PPARγ Deficiency Research Articles

Adipose tissue-specific PPARγ deficiency increases resistance to oxidative stress

The nuclear hormone receptor peroxisome proliferator activated receptor gamma (PPARγ) critically regulates adipogenesis and lipogenesis. Obesity is closely associated with increased oxidative stress, and pharmacological activation of PPARγ by its ligands significantly suppresses oxidative stress in cultured adipocytes. On the other hand, a PPARγ2 Pro12Ala polymorphism, which decreases receptor transcription activity, is associated with lower body mass index and increased insulin sensitivity in humans. This mutation is also found to be positively associated with increased human lifespan. Here we show that adipose tissue-specific PPARγ heterozygous mice, which exhibit significant improvement in insulin sensitivity in skeletal muscle, show increased resistance to paraquat-induced oxidative stress. The enhanced oxidative stress tolerance is associated with significant upregulation of antioxidant genes in white adipose tissue and skeletal muscle whereas prooxidant genes are not changed. This is also associated with a significant increase in adipose tissue of Foxo3a, a transcription factor that is known to regulate clearance of reactive oxygen species. Consistently, Foxo3a dependent genes are significantly upregulated in adipose tissue. These data implicate adipose tissue PPARγ in the regulation of oxidative stress, which may underlie extended lifespan in humans bearing PPARγ2 Pro12Ala mutation.

Maternal PPAR gamma protects nursing neonates by suppressing the production of inflammatory milk.

Lactation is a highly demanding lipid synthesis and transport process that is crucial for the development of newborn mammals. While PPAR gamma is known to promote adipogenesis and lipogenesis in adipose tissue, its role in the lactating mammary gland is unexplored. Here, we report that a targeted deletion of PPAR gamma in mice results in the production of "toxic milk" containing elevated levels of inflammatory lipids. Surprisingly, ingestion of this "toxic milk" causes inflammation, alopecia, and growth retardation in the nursing neonates. Genomic profiling reveals that PPAR gamma deficiency leads to increased expression of lipid oxidation enzymes in the lactating mammary gland. Consistently, metabolomic profiling detects increased levels of oxidized free fatty acids in the pups nursed by PPAR gamma-deficient mothers. Therefore, maternal PPAR gamma is pivotal for maintaining the quality of milk and protecting the nursing newborns by suppressing the production of inflammatory lipids in the lactating mammary gland.

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

Abscisic acid (ABA) is a natural phytohormone and peroxisome proliferator-activated receptor γ (PPARγ) agonist that significantly improves insulin sensitivity in db/db mice. Although it has become clear that obesity is associated with macrophage infiltration into white adipose tissue (WAT), the phenotype of adipose tissue macrophages (ATMs) and the mechanisms by which insulin-sensitizing compounds modulate their infiltration remain unknown. We used a loss-of-function approach to investigate whether ABA ameliorates insulin resistance through a mechanism dependent on immune cell PPARγ. We characterized two phenotypically distinct ATM subsets in db/db mice based on their surface expression of F4/80. F4/80 hi ATMs were more abundant and expressed greater concentrations of chemokine receptor (CCR) 2 and CCR5 when compared to F4/80 lo ATMs. ABA significantly decreased CCR2 + F4/80 hi infiltration into WAT and suppressed monocyte chemoattractant protein-1 (MCP-1) expression in WAT and plasma. Furthermore, the deficiency of PPARγ in immune cells, including macrophages, impaired the ability of ABA to suppress the infiltration of F4/80 hi ATMs into WAT, to repress WAT MCP-1 expression and to improve glucose tolerance. We provide molecular evidence in vivo demonstrating that ABA improves insulin sensitivity and obesity-related inflammation by inhibiting MCP-1 expression and F4/80 hi ATM infiltration through a PPARγ-dependent mechanism.

Peroxisome Proliferator-Activated Receptor γ Is Required for Regulatory CD4+ T Cell-Mediated Protection against Colitis

Peroxisome proliferator-activated receptor (PPAR) gamma activation has been implicated in the prevention of immunoinflammatory disorders; however, the mechanisms of regulation of effector and regulatory CD4+ T cell functions by endogenously activated PPAR-gamma remain unclear. We have used PPAR-gamma-deficient CD4+ T cells obtained from tissue-specific PPAR-gamma null mice (i.e., PPAR-gamma fl/fl; MMTV-Cre+) to investigate the role of endogenous PPAR-gamma on regulatory T cell (Treg) and effector CD4+ T cell function. Overall, we show that the loss of PPAR-gamma results in enhanced Ag-specific proliferation and overproduction of IFN-gamma in response to IL-12. These findings correlate in vivo with enhanced susceptibility of tissue-specific PPAR-gamma null mice to trinitrobenzene sulfonic acid-induced colitis. Furthermore, the transfer of purified PPAR-gamma null CD4+ T cells into SCID recipients results in enteric disease. To test the assertion that the deficiency of PPAR-gamma in Treg impairs their ability to prevent effector T cell-induced colitis, we performed cotransfer studies. These studies demonstrate that PPAR-gamma-expressing, but not PPAR-gamma null Treg, prevent colitis induced by transfer of naive CD4+ T cells into SCID recipients. In line with these findings, the production of IFN-gamma by spleen and mesenteric lymph node-derived CD4+ T cells was down-regulated following transfer of PPAR-gamma-expressing, but not PPAR-gamma null, Treg. In conclusion, our data suggest that endogenous PPAR-gamma activation represents a Treg intrinsic mechanism of down-regulation of effector CD4+ T cell function and prevention of colitis.

Epithelial Cell PPARγ Deficiency Results in Abnormalities in Lung Structure and Function

Peroxisome proliferator-activated receptor (PPAR)-γ is a nuclear hormone receptor that regulates gene expression, cell proliferation and differentiation. We previously described mice with conditional PPARγ deficiency within the conducting airway epithelium. These conditionally targeted mice develop airspace enlargement coincident with alveolarization that is not progressive with ageing. Measurement of lung mechanics in these animals revealed decreased tissue resistance and increased lung volumes. Analysis of air-saline filled pressure volume profiles demonstrated loss of elastic recoil. No significant differences in surfactant quantity (phospholipids content) or function (surfactometry) were observed between targeted animals and littermate controls. Elastin and collagen histochemistry and biochemistry also showed no gross changes between targeted and control animals. We analyzed parenchymal structure by measuring the distribution of alveoli (defined as airspaces of 250–2000 _m2) and alveolar ducts (>5000 _m2). At 8 weeks of age, there were fewer alveoli (mean number of 126 vs 154, p<0.001) and more alveolar ducts (mean number of 11.1 vs 8.4, p<0.001). Additionally, the alveolar ducts were larger (mean size of 10,274 _m2 vs 8,658 _m2, p<0.001) in the targeted animals. Radial alveolar counts were reduced in the targeted animals (mean number of 8.2 vs. 9.9, p=0.045). These data suggest that the functional abnormalities, including loss of recoil contributed by the air-liquid interface are secondary to altered force transmission due to differences in structure of the alveolar duct, rather than changes in surfactant function or elastin or collagen content. Supported by CF Foundation and NIH HL71885

PPAR GAMMA IS EXPRESSED AND REGULATES PLACENTAL DEVELOPMENT AND TROPHOBLAST DIFFERENTIATION IN BOTH HUMANS AND MICE

Peroxisome proliferator activated receptor gamma (PPARγ) deficiency causes placental vascular failure with abortive branching morphogenesis of chorionic epithelium, a grossly deficient vascular exchange interface and embryonic lethality. PPARγ is expressed in murine chorion, ectoplacental cone (EPC), labyrinth, trophoblast giant cells (TGC), clusters of labyrinthine cells later in gestation and in term human placenta. Placental VEGFR1, Tpbpa and MASH2, (but not VEGF, PECAM and E-selectin) are diminished and placental lactogen-1 is increased by PPARγ deficiency. PPARγ conditional inactivation in EPC and its derivatives (using Tpbpa-Cre transgenic mice) produces live-born neonates, suggesting that chorion and/or TGC expression is critical. PPARγ-deficient trophoblast stem cells proliferate and differentiate normally in vitro. Abnormal spatial expression of GCM-1, a transcription factor required for syncytial trophoblast formation and branching morphogenesis, is the earliest abnormality observed to date in the absence of PPARγ. Therefore, PPARγ may act downstream of inductive interactions between allantoic mesoderm and/or endothelium and precursors of vascular trophoblast.

Peroxisomal proliferator activated receptor-γ deficiency in a Canadian kindred with familial partial lipodystrophy type 3 (FPLD3)

BackgroundFamilial partial lipodystrophy (Dunnigan) type 3 (FPLD3, Mendelian Inheritance in Man [MIM] 604367) results from heterozygous mutations in PPARG encoding peroxisomal proliferator-activated receptor-γ. Both dominant-negative and haploinsufficiency mechanisms have been suggested for this condition.MethodsWe present a Canadian FPLD3 kindred with an affected mother who had loss of fat on arms and legs, but no increase in facial, neck, suprascapular or abdominal fat. She had profound insulin resistance, diabetes, severe hypertriglyceridemia and relapsing pancreatitis, while her pre-pubescent daughter had normal fat distribution but elevated plasma triglycerides and C-peptide and depressed high-density lipoprotein cholesterol.ResultsThe mother and daughter were each heterozygous for PPARG nonsense mutation Y355X, whose protein product in vitro was transcriptionally inactive with no dominant-negative activity against the wild-type receptor. In addition the mutant protein appeared to be markedly unstable.ConclusionTaken together with previous studies of human PPARG mutations, these findings suggest that PPAR-γ deficiency due either to haploinsufficiency or to substantial activity loss due to dominant negative interference of the normal allele product's function can each contribute to the FPLD3 phenotype.

Mouse models of PPAR-γ deficiency: dissecting PPAR-γ's role in metabolic homoeostasis

The identification of humans with mutations in PPAR-gamma (peroxisome-proliferator-activated receptor-gamma) has underlined its importance in the pathogenesis of the metabolic syndrome. Genetically modified mice provide powerful tools to dissect the mechanisms by which PPAR-gamma regulates metabolic processes. Ablation of PPAR-gamma in vivo is lethal and thus dissection of PPAR-gamma function using mouse models has relied on the development of tissue and isoform-specific ablation and mouse models of human mutations. These models exhibit phenotypes of partial PPAR-gamma impairment and are useful to elucidate how PPAR-gamma regulates specific metabolic processes. These murine models have confirmed the involvement of PPAR-gamma in adipose tissue development, maintenance and distribution. The mechanism involved in PPAR-gamma regulation of glucose homoeostasis is obscure as both agonism and partial impairment of PPAR-gamma increase insulin sensitivity. While adipose tissue is likely to be the primary target for the insulin-sensitizing effects of PPAR-gamma, some murine models suggest PPAR-gamma expressed outside adipose tissue may also contribute actively to maintain glucose homoeostasis. Interestingly, mutations in PPAR-gamma that cause severe insulin resistance in humans when expressed in mice do not result in insulin insensitivity. However, these murine models can recapitulate the effects in fuel partitioning, post-prandial lipid handling and vasculature dysfunction observed in humans. In summary, these murine models of PPAR-gamma have provided useful in vivo systems to dissect the function of PPAR-gamma, but additionally have revealed a picture of complexity. These models have confirmed a key role for PPAR-gamma in the metabolic syndrome; however, they challenge the concept that insulin resistance is the main factor linking the clinical manifestations of the metabolic syndrome.

Mouse models of PPAR-γ deficiency: dissecting PPAR-γ's role in metabolic homoeostasis

Mouse models of PPAR-γ deficiency: dissecting PPAR-γ's role in metabolic homoeostasis

Conditional Knockout of Macrophage PPARγIncreases Atherosclerosis in C57BL/6 and Low-Density Lipoprotein Receptor–Deficient Mice

Peroxisome proliferator-activated receptor gamma (PPARgamma) is highly expressed in macrophage-derived foam cells of atherosclerotic lesions, and its expression may have a dramatic impact on atherosclerosis. To investigate the contribution of macrophage PPARgamma expression on atherogenesis in vivo, we generated macrophage-specific PPARgamma knockout (MacPPARgammaKO) mice. C57BL/6 and low-density lipoprotein (LDL) receptor-deficient (LDLR(-/-)) mice were reconstituted with MacPPARgammaKO or wild-type marrow and challenged with an atherogenic diet. No differences were found in serum lipids between recipients reconstituted with MacPPARgammaKO and wild-type marrow. In contrast, both C57BL/6 and LDLR(-/-) mice transplanted with MacPPARgammaKO marrow had significantly larger atherosclerotic lesions than control recipients. In addition, MacPPARgammaKO-->LDLR(-/-) mice had higher numbers of macrophages in atherosclerotic lesions compared with controls. Peritoneal macrophages isolated from the MacPPARgammaKO mice had decreased uptake of oxidized but not acetylated LDL and showed no changes in either cholesterol efflux or inflammatory cytokine expression. Macrophages from MacPPARgammaKO mice had increased levels of migration and CC chemokine receptor 2 (CCR2) expression compared with wild-type macrophages. Thus, macrophage PPARgamma deficiency increases atherosclerosis under conditions of mild and severe hypercholesterolemia, indicating an antiatherogenic role for PPARgamma, which may be caused, at least in part, by modulation of CCR2 expression and monocyte recruitment.

Nuclear Receptors as Targets for Drug Development: Molecular Mechanisms for Regulation of Obesity and Insulin Resistance by Peroxisome Proliferator-Activated Receptor , CREB-Binding Protein, and Adiponectin

Obesity is defined as increased mass of adipose tissue, conferring a higher risk of cardiovascular and metabolic disorders such as diabetes, hyperlipidemia, and coronary heart disease. To investigate the role of transcriptional factors, which are involved in adipocytes differentiation and adiposity, we have generated peroxisome proliferator-activated receptor (PPAR) γ or CREB-binding protein (CBP)-deficient mice by gene targeting. Heterozygous PPARγ-deficient mice were protected from the development of insulin resistance due to adipocyte hypertrophy under a high-fat diet. Heterozygous CBP-deficient mice showed increased insulin sensitivity and were completely protected from body weight gain induced by a high-fat diet. PPARγ or CBP deficiency results in increased effects of hormones such as adiponectin and leptin. Adiponectin was decreased in obesity and lipoatrophy, and replenishment of adiponectin ameliorated insulin resistance. Moreover, adiponectin-deficient mice showed insulin resistance and atherogenic phenotype. Finally, cDNA encoding adiponectin receptors (AdipoR1/R2) have been identified by expression cloning. The expression of AdipoR1/R2 appears to be inversely regulated by insulin in physiological and pathophysiological states such as fasting/refeeding, insulin deficiency, and hyperinsulinemia models, and it is correlated with adiponectin sensitivity. These results facilitate the understanding of molecular mechanisms of adiponectin actions and obesity-linked diseases such as diabetes and atherosclerosis and propose the molecular targets for anti-diabetic and anti-atherogenic drugs.

The Pro12Ala Polymorphism in PPAR γ2 May Confer Resistance to Type 2 Diabetes

Peroxisome proliferator-activated receptor γ (PPARγ) has been implicated in adipocyte differentiation. Recently it was reported that heterozygous deficiency of PPARγ led to the protection from high-fat diet-induced insulin resistance in an animal model. A Pro12Ala polymorphism has been detected in the human PPARγ2 gene. Since this amino acid substitution may cause a reduction in the transcriptional activity of PPARγ, this polymorphism may be associated with decreased insulin resistance and decreased risk of type 2 diabetes. To investigate this hypothesis, we performed a case-control study of the Pro12Ala PPARγ2 polymorphism in Japanese diabetic and non-diabetic subjects. The frequency of Ala12 was significantly lower in the diabetic group. In an overweight or obese group, subjects with Ala12 were more insulin sensitive than those without. These results suggest that the PPARγ is a thrifty gene and that the Pro12Ala PPARγ2 polymorphism protects against type 2 diabetes in the Japanese.

Regulation by PPARγ: Expanding its Biological Functions

The peroxisome proliferator-activated receptor (PPAR) family is comprised of ligand-regulated transcription factors that have been implicated in controlling lipid metabolism. When stimulated, PPARγ induces the expression of genes that mediate fatty acid uptake, metabolism, and storage. PPARγ activation also induces adipocyte differentiation and sensitivity to insulin. Barrak et al. and Kubota et al. have now generated PPARγ-deficient mice. Both groups show that PPARγ-deficient mice die during prenatal development due to a defect in placenta vacularization and heart development. Those mutant mice that survived following a placental reconstitution procedure displayed no adipose tissue and died shortly after birth. Heterozygote mutant mice did not exhibit adipocyte hypertrophy from a high fat diet nor did they show insulin resistance, indicating that both PPARγ alleles are required for obesity and insulin resistance to occur as the result of such a diet. Rosen et al. generated chimeric mice by injecting ES cells homozygous for PPARγ deficiency into wild type blastocysts, and also found that the gene was necessary for normal development of adipocytes. The expanse in PPARγ function revealed in these studies and the proposed and molecular control of these various processes by PPARγ are further discussed by Lowell. Lowell, B.B. (1999) PPARγ: An essential regulator of adipogenesis and modulator of fat cell function. Cell 99 : 239-242. [Online Journal] Barak,Y., Nelson, M.C., Ong, E.S., Jones,Y.Z.,Ruiz-Lozano, P., Chien, K.R., Koder, A., and Evans, R.A. (1999) PPARγ is required for placental, cardiac, and adipose tissue development. Mol. Cell 4 : 585-595. [Online Journal] Kubota, N., Terauchi,Y., Miki, H., Tamemoto, H., Yamauchi, T., Komeda, K., Satoh, S., Nakano, R., Ishii, C., Sugiyama, T., Eto, K., Tsubamoto, Y., Okuno, A., Murakami, K., Sekihara, H., Hasegawa, G., Naito, M., Toyoshima, Y., Tanaka, S., Shiota, K., Kitamura, T., Fujita, T., Ezaki, O., Aizawa, S., Nagai, R., Tobe, K., Kimura, S., and Kadowaki, T. (1999) PPARγ mediates high-fat diet-induced adipocyte hypertrophy and insulin resistance. Mol. Cell 4 : 597-609. [Online Journal] Rosen, E.D., Sarraf, P., Troy, A.E., Bradwin, G., Moore, K., Milstone, D.S., Spiegelman, B.M., and Mortensen, R.M. (1999) PPARγ is required for the differentiation of adipose tissue in vivo and in vitro. Mol. Cell 4 : 611-617. [Online Journal]

PPARγ Is Required for Placental, Cardiac, and Adipose Tissue Development

The nuclear hormone receptor PPARγ promotes adipogenesis and macrophage differentiation and is a primary pharmacological target in the treatment of type II diabetes. Here, we show that PPARγ gene knockout results in two independent lethal phases. Initially, PPARγ deficiency interferes with terminal differentiation of the trophoblast and placental vascularization, leading to severe myocardial thinning and death by E10.0. Supplementing PPARγ null embryos with wild-type placentas via aggregation with tetraploid embryos corrects the cardiac defect, implicating a previously unrecognized dependence of the developing heart on a functional placenta. A tetraploid-rescued mutant surviving to term exhibited another lethal combination of pathologies, including lipodystrophy and multiple hemorrhages. These findings both confirm and expand the current known spectrum of physiological functions regulated by PPARγ.