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Free access

Wo-Shing Au, Liwei Lu, Chung-Man Yeung, Ching-Chiu Liu, Oscar G Wong, Lihui Lai, Hsiang-fu Kung and Marie C Lin

Insulin inhibits the transcription of the microsomal triglyceride transfer protein (MTTP), which plays a pivotal role in lipoprotein assembly and secretion. Here, we provide evidence that a hepatocyte nuclear factor 1 binding element (HNF1A element) within the MTTP promoter serves as a novel negative insulin-responsive element. Deletion/mutation mapping of the MTTP gene promoter identified a modified HNF1A element that is crucial to the negative insulin effect. Chimeric promoter containing this HNF1A element and minimal TEAD1 promoter also responded negatively toward insulin treatment. Gel shift assay demonstrated that HNF1A but not HNF1B binds to this element. Enforced expression of HNF1A was sufficient to reconstitute the negative insulin responsiveness of MTTP promoter in TM4SF1 myocytes that are HNF1A negative. Furthermore, replacing this element with consensus HNF1A element preserved the negative insulin response, suggesting that negative insulin responsiveness is a generic characteristic of HNF1A element. Given that many genes implicated in diabetes contain HNF1A element, the potential regulation of these genes by insulin via HNF1A element may provide important clues for the manifestation and treatment of diabetic metabolic syndromes.

Free access

Charit Taneja, Sakshi Gera, Se-Min Kim, Jameel Iqbal, Tony Yuen and Mone Zaidi

FSH has a primary function in procreation, wherein it induces estrogen production in females and regulates spermatogenesis in males. However, in line with our discoveries over the past decade of non-unitary functions of pituitary hormones, we and others have described hitherto uncharacterized functions of FSH. Through high-affinity receptors, some of which are variants of the ovarian FSH receptor (FSHR), FSH regulates bone mass, adipose tissue function, energy metabolism, and cholesterol production in both sexes. These newly described actions of FSH may indeed be relevant to the pathogenesis of bone loss, dysregulated energy homeostasis, and disordered lipid metabolism that accompany the menopause in females and aging in both genders. We are therefore excited about the possibility of modulating circulating FSH levels toward a therapeutic benefit for a host of age-associated diseases, including osteoporosis, obesity and dyslipidemia, among other future possibilities.

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J Rasschaert, M-H Giroix, I Conget, D Mercan, V Leclercq-Meyer, A Sener, B Portha and W J Malaisse

ABSTRACT

This study aimed to compare the metabolic and secretory responses of pancreatic islets from animals with non-insulin-dependent diabetes to d-glucose with the effects of the methyl esters of succinic acid (SME) and glutamic acid (GME). The insulin secretory response to d-glucose was impaired in islets from rats with diabetes which was either inherited (Goto—Kakizaki (GK) rats) or acquired (streptozotocin-treated (STZ) rats). This coincided with a preferential alteration of oxidative relative to total glycolysis in intact islets and a selective defect of FAD-linked mitochondrial glycerophosphate dehydrogenase (m-GDH) in islet homogenates. This enzymatic defect was also found in purified B cells from STZ rats. It contrasted both with unaltered activities of glutamate dehydrogenase and succinate dehydrogenase in the islets of diabetic animals and with a normal or even increased activity of m-GDH in the livers of GK and STZ rats. The oxidation of [1,4-14C]SME and [U-14C]GME appeared decreased in islets of GK or STZ animals when compared with control rats, but no significant difference between control and diabetic rats was observed when the oxidative data were expressed relative to the rate of [U-14C]GME hydrolysis. Nevertheless, the absolute values for insulin release evoked by a non-metabolized analogue of l-leucine (BCH), by SME and by the association of BCH with either SME or GME were invariably lower in islets of GK and STZ rats than in those of control animals. These findings indicate that the enzymatic and metabolic situation in islets of GK and STZ rats could allow the expression of the insulinotropic potential of SME and GME, even if their immediate secretory effects are impaired in the islets of these diabetic animals.

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E. Uchida, T. Hayakawa, S. Niimi, A. Tanaka and M. Morikawa

ABSTRACT

Cultured 3T3-F442A preadipocytes are able to undergo GH-promoted differentiation into adipocytes. The relationship between the structure and function of GH receptors on 3T3 cells (3T3-F442A preadipocytes, differentiated adipocytes and 3T3-C2 cells, which vary in susceptibility to adipose conversion or with respect to carbohydrate and lipid metabolism) was studied by the covalent cross-linking of 125I-labelled human (h) GH to intact cells with the bifunctional reagent disuccinimidyl suberate. When preadipocytes were cross-linked and analysed using sodium dodecylsulphate-polyacrylamide gel electrophoresis, a prominent 125I-labelled hGH-receptor complex of M r 130 000 was observed along with minor complexes (M r 300 000, 230 000 and 60 000) on autoradiography. Non-reducing—reducing two-dimensional gel electrophoresis revealed that the higher molecular weight complexes also contained the M r 130 000 complex. Neuraminidase and tunicamycin treatment demonstrated that the GH receptor on F442A preadipocytes is a sialo-glycoprotein with N-linked carbohydrate chains. When the differentiated 3T3-F442A adipocytes and 3T3-C2 cells (a sub-line with no susceptibility to adipose conversion with GH) were examined in the same way as 3T3-F442A preadipocytes, no differences were observed in the specificity of GH binding and in the molecular size of the 125I-labelled hGH-receptor complexes and their glycosylation characteristics. This suggests that the structural characteristics of the GH receptor are closely related in each cell type, but that the hormonal signals produced after GH binding to the receptor may cause different effects according to the cell type.

Free access

Penny Ahlstrom, Esther Rai, Suharto Chakma, Hee Ho Cho, Palanivel Rengasamy and Gary Sweeney

Skeletal muscle insulin resistance is known to play an important role in the pathogenesis of diabetes, and one potential causative cellular mechanism is endoplasmic reticulum (ER) stress. Adiponectin mediates anti-diabetic effects via direct metabolic actions and by improving insulin sensitivity, and we recently demonstrated an important role in stimulation of autophagy by adiponectin. However, there is limited knowledge on crosstalk between autophagy and ER stress in skeletal muscle and in particular how they are regulated by adiponectin. Here, we utilized the model of high insulin/glucose (HIHG)-induced insulin resistance, determined by measuring Akt phosphorylation (T308 and S473) and glucose uptake in L6 skeletal muscle cells. HIHG reduced autophagic flux measured by LC3 and p62 Western blotting and tandem fluorescent RFP/GFP-LC3 immunofluorescence (IF). HIHG also induced ER stress assessed by thioflavin T/KDEL IF, pIRE1, pPERK, peIF2α and ATF6 Western blotting and induction of a GRP78-mCherry reporter. Induction of autophagy by adiponectin or rapamycin attenuated HIHG-induced ER stress and improved insulin sensitivity. The functional significance of enhanced autophagy was validated by demonstrating a lack of improved insulin sensitivity in response to adiponectin in autophagy-deficient cells generated by overexpression of dominant negative mutant of Atg5. In summary, adiponectin-induced autophagy in skeletal muscle cells alleviated HIHG-induced ER stress and insulin resistance.

Free access

Eva Tudurí, Monica Imbernon, Rene Javier Hernández-Bautista, Marta Tojo, Johan Fernø, Carlos Diéguez and Rubén Nogueiras

GPR55 is a G-protein-coupled receptor (GPCR) that has been identified as a new cannabinoid receptor. Given the wide localization of GPR55 in brain and peripheral tissues, this receptor has emerged as a regulator of multiple biological actions. Lysophosphatidylinositol (LPI) is generally accepted as the endogenous ligand of GPR55. In this review, we will focus on the role of GPR55 in energy balance and glucose metabolism. We will summarize its actions on feeding, nutrient partitioning, gastrointestinal motility and insulin secretion in preclinical models and the scarce data available in humans. The potential of GPR55 to become a new pharmaceutical target to treat obesity and type 2 diabetes, as well as the foreseeing difficulties are also discussed.

Free access

Inagadapa J N Padmavathi, Kalashikam Rajender Rao and Manchala Raghunath

Robust evidence suggests that nutritional insult during fetal development could program the offspring to glucose intolerance, impaired insulin response and insulin resistance (IR). Considering the importance of chromium (Cr) in maintaining carbohydrate metabolism, this study determined the effect of maternal Cr restriction (CrR) on glucose metabolism and plasma insulin in Wistar/NIN (WNIN) rat offspring and the associated biochemical and/or molecular mechanisms. Female, weanling WNIN rats received ad libitum for 12 weeks, a control diet or the same with 65% restriction of Cr and mated with control males. Some of the Cr-restricted dams were rehabilitated from conception or parturition and their pups weaned on to control diet. At the time of weaning, half of the Cr restricted offspring were rehabilitated to control diet while others continued on Cr-restricted diet. Maternal CrR increased fasting plasma glucose, fasting insulin, homeostasis model assessment of IR, and area under the curve of glucose and insulin during oral glucose tolerance test in the offspring. Expression and activity of rate-limiting enzymes of glucose metabolism were comparable among different groups and expression of genes involved in insulin secretion was increased albeit in male offspring whereas antioxidant enzyme activities were decreased in offspring of both genders. Rehabilitation, in general, corrected the changes albeit partially. Maternal dietary CrR induced IR, impaired glucose tolerance in WNIN rat offspring and was associated with increased oxidative stress, which may predispose them to type 2 diabetes in their later life.

Free access

Katherine A Robinson, Jonathan W Brock and Maria G Buse

Thioredoxin-interacting protein (Txnip) is a metabolic regulator, which modulates insulin sensitivity and likely plays a role in type 2 diabetes. We studied the regulation of Txnip in 3T3-L1 adipocytes. Cells were incubated under different conditions and Txnip was measured by immunoblotting. We confirmed that high glucose markedly increases Txnip expression by promoting transcription. Insulin decreases Txnip protein levels. Rapamycin under most conditions decreased Txnip, suggesting that mTOR complex-1 is involved. The acute effects of insulin are mainly posttranscriptional; insulin (100 nM) accelerates Txnip degradation more than tenfold. This effect is cell type specific. It works in adipocytes, preadipocytes and in L6 myotubes but not in HepG2 or in HEK 293 cells or in a pancreatic β-cell line. The ubiquitin/proteasome pathway is involved. Degradation of Txnip occurred within 15 min in the presence of 3 nM insulin and overnight with 0.6 nM insulin. Proteasomal Txnip degradation is not mediated by a cysteine protease or an anti-calpain enzyme. Okadaic acid (OKA), an inhibitor of phosphoprotein phosphatases (pp), markedly reduced Txnip protein and stimulated its further decrease by insulin. The latter occurred after incubation with 1 or 1000 nM OKA, suggesting that insulin enhances the phosphorylation of a pp2A substrate. Incubation with 0.1 μM Wortmannin, a PI3 kinase inhibitor, increased Txnip protein twofold and significantly inhibited its insulin-induced decrease. Thus, while OKA mimics the effect of insulin, Wortmannin opposes it. In summary, insulin stimulates Txnip degradation by a PI3 kinase-dependent mechanism, which activates the ubiquitin/proteasome pathway and likely serves to mitigate insulin resistance.

Free access

Hanze Du, Limei Liu, Ying Wang, Yuichi Nakagawa, Alexei Lyzlov, Kabirullah Lutfy, Theodore C Friedman, Xiaozhong Peng and Yanjun Liu

Pre-receptor activation of glucocorticoids via 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1 (HSD11B1)) has been identified as an important mediator of the metabolic syndrome. Hexose-6-phosphate dehydrogenase (H6PDH) mediates 11β-HSD1 amplifying tissue glucocorticoid production by driving intracellular NADPH exposure to 11β-HSD1 and requires glucose-6-phosphate transporter (G6PT (SLC37A4)) to maintain its activity. However, the potential effects of G6PT on tissue glucocorticoid production in type 2 diabetes and obesity have not yet been defined. Here, we evaluated the possible role of G6PT antisense oligonucleotides (G6PT ASO) in the pre-receptor metabolism of glucocorticoids as related to glucose homeostasis and insulin tolerance by examining the production of 11β-HSD1 and H6PDH in both male db/ + and db/db mouse liver tissue. We observed that G6PT ASO treatment of db/db mice markedly reduced hepatic G6PT mRNA and protein levels and substantially diminished the activation of hepatic 11β-HSD1 and H6PDH. Reduction of G6pt expression was correlated with the suppression of both hepatic gluconeogenic enzymes G6Pase and PEPCK and corresponded to the improvement of hyperglycemia and insulin resistance in db/db mice. Addition of G6PT ASO to mouse hepa1–6 cells led to a dose-dependent decrease in 11B-Hsd1 production. Knockdown of G6PT with RNA interference also impaired 11B-Hsd1 expression and showed comparable effects to H6pdh siRNA on silencing of H6pdh and 11B-Hsd1 expression in these intact cells. These findings suggest that G6PT plays an important role in the modulation of pre-receptor activation of glucocorticoids and provides new insights into the role of G6PT in the development of type 2 diabetes.

Free access

Sufang Chen, Wei Wei, Minjie Chen, Xiaobo Qin, Lianglin Qiu, Li Zhang, Yuhao Zhang, Qi Cao and Zhekang Ying

Numerous studies have implicated tumor necrosis factor α (TNFα) in the pathogenesis of type 2 diabetes. However, the role of its primary receptor, TNF receptor 1 (TNFR1), in homeostatic regulation of glucose metabolism is still controversial. In addition to TNFα, lymphotoxin α (LTα) binds to and activates TNFR1. Thus, TNFα and LTα together are known as TNF. To delineate the role of TNF signaling in glucose homeostasis, the present study ascertained how TNF signaling deficiency affects major regulatory components of glucose homeostasis. To this end, normal diet-fed male TNFR1-deficient mice (TNFR1−/−), TNFα/LTα/LTβ triple-deficient mice (TNF/LT∆3) and their littermate controls were subjected to intraperitoneal glucose tolerance test, insulin tolerance test and oral glucose tolerance test. The present results showed that TNFR1−/− and TNF/LT∆3 mice vs their controls had comparable body weight, tolerance to intraperitoneal glucose and sensitivity to insulin. However, their tolerance to oral glucose was significantly increased. Additionally, glucose-induced insulin secretion assessments revealed that TNFR1 or TNF/LT deficiency significantly increased oral but not intraperitoneal glucose-induced insulin secretion. Consistently, qPCR and immunohistochemistry analyses showed that TNFR1−/− and TNF/LT∆3 mice vs their controls had significantly increased ileal expression of glucagon-like peptide-1 (GLP-1), one of the primary incretins. Their oral glucose-induced secretion of GLP-1 was also significantly increased. These data collectively suggest that physiological TNF signaling regulates glucose metabolism primarily through effects on GLP-1 expression and secretion and subsequently insulin secretion.