A Link Between Cholesterol Levels and Phenobarbital Induction of Cytochromes P450

Biochemical and Biophysical Research Communications. Feb, 2002  |  Pubmed ID: 11846416

Squalestatin1 (SQ1), a potent inhibitor of squalene synthase produced a dose-dependent induction of cytochromes P450 CYP2H1 and CYP3A37 mRNAs in chicken hepatoma cells. The effect of SQ1 was completely reversed by 25-hydroxycholesterol. Bile acids elicited an induction of CYP3A37 and CYP2H1 mRNA. Bile acids also reduced the phenobarbital induction of CYP2H1 but not of CYP3A37 mRNA. The effects of SQ1 and its reversal by 25-hydroxycholesterol and the effects of bile acids were reproduced in reporter gene assays with a phenobarbital-responsive enhancer unit of CYP2H1. These data suggest that an endogenous molecule related to cholesterol homeostasis regulates induction of drug-inducible CYPs.

NUBIScan, an in Silico Approach for Prediction of Nuclear Receptor Response Elements

Molecular Endocrinology (Baltimore, Md.). Jun, 2002  |  Pubmed ID: 12040014

Nuclear receptors (NRs) are transcription factors activated by a multitude of hormones, other endogenous substances, and exogenous molecules. These proteins modulate the regulation of target genes by contacting their promoter or enhancer sequences at specific recognition sites. The identification of these response elements is the first step toward detailed insight into the regulatory mechanisms affecting a gene. We have developed NUBIScan, a computer algorithm to predict DNA recognition sites for NRs in the regulatory regions of genes. The algorithm is based on weighted nucleotide distribution matrices and combines scores from both half-sites necessary for NR dimer binding. It provides more specific identification of functional sites than previous in silico approaches, as evidenced by scanning published regulatory regions of drug-inducible genes and comparing the obtained predictions with experimental results. In prospective analyses, NUBIScan consistently identified new functional NR binding sites in sets of large sequences, which had eluded previous analyses. This is exemplified by the detailed functional analysis of the flanking region of two genes. This approach therefore facilitates the selection of likely sites of gene regulation for subsequent experimental analysis.

Cholesterol and Bile Acids Regulate Xenosensor Signaling in Drug-mediated Induction of Cytochromes P450

The Journal of Biological Chemistry. Aug, 2002  |  Pubmed ID: 12045201

Cytochromes P450 (CYP) constitute the major enzymatic system for metabolism of xenobiotics. Here we demonstrate that transcriptional activation of CYPs by the drug-sensing nuclear receptors pregnane X receptor, constitutive androstane receptor, and the chicken xenobiotic receptor (CXR) can be modulated by endogenous cholesterol and bile acids. Bile acids induce the chicken drug-activated CYP2H1 via CXR, whereas the hydroxylated metabolites of bile acids and oxysterols inhibit drug induction. The cholesterol-sensing liver X receptor competes with CXR, pregnane X receptor, or constitutive androstane receptor for regulation of drug-responsive enhancers from chicken CYP2H1, human CYP3A4, or human CYP2B6, respectively. Thus, not only cholesterol 7 alpha-hydroxylase (CYP7A1), but also drug-inducible CYPs, are diametrically affected by these receptors. Our findings reveal new insights into the increasingly complex network of nuclear receptors regulating lipid homeostasis and drug metabolism.

An Autoregulatory Loop Controls Peroxisome Proliferator-activated Receptor Gamma Coactivator 1alpha Expression in Muscle

Proceedings of the National Academy of Sciences of the United States of America. Jun, 2003  |  Pubmed ID: 12764228

Skeletal muscle adapts to chronic physical activity by inducing mitochondrial biogenesis and switching proportions of muscle fibers from type II to type I. Several major factors involved in this process have been identified, such as the calcium/calmodulin-dependent protein kinase IV (CaMKIV), calcineurin A (CnA), and the transcriptional component peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha). Transgenic expression of PGC-1alpha recently has been shown to dramatically increase the content of type I muscle fibers in skeletal muscle, but the relationship between PGC-1alpha expression and the key components in calcium signaling is not clear. In this report, we show that the PGC-1alpha promoter is regulated by both CaMKIV and CnA activity. CaMKIV activates PGC-1alpha largely through the binding of cAMP response element-binding protein to the PGC-1alpha promoter. Moreover, we show that a positive feedback loop exists between PGC-1alpha and members of the myocyte enhancer factor 2 (MEF2) family of transcription factors. MEF2s bind to the PGC-1alpha promoter and activate it, predominantly when coactivated by PGC-1alpha. MEF2 activity is stimulated further by CnA signaling. These findings imply a unified pathway, integrating key regulators of calcium signaling with the transcriptional switch PGC-1alpha. Furthermore, these data suggest an autofeedback loop whereby the calcium-signaling pathway may result in a stable induction of PGC-1alpha, contributing to the relatively stable nature of muscle fiber-type determination.

Induction of Drug Metabolism: the Role of Nuclear Receptors

Pharmacological Reviews. Dec, 2003  |  Pubmed ID: 14657421

Induction of drug metabolism was described more than 40 years ago. Progress in understanding the molecular mechanism of induction of drug-metabolizing enzymes was made recently when the important roles of the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), two members of the nuclear receptor superfamily of transcription factors, were discovered to act as sensors for lipophilic xenobiotics, including drugs. CAR and PXR bind as heterodimeric complexes with the retinoid X receptor to response elements in the regulatory regions of the induced genes. PXR is directly activated by xenobiotic ligands, whereas CAR is involved in a more complex and less well understood mechanism of signal transduction triggered by drugs. Most recently, analysis of these xenobiotic-sensing nuclear receptors and their nonmammalian precursors such as the chicken xenobiotic receptor suggests an important role of PXR and CAR also in endogenous pathways, such as cholesterol and bile acid biosynthesis and metabolism. In this review, recent findings regarding xenosensors and their target genes are summarized and are put into an evolutionary perspective in regard to how a living organism has derived a system that is able to deal with potentially toxic compounds it has not encountered before.

In Silico Approaches, and in Vitro and in Vivo Experiments to Predict Induction of Drug Metabolism

Drug News & Perspectives. Sep, 2003  |  Pubmed ID: 14668938

Despite being described more than 40 years ago, the molecular mechanism that regulates hepatic induction of cytochromes P450 and other drug-metabolizing enzymes and drug transporters by xenobiotics has remained enigmatic until recently. A major breakthrough was the discovery of the orphan nuclear receptors pregnane X receptor and constitutive androstane receptor playing key roles as species-specific xenosensors in this induction response. Using this newly acquired knowledge, the human induction response can now be more accurately predicted. This is of considerable clinical importance, since induction of cytochrome P450s and other enzymes can lead to unwanted drug-drug interactions, adverse drug reactions and drug toxicity. In this review, in vitro, in vivo and in silico techniques are discussed that can identify troublesome compounds at an early stage and that can help to design new, safer medicines faster.

Suppression of Mitochondrial Respiration Through Recruitment of P160 Myb Binding Protein to PGC-1alpha: Modulation by P38 MAPK

Genes & Development. Feb, 2004  |  Pubmed ID: 14744933

The transcriptional coactivator PPAR gamma coactivator 1 alpha (PGC-1alpha) is a key regulator of metabolic processes such as mitochondrial biogenesis and respiration in muscle and gluconeogenesis in liver. Reduced levels of PGC-1alpha in humans have been associated with type II diabetes. PGC-1alpha contains a negative regulatory domain that attenuates its transcriptional activity. This negative regulation is removed by phosphorylation of PGC-1alpha by p38 MAPK, an important kinase downstream of cytokine signaling in muscle and beta-adrenergic signaling in brown fat. We describe here the identification of p160 myb binding protein (p160MBP) as a repressor of PGC-1alpha. The binding and repression of PGC-1alpha by p160MBP is disrupted by p38 MAPK phosphorylation of PGC-1alpha. Adenoviral expression of p160MBP in myoblasts strongly reduces PGC-1alpha's ability to stimulate mitochondrial respiration and the expression of the genes of the electron transport system. This repression does not require removal of PGC-1alpha from chromatin, suggesting that p160MBP is or recruits a direct transcriptional suppressor. Overall, these data indicate that p160MBP is a powerful negative regulator of PGC-1alpha function and provide a molecular mechanism for the activation of PGC-1alpha by p38 MAPK. The discovery of p160MBP as a PGC-1alpha regulator has important implications for the understanding of energy balance and diabetes.

Erralpha and Gabpa/b Specify PGC-1alpha-dependent Oxidative Phosphorylation Gene Expression That is Altered in Diabetic Muscle

Proceedings of the National Academy of Sciences of the United States of America. Apr, 2004  |  Pubmed ID: 15100410

Recent studies have shown that genes involved in oxidative phosphorylation (OXPHOS) exhibit reduced expression in skeletal muscle of diabetic and prediabetic humans. Moreover, these changes may be mediated by the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha). By combining PGC-1alpha-induced genome-wide transcriptional profiles with a computational strategy to detect cis-regulatory motifs, we identified estrogen-related receptor alpha (Erralpha) and GA repeat-binding protein alpha as key transcription factors regulating the OXPHOS pathway. Interestingly, the genes encoding these two transcription factors are themselves PGC-1alpha-inducible and contain variants of both motifs near their promoters. Cellular assays confirmed that Erralpha and GA-binding protein a partner with PGC-1alpha in muscle to form a double-positive-feedback loop that drives the expression of many OXPHOS genes. By using a synthetic inhibitor of Erralpha, we demonstrated its key role in PGC-1alpha-mediated effects on gene regulation and cellular respiration. These results illustrate the dissection of gene regulatory networks in a complex mammalian system, elucidate the mechanism of PGC-1alpha action in the OXPHOS pathway, and suggest that Erralpha agonists may ameliorate insulin-resistance in individuals with type 2 diabetes mellitus.

Identification of the Xenosensors Regulating Human 5-aminolevulinate Synthase

Proceedings of the National Academy of Sciences of the United States of America. Jun, 2004  |  Pubmed ID: 15178759

Heme is an essential component of numerous hemoproteins with functions including oxygen transport, energy metabolism, and drug biotransformation. In nonerythropoietic cells, 5-aminolevulinate synthase (ALAS1) is the rate-limiting enzyme in heme biosynthesis. Upon exposure to drugs that induce cytochromes P450 and other drug-metabolizing enzymes, ALAS1 is transcriptionally up-regulated, increasing the rate of heme biosynthesis to provide heme for cytochrome P450 hemoproteins. We used a combined in silico-in vitro approach to identify sequences in the ALAS1 gene that mediate direct transcriptional response to xenobiotic challenge. We have characterized two enhancer elements, located 20 and 16 kb upstream of the transcriptional start site. Both elements respond to prototypic inducer drugs and interact with the human pregnane X receptor NR1I2 and the human constitutive androstane receptor NR1I3. Our results suggest that the fundamental mechanism of drug induction is the same for cytochromes P450 and ALAS1. Transcriptional activation of the ALAS1 gene is the first step in the coordinated up-regulation of apoprotein and heme synthesis in response to exogenous and endogenous signals controlling heme levels. Understanding the direct effects of drugs on heme synthesis is of clinical interest, particularly in patients with hepatic porphyrias.

The Evolution of Drug-activated Nuclear Receptors: One Ancestral Gene Diverged into Two Xenosensor Genes in Mammals

Nuclear Receptor. 10, 2004  |  Pubmed ID: 15479477

BACKGROUND: Drugs and other xenobiotics alter gene expression of cytochromes P450 (CYP) by activating the pregnane X receptor (PXR) and constitutive androstane receptor (CAR) in mammals. In non-mammalian species, only one xenosensor gene has been found. Using chicken as a model organism, the aim of our study was to elucidate whether non-mammalian species only have one or two xenosensors like mammals. RESULTS: To explore the evolutionary aspect of this divergence, we tried to identify additional xenobiotic sensing nuclear receptors in chicken using various experimental approaches. However, none of those revealed novel candidates. Ablation of chicken xenobiotic receptor (CXR) function by RNAi or dominant-negative alleles drastically reduced drug-induction in a chicken hepatoma cell line. Subsequently, we functionally and structurally characterized CXR and compared our results to PXR and CAR. Despite the high similarity in their amino acid sequence, PXR and CAR have very distinct modes of activation. Some aspects of CXR function, e.g. direct ligand activation and high promiscuity are very reminiscent of PXR. On the other hand, cellular localization studies revealed common characteristics of CXR and CAR in terms of cytoplasmic-nuclear distribution. Finally, CXR has unique properties regarding its regulation in comparison to PXR and CAR. CONCLUSION: Our finding thus strongly suggest that CXR constitutes an ancestral gene which has evolved into PXR and CAR in mammals. Future studies should elucidate the reason for this divergence in mammalian versus non-mammalian species.

Regulatory Network of Lipid-sensing Nuclear Receptors: Roles for CAR, PXR, LXR, and FXR

Archives of Biochemistry and Biophysics. Jan, 2005  |  Pubmed ID: 15581595

Cloning and characterization of the orphan nuclear receptors constitutive androstane receptor (CAR, NR1I3) and pregnane X receptor (PXR, NR1I2) led to major breakthroughs in studying drug-mediated transcriptional induction of drug-metabolizing cytochromes P450 (CYPs). More recently, additional roles for CAR and PXR have been discovered. As examples, these xenosensors are involved in the homeostasis of cholesterol, bile acids, bilirubin, and other endogenous hydrophobic molecules in the liver: CAR and PXR thus form an intricate regulatory network with other members of the nuclear receptor superfamily, foremost the cholesterol-sensing liver X receptor (LXR, NR1H2/3) and the bile-acid-activated farnesoid X receptor (FXR, NR1H4). In this review, functional interactions between these nuclear receptors as well as the consequences on physiology and pathophysiology of the liver are discussed.

Species-specific Mechanisms for Cholesterol 7alpha-hydroxylase (CYP7A1) Regulation by Drugs and Bile Acids

Archives of Biochemistry and Biophysics. Feb, 2005  |  Pubmed ID: 15629111

The gene encoding cholesterol 7alpha-hydroxylase (CYP7A1) is tightly regulated in order to control intrahepatic cholesterol and bile acid levels. Ligands of the xenobiotic-sensing pregnane X receptor inhibit CYP7A1 expression. To retrace the evolution of the molecular mechanisms underlying CYP7A1 inhibition, we used a chicken hepatoma cell system that retains the ability to be induced by phenobarbital and other drugs. Whereas bile acids regulate CYP7A1 via small heterodimer partner and liver receptor homolog-1, mRNA expression of these nuclear receptors is unchanged by xenobiotics. Instead, drugs repress chicken hepatic nuclear factor 4alpha (HNF4alpha) transcript levels concomitant with a reduction in CYP7A1 expression. Importantly, no reduction of HNF4alpha levels is found in mouse liver in vivo and in human primary hepatocyte cultures, respectively. Thus, besides the importance of HNF4alpha in CYP7A1 regulation in all species, birds and mammals use different signaling pathways to adjust CYP7A1 levels after exposure to xenobiotics.

Hyperlipidemic Effects of Dietary Saturated Fats Mediated Through PGC-1beta Coactivation of SREBP

Cell. Jan, 2005  |  Pubmed ID: 15680331

The PGC-1 family of coactivators stimulates the activity of certain transcription factors and nuclear receptors. Transcription factors in the sterol responsive element binding protein (SREBP) family are key regulators of the lipogenic genes in the liver. We show here that high-fat feeding, which induces hyperlipidemia and atherogenesis, stimulates the expression of both PGC-1beta and SREBP1c and 1a in liver. PGC-1beta coactivates the SREBP transcription factor family and stimulates lipogenic gene expression. Further, PGC-1beta is required for SREBP-mediated lipogenic gene expression. However, unlike SREBP itself, PGC-1beta reduces fat accumulation in the liver while greatly increasing circulating triglycerides and cholesterol in VLDL particles. The stimulation of lipoprotein transport upon PGC-1beta expression is likely due to the simultaneous coactivation of the liver X receptor, LXRalpha, a nuclear hormone receptor with known roles in hepatic lipid transport. These data suggest a mechanism through which dietary saturated fats can stimulate hyperlipidemia and atherogenesis.

LXR Deficiency and Cholesterol Feeding Affect the Expression and Phenobarbital-mediated Induction of Cytochromes P450 in Mouse Liver

Journal of Lipid Research. Aug, 2005  |  Pubmed ID: 15930522

Metabolic transformation by the superfamily of cytochromes P450 (CYPs) plays an important role in the detoxification of xenobiotics such as drugs, environmental pollutants, and food additives. Endogenous substrates of CYPs include fatty acids, sterols, steroids, and bile acids. Induction of CYPs via transcriptional activation by substrates and other xenobiotics is an important adaptive mechanism that increases the organism's defense capability against toxicity. Numerous in vivo and in vitro data have highlighted the concept that the molecular mechanism of hepatic drug induction is linked to endogenous regulatory pathways. In particular, in vitro data suggest that oxysterols via the liver X receptor (LXR) inhibit phenobarbital (PB)-mediated induction of CYPs. To study the link between LXR, cholesterol homeostasis, and drug induction in vivo, we designed experiments in wild-type, LXRalpha-, LXRbeta-, and LXRalpha/beta-deficient mice. Our data expose differential regulatory patterns for Cyp2b10 and Cyp3a11 dependent on the expression of LXR isoforms and on challenge of cholesterol homeostasis by excess dietary cholesterol. Our results suggest that, in the mouse, liver cholesterol status significantly alters the pattern of expression of Cyp3a11, whereas the absence of LXR leads to an increase in PB-mediated activation of Cyp2b10.

Transcriptional Coactivator PGC-1 Alpha Controls the Energy State and Contractile Function of Cardiac Muscle

Cell Metabolism. Apr, 2005  |  Pubmed ID: 16054070

Skeletal and cardiac muscle depend on high turnover of ATP made by mitochondria in order to contract efficiently. The transcriptional coactivator PGC-1alpha has been shown to function as a major regulator of mitochondrial biogenesis and respiration in both skeletal and cardiac muscle, but this has been based only on gain-of-function studies. Using genetic knockout mice, we show here that, while PGC-1alpha KO mice appear to retain normal mitochondrial volume in both muscle beds, expression of genes of oxidative phosphorylation is markedly blunted. Hearts from these mice have reduced mitochondrial enzymatic activities and decreased levels of ATP. Importantly, isolated hearts lacking PGC-1alpha have a diminished ability to increase work output in response to chemical or electrical stimulation. As mice lacking PGC-1alpha age, cardiac dysfunction becomes evident in vivo. These data indicate that PGC-1alpha is vital for the heart to meet increased demands for ATP and work in response to physiological stimuli.

Metabolic Control Through the PGC-1 Family of Transcription Coactivators

Cell Metabolism. Jun, 2005  |  Pubmed ID: 16054085

Many complex biological programs are controlled at the level of gene transcription by DNA binding transcription factors. Recent studies have revealed a novel mode of regulation by coactivator proteins, best illustrated by the PGC-1 family of coactivators. These factors are highly responsive to a variety of environmental cues, from temperature to nutritional status to physical activity, and they coordinately regulate metabolic pathways and biological processes in a tissue-specific manner. Notably, the PGC-1 coactivators play a critical role in the maintenance of glucose, lipid, and energy homeostasis and are likely involved in the pathogenic conditions such as obesity, diabetes, neurodegeneration, and cardiomyopathy. These actions also raise new opportunities for the development of novel therapeutics.

Nutritional Regulation of Hepatic Heme Biosynthesis and Porphyria Through PGC-1alpha

Cell. Aug, 2005  |  Pubmed ID: 16122419

Inducible hepatic porphyrias are inherited genetic disorders of enzymes of heme biosynthesis. The main clinical manifestations are acute attacks of neuropsychiatric symptoms frequently precipitated by drugs, hormones, or fasting, associated with increased urinary excretion of delta-aminolevulinic acid (ALA). Acute attacks are treated by heme infusion and glucose administration, but the mechanisms underlying the precipitating effects of fasting and the beneficial effects of glucose are unknown. We show that the rate-limiting enzyme in hepatic heme biosynthesis, 5-aminolevulinate synthase (ALAS-1), is regulated by the peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha). Elevation of PGC-1alpha in mice via adenoviral vectors increases the levels of heme precursors in vivo as observed in acute attacks. The induction of ALAS-1 by fasting is lost in liver-specific PGC-1alpha knockout animals, as is the ability of porphyrogenic drugs to dysregulate heme biosynthesis. These data show that PGC-1alpha links nutritional status to heme biosynthesis and acute hepatic porphyria.

Partnership of PGC-1alpha and HNF4alpha in the Regulation of Lipoprotein Metabolism

The Journal of Biological Chemistry. May, 2006  |  Pubmed ID: 16574644

Peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) is a transcriptional coactivator involved in several aspects of energy metabolism. It is induced or activated under different stimuli in a highly tissue-specific manner and subsequently partners with certain transcription factors in those tissues to execute various biological programs. In the fasted liver, PGC-1alpha is induced and interacts with hepatocyte nuclear factor 4alpha (HNF4alpha) and other transcription factors to activate gluconeogenesis and increase hepatic glucose output. Given the broad spectrum of liver genes responsive to HNF4alpha, we sought to determine those that were specifically targeted by the combination of PGC-1alpha and HNF4alpha. Coexpression of these two molecules in murine stem cells reveals a high induction of mRNA for apolipoproteins A-IV and C-II. Forced expression of PGC-1alpha in mouse and human hepatoma cells increases the mRNA of a subset of apolipoproteins implicated in very low density lipoprotein and triglyceride metabolism, including apolipoproteins A-IV, C-II, and C-III. Coactivation of the apoC-III/A-IV promoter region by PGC-1alpha occurs through a highly conserved HNF4alpha response element, the loss of which completely abolishes activation by PGC-1alpha and HNF4alpha. Adenoviral infusion of PGC-1alpha into live mice increases hepatic expression of apolipoproteins A-IV, C-II, and C-III and increases serum and very low density lipoprotein triglyceride levels. Conversely, knock down of PGC-1alpha in vivo causes a decrease in both apolipoprotein expression and serum triglyceride levels. These data point to a crucial role for the PGC-1alpha/HNF4alpha partnership in hepatic lipoprotein metabolism.

Transducer of Regulated CREB-binding Proteins (TORCs) Induce PGC-1alpha Transcription and Mitochondrial Biogenesis in Muscle Cells

Proceedings of the National Academy of Sciences of the United States of America. Sep, 2006  |  Pubmed ID: 16980408

PGC-1alpha (peroxisome proliferator-activated receptor gamma coactivator 1alpha) is a master regulator of mitochondrial biogenesis and plays an important role in several other aspects of energy metabolism. To identify upstream regulators of PGC-1alpha gene transcription, 10,000 human full-length cDNAs were screened for induction of the PGC-1alpha promoter. A number of activators of PGC-1alpha transcription were found; the most potent activator was the transducer of regulated CREB (cAMP response element-binding protein) binding protein (TORC) 1, a coactivator of CREB. The other two members of the TORC family, TORC2 and TORC3, also strongly activated PGC-1alpha transcription. TORCs dramatically induced PGC-1alpha gene transcription through CREB. Forced expression of TORCs in primary muscle cells induced the endogenous mRNA of PGC-1alpha and its downstream target genes in the mitochondrial respiratory chain and TCA cycle. Importantly, these changes in gene expression resulted in increased mitochondrial oxidative capacity measured by cellular respiration and fatty acid oxidation. Finally, we demonstrated that the action of TORCs in promoting mitochondrial gene expression and function requires PGC-1alpha. Previous studies had indicated that TORCs function as a calcium- and cAMP-sensitive coincidence detector and mediate individual and synergistic effects of these two pathways. Our results, together with previous findings, strongly suggest that TORCs play a key role in linking these external signals to the transcriptional program of adaptive mitochondrial biogenesis by activating PGC-1alpha gene transcription.

PGC-1alpha Protects Skeletal Muscle from Atrophy by Suppressing FoxO3 Action and Atrophy-specific Gene Transcription

Proceedings of the National Academy of Sciences of the United States of America. Oct, 2006  |  Pubmed ID: 17053067

Maintaining muscle size and fiber composition requires contractile activity. Increased activity stimulates expression of the transcriptional coactivator PGC-1alpha (peroxisome proliferator-activated receptor gamma coactivator 1alpha), which promotes fiber-type switching from glycolytic toward more oxidative fibers. In response to disuse or denervation, but also in fasting and many systemic diseases, muscles undergo marked atrophy through a common set of transcriptional changes. FoxO family transcription factors play a critical role in this loss of cell protein, and when activated, FoxO3 causes expression of the atrophy-related ubiquitin ligases atrogin-1 and MuRF-1 and profound loss of muscle mass. To understand how exercise might retard muscle atrophy, we investigated the possible interplay between PGC-1alpha and the FoxO family in regulation of muscle size. Rodent muscles showed a large decrease in PGC-1alpha mRNA during atrophy induced by denervation as well as by cancer cachexia, diabetes, and renal failure. Furthermore, in transgenic mice overexpressing PGC-1alpha, denervation and fasting caused a much smaller decrease in muscle fiber diameter and a smaller induction of atrogin-1 and MuRF-1 than in control mice. Increased expression of PGC-1alpha also increased mRNA for several genes involved in energy metabolism whose expression decreases during atrophy. Transfection of PGC-1alpha into adult fibers reduced the capacity of FoxO3 to cause fiber atrophy and to bind to and transcribe from the atrogin-1 promoter. Thus, the high levels of PGC-1alpha in dark and exercising muscles can explain their resistance to atrophy, and the rapid fall in PGC-1alpha during atrophy should enhance the FoxO-dependent loss of muscle mass.

Suppression of Reactive Oxygen Species and Neurodegeneration by the PGC-1 Transcriptional Coactivators

Cell. Oct, 2006  |  Pubmed ID: 17055439

PPARgamma coactivator 1alpha (PGC-1alpha) is a potent stimulator of mitochondrial biogenesis and respiration. Since the mitochondrial electron transport chain is the main producer of reactive oxygen species (ROS) in most cells, we examined the effect of PGC-1alpha on the metabolism of ROS. PGC-1alpha is coinduced with several key ROS-detoxifying enzymes upon treatment of cells with an oxidative stressor; studies with RNAi or null cells indicate that PGC-1alpha is required for the induction of many ROS-detoxifying enzymes, including GPx1 and SOD2. PGC-1alpha null mice are much more sensitive to the neurodegenerative effects of MPTP and kainic acid, oxidative stressors affecting the substantia nigra and hippocampus, respectively. Increasing PGC-1alpha levels dramatically protects neural cells in culture from oxidative-stressor-mediated death. These studies reveal that PGC-1alpha is a broad and powerful regulator of ROS metabolism, providing a potential target for the therapeutic manipulation of these important endogenous toxins.

RANTES (regulated on Activation, Normal T Cell Expressed and Secreted), Inflammation, Obesity, and the Metabolic Syndrome

Circulation. Feb, 2007  |  Pubmed ID: 17325252

PGC-1alpha Regulates the Neuromuscular Junction Program and Ameliorates Duchenne Muscular Dystrophy

Genes & Development. Apr, 2007  |  Pubmed ID: 17403779

The coactivator PGC-1alpha mediates key responses of skeletal muscle to motor nerve activity. We show here that neuregulin-stimulated phosphorylation of PGC-1alpha and GA-binding protein (GABP) allows recruitment of PGC-1alpha to the GABP complex and enhances transcription of a broad neuromuscular junction gene program. Since a subset of genes controlled by PGC-1alpha and GABP is dysregulated in Duchenne muscular dystrophy (DMD), we examined the effects of transgenic PGC-1alpha in muscle of mdx mice. These animals show improvement in parameters characteristic of DMD, including muscle histology, running performance, and plasma creatine kinase levels. Thus, control of PGC-1alpha levels in skeletal muscle could represent a novel avenue to prevent or treat DMD.

A Fundamental System of Cellular Energy Homeostasis Regulated by PGC-1alpha

Proceedings of the National Academy of Sciences of the United States of America. May, 2007  |  Pubmed ID: 17470778

Maintenance of ATP levels is a critical feature of all cells. Mitochondria are responsible for most ATP synthesis in eukaryotes. We show here that mammalian cells respond to a partial chemical uncoupling of mitochondrial oxidative phosphorylation with a decrease in ATP levels, which recovers over several hours to control levels. This recovery occurs through an increased expression of the transcriptional coactivator peroxisome proliferator-activated receptor-coactivator 1alpha (PGC-1alpha) and mitochondrial genes. Cells and animals lacking PGC-1alpha lose this compensatory mechanism and cannot defend their ATP levels or increase mitochondrial gene expression in response to reduced oxidative phosphorylation. The induction of PGC-1alpha and its mitochondrial target genes is triggered by a burst of intracellular calcium, which causes an increase in cAMP-response-element-binding protein and transducer of regulated cAMP-response-element-binding proteins actions on the PGC-1alpha promoter. These data illustrate a fundamental transcriptional cycle that provides homeostatic control of cellular ATP. In light of this compensatory system that limits the toxicity of mild uncoupling, the use of chemical uncoupling of mitochondria as a means of treating obesity should be re-evaluated.

AMP-activated Protein Kinase (AMPK) Action in Skeletal Muscle Via Direct Phosphorylation of PGC-1alpha

Proceedings of the National Academy of Sciences of the United States of America. Jul, 2007  |  Pubmed ID: 17609368

Activation of AMP-activated kinase (AMPK) in skeletal muscle increases glucose uptake, fatty acid oxidation, and mitochondrial biogenesis by increasing gene expression in these pathways. However, the transcriptional components that are directly targeted by AMPK are still elusive. The peroxisome-proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) has emerged as a master regulator of mitochondrial biogenesis; furthermore, it has been shown that PGC-1alpha gene expression is induced by exercise and by chemical activation of AMPK in skeletal muscle. Using primary muscle cells and mice deficient in PGC-1alpha, we found that the effects of AMPK on gene expression of glucose transporter 4, mitochondrial genes, and PGC-1alpha itself are almost entirely dependent on the function of PGC-1alpha protein. Furthermore, AMPK phosphorylates PGC-1alpha directly both in vitro and in cells. These direct phosphorylations of the PGC-1alpha protein at threonine-177 and serine-538 are required for the PGC-1alpha-dependent induction of the PGC-1alpha promoter. These data indicate that AMPK phosphorylation of PGC-1alpha initiates many of the important gene regulatory functions of AMPK in skeletal muscle.

Skeletal Muscle Fiber-type Switching, Exercise Intolerance, and Myopathy in PGC-1alpha Muscle-specific Knock-out Animals

The Journal of Biological Chemistry. Oct, 2007  |  Pubmed ID: 17702743

The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) is a key integrator of neuromuscular activity in skeletal muscle. Ectopic expression of PGC-1alpha in muscle results in increased mitochondrial number and function as well as an increase in oxidative, fatigue-resistant muscle fibers. Whole body PGC-1alpha knock-out mice have a very complex phenotype but do not have a marked skeletal muscle phenotype. We thus analyzed skeletal muscle-specific PGC-1alpha knock-out mice to identify a specific role for PGC-1alpha in skeletal muscle function. These mice exhibit a shift from oxidative type I and IIa toward type IIx and IIb muscle fibers. Moreover, skeletal muscle-specific PGC-1alpha knock-out animals have reduced endurance capacity and exhibit fiber damage and elevated markers of inflammation following treadmill running. Our data demonstrate a critical role for PGC-1alpha in maintenance of normal fiber type composition and of muscle fiber integrity following exertion.

Abnormal Glucose Homeostasis in Skeletal Muscle-specific PGC-1alpha Knockout Mice Reveals Skeletal Muscle-pancreatic Beta Cell Crosstalk

The Journal of Clinical Investigation. Nov, 2007  |  Pubmed ID: 17932564

The transcriptional coactivator PPARgamma coactivator 1alpha (PGC-1alpha) is a strong activator of mitochondrial biogenesis and oxidative metabolism. While expression of PGC-1alpha and many of its mitochondrial target genes are decreased in the skeletal muscle of patients with type 2 diabetes, no causal relationship between decreased PGC-1alpha expression and abnormal glucose metabolism has been established. To address this question, we generated skeletal muscle-specific PGC-1alpha knockout mice (MKOs), which developed significantly impaired glucose tolerance but showed normal peripheral insulin sensitivity. Surprisingly, MKOs had expanded pancreatic beta cell mass, but markedly reduced plasma insulin levels, in both fed and fasted conditions. Muscle tissue from MKOs showed increased expression of several proinflammatory genes, and these mice also had elevated levels of the circulating IL-6. We further demonstrated that IL-6 treatment of isolated mouse islets suppressed glucose-stimulated insulin secretion. These data clearly illustrate a causal role for muscle PGC-1alpha in maintenance of glucose homeostasis and highlight an unexpected cytokine-mediated crosstalk between skeletal muscle and pancreatic islets.

The Role of Exercise and PGC1alpha in Inflammation and Chronic Disease

Nature. Jul, 2008  |  Pubmed ID: 18650917

Inadequate physical activity is linked to many chronic diseases. But the mechanisms that tie muscle activity to health are unclear. The transcriptional coactivator PGC1alpha has recently been shown to regulate several exercise-associated aspects of muscle function. We propose that this protein controls muscle plasticity, suppresses a broad inflammatory response and mediates the beneficial effects of exercise.

Paradoxical Effects of Increased Expression of PGC-1alpha on Muscle Mitochondrial Function and Insulin-stimulated Muscle Glucose Metabolism

Proceedings of the National Academy of Sciences of the United States of America. Dec, 2008  |  Pubmed ID: 19066218

Peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1alpha has been shown to play critical roles in regulating mitochondria biogenesis, respiration, and muscle oxidative phenotype. Furthermore, reductions in the expression of PGC-1alpha in muscle have been implicated in the pathogenesis of type 2 diabetes. To determine the effect of increased muscle-specific PGC-1alpha expression on muscle mitochondrial function and glucose and lipid metabolism in vivo, we examined body composition, energy balance, and liver and muscle insulin sensitivity by hyperinsulinemic-euglycemic clamp studies and muscle energetics by using (31)P magnetic resonance spectroscopy in transgenic mice. Increased expression of PGC-1alpha in muscle resulted in a 2.4-fold increase in mitochondrial density, which was associated with an approximately 60% increase in the unidirectional rate of ATP synthesis. Surprisingly, there was no effect of increased muscle PGC-1alpha expression on whole-body energy expenditure, and PGC-1alpha transgenic mice were more prone to fat-induced insulin resistance because of decreased insulin-stimulated muscle glucose uptake. The reduced insulin-stimulated muscle glucose uptake could most likely be attributed to a relative increase in fatty acid delivery/triglyceride reesterfication, as reflected by increased expression of CD36, acyl-CoA:diacylglycerol acyltransferase1, and mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase, that may have exceeded mitochondrial fatty acid oxidation, resulting in increased intracellular lipid accumulation and an increase in the membrane to cytosol diacylglycerol content. This, in turn, caused activation of PKC, decreased insulin signaling at the level of insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, and skeletal muscle insulin resistance.

The Biology of PGC-1α and Its Therapeutic Potential

Trends in Pharmacological Sciences. Jun, 2009  |  Pubmed ID: 19446346

In eukaryotes, cellular and systemic metabolism is primarily controlled by mitochondrial activity. The peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) is an important regulator of mitochondrial biogenesis and function. Furthermore, PGC-1alpha controls many of the phenotypic adaptations of oxidative tissues to external and internal perturbations. By contrast, dysregulated metabolic plasticity is involved in the etiology of numerous diseases. Accordingly, modulation of PGC-1alpha levels and activity has recently been proposed as a therapeutic option for several pathologies. However, pharmacological interventions aimed at PGC-1alpha have to overcome inherent limitations of targeting a coactivator protein. Here, I focus on the recent breakthroughs in the identification of physiological and pathophysiological contexts involving PGC-1alpha. In addition, perspectives regarding the therapeutic importance of PGC-1alpha-controlled cellular and systemic metabolism are outlined.

Peroxisome Proliferator-activated Receptor-gamma Coactivator-1alpha in Muscle Links Metabolism to Inflammation

Clinical and Experimental Pharmacology & Physiology. Dec, 2009  |  Pubmed ID: 19671064

1. In higher eukaryotes, metabolism and immunity are tightly coupled. However, whereas in evolutionary terms a compromised immune response due to undernourishment has been the predominant problem, the inflammatory response to obesity and other lifestyle-associated diseases has increased in relevance in Western societies in the past 100 years. 2. Traditionally, fat tissue has been considered as the major source of pro-inflammatory secreted factors in these pathologies. However, in recent years the contribution of other tissues to disease-causing chronic inflammation has been increasingly appreciated. 3. Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) is one of the key regulatory factors in active skeletal muscle. Aberrant expression of PGC-1alpha in inactive muscle fibres could be linked to a sedentary lifestyle, persistent systemic inflammation and a higher risk for many chronic diseases. Accordingly, modulation of PGC-1alpha activity in skeletal muscle may have a broad range of therapeutic effects. Here, recent advances in the understanding of the role of muscle PGC-1alpha in health and disease are reviewed.

A High-mobility, Low-cost Phenotype Defines Human Effector-memory CD8+ T Cells

Blood. Jan, 2009  |  Pubmed ID: 18845792

T cells move randomly ("random-walk"), a characteristic thought to be integral to their function. Using migration assays and time-lapse microscopy, we found that CD8+ T cells lacking the lymph node homing receptors CCR7 and CD62L migrate more efficiently in transwell assays, and that these same cells are characterized by a high frequency of cells exhibiting random crawling activity under culture conditions mimicking the interstitial/extravascular milieu, but not when examined on endothelial cells. To assess the energy efficiency of cells crawling at a high frequency, we measured mRNA expression of genes key to mitochondrial energy metabolism (peroxisome proliferator-activated receptor gamma coactivator 1beta [PGC-1beta], estrogen-related receptor alpha [ERRalpha], cytochrome C, ATP synthase, and the uncoupling proteins [UCPs] UCP-2 and -3), quantified ATP contents, and performed calorimetric analyses. Together these assays indicated a high energy efficiency of the high crawling frequency CD8+ T-cell population, and identified differentially regulated heat production among nonlymphoid versus lymphoid homing CD8+ T cells.

For a Pragmatic Approach to Exercise Studies

Journal of Applied Physiology (Bethesda, Md. : 1985). Jan, 2010  |  Pubmed ID: 20066760

Regulation of Skeletal Muscle Cell Plasticity by the Peroxisome Proliferator-activated Receptor γ Coactivator 1α

Journal of Receptor and Signal Transduction Research. Dec, 2010  |  Pubmed ID: 20178454

Exercise triggers a pleiotropic response in skeletal muscle, which results in a profound remodeling of this tissue. Physical activity-dependent muscle fiber plasticity is regulated by a number of distinct signaling pathways. Even though most of these pathways are activated by different stimuli and in a temporally and spatially separated manner during exercise, many of the major signal transduction events converge on the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) by post-translationally modifying the PGC-1α protein, modulating PGC-1α gene expression or both. In turn, depending on the cellular context, PGC-1α regulates specific gene programs. Ultimately, PGC-1α modulates most of the transcriptional adaptations of skeletal muscle to exercise. In this review, the regulation and function of this pivotal transcriptional coactivator in muscle are discussed.

Electric Pulse Stimulation of Cultured Murine Muscle Cells Reproduces Gene Expression Changes of Trained Mouse Muscle

PloS One. 2010  |  Pubmed ID: 20532042

Adequate levels of physical activity are at the center of a healthy lifestyle. However, the molecular mechanisms that mediate the beneficial effects of exercise remain enigmatic. This gap in knowledge is caused by the lack of an amenable experimental model system. Therefore, we optimized electric pulse stimulation of muscle cells to closely recapitulate the plastic changes in gene expression observed in a trained skeletal muscle. The exact experimental conditions were established using the peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) as a marker for an endurance-trained muscle fiber. We subsequently compared the changes in the relative expression of metabolic and myofibrillar genes in the muscle cell system with those observed in mouse muscle in vivo following either an acute or repeated bouts of treadmill exercise. Importantly, in electrically stimulated C2C12 mouse muscle cells, the qualitative transcriptional adaptations were almost identical to those in trained muscle, but differ from the acute effects of exercise on muscle gene expression. In addition, significant alterations in the expression of myofibrillar proteins indicate that this stimulation could be used to modulate the fiber-type of muscle cells in culture. Our data thus describe an experimental cell culture model for the study of at least some of the transcriptional aspects of skeletal muscle adaptation to physical activity. This system will be useful for the study of the molecular mechanisms that regulate exercise adaptation in muscle.

SIRT1 Reduces Endothelial Activation Without Affecting Vascular Function in ApoE-/- Mice

Aging. Jun, 2010  |  Pubmed ID: 20606253

Excessive production of reactive oxygen species (ROS) contributes to progression of atherosclerosis, at least in part by causing endothelial dysfunction and inflammatory activation. The class III histone deacetylase SIRT1 has been implicated in extension of lifespan. In the vasculature,SIRT1 gain-of-function using SIRT1 overexpression or activation has been shown to improve endothelial function in mice and rats via stimulation of endothelial nitric oxide (NO) synthase (eNOS). However, the effects of SIRT1 loss-of-function on the endothelium in atherosclerosis remain to be characterized. Thus, we have investigated the endothelial effects of decreased endogenous SIRT1 in hypercholesterolemic ApoE-/- mice. We observed no difference in endothelial relaxation and eNOS (Ser1177) phosphorylation between 20-week old male atherosclerotic ApoE-/- SIRT1+/- and ApoE-/- SIRT1+/+ mice. However, SIRT1 prevented endothelial superoxide production, inhibited NF-kappaB signaling, and diminished expression of adhesion molecules. Treatment of young hypercholesterolemic ApoE-/- SIRT1+/- mice with lipopolysaccharide to boost NF-kappaB signaling led to a more pronounced endothelial expression of ICAM-1 and VCAM-1 as compared to ApoE-/- SIRT1+/+ mice. In conclusion, endogenous SIRT1 diminishes endothelial activation in ApoE-/- mice, but does not affect endothelium-dependent vasodilatation.

Peroxisome Proliferator-activated Receptor {gamma} Coactivator 1{alpha} (PGC-1{alpha}) Promotes Skeletal Muscle Lipid Refueling in Vivo by Activating De Novo Lipogenesis and the Pentose Phosphate Pathway

The Journal of Biological Chemistry. Oct, 2010  |  Pubmed ID: 20716531

Exercise induces a pleiotropic adaptive response in skeletal muscle, largely through peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α). PGC-1α enhances lipid oxidation and thereby provides energy for sustained muscle contraction. Its potential implication in promoting muscle refueling remains unresolved, however. Here, we investigated a possible role of elevated PGC-1α levels in skeletal muscle lipogenesis in vivo and the molecular mechanisms that underlie PGC-1α-mediated de novo lipogenesis. To this end, we studied transgenic mice with physiological overexpression of PGC-1α and human muscle biopsies pre- and post-exercise. We demonstrate that PGC-1α enhances lipogenesis in skeletal muscle through liver X receptor α-dependent activation of the fatty acid synthase (FAS) promoter and by increasing FAS activity. Using chromatin immunoprecipitation, we establish a direct interaction between PGC-1α and the liver X receptor-responsive element in the FAS promoter. Moreover, we show for the first time that increased glucose uptake and activation of the pentose phosphate pathway provide substrates for RNA synthesis and cofactors for de novo lipogenesis. Similarly, we observed increased lipogenesis and lipid levels in human muscle biopsies that were obtained post-exercise. Our findings suggest that PGC-1α coordinates lipogenesis, intramyocellular lipid accumulation, and substrate oxidation in exercised skeletal muscle in vivo.

ApoE-/- PGC-1α-/- Mice Display Reduced IL-18 Levels and Do Not Develop Enhanced Atherosclerosis

PloS One. 2010  |  Pubmed ID: 21042583

Atherosclerosis is a chronic inflammatory disease that evolves from the interaction of activated endothelial cells, macrophages, lymphocytes and modified lipoproteins (LDLs). In the last years many molecules with crucial metabolic functions have been shown to prevent important steps in the progression of atherogenesis, including peroxisome proliferator activated receptors (PPARs) and the class III histone deacetylase (HDAC) SIRT1. The PPARγ coactivator 1 alpha (Ppargc1a or PGC-1α) was identified as an important transcriptional cofactor of PPARγ and is activated by SIRT1. The aim of this study was to analyze total PGC-1α deficiency in an atherosclerotic mouse model.

PGC-1 Coactivators and the Regulation of Skeletal Muscle Fiber-type Determination

Cell Metabolism. Apr, 2011  |  Pubmed ID: 21459315

Coordinated Balancing of Muscle Oxidative Metabolism Through PGC-1α Increases Metabolic Flexibility and Preserves Insulin Sensitivity

Biochemical and Biophysical Research Communications. Apr, 2011  |  Pubmed ID: 21501593

The peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) enhances oxidative metabolism in skeletal muscle. Excessive lipid oxidation and electron transport chain activity can, however, lead to the accumulation of harmful metabolites and impair glucose homeostasis. Here, we investigated the effect of over-expression of PGC-1α on metabolic control and generation of insulin desensitizing agents in extensor digitorum longus (EDL), a muscle that exhibits low levels of PGC-1α in the untrained state and minimally relies on oxidative metabolism. We demonstrate that PGC-1α induces a strictly balanced substrate oxidation in EDL by concomitantly promoting the transcription of activators and inhibitors of lipid oxidation. Moreover, we show that PGC-1α enhances the potential to uncouple oxidative phosphorylation. Thereby, PGC-1α boosts elevated, yet tightly regulated oxidative metabolism devoid of side products that are detrimental for glucose homeostasis. Accordingly, PI3K activity, an early phase marker for insulin resistance, is preserved in EDL muscle. Our findings suggest that PGC-1α coordinately coactivates the simultaneous transcription of gene clusters implicated in the positive and negative regulation of oxidative metabolism and thereby increases metabolic flexibility. Thus, in mice fed a normal chow diet, over-expression of PGC-1α does not alter insulin sensitivity and the metabolic adaptations elicited by PGC-1α mimic the beneficial effects of endurance training on muscle metabolism in this context.

PGC-1α and Myokines in the Aging Muscle - a Mini-review

Gerontology. 2011  |  Pubmed ID: 20134150

Aging is associated with far-reaching changes in physiological functions resulting in morbidity and ultimately death. Age-related frailty, insecurity and reduced physical activity contribute to a progressive loss of muscle mass and function, commonly referred to as sarcopenia. Due to the increase in life expectancy in many countries, loss of muscle mass and its consequences gain in relevance for public health. At the same time, the molecular mechanisms that underlie sarcopenia are poorly understood and therefore, therapeutic approaches are limited. Interestingly though, endurance, strength and stretching exercise is significantly superior to all known pharmacological, nutritional and hormonal interventions for stabilizing, alleviating and reversing sarcopenia. Thus, increased knowledge about the plastic changes of skeletal muscle after physical activity and the signaling factors that mediate the beneficial effects of exercise on other organs might yield a better understanding of the disease and open new avenues for treatment. Here, we discuss how current discoveries about the peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), a key exercise factor in muscle, and myokines, factors produced and secreted by active muscle fibers, expand our view of the pathological changes and the therapeutic options for sarcopenia.

Myopathy Caused by Mammalian Target of Rapamycin Complex 1 (mTORC1) Inactivation is Not Reversed by Restoring Mitochondrial Function

Proceedings of the National Academy of Sciences of the United States of America. Dec, 2011  |  Pubmed ID: 22143799

Mammalian target of rapamycin complex 1 (mTORC1) is central to the control of cell, organ, and body size. Skeletal muscle-specific inactivation of mTORC1 in mice results in smaller muscle fibers, fewer mitochondria, increased glycogen stores, and a progressive myopathy that causes premature death. In mTORC1-deficient muscles, peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α), which regulates mitochondrial biogenesis and glucose homeostasis, is strongly down-regulated. Here we tested whether induction of mitochondrial biogenesis pharmacologically or by the overexpression of PGC-1α is sufficient to reverse the phenotype of mice deficient for mTORC1. We show that both approaches normalize mitochondrial function, such as oxidative capacity and expression of mitochondrial genes. However, they do not prevent or delay the progressive myopathy. In addition, we find that mTORC1 has a much stronger effect than PGC-1α on the glycogen content in muscle. This effect is based on the strong activation of PKB/Akt in mTORC1-deficient mice. We also show that activation of PKB/Akt not only affects glycogen synthesis but also diminishes glycogen degradation. Thus, our work provides strong functional evidence that mitochondrial dysfunction in mice with inactivated mTORC1 signaling is caused by the down-regulation of PGC-1α. However, our data also show that the impairment of mitochondria does not lead directly to the lethal myopathy.

PGC-1α Determines Light Damage Susceptibility of the Murine Retina

PloS One. 2012  |  Pubmed ID: 22348062

The peroxisome proliferator-activated receptor γ coactivator 1 (PGC-1) proteins are key regulators of cellular bioenergetics and are accordingly expressed in tissues with a high energetic demand. For example, PGC-1α and PGC-1β control organ function of brown adipose tissue, heart, brain, liver and skeletal muscle. Surprisingly, despite their prominent role in the control of mitochondrial biogenesis and oxidative metabolism, expression and function of the PGC-1 coactivators in the retina, an organ with one of the highest energy demands per tissue weight, are completely unknown. Moreover, the molecular mechanisms that coordinate energy production with repair processes in the damaged retina remain enigmatic. In the present study, we thus investigated the expression and function of the PGC-1 coactivators in the healthy and the damaged retina. We show that PGC-1α and PGC-1β are found at high levels in different structures of the mouse retina, most prominently in the photoreceptors. Furthermore, PGC-1α knockout mice suffer from a striking deterioration in retinal morphology and function upon detrimental light exposure. Gene expression studies revealed dysregulation of all major pathways involved in retinal damage and apoptosis, repair and renewal in the PGC-1α knockouts. The light-induced increase in apoptosis in vivo in the absence of PGC-1α was substantiated in vitro, where overexpression of PGC-1α evoked strong anti-apoptotic effects. Finally, we found that retinal levels of PGC-1 expression are reduced in different mouse models for retinitis pigmentosa. We demonstrate that PGC-1α is a central coordinator of energy production and, importantly, all of the major processes involved in retinal damage and subsequent repair. Together with the observed dysregulation of PGC-1α and PGC-1β in retinitis pigmentosa mouse models, these findings thus imply that PGC-1α might be an attractive target for therapeutic approaches aimed at retinal degeneration diseases.

Remodeling of Calcium Handling in Skeletal Muscle Through PGC-1α: Impact on Force, Fatigability, and Fiber Type

American Journal of Physiology. Cell Physiology. Jan, 2012  |  Pubmed ID: 21918181

Regular endurance exercise remodels skeletal muscle, largely through the peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). PGC-1α promotes fiber type switching and resistance to fatigue. Intracellular calcium levels might play a role in both adaptive phenomena, yet a role for PGC-1α in the adaptation of calcium handling in skeletal muscle remains unknown. Using mice with transgenic overexpression of PGC-1α, we now investigated the effect of PGC-1α on calcium handling in skeletal muscle. We demonstrate that PGC-1α induces a quantitative reduction in calcium release from the sarcoplasmic reticulum by diminishing the expression of calcium-releasing molecules. Concomitantly, maximal muscle force is reduced in vivo and ex vivo. In addition, PGC-1α overexpression delays calcium clearance from the myoplasm by interfering with multiple mechanisms involved in calcium removal, leading to higher myoplasmic calcium levels following contraction. During prolonged muscle activity, the delayed calcium clearance might facilitate force production in mice overexpressing PGC-1α. Our results reveal a novel role of PGC-1α in altering the contractile properties of skeletal muscle by modulating calcium handling. Importantly, our findings indicate PGC-1α to be both down- as well as upstream of calcium signaling in this tissue. Overall, our findings suggest that in the adaptation to chronic exercise, PGC-1α reduces maximal force, increases resistance to fatigue, and drives fiber type switching partly through remodeling of calcium transients, in addition to promoting slow-type myofibrillar protein expression and adequate energy supply.