Importin-alpha protein binding to a nuclear localization signal of carbohydrate response element-binding protein (ChREBP)

Molecular glues of the regulatory ChREBP/14-3-3 complex protect beta cells from glucolipotoxicity

The Carbohydrate Response Element Binding Protein (ChREBP) is a glucose-responsive transcription factor (TF) that is characterized by two major splice isoforms (α and β). In acute hyperglycemia, both ChREBP isoforms regulate adaptive β-expansion; however, during chronic hyperglycemia and glucolipotoxicity, ChREBPβ expression surges, leading to β-cell dedifferentiation and death. 14-3-3 binding to ChREBPα results in its cytoplasmic retention and concomitant suppression of transcriptional activity, suggesting that small molecule-mediated stabilization of this protein-protein interaction (PPI) via molecular glues may represent an attractive entry for the treatment of metabolic disease. Here, we show that structure-based optimizations of a molecular glue tool compound led not only to more potent ChREBPα/14-3-3 PPI stabilizers but also for the first time cellular active compounds. In primary human β-cells, the most active compound stabilized the ChREBPα/14-3-3 interaction and thus induced cytoplasmic retention of ChREBPα, resulting in highly efficient β-cell protection from glucolipotoxicity while maintaining β-cell identity. This study may thus not only provide the basis for the development of a unique class of compounds for the treatment of Type 2 Diabetes but also showcases an alternative 'molecular glue' approach for achieving small molecule control of notoriously difficult targetable TFs.

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.

The Roles of Carbohydrate Response Element Binding Protein in the Relationship between Carbohydrate Intake and Diseases

Carbohydrates are macronutrients that serve as energy sources. Many studies have shown that carbohydrate intake is nonlinearly associated with mortality. Moreover, high-fructose corn syrup (HFCS) consumption is positively associated with obesity, cardiovascular disease, and type 2 diabetes mellitus (T2DM). Accordingly, products with equal amounts of glucose and fructose have the worst effects on caloric intake, body weight gain, and glucose intolerance, suggesting that carbohydrate amount, kind, and form determine mortality. Understanding the role of carbohydrate response element binding protein (ChREBP) in glucose and lipid metabolism will be beneficial for elucidating the harmful effects of high-fructose corn syrup (HFCS), as this glucose-activated transcription factor regulates glycolytic and lipogenic gene expression. Glucose and fructose coordinately supply the metabolites necessary for ChREBP activation and de novo lipogenesis. Chrebp overexpression causes fatty liver and lower plasma glucose levels, and ChREBP deletion prevents obesity and fatty liver. Intestinal ChREBP regulates fructose absorption and catabolism, and adipose-specific Chrebp-knockout mice show insulin resistance. ChREBP also regulates the appetite for sweets by controlling fibroblast growth factor 21, which promotes energy expenditure. Thus, ChREBP partly mimics the effects of carbohydrate, especially HFCS. The relationship between carbohydrate intake and diseases partly resembles those between ChREBP activity and diseases.

Short- and Long-Term Adaptation to Altered Levels of Glucose: Fifty Years of Scientific Adventure

My graduate and postdoctoral training in metabolism and enzymology eventually led me to study the short- and long-term regulation of glucose and lipid metabolism. In the early phase of my career, my trainees and I identified, purified, and characterized a variety of phosphofructokinase enzymes from mammalian tissues. These studies led us to discover fructose 2,6-P₂, the most potent activator of phosphofructokinase and glycolysis. The discovery of fructose 2,6-P₂ led to the identification and characterization of the tissue-specific bifunctional enzyme 6-phosphofructo-2-kinase:fructose 2,6-bisphosphatase. We discovered a glucose signaling mechanism by which the liver maintains glucose homeostasis by regulating the activities of this bifunctional enzyme. With a rise in glucose, a signaling metabolite, xylulose 5-phosphate, triggers rapid activation of a specific protein phosphatase (PP2ABδC), which dephosphorylates the bifunctional enzyme, thereby increasing fructose 2,6-P₂ levels and upregulating glycolysis. These endeavors paved the way for us to initiate the later phase of my career in which we discovered a new transcription factor termed the carbohydrate response element binding protein (ChREBP). Now ChREBP is recognized as the masterregulator controlling conversion of excess carbohydrates to storage of fat in the liver. ChREBP functions as a central metabolic coordinator that responds to nutrients independently of insulin. The ChREBP transcription factor facilitates metabolic adaptation to excess glucose, leading to obesity and its associated diseases.

A lack of ChREBP inhibits mitochondrial cristae formation in brown adipose tissue

The carbohydrate response element binding protein (ChREBP) is a glucose-responsive transcription factor that increases the transcription of multiple genes. ChREBP is highly localized in the liver, where it upregulates the expression of genes that code for glycolytic and lipogenic enzymes, resulting in the conversion of excess carbohydrate into storage fat. ChREBP knockout (KO) mice display an anti-obese phenotype. However, at this time, role of ChREBP in adipose tissue remains unclear. Therefore, the energy metabolism and morphology of mitochondrial brown adipose tissue (BAT) in ChREBP KO mice was examined. We found increased expression levels of electron transport system proteins including the mitochondrial uncoupling protein (UCP1), and mitochondrial structural alterations such as dysplasia of the cristae and the presence of small mitochondria in BAT of ChREBP KO mice. Mass spectrometry analyses revealed that fatty acid synthase was absent in the BAT of ChREBP KO mice, which probably led to a reduction in fatty acids and cardiolipin, a regulator of various mitochondrial events. Our study clarified the new role of ChREBP in adipose tissue and its involvement in mitochondrial function. A clearer understanding of ChREBP in mitochondria could pave the way for improvements in obesity management.

Adaptive and maladaptive roles for ChREBP in the liver and pancreatic islets

Excessive sugar consumption is a contributor to the worldwide epidemic of cardiometabolic disease. Understanding mechanisms by which sugar is sensed and regulates metabolic processes may provide new opportunities to prevent and treat these epidemics. Carbohydrate Responsive-Element Binding Protein (ChREBP) is a sugar-sensing transcription factor that mediates genomic responses to changes in carbohydrate abundance in key metabolic tissues. Carbohydrate metabolites activate the canonical form of ChREBP, ChREBP-alpha, which stimulates production of a potent, constitutively active ChREBP isoform called ChREBP-beta. Carbohydrate metabolites and other metabolic signals may also regulate ChREBP activity via posttranslational modifications including phosphorylation, acetylation, and O-GlcNAcylation that can affect ChREBP’s cellular localization, stability, binding to cofactors, and transcriptional activity. In this review, we discuss mechanisms regulating ChREBP activity and highlight phenotypes and controversies in ChREBP gain- and loss-of-function genetic rodent models focused on the liver and pancreatic islets.

Sugar and Nitrate Sensing: A Multi-Billion-Year Story

Sugars and nitrate play a major role in providing carbon and nitrogen in plants. Understanding how plants sense these nutrients is crucial, most notably for crop improvement. The mechanisms underlying sugar and nitrate sensing are complex and involve moonlighting proteins such as the nitrate transporter NRT1.1/NFP6.3 or the glycolytic enzyme HXK1. Major components of nutrient signaling, such as SnRK1, TOR, and HXK1, are relatively well conserved across eukaryotes, and the diversification of components such as the NRT1 family and the SWEET sugar transporters correlates with plant terrestrialization. In plants, Tre6P plays a hormone-like role in plant development. In addition, nutrient signaling has evolved to interact with the more recent hormone signaling, allowing fine-tuning of physiological and developmental responses.

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.

The structure of importin α and the nuclear localization peptide of ChREBP, and small compound inhibitors of ChREBP-importin α interactions

The carbohydrate response element binding protein (ChREBP) is a glucose-responsive transcription factor that plays a critical role in glucose-mediated induction of genes involved in hepatic glycolysis and lipogenesis. In response to fluctuating blood glucose levels ChREBP activity is regulated mainly by nucleocytoplasmic shuttling of ChREBP. Under high glucose ChREBP binds to importin α and importin β and translocates into the nucleus to initiate transcription. We have previously shown that the nuclear localization signal site (NLS) for ChREBP is bipartite with the NLS extending from Arg158 to Lys190. Here, we report the 2.5 Å crystal structure of the ChREBP-NLS peptide bound to importin α. The structure revealed that the NLS binding is monopartite, with the amino acid residues K¹⁷¹RRI¹⁷⁴ from the ChREBP-NLS interacting with ARM2–ARM5 on importin α. We discovered that importin α also binds to the primary binding site of the 14-3-3 proteins with high affinity, which suggests that both importin α and 14-3-3 are each competing with the other for this broad-binding region (residues 117–196) on ChREBP. We screened a small compound library and identified two novel compounds that inhibit the ChREBP-NLS/importin α interaction, nuclear localization, and transcription activities of ChREBP. These candidate molecules support developing inhibitors of ChREBP that may be useful in treatment of obesity and the associated diseases.

Glucose-Sensing Transcription Factor MondoA/ChREBP as Targets for Type 2 Diabetes: Opportunities and Challenges

The worldwide increase in type 2 diabetes (T2D) is becoming a major health concern, thus searching for novel preventive and therapeutic strategies has become urgent. In last decade, the paralogous transcription factors MondoA and carbohydrate response element-binding protein (ChREBP) have been revealed to be central mediators of glucose sensing in multiple metabolic organs. Under normal nutrient conditions, MondoA/ChREBP plays vital roles in maintaining glucose homeostasis. However, under chronic nutrient overload, the dysregulation of MondoA/ChREBP contributes to metabolic disorders, such as insulin resistance (IR) and T2D. In this review, we aim to provide an overview of recent advances in the understanding of MondoA/ChREBP and its roles in T2D development. Specifically, we will briefly summarize the functional similarities and differences between MondoA and ChREBP. Then, we will update the roles of MondoA/ChREBP in four T2D-associated metabolic organs (i.e., the skeletal muscle, liver, adipose tissue, and pancreas) in physiological and pathological conditions. Finally, we will discuss the opportunities and challenges of MondoA/ChREBP as drug targets for anti-diabetes. By doing so, we highlight the potential use of therapies targeting MondoA/ChREBP to counteract T2D and its complications.

Sugar starvation-regulated MYBS2 and 14-3-3 protein interactions enhance plant growth, stress tolerance, and grain weight in rice

Autotrophic plants have evolved distinctive mechanisms for maintaining a range of homeostatic states for sugars. The on/off switch of reversible gene expression by sugar starvation/provision represents one of the major mechanisms by which sugar levels are maintained, but the details remain unclear. α-Amylase (αAmy) is the key enzyme for hydrolyzing starch into sugars for plant growth, and it is induced by sugar starvation and repressed by sugar provision. αAmy can also be induced by various other stresses, but the physiological significance is unclear. Here, we reveal that the on/off switch of αAmy expression is regulated by 2 MYB transcription factors competing for the same promoter element. MYBS1 promotes αAmy expression under sugar starvation, whereas MYBS2 represses it. Sugar starvation promotes nuclear import of MYBS1 and nuclear export of MYBS2, whereas sugar provision has the opposite effects. Phosphorylation of MYBS2 at distinct serine residues plays important roles in regulating its sugar-dependent nucleocytoplasmic shuttling and maintenance in cytoplasm by 14-3-3 proteins. Moreover, dehydration, heat, and osmotic stress repress MYBS2 expression, thereby inducing αAmy3 Importantly, activation of αAmy3 and suppression of MYBS2 enhances plant growth, stress tolerance, and total grain weight per plant in rice. Our findings reveal insights into a unique regulatory mechanism for an on/off switch of reversible gene expression in maintaining sugar homeostatic states, which tightly regulates plant growth and development, and also highlight MYBS2 and αAmy3 as potential targets for crop improvement.

Recent insights into the role of ChREBP in intestinal fructose absorption and metabolism

Fructose in the form of sucrose and high fructose corn syrup is absorbed by the intestinal transporter and mainly metabolized in the small intestine. However, excess intake of fructose overwhelms the absorptive capacity of the small intestine, leading to fructose malabsorption. Carbohydrate response element-binding protein (ChREBP) is a basic helix-loop-helix leucine zipper transcription factor that plays a key role in glycolytic and lipogenic gene expression in response to carbohydrate consumption. While ChREBP was initially identified as a glucose-responsive factor in the liver, recent evidence suggests that ChREBP is essential for fructoseinduced lipogenesis and gluconeogenesis in the small intestine as well as in the liver. We recently identified that the loss of ChREBP leads to fructose intolerance via insufficient induction of genes involved in fructose transport and metabolism in the intestine. As fructose consumption is increasing and closely associated with metabolic and gastrointestinal diseases, a comprehensive understanding of cellular fructose sensing and metabolism via ChREBP may uncover new therapeutic opportunities. In this mini review, we briefly summarize recent progress in intestinal fructose metabolism, regulation and function of ChREBP by fructose, and delineate the potential mechanisms by which excessive fructose consumption may lead to irritable bowel syndrome.

Variations in nuclear localization strategies among pol X family enzymes

Despite the essential roles of pol X family enzymes in DNA repair, information about the structural basis of their nuclear import is limited. Recent studies revealed the unexpected presence of a functional nuclear localization signal (NLS) in DNA polymerase β, indicating the importance of active nuclear targeting, even for enzymes likely to leak into and out of the nucleus. The current studies further explore the active nuclear transport of these enzymes by identifying and structurally characterizing the functional NLS sequences in the three remaining human pol X enzymes: terminal deoxynucleotidyl transferase (TdT), DNA polymerase mu (pol μ) and DNA polymerase lambda (pol λ). NLS identifications are based on Importin α (Impα) binding affinity determined by fluorescence polarization of fluorescein-labeled NLS peptides, X-ray crystallographic analysis of the Impα∆IBB•NLS complexes and fluorescence-based subcellular localization studies. All three polymerases use NLS sequences located near their N-terminus; TdT and pol μ utilize monopartite NLS sequences, while pol λ utilizes a bipartite sequence, unique among the pol X family members. The pol μ NLS has relatively weak measured affinity for Impα, due in part to its proximity to the N-terminus that limits non-specific interactions of flanking residues preceding the NLS. However, this effect is partially mitigated by an N-terminal sequence unsupportive of Met1 removal by methionine aminopeptidase, leading to a 3-fold increase in affinity when the N-terminal methionine is present. Nuclear targeting is unique to each pol X family enzyme with variations dependent on the structure and unique functional role of each polymerase.

Nuclear transport of the Neurospora crassa NIT-2 transcription factor is mediated by importin-α

The Neurospora crassa NIT-2 transcription factor belongs to the GATA transcription factor family and plays a fundamental role in the regulation of nitrogen metabolism. Because NIT-2 acts by accessing DNA inside the nucleus, understanding the nuclear import process of NIT-2 is necessary to characterize its function. Thus, in the present study, NIT-2 nuclear transport was investigated using a combination of biochemical, cellular, and biophysical methods. A complemented strain that produced an sfGFP–NIT-2 fusion protein was constructed, and nuclear localization assessments were made under conditions that favored protein translocation to the nucleus. Nuclear translocation was also investigated using HeLa cells, which showed that the putative NIT-2 nuclear localization sequence (NLS; 915TISSKRQRRHSKS927) was recognized by importin-α and that subsequent transport occurred via the classical import pathway. The interaction between the N. crassa importin-α (NcImpα) and the NIT-2 NLS was quantified with calorimetric assays, leading to the observation that the peptide bound to two sites with different affinities, which is typical of a monopartite NLS sequence. The crystal structure of the NcImpα/NIT-2 NLS complex was solved and revealed that the NIT-2 peptide binds to NcImpα with the major NLS-binding site playing a primary role. This result contrasts other recent studies that suggested a major role for the minor NLS-binding site in importin-α from the α2 family, indicating that both sites can be used for different cargo proteins according to specific metabolic requirements.

DNA mismatch repair proteins MLH1 and PMS2 can be imported to the nucleus by a classical nuclear import pathway

MLH1 and PMS2 proteins form the MutLα heterodimer, which plays a major role in DNA mismatch repair (MMR) in humans. Mutations in MMR-related proteins are associated with cancer, especially with colon cancer. The N-terminal region of MutLα comprises the N-termini of PMS2 and MLH1 and, similarly, the C-terminal region of MutLα is composed by the C-termini of PMS2 and MLH1, and the two are connected by linker region. The nuclear localization sequences (NLSs) necessary for the nuclear transport of the two proteins are found in this linker region. However, the exact NLS sequences have been controversial, with different sequences reported, particularly for MLH1. The individual components are not imported efficiently, presumably due to their C-termini masking their NLSs. In order to gain insights into the nuclear transport of these proteins, we solved the crystal structures of importin-α bound to peptides corresponding to the supposed NLSs of MLH1 and PMS2 and performed isothermal titration calorimetry to study their binding affinities. Both putative MLH1 and PMS2 NLSs can bind to importin-α as monopartite NLSs, which is in agreement with some previous studies. However, MLH1-NLS has the highest affinity measured by a natural NLS peptide, suggesting a major role of MLH1 protein in nuclear import compared to PMS2. Finally, the role of MLH1 and PMS2 in the nuclear transport of the MutLα heterodimer is discussed.

MondoA/ChREBP: The usual suspects of transcriptional glucose sensing; Implication in pathophysiology

Identification of the Mondo glucose-responsive transcription factors family, including the MondoA and MondoB/ChREBP paralogs, has shed light on the mechanism whereby glucose affects gene transcription. They have clearly emerged, in recent years, as key mediators of glucose sensing by multiple cell types. MondoA and ChREBP have overlapping yet distinct expression profiles, which underlie their downstream targets and separate roles in regulating genes involved in glucose metabolism. MondoA can restrict glucose uptake and influences energy utilization in skeletal muscle, while ChREBP signals energy storage through de novo lipogenesis in liver and white adipose tissue. Because Mondo proteins mediate metabolic adaptations to changing glucose levels, a better understanding of cellular glucose sensing through Mondo proteins will likely uncover new therapeutic opportunities in the context of the imbalanced glucose homeostasis that accompanies metabolic diseases such as type 2 diabetes and cancer. Here, we provide an overview of structural homologies, transcriptional partners as well as the nutrient and hormonal mechanisms underlying Mondo proteins regulation. We next summarize their relative contribution to energy metabolism changes in physiological states and the evolutionary conservation of these pathways. Finally, we discuss their possible targeting in human pathologies.

Metabolite Regulation of Nuclear Localization of Carbohydrate-response Element-binding Protein (ChREBP): ROLE OF AMP AS AN ALLOSTERIC INHIBITOR

The carbohydrate-response element-binding protein (ChREBP) is a glucose-responsive transcription factor that plays an essential role in converting excess carbohydrate to fat storage in the liver. In response to glucose levels, ChREBP is regulated by nuclear/cytosol trafficking via interaction with 14-3-3 proteins, CRM-1 (exportin-1 or XPO-1), or importins. Nuclear localization of ChREBP was rapidly inhibited when incubated in branched-chain α-ketoacids, saturated and unsaturated fatty acids, or 5-aminoimidazole-4-carboxamide ribonucleotide. Here, we discovered that protein-free extracts of high fat-fed livers contained, in addition to ketone bodies, a new metabolite, identified as AMP, which specifically activates the interaction between ChREBP and 14-3-3. The crystal structure showed that AMP binds directly to the N terminus of ChREBP-α2 helix. Our results suggest that AMP inhibits the nuclear localization of ChREBP through an allosteric activation of ChREBP/14-3-3 interactions and not by activation of AMPK. AMP and ketone bodies together can therefore inhibit lipogenesis by restricting localization of ChREBP to the cytoplasm during periods of ketosis.

The Interaction of FABP with Kapα

Gene-activating lipophilic compounds are carried into the nucleus when loaded on fatty-acid-binding proteins (FABP). Some of these proteins are recognized by the α-Karyopherin (Kapα) through its nuclear localization signal (NLS) consisting of three positive residues that are not in a continuous sequence. The Importin system can distinguish between FABP loaded with activating and non-activating compounds. In the present study, we introduced molecular dynamics as a tool for clarifying the mechanism by which FABP4, loaded with activating ligand (linoleate) is recognized by Kapα. In the first phase, we simulated the complex between Kapα^ΔIBB (termed “Armadillo”) that was crystallized with two NLS hepta-peptides. The trajectory revealed that the crystal-structure orientation of the peptides is rapidly lost and new interactions dominate. Though, the NLS sequence of FABP4 is cryptic, since the functional residues are not in direct sequence, implicating more than one possible conformation. Therefore, four possible docked conformations were generated, in which the NLS of FABP4 is interacting with either the major or the minor sites of Kapα, and the N → C vectors are parallel or anti-parallel. Out of these four basic starting positions, only the FABP4-minor site complex exhibited a large number of contact points. In this complex, the FABP interacts with the minor and the major sites, suppressing the self-inhibitory interaction of the Kapα, rendering it free to react with Kapβ. Finally, we propose that the transportable conformation generated an extended hydrophobic domain which expanded out of the boundary of the FABP4, allowing the loaded linoleate to partially migrate out of the FABP into a joint complex in which the Kapα contributes part of a combined binding pocket.

Structure of Importin-α from a Filamentous Fungus in Complex with a Classical Nuclear Localization Signal

Neurospora crassa is a filamentous fungus that has been extensively studied as a model organism for eukaryotic biology, providing fundamental insights into cellular processes such as cell signaling, growth and differentiation. To advance in the study of this multicellular organism, an understanding of the specific mechanisms for protein transport into the cell nucleus is essential. Importin-α (Imp-α) is the receptor for cargo proteins that contain specific nuclear localization signals (NLSs) that play a key role in the classical nuclear import pathway. Structures of Imp-α from different organisms (yeast, rice, mouse, and human) have been determined, revealing that this receptor possesses a conserved structural scaffold. However, recent studies have demonstrated that the Impα mechanism of action may vary significantly for different organisms or for different isoforms from the same organism. Therefore, structural, functional, and biophysical characterization of different Impα proteins is necessary to understand the selectivity of nuclear transport. Here, we determined the first crystal structure of an Impα from a filamentous fungus which is also the highest resolution Impα structure already solved to date (1.75 Å). In addition, we performed calorimetric analysis to determine the affinity and thermodynamic parameters of the interaction between Imp-α and the classical SV40 NLS peptide. The comparison of these data with previous studies on Impα proteins led us to demonstrate that N. crassa Imp-α possess specific features that are distinct from mammalian Imp-α but exhibit important similarities to rice Imp-α, particularly at the minor NLS binding site.

Probing formation of cargo/importin-α transport complexes in plant cells using a pathogen effector

Importin-αs are essential adapter proteins that recruit cytoplasmic proteins destined for active nuclear import to the nuclear transport machinery. Cargo proteins interact with the importin-α armadillo repeat domain via nuclear localization sequences (NLSs), short amino acids motifs enriched in Lys and Arg residues. Plant genomes typically encode several importin-α paralogs that can have both specific and partially redundant functions. Although some cargos are preferentially imported by a distinct importin-α it remains unknown how this specificity is generated and to what extent cargos compete for binding to nuclear transport receptors. Here we report that the effector protein HaRxL106 from the oomycete pathogen Hyaloperonospora arabidopsidis co-opts the host cell's nuclear import machinery. We use HaRxL106 as a probe to determine redundant and specific functions of importin-α paralogs from Arabidopsis thaliana. A crystal structure of the importin-α3/MOS6 armadillo repeat domain suggests that five of the six Arabidopsis importin-αs expressed in rosette leaves have an almost identical NLS-binding site. Comparison of the importin-α binding affinities of HaRxL106 and other cargos in vitro and in plant cells suggests that relatively small affinity differences in vitro affect the rate of transport complex formation in vivo. Our results suggest that cargo affinity for importin-α, sequence variation at the importin-α NLS-binding sites and tissue-specific expression levels of importin-αs determine formation of cargo/importin-α transport complexes in plant cells.

Mutations in SLC2A2 gene reveal hGLUT2 function in pancreatic β cell development

The structure-function relationships of sugar transporter-receptor hGLUT2 coded by SLC2A2 and their impact on insulin secretion and β cell differentiation were investigated through the detailed characterization of a panel of mutations along the protein. We studied naturally occurring SLC2A2 variants or mutants: two single-nucleotide polymorphisms and four proposed inactivating mutations associated to Fanconi-Bickel syndrome. We also engineered mutations based on sequence alignment and conserved amino acids in selected domains. The single-nucleotide polymorphisms P68L and T110I did not impact on sugar transport as assayed in Xenopus oocytes. All the Fanconi-Bickel syndrome-associated mutations invalidated glucose transport by hGLUT2 either through absence of protein at the plasma membrane (G20D and S242R) or through loss of transport capacity despite membrane targeting (P417L and W444R), pointing out crucial amino acids for hGLUT2 transport function. In contrast, engineered mutants were located at the plasma membrane and able to transport sugar, albeit with modified kinetic parameters. Notably, these mutations resulted in gain of function. G20S and L368P mutations increased insulin secretion in the absence of glucose. In addition, these mutants increased insulin-positive cell differentiation when expressed in cultured rat embryonic pancreas. F295Y mutation induced β cell differentiation even in the absence of glucose, suggesting that mutated GLUT2, as a sugar receptor, triggers a signaling pathway independently of glucose transport and metabolism. Our results describe the first gain of function mutations for hGLUT2, revealing the importance of its receptor versus transporter function in pancreatic β cell development and insulin secretion.

Metabolite regulation of nucleo-cytosolic trafficking of carbohydrate response element-binding protein (ChREBP): role of ketone bodies

The carbohydrate response element-binding protein (ChREBP) is a glucose-responsive transcription factor that plays a critical role in converting excess carbohydrate to storage fat in liver. In response to changing glucose levels, ChREBP activity is regulated by nucleo-cytoplasmic shuttling of ChREBP via interactions with 14-3-3 proteins and importins. The nuclear/cytosol trafficking is regulated partly by phosphorylation/dephosphorylation of serine 196 mediated by cAMP-dependent protein kinase and protein phosphatase. We show here that protein-free extracts of starved and high fat-fed livers contain metabolites that activate interaction of ChREBP·14-3-3 and inhibit the ChREBP/importin α interaction, resulting in cytosolic localization. These metabolites were identified as β-hydroxybutyrate and acetoacetate. Nuclear localization of GFP-ChREBP is rapidly inhibited in hepatocytes incubated in β-hydroxybutyrate or fatty acids, and the observed inhibition is closely correlated with the production of ketone bodies. These observations show that ketone bodies play an important role in the regulation of ChREBP activity by restricting ChREBP localization to the cytoplasm, thus inhibiting fat synthesis during periods of ketosis.

Nuclear transport modulation reduces hypercholesterolemia, atherosclerosis, and fatty liver

Background

Elevated cholesterol and triglycerides in blood lead to atherosclerosis and fatty liver, contributing to rising cardiovascular and hepatobiliary morbidity and mortality worldwide.

Methods and Results

A cell‐penetrating nuclear transport modifier (NTM) reduced hyperlipidemia, atherosclerosis, and fatty liver in low‐density lipoprotein receptor‐deficient mice fed a Western diet. NTM treatment led to lower cholesterol and triglyceride levels in blood compared with control animals (36% and 53%, respectively; P<0.005) and liver (41% and 34%, respectively; P<0.05) after 8 weeks. Atherosclerosis was reduced by 63% (P<0.0005), and liver function improved compared with saline‐treated controls. In addition, fasting blood glucose levels were reduced from 209 to 138 mg/dL (P<0.005), and body weight gain was ameliorated (P<0.005) in NTM‐treated mice, although food intake remained the same as that in control animals. The NTM used in this study, cSN50.1 peptide, is known to modulate nuclear transport of stress‐responsive transcription factors such as nuclear factor kappa B, the master regulator of inflammation. This NTM has now been demonstrated to also modulate nuclear transport of sterol regulatory element‐binding protein (SREBP) transcription factors, the master regulators of cholesterol, triglyceride, and fatty acid synthesis. NTM‐modulated translocation of SREBPs to the nucleus was associated with attenuated transactivation of their cognate genes that contribute to hyperlipidemia.

Conclusions

Two‐pronged control of inflammation and dyslipidemia by modulating nuclear transport of their critical regulators offers a new approach to comprehensive amelioration of hyperlipidemia, atherosclerosis, fatty liver, and their potential complications.

Thioredoxin-interacting protein gene expression via MondoA is rapidly and transiently suppressed during inflammatory responses

Whereas accumulating evidence indicates that a number of inflammatory genes are induced by activation of nuclear factor-κB and other transcription factors, less is known about genes that are suppressed by proinflammatory stimuli. Here we show that expression of thioredoxin-interacting protein (Txnip) is dramatically suppressed both in mRNA and protein levels upon stimulation with lipopolysaccharide in mouse and human macrophages. In addition to lipopolysaccharide, a Toll-like receptor 4 ligand, stimulation with other Toll-like receptor ligands such as CpG DNA also suppressed Txnip expression. Not only the Toll-like receptor ligands, but also other proinflammatory stimulators, such as interleukin-1β and tumor necrosis factor-α elicited the similar response in fibroblasts. Suppression of Txnip by lipopolysaccharide is accompanied by a decrease of the glucose sensing transcription factor MondoA in the nuclei and dissociation of the MondoA:Mlx complex that bound to the carbohydrate-response elements in the Txnip promoter in unstimulated cells. Lipopolysaccharide-mediated decrease of nuclear MondoA was inhibited in the presence of 2-deoxyglucose. Furthermore, blockage of glyceraldehyde-3-phosphate dehydrogenase by iodoacetate alleviated the suppression of Txnip mRNA by lipopolysaccharide, suggesting the involvement of glucose-metabolites in the regulation. Since Txnip is implicated in the regulation of glucose metabolism, this observation links between inflammatory responses and metabolic regulation.

Hop-on hop-off: importin-α-guided tours to the nucleus in innate immune signaling

Nuclear translocation of immune regulatory proteins and signal transducers is an essential process in animal and plant defense signaling against pathogenic microbes. Import of proteins containing a nuclear localization signal (NLS) into the nucleus is mediated by nuclear transport receptors termed importins, typically dimers of a cargo-binding α-subunit and a β-subunit that mediates translocation through the nuclear pore complex. Here, we review recent reports of importin-α cargo specificity and mutant phenotypes in plant- and animal-microbe interactions. Using homology modeling of the NLS-binding cleft of nine predicted Arabidopsis α-importins and analyses of their gene expression patterns, we discuss functional redundancy and specialization within this transport receptor family. In addition, we consider how pathogen effector proteins that promote infection by manipulating host cell nuclear processes might compete with endogenous cargo proteins for nuclear uptake.

Recent progress on the role of ChREBP in glucose and lipid metabolism

Carbohydrate response element binding protein (ChREBP) is a transcription factor activated by glucose that is highly expressed in liver, pancreatic β-cells, brown and white adipose tissues, and muscle. We reported that hepatic suppression of the Chrebp gene improves hepatic steatosis, glucose intolerance, and obesity in genetically obese mice. Moreover, we have studied the role of ChREBP with special reference to feedforward and feedback looping in liver and pancreatic β-cells. Recently, several groups reported that (1) glucose activates ChREBP-α transactivity and in turn ChREBP-α induces ChREBP-β on both transcriptional and translational levels in adipose tissues, and (2) ChREBP regulates glucose transporter type 4 mRNA levels, which may affect glucose uptake in adipose tissues. Moreover, in adipose tissues of obese patients, Chrebpb mRNA levels were much lower than those in lean subjects, while the levels were much higher in liver of obese patients than those in lean subjects. These findings suggest that Chrebpb mRNA levels are different in various tissues and probably in the stages of diabetes mellitus. Herein, we review recent progress in the study of ChREBP with special references to (1) the mechanisms regulating ChREBP transactivity (posttranslational modifications, intramolecular glucose sensing module, feedforward mechanism, and the feedback loop between ChREBP and its target genes), and (2) the role of ChREBP in liver, pancreatic islets and adipose tissues. Understanding the role of ChREBP in each tissue will provide important insight into the pathogenesis of metabolic syndrome.

Nuclear import of chromatin remodeler Isw1 is mediated by atypical bipartite cNLS and classical import pathway

The protein Isw1 of Saccharomyces cerevisiae is an imitation-switch chromatin-remodeling factor. We studied the mechanisms of its nuclear import and found that the nuclear localization signal (NLS) mediating the transport of Isw1 into the nucleus is located at the end of the C-terminus of the protein (aa1079-1105). We show that it is an atypical bipartite signal with an unconventional linker of 19 aa (KRIR X(19) KKAK) and the only nuclear targeting signal within the Isw1 molecule. The efficiency of Isw1 nuclear import was found to be modulated by changes to the amino acid composition in the vicinity of the KRIR motif, but not by the linker length. Live-cell imaging of various karyopherin mutants and in vitro binding assays of Isw1NLS to importin-α revealed that the nuclear translocation of Isw1 is mediated by the classical import pathway. Analogous motifs to Isw1NLS are highly conserved in Isw1 homologues of other yeast species, and putative bipartite cNLS were identified in silico at the end of the C-termini of imitation switch (ISWI) proteins from higher eukaryotes. We suggest that the C-termini of the ISWI family proteins play an important role in their nuclear import.

Structural characterization of a unique interface between carbohydrate response element-binding protein (ChREBP) and 14-3-3β protein

Carbohydrate response element-binding protein (ChREBP) is an insulin-independent, glucose-responsive transcription factor that is expressed at high levels in liver hepatocytes where it plays a critical role in converting excess carbohydrates to fat for storage. In response to fluctuating glucose levels, hepatic ChREBP activity is regulated in large part by nucleocytoplasmic shuttling of ChREBP protein via interactions with 14-3-3 proteins. The N-terminal ChREBP regulatory region is necessary and sufficient for glucose-responsive ChREBP nuclear import and export. Here, we report the crystal structure of a complex of 14-3-3β bound to the N-terminal regulatory region of ChREBP at 2.4 Å resolution. The crystal structure revealed that the α2 helix of ChREBP (residues 117-137) adopts a well defined α-helical conformation and binds 14-3-3 in a phosphorylation-independent manner that is different from all previously characterized 14-3-3 and target protein-binding modes. ChREBP α2 interacts with 14-3-3 through both electrostatic and van der Waals interactions, and the binding is partially mediated by a free sulfate or phosphate. Structure-based mutagenesis and binding assays indicated that disrupting the observed 14-3-3 and ChREBP α2 interface resulted in a loss of complex formation, thus validating the novel protein interaction mode in the 14-3-3β·ChREBP α2 complex.

Roles of Ca2+ ions in the control of ChREBP nuclear translocation

Carbohydrate-responsive element binding protein (ChREBP (MLXIPL)) is emerging as an important mediator of glucotoxity both in the liver and in the pancreatic β-cells. Although the regulation of its nuclear translocation and transcriptional activation by glucose has been the subject of intensive research, it is still not fully understood. We have recently uncovered a novel mechanism in the excitable pancreatic β-cell where ChREBP interacts with sorcin, a penta-EF-hand Ca(2)(+)-binding protein, and is sequestered in the cytosol at low glucose concentrations. Upon stimulation with glucose and activation of Ca(2)(+) influx, or application of ATP as an intracellular Ca(2)(+)-mobilising agent, ChREBP rapidly translocates to the nucleus. In sorcin-silenced cells, ChREBP is constitutively present in the nucleus, and both glucose and Ca(2)(+) are ineffective in stimulating further ChREBP nuclear shuttling. Whether an active Ca(2)(+)-sorcin element of ChREBP activation also exists in non-excitable cells is discussed.

Glucose sensing by ChREBP/MondoA-Mlx transcription factors

The paralogous transcription factors ChREBP and MondoA, together with their common binding partner Mlx, have emerged as key mediators of intracellular glucose sensing. By regulating target genes involved in glycolysis and lipogenesis, they mediate metabolic adaptation to changing glucose levels. As disturbed glucose homeostasis plays a central role in human metabolic diseases and as cancer cells often display altered glucose metabolism, better understanding of cellular glucose sensing will likely uncover new therapeutic opportunities. Here we review the regulation, function and evolutionary conservation of the ChREBP/MondoA-Mlx glucose sensing system and discuss possible directions for future research.