Total skeletal muscle PGC-1 deficiency uncouples mitochondrial derangements from fiber type determination and insulin sensitivity

Evidence is emerging that the PGC-1 coactivators serve a critical role in skeletal muscle metabolism, function, and disease. Mice with total PGC-1 deficiency in skeletal muscle (PGC-1α(-/-)β(f/f/MLC-Cre) mice) were generated and characterized. PGC-1α(-/-)β(f/f/MLC-Cre) mice exhibit a dramatic reduction in exercise performance compared to single PGC-1α- or PGC-1β-deficient mice and wild-type controls. The exercise phenotype of the PGC-1α(-/-)β(f/f/MLC-Cre) mice was associated with a marked diminution in muscle oxidative capacity, together with rapid depletion of muscle glycogen stores. In addition, the PGC-1α/β-deficient muscle exhibited mitochondrial structural derangements consistent with fusion/fission and biogenic defects. Surprisingly, the proportion of oxidative muscle fiber types (I, IIa) was not reduced in the PGC-1α(-/-)β(f/f/MLC-Cre) mice. Moreover, insulin sensitivity and glucose tolerance were not altered in the PGC-1α(-/-)β(f/f/MLC-Cre) mice. Taken together, we conclude that PGC-1 coactivators are necessary for the oxidative and mitochondrial programs of skeletal muscle but are dispensable for fundamental fiber type determination and insulin sensitivity.

The PGC-1 cascade as a therapeutic target for heart failure

The PPARγ coactivator-1 (PGC-1) family of transcriptional coactivators, together with estrogen related receptors (ERRs), plays a key role in regulating genes involved in myocardial fuel metabolism and cardiac function. Increasing evidence implicates dysregulation of this transcriptional regulatory circuit in the metabolic and functional disturbances that presage heart failure due to common diseases such as hypertension and diabetes. Accordingly, the PGC-1/ERR axis is a plausible candidate therapeutic target for novel therapeutics aimed at reversing the energy metabolic disturbances that contribute to heart failure. This review describes the biologic actions of the PGC-1 and ERR cascade and summarizes the evidence that dysregulation of this transcriptional regulatory circuit contributes to heart failure. Potential strategies to modulate this target pathway are reviewed. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

A PPARα promoter variant impairs ERR-dependent transactivation and decreases mortality after acute coronary ischemia in patients with diabetes

Activation of peroxisome proliferator-activated receptor alpha (PPARα) occurs in animal models of diabetes (DM) and is implicated in pathological responses to myocardial ischemia. Using bioinformatics, we identified a single nucleotide polymorphism (SNP) in the PPARα gene promoter (PPARA -54,642 G>A; rs135561) that altered the consensus sequence for a nuclear receptor binding site. Electrophoretic mobility shift assays showed that the domain bound two known PPARA transcriptional activators, estrogen-related receptor (ERR)-α and -γ and that PPARA G bound with greater affinity than PPARA A (>2-fold; P<0.05). Likewise, promoter-reporter analyses showed enhanced transcriptional activity for PPARA G vs. PPARA A for both ERR-α and -γ (3.1 vs.1.9-fold; P<0.05). Since PPARα activation impairs post-ischemic cardiac function in experimental models of DM, we tested whether decreased PPARA transcription in PPARA A carriers favorably impacted outcome after acute coronary ischemia in 705 patients hospitalized with acute coronary syndromes (ACS; 552 Caucasian, 106 African American). PPARA A allele frequencies were similar to non-diseased subjects. However, PPARA genotype correlated with 5-year mortality in diabetic (22.2% AA vs. 18.8% AG vs. 39.5% GG; P = 0.008), but not non-diabetic (P = 0.96) subjects (genotype by diabetes interaction P = 0.008). In the diabetic ACS subjects, PPARA A carriers had strikingly reduced all-cause mortality compared to PPARA G homozygotes, (unadjusted HR 0.44, 95% CI 0.26-0.75; P = 0.003; adjusted HR 0.48, 95% CI 0.27-0.83; P = 0.009). Consistent with previous descriptions of PPARα in experimental models and human disease, we describe a novel PPARA promoter SNP that decreases transcriptional activation of PPARA and protects against mortality in diabetic patients after ACS.

Ubiquitin proteasome-dependent degradation of the transcriptional coactivator PGC-1{alpha} via the N-terminal pathway

PGC-1α is a potent, inducible transcriptional coactivator that exerts control on mitochondrial biogenesis and multiple cellular energy metabolic pathways. PGC-1α levels are controlled in a highly dynamic manner reflecting regulation at both transcriptional and post-transcriptional levels. Here, we demonstrate that PGC-1α is rapidly degraded in the nucleus (t(½ 0.3 h) via the ubiquitin proteasome system. An N-terminal deletion mutant of 182 residues, PGC182, as well as a lysine-less mutant form, are nuclear and rapidly degraded (t(½) 0.5 h), consistent with degradation via the N terminus-dependent ubiquitin subpathway. Both PGC-1α and PGC182 degradation rates are increased in cells under low serum conditions. However, a naturally occurring N-terminal splice variant of 270 residues, NT-PGC-1α is cytoplasmic and stable (t(½>7 h), providing additional evidence that PGC-1α is degraded in the nucleus. These results strongly suggest that the nuclear N terminus-dependent ubiquitin proteasome pathway governs PGC-1α cellular degradation. In contrast, the cellular localization of NT-PCG-1α results in a longer-half-life and possible distinct temporal and potentially biological actions.

Cardiovascular drug discovery in the academic setting: building infrastructure, harnessing strengths, and seeking synergies

The flow of innovative, effective, and safe new drugs from pharmaceutical laboratories for the treatment and prevention of cardiovascular disease has slowed to a trickle. While the need for breakthrough cardiovascular disease drugs is still paramount, the incentive to develop these agents has been blunted by burgeoning clinical development costs coupled with a heightened risk of failure due to the unprecedented nature of the emerging drug targets and increasingly challenging regulatory environment. A fuller understanding of the drug targets and employing novel biomarker strategies in clinical trials should serve to mitigate the risk. In any event, these current challenges have evoked changing trends in the pharmaceutical industry, which have created an opportunity for non-profit biomedical research institutions to play a pivotal partnering role in early stage drug discovery. The obvious strengths of academic research institutions is the breadth of their scientific programs and the ability and motivation to "go deep" to identify and characterize new target pathways that unlock the specific mysteries of cardiovascular diseases--leading to a bounty of novel therapeutic targets and prescient biomarkers. However, success in the drug discovery arena within the academic environment is contingent upon assembling the requisite infrastructure, annexing the talent to interrogate and validate the drug targets, and building translational bridges with pharmaceutical organizations and patient-oriented researchers.

Structural effects in solvolytic reactions; carbon-13 NMR studies of carbocations: Effect of increasing electron demand on the carbon-13 NMR shifts in substituted tert-cumyl and 1-aryl-1-cyclopentyl carbocations-correlation of the data by a new set of substituent constants, sigma

The cationic carbon substituent chemical shifts (DeltadeltaC(+)) for nine representative meta-substituted tert-cumyl carbocations are correlated satisfactorily by the sigma(m) (+) substituent constants (slope rho+ = -18.18, correlation coefficient r = 0.990). However, the substituent chemical shifts (DeltadeltaC(+)) for the corresponding para derivatives are not correlated by the sigma(p) (+) substituent constants. The possibility of developing a set of substituent constants capable of correlating such (13)C NMR shifts was examined. The slope of the line defined by the meta substituents (rho+ = -18.18) was utilized to calculate sigma(C+) constants for both meta and para substituents. The utility of these constants was then tested by their ability to correlate the (13)C NMR shifts in the cations for a different system, the 1-aryl-1-cyclopentyl cations. Indeed, these sigma(C+) values correlate very well with the DeltadeltaC(+) values, yielding rho(C+) = -16.84, r = 0.999.

Preferential oxidation of triacylglyceride-derived fatty acids in heart is augmented by the nuclear receptor PPARalpha

RATIONALE

Long chain fatty acids (LCFAs) are the preferred substrate for energy provision in hearts. However, the contribution of endogenous triacylglyceride (TAG) turnover to LCFA oxidation and the overall dependence of mitochondrial oxidation on endogenous lipid is largely unstudied.

OBJECTIVE

We sought to determine the role of TAG turnover in supporting LCFA oxidation and the influence of the lipid-activated nuclear receptor, proliferator-activated receptor (PPAR)alpha, on this balance.

METHODS AND RESULTS

Palmitoyl turnover within TAG and palmitate oxidation rates were quantified in isolated hearts, from normal mice (nontransgenic) and mice with cardiac-specific overexpression of PPARalpha (MHC-PPARalpha). Turnover of palmitoyl units within TAG, and thus palmitoyl-coenzyme A recycling, in nontransgenic (4.5+/-2.3 micromol/min per gram dry weight) was 3.75-fold faster than palmitate oxidation (1.2+/-0.4). This high rate of palmitoyl unit turnover indicates preferential oxidation of palmitoyl units derived from TAG in normal hearts. PPARalpha overexpression augmented TAG turnover 3-fold over nontransgenic hearts, despite similar fractions of acetyl-coenzyme A synthesis from palmitate and oxygen use at the same workload. Palmitoyl turnover within TAG of MHC-PPARalpha hearts (16.2+/-2.9, P<0.05) was 12.5-fold faster than oxidation (1.3+/-0.2). Elevated TAG turnover in MHC-PPARalpha correlated with increased mRNA for enzymes involved in both TAG synthesis, Gpam (glycerol-3-phosphate acyltransferase, mitochondrial), Dgat1 (diacylglycerol acetyltransferase 1), and Agpat3 (1-acylglycerol-3-phospate O-acyltransferase 3), and lipolysis, Pnliprp1 (pancreatic lipase related protein 1).

CONCLUSIONS

The role of endogenous TAG in supporting beta-oxidation in the normal heart is much more dynamic than previously thought, and lipolysis provides the bulk of LCFA for oxidation. Accelerated palmitoyl turnover in TAG, attributable to chronic PPARalpha activation, results in near requisite oxidation of LCFAs from TAG.

Rescue of cardiomyopathy in peroxisome proliferator-activated receptor-alpha transgenic mice by deletion of lipoprotein lipase identifies sources of cardiac lipids and peroxisome proliferator-activated receptor-alpha activators

BACKGROUND

Emerging evidence in obesity and diabetes mellitus demonstrates that excessive myocardial fatty acid uptake and oxidation contribute to cardiac dysfunction. Transgenic mice with cardiac-specific overexpression of the fatty acid-activated nuclear receptor peroxisome proliferator-activated receptor-alpha (myosin heavy chain [MHC]-PPARalpha mice) exhibit phenotypic features of the diabetic heart, which are rescued by deletion of CD36, a fatty acid transporter, despite persistent activation of PPARalpha gene targets involved in fatty acid oxidation.

METHODS AND RESULTS

To further define the source of fatty acid that leads to cardiomyopathy associated with lipid excess, we crossed MHC-PPARalpha mice with mice deficient for cardiac lipoprotein lipase (hsLpLko). MHC-PPARalpha/hsLpLko mice exhibit improved cardiac function and reduced myocardial triglyceride content compared with MHC-PPARalpha mice. Surprisingly, in contrast to MHC-PPARalpha/CD36ko mice, the activity of the cardiac PPARalpha gene regulatory pathway is normalized in MHC-PPARalpha/hsLpLko mice, suggesting that PPARalpha ligand activity exists in the lipoprotein particle. Indeed, LpL mediated hydrolysis of very-low-density lipoprotein activated PPARalpha in cardiac myocytes in culture. The rescue of cardiac function in both models was associated with improved mitochondrial ultrastructure and reactivation of transcriptional regulators of mitochondrial function.

CONCLUSIONS

MHC-PPARalpha mouse hearts acquire excess lipoprotein-derived lipids. LpL deficiency rescues myocyte triglyceride accumulation, mitochondrial gene regulatory derangements, and contractile function in MHC-PPARalpha mice. Finally, LpL serves as a source of activating ligand for PPARalpha in the cardiomyocyte.

Impaired contractile function and calcium handling in hearts of cardiac-specific calcineurin b1-deficient mice

To define the necessity of calcineurin (Cn) signaling for cardiac maturation and function, the postnatal phenotype of mice with cardiac-specific targeted ablation of the Cn B1 regulatory subunit (Ppp3r1) gene (csCnb1(-/-) mice) was characterized. csCnb1(-/-) mice develop a lethal cardiomyopathy, characterized by impaired postnatal growth of the heart and combined systolic and diastolic relaxation abnormalities, despite a lack of structural derangements. Notably, the csCnb1(-/-) hearts did not exhibit diastolic dilatation, despite the severe functional phenotype. Myocytes isolated from the mutant mice exhibited reduced rates of contraction/relaxation and abnormalities in calcium transients, consistent with altered sarcoplasmic reticulum loading. Levels of sarco(endo) plasmic reticulum Ca-ATPase 2a (Atp2a2) and phospholamban were normal, but phospholamban phosphorylation was markedly reduced at Ser(16) and Thr(17). In addition, levels of the Na/Ca exchanger (Slc8a1) were modestly reduced. These results define a novel mouse model of cardiac-specific Cn deficiency and demonstrate novel links between Cn signaling, postnatal growth of the heart, pathological ventricular remodeling, and excitation-contraction coupling.

Interaction between PPARA genotype and beta-blocker treatment influences clinical outcomes following acute coronary syndromes

AIMS

beta-blockers (BB) are strongly recommended after an acute coronary syndrome (ACS), although all patients may not benefit. Causes for variable patient responses to BB are unknown. Given that myocardial ischemia and BB influence metabolic processes regulated by peroxisome proliferator-activated receptor alpha (PPARalpha), we hypothesized that interactions between polymorphisms of the PPARalpha gene (PPARA) and BB treatment would influence clinical outcome following ACS.

PATIENTS & METHODS

Patients were prospectively enrolled into an ACS registry. A total of 735 ACS patients were genotyped. Mortality and cardiac rehospitalization through 1 year were analyzed in relation to PPARA genotype and BB prescription (597 BB; 138 no BB) at discharge.

RESULTS

Significantly different outcomes associated with BB therapy were observed according to PPARA IVS7 2498 genotype (p = 0.002 for interaction). PPARA IVS7 2498 GG homozygous patients discharged on BB had decreased cardiac rehospitalization (hazard ratio [HR]: 0.52; 95% CI: 0.32-0.86; p = 0.011), while C allele carriers discharged on BB had nearly threefold increased cardiac rehospitalization (HR: 2.92; 95% CI: 1.32-6.92; p = 0.015; genotype interaction p = 0.0005) compared with patients not on BB. PPARA genotype was also associated with differences in PPARalpha expression, with significantly increased mRNA levels in myocardial samples from normal hearts among GC heterozygotes compared with GG homozygotes (p = 0.04). Transgenic mice with cardiac-specific overexpression of PPARalpha showed significantly reduced myocardial contractile and chronotropic responses to the beta-sympathomimetic dobutamine (p < 0.05) compared with wild-type littermates, supporting the hypothesis that increased PPARalpha levels result in a blunted beta-adrenergic response.

CONCLUSIONS

PPARA IVS7 2498 genotype is associated with heterogeneity in 1-year outcome in response to BB among patients following ACS, and may predict which patients benefit from BB therapy, putatively related to the effect of myocardial PPARalpha expression on beta-adrenergic responsiveness.

Differential effects of luminal arginine and glutamine on metalloproteinase production in the postischemic gut

BACKGROUND

Matrix metalloproteinases (MMPs) are a group of endopeptidases induced under inflammatory conditions in the intestine which possess the capacity to degrade components of the extracellular matrix. We have previously demonstrated that MMP-2 expression correlates with increased inducible nitric oxide synthase (iNOS) production in the stomach and that iNOS is upregulated in the postischemic gut by the luminal nutrient arginine and repressed by luminal glutamine. We therefore hypothesized that arginine would enhance expression of MMP-2 in the postischemic gut.

METHODS

Jejunal sacs were created in rats at laparotomy and filled with either 60 mM glutamine, arginine, or magnesium sulfate (osmotic control) followed by 60 minutes of superior mesenteric artery occlusion (SMAO) and 6 hours of reperfusion and compared with shams. Jejunum was harvested, and membrane type-1 matrix metalloproteinase (MT1-MMP), MMP-2, and iNOS protein expression was determined by Western analysis and MMP-9 production by gelatin zymography.

RESULTS

MMP-2, MT1-MMP, MMP-9, and iNOS were all increased after SMAO compared with shams. Arginine maintained while glutamine inhibited the increase in iNOS, MT1-MMP, and MMP-2 expression in the postischemic gut. Pretreatment of the arginine group with a selective iNOS inhibitor blunted the induction of MMP-2 in the postischemic gut. There was no differential modulation of MMP-9 by the luminal nutrients.

CONCLUSIONS

The arginine-induced upregulation of iNOS may contribute to increased activity of MT1-MMP and MMP-2. The mechanism for this differential regulation by arginine warrants further investigation.

PPARalpha-mediated remodeling of repolarizing voltage-gated K+ (Kv) channels in a mouse model of metabolic cardiomyopathy

Diabetes is associated with increased risk of diastolic dysfunction, heart failure, QT prolongation and rhythm disturbances independent of age, hypertension or coronary artery disease. Although these observations suggest electrical remodeling in the heart with diabetes, the relationship between the metabolic and the functional derangements is poorly understood. Exploiting a mouse model (MHC-PPARalpha) with cardiac-specific overexpression of the peroxisome proliferator-activated receptor alpha (PPARalpha), a key driver of diabetes-related lipid metabolic dysregulation, the experiments here were aimed at examining directly the link(s) between alterations in cardiac fatty acid metabolism and the functioning of repolarizing, voltage-gated K(+) (Kv) channels. Electrophysiological experiments on left (LV) and right (RV) ventricular myocytes isolated from young (5-6 week) MHC-PPARalpha mice revealed marked K(+) current remodeling: I(to,f) densities are significantly (P<0.01) lower, whereas I(ss) densities are significantly (P<0.001) higher in MHC-PPARalpha, compared with age-matched wild type (WT), LV and RV myocytes. Consistent with the observed reductions in I(to,f) density, expression of the KCND2 (Kv4.2) transcript is significantly (P<0.001) lower in MHC-PPARalpha, compared with WT, ventricles. Western blot analyses revealed that expression of the Kv accessory protein, KChIP2, is also reduced in MHC-PPARalpha ventricles in parallel with the decrease in Kv4.2. Although the properties of the endogenous and the "augmented" I(ss) suggest a role(s) for two pore domain K(+) channel (K2P) pore-forming subunits, the expression levels of KCNK2 (TREK1), KCNK3 (TASK1) and KCNK5 (TASK2) in MHC-PPARalpha and WT ventricles are not significantly different. The molecular mechanisms underlying I(to,f) and I(ss) remodeling in MHC-PPARalpha ventricular myocytes, therefore, are distinct.

The PPAR trio: regulators of myocardial energy metabolism in health and disease

Common causes of heart failure are associated with derangements in myocardial fuel utilization. Evidence is emerging that metabolic abnormalities may contribute to the development and progression of myocardial disease. The peroxisome proliferator-activated receptor (PPAR) family of nuclear receptor transcription factors has been shown to regulate cardiac fuel metabolism at the gene expression level. The three PPAR family members (alpha, beta/delta and gamma) are uniquely suited to serve as transducers of developmental, physiological, and dietary cues that influence cardiac fatty acid and glucose metabolism. This review describes murine PPAR loss- and gain-of-function models that have shed light on the roles of these receptors in regulating myocardial metabolic pathways and have defined key links to disease states including the hypertensive and diabetic heart.

The sulphur cycle: definitions, mechanisms and dynamics

The principal biochemical processes of the sulphur cycle are described and the types of organisms known to catalyse the reductive and oxidative phases of the cycle outlined. Attention is drawn to the shortcomings in our current knowledge of the scale of turnover of the sulphur cycle and of our understanding of the microorganisms involved in specialized environments. Examples of some special habitats are used to illustrate these points. The role of sulphate-reducing bacteria and sulphur-oxidizing chemolithotrophs in the formation and recycling of sulphide minerals is described.

The transcriptional coactivator PGC-1alpha is essential for maximal and efficient cardiac mitochondrial fatty acid oxidation and lipid homeostasis

High-capacity mitochondrial ATP production is essential for normal function of the adult heart, and evidence is emerging that mitochondrial derangements occur in common myocardial diseases. Previous overexpression studies have shown that the inducible transcriptional coactivator peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1alpha is capable of activating postnatal cardiac myocyte mitochondrial biogenesis. Recently, we generated mice deficient in PGC-1alpha (PGC-1alpha(-/-) mice), which survive with modestly blunted postnatal cardiac growth. To determine if PGC-1alpha is essential for normal cardiac energy metabolic capacity, mitochondrial function experiments were performed on saponin-permeabilized myocardial fibers from PGC-1alpha(-/-) mice. These experiments demonstrated reduced maximal (state 3) palmitoyl-l-carnitine respiration and increased maximal (state 3) pyruvate respiration in PGC-1alpha(-/-) mice compared with PGC-1alpha(+/+) controls. ATP synthesis rates obtained during maximal (state 3) respiration in permeabilized myocardial fibers were reduced for PGC-1alpha(-/-) mice, whereas ATP produced per oxygen consumed (ATP/O), a measure of metabolic efficiency, was decreased by 58% for PGC-1alpha(-/-) fibers. Ex vivo isolated working heart experiments demonstrated that PGC-1alpha(-/-) mice exhibited lower cardiac power, reduced palmitate oxidation, and increased reliance on glucose oxidation, with the latter likely a compensatory response. (13)C NMR revealed that hearts from PGC-1alpha(-/-) mice exhibited a limited capacity to recruit triglyceride as a source for lipid oxidation during beta-adrenergic challenge. Consistent with reduced mitochondrial fatty acid oxidative enzyme gene expression, the total triglyceride content was greater in hearts of PGC-1alpha(-/-) mice relative to PGC-1alpha(+/+) following a fast. Overall, these results demonstrate that PGC-1alpha is essential for the maintenance of maximal, efficient cardiac mitochondrial fatty acid oxidation, ATP synthesis, and myocardial lipid homeostasis.

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.

Isolation and immunological detection of Mycoplasma ovipneumoniae in sheep with atypical pneumonia, and lack of a role for Mycoplasma arginini

Mycoplasma ovipneumoniae NCTC 10151(T) and four new isolates from UK sheep flocks were compared. Only glucose and pyruvate were used as energy sources by the five strains: glucose was the best energy source for the type strain, pyruvate supported better growth of the new strains. Whole cell protein patterns and antigenic profiles showed high similarity between all five strains. The new isolates fell into two groups in ELISA tests. Serum samples from 30 pneumonic sheep were assessed for M. ovipneumoniae infection and Mycoplasma arginini co-infection. Fourteen (out of 30) serum samples were positive for M. ovipneumoniae both by ELISA and immunoblotting. Twelve antigenic proteins of M. ovipneumoniae were detected in infected serum samples: the antigen patterns were unique, with between one and at least seven occurring in any one sample. All serum samples were designated as negative for M. arginini antibodies by both ELISA and immunoblotting.

The nuclear receptor ERRalpha is required for the bioenergetic and functional adaptation to cardiac pressure overload

Downregulation and functional deactivation of the transcriptional coactivator PGC-1alpha has been implicated in heart failure pathogenesis. We hypothesized that the estrogen-related receptor alpha (ERRalpha), which recruits PGC-1alpha to metabolic target genes in heart, exerts protective effects in the context of stressors known to cause heart failure. ERRalpha(-/-) mice subjected to left ventricular (LV) pressure overload developed signatures of heart failure including chamber dilatation and reduced LV fractional shortening. (31)P-NMR studies revealed abnormal phosphocreatine depletion in ERRalpha(-/-) hearts subjected to hemodynamic stress, indicative of a defect in ATP reserve. Mitochondrial respiration studies demonstrated reduced maximal ATP synthesis rates in ERRalpha(-/-) hearts. Cardiac ERRalpha target genes involved in energy substrate oxidation, ATP synthesis, and phosphate transfer were downregulated in ERRalpha(-/-) mice at baseline or with pressure overload. These results demonstrate that the nuclear receptor ERRalpha is required for the adaptive bioenergetic response to hemodynamic stressors known to cause heart failure.

Spread-space spread-spectrum technique for secure multiplexing

A novel technique for multiplexing complex images is proposed in which each image may be demultiplexed only if a set of random encryption keys is known. The technique utilizes the ability of the double random phase encoding method to spread a signals' energy in both the space and the spatial frequency domains in a controlled manner. To multiplex, images are independently encrypted with different phase keys and then superimposed by recording sequentially on the same material. Each image is extracted by using the particular key associated with it. During decryption the energy from the other images is further spread, making it possible to minimize its effects by using suitable filters. Wigner analysis is applied to the technique, and numerical results are presented.

Genome-wide orchestration of cardiac functions by the orphan nuclear receptors ERRalpha and gamma

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.

CD36 deficiency rescues lipotoxic cardiomyopathy

Obesity-related diabetes mellitus leads to increased myocardial uptake of fatty acids (FAs), resulting in a form of cardiac dysfunction referred to as lipotoxic cardiomyopathy. We have shown previously that chronic activation of the FA-activated nuclear receptor, peroxisome proliferator-activated receptor alpha (PPARalpha), is sufficient to drive the metabolic and functional abnormalities of the diabetic heart. Mice with cardiac-restricted overexpression of PPARalpha (myosin heavy chain [MHC]-PPARalpha) exhibit myocyte lipid accumulation and cardiac dysfunction. We sought to define the role of the long-chain FA transporter CD36 in the pathophysiology of lipotoxic forms of cardiomyopathy. MHC-PPARalpha mice were crossed with CD36-deficient mice (MHC-PPARalpha/CD36-/- mice). The absence of CD36 prevented myocyte triacylglyceride accumulation and cardiac dysfunction in the MHC-PPARalpha mice under basal conditions and following administration of high-fat diet. Surprisingly, the rescue of the MHC-PPARalpha phenotype by CD36 deficiency was associated with increased glucose uptake and oxidation rather than changes in FA utilization. As predicted by the metabolic changes, the activation of PPARalpha target genes involved in myocardial FA-oxidation pathways in the hearts of the MHC-PPARalpha mice was unchanged in the CD36-deficient background. However, PPARalpha-mediated suppression of genes involved in glucose uptake and oxidation was reversed in the MHC-PPARalpha/ CD36-/- mice. We conclude that CD36 is necessary for the development of lipotoxic cardiomyopathy in MHC-PPARalpha mice and that novel therapeutic strategies aimed at reducing CD36-mediated FA uptake show promise for the prevention or treatment of cardiac dysfunction related to obesity and diabetes.

Insulin-resistant heart exhibits a mitochondrial biogenic response driven by the peroxisome proliferator-activated receptor-alpha/PGC-1alpha gene regulatory pathway

BACKGROUND

Obesity and diabetes mellitus are complex metabolic problems of pandemic proportion, contributing to significant cardiovascular mortality. Recent studies have shown altered mitochondrial function in the hearts of diabetic animals. We hypothesized that regulatory events involved in the control of mitochondrial function are activated in the prediabetic, insulin-resistant stage.

METHODS AND RESULTS

Morphometric analyses demonstrated that cardiac myocyte mitochondrial volume density was increased in insulin-resistant uncoupling protein-diphtheria toxin A (UCP-DTA) transgenic mice, a murine model of metabolic syndrome, compared with littermate controls. Mitochondrial DNA content and expression of genes involved in multiple mitochondrial pathways were also increased in insulin-resistant UCP-DTA hearts. The nuclear receptor, peroxisome proliferator-activated receptor-alpha (PPARalpha), is known to activate metabolic genes in the diabetic heart. Therefore, we evaluated the role of PPARalpha in the observed mitochondrial biogenesis response in the insulin-resistant heart. Insulin-resistant UCP-DTA mice crossed into a PPARalpha-null background did not exhibit evidence of mitochondrial biogenesis or induction of mitochondrial gene expression. Conversely, transgenic mice with cardiac-specific overexpression of PPARalpha exhibited signatures of cardiac mitochondrial biogenesis. A screen for candidate mediators of the PPARalpha-driven mitochondrial biogenic response revealed that expression of PPARgamma coactivator-1alpha (PGC-1alpha), a known regulator of mitochondrial biogenesis, was activated in wild-type UCP-DTA mice but not in PPARalpha-deficient UCP-DTA mice.

CONCLUSIONS

These results demonstrate that mitochondrial biogenesis occurs early in the development of diabetic cardiac dysfunction through a transcriptional regulatory circuit that involves activation of PGC-1alpha gene expression by the fatty acid-activated nuclear receptor PPARalpha.

Lipin 1 is an inducible amplifier of the hepatic PGC-1alpha/PPARalpha regulatory pathway

Perturbations in hepatic lipid homeostasis are linked to the development of obesity-related steatohepatitis. Mutations in the gene encoding lipin 1 cause hepatic steatosis in fld mice, a genetic model of lipodystrophy. However, the molecular function of lipin 1 is unclear. Herein, we demonstrate that the expression of lipin 1 is induced by peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha (PGC-1alpha), a transcriptional coactivator controlling several key hepatic metabolic pathways. Gain-of-function and loss-of-function strategies demonstrated that lipin selectively activates a subset of PGC-1alpha target pathways, including fatty acid oxidation and mitochondrial oxidative phosphorylation, while suppressing the lipogenic program and lowering circulating lipid levels. Lipin activates mitochondrial fatty acid oxidative metabolism by inducing expression of the nuclear receptor PPARalpha, a known PGC-1alpha target, and via direct physical interactions with PPARalpha and PGC-1alpha. These results identify lipin 1 as a selective physiological amplifier of the PGC-1alpha/PPARalpha-mediated control of hepatic lipid metabolism.

PGC-1 coactivators: inducible regulators of energy metabolism in health and disease

Members of the PPARgamma coactivator-1 (PGC-1) family of transcriptional coactivators serve as inducible coregulators of nuclear receptors in the control of cellular energy metabolic pathways. This Review focuses on the biologic and physiologic functions of the PGC-1 coactivators, with particular emphasis on striated muscle, liver, and other organ systems relevant to common diseases such as diabetes and heart failure.

Cost of transport is increased after cold exposure in Monodelphis domestica: training for inefficiency

Monodelphis domestica (Didelphidae: Marsupialia) lacks brown adipose tissue and thus relies on skeletal muscle as its primary thermogenic organ. Following cold exposure, the aerobic capacity of skeletal muscle in these animals is greatly increased. We investigated the effects of this plastic response to thermogenesis on locomotion and muscle mechanics. In cold-exposed animals, cost of transport was 15% higher than in controls but was unaffected by exercise training. Twitch kinetics in isolated semitendinosus muscles of cold-exposed animals were characteristic of slow-oxidative fiber types. Both time-to-peak tension and half-relaxation time were longer and maximal shortening velocity was slower following cold exposure compared to either thermoneutral controls or exercise-trained animals. Further, muscles from the cold-exposed animals had greater fatigue resistance than either control or exercise-trained animals, indicating greater oxidative capacity. Finally, we identified an uncoupling protein 3 homologue, whose gene expression was upregulated in skeletal muscle of cold-exposed Monodelphis domestica. Cold exposure provided a potent stimulus for muscle plasticity, driving a fast-to-slow transition more effectively than exercise training. However, linked to the dramatic shift in muscle properties is an equally dramatic increase in whole animal muscle energetics during locomotion, suggesting an uncoupled state, or 'training for inefficiency'.

Chronic activation of PPARα is detrimental to cardiac recovery after ischemia

High fatty acid oxidation (FAO) rates contribute to ischemia-reperfusion injury of the myocardium. Because peroxisome proliferator-activated receptor (PPAR)α regulates transcription of several FAO enzymes in the heart, we examined the response of mice with cardiac-restricted overexpression of PPARα (MHC-PPARα) or whole body PPARα deletion including the heart (PPARα−/−) to myocardial ischemia-reperfusion injury. Isolated working hearts from MHC-PPARα and nontransgenic (NTG) littermates were subjected to no-flow global ischemia followed by reperfusion. MHC-PPARα hearts had significantly higher FAO rates during aerobic and postischemic reperfusion (aerobic 1,479 ± 171 vs. 699 ± 117, reperfusion 1,062 ± 214 vs. 601 ± 70 nmol·g dry wt−1·min−1; P < 0.05) and significantly lower glucose oxidation rates compared with NTG hearts (aerobic 225 ± 36 vs. 1,563 ± 165, reperfusion 402 ± 54 vs. 1,758 ± 165 nmol·g dry wt−1·min−1; P < 0.05). In hearts from PPARα−/−mice, FAO was significantly lower during aerobic and reperfusion (aerobic 235 ± 36 vs. 442 ± 75, reperfusion 205 ± 25 vs. 346 ± 38 nmol·g dry wt−1·min−1; P < 0.05) whereas glucose oxidation was significantly higher compared with wild-type (WT) hearts (aerobic 2,491 ± 631 vs. 901 ± 119, reperfusion 2,690 ± 562 vs. 1,315 ± 172 nmol·g dry wt−1·min−1; P < 0.05). Increased FAO rates in MHC-PPARα hearts were associated with a markedly lower recovery of cardiac power (45 ± 9% vs. 71 ± 6% of preischemic levels in NTG hearts; P < 0.05). In contrast, the percent recovery of cardiac power of PPARα−/−hearts was not significantly different from that of WT hearts (80 ± 8% vs. 75 ± 9%). This study demonstrates that chronic activation of PPARα is detrimental to the cardiac recovery during reperfusion after ischemia.

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.

Decreased contractile and metabolic reserve in peroxisome proliferator-activated receptor-alpha-null hearts can be rescued by increasing glucose transport and utilization

BACKGROUND

Downregulation of peroxisome proliferator-activated receptor-alpha (PPARalpha) in hypertrophied and failing hearts leads to the reappearance of the fetal metabolic pattern, ie, decreased fatty acid oxidation and increased reliance on carbohydrates. Here, we sought to elucidate the functional significance of this shift in substrate preference.

METHODS AND RESULTS

We assessed contractile function and substrate utilization using 13C nuclear magnetic resonance spectroscopy and high-energy phosphate metabolism using 31P nuclear magnetic resonance spectroscopy in perfused hearts isolated from genetically modified mice (PPARalpha(-/-)) that mimic the metabolic profile in myocardial hypertrophy. We found that the substrate switch from fatty acid to glucose (3-fold down) and lactate (3-fold up) in PPARalpha(-/-) hearts was sufficient for sustaining normal energy metabolism and contractile function at baseline but depleted the metabolic reserve for supporting high workload. Decreased ATP synthesis (measured by 31P magnetization transfer) during high workload challenge resulted in progressive depletion of high-energy phosphate content and failure to sustain high contractile performance. Interestingly, the metabolic and functional defects in PPARalpha(-/-) hearts could be corrected by overexpressing the insulin-independent glucose transporter GLUT1, which increased the capacity for glucose utilization beyond the intrinsic response to PPARalpha deficiency.

CONCLUSIONS

These findings demonstrate that metabolic remodeling in hearts deficient in PPARalpha increases the susceptibility to functional deterioration during hemodynamic overload. Moreover, our results suggest that normalization of myocardial energetics by further enhancing myocardial glucose utilization is an effective strategy for preventing the progression of cardiac dysfunction in hearts with impaired PPARalpha activity such as hearts with pathological hypertrophy.

Cardiac-specific overexpression of peroxisome proliferator-activated receptor-alpha causes insulin resistance in heart and liver

Diabetic heart failure may be causally associated with alterations in cardiac energy metabolism and insulin resistance. Mice with heart-specific overexpression of peroxisome proliferator-activated receptor (PPAR)alpha showed a metabolic and cardiomyopathic phenotype similar to the diabetic heart, and we determined tissue-specific glucose metabolism and insulin action in vivo during hyperinsulinemic-euglycemic clamps in awake myosin heavy chain (MHC)-PPARalpha mice (12-14 weeks of age). Basal and insulin-stimulated glucose uptake in heart was significantly reduced in the MHC-PPARalpha mice, and cardiac insulin resistance was mostly attributed to defects in insulin-stimulated activities of insulin receptor substrate (IRS)-1-associated phosphatidylinositol (PI) 3-kinase, Akt, and tyrosine phosphorylation of signal transducer and activator of transcription 3 (STAT3). Interestingly, MHC-PPARalpha mice developed hepatic insulin resistance associated with defects in insulin-mediated IRS-2-associated PI 3-kinase activity, increased hepatic triglyceride, and circulating interleukin-6 levels. To determine the underlying mechanism, insulin clamps were conducted in 8-week-old MHC-PPARalpha mice. Insulin-stimulated cardiac glucose uptake was similarly reduced in 8-week-old MHC-PPARalpha mice without changes in cardiac function and hepatic insulin action compared with the age-matched wild-type littermates. Overall, these findings indicate that increased activity of PPARalpha, as occurs in the diabetic heart, leads to cardiac insulin resistance associated with defects in insulin signaling and STAT3 activity, subsequently leading to reduced cardiac function. Additionally, age-associated hepatic insulin resistance develops in MHC-PPARalpha mice that may be due to altered cardiac metabolism, functions, and/or inflammatory cytokines.

Role of adiponectin in the protective action of dietary saturated fat against alcoholic fatty liver in mice

The protective effect of dietary saturated fatty acids against the development of alcoholic liver disease has long been known, but the underlying mechanism is not completely understood. We examined the involvement of the adipocyte hormone adiponectin. Circulating adiponectin levels were significantly elevated by chronic ethanol administration to mice consuming a diet high in saturated fat. The increase in circulating adiponectin was associated with the activation a set of hepatic signaling pathways mediated through AMP-activated protein kinase, PPAR-alpha, and PPAR-gamma coactivator alpha, which in turn led to markedly increased rates of fatty acid oxidation, prevention of hepatic steatosis, and alleviation of liver enzyme changes. Furthermore, treatment of rat 3T3-L1 adipocytes with saturated fatty acids (palmitic or stearic acids) in the presence of ethanol increased secretion of adiponectin and enhanced activity of a mouse adiponectin promoter. In conclusion, the protective action of saturated fat against the development of alcoholic fatty liver in mice is partially mediated through induction of adiponectin. The present findings suggest a novel paradigm for dietary fatty acids in the pathogenesis of alcoholic liver disease and provide a promising therapeutic strategy-nutritional modulation of adiponectin-in treating human alcoholic fatty liver disease.

Mitochondrial energy metabolism in heart failure: a question of balance

The mitochondrion serves a critical role as a platform for energy transduction, signaling, and cell death pathways relevant to common diseases of the myocardium such as heart failure. This review focuses on the molecular regulatory events and downstream effector pathways involved in mitochondrial energy metabolic derangements known to occur during the development of heart failure.

Mitochondrial energy metabolism in heart failure: a question of balance

The mitochondrion serves a critical role as a platform for energy transduction, signaling, and cell death pathways relevant to common diseases of the myocardium such as heart failure. This review focuses on the molecular regulatory events and downstream effector pathways involved in mitochondrial energy metabolic derangements known to occur during the development of heart failure.

PGC-1alpha deficiency causes multi-system energy metabolic derangements: muscle dysfunction, abnormal weight control and hepatic steatosis

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.

Eliminating the activity of the gene PGC-1 α in mice reveals its role in post-natal metabolism and provides a link to obesity and some intriguing differences with another report of this knockout

A potential link between muscle peroxisome proliferator- activated receptor-alpha signaling and obesity-related diabetes

The role of the peroxisome proliferator-activated receptor-alpha (PPARalpha) in the development of insulin-resistant diabetes was evaluated using gain- and loss-of-function approaches. Transgenic mice overexpressing PPARalpha in muscle (MCK-PPARalpha mice) developed glucose intolerance despite being protected from diet-induced obesity. Conversely, PPARalpha null mice were protected from diet-induced insulin resistance in the context of obesity. In skeletal muscle, MCK-PPARalpha mice exhibited increased fatty acid oxidation rates, diminished AMP-activated protein kinase activity, and reduced insulin-stimulated glucose uptake without alterations in the phosphorylation status of key insulin-signaling proteins. These effects on muscle glucose uptake involved transcriptional repression of the GLUT4 gene. Pharmacologic inhibition of fatty acid oxidation or mitochondrial respiratory coupling prevented the effects of PPARalpha on GLUT4 expression and glucose homeostasis. These results identify PPARalpha-driven alterations in muscle fatty acid oxidation and energetics as a potential link between obesity and the development of glucose intolerance and insulin resistance.

Mouse models of mitochondrial dysfunction and heart failure

Mitochondria in the adult mammalian heart have a tremendous capacity for oxidative metabolism, and the conversion of energy by these pathways is critical for proper cardiac function. This review describes mouse models relating mitochondrial metabolism to cardiac function through gain- or loss-of-function approaches that manipulate mitochondrial energy transduction or ATP synthetic pathways. Mouse models of mitochondrial defects are relevant to genetic and acquired forms of human cardiomyopathy. Examples include inborn errors in mitochondrial metabolism or end-stage heart failure. Conversely, chronic reliance on energy production via mitochondrial fatty acid oxidation, such as occurs in the diabetic heart, likely leads to maladaptive sequelae including cellular lipotoxicity and mitochondrial dysfunction. Collectively, these model systems have allowed us to begin to dissect the relationship between mitochondrial metabolism and the development of cardiomyopathy and to define the molecular pathways regulating cardiac mitochondrial number and function.

G-protein signaling participates in the development of diabetic cardiomyopathy

Diabetic patients develop a cardiomyopathy that consists of ventricular hypertrophy and diastolic dysfunction. Although the pathogenesis of this condition is poorly understood, previous studies implicated abnormal G-protein activation. In this work, mice with cardiac overexpression of the transcription factor peroxisome proliferator-activated receptor-alpha (PPAR-alpha) were examined as a model of diabetic cardiomyopathy. PPAR-alpha transgenic mice develop spontaneous cardiac hypertrophy, contractile dysfunction, and "fetal" gene induction. We examined the role of abnormal G-protein activation in the pathogenesis of cardiac dysfunction by crossing PPAR-alpha mice with transgenic mice with cardiac-specific overexpression of regulator of G-protein signaling subtype 4 (RGS4), a GTPase activating protein for Gq and Gi. Generation of compound transgenic mice demonstrated that cardiac RGS4 overexpression ameliorated the cardiomyopathic phenotype that occurred as a result of PPAR-alpha overexpression without affecting the metabolic abnormalities seen in these hearts. Next, transgenic mice with increased or decreased cardiac Gq signaling were made diabetic by injection with streptozotocin (STZ). RGS4 transgenic mice were resistant to STZ-induced cardiac fetal gene induction. Transgenic mice with cardiac-specific expression of mutant Galphaq, Galphaq-G188S, that is resistant to RGS protein action were sensitized to the development of STZ-induced cardiac fetal gene induction and bradycardia. These results establish that Gq-mediated signaling plays a critical role in the pathogenesis of diabetic cardiomyopathy.

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.

Cardiac-specific induction of the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha promotes mitochondrial biogenesis and reversible cardiomyopathy in a developmental stage-dependent manner

Recent evidence has identified the peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) 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-1alpha. Expression of the PGC-1alpha transgene in this system (tet-on PGC-1alpha) is cardiac-specific in the presence of doxycycline (dox) and is not leaky in the absence of dox. Overexpression of PGC-1alpha in tet-on PGC-1alpha 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-1alpha 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-1alpha mice is characterized by an increase in ventricular mass and chamber dilatation. Surprisingly, removal of dox and cessation of PGC-1alpha overexpression in adult mice results in complete reversal of cardiac dysfunction within 4 weeks. These results indicate that PGC-1alpha 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.

Liver fatty acid binding protein is required for high rates of hepatic fatty acid oxidation but not for the action of PPARalpha in fasting mice

Liver fatty acid binding protein (L-FABP) has been proposed to limit the availability of long-chain fatty acids (LCFA) for oxidation and for peroxisome proliferator-activated receptor alpha (PPAR-alpha), a fatty acid binding transcription factor that determines the capacity of hepatic fatty acid oxidation. Here, we used L-FABP null mice to test this hypothesis. Under fasting conditions, this mutation reduced beta-hydroxybutyrate (BHB) plasma levels as well as BHB release and palmitic acid oxidation by isolated hepatocytes. However, the capacity for ketogenesis was not reduced: BHB plasma levels were restored by octanoate injection; BHB production and palmitic acid oxidation were normal in liver homogenates; and hepatic expression of key PPAR-alpha target (MCAD, mitochondrial HMG CoA synthase, ACO, CYP4A3) and other (CPT1, LCAD) genes of mitochondrial and extramitochondrial LCFA oxidation and ketogenesis remained at wild-type levels. During standard diet, mitochondrial HMG CoA synthase mRNA was selectively reduced in L-FABP null liver. These results suggest that under fasting conditions, hepatic L-FABP contributes to hepatic LCFA oxidation and ketogenesis by a nontranscriptional mechanism, whereas L-FABP can activate ketogenic gene expression in fed mice. Thus, the mechanisms whereby L-FABP affects fatty acid oxidation may vary with physiological condition.