The peroxisome proliferator-activated receptor ␣ (PPAR␣) is a fatty acid-activated nuclear receptor that plays a key role in the transcriptional regulation of genes involved in cellular lipid and energy metabolism. PPAR␣ together with PPAR␦ and PPAR␥ form a subgroup within the nuclear receptor superfamily (12, 17). In contrast to PPAR␣ which is involved in the control of cellular lipid utilization, PPAR␥ has been shown to be a necessary component of the adipocyte differentiation program (22,36). The biological function of PPAR␦ is unknown. A diverse group of compounds can act as activating ligands for PPAR␣ including several prostaglandin derivatives, eicosanoids, and long-chain unsaturated fatty acids (8,18,39). To date, the majority of PPAR␣ target genes identified are involved in cellular fatty acid oxidation (FAO) (22). We and others have previously demonstrated that PPAR␣ mediates fatty acid-induced transcriptional control of several nuclear genes encoding mitochondrial FAO enzymes, including mediumchain acyl coenzyme A (acyl-CoA) dehydrogenase (MCAD) (9) and muscle carnitine palmitoyltransferase I (M-CPT I or CPT I␤) (2, 9, 26, 41). PPAR␣ is enriched in tissues with high oxidative energy demands that depend on mitochondrial FAO as a primary energy source such as heart and liver (17). PPAR␣ is also expressed at high levels in brown adipose tissue (BAT), a specialized tissue in which mitochondrial FAO provides the reducing equivalents necessary for the generation of heat via the uncoupling of oxidative phosphorylation. Consistent with its regulatory role in mitochondrial FAO, the expression of PPAR␣ is much higher in BAT than in white adipose tissue, which is a lipid storage tissue (15,36). Recent studies of PPAR␣-null mice have confirmed that PPAR␣ is necessary in vivo for high-level expression of mitochondrial and peroxisomal FAO enzyme genes in heart and liver under basal and stimulated conditions (1,7,24).Evidence has emerged that nuclear receptors regulate transcription, in large part, via interactions with coactivator (e.g., CBP/p300, SRC-1, GRIP1, pCIP) or corepressor (e.g., N-CoR, SMRT) molecules (4,5,10,11,14,20). Nuclear receptor interacting proteins regulate transcriptional activity by affecting chromatin structure through changes in the acetylation status of histones. Most coactivators are recruited to nuclear receptors upon ligand binding. Several coactivators such as SRC-1, which possesses intrinsic histone acetylase activity, also serve as adaptor molecules to link nuclear receptors to multiprotein complexes containing larger pleiotropic activator proteins such as CBP or p300 (35,37,40). The ligand-mediated activation of PPARs also involves coactivator networks (28, 44). Crystallographic studies have demonstrated that the binding of ligand to PPAR stabilizes the position of an alpha-helical domain (the AF2 helix) forming a "charge clamp" that interacts with an LXXLL motif within coactivator molecules (28). Indeed, SRC-1 has been shown to interact with the PPARs upon ligand binding leading to t...

Orphan nuclear receptor ERRalpha (NR3B1) is recognized as a key regulator of mitochondrial biogenesis, but it is not known whether ERRalpha and other ERR isoforms play a broader role in cardiac energetics and function. We used genome-wide location analysis and expression profiling to appraise the role of ERRalpha and gamma (NR3B3) in the adult heart. Our data indicate that the two receptors, acting as nonobligatory heterodimers, target a common set of promoters involved in the uptake of energy substrates, production and transport of ATP across the mitochondrial membranes, and intracellular fuel sensing, as well as Ca(2+) handling and contractile work. Motif-finding algorithms assisted by functional studies indicated that ERR target promoters are enriched for NRF-1, CREB, and STAT3 binding sites. Our study thus reveals that the ERRs orchestrate a comprehensive cardiac transcriptional program and further suggests that modulation of ERR activities could be used to manage cardiomyopathies.

The transcriptional coactivator PPAR␥ coactivator-1␣ (PGC-1␣) has been characterized as a broad regulator of cellular energy metabolism. Although PGC-1␣ functions through many transcription factors, the PGC-1␣ partners identified to date are unlikely to account for all of its biologic actions. The orphan nuclear receptor estrogen-related receptor ␣ (ERR␣) was identified in a yeast two-hybrid screen of a cardiac cDNA library as a novel PGC-1␣-binding protein. Cellular energy production is tightly linked to metabolic demand, which is, in turn, dictated by diverse developmental, physiologic, and environmental conditions. The capacity for cellular ATP production is controlled, in part, by the expression levels of nuclear genes involved in mitochondrial oxidative metabolism. Thus, tight regulation of cellular energy metabolism necessitates transduction of diverse signals related to cellular energy demands to the nucleus. Although numerous factors involved in the transcriptional regulation of metabolic gene expression have been identified, the precise pathways involved in the physiologic control of cellular energy metabolism have not been delineated. The recent discovery of PPAR␥ coactivator-1␣ (PGC-1␣), 1 PGC-1␤, and the PGC-1-related protein, a family of inducible transcriptional coactivators responsive to selective physiological stimuli, have provided new insights into the link between extracellular events and the regulation of genes involved in energy metabolism. PGC-1␣, the first member of this novel coactivator family to be identified, was initially characterized as a key regulator of thermogenesis in brown adipose tissue (BAT) and skeletal muscle via its coactivation of the adipose-enriched nuclear receptor, PPAR␥ (1, 2). Subsequent studies have revealed a broader role for PGC-1␣ in a variety of cellular energy metabolic processes including mitochondrial biogenesis, mitochondrial fatty acid oxidation (FAO), and gluconeogenesis (2-6). The function of PGC-1␤ and PGC-1-related protein remain to be defined.PGC-1␣ is unique from the p160 and p300/cAMP response element-binding protein-binding protein classes of transcriptional coactivators in its tissue-restricted expression pattern, its developmental regulation, and its inducibility by specific physiological stimuli. PGC-1␣ is enriched in tissues reliant on oxidative metabolism for ATP generation (heart, skeletal muscle) or heat (BAT) but is also expressed in liver, brain, and kidney (1). Immediately after birth, PGC-1␣ expression increases in heart coincident with a shift from reliance on glycolysis to mitochondrial FAO as the chief energy source in the adult myocardium (4). PGC-1␣ expression is induced in adult skeletal muscle, BAT, and heart in response to stimuli that increase energy demands. For example, cold exposure leads to

Estrogen-related receptors (ERRs) are orphan nuclear receptors activated by the transcriptional coactivator peroxisome proliferator-activated receptor ␥ (PPAR␥) coactivator 1␣ (PGC-1␣), a critical regulator of cellular energy metabolism. However, metabolic target genes downstream of ERR␣ have not been well defined. To identify ERR␣-regulated pathways in tissues with high energy demand such as the heart, gene expression profiling was performed with primary neonatal cardiac myocytes overexpressing ERR␣. ERR␣ upregulated a subset of PGC-1␣ 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, C 2 C 12 myotubes, and cardiac and skeletal muscle of ERR␣ ؊/؊ mice. Consistent with the gene expression results, ERR␣ increased myocyte lipid accumulation and fatty acid oxidation rates. Many of the genes regulated by ERR␣ are known targets for the nuclear receptor PPAR␣, and therefore, the interaction between these regulatory pathways was explored. ERR␣ activated PPAR␣ gene expression via direct binding of ERR␣ to the PPAR␣ gene promoter. Furthermore, in fibroblasts null for PPAR␣ and ERR␣, the ability of ERR␣ to activate several PPAR␣ targets and to increase cellular fatty acid oxidation rates was abolished. PGC-1␣ was also shown to activate ERR␣ gene expression. We conclude that ERR␣ serves as a critical nodal point in the regulatory circuitry downstream of PGC-1␣ to direct the transcription of genes involved in mitochondrial energy-producing pathways in cardiac and skeletal muscle.The essential role of nuclear receptors in regulating various cellular metabolic pathways is becoming increasingly evident. In recent years, various nuclear receptors that do not respond to classical endocrine ligands, including peroxisome proliferator-activated receptors (PPARs), liver X receptors, farnesoid X receptors, and retinoid X receptors, have been shown to be activated by low-affinity diet-derived ligands (6,11,26,44). Activation of these receptors by metabolite ligands such as fatty acids, oxysterols, and bile acids elicits downstream transcriptional regulation of pathways involved in synthesis and catabolism of these ligands. The remaining receptors, designated orphan receptors because endogenous ligands have not been identified, comprise the largest subcategory of nuclear receptors. It is likely that orphan receptors serve additional roles in regulating intermediary metabolism. Linking orphan receptors to target genes is an important goal in the field of nuclear receptor biology. Target gene profiling will also provide insights for determining what metabolites serve as endogenous ligands for these receptors and, in turn, for developing pharmacologic interventions designed to regulate cellular metabolism.One group of orphan receptors recently identified as candidate regulators of cellular metabolism are the estrogen-related recepto...

The transcriptional coactivator PGC-1␣ 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-1␣ expression in skeletal muscle coordinately upregulate the expression of pyruvate dehydrogenase kinase 4 (PDK4), a negative regulator of glucose oxidation. Forced expression of PGC-1␣ in C 2 C 12 myotubes induced PDK4 mRNA and protein expression. PGC-1␣-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-1␣-responsive element bound the estrogen-related receptor ␣ (ERR␣), a recently identified component of the PGC-1␣ signaling pathway. In addition, PGC-1␣ was shown to activate ERR␣ expression. Chromatin immunoprecipitation assays confirmed that PGC-1␣ and ERR␣ occupied the mPDK4 promoter in C 2 C 12 myotubes. Additionally, transfection studies using ERR␣-null primary fibroblasts demonstrated that ERR␣ is required for PGC-1␣-mediated activation of the mPDK4 promoter. As predicted by the effects of PGC-1␣ on PDK4 gene transcription, overexpression of PGC-1␣ in C 2 C 12 myotubes decreased glucose oxidation rates. These results identify the PDK4 gene as a new PGC-1␣/ERR␣ target and suggest a mechanism whereby PGC-1␣ exerts reciprocal inhibitory influences on glucose catabolism while increasing alternate mitochondrial oxidative pathways in skeletal muscle.