In 1998, Pederson et al.18 studied the oral administration of the DPP-4 inhibitor iso-thiazolidide (20 μmol/300 g bodyweight) together with glucose (1 g/kg) in obese and lean Zucker rats. They found that the insulin response to oral glucose was augmented with the DPP-4 inhibitor along with improved glucose tolerance and that the effects were more pronounced in the obese versus the lean rats. Also Balkan et al.19 explored the potential of DPP-4 inhibition to stimulate insulin secretion after oral glucose in obese and lean Zucker rats, as shown in a study published in 1999. They administered the DPP-4 inhibitor NVP-DPP728 at 10 μmol/kg through an oral tube 30 min before administration of glucose (1 g/kg). They found that in obese Zucker rats, NVP-DPP728 augmented the insulin response to oral glucose in association with improved glucose tolerance. These effects were also observed in the lean Zucker rats, although to a lesser degree. Results from a paracetamol test showed that gastric emptying was not affected by NVP-DPP728, suggesting that the primary reason for the improved glucose tolerance was the increased insulin response. The authors also showed that the active GLP-1 concentrations were augmented by DPP-4 inhibition; therefore, the authors concluded that NVP-DPP728, through increasing GLP-1 levels, stimulates insulin secretion in rats19. The following year, it was also shown that the DPP-4 inhibitor, valine pyrrolidide (100 μmol/kg), when given through oral gavage, augmented the insulin response and improved glucose tolerance after oral glucose (150 mg) in both control mice and in mice rendered insulin resistant by a high-fat diet20. Also in that study, the GLP-1 response to oral glucose was increased by DPP-4 inhibition, again suggesting that DPP-4 inhibition augments insulin secretion after oral glucose by increasing the active concentrations of GLP-120. Similar augmentation of the insulin response to glucose gavage has been shown for LAF 237 (vildagliptin) in control and high-fat fed mice21. Furthermore, sitagliptin has been shown to stimulate insulin secretion from isolated islets and in the perfused pancreas after 10 weeks of treatment in a diabetes model in mice consisting of a combination of streptozotocin with high-fat feeding22. Also, BI1356 (linagliptin; 3 mg/kg) increased the insulin response to oral glucose in Zucker fatty rats along with improved glucose tolerance23. Hence, it is now well documented in animal studies that DPP-4 inhibition stimulates insulin secretion, and that the effect is pronounced in insulin-resistant animals. The latter finding is corroborated by the demonstration that the insulin secretory response to GLP-1 is augmented in insulin-resistant mice compared with normal mice24. The hypothesis explaining these findings is that the islet adaptation to insulin resistance that involves upregulated insulin secretion also involves an increased sensitivity to GLP-1.
Several animal studies have explored the mechanisms of improved β-cell function after DPP-4 inhibition in animal studies. A most likely explanation is that GLP-1 contributes. However, there are also other potential substrates for DPP-4 that might contribute. One such substrate is GIP, the concentration of which is also increased after DPP-4 inhibition25, and another potential substrate for DPP-4 is the neuropeptide, pituitary adenylate cylcase activating polypeptide (PACAP)26. Interestingly, the insulin secretory responses to all these three bioactive peptides (GLP-1, GIP and PACAP) have been shown to be augmented by the DPP-4 inhibitor, valine-pyrrolidide, in model experiments in mice27. However, a study in mice with genetic deletion of GIP and GLP-1 receptors (DIRKO; double incretin receptor knockout mice) showed that after oral glucose (1.5 mg/g bodyweight), neither valine pyrrolidide nor the DPP-4 inhibitor, SYR106124, augmented the insulin response, and neither of these two DPP-4 inhibitors nor the DPP-4 inhibitors TP8211 or LAF237 (vildagliptin) had an effect on glucose tolerance28. This would suggest that the acute influences on insulin secretion by DPP-4 inhibition are mediated only by the incretin hormones. Nevertheless, a contribution by other bioactive peptides can at present not be excluded, especially on a long-term basis, and needs to be explored further.
Although incretin hormone receptors, such as GLP-1 receptors, are expressed in the β-cells, a recent study suggested that a direct β-cell action of the incretin hormones might not entirely explain the improved islet function after DPP-4 inhibition29. The alternate mechanism might be an indirect action through stimulation of afferent nerves in the gut. The evidence for this is that oral administration of a low dose of the DPP-4 inhibitor, sitagliptin, inhibited DPP-4 activity in the gut, but not in the circulation, and this low-dose sitagliptin was sufficient to augment the insulin response to oral glucose; furthermore, the effect was associated with increased nerve activity and absent in mice with deletion of the GIP or GLP-1 receptors. The results are compatible with the view that incretin hormones released after oral glucose are stabilized locally in the gut by DPP-4 inhibition and that this local DPP-4 inhibition, through prevented local incretin hormone inactivation, is sufficient to activate local afferent nerves that mediate the signal to stimulated insulin secretion29. Such an indirect effect seems consistent with previous data after GLP-1 administration in rodents. Thus, it has been shown first that in rats a ganglionic blockade impairs the insulinotropic action of GLP-130, and in mice a sensory deafferentation by capsaicin has been shown to prevent a low-dose GLP-1 administration from stimulating insulin secretion31. These results together suggest that GLP-1 already within the gut after its release activates afferent nerves, which after central relaying activates efferent nerves that stimulate insulin secretion32. DPP-4 inhibition might, through prevention of the local inactivation by GLP-1 in the gut, activate this neural circuit. Whether this is a mechanism that also improves glucose metabolism in humans remains to be established.
It was recently also shown that insulin secretion in response to DPP-4 inhibition requires normal β-cell glucose signaling. The evidence for this was obtained in a study in mice with dominant negative overexpression of the hepatic nuclear factor 1α (HNF-1α) in β-cells, which disrupts glucose signaling. In these mice, DPP-4 inhibition only marginally increased insulin secretion after oral glucose compared with wild-type mice33. Hence, there are several potential mechanistic explanations for the augmented insulin secretion by DPP-4 inhibition (Table 1).
Table 1. Potential mechanistic explanation for stimulated insulin secretion by dipeptidyl peptidase-4 inhibition
|1||Stimulation of GLP-1 receptors on β-cells through prevented inactivation of GLP-1|
|2||Stimulation of GIP receptors on β-cells through prevented inactivation of GIP|
|3||Stimulation of PACAP receptors on β-cells through prevented inactivation of PACAP|
|4||Augmentation of β-cell glucose signaling through β-cell receptor activation|
|5||Activation of GLP-1 receptors on enteric afferent autonomic nerves eliciting neurally-induced insulin secretion|
In 2002, an 8-week long-term study was reported in which control and insulin-resistant high-fat fed mice were given the DPP-4 inhibitor, NVP-DPP728 (daily dose 0.12 mμmol/g bodyweight), in the drinking water for the 8-week period. It was found that glucose tolerance after gastric glucose gavage was still increased after 8-week DPP-4 inhibition in both groups of mice, and this was accompanied by increased plasma levels of insulin and intact GLP-1. Also, glucose-stimulated insulin secretion from islets isolated from NVP-DPP728-treated animals was increased as compared with islets from control animals34. Similar results were evident after 8 weeks of treatment with vildagliptin of mice overexpressing human islet amyloid polypeptide in β-cells: DPP-4 inhibition after 8 weeks still augmented the insulin response to oral glucose, and it was also shown that islets isolated from vildagliptin-treated animals had a marked improvement of glucose-stimulated insulin secretion35. Furthermore, sitagliptin increased insulin secretion from isolated islets and in the perfused pancreas after 10 weeks of treatment in a diabetes model in mice consisting of a combination of streptozotocin with high-fat feeding22. Thus, these studies show that increased insulin secretion persists during long-term DPP-4 inhibition in mice.