Endocrine pharmacology
Effects of long-term treatment with the dipeptidyl peptidase-4 inhibitor vildagliptin on islet endocrine cells in non-obese type 2 diabetic Goto-Kakizaki rats

https://doi.org/10.1016/j.ejphar.2012.07.030Get rights and content

Abstract

Reduced β cell mass is a characteristic feature of type 2 diabetes and incretin therapy is expected to prevent this condition. However, it is unknown whether dipeptidyl peptidase-4 inhibitors influence β and α cell mass in animal models of diabetes that can be translated to humans. Therefore, we examined the long-term effects of treatment with the dipeptidyl peptidase-4 inhibitor vildagliptin on islet morphology in Goto-Kakizaki (GK) rats, a spontaneous, non-obese model of type 2 diabetes, and explored the underlying mechanisms. Four-week-old GK rats were orally administered with vildagliptin (15 mg/kg) twice daily for 18 weeks. Glucose tolerance was monitored during the study. After 18 weeks, β and α cell morphology and the expression of molecules involved in cell proliferation and cell death were examined by immunohistochemistry and morphometric analysis. We found that vildagliptin improved glucose tolerance and insulin secretion, and suppressed hyperglucagonemia by increasing plasma active glucagon-like peptide-1 concentrations. β cell mass was reduced in GK rats to 40% of that in Wistar rats, but was restored to 80% by vildagliptin. Vildagliptin enhanced β and α cell proliferation, and increased the number of small neogenetic islets. Vildagliptin also reduced the number of 8-hydroxy-2′-deoxyguanosine-positive cells and forkhead box protein O1 expression, inhibited macrophage infiltration, and enhanced S6 ribosomal protein, molecule of target of rapamycin, and pancreatic duodenal homeobox 1 expression. These results indicate that starting vildagliptin treatment from an early age improved glucose tolerance and preserved islet β cell mass in GK rats by facilitating the proliferation of islet endocrine cells.

Introduction

The emergence of incretin therapies, exemplified by glucagon-like peptide (GLP)-1 and dipeptidyl peptidase (DPP)-4 inhibitors, has drastically changed the direction of diabetes treatment. Studies within clinical practice have confirmed that incretin therapies suppress hyperglycemia by promoting insulin secretion in patients with type 2 diabetes (Bosi et al., 2008, Nauck et al., 2009). However, there is no convincing proof showing that incretin therapies promote the regeneration of islet β cells or protect against β cell death in patients with diabetes.

Type 2 diabetes is characterized by insulin resistance and poor insulin secretion, which predominate in the obese and lean phenotypes, respectively (Funakoshi et al., 2008, SEARCH for Diabetes in Youth Study Group). The response of islet β cells to DPP-4 inhibitors may therefore vary among patients with type 2 diabetes. However, no studies have examined the possible differences in incretin effects between obese and lean patients with type 2 diabetes. To address this issue, studies using appropriate animal models may be useful to translate the findings to humans. Some reports have described beneficial effects of DPP-4 inhibitors on islet structures in obese animal models of type 2 diabetes, including Zucker diabetic fatty rats (Brand et al., 2009, Sudre et al., 2002), db/db mice (Moritoh et al., 2010), high-fat diet-induced obese animals (Lamont and Drucker, 2008), and low-dose streptozotocin-induced diabetic mice (Mu et al., 2006). Unfortunately, information is lacking on the effects of DPP-4 inhibitors on non-obese diabetic animals. Similarly, their effects on other islet endocrine cells, especially α cells, have yet to be determined.

The Goto-Kakizaki (GK) rat is a non-obese animal model of type 2 diabetes characterized by low insulin secretion, similar to that in Asian patients with type 2 diabetes (Goto et al., 1988, Portha et al., 1991). The islet pathology in this model involves a progressive decline of β cell mass (Koyama et al., 1998, Koyama et al., 2000). Only a few studies have addressed the effects of GLP-1 or exenatide on β cell growth (Svensson et al., 2007, Tourrel et al., 2002) and function in GK rats (Mukai et al., 2011, Simonsen et al., 2009), but no studies have examined the effects of DPP-4 inhibitors in this model. Under these circumstances, we examined the effects of treatment with a DPP-4 inhibitor on islet morphology in GK rats. We also explored the mechanisms involved in the changes induced by the DPP-4 inhibitor.

Section snippets

Rats, vildagliptin treatment, and tissue processing

Male GK and Wistar rats were purchased from CLEA Japan Inc. (Tokyo, Japan). Based on the blood levels of glucose and body weight, 4-week-old rats (about 1 week after weaning) were assigned into two balanced groups and were orally administered with the DPP-4 inhibitor vildagliptin (provided by Novartis, Basel, Switzerland) at a dose of 15 mg/kg twice daily (09:00 h and 18:00 h), or with vehicle alone (untreated group). The animals were fed a standard diet (CE-2, CLEA Japan) ad libitum. Wistar rats

Laboratory findings and food intake

From the start of the experiment, the body weight of GK rats at 4 weeks of age was 25% less than that of Wistar rats (Table 1). The GK rats at this age already showed marked hyperglycemia compared with Wistar rats, although there was no significant reduction in β cell volume density (Table 2). The difference in body weight between GK rats and Wistar rats remained constant throughout the study. Treatment with vildagliptin did not affect body weight of GK or Wistar rats. Fasting and non-fasting

Discussion

In the present study, long-term treatment with the DPP-4 inhibitor vildagliptin for 18 weeks from an early age improved glucose intolerance, promoted glucose-stimulated insulin secretion, and suppressed hyperglucagonemia in GK rats. Vildagliptin also enhanced the preservation of pancreatic insulin content, β cell volume, and β and α cell proliferation, and increased the number of small neogenetic islets. Previous studies have documented the expansion of β cell mass in 2-month-old GK rats

Contribution statement

WI, HM, and SY conceived and designed the experiments, or analyzed and interpreted the data. SY drafted the article and revised it critically for intellectual content. All of the authors read and approved the final version of the manuscript to be published.

Acknowledgment

The authors thank Ms. Shiho Fujiwara, Mari Tsujii, Mrs. Saori Ogasawara, and Mrs. Hiroko Mori for their technical assistance. The study was partly supported by a Research Grant from Novartis. Some of this work was presented at the 46th Annual Meeting of the European Association for the Study of Diabetes in Stockholm, September 2123, 2010.

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