PPAR-gamma dependent and independent effects on macrophage-gene expression in lipid metabolism and inflammation

Conditional disruption of the peroxisome proliferator-activated receptor gamma gene in mice results in lowered expression of ABCA1, ABCG1, and apoE in macrophages and reduced cholesterol efflux

Disruption of the peroxisome proliferator-activated receptor gamma (PPAR gamma) gene causes embryonic lethality due to placental dysfunction. To circumvent this, a PPAR gamma conditional gene knockout mouse was produced by using the Cre-loxP system. The targeted allele, containing loxP sites flanking exon 2 of the PPAR gamma gene, was crossed into a transgenic mouse line expressing Cre recombinase under the control of the alpha/beta interferon-inducible (MX) promoter. Induction of the MX promoter by pIpC resulted in nearly complete deletion of the targeted exon, a corresponding loss of full-length PPAR gamma mRNA transcript and protein, and marked reductions in basal and troglitazone-stimulated expression of the genes encoding lipoprotein lipase, CD36, LXR alpha, and ABCG1 in thioglycolate-elicited peritoneal macrophages. Reductions in the basal levels of apolipoprotein E (apoE) mRNA in macrophages and apoE protein in total plasma and high-density lipoprotein (HDL) were also observed in pIpC-treated PPAR gamma-MXCre(+) mice. Basal cholesterol efflux from cholesterol-loaded macrophages to HDL was significantly reduced after disruption of the PPAR gamma gene. Troglitazone selectively inhibited ABCA1 expression (while rosiglitazone, ciglitazone, and pioglitazone had little effect) and cholesterol efflux in both PPAR gamma-deficient and control macrophages, indicating that this drug can exert paradoxical effects on cholesterol homeostasis that are independent of PPAR gamma. Together, these data indicate that PPAR gamma plays a critical role in the regulation of cholesterol homeostasis by controlling the expression of a network of genes that mediate cholesterol efflux from cells and its transport in plasma.

Vascular signaling pathways in the metabolic syndrome

There are several potential cellular and molecular pathways whereby cardiovascular risk factors act through very specific signal transduction pathways in the formation of atherosclerosis, as seen often in the metabolic syndrome. Many examples point to multiple postreceptor defects in the insulin signaling pathway in vascular tissue, however, there are differences in the insulin receptor pathway in vascular tissue compared with skeletal muscle or fat. In addition to insulin receptors, insulin may affect atherosclerotic changes in the vascular cells via stimulation of insulin-like growth factor-1 receptors and their signaling pathway. Insulin also causes activation of the vascular renin-angiotensin system in both vascular smooth muscle cells and endothelial cells. Insulin-activated tissue renin-angiotensin system leads to increased cell growth and contributes to the cause of atherosclerosis. The fact that agents that inhibit the renin-angiotensin system also block insulin-mediated renin-angiotensin system expression and cell growth reinforces the potential implication of a vascular insulin-renin-angiotensin system pathway. Finally, novel substances such as the adipokines, factors produced from fat cells, reveal new risk factors in the metabolic syndrome and offer further evidence for a link between insulin resistance and accelerated atherosclerosis.

New mouse models for lipoprotein metabolism and atherosclerosis

Transgenic mouse models have been crucial to our current understanding of the mechanisms of lipoprotein metabolism. Moreover, these models have greatly advanced our understanding of the pathology associated with altered lipoprotein levels. Recent progress has been made in cellular uptake, intracellular metabolism, cellular efflux mechanisms and transcriptional regulation. In particular, much progress has been made in our understanding of events that take place in the vessel wall. In addition, the transgenic mouse model is becoming a crucial tool in genomic studies to evaluate gene function, as well as a subject of genome-wide expression studies. The present review describes progress in all of these areas and shows that animal models are likely to remain important to our view of gene function in the context of the whole organism.

Peroxisome proliferator-activated receptor-gamma agonists inhibit experimental allergic encephalomyelitis by blocking IL-12 production, IL-12 signaling and Th1 differentiation

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear receptor transcription factor that regulates adipocyte differentiation and glucose homeostasis. PPARgamma agonists are potent therapeutic agents for the treatment of type 2 diabetes and obesity. PPARgamma agonists also prevent inflammation in animal models, suggesting their use for the treatment of human inflammatory diseases. Experimental allergic encephalomyelitis (EAE) is a Th1 cell-mediated inflammatory demyelinating disease model of multiple sclerosis (MS) and IL-12 plays a crucial role in the pathogenesis of EAE and MS. In this study we have examined the effect of PPARgamma agonists on the pathogenesis of EAE. In vivo treatment of SJL/J mice with PPARgamma agonists, 15-deoxydelta(12,14) prostaglandin J2 or Ciglitazone, decreased the duration and clinical severity of active immunization and adoptive transfer models of EAE. PPARgamma agonists inhibited EAE in association with a decrease in IL-12 production and differentiation of neural antigen-specific Th1 cells. In vitro treatment of activated T cells with PPARgamma agonists inhibited IL-12-induced activation of JAK-STAT signaling pathway and Th1 differentiation. These findings highlight the fact that PPARgamma agonists regulate central nervous system inflammation and demyelination by inhibiting IL-12 production, IL-12 signaling and Th1 differentiation in EAE.

15-Deoxy-delta 12,14-prostaglandins D2 and J2 are potent activators of human eosinophils

15-Deoxy-Delta(12,14)-PDJ(2) (15d-PGJ(2)) is a degradation product of PGD(2) that has been proposed as an anti-inflammatory compound because of its various inhibitory effects, some of which are mediated by peroxisome proliferator-activated receptor-gamma. In contrast to its reported inhibitory effects on macrophages and other cells, we found that this compound is a potent activator of eosinophils, inducing calcium mobilization, actin polymerization, and CD11b expression. It is selective for eosinophils, having little or no effect on neutrophils or monocytes. 15d-PGJ(2) has an EC(50) of approximately 10 nM, similar to that of its precursor, PGD(2). The concentrations of 15d-PGJ(2) required to activate eosinophils are thus much lower than those required for its anti-inflammatory effects (usually micromolar). 15-Deoxy-Delta(12,14)-prostaglandin D(2) (15d-PGD(2)) is also a potent activator of eosinophils, with an EC(50) about the same as that of PGD(2), whereas Delta(12)-PGJ(2) is slightly less potent. Eosinophils pretreated with PGD(2) no longer respond to 15d-PGJ(2), and vice versa, but in both cases the cells still respond to another eicosanoid proinflammatory mediator, 5-oxo-6,8,11,14-eicosatetraenoic acid. This indicates that the effects of 15d-PGJ(2) are mediated by the DP(2)/chemoattractant receptor-homologous molecule expressed on Th2 cells that has recently been identified in eosinophils. 15d-PGJ(2) is selective for the DP(2) receptor, in that it has no effect on DP(1) receptor-mediated adenylyl cyclase activity in platelets. We conclude that 15d-PGJ(2) and 15d-PGD(2) are selective DP(2) receptor agonists that activate human eosinophils with potencies at least 100 times greater than those for the proposed anti-inflammatory effects of 15d-PGJ(2) on other cells.

Peroxisome proliferator-activated receptor gamma and metabolic disease

The nuclear peroxisome proliferator-activated receptor gamma (PPAR gamma) is a transcription factor that is activated by polyunsaturated fatty acids and their metabolites and is essential for fat cell formation. Although obesity is a strong risk factor for type 2 diabetes mellitus and other metabolic diseases, potent PPAR gamma activators such as the glitazone drugs lower glucose and lipid levels in patients with type 2 diabetes and also have antiatherosclerotic and antihypertensive effects. We review recent studies providing insight into the paradoxical relationship between PPAR gamma and metabolic disease. We also review recent advances in understanding the structural basis for PPAR gamma activation by ligands. The unusual ligand-binding properties of PPAR gamma suggest that it will be possible to discover new chemical classes of receptor "modulators" with distinct pharmacological activities for the treatment of type 2 diabetes and other metabolic diseases.

Regulation of cytokine expression by ligands of peroxisome proliferator activated receptors

Peroxisome proliferator activated receptors (PPARs) are ligand-activated transcription factors with diverse actions including adipocyte differentiation and lipid metabolism. Recent studies have revealed anti-inflammatory activities, but the majority of these studies have been performed in monocyte/macrophages. In these studies, we investigate the effects of PPAR ligands in murine mitogen-activated splenocytes. Ciglitazone, a PPARgamma ligand, consistently decreased IFN-gamma and IL-2 production by mitogen-activated splenocytes and had modest effects on splenocyte proliferation. The effects of WY14,643, a representative of the fibrate class of PPARalpha ligands, on splenocyte proliferation and IL-2 levels are less marked than those observed with the PPARgamma ligand. In addition, treatment with WY14,643 and other fibrates led to marked increases in supernatant concentrations of IL-4. However, treatment with a potent and specific PPARalpha ligand (GW7,647) did not augment IL-4. Also, WY14,643 induced IL-4 expression in splenocytes from PPARalpha knockout mice, suggesting that the fibrate effect on IL-4 was largely through a PPARalpha-independent mechanism. This increase in IL-4 was associated with and causatively related to augmented expression of CD23 by CD45R/B220(+) cells. We also demonstrate that PPARgamma gene expression is up-regulated in T cells by mitogen activation, that it is positively regulated by IL-4 and WY14,643, and that it is blocked by anti-IL-4. Finally, we demonstrate that WY14,643 can modestly augment IL-4 promoter activity in a PPARalpha-independent manner. In concert, these findings support the roles of PPAR ligands in modulating inflammatory responses involving lymphocytes but also establish potent effects of the fibrate class of PPARalpha ligands on IL-4 expression that are receptor independent.

Bisphenol A diglycidyl ether induces apoptosis in tumour cells independently of peroxisome proliferator-activated receptor-gamma, in caspase-dependent and -independent manners

Peroxisome proliferator-activated receptors (PPARs) are nuclear transcription factors which are involved in many biological processes, such as regulation of cell differentiation, lipid metabolism, inflammation and cell death. PPARs consist of three families, PPAR-alpha, PPAR-delta and PPAR-gamma. Bisphenol A diglycidyl ether (BADGE) has been described as a pure antagonist of PPAR-gamma. However, recent data also revealed PPAR-gamma-agonistic activities of BADGE. Here we show that BADGE kills transformed cells by apoptosis and promotes the cytotoxic effects of tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) and indomethacin. The cytotoxic effect of BADGE does not require PPAR-gamma expression and is mediated in caspase-dependent and caspase-independent manners.

Role of CD36 in membrane transport of long-chain fatty acids

CD36 is a multispecific membrane glycoprotein that has been postulated to have a variety of functions. Evidence generated in isolated cells and in mice and rat models of altered CD36 expression has indicated an important role for CD36 in membrane transport of long-chain fatty acids. The cumulative data indicate that CD36 facilitates a major fraction of fatty acid uptake by muscle and fat, and that CD36 deficiency is associated with a large (60-80%) defect in fatty acid uptake by those tissues. In humans, polymorphisms in the CD36 gene may underlie defective fatty acid metabolism and some forms of heart disease. Herein we review our current understanding of the transport function and regulation of CD36. The realization that the transport step rate limits cellular fatty acid utilization suggests that abnormalities in CD36 expression or function may impact on susceptibility to certain metabolic diseases such as obesity and insulin resistance.

The dawn of the SPPARMs?

Thiazolidinediones (TZDs) are used as antidiabetic agents in the treatment of type II diabetes. These compounds are ligands for the nuclear hormone receptor PPARgamma, which is highly expressed in adipose tissue. PPARgamma acts as a molecular switch in the process of fat cell development. The quest for the ideal antidiabetic agent is challenged by the need to develop PPARgamma ligands that improve insulin sensitivity, but do not promote fat cell formation. A newly described PPARgamma ligand may represent an initial step in this direction and could lead to improved agents for treating insulin resistance in type II diabetes.

PPARgamma but not PPARalpha ligands are potent repressors of major histocompatibility complex class II induction in atheroma-associated cells

Peroxisome proliferator-activated receptors (PPARs) are essential in glucose and lipid metabolism and are implicated in metabolic disorders predisposing to atherosclerosis, such as diabetes and dyslipidemia. Conversely, antidiabetic glitazones and hypolipidemic fibrate drugs, known as PPARgamma and PPARalpha ligands, respectively, reduce the process of atherosclerotic lesion formation, which involves chronic immunoinflammatory processes. Major histocompatibility complex class II (MHC-II) molecules, expressed on the surface of specialized cells, are directly involved in the activation of T lymphocytes and in the control of the immune response. Interestingly, expression of MHC-II has recently been observed in atherosclerotic plaques, and it can be induced by the proinflammatory cytokine interferon-gamma (IFN-gamma) in vascular cells. To explore a possible role for PPAR ligands in the regulation of the immune response, we investigated whether PPAR activation affects MHC-II expression in atheroma-associated cells. In the present study, we demonstrate that PPARgamma but not PPARalpha ligands act as inhibitors of IFN-gamma-induced MHC-II expression and thus as repressors of MHC-II-mediated T-cell activation. All different types of PPARgamma ligands tested inhibit MHC-II. This effect of PPARgamma ligands is due to a specific inhibition of promoter IV of CIITA and does not concern constitutive expression of MHC-II. Thus, the beneficial effects of antidiabetic PPARgamma activators on atherosclerotic plaque development may be partly explained by their repression of MHC-II expression and subsequent inhibition of T-lymphocyte activation.

PPARgamma ligand inhibits osteopontin gene expression through interference with binding of nuclear factors to A/T-rich sequence in THP-1 cells

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear receptor superfamily that acts as a key player in adipocyte differentiation, glucose metabolism, and macrophage differentiation. Osteopontin (OPN), a component of extracellular matrix, is elevated during neointimal formation in the vessel wall and is synthesized by macrophages in atherosclerotic plaques. In the present study, we investigated the molecular mechanisms regulating OPN gene expression by PPARgamma in THP-1 cells, a cell line derived from human monocytic leukemia cells. Northern and Western blot analyses showed that exposure of THP-1 cells to PMA (phorbol 12-myristate 13-acetate) increases OPN mRNA and protein levels in a time-dependent manner. PMA-induced OPN expression was significantly decreased by troglitazone (Tro) and other PPARgamma ligands. Transient transfection assays of the human OPN promoter/luciferase construct showed that PPARgamma represses OPN promoter activity, and the PPARgamma-responsive region within the OPN promoter lies between -1000 and -970 relative to the transcription start site. Site-specific mutation analysis and electrophoretic mobility shift assays indicated that a homeobox-like A/T-rich sequence between -990 and -981, which functions as a binding site for PMA-induced nuclear factors other than PPARgamma, mediates the repression of OPN expression by Tro. Furthermore, concatenated A/T-rich sequences conferred the PPARgamma responsiveness on the heterologous promoter. Taken together, these data suggest that PPARgamma ligand inhibits OPN gene expression through the interference with the binding of nuclear factors to A/T-rich sequence in THP-1 cells.

PPARγ Ligand Inhibits Osteopontin Gene Expression Through Interference With Binding of Nuclear Factors to A/T-Rich Sequence in THP-1 Cells

Peroxisome proliferator-activated receptor γ (PPARγ) is a member of the nuclear receptor superfamily that acts as a key player in adipocyte differentiation, glucose metabolism, and macrophage differentiation. Osteopontin (OPN), a component of extracellular matrix, is elevated during neointimal formation in the vessel wall and is synthesized by macrophages in atherosclerotic plaques. In the present study, we investigated the molecular mechanisms regulating OPN gene expression by PPARγ in THP-1 cells, a cell line derived from human monocytic leukemia cells. Northern and Western blot analyses showed that exposure of THP-1 cells to PMA (phorbol 12-myristate 13-acetate) increases OPN mRNA and protein levels in a time-dependent manner. PMA-induced OPN expression was significantly decreased by troglitazone (Tro) and other PPARγ ligands. Transient transfection assays of the human OPN promoter/luciferase construct showed that PPARγ represses OPN promoter activity, and the PPARγ-responsive region within the OPN promoter lies between −1000 and −970 relative to the transcription start site. Site-specific mutation analysis and electrophoretic mobility shift assays indicated that a homeobox-like A/T-rich sequence between −990 and −981, which functions as a binding site for PMA-induced nuclear factors other than PPARγ, mediates the repression of OPN expression by Tro. Furthermore, concatenated A/T-rich sequences conferred the PPARγ responsiveness on the heterologous promoter. Taken together, these data suggest that PPARγ ligand inhibits OPN gene expression through the interference with the binding of nuclear factors to A/T-rich sequence in THP-1 cells.

Oxysterol activators of liver X receptor and 9-cis-retinoic acid promote sequential steps in the synthesis and secretion of tumor necrosis factor-alpha from human monocytes

Liver X receptor alpha (LXRalpha), is a nuclear hormone receptor that is activated by oxysterols and plays a crucial role in regulating cholesterol and lipid metabolism in liver and cholesterol efflux from lipid-loaded macrophages. Here we show that treatment of human peripheral blood monocytes or monocytic THP-1 cells with the LXR ligand 22(R)-hydroxycholesterol (22(R)-HC), in combination with 9-cis-retinoic acid (9cRA), a ligand for the LXR heterodimerization partner retinoid X receptor (RXR), results in the specific induction of the potent pro-apoptotic and pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha). Promoter analysis, inhibitor studies, and order-of-addition experiments demonstrated that TNF-alpha induction by 22(R)-HC and 9cRA occurs by a novel two-step process. The initial step involves 22(R)-HC-dependent induction of TNF-alpha mRNA, and intracellular accumulation of TNF-alpha protein, mediated by binding of LXRalpha/RXRalpha to an LXR response element at position -879 of the TNF-alpha promoter. Subsequent cell release of TNF-alpha protein occurs via a separable 9cRA-dependent, LXRalpha-independent step that requires de novo transcription and protein synthesis. Our findings reveal a potentially new dimension of the physiological role of LXRalpha and identify a unique multistep pathway of TNF-alpha production that may be of consequence to the normal function of LXR in monocyte/macrophages and in disease conditions such as atherosclerosis.

Effects of differently oxidized LDL on the expression of pro-inflammatory molecules in human monocytes in vitro

The oxidation of low-density lipoprotein (LDL) is thought to be a major contributor to the development of atherosclerosis and considerable evidence has accumulated showing that oxidized LDL (ox LDL) induces cell damage and pro-atherogenic events. However, evidence that oxidized LDL directly causes atherosclerosis is lacking. We studied whether native and enzymatically or chemically ox LDL at concentrations of 5 and 100 microg/mL is cytotoxic to or promotes pro-atherogenic activation of human primary monocytes in culture. Both types of ox LDL (100 microg/mL), but not native LDL added to monocytes for 24 h significantly diminish DNA synthesis and increase cell death. In addition, both preparations of ox LDL inhibit cytokine and metalloproteinase production, diminish cellular oxygen consumption and induce PPAR gamma expression. Enzymatically ox LDL, but not LDL oxidized by copper sulfate, also increases the monocyte metabolic rate and induces intracellular lipid accumulation. Low concentrations of either preparation of oxidized and native LDL did not show significant effects on all parameters measured. These data establish a direct link between ox LDL concentration and cytotoxicity and suggest that oxidation by copper of the lipid moiety in LDL and of the protein moiety by enzyme creates ox LDL, which can damage monocytes without release of pro-inflammatory molecular species. In contrast to native and enzymatically ox LDL, copper ox LDL does not induce intracellular lipid accumulation. Differently oxidized LDL molecules may exert distinct effects in lesion development in atherosclerosis.

Atorvastatin activates PPAR-gamma and attenuates the inflammatory response in human monocytes

OBJECTIVE

To investigate the ability of statins to activate the nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR-gamma) in primary human monocytes in culture.

MATERIALS AND METHODS

Human peripheral monocytes were incubated with atorvastatin (0.1-10 micromol/1) for up to 24 hours. PPAR-gamma expression was analysed by electrophoretic mobility shift assay. Pro-inflammatory cytokines were measured by enzyme-linked immunosorbent assays, and oxygen consumption was determined polarographically with a Clark-type oxygen electrode.

RESULTS

We found that atorvastatin activates PPAR-gamma and inhibits the production of tumour necrosis factor-alpha up to 38% (p < 0.05), monocyte chemoattractant protein-1 up to 85% (p < 0.05), and gelatinase B up to 73% (p < 0.05), in a concentration-dependent manner. Moreover, atorvastatin shows concentration-dependent inhibition of cellular oxygen consumption up to 41%.

CONCLUSIONS

These findings contribute to the growing knowledge of the anti-inflammatory effects of statins, and have led us to the suggestion that statins may control inflammatory responses by the regulation of intracellular lipid homeostasis.

Peroxisome proliferator-activated receptor gamma and atherosclerosis

Within the past couple of years, peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear transcription factor expressed in vascular cells, has received growing interest in vascular biology and atherosclerosis research. Initially, PPARgamma was identified as a regulator of gene expression in lipid metabolism and adipogenesis, but recent data in monocyte/macrophages, endothelial cells, and vascular smooth muscle cells suggest that PPARgamma might have anti-inflammatory effects in atherogenesis. However, with growing insight, there is much controversy about potential anti- or proatherosclerotic effects. This review will focus on these aspects and discuss what PPARgamma activation in vascular cells could mean for atherogenesis and the development of atherosclerosis.

Peroxisome proliferator-activated receptor gamma activators inhibit MMP-1 production in human synovial fibroblasts likely by reducing the binding of the activator protein 1

OBJECTIVE

To investigate the expression and activity of PPARgamma in human synovial fibroblasts and the effects of PPARgamma agonists on the expression of MMP-1. The molecular mechanisms by which PPARgamma agonists modulate MMP-1 expression were also examined.

METHODS

PPARgamma expression and activity were measured using reverse-transcription polymerase chain reaction (RT-PCR) and transient transfection assays. Human synovial fibroblasts were cultured with IL-1beta in the absence or presence of PPARgamma activators, and the expression and production of MMP-1 were evaluated by Northern blot and ELISA, respectively. The effect of 15d-PGJ(2) on MMP-1 promoter activation was analysed in transient transfection experiments, while electrophoretic mobility shift assays were performed to study the binding activity of the transcription factor AP-1.

RESULTS

PPARgamma was expressed and transcriptionally functional in human synovial fibroblasts. PPARgamma activators (15d-PGJ(2) and BRL 49653) inhibited IL-1beta-induced MMP-1 synthesis in a dose-dependent manner. Similarly, both activators inhibited IL-1-induced MMP-1 mRNA expression. Activation of the human MMP-1 promoter was also attenuated by 15d-PGJ(2), indicating that the inhibitory effect of 15d-PGJ(2) occurs at the transcriptional level. Interestingly, 15d-PGJ(2) reduced both basal and IL-1beta-induced AP-1 binding activity.

CONCLUSIONS

These data indicate that PPARgamma agonists inhibit MMP-1 gene expression by transcriptional mechanisms, and suggest that they may be useful in reducing joint tissue destruction.

PPARgamma is not a critical mediator of primary monocyte differentiation or foam cell formation

In the present report we clarify the role of PPARgamma in differentiation and function of human-derived monocyte/macrophages in vitro. Rosiglitazone, a selective PPARgamma activator, had no effect on the kinetics of appearance of monocyte/macrophage differentiation markers or on cell size or granularity. Depletion of PPARgamma by more than 90% using antisense oligonucleotides did not influence accumulation of oxidized LDL or prevent the upregulation of CD36 that normally accompanies oxLDL treatment. In contrast, PPARgamma depletion reduced the expression of ABCA1 and LXRalpha mRNAs. Metalloproteinase-9 expression, a marker of atherosclerotic plaque vulnerability, was suppressed by rosiglitazone. We conclude that activation of PPARgamma does not affect monocyte/macrophage differentiation. In addition, PPARgamma is not absolutely required for oxLDL-driven lipid accumulation, but is required for full expression of ABCA1 and LXRalpha. Our data support a role for rosiglitazone as a potential directly acting antiatherosclerotic agent.

Peroxisome proliferator-activated receptors

An exciting and rapidly evolving area in vascular biology and atherosclerosis research over the past 3 years has been the establishment of peroxisome proliferator-activated receptor (PPAR) expression in the vascular and inflammatory cells, and the emerging picture of the roles these ligand-activated nuclear receptor/transcription factors might play in vascular biology and atherosclerosis. Such work is all the more compelling given the ongoing clinical use of PPAR activators in patients. Thiazolidinediones (PPAR-g agonists) are used as insulin sensitizers in diabetic patients known to be at extraordinarily high risk for cardiovascular disease, whereas fibrates (PPAR-a agonists) are used to treat dyslipidemia, particularly in the case of high triglycerides and low high-density lipoprotein cholesterol.

Toxicological consequences of altered peroxisome proliferator-activated receptor gamma (PPARgamma) expression in the liver: insights from models of obesity and type 2 diabetes

The pivotal role of peroxisome proliferator-activated receptor gamma (PPARgamma) in the liver, although important for the regulation of genes involved in glucose and lipid metabolism, has generally not been fully appreciated. This may be due to the fact that PPARgamma, in contrast to PPARalpha or PPARdelta, is not abundantly expressed in liver under normal conditions. However, recent findings have revealed that in several murine models of obesity and type 2 diabetes mellitus (T2DM), PPARgamma mRNA and receptor protein are highly up-regulated in the liver, and that the receptor causes increased transcriptional activity as demonstrated by the activation of PPARgamma-responsive genes in the liver. Prolonged treatment of obese and diabetic mice, but not of lean control mice, with the selective PPARgamma ligands and activators, thiazolidinediones (TZDs), including troglitazone, rosiglitazone, or pioglitazone, has resulted in the development of severe hepatic centrilobular steatosis. In contrast to these effects in hepatocytes, TZD-mediated effects on Kupffer cells (down-regulation of proinflammatory cytokines) seem to be PPARgamma-independent. In view of the findings that sustained hepatic steatosis can lead to steatohepatitis and/or fibrosis and that troglitazone (but not the other TZDs) has been associated with rare but serious hepatotoxicity in patients, further insight into PPARgamma-mediated versus non-PPARgamma-mediated effects of TZDs is desirable. It is concluded that liver-specific effects associated with TZD antidiabetics may become relevant under conditions of selective PPARgamma up-regulation in the liver. Therefore, receptor expression in human liver tissue of obese and T2DM patients should deserve increased consideration in the future.

Peroxisome proliferator-activated receptor gamma inhibits transforming growth factor beta-induced connective tissue growth factor expression in human aortic smooth muscle cells by interfering with Smad3

Activation of peroxisome proliferator-activated receptor gamma (PPAR gamma) after balloon injury significantly inhibits VSMC proliferation and neointima formation. However, the precise mechanisms of this inhibition have not been determined. We hypothesized that activation of PPAR gamma in vascular injury could attenuate VSMC growth and matrix production during vascular lesion formation. Since connective tissue growth factor (CTGF) is a key factor regulating extracellular matrix production, abrogation of transforming growth factor beta (TGF-beta)-induced CTGF production by PPAR gamma activation may be one of the mechanisms through which PPAR gamma agonists inhibit neointima formation after vascular injury. In this study, we demonstrate that the PPAR gamma natural ligand (15-deoxyprostaglandin J(2)) and a synthetic ligand (GW7845) significantly inhibit TGF-beta-induced CTGF production in a dose-dependent manner in HASMCs. In addition, suppression of CTGF mRNA expression is relieved by pretreatment with an antagonist of PPAR gamma (GW9662), suggesting that the inhibition of CTGF expression is mediated by PPAR gamma. To elucidate further the molecular mechanism by which PPAR gamma inhibits CTGF expression, an approximately 2-kilobase pair CTGF promoter was cloned. We found that PPAR gamma activation inhibits TGF-beta-induced CTGF promoter activity in a dose-dependent manner, and suppression of CTGF promoter activity by PPAR gamma activation is completely rescued by overexpression of Smad3, but not by Smad4. Furthermore, PPAR gamma physically interacts with Smad3 but not Smad4 in vitro in glutathione S-transferase pull-down experiments. Taken together, the data suggest that PPAR gamma inhibits TGF-beta-induced CTGF expression in HASMCs by directly interfering with the Smad3 signaling pathway.

Oxidized low density lipoprotein exposure alters the transcriptional response of macrophages to inflammatory stimulus

Macrophage-derived foam cells in atherosclerotic lesions are generally thought to play a major role in the pathology of the disease. Because macrophages play a central role in the inflammatory response, and the atherosclerotic lesion has features associated with chronic inflammatory settings, we investigated foam cell inflammatory potential. THP-1-derived macrophages were treated with oxidized low density lipoprotein (OxLDL) for 3 days to lipid load the macrophages and establish a foam cell-like phenotype. The cells were then activated by treatment with lipopolysaccharide (LPS), and RNA was harvested at 0, 1, and 6 h after LPS addition. RNA from treated and control cells was hybridized to microarrays containing approximately 16,000 human cDNAs. Genes that exhibited a 4-fold or greater increase or decrease at either 1 or 6 h after LPS treatment were counted as LPS-responsive genes. Employing these criteria, 127 LPS-responsive genes were identified. Prior treatment of THP-1 macrophages with OxLDL affected the expression of 57 of these 127 genes. Among these 57 genes was a group of chemokine, cytokine, and signal transduction genes with pronounced expression changes. OxLDL pretreatment resulted in a significant perturbation of LPS-induced NF kappa B activation. Furthermore, some of the OxLDL effects appear to be mediated by the nuclear receptors retinoid X receptor and peroxisomal proliferator-activated receptor gamma because pretreatment of THP-1 macrophages with ligands for these receptors, followed by LPS treatment, recapitulates the OxLDL plus LPS results for several of the most significantly modulated genes.

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

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

PPARgamma and atherosclerosis: effects on cell growth and movement

Atherosclerosis is a major vascular complication of diabetes and the primary cause of mortality in persons with this disease. Metabolic abnormalities related to the Insulin Resistance Syndrome or Metabolic Syndrome may importantly contribute to the increased risk of atherosclerosis associated with diabetes. Thiazolidinediones (TZDs) are oral insulin sensitizers in broad clinical use that enhance insulin-stimulated glucose uptake into skeletal muscle. TZDs can also improve cardiovascular risk factors and exert direct effects on vascular cells to potentially retard the atherosclerotic process. Direct vascular effects of TZDs likely result from their activity as ligands for the nuclear receptor, PPARgamma. All of the major cell types in the vasculature express PPARgamma, including intimal macrophages and vascular smooth muscle cells (VSMCs) in human atheroma. TZDs block VSMC growth by inducing cell cycle arrest in G1 through an inhibition of retinoblastoma protein phosphorylation. Migration of monocytes and VSMCs is also inhibited by TZDs, possibly through decreased matrix metalloproteinase production. Activation of PPARgamma by TZDs in macrophages induces ABCA1 transporter expression to promote reverse cholesterol transport. These antiatherogenic activities may also occur in vivo because TZDs have been shown to inhibit lesion formation in several animal models. Thus, TZD activation of PPARgamma may protect against atherosclerosis both by normalizing proatherogenic metabolic abnormalities of the insulin resistance/diabetes milieu and through an inhibition of vascular cell growth and movement.

Peroxisome proliferator-activated receptor-gamma haploinsufficiency enhances B cell proliferative responses and exacerbates experimentally induced arthritis

Peroxisome proliferator-activated receptor-gamma (PPAR gamma) controls adipogenesis and glucose metabolism. It was reported recently that PPAR gamma activation by its agonistic ligands modifies lymphocyte function. Since synthetic ligands are known to exert their effect via PPAR gamma-dependent and -independent pathways, we examined the physiological role of PPAR gamma in lymphocytes by using heterozygote mutant mice in which one allele of PPAR gamma is deleted (PPAR gamma(+/-)). In contrast to T cells, which did not exhibit a significant difference, B cells from PPAR gamma(+/-) showed an enhanced proliferative response to stimulation by either lipopolysaccharide or cross-linking of antigen receptors. Dysregulation of the NF-kappa B pathway in B cells from PPAR gamma(+/-) was indicated by spontaneous NF-kappa B activation, as well as increased I kappa B alpha phosphorylation and gel-shift activity following LPS stimulation. Mice primed with either ovalbumin or methylated BSA also showed enhanced antigen-specific immune response of both T and B cells, an immunological abnormality that exacerbated antigen-induced arthritis. These findings indicate that PPAR gamma plays a critical role in the control of B cell response and imply a role in diseases in which B cell hyperreactivity is involved, such as arthritis and autoimmunity.

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

The peroxisome proliferator-activated receptors (PPARs) are a family of fatty acid-activated transcription factors which control lipid homeostasis and cellular differentiation. PPARalpha (NR1C1) controls lipid oxidation and clearance in hepatocytes and PPARgamma (NR1C3) promotes preadipocyte differentiation and lipogenesis. Drugs that activate PPARalpha are effective in lowering plasma levels of lipids and have been used in the management of hyperlipidemia. PPARgamma agonists increase insulin sensitivity and are used in the management of type 2 diabetes. In contrast, there are no marketed drugs that selectively target PPARdelta (NR1C2) and the physiological roles of PPARdelta are unclear. In this report we demonstrate that the expression of PPARdelta is increased during the differentiation of human macrophages in vitro. In addition, a highly selective agonist of PPARdelta (compound F) promotes lipid accumulation in primary human macrophages and in macrophages derived from the human monocytic cell line, THP-1. Compound F increases the expression of genes involved in lipid uptake and storage such as the class A and B scavenger receptors (SRA, CD36) and adipophilin. PPARdelta activation also represses key genes involved in lipid metabolism and efflux, i.e. cholesterol 27-hydroxylase and apolipoprotein E. We have generated THP-1 sublines that overexpress PPARdelta and have confirmed that PPARdelta is a powerful promoter of macrophage lipid accumulation. These data suggest that PPARdelta may play a role in the pathology of diseases associated with lipid-filled macrophages, such as atherosclerosis, arthritis, and neurodegeneration.

In vivo myocardial protection from ischemia/reperfusion injury by the peroxisome proliferator-activated receptor-gamma agonist rosiglitazone

BACKGROUND

Diabetes is associated with increased risk of mortality as a consequence of acute myocardial infarction. This study determined whether rosiglitazone (ROSI) could reduce myocardial infarction after ischemia/reperfusion injury.

METHODS AND RESULTS

Male Lewis rats were anesthetized, and the left anterior descending coronary artery was ligated for 30 minutes. After reperfusion for 24 hours, the ischemic and infarct sizes were determined. ROSI at 1 and 3 mg/kg IV reduced infarct size by 30% and 37%, respectively (P<0.01 versus vehicle). Pretreatment with ROSI (3 mg. kg(-1). d(-1) PO) for 7 days also reduced infarct size by 24% (P<0.01). ROSI also improved ischemia/reperfusion-induced myocardial contractile dysfunction. Left ventricular systolic pressure and positive and negative maximal values of the first derivative of left ventricular pressure (dP/dt) were significantly improved in ROSI-treated rats. ROSI reduced the accumulation of neutrophils and macrophages in the ischemic heart by 40% and 43%, respectively (P<0.01). Ischemia/reperfusion induced upregulation of CD11b/CD18 and downregulation of L-selectin on neutrophils and monocytes; these effects were significantly attenuated in ROSI-treated animals. Likewise, intercellular adhesion molecule-1 expression in ischemic hearts was markedly diminished by ROSI, as was the ischemia/reperfusion-stimulated upregulation of monocyte chemoattractant protein-1.

CONCLUSIONS

ROSI reduced myocardial infarction and improved contractile dysfunction caused by ischemia/reperfusion injury. The cardioprotective effect of ROSI was most likely due to inhibition of the inflammatory response.

A unique PPARgamma ligand with potent insulin-sensitizing yet weak adipogenic activity

FMOC-L-Leucine (F-L-Leu) is a chemically distinct PPARgamma ligand. Two molecules of F-L-Leu bind to the ligand binding domain of a single PPARgamma molecule, making its mode of receptor interaction distinct from that of other nuclear receptor ligands. F-L-Leu induces a particular allosteric configuration of PPARgamma, resulting in differential cofactor recruitment and translating in distinct pharmacological properties. F-L-Leu activates PPARgamma with a lower potency, but a similar maximal efficacy, than rosiglitazone. The particular PPARgamma configuration induced by F-L-Leu leads to a modified pattern of target gene activation. F-L-Leu improves insulin sensitivity in normal, diet-induced glucose-intolerant, and in diabetic db/db mice, yet it has a lower adipogenic activity. These biological effects suggest that F-L-Leu is a selective PPARgamma modulator that activates some (insulin sensitization), but not all (adipogenesis), PPARgamma-signaling pathways.

A single element in the phosphoenolpyruvate carboxykinase gene mediates thiazolidinedione action specifically in adipocytes

Phosphoenolpyruvate carboxykinase (PEPCK) is the key enzyme in glyceroneogenesis, an important metabolic pathway that functions to restrain the release of non-esterified fatty acids (NEFAs) from adipocytes. The antidiabetic drugs known as thiazolidinediones (TZDs) are thought to achieve some of their benefits by lowering elevated plasma NEFAs. Moreover, peroxisome proliferator activated receptor gamma (PPARgamma) mediates the antidiabetic effects of TZDs, though many TZD responses appear to occur via PPARgamma-independent pathways. PPARgamma is required for adipocyte PEPCK expression, hence PEPCK could be a major target gene for the antidiabetic actions of TZDs. Here we used tissue culture and transfection assays to confirm that the TZD, rosiglitazone, stimulates PEPCK gene transcription specifically in adipocytes. We made the novel observation that this effect was by far the most rapid and robust among several other genes expressed in adipocytes. Adipocytes were transfected with a PEPCK/chloramphenicol acetyltransferase chimeric gene, in which either of the two previously discovered PPARgamma/retinoid X receptor alpha response elements, PCK2 and gAF1/PCK1, had been inactivated by mutagenesis. We demonstrate that PCK2 alone is a bona fide thiazolidinedione response element. We show also that the regulation of PEPCK by PPARs is cell-specific and isotype-specific since rosiglitazone induces PEPCK gene expression selectively in adipocytes, and PPARalpha- and PPARbeta-specific activators are inefficient. Hence, TZDs could lower plasma NEFAs via PPARgamma and PEPCK by enhancing adipocyte glyceroneogenesis.

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

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

Peroxisome proliferator-activated receptor gamma activators affect the maturation of human monocyte-derived dendritic cells

Peroxisome proliferator-activated receptor gamma (PPARgamma ), a member of the nuclear receptor superfamily, has recently been described as a modulator of macrophage functions and as an inhibitor of T cell proliferation. Here, we investigated the role of PPARgamma in dendritic cells (DC), the most potent antigen-presenting cells. We showed that PPARgamma is highly expressed in immature human monocyte-derived DC (MDDC) and that it may affect the immunostimulatory function of MDDC stimulated with lipopolysaccharide (LPS) or via CD40 ligand (CD40L). We found that the synthetic PPARgamma agonist rosiglitazone (as well as pioglitazone and troglitazone) significantly increases on LPS- and CD40L-activated MDDC, the surface expression of CD36 (by 184% and 104%, respectively) and CD86 (by 54% and 48%), whereas it reduces the synthesis of CD80 (by 42% and 42%). Moreover, activation of PPARgamma resulted in a dramatic decreased secretion of the Th1-promoting factor IL-12 in LPS- and CD40L-stimulated cells (by 47% and 62%), while the production of IL-1beta, TNF-alpha, IL-6 and IL-10 was unaffected. Finally, PPARgamma ligands down-modulate the synthesis of IFN-gamma -inducible protein-10 (recently termed as CXCL10) and RANTES (CCL5), both chemokines involved in the recruitment of Th1 lymphocytes (by 49% and 30%), but not the levels of the Th2 cell-attracting chemokines,macrophage-derived chemokine (CCL22) and thymus and activation regulated chemokine (CCL17), in mature MDDC. Taken together, our data suggest that activation of PPARgamma in human DC may have an impact in the orientation of primary and secondary immune responses by favoring type 2 responses.

Prostaglandin-J2 induces synthesis of interleukin-8 by endothelial cells in a PPAR-gamma-independent manner

PPARgamma is a transcription factor of nuclear receptor superfamily, involved in the regulation of inflammation. We investigated the influence of PPARgamma-ligands, 15-deoxy-delta12,14 prostaglandin-J2 (15d-PGJ2), and ciglitazone, on the generation of interleukin-8 (IL-8) by the human microvascular endothelial cell line (HMEC- 1). Expression of PPARgamma in HMEC-1 was confirmed by RT-PCR. Both PPARgamma-ligands tested induced the activation of PPAR, but the potency of ciglitazone was higher, as evidenced by luciferase assay. Resting HMEC-1 released about 150 pg/ml of IL-8 protein. Treatment with LPS increased the IL-8 secretion up to 1 ng/ml. 15d-PGJ2 potently and dose-dependently increased both the steady-state and LPS-induced generation of IL-8 mRNA and IL-8 protein. In contrast, neither basal nor LPS-elicited expression of IL-8 was influenced by ciglitazone. We conclude, that 15d-PGJ2 is a potent inducer of IL-8 production and can be a mediator of inflammatory response, but this effect is independent of PPARgamma activation.

Peroxisome proliferator-activated receptors in endothelial cell biology

New insights into the endothelium as a dynamic, interactive organ have generated increased interest in endothelial cell transcriptional regulation. Peroxisomal proliferator-activated receptors (PPARs), as ligand-activated nuclear receptors expressed in endothelial cells, represent one important pathway that likely influences vascular responses both directly and indirectly by altering gene expression. PPAR ligands such as fibrates (PPAR-alpha) and insulin-sensitizing thiazolidinediones (PPAR-gamma) are in clinical use and may alter the process of atherosclerosis. The present review highlights the emerging evidence for PPAR-alpha and PPAR-gamma expression in the vasculature, as well as their potential roles in endothelial cell biology.

Inhibition of RXR and PPARgamma ameliorates diet-induced obesity and type 2 diabetes

PPARgamma is a ligand-activated transcription factor and functions as a heterodimer with a retinoid X receptor (RXR). Supraphysiological activation of PPARgamma by thiazolidinediones can reduce insulin resistance and hyperglycemia in type 2 diabetes, but these drugs can also cause weight gain. Quite unexpectedly, a moderate reduction of PPARgamma activity observed in heterozygous PPARgamma-deficient mice or the Pro12Ala polymorphism in human PPARgamma, has been shown to prevent insulin resistance and obesity induced by a high-fat diet. In this study, we investigated whether functional antagonism toward PPARgamma/RXR could be used to treat obesity and type 2 diabetes. We show herein that an RXR antagonist and a PPARgamma antagonist decrease triglyceride (TG) content in white adipose tissue, skeletal muscle, and liver. These inhibitors potentiated leptin's effects and increased fatty acid combustion and energy dissipation, thereby ameliorating HF diet-induced obesity and insulin resistance. Paradoxically, treatment of heterozygous PPARgamma-deficient mice with an RXR antagonist or a PPARgamma antagonist depletes white adipose tissue and markedly decreases leptin levels and energy dissipation, which increases TG content in skeletal muscle and the liver, thereby leading to the re-emergence of insulin resistance. Our data suggested that appropriate functional antagonism of PPARgamma/RXR may be a logical approach to protection against obesity and related diseases such as type 2 diabetes.

Prospects for prevention and treatment of cancer with selective PPARgamma modulators (SPARMs)

Peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor and transcription factor that regulates the expression of many genes relevant to carcinogenesis, is now an important target for development of new drugs for the prevention and treatment of cancer. Deficient expression of PPARgamma can be a significant risk factor for carcinogenesis, although in some cases overexpression enhances carcinogenesis. Ligands for PPARgamma suppress breast carcinogenesis in experimental models and induce differentiation of human liposarcoma cells. By analogy to the selective estrogen receptor modulator (SERM) concept, it is suggested that selective PPARgamma modulators (SPARMs), designed to have desired effects on specific genes and target tissues without undesirable effects on others, will be clinically important in the future for chemoprevention and chemotherapy of cancer.

Peroxisome proliferator-activated receptor gamma-retinoid X receptor agonists increase CD36-dependent phagocytosis of Plasmodium falciparum-parasitized erythrocytes and decrease malaria-induced TNF-alpha secretion by monocytes/macrophages

Severe and fatal malaria is associated with the failure of host defenses to control parasite replication, excessive secretion of proinflammatory cytokines such as TNF-alpha, and sequestration of parasitized erythrocytes (PEs) in vital organs. The identification of CD36 as a major sequestration receptor has led to the assumption that it contributes to the pathophysiology of severe malaria and has prompted the development of antiadherence therapies to disrupt the CD36-PE interaction. This concept has been challenged by unexpected evidence that individuals deficient in CD36 are more susceptible to severe and cerebral malaria. In this study, we demonstrate that CD36 is the major receptor mediating nonopsonic phagocytosis of PEs by macrophages, a clearance mechanism of potential importance in nonimmune hosts at the greatest risk of severe malaria. CD36-mediated uptake of PEs occurs via a novel pathway that does not involve thrombospondin, the vitronectin receptor, or phosphatidylserine recognition. Furthermore, we show that proliferator-activated receptor gamma-retinoid X receptor agonists induce an increase in CD36-mediated phagocytosis and a decrease in parasite-induced TNF-alpha secretion. Specific up-regulation of monocyte/macrophage CD36 may represent a novel therapeutic strategy to prevent or treat severe malaria.

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

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

PPARgamma and inflammatory bowel disease: a new therapeutic target for ulcerative colitis and Crohn's disease

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor that is known to play a central role in adipocyte differentiation and insulin sensitivity. Through work in several animal models of intestinal inflammation, it is now recognized that PPARgamma also inhibits tissue injury associated with immune activation. These studies point to PPARgamma as a novel anti-inflammatory mediator with broad therapeutic potential.

Expression and characterization of recombinant rat Acyl-CoA synthetases 1, 4, and 5. Selective inhibition by triacsin C and thiazolidinediones

Inhibition by triacsins and troglitazone of long chain fatty acid incorporation into cellular lipids suggests the existence of inhibitor-sensitive and -resistant acyl-CoA synthetases (ACS, EC ) that are linked to specific metabolic pathways. In order to test this hypothesis, we cloned and purified rat ACS1, ACS4, and ACS5, the isoforms present in liver and fat cells, expressed the isoforms as ACS-Flag fusion proteins in Escherichia coli, and purified them by Flag affinity chromatography. The Flag epitope at the C terminus did not alter the kinetic properties of the enzyme. Purified ACS1-, 4-, and 5-Flag isoforms differed in their apparent K(m) values for ATP, thermolability, pH optima, requirement for Triton X-100, and sensitivity to N-ethylmaleimide and phenylglyoxal. The ACS inhibitor triacsin C strongly inhibited ACS1 and ACS4, but not ACS5. The thiazolidinedione (TZD) insulin-sensitizing drugs and peroxisome proliferator-activated receptor gamma (PPARgamma) ligands, troglitazone, rosiglitazone, and pioglitazone, strongly and specifically inhibited only ACS4, with an IC(50) of less than 1.5 microm. Troglitazone exhibited a mixed type inhibition of ACS4. alpha-Tocopherol, whose ring structure forms the non-TZD portion of troglitazone, did not inhibit ACS4, indicating that the thiazolidine-2,4-dione moiety is the critical component for inhibition. A non-TZD PPARgamma ligand, GW1929, which is 7-fold more potent than rosiglitazone, inhibited ACS1 and ACS4 poorly with an IC(50) of greater than 50 microm, more than 100-fold higher than was required for rosiglitazone, thereby demonstrating the specificity of TZD inhibition. Further, the PPARalpha ligands, clofibrate and GW4647, and various xenobiotic carboxylic acids known to be incorporated into complex lipids had no effect on ACS1, -4, or -5. These results, together with previous data showing that triacsin C and troglitazone strongly inhibit triacylglycerol synthesis compared with other metabolic pathways, suggest that ACS1 and ACS4 catalyze the synthesis of acyl-CoAs used for triacylglycerol synthesis and that lack of inhibition of a metabolic pathway by triacsin C does not prove lack of acyl-CoA involvement. The results further suggest the possibility that the insulin-sensitizing effects of the thiazolidinedione drugs might be achieved, in part, through direct interaction with ACS4 in a PPARgamma-independent manner.