Loss of PPAR gamma in immune cells impairs the ability of abscisic acid to improve insulin sensitivity by suppressing monocyte chemoattractant protein-1 expression and macrophage infiltration into white adipose tissue

Dietary α-eleostearic acid ameliorates experimental inflammatory bowel disease in mice by activating peroxisome proliferator-activated receptor-γ

Background

Treatments for inflammatory bowel disease (IBD) are modestly effective and associated with side effects from prolonged use. As there is no known cure for IBD, alternative therapeutic options are needed. Peroxisome proliferator-activated receptor-gamma (PPARγ) has been identified as a potential target for novel therapeutics against IBD. For this project, compounds were screened to identify naturally occurring PPARγ agonists as a means to identify novel anti-inflammatory therapeutics for experimental assessment of efficacy.

Methodology/Principal Findings

Here we provide complementary computational and experimental methods to efficiently screen for PPARγ agonists and demonstrate amelioration of experimental IBD in mice, respectively. Computational docking as part of virtual screening (VS) was used to test binding between a total of eighty-one compounds and PPARγ. The test compounds included known agonists, known inactive compounds, derivatives and stereoisomers of known agonists with unknown activity, and conjugated trienes. The compound identified through VS as possessing the most favorable docked pose was used as the test compound for experimental work. With our combined methods, we have identified α-eleostearic acid (ESA) as a natural PPARγ agonist. Results of ligand-binding assays complemented the screening prediction. In addition, ESA decreased macrophage infiltration and significantly impeded the progression of IBD-related phenotypes through both PPARγ-dependent and –independent mechanisms in mice with experimental IBD.

Conclusions/Significance

This study serves as the first significant step toward a large-scale VS protocol for natural PPARγ agonist screening that includes a massively diverse ligand library and structures that represent multiple known target pharmacophores.

Fats, inflammation and insulin resistance: insights to the role of macrophage and T-cell accumulation in adipose tissue

High-fat diet-induced obesity is associated with a chronic state of low-grade inflammation, which pre-disposes to insulin resistance (IR), which can subsequently lead to type 2 diabetes mellitus. Macrophages represent a heterogeneous population of cells that are instrumental in initiating the innate immune response. Recent studies have shown that macrophages are key mediators of obesity-induced IR, with a progressive infiltration of macrophages into obese adipose tissue. These adipose tissue macrophages are referred to as classically activated (M1) macrophages. They release cytokines such as IL-1β, IL-6 and TNFα creating a pro-inflammatory environment that blocks adipocyte insulin action, contributing to the development of IR and type 2 diabetes mellitus. In lean individuals macrophages are in an alternatively activated (M2) state. M2 macrophages are involved in wound healing and immunoregulation. Wound-healing macrophages play a major role in tissue repair and homoeostasis, while immunoregulatory macrophages produce IL-10, an anti-inflammatory cytokine, which may protect against inflammation. The functional role of T-cell accumulation has recently been characterised in adipose tissue. Cytotoxic T-cells are effector T-cells and have been implicated in macrophage differentiation, activation and migration. Infiltration of cytotoxic T-cells into obese adipose tissue is thought to precede macrophage accumulation. T-cell-derived cytokines such as interferon γ promote the recruitment and activation of M1 macrophages augmenting adipose tissue inflammation and IR. Manipulating adipose tissue macrophages/T-cell activity and accumulation in vivo through dietary fat modification may attenuate adipose tissue inflammation, representing a therapeutic target for ameliorating obesity-induced IR.

Molecular modeling of lanthionine synthetase component C-like protein 2: a potential target for the discovery of novel type 2 diabetes prophylactics and therapeutics

The rates of type 2 diabetes (T2D) are rising to epidemic proportions in the US and worldwide. While current T2D medications are efficacious, significant side effects have limited their use and availability. Our laboratory has discovered that abscisic acid (ABA) exerts anti-diabetic effects, in part, by activating peroxisome proliferator-activated receptor γ (PPAR γ). However, since ABA does not bind to the ligand-binding domain (LBD) of PPAR γ, the mechanism of activation of PPAR γ by ABA remains unknown. Lanthionine synthetase component C-like protein 2 (LANCL2) was predicted to be a novel target for the binding and signaling of ABA in human granulocytes and rat insulinoma cells. The goal of this study was to determine whether LANCL2 is a molecular target of ABA and other PPAR γ agonists. To this end we performed homology modeling to construct a three-dimensional structure of LANCL2 using the crystal structure of LANCL1 as a template. Our molecular docking studies predicted that ABA and other PPAR γ agonists (e.g., rosiglitazone and pioglitazone) share a binding site on the surface of LANCL2. The identification of a binding site for PPAR γ agonists will facilitate the high-throughput virtual screening of large compound libraries and may shed new light on alternative mechanisms of PPAR γ activation.

Suppression of intestinal inflammation and inflammation-driven colon cancer in mice by dietary sphingomyelin: importance of peroxisome proliferator-activated receptor γ expression

Inflammation of the gastrointestinal tract increases the risk of developing colon cancer especially in younger adults. Dietary compounds are not only associated with the etiology of inflammation and colon cancer but also in their prevention. Sphingolipid metabolites have been shown to play a role in the initiation and perpetuation of inflammatory responses. In the present study, we investigated the suppression of dextran sodium sulfate-induced colitis and azoxymethane-induced colon cancer by dietary sphingomyelin (SM) in mice that lack functional peroxisome proliferator-activated receptor γ (PPAR-γ) in intestinal epithelial and immune cells. Dietary SM decreased disease activity and colonic inflammatory lesions in mice of both genotypes but more efficiently in mice expressing PPAR-γ. The increased survival and suppression of tumor formation in the SM-fed mice appeared to be independent of PPAR-γ expression in immune and epithelial cells. Using a real-time polymerase chain reaction array, we detected an up-regulation in genes involved in Th1 (interferon γ) and Th17 (interleukin [IL]-17 and IL-23) responses despite the reduced inflammation scores. However, the genes involved in Th2 (IL-4, IL-13 and IL-13ra2) and Treg (IL-10rb) anti-inflammatory responses were up-regulated in a PPAR-γ-dependent manner. In line with the PPAR-γ dependency of our in vivo findings, treatment of RAW macrophages with sphingosine increased the PPAR-γ reporter activity. In conclusion, dietary SM modulated inflammatory responses at the early stages of the disease by activating PPAR-γ, but its anticarcinogenic effects followed a PPAR-γ-independent pattern.

Abscisic acid regulates inflammation via ligand-binding domain-independent activation of peroxisome proliferator-activated receptor gamma

Abscisic acid (ABA) has shown efficacy in the treatment of diabetes and inflammation; however, its molecular targets and the mechanisms of action underlying its immunomodulatory effects remain unclear. This study investigates the role of peroxisome proliferator-activated receptor γ (PPAR γ) and lanthionine synthetase C-like 2 (LANCL2) as molecular targets for ABA. We demonstrate that ABA increases PPAR γ reporter activity in RAW 264.7 macrophages and increases ppar γ expression in vivo, although it does not bind to the ligand-binding domain of PPAR γ. LANCL2 knockdown studies provide evidence that ABA-mediated activation of macrophage PPAR γ is dependent on lancl2 expression. Consistent with the association of LANCL2 with G proteins, we provide evidence that ABA increases cAMP accumulation in immune cells. ABA suppresses LPS-induced prostaglandin E(2) and MCP-1 production via a PPAR γ-dependent mechanism possibly involving activation of PPAR γ and suppression of NF-κB and nuclear factor of activated T cells. LPS challenge studies in PPAR γ-expressing and immune cell-specific PPAR γ null mice demonstrate that ABA down-regulates toll-like receptor 4 expression in macrophages and T cells in vivo through a PPAR γ-dependent mechanism. Global transcriptomic profiling and confirmatory quantitative RT-PCR suggest novel candidate targets and demonstrate that ABA treatment mitigates the effect of LPS on the expression of genes involved in inflammation, metabolism, and cell signaling, in part, through PPAR γ. In conclusion, ABA decreases LPS-mediated inflammation and regulates innate immune responses through a bifurcating pathway involving LANCL2 and an alternative, ligand-binding domain-independent mechanism of PPAR γ activation.

Treatment of Obesity-Related Complications with Novel Classes of Naturally Occurring PPAR Agonists

The prevalence of obesity and its associated comorbidities has grown to epidemic proportions in the US and worldwide. Thus, developing safe and effective therapeutic approaches against these widespread and debilitating diseases is important and timely. Activation of peroxisome proliferator-activated receptors (PPARs) α, γ, and δ through several classes of pharmaceuticals can prevent or treat a variety of metabolic and inflammatory diseases, including type II diabetes (T2D). Thus, PPARs represent important molecular targets for developing novel and better treatments for a wide range of debilitating and widespread obesity-related diseases and disorders. However, available PPAR γ agonistic drugs such as Avandia have significant adverse side effects, including weight gain, fluid retention, hepatotoxicity, and congestive heart failure. An alternative to synthetic agonists of PPAR γ is the discovery and development of naturally occurring and safer nutraceuticals that may be dual or pan PPAR agonists. The purpose of this paper is to summarize the health effects of three plant-derived PPAR agonists: abscisic acid (ABA), punicic acid (PUA), and catalpic acid (CAA) in the prevention and treatment of chronic inflammatory and metabolic diseases and disorders.

Liver X receptors as regulators of macrophage inflammatory and metabolic pathways

The liver X receptors (LXRα and LXRβ) are members of the nuclear receptor family of transcription factors that play essential roles in the transcriptional control of lipid metabolism. LXRs are endogenously activated by modified forms of cholesterol known as oxysterols and control the expression of genes important for cholesterol uptake, efflux, transport, and excretion in multiple tissues. In addition to their role as cholesterol sensors, a number of studies have implicated LXRs in the modulation of innate and adaptive immune responses. Both through activation and repression mechanisms, LXRs regulate diverse aspects of inflammatory gene expression in macrophages. The ability of LXRs to coordinate metabolic and immune responses constitutes an attractive therapeutic target for the treatment of chronic inflammatory disorders. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Deletion of PPAR-γ in immune cells enhances susceptibility to antiglomerular basement membrane disease

Activation of the nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPAR-γ) has been shown to be immunoregulatory in autoimmune diseases by inhibiting production of a number of inflammatory mediators. We investigated whether PPAR-γ gene deletion in hematopoietic cells would alter disease pathogenesis in the antiglomerular basement membrane (anti-GBM) mouse model. PPAR-γ^+/+ and PPAR-γ^−/− mice were immunized with rabbit antimouse GBM antibodies and lipopolysaccharide and evaluated for two weeks. Although both the PPAR-γ^+/+ and PPAR-γ^−/− mice had IgG deposition in the glomerulus and showed proteinuria two weeks after injection, glomerular and tubulointerstitial disease in PPAR-γ^−/− mice were significantly more severe compared with the PPAR-γ^+/+ animals. We observed that the PPAR-γ^−/− mice had decreased CD4⁺CD25⁺ regulatory T cells and an increased CD8⁺:CD4⁺ ratio as compared with the PPAR-γ^+/+ mice, suggesting that PPAR-γ has a role in the regulation of T cells. Furthermore, plasma interleukin-6 levels were significantly increased in the PPAR-γ^−/− mice at two weeks as compared with the PPAR-γ^+/+ animals. Taken together, these studies show that the lack of PPAR-γ expression enhances inflammatory renal disease in the anti-GBM antibody-induced glomerulonephritis mouse model and suggests targeting PPAR-γ may have therapeutic efficacy.

Adipose tissue recruitment of leukocytes

PURPOSE OF REVIEW

In December of 2003, two seminal articles describing the presence of macrophages in obese adipose tissue were published. These adipose tissue macrophages (ATMs) are inflammatory and promote local and systemic insulin resistance. Due to the continuing rise in obesity around the world, understanding how these ATMs contribute to metabolic disorders is of much interest.

RECENT FINDINGS

Chemokines have been extensively studied for their role in ATM recruitment. Deficiency or antagonism of chemokine receptors that interact with multiple chemokine ligands reduces ATM accumulation. ATMs are now defined as either classically (M1) or alternatively (M2) activated. Peroxisome proliferator-activated receptor activation and adiponectin promote an M2-polarized state resulting in improved insulin sensitivity. Finally, recent studies have provided evidence that T lymphocytes, natural killer T cells, mast cells, and B cells also enter adipose tissue and may interact with macrophages and adipocytes.

SUMMARY

Literature published during the past year has shown that macrophage recruitment to adipose tissue is only one of the important mediators of obesity-related insulin resistance. The phenotype of ATMs and recruitment of other immune cells to the adipose tissue play key roles in the overall contribution of adipose tissue to systemic metabolic outcomes of obesity.

Abscisic acid ameliorates experimental IBD by downregulating cellular adhesion molecule expression and suppressing immune cell infiltration

BACKGROUND & AIMS

Abscisic acid (ABA) has shown effectiveness in ameliorating inflammation in obesity, diabetes and cardiovascular disease models. The objective of this study was to determine whether ABA prevents or ameliorates experimental inflammatory bowel disease (IBD).

METHODS

C57BL/6J mice were fed diets with or without ABA (100mg/kg) for 35 days prior to challenge with 2.5% dextran sodium sulfate (DSS). The severity of clinical disease was assessed daily. Colonic mucosal lesions were evaluated by histopathology, and cellular adhesion molecular and inflammatory markers were assayed by real-time quantitative PCR. Flow cytometry was used to quantify leukocyte populations in the blood, spleen, and mesenteric lymph nodes (MLN). The effect of ABA on cytotoxic T-lymphocyte antigen 4 (CTLA-4) expression in splenocytes was also investigated.

RESULTS

ABA significantly ameliorated disease activity, colitis and reduced colonic leukocyte infiltration and inflammation. These improvements were associated with downregulation in vascular cell adhesion marker-1 (VCAM-1), E-selectin, and mucosal addressin adhesion marker-1 (MAdCAM-1) expression. ABA also increased CD4(+) and CD8(+) T-lymphocytes in blood and MLN and regulatory T cells in blood. In vitro, ABA increased CTLA-4 expression through a PPAR γ-dependent mechanism.

CONCLUSIONS

We conclude that ABA ameliorates gut inflammation by modulating T cell distribution and adhesion molecule expression.

Abscisic acid synergizes with rosiglitazone to improve glucose tolerance and down-modulate macrophage accumulation in adipose tissue: possible action of the cAMP/PKA/PPAR γ axis

BACKGROUND & AIMS

Abscisic acid (ABA) is effective in preventing insulin resistance and obesity-related inflammation through a PPAR γ-dependent mechanism. The objective of this study was to assess the efficacy ABA in improving glucose homeostasis and suppress inflammation when administered in combination with rosiglitazone (Ros) and to determine whether PPAR γ activation by ABA is initiated via cAMP/protein kinase A (PKA) signaling.

METHODS

Obese db/db mice were fed high-fat diets containing 0, 10, or 70 mg/kg Ros with and without racemic ABA (100 mg/kg) for 60 days. Glucose tolerance and fasting insulin levels were assessed at 6 and 8 weeks, respectively, and adipose tissue macrophage (ATM) infiltration was examined by flow cytometry. Gene expression was examined on white adipose tissue (WAT) and stromal vascular cells (SVCs) cultured with ABA, Ros, or an ABA/Ros combination.

RESULTS

Both Ros and ABA improved glucose tolerance, and ABA decreased plasma insulin levels while having no effect on Ros-induced weight gain. ABA in combination with low-dose Ros (10 mg/kg; Roslo) synergistically inhibited ATM infiltration. Treatment of SVCs with Ros, ABA or ABA/Ros suppressed expression of the M1 marker CCL17. ABA and Ros synergistically increased PPAR γ activity and pretreatment with a cAMP-inhibitor or a PKA-inhibitor abrogated ABA-induced PPAR γ activation.

CONCLUSIONS

ABA and Ros act synergistically to modulate PPAR γ activity and macrophage accumulation in WAT and ABA enhances PPAR γ activity through a membrane-initiated mechanism dependent on cAMP/PKA signaling.

Abscisic acid ameliorates atherosclerosis by suppressing macrophage and CD4+ T cell recruitment into the aortic wall

Abscisic acid (ABA) is a natural phytohormone which improves insulin sensitivity and reduces adipose tissue inflammation when supplemented into diets of obese mice. The objective of this study was to investigate the mechanisms by which ABA prevents or ameliorates atherosclerosis. apolipoprotein E-deficient (ApoE(-/-)) mice were fed high-fat diets with or without ABA for 84 days. Systolic blood pressure was assessed on Days 0, 28, 56 and 72. Gene expression, immune cell infiltration and histological lesions were evaluated in the aortic root wall. Human aortic endothelial cells were used to examine the effect of ABA on 3',5'-cyclic adenosine monophosphate (cAMP) and nitric oxide (NO) production in vitro. We report that ABA-treated mice had significantly improved systolic blood pressure and decreased accumulation of F4/80(+)CD11b(+) macrophages and CD4(+) T cells in aortic root walls. At the molecular level, ABA significantly enhanced aortic endothelial nitric oxide synthase (eNOS) and tended to suppress aortic vascular cell adhesion molecule-1 (VCAM-1) and monocyte chemoattractant protein-1 (MCP-1) expression and plasma MCP-1 concentrations. ABA also caused a dose-dependent increase in intracellular concentrations of cAMP and NO and up-regulated eNOS mRNA expression in human aortic endothelial cells. This is the first report showing that ABA prevents or ameliorates atherosclerosis-induced hypertension, immune cell recruitment into the aortic root wall and up-regulates aortic eNOS expression in ApoE(-/-) mice.

PPAR gamma is highly expressed in F4/80(hi) adipose tissue macrophages and dampens adipose-tissue inflammation

Macrophage infiltration into adipose tissue is a hallmark of obesity. We recently reported two phenotypically distinct subsets of adipose tissue macrophages (ATM) based on the surface expression of the glycoprotein F4/80 and responsiveness to treatment with a peroxisome proliferator-activated receptor (PPAR) gamma agonist. Hence, we hypothesized that F4/80(hi) and F4/80(lo) ATM differentially express PPAR gamma. This study phenotypically and functionally characterizes F4/80(hi) and F4/80(lo) ATM subsets during obesity. Changes in gene expression were also examined on sorted F4/80(lo) and F4/80(hi) ATM by quantitative real-time RT-PCR. We show that while F4/80(lo) macrophages predominate in adipose tissue of lean mice, obesity causes accumulation of both F4/80(lo) and F4/80(hi) ATM. Moreover, accumulation of F4/80(hi) ATM in adipose tissue is associated with impaired glucose tolerance. Phenotypically, F4/80(hi) ATM express greater amounts of CD11c, MHC II, CD49b, and CX3CR1 and produce more TNF-alpha, MCP-1, and IL-10 than F4/80(lo) ATM. Gene expression analyses of the sorted populations revealed that only the F4/80(lo) population produced IL-4, whereas the F4/80(hi) ATM expressed greater amounts of PPAR gamma, delta, CD36 and toll-like receptor-4. In addition, the deficiency of PPAR gamma in immune cells favors expression of M1 and impairs M2 macrophage marker expression in adipose tissue. Thus, PPAR gamma is differentially expressed in F4/80(hi) versus F4/80(low) ATM subsets and its deficiency favors a predominance of M1 markers in WAT.

Abscisic acid released by human monocytes activates monocytes and vascular smooth muscle cell responses involved in atherogenesis

Abscisic acid (ABA) is a phytohormone recently identified as a new endogenous pro-inflammatory hormone in human granulocytes. Here we report the functional activation of human monocytes and vascular smooth muscle cells by ABA. Incubation of monocytes with ABA evokes an intracellular Ca2+ rise through the second messenger cyclic ADP-ribose, leading to NF-kappaB activation and consequent increase of cyclooxygenase-2 expression and prostaglandin E2 production and enhanced release of MCP-1 (monocyte chemoattractant protein-1) and of metalloprotease-9, all events reportedly involved in atherogenesis. Moreover, monocytes release ABA when exposed to thrombin-activated platelets, a condition occurring at the injured vascular endothelium; monocyte-derived ABA behaves as an autocrine and paracrine pro-inflammatory hormone-stimulating monocyte migration and MCP-1 release, as well as vascular smooth muscle cells migration and proliferation. These results, and the presence of ABA in human arterial plaques at a 10-fold higher concentration compared with normal arterial tissue, identify ABA as a new signal molecule involved in the development of atherosclerosis and suggest a possible new target for anti-atherosclerotic therapy.

Catalpic acid decreases abdominal fat deposition, improves glucose homeostasis and upregulates PPAR alpha expression in adipose tissue

BACKGROUND & AIMS

Catalpic acid (CAT) is a conjugated linolenic acid (CLN) isomer containing trans-9, trans-11, cis-13 double bonds in an 18-carbon chain and it is found primarily in the seed oil of ornamental and medicinal trees and shrubs of the family Bignoniaceae. The objective of this study was to investigate whether CAT decreases obesity and ameliorates insulin sensitivity and glucose tolerance in mice fed high-fat diets.

METHODS

To test the efficacy of CAT in decreasing obesity and diabetes we used both a model of diet-induced obesity (DIO) and a genetic model of obesity (i.e., mice lacking the leptin receptor). Blood was collected on days 0, 7, 14, 21 and 28 for determining fasting glucose and insulin concentrations in plasma. In addition, a glucose tolerance test was administered on day 28.

RESULTS

We found that dietary CAT (1g/100g) decreased fasting plasma glucose and insulin concentrations, ameliorated the glucose normalizing ability following glucose challenge and decreased abdominal white adipose tissue accumulation. In white adipose tissue (WAT), CAT upregulated peroxisome proliferator-activated receptor (PPAR) alpha and its responsive genes [i.e., stearoyl-coenzyme A desaturase (SCD1) and enoyl-coenzyme A hydratase (ECH)], increased concentrations of high-density lipoprotein (HDL) cholesterol and decreased plasma triglyceride (TG) levels.

CONCLUSIONS

CAT decreased abdominal fat deposition, increased HDL cholesterol, decreased TG concentrations, decreased glucose and insulin homeostasis and modulated WAT gene expression in a manner reminiscent of the actions of the PPAR alpha-activating fibrate class of lipid-lowering drugs.