Jiandie Lin

Transcriptional co-activator PGC-1α drives the formation of slow-twitch muscle fibres

The biochemical basis for the regulation of fibre-type determination in skeletal muscle is not well understood. In addition to the expression of particular myofibrillar proteins, type I (slow-twitch) fibres are much higher in mitochondrial content and are more dependent on oxidative metabolism than type II (fast-twitch) fibres. We have previously identified a transcriptional co-activator, peroxisome-proliferator-activated receptor-γ co-activator-1 (PGC-1α), which is expressed in several tissues including brown fat and skeletal muscle, and that activates mitochondrial biogenesis and oxidative metabolism. We show here that PGC-1α is expressed preferentially in muscle enriched in type I fibres. When PGC-1α is expressed at physiological levels in transgenic mice driven by a muscle creatine kinase (MCK) promoter, a fibre type conversion is observed: muscles normally rich in type II fibres are redder and activate genes of mitochondrial oxidative metabolism. Notably, putative type II muscles from PGC-1α transgenic mice also express proteins characteristic of type I fibres, such as troponin I (slow) and myoglobin, and show a much greater resistance to electrically stimulated fatigue. Using fibre-type-specific promoters, we show in cultured muscle cells that PGC-1α activates transcription in cooperation with Mef2 proteins and serves as a target for calcineurin signalling, which has been implicated in slow fibre gene expression. These data indicate that PGC-1α is a principal factor regulating muscle fibre type determination.

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

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.

Defects in Adaptive Energy Metabolism with CNS-Linked Hyperactivity in PGC-1α Null Mice

PGC-1alpha is a coactivator of nuclear receptors and other transcription factors that regulates several metabolic processes, including mitochondrial biogenesis and respiration, hepatic gluconeogenesis, and muscle fiber-type switching. We show here that, while hepatocytes lacking PGC-1alpha are defective in the program of hormone-stimulated gluconeogenesis, the mice have constitutively activated gluconeogenic gene expression that is completely insensitive to normal feeding controls. C/EBPbeta is elevated in the livers of these mice and activates the gluconeogenic genes in a PGC-1alpha-independent manner. Despite having reduced mitochondrial function, PGC-1alpha null mice are paradoxically lean and resistant to diet-induced obesity. This is largely due to a profound hyperactivity displayed by the null animals and is associated with lesions in the striatal region of the brain that controls movement. These data illustrate a central role for PGC-1alpha in the control of energy metabolism but also reveal novel systemic compensatory mechanisms and pathogenic effects of impaired energy homeostasis.

PGC-1α protects skeletal muscle from atrophy by suppressing FoxO3 action and atrophy-specific gene transcription

Maintaining muscle size and fiber composition requires contractile activity. Increased activity stimulates expression of the transcriptional coactivator PGC-1α (peroxisome proliferator-activated receptor γ coactivator 1α), 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-1α and the FoxO family in regulation of muscle size. Rodent muscles showed a large decrease in PGC-1α mRNA during atrophy induced by denervation as well as by cancer cachexia, diabetes, and renal failure. Furthermore, in transgenic mice overexpressing PGC-1α, 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-1α also increased mRNA for several genes involved in energy metabolism whose expression decreases during atrophy. Transfection of PGC-1α 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-1α in dark and exercising muscles can explain their resistance to atrophy, and the rapid fall in PGC-1α during atrophy should enhance the FoxO-dependent loss of muscle mass.

Cytokine Stimulation of Energy Expenditure through p38 MAP Kinase Activation of PPARγ Coactivator-1

Cachexia is a chronic state of negative energy balance and muscle wasting that is a severe complication of cancer and chronic infection. While cytokines such as IL-1alpha, IL-1beta, and TNFalpha can mediate cachectic states, how these molecules affect energy expenditure is unknown. We show here that many cytokines activate the transcriptional PPAR gamma coactivator-1 (PGC-1) through phosphorylation by p38 kinase, resulting in stabilization and activation of PGC-1 protein. Cytokine or lipopolysaccharide (LPS)-induced activation of PGC-1 in cultured muscle cells or muscle in vivo causes increased respiration and expression of genes linked to mitochondrial uncoupling and energy expenditure. These data illustrate a direct thermogenic action of cytokines and p38 MAP kinase through the transcriptional coactivator PGC-1.

An autoregulatory loop controls peroxisome proliferator-activated receptor γ coactivator 1α expression in muscle

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 γ coactivator 1α (PGC-1α). Transgenic expression of PGC-1α recently has been shown to dramatically increase the content of type I muscle fibers in skeletal muscle, but the relationship between PGC-1α expression and the key components in calcium signaling is not clear. In this report, we show that the PGC-1α promoter is regulated by both CaMKIV and CnA activity. CaMKIV activates PGC-1α largely through the binding of cAMP response element-binding protein to the PGC-1α promoter. Moreover, we show that a positive feedback loop exists between PGC-1α and members of the myocyte enhancer factor 2 (MEF2) family of transcription factors. MEF2s bind to the PGC-1α promoter and activate it, predominantly when coactivated by PGC-1α. 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-1α. Furthermore, these data suggest an autofeedback loop whereby the calcium-signaling pathway may result in a stable induction of PGC-1α, contributing to the relatively stable nature of muscle fiber-type determination.

Hyperlipidemic effects of dietary saturated fats mediated through PGC-1β coactivation of SREBP

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.

Muscle-specific expression of PPARγ coactivator-1α improves exercise performance and increases peak oxygen uptake

The induction of peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), a key regulator of mitochondriogenesis, is well-established under multiple physical exercise regimens, including, endurance, resistance, and sprint training. We wanted to determine if increased expression of PGC-1alpha in muscle is sufficient to improve performance during exercise in vivo. We demonstrate that muscle-specific expression of PGC-1alpha improves the performance during voluntary as well as forced exercise challenges. Additionally, PGC-1alpha transgenic mice exhibit an enhanced performance during a peak oxygen uptake exercise test, demonstrating an increased peak oxidative capacity, or whole body oxygen uptake. This increased ability to perform in multiple exercise paradigms is supported by enhanced mitochondrial function as suggested by increased mitochondrial gene expression, mitochondrial DNA, and mitochondrial enzyme activity. Thus this study demonstrates that upregulation of PGC-1alpha in muscle in vivo is sufficient to greatly improve exercise performance under various exercise paradigms as well as increase peak oxygen uptake.

Paradoxical effects of increased expression of PGC-1α on muscle mitochondrial function and insulin-stimulated muscle glucose metabolism

Peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α has been shown to play critical roles in regulating mitochondria biogenesis, respiration, and muscle oxidative phenotype. Furthermore, reductions in the expression of PGC-1α in muscle have been implicated in the pathogenesis of type 2 diabetes. To determine the effect of increased muscle-specific PGC-1α 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 31P magnetic resonance spectroscopy in transgenic mice. Increased expression of PGC-1α in muscle resulted in a 2.4-fold increase in mitochondrial density, which was associated with an ≈60% increase in the unidirectional rate of ATP synthesis. Surprisingly, there was no effect of increased muscle PGC-1α expression on whole-body energy expenditure, and PGC-1α 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 PKCθ, decreased insulin signaling at the level of insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, and skeletal muscle insulin resistance.

Nutritional Regulation of Hepatic Heme Biosynthesis and Porphyria through PGC-1α

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.