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 γ 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/energyproduction 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.

The gene encoding the transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) was targeted in mice. PGC-1α null (PGC-1α−/−) mice were viable. However, extensive phenotyping revealed multi-system abnormalities indicative of an abnormal energy metabolic phenotype. The postnatal growth of heart and slow-twitch skeletal muscle, organs with high mitochondrial energy demands, is blunted in PGC-1α−/− mice. With age, the PGC-1α−/− mice develop abnormally increased body fat, a phenotype that is more severe in females. Mitochondrial number and respiratory capacity is diminished in slow-twitch skeletal muscle of PGC-1α−/− mice, leading to reduced muscle performance and exercise capacity. PGC-1α−/− mice exhibit a modest diminution in cardiac function related largely to abnormal control of heart rate. The PGC-1α−/− mice were unable to maintain core body temperature following exposure to cold, consistent with an altered thermogenic response. Following short-term starvation, PGC-1α−/− mice develop hepatic steatosis due to a combination of reduced mitochondrial respiratory capacity and an increased expression of lipogenic genes. Surprisingly, PGC-1α−/− mice were less susceptible to diet-induced insulin resistance than wild-type controls. Lastly, vacuolar lesions were detected in the central nervous system of PGC-1α−/− mice. These results demonstrate that PGC-1α is necessary for appropriate adaptation to the metabolic and physiologic stressors of postnatal life.

Muscle tissue is the major site for insulin-stimulated glucose uptake in vivo, due primarily to the recruitment of the insulinsensitive glucose transporter (GLUT4) to the plasma membrane. Surprisingly, virtually all cultured muscle cells express little or no GLUT4. We show here that adenovirus-mediated expression of the transcriptional coactivator PGC-1, which is expressed in muscle in vivo but is also deficient in cultured muscle cells, causes the total restoration of GLUT4 mRNA levels to those observed in vivo. This increased GLUT4 expression correlates with a 3-fold increase in glucose transport, although much of this protein is transported to the plasma membrane even in the absence of insulin. PGC-1 mediates this increased GLUT4 expression, in large part, by binding to and coactivating the muscle-selective transcription factor MEF2C. These data indicate that PGC-1 is a coactivator of MEF2C and can control the level of endogenous GLUT4 gene expression in muscle.MEF2 ͉ diabetes T ype 2 diabetes mellitus, the most common endocrine disorder, potentially affects up to 5% of the western population (1). Patients with type 2 diabetes generally suffer both from reduced insulin secretion and from resistance to the actions of insulin. Hyperglycemic-hyperinsulinemic clamp analyses of human type 2 diabetic patients show that insulin resistance in muscle is caused by a defect in glucose transport (2). The principal insulin-sensitive glucose transporter in muscle is the insulin-sensitive glucose transporter (GLUT4), which is recruited to the sarcolemma following insulin stimulation. Definitive evidence that GLUT4 is the primary mediator both of basal and insulin-stimulated glucose transport in muscle comes from muscle-specific GLUT4 knockout mice (3). Ex vivo glucose transport assays of isolated muscle tissue from these mice demonstrate an 80% reduction in basal glucose transport and a complete loss of insulin-stimulated glucose uptake. Overexpression of GLUT4 in muscle of genetically diabetic mice (db͞db) alleviates insulin resistance and improves glycemic control by elevating both basal and insulin-stimulated glucose transport (4). Together, these data point to skeletal muscle glucose transport as the rate-limiting step for whole body glucose disposal and suggest that the regulation of GLUT4 expression is a potential target for treatment of diabetes mellitus.We have previously described a coactivator of PPAR␥ and other nuclear receptors termed PGC-1 that plays a key role in several aspects of thermogenesis and oxidative metabolism. PGC-1 also stimulates mitochondrial biogenesis per se through coactivation of nuclear respiratory factor-1 (NRF-1) (5, 6). Indeed, PGC-1 expression in both muscle and fat cells activates the expression of several genes of the oxidative phosphorylation pathway, including cytochrome c oxidase (COX) subunits II and IV, and ATP synthetase. This coactivator also stimulates the induction of a mitochondrial uncoupling protein (UCP), UCP-1 in fat cells and UCP-2 in muscle cells (6).The chronically inc...

Recent evidence has defined an important role for PPARα in the transcriptional control of cardiac energy metabolism. To investigate the role of PPARα in the genesis of the metabolic and functional derangements of diabetic cardiomyopathy, mice with cardiac-restricted overexpression of PPARα (MHC-PPAR) were produced and characterized. The expression of PPARα target genes involved in cardiac fatty acid uptake and oxidation pathways was increased in MHC-PPAR mice. Surprisingly, the expression of genes involved in glucose transport and utilization was reciprocally repressed in MHC-PPAR hearts. Consistent with the gene expression profile, myocardial fatty acid oxidation rates were increased and glucose uptake and oxidation decreased in MHC-PPAR mice, a metabolic phenotype strikingly similar to that of the diabetic heart. MHC-PPAR hearts exhibited signatures of diabetic cardiomyopathy including ventricular hypertrophy, activation of gene markers of pathologic hypertrophic growth, and transgene expression-dependent alteration in systolic ventricular dysfunction. These results demonstrate that (a) PPARα is a critical regulator of myocardial fatty acid uptake and utilization, (b) activation of cardiac PPARα regulatory pathways results in a reciprocal repression of glucose uptake and utilization pathways, and (c) derangements in myocardial energy metabolism typical of the diabetic heart can become maladaptive, leading to cardiomyopathy.

Recent evidence has defined an important role for PPARα in the transcriptional control of cardiac energy metabolism. To investigate the role of PPARα in the genesis of the metabolic and functional derangements of diabetic cardiomyopathy, mice with cardiac-restricted overexpression of PPARα (MHC-PPAR) were produced and characterized. The expression of PPARα target genes involved in cardiac fatty acid uptake and oxidation pathways was increased in MHC-PPAR mice. Surprisingly, the expression of genes involved in glucose transport and utilization was reciprocally repressed in MHC-PPAR hearts. Consistent with the gene expression profile, myocardial fatty acid oxidation rates were increased and glucose uptake and oxidation decreased in MHC-PPAR mice, a metabolic phenotype strikingly similar to that of the diabetic heart. MHC-PPAR hearts exhibited signatures of diabetic cardiomyopathy including ventricular hypertrophy, activation of gene markers of pathologic hypertrophic growth, and transgene expression-dependent alteration in systolic ventricular dysfunction. These results demonstrate that (a) PPARα is a critical regulator of myocardial fatty acid uptake and utilization, (b) activation of cardiac PPARα regulatory pathways results in a reciprocal repression of glucose uptake and utilization pathways, and (c) derangements in myocardial energy metabolism typical of the diabetic heart can become maladaptive, leading to cardiomyopathy.

Recent evidence has defined an important role for PPARα in the transcriptional control of cardiac energy metabolism. To investigate the role of PPARα in the genesis of the metabolic and functional derangements of diabetic cardiomyopathy, mice with cardiac-restricted overexpression of PPARα (MHC-PPAR) were produced and characterized. The expression of PPARα target genes involved in cardiac fatty acid uptake and oxidation pathways was increased in MHC-PPAR mice. Surprisingly, the expression of genes involved in glucose transport and utilization was reciprocally repressed in MHC-PPAR hearts. Consistent with the gene expression profile, myocardial fatty acid oxidation rates were increased and glucose uptake and oxidation decreased in MHC-PPAR mice, a metabolic phenotype strikingly similar to that of the diabetic heart. MHC-PPAR hearts exhibited signatures of diabetic cardiomyopathy including ventricular hypertrophy, activation of gene markers of pathologic hypertrophic growth, and transgene expression-dependent alteration in systolic ventricular dysfunction. These results demonstrate that (a) PPARα is a critical regulator of myocardial fatty acid uptake and utilization, (b) activation of cardiac PPARα regulatory pathways results in a reciprocal repression of glucose uptake and utilization pathways, and (c) derangements in myocardial energy metabolism typical of the diabetic heart can become maladaptive, leading to cardiomyopathy.

Abstract-Recent evidence has identified the peroxisome proliferator-activated receptor ␥ coactivator-1␣ (PGC-1␣) as a regulator of cardiac energy metabolism and mitochondrial biogenesis. We describe the development of a transgenic system that permits inducible, cardiac-specific overexpression of PGC-1␣. Expression of the PGC-1␣ transgene in this system (tet-on PGC-1␣) is cardiac-specific in the presence of doxycycline (dox) and is not leaky in the absence of dox.Overexpression of PGC-1␣ in tet-on PGC-1␣ mice during the neonatal stages leads to a dramatic increase in cardiac mitochondrial number and size coincident with upregulation of gene markers associated with mitochondrial biogenesis. In contrast, overexpression of PGC-1␣ in the hearts of adult mice leads to a modest increase in mitochondrial number, derangements of mitochondrial ultrastructure, and development of cardiomyopathy. The cardiomyopathy in adult tet-on PGC-1␣ mice is characterized by an increase in ventricular mass and chamber dilatation. Surprisingly, removal of dox and cessation of PGC-1␣ overexpression in adult mice results in complete reversal of cardiac dysfunction within 4 weeks. These results indicate that PGC-1␣ drives mitochondrial biogenesis in a developmental stage-dependent manner permissive during the neonatal period. This unique murine model should prove useful for the study of the molecular regulatory programs governing mitochondrial biogenesis and characterization of the relationship between mitochondrial dysfunction and cardiomyopathy and as a general model of inducible, reversible cardiomyopathy. Key Words: mitochondria Ⅲ metabolism Ⅲ transgenic mice Ⅲ cardiomyopathy Ⅲ transcription factors T he heart has an extraordinarily high capacity for mitochondrial ATP production to meet the rigorous and dynamic energy demands of the postnatal environment. The importance of the mitochondrion for cardiac function is underscored by the development of cardiomyopathy in inherited and acquired forms of mitochondrial dysfunction in humans. [1][2][3] Because mitochondria contain proteins encoded by both nuclear and mitochondrial genes, mitochondrial biogenesis requires the coordinate regulation of these 2 genomes. The transcriptional regulatory network controlling the expression of nuclear and mitochondrial genes includes nuclear respiratory factor (NRF)-1 and NRF-2 and mitochondrial transcription factor A (mtTFA). The regulatory pathways upstream of these factors are a focus of intense investigation. 4 -7 Recently, the peroxisome proliferator-activated receptor (PPAR) ␥ coactivator-1␣ (PGC-1␣) has been identified as an inducible upstream regulator of mitochondrial number and function. 8 -10 PGC-1␣ is a transcriptional coactivator that lacks DNA-binding activity but interacts with and coactivates numerous transcription factors, including nuclear receptors such as PPAR␥ and PPAR␣, estrogen receptor ␣, thyroid hormone, retinoid receptors, and hepatocyte nuclear factor-4␣. 8,11-16 PGC-1␣ also coactivates nonnuclear receptor transcription factors, ...

The transcriptional coactivator peroxisome proliferator-activated receptor ␥ coactivator-1␣ (PGC-1␣) has been identified as an inducible regulator of mitochondrial function. Skeletal muscle PGC-1␣ expression is induced post-exercise. Therefore, we sought to determine its role in the regulation of muscle fuel metabolism. Studies were performed using conditional, musclespecific, PGC-1␣ gain-of-function and constitutive, generalized, loss-of-function mice. Forced expression of PGC-1␣ increased muscle glucose uptake concomitant with augmentation of glycogen stores, a metabolic response similar to postexercise recovery. Induction of muscle PGC-1␣ expression prevented muscle glycogen depletion during exercise. Conversely, PGC-1␣-deficient animals exhibited reduced rates of muscle glycogen repletion post-exercise. PGC-1␣ was shown to increase muscle glycogen stores via several mechanisms including stimulation of glucose import, suppression of glycolytic flux, and by down-regulation of the expression of glycogen phosphorylase and its activating kinase, phosphorylase kinase ␣. These findings identify PGC-1␣ as a critical regulator of skeletal muscle fuel stores.Glucose and fatty acids are the chief fuel sources for skeletal muscle. During prolonged bouts of low intensity exercise, muscle energy needs are met through utilization of both substrates with mitochondrial fatty acid oxidation serving a "glucose sparing" function (1, 2). During acute high intensity exercise, glucose derived from hepatic and muscle glycogen stores serves as the chief energy source (reviewed in Refs. 3-5). Rapid glycogen repletion following a bout of exhausting intense exercise is an important adaptive response, preparing the muscle for subsequent bouts of activity. With endurance exercise training, the capacity for mitochondrial oxidation of fatty acids is augmented and muscle glycogen reserves increase (2). In disease states such as diabetes and heart failure, the capacity for muscle energy substrate utilization is reduced due to alterations in glucose metabolism and derangements in mitochondrial function (6, 7) (reviewed in Ref. 8).The molecular regulatory mechanisms involved in the control of muscle fuel metabolism are incompletely understood. Recent evidence implicates the transcriptional coactivator, peroxisome proliferator-activated receptor (PPAR) 5 -␥ coactivator 1␣ (PGC-1␣), in the regulation of striated muscle energy metabolism and function (9 -13). PGC-1␣ levels are rapidly induced in skeletal muscle following bouts of activity in rodents and humans (14 -22). PGC-1␣ coactivates multiple transcription factors involved in mitochondrial biogenesis, oxidative phosphorylation, and fatty acid oxidation, including the estrogen-related receptor ␣, PPAR␣, and nuclear respiratory factors 1 and 2 (6, 23-26). PGC-1␣ gain-and loss-of-function studies conducted in cells and in mice have demonstrated that PGC-1␣ stimulates gene regulatory programs that augment mitochondrial oxidative capacity in tissues with high energy demands, such as heart and ske...