The subcellular localization of the ChoRE-binding protein, encoded by the Williams–Beuren syndrome critical region gene 14, is regulated by 14-3-3

Age-related retinal degeneration resulting from the deletion of Shp2 tyrosine phosphatase in photoreceptor neurons

Shp2, a critical SH2-domain-containing tyrosine phosphatase, is essential for cellular regulation and implicated in metabolic disruptions, obesity, diabetes, Noonan syndrome, LEOPARD syndrome, and cancers. This study focuses on Shp2 in rod photoreceptor cells, revealing its enrichment, particularly in rods. Deletion of Shp2 in rods leads to age-dependent photoreceptor degeneration. Shp2 targets occludin (OCLN), a tight junction protein, and its deletion reduces OCLN expression in the retina and retinal pigment epithelium (RPE). The isolation of actively translating mRNAs from rods lacking Shp2, followed by RNA sequencing, reveals alterations in cell cycle regulation. Additionally, altered retinal metabolism is observed in retinal cells lacking Shp2. Our studies indicate that Shp2 is crucial for maintaining the structure and function of photoreceptors.

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.

MondoA drives malignancy in B-ALL through enhanced adaptation to metabolic stress

Cancer cells are in most instances characterized by rapid proliferation and uncontrolled cell division. Hence, they must adapt to proliferation-induced metabolic stress through intrinsic or acquired antimetabolic stress responses to maintain homeostasis and survival. One mechanism to achieve this is reprogramming gene expression in a metabolism-dependent manner. MondoA (also known as Myc-associated factor X-like protein X-interacting protein [MLXIP]), a member of the MYC interactome, has been described as an example of such a metabolic sensor. However, the role of MondoA in malignancy is not fully understood and the underlying mechanism in metabolic responses remains elusive. By assessing patient data sets, we found that MondoA overexpression is associated with worse survival in pediatric common acute lymphoblastic leukemia (ALL; B-precursor ALL [B-ALL]). Using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) and RNA-interference approaches, we observed that MondoA depletion reduces the transformational capacity of B-ALL cells in vitro and dramatically inhibits malignant potential in an in vivo mouse model. Interestingly, reduced expression of MondoA in patient data sets correlated with enrichment in metabolic pathways. The loss of MondoA correlated with increased tricarboxylic acid cycle activity. Mechanistically, MondoA senses metabolic stress in B-ALL cells by restricting oxidative phosphorylation through reduced pyruvate dehydrogenase activity. Glutamine starvation conditions greatly enhance this effect and highlight the inability to mitigate metabolic stress upon loss of MondoA in B-ALL. Our findings give novel insight into the function of MondoA in pediatric B-ALL and support the notion that MondoA inhibition in this entity offers a therapeutic opportunity and should be further explored.

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.

Comparative Genomic Analysis of the DUF34 Protein Family Suggests Role as a Metal Ion Chaperone or Insertase

Members of the DUF34 (domain of unknown function 34) family, also known as the NIF3 protein superfamily, are ubiquitous across superkingdoms. Proteins of this family have been widely annotated as "GTP cyclohydrolase I type 2" through electronic propagation based on one study. Here, the annotation status of this protein family was examined through a comprehensive literature review and integrative bioinformatic analyses that revealed varied pleiotropic associations and phenotypes. This analysis combined with functional complementation studies strongly challenges the current annotation and suggests that DUF34 family members may serve as metal ion insertases, chaperones, or metallocofactor maturases. This general molecular function could explain how DUF34 subgroups participate in highly diversified pathways such as cell differentiation, metal ion homeostasis, pathogen virulence, redox, and universal stress responses.

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.

Role for carbohydrate response element-binding protein (ChREBP) in high glucose-mediated repression of long noncoding RNA Tug1

Long noncoding RNAs (lncRNAs) have been shown to play key roles in a variety of biological activities of the cell. However, less is known about how lncRNAs respond to environmental cues and what transcriptional mechanisms regulate their expression. Studies from our laboratory have shown that the lncRNA Tug1 (taurine upregulated gene 1) is crucial for the progression of diabetic kidney disease, a major microvascular complication of diabetes. Using a combination of proximity labeling with the engineered soybean ascorbate peroxidase (APEX2), ChIP-qPCR, biotin-labeled oligonucleotide pulldown, and classical promoter luciferase assays in kidney podocytes, we extend our initial observations in the current study and now provide a detailed analysis on a how high-glucose milieu downregulates Tug1 expression in podocytes. Our results revealed an essential role for the transcription factor carbohydrate response element binding protein (ChREBP) in controlling Tug1 transcription in the podocytes in response to increased glucose levels. Along with ChREBP, other coregulators, including MAX dimerization protein (MLX), MAX dimerization protein 1 (MXD1), and histone deacetylase 1 (HDAC1), were enriched at the Tug1 promoter under high-glucose conditions. These observations provide the first characterization of the mouse Tug1 promoter's response to the high-glucose milieu. Our findings illustrate a molecular mechanism by which ChREBP can coordinate glucose homeostasis with the expression of the lncRNA Tug1 and further our understanding of dynamic transcriptional regulation of lncRNAs in a disease state.

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.

Nutritional Regulation of Gene Expression: Carbohydrate-, Fat- and Amino Acid-Dependent Modulation of Transcriptional Activity

The ability to detect changes in nutrient levels and generate an adequate response to these changes is essential for the proper functioning of living organisms. Adaptation to the high degree of variability in nutrient intake requires precise control of metabolic pathways. Mammals have developed different mechanisms to detect the abundance of nutrients such as sugars, lipids and amino acids and provide an integrated response. These mechanisms include the control of gene expression (from transcription to translation). This review reports the main molecular mechanisms that connect nutrients’ levels, gene expression and metabolism in health. The manuscript is focused on sugars’ signaling through the carbohydrate-responsive element binding protein (ChREBP), the role of peroxisome proliferator-activated receptors (PPARs) in the response to fat and GCN2/activating transcription factor 4 (ATF4) and mTORC1 pathways that sense amino acid concentrations. Frequently, alterations in these pathways underlie the onset of several metabolic pathologies such as obesity, insulin resistance, type 2 diabetes, cardiovascular diseases or cancer. In this context, the complete understanding of these mechanisms may improve our knowledge of metabolic diseases and may offer new therapeutic approaches based on nutritional interventions and individual genetic makeup.

Sugar sensing by ChREBP/Mondo-Mlx-new insight into downstream regulatory networks and integration of nutrient-derived signals

Animals regulate their physiology with respect to nutrient status, which requires nutrient sensing pathways. Simple carbohydrates, sugars, are sensed by the basic-helix-loop-helix leucine zipper transcription factors ChREBP/Mondo, together with their heterodimerization partner Mlx, which are well-established activators of sugar-induced lipogenesis. Loss of ChREBP/Mondo-Mlx in mouse and Drosophila leads to sugar intolerance, that is, inability to survive on sugar containing diet. Recent evidence has revealed that ChREBP/Mondo-Mlx responds to sugar and fatty acid-derived metabolites through several mechanisms and cross-connects with other nutrient sensing pathways. ChREBP/Mondo-Mlx controls several downstream transcription factors and hormones, which mediate not only readjustment of metabolic pathways, but also control feeding behavior, intestinal digestion, and circadian rhythm.

Impaired glucose metabolism in subjects with the Williams-Beuren syndrome: A five-year follow-up cohort study

Objective

The Williams-Beuren syndrome (WS) is associated with impaired glucose metabolism (IGM) early in adulthood. However, the pathophysiology of IGM remains poorly defined, due to the lack of longitudinal studies investigating the contribution of β-cell dysfunction and impaired insulin sensitivity. This study aimed at assessing incidence of IGM and the underlying mechanisms in WS adults.

Methods

This observational, longitudinal (5-year), cohort study enrolled thirty-one consecutive WS subjects attending a tertiary referral center. An oral glucose tolerance test (OGTT) was performed yearly and used to classify patients as normal or IGM, including impaired fasting glucose (IFG) and/or impaired glucose tolerance (IGT) and diabetes mellitus (DM), and to calculate surrogate measures of insulin secretion and/or sensitivity.

Results

IGM patients were 18 (58.1%, three DM) at baseline and 19 (61.3%, five DM) at end-of-follow-up. However, 13 individuals changed category of glucose homeostasis in both directions during follow-up (8 progressors, 5 regressors) and 18 did not (8 non-progressors, 10 non-regressors). New cases of IGM and DM were 11.1 and 2.53 per 100 persons-year, respectively, and were treated non-pharmacologically. In the whole cohort and, to a higher extent, in progressors, indices of early-phase insulin secretion and insulin sensitivity decreased significantly from baseline to end-of-follow-up, with concurrent reduction of the oral disposition index and insulin secretion-sensitivity index-2 (ISSI-2), compensating insulin secretion for the level of insulin resistance. No baseline measure independently predicted progression, which correlated with change from baseline in ISSI-2. Compared with patients with normal glucose homeostasis, IGT subjects had impaired insulin sensitivity, whereas insulin secretion was reduced only in those with IFG+IGT or DM.

Conclusions

IGM incidence is high in young adults with WS, suggesting the need of early screening and timed intervention. As in classical type 2 diabetes, impaired insulin sensitivity and β-cell dysfunction contribute, in this sequence, to progression to IGM and DM.

The regulation and role of carbohydrate response element-binding protein in metabolic homeostasis and disease

The transcription factor carbohydrate response element-binding protein (ChREBP) is a member of the basic helix-loop-helix leucine zipper transcription factor family. Under high-glucose conditions, it has a role in regulating the expression of key genes involved in various pathways, including glycolysis, gluconeogenesis and lipogenesis. It does this by forming a tetrameric complex made up of two ChREBP/Mlx heterodimers, which enables it to bind to the carbohydrate response element (ChoRE) in the promoter region of its target genes to regulate transcription. Because ChREBP plays a key role in glucose signalling and metabolism, and aberrations in glucose homeostasis are often present in metabolic diseases, this transcription factor presents itself as an enticing target with respect to further understanding metabolic disease mechanisms and potentially uncovering new therapeutic targets.

Sweet Sixteenth for ChREBP: Established Roles and Future Goals

With the identification of ChREBP in 2001, our interest in understanding the molecular control of carbohydrate sensing has surged. While ChREBP was initially studied as a master regulator of lipogenesis in liver and fat tissue, it is now clear that ChREBP functions as a central metabolic coordinator in a variety of cell types in response to environmental and hormonal signals, with wide implications in health and disease. Celebrating its sweet sixteenth birthday, we review here the current knowledge about the function and regulation of ChREBP throughout usual and less explored tissues, to recapitulate ChREBP's role as a whole-body glucose sensor.

Validation, Identification, and Biological Consequences of the Site-specific O-GlcNAcylation Dynamics of Carbohydrate-responsive Element-binding Protein (ChREBP)

O-GlcNAcylation of carbohydrate-responsive element-binding protein (ChREBP) is believed as an important modulator of ChREBP activities, however little direct evidence of O-GlcNAcylation on ChREBP and no exact O-GlcNAcylation sites have been reported so far. Here, we validate O-GlcNAcylation on ChREBP in cell-free coupled transcription/translation system and in cells by chemoenzymatic and metabolic labeling, respectively. Moreover, for the first time, we identify O-GlcNAcylation on Ser614 in the C-terminus of ChREBP by mass spectrometry and validate two important sites, Thr517 and Ser839 for O-GlcNAcylation and their function via molecular and chemical biological method. Under high glucose conditions, Ser514 phosphorylation enhances ChREBP O-GlcNAcylation, maintaining the transcriptional activity of ChREBP; Ser839 O-GlcNAcylation is essential for Mlx-heterodimerization and DNA-binding activity enhancement, consequently inducing transcriptional activity. Ser839 O-GlcNAcylation is also crucial for ChREBP nuclear export partially by strengthening interactions with CRM1 and 14-3-3. This work is a detailed study of ChREBP O-GlcNAcylation and highlights the biological consequences of the site-specific O-GlcNAcylation dynamics of ChREBP.

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.

A proteomic adaptation of small intestinal mucosa in response to dietary protein limitation

Dietary protein limitation (PL) is not only beneficial to human health but also applied to minimize nitrogen excretion in livestock production. However, the impact of PL on intestinal physiology is largely unknown. In this study, we identified 5275 quantitative proteins using a porcine model in which pigs suffered PL. A total of 202 proteins |log₂ fold-change| > 1 were taken as differentially expressed proteins and subjected to functional and pathway enrichment analysis to reveal proteomic alterations of the jejunal mucosa. Combining with the results of western blotting analysis, we found that protein/carbohydrate digestion, intestinal mucosal tight junction and cell adhesion molecules, and the immune response to foreign antigens were increased in the jejunal mucosa of the pigs upon PL. In contrast, amino acid transport, innate and auto immunity, as well as cell proliferation and apoptosis were reduced. In addition, the expression of functional proteins that involved in DNA replication, transcription and mRNA splicing as well as translation were altered in the jejunal mucosa in response to PL. Furthermore, PL may reduce amino acid transport and cell proliferation through the depression of mTOR pathway. This study provides new insights into the molecular mechanisms underlying the small intestinal response to PL.

Williams-beuren syndrome is a genetic disorder associated with impaired glucose tolerance and diabetes in childhood and adolescence: new insights from a longitudinal study

BACKGROUND

In adults with Williams-Beuren syndrome (WBS), a common endocrine abnormality is type 2 diabetes mellitus (T2DM) or impaired glucose tolerance (IGT). However, few and sporadic data are available in children, adolescents, and young adults with WBS.

AIM

To evaluate the frequency of IGT and T2DM in a cohort of children and young patients with WBS.

PATIENTS AND METHODS

We longitudinally evaluated 27 patients (9 males and 18 females, median age at study onset 13.6 years) with WBS. The median follow-up was 3.6 years. Variables of insulin resistance and β-cell function were evaluated in all subjects using an oral glucose tolerance test. The homeostasis model assessment (HOMA) of insulin resistance and the Matsuda index of insulin sensitivity were calculated. The study of the GCK and HNF1Α genes was performed in patients with glucose metabolism abnormalities. 45 age- and sex-matched healthy subjects and 51 age-, sex- and BMI-matched subjects were recruited as two control groups.

RESULTS

Considering nutritional status, 7 (25.9%) patients were obese, 9 (33.3%) overweight, and 11 (40.8%) normal-weight. One (3.1%) patient had acanthosis nigricans. IGT was diagnosed in 7 (25.9%) WBS patients and T2DM in 3 (11.1%). Considering all WBS patients, the median value of HOMA was 5.23 (range 2.93-14.89; insulin 24.73 ± 14.67 μU/ml; glucose 104.98 ± 16.06 mg/dl). Considering BMI values, HOMA was 11.00 (range 6.53-12.56), 5.64 (range 3.54-7.95), and 4.54 (range 3.21-5.43), and insulin was 34.53 ± 6.84, 22.76 ± 8.91, and 19.47 ± 6.01 μU/ml in obese, overweight, and normal-weight WBS patients, respectively. Comparing the results with the two control groups, WBS patients showed higher insulin values than healthy controls (p < 0.001), but similar values as the BMI-matched control group (p = n.s.). However, WBS patients showed significantly higher values of glycemia (healthy control group, p < 0.001; BMI-matched control group, p < 0.05) and HOMA (healthy control group, p < 0.001; BMI-matched control group, p < 0.05) than the two control groups. Finally, among WBS patients there was a higher number of subjects with IGT and T2DM than among healthy controls (p < 0.0001) and the BMI-matched control group (p = 0.0002).

CONCLUSION

Our data strongly suggest that IGT and T2DM may be frequently discovered in children, adolescents, and young adults with WBS. WBS should be included among the genetic syndromes associated with T2DM. Further studies are necessary to evaluate the etiopathogenesis of this aspect.

Energy metabolism in the liver

The liver is an essential metabolic organ, and its metabolic function is controlled by insulin and other metabolic hormones. Glucose is converted into pyruvate through glycolysis in the cytoplasm, and pyruvate is subsequently oxidized in the mitochondria to generate ATP through the TCA cycle and oxidative phosphorylation. In the fed state, glycolytic products are used to synthesize fatty acids through de novo lipogenesis. Long-chain fatty acids are incorporated into triacylglycerol, phospholipids, and/or cholesterol esters in hepatocytes. These complex lipids are stored in lipid droplets and membrane structures, or secreted into the circulation as very low-density lipoprotein particles. In the fasted state, the liver secretes glucose through both glycogenolysis and gluconeogenesis. During pronged fasting, hepatic gluconeogenesis is the primary source for endogenous glucose production. Fasting also promotes lipolysis in adipose tissue, resulting in release of nonesterified fatty acids which are converted into ketone bodies in hepatic mitochondria though β-oxidation and ketogenesis. Ketone bodies provide a metabolic fuel for extrahepatic tissues. Liver energy metabolism is tightly regulated by neuronal and hormonal signals. The sympathetic system stimulates, whereas the parasympathetic system suppresses, hepatic gluconeogenesis. Insulin stimulates glycolysis and lipogenesis but suppresses gluconeogenesis, and glucagon counteracts insulin action. Numerous transcription factors and coactivators, including CREB, FOXO1, ChREBP, SREBP, PGC-1α, and CRTC2, control the expression of the enzymes which catalyze key steps of metabolic pathways, thus controlling liver energy metabolism. Aberrant energy metabolism in the liver promotes insulin resistance, diabetes, and nonalcoholic fatty liver diseases.

Flightless I homolog negatively regulates ChREBP activity in cancer cells

The glucose-responsive transcription factor carbohydrate responsive element binding protein (ChREBP) plays an important role in regulating glucose metabolism in support of anabolic synthesis in both hepatocytes and cancer cells. In order to further investigate the molecular mechanism by which ChREBP regulates transcription, we used a proteomic approach to identify proteins interacting with ChREBP. We found several potential ChREBP-interacting partners, one of which, flightless I homolog (FLII) was verified to interact and co-localize with ChREBP in HCT116 colorectal cancer and HepG2 hepatocellular carcinoma cells. FLII is a member of the gelsolin superfamily of actin-remodeling proteins and can function as a transcriptional co-regulator. The C-terminal 227 amino acid region of ChREBP containing the DNA-binding domain interacted with FLII. Both the N-terminal leucine-rich repeat (LRR) domain and C-terminal gelsolin homolog domain (GLD) of FLII interacted and co-localized with ChREBP. ChREBP and FLII localized in both the cytoplasm and nucleus of cancer cells. Glucose increased expression and nuclear localization of ChREBP, and had minimal effect on the level and distribution of FLII. FLII knockdown using siRNAs increased mRNA and protein levels of ChREBP-activated genes and decreased transcription of ChREBP-repressed genes in cancer cells. Conversely, FLII overexpression negatively regulated ChREBP-mediated transcription in cancer cells. Our findings suggest that FLII is a component of the ChREBP transcriptional complex and negatively regulates ChREBP function in cancer cells.

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.

Prevalence of diabetes and pre-diabetes in a cohort of Italian young adults with Williams syndrome

Williams syndrome (WS) is a rare, multisystemic genomic disorder showing a high prevalence of impaired glucose metabolism in adulthood. The reason for this association is unknown, though hemizygosity for genes mapping to the WS chromosome region has been implicated. Twenty-two Italian young adults with WS (13 females, 9 males) were studied. A 75 g oral glucose tolerance test (OGTT) was performed and β-cell function was estimated with Homeostasis Model Assessment (HOMA)-B%, Insulinogenic Index, and corrected insulin response whereas insulin sensitivity was assessed with HOMA-Insulin Resistance Index, Quantitative Insulin Check Index, and composite Insulin Sensitivity Index. One patient had known diabetes mellitus (DM), whereas impaired glucose tolerance (IGT) was diagnosed in 12 patients and DM in one (63.6% prevalence of impaired glucose metabolism). IGT patients were more insulin resistant than those with normal glucose tolerance (NGT), whereas β-cell function was unchanged or increased. Islet autoimmunity was absent. Logistic regression showed that impaired glucose metabolism was not associated with age, body mass index (BMI), or family history of DM. β-cell function, insulin sensitivity, and post-load insulin levels did not differ between WS patients with NGT and healthy controls comparable for gender, age, and BMI, though WS-NGT patients had higher post-load glucose values. These data confirm the high prevalence of impaired glucose metabolism in WS young adults, thus suggesting the need for screening these patients with OGTT. IGT is associated with reduced insulin sensitivity, but not with impaired β-cell function, islet autoimmunity, and traditional risk factors for type 2 DM.

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.

Insulin Signaling, GSK-3, Heat Shock Proteins and the Natural History of Type 2 Diabetes Mellitus: A Hypothesis

Metabolic syndrome and type 2 diabetes are progressive, indolent, multi-organ diseases. Understanding the abnormalities of heat shock proteins (HSPs) in these diseases is paramount to understanding their pathogenesis. In insulin resistant states and diabetes, heat shock factor 1(HSF-1) is low in insulin sensitive tissues, resulting in low Hsp 60, 70, and 90 levels. We propose that low Hsps levels are the result of decreased insulin action leading to less phosphorylation of PI3K, PKB, and glycogen synthase kinase-3 (GSK-3). Importantly, less GSK-3 phosphorylation (and thus more GSK-3 activity) will lower HSF-1. Low Hsps make organs vulnerable to injury, impair the stress response, accelerate systemic inflammation, raise islet amyloid polypeptide, and increase insulin resistance. Feeding this cycle is excess saturated fat and calorie consumption, hypertension, inactivity, aging, and genetic predisposition- all of which are a associated with high GSK-3 activity and low Hsps. Support for the proposed "vicious" cycle is based on the observation that GSK-3 inhibition and Hsp stimulation result in increased insulin sensitivity, reduced accumulation of degenerative proteins with in the cell, improved wound healing, decreased organ damage and improved recovery from vascular ischemia. Recognizing GSK-3 and Hsps in the pathogenesis of insulin resistance, the central common feature of the metabolic syndrome, and type 2 diabetes will expand our understanding of the disease, offering new therapeutic options.

The E3-ubiquitin ligase TRIM50 interacts with HDAC6 and p62, and promotes the sequestration and clearance of ubiquitinated proteins into the aggresome

In this study we report that, in response to proteasome inhibition, the E3-Ubiquitin ligase TRIM50 localizes to and promotes the recruitment and aggregation of polyubiquitinated proteins to the aggresome. Using Hdac6-deficient mouse embryo fibroblasts (MEF) we show that this localization is mediated by the histone deacetylase 6, HDAC6. Whereas Trim50-deficient MEFs allow pinpointing that the TRIM50 ubiquitin-ligase regulates the clearance of polyubiquitinated proteins localized to the aggresome. Finally we demonstrate that TRIM50 colocalizes, interacts with and increases the level of p62, a multifunctional adaptor protein implicated in various cellular processes including the autophagy clearance of polyubiquitinated protein aggregates. We speculate that when the proteasome activity is impaired, TRIM50 fails to drive its substrates to the proteasome-mediated degradation, and promotes their storage in the aggresome for successive clearance.

Adaptive metabolic response to 4 weeks of sugar-sweetened beverage consumption in healthy, lightly active individuals and chronic high glucose availability in primary human myotubes

PURPOSE

Chronic sugar-sweetened beverage (SSB) consumption is associated with obesity and type 2 diabetes mellitus (T2DM). Hyperglycaemia contributes to metabolic alterations observed in T2DM, such as reduced oxidative capacity and elevated glycolytic and lipogenic enzyme expressions in skeletal muscle tissue. We aimed to investigate the metabolic alterations induced by SSB supplementation in healthy individuals and to compare these with the effects of chronic hyperglycaemia on primary muscle cell cultures.

METHODS

Lightly active, healthy, lean subjects (n = 11) with sporadic soft drink consumption underwent a 4-week SSB supplementation (140 ± 15 g/day, ~2 g glucose/kg body weight/day, glucose syrup). Before and after the intervention, body composition, respiratory exchange ratio (RER), insulin sensitivity, muscle metabolic gene and protein expression were assessed. Adaptive responses to hyperglycaemia (7 days, 15 mM) were tested in primary human myotubes.

RESULTS

SSB supplementation increased fat mass (+1.0 kg, P < 0.05), fasting RER (+0.12, P < 0.05), fasting glucose (+0.3 mmol/L, P < 0.05) and muscle GAPDH mRNA expressions (+0.94 AU, P < 0.05). PGC1α mRNA was reduced (-0.20 AU, P < 0.05). Trends were found for insulin resistance (+0.16 mU/L, P = 0.09), and MondoA protein levels (+1.58 AU, P = 0.08). Primary myotubes showed elevations in GAPDH, ACC, MondoA and TXNIP protein expressions (P < 0.05).

CONCLUSION

Four weeks of SSB supplementation in healthy individuals shifted substrate metabolism towards carbohydrates, increasing glycolytic and lipogenic gene expression and reducing mitochondrial markers. Glucose-sensing protein MondoA might contribute to this shift, although further in vivo evidence is needed to corroborate this.

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.

A novel N-terminal domain may dictate the glucose response of Mondo proteins

Glucose is a fundamental energy source for both prokaryotes and eukaryotes. The balance between glucose utilization and storage is integral for proper energy homeostasis, and defects are associated with several diseases, e.g. type II diabetes. In vertebrates, the transcription factor ChREBP is a major component in glucose metabolism, while its ortholog MondoA is involved in glucose uptake. Both MondoA and ChREBP contain five Mondo conserved regions (MCRI-V) that affect their cellular localization and transactivation ability. While phosphorylation has been shown to affect ChREBP function, the mechanisms controlling glucose response of both ChREBP and MondoA remain elusive. By incorporating sequence analysis techniques, structure predictions, and functional annotations, we synthesized data surrounding Mondo family proteins into a cohesive, accurate, and general model involving the MCRs and two additional domains that determine ChREBP and MondoA glucose response. Paramount, we identified a conserved motif within the transactivation region of Mondo family proteins and propose that this motif interacts with the phosphorylated form of glucose. In addition, we discovered a putative nuclear receptor box in non-vertebrate Mondo and vertebrate ChREBP sequences that reveals a potentially novel interaction with nuclear receptors. These interactions are likely involved in altering ChREBP and MondoA conformation to form an active complex and induce transcription of genes involved in glucose metabolism and lipogenesis.

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.

Glucose-induced nuclear shuttling of ChREBP is mediated by sorcin and Ca(2+) ions in pancreatic β-cells

Carbohydrate-responsive element-binding protein (ChREBP) is a regulator of pancreatic β-cell gene expression and an important mediator of glucotoxicity. Glucose increases the activity and nuclear localization of ChREBP by still ill-defined mechanisms. Here we reveal, using both MIN6 and primary mouse β-cells, a unique mechanism behind ChREBP nuclear translocation. At low glucose concentrations, ChREBP interacts with sorcin, a penta EF hand Ca(2+) binding protein, and is sequestered in the cytosol. Sorcin overexpression inhibits ChREBP nuclear accumulation at high glucose and reduced the activity of L-type pyruvate kinase (L-PK) and TxNIP promoters, two well-characterized ChREBP target genes. Sorcin inactivation by RNA interference increases ChREBP nuclear localization and in vivo binding to the L-PK promoter at low glucose concentrations. Ca(2+) influx was essential for this process since Ca(2+) chelation with EGTA, or pharmacological inhibition with diazoxide and nifedipine, blocked the effects of glucose. Conversely, mobilization of intracellular Ca(2+) with ATP caused the nuclear accumulation of ChREBP. Finally, sorcin silencing inhibited ATP-induced increases in intracellular Ca(2+) and glucose-stimulated insulin secretion. We therefore conclude that sorcin retains ChREBP in the cytosol at low glucose concentrations and may act as a Ca(2+) sensor for glucose-induced nuclear translocation and the activation of ChREBP-dependent genes.

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

Carbohydrate response element-binding protein (ChREBP) is a glucose-responsive transcription factor that plays a critical role in the glucose-mediated induction of genes involved in hepatic glycolysis and lipogenesis. Circulating blood glucose levels affect ChREBP activity in hepatocytes largely by post-translational mechanisms that include phosphorylation-dependent subcellular localization. Previously, we showed that ChREBP is retained in the cytosol by phosphorylation-dependent binding to 14-3-3 protein dimers and identified the α2 helix (residues 125-135) phospho-Ser(140) domain as the primary 14-3-3 binding site (Sakiyama, H., Wynn, R. M., Lee, W. R., Fukasawa, M., Mizuguchi, H., Gardner, K. H., Repa, J. J., and Uyeda, K. (2008) J. Biol. Chem. 283, 24899-24908). To enter the nucleus in response to high glucose, ChREBP must bind importin-α; this heterodimer then forms a complex with importin-β to interact with the nuclear pore complex. In this work, we recharacterized the importin-α binding nuclear localization signal (NLS) of rat ChREBP, identifying it as an extended classical bipartite NLS encompassing minimally residues 158-190. Replacing Lys(159)/Lys(190) residues of ChREBP with alanine resulted in loss of importin-α binding, glucose-stimulated transcriptional activity and nuclear localization. A secondary 14-3-3 protein binding site also was identified, the α3 helix (residues 170-190) phospho-Ser(196) domain. Importin-α and 14-3-3 were found to bind competitively to this secondary site. These results suggest an important mechanism by which importin-α and 14-3-3 control movement of ChREBP in and out of the nucleus in response to changes in glucose levels in liver and thus further suggest that the extended NLS of ChREBP is a critical glucose-sensing, glucose-responsive site.

Cross-regulation of hepatic glucose metabolism via ChREBP and nuclear receptors

There is a worldwide epidemic of obesity and type 2 diabetes, two major public health concerns associated with alterations in both insulin and glucose signaling pathways. Glucose is not only an energy source but also controls the expression of key genes involved in energetic metabolism, through the glucose-signaling transcription factor, Carbohydrate Responsive Element Binding Protein (ChREBP). ChREBP has emerged as a central regulator of de novo fatty acid synthesis (lipogenesis) in response to glucose under both physiological and physiopathological conditions. Glucose activates ChREBP by regulating its entry from the cytosol to the nucleus, thereby promoting its binding to carbohydrate responsive element (ChoRE) in the promoter regions of glycolytic (L-PK) and lipogenic genes (ACC and FAS). We have previously reported that the inhibition of ChREBP in liver of obese ob/ob mice improves the metabolic alterations linked to obesity, fatty liver and insulin-resistance. Therefore, regulating ChREBP activity could be an attractive target for lipid-lowering therapies in obesity and diabetes. However, before this is possible, a better understanding of the mechanism(s) regulating its activity is needed. In this review, we summarize recent findings on the role and regulation of ChREBP and particularly emphasize on the cross-regulations that may exist between key nuclear receptors (LXR, TR, HNF4α) and ChREBP for the control of hepatic glucose metabolism. These novel molecular cross-talks may open the way to new pharmacological opportunities. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

ChREBP mediates glucose repression of peroxisome proliferator-activated receptor alpha expression in pancreatic beta-cells

Chronic exposure to elevated levels of glucose and fatty acids leads to dysfunction of pancreatic β-cells by mechanisms that are only partly understood. The transcription factor peroxisome proliferator-activated receptor α (PPARα) is an important regulator of genes involved in fatty acid metabolism and has been shown to protect against lipid-induced β-cell dysfunction. We and others have previously shown that expression of the PPARα gene in β-cells is rapidly repressed by glucose. Here we show that the PPARα gene is transcribed from five alternative transcription start sites, resulting in three alternative first exons that are spliced to exon 2. Expression of all PPARα transcripts is repressed by glucose both in insulinoma cells and in isolated pancreatic islets. The observation that the dynamics of glucose repression of PPARα transcription are very similar to those of glucose activation of target genes by the carbohydrate response element-binding protein (ChREBP) prompted us to investigate the potential role of ChREBP in the regulation of PPARα expression. We show that a constitutively active ChREBP lacking the N-terminal domain efficiently represses PPARα expression in insulinoma cells and in rodent and human islets. In addition, we demonstrate that siRNA-mediated knockdown of ChREBP abrogates glucose repression of PPARα expression as well as induction of well established ChREBP target genes in insulinoma cells. In conclusion, this work shows that ChREBP is a critical and direct mediator of glucose repression of PPARα gene expression in pancreatic β-cells, suggesting that ChREBP may be important for glucose suppression of the fatty acid oxidation capacity of β-cells.

Using transcription modules to identify expression clusters perturbed in Williams-Beuren syndrome

The genetic dissection of the phenotypes associated with Williams-Beuren Syndrome (WBS) is advancing thanks to the study of individuals carrying typical or atypical structural rearrangements, as well as in vitro and animal studies. However, little is known about the global dysregulations caused by the WBS deletion. We profiled the transcriptomes of skin fibroblasts from WBS patients and compared them to matched controls. We identified 868 differentially expressed genes that were significantly enriched in extracellular matrix genes, major histocompatibility complex (MHC) genes, as well as genes in which the products localize to the postsynaptic membrane. We then used public expression datasets from human fibroblasts to establish transcription modules, sets of genes coexpressed in this cell type. We identified those sets in which the average gene expression was altered in WBS samples. Dysregulated modules are often interconnected and share multiple common genes, suggesting that intricate regulatory networks connected by a few central genes are disturbed in WBS. This modular approach increases the power to identify pathways dysregulated in WBS patients, thus providing a testable set of additional candidates for genes and their interactions that modulate the WBS phenotypes.

Serum 14-3-3; matrix metalloproteinases and their inhibitors' levels before and after cardiovascular surgery in diabetic and non-diabetic patients

AIMS

We sought to assess the serum levels of the main extracellular matrix components before and after surgery in order to differentiate the response of diabetic patients to acute wounds from that of non-diabetic patients.

METHODS

The serum levels of 14-3-3, Pro-MMP-1, MMP-3, and TIMP-1 were measured in diabetics (18 patients) and non-diabetics (22 patients) in samples obtained before a coronary artery bypass grafting operation and on the 1st, 3rd, and 5th postoperative days.

RESULTS

The diabetics had higher serum levels of 14-3-3 both in the pre- and postoperative phases. Nevertheless, there was a postoperative drop in these amounts in all the patients. There was no difference in the serum levels of Pro-MMP-1 between the two groups. In addition, the serum levels of MMP-3 on the 3rd and 5th postoperative days and also TIMP-1 (inhibitor of both MMPs) on all postoperative days were higher in the diabetics.

CONCLUSIONS

There was perfect synchronicity between the changes in the serum levels of these proteins and their functional nature in the injured tissue. Furthermore, the diabetic patients exhibited more changes in the levels of some of their extracellular enzymes in the wake of acute wounds; these changes were also traceable in the serum.

Activation and repression of glucose-stimulated ChREBP requires the concerted action of multiple domains within the MondoA conserved region

Carbohydrate response element-binding protein (ChREBP) is a glucose-dependent transcription factor that stimulates the expression of glycolytic and lipogenic genes in mammals. Glucose regulation of ChREBP has been mapped to its conserved NH(2)-terminal region of 300 amino acids, designated the MondoA conserved region (MCR). Within the MCR, five domains (MCR1-5) have a particularly high level of conservation and are likely to be important for glucose regulation. We carried out a large-scale deletion and substitution mutational analysis of the MCR domain of ChREBP. This analysis revealed that MCRs 1-4 function in a concerted fashion to repress ChREBP activity in basal (nonstimulatory) conditions. Deletion of the entire MCR1-4 segment or the combination of four specific point mutations located across this region leads to a highly active, glucose-independent form of ChREBP. However, deletion of any individual MCR domain and the majority of point mutations throughout MCR1-4 rendered ChREBP inactive. These observations suggest that the MCR1-4 region interacts with an additional coregulatory factor required for activation. This possibility is supported by the observation that the MCR1-4 region can compete for activity with wild-type ChREBP in stimulatory conditions. In contrast, mutations in the MCR5 domain result in increased activity, suggesting that this domain may be the target of intramolecular repression in basal conditions. Thus, the MCR domains act in a complex and coordinated manner to regulate ChREBP activity in response to glucose.

High prevalence of diabetes and pre-diabetes in adults with Williams syndrome

A standard oral glucose tolerance test (OGTT) was administered to 28 adults with Williams syndrome (WS). Three quarters of the WS subjects showed abnormal glucose curves, meeting diagnostic criteria for either diabetes or the pre-diabetic state of impaired glucose tolerance. Fasting mean glucose and median insulin levels did not differ significantly in the total WS cohort versus age-gender-BMI matched controls, though the glucose area under the curve was greater in the WS subjects. HbA1c levels were not as reliable as the OGTT in diagnosing the presence of diabetes. Given the high prevalence of impaired glucose regulation, adults with WS should be screened for diabetes, and when present should be treated in accordance with standard medical practice. Hemizygosity for a gene mapping to the Williams syndrome chromosome region (WSCR) is likely the major factor responsible for the high frequency of diabetes in WS. Syntaxin-1A is a prime candidate gene based on its location in the WSCR, its role in insulin release, and the presence of abnormal glucose metabolism in mouse models with aberrantly expressed Stx-1a.

All-trans retinoic acid affects subcellular localization of a novel BmNIF3l protein: functional deduce and tissue distribution of NIF3l gene from silkworm (Bombyx mori)

A novel cDNA sequence encoding a predicted protein of 271 amino acids containing a conserved NIF3 domain was found from a pupal cDNA library of silkworm. The corresponding gene was named BmNIF3l (Bombyx mori NGG1p interacting factor 3-like). It was found by bioinformatics that BmNIF3l gene consisted of five exons and four introns and BmNIF3l had a high degree of homology to other NIF3-like proteins, especially in the N-terminal and C-terminal regions. A His-tagged BmNIF3l fusion protein with a molecular weight of approximately 33.6 kDa was expressed and purified to homogeneity. We have used the purified fusion protein to produce polyclonal antibodies against BmNIF3l for histochemical analysis. Subcellular localization revealed that BmNIF3l is a cytoplasmic protein that responds to all-trans retinoic acid (ATRA). Western blotting and real-time reverse transcription polymerase chain reaction showed that the expression level of BmNIF3l is higher in tissues undergoing differentiation. Taken together, the results suggest that BmNIF3l functions in transcription.

Coordinate regulation/localization of the carbohydrate responsive binding protein (ChREBP) by two nuclear export signal sites: discovery of a new leucine-rich nuclear export signal site

Carbohydrate response element binding protein (ChREBP) is responsible for conversion of dietary carbohydrate to storage fat in liver by coordinating expression of the enzymes that channel glycolytic pyruvate into lipogenesis. The activation of ChREBP in response to high glucose is nuclear localization and transcription, and the inactivation of ChREBP under low glucose involves export from the nucleus to the cytosol. Here we report a new nuclear export signal site ("NES1") of ChREBP. Together these signals provide ChREBP with two NES sequences, both the previously reported NES2 and now the new NES1 coordinate to interact together with CRM1 (exportin) for nuclear export of the carbohydrate response element binding protein.

Regulation of nuclear import/export of carbohydrate response element-binding protein (ChREBP): interaction of an alpha-helix of ChREBP with the 14-3-3 proteins and regulation by phosphorylation

Carbohydrate response element-binding protein (ChREBP) is a glucose-responsive transcription factor that plays a critical role in the glucose-mediated induction of gene products involved in hepatic glycolysis and lipogenesis. Glucose affects the activity of ChREBP largely through post-translational mechanisms involving phosphorylation-dependent cellular localization. In this work we show that the N-terminal region of ChREBP (residues 1-251) regulates its subcellular localization via an interaction with 14-3-3. 14-3-3 binds an alpha-helix in this region (residues 125-135) to retain ChREBP in the cytosol, and binding of 14-3-3 is facilitated by phosphorylation of nearby Ser-140 and Ser-196. Phosphorylation of ChREBP at these sites was essential for its interaction with CRM1 for export to the cytosol, whereas nuclear import of ChREBP requires dephosphorylated ChREBP to interact with importin alpha. Notably, 14-3-3 appears to compete with importin alpha for ChREBP binding. 14-3-3beta bound to a synthetic peptide spanning residues 125-144 and bearing a phosphate at Ser-140 with a dissociation constant of 1.1 microm, as determined by isothermal calorimetry. The interaction caused a shift in the fluorescence maximum of the tryptophan residues of the peptide. The corresponding unphosphorylated peptide failed to bind 14-3-3beta. These results suggest that interactions with importin alpha and 14-3-3 regulate movement of ChREBP into and out of the nucleus, respectively, and that these interactions are regulated by the ChREBP phosphorylation status.

Carbohydrate responsive element binding protein and lipid homeostasis

PURPOSE OF REVIEW

The liver is responsible for the conversion of excess dietary carbohydrates into fatty acids, through de-novo lipogenesis. A clear understanding of the control of lipogenesis is crucial since excess fatty acids leads to hepatic steatosis and associated metabolic diseases. The transcription factor sterol regulatory element binding protein 1c and the nuclear receptor liver X receptor are implicated in the insulin-mediated induction of lipogenic genes. Recently, the transcription factor carbohydrate responsive element binding protein has emerged as the hepatic glucose sensor required for the induction of lipogenic genes in response to glucose.

RECENT FINDINGS

We have recently demonstrated that the liver-specific inhibition of carbohydrate responsive element binding protein decreases the rate of lipogenesis and improves hepatic steatosis and insulin resistance in obese ob/ob mice. These results suggest that carbohydrate responsive element binding protein is a potential therapeutic target, and an accurate knowledge of the mechanisms involved in regulating its expression or activation is needed for the development of pharmacological approaches for the treatment of metabolic diseases. Recent studies report that carbohydrate responsive element binding protein is regulated at the transcriptional level by glucose and by liver X receptor but that posttranslational modifications are needed for carbohydrate responsive element binding protein to become active.

SUMMARY

Here we review some of the studies that provided a better understanding of the role and regulation of the newly identified transcription factor carbohydrate responsive element binding protein in lipid homeostasis.

Glucose-mediated transactivation of carbohydrate response element-binding protein requires cooperative actions from Mondo conserved regions and essential trans-acting factor 14-3-3

Carbohydrate response element-binding protein (ChREBP) is a basic helix-loop-helix/leucine zipper transcription factor that binds to the carbohydrate response element in the promoter of certain lipogenic and glycolytic genes. High glucose can activate ChREBP by releasing an intramolecular inhibition within the glucose-sensing module (GSM) that occurs in low glucose. We report here that the glucose response of GSM is mediated by cooperation between five conserved submodules known as Mondo conserved regions (MCRs) I through V within GSM. Deletion of individual MCRs leads to complete (for MCR II, III, and IV) or partial (MCR I) loss of glucose response of ChREBP. MCR IV is necessary and sufficient for inhibiting the transcriptional activity of ChREBP under low glucose. The roles of MCR II and III in glucose response of ChREBP are independent of and distinct from their function in controlling subcellular localization. We further demonstrate that, instead of inhibiting ChREBP activity as would be predicted from its cytoplasmic retentive function, 14-3-3 binding with MCR III is essential for the glucose responsiveness of ChREBP. The interaction between 14-3-3 and ChREBP is constitutive, indicating a permissive role of 14-3-3 in the glucose response of ChREBP. We further uncovered an unconventional 14-3-3 binding motif (residues 116-135) lacking phosphor-serine/threonine within MCR III, a predicted alpha-helix highly conserved in all Mondo proteins. We conclude that individual subdomains in the GSM (MCR I through V) play diverse but crucial roles in cooperation with essential trans-acting cofactors such as 14-3-3 proteins to mediate the glucose response of ChREBP.

Williams-Beuren syndrome TRIM50 encodes an E3 ubiquitin ligase

Williams-Beuren syndrome (WBS) is a neurodevelopmental and multisystemic disease that results from hemizygosity of approximately 25 genes mapping to chromosomal region 7q11.23. We report here the preliminary description of eight novel genes mapping within the WBS critical region and/or its syntenic mouse region. Three of these genes, TRIM50, TRIM73 and TRIM74, belong to the TRIpartite motif gene family, members of which were shown to be associated to several human genetic diseases. We describe the preliminary functional characterization of these genes and show that Trim50 encodes an E3 ubiquitin ligase, opening the interesting hypothesis that the ubiquitin-mediated proteasome pathway might be involved in the WBS phenotype.

Sex specific gene regulation and expression QTLs in mouse macrophages from a strain intercross

BACKGROUND

A powerful way to identify genes for complex traits it to combine genetic and genomic methods. Many trait quantitative trait loci (QTLs) for complex traits are sex specific, but the reason for this is not well understood.

METHODOLOGY/PRINCIPAL FINDINGS

RNA was prepared from bone marrow derived macrophages of 93 female and 114 male F(2) mice derived from a strain intercross between apoE-deficient mice on the AKR and DBA/2 genetic backgrounds, and was subjected to transcriptome profiling using microarrays. A high density genome scan was performed using a mouse SNP chip, and expression QTLs (eQTLs) were located for expressed transcripts. Using suggestive and significant LOD score cutoffs of 3.0 and 4.3, respectively, thousands of eQTLs in the female and male cohorts were identified. At the suggestive LOD threshold the majority of the eQTLs were trans eQTLs, mapping unlinked to the position of the gene. Cis eQTLs, which mapped to the location of the gene, had much higher LOD scores than trans eQTLs, indicating their more direct effect on gene expression. The majority of cis eQTLs were common to both males and females, but only approximately 1% of the trans eQTLs were shared by both sexes. At the significant LOD threshold, the majority of eQTLs were cis eQTLs, which were mostly sex-shared, while the trans eQTLs were overwhelmingly sex-specific. Pooling the male and female data, 31% of expressed transcripts were expressed at different levels in males vs. females after correction for multiple testing.

CONCLUSIONS/SIGNIFICANCE

These studies demonstrate a large sex effect on gene expression and trans regulation, under conditions where male and female derived cells were cultured ex vivo and thus without the influence of endogenous sex steroids. These data suggest that eQTL data from male and female mice should be analyzed separately, as many effects, such as trans regulation are sex specific.

The liver X receptor (LXR) and hepatic lipogenesis. The carbohydrate-response element-binding protein is a target gene of LXR

The liver X receptors, LXRalpha (NR1H3) and LXRbeta (NR1H2), are ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily. LXRs play a critical role in cholesterol homeostasis and bile acid metabolism. In addition, oral administration of LXR agonists to mice results in elevated hepatic fatty acid synthesis and steatosis and increased secretion of triglyceride-rich very low density lipoprotein resulting in hypertriglyceridemia. This increased hepatic lipogenesis has been largely attributed to the LXR-dependent up-regulation of sterol regulatory element-binding protein 1c (SREBP-1c) expression. However, it has been reported that treating Srebp-1c null mice with the synthetic LXR agonist T0901317 still results in enhanced expression of many lipogenic genes, suggesting additional mechanisms by which LXR can enhance hepatic lipogenesis. In this report, we identify the carbohydrate response element-binding protein (ChREBP) as an LXR target that independently enhances the up-regulation of select lipogenic genes. The ChREBP promoter contains functional LXR-binding sites that confer receptor-dependent binding and transactivation. We show that T0901317 treatment of mice is associated with up-regulation of the ChREBP target gene, liver-type pyruvate kinase. Therefore, activation of LXR not only increases ChREBP mRNA via enhanced transcription but also modulates ChREBP activity. This establishes LXR as a master lipogenic transcription factor, as it directly regulates both SREBP-1c and ChREBP to enhance hepatic fatty acid synthesis.

Carbohydrate response element binding protein, ChREBP, a transcription factor coupling hepatic glucose utilization and lipid synthesis

The ability of an organism to sense and store nutrients is vital to survival. The liver is the major organ responsible for converting excess dietary carbohydrate to lipid for storage. An elegant molecular pathway has evolved that allows increased glucose flux into hepatocytes to generate a signaling molecule, xylulose 5-phosphate, that triggers rapid changes in glycolytic enzyme activities and nuclear import of a transcription factor, ChREBP, which coordinates the transcriptional regulation of enzymes that channel the glycolytic end-products into lipogenesis. Further understanding of this metabolic cascade should provide insights on conditions such as fatty liver, obesity, and the metabolic syndrome.

Submicroscopic deletion in patients with Williams-Beuren syndrome influences expression levels of the nonhemizygous flanking genes

Genomic imbalance is a common cause of phenotypic abnormalities. We measured the relative expression level of genes that map within the microdeletion that causes Williams-Beuren syndrome and within its flanking regions. We found, unexpectedly, that not only hemizygous genes but also normal-copy neighboring genes show decreased relative levels of expression. Our results suggest that not only the aneuploid genes but also the flanking genes that map several megabases away from a genomic rearrangement should be considered possible contributors to the phenotypic variation in genomic disorders.