Insight into the "odd" hexose transporters GLUT3, GLUT5, and GLUT7

Genome-wide identification and characterization of glucose transporter (glut) genes in spotted sea bass (Lateolabrax maculatus) and their regulated hepatic expression during short-term starvation

The glucose transporters (GLUTs) are well known for their essential roles in moving the key metabolites, glucose, galactose, fructose and a number of other important substrates in and out of cells. In this study, we identified a total of 21 glut genes in spotted sea bass (Lateolabrax maculatus) through extensive data mining of existing genomic and transcriptomic databases. Glut genes of spotted sea bass were classified into three subfamilies (Class I, Class II and Class III) according to the phylogenetic analysis. Glut genes of spotted sea bass were distributed in 15 out of 24 chromosomes. Deduced gene structure analysis including the secondary structure and the three-dimensional structures, as well as the syntenic analysis further supported their annotations and orthologies. Expression profile in healthy tissues indicated that 9 of 21 glut genes were expressed in liver of spotted sea bass. During short-term starvation, the mRNA expression levels of 3 glut genes (glut2, glut5, and glut10) were significantly up-regulated in liver (P < 0.05), indicating their potential roles in sugar transport and consumption. These findings in our study will facilitate the further evolutionary characterization of glut genes in fish species and provide a theoretical basis for their functional study.

Honey as a Potential Natural Antioxidant Medicine: An Insight into Its Molecular Mechanisms of Action

Honey clasps several medicinal and health effects as a natural food supplement. It has been established as a potential therapeutic antioxidant agent for various biodiverse ailments. Data report that it exhibits strong wound healing, antibacterial, anti-inflammatory, antifungal, antiviral, and antidiabetic effects. It also retains immunomodulatory, estrogenic regulatory, antimutagenic, anticancer, and numerous other vigor effects. Data also show that honey, as a conventional therapy, might be a novel antioxidant to abate many of the diseases directly or indirectly associated with oxidative stress. In this review, these wholesome effects have been thoroughly reviewed to underscore the mode of action of honey exploring various possible mechanisms. Evidence-based research intends that honey acts through a modulatory road of multiple signaling pathways and molecular targets. This road contemplates through various pathways such as induction of caspases in apoptosis; stimulation of TNF-α, IL-1β, IFN-γ, IFNGR1, and p53; inhibition of cell proliferation and cell cycle arrest; inhibition of lipoprotein oxidation, IL-1, IL-10, COX-2, and LOXs; and modulation of other diverse targets. The review highlights the research done as well as the apertures to be investigated. The literature suggests that honey administered alone or as adjuvant therapy might be a potential natural antioxidant medicinal agent warranting further experimental and clinical research.

Fibromyalgia Syndrome: A Metabolic Approach Grounded in Biochemistry for the Remission of Symptoms

Fibromyalgia syndrome (FMS) is a chronic, complex, and heterogeneous disorder of still poorly understood etiopathophysiology associated with important musculoskeletal widespread pain, fatigue, non-restorative sleep, and mood disturbances. It is estimated to afflict 2–3% of the worldwide population, with clean prevalence among women. The objective of this paper is to propose a novel treatment for symptomatic remission of FMS, grounded in biochemistry and consisting in the withdrawal from the diet of molecules that can indirectly trigger the symptoms. The hypothesis develops from the evidence that low serotonin levels are involved in FMS. Serotonin is synthesized starting from the essential amino acid tryptophan. The presence of non-absorbed molecules in the gut, primarily fructose, reduces tryptophan absorption. Low tryptophan absorption leads to low serotonin synthesis that triggers FMS symptoms. Moreover not-absorbed sugars could also produce a microbiota deterioration activating a positive feedback loop: the increasing microbiota deterioration reduces the functionality of absorption both of fructose and tryptophan in the gut, entering a vicious circle. The therapeutic idea is to sustain serotonin synthesis allowing the proper tryptophan absorption. The core of the cure treatment is the exclusion from the diet of some carbohydrates and the marked reduction of some others. The main target is the limitation of total dietary fructose as marked as possible. It could be an effective strategy to get the remission of symptoms acting on the impaired biochemical pathways. The straying from the treatment is expected to cause the reappear of the symptoms.

Fructose transporters GLUT5 and GLUT2 expression in adult patients with fructose intolerance

BACKGROUND

Gastrointestinal symptoms and malabsorption following fructose ingestion (fructose intolerance) are common in functional gastrointestinal disorders (FGID). The underlying mechanism is unclear, but is hypothesized to be related an abnormality of intestinal fructose transporter proteins.

OBJECTIVE

To assess the expression of the main intestinal fructose transporter proteins, glucose transport protein 5 (GLUT5) and 2 (GLUT2), in FGID.

METHODS

The expression of GLUT5 and GLUT2 protein and mRNA in small intestinal biopsy tissue was investigated using real-time reverse-transcription PCR and Western immunoblotting in 11 adults with FGID and fructose intolerance ascertained by breath testing and in 15 controls.

RESULTS

Median expression levels of GLUT5 mRNA normalized to beta-actin were 0.18 (interquartile range, IQR, 0.13-0.21) in patients and 0.17 (IQR 0.12-0.19) in controls (p > 0.05). Respective levels of GLUT2 mRNA were 0.26 (IQR 0.20-0.31) and 0.26 (IQR 0.19-0.31) (p > 0.05). Median expression levels of GLUT5 protein normalized to alpha-tubulin were 0.95 (IQR 0.52-1.68) in patients and 0.95 (IQR 0.59-1.15) in controls (p > 0.05). Respective protein expression levels for GLUT2 were 1.56 (IQR 1.06-2.14) and 1.35 (IQR 0.96-1.79) (p > 0.05).

CONCLUSIONS

Human fructose intolerance may not be associated with marked changes in GLUT5 and GLUT2 expression. Replication of these results in a larger subject group, including measures of transporter activation and membrane and subcellular localization, is warranted.

Correlation between Expression of Glucose Transporters in Granulosa Cells and Oocyte Quality in Women with Polycystic Ovary Syndrome

BACKGROUND

The glucose transporters (GLUTs) exhibit different tissue-specific expression. This study aimed to investigate the types of GLUTs expressed in human granulosa cells (GCs) obtained from women with polycystic ovary syndrome (PCOS) and their relationship with insulin resistance (IR) and the outcomes of in vitro maturation (IVM) of immature oocytes.

METHODS

Expression of GLUTs was evaluated in GCs from women with PCOS with or without IR. Thirty-six women with PCOS undergoing an IVM program were included. Differential gene expression between the insulin sensitive (IS) and IR group was measured by reverse transcription polymerase chain reaction.

RESULTS

Expression of GLUTs 1, 3, 5, 8, and 13 was constitutive, whereas expression of GLUTs 2 and 7 was not observed in human GCs. The remaining GLUTs, 4, 6, 9, 10, 11, and 12, were differentially expressed among patients according to metabolic status, such as insulin sensitivity. A higher number of GCs from patients with IR (92%) expressed GLUT6 than GCs from IS PCOS patients (46.3%). Logistic regression showed that expression of GLUTs 9, 11, and 12 correlates with rates of IVM at 48 hours, fertilization, and implantation, respectively.

CONCLUSION

This is the first report describing the expression pattern of all 13 members of the GLUT family in human GCs. Results of the present study suggest that patients' insulin sensitivity regulates GLUT expression in GCs in PCOS patients, and this may control oocyte quality for IVM and subsequent processes such as fertilization and implantation in patients taking part in an in vitro fertilization program.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

Phytanic acid stimulates glucose uptake in a model of skeletal muscles, the primary porcine myotubes

BACKGROUND

Phytanic acid (PA) is a chlorophyll metabolite with potentials in regulating glucose metabolism, as it is a natural ligand of the peroxisome proliferator-activated receptor (PPAR) that is known to regulate hepatic glucose homeostasis. This study aimed to establish primary porcine myotubes as a model for measuring glucose uptake and glycogen synthesis, and to examine the impact of physiological amounts of PA on glucose uptake and glycogen synthesis either alone or in combination with insulin.

METHODS

Porcine satellite cells were cultured into differentiated myotubes and tritiated 2-deoxyglucose (2-DOG) was used to measure glucose uptake, in relation to PA and 2-DOG exposure times and also in relation to PA and insulin concentrations. The MIXED procedure model of SAS was used for statistical analysis of data.

RESULTS

PA increased glucose uptake by approximately 35%, and the presence of insulin further increased the uptake, but this further increase in uptake was non- additive and less pronounced at high insulin concentrations. There was no effect of PA alone on glycogen synthesis, while the insulin stimulation of glycogen was increased by 20% in the presence of PA. PA neither stimulated glucose uptake nor glycogen synthesis in insulin-resistant myotubes generated by excess glucose exposure.

CONCLUSIONS

Primary porcine myotubes were established as a model of skeletal muscles for measuring glucose uptake and glycogen synthesis, and we showed that PA can play a role in stimulating glucose uptake at no or inadequate insulin concentrations.

Hepatic expression and cellular distribution of the glucose transporter family

Glucose and other carbohydrates are transported into cells using members of a family of integral membrane glucose transporter (GLUT) molecules. To date 14 members of this family, also called the solute carrier 2A proteins have been identified which are divided on the basis of transport characteristics and sequence similarities into several families (Classes 1 to 3). The expression of these different receptor subtypes varies between different species, tissues and cellular subtypes and each has differential sensitivities to stimuli such as insulin. The liver is a contributor to metabolic carbohydrate homeostasis and is a major site for synthesis, storage and redistribution of carbohydrates. Situations in which the balance of glucose homeostasis is upset such as diabetes or the metabolic syndrome can lead metabolic disturbances that drive chronic organ damage and failure, confirming the importance of understanding the molecular regulation of hepatic glucose homeostasis. There is a considerable literature describing the expression and function of receptors that regulate glucose uptake and release by hepatocytes, the most import cells in glucose regulation and glycogen storage. However there is less appreciation of the roles of GLUTs expressed by non parenchymal cell types within the liver, all of which require carbohydrate to function. A better understanding of the detailed cellular distribution of GLUTs in human liver tissue may shed light on mechanisms underlying disease pathogenesis. This review summarises the available literature on hepatocellular expression of GLUTs in health and disease and highlights areas where further investigation is required.

Fructose might contribute to the hypoglycemic effect of honey

Honey is a natural substance with many medicinal properties, including antibacterial, hepatoprotective, hypoglycemic, antioxidant and antihypertensive effects. It reduces hyperglycemia in diabetic rats and humans. However, the mechanism(s) of its hypoglycemic effect remain(s) unknown. Honey comprises many constituents, making it difficult to ascertain which component(s) contribute(s) to its hypoglycemic effect. Nevertheless, available evidence indicates that honey consists of predominantly fructose and glucose. The objective of this review is to summarize findings which indicate that fructose exerts a hypoglycemic effect. The data show that glucose and fructose exert a synergistic effect in the gastrointestinal tract and pancreas. This synergistic effect might enhance intestinal fructose absorption and/or stimulate insulin secretion. The results indicate that fructose enhances hepatic glucose uptake and glycogen synthesis and storage via activation of hepatic glucokinase and glycogen synthase, respectively. The data also demonstrate the beneficial effects of fructose on glycemic control, glucose- and appetite-regulating hormones, body weight, food intake, oxidation of carbohydrate and energy expenditure. In view of the similarities of these effects of fructose with those of honey, the evidence may support the role of fructose in honey in mediating the hypoglycemic effect of honey.

Slc2a5 (Glut5) is essential for the absorption of fructose in the intestine and generation of fructose-induced hypertension

The identity of the transporter responsible for fructose absorption in the intestine in vivo and its potential role in fructose-induced hypertension remain speculative. Here we demonstrate that Glut5 (Slc2a5) deletion reduced fructose absorption by approximately 75% in the jejunum and decreased the concentration of serum fructose by approximately 90% relative to wild-type mice on increased dietary fructose. When fed a control (60% starch) diet, Glut5(-/-) mice had normal blood pressure and displayed normal weight gain. However, whereas Glut5(+/+) mice showed enhanced salt absorption in their jejuna in response to luminal fructose and developed systemic hypertension when fed a high fructose (60% fructose) diet for 14 weeks, Glut5(-/-) mice did not display fructose-stimulated salt absorption in their jejuna, and they experienced a significant impairment of nutrient absorption in their intestine with accompanying hypotension as early as 3-5 days after the start of a high fructose diet. Examination of the intestinal tract of Glut5(-/-) mice fed a high fructose diet revealed massive dilatation of the caecum and colon, consistent with severe malabsorption, along with a unique adaptive up-regulation of ion transporters. In contrast to the malabsorption of fructose, Glut5(-/-) mice did not exhibit an absorption defect when fed a high glucose (60% glucose) diet. We conclude that Glut5 is essential for the absorption of fructose in the intestine and plays a fundamental role in the generation of fructose-induced hypertension. Deletion of Glut5 results in a serious nutrient-absorptive defect and volume depletion only when the animals are fed a high fructose diet and is associated with compensatory adaptive up-regulation of ion-absorbing transporters in the colon.