Fructose modulates GLUT5 mRNA stability in differentiated Caco-2 cells: role of cAMP-signalling pathway and PABP (polyadenylated-binding protein)-interacting protein (Paip) 2

Intestinal fructose transporters GLUT5 and GLUT2 in children and adolescents with obesity and metabolic disorders

PURPOSE

The excessive fructose intake including high-fructose corn syrup (HFCS) may be responsible for increase of obesity occurrence. This study was designed to find potential differences in duodenal fructose transporters on mRNA and protein levels between obese and normal weight children and adolescents.

MATERIALS/METHODS

We performed a cross-sectional study on a group of 106 hospitalized patients aged 12 to 18. Glucose transporter 2 (GLUT2) and glucose transporter 5 (GLUT5) mRNA as well as protein levels (ELISA and Western blot methods) were assessed in duodenal mucosa biopsies of the patients categorized as obese or normal weight. Additionally, the expression of the aforementioned transporters was analyzed in patients based on the presence of insulin resistance (IR) and metabolic syndrome (MS).

RESULTS

In children with obesity, increased duodenal protein levels of GLUT5 (Relative protein GLUT5 expression/ACTB) (0.027 ​± ​0.009 vs. 0.011 ​± ​0.006, p ​< ​0.05) but not GLUT2 as compared with the normal weight group, were revealed. No significant differences in duodenal relative GLUT2 and GLUT5 genes expression between the studied groups were found. There was no relationship between the presence of IR or MS and intestinal mRNA GLUT2 and GLUT5 as well as GLUT2 protein expression.

CONCLUSION

The upregulation of the duodenal GLUT5 may contribute to obesity occurrence in children and adolescents.

Genetic Variants in Carbohydrate Digestive Enzyme and Transport Genes Associated with Risk of Irritable Bowel Syndrome

Irritable Bowel Syndrome (IBS) is characterized by abdominal pain and alterations in bowel pattern, such as constipation (IBS-C), diarrhea (IBS-D), or mixed (IBS-M). Since malabsorption of ingested carbohydrates (CHO) can cause abdominal symptoms that closely mimic those of IBS, identifying genetic mutations in CHO digestive enzymes associated with IBS symptoms is critical to ascertain IBS pathophysiology. Through candidate gene association studies, we identify several common variants in TREH, SI, SLC5A1 and SLC2A5 that are associated with IBS symptoms. By investigating rare recessive Mendelian or oligogenic inheritance patterns, we identify case-exclusive rare deleterious variation in known disease genes (SI, LCT, ALDOB, and SLC5A1) as well as candidate disease genes (MGAM and SLC5A2), providing potential evidence of monogenic or oligogenic inheritance in a subset of IBS cases. Finally, our data highlight that moderate to severe IBS-associated gastrointestinal symptoms are often observed in IBS cases carrying one or more of deleterious rare variants.

Trichostatin a inhibits expression of the human SLC2A5 gene via SNAI1/SNAI2 transcription factors and sensitizes colon cancer cells to platinum compounds

GLUT5, a key protein encoded by the SLC2A5 gene, is involved in the uptake of fructose from the intestine. Currently, with the increased consumption of this sugar and the associated increased incidence of obesity, diabetes and cancer, GLUT5 may represent an important molecular target in the prevention and treatment of these diseases. Here, we demonstrate that overexpression of the SNAI1 and SNAI2 transcription factors in cells expressing high levels of SLC2A5 mRNA reduced SLC2A5 gene expression. Furthermore, a histone deacetylase inhibitor, trichostatin A, which induces SNAI1 and SNAI2 expression, inhibits SLC2A5/GLUT5 expression and sensitizes colon cancer cells to cisplatin and oxaliplatin. This finding might have potential relevance for the development of therapeutic treatments aimed at modulating fructose transport or genes involved in this process for use with certain cancers.

Recent Progress on Fructose Metabolism-Chrebp, Fructolysis, and Polyol Pathway

Excess fructose intake is associated with obesity, fatty liver, tooth decay, cancer, and cardiovascular diseases. Even after the ingestion of fructose, fructose concentration in the portal blood is never high; fructose is further metabolized in the liver, and the blood fructose concentration is 1/100th of the glucose concentration. It was previously thought that fructose was metabolized in the liver and not in the small intestine, but it has been reported that metabolism in the small intestine also plays an important role in fructose metabolism. Glut5 knockout mice exhibit poor fructose absorption. In addition, endogenous fructose production via the polyol pathway has also received attention; gene deletion of aldose reductase (Ar), ketohexokinase (Khk), and triokinase (Tkfc) has been found to prevent the development of fructose-induced liver lipidosis. Carbohydrate response element-binding protein (Chrebp) regulates the expression of Glut5, Khk, aldolase b, and Tkfc. We review fructose metabolism with a focus on the roles of the glucose-activating transcription factor Chrebp, fructolysis, and the polyol pathway.

Late-Stage Functionalization through Click Chemistry Provides GLUT5-Targeting Glycoconjugate as a Potential PET Imaging Probe

The targeting of facilitative sugar transporters (GLUTs) has been utilized in the development of tools for diagnostics and therapy. The interest in this area is promoted by the phenomenon of alterations in cellular metabolic processes that are linked to multitudes of metabolic disorders and diseases. However, nonspecific targeting (e.g., glucose-transporting GLUTs) leads to a lack of disease detection efficiency. Among GLUTs, GLUT5 stands out as a prominent target for developing specific molecular tools due to its association with metabolic diseases, including cancer. This work reports a non-radiolabeled fluoride (19F) coumarin-based glycoconjugate of 2,5-anhydro-D-mannitol as a potential PET imaging probe that targets the GLUT5 transporter. Inherent fluorescent properties of the coumarin fluorophore allowed us to establish the probe's uptake efficiency and GLUT5-specificity in a GLUT5-positive breast cell line using fluorescence detection techniques. The click chemistry approach employed in the design of the probe enables late-stage functionalization, an essential requirement for obtaining the radiolabeled analog of the probe for future in vivo cancer imaging applications. The high affinity of the probe to GLUT5 allowed for the effective uptake in nutrition-rich media.

Gene variants of the SLC2A5 gene encoding GLUT5, the major fructose transporter, do not contribute to clinical presentation of acquired fructose malabsorption

Background

While role of ALDOB-related gene variants for hereditary fructose intolerance is well established, contribution of gene variants for acquired fructose malabsorption (e.g. SLC2A5, GLUT5) is not well understood.

Methods

Patients referred to fructose breath test were further selected to identify those having acquired fructose malabsorption. Molecular analysis of genomic DNA included (I) exclusion of 3 main ALDOB gene variants causing hereditary fructose intolerance and (II) sequencing analysis of SLC2A5 gene comprising complete coding region, at least 20 bp of adjacent intronic regions and 700 bp of proximal promoter.

Results

Among 494 patients, 35 individuals with acquired fructose malabsorption were identified based on pathological fructose-breath test and normal lactose-breath test. Thirty four of them (97%) had negative tissue anti-transglutaminase and/or deamidated gliadin antibodies in their medical records. Molecular analysis of SLC2A5 gene of all 35 subjects identified 5 frequent and 5 singular gene variants mostly in noncoding regions (promoter and intron). Allele frequencies of gene variants were similar to those reported in public databases strongly implying that none of them was associated with acquired fructose malabsorption.

Conclusions

Gene variants of coding exons, adjacent intronic regions and proximal promoter region of SLC2A5 gene are unlikely to contribute to genetic predisposition of acquired fructose malabsorption.

Insight on Glucose and Fructose Absorption and Relevance in the Enterocyte Milieu

Although epidemiological studies indicate a strong correlation between high sugar intake and metabolic diseases, the biological mechanisms underlying this link are still controversial. To further examine the modification and crosstalk occurring in enterocyte metabolism during sugar absorption, in this study we evaluate the diffusion and intestinal metabolism of glucose, fructose and sucrose, which were supplemented in equimolar concentration to Caco-2 cells grown on polyester membrane inserts. At different time points after supplementation, changes in metabolite concentration were evaluated in the apical and basolateral chambers by nuclear magnetic resonance (NMR) and gas-chromatography (GC). Sucrose was only minimally hydrolyzed by Caco-2 cells. Upon supplementation, we observed a faster uptake of fructose than glucose, the pentose sugar being also faster catabolized. Monosaccharide absorption was concomitant to the synthesis/transport of other metabolites, which occurred differently in glucose and fructose supplemented cells. Our results confirm the prominent role of intestinal cells in fructose metabolism and clearance after absorption, representing a further step forward in the understanding of the role of dietary sugars. Future research, including targeted analysis on specific transporters/enzymes and the use of labeled substrates, will be helpful to confirm the present results and their interpretation.

The Contribution of Dietary Fructose to Non-alcoholic Fatty Liver Disease

Fructose, especially industrial fructose (sucrose and high fructose corn syrup) is commonly used in all kinds of beverages and processed foods. Liver is the primary organ for fructose metabolism, recent studies suggest that excessive fructose intake is a driving force in non-alcoholic fatty liver disease (NAFLD). Dietary fructose metabolism begins at the intestine, along with its metabolites, may influence gut barrier and microbiota community, and contribute to increased nutrient absorption and lipogenic substrates overflow to the liver. Overwhelming fructose and the gut microbiota-derived fructose metabolites (e.g., acetate, butyric acid, butyrate and propionate) trigger the de novo lipogenesis in the liver, and result in lipid accumulation and hepatic steatosis. Fructose also reprograms the metabolic phenotype of liver cells (hepatocytes, macrophages, NK cells, etc.), and induces the occurrence of inflammation in the liver. Besides, there is endogenous fructose production that expands the fructose pool. Considering the close association of fructose metabolism and NAFLD, the drug development that focuses on blocking the absorption and metabolism of fructose might be promising strategies for NAFLD. Here we provide a systematic discussion of the underlying mechanisms of dietary fructose in contributing to the development and progression of NAFLD, and suggest the possible targets to prevent the pathogenetic process.

Characterization of SGLT1-mediated glucose transport in Caco-2 cell monolayers, and absence of its regulation by sugar or epinephrine

Caco-2 cells are increasingly used to study the absorption of drugs and nutrients, including D-glucose, an important nutrient that mainly gets absorbed from the intestine by the sodium/glucose cotransporter 1 (SGLT1). However, disadvantages of Caco-2 cells for such studies have been reported, e.g., D-glucose cannot elicit translocation of the intracellular pool of SGLT1 to the apical membrane, the origin of the cells affects glucose uptake, and Caco-2 cells exhibit heterogeneity. This study aimed to characterize SGLT1-mediated glucose transport across Caco-2 cell monolayers. We found that at lower glucose concentrations (5 mM) SGLT1 contributes more to total glucose transport than at higher (10 mM) glucose concentrations, suggesting contributions by another transporter at higher glucose concentrations. This contrasts with the in vivo situation, where SGLT1 dominant glucose transporter at all glucose concentrations. We also tested whether known regulators like sugars or catecholamines can stimulate glucose transport across Caco-2 cell monolayers. Neither epinephrine nor 2-deoxy-D-glucose could stimulate glucose transport. Moreover, the epinephrine could not induce accumulation of cyclic adenosine monophosphate (cAMP) in Caco-2 cells, indicating the absence of a functional β₂-adrenoceptor in Caco-2 cells, which could explain the lack of epinephrine effect on glucose transport. Also, Caco-2 cells may lack some kinases required for increased SGLT1 transport. Overall, SGLT1-mediated glucose transport and its regulation in Caco-2 cells differ from that in vivo, and caution is advised when extrapolating glucose transport results obtained with this model to the in vivo situation.

Glucose transporters in the small intestine in health and disease

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facititative transporters GLUT2 and GLUT5. SGLT1 and GLUT2 are relevant for absorption of D-glucose and D-galactose while GLUT5 is relevant for D-fructose absorption. SGLT1 and GLUT5 are constantly localized in the brush border membrane (BBM) of enterocytes, whereas GLUT2 is localized in the basolateral membrane (BLM) or the BBM plus BLM at low and high luminal D-glucose concentrations, respectively. At high luminal D-glucose, the abundance SGLT1 in the BBM is increased. Hence, D-glucose absorption at low luminal glucose is mediated via SGLT1 in the BBM and GLUT2 in the BLM whereas high-capacity D-glucose absorption at high luminal glucose is mediated by SGLT1 plus GLUT2 in the BBM and GLUT2 in the BLM. The review describes functions and regulations of SGLT1, GLUT2, and GLUT5 in the small intestine including diurnal variations and carbohydrate-dependent regulations. Also, the roles of SGLT1 and GLUT2 for secretion of enterohormones are discussed. Furthermore, diseases are described that are caused by malfunctions of small intestinal monosaccharide transporters, such as glucose-galactose malabsorption, Fanconi syndrome, and fructose intolerance. Moreover, it is reported how diabetes, small intestinal inflammation, parental nutrition, bariatric surgery, and metformin treatment affect expression of monosaccharide transporters in the small intestine. Finally, food components that decrease D-glucose absorption and drugs in development that inhibit or downregulate SGLT1 in the small intestine are compiled. Models for regulations and combined functions of glucose transporters, and for interplay between D-fructose transport and metabolism, are discussed.

Adenylyl Cyclase 6 Expression Is Essential for Cholera Toxin-Induced Diarrhea

BACKGROUND

Cholera toxin (CT)-induced diarrhea is mediated by cyclic adenosine monophosphate (cAMP)-mediated active Cl- secretion via the cystic fibrosis transmembrane conductance regulator (CFTR). Although the constitutive activation of adenylyl cyclase (AC) in response to CT is due to adenosine diphosphate ribosylation of the small G protein α-subunit activating CFTR with consequent secretory diarrhea, the AC isoform(s) involved remain unknown.

METHODS

We generated intestinal epithelial cell-specific adenylyl cyclase 6 (AC6) knockout mice to study its role in CT-induced diarrhea.

RESULTS

AC6 messenger RNA levels were the highest of all 9 membrane-bound AC isoforms in mouse intestinal epithelial cells. Intestinal epithelial-specific AC6 knockout mice (AC6loxloxVillinCre) had undetectable AC6 levels in small intestinal and colonic epithelial cells. No significant differences in fluid and food intake, plasma electrolytes, intestinal/colon anatomy and morphology, or fecal water content were observed between genotypes. Nevertheless, CT-induced fluid accumulation in vivo was completely absent in AC6loxloxVillinCre mice, associated with a lack of forskolin- and CT-induced changes in the short-circuit current (ISC) of the intestinal mucosa, impaired cAMP generation in acutely isolated small intestinal epithelial cells, and significantly impaired apical CFTR levels in response to forskolin.

CONCLUSIONS

AC6 is a novel target for the treatment of CT-induced diarrhea.

Intestinal Fructose and Glucose Metabolism in Health and Disease

The worldwide epidemics of obesity and diabetes have been linked to increased sugar consumption in humans. Here, we review fructose and glucose metabolism, as well as potential molecular mechanisms by which excessive sugar consumption is associated to metabolic diseases and insulin resistance in humans. To this end, we focus on understanding molecular and cellular mechanisms of fructose and glucose transport and sensing in the intestine, the intracellular signaling effects of dietary sugar metabolism, and its impact on glucose homeostasis in health and disease. Finally, the peripheral and central effects of dietary sugars on the gut-brain axis will be reviewed.

Circadian rhythm-dependent induction of hepatic lipogenic gene expression in rats fed a high-sucrose diet

Metabolic syndrome has become a global health challenge and was recently reported to be positively correlated with increased sucrose consumption. Mechanistic analyses of excess sucrose-induced progression of metabolic syndrome have been focused mainly on abnormal hepatic lipogenesis, and the exact contribution of excess sucrose to metabolic disorders remains controversial. Considering that carbohydrate and lipid metabolisms exhibit clear circadian rhythms, here we investigated the possible contribution of diurnal oscillations to responses of hepatic lipid metabolism to excess sucrose. We found that excess sucrose dose-dependently promotes fatty liver and hyperlipidemia in in rats fed a high-sucrose diet (HSD). We observed that excess sucrose enhances the oscillation amplitudes of the expression of clock genes along with the levels of hepatic lipid and carbohydrate metabolism-related mRNAs that increase lipogenesis. We did not observe similar changes in the levels of the transcription factors regulating the expression of these genes. This suggested that the excess sucrose-induced, circadian rhythm-dependent amplification of lipogenesis is post-transcriptionally regulated via the stability of metabolic gene transcripts. Of note, our findings also provide evidence that fructose causes some of the HSD-induced, circadian rhythm-dependent alterations in lipogenic gene expression. Our discovery of HSD-induced circadian rhythm-dependent alterations in lipogenesis at the post-transcriptional level may inform future studies investigating the complex relationships among sucrose uptake, circadian rhythm, and metabolic enzyme expression. Our findings could contribute to the design of chrono-nutritional interventions to prevent or manage the development of fatty liver and hyperlipidemia in sucrose-induced metabolic syndrome.

Effect of chrysin on changes in intestinal environment and microbiome induced by fructose-feeding in rats

Intake of fructose-containing sugars is epidemiological and experimentally linked to metabolic syndrome (MS). We recently verified that the dietary polyphenol chrysin was able to abolish some of the metabolic changes induced by fructose-feeding in the rat. Because the role of the intestine upon fructose-induced MS is poorly understood, we decided to investigate the influence of fructose, in vivo, on the intestinal environment and the ability of chrysin to interfere with the putative observed changes. For this, adult male Sprague-Dawley rats were treated for 18 weeks as follows: (A) tap water (CONT), (B) tap water and chrysin (100 mg kg^-1 day^-1) (CHRY), (C) 10% fructose in tap water (FRUCT), and (D) 10% fructose in tap water and chrysin (100 mg kg^-1 day^-1) (FRUCT + CHRY). Our findings show that the relative expression of SGLT1 and GLUT2 mRNA were not affected by fructose-feeding and/or chrysin. In contrast, GLUT5 mRNA expression was markedly increased in fructose-fed animals, and this effect was reduced by chrysin. However, the apparent permeability to ¹⁴C-FRUCT was markedly and similarly decreased in FRUCT, CHRY and FRUCT + CHRY rats. Jejunal villus width and crypt depth were significantly higher in FRUCT and FRUCT + CHRYS rats, respectively. Finally, chrysin did not alter gut microbiota composition, but fructose significantly increased Lactobacillus and E. coli. Moreover, FRUCT + CHRY rats had an increase on the Firmicutes to Bacteroidetes ratio. This is the first report showing that chrysin is able to interfere with the effects of fructose at the intestinal level, which may contribute to the fructose-induced MS features.

Paip2 cooperates with Cbp80 at an active promoter and participates in RNA Polymerase II phosphorylation in Drosophila

The Paip2 protein is a factor regulating mRNA translation and stability in the cytoplasm. It has also been found in the nuclei of several cell types in Drosophila. Here, we aim to elucidate the functions of Paip2 in the cell nucleus. We find that nuclear Paip2 is a component of an ~300-kDa protein complex. Paip2 interacts with mRNA capping factor and factors of RNA polymerase II (Pol II) transcription initiation and early elongation. Paip2 functionally cooperates with the Cbp80 subunit of the cap-binding complex, with both proteins ensuring proper Pol II C-terminal domain (CTD) Ser5 phosphorylation at the promoter. Thus, Paip2 is a novel player at the stage of mRNA capping and early Pol II elongation.

Paip2 is localized to active promoters and loaded onto nascent mRNA in Drosophila

Paip2 (Poly(A)-binding protein - interacting protein 2) is a conserved metazoan-specific protein that has been implicated in regulating the translation and stability of mRNAs. However, we have found that Paip2 is not restricted to the cytoplasm but is also found in the nucleus in Drosophila embryos, salivary glands, testes, and tissue culture cells. Nuclear Paip2 is associated with chromatin, and in chromatin immunoprecipitation experiments it maps to the promoter regions of active genes. However, this chromatin association is indirect, as it is RNA-dependent. Thus, Paip2 is one more item in the growing list of translation factors that are recruited to mRNAs co-transcriptionally.

SLC2A5 promotes lung adenocarcinoma cell growth and metastasis by enhancing fructose utilization

The metabolism of cancer cells is highly plastic. Cancer cells can change their preference for nutrient uptake under nutrient stress. Fructose is one of the most common carbohydrates in diet and its metabolism is also involved in the development and progression of tumors. GLUT5, encoded by SLC2A5, is the specific fructose transporter in mammalian cells. In this study, we found that SLC2A5 is significantly upregulated in lung adenocarcinoma (LUAD) patients and overexpression of SLC2A5 is highly correlated with poor prognosis of LUAD patients. The expression of SLC2A5 determined fructose uptake and utilization efficacy in LUAD cells. GLUT5 is critical for the survival of LUAD cells in fructose-containing culture medium. Depletion of SLC2A5 undermined cell proliferation and invasion meanwhile increased cell apoptosis. Overexpression of SLC2A5 enhances cell proliferation, migration, invasion, and tumorigenic. Compared to glucose, fructose is prone to strengthen intracellular-free fatty acid accumulation and ATP production. Moreover, inhibition of GLUT5 by specific small chemical inhibitor sensitizes LUAD cells to paclitaxel treatment. Taken together, our results suggest that GLUT5 could be a potential target alone or combination with other treatment for lung cancer therapy.

Transporter Gene Expression and Transference of Fructose in Broiler Chick Intestine

Recent studies have suggested that a high-fructose diet leads to the development of metabolic syndrome in mammals. However, relatively little information is available regarding the absorption of fructose in the chicken intestine. We therefore investigated fructose absorption and its transporters in the chicken small intestine. The gene expression of three transporters (glucose transporter protein member 2 and 5 and sodium-dependent glucose transporter protein 1) in the jejunum of fasted chicks were lower than those in chicks fed ad libitum. The everted intestinal sacs (in vitro method for investigating intestinal absorption) showed that the concentration of fructose uptake rapidly increased within 15 min after incubation, and then gradually increased until 60 min. After 15 min of incubation, fructose uptake in the ad libitum chick intestine was approximately 2-fold that in the fasted intestine and was less than half of the glucose uptake in the ad libitum chick intestine. Our results suggest that fructose is absorbed in the small intestine of chicks and that uptake is decreased by fasting treatment with decreases in the mRNA expression of related transporters.

The Role of Carbohydrate Response Element Binding Protein in Intestinal and Hepatic Fructose Metabolism

Many articles have discussed the relationship between fructose consumption and the incidence of obesity and related diseases. Fructose is absorbed in the intestine and metabolized in the liver to glucose, lactate, glycogen, and, to a lesser extent, lipids. Unabsorbed fructose causes bacterial fermentation, resulting in irritable bowl syndrome. Therefore, understanding the mechanisms underlying intestinal and hepatic fructose metabolism is important for the treatment of metabolic syndrome and fructose malabsorption. Carbohydrate response element binding protein (ChREBP) is a glucose-activated transcription factor that controls approximately 50% of de novo lipogenesis in the liver. ChREBP target genes are involved in glycolysis (Glut2, liver pyruvate kinase), fructolysis (Glut5, ketohexokinase), and lipogenesis (acetyl CoA carboxylase, fatty acid synthase). ChREBP gene deletion protects against high sucrose diet-induced and leptin-deficient obesity, because Chrebp^−/− mice cannot consume fructose or sucrose. Moreover, ChREBP contributes to some of the physiological effects of fructose on sweet taste preference and glucose production through regulation of ChREBP target genes, such as fibroblast growth factor-21 and glucose-6-phosphatase catalytic subunits. Thus, ChREBP might play roles in fructose metabolism. Restriction of excess fructose intake will be beneficial for preventing not only metabolic syndrome but also irritable bowl syndrome.

Fructose Malabsorption in Systemic Sclerosis

The deleterious effect of fructose, which is increasingly incorporated in many beverages, dairy products, and processed foods, has been described; fructose malabsorption has thus been reported in up to 2.4% of healthy subjects, leading to digestive clinical symptoms (eg, pain, distension, diarrhea). Because digestive involvement is frequent in patients with systemic sclerosis (SSc), we hypothesized that fructose malabsorption could be responsible for intestinal manifestations in these patients. The aims of this prospective study were to: determine the prevalence of fructose malabsorption, in SSc; predict which SSc patients are at risk of developing fructose malabsorption; and assess the outcome of digestive symptoms in SSc patients after initiation of standardized low-fructose diet. Eighty consecutive patients with SSc underwent fructose breath test. All SSc patients also completed a questionnaire on digestive symptoms, and a global symptom score (GSS) was calculated. The prevalence of fructose malabsorption was as high as 40% in SSc patients. We also observed a marked correlation between the presence of fructose malabsorption and: higher values of GSS score of digestive symptoms (P = 0.000004); and absence of delayed gastric emptying (P = 0.007). Furthermore, in SSc patients with fructose malabsorption, the median value of GSS score of digestive symptoms was lower after initiation of standardized low-fructose diet (4 before vs. 1 after; P = 0.0009). Our study underscores that fructose malabsorption often occurs in SSc patients. Our findings are thus relevant for clinical practice, highlighting that fructose breath test is a helpful, noninvasive method by: demonstrating fructose intolerance in patients with SSc; and identifying the group of SSc patients with fructose intolerance who may benefit from low-fructose diet. Interestingly, because the present series also shows that low-fructose diet resulted in a marked decrease of gastrointestinal clinical manifestations in SSc patients with fructose malabsorption, our findings underscore that fructose malabsorption may play a significant role in the onset of gastrointestinal symptoms in these patients. Finally, we suggest that fructose malabsorption may be due to reduced fructose absorption by enterocytes, impaired enteric microbiome, and decreased intestinal permeability.

High consumption of fructose rather than glucose promotes a diet-induced obese phenotype in Drosophila melanogaster

During the last 20 years, there has been a considerable scientific debate about the possible mechanisms of induction of metabolic disorders by reducing monosaccharides such as glucose or fructose. In this study, we report the metabolic rearrangement in response to consumption of these monosaccharides at concentrations ranging from 0.25% to 20% in a Drosophila model. Flies raised on high-glucose diet displayed delay in pupation and increased developmental mortality compared with fructose consumers. Both monosaccharides at high concentrations promoted an obese-like phenotype indicated by increased fly body mass, levels of uric acid, and circulating and stored carbohydrates and lipids; and decreased percentage of water in the body. However, flies raised on fructose showed lower levels of circulating glucose and higher concentrations of stored carbohydrates, lipids, and uric acid. The preferential induction of obesity caused by fructose in Drosophila was associated with increased food consumption and reduced mRNA levels of DILP2 and DILP5 in the brain of adult flies. Our data show that glucose and fructose differently affect carbohydrate and lipid metabolism in Drosophila in part by modulation of insulin/insulin-like growth factor signaling. Some reported similarities with effects observed in mammals make Drosophila as a useful model to study carbohydrate influence on metabolism and development of metabolic disorders.

Involvement of intracellular signaling in the IL-1β inhibitory effect on fructose intestinal absorption

A variety of bacteria and their excreted/secreted products having direct effects on epithelial ion transport and permeability and the release of cytokines during bacterial infection may impact directly on epithelial function. Interleukin-1β (IL-1β) is a pleiotropic cytokine that affects the intestinal absorption of nutrients. The aim of this work was to study the intracellular signaling pathways involved in the inhibitory effect of IL-1β on D-fructose intestinal transport in rabbit jejunum and Caco-2 cells. The results show that the cytokine inhibitory effect was completely reversed in presence of proteasome or PKC selective inhibitors in IL-1β treated rabbits. In addition, the activation of PI3K abolished the IL-1β effect. Likewise, these results were confirmed in Caco-2 cells. In addition, p-PI3K expression was increased by IL-1β-treatment whereas the expression of p-PKCα was not significantly affected. In summary, the results suggest that IL-1β could regulate the activation of pPKCα 73, pPI3K 55, and NF-kB proteins. These events could exert an inhibitory effect on fructose intestinal absorption by a modification of GLUT5 insertion to brush-border membrane and/or the functional transporter activity. This effect is independent of hormonal milieu and nervous stimuli.

Retinoic Acid Improves a Hybridoma Culture in a Fructose-Based Medium by Up-Regulation of Fructose IncorporationviaRetinoid Nuclear Receptors

Fructose was focused on as an alternative sugar source to glucose in a hybridoma culture medium because it decreases lactate production during cultivation, leading to cell and product stability. But, not all human hybridoma cell lines grew well in a fructose-based serum-free medium. We found that the addition of all-trans-retinoic acid to the fructose-based medium improved the growth and monoclonal antibody production of hybridoma cell lines by up-regulation of fructose incorporation that represented increased expression of the fructose transporter, GLUT5. Selective activation of retinoid nuclear receptor by synthetic ligands showed that both retinoic acid receptors and retinoid X receptors might be related to the improvement of the fructose-based hybridoma culture. This study might be applicable to cell cultures susceptible to lactate and pH changes as well as hybridoma cultures.

Trimethylation of histone H3K4 is associated with the induction of fructose-inducible genes in rat jejunum

We previously reported that fructose force-feeding rapidly induces jejunal Slc2a5 gene expression in rats. In this study, we conducted microarray analyses using total RNA to identify genes upregulated in rat jejunum by fructose force-feeding. Rats were force-fed fructose, glucose or distilled water for 6h. Genes such as Slc2a5, Cdkn1c, Cabp2, Ranbp3, Vwce and Gcgr were induced by force-feeding with fructose compared with glucose or distilled water. Chromatin immunoprecipitation assays revealed that trimethylation of histone H3K4, and acetylation of histones H3 and H4, on the transcribed region of these fructose-inducible genes were enhanced by force-feeding of fructose, but not glucose or distilled water. These results suggest that the induction of genes in the rat jejunum by fructose force-feeding is coordinately regulated by histone modifications, particularly trimethylation of histone H3K4.

Histone H3 methylation at lysine 4 on the SLC2A5 gene in intestinal Caco-2 cells is involved in SLC2A5 expression

Histone H3 methylation at lysine 4 (K4) is associated with euchromatic regions and is thought to be important for the transcriptional activation of genes during differentiation. In this study, we found that di- and tri-methylation of histone H3 at K4 and acetylation of histones H3 and H4 from the promoter/enhancer to the transcribed region close to the transcription initiation site of the solute carrier family 2, member 5 (SLC2A5) gene, and its expression, were induced by differentiation of intestine-like Caco-2 cells. These effects were accompanied by contact inhibition of cell growth of these cells. Furthermore, these modifications were induced by co-treatment with a synthetic glucocorticoid hormone dexamethasone and a p44/42 mitogen-activated protein kinase inhibitor PD89059. Our results suggest that methylation of histone H3 at K4 and acetylation of histones H3 and H4 are involved in SLC2A5 gene induction associated with intestinal differentiation of Caco-2 cells.

Positive regulatory control loop between gut leptin and intestinal GLUT2/GLUT5 transporters links to hepatic metabolic functions in rodents

Background and Aims

The small intestine is the major site of absorption of dietary sugars. The rate at which they enter and exit the intestine has a major effect on blood glucose homeostasis. In this study, we determine the effects of luminal leptin on activity/expression of GLUT2 and GLUT5 transporters in response to sugars intake and analyse their physiological consequences.

Methodology

Wistar rats, wild type and AMPKα₂^−/− mice were used. In vitro and in vivo isolated jejunal loops were used to quantify transport of fructose and galactose in the absence and the presence of leptin. The effects of fructose and galactose on gastric leptin release were determined. The effects of leptin given orally without or with fructose were determined on the expression of GLUT2/5, on some gluconeogenesis and lipogenic enzymes in the intestine and the liver.

Principal Findings

First, in vitro luminal leptin activating its receptors coupled to PKCβII and AMPKα, increased insertion of GLUT2/5 into the brush-border membrane leading to enhanced galactose and fructose transport. Second in vivo, oral fructose but not galactose induced in mice a rapid and potent release of gastric leptin in gastric juice without significant changes in plasma leptin levels. Moreover, leptin given orally at a dose reproducing comparable levels to those induced by fructose, stimulated GLUT5-fructose transport, and potentiated fructose-induced: i) increase in blood glucose and mRNA levels of key gluconeogenesis enzymes; ii) increase in blood triglycerides and reduction of mRNA levels of intestinal and hepatic Fasting-induced adipocyte factor (Fiaf) and iii) increase in SREBP-1c, ACC-1, FAS mRNA levels and dephosphorylation/activation of ACC-1 in liver.

Conclusion/Significance

These data identify for the first time a positive regulatory control loop between gut leptin and fructose in which fructose triggers release of gastric leptin which, in turn, up-regulates GLUT5 and concurrently modulates metabolic functions in the liver. This loop appears to be a new mechanism (possibly pathogenic) by which fructose consumption rapidly becomes highly lipogenic and deleterious.

Sugar absorption in the intestine: the role of GLUT2

Intestinal glucose absorption comprises two components. One is classical active absorption mediated by the Na+/glucose cotransporter. The other is a diffusive component, formerly attributed to paracellular flow. Recent evidence, however, indicates that the diffusive component is mediated by the transient insertion of glucose transporter type 2 (GLUT2) into the apical membrane. This apical GLUT2 pathway of intestinal sugar absorption is present in species from insect to human, providing a major route at high sugar concentrations. The pathway is regulated by rapid trafficking of GLUT2 to the apical membrane induced by glucose during assimilation of a meal. Apical GLUT2 is therefore a target for multiple short-term and long-term nutrient-sensing mechanisms. These include regulation by a newly recognized pathway of calcium absorption through the nonclassical neuroendocrine l-type channel Cav1.3 operating during digestion, activation of intestinal sweet taste receptors by natural sugars and artificial sweeteners, paracrine and endocrine hormones, especially insulin and GLP-2, and stress. Permanent apical GLUT2, resulting in increased sugar absorption, is a characteristic of experimental diabetes and of insulin-resistant states induced by fructose and fat. The nutritional consequences of apical and basolateral GLUT2 regulation are discussed in the context of Western diet, processed foods containing artificial sweeteners, obesity, and diabetes.

Regulation of the fructose transporter GLUT5 in health and disease

Fructose is now such an important component of human diets that increasing attention is being focused on the fructose transporter GLUT5. In this review, we describe the regulation of GLUT5 not only in the intestine and testis, where it was first discovered, but also in the kidney, skeletal muscle, fat tissue, and brain where increasing numbers of cell types have been found to have GLUT5. GLUT5 expression levels and fructose uptake rates are also significantly affected by diabetes, hypertension, obesity, and inflammation and seem to be induced during carcinogenesis, particularly in the mammary glands. We end by highlighting research areas that should yield information needed to better understand the role of GLUT5 during normal development, metabolic disturbances, and cancer.

Protein kinases, TNF-{alpha}, and proteasome contribute in the inhibition of fructose intestinal transport by sepsis in vivo

Lipopolysaccharide (LPS) endotoxin is a causative agent of sepsis. The aim of this study was to examine LPS effects on intestinal fructose absorption and to decipher mechanisms. Sepsis was induced by intravenous injection of LPS in rabbits. The ultrastructural study and DNA fragmentation patterns were identical in the intestine of LPS and sham animals. LPS treatment reduced fructose absorption altering both mucosal-to-serosal transepithelial fluxes and uptake into brush border membrane vesicles (BBMVs). Cytochalasin B was ineffective on fructose uptake, indicating that GLUT5, but not GLUT2, transport activity was targeted. GLUT5 protein levels in BBMvs were lower in LPS than in sham-injected rabbits. Thus lower fructose transport resulted from lower levels of GLUT5 protein. LPS treatment decreased GLUT5 levels by proteasome-dependent degradation. Specific inhibitors of PKC, PKA, and MAP kinases (p38MAPK, JNK, MEK1/2) protected fructose uptake from adverse LPS effect. Moreover, a TNF-alpha antagonist blocked LPS action on fructose uptake. We conclude that intestinal fructose transport inhibition by LPS is associated with diminished GLUT5 numbers in the brush border membrane of enterocytes triggered by activation of several interrelated signaling cascades and proteasome degradation.

Sugar sensing by enterocytes combines polarity, membrane bound detectors and sugar metabolism

Sugar consumption and subsequent sugar metabolism are known to regulate the expression of genes involved in intestinal sugar absorption and delivery. Here we investigate the hypothesis that sugar-sensing detectors in membranes facing the intestinal lumen or the bloodstream can also modulate intestinal sugar absorption. We used wild-type and GLUT2-null mice, to show that dietary sugars stimulate the expression of sucrase-isomaltase (SI) and L-pyruvate kinase (L-PK) by GLUT2-dependent mechanisms, whereas the expression of GLUT5 and SGLT1, did not rely on the presence of GLUT2. By providing sugar metabolites, sugar transporters, including GLUT2, fuelled a sensing pathway. In Caco2/TC7 enterocytes, we could disconnect the sensing triggered by detector from that produced by metabolism, and found that GLUT2 generated a metabolism-independent pathway to stimulate the expression of SI and L-PK. In cultured enterocytes, both apical and basolateral fructose could increase the expression of GLUT5, conversely, basolateral sugar administration could stimulate the expression of GLUT2. Finally, we located the sweet-taste receptors T1R3 and T1R2 in plasma membranes, and we measured their cognate G alpha Gustducin mRNA levels. Furthermore, we showed that a T1R3 inhibitor altered the fructose-induced expression of SGLT1, GLUT5, and L-PK. Intestinal gene expression is thus controlled by a combination of at least three sugar-signaling pathways triggered by sugar metabolites and membrane sugar receptors that, according to membrane location, determine sugar-sensing polarity. This provides a rationale for how intestine adapts sugar delivery to blood and dietary sugar provision.

Regulation of poly(A)-binding protein through PABP-interacting proteins

Translation initiation requires the participation of eukaryotic translation initiation factors (eIFs). The poly(A)-binding protein (PABP) is thought to stimulate translation by promoting mRNA circularization through simultaneous interactions with eIF4G and the 3' poly(A) tail. PABP activity is regulated by the PABP-interacting proteins (Paips), a family of proteins consisting of Paip1, a translational stimulator, and Paip2A and Paip2B, two translational inhibitors. Paip2A controls PABP homeostasis via ubiquitination. When the cellular concentration of PABP is reduced, Paip2A becomes ubiquitinated and degraded, resulting in the relief of PABP repression. Paip1 interacts with eIF4A and eIF3, which promotes translation. The regulation of PABP activity by Paips represents the first known mechanism for controlling PABP, adding a new layer to the existing knowledge of PABP function.

Triiodothyronine (T3) and fructose coordinately enhance expression of the GLUT5 gene in the small intestine of rats during weaning period

Jejunal GLUT5 is elevated with triiodothyronine (T(3)) during weaning of rats. A perfusion of fructose into the small intestine of T(3)-injected rats at 21 d induced expression of the GLUT5 gene, but one into that of vehicle-injected rats did not. These results suggest that T(3) and fructose coordinately enhance jejunal expression of the GLUT5 gene in rats during weaning period.

Regulation of poly(A) binding protein function in translation: Characterization of the Paip2 homolog, Paip2B

The 5' cap and 3' poly(A) tail of eukaryotic mRNAs act synergistically to enhance translation. This synergy is mediated via interactions between eIF4G (a component of the eIF4F cap binding complex) and poly(A) binding protein (PABP). Paip2 (PABP-interacting protein 2) binds PABP and inhibits translation both in vitro and in vivo by decreasing the affinity of PABP for polyadenylated RNA. Here, we describe the functional characteristics of Paip2B, a Paip2 homolog. A full-length brain cDNA of Paip2B encodes a protein that shares 59% identity and 80% similarity with Paip2 (Paip2A), with the highest conservation in the two PABP binding domains. Paip2B acts in a manner similar to Paip2A to inhibit translation of capped and polyadenylated mRNAs both in vitro and in vivo by displacing PABP from the poly(A) tail. Also, similar to Paip2A, Paip2B does not affect the translation mediated by the internal ribosome entry site (IRES) of hepatitis C virus (HCV). However, Paip2A and Paip2B differ with respect to both mRNA and protein distribution in different tissues and cell lines. Paip2A is more highly ubiquitinated than is Paip2B and is degraded more rapidly by the proteasome. Paip2 protein degradation may constitute a primary mechanism by which cells regulate PABP activity in translation.

Intestinal sugar transport

Carbohydrates are an important component of the diet. The carbohydrates that we ingest range from simple monosaccharides (glucose, fructose and galactose) to disaccharides (lactose, sucrose) to complex polysaccharides. Most carbohydrates are digested by salivary and pancreatic amylases, and are further broken down into monosaccharides by enzymes in the brush border membrane (BBM) of enterocytes. For example, lactase-phloridzin hydrolase and sucrase-isomaltase are two disaccharidases involved in the hydrolysis of nutritionally important disaccharides. Once monosaccharides are presented to the BBM, mature enterocytes expressing nutrient transporters transport the sugars into the enterocytes. This paper reviews the early studies that contributed to the development of a working model of intestinal sugar transport, and details the recent advances made in understanding the process by which sugars are absorbed in the intestine.

Intestinal gluconeogenesis and glucose transport according to body fuel availability in rats

Intestinal hexose absorption and gluconeogenesis have been studied in relation to refeeding after two different fasting phases: a long period of protein sparing during which energy expenditure is derived from lipid oxidation (phase II), and a later phase characterized by a rise in plasma corticosterone triggering protein catabolism (phase III). Such a switch in body fuel uses, leading to changes in body reserves and gluconeogenic precursors, could modulate intestinal gluconeogenesis and glucose transport. The gene and protein levels, and the cellular localization of the sodium-glucose cotransporter SGLT1, and of GLUT5 and GLUT2, as well as that of the key gluconeogenic enzymes phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (Glc6Pase) were measured. PEPCK and Glc6Pase activities were also determined. In phase III fasted rats, SGLT1 was up-regulated and intestinal glucose uptake rates were higher than in phase II fasted and fed rats. PEPCK and Glc6Pase mRNA, protein levels and activities also increased in phase III. GLUT5 and GLUT2 were down-regulated throughout the fast, but increased after refeeding, with GLUT2 recruited to the apical membrane. The increase in SGLT1 expression during phase III may allow glucose absorption at low concentrations as soon as food is available. Furthermore, an increased epithelial permeability due to fasting may induce a paracellular movement of glucose. In the absence of intestinal GLUT2 during fasting, Glc6Pase could be involved in glucose release to the bloodstream via membrane trafficking. Finally, refeeding triggered GLUT2 and GLUT5 synthesis and apical recruitment of GLUT2, to absorb larger amounts of hexoses.

Dysregulation of cytochrome P450 17alpha-hydroxylase messenger ribonucleic acid stability in theca cells isolated from women with polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a common reproductive endocrine disorder characterized by ovarian hyperandrogenism. Theca interna cells isolated from the ovaries of women with PCOS are characterized by increased expression of cytochrome P450 17alpha-hydroxylase (CYP17) [steroid 17alpha-hydroxylase/17,20 lyase (P450c17)], a steroidogenic enzyme obligatory for the biosynthesis of androgens. Augmented expression of the gene encoding P450c17 (CYP17) in PCOS theca has been attributed, in part, to differential transcriptional regulation of the CYP17 promoter in normal and PCOS cells. The present studies examine whether CYP17 gene expression is also posttranscriptionally regulated at the level of mRNA stability in normal and PCOS theca cells maintained in long-term culture. Determination of endogenous CYP17 mRNA half-life by pharmacological inhibition of transcription demonstrated that the half-life of CYP17 mRNA increased 2-fold in PCOS theca cells, compared with normal theca cells. Forskolin treatment also prolonged CYP17 mRNA half-life in both normal and PCOS theca cells. In vitro mRNA degradation studies demonstrated that the 5'-untranslated region confers increased stability to CYP17 mRNA in PCOS theca cells and showed that the 5'-untranslated region of CYP17 also confers forskolin-stimulated stabilization of CYP17 mRNA. These studies indicate that a slower rate of CYP17 mRNA decay contributes to increased steady-state mRNA accumulation and augmented CYP17 gene expression in PCOS theca cells.

Poly(A)-binding protein-interacting protein 2, a strong regulator of vascular endothelial growth factor mRNA

Expression of vascular endothelial growth factor (VEGF) is tightly regulated, particularly at the level of its mRNA stability, which is essentially mediated through the 3'-untranslated region (3'-UTR) of VEGF mRNA. To identify new protein partners regulating VEGF mRNA stability, we screened a cDNA expression library with an RNA probe corresponding to the entire VEGF mRNA 3'-UTR. We identified the "poly(A)-binding protein-interacting protein 2" (PAIP2) as a new VEGF mRNA 3'-UTR interacting protein. By RNA electromobility shift assays, we showed that PAIP2 binds to two distinct regions of a domain encompassing base 1 to 1280 of the VEGF 3'-UTR. Such in vitro interaction was confirmed using cell extracts in which PAIP2 expression is induced by tetracycline (Tet-on cells). Moreover, we demonstrated by RNA affinity purification as well as by ribonucleoprotein complexes immunoprecipitation, that PAIP2 interacts with VEGF mRNA in vivo. Using an in vitro RNA degradation assay, the half-life of VEGF 3'-UTR was found to be increased by overexpressing PAIP2. PAIP2 stabilizes endogenous VEGF mRNA in Tet-on cells, leading to increased VEGF secretion. Moreover, RNAi-mediated knock-down of PAIP2 significantly reduces the steady-state levels of endogenous VEGF mRNA. We also showed, by in vitro protein-protein interactions and co-immunoprecipitation experiments, that PAIP2 interacts with HuR, an already known VEGF mRNA-binding protein, suggesting cooperation of both proteins for VEGF mRNA stabilization. Hence, PAIP2 appears to be a crucial regulator of VEGF mRNA and as a consequence, any variation in its expression could modulate angiogenesis.

Carbohydrate intolerance

PURPOSE OF REVIEW

The small intestinal mucosa is highly specialized for terminal digestion of nutrient polysaccharides and disaccharides and absorption of monosaccharides. However, in the case of digestive or absorptive deficiency, symptoms of carbohydrate intolerance result. Significant progress has been made toward defining the molecular genetic mechanisms responsible for several carbohydrate intolerances.

RECENT FINDINGS

This review summarizes monosaccharide and disaccharide intolerance conditions and recent clinical and basic science reports related to carbohydrate digestion and membrane transport. Genetic polymorphisms closely associated with lactase persistence/nonpersistence have been identified. Lactose intolerance is capable of preventing the achievement of adequate peak bone mass in susceptible young adults and may predispose to osteoporosis. Recent studies support previous reports that fructose malabsorption is associated with unexplained gastrointestinal symptoms. GLUT2 may be recruited from the basolateral to the apical membrane of enterocytes to facilitate small intestinal fructose absorption.

SUMMARY

Knowledge regarding the clinical aspects of and the physiologic mechanisms responsible for specific carbohydrate intolerances has allowed for improved diagnostic and treatment options and has contributed to continuing investigation of intestinal gene expression.