Transcription Factor Tfe3 Directly Regulates Pgc-1alpha in Muscle

The role of TFE3 in mediating skeletal muscle mitochondrial adaptations to exercise training

Transcription factor E3 (TFE3) is a transcription factor that activates the expression of lysosomal genes involved in the clearance of dysfunctional mitochondria, termed mitophagy. With exercise, TFE3 is presumed to optimize the mitochondrial pool through the removal of organelles via lysosomes. However, the molecular mechanisms of the involved pathways remain unknown. Wild-type (WT) and TFE3 knockout (KO) mice were subjected to 6 wk of voluntary wheel running as an endurance training regimen. This was followed by a 45-min bout of in situ stimulation of the sciatic nerve innervating hindlimb muscles to evaluate muscle fatigue and contractile properties. A subset of animals was treated with colchicine to measure autophagy and mitophagy flux. Fatigability during stimulation was reduced with training in WT animals, as seen by a 13% increase in the percentage of maximum force at 5 min of stimulation, and a 30% increase at 30 minutes. Permeabilized fiber oxygen consumption was also improved with training. Concurrent with improved muscle and mitochondrial function, cytochrome c oxidase (COX) activity and COX I protein expression were increased in trained WT animals compared to untrained animals, signifying an increase in mitochondrial content. These training adaptations were abolished with the loss of TFE3. Surprisingly, the absence of TFE3 did not affect lysosomal content nor did it blunt the induction of mitophagy flux with contractile activity compared to WT mice. Our results suggest that the loss of TFE3 compromises beneficial training adaptations that lead to improved muscle endurance and mitochondrial function.NEW & NOTEWORTHY Our understanding of the role of transcription factor E3 (TFE3) in skeletal muscle is very limited. This research shows that TFE3 plays a direct role in skeletal muscle mitochondrial enhancement with exercise training, thereby introducing a paradigm shift in our perception of the function of TFE3 in mitochondrial maintenance, beyond mitophagy. This research serves to introduce TFE3 as a protein that holds promise as a future therapeutic target for metabolic diseases and skeletal muscle dysfunction.

Role of Flavonoids in Modulation of Mitochondria Dynamics during Oxidative Stress

BACKGROUND

Flavonoids are a widespread category of naturally occurring polyphenols distinguished by the flavan nucleus in plant-based foods and beverages, known for their various health benefits. Studies have suggested that consuming 150-500 mg of flavonoids daily is beneficial for health. Recent studies suggest that flavonoids are involved in maintaining mitochondrial activity and preventing impairment of mitochondrial dynamics by oxidative stress.

OBJECTIVE

This review emphasized the significance of studying the impact of flavonoids on mitochondrial dynamics, oxidative stress, and inflammatory response.

METHODS

This review analysed and summarised the findings related to the impact of flavonoids on mitochondria from publicly available search engines namely Pubmed, Scopus, and Web of Science.

DESCRIPTION

Any disruption in mitochondrial dynamics can contribute to cellular dysfunction and diseases, including cancer, cardiac conditions, and neurodegeneration. Flavonoids have been shown to modulate mitochondrial dynamics by regulating protein expression involved in fission and fusion events. Furthermore, flavonoids exhibit potent antioxidant properties by lowering the production of ROS and boosting the performance of antioxidant enzymes. Persistent inflammation is a characteristic of many different disorders. This is because flavonoids also alter the inflammatory response by controlling the expression of numerous cytokines and chemokines involved in the inflammatory process. Flavonoids exhibit an impressive array of significant health effects, making them an effective therapeutic agent for managing various disorders. Further this review summarised available mechanisms underlying flavonoids' actions on mitochondrial dynamics and oxidative stress to recognize the optimal dose and duration of flavonoid intake for therapeutic purposes.

CONCLUSION

This review may provide a solid foundation for developing targeted therapeutic interventions utilizing flavonoids, ultimately benefiting individuals afflicted with various disorders.

Effects of feeding restriction on skeletal muscle development and functional analysis of TNNI1 in New Zealand white rabbits

While restricting nutrition can improve diseases related to the digestive tract, excessive restriction of food intake can also lead to malnutrition and delayed physical growth. Therefore, this brings the demand to study the effect and potential mechanism of restricted feeding on skeletal muscle development in rabbits. This study utilized hematoxylin-eosin (HE) staining to detect muscle fiber area which depicted significant reduction in skeletal muscle fiber upon 30% feed restriction (p < 0.05). The control group and 30% feed restricted group showed 615 deferentially expressed genes (DEGs). Through the GO and KEGG functional enrichment analysis demonstrated 28 DEGs related to muscle development. KEGG analysis showed enrichment of pathways including PI3K/Akt signaling pathway, MAPK signaling pathway, and Hedgehog signaling pathway. Further, the full length of troponin I1, slow skeletal type (TNNI1) was cloned. We studied the expression of skeletal muscle differentiation-related genes such as MyoD, Myf5 gene and Desmin. Specifically, the TNNI1 gene overexpression and knockdown studies were conducted. The over-expression of TNNI1 significantly enhanced the expression of the skeletal muscle development-related genes. Contrastingly, the silencing of TNNI1 gene reduced the expression significantly. These findings showed that TNNI1 may be a regulator for regulating the expression of muscle development-related genes.

Emerging roles of the MiT/TFE factors in cancer

The microphthalmia/transcription factor E (MiT/TFE) transcription factors (TFs; TFEB, TFE3, MITF, and TFEC) play a central role in cellular catabolism and quality control and are subject to extensive layers of regulation that influence their localization, stability, and activity. Recent studies have highlighted a broader role for these TFs in driving diverse stress-adaptation pathways, which manifest in a context- and tissue-dependent manner. Several human cancers upregulate the MiT/TFE factors to survive extreme fluctuations in nutrients, energy, and pharmacological challenges. Emerging data suggest that reduced activity of the MiT/TFE factors can also promote tumorigenesis. Here, we outline recent findings relating to novel mechanisms of regulation and activity of MiT/TFE proteins across some of the most aggressive human cancers.

PGC-1α Is a Master Regulator of Mitochondrial Lifecycle and ROS Stress Response

Mitochondria play a major role in ROS production and defense during their life cycle. The transcriptional activator PGC-1α is a key player in the homeostasis of energy metabolism and is therefore closely linked to mitochondrial function. PGC-1α responds to environmental and intracellular conditions and is regulated by SIRT1/3, TFAM, and AMPK, which are also important regulators of mitochondrial biogenesis and function. In this review, we highlight the functions and regulatory mechanisms of PGC-1α within this framework, with a focus on its involvement in the mitochondrial lifecycle and ROS metabolism. As an example, we show the role of PGC-1α in ROS scavenging under inflammatory conditions. Interestingly, PGC-1α and the stress response factor NF-κB, which regulates the immune response, are reciprocally regulated. During inflammation, NF-κB reduces PGC-1α expression and activity. Low PGC-1α activity leads to the downregulation of antioxidant target genes resulting in oxidative stress. Additionally, low PGC-1α levels and concomitant oxidative stress promote NF-κB activity, which exacerbates the inflammatory response.

Induction of lysosomal and mitochondrial biogenesis by AMPK phosphorylation of FNIP1

Cells respond to mitochondrial poisons with rapid activation of the adenosine monophosphate-activated protein kinase (AMPK), causing acute metabolic changes through phosphorylation and prolonged adaptation of metabolism through transcriptional effects. Transcription factor EB (TFEB) is a major effector of AMPK that increases expression of lysosome genes in response to energetic stress, but how AMPK activates TFEB remains unresolved. We demonstrate that AMPK directly phosphorylates five conserved serine residues in folliculin-interacting protein 1 (FNIP1), suppressing the function of the folliculin (FLCN)-FNIP1 complex. FNIP1 phosphorylation is required for AMPK to induce nuclear translocation of TFEB and TFEB-dependent increases of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) and estrogen-related receptor alpha (ERRα) messenger RNAs. Thus, mitochondrial damage triggers AMPK-FNIP1-dependent nuclear translocation of TFEB, inducing sequential waves of lysosomal and mitochondrial biogenesis.

Emerging roles of TFE3 in metabolic regulation

TFE3 is a member of the MiT family of the bHLH-leucine zipper transcription factor. We previously focused on the role of TFE3 in autophagy and cancer. Recently, an increasing number of studies have revealed that TFE3 plays an important role in metabolic regulation. TFE3 participates in the metabolism of energy in the body by regulating pathways such as glucose and lipid metabolism, mitochondrial metabolism, and autophagy. This review summarizes and discusses the specific regulatory mechanisms of TFE3 in metabolism. We determined both the direct regulation of TFE3 on metabolically active cells, such as hepatocytes and skeletal muscle cells, and the indirect regulation of TFE3 through mitochondrial quality control and the autophagy-lysosome pathway. The role of TFE3 in tumor cell metabolism is also summarized in this review. Understanding the diverse roles of TFE3 in metabolic processes can provide new avenues for the treatment of some metabolism-related disorders.

TFEB regulates sulfur amino acid and coenzyme A metabolism to support hepatic metabolic adaptation and redox homeostasis

Fatty liver is a highly heterogenous condition driven by various pathogenic factors in addition to the severity of steatosis. Protein insufficiency has been causally linked to fatty liver with incompletely defined mechanisms. Here we report that fatty liver is a sulfur amino acid insufficient state that promotes metabolic inflexibility via limiting coenzyme A availability. We demonstrate that the nutrient-sensing transcriptional factor EB synergistically stimulates lysosome proteolysis and methionine adenosyltransferase to increase cysteine pool that drives the production of coenzyme A and glutathione, which support metabolic adaptation and antioxidant defense during increased lipid influx. Intriguingly, mice consuming an isocaloric protein-deficient Western diet exhibit selective hepatic cysteine, coenzyme A and glutathione deficiency and acylcarnitine accumulation, which are reversed by cystine supplementation without normalizing dietary protein intake. These findings support a pathogenic link of dysregulated sulfur amino acid metabolism to metabolic inflexibility that underlies both overnutrition and protein malnutrition-associated fatty liver development.

Crosstalk of organelles in Parkinson's disease - MiT family transcription factors as central players in signaling pathways connecting mitochondria and lysosomes

Living organisms constantly need to adapt to their surrounding environment and have evolved sophisticated mechanisms to deal with stress. Mitochondria and lysosomes are central organelles in the response to energy and nutrient availability within a cell and act through interconnected mechanisms. However, when such processes become overwhelmed, it can lead to pathologies. Parkinson's disease (PD) is a common neurodegenerative disorder (NDD) characterized by proteinaceous intracellular inclusions and progressive loss of dopaminergic neurons, which causes motor and non-motor symptoms. Genetic and environmental factors may contribute to the disease etiology. Mitochondrial dysfunction has long been recognized as a hallmark of PD pathogenesis, and several aspects of mitochondrial biology are impaired in PD patients and models. In addition, defects of the autophagy-lysosomal pathway have extensively been observed in cell and animal models as well as PD patients' brains, where constitutive autophagy is indispensable for adaptation to stress and energy deficiency. Genetic and molecular studies have shown that the functions of mitochondria and lysosomal compartments are tightly linked and influence each other. Connections between these organelles are constituted among others by mitophagy, organellar dynamics and cellular signaling cascades, such as calcium (Ca²⁺) and mTOR (mammalian target of rapamycin) signaling and the activation of transcription factors. Members of the Microphthalmia-associated transcription factor family (MiT), including MITF, TFE3 and TFEB, play a central role in regulating cellular homeostasis in response to metabolic pressure and are considered master regulators of lysosomal biogenesis. As such, they are part of the interconnection between mitochondria and lysosome functions and therefore represent attractive targets for therapeutic approaches against NDD, including PD. The activation of MiT transcription factors through genetic and pharmacological approaches have shown encouraging results at ameliorating PD-related phenotypes in in vitro and in vivo models. In this review, we summarize the relationship between mitochondrial and autophagy-lysosomal functions in the context of PD etiology and focus on the role of the MiT pathway and its potential as pharmacological target against PD.

Folliculin promotes substrate-selective mTORC1 activity by activating RagC to recruit TFE3

Mechanistic target of rapamycin complex I (mTORC1) is central to cellular metabolic regulation. mTORC1 phosphorylates a myriad of substrates, but how different substrate specificity is conferred on mTORC1 by different conditions remains poorly defined. Here, we show how loss of the mTORC1 regulator folliculin (FLCN) renders mTORC1 specifically incompetent to phosphorylate TFE3, a master regulator of lysosome biogenesis, without affecting phosphorylation of other canonical mTORC1 substrates, such as S6 kinase. FLCN is a GTPase-activating protein (GAP) for RagC, a component of the mTORC1 amino acid (AA) sensing pathway, and we show that active RagC is necessary and sufficient to recruit TFE3 onto the lysosomal surface, allowing subsequent phosphorylation of TFE3 by mTORC1. Active mutants of RagC, but not of RagA, rescue both phosphorylation and lysosomal recruitment of TFE3 in the absence of FLCN. These data thus advance the paradigm that mTORC1 substrate specificity is in part conferred by direct recruitment of substrates to the subcellular compartments where mTORC1 resides and identify potential targets for specific modulation of specific branches of the mTOR pathway.

How does the mTORC1 complex, which influences myriad cellular processes, achieve specificity? This study shows that substrate specificity is in part conferred by modulating the recruitment of substrates, in this case the transcription factor TFE3, to mTORC1.

Therapeutic potential of PGC-1α in age-related macular degeneration (AMD) - the involvement of mitochondrial quality control, autophagy, and antioxidant response

INTRODUCTION

Age-related macular degeneration (AMD) is the leading, cause of sight loss in the elderly in the Western world. Most patients remain still without any treatment options. The targeting of Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), a transcription co-factor, is a putative therapy against AMD.

AREAS COVERED

The characteristics of AMD and their possible connection with PGC-1α as well as the transcriptional and post-transcriptional control of PGC-1α are discussed. The PGC-1α-driven control of mitochondrial functions, and its involvement in autophagy and antioxidant responses are also examined. Therapeutic possibilities via drugs and epigenetic approaches to enhance PGC-1α expression are discussed. Authors conducted a search of literature mainly from the recent decade from the PubMed database.

EXPERT OPINION

Therapy options in AMD could include PGC-1α activation or stabilization. This could be achieved by a direct elevation of PGC-1α activity, a stabilization or modification of its upstream activators and inhibitors by chemical compounds, like 5-Aminoimidazole-4-carboxamide riboside, metformin, and resveratrol. Furthermore, manipulations with epigenetic modifiers of PGC-1α expression, including miRNAs, e.g. miR-204, are considered. A therapy aimed at PGC-1α up-regulation may be possible in other disorders besides AMD, if they are associated with disturbances in the mitochondria-antioxidant response-autophagy axis.

TFEB deficiency attenuates mitochondrial degradation upon brown adipose tissue whitening at thermoneutrality

OBJECTIVE

Brown adipose tissue (BAT) thermogenesis offers the potential to improve metabolic health in mice and humans. However, humans predominantly live under thermoneutral conditions, leading to BAT whitening, a reduction in BAT mitochondrial content and metabolic activity. Recent studies have established mitophagy as a major driver of mitochondrial degradation in the whitening of thermogenic brite/beige adipocytes, yet the pathways mediating mitochondrial breakdown in whitening of classical BAT remain largely elusive. The transcription factor EB (TFEB), a master regulator of lysosomal biogenesis and autophagy belonging to the MiT family of transcription factors, is the only member of this family that is upregulated during whitening, pointing toward a role of TFEB in whitening-associated mitochondrial breakdown.

METHODS

We generated brown adipocyte-specific TFEB knockout mice, and induced BAT whitening by thermoneutral housing. We characterized gene and protein expression patterns, BAT metabolic activity, systemic metabolism, and mitochondrial localization using in vivo and in vitro approaches.

RESULTS

Under low thermogenic activation conditions, deletion of TFEB preserves mitochondrial mass independently of mitochondriogenesis in BAT and primary brown adipocytes. However, this does not translate into elevated thermogenic capacity or protection from diet-induced obesity. Autophagosomal/lysosomal marker levels are altered in TFEB-deficient BAT and primary adipocytes, and lysosomal markers co-localize and co-purify with mitochondria in TFEB-deficient BAT, indicating trapping of mitochondria in late stages of mitophagy.

CONCLUSION

We identify TFEB as a driver of BAT whitening, mediating mitochondrial degradation via the autophagosomal and lysosomal machinery. This study provides proof of concept that interfering with the mitochondrial degradation machinery can increase mitochondrial mass in classical BAT under human-relevant conditions. However, it must be considered that interfering with autophagy may result in accumulation of non-functional mitochondria. Future studies targeting earlier steps of mitophagy or target recognition are therefore warranted.

Folliculin: A Regulator of Transcription Through AMPK and mTOR Signaling Pathways

Folliculin (FLCN) is a tumor suppressor gene responsible for the inherited Birt-Hogg-Dubé (BHD) syndrome, which affects kidneys, skin and lungs. FLCN is a highly conserved protein that forms a complex with folliculin interacting proteins 1 and 2 (FNIP1/2). Although its sequence does not show homology to known functional domains, structural studies have determined a role of FLCN as a GTPase activating protein (GAP) for small GTPases such as Rag GTPases. FLCN GAP activity on the Rags is required for the recruitment of mTORC1 and the transcriptional factors TFEB and TFE3 on the lysosome, where mTORC1 phosphorylates and inactivates these factors. TFEB/TFE3 are master regulators of lysosomal biogenesis and function, and autophagy. By this mechanism, FLCN/FNIP complex participates in the control of metabolic processes. AMPK, a key regulator of catabolism, interacts with FLCN/FNIP complex. FLCN loss results in constitutive activation of AMPK, which suggests an additional mechanism by which FLCN/FNIP may control metabolism. AMPK regulates the expression and activity of the transcriptional cofactors PGC1α/β, implicated in the control of mitochondrial biogenesis and oxidative metabolism. In this review, we summarize our current knowledge of the interplay between mTORC1, FLCN/FNIP, and AMPK and their implications in the control of cellular homeostasis through the transcriptional activity of TFEB/TFE3 and PGC1α/β. Other pathways and cellular processes regulated by FLCN will be briefly discussed.

Exercise Is Muscle Mitochondrial Medicine

Exercise stimulates the biogenesis of mitochondria in muscle. Some literature supports the use of pharmaceuticals to enhance mitochondria as a substitute for exercise. We provide evidence that exercise rejuvenates mitochondrial function, thereby augmenting muscle health with age, in disease, and in the absence of cellular regulators. This illustrates the power of exercise to act as mitochondrial medicine in muscle.

MiT/TFE Family of Transcription Factors: An Evolutionary Perspective

Response and adaptation to stress are critical for the survival of all living organisms. The regulation of the transcriptional machinery is an important aspect of these complex processes. The members of the microphthalmia (MiT/TFE) family of transcription factors, apart from their involvement in melanocyte biology, are emerging as key players in a wide range of cellular functions in response to a plethora of internal and external stresses. The MiT/TFE proteins are structurally related and conserved through evolution. Their tissue expression and activities are highly regulated by alternative splicing, promoter usage, and posttranslational modifications. Here, we summarize the functions of MiT/TFE proteins as master transcriptional regulators across evolution and discuss the contribution of animal models to our understanding of the various roles of these transcription factors. We also highlight the importance of deciphering transcriptional regulatory mechanisms in the quest for potential therapeutic targets for human diseases, such as lysosomal storage disorders, neurodegeneration, and cancer.

Transcription factor EB and TFE3: new metabolic coordinators mediating adaptive responses to exercise in skeletal muscle?

In response to the increased energy demands of contractions, skeletal muscle adapts remarkably well through acutely regulating metabolic pathways to maintain energy balance and in the longer term by regulating metabolic reprogramming, such as remodeling and expanding the mitochondrial network. This long-term adaptive response involves modulation of gene expression at least partly through the regulation of specific transcription factors and transcriptional coactivators. The AMPK-peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α) pathway has long been known to orchestrate contraction-mediated adaptive responses, although AMPK- and PGC1α-independent pathways have also been proposed. Transcription factor EB (TFEB) and TFE3, known as important regulators of lysosomal biogenesis and autophagic processes, have emerged as new metabolic coordinators. The activity of TFEB/TFE3 is regulated through posttranslational modifications (i.e., phosphorylation) and spatial organization. Under nutrient and energy stress, TFEB and TFE3 are dephosphorylated and translocate to the nucleus, where they activate transcription of their target genes. It has recently been reported that exercise promotes nuclear translocation and activation of TFEB/TFE3 in mouse skeletal muscle through the Ca²⁺-stimulated protein phosphatase calcineurin. Skeletal muscle-specific ablation of TFEB exhibits impaired glucose homeostasis and mitochondrial biogenesis with reduced metabolic flexibility during exercise, and global TFE3 depletion results in diminished endurance and abolished exercise-induced metabolic benefits. Transcriptomic analysis of the muscle-specific TFEB-null mice has demonstrated that TFEB regulates the expression of genes involved in glucose metabolism and mitochondrial homeostasis. This review aims to summarize and discuss emerging roles for TFEB/TFE3 in metabolic and adaptive responses to exercise and contractile activity in skeletal muscle.

Dual role of G-quadruplex in translocation renal cell carcinoma: Exploring plausible Cancer therapeutic innovation

BACKGROUND

Renal Cell Carcinoma (RCC) is the ninth leading cause of death among kidney cancer. Xp11.2 translocation harboring TFE3 fusion proteins, act as an oncogene in translocation cancers that constitute the hallmark of translocation renal cell carcinoma (tRCC). G-quadruplex (G4), an alternative nucleic acid structure is an emerging and promising factor in cancer. The presence of G4 within the genome plays a pioneering role in cancer as it contributes to genomic aberration as well as inhibition in cell proliferation.

SCOPE OF REVIEW

Here we discuss the link between G4 and tRCC. We compile the available information of G-quadruplex & propose their dual role in tRCC, suggesting both stabilization and destabilization of G-quadruplex could be considered targets for tRCC.

MAJOR CONCLUSIONS

Our in Silico analysis of TFE3 and their three fusions partner's PRCC, SFPQ, and ASPSCR1 discloses a few putative G4 forming sequences (PQS) in their corresponding fusion gene or fusion transcript. Stabilization of G4 structure within fusion gene/transcript can be of great use towards potential therapeutics targeting fusion protein derived oncogenesis, as G4 is a serious menace for DNA polymerization, transcription & translation. G-quadruplex at intron-2 of the TFE3 has been reported to mediate its translocation also. Both stabilization and destabilization of the G4 structure would be a promising approach in the suppression of cancerous cell proliferation.

GENERAL SIGNIFICANCE

Pioneering studies discovered the relevance of G4 in cancer therapy and explore our approaches towards therapeutic innovation against oncogenic fusion protein and tRCC. Selectively targeting G4 in oncogenic fusion transcript will emerge as potential druggable structures.

Flavonoids and Mitochondria: Activation of Cytoprotective Pathways?

A large number of diverse mechanisms that lead to cytoprotection have been described to date. Perhaps, not surprisingly, the role of mitochondria in these phenomena is notable. In addition to being metabolic centers, due to their role in cell catabolism, ATP synthesis, and biosynthesis these organelles are triggers and/or end-effectors of a large number of signaling pathways. Their role in the regulation of the intrinsic apoptotic pathway, calcium homeostasis, and reactive oxygen species signaling is well documented. In this review, we aim to characterize the prospects of influencing cytoprotective mitochondrial signaling routes by natural substances of plant origin, namely, flavonoids (e.g., flavanones, flavones, flavonols, flavan-3-ols, anthocyanidins, and isoflavones). Flavonoids are a family of widely distributed plant secondary metabolites known for their beneficial effects on human health and are widely applied in traditional medicine. Their pharmacological characteristics include antioxidative, anticarcinogenic, anti-inflammatory, antibacterial, and antidiabetic properties. Here, we focus on presenting mitochondria-mediated cytoprotection against various insults. Thus, the role of flavonoids as antioxidants and modulators of antioxidant cellular response, apoptosis, mitochondrial biogenesis, autophagy, and fission and fusion is reported. Finally, an emerging field of flavonoid-mediated changes in the activity of mitochondrial ion channels and their role in cytoprotection is outlined.

Regulation of PGC-1α expression by a GSK-3β-TFEB signaling axis in skeletal muscle

OBJECTIVE

In muscle cells, the peroxisome proliferator-activated receptor γ co-activator 1 (PGC-1) signaling network, which has been shown to be disturbed in the skeletal muscle in several chronic diseases, tightly controls mitochondrial biogenesis and oxidative substrate metabolism. Previously, we showed that inactivation of glycogen synthase kinase (GSK)-3β potently increased Pgc-1α abundance and oxidative metabolism in skeletal muscle cells. The current study aims to unravel the molecular mechanism driving the increase in Pgc-1α mediated by GSK-3β inactivation.

METHODS

GSK-3β was inactivated genetically or pharmacologically in C2C12 myotubes and the requirement of transcription factors known to be involved in Pgc-1α transcription for increases in Pgc-1α abundance mediated by inactivation of GSK-3β was examined.

RESULTS

Enhanced PGC-1α promoter activation after GSK-3β inhibition suggested a transcriptionally-controlled mechanism. While myocyte enhancer factor (MEF)2 transcriptional activity was unaltered, GSK-3β inactivation increased the abundance and activity of the transcription factors estrogen-related receptor (ERR)α and ERRγ. Pharmacological inhibition or knock-down of ERRα and ERRγ however failed to prevent increases in Pgc-1α mRNA mediated by GSK-3β inactivation. Interestingly, GSK-3β inactivation activated transcription factor EB (TFEB), evidenced by decreased phosphorylation and enhanced nuclear localization of the TFEB protein. Moreover, knock-down of TFEB completely prevented increases in Pgc-1α gene expression, PGC-1α promoter activity and PGC-1α protein abundance induced by GSK-3β inactivation. Furthermore, mutation of a specific TFEB binding site on the PGC-1α promoter blocked promoter activation upon inhibition of GSK-3β.

CONCLUSIONS

In skeletal muscle, GSK-3β inactivation causes dephosphorylation and nuclear translocation of TFEB resulting in TFEB-dependent induction of Pgc-1α expression.

Auranofin Mediates Mitochondrial Dysregulation and Inflammatory Cell Death in Human Retinal Pigment Epithelial Cells: Implications of Retinal Neurodegenerative Diseases

Purpose

Photoreceptor degeneration occurs in various retinal diseases including age-related macular degeneration (AMD), Retinitis pigmentosa (RP), and diabetic retinopathy (DR). However, molecular mechanisms are not fully understood yet. The retinal pigment epithelium (RPE) forms the outer blood retinal barrier (oBRB) and supplies glucose, oxygen and nutrients from the fenestrated choriocapillaris to photoreceptors for visual function. Therefore, RPE dysfunction leads to photoreceptor injury/death and progression of blinding eye diseases. This study aims to understand the role of the thioredoxin (Trx) and its reductase (TrxR) redox signaling in human RPE dysfunction and cell death mechanism(s) in an in vitro system.

Methods

A human RPE cell line (APRE-19) was cultured in DMEM/F12 medium and treated with auranofin (AF - 4 μM, an inhibitor of TrxR) for 4 and 24 h. Mitochondrial and lysosomal function, cellular oxidative stress and NLRP3 inflammasome activity were measured using cell assays, Western blotting, and confocal microscopy. Antioxidants and anti-inflammatory compounds were tested for blocking AF effects on RPE damage. Cell death mechanisms (LDH release to culture media) were determined using necroptosis, ferroptosis and pyroptosis inhibitors. P < 0.05 was considered significant in statistical analysis.

Results

Auranofin causes mitochondrial dysfunction (Δψm↓ and ATP↓), oxidative stress (H₂O₂↑) and mitophagic flux to lysosomes. Furthermore, the lysosomal enzyme (cathepsin L) activity is reduced while that of pro-inflammatory caspase-1 (NLRP3 inflammasome) is enhanced in ARPE-19. These effects of AF on ARPE-19 are inhibited by antioxidant N-acetylcysteine (5 mM, NAC) and significantly by a combination of SS31 (mitochondrial antioxidant) and anti-inflammatory drugs (amlexanox and tranilast). AF also causes cell death as measured by cytosolic LDH release/leakage, which is not inhibited by either ferrostatin-1 or necrostatin-1 (ferroptosis and necroptosis inhibitors, respectively). Conversely, AF-induced LDH release is significantly reduced by MCC950 and Ac-YVAD-cmk (NLRP3 and Caspase-1 inhibitors, respectively), suggesting a pro-inflammatory cell death by pyroptosis.

Conclusion

The Trx/TrxR redox system is critical for RPE function and viability. We previously showed that thioredoxin-interacting protein (TXNIP) is strongly induced in DR inhibiting the Trx/TrxR system and RPE dysfunction. Therefore, our results suggest that the TXNIP-Trx-TrxR redox pathway may participate in RPE dysfunction in DR and other retinal neurodegenerative diseases.

Cellular and molecular mechanisms of sarcopenia: the S100B perspective

Primary sarcopenia is a condition of reduced skeletal muscle mass and strength, reduced agility, and increased fatigability and risk of bone fractures characteristic of aged, otherwise healthy people. The pathogenesis of primary sarcopenia is not completely understood. Herein, we review the essentials of the cellular and molecular mechanisms of skeletal mass maintenance; the alterations of myofiber metabolism and deranged properties of muscle satellite cells (the adult stem cells of skeletal muscles) that underpin the pathophysiology of primary sarcopenia; the role of the Ca²⁺ -sensor protein, S100B, as an intracellular factor and an extracellular signal regulating cell functions; and the functional role of S100B in muscle tissue. Lastly, building on recent results pointing to S100B as to a molecular determinant of myoblast-brown adipocyte transition, we propose S100B as a transducer of the deleterious effects of accumulation of reactive oxygen species in myoblasts and, potentially, myofibers concurring to the pathophysiology of sarcopenia.

Folliculin Regulates Osteoclastogenesis Through Metabolic Regulation

Osteoclast differentiation is a dynamic differentiation process, which is accompanied by dramatic changes in metabolic status as well as in gene expression. Recent findings have revealed an essential connection between metabolic reprogramming and dynamic gene expression changes during osteoclast differentiation. However, the upstream regulatory mechanisms that drive these metabolic changes in osteoclastogenesis remain to be elucidated. Here, we demonstrate that induced deletion of a tumor suppressor gene, Folliculin (Flcn), in mouse osteoclast precursors causes severe osteoporosis in 3 weeks through excess osteoclastogenesis. Flcn-deficient osteoclast precursors reveal cell autonomous accelerated osteoclastogenesis with increased sensitivity to receptor activator of NF-κB ligand (RANKL). We demonstrate that Flcn regulates oxidative phosphorylation and purine metabolism through suppression of nuclear localization of the transcription factor Tfe3, thereby inhibiting expression of its target gene Pgc1. Metabolome studies revealed that Flcn-deficient osteoclast precursors exhibit significant augmentation of oxidative phosphorylation and nucleotide production, resulting in an enhanced purinergic signaling loop that is composed of controlled ATP release and autocrine/paracrine purinergic receptor stimulation. Inhibition of this purinergic signaling loop efficiently blocks accelerated osteoclastogenesis in Flcn-deficient osteoclast precursors. Here, we demonstrate an essential and novel role of the Flcn-Tfe3-Pgc1 axis in osteoclastogenesis through the metabolic reprogramming of oxidative phosphorylation and purine metabolism. © 2018 The Authors Journal of Bone and Mineral Research published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research (ASBMR).

Emerging roles and regulation of MiT/TFE transcriptional factors

The MiT/TFE transcription factors play a pivotal role in the regulation of autophagy and lysosomal biogenesis. The subcellular localization and activity of MiT/TFE proteins are primarily regulated through phosphorylation. And the phosphorylated protein is retained in the cytoplasm and subsequently translocates to the nucleus upon dephosphorylation, where it stimulates the expression of hundreds of genes, leading to lysosomal biogenesis and autophagy induction. The transcription factor-mediated lysosome-to-nucleus signaling can be directly controlled by several signaling molecules involved in the mTORC1, PKC, and AKT pathways. MiT/TFE family members have attracted much attention owing to their intracellular clearance of pathogenic factors in numerous diseases. Recently, multiple studies have also revealed the MiT/TFE proteins as master regulators of cellular metabolic reprogramming, converging on autophagic and lysosomal function and playing a critical role in cancer, suggesting that novel therapeutic strategies could be based on the modulation of MiT/TFE family member activity. Here, we present an overview of the latest research on MiT/TFE transcriptional factors and their potential mechanisms in cancer.

TFE3 regulates whole-body energy metabolism in cooperation with TFEB

TFE3 and TFEB are members of the MiT family of HLH–leucine zipper transcription factors. Recent studies demonstrated that they bind overlapping sets of promoters and are post‐transcriptionally regulated through a similar mechanism. However, while Tcfeb knockout (KO) mice die during early embryonic development, no apparent phenotype was reported in Tfe3 KO mice. Thus raising the need to characterize the physiological role of TFE3 and elucidate its relationship with TFEB. TFE3 deficiency resulted in altered mitochondrial morphology and function both in vitro and in vivo due to compromised mitochondrial dynamics. In addition, Tfe3 KO mice showed significant abnormalities in energy balance and alterations in systemic glucose and lipid metabolism, resulting in enhanced diet‐induced obesity and diabetes. Conversely, viral‐mediated TFE3 overexpression improved the metabolic abnormalities induced by high‐fat diet (HFD). Both TFEB overexpression in Tfe3 KO mice and TFE3 overexpression in Tcfeb liver‐specific KO mice (Tcfeb LiKO) rescued HFD‐induced obesity, indicating that TFEB can compensate for TFE3 deficiency and vice versa. Analysis of Tcfeb LiKO/Tfe3 double KO mice demonstrated that depletion of both TFE3 and TFEB results in additive effects with an exacerbation of the hepatic phenotype. These data indicate that TFE3 and TFEB play a cooperative, rather than redundant, role in the control of the adaptive response of whole‐body metabolism to environmental cues such as diet and physical exercise.

TFEB and TFE3: The art of multi-tasking under stress conditions

Cellular adaptation response to a myriad of stressors is key for survival. The lysosomal/autophagy pathway is inextricably connected to the stress response regulation. Two transcription factors, TFEB and TFE3, have recently emerged as master regulators of this degradative pathway. Their function modulating different cellular pathways will be discussed.

The tumor suppressor FLCN mediates an alternate mTOR pathway to regulate browning of adipose tissue

Noncanonical mechanistic target of rapamycin (mTOR) pathways remain poorly understood. Mutations in the tumor suppressor folliculin (FLCN) cause Birt-Hogg-Dubé syndrome, a hamartomatous disease marked by mitochondria-rich kidney tumors. FLCN functionally interacts with mTOR and is expressed in most tissues, but its role in fat has not been explored. We show here that FLCN regulates adipose tissue browning via mTOR and the transcription factor TFE3. Adipose-specific deletion of FLCN relieves mTOR-dependent cytoplasmic retention of TFE3, leading to direct induction of the PGC-1 transcriptional coactivators, drivers of mitochondrial biogenesis and the browning program. Cytoplasmic retention of TFE3 by mTOR is sensitive to ambient amino acids, is independent of growth factor and tuberous sclerosis complex (TSC) signaling, is driven by RagC/D, and is separable from canonical mTOR signaling to S6K. Codeletion of TFE3 in adipose-specific FLCN knockout animals rescues adipose tissue browning, as does codeletion of PGC-1β. Conversely, inducible expression of PGC-1β in white adipose tissue is sufficient to induce beige fat gene expression in vivo. These data thus unveil a novel FLCN-mTOR-TFE3-PGC-1β pathway-separate from the canonical TSC-mTOR-S6K pathway-that regulates browning of adipose tissue.

Mitochondrial biogenesis and clearance: a balancing act

Mitochondria are semi-autonomous organelles of prokaryotic origin that are postulated to have been acquired by eukaryotic cells through an early endosymbiotic event. Except for their main role in energy production, they are also implicated in fundamental cellular processes, including ion homeostasis, lipid metabolism, and initiation of apoptotic cell death. Perturbed mitochondrial function has been correlated with severe human pathologies such as type-2 diabetes, cardiovascular, and neurodegenerative diseases. Thus, proper mitochondrial physiology is a prerequisite for health and survival. Cells have developed sophisticated and elaborate mechanisms to adapt to stress conditions and alterations in metabolic demands, by regulating mitochondrial number and function. Hence, the generation of new and the removal of damaged or unwanted mitochondria are highly regulated processes that need to be accurately coordinated for the maintenance of mitochondrial and cellular homeostasis. Here, we survey recent research findings that advance our understanding and highlight the importance of the underlying molecular mechanisms.

TFEB and TFE3: Linking Lysosomes to Cellular Adaptation to Stress

In recent years, our vision of lysosomes has drastically changed. Formerly considered to be mere degradative compartments, they are now recognized as key players in many cellular processes. The ability of lysosomes to respond to different stimuli revealed a complex and coordinated regulation of lysosomal gene expression. This review discusses the participation of the transcription factors TFEB and TFE3 in the regulation of lysosomal function and biogenesis, as well as the role of the lysosomal pathway in cellular adaptation to a variety of stress conditions, including nutrient deprivation, mitochondrial dysfunction, protein misfolding, and pathogen infection. We also describe how cancer cells make use of TFEB and TFE3 to promote their own survival and highlight the potential of these transcription factors as therapeutic targets for the treatment of neurological and lysosomal diseases.

Transcription factor EB: from master coordinator of lysosomal pathways to candidate therapeutic target in degenerative storage diseases

The lysosome is the main catabolic hub of the cell. Owing to its role in fundamental processes such as autophagy, plasma membrane repair, mTOR signaling, and maintenance of cellular homeostasis, the lysosome has a profound influence on cellular metabolism and human health. Indeed, inefficient or impaired lysosomal function has been implicated in the pathogenesis of a number of degenerative diseases affecting various organs and tissues, most notably the brain, liver, and muscle. The discovery of the coordinated lysosomal expression and regulation (CLEAR) genetic program and its master controller, transcription factor EB (TFEB), has provided an unprecedented tool to study and manipulate lysosomal function. Most lysosome-based processes-including macromolecule degradation, autophagy, lysosomal exocytosis, and proteostasis-are under the transcriptional control of TFEB. Interestingly, impaired TFEB signaling has been suggested to be a contributing factor in the pathogenesis of several degenerative storage diseases. Preclinical studies based on TFEB exogenous expression to reinstate TFEB activity or promote CLEAR network-based lysosomal enhancement have highlighted TFEB as a candidate therapeutic target for the treatment of various degenerative storage diseases.