Oral metformin effectively inhibits DPP IV activity in Type 2 diabetic patients, suggesting that the drug may have potential for future combination therapy with incretin hormones.

Glucagon-like peptide-1(7-36)amide (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are gastrointestinal insulin-releasing hormones involved in the regulation of postprandial nutrient homeostasis. These two incretin hormones are glucose-dependent stimulators of pancreatic beta-cell function, exhibiting a spectrum of secondary extrapancreatic activities, which favour the efficient control of blood glucose homeostasis. Such actions of GLP-1 and GIP have generated considerable interest in their possible exploitation as novel agents for the treatment of type 2 diabetes. Despite the many attributes of GLP-1 and GIP as possible future antidiabetic agents, their rapid degradation in the circulation by dipeptidyl peptidase IV (DPP IV) to inactive truncated forms GLP-1(9-36)amide and GIP(3-42), severely limits their therapeutic usefulness. This review will consider recent developments in the design and effectiveness of synthetic DPP IV-resistant analogues of GLP-1 and GIP. Consideration will be given to the effects of N-terminal modification and amino acid substitution of GLP-1 and GIP either side of the DPP IV cleavage site on (i) susceptibility to enzymatic degradation, (ii) binding to native hormone receptor, (iii) ability to elevate intracellular cyclic AMP, (iv) potency as insulin secretagogues, and (v) antihyperglycaemic activity in type 2 diabetes. It will be shown that structural modification can produce a varied set of biological activities, ranging from more efficacious analogues to those which antagonise the activity of the native hormone. The antidiabetic properties of the best GLP-1 and GIP analogues indeed promise to provide the basis for novel, effective and long-acting drugs for type 2 diabetes therapy. This approach is currently being pursued actively by the pharmaceutical industry.

The effects of different classes of amino acids known to be transported and utilized by pancreatic B-cells were examined using the novel glucose-responsive pancreatic B-cell line, BRIN-BD11. Amino acids tested included alpha-aminoisobutyric acid, L-alanine, L-arginine, L-glutamine, glycine, L-leucine, L-lysine, L-proline and L-serine. At non-stimulatory (1.1 mmol/l) glucose, acute incubations with either 1 or 10 mmol/l amino acid evoked 1.3- to 4.7-fold increases of insulin release. Raising glucose to 16.7 mmol/l enhanced the effects of all amino acids except L-glutamine, and increased insulin output at 10 mmol/l compared with 1 mmol/l amino acid. Glyceraldehyde (10 mmol/l) also served to promote 10 mmol/l amino acid-induced insulin secretion with the exceptions of L-arginine, glycine, L-lysine and L-proline. At 16.7 mmol/l glucose, diazoxide (300 mumol/l) significantly decreased the secretory response to all amino acids except L-glutamine. Likewise, verapamil (20 mumol/l) or depletion of extracellular Ca2+ reduced insulin output indicating the importance of Ca2+ influx in the actions of amino acids. These data indicate that BRIN-BD11 cells transport and utilize amino acids, acting in association with glycolysis, K(+)-ATP channels and/or voltage-dependent Ca2+ channels to promote Ca2+ influx and insulin secretion. The response of BRIN-BD11 cells to glucose and amino acids indicates that this is a useful cell line for future research on the mechanisms of nutrient regulation of insulin secretion.

Aims/hypothesis Cholecystokinin (CCK) is a rapidly degraded gastrointestinal peptide that stimulates satiety and insulin secretion. We aimed to investigate the beneficial weight-lowering and metabolic effects of the novel N-terminally modified CCK analogue, (pGlu-Gln)-CCK-8. Methods The biological actions of (pGlu-Gln)-CCK-8 were comprehensively evaluated in pancreatic clonal BRIN BD11 cells and in vivo in high-fat-fed and ob/ob mice. Results (pGlu-Gln)-CCK-8 was completely resistant to enzymatic degradation and its satiating effects were significantly (p<0.05 to p<0.001) more potent than CCK-8. In BRIN-BD11 cells, (pGlu-Gln)-CCK-8 exhibited enhanced (p<0.01 to p<0.001) insulinotropic actions compared with CCK-8. When administered acutely to high-fat-fed or ob/ob mice, (pGlu-Gln)-CCK-8 improved glucose homeostasis. Sub-chronic twice daily injections of (pGlu-Gln)-CCK-8 in high-fat-fed mice for 28 days significantly decreased body weight (p<0.05 to p<0.001), accumulated food intake (p< 0.05 to p<0.001), non-fasting glucose (p<0.05) and triacylglycerol deposition in pancreatic (p<0.01), adipose (p< 0.05) and liver (p<0.001) tissue, and improved oral (p< 0.05) and i.p. (p<0.05) glucose tolerance and insulin sensitivity (p<0.001). Similar observations were noted in ob/ob mice given twice daily injections of (pGlu-Gln)-CCK-8. In addition, these beneficial effects were not reproduced by simple dietary restriction and were not associated with changes in energy expenditure. There was no evidence for development of tolerance to (pGlu-Gln)-CCK-8, and analysis of histology or blood-borne markers for pancreatic, liver and renal function in mice treated with (pGlu-Gln)-CCK-8 suggested little abnormal pathology. Conclusions/interpretation These studies emphasise the potential of (pGlu-Gln)-CCK-8 for the alleviation of obesity and insulin resistance.

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The incretin hormone glucose-dependent insulinotropic polypeptide (GIP) is rapidly degraded in the circulation by dipeptidyl peptidase IV forming the N-terminally truncated peptide GIP(3-42). The present study examined the biological activity of this abundant circulating fragment peptide to establish its possible role in GIP action. Human GIP and GIP(3-42) were synthesised by Fmoc solid-phase peptide synthesis, purified by HPLC and characterised by electrospray ionisation-mass spectrometry. In GIP receptor-transfected Chinese hamster lung fibroblasts, GIP(3-42) dose dependently inhibited GIP-stimulated (10 7 M) cAMP production (up to 75·4 5·4%; P<0·001). In BRIN-BD11 cells, GIP(3-42) was significantly less potent at stimulating insulin secretion (1·9-to 3·2-fold; P<0·001), compared with native GIP and significantly inhibited GIP-stimulated (10 7 M) insulin secretion with maximal inhibition (48·8 6·2%; P<0·001) observed at 10 7 M. In (ob/ob) mice, administration of GIP(3-42) significantly inhibited GIPstimulated insulin release (2·1-fold decrease; P<0·001) and exaggerated the glycaemic excursion (1·4-fold; P<0·001) induced by a conjoint glucose load. These data indicate that the N-terminally truncated GIP(3-42) fragment acts as a GIP receptor antagonist, moderating the insulin secreting and metabolic actions of GIP in vivo.

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Aims/hypothesis Ablation of gastric inhibitory polypeptide (GIP) receptor action is reported to protect against obesity and associated metabolic abnormalities. The aim of this study was to use prediabetic ob/ob mice to examine whether 60 days of chemical GIP receptor ablation with (Pro 3 )GIP is able to counter the development of genetic obesityrelated diabetes. Materials and methods Young (5-7 weeks) ob/ob mice received once daily i.p. injections of either saline vehicle or (Pro 3 )GIP (25 nmol kg −1 day −1 ) over a 60 day period. Food intake, body weight and circulating glucose and insulin were measured at frequent intervals. At 60 days, glucose tolerance, response to native GIP, postprandial responses, insulin sensitivity, HbA 1c , circulating hormones and plasma lipids were assessed. Results Body weight and food intake in (Pro 3 )GIP-treated mice did not differ from ob/ob controls. GIP receptor blockade significantly improved non-fasting glucose (p< 0.001), HbA 1c (p<0.05), glucose tolerance (p<0.001), meal tolerance (p<0.001) and insulin sensitivity (p<0.05). Remarkably, (Pro 3 )GIP treatment prevented the age-related development of diabetes, as none of these parameters differed significantly between treated ob/ob mice and normal age-matched lean controls. Circulating levels of glucagon, corticosterone, adiponectin and total cholesterol were unchanged by (Pro 3 )GIP, while levels of triacylglycerol, LDLcholesterol and resistin were decreased (p<0.05) compared with those in control ob/ob mice. Plasma and pancreatic insulin concentrations were generally lower after (Pro 3 )GIP treatment than in control ob/ob mice (p<0.01), but plasma insulin levels remained substantially raised (p<0.001) compared with those observed in lean controls. Conclusions/interpretation These data indicate that sustained GIP receptor antagonism provides an effective means of preventing the development of many of the metabolic abnormalities of obesity-driven diabetes.