Tungstate stimulates insulin release and inhibits somatostatin output in the perfused rat pancreas

Gluconeogenic Enzymes in β-Cells: Pharmacological Targets for Improving Insulin Secretion

Pancreatic β-cells express the gluconeogenic enzymes glucose 6-phosphatase (G6Pase), fructose 1,6-bisphosphatase (FBP), and phosphoenolpyruvate (PEP) carboxykinase (PCK), which modulate glucose-stimulated insulin secretion (GSIS) through their ability to reverse otherwise irreversible glycolytic steps. Here, we review current knowledge about the expression and regulation of these enzymes in the context of manipulating them to improve insulin secretion in diabetics. Because the regulation of gluconeogenic enzymes in β-cells is so poorly understood, we propose novel research avenues to study these enzymes as modulators of insulin secretion and β-cell dysfunction, with especial attention to FBP, which constitutes an attractive target with an inhibitor under clinical evaluation at present.

Antidiabetic potential of polyoxotungstates: in vitro and in vivo studies

Diabetes mellitus is a chronic metabolic disorder continuously affecting people all over the world. A common way to treat diabetes mellitus is to limit the conversion of carbohydrates into glucose which is mediated by glucosidase enzymes. Diabetes mellitus is also famous for its life-threatening microvascular (retinopathy, neuropathy and nephropathy) and macrovascular (atherosclerosis) complications. Aldose reductases present in eye lens (ALR1) and kidney (ALR2) are responsible for microvascular complications. The production of advanced glycation end products (AGEs) is involved in the development of atherosclerosis. The present work was aimed at the synthesis and in vitro/in vivo evaluation of different polyoxotungstates against glucosidases (α- and β), aldose reductases (ALR1 and ALR2) and AGEs to discover a new treatment which may limit the complications associated with diabetes mellitus. The polyanion [P6W18O79](20-) was found to be the most potent inhibitor of α-glucosidase (IC50 = 1.33 ± 0.41 μM), ALR1 (IC50 = 0.4 ± 0.009 μM) and ALR2 (IC50 = 0.38 ± 0.02 μM). Animal studies showed that the polyanion [H2W12O40](6-) was very effective in reducing the blood glucose level to 84.25 ± 5.07 mg dL(-1) when compared with standard antidiabetic drug glibenclamide (150.62 ± 9.35 mg dL(-1)) measured after maximum 8 h of dose administration. The data obtained from in vitro and in vivo experiments confirm that [P6W18O79](20-) and [H2W12O40](6-) could be used as a new treatment of diabetes mellitus.

Cytotoxicity and enzyme inhibition studies of polyoxometalates and their chitosan nanoassemblies

Polyoxometalates (POMs) have become very significant in biomedical research for their structural diversity which renders them highly active against bacterial, viral and cancer diseases. In this study three different POMs were synthesized and nanoassemblies were made with chitosan (CTS), a natural biodegradable polymer with excellent drug carrier properties. The compounds were tested on two isoenzymes of alkaline phosphatases including tissue specific calf intestine alkaline phosphatase (CIAP) and tissue non-specific alkaline phosphatase (TNAP). Compound [TeW₆O₂₄]⁶⁻ (TeW₆) showed the highest activity (45.4 ± 11.3 nM) among tested compounds against TNAP. Similarly, chitosan-[TeW₆O₂₄]⁶⁻ (CTS-TeW₆) was proved to be a potent inhibitor of CIAP with K_i value of 22 ± 7 nM. A comparative study was made to evaluate their cytotoxic potential against HeLa cells. Among all tested compounds, Chitosan-[NaP₅W₃₀O₁₁₀]¹⁴⁻ (CTS-P₅W₃₀) has showed higher percent cytotoxicity (88 ± 10%) at 10 μM when compared with the standard anticancer drug vincristine (72 ± 7%). The study revealed that selected POMs proved excellent anticancer potential and were equally effective against alkaline phosphatase enzyme, an increased level of which may indicate cancer metastasis.

GPR120 (FFAR4) is preferentially expressed in pancreatic delta cells and regulates somatostatin secretion from murine islets of Langerhans

AIMS/HYPOTHESIS

The NEFA-responsive G-protein coupled receptor 120 (GPR120) has been implicated in the regulation of inflammation, in the control of incretin secretion and as a predisposing factor influencing the development of type 2 diabetes by regulation of islet cell apoptosis. However, there is still considerable controversy about the tissue distribution of GPR120 and, in particular, it remains unclear which islet cell types express this molecule. In the present study, we have addressed this issue by constructing a Gpr120-knockout/β-galactosidase (LacZ) knock-in (KO/KI) mouse to examine the distribution and functional role of GPR120 in the endocrine pancreas.

METHODS

A KO/KI mouse was generated in which exon 1 of the Gpr120 gene (also known as Ffar4) was replaced in frame by LacZ, thereby allowing for regulated expression of β-galactosidase under the control of the endogenous GPR120 promoter. The distribution of GPR120 was inferred from expression studies detecting β-galactosidase activity and protein production. Islet hormone secretion was measured from isolated mouse islets treated with selective GPR120 agonists.

RESULTS

β-galactosidase activity was detected as a surrogate for GPR120 expression exclusively in a small population of islet endocrine cells located peripherally within the islet mantle. Immunofluorescence analysis revealed co-localisation with somatostatin suggesting that GPR120 is preferentially produced in islet delta cells. In confirmation of this, glucose-induced somatostatin secretion was inhibited by a range of selective GPR120 agonists. This response was lost in GPR120-knockout mice.

CONCLUSIONS/INTERPRETATION

The results imply that GPR120 is selectively present within the delta cells of murine islets and that it regulates somatostatin secretion.

Anti-diabetic and anti-obesity agent sodium tungstate enhances GCN pathway activation through Glc7p inhibition

Tungstate counteracts diabetes and obesity in animal models, but its molecular mechanisms remain elusive. Our Saccharomyces cerevisiae-based approach has found that tungstate alleviated the growth defect induced by nutrient stress and enhanced the activation of the GCN pathway. Tungstate relieved the sensitivity to starvation of a gcn2-507 yeast hypomorphic mutant, indicating that tungstate modulated the GCN pathway downstream of Gcn2p. Interestingly, tungstate inhibited Glc7p and PP1 phosphatase activity, both negative regulators of the GCN pathway in yeast and humans, respectively. Accordingly, overexpression of a dominant-negative Glc7p mutant in yeast mimicked tungstate effects. Therefore tungstate alleviates nutrient stress in yeast by in vivo inhibition of Glc7p. These data uncover a potential role for tungstate in the treatment of PP1 and GCN related diseases.

Insulin mimetic effect of tungsten compounds on isolated rat adipocytes

Investigations of effective, orally active, and safe antidiabetic metallopharmaceuticals have been carried out during the last two decades. It has been reported that tungsten compounds mimic the action of insulin in intact cell systems. As insulin mimetics, the most investigated tungsten compound was sodium tungstate (ST), rarely investigated was tungstophosphoric acid (WPA), but never alanine complex of tungstophosphoric acid (WPA-A). In this study, the insulin mimetic activity of three different tungsten compounds, ST, WPA, and WPA-A, was evaluated by means of in vitro measurements of the glucose uptake and inhibition of free fatty acids release from epinephrine-treated isolated rat white adipocytes. We investigated the influence of concentration (lower and higher, 0.1 and 1.0 mM, respectively) and solvent: isotonic salt solution-saline (0.9% w/v of NaCl) and dimethyl sulfoxide (DMSO; 2% v/v), on the biological effect of tested compounds. Our experimental data showed that all of the three investigated tungsten compounds possess insulin mimetic activity in vitro on the isolated adipocytes. Influence of concentration and solvents on insulin mimetic effect for the certain tungsten compounds were: WPA was shown effect independently of concentration and solvents; higher concentration and DMSO were significant decreasing insulin mimetic effect of ST; lower concentration and saline led to decreasing effect of WPA-A. Generally, there were no differences in insulin mimetic effect of three tungsten compounds in lower concentration and dissolved in DMSO. When saline was used as solvent, it was needed higher concentration of investigated compounds to accomplish the same effect. In conclusion, our results suggest that low concentration (0.1 mM) of ST, WPA, and WPA-A dissolved in 2% DMSO could be the good candidates for in vivo investigation of their antidiabetic properties.

Molecular mechanisms of tungstate-induced pancreatic plasticity: a transcriptomics approach

Background

Sodium tungstate is known to be an effective anti-diabetic agent, able to increase beta cell mass in animal models of diabetes, although the molecular mechanisms of this treatment and the genes that control pancreas plasticity are yet to be identified. Using a transcriptomics approach, the aim of the study is to unravel the molecular mechanisms which participate in the recovery of exocrine and endocrine function of streptozotocin (STZ) diabetic rats treated with tungstate, determining the hyperglycemia contribution and the direct effect of tungstate.

Results

Streptozotocin (STZ)-diabetic rats were treated orally with tungstate for five weeks. Treated (STZ)-diabetic rats showed a partial recovery of exocrine and endocrine function, with lower glycemia, increased insulinemia and amylasemia, and increased beta cell mass achieved by reducing beta cell apoptosis and raising beta cell proliferation. The microarray analysis of the pancreases led to the identification of three groups of differentially expressed genes: genes altered due to diabetes, genes restored by the treatment, and genes specifically induced by tungstate in the diabetic animals. The results were corroborated by quantitative PCR. A detailed description of the pathways involved in the pancreatic effects of tungstate is provided in this paper. Hyperglycemia contribution was studied in STZ-diabetic rats treated with phloridzin, and the direct effect of tungstate was determined in INS-1E cells treated with tungstate or serum from untreated or treated STZ-rats, observing that tungstate action in the pancreas takes places via hyperglycemia-independent pathways and via a combination of tungstate direct and indirect (through the serum profile modification) effects. Finally, the MAPK pathway was evaluated, observing that it has a key role in the tungstate-induced increase of beta cell proliferation as tungstate activates the mitogen-activated protein kinase (MAPK) pathway directly by increasing p42/p44 phosphorylation and indirectly by decreasing the expression of raf kinase inhibitor protein (Rkip), a negative modulator of the pathway.

Conclusion

In conclusion, tungstate improves pancreatic function through a combination of hyperglycemia-independent pathways and through its own direct and indirect effects, whereas the MAPK pathway has a key role in the tungstate-induced increase of beta cell proliferation.

Hypoglycemic and antioxidant effects of phenolic extracts and purified hydroxytyrosol from olive mill waste in vitro and in rats

This study aimed to evaluate the effect of phenolic extract and purified hydroxytyrosol (HT) from olive mill waste (OMW) on oxidative stress and hyperglycemia in alloxan-induced diabetic rats. The OMW biophenols were extracted using ethyl acetate. The obtained extract was fractionated by solid phase extraction (SPE) experimentation to generate two fractions: (F1) and (F2). HPLC-UV and HPLC-MS analysis showed that (F1) was made of known OMW monomeric phenolics mainly hydroxytyrosol (HT) while (F2) contained oligomeric and polymeric phenols such as verbascosid and ligstrosid. (HT) was purified from (F1) using silica gel-column chromatography and silica gel-TLC techniques. In incubated pancreas, supplementation of OMW fractions enhanced insulin secretion. The administration of OMW extract fractions (F1) and (F2) as well as purified (HT) in diabetic rats caused a decrease in glucose level in plasma and an increase in renal superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) activities in liver and kidney. Furthermore, a protective action against hepatic and renal toxicity in diabetic rats was clearly observed. Furthermore, a significant decrease in hepatic and renal indices toxicity was observed, i.e. alkalines phosphatases (ALP), aspartate and lactate transaminases (AST and ALT) activities and the thiobarbituric acid-reactive substances (TBARs), total and direct bilirubin, creatinine and urea levels. In addition, (F1), (F2) and especially (HT) decreased triglycerides (TG), total-cholesterol (T-Ch) and higher HDL-cholesterol (HDL-Ch) in serum. These beneficial effects of OMW biophenols were confirmed by histological findings in hepatic, renal and pancreatic tissues of diabetic rats. This study demonstrates for the first time that OMW polyphenols and especially (HT) are efficient in inhibiting hyperglycemia and oxidative stress induced by diabetes and suggests that administration of HT may be helpful in the prevention of diabetic complications associated with oxidative stress.