 2 point cut-off.

CONCLUSIONS

Inflammatory scores obtained before sorafenib treatment initiation are associated with OS in advanced HCC. Their combination with other risk factors improves prediction of 3- and 12-month survival, which could guide treatment decisions in selected patient subgroups.

Insights into the Role of PPARβ/δ in NAFLD

Non-alcoholic fatty liver disease (NAFLD) is a major health issue in developed countries. Although usually associated with obesity, NAFLD is also diagnosed in individuals with low body mass index (BMI) values, especially in Asia. NAFLD can progress from steatosis to non-alcoholic steatohepatitis (NASH), which is characterized by liver damage and inflammation, leading to cirrhosis and hepatocellular carcinoma (HCC). NAFLD development can be induced by lipid metabolism alterations; imbalances of pro- and anti-inflammatory molecules; and changes in various other factors, such as gut nutrient-derived signals and adipokines. Obesity-related metabolic disorders may be improved by activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)β/δ, which is involved in metabolic processes and other functions. This review is focused on research findings related to PPARβ/δ-mediated regulation of hepatic lipid and glucose metabolism and NAFLD development. It also discusses the potential use of pharmacological PPARβ/δ activation for NAFLD treatment.

Ligand activation of peroxisome proliferator-activated receptor-β/δ suppresses liver tumorigenesis in hepatitis B transgenic mice

Peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) inhibits steatosis and inflammation, known risk factors for liver cancer. In this study, the effect of ligand activation of PPARβ/δ in modulating liver tumorigenesis in transgenic hepatitis B virus (HBV) mice was examined. Activation of PPARβ/δ in HBV mice reduced steatosis, the average number of liver foci, and tumor multiplicity. Reduced expression of hepatic CYCLIN D1 and c-MYC, tumor necrosis factor alpha (Tnfa) mRNA, serum levels of alanine aminotransaminase, and an increase in apoptotic signaling was also observed following ligand activation of PPARβ/δ in HBV mice compared to controls. Inhibition of Tnfa mRNA expression was not observed in wild-type hepatocytes. Ligand activation of PPARβ/δ inhibited lipopolysaccharide (LPS)-induced mRNA expression of Tnfa in wild-type, but not in Pparβ/δ-null Kupffer cells. Interestingly, LPS-induced expression of Tnfa mRNA was also inhibited in Kupffer cells from a transgenic mouse line that expressed a DNA binding mutant form of PPARβ/δ compared to controls. Combined, these results suggest that ligand activation of PPARβ/δ attenuates hepatic tumorigenesis in HBV transgenic mice by inhibiting steatosis and cell proliferation, enhancing hepatocyte apoptosis, and modulating anti-inflammatory activity in Kupffer cells.

The transcriptional PPARβ/δ network in human macrophages defines a unique agonist-induced activation state

Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is a lipid ligand-inducible transcription factor with established metabolic functions, whereas its anti-inflammatory function is poorly understood. To address this issue, we determined the global PPARβ/δ-regulated signaling network in human monocyte-derived macrophages. Besides cell type-independent, canonical target genes with metabolic and immune regulatory functions we identified a large number of inflammation-associated NFκB and STAT1 target genes that are repressed by agonists. Accordingly, PPARβ/δ agonists inhibited the expression of multiple pro-inflammatory mediators and induced an anti-inflammatory, IL-4-like morphological phenotype. Surprisingly, bioinformatic analyses also identified immune stimulatory effects. Consistent with this prediction, PPARβ/δ agonists enhanced macrophage survival under hypoxic stress and stimulated CD8⁺ T cell activation, concomitantly with the repression of immune suppressive target genes and their encoded products CD274 (PD-1 ligand), CD32B (inhibitory Fcγ receptor IIB) and indoleamine 2,3-dioxygenase 1 (IDO-1), as well as a diminished release of the immune suppressive IDO-1 metabolite kynurenine. Comparison with published data revealed a significant overlap of the PPARβ/δ transcriptome with coexpression modules characteristic of both anti-inflammatory and pro-inflammatory cytokines. Our findings indicate that PPARβ/δ agonists induce a unique macrophage activation state with strong anti-inflammatory but also specific immune stimulatory components, pointing to a context-dependent function of PPARβ/δ in immune regulation.

PPARβ/δ prevents endoplasmic reticulum stress-associated inflammation and insulin resistance in skeletal muscle cells through an AMPK-dependent mechanism

AIM/HYPOTHESIS

Endoplasmic reticulum (ER) stress, which is involved in the link between inflammation and insulin resistance, contributes to the development of type 2 diabetes mellitus. In this study, we assessed whether peroxisome proliferator-activated receptor (PPAR)β/δ prevented ER stress-associated inflammation and insulin resistance in skeletal muscle cells.

METHODS

Studies were conducted in mouse C2C12 myotubes, in the human myogenic cell line LHCN-M2 and in skeletal muscle from wild-type and PPARβ/δ-deficient mice and mice exposed to a high-fat diet.

RESULTS

The PPARβ/δ agonist GW501516 prevented lipid-induced ER stress in mouse and human myotubes and in skeletal muscle of mice fed a high-fat diet. PPARβ/δ activation also prevented thapsigargin- and tunicamycin-induced ER stress in human and murine skeletal muscle cells. In agreement with this, PPARβ/δ activation prevented ER stress-associated inflammation and insulin resistance, and glucose-intolerant PPARβ/δ-deficient mice showed increased phosphorylated levels of inositol-requiring 1 transmembrane kinase/endonuclease-1α in skeletal muscle. Our findings demonstrate that PPARβ/δ activation prevents ER stress through the activation of AMP-activated protein kinase (AMPK), and the subsequent inhibition of extracellular-signal-regulated kinase (ERK)1/2 due to the inhibitory crosstalk between AMPK and ERK1/2, since overexpression of a dominant negative AMPK construct (K45R) reversed the effects attained by PPARβ/δ activation.

CONCLUSIONS/INTERPRETATION

Overall, these findings indicate that PPARβ/δ prevents ER stress, inflammation and insulin resistance in skeletal muscle cells by activating AMPK.

Identification of FHL2-regulated genes in liver by microarray and bioinformatics analysis

FHL2 is a LIM domain protein that is able to form various protein complexes and regulate gene transcription. Recent findings showed that FHL2 is a potential tumor suppressor gene that was down-regulated in hepatocellular carcinoma. In the present study, microarray profiling of gene expression was performed to identify the genes regulated by FHL2 in mouse livers. The differentially expressed genes were further analyzed by bioinformatics tools including DAVID, KEGG, and STRING. Our data illustrate that FHL2 affects genes involved in various functions including signal transduction, responses to external stimulus, cancer-related pathways, cardiovascular function and regulation of actin cytoskeleton. Moreover, a network of differentially expressed genes identified in this study and known FHL2-interacting proteins was constructed. Then, genes identified by bioinformatics tools and most functional relevant to FHL2 were selected for further validation. Finally, the differential expression of Ar, Id3, Inhbe, Alas1, Bcl6, Pparδ, Angptl4, and Erbb4 were confirmed by quantitative real-time PCR. In summary, we have established a database of genes that are potentially regulated by FHL2 and these genes should be future targets for the elucidation of functional roles of FHL2.

Clustering nuclear receptors in liver regeneration identifies candidate modulators of hepatocyte proliferation and hepatocarcinoma

Background & Aims

Liver regeneration (LR) is a valuable model for studying mechanisms modulating hepatocyte proliferation. Nuclear receptors (NRs) are key players in the control of cellular functions, being ideal modulators of hepatic proliferation and carcinogenesis.

Methods & Results

We used a previously validated RT-qPCR platform to profile modifications in the expression of all 49 members of the NR superfamily in mouse liver during LR. Twenty-nine NR transcripts were significantly modified in their expression during LR, including fatty acid (peroxisome proliferator-activated receptors, PPARs) and oxysterol (liver X receptors, Lxrs) sensors, circadian masters RevErbα and RevErbβ, glucocorticoid receptor (Gr) and constitutive androxane receptor (Car). In order to detect the NRs that better characterize proliferative status vs. proliferating liver, we used the novel Random Forest (RF) analysis to selected a trio of down-regulated NRs (thyroid receptor alpha, Trα; farsenoid X receptor beta, Fxrβ; Pparδ) as best discriminators of the proliferating status. To validate our approach, we further studied PPARδ role in modulating hepatic proliferation. We first confirmed the suppression of PPARδ both in LR and human hepatocellular carcinoma at protein level, and then demonstrated that PPARδ agonist GW501516 reduces the proliferative potential of hepatoma cells.

Conclusions

Our data suggest that NR transcriptome is modulated in proliferating liver and is a source of biomarkers and bona fide pharmacological targets for the management of liver disease affecting hepatocyte proliferation.

Genetic and environmental influences on the prospective correlation between systemic inflammation and coronary heart disease death in male twins

OBJECTIVE

Because of lack of evidence, we aimed to examine to what degree low-grade systemic inflammation and coronary heart disease (CHD) death shared common genetic and environmental substrates.

APPROACH AND RESULTS

From the 41-year prospective National Heart, Lung, and Blood Institute Twin Study, we included 950 middle-aged male twins at baseline (1969-1973). Low-grade systemic inflammation was measured with plasma levels of interleukin-6 (IL-6) and C-reactive protein. Univariate and bivariate structural equation models were used, adjusted for a risk score for CHD death. The score-adjusted heritability was 19% for IL-6, 27% for C-reactive protein, and 22% for CHD death. The positive phenotypic correlation of IL-6 with CHD death (radjusted=0.27; 95% confidence interval [CI], 0.08-0.43) was driven by additive genetic factors (contribution [relative contribution], 0.30 [111%]) but attenuated by unique environment (-0.03 [-11%]). The genetic correlation between IL-6 and CHD death was 0.74 (95% CI, 0.21-1.00), whereas the unique environmental correlation was -0.05 (95% CI, -0.35 to 0.25). The proportion of genetic variance for CHD death shared with that for IL-6 was 74%. The phenotypic correlation of C-reactive protein with CHD death (radjusted=0.10; 95% CI, -0.02 to 0.22) was explained by additive genetic factors (0.20 [149%]) but was attenuated by the unique environment (-0.09 [-49%]). The genetic correlation of C-reactive protein with CHD death was 0.63 (95% CI, -0.07 to 1.00), whereas the unique environmental correlation was -0.07 (95% CI, -0.29 to 0.17).

CONCLUSIONS

Low-grade systemic inflammation, measured by IL-6, and long-term CHD death share moderate genetic substrates that augment both traits.

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

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

Conditional Expression of Human PPARδ and a Dominant Negative Variant of hPPARδ In Vivo

The nuclear receptor, NR1C2 or peroxisome proliferator-activated receptor (PPAR)-δ, is ubiquitously expressed and important for placental development, fatty acid metabolism, wound healing, inflammation, and tumour development. PPARδ has been hypothesized to function as both a ligand activated transcription factor and a repressor of transcription in the absence of agonist. In this paper, treatment of mice conditionally expressing human PPARδ with GW501516 resulted in a marked loss in body weight that was not evident in nontransgenic animals or animals expressing a dominant negative derivative of PPARδ. Expression of either functional or dominant negative hPPARδ blocked bezafibrate-induced PPARα-dependent hepatomegaly and blocked the effect of bezafibrate on the transcription of PPARα target genes. These data demonstrate, for the first time, that PPARδ could inhibit the activation of PPARα in vivo and provide novel models for the investigation of the role of PPARδ in pathophysiology.

The peroxisome proliferator-activated receptor (PPAR) β/δ agonist GW501516 inhibits IL-6-induced signal transducer and activator of transcription 3 (STAT3) activation and insulin resistance in human liver cells

AIM/HYPOTHESIS

IL-6 induces insulin resistance by activating signal transducer and activator of transcription 3 (STAT3) and upregulating the transcription of its target gene SOCS3. Here we examined whether the peroxisome proliferator-activated receptor (PPAR)β/δ agonist GW501516 prevented activation of the IL-6-STAT3-suppressor of cytokine signalling 3 (SOCS3) pathway and insulin resistance in human hepatic HepG2 cells.

METHODS

Studies were conducted with human HepG2 cells and livers from mice null for Pparβ/δ (also known as Ppard) and wild-type mice.

RESULTS

GW501516 prevented IL-6-dependent reduction in insulin-stimulated v-akt murine thymoma viral oncogene homologue 1 (AKT) phosphorylation and in IRS-1 and IRS-2 protein levels. In addition, treatment with this drug abolished IL-6-induced STAT3 phosphorylation of Tyr⁷⁰⁵ and Ser⁷²⁷ and prevented the increase in SOCS3 caused by this cytokine. Moreover, GW501516 prevented IL-6-dependent induction of extracellular-related kinase 1/2 (ERK1/2), a serine-threonine protein kinase involved in serine STAT3 phosphorylation; the livers of Pparβ/δ-null mice showed increased Tyr⁷⁰⁵- and Ser⁷²⁷-STAT3 as well as phospho-ERK1/2 levels. Furthermore, drug treatment prevented the IL-6-dependent reduction in phosphorylated AMP-activated protein kinase (AMPK), a kinase reported to inhibit STAT3 phosphorylation on Tyr⁷⁰⁵. In agreement with the recovery in phospho-AMPK levels observed following GW501516 treatment, this drug increased the AMP/ATP ratio and decreased the ATP/ADP ratio.

CONCLUSIONS/INTERPRETATION

Overall, our findings show that the PPARβ/δ activator GW501516 prevents IL-6-induced STAT3 activation by inhibiting ERK1/2 phosphorylation and preventing the reduction in phospho-AMPK levels. These effects of GW501516 may contribute to the prevention of cytokine-induced insulin resistance in hepatic cells.

The use of rat lens explant cultures to study the mechanism of drug-induced cataractogenesis

Lens explant cultures were used to assess the mechanism of drug-induced cataractogenic potential of NVS001, a peroxisome proliferator-activated receptor delta (PPARδ) agonist, which resulted in cataract in all treated animals during a 13-week rat study. Ciglitazone, a PPARγ agonist and cataractogenic compound, was used as a positive control to validate this model. Rat lenses were extracted and cultured in medium supplemented with antibiotics for 24-h preincubation pretreatment. Lenses showing no signs of damage at the end of the preincubation pretreatment period were randomized into five experimental groups, (1) untreated control, (2) 0.1% dimethyl sulphoxide control, (3) 10μM NVS001, (4) 10μM ciglitazone, and (5) 10μM acetaminophen (negative control). Lenses were treated every 24 h after preincubation pretreatment for up to 48 h. Samples for viability, histology, and gene expression profiling were collected at 4, 24, and 48 h. There was a time-dependent increase in opacity, which correlated to a decrease in viability measured by adenosine triphosphate levels in NVS001 and ciglitazone-treated lenses compared with controls. NVS001 and ciglitazone had comparable cataractogenic effects after 48 h with histology showing rupture of the lens capsule, lens fiber degeneration, cortical lens vacuolation, and lens epithelial degeneration. Furthermore, no changes were seen when lenses were treated with acetaminophen. Gene expression analysis supported oxidative and osmotic stress, along with decreases in membrane and epithelial cell integrity as key factors in NVS001-induced cataracts. This study suggests that in vitro lens cultures can be used to assess cataractogenic potential of PPAR agonists and to study/understand the underlying molecular mechanism of cataractogenesis in rat.

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

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

NO-Donating NSAIDs, PPARdelta, and Cancer: Does PPARdelta Contribute to Colon Carcinogenesis?

The chemopreventive NO-donating NSAIDs (NO-NSAIDs; NSAIDs with an NO-releasing moiety) modulate PPARδ and offer the opportunity to revisit the controversial role of PPARδ in carcinogenesis (several papers report that PPARδ either promotes or inhibits cancer). This review summarizes the pharmacology of NO-NSAIDs, PPARδ cancer biology, and the relationship between the two. In particular, a study of the chemopreventive effect of two isomers of NO-aspirin on intestinal neoplasia in Min mice showed that, compared to wild-type controls, PPARδ is overexpressed in the intestinal mucosa of Min mice; PPARδ responds to m- and p-NO-ASA proportionally to their antitumor effect (p- > m-). This effect is accompanied by the induction of epithelial cell death, which correlates with the antineoplastic effect of NO-aspirin; and NO-aspirin's effect on PPARδ is specific (no changes in PPARα or PPARγ). Although these data support the notion that PPARδ promotes intestinal carcinogenesis and its inhibition could be therapeutically useful, more work is needed before a firm conclusion is reached.

Activation of peroxisome proliferator-activated receptor-β/-δ (PPAR-β/-δ) ameliorates insulin signaling and reduces SOCS3 levels by inhibiting STAT3 in interleukin-6-stimulated adipocytes

OBJECTIVE

It has been suggested that interleukin (IL)-6 is one of the mediators linking obesity-derived chronic inflammation with insulin resistance through activation of STAT3, with subsequent upregulation of suppressor of cytokine signaling 3 (SOCS3). We evaluated whether peroxisome proliferator-activated receptor (PPAR)-β/-δ prevented activation of the IL-6-STAT3-SOCS3 pathway and insulin resistance in adipocytes.

RESEARCH DESIGN AND METHODS

Adipocytes and white adipose tissue from wild-type and PPAR-β/-δ-null mice were used to evaluate the effect of PPAR-β/-δ on the IL-6-STAT3-SOCS3 pathway.

RESULTS

First, we observed that the PPAR-β/-δ agonist GW501516 prevented both IL-6-dependent reduction in insulin-stimulated Akt phosphorylation and glucose uptake in adipocytes. In addition, this drug treatment abolished IL-6-induced SOCS3 expression in differentiated 3T3-L1 adipocytes. This effect was associated with the capacity of the drug to prevent IL-6-induced STAT3 phosphorylation on Tyr(705) and Ser(727) residues in vitro and in vivo. Moreover, GW501516 prevented IL-6-dependent induction of extracellular signal-related kinase (ERK)1/2, a serine-threonine-protein kinase involved in serine STAT3 phosphorylation. Furthermore, in white adipose tissue from PPAR-β/-δ-null mice, STAT3 phosphorylation (Tyr(705) and Ser(727)), STAT3 DNA-binding activity, and SOCS3 protein levels were higher than in wild-type mice. Several steps in STAT3 activation require its association with heat shock protein 90 (Hsp90), which was prevented by GW501516 as revealed in immunoprecipitation studies. Consistent with this finding, the STAT3-Hsp90 association was enhanced in white adipose tissue from PPAR-β/-δ-null mice compared with wild-type mice.

CONCLUSIONS

Collectively, our findings indicate that PPAR-β/-δ activation prevents IL-6-induced STAT3 activation by inhibiting ERK1/2 and preventing the STAT3-Hsp90 association, an effect that may contribute to the prevention of cytokine-induced insulin resistance in adipocytes.

Peroxisome proliferator-activated receptor β/δ agonism protects the kidney against ischemia/reperfusion injury in diabetic rats

Diabetes is an important risk factor for ischemic acute kidney injury, whose pharmacological treatment remains an unmet medical need. The peroxisome proliferator-activated receptor (PPAR) β/δ is highly expressed in the kidney, although its role has not yet been elucidated. Here, we used an in vivo model of renal ischemia/reperfusion (I/R) in streptozotocin-induced diabetic rats (i) to evaluate whether diabetes increases kidney susceptibility to I/R injury and (ii) to investigate the effects of PPARβ/δ activation. The degree of renal injury (1h ischemia/6h reperfusion) was significantly increased in diabetic rats compared with nondiabetic littermates. PPARβ/δ expression was increased after I/R, with the highest levels in diabetic rats. Administration of the selective PPARβ/δ agonist GW0742 attenuated the renal dysfunction, leukocyte infiltration, and formation of interleukin-6 and tumor necrosis factor-α. These effects were accompanied by an increased expression of the suppressor of cytokine signaling (SOCS)-3, which plays a critical role in the cytokine-activated signaling pathway. The beneficial effects of GW0742 were attenuated by the selective PPARβ/δ antagonist GSK0660. Thus, we report herein that PPARβ/δ activation protects the diabetic kidney against I/R injury by a mechanism that may involve changes in renal expression of SOCS-3 resulting in a reduced local inflammatory response.

PPARβ/δ activation blocks lipid-induced inflammatory pathways in mouse heart and human cardiac cells

Owing to its high fat content, the classical Western diet has a range of adverse effects on the heart, including enhanced inflammation, hypertrophy, and contractile dysfunction. Proinflammatory factors secreted by cardiac cells, which are under the transcriptional control of nuclear factor-κB (NF-κB), may contribute to heart failure and dilated cardiomyopathy. The underlying mechanisms are complex, since they are linked to systemic metabolic abnormalities and changes in cardiomyocyte phenotype. Peroxisome proliferator-activated receptors (PPARs) are transcription factors that regulate metabolism and are capable of limiting myocardial inflammation and hypertrophy via inhibition of NF-κB. Since PPARβ/δ is the most prevalent PPAR isoform in the heart, we analyzed the effects of the PPARβ/δ agonist GW501516 on inflammatory parameters. A high-fat diet induced the expression of tumor necrosis factor-α, monocyte chemoattractant protein-1, and interleukin-6, and enhanced the activity of NF-κB in the heart of mice. GW501516 abrogated this enhanced proinflammatory profile. Similar results were obtained when human cardiac AC16 cells exposed to palmitate were coincubated with GW501516. PPARβ/δ activation by GW501516 enhanced the physical interaction between PPARβ/δ and p65, which suggests that this mechanism may also interfere NF-κB transactivation capacity in the heart. GW501516-induced PPARβ/δ activation can attenuate the inflammatory response induced in human cardiac AC16 cells exposed to the saturated fatty acid palmitate and in mice fed a high-fat diet. This is relevant, especially taking into account that PPARβ/δ has been postulated as a potential target in the treatment of obesity and the insulin resistance state.

Protective role of peroxisome proliferator-activated receptor-β/δ in septic shock

RATIONALE

Peroxisome proliferator-activated receptor (PPAR)-β/δ is a transcription factor that belongs to the PPAR nuclear hormone receptor family, but the role of PPAR-β/δ in sepsis is unknown.

OBJECTIVES

We investigated the role of PPAR-β/δ in murine models of LPS-induced organ injury and dysfunction and cecal ligation and puncture (CLP)-induced polymicrobial sepsis.

METHODS

Wild-type (WT) and PPAR-β/δ knockout (KO) mice and C57BL/6 mice were subjected to LPS for 16 hours. C57BL/6 mice received the PPAR-β/δ agonist GW0742 (0.03 mg/kg intravenously, 1 h after LPS) or GW0742 plus the PPAR-β/δ antagonist GSK0660 (0.1 mg/kg intravenously, 30 min before LPS). CD-1 mice subjected to CLP received GW0742 or GW0742 plus GSK0660.

MEASUREMENTS AND MAIN RESULTS

In PPAR-β/δ KO mice, endotoxemia exacerbated organ injury and dysfunction (cardiac, renal, and hepatic) and inflammation (lung) compared with WT mice. In C57BL/6 mice subjected to endotoxemia, GW0742 significantly (1) attenuated organ (cardiac and renal) dysfunction and inflammation (lung); (2) increased the phosphorylation of Akt and glycogen synthase kinase (GSK)-3β; (3) attenuated the increase in extracellular signal-regulated kinase (ERK)1/2 and signal transducer and activator of transcription (STAT)-3 phosphorylation; and (4) attenuated the activation of nuclear factor (NF)-κB and the expression of inducible nitric oxide synthase (iNOS). In CD-1 mice subjected to CLP, GW0742 improved 10-day survival. All the observed beneficial effects of GW0742 were attenuated by the PPAR-β/δ antagonist GSK0660.

CONCLUSIONS

PPAR-β/δ protects against multiple organ injury and dysfunction, and inflammation caused by endotoxic shock and improves survival in polymicrobial sepsis by a mechanism that may involve activation of Akt and inhibition of GSK-3β and NF-κB.

Transcriptional profiling identifies functional interactions of TGF β and PPAR β/δ signaling: synergistic induction of ANGPTL4 transcription

Peroxisome proliferator-activated receptors (PPARs) not only play a key role in regulating metabolic pathways but also modulate inflammatory processes, pointing to a functional interaction between PPAR and cytokine signaling pathways. In this study, we show by genome-wide transcriptional profiling that PPARβ/δ and transforming growth factor-β (TGFβ) pathways functionally interact in human myofibroblasts and that a subset of these genes is cooperatively activated by TGFβ and PPARβ/δ. Using the angiopoietin-like 4 (ANGPTL4) gene as a model, we demonstrate that two enhancer regions cooperate to mediate the observed synergistic response. A TGFβ-responsive enhancer located ∼8 kb upstream of the transcriptional start site is regulated by a mechanism involving SMAD3, ETS1, RUNX, and AP-1 transcription factors that interact with multiple contiguous binding sites. A second enhancer (PPAR-E) consisting of three juxtaposed PPAR response elements is located in the third intron ∼3.5 kb downstream of the transcriptional start site. The PPAR-E is strongly activated by all three PPAR subtypes, with a novel type of PPAR response element motif playing a central role. Although the PPAR-E is not regulated by TGFβ, it interacts with SMAD3, ETS1, RUNX2, and AP-1 in vivo, providing a possible mechanistic explanation for the observed synergism.

Iron restriction improves type 2 diabetes mellitus in Otsuka Long-Evans Tokushima fatty rats

Accumulating evidence suggests that alcohol, hepatitis C virus infection, steatosis with obesity, and insulin resistance are accompanied by iron overload states. Phlebotomy and oral iron chelators are effective treatments for these conditions and for hemochromatosis. However, the mechanisms by which iron depletion improves clinical factors remain unclear. We examined the effect of iron depletion in a model of type 2 diabetes, Otsuka Long-Evans Tokushima Fatty (OLETF) rats. Age-matched Long-Evans Tokushima Otsuka (LETO) rats were used as controls for all experiments. Iron restriction was performed by eliminating iron in the diet from 15 wk of age or by phlebotomy. Phlebotomy was commenced at 29 wk of age by removing 4 and 3 ml of blood from the tail vein every week in OLETF and LETO rats, respectively. Rats were euthanized at 43 wk of age, and detailed analyses were performed. The plasma ferritin concentration was markedly higher in OLETF rats and decreased in iron-deficient (ID) diet and phlebotomy rats. Hemoglobin A(1c) (Hb A(1c)) was decreased significantly in OLETF rats fed the ID diet and in the phlebotomy group. Increased levels of triglycerides, glucose, free fatty acids, and total cholesterol were found in ID OLETF rats. Plasma, liver, and pancreas lipid peroxidation and hepatic superoxide production decreased in both groups. Pancreatic fibrosis and insulin levels improved in both groups of OLETF rats. Pancreatic levels of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands and hypoxia-inducible factor (HIF)-1alpha were decreased significantly in OLETF rats. These factors were normalized in both rats fed ID and phlebotomy groups of OLETF rats. In conclusion, iron depletion improved diabetic complications by inhibition of oxidative stress and TGFbeta signal pathways and the maintenance of pancreatic PPARbeta/delta and HIF-1alpha pathways.

Profiling of promoter occupancy by PPARalpha in human hepatoma cells via ChIP-chip analysis

The transcription factor peroxisome proliferator-activated receptor alpha (PPARalpha) is an important regulator of hepatic lipid metabolism. While PPARalpha is known to activate transcription of numerous genes, no comprehensive picture of PPARalpha binding to endogenous genes has yet been reported. To fill this gap, we performed Chromatin immunoprecipitation (ChIP)-chip in combination with transcriptional profiling on HepG2 human hepatoma cells treated with the PPARalpha agonist GW7647. We found that GW7647 increased PPARalpha binding to 4220 binding regions. GW7647-induced binding regions showed a bias around the transcription start site and most contained a predicted PPAR binding motif. Several genes known to be regulated by PPARalpha, such as ACOX1, SULT2A1, ACADL, CD36, IGFBP1 and G0S2, showed GW7647-induced PPARalpha binding to their promoter. A GW7647-induced PPARalpha-binding region was also assigned to SREBP-targets HMGCS1, HMGCR, FDFT1, SC4MOL, and LPIN1, expression of which was induced by GW7647, suggesting cross-talk between PPARalpha and SREBP signaling. Our data furthermore demonstrate interaction between PPARalpha and STAT transcription factors in PPARalpha-mediated transcriptional repression, and suggest interaction between PPARalpha and TBP, and PPARalpha and C/EBPalpha in PPARalpha-mediated transcriptional activation. Overall, our analysis leads to important new insights into the mechanisms and impact of transcriptional regulation by PPARalpha in human liver and highlight the importance of cross-talk with other transcription factors.

The epidermal growth factor receptor ligand amphiregulin is a negative regulator of hepatic acute-phase gene expression

BACKGROUND/AIMS

The modulation of the hepatic acute-phase reaction (APR) that occurs during inflammation and liver regeneration is important for allowing normal hepatocellular proliferation and the restoration of homeostasis. Activation of acute-phase protein (APP) gene expression by interleukin-6 (IL-6)-type cytokines is thought to be counteracted by growth factors released during hepatic inflammation and regeneration. The epidermal growth factor receptor (EGFR) ligand amphiregulin (AR) is readily induced by inflammatory signals and plays a nonredundant protective role during liver injury. In this paper, we investigated the role of AR as a modulator of liver APP gene expression.

METHODS

Expression of APP genes was measured in the livers of AR(+/+) and AR(-/-)mice during inflammation and regeneration and in cultured liver cells treated with AR and oncostatin M (OSM). Crosstalk between AR and OSM signalling was studied.

RESULTS

APP genes were overexpressed in the livers of AR(-/-) mice during inflammation and hepatocellular regeneration. In cultured AR-null hepatocytes and human hepatocellular carcinoma (HCC) cells after AR knockdown, APP gene expression is enhanced. AR counteracts OSM-triggered signal transducer and activator of transcription 3 signalling in hepatocytes and attenuates APP gene transcription.

CONCLUSIONS

Our data support the relevance of EGFR-mediated signalling in the modulation of cytokine-activated pathways. We have identified AR as a key regulator of hepatic APP gene expression during inflammation and liver regeneration.

PPARdelta ligand L-165041 ameliorates Western diet-induced hepatic lipid accumulation and inflammation in LDLR-/- mice

Although peroxisome proliferator-activated receptor delta (PPARdelta) has been implicated in energy metabolism and lipid oxidation process, detailed roles of PPARdelta in lipid homeostasis under pathologic conditions still remain controversial. Thus, we investigated the effect of PPARdelta ligand L-165041 on Western diet-induced fatty liver using low-density lipoprotein receptor-deficient (LDLR(-/-)) mice. LDLR(-/-) mice received either L-165041 (5mg/kg/day) or vehicle (0.1N NaOH) with Western diet for 16 weeks. According to our data, L-165041 drastically reduced lipid accumulation in the liver, decreasing total hepatic cholesterol and triglyceride content compared to the vehicle group. Gene expression analysis demonstrated that L-165041 lowered hepatic expression of PPARgamma, apolipoprotein B, interleukin 1 beta (IL-1beta), and interleukin-6. In contrast, L-165041 increased hepatic expressions of PPARdelta, lipoprotein lipase (LPL), and ATP-binding cassette transporter G1 (ABCG1). Our data suggest that L-165041 might be effective in preventing Western diet-induced hepatic steatosis by regulating genes involved in lipid metabolism and the inflammatory response.

Peroxisome proliferator-activated receptor agonists: do they increase cardiovascular risk?

Cardiovascular disease is a major cause of morbidity and mortality among people with type 2 diabetes mellitus. The peroxisome proliferator-activated receptor (PPAR) agonists have a significant role on glucose and fat metabolism. Thiazolidinediones (TZDs) are predominantly PPARγ agonists, and their primary benefit appears to be the prevention of diabetic complications by improving glycemic control and lipid profile. Recently, the cardiovascular safety of rosiglitazone was brought to center stage following meta analyses and the interim analysis of the RECORD trial. Current evidence points to rosiglitazone having a greater risk of myocardial ischemic events than placebo, metformin, or sulfonylureas. This review article discusses the mechanism of action of PPAR agonists and correlates it with clinical and laboratory outcomes in the published literature. In addition, this review article attempts to discuss some of the molecular mechanisms regarding the association between TZDs therapy and the nontraditional cardiovascular risks.

Sorting out the functional role(s) of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) in cell proliferation and cancer

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) has many beneficial physiological functions ranging from enhancing fatty acid catabolism, improving insulin sensitivity, inhibiting inflammation and increasing oxidative myofibers allowing for improved athletic performance. Thus, given the potential for targeting PPARbeta/delta for the prevention and/or treatment of diseases including diabetes, dyslipidemias, metabolic syndrome and cancer, it is critical to clarify the functional role of PPARbeta/delta in cell proliferation and associated disorders such as cancer. However, there is considerable controversy whether PPARbeta/delta stimulates or inhibits cell proliferation. This review summarizes the literature describing the influence of PPARbeta/delta on cell proliferation, with an emphasis toward dissecting the data that give rise to opposing hypotheses. Suggestions are offered to standardize measurements associated with these studies so that interlaboratory comparisons can be accurately assessed.

Peroxisome-proliferator receptor gamma represses hepatic sex hormone-binding globulin expression

Plasma SHBG production by the liver is influenced by its metabolic state, and hepatocyte nuclear factor-4alpha regulates SHBG expression in response to changes in lipogenesis. Peroxisome-proliferator receptors (PPARs) also regulate glucose homeostasis and fatty acid metabolism. The human SHBG promoter contains a PPAR-response element (PPAR-RE), and plasma SHBG levels increase in polycystic ovarian syndrome patients treated with the PPARgamma agonist, rosiglitazone. In addition, plasma SHBG levels are associated with a genetic polymorphism in the PPARgamma-2 coding sequence that alters its transcriptional activity. Therefore, we set out to determine whether PPARgamma influences hepatic production of SHBG by using human HepG2 hepatoblastoma cells as an in vitro model. Surprisingly, treatment of HepG2 cells with rosiglitazone reduced SHBG production and SHBG promoter activity (as assessed in a luciferase reporter gene assay) by 20-25%, whereas the PPARgamma antagonist, GW9662, increased both by 2- to 3-fold. The effects of PPARgamma agonists and antagonists on SHBG promoter activity were substantially diminished when the PPAR-RE in the SHBG promoter was mutated. A PPARgamma small interfering RNA also increased SHBG production by HepG2 cells as well as SHBG promoter activity, and the latter was accentuated by cotreatment with GW9662. Importantly, overexpression of a PPARgamma-2 Pro12 variant in HepG2 cells was more effective at reducing SHBG promoter activity, when compared with PPARgamma-2 Ala12, consistent with its superior PPAR-RE binding activity. We conclude that PPARgamma represses human SHBG expression in liver cells, and that differences in PPARgamma levels and activity contribute directly to variations in plasma SHBG levels.

Peroxisome proliferator activated receptor beta/delta activation prevents extracellular regulated kinase 1/2 phosphorylation and protects the testis from ischemia and reperfusion injury

PURPOSE

Testicular torsion is a medical emergency that requires immediate diagnosis and treatment to avoid subsequent testicular injury and infertility. PPARs are a family of nuclear hormone receptors belonging to the steroid receptor superfamily. Three PPAR isotypes (alpha, beta/delta and gamma) encoded by separate genes and showing different tissue distribution patterns have been identified. PPARbeta/delta is expressed in testis and its role is largely unknown. We tested whether pharmacological activation of PPARbeta/delta might protect the testis from ischemia and reperfusion injury.

MATERIALS AND METHODS

Adult male Sprague-Dawley rats were subjected to 1-hour testicular ischemia, followed by 24 hours of reperfusion. Sham testicular ischemia-reperfusion rats served as controls. The animals were randomized to receive immediately after detorsion 1) L-165,041 (4 mg/kg intraperitoneally), a potent agonist of PPARbeta/delta, 2) GW9662 (Calbiochem(R)) (4 mg/kg intraperitoneally), an antagonist of PPAR, 3) L-165,041 (4 mg/kg intraperitoneally) plus GW9662 (4 mg/kg intraperitoneally) concomitantly or 4) vehicle (1 ml/kg 10% dimethyl sulfoxide/NaCl solution). We evaluated testicular extracellular signal regulated kinase, tumor necrosis factor-alpha and interleukin-6 by Western blot. We also investigated PPARbeta/delta activation by Western blot, mRNA expression and organ damage.

RESULTS

Testicular ischemia-reperfusion injury caused a significant increase in extracellular signal regulated kinase, tumor necrosis factor-alpha and interleukin-6 expression in each testis. Furthermore, histological examination revealed marked damage. L-165,041 administration increased the PPARbeta/delta message and protein, inhibited extracellular signal regulated kinase, tumor necrosis factor-alpha and interleukin-6 expression, and decreased histological damage. Concomitant administration of GW9662 reversed the protection exerted by PPARbeta/delta agonist.

CONCLUSIONS

These findings indicate that PPARbeta/delta agonists might be an attractive therapeutic candidate for managing testicular torsion.

Role of peroxisome-proliferator-activated receptor beta/delta (PPARbeta/delta) in gastrointestinal tract function and disease

PPARbeta/delta (peroxisome-proliferator-activated receptor beta/delta) is one of three PPARs in the nuclear hormone receptor superfamily that are collectively involved in the control of lipid homoeostasis among other functions. PPARbeta/delta not only acts as a ligand-activated transcription factor, but also affects signal transduction by interacting with other transcription factors such as NF-kappaB (nuclear factor kappaB). Constitutive expression of PPARbeta/delta in the gastrointestinal tract is very high compared with other tissues and its potential physiological roles in this tissue include homoeostatic regulation of intestinal cell proliferation/differentiation and modulation of inflammation associated with inflammatory bowel disease and colon cancer. Analysis of mouse epithelial cells in the intestine and colon has clearly demonstrated that ligand activation of PPARbeta/delta induces terminal differentiation. The PPARbeta/delta target genes mediating this effect are currently unknown. Emerging evidence suggests that PPARbeta/delta can suppress inflammatory bowel disease through PPARbeta/delta-dependent and ligand-independent down-regulation of inflammatory signalling. However, the role of PPARbeta/delta in colon carcinogenesis remains controversial, as conflicting evidence suggests that ligand activation of PPARbeta/delta can either potentiate or attenuate this disease. In the present review, we summarize the role of PPARbeta/delta in gastrointestinal physiology and disease with an emphasis on findings in experimental models using both high-affinity ligands and null-mouse models.

Ligand activation of peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) attenuates carbon tetrachloride hepatotoxicity by downregulating proinflammatory gene expression

Peroxisome proliferator-activated receptor (PPAR) beta/delta-null mice exhibit exacerbated hepatotoxicity in response to administration of carbon tetrachloride (CCl(4)). To determine whether ligand activation of the receptor protects against chemical toxicity in the liver, wild-type and PPARbeta/delta-null mice were administered CCl(4) with or without coadministration of the highly specific PPARbeta/delta ligand GW0742. Biomarkers of liver toxicity, including serum alanine aminotransferase (ALT) and hepatic tumor necrosis factor (TNF) alpha mRNA, were significantly higher in CCl(4)-treated PPARbeta/delta-null mice compared to wild-type mice. Hepatic expression of TNF-like weak inducer of apoptosis receptor (TWEAKr) and S100 calcium-binding protein A6 (S100A6/calcyclin), genes involved in nuclear factor kappa B signaling, was higher in the CCl(4)-treated PPARbeta/delta-null mice compared to wild-type mice. GW0742 treatment resulted in reduced serum ALT concentration and lower expression of CCl(4)-induced TNF-alpha, S100A6, monocyte chemoattractant protein-1 (MCP1), and TWEAKr in wild-type mice, and these effects were not observed in PPARbeta/delta-null mice. Expression of TNF-alpha was higher in PPARbeta/delta-null primary hepatocytes in response to interleukin-1beta treatment compared to wild-type hepatocytes, but GW0742 did not significantly modulate TNF-alpha expression in hepatocytes from either genotype. While PPARbeta/delta-null hepatic stellate exhibited higher rates of proliferation compared to wild-type cells, GW0742 did not affect alpha-smooth muscle actin expression in these cells. Combined, these findings demonstrate that ligand activation of PPARbeta/delta protects against chemically induced hepatotoxicity by downregulating expression of proinflammatory genes. Hepatocytes and hepatic stellate cells do not appear to directly mediate the inhibitory effects of ligand activation of PPARbeta/delta in liver, suggesting the involvement of paracrine and autocrine events mediated by hepatic cells.

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands do not potentiate growth of human cancer cell lines

Ligands for peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) increase skeletal muscle fatty acid catabolism, improve insulin sensitivity, increase serum high-density lipoprotein cholesterol, elicit anti-inflammatory activity and induce terminal differentiation. Contradictory findings are also reported suggesting that PPARbeta/delta ligands potentiate tumorigenesis by increasing cell proliferation, by inhibiting apoptosis through phosphorylation of Akt and by increasing cyclooxygenase-2 (COX2) and vascular endothelial growth factor (VEGF) expression. The contradictory findings could be due to differences in the model system (cancer cell line versus in vivo), differences in cell culture conditions (with and without serum) or differences in ligands. The present study examined the effect of two different PPARbeta/delta ligands (GW0742 and GW501516) in human cancer cell lines (HT29, HCT116, LS-174T, HepG2 and HuH7) cultured in the presence or absence of serum and compared in vitro analysis with in vivo analysis. Neither PPARbeta/delta ligand increased cell growth or phosphorylation of Akt and no increase in the expression of VEGF or COX2 were detected in any cancer cell line in the presence or absence of serum. Similarly, liver, colon and colon polyps from mice administered these PPARbeta/delta ligands in vivo did not exhibit changes in these markers. Results from these studies demonstrate that serum withdrawal and/or differences in ligands do not underlie the disparity in responses reported in the literature. The quantitative nature of the present findings are inconsistent with the hypothesis that cancer cell lines respond differentially as compared with normal cells, and provide further evidence that PPARbeta/delta ligands do not potentiate tumorigenesis.

3-O-Formyl-20R,21-epoxyresibufogenin suppresses IL-6–type cytokine actions by targeting the glycoprotein 130 subunit: Potential clinical implications

BACKGROUND

The multifunctional inflammatory cytokine IL-6 regulates the acute phase reaction and plays central roles in the pathogenesis of chronic inflammatory disorders.

OBJECTIVES

Two small chemical compounds, 3-O-formyl-20R,21-epoxyresibufogenin (TB-2-081) and 3-O-formyl-20S,21-epoxyresibufogenin (TB-2-082), known isolates from the Chinese toad skin extract drug Ch'an Su, were synthesized and tested on the IL-6-induced hepatic acute-phase reaction.

METHODS

HepG2 cells or rat primary hepatocytes were incubated with the compounds, and the effects on IL-6-induced expression of acute-phase molecules were tested.

RESULTS

TB-2-081, and to a lesser extent TB-2-082, suppressed IL-6-induced alpha1-antichymotrypsin (AACT) mRNA expression in HepG2 cells, whereas TB-2-081 failed to influence the mRNA expression of the TNF-alpha-induced mRNA expression of the methionine adenosyltransferase 2A gene in these cells. TB-2-081 suppressed IL-6-induced mRNA expression of alpha1-acid glycoprotein, alpha2-macroglobulin, and beta-fibrinogen in and secretion of the C-reactive protein by rat primary hepatocytes. TB-2-081 shifted the IL-6 dose-response curve of the AACT mRNA expression right and downward and inhibited IL-6-induced phosphorylation of signal transducer and activator of transcription 3. In addition to IL-6, TB-2-081 inhibited IL-11-stimulated and oncostatin M-stimulated AACT mRNA expression independently of the IL-6 receptor subunit. The soluble glycoprotein 130, but not the soluble IL-6 receptor, antagonized TB-2-081-induced suppression of IL-6-stimulated AACT mRNA expression.

CONCLUSION

TB-2-081 inhibits IL-6-type cytokine action by attenuating the function of the common receptor subunit glycoprotein 130.

CLINICAL IMPLICATIONS

This class of compounds may be beneficial for the treatment of diseases in which excessive circulation/production/action of IL-6-type cytokines play pathologic roles.