Dietary fructose improves intestinal cell survival and nutrient absorption.

Fructose consumption is linked to the rising incidence of obesity and cancer, which are two of the leading causes of morbidity and mortality globally1,2. Dietary fructose metabolism begins at the epithelium of the small intestine, where fructose is transported by glucose transporter type 5 (GLUT5; encoded by SLC2A5) and phosphorylated by ketohexokinase to form fructose 1-phosphate, which accumulates to high levels in the cell3,4. Although this pathway has been implicated in obesity and tumour promotion, the exact mechanism that drives these pathologies in the intestine remains unclear. Here we show that dietary fructose improves the survival of intestinal cells and increases intestinal villus length in several mouse models. The increase in villus length expands the surface area of the gut and increases nutrient absorption and adiposity in mice that are fed a high-fat diet. In hypoxic intestinal cells, fructose 1-phosphate inhibits the M2 isoform of pyruvate kinase to promote cell survival5-7. Genetic ablation of ketohexokinase or stimulation of pyruvate kinase prevents villus elongation and abolishes the nutrient absorption and tumour growth that are induced by feeding mice with high-fructose corn syrup. The ability of fructose to promote cell survival through an allosteric metabolite thus provides additional insights into the excess adiposity generated by a Western diet, and a compelling explanation for the promotion of tumour growth by high-fructose corn syrup.

Ketohexokinase-A deficiency attenuates the proliferation via reducing ß-catenin in gastric cancer cells.

Overconsumption of fructose is closely related to cancer. Ketohexokinase (KHK) catalyzes the conversion from fructose to fructose-1-phosphate (F1P), which is the first and committed step of fructose metabolism. Recently, aberrant KHK activation has been identified in multiple malignancies. However, the roles of KHK in gastric cancer (GC) cells are largely unclear. Herein, we reveal that the expression of ketohexokinase-A (KHK-A), one alternatively spliced KHK isoform that possesses low affinity for fructose, was markedly increased in GC cells. Depletion of endogenous KHK-A expression using lentiviruses encoding short hairpin RNAs (shRNAs) or pharmaceutical disruption of KHK-A activity using KHK-IN-1 hydrochloride in GC NCI-N87 and HGC-27 cells inhibited the proliferation in vitro and in vivo. Additionally, the mitochondrial respiration in the GC cells with KHK-A deficiency compared with the control cells was significantly impaired. One commercially-available antibody array was used to explore the effects of KHK-A knockdown on signaling pathways, showing that ß-catenin was remarkably reduced in the KHK-A deficient GC cells compared with the control ones. Pharmaceutical reduction in ß-catenin levels slowed down the proliferation of GC cells. These data uncover that KHK-A promotes the proliferation in GC cells, indicating that this enzyme might be a promising therapeutical target for GC treatment.

Ketohexokinase-C regulates global protein acetylation to decrease carnitine palmitoyltransferase 1a-mediated fatty acid oxidation.

BACKGROUND & AIMS: The consumption of sugar and a high-fat diet (HFD) promotes the development of obesity and metabolic dysfunction. Despite their well-known synergy, the mechanisms by which sugar worsens the outcomes associated with a HFD are largely elusive. METHODS: Six-week-old, male, C57Bl/6 J mice were fed either chow or a HFD and were provided with regular, fructose- or glucose-sweetened water. Moreover, cultured AML12 hepatocytes were engineered to overexpress ketohexokinase-C (KHK-C) using a lentivirus vector, while CRISPR-Cas9 was used to knockdown CPT1α. The cell culture experiments were complemented with in vivo studies using mice with hepatic overexpression of KHK-C and in mice with liver-specific CPT1α knockout. We used comprehensive metabolomics, electron microscopy, mitochondrial substrate phenotyping, proteomics and acetylome analysis to investigate underlying mechanisms. RESULTS: Fructose supplementation in mice fed normal chow and fructose or glucose supplementation in mice fed a HFD increase KHK-C, an enzyme that catalyzes the first step of fructolysis. Elevated KHK-C is associated with an increase in lipogenic proteins, such as ACLY, without affecting their mRNA expression. An increase in KHK-C also correlates with acetylation of CPT1α at K508, and lower CPT1α protein in vivo. In vitro, KHK-C overexpression lowers CPT1α and increases triglyceride accumulation. The effects of KHK-C are, in part, replicated by a knockdown of CPT1α. An increase in KHK-C correlates negatively with CPT1α protein levels in mice fed sugar and a HFD, but also in genetically obese db/db and lipodystrophic FIRKO mice. Mechanistically, overexpression of KHK-C in vitro increases global protein acetylation and decreases levels of the major cytoplasmic deacetylase, SIRT2. CONCLUSIONS: KHK-C-induced acetylation is a novel mechanism by which dietary fructose augments lipogenesis and decreases fatty acid oxidation to promote the development of metabolic complications. IMPACT AND IMPLICATIONS: Fructose is a highly lipogenic nutrient whose negative consequences have been largely attributed to increased de novo lipogenesis. Herein, we show that fructose upregulates ketohexokinase, which in turn modifies global protein acetylation, including acetylation of CPT1a, to decrease fatty acid oxidation. Our findings broaden the impact of dietary sugar beyond its lipogenic role and have implications on drug development aimed at reducing the harmful effects attributed to sugar metabolism.

Sequential uptake of aldoses over fructose and enhanced phosphate solubilization in Rhizobium sp. RM.

Rhizobium sp. RM solubilized tri-calcium phosphate (TCP: 324-463 µg ml-1) and rock phosphate (RP: 36-46.58 µg ml-1) in the presence of common rhizospheric sugars-glucose, arabinose, xylose and their combinations. Fructose, though did not support RP solubilization individually, surprisingly solubilized significantly higher phosphate when combined with aldoses. The highest TCP (644 µg ml-1) and RP (75 µg ml-1) solubilization was achieved in fructose + glucose combination. Presence of gluconate, malate and oxalate in culture supernatant indicated functioning of periplasmic glucose oxidation, the non-phosphorylative arabinose dehydrogenase pathway and the tricarboxylate (TCA) cycle, respectively. Aldoses, when present together, were co-utilized (monoauxic growth) however, when added with fructose, prevented the uptake of fructose yielding a typical diauxic growth. This presented an unusual sequential utilization of aldoses over a ketose (fructose) in strain RM. The prevention of fructose uptake by aldoses was investigated through real-time expression of key genes coding fructose transport proteins and initial enzymes of sugar metabolism. Fructose was actively transported via fructose-specific ABC transporters as suggested by upregulation of frcB and frcC only in fructose and fructose growth phases of fructose + aldose combinations. The probable route of initial fructose metabolism involved either fructokinase and/or xylose isomerase, as confirmed by enzyme activities. The upregulation of hfq and hprK genes only in aldose phase of fructose + aldose combinations suggested their possible involvement in governing the preferential utilization. The novel aspects of this study are enhanced organic acid mediated P solubilization in fructose + aldose combinations and a rare hierarchy of aldoses over fructose which is possibly regulated at the level of fructose transport and fructokinase. KEY POINTS: • Sugars when provided in different dual combinations, supported enhanced P solubilization from complex phosphate sources like TCP and RP in Rhizobium sp. RM. • Transcriptional status of genes in cells of RM when grown in different individual sugars and their combinations suggested that fructose might be a less preferred carbon source and hence was utilized after aldoses with the possible regulation by Hfq and HPrK. • First study to present a unique phenomenon of sequential utilization of aldoses (glucose, arabinose and xylose) over fructose in a concentration-independent manner in Rhizobium sp. RM. and to present the effect of dual combinations of sugars on organic acid mediated P solubilization trait of rhizobia.

A Comparative Characterization and Expression Profiling Analysis of Fructokinase and Fructokinase-like Genes: Exploring Their Roles in Cucumber Development and Chlorophyll Biosynthesis.

Fructokinase (FRK) and fructokinase-like (FLN), belonging to the phosphofructokinase B type subfamily, share substantial sequence similarity, and are crucial in various plant physiological processes. However, there is limited information regarding what functionally differentiates plant FRKs from FLNs. Here, a total of three CsFRKs and two CsFLNs were identified from the cucumber genome. Their significant difference lay in the structure of their G/AXGD motif, which existed as GAGD in CsFRKs, but as G/ASGD in CsFLNs. Comparative phylogenetic analysis classified CsFRKs and CsFLNs into five sub-branches consistent with their quite different exon/intron organizations. Both transcriptome data and RT-qPCR analyses revealed that CsFRK3 was the most active gene, with the highest expression in the majority of tissues tested. Moreover, the expression levels of two putative plastidic genes, CsFRK1 and CsFLN2, were significantly positively associated with chlorophyll accumulation in the chlorophyll-reduced cucumber mutant. Briefly, both CsFRK and CsFLN genes were involved in the development of sink tissues, especially CsFRK3. CsFRK1 and CsFLN2 were recognized as candidates in the chlorophyll biosynthesis pathway of cucumber. These results would greatly assist in further investigation on functional characterization of FRKs and FLNs, especially in the development and chlorophyll biosynthesis of cucumber.

Comparative Proteomics of Potato Cultivars with a Variable Dormancy Period.

The control of the duration of the dormancy phase is a significant challenge in the potato industry and for seed producers. However, the proteome landscape involved in the regulation of the length of the dormancy period over potato cultivars remains largely unexplored. In this study, we performed for the first time a comparative proteome profiling of potato cultivars with differential duration of tuber dormancy. More specifically, the proteome profiling of Agata, Kennebec and Agria commercial potato varieties with short, medium and medium-long dormancy, respectively, was assessed at the endodormancy stage using high-resolution two-dimensional electrophoresis (2-DE) coupled to reversed-phase liquid chromatography-tandem mass spectrometry (LC-TripleTOF MS/MS). A total of 11 proteins/isoforms with statistically significant differential abundance among cultivars were detected on 2-DE gels and confidently identified by LC-TripleTOF MS/MS. Identified proteins have known functions related to tuber development, sprouting and the oxylipins biosynthesis pathway. Fructokinase, a mitochondrial ADP/ATP carrier, catalase isozyme 2 and heat shock 70 kDa were the proteins with the strongest response to dormancy variations. To the best of our knowledge, this study reports the first candidate proteins underlying variable dormancy length in potato cultivars.

Hypoxic naked mole-rat brains use microRNA to coordinate hypometabolic fuels and neuroprotective defenses.

Naked mole-rats are among the mammalian champions of hypoxia tolerance. They evolved adaptations centered around reducing metabolic rate to overcome the challenges experienced in their underground burrows. In this study, we used next-generation sequencing to investigate one of the factors likely supporting hypoxia tolerance in naked mole-rat brains, posttranscriptional microRNAs (miRNAs). Of the 212 conserved miRNAs identified using small RNA sequencing, 18 displayed significant differential expression during hypoxia. Bioinformatic enrichment revealed that hypoxia-mediated miRNAs were suppressing energy expensive processes including de novo protein translation and cellular proliferation. This suppression occurred alongside the activation of neuroprotective and neuroinflammatory pathways, and the induction of central signal transduction pathways including HIF-1α and NFκB via miR-335, miR-101, and miR-155. MiRNAs also coordinated anaerobic glycolytic fuel sources, where hypoxia-upregulated miR-365 likely suppressed protein levels of ketohexokinase, the enzyme responsible for catalyzing the first committed step of fructose catabolism. This was further supported by a hypoxia-mediated reduction in glucose transporter 5 proteins that import fructose into the cell. Yet, messenger RNA and protein levels of lactate dehydrogenase, which converts pyruvate to lactate in the absence of oxygen, were elevated during hypoxia. Together, this demonstrated the induction of anaerobic glycolysis despite a lack of reliance on fructose as the primary fuel source, suggesting that hypoxic brains are metabolically different than anoxic naked mole-rat brains that were previously found to shift to fructose-based glycolysis. Our findings contribute to the growing body of oxygen-responsive miRNAs "OxymiRs" that facilitate natural miRNA-mediated mechanisms for successful hypoxic exposures.

HIF-driven SF3B1 induces KHK-C to enforce fructolysis and heart disease.

Fructose is a major component of dietary sugar and its overconsumption exacerbates key pathological features of metabolic syndrome. The central fructose-metabolising enzyme is ketohexokinase (KHK), which exists in two isoforms: KHK-A and KHK-C, generated through mutually exclusive alternative splicing of KHK pre-mRNAs. KHK-C displays superior affinity for fructose compared with KHK-A and is produced primarily in the liver, thus restricting fructose metabolism almost exclusively to this organ. Here we show that myocardial hypoxia actuates fructose metabolism in human and mouse models of pathological cardiac hypertrophy through hypoxia-inducible factor 1α (HIF1α) activation of SF3B1 and SF3B1-mediated splice switching of KHK-A to KHK-C. Heart-specific depletion of SF3B1 or genetic ablation of Khk, but not Khk-A alone, in mice, suppresses pathological stress-induced fructose metabolism, growth and contractile dysfunction, thus defining signalling components and molecular underpinnings of a fructose metabolism regulatory system crucial for pathological growth.

Osthol Ameliorates Kidney Damage and Metabolic Syndrome Induced by a High-Fat/High-Sugar Diet.

Excessive intake of fructose results in metabolic syndrome (MS) and kidney damage, partly mediated by its metabolism by fructokinase-C or ketohexokinase-C (KHK-C). Osthol has antioxidant properties, is capable of regulating adipogenesis, and inhibits KHK-C activity. Here, we examined the potential protective role of osthol in the development of kidney disease induced by a Western (high-fat/high-sugar) diet. Control rats fed with a high-fat/high-sugar diet were compared with two groups that also received two different doses of osthol (30 mg/kg/d or 40 mg/kg/d body weight BW). A fourth group served as a normal control and received regular chow. At the end of the follow-up, kidney function, metabolic markers, oxidative stress, and lipogenic enzymes were evaluated. The Western diet induced MS (hypertension, hyperglycemia, hypertriglyceridemia, obesity, hyperuricemia), a fall in the glomerular filtration rate, renal tubular damage, and increased oxidative stress in the kidney cortex, with increased expression of lipogenic enzymes and increased kidney KHK expression. Osthol treatment prevented the development of MS and ameliorated kidney damage by inhibiting KHK activity, preventing oxidative stress via nuclear factor erythroid 2-related factor (Nrf2) activation, and reducing renal lipotoxicity. These data suggest that the nutraceutical osthol might be an ancillary therapy to slow the progression of MS and kidney damage induced by a Western diet.

Identification of miRNAs and Their Targets Involved in Flower and Fruit Development across Domesticated and Wild Capsicum Species.

MicroRNAs (miRNAs) are regulators of the post-transcription stage of gene activity documented to play central roles in flower and fruit development in model plant species. However, little is known about their roles and differences in domesticated and wild Capsicum species. In this study, we used high-throughput sequencing to analyze the miRNA content at three developmental stages (flower, small fruit, and middle fruit) from two cultivated (C. baccatum and C. annuum) and two wild (C. chacoense and C. eximium) pepper species. This analysis revealed 22 known and 27 novel miRNAs differentially expressed across species and tissues. A number of stage- and species-specific miRNAs were identified, and Gene Ontology terms were assigned to 138 genes targeted by the miRNAs. Most Gene Ontology terms were for the categories "genetic information processing", "signaling and cellular processes", "amino acid metabolism", and "carbohydrate metabolism". Enriched KEGG analysis revealed the pathways amino acids, sugar and nucleotide metabolism, starch and sucrose metabolism, and fructose-mannose metabolism among the principal ones regulated by miRNAs during pepper fruit ripening. We predicted miRNA-target gene interactions regulating flowering time and fruit development, including miR156/157 with SPL genes, miR159 with GaMYB proteins, miR160 with ARF genes, miR172 with AP2-like transcription factors, and miR408 with CLAVATA1 gene across the different Capsicum species. In addition, novel miRNAs play an important role in regulating interactions potentially controlling plant pathogen defense and fruit quality via fructokinase, alpha-L-arabinofuranosidase, and aromatic and neutral amino acid transporter. Overall, the small RNA-sequencing results from this study represent valuable information that provides a solid foundation for uncovering the miRNA-mediated mechanisms of flower and fruit development between domesticated and wild Capsicum species.

Cell-Type-Specific, Ketohexokinase-Dependent Induction by Fructose of Lipogenic Gene Expression in Mouse Small Intestine.

BACKGROUND: High intakes of fructose are associated with metabolic diseases, including hypertriglyceridemia and intestinal tumor growth. Although small intestinal epithelia consist of many different cell types, express lipogenic genes, and convert dietary fructose to fatty acids, there is no information on the identity of the cell type(s) mediating this conversion and on the effects of fructose on lipogenic gene expression. OBJECTIVES: We hypothesized that fructose regulates the intestinal expression of genes involved in lipid and apolipoprotein synthesis, that regulation depends on the fructose transporter solute carrier family 2 member a5 [Slc2a5 (glucose transporter 5)] and on ketohexokinase (Khk), and that regulation occurs only in enterocytes. METHODS: We compared lipogenic gene expression among different organs from wild-type adult male C57BL mice consuming a standard vivarium nonpurified diet. We then gavaged twice daily for 2.5 d fructose or glucose solutions (15%, 0.3 mL per mouse) into wild-type, Slc2a5-knockout (KO), and Khk-KO mice with free access to the nonpurified diet and determined expression of representative lipogenic genes. Finally, from mice fed the nonpurified diet, we made organoids highly enriched in enterocyte, goblet, Paneth, or stem cells and then incubated them overnight in 10 mM fructose or glucose. RESULTS: Most lipogenic genes were significantly expressed in the intestine relative to the kidney, liver, lung, and skeletal muscle. In vivo expression of Srebf1, Acaca, Fasn, Scd1, Dgat1, Gk, Apoa4, and Apob mRNA and of Scd1 protein increased (P < 0.05) by 3- to 20-fold in wild-type, but not in Slc2a5-KO and Khk-KO, mice gavaged with fructose. In vitro, Slc2a5- and Khk-dependent, fructose-induced increases, which ranged from 1.5- to 4-fold (P < 0.05), in mRNA concentrations of all these genes were observed only in organoids enriched in enterocytes. CONCLUSIONS: Fructose specifically stimulates expression of mouse small intestinal genes for lipid and apolipoprotein synthesis. Secretory and stem cells seem incapable of transport- and metabolism-dependent lipogenesis, occurring only in absorptive enterocytes.

Fructose malabsorption induces cholecystokinin expression in the ileum and cecum by changing microbiota composition and metabolism.

Current fructose consumption levels often overwhelm the intestinal capacity to absorb fructose. We investigated the impact of fructose malabsorption on intestinal endocrine function and addressed the role of the microbiota in this process. To answer this question, a mouse model of moderate fructose malabsorption [ketohexokinase mutant (KHK)-/-] and wild-type (WT) littermate mice were used and received a 20%-fructose (KHK-F and WT-F) or 20%-glucose diet. Cholecystokinin (Cck) mRNA and protein expression in the ileum and cecum, as well as preproglucagon (Gcg) and neurotensin (Nts) mRNA expression in the cecum, increased in KHK-F mice. In KHK-F mice, triple-label immunohistochemistry showed major up-regulation of CCK in enteroendocrine cells (EECs) that were glucagon-like peptide-1 (GLP-1)+/Peptide YY (PYY-) in the ileum and colon and GLP-1-/PYY- in the cecum. The cecal microbiota composition was drastically modified in the KHK-F in association with an increase in glucose, propionate, succinate, and lactate concentrations. Antibiotic treatment abolished fructose malabsorption-dependent induction of cecal Cck mRNA expression and, in mouse GLUTag and human NCI-H716 cells, Cck mRNA expression levels increased in response to propionate, both suggesting a microbiota-dependent process. Fructose reaching the lower intestine can modify the composition and metabolism of the microbiota, thereby stimulating the production of CCK from the EECs possibly in response to propionate.-Zhang, X., Grosfeld, A., Williams, E., Vasiliauskas, D., Barretto, S., Smith, L., Mariadassou, M., Philippe, C., Devime, F., Melchior, C., Gourcerol, G., Dourmap, N., Lapaque, N., Larraufie, P., Blottière, H. M., Herberden, C., Gerard, P., Rehfeld, J. F., Ferraris, R. P., Fritton, J. C., Ellero-Simatos, S., Douard, V. Fructose malabsorption induces cholecystokinin expression in the ileum and cecum by changing microbiota composition and metabolism.

Sucrose Metabolism in Haloarchaea: Reassessment Using Genomics, Proteomics, and Metagenomics.

The canonical pathway for sucrose metabolism in haloarchaea utilizes a modified Embden-Meyerhof-Parnas pathway (EMP), in which ketohexokinase and 1-phosphofructokinase phosphorylate fructose released from sucrose hydrolysis. However, our survey of haloarchaeal genomes determined that ketohexokinase and 1-phosphofructokinase genes were not present in all species known to utilize fructose and sucrose, thereby indicating that alternative mechanisms exist for fructose metabolism. A fructokinase gene was identified in the majority of fructose- and sucrose-utilizing species, whereas only a small number possessed a ketohexokinase gene. Analysis of a range of hypersaline metagenomes revealed that haloarchaeal fructokinase genes were far more abundant (37 times) than haloarchaeal ketohexokinase genes. We used proteomic analysis of Halohasta litchfieldiae (which encodes fructokinase) and identified changes in protein abundance that relate to growth on sucrose. Proteins inferred to be involved in sucrose metabolism included fructokinase, a carbohydrate primary transporter, a putative sucrose hydrolase, and two uncharacterized carbohydrate-related proteins encoded in the same gene cluster as fructokinase and the transporter. Homologs of these proteins were present in the genomes of all haloarchaea that use sugars for growth. Enzymes involved in the semiphosphorylative Entner-Doudoroff pathway also had higher abundances in sucrose-grown H. litchfieldiae cells, consistent with this pathway functioning in the catabolism of the glucose moiety of sucrose. The study revises the current understanding of fundamental pathways for sugar utilization in haloarchaea and proposes alternatives to the modified EMP pathway used by haloarchaea for sucrose and fructose utilization.IMPORTANCE Our ability to infer the function that microorganisms perform in the environment is predicated on assumptions about metabolic capacity. When genomic or metagenomic data are used, metabolic capacity is inferred from genetic potential. Here, we investigate the pathways by which haloarchaea utilize sucrose. The canonical haloarchaeal pathway for fructose metabolism involving ketohexokinase occurs only in a small proportion of haloarchaeal genomes and is underrepresented in metagenomes. Instead, fructokinase genes are present in the majority of genomes/metagenomes. In addition to genomic and metagenomic analyses, we used proteomic analysis of Halohasta litchfieldiae (which encodes fructokinase but lacks ketohexokinase) and identified changes in protein abundance that related to growth on sucrose. In this way, we identified novel proteins implicated in sucrose metabolism in haloarchaea, comprising a transporter and various catabolic enzymes (including proteins that are annotated as hypothetical).

Endogenous fructose production: what do we know and how relevant is it?

PURPOSE OF REVIEW: Excessive sugar and particularly fructose consumption has been proposed to be a key player in the pathogenesis of metabolic syndrome and kidney disease in humans and animal models. However, besides its dietary source, fructose can be endogenously produced in the body from glucose via the activation of the polyol pathway. In this review, we aim to describe the most recent findings and current knowledge on the potential role of endogenous fructose production and metabolism in disease. RECENT FINDINGS: Over the recent years, the activation of the polyol pathway and endogenous fructose production has been observed in multiple tissues including the liver, renal cortex, and hypothalamic areas of the brain. The activation occurs during the development and progression of metabolic syndrome and kidney disease and results from different stimuli including osmotic effects, diabetes, and ischemia. Even though the potential toxicity of the activation of the polyol pathway can be attributed to several intermediate products, the blockade of endogenous fructose metabolism either by using fructokinase deficient mice or specific inhibitors resulted in marked amelioration of multiple metabolic diseases. SUMMARY: New findings suggest that fructose can be produced in the body and that the blockade of tis metabolism could be clinically relevant for the prevention and treatment of metabolic syndrome and kidney disease.

Fructose metabolism, cardiometabolic risk, and the epidemic of coronary artery disease.

Despite strong indications that increased consumption of added sugars correlates with greater risks of developing cardiometabolic syndrome (CMS) and cardiovascular disease (CVD), independent of the caloric intake, the worldwide sugar consumption remains high. In considering the negative health impact of overconsumption of dietary sugars, increased attention is recently being given to the role of the fructose component of high-sugar foods in driving CMS. The primary organs capable of metabolizing fructose include liver, small intestine, and kidneys. In these organs, fructose metabolism is initiated by ketohexokinase (KHK) isoform C of the central fructose-metabolizing enzyme KHK. Emerging data suggest that this tissue restriction of fructose metabolism can be rescinded in oxygen-deprived environments. In this review, we highlight recent progress in understanding how fructose metabolism contributes to the development of major systemic pathologies that cooperatively promote CMS and CVD, reference recent insights into microenvironmental control of fructose metabolism under stress conditions and discuss how this understanding is shaping preventive actions and therapeutic approaches.

Salinity Effects on Sugar Homeostasis and Vascular Anatomy in the Stem of the Arabidopsis Thaliana Inflorescence.

The regulation of sugar metabolism and partitioning plays an essential role for a plant's acclimation to its environment, with specific responses in autotrophic and heterotrophic organs. In this work, we analyzed the effects of high salinity on sugar partitioning and vascular anatomy within the floral stem. Stem sucrose and fructose content increased, while starch reduced, in contrast to the response observed in rosette leaves of the same plants. In the stem, the effects were associated with changes in the expression of SWEET and TMT2 genes encoding sugar transporters, SUSY1 encoding a sucrose synthase and several FRK encoding fructokinases. By contrast, the expression of SUC2, SWEET11 and SWEET12, encoding sugar transporters for phloem loading, remained unchanged in the stem. Both the anatomy of vascular tissues and the composition of xylem secondary cell walls were altered, suggesting that high salinity triggered major readjustments of sugar partitioning in this heterotrophic organ. There were changes in the composition of xylem cell walls, associated with the collapse and deformation of xylem vessels. The data are discussed regarding sugar partitioning and homeostasis of sugars in the vascular tissues of the stem.

Effects of Quercetin on Expression of Genes of Carbohydrate and Lipid Metabolism Enzymes in the Liver of Rats Receiving High-Fructose Ration.

We studied the expression of genes encoding enzymes of carbohydrate and lipid metabolism ketohexokinase (Khk), glucokinase (Gck), pyruvate kinase (Pklr), acetyl-Co-carboxylase (Acaca), fatty acid synthase (Fasn), stearoyl-CoA desaturase (Scd), and their transcription regulators ChREBP (Mlxipl), SREBP-1c (Srebf1), and PPARα (Ppara) in rat liver. Control group rats received a semisynthetic ration over 20 weeks. Experimental group 1 received a semisynthetic ration and 20% fructose solution instead of drinking water. Experimental group 2 rats received a semisynthetic ration with quercetin (0.1% fodder weight) and 20% fructose solution. Consumption of 20% fructose solution (experimental group 1) led to an increase in Scd expression in comparison with the control and did not affect the expression of other genes. Addition of quercetin to the ration (experimental group 2) led to a decrease in the expression of Khk, Gck, Fasn, Scd, Mlxipl, and Ppara genes in comparison with experimental group 1. The results suggest that quercetin reduced the expression of genes of carbohydrate and lipid metabolism enzymes in the liver of rats receiving high-fructose ration.

A mathematical analysis of adaptations to the metabolic fate of fructose in essential fructosuria subjects.

Fructose is a major component of Western diets and is implicated in the pathogenesis of obesity and type 2 diabetes. In response to an oral challenge, the majority of fructose is cleared during "first-pass" liver metabolism, primarily via phosphorylation by ketohexokinase (KHK). A rare benign genetic deficiency in KHK, called essential fructosuria (EF), leads to altered fructose metabolism. The only reported symptom of EF is the appearance of fructose in the urine following either oral or intravenous fructose administration. Here we develop and use a mathematical model to investigate the adaptations to altered fructose metabolism in people with EF. First, the model is calibrated to fit available data in normal healthy subjects. Then, to mathematically represent EF subjects, we systematically implement metabolic adaptations such that model simulations match available data for this phenotype. We hypothesize that these modifications represent the major metabolic adaptations present in these subjects. This modeling approach suggests that several other aspects of fructose metabolism, beyond hepatic KHK deficiency, are altered and contribute to the etiology of this benign condition. Specifically, we predict that fructose absorption into the portal vein is altered, peripheral metabolism is slowed, renal reabsorption of fructose is mostly ablated, and alternate pathways for hepatic metabolism of fructose are upregulated. Moreover, these findings have implications for drug discovery and development, suggesting that the therapeutic targeting of fructose metabolism could lead to unexpected metabolic adaptations, potentially due to a physiological response to high-fructose conditions.

Ketohexokinase knockout mice, a model for essential fructosuria, exhibit altered fructose metabolism and are protected from diet-induced metabolic defects.

Fructose consumption in humans and animals has been linked to enhanced de novo lipogenesis, dyslipidemia, and insulin resistance. Hereditary deficiency of ketohexokinase (KHK), the first enzymatic step in fructose metabolism, leads to essential fructosuria in humans, characterized by elevated levels of blood and urinary fructose following fructose ingestion but is otherwise clinically benign. To address whether KHK deficiency is associated with altered glucose and lipid metabolism, a Khk knockout (KO) mouse line was generated and characterized. NMR spectroscopic analysis of plasma following ingestion of [6-13C] fructose revealed striking differences in biomarkers of fructose metabolism. Significantly elevated urine and plasma 13C-fructose levels were observed in Khk KO vs. wild-type (WT) control mice, as was reduced conversion of 13C-fructose into plasma 13C-glucose and 13C-lactate. In addition, the observation of significant levels of fructose-6-phosphate in skeletal muscle tissue of Khk KO, but not WT, mice suggests a potential mechanism, whereby fructose is metabolized via muscle hexokinase in the absence of KHK. Khk KO mice on a standard chow diet displayed no metabolic abnormalities with respect to ambient glucose, glucose tolerance, body weight, food intake, and circulating trigylcerides, ß-hydroxybutyrate, and lactate. When placed on a high-fat and high-fructose (HF/HFruc) diet, Khk KO mice had markedly reduced liver weight, triglyceride levels, and insulin levels. Together, these results suggest that Khk KO mice may serve as a good model for essential fructosuria in humans and that inhibition of KHK offers the potential to protect from diet-induced hepatic steatosis and insulin resistance.