The role of the CCL25-CCR9 axis in beta-cell function: potential for therapeutic intervention in type 2 diabetes

Highlights

• Islets from lean and obese donors demonstrate altered expression of several GPCRs.

• CCR9 is the most up-regulated GPCR mRNA in islets from high BMI donors.

• Activation of islet CCR9 with CCL25 impairs beta-cell function by Gαi coupling.

• CCL25 reduces insulin secretion and enhances cytokine-induced apoptosis.

• Thus, CCL25 down-regulation or CCR9 inhibition could be explored to treat T2D.

Abstract

Background and purpose

Chemokines are known to play essential roles mediating immunity and inflammation in many physiological and pathophysiological processes, with reports linking their action to the development of obesity, insulin resistance and type 2 diabetes (T2D). Given our findings of highly upregulated mRNA expression of the chemokine receptor CCR9 in islets from obese human donors, we have determined the effects of CCR9 activation by CCL25 on islet function and viability.

Basic procedures

RT-qPCR was used to measure expression of 384 GPCR mRNAs in human islets from organ donors with normal and elevated BMI. mRNA encoding CCR9, a receptor that was highly upregulated in islets from obese donors, was also quantified in islets from lean and high-fat diet (HFD) mice. The effects of CCR9 activation by exogenous CCL25 in human and mouse islets and its inhibition by the CCR9 antagonist vercirnon on insulin secretion, apoptosis and cAMP accumulation were examined using standard techniques.

Main findings

The qPCR analysis showed altered expression of several GPCRs in islets isolated from lean and obese donors. CCR9 displayed over 90-fold upregulation in islets from obese individuals, and it was also significantly upregulated in islets from obese mice. In isolated human and mouse islets exogenous CCL25 inhibited glucose-induced insulin secretion in a concentration-dependent manner, enhanced cytokine-induced apoptosis and significantly reduced forskolin-induced elevation in cAMP levels. These detrimental effects of CCL25 in islets were blocked by vercirnon, which had no effect on its own.

Principal conclusions

We have shown that CCL25 acts via the Gαi-coupled receptor CCR9 to impair beta-cell function by inhibiting insulin secretion and promoting cytokine-induced apoptosis. Upregulation of CCR9 in islets in obesity, possibly secondary to accumulation of passenger immune cells, may predispose to metabolic dysfunction and our data suggest that CCL25 downregulation or CCR9 inhibition could be explored to treat T2D.

Introduction

G protein-coupled receptors (GPCRs) have been successfully exploited as targets for some of the most commonly used prescription medications [1,2], but the full potential of these receptors in drug discovery has not yet been thoroughly elucidated. This is, at least in part, a consequence of there being close structural homology in ligand binding sites within GPCRs, difficulties in developing reproducible protocols for purification and crystallisation of membrane proteins, challenges in generating specific GPCR antibodies and incomplete information on the GPCR expression patterns of therapeutically relevant target tissues [3]. We have previously identified that human islets express 293 GPCRs, which suggests that there are tractable targets for therapeutic intervention in type 2 diabetes (T2D) by potentially increasing beta-cell number and improving insulin secretory capacity [4]. Surprisingly, however, there is only one T2D GPCR-based therapy available for clinical use, the GLP-1 agonists, which act at islet GLP-1 receptors to potentiate glucose-induced insulin secretion and promote beta-cell survival [5,6]. The success of these incretin mimetics as T2D therapeutics has led to an increased interest in islet GPCRs as promising targets to treat diabetes, but current focus is largely on the other intestinal-derived incretin, GIP, and on agonists of the long chain fatty acid receptor FFAR1 or the novel cannabinoid receptor GPR119 [7].

It is well-established that there is a close link between increased body mass index (BMI) and insulin resistance, and it has been suggested that obesity accounts for 80–85% of the risk of developing T2D [8]. In addition, chronic low-grade inflammation in obesity, which is linked with elevated pro-inflammatory cytokines, fatty acids and reactive oxygen species, is associated with reduced insulin sensitivity and decreased beta-cell mass as a consequence of induction of beta-cell apoptosis [9,10]. Of particular interest is the observation that more than 30 GPCRs have been implicated in triggering insulin resistance, activating obesity-induced T2D inflammatory processes and reducing beta-cell function [11]. We have therefore determined whether there are changes in islet GPCR mRNA expression that are dependent on the BMI of the donor, and this led us to identify that the chemokine receptor CCR9 is highly upregulated in islets isolated from obese donors compared to lean ones. Although there is no reported information on the role of this receptor in beta-cell function, activation of CCR9 by its natural ligand, CCL25, has been implicated in inflammatory responses related to Crohn's disease and ulcerative colitis [12,13]. If a similar scenario occurs in islets, CCR9 activation would be expected to be associated with impaired islet viability and function, consistent with the main role of chemokines as disruptors of cell function. Based on this, in the current study we have determined the effects of CCL25 on islet function and viability and explored its signalling via Gαi.