Energizer: PGC-1 alpha keeps the heart going

Global Trends in Research of Mitochondrial Biogenesis over past 20 Years: A Bibliometric Analysis

Background

Mitochondrial biogenesis-related studies have increased rapidly within the last 20 years, whereas there has been no bibliometric analysis on this topic to reveal relevant progress and development trends.

Objectives

In this study, a bibliometric approach was adopted to summarize and analyze the published literature in this field of mitochondrial biogenesis over the past 20 years to reveal the major countries/regions, institutions and authors, core literature and journal, research hotspots and frontiers in this field.

Methods

The Web of Science Core Collection database was used for literature retrieval and dataset export. The CiteSpace and VOSviewer visual mapping software were used to explore research collaboration between countries/regions, institutions and authors, distribution of subject categories, core journals, research hotspots, and frontiers in this field.

Results

In the last 20 years, the annual number of publications has shown an increasing trend yearly. The USA, China, and South Korea have achieved fruitful research results in this field, among which Duke University and Chinese Academy of Sciences are the main research institutions. Rick G Schnellmann, Claude A Piantadosi, and Hagir B Suliman are the top three authors in terms of number of publications, while RC Scarpulla, ZD Wu, and P Puigserver are the top three authors in terms of cocitation frequency. PLOS One, Biochemical and Biophysical Research Communications, and Journal of Biological Chemistry are the top three journals in terms of number of articles published. Three papers published by Richard C Scarpulla have advanced this field and are important literature for understanding the field. Mechanistic studies on mitochondrial biosynthesis have been a long-standing hot topic; the main keywords include skeletal muscle, oxidative stress, gene expression, activation, and nitric oxide, and autophagy and apoptosis have been important research directions in recent years.

Conclusion

These results summarize the major research findings in the field of mitochondrial biogenesis over the past 20 years in various aspects, highlighting the major research hotspots and possible future research directions and helping researchers to quickly grasp the overview of the developments in this field.

Inter-tissue expression patterns of the key metabolic biomarker PGC-1α in severely obese individuals: Implication in obesity-induced disease

OBJECTIVE

PGC-1α is already known as a significant regulator of mitochondrial biogenesis, oxidative phosphorylation and fatty acid metabolism. Our study focuses on the role of PGC1α in morbid obesity, in five different tissues, collected from 50 severely obese patients during planned bariatric surgery.

METHODS

The investigated tissues included subcutaneous adipose tissue (SAT), visceral adipose tissue (VAT), skeletal muscle (SM), extramyocellular adipose tissue (EMAT) and liver. PGC1α expression was investigated with immunohistochemistry and evaluated with microscopy.

RESULTS

Our findings highlighted significant positive inter-tissue correlations regarding PGC-1α expression between several tissue pairs (VAT-SAT, VAT-SM, VAT-EMAT, SAT-SM, SAT-EMAT, SM-EMAT). Moreover, we found significant negative correlations between PGC1α expression in VAT with CD68 expression in skeletal muscle and EMAT, implying a possible protective role of PGC1α against obesity-induced inflammation.

CONCLUSION

Unmasking the inter-tissue communication networks regarding PGC-1α expression in morbid obesity, will give more insight into its significant role in obesity-induced diseases. PGC1α could potentially represent a future preventive and therapeutic target against obesity-induced disease, probably through enhancing mitochondrial biogenesis and metabolism.

Peroxisome proliferator-activated receptor γ coactivator 1α and FoxO3A mediate chondroprotection by AMP-activated protein kinase

OBJECTIVE

AMP-activated protein kinase (AMPK) inhibits chondrocyte procatabolic responses to inflammation and biomechanical injury. This study was undertaken to test the hypothesis that peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and FoxO3A, 2 major AMPK downstream targets, mediate the chondroprotective effect of AMPK activation.

METHODS

We assessed the activity of AMPKα (threonine 172 phosphorylation) and the expression of PGC-1α and FoxO3A in human chondrocytes and AMPKα1- or AMPKα2-knockout mouse chondrocytes by Western blotting, and in mouse knee cartilage by immunohistochemistry. We also knocked down or overexpressed PGC-1α and FoxO3A by small interfering RNA or plasmid DNA transfection, respectively. We assessed mitochondrial superoxide generation using MitoSOX Red.

RESULTS

Expression of PGC-1α and FoxO3A was enhanced by pharmacologic AMPK activator A-769662 but impaired in AMPKα1(-/-) or AMPKα2(-/-) mouse chondrocytes. Reduced expression of PGC-1α and FoxO3A was observed in mouse knee instability-induced osteoarthritis (OA) cartilage and in aged C57BL/6 mouse knee cartilage. Knockdown of PGC-1α and FoxO3A enhanced, but limited the ability of A-769662 to inhibit, phosphorylation of p65 NF-κB (Ser(536) ) and procatabolic responses induced by inflammatory cytokines. Forced expression of PGC-1α and FoxO3A induced increased expression of superoxide dismutase 2 (SOD2) and catalase, but A-769662 failed to increase the expression of SOD2 and catalase in either PGC-1α- or FoxO3A-knockdown chondrocytes. Last, menadione-induced superoxide generation was inhibited by AMPK pharmacologic activators and by overexpression of PGC-1α or FoxO3A.

CONCLUSION

PGC-1α and FoxO3A limit oxidative stress and at least partially mediate the capacity of AMPK activity to block procatabolic responses in chondrocytes, and therefore have the potential to inhibit the progression of cartilage damage in OA.

SOCS3: A novel therapeutic target for cardioprotection

The suppressors of cytokine signaling (SOCS) family of proteins are cytokine-inducible inhibitors of Janus kinase (JAK)-signal transducer and activator of the transcription (STAT) signaling pathways. Among the family, SOCS1 and SOCS3 potently suppress cytokine actions by inhibiting JAK kinase activities. The generation of mice lacking individual SOCS genes has been instrumental in defining the role of individual SOCS proteins in specific cytokine pathways in vivo; SOCS1 is an essential negative regulator of interferon-γ (IFNγ) and SOCS3 is an essential negative regulator of leukemia inhibitory factor (LIF). JAK-STAT3 activating cytokines have exhibited cardioprotective roles in the heart. The cardiac-specific deletion of SOCS3 enhances the activation of cardioprotective signaling pathways, inhibits myocardial apoptosis and fibrosis and results in the inhibition of left ventricular remodeling after myocardial infarction (MI). We propose that myocardial SOCS3 is a key determinant of left ventricular remodeling after MI, and SOCS3 may serve as a novel therapeutic target to prevent left ventricular remodeling after MI. In this review, we discuss the signaling pathways mediated by JAK-STAT and SOCS proteins and their roles in the development of myocardial injury under stress (e.g., pressure overload, viral infection and ischemia).

Cardiac-specific deletion of SOCS-3 prevents development of left ventricular remodeling after acute myocardial infarction

OBJECTIVES

The study investigated the role of myocardial suppressor of cytokine signaling-3 (SOCS3), an intrinsic negative feedback regulator of the janus kinase and signal transducer and activator of transcription (JAK-STAT) signaling pathway, in the development of left ventricular (LV) remodeling after acute myocardial infarction (AMI).

BACKGROUND

LV remodeling after AMI results in poor cardiac performance leading to heart failure. Although it has been shown that JAK-STAT-activating cytokines prevent LV remodeling after AMI in animals, little is known about the role of SOCS3 in this process.

METHODS

Cardiac-specific SOCS3 knockout mice (SOCS3-CKO) were generated and subjected to AMI induced by permanent ligation of the left anterior descending coronary artery.

RESULTS

Although the initial infarct size after coronary occlusion measured by triphenyltetrazolium chloride staining was comparable between SOCS3-CKO and control mice, the infarct size 14 days after AMI was remarkably inhibited in SOCS3-CKO, indicating that progression of LV remodeling after AMI was prevented in SOCS3-CKO hearts. Prompt and marked up-regulations of multiple JAK-STAT-activating cytokines including leukemia inhibitory factor and granulocyte colony-stimulating factor (G-CSF) were observed within the heart following AMI. Cardiac-specific SOCS3 deletion enhanced multiple cardioprotective signaling pathways including STAT3, AKT, and extracellular signal-regulated kinase (ERK)-1/2, while inhibiting myocardial apoptosis and fibrosis as well as augmenting antioxidant expression.

CONCLUSIONS

Enhanced activation of cardioprotective signaling pathways by inhibiting myocardial SOCS3 expression prevented LV remodeling after AMI. Our data suggest that myocardial SOCS3 may be a key molecule in the development of LV remodeling after AMI.

AMP activated protein kinase-α2 regulates expression of estrogen-related receptor-α, a metabolic transcription factor related to heart failure development

The normal expression of myocardial mitochondrial enzymes is essential to maintain the cardiac energy reserve and facilitate responses to stress, but the molecular mechanisms to maintain myocardial mitochondrial enzyme expression have been elusive. Here we report that congestive heart failure is associated with a significant decrease of myocardial estrogen-related receptor-α (ERRα), but not peroxisome proliferator-activated receptor-γ coactivator 1α, in human heart failure samples. In addition, chronic pressure overload in mice caused a decrease of ERRα expression that was significantly correlated to the degree of left ventricular dysfunction, pulmonary congestion, and decreases of a group of myocardial energy metabolism-related genes. We found that the metabolic sensor AMP activated protein kinase (AMPK) regulates ERRα expression in vivo and in vitro. AMPKα2 knockout decreased myocardial ERRα (both mRNA and protein) and its downstream targets under basal conditions, with no change in myocardial peroxisome proliferator-activated receptor-γ coactivator 1α expression. Using cultured rat neonatal cardiac myocytes, we found that overexpression of constitutively active AMPKα significantly induced ERRα mRNA, protein, and promoter activity. Conversely, selective gene silencing of AMPKα2 repressed ERRα and its target gene levels, indicating that AMPKα2 is involved in the regulation of ERRα expression. In addition, overexpression of ERRα in AMPKα2 knockout neonatal cardiac myocytes partially rescued the repressed expression of some energy metabolism-related genes. These data support an important role for AMPKα2 in regulating the expression of myocardial ERRα and its downstream mitochondrial enzymes.

Mitochondrial transcription factors TFAM and TFB2M regulate Serca2 gene transcription

AIMS

Sarco(endo)plasmic reticulum Ca²(+)-ATPase 2a (SERCA2a) transports Ca²(+) by consuming ATP produced by mitochondrial respiratory chain enzymes. Messenger RNA (mRNA) for these enzymes is transcribed by mitochondrial transcription factors A (TFAM) and B2 (TFB2M). This study examined whether TFAM and TFB2M coordinately regulate the transcription of the Serca2 gene and mitochondrial genes.

METHODS AND RESULTS

Nuclear localization of TFAM and TFB2M was demonstrated by immunostaining in rat neonatal cardiac myocytes. Chromatin immunoprecipitation assay and fluorescence correlation spectroscopy revealed that TFAM and TFB2M bind to the -122 to -114 nt and -122 to -117 nt regions of the rat Serca2 gene promoter, respectively. Mutation of these sites resulted in decreased Serca2 gene transcription. In a rat myocardial infarction model, Serca2a mRNA levels significantly correlated with those of Tfam (r = 0.54, P < 0.001) and Tfb2m (r = 0.73, P < 0.001). Overexpression of TFAM and TFB2M blocked hydrogen peroxide- and norepinephrine-induced decreases in Serca2a mRNA levels. In addition, overexpression of TFAM and TFB2M increased the mitochondrial DNA (mtDNA) copy number and mRNA levels of mitochondrial enzymes.

CONCLUSION

Although TFAM and TFB2M are recognized as mtDNA-specific transcription factors, they also regulate transcription of nuclear DNA, i.e. the Serca2 gene. Our findings suggest a novel paradigm in which the transcription of genes for mitochondrial enzymes that produce ATP and the gene for SERCA2a that consumes ATP is coordinately regulated by the same transcription factors. This mechanism may contribute to maintaining proper cardiac function.

Control by Circulating Factors of Mitochondrial Function and Transcription Cascade in Heart Failure

Background— Evidence is emerging to support the concept that the failing heart is “energy depleted” and that defects in energy metabolism are important determinants in the development and the progression of the disease. We have shown previously that depressed mitochondrial function in cardiac and skeletal muscles in chronic heart failure is linked to decreased expression of the gene encoding transcriptional proliferator-activated receptor-γ coactivator-1α, the inducible regulator of mitochondrial biogenesis and its transcription cascade, leading to altered expression of mitochondrial proteins. However, oxidative capacity of the myocardium of patients treated for chronic heart failure and pathophysiological mechanisms of mitochondrial dysfunction are still largely unknown.

Methods and Results— In patients with chronic heart failure treated with angiotensin-converting enzyme inhibition, cardiac oxidative capacity, measured in saponin-permeabilized fibers, was 25% lower, and proliferator-activated receptor-γ coactivator-1α protein content was 34% lower compared with nonfailing controls. In a rat model of myocardial infarction, angiotensin-converting enzyme inhibition therapy was only partially able to protect cardiac mitochondrial function and transcription cascade. Expression of proliferator-activated receptor-γ coactivator-1α and its transcription cascade were evaluated after a 48-hour exposure of cultured adult rat ventricular myocytes to endothelin-1, angiotensin II, aldosterone, phenylephrine, or isoprenaline. Endothelin-1 (−30%) and, to a lesser degree, angiotensin II (−20%) decreased proliferator-activated receptor-γ coactivator-1α mRNA content, whereas other hormones had no effect (phenylephrine) or even increased it (aldosterone, isoprenaline).

Conclusions— Taken together, these results show that, despite angiotensin-converting enzyme inhibition treatment, oxidative capacity is reduced in human and experimental heart failure and that endothelin-1 and angiotensin II could be involved in the downregulation of the mitochondrial transcription cascade.

Mutual dependence of Foxo3a and PGC-1alpha in the induction of oxidative stress genes

Oxidative stress is a hallmark of metabolism-related diseases and a risk factor for atherosclerosis. FoxO factors have been shown to play a key role in vascular endothelial development and homeostasis. Foxo3a can protect quiescent cells from oxidative stress through the regulation of detoxification genes such as sod2 and catalase. Here we show that Foxo3a is a direct transcriptional regulator of a group of oxidative stress protection genes in vascular endothelial cells. Importantly, Foxo3a activity requires the transcriptional co-activator PGC-1alpha, because it is severely curtailed in PGC-1alpha-deficient endothelial cells. Foxo3a and PGC-1alpha appear to interact directly, as shown by co-immunoprecipitation and in vitro interaction assays, and are recruited to the same promoter regions. The notion that Foxo3a and PGC-1alpha interact directly to regulate oxidative stress protection genes in the vascular endothelium is supported by the observation that PGC-1alpha transcriptional activity at the sod2 (manganese superoxide dismutase) promoter requires a functional FoxO site. We also demonstrate that Foxo3a is a direct transcriptional regulator of PGC-1alpha, suggesting that an auto-regulatory cycle regulates Foxo3a/PGC-1alpha control of the oxidative stress response.

Intramolecular control of protein stability, subnuclear compartmentalization, and coactivator function of peroxisome proliferator-activated receptor gamma coactivator 1alpha

Peroxisome proliferator-activated receptor gamma coactivator (PGC)-1 is a critical transcriptional regulator of energy metabolism. Here we found that PGC-1alpha is a short lived and aggregation-prone protein. PGC-1alpha localized throughout the nucleoplasm and was rapidly destroyed via the ubiquitin-proteasome pathway. Upon proteasome inhibition, PGC-1alpha formed insoluble polyubiquitinated aggregates. Ubiquitination of PGC-1alpha depended on the integrity of the C terminus-containing arginine-serine-rich domains and an RNA recognition motif. Interestingly, ectopically expressed C-terminal fragment of PGC-1alpha was autonomously ubiquitinated and aggregated with promyelocytic leukemia protein. Cooperation of the N-terminal region containing two PEST-like motifs was required for prevention of aggregation and targeting of the polyubiquitinated PGC-1alpha for degradation. This region thereby negatively controlled the aggregation properties of the C-terminal region to regulate protein turnover and intranuclear compartmentalization of PGC-1alpha. Exogenous expression of the PGC-1alpha C-terminal fragment interfered with degradation of full-length PGC-1alpha and enhanced its coactivation properties. We concluded that PGC-1alpha function is critically regulated at multiple steps via intramolecular cooperation among several distinct structural domains of the protein.

Ménage-à-trois 1 is critical for the transcriptional function of PPARgamma coactivator 1

The Cdk7/cyclin H/ménage-à-trois 1 (MAT1) heterotrimer has proposed functions in transcription as the kinase component of basal transcription factor TFIIH and is activated in adult hearts by Gq-, calcineurin-, and biomechanical stress-dependent pathways for hypertrophic growth. Using cardiac-specific Cre, we have ablated MAT1 in myocardium. Despite reduced Cdk7 activity, MAT1-deficient hearts grew normally, but fatal heart failure ensued at 6-8 weeks. By microarray profiling, quantitative RT-PCR, and western blotting at 4 weeks, genes for energy metabolism were found to be suppressed selectively, including targets of peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1). Cardiac metabolic defects were substantiated in isolated perfused hearts and isolated mitochondria. In culture, deleting MAT1 with Cre disrupted PGC-1 function: PGC-1alpha failed to activate PGC-1-responsive promoters and nuclear receptors, GAL4-PGC-1alpha was functionally defective, and PGC-1beta was likewise deficient. PGC-1 bound to both MAT1 and Cdk7 in coprecipitation assays. Thus, we demonstrate a requirement for MAT1 in the operation of PGC-1 coactivators that control cell metabolism.

[Transcriptional regulation of metabolic switching PDK4 gene under various physiological conditions]

Pyruvate dehydrogenase kinase 4 (PDK4) phosphorylates and inactivates the pyruvate dehydrogenase complex to respond to physiologic conditions. This response switches the energy source from glucose to fatty acids to maintain blood glucose levels. Transcription of the PDK4 gene is activated by fasting or by the administration of a peroxisome proliferator-activated receptor alpha (PPARalpha) ligand in a tissue-specific manner. However, the two mechanisms to induce PDK4 mRNA as well as the relationship between the two have not been studied in detail. In this study, we show that the two mechanisms are independent, at least in the mouse skeletal muscle, and that estrogen-related receptor alpha (ERRalpha) is directly involved in the PPARalpha-independent transcriptional activation of the PDK4 gene with peroxisome proliferator-activated receptor gamma co-activator 1alpha (PGC-1alpha) as a specific partner. The latter conclusion is based on the following evidence: 1) Deletion and point mutation analyses of the cloned mouse PDK4 gene promoter sequence identified an exact possible ERRalpha-binding motif as the PGC-1alpha responsive element. 2) The overexpression of ERRalpha by cotransfection enhanced, and the knocking down of it by specific shRNAs diminished, the PGC-1alpha-dependent activation. 3) Specific binding of ERRalpha to the identified PGC-1alpha-responsive sequence of the mouse PDK4 promoter was confirmed in the electrophoresis mobility shift assay using anti-ERRalpha antibodies. These results suggest that PGC-1alpha plays an essential role not only in regulating the amounts of energy creating enzymes, but also at the step of metabolic switching with unevenly distributed tissue transcription factors such as ERRalpha in the skeletal muscle, thus harmonizing tissue-specific functions and energy metabolism.

Identification of ERRalpha as a specific partner of PGC-1alpha for the activation of PDK4 gene expression in muscle

Pyruvate dehydrogenase kinase 4 (PDK4) is a key regulatory enzyme involved in switching the energy source from glucose to fatty acids in response to physiological conditions. Transcription of the PDK4 gene is activated by fasting or by the administration of a PPARalpha ligand in a tissue-specific manner. Here, we show that the two mechanisms are independent, and that ERRalpha is directly involved in PPARalpha-independent transcriptional activation of the PDK4 gene with PGC-1alpha as a specific partner. This conclusion is based on the following evidence. First, detailed mutation analyses of the cloned PDK4 gene promoter sequence identified a possible ERRalpha-binding motif as the PGC-1alpha responsive element. Second, overexpression of ERRalpha by cotransfection enhanced, and the knockout of it by shRNAs diminished, PGC-1alpha-dependent activation. Third, specific binding of ERRalpha to the identified PGC-1alpha responsive sequence was confirmed by the electrophoresis mobility shift assay. Finally, cell-type-specific responsiveness to PGC-1alpha was observed and this could be explained by differences in the expression levels of ERRalpha, however, ectopic expression of ERRalpha in poorly responsive cells did not restore PGC-1alpha responsiveness, indicating that ERRalpha is necessary, but not sufficient for the response.