miRNAs in the Beta Cell—Friends or Foes?

Type 2 diabetes (T2D) develops due to insulin resistance and an inability of the pancreatic β-cells to increase secretion of insulin and reduce elevated blood glucose levels. Diminished β-cell function and mass have been implicated in impaired β-cell secretory capacity and several microRNAs (miRNAs) have been reported to be involved in regulating β-cell processes. We believe miRNAs are nodes in important miRNA-mRNA networks regulating β-cell function and that miRNAs therefore can be targets for the treatment of T2D. MicroRNAs are short (≈19-23 nucleotides [nt]) endogenous noncoding RNAs which regulate gene expression by directly binding to the mRNA of their target genes. Under normal circumstances, miRNAs act as rheostats to keep expression of their gene targets at optimal levels for different β-cell outputs. In T2D, levels of some miRNAs are altered as part of the compensatory mechanism to improve insulin secretion. Other miRNAs are differentially expressed as part of the process of T2D pathogenesis, which results in reduced insulin secretion and increased blood glucose. In this review, we present recent findings concerning miRNAs in islets and in insulin-secreting cells, and their differential expression in diabetes, with a specific focus on miRNAs involved in β-cell apoptosis/proliferation and glucose-stimulated insulin secretion. We present thoughts around miRNA-mRNA networks and miRNAs as both therapeutic targets to improve insulin secretion and as circulating biomarkers of diabetes. Overall, we hope to convince you that miRNAs in β-cells are essential for regulating β-cell function and can in the future be of clinical use in the treatment and/or prevention of diabetes.