Investigation of the effects of sulfonylurea exposure on pancreatic beta cell metabolism

Beta-cell function in type 2 diabetic patients who failed to maintain good glycemic status with a combination of maximum dosages of metformin and sulfonylurea

Background: The aim of this study was to investigate beta-cell function and examine whether sulfonylureas (SUs) are still useful in patients with type 2 diabetes (T2DM) who failed to maintain optimal glycemic control with a combination of maximum dosages of metformin and SU. Method: T2DM who had Hb_A1c >8% during treatment with a combination of maximum dosages of metformin and SU were studied. After enrollment, the patients were assigned to continue maximum dosages of SU and metformin for 2 weeks and then underwent the first oral glucose tolerance test (OGTT), the Max-SU OGTT. After the Max-SU OGTT, SUs were discontinued for 4 weeks and the second OGTT, the Discont-SU OGTT, was performed. After the Discont-SU OGTT, the same SU was restarted at 25% of the maximum dosage (25%Max-SU). After taking 25%Max-SU for 4 weeks, the third OGTT, the 25%Max-SU OGTT, was performed. Metformin at the same dosage was continued throughout the study. Normal OGTT (NGT) subjects, matched for age and body mass index (BMI), were also studied. Results: There were 25 T2DM and 28 NGT subjects. There was no difference in age and BMI between the two groups. The beta-cell function during Max-SU was 0.1, which was higher than 0.06 during Discont-SU (p<0.001) and also higher than 0.09 during 25%Max-SU (p=0.269). The beta-cell function during 25%Max-SU was higher than during Discont-SU (p<0.001). The beta-cell function of the NGT group was 0.34 and higher than during Max-SU (p<0.001). Fasting capillary blood glucose (FCBG) levels during Discont-SU (14.2±3.7 mmol/L) were higher than during 25%Max-SU (12.3±3.4 mmol/L) and during Max-SU (10.3±2.4 mmol/L) (p<0.05). In addition, the FCBG during Discont-SU was higher than that during 25%Max-SU (p<0.05). Conclusion: In T2DM patients who failed to achieve glycemic control with a combination of maximum dosages of metformin and SU, the beta-cell function declined compared to NGT subjects. However, the beta-cells were still responsive to SUs, which play a significant role in glycemic control.

The Pancreatic β-Cell: A Bioenergetic Perspective

The pancreatic β-cell secretes insulin in response to elevated plasma glucose. This review applies an external bioenergetic critique to the central processes of glucose-stimulated insulin secretion, including glycolytic and mitochondrial metabolism, the cytosolic adenine nucleotide pool, and its interaction with plasma membrane ion channels. The control mechanisms responsible for the unique responsiveness of the cell to glucose availability are discussed from bioenergetic and metabolic control standpoints. The concept of coupling factor facilitation of secretion is critiqued, and an attempt is made to unravel the bioenergetic basis of the oscillatory mechanisms controlling secretion. The need to consider the physiological constraints operating in the intact cell is emphasized throughout. The aim is to provide a coherent pathway through an extensive, complex, and sometimes bewildering literature, particularly for those unfamiliar with the field.

Metabolomic analysis of pancreatic beta cells following exposure to high glucose

BACKGROUND

Chronic exposure to hyperglycaemic conditions has been shown to have detrimental effects on beta cell function. The resulting glucotoxicity is a contributing factor to the development of type 2 diabetes. The objective of this study was to combine a metabolomics approach with functional assays to gain insight into the mechanism by which glucotoxicity exerts its effects.

METHODS

The BRIN-BD11 and INS-1E beta cell lines were cultured in 25 mM glucose for 20 h to mimic glucotoxic effects. PDK-2 protein expression, intracellular glutathione levels and the change in mitochondrial membrane potential and intracellular calcium following glucose stimulation were determined. Metabolomic analysis of beta cell metabolite extracts was performed using GC-MS, 1H NMR and 13C NMR.

RESULTS

Conditions to mimic glucotoxicity were established and resulted in no loss of cellular viability in either cell line while causing a decrease in insulin secretion. Metabolomic analysis of beta cells following exposure to high glucose revealed a change in amino acids, an increase in glucose and a decrease in phospho-choline, n-3 and n-6 PUFAs during glucose stimulated insulin secretion relative to cells cultured under control conditions. However, no changes in calcium handling or mitochondrial membrane potential were evident.

CONCLUSIONS

Results indicate that a decrease in TCA cycle metabolism in combination with an alteration in fatty acid composition and phosphocholine levels may play a role in glucotoxicity induced impairment of glucose stimulated insulin secretion.

GENERAL SIGNIFICANCE

Alterations in certain metabolic pathways play a role in glucotoxicity in the pancreatic beta cell.

The pyruvate carboxylase-pyruvate dehydrogenase axis in islet pyruvate metabolism: Going round in circles?

Pyruvate is the major product of glycolysis in pancreatic β-cells, and its ultimate metabolic fate depends on the relative activities of two enzymes. The first, pyruvate carboxylase (PC) replenishes oxaloacetate withdrawn from the tricarboxylic acid (TCA) cycle via the carboxylation of pyruvate to form oxaloacetate. Flux via PC is also involved in the formation of NADPH, one of several important coupling factors for insulin secretion. In most tissues, PC activity is enhanced by increased acetyl-CoA. The alternative fate of pyruvate is its oxidative decarboxylation to form acetyl-CoA via the pyruvate dehydrogenase complex (PDC). The ultimate fate of acetyl-CoA carbon is oxidation to CO2 via the TCA cycle, and so the PDC reaction results of the irreversible loss of glucose-derived carbon. Thus, PDC activity is stringently regulated. The mechanisms controlling PDC activity include end-product inhibition by increased acetyl-CoA, NADH and ATP, and its phosphorylation (inactivation) by a family of pyruvate dehydrogenase kinases (PDHKs 1-4). Here we review new developments in the regulation of the activities and expression of PC, PDC and the PDHKs in the pancreatic islet in relation to islet pyruvate disposition and glucose-stimulated insulin secretion (GSIS).

NMR metabonomics for mammalian cell metabolism studies

The detailed knowledge of mammalian cell metabolism and its adjustments to different cell properties and perturbations, such as disease and drug exposure, is of enormous value in the deeper understanding of pathological processes and drug mechanisms, as well as in the development of new and improved methods for diagnosis, follow-up of disease progression and treatment response. This review covers recent developments in the use of NMR-based metabonomics to characterize cellular metabolomes and interpret them in terms of metabolic changes taking place in a wide range of situations. The analytical methodology available is briefly presented and the applications developed so far are reviewed. These include differences in cell properties (e.g., drug resistance, cell cycle stage, specific growth conditions and genetic characteristics) and changes induced in response to different perturbations (e.g., disease, drug exposure and irradiation).

Effects of metformin on BRIN-BD11 beta-cell insulin secretory desensitization induced by prolonged exposure to sulphonylureas

AIMS

Prolonged exposure of pancreatic beta-cells in vitro to the sulphonylureas tolbutamide and glibenclamide induces subsequent desensitization of insulinotropic pathways. Clinically, the insulin-sensitizing biguanide drug metformin is often administered alongside sulphonylurea as antidiabetic therapy. The present study examines the functional effects of metformin (200 µM) on tolbutamide- and glibenclamide-induced desensitisation.

METHODS

Acute and prolonged (18 h) effects of exposure to tolbutamide and glibenclamide alone, or in the presence of metformin, were examined in insulin-secreting BRIN-BD11 cells.

RESULTS

In acute 20 min incubations at 1.1 mM glucose, metformin increased (1.2-1.7-fold; p < 0.001) the insulin-releasing actions of tolbutamide and glibenclamide. At 16.7 mM glucose, metformin significantly enhanced glibenclamide-induced insulin release at all concentrations (50-400 µM) examined, but tolbutamide-stimulated insulin secretion was only augmented at higher concentrations (300-400 µM). Exposure for 18 h to 100 µM tolbutamide or glibenclamide significantly impaired insulin release in response to glucose and a broad range of insulin secretagogues. Concomitant culture with metformin (200 µM) prevented or partially reversed many of the adverse effects on K(ATP) channel dependent and independent insulinotropic pathways. Beneficial effects of metformin were also observed in cells exposed to glibenclamide for 18 h with significant improvements in the insulin secretory responsiveness to alanine, GLP-1 and sulphonylureas. The decrease of viable cell numbers observed with glibenclamide was reversed by co-culture with metformin, but cellular insulin content was depressed.

CONCLUSIONS

The results suggest that metformin can prevent the aspects of sulphonylurea-induced beta-cell desensitization.

Prolonged L-alanine exposure induces changes in metabolism, Ca2+ handling and desensitization of insulin secretion in clonal pancreatic β-cells

Acute insulin-releasing actions of amino acids have been studied in detail, but comparatively little is known about the β-cell effects of long-term exposure to amino acids. The present study examined the effects of prolonged exposure of β-cells to the metabolizable amino acid L-alanine. Basal insulin release or cellular insulin content were not significantly altered by alanine culture, but acute alanine-induced insulin secretion was suppressed by 74% (P<0.001). Acute stimulation of insulin secretion with glucose, KCl or KIC (2-oxoisocaproic acid) following alanine culture was not affected. Acute alanine exposure evoked strong cellular depolarization after control culture, whereas AUC (area under the curve) analysis revealed significant (P<0.01) suppression of this action after culture with alanine. Compared with control cells, prior exposure to alanine also markedly decreased (P<0.01) the acute elevation of [Ca2+]i (intracellular [Ca2+]) induced by acute alanine exposure. These diminished stimulatory responses were partially restored after 18 h of culture in the absence of alanine, indicating reversible amino-acid-induced desensitization. 13C NMR spectra revealed that alanine culture increased glutamate labelling at position C4 (by 60%; P<0.01), as a result of an increase in the singlet peak, indicating increased flux through pyruvate dehydrogenase. Consistent with this, protein expression of the pyruvate dehydrogenase kinases PDK2 and PDK4 was significantly reduced. This was accompanied by a decrease in cellular ATP (P<0.05), consistent with diminished insulin-releasing actions of this amino acid. Collectively, these results illustrate the phenomenon of β-cell desensitization by amino acids, indicating that prolonged exposure to alanine can induce reversible alterations to metabolic flux, Ca2+ handling and insulin secretion.

Polymer-phloridzin conjugates as an anti-diabetic drug that inhibits glucose absorption through the Na+/glucose cotransporter (SGLT1) in the small intestine

Poly(gamma-glutamic acid)s (gamma-PGA) modified with phloridzin, which is an inhibitor of the Na(+)/glucose cotransporter (SGLT1), via a omega-amino triethylene glycol linker were synthesized. The potential of gamma-PGA-phloridzin conjugates (PGA-PRZs) obtained as a novel oral anti-diabetic drug was examined by in vitro and in vivo experiments. A PGA-PRZ with a 15% phloridzin content inhibited glucose transport from mucosal to serosal sides of the everted rat's small intestine, and its inhibitory effect was as strong as that of intact phloridzin. When the PGA-PRZ was given orally to rats before glucose administration, the glucose-induced hyperglycemic effect was significantly suppressed. On the other hand, reduction of an increase in the blood glucose concentration was scarcely observed when the PGA-PRZ was substituted with a double amount of intact phloridzin. This difference in the biological activity between PGA-PRZ and intact phloridzin might have resulted from the improved stability of a glucoside bond of phloridzin through the conjugation with gamma-PGA. These results suggest that the gamma-PGA-phloridzin conjugates have potential as oral anti-diabetic drugs.