Adam R. Wende

Adaptations of skeletal muscle to exercise: rapid increase in the transcriptional coactivator PGC-1

Endurance exercise induces increases in mitochondria and the GLUT4 isoform of the glucose transporter in muscle. Although little is known about the mechanisms underlying these adaptations, new information has accumulated regarding how mitochondrial biogenesis and GLUT4 expression are regulated. This includes the findings that the transcriptional coactivator PGC‐1 promotes mitochondrial biogenesis and that NRF‐1 and NRF‐2 act as transcriptional activators of genes encoding mitochondrial enzymes. We tested the hypothesis that increases in PGC‐1, NRF‐1, and NRF‐2 are involved in the initial adaptive response of muscle to exercise. Five daily bouts of swimming induced increases in mitochondrial enzymes and GLUT4 in skeletal muscle in rats. One exercise bout resulted in ~ twofold increases in full‐length muscle PGC‐1 mRNA and PGC‐1 protein, which were evident 18 h after exercise. A smaller form of PGC‐1 increased after exercise. The exercise induced increases in muscle NRF‐1 and NRF‐2 that were evident 12 to 18 h after one exercise bout. These findings suggest that increases in PGC‐1, NRF‐1, and NRF‐2 represent key regulatory components of the stimulation of mitochondrial biogenesis by exercise and that PGC‐1 mediates the coordinated increases in GLUT4 and mitochondria.—Baar, K., Wende, A. R., Jones, T. E., Marison, M., Nolte, L. A., Chen, M., Kelly, D. P., Holloszy, J. O. Adaptations of skeletal muscle to exercise: rapid increase in the transcriptional coactivator PGC‐1. FASEB J. 16, 1879–1886 (2002)

PGC-1α 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.

PGC-1α Coactivates PDK4 Gene Expression via the Orphan Nuclear Receptor ERRα: a Mechanism for Transcriptional Control of Muscle Glucose Metabolism

ABSTRACT The transcriptional coactivator PGC-1α 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-1α expression in skeletal muscle coordinately upregulate the expression of pyruvate dehydrogenase kinase 4 (PDK4), a negative regulator of glucose oxidation. Forced expression of PGC-1α in C2C12 myotubes induced PDK4 mRNA and protein expression. PGC-1α-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-1α-responsive element bound the estrogen-related receptor α (ERRα), a recently identified component of the PGC-1α signaling pathway. In addition, PGC-1α was shown to activate ERRα expression. Chromatin immunoprecipitation assays confirmed that PGC-1α and ERRα occupied the mPDK4 promoter in C2C12 myotubes. Additionally, transfection studies using ERRα-null primary fibroblasts demonstrated that ERRα is required for PGC-1α-mediated activation of the mPDK4 promoter. As predicted by the effects of PGC-1α on PDK4 gene transcription, overexpression of PGC-1α in C2C12 myotubes decreased glucose oxidation rates. These results identify the PDK4 gene as a new PGC-1α/ERRα target and suggest a mechanism whereby PGC-1α exerts reciprocal inhibitory influences on glucose catabolism while increasing alternate mitochondrial oxidative pathways in skeletal muscle.

A Role for the Transcriptional Coactivator PGC-1α in Muscle Refueling

The transcriptional coactivator peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) has been identified as an inducible regulator of mitochondrial function. Skeletal muscle PGC-1α expression is induced post-exercise. Therefore, we sought to determine its role in the regulation of muscle fuel metabolism. Studies were performed using conditional, muscle-specific, PGC-1α gain-of-function and constitutive, generalized, loss-of-function mice. Forced expression of PGC-1α increased muscle glucose uptake concomitant with augmentation of glycogen stores, a metabolic response similar to post-exercise recovery. Induction of muscle PGC-1α expression prevented muscle glycogen depletion during exercise. Conversely, PGC-1α-deficient animals exhibited reduced rates of muscle glycogen repletion post-exercise. PGC-1α was shown to increase muscle glycogen stores via several mechanisms including stimulation of glucose import, suppression of glycolytic flux, and by down-regulation of the expression of glycogen phosphorylase and its activating kinase, phosphorylase kinase α. These findings identify PGC-1α as a critical regulator of skeletal muscle fuel stores.

Lipotoxicity in the heart

Obesity and insulin resistance are associated with ectopic lipid deposition in multiple tissues, including the heart. Excess lipid may be stored as triglycerides, but are also shunted into non-oxidative pathways that disrupt normal cellular signaling leading to organ dysfunction and in some cases apoptosis, a process termed lipotoxicity. Various pathophysiological mechanisms have been proposed to lead to lipotoxic tissue injury, which might vary by cell type. Specific mechanisms by which lipotoxicity alter cardiac structure and function are incompletely understood, but are beginning to be elucidated. This review will focus on mechanisms that have been proposed to lead to lipotoxic injury in the heart and will review the state of knowledge regarding potential causes and correlates of increased myocardial lipid content in animal models and humans. We will seek to highlight those areas where additional research is warranted.

Insulin-like growth factor I receptor signaling is required for exercise-induced cardiac hypertrophy.

The receptors for IGF-I (IGF-IR) and insulin (IR) have been implicated in physiological cardiac growth, but it is unknown whether IGF-IR or IR signaling are critically required. We generated mice with cardiomyocyte-specific knockout of IGF-IR (CIGF1RKO) and compared them with cardiomyocyte-specific insulin receptor knockout (CIRKO) mice in response to 5 wk exercise swim training. Cardiac development was normal in CIGF1RKO mice, but the hypertrophic response to exercise was prevented. In contrast, despite reduced baseline heart size, the hypertrophic response of CIRKO hearts to exercise was preserved. Exercise increased IGF-IR content in control and CIRKO hearts. Akt phosphorylation increased in exercise-trained control and CIRKO hearts and, surprisingly, in CIGF1RKO hearts as well. In exercise-trained control and CIRKO mice, expression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) and glycogen content were both increased but were unchanged in trained CIGF1RKO mice. Activation of AMP-activated protein kinase (AMPK) and its downstream target eukaryotic elongation factor-2 was increased in exercise-trained CIGF1RKO but not in CIRKO or control hearts. In cultured neonatal rat cardiomyocytes, activation of AMPK with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) prevented IGF-I/insulin-induced cardiomyocyte hypertrophy. These studies identify an essential role for IGF-IR in mediating physiological cardiomyocyte hypertrophy. IGF-IR deficiency promotes energetic stress in response to exercise, thereby activating AMPK, which leads to phosphorylation of eukaryotic elongation factor-2. These signaling events antagonize Akt signaling, which although necessary for mediating physiological cardiac hypertrophy, is insufficient to promote cardiac hypertrophy in the absence of myocardial IGF-I signaling.

PGC-1β Deficiency Accelerates the Transition to Heart Failure in Pressure Overload Hypertrophy

Rationale: Pressure overload cardiac hypertrophy, a risk factor for heart failure, is associated with reduced mitochondrial fatty acid oxidation (FAO) and oxidative phosphorylation (OXPHOS) proteins that correlate in rodents with reduced PGC-1&agr; expression. Objective: To determine the role of PGC-1&bgr; in maintaining mitochondrial energy metabolism and contractile function in pressure overload hypertrophy. Methods and Results: PGC-1&bgr; deficient (KO) mice and wildtype (WT) controls were subjected to transverse aortic constriction (TAC). Although LV function was modestly reduced in young KO hearts, there was no further decline with age so that LV function was similar between KO and WT when TAC was performed. WT-TAC mice developed relatively compensated LVH, despite reduced mitochondrial function and repression of OXPHOS and FAO genes. In nonstressed KO hearts, OXPHOS gene expression and palmitoyl-carnitine-supported mitochondrial function were reduced to the same extent as banded WT, but FAO gene expression was normal. Following TAC, KO mice progressed more rapidly to heart failure and developed more severe mitochondrial dysfunction, despite a similar overall pattern of repression of OXPHOS and FAO genes as WT-TAC. However, in relation to WT-TAC, PGC-1&bgr; deficient mice exhibited greater degrees of oxidative stress, decreased cardiac efficiency, lower rates of glucose metabolism, and repression of hexokinase II protein. Conclusions: PGC-1&bgr; plays an important role in maintaining baseline mitochondrial function and cardiac contractile function following pressure overload hypertrophy by preserving glucose metabolism and preventing oxidative stress.

Diminished Hepatic Gluconeogenesis via Defects in Tricarboxylic Acid Cycle Flux in Peroxisome Proliferator-activated Receptor γ Coactivator-1α (PGC-1α)-deficient Mice

The peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1α (PGC-1α) is a highly inducible transcriptional coactivator implicated in the coordinate regulation of genes encoding enzymes involved in hepatic fatty acid oxidation, oxidative phosphorylation, and gluconeogenesis. The present study sought to assess the effects of chronic PGC-1α deficiency on metabolic flux through the hepatic gluconeogenic, fatty acid oxidation, and tricarboxylic acid cycle pathways. To this end, hepatic metabolism was assessed in wild-type (WT) and PGC-1α–/– mice using isotopomer-based NMR with complementary gene expression analyses. Hepatic glucose production was diminished in PGC-1α–/– livers coincident with reduced gluconeogenic flux from phosphoenolpyruvate. Surprisingly, the expression of PGC-1α target genes involved in gluconeogenesis was unaltered in PGC-1α–/– compared with WT mice under fed and fasted conditions. Flux through tricarboxylic acid cycle and mitochondrial fatty acid β-oxidation pathways was also diminished in PGC-1α–/– livers. The expression of multiple genes encoding tricarboxylic acid cycle and oxidative phosphorylation enzymes was significantly depressed in PGC-1α–/– mice and was activated by PGC-1α overexpression in the livers of WT mice. Collectively, these findings suggest that chronic whole-animal PGC-1α deficiency results in defects in hepatic glucose production that are secondary to diminished fatty acid β-oxidation and tricarboxylic acid cycle flux rather than abnormalities in gluconeogenic enzyme gene expression per se.

Insulin receptor substrate signaling suppresses neonatal autophagy in the heart

The induction of autophagy in the mammalian heart during the perinatal period is an essential adaptation required to survive early neonatal starvation; however, the mechanisms that mediate autophagy suppression once feeding is established are not known. Insulin signaling in the heart is transduced via insulin and IGF-1 receptors (IGF-1Rs). We disrupted insulin and IGF-1R signaling by generating mice with combined cardiomyocyte-specific deletion of Irs1 and Irs2. Here we show that loss of IRS signaling prevented the physiological suppression of autophagy that normally parallels the postnatal increase in circulating insulin. This resulted in unrestrained autophagy in cardiomyocytes, which contributed to myocyte loss, heart failure, and premature death. This process was ameliorated either by activation of mTOR with aa supplementation or by genetic suppression of autophagic activation. Loss of IRS1 and IRS2 signaling also increased apoptosis and precipitated mitochondrial dysfunction, which were not reduced when autophagic flux was normalized. Together, these data indicate that in addition to prosurvival signaling, insulin action in early life mediates the physiological postnatal suppression of autophagy, thereby linking nutrient sensing to postnatal cardiac development.

Mechanisms of Lipotoxicity in the Cardiovascular System

Cardiovascular diseases account for approximately one third of all deaths globally. Obese and diabetic patients have a high likelihood of dying from complications associated with cardiovascular dysfunction. Obesity and diabetes increase circulating lipids that upon tissue uptake, may be stored as triglyceride, or may be metabolized in other pathways, leading to the generation of toxic intermediates. Excess lipid utilization or activation of signaling pathways by lipid metabolites may disrupt cellular homeostasis and contribute to cell death, defining the concept of lipotoxicity. Lipotoxicity occurs in multiple organs, including cardiac and vascular tissues, and a number of specific mechanisms have been proposed to explain lipotoxic tissue injury. In addition, recent data suggests that increased tissue lipids may also be protective in certain contexts. This review will highlight recent progress toward elucidating the relationship between nutrient oversupply, lipotoxicity, and cardiovascular dysfunction. The review will focus in two sections on the vasculature and cardiomyocytes respectively.