Evaluating insulin secretagogues in a humanized mouse model with functional human islets

Abstract

Objective

To develop a rapid, easy and clinically relevant in vivo model to evaluate novel insulin secretagogues on human islets, we investigated the effect of insulin secretagogues on functional human islets in a humanized mouse model.

Materials/Methods

Human islets were transplanted under the kidney capsule of streptozotocin (STZ)-induced diabetic mice with immunodeficiency. Human islet graft function was monitored by measuring non-fasting blood glucose levels. After diabetes was reversed, human islet transplanted mice were characterized physiologically by oral glucose tolerance and pharmacologically with clinically proven insulin secretagogues, glucagon-like peptide-1 (GLP-1), exenatide, glyburide, nateglinide and sitagliptin. Additionally, G protein-coupled receptor 40 (GPR40) agonists were evaluated in this model.

Results

Long-term human islet graft survival could be achieved in immunodeficient mice. Oral glucose challenge in human islet transplanted mice resulted in an immediate incremental increase of plasma human C-peptide, while the plasma mouse C-peptide was undetectable. Treatments with GLP-1, exenatide, glyburide, nateglinide and sitagliptin effectively increased plasma human C-peptide levels and improved postprandial glucose concentrations. GPR40 agonists also stimulated human C-peptide secretion and significantly improved postprandial glucose in the human islet transplanted mice.

Conclusions

Our studies indicate that a humanized mouse model with human islet grafts could mimic the in vivo characteristics of human islets and could be a powerful tool for the evaluation of novel insulin secretagogues or other therapeutic agents that directly and/or indirectly target human β cells.

Introduction

Pancreatic β-cell dysfunction is responsible for the pathogenesis and progression of type 2 diabetes [1], [2], [3]. Inadequate secretion of insulin is a very early element in the development of type 2 diabetes. This β-cell defect is a consequence of β-cell loss and/or endocrine dysregulation of islet function [2]. Quantitation of insulin sensitivity and the major parameters of β-cell function (glucose sensitivity, rate sensitivity and potentiating factor) in a large number of individuals spanning the range from normal glucose tolerance to overt diabetes demonstrated that impaired β-cell glucose sensitivity is a characteristic feature of even minimally impaired glucose tolerance [4], [5]. The use of insulin secretagogues such as sulfonylureas and related ATP-sensitive K⁺ channel blockers has been a critical part of treatment for patients with type 2 diabetes for a long time [6], [7]. With the introduction of incretin based therapies [3], [8], [9], [10], such as exenatide, a glucagon-like peptide-1 (GLP-1) analogue, and sitagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor, there has been continued interest in novel drug targets that stimulate glucose dependent insulin secretion. Recently, several G protein-coupled receptors including GPR40, GPR119 and GPR120 have emerged as possible targets for treating diabetes [11]. Agonists of those GPRs stimulate not only pancreatic β cells to secrete insulin but also gut cells to secrete incretin hormones [12], [13], [14], [15], [16], [17], [18], [19].

In drug discovery and academic research, rodent models are commonly used for in vivo evaluation because they are readily available [20], [21]. However, studies have shown that there are striking species differences in islets with regard to both cytoarchitecture and function, especially between rodents and primates [22]. For a novel insulin secretagogue, the results generated from rodent models may be misleading. Thus, it is critical to demonstrate a secretagogue's specificity and function in relation to human islets, particularly in an in vivo setting.

Transplanting isolated human islets into immunodeficient mice to evaluate their ability to revert the hyperglycemia induced by streptozotocin (STZ) has often been used to assess the quality and function of isolated human islets [23], [24]. In light of this method, we explored and established the use of a humanized mouse model with functional human islets as a means for evaluating insulin secretagogues.

GLP-1 is an incretin peptide secreted from intestinal L-cells in response to an oral glucose challenge or meal [25], [26], [27]. It directly stimulates glucose-dependent insulin secretion from β cells and improves glucose regulation. Exenatide is a currently marketed GLP-1 analog and has the same pharmacological actions as GLP-1 [28], [29]. Nateglinide and glyburide are different pharmaceutical agents that directly stimulate insulin secretion from pancreatic islets [30]. Sitagliptin, a recently marketed anti-diabetic drug, indirectly increases insulin levels by slowing down degradation of GLP-1 through inhibition of DPP-4 [31], [32]. Metformin is a glucose lowering agent that reduces plasma glucose by suppression of hepatic glucose production and enhancement of insulin sensitivity in peripheral tissues [33]. GPR40, which is predominantly expressed in pancreatic β cells, regulates free fatty acid-induced insulin secretion [34], [35]. GPR40 agonists have been investigated for discovery and development of novel glucose dependent insulin secretagogues [15], [36], [37], [38]. Using humanized mice with functional human islets, we evaluated both these clinically proven drugs and novel insulin secretagogues on human insulin secretion in response to oral glucose challenge.