Adenosine-containing molecules amplify glucose signaling and enhance txnip expression

The Function of MondoA and ChREBP Nutrient-Sensing Factors in Metabolic Disease

Obesity is a major global public health concern associated with an increased risk of many health problems, including type 2 diabetes, heart disease, stroke, and some types of cancer. Obesity is also a critical factor in the development of insulin resistance and type 2 diabetes. Insulin resistance is associated with metabolic inflexibility, which interferes with the body's ability to switch from free fatty acids to carbohydrate substrates, as well as with the ectopic accumulation of triglycerides in non-adipose tissue, such as that of skeletal muscle, the liver, heart, and pancreas. Recent studies have demonstrated that MondoA (MLX-interacting protein or MLXIP) and the carbohydrate response element-binding protein (ChREBP, also known as MLXIPL and MondoB) play crucial roles in the regulation of nutrient metabolism and energy homeostasis in the body. This review summarizes recent advances in elucidating the function of MondoA and ChREBP in insulin resistance and related pathological conditions. This review provides an overview of the mechanisms by which MondoA and ChREBP transcription factors regulate glucose and lipid metabolism in metabolically active organs. Understanding the underlying mechanism of MondoA and ChREBP in insulin resistance and obesity can foster the development of new therapeutic strategies for treating metabolic diseases.

Normal and Neoplastic Growth Suppression by the Extended Myc Network

Among the first discovered and most prominent cellular oncogenes is MYC, which encodes a bHLH-ZIP transcription factor (Myc) that both activates and suppresses numerous genes involved in proliferation, energy production, metabolism and translation. Myc belongs to a small group of bHLH-ZIP transcriptional regulators (the Myc Network) that includes its obligate heterodimerization partner Max and six "Mxd proteins" (Mxd1-4, Mnt and Mga), each of which heterodimerizes with Max and largely opposes Myc's functions. More recently, a second group of bHLH-ZIP proteins (the Mlx Network) has emerged that bears many parallels with the Myc Network. It is comprised of the Myc-like factors ChREBP and MondoA, which, in association with the Max-like member Mlx, regulate smaller and more functionally restricted repertoires of target genes, some of which are shared with Myc. Opposing ChREBP and MondoA are heterodimers comprised of Mlx and Mxd1, Mxd4 and Mnt, which also structurally and operationally link the two Networks. We discuss here the functions of these "Extended Myc Network" members, with particular emphasis on their roles in suppressing normal and neoplastic growth. These roles are complex due to the temporal- and tissue-restricted expression of Extended Myc Network proteins in normal cells, their regulation of both common and unique target genes and, in some cases, their functional redundancy.

The Role of Mondo Family Transcription Factors in Nutrient-Sensing and Obesity

In the past several decades obesity has become one of the greatest health burdens worldwide. Diet high in fats and fructose is one of the main causes for the prevalence of metabolic disorders including obesity. Promoting brown or beige adipocyte development and activity is regarded as a potential treatment of obesity. Mondo family transcription factors including MondoA and carbohydrate response element binding protein (ChREBP) are critical for nutrient-sensing in multiple metabolic organs including the skeletal muscle, liver, adipose tissue and pancreas. Under normal nutrient conditions, MondoA and ChREBP contribute to maintaining metabolic homeostasis. When nutrient is overloaded, Mondo family transcription factors directly regulate glucose and lipid metabolism in brown and beige adipocytes or modulate the crosstalk between metabolic organs. In this review, we aim to provide an overview of recent advances in the understanding of MondoA and ChREBP in sensing nutrients and regulating obesity or related pathological conditions.

Thioredoxin-Interacting Protein (TXNIP) with Focus on Brain and Neurodegenerative Diseases

The development of new therapeutic approaches to diseases relies on the identification of key molecular targets involved in amplifying disease processes. One such molecule is thioredoxin-interacting protein (TXNIP), also designated thioredoxin-binding protein-2 (TBP-2), a member of the α-arrestin family of proteins and a central regulator of glucose and lipid metabolism, involved in diabetes-associated vascular endothelial dysfunction and inflammation. TXNIP sequesters reduced thioredoxin (TRX), inhibiting its function, resulting in increased oxidative stress. Many different cellular stress factors regulate TXNIP expression, including high glucose, endoplasmic reticulum stress, free radicals, hypoxia, nitric oxide, insulin, and adenosine-containing molecules. TXNIP is also directly involved in inflammatory activation through its interaction with the nucleotide-binding domain, leucine-rich-containing family, and pyrin domain-containing-3 (NLRP3) inflammasome complex. Neurodegenerative diseases such as Alzheimer’s disease have significant pathologies associated with increased oxidative stress, inflammation, and vascular dysfunctions. In addition, as dysfunctions in glucose and cellular metabolism have been associated with such brain diseases, a role for TXNIP in neurodegeneration has actively been investigated. In this review, we will focus on the current state of the understanding of possible normal and pathological functions of TXNIP in the central nervous system from studies of in vitro neural cells and the brains of humans and experimental animals with reference to other studies. As TXNIP can be expressed by neurons, microglia, astrocytes, and endothelial cells, a complex pattern of regulation and function in the brain is suggested. We will examine data suggesting TXNIP as a therapeutic target for neurodegenerative diseases where further research is needed.

Expression of recombinant classical swine fever virus E2 glycoprotein by endogenous Txnip promoter in stable transgenic CHO cells

As the main immunogen that could stimulate neutralized antibody in pigs, recombinant E2 protein of CSFV was expressed in CHO-dhfr-cells driven by endogenous Txnip promoter from Chinese hamster. Different fragments of Txnip promoter were amplified by PCR from isolated genomic DNA of CHO cells and cloned into different expression vectors. Compared with CMV promoter, CHO-pTxnip-4-rE2 (F12) cell clone with the highest yield of rE2 protein was established by random insertion of the expression cassette driven by 860 bp sequences of Txnip promoter. In combination with treatment of 800 nM MTX for copy amplification of inserted expression cassette, the dynamic expression profile of rE2 protein was observed. Then inducible expression strategy of balance between viable cell density and product yield was conducted by mixed addition of 0.1 mM NADH and 0.1 mM ATP in culture medium at day 3 of batch-wise culture. It could be concluded that Txnip promoter would be a promising alternative promoter for recombinant antigen protein expression in transgenic cells.

Glucose-6-Phosphate Upregulates Txnip Expression by Interacting With MondoA

The major metabolic fates of glucose in cells are glycolysis and the pentose phosphate pathway, and they share the first step: converting glucose to glucose-6-phosphate (G6P). Here, we show that G6P can be sensed by the transcription factor MondoA/Mlx to modulate Txnip expression. Endogenous knockdown and EMSA (gel migration assay) analyses both confirmed that G6P is the metabolic intermediate that activates the heterocomplex MondoA/Mlx to elicit the expression of Txnip. Additionally, the three-dimensional structure of MondoA is modeled, and the binding mode of G6P to MondoA is also predicted by in silico molecular docking and binding free energy calculation. Finally, free energy decomposition and mutational analyses suggest that certain residues in MondoA, GKL139-141 in particular, mediate its binding with G6P to activate MondoA, which signals the upregulation of the expression of Txnip.

Effect of Taurine on Cell Function via TXNIP Induction in Caco-2 Cells

Taurine (2-aminoethanesulfonic acid), a sulfur-containing β-amino acid, is a free amino acid present in high concentrations in mammalian tissues. Taurine has pivotal roles in anti-oxidation, membrane stabilization, osmoregulation, anti-inflammation, and other process. In a DNA microarray analysis, we previously found that taurine markedly increases the mRNA expression of thioredoxin interacting protein (TXNIP) in Caco-2 cells. In this study, we investigated the effect of these taurine-induced changes in TXNIP on the function of Caco-2 cells. We found that taurine decreases glucose uptake in a dose-dependent manner. The taurine-induced decrease in glucose uptake was completely abolished by transfection with siRNA against TXNIP, suggesting that TXNIP is involved in the taurine-induced down-regulation of glucose uptake. We also revealed that taurine induces AMPK activation and further increases the intracellular ATP content in Caco-2 cells. These results suggest that taurine could regulate the function of Caco-2 cells via TXNIP induction, leading to extend our understanding of the functions of taurine.

The Function of Thioredoxin-Binding Protein-2 (TBP-2) in Different Diseases

Thioredoxin-binding protein-2 (TBP-2) has an important role in the redox system, but it plays a different role in many different diseases (e.g., various cancers, diabetes mellitus (DM), cardiovascular disease, and cataracts) by influencing cell proliferation, differentiation, apoptosis, autophagy, and metabolism. Distinct transcription factors (TFs) stimulated by different factors combine with binding sites or proteins to upregulate or downregulate TBP-2 expression, in order to respond to the change in the internal environment. Most research disclosed that the main function of TBP-2 is associating with thioredoxin (Trx) to inhibit the antioxidant capacity of Trx. Furthermore, the TBP-2 located in tissues, whether normal or abnormal, has the ability to cause the dysfunctioning of cells and even death through different pathways, such as shortening the cell cycle and inducing apoptosis or autophagy. Through these studies, we found that TBP-2 promoted the development of diseases which are involved in inflammatory and oxidative damage. To a certain extent, we believe that there is some hidden connection between the biological functions which TBP-2 participates in and some distinct diseases. This review presents only a summary of the roles that TBP-2 plays in cancer, DM, cataracts, and so on, as well as its universal mechanisms. Further investigations are needed for the cell signaling pathways of the effects caused by TBP-2. A greater understanding of the mechanisms of TBP-2 could produce potential new targets for the treatment of diseases, including cancer and diabetes, cardiovascular disease, and cataracts.

Investigation of the influence of high glucose on molecular and genetic responses: an in vitro study using a human intestine model

Background

Dietary glucose consumption has increased worldwide. Long-term high glucose intake contributes to the development of obesity and type 2 diabetes mellitus (T2DM). Obese people tend to eat glucose-containing foods, which can lead to an addiction to glucose, increased glucose levels in the blood and intestine lumen, and exposure of intestinal enterocytes to high dietary glucose. Recent studies have documented a role for enterocytes in glucose sensing. However, the molecular and genetic relationship between high glucose levels and intestinal enterocytes has not been determined. We aimed to identify relevant target genes and molecular pathways regulated by high glucose in a well-established in vitro epithelial cell culture model of the human intestinal system (Caco-2 cells).

Methods

Cells were grown in a medium containing 5.5 and 25 mM glucose in a bicameral culture system for 21 days to mimic the human intestine. Transepithelial electrical resistance was used to control monolayer formation and polarization of the cells. Total RNA was isolated, and genome-wide mRNA expression profiles were determined. Molecular pathways were analyzed using the DAVID bioinformatics program. Gene expression levels were confirmed by quantitative reverse transcription polymerase chain reaction (RT-qPCR).

Results

Microarray gene expression data demonstrated that 679 genes (297 upregulated, 382 downregulated) were affected by high glucose treatment. Bioinformatics analysis indicated that intracellular protein export (p = 0.0069) and ubiquitin-mediated proteolysis (p = 0.024) pathways were induced, whereas glycolysis/gluconeogenesis (p < 0.0001), pentose phosphate (p = 0.0043), and fructose-mannose metabolism (p = 0.013) pathways were downregulated, in response to high glucose. Microarray analysis of gene expression showed that high glucose significantly induced mRNA expression levels of thioredoxin-interacting protein (TXNIP, p = 0.0001) and lipocalin 15 (LCN15, p = 0.0016) and reduced those of ATP-binding cassette, sub-family A member 1 (ABCA1, p = 0.0004), and iroquois homeobox 3 (IRX3, p = 0.0001).

Conclusions

To our knowledge, this is the first investigation of high glucose-regulated molecular responses in an intestinal enterocyte model. Our findings identify new target genes that may be important in the intestinal glucose absorption and metabolism during high glucose consumption.

Expression of TXNIP in Cancer Cells and Regulation by 1,25(OH)₂D₃: Is It Really the Vitamin D₃ Upregulated Protein?

Thioredoxin-interacting protein (TXNIP) was originally identified in HL-60 cells as the vitamin D₃ upregulated protein 1, and is now known to be involved in diverse cellular processes, such as maintenance of glucose homeostasis, redox balance, and apoptosis. Besides the initial characterization, little is known about if and how 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] induces TXNIP expression. We therefore screened multiple cancerous cell lines of different tissue origins, and observed induction, repression, or no change in TXNIP expression in response to 1,25(OH)₂D₃. In-depth analyses on HL-60 cells revealed a rapid and transient increase in TXNIP mRNA levels by 1,25(OH)₂D₃ (3-24 h), followed by a clear reduction at later time points. Furthermore, a strong induction in protein levels was observed only after 96 h of 1,25(OH)₂D₃ treatment. Induction of TXNIP expression by 1,25(OH)₂D₃ was found to be dependent on the availability of glucose in the culture medium, as well as the presence of a functional glucose transport system, indicating an inter-dependence of 1,25(OH)₂D₃ actions and glucose-sensing mechanisms. Moreover, the inhibition of de novo protein synthesis by cycloheximide reduced TXNIP half-life in 24 h, but not in 96 h-1,25(OH)₂D₃-treated HL-60 cells, demonstrating a possible influence of 1,25(OH)₂D₃ on TXNIP stability in long-term treatment.

Phosphorylation of TXNIP by AKT Mediates Acute Influx of Glucose in Response to Insulin

Growth factors, such as insulin, can induce both acute and long-term glucose uptake into cells. Apart from the rapid, insulin-induced fusion of glucose transporter (GLUT)4 storage vesicles with the cell surface that occurs in muscle and adipose tissues, the mechanism behind acute induction has been unclear in other systems. Thioredoxin interacting protein (TXNIP) has been shown to be a negative regulator of cellular glucose uptake. TXNIP is transcriptionally induced by glucose and reduces glucose influx by promoting GLUT1 endocytosis. Here, we report that TXNIP is a direct substrate of protein kinase B (AKT) and is responsible for mediating AKT-dependent acute glucose influx after growth factor stimulation. Furthermore, TXNIP functions as an adaptor for the basal endocytosis of GLUT4 in vivo, its absence allows excess glucose uptake in muscle and adipose tissues, causing hypoglycemia during fasting. Altogether, TXNIP serves as a key node of signal regulation and response for modulating glucose influx through GLUT1 and GLUT4.

Mapping Txnip: Key connexions in progression of diabetic nephropathy

Studies demonstrates the major involvement of inflammatory and apoptotic pathway in the pathophysiology of diabetic nephropathy. The cross talk between inflammatory and apoptotic pathway suggests Txnip as a molecular connexion in progression of disease state. Txnip modulates inflammatory pathway (via ROS production and NLRP3 inflammasome activity) and apoptotic pathway (via mTOR pathway). The key contribution of Txnip in both the pathways, reflects, its crucial role in diabetic nephropathy. In the present review, we have first provided an overview of diabetic nephropathy and Txnip system, followed by the mechanistic insight of Txnip in the progression of diabetic nephropathy. This new mechanistic approach suggests to explore Txnip modulators as a promising therapeutic drug target in diabetic nephropathy.

Physiological, metabolic and transcriptional postnatal phenotypes of in vitro fertilization (IVF) in the mouse

Approximately 1-4% of children today are conceived using assisted reproductive technologies (ARTs), including in vitro fertilization (IVF). IVF is considered safe and the great majority of these children are healthy, yet there is increasing physiological and molecular evidence from animal models that ART is associated with postnatal metabolic and cardiovascular alterations. Understanding the mechanisms underlying these changes and determining whether they have biological significance is of paramount importance for optimizing the design of culture conditions and improving the health of ART children across the life course. In this review, we examine the evidence of molecular changes present in adult tissues of rodent offspring generated by preimplantation manipulation of gametes and embryos. Although embryo manipulation in vitro can induce common transcriptional effects in the blastocyst, transcriptional and metabolomic signatures in adult IVF tissues are largely tissue-specific. However, there is pervasive evidence of oxidative stress and metabolic dysfunction, indicating a lasting effect of IVF on molecular physiology.

The glucose-sensing transcription factor MLX promotes myogenesis via myokine signaling

In this study, Hunt et. al. provide novel insight into the regulation of glucose-induced myogenesis. They demonstrate that changes in glucose levels regulate myogenesis by increasing the activity of the glucose-responsive transcription factor MLX, which is necessary and sufficient for myoblast fusion and differentiation.

Metabolic stress and changes in nutrient levels modulate many aspects of skeletal muscle function during aging and disease. Growth factors and cytokines secreted by skeletal muscle, known as myokines, are important signaling factors, but it is largely unknown whether they modulate muscle growth and differentiation in response to nutrients. Here, we found that changes in glucose levels increase the activity of the glucose-responsive transcription factor MLX (Max-like protein X), which promotes and is necessary for myoblast fusion. MLX promotes myogenesis not via an adjustment of glucose metabolism but rather by inducing the expression of several myokines, including insulin-like growth factor 2 (IGF2), whereas RNAi and dominant-negative MLX reduce IGF2 expression and block myogenesis. This phenotype is rescued by conditioned medium from control muscle cells and by recombinant IGF2, which activates the myogenic kinase Akt. Importantly, MLX-null mice display decreased IGF2 induction and diminished muscle regeneration in response to injury, indicating that the myogenic function of MLX is manifested in vivo. Thus, glucose is a signaling molecule that regulates myogenesis and muscle regeneration via MLX/IGF2/Akt signaling.

New Insight Into Metformin Action: Regulation of ChREBP and FOXO1 Activities in Endothelial Cells

Metformin has been considered a potential adjunctive therapy in treating poorly controlled type 1 diabetes with obesity and insulin resistance, owing to its potent effects on improving insulin sensitivity. However, the underlying mechanism of metformin's vascular protective effects remains obscure. Thioredoxin-interacting protein (TXNIP), a key regulator of cellular redox state induced by high-glucose concentration, decreases thioredoxin reductase activity and mediates apoptosis induced by oxidative stress. Here we report that high glucose-induced endothelial dysfunction is associated with induction of TXNIP expression in primary human aortic endothelial cells exposed to high-glucose conditions, whereas the metformin treatment suppresses high-glucose-induced TXNIP expression at mRNA and protein levels. We further show that metformin decreases the high-glucose-stimulated nuclear entry rate of two transcription factors, carbohydrate response element-binding protein (ChREBP) and forkhead box O1 (FOXO1), as well as their recruitment on the TXNIP promoter. An AMP-activated protein kinase inhibitor partially compromised these metformin effects. Our data suggest that endothelial dysfunction resulting from high-glucose concentrations is associated with TXNIP expression. Metformin down-regulates high-glucose-induced TXNIP transcription by inactivating ChREBP and FOXO1 in endothelial cells, partially through AMP-activated protein kinase activation.

Nitrosative/oxidative stress conditions regulate thioredoxin-interacting protein (TXNIP) expression and thioredoxin-1 (TRX-1) nuclear localization

Thioredoxin (TRX-1) is a multifunctional protein that controls the redox status of other proteins. TRX-1 can be found in the extracellular milieu, cytoplasm and nucleus, and it has distinct functions in each environment. Previously, we studied the intracellular localization of TRX-1 and its relationship with the activation of the p21Ras - ERK1/2 MAP Kinases signaling pathway. In situations where this pathway was activated by stress conditions evoked by a nitrosothiol, S-nitroso-N-acetylpenicillamine (SNAP), TRX-1 accumulated in the nuclear compartment due to nitrosylation of p21Ras and activation of downstream ERK1/2 MAP kinases. Presently, we demonstrate that ERK1/2 MAP Kinases activation and spatial distribution within cells trigger TRX-1 nuclear translocation through down-regulation of the physiological inhibitor of TRX-1, Thioredoxin Interacting Protein (TXNIP). Once activated by the oxidants, SNAP and H₂O₂, the ERK1/2 MAP kinases migrate to the nucleus. This is correlated with down-regulation of TXNIP. In the presence of the MEK inhibitors (PD98059 or UO126), or in cells transfected with the Protein Enriched in Astrocytes (PEA-15), a cytoplasmic anchor of ERK1/2 MAP kinases, TRX-1 nuclear migration and TXNIP down-regulation are no longer observed in cells exposed to oxidants. On the other hand, over-expression of TXNIP abolishes nuclear migration of TRX-1 under nitrosative/oxidative stress conditions, whereas gene silencing of TXNIP facilitates nuclear migration even in the absence of stress conditions. Studies based on the TXNIP promoter support this regulation. In conclusion, changes in TRX-1 compartmentalization under nitrosative/oxidative stress conditions are dependent on the expression levels of TXNIP, which are regulated by cellular compartmentalization and activation of the ERK1/2 MAP kinases.

The synergism of MCL1 and glycolysis on pediatric acute lymphoblastic leukemia cell survival and prednisolone resistance

In vitro and in vivo resistance to prednisolone are predictive for an adverse prognosis in pediatric precursor B-acute lymphoblastic leukemia. Causes of resistance are still poorly understood. In this study, we observed that prednisolone exposure of prednisolone-sensitive patients' leukemic cells decreased anti-apoptotic MCL1 protein levels by 2.9-fold, while MCL1 protein expression in prednisolone-resistant leukemic patients' cells was unaffected (P<0.01). Locked nucleic acid oligonucleotides directed against MCL1 reduced MCL1 protein levels by 82±16% (P<0.05) in leukemic cells, decreased proliferation by 9-fold and sensitized to prednisolone up to 80.8-fold, compared to a non-silencing-control locked nucleic acid (P<0.05). Remarkably, we discovered that MCL1-silencing up-regulated the glucose consumption of leukemic cells by 2.5-fold (P<0.05), suggesting a potential rescue mechanism mediated by glycolysis. Targeting glycolysis by 2-deoxyglucose synergistically inhibited leukemic survival by 23.2-fold in MCL1-silenced cells (P<0.05). Moreover, 2-deoxyglucose and MCL1 locked nucleic acid concomitantly sensitized leukemic cells to prednisolone compared to MCL1 locked nucleic acid or 2-deoxyglucose alone (P<0.05). In conclusion, these results indicate the need to target both MCL1 and glycolysis simultaneously to inhibit leukemic survival and sensitize acute leukemia patients towards prednisolone.

MondoA senses adenine nucleotides: transcriptional induction of thioredoxin-interacting protein

The MondoA-Mlx transcription complex plays a pivotal role in glucose homoeostasis by activating target gene expression in response to G6P (glucose 6-phosphate), the first reaction intermediate in glycolysis. TXNIP (thioredoxin-interacting protein) is a direct and glucose-responsive target of MondoA that triggers a negative-feedback loop by restricting glucose uptake when G6P levels increase. We show in the present study that TXNIP expression is also activated by AICAR (5-amino-4-imidazolecarboxamide ribofuranoside) and adenosine. Using pharmacological inhibitors and genetic knockdowns of purine metabolic enzymes, we establish that TXNIP induction by AICAR and adenosine requires their cellular uptake and metabolism to adenine nucleotides. AICAR induction of TXNIP depended on MondoA, but was independent of AMPK (AMP-activated protein kinase) activation and calcium. The findings of the present study have two important implications. First, in addition to activating AMPK, AICAR may have AMPK-independent effects on gene expression by regulating MondoA-Mlx activity following its flux into the adenine nucleotide pool. Secondly, MondoA-Mlx complexes sense elevated levels of G6P and adenine nucleotides to trigger a TXNIP-dependent feedback inhibition of glycolysis. We propose that this mechanism serves as a checkpoint to restore metabolic homoeostasis.

Thioredoxin and thioredoxin target proteins: from molecular mechanisms to functional significance

The thioredoxin (Trx) system is one of the central antioxidant systems in mammalian cells, maintaining a reducing environment by catalyzing electron flux from nicotinamide adenine dinucleotide phosphate through Trx reductase to Trx, which reduces its target proteins using highly conserved thiol groups. While the importance of protecting cells from the detrimental effects of reactive oxygen species is clear, decades of research in this field revealed that there is a network of redox-sensitive proteins forming redox-dependent signaling pathways that are crucial for fundamental cellular processes, including metabolism, proliferation, differentiation, migration, and apoptosis. Trx participates in signaling pathways interacting with different proteins to control their dynamic regulation of structure and function. In this review, we focus on Trx target proteins that are involved in redox-dependent signaling pathways. Specifically, Trx-dependent reductive enzymes that participate in classical redox reactions and redox-sensitive signaling molecules are discussed in greater detail. The latter are extensively discussed, as ongoing research unveils more and more details about the complex signaling networks of Trx-sensitive signaling molecules such as apoptosis signal-regulating kinase 1, Trx interacting protein, and phosphatase and tensin homolog, thus highlighting the potential direct and indirect impact of their redox-dependent interaction with Trx. Overall, the findings that are described here illustrate the importance and complexity of Trx-dependent, redox-sensitive signaling in the cell. Our increasing understanding of the components and mechanisms of these signaling pathways could lead to the identification of new potential targets for the treatment of diseases, including cancer and diabetes.

AMPK-dependent degradation of TXNIP upon energy stress leads to enhanced glucose uptake via GLUT1

Thioredoxin-interacting protein (TXNIP) is an α-arrestin family protein that is induced in response to glucose elevation. It has been shown to provide a negative feedback loop to regulate glucose uptake into cells, though the biochemical mechanism of action has been obscure. Here, we report that TXNIP suppresses glucose uptake directly, by binding to the glucose transporter GLUT1 and inducing GLUT1 internalization through clathrin-coated pits, as well as indirectly, by reducing the level of GLUT1 messenger RNA (mRNA). In addition, we show that energy stress results in the phosphorylation of TXNIP by AMP-dependent protein kinase (AMPK), leading to its rapid degradation. This suppression of TXNIP results in an acute increase in GLUT1 function and an increase in GLUT1 mRNA (hence the total protein levels) for long-term adaptation. The glucose influx through GLUT1 restores ATP-to-ADP ratios in the short run and ultimately induces TXNIP protein production to suppress glucose uptake once energy homeostasis is reestablished.

Thioredoxin Interacting Protein (TXNIP) and Pathogenesis of Diabetic Retinopathy

Chronic hyperglycemia (HG)-associated reactive oxygen/nitrogen species (ROS/RNS) stress and low grade inflammation are considered to play critical roles in the development of diabetic retinopathy (DR). Excess glucose metabolic flux through the aldose reductase/polyol pathway, advanced glycation end product (AGE) formation, elevated hexosamine biosynthesis pathway (HBP), diacyl glycerol/PKC activation, and mitochondrial ROS generation are all implicated in DR. In addition, endoplasmic reticulum stress/unfolded protein response (er-UPR) and deregulation of mitochondrial quality control by autophagy/mitophagy are observed causing cellular bioenergetic deficiency and injury. Recently, a pro-oxidant and pro-apoptotic thioredoxin interacting protein (TXNIP) was shown to be highly upregulated in DR and by HG in retinal cells in culture. TXNIP binds to thioredoxin (Trx) inhibiting its oxidant scavenging and thiolreducing capacity. Hence, prolonged overexpression of TXNIP causes ROS/RNS stress, mitochondrial dysfunction, inflammation and premature cell death in DR. Initially, DR was considered as microvascular complications of endothelial dysfunction and pericyte loss characterized by capillary basement membrane thickening, pericyte ghost, blood retinal barrier leakage, acellular capillary and neovascularization. However, it is currently acknowledged that neuro-glia are also affected by HG in diabetes and that neuronal injury, glial activation, innate immunity/sterile inflammation, and ganglion apoptosis occur early in DR. In addition, retinal pigment epithelium (RPE) becomes dysfunctional in DR. Since TXNIP is induced by HG in most cells, its effects are not restricted to a particular cell type in DR. However, depending on the metabolic activity and anti-oxidant capacity, some cells may be affected earlier by TXNIP than others. Identification of TXNIP sensitive cells and elucidating the underlying mechanism(s) will be critical for preventing pre-mature cell death and progression of DR.

Glucose induces protein targeting to glycogen in hepatocytes by fructose 2,6-bisphosphate-mediated recruitment of MondoA to the promoter

In the liver, a high glucose concentration activates transcription of genes encoding glucose 6-phosphatase and enzymes for glycolysis and lipogenesis by elevation in phosphorylated intermediates and recruitment of the transcription factor ChREBP (carbohydrate response element binding protein) and its partner, Mlx, to gene promoters. A proposed function for this mechanism is intracellular phosphate homeostasis. In extrahepatic tissues, MondoA, the paralog of ChREBP, partners with Mlx in transcriptional induction by glucose. We tested for glucose induction of regulatory proteins of the glycogenic pathway in hepatocytes and identified the glycogen-targeting proteins, G(L) and PTG (protein targeting to glycogen), as being encoded by Mlx-dependent glucose-inducible genes. PTG induction by glucose was MondoA dependent but ChREBP independent and was enhanced by forced elevation of fructose 2,6-bisphosphate and by additional xylitol-derived metabolites. It was counteracted by selective depletion of fructose 2,6-bisphosphate with a bisphosphatase-active kinase-deficient variant of phosphofructokinase 2/fructosebisphosphatase 2, which prevented translocation of MondoA to the nucleus and recruitment to the PTG promoter. We identify a novel role for MondoA in the liver and demonstrate that elevated fructose 2,6-bisphosphate is essential for recruitment of MondoA to the PTG promoter. Phosphometabolite activation of MondoA and ChREBP and their recruitment to target genes is consistent with a mechanism for gene regulation to maintain intracellular phosphate homeostasis.

Riboflavin carrier protein-targeted fluorescent USPIO for the assessment of vascular metabolism in tumors

Riboflavin (Rf) and its metabolic analogs flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are essential for normal cellular growth and function. Their intracellular transport is regulated by the riboflavin carrier protein (RCP), which has been shown to be over-expressed by metabolically active cancer cells. Therefore, FAD-decorated ultrasmall superparamagnetic iron oxide nanoparticles (FAD USPIO) were developed as the first carrier-protein-targeted molecular MR agents for visualizing tumor metabolism. FAD USPIO were synthesized using an adsorptive, fluorescent and non-polymeric coating method, and their physicochemical properties were characterized using TEM, SEM, FTIR, MRI and fluorescence spectroscopy. In vitro analyses showed the biocompatibility of FAD USPIO, and confirmed that they were strongly and specifically taken up by cancer (LnCap) and endothelial (HUVEC) cells. In vivo molecular MRI together with subsequent histological validation finally demonstrated that FAD USPIO efficiently accumulate in tumors and tumor blood vessels, indicating that RCP-targeted diagnostic nanoparticles are interesting new materials for the assessment of vascular metabolism in tumors.

Effect of taurine on mRNA expression of thioredoxin interacting protein in Caco-2 cells

Taurine (2-aminoethanesulfonic acid), a sulfur-containing β-amino acid, plays an important role in several essential biological processes; although, the underlying mechanisms for these regulatory functions remain to be elucidated, especially at the genetic level. We investigated the effects of taurine on the gene expression profile in Caco-2 cells using DNA microarray. Taurine increased the mRNA expression of thioredoxin interacting protein (TXNIP), which is involved in various metabolisms and diseases. β-Alanine or γ-aminobutyric acid (GABA), which are structurally or functionally related to taurine, did not increase TXNIP mRNA expression. These suggest the expression of TXNIP mRNA is induced specifically by taurine. β-Alanine is also known to be a substrate of taurine transporter (TAUT) and competitively inhibits taurine uptake. Inhibition of taurine uptake by β-alanine eliminated the up-regulation of TXNIP, which suggests TAUT is involved in inducing TXNIP mRNA expression. The up-regulation of TXNIP mRNA expression by taurine was also observed at the protein level. Furthermore, taurine significantly increased TXNIP promoter activity. Our present study demonstrated the taurine-specific phenomenon of TXNIP up-regulation, which sheds light on the physiological function of taurine.

Redox regulation by nuclear factor erythroid 2-related factor 2: gatekeeping for the basal and diabetes-induced expression of thioredoxin-interacting protein

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor activated by a range of oxidants and electrophiles. The transcriptional response to endogenous oxidative cues by Nrf2 plays an important role in mammalian redox physiology and oxidative pathology. Hyperglycemia induces oxidative stress in the heart where it leads to apoptosis and ultimately cardiomyopathy. Here we investigated the mechanism by which Nrf2 suppresses oxidative stress in diabetic mouse heart. Knockout (KO) of Nrf2 induced oxidative stress and apoptosis in KO heart; diabetes further increased oxidative damage. A pathway-focused gene array revealed that Nrf2 controls the expression of 24 genes in the heart, including the gene encoding thioredoxin-interacting protein (TXNIP). Nrf2 suppressed the basal expression of Txnip in the heart and blocked induction of Txnip by high glucose by binding to an antioxidant response element (ARE) (-1286 to -1276) of the Txnip promoter. Binding of Nrf2 to ARE also suppressed the binding of MondoA to the carbohydrate response element with or without high glucose. TXNIP promoted reactive oxygen species production and apoptosis by inhibiting thioredoxin. On the other hand, Nrf2 boosted thioredoxin activity by inhibiting Txnip. The findings revealed, for the first time, that Nrf2 is a key gatekeeper of Txnip transcription, suppressing both its basal expression and MondoA-driven induction to control the thioredoxin redox signaling in diabetes.

A potential mechanism of metformin-mediated regulation of glucose homeostasis: inhibition of Thioredoxin-interacting protein (Txnip) gene expression

Metformin (dimethylbiguanide) is widely used among diabetic patients to lower the blood sugar level. Although several mechanisms have been proposed, its mode of action in enhancing peripheral glucose uptake and inhibiting hepatic glucose production is not fully understood. Thioredoxin-interacting protein (Txnip) is known to play important roles in glucose metabolism by inhibiting cellular glucose uptake and metabolism and promoting hepatic gluconeogenesis. The expression of the gene encoding Txnip is regulated in a glucose dependent manner via the Mondo:MLX transcription factor complex. In the present study, we report that Txnip mRNA as well as protein expression in cultured cells is markedly reduced upon metformin administration. The binding of Mondo:MLX to the Txnip gene promoter is reduced, suggesting that the transcription of the Txnip gene is repressed by metformin. Moreover, we show that the effect of metformin on Txnip gene transcription is due to the inhibition of mitochondrial complex I and increased glycolysis, and is partially mediated by the AMP activated kinase (AMPK). These observations prompt us to propose that the novel action of metformin on the Txnip gene expression may contribute to its therapeutic effects in the treatment of type II diabetes.

Thioredoxin interacting protein: redox dependent and independent regulatory mechanisms

SIGNIFICANCE

The thioredoxin-interacting protein (TXNIP, also termed VDUP1 for vitamin D upregulated protein or TBP2 for thioredoxin-binding protein) was originally discovered by virtue of its strong regulation by vitamin D. Recently, TXNIP has been found to regulate the cellular reduction-oxidation (redox) state by binding to and inhibiting thioredoxin (TRX) in a redox-dependent fashion.

RECENT ADVANCES

Studies of the Hcb-19 mouse, TXNIP nonsense mutated mouse, demonstrate redox-mediated roles in lipid and glucose metabolism, cardiac function, inflammation, and carcinogenesis. Exciting recent data indicate important roles for TXNIP in redox independent signaling. Specifically, sequence analysis revealed that TXNIP is a member of the classical visual/β-arrestin superfamily, and is one of the six members of the arrestin domain-containing (ARRDC, or α-arrestin) family.

CRITICAL ISSUES

Although the function of α-arrestins is not well known, recent studies suggest roles in endocytosis and protein ubiquitination through PPxY motifs in their C-terminal tails. Importantly, the ability of TXNIP to inhibit glucose uptake was found to be independent of TRX binding. Further investigation showed that several metabolic functions of TXNIP were due to the arrestin domains, thus further supporting the importance of redox independent functions of TXNIP.

FUTURE DIRECTIONS

Since TXNIP transcription and protein stability are highly regulated by multiple tissue-specific stimuli, it appears that TXNIP should be a good therapeutic target for metabolic diseases.

Thioredoxin binding protein (TBP)-2/Txnip and α-arrestin proteins in cancer and diabetes mellitus

Thioredoxin binding protein -2/ thioredoxin interacting protein is an α-arrestin protein that has attracted much attention as a multifunctional regulator. Thioredoxin binding protein -2 expression is downregulated in tumor cells and the level of thioredoxin binding protein is correlated with clinical stage of cancer. Mice with mutations or knockout of the thioredoxin binding protein -2 gene are much more susceptible to carcinogenesis than wild-type mice, indicating a role for thioredoxin binding protein -2 in cancer suppression. Studies have also revealed roles for thioredoxin binding protein -2 in metabolic control. Enhancement of thioredoxin binding protein -2 expression causes impairment of insulin sensitivity and glucose-induced insulin secretion, and β-cell apoptosis. These changes are important characteristics of type 2 diabetes mellitus. Thioredoxin binding protein -2 regulates transcription of metabolic regulating genes. Thioredoxin binding protein -2-like inducible membrane protein/ arrestin domain containing 3 regulates endocytosis of receptors such as the β(2)-adrenergic receptor. The α-arrestin family possesses PPXY motifs and may function as an adaptor/scaffold for NEDD family ubiquitin ligases. Elucidation of the molecular mechanisms of α-arrestin proteins would provide a new pharmacological basis for developing approaches against cancer and type 2 diabetes mellitus.

Purinergic regulation of high-glucose-induced caspase-1 activation in the rat retinal Müller cell line rMC-1

Chronic activation of proinflammatory caspase-1 in the retinas of diabetic animals and patients in vivo and retinal Müller cells in vitro is well documented. In this study we characterized how elevated glucose and extracellular purines contribute to the activation of caspase-1 in a cultured rat Müller cell (rMC-1) model. The ability of high glucose (25 mM, 24 h) to activate caspase-1 was attenuated by either apyrase, which metabolizes extracellular ATP to AMP, or adenosine deaminase (ADA), which metabolizes extracellular adenosine to inosine. This suggested that autocrine stimulation of ATP-sensing P2 receptors and adenosine-sensing P1 receptors may in part mediate the response to high glucose. Exogenous ATP, 5'-N-ethylcarboxamido-adenosine (NECA), a nonselective P1 receptor agonist, or forskolin (FSK) increased caspase-1 activity in rMC-1 cells cultured in control glucose (5 mM) medium. Accumulation of active caspase-1 was also increased by dipyridamole, which suppresses adenosine reuptake. High-glucose stimulation of caspase-1 was attenuated by suramin, a nonselective P2 antagonist, or A2 adenosine receptor antagonists, but not by antagonism of P2X7 ATP-gated ion channel receptors. Although high glucose increased P2X7 mRNA, neither P2X7 protein nor function was detected in rMC-1 cells. The increased caspase-1 activity stimulated by high glucose, FSK, NECA, or ATP was correlated with increased gene expression of caspase-1 and thioredoxin-interacting-protein (TXNIP). These findings support a novel role for autocrine P1 and P2 purinergic receptors coupled to cAMP signaling cascades and transcriptional induction of caspase-1 in mediating the high-glucose-induced activation of caspase-1 and secretion of IL-1β in a cell culture model of nonhematopoietic retinal Müller cells.

Gene regulation mediating fiber-type transformation in skeletal muscle cells is partly glucose- and ChREBP-dependent

Adaptations in the oxidative capacity of skeletal muscle cells can occur under several physiological or pathological conditions. We investigated the effect of increasing extracellular glucose concentration on the expression of markers of energy metabolism in primary skeletal muscle cells and the C2C12 muscle cell line. Growth of myotubes in 25mM glucose (high glucose, HG) compared with 5.55mM led to increases in the expression and activity of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a marker of glycolytic energy metabolism, while oxidative markers peroxisome proliferator-activated receptor γ coactivator 1α and citrate synthase decreased. HG induced metabolic adaptations as are seen during a slow-to-fast fiber transformation. Furthermore, HG increased fast myosin heavy chain (MHC) IId/x but did not change slow MHCI/β expression. Protein phosphatase 2A (PP2A) was shown to mediate the effects of HG on GAPDH and MHCIId/x. Carbohydrate response element-binding protein (ChREBP), a glucose-dependent transcription factor downstream of PP2A, partially mediated the effects of glucose on metabolic markers. The glucose-induced increase in PP2A activity was associated with an increase in p38 mitogen-activated protein kinase activity, which presumably mediates the increase in MHCIId/x promoter activity. Liver X receptor, another possible mediator of glucose effects, induced only an incomplete metabolic shift, mainly increasing the expression of the glycolytic marker. Taken together, HG induces a partial slow-to-fast transformation comprising metabolic enzymes together with an increased expression of MHCIId/x. This work demonstrates a functional role for ChREBP in determining the metabolic type of muscle fibers and highlights the importance of glucose as a signaling molecule in muscle.

Hypoxia-inducible factor independent down-regulation of thioredoxin-interacting protein in hypoxia

Thioredoxin-Interacting Protein (Txnip) is an important regulator of glucose metabolism and functions by inhibiting cellular glucose uptake. The expression of the Txnip gene is sensitive to glucose availability and is negatively correlated with the glycolytic rate. Here we show that hypoxia induces a rapid decrease in Txnip mRNA and protein expression in a Hypoxia-Inducible Factor independent manner. Hypoxia caused reduced binding of the glucose responsive MondoA:Mlx transcription factor to the carbohydrate response elements (ChoREs) in the Txnip promoter. Our data suggest that hypoxia decreases MondoA:Mlx activity by increasing glycolytic flux, leading to the depletion of glycolytic intermediates which normally activate MondoA:Mlx. Hypoxia dependent Txnip down-regulation may be an important compensatory mechanism through which cancer cells adapt their metabolism to low oxygen concentrations.

Thioredoxin-interacting protein (Txnip) gene expression: sensing oxidative phosphorylation status and glycolytic rate

Thioredoxin-interacting protein (Txnip) has important functions in regulating cellular metabolism including glucose utilization; the expression of the Txnip gene is sensitive to the availability of glucose and other fuels. Here, we show that Txnip expression is down-regulated at the transcriptional level by diverse inhibitors of mitochondrial oxidative phosphorylation (OXPHOS). The effect of these OXPHOS inhibitors is mediated by earlier identified carbohydrate-response elements (ChoREs) on the Txnip promoter and the ChoRE-associated transcription factors Max-like protein X (MLX) and MondoA (or carbohydrate-response element-binding protein (ChREBP)) involved in glucose-induced Txnip expression, suggesting that inhibited oxidative phosphorylation compromises glucose-induced effects on Txnip expression. We also show that the OXPHOS inhibitors repress the Txnip transcription most likely by inducing the glycolytic rate, and increased glycolytic flux decreases the levels of glycolytic intermediates important for the function of MLX and MondoA (or ChREBP). Our findings suggest that the Txnip expression is tightly correlated with glycolytic flux, which is regulated by oxidative phosphorylation status. The identified link between the Txnip expression and glycolytic activity implies a mechanism by which the cellular glucose uptake/homeostasis is regulated in response to various metabolic cues, oxidative phosphorylation status, and other physiological signals, and this may facilitate our efforts toward understanding metabolism in normal or cancer cells.

Cyclic AMP signaling stimulates proteasome degradation of thioredoxin interacting protein (TxNIP) in pancreatic beta-cells

Thioredoxin interacting protein (TxNIP) functions as an effector of glucotoxicity in pancreatic beta-cells. Exendin-4 (Ex-4), a long-term effective GLP-1 receptor agonist, reduces TxNIP level in pancreatic beta-cells. Mechanisms underlying this reduction, however, remain largely unknown. We show here that Ex-4, 8-bromo-cAMP, the cAMP promoting agent forskolin, as well as activators of protein kinase A (PKA) and exchange protein activated by cAMP (Epac), all attenuated the effect of high glucose (20mM) on TxNIP level in the pancreatic beta-cell line Ins-1. Forskolin and Ex-4 also reduced TxNIP level in cultured primary rat islets. This repressive effect is at least partially mediated via stimulating proteasome-dependent TxNIP degradation, since the proteasomal inhibitor MG132, but not the lysosomal inhibitor chloroquine, significantly blocked the repressive effect of forskolin. Furthermore, forskolin enhanced TxNIP ubiquitination. Both PKA inhibition and Epac inhibition partially blocked the repressive effect of forskolin on TxNIP level. In addition, forskolin and Ex-4 protected Ins-1 cells from high glucose-induced apoptotic activity, assessed by measuring caspase 3 activity. Finally, knockdown of TxNIP expression led to reduced caspase 3 expression levels and blunted response to forskolin treatment. We suggest that proteasome-dependent TxNIP degradation is a novel mechanism by which Ex-4-cAMP signaling protects pancreatic beta cells.

Evolutionary lineages and functional diversification of plant hexokinases

Sequencing data from 10 species show that a plant hexokinase (HXK) family contains 5-11 genes. Functionally, a given family can include metabolic catalysts, glucose signaling proteins, and non-catalytic, apparent regulatory enzyme homologs. This study has two goals. The first aim is to develop a predictive method to determine which HXK proteins within a species have which type of function. The second aim is to determine whether HXK-dependent glucose signaling proteins occur among more primitive plants, as well as among angiosperms. Using a molecular phylogeny approach, combined with selective experimental testing, we found that non-catalytic HXK homologs might occur in all plants, including the relatively primitive Selaginella moellendorffi. We also found that different lineages of angiosperm HXKs have apparent conserved features for catalytic activity and for sub-cellular targeting. Most higher-plant HXKs are expressed predominantly at mitochondria, with HXKs of one lineage occurring in the plastid, and HXKs of one monocot lineage occurring in the cytosol. Using protoplast transient expression assays, we found that HXK glucose signaling proteins occur likely in all higher plants and in S. moellendorffi as well. Thus, the use of glucose by plant HXK isoforms in metabolism and/or as a regulatory metabolite occurs as widespread, conserved processes.

Tandem ChoRE and CCAAT motifs and associated factors regulate Txnip expression in response to glucose or adenosine-containing molecules

Background

Thioredoxin interacting protein (Txnip) is a multifunctional protein involved in regulation of cell cycle events and cellular metabolism. The expression of Txnip is known to be induced by glucose, adenosine-containing molecules, and other physiological cues; however, the underlying regulatory mechanisms remain elusive.

Methodology/Principal Findings

In this study, using promoter reporter, electrophoresis mobility shift (EMSA), and chromatin immuno-precipitation (ChIP) assays, we have identified an additional carbohydrate response element (ChoRE) on the promoter of Txnip gene, which functions cooperatively with the earlier identified ChoRE to mediate optimal Txnip expression. However, these two ChoREs are not sufficient to mediate the induction of Txnip expression by glucose or adenosine-containing molecules; and two CCAAT boxes, both of which can recruit nuclear factor Y (NF-Y) to the Txnip promoter, are also required for the induction. Accordingly, we have found that the function of ChoREs and associated factors is contingent on tandem CCAAT boxes, in that occupancy of the Txnip promoter by NF-Y is a prerequisite for efficacious recruitment of Mondo/MLX to ChoREs under glucose stimulation.

Conclusions/Significance

Our findings suggest a synergy between the tandem CCAAT and ChoRE motifs and associated NF-Y and Mondo/MLX transcription factors in enhancing transcription from the Txnip promoter. This piece of information will be helpful for future dissection of molecular mechanisms governing the transcriptional regulation of Txnip, a glucose responsive gene.