PTP1B deficiency increases glucose uptake in neonatal hepatocytes: involvement of IRA/GLUT2 complexes

Fanconi-Bickel Syndrome: A Review of the Mechanisms That Lead to Dysglycaemia

Accumulation of glycogen in the kidney and liver is the main feature of Fanconi-Bickel Syndrome (FBS), a rare disorder of carbohydrate metabolism inherited in an autosomal recessive manner due to SLC2A2 gene mutations. Missense, nonsense, frame-shift (fs), in-frame indels, splice site, and compound heterozygous variants have all been identified in SLC2A2 gene of FBS cases. Approximately 144 FBS cases with 70 different SLC2A2 gene variants have been reported so far. SLC2A2 encodes for glucose transporter 2 (GLUT2) a low affinity facilitative transporter of glucose mainly expressed in tissues playing important roles in glucose homeostasis, such as renal tubular cells, enterocytes, pancreatic β-cells, hepatocytes and discrete regions of the brain. Dysfunctional mutations and decreased GLUT2 expression leads to dysglycaemia (fasting hypoglycemia, postprandial hyperglycemia, glucose intolerance, and rarely diabetes mellitus), hepatomegaly, galactose intolerance, rickets, and poor growth. The molecular mechanisms of dysglycaemia in FBS are still not clearly understood. In this review, we discuss the physiological roles of GLUT2 and the pathophysiology of mutants, highlight all of the previously reported SLC2A2 mutations associated with dysglycaemia, and review the potential molecular mechanisms leading to dysglycaemia and diabetes mellitus in FBS patients.

Protein tyrosine phosphatase 1B inhibition as a potential therapeutic target for chronic wounds in diabetes

Non-healing diabetic foot ulcers (DFUs) are a serious complication in diabetic patients. Their incidence has increased in recent years. Although there are several treatments for DFUs, they are often not effective enough to avoid amputation. Protein tyrosine phosphatase 1B (PTP1B) is expressed in most tissues and is a negative regulator of important metabolic pathways. PTP1B is overexpressed in tissues under diabetic conditions. Recently, PTP1B inhibition has been found to enhance wound healing. PTP1B inhibition decreases inflammation and bacterial infection at the wound site and promotes angiogenesis and tissue regeneration, thereby facilitating diabetic wound healing. In summary, the pharmacological modulation of PTP1B activity may help treat DFUs, suggesting that PTP1B inhibition is an outstanding therapeutic target.

Edgeworthia gardneri (Wall.) Meisn. Water Extract Ameliorates Palmitate Induced Insulin Resistance by Regulating IRS1/GSK3β/FoxO1 Signaling Pathway in Human HepG2 Hepatocytes

The flower of Edgeworthia gardneri (Wall.) Meisn is commonly used in beverage products in Tibet and has potential health benefits for diabetes. However, the mechanisms underlying anti-insulin resistance (IR) action of the flower of E. gardneri are not fully understood. This study aims to investigate the effects of the water extract of the flower of E. gardneri (WEE) on IR in palmitate (PA)-exposed HepG2 hepatocytes. WEE was characterized by UPLC analysis. PA-treated HepG2 cells were selected as the IR cell model. The cell viability was determined using MTT assay. Moreover, the glucose consumption and production were measured by glucose oxidase method. The glucose uptake and glycogen content were determined by the 2-NBDG (2-deoxy-2-[(7-nitro-2,1,3-benzoxadiazol-4-yl) amino]-D-glucose) glucose uptake assay and anthrone-sulfuric acid assay, respectively. The intracellular triglyceride content was detected by oxidative enzymic method. Protein levels were examined by Western blotting. Nuclear localization of FoxO1 was detected using immunofluorescence analyses and Western blotting. The expression of FoxO1 target genes was detected by quantitative real-time polymerase chain reaction (qRT-PCR). The viability of PA-treated HepG2 cells was concentration-dependently increased by incubation with WEE for 24 h. WEE treatment remarkably increased the consumption and uptake of glucose in PA-exposed HepG2 cells. Moreover, treatment with WEE significantly decreased the PA-induced over-production of glucose in HepG2 cells. After exposure of HepG2 cells with PA and WEE, the glycogen content was significantly elevated. The phosphorylation and total levels of IRβ, IRS1, and Akt were upregulated by WEE treatment in PA-exposed HepG2 cells. The phosphorylation of GSK3β was elevated after WEE treatment in PA-treated cells. WEE treatment also concentration-dependently downregulated the phosphorylated CREB, ERK, c-Jun, p38 and JNK in PA-exposed HepG2 cells. Furthermore, the nuclear protein level and nuclear translocation of FoxO1 were also suppressed by WEE. Additionally, PA-induced changes of FoxO1 targeted genes were also attenuated by WEE treatment. The GLUT2 and GLUT4 translocation were also promoted by WEE treatment in PA-treated HepG2 cells. Taken together, WEE has potential anti-IR effect in PA-exposed HepG2 cells; the underlying mechanism of this action may be associated with the regulation of IRS1/GSK3β/FoxO1 signaling pathway. This study provides a pharmacological basis for the application of WEE in the treatment of metabolic diseases such as type 2 diabetes mellitus.

Aberrant liver insulin receptor isoform a expression normalises with remission of type 2 diabetes after gastric bypass surgery

Type 2 diabetes mellitus (T2DM) results from a combination of progressive insulin resistance and loss of pancreatic beta cell function and/or mass. Insulin signalling occurs through the insulin receptor, (INSR) which is alternatively spliced into two isoforms: INSRA (-exon 11) and INSRB (+exon 11). Because the INSR isoforms have different functional characteristics, their relative expression ratio has been implicated in the pathogenesis of insulin resistance and T2DM. We studied levels of INSR isoform mRNA in liver samples taken from 46 individuals with or without T2DM at Roux-en-Y (RYGB) surgery, and on average 17 (± 5.6) months later in 16 of the same individuals (8 diabetic and non-diabetic patients). INSRA or INSRB was also overexpressed in HepG2 cells to ascertain their effect on AKT phosphorylation and PCK1 expression as markers of insulin-mediated metabolic signalling. We found the INSRB:A isoform ratio was reduced in individuals with T2DM in comparison to those with normal glucose tolerance and normalised with remission of diabetes. The INSRB:A ratio increased due to a reduction in the alternatively spliced INSRA isoform following remission of diabetes. Overexpressing INSRA isoform in HepG2 hepatoma cells reduced inhibition of PCK1 transcription and did not increase AKT phosphorylation in response to insulin load compared to the effect of overexpressing the B isoform. Data presented here revitalizes the role of the INSR isoforms in the pathogenesis of T2DM, and suggests that an abrogated INSRB:A ratio that favours the INSRA isoform may negatively impact insulin-mediated metabolic signalling.

Adipocyte SIRT1 knockout promotes PPARγ activity, adipogenesis and insulin sensitivity in chronic-HFD and obesity

OBJECTIVE

Adipose tissue is the primary site for lipid deposition that protects the organisms in cases of nutrient excess during obesogenic diets. The histone deacetylase Sirtuin 1 (SIRT1) inhibits adipocyte differentiation by targeting the transcription factor peroxisome proliferator activated-receptor gamma (PPARγ).

METHODS

To assess the specific role of SIRT1 in adipocytes, we generated Sirt1 adipocyte-specific knockout mice (ATKO) driven by aP2 promoter onto C57BL/6 background. Sirt1 (flx/flx) aP2Cre (+) (ATKO) and Sirt1 (flx/flx) aP2Cre (-) (WT) mice were fed high-fat diet for 5 weeks (short-term) or 15 weeks (chronic-term). Metabolic studies were combined with gene expression analysis and phosphorylation/acetylation patterns in adipose tissue.

RESULTS

On standard chow, ATKO mice exhibit low-grade chronic inflammation in adipose tissue, along with glucose intolerance and insulin resistance compared with control fed mice. On short-term HFD, ATKO mice become more glucose intolerant, hyperinsulinemic, insulin resistant and display increased inflammation. During chronic HFD, WT mice developed a metabolic dysfunction, higher than ATKO mice, and thereby, knockout mice are more glucose tolerant, insulin sensitive and less inflamed relative to control mice. SIRT1 attenuates adipogenesis through PPARγ repressive acetylation and, in the ATKO mice adipocyte PPARγ was hyperacetylated. This high acetylation was associated with a decrease in Ser273-PPARγ phosphorylation. Dephosphorylated PPARγ is constitutively active and results in higher expression of genes associated with increased insulin sensitivity.

CONCLUSION

Together, these data establish that SIRT1 downregulation in adipose tissue plays a previously unknown role in long-term inflammation resolution mediated by PPARγ activation. Therefore, in the context of obesity, the development of new therapeutics that activate PPARγ by targeting SIRT1 may provide novel approaches to the treatment of T2DM.

Dysregulated hepatic expression of glucose transporters in chronic disease: contribution of semicarbazide-sensitive amine oxidase to hepatic glucose uptake

Insulin resistance is common in patients with chronic liver disease (CLD). Serum levels of soluble vascular adhesion protein-1 (VAP-1) are also increased in these patients. The amine oxidase activity of VAP-1 stimulates glucose uptake via translocation of transporters to the cell membrane in adipocytes and smooth muscle cells. We aimed to document human hepatocellular expression of glucose transporters (GLUTs) and to determine if VAP-1 activity influences receptor expression and hepatic glucose uptake. Quantitative PCR and immunocytochemistry were used to study human liver tissue and cultured cells. We also used tissue slices from humans and VAP-1-deficient mice to assay glucose uptake and measure hepatocellular responses to stimulation. We report upregulation of GLUT1, -3, -5, -6, -7, -8, -9, -10, -11, -12, and -13 in CLD. VAP-1 expression and enzyme activity increased in disease, and provision of substrate to hepatic VAP-1 drives hepatic glucose uptake. This effect was sensitive to inhibition of VAP-1 and could be recapitulated by H2O2. VAP-1 activity also altered expression and subcellular localization of GLUT2, -4, -9, -10, and -13. Therefore, we show, for the first time, alterations in hepatocellular expression of glucose and fructose transporters in CLD and provide evidence that the semicarbazide-sensitive amine oxidase activity of VAP-1 modifies hepatic glucose homeostasis and may contribute to patterns of GLUT expression in chronic disease.

Sesquiterpenes from the rhizomes of Curcuma heyneana

Four new germacranes [heyneanones A-D (1-4)], three new guaianes [4,10-epizedoarondiol (5), 15-hydroxyprocurcumenol (6), 12-hydroxycurcumenol (7)], and two new spirolactones [curcumanolides C (8) and D (9)] were isolated from the rhizomes of Curcuma heyneana together with 13 known sesquiterpenes and two known labdane-type diterpenes. Among the isolated compounds, heyneanone A (1), heyneanone C (3), 4,10-epizedoarondiol (5), procurcumenol (16), aerugidiol (17), zerumin A (23), and (E)-15,16-bisnorlabda-8(17),11-dien-13-one (24) inhibited protein tyrosine phosphatase 1B (PTP1B) with IC(50) values of 42.5, 35.2, 35.1, 45.6, 35.7, 10.4, and 14.7 μM, respectively.

Gut-derived serotonin is a multifunctional determinant to fasting adaptation

Energy release from cellular storage is mandatory for survival during fasting. This is achieved through lipolysis and liver gluconeogenesis. We show here that in the mouse, gut-derived serotonin (GDS) is upregulated during fasting and that it favors both mechanisms. In adipocytes, GDS signals through the Htr2b receptor to favor lipolysis by increasing phosphorylation and activity of hormone-sensitive lipase. In hepatocytes, GDS signaling through Htr2b promotes gluconeogenesis by enhancing activity of two rate-limiting gluconeogenic enzymes, FBPase and G6Pase. In addition, GDS signaling in hepatocytes prevents glucose uptake in a Glut2-dependent manner, thereby further favoring maintenance of blood glucose levels. As a result, inhibition of GDS synthesis can improve glucose intolerance caused by high-fat diet. Hence, GDS opposes deleterious consequences of food deprivation by favoring lipolysis and liver gluconeogenesis while preventing glucose uptake by hepatocytes. As a result, pharmacological inhibition of its synthesis may contribute to improve type 2 diabetes.

IRS2 and PTP1B: Two opposite modulators of hepatic insulin signalling

Type 2 Diabetes mellitus (T2D) is the most common endocrine disorder associated to metabolic syndrome (MS) and occurs when insulin secretion can no compensate peripheral insulin resistance. Among peripheral tissues, the liver controls glucose homeostasis due to its ability to consume and produce glucose. The molecular mechanism underlying hepatic insulin resistance is not completely understood; however, it involves the impairment of the insulin signalling network. Among the critical nodes of hepatic insulin signalling, insulin receptor substrate 2 (IRS2) and protein tyrosine phosphatase 1B (PTP1B) modulate the phosphatidylinositol (PI) 3-kinase/Akt/Foxo1 pathway that controls the suppression of gluconeogenic genes. In this review, we will focus on recent findings regarding the molecular mechanism by which IRS2 and PTP1B elicit opposite effects on carbohydrate metabolism in the liver in response to insulin. Finally, we will discuss the involvement of the critical nodes of insulin signalling in non-alcoholic fatty liver disease (NAFLD) in humans.

Tissue-specificity of insulin action and resistance

Insulin resistance is the most important pathophysiological feature in many pre-diabetic states. Type 2 diabetes mellitus is a complex metabolic disease and its pathogenesis involves abnormalities in both peripheral insulin action and insulin secretion by pancreatic beta cells. The creation of monogenic or polygenic genetically manipulated mice models in a tissue-specific manner was of great help to elucidate the tissue-specificity of insulin action and its contribution to the overall insulin resistance. However, complete understanding of the molecular bases of the insulin action and resistance requires the identification of the intracellular pathways that regulate insulin-stimulated proliferation, differentiation and metabolism. Accordingly, cell lines derived from insulin target tissues such as brown adipose tissue, liver and beta islets lacking insulin receptors or sensitive candidate genes such as IRS-1, IRS-2, IRS-3, IR and PTP1B were developed. Indeed, these cell lines have been also very useful to understand the tissue-specificity of insulin action and inaction.

Protein Tyrosine Phosphatase 1B (PTP1B) deficiency accelerates hepatic regeneration in mice

Protein tyrosine phosphatase 1B (PTP1B) is a key regulator of metabolism and cell growth by its ability to dephosphorylate tyrosine kinase receptors and modulate the intensity of their signaling cascades. Because liver regeneration involves tyrosine phosphorylation-mediated signaling, we investigated the role of PTP1B in this process by performing partial hepatectomy in wild-type (PTP1B(+/+)) and PTP1B-deficient (PTP1B(-/-)) mice. The expression of PCNA and cyclins D1 and E (cell proliferation markers) was enhanced in PTP1B(-/-) regenerating livers, in parallel with 5'-bromo-2'-deoxyuridine incorporation. Phosphorylation of JNK1/2 and STAT3, early triggers of hepatic regeneration in response to TNF-α and IL-6, was accelerated in PTP1B(-/-) mice compared with PTP1B(+/+) mice. These phosphorylations were increased in PTP1B(-/-) hepatocytes or by silencing PTP1B in wild-type cells and decreased further after the addition of recombinant PTP1B. Enhanced EGF- and HGF receptor-mediated signaling was observed in regenerating livers lacking PTP1B and in EGF- or HGF-stimulated PTP1B(-/-) hepatocytes. Moreover, PTP1B(-/-) mice displayed a more rapid increase in intrahepatic lipid accumulation than PTP1B(+/+) control mice. Late responses to partial hepatectomy revealed additional divergences because stress-mediated signaling was attenuated at 24 to 96 hours in PTP1B(-/-) mice compared with PTP1B(+/+) mice. Finally, PTP1B deficiency also improves hepatic regeneration in mice fed a high-fat diet. These results suggest that pharmacological inhibition of PTP1B would improve liver regeneration in patients with acute or chronic liver injury.

Tissue specificity on insulin action and resistance: past to recent mechanisms

Insulin resistance is the most important pathophysiological feature in many pre-diabetic states. Type 2 diabetes mellitus is a complex metabolic disease and its pathogenesis involves abnormalities in both peripheral insulin action and insulin secretion by pancreatic β-cells. The creation of monogenic or polygenic genetically manipulated mice models in a tissue-specific manner was of great help to elucidate the tissue specificity of insulin action and its contribution to the overall insulin resistance. However, a complete understanding of the molecular bases of insulin action and resistance requires the identification of intracellular pathways that regulate insulin-stimulated proliferation, differentiation and metabolism. Accordingly, cell lines derived from insulin target tissues such as brown adipose tissue, liver and beta islets lacking insulin resistance or sensitive candidate genes such as IRS-1, IRS-2, IRS-3, IR and PTP1B have been developed. Indeed, these cell lines have also been very useful to understand the tissue specificity of insulin action and inaction. Obesity is a risk factor for several components of the metabolic syndromes such as type 2 diabetes, dyslipidaemia and systolic hypertension, because white and brown adipose tissues as endocrine organs express and secrete a variety of adipocytokines that can act at both local and systemic levels, modulating the insulin sensitivity. Recent studies revealed that the subjects with the highest transcription rates of genes encoding TNF-α and IL-6 were prone to develop obesity, insulin resistance and type 2 diabetes. Accordingly, we specifically focus in this review on the impact of those adipocytokines on the modulation of insulin action in skeletal muscle.

Snail1 suppresses TGF-beta-induced apoptosis and is sufficient to trigger EMT in hepatocytes

Although TGF-β suppresses early stages of tumour development, it later contributes to tumour progression when cells become resistant to its suppressive effects. In addition to circumventing TGF-β-induced growth arrest and apoptosis, malignant tumour cells become capable of undergoing epithelial-to-mesenchymal transition (EMT), favouring invasion and metastasis. Therefore, defining the mechanisms that allow cancer cells to escape from the suppressive effects of TGF-β is fundamental to understand tumour progression and to design specific therapies. Here, we have examined the role of Snail1 as a suppressor of TGF-β-induced apoptosis in murine non-transformed hepatocytes, rat and human hepatocarcinoma cell lines and transgenic mice. We show that Snail1 confers resistance to TGF-β-induced cell death and that it is sufficient to induce EMT in adult hepatocytes, cells otherwise refractory to this transition upon exposure to TGF-β. Furthermore, we show that Snail1 silencing prevents EMT and restores the cell death response induced by TGF-β. As Snail1 is a known target of TGF-β signalling, our data indicate that Snail1 might transduce the tumour-promoting effects of TGF-β, namely the EMT concomitant with the resistance to cell death.

Pparg-P465L mutation worsens hyperglycemia in Ins2-Akita female mice via adipose-specific insulin resistance and storage dysfunction

OBJECTIVE

The dominant-negative P467L mutation in peroxisome proliferator activated receptor-γ (PPARγ) was identified in insulin-resistant patients with hyperglycemia and lipodystrophy. In contrast, mice carrying the corresponding Pparg-P465L mutation have normal insulin sensitivity, with mild hyperinsulinemia. We hypothesized that murine Pparg-P465L mutation leads to covert insulin resistance, which is masked by hyperinsulinemia and increased pancreatic islet mass, to retain normal plasma glucose.

RESEARCH DESIGN AND METHODS

We introduced in Pparg(P465L/+) mice an Ins2-Akita mutation that causes improper protein folding and islet apoptosis to lower plasma insulin.

RESULTS

Unlike Ins2(Akita/+) littermates, male Pparg(P465L/+)Ins2(Akita/+) mice have drastically reduced life span with enhanced type 1 diabetes. Hyperglycemia in Ins2(Akita/+) females is mild. However, Pparg(P465L/+)Ins2(Akita/+) females have aggravated hyperglycemia, smaller islets, and reduced plasma insulin. In an insulin tolerance test, they showed smaller reduction in plasma glucose, indicating impaired insulin sensitivity. Although gluconeogenesis is enhanced in Pparg(P465L/+)Ins2(Akita/+) mice compared with Ins2(Akita/+), exogenous insulin equally suppressed gluconeogenesis in hepatocytes, suggesting that Pparg(P465L/+)Ins2(Akita/+) livers are insulin sensitive. Expression of genes regulating insulin sensitivity and glycogen and triglyceride contents suggest that skeletal muscles are equally insulin sensitive. In contrast, adipose tissue and isolated adipocytes from Pparg(P465L/+)Ins2(Akita/+) mice have impaired glucose uptake in response to exogenous insulin. Pparg(P465L/+)Ins2(Akita/+) mice have smaller fat depots composed of larger adipocytes, suggesting impaired lipid storage with subsequent hepatomegaly and hypertriglyceridemia.

CONCLUSIONS

PPARg-P465L mutation worsens hyperglycemia in Ins2(Akita/+) mice primarily because of adipose-specific insulin resistance and altered storage function. This underscores the important interplay between insulin and PPARγ in adipose tissues in diabetes.

Glucose binds to the insulin receptor affecting the mutual affinity of insulin and its receptor

Insulin activity is sensitive to glucose concentration but the mechanisms are still unclear. An unexamined possibility is that the insulin receptor (IR) is sensitive to glucose concentration. We demonstrate here that insulin-like peptides derived from the IR bind glucose at low millimolar, and cytochalasin B at low micromolar, concentrations; several insulin-like IR peptides bind insulin at nanomolar Kd; and this binding is antagonized by increasing glucose concentrations. In addition, glucose and cytochalasin B bind to IR isolated from rat liver and increasing glucose decreases insulin binding to this IR preparation. The presence of GLUT 1 in our IR preparation suggests the possibility of additional glucose-mediated allosteric control. We propose a model in which glucose binds to insulin, the IR, and GLUT; insulin binds to the IR; and the IR binds to GLUT. This set of interactions produces an integrated system of insulin-dependent interactions that is highly sensitive to glucose concentration.

GLUT2 mutations, translocation, and receptor function in diet sugar managing

Cloned 20 years ago, GLUT2 is a facilitative glucose transporter in the liver, pancreas, intestine, kidney, and brain. It ensures large bidirectional fluxes of glucose in and out the cell due to its low affinity and high capacity. It also transports other dietary sugars, such as fructose and galactose, within the range of physiological concentrations. Sugars and hormones regulate its gene expression. The contribution of GLUT2 to human metabolic diseases previously appeared modest. However, in the past decade, three major features of the GLUT2 protein have been revealed. First, GLUT2 mutations cause the severe but rare Fanconi-Bickel syndrome, mainly characterized by glycogenosis. Recently, a GLUT2 polymorphism has been associated with preferences for sugary food. Second, the GLUT2 location at the cell surface is regulated; this governs cellular activities dependent on glucose in the intestine and possibly those in the liver and pancreas. For instance, GLUT2 translocation from an intracellular pool to the apical membrane after a sugar meal transiently increases sugar uptake by enterocytes (reviewed in 32). Third, GLUT2 functions as a membrane receptor of sugar. Independently of glucose metabolism, GLUT2 detects the presence of extracellular sugar and transduces a signal to modulate cell functions, including beta-cell insulin secretion, renal reabsorption, and intestinal absorption according to the sugar environment. These recent developments are examined here in heath and metabolic disease, highlighting various unanswered questions.