Biochemical properties and biological actions of obestatin and its relevence in type 2 diabetes

Obestatin was initially discovered in rat stomach extract, and although it is principally produced in the gastric mucosa, it can be found throughout the gastrointestinal tract. This 23-amino acid C-terminally amidated peptide is derived from preproghrelin and has been ascribed a wide range of metabolic effects relevant to type 2 diabetes. Obestatin reportedly inhibits gastrointestinal motility, reduces food intake and lowers body weight and improves lipid metabolism. Furthermore, it appears to exert actions on the pancreatic β-cell, most notably increasing β-cell mass and upregulating genes associated with insulin production and β-cell regeneration, with relevance to type 2 diabetes. It is becoming evident that obestatin also exerts pleiotropic effects on the cardiovascular system, possibly modulating blood pressure, endothelial function and triggering cardioprotective mechanisms, which may be important in determining cardiovascular outcomes in type 2 diabetes. Furthermore, it seems that like other gut peptides obestatin has neuroprotective properties. This review examines the biochemical properties of the obestatin peptide (its structure, sequence, stability and distribution) and the candidate receptors through which it may act. It provides a balanced examination of the reported pancreatic and extrapancreatic actions of obestatin and evaluates its potential relevance with respect to diabetes therapy, together with discussion of direct evidence linking alterations in obestatin signalling with obesity/diabetes and other diseases.

The incretin mimetic, exenatide: a novel treatment option for type 2 diabetes

Exenatide, (BYETTA TM), is the first in a new class of agents termed incretin mimetics, which replicate several glucoregulatory effects of the endogenous incretin hormone, glucagon-like peptide-1 (GLP-1). Currently approved (April 2005) in the United States as an injectable adjunct to metformin and/or sulphonylurea therapy, exenatide improves glycaemic control through multiple mechanisms of action including: enhancement of glucose-dependent insulin secretion, restoration of first-phase insulin response, suppression of inappropriately elevated glucagon secretion, slowing of gastric emptying and reduction in food intake. Subcutaneous exenatide injected twice-daily before morning and evening meals shows immediate and sustained effects on both postprandial and fasting glucose concentrations and is accompanied by reductions in body weight. Since the actions of exenatide on insulin and glucagon secretion are glucose dependent, the risk of hypoglycaemia is minimised except when exenatide is used in conjunction with agents that induce hypoglycaemia, such as sulphonylureas. Mild-to-moderate nausea is the most common side effect, which can be reduced with dose titration upon initiation of therapy. The incretin mimetic, exenatide, is a novel therapeutic option to improve glycaemic control with potential weight reduction in patients with type 2 diabetes suboptimally controlled with metformin or a sulphonylurea.

Review: Saxagliptin: a review

Modulation of the effects of incretin hormones provides a novel mechanism of action for some of the newer therapies for patients with type 2 diabetes. The selective, reversible dipeptidyl peptidase-4 inhibitor saxagliptin has demonstrated robust improvements in glycaemic control, as monotherapy or as add-on therapy to metformin, sulphonylureas and thiazolidinediones, without significant change in body weight and while exhibiting a low risk of hypoglycaemia.

Fatty acid derivatised analogues of glucose-dependent insulinotropic polypeptide with improved antihyperglycaemic and insulinotropic properties

C-terminal acylation of Lys(37) with myristic (MYR; tetradecanoic acid), palmitic (PAL; hexadecanoic acid) and stearic (octadecanoic acid) fatty acids with or without N-terminal acetylation was employed to develop long-acting analogues of the glucoregulatory hormone, glucose-dependent insulinotropic polypeptide (GIP). All GIP analogues exhibited resistance to dipeptidylpeptidase-IV (DPP-IV) and significantly improved in vitro cAMP production and insulin secretion. Administration of GIP analogues to ob/ob mice significantly lowered plasma glucose-GIP(Lys(37)MYR), N-AcGIP(Lys(37)MYR) and GIP(Lys(37)PAL) increased plasma insulin concentrations. GIP(Lys(37)MYR) and N-AcGIP(Lys(37)MYR) elicited protracted glucose-lowering effects when administered 24h prior to an intraperitoneal glucose load. Daily administration of GIP(Lys(37)MYR) and N-AcGIP(Lys(37)MYR) to ob/ob mice for 24 days decreased glucose and significantly improved plasma insulin, glucose tolerance and beta-cell glucose responsiveness. Insulin sensitivity, pancreatic insulin content and triglyceride levels were not changed. These data demonstrate that C-terminal acylation particularly with myristic acid provides a class of stable, longer-acting forms of GIP for further evaluation in diabetes therapy.

Incretin hormone mimetics and analogues in diabetes therapeutics

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are physiological gut peptides with insulin-releasing and extrapancreatic glucoregulatory actions. Incretin analogues/mimetics activate GLP-1 or GIP receptors whilst avoiding physiological inactivation by dipeptidyl peptidase 4 (DPP-4), and they represent one of the newest classes of antidiabetic drug. The first clinically approved GLP-1 mimetic for the treatment of type-2 diabetes is exenatide (Byetta/exendin) which is administered subcutaneously twice daily. Clinical trials of liraglutide, a GLP-1 analogue suitable for once-daily administration, are ongoing. A number of other incretin molecules are at earlier stages of development. This review discusses the various attributes of GLP-1 and GIP for diabetes treatment and summarises current clinical data. Additionally, it explores the therapeutic possibilities offered by preclinical agents, such as non-peptide GLP-1 mimetics, GLP-1/glucagon hybrid peptides, and specific GIP receptor antagonists.

Effects of subchronic treatment with the long-acting glucose-dependent insulinotropic polypeptide receptor agonist, N-AcGIP, on glucose homeostasis in streptozotocin-induced diabetes

OBJECTIVES

N-AcGIP is a potent and dipeptidylpeptidase IV-resistant analogue of glucose-dependent insulinotropic polypeptide with significantly improved antidiabetic actions in type 2 diabetes. The present study investigated the effects of subchronic treatment with N-AcGIP on glucose homeostasis in a type 1 model, namely, streptozotocin (STZ)-induced diabetic mice.

METHODS

Swiss TO mice given a single intraperitoneal injection of STZ (150 mg/kg body weight) received once-daily injection of N-AcGIP (25 nmol/kg body weight) or saline for 20 days and effects on metabolic parameters and islet architecture assessed.

RESULTS

Daily injection of N-AcGIP for 20 days did not significantly alter the characteristic STZ-induced changes of pancreatic insulin content, body weight, food intake, glucose, and glycated hemoglobin levels. Glucose tolerance and insulin sensitivity were also unchanged by N-AcGIP treatment. Circulating insulin was undetectable, and the number of intact islets and insulin expression was greatly reduced in both groups. Some proliferative activity was identified by 5-bromo-2-deoxyuridine staining in the pancreas, but this and expression of glucagon and somatostatin were similar in the 2 groups.

CONCLUSIONS

These data indicate that subchronic treatment with the long-acting glucose-dependent insulinotropic polypeptide receptor agonist, N-AcGIP, does not have beneficial effects in insulin-deficient STZ-diabetic mice. This supports the primary antidiabetic action of this analogue in type 2 diabetes as stimulation of beta-cell function and insulin secretion.

Characterisation and biological activity of Glu3 amino acid substituted GIP receptor antagonists

Glucose-dependent insulinotropic polypeptide (GIP) is an important gastrointestinal hormone, which regulates insulin release and glucose homeostasis, but is rapidly inactivated by enzymatic N-terminal truncation. Here we report the enzyme resistance and biological activity of several Glu(3)-substituted analogues of GIP namely; (Ala(3))GIP, (Lys(3))GIP, (Phe(3))GIP, (Trp(3))GIP and (Tyr(3))GIP. Only (Lys(3))GIP demonstrated moderately enhanced resistance to DPP-IV (p<0.05 to p<0.01) compared to native GIP. All analogues demonstrated a decreased potency in cAMP production (EC(50) 1.47 to 11.02 nM; p<0.01 to p<0.001) with (Lys(3))GIP and (Phe(3))GIP significantly inhibiting GIP-stimulated cAMP production (p<0.05). In BRIN-BD11 cells, (Lys(3))GIP, (Phe(3))GIP, (Trp(3))GIP and (Tyr(3))GIP did not stimulate insulin secretion with both (Lys(3))GIP and (Phe(3))GIP significantly inhibiting GIP-stimulated insulin secretion (p<0.05). Injection of each GIP analogue together with glucose in ob/ob mice significantly increased the glycaemic excursion compared to control (p<0.05 to p<0.001). This was associated with lack of significant insulin responses. (Ala(3))GIP, (Phe(3))GIP and (Tyr(3))GIP, when administered together with GIP, significantly reduced plasma insulin (p<0.05 to p<0.01) and impaired the glucose-lowering ability (p<0.05 to p<0.01) of the native peptide. The DPP-IV resistance and GIP antagonism observed were similar but less pronounced than (Pro(3))GIP. These data demonstrate that position 3 amino acid substitution of GIP with (Ala(3)), (Phe(3)), (Tyr(3)) or (Pro(3)) provides a new class of functional GIP receptor antagonists.

Soluble dietary fibre fraction of Trigonella foenum-graecum (fenugreek) seed improves glucose homeostasis in animal models of type 1 and type 2 diabetes by delaying carbohydrate digestion and absorption, and enhancing insulin action

Trigonella foenum-graecum (fenugreek) seeds have been documented as a traditional plant treatment for diabetes. In the present study, the antidiabetic properties of a soluble dietary fibre (SDF) fraction of T. foenum-graecum were evaluated. Administration of SDF fraction (0 x 5 g/kg body weight) to normal, type 1 or type 2 diabetic rats significantly improved oral glucose tolerance. Total remaining unabsorbed sucrose in the gastrointestinal tract of non-diabetic and type 2 diabetic rats, following oral sucrose loading (2 x 5 g/kg body weight) was significantly increased by T. foenum-graecum (0 x 5 g/kg body weight). The SDF fraction suppressed the elevation of blood glucose after oral sucrose ingestion in both non-diabetic and type 2 diabetic rats. Intestinal disaccharidase activity and glucose absorption were decreased and gastrointestinal motility increased by the SDF fraction. Daily oral administration of SDF to type 2 diabetic rats for 28 d decreased serum glucose, increased liver glycogen content and enhanced total antioxidant status. Serum insulin and insulin secretion were not affected by the SDF fraction. Glucose transport in 3T3-L1 adipocytes and insulin action were increased by T. foenum-graecum. The present findings indicate that the SDF fraction of T. foenum-graecum seeds exerts antidiabetic effects mediated through inhibition of carbohydrate digestion and absorption, and enhancement of peripheral insulin action.

Dipeptidyl peptidase IV (DPP IV) inhibitors: A newly emerging drug class for the treatment of type 2 diabetes

Inhibitors of the enzyme dipeptidyl peptidase IV (DPP IV) provide a strategy for the treatment of type 2 diabetes. DPP IV rapidly inactivates the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Inhibition of DPP IV prolongs and enhances the activity of endogenous GLP-1 and GIP, which serve as important prandial stimulators of insulin secretion and regulators of blood glucose control. In clinical trials DPP IV inhibitors (or 'gliptins') have shown efficacy and tolerability in the management of hyperglycaemia in type 2 diabetes, without causing weight gain or hypoglycaemia.

Inhibition of dipeptidyl peptidase IV activity as a therapy of Type 2 diabetes

Dipeptidyl peptidase IV (DPP IV) is a ubiquitous, multifunctional, serine protease enzyme and receptor with roles in the control of endocrine and immune function, cell metabolism, growth and adhesion. As an enzyme, DPP IV cleaves the N-terminal dipeptide from the incretin hormones glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide. This inactivates the hormones, thereby cancelling their prandial insulinotropic effect. One approach to restore incretin activity as a therapy for Type 2 diabetes has been the development of DPP IV inhibitors. Inhibitors of DPP IV have shown efficacy and tolerability when used to control the hyperglycaemia of noninsulin-dependent animal models and human Type 2 diabetes. These DPP IV inhibitors prolong active incretin hormone concentrations and may exert additional antidiabetic effects. If long-term clinical trials confirm sustained and safe control of blood glucose, DPP IV inhibitors (known as 'gliptins') may be expected to provide a new treatment modality for Type 2 diabetes.

Inhibition of dipeptidyl peptidase-IV activity by metformin enhances the antidiabetic effects of glucagon-like peptide-1

GLP-1 and GIP are insulin-releasing 'incretin' hormones inactivated following degradation by dipeptidyl peptidase IV. Incretin hormone analogues resistant to degradation by DPP IV, as well as, inhibitors of DPP IV are in development as novel treatments for type 2 diabetes. The biguanide metformin is an oral agent commonly prescribed to treat type 2 diabetes. Antidiabetic actions of metformin involve the reduction of hepatic glucose production and/or insulin resistance. Recent reports indicate that metformin may have the additional property of inhibiting DPP IV activity. Here we examine the effects of metformin on plasma DPP IV activity of normal and ob/ob diabetic mice. DPP IV activity present in mouse plasma was concentration-dependently inhibited by metformin generating IC(50) values of 38 microM for normal mice and 29 microM for ob/ob mice. In vivo metformin lowered plasma DPP IV activity in ob/ob mice, and improved glucose-lowering and insulin-releasing effects of exogenous GLP-1 administration. This was associated with increased circulating concentrations of active GLP-1(7-36)amide. In contrast metformin had minor effects on in vitro GLP-1-stimulated insulin release from clonal beta cells. Long-term (12 day) oral metformin administration to ob/ob mice resulted in lower DPP IV activity but had no effect on basal glucose and insulin levels. These findings indicate that metformin decreases the plasma DPP IV activity, limiting the inactivation of exogenously administered GLP-1 and improving glycaemic control.

Drugs on the horizon for diabesity

The coexistence of type 2 diabetes and obesity presents a complex therapeutic challenge. Future combination tablets may include agents to address diabetes and any accompanying cardiovascular risk factors. Injectable agents that improve glycemic control and facilitate weight loss have recently become available: the soluble amylin analogue pramlintide provides an adjunct to insulin therapy in type 1 and type 2 diabetes, and the incretin mimetic exenatide can enhance prandial insulin release in type 2 diabetes. Orally active inhibitors of the incretin-degrading enzyme dipeptidyl peptidase-IV, agonists of peroxisome proliferator-activated receptor (PPAR)-a and PPAR-g ("dual PPARs"), and the CB1 cannabinoid receptor inhibitor rimonabant are advanced in clinical development. Many novel antidiabetic and antiobesity compounds are emerging in preclinical development.