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.

IntroductionMyocardial energy substrate preference is tightly controlled in mammalian organisms during development and in response to diverse dietary, physiologic, and pathophysiologic conditions (1, 2). During the fetal period, glucose and lactate serve as the chief myocardial energy substrates. After birth and during the postnatal period, myocardial energy is derived increasingly from reducing equivalents generated by mitochondrial β-oxidation of long-chain fatty acids (3). In the normal adult heart, mitochondrial fatty acid oxidation (FAO) accounts for the majority of ATP production (1). The importance of the FAO pathway as a source of energy in the postnatal human heart is underscored by the severe clinical manifestations of genetic defects in mitochondrial FAO enzymes, including childhood cardiomyopathy and sudden death, presumably due to the accumulation of myocardial long-chain fatty acid intermediates coupled with depletion of energy stores (4).The results of studies performed in cell culture and in vivo have established a critical role for members of the nuclear receptor superfamily in the transcriptional control of genes encoding cardiac FAO enzymes (5-7).Peroxisome proliferator-activated receptor-α (PPARα), a lipid-activated nuclear receptor (8), has been shown to regulate basal and fatty acid-induced transcription of FAO enzyme genes, including medium-chain acylCoA dehydrogenase (5) and muscle carnitine palmitoyltransferase I (M-CPT I or CPT Iβ) (9, 10). PPARα binds to target DNA elements as a heterodimeric partner with the retinoid X receptor, and is activated by a variety of ligands, including long-chain fatty acids (11). The expression of mitochondrial and peroxisomal FAO enzymes are reduced in postnatal liver and heart of PPARα-null (PPARα -/-) mice (12, 13). Moreover, PPARα -/-mice accumulate myocardial lipid in the context of conditions known to increase FAO rates, such as fasting, indicating that PPARα plays a critical role in the maintenance of cardiac energy and lipid homeostasis by its regulatory influence on cellular fatty acid utilization pathways (14,15).During the development of pressure overloadinduced ventricular hypertrophy, myocardial FAO rates decrease and glucose utilization increases, a reversion to the fetal pattern of energy substrate utilization (16-18). The expression of mitochondrial We sought to delineate the molecular regulatory events involved in the energy substrate preference switch from fatty acids to glucose during cardiac hypertrophic growth. α 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-α (PPARα) response element...

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Medical and device therapies that reduce heart failure morbidity and mortality also lead to decreased left ventricular (LV) volume and mass, and a more normal elliptical shape of the ventricle. These are due to changes in myocyte size, structure and organization that have been referred to collectively as “reverse remodeling.” Moreover, there are subsets of patients whose hearts have undergone reverse remodeling either spontaneously, or following medical or device therapies, and whose clinical course is associated with freedom from future heart failure events. This phenomenon has been referred to as “myocardial recovery.” Despite the frequent interchangeable use of the terms myocardial recovery and reverse remodeling to describe the reversal of various aspects of the heart failure phenotype following medical and device therapy, the literature suggests that there are important differences between these two phenomenon, and that myocardial recovery and reverse remodeling are not synonymous. In the following review, we will discuss the biology of cardiac remodeling, cardiac reverse remodeling and myocardial recovery, with the intent of providing a conceptual framework for understanding myocardial recovery.

The expression of enzymes involved in fatty acid ␤-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 ␣ (PPAR␣). Evidence has emerged that PPAR␣ 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 PPAR␣ 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 PPAR␣ target element reporters and mammalian one-hybrid transcription assays revealed that p38 MAPK phosphorylation of PPAR␣ significantly enhanced ligand-dependent transactivation. Cotransfection studies performed with several known coactivators of PPAR␣ 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 PPAR␣ as a downstream effector of p38 kinase-dependent stress-activated signaling in the heart, linking extracellular stressors to alterations in energy metabolic gene expression.The expression of enzymes involved in fatty acid ␤-oxidation (FAO), 1 the principal source of energy production in the adult mammalian heart, is tightly controlled at the transcriptional level during cardiac development and in response to physiologic and pathophysiologic stimuli (1-7). The nuclear receptor PPAR␣ has been shown to serve as a key transcriptional regulator of this energy metabolic pathway (Ref. 8; reviewed in Ref. 9). PPAR␣ is a member of the nuclear receptor superfamily of transcription factors and binds cognate response elements as an obligate heterodimer with the retinoid X receptor (RXR). PPAR␣ is ligand-activated by a variety of natural and synthetic agonists, including arachidonic acid derivatives, fibrates, and long-chain fatty acids: metabolic substrates for cardiac FAO enzymes. The important role played by PPAR␣ in cardiac metabolism is underscored by the marked reduction in the basal level of cardiac FAO enzyme gene expression in PPAR␣ Ϫ/Ϫ mice (10, 11), leading to reduced long-chain fatty acid uptake and oxidation (12).Evidence has emerged that PPAR␣ plays a critical role in the energy metabolic stress response in tissues that rely largely on mitochondrial fat oxidation for energy production, such as heart and liver. Under normal physiologic conditions, the expression of cardiac FAO enzyme genes are induced after a short term fast coincident with increased use of fatty acids for myocardial energy production (1, 3). In contrast, PPAR␣ Ϫ/Ϫ mice do not exhibit the expecte...

Hypertrophy allows the heart to adapt to workload but culminates in later pump failure; how it is achieved remains uncertain. Previously, we showed that hypertrophy is accompanied by activation of cyclin T/Cdk9, which phosphorylates the C-terminal domain of the large subunit of RNA polymerase II, stimulating transcription elongation and pre-mRNA processing; Cdk9 activity was required for hypertrophy in culture, whereas heart-specific activation of Cdk9 by cyclin T1 provoked hypertrophy in mice. Here, we report that aMHC-cyclin T1 mice appear normal at baseline yet suffer fulminant apoptotic cardiomyopathy when challenged by mechanical stress or signaling by the G-protein Gq. At pathophysiological levels, Cdk9 activity suppresses many genes for mitochondrial proteins including master regulators of mitochondrial function (peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1), nuclear respiratory factor-1). In culture, cyclin T1/Cdk9 suppresses PGC-1, decreases mitochondrial membrane potential, and sensitizes cardiomyocytes to apoptosis, effects rescued by exogenous PGC-1. Cyclin T1/Cdk9 inhibits PGC-1 promoter activity and preinitiation complex assembly. Thus, chronic activation of Cdk9 causes not only cardiomyocyte enlargement but also defective mitochondrial function, via diminished PGC-1 transcription, and a resulting susceptibility to apoptotic cardiomyopathy.

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BackgroundTissue injury triggers inflammatory responses that promote tissue fibrosis; however, the mechanisms that couple tissue injury, inflammation, and fibroblast activation are not known. Given that dying cells release proinflammatory “damage-associated molecular patterns” (DAMPs), we asked whether proteins released by necrotic myocardial cells (NMCs) were sufficient to activate fibroblasts in vitro by examining fibroblast activation after stimulation with proteins released by necrotic myocardial tissue, as well as in vivo by injecting proteins released by necrotic myocardial tissue into the hearts of mice and determining the extent of myocardial inflammation and fibrosis at 72 hours.Methods and ResultsThe freeze–thaw technique was used to induce myocardial necrosis in freshly excised mouse hearts. Supernatants from NMCs contained multiple DAMPs, including high mobility group box-1 (HMGB1), galectin-3, S100β, S100A8, S100A9, and interleukin-1α. NMCs provoked a significant increase in fibroblast proliferation, α–smooth muscle actin activation, and collagen 1A1 and 3A1 mRNA expression and significantly increased fibroblast motility in a cell-wounding assay in a Toll-like receptor 4 (TLR4)- and receptor for advanced glycation end products–dependent manner. NMC stimulation resulted in a significant 3- to 4-fold activation of Akt and Erk, whereas pretreatment with Akt (A6730) and Erk (U0126) inhibitors decreased NMC-induced fibroblast proliferation dose-dependently. The effects of NMCs on cell proliferation and collagen gene expression were mimicked by several recombinant DAMPs, including HMGB1 and galectin-3. Moreover, immunodepletion of HMGB1 in NMC supernatants abrogated NMC-induced cell proliferation. Finally, injection of NMC supernatants or recombinant HMGB1 into the heart provoked increased myocardial inflammation and fibrosis in wild-type mice but not in TLR4-deficient mice.ConclusionsThese studies constitute the initial demonstration that DAMPs released by NMCs induce fibroblast activation in vitro, as well as myocardial inflammation and fibrosis in vivo, at least in part, through TLR4-dependent signaling.