Chlorogenic acid analogue S 3483: a potent competitive inhibitor of the hepatic and renal glucose-6-phosphatase systems

Elucidation of intermolecular interactions between chlorogenic acid and glucose-6-phosphate translocase: A step towards chemically induced glycogen storage disease type 1b model

Glucose-6-phosphate translocase enzyme, encoded by SLC37A4 gene, is a crucial enzyme involved in transporting glucose-6-phosphate into the endoplasmic reticulum. Inhibition of this enzyme can cause Von-Gierke's/glycogen storage disease sub-type 1b. The current study dealt to elucidate the intermolecular interactions to assess the inhibitory activity of Chlorogenic acid (CGA) against SLC37A4 was assessed by molecular docking and dynamic simulation. The alpha folded model of SLC37A4 and CGA 3D structure were optimized using CHARMM force field, using energy minimization protocol in the Discovery Studio software. Glucose-6-phosphate (G6P) and CGA molecular docking, Molecular dynamics (MD) simulation, analysis of binding free energy of G6P-SLC37A4 and CGA-SLC37A4 complexes was performed for 100 ns using GROMACS, followed by principal component analysis (PCA). The docking score of the CGA-SLC37A4 complex exhibited a higher docking score (- 8.2 kcal/mol) when compared to the G6P-SLC37A4 complex (- 6.5 kcal/mol), suggesting a stronger binding interaction between CGA and SLC37A4. Further, the MD simulation demonstrated a stable backbone and complex Root Mean Square Deviation (RMSD), the least RMS fluctuation, and stable active site residue interactions throughout the 100 ns production run. The CGA complex with SLC37A4 exhibits higher compactness and formed 8 hydrogen bonds to achieve stability. The binding free energy of the G6P-SLC37A4 and CGA-SLC37A4 complex was found to be - 12.73 and - 31.493 kcal/mol. Lys29 formed stable contact for both G6P (- 4.73 kJ/mol) and SLC37A4 (- 2.18 kJ/mol). This study imparts structural insights into the competitive inhibition of SLC37A4 by CGA. CGA shows potential as a candidate to induce manifestations of GSD1b by inhibiting glycogenolysis, and gluconeogenesis.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03661-5.

Effects of Dietary Ramie Powder at Various Levels on the Production Performance, Serum Biochemical Indices, Antioxidative Capacity, and Intestinal Development of Laying Hens

The purpose of this study was to investigate the effects of ramie (0, 3, 6, and 9%) included in diets on production performance, antioxidative capacity, serum biochemical indices, and intestinal development of laying hens. A total of 432 Lohmann commercial laying hens were randomly allotted to one of four dietary treatments and fed for 6 weeks. The results showed that the inclusion of ramie had no negative effects on laying performance, and increased (quadratic, P < 0.05) the laying rate with the highest value in the 6% ramie group. However, ramie content in the diet up to 9% reduced the apparent metabolic energy, dry matter, and organic matter apparent digestibility of laying hens compared with those in the 3% ramie group. The content of high-density lipoprotein (HDL-C) in serum was increased (P < 0.05), but the activity of aspartate aminotransferase (AST) was decreased (P < 0.05) by dietary ramie supplementation. As the dietary ramie level increased, the activity of serum glutathione peroxidase (GSH-Px) was increased quadratically (P < 0.05). Compared with control, 3% ramie group significantly increased (P < 0.01) liver total superoxide dismutase (SOD) activity. Meanwhile, the addition of 3∼6% ramie powder increased (P < 0.05) villus height of jejunum and villus height/crypt depth (V/C) of ileum, which reflected the intestinal promotional effect of ramie powder. In conclusion, ramie in a diet of less than 9% might protect the liver and improve the antioxidative capacity with no detrimental impacts on the laying hens. Moreover, it could promote the intestinal mucosal structure and have a positive impact on the intestine health of the laying hens.

In Vitro Tests for a Rapid Evaluation of Antidiabetic Potential of Plant Species Containing Caffeic Acid Derivatives: A Validation by Two Well-Known Antidiabetic Plants, Ocimum gratissimum L. Leaf and Musanga cecropioides R. Br. ex Tedlie (Mu) Stem Bark

Plant bioactive extracts represent a major resource for identifying drugs and adjuvant therapy for type 2 diabetes. To promote early screening of plants' antidiabetic potential, we designed a four in vitro tests strategy to anticipate in vivo bioactivity. Two antidiabetic plants were studied: Ocimum gratissimum L. (Oc) leaf extract and Musanga cecropoides R. Br. ex Tedlie (Mu) stem bark extract. Chemical compositions were analyzed by LCMS and HPLC. Antidiabetic properties were measured based on (1) INS-1 cells for insulin secretion, (2) L6 myoblast cells for insulin sensitization (Glut-4 translocation), (3) L6 myoblast cells for protection against hydrogen peroxide (H2O2) oxidative stress (cell mortality), and (4) liver microsomial fraction for glucose-6-phosphastase activity (G6P). Oc extract increased insulin secretion and insulin sensitivity, whereas it decreased oxidative stress-induced cell mortality and G6P activity. Mu extract decreased insulin secretion and had no effect on insulin sensitivity or G6P activity, but it increased oxidative stress-induced cell mortality. Results were compared with NCRAE, an antidiabetic plant extract used as reference, previously characterized and reported with increased insulin secretion and insulin sensitivity, protection against oxidative stress, and decreased G6P activity. The proposed set of four in vitro tests combined with chemical analysis provided insight into the interest in rapid early screening of plant extract antidiabetic potential to anticipate pharmaco-toxicological in vivo effects.

Inter- and Transgenerational Effects of Paternal Exposure to Inorganic Arsenic

The rise of metabolic disorders in modern times is mainly attributed to the environment. However, heritable effects of environmental chemicals on mammalian offsprings' metabolic health are unclear. Inorganic arsenic (iAs) is the top chemical on the Agency for Toxic Substances and Disease Registry priority list of hazardous substances. Here, we assess cross-generational effects of iAs in an exclusive male-lineage transmission paradigm. The exposure of male mice to 250 ppb iAs causes glucose intolerance and hepatic insulin resistance in F1 females, but not males, without affecting body weight. Hepatic expression of glucose metabolic genes, glucose output, and insulin signaling are disrupted in F1 females. Inhibition of the glucose 6-phosphatase complex masks the intergenerational effect of iAs, demonstrating a causative role of hepatic glucose production. F2 offspring from grandpaternal iAs exposure show temporary growth retardation at an early age, which diminishes in adults. However, reduced adiposity persists into middle age and is associated with altered gut microbiome and increased brown adipose thermogenesis. In contrast, F3 offspring of the male-lineage iAs exposure show increased adiposity, especially on a high-calorie diet. These findings have unveiled sex- and generation-specific heritable effects of iAs on metabolic physiology, which has broad implications in understanding gene-environment interactions.

The Metabolic Effects of Cynara Supplementation in Overweight and Obese Class I Subjects with Newly Detected Impaired Fasting Glycemia: A Double-Blind, Placebo-Controlled, Randomized Clinical Trial

Impaired fasting glucose (IFG) is a condition that precedes diabetes and increases the risk of developing it. Studies support the hypoglycemic effect of Cynarascolymus (Cs) extracts due to the content of chlorogenic acid, which is a potent inhibitor of glucose 6-phosphate translocase and of dicaffeoylquinic acid derivatives that modulate the activity of alpha-glucosidase. Given this background, we investigated whether a new highly standardized Cs extract could improve glycemic control, insulin sensitivity and other metabolic parameters (total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-C), high-density lipoprotein-cholesterol (HDL-C) Triglycerides, Apolipo protein B (ApoB), Apolipo protein A (ApoA), waist circumference, visceral adipose tissue (VAT) by dual-energy X-ray absorptiometry (DXA) in overweight subjects with newly diagnosed IFG. Fifty-four subjects (females/males 26/28, mean ± SD age 51.5 ± 6.2) were randomly assigned to the supplemented group (n = 27) and placebo (n = 27). After multiple testing correction, statistically significant interactions between time and group were observed for the primary endpoint glycemia (β = 0.36, p < 0.0001) and for the secondary endpoints HDL (β = −0.10, p < 0.0001), total cholesterol/HDL (β = 0.27, p < 0.0001), LDL (β = 0.15, p = 0.005), LDL/HDL (β = 0.23, p = 0.001), insulin (β = 1.28, p = 0.04), glycated hemoglobin (β = 0.21, p = 0.0002), A1c-derived average glucose (β = 0.34, p = 0.0002), ApoB (β = 6.00, p = 0.01), ApoA (β = −4.50, p = 0.04), ApoB/ApoA (β = 0.08, p = 0.003), waist circumference (β = 1.89, p = 0.05), VATβ = 222.37, p = 0.005). In conclusion, these results confirm that Cs supplementation has a significant effect on metabolic parameters in IFG patients.

Synthesis, characterization, and pharmacological evaluation of thiourea derivatives

Thioureas and their derivatives are organosulfur compounds having applications in numerous fields such as organic synthesis and pharmaceutical industries. Symmetric thiourea derivatives were synthesized by the reaction of various anilines with CS2. The synthesized compounds were characterized using the UV-visible and nuclear magnetic resonance (NMR) spectroscopic techniques. The compounds were screened forin vitroinhibition of α-amylase, α-glucosidase, acetylcholinesterase (AChE), and butyrylcholinesterase (BuChE) enzymes and for their antibacterial and antioxidant potentials. These compounds were fed to Swiss male albino mice to evaluate their toxicological effects and potential to inhibit glucose-6-phosphatase (G6Pase) inhibition. The antibacterial studies revealed that compound4was more active against the selected bacterial strains. Compound1was more active against 2,2-diphenyl-1-picrylhydrazyl and 2,2’-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) free radicals, AChE, BuChE, and α-glucosidase. Compound2was more potent against α-amylase and G6Pase. Toxicity studies showed that compound4is safe as it exerted no toxic effect on any of the hematological and biochemical parameters or on liver histology of the experimental animals at any studied dose rate. The synthesized compounds showed promising antibacterial and antioxidant potential and were very active (bothin vitroandin vivo) against G6Pase and moderately active against the other selected enzymes used in this study.

Failure to eliminate a phosphorylated glucose analog leads to neutropenia in patients with G6PT and G6PC3 deficiency

Neutropenia presents an important clinical problem in patients with G6PC3 or G6PT deficiency, yet why neutropenia occurs is unclear. We discovered that G6PC3 and G6PT collaborate to dephosphorylate a noncanonical metabolite (1,5-anhydroglucitol-6-phosphate; 1,5AG6P) which is produced when glucose-phosphorylating enzymes erroneously act on 1,5-anhydroglucitol, a food-derived polyol present in blood. In patients or mice with G6PC3 or G6PT deficiency, 1,5AG6P accumulates and inhibits the first step of glycolysis. This is particularly detrimental in neutrophils, since their energy metabolism depends almost entirely on glycolysis. Consistent with our findings, we observed that treatment with a 1,5-anhydroglucitol-lowering drug treats neutropenia in G6PC3-deficient mice. Our findings highlight that the elimination of noncanonical side products by metabolite-repair enzymes makes an important contribution to mammalian physiology.

Neutropenia represents an important problem in patients with genetic deficiency in either the glucose-6-phosphate transporter of the endoplasmic reticulum (G6PT/SLC37A4) or G6PC3, an endoplasmic reticulum phosphatase homologous to glucose-6-phosphatase. While affected granulocytes show reduced glucose utilization, the underlying mechanism is unknown and causal therapies are lacking. Using a combination of enzymological, cell-culture, and in vivo approaches, we demonstrate that G6PT and G6PC3 collaborate to destroy 1,5-anhydroglucitol-6-phosphate (1,5AG6P), a close structural analog of glucose-6-phosphate and an inhibitor of low-K_M hexokinases, which catalyze the first step in glycolysis in most tissues. We show that 1,5AG6P is made by phosphorylation of 1,5-anhydroglucitol, a compound normally present in human plasma, by side activities of ADP-glucokinase and low-K_M hexokinases. Granulocytes from patients deficient in G6PC3 or G6PT accumulate 1,5AG6P to concentrations (∼3 mM) that strongly inhibit hexokinase activity. In a model of G6PC3-deficient mouse neutrophils, physiological concentrations of 1,5-anhydroglucitol caused massive accumulation of 1,5AG6P, a decrease in glucose utilization, and cell death. Treating G6PC3-deficient mice with an inhibitor of the kidney glucose transporter SGLT2 to lower their blood level of 1,5-anhydroglucitol restored a normal neutrophil count, while administration of 1,5-anhydroglucitol had the opposite effect. In conclusion, we show that the neutropenia in patients with G6PC3 or G6PT mutations is a metabolite-repair deficiency, caused by a failure to eliminate the nonclassical metabolite 1,5AG6P.

The Physiopathological Role of the Exchangers Belonging to the SLC37 Family

The human SLC37 gene family includes four proteins SLC37A1-4, localized in the endoplasmic reticulum (ER) membrane. They have been grouped into the SLC37 family due to their sequence homology to the bacterial organophosphate/phosphate (Pi) antiporter. SLC37A1-3 are the less characterized isoforms. SLC37A1 and SLC37A2 are Pi-linked glucose-6-phosphate (G6P) antiporters, catalyzing both homologous (Pi/Pi) and heterologous (G6P/Pi) exchanges, whereas SLC37A3 transport properties remain to be clarified. Furthermore, SLC37A1 is highly homologous to the bacterial glycerol 3-phosphate permeases, so it is supposed to transport also glycerol-3-phosphate. The physiological role of SLC37A1-3 is yet to be further investigated. SLC37A1 seems to be required for lipid biosynthesis in cancer cell lines, SLC37A2 has been proposed as a vitamin D and a phospho-progesterone receptor target gene, while mutations in the SLC37A3 gene appear to be associated with congenital hyperinsulinism of infancy. SLC37A4, also known as glucose-6-phosphate translocase (G6PT), transports G6P from the cytoplasm into the ER lumen, working in complex with either glucose-6-phosphatase-α (G6Pase-α) or G6Pase-β to hydrolyze intraluminal G6P to Pi and glucose. G6PT and G6Pase-β are ubiquitously expressed, whereas G6Pase-α is specifically expressed in the liver, kidney and intestine. G6PT/G6Pase-α complex activity regulates fasting blood glucose levels, whereas G6PT/G6Pase-β is required for neutrophil functions. G6PT deficiency is responsible for glycogen storage disease type Ib (GSD-Ib), an autosomal recessive disorder associated with both defective metabolic and myeloid phenotypes. Several kinds of mutations have been identified in the SLC37A4 gene, affecting G6PT function. An increased autoimmunity risk for GSD-Ib patients has also been reported, moreover, SLC37A4 seems to be involved in autophagy.

Yacon (Smallanthus sonchifolius) Leaf Extract Attenuates Hyperglycemia and Skeletal Muscle Oxidative Stress and Inflammation in Diabetic Rats

The effects of hydroethanolic extract of Yacon leaves (HEYL) on antioxidant, glycemic, and inflammatory biomarkers were tested in diabetic rats. Outcome parameters included glucose, insulin, interleukin-6 (IL-6), and hydrophilic antioxidant capacity (HAC) in serum and IL-6, HAC, malondialdehyde (MDA), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) in soleus. The rats (10/group) were divided as follows: C, controls; C + Y, HEYL treated; DM, diabetic controls; and DM + Y, diabetic rats treated with HEYL. Diabetes mellitus was induced by administration of streptozotocin. C + Y and DM + Y groups received 100 mg/kg HEYL daily via gavage for 30 d. Hyperglycemia was improved in the DM + Y versus DM group. Insulin was reduced in DM versus C group. DM rats had higher IL-6 and MDA and lower HAC in the soleus muscle. HEYL treatment decreased IL-6 and MDA and increased HAC in DM rats. DM + Y rats had the highest CAT activity versus the other groups; GPx was higher in C + Y and DM + Y versus their respective controls. The apparent benefit of HEYL may be mediated via improving glucoregulation and ameliorating oxidative stress and inflammation, particularly in diabetic rats.

The potential beneficial effects of phenolic compounds isolated from A. pilosa Ledeb on insulin-resistant hepatic HepG2 cells

Agrimonia pilosaLedeb (AP) has already been applied in practice for the treatment of different disorders and is available to access without the provision of a medical prescription.

Acute effects of light and dark roasted coffee on glucose tolerance: a randomized, controlled crossover trial in healthy volunteers

PURPOSE

Epidemiological evidence suggests that coffee consumption is associated with a lower risk of type 2 diabetes. Coffee contains caffeine and several other components that may modulate glucose regulation. The chlorogenic acids (CGA) in coffee have been indicated as constituents that may help to normalize the acute glucose response after a carbohydrate challenge. The aim of this study was to investigate whether two coffee beverages that differ in CGA content due to different roasting degrees will differentially affect glucose regulation.

METHODS

In a controlled crossover trial, 11 healthy fasted volunteers consumed 300 mL of either light (LIR) or dark (DAR) roasted coffee, or water, followed 30 min later by a 75-g oral glucose tolerance test (OGTT). Blood samples were drawn at baseline, 30, 60, and 120 min. Differences in glucose and insulin responses and insulin sensitivity index (ISI) were analyzed. The CGA and caffeine contents in the coffees were analyzed using UPLC-MS/MS.

RESULTS

No differences in glucose area under the curve (AUC) were found between treatments. Glucose concentrations were higher at 60 min after ingestion of DAR compared with water, while ingestion of LIR showed similar glucose concentrations as ingestion of water. Insulin AUC was higher after ingestion of DAR compared with water, and both coffees raised insulin concentrations and reduced ISI compared with water, with no difference between the two coffees.

CONCLUSION

Two coffees with different CGA contents did not differentially affect glucose or insulin responses during an OGTT, but both increased the insulin response compared with water.

The SLC37 family of sugar-phosphate/phosphate exchangers

The SLC37 family members are endoplasmic reticulum (ER)-associated sugar-phosphate/phosphate (P(i)) exchangers. Three of the four members, SLC37A1, SLC37A2, and SLC37A4, function as Pi-linked glucose-6-phosphate (G6P) antiporters catalyzing G6P:P(i) and P(i):P(i) exchanges. The activity of SLC37A3 is unknown. SLC37A4, better known as the G6P transporter (G6PT), has been extensively characterized, functionally and structurally, and is the best characterized family member. G6PT contains 10 transmembrane helices with both N and C termini facing the cytoplasm. The primary in vivo function of the G6PT protein is to translocate G6P from the cytoplasm into the ER lumen where it couples with either the liver/kidney/intestine-restricted glucose-6-phosphatase-α (G6Pase-α or G6PC) or the ubiquitously expressed G6Pase-β (or G6PC3) to hydrolyze G6P to glucose and P(i). The G6PT/G6Pase-α complex maintains interprandial glucose homeostasis, and the G6PT/G6Pase-β complex maintains neutrophil energy homeostasis and functionality. G6PT is highly selective for G6P and is competitively inhibited by cholorogenic acid and its derivatives. Neither SLC37A1 nor SLC37A2 can couple functionally with G6Pase-α or G6Pase-β, and the antiporter activities of SLC37A1 or SLC37A2 are not inhibited by cholorogenic acid. Deficiencies in G6PT cause glycogen storage disease type Ib (GSD-Ib), a metabolic and immune disorder. To date, 91 separate SLC37A4 mutations, including 39 missense mutations, have been identified in GSD-Ib patients. Characterization of missense mutations has yielded valuable information on functionally important residues in the G6PT protein. The biological roles of the other SLC37 proteins remain to be determined and deficiencies have not yet been correlated to diseases.

Antihyperglycemic effect of a natural chicoric acid extract of chicory (Cichorium intybus L.): a comparative in vitro study with the effects of caffeic and ferulic acids

ETHNOPHARMACOLOGICAL RELEVANCE

In Eurasia folk medicine, roots of chicory (Cichorium intybus L.) have been reported to exert antidiabetic benefits. In vitro, a natural chicoric acid extract (NCRAE) from Cichorium intybus root has been shown to increase insulin secretion by pancreatic β-cells and glucose uptake by muscle cells.

MATERIALS AND METHODS

In vitro experiments were designed to compare the effects of two hydroxycinnamic acids, caffeic and ferulic acids, to those obtained with NCRAE (50 and 100 µg.mL(-1)) on the three major tissues implicated in glycemic regulation (pancreas, muscle and liver). In vivo experiments were performed in Wistar rats submitted to a daily intraperitoneal injection of NCRAE (3, 15 or 30 mg kg(-1)) for 4 days. On the fourth day, an intraperitoneal glucose tolerance test (IPGTT; 1 g kg(-1)) was carried out.

RESULTS

Our results show that the three compounds we used are able each to induce an original response. Caffeic acid mainly promotes a decrease in hepatic glycogenolysis. Ferulic acid elicits a clear increase of insulin release and a reduction of hepatic glycogenolysis. However, this compound induces an inhibition of muscle glucose uptake. NCRAE provokes an increase of insulin release and glucose uptake without any effect on hepatic glycogenolysis. We could also show that none of these compounds implicates hepatic glucose 6-phosphatase in contrast to chlorogenic acid, known as an inhibitor of glucose 6-phosphatase and which is able to decrease glucose output from hepatocytes. Our results point out that NCRAE is able to decrease blood glucose without any effect hepatic effect. Our in vivo experiments bring evidence that 4 daily IP administrations of NCRAE improve IP glucose tolerance in a dose-dependent manner and mainly via an insulin sensitizing effect.

CONCLUSIONS

We conclude that NCRAE presents an antihyperglycemic effect essentially due to a peripheral effect on muscle glucose uptake.

Phosphoenolpyruvate carboxykinase and glucose-6-phosphatase are required for steroidogenesis in testicular Leydig cells

Cyclic AMP (cAMP) induces steroidogenic enzyme gene expression and stimulates testosterone production in Leydig cells. Phosphoenolpyruvate carboxykinase (PEPCK) is expressed in Leydig cells, but its role has not been defined. In this study, we found that PEPCK and glucose-6-phosphatase (Glc-6-Pase) are increased significantly following cAMP treatment of mouse Leydig cells. Moreover, cAMP treatment increased recruitment of the cAMP-response element-binding transcription factor and decreased recruitment of the corepressor DAX-1 on the pepck promoter. Furthermore, cAMP induced an increase in ATP that correlated with a decrease in phospho-AMP-activated protein kinase (AMPK). In contrast, knockdown or inhibition of PEPCK decreased ATP and increased phospho-AMPK. Treatment with an AMPK activator or overexpression of the constitutively active form of AMPK inhibited cAMP-induced steroidogenic enzyme promoter activities and gene expression. Liver receptor homolog-1 (LRH-1) was involved in cAMP-induced steroidogenic enzyme gene expression but was inhibited by AMPK activation in Leydig cells. Additionally, inhibition or knockdown of PEPCK and Glc-6-Pase decreased cAMP-mediated induction of steroidogenic enzyme gene expression and steroidogenesis. Finally, pubertal mouse (8-week-old) testes and human chorionic gonadotropin-induced prepubertal mouse testes showed increased PEPCK and Glc-6-Pase gene expression. Taken together, these results suggest that induction of PEPCK and Glc-6-Pase by cAMP plays an important role in Leydig cell steroidogenesis.

The glucose-6-phosphate transport is not mediated by a glucose-6-phosphate/phosphate exchange in liver microsomes

A phosphate-linked antiporter activity of the glucose-6-phosphate transporter (G6PT) has been recently described in liposomes including the reconstituded transporter protein. We directly investigated the mechanism of glucose-6-phosphate (G6P) transport in rat liver microsomal vesicles. Pre-loading with inorganic phosphate (Pi) did not stimulate G6P or Pi microsomal inward transport. Pi efflux from pre-loaded microsomes could not be enhanced by G6P or Pi addition. Rapid G6P or Pi influx was registered by light-scattering in microsomes not containing G6P or Pi. The G6PT inhibitor, S3483, blocked G6P transport irrespectively of experimental conditions. We conclude that hepatic G6PT functions as an uniporter.

Protective effect of yacon leaves decoction against early nephropathy in experimental diabetic rats

Nephropathy is the most common cause of morbidity and mortality in diabetic patients. Prevention of this complication has a major relevance. Smallanthus sonchifolius (yacon) leaves have been shown to ameliorate hyperglycemia in streptozotocin-induced diabetic rats. We examined the beneficial effects of yacon leaves decoction on diabetic nephropathy and explored the possible underlying action mechanism. Streptozotocin-diabetic rats were orally administered 10% yacon leaves water decoction (70mg dry extract/kg body weight) once a day for 4weeks. Biochemical parameters in blood and urine were analyzed and immunohistochemistry staining, western immunoblotting and qRT-PCR were assessed. Yacon decoction significantly decreased high blood glucose level in diabetic rats and improved insulin production. Diabetic-dependent alterations in urinary albumin excretion, creatinine clearance, kidney hypertrophy and basement membrane thickening were attenuated by yacon decoction. These findings were associated with a marked decrease in TGF-β1/Smad2/3 signaling. The expression of molecular markers of diabetic nephropathy such as collagen IV, laminin-1, fibronectin and collagen III were also diminished in the yacon-treated group compared to control diabetic group. These results suggest that yacon leaves decoction is a protective agent against renal damage in diabetic nephropathy, whose action can be mediated by TGF-β/Smads signals.

Contribution of fructose-6-phosphate to glucocorticoid activation in the endoplasmic reticulum: possible implication in the metabolic syndrome

Both fructose consumption and increased intracellular glucocorticoid activation have been implicated in the pathogenesis of the metabolic syndrome. Glucocorticoid activation by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) depends on hexose-6-phosphate dehydrogenase (H6PD), which physically interacts with 11β-HSD1 at the luminal surface of the endoplasmic reticulum (ER) membrane and generates reduced nicotinamide adenine dinucleotide phosphate for the reduction of glucocorticoids. The reducing equivalents for the reaction are provided by glucose-6-phosphate (G6P) that is transported by G6P translocase into the ER. Here, we show that fructose-6-phosphate (F6P) can substitute for G6P and is sufficient to maintain reductase activity of 11β-HSD1 in isolated microsomes. Our findings indicate that the mechanisms of F6P and G6P transport across the ER membrane are distinct and provide evidence that F6P is converted to G6P in the ER lumen, thus yielding substrate for H6PD-dependent reduced nicotinamide adenine dinucleotide phosphate generation. Using the purified enzyme, we show that F6P cannot be directly dehydrogenated by H6PD, and we also excluded H6PD as a phosphohexose isomerase. Therefore, we postulate the existence of an ER luminal hexose-phosphate isomerase different from the cytosolic enzyme. The results suggest that cytosolic F6P promotes prereceptor glucocorticoid activation in white adipose tissue, which might have a role in the pathophysiology of the metabolic syndrome.

Contribution of chlorogenic acids to the inhibition of human hepatic glucose-6-phosphatase activity in vitro by Svetol, a standardized decaffeinated green coffee extract

Glucose-6-phosphatase (Glc-6-Pase) is a multicomponent system that exists primarily in the liver and catalyzes the terminal step in gluconeogenesis and glycogenolysis. Several studies have attempted to identify synthetic or natural compounds that inhibit this enzyme complex for therapeutic use in regulating blood glucose and type 2 diabetes. For this paper an in vitro structure-activity relationship study of several natural chlorogenic acids was conducted, and the active components of the natural decaffeinated green coffee extract Svetol were identified. Glucose-6-phosphate (Glc-6-P) hydrolysis was measured in the presence of Svetol or chlorogenic acids in intact human liver microsomes. Svetol significantly inhibited Glc-6-P hydrolysis in intact human liver microsomes in a competitive manner, and it was determined that chlorogenic acids (caffeoylquinic acids and dicaffeoylquinic acids) were the chief compounds mediating this activity. In addition, the structure-activity analysis showed that variation in the position of the caffeoyl residue is an important determinant of inhibition of Glc-6-P hydrolysis. This inhibition by Svetol contributes to its antidiabetic, glucose-lowering effects by reducing hepatic glucose production.

Chicoric acid, a new compound able to enhance insulin release and glucose uptake

Caffeic acid and chlorogenic acid (CGA), a mono-caffeoyl ester, have been described as potential antidiabetic agents. Using in vitro studies, we report the effects of a dicaffeoyl ester, chicoric acid (CRA) purified from Cichorium intybus, on glucose uptake and insulin secretion. Our results show that CRA and CGA increased glucose uptake in L6 muscular cells, an effect only observed in the presence of stimulating concentrations of insulin. Moreover, we found that both CRA and CGA were able to stimulate insulin secretion from the INS-1E insulin-secreting cell line and rat islets of Langerhans. In the later case, the effect of CRA is only observed in the presence of subnormal glucose levels. Patch clamps studies show that the mechanism of CRA and CGA was different from that of sulfonylureas, as they did not close K(ATP) channels. Chicoric acid is a new potential antidiabetic agent carrying both insulin sensitizing and insulin-secreting properties.

Yerba Maté (Ilex paraguariensis) aqueous extract decreases intestinal SGLT1 gene expression but does not affect other biochemical parameters in alloxan-diabetic Wistar rats

Yerba maté (Ilex paraguariensis) is rich in polyphenols, especially chlorogenic acids. Evidence suggests that dietary polyphenols could play a role in glucose absorption and metabolism. The aim of this study was to evaluate the antidiabetic properties of yerba maté extract in alloxan-induced diabetic Wistar rats. Animals (n = 41) were divided in four groups: nondiabetic control (NDC, n = 10), nondiabetic yerba maté (NDY, n = 10), diabetic control (DC, n = 11), and diabetic yerba maté (DY, n = 10). The intervention consisted in the administration of yerba maté extract in a 1 g extract/kg body weight dose for 28 days; controls received saline solution only. There were no significant differences in serum glucose, insulin, and hepatic glucose-6-phosphatase activity between the groups that ingested yerba maté extract (NDY and DY) and the controls (NDC and DC). However, the intestinal SGLT1 gene expression was significantly lower in animals that received yerba maté both in upper (p = 0.007) and middle (p < 0.001) small intestine. These results indicate that bioactive compounds present in yerba maté might be capable of interfering in glucose absorption, by decreasing SGLT1 expression.

Induction of glucokinase mRNA by dietary phenolic compounds in rat liver cells in vitro

Diabetes and its complications, including oxidative stress, are major reasons for medical intervention and one of the most frequent causes of death in developed countries. Several lines of data suggest that the use of certain dietary polyphenolic compounds may alter glucose metabolism, thus decreasing the risk for type 2 diabetes. In this paper, we present the effect of phenolic acids (caffeic, chlorogenic, rosmarinic, and ferulic) and extracts from Smallanthus sonchifolius and Prunella vulgaris on glucose production in rat hepatocytes and on glucokinase, glucose-6-phosphatase, and phosphoenol-pyruvate carboxykinase mRNA expression in rat hepatoma Fao cells. The phenolics at 500 microM and after 1 h incubation lowered glucose production via both gluconeogenesis (10 mM alanine or dihydroxyacetone as precursors) and glycogenolysis compared with metformin. Most of the phenolics increased the level of glucokinase mRNA after 24 h in the same way as insulin (10(-7) M).

Structure-activity relationships of semisynthetic mumbaistatin analogs

Mumbaistatin (1), a new anthraquinone natural product, is one of the most potent known inhibitors of hepatic glucose-6-phosphate translocase, an important target for the treatment of type II diabetes. Its availability, however, has been limited due to its extremely low yield from the natural source. Starting from DMAC (5, 3,8-dihydroxyanthraquinone-2-carboxylic acid), a structurally related polyketide product of engineered biosynthesis, we developed a facile semisynthetic method that afforded a variety of mumbaistatin analogs within five steps. This work was facilitated by the initial development of a DMAC overproduction system. In addition to reinforcing the biological significance of the anthraquinone moiety of mumbaistatin, several semisynthetic analogs were found to have low micromolar potency against the translocase in vitro. Two of them were also active in glucose release assays from primary hepatocytes. The synergistic combination of biosynthesis and synthesis is a promising avenue for the discovery of new bioactive substances.

Specific reduction of hepatic glucose 6-phosphate transporter-1 ameliorates diabetes while avoiding complications of glycogen storage disease

D-Glucose-6-phosphatase is a key regulator of endogenous glucose production, and its inhibition may improve glucose control in type 2 diabetes. Herein, 2'-O-(2-methoxy)ethyl-modified phosphorothioate antisense oligonucleotides (ASOs) specific to the glucose 6-phosphate transporter-1 (G6PT1) enabled reduction of hepatic D-Glu-6-phosphatase activity in diabetic ob/ob mice. Treatment with G6PT1 ASOs decreased G6PT1 expression, reduced G6PT1 activity, blunted glucagon-stimulated glucose production, and lowered plasma glucose concentration in a dose-dependent manner. In contrast to G6PT1 knock-out mice and patients with glycogen storage disease, excess hepatic and renal glycogen accumulation, hyperlipidemia, neutropenia, and elevations in plasma lactate and uric acid did not occur. In addition, hypoglycemia was not observed in animals during extended periods of fasting, and the ability of G6PT1 ASO-treated mice to recover from an exogenous insulin challenge was not impaired. Together, these results demonstrate that effective glucose lowering by G6PT1 inhibitors can be achieved without adversely affecting carbohydrate and lipid metabolism.

Exaggeration of Postprandial Hyperglycemia in Patients with type 2 Diabetes by Administration of Caffeine in Coffee

OBJECTIVE

To test whether caffeine administered in coffee increases postprandial hyperglycemia in patients with type 2 diabetes who are habitual coffee drinkers.

METHODS

The study used a within-subject, double-blind, placebo-controlled experimental design. Twenty adult coffee drinkers (11 women and 9 men) with type 2 diabetes treated with diet, exercise, orally administered antidiabetic agents, or some combination of these factors completed two mixed-meal tolerance tests (MMTT) after an overnight fast. Before the MMTT, each study participant received 250 mg of caffeine in 16 oz (475 mL) of decaffeinated coffee or decaffeinated coffee alone, with the treatment order counterbalanced in the group. Fasting and 1-hour and 2-hour postprandial blood samples were collected for measurement of plasma glucose and insulin concentrations.

RESULTS

Glucose and insulin responses to the MMTT were quantified by the incremental areas under the 2-hour concentration-time curves (AUC2h). Administration of caffeine in decaffeinated coffee increased postprandial glucose and insulin responses (both P = 0.02). The mean plasma glucose AUC2h was 28% larger and the mean plasma insulin AUC2h was 19% larger after administration of caffeine than after administration of placebo.

CONCLUSION

Other constituents in coffee did not prevent the exaggeration of postprandial hyperglycemia by caffeine in these patients with type 2 diabetes, who were habitual coffee drinkers. Repeated on a daily basis, such effects could impair long-term glucose control in those patients with type 2 diabetes who habitually drink coffee or other caffeinated beverages.

Transport and transporters in the endoplasmic reticulum

Enzyme activities localized in the luminal compartment of the endoplasmic reticulum are integrated into the cellular metabolism by transmembrane fluxes of their substrates, products and/or cofactors. Most compounds involved are bulky, polar or even charged; hence, they cannot be expected to diffuse through lipid bilayers. Accordingly, transport processes investigated so far have been found protein-mediated. The selective and often rate-limiting transport processes greatly influence the activity, kinetic features and substrate specificity of the corresponding luminal enzymes. Therefore, the phenomenological characterization of endoplasmic reticulum transport contributes largely to the understanding of the metabolic functions of this organelle. Attempts to identify the transporter proteins have only been successful in a few cases, but recent development in molecular biology promises a better progress in this field.

Inhibition of fructose 1,6-bisphosphatase reduces excessive endogenous glucose production and attenuates hyperglycemia in Zucker diabetic fatty rats

Gluconeogenesis is increased in type 2 diabetes and contributes significantly to fasting and postprandial hyperglycemia. We recently reported the discovery of the first potent and selective inhibitors of fructose 1,6-bisphosphatase (FBPase), a rate-controlling enzyme of gluconeogenesis. Herein we describe acute and chronic effects of the lead inhibitor, MB06322 (CS-917), in rodent models of type 2 diabetes. In fasting male ZDF rats with overt diabetes, a single dose of MB06322 inhibited gluconeogenesis by 70% and overall endogenous glucose production by 46%, leading to a reduction in blood glucose of >200 mg/dl. Chronic treatment of freely feeding 6-week-old male Zucker diabetic fatty (ZDF) rats delayed the development of hyperglycemia and preserved pancreatic function. Elevation of lactate ( approximately 1.5-fold) occurred after 4 weeks of treatment, as did the apparent shunting of precursors into triglycerides. Profound glucose lowering ( approximately 44%) and similar metabolic ramifications were associated with 2-week intervention therapy of 10-week-old male ZDF rats. In high-fat diet-fed female ZDF rats, MB06322 treatment for 2 weeks fully attenuated hyperglycemia without evidence of metabolic perturbation other than a modest reduction in glycogen stores ( approximately 20%). The studies confirm that excessive gluconeogenesis plays an integral role in the pathophysiology of type 2 diabetes and suggest that FBPase inhibitors may provide a future treatment option.

The chemopreventive properties of chlorogenic acid reveal a potential new role for the microsomal glucose-6-phosphate translocase in brain tumor progression

Background

Chlorogenic acid (CHL), the most potent functional inhibitor of the microsomal glucose-6-phosphate translocase (G6PT), is thought to possess cancer chemopreventive properties. It is not known, however, whether any G6PT functions are involved in tumorigenesis. We investigated the effects of CHL and the potential role of G6PT in regulating the invasive phenotype of brain tumor-derived glioma cells.

Results

RT-PCR was used to show that, among the adult and pediatric brain tumor-derived cells tested, U-87 glioma cells expressed the highest levels of G6PT mRNA. U-87 cells lacked the microsomal catalytic subunit glucose-6-phosphatase (G6Pase)-α but expressed G6Pase-β which, when coupled to G6PT, allows G6P hydrolysis into glucose to occur in non-glyconeogenic tissues such as brain. CHL inhibited U-87 cell migration and matrix metalloproteinase (MMP)-2 secretion, two prerequisites for tumor cell invasion. Moreover, CHL also inhibited cell migration induced by sphingosine-1-phosphate (S1P), a potent mitogen for glioblastoma multiform cells, as well as the rapid, S1P-induced extracellular signal-regulated protein kinase phosphorylation potentially mediated through intracellular calcium mobilization, suggesting that G6PT may also perform crucial functions in regulating intracellular signalling. Overexpression of the recombinant G6PT protein induced U-87 glioma cell migration that was, in turn, antagonized by CHL. MMP-2 secretion was also inhibited by the adenosine triphosphate (ATP)-depleting agents 2-deoxyglucose and 5-thioglucose, a mechanism that may inhibit ATP-mediated calcium sequestration by G6PT.

Conclusion

We illustrate a new G6PT function in glioma cells that could regulate the intracellular signalling and invasive phenotype of brain tumor cells, and that can be targeted by the anticancer properties of CHL.

Immunodetection of the expression of microsomal proteins encoded by the glucose 6-phosphate transporter gene

Glucose 6-phosphate transport has been well characterized in liver microsomes. The transport is required for the functioning of the glucose-6-phosphatase enzyme that is situated in the lumen of the hepatic endoplasmic reticulum. The genetic deficiency of the glucose 6-phosphate transport activity causes a severe metabolic disease termed type 1b glycogen storage disease. The cDNA encoding a liver transporter for glucose 6-phosphate was cloned and was found to be mutated in patients suffering from glycogen storage disease 1b. While related mRNAs have been described in liver and other tissues, the encoded protein(s) has not been immunologically characterized yet. In the present study, we report (using antibodies against three different peptides of the predicted amino acid sequence) that a major protein encoded by the glucose 6-phosphate transporter gene is expressed in the endoplasmic reticulum membranes of rat and human liver. The protein has an apparent molecular mass of approx. 33 kDa using SDS/PAGE, but several lines of evidence indicate that its real molecular mass is 46 kDa, as expected. The glucose 6-phosphate transporter protein was also immunodetected in kidney microsomes, but not in microsomes derived from human fibrocytes, rat spleen and lung, and a variety of cell lines. Moreover, little or no expression of the glucose 6-phosphate transporter protein was found in liver microsomes obtained from three glycogen storage disease 1b patients, even bearing mutations that do not directly interfere with protein translation, which can be explained by a (proteasome-mediated) degradation of the mutated transporter.

Evolutionary Chemistry Approach toward Finding Novel Inhibitors of the Type 2 Diabetes Target Glucose-6-phosphate Translocase

A genetic algorithm (GA), driven by experimentally determined biological activities as a feedback fitness function, was used to propose novel small molecules as inhibitors of glucose-6-phosphate translocase (G6PT) in iterative rounds of evolutionary optimization. A straightforward polymer-supported synthetic sequence was implemented to synthesize molecules proposed by the GA, and the biological activities of the compounds were determined by a microsomal assay. Additional compound design strategies were integrated, such as Tanimoto similarity-based selection of starting materials and transfer of favored structure elements into a new chemical scaffold to identify more active and selective inhibitors.

Aminopeptidase N (CD13) is a molecular target of the cholesterol absorption inhibitor ezetimibe in the enterocyte brush border membrane

Intestinal cholesterol absorption is an important regulator of serum cholesterol levels. Ezetimibe is a specific inhibitor of intestinal cholesterol absorption recently introduced into medical practice; its mechanism of action, however, is still unknown. Ezetimibe neither influences the release of cholesterol from mixed micelles in the gut lumen nor the transfer of cholesterol to the enterocyte brush border membrane. With membrane-impermeable Ezetimibe analogues we could demonstrate that binding of cholesterol absorption inhibitors to the brush border membrane of small intestinal enterocytes from the gut lumen is sufficient for inhibition of cholesterol absorption. A 145-kDa integral membrane protein was identified as the molecular target for cholesterol absorption inhibitors in the enterocyte brush border membrane by photoaffinity labeling with photoreactive Ezetimibe analogues (Kramer, W., Glombik, H., Petry, S., Heuer, H., Schafer, H. L., Wendler, W., Corsiero, D., Girbig, F., and Weyland, C. (2000) FEBS Lett. 487, 293-297). The 145-kDa Ezetimibe-binding protein was purified by three different methods and sequencing revealed its identity with the membrane-bound ectoenzyme aminopeptidase N ((alanyl)aminopeptidase; EC 3.4.11.2; APN; leukemia antigen CD13). The enzymatic activity of APN was not influenced by Ezetimibe (analogues). The uptake of cholesterol delivered by mixed micelles by confluent CaCo-2 cells was partially inhibited by Ezetimibe and nonabsorbable Ezetimibe analogues. Preincubation of confluent CaCo-2 cells with Ezetimibe led to a strong decrease of fluorescent APN staining with a monoclonal antibody in the plasma membrane. Independent on its enzymatic activity, aminopeptidase N is involved in endocytotic processes like the uptake of viruses. Our findings suggest that binding of Ezetimibe to APN from the lumen of the small intestine blocks endocytosis of cholesterol-rich membrane microdomains, thereby limiting intestinal cholesterol absorption.

Quinides of roasted coffee enhance insulin action in conscious rats

Consumption of large amounts of coffee has been shown to decrease the incidence of type 2 diabetes. However, the specific compounds and mechanisms responsible for this effect are not known. The aim of this study was to determine the effects of a decaffeinated coffee extract and a synthetic quinide, representative of those found in roasted coffee, 3,4-diferuloyl-1,5-quinolactone, on insulin-stimulated glucose disposal and muscle glucose uptake. Experiments were performed on conscious rats during hyperinsulinemic, euglycemic clamps receiving gastric infusions of saline, a decaffeinated coffee extract (DECAF) (220 mg/kg), or 3,4-diferuloyl-1,5-quinide (DIFEQ) (110 mg/kg). Following treatment, rats received an intravenous bolus of deoxy-[2-3H] glucose to assess muscle glucose uptake (Rg, micromol x 100 g(-1) x min(-1)). Glucose infusions [mg/(kg x min)] required to maintain euglycemia during the tracer period were higher with DIFEQ (14.6 +/- 0.7) than with saline (10.8 +/- 0.7) and DECAF (11.5 +/- 1.1). Despite increased glucose requirements, Rg in skeletal (soleus, gastrocnemius, superficial vastus lateralis) and cardiac muscle were unchanged. DECAF or DIFEQ did not affect heart rate, blood pressure, plasma nonesterified fatty acids or liver aminotransferase activity. These results demonstrate that DIFEQ increases whole-body glucose disposal independently of skeletal muscle Rg.

Intestinal cholesterol absorption: identification of different binding proteins for cholesterol and cholesterol absorption inhibitors in the enterocyte brush border membrane

Absorption of cholesterol from the intestine is a central part of body cholesterol homeostasis. The molecular mechanisms of intestinal cholesterol absorption and the proteins mediating membrane transport are not known. We therefore aimed to identify the proteins involved in intestinal cholesterol absorption across the luminal brush border membrane of small intestinal enterocytes. By photoaffinity labeling using photoreactive derivatives of cholesterol and 2-azetidinone cholesterol absorption inhibitors, an 80-kDa and a 145-kDa integral membrane protein were identified as specific binding proteins for cholesterol and cholesterol absorption inhibitors, respectively, in the brush border membrane of small intestinal enterocytes. The 80-kDa cholesterol-binding protein did not interact with cholesterol absorption inhibitors and vice versa; cholesterol or plant sterols did not interfere with the 145-kDa molecular target for cholesterol absorption inhibitors. Both proteins showed an identical tissue distribution and were exclusively found at the anatomical sites of cholesterol absorption-duodenum, jejunum and ileum. Neither stomach, cecum, colon, rectum, kidney, liver nor fat tissue expressed the 80- or 145-kDa binding proteins for cholesterol and cholesterol absorption inhibitors. Both proteins are different from the hitherto described candidate proteins for the intestinal cholesterol transporter,-SR-BI, ABC G5/ABC G8 or ABC A1. Our data strongly suggest that intestinal cholesterol absorption is not facilitated by a single transporter protein but occurs by a complex machinery. Two specific binding proteins for cholesterol (80 kDa) and cholesterol absorption inhibitors (145 kDa) of the enterocyte brush border membrane are probable protein constituents of the mechanism responsible for the intestinal absorption of cholesterol.

Inhibition of microsomal glucose-6-phosphate transport in human neutrophils results in apoptosis: a potential explanation for neutrophil dysfunction in glycogen storage disease type 1b

Mutations in the gene of the hepatic glucose-6-phosphate transporter cause glycogen storage disease type 1b. In this disease, the altered glucose homeostasis and liver functions are accompanied by an impairment of neutrophils/monocytes. However, neither the existence of a microsomal glucose-6-phosphate transport, nor the connection between its defect and cell dysfunction has been demonstrated in neutrophils/monocytes. In this study we have characterized the microsomal glucose-6-phosphate transport of human neutrophils and differentiated HL-60 cells. The transport of glucose-6-phosphate was sensitive to the chlorogenic acid derivative S3483, N-ethylmaleimide, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, known inhibitors of the hepatic microsomal glucose-6-phosphate transporter. A glucose-6-phosphate uptake was also present in microsomes from undifferentiated HL-60 and Jurkat cells, but it was insensitive to S3483. The treatment with S3484 of intact human neutrophils and differentiated HL-60 cells mimicked some leukocyte defects of glycogen storage disease type 1b patients (ie, the drug inhibited phorbol myristate acetate-induced superoxide anion production and reduced the size of endoplasmic reticulum Ca(2+) stores). Importantly, the treatment with S3484 also resulted in apoptosis of human neutrophils and differentiated HL-60 cells, while undifferentiated HL-60 and Jurkat cells were unaffected by the drug. The proapoptotic effect of S3483 was prevented by the inhibition of nicotinamide adenine dinucleotide phosphate oxidase or by antioxidant treatment. These results suggest that microsomal glucose-6-phosphate transport has a role in the antioxidant protection of neutrophils, and that the genetic defect of the transporter leads to the impairment of cellular functions and apoptosis.

Studies toward the total synthesis of mumbaistatin, a highly potent glucose-6-phosphate translocase inhibitor. Synthesis of a mumbaistatin analogue

A strategy for the total synthesis of the highly potent glucose-6-phosphate translocase inhibitor mumbaistatin (1) and structural analogues was elaborated. Such compounds represent a lead structure in the development of potential new drugs for the treatment of diabetes. To evaluate the general strategy, the close mumbaistatin analogue 10 was synthesized in a convergent manner. The anthraquinone building block 20 was efficiently prepared via aryne/phthalide annulation. After conversion of 20 into the corresponding 9,10-dimethoxyanthracene-1-carbaldehyde derivative (13), coupling with a lithiated arene (12) and subsequent multiple oxidation under Jones conditions yielded the mumbaistatin analogue 10. The preparation of the functionalized arene intermediates was achieved exploiting highly regioselective bromination and ortho-lithiation reactions.

Evidence for glucose-6-phosphate transport in rat liver microsomes

The existence of glucose-6-phosphate transport across the liver microsomal membrane is still controversial. In this paper, we show that S3483, a chlorogenic acid derivative known to inhibit glucose-6-phosphatase in intact microsomes, caused the intravesicular accumulation of glucose-6-phosphate when the latter was produced by glucose-6-phosphatase from glucose and carbamoyl-phosphate. S3483 also inhibited the conversion of glucose-6-phosphate to 6-phosphogluconate occurring inside microsomes in the presence of electron acceptors (NADP or metyrapone). These data indicate that liver microsomal membranes contain a reversible glucose-6-phosphate transporter, which furnishes substrate not only to glucose-6-phosphatase, but also to hexose-6-phosphate dehydrogenase.

Glucose release from GLUT2-null hepatocytes: characterization of a major and a minor pathway

We previously reported that glucose can be released from GLUT2-null hepatocytes through a membrane traffic-based pathway issued from the endoplasmic reticulum. Here, we further characterized this glucose release mechanism using biosynthetic labeling protocols. In continuous pulse-labeling experiments, we determined that glucose secretion proceeded linearly and with the same kinetics in control and GLUT2-null hepatocytes. In GLUT2-deficient hepatocytes, however, a fraction of newly synthesized glucose accumulated intracellularly. The linear accumulation of glucose in the medium was inhibited in mutant, but not in control, hepatocytes by progesterone and low temperature, as previously reported, but, importantly, also by microtubule disruption. The intracellular pool of glucose was shown to be present in the cytosol, and, in pulse-chase experiments, it was shown to be released at a relatively slow rate. Release was not inhibited by S-4048 (an inhibitor of glucose-6-phosphate translocase), cytochalasin B, or progesterone. It was inhibited by phloretin, carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, and low temperature. We conclude that the major release pathway segregates glucose away from the cytosol by use of a membrane traffic-based, microtubule-dependent mechanism and that the release of the cytosolic pool of newly synthesized glucose, through an as yet unidentified plasma membrane transport system, cannot account for the bulk of glucose release.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.

Prolonged blood glucose reduction in mrp-2 deficient rats (GY/TR(-)) by the glucose-6-phosphate translocase inhibitor S 3025

Chlorogenic acid derivatives are potent inhibitors of hepatic glucose production by inhibition of the glucose-6-phosphate translocase component of the hepatic glucose-6-phosphatase system. The pharmacological proof of concept was clearly demonstrated during i.v. infusion of potent derivatives (S 4048, S 3483) in rats. However, the blood glucose lowering effect of S 4048 after bolus i.v. injection lasted only 60-90 min. Plasma clearance of S 4048 was very high, and the parent compound was rapidly and efficiently excreted into the bile of Wistar and GY/TR(-) rats, indicating that mrp-2 was not involved in this hepatobiliary elimination process. About 72% of the total administered radioactivity appeared in the bile within 20 min after i.v. bolus injection of the radiolabeled analogue [(3)H]S 1743 in a Wistar rat. However, in GY/TR(-) rats the dicarboxylic analogue of S 4048, S 3025, was cleared from the plasma less rapidly than its parent compound and its biliary elimination was comparatively low. In contrast, S 3025 exhibited comparable pharmacokinetics and biliary elimination profile as S 4048 in Wistar rats, suggesting that biliary elimination of S 3025 is facilitated by mrp-2, functionally absent in GY/TR(-) rats. Targeting to mrp-2 resulted in a significantly prolonged reduction of blood glucose levels in GY/TR(-) rats after i.v. bolus administration of S 3025.

Normal kinetics of intestinal glucose absorption in the absence of GLUT2: evidence for a transport pathway requiring glucose phosphorylation and transfer into the endoplasmic reticulum

Glucose is absorbed through the intestine by a transepithelial transport system initiated at the apical membrane by the cotransporter SGLT-1; intracellular glucose is then assumed to diffuse across the basolateral membrane through GLUT2. Here, we evaluated the impact of GLUT2 gene inactivation on this transepithelial transport process. We report that the kinetics of transepithelial glucose transport, as assessed in oral glucose tolerance tests, was identical in the presence or absence of GLUT2; that the transport was transcellular because it could be inhibited by the SGLT-1 inhibitor phlorizin, and that it could not be explained by overexpression of another known glucose transporter. By using an isolated intestine perfusion system, we demonstrated that the rate of transepithelial transport was similar in control and GLUT2(-/-) intestine and that it was increased to the same extent by cAMP in both situations. However, in the absence, but not in the presence, of GLUT2, the transport was inhibited dose-dependently by the glucose-6-phosphate translocase inhibitor S4048. Furthermore, whereas transport of [(14)C]glucose proceeded with the same kinetics in control and GLUT2(-/-) intestine, [(14)C]3-O-methylglucose was transported in intestine of control but not of mutant mice. Together our data demonstrate the existence of a transepithelial glucose transport system in GLUT2(-/-) intestine that requires glucose phosphorylation and transfer of glucose-6-phosphate into the endoplasmic reticulum. Glucose may then be released out of the cells by a membrane traffic-based pathway similar to the one we previously described in GLUT2-null hepatocytes.

Novel arguments in favor of the substrate-transport model of glucose-6-phosphatase

The purpose of this work was to discriminate between two models for glucose-6-phosphatase: one in which the enzyme has its catalytic site oriented toward the lumen of the endoplasmic reticulum, requiring transporters for glucose-6-phosphate, inorganic phosphate (Pi), and glucose (substrate-transport model), and a second one in which the hydrolysis of glucose-6-phosphate occurs inside the membrane (conformational model). We show that microsomes preloaded with yeast phosphoglucose isomerase catalyzed the detritiation of [2-(3)H]glucose-6-phosphate and that this reaction was inhibited by up to 90% by S3483, a compound known to inhibit glucose-6-phosphate hydrolysis in intact but not in detergent-treated microsomes. These results indicate that glucose-6-phosphate is transported to the lumen of the microsomes in an S3483-sensitive manner. Detritiation by intramicrosomal phosphoglucose isomerase was stimulated twofold by 1 mmol/l vanadate, a phosphatase inhibitor, indicating that glucose-6-phosphatase and the isomerase compete for the same intravesicular pool of glucose-6-phosphate. To investigate the site of release of Pi from glucose-6-phosphate, we incubated microsomes with Pb(2+), which forms an insoluble complex with Pi, preventing its rapid exit from the microsomes. Under these conditions, approximately 80% of the Pi that was formed after 5 min was intramicrosomal, compared with <10% in the absence of Pb(2+). We also show that, when incubated with glucose-6-phosphate and mannitol, glucose-6-phosphatase formed mannitol-1-phosphate and that this nonphysiological product was initially present within the microsomes before being released to the medium. These results indicate that the primary site of product release by glucose-6-phosphatase is the lumen of the endoplasmic reticulum.

Upregulation of hepatic glucose 6-phosphatase gene expression in rats treated with an inhibitor of glucose-6-phosphate translocase

The multicomponent hepatic glucose 6-phosphatase (Glc-6-Pase) system catalyzes the terminal step of hepatic glucose production and plays a key role in the regulation of blood glucose. We used the chlorogenic acid derivative S 3483, a reversible inhibitor of the glucose-6-phosphate (Glc-6-P) translocase component, to demonstrate for the first time upregulation of Glc-6-Pase expression in rat liver in vivo after inhibition of Glc-6-P translocase. In accordance with its mode of action, S 3483-treatment of overnight-fasted rats induced hypoglycemia and increased blood lactate, hepatic Glc-6-P, and glycogen. The metabolic changes were accompanied by rapid and marked increases in Glc-6-Pase mRNA (above 35-fold), protein (about 2-fold), and enzymatic activity (about 2-fold). Maximal mRNA levels were reached after 4 h of treatment. Glycemia, blood lactate, and Glc-6-Pase mRNA levels returned to control values, whereas Glc-6-P and glycogen levels decreased but were still elevated 2 h after S 3483 withdrawal. The capacity for Glc-6-P influx was only marginally increased after 8.5 h of treatment. Prevention of hypoglycemia by euglycemic clamp did not abolish the increase in Glc-6-Pase mRNA induced by S 3483 treatment. A similar pattern of hypoglycemia and possibly of associated counterregulatory responses elicited by treatment with the phosphoenolpyruvate carboxykinase inhibitor 3-mercaptopicolinic acid could account for only a 2-fold induction of Glc-6-Pase mRNA. These findings suggest that the significant upregulation of Glc-6-Pase gene expression observed after treatment of rats in vivo with an inhibitor of Glc-6-P translocase is caused predominantly either by S 3483 per se or by the compound-induced changes of intracellular carbohydrate metabolism.

Alterations of carbohydrate and lipid intermediary metabolism during inhibition of glucose-6-phosphatase in rats

S 4048 (1-[2-(4-Chloro-phenyl)-cyclopropylmethoxy]-3, 4-dihydroxy-5-(3-imidazo[4, 5-b]pyridin-1-yl-3-phenyl-acryloyloxy)-cyclohexanecarboxylic acid), a derivative of chlorogenic acid, specifically inhibits the glucose-6-phosphate translocating component T1 of the glucose-6-phosphatase system. Its pharmacological effect was studied on carbohydrate and lipid parameters in rats. In starved and fed rats, S 4048 caused a dose-dependent reduction of blood glucose levels with a corresponding increase in hepatic and renal glycogen and glucose-6-phosphate. The major quantitative route of carbon flux in the liver during S 4048-induced inhibition of the glucose-6-phosphatase activity seemed to be glycogenesis. Plasma free fatty acids were increased secondarily due to the S 4048-induced hypoglycemia. Hepatic triglycerides were increased possibly due to increased re-esterification of the readily available free fatty acids. Glucose-6-phosphate translocase inhibitors may be useful for experimentally studying aspects of type 1 glycogen storage disease in laboratory animals as well as for the therapeutic modulation of inappropriately high rates of hepatic glucose production in type 2 diabetes.