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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 GIP-stimulated 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.

The therapeutic potential of glucagon-like peptide-1 (GLP-1) in improving glycaemic control in diabetes has been widely studied, but the potential beneficial effects of glucose-dependent insulinotropic polypeptide (GIP) have until recently been almost overlooked. One of the major problems, however, in exploiting either GIP or GLP-1 as potential therapeutic agents is their short duration of action, due to enzymatic degradation in vivo by dipeptidylpeptidase IV (DPP IV). Therefore, this study examined the plasma stability, biological activity and antidiabetic potential of two novel NH2-terminal Ala2-substituted analogues of GIP, containing glycine (Gly) or serine (Ser). Following incubation in plasma, (Ser2)GIP had a reduced hydrolysis rate compared with native GIP, while (Gly2)GIP was completely stable. In Chinese hamster lung fibroblasts stably transfected with the human GIP receptor, GIP, (Gly2)GIP and (Ser2)GIP stimulated cAMP production with EC(50) values of 18.2, 14.9 and 15.0 nM respectively. In the pancreatic BRIN-BD11 beta-cell line, (Gly2)GIP and (Ser2)GIP (10(-8) M) evoked significant increases (1.2- and 1.5-fold respectively; P<0.01 to P<0.001) in insulinotropic activity compared with GIP. In obese diabetic ob/ob mice, both analogues significantly lowered (P<0.001) the glycaemic excursion in response to i.p. glucose. This enhanced glucose-lowering ability was coupled to a significantly raised (P<0.01) and more protracted insulin response compared with GIP. These data indicate that substitution of the penultimate Ala2 in GIP by Gly or Ser confers resistance to plasma DPP IV degradation, resulting in enhanced biological activity, therefore raising the possibility of their use in the treatment of type 2 diabetes.

Glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are important enteroendocrine hormones that are rapidly degraded by an ubiquitous enzyme dipeptidyl peptidase IV to yield truncated metabolites GIP(3–42) and GLP-1(9–36)amide. In this study, we investigated the effects of sub-chronic exposure to these major circulating forms of GIP and GLP-1 on blood glucose control and endocrine pancreatic function in obese diabetic (ob/ob) mice. A once daily injection of either peptide for 14 days had no effect on body weight, food intake or pancreatic insulin content or islet morphology. GLP-1(9–36)amide also had no effect on plasma glucose homeostasis or insulin secretion. Mice receiving GIP(3–42) exhibited small but significant improvements in non-fasting plasma glucose, glucose tolerance and glycaemic response to feeding. Accordingly, plasma insulin responses were unchanged suggesting that the observed enhancement of insulin sensitivity was responsible for the improvement in glycaemic control. These data indicate that sub-chronic exposure to GIP and GLP-1 metabolites does not result in physiological impairment of insulin secretion or blood glucose control. GIP(3–42) might exert an overall beneficial effect by improving insulin sensitivity through extrapancreatic action.

Glucagon-like peptide-1(7-36)amide (GLP-1) possesses several unique and beneficial effects for the potential treatment of type 2 diabetes. However, the rapid inactivation of GLP-1 by dipeptidyl peptidase IV (DPP IV) results in a short half-life in vivo (less than 2 min) hindering therapeutic development. In the present study, a novel His(7)-modified analogue of GLP-1, N-pyroglutamyl-GLP-1, as well as N-acetyl-GLP-1 were synthesised and tested for DPP IV stability and biological activity. Incubation of GLP-1 with either DPP IV or human plasma resulted in rapid degradation of native GLP-1 to GLP-1(9-36)amide, while N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 were completely resistant to degradation. N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 bound to the GLP-1 receptor but had reduced affinities (IC(50) values 32.9 and 6.7 nM, respectively) compared with native GLP-1 (IC(50) 0.37 nM). Similarly, both analogues stimulated cAMP production with EC(50) values of 16.3 and 27 nM respectively compared with GLP-1 (EC(50) 4.7 nM). However, N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 exhibited potent insulinotropic activity in vitro at 5.6 mM glucose (P<0.05 to P<0.001) similar to native GLP-1. Both analogues (25 nM/kg body weight) lowered plasma glucose and increased plasma insulin levels when administered in conjunction with glucose (18 nM/kg body weight) to adult obese diabetic (ob/ob) mice. N-pyroglutamyl-GLP-1 was substantially better at lowering plasma glucose compared with the native peptide, while N-acetyl-GLP-1 was significantly more potent at stimulating insulin secretion. These studies indicate that N-terminal modification of GLP-1 results in DPP IV-resistant and biologically potent forms of GLP-1. The particularly powerful antihyperglycaemic action of N-pyroglutamyl-GLP-1 shows potential for the treatment of type 2 diabetes.