The peroxisome proliferator-activated receptor delta promotes lipid accumulation in human macrophages

Exercise might favor reverse cholesterol transport and lipoprotein clearance: potential mechanism for its anti-atherosclerotic effects

Livers of C57 BL/6 mice exercised for 2 weeks showed a dramatic increase in scavenger receptor B1 (SR-B1), CD36 and low density lipoprotein (LDL) receptor and a decrease in acetyl LDL receptor gene expression. These effects on lipoprotein receptors are reminiscent of the effects mediated by peroxisome proliferator-activated receptor (PPARgamma) ligands.

Retinoids potentiate peroxisome proliferator-activated receptor gamma action in differentiation, gene expression, and lipid metabolic processes in developing myeloid cells

Nuclear hormone receptors have been shown to be important transcription factors for regulating lipid metabolism in myeloid cells and were also implicated in differentiation processes of the myeloid lineage and macrophages. Peroxisome proliferator-activated receptor gamma (PPARgamma) seems to be a key component of lipid uptake by inducing the scavenger receptor CD36 that mediates oxidized low-density lipoprotein uptake in macrophages. Retinoic acid receptors, on the other hand, were also shown to play important roles in myeloid cell differentiation. In this study, we present evidence for a cross-talk between these two nuclear receptor pathways in myeloid cells. We show that expression level of PPARgamma increases with the degree of monocyte/macrophage commitment during maturation. Activation of PPARgamma leads to the increased expression of maturation markers (e.g., CD14, CD36). It is interesting that retinoid treatment potentiates PPARgamma's ability to induce transcription of its target genes. Retinoid-increased PPARgamma response is sufficient for enhancing lipid uptake. Our data, taken together, indicate that the expression level of PPARgamma increases during monocyte/macrophage development. PPARgamma activity can be enhanced by retinoids at least in part via increasing PPARgamma expression level. These observations can be exploited to enhance therapeutically beneficial PPAR responses in myeloid cells.

HDL as a target in the treatment of atherosclerotic cardiovascular disease

Lipid abnormalities are among the key risk factors for cardiovascular disease. Indeed, lipid-modifying drugs - in particular, the statins, which primarily lower plasma levels of low-density lipoprotein (LDL) cholesterol - considerably reduce the risk of cardiovascular events, leading to their widespread use. Nevertheless, it seems that there might be limits to the degree of benefit that can be achieved by lowering LDL-cholesterol levels alone, which has led to increased interest in targeting other lipid-related risk factors for cardiovascular disease, such as low levels of high-density lipoprotein (HDL) cholesterol. In this article, we first consider the mechanisms that underlie the protective effect of HDL cholesterol, and then discuss several strategies that have recently emerged to increase levels of HDL cholesterol to treat cardiovascular disease, including nuclear receptor modulation, inhibition of cholesteryl ester transfer protein and infusion of apolipoprotein/phospholipid complexes.

Effects of peroxisome proliferator-activated receptor alpha/delta agonists on HDL-cholesterol in vervet monkeys

The objective of this study was to demonstrate the efficacy of a novel peroxisome proliferator-activated receptor (PPAR) agonist and known PPARalpha and PPARdelta agonists to increase HDL-cholesterol (HDL-C) in the St. Kitts vervet, a nonhuman primate model of atherosclerosis. Four groups (n = 6) were studied and each group was assigned one of the following "treatments": a) vehicle only (vehicle); b) the PPARdelta selective agonist GW501516 (GW); c) the PPARalpha/delta agonist T913659 (T659); and d) the PPARalpha agonist TriCor (fenofibrate). No statistically significant changes were seen in body weight, total plasma cholesterol, plasma triglycerides, VLDL-C, LDL-C, or apolipoprotein B (apoB) concentrations. Each of the PPARalpha and PPARdelta agonists investigated in this study increased plasma HDL-C, apoA-I, and apoA-II concentrations and increased HDL particle size in St. Kitts vervets. The maximum percentage increase in HDL-C from baseline for each group was as follows: vehicle, 5%; GW, 43%; T659, 43%; and fenofibrate, 20%. Treatment with GW and T659 resulted in an increase in medium-sized HDL particles, whereas fenofibrate showed increases in large HDL particles. These data provide additional evidence that PPARalpha and PPARdelta agonists (both mixed and selective) have beneficial effects on HDL-C in these experimental primates.

Differential inhibition of macrophage foam-cell formation and atherosclerosis in mice by PPARalpha, beta/delta, and gamma

PPARalpha, beta/delta, and gamma regulate genes involved in the control of lipid metabolism and inflammation and are expressed in all major cell types of atherosclerotic lesions. In vitro studies have suggested that PPARs exert antiatherogenic effects by inhibiting the expression of proinflammatory genes and enhancing cholesterol efflux via activation of the liver X receptor-ABCA1 (LXR-ABCA1) pathway. To investigate the potential importance of these activities in vivo, we performed a systematic analysis of the effects of PPARalpha, beta, and gamma agonists on foam-cell formation and atherosclerosis in male LDL receptor-deficient (LDLR(-/-)) mice. Like the PPARgamma agonist, a PPARalpha-specific agonist strongly inhibited atherosclerosis, whereas a PPARbeta-specific agonist failed to inhibit lesion formation. In concert with their effects on atherosclerosis, PPARalpha and PPARgamma agonists, but not the PPARbeta agonist, inhibited the formation of macrophage foam cells in the peritoneal cavity. Unexpectedly, PPARalpha and PPARgamma agonists inhibited foam-cell formation in vivo through distinct ABCA1-independent pathways. While inhibition of foam-cell formation by PPARalpha required LXRs, activation of PPARgamma reduced cholesterol esterification, induced expression of ABCG1, and stimulated HDL-dependent cholesterol efflux in an LXR-independent manner. In concert, these findings reveal receptor-specific mechanisms by which PPARs influence macrophage cholesterol homeostasis. In the future, these mechanisms may be exploited pharmacologically to inhibit the development of atherosclerosis.

Nuclear receptors in macrophage biology: at the crossroads of lipid metabolism and inflammation

Macrophages are essential modulators of lipid metabolism and the innate immune system. Lipid and inflammatory pathways induced in activated macrophages are central to the pathogenesis of human diseases including atherosclerosis. Recent work has shown that expression of genes involved in lipid uptake and cholesterol efflux in macrophages is controlled by peroxisome proliferator-activated receptors (PPARs) and liver X receptors (LXRs). Other studies have implicated these same receptors in the modulation of macrophage inflammatory gene expression. Together, these observations position PPARs and LXRs at the crossroads of lipid metabolism and inflammation and suggest that these receptors may serve to integrate these pathways in the control of macrophage gene expression. In this review, we summarize recent work that has advanced our understanding of the roles of PPARs and LXRs in macrophage biology and discuss the implication of these results for cardiovascular physiology and disease.

Multiple expression control mechanisms of peroxisome proliferator-activated receptors and their target genes

The peroxisome proliferator-activated receptors (PPAR) alpha, beta/delta and gamma belong to the nuclear hormone receptor superfamily. As ligand-activated receptors, they form a functional transcriptional unit upon heterodimerization with retinoid X receptors (RXRs). PPARs are activated by fatty acids and their derivatives, whereas RXR is activated by 9-cis retinoic acid. This heterodimer binds to peroxisome proliferator response elements (PPRE) residing in target genes and stimulates their expression. Recent reports now indicate that PPARs and RXRs can function independently, in the absence of a hetero-partner, to modulate gene expression. Of importance, these non-canonical mechanisms underscore the impact of both cofactors and DNA on gene expression. Furthermore, these different mechanisms reveal the increasing repertoire of PPAR 'target' genes that now encompasses non-PPREs containing genes. It is also becoming apparent that understanding the regulation of PPAR expression and activity, can itself have a significant influence on how the expression of subgroups of target genes is studied and integrated in current knowledge.

PPAR- and LXR-dependent pathways controlling lipid metabolism and the development of atherosclerosis

The nuclear receptor superfamily is composed of transcription factors that positively and negatively regulate gene expression in response to the binding of a diverse array of lipid-derived hormones and metabolites. Intense efforts are currently being directed at defining the biological roles and mechanisms of action of liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs). LXRs have been found to play essential roles in the regulation of whole body cholesterol absorption and excretion, in the efflux of cholesterol from peripheral cells, and in the biosynthesis and metabolism of very low density lipoproteins. PPARs have been found to regulate diverse aspects of lipid metabolism, including fatty acid oxidation, fat cell development, lipoprotein metabolism, and glucose homeostasis. Intervention studies indicate that activation of PPARalpha, PPARgamma, and LXRs by specific synthetic ligands can inhibit the development of atherosclerosis in animal models. Here, we review recent studies that provide new insights into the mechanisms by which these subclasses of nuclear receptors act to systemically influence lipid and glucose metabolism and regulate gene expression within the artery wall.

Peroxisome proliferator-activated receptors and atherogenesis: regulators of gene expression in vascular cells

A large body of data gathered over the past couple of years has identified the peroxisome proliferator-activated receptors (PPAR) alpha, gamma, and beta/delta as transcription factors exerting modulatory actions in vascular cells. PPARs, which belong to the nuclear receptor family of ligand-activated transcription factors, were originally described as gene regulators of various metabolic pathways. Although the PPARalpha, gamma, and beta/delta subtypes are approximately 60% to 80% homologous in their ligand- and DNA-binding domains, significant differences in ligand and target gene specificities are observed. PPARalpha is activated by polyunsaturated fatty acids and oxidized derivatives and by lipid-modifying drugs of the fibrate family, including fenofibrate or gemfibrozil. PPARalpha controls expression of genes implicated in lipid metabolism. PPARgamma, in contrast, is a key regulator of glucose homeostasis and adipogenesis. Ligands of PPARgamma include naturally occurring FA derivatives, such as hydroxyoctadecadienoic acids (HODEs), prostaglandin derivatives such as 15-deoxyDelta12,14-prostaglandin J2, and glitazones, insulin-sensitizing drugs presently used to treat patients with type 2 diabetes. Ligands for PPARbeta/delta are polyunsaturated fatty acids, prostaglandins, and synthetic compounds, some of which are presently in clinical development. PPARbeta/delta stimulates fatty acid oxidation predominantly acting in muscle. All PPARs are expressed in vascular cells, where they exhibit antiinflammatory and antiatherogenic properties. In addition, studies in various animal models as well as clinical data suggest that PPARalpha and PPARgamma activators can modulate atherogenesis in vivo. At present, no data are available relating to possible effects of PPARbeta/delta agonists on atherogenesis. Given the widespread use of PPARalpha and PPARgamma agonists in patients at high risk for cardiovascular disease, the understanding of their function in the vasculature is not only of basic interest but also has important clinical implications. This review will focus on the role of PPARs in the vasculature and summarize the present understanding of their effects on atherogenesis and its cardiovascular complications.

Potency of arachidonic acid in polyunsaturated fatty acid-induced death of human monocyte-macrophages: implications for atherosclerosis

Evidence suggests that oxidation of LDL is involved in the progression of atherosclerosis by inducing apoptosis in macrophages. Polyunsaturated fatty acids (PUFAs) are prominent components of LDL and are highly peroxidisable. We therefore tested PUFAs for induction of apoptosis in human monocyte-macrophages in vitro. Arachidonic acid (AA) induced the highest levels of apoptosis followed by docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), despite DHA and EPA being more peroxidisable than AA. alpha-Linolenic acid induced lower levels of apoptosis. Linoleic and oleic acids were innocuous. Results of experiments with AA products and enzyme inhibitors suggest roles for peroxidation, cyclooxygenase and lipoxygenase in AA-induced apoptosis. Our results further suggest activation of PPARgamma by AA and DHA associated with apoptosis induction. These findings may be relevant to potential mechanisms of fatty acid influences on plaques and may suggest strategies for combating atherosclerosis progression.

Peroxisome proliferator-activated receptor (PPAR)-beta/delta stimulates differentiation and lipid accumulation in keratinocytes

Peroxisome proliferator-activated receptor (PPAR) are nuclear hormone receptors that are activated by endogenous lipid metabolites. Previous studies have demonstrated that PPAR-alpha activation stimulates keratinocyte differentiation in vitro and in vivo, is anti-inflammatory, and improves barrier homeostasis. Recent studies have shown that PPAR-beta/delta activation induces keratinocyte differentiation in vitro. This study demonstrated that topical treatment of mice with a selective PPAR-beta/delta agonist (GW1514) in vivo had pro-differentiating effects, was anti-inflammatory, improved barrier homeostasis, and stimulated differentiation in a disease model of epidermal hyperproliferation [corrected]. In contrast to PPAR-alpha activation, PPAR-beta/deltain vivo did not display anti-proliferative or pro-apoptotic effects. The pro-differentiating effects persisted in mice lacking PPAR-alpha, but were decreased in mice deficient in retinoid X receptor-alpha, the major heterodimerization partner of PPAR. Furthermore, in vitro PPAR-beta/delta activation, aside from stimulating differentiation-related genes, additionally induced adipose differentiation-related protein (ADRP) and fasting induced adipose factor (FIAF) mRNA in cultures keratinocytes, which was paralleled by increased oil red O staining indicative of lipid accumulation, the bulk of which were triglycerides (TG). Comparison of differentially expressed genes between PPAR-beta/delta and PPAR-alpha activation revealed distinct profiles. Together, these studies indicate that PPAR-beta/delta activation stimulates keratinocyte differentiation, is anti-inflammatory, improves barrier homeostasis, and stimulates TG accumulation in keratinocytes.

Activation of peroxisome proliferator-activated receptor delta stimulates the proliferation of human breast and prostate cancer cell lines

The nuclear receptor peroxisome proliferator-activated receptor delta [PPARdelta/beta (NR1C2)] has been implicated in colorectal carcinogenesis by various molecular genetic observations. These observations have recently been supported by studies of activation of PPARdelta by pharmacological agents. Here we present the first report of the stimulation of breast and prostate cancer cell growth using PPARdelta selective agonists. Activation of PPARdelta with compound F stimulated proliferation in breast (T47D, MCF7) and prostate (LNCaP, PNT1A) cell lines, which are responsive to sex hormones. Conversely, we have found that several steroid-independent cell lines, including colon lines, were unresponsive to compound F. These findings were confirmed with an additional high-affinity PPARdelta agonist, GW501516. Conditional expression of PPARdelta in MCF7 Tet-On cells resulted in a doxycycline-enhanced response to GW501516, thus providing direct genetic evidence for the role of PPARdelta in the proliferative response to this drug. Activation of PPARdelta in T47D cells resulted in increased expression of the proliferation marker Cdk2 and also vascular endothelial growth factor alpha (VEGFalpha) and its receptor, FLT-1, thus, suggesting that PPARdelta may initiate an autocrine loop for cellular proliferation and possibly angiogenesis. Consistent with this hypothesis, we demonstrated a pro-proliferative effect of GW501516 on human umbilical vein endothelial cell cultures and found that GW501516 also regulated the expression of VEGFalpha and FLT-1 in these cells. Our observations provide the first evidence that activation of PPARdelta can result in increased growth in breast and prostate cancer cell lines and primary endothelial cells and supports the possibility that PPARdelta antagonists may be of therapeutic value in the treatment of breast and prostate cancer.

PPARs and LXRs: atherosclerosis goes nuclear

Atherosclerosis is the leading cause of mortality in the Western world, and new therapeutics to target the metabolic and inflammatory factors that underlie its pathogenesis are needed. Peroxisome proliferator-activated receptors and liver X receptors are lipid-activated nuclear receptors that regulate systemic glucose and lipid metabolism, and modulate inflammation within the vascular wall. New understanding of their functions in physiology and the development of high-affinity synthetic ligands highlight their potential as targets for the treatment of cardiovascular disease.

A short and efficient synthesis of the pharmacological research tool GW501516 for the peroxisome proliferator-activated receptor delta

The most potent and selective peroxisome proliferator-activated receptor delta (PPARdelta) agonist GW501516 (1) was synthesized in 4 steps and 78% overall yield starting from o-cresol by using a one-pot regiocontrolled dialkylation of mercaptophenol 5 as the key step.

Adipophilin Enhances Lipid Accumulation and Prevents Lipid Efflux From THP-1 Macrophages

Objective— Uptake of modified low-density lipoprotein (LDL) by macrophages through scavenger receptors results in lipid droplets accumulation and foam cell formation. Excess lipid deposition in macrophages has been reported to modulate expression of several genes including adipophilin. In this study, we investigated the function of adipophilin in lipid accumulation and cholesterol efflux in THP-1 macrophages.

Methods and Results— Adipophilin mRNA expression was 3.5-fold higher in human atherosclerotic plaques compared with healthy areas of the same arteries. Moreover, in the presence of acetylated LDL (AcLDL), triglycerides and cholesteryl esters were increased in macrophages overexpressing adipophilin by 40% and 67%, respectively, whereas their accumulation was reduced when endogenous cellular adipophilin was depleted using siRNA approach. In addition, neither overexpression nor downregulation of adipophilin altered expression of genes involved in lipid efflux. However, the affinity and the number of AcLDL receptors were not affected. After 24-hour incubation of lipid-loaded macrophages with apolipoprotein A-I, cholesterol efflux was reduced by 47% in adipophilin transfected cells versus control cells.

Conclusion— Our results showed that stimulation of adipophilin expression in macrophages by modified LDL promotes triglycerides and cholesterol storage and reduces cholesterol efflux. Therefore, adipophilin might contribute, in vivo, to lipid accumulation in the intima of the arterial wall.

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.

Induction of Adipophilin-Specific Cytotoxic T Lymphocytes Using a Novel HLA-A2-Binding Peptide That Mediates Tumor Cell Lysis

Identification of tumor-associated antigens and advances in tumor immunology resulted in the development of vaccination strategies to treat patients with malignant diseases. Using a novel approach that combines DNA chip analysis of tumor samples with isolation of peptides on the surface of tumor cells, a HLA-A*0201-binding peptide derived from the adipophilin protein was identified. Adipophilin is involved in lipid storage and was thought to be expressed only in adipocytes, but it can be found in other cell types such as macrophages or tumor cells. In the present study, we analyzed the possible use of this peptide as a T-cell epitope presented by malignant cells. To accomplish this, we induced CTL responses using this HLA-A*0201-binding peptide. The in vitro-induced CTLs efficiently lysed cells pulsed with the adipophilin peptide and HLA-matched tumor cell lines in an antigen-specific and HLA-restricted manner. Finally, the induced CTLs recognized autologous dendritic cells (DCs) pulsed with the antigenic peptide or transfected with tumor RNA purified from an adipophilin-expressing tumor cell line. To further analyze the possible use of this peptide in immunotherapies of human malignancies, we induced adipophilin-specific CTLs using peripheral blood mononuclear cells and DCs from HLA-A*0201-positive patients with chronic lymphatic leukemia and plasma cell leukemia. The in vitro-generated CTLs recognized autologous chronic lymphatic leukemia cells and malignant plasma cells, whereas they spared nonmalignant resting or activated B and T lymphocytes, monocytes, or DCs. Our results demonstrate that this peptide might represent an interesting candidate for the development of cancer vaccines designed to target adipophilin-derived epitopes in a wide range of malignancies.

Decoding Transcriptional Programs Regulated by PPARs and LXRs in the Macrophage

Macrophages play essential roles in immunity and homeostasis. As professional scavengers, macrophages phagocytose microbes and apoptotic and necrotic cells and take up modified lipoprotein particles. These functions require tightly regulated mechanisms for the processing and disposal of cellular lipids. Under pathological conditions, arterial wall macrophages become foam cells by accumulating large amounts of cholesterol, contributing to the development of atherosclerosis. Peroxisome proliferator–activated receptors (PPARs) and liver X receptors (LXRs) are members of the nuclear receptor superfamily of transcription factors that have emerged as key regulators of macrophage homeostasis. PPARs and LXRs control transcriptional programs involved in processes of lipid uptake and efflux, lipogenesis, and lipoprotein metabolism. In addition, PPARs and LXRs negatively regulate transcriptional programs involved in the development of inflammatory responses. This review summarizes recent efforts to decode the differential and overlapping roles of PPARs and LXRs in the context of macrophage lipid homeostasis and the control of inflammation.

Effects of PPAR??, ?? and ?? haplotypes on plasma levels of lipids, severity and progression of coronary atherosclerosis and response to statin therapy in the lipoprotein coronary atherosclerosis study

Peroxisome proliferator-activated receptors (PPARs) alpha, delta and gamma are nuclear transcription factors that regulate fatty acid biosynthesis. Our objectives were to determine the effects of PPAR haplotypes on biochemical, angiographic, clinical phenotypes and their responses to treatment with fluvastatin. We genotyped 372 Lipoprotein and Coronary Atherosclerosis Study subjects for seven single nucleotide polymorphisms (SNPs) in PPARalpha (-35 089A>C, 484C>G), delta (-4401C>T, 294T>C) and gamma (34C>G, 25 506C>T, 161C>T) by restriction mapping or 5' exonuclease assay. We reconstructed and estimated haplotypes frequencies using four algorithms. Linkage disequilibrium (LD) was calculated by D' and haplotype effects by permutation and regression analyses. The PPARD and PPARG SNPs were in LD. The baseline plasma triglyceride levels and their responses to treatment with fluvastatin were associated with PPARD haplotypes (P = 0.01). Triglyceride levels were lowest and highest in homozygotes with diplotypes 3 and 4 (130.1 +/- 40.8 and 194.2 +/- 44.6 mg/dl, P < 0.001), respectively. PPARD haplotype 3 was also an independent determinant of plasma apolipoprotein (apo)B (P = 0.021) and apoC-III (P = 0.001) levels, mean number of coronary lesions (P = 0.046) and changes in triglyceride (P = 0.01) and apoC-III (P = 0.047) levels in response to fluvastatin. Plasma triglyceride levels (P = 0.044), the mean number of coronary lesions (P = 0.026) and changes in minimum lumen diameter in response to fluvastatin (P = 0.022) were also associated with PPARG haplotypes. No significant associations between PPARA haplotypes and the phenotypes or significant interactions between PPAR haplotypes and the occurrence of new clinical events were detected. PPARD and PPARG haplotypes are independent determinants of plasma levels of lipids, severity of coronary atherosclerosis and its response to therapy.

Regulation of Milk Lipid Formation and Secretion in the Mouse Mammary Gland

Cytosolic lipid droplets (CLDs), the immediate precursors of milk lipids in lactating animals, undergo cell-specific changes in their formation and intracellular distribution during mammary gland differentiation. Cell biological studies indicate that CLD formation in mammary epithelial cells is regulated in part by AKT-dependent increases in glucose uptake. Proteomic studies show that CLDs from lactating mammary epithelial cells possess a distinct protein composition enriched in molecules involved in their secretion and intracellular transport. CLD secretion is dependent on lactation and requires the purine catabolic enzyme xanthine oxidoreductase (XOR). Confocal immunofluorescence microscopy of XOR in lactating and nonlactating mammary glands and biochemical analysis of secreted CLDs link the secretion process to the formation of a stable tripartite complex between XOR, adipophilin (ADPH), and butyrophilin (Btn). Together these studies provide a molecular and cellular framework for understanding the process of milk lipid formation.

The peroxisome proliferator activated receptor δ is required for the differentiation of THP-1 monocytic cells by phorbol ester

Background

PPARδ (NR1C2) promotes lipid accumulation in human macrophages in vitro and has been implicated in the response of macrophages to vLDL. We have investigated the role of PPARδ in PMA-stimulated macrophage differentiation.

The THP-1 monocytic cell line which displays macrophage like differentiation in response to phorbol esters was used as a model system. We manipulated the response to PMA using a potent synthetic agonist of PPARδ , compound F. THP-1 sub-lines that either over-expressed PPARδ protein, or expressed PPARδ anti-sense RNA were generated. We then explored the effects of these genetic modulations on the differentiation process.

Results

The PPARδ agonist, compound F, stimulated differentiation in the presence of sub-nanomolar concentrations of phorbol ester. Several markers of differentiation were induced by compound F in a synergistic fashion with phorbol ester, including CD68 and IL8. Over-expression of PPARδ also sensitised THP-1 cells to phorbol ester and correspondingly, inhibition of PPARδ by anti-sense RNA completely abolished this response.

Conclusions

These data collectively demonstrate that PPARδ plays a fundamental role in mediating a subset of cellular effects of phorbol ester and supports observations from mouse knockout models that PPARδ is involved in macrophage-mediated inflammatory responses.

The lipid droplet-associated protein adipophilin is expressed in human trophoblasts and is regulated by peroxisomal proliferator-activated receptor-gamma/retinoid X receptor

Uptake and transplacental transfer of fatty acids is essential for fetal development. Human adipophilin and its murine ortholog adipocyte differentiation-related protein are lipid droplet-associated proteins that are implicated in cellular fatty acid uptake in adipocytes. The nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARgamma) stimulates lipid uptake by adipocytes and enhances differentiation of placental trophoblasts. We therefore hypothesized that adipophilin is expressed in human trophoblasts and that its expression is regulated by PPARgamma. We initially determined that adipophilin is expressed in human villous trophoblasts and that adipophilin expression is enhanced during differentiation of cultured primary term human trophoblasts. We also found that exposure of cultured human trophoblasts to the PPARgamma ligand troglitazone resulted in a concentration-dependent increase in adipophilin expression. We observed a similar increase with LG268, a ligand for retinoid X receptor (RXR), the heterodimeric partner of PPARgamma. Lastly, we demonstrated that ligand-activated PPARgamma and RXR stimulated the transcriptional activity of adipophilin promoter in CV-1 cells and in the placental JEG3 cell line. We conclude that the expression of adipophilin is enhanced during trophoblast differentiation and is up-regulated by ligand-activated PPARgamma/RXR. Enhanced adipophilin expression may contribute to fatty acid uptake by the placenta.

Discovery of gene networks regulating cytokine-induced dysfunction and apoptosis in insulin-producing INS-1 cells

Locally released cytokines contribute to beta-cell dysfunction and apoptosis in type 1 diabetes. In vitro exposure of insulin-producing INS-1E cells to the cytokines interleukin (IL)-1beta + interferon (IFN)-gamma leads to a significant increase in apoptosis. To characterize the genetic networks implicated in beta-cell dysfunction and apoptosis and its dependence on nitric oxide (NO) production, we performed a time-course microarray analysis of cytokine-induced genes in insulin-producing INS-1E cells. INS-1E cells were exposed in duplicate to IL-1beta + IFN-gamma for six different time points (1, 2, 4, 8, 12, and 24 h) with or without the inducible NO synthase (iNOS) blocker N(G)-monomethyl-L-arginine (NMA). The microarray analysis identified 698 genes as cytokine modified (>or=2.5-fold change compared with control) in at least one time point. Based on their temporal pattern of variation, the cytokine-regulated genes were classified into 15 clusters by the k-means method. These genes were further classified into 14 different groups according to their putative function. Changes in the expression of genes related to metabolism, signal transduction, and transcription factors at all time points studied indicate beta-cell attempts to adapt to the effects of continuous cytokine exposure. Notably, several apoptosis-related genes were modified at early time points (2-4 h) preceding iNOS expression. On the other hand, 46% of the genes modified by cytokines after 8-24 h were NO dependent, indicating the important role of this radical for the late effects of cytokines. The present results increase by more than twofold the number of known cytokine-modified genes in insulin-producing cells and yield comprehensive information on the role of NO for these modifications in gene expression. These data provide novel and detailed insights into the gene networks activated in beta-cells facing a prolonged immune assault.

Transcriptional repression of atherogenic inflammation: modulation by PPARdelta

The formation of an atherosclerotic lesion is mediated by lipid-laden macrophages (foam cells), which also establish chronic inflammation associated with lesion progression. The peroxisome proliferator-activated receptor (PPAR) gamma promotes lipid uptake and efflux in these atherogenic cells. In contrast, we found that the closely related receptor PPARdelta controls the inflammatory status of the macrophage. Deletion of PPARdelta from foam cells increased the availability of inflammatory suppressors, which in turn reduced atherosclerotic lesion area by more than 50%. We propose an unconventional ligand-dependent transcriptional pathway in which PPARdelta controls an inflammatory switch through its association and disassociation with transcriptional repressors. PPARdelta and its ligands may thus serve as therapeutic targets to attenuate inflammation and slow the progression of atherosclerosis.

The peroxisome proliferator-activated receptor beta/delta agonist, GW501516, regulates the expression of genes involved in lipid catabolism and energy uncoupling in skeletal muscle cells

Lipid homeostasis is controlled by the peroxisome proliferator-activated receptors (PPARalpha, -beta/delta, and -gamma) that function as fatty acid-dependent DNA-binding proteins that regulate lipid metabolism. In vitro and in vivo genetic and pharmacological studies have demonstrated PPARalpha regulates lipid catabolism. In contrast, PPARgamma regulates the conflicting process of lipid storage. However, relatively little is known about PPARbeta/delta in the context of target tissues, target genes, lipid homeostasis, and functional overlap with PPARalpha and -gamma. PPARbeta/delta, a very low-density lipoprotein sensor, is abundantly expressed in skeletal muscle, a major mass peripheral tissue that accounts for approximately 40% of total body weight. Skeletal muscle is a metabolically active tissue, and a primary site of glucose metabolism, fatty acid oxidation, and cholesterol efflux. Consequently, it has a significant role in insulin sensitivity, the blood-lipid profile, and lipid homeostasis. Surprisingly, the role of PPARbeta/delta in skeletal muscle has not been investigated. We utilize selective PPARalpha, -beta/delta, -gamma, and liver X receptor agonists in skeletal muscle cells to understand the functional role of PPARbeta/delta, and the complementary and/or contrasting roles of PPARs in this major mass peripheral tissue. Activation of PPARbeta/delta by GW501516 in skeletal muscle cells induces the expression of genes involved in preferential lipid utilization, beta-oxidation, cholesterol efflux, and energy uncoupling. Furthermore, we show that treatment of muscle cells with GW501516 increases apolipoprotein-A1 specific efflux of intracellular cholesterol, thus identifying this tissue as an important target of PPARbeta/delta agonists. Interestingly, fenofibrate induces genes involved in fructose uptake, and glycogen formation. In contrast, rosiglitazone-mediated activation of PPARgamma induces gene expression associated with glucose uptake, fatty acid synthesis, and lipid storage. Furthermore, we show that the PPAR-dependent reporter in the muscle carnitine palmitoyl-transferase-1 promoter is directly regulated by PPARbeta/delta, and not PPARalpha in skeletal muscle cells in a PPARgamma coactivator-1-dependent manner. This study demonstrates that PPARs have distinct roles in skeletal muscle cells with respect to the regulation of lipid, carbohydrate, and energy homeostasis. Moreover, we surmise that PPARbeta/delta agonists would increase fatty acid catabolism, cholesterol efflux, and energy expenditure in muscle, and speculate selective activators of PPARbeta/delta may have therapeutic utility in the treatment of hyperlipidemia, atherosclerosis, and obesity.

Peroxisome proliferator-activated receptors: new targets for the pharmacological modulation of macrophage gene expression and function

PURPOSE OF REVIEW

This review focuses on recent advances on the roles of peroxisome proliferator-activated receptors in the control of lipid metabolism, and the inflammatory response of macrophages and the potential participation of these actions in the modulation of atherogenesis.

RECENT FINDINGS

Altered macrophage functions contribute to the pathogenesis of many infectious, immunological and inflammatory disease processes. Pharmacological modulation of macrophage gene expression therefore represents an important strategy for the prevention and treatment of inflammation-related diseases, such as atherosclerosis. Peroxisome proliferator-activated receptors are lipid-activated transcription factors that control lipid and lipoprotein metabolism, glucose and energy homeostasis, as well as cellular differentiation and proliferation. Recent data suggest that peroxisome proliferator-activated receptor alpha and peroxisome proliferator-activated receptor gamma activators may decrease the incidence of cardiovascular disease, not only by correcting metabolic disorders, but also by directly acting at the level of the vascular wall. In this context, ligand-activated peroxisome proliferator-activated receptors control cellular functions by regulating gene expression in different cell types, including monocytes, macrophages and foam cells.

SUMMARY

These findings identify the crucial roles of peroxisome proliferator-activated receptors in macrophages, improving the comprehension of the patho-physiological mechanisms of atherogenesis. Moreover, a scientific rationale for the evaluation of peroxisome proliferator-activated receptor activators in the treatment of inflammatory disorders such as atherosclerosis is thus provided.

PPARgamma and metabolism: insights from the study of human genetic variants

Diabetes, obesity, atherosclerosis and cancer are the principal contributors to morbidity and mortality in Western society. Emerging evidence indicates that a nuclear receptor, the peroxisome proliferator-activated receptor gamma (PPARgamma), plays a role in these pathological processes. Furthermore, modulation of receptor action in these diseases may be of therapeutic value, as exemplified by the recent introduction of the thiazolidinediones, a novel class of insulin-sensitizing agent for the treatment of type 2 diabetes mellitus. The availability of such high-affinity ligands has facilitated the study of signalling pathways through which PPARgamma regulates metabolic processes; these analyses have been complemented by the study of human subjects harbouring (naturally occurring) mutations and polymorphisms within the receptor. The latter have provided unique genetic evidence for a link between PPARgamma and mammalian glucose homeostasis, lipid metabolism and regulation of fat mass. This review highlights recent studies which have advanced our understanding of the pivotal role that this receptor plays in metabolism, with particular reference to the consequences of inherited variation in the human receptor gene.

Microarray analysis of peroxisome proliferator-activated receptor-gamma induced changes in gene expression in macrophages

We used a combination of expression microarray and Northern blot analyses to identify target genes for peroxisome proliferator-activated receptor (PPAR) gamma in RAW264.7 macrophages. PPARgamma natural ligand 15-deoxy-Delta(12,14) prostaglandin and synthetic ligands ciglitazone and rosiglitazone increased the expression of scavenger receptor CD36 and ATP-binding cassette transporter A1, as well as adipophilin (a lipid droplet coating protein involved in intracellular lipid storage and transport), calpain (a protease implicated in ABCA1 protein degradation), and ADAM8 (a disintegrin and metalloprotease protein involved in cell adhesion). These findings are relevant to understanding the effect of PPARgamma activation on gene expression and cognate pathways in macrophages.

PPARdelta activation induces COX-2 gene expression and cell proliferation in human hepatocellular carcinoma cells

Cyclooxygenase-2 (COX-2) has been suggested to be associated with carcinogenesis. Recently, many studies have shown increased expression of COX-2 in a variety of human malignancies, including hepatocellular carcinoma (HCC). Therefore, it becomes important to know more about what determines COX-2 expression. In this work, we have studied the effect of PPARdelta activation on COX-2 expression using a selective agonist (GW501516) in human hepatocellular carcinoma (HepG2) cells. Activation of PPARdelta resulted in increased COX-2 mRNA and protein expression. The mechanism behind the induction seems to be increased activity of the proximal promoter of the COX-2 gene, spanning nucleotides -327 to +59. The increased COX-2 protein expression and promoter activity induced by the GW501516 was also confirmed in the monocytic cell line THP-1. Induced levels of COX-2 have previously been associated with resistance to apoptosis and increased cell proliferation in many cell types. In HepG2 cells, we observed a dose-dependent increase in cell number by GW501516 treatment for 72h. The levels of PCNA, used as an indicator of cell division were induced, and the cell survival promoting complex p65 (NF-kappaB) was phosphorylated under GW501516 treatment. We conclude that PPARdelta activation in HepG2 cells results in induced COX-2 expression and increased cellular proliferation. These results may suggest that PPARdelta plays an important role in the development of HCC by modulating expression of COX-2.

Peroxisome proliferator-activated receptors and the cardiovascular system

Insulin resistance syndrome (also called syndrome X) includes obesity, diabetes, hypertension, and dyslipidemia and is a complex phenotype of metabolic abnormalities. The disorder poses a major public health problem by predisposing individuals to coronary heart disease and stroke, the leading causes of mortality in Western countries. Given that hypertension, diabetes, dyslipidemia, and obesity exhibit a substantial heritable component, it is postulated that certain genes may predispose some individuals to this cluster of cardiovascular risk factors. Emerging data suggest that peroxisome proliferator-activated receptors (PPARs), including alpha, gamma, and delta, are important determinants that may provide a functional link between obesity, hypertension, and diabetes. It has been well documented that hypolipidemic fibrates and antidiabetic thiazolidinediones are synthetic ligands for PPAR alpha and PPAR gamma, respectively. In addition, PPAR natural ligands, such as leukotriene B4 for PPAR alpha, 15-deoxy-delta 12,14-prostaglandin J2 for PPAR gamma, and prostacyclin for PPAR delta, are known to be eicosanoids and fatty acids. Studies have documented that PPARs are present in all critical vascular cells: endothelial cells, vascular smooth muscle cells, and monocyte-macrophages. These observations suggest that PPARs not only control lipid metabolism but also regulate vascular diseases such as atherosclerosis and hypertension. In this review, we present structure and tissue distribution of PPAR nuclear receptors, discuss the mechanisms of action and regulation, and summarize the rapid progress made in this area of study and its impact on the cardiovascular system.

Peroxisome proliferator-activated receptors: are they involved in atherosclerosis progression?

Peroxisome proliferator-activated receptors (PPAR) are nuclear receptors present in several organs and cell types. They are subdivided into PPAR alpha, PPAR gamma and PPAR delta (or beta). PPAR alpha and gamma are the two main categories of these receptors, which are both characterized by their ability to influence lipid metabolism, glucose homeostasis, cell proliferation, differentiation and apoptosis, as well as the inflammatory response, by transcriptional activation of target genes. PPAR alpha are activated by fatty acids, eicosanoids and fibrates, while PPAR gamma activators include arachidonic acid metabolites, oxidized low density lipoprotein and thiazolidinediones. Atherosclerosis is now considered a chronic inflammatory condition. Thus, PPAR activation appears a promising approach to favorably affect atherosclerosis development through both metabolic and anti-inflammatory effects. However, the clinical data in favor of an anti-atherosclerotic action of PPAR agonists are still scanty, and some experimental data would even indicate possible pro-atherogenic effects, or a lack of effect in the female sex. New controlled clinical studies will provide the information necessary to understand the true significance and usefulness of PPAR alpha, gamma and delta activators in the control of atherosclerotic disease.

Nutritional regulation and role of peroxisome proliferator-activated receptor delta in fatty acid catabolism in skeletal muscle

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors primarily involved in lipid homeostasis. PPARdelta displays strong expression in tissues with high lipid metabolism, such as adipose, intestine and muscle. Its role in skeletal muscle remains largely unknown. After a 24-h starvation period, PPARdelta mRNA levels are dramatically up-regulated in gastrocnemius muscle of mice and restored to control level upon refeeding. The rise of PPARdelta is accompanied by parallel up-regulations of fatty acid translocase/CD36 (FAT/CD36) and heart fatty acid binding protein (H-FABP), while refeeding promotes down-regulation of both genes. To directly access the role of PPARdelta in muscle cells, we forced its expression and that of a dominant-negative PPARdelta mutant in C2C12 myogenic cells. Differentiated C2C12 cells responds to 2-bromopalmitate or synthetic PPARdelta agonist by induction of genes involved in lipid metabolism and increment of fatty acid oxidation. Overexpression of PPARdelta enhanced these cellular responses, whereas expression of the dominant-negative mutant exerts opposite effects. These data strongly support a role for PPARdelta in the regulation of fatty acid oxidation in skeletal muscle and in adaptive response of this tissue to lipid catabolism.

Novel selective small molecule agonists for peroxisome proliferator-activated receptor delta (PPARdelta)--synthesis and biological activity

We report the synthesis and biological activity of a new series of small molecule agonists of the human Peroxisome Proliferator-Activated Receptor delta (PPARdelta). Several hits were identified from our original libraries containing lipophilic carboxylic acids. Optimization of these hits by structure-guided design led to 7k (GW501516) and 7l (GW0742), which shows an EC(50) of 1.1 nM against PPARdelta with 1000-fold selectivity over the other human subtypes.

Differential effects of peroxisome proliferator-activated receptor activators on the mRNA levels of genes involved in lipid metabolism in primary human monocyte-derived macrophages

Peroxisome proliferator-activated receptors (PPARs) are key regulators of macrophage lipid metabolism. We compared the effects of three PPAR activators (bezafibrate, fenofibrate, and troglitazone) on the mRNA levels of genes involved in lipid metabolism in primary human macrophages and macrophage-derived foam cells. Treatment of human macrophages for 24 hours with 100 micro mol/L bezafibrate, a nonselective drug that activates the 3 PPAR subtypes (PPARalpha, PPARbeta/delta, and PPARgamma), caused an 87% (P <.01) and a 230% rise in CD36 and adipocyte fatty acid-binding protein (aP2) mRNA levels, respectively, whereas the expressions of PPARgamma, PPARalpha, acyl-CoA oxidase, carnitine palmitoyltransferase I (CPT-I), adenosine triphosphate (ATP)-binding cassette transporter 1 (ABCA1), neutral cholesteryl ester hydrolase, and lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) were not modified. However, treatment with selective PPARalpha (fenofibrate at 100 micro mol/L) and PPARgamma (troglitazone at 5 micro mol/L) activators had different effects. Fenofibrate increased PPARalpha (62%, P <.05) and LOX-1 (180%, P <.05) mRNA levels; and troglitazone upregulated CPT-I expression (75%, P <.05). When the effects of these drugs were assessed in macrophage-derived foam cells, we found that troglitazone caused a 134% (P <.05) and a 66% (P <.01) rise in ABCA1 and CPT-I mRNA levels, respectively, whereas the 3 drugs significantly increased aP2 transcripts (about 100% induction). Given that troglitazone treatment resulted in the upregulation of genes involved in the mitochondrial beta-oxidation of fatty acids (CPT-I) and in the reverse-cholesterol-transport pathway (ABCA1), we subsequently determined whether these changes affected intracellular cholesterol ester accumulation. In macrophage-derived foam cells a significant reduction (32%, P <.01) was observed in intracellular cholesterol accumulation after troglitazone, but not after bezafibrate or fenofibrate treatment. Since CPT-I inhibition promotes cholesterol incorporation into cholesteryl esters in macrophages, study is now needed on whether CPT-I induction by troglitazone may reduce the availability of fatty acids for synthesizing cholesterol esters, leading to less foam cell formation.

Peroxisome-proliferator-activated receptor delta activates fat metabolism to prevent obesity

In contrast to the well-established roles of PPARgamma and PPARalpha in lipid metabolism, little is known for PPARdelta in this process. We show here that targeted activation of PPARdelta in adipose tissue specifically induces expression of genes required for fatty acid oxidation and energy dissipation, which in turn leads to improved lipid profiles and reduced adiposity. Importantly, these animals are completely resistant to both high-fat diet-induced and genetically predisposed (Lepr(db/db)) obesity. As predicted, acute treatment of Lepr(db/db) mice with a PPARdelta agonist depletes lipid accumulation. In parallel, PPARdelta-deficient mice challenged with high-fat diet show reduced energy uncoupling and are prone to obesity. In vitro, activation of PPARdelta in adipocytes and skeletal muscle cells promotes fatty acid oxidation and utilization. Our findings suggest that PPARdelta serves as a widespread regulator of fat burning and identify PPARdelta as a potential target in treatment of obesity and its associated disorders.

Evidence that peroxisome proliferator-activated receptor delta influences cholesterol metabolism in men

OBJECTIVE

The objective of this work was to explore the role of peroxisome proliferator-activated receptor delta (PPARD) in lipid metabolism in humans.

METHODS AND RESULTS

PPARD is a nuclear receptor involved in lipid metabolism in primates and mice. We screened the 5'-region of the human gene for polymorphisms to be used as tools in association studies. Four polymorphisms were detected: -409C/T in the promoter region, +73C/T in exon 1, +255A/G in exon 3, and +294T/C in exon 4. The frequencies of the rare alleles were 4.2%, 4.2%, 1.2% and 15.6%, respectively, in a population-based group of 543 healthy men. Only the +294T/C polymorphism showed significant association with a metabolic trait. Homozygotes for the rare C allele had a higher plasma LDL-cholesterol concentration than homozygotes for the common T allele, which was verified in an independent cohort consisting of 282 healthy men. Transfection studies showed that the rare C allele had higher transcriptional activity than the common T allele. Electrophoretic mobility shift assays demonstrated that the +294T/C polymorphism influenced binding of Sp-1. An interaction with the PPAR alpha L162V polymorphism was also detected for several lipid parameters.

CONCLUSIONS

These findings suggest that PPARD plays a role in cholesterol metabolism in humans.

Peroxisome proliferator-activated receptors beta/delta: emerging roles for a previously neglected third family member

PURPOSE OF REVIEW

Peroxisome proliferator-activated receptors alpha, beta/delta and gamma are members of the nuclear receptor superfamily. They mediate the effects of fatty acids and their derivatives at the transcriptional level, and are considered to be lipid sensors that participate in the regulation of energy homeostasis. Compared with the alpha and gamma peroxisome proliferator-activated receptor isotypes, peroxisome proliferator-activated receptor beta functions have long remained an enigma. In this review, we focus on emerging knowledge about peroxisome proliferator-activated receptor beta activation and roles.

RECENT FINDINGS

We review recent data that suggest key roles in basic cell functions, such as proliferation, differentiation and survival, and in embryonic development and lipid metabolism in peripheral tissues.

SUMMARY

The newly unveiled roles of peroxisome proliferator-activated receptor beta in important basic cell functions certainly justify a further exploration of its potential as a therapeutic target in pathologies such as metabolic syndrome X or skin diseases.

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.

The generation of monoclonal antibodies against human peroxisome proliferator-activated receptors (PPARs)

Monoclonal antibodies (Mabs) are valuable reagents for the purification, characterization and immunolocalization of proteins. In this study, we raised Mabs against human peroxisome proliferator-activated receptors (PPARs) using baculovirus particles displaying surface glycoprotein gp64-fusion proteins as the immunizing agent. In this system, to display fusion proteins on the viral surface, the amino terminal sequences of human PPARd and PPARg2 are inserted in-frame between the signal sequence and the mature domain of the gp64 nucleotide sequence.Mabs were raised by immunization with whole virus without a purification of the target antigens. The Mabs generated by this novel method were shown to recognize not only the gp64-PPARs fusion protein, but also mature, expressed proteins by a wide variety of techniques, including immunohistochemistry, immunoblotting, and electrophoretic mobility shift assays (EMSAs). Transfection of the transfer vector containing a nucleotide sequence encoding less than 30 amino acids along with linearized baculovirus DNA allows for the production of a high affinity antibody against the corresponding mature form. This method is of potential utility in that it allows the production of valuable antibodies without the requirement of a protein purification step.

Lipids and the immune response: from molecular mechanisms to clinical applications

PURPOSE OF REVIEW

This review critically evaluates recent studies investigating the effects of fatty acids on immune and inflammatory responses in both healthy individuals and in patients with inflammatory diseases, with some reference to animal studies where relevant. It examines recent findings describing the cellular and molecular basis for the modulation of immune function by fatty acids. The newly emerging area of diet-genotype interactions will also be discussed, with specific reference to the anti-inflammatory effects of fish oil.

RECENT FINDINGS

Fatty acids are participants in many intracellular signalling pathways. They act as ligands for nuclear receptors regulating a host of cell responses, they influence the stability of lipid rafts, and modulate eicosanoid metabolism in cells of the immune system. Recent findings suggest that some or all of these mechanisms may be involved in the modulation of immune function by fatty acids.

SUMMARY

Human studies investigating the relationship between dietary fatty acids and some aspects of the immune response have been disappointingly inconsistent. This review presents the argument that most studies have not been adequately powered to take into account the influence of variation (genotypic or otherwise) on parameters of immune function. There is well-documented evidence that fatty acids modulate T lymphocyte activation, and recent findings describe a range of potential cellular and molecular mechanisms. However, there are still many questions remaining, particularly with respect to the roles of nuclear receptors, for which fatty acids act as ligands, and the modulation of eicosanoid synthesis, for which fatty acids act as precursors.

PPARdelta is a very low-density lipoprotein sensor in macrophages

Although triglyceride-rich particles, such as very low-density lipoprotein (VLDL), contribute significantly to human atherogenesis, the molecular basis for lipoprotein-driven pathogenicity is poorly understood. We demonstrate that in macrophages, VLDL functions as a transcriptional regulator via the activation of the nuclear receptor peroxisome proliferator-activated receptor delta. The signaling components of native VLDL are its triglycerides, whose activity is enhanced by lipoprotein lipase. Generation of peroxisome proliferator-activated receptor delta null macrophages verifies the absolute requirement of this transcription factor in mediating the VLDL response. Thus, our data reveal a pathway through which dietary triglycerides and VLDL can directly regulate gene expression in atherosclerotic lesions.

Advances in understanding the regulation of apoptosis and mitosis by peroxisome-proliferator activated receptors in pre-clinical models: relevance for human health and disease

Peroxisome proliferator activated receptors (PPARs) are a family of related receptors implicated in a diverse array of biological processes. There are 3 main isotypes of PPARs known as PPARalpha, PPARbeta and PPARgamma and each is organized into domains associated with a function such as ligand binding, activation and DNA binding. PPARs are activated by ligands, which can be both endogenous such as fatty acids or their derivatives, or synthetic, such as peroxisome proliferators, hypolipidaemic drugs, anti-inflammatory or insulin-sensitizing drugs. Once activated, PPARs bind to DNA and regulate gene transcription. The different isotypes differ in their expression patterns, lending clues on their function. PPARalpha is expressed mainly in liver whereas PPARgamma is expressed in fat and in some macrophages. Activation of PPARalpha in rodent liver is associated with peroxisome proliferation and with suppression of apoptosis and induction of cell proliferation. The mechanism by which activation of PPARalpha regulates apoptosis and proliferation is unclear but is likely to involve target gene transcription. Similarly, PPARgamma is involved in the induction of cell growth arrest occurring during the differentiation process of fibroblasts to adipocytes. However, it has been implicated in the regulation of cell cycle and cell proliferation in colon cancer models. Less in known concerning PPARbeta but it was identified as a downstream target gene for APC/beta-catenin/T cell factor-4 tumor suppressor pathway, which is involved in the regulation of growth promoting genes such as c-myc and cyclin D1. Marked species and tissue differences in the expression of PPARs complicate the extrapolation of pre-clinical data to humans. For example, PPARalpha ligands such as the hypolipidaemic fibrates have been used extensively in the clinic over the past 20 years to treat cardiovascular disease and side effects of clinical fibrate use are rare, despite the observation that these compounds are rodent carcinogens. Similarly, adverse clinical responses have been seen with PPARgamma ligands that were not predicted by pre-clinical models. Here, we consider the response to PPAR ligands seen in pre-clinical models of efficacy and safety in the context of human health and disease.

Fatty acid transport across membranes: relevance to nutrition and metabolic pathology

Long-chain fatty acids are an important constituent of the diet and they contribute to a multitude of cellular pathways and functions. Uptake of long-chain fatty acids across plasma membranes is the first step in fatty acid utilization, and recent evidence supports an important regulatory role for this process. Although uptake of fatty acids involves two components, passive diffusion through the lipid bilayer and protein-facilitated transfer, the latter component appears to play the major role in mediating uptake by key tissues. Identification of several proteins as fatty acid transporters, and emerging evidence from genetically altered animal models for some of these proteins, has contributed significant insight towards understanding the limiting role of transport in the regulation of fatty acid utilization. We are also beginning to better appreciate how disturbances in fatty acid utilization influence general metabolism and contribute to metabolic pathology.

Emerging roles of PPARs in inflammation and immunity

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

A novel pathway of prostacyclin signaling-hanging out with nuclear receptors

Prostacylin (PGI(2)), one of the major prostaglandins, is derived from arachidonic acid by the action of the cyclooxygenase (COX) system coupled to PGI(2) synthase (PGIS). The presence of the COX-2/PGIS at the nuclear and endoplasmic reticular membrane suggests differential signaling pathways of PGI(2) actions involving both cell surface and nuclear receptors. Although the signaling of PGI(2) via its cell surface receptor, prostacyclin receptor (IP), is well documented in vascular biology, its action via nuclear receptors in other physiological responses is gradually being more appreciated. Peroxisomal proliferator-activated receptors (PPARs), PPARalpha, PPARgamma, and PPARdelta, though initially cloned as a family of orphan receptors, are now known for their ligand promiscuity. The ligands range from free fatty acids and their derivatives produced by the cyclooxygenase or lipoxygenase pathway to certain hypolipidemic drugs. The predisposition of PPARs to use a wide spectrum of ligands is well explained by their unusually large ligand-binding pocket. The promiscuous ligand usage by PPARs is also reflected by their involvement in various pathophysiological events. Several recent independent reports show that endogenously produced PGI(2) indeed activates PPARdelta in vivo, indicating that a novel signaling mechanism for this abundant eicosanoid is operative in certain systems. This review attempts to cover recent developments in nuclear actions of PGI(2) in diverse biological functions.

The role of PPARs in atherosclerosis

Peroxisome proliferator-activated receptors (PPARs) are lipid-activated transcription factors that regulate lipid and lipoprotein metabolism, glucose homeostasis and inflammation. The PPAR family consists of three proteins, alpha, beta/delta and gamma. Recent data suggest that PPAR alpha and gamma activation decreases atherosclerosis progression not only by correcting metabolic disorders, but also through direct effects on the vascular wall. PPARs modulate the recruitment of leukocytes to endothelial cells, control the inflammatory response and lipid homeostasis of monocytes/macrophages and regulate inflammatory cytokine production by smooth muscle cells. Experiments using animal models of atherosclerosis and clinical studies in humans strongly support an anti-atherosclerotic role for PPAR alpha and gamma in vivo. Thus, PPARs remain attractive therapeutic targets for the development of drugs used in the treatment of chronic inflammatory diseases such as atherosclerosis. Future research will aim for the development of more potent drugs with co-agonist activity on PPAR alpha, PPAR beta/delta and/or PPAR gamma as well as tissue and target gene-selective PPAR receptor modulators (SPPARMs).

Unsaturated fatty acids and their oxidation products stimulate CD36 gene expression in human macrophages

Fatty acids (FA) have been implicated in the control of expression of several atherosclerosis-related genes. Similarly, the CD36 receptor has recently been shown to play an important role in atherosclerosis and other pathologies. The aim of the present study was to evaluate the direct effect of FA and their oxidation products (aldehydes), on the expression of CD36 in both THP-1 macrophages and human monocyte-derived macrophages (HMDM). The FA tested included the saturated FA (SFA) lauric, myristic, palmitic and stearic acid; the monounsaturated FA oleic acid; and the unsaturated FA (UFA) linoleic, arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Aldehydes used were malondialdehyde (MDA), hexanal, 2,4-decadienal (DDE) and 4-hydroxynonenal (HNE). CD36 expression was measured by RT-PCR, Western blot and immunofluorescence. Incubation of THP-1 macrophages for 24 h with non-cytotoxic concentrations of UFA significantly increased CD36 mRNA expression. By contrast, exposure of THP-1 macrophages to SFA did not affect the levels of CD36 mRNA. Among all UFAs tested, EPA and DHA were the strongest inducers of CD36 mRNA levels, followed by oleic and linoleic acid. Incubation of HMDM with either oleic or linoleic acid significantly increased steady-state CD36 mRNA in a dose-dependent manner. Consistent with the increase of CD36 mRNA expression, incubation of THP-1 macrophages with oleic and linoleic acid for 24 h markedly increased CD36 protein expression. Treatment of THP-1 macrophages with MDA or hexanal for 24 h significantly increased CD36 mRNA expression in a dose dependent manner. In contrast, DDE and HNE significantly decreased this parameter. The data provide evidence for a direct regulatory effect of UFA on CD36 gene expression and support a role for aldehydes in the regulation of CD36 expression by FA.

Elimination of cholesterol ester from macrophage foam cells by adenovirus-mediated gene transfer of hormone-sensitive lipase

Cholesterol ester (CE)-laden foam cells are a hallmark of atherosclerosis. To determine whether stimulation of the hydrolysis of cytosolic CE can be used as a novel therapeutic modality of atherosclerosis, we overexpressed hormone-sensitive lipase (HSL) in THP-1 macrophage-like cells by adenovirus-mediated gene delivery, and we examined its effects on the cellular cholesterol trafficking. We show here that the overexpression of HSL robustly increased neutral CE hydrolase activity and completely eliminated CE in the cells that had been preloaded with CE by incubation with acetylated low density lipoprotein. In these cells, cholesterol efflux was stimulated in the absence or presence of high density lipoproteins, which might be at least partially explained by the increase in the expression of ABCA1. Importantly, these effects were achieved without the addition of acyl-CoA:cholesterol acyltransferase inhibitor, cAMP, or even high density lipoproteins. Furthermore, the uptake and degradation of acetylated low density lipoprotein was significantly reduced probably by decreased expression of scavenger receptor A and CD36. Notably, the cells with stimulated CE hydrolysis did not exhibit either buildup of free cholesterol or cytotoxicity. In conclusion, increased hydrolysis of CE by the overexpression of HSL leads to complete elimination of CE from THP-1 foam cells not only by increasing efflux but also by decreasing influx of cholesterol.

The peroxisome proliferator-activated receptor alpha (PPARalpha): role in hepatocarcinogenesis

The peroxisome proliferator-activated receptor alpha (PPARalpha) is a member of the nuclear receptor superfamily and mediates most of the known biological effects of peroxisome proliferators. The latter represents a large group of chemicals that include the fibrate hyperlipidemic drugs, the pthalate plasticizers, various solvents and degreasing agents, and endogenous hormones and fatty acids. Peroxisome proliferators are classical members of the nongenotoxic group of chemical carcinogens that do not require metabolic activation to electrophiles in order to exert their harmful effects. These chemicals are of particular concern to regulatory agencies since they can only be detected by long-term carcinogen bioassays using rodents. The mechanism of the carcinogenic action of peroxisome proliferators is beginning to emerge. PPARalpha-null mice are resistant to hepatocarcinogenesis indicating that this receptor is necessary for cancer. However, recent studies indicate that Kupffer cells, in a PPARalpha independent manor, are required for the major effects of peroxisome proliferators on cell proliferation. An interaction between PPARalpha and estrogen carcinogenesis has also been elucidated.