Acute inhibition of glucose-6-phosphate translocator activity leads to increased de novo lipogenesis and development of hepatic steatosis without affecting VLDL production in rats

Biochemical and metabolic characterization of a G6PC2 inhibitor

Three glucose-6-phosphatase catalytic subunits, that hydrolyze glucose-6-phosphate (G6P) to glucose and inorganic phosphate, have been identified, designated G6PC1-3, but only G6PC1 and G6PC2 have been implicated in the regulation of fasting blood glucose (FBG). Elevated FBG has been associated with multiple adverse clinical outcomes, including increased risk for type 2 diabetes and various cancers. Therefore, G6PC1 and G6PC2 inhibitors that lower FBG may be of prophylactic value for the prevention of multiple conditions. The studies described here characterize a G6PC2 inhibitor, designated VU0945627, previously identified as Compound 3. We show that VU0945627 preferentially inhibits human G6PC2 versus human G6PC1 but activates human G6PC3. VU0945627 is a mixed G6PC2 inhibitor, increasing the Km but reducing the Vmax for G6P hydrolysis. PyRx virtual docking to an AlphaFold2-derived G6PC2 structural model suggests VU0945627 binds two sites in human G6PC2. Mutation of residues in these sites reduces the inhibitory effect of VU0945627. VU0945627 does not inhibit mouse G6PC2 despite its 84% sequence identity with human G6PC2. Mutagenesis studies suggest this lack of inhibition of mouse G6PC2 is due, in part, to a change in residue 318 from histidine in human G6PC2 to proline in mouse G6PC2. Surprisingly, VU0945627 still inhibited glucose cycling in the mouse islet-derived βTC-3 cell line. Studies using intact mouse liver microsomes and PyRx docking suggest that this observation can be explained by an ability of VU0945627 to also inhibit the G6P transporter SLC37A4. These data will inform future computational modeling studies designed to identify G6PC isoform-specific inhibitors.

Hepatic ChREBP orchestrates intrahepatic carbohydrate metabolism to limit hepatic glucose 6-phosphate and glycogen accumulation in a mouse model for acute Glycogen Storage Disease type Ib

OBJECTIVE

Carbohydrate Response Element Binding Protein (ChREBP) is a glucose 6-phosphate (G6P)-sensitive transcription factor that acts as a metabolic switch to maintain intracellular glucose and phosphate homeostasis. Hepatic ChREBP is well-known for its regulatory role in glycolysis, the pentose phosphate pathway, and de novo lipogenesis. The physiological role of ChREBP in hepatic glycogen metabolism and blood glucose regulation has not been assessed in detail, and ChREBP's contribution to carbohydrate flux adaptations in hepatic Glycogen Storage Disease type 1 (GSD I) requires further investigation.

METHODS

The current study aimed to investigate the role of ChREBP as a regulator of glycogen metabolism in response to hepatic G6P accumulation, using a model for acute hepatic GSD type Ib. The immediate biochemical and regulatory responses to hepatic G6P accumulation were evaluated upon G6P transporter inhibition by the chlorogenic acid S4048 in mice that were either treated with a short hairpin RNA (shRNA) directed against ChREBP (shChREBP) or a scrambled shRNA (shSCR). Complementary stable isotope experiments were performed to quantify hepatic carbohydrate fluxes in vivo.

RESULTS

ShChREBP treatment normalized the S4048-mediated induction of hepatic ChREBP target genes to levels observed in vehicle- and shSCR-treated controls. In parallel, hepatic shChREBP treatment in S4048-infused mice resulted in a more pronounced accumulation of hepatic glycogen and further reduction of blood glucose levels compared to shSCR treatment. Hepatic ChREBP knockdown modestly increased glucokinase (GCK) flux in S4048-treated mice while it enhanced UDP-glucose turnover as well as glycogen synthase and phosphorylase fluxes. Hepatic GCK mRNA and protein levels were induced by shChREBP treatment in both vehicle- and S4048-treated mice, while glycogen synthase 2 (GYS2) and glycogen phosphorylase (PYGL) mRNA and protein levels were reduced. Finally, knockdown of hepatic ChREBP expression reduced starch domain binding protein 1 (STBD1) mRNA and protein levels while it inhibited acid alpha-glucosidase (GAA) activity, suggesting reduced capacity for lysosomal glycogen breakdown.

CONCLUSIONS

Our data show that ChREBP activation controls hepatic glycogen and blood glucose levels in acute hepatic GSD Ib through concomitant regulation of glucose phosphorylation, glycogenesis, and glycogenolysis. ChREBP-mediated control of GCK enzyme levels aligns with corresponding adaptations in GCK flux. In contrast, ChREBP activation in response to acute hepatic GSD Ib exerts opposite effects on GYS2/PYGL enzyme levels and their corresponding fluxes, indicating that GYS2/PYGL expression levels are not limiting to their respective fluxes under these conditions.

Dynamic glucose uptake, storage, and release by human microvascular endothelial cells

Endothelia determine blood-to-tissue solute delivery, yet glucose transit is poorly understood. To illuminate mechanisms, we tracked [³H]-2-deoxyglucose (2-DG) in human adipose-tissue microvascular endothelial cells. 2-DG uptake was largely facilitated by the glucose transporters GLUT1 and GLUT3. Once in the cytosol, >80% of 2-DG became phosphorylated and ∼20% incorporated into glycogen, suggesting that transported glucose is readily accessible to cytosolic enzymes. Interestingly, a fraction of intracellular 2-DG was released over time (15-20% over 30 min) with slower kinetics than for uptake, involving GLUT3. In contrast to intracellular 2-DG, the released 2-DG was largely unphosphorylated. Glucose release involved endoplasmic reticulum-resident translocases/phosphatases and was stimulated by adrenaline, consistent with participation of glycogenolysis and glucose dephosphorylation. Surprisingly, the fluorescent glucose derivative 2-NBD-glucose (2-NBDG) entered cells largely via fluid phase endocytosis and exited by recycling. 2-NBDG uptake was insensitive to GLUT1/GLUT3 inhibition, suggesting poor influx across membranes. 2-NBDG recycling, but not 2-DG efflux, was sensitive to N-ethyl maleimide. In sum, by utilizing radioactive and fluorescent glucose derivatives, we identified two parallel routes of entry: uptake into the cytosol through dedicated glucose transporters and endocytosis. This reveals the complex glucose handling by endothelial cells that may contribute to glucose delivery to tissues.

Untargeted plasma metabolomics identifies broad metabolic perturbations in glycogen storage disease type I

BACKGROUND

The metabolic defect in glycogen storage disease type I (GSDI) results in fasting hypoglycemia and typical secondary metabolic abnormalities (eg, hypertriglyceridemia, hyperlactatemia, hyperuricemia). The aim of this study was to assess further perturbations of the metabolic network in GSDI patients under ongoing treatment.

METHODS

In this prospective observational study, plasma samples of 14 adult patients (11 GSDIa, 3 GSDIb. Mean age 26.4 years, range 16-46 years) on standard treatment were compared to a cohort of 31 healthy controls utilizing ultra-high performance liquid chromatography (UHPLC) in combination with high resolution tandem mass spectrometry (HR-MS/MS) and subsequent statistical multivariate analysis. In addition, plasma fatty acid profiling was performed by GC/EI-MS.

RESULTS

The metabolomic profile showed alterations of metabolites in different areas of the metabolic network in both GSD subtypes, including pathways of fuel metabolism and energy generation, lipids and fatty acids, amino acid and methyl-group metabolism, the urea cycle, and purine/pyrimidine metabolism. These alterations were present despite adequate dietary treatment, did not correlate with plasma triglycerides or lactate, both parameters typically used to assess the quality of metabolic control in clinical practice, and were not related to the presence or absence of complications (ie, nephropathy or liver adenomas).

CONCLUSION

The metabolic defect of GSDI has profound effects on a variety of metabolic pathways in addition to the known typical abnormalities. These alterations are present despite optimized dietary treatment, which may contribute to the risk of developing long-term complications, an inherent problem of GSDI which appears to be only partly modified by current therapy.

Impaired Very-Low-Density Lipoprotein catabolism links hypoglycemia to hypertriglyceridemia in Glycogen Storage Disease type Ia

Prevention of hypertriglyceridemia is one of the biomedical targets in Glycogen Storage Disease type Ia (GSD Ia) patients, yet it is unclear how hypoglycemia links to plasma triglyceride (TG) levels. We analyzed whole-body TG metabolism in normoglycemic (fed) and hypoglycemic (fasted) hepatocyte-specific glucose-6-phosphatase deficient (L-G6pc^-/- ) mice. De novo fatty acid synthesis contributed substantially to hepatic TG accumulation in normoglycemic L-G6pc^-/- mice. In hypoglycemic conditions, enhanced adipose tissue lipolysis was the main driver of liver steatosis, supported by elevated free fatty acid concentrations in GSD Ia mice and GSD Ia patients. Plasma very-low-density lipoprotein (VLDL) levels were increased in GSD Ia patients and in normoglycemic L-G6pc^-/- mice, and further elevated in hypoglycemic L-G6pc^-/- mice. VLDL-TG secretion rates were doubled in normo- and hypoglycemic L-G6pc^-/- mice, while VLDL-TG catabolism was selectively inhibited in hypoglycemic L-G6pc^-/- mice. In conclusion, fasting-induced hypoglycemia in L-G6pc^-/- mice promotes adipose tissue lipolysis and arrests VLDL catabolism. This mechanism likely contributes to aggravated liver steatosis and dyslipidemia in GSD Ia patients with poor glycemic control and may explain clinical heterogeneity in hypertriglyceridemia between GSD Ia patients.

Regional functional and structural abnormalities within the aorta as a potential driver of vascular disease in metabolic syndrome

NEW FINDINGS

What is the central question of this study? Is aortic dysfunction, a significant contributor to cardiovascular disease in metabolic syndrome, expressed uniformly across both the thoracic and abdominal aorta? What is the main finding and its importance? Our study shows that, in the setting of metabolic syndrome, functional and structural deficits in the aorta are differentially expressed along its length, with the abdominal portion displaying more extensive vascular abnormalities. It is, therefore, likely that early interventional strategies targeting the abdominal aorta might alleviate cardiovascular pathologies driven by the metabolic syndrome.

ABSTRACT

The extent of vascular dysfunction associated with metabolic syndrome might vary along the length of the aorta. In this study, we investigated regional functional and structural changes in the thoracic and abdominal aorta of a rat model of metabolic syndrome, namely, high-fat diet (HFD) streptozotocin-induced diabetes mellitus (HFD-D). Four-week-old male Wistar albino rats were fed with either HFD or control diet (CD) for 10 weeks. At week 6, 40 mg/kg streptozotocin and its vehicle were injected i.p. into HFD and CD groups, respectively. At the end of the feeding period, rats were euthanised and aortic segments collected for assessment of vascular functional responses and histomorphometry. Tail-cuff systolic blood pressures (154 ± 6  vs. 110 ± 4 mmHg) and areas under the curve for oral glucose and i.p. insulin tolerance tests were greater in HFD-D versus CD rats. Abdominal aortic vasoconstriction in response to noradrenaline and KCl was greater in HFD-D compared with CD rats. Thoracic vasoconstrictor responses to noradrenaline, but not KCl, were greater in the HFD-D group. Abdominal, but not thoracic, endothelium-dependent vasorelaxation in response to acetylcholine was blunted in HFD-D relative to CD rats; however, nitric oxide-dependent vasorelaxation in HFD-D rats was impaired in both thoracic and abdominal segments. The abdominal aorta of HFD-D rats showed deranged interlamellar spacing and increased lipid plaque deposition. In conclusion, vascular dysfunction in metabolic syndrome is expressed differentially along the length of the aorta, with the abdominal aorta exhibiting increased susceptibility to vasoconstrictors and greater deficits in endothelium-dependent relaxation. These vascular functional abnormalities could potentially underlie the development of hypertensive cardiovascular disease associated with the metabolic syndrome.

Is serum biotinidase enzyme activity a potential marker of perturbed glucose and lipid metabolism?

Glycogen storage diseases (GSDs) belong to the group of inborn errors of carbohydrate metabolism. Hepatic GSDs predominantly involve the liver and most present with hepatomegaly. Biochemically they show known disturbances in glucose and fatty acids metabolism, namely fasting hypoglycaemia and increased triglycerides. Additionally, increased biotinidase (BTD) enzyme activity has been shown to be associated with many GSD types, whereas the mechanism by which BTD enzyme activity is altered remains unknown so far. We aimed to delineate changes in gluconeogenesis and fatty acid synthesis, potentially explaining raised BTD enzyme activity, by using liver (specimens from 2 patients) and serum samples of GSD Ia and GSD IV patients. By expression analysis of genes involved in gluconeogenesis, we ascertained increased levels of phosphoenolpyruvate carboxykinase and fructose-1,6-biphosphatase, indicating an increased flux through the gluconeogenic pathway. Additionally, we found increased gene expression of the biotin-dependent pyruvate and acetyl-CoA carboxylases, providing substrate for gluconeogenesis and increased fatty acid synthesis. We also observed a significant linear correlation between BTD enzyme activity and triglyceride levels in a cohort of GSD Ia patients. The results of this pilot study suggest that enhancement of BTD activity might serve the purpose of providing extra cofactor to the carboxylase enzymes as an adjustment to disturbed glucose and fatty acid metabolism. Future studies involving a higher number of samples should aim at confirming the here proposed mechanism, which extends the application of BTD enzyme activity measurement beyond its diagnostic purpose in suspected GSD, and opens up possibilities for its use as a sensor for increased gluconeogenesis and fatty acid synthesis.

An early-life diet containing large phospholipid-coated lipid globules programmes later-life postabsorptive lipid trafficking in high-fat diet- but not in low-fat diet-fed mice

Feeding mice in early life a diet containing an experimental infant milk formula (Nuturis®; eIMF), with a lipid structure similar to human milk, transiently lowered body weight (BW) and fat mass gain upon Western-style diet later in life, when compared with mice fed diets based on control IMF (cIMF). We tested the hypothesis that early-life eIMF feeding alters the absorption or the postabsorptive trafficking of dietary lipids in later life. Male C57BL/6JOlaHsd mice were fed eIMF/cIMF from postnatal day 16-42, followed by low- (LFD, American Institute of Nutrition (AIN)-93 G, 7 wt% fat) or high-fat diet (HFD, D12451, 24 wt% fat) until day 63-70. Lipid absorption rate and tissue concentrations were determined after intragastric administration of stable isotope (2H or 13C) labelled lipids in separate groups. Lipid enrichments in plasma and tissues were analysed using GC-MS. The rate of triolein absorption was similar between eIMF and cIMF fed LFD: 3·2 (sd 1·8) and 3·9 (sd 2·1) and HFD: 2·6 (sd 1·7) and 3·8 (sd 3·0) % dose/ml per h. Postabsorptive lipid trafficking, that is, concentrations of absorbed lipids in tissues, was similar in the eIMF and cIMF groups after LFD. Tissue levels of absorbed TAG after HFD feeding were lower in heart (-42 %) and liver (-46 %), and higher in muscle (+81 %, all P < 0·05) in eIMF-fed mice. In conclusion, early-life IMF diet affected postabsorptive trafficking of absorbed lipids after HFD, but not LFD. Changes in postabsorptive lipid trafficking could underlie the observed lower BW and body fat accumulation in later life upon a persistent long-term obesogenic challenge.

Flux analysis of inborn errors of metabolism

Patients with an inborn error of metabolism (IEM) are deficient of an enzyme involved in metabolism, and as a consequence metabolism reprograms itself to reach a new steady state. This new steady state underlies the clinical phenotype associated with the deficiency. Hence, we need to know the flux of metabolites through the different metabolic pathways in this new steady state of the reprogrammed metabolism. Stable isotope technology is best suited to study this. In this review the progress made in characterizing the altered metabolism will be presented. Studies done in patients to estimate the residual flux through the metabolic pathway affected by enzyme deficiencies will be discussed. After this, studies done in model systems will be reviewed. The focus will be on glycogen storage disease type I, medium-chain acyl-CoA dehydrogenase deficiency, propionic and methylmalonic aciduria, urea cycle defects, phenylketonuria, and combined D,L-2-hydroxyglutaric aciduria. Finally, new developments are discussed, which allow the tracing of metabolic reprogramming in IEM on a genome-wide scale. In conclusion, the outlook for flux analysis of metabolic derangement in IEMs looks promising.

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.

Targeting hepatic glucose metabolism in the treatment of type 2 diabetes

Type 2 diabetes mellitus is characterized by the dysregulation of glucose homeostasis, resulting in hyperglycaemia. Although current diabetes treatments have exhibited some success in lowering blood glucose levels, their effect is not always sustained and their use may be associated with undesirable side effects, such as hypoglycaemia. Novel antidiabetic drugs, which may be used in combination with existing therapies, are therefore needed. The potential of specifically targeting the liver to normalize blood glucose levels has not been fully exploited. Here, we review the molecular mechanisms controlling hepatic gluconeogenesis and glycogen storage, and assess the prospect of therapeutically targeting associated pathways to treat type 2 diabetes.

Apigenin Ameliorates Dyslipidemia, Hepatic Steatosis and Insulin Resistance by Modulating Metabolic and Transcriptional Profiles in the Liver of High-Fat Diet-Induced Obese Mice

Several in vitro and in vivo studies have reported the anti-inflammatory, anti-diabetic and anti-obesity effects of the flavonoid apigenin. However, the long-term supplementary effects of low-dose apigenin on obesity are unclear. Therefore, we investigated the protective effects of apigenin against obesity and related metabolic disturbances by exploring the metabolic and transcriptional responses in high-fat diet (HFD)-induced obese mice. C57BL/6J mice were fed an HFD or apigenin (0.005%, w/w)-supplemented HFD for 16 weeks. In HFD-fed mice, apigenin lowered plasma levels of free fatty acid, total cholesterol, apolipoprotein B and hepatic dysfunction markers and ameliorated hepatic steatosis and hepatomegaly, without altering food intake and adiposity. These effects were partly attributed to upregulated expression of genes regulating fatty acid oxidation, tricarboxylic acid cycle, oxidative phosphorylation, electron transport chain and cholesterol homeostasis, downregulated expression of lipolytic and lipogenic genes and decreased activities of enzymes responsible for triglyceride and cholesterol ester synthesis in the liver. Moreover, apigenin lowered plasma levels of pro-inflammatory mediators and fasting blood glucose. The anti-hyperglycemic effect of apigenin appeared to be related to decreased insulin resistance, hyperinsulinemia and hepatic gluconeogenic enzymes activities. Thus, apigenin can ameliorate HFD-induced comorbidities via metabolic and transcriptional modulations in the liver.

Cortisol promotes endoplasmic glucose production via pyridine nucleotide redox

Both increased adrenal and peripheral cortisol production, the latter governed by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), contribute to the maintenance of fasting blood glucose. In the endoplasmic reticulum (ER), the pyridine nucleotide redox state (NADP/NADPH) is dictated by the concentration of glucose-6-phosphate (G6P) and the coordinated activities of two enzymes, hexose-6-phosphate dehydrogenase (H6PDH) and 11β-HSD1. However, luminal G6P may similarly serve as a substrate for hepatic glucose-6-phophatase (G6Pase). A tacit belief is that the G6P pool in the ER is equally accessible to both H6PDH and G6Pase. Based on our inhibition studies and kinetic analysis in isolated rat liver microsomes, these two aforesaid luminal enzymes do share the G6P pool in the ER, but not equally. Based on the kinetic modeling of G6P flux, the ER transporter for G6P (T1) preferentially delivers this substrate to G6Pase; hence, the luminal enzymes do not share G6P equally. Moreover, cortisol, acting through 11β-HSD1, begets a more reduced pyridine redox ratio. By altering this luminal redox ratio, G6P flux through H6PDH is restrained, allowing more G6P for the competing enzyme G6Pase. And, at low G6P concentrations in the ER lumen, which occur during fasting, this acute cortisol-induced redox adjustment promotes glucose production. This reproducible cortisol-driven mechanism has been heretofore unrecognized.

Induction of autophagy improves hepatic lipid metabolism in glucose-6-phosphatase deficiency

BACKGROUND & AIMS

Glucose-6-phosphatase (G6Pase α, G6PC) deficiency, also known as von Gierke's disease or GSDIa, is the most common glycogen storage disorder. It is characterized by a decreased ability of the liver to convert glucose-6-phosphate (G6P) to glucose leading to glycogen and lipid over-accumulation progressing to liver failure and/or hepatomas and carcinomas. Autophagy of intracellular lipid stores (lipophagy) has been shown to stimulate fatty acid β-oxidation in hepatic cells. Thus, we examined autophagy and its effects on reducing hepatic lipid over-accumulation in several cell culture and animal models of GSDIa.

METHODS

Autophagy in G6PC-deficient hepatic cell lines, mice, and dogs was measured by Western blotting for key autophagy markers. Pro-autophagic Unc51-like kinase 1 (ULK1/ATG1) was overexpressed in G6PC-deficient hepatic cells, and lipid clearance and oxidative phosphorylation measured. G6PC(-/-) mice and GSDIa dogs were treated with rapamycin and assessed for liver function.

RESULTS

Autophagy was impaired in the cell culture, mouse, and canine models of GSDIa. Stimulation of the anti-autophagic mTOR, and inhibition of the pro-autophagic AMPK pathways occurred both in vitro and in vivo. Induction of autophagy by ULK1/ATG1 overexpression decreased lipid accumulation and increased oxidative phosphorylation in G6PC-deficient hepatic cells. Rapamycin treatment induced autophagy and decreased hepatic triglyceride and glycogen content in G6PC(-/-) mice, as well as reduced liver size and improved circulating markers of liver damage in GSDIa dogs.

CONCLUSIONS

Autophagy is impaired in GSDIa. Pharmacological induction of autophagy corrects hepatic lipid over-accumulation and may represent a new therapeutic strategy for GSDIa.

A mutant allele encoding DNA binding-deficient FoxO1 differentially regulates hepatic glucose and lipid metabolism

Insulin signaling in the liver blunts glucose production and stimulates triglyceride biosynthesis. FoxO1 is required for cAMP induction of hepatic glucose production and is permissive for the effect of insulin to suppress this process. Moreover, FoxO1 ablation increases lipogenesis. In this study, we investigated the pleiotropic actions of FoxO1 on glucose and lipid metabolism. To this end, we reconstituted FoxO1 function in mice with a liver-specific deletion of Foxo1 using targeted knock-in of an allele encoding a DNA binding-deficient FoxO1 mutant (L-DBD). Chow-reared L-DBD mice showed defects in hepatic glucose production but normal liver triglyceride content despite increased rates of de novo lipogenesis and impaired fatty acid oxidation in isolated hepatocytes. Gene expression studies indicated that FoxO1 regulates the expression of glucokinase via a cell-nonautonomous coregulatory mechanism, while its regulation of glucose-6-phosphatase proceeds via a cell-autonomous action as a direct transcriptional activator. These conclusions support a differential regulation of hepatic glucose and lipid metabolism by FoxO1 based on the mechanism by which it alters the expression of key target genes involved in each process.

Ileal transposition in rats influenced glucose metabolism and HSP70 levels

Objective: Ileal transposition procedure (IT), in combination with sleeve gastrectomy, is widely used to induce diabetes remission and to control related metabolic abnormalities. A transposition of a long segment of distal ileum in obese Zucker rats improved glucose tolerance 6 months after IT. The premise of our study was to to examine the long - term effects of ileum transposition on the liver glycolytic enzymes content in a euglycemic group of operated Zucker rats. Methods: Twenty male Zucker rats underwent either the transposition of 50% distal ileum or a sham surgery. Six months after surgery, liver tissue concentrations of glycogen synthase kinase alpha (GSK-3α), glucose 6-phosphatase (G6PC), glycogen phosphorylase (PYGM) and phosphofructokinase (PFK) and HSP70 were assessed by immunoenzymatic methods. Results: HSP70 values were significantly higher in the IT group compared to SHAM. G6PC liver concentrations in the IT group were almost 1.45-fold lower than in the SHAM operated rats. Statistical analyses (F-test) showed HSP70 levels were significantly related to caveolin-1and SHAM group. Conclusions: Lowered glycolytic enzyme concentrations assessed in the liver suggest positive effects on glucose metabolism in long-term observations.

Pathway-selective insulin resistance and metabolic disease: the importance of nutrient flux

Hepatic glucose and lipid metabolism are altered in metabolic disease (e.g. obesity, metabolic syndrome, and Type 2 diabetes). Insulin-dependent regulation of glucose metabolism is impaired. In contrast, lipogenesis, hypertriglyceridemia, and hepatic steatosis are increased. Because insulin promotes lipogenesis and liver fat accumulation, to explain the elevation in plasma and tissue lipids, investigators have suggested the presence of pathway-selective insulin resistance. In this model, insulin signaling to glucose metabolism is impaired, but insulin signaling to lipid metabolism is intact. We discuss the evidence for the differential regulation of hepatic lipid and glucose metabolism. We suggest that the primary phenotypic driver is altered substrate delivery to the liver, as well as the repartitioning of hepatic nutrient handling. Specific alterations in insulin signaling serve to amplify the alterations in hepatic substrate metabolism. Thus, hyperinsulinemia and its resultant increased signaling may facilitate lipogenesis, but are not the major drivers of the phenotype of pathway-selective insulin resistance.

Hepatic glucose sensing and integrative pathways in the liver

The hepatic glucose-sensing system is a functional network of enzymes and transcription factors that is critical for the maintenance of energy homeostasis and systemic glycemia. Here we review the recent literature on its components and metabolic actions. Glucokinase (GCK) is generally considered as the initial postprandial glucose-sensing component, which acts as the gatekeeper for hepatic glucose metabolism and provides metabolites that activate the transcription factor carbohydrate response element binding protein (ChREBP). Recently, liver receptor homolog 1 (LRH-1) has emerged as an upstream regulator of the central GCK-ChREBP axis, with a critical role in the integration of hepatic intermediary metabolism in response to glucose. Evidence is also accumulating that O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) and acetylation can act as glucose-sensitive modifications that may contribute to hepatic glucose sensing by targeting regulatory proteins and the epigenome. Further elucidation of the components and functional roles of the hepatic glucose-sensing system may contribute to the future treatment of liver diseases associated with deregulated glucose sensors.

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.

Overview of food products and dietary constituents with antidiabetic properties and their putative mechanisms of action: a natural approach to complement pharmacotherapy in the management of diabetes

Diabetes is one of the fastest growing chronic, noncommunicable diseases worldwide. Currently, 11 major classes of pharmacotherapy are available for the management of this metabolic disorder. However, the usage of these drugs is often associated with undesirable side effects, including weight gain and hypoglycemia. There is thus a need for new, safe and effective treatment strategies. Diet is known to play a major role in the prevention and management of diabetes. Numerous studies have reported the putative association of the consumption of specific food products, or their constituents, with the incidence of diabetes, and mounting evidence now suggests that some dietary factors can improve glycemic regulation. Foods and dietary constituents, similar to synthetic drugs, have been shown to modulate hormones, enzymes, and organ systems involved in carbohydrate metabolism. The present article reviews the major classes and modes of action of antidiabetic drugs, and examines the evidence on food products and dietary factors with antidiabetic properties as well as their plausible mechanisms of action. The findings suggest potential use of dietary constituents as a complementary approach to pharmacotherapy in the prevention and/or management of diabetes, but further research is necessary to identify the active components and evaluate their efficacy and safety.

Targeting pyruvate carboxylase reduces gluconeogenesis and adiposity and improves insulin resistance

We measured the mRNA and protein expression of the key gluconeogenic enzymes in human liver biopsy specimens and found that only hepatic pyruvate carboxylase protein levels related strongly with glycemia. We assessed the role of pyruvate carboxylase in regulating glucose and lipid metabolism in rats through a loss-of-function approach using a specific antisense oligonucleotide (ASO) to decrease expression predominantly in liver and adipose tissue. Pyruvate carboxylase ASO reduced plasma glucose concentrations and the rate of endogenous glucose production in vivo. Interestingly, pyruvate carboxylase ASO also reduced adiposity, plasma lipid concentrations, and hepatic steatosis in high fat-fed rats and improved hepatic insulin sensitivity. Pyruvate carboxylase ASO had similar effects in Zucker Diabetic Fatty rats. Pyruvate carboxylase ASO did not alter de novo fatty acid synthesis, lipolysis, or hepatocyte fatty acid oxidation. In contrast, the lipid phenotype was attributed to a decrease in hepatic and adipose glycerol synthesis, which is important for fatty acid esterification when dietary fat is in excess. Tissue-specific inhibition of pyruvate carboxylase is a potential therapeutic approach for nonalcoholic fatty liver disease, hepatic insulin resistance, and type 2 diabetes.

Noninvasive measurement of murine hepatic acetyl-CoA ¹³C-enrichment following overnight feeding with ¹³C-enriched fructose and glucose

The ¹³C-isotopomer enrichment of hepatic cytosolic acetyl-CoA of overnight-fed mice whose drinking water was supplemented with [U-¹³C]fructose, and [1-¹³C]glucose and p-amino benzoic acid (PABA) was quantified by ¹³C NMR analysis of urinary N-acetyl-PABA. Four mice were given normal chow plus drinking water supplemented with 5% [1-¹³C]glucose, 2.5% [U-¹³C]fructose, and 2.5% fructose (Solution 1) overnight. Four were given chow and water containing 17.5% [1-¹³C]glucose, 8.75% [U-¹³C]fructose and 8.75% fructose (Solution 2). PABA (0.25%) was present in both studies. Urinary N-acetyl-PABA was analyzed by ¹³C NMR. In addition to [2-¹³C]- and [1,2-¹³C]acetyl isotopomers from catabolism of [U-¹³C]fructose and [1-¹³C]glucose to acetyl-CoA, [1-¹³C]acetyl was also found indicating pyruvate recycling activity. This precluded precise estimates of [1-¹³C]glucose contribution to acetyl-CoA while that of [U-¹³C]fructose was unaffected. The fructose contribution to acetyl-CoA from Solutions 1 and 2 was 4.0 ± 0.4% and 10.6 ± 0.6%, respectively, indicating that it contributed to a minor fraction of lipogenic acetyl-CoA under these conditions.

Downregulation of hepatic glucose-6-phosphatase-α in patients with hepatic steatosis

Glucose-6-phosphate transporter (G6PT) and microsomal glucose-6-phosphatase-α (G6Pase-α) perform the terminal step in glycogenolysis and gluconeogenesis. Deficiency of these proteins leads to glycogen storage diseases. Partial inhibition of G6Pase in rats results in increased hepatic triglyceride content and de novo lipogenesis leading to hepatic steatosis. Hepatic steatosis represents hepatic manifestation of the metabolic syndrome. We investigated molecular mechanisms that may explain the relationship between fatty liver and G6Pase-α in humans in detail. A total of 27 patients (11 men, 16 women) underwent liver biopsy. Histological diagnosis identified nonfatty liver in seven patients and nonalcoholic fatty liver in 20 patients. We quantified G6Pase-α and G6PT mRNA expression by real-time PCR. Anthropometric measurements and analysis of plasma lipids and liver enzymes were performed. Patients with fatty liver showed no significant differences in age, HOMA(IR) (homeostasis model assessment of insulin resistance), BMI, liver enzymes or waist-to-hip ratio compared to those with nonfatty liver, but total plasma cholesterol levels and liver fat content were higher in patients with fatty liver (P < 0.05). G6Pase-α and G6PT mRNA expressions were significantly downregulated in fatty compared to histologically normal liver (P < 0.05). G6Pase-α and G6PT mRNA expressions correlated positively (R(2) = 0.406 P < 0.05). Both expressions did not correlate with age, BMI, aspartate transaminase, alanine transaminase, alkaline phosphatase, γ-glutamyl transferase, triglycerides or glucose levels. Our data suggest that expression of hepatic G6Pase-α and G6PT are closely interlinked. Downregulation of G6Pase-α in fatty liver might be associated with hepatic fat accumulation and pathogenesis of hepatic steatosis.

Effect of liver X receptor activation on the very low density lipoprotein secretion and messenger ribonucleic acid level of related genes in goose primary hepatocytes

In this study, we investigated the role of liver X receptor (LXR) activation in hepatic assembly and in the secretion of very low density lipoprotein-triglycerides in goose primary hepatocytes. Goose primary hepatocytes were isolated and treated with the LXR agonist T0901317. Total triglyceride accumulation, intracellular and extracellular triglyceride concentrations, extracellular very low density lipoprotein concentration, and gene expression levels of LXRα, microsomal triglyceride transfer protein, acyl coenzyme A:diacylglycerol acyltransferase (DGAT) 1, and DGAT2 were measured in primary hepatocytes. We found a dose-dependent upregulation of total and intracellular TG accumulation when using 0, 0.01, 0.1, 1, and 10 μM T0901317, but the extracellular triglyceride and very low density lipoprotein concentrations were dose dependent only when the T0901317 concentration was below 1 μM; as compared with 1 μM T0901317, 10 μM T0901317 had an inhibiting effect (P < 0.05). The mRNA levels of all the detected genes increased in the presence of T0901317. The change in LXRα and DGAT1 was dose dependent, and the mRNA levels of microsomal triglyceride transfer protein and DGAT2 increased with a T0901317 concentration up to 1 μM, but decreased when treated with 10 μM T0901317 (P < 0.05). In conclusion, the secretion of very low density lipoprotein plays a role in pharmacologically activating the LXR-induced development of hepatocellular steatosis in geese.

Carbohydrate-response-element-binding protein (ChREBP) and not the liver X receptor α (LXRα) mediates elevated hepatic lipogenic gene expression in a mouse model of glycogen storage disease type 1

GSD-1 (glycogen storage disease type 1) is caused by an inherited defect in glucose-6-phosphatase activity, resulting in a massive accumulation of hepatic glycogen content and an induction of de novo lipogenesis. The chlorogenic acid derivative S4048 is a pharmacological inhibitor of the glucose 6-phosphate transporter, which is part of glucose-6-phosphatase, and allows for mechanistic studies concerning metabolic defects in GSD-1. Treatment of mice with S4048 resulted in an ~60% reduction in blood glucose, increased hepatic glycogen and triacylglycerol (triglyceride) content, and a markedly enhanced hepatic lipogenic gene expression. In mammals, hepatic expression of lipogenic genes is regulated by the co-ordinated action of the transcription factors SREBP (sterol-regulatory-element-binding protein)-1c, LXRα (liver X receptor α) and ChREBP (carbohydrate-response-element-binding protein). Treatment of Lxra-/- mice and Chrebp-/- mice with S4048 demonstrated that ChREBP, but not LXRα, mediates the induction of hepatic lipogenic gene expression in this murine model of GSD-1. Thus ChREBP is an attractive target to alleviate derangements in lipid metabolism observed in patients with GSD-1.

Mechanisms underlying different responses of plasma triglyceride to high-fat diets in hamsters and mice: roles of hepatic MTP and triglyceride secretion

Hypertriglyceridemia, closely associated with insulin resistance, is induced on high-fat diets (HFD) in humans but not in mouse models. Mechanisms underlying this species difference are still unclear. Hamsters resemble humans in lipoprotein metabolism. Here by comparing the responses to HFD in hamsters and mice, we found that hepatic TG secretion, MTP expression and plasma free fatty acid (FFA) level were increased in hamsters on HFD feeding but decreased in mice. Although hepatic steatosis and de novo lipogenesis were induced by HFD feeding in both models, cholesterol biosynthesis was inhibited in mice but not in hamsters. Moreover, in insulin deficient state, HFD increased plasma TG level, hepatic TG secretion, MTP expression and plasma FFA level in both models. In summary, distinct changes of MTP expression, in correlation with hepatic TG secretion, underlie the opposite responses of plasma TG levels to high-fat diets in hamsters and mice. Furthermore, hepatic TG secretion and MTP expression seems to be associated with plasma FFA level and cholesterol biosynthesis but not hepatic steatosis or de novo lipogenesis.

High fat feeding induces hepatic fatty acid elongation in mice

Background

High-fat diets promote hepatic lipid accumulation. Paradoxically, these diets also induce lipogenic gene expression in rodent liver. Whether high expression of these genes actually results in an increased flux through the de novo lipogenic pathway in vivo has not been demonstrated.

Methodology/Principal Findings

To interrogate this apparent paradox, we have quantified de novo lipogenesis in C57Bl/6J mice fed either chow, a high-fat or a n-3 polyunsaturated fatty acid (PUFA)-enriched high-fat diet. A novel approach based on mass isotopomer distribution analysis (MIDA) following 1-¹³C acetate infusion was applied to simultaneously determine de novo lipogenesis, fatty acid elongation as well as cholesterol synthesis. Furthermore, we measured very low density lipoprotein-triglyceride (VLDL-TG) production rates. High-fat feeding promoted hepatic lipid accumulation and induced the expression of lipogenic and cholesterogenic genes compared to chow-fed mice: induction of gene expression was found to translate into increased oleate synthesis. Interestingly, this higher lipogenic flux (+74 µg/g/h for oleic acid) in mice fed the high-fat diet was mainly due to an increased hepatic elongation of unlabeled palmitate (+66 µg/g/h) rather than to elongation of de novo synthesized palmitate. In addition, fractional cholesterol synthesis was increased, i.e. 5.8±0.4% vs. 8.1±0.6% for control and high fat-fed animals, respectively. Hepatic VLDL-TG production was not affected by high-fat feeding. Partial replacement of saturated fat by fish oil completely reversed the lipogenic effects of high-fat feeding: hepatic lipogenic and cholesterogenic gene expression levels as well as fatty acid and cholesterol synthesis rates were normalized.

Conclusions/Significance

High-fat feeding induces hepatic fatty acid synthesis in mice, by chain elongation and subsequent desaturation rather than de novo synthesis, while VLDL-TG output remains unaffected. Suppression of lipogenic fluxes by fish oil prevents from high fat diet-induced hepatic steatosis in mice.

Role of bile acids and bile acid receptors in metabolic regulation

The incidence of the metabolic syndrome has taken epidemic proportions in the past decades, contributing to an increased risk of cardiovascular disease and diabetes. The metabolic syndrome can be defined as a cluster of cardiovascular disease risk factors including visceral obesity, insulin resistance, dyslipidemia, increased blood pressure, and hypercoagulability. The farnesoid X receptor (FXR) belongs to the superfamily of ligand-activated nuclear receptor transcription factors. FXR is activated by bile acids, and FXR-deficient (FXR(-/-)) mice display elevated serum levels of triglycerides and high-density lipoprotein cholesterol, demonstrating a critical role of FXR in lipid metabolism. In an opposite manner, activation of FXR by bile acids (BAs) or nonsteroidal synthetic FXR agonists lowers plasma triglycerides by a mechanism that may involve the repression of hepatic SREBP-1c expression and/or the modulation of glucose-induced lipogenic genes. A cross-talk between BA and glucose metabolism was recently identified, implicating both FXR-dependent and FXR-independent pathways. The first indication for a potential role of FXR in diabetes came from the observation that hepatic FXR expression is reduced in animal models of diabetes. While FXR(-/-) mice display both impaired glucose tolerance and decreased insulin sensitivity, activation of FXR improves hyperglycemia and dyslipidemia in vivo in diabetic mice. Finally, a recent report also indicates that BA may regulate energy expenditure in a FXR-independent manner in mice, via activation of the G protein-coupled receptor TGR5. Taken together, these findings suggest that modulation of FXR activity and BA metabolism may open new attractive pharmacological approaches for the treatment of the metabolic syndrome and type 2 diabetes.

Transcriptome analysis of a spontaneous mutant in sweet orange [Citrus sinensis (L.) Osbeck] during fruit development

Bud mutations often arise in citrus. The selection of mutants is one of the most important breeding channels in citrus. However, the molecular basis of bud mutation has rarely been studied. To identify differentially expressed genes in a spontaneous sweet orange [C. sinensis (L.) Osbeck] bud mutation which causes lycopene accumulation, low citric acid, and high sucrose in fruit, suppression subtractive hybridization and microarray analysis were performed to decipher this bud mutation during fruit development. After sequencing of the differentially expressed clones, a total of 267 non-redundant transcripts were obtained and 182 (68.2%) of them shared homology (E-value < or = 1x10(-10)) with known gene products. Few genes were constitutively up- or down-regulated (fold change > or = 2) in the bud mutation during fruit development. Self-organizing tree algorithm analysis results showed that 95.1% of the differentially expressed genes were extensively coordinated with the initiation of lycopene accumulation. Metabolic process, cellular process, establishment of localization, response to stimulus, and biological regulation-related transcripts were among the most regulated genes. These genes were involved in many biological processes such as organic acid metabolism, lipid metabolism, transport, and pyruvate metabolism, etc. Moreover, 13 genes which were differentially regulated at 170 d after flowering shared homology with previously described signal transduction or transcription factors. The information generated in this study provides new clues to aid in the understanding of bud mutation in citrus.

Liver lipid metabolism

The liver plays a key role in lipid metabolism. Depending on species it is, more or less, the hub of fatty acid synthesis and lipid circulation through lipoprotein synthesis. Eventually the accumulation of lipid droplets into the hepatocytes results in hepatic steatosis, which may develop as a consequence of multiple dysfunctions such as alterations in beta-oxidation, very low density lipoprotein secretion, and pathways involved in the synthesis of fatty acids. In addition an increased circulating pool of non-esterified fatty acid may also to be a major determinant in the pathogenesis fatty liver disease. This review also focuses on transcription factors such as sterol-regulatory-element-binding protein-1c and peroxisome proliferator-activated receptor alpha, which promote either hepatic fatty acid synthesis or oxidation.

Increased de novo lipogenesis and delayed conversion of large VLDL into intermediate density lipoprotein particles contribute to hyperlipidemia in glycogen storage disease type 1a

Glycogen storage disease type 1a (GSD-1a) is a metabolic disorder characterized by fasting-induced hypoglycemia, hepatic steatosis, and hyperlipidemia. The mechanisms underlying the lipid abnormalities are largely unknown. To investigate these mechanisms seven GSD-1a patients and four healthy control subjects received an infusion of [1-(13)C]acetate to quantify cholesterogenesis and lipogenesis. In a subset of patients, [1-(13)C]valine was given to assess lipoprotein metabolism and [2-(13)C]glycerol to determine whole body lipolysis. Cholesterogenesis was 274 +/- 112 mg/d in controls and 641 +/- 201 mg/d in GSD-1a patients (p < 0.01). Plasma triglyceride-palmitate derived from de novo lipogenesis was 7.1 +/- 9.4 and 86.3 +/- 42.5 micromol/h in controls and patients, respectively (p < 0.01). Production of VLDL did not show a consistent difference between the groups, but conversion of VLDL into intermediate density lipoproteins was relatively retarded in all patients (0.6 +/- 0.5 pools/d) compared with controls (4.3 +/- 1.8 pools/d). Fractional catabolic rate of intermediate density lipoproteins was lower in patients (0.8 +/- 0.6 pools/d) compared with controls (3.1 +/- 1.5 pools/d). Whole body lipolysis was similar, i.e., 4.5 +/- 1.9 micromol/kg/min in patients and 3.8 +/- 1.9 micromol/kg/min in controls. Hyperlipidemia in GSD-1a is associated with strongly increased lipid production and a slower relative conversion of VLDL to LDL.

New hepatic targets for glycaemic control in diabetes

Type-2 diabetes is associated with impaired glucose clearance by the liver in the postprandial state, and with elevated glucose production in the post-absorptive state. New targets within the liver are currently being investigated for development of antihyperglycaemic drugs for type-2 diabetes. They include glucokinase, which catalyses the first step in glucose metabolism, the glucagon receptor, and enzymes of gluconeogenesis and/or glycogenolysis such as glucose 6-phosphatase, fructose 1,6-bisphosphatase and glycogen phosphorylase. Preclinical studies with candidate drugs on animal models or cell-based assays suggest that these targets have the potential for pharmacological glycaemic control. Data from clinical studies is awaited. Further work is required for better understanding of the implications of targeting these sites in terms of possible side-effects or tachyphylaxis. The advantage of combined targeting of two or more sites within the liver for minimizing side-effects and tachyphylaxis caused by single-site targeting is discussed.

Characterization of hepatic and brain metabolism in young adults with glycogen storage disease type 1: a magnetic resonance spectroscopy study

In glycogen storage disease type 1 (GSD1), children present with severe hypoglycemia, whereas the propensity for hypoglycemia may decrease with age in these patients. It was the aim of this study to elucidate the mechanisms for milder hypoglycemia symptoms in young adult GSD1 patients. Four patients with GSD1 [body mass index (BMI) 23.2 +/- 6.3 kg/m, age 21.3 +/- 2.9 yr] and four healthy controls matched for BMI (23.1 +/- 3.0 kg/m) and age (24.0 +/- 3.1 yr) were studied. Combined (1)H/(31)P nuclear magnetic resonance spectroscopy (NMRS) was used to assess brain metabolism. Before and after administration of 1 mg glucagon, endogenous glucose production (EGP) was measured with d-[6,6-(2)H(2)]glucose and hepatic glucose metabolism was examined by (1)H/(13)C/(31)P NMRS. At baseline, GSD1 patients exhibited significantly lower rates of EGP (0.53 +/- 0.04 vs. 1.74 +/- 0.03 mg.kg(-1).min(-1); P < 0.01) but an increased intrahepatic glycogen (502 +/- 89 vs. 236 +/- 11 mmol/l; P = 0.05) and lipid content (16.3 +/- 1.1 vs. 1.4 +/- 0.4%; P < 0.001). After glucagon challenge, EGP did not change in GSD1 patients (0.53 +/- 0.04 vs. 0.59 +/- 0.24 mg.kg(-1).min(-1); P = not significant) but increased in healthy controls (1.74 +/- 0.03 vs. 3.95 +/- 1.34; P < 0.0001). In GSD1 patients, we found an exaggerated increase of intrahepatic phosphomonoesters (0.23 +/- 0.08 vs. 0.86 +/- 0.19 arbitrary units; P < 0.001), whereas inorganic phosphate decreased (0.36 +/- 0.08 vs. -0.43 +/- 0.17 arbitrary units; P < 0.01). Intracerebral ratios of glucose and lactate to creatine were higher in GSD1 patients (P < 0.05 vs. control). Therefore, hepatic defects of glucose metabolism persist in young adult GSD1 patients. Upregulation of the glucose and lactate transport at the blood-brain barrier could be responsible for the amelioration of hypoglycemic symptoms.

Bile acids, farnesoid X receptor, atherosclerosis and metabolic control

PURPOSE OF REVIEW

Bile acids are amphiphilic molecules synthesized from cholesterol exclusively in the liver that are essential for effective absorption of dietary fat. In addition to this 'classical role', bile acids act as signalling molecules that control their own metabolism by activating the nuclear receptor, farnesoid X receptor.

RECENT FINDINGS

Recent work demonstrates that farnesoid X receptor exerts metabolic control beyond bile acid homeostasis, notably effects on HDL, triglyceride and glucose metabolism. Farnesoid X receptor influences insulin sensitivity of tissues that are not part of the enterohepatic circulation, for example, adipose tissue. Certain metabolic effects in the liver appear to be mediated via farnesoid X receptor-stimulated release of an intestinal growth factor. In addition, novel signalling pathways independent of farnesoid X receptor have been identified that may contribute to bile acid-mediated metabolic regulation.

SUMMARY

Farnesoid X receptor represents a potentially attractive target for treatment of various aspects of the metabolic syndrome and for prevention of atherosclerosis. Yet, in view of its pleiotropic effects and apparent species-specificity, it is evident that successful interference of the farnesoid X receptor signalling system will require the development of gene-specific and/or organ-specific farnesoid X receptor modulators and extensive testing in human models of disease.

Metabolic characterisation of plasma in juveniles with glycogen storage disease type 1a (GSD1a) by high-resolution (1)H NMR spectroscopy

This paper reports the first application of high-resolution (1)H NMR spectroscopy to the plasma of five juveniles with glycogen storage disease type 1a (GSD1a), permitting the characterisation of the plasma metabolic profile and the identification of alterations relative to a set of control samples. The relaxation-weighted spectra allowed changes in low molecular weight compounds to be detected more clearly, whereas diffusion-edited spectra were used to characterise the plasma lipoprotein profile. Low molecular weight metabolites with altered levels in most patients were lactate, ketone bodies, acetate, creatine/creatinine and glucose. One of the patients showed distinctively lower glucose levels and higher lactate and ketone body contents, suggesting poorer metabolic control of the disease compared with other patients. In addition, a metabolite tentatively identified as alpha-hydroxyisobutyrate was only detected in the spectra of GSD1a plasmas, representing, therefore, a possible novel GSD1a biomarker. Total lipoprotein contents were higher in the plasma from GSD1a patients. Furthermore, lower HDL and higher VLDL + LDL levels also characterised the plasma of these patients. Preliminary results on principal component analysis of (1)H NMR spectra allowed a clear separation between GSD1a and control plasmas. The specificity of the changes observed to GSD1a is discussed, together with the recognised potential of NMR and pattern recognition methods for aiding the diagnosis of GSD1a.

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.

HNF1alpha inactivation promotes lipogenesis in human hepatocellular adenoma independently of SREBP-1 and carbohydrate-response element-binding protein (ChREBP) activation

Biallelic inactivating mutations of the transcription factor 1 gene (TCF1), encoding hepatocyte nuclear factor 1alpha (HNF1alpha) were identified in 50% of hepatocellular adenomas (HCA) phenotypically characterized by a striking steatosis. To understand the molecular basis of this aberrant lipid storage, we performed a microarray transcriptome analysis validated by quantitative reverse transcription-PCR, Western blotting, and lipid profiling. In mutated HCA, we showed a repression of gluconeogenesis coordinated with an activation of glycolysis, citrate shuttle, and fatty acid synthesis predicting elevated rates of lipogenesis. Moreover, the strong down-regulation of liver fatty acid-binding protein suggests that impaired fatty acid trafficking may also contribute to the fatty phenotype. In addition, transcriptional profile analysis of the observed deregulated genes in non-HNF1alpha-mutated HCA as well as in non-tumor livers allowed us to define a specific signature of the HNF1alpha-mutated HCA. In these tumors, lipid composition was dramatically modified according to the transcriptional deregulations identified in the fatty acid synthetic pathway. Surprisingly, lipogenesis activation did not operate through sterol regulatory element-binding protein-1 (SREBP-1) and carbohydrate-response element-binding protein (ChREBP) that were repressed. We conclude that steatosis in HNF1alpha-mutated HCA results mainly from an aberrant promotion of lipogenesis that is linked to HNF1alpha inactivation and that is independent of both SREBP-1 and ChREBP activation. Finally, our findings have potential clinical implications since lipogenesis can be efficiently inhibited by targeted therapies.

Deletion of NEMO/IKKgamma in liver parenchymal cells causes steatohepatitis and hepatocellular carcinoma

The IkappaB kinase (IKK) subunit NEMO/IKKgamma is essential for activation of the transcription factor NF-kappaB, which regulates cellular responses to inflammation. The function of NEMO in the adult liver remains elusive. Here we show that ablation of NEMO in liver parenchymal cells caused the spontaneous development of hepatocellular carcinoma in mice. Tumor development was preceded by chronic liver disease resembling human nonalcoholic steatohepatitis (NASH). Antioxidant treatment and genetic ablation of FADD demonstrated that death receptor-mediated and oxidative stress-dependent death of NEMO-deficient hepatocytes triggered disease pathogenesis in this model. These results reveal that NEMO-mediated NF-kappaB activation in hepatocytes has an essential physiological function to prevent the spontaneous development of steatohepatitis and hepatocellular carcinoma, identifying NEMO as a tumor suppressor in the liver.

Amelioration of glucose tolerance by hepatic inhibition of nuclear factor kappaB in db/db mice

AIMS/HYPOTHESIS

Recent studies have identified the involvement of inhibitor IkappaB kinase (IKK) in the pathogenesis of insulin resistance. To investigate the mechanism involved, we examined the role of nuclear factor kappaB (NF-kappaB), the distal target of IKK, in hepatic glucose metabolism.

METHODS

To inhibit NF-kappaB activity, db/db mice were infected with adenovirus expressing the IkappaBalpha super-repressor.

RESULTS

The IkappaBalpha super-repressor adenovirus infection caused a moderate reduction of NF-kappaB activity in liver. The treatment was associated with improved glucose tolerance, reduction in the serum insulin level, and increased hepatic triacylglycerol and glycogen contents, but had no effect on insulin-stimulated phosphorylation of Akt. On the other hand, quantification of mRNA in the liver revealed marked reduction of expression of gluconeogenic genes, such as those encoding phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase, concurrent with reduced expression of gene encoding peroxisome proliferator-activated receptor gamma coactivator-1alpha (PPARGC1A, also known as PGC-1alpha). Furthermore, the production of super-repressor IkappaBalpha suppressed the increase in blood glucose level after pyruvate injection.

CONCLUSIONS/INTERPRETATION

Our results indicate that moderate inhibition of NF-kappaB improved glucose tolerance through decreased gluconeogenesis associated with reduced PGC-1alpha gene expression in db/db mice, and suggest that inhibition of NF-kappaB activity in liver is a potentially suitable strategy for the normalisation of blood glucose concentration in type 2 diabetes.

Gene expression profiles of hepatic cell-type specific marker genes in progression of liver fibrosis

AIM: To determine the gene expression profile data for the whole liver during development of dimethylni-trosamine (DMN)-induced hepatic fibrosis.

METHODS: Marker genes were identified for different types of hepatic cells, including hepatic stellate cells (HSCs), Kupffer cells (including other inflammatory cells), and hepatocytes, using independent temporal DNA microarray data obtained from isolated hepatic cells.

RESULTS: The cell-type analysis of gene expression gave several key results and led to formation of three hypotheses: (1) changes in the expression of HSC-specific marker genes during fibrosis were similar to gene expression data in in vitro cultured HSCs, suggesting a major role of the self-activating characteristics of HSCs in formation of fibrosis; (2) expression of mast cell-specific marker genes reached a peak during liver fibrosis, suggesting a possible role of mast cells in formation of fibrosis; and (3) abnormal expression of hepatocyte-specific marker genes was found across several metabolic pathways during fibrosis, including sulfur-containing amino acid metabolism, fatty acid metabolism, and drug metabolism, suggesting a mechanistic relationship between these abnormalities and symptoms of liver fibrosis.

CONCLUSION: Analysis of marker genes for specific hepatic cell types can identify the key aspects of fibrogenesis. Sequential activation of inflammatory cells and the self-supporting properties of HSCs play an important role in development of fibrosis.

Critical role of stearoyl-CoA desaturase-1 (SCD1) in the onset of diet-induced hepatic insulin resistance

Stearoyl-CoA desaturase-1 (SCD1) catalyzes the synthesis of monounsaturated fatty acids from saturated fatty acids. Mice with a targeted disruption of Scd1 gene locus are lean and display increased insulin sensitivity. To examine whether Scd1 activity is required for the development of diet-induced hepatic insulin resistance, we used a sequence-specific antisense oligodeoxynucleotide (ASO) to lower hepatic Scd1 expression in rats and mice with diet-induced insulin resistance. Treatment of rats with Scd1 ASO markedly decreased liver Scd1 expression (approximately 80%) and total Scd activity (approximately 50%) compared with that in rats treated with scrambled ASO (control). Insulin clamp studies revealed severe hepatic insulin resistance in high-fat-fed rats and mice that was completely reversed by 5 days of treatment with Scd1 ASO. The latter treatment decreased glucose production (by approximately 75%), gluconeogenesis, and glycogenolysis. Downregulation of Scd1 also led to increased Akt phosphorylation and marked decreases in the expression of glucose-6-phosphatase (Glc-6-Pase) and phosphoenolpyruvate carboxykinase (PEPCK). Thus, Scd1 is required for the onset of diet-induced hepatic insulin resistance.

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)-alpha but expressed G6Pase-beta 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.

Acute hepatic steatosis in mice by blocking beta-oxidation does not reduce insulin sensitivity of very-low-density lipoprotein production

Accumulation of triglycerides (TG) in the liver is generally associated with hepatic insulin resistance. We questioned whether acute hepatic steatosis induced by pharmacological blockade of beta-oxidation affects hepatic insulin sensitivity, i.e., insulin-mediated suppression of VLDL production and insulin-induced activation of phosphatidylinositol 3-kinase (PI3-kinase) and PKB. Tetradecylglycidic acid (TDGA), an inhibitor of carnitine palmitoyl transferase-1 (CPT1), was used for this purpose. Male C57BL/6J mice received 30 mg/kg TDGA or its solvent intraperitoneally and were subsequently fasted for 12 h. CPT1 inhibition resulted in severe microvesicular hepatic steatosis (19.9 +/- 8.3 vs. 112.4 +/- 25.2 nmol TG/mg liver, control vs. treated, P < 0.05) with elevated plasma nonesterified fatty acid (0.68 +/- 0.25 vs. 1.21 +/- 0.41 mM, P < 0.05) and plasma TG (0.39 +/- 0.16 vs. 0.60 +/- 0.10 mM, P < 0.05) concentrations. VLDL-TG production rate was not affected on CPT1 inhibition (74.9 +/- 15.2 vs. 79.1 +/- 12.8 mumol TG.kg(-1).min(-1), control vs. treated) although treated mice secreted larger VLDL particles (59.3 +/- 3.6 vs. 66.6 +/- 4.5 nm diameter, P < 0.05). Infusion of insulin under euglycemic conditions suppressed VLDL production rate in control and treated mice by 43 and 54%, respectively, with formation of smaller VLDL particles (51.2 +/- 2.5 and 53.2 +/- 2.8 nm diameter). Insulin-induced insulin receptor substrate (IRS)1- and IRS2-associated PI3-kinase activity and PKB-phosphorylation were not affected on TDGA treatment. In conclusion, acute hepatic steatosis caused by pharmacological inhibition of beta-oxidation is not associated with reduced hepatic insulin sensitivity, indicating that hepatocellular fat content per se is not causally related to insulin resistance.

Intestinal glucose-dependent expression of glucose-6-phosphatase: involvement of the aryl receptor nuclear translocator transcription factor

Glucose-6-phosphatase (G6Pase) catalyzes the release of glucose from glucose 6-phosphate. This enzyme was mainly studied in the liver, but while detected in the small intestine little is known about the regulation of its intestinal expression. This study describes the mechanisms of the glucose-dependent regulation of G6Pase expression in intestinal cells. Results obtained in vivo and in Caco-2/TC7 enterocytes showed that glucose increases the G6Pase mRNA level. In Caco-2/TC7 cells, glucose stabilized G6Pase mRNA and activated the transcription of the gene, meaning that glucose-dependent G6Pase expression involved both transcriptional and post-transcriptional mechanisms. Reporter-gene studies showed that, although the -299/+57 region of the human G6Pase promoter was sufficient to trigger the glucose response in the hepatoma cell line HepG2, the -1157/-1133 fragment was required for maximal activation of glucose-6-phosphatase gene transcription in Caco-2/TC7 cells. This fragment binds the aryl receptor nuclear translocator (ARNT), cAMP-responsive element-binding protein, and upstream stimulatory factor transcription factors. The DNA binding activity of these transcription factors was increased in nuclear extracts of differentiated cells from the intestinal villus of mice fed sugar-rich diets as compared with mice fed a no-sugar diet. A direct implication of ARNT in the activation of G6Pase gene transcription by glucose has been observed in Caco-2/TC7 cells using RNA interference experiments. These results support a physiological role for G6Pase in the control of nutrient absorption in the small intestine.

Hepatic steatosis: a mediator of the metabolic syndrome. Lessons from animal models

Epidemiological studies in humans, as well as experimental studies in animal models, have shown an association between visceral obesity and dyslipidemia, insulin resistance, and type 2 diabetes mellitus. Recently, attention has been focused on the excessive accumulation of triglycerides (TG) in the liver as part of this syndrome. In this review, important principles of the pathophysiological involvement of the liver in the metabolic syndrome obtained in rodent models are summarized. We focus on non-alcoholic causes of steatosis, because the animal experiments we refer to did not include alcohol as an experimental condition. In general, there is continuous cycling and redistribution of non-oxidized fatty acids between different organs. The amount of TG in an intrinsically normal liver is not fixed but can readily be increased by nutritional, metabolic, and endocrine interactions involving TG/free fatty acid (FFA) partitioning and TG/FFA metabolism. Several lines of evidence indicate that hepatic TG accumulation is also a causative factor involved in hepatic insulin resistance. Complex interactions between endocrine, metabolic, and transcriptional pathways are involved in TG-induced hepatic insulin resistance. Therefore, the liver participates passively and actively in the metabolic derangements of the metabolic syndrome. We speculate that similar mechanisms may also be involved in human pathophysiology.

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.

Stimulation of lipogenesis by pharmacological activation of the liver X receptor leads to production of large, triglyceride-rich very low density lipoprotein particles

The oxysterol-activated liver X receptor (LXR) provides a link between sterol and fatty acid metabolism; activation of LXR induces transcription of lipogenic genes. This study shows that induction of the lipogenic genes Srebp-1c, Fas, and Acc1 upon administration of the synthetic LXR agonist T0901317 to C57BL/6J mice (10 mg/kg/day, 4 days) is associated with massive hepatic steatosis along the entire liver lobule and a 2.5-fold increase in very low density lipoprotein-triglyceride (VLDL-TG) secretion. The increased VLDL-TG secretion was fully accounted for by formation of larger (129 +/- 9 nm versus 94 +/- 12 nm, a 2.5-fold increase of particle volume) TG-rich particles. Stimulation of VLDL-TG secretion did not lead to elevated plasma TG levels in C57BL/6J mice, indicating efficient particle metabolism and clearance. However, T0901317 treatment did lead to severe hypertriglyceridemia in mouse models of defective TG-rich lipoprotein clearance, i.e. APOE*3-Leiden transgenic mice (3.2-fold increase) and apoE-/- LDLr-/- double knockouts (12-fold increase). Incubation of rat hepatoma McA-RH7777 cells with T0901317 also resulted in intracellular TG accumulation and enhanced TG secretion. We conclude that, in addition to raising high density lipoprotein cholesterol concentrations, pharmacological LXR activation in mice leads to development of hepatic steatosis and secretion of atherogenic, large TG-rich VLDL particles.