MicroRNA expression in alpha and beta cells of human pancreatic islets

The pancreatic β-cell transcriptome and integrated-omics

PURPOSE OF REVIEW

β Cells represent one of many cell types in heterogeneous pancreatic islets and play the central role in maintaining glucose homeostasis, such that disrupting β-cell function leads to diabetes. This review summarizes the methods for isolating and characterizing β cells, and describes integrated 'omics' approaches used to define the β cell by its transcriptome and proteome.

RECENT FINDINGS

RNA sequencing and mass spectrometry-based protein identification have now identified RNA and protein profiles for mouse and human pancreatic islets and β cells, and for β-cell lines. Recent publications have outlined these profiles and, more importantly, have begun to assign the presence or absence of specific genes and regulatory molecules to β-cell function and dysfunction. Overall, researchers have focused on understanding the pathophysiology of diabetes by connecting genome, transcriptome, proteome, and regulatory RNA profiles with findings from genome-wide association studies.

SUMMARY

Studies employing these relatively new techniques promise to identify specific genes or regulatory RNAs with altered expression as β-cell function begins to deteriorate in the spiral toward the development of diabetes. The ultimate goal is to identify the potential therapeutic targets to prevent β-cell dysfunction and thereby better treat the individual with diabetes.

VIDEO ABSTRACT

http://links.lww.com/COE/A5.

Epigenetic regulation of the DLK1-MEG3 microRNA cluster in human type 2 diabetic islets

Type 2 diabetes mellitus (T2DM) is a complex disease characterized by the inability of the insulin-producing β cells in the endocrine pancreas to overcome insulin resistance in peripheral tissues. To determine if microRNAs are involved in the pathogenesis of human T2DM, we sequenced the small RNAs of human islets from diabetic and nondiabetic organ donors. We identified a cluster of microRNAs in an imprinted locus on human chromosome 14q32 that is highly and specifically expressed in human β cells and dramatically downregulated in islets from T2DM organ donors. The downregulation of this locus strongly correlates with hypermethylation of its promoter. Using HITS-CLIP for the essential RISC-component Argonaute, we identified disease-relevant targets of the chromosome 14q32 microRNAs, such as IAPP and TP53INP1, that cause increased β cell apoptosis upon overexpression in human islets. Our results support a role for microRNAs and their epigenetic control by DNA methylation in the pathogenesis of T2DM.

Profiling of circulating microRNAs reveals common microRNAs linked to type 2 diabetes that change with insulin sensitization

OBJECTIVE

This study sought to identify the profile of circulating microRNAs (miRNAs) in type 2 diabetes (T2D) and its response to changes in insulin sensitivity.

RESEARCH DESIGN AND METHODS

The circulating miRNA profile was assessed in a pilot study of 12 men: 6 with normal glucose tolerance (NGT) and 6 T2D patients. The association of 10 circulating miRNAs with T2D was cross-sectionally validated in an extended sample of 45 NGT vs. 48 T2D subjects (65 nonobese and 28 obese men) and longitudinally in 35 T2D patients who were recruited in a randomized, double-blinded, and placebo-controlled 3-month trial of metformin treatment. Circulating miRNAs were also measured in seven healthy volunteers before and after a 6-h hyperinsulinemic-euglycemic clamp and insulin plus intralipid/heparin infusion.

RESULTS

Cross-sectional studies disclosed a marked increase of miR-140-5p, miR-142-3p, and miR-222 and decreased miR-423-5p, miR-125b, miR-192, miR-195, miR-130b, miR-532-5p, and miR-126 in T2D patients. Multiple linear regression analyses revealed that miR-140-5p and miR-423-5p contributed independently to explain 49.5% (P < 0.0001) of fasting glucose variance after controlling for confounders. A discriminant function of four miRNAs (miR-140-5p, miR-423-5p, miR-195, and miR-126) was specific for T2D with an accuracy of 89.2% (P < 0.0001). Metformin (but not placebo) led to significant changes in circulating miR-192 (49.5%; P = 0.022), miR-140-5p (-15.8%; P = 0.004), and miR-222 (-47.2%; P = 0.03), in parallel to decreased fasting glucose and HbA1c. Furthermore, while insulin infusion during clamp decreased miR-222 (-62%; P = 0.002), the intralipid/heparin mixture increased circulating miR-222 (163%; P = 0.015) and miR-140-5p (67.5%; P = 0.05).

CONCLUSIONS

This study depicts the close association between variations in circulating miRNAs and T2D and their potential relevance in insulin sensitivity.

Increased expression of miR-187 in human islets from individuals with type 2 diabetes is associated with reduced glucose-stimulated insulin secretion

AIMS/HYPOTHESIS

Type 2 diabetes is characterised by progressive beta cell dysfunction, with changes in gene expression playing a crucial role in its development. MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression and therefore alterations in miRNA levels may be involved in the deterioration of beta cell function.

METHODS

Global TaqMan arrays and individual TaqMan assays were used to measure islet miRNA expression in discovery (n = 20) and replication (n = 20) cohorts from individuals with and without type 2 diabetes. The role of specific dysregulated miRNAs in regulating insulin secretion, content and apoptosis was subsequently investigated in primary rat islets and INS-1 cells. Identification of miRNA targets was assessed using luciferase assays and by measuring mRNA levels.

RESULTS

In the discovery and replication cohorts miR-187 expression was found to be significantly increased in islets from individuals with type 2 diabetes compared with matched controls. An inverse correlation between miR-187 levels and glucose-stimulated insulin secretion (GSIS) was observed in islets from normoglycaemic donors. This correlation paralleled findings in primary rat islets and INS-1 cells where overexpression of miR-187 markedly decreased GSIS without affecting insulin content or apoptotic index. Finally, the gene encoding homeodomain-interacting protein kinase-3 (HIPK3), a known regulator of insulin secretion, was identified as a direct target of miR-187 and displayed reduced expression in islets from individuals with type 2 diabetes.

CONCLUSIONS/INTERPRETATION

Our findings suggest a role for miR-187 in the blunting of insulin secretion, potentially involving regulation of HIPK3, which occurs during the pathogenesis of type 2 diabetes.

Downregulation of Bcl-2 expression by miR-34a mediates palmitate-induced Min6 cells apoptosis

Recent studies have demonstrated that the expression of miR-34a is significantly upregulated and associated with cell apoptosis in pancreatic β -cell treated with palmitate. Nevertheless, the underlying detailed mechanism is largely unknown. Here, we showed that miR-34a was significantly induced in Min6 pancreatic β -cell upon palmitate treatment. Elevated miR-34a promoted Min6 cell apoptosis. Intriguingly, ectopic expression of miR-34a lowered the expression of Bcl-2, an antiapoptotic protein. Luciferase reporter assay indicated the direct interaction of miR-34a with the Bcl-2 3'-UTR. Moreover, downregulated expression of Bcl-2 induced by palmitate could be restored by inhibition of miR-34a. We conclude that direct suppression of Bcl-2 by miR-34a accounts for palmitate-induced increased apoptosis rate in pancreatic β -cell.

Recent progress in generation of human surrogate β cells

PURPOSE OF REVIEW

This review evaluates recent progress in several approaches aimed at developing human surrogate β cells, and identifies gaps that need to be filled for bringing them closer to clinical application.

RECENT FINDINGS

Cells expanded in vitro from human cadaver donor β cells under conditions causing dedifferentiation have been shown to undergo redifferentiation following inhibition of the Notch pathway. Efforts for differentiation of insulin-producing cells from human pluripotent stem cells have focused on isolation and expansion of intermediate-stage cells. The role of mesenchyme in expansion of pancreas progenitors has been emphasized by mouse cell ablation, and co-culture of human embryonic stem cell-derived definitive endoderm with mesenchyme. Incomplete removal of Polycomb-mediated repression of endocrine genes in embryonic stem cell-derived insulin-producing cells generated in vitro has been suggested to be responsible for their immature phenotype. Induced pluripotent stem cells reprogrammed from β cells have been shown to exhibit an enhanced differentiation capacity toward insulin-producing cells, compared with other pluripotent stem cells. A new approach for reprogramming non-β into β-like cells involving transcription factor gene ablation has been demonstrated in mouse enteroendocrine cells in vivo.

SUMMARY

New insights into the stumbling blocks in expansion of human donor islet cells, differentiation of pluripotent stem cells, and reprogramming of non-β cell types are shaping improved strategies, which are likely to bring us closer to the goal of generating abundant human surrogate β cells.