Differences in islet-enriched miRNAs in healthy and glucose intolerant human subjects

Abstract

Many microRNAs (miRNAs) are known to be cell-type specific and are implicated in development of diseases. We investigated the global expression pattern of miRNAs in human pancreatic islets compared to liver and skeletal muscle, using bead-based technology and quantitative RT-PCR. In addition to the known islet-specific miR-375, we also found enrichment of miR-127-3p, miR-184, miR-195 and miR-493∗ in the pancreatic islets. The expression of miR-375, miR-127-3p, miR-184 and the liver-enriched miR-122 is positively correlated to insulin biosynthesis, while the expression of miR-127-3p and miR-184 is negatively correlated to glucose-stimulated insulin secretion (GSIS). These correlations were absent in islets of glucose intolerant donors (HbA1c ⩾ 6.1). We suggest that the presence of an islet-specific miRNA network, which consists of at least miR-375, miR-127-3p and miR-184, potentially involved in insulin secretion. Our results provide new insight into miRNA-mediated regulation of insulin secretion in healthy and glucose intolerant subjects.

Introduction

MicroRNAs (miRNAs) are short (21–23 nucleotides) non-coding RNA molecules which regulate gene expression by binding to the 3′-untranslated regions (UTR) of messenger RNAs (mRNAs) [1], resulting in mRNA degradation, mRNA deadenylation and/or translational repression [2], [3]. In recent years, a number of miRNAs have been implicated in the aetiology of several diseases including cancer [4], autoimmunity [5] and diabetes [6].

Type 2 diabetes is a complex, polygenic disease influenced by both genetic and environmental factors. It is characterized by hyperglycemia due to insulin resistance and/or impaired insulin secretion in the pancreatic β-cells. In the latter scenario, miR-375 [7], [8], miR-9 [9] and miR-7 [10] have been suggested to be important regulators of insulin secretion. Over-expression of miR-375 reduces insulin secretion through inhibition of exocytosis of insulin granules via translational repression of the cytoplasmic protein myotrophin [7]. Mice lacking miR-375 (375KO) are hyperglycemic, and pancreatic β-cell mass is decreased due to impaired proliferation [8]. These studies indicate that optimal insulin secretion requires an optimal balance between the levels of miR-375 and the target proteins it controls. Indeed, the relationship between miR-375 and its target myotrophin, in particular has been suggested to be a prime example of a tuning interaction, where the miRNA acts as a rheostat, keeping the protein within a functional range of expression [11].

Although a number of miRNAs have been shown to be capable of regulating insulin secretion, and are therefore important in the development of type 2 diabetes, their expression has been poorly investigated in human glucose-sensitive tissues. In this study, we have investigated the expression of a set of miRNAs in human pancreatic islets from non-diabetic donors and compared it to (1) the expression pattern in liver and skeletal muscle to identify miRNAs important for islet function and (2) the expression pattern in islets from glucose tolerant human donors (HbA1c ⩾ 6.1) to reveal abnormalities in islet miRNA expression in this group. We have correlated miRNA expression with insulin biosynthesis and GSIS, in islets from both healthy and glucose in tolerant human donors, to investigate potential regulation of insulin secretion by miRNAs. Finally, we performed computational miRNA target analysis to generate a putative target gene list for further characterization.