PGC-1beta in the regulation of hepatic glucose and energy metabolism

Direct link between metabolic regulation and the heat-shock response through the transcriptional regulator PGC-1α

In recent years an extensive effort has been made to elucidate the molecular pathways involved in metabolic signaling in health and disease. Here we show, surprisingly, that metabolic regulation and the heat-shock/stress response are directly linked. Peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), a critical transcriptional coactivator of metabolic genes, acts as a direct transcriptional repressor of heat-shock factor 1 (HSF1), a key regulator of the heat-shock/stress response. Our findings reveal that heat-shock protein (HSP) gene expression is suppressed during fasting in mouse liver and in primary hepatocytes dependent on PGC-1α. HSF1 and PGC-1α associate physically and are colocalized on several HSP promoters. These observations are extended to several cancer cell lines in which PGC-1α is shown to repress the ability of HSF1 to activate gene-expression programs necessary for cancer survival. Our study reveals a surprising direct link between two major cellular transcriptional networks, highlighting a previously unrecognized facet of the activity of the central metabolic regulator PGC-1α beyond its well-established ability to boost metabolic genes via its interactions with nuclear hormone receptors and nuclear respiratory factors. Our data point to PGC-1α as a critical repressor of HSF1-mediated transcriptional programs, a finding with possible implications both for our understanding of the full scope of metabolically regulated target genes in vivo and, conceivably, for therapeutics.

The Gly482Ser polymorphism of the peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) is associated with type 2 diabetes in Tunisian population

BACKGROUND AND AIMS

Peroxisome proliferator-activated receptor-γ co-activator-1α (PGC-1α) is a transcriptional co-activator involved in adaptive thermogenesis, skeletal muscle metabolism, fatty acid oxidation, and gluconeogenesis. Several studies have suggested that the common PGC-1α polymorphism Gly482Ser (rs8192678) may be associated with risk of type 2 diabetes (T2D), with conflicting results. The aim of this study was to analyze whether the Gly482Ser variant is a risk factor for development of T2D in Tunisian population.

METHODS

In a case-control study 487 unrelated patients with type 2 diabetes and 402 apparently healthy controls were recruited from January 2008 to August 2010. The Gly482Ser polymorphism was determined by PCR-RFLP analysis.

RESULTS

A significant difference in genotypes distribution was observed between patients (Gly/Gly: 34.1%; Gly/Ser: 47.1%; Ser/Ser: 18.5%) and controls (Gly/Gly: 43.8%; Gly/Ser: 42.3%; Ser/Ser: 13.9%) (χ(2)=9.44, p=0.009). The T2D patient group showed a significant higher frequency of the Ser allele compared to the controls (43% vs. 34%; OR: 1.35, 95% [CI]: 1.11-1.65, p=0.002). The association between the Gly482Ser polymorphism and T2D remained significant after adjustment for other well-established cardiovascular risk factors.

CONCLUSIONS

In the current study, a significant and independent association between the Gly482Ser polymorphism (rs8192678) of the PGC-1α gene and T2D in the Tunisian population was found.

Combined inhibition of glycolysis and AMPK induces synergistic breast cancer cell killing

Targeting glycolysis for cancer treatment has been investigated as a therapeutic method but has not offered a feasible chemotherapeutic strategy. Our aim was to examine whether AMP-activated protein kinase (AMPK), a conditional oncogene, rescues the energetic stress and cytotoxicity induced by 2-deoxyglucose (2-DG), a glycolytic inhibitor, and the related mechanisms. Luciferin/luciferase adenosine triphosphate (ATP) determination, Western analysis, qRT-PCR analyses, MTT growth assay, clonogenic assay, and statistical analysis were performed in this study. 2-DG decreased ATP levels and subsequently activated AMPK, which contribute to intracellular ATP recovery in MCF-7 cells thus exhibiting no apparent cytotoxicity. Compound C, an AMPK inhibitor, further potentiates 2-DG-induced decrease in ATP levels and inhibits their recovery. 2-DG, via AMPK activation, stimulated cAMP response element-binding protein (CREB) phosphorylation and activity and promoted nuclear peroxisome proliferator-activated receptor gamma coactivator-1-beta (PGC-1β) and estrogen-related receptor α (ERRα) protein expression, leading to augmented mitochondrial biogenesis and expression of fatty acid oxidation (FAO) genes including PPARα, MCAD, CPT1C, and ACO. This metabolic adaptation elicited by AMPK counteracts the ATP-depleting and cancer cell-killing effect of 2-DG. However, 2-DG in combination with AMPK antagonists or small interfering RNA caused a dramatic increase in cytotoxicity in MCF-7 but not in MCF-10A cells. Similarly, when combined with inhibition of CREB/PGC-1β/ERRα pathway, 2-DG saliently suppressed mitochondrial biogenesis and the expression of FAO genes, depleted ATP production, and enhanced cytotoxicity in cancer cells. Collectively, the combination of 2-DG and AMPK inhibition synergistically enhanced the cytotoxic potential in breast cancer cells with a relative nontoxicity to normal cells and may offer a promising, safe, and effective breast cancer therapeutic strategy.

Regulation of lipid droplet size in mammary epithelial cells by remodeling of membrane lipid composition-a potential mechanism

Milk fat globule size is determined by the size of its precursors—intracellular lipid droplets—and is tightly associated with its composition. We examined the relationship between phospholipid composition of mammary epithelial cells and the size of both intracellular and secreted milk fat globules. Primary culture of mammary epithelial cells was cultured in medium without free fatty acids (control) or with 0.1 mM free capric, palmitic or oleic acid for 24 h. The amount and composition of the cellular lipids and the size of the lipid droplets were determined in the cells and medium. Mitochondrial quantity and expression levels of genes associated with mitochondrial biogenesis and polar lipid composition were determined. Cells cultured with oleic and palmitic acids contained similar quantities of triglycerides, 3.1- and 3.8-fold higher than in controls, respectively (P < 0.0001). When cultured with oleic acid, 22% of the cells contained large lipid droplets (>3 μm) and phosphatidylethanolamine concentration was higher by 23 and 63% compared with that in the control and palmitic acid treatments, respectively (P < 0.0001). In the presence of palmitic acid, only 4% of the cells contained large lipid droplets and the membrane phosphatidylcholine concentration was 22% and 16% higher than that in the control and oleic acid treatments, respectively (P < 0.0001). In the oleic acid treatment, approximately 40% of the lipid droplets were larger than 5 μm whereas in that of the palmitic acid treatment, only 16% of the droplets were in this size range. Triglyceride secretion in the oleic acid treatment was 2- and 12-fold higher compared with that in the palmitic acid and control treatments, respectively. Results imply that membrane composition of bovine mammary epithelial cells plays a role in controlling intracellular and secreted lipid droplets size, and that this process is not associated with cellular triglyceride content.

Metabolic roles of PGC-1α and its implications for type 2 diabetes

PGC-1α is a transcriptional coactivator expressed in brown adipose tissue, liver, pancreas, kidney, skeletal and cardiac muscles, and the brain. This review presents data illustrating how PGC-1α regulates metabolic adaptations and participates in the aetiology of type 2 diabetes (T2D). Studies in mice have shown that increased PGC-1α expression may be beneficial or deleterious, depending on the tissue: in adipose tissue, it promotes thermogenesis and thus protects against energy overload, such as seen in diabetes and obesity; in muscle, PGC-1α induces a change of phenotype towards oxidative metabolism. In contrast, its role is clearly deleterious in the liver and pancreas, where it induces hepatic glucose production and inhibits insulin secretion, changes that promote diabetes. Previous studies by our group have also demonstrated the role of PGC-1α in the fetal origins of T2D. Overexpression of PGC-1α in β cells during fetal life in mice is sufficient to induce β-cell dysfunction in adults, leading to glucose intolerance. PGC-1α also is associated with glucocorticoid receptors in repressing expression of Pdx1, a key β-cell transcription factor. In conclusion, PGC-1α participates in the onset of diabetes through regulation of major metabolic tissues. Yet, it may not represent a useful target for therapeutic strategies against diabetes as it exerts both beneficial and deleterious actions on glucose homoeostasis, and because PGC-1α modulation is involved in neurodegenerative diseases. However, its role in cellular adaptation shows that greater comprehension of PGC-1α actions is needed.

The E3 ligase synoviolin controls body weight and mitochondrial biogenesis through negative regulation of PGC-1β

Obesity is a major global public health problem, and understanding its pathogenesis is critical for identifying a cure. In this study, a gene knockout strategy was used in post-neonatal mice to delete synoviolin (Syvn)1/Hrd1/Der3, an ER-resident E3 ubiquitin ligase with known roles in homeostasis maintenance. Syvn1 deficiency resulted in weight loss and lower accumulation of white adipose tissue in otherwise wild-type animals as well as in genetically obese (ob/ob and db/db) and adipose tissue-specific knockout mice as compared to control animals. SYVN1 interacted with and ubiquitinated the thermogenic coactivator peroxisome proliferator-activated receptor coactivator (PGC)-1β, and Syvn1 mutants showed upregulation of PGC-1β target genes and increase in mitochondrion number, respiration, and basal energy expenditure in adipose tissue relative to control animals. Moreover, the selective SYVN1 inhibitor LS-102 abolished the negative regulation of PGC-1β by SYVN1 and prevented weight gain in mice. Thus, SYVN1 is a novel post-translational regulator of PGC-1β and a potential therapeutic target in obesity treatment.

The Role of miR-378a in Metabolism, Angiogenesis, and Muscle Biology

MicroRNA-378a (miR-378a, previously known as miR-378) is one of the small noncoding RNA molecules able to regulate gene expression at posttranscriptional level. Its two mature strands, miR-378a-3p and miR-378a-5p, originate from the first intron of the peroxisome proliferator-activated receptor gamma, coactivator 1 beta (ppargc1b) gene encoding PGC-1β. Embedding in the sequence of this transcriptional regulator of oxidative energy metabolism implies involvement of miR-378a in metabolic pathways, mitochondrial energy homeostasis, and related biological processes such as muscle development, differentiation, and regeneration. On the other hand, modulating the expression of proangiogenic factors such as vascular endothelial growth factor, angiopoietin-1, or interleukin-8, influencing inflammatory reaction, and affecting tumor suppressors, such as SuFu and Fus-1, miR-378a is considered as a part of an angiogenic network in tumors. In the latter, miR-378a can evoke broader actions by enhancing cell survival, reducing apoptosis, and promoting cell migration and invasion. This review describes the current knowledge on miR-378a linking oxidative/lipid metabolism, muscle biology, and blood vessel formation.

PGC-1β promotes enterocyte lifespan and tumorigenesis in the intestine

The mucosa of the small intestine is renewed completely every 3-5 d throughout the entire lifetime by small populations of adult stem cells that are believed to reside in the bottom of the crypts and to migrate and differentiate into all the different populations of intestinal cells. When the cells reach the apex of the villi and are fully differentiated, they undergo cell death and are shed into the lumen. Reactive oxygen species (ROS) production is proportional to the electron transfer activity of the mitochondrial respiration chain. ROS homeostasis is maintained to control cell death and is finely tuned by an inducible antioxidant program. Here we show that peroxisome proliferator-activated receptor-γ coactivator-1β (PGC-1β) is highly expressed in the intestinal epithelium and possesses dual activity, stimulating mitochondrial biogenesis and oxygen consumption while inducing antioxidant enzymes. To study the role of PGC-1β gain and loss of function in the gut, we generated both intestinal-specific PGC-1β transgenic and PGC-1β knockout mice. Mice overexpressing PGC-1β present a peculiar intestinal morphology with very long villi resulting from increased enterocyte lifespan and also demonstrate greater tumor susceptibility, with increased tumor number and size when exposed to carcinogens. PGC-1β knockout mice are protected from carcinogenesis. We show that PGC-1β triggers mitochondrial respiration while protecting enterocytes from ROS-driven macromolecule damage and consequent apoptosis in both normal and dysplastic mucosa. Therefore, PGC-1β in the gut acts as an adaptive self-point regulator, capable of providing a balance between enhanced mitochondrial activity and protection from increased ROS production.

Effects of salinity on metabolic rate and branchial expression of genes involved in ion transport and metabolism in Mozambique tilapia (Oreochromis mossambicus)

This study investigated the effects of two rearing salinities, and acute salinity transfer, on the energetic costs of osmoregulation and the expression of metabolic and osmoregulatory genes in the gill of Mozambique tilapia. Using automated, intermittent-flow respirometry, measured standard metabolic rates (SMRs) of tilapia reared in seawater (SW, 130 mg O₂ kg⁻¹ h⁻¹) were greater than those reared in fresh water (FW, 103 mg O₂ kg⁻¹ h⁻¹), when normalized to a common mass of 0.05 kg and at 25±1°C. Transfer from FW to 75% SW increased SMR within 18h, to levels similar to SW-reared fish, while transfer from SW to FW decreased SMR to levels similar to FW-reared fish. Branchial gene expression of Na⁺-K⁺-2Cl⁻ cotransporter (NKCC), an indicator of SW-type mitochondria-rich (MR) cells, was positively correlated with SMR, while Na⁺-Cl⁻ cotransporter (NCC), an indicator of FW-type MR cells, was negatively correlated. Principal Components Analysis also revealed that branchial expression of cytochrome c oxidase subunit IV (COX-IV), glycogen phosphorylase (GP), and a putative mitochondrial biogenesis regulator in fish, peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), were correlated with a higher SMR, plasma osmolality, and environmental salinity, while expression of glycogen synthase (GS), PGC-1β, and nuclear respiratory factor 1 (NRF-1) had negative correlations. These results suggest that the energetic costs of osmoregulation are higher in SW than in FW, which may be related to the salinity-dependent differences in osmoregulatory mechanisms found in the gills of Mozambique tilapia.

Effect of one month duration ketogenic and non-ketogenic high fat diets on mouse brain bioenergetic infrastructure

Diet composition may affect energy metabolism in a tissue-specific manner. Using C57Bl/6J mice, we tested the effect of ketosis-inducing and non-inducing high fat diets on genes relevant to brain bioenergetic infrastructures, and on proteins that constitute and regulate that infrastructure. At the end of a one-month study period the two high fat diets appeared to differentially affect peripheral insulin signaling, but brain insulin signaling was not obviously altered. Some bioenergetic infrastructure parameters were similarly impacted by both high fat diets, while other parameters were only impacted by the ketogenic diet. For both diets, mRNA levels for CREB, PGC1α, and NRF2 increased while NRF1, TFAM, and COX4I1 mRNA levels decreased. PGC1β mRNA increased and TNFα mRNA decreased only with the ketogenic diet. Brain mtDNA levels fell in both the ketogenic and non-ketogenic high fat diet groups, although TOMM20 and COX4I1 protein levels were maintained, and mRNA and protein levels of the mtDNA-encoded COX2 subunit were also preserved. Overall, the pattern of changes observed in mice fed ketogenic and non-ketogenic high fat diets over a one month time period suggests these interventions enhance some aspects of the brain's aerobic infrastructure, and may enhance mtDNA transcription efficiency. Further studies to determine which diet effects are due to changes in brain ketone body levels, fatty acid levels, glucose levels, altered brain insulin signaling, or other factors such as adipose tissue-associated hormones are indicated.

Energy metabolism and cytochrome oxidase activity: linking metabolism to gene expression

Modification of mitochondrial content demands the synthesis of hundreds of proteins encoded by nuclear and mitochondrial genomes. The responsibility for coordination of this process falls to nuclear-encoded master regulators of transcription. DNA-binding proteins and coactivators integrate information from energy-sensing pathways and hormones to alter mitochondrial gene expression. In mammals, the signaling cascade for mitochondrial biogenesis can be described as follows: hormonal signals and energetic information are sensed by protein-modifying enzymes that in turn regulate the post-translational modification of transcription factors. Once activated, transcription-factor complexes form on promoter elements of many of the nuclear-encoded mitochondrial genes, recruiting proteins that remodel chromatin and initiate transcription. One master regulator in mammals, PGC-1α, is well studied because of its role in determining the metabolic phenotype of muscles, but also due to its importance in mitochondria-related metabolic diseases. However, relatively little is known about the role of this pathway in other vertebrates. These uncertainties raise broader questions about the evolutionary origins of the pathway and its role in generating the diversity in muscle metabolic phenotypes seen in nature.

Genetic models of PGC-1 and glucose metabolism and homeostasis

Type II diabetes and its complications are a tremendous health burden throughout the world. Our understanding of the changes that lead to glucose imbalance and insulin resistance and ultimately diabetes remain incompletely understood. Many signaling and transcriptional pathways have been identified as being important to maintain normal glucose balance, including that of the peroxisome proliferator activated receptor gamma coactivator (PGC-1) family. This family of transcriptional coactivators strongly regulates mitochondrial and metabolic biology in numerous organs. The use of genetic models of PGC-1s, including both tissue-specific overexpression and knock-out models, has helped to reveal the specific roles that these coactivators play in each tissue. This review will thus focus on the PGC-1s and recently developed genetic rodent models that have highlighted the importance of these molecules in maintaining normal glucose homeostasis.

Exercise training decreases mitogen-activated protein kinase phosphatase-3 expression and suppresses hepatic gluconeogenesis in obese mice

Insulin plays an important role in the control of hepatic glucose production. Insulin resistant states are commonly associated with excessive hepatic glucose production, which contributes to both fasting hyperglycaemia and exaggerated postprandial hyperglycaemia. In this regard, increased activity of phosphatases may contribute to the dysregulation of gluconeogenesis. Mitogen-activated protein kinase phosphatase-3 (MKP-3) is a key protein involved in the control of gluconeogenesis. MKP-3-mediated dephosphorylation activates FoxO1 (a member of the forkhead family of transcription factors) and subsequently promotes its nuclear translocation and binding to the promoters of gluconeogenic genes such as phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). In this study, we investigated the effects of exercise training on the expression of MKP-3 and its interaction with FoxO1 in the livers of obese animals. We found that exercised obese mice had a lower expression of MKP-3 and FoxO1/MKP-3 association in the liver. Further, the exercise training decreased FoxO1 phosphorylation and protein levels of Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and gluconeogenic enzymes (PEPCK and G6Pase). These molecular results were accompanied by physiological changes, including increased insulin sensitivity and reduced hyperglycaemia, which were not caused by reductions in total body mass. Similar results were also observed with oligonucleotide antisense (ASO) treatment. However, our results showed that only exercise training could reduce an obesity-induced increase in HNF-4α protein levels while ASO treatment alone had no effect. These findings could explain, at least in part, why additive effects of exercise training treatment and ASO treatment were not observed. Finally, the suppressive effects of exercise training on MKP-3 protein levels appear to be related, at least in part, to the reduced phosphorylation of Extracellular signal-regulated kinases (ERK) in the livers of obese mice.

Genomic and non-genomic regulation of PGC1 isoforms by estrogen to increase cerebral vascular mitochondrial biogenesis and reactive oxygen species protection

We previously found that estrogen exerts a novel protective effect on mitochondria in brain vasculature. Here we demonstrate in rat cerebral blood vessels that 17β-estradiol (estrogen), both in vivo and ex vivo, affects key transcriptional coactivators responsible for mitochondrial regulation. Treatment of ovariectomized rats with estrogen in vivo lowered mRNA levels of peroxisome proliferator-activated receptor-γ coactivator-1 alpha (PGC-1α) but increased levels of the other PGC-1 isoforms: PGC-1β and PGC-1 related coactivator (PRC). In vessels ex vivo, estrogen decreased protein levels of PGC-1α via activation of phosphatidylinositol 3-kinase (PI3K). Estrogen treatment also increased phosphorylation of forkhead transcription factor, FoxO1, a known pathway for PGC-1α downregulation. In contrast to the decrease in PGC-1α, estrogen increased protein levels of nuclear respiratory factor 1, a known PGC target and mediator of mitochondrial biogenesis. The latter effect of estrogen was independent of PI3K, suggesting a separate mechanism consistent with increased expression of PGC-1β and PRC. We demonstrated increased mitochondrial biogenesis following estrogen treatment in vivo; cerebrovascular levels of mitochondrial transcription factor A and electron transport chain subunits as well as the mitochondrial/nuclear DNA ratio were increased. We examined a downstream target of PGC-1β, glutamate-cysteine ligase (GCL), the rate-limiting enzyme for glutathione synthesis. In vivo estrogen increased protein levels of both GCL subunits and total glutathione levels. Together these data show estrogen differentially regulates PGC-1 isoforms in brain vasculature, underscoring the importance of these coactivators in adapting mitochondria in specific tissues. By upregulating PGC-1β and/or PRC, estrogen appears to enhance mitochondrial biogenesis, function and reactive oxygen species protection.

The longevity effect of cranberry extract in Caenorhabditis elegans is modulated by daf-16 and osr-1

Nutraceuticals are known to have numerous health and disease preventing properties. Recent studies suggest that extracts containing cranberry may have anti-aging benefits. However, little is known about whether and how cranberry by itself promotes longevity and healthspan in any organism. Here we examined the effect of a cranberry only extract on lifespan and healthspan in Caenorhabditis elegans. Supplementation of the diet with cranberry extract (CBE) increased the lifespan in C. elegans in a concentration-dependent manner. Cranberry also increased tolerance of C. elegans to heat shock, but not to oxidative stress or ultraviolet irradiation. In addition, we tested the effect of cranberry on brood size and motility and found that cranberry did not influence these behaviors. Our mechanistic studies indicated that lifespan extension induced by CBE requires the insulin/IGF signaling pathway and DAF-16. We also found that cranberry promotes longevity through osmotic stress resistant-1 (OSR-1) and one of its downstream effectors, UNC-43, but not through SEK-1, a component of the p38 MAP kinase pathway. However, SIR-2.1 and JNK signaling pathways are not required for cranberry to promote longevity. Our findings suggest that cranberry supplementation confers increased longevity and stress resistance in C. elegans through pathways modulated by daf-16 and osr-1. This study reveals the anti-aging property of widely consumed cranberry and elucidates the underpinning mechanisms.

Three-dimensional culture and cAMP signaling promote the maturation of human pluripotent stem cell-derived hepatocytes

Human pluripotent stem cells (hPSCs) represent a novel source of hepatocytes for drug metabolism studies and cell-based therapy for the treatment of liver diseases. These applications are, however, dependent on the ability to generate mature metabolically functional cells from the hPSCs. Reproducible and efficient generation of such cells has been challenging to date, owing to the fact that the regulatory pathways that control hepatocyte maturation are poorly understood. Here, we show that the combination of three-dimensional cell aggregation and cAMP signaling enhance the maturation of hPSC-derived hepatoblasts to a hepatocyte-like population that displays expression profiles and metabolic enzyme levels comparable to those of primary human hepatocytes. Importantly, we also demonstrate that generation of the hepatoblast population capable of responding to cAMP is dependent on appropriate activin/nodal signaling in the definitive endoderm at early stages of differentiation. Together, these findings provide new insights into the pathways that regulate maturation of hPSC-derived hepatocytes and in doing so provide a simple and reproducible approach for generating metabolically functional cell populations.

Temporal clustering of gene expression links the metabolic transcription factor HNF4α to the ER stress-dependent gene regulatory network

The unfolded protein response (UPR) responds to disruption of endoplasmic reticulum (ER) function by initiating signaling cascades that ultimately culminate in extensive transcriptional regulation. Classically, this regulation includes genes encoding ER chaperones, ER-associated degradation factors, and others involved in secretory protein folding and processing, and is carried out by the transcriptional activators that are produced as a consequence of UPR activation. However, up to half of the mRNAs regulated by ER stress are downregulated rather than upregulated, and the mechanisms linking ER stress and UPR activation to mRNA suppression are poorly understood. To begin to address this issue, we used a "bottom-up" approach to study the metabolic gene regulatory network controlled by the UPR in the liver, because ER stress in the liver leads to lipid accumulation, and fatty liver disease is the most common liver disease in the western world. qRT-PCR profiling of mouse liver mRNAs during ER stress revealed that suppression of the transcriptional regulators C/EBPα, PPARα, and PGC-1α preceded lipid accumulation, and was then followed by suppression of mRNAs encoding key enzymes involved in fatty acid oxidation and lipoprotein biogenesis and transport. Mice lacking the ER stress sensor ATF6α, which experience persistent ER stress and profound lipid accumulation during challenge, were then used as the basis for a functional genomics approach that allowed genes to be grouped into distinct expression profiles. This clustering predicted that ER stress would suppress the activity of the metabolic transcriptional regulator HNF4α-a finding subsequently confirmed by chromatin immunopreciptation at the Cebpa and Pgc1a promoters. Our results establish a framework for hepatic gene regulation during ER stress and suggest that HNF4α occupies the apex of that framework. They also provide a unique resource for the community to further explore the temporal regulation of gene expression during ER stress in vivo.

Lactate administration reproduces specific brain and liver exercise-related changes

The effects of exercise are not limited to muscle, and its ability to mitigate some chronic diseases is under study. A more complete understanding of how exercise impacts non-muscle tissues might facilitate design of clinical trials and exercise mimetics. Here, we focused on lactate's ability to mediate changes in liver and brain bioenergetic-associated parameters. In one group of experiments, C57BL/6 mice underwent 7 weeks of treadmill exercise sessions at intensities intended to exceed the lactate threshold. Over time, the mice dramatically increased their lactate threshold. To ensure that plasma lactate accumulated during the final week, the mice were run to exhaustion. In the liver, mRNA levels of gluconeogenesis-promoting genes increased. While peroxisome proliferator-activated receptor-gamma co-activator 1 alpha (PGC-1α) expression increased, there was a decrease in PGC-1β expression, and overall gene expression changes favored respiratory chain down-regulation. In the brain, PGC-1α and PGC-1β were unchanged, but PGC-1-related co-activator expression and mitochondrial DNA copy number increased. Brain tumor necrosis factor alpha expression fell, whereas vascular endothelial growth factor A expression rose. In another group of experiments, exogenously administered lactate was found to reproduce some but not all of these observed liver and brain changes. Our data suggest that lactate, an exercise byproduct, could mediate some of the effects exercise has on the liver and the brain, and that lactate itself can act as a partial exercise mimetic.

Apoptosis induced by PGC-1β in breast cancer cells is mediated by the mTOR pathway

The peroxisome proliferator-activated receptor-γ (PPAR-γ) coactivator-1β (PGC-1β) is a well-established regulator of mitochondrial biogenesis. However, the underlying mechanism of PGC-1β action remains elusive. This study reveals that knockdown of endogenous PGC-1β by short-hairpin RNA (shRNA) leads to a decrease in the expression of mammalian target of rapamycin (mTOR) pathway-related genes in MDA-MB-231 cells. After knockdown of PGC-1β, phosphorylation of AMP-activated protein kinase (AMPK), phosphorylation of Rictor on Thr1135, Raptor and S6 protein was inhibited. However, Akt phosphorylation on Ser473 was upregulated and cell apoptosis occurred. In particular, we demonstrate that the levels of PGC-1β and mTOR correlated with overall mitochondrial activity. These results provide new evidence that cell apoptosis is orchestrated by the balance between several signaling pathways, and that PGC-1β takes part in these events in breast cancer cells mediated by the mTOR signaling pathway.

The optic nerve: a "mito-window" on mitochondrial neurodegeneration

Retinal ganglion cells (RGCs) project their long axons, composing the optic nerve, to the brain, transmitting the visual information gathered by the retina, ultimately leading to formed vision in the visual cortex. The RGC cellular system, representing the anterior part of the visual pathway, is vulnerable to mitochondrial dysfunction and optic atrophy is a very frequent feature of mitochondrial and neurodegenerative diseases. The start of the molecular era of mitochondrial medicine, the year 1988, was marked by the identification of a maternally inherited form of optic atrophy, Leber's hereditary optic neuropathy, as the first disease due to mitochondrial DNA point mutations. The field of mitochondrial medicine has expanded enormously over the last two decades and many neurodegenerative diseases are now known to have a primary mitochondrial etiology or mitochondrial dysfunction plays a relevant role in their pathogenic mechanism. Recent technical advancements in neuro-ophthalmology, such as optical coherence tomography, prompted a still ongoing systematic re-investigation of retinal and optic nerve involvement in neurodegenerative disorders. In addition to inherited optic neuropathies, such as Leber's hereditary optic neuropathy and dominant optic atrophy, and in addition to the syndromic mitochondrial encephalomyopathies or mitochondrial neurodegenerative disorders such as some spinocerebellar ataxias or familial spastic paraparesis and other disorders, we draw attention to the involvement of the optic nerve in classic age-related neurodegenerative disorders such as Parkinson and Alzheimer disease. We here provide an overview of optic nerve pathology in these different clinical settings, and we review the possible mechanisms involved in the pathogenesis of optic atrophy. This may be a model of general value for the field of neurodegeneration. This article is part of a Special Issue entitled 'Mitochondrial function and dysfunction in neurodegeneration'.

Mice lacking PGC-1β in adipose tissues reveal a dissociation between mitochondrial dysfunction and insulin resistance

Proper development and function of white adipose tissue (WAT), which are regulated by multiple transcription factors and coregulators, are crucial for glucose homeostasis. WAT is also the main target of thiazolidinediones, which are thought to exert their insulin-sensitizing effects by promoting mitochondrial biogenesis in adipocytes. Besides being expressed in WAT, the role of the coactivator PGC-1β in this tissue has not been addressed. To study its function in WAT, we have generated mice that lack PGC-1β in adipose tissues. Gene expression profiling analysis of WAT reveals that PGC-1β regulates mitochondrial genes involved in oxidative metabolism. Furthermore, lack of PGC-1β prevents the induction of mitochondrial genes by rosiglitazone in WAT without affecting the capacity of thiazolidinediones to enhance insulin sensitivity. Our findings indicate that PGC-1β is important for basal and rosiglitazone-induced mitochondrial function in WAT, and that induction of mitochondrial oxidative capacity is not essential for the insulin-sensitizing effects of thiazolidinediones.

Oxidized eicosapentaenoic acids more potently reduce LXRα-induced cellular triacylglycerol via suppression of SREBP-1c, PGC-1β and GPA than its intact form

Dietary polyunsaturated fatty acids (PUFA), especially eicosapentaenoic acid (EPA), improve lipid metabolism and contribute to the prevention of vascular diseases such as atherosclerosis. However, EPA in the diet is easily oxidized at room temperature and several types of oxidized EPA (OEPA) derivatives are generated. To compare the efficiencies of OEPAs on lipid metabolism with EPA, human hepatocellular liver carcinoma cell line (HepG2) was treated with EPA or OEPAs and their effects on lipid metabolism related genes were studied. OEPAs more potently suppressed the expression of sterol-responsive element-binding protein (SREBP)-1c, a major transcription factor that activates the expression of lipogenic genes, and its downstream target genes than did EPA under conditions of lipid synthesis enhanced by T0901317, a synthetic liver X receptor (LXR) agonist. Furthermore, PGC-1β, a coactivator of both LXRα and SREBP-1, was markedly down-regulated by OEPAs compared with EPA. The treatment of OEPAs also significantly down-regulated the expression of glycerol-3-phosphate acyltransferase (GPA), the initiating enzyme in triacylglycerol (TG) synthesis, more than EPA. Therefore, the advantageous effects of OEPAs on cardiovascular diseases might be due to their SREBP-1c, PGC-1β and GPA mediated ameliorating effects.

PGC-1β and ChREBP partner to cooperatively regulate hepatic lipogenesis in a glucose concentration-dependent manner

Peroxisome proliferator-activated receptorγ coactivators (PGC-1α and PGC-1β) play important roles in the transcriptional regulation of intermediary metabolism. To evaluate the effects of overexpressing PGC-1α or PGC-1β at physiologic levels in liver, we generated transgenic mice with inducible overexpression of PGC-1α or PGC-1β. Gene expression array profiling revealed that whereas both PGC-1 family proteins induced mitochondrial oxidative enzymes, the expression of several genes involved in converting glucose to fatty acid was induced by PGC-1β, but not PGC-1α. The increased expression of enzymes involved in carbohydrate utilization and de novo lipogenesis by PGC-1β required carbohydrate response element binding protein (ChREBP). The interaction between PGC-1β and ChREBP, as well as PGC-1β occupancy of the liver-type pyruvate kinase promoter, was influenced by glucose concentration and liver-specific PGC-1β(-/-) hepatocytes were refractory to the lipogenic response to high glucose conditions. These data suggest that PGC-1β-mediated coactivation of ChREBP is involved in the lipogenic response to hyperglycemia.

Hepatic-specific activation of peroxisome proliferator-activated receptor γ coactivator-1β protects against steatohepatitis

Development of hepatic steatosis and its progression to steatohepatitis may be the consequence of dysfunction of several metabolic pathways, such as triglyceride synthesis, very low-density lipoprotein (VLDL) secretion, and fatty acid β-oxidation. Peroxisome proliferator-activated receptor γ coactivator-1β (PGC-1β) is a master regulator of mitochondrial biogenesis and oxidative metabolism, lipogenesis, and triglyceride (TG) secretion. Here we generated a novel mouse model with constitutive hepatic activation of PGC-1β and studied the role of this transcriptional coactivator in dietary-induced steatosis and steatohepatitis. Selective activation of PGC-1β within hepatocytes is able to protect the liver from lipid overload and from progression to fibrosis. The protective function exerted by PGC-1β is due to its ability to induce mitochondrial oxidative phosphorylation, fatty acid β-oxidation, and citrate cycle, as well as to decrease oxidative stress and promote TG secretion in the blood stream. These findings bolster the concept that a combined hepatic specific action of PGC-1β on lipid synthesis and secretion, as well as on mitochondrial biogenesis and function, could protect against steatohepatitis.

Metabolism and the circadian clock converge

Circadian rhythms occur in almost all species and control vital aspects of our physiology, from sleeping and waking to neurotransmitter secretion and cellular metabolism. Epidemiological studies from recent decades have supported a unique role for circadian rhythm in metabolism. As evidenced by individuals working night or rotating shifts, but also by rodent models of circadian arrhythmia, disruption of the circadian cycle is strongly associated with metabolic imbalance. Some genetically engineered mouse models of circadian rhythmicity are obese and show hallmark signs of the metabolic syndrome. Whether these phenotypes are due to the loss of distinct circadian clock genes within a specific tissue versus the disruption of rhythmic physiological activities (such as eating and sleeping) remains a cynosure within the fields of chronobiology and metabolism. Becoming more apparent is that from metabolites to transcription factors, the circadian clock interfaces with metabolism in numerous ways that are essential for maintaining metabolic homeostasis.

Peroxisome proliferator-activated receptor γ coactivator 1β (PGC-1β) protein attenuates vascular lesion formation by inhibition of chromatin loading of minichromosome maintenance complex in smooth muscle cells

Proliferation of vascular smooth muscle cells (VSMCs) in response to vascular injury plays a critical role in vascular lesion formation. Emerging data suggest that peroxisome proliferator-activated receptor γ coactivator 1 (PGC-1) is a key regulator of energy metabolism and other biological processes. However, the physiological role of PGC-1β in VSMCs remains unknown. A decrease in PGC-1β expression was observed in balloon-injured rat carotid arteries. PGC-1β overexpression substantially inhibited neointima formation in vivo and markedly inhibited VSMC proliferation and induced cell cycle arrest at the G(1)/S transition phase in vitro. Accordingly, overexpression of PGC-1β decreased the expression of minichromosome maintenance 4 (MCM4), which leads to a decreased loading of the MCM complex onto chromatin at the replication origins and decreased cyclin D1 levels, whereas PGC-1β loss of function by adenovirus containing PGC-1β shRNA resulted in the opposite effect. The transcription factor AP-1 was involved in the down-regulation of MCM4 expression. Furthermore, PGC-1β is up-regulated by metformin, and metformin-associated anti-proliferative activity in VSMCs is at least partially dependent on PGC-1β. Our data show that PGC-1β is a critical component in regulating DNA replication, VSMC proliferation, and vascular lesion formation, suggesting that PGC-1β may emerge as a novel therapeutic target for control of proliferative vascular diseases.

Complex interplay between the lipin 1 and the hepatocyte nuclear factor 4 α (HNF4α) pathways to regulate liver lipid metabolism

Lipin 1 is a bifunctional protein that serves as a metabolic enzyme in the triglyceride synthesis pathway and regulates gene expression through direct protein-protein interactions with DNA-bound transcription factors in liver. Herein, we demonstrate that lipin 1 is a target gene of the hepatocyte nuclear factor 4α (HNF4α), which induces lipin 1 gene expression in cooperation with peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) through a nuclear receptor response element in the first intron of the lipin 1 gene. The results of a series of gain-of-function and loss-of-function studies demonstrate that lipin 1 coactivates HNF4α to activate the expression of a variety of genes encoding enzymes involved in fatty acid catabolism. In contrast, lipin 1 reduces the ability of HNF4α to induce the expression of genes encoding apoproteins A4 and C3. Although the ability of lipin to diminish HNF4α activity on these promoters required a direct physical interaction between the two proteins, lipin 1 did not occupy the promoters of the repressed genes and enhances the intrinsic activity of HNF4α in a promoter-independent context. Thus, the induction of lipin 1 by HNF4α may serve as a mechanism to affect promoter selection to direct HNF4α to promoters of genes encoding fatty acid oxidation enzymes.

Nuclear respiratory factor 1 mediates the transcription initiation of insulin-degrading enzyme in a TATA box-binding protein-independent manner

CpG island promoters often lack canonical core promoter elements such as the TATA box, and have dispersed transcription initiation sites. Despite the prevalence of CpG islands associated with mammalian genes, the mechanism of transcription initiation from CpG island promoters remains to be clarified. Here we investigate the mechanism of transcription initiation of the CpG island-associated gene, insulin-degrading enzyme (IDE). IDE is ubiquitously expressed, and has dispersed transcription initiation sites. The IDE core promoter locates within a 32-bp region, which contains three CGGCG repeats and a nuclear respiratory factor 1 (NRF-1) binding motif. Sequential mutation analysis indicates that the NRF-1 binding motif is critical for IDE transcription initiation. The NRF-1 binding motif is functional, because NRF-1 binds to this motif in vivo and this motif is required for the regulation of IDE promoter activity by NRF-1. Furthermore, the NRF-1 binding site in the IDE promoter is conserved among different species, and dominant negative NRF-1 represses endogenous IDE expression. Finally, TATA-box binding protein (TBP) is not associated with the IDE promoter, and inactivation of TBP does not abolish IDE transcription, suggesting that TBP is not essential for IDE transcription initiation. Our studies indicate that NRF-1 mediates IDE transcription initiation in a TBP-independent manner, and provide insights into the potential mechanism of transcription initiation for other CpG island-associated genes.

Crosstalk between mitochondrial (dys)function and mitochondrial abundance

A controlled regulation of mitochondrial mass through either the production (biogenesis) or the degradation (mitochondrial quality control) of the organelle represents a crucial step for proper mitochondrial and cell function. Key steps of mitochondrial biogenesis and quality control are overviewed, with an emphasis on the role of mitochondrial chaperones and proteases that keep mitochondria fully functional, provided the mitochondrial activity impairment is not excessive. In this case, the whole organelle is degraded by mitochondrial autophagy or "mitophagy." Beside the maintenance of adequate mitochondrial abundance and functions for cell homeostasis, mitochondrial biogenesis might be enhanced, through discussed signaling pathways, in response to various physiological stimuli, like contractile activity, exposure to low temperatures, caloric restriction, and stem cells differentiation. In addition, mitochondrial dysfunction might also initiate a retrograde response, enabling cell adaptation through increased mitochondrial biogenesis.

Peri-implantation lethality in mice lacking the PGC-1-related coactivator protein

BACKGROUND

Members of the PPARγ coactivator-1 (PGC-1) family are central transcriptional coactivators that regulate cell metabolic processes ranging from mitochondrial biogenesis to oxidative respiration. PGC-1-related coactivator (PPRC1 or PRC), initially identified as a member of the PGC-1 family, is believed to regulate mitochondria biogenesis, respiration pathways, and cell proliferation. However, its physiological role is not clearly understood. Here, we investigate the biological functions of PPRC1 in vivo using PPRC1 deficient mice generated by gene targeting.

RESULTS

Homozygous deficient PPRC1 mice failed to form egg cylinders and died after implantation but before embryonic day 6.5, whereas mice heterozygous for PPRC1 were viable, fertile and indistinguishable from their wild-type littermates. Furthermore, PPRC1 mRNA was expressed at the embryonic stage before implantation and was rapidly up-regulated during the first day of embryoid body formation. The PPRC1 mRNA was then subsequently down-regulated, although its precise function at this stage of development was unclear.

CONCLUSIONS

This is the first study to suggest a nonredundant role for PPRC1 in mouse early embryonic development.

Lipoic acid administration prevents nonalcoholic steatosis linked to long-term high-fat feeding by modulating mitochondrial function

Nonalcoholic steatosis is an important hepatic complication of obesity linked to mitochondrial dysfunction and insulin resistance. Furthermore, lipoic acid has been reported to have beneficial effects on mitochondrial function. In this study, we analyzed the potential protective effect of lipoic acid supplementation against the development of nonalcoholic steatosis associated with a long-term high-fat diet feeding and the potential mechanism of this effect. Wistar rats were fed on a standard diet (n=10), a high-fat diet (n=10) and a high-fat diet supplemented with lipoic acid (n=10). A group pair-fed to the latter group (n=6) was also included. Lipoic acid prevented hepatic triglyceride accumulation and liver damage in rats fed a high-fat diet (-68%±11.3% vs. obese group) through the modulation of genes involved in lipogenesis and mitochondrial β-oxidation and by improving insulin sensitivity. Moreover, this molecule showed an inhibitory action on electron transport chain complexes activities (P<.01-P<.001) and adenosine triphosphate synthesis (P<.05), and reduced significantly energy efficiency. By contrast, lipoic acid induced an increase in mitochondrial copy number and in Ucp2 gene expression (P<.001 vs. obese). In summary, this investigation demonstrated the ability of lipoic acid to prevent nonalcoholic steatosis induced by a high-fat intake. Finally, the novelty and importance of this study are the finding of how lipoic acid modulates some of the mitochondrial processes involved in energy homeostasis. The reduction in mitochondrial energy efficiency could also explain, at least in part, the beneficial effects of lipoic acid not only in fatty liver but also in preventing excessive body weight gain.

Clenbuterol, a β2-adrenergic agonist, reciprocally alters PGC-1 alpha and RIP140 and reduces fatty acid and pyruvate oxidation in rat skeletal muscle

Clenbuterol, a β2-adrenergic agonist, reduces mitochondrial content and enzyme activities in skeletal muscle, but the mechanism involved has yet to be identified. We examined whether clenbuterol-induced changes in the muscles' metabolic profile and the intrinsic capacity of mitochondria to oxidize substrates are associated with reductions in the nuclear receptor coactivator PGC-1 alpha and/or an increase in the nuclear corepressor RIP140. In rats, clenbuterol was provided in the drinking water (30 mg/l). In 3 wk, this increased body (8%) and muscle weights (12–17%). In red (R) and white (W) muscles, clenbuterol induced reductions in mitochondrial content (citrate synthase: R, 27%; W, 52%; cytochrome- c oxidase: R, 24%; W, 34%), proteins involved in fatty acid transport (fatty acid translocase/CD36: R, 36%; W, 35%) and oxidation [β-hydroxyacyl CoA dehydrogenase (β-HAD): R, 33%; W, 62%], glucose transport (GLUT4: R, 8%; W, 13%), lactate transport monocarboxylate transporter (MCT1: R, 61%; W, 37%), and pyruvate oxidation (PDHE1α, R, 18%; W, 12%). Concurrently, only red muscle lactate dehydrogenase activity (25%) and MCT4 (31%) were increased. Palmitate oxidation was reduced in subsarcolemmal (SS) (R, 30%; W, 52%) and intermyofibrillar (IMF) mitochondria (R, 17%; W, 44%) along with reductions in β-HAD activity (SS: R, 17%; W, 51%; IMF: R, 20%; W, 57%). Pyruvate oxidation was only reduced in SS mitochondria (R, 20%; W, 28%), but this was not attributable solely to PDHE1α, which was reduced in both SS (R, 21%; W, 20%) and IMF mitochondria (R, 15%; W, 43%). These extensive metabolic changes induced by clenbuterol were associated with reductions in PGC-1α (R, 37%; W, 32%) and increases in RIP140 (R, 23%; W, 21%). This is the first evidence that clenbuterol appears to exert its metabolic effects via simultaneous and reciprocal changes in the nuclear receptor coactivator PGC-1α and the nuclear corepressor RIP140.

Prophylactic treatment with telmisartan induces tissue-specific gene modulation favoring normal glucose homeostasis in Cohen-Rosenthal diabetic hypertensive rats

The objectives were to assess the potential of long-term prophylactic administration of telmisartan, an angiotensin II receptor antagonist and a partial peroxisome proliferator activator receptor (PPAR)γ agonist, in preventing the development of hypertension and hyperglycemia and to demonstrate the alteration in gene expression associated with the development of hyperglycemia and insulin resistance in Cohen-Rosenthal diabetic hypertensive rat, a unique model of hypertension and type 2 diabetes mellitus comorbidity. Cohen-Rosenthal diabetic hypertensive rats were continuously treated with telmisartan (3 mg/[kg d]) starting at age 6 to 8 weeks before developing hypertension or diabetes. Weight changes, blood pressure, blood insulin, adiponectin, glucose tolerance, and insulin sensitivity were monitored. Fat, liver, and muscle messenger RNAs were examined for the expression of genes potentially involved in the onset of insulin resistance. In addition to the expected antihypertensive effect of prophylactic telmisartan, diabetes was blunted, evidenced at the end of the study by a significantly lower glucose level. This was accompanied by improved glucose tolerance, increased sensitivity to insulin, reduction in fasting insulin levels and homeostasis model assessment index, as well as an increase in serum adiponectin. Telmisartan also prevented the increase in serum triglycerides and the associated appearance of lipid droplets in the liver. Diabetes induced tissue-specific changes in messenger RNAs expression of the following selected genes, which were restored by telmisartan treatment: PPARγ, PPARδ, PPARγ coactivator 1α, adiponectin, adiponectin receptor 1, adiponectin receptor 2, phosphotyrosine binding domain and a pleckstrin homology domain-containing adaptor protein, adenosine monophosphate kinase, and glucose translocator 4. Telmisartan blunted the development of hypertension, insulin resistance, and diabetes in prediabetic Cohen-Rosenthal diabetic hypertensive rats through pleiotropic activity, involving specific gene regulation of target organs.

The diverse role of the PPARγ coactivator 1 family of transcriptional coactivators in cancer

The critical role that altered cellular metabolism plays in promoting and maintaining the cancer phenotype has received considerable attention in recent years. For many years it was believed that aerobic glycolysis, also known as the Warburg Effect, played an important role in cancer. However, recent studies highlight the requirement of mitochondrial function, oxidative phosphorylation and biosynthetic pathways in cancer. This has promoted interest into mechanisms controlling these metabolic pathways. The PPARγ coactivator (PGC)-1 family of transcriptional coactivators have emerged as key regulators of several metabolic pathways including oxidative metabolism, energy homeostasis and glucose and lipid metabolism. While PGC-1s have been implicated in a number of metabolic diseases, recent studies highlight an important role in cancer. Studies show that PGC-1s have both pro and anticancer functions and suggests a dynamic role for the PGC-1s in cancer. We discuss in this review the links between PGC-1s and cancer, with a focus on the most well studied family member, PGC-1α.

Lipopolysaccharide-induced mitochondrial DNA depletion

Hepatic energy depletion has been described in severe sepsis, and lipopolysaccharide (LPS) has been shown to cause mitochondrial DNA (mtDNA) damage. To clarify the mechanisms of LPS-induced mtDNA damage and mitochondrial alterations, we treated wild-type (WT) or transgenic manganese superoxide dismutase-overerexpressing (MnSOD(+++)) mice with a single dose of LPS (5 mg/kg). In WT mice, LPS increased mitochondrial reactive oxygen species formation, hepatic inducible nitric oxide synthase (NOS) mRNA and protein, tumor necrosis factor-alpha, interleukin-1 beta, and high-mobility group protein B1 concentrations. Six to 48 h after LPS administration (5 mg/kg), liver mtDNA levels, respiratory complex I activity, and adenosine triphosphate (ATP) contents were decreased. In addition, LPS increased interferon-β concentration and decreased mitochondrial transcription factor A (Tfam) mRNA, Tfam protein, and mtDNA-encoded mRNAs. Morphological studies showed mild hepatic inflammation. The LPS (5 mg/kg)-induced mtDNA depletion, complex I inactivation, ATP depletion, and alanine aminotransferase increase were prevented in MnSOD(+++) mice or in WT mice cotreated with 1400W (a NOS inhibitor), (2-(2,2,6,6-tetramethylpiperidin-1-oxyl-4-ylamino)-2-oxoethyl)triphenylphosphonium chloride, monohydrate (a superoxide scavenger) or uric acid (a peroxynitrite scavenger). The MnSOD overexpression delayed death in mice challenged by a higher, lethal dose of LPS (25 mg/kg). In conclusion, LPS administration damages mtDNA and alters mitochondrial function. The protective effects of MnSOD, NOS inhibitors, and superoxide or peroxynitrite scavengers point out a role of the superoxide anion reacting with NO to form mtDNA- and protein-damaging peroxynitrite. In addition to the acute damage caused by reactive species, decreased levels of mitochondrial transcripts contribute to mitochondrial dysfunction.

Nucleus-encoded regulators of mitochondrial function: integration of respiratory chain expression, nutrient sensing and metabolic stress

Nucleus-encoded regulatory factors are major contributors to mitochondrial biogenesis and function. Several act within the organelle to regulate mitochondrial transcription and translation while others direct the expression of nuclear genes encoding the respiratory chain and other oxidative functions. Loss-of-function studies for many of these factors reveal a wide spectrum of phenotypes. These range from embryonic lethality and severe respiratory chain deficiency to relatively mild mitochondrial defects seen only under conditions of physiological stress. The PGC-1 family of regulated coactivators (PGC-1α, PGC-1β and PRC) plays an important integrative role through their interactions with transcription factors (NRF-1, NRF-2, ERRα, CREB, YY1 and others) that control respiratory gene expression. In addition, recent evidence suggests that PGC-1 coactivators may balance the cellular response to oxidant stress by promoting a pro-oxidant environment or by orchestrating an inflammatory response to severe metabolic stress. These pathways may serve as essential links between the energy generating functions of mitochondria and the cellular REDOX environment associated with longevity, senescence and disease. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.

Effects of nicotinic acid on gene expression: potential mechanisms and implications for wanted and unwanted effects of the lipid-lowering drug

CONTEXT

Nicotinic acid (NA), or niacin, lowers circulating levels of lipids, including triglycerides, very low-density lipoprotein-cholesterol, and low-density lipoprotein-cholesterol. The lipid-lowering effects have been attributed to its effect to inhibit lipolysis in adipocytes and thus lower plasma free fatty acid (FFA) level. However, evidence accumulates that the FFA-lowering effect may account for only a fraction of NA effects on plasma lipids, and other mechanisms may be involved. Recent studies have reported NA effects on gene expression in various tissues in vivo and in cultured cells in vitro.

EVIDENCE ACQUISITION

We reviewed articles reporting NA effects on gene expression, identified by searching PubMed, focusing on potential underlying mechanisms and implications for unexplained NA effects.

CONCLUSION

The effects of NA on gene expression may be mediated directly via the NA receptor in the affected cells, indirectly via changes in circulating FFA or hormone levels induced by NA, or by activating the transcription factor FOXO1 in insulin-sensitive tissues. NA effects on gene expression provide new insights into previously unexplained NA effects, such as FFA-independent lipid-lowering effects, FFA rebound, and insulin resistance observed in clinics during NA treatment.

PGC-1-related coactivator (PRC), a sensor of metabolic stress, orchestrates a redox-sensitive program of inflammatory gene expression

PGC-1-related coactivator (PRC) is a growth-regulated transcriptional cofactor that activates many nuclear genes specifying mitochondrial respiratory function. Stable PRC silencing in U2OS cells results in a complex phenotype typical of mitochondrial dysfunction including abundant abnormal mitochondria, reduced respiratory subunit expression, diminished respiratory enzymes and ATP levels, and elevated lactate production. The PRC response to metabolic stress was investigated by subjecting cells to metabolic insults including treatment with the uncoupler carbonyl cyanide 3-chlorophenylhydrazone (CCCP), expression of a dominant negative allele of nuclear respiratory factor 1 (NRF-1), and glucose deprivation. These treatments led to constitutively elevated PRC protein levels, a departure from its normal transient expression upon the initiation of cell growth. A microarray screen identified 45 genes that require PRC for their induction by CCCP. A subset of these genes specific to inflammation and cell stress was also induced by dominant negative NRF-1 and by glucose deprivation, suggesting that diverse metabolic insults converge on the same PRC-dependent inflammatory program. The PRC-dependent inflammatory response was inhibited by N-acetylcysteine, suggesting that PRC may contribute to the inflammatory microenvironment linked to oxidant signaling. The induction of this PRC-dependent program may be an early event in adaptations linked to cancer and degenerative diseases.

Exposure to atrazine affects the expression of key genes in metabolic pathways integral to energy homeostasis in Xenopus laevis tadpoles

In our laboratory, Xenopus laevis tadpoles exposed throughout development to 200 or 400 μg/L atrazine, concentrations reported to periodically occur in puddles, vernal ponds and runoff soon after application, were smaller and had smaller fat bodies (the tadpole's lipid storage organ) than controls. It was hypothesized that these changes were due to atrazine-related perturbations of energy homeostasis. To investigate this hypothesis, selected metabolic responses to exposure at the transcriptional and biochemical levels in atrazine-exposed tadpoles were measured. DNA microarray technology was used to determine which metabolic pathways were affected after developmental exposure to 400 μg/L atrazine. From these data, genes representative of the affected pathways were selected for assay using quantitative real time polymerase chain reaction (qRT-PCR) to measure changes in expression during a 2-week exposure to 400 μg/L. Finally, ATP levels were measured from tadpoles both early in and at termination of exposure to 200 and 400 μg/L. Microarray analysis revealed significant differential gene expression in metabolic pathways involved with energy homeostasis. Pathways with increased transcription were associated with the conversion of lipids and proteins into energy. Pathways with decreased transcription were associated with carbohydrate metabolism, fat storage, and protein synthesis. Using qRT-PCR, changes in gene expression indicative of an early stress response to atrazine were noted. Exposed tadpoles had significant decreases in acyl-CoA dehydrogenase (AD) and glucocorticoid receptor protein (GR) mRNA after 24 h of exposure, and near-significant (p=0.07) increases in peroxisome proliferator-activated receptor β (PPAR-β) mRNA by 72 h. Decreases in AD suggested decreases in fatty acid β-oxidation while decreases in GR may have been a receptor desensitization response to a glucocorticoid surge. Involvement of PPAR-β, an energy homeostasis regulatory molecule, also suggested changes in energy status. Despite, or possibly because of, these early gene changes, there were no differences in either absolute ATP levels or ADP:ATP ratios early in the exposure. However, livers from animals exposed to 200 μg/L atrazine had near-significant (p=0.06) increases in ADP:ATP ratios at the end of exposure suggesting tadpoles may have had difficulty maintaining energy homeostasis. Perturbations in the expression of genes regulating energy metabolism by 24 h into exposure to 400 μg/L atrazine was noteworthy, especially since these tadpoles were significantly smaller than controls by 72 h of exposure.

A reduction in ATP demand and mitochondrial activity with neural differentiation of human embryonic stem cells

Here, we have investigated mitochondrial biology and energy metabolism in human embryonic stem cells (hESCs) and hESC-derived neural stem cells (NSCs). Although stem cells collectively in vivo might be expected to rely primarily on anaerobic glycolysis for ATP supply, to minimise production of reactive oxygen species, we show that in vitro this is not so: hESCs generate an estimated 77% of their ATP through oxidative phosphorylation. Upon differentiation of hESCs into NSCs, oxidative phosphorylation declines both in absolute rate and in importance relative to glycolysis. A bias towards ATP supply from oxidative phosphorylation in hESCs is consistent with the expression levels of the mitochondrial gene regulators peroxisome-proliferator-activated receptor γ coactivator (PGC)-1α, PGC-1β and receptor-interacting protein 140 (RIP140) in hESCs when compared with a panel of differentiated cell types. Analysis of the ATP demand showed that the slower ATP turnover in NSCs was associated with a slower rate of most energy-demanding processes but occurred without a reduction in the cellular growth rate. This mismatch is probably explained by a higher rate of macromolecule secretion in hESCs, on the basis of evidence from electron microscopy and an analysis of conditioned media. Taken together, our developmental model provides an understanding of the metabolic transition from hESCs to more quiescent somatic cell types, and supports important roles for mitochondria and secretion in hESC biology.

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-1α expression.

OBJECTIVE

To determine the role of PGC-1β in maintaining mitochondrial energy metabolism and contractile function in pressure overload hypertrophy.

METHODS AND RESULTS

PGC-1β 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-1β deficient mice exhibited greater degrees of oxidative stress, decreased cardiac efficiency, lower rates of glucose metabolism, and repression of hexokinase II protein.

CONCLUSIONS

PGC-1β 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.

Nuclear Receptor Cofactors in PPARgamma-Mediated Adipogenesis and Adipocyte Energy Metabolism

Transcriptional cofactors are integral to the proper function and regulation of nuclear receptors. Members of the peroxisome proliferator-activated receptor (PPAR) family of nuclear receptors are involved in the regulation of lipid and carbohydrate metabolism. They modulate gene transcription in response to a wide variety of ligands, a process that is mediated by transcriptional coactivators and corepressors. The mechanisms by which these cofactors mediate transcriptional regulation of nuclear receptor function are still being elucidated. The rapidly increasing array of cofactors has brought into focus the need for a clear understanding of how these cofactors interact in ligand- and cell-specific manners. This review highlights the differential effects of the assorted cofactors regulating the transcriptional action of PPARγ and summarizes the recent advances in understanding the physiological functions of corepressors and coactivators.

Metabolic control of mitochondrial biogenesis through the PGC-1 family regulatory network

The PGC-1 family of regulated coactivators, consisting of PGC-1α, PGC-1β and PRC, plays a central role in a regulatory network governing the transcriptional control of mitochondrial biogenesis and respiratory function. These coactivators target multiple transcription factors including NRF-1, NRF-2 and the orphan nuclear hormone receptor, ERRα, among others. In addition, they themselves are the targets of coactivator and co-repressor complexes that regulate gene expression through chromatin remodeling. The expression of PGC-1 family members is modulated by extracellular signals controlling metabolism, differentiation or cell growth and in some cases their activities are known to be regulated by post-translational modification by the energy sensors, AMPK and SIRT1. Recent gene knockout and silencing studies of many members of the PGC-1 network have revealed phenotypes of wide ranging severity suggestive of complex compensatory interactions or broadly integrative functions that are not exclusive to mitochondrial biogenesis. The results point to a central role for the PGC-1 family in integrating mitochondrial biogenesis and energy production with many diverse cellular functions. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

PGC-1α mRNA level and oxidative capacity of the plantaris muscle in rats with metabolic syndrome, hypertension, and type 2 diabetes

We examined the fiber profiles and the mRNA levels of peroxisome proliferator-activated receptors (PPARα and PPARδ/β) and of the PPARγ coactivator-1α (PGC-1α) in the plantaris muscles of 15-week-old control (WR), metabolic syndrome (CP), hypertensive (SHR), and type 2 diabetic (GK) rats. The deep regions in the muscles of SHR and GK rats exhibited lower percentages of high-oxidative type I and IIA fibers and higher percentages of low-oxidative type IIB fibers compared with WR and CP rats. The surface regions in the muscles of CP, SHR, and GK rats exhibited lower percentages of high-oxidative type IIA fibers and higher percentages of low-oxidative type IIB fibers compared with WR rats. The muscles of SHR and GK rats had lower oxidative enzyme activity compared with WR rats. The muscles of SHR rats had the lowest PPARδ/β mRNA level. In addition, the muscles of SHR and GK rats had lower PGC-1α mRNA level compared with WR and CP rats. We concluded that the plantaris muscles of rats with hypertension and type 2 diabetes have lower oxidative capacity, which is associated with the decreased level of PGC-1α mRNA.

MnSOD overexpression prevents liver mitochondrial DNA depletion after an alcohol binge but worsens this effect after prolonged alcohol consumption in mice

Both acute and chronic alcohol consumption increase reactive oxygen species (ROS) formation and lipid peroxidation, whose products damage hepatic mitochondrial DNA (mtDNA). To test whether manganese superoxide dismutase (MnSOD) overexpression modulates acute and chronic alcohol-induced mtDNA lesions, transgenic MnSOD-overexpressing (TgMnSOD(+++)) mice and wild-type (WT) mice were treated by alcohol, either chronically (7 weeks in drinking water) or acutely (single intragastric dose of 5 g/kg). Acute alcohol administration increased mitochondrial ROS formation, decreased mitochondrial glutathione, depleted and damaged mtDNA, durably increased inducible nitric oxide synthase (NOS) expression, plasma nitrites/nitrates and the nitration of tyrosine residues in complex V proteins and decreased complex V activity in WT mice. These effects were prevented in TgMnSOD(+++) mice. In acutely alcoholized WT mice, mtDNA depletion was prevented by tempol, a superoxide scavenger, L-NAME and 1400W, two NOS inhibitors, or uric acid, a peroxynitrite scavenger. In contrast, chronic alcohol consumption decreased cytosolic glutathione and increased hepatic iron, lipid peroxidation products and respiratory complex I protein carbonyls only in ethanol-treated TgMnSOD(+++) mice but not in WT mice. In chronic ethanol-fed TgMnSOD(+++) mice, but not WT mice, mtDNA was damaged and depleted, and the iron chelator, deferoxamine (DFO), prevented this effect. In conclusion, MnSOD overexpression prevents mtDNA depletion after an acute alcohol binge but aggravates this effect after prolonged alcohol consumption, which selectively triggers iron accumulation in TgMnSOD(+++) mice but not in WT mice. In the model of acute alcohol binge, the protective effects of MnSOD, tempol, NOS inhibitors and uric acid suggested a role of the superoxide anion reacting with NO to form mtDNA-damaging peroxynitrite. In the model of prolonged ethanol consumption, the protective effects of DFO suggested the role of iron reacting with hydrogen peroxide to form mtDNA-damaging hydroxyl radical.

PGC-1 coactivators in the control of energy metabolism

Chronic disruption of energy balance, where energy intake exceeds expenditure, is a major risk factor for the development of metabolic syndrome. The latter is characterized by a constellation of symptoms including obesity, dyslipidemia, insulin resistance, hypertension, and non-alcoholic fatty liver disease. Altered expression of genes involved in glucose and lipid metabolism as well as mitochondrial oxidative phosphorylation has been implicated in the pathogenesis of these disorders. The peroxisome proliferator-activated receptor γ coactivator-1 (PGC-1) family of transcriptional coactivators is emerging as a hub linking nutritional and hormonal signals and energy metabolism. PGC-1α and PGC-1β are highly responsive to environmental cues and coordinate metabolic gene programs through interaction with transcription factors and chromatin-remodeling proteins. PGC-1α has been implicated in the pathogenic conditions including obesity, type 2 diabetes, neurodegeneration, and cardiomyopathy, whereas PGC-1β plays an important role in plasma lipoprotein homeostasis and serves as a hepatic target for niacin, a potent hypotriglyceridemic drug. Here, we review recent advances in the identification of physiological and pathophysiological contexts involving PGC-1 coactivators, and also discuss their implications for therapeutic development.

Transcriptional co-factors and hepatic energy metabolism

After binding to their cognate DNA-binding partner, transcriptional co-factors exert their function through the recruitment of enzymatic, chromatin-modifying activities. In turn, the assembly of co-factor-associated multi-protein complexes efficiently impacts target gene expression. Recent advances have established transcriptional co-factor complexes as a critical regulatory level in energy homeostasis and aberrant co-factor activity has been linked to the pathogenesis of severe metabolic disorders including obesity, type 2 diabetes and other components of the Metabolic Syndrome. The liver represents the key peripheral organ for the maintenance of systemic energy homeostasis, and aberrations in hepatic glucose and lipid metabolism have been causally linked to the manifestation of disorders associated with the Metabolic Syndrome. Therefore, this review focuses on the role of distinct classes of transcriptional co-factors in hepatic glucose and lipid homeostasis, emphasizing pathway-specific functions of these co-factors under physiological and pathophysiological conditions.