Hypertension and abnormal fat distribution but not insulin resistance in mice with P465L PPARgamma

Impaired Remodeling of White Adipose Tissue in Obesity and Aging: From Defective Adipogenesis to Adipose Organ Dysfunction

The adipose organ adapts and responds to internal and environmental stimuli by remodeling both its cellular and extracellular components. Under conditions of energy surplus, the subcutaneous white adipose tissue (WAT) is capable of expanding through the enlargement of existing adipocytes (hypertrophy), followed by de novo adipogenesis (hyperplasia), which is impaired in hypertrophic obesity. However, an impaired hyperplastic response may result from various defects in adipogenesis, leading to different WAT features and metabolic consequences, as discussed here by reviewing the results of the studies in animal models with either overexpression or knockdown of the main molecular regulators of the two steps of the adipogenesis process. Moreover, impaired WAT remodeling with aging has been associated with various age-related conditions and reduced lifespan expectancy. Here, we delve into the latest advancements in comprehending the molecular and cellular processes underlying age-related changes in WAT function, their involvement in common aging pathologies, and their potential as therapeutic targets to influence both the health of elderly people and longevity. Overall, this review aims to encourage research on the mechanisms of WAT maladaptation common to conditions of both excessive and insufficient fat tissue. The goal is to devise adipocyte-targeted therapies that are effective against both obesity- and age-related disorders.

Not Enough Fat: Mouse Models of Inherited Lipodystrophy

Lipodystrophies belong to the heterogenous group of syndromes in which the primary defect is a generalized or partial absence of adipose tissue, which may be congenital or acquired in origin. Lipodystrophy should be considered in patients manifesting the combination of insulin resistance (with or without overt diabetes), dyslipidemia and fatty liver. Lipodystrophies are classified according to the etiology of the disease (genetic or acquired) and to the anatomical distribution of adipose tissue (generalized or partial). The mechanism of adipose tissue loss is specific to each syndrome, depending on the biological function of the mutated gene. Mice models, together with cellular studies have permitted clarification of the mechanisms by which human mutations deeply compromise adipocyte homeostasis. In addition, rodent models have proven to be crucial in deciphering the cardiometabolic consequences of the lack of adipose tissue such as NAFLD, muscle insulin resistance and cardiomyopathy. More precisely, tissue-specific transgenic and knockout mice have brought new tools to distinguish phenotypic traits that are the consequences of lipodystrophy from those that are cell-autonomous. In this review, we discuss the mice models of lipodystrophy including those of inherited human syndromes of generalized and partial lipodystrophy. We present how these models have demonstrated the central role of white adipose tissue in energetic homeostasis in general, including insulin sensitivity and lipid handling in particular. We underscore the differences reported with the human phenotype and discuss the limit of rodent models in recapitulating adipose tissue primary default. Finally, we present how these mice models have highlighted the function of the causative-genes and brought new insights into the pathophysiology of the cardiometabolic complications associated with lipodystrophy.

Role of the Peroxisome Proliferator Activated Receptors in Hypertension

Nuclear receptors represent a large family of ligand-activated transcription factors which sense the physiological environment and make long-term adaptations by mediating changes in gene expression. In this review, we will first discuss the fundamental mechanisms by which nuclear receptors mediate their transcriptional responses. We will focus on the PPAR (peroxisome proliferator-activated receptor) family of adopted orphan receptors paying special attention to PPARγ, the isoform with the most compelling evidence as an important regulator of arterial blood pressure. We will review genetic data showing that rare mutations in PPARγ cause severe hypertension and clinical trial data which show that PPARγ activators have beneficial effects on blood pressure. We will detail the tissue- and cell-specific molecular mechanisms by which PPARs in the brain, kidney, vasculature, and immune system modulate blood pressure and related phenotypes, such as endothelial function. Finally, we will discuss the role of placental PPARs in preeclampsia, a life threatening form of hypertension during pregnancy. We will close with a viewpoint on future research directions and implications for developing novel therapies.

PPARγ activation improves the microenvironment of perivascular adipose tissue and attenuates aortic stiffening in obesity

Background

Obesity-related cardiovascular risk, end points, and mortality are strongly related to arterial stiffening. Current therapeutic approaches for arterial stiffening are not focused on direct targeting within the vessel. Perivascular adipose tissue (PVAT) surrounding the artery has been shown to modulate vascular function and inflammation. Peroxisome proliferator-activated receptor γ (PPARγ) activation significantly decreases arterial stiffness and inflammation in diabetic patients with coronary artery disease. Thus, we hypothesized that PPARγ activation alters the PVAT microenvironment, thereby creating a favorable environment for the attenuation of arterial stiffening in obesity.

Methods

Obese ob/ob mice were used to investigate the effect of PPARγ activation on the attenuation of arterial stiffening. Various cell types, including macrophages, fibroblasts, adipocytes, and vascular smooth muscle cells, were used to test the inhibitory effect of pioglitazone, a PPARγ agonist, on the expression of elastolytic enzymes.

Results

PPARγ activation by pioglitazone effectively attenuated arterial stiffening in ob/ob mice. This beneficial effect was not associated with the repartitioning of fat from or changes in the browning of the PVAT depot but was strongly related to improvement of the PVAT microenvironment, as evidenced by reduction in the expression of pro-inflammatory and pro-oxidative factors. Pioglitazone treatment attenuated obesity-induced elastin fiber fragmentation and elastolytic activity and ameliorated the obesity-induced upregulation of cathepsin S and metalloproteinase 12, predominantly in the PVAT. In vitro, pioglitazone downregulated Ctss and Mmp12 in macrophages, fibroblasts, and adipocytes—cell types residing within the adventitia and PVAT. Ultimately, several PPARγ binding sites were found in Ctss and Mmp12 in Raw 264.7 and 3T3-L1 cells, suggesting a direct regulatory mechanism by which PPARγ activation repressed the expression of Ctss and Mmp-12 in macrophages and fibroblasts.

Conclusions

PPARγ activation attenuated obesity-induced arterial stiffening and reduced the inflammatory and oxidative status of PVAT. The improvement of the PVAT microenvironment further contributed to the amelioration of elastin fiber fragmentation, elastolytic activity, and upregulated expression of Ctss and Mmp12. Our data highlight the PVAT microenvironment as an important target against arterial stiffening in obesity and provide a novel strategy for the potential clinical use of PPARγ agonists as a therapeutic against arterial stiffness through modulation of PVAT function.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-021-00720-y.

Inhibitory effect of PPARγ on NLRP3 inflammasome activation

Rationale: Stimulation of the NLRP3 inflammasome by metabolic byproducts is known to result in inflammatory responses and metabolic diseases. However, how the host controls aberrant NLRP3 inflammasome activation remains unclear. PPARγ, a known regulator of energy metabolism, plays an anti-inflammatory role through the inhibition of NF-κB activation and additionally attenuates NLRP3-dependent IL-1β and IL-18 production. Therefore, we hypothesized that PPARγ serves as an endogenous modulator that attenuates NLRP3 inflammasome activation in macrophages.

Methods: Mouse peritoneal macrophages with exposure to a PPARγ agonist at different stages and the NLRP3 inflammasome-reconstituted system in HEK293T cells were used to investigate the additional anti-inflammatory effect of PPARγ on NLRP3 inflammasome regulation. Circulating mononuclear cells of obese patients with weight-loss surgery were used to identify the in vivo correlation between PPARγ and the NLRP3 inflammasome.

Results: Exposure to the PPARγ agonist, rosiglitazone, during the second signal of NLRP3 inflammasome activation attenuated caspase-1 and IL-1β maturation. Moreover, PPARγ interfered with NLRP3 inflammasome formation by decreasing NLRP3-ASC and NLRP3-NLRP3 interactions as well as NLRP3-dependent ASC oligomerization, which is mediated through interaction between the PPARγ DNA-binding domain and the nucleotide-binding and leucine-rich repeat domains of NLRP3. Furthermore, PPARγ was required to limit metabolic damage-associated molecular pattern-induced NLRP3 inflammasome activation in mouse macrophages. Finally, the mature caspase-1/PPARγ ratio was reduced in circulating mononuclear cells of obese patients after weight-loss surgery, which we define as an “NLRP3 accelerating index”.

Conclusions: These results revealed an additional anti-inflammatory role for PPARγ in suppressing NLRP3 inflammasome activation through interaction with NLRP3. Thus, our study highlights that PPARγ agonism may be a therapeutic option for targeting NLRP3-related metabolic diseases.

Loss of EGR-1 uncouples compensatory responses of pancreatic β cells

Rationale: Subjects unable to sustain β-cell compensation develop type 2 diabetes. Early growth response-1 protein (EGR-1), implicated in the regulation of cell differentiation, proliferation, and apoptosis, is induced by diverse metabolic challenges, such as glucose or other nutrients. Therefore, we hypothesized that deficiency of EGR-1 might influence β-cell compensation in response to metabolic overload.

Methods: Mice deficient in EGR-1 (Egr1^-/-) were used to investigate the in vivo roles of EGR-1 in regulation of glucose homeostasis and beta-cell compensatory responses.

Results: In response to a high-fat diet, Egr1^-/- mice failed to secrete sufficient insulin to clear glucose, which was associated with lower insulin content and attenuated hypertrophic response of islets. High-fat feeding caused a dramatic impairment in glucose-stimulated insulin secretion and downregulated the expression of genes encoding glucose sensing proteins. The cells co-expressing both insulin and glucagon were dramatically upregulated in islets of high-fat-fed Egr1^-/- mice. EGR-1-deficient islets failed to maintain the transcriptional network for β-cell compensatory response. In human pancreatic tissues, EGR1 expression correlated with the expression of β-cell compensatory genes in the non-diabetic group, but not in the diabetic group.

Conclusion: These results suggest that EGR-1 couples the transcriptional network to compensation for the loss of β-cell function and identity. Thus, our study highlights the early stress coupler EGR-1 as a critical factor in the development of pancreatic islet failure.

[Role of PPARγ, a transcription factor in cardiovascular disease]

Peroxisome proliferator-activated receptor γ (PPARγ) is a ligand activated transcription factor known to regulate fatty acid metabolism. Thiazolidinediones (TZDs), PPARγ synthetic agonists, currently used to treat patients with type 2 diabetes, have been shown to lower the blood pressure and protect against vascular diseases such as atherosclerosis. In line with these findings, it has been reported that individuals with loss-of-function mutations of PPARγ developed sever early-onset hypertension in addition to metabolic abnormalities. Accumulating evidences suggest PPARγ in the vasculature has protective effects on cardiovascular disease despite unclear mechanism. Because of ubiquitous expression of PPARγ, TZDs are well-known to be associated with serious side effects such as weight gain, fluid retention, and bone fractures. Thus identification of mechanisms on tissue-specific PPARγ activity may lead to the development of targeted treatment which is characterized by no deleterious effects. This review discusses role of PPARγ in cardiovascular disease.

What lipodystrophies teach us about the metabolic syndrome

Lipodystrophies are the result of a range of inherited and acquired causes, but all are characterized by perturbations in white adipose tissue function and, in many instances, its mass or distribution. Though patients are often nonobese, they typically manifest a severe form of the metabolic syndrome, highlighting the importance of white fat in the "safe" storage of surplus energy. Understanding the molecular pathophysiology of congenital lipodystrophies has yielded useful insights into the biology of adipocytes and informed therapeutic strategies. More recently, genome-wide association studies focused on insulin resistance have linked common variants to genes implicated in adipose biology and suggested that subtle forms of lipodystrophy contribute to cardiometabolic disease risk at a population level. These observations underpin the use of aligned treatment strategies in insulin-resistant obese and lipodystrophic patients, the major goal being to alleviate the energetic burden on adipose tissue.

Efficacy of Metreleptin Treatment in Familial Partial Lipodystrophy Due to PPARG vs LMNA Pathogenic Variants

CONTEXT

Familial partial lipodystrophy (FPLD) is most commonly caused by pathogenic variants in LMNA and PPARG. Leptin replacement with metreleptin has largely been studied in the LMNA group.

OBJECTIVE

To understand the efficacy of metreleptin in PPARG vs LMNA pathogenic variants and investigate predictors of metreleptin responsiveness.

DESIGN

Subgroup analysis of a prospective open-label study of metreleptin in lipodystrophy.

SETTING

National Institutes of Health, Bethesda, Maryland.

PARTICIPANTS

Patients with LMNA (n = 22) or PPARG pathogenic variants (n = 7), leptin <12 ng/mL, and diabetes, insulin resistance, or high triglycerides.

INTERVENTION

Metreleptin (0.08 to 0.16 mg/kg) for 12 months.

OUTCOME

Hemoglobin A1c (HbA1c), lipids, and medication use at baseline and after 12 months.

RESULTS

Baseline characteristics were comparable in patients with PPARG and LMNA: HbA1c, 9.2 ± 2.3 vs 7.8 ± 2.1%; median [25th, 75th percentile] triglycerides, 1377 [278, 5577] vs 332 [198, 562] mg/dL; leptin, 6.3 ± 3.8 vs 5.5 ± 2.5 ng/mL (P > 0.05). After 12 months of metreleptin, HbA1c declined to 7.7 ± 2.4 in PPARG and 7.3 ± 1.7% in LMNA; insulin requirement decreased from 3.8 [2.7, 4.3] to 2.1 [1.6, 3.0] U/kg/d in PPARG and from 1.7 [1.3, 4.4] to 1.2 [1.0, 2.3] U/kg/d in LMNA (P < 0.05). Triglycerides decreased to 293 [148, 406] mg/dL in LMNA (P < 0.05), but changes were not significant in PPARG: 680 [296, 783] mg/dL at 12 months (P = 0.2). Both groups were more likely to experience clinically relevant triglyceride (≥30%) or HbA1c (≥1%) reduction with metreleptin if they had baseline triglycerides ≥500 mg/dL or HbA1c >8%.

CONCLUSION

Metreleptin resulted in similar metabolic improvements in patients with LMNA and PPARG pathogenic variants. Our findings support the efficacy of metreleptin in patients with the two most common genetic causes of FPLD.

Endothelial PPARγ Is Crucial for Averting Age-Related Vascular Dysfunction by Stalling Oxidative Stress and ROCK

Aging plays a significant role in the progression of vascular diseases and vascular dysfunction. Activation of the ADP-ribosylation factor 6 and small GTPases by inflammatory signals may cause vascular permeability and endothelial leakage. Pro-inflammatory molecules have a significant effect on smooth muscle cells (SMC). The migration and proliferation of SMC can be promoted by tumor necrosis factor alpha (TNF-α). TNF-α can also increase oxidative stress in SMCs, which has been identified to persuade DNA damage resulting in apoptosis and cellular senescence. Peroxisome proliferator-activated receptor (PPAR) acts as a ligand-dependent transcription factor and a member of the nuclear receptor superfamily. They play key roles in a wide range of biological processes, including cell differentiation and proliferation, bone formation, cell metabolism, tissue remodeling, insulin sensitivity, and eicosanoid signaling. The PPARγ activation regulates inflammatory responses, which can exert protective effects in the vasculature. In addition, loss of function of PPARγ enhances cardiovascular events and atherosclerosis in the vascular endothelium. This appraisal, therefore, discusses the critical linkage of PPARγ in the inflammatory process and highlights a crucial defensive role for endothelial PPARγ in vascular dysfunction and disease, as well as therapy for vascular aging.

The Protective Role of Peroxisome Proliferator-Activated Receptor-Gamma in Seizure and Neuronal Excitotoxicity

The peroxisome proliferator-activated receptor (PPAR) family, type II nucleus receptors have been successfully tested for their neuroprotective potential in certain central nervous system diseases. The aim of the present study was to determine if modulation by PPAR-γ could attenuate pilocarpine-induced seizures and decrease neuronal excitability. Adult male C57BL/6 mice were divided into two groups: one group received pretreatment with pioglitazone and the other received dimethyl sulfoxide (DMSO) for a period of 2 weeks. Status epilepticus was then induced in both groups by lithium-pilocarpine, after which seizure susceptibility, severity, and mortality were evaluated. Hippocampal histopathology was carried out on all mice at 24 h post-status epilepticus as well as blood-brain barrier (BBB) damage analysis. With the aid of patch clamp technology, the hippocampal neuronal excitability from mice with PPAR-γ 50% expression (Pparg^C/C) and PPAR-γ 25% expression (Pparg^C/-), as well as the effect of pioglitazone on the sodium currents in hippocampal neurons, were evaluated. It was found that pioglitazone, a PPAR-γ agonist, could attenuate pilocarpine-induced seizure severity in mice. Pathological examination showed that pioglitazone significantly attenuated pilocarpine-induced status epilepticus-related hippocampal neuronal loss and BBB damage. Further characterization of neuronal excitability revealed higher excitability in the brain slices from mice with Pparg^C/- expression, compared with the Pparg^C/C group. It was also found that pioglitazone could decrease sodium currents in hippocampal neurons. In conclusion, PPAR-γ deficiency aggravated neuronal excitability and excitotoxicity. PPAR-γ attenuated pilocarpine-induced seizure severity, neuronal loss, BBB damage, and sodium currents in hippocampal neurons. Modulation of PPAR-γ could be a potential novel treatment for epileptic seizures.

Serum PPARγ level and PPARγ gene polymorphism as well as severity and prognosis of brain injury in patients with arteriosclotic cerebral infarction

The aim of the study was to study the serum peroxisome proliferator-activated receptor gamma (PPARγ) level and PPARγ gene polymorphism as well as the severity and prognosis of brain injury in patients with arteriosclotic cerebral infarction (ACI). A total of 246 ACI patients presenting at the Department of Neurology of Zengcheng District People's Hospital of Guangzhou between April 2009 and July 2015 were selected as the case group, and 382 control subjects were enrolled as the control group. The hepatic and renal functions and homocysteine (Hcy) expression levels were measured. Enzyme-linked immunosorbent assay (ELISA) kit was used to detect the serum PPARγ levels of the ACI patients. Polymerase chain reaction-restriction fragment length polymorphism method was applied to measure the PPARγ gene polymorphism. The proportions of hypertension patients, diabetes patients and smoking people in the case group were significantly higher than those in the control group. The levels of cholesterol and fasting blood glucose in the case group were elevated obviously compared with those in the control group. The levels of indexes related to the hepatic function and renal function in the case group were remarkably higher than those in the control group. The serum PPARγ levels were increased progressively at acute stage. The distribution frequencies of PPARγ genotypes CC, CT and TT in the case group were higher than those in the control group; compared with that in the control group, the proportion of C allele in the case group was raised obviously, while that of T allele was significantly decreased. The serum PPARγ level has a close correlation with the PPARγ gene polymorphism in ACI patients, and PPARγ is also remarkably related to the severity of brain injury; therefore, PPARγ has great significance in the diagnosis and treatment of cerebral infarction.

Endothelial PPARγ (Peroxisome Proliferator-Activated Receptor-γ) Is Essential for Preventing Endothelial Dysfunction With Aging

Little is known about mechanisms that control vascular aging, particularly at the cell-specific level. PPARγ (peroxisome proliferator-activated receptor-γ) exerts protective effects in the vasculature when activated pharmacologically. To gain insight into the cell-specific impact of PPARγ, we examined the hypothesis that genetic interference with endothelial PPARγ would augment age-induced vascular dysfunction. We studied carotid arteries from adult (11.6±0.3 months) and old (24.7±0.6 months) mice with endothelial-specific expression of a human dominant negative mutation in PPARγ driven by the vascular cadherin promoter (E-V290M), along with age-matched, nontransgenic littermates. Acetylcholine (an endothelium-dependent agonist) produced similar relaxation in arteries from adult nontransgenic and E-V290M mice and old nontransgenic mice. In contrast, responses to acetylcholine were reduced by >50% in old male and female E-V290M mice (P<0.01). Endothelial function in old E-V290M mice was not altered by an inhibitor of COX (cyclooxygenase) but was restored to normal by a superoxide scavenger, an inhibitor of NADPH oxidase, or inhibition of ROCK (Rho kinase). Relaxation of arteries to nitroprusside, which acts directly on vascular muscle, was similar in all groups. Vascular expression of IL (interleukin)-6, Nox-2, and CDKN2A (a marker of senescence) was significantly increased in old E-V290M mice compared with controls (P<0.05). These findings provide the first evidence that age-related vascular dysfunction, inflammation, and senescence is accelerated after interference with endothelial PPARγ via mechanisms involving oxidative stress and ROCK. The finding of an essential protective role for endothelial PPARγ has implications for vascular disease and therapy for vascular aging.

Peroxisome proliferator activating receptor-γ and the podocyte

Over the past two decades it has become clear that the glomerular podocyte is a key cell in preventing albuminuria, kidney failure and cardiovascular morbidity. Understanding the key pathways that protect the podocyte in times of glomerular stress, which can also be therapeutically manipulated, are highly attractive. In the following review we assess the evidence that the peroxisome proliferator activating receptor (PPAR) agonists are beneficial for podocyte and kidney function with a focus on PPAR-γ. We explain our current understanding of the mechanisms of action of these agonists and the evidence they are beneficial in diabetic and non-diabetic kidney disease. We also outline why these drugs have not been widely used for kidney disease in the past but they may be in the future.

Lactobacillus gasseri attenuates allergic airway inflammation through PPARγ activation in dendritic cells

Lactobacilli prevent the early development of allergic diseases in children and experimental asthma in mice. However, the detailed mechanism underlying this action remains unknown. We aimed to explore the activation pathway in the host by Lactobacillus and identify its immunomodulation mechanism in allergic asthma. Continuous administration of 10⁷ cfu, but not 10⁹ cfu, of L. gasseri for 4 weeks prevented Dermatophagoides pteronyssinus (Der p)-induced airway hypersensitivity and inflammation in a mouse model of allergic asthma. DNA microarray analysis of the mesenteric and lung draining lymph nodes revealed a significant decrease in inflammatory chemokines and increase in gene expression in carbohydrate and lipid metabolism, particularly of PPARγ, in 10⁷ cfu L. gasseri-administered mice compared with untreated mice. Compared with WT mice, Der p-sensitized PPARγ^L/+ mice showed increased airway hyperresponsiveness to methacholine, inflammatory cell infiltration, and inflammatory cytokine secretion in bronchoalveolar fluid. Moreover, the protective effects of L. gasseri were lost in Der p-induced airway inflammation in PPARγ^L/+ mice, and L. gasseri-induced PPARγ activation in BMDCs inhibited the development of allergic airway inflammation in both PPARγ WT and PPARγ^L/+ mice. L. gasseri may act via a novel PPARγ activation pathway in DCs to alleviate allergen-induced airway inflammation in allergic asthma.

KEY MESSAGES

L. gasseri prevents mite allergen (Der p)-induced airway inflammation. Prevention of airway inflammation occurs via activation of PPARγ in dendritic cells. L. gasseri administration does not reverse Der p-induced airway inflammation in PPARγ^(+/-) mice. L. gasseri-induced PPARγ activation inhibits development of airway inflammation in WT and PPARγ^(+/-) mice.

Is the Mouse a Good Model of Human PPARγ-Related Metabolic Diseases?

With the increasing number of patients affected with metabolic diseases such as type 2 diabetes, obesity, atherosclerosis and insulin resistance, academic researchers and pharmaceutical companies are eager to better understand metabolic syndrome and develop new drugs for its treatment. Many studies have focused on the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ), which plays a crucial role in adipogenesis and lipid metabolism. These studies have been able to connect this transcription factor to several human metabolic diseases. Due to obvious limitations concerning experimentation in humans, animal models—mainly mouse models—have been generated to investigate the role of PPARγ in different tissues. This review focuses on the metabolic features of human and mouse PPARγ-related diseases and the utility of the mouse as a model.

Adipocyte (Pro)Renin-Receptor Deficiency Induces Lipodystrophy, Liver Steatosis and Increases Blood Pressure in Male Mice

Adipose tissue dysfunction related to obesity is overwhelmingly associated with increased risk of developing cardiovascular diseases. In the setting of obesity, (pro)renin receptor (PRR) is increased in adipose tissue of mice. We sought to determine the physiological consequences of adipocyte-PRR deficiency using adiponectin-Cre mice. We report a unique model of adipocyte-PRR-deficient mice (PRR(Adi/Y)) with almost no detectable white adipose tissues. As a consequence, the livers of PRR(Adi/Y) mice were enlarged and demonstrated a marked accumulation of lipids. Adipocyte-specific deficiency of PRR increased systolic blood pressure and the concentration of soluble PRR in plasma. To determine whether adipocyte-PRR was involved in the development of obesity-induced hypertension, mice were fed a low-fat or a high-fat diet for 16 weeks. Adipocyte-PRR-deficient mice were resistant to diet-induced obesity. Both high-fat- and low-fat-fed PRR(Adi/Y) mice had elevated insulin levels. Interestingly, adipocyte-PRR deficiency improved glucose tolerance in high-fat-fed PRR(Adi/Y) mice. In response to feeding either low-fat or high-fat diets, systolic blood pressure was greater in PRR(Adi/Y) mice than in control mice. High-fat feeding elevated soluble PRR concentration in control and PRR(Adi/Y) mice. In vitro knockdown of PRR by siRNA significantly decreased mRNA abundance of PPARγ (peroxisome proliferator-activated receptor gamma), suggesting an important role for PRR in adipogenesis. Our data indicate that adipocyte-PRR is involved in lipid homeostasis and glucose and insulin homeostasis, and that soluble PRR may be a predictor of metabolic disturbances and play a role in systolic blood pressure regulation.

Effect of selective expression of dominant-negative PPARγ in pro-opiomelanocortin neurons on the control of energy balance

Peroxisome proliferator-activated receptor-γ (PPARγ), a master regulator of adipogenesis, was recently shown to affect energy homeostasis through its actions in the brain. Deletion of PPARγ in mouse brain, and specifically in the pro-opiomelanocortin (POMC) neurons, results in resistance to diet-induced obesity. To study the mechanisms by which PPARγ in POMC neurons controls energy balance, we constructed a Cre-recombinase-dependent conditionally activatable transgene expressing either wild-type (WT) or dominant-negative (P467L) PPARγ and the tdTomato reporter. Inducible expression of both forms of PPARγ was validated in cells in culture, in liver of mice infected with an adenovirus expressing Cre-recombinase (AdCre), and in the brain of mice expressing Cre-recombinase either in all neurons (NES(Cre)/PPARγ-P467L) or selectively in POMC neurons (POMC(Cre)/PPARγ-P467L). Whereas POMC(Cre)/PPARγ-P467L mice exhibited a normal pattern of weight gain when fed 60% high-fat diet, they exhibited increased weight gain and fat mass accumulation in response to a 10% fat isocaloric-matched control diet. POMC(Cre)/PPARγ-P467L mice were leptin sensitive on control diet but became leptin resistant when fed 60% high-fat diet. There was no difference in body weight between POMC(Cre)/PPARγ-WT mice and controls in response to 60% high-fat diet. However, POMC(Cre)/PPARγ-WT, but not POMC(Cre)/PPARγ-P467L, mice increased body weight in response to rosiglitazone, a PPARγ agonist. These observations support the concept that alterations in PPARγ-driven mechanisms in POMC neurons can play a role in the regulation of metabolic homeostasis under certain dietary conditions.

PPARγ Regulation in Hypertension and Metabolic Syndrome

Dysregulation of peroxisome proliferator-activated receptor gamma (PPARγ) activity leads to significant alterations in cardiovascular and metabolic regulation. This is most keenly observed by the metabolic syndrome-like phenotypes exhibited by patients carrying mutations in PPARγ. We will summarize recent findings regarding mechanisms of PPARγ regulation in the cardiovascular and nervous systems focusing largely on PPARγ in the smooth muscle, endothelium, and brain. Canonically, PPARγ exerts its effects by regulating the expression of target genes in these cells, and we will discuss mechanisms by which PPARγ targets in the vasculature regulate cardiovascular function. We will also discuss emerging evidence that PPARγ in the brain is a mediator of appetite and obesity. Finally, we will briefly review how novel PPARγ activators control posttranslational modifications of PPARγ and their prospects to offer new therapeutic options for treatment of metabolic diseases without the adverse side effects of thiazolidinediones which strongly activate transcriptional activity of PPARγ.

Case-control study on peroxisome proliferator-activated receptor gamma polymorphism and interaction with HDL on essential hypertension in Chinese Han

OBJECTIVES

To investigate the association of single nucleotide polymorphisms (SNPs) in the peroxisome proliferator-activated receptors gamma (PPARG) with essential hypertension (EH) and additional role of gene- high-density lipoprotein cholesterol (HDL) interaction.

MATERIALS AND METHODS

A total of 1640 patients with EH (806 males, 834 females), with a mean age of 52.5±12.6 years, were selected, including 816 EH patients and 824 controls, who were enrolled from the community. Three SNPs were selected for genotyping in the case-control study: rs10865710, rs709158, rs1805192. Logistic regression model was used to examine the interaction between SNP and HDL on EH, odds ratio (OR) and 95% confidence interval (95% CI) were also calculated.

RESULTS

All genotypes were distributed according to Hardy-Weinberg equilibrium in controls. Logistic regression analysis showed an association between genotypes of variants in rs1805192 and decreased EH risk, EH risk was significantly lower in carriers of Ala allele of the rs1805192 polymorphism than those with Pro/Pro (Pro/Ala+ Ala/Ala versus Pro/Pro, adjusted OR (95% CI) =0.65 (0.53-0.83), after covariate adjustment. In addition, the Ala allele of the rs1805192 polymorphism was also associated with diastolic blood pressure (DBP), but not systolic blood pressure (SBP), we also found, by interaction analysis, combined effect of rs1805192 and HDL on EH risk after covariate adjustment.

CONCLUSION

Our results support an important association between rs1805192 minor allele (Ala allele) of PPARG and lower EH risk, the interaction analysis showed a combined effect of Ala- HDL on lower EH risk.

Role of peroxisome proliferator-activated receptor-γ in vascular muscle in the cerebral circulation

Although peroxisome proliferator-activated receptor-γ (PPARγ) is thought to play a protective role in the vasculature, its cell-specific effect, particularly in resistance vessels, is poorly defined. Nitric oxide (NO) plays a major role in vascular biology in the brain. We examined the hypothesis that selective interference with PPARγ in vascular muscle would impair NO-dependent responses and augment vasoconstrictor responses in the cerebral circulation. We studied mice expressing a dominant negative mutation in human PPARγ (P467L) under the control of the smooth muscle myosin heavy chain promoter (S-P467L). In S-P467L mice, dilator responses to exogenously applied or endogenously produced NO were greatly impaired in cerebral arteries in vitro and in small cerebral arterioles in vivo. Select NO-independent responses, including vasodilation to low concentrations of potassium, were also impaired in S-P467L mice. In contrast, increased expression of wild-type PPARγ in smooth muscle had little effect on vasomotor responses. Mechanisms underlying impairment of both NO-dependent and NO-independent vasodilator responses after interference with PPARγ involved Rho kinase with no apparent contribution by oxidative stress-related mechanisms. These findings support the concept that via effects on Rho kinase-dependent signaling, PPARγ in vascular muscle is a major determinant of vascular tone in resistance vessels and, in particular, NO-mediated signaling in cerebral arteries and brain microvessels. Considering the importance of NO and Rho kinase, these findings have implications for regulation of cerebral blood flow and the pathogenesis of large and small vessel disease in brain.

Role of vascular smooth muscle PPARγ in regulating AT1 receptor signaling and angiotensin II-dependent hypertension

Peroxisome proliferator activated receptor γ (PPARγ) has been reported to play a protective role in the vasculature; however, the underlying mechanisms involved are not entirely known. We previously showed that vascular smooth muscle-specific overexpression of a dominant negative human PPARγ mutation in mice (S-P467L) leads to enhanced myogenic tone and increased angiotensin-II-dependent vasoconstriction. S-P467L mice also exhibit increased arterial blood pressure. Here we tested the hypotheses that a) mesenteric smooth muscle cells isolated from S-P467L mice exhibit enhanced angiotensin-II AT₁ receptor signaling, and b) the increased arterial pressure of S-P467L mice is angiotensin-II AT₁ receptor dependent. Phosphorylation of mitogen-activated protein/extracellular signal-regulated kinase (ERK1/2) was robustly increased in mesenteric artery smooth muscle cell cultures from S-P467L in response to angiotensin-II. The increase in ERK1/2 activation by angiotensin-II was blocked by losartan, a blocker of AT₁ receptors. Angiotensin-II-induced ERK1/2 activation was also blocked by Tempol, a scavenger of reactive oxygen species, and correlated with increased Nox4 protein expression. To investigate whether endogenous renin-angiotensin system activity contributes to the elevated arterial pressure in S-P467L, non-transgenic and S-P467L mice were treated with the AT₁ receptor blocker, losartan (30 mg/kg per day), for 14-days and arterial pressure was assessed by radiotelemetry. At baseline S-P467L mice showed a significant increase of systolic arterial pressure (142.0±10.2 vs 129.1±3.0 mmHg, p<0.05). Treatment with losartan lowered systolic arterial pressure in S-P467L (132.2±6.9 mmHg) to a level similar to untreated non-transgenic mice. Losartan also lowered arterial pressure in non-transgenic (113.0±3.9 mmHg) mice, such that there was no difference in the losartan-induced depressor response between groups (−13.53±1.39 in S-P467L vs −16.16±3.14 mmHg in non-transgenic). Our results suggest that interference with PPARγ in smooth muscle: a) causes enhanced angiotensin-II AT₁ receptor-mediated ERK1/2 activation in resistance vessels, b) and may elevate arterial pressure through both angiotensin-II AT₁ receptor-dependent and -independent mechanisms.

Diuretics prevent thiazolidinedione-induced cardiac hypertrophy without compromising insulin-sensitizing effects in mice

Much concern has arisen regarding critical adverse effects of thiazolidinediones (TZDs), including rosiglitazone and pioglitazone, on cardiac tissue. Although TZD-induced cardiac hypertrophy (CH) has been attributed to an increase in plasma volume or a change in cardiac nutrient preference, causative roles have not been established. To test the hypothesis that volume expansion directly mediates rosiglitazone-induced CH, mice were fed a high-fat diet with rosiglitazone, and cardiac and metabolic consequences were examined. Rosiglitazone treatment induced volume expansion and CH in wild-type and PPARγ heterozygous knockout (Pparg(+/-)) mice, but not in mice defective for ligand binding (Pparg(P465L/+)). Cotreatment with the diuretic furosemide in wild-type mice attenuated rosiglitazone-induced CH, hypertrophic gene reprogramming, cardiomyocyte apoptosis, hypertrophy-related signal activation, and left ventricular dysfunction. Similar changes were observed in mice treated with pioglitazone. The diuretics spironolactone and trichlormethiazide, but not amiloride, attenuated rosiglitazone effects on volume expansion and CH. Interestingly, expression of glucose and lipid metabolism genes in the heart was altered by rosiglitazone, but these changes were not attenuated by furosemide cotreatment. Importantly, rosiglitazone-mediated whole-body metabolic improvements were not affected by furosemide cotreatment. We conclude that releasing plasma volume reduces adverse effects of TZD-induced volume expansion and cardiac events without compromising TZD actions in metabolic switch in the heart and whole-body insulin sensitivity.

Dominant negative PPARγ promotes atherosclerosis, vascular dysfunction, and hypertension through distinct effects in endothelium and vascular muscle

Agonists of the nuclear hormone receptor peroxisome proliferator-activated receptor γ (PPARγ) have potent insulin-sensitizing effects and inhibit atherosclerosis progression in patients with Type II diabetes. Conversely, missense mutations in the ligand-binding domain of PPARγ that render the transcription factor dominant negative (DN) cause early-onset hypertension and Type II diabetes. We tested the hypothesis that DN PPARγ-mediated interference of endogenous wild-type PPARγ in the endothelium and vascular smooth muscle exacerbates atherosclerosis in apolipoprotein E-deficient (ApoE(-/-)) mice. Endothelium-specific expression of DN PPARγ on the ApoE(-/-) background unmasked significant impairment of endothelium-dependent relaxation in aortic rings, increased systolic blood pressure, altered expression of atherogenic markers (e.g., Cd36, Mcp1, Catalase), and enhanced diet-induced atherosclerotic lesion formation in aorta. Smooth muscle-specific expression of DN PPARγ, which induces aortic dysfunction and increased systolic blood pressure at baseline, also resulted in enhanced diet-induced atherosclerotic lesion formation in aorta on the ApoE(-/-) background that was associated with altered expression of a shared, yet distinct, set of atherogenic markers (e.g., Cd36, Mcp1, Osteopontin, Vcam1). In particular, induction of Osteopontin expression by smooth muscle-specific DN PPARγ correlated with increased plaque calcification. These data demonstrate that inhibition of PPARγ function specifically in the vascular endothelium or smooth muscle may contribute to cardiovascular disease.

Cullin-3 regulates vascular smooth muscle function and arterial blood pressure via PPARγ and RhoA/Rho-kinase

Dominant-negative (DN) mutations in the nuclear hormone receptor peroxisome proliferator-activated receptor-γ (PPARγ) cause hypertension by an unknown mechanism. Hypertension and vascular dysfunction are recapitulated by expression of DN PPARγ specifically in vascular smooth muscle of transgenic mice. DN PPARγ increases RhoA and Rho-kinase activity, and inhibition of Rho-kinase restores normal reactivity and reduces arterial pressure. RhoBTB1, a component of the Cullin-3 RING E3 ubiquitin ligase complex, is a PPARγ target gene. Decreased RhoBTB1, Cullin-3, and neddylated Cullin-3 correlated with increased levels of the Cullin-3 substrate RhoA. Knockdown of Cullin-3 or inhibition of cullin-RING ligase activity in aortic smooth muscle cells increased RhoA. Cullin-RING ligase inhibition enhanced agonist-mediated contraction in aortic rings from normal mice by a Rho-kinase-dependent mechanism, and it increased arterial pressure in vivo. We conclude that Cullin-3 regulates vascular function and arterial pressure, thus providing a mechanistic link between mutations in Cullin-3 and hypertension in humans.

Rosiglitazone improves insulin sensitivity and baroreflex gain in rats with diet-induced obesity

Obesity decreases baroreflex gain (BRG); however, the mechanisms are unknown. We tested the hypothesis that impaired BRG is related to the concurrent insulin resistance, and, therefore, BRG would be improved after treatment with the insulin-sensitizing drug rosiglitazone. Male rats fed a high-fat diet diverged into obesity-prone (OP) and obesity-resistant (OR) groups after 2 weeks. Then, OP and OR rats, as well as control (CON) rats fed a standard diet, were treated daily for 2 to 3 weeks with rosiglitazone (3 or 6 mg/kg) or its vehicle by gavage. Compared with OR and CON rats, conscious OP rats exhibited reductions in BRG (OP, 2.9 ± 0.1 bpm/mm Hg; OR, 4.0 ± 0.2 bpm/mm Hg; CON, 3.9 ± 0.2 bpm/mm Hg; P < 0.05) and insulin sensitivity (hyperinsulinemic euglycemic clamp; OP, 6.8 ± 0.9 mg/kg · min; OR, 22.2 ± 1.2 mg/kg · min; CON, 17.7 ± 0.8 mg/kg · min; P < 0.05), which were well correlated (r(2) = 0.49; P < 0.01). In OP rats, rosiglitazone dose-dependently improved (P < 0.05) insulin sensitivity (12.8 ± 0.6 mg/kg · min at 3 mg/kg; 16.0 ± 1.5 mg/kg · min at 6 mg/kg) and BRG (3.8 ± 0.4 bpm/mm Hg at 3 mg/kg; 5.3 ± 0.7 bpm/mm Hg at 6 mg/kg). However, 6 mg/kg rosiglitazone also increased BRG in OR rats without increasing insulin sensitivity, disrupted the correlation between BRG and insulin sensitivity (r(2) = 0.08), and, in OP and OR rats, elevated BRG relative to insulin sensitivity (analysis of covariance; P < 0.05). Moreover, in OP rats, stimulation of the aortic depressor nerve, to activate central baroreflex pathways, elicited markedly reduced decreases in heart rate and arterial pressure, but these responses were not improved by rosiglitazone. In conclusion, diet-induced obesity impairs BRG via a central mechanism that is related to the concurrent insulin resistance. Rosiglitazone normalizes BRG, but not by improving brain baroreflex processing or insulin sensitivity.

PPARγ regulates resistance vessel tone through a mechanism involving RGS5-mediated control of protein kinase C and BKCa channel activity

RATIONALE

Activation of peroxisome proliferator-activated receptor-γ (PPARγ) by thiazolidinediones lowers blood pressure, whereas PPARγ mutations cause hypertension. Previous studies suggest these effects may be mediated through the vasculature, but the underlying mechanisms remain unclear.

OBJECTIVE

To identify PPARγ mechanisms and transcriptional targets in vascular smooth muscle and their role in regulating resistance artery tone.

METHODS AND RESULTS

We studied mesenteric artery (MA) from transgenic mice expressing dominant-negative (DN) mutant PPARγ driven by a smooth muscle cell-specific promoter. MA from transgenic mice exhibited a robust increase in myogenic tone. Patch clamp analysis revealed a reduced large conductance Ca(2+)-activated K(+) (BKCa) current in freshly dissociated smooth muscle cell from transgenic MA. Inhibition of protein kinase C corrected both enhanced myogenic constriction and impaired the large conductance Ca(2+)-activated K(+) channel function. Gene expression profiling revealed a marked loss of the regulator of G protein signaling 5 (RGS5) mRNA in transgenic MA, which was accompanied by a substantial increase in angiotensin II-induced constriction in MA. Small interfering RNA targeting RGS5 caused augmented myogenic tone in intact mesenteric arteries and increased activation of protein kinase C in smooth muscle cell cultures. PPARγ and PPARδ each bind to a PPAR response element close to the RGS5 promoter. RGS5 expression in nontransgenic MA was induced after activation of either PPARγ or PPARδ, an effect that was markedly blunted by DN PPARγ.

CONCLUSIONS

We conclude that RGS5 in smooth muscle is a PPARγ and PPARδ target, which when activated blunts angiotensin II-mediated activation of protein kinase C, and preserves the large conductance Ca(2+)-activated K(+) channel activity, thus providing tight control of myogenic tone in the microcirculation.

Consequences of stopping and restarting leptin in an adolescent with lipodystrophy

BACKGROUND/AIMS

Lipodystrophy encompasses a group of rare disorders characterized by deficiency of adipose tissue resulting in hypoleptinemia, and metabolic abnormalities including insulin resistance, diabetes, dyslipidemia, and nonalcoholic steatohepatitis. Leptin replacement effectively ameliorates these metabolic derangements. We report effects of leptin discontinuation and resumption in a child with acquired generalized lipodystrophy.

METHODS

Intermittent treatment with leptin with follow-up over 5 years.

RESULTS

Pretreatment metabolic abnormalities included insulin resistance, hypertriglyceridemia and steatohepatitis. Leptin was started at the age of 10 years. After 2 years, the family requested discontinuation of leptin due to lack of visible physical changes. Nine months later, worsened metabolic abnormalities and arrest of pubertal development were observed. Leptin was restarted, followed by improvements in metabolic parameters. Laboratory changes (before vs. 6 months after restarting leptin) were: fasting glucose from 232 to 85 mg/dl, insulin from 232 to 38.9 µU/ml, HbA(1c) from 7.5 to 4.8%, triglycerides from 622 to 96 mg/dl, ALT from 229 to 61 U/l, AST from 91 to 18 U/l, and urine protein:creatinine ratio from 5.4 to 0.3. Progression of puberty was observed 1 year after restarting leptin.

CONCLUSION

Initial leptin therapy likely prevented progression of metabolic abnormalities. Treatment discontinuation led to rapid metabolic decomposition and pubertal arrest. Reintroduction of leptin reversed metabolic abnormalities and allowed normal pubertal progression.

Pro- and antiatherogenic effects of a dominant-negative P465L mutation of peroxisome proliferator-activated receptor-γ in apolipoprotein E-Null mice

OBJECTIVE

The dominant-negative mutation, P467L, in peroxisome proliferator-activated receptor-γ (PPARγ) affects adipose tissue distribution, insulin sensitivity, and blood pressure in heterozygous humans. We hypothesized that the equivalent mutation, PPARγ-P465L, in mice will worsen atherosclerosis.

METHODS AND RESULTS

Apolipoprotein E-null mice with and without PPARγ-P465L mutation were bred in 129S6 inbred genetic background. Mild hypertension and lipodystrophy of PPARγ-P465L persisted in the apolipoprotein E-null background. Glucose homeostasis was normal, but plasma adiponectin was significantly lower and resistin was higher in PPARγ-P465L mice. Plasma cholesterol and lipoprotein distribution were not different, but plasma triglycerides tended to be reduced. Surprisingly, there were no overall changes in the atherosclerotic plaque size or composition. PPARγ-P465L macrophages had a small decrease in CD36 mRNA and a small yet significant reduction in very-low-density lipoprotein uptake in culture. In unloaded apolipoprotein E-null macrophages with PPARγ-P465L, cholesterol uptake was reduced whereas apolipoprotein AI-mediated efflux was increased. However, when cells were cholesterol loaded in the presence of acetylated low-density lipoprotein, no genotype difference in uptake or efflux was apparent. A reduction of vascular cell adhesion molecule-1 expression in aorta suggests a relatively antiatherogenic vascular environment in mice with PPARγ-P465L.

CONCLUSIONS

Small, competing pro- and antiatherogenic effects of PPARγ-P465L mutation result in unchanged plaque development in apolipoprotein E-deficient mice.

Peroxisome proliferator-activated receptor-γ protects against vascular aging

Vascular disease occurs commonly during aging. Carotid artery and cerebrovascular disease are major causes of stroke and contributors to dementia. Recent evidence suggests that peroxisome proliferator-activated receptor-γ (PPARγ) may play a protective role in the vasculature, but the potential importance of PPARγ in vascular aging is unknown. To examine the hypothesis that PPARγ normally protects against vascular aging, we studied heterozygous knockin mice expressing a human dominant-negative mutation in PPARγ (P465L, designated L/+). Endothelial dysfunction, a major contributor to vascular disease, was studied using carotid arteries from adult (8 ± 1 mo) and old (24 ± 1 mo) L/+ mice and wild-type littermates. In arteries from wild-type mice, responses to the endothelium-dependent agonist ACh were similar in adult and old wild-type mice but were reduced by ∼50% in old L/+ mice (n = 7-10, P < 0.05). Impaired responses in arteries from old L/+ mice were restored to normal by a scavenger of superoxide. Relaxation of arteries to nitroprusside (an NO donor) was similar in all groups. Contraction of arteries to U46619 was not affected by age or genotype, while maximal responses to endothelin-1 were reduced with age in both wild-type and L/+ mice. Vascular expression (mRNA) of the catalytic component of NADPH oxidase (Nox2) was not altered in wild-type mice but was increased significantly in old L/+ mice. These findings provide the first evidence that interference with PPARγ function accelerates vascular aging, suggesting a novel role for PPARγ in protecting against age-induced oxidative stress and endothelial dysfunction.

Peroxisome proliferator-activated receptor gamma (PPAR-γ) and neurodegenerative disorders

As the growth of the aging population continues to accelerate globally, increased prevalence of neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and stroke, has generated substantial public concern. Unfortunately, despite of discoveries of common factors underlying these diseases, few drugs are available to effectively treat these diseases. Peroxisome proliferator-activated receptor gamma (PPAR-γ) is a ligand-activated transcriptional factor that belongs to the nuclear hormone receptor superfamily. PPAR-γ has been shown to influence the expression or activity of a large number of genes in a variety of signaling networks, including regulation of insulin sensitivity, glucose homeostasis, fatty acid oxidation, immune responses, redox balance, cardiovascular integrity, and cell fates. Recent epidemiological, preclinical animal, and clinical studies also show that PPAR-γ agonists can lower the incidence of a number of neurological disorders, despite of multiple etiological factors involved in the development of these disorders. In this manuscript, we review current knowledge on mechanisms underlying the beneficial effect of PPAR-γ in different neurodegenerative diseases, in particular, AD, PD, and stroke, and attempt to analyze common and overlapping features among these diseases. Our investigation unveiled information suggesting the ability for PPAR-γ to inhibit NF-κB-mediated inflammatory signaling at multiple sites, and conclude that PPAR-γ agonists represent a novel class of drugs for treating neuroinflammatory diseases.

Carbon-nanoparticle-triggered acute lung inflammation and its resolution are not altered in PPARγ-defective (P465L) mice

BACKGROUND

The alveolar macrophage (AM) - first line of innate immune defence against pathogens and environmental irritants - constitutively expresses peroxisome-proliferator activated receptor γ (PPARγ). PPARγ ligand-induced activation keeps the AM quiescent, and thereby contributes to combat invaders and resolve inflammation by augmenting the phagocytosis of apoptotic neutrophils and inhibiting an excessive expression of inflammatory genes. Because of these presumed anti-inflammatory functions of PPARγ we tested the hypothesis, whether reduced functional receptor availability in mutant mice resulted in increased cellular and molecular inflammatory response during acute inflammation and/or in an impairment of its resolution.

METHODS

To address this hypothesis we examined the effects of a carbon-nanoparticle (CNP) lung challenge, as surrogate for non-infectious environmental irritants, in a murine model carrying a dominant-negative point mutation in the ligand-binding domain of PPARγ (P465L/wt). Animals were instilled intratracheally with Printex 90 CNPs and bronchoalveolar lavage (BAL) was gained 24 h or 72 h after instillation to investigate its cellular and protein composition.

RESULTS

Higher BAL cell numbers - due to higher macrophage counts - were found in mutants irrespective of treatment. Neutrophil numbers in contrast were slightly lower in mutants. Intratracheal CNP instillation resulted in a profound recruitment of inflammatory neutrophils into the alveolus, but genotype related differences at acute inflammation (24 h) and resolution (72 h) were not observed. There were no signs for increased alveolar-capillary membrane damage or necrotic cell death in mutants as determined by BAL protein and lactate-dehydrogenase content. Pro-inflammatory macrophage-derived cytokine osteopontin was higher, but galectin-3 lower in female mutants. CXCL5 and lipocalin-2 markers, attributed to epithelial cell stimulation did not differ.

CONCLUSIONS

Despite general genotype-related differences, we had to reject our hypothesis of an increased CNP induced lung inflammation and an impairment of its resolution in PPARγ defective mice. Although earlier studies showed ligand-induced activation of nuclear receptor PPARγ to promote resolution of lung inflammation, its reduced activity did not provide signs of resolution impairment in the settings investigated here.

'Striking the Right Balance' in Targeting PPARgamma in the Metabolic Syndrome: Novel Insights from Human Genetic Studies

At a time when the twin epidemics of obesity and type 2 diabetes threaten to engulf even the most well-resourced Western healthcare systems, the nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) has emerged as a bona fide therapeutic target for treating human metabolic disease. The novel insulin-sensitizing antidiabetic thiazolidinediones (TZDs, e.g., rosiglitazone, pioglitazone), which are licensed for use in the treatment of type 2 diabetes, are high-affinity PPARγ ligands, whose beneficial effects extend beyond improvement in glycaemic control to include amelioration of dyslipidaemia, lowering of blood pressure, and favourable modulation of macrophage lipid handling and inflammatory responses. However, a major drawback to the clinical use of exisiting TZDs is weight gain, reflecting both enhanced adipogenesis and fluid retention, neither of which is desirable in a population that is already overweight and prone to cardiovascular disease. Accordingly, the “search is on” to identify the next generation of PPARγ modulators that will promote maximal clinical benefit by targeting specific facets of the metabolic syndrome (glucose intolerance/diabetes, dyslipidaemia, and hypertension), while simultaneously avoiding undesirable side effects of PPARγ activation (e.g., weight gain). This paper outlines the important clinical and laboratory observations made in human subjects harboring genetic variations in PPARγ that support such a therapeutic strategy.

PPAR-gamma in the Cardiovascular System

Peroxisome proliferator-activated receptor-γ (PPAR-γ), an essential transcriptional mediator of adipogenesis, lipid metabolism, insulin sensitivity, and glucose homeostasis, is increasingly recognized as a key player in inflammatory cells and in cardiovascular diseases (CVD) such as hypertension, cardiac hypertrophy, congestive heart failure, and atherosclerosis. PPAR-γ agonists, the thiazolidinediones (TZDs), increase insulin sensitivity, lower blood glucose, decrease circulating free fatty acids and triglycerides, lower blood pressure, reduce inflammatory markers, and reduce atherosclerosis in insulin-resistant patients and animal models. Human genetic studies on PPAR-γ have revealed that functional changes in this nuclear receptor are associated with CVD. Recent controversial clinical studies raise the question of deleterious action of PPAR-γ agonists on the cardiovascular system. These complex interactions of metabolic responsive factors and cardiovascular disease promise to be important areas of focus for the future.

Genetic manipulations of PPARs: effects on obesity and metabolic disease

The interest in genetic manipulations of PPARs is as old as their discovery as receptors of ligands with beneficial clinical activities. Considering the effects of PPAR ligands on critical aspects of systemic physiology, including obesity, lipid metabolism, insulin resistance, and diabetes, gene knockout (KO) in mice is the ideal platform for both hypothesis testing and discovery of new PPAR functions in vivo. With the fervent pursuit of the magic bullet to eradicate the obesity epidemic, special emphasis has been placed on the impacts of PPARs on obesity and its associated diseases. As detailed in this review, understanding how PPARs regulate gene expression and basic metabolic pathways is a necessary intermediate en route to deciphering their effects on obesity. Over a decade and dozens of genetic modifications of PPARs into this effort, valuable lessons have been learned, but we are left with more questions to be answered. These lessons and future prospects are the subject of this review.

Relaxin causes selective outward remodeling of brain parenchymal arterioles via activation of peroxisome proliferator-activated receptor-γ

Brain parenchymal arterioles (PAs), but not pial arteries, undergo hypotrophic outward remodeling during pregnancy that involves peroxisome proliferator-activated receptor-γ (PPARγ) activation. Relaxin, a peptide hormone produced during pregnancy, is involved in systemic and renal artery remodeling and activates PPARγ in vitro. Thus, we hypothesized that relaxin is involved in the selective outward remodeling of PAs through a PPARγ-dependent mechanism. Nonpregnant rats were treated with relaxin (4 μg/h, osmotic minipump), relaxin plus PPARγ inhibitor GW9662 (10 mg/kg/d), or vehicle for 10 d. Vascular function and structure were compared in isolated and pressurized middle cerebral arteries (MCAs) and PAs taken from the same animals. Relaxin treatment increased serum relaxin to the level of pregnancy (54 ng/ml) and increased passive wall thickness (hypertrophy; 70 ± 5 vs. 54 ± 4 μm in vehicle; P<0.05) and inner diameter (outward remodeling; 10.6 ± 0.5 vs. 8.0 ± 0.6 μm in vehicle; P<0.05) in PAs, but not in MCAs. This hypertrophic outward remodeling was prevented by GW9662 that had diameters (57 ± 3 μm) and wall thickness (8.6 ± 1.0 μm) similar to vehicle. GW9662 also prevented relaxin-induced changes in PPARγ target gene expression. These results suggest that relaxin produced during pregnancy may be partly responsible for selective remodeling of PAs during pregnancy through a mechanism involving PPARγ

Protecting against vascular disease in brain

Endothelial cells exert an enormous influence on blood vessels throughout the circulation, but their impact is particularly pronounced in the brain. New concepts have emerged recently regarding the role of this cell type and mechanisms that contribute to endothelial dysfunction and vascular disease. Activation of the renin-angiotensin system plays a prominent role in producing these abnormalities. Both oxidative stress and local inflammation are key mechanisms that underlie vascular disease of diverse etiology. Endogenous mechanisms of vascular protection are also present, including antioxidants, anti-inflammatory molecules, and peroxisome proliferator-activated receptor-γ. Despite their clear importance, studies of mechanisms that underlie cerebrovascular disease continue to lag behind studies of vascular biology in general. Identification of endogenous molecules and pathways that protect the vasculature may result in targeted approaches to prevent or slow the progression of vascular disease that causes stroke and contributes to the vascular component of dementia and Alzheimer's disease.

PPARγ agonist beyond glucose lowering effect

The nuclear hormone receptor PPARγ is activated by several agonists, including members of the thiazolidinedione group of insulin sensitizers. Pleiotropic beneficial effects of these agonists, independent of their blood glucose-lowering effects, have recently been demonstrated in the vasculature. PPARγ agonists have been shown to lower blood pressure in animals and humans, perhaps by suppressing the renin-angiotensin (Ang)-aldosterone system (RAAS), including the inhibition of Ang II type 1 receptor expression, Ang-II-mediated signaling pathways, and Ang-II-induced adrenal aldosterone synthesis/secretion. PPARγ agonists also inhibit the progression of atherosclerosis in animals and humans, possibly through a pathway involving the suppression of RAAS and the thromboxane A₂ system, as well as the protection of endothelial function. Moreover, PPARγ-agonist-mediated renal protection, especially the reduction of albuminuria, has been observed in diabetic nephropathy, including animal models of the disease, and in non-diabetic renal dysfunction. The renal protective activities may reflect, at least in part, the ability of PPARγ agonists to lower blood pressure, protect endothelial function, and cause vasodilation of the glomerular efferent arterioles. Additionally, anti-neoplastic effects of PPARγ agonists have recently been described. Based on the multiple therapeutic actions of PPARγ agonists, they will no doubt lead to novel approaches in the treatment of lifestyle-related and other diseases.

Pparg-P465L mutation worsens hyperglycemia in Ins2-Akita female mice via adipose-specific insulin resistance and storage dysfunction

OBJECTIVE

The dominant-negative P467L mutation in peroxisome proliferator activated receptor-γ (PPARγ) was identified in insulin-resistant patients with hyperglycemia and lipodystrophy. In contrast, mice carrying the corresponding Pparg-P465L mutation have normal insulin sensitivity, with mild hyperinsulinemia. We hypothesized that murine Pparg-P465L mutation leads to covert insulin resistance, which is masked by hyperinsulinemia and increased pancreatic islet mass, to retain normal plasma glucose.

RESEARCH DESIGN AND METHODS

We introduced in Pparg(P465L/+) mice an Ins2-Akita mutation that causes improper protein folding and islet apoptosis to lower plasma insulin.

RESULTS

Unlike Ins2(Akita/+) littermates, male Pparg(P465L/+)Ins2(Akita/+) mice have drastically reduced life span with enhanced type 1 diabetes. Hyperglycemia in Ins2(Akita/+) females is mild. However, Pparg(P465L/+)Ins2(Akita/+) females have aggravated hyperglycemia, smaller islets, and reduced plasma insulin. In an insulin tolerance test, they showed smaller reduction in plasma glucose, indicating impaired insulin sensitivity. Although gluconeogenesis is enhanced in Pparg(P465L/+)Ins2(Akita/+) mice compared with Ins2(Akita/+), exogenous insulin equally suppressed gluconeogenesis in hepatocytes, suggesting that Pparg(P465L/+)Ins2(Akita/+) livers are insulin sensitive. Expression of genes regulating insulin sensitivity and glycogen and triglyceride contents suggest that skeletal muscles are equally insulin sensitive. In contrast, adipose tissue and isolated adipocytes from Pparg(P465L/+)Ins2(Akita/+) mice have impaired glucose uptake in response to exogenous insulin. Pparg(P465L/+)Ins2(Akita/+) mice have smaller fat depots composed of larger adipocytes, suggesting impaired lipid storage with subsequent hepatomegaly and hypertriglyceridemia.

CONCLUSIONS

PPARg-P465L mutation worsens hyperglycemia in Ins2(Akita/+) mice primarily because of adipose-specific insulin resistance and altered storage function. This underscores the important interplay between insulin and PPARγ in adipose tissues in diabetes.

The role of peroxisome proliferator-activated receptor gamma in blood pressure regulation

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a member of the nuclear hormone receptor superfamily. It is expressed in adipocytes, immune cells, and cardiovascular cells that include cardiomyocytes, endothelial cells, and smooth muscle cells. PPARgamma plays a role in regulating cellular anti-inflammatory responses and is a mediator of insulin sensitization induced by thiazolidinediones, which also can reduce elevated blood pressure both clinically and experimentally. As a result, research regarding the role of PPARgamma in blood pressure homeostasis is ongoing. Recent studies have demonstrated that increases or decreases in blood pressure phenotype may be PPARgamma-dependent and involve a number of different signaling pathways. Furthermore, studies using PPARgamma mutant transgenic and knockout animal models provide further evidence regarding a role for endothelial-cell and vascular smooth muscle-cell PPARgamma in blood pressure regulation. However, there is a need for further research regarding PPARgamma--mediated mechanisms involved in maintaining physiologic control of blood pressure.

Peroxisome proliferator-activated receptors, metabolic syndrome and cardiovascular disease

Metabolic syndrome (MetS) is a constellation of risk factors including insulin resistance, central obesity, dyslipidemia and hypertension that markedly increase the risk of Type 2 diabetes (T2DM) and cardiovascular disease (CVD). The peroxisome proliferators-activated receptor (PPAR) isotypes, PPARα, PPARδ/ß and PPARγ are ligand-activated nuclear transcription factors, which modulate the expression of an array of genes that play a central role in regulating glucose, lipid and cholesterol metabolism, where imbalance can lead to obesity, T2DM and CVD. They are also drug targets, and currently, PPARα (fibrates) and PPARγ (thiazolodinediones) agonists are in clinical use for treating dyslipidemia and T2DM, respectively. These metabolic characteristics of the PPARs, coupled with their involvement in metabolic diseases, mean extensive efforts are underway worldwide to develop new and efficacious PPAR-based therapies for the treatment of additional maladies associated with the MetS. This article presents an overview of the functional characteristics of three PPAR isotypes, discusses recent advances in our understanding of the diverse biological actions of PPARs, particularly in the vascular system, and summarizes the developmental status of new single, dual, pan (multiple) and partial PPAR agonists for the clinical management of key components of MetS, T2DM and CVD. It also summarizes the clinical outcomes from various clinical trials aimed at evaluating the atheroprotective actions of currently used fibrates and thiazolodinediones.

Pioglitazone-induced reductions in atherosclerosis occur via smooth muscle cell-specific interaction with PPAR{gamma}

RATIONALE

Peroxisome proliferator-activated receptor (PPAR)γ agonists attenuate atherosclerosis and abdominal aortic aneurysms (AAAs). PPARγ, a nuclear receptor, is expressed on many cell types including smooth muscle cells (SMCs).

OBJECTIVE

To determine whether a PPARγ agonist reduces angiotensin II (Ang II)-induced atherosclerosis and AAAs via interaction with SMC-specific PPARγ.

METHODS AND RESULTS

Low-density lipoprotein receptor (LDLR)(-/-) mice with SMC-specific PPARγ deficiency were developed using PPARγ floxed (PPARγ(f/f)) and SM22 Cre(+) mice. PPARγ(f/f) littermates were generated that did not express Cre (Cre(0/0)) or were hemizygous for Cre (Cre(+/0)). To assess the contribution of SMC-specific PPARγ in ligand-mediated attenuation of Ang II-induced atherosclerosis and AAAs, both male and female Cre(0/0) and Cre(+/0) mice were fed a fat-enriched diet with or without the PPARγ agonist pioglitazone (Pio) (20 mg/kg per day) for 5 weeks. After 1 week of feeding modified diets, mice were infused with Ang II (1000 ng/kg per minute) for 4 weeks. SMC-specific PPARγ deficiency or Pio administration had no effect on plasma cholesterol concentrations. Pio administration attenuated Ang II-increased systolic blood pressure equivalently in both Cre(0/0) and Cre(+/0) groups. SMC-specific PPARγ deficiency increased atherosclerosis in male mice. Pio administration reduced atherosclerosis in only the Cre(0/0) mice, but not in mice with SMC-specific PPARγ deficiency. SMC-specific PPARγ deficiency or Pio administration had no effect on Ang II-induced AAA development. Pio also did not attenuate Ang II-induced monocyte chemoattractant protein-1 production in PPARγ-deficient SMCs.

CONCLUSIONS

Pio attenuates Ang II-induced atherosclerosis via the interaction with SMC-specific PPARγ, but has no effect on the development of AAAs.

The adipose renin-angiotensin system modulates systemic markers of insulin sensitivity and activates the intrarenal renin-angiotensin system

Background. The adipose tissue renin-angiotensin system (RAS) contributes to regulation of fat mass and may also impact systemic functions such as blood pressure and metabolism. Methods and results. A panel of mouse models including mice lacking angiotensinogen, Agt (Agt-KO), mice expressing Agt solely in adipose tissue (aP2-Agt/Agt-KO), and mice overexpressing Agt in adipose tissue (aP2-Agt) was studied. Total body weight, epididymal fat pad weight, and circulating levels of leptin, insulin, and resistin were significantly decreased in Agt-KO mice, while plasma adiponectin levels were increased. aP2-Agt mice exhibited increased adiposity and plasma leptin and insulin levels compared to wild type (WT) controls. Angiotensinogen and type I Ang II receptor protein levels were also elevated in kidney of aP2-Agt mice. Conclusion. These findings demonstrate that alterations in adipose RAS activity significantly impact both local and systemic physiology in a way that may contribute to the detrimental health effects of obesity.

PPARs in the Renal Regulation of Systemic Blood Pressure

Recent research has revealed roles for the peroxisome proliferator activated receptor (PPAR) family of transcription factors in blood pressure regulation, expanding the possible therapeutic use of PPAR ligands. PPARalpha and PPARgamma modulate the renin-angiotensin-aldosterone system (RAAS), a major regulator of systemic blood pressure and interstitial fluid volume by transcriptional control of renin, angiotensinogen, angiotensin converting enzyme (ACE) and angiotensin II receptor 1 (AT-R1). Blockade of RAAS is an important therapeutic target in hypertension management and attenuates microvascular damage, glomerular inflammation and left ventricular hypertrophy in hypertensive patients and also show antidiabetic effects. The mechanisms underlying the benefits of RAAS inhibition appear to involve PPARgamma-regulated pathways. This review summarizes current knowledge on the role of PPARs in the transcriptional control of the RAAS and the possible use of PPAR ligands in the treatment of RAAS dependent hypertension.

Inhibitor of DNA binding 4 (ID4) regulation of adipocyte differentiation and adipose tissue formation in mice

Inhibitor of DNA binding 4 (ID4) is a helix-loop-helix protein that heterodimerizes with basic helix-loop-helix transcription factors inhibiting their function. ID4 expression is important for adipogenic differentiation of the 3T3-L1 cell line, and inhibition of ID4 is associated with a concomitant decrease in CCAAT/enhancer-binding protein alpha and peroxisome proliferator-activated receptor gamma mRNA and protein expression. Mice with a homozygous deletion of Id4 (Id4(-/-)) have reduced body fat and gain much less weight compared with wild-type littermates when placed on diets with high fat content. Mouse embryonic fibroblasts (MEFs) isolated from Id4(-/-) mice have reduced adipogenic potential when compared with wild-type MEFs. In agreement with changes in morphological differentiation, the levels of CCAAT/enhancer-binding protein alpha and peroxisome proliferator-activated receptor gamma were also reduced in MEFs from Id4(-/-) mice. Our results demonstrate the importance of ID4 in adipocyte differentiation and the implications of this regulation for adipose tissue formation.

Does peroxisome proliferator-activated receptor-gamma (PPAR gamma) protect from hypertension directly through effects in the vasculature?

Peroxisome proliferator-activated receptor-gamma (PPAR gamma) is a ligand-activated transcription factor of the nuclear hormone receptor superfamily. Increasing evidence suggests that PPAR gamma is involved in the regulation of vascular function and blood pressure in addition to its well recognized role in metabolism. Thiazolidinediones, PPAR gamma agonists, lower blood pressure and have protective vascular effects through largely unknown mechanisms. In contrast, loss-of-function dominant-negative mutations in human PPAR gamma cause insulin resistance and severe early onset hypertension. Recent studies using genetically manipulated mouse models have begun to specifically address the importance of PPAR gamma in the vasculature. In this minireview, evidence for a protective role of PPAR gamma in the endothelium and vascular smooth muscle, derived largely from studies of genetically manipulated mice, will be discussed.

Bioinformatic analysis of gene sets regulated by ligand-activated and dominant-negative peroxisome proliferator-activated receptor gamma in mouse aorta

OBJECTIVE

Drugs that activate peroxisome proliferator-activated receptor (PPAR) gamma improve glucose sensitivity and lower blood pressure, whereas dominant-negative mutations in PPARgamma cause severe insulin resistance and hypertension. We hypothesize that these PPARgamma mutants regulate target genes opposite to those of ligand-mediated activation, and we tested this hypothesis on a genomewide scale.

METHODS AND RESULTS

We integrated gene expression data in aorta specimens from mice treated with the PPARgamma ligand rosiglitazone with data from mice containing a globally expressed knockin of the PPARgamma P465L dominant-negative mutation. We also integrated our data with publicly available data sets containing the following: (1) gene expression profiles in many human tissues, (2) PPARgamma target genes in 3T3-L1 adipocytes, and (3) experimentally validated PPARgamma binding sites throughout the genome. Many classic PPARgamma target genes were induced by rosiglitazone and repressed by dominant-negative PPARgamma. A similar pattern was observed for about 90% of the gene sets regulated by both rosiglitazone and dominant-negative PPARgamma. Genes exhibiting this pattern of contrasting regulation were significantly enriched for nearby PPARgamma binding sites.

CONCLUSIONS

These results provide convincing evidence that the PPARgamma P465L mutation causes transcriptional effects that are opposite to those mediated by PPARgamma ligand, thus validating mice carrying the mutation as a model of PPARgamma interference.