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(Circulation Research. 2004;95:568.)
© 2004 American Heart Association, Inc.

Reviews

Nuclear Receptor Signaling and Cardiac Energetics

Janice M. Huss, Daniel P. Kelly

From the Center for Cardiovascular Research and Departments of Medicine (J.M.H., D.P.K.), Molecular Biology and Pharmacology (D.P.K.), and Pediatrics (D.P.K.), Washington University School of Medicine, St Louis, Mo.

Correspondence to Daniel P. Kelly, MD, Center for Cardiovascular Research Washington University School of Medicine, St. Louis, MO 63110. E-mail dkelly{at}im.wustl.edu

This Review is part of a thematic series on Nuclear Receptor Signaling, which includes the following articles:

Nuclear Receptor Signaling and Cardiac Energetics

PPARs and Atherogenesis: Regulators of Gene Expression in Vascular Cells

Nuclear Receptor Signaling in the Control of Cholesterol Homeostasis
Daniel Kelly Guest Editor

	Abstract

Top Abstract Introduction Nuclear Receptors: A Brief... PPARs: Regulators of Cardiac... PGC-1 Family: Inducible NR... Estrogen-Related Receptors:... Altered NR Signaling in... Future Directions References

 
The heart has a tremendous capacity for ATP generation, allowing it to function as an efficient pump throughout the life of the organism. The adult myocardium uses either fatty acid or glucose oxidation as its main energy source. Under normal conditions, the adult heart derives most of its energy through oxidation of fatty acids in mitochondria. However, the myocardium has a remarkable ability to switch between carbohydrate and fat fuel sources so that ATP production is maintained at a constant rate in diverse physiological and dietary conditions. This fuel selection flexibility is important for normal cardiac function. Although cardiac energy conversion capacity and metabolic flux is modulated at many levels, an important mechanism of regulation occurs at the level of gene expression. The expression of genes involved in multiple energy transduction pathways is dynamically regulated in response to developmental, physiological, and pathophysiological cues. This review is focused on gene transcription pathways involved in short- and long-term regulation of myocardial energy metabolism. Much of our knowledge about cardiac metabolic regulation comes from studies focused on mitochondrial fatty acid oxidation. The genes involved in this key energy metabolic pathway are transcriptionally regulated by members of the nuclear receptor superfamily, specifically the fatty acid-activated peroxisome proliferator-activated receptors (PPARs) and the nuclear receptor coactivator, PPAR coactivator-1 (PGC-1). The dynamic regulation of the cardiac PPAR/PGC-1 complex in accordance with physiological and pathophysiological states will be described.

Key Words: mitochondria • fatty acids • peroxisome proliferator-activated receptor • gene regulation • PPAR

	Introduction

Top Abstract Introduction Nuclear Receptors: A Brief... PPARs: Regulators of Cardiac... PGC-1 Family: Inducible NR... Estrogen-Related Receptors:... Altered NR Signaling in... Future Directions References

 
The normal adult heart uses two main energy substrates, fatty acids (FA) and glucose, to generate ATP for cardiac work.^1,2 The major "work" functions of the cardiac myocyte are contraction (pump function), Ca²⁺ uptake into the sarcoplasmic reticulum, and maintenance of the sarcolemmal ion gradients. Under normal conditions, nearly all of the ATP is generated from mitochondrial oxidation of FA and glucose; 2% or less is derived from anaerobic glycolysis. FA oxidation (FAO) supplies 60% to 90% of myocardial ATP in the healthy adult mammalian heart, whereas the balance (10% to 40%) comes from glucose and lactate^3–5 (Figure 1). FAs are catabolized in the mitochondrial matrix by the process of ß-oxidation, whereas pyruvate derived from glucose and lactate is oxidized by the pyruvate dehydrogenase (PDH) complex, localized within the inner mitochondrial membrane. Acetyl-CoA derived from both pathways is oxidized by the tricarboxylic acid (TCA) cycle to generate NADH and FADH₂, which are used by the electron transport chain to establish a proton gradient that when coupled to oxidative phosphorylation drives ATP formation.

View larger version (74K): [in this window] [in a new window]   Figure 1. Pathways involved in cardiac energy metabolism. FA and glucose oxidation are the main cardiac ATP producing pathways. Genes encoding enzymes involved in multiple stages of these metabolic pathways (ie, uptake, esterification, mitochondrial transport, oxidation) are regulated transcriptionally by NRs. The 4 mitochondrial ß-oxidation reactions are (1) Acyl-CoA dehydrogenase, (2) Enoyl-CoA hydratase, (3) 3-hydroxyacyl-CoA dehydrogenase, (4) 3-Ketoacyl-CoA thiolase. Acetyl-CoA produced by FA and glucose oxidation pathways is further oxidized in the TCA cycle to generate NADH and FADH2, which enter the electron transport/oxidative phosphorylation pathway. FA indicates fatty acid; TCA, tricarboxylic acid; FAO, fatty acid oxidation; Cytc, cytochrome c.

Although FAO serves as the primary myocardial ATP-generating pathway, the heart has relatively limited lipid storage capacity. Thus, the pathways of cellular FA uptake and oxidation must be tightly coupled. Uptake of long-chain FAs (LCFAs) into cells is facilitated by the transport proteins, FA transport protein 1 (FATP1), and CD36/FAT.^6,7 LCFA uptake is coupled to esterification by fatty acyl-CoA synthetase (FACS).⁶ The resulting long-chain acyl-CoAs are converted by carnitine palmitoyltransferase I (CPT I) to their carnitine derivatives, which are transported into the mitochondria and enter the ß-oxidation pathway (Figure 1).

Myocardial fuel selection is highly influenced by developmental stage and physiological/pathophysiological conditions (see review²). Much of this regulation is achieved by coordinated changes in the expression of genes involved in cellular FA utilization. For example, a switch in cardiac fuel preference from glucose to FAs occurs during the fetal to newborn transition when O₂ availability and dietary fat content abruptly increase.^8,9 The expression of genes encoding enzymes of FA uptake and mitochondrial ß-oxidation is coordinately increased in parallel with this perinatal energy substrate switch. The critical importance of this metabolic switch in postnatal adaptation is evidenced in children with mutations in the FA ß-oxidation enzyme genes, MCAD and VLCAD, who develop hepatic dysfunction and cardiomyopathy during the postnatal period under conditions of illness or metabolic stress.^10–12 Acquired forms of heart failure are also associated with energy substrate "switches." In pressure- or volume overload-induced hypertrophy, mitochondrial oxidative capacity is reduced and the heart shifts to reliance on glucose metabolism, resembling the fetal metabolic program.^13–16 Conversely, in the uncontrolled diabetic state, because of the combined effects of myocyte insulin resistance and high circulating free FAs, the myocardium uses fats almost exclusively to support ATP synthesis.^17,18 In these disease states, the profile of metabolic gene expression parallels the metabolic shift; thus expression of ß-oxidation genes are downregulated in the hypertrophied heart and induced in the diabetic state.^14,19,20

	Nuclear Receptors: A Brief Primer

Top Abstract Introduction Nuclear Receptors: A Brief... PPARs: Regulators of Cardiac... PGC-1 Family: Inducible NR... Estrogen-Related Receptors:... Altered NR Signaling in... Future Directions References

 
Nuclear receptor (NR) transcription factors are particularly well-suited for regulating the cardiac metabolic gene program. NRs were originally described as ligand-dependent transcription factors, which is the case for roughly half the mammalian NRs.²¹ Ligand-activated receptors include the classical endocrine receptors that respond to steroid hormones or thyroid hormone. In recent years, a number of receptors identified without prior insight to their ligands have been shown to respond to dietary-derived lipid intermediates, including long-chain FAs, oxysterols, and bile acids.^22–24 These receptors are generally involved in feed-forward regulation of pathways involved in the metabolism of these activating ligands. The remaining "orphan" NRs have no identified ligands, although it is likely that modulating ligands will be identified for some of these receptors. Because the heart must adapt to continuously changing energy demands but has limited capacity for storing FAs or glucose, myocardial energy substrate flux must be tightly matched with demand. Ligand-activated NRs are poised to rapidly respond to fluctuating substrate levels. As will be described later, the peroxisome proliferator-activated receptors (PPARs), as FA-activated NRs, are now recognized as key regulators of cardiac FA metabolism. In addition, evidence is emerging that a select group of orphan NRs serve novel roles in regulating cardiac energy metabolism.

NRs have a conserved modular domain structure (Figure 2A). These proteins contain an NH₂-terminal ligand-independent transcriptional activation (AF-1) domain, a conserved zinc-finger DNA binding domain (DBD), and a composite COOH-terminal region that includes the ligand binding domain (LBD) and a conserved ligand-dependent activation function (AF-2). NRs bind to regulatory DNA elements in target genes as homodimers, heterodimers, or in some cases as monomers. Unlike the classic steroid receptors that function as homodimers, many of the NRs involved in nutrient sensing and metabolic regulation heterodimerize with the retinoid X receptor (RXR).²³ NR regulatory DNA elements are composed of variably spaced hexameric half-sites (AGGTCA) arranged as direct, indirect, or everted repeats. Once bound to their specific response element, receptors recruit coactivator proteins often coincident with displacement of corepressor proteins (Figure 2B). For ligand-activated receptors, it is through ligand binding that the LBD adopts a permissive conformation for coactivator interaction. More than 100 coactivator proteins have been identified for NRs, including ATP-dependent chromatin remodeling complexes, histone acetylases, histone methyltransferases, and RNA polymerase II recruiting complexes, that open up the chromatin structure and facilitate binding of basal transcription factors and RNA polymerase II.²⁵ Histone-modifying proteins are often recruited into complexes by adapter proteins, which lack catalytic activity, bound to the NR LBD. One such adapter/coactivator, the PPAR coactivator-1 (PGC-1), serves as a key link between physiological cues and metabolic regulation in heart.

View larger version (33K): [in this window] [in a new window]   Figure 2. NR structure and mechanism of action. A, Members of the NR superfamily share a similar modular structure with functionally distinct domains. NH2-terminal A/B domain mediates ligand-independent transcriptional activation. DNA binding domain (C) dictates specific response element recognition. COOH-terminal E/F domain encompasses the ligand binding domain, which mediates ligand-dependent coactivator interactions. B, Many NRs involved in metabolic regulation, such as PPARs, bind as heterodimers with RXR; others, such as ERR, bind as homodimers. Ligand-activated NR complex recruits coactivator proteins that increase transcriptional activity of the gene. PGC-1 plays a central role in coactivator complexes regulating cardiac energy metabolic genes.

	PPARs: Regulators of Cardiac FA Metabolism

Top Abstract Introduction Nuclear Receptors: A Brief... PPARs: Regulators of Cardiac... PGC-1 Family: Inducible NR... Estrogen-Related Receptors:... Altered NR Signaling in... Future Directions References

 
The peroxisome proliferator-activated receptors (PPAR) are involved in various aspects of lipid metabolism.^26,27 Three PPAR isoforms have been identified: PPAR, PPARß/ (hereafter PPARß), and PPAR. All three PPARs are activated by FAs and bind as obligate heterodimers with RXR to the consensus response element, AGGTCANAGGTCA (direct repeat with a single nucleotide spacing), within the regulatory regions of target genes. Functional specificity among the PPARs is achieved by isoform-specific tissue distribution, ligands, and cofactor interactions.^28,29 PPAR has been characterized as the central regulator of mitochondrial FA catabolism, whereas PPAR is thought to primarily regulate lipid storage. Until recently, the function of PPARß was relatively unexplored. However, several lines of evidence suggest that all three isoforms modulate cardiac energy metabolism.

PPAR
PPAR is thought to be the primary transcriptional regulator of fat metabolism in tissues with high FA oxidation rates, such as heart, liver, kidney, and skeletal muscle.³⁰ Although the endogenous ligand for PPAR has not been identified, PPAR is activated by a number of lipid-derived molecules, including long-chain FAs, eicosanoids, and leukotriene B4.^31,32 The fibrate class of hyperlipidemic drugs, including fenofibrate and gemfibrozil, are synthetic PPAR ligands.³¹ In studies using gain- and loss-of-function strategies, genetically altered mice have shown that PPAR regulates genes involved in virtually every step of cardiac FA utilization including (1) FA uptake, (2) thioesterification to fatty acyl-CoA, (3) transport into the mitochondria, and (4) mitochondrial ß-oxidation (Figure 1 and Table). The gene encoding medium-chain acyl-CoA dehydrogenase (MCAD), which catalyzes the first step in the ß-oxidation pathway, was the first energy metabolic target identified for PPAR.³³ The complex response element (NRRE-1) conferring PPAR responsiveness binds several NRs and dictates the developmental and FA responsiveness of the MCAD gene.³⁴

View this table: [in this window] [in a new window]   Table 1. PPAR-Regulated Genes Involved in Cardiac Energy Metabolism

Definitive evidence for PPAR as a key regulator of cardiac energy metabolism has been provided by the PPAR "knockout" (PPAR^–/–) mouse studies.^35,36 Constitutive expression of genes involved in FA uptake (CD36/FAT, FATP, FACS-1), mitochondrial transport (CPT I, CPT II), and ß-oxidation (MCAD, VLCAD, SCAD, SCHAD, trifunctional protein ) are decreased in hearts of PPAR^–/– mice.^37,38 Correspondingly, myocardial long chain FA uptake and oxidation rates are diminished in PPAR^–/– hearts.^37,39 Despite these metabolic derangements, cardiac function is maintained in unstressed adult animals. However, a fasting stress, which in wild-type mice induces cardiac ß-oxidation enzyme gene expression,^40,41 causes hypoglycemia and hepatic and cardiac triglyceride accumulation in PPAR^–/– mice.⁴⁰ With aging, PPAR^–/– mice develop cardiac fibrosis and myofibrillar fragmentation associated with abnormal mitochondrial ultrastructure.³⁷

Murine PPAR gain-of-function strategies have also provided important evidence for the role of PPAR as a direct regulator of myocardial energy metabolic genes. Cardiac-specific PPAR overexpression (MHC-PPAR mice) activates expression of FA utilization genes.²⁰ Metabolic studies showed that FA uptake and oxidation are increased in MHC-PPAR hearts, whereas glucose utilization is reciprocally decreased.^20,42 Diminished glucose utilization is explained, in part, by well-described inhibitory effects of acetyl-CoA/CoASH, ATP/ADP, and NADH/NAD⁺ ratios, increased during high rates of FA oxidation, on the activity of the PDH complex.^43,44 However, the reciprocal reduction in myocardial glucose oxidation in MHC-PPAR mice also involves gene regulatory effects. Expression of pyruvate dehydrogenase kinase 4 (PDK4), which decreases glucose oxidation through PDH inhibition, is markedly induced in MHC-PPAR hearts. In addition, the glucose transporter, GLUT4, and the glycolytic enzyme phosphofructokinase are downregulated in MHC-PPAR hearts.²⁰ These results suggest that PPAR links circuits involved in reciprocal gene regulatory "crosstalk" between myocardial FA and glucose utilization. Collectively, the results of in vivo studies indicate that PPAR activates expression of genes involved in cardiac FA utilization and mediates dynamic metabolic regulation in response to diverse physiological stimuli allowing the heart to meet energy demands and maintain tight lipid balance.

PPARß: An Emerging Player in the Regulation of Cardiac Energetics
Among the PPAR isoforms, PPARß displays an expression pattern least suggestive of a distinct role in lipid metabolism. It is detectable in numerous cell types within all major organ systems, in contrast to PPAR and PPAR, which are highly expressed in fat utilization or fat storage tissues, respectively.^28,29,45 However, PPARß is activated by FAs and the triglyceride component of VLDL particles implicating this NR in the regulation of lipid metabolism.^46,47 Independent analyses of PPARß^–/– gene deletion have revealed varied phenotypic effects in skin, adipose, placenta formation and brain.^48–50 Notably, one line of PPARß^–/– mice was deficient in brown and white adipose formation, supporting a role for PPARß in lipid homeostasis.⁴⁹

Gain-of-function studies involving PPARß overexpression or treatment with selective PPARß agonists have demonstrated a role for PPARß in regulating expression of FA utilization enzymes and increasing FAO rates in skeletal muscle cells.^51–53 Activation of the PPARß regulatory pathway in vivo has been shown to increase lipid catabolism when ectopically expressed white adipose,⁵⁴ to enrich slow-twitch oxidative fiber in skeletal muscle,⁵⁵ and to increase skeletal muscle FAO and improve serum lipid profiles and insulin sensitivity in several obesity models in mice.^56–58

Recent studies focusing on the role of PPARß in cardiac metabolism have shown that PPARß selective ligands induce expression of mitochondrial FAO enzymes and increase palmitate oxidation rates in neonatal and adult cardiac myocytes as effectively as PPAR-selective ligands.^59,60 Furthermore, PPARß activation rescues expression of FAO enzyme genes that are reduced at baseline in PPAR^–/– cardiomyocytes.^60,61 These results highlight the regulatory overlap between PPAR and PPARß. It is unclear, however, whether PPAR and PPARß regulate discrete, albeit overlapping, sets of genes or superimposed pathways. PPAR and PPARß are not functionally redundant. PPAR^–/– mice are phenotypically normal at baseline, but display myocardial lipid accumulation subacutely when subjected to a metabolic stress, and progressively with age, suggesting that PPARß cannot compensate for PPAR under all physiological circumstances.^37,40,62

PPAR
PPAR is primarily a regulator of lipid storage and is essential for adipose formation.^22,63 Although its expression is adipose tissue-enriched, PPAR is also expressed at low levels in extra-adipose tissues including the vascular wall, skeletal muscle, pancreatic ß-cell, and heart. Thiazolidinediones (TZDs), such as pioglitazone, are PPAR ligands used as insulin sensitizers to treat type II diabetes.⁶⁴ TZDs are thought to mediate their effects largely through PPAR activation in adipose tissue and skeletal muscle. In adipose tissue, PPAR activation promotes glucose uptake and triglyceride synthesis/storage and inhibits lipolysis, which concurrently increase adipose mass and reduce serum glucose and free FAs. In skeletal muscle, the role of PPAR remains controversial. Murine loss-of-function studies have provided conflicting results regarding the direct role of the muscle PPAR pathway in the development of whole body insulin resistance and in insulin-sensitization by TZDs.^65,66

It is generally thought that PPAR modulates cardiac FA utilization through its effects on extra-cardiac tissues. Changes in circulating FAs resulting from PPAR mediated effects on lipid storage will directly modulate PPAR and PPARß activity and affect insulin sensitivity in the heart. Adipose tissue also secretes various signaling factors, such as tumor necrosis factor-, leptin, and adiponectin that affect insulin sensitivity and metabolism in heart and other tissues.^67,68 Direct regulation of cardiac metabolism by PPAR is a subject of considerable debate. PPAR is expressed in the heart at levels far below PPAR and ß, and PPAR ligands do not affect metabolic gene expression or FAO rates in cultured cardiac myocytes.⁵⁹ However, PPAR activation has been shown to inhibit the induction of hypertrophy markers in cardiac myocytes in response hypertrophic stimuli, suggesting that PPAR is functional in these cells. Further studies using cardiac-specific PPAR deletion will be required to determine PPAR regulation of cardiac gene expression is mediated by direct or indirect mechanisms.

	PGC-1 Family: Inducible NR Coactivators and Integrators of Cardiac Metabolic Gene Regulatory Pathways

Top Abstract Introduction Nuclear Receptors: A Brief... PPARs: Regulators of Cardiac... PGC-1 Family: Inducible NR... Estrogen-Related Receptors:... Altered NR Signaling in... Future Directions References

 
PGC-1 was initially identified as a PPAR coactivator, linked to adaptive thermogenesis in brown adipose.⁶⁹ Two structurally related proteins, PGC-1ß and PRC, have subsequently been cloned and shown to also regulate energy metabolic pathways.^70–74 The tissue-specific and inducible nature of PGC-1 expression reflects its role in the dynamic regulation of cellular energy metabolic processes, including mitochondrial biogenesis and oxidation, hepatic gluconeogenesis, and skeletal muscle glucose uptake.^75–78 PGC-1 is selectively expressed in highly oxidative tissues such as heart, skeletal muscle, brown adipose, and liver.⁶⁹ In heart, PGC-1 expression increases sharply at birth coincident with a perinatal shift from glucose metabolism to fat oxidation.⁷⁹ PGC-1 activity and expression levels are induced by cold exposure, fasting, and exercise; stimuli known to promote oxidative metabolism.^79–81 Signaling pathways associated with these stimuli, including p38 MAP kinase, ß-adrenergic/cAMP, nitric oxide, AMP kinase, and Ca²⁺-calmodulin kinase, activate PGC-1, and its downstream target genes either by increasing PGC-1 expression or transactivation function (Figure 3).^82–88

View larger version (27K): [in this window] [in a new window]   Figure 3. PGC-1 transduces cell signals associated with physiologic stimuli to regulate cardiac metabolic genes. Numerous signaling pathways downstream of physiologic stimuli, like fasting and exercise, activate the PGC-1 cascade either by increasing PGC-1 expression or activity. PGC-1, in turn, coactivates transcriptional partners, including nuclear respiratory factor-1 and 2 (NRF-1/2) and PPAR, resulting in downstream activation of mitochondrial biogenesis and fatty acid oxidation pathways, respectively.

PGC-1 mediates its broad metabolic regulatory effects through coactivation of numerous transcription factor partners, including many NRs. Most relevant to the current focus are those linked to cardiac energy metabolism. PGC-1 coactivates PPAR and enhances FA-dependent regulation of PPAR responsive genes involved in the FAO pathway.⁸⁹ PPARß is also a transcriptional partner for PGC-1.⁵³ Consistent with its functional interaction with PPARs, PGC-1 activates expression of genes involved in FA uptake and oxidation when overexpressed in cardiac myocytes.⁷⁹ Recent studies, discussed later, have implicated additional orphan NRs in mediating PGC-1 regulation of cardiac energy metabolism.

In addition to activating FA metabolic pathways via PPARs, PGC-1 has been shown to increase mitochondrial oxidative capacity in multiple cell and tissue models.⁹⁰ PGC-1 overexpression increases mitochondrial number in brown adipocytes and skeletal and cardiac myocytes.^69,79,91 In skeletal myocytes, PGC-1 primarily increases uncoupled respiration associated with induction of uncoupling proteins 2 (UCP2) and 3 (UCP3).⁹¹ In vivo PGC-1 effects on mitochondrial number and respiration causes a skeletal muscle fiber-type switch from fast glycolytic to slow oxidative.⁹² In cardiac myocytes, PGC-1 drives mitochondrial biogenesis, increases FAO and overall mitochondrial oxidative capacity in the form of ATP-generating, coupled respiration.⁷⁹ (J. Huss, unpublished observations, 2004). Global gene expression analyses in cardiac myocytes overexpressing PGC-1 have shown that PGC-1 activates genes encoding enzymes at every level of oxidative energy metabolism, including FA uptake, mitochondrial ß-oxidation, TCA cycle, electron transport, and oxidative phosphorylation⁷⁹ (L. Russell, unpublished observations, 2004). Finally, in an inducible transgenic model to investigate cardiac-specific effects of PGC-1, PGC-1 overexpression triggered robust mitochondrial biogenesis during the neonatal period. In adult mice, PGC-1 expression led to modest mitochondrial proliferation, mitochondrial ultrastructural abnormalities, and reduced myofibrillar density.⁹³ These metabolic and structural changes resulted in dilated cardiomyopathy and diastolic dysfunction. Interestingly, the mitochondrial proliferation was reversible and the cardiomyopathy rescued on reduction in transgene expression. Collectively, these studies suggest that, in addition to serving as an activator of cellular FA metabolism through PPARs, PGC-1 is linked to the mitochondrial biogenic program. Therefore, PGC-1 serves as a master modulator of oxidative energy metabolism, responsive to changes in the cellular energy status.

The role of PGC-1 in driving mitochondrial biogenesis and oxidative metabolism is consistent with the array of transcription factor partners with which PGC-1 functionally interacts.⁹⁴ In addition to NRs, PGC-1 coactivates transcription factors involved in regulating mitochondrial biogenesis and oxidative metabolism, including nuclear respiratory factor-1 (NRF-1) and NRF-2. The NRFs directly regulate downstream genes involved in mitochondrial respiratory function and biogenesis, including mitochondrial transcription factor A (Tfam), which is involved in mitochondrial DNA maintenance, replication, and transcription.^95,96 NRF-1 and NRF-2 also regulate expression of nuclear genes encoding components of mitochondrial electron transport and oxidative phosphorylation.⁹⁷ It is likely that additional PGC-1 partners exist in the mitochondrial biogenic cascade.⁹⁴

	Estrogen-Related Receptors: Emerging Role for Orphan NRs in Regulating Cardiac Energy Metabolism

Top Abstract Introduction Nuclear Receptors: A Brief... PPARs: Regulators of Cardiac... PGC-1 Family: Inducible NR... Estrogen-Related Receptors:... Altered NR Signaling in... Future Directions References

 
Evidence is emerging that the estrogen-related receptor (ERR) family of orphan NRs function as PGC-1-activated regulators of cardiac and skeletal muscle energy metabolism. There are three members of the ERR family: ERR, ERRß, and ERR.^98–100 ERR and ERR expression is enriched in adult tissues that rely primarily on mitochondrial oxidative metabolism for ATP production, such as heart and slow-twitch skeletal muscle.^99,101,102 ERR expression dramatically increases in heart after birth, in parallel with the global upregulation of enzymes involved in cellular FA uptake and mitochondrial oxidation.¹⁰³ Recently, ERR and ERR were identified as novel partners for the PGC-1 family of coactivators.^74,103–105 This functional relationship between ERR isoforms and PGC-1 have stimulated interest in the role of ERRs in energy metabolism.

Deletion of the ERR gene reveals a tissue-specific role for ERR in constitutive regulation of lipid metabolism.¹⁰⁶ White adipose mass is decreased in ERR^–/– mice coincident with decreased adipocyte size and lipid synthesis rates. In contrast, ERR likely plays a role in lipid catabolism in heart, consistent with its functional interaction with PGC-1. ERR^–/– mice, which do not display an overt cardiac phenotype, exhibit a compensatory increase in cardiac PGC-1 and ERR expression (J. Huss, unpublished data, 2004). These results suggest that ERR isoforms contribute to constitutive expression of FA metabolic genes in heart. However, the metabolic effects of the observed gene expression changes remain to be explored.

Gene expression profiling in cardiac myocytes overexpressing ERR have begun to identify cardiac ERR target genes. ERR activates genes involved in energy production pathways, including cellular FA uptake (LPL, CD36/FAT, H-FABP, FACS-1), ß-oxidation (MCAD, VLCAD, LCHAD), and mitochondrial electron transport/oxidative phosphorylation (cytochrome c, COXIV, COXVIII, NADH ubiquinone dehydrogenase, flavoprotein-ubiquinone oxidoreductase, ATP synthase ß). ERR also increases palmitate oxidation rates in cardiac myocytes. Activation of ß-oxidation enzymes genes by ERR involves the PPAR signaling pathway. ERR directly activates PPAR gene expression, and ERR-mediated regulation of MCAD and M-CPT I is abolished in cells derived from PPAR^–/– mice (J. Huss, unpublished data, 2004). Recently, ERR has also been shown to be involved in the PGC-1 regulation of mitochondrial biogenesis.^107,108 ERR was found to mediate PGC-1 activation of the NRF pathway through regulation of the Gapba gene, which encodes a subunit of the NRF-2 complex.¹⁰⁸ ERR also directly activates genes involved in mitochondrial oxidative metabolism at the level of transcription. ERR with its coactivator PGC-1 activates the MCAD,^101–103 cytochrome c, and ATP synthase ß gene promoters.¹⁰⁷ Collectively, these results identify ERR as a regulator of cardiac oxidative energy metabolism through its involvement in the PGC-1 regulatory circuit. Studies using animal models of cardiac-specific ERR and ERR activation or inactivation will be essential to delineate the precise biologic roles of ERRs in heart.

	Altered NR Signaling in the Diseased Heart: Driver or Passenger?

Top Abstract Introduction Nuclear Receptors: A Brief... PPARs: Regulators of Cardiac... PGC-1 Family: Inducible NR... Estrogen-Related Receptors:... Altered NR Signaling in... Future Directions References

 
A central question regarding cardiac metabolism is the role that perturbations in cardiac energy transfer pathways play in the development of pathological cardiac remodeling and heart failure. Inherited and acquired forms of heart failure are associated with an overall decrease in mitochondrial oxidative capacity and a shift away from FA oxidation toward glucose utilization (Figure 4).^19,109,110 It is unclear whether this metabolic shift is a protective response allowing the heart to maintain contractile function or an initial step in progressive decompensation. Conversely, diabetic cardiomyopathy develops in the context of chronically high FAO and inhibited glucose uptake and oxidation. Although this switch in fuel utilization may initially be adaptive, the drive on FA utilization and loss of substrate utilization flexibility may become an etiological component of the disease.^111,112

View larger version (39K): [in this window] [in a new window]   Figure 4. Dynamic regulation of myocardial energy metabolism by developmental, dietary, and pathophysiological changes. During fetal development glucose metabolism is favored, whereas FAs are the primary energy substrate in the adult myocardium. Significant shifts in substrate preference occur in response to dietary (fasting/refeeding) and physiological (exercise) stimuli. Certain pathophysiological contexts, like hypertrophy, hypoxia, and ischemia, drive metabolism toward a fetal-like state, whereas in diabetes, the heart es FAs almost exclusively. These metabolic alterations are accompanied by changes in expression of enzymes involved in FA utilization mediated by NRs (PPARs and ERRs) and the NR coactivator PGC-1.

Altered PPAR Signaling in Pressure Overload Induced-Hypertrophy and Heart Failure
During the progression of pathological cardiac hypertrophy in animal models and humans, the heart undergoes a shift from FAO toward increased glucose utilization (Figure 4).^13,113 This metabolic switch is driven by the coordinated counter-regulation of FAO and glucose-metabolizing enzyme genes^14,19 Evidence implicates deactivation of the PPAR signaling pathway as the mechanism driving downregulation of FAO genes in the hypertrophied heart. Reduced expression and activity of PPAR closely correlates with decreased ß-oxidation observed in cardiac hypertrophy.^114–116 PPAR-dependent activity of the M-CPT I gene promoter is inhibited by the ₁-agonist, phenylephrine, which induces hypertrophic growth and decreases FAO rates in cardiac myocytes.¹¹⁴ Inhibition of the PPAR signaling pathway involves both early and late events. PPAR is rapidly deactivated by extracellular signal-regulated MAP kinase phosphorylation.¹¹⁴ At later stages of hypertrophic growth, transcriptional repressors of FAO genes, such as the orphan NR COUP-TF, are induced and inhibit PPAR activation of FAO target genes.¹¹⁵

Although PPAR activity and cardiac substrate utilization influence the response to hypertrophic stimuli, decreased PPAR activity and FA metabolism are not thought to drive progression of pathological hypertrophy to heart failure. In fact, the metabolic switch in early stages of pathological hypertrophy may be protective. PPAR activation in rats subjected to pressure-overload induced hypertrophy prevents the metabolic switch and results in mild contractile dysfunction.¹¹⁷ PPAR activity also influences hypertrophic growth. The PPAR ligands, fenofibrate and Wy14,463, inhibited cardiac myocyte hypertrophy in response to endothelin.¹¹⁸ Human genetic studies identified a significant association between a PPAR gene polymorphism and development of physiological or pathological left ventricular hypertrophy though did not establish that the anti-hypertrophic effects of PPAR were linked to metabolic changes.¹¹⁹ Similar antihypertrophic action has been observed for other NR ligands, including 1,25 dihydroxyvitamin D and retinoic acid, which act through vitamin D receptor (VDR)/RXR heterodimers.¹²⁰ Finally, recent studies demonstrated that the PPAR ligands, TZDs, inhibited hypertrophic growth in cardiac myocytes in response to mechanical stress and angiotensin II treatment.^121,122 Pioglitazone treatment in mice inhibited pressure-overload induced increases in heart weight, wall thickness, and myocyte size.¹²²

Diabetic Cardiomyopathy
Epidemiological studies have demonstrated that the incidence of cardiomyopathy in diabetics is significantly increased independent of additional risk factors, such as hypertension and vascular disease, present in diabetics.¹²³ Therefore, attention has focused on the role diabetes-related metabolic dysregulation plays in the development of cardiomyopathy.^112,124 In the diabetic state, cardiac energy demands are almost entirely fueled by FAO, a consequence of impaired glucose uptake and metabolism attributable to myocardial insulin resistance and increased circulating FAs.^17,18 Leptin-deficient obese (ob/ob) mice and Zucker fatty rats exhibit myocardial triglyceride accumulation and increased expression of genes involved in lipid uptake and triglyceride synthesis.^125,126 In both models, ventricular mass was increased. Functionally, decreased contractile function observed in Zucker fatty rats was attributed to lipotoxic effects of the accumulated lipid species.¹²⁷ These studies implicate defective PPAR signaling in these models, causing a mismatch between FA uptake and metabolism.¹²⁷ Indeed, the cardiac PPAR signaling pathway is activated in diabetic rodent models, including streptozotocin (STZ)-induced diabetes and the obese db/db mice, as evidenced by increased expression of PGC-1, PPAR, and downstream metabolic target genes.²⁰ Furthermore, PPAR^–/– mice are resistant to cardiomyopathy that develops in STZ-induced diabetic wild-type mice.¹²⁸ The observed induction of cardiac PGC-1 expression in diabetic mice contrasts with the downregulation of PGC-1 and mitochondrial oxidative phosphorylation enzymes reported in insulin resistant skeletal muscle.^129,130

The MHC-PPAR model has demonstrated a direct relationship between the chronic drive on myocardial FA metabolism and the development of cardiomyopathy.²⁰ Cardiac PPAR overexpression replicates the diabetic metabolic profile: myocardial triglyceride accumulation, increased FAO rates, and decreased glucose uptake and metabolism. MHC-PPAR mice develop ventricular hypertrophy and dysfunction that is exacerbated with high fat feeding.¹²⁸ These studies demonstrate the relationship between cardiac metabolic derangement and dysfunction and suggest that PPAR-driven increases in FA uptake and oxidation contribute to diabetic cardiomyopathy. Cardiac dysfunction in MHC-PPAR mice may be caused by toxic effects of myocardial lipid accumulation or free radical damage from chronically high oxidative flux. This model has important implications for human disease because it mimics the cardiac metabolic derangement that is chronic in obesity-related disorders, particularly type II diabetes.

Implications from studies of the MHC-PPAR mice are that activation of PPAR and downstream FA metabolism may contribute to heart failure progression in the diabetic. Thus, PPAR inhibition may be a means to delay or prevent heart failure in diabetic patients. Inhibiting mitochondrial ß-oxidation with 3-ketoacyl-CoA thiolase inhibitors, such as trimetazidine, increased cardiac efficiency in patients with diabetes and ischemic cardiomyopathy by shifting metabolism toward glucose oxidation.^131,132 However, chronic FAO inhibition may also predispose the heart to lipid accumulation and subsequent lipotoxicity. Furthermore, PPAR agonists are used to treat hyperlipidemia, a major risk factor for atherosclerosis. Bezafibrate and gemfibrozil improve serum lipid profiles in type II diabetics, reducing serum triglycerides and increasing HDL levels.^133,134 The VA-HIT study, which investigated the link between diabetes and cardiovascular outcomes, showed that gemfibrozil treatment reversed the increased incidence of major cardiovascular events associated with diabetes and low HDL cholesterol.¹³⁵ The weight of clinical data suggests that systemic activation of the PPAR pathway in diabetics improves cardiovascular outcome likely via extracardiac effects. Clearly, additional studies are necessary to predict the effects of modulating lipid metabolism in the prevention and treatment of cardiomyopathy in the diabetic.

	Future Directions

Top Abstract Introduction Nuclear Receptors: A Brief... PPARs: Regulators of Cardiac... PGC-1 Family: Inducible NR... Estrogen-Related Receptors:... Altered NR Signaling in... Future Directions References

 
This review sought to provide a view of the current state of knowledge regarding the regulation of cardiac metabolism by NRs. As ligand-responsive transcription factors, NRs are well suited to respond to fluctuations in substrates and metabolic intermediates and to regulate pathways involved in their catabolism. Given the explosion of new information about PPARs and the development of isoform-specific PPAR agonists, this group comprises an important potential therapeutic target to modulate cardiac energy metabolism. Several challenges must be addressed to use NR modulators as therapeutic agents for myocardial disease. We must first determine whether NR-driven alterations in cardiac metabolism are adaptive or maladaptive in the hypertrophied or diabetic state. Another important consideration is the lack of tissue specificity of the compounds. Future studies combining tissue-selective genetic mouse models and pathophysiological models of hypertrophy and diabetes will test the utility of existing and novel NR ligands as potential therapies for heart failure.

	Acknowledgments

 
This work was supported by NIH grants R01 DK45416.R01 HL58493, PO1 HL57278, and the Digestive Diseases Core Center Grant P30 DK52574 (D.P.K). J.M.H. is supported by NIH grant K01 DK063051 and the Washington University School of Medicine Diabetes Research Training Center P60 DK20579 (J.M.H.). We thank Laurie K. Russell for allowing discussion of unpublished data. We also thank Mary Wingate for expert assistance in manuscript preparation.

	Footnotes

 
Original received April 16, 2004; revision received July 2, 2004; accepted July 2, 2004.

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Top Abstract Introduction Nuclear Receptors: A Brief... PPARs: Regulators of Cardiac... PGC-1 Family: Inducible NR... Estrogen-Related Receptors:... Altered NR Signaling in... Future Directions References

 

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