Transcriptional control of a nuclear gene encoding a mitochondrial fatty acid oxidation enzyme in transgenic mice: role for nuclear receptors in cardiac and brown adipose expression

Adriamycin downregulates the expression of KLF4 in cardiomyocytes in vitro and contributes to impaired cardiac energy metabolism in Adriamycin-induced cardiomyopathy

Adriamycin is a well-known anthracycline chemotherapeutic agent widely used in treating a variety of malignancies. However, Adriamycin's clinical use is limited due to its adverse side-effects, most importantly cardiomyopathy. Adriamycin-induced cardiotoxicity reportedly includes mitochondrial dysfunction. We hypothesize that modulation of KLF4, a key regulator of cardiac mitochondrial homeostasis might play a role in the development of Adriamycin-induced cardiomyopathy. Therefore, in the current work, we evaluated the interaction of Adriamycin with KLF4 and its subsequent downstream targets. Molecular docking revealed that Adriamycin interacts strongly with KLF4 at residues Thr 448, Arg 452, Ser 444 falls within C2H2 motif which is the active site. Quantitative real-time PCR also revealed that KLF4 is downregulated by Adriamycin in cardiomyocytes in vitro. The expression of KLF4 is downregulated in a dose-dependent manner, with a 0.12 ± 0.09-fold (p ≤ 0.05, n = 3) downregulation at a low dosage and 0.21 ± 0.02-fold (p ≤ 0.05, n = 3) downregulation at high dosage. Deficiency of KLF4 leads to an impairment of PPARγ that consequently supresses the proteins/enzymes involved in the fatty acid metabolism. Adriamycin-mediated suppression of KLF4 also affected the expression of PPARα in vitro. PPARα dysfunction is likely to cause defects in β-oxidation which ultimately results in impaired ATP synthesis. Cardiac cells are thus forced to switch over the substrate from free fatty acid to glucose. Moreover, Adriamycin elevates the expression of PPARβ due to downregulation of KLF4 leads to increased myocardial glucose utilization. Thus, a change in substrate preference affects the flexibility of metabolic network culminating in diminished energy production and other regulatory activities, altogether contributing to the development of cardiomyopathy. Thus, we conclude that the effect of Adriamycin on KLF4 disrupts mitochondrial homeostasis and lipid/glucose homeostasis resulting in a reduction of ATP synthesis which ultimately results in dilated cardiomyopathy.

Conserved transcriptional activity and ligand responsiveness of avian PPARs: Potential role in regulating lipid metabolism in mirgratory birds

Migratory birds undergo metabolic remodeling in tissues, including increased lipid storage in white adipose and fatty acid uptake and oxidation in skeletal muscle, to optimize energy substrate availability and utilization in preparation for long-distance flight. Different tissues undergo gene expression changes in keeping with their specialized functions and driven by tissue specific transcriptional pathways. Peroxisome proliferator-activated receptors (PPARs) are lipid-activated nuclear receptors that regulate metabolic pathways involved in lipid and glucose utilization or storage in mammals. To examine whether PPARs might mediate fatty acid activation of metabolic gene programs that would be relevant during pre-migratory fattening, we used gray catbird as the focal species. PPAR isoforms cloned from catbird share high amino acid identity with mammalian homologs (% vs human): gcPPARα (88.1%), gcPPARδ (87.3%), gcPPARγ (91.2%). We tested whether gcPPARs activated fatty acid (FA) utilization genes using Lpl and Cpt1b gene promoter-luciferase reporters in mammalian cell lines. In C2C12 mouse myocytes gcPPARα was broadly activated by the saturated and unsaturated FAs tested; while gcPPARδ showed highest activation by the mono-unsaturated FA, 18:1 oleic acid (+80%). In CV-1 monkey kidney cells gcPPARγ responded to the poly-unsaturated fatty acid, 20:5 eicosapentaenoic acid (+60%). Moreover, in agreement with their structural conservation, gcPPARs were activated by isoform selective synthetic agonists similar to the respective mammalian isoform. Adenoviral mediated over-expression of PPARα in C2C12 myocytes induced expression of genes involved in fatty acid transport, including Cd36/Fat, as well as Cpt1b, which mediates a key rate limiting step of mitochondrial β-oxidation. These gene expression changes correlated with increased lipid droplet accumulation in C2C12 myoblasts and differentiated myotubes and enhanced β-oxidation in myotubes. Collectively, the data predict that the PPARs play a conserved role in gray catbirds to regulate lipid metabolism in target tissues that undergo metabolic remodeling throughout the annual migratory cycle.

Effects of PPARα/PGC-1α on the energy metabolism remodeling and apoptosis in the doxorubicin induced mice cardiomyocytes in vitro

Dilated cardiomyopathy is the most frequent form of myocardial disease. Many factors contribute to dilated cardiomyopathy, for instance, long-term use of doxorubicin, one of the anthracyclines clinically used for cancer chemotherapy, result in dilated cardiomyopathy and congestive heart failure. However, the mechanism underlining doxorubicin-induced cardiomyocyte is still not fully understood. In this study, we evaluate the effects and their mechanisms of PPARα and PGC-1α pathways in doxorubicin induced mice cardiomyocytes. In vitro, cardiomyocytes isolated from hearts of adult FVB/NJ mice were treated with doxorubicin, GW 6471 (PPARα inhibitors) and WY14643 (PPARα agonists). The expression of PPARα and PGC-1α were detected via western blotting and Quantitative Real-Time PCR methods. Changes in energy and substrate metabolism were analyzed. MTT and flow cytometry were used for cell proliferation and apoptosis analysis. We detected expression of PPARα and PGC-1α was significantly higher in control group than doxorubicin group. Mitochondrial dysfunction was found in doxorubicin group including lower content of high-energy phosphates, significantly decreased mitochondrial ANT transport activity and markedly reduced mitochondrial membrane potential compared with control group. Metabolic remodeling existed in doxorubicin group because of higher concentration of free fatty acid and glucose consumption than of control group. More accumulations of reactive oxygen species were detected in doxorubicin group. The decreased cell viability and increased cell apoptosis observed in doxorubicin group. Severe apoptosis in doxorubicin group was verified by a set of markers including Bax, Bcl-2, cytosolic cytochrome c and caspase-3 up-regulation expression. These findings indicate that the PPARα and PGC-1α are closely involved in energy metabolism remodeling and apoptosis in cardiomyocytes.

Gene expression profiling of murine hepatic steatosis induced by tamoxifen

Tamoxifen is an antiestrogenic agent used widely in the treatment of estrogen receptor-positive breast cancer. However, hepatic steatosis has been reported during clinical trials of tamoxifen. To explore the mechanism responsible for this tamoxifen-induced hepatic steatosis, we used microarray analysis to profile the gene expression pattern of mouse liver after tamoxifen treatment. Tamoxifen was administered orally as a single dose of 10mg/kg (low dose), 50mg/kg (medium dose), or 100mg/kg (high dose) to C57BL/6 mice, and the livers were removed 2h, 4h, 8h, and 24h later. From microarray data obtained from the liver samples, 414 genes were selected as tamoxifen-responsive genes (P<0.05, two-way ANOVA; cutoff ≥ 1.5-fold response). These genes were classified into three groups: 308 of the 414 genes showed a time-dependent response, nine genes showed a dose-dependent response, and 97 genes showed a time- and dose-dependent response. Most of the 308 time-dependent-responsive genes were associated predominantly with the biological processes involved in lipid metabolism. Overrepresented transcription factor binding site analysis showed that the following nuclear receptors that are important in lipid and carbohydrate metabolism were overrepresented: the androgen receptor (AR), nuclear receptor subfamily 2 group F member 1 (NR2F1), hepatocyte nuclear factor 4α (HNF4α), and retinoic acid receptor-related orphan receptor alpha 1 (RORα1). Reporter gene analysis further revealed that tamoxifen repressed the 5α-dihydrotestosterone-induced activation of the AR and the intrinsic transactivation function of RORα1, HNF4α, and NR2F1. Taken together, these data provide a better understanding of the molecular mechanism underlying tamoxifen-induced steatogenic hepatotoxicity and useful information for predicting steatogenic hepatotoxicity.

Hypoxia induces PGC-1α expression and mitochondrial biogenesis in the myocardium of TOF patients

PGC-1alpha, a potent transcriptional coactivator, is the major regulator of mitochondrial biogenesis and activity in the cardiac muscle. The dysregulation of PGC-1alpha and its target genes has been reported to be associated with congenital and acquired heart diseases. By examining myocardium samples from patients with Tetralogy of Fallot, we show here that PGC-1alpha expression levels are markedly increased in patients compared with healthy controls and positively correlated with the severity of cyanosis. Furthermore, hypoxia significantly induced the expression of PGC-1alpha and mitochondrial biogenesis in cultured cardiac myocytes. Mechanistic studies suggest that hypoxia-induced PGC-1alpha expression is regulated through the AMPK signaling pathway. Together, our data indicate that hypoxia can stimulate the expression of PGC-1alpha and mitochondrial biogenesis in the cardiac myocytes, and this process might provide a potential adaptive mechanism for cardiac myocytes to increase ATP output and minimize hypoxic damage to the heart.

Bcl3 interacts cooperatively with peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha to coactivate nuclear receptors estrogen-related receptor alpha and PPARalpha

Estrogen-related receptors (ERRs) play critical roles in regulation of cellular energy metabolism in response to inducible coactivators such as peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha (PGC-1alpha). A yeast two-hybrid screen led to the identification of the cytokine-stimulated transcriptional regulator, Bcl3, as an ERRalpha coactivator. Bcl3 was shown to synergize with PGC-1alpha to coactivate ERRalpha. Chromatin immunoprecipitation studies demonstrated that ERRalpha, PGC-1alpha, and Bcl3 form a complex on an ERRalpha-responsive element within the pyruvate dehydrogenase kinase 4 gene promoter in cardiac myocytes. Mapping studies demonstrated that Bc13 interacts with PGC-1alpha and ERRalpha, allowing for interaction with both proteins. Transcriptional profiling demonstrated that Bcl3 activates genes involved in diverse pathways including a subset involved in cellular energy metabolism known to be regulated by PGC-1alpha, ERRalpha, and a second nuclear receptor, PPARalpha. Consistent with the gene expression profiling results, Bcl3 was shown to synergistically coactivate PPARalpha with PGC-1alpha in a manner similar to ERRalpha. We propose that the cooperativity between Bcl3 and PGC-1alpha may serve as a point of convergence on nuclear receptor targets to direct programs orchestrating inflammatory and energy metabolism responses in heart and other tissues.

Transcriptional control of energy homeostasis by the estrogen-related receptors

Transcriptional control of cellular energy metabolic pathways is achieved by the coordinated action of numerous transcription factors and associated coregulators. Several members of the nuclear receptor superfamily have been shown to play important roles in this process because they can translate hormonal, nutrient, and metabolite signals into specific gene expression networks to satisfy energy demands in response to distinct physiological cues. Estrogen-related receptor (ERR) alpha, ERRbeta, and ERRgamma are nuclear receptors that have yet to be associated with a natural ligand and are thus considered as orphan receptors. However, the transcriptional activity of the ERRs is exquisitely sensitive to the presence of coregulatory proteins known to be essential for the control of energy homeostasis, and for all intents and purposes, these coregulators function as protein ligands for the ERRs. In particular, functional genomics and biochemical studies have shown that ERRalpha and ERRgamma operate as the primary conduits for the activity of members of the family of PGC-1 coactivators. As transcription factors, the ERRs control vast gene networks involved in all aspects of energy homeostasis, including fat and glucose metabolism as well as mitochondrial biogenesis and function. Phenotypic analyses of knockout mouse models have shown that all three ERRs are indispensable for proper development and/or survival of the organism when subjected to a variety of physiological challenges. The focus of this review is on the recent and rapid advances in understanding the functions of the ERRs in regulating bioenergetic pathways, with an emphasis on their roles in the specification of energetic properties required for cell- and tissue-specific functions.

Metabolic reserve of the heart: the forgotten link between contraction and coronary flow

Myocardial energy substrate metabolism entails a complex system of enzyme catalyzed reactions, in which the heart efficiently converts chemical to mechanical energy. The system is highly regulated and responsive to changes in workload as well as in substrate and hormone supply to the heart. Akin to the terms "contractile reserve" and "coronary flow reserve" we propose the term "metabolic reserve" to reflect the heart's capacity to respond to increases in workload. The heart's metabolic response to inotropic stimulation involves the ability to increase oxidative metabolism over a wide range, by activating the oxidation of glycogen and carbohydrate substrates. Here we review the known biochemical mechanisms responsible for those changes. Specifically, we explore the notion that disturbances in the metabolic reserve result in contractile dysfunction of the stressed heart.

Nuclear receptors PPARbeta/delta and PPARalpha direct distinct metabolic regulatory programs in the mouse heart

In the diabetic heart, chronic activation of the PPARalpha pathway drives excessive fatty acid (FA) oxidation, lipid accumulation, reduced glucose utilization, and cardiomyopathy. The related nuclear receptor, PPARbeta/delta, is also highly expressed in the heart, yet its function has not been fully delineated. To address its role in myocardial metabolism, we generated transgenic mice with cardiac-specific expression of PPARbeta/delta, driven by the myosin heavy chain (MHC-PPARbeta/delta mice). In striking contrast to MHC-PPARalpha mice, MHC-PPARbeta/delta mice had increased myocardial glucose utilization, did not accumulate myocardial lipid, and had normal cardiac function. Consistent with these observed metabolic phenotypes, we found that expression of genes involved in cellular FA transport were activated by PPARalpha but not by PPARbeta/delta. Conversely, cardiac glucose transport and glycolytic genes were activated in MHC-PPARbeta/delta mice, but repressed in MHC-PPARalpha mice. In reporter assays, we showed that PPARbeta/delta and PPARalpha exerted differential transcriptional control of the GLUT4 promoter, which may explain the observed isotype-specific effects on glucose uptake. Furthermore, myocardial injury due to ischemia/reperfusion injury was significantly reduced in the MHC-PPARbeta/delta mice compared with control or MHC-PPARalpha mice, consistent with an increased capacity for myocardial glucose utilization. These results demonstrate that PPARalpha and PPARbeta/delta drive distinct cardiac metabolic regulatory programs and identify PPARbeta/delta as a potential target for metabolic modulation therapy aimed at cardiac dysfunction caused by diabetes and ischemia.

Hypoxia-inducible factor 1 mediates hypoxia-induced cardiomyocyte lipid accumulation by reducing the DNA binding activity of peroxisome proliferator-activated receptor alpha/retinoid X receptor

In response to cellular hypoxia, cardiomyocytes adapt to consume less oxygen by shifting ATP production from mitochondrial fatty acid beta-oxidation to glycolysis. The transcriptional activation of glucose transporters and glycolytic enzymes by hypoxia is mediated by hypoxia-inducible factor 1 (HIF-1). In this study, we examined whether HIF-1 was involved in the suppression of mitochondrial fatty acid beta-oxidation in hypoxic cardiomyocytes. We showed that either hypoxia or adenovirus-mediated expression of a constitutively stable hybrid form (HIF-1alpha/VP16) suppressed mitochondrial fatty acid metabolism, as indicated by an accumulation of intracellular neutral lipid. Both treatments also reduced the mRNA levels of muscle carnitine palmitoyltransferase I which catalyzes the rate-limiting step in the mitochondrial import of fatty acids for beta-oxidation. Furthermore, adenovirus-mediated expression of HIF-1alpha/VP16 in cardiomyocytes under normoxic conditions also mimicked the reduction in the DNA binding activity of peroxisome proliferator-activated receptor alpha (PPARalpha)/retinoid X receptor (RXR), in the presence or absence of a PPARalpha ligand. These results suggest that HIF-1 may be involved in hypoxia-induced suppression of fatty acid metabolism in cardiomyocytes by reducing the DNA binding activity of PPARalpha/RXR.

Inhibition of PPAR-α activity in mice with cardiac-restricted expression of tumor necrosis factor: potential role of TGF-β/Smad3

A shift in energy substrate utilization from fatty acids to glucose has been reported in failing hearts, resulting in improved oxygen efficiency yet perhaps also contributing to a state of energy deficiency. Peroxisome proliferator-activated receptor (PPAR)-α, the principal transcriptional regulator of cardiac fatty acid β-oxidation (FAO) genes, is downregulated in heart failure, and this may contribute to reduced fatty acid utilization. Cardiomyopathic states are also accompanied by elevated levels of circulating cytokines, such as tumor necrosis factor (TNF), as well as increased local production of cytokines and profibrotic factors, such as transforming growth factor (TGF)-β. However, whether these molecular pathways directly modulate cardiac energy metabolism and PPAR-α activity is not known. Therefore, FAO capacity and FAO gene expression were determined in mice with cardiac-restricted overexpression of TNF (MHCsTNF3). MHCsTNF3 hearts had significantly lower FAO capacity and decreased expression of PPAR-α and FAO target genes compared with control hearts. Surprisingly, TNF had little effect on PPAR-α activity and FAO rates in cultured ventricular myocytes, suggesting that TNF acts indirectly on myocyte FAO in vivo. We found that TGF-β expression was upregulated in MHCsTNF3 hearts and that treatment of cultured myocytes with TGF-β significantly suppressed FAO rates and directly impaired PPAR-α activity, a result reproduced by Smad3 overexpression. This work demonstrates that TGF-β signaling pathways directly suppress PPAR-α activity and reduce FAO in cardiac myocytes, perhaps in response to locally elevated TNF. Although speculative, TGF-β-driven repair mechanisms may also include the additional benefit of limiting FAO in injured myocardium.

Interplay between estrogen-related receptor alpha (ERRalpha) and gamma (ERRgamma) on the regulation of ERRalpha gene expression

Estrogen-related receptor alpha (ERRalpha) modulates estrogen receptor (ER)-mediated activity and is participating in the energy homeostasis by regulation of downstream target genes. The ERRalpha gene itself is proposed to be regulated by peroxisome proliferator-activated receptor gamma coactivator (PGC-1alpha) through an autoregulatory loop under physiological stimulation. We have previously shown that the close family member ERRgamma is a positive regulator of ERRalpha gene expression. ERRalpha and ERRgamma are coexpressed in metabolically active tissues such as heart, kidney and muscle, yet the physiological role of ERRgamma and its relationship with ERRalpha in gene regulation are currently unknown. The present study examined the interplay of ERRgamma and ERRalpha in regulation of ERRalpha gene expression. Using real-time PCR analyses we found that ERRgamma, like the ERRalpha and PGC-1alpha is induced in mouse liver during fasting. Overexpression of ERRgamma in the HEC-1B cells robustly stimulated the multi-hormone response element (MHRE) of the ERRalpha gene promoter and this activity was repressed by increasing expression of ERRalpha. The two ERRs bind MHRE simultaneously in electrophoretic mobility shift assay (EMSA) and they were detected as multimeric complexes in cells by coimmunoprecipitation. Although ERRalpha and ERRgamma share high sequence identity, they differ in biochemical and molecular characteristics as examined by trypsin digestion, reporter activation and coactivator interaction and utilization. Using chromatin immunoprecipitation (ChIP) assay, we showed that ectopic expression of both ERRalpha and ERRgamma modifies chromatin structure at the MHRE region while ectopic expression of PGC-1alpha in HEC-1B cells promotes ERRgamma but not ERRalpha occupancy at the MHRE region of the ERRalpha gene promoter and enhances the recruitment of coactivator SRC1. These data suggested that ERRalpha and ERRgamma regulate ERRalpha gene expression with different molecular mechanisms.

Ontogeny of mRNA expression and activity of long-chain acyl-CoA synthetase (ACSL) isoforms in Mus musculus heart

Long-chain acyl-CoA synthetases (ACSL) activate fatty acids (FA) and provide substrates for virtually every metabolic pathway that catabolizes FA or synthesizes complex lipids. We have hypothesized that each of the five cloned ACSL isoforms partitions FA towards specific downstream pathways. Adult heart expresses all five cloned ACSL isoforms, but their independent functional roles have not been elucidated. Studies implicate ACSL1 in both oxidative and lipid synthetic pathways. To clarify the functional role of ACSL1 and the other ACSL isoforms (3-6), we examined ACS specific activity and Acsl mRNA expression in the developing mouse heart which increases FA oxidative pathways for energy production after birth. Compared to the embryonic heart, ACS specific activity was 14-fold higher on post-natal day 1 (P1). On P1, as compared to the fetus, only Acsl1 mRNA increased, whereas transcripts for the other Acsl isoforms remained the same, suggesting that ACSL1 is the major isoform responsible for activating long-chain FA for myocardial oxidation after birth. In contrast, the mRNA abundance of Acsl3 was highest on E16, and decreased dramatically by P7, suggesting that ACSL3 may play a critical role during the development of the fetal heart. Our data support the hypothesis that each ACSL has a specific role in the channeling of FA towards distinct metabolic fates.

Orphan nuclear receptors: therapeutic opportunities in skeletal muscle

Nuclear hormone receptors (NRs) are ligand-dependent transcription factors that bind DNA and translate physiological signals into gene regulation. The therapeutic utility of NRs is underscored by the diversity of drugs created to manage dysfunctional hormone signaling in the context of reproductive biology, inflammation, dermatology, cancer, and metabolic disease. For example, drugs that target nuclear receptors generate over $10 billion in annual sales. Almost two decades ago, gene products were identified that belonged to the NR superfamily on the basis of DNA and protein sequence identity. However, the endogenous and synthetic small molecules that modulate their action were not known, and they were denoted orphan NRs. Many of the remaining orphan NRs are highly enriched in energy-demanding major mass tissues, including skeletal muscle, brown and white adipose, brain, liver, and kidney. This review focuses on recently adopted and orphan NR function in skeletal muscle, a tissue that accounts for approximately 35% of the total body mass and energy expenditure, and is a major site of fatty acid and glucose utilization. Moreover, this lean tissue is involved in cholesterol efflux and secretes that control energy expenditure and adiposity. Consequently, muscle has a significant role in insulin sensitivity, the blood lipid profile, and energy balance. Accordingly, skeletal muscle plays a considerable role in the progression of dyslipidemia, diabetes, and obesity. These are risk factors for cardiovascular disease, which is the the foremost cause of global mortality (>16.7 million deaths in 2003). Therefore, it is not surprising that orphan NRs and skeletal muscle are emerging as therapeutic candidates in the battle against dyslipidemia, diabetes, obesity, and cardiovascular disease.

Transverse aortic constriction leads to accelerated heart failure in mice lacking PPAR-gamma coactivator 1alpha

Heart failure is accompanied by important defects in metabolism. The transcriptional coactivator peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha) is a powerful regulator of mitochondrial biology and metabolism. PGC-1alpha and numerous genes regulated by PGC-1alpha are repressed in models of cardiac stress, such as that generated by transverse aortic constriction (TAC). This finding has suggested that PGC-1alpha repression may contribute to the maladaptive response of the heart to chronic hemodynamic loads. We show here that TAC in mice genetically engineered to lack PGC-1alpha leads to accelerated cardiac dysfunction, which is accompanied by signs of significant clinical heart failure. Treating cardiac cells in tissue culture with the catecholamine epinephrine leads to repression of PGC-1alpha and many of its target genes, recapitulating the findings in vivo in response to TAC. Importantly, introduction of ectopic PGC-1alpha can reverse the repression of most of these genes by epinephrine. Together, these data indicate that endogenous PGC-1alpha serves a cardioprotective function and suggest that repression of PGC-1alpha significantly contributes to the development of heart failure. Moreover, the data suggest that elevating PGC-1alpha activity may have therapeutic potential in the treatment of heart failure.

The chicken ovalbumin upstream promoter-transcription factors modulate genes and pathways involved in skeletal muscle cell metabolism

The chicken ovalbumin upstream promoter-transcription factors (COUP-TFs) are "orphan" members of the nuclear hormone receptor (NR) superfamily. COUP-TFs are involved in organogenesis and neurogenesis. However, their role in skeletal muscle (and other major mass tissues) and metabolism remains obscure. Skeletal muscle accounts for approximately 40% of total body mass and energy expenditure. Moreover, this peripheral tissue is a primary site of glucose and fatty acid utilization. We utilize small interfering RNA (siRNA)-mediated attenuation of Coup-TfI and II (mRNA and protein) in a skeletal muscle cell culture model to understand the regulatory role of Coup-Tfs in this energy demanding tissue. This targeted NR repression resulted in the significant attenuation of genes that regulate lipid mobilization and utilization (including Pparalpha, Fabp3, and Cpt-1). This was coupled to reduced fatty acid beta-oxidation. Additionally we observed significant attenuation of Ucp1, a gene involved in energy expenditure. Concordantly, we observed a 5-fold increase in ATP levels in cells with siRNA-mediated repression of Coup-TfI and II. Furthermore, the expression of "classical" liver X receptor (LXR) target genes involved in reverse cholesterol transport (Abca1 and Abcg1) were both significantly repressed. Moreover, we observed that repression of the Coup-Tfs ablated the activation of Abca1, and Abcg1 mRNA expression by the selective LXR agonist, T0901317. In concordance, Coup-Tf-siRNA-transfected cells were refractory to Lxr-mediated reduction of total intracellular cholesterol levels in contrast to the negative control cells. In agreement Lxr-mediated activation of the Abca1 promoter in Coup-Tf-siRNA cells was attenuated. Collectively, these data suggest a pivotal role for Coup-Tfs in the regulation of lipid utilization/cholesterol homeostasis in skeletal muscle cells and the modulation of Lxr-dependent gene regulation.

PGC-1alpha coactivates PDK4 gene expression via the orphan nuclear receptor ERRalpha: a mechanism for transcriptional control of muscle glucose metabolism

The transcriptional coactivator PGC-1alpha is a key regulator of energy metabolism, yet little is known about its role in control of substrate selection. We found that physiological stimuli known to induce PGC-1alpha expression in skeletal muscle coordinately upregulate the expression of pyruvate dehydrogenase kinase 4 (PDK4), a negative regulator of glucose oxidation. Forced expression of PGC-1alpha in C(2)C(12) myotubes induced PDK4 mRNA and protein expression. PGC-1alpha-mediated activation of PDK4 expression was shown to occur at the transcriptional level and was mapped to a putative nuclear receptor binding site. Gel shift assays demonstrated that the PGC-1alpha-responsive element bound the estrogen-related receptor alpha (ERRalpha), a recently identified component of the PGC-1alpha signaling pathway. In addition, PGC-1alpha was shown to activate ERRalpha expression. Chromatin immunoprecipitation assays confirmed that PGC-1alpha and ERRalpha occupied the mPDK4 promoter in C(2)C(12) myotubes. Additionally, transfection studies using ERRalpha-null primary fibroblasts demonstrated that ERRalpha is required for PGC-1alpha-mediated activation of the mPDK4 promoter. As predicted by the effects of PGC-1alpha on PDK4 gene transcription, overexpression of PGC-1alpha in C(2)C(12) myotubes decreased glucose oxidation rates. These results identify the PDK4 gene as a new PGC-1alpha/ERRalpha target and suggest a mechanism whereby PGC-1alpha exerts reciprocal inhibitory influences on glucose catabolism while increasing alternate mitochondrial oxidative pathways in skeletal muscle.

Fat to the fire: the regulation of lipid oxidation with exercise and environmental stress

Lipids are an important fuel for submaximal aerobic exercise. The ways in which lipid oxidation is regulated during locomotion is an area of active investigation. Indeed, the integration between cellular regulation of lipid metabolism and whole-body exercise performance is a fascinating but often overlooked research area. Additionally, the interaction between environmental stress, exercise, and lipid oxidation has not been sufficiently examined. There are many functional and structural steps as fatty acids are mobilized, transported, and oxidized in working muscle, which may serve either as regulatory points for responding to acute or chronic stimuli or as raw material for natural selection. At the whole-animal level, the partitioning of lipids and carbohydrates across exercise intensities is remarkably similar among mammals, which suggests that there is conservation in regulatory mechanisms. Conversely, the proportions of circulatory and intramuscular fuels differ between species and across exercise intensities. Responses to acute and chronic environmental stress likely involve the interaction of genetic and nongenetic changes in the fatty acid pathway. Determining which of these factors help regulate the fatty acid pathway and what impact they have on whole-animal lipid oxidation and performance is an important area of future research. Using an integrative approach to complete the information loop from gene to physiological function provides the most powerful mode of analysis.

Estrogen-related receptor alpha directs peroxisome proliferator-activated receptor alpha signaling in the transcriptional control of energy metabolism in cardiac and skeletal muscle

Estrogen-related receptors (ERRs) are orphan nuclear receptors activated by the transcriptional coactivator peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha (PGC-1alpha), a critical regulator of cellular energy metabolism. However, metabolic target genes downstream of ERRalpha have not been well defined. To identify ERRalpha-regulated pathways in tissues with high energy demand such as the heart, gene expression profiling was performed with primary neonatal cardiac myocytes overexpressing ERRalpha. ERRalpha upregulated a subset of PGC-1alpha target genes involved in multiple energy production pathways, including cellular fatty acid transport, mitochondrial and peroxisomal fatty acid oxidation, and mitochondrial respiration. These results were validated by independent analyses in cardiac myocytes, C2C12 myotubes, and cardiac and skeletal muscle of ERRalpha-/- mice. Consistent with the gene expression results, ERRalpha increased myocyte lipid accumulation and fatty acid oxidation rates. Many of the genes regulated by ERRalpha are known targets for the nuclear receptor PPARalpha, and therefore, the interaction between these regulatory pathways was explored. ERRalpha activated PPARalpha gene expression via direct binding of ERRalpha to the PPARalpha gene promoter. Furthermore, in fibroblasts null for PPARalpha and ERRalpha, the ability of ERRalpha to activate several PPARalpha targets and to increase cellular fatty acid oxidation rates was abolished. PGC-1alpha was also shown to activate ERRalpha gene expression. We conclude that ERRalpha serves as a critical nodal point in the regulatory circuitry downstream of PGC-1alpha to direct the transcription of genes involved in mitochondrial energy-producing pathways in cardiac and skeletal muscle.

Nuclear receptor signaling and cardiac energetics

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, PPARgamma coactivator-1alpha (PGC-1alpha). The dynamic regulation of the cardiac PPAR/PGC-1 complex in accordance with physiological and pathophysiological states will be described.

Calcineurin and calcium/calmodulin-dependent protein kinase activate distinct metabolic gene regulatory programs in cardiac muscle

To learn more about the targets of Cn (Cn) and calcium/calmodulin-dependent protein kinase in cardiac muscle, we investigated their actions in cultured cardiac myocytes and the hearts of mice in vivo. Adenoviral-mediated expression of constitutively active forms of either pathway induced expression of peroxisome proliferator-activated receptor gamma coactivator 1alpha, a transcriptional coactivator involved in the control of multiple cellular energy metabolic pathways in cardiac myocytes. Transcriptional profiling studies demonstrated that Cn and calcium/calmodulin-dependent protein kinase activate distinct but overlapping metabolic gene regulatory programs. Expression of the nuclear receptor, peroxisome proliferator-activated receptor alpha, was markedly increased by Cn, but not calcium/calmodulin-dependent protein kinase, providing one mechanism whereby cellular fatty acid utilization genes are selectively activated by Cn. Transfection experiments demonstrated that Cn directly activates the mouse peroxisome proliferator-activated receptor alpha gene promoter. Co-transfection "add-back" experiments demonstrated that the transcription factors, myocyte enhancer factors 2C or 2D, were sufficient to confer Cn-mediated activation of the peroxisome proliferator-activated receptor alpha gene. Cn was also shown to directly activate a known peroxisome proliferator-activated receptor alpha target, muscle-type carnitine palmitoyltransferase I, providing a second mechanism by which Cn activates genes of cellular fatty acid utilization. Lastly, the gene expression of peroxisome proliferator-activated receptor gamma coactivator 1alpha and peroxisome proliferator-activated receptor alpha was reduced in the hearts of mice with cardiac-specific ablation of the Cn regulatory subunit. These data support a role for calcium-triggered signaling pathways in the regulation of cardiac energetics and identify pathway-specific control of metabolic targets.

Counter-regulatory effects of incremental hypoxia on the transcription of a cardiac fatty acid oxidation enzyme-encoding gene

Cardiac fatty acid oxidation (FAO) enzyme gene expression is known to be downregulated during hypoxia in concordance with reduced FAO rates. To evaluate this metabolic switch, the transcriptional control of a cardiac FAO enzyme-encoding gene (medium-chain acyl-CoA dehydrogenase, MCAD) was characterized in response to hypobaric hypoxia. Transgenic mice harboring 560-bp of the human MCAD gene promoter fused to the bacterial chloramphenicol acetyl transferase (CAT) reporter gene were exposed to moderate (14% O2) or severe (8% O2) hypoxia for 2 or 7 days. MCAD-CAT activity and gene expression were significantly downregulated following 7 days of moderate hypoxia versus normoxic controls (p < 0.05). In parallel two known transcriptional regulators of MCAD expression, PPARalpha and Sp3, were concordantly downregulated at 7 days hypoxia. In contrast, severe hypoxia increased MCAD-CAT activity by 31 +/- 1.4% after 2 days hypoxia, returning to base +/- 4% after 2 days (p < 0.001) and returned to control levels after 7 days of hypoxia. These data demonstrate that MCAD gene expression is downregulated after 7 days of moderate hypoxia and inversely regulated with severe hypoxia. The known MCAD transcriptional regulators PPARalpha and Sp3 mirror MCAD expression. These data indicate that the transcriptional regulatory circuits involved in the control of MCAD gene expression under hypoxic conditions are modulated by upstream factors that are sensitive to the levels of oxygen.

Pitavastatin ameliorates severe hepatic steatosis in aromatase-deficient (Ar-/-) mice

Tamoxifen is a potent antagonist of estrogen, and hepatic steatosis is a frequent complication in adjuvant tamoxifen for breast cancer. Impaired hepatic FA beta-oxidation in peroxisomes, microsomes, and mitochondria results in progression of massive hepatic steatosis in estrogen deficiency. This impairment, although latent, is potentially serious: About 3% of the general population in the United States is now suffering from nonalcoholic steatohepatitis associated with obesity and hyperlipidemia. Therefore, in the present study we tried to restore impaired hepatic FA beta-oxidation by administering a novel statin, pitavastatin, to aromatase-deficient (Ar-/-) mice defective in intrinsic estrogen synthesis. Northern blot analysis of Ar-/- mice liver revealed a significant restoration of mRNA expression of essential enzymes involved in FA beta-oxidation such as very long fatty acyl-CoA synthetase in peroxisome, peroxisomal fatty acyl-CoA oxidase, and medium-chain acyl-CoA dehydrogenase. Severe hepatic steatosis observed in Ar-/- mice substantially regressed. Consistent findings were obtained in the in vitro assays of FA beta-oxidation activity. These findings demonstrate that pitavastatin is capable of restoring impaired FA beta-oxidation in vivo via the peroxisome proliferator-activated receptor-alpha-mediated signaling pathway and is potent enough to ameliorate severe hepatic steatosis in mice deficient in intrinsic estrogen.

Hypoxia-induced switches of myosin heavy chain iso-gene expression in rat heart

Cardiac hypertrophy and atrophy increase expression of fetal iso-genes. A common factor is a decrease in cellular oxygen tension. To test the hypothesis that hypoxia changes cardiac MHC iso-gene expression Wistar rats were exposed to 24 and 48 h of hypobaric hypoxia (11% oxygen) and mRNA was isolated from the left ventricle. In addition, neonatal rat cardiomyocytes were incubated for up to 48 h in a hypoxic chamber. Transcript levels of MHCalpha (adult isoform), MHCbeta (fetal isoform), and Nkx2.5, the earliest known marker for cardiogenesis, were measured by real-time quantitative RT-PCR and normalized to levels of 18S rRNA. Expression of the transcription factor Nkx2.5 increased with hypoxia. Hypoxia decreased MHCalpha and increased MHCbeta transcript levels, both in vivo and in vitro. We conclude that hypoxia per se induces a pattern of isoform gene expression associated with early cardiac development.

A thyroid hormone response unit formed between the promoter and first intron of the carnitine palmitoyltransferase-Ialpha gene mediates the liver-specific induction by thyroid hormone

Carnitine palmitoyltransferase-I (CPT-I) catalyzes the rate-controlling step of fatty acid oxidation. CPT-I converts long-chain fatty acyl-CoAs to acylcarnitines for translocation across the mitochondrial membrane. The mRNA levels and enzyme activity of the liver isoform, CPT-Ialpha, are greatly increased in the liver of hyperthyroid animals. Thyroid hormone (T3) stimulates CPT-Ialpha transcription far more robustly in the liver than in non-hepatic tissues. We have shown that the thyroid hormone receptor (TR) binds to a thyroid hormone response element (TRE) located in the CPT-Ialpha promoter. In addition, elements in the first intron participate in the T3 induction of CPT-Ialpha gene expression, but the CPT-Ialpha intron alone cannot confer a T3 response. We found that deletion of sequences in the first intron between +653 and +744 decreased the T3 induction of CPT-Ialpha. Upstream stimulatory factor (USF) and CCAAT enhancer binding proteins (C/EBPs) bind to elements within this region, and these factors are required for the T3 response. The binding of TR and C/EBP to the CPT-Ialpha gene in vivo was shown by the chromatin immunoprecipitation assay. We determined that TR can physically interact with USF-1, USF-2, and C/EBPalpha. Transgenic mice were created that carry CPT-Ialpha-luciferase transgenes with or without the first intron of the CPT-Ialpha gene. In these mouse lines, the first intron is required for T3 induction as well as high levels of hepatic expression. Our data indicate that the T3 stimulates CPT-Ialpha gene expression in the liver through a T3 response unit consisting of the TRE in the promoter and additional factors, C/EBP and USF, bound in the first intron.

Peroxisome proliferator-activated receptor coactivator-1alpha (PGC-1alpha) coactivates the cardiac-enriched nuclear receptors estrogen-related receptor-alpha and -gamma. Identification of novel leucine-rich interaction motif within PGC-1alpha

The transcriptional coactivator PPARgamma coactivator-1alpha (PGC-1alpha) has been characterized as a broad regulator of cellular energy metabolism. Although PGC-1alpha functions through many transcription factors, the PGC-1alpha partners identified to date are unlikely to account for all of its biologic actions. The orphan nuclear receptor estrogen-related receptor alpha (ERRalpha) was identified in a yeast two-hybrid screen of a cardiac cDNA library as a novel PGC-1alpha-binding protein. ERRalpha was implicated previously in regulating the gene encoding medium-chain acyl-CoA dehydrogenase (MCAD), which catalyzes the initial step in mitochondrial fatty acid oxidation. The cardiac perinatal expression pattern of ERRalpha paralleled that of PGC-1alpha and MCAD. Adenoviral-mediated ERRalpha overexpression in primary neonatal cardiac mycoytes induced endogenous MCAD expression. Furthermore, PGC-1alpha enhanced the transactivation of reporter plasmids containing an estrogen response element or the MCAD gene promoter by ERRalpha and the related isoform ERRgamma. In vitro binding experiments demonstrated that ERRalpha interacts with PGC-1alpha via its activation function-2 homology region. Mutagenesis studies revealed that the LXXLL motif at amino acid position 142-146 of PGC-1alpha (L2), necessary for PGC-1alpha interactions with other nuclear receptors, is not required for the PGC-1alpha.ERRalpha interaction. Rather, ERRalpha binds PGC-1alpha primarily through a Leu-rich motif at amino acids 209-213 (Leu-3) and utilizes additional LXXLL-containing domains as accessory binding sites. Thus, the PGC-1alpha.ERRalpha interaction is distinct from that of other nuclear receptor PGC-1alpha partners, including PPARalpha, hepatocyte nuclear factor-4alpha, and estrogen receptor alpha. These results identify ERRalpha and ERRgamma as novel PGC-1alpha interacting proteins, implicate ERR isoforms in the regulation of mitochondrial energy metabolism, and suggest a potential mechanism whereby PGC-1alpha selectively binds transcription factor partners.

Gene regulatory mechanisms governing energy metabolism during cardiac hypertrophic growth

Studies in a variety of mammalian species, including humans, have demonstrated a reduction in fatty acid oxidation (FAO) and increased glucose utilization in pathologic cardiac hypertrophy, consistent with reinduction of the fetal energy metabolic program. This review describes results of recent molecular studies aimed at delineating the gene regulatory events which facilitate myocardial energy substrate switches during hypertrophic growth of the heart. Studies aimed at the characterization of transcriptional control mechanisms governing FAO enzyme gene expression in the cardiac myocyte have defined a central role for the fatty acid-activated nuclear receptor peroxisome proliferator-activated receptor alpha (PPAR(alpha)). Cardiac FAO enzyme gene expression was shown to be coordinately downregulated in murine models of ventricular pressure overload, consistent with the energy substrate switch away from fatty acid utilization in the hypertrophied heart. Nuclear protein levels of PPAR(alpha) decline in the ventricle in response to pressure overload, while several Sp and nuclear receptor transcription factors are induced to fetal levels, consistent with their binding to DNA as transcriptional repressors of rate-limiting FAO enzyme genes with hypertrophy. Knowledge of key components of this transcriptional regulatory pathway will allow for the development of genetic engineering strategies in mice that will modulate fatty acid oxidative flux and assist in defining whether energy metabolic derangements play a primary role in the development of pathologic cardiac hypertrophy and eventual progression to heart failure.

Transcriptional activation of energy metabolic switches in the developing and hypertrophied heart

1. The present review focuses on the gene regulatory mechanisms involved in the control of cardiac mitochondrial energy production in the developing heart and following the onset of pathological cardiac hypertrophy. Particular emphasis has been given to the mitochondrial fatty acid oxidation (FAO) pathway and its control by members of the nuclear receptor transcription factor superfamily. 2. During perinatal cardiac development, the heart undergoes a switch in energy substrate preference from glucose in the fetal period to fatty acids following birth. This energy metabolic switch is paralleled by changes in the expression of the enzymes and protein involved in the respective pathways. 3. The postnatal activation of the mitochondrial energy production pathway involves the induced expression of nuclear genes encoding FAO enzymes, as well as other proteins important in mitochondrial energy transduction/production pathways. Recent evidence indicates that this postnatal gene regulatory effect involves the actions of the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha) and its coactivator the PPARgamma coactivator 1 (PGC-1). 4. The PGC-1 not only activates PPARalpha to induce FAO pathway enzymes in the postnatal heart, but it also plays a pivotal role in the control of cardiac mitochondrial number and function. Thus, PGC-1 plays a master regulatory role in the high-capacity mitochondrial energy production system in the adult mammalian heart. 5. During the development of pathological forms of cardiac hypertrophy, such as that due to pressure overload, the myocardial energy substrate preference shifts back towards the fetal pattern, with a corresponding reduction in the expression of FAO enzyme genes. This metabolic shift is due to the deactivation of the PPARalpha/PGC-1 complex. 6. The deactivation of PPARalpha and PGC-1 during the development of cardiac hypertrophy involves regulation at several levels, including a reduction in the expression of these genes, as well as post-translational effects due to the mitogen-activated protein kinase pathway. Future studies aim at defining whether this transcriptional 'switch' and its effects on myocardial metabolism are adaptive or maladaptive in the hypertrophied heart.

p38 mitogen-activated protein kinase activates peroxisome proliferator-activated receptor alpha: a potential role in the cardiac metabolic stress response

The expression of enzymes involved in fatty acid beta-oxidation (FAO), the principal source of energy production in the adult mammalian heart, is controlled at the transcriptional level via the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha). Evidence has emerged that PPARalpha activity is activated as a component of an energy metabolic stress response. The p38 mitogen-activated protein kinase (MAPK) pathway is activated by cellular stressors in the heart, including ischemia, hypoxia, and hypertrophic growth stimuli. We show here that PPARalpha is phosphorylated in response to stress stimuli in rat neonatal cardiac myocytes; in vitro kinase assays demonstrated that p38 MAPK phosphorylates serine residues located within the NH(2)-terminal A/B domain of the protein. Transient transfection studies in cardiac myocytes and in CV-1 cells utilizing homologous and heterologous PPARalpha target element reporters and mammalian one-hybrid transcription assays revealed that p38 MAPK phosphorylation of PPARalpha significantly enhanced ligand-dependent transactivation. Cotransfection studies performed with several known coactivators of PPARalpha demonstrated that p38 MAPK markedly increased coactivation specifically by PGC-1, a transcriptional coactivator implicated in myocyte energy metabolic gene regulation and mitochondrial biogenesis. These results identify PPARalpha as a downstream effector of p38 kinase-dependent stress-activated signaling in the heart, linking extracellular stressors to alterations in energy metabolic gene expression.

TNF alpha is required for hypoxia-mediated right ventricular hypertrophy

Hypoxia has been shown to activate the pleiotropic cytokine TNFalpha in the lung. TNFalpha in turn, is known to induce pulmonary vasoconstriction. Additional effects of this cytokine in hypoxia mediated cardiopulmonary remodeling are poorly understood. To further evaluate the role of TNFalpha in chronic hypoxia we exposed TNFalpha null (TNFalpha-/-) and wild-type mice to three weeks of hypobaric hypoxia (10% O2). Equivalent erythocytosis (Hematocrit increased by > 40%) developed in both genetic backgrounds. In contrast, right ventricular systolic pressure increased in response to three weeks of hypoxia in the wild-type mice (> or = 75%), yet was unaltered in the TNFalpha-/- mice. Concomitantly right ventricular hypertrophy was attenuated in the TNFalpha-/- mice (35 +/- 5% increase) when compared to wild-type mice (124 +/- 6% increase p < 0.001, n > or = 20). Interestingly in both strains the lung wet weights increased to a similar degree in response to hypoxia. In conclusion, our data demonstrate that TNFalpha is an integral autocoid in chronic hypoxia mediated right ventricular hypertrophy. Moreover, additional components of cardiopulmonary remodeling may be regulated by TNFalpha signaling as suggested by the negligible right ventricular systolic pressure response to hypoxia in the absence of TNFalpha.

Biochemical properties of porcine white adipose tissue mitochondria and relevance to fatty acid oxidation

The capacity of white adipose tissue mitochondria to support a high beta-oxidative flux was investigated by comparison to liver mitochondria. Based on marker enzyme activities and electron microscopy, the relative purity of the isolated mitochondria was similar thus allowing a direct comparison on a protein basis. The results confirm the comparable capacity of adipose tissue and liver mitochondria for palmitoyl-carnitine oxidation. Relative to liver, both citrate synthase and alpha-ketoglutarate dehydrogenase were increased 7.87- and 10.38-fold, respectively. In contrast, adipose tissue NAD-isocitrate dehydrogenase was decreased (2.85-fold). Such modifications in the citric acid cycle are expected to severely restrict citrate oxidation in porcine adipose tissue. Except for cytochrome c oxidase, activities of the enzyme complexes comprising the electron transport chain were not significantly different. The decrease in adipose cytochrome c oxidase activity could partly be attributed to a decreased inner membrane as suggested by lipid and enzyme analysis. In addition, Western blotting indicated that adipose and liver mitochondria possess similar quantities of cytochrome c oxidase protein. Taken together these results indicate that not only is the white adipose tissue protoplasm relatively rich in mitochondria, but that these mitochondria contain comparable enzymatic machinery to support a relatively high beta-oxidative rate.

Hypoxia inhibits the peroxisome proliferator-activated receptor alpha/retinoid X receptor gene regulatory pathway in cardiac myocytes: a mechanism for O2-dependent modulation of mitochondrial fatty acid oxidation

Hypoxia triggers a cascade of cellular energy metabolic responses including a decrease in mitochondrial oxidative flux. To characterize gene regulatory mechanisms by which mitochondrial fatty acid oxidative capacity is diminished in response to hypoxia, cardiac myocytes in culture were exposed to long-chain fatty acids (LCFA) under normoxic or hypoxic conditions. Hypoxia prevented the known LCFA-induced accumulation of mRNA encoding muscle carnitine palmitoyltransferase I (M-CPT I), an enzyme that catalyzes the rate-limiting step in mitochondrial fatty acid oxidation (FAO). Under hypoxic conditions, myocytes exhibited significant accumulation of intracellular neutral lipid consistent with reduced CPT I activity and diminished FAO capacity. Transient transfection experiments demonstrated that the hypoxia-mediated blunting of M-CPT I gene expression occurs at the transcriptional level, is localized to an LCFA/peroxisome proliferator-activated receptor alpha (PPARalpha)/retinoid X receptor (RXR) response element within the M-CPT I gene promoter, and is PPARalpha-dependent. DNA-protein binding studies demonstrated that exposure to hypoxia reduces PPARalpha/RXR binding activity. Immunoblotting studies demonstrated that whereas hypoxia had no effect on nuclear levels of PPARalpha protein, nuclear and cellular RXRalpha levels were reduced. Hypoxia also diminished the 9-cis-retinoic acid-mediated activation of a reporter containing an RXR homodimer response element. These results demonstrate that hypoxia deactivates PPARalpha by reducing the availability of its obligate partner RXR.

Hepatocyte nuclear factor 4alpha (nuclear receptor 2A1) is essential for maintenance of hepatic gene expression and lipid homeostasis

The numerous functions of the liver are controlled primarily at the transcriptional level by the concerted actions of a limited number of hepatocyte-enriched transcription factors (hepatocyte nuclear factor 1alpha [HNF1alpha], -1beta, -3alpha, -3beta, -3gamma, -4alpha, and -6 and members of the c/ebp family). Of these, only HNF4alpha (nuclear receptor 2A1) and HNF1alpha appear to be correlated with the differentiated phenotype of cultured hepatoma cells. HNF1alpha-null mice are viable, indicating that this factor is not an absolute requirement for the formation of an active hepatic parenchyma. In contrast, HNF4alpha-null mice die during embryogenesis. Moreover, recent in vitro experiments using tetraploid aggregation suggest that HNF4alpha is indispensable for hepatocyte differentiation. However, the function of HNF4alpha in the maintenance of hepatocyte differentiation and function is less well understood. To address the function of HNF4alpha in the mature hepatocyte, a conditional gene knockout was produced using the Cre-loxP system. Mice lacking hepatic HNF4alpha expression accumulated lipid in the liver and exhibited greatly reduced serum cholesterol and triglyceride levels and increased serum bile acid concentrations. The observed phenotypes may be explained by (i) a selective disruption of very-low-density lipoprotein secretion due to decreased expression of genes encoding apolipoprotein B and microsomal triglyceride transfer protein, (ii) an increase in hepatic cholesterol uptake due to increased expression of the major high-density lipoprotein receptor, scavenger receptor BI, and (iii) a decrease in bile acid uptake to the liver due to down-regulation of the major basolateral bile acid transporters sodium taurocholate cotransporter protein and organic anion transporter protein 1. These data indicate that HNF4alpha is central to the maintenance of hepatocyte differentiation and is a major in vivo regulator of genes involved in the control of lipid homeostasis.

Peroxisome proliferator-activated receptor gamma coactivator-1 promotes cardiac mitochondrial biogenesis

Cardiac mitochondrial function is altered in a variety of inherited and acquired cardiovascular diseases. Recent studies have identified the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) as a regulator of mitochondrial function in tissues specialized for thermogenesis, such as brown adipose. We sought to determine whether PGC-1 controlled mitochondrial biogenesis and energy-producing capacity in the heart, a tissue specialized for high-capacity ATP production. We found that PGC-1 gene expression is induced in the mouse heart after birth and in response to short-term fasting, conditions known to increase cardiac mitochondrial energy production. Forced expression of PGC-1 in cardiac myocytes in culture induced the expression of nuclear and mitochondrial genes involved in multiple mitochondrial energy-transduction/energy-production pathways, increased cellular mitochondrial number, and stimulated coupled respiration. Cardiac-specific overexpression of PGC-1 in transgenic mice resulted in uncontrolled mitochondrial proliferation in cardiac myocytes leading to loss of sarcomeric structure and a dilated cardiomyopathy. These results identify PGC-1 as a critical regulatory molecule in the control of cardiac mitochondrial number and function in response to energy demands.

PPAR signaling in the control of cardiac energy metabolism

Cardiac energy metabolic shifts occur as a normal response to diverse physiologic and dietary conditions and as a component of the pathophysiologic processes which accompany cardiac hypertrophy, heart failure, and myocardial ischemia. The capacity to produce energy via the utilization of fats by the mammalian postnatal heart is controlled in part at the level of expression of nuclear genes encoding enzymes involved in mitochondrial fatty acid beta-oxidation (FAO). The principal transcriptional regulator of FAO enzyme genes is the peroxisome proliferator-activated receptor alpha (PPARalpha), a member of the ligand-activated nuclear receptor superfamily. Among the ligand activators of PPARalpha are long-chain fatty acids; therefore, increased uptake of fatty acid substrate into the cardiac myocyte induces a transcriptional response leading to increased expression of FAO enzymes. PPARalpha-mediated control of cardiac metabolic gene expression is activated during postnatal development, short-term starvation, and in response to exercise training. In contrast, certain pathophysiologic states, such as pressure overload-induced hypertrophy, result in deactivation of PPARalpha and subsequent dysregulation of FAO enzyme gene expression, which sets the stage for abnormalities in cardiac lipid homeostasis and energy production, some of which are influenced by gender. Thus, PPARalpha not only serves a critical role in normal cardiac metabolic homeostasis, but alterations in PPARalpha signaling likely contribute to the pathogenesis of a variety of disease states. PPARalpha as a ligand-activated transcription factor is a potential target for the development of new therapeutic strategies aimed at the prevention of pathologic cardiac remodeling.

Deactivation of peroxisome proliferator-activated receptor-alpha during cardiac hypertrophic growth

We sought to delineate the molecular regulatory events involved in the energy substrate preference switch from fatty acids to glucose during cardiac hypertrophic growth. alpha(1)-adrenergic agonist-induced hypertrophy of cardiac myocytes in culture resulted in a significant decrease in palmitate oxidation rates and a reduction in the expression of the gene encoding muscle carnitine palmitoyltransferase I (M-CPT I), an enzyme involved in mitochondrial fatty acid uptake. Cardiac myocyte transfection studies demonstrated that M-CPT I promoter activity is repressed during cardiac myocyte hypertrophic growth, an effect that mapped to a peroxisome proliferator-activated receptor-alpha (PPARalpha) response element. Ventricular pressure overload studies in mice, together with PPARalpha overexpression studies in cardiac myocytes, demonstrated that, during hypertrophic growth, cardiac PPARalpha gene expression falls and its activity is altered at the posttranscriptional level via the extracellular signal-regulated kinase mitogen-activated protein kinase pathway. Hypertrophied myocytes exhibited reduced capacity for cellular lipid homeostasis, as evidenced by intracellular fat accumulation in response to oleate loading. These results indicate that during cardiac hypertrophic growth, PPARalpha is deactivated at several levels, leading to diminished capacity for myocardial lipid and energy homeostasis.

Specific elevation of transcript levels of particular protein subtypes induced in brown adipose tissue by cold exposure

To understand the difference in metabolic flow in rat brown adipose tissue (BAT) from that in white adipose tissue (WAT) at the molecular level, we examined the steady-state transcript levels of 39 proteins in both adipose tissues with and without cold exposure by Northern blot analysis. In addition to the transcript levels of uncoupling protein isoforms, those of proteins involved in the transport and catabolism of fatty acids and glucose in BAT were elevated by cold exposure, suggesting the stimulation of utilization of fatty acids and glucose as fuels in BAT. As to these changes, the muscle-type subtypes were remarkable; and therefore, they were suggested to be responsible for the cold exposure-induced acceleration of energy expenditure in BAT. Furthermore, of the isoforms of beta-adrenergic receptor (beta-AR) and CCAAT enhancer binding protein (C/EBP), transcript levels of beta(1)-AR and C/EBPbeta in BAT were increased by the cold exposure. Possible roles of these proteins in energy metabolism in BAT were discussed.

Characterization of the human very-long-chain acyl-CoA dehydrogenase gene promoter region: a role for activator protein 2

Very-long-chain acyl-CoA dehydrogenase (VLCAD) is one of a family of nuclear-encoded enzymes that catalyze the initial step in mitochondrial fatty acid beta-oxidation (FAO). Previous studies have indicated that two other members of the AD gene family (medium-chain AD and long-chain AD) are controlled at the transcriptional level by nuclear hormone receptors. In this study, we have cloned and characterized the human VLCAD gene promoter region to identify cis-acting elements involved in its transcriptional control. VLCAD gene promoter-luciferase reporter (VLCAD-Luc) constructs were found to be transcriptionally active in a variety of mammalian cell lines and in primary rat cardiomyocytes when driven by varying lengths of the VLCAD promoter region. Removal of a 20-bp DNA segment of the proximal VLCAD gene promoter markedly reduced the transcriptional activity of VLCAD-Luc constructs. Gel mobility shift assays identified a DNA-binding activity in nuclear extracts prepared from human hepatoma G2 cells that interacted with the 20-bp regulatory region. Competition studies revealed that this DNA-binding activity could be abolished by a molar excess of unlabeled specific oligonucleotide as well as a DNA fragment containing an activator protein 2 (AP-2)-binding site but not by an unrelated nonspecific DNA fragment. These results provide an initial characterization of the human VLCAD gene promoter, identify AP-2 as a candidate activator of VLCAD gene transcription, and suggest that VLCAD gene transcription may be regulated by pathways distinct from that of other AD genes.

Evidence for a novel cardiac-enriched retinoid X receptor partner

Recent studies indicate that retinoid-mediated pathways play a pivotal role in cardiac morphogenesis and function. To identify proteins that serve as interacting partners of the retinoid X receptor alpha (RXRalpha) in heart, DNA-protein binding studies were performed with an RXR-responsive element (NRRE-1) derived from the medium chain acyl-CoA dehydrogenase gene promoter and nuclear protein extracts prepared from adult rat heart. NRRE-1 is a pleiotropic RXR-responsive element comprised of three potential recognition sites for class II members of the nuclear receptor superfamily. Gel mobility shift assays performed with an NRRE-1 probe in the absence or presence of bacterially overproduced RXRalpha and nuclear protein extracts prepared from adult rat heart, liver, or brain identified a cardiac-specific, RXR-dependent DNA-protein interaction. The NRRE-1-RXR.cardiac-enriched RXR-interacting protein (CERIP) complex exhibited a distinct mobility compared with NRRE-1-RXR.peroxisome proliferator-activated receptor, NRRE-1-RXR.retinoic acid receptor, or NRRE-1-RXR.thyroid receptor complexes. Mutational analysis demonstrated that two of the three potential binding half-sites of NRRE-1 (an everted repeat separated by an 8-base pair spacer) are required for the NRRE-1-RXR. CERIP interaction. Gel mobility shift assays demonstrated that CERIP interacted with RXRalpha and RXRgamma but not with RXRbeta, indicating a receptor subtypespecific binding preference and suggesting an RXR AB region-dependent interaction. The RXR.CERIP complex did not form on NRRE-1 when a mutant GST-RXRalpha fusion protein lacking the NH(2)-terminal AB region (but containing the receptor dimerization domain) of RXRalpha was added in place of the full-length RXRalpha, confirming a role for the AB region in the RXR. CERIP interaction. DNA-protein cross-linking studies demonstrated that CERIP is a DNA-binding protein of approximately 110 kDa. These results provide evidence for the existence of a cardiac-enriched DNA-binding protein that interacts with RXRalpha via the AB region and suggest a mechanism whereby cardiac retinoid signaling is controlled in an RXR subtype-specific manner.

RGS4 causes increased mortality and reduced cardiac hypertrophy in response to pressure overload

RGS family members are GTPase-activating proteins (GAPs) for heterotrimeric G proteins. There is evidence that altered RGS gene expression may contribute to the pathogenesis of cardiac hypertrophy and failure. We investigated the ability of RGS4 to modulate cardiac physiology using a transgenic mouse model. Overexpression of RGS4 in postnatal ventricular tissue did not affect cardiac morphology or basal cardiac function, but markedly compromised the ability of the heart to adapt to transverse aortic constriction (TAC). In contrast to wild-type mice, the transgenic animals developed significantly reduced ventricular hypertrophy in response to pressure overload and also did not exhibit induction of the cardiac "fetal" gene program. TAC of the transgenic mice caused a rapid decompensation in most animals characterized by left ventricular dilatation, depressed systolic function, and increased postoperative mortality when compared with nontransgenic littermates. These results implicate RGS proteins as a crucial component of the signaling pathway involved in both the cardiac response to acute ventricular pressure overload and the cardiac hypertrophic program.

Fatty acid utilization in the hypertrophied and failing heart: molecular regulatory mechanisms

During the development of cardiac hypertrophy and in the failing heart, the chief myocardial energy source switches from fatty acid beta-oxidation to glycolysis: a reversion to the fetal energy substrate preference pattern. This review describes recent molecular studies aimed at delineating the gene regulatory pathway involved in the energy metabolic switch in the hypertrophied heart and the potential role of the attendant metabolic consequences in the pathogenesis of heart failure. Studies have been performed with the 'spontaneous hypertensive and heart failure' rat strain and with human cardiomyopathic tissue. These studies have demonstrated that expression of the gene that encodes medium-chain acyl-coenzyme A dehydrogenase (MCAD), a key fatty acid beta-oxidation enzyme, is down-regulated during the progression from cardiac hypertrophy to ventricular dysfunction. A series of studies performed in mice transgenic for the human MCAD gene promoter have identified a transcriptional regulatory pathway involved in the repression of MCAD gene expression in the hypertrophied mouse heart. Two categories of transcription factors, nuclear hormone receptors and Sp factors, bind MCAD gene promoter regulatory elements in response to pressure overload to reactivate a fetal metabolic gene program. Studies are under way to manipulate this transcriptional regulatory pathway in mice using genetic engineering strategies to determine whether this energy metabolic derangement plays a primary role in the development of cardiac hypertrophy and heart failure.

A critical role for the peroxisome proliferator-activated receptor alpha (PPARalpha) in the cellular fasting response: the PPARalpha-null mouse as a model of fatty acid oxidation disorders

We hypothesized that the lipid-activated transcription factor, the peroxisome proliferator-activated receptor alpha (PPARalpha), plays a pivotal role in the cellular metabolic response to fasting. Short-term starvation caused hepatic steatosis, myocardial lipid accumulation, and hypoglycemia, with an inadequate ketogenic response in adult mice lacking PPARalpha (PPARalpha-/-), a phenotype that bears remarkable similarity to that of humans with genetic defects in mitochondrial fatty acid oxidation enzymes. In PPARalpha+/+ mice, fasting induced the hepatic and cardiac expression of PPARalpha target genes encoding key mitochondrial (medium-chain acyl-CoA dehydrogenase, carnitine palmitoyltransferase I) and extramitochondrial (acyl-CoA oxidase, cytochrome P450 4A3) enzymes. In striking contrast, the hepatic and cardiac expression of most PPARalpha target genes was not induced by fasting in PPARalpha-/- mice. These results define a critical role for PPARalpha in a transcriptional regulatory response to fasting and identify the PPARalpha-/- mouse as a potentially useful murine model of inborn and acquired abnormalities of human fatty acid utilization.

The role of the peroxisome proliferator-activated receptor alpha (PPAR alpha) in the control of cardiac lipid metabolism

The postnatal mammalian heart uses mitochondrial fatty acid oxidation (FAO) as the chief source of energy to meet the high energy demands necessary for pump function. Flux through the cardiac FAO pathway is tightly controlled in accordance with energy demands dictated by diverse physiologic and dietary conditions. In this report, we demonstrate that the lipid-activated nuclear receptor, peroxisome proliferator-activated receptor alpha (PPARalpha), regulates the expression of several key enzymes involved in cardiac mitochondrial FAO. In response to the metabolic stress imposed by pharmacologic inhibition of mitochondrial long-chain fatty acid import with etomoxir, PPARa serves as a molecular 'lipostat' factor by inducing the expression of target genes involved in fatty acid utilization including enzymes involved in mitochondrial and peroxisomal beta-oxidation pathways. In mice lacking PPARalpha (PPARalpha-/- mice), etomoxir precipitates a cardiac phenotype characterized by myocyte lipid accumulation. Surprisingly, this metabolic regulatory response is influenced by gender as demonstrated by the observation that male PPARalpha-/- mice are more susceptible to the metabolic stress compared to female animals. These results identify an important role for PPARalpha in the control of cardiac lipid metabolism.

A corepressor and chicken ovalbumin upstream promoter transcriptional factor proteins modulate peroxisome proliferator-activated receptor-gamma2/retinoid X receptor alpha-activated transcription from the murine lipoprotein lipase promoter

Complex physiological stimuli differentially regulate the tissue-specific transcription of the lipoprotein lipase (LPL) gene. A conserved DNA recognition element (-171 to -149 bp) within the promoter functions as a transcriptional enhancer when bound by the peroxisome proliferator-activated receptor-gamma2 (PPARgamma2)/retinoid X receptor alpha (RXRalpha) heterodimer, but serves as a transcriptional silencer in the presence of unidentified double and single stranded DNA-binding proteins. To address this apparent paradox, the current study examined the effect of two classes of candidate comodulatory proteins, COUP-TF (chicken ovalbumin upstream promoter transcriptional factor) and the corepressor SMRT (silencing mediator of retinoic acid receptor and thyroid receptor). The expression of COUP-TF was detected by Western and Northern blots in a preadipocyte 3T3-L1 cell model during periods corresponding to increased LPL transcription. Cotransfection of COUP-TF expression constructs in the renal epithelial 293T cell line significantly increased transcription from the LPL promoter in synergy with PPARgamma2/RXRalpha heterodimers. The COUP-TFII (ARP-1) protein specifically bound the LPL PPAR recognition element inelectromobility shift assays and interacted directly with the ligand-binding domain of PPARgamma in pull-down experiments. In contrast, cotransfection of SMRT repressed PPARgamma2/ RXRalpha-mediated LPL transcription in the absence or presence of COUP-TFII (ARP-1). The interaction between PPARgamma2 and SMRT localized to the receptor-interactive domain 2 (amino acids 1260-1495) of the SMRT protein based on cotransfection and pull-down assays. These in vitro data indicate that COUP-TF proteins and SMRT modulate PPARgamma-mediated LPL transcription in the 293T cell line.

Defects in mitochondrial beta-oxidation of fatty acids

Mitochondrial beta-oxidation of fatty acids generates energy by direct electron transfer at the dehydrogenase steps along with the ultimate product of acetyl-coenzyme A that can be further oxidized for ATP synthesis, or conversion to ketone bodies. This review describes the human inborn errors of this pathway and recent results concerning the development and use of mouse models of these inherited enzyme deficiencies.