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

MYC-an emerging player in mitochondrial diseases

The mitochondrion is a major hub of cellular metabolism and involved directly or indirectly in almost all biological processes of the cell. In mitochondrial diseases, compromised respiratory electron transfer and oxidative phosphorylation (OXPHOS) lead to compensatory rewiring of metabolism with resemblance to the Warburg-like metabolic state of cancer cells. The transcription factor MYC (or c-MYC) is a major regulator of metabolic rewiring in cancer, stimulating glycolysis, nucleotide biosynthesis, and glutamine utilization, which are known or predicted to be affected also in mitochondrial diseases. Albeit not widely acknowledged thus far, several cell and mouse models of mitochondrial disease show upregulation of MYC and/or its typical transcriptional signatures. Moreover, gene expression and metabolite-level changes associated with mitochondrial integrated stress response (mt-ISR) show remarkable overlap with those of MYC overexpression. In addition to being a metabolic regulator, MYC promotes cellular proliferation and modifies the cell cycle kinetics and, especially at high expression levels, promotes replication stress and genomic instability, and sensitizes cells to apoptosis. Because cell proliferation requires energy and doubling of the cellular biomass, replicating cells should be particularly sensitive to defective OXPHOS. On the other hand, OXPHOS-defective replicating cells are predicted to be especially vulnerable to high levels of MYC as it facilitates evasion of metabolic checkpoints and accelerates cell cycle progression. Indeed, a few recent studies demonstrate cell cycle defects and nuclear DNA damage in OXPHOS deficiency. Here, we give an overview of key mitochondria-dependent metabolic pathways known to be regulated by MYC, review the current literature on MYC expression in mitochondrial diseases, and speculate how its upregulation may be triggered by OXPHOS deficiency and what implications this has for the pathogenesis of these diseases.

Macrophage Xanthine Oxidoreductase Links LPS Induced Lung Inflammatory Injury to NLRP3 Inflammasome Expression and Mitochondrial Respiration

Acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS) remain poorly treated inflammatory lung disorders. Both reactive oxygen species (ROS) and macrophages are involved in the pathogenesis of ALI/ARDS. Xanthine oxidoreductase (XOR) is an ROS generator that plays a central role in the inflammation that contributes to ALI. To elucidate the role of macrophage-specific XOR in endotoxin induced ALI, we developed a conditional myeloid specific XOR knockout in mice. Myeloid specific ablation of XOR in LPS insufflated mice markedly attenuated lung injury demonstrating the essential role of XOR in this response. Macrophages from myeloid specific XOR knockout exhibited loss of inflammatory activation and increased expression of anti-inflammatory genes/proteins. Transcriptional profiling of whole lung tissue of LPS insufflated XOR fl/fl//LysM-Cre mice demonstrated an important role for XOR in expression and activation of the NLRP3 inflammasome and acquisition of a glycolytic phenotype by inflammatory macrophages. These results identify XOR as an unexpected link between macrophage redox status, mitochondrial respiration and inflammatory activation.

Protective effects of Shen Yuan Dan on myocardial ischemia-reperfusion injury via the regulation of mitochondrial quality control

Background

Myocardial cell death resulting from ischemia-reperfusion (I/R) injury has been a predominant contributor to morbidity and mortality globally. The mitochondria-centered mechanism plays an important role in the formation of I/R injury. This study intended to discuss the protective mechanism of Shen Yuan Dan (SYD) on cardiomyocytes hypoxia-reoxygenation (H/R) injury via the regulation of mitochondrial quality control (MQC). Additionally, this study clarified the mechanism by which SYD suppressed mitophagy activity through the suppression of the PTEN-induced kinase 1 (PINK1)/Parkin pathway.

Methods

To induce cellular injury, H9c2 cardiomyocytes were exposed to H/R stimulation. Following the pretreatment with SYD, cardiomyocytes were subjected to H/R stimulation. Mitochondrial membrane potential (MMP), adenosine triphosphate (ATP), superoxide dismutase (SOD), and methane dicarboxylic aldehyde (MDA) were detected to evaluate the degree of cardiomyocyte mitochondrial damage. Laser confocal microscopy was applied to observe the mitochondrial quality, and the messenger (mRNA) levels of mitofusin 1 (Mfn1), mitofusin 2 (Mfn2), optic atrophy protein 1 (Opa1), dynamin-related protein 1 (Drp1), fission 1 (Fis1), and peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) in cardiomyocytes were assessed using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Western blotting was employed for the estimation of light chain 3 (LC3)-I, LC3-II, PINK1, and Parkin in cardiomyocytes.

Results

It was discovered that SYD pretreatment elevated MMP in H/R injury cardiomyocytes, enhanced ATP content, activated SOD activity, and reduced MDA level. SYD treatment increased the mRNA levels of Mfn1, Mfn2, Opa1 and PGC-1α decreased the mRNA levels of Drp1 and Fis1, and reduced the protein levels of LC3, PINK1, and Parkin.

Conclusions

SYD plays a protective role in H/R injury to cardiomyocytes by regulating mitochondrial quality. Meanwhile, SYD may inhibit mitophagy activity through inhibiting the PINK1/Parkin pathway. This study provides insights into the underlying mechanism of SYD in alleviating myocardial I/R injury.

The Role of Mitophagy in Skeletal Muscle Damage and Regeneration

Mitochondria are cellular organelles that play an essential role in generating the chemical energy needed for the biochemical reactions in cells. Mitochondrial biogenesis, i.e., de novo mitochondria formation, results in enhanced cellular respiration, metabolic processes, and ATP generation, while autophagic clearance of mitochondria (mitophagy) is required to remove damaged or useless mitochondria. The balance between the opposing processes of mitochondrial biogenesis and mitophagy is highly regulated and crucial for the maintenance of the number and function of mitochondria as well as for the cellular homeostasis and adaptations to metabolic demands and extracellular stimuli. In skeletal muscle, mitochondria are essential for maintaining energy homeostasis, and the mitochondrial network exhibits complex behaviors and undergoes dynamic remodeling in response to various conditions and pathologies characterized by changes in muscle cell structure and metabolism, such as exercise, muscle damage, and myopathies. In particular, the involvement of mitochondrial remodeling in mediating skeletal muscle regeneration following damage has received increased attention, as modifications in mitophagy-related signals arise from exercise, while variations in mitochondrial restructuring pathways can lead to partial regeneration and impaired muscle function. Muscle regeneration (through myogenesis) following exercise-induced damage is characterized by a highly regulated, rapid turnover of poor-functioning mitochondria, permitting the synthesis of better-functioning mitochondria to occur. Nevertheless, essential aspects of mitochondrial remodeling during muscle regeneration remain poorly understood and warrant further characterization. In this review, we focus on the critical role of mitophagy for proper muscle cell regeneration following damage, highlighting the molecular mechanisms of the mitophagy-associated mitochondrial dynamics and network reformation.

Metformin attenuated sepsis-associated liver injury and inflammatory response in aged mice

Sepsis-associated liver injury is with poor survival in intensive care units. Metformin is well known for its therapeutic effects; however, its impact on treating liver injury due to sepsis remains poorly understood. This study investigated the therapeutic effects of metformin on aged mice suffering from sepsis-associated liver injury. Male C57BL/6 J mice aged (18–19 months) were divided into 3 groups: 1) intraperitoneal injection of sterile normal saline (C group), 12.5 mg/kg lipopolysaccharide (LPS) to induce sepsis-associated liver injury (LPS group), and 25 mg/kg metformin (MET) at 1 h after LPS injection (MET group). After 24 h, blood samples and liver tissue were collected for biochemical analysis. Histological assays revealed significantly elevated inflammatory infiltration and apoptosis in the liver, while metformin was found to relieve these aberrant features. The percentage of apoptotic cells decreased after metformin treatment (P < 0.05). Additionally, MET group had significantly reduced plasma alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels compared to the LPS group (P < 0.05). Furthermore, in the MET group, the mRNA levels of chemokines and inflammatory factors, TNF-α, IL-6, caspase-1, decreased markedly (P < 0.05). Metformin notably reversed the decreased phosphorylated AMP-activated protein kinase (p-AMPK) and PGC-1α expressions in the liver of septic rats. Metformin also inhibited PDK1, HIF-1α expression, including downstream inflammatory mediators, HMGB1 and TNF-α. Metformin attenuated inflammation and liver injury in septic aged mice. Most importantly, we report the effect of metformin on liver injury via the AMPK–PGC1α axis in septic aged mice for the first time.

Mitochondrial network remodeling: an important feature of myogenesis and skeletal muscle regeneration

The remodeling of the mitochondrial network is a critical process in maintaining cellular homeostasis and is intimately related to mitochondrial function. The interplay between the formation of new mitochondria (biogenesis) and the removal of damaged mitochondria (mitophagy) provide a means for the repopulation of the mitochondrial network. Additionally, mitochondrial fission and fusion serve as a bridge between biogenesis and mitophagy. In recent years, the importance of these processes has been characterised in multiple tissue- and cell-types, and under various conditions. In skeletal muscle, the robust remodeling of the mitochondrial network is observed, particularly after injury where large portions of the tissue/cell structures are damaged. The significance of mitochondrial remodeling in regulating skeletal muscle regeneration has been widely studied, with alterations in mitochondrial remodeling processes leading to incomplete regeneration and impaired skeletal muscle function. Needless to say, important questions related to mitochondrial remodeling and skeletal muscle regeneration still remain unanswered and require further investigation. Therefore, this review will discuss the known molecular mechanisms of mitochondrial network remodeling, as well as integrate these mechanisms and discuss their relevance in myogenesis and regenerating skeletal muscle.

Quality Matters? The Involvement of Mitochondrial Quality Control in Cardiovascular Disease

Cardiovascular diseases are one of the leading causes of death and global health problems worldwide. Multiple factors are known to affect the cardiovascular system from lifestyles, genes, underlying comorbidities, and age. Requiring high workload, metabolism of the heart is largely dependent on continuous power supply via mitochondria through effective oxidative respiration. Mitochondria not only serve as cellular power plants, but are also involved in many critical cellular processes, including the generation of intracellular reactive oxygen species (ROS) and regulating cellular survival. To cope with environmental stress, mitochondrial function has been suggested to be essential during bioenergetics adaptation resulting in cardiac pathological remodeling. Thus, mitochondrial dysfunction has been advocated in various aspects of cardiovascular pathology including the response to ischemia/reperfusion (I/R) injury, hypertension (HTN), and cardiovascular complications related to type 2 diabetes mellitus (DM). Therefore, mitochondrial homeostasis through mitochondrial dynamics and quality control is pivotal in the maintenance of cardiac health. Impairment of the segregation of damaged components and degradation of unhealthy mitochondria through autophagic mechanisms may play a crucial role in the pathogenesis of various cardiac disorders. This article provides in-depth understanding of the current literature regarding mitochondrial remodeling and dynamics in cardiovascular diseases.

PDE5 inhibition rescues mitochondrial dysfunction and angiogenic responses induced by Akt3 inhibition by promotion of PRC expression

Akt3 regulates mitochondrial content in endothelial cells through the inhibition of PGC-1α nuclear localization and is also required for angiogenesis. However, whether there is a direct link between mitochondrial function and angiogenesis is unknown. Here we show that Akt3 depletion in primary endothelial cells results in decreased uncoupled oxygen consumption, increased fission, decreased membrane potential, and increased expression of the mitochondria-specific protein chaperones, HSP60 and HSP10, suggesting that Akt3 is required for mitochondrial homeostasis. Direct inhibition of mitochondrial homeostasis by the model oxidant paraquat results in decreased angiogenesis, showing a direct link between angiogenesis and mitochondrial function. Next, in exploring functional links to PGC-1α, the master regulator of mitochondrial biogenesis, we searched for compounds that induce this process. We found that, sildenafil, a phosphodiesterase 5 inhibitor, induced mitochondrial biogenesis as measured by increased uncoupled oxygen consumption, mitochondrial DNA content, and voltage-dependent anion channel protein expression. Sildenafil rescued the effects on mitochondria by Akt3 depletion or pharmacological inhibition and promoted angiogenesis, further supporting that mitochondrial homeostasis is required for angiogenesis. Sildenafil also induces the expression of PGC-1 family member PRC and can compensate for PGC-1α activity during mitochondrial stress by an Akt3-independent mechanism. The induction of PRC by sildenafil depends upon cAMP and the transcription factor CREB. Thus, PRC can functionally substitute during Akt3 depletion for absent PGC-1α activity to restore mitochondrial homeostasis and promote angiogenesis. These findings show that mitochondrial homeostasis as controlled by the PGC family of transcriptional activators is required for angiogenic responses.

When Friendship Turns Sour: Effective Communication Between Mitochondria and Intracellular Organelles in Parkinson's Disease

Parkinson's disease (PD) is a complex neurodegenerative disease with pathological hallmarks including progressive neuronal loss from the substantia nigra pars compacta and α-synuclein intraneuronal inclusions, known as Lewy bodies. Although the etiology of PD remains elusive, mitochondrial damage has been established to take center stage in the pathogenesis of PD. Mitochondria are critical to cellular energy production, metabolism, homeostasis, and stress responses; the association with PD emphasizes the importance of maintenance of mitochondrial network integrity. To accomplish the pleiotropic functions, mitochondria are dynamic not only within their own network but also in orchestrated coordination with other organelles in the cellular community. Through physical contact sites, signal transduction, and vesicle transport, mitochondria and intracellular organelles achieve the goals of calcium homeostasis, redox homeostasis, protein homeostasis, autophagy, and apoptosis. Herein, we review the finely tuned interactions between mitochondria and surrounding intracellular organelles, with focus on the nucleus, endoplasmic reticulum, Golgi apparatus, peroxisomes, and lysosomes. Participants that may contribute to the pathogenic mechanisms of PD will be highlighted in this review.

NRF-1 and HIF-1α contribute to modulation of human VDAC1 gene promoter during starvation and hypoxia in HeLa cells

VDAC (Voltage Dependent Anion Channel) is a family of pore forming protein located in the outer mitochondrial membrane. Its channel property ensures metabolites exchange between mitochondria and the rest of the cell resulting in metabolism and bioenergetics regulation, and in cell death and life switch. VDAC1 is the best characterized and most abundant isoform, and is involved in many pathologies, as cancer or neurodegenerative diseases. However, little information is available about its gene expression regulation in normal and/or pathological conditions. In this work, we explored VDAC1 gene expression regulation in normal conditions and in the contest of some metabolic and energetic mitochondrial dysfunction and cell stress as example. The core of the putative promoter region was characterized in terms of transcription factors responsive elements both by bioinformatic studies and promoter activity experiments. In particular, we found an abundant presence of NRF-1 sites, together with other transcription factors binding sites involved in cell growth, proliferation, development, and we studied their prevalence in gene activity. Furthermore, upon depletion of nutrients or controlled hypoxia, as detected in various pathologies, we found that VDAC1 transcripts levels were significantly increased in a time related manner. VDAC1 promoter activity was also validated by gene reporter assays. According to PCR real-time experiments, it was confirmed that VDAC1 promoter activity is further stimulated when cells are exposed to stress. A bioinformatic survey suggested HIF-1α, besides NRF-1, as a most active TFBS. Their validation was obtained by TFBS mutagenesis and TF overexpression experiments. In conclusion, we experimentally demonstrated the involvement of both NRF-1 and HIF-1α in the regulation of VDAC1 promoter activation at basal level and in some peculiar cell stress conditions.

SUMOylation-Mediated Response to Mitochondrial Stress

Mitochondrial stress is considered as a factor that reprograms the mitochondrial biogenesis and metabolism. As known, SUMOylation occurs through a series of stress-induced biochemical reactions. During the process of SUMOylation, the small ubiquitin-like modifier (SUMO) and its specific proteases (SENPs) are key signal molecules. Furthermore, they are considered as novel mitochondrial stress sensors that respond to the signals produced by various stresses. The responses are critical for mitochondrial homeostasis. The scope of this review is to provide an overview of the function of SUMOylation in the mitochondrial stress response, to delineate a SUMOylation-involved signal network diagram, and to highlight a number of key questions that remain answered.

The regulatory role of PGC1α-related coactivator in response to drug-induced liver injury

PGC1α-Related Coactivator (PRC) is a transcriptional coactivator promoting cytokine expression in vitro in response to mitochondrial injury and oxidative stress, however, its physiological role has remained elusive. Herein we investigate aspects of the immune response function of PRC, first in an in vivo thioacetamide (TAA)-induced mouse model of drug-induced liver injury (DILI), and subsequently in vitro in human monocytes, HepG2, and dendritic (DC) cells. TAA treatment resulted in the dose-dependent induction of PRC mRNA and protein, both of which were shown to correlate with liver injury markers. Conversely, an adenovirus-mediated knockdown of PRC attenuated this response, thereby reducing hepatic cytokine mRNA expression and monocyte infiltration. Subsequent in vitro studies with conditioned media from HepG2 cells overexpressing PRC, activated human monocytes and monocyte-derived DC, demonstrated up to 20% elevated expression of CD86, CD40, and HLA-DR. Similarly, siRNA-mediated knockdown of PRC abolished this response in oligomycin stressed HepG2 cells. A putative mechanism was suggested by the co-immunoprecipitation of Signal Transducer and Activator of Transcription 1 (STAT1) with PRC, and induction of a STAT-dependent reporter. Furthermore, PRC co-activated an NF-κB-dependent reporter, indicating interaction with known major inflammatory factors. In summary, our study indicates PRC as a novel factor modulating inflammation in DILI.

Alzheimer's and Parkinson's brain tissues have reduced expression of genes for mtDNA OXPHOS Proteins, mitobiogenesis regulator PGC-1α protein and mtRNA stabilizing protein LRPPRC (LRP130)

We used RNA sequencing (RNA-seq) to quantitate gene expression in total RNA extracts of vulnerable brain tissues from Alzheimer's disease (AD, frontal cortical ribbon) and Parkinson's disease (PD, ventral midbrain) subjects and phenotypically negative control subjects. Paired-end sequencing files were processed with HISAT2 aligner/Cufflinks quantitation against the hg38 human genome. We observed a significant decrease in gene expression of all mtDNA OXPHOS genes in AD and PD tissues. Gene expression of the master mitochondrial biogenesis regulator PGC-1α (PPARGC1A) was significantly reduced in AD; expression of genes for mitochondrial transcription factors A (TFAM) and B1/B2 (TFB1M/TFB2M) were not significantly changed in AD and PD tissues. 2-way ANOVAs showed significant reduction in AD brain Complex I subunits' expressions and nearly significant reductions in PD brain. We found a significant reduction in both AD and PD brain samples of expression of genes for leucine-rich pentatricopeptide repeat containing (LRPPRC, a.k.a. LRP130), a known mtRNA-stabilizing protein. Our findings suggest that AD and PD brain tissues have a reduction in mitochondrial ATP production derived from a reduction of mitobiogenesis and mtRNA stability. If true, increased brain expression of PGC-1α and/or LRPPRC may improve bioenergetics of AD and PD and alter the course of neurodegeneration in both conditions. (201 words).

NAMPT and NAPRT: Two Metabolic Enzymes With Key Roles in Inflammation

Nicotinamide phosphoribosyltransferase (NAMPT) and nicotinate phosphoribosyltransferase (NAPRT) are two intracellular enzymes that catalyze the first step in the biosynthesis of NAD from nicotinamide and nicotinic acid, respectively. By fine tuning intracellular NAD levels, they are involved in the regulation/reprogramming of cellular metabolism and in the control of the activity of NAD-dependent enzymes, including sirtuins, PARPs, and NADases. However, during evolution they both acquired novel functions as extracellular endogenous mediators of inflammation. It is well-known that cellular stress and/or damage induce release in the extracellular milieu of endogenous molecules, called alarmins or damage-associated molecular patterns (DAMPs), which modulate immune functions through binding pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), and activate inflammatory responses. Increasing evidence suggests that extracellular (e)NAMPT and eNAPRT are novel soluble factors with cytokine/adipokine/DAMP-like actions. Elevated eNAMPT were reported in several metabolic and inflammatory disorders, including obesity, diabetes, and cancer, while eNAPRT is emerging as a biomarker of sepsis and septic shock. This review will discuss available data concerning the dual role of this unique family of enzymes.

Association of Elite Sports Status with Gene Variants of Peroxisome Proliferator Activated Receptors and Their Transcriptional Coactivator

BACKGROUND

Although the scientific literature regarding sports genomics has grown during the last decade, some genes, such as peroxisome proliferator activated receptors (PPARs), have not been fully described in terms of their role in achieving extraordinary sports performance. Therefore, the purpose of this systematic review was to determine which elite sports performance constraints are positively influenced by PPARs and their coactivators.

METHODS

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were used, with a combination of PPAR and sports keywords.

RESULTS

In total, 27 studies that referred to PPARs in elite athletes were included, where the Ala allele in PPARG rs1801282 was associated with strength and power elite athlete status in comparison to subelite athlete status. The C allele in PPARA rs4253778 was associated with soccer, and the G allele PPARA rs4253778 was associated with endurance elite athlete status. Other elite status endurance alleles were the Gly allele in PPARGC1A rs8192678 and the C allele PPARD rs2016520.

CONCLUSIONS

PPARs can be used for estimating the potential to achieve elite status in human physical performance in strength and power, team, and aerobic sports disciplines. Carrying specific PPAR alleles can provide a partial benefit to achieving elite sports status, but does not preclude achieving elite status if they are absent.

Germline knockdown of spargel (PGC-1) produces embryonic lethality in Drosophila

The PGC-1 transcriptional coactivators have been proposed as master regulators of mitochondrial biogenesis and energy metabolism. Here we show that the single member of the family in Drosophila, spargel (srl) has an essential role in early development. Female germline-specific RNAi knockdown resulted in embryonic semilethality. Embryos were small, with most suffering a catastrophic derangement of cellularization and gastrulation, although genes dependent on localized determinants were expressed normally. The abundance of mtDNA, representative mitochondrial proteins and mRNAs were not decreased in knockdown ovaries or embryos, indicating that srl has a more general role in early development than specifically promoting mitochondrial biogenesis.

NAD-Biosynthetic and Consuming Enzymes as Central Players of Metabolic Regulation of Innate and Adaptive Immune Responses in Cancer

Cancer cells, particularly in solid tumors, are surrounded by non-neoplastic elements, including endothelial and stromal cells, as well as cells of immune origin, which can support tumor growth by providing the right conditions. On the other hand, local hypoxia, and lack of nutrients induce tumor cells to reprogram their metabolism in order to survive, proliferate, and disseminate: the same conditions are also responsible for building a tumor-suppressive microenvironment. In addition to tumor cells, it is now well-recognized that metabolic rewiring occurs in all cellular components of the tumor microenvironment, affecting epigenetic regulation of gene expression and influencing differentiation/proliferation decisions of these cells. Nicotinamide adenine dinucleotide (NAD) is an essential co-factor for energy transduction in metabolic processes. It is also a key component of signaling pathways, through the regulation of NAD-consuming enzymes, including sirtuins and PARPs, which can affect DNA plasticity and accessibility. In addition, both NAD-biosynthetic and NAD-consuming enzymes can be present in the extracellular environment, adding a new layer of complexity to the system. In this review we will discuss the role of the “NADome” in the metabolic cross-talk between cancer and infiltrating immune cells, contributing to cancer growth and immune evasion, with an eye to therapeutic implications.

The PGC-1/ERR network and its role in precision oncology

Transcriptional regulators include a superfamily of nuclear proteins referred to as co-activators and co-repressors, both of which are involved in controlling the functions of several nuclear receptors (NRs). The Nuclear Receptor Signaling Atlas (NURSA) has cataloged the composition of NRs, co-regulators, and ligands present in the human cell and their effort has been identified in more than 600 potential molecules. Given the importance of co-regulators in steroid, retinoid, and thyroid hormone signaling networks, hypothesizing that NRs/co-regulators are implicated in a wide range of pathologies are tempting. The co-activators known as peroxisome proliferator-activated receptor gamma co-activator 1 (PGC-1) and their key nuclear partner, the estrogen-related receptor (ERR), are emerging as pivotal transcriptional signatures that regulate an extremely broad repertoire of mitochondrial and metabolic genes, making them very attractive drug targets for cancer. Several studies have provided an increased understanding of the functional and structural biology of nuclear complexes. However, more comprehensive work is needed to create different avenues to explore the therapeutic potential of NRs/co-activators in precision oncology. Here, we discuss the emerging data associated with the structure, function, and molecular biology of the PGC-1/ERR network and address how the concepts evolving from these studies have deepened our understanding of how to develop more effective treatment strategies. We present an overview that underscores new biological insights into PGC-1/ERR to improve cancer outcomes against therapeutic resistance. Finally, we discuss the importance of exploiting new technologies such as single-particle cryo-electron microscopy (cryo-EM) to develop a high-resolution biological structure of PGC-1/ERR, focusing on novel drug discovery for precision oncology.

Metabolic aspects in NAFLD, NASH and hepatocellular carcinoma: the role of PGC1 coactivators

Alterations of hepatic metabolism are critical to the development of liver disease. The peroxisome proliferator-activated receptor-γ coactivators (PGC1s) are able to orchestrate, on a transcriptional level, different aspects of liver metabolism, such as mitochondrial oxidative phosphorylation, gluconeogenesis and fatty acid synthesis. As modifications affecting both mitochondrial and lipid metabolism contribute to the initiation and/or progression of liver steatosis, nonalcoholic fatty liver disease (NAFLD), nonalcoholic steatohepatitis (NASH) and hepatocellular carcinoma (HCC), a link between disrupted PGC1 pathways and onset of these pathological conditions has been postulated. However, despite the large quantity of studies, the scenario is still not completely understood, and some issues remain controversial. Here, we discuss the roles of PGC1s in healthy liver and explore their contribution to the pathogenesis and future therapy of NASH and HCC.

Endothelial cell senescence in aging-related vascular dysfunction

Increased cardiovascular disease in aging is partly a consequence of the vascular endothelial cell (EC) senescence and associated vascular dysfunction. In this contest, EC senescence is a pathophysiological process of structural and functional changes including dysregulation of vascular tone, increased endothelium permeability, arterial stiffness, impairment of angiogenesis and vascular repair, and a reduction of EC mitochondrial biogenesis. Dysregulation of cell cycle, oxidative stress, altered calcium signaling, hyperuricemia, and vascular inflammation have been implicated in the development and progression of EC senescence and vascular disease in aging. A number of abnormal molecular pathways are associated with these underlying pathophysiological changes including Sirtuin 1, Klotho, fibroblast growth factor 21, and activation of the renin angiotensin-aldosterone system. However, the molecular mechanisms of EC senescence and associated vascular impairment in aging are not completely understood. This review provides a contemporary update on molecular mechanisms, pathophysiological events, as well functional changes in EC senescence and age-associated cardiovascular disease. This article is part of a Special Issue entitled: Genetic and epigenetic regulation of aging and longevity edited by Jun Ren & Megan Yingmei Zhang.

Growth differentiation factor 15 is a myomitokine governing systemic energy homeostasis

Chung et al. show that the myomitokine GDF15 can act to modulate oxidative and lipolytic function in a non–cell-autonomous manner, thereby regulating systemic energy homeostasis in skeletal muscle-specific Crif1-deficient mice. This pathway may be a potential therapeutic target for preventing the onset of obesity and insulin resistance.

Reduced mitochondrial electron transport chain activity promotes longevity and improves energy homeostasis via cell-autonomous and –non-autonomous factors in multiple model systems. This mitohormetic effect is thought to involve the mitochondrial unfolded protein response (UPR^mt), an adaptive stress-response pathway activated by mitochondrial proteotoxic stress. Using mice with skeletal muscle–specific deficiency of Crif1 (muscle-specific knockout [MKO]), an integral protein of the large mitoribosomal subunit (39S), we identified growth differentiation factor 15 (GDF15) as a UPR^mt-associated cell–non-autonomous myomitokine that regulates systemic energy homeostasis. MKO mice were protected against obesity and sensitized to insulin, an effect associated with elevated GDF15 secretion after UPR^mt activation. In ob/ob mice, administration of recombinant GDF15 decreased body weight and improved insulin sensitivity, which was attributed to elevated oxidative metabolism and lipid mobilization in the liver, muscle, and adipose tissue. Thus, GDF15 is a potent mitohormetic signal that safeguards against the onset of obesity and insulin resistance.

Concerted Action of PGC-1-related Coactivator (PRC) and c-MYC in the Stress Response to Mitochondrial Dysfunction

PGC-1-related coactivator (PRC) has a dual function in growth-regulated mitochondrial biogenesis and as a sensor of metabolic stress. PRC induction by mitochondrial inhibitors, intracellular ROS, or topoisomerase I inhibition orchestrates an inflammatory program associated with the adaptation to cellular stress. Activation of this program is accompanied by the coordinate expression of c-MYC, which is linked kinetically to that of PRC in response to multiple stress inducers. Here, we show that the c-MYC inhibitor 10058-F4 blocks the induction of c-MYC, PRC, and representative PRC-dependent stress genes by the respiratory chain uncoupler, carbonyl cyanide m-chlorophenyl hydrazine (CCCP). This result, confirmed by the suppression of PRC induction by c-MYC siRNA silencing, demonstrates a requirement for c-MYC in orchestrating the stress program. PRC steady-state expression was markedly increased upon mutation of two GSK-3 serine phosphorylation sites within the carboxyl-terminal domain. The negative control of PRC expression by GSK-3 was consistent with the phosphor-inactivation of GSK-3β by CCCP and by the induction of PRC by the GSK-3 inhibitor AZD2858. Unlike PRC, which was induced post-translationally through increased protein half-life, c-MYC was induced predominantly at the mRNA level. Moreover, suppression of Akt activation by the Akt inhibitor MK-2206 blocked the CCCP induction of PRC, c-MYC, and representative PRC stress genes, demonstrating a requirement for Akt signaling. MK-2206 also inhibited the phosphor-inactivation of GSK-3β by CCCP, a result consistent with the ability of Akt to phosphorylate, and thereby suppress GSK-3 activity. Thus, PRC and c-MYC can act in concert through Akt-GSK-3 signaling to reprogram gene expression in response to mitochondrial stress.

Mitochondrial Alterations in Peripheral Mononuclear Blood Cells from Alzheimer's Disease and Mild Cognitive Impairment Patients

It is well recognized that mitochondrial dysfunction contributes to neurodegeneration occurring in Alzheimer's disease (AD). However, evidences of mitochondrial defects in AD peripheral cells are still inconclusive. Here, some mitochondrial-encoded and nuclear-encoded proteins, involved in maintaining the correct mitochondria machine, were investigated in terms of protein expression and enzymatic activity in peripheral blood mononuclear cells (PBMCs) isolated from AD and Mild Cognitive Impairment (MCI) patients and healthy subjects. In addition mitochondrial DNA copy number was measured by real time PCR. We found some differences and some similarities between AD and MCI patients when compared with healthy subjects. For example, cytochrome C and cytochrome B were decreased in AD, while MCI showed only a statistical reduction of cytochrome C. On the other hand, both AD and MCI blood cells exhibited highly nitrated MnSOD, index of a prooxidant environment inside the mitochondria. TFAM, a regulator of mitochondrial genome replication and transcription, was decreased in both AD and MCI patients' blood cells. Moreover also the mitochondrial DNA amount was reduced in PBMCs from both patient groups. In conclusion these data confirmed peripheral mitochondria impairment in AD and demonstrated that TFAM and mtDNA amount reduction could be two features of early events occurring in AD pathogenesis.

Aging-dependent alterations in gene expression and a mitochondrial signature of responsiveness to human influenza vaccination

To elucidate gene expression pathways underlying age-associated impairment in influenza vaccine response, we screened young (age 21-30) and older (age ≥65) adults receiving influenza vaccine in two consecutive seasons and identified those with strong or absent response to vaccine, including a subset of older adults meeting criteria for frailty. PBMCs obtained prior to vaccination (Day 0) and at day 2 or 4, day 7 and day 28 post-vaccine were subjected to gene expression microarray analysis. We defined a response signature and also detected induction of a type I interferon response at day 2 and a plasma cell signature at day 7 post-vaccine in young responders. The response signature was dysregulated in older adults, with the plasma cell signature induced at day 2, and was never induced in frail subjects (who were all non-responders). We also identified a mitochondrial signature in young vaccine responders containing genes mediating mitochondrial biogenesis and oxidative phosphorylation that was consistent in two different vaccine seasons and verified by analyses of mitochondrial content and protein expression. These results represent the first genome-wide transcriptional profiling analysis of age-associated dynamics following influenza vaccination, and implicate changes in mitochondrial biogenesis and function as a critical factor in human vaccine responsiveness.

Intercellular: local and systemic actions of skeletal muscle PGC-1s

Physical exercise promotes complex adaptations in skeletal muscle that benefit various aspects of human health. Many of these adaptations are coordinated at the gene expression level by the concerted action of transcriptional regulators. Peroxisome proliferator-activated receptor gamma (PPARγ) coactivator-1 (PGC-1) proteins play a prominent role in skeletal muscle transcriptional reprogramming induced by numerous stimuli. PGC-1s are master coactivators that orchestrate broad gene programs to modulate fuel supply and mitochondrial function, thus improving cellular energy metabolism. Recent studies unveiled novel biological functions for PGC-1s that extend well beyond skeletal muscle bioenergetics. Here we review recent advances in our understanding of PGC-1 actions in skeletal muscle, with special focus on their systemic effects.

PGC-1/Spargel Counteracts High-Fat-Diet-Induced Obesity and Cardiac Lipotoxicity Downstream of TOR and Brummer ATGL Lipase

Obesity and metabolic syndrome are associated with an increased risk for lipotoxic cardiomyopathy, which is strongly correlated with excessive accumulation of lipids in the heart. Obesity- and type-2-diabetes-related disorders have been linked to altered expression of the transcriptional cofactor PGC-1α, which regulates the expression of genes involved in energy metabolism. Using Drosophila, we identify PGC-1/spargel (PGC-1/srl) as a key antagonist of high-fat diet (HFD)-induced lipotoxic cardiomyopathy. We find that HFD-induced lipid accumulation and cardiac dysfunction are mimicked by reduced PGC-1/srl function and reversed by PGC-1/srl overexpression. Moreover, HFD feeding lowers PGC-1/srl expression by elevating TOR signaling and inhibiting expression of the Drosophila adipocyte triglyceride lipase (ATGL) (Brummer), both of which function as upstream modulators of PGC-1/srl. The lipogenic transcription factor SREBP also contributes to HFD-induced cardiac lipotoxicity, likely in parallel with PGC-1/srl. These results suggest a regulatory network of key metabolic genes that modulates lipotoxic heart dysfunction.

Resistance to high-fat diet-induced obesity in male heterozygous Pprc1 knockout mice

Peroxisome proliferator-activated receptor gamma, co-activator-related 1 (Pprc1) is the third member of the Pgc1 family. Other than the well-characterized Pgc1a and Pgc1b that act as regulators of mitochondrial biogenesis and oxidative metabolism, the function of Pprc1 in vivo is rarely reported, due to embryonic lethality of whole-body Pprc1 knockout mice. To investigate the biological and physiological function of Pprc1 in metabolic processes, male Pprc1(+/-) mice fed with a high fat diet (HFD) showed resistance to diet-induced obesity with a decrease of adipose tissue in Pprc1(+/-) mice, which was a result of elevated energy expenditure. In skeletal muscle of Pprc1(+/-) mice, Pprc1 level showed haplo-insufficiency with down-regulation of Pgc1b and Pgc1a, whereas in adipose tissue, Pprc1 expression remained normal, with significant compensatory increase of other Pgc1 family members to induce an up-regulation of respiratory chain genes. Taken together, as the first report on the metabolic roles of Pprc1 in vivo, these results indicated an elevated basal metabolic rate and lipid metabolic alteration of male Pprc1(+/-) mice on HFD, suggesting the significant role of Pprc1 in controlling mitochondrial gene expression and energy metabolic processes, synergistically with Pgc1a and Pgc1b.

MYC and mitochondrial biogenesis

Mitochondria, the powerhouses of the cell, face two imperatives concerning biogenesis. The first is the requirement for dividing cells to replicate their mitochondrial content by growth of existing mitochondria. The second is the dynamic regulation of mitochondrial content in response to organismal and cellular cues (e.g., exercise, caloric restriction, energy status, temperature). MYC provides the clearest example of a programmed expansion of mitochondrial content linked to the cell cycle. As an oncogene, MYC also presents intriguing questions about the role of its mitochondrial targets in cancer-related phenotypes, such as the Warburg effect and MYC-dependent apoptosis.

Estrogen and mitochondria function in cardiorenal metabolic syndrome

The cardiorenal metabolic syndrome (CRS) consists of a constellation of cardiac, renal, and metabolic disorders including insulin resistance (IR), obesity, metabolic dyslipidemia, high-blood pressure, and evidence of early cardiac and renal disease. Mitochondria dysfunction often occurs in the CRS, and this dysfunction is promoted by excess reactive oxygen species, genetic factors, IR, aging, and altered mitochondrial biogenesis. Recently, it has been shown that there are important sex-related differences in mitochondria function and metabolic, cardiovascular, and renal components. Sex differences in the CRS have mainly been attributed to the estrogen's effects that are mainly mediated by estrogen receptor (ER) α, ERβ, and G-protein coupled receptor 30. In this review, we discuss the effects of estrogen on the mitochondrial function, insulin metabolic signaling, glucose transport, lipid metabolism, and inflammatory responses from liver, pancreatic β cells, adipocytes, skeletal muscle, and cardiovascular tissue.

Mitochondrial quality control and communications with the nucleus are important in maintaining mitochondrial function and cell health

BACKGROUND

The maintenance of cell metabolism and homeostasis is a fundamental characteristic of living organisms. In eukaryotes, mitochondria are the cornerstone of these life supporting processes, playing leading roles in a host of core cellular functions, including energy transduction, metabolic and calcium signalling, and supporting roles in a number of biosynthetic pathways. The possession of a discrete mitochondrial genome dictates that the maintenance of mitochondrial 'fitness' requires quality control mechanisms which involve close communication with the nucleus.

SCOPE OF REVIEW

This review explores the synergistic mechanisms that control mitochondrial quality and function and ensure cellular bioenergetic homeostasis. These include antioxidant defence mechanisms that protect against oxidative damage caused by reactive oxygen species, while regulating signals transduced through such free radicals. Protein homeostasis controls import, folding, and degradation of proteins underpinned by mechanisms that regulate bioenergetic capacity through the mitochondrial unfolded protein response. Autophagic machinery is recruited for mitochondrial turnover through the process of mitophagy. Mitochondria also communicate with the nucleus to exact specific transcriptional responses through retrograde signalling pathways.

MAJOR CONCLUSIONS

The outcome of mitochondrial quality control is not only reliant on the efficient operation of the core homeostatic mechanisms but also in the effective interaction of mitochondria with other cellular components, namely the nucleus.

GENERAL SIGNIFICANCE

Understanding mitochondrial quality control and the interactions between the organelle and the nucleus will be crucial in developing therapies for the plethora of diseases in which the pathophysiology is determined by mitochondrial dysfunction. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Estradiol and tamoxifen regulate NRF-1 and mitochondrial function in mouse mammary gland and uterus

Nuclear respiratory factor-1 (NRF-1) stimulates the transcription of nuclear-encoded genes that regulate mitochondrial (mt) genome transcription and biogenesis. We reported that estradiol (E2) and 4-hydroxytamoxifen (4-OHT) stimulate NRF-1 transcription in an estrogen receptor α (ERα)- and ERβ-dependent manner in human breast cancer cells. The aim of this study was to determine whether E2 and 4-OHT increase NRF-1 in vivo. Here, we report that E2 and 4-OHT increase NRF-1 expression in mammary gland (MG) and uterus of ovariectomized C57BL/6 mice in a time-dependent manner. E2 increased NRF-1 protein in the uterus and MG; however, in MG, 4-OHT increased Nrf1 mRNA but not protein. Chromatin immunoprecipitation assays revealed increased in vivo recruitment of ERα to the Nrf1 promoter and intron 3 in MG and uterus 6 h after E2 and 4-OHT treatment, commensurate with increased NRF-1 expression. E2- and 4-OHT-induced increases in NRF-1 and its target genes Tfam, Tfb1m, and Tfb2m were coordinated in MG but not in uterus due to uterine-selective inhibition of the expression of the NRF-1 coactivators Ppargc1a and Ppargc1b by E2 and 4-OHT. E2 transiently increased NRF-1 and PGC-1α nuclear staining while reducing PGC-1α in uterus. E2, not 4-OHT, activates mt biogenesis in MG and uterus in a time-dependent manner. E2 increased mt outer membrane Tomm40 protein levels in MG and uterus whereas 4-OHT increased Tomm40 only in uterus. These data support the hypothesis of tissue-selective regulation of NRF-1 and its downstream targets by E2 and 4-OHT in vivo.

Dealing with hunger: Metabolic stress responses in tumors

Increased nutrient uptake and usage is a hallmark of many human malignancies. During the course of tumorigenesis, cancer cells often outstrip their local nutrient supply leading to periods of nutrient deprivation. Interestingly, cancer cells often develop strategies to adapt and survive these challenging conditions. Accordingly, understanding these processes is critical for developing therapies that target cancer metabolism. Exciting new progress has been made in elucidating the mechanisms used by cancer cells under nutrient restricted conditions. In this review, we highlight recent studies that have brought insight into how cancer cells deal with low nutrient environments.

Activation of a PGC-1-related coactivator (PRC)-dependent inflammatory stress program linked to apoptosis and premature senescence

PGC-1-related coactivator (PRC), a growth-regulated member of the PGC-1 coactivator family, contributes to the expression of the mitochondrial respiratory apparatus. PRC also orchestrates a robust response to metabolic stress by promoting the expression of multiple genes specifying inflammation, proliferation, and metabolic reprogramming. Here, we demonstrate that this PRC-dependent stress program is activated during apoptosis and senescence, two major protective mechanisms against cellular dysfunction. Both PRC and its targets (IL1α, SPRR2D, and SPRR2F) were rapidly induced by menadione, an agent that promotes apoptosis through the generation of intracellular oxidants. Menadione-induced apoptosis and the PRC stress program were blocked by the antioxidant N-acetylcysteine. The PRC stress response was also activated by the topoisomerase I inhibitor 7-ethyl-10-hydroxycamptothecin (SN-38), an inducer of premature senescence in tumor cells. Cells treated with SN-38 displayed morphological characteristics of senescence and express senescence-associated β-galactosidase activity. In contrast to menadione, the SN-38 induction of the PRC program occurred over an extended time course and was antioxidant-insensitive. The potential adaptive function of the PRC stress response was investigated by treating cells with meclizine, a drug that promotes glycolytic energy metabolism and has been linked to cardio- and neuroprotection against ischemia-reperfusion injury. Meclizine increased lactate production and was a potent inducer of the PRC stress program, suggesting that PRC may contribute to the protective effects of meclizine. Finally, c-MYC and PRC were coordinately induced under all conditions tested, implicating c-MYC in the biological response to metabolic stress. The results suggest a general role for PRC in the adaptive response to cellular dysfunction.

Transcriptional integration of mitochondrial biogenesis

Gene regulatory factors encoded by the nuclear genome are essential for mitochondrial biogenesis and function. Some of these factors act exclusively within the mitochondria to regulate the control of mitochondrial transcription, translation, and other functions. Others govern the expression of nuclear genes required for mitochondrial metabolism and organelle biogenesis. The peroxisome proliferator-activated receptor γ coactivator-1 (PGC-1) family of transcriptional coactivators play a major role in transducing and integrating physiological signals governing metabolism, differentiation, and cell growth to the transcriptional machinery controlling mitochondrial functional capacity. Thus, the PGC-1 coactivators serve as a central component of the transcriptional regulatory circuitry that coordinately controls the energy-generating functions of mitochondria in accordance with the metabolic demands imposed by changing physiological conditions, senescence, and disease.

NAD+-dependent sirtuin 1 and 6 proteins coordinate a switch from glucose to fatty acid oxidation during the acute inflammatory response

The early initiation phase of acute inflammation is anabolic and primarily requires glycolysis with reduced mitochondrial glucose oxidation for energy, whereas the later adaptation phase is catabolic and primarily requires fatty acid oxidation for energy. We reported previously that switching from the early to the late acute inflammatory response following TLR4 stimulation depends on NAD(+) activation of deacetylase sirtuin 1 (SirT1). Here, we tested whether NAD(+) sensing by sirtuins couples metabolic polarity with the acute inflammatory response. We found in TLR4-stimulated THP-1 promonocytes that SirT1 and SirT 6 support a switch from increased glycolysis to increased fatty acid oxidation as early inflammation converts to late inflammation. Glycolysis enhancement required hypoxia-inducing factor-1α to up-regulate glucose transporter Glut1, phospho-fructose kinase, and pyruvate dehydrogenase kinase 1, which interrupted pyruvate dehydrogenase and reduced mitochondrial glucose oxidation. The shift to late acute inflammation and elevated fatty acid oxidation required peroxisome proliferator-activated receptor γ coactivators PGC-1α and β to increase external membrane CD36 and fatty acid mitochondrial transporter carnitine palmitoyl transferase 1. Metabolic coupling between early and late responses also required NAD(+) production from nicotinamide phosphoryltransferase (Nampt) and activation of SirT6 to reduce glycolysis and SirT1 to increase fatty oxidation. We confirmed similar shifts in metabolic polarity during the late immunosuppressed stage of human sepsis blood leukocytes and murine sepsis splenocytes. We conclude that NAD(+)-dependent bioenergy shifts link metabolism with the early and late stages of acute inflammation.

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.