PPARdelta activation normalizes cardiac substrate metabolism and reduces right ventricular hypertrophy in congestive heart failure

Pharmacological Utility of PPAR Modulation for Angiogenesis in Cardiovascular Disease

Peroxisome proliferator activated receptors, including PPARα, PPARβ/δ, and PPARγ, are ligand-activated transcription factors belonging to the nuclear receptor superfamily. They play important roles in glucose and lipid metabolism and are also supposed to reduce inflammation and atherosclerosis. All PPARs are involved in angiogenesis, a process critically involved in cardiovascular pathology. Synthetic specific agonists exist for all PPARs. PPARα agonists (fibrates) are used to treat dyslipidemia by decreasing triglyceride and increasing high-density lipoprotein (HDL) levels. PPARγ agonists (thiazolidinediones) are used to treat Type 2 diabetes mellitus by improving insulin sensitivity. PPARα/γ (dual) agonists are supposed to treat both pathological conditions at once. In contrast, PPARβ/δ agonists are not in clinical use. Although activators of PPARs were initially considered to have favorable effects on the risk factors for cardiovascular disease, their cardiovascular safety is controversial. Here, we discuss the implications of PPARs in vascular biology regarding cardiac pathology and focus on the outcomes of clinical studies evaluating their benefits in cardiovascular diseases.

Important Functions and Molecular Mechanisms of Mitochondrial Redox Signaling in Pulmonary Hypertension

Mitochondria are important organelles that act as a primary site to produce reactive oxygen species (ROS). Additionally, mitochondria play a pivotal role in the regulation of Ca²⁺ signaling, fatty acid oxidation, and ketone synthesis. Dysfunction of these signaling molecules leads to the development of pulmonary hypertension (PH), atherosclerosis, and other vascular diseases. Features of PH include vasoconstriction and pulmonary artery (PA) remodeling, which can result from abnormal proliferation, apoptosis, and migration of PA smooth muscle cells (PASMCs). These responses are mediated by increased Rieske iron–sulfur protein (RISP)-dependent mitochondrial ROS production and increased mitochondrial Ca²⁺ levels. Mitochondrial ROS and Ca²⁺ can both synergistically activate nuclear factor κB (NF-κB) to trigger inflammatory responses leading to PH, right ventricular failure, and death. Evidence suggests that increased mitochondrial ROS and Ca²⁺ signaling leads to abnormal synthesis of ketones, which play a critical role in the development of PH. In this review, we discuss some of the recent findings on the important interactive role and molecular mechanisms of mitochondrial ROS and Ca²⁺ in the development and progression of PH. We also address the contributions of NF-κB-dependent inflammatory responses and ketone-mediated oxidative stress due to abnormal regulation of mitochondrial ROS and Ca²⁺ signaling in PH.

Targeting fatty acid metabolism for fibrotic disorders

Fibrosis is defined by abnormal accumulation of extracellular matrix, which can affect virtually every organ system under diseased conditions. Fibrotic tissue remodeling often leads to organ dysfunction and is highly associated with increased morbidity and mortality. The disease burden caused by fibrosis is substantial, and the medical need for effective antifibrotic therapies is essential. Significant progress has been made in understanding the molecular mechanism and pathobiology of fibrosis, such as transforming growth factor-β (TGF-β)-mediated signaling pathways. However, owing to the complex and dynamic properties of fibrotic disorders, there are currently no therapeutic options that can prevent or reverse fibrosis. Recent studies have revealed that alterations in fatty acid metabolic processes are common mechanisms and core pathways that play a central role in different fibrotic disorders. Excessive lipid accumulation or defective fatty acid oxidation is associated with increased lipotoxicity, which directly contributes to the development of fibrosis. Genetic alterations or pharmacologic targeting of fatty acid metabolic processes have great potential for the inhibition of fibrosis development. Furthermore, mechanistic studies have revealed active interactions between altered metabolic processes and fibrosis development. Several well-known fibrotic factors change the lipid metabolic processes, while altered metabolic processes actively participate in fibrosis development. This review summarizes the recent evidence linking fatty acid metabolism and fibrosis, and provides new insights into the pathogenesis of fibrotic diseases for the development of drugs for fibrosis prevention and treatment.

Untangling the Cooperative Role of Nuclear Receptors in Cardiovascular Physiology and Disease

The heart is the first organ to acquire its physiological function during development, enabling it to supply the organism with oxygen and nutrients. Given this early commitment, cardiomyocytes were traditionally considered transcriptionally stable cells fully committed to contractile function. However, growing evidence suggests that the maintenance of cardiac function in health and disease depends on transcriptional and epigenetic regulation. Several studies have revealed that the complex transcriptional alterations underlying cardiovascular disease (CVD) manifestations such as myocardial infarction and hypertrophy is mediated by cardiac retinoid X receptors (RXR) and their partners. RXRs are members of the nuclear receptor (NR) superfamily of ligand-activated transcription factors and drive essential biological processes such as ion handling, mitochondrial biogenesis, and glucose and lipid metabolism. RXRs are thus attractive molecular targets for the development of effective pharmacological strategies for CVD treatment and prevention. In this review, we summarize current knowledge of RXR partnership biology in cardiac homeostasis and disease, providing an up-to-date view of the molecular mechanisms and cellular pathways that sustain cardiomyocyte physiology.

CD36 as a target for metabolic modulation therapy in cardiac disease

Introduction: Disturbances in myocardial lipid metabolism are increasingly being recognized as drivers of the development and progression of heart disease. Therefore, there is a need for treatments that can directly target lipid metabolic defects in heart failure. The membrane-associated glycoprotein CD36 plays a pivotal role in governing myocardial lipid metabolism by mediating lipid signaling and facilitating the cellular uptake of long-chain fatty acids. Emerging evidence suggests that CD36 is a prominent target in the treatment of heart failure.Areas covered: This article provides an overview of the key role of CD36 for proper contractile functioning of a healthy heart, its implications in the development of cardiac disease (ischemia/reperfusion, cardiac hypertrophy, and diabetic cardiomyopathy), and its application as a target to normalize cardiac metabolism as part of so-called metabolic modulation therapy.Expert opinion: CD36 appears a promising and effective therapeutic target in the treatment of heart failure. Natural compounds and chemical agents known to alter the amount or subcellular distribution of CD36 or inhibit its functioning, should be evaluated for their potency to correct cardiac metabolism and cure heart disease.

The nutrient sensing pathways FoxO1/3 and mTOR in the heart are coordinately regulated by central leptin through PPARβ/δ. Implications in cardiac remodeling

BACKGROUND

Cardiovascular disease in obese individuals with type 2 diabetes is often associated with hyperleptinemia and leptin resistance, while other studies support that leptin has cardioprotective effects. Besides, the role of leptin in regulating cardiac atrophy or hypertrophy remains to be clearly defined. In fact, in rats with normal leptin sensitivity, the molecular underpinnings of the effects of central leptin regulating cardiac structural pathways remain poorly understood.

OBJECTIVE

Hence, we assessed the effects of intracerebroventricular (icv) leptin infusion on cardiac remodeling analyzing FOXO1/3 and mTORC1 pathways, focusing special attention to PPARβ/δ as mediator of central leptin's effects on cardiac metabolism.

METHODS

Male 3-months-old Wistar rats, infused with icv leptin (0.2 μg/day) for 7 days, were daily co-treated intraperitoneally with the specific PPARβ/δ antagonist GSK0660, at 1 mg/kg per day along leptin treatment.

RESULTS

Central leptin regulated dynamically, in an opposite manner, the network between FOXOs and mTORC1 and induced an atrophy-related gene program in cardiac tissue. Leptin activated the anti-hypertrophic kinase GSK3β and increased the protein levels of muscle-specific ubiquitin ligases, muscle RING finger 1 (MuRF1) and muscle atrophy F-box (MAFbx)/Atrogin-1 involved in limiting cardiac hypertrophy. FOXO1 activity and the expression of their target genes, Sod2 and Lpl, were also increased in the heart upon central leptin infusion. Besides, Beclin-1 and LC3B-II, gene products of the autophagic pathway response, were upregulated, while the content and expression levels of phenotypic markers of cardiac hypertrophy as ANP and β-myosin heavy chain, gene product of Myh7 were significantly decreased. On the other hand, mTORC1 activity and OXPHOS protein levels were decreased suggesting a key role of central leptin preventing cardiac oxidative stress. In fact, the content of carbonylated proteins, TBARS and ROS/RSN were not increased in cardiac tissue in response to central leptin infusion. Finally, the pharmacological inhibition of PPARβ/δ, via in vivo administration of the selective antagonist GSK0660, blunted the induction of FOXO1/3, Atrogin-1, MuRF1 and GSK3β in the heart mediated by icv leptin infusion.

CONCLUSIONS

Our results demonstrate that, in lean rats with normal leptin sensitivity, central leptin regulates nutrient sensing pathways in heart contributing to balance cardiac remodeling through the anti- and pro-hypertrophic programs, and in this process is involved PPARβ/δ.

PPARs and Myocardial Infarction

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone receptor family. They are ligand-activated transcription factors and exist in three different isoforms, PPARα (NR1C1), PPARβ/δ (NR1C2), and PPARγ (NR1C3). PPARs regulate a variety of functions, including glucose and lipid homeostasis, inflammation, and development. They exhibit tissue and cell type-specific expression patterns and functions. Besides the established notion of the therapeutic potential of PPAR agonists for the treatment of glucose and lipid disorders, more recent data propose specific PPAR ligands as potential therapies for cardiovascular diseases. In this review, we focus on the knowledge of PPAR function in myocardial infarction, a severe pathological condition for which therapeutic use of PPAR modulation has been suggested.

Elucidating the Beneficial Role of PPAR Agonists in Cardiac Diseases

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that bind to DNA and regulate transcription of genes involved in lipid and glucose metabolism. A growing number of studies provide strong evidence that PPARs are the promising pharmacological targets for therapeutic intervention in various diseases including cardiovascular disorders caused by compromised energy metabolism. PPAR agonists have been widely used for decades as lipid-lowering and anti-inflammatory drugs. Existing studies are mainly focused on the anti-atherosclerotic effects of PPAR agonists; however, their role in the maintenance of cellular bioenergetics remains unclear. Recent studies on animal models and patients suggest that PPAR agonists can normalize lipid metabolism by stimulating fatty acid oxidation. These studies indicate the importance of elucidation of PPAR agonists as potential pharmacological agents for protection of the heart from energy deprivation. Here, we summarize and provide a comprehensive analysis of previous studies on the role of PPARs in the heart under normal and pathological conditions. In addition, the review discusses the PPARs as a therapeutic target and the beneficial effects of PPAR agonists, particularly bezafibrate, to attenuate cardiomyopathy and heart failure in patients and animal models.

The PPAR pan-agonist bezafibrate ameliorates cardiomyopathy in a mouse model of Barth syndrome

BACKGROUND

The PGC-1α/PPAR axis has been proposed as a potential therapeutic target for several metabolic disorders. The aim was to evaluate the efficacy of the pan-PPAR agonist, bezafibrate, in tafazzin knockdown mice (TazKD), a mouse model of Barth syndrome that exhibits age-dependent dilated cardiomyopathy with left ventricular (LV) dysfunction.

RESULTS

The effect of bezafibrate on cardiac function was evaluated by echocardiography in TazKD mice with or without beta-adrenergic stress. Adrenergic stress by chronic isoproterenol infusion exacerbates the cardiac phenotype in TazKD mice, significantly depressing LV systolic function by 4.5 months of age. Bezafibrate intake over 2 months substantially ameliorates the development of LV systolic dysfunction in isoproterenol-stressed TazKD mice. Without beta-adrenergic stress, TazKD mice develop dilated cardiomyopathy by 7 months of age. Prolonged treatment with suprapharmacological dose of bezafibrate (0.5% in rodent diet) over a 4-month period effectively prevented LV dilation in mice isoproterenol treatment. Bezafibrate increased mitochondrial biogenesis, however also promoted oxidative stress in cardiomyocytes. Surprisingly, improvement of systolic function in bezafibrate-treated mice was accompanied with simultaneous reduction of cardiolipin content and increase of monolysocardiolipin levels in cardiac muscle.

CONCLUSIONS

Thus, we demonstrate that bezafibrate has a potent therapeutic effect on preventing cardiac dysfunction in a mouse model of Barth syndrome with obvious implications for treating the human disease. Additional studies are needed to assess the potential benefits of PPAR agonists in humans with Barth syndrome.

Preventive aerobic training exerts a cardioprotective effect on rats treated with monocrotaline

Pulmonary arterial hypertension (PAH) is a chronic disease which causes overload to the right ventricle. The effect of preventive training on cardiac remodelling in this condition is still unknown. This study aimed to evaluate the influence of preventive training on hypertrophy, heart function and gene expression of calcium transport proteins in rats with monocrotaline-induced PAH. Thirty-two male Wistar rats were randomly divided into four groups: S, sedentary control; T, trained control; SM, sedentary monocrotaline; and TM, trained monocrotaline. The preventive training protocol was performed on a treadmill for 13 weeks, five times/week. The first two weeks were adopted for adaptation to training with gradual increases in speed/time. The speed of the physical training from the third to tenth weeks was gradually increased from 0.9 to 1.1 km/h for 60 min. Next, monocrotaline was applied (60 mg/kg) to induce PAH and lactate threshold analysis performed to determine the training speeds. The training speed of the TM group in the following two weeks was 0.8 km/h for 60 min and the T = 0.9 km/h for 60 min; in the final two weeks, both groups trained at the same speed and duration 0.9 km/h, 60 min. Cardiac function was assessed through echocardiography, ventricular hypertrophy through histomorphometric analysis and gene expression through RT-qPCR. Right cardiac function assessed through the peak flow velocity was SM = 75.5 cm/s vs. TM = 92.0 cm/s (P = 0.001), and ventricular hypertrophy was SM = 106.4 μm² vs. TM = 77.7 μm² (P = 0.004). There was a decrease in the gene expression of ryanodine S = 1.12 au vs. SM = 0.60 au (P = 0.02) without alterations due to training. Thus, we conclude that prior physical training exerts a cardioprotective effect on the right ventricle in the monocrotaline rat model.

Assessing Cardiac Metabolism: A Scientific Statement From the American Heart Association

In a complex system of interrelated reactions, the heart converts chemical energy to mechanical energy. Energy transfer is achieved through coordinated activation of enzymes, ion channels, and contractile elements, as well as structural and membrane proteins. The heart's needs for energy are difficult to overestimate. At a time when the cardiovascular research community is discovering a plethora of new molecular methods to assess cardiac metabolism, the methods remain scattered in the literature. The present statement on "Assessing Cardiac Metabolism" seeks to provide a collective and curated resource on methods and models used to investigate established and emerging aspects of cardiac metabolism. Some of those methods are refinements of classic biochemical tools, whereas most others are recent additions from the powerful tools of molecular biology. The aim of this statement is to be useful to many and to do justice to a dynamic field of great complexity.

PPARs: Protectors or Opponents of Myocardial Function?

Over 5 million people in the United States suffer from the complications of heart failure (HF), which is a rapidly expanding health complication. Disorders that contribute to HF include ischemic cardiac disease, cardiomyopathies, and hypertension. Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor family. There are three PPAR isoforms: PPARα, PPARγ, and PPARδ. They can be activated by endogenous ligands, such as fatty acids, as well as by pharmacologic agents. Activators of PPARs are used for treating several metabolic complications, such as diabetes and hyperlipidemia that are directly or indirectly associated with HF. However, some of these drugs have adverse effects that compromise cardiac function. This review article aims to summarize the current basic and clinical research findings of the beneficial or detrimental effects of PPAR biology on myocardial function.

Fatty acid metabolism in pulmonary arterial hypertension: role in right ventricular dysfunction and hypertrophy

Pulmonary arterial hypertension (PAH) is a complex, multifactorial disease in which an increase in pulmonary vascular resistance leads to increased afterload on the right ventricle (RV), causing right heart failure and death. Our understanding of the pathophysiology of RV dysfunction in PAH is limited but is constantly improving. Increasing evidence suggests that in PAH RV dysfunction is associated with various components of metabolic syndrome, such as insulin resistance, hyperglycemia, and dyslipidemia. The relationship between RV dysfunction and fatty acid/glucose metabolites is multifaceted, and in PAH it is characterized by a shift in utilization of energy sources toward increased glucose utilization and reduced fatty acid consumption. RV dysfunction may be caused by maladaptive fatty acid metabolism resulting from an increase in fatty acid uptake by fatty acid transporter molecule CD36 and an imbalance between glucose and fatty acid oxidation in mitochondria. This leads to lipid accumulation in the form of triglycerides, diacylglycerol, and ceramides in the cytoplasm, hallmarks of lipotoxicity. Current interventions in animal models focus on improving RV dysfunction through altering fatty acid oxidation rates and limiting lipid accumulation, but more specific and effective therapies may be available in the coming years based on current research. In conclusion, a deeper understanding of the complex mechanisms of the metabolic remodeling of the RV will aid in the development of targeted treatments for RV failure in PAH.

The role of PPARδ signaling in the cardiovascular system

Peroxisome proliferator-activated receptors (PPARα, β/δ, and γ), members of the nuclear receptor transcription factor superfamily, play important roles in the regulation of metabolism, inflammation, and cell differentiation. All three PPAR subtypes are expressed in the cardiovascular system with various expression patterns. Among the three PPAR subtypes, PPARδ is the least studied but has arisen as a potential therapeutic target for cardiovascular and many other diseases. It is known that PPARδ is ubiquitously expressed and abundantly expressed in cardiomyocytes. Accumulated evidence illustrates the role of PPARδ in regulating cardiovascular function and determining pathological progression. In this chapter, we will discuss the current knowledge in the role of PPARδ in the cardiovascular system, the mechanistic insights, and the potential therapeutic utilization for treating cardiovascular disease.

Enalaprilat increases PPARβ/δ expression, without influence on PPARα and PPARγ, and modulate cardiac function in sub-acute model of daunorubicin-induced cardiomyopathy

Anthracycline therapy is limited by a cardiotoxicity that may eventually lead to chronic heart failure which is thought to be prevented by ACE inhibitors (ACEi). However, the protective effect of ACEi in early stages of this specific injury remains elusive. Activated nuclear transcription factors peroxisome proliferator-activated receptors (PPAR) regulate cellular metabolism, but their involvement in anthracycline cardiomyopathy has not been investigated yet. For this purpose, Wistar rats were administered with daunorubicin (i.p., 3 mg/kg, in 48 h intervals) or co-administered with daunorubicine and enalaprilat (i.p., 5 mg/kg in 12 h intervals). Control animals received vehicle. Left ventricular function was measured invasively under anesthesia. Cell-shortening was measured by videomicroscopy in isolated cardiomyocytes. Expression of PPARs mRNA in cardiac tissue was measured by Real-Time PCR. Although the hemodynamic parameters of daunorubicin-treated rats remained altered upon ACEi co-administration, ACEi normalized daunorubicin-induced QT prolongation. On cellular level, ACEi normalized altered basal and isoproterenol-stimulated cardiac cell shortening in daunorubicine-treated group. Moreover, anthracycline administration significantly up-regulated heart PPARα mRNA and its expression remained increased after ACEi co-administration. On the other hand, the expression of cardiac PPARβ/δ was not altered in anthracycline-treated animals, whereas co-administration of ACEi increased its expression. Conclusively, effect of ACEi can be already detected in sub-acute phase of anthracycline-induced cardiotoxicity. Altered expression of heart PPARs may suggest these nuclear receptors as a novel target in anthracycline cardiomyopathy.

Peroxisome-proliferator-activated receptors regulate redox signaling in the cardiovascular system

Peroxisome-proliferator-activated receptors (PPARs) comprise three subtypes (PPARα, δ and γ) to form a nuclear receptor superfamily. PPARs act as key transcriptional regulators of lipid metabolism, mitochondrial biogenesis, and anti-oxidant defense. While their roles in regulating lipid metabolism have been well established, the role of PPARs in regulating redox activity remains incompletely understood. Since redox activity is an integral part of oxidative metabolism, it is not surprising that changes in PPAR signaling in a specific cell or tissue will lead to alteration of redox state. The effects of PPAR signaling are directly related to PPAR expression, protein activities and PPAR interactions with their coregulators. The three subtypes of PPARs regulate cellular lipid and energy metabolism in most tissues in the body with overlapping and preferential effects on different metabolic steps depending on a specific tissue. Adding to the complexity, specific ligands of each PPAR subtype may also display different potencies and specificities of their role on regulating the redox pathways. Moreover, the intensity and extension of redox regulation by each PPAR subtype are varied depending on different tissues and cell types. Both beneficial and adverse effects of PPAR ligands against cardiovascular disorders have been extensively studied by many groups. The purpose of the review is to summarize the effects of each PPAR on regulating redox and the underlying mechanisms, as well as to discuss the implications in the cardiovascular system.

Dynamic microRNA expression during the transition from right ventricular hypertrophy to failure

MicroRNAs (miRs) are small, noncoding RNAs that are emerging as crucial regulators of cardiac remodeling in left ventricular hypertrophy (LVH) and failure (LVF). However, there are no data on their role in right ventricular hypertrophy (RVH) and failure (RVF). This is a critical question given that the RV is uniquely at risk in patients with congenital right-sided obstructive lesions and in those with systemic RVs. We have developed a murine model of RVH and RVF using pulmonary artery constriction (PAC). miR microarray analysis of RV from PAC vs. control demonstrates altered miR expression with gene targets associated with cardiomyocyte survival and growth during hypertrophy (miR 199a-3p) and reactivation of the fetal gene program during heart failure (miR-208b). The transition from hypertrophy to heart failure is characterized by apoptosis and fibrosis (miRs-34, 21, 1). Most are similar to LVH/LVF. However, there are several key differences between RV and LV: four miRs (34a, 28, 148a, and 93) were upregulated in RVH/RVF that are downregulated or unchanged in LVH/LVF. Furthermore, there is a corresponding downregulation of their putative target genes involving cell survival, proliferation, metabolism, extracellular matrix turnover, and impaired proteosomal function. The current study demonstrates, for the first time, alterations in miRs during the process of RV remodeling and the gene regulatory pathways leading to RVH and RVF. Many of these alterations are similar to those in the afterload-stressed LV. miRs differentially regulated between the RV and LV may contribute to the RVs increased susceptibility to heart failure.

Dietary fat and heart failure: moving from lipotoxicity to lipoprotection

There is growing evidence suggesting that dietary fat intake affects the development and progression of heart failure. Studies in rodents show that in the absence of obesity, replacing refined carbohydrate with fat can attenuate or prevent ventricular expansion and contractile dysfunction in response to hypertension, infarction, or genetic cardiomyopathy. Relatively low intake of n-3 polyunsaturated fatty acids from marine sources alters cardiac membrane phospholipid fatty acid composition, decreases the onset of new heart failure, and slows the progression of established heart failure. This effect is associated with decreased inflammation and improved resistance to mitochondrial permeability transition. High intake of saturated, monounsaturated, or n-6 polyunsaturated fatty acids has also shown beneficial effects in rodent studies. The underlying mechanisms are complex, and a more thorough understanding is needed of the effects on cardiac phospholipids, lipid metabolites, and metabolic flux in the normal and failing heart. In summary, manipulation of dietary fat intake shows promise in the prevention and treatment of heart failure. Clinical studies generally support high intake of n-3 polyunsaturated fatty acids from marine sources to prevent and treat heart failure. Additional clinical and animals studies are needed to determine the optimal diet in terms of saturated, monounsaturated, and n-6 polyunsaturated fatty acids intake for this vulnerable patient population.

Mitochondrial protein phosphorylation as a regulatory modality: implications for mitochondrial dysfunction in heart failure

Phosphorylation of mitochondrial proteins has been recognized for decades, and the regulation of pyruvate- and branched-chain α-ketoacid dehydrogenases by an atypical kinase/phosphatase cascade is well established. More recently, the development of new mass spectrometry-based technologies has led to the discovery of many novel phosphorylation sites on a variety of mitochondrial targets. The evidence suggests that the major classes of kinase and several phosphatases may be present at the mitochondrial outer membrane, intermembrane space, inner membrane, and matrix, but many questions remain to be answered as to the location, timing, and reversibility of these phosphorylation events and whether they are functionally relevant. The authors review phosphorylation as a mitochondrial regulatory strategy and highlight its possible role in the pathophysiology of cardiac hypertrophy and failure.

Chronic and acute exposure of mouse hearts to fatty acids increases oxygen cost of excitation-contraction coupling

The aim of the present study was to evaluate the underlying processes involved in the oxygen wasting induced by inotropic drugs and acute and chronic elevation of fatty acid (FA) supply, using unloaded perfused mouse hearts from normal and type 2 diabetic ( db/db) mice. We found that an acute elevation of the FA supply in normal hearts, as well as a chronic (in vivo) exposure to elevated FA as in db/db hearts, increased myocardial oxygen consumption (MV̇o2unloaded) due to increased oxygen cost for basal metabolism and for excitation-contraction (EC) coupling. Isoproterenol stimulation, on top of a high FA supply, led to an additive increase in MV̇o2unloaded, because of a further increase in oxygen cost for EC coupling. In db/db hearts, the acute elevation of FA did not further increase MV̇o2. Since the elevation in the FA supply is accompanied by increased rates of myocardial FA oxidation, the present study compared MV̇o2 following increased FA load versus FA oxidation rate by exposing normal hearts to normal and high FA concentration (NF and HF, respectively) and to compounds that either stimulate (GW-610742) or inhibit [dichloroacetate (DCA)] FA oxidation. While HF and NF + GW-610742 increased FA oxidation to the same extent, only HF increased MV̇o2unloaded. Although DCA counteracted the HF-induced increase in FA oxidation, DCA did not reduce MV̇o2unloaded. Thus, in normal hearts, acute FA-induced oxygen waste is 1) due to an increase in the oxygen cost for both basal metabolism and EC coupling and 2) not dependent on the myocardial FA oxidation rate per se, but on processes initiated by the presence of FAs. In diabetic hearts, chronic exposure to elevated circulating FAs leads to adaptations that afford protection against the detrimental effect of an acute FA load, suggesting different underlying mechanisms behind the increased MV̇o2 following acute and chronic FA load.

PPARs as therapeutic targets in cardiovascular disease

IMPORTANCE OF THE FIELD

The role of peroxisome proliferator-activated receptors PPARα, PPARδ and PPARγ in cardiovascular disease is receiving widespread attention. As ligand-activated nuclear receptors, they play a role in regulation of lipid and glucose metabolism. This feature of the PPARs has been successfully exploited to treat systemic metabolic diseases, like hyperlipidemia and type-2 diabetes. Indirectly, their lipid lowering effect also leads to a reduction of the risk for cardiovascular diseases, primarily atherosclerosis.

AREAS COVERED IN THIS REVIEW

The pleiotropic effects of each of the PPAR isotypes on vascular and cardiac disease are discussed, with special emphasis on the molecular mechanism of action and on preclinical observations. The mechanism underlying the beneficial effect of PPARs is not confined to whole body metabolism, but also includes modulation of other vital processes, such as inflammation and cell fate (proliferation, differentiation, apoptosis).

WHAT THE READER WILL GAIN

A large body of preclinical studies indicates that, in addition to their effect on atherogenesis, PPAR ligands also impact on ischemic heart disease and the development of cardiac failure. It remains to be established to what extent these intriguing observations can be translated into clinical practice.

TAKE HOME MESSAGE

The versatile mechanism of action extends the potential therapeutic profile of the PPARs enormously. Conversely, this versatility makes it harder to attain a specific therapeutic effect, without increasing the risk of undesirable side effects. The future challenge will be to design PPAR-based therapeutic strategies that minimize the detrimental side effects.

Transcriptional profiling reveals divergent roles of PPARα and PPARβ/δ in regulation of gene expression in mouse liver

Little is known about the role of the transcription factor peroxisome proliferator-activated receptor (PPAR) β/δ in liver. Here we set out to better elucidate the function of PPARβ/δ in liver by comparing the effect of PPARα and PPARβ/δ deletion using whole genome transcriptional profiling and analysis of plasma and liver metabolites. In fed state, the number of genes altered by PPARα and PPARβ/δ deletion was similar, whereas in fasted state the effect of PPARα deletion was much more pronounced, consistent with the pattern of gene expression of PPARα and PPARβ/δ. Minor overlap was found between PPARα- and PPARβ/δ-dependent gene regulation in liver. Pathways upregulated by PPARβ/δ deletion were connected to innate immunity and inflammation. Pathways downregulated by PPARβ/δ deletion included lipoprotein metabolism and various pathways related to glucose utilization, which correlated with elevated plasma glucose and triglycerides and reduced plasma cholesterol in PPARβ/δ−/− mice. Downregulated genes that may underlie these metabolic alterations included Pklr, Fbp1, Apoa4, Vldlr, Lipg, and Pcsk9, which may represent novel PPARβ/δ target genes. In contrast to PPARα−/− mice, no changes in plasma free fatty acid, plasma β-hydroxybutyrate, liver triglycerides, and liver glycogen were observed in PPARβ/δ−/− mice. Our data indicate that PPARβ/δ governs glucose utilization and lipoprotein metabolism and has an important anti-inflammatory role in liver. Overall, our analysis reveals divergent roles of PPARα and PPARβ/δ in regulation of gene expression in mouse liver.

Myocardial fatty acid metabolism in health and disease

There is a constant high demand for energy to sustain the continuous contractile activity of the heart, which is met primarily by the beta-oxidation of long-chain fatty acids. The control of fatty acid beta-oxidation is complex and is aimed at ensuring that the supply and oxidation of the fatty acids is sufficient to meet the energy demands of the heart. The metabolism of fatty acids via beta-oxidation is not regulated in isolation; rather, it occurs in response to alterations in contractile work, the presence of competing substrates (i.e., glucose, lactate, ketones, amino acids), changes in hormonal milieu, and limitations in oxygen supply. Alterations in fatty acid metabolism can contribute to cardiac pathology. For instance, the excessive uptake and beta-oxidation of fatty acids in obesity and diabetes can compromise cardiac function. Furthermore, alterations in fatty acid beta-oxidation both during and after ischemia and in the failing heart can also contribute to cardiac pathology. This paper reviews the regulation of myocardial fatty acid beta-oxidation and how alterations in fatty acid beta-oxidation can contribute to heart disease. The implications of inhibiting fatty acid beta-oxidation as a potential novel therapeutic approach for the treatment of various forms of heart disease are also discussed.

PPARs and the cardiovascular system

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone-receptor superfamily. Originally cloned in 1990, PPARs were found to be mediators of pharmacologic agents that induce hepatocyte peroxisome proliferation. PPARs also are expressed in cells of the cardiovascular system. PPAR gamma appears to be highly expressed during atherosclerotic lesion formation, suggesting that increased PPAR gamma expression may be a vascular compensatory response. Also, ligand-activated PPAR gamma decreases the inflammatory response in cardiovascular cells, particularly in endothelial cells. PPAR alpha, similar to PPAR gamma, also has pleiotropic effects in the cardiovascular system, including antiinflammatory and antiatherosclerotic properties. PPAR alpha activation inhibits vascular smooth muscle proinflammatory responses, attenuating the development of atherosclerosis. However, PPAR delta overexpression may lead to elevated macrophage inflammation and atherosclerosis. Conversely, PPAR delta ligands are shown to attenuate the pathogenesis of atherosclerosis by improving endothelial cell proliferation and survival while decreasing endothelial cell inflammation and vascular smooth muscle cell proliferation. Furthermore, the administration of PPAR ligands in the form of TZDs and fibrates has been disappointing in terms of markedly reducing cardiovascular events in the clinical setting. Therefore, a better understanding of PPAR-dependent and -independent signaling will provide the foundation for future research on the role of PPARs in human cardiovascular biology.

Nuclear receptors, mitochondria and lipid metabolism

Lipid metabolism is a continuum from emulsification and uptake of lipids in the intestine to cellular uptake and transport to compartments such as mitochondria. Whether fats are shuttled into lipid droplets in adipose tissue or oxidized in mitochondria and peroxisomes depends on metabolic substrate availability, energy balance and endocrine signaling of the organism. Several members of the nuclear hormone receptor superfamily are lipid-sensing factors that affect all aspects of lipid metabolism. The physiologic actions of glandular hormones (e.g. thyroid, mineralocorticoid and glucocorticoid), vitamins (e.g. vitamins A and D) and reproductive hormones (e.g. progesterone, estrogen and testosterone) and their cognate receptors are well established. The peroxisome-proliferator activated receptors (PPARs) and liver X receptors (LXRs), acting in concert with PPARgamma Coactivator 1alpha (PGC-1alpha), have been shown to regulate insulin sensitivity and lipid handling. These receptors are the focus of intense pharmacologic studies to expand the armamentarium of small molecule ligands to treat diabetes and the metabolic syndrome (hypertension, insulin resistance, hyperglycemia, dyslipidemia and obesity). Recently, additional partners of PGC-1alpha have moved to the forefront of metabolic research, the estrogen-related receptors (ERRs). Although no endogenous ligands for these receptors have been identified, phenotypic analyses of knockout mouse models demonstrate an important role for these molecules in substrate sensing and handling as well as mitochondrial function.

Inflammatory pathways are activated during cardiomyocyte hypertrophy and attenuated by peroxisome proliferator-activated receptors PPARalpha and PPARdelta

Accumulating evidence indicates an important role for inflammation in cardiac hypertrophy and failure. Peroxisome proliferator-activated receptors (PPARs) have been reported to attenuate inflammatory signaling pathways and, as such, may interfere with cardiac remodeling. Accordingly, the objectives of the present study were to explore the relationship between cardiomyocyte hypertrophy and inflammation and to investigate whether PPARalpha and PPARdelta are able to inhibit NF-kappaB activation and, consequently, the hypertrophic growth response of neonatal rat cardiomyocytes (NCM). mRNA levels of markers of both hypertrophy and inflammation were increased following treatment with the pro-hypertrophic factor phenylephrine (PE) or the chemokine TNF-alpha. Induction of inflammatory genes was found to be fast (within 2 h after stimulation) and transient, while induction of hypertrophic marker genes was more gradual (peaking at 24-48 h). Inflammatory and hypertrophic pathways appeared to converge on NF-kappaB as both PE and TNF-alpha increased NF-kappaB binding activity as measured by electrophoretic mobility shift assay. Following transient transfection, the p65-induced transcriptional activation of a NF-kappaB reporter construct was significantly blunted after co-transfection of PPARalpha or PPARdelta in the presence of their respective ligands. Finally, adenoviral overexpression of PPARalpha and PPARdelta markedly attenuated cell enlargement and the expression of hypertrophic marker genes in PE-stimulated NCM. The collective findings reveal a close relationship between hypertrophic and inflammatory signaling pathways in the cardiomyocyte. It was shown that both PPARalpha and PPARdelta are able to mitigate cardiomyocyte hypertrophy in vitro by inhibiting NF-kappaB activation.

Muscle unloading-induced metabolic remodeling is associated with acute alterations in PPARδ and UCP-3 expression

A number of physiological changes follow prolonged skeletal muscle unloading as occurs in spaceflight, bed rest, and hindlimb suspension (HLS) and also in aging. These include muscle atrophy, fiber type switching, and loss of the ability to switch between lipid and glucose usage, or metabolic inflexibility. The signaling and genomic events that precede these physiological manifestations have not been investigated in detail, particularly in regard to loss of metabolic flexibility. Here we used gene arrays to determine the effects of 24-h HLS on metabolic remodeling in mouse muscle. Acute unloading resulted in differential expression of a number of transcripts in soleus and gastrocnemius muscle, including many involved in lipid and glucose metabolism. These include the peroxisome proliferator-activated receptors (PPARs). In contrast to Ppar-α and Ppar-γ, which were downregulated by acute HLS, Ppar-δ was upregulated concomitant with increased expression of its downstream target, uncoupling protein-3 ( Ucp-3). However, differential expression of Ppar-δ was both acute and transient in nature, suggesting that regulation of PPARδ may represent an adaptive, compensatory response aimed at regulating fuel utilization and maintaining metabolic flexibility.

Decrease of peroxisome proliferator-activated receptor delta expression in cardiomyopathy of streptozotocin-induced diabetic rats

AIMS

The role of peroxisome proliferator-activated receptor delta (PPARdelta) in the development of cardiomyopathy, which is widely observed in diabetic disorders, is likely because cardiomyocyte-restricted PPARdelta deletion causes cardiac hypertrophy. Thus, we investigated the effect of hyperglycaemia-induced oxidative stress on the expression of cardiac PPARdelta both in vivo and in vitro.

METHODS AND RESULTS

We used male Wistar rats to examine the effect of hyperglycaemia on PPARdelta expression in streptozotocin-induced diabetic rats, primary neonatal rat cardiomyocytes, and H9c2 embryonic rat cardiomyocytes. PPARdelta mRNA (messenger ribonucleic acid) and protein levels were measured using northern and western blotting, respectively. The lipid deposition within the heart section was assessed by oil red O staining. The formation of reactive oxygen species (ROS) and changes in morphology, protein synthesis, and alpha-actinin content in hyperglycaemic cells were also examined. Inhibitors of ROS production or mitogen-activated protein kinase (MAPK) activation were employed to investigate the possible mechanisms. Cardiomyopathy induced in streptozotocin-diabetic rats was associated with a marked decrease in cardiac PPARdelta expression. Also, ROS production, cell size, and protein synthesis were increased while PPARdelta expression was reduced in cells exposed to hyperglycaemia in vitro. However, these glucose-induced changes were abolished in the presence of tiron or PD98059 (MEK/ERK inhibitor).

CONCLUSION

Our results suggest that inhibitors of ROS production or MAPK activation are involved in reduction of cardiac PPARdelta expression in response to hyperglycaemia.

Selective PPARdelta agonist treatment increases skeletal muscle lipid metabolism without altering mitochondrial energy coupling: an in vivo magnetic resonance spectroscopy study

Peroxisome proliferator-activated receptor-delta (PPARdelta) activation results in upregulation of genes associated with skeletal muscle fatty acid oxidation and mitochondrial uncoupling. However, direct, noninvasive assessment of lipid metabolism and mitochondrial energy coupling in skeletal muscle following PPARdelta stimulation has not been examined. Therefore, in this study we examined the response of a selective PPARdelta agonist (GW610742X at 5 or 100 mg.kg(-1).day(-1) for 8 days) on skeletal-muscle lipid metabolism and mitochondrial coupling efficiency in rats by using in vivo magnetic resonance spectroscopy (MRS). There was a decrease in the intramyocellular lipid-to-total creatine ratio as assessed by in vivo (1)H-MRS in soleus and tibialis anterior muscles by day 7 (reduced by 49 and 46%, respectively; P < 0.01) at the high dose. Following the (1)H-MRS experiment (day 8), [1-(13)C]glucose was administered to conscious rats to assess metabolism in the soleus muscle. The relative fat-vs.-carbohydrate oxidation rate increased in a dose-dependent manner (increased by 52 and 93% in the 5 and 100 mg.kg(-1).day(-1) groups, respectively; P < 0.05). In separate experiments where mitochondrial coupling was assessed in vivo (day 7), (31)P-MRS was used to measure hindlimb ATP synthesis and (13)C-MRS was used to measure the hindlimb tricarboxylic acid cycle flux (V(tca)). There was no alteration, at either dose, in mitochondrial coupling efficiency measured as the ratio of unidirectional ATP synthesis flux to V(tca). Soleus muscle GLUT4 expression was decreased by twofold, whereas pyruvate dehydrogenase kinase 4, carnitine palmitoyl transferase 1a, and uncoupling protein 2 and 3 expression was increased by two- to threefold at the high dose (P < 0.05). In summary, these are the first noninvasive measurements illustrating a selective PPARdelta-mediated decrease in muscle lipid content that was consistent with a shift in metabolic substrate utilization from carbohydrate to lipid. However, the mitochondrial-energy coupling efficiency was not altered in the presence of increased uncoupling protein expression.