Peroxisome proliferator-activated receptor alpha and hypertensive heart disease

Peroxisome Proliferator–Activated Receptor-α: A Pivotal Regulator of the Gastrointestinal Tract

Peroxisome proliferator–activated receptor (PPAR)-α is a ligand-activated transcription factor distributed in various tissues and cells. It regulates lipid metabolism and plays vital roles in the pathology of the cardiovascular system. However, its roles in the gastrointestinal tract (GIT) are relatively less known. In this review, after summarizing the expression profile of PPAR-α in the GIT, we analyzed its functions in the GIT, including physiological control of the lipid metabolism and pathologic mediation in the progress of inflammation. The mechanism of this regulation could be achieved via interactions with gut microbes and further impact the maintenance of body circadian rhythms and the secretion of nitric oxide. These are also targets of PPAR-α and are well-described in this review. In addition, we also highlighted the potential use of PPAR-α in treating GIT diseases and the inadequacy of clinical trials in this field.

Inhibition of the canonical Wnt signaling pathway by a β-catenin/CBP inhibitor prevents heart failure by ameliorating cardiac hypertrophy and fibrosis

In heart failure (HF) caused by hypertension, the myocyte size increases, and the cardiac wall thickens. A low-molecular-weight compound called ICG001 impedes β-catenin-mediated gene transcription, thereby protecting both the heart and kidney. However, the HF-preventive mechanisms of ICG001 remain unclear. Hence, we investigated how ICG001 can prevent cardiac hypertrophy and fibrosis induced by transverse aortic constriction (TAC). Four weeks after TAC, ICG001 attenuated cardiac hypertrophy and fibrosis in the left ventricular wall. The TAC mice treated with ICG001 showed a decrease in the following: mRNA expression of brain natriuretic peptide (Bnp), Klf5, fibronectin, β-MHC, and β-catenin, number of cells expressing the macrophage marker CD68 shown in immunohistochemistry, and macrophage accumulation shown in flow cytometry. Moreover, ICG001 may mediate the substrates in the glycolysis pathway and the distinct alteration of oxidative stress during cardiac hypertrophy and HF. In conclusion, ICG001 is a potential drug that may prevent cardiac hypertrophy and fibrosis by regulating KLF5, immune activation, and the Wnt/β-catenin signaling pathway and inhibiting the inflammatory response involving macrophages.

Peroxisome Proliferator-Activated Receptor α Reduces Endothelin-1-Caused Cardiomyocyte Hypertrophy by Inhibiting Nuclear Factor-κB and Adiponectin

Peroxisome proliferator-activated receptor α (PPARα) plays a role in the pathogenesis of cardiac hypertrophy, although its underlying mechanism remains unclear. The purpose of this study was to evaluate the effect of PPARα activation on endothelin-1- (ET-1-) caused cardiomyocyte hypertrophy and explore its underlying mechanisms. Human cardiomyocytes (HCMs) were cultured with or without ET-1, whereafter the inhibitory effects of fenofibrate, a PPARα activator, on cell size and adiponectin protein were tested. We examined the activation of extracellular signal-regulated kinase (ERK) and p38 proteins caused by ET-1 and the inhibition of the ERK and p38 pathways on ET-1-induced cell size and adiponectin expression. Moreover, we investigated the interaction of PPARα with adiponectin and nuclear factor-κB (NF-κB) by electrophoretic mobility shift assays and coimmunoprecipitation. ET-1 treatment significantly increased cell size, suppressed PPARα expression, and enhanced the expression of adiponectin. Pretreatment with fenofibrate inhibited the increase in cell size and enhancement of adiponectin expression. ET-1 significantly activated the ERK and p38 pathways, whereas PD98059 and SB205380, respectively, inhibited them. Our results suggest that activated PPARα can decrease activation of adiponectin and NF-κB and inhibit ET-1-induced cardiomyocyte hypertrophy.

Effects of short-chain acyl-CoA dehydrogenase on cardiomyocyte apoptosis

Short-chain acyl-CoA dehydrogenase (SCAD), a key enzyme of fatty acid β-oxidation, plays an important role in cardiac hypertrophy. However, its effect on the cardiomyocyte apoptosis remains unknown. We aimed to determine the role of SCAD in tert-butyl hydroperoxide (tBHP)-induced cardiomyocyte apoptosis. The mRNA and protein expression of SCAD were significantly down-regulated in the cardiomyocyte apoptosis model. Inhibition of SCAD with siRNA-1186 significantly decreased SCAD expression, enzyme activity and ATP content, but obviously increased the content of free fatty acids. Meanwhile, SCAD siRNA treatment triggered the same apoptosis as cardiomyocytes treated with tBHP, such as the increase in cell apoptotic rate, the activation of caspase3 and the decrease in the Bcl-2/Bax ratio, which showed that SCAD may play an important role in primary cardiomyocyte apoptosis. The changes of phosphonate AMP-activated protein kinase α (p-AMPKα) and Peroxisome proliferator-activated receptor α (PPARα) in cardiomyocyte apoptosis were consistent with that of SCAD. Furthermore, PPARα activator fenofibrate and AMPKα activator AICAR treatment significantly increased the expression of SCAD and inhibited cardiomyocyte apoptosis. In conclusion, for the first time our findings directly demonstrated that SCAD may be as a new target to prevent cardiomyocyte apoptosis through the AMPK/PPARα/SCAD signal pathways.

The pathological roles of environmental and redox stresses in cardiovascular diseases

Oxidative stress and inflammation are implicated in cardiovascular diseases such as atherosclerosis, reperfusion injury, hypertension, and heart failure. High levels of oxidative stress resulting from increased cardiac generation of reactive oxygen species (ROS) is thought to contribute to contractile and endothelial dysfunction, apoptosis and necrosis of myocytes, and extracellular matrix remodeling in the heart. ROS activate several transcription factors known as redox-regulated transcription factors, and these transcription factors play important roles in the pathophysiology of cardiovascular diseases. This review focuses on the pathological roles of environmental and redox stresses in cardiovascular diseases, especially severe cardiac dysfunction and the transition from compensated hypertrophy to heart failure. The aryl hydrocarbon receptor (AHR) and NF-E2 p45-related factor (Nrf2) are transcription factors involved in the regulation of drug-metabolizing enzymes. AHR has been studied as a receptor for environmental contaminants and as a mediator of chemical toxicity. However, other roles for AHR in cardiac and vascular development have recently been described. Moreover, Nrf2 protects against oxidative stress by increasing the transcription of genes, including those for several antioxidant enzymes. The roles of these transcription factors, AHR and Nrf2 in angiogenesis are also discussed in this review.

PPARα activation inhibits endothelin-1-induced cardiomyocyte hypertrophy by prevention of NFATc4 binding to GATA-4

Peroxisome proliferator-activated receptor alpha (PPARα) has been implicated in the pathogenesis of cardiac hypertrophy, although its mechanism of action remains largely unknown. To determine the effect of PPARα activation on endothelin-1 (ET-1)-induced cardiomyocyte hypertrophy and explore its molecular mechanisms, we evaluated the interaction of PPARα with nuclear factor of activated T-cells c4 (NFATc4) in nuclei of cardiomyocytes from neonatal rats in primary culture. In ET-1-stimulated cardiomyocytes, data from electrophoretic mobility-shift assays (EMSA) and co-immunoprecipitation (co-IP) revealed that fenofibrate (Fen), a PPARα activator, in a concentration-dependent manner, enhanced the association of NFATc4 with PPARα and decreased its interaction with GATA-4, in promoter complexes involved in activation of the rat brain natriuretic peptide (rBNP) gene. Effects of PPARα overexpression were similar to those of its activation by Fen. PPARα depletion by small interfering RNA abolished inhibitory effects of Fen on NFATc4 binding to GATA-4 and the rBNP DNA. Quantitative RT-PCR and confocal microscopy confirmed inhibitory effects of PPARα activation on elevation of rBNP mRNA levels and ET-1-induced cardiomyocyte hypertrophy. Our results suggest that activated PPARα can compete with GATA-4 binding to NFATc4, thereby decreasing transactivation of NFATc4, and interfering with ET-1 induced cardiomyocyte hypertrophy.

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.

Linoleate-rich high-fat diet decreases mortality in hypertensive heart failure rats compared with lard and low-fat diets

Recent studies indicate that high-fat diets may attenuate cardiac hypertrophy and contractile dysfunction in chronic hypertension. However, it is unclear whether consuming a high-fat diet improves prognosis in aged individuals with advanced hypertensive heart disease or the extent to which differences in its fatty acid composition modulate its effects in this setting. In this study, aged spontaneously hypertensive heart failure rats were administered a standard high-carbohydrate diet or high-fat diet (42% of kilocalories) supplemented with high-linoleate safflower oil or lard until death to determine their effects on disease progression and mortality. Both high-fat diets attenuated cardiac hypertrophy, left ventricular chamber dilation, and systolic dysfunction observed in rats consuming the high-carbohydrate diet. However, the lard diet significantly hastened heart failure mortality compared with the high-carbohydrate diet, whereas the linoleate diet significantly delayed mortality. Both high-fat diets elicited changes in the myocardial fatty acid profile, but neither had any effect on thromboxane excretion or blood pressure. The prosurvival effect of the linoleate diet was associated with a greater myocardial content and linoleate-enrichment of cardiolipin, an essential mitochondrial phospholipid known to be deficient in the failing heart. This study demonstrates that, despite having favorable effects on cardiac morphology and function in hypertension, a high-fat diet may accelerate or attenuate mortality in advanced hypertensive heart disease depending on its fatty acid composition. The precise mechanisms responsible for the divergent effects of the lard and linoleate-enriched diets merit further investigation but may involve diet-induced changes in the content and/or composition of cardiolipin in the heart.

The PPAR trio: regulators of myocardial energy metabolism in health and disease

Common causes of heart failure are associated with derangements in myocardial fuel utilization. Evidence is emerging that metabolic abnormalities may contribute to the development and progression of myocardial disease. The peroxisome proliferator-activated receptor (PPAR) family of nuclear receptor transcription factors has been shown to regulate cardiac fuel metabolism at the gene expression level. The three PPAR family members (alpha, beta/delta and gamma) are uniquely suited to serve as transducers of developmental, physiological, and dietary cues that influence cardiac fatty acid and glucose metabolism. This review describes murine PPAR loss- and gain-of-function models that have shed light on the roles of these receptors in regulating myocardial metabolic pathways and have defined key links to disease states including the hypertensive and diabetic heart.

Overexpression of human truncated peroxisome proliferator-activated receptor alpha induces apoptosis in HL-1 cardiomyocytes

AIMS

Our goal was to analyse whether truncated peroxisome proliferator-activated receptor alpha (PPARalpha) overexpression induces apoptosis of cardiomyocytes.

METHODS AND RESULTS

We constructed a recombinant vector of human truncated PPARalpha and a mammalian expression vector to transfect PPARalpha into a line of murine cardiomyocytes designated HL-1. Four hallmarks of apoptosis were measured in these transfected cells: depolarization of mitochondrial membrane, activation of caspase-3, phosphatidylserine (PS) externalization, and DNA fragmentation. Co-transfection with human cyclic adenosine monophosphate response element-binding protein (CREB) and human CREB binding protein (CBP) and analysis of apoptosis regulatory proteins, Bcl-2 and Bax, were also performed in truncated PPARalpha-transfected cells to determine the potential mechanisms by which truncated PPARalpha may influence apoptosis. Progressive depolarization of mitochondrial membrane, activation of caspase-3, PS externalization, DNA fragmentation, and cell death were observed in HL-1 cells upon increasing levels of transfected truncated PPARalpha. The expression of the antiapoptotic protein Bcl-2 decreased in transfected HL-1 cardiomyocytes, whereas no changes in the proapoptotic protein Bax were observed in these cells. Overexpression of CREB plus CBP abolished the inhibitory effect of truncated PPARalpha on Bcl-2 protein.

CONCLUSION

These results demonstrate that human truncated PPARalpha overexpression induces apoptosis in HL-1 cardiomyocytes. In addition, our findings suggest that truncated PPARalpha may induce cardiomyocyte apoptosis through the inhibition of the antiapoptotic protein, Bcl-2. It is proposed that competition with CREB for coactivators like CBP could be involved in this inhibitory effect.

Activation of peroxisome proliferator-activated receptor-alpha prevents glycogen synthase 3beta phosphorylation and inhibits cardiac hypertrophy

Activation of peroxisome proliferator-activated receptor-alpha (PPAR-alpha) has been recently reported to inhibit vascular inflammatory response and prevent cardiac hypertrophy. However, it is unclear how the activation of PPAR-alpha regulates hypertrophic response. In the present study, we found that application of fenofibrate and overexpression of PPAR-alpha inhibited endothelin-1 (ET-1)-induced phosphorylation of protein kinase B (Akt) at Ser473 and glycogen synthase kinase3beta (GSK3beta) at Ser9, and prevented ET-1-induced nuclear translocation of NFATc4 in cardiomyocytes. Moreover, co-immunoprecipitation studies showed that fenofibrate strongly induced the association of nuclear factor of activated T cells (NFATc4) with PPAR-alpha. These results suggest that activation of PPAR-alpha inhibits ET-1-induced cardiac hypertrophy through regulating PI3K/Akt/GSK3beta and NFAT signaling pathways.

Attenuation of cardiac dysfunction by a PPAR-alpha agonist is associated with down-regulation of redox-regulated transcription factors

Peroxisome proliferator-activated receptor-alpha (PPAR-alpha) is a key regulator of lipid and glucose metabolism and is implicated in inflammation. We investigated the effects of the PPAR-alpha activator fenofibrate on, as well as the role of redox-regulated transcription factors, in the development of left ventricular (LV) hypertrophy and heart failure in Dahl salt-sensitive (DS) rats. DS rats were fed a high-salt diet and treated with either fenofibrate (30 or 50 mg/kg per day) or vehicle from 7 weeks of age. Fenofibrate inhibited the development of compensated hypertensive LV hypertrophy, attenuated the LV relaxation abnormality and systolic dysfunction, and improved the survival rate in DS rats. It also prevented a decrease in the ratio of reduced to oxidized glutathione and inhibited up-regulation of the DNA binding activities of the redox-regulated transcription factors NF-kappaB, AP-1, Egr-1, SP1, and Ets-1 induced in the left ventricle by the high-salt diet. Expression of target genes for these transcription factors, including those for adhesion molecules (VCAM-1, ICAM-1), cytokines (MCP-1), growth factors (TGF-beta, PDGF-B), and osteopontin, was also increased by the high-salt diet in a manner sensitive to treatment with fenofibrate. Furthermore, the infiltration of macrophages and T lymphocytes into the left ventricle and the increase in the plasma concentration of C-reactive protein were inhibited by fenofibrate. The PPAR-alpha activator fenofibrate thus attenuated the progression of heart failure and improved the survival rate in this rat model. These effects were associated with inhibition of the inflammatory response and of activation of redox-regulated transcription factors in the left ventricle.

Malonyl-CoA decarboxylase is a major regulator of myocardial fatty acid oxidation

The energy demands of the heart are normally met by oxidation of both glucose and fatty acids. Fatty acid oxidation is limited by the uptake of fatty acyl coenzyme A (CoA) into the mitochondria, a process regulated by carnitine palmitoyltransferase (CPT)1. Malonyl CoA is a potent endogenous inhibitor of CPT1, and therefore plays an integral role in the control of myocardial fatty acid oxidation. Malonyl-CoA decarboxylase (MCD) is responsible for the removal of malonyl CoA and may control myocardial fatty acid oxidation. Indeed, strategies using MCD inhibitors and MCD knockout mice have provided the first evidence for a direct role of MCD in the control of myocardial fatty acid oxidation. Based on these studies, pharmacologic inhibition of MCD has been proposed to be a viable approach for the treatment of ischemic heart disease resulting from a variety of pathologic conditions, including coronary artery diseases, pathologic hypertrophy, and hypertension.

Modulation of Hepatic Inflammatory Risk Markers of Cardiovascular Diseases by PPAR–α Activators

Atherosclerosis is a long-term chronic inflammatory disease associated with increased concentrations of inflammatory hepatic markers, such as CRP and fibrinogen, and of peripheral origin, such as tumor necrosis factor (TNF)-α and interleukin (IL)-6. Peroxisome proliferator-activated receptor (PPAR-)-α is a ligand-activated transcription factor that regulates expression of key genes involved in lipid homeostasis and modulates the inflammatory response both in the vascular wall and the liver. PPAR-α is activated by natural ligands, such as fatty acids, as well as the lipid-lowering fibrates. PPAR-α agonists impact on different steps of atherogenesis: (1) early markers of atherosclerosis, such as vascular wall reactivity, are improved, (2) however, reduced expression of adhesion molecules on the surface of endothelial cells, accompanied by decreased levels of inflammatory cytokines, such as TNF-α, IL-1, and IL-6, leads to a decreased leukocyte recruitment into the arterial wall; (3) in later stages of the atherosclerotic process, PPAR-α agonists may promote plaque stabilization and reduce cardiovascular events, via effects on metalloproteinases, such as MMP9. Moreover, PPAR-α activation by fibrates also impairs proinflammatory cytokine-signaling pathways in the liver resulting in the modulation of the acute phase response reaction via mechanisms independent of changes in lipoprotein levels. Effective coronary artery disease (CAD) prevention requires the use of agents that act beyond low-density lipoprotein cholesterol-lowering. PPAR-α agonists appear to comprehensively address some of the abnormalities of the most common clinical phenotypes of the high CAD risk patient of the 21st century such as in the metabolic syndrome and type 2 diabetes: low high-density lipoprotein cholesterol, high triglycerides, small, dense low-density lipoprotein, and a proinflammatory, procoagulant state.

Regulation of drug transporters by PDZ adaptor proteins and nuclear receptors

Drug transporters have been suggested to be involved in various aspects of pharmacokinetics. Identification and characterization of drug transporters have given us a scientific basis for understanding drug disposition, as well as the molecular mechanisms of drug interaction and inter-individual/inter-species differences. On the other hand, regulatory mechanisms of drug transporters are still poorly understood, and information is limited to induction and down-regulation of drug transporters by various microsomal enzyme inducers. Little is known about the molecular machinery that directly interacts with the drug transporters. As a first step to clarify such molecular mechanisms, recent studies have identified PDZ (PSD-95/Discs-large/ZO-1) domain-containing proteins that directly interact with the so-called PDZ binding motif located at the C-terminus of drug transporters. Some of the PDZ proteins have been suggested to regulate transporters via at least two pathways, i.e. stabilization at the cell-surface and direct modulation of transporter function. Therefore, it is possible that membrane transport of therapeutic agents is not only governed by the drug transporters themselves, but also indirectly by PDZ proteins. The PDZ proteins are classified as a family, the members of which are thought to have distinct, but also redundant physiological roles. The purpose of this review article is to summarize the available knowledge on protein interactions and functional modulation of drug transporters.