Short chain fatty acids in human gut and metabolic health.

Evidence is accumulating that short chain fatty acids (SCFA) play an important role in the maintenance of gut and metabolic health. The SCFA acetate, propionate and butyrate are produced from the microbial fermentation of indigestible carbohydrates and appear to be key mediators of the beneficial effects elicited by the gut microbiome. Microbial SCFA production is essential for gut integrity by regulating the luminal pH, mucus production, providing fuel for epithelial cells and effects on mucosal immune function. SCFA also directly modulate host metabolic health through a range of tissue-specific mechanisms related to appetite regulation, energy expenditure, glucose homeostasis and immunomodulation. Therefore, an increased microbial SCFA production can be considered as a health benefit, but data are mainly based on animal studies, whereas well-controlled human studies are limited. In this review an expert group by ILSI Europe's Prebiotics Task Force discussed the current scientific knowledge on SCFA to consider the relationship between SCFA and gut and metabolic health with a particular focus on human evidence. Overall, the available mechanistic data and limited human data on the metabolic consequences of elevated gut-derived SCFA production strongly suggest that increasing SCFA production could be a valuable strategy in the preventing gastro-intestinal dysfunction, obesity and type 2 diabetes mellitus. Nevertheless, there is an urgent need for well controlled longer term human SCFA intervention studies, including measurement of SCFA fluxes and kinetics, the heterogeneity in response based on metabolic phenotype, the type of dietary fibre and fermentation site in fibre intervention studies and the control for factors that could shape the microbiome like diet, physical activity and use of medication.

Vasoactive intestinal peptide is a local mediator in a gut-brain neural axis activating intestinal gluconeogenesis.

Intestinal gluconeogenesis (IGN) promotes metabolic benefits through activation of a gut-brain neural axis. However, the local mediator activating gluconeogenic genes in the enterocytes remains unknown. We show that (i) vasoactive intestinal peptide (VIP) signaling through VPAC1 receptor activates the intestinal glucose-6-phosphatase gene in vivo, (ii) the activation of IGN by propionate is counteracted by VPAC1 antagonism, and (iii) VIP-positive intrinsic neurons in the submucosal plexus are increased under the action of propionate. These data support the role of VIP as a local neuromodulator released by intrinsic enteric neurons and responsible for the induction of IGN through a VPAC1 receptor-dependent mechanism in enterocytes.

Metabolic effects of portal vein sensing.

The extrinsic gastrointestinal nerves are crucial in the sensing of nutrients and hormones and its translation in terms of control of food intake. Major macronutrients like glucose and protein are sensed by the extrinsic nerves located in the portal vein walls, which signal to the brain and account for the satiety phenomenon they promote. Glucose is sensed in the portal vein by neurons expressing the glucose receptor SGLT3, which activate the main regions of the brain involved in the control of food intake. Proteins indirectly act on food intake by inducing intestinal gluconeogenesis and its sensing by the portal glucose sensor. The mechanism involves a prior antagonism by peptides of the µ-opioid receptors present in the portal vein nervous system and a reflex arc with the brain inducing intestinal gluconeogenesis. In a comparable manner, short-chain fatty acids produced from soluble fibre act via intestinal gluconeogenesis to exert anti-obesity and anti-diabetic effects. In the case of propionate, the mechanism involves a prior activation of the free fatty acid receptor FFAR3 present in the portal nerves and a reflex arc initiating intestinal gluconeogenesis.

Influence of diabetes surgery on a gut-brain-liver axis regulating food intake and internal glucose production.

It has long been known that the brain, especially the hypothalamus, can modulate both insulin secretion and hepatic glucose fluxes, via the modulation of the sympathetic system (promoting glycogen breakdown) and the parasympathetic system (stimulating glycogen deposition). Central insulin signalling or hypothalamic long-chain fatty acid oxidation can also control insulin's suppression of endogenous glucose production. Interestingly, intestinal gluconeogenesis can initiate a portal glucose signal, transmitted to the hypothalamus via the gastrointestinal nervous system. This signal may modulate the sensation of hunger and satiety and insulin sensitivity of hepatic glucose fluxes as well. The rapid improvements of glucose control taking place after gastric bypass surgery in obese diabetics has long been mysterious. Actually, the specificity of gastric bypass in obese diabetic mice relates to major changes in the sensations of hunger and to rapid improvement in insulin sensitivity of endogenous glucose production. We have shown that an induction of intestinal gluconeogenesis plays a major role in these phenomena. In addition, the restoration of the secretion of glucagon like peptide 1 and consequently of insulin plays a key additional role to improve postprandial glucose tolerance. Therefore, a synergy between incretin effects and intestinal gluconeogenesis might be a key feature explaining the rapid improvement of glucose control in obese diabetics after bypass surgery.

Se sabe desde hace tiempo que el cerebro, especialmente el hipotálamo, puede modular la secreción de insulina y los flujos hepáticos de glucosa mediante la modulación del sistema simpático (promoviendo la degradación del glucógeno) y el sistema parasimpático (estimulando el depósito de glucógeno). La señalización central de la insulina o la oxidación hipotalámica de los ácidos grasos de cadena larga también pueden controlar la producción de la glucosa endógena por la supresión de la insulina. De forma interesante, la gluconeogénesis intestinal puede iniciar una señal de glucosa portal, que se transmite al hipotálamo a través del sistema nervioso gastrointestinal. Esta señal puede modular la sensación de hambre y la saciedad, así como la sensibilidad a la insulina de los flujos hepáticos de glucosa. Las mejorías rápidas del control de la glucosa que ocurren tras la cirugía de derivación gástrica en los diabéticos obesos siguen siendo un misterio. En realidad, la especificidad de la derivación gástrica en ratones obesos diabéticos se relaciona con cambios importantes en las sensaciones de hambre y con una mejoría rápida de la sensibilidad a la insulina de la producción endógena de glucosa. Hemos demostrado que la inducción de la gluconeogénesis intestinal desempeña un papel principal en estos fenómenos. Además, la restauración de la secreción del péptido 1 de tipo glucagón y, por consiguiente, de la insulina, desempeña un papel clave adicional en la mejora de la tolerancia a la glucosa postprandial. Por lo tanto, la sinergia entre los efectos de la incretina y la gluconeogénesis intestinal podría ser un elemento clave en la mejora rápida del control de la glucosa en los diabéticos obesos tras la cirugía de derivación.

Glycogen storage disease type 1 and diabetes: learning by comparing and contrasting the two disorders.

Glycogen storage disease type 1 (GSD1) and diabetes may look at first like totally opposite disorders, as diabetes is characterized by uncontrolled hyperglycaemia, whereas GSD1 is characterized by severe fasting hypoglycaemia. Diabetes is due to a failure to suppress endogenous glucose production (EGP) in the postprandial state because of either a lack of insulin or insulin resistance. In contrast, GSD1 is characterized by a lack of EGP. However, both diseases share remarkably similar patterns in terms of pathophysiology such as the long-term progression of renal dysfunction and hepatic steatosis leading to renal failure and the development of hepatic tumours, respectively. Thus, much may be learned from considering the similarities between GSD1 and diabetes, especially in the metabolic pathways underlying nephropathy and fatty liver, and perhaps even more from their differences. In this review, the differences between diabetes and GSD1 are first highlighted, as both are characterized by alterations in EGP. The molecular pathways involved in liver pathologies, including steatosis, hepatomegaly (glycogenic hepatopathy) and the development of liver tumours are also compared. These pathologies are mainly due to the accumulation of lipids and/or glycogen in hepatocytes. Finally, the similar pathways leading to nephropathy in both diabetic and GSD1 patients are described. In conclusion, comparisons of these pathologies should lead to a better understanding of the crucial role of EGP in the control of glucose and energy homoeostasis. Moreover, it may highlight similar therapeutic targets for the two disorders. Thus, this review suggests that the treatment of adult patients with either GSD1 or diabetes could be carried out by the same specialists-diabetologists.

Influence of diabetes surgery on a gut-brain-liver axis regulating foodintake and internal glucose production / Influencia de la cirugía de diabetes sobre el eje intestino-cerebro-hígado que regula ingesta alimentaria y producción interna de glucosa

It has long been known that the brain, especially the hypothalamus, can modulate both insulin secretion and hepatic glucose fluxes, via the modulation of the sympathetic system (promoting glycogen breakdown) and the parasympathetic system (stimulating glycogen deposition). Central insulin signalling or hypothalamic long-chain fatty acid oxidation can also control insulin's suppression of endogenous glucose production. Interestingly, intestinal gluconeogenesis can initiate a portal glucose signal, transmitted to the hypothalamus via the gastrointestinal nervous system. This signal may modulate the sensation of hunger and satiety and insulin sensitivity of hepatic glucose fluxes as well. The rapid improvements of glucose control taking place after gastric bypass surgery in obese diabetics has long been mysterious. Actually, the specificity of gastric bypass in obese diabetic mice relates to major changes in the sensations of hunger and to rapid improvement in insulin sensitivity of endogenous glucose production. We have shown that an induction of intestinal gluconeogenesis plays a major role in these phenomena. In addition, the restoration of the secretion of glucagon like peptide 1 and consequently of insulin plays a key additional role to improve postprandial glucose tolerance. Therefore, a synergy between incretin effects and intestinal gluconeogenesis might be a key feature explaining the rapid improvement of glucose control in obese diabetics after bypass surgery (AU)

Se sabe desde hace tiempo que el cerebro, especialmente el hipotálamo, puede modular la secreción de insulina y los flujos hepáticos de glucosa mediante la modulación del sistema simpático (promoviendo la degradación del glucógeno) y el sistema parasimpático (estimulando el depósito de glucógeno). La señalización central de la insulina o la oxidación hipotalámica de los ácidos grasos de cadena larga también pueden controlar la producción de la glucosa endógena por la supresión de la insulina. De forma interesante, la gluconeogénesis intestinal puede iniciar una señal de glucosa portal, que se transmite al hipotálamo a través del sistema nervioso gastrointestinal. Esta señal puede modular la sensación de hambre y la saciedad, así como la sensibilidad a la insulina de los flujos hepáticos de glucosa. Las mejorías rápidas del control de la glucosa que ocurren tras la cirugía de derivación gástrica en los diabéticos obesos siguen siendo un misterio. En realidad, la especificidad de la derivación gástrica en ratones obesos diabéticos se relaciona con cambios importantes en las sensaciones de hambre y con una mejoría rápida de la sensibilidad a la insulina de la producción endógena de glucosa. Hemos demostrado que la inducción de la gluconeogénesis intestinal desempeña un papel principal en estos fenómenos. Además, la restauración de la secreción del péptido 1 de tipo glucagón y, por consiguiente, de la insulina, desempeña un papel clave adicional en la mejora de la tolerancia a la glucosa postprandial. Por lo tanto, la sinergia entre los efectos de la incretina y la gluconeogénesis intestinal podría ser un elemento clave en la mejora rápida del control de la glucosa en los diabéticos obesos tras la cirugía de derivación (AU)

Hypothalamic integration of portal glucose signals and control of food intake and insulin sensitivity.

Glycolysis is an essential metabolic function that lies at the core of any cellular life. Glucose homoeostasis is, thus, a crucial physiological function of living organisms. A system of plasma glucose-sensing in the portal vein plays a key role in this homoeostasis. Connected to the hypothalamus via the peripheral nervous system, the system allows the body to adapt its response to any variation of portal glycaemia. The hypothalamus controls food intake (exogenous glucose supply) and hepatic glycogenolysis (endogenous glucose supply). Intestinal gluconeogenesis, via the release of glucose into the portal vein, plays a key role in the control of hunger and satiety, and of endogenous glucose production through the modulation of liver insulin sensitivity. The induction of intestinal gluconeogenesis provides a physiological explanation for the satiety effects induced by protein-enriched diets. In particular, the influence of protein-enriched diets on the hypothalamus is comparable to the activation observed after glucose infusion into the portal vein. The induction of intestinal gluconeogenesis also offers an explanation for the early improvement in glycaemia control observed in obese diabetic patients treated by gastric-bypass surgery. In addition to intestinal gluconeogenesis, a number of gastrointestinal hormones involved in the control of food intake exert their effects, at least in part, via the peripheral afferent nervous system. These data emphasize the importance of the gut-brain axis in the understanding and treatment of obesity and type 2 diabetes.

Brain, liver, intestine: a triumvirate to coordinate insulin sensitivity of endogenous glucose production.

The brain, especially the hypothalamus, can modulate hepatic glucose fluxes. The sympathetic system promotes glycogen breakdown. The parasympathetic system stimulates glycogen deposition. Central insulin signalling or hypothalamic long-chain fatty acid oxidation can both control insulin's suppression of endogenous glucose production. Intestinal gluconeogenesis initiates a portal glucose signal, transmitted to the brain via the gastrointestinal nervous system. This signal may modulate the sensation of hunger and satiety and insulin sensitivity of hepatic glucose fluxes as well, via the modulation of hypothalamic activity.

What can bariatric surgery teach us about the pathophysiology of type 2 diabetes?

Bariatric surgery is indicated in cases of severe obesity. However, malabsorption-based techniques (gastric bypass and biliopancreatic diversion, both of which exclude the duodenum and jejunum from the alimentary circuit), but not restrictive techniques, can abolish type 2 diabetes within days of surgery, even before any significant weight loss has occurred. This means that calorie restriction alone cannot entirely account for this effect. In Goto-Kakizaki rats, a type 2 diabetes model, glycaemic equilibrium is improved by surgical exclusion of the proximal intestine, but deteriorates again when the proximal intestine is reconnected to the circuit in the same animals. This effect is independent of weight, suggesting that the intestine is itself involved in the immediate regulation of carbohydrate homoeostasis. In humans, the rapid improvement in carbohydrate homoeostasis observed after bypass surgery is secondary to an increase in insulin sensitivity rather than an increase in insulin secretion, which occurs later. Several mechanisms are involved--disappearance of hypertriglyceridaemia and decrease in levels of circulating fatty acids, disappearance of the mechanisms of lipotoxicity in the liver and skeletal muscle, and increases in secretion of GLP-1 and PYY--and may be intricately linked. In the medium term and in parallel with weight loss, a decrease in fatty tissue inflammation (which is also seen with restrictive techniques) may also be involved in metabolic improvement. Other mechanisms specific to malabsorption-based techniques (due to the required exclusion of part of the intestine), such as changes in the activity of digestive vagal afferents, changes in intestinal flora and stimulation of intestinal neoglucogenesis, also need to be studied in greater detail. The intestine is, thus, a key organ in the regulation of glycaemic equilibrium and may even be involved in the pathophysiology of type 2 diabetes.

Protein hydrolysates stimulate proglucagon gene transcription in intestinal endocrine cells via two elements related to cyclic AMP response element.

AIMS/HYPOTHESIS: Protein hydrolysates (peptones) increase not only glucagon-like peptide-1 (GLP-1) secretion but also transcription of the proglucagon ( PG) gene in the intestine. The critical physiological roles of gut-derived GLPs raised hope for their therapeutic use in several disorders, especially GLP-1 in diabetes. We aimed to investigate the molecular mechanisms involved in this nutrient- PG gene interaction. METHODS: Wild-type and mutated PG promoter fragments fused to the luciferase reporter gene were transfected into enteroendocrine STC-1 cells, which were then either treated or not with peptones. Co-transfection with expression vectors of dominant-negative forms of cAMP response element binding protein (CREB) and protein kinase A (PKA) proteins were performed, as well as electrophoresis mobility shift assays. RESULTS: Deletion analysis showed that the promoter region spanning between -350 and -292 bp was crucial for the transcriptional stimulation induced by peptones. Site-directed mutagenesis of the canonical cAMP response element (CRE(PG)) and of the adjacent putative CRE site (CRE-like1) led to a dramatic inhibition of the promoter responsiveness to peptones. Over expression of a dominant-negative mutant of CREB or of PKA produced a comparable and selective inhibitory effect on the activity of transfected promoter fragment containing the -350/-292 sequence. EMSA showed that CREB and fra2 transcription factors bound to CRE(PG) and CRE-like1 elements respectively, independently of peptone treatment. CONCLUSIONS/INTERPRETATION: Our report identified cis- and trans-regulatory elements implicated in the transcriptional control of PG gene by nutrients in enteroendocrine cells. It highlights the role of a previously unsuspected CRE-like1 element, and emphasises the importance of CRE-related sequences in the regulation of PG gene transcription in the intestine.

New data and concepts on glutamine and glucose metabolism in the gut.

Both glutamine and glucose are highly utilized by the small intestine in various animal species. They are, however, very partially oxidized, the major known fate of glucose being lactate and alanine, and that of glutamine being citrulline or proline. At variance with the current view that only the liver and kidney are gluconeogenic organs, because both are the only tissues to express the glucose-6 phosphatase gene, this gene is also expressed in the small intestine in rats and humans, and is strongly induced in insulinopenic states, such as fasting and diabetes. Under the latter conditions, the small intestine contributes 20-25% of whole-body endogenous glucose production. The main small intestine gluconeogenic substrate is glutamine and, to a lesser extent, glycerol. Accounting for these fluxes, the phosphoenolpyruvate carboxykinase gene is strongly induced in insulinopenia and, although up to now it had been considered absent from this tissue, the glycerokinase gene is expressed in the small intestine. The production of glucose by the small intestine may be acutely blunted upon insulin infusion. These new data also emphasize the central role of alanine aminotransferase in the coupling of glutamine and glucose metabolisms in the small intestine.

Glucose 6-phosphate hydrolysis is activated by glucagon in a low temperature-sensitive manner.

Glucagon affects liver glucose metabolism mainly by activating glycogen breakdown and by inhibiting pyruvate kinase, whereas a possible effect on glucose-6-phosphatase has also been suggested. Although such a target is of physiological importance for liver glucose production it was never proven. By using a model of liver cells, perifused with dihydroxyacetone, we show here that the acute stimulation of gluconeogenesis by glucagon (10(-7) m) was not related to the significant inhibition of pyruvate kinase but to a dramatic activation of the hydrolysis of glucose 6-phosphate. We failed to find an acute change in glucose-6-phosphatase activity by glucagon, but the increase in glucose 6-phosphate hydrolysis was abolished at 21 degrees C; conversely the effect on pyruvate kinase was not affected by temperature. The activation of glucose 6-phosphate hydrolysis by glucagon was confirmed in vivo, in postabsorptive rats receiving a constant infusion of glucagon, by the combination of a 2-fold increase in hepatic glucose production and a 60% decrease in liver glucose 6-phosphate concentration. Besides the description of a novel effect of glucagon on glucose 6-phosphate hydrolysis by a temperature-sensitive mechanism, this finding could represent an important breakthrough in the understanding of type II diabetes, because glucose 6-phosphate is proposed to be a key molecule in the transcriptional effect of glucose.

Rat small intestine is an insulin-sensitive gluconeogenic organ.

At variance with the current view that only liver and kidney are gluconeogenic organs, because both are the only tissues to express glucose-6-phosphatase (Glc6Pase), we have recently demonstrated that the Glc6Pase gene is expressed in the small intestine in rats and humans and that it is induced in insulinopenic states such as fasting and diabetes. We used a combination of arteriovenous balance and isotopic techniques, reverse transcription-polymerase chain reaction, Northern blot analysis, and enzymatic activity assays. We report that rat small intestine can release neosynthesized glucose in mesenteric blood in insulinopenia, contributing 20-25% of total endogenous glucose production. Like liver glucose production, small intestine glucose production is acutely suppressed by insulin infusion. In the small intestine, glutamine and, to a much lesser extent, glycerol are the precursors of glucose, whereas alanine and lactate are the main precursors in liver. Accounting for these metabolic fluxes: 1) the phosphoenolpyruvate carboxykinase gene (required for the utilization of glutamine) is strongly induced at the mRNA and enzyme levels in insulinopenia; 2) the glycerokinase gene is expressed, but not induced; 3) the pyruvate carboxylase gene (required for the utilization of alanine and lactate) is repressed by 80% at the enzyme level in insulinopenia. These studies identify small intestine as a new insulin-sensitive tissue and a third gluconeogenic organ, possibly involved in the pathophysiology of diabetes.

beta-cell function and viability in the spontaneously diabetic GK rat: information from the GK/Par colony.

The GK rat model of type 2 diabetes is especially convenient to dissect the pathogenic mechanism necessary for the emergence of overt diabetes because all adult rats obtained in our department (GK/Par colony) to date have stable basal mild hyperglycemia and because overt diabetes is preceded by a period of normoglycemia, ranging from birth to weaning. The purpose of this article is to sum up the information so far available related to the biology of the beta-cell in the GK/Par rat. In terms of beta-cell function, there is no major intrinsic secretory defect in the prediabetic GK/Par beta-cell, and the lack of beta-cell reactivity to glucose (which reflects multiple intracellular abnormalities), as seen during the adult period when the GK/Par rats are overtly diabetic, represents an acquired defect (perhaps glucotoxicity). In terms of beta-cell population, the earliest alteration so far detected in the GK/Par rat targets the size of the beta-cell population. Several convergent data suggest that the permanently reduced beta-cell mass in the GK/Par rat reflects a limitation of beta-cell neogenesis during early fetal life, and it is conceivable that some genes among the set involved in GK diabetes belong to the subset of genes controlling early beta-cell development.

Induction of PEPCK gene expression in insulinopenia in rat small intestine.

PEPCK is a key enzyme of gluconeogenesis in liver and kidney. Recently, we have shown that small intestine also contributes to the endogenous glucose production in insulinopenia in rats and that glutamine is the main precursor of glucose synthesized in this tissue. The expression of the PEPCK gene in rat and human small intestine and the effect of streptozotocin-induced diabetes and fasting have been studied using reverse transcriptase-polymerase chain reaction, Northern blot analysis, and determination of enzyme activity. The PEPCK gene is expressed along the whole small intestine in adult rat and human. The abundance of PEPCK mRNA was increased approximately 30 times in the duodenum, 15 times in the jejunum, and only 3 times in the ileum from diabetic rats. PEPCK mRNA was downregulated in all parts of the tissue upon insulin treatment for 10 h. In 48-h fasted rats, the PEPCK mRNA abundance was increased 17 times in the duodenum and the jejunum and 3 times in the ileum, and it was normalized upon refeeding for 7 h. PEPCK activity was also increased 2-5 times in diabetic and fasted rats in the duodenum and jejunum but not in the ileum. We conclude that PEPCK is a crucial enzyme contributing to the induction of gluconeogenesis in small intestine, just as it is well known to be in the liver and kidney.

Mechanisms by which insulin, associated or not with glucose, may inhibit hepatic glucose production in the rat.

We investigated the intrahepatic mechanisms by which insulin, associated or not with hyperglycemia, may inhibit hepatic glucose production (HGP) in the rat. After a hyperinsulinemic euglycemic clamp in postabsorptive (PA) anesthetized rats, the 70% inhibition of HGP could be explained by a dramatic decrease in the glucose 6-phosphate (G-6-P) concentration, whereas the glucose-6-phosphatase (G-6-Pase) and glucokinase (GK) activities were unchanged. Under hyperinsulinemic hyperglycemic condition, the GK flux was increased. The G-6-P concentration was not or only weakly decreased. The inhibition of HGP involved a significant 25% inhibition of the G-6-Pase activity. Under similar conditions in fasted rats, the GK flux was very low. The suppression of G-6-Pase and HGP did not occur, despite plasma insulin and glucose concentrations similar to those in PA rats. Therefore, 1) insulin suppresses HGP in euglycemia by solely decreasing the G-6-P concentration; 2) when combining both hyperinsulinemia and hyperglycemia, the suppression of HGP involves the inhibition of the G-6-Pase activity; and 3) a sustained glucose-phosphorylation flux might be a crucial determinant in the inhibition of G-6-Pase and of HGP.

Regulatory role of glucose-6 phosphatase in the repletion of liver glycogen during refeeding in fasted rats.

We analyzed the biochemical mechanisms involved in the liver glycogen repletion upon refeeding for 360 min in 48 and 96 h-fasted rats. In 48 h-fasted rats, the glycogen synthesis involved a rapid and further sustained induction of glucokinase (GK) (increased twice from 90 min) and a rapid but transient activation of glycogen synthase a (GSa) (maximal increase by 150% at 90 min). It did not involve the inhibition of glycogen phosphorylase a (GPa). In 96 h-fasted rats, the glycogen repletion did not involve the induction of GK for the first 180 min of refeeding. It involved a slow activation of GSa (maximal 150% increase at 180 min) and a rapid inhibition of GPa (significant from 90 min, maximal 50% inhibition by 180 min). In both groups of rats, there was a progressive inhibition of the glucose-6 phosphatase (Glc6Pase) activity (maximal suppression by 30% in both groups at 360 min). These results highlighted a key role for the inhibition of Glc6Pase activity in the liver glycogen repletion upon refeeding.

The glucose-6 phosphatase gene is expressed in human and rat small intestine: regulation of expression in fasted and diabetic rats.

BACKGROUND & AIMS: Glucose-6 phosphatase (Glc6Pase) is the last enzyme of gluconeogenesis and glycogenolysis, previously assumed to be expressed in the liver and kidney only, conferring on both tissues the capacity to produce endogenous glucose in blood. METHODS: Using Northern blotting and reverse-transcription polymerase chain reaction and a highly specific Glc6Pase assay, we studied expression of the Glc6Pase gene in human and in rat tissues (fasted and diabetic). RESULTS: The Glc6Pase gene is expressed in the duodenum and jejunum in normal fed rats and in the duodenum, jejunum, and ileum in humans. The Glc6Pase messenger RNA (mRNA) abundance was increased eightfold and sixfold in the duodenum and jejunum of streptozotocin diabetic rats. It was normalized in both tissues after 10 hours of insulin treatment. Glc6Pase activity was increased by 300% in the duodenum and jejunum in diabetic rats compared with normal rats. The Glc6Pase mRNA abundances and enzymatic activities were increased in a similar manner in both tissues in 48-hour-fasted rats. Normalization of mRNA abundance was achieved after refeeding for 7 hours. In addition, Glc6Pase mRNA and activity were also expressed in the ileum during fasting in rats. CONCLUSIONS: These data show that the small intestine has the ability to release endogenous glucose and strongly suggest that its contribution to systemic glucose production might be increased in situations of insulinopenia (type 1 diabetes) and insulin resistance (type 2 diabetes and others).

Phosphatidylinositol 3-kinase translocates onto liver endoplasmic reticulum and may account for the inhibition of glucose-6-phosphatase during refeeding.

By using a rapid procedure of isolation of microsomes, we have shown that the liver glucose-6-phosphatase activity was lowered by about 30% (p < 0.001) after refeeding for 360 min rats previously unfed for 48 h, whereas the amount of glucose-6-phosphatase protein was not lowered during the same time. The amount of the regulatory subunit (p85) and the catalytic activity of phosphatidylinositol 3-kinase (PI3K) were higher by a factor of 2.6 and 2.4, respectively (p < 0.01), in microsomes from refed as compared with fasted rats. This resulted from a translocation process because the total amount of p85 was the same in the whole liver homogenates from fasted and refed rats. The amount of insulin receptor substrate 1 (IRS1) was also higher by a factor of 2.6 in microsomes from refed rats (p < 0. 01). Microsome-bound IRS1 was only detected in p85 immunoprecipitates. These results strongly suggest that an insulin-triggered mechanism of translocation of PI3K onto microsomes occurs in the liver of rats during refeeding. This process, via the lipid products of PI3K, which are potent inhibitors of glucose-6-phosphatase (Mithieux, G., Danièle, N., Payrastre, B., and Zitoun, C. (1998) J. Biol. Chem. 273, 17-19), may account for the inhibition of the enzyme and participate to the inhibition of hepatic glucose production occurring in this situation.