MicroRNA-9 controls the expression of Granuphilin/Slp4 and the secretory response of insulin-producing cells

MicroRNA let-7: an emerging next-generation cancer therapeutic

In recent years, various RNA-based technologies have been under evaluation as potential next-generation cancer therapeutics. Micrornas (miRNAS), known to regulate the cell cycle and development, are deregulated in various cancers. Thus, they might serve as good targets or candidates in an exploration of anticancer therapeutics. One attractive candidate for this purpose is let-7 ("lethal-7"). Let-7 is underexpressed in various cancers, and restoration of its normal expression is found to inhibit cancer growth by targeting various oncogenes and inhibiting key regulators of several mitogenic pathways. In vivo, let-7 administration was found effective against mouse-model lung and breast cancers, and our computational prediction supports the possible effectiveness of let-7 in estrogen receptor (ER)-positive metastatic breast cancer. Data also suggest that let-7 regulates apoptosis and cancer stem cell (CSC) differentiation and can therefore be tested as a potential therapeutic in cancer treatment. However, the exact role of let-7 in cancer is not yet fully understood. There is a need to understand the causative molecular basis of let-7 alterations in cancer and to develop proper delivery systems before proceeding to therapeutic applications. This article attempts to highlight certain critical aspects of let-7's therapeutic potential in cancer.

MicroRNAs in Development and Disease

MicroRNAs (miRNAs) are a class of posttranscriptional regulators that have recently introduced an additional level of intricacy to our understanding of gene regulation. There are currently over 10,000 miRNAs that have been identified in a range of species including metazoa, mycetozoa, viridiplantae, and viruses, of which 940, to date, are found in humans. It is estimated that more than 60% of human protein-coding genes harbor miRNA target sites in their 3′ untranslated region and, thus, are potentially regulated by these molecules in health and disease. This review will first briefly describe the discovery, structure, and mode of function of miRNAs in mammalian cells, before elaborating on their roles and significance during development and pathogenesis in the various mammalian organs, while attempting to reconcile their functions with our existing knowledge of their targets. Finally, we will summarize some of the advances made in utilizing miRNAs in therapeutics.

miR-29a and miR-29b contribute to pancreatic beta-cell-specific silencing of monocarboxylate transporter 1 (Mct1)

In pancreatic β cells, elevated glucose concentrations stimulate mitochondrial oxidative metabolism to raise intracellular ATP/ADP levels, prompting insulin secretion. Unusually low levels of expression of genes encoding the plasma membrane monocarboxylate transporter, MCT1 (SLC16A1), as well as lactate dehydrogenase A (LDHA) ensure that glucose-derived pyruvate is efficiently metabolized by mitochondria, while exogenous lactate or pyruvate is unable to stimulate metabolism and hence insulin secretion inappropriately. We show here that whereas DNA methylation at the Mct1 promoter is unlikely to be involved in cell-type-specific transcriptional repression, three microRNAs (miRNAs), miR-29a, miR-29b, and miR-124, selectively target both human and mouse MCT1 3' untranslated regions. Mutation of the cognate miR-29 or miR-124 binding sites abolishes the effects of the corresponding miRNAs, demonstrating a direct action of these miRNAs on the MCT1 message. However, despite reports of its expression in the mouse β-cell line MIN6, miR-124 was not detectably expressed in mature mouse islets. In contrast, the three isoforms of miR-29 are highly expressed and enriched in mouse islets. We show that inhibition of miR-29a in primary mouse islets increases Mct1 mRNA levels, demonstrating that miR-29 isoforms contribute to the β-cell-specific silencing of the MCT1 transporter and may thus affect insulin release.

Chromatin-associated protein kinase C-θ regulates an inducible gene expression program and microRNAs in human T lymphocytes

Studies in yeast demonstrate that signaling kinases have a surprisingly active role in the nucleus, where they tether to chromatin and modulate gene expression programs. Despite these seminal studies, the nuclear mechanism of how signaling kinases control transcription of mammalian genes is in its infancy. Here, we provide evidence for a hitherto unknown function of protein kinase C-theta (PKC-θ), which physically associates with the regulatory regions of inducible immune response genes in human T cells. Chromatin-anchored PKC-θ forms an active nuclear complex by interacting with RNA polymerase II, the histone kinase MSK-1, and the adaptor molecule 14-3-3ζ. ChIP-on-chip reveals that PKC-θ binds to promoters and transcribed regions of genes, as well as to microRNA promoters that are crucial for cytokine regulation. Our results provide a molecular explanation for the role of PKC-θ not only in normal T cell function, but also in circumstances of its ectopic expression in cancer.

MicroRNAs as biomarkers of disease onset

MicroRNAs (miRNAs) are small, noncoding RNA molecules with the ability to posttranscriptionally regulate gene expression via targeting the 3' untranslated region of messenger RNAs. miRNAs are critical for normal cellular functions such as the regulation of the cell cycle, differentiation, and apoptosis, and they target genes during embryonal and postnatal development, whereas their expression is unbalanced in various pathological states. Importantly, miRNAs are abundantly present in body fluids (e.g., blood), which are routinely examined in patients. These molecules circulate in free and exosome encapsulated forms, and can be efficiently detected and amplified by means of molecular biology tools such as real-time PCR. Together with relative stability, specificity, and reproducibility, they are seen as good candidates for early recognition of the onset of disease. Thus, miRNAs might be considered as biomarkers for many pathological states.

Diabetes mellitus, a microRNA-related disease?

Diabetes mellitus is a complex disease resulting in altered glucose homeostasis. In both type 1 and type 2 diabetes mellitus, pancreatic β cells cannot secrete appropriate amounts of insulin to regulate blood glucose level. Moreover, in type 2 diabetes mellitus, altered insulin secretion is combined with a resistance of insulin-target tissues, mainly liver, adipose tissue, and skeletal muscle. Both environmental and genetic factors are known to contribute to the development of the disease. Growing evidence indicates that microRNAs (miRNAs), a class of small noncoding RNA molecules, are involved in the pathogenesis of diabetes. miRNAs function as translational repressors and are emerging as important regulators of key biological processes. Here, we review recent studies reporting changes in miRNA expression in tissues isolated from different diabetic animal models. We also describe the role of several miRNAs in pancreatic β cells and insulin-target tissues. Finally, we discuss the possible use of miRNAs as blood biomarkers to prevent diabetes development and as tools for gene-based therapy to treat both type 1 and type 2 diabetes mellitus.

Gene-chip studies of adipogenesis-regulated microRNAs in mouse primary adipocytes and human obesity

BACKGROUND

Adipose tissue abundance relies partly on the factors that regulate adipogenesis, i.e. proliferation and differentiation of adipocytes. While components of the transcriptional program that initiates adipogenesis is well-known, the importance of microRNAs in adipogenesis is less well studied. We thus set out to investigate whether miRNAs would be actively modulated during adipogenesis and obesity.

METHODS

Several models exist to study adipogenesis in vitro, of which the cell line 3T3-L1 is the most well known, albeit not the most physiologically appropriate. Thus, as an alternative, we produced EXIQON microarray of brown and white primary murine adipocytes (prior to and following differentiation) to yield global profiles of miRNAs.

RESULTS

We found 65 miRNAs regulated during in vitro adipogenesis in primary adipocytes. We evaluated the similarity of our responses to those found in non-primary cell models, through literature data-mining. When comparing primary adipocyte profiles, with those of cell lines reported in the literature, we found a high degree of difference in 'adipogenesis' regulated miRNAs suggesting that the model systems may not be accurately representing adipogenesis. The expression of 10 adipogenesis-regulated miRNAs were studied using real-time qPCR and then we selected 5 miRNAs, that showed robust expression, were profiled in subcutaneous adipose tissue obtained from 20 humans with a range of body mass indices (BMI, range = 21-48, and all samples have U133+2 Affymetrix profiles provided). Of the miRNAs tested, mir-21 was robustly expressed in human adipose tissue and positively correlated with BMI (R2 = 0.49, p < 0.001).

CONCLUSION

In conclusion, we provide a preliminary analysis of miRNAs associated with primary cell in vitro adipogenesis and demonstrate that the inflammation-associated miRNA, mir-21 is up-regulated in subcutaneous adipose tissue in human obesity. Further, we provide a novel transcriptomics database of EXIQON and Affymetrix adipocyte profiles to facilitate data mining.

Sirt1 and mir-9 expression is regulated during glucose-stimulated insulin secretion in pancreatic β-islets

MicroRNA mir-9 is speculated to be involved in insulin secretion because of its ability to regulate exocytosis. Sirt1 is an NAD-dependent protein deacetylase and a critical factor in the modulation of cellular responses to altered metabolic flux. It has also been shown recently to control insulin secretion from pancreatic β-islets. However, little is known about the regulation of Sirt1 and mir-9 levels in pancreatic β-cells, particularly during glucose-dependent insulin secretion. In this article, we report that mir-9 and Sirt1 protein levels are actively regulated in vivo in β-islets during glucose-dependent insulin secretion. Our data also demonstrates that mir-9 targets and regulates Sirt1 expression in insulin-secreting cells. This targeting is relevant in pancreatic β-islets, where we show a reduction in Sirt1 protein levels when mir-9 expression is high during glucose-dependent insulin secretion. This functional interplay between insulin secretion, mir-9 and Sirt1 expression could be relevant in diabetes. It also highlights the crosstalk between an NAD-dependent protein deacetylase and microRNA in pancreatic β-cells.

Significance of serum microRNAs in pre-diabetes and newly diagnosed type 2 diabetes: a clinical study

To explore the clinical significance of seven diabetes-related serum microRNAs (miR-9, miR-29a, miR-30d, miR34a, miR-124a, miR146a and miR375) during the pathogenesis of type 2 diabetes (T2D), 56 subjects were recruited to this study: 18 cases of newly diagnosed T2D (n-T2D) patients, 19 cases of pre-diabetes individuals (impaired glucose tolerance [IGT] and/or impaired fasting glucose [IFG]) and 19 cases of T2D-susceptible individuals with normal glucose tolerance (s-NGT). Serum miRNAs were determined by real-time RT-PCR. Expression levels of single miRNAs and the expression signatures of miRNAs as a panel were analysed among the three groups. In n-T2D, all 7 miRNAs were significantly up-regulated compared with s-NGT and five were significantly up-regulated compared with pre-diabetes, while miRNA expression was not significantly different between s-NGT and pre-diabetes. By Canonical discriminant analysis, 70.6% of n-T2D subjects (12/17) were recognized by canonical discriminant function, while s-NGT and pre-diabetes subjects could not be discriminated from each other. Similar results were found in Hierarchical Clustering analysis based on the expression levels of all seven miRNAs. In different statistical analysis, miR-34a always showed the most significant differences. We conclude that the expression levels of seven diabetes-related miRNAs in serum were significantly elevated in n-T2D compared with pre-diabetes and/or s-NGT, and the latter two groups featured similar expression patterns of these miRNAs, suggesting that during the pathogenesis of T2D, the peripheral diabetes-related miRNAs have not changed significantly from s-NGT at pre-diabetic stage.

MicroRNA-9 reveals regional diversity of neural progenitors along the anterior-posterior axis

Neural progenitors self-renew and generate neurons throughout the central nervous system. Here, we uncover an unexpected regional specificity in the properties of neural progenitor cells, revealed by the function of a microRNA—miR-9. miR-9 is expressed in neural progenitors, and its knockdown results in an inhibition of neurogenesis along the anterior-posterior axis. However, the underlying mechanism differs—in the hindbrain, progenitors fail to exit the cell cycle, whereas in the forebrain they undergo apoptosis, counteracting the proliferative effect. Among several targets, we functionally identify hairy1 as a primary target of miR-9, regulated at the mRNA level. hairy1 mediates the effects of miR-9 on proliferation, through Fgf8 signaling in the forebrain and Wnt signaling in the hindbrain, but affects apoptosis only in the forebrain, via the p53 pathway. Our findings show a positional difference in the responsiveness of progenitors to miR-9 depletion, revealing an underlying divergence of their properties.

miRNAs control insulin content in pancreatic β-cells via downregulation of transcriptional repressors

MicroRNAs (miRNAs) were shown to be important for pancreas development, yet their roles in differentiated β-cells remain unclear. Here, we show that miRNA inactivation in β-cells of adult mice results in a striking diabetic phenotype. While islet architecture is intact and differentiation markers are maintained, Dicer1-deficient β-cells show a dramatic decrease in insulin content and insulin mRNA. As a consequence of the change in insulin content, the animals become diabetic. We provide evidence for involvement of a set of miRNAs in regulating insulin synthesis. The specific knockdown of miR-24, miR-26, miR-182 or miR-148 in cultured β-cells or in isolated primary islets downregulates insulin promoter activity and insulin mRNA levels. Further, miRNA-dependent regulation of insulin expression is associated with upregulation of transcriptional repressors, including Bhlhe22 and Sox6. Thus, miRNAs in the adult pancreas act in a new network that reinforces insulin expression by reducing the expression of insulin transcriptional repressors.

Insertion of microRNA targets into the flavivirus genome alters its highly neurovirulent phenotype

Flaviviruses such as West Nile, Japanese encephalitis, and tick-borne encephalitis (TBEV) viruses are important neurotropic human pathogens, causing a devastating and often fatal neuroinfection. Here, we demonstrate that incorporation into the viral genome of a target sequence for cellular microRNAs expressed in the central nervous system (CNS) enables alteration of the neurovirulence of the virus and control of the neuropathogenesis of flavivirus infection. As a model virus for this type of modification, we used a neurovirulent chimeric tick-borne encephalitis/dengue virus (TBEV/DEN4) that contained the structural protein genes of a highly pathogenic TBEV. The inclusion of just a single target copy for a brain tissue-expressed mir-9, mir-124a, mir-128a, mir-218, or let-7c microRNA into the TBEV/DEN4 genome was sufficient to prevent the development of otherwise lethal encephalitis in mice infected intracerebrally with a large dose of virus. Viruses bearing a complementary target for mir-9 or mir-124a were highly restricted in replication in primary neuronal cells, had limited access into the CNS of immunodeficient mice, and retained the ability to induce a strong humoral immune response in monkeys. This work suggests that microRNA targeting to control flavivirus tissue tropism and pathogenesis might represent a rational approach for virus attenuation and vaccine development.

Roles of myosin Va and Rab3D in membrane remodeling of immature secretory granules

Neuroendocrine secretory granules (SGs) are formed at the trans-Golgi network (TGN) as immature intermediates. In PC12 cells, these immature SGs (ISGs) are transported within seconds to the cell cortex, where they move along actin filaments and complete maturation. This maturation process comprises acidification-dependent processing of cargo proteins, condensation of the SG matrix, and removal of membrane and proteins not destined to mature SGs (MSGs) into ISG-derived vesicles (IDVs). We investigated the roles of myosin Va and Rab3 isoforms in the maturation of ISGs in neuroendocrine PC12 cells. The expression of dominant-negative mutants of myosin Va or Rab3D blocked the removal of the endoprotease furin from ISGs. Furthermore, expression of mutant Rab3D, but not of mutant myosin Va, impaired cargo processing of SGs. In conclusion, our data suggest an implication of myosin Va and Rab3D in the maturation of SGs where they participate in overlapping but not identical tasks.

Global microRNA expression profiles in insulin target tissues in a spontaneous rat model of type 2 diabetes

AIMS/HYPOTHESIS

MicroRNAs regulate a broad range of biological mechanisms. To investigate the relationship between microRNA expression and type 2 diabetes, we compared global microRNA expression in insulin target tissues from three inbred rat strains that differ in diabetes susceptibility.

METHODS

Using microarrays, we measured the expression of 283 microRNAs in adipose, liver and muscle tissue from hyperglycaemic (Goto-Kakizaki), intermediate glycaemic (Wistar Kyoto) and normoglycaemic (Brown Norway) rats (n = 5 for each strain). Expression was compared across strains and validated using quantitative RT-PCR. Furthermore, microRNA expression variation in adipose tissue was investigated in 3T3-L1 adipocytes exposed to hyperglycaemic conditions.

RESULTS

We found 29 significantly differentiated microRNAs (p(adjusted) < 0.05): nine in adipose tissue, 18 in liver and two in muscle. Of these, five microRNAs had expression patterns that correlated with the strain-specific glycaemic phenotype. MiR-222 (p(adjusted) = 0.0005) and miR-27a (p(adjusted) = 0.006) were upregulated in adipose tissue; miR-195 (p(adjusted) = 0.006) and miR-103 (p(adjusted) = 0.04) were upregulated in liver; and miR-10b (p(adjusted) = 0.004) was downregulated in muscle. Exposure of 3T3-L1 adipocytes to increased glucose concentration upregulated the expression of miR-222 (p = 0.008), miR-27a (p = 0.02) and the previously reported miR-29a (p = 0.02). Predicted target genes of these differentially expressed microRNAs are involved in pathways relevant to type 2 diabetes.

CONCLUSION

The expression patterns of miR-222, miR-27a, miR-195, miR-103 and miR-10b varied with hyperglycaemia, suggesting a role for these microRNAs in the pathophysiology of type 2 diabetes, as modelled by the Gyoto-Kakizaki rat. We observed similar patterns of expression of miR-222, miR-27a and miR-29a in adipocytes as a response to increased glucose levels, which supports our hypothesis that altered expression of microRNAs accompanies primary events related to the pathogenesis of type 2 diabetes.

Role of microRNAs in obesity and the metabolic syndrome

Obesity and the metabolic syndrome are major public health concerns, and present a formidable therapeutic challenge. Many patients remain recalcitrant to conventional lifestyle changes and medical therapies. Bariatric surgery has made laudable progress in the treatment of obesity and its related metabolic disorders, yet carries inherent risks. Unravelling the molecular mechanisms of metabolic disorders is essential in order to develop novel, valid therapeutic strategies. Mi(cro)RNAs play important regulatory roles in a variety of biological processes including adipocyte differentiation, metabolic integration, insulin resistance and appetite regulation. Investigation of these molecules and their genetic targets may potentially identify new pathways involved in complex metabolic disease processes, improving our understanding of metabolic disorders and influence future approaches to the treatment of obesity. This review discusses the role of miRNAs in obesity and related components of the metabolic syndrome, and highlights the potential of using miRNAs as novel biomarkers and therapeutic targets for these diseases.

microRNAs: critical regulators in Th17 cells and players in diseases

microRNAs are a novel group of small, conserved, non-coding RNA molecules that are present in all species. These molecules post-transcriptionally regulate gene expression by targeting mRNAs for degradation or by repressing the translation of the mRNAs. A good understanding of miRNA-mediated gene regulation is critical to gain a comprehensive view of many physiological processes and disease states. Emerging evidence demonstrates that miRNAs play an important role in the differentiation and function of the adaptive immune system. This review provides an overview of the diverse functions of miRNAs in modulating immune responses and in immune cell development, particularly the development of Th17 cells, and explores the involvement of miRNAs in several autoimmune diseases including multiple sclerosis (MS), rheumatoid arthritis (RA), inflammatory bowel disease (IBD) and diabetes.

Involvement of microRNAs in the cytotoxic effects exerted by proinflammatory cytokines on pancreatic beta-cells

OBJECTIVE

Pancreatic beta-cells exposed to proinflammatory cytokines display alterations in gene expression resulting in defective insulin secretion and apoptosis. MicroRNAs are small noncoding RNAs emerging as key regulators of gene expression. Here, we evaluated the contribution of microRNAs to cytokine-mediated beta-cell cytotoxicity.

RESEARCH DESIGN AND METHODS

We used global microarray profiling and real-time PCR analysis to detect changes in microRNA expression in beta-cells exposed to cytokines and in islets of pre-diabetic NOD mice. We assessed the involvement of the microRNAs affected in cytokine-mediated beta-cell failure by modifying their expression in insulin-secreting MIN6 cells.

RESULTS

We found that IL-1beta and TNF-alpha induce the expression of miR-21, miR-34a, and miR-146a both in MIN6 cells and human pancreatic islets. We further show an increase of these microRNAs in islets of NOD mice during development of pre-diabetic insulitis. Blocking miR-21, miR-34a, or miR-146a function using antisense molecules did not restore insulin-promoter activity but prevented the reduction in glucose-induced insulin secretion observed upon IL-1beta exposure. Moreover, anti-miR-34a and anti-miR-146a treatment protected MIN6 cells from cytokine-triggered cell death.

CONCLUSIONS

Our data identify miR-21, miR-34a, and miR-146a as novel players in beta-cell failure elicited in vitro and in vivo by proinflammatory cytokines, notably during the development of peri-insulitis that precedes overt diabetes in NOD mice.

Versatile roles for myosin Va in dense core vesicle biogenesis and function

The motor protein myosin Va is involved in multiple successive steps in the development of dense-core vesicles, such as in the membrane remodelling during their maturation, their transport along actin filaments and the regulation of their exocytosis. In the present paper, we summarize the current knowledge on the roles of myosin Va in the different steps of dense-core vesicle biogenesis and exocytosis, and compare findings obtained from different cell types and experimental systems.

MicroRNA-370 controls the expression of microRNA-122 and Cpt1alpha and affects lipid metabolism

We previously observed that treatment of mice with a dominant negative form of cJun (dn-cJun) increased the expression of genes involved in lipid metabolism and modulated the expression of nine microRNAs (miR). To investigate the potential effect of these miRs on the expression of the genes of lipid metabolism, we performed studies in cultured HepG2 cells. Transfection of HepG2 cells with sense or antisense miR-370 or miR-122 upregulated and downregulated, respectively, the transcription factor sterol-regulatory element binding protein 1c (SREBP-1c) and the enzymes diacylglycerol acyltransferase-2 (DGAT2), fatty acid synthase (FAS), and acyl-CoA carboxylase 1 (ACC1) that regulate fatty acid and triglyceride biosynthesis. The other seven miRs identified by the miR array screening did not affect the expression of lipogenic genes. miR-370 upregulated the expression of miR-122. Furthermore, the effect of miR-370 on the expression of the lipogenic genes was abolished by antisense miR-122. miR-370 targets the 3' untranslated region (UTR) of Cpt1alpha, and it downregulated the expression of the carnitine palmitoyl transferase 1alpha (Cpt1alpha) gene as well as the rate of beta oxidation. Our data suggest that miR-370 acting via miR-122 may have a causative role in the accumulation of hepatic triglycerides by modulating initially the expression of SREBP-1c, DGAT2, and Cpt1alpha and, subsequently, the expression of other genes that affect lipid metabolism.

Obesity and genetics regulate microRNAs in islets, liver, and adipose of diabetic mice

Type 2 diabetes results from severe insulin resistance coupled with a failure of b cells to compensate by secreting sufficient insulin. Multiple genetic loci are involved in the development of diabetes, although the effect of each gene on diabetes susceptibility is thought to be small. MicroRNAs (miRNAs) are noncoding 19-22-nucleotide RNA molecules that potentially regulate the expression of thousands of genes. To understand the relationship between miRNA regulation and obesity-induced diabetes, we quantitatively profiled approximately 220 miRNAs in pancreatic islets, adipose tissue, and liver from diabetes-resistant (B6) and diabetes-susceptible (BTBR) mice. More than half of the miRNAs profiled were expressed in all three tissues, with many miRNAs in each tissue showing significant changes in response to genetic obesity. Furthermore, several miRNAs in each tissue were differentially responsive to obesity in B6 versus BTBR mice, suggesting that they may be involved in the pathogenesis of diabetes. In liver there were approximately 40 miRNAs that were downregulated in response to obesity in B6 but not BTBR mice, indicating that genetic differences between the mouse strains play a critical role in miRNA regulation. In order to elucidate the genetic architecture of hepatic miRNA expression, we measured the expression of miRNAs in genetically obese F2 mice. Approximately 10% of the miRNAs measured showed significant linkage (miR-eQTLs), identifying loci that control miRNA abundance. Understanding the influence that obesity and genetics exert on the regulation of miRNA expression will reveal the role miRNAs play in the context of obesity-induced type 2 diabetes.

Obesity and genetics regulate microRNAs in islets, liver, and adipose of diabetic mice

Type 2 diabetes results from severe insulin resistance coupled with a failure of b cells to compensate by secreting sufficient insulin. Multiple genetic loci are involved in the development of diabetes, although the effect of each gene on diabetes susceptibility is thought to be small. MicroRNAs (miRNAs) are noncoding 19-22-nucleotide RNA molecules that potentially regulate the expression of thousands of genes. To understand the relationship between miRNA regulation and obesity-induced diabetes, we quantitatively profiled approximately 220 miRNAs in pancreatic islets, adipose tissue, and liver from diabetes-resistant (B6) and diabetes-susceptible (BTBR) mice. More than half of the miRNAs profiled were expressed in all three tissues, with many miRNAs in each tissue showing significant changes in response to genetic obesity. Furthermore, several miRNAs in each tissue were differentially responsive to obesity in B6 versus BTBR mice, suggesting that they may be involved in the pathogenesis of diabetes. In liver there were approximately 40 miRNAs that were downregulated in response to obesity in B6 but not BTBR mice, indicating that genetic differences between the mouse strains play a critical role in miRNA regulation. In order to elucidate the genetic architecture of hepatic miRNA expression, we measured the expression of miRNAs in genetically obese F2 mice. Approximately 10% of the miRNAs measured showed significant linkage (miR-eQTLs), identifying loci that control miRNA abundance. Understanding the influence that obesity and genetics exert on the regulation of miRNA expression will reveal the role miRNAs play in the context of obesity-induced type 2 diabetes.

Exocytosis in islet beta-cells

The development of technologies that allow for live optical imaging of exocytosis from beta-cells has greatly improved our understanding of insulin secretion. Two-photon imaging, in particular, has enabled researchers to visualize the exocytosis of large dense-core vesicles (LDCVs) containing insulin from beta-cells in intact islets of Langerhans. These studies have revealed that high glucose levels induce two phases of insulin secretion and that this release is dependent upon cytosolic Ca(2+) and cAMP. This technology has also made it possible to examine the spatial profile of insulin exocytosis in these tissues and compare that profile with those of other secretory glands. Such studies have led to the discovery of the massive exocytosis of synaptic-like microvesicles (SLMVs) in beta-cells. These imaging studies have also helped clarify facets of insulin exocytosis that cannot be properly addressed using the currently available electrophysiological techniques. This chapter provides a concise introduction to the field of optical imaging for those researchers who wish to characterize exocytosis from beta-cells in the islets of Langerhans.

Negative regulation of TLR4 via targeting of the proinflammatory tumor suppressor PDCD4 by the microRNA miR-21

The tumor suppressor PDCD4 is a proinflammatory protein that promotes activation of the transcription factor NF-kappaB and suppresses interleukin 10 (IL-10). Here we found that mice deficient in PDCD4 were protected from lipopolysaccharide (LPS)-induced death. The induction of NF-kappaB and IL-6 by LPS required PDCD4, whereas LPS enhanced IL-10 induction in cells lacking PDCD4. Treatment of human peripheral blood mononuclear cells with LPS resulted in lower PDCD4 expression, which was due to induction of the microRNA miR-21 via the adaptor MyD88 and NF-kappaB. Transfection of cells with a miR-21 precursor blocked NF-kappaB activity and promoted IL-10 production in response to LPS, whereas transfection with antisense oligonucleotides to miR-21 or targeted protection of the miR-21 site in Pdcd4 mRNA had the opposite effect. Thus, miR-21 regulates PDCD4 expression after LPS stimulation.

Role and therapeutic potential of microRNAs in diabetes

The discovery in mammalian cells of hundreds of small RNA molecules, called microRNAs, with the potential to modulate the expression of the majority of the protein-coding genes has revolutionized many areas of biomedical research, including the diabetes field. MicroRNAs function as translational repressors and are emerging as key regulators of most, if not all, physiological processes. Moreover, alterations in the level or function of microRNAs are associated with an increasing number of diseases. Here, we describe the mechanisms governing the biogenesis and activities of microRNAs. We present evidence for the involvement of microRNAs in diabetes mellitus, by outlining the contribution of these small RNA molecules in the control of pancreatic beta-cell functions and by reviewing recent studies reporting changes in microRNA expression in tissues isolated from diabetes animal models. MicroRNAs hold great potential as therapeutic targets. We describe the strategies developed for the delivery of molecules mimicking or blocking the function of these tiny regulators of gene expression in living animals. In addition, because changes in serum microRNA profiles have been shown to occur in association with different human diseases, we also discuss the potential use of microRNAs as blood biomarkers for prevention and management of diabetes.

Meta-regulation: microRNA regulation of glucose and lipid metabolism

Maintenance of homeostasis during environmental flux requires constant metabolic adjustment, achieved partly through the fine regulation of gene expression. MicroRNAs are key players in this regulatory milieu; they have been implicated in regulating gene expression within several metabolically active tissues including the endocrine pancreas, liver and adipose tissue. Recent studies, for example, implicate miR-375 in pancreatic islet cell viability and function, and removal or overexpression of miR-375 profoundly affects glucose metabolism. In the liver, miR-122 is important for normal lipid metabolism. In fact, misexpression of miRNAs can occur in some diseases, suggesting that restoring miRNA expression is a potential therapeutic approach for both metabolic syndrome and diabetes.

MicroRNAs and their role in progressive kidney diseases

MicroRNAs (miRs) are a family of short non-coding RNAs. These endogenously produced factors have been shown to play important roles in gene regulation. The discovery of miRs has greatly expanded our knowledge of gene regulation at the posttranscriptional level. miRs inhibit target gene expression by blocking protein translation or by inducing mRNA degradation and therefore have the potential to modulate physiologic and pathologic processes. The imperative need to determine their cellular targets and disease relevance has sparked an unprecedented explosion of research in the miR field. Recent findings have revealed critical functions for specific miRs in cellular events such as proliferation, differentiation, development, and immune responses and in the regulation of genes relevant to human diseases. Of particular interest to renal researchers are recent reports that key miRs are highly expressed in the kidney and can act as effectors of TGF-beta actions and high glucose in diabetic kidney disease. Moreover, podocyte-specific deletion of Dicer, a key enzyme involved in miR biogenesis, led to proteinuria and severe renal dysfunction in mice. Hence, studies aimed at determining the in vitro and in vivo functions of miRs in the kidney could determine their value as therapeutic targets for progressive renal glomerular and tubular diseases. Translational approaches could be facilitated by the development of effective inhibitors of specific miRs and methods for optimal delivery of anti-miRs to the kidney. The major goal of this review is to highlight key functions of these miRs and their relationships to human diseases, with special emphasis on diabetic kidney disease.

MicroRNA-125a is over-expressed in insulin target tissues in a spontaneous rat model of Type 2 Diabetes

Background

MicroRNAs (miRNAs) are non-coding RNA molecules involved in post-transcriptional control of gene expression of a wide number of genes, including those involved in glucose homeostasis. Type 2 diabetes (T2D) is characterized by hyperglycaemia and defects in insulin secretion and action at target tissues. We sought to establish differences in global miRNA expression in two insulin-target tissues from inbred rats of spontaneously diabetic and normoglycaemic strains.

Methods

We used a miRNA microarray platform to measure global miRNA expression in two insulin-target tissues: liver and adipose tissue from inbred rats of spontaneously diabetic (Goto-Kakizaki [GK]) and normoglycaemic (Brown-Norway [BN]) strains which are extensively used in genetic studies of T2D. MiRNA data were integrated with gene expression data from the same rats to investigate how differentially expressed miRNAs affect the expression of predicted target gene transcripts.

Results

The expression of 170 miRNAs was measured in liver and adipose tissue of GK and BN rats. Based on a p-value for differential expression between GK and BN, the most significant change in expression was observed for miR-125a in liver (FC = 5.61, P = 0.001, P_adjusted = 0.10); this overexpression was validated using quantitative RT-PCR (FC = 13.15, P = 0.0005). MiR-125a also showed over-expression in the GK vs. BN analysis within adipose tissue (FC = 1.97, P = 0.078, P_adjusted = 0.99), as did the previously reported miR-29a (FC = 1.51, P = 0.05, P_adjusted = 0.99). In-silico tools assessing the biological role of predicted miR-125a target genes suggest an over-representation of genes involved in the MAPK signaling pathway. Gene expression analysis identified 1308 genes with significantly different expression between GK and BN rats (P_adjusted < 0.05): 233 in liver and 1075 in adipose tissue. Pathways related to glucose and lipid metabolism were significantly over-represented among these genes. Enrichment analysis suggested that differentially expressed genes in GK compared to BN included more predicted miR-125a target genes than would be expected by chance in adipose tissue (FDR = 0.006 for up-regulated genes; FDR = 0.036 for down-regulated genes) but not in liver (FDR = 0.074 for up-regulated genes; FDR = 0.248 for down-regulated genes).

Conclusion

MiR-125a is over-expressed in liver in hyperglycaemic GK rats relative to normoglycaemic BN rats, and our array data also suggest miR-125a is over-expressed in adipose tissue. We demonstrate the use of in-silico tools to provide the basis for further investigation of the potential role of miR-125a in T2D. In particular, the enrichment of predicted miR-125a target genes among differentially expressed genes has identified likely target genes and indicates that integrating global miRNA and mRNA expression data may give further insights into miRNA-mediated regulation of gene expression.

MicroRNA and cancer--focus on apoptosis

MicroRNAs (miRs) are small non-coding RNAs regulating gene expression at the post-transcriptional and/or translational levels. miRs play important roles in diverse biological processes, including development, cell differentiation, proliferation and apoptosis. Recent evidence has shown that miR loci frequently map to cancer-associated genomic regions and deregulated miR expression profiles are associated with many cancer types, implicating miRs in crucial processes that lead to tumourigenesis. Here, we review the current findings about miRs and tumourigenesis, focusing on their involvement in the apoptosis pathway. A significant observation is that greater than one-quarter of all known human miRs were reported to be deregulated in at least one cancer type. The expression of a subset of miRs (e.g. miR-21 and miR-155) was found to be consistently up-regulated, whereas another subset of miRs (e.g.miR-143 and miR-145) was consistently down-regulated across different cancer types suggesting their involvement in regulating common cellular processes whose deregulation may lead to tumourigenesis. Several miRs were implicated to play roles in cell proliferation and apoptosis. Some miRs, such as miR-29b and miR-15–16, influence only the apoptotic pathway, whereas others including let-7/miR-98 and miR-17–92 may play roles in both the apoptotic and cell-proliferation pathways. In conclusion, although our current understanding of the functions of miRs is still fragmentary, taken together, this review highlights the complex and intricate roles that miRs play in the regulation of cellular processes. Perturbation of the expression of miRs may thus lead to tumourigenesis.

CHANGES IN microRNA (miR) profile and effects of miR-320 in insulin-resistant 3T3-L1 adipocytes

1. MicroRNAs (miRNAs) play essential roles in many biological processes. It is known that aberrant miRNA expression contributes to some pathological conditions. However, it is not known whether miRNAs play any role in the development of insulin resistance in adipocytes, a key pathophysiological link between obesity and diabetes. 2. To investigate the function of miRNAs in the development of insulin resistance, using miRNA microarray analysis we compared miRNA expression profiles between normal insulinsensitive 3T3-L1 adipocytes and 3T3-L1 adipocytes rendered insulin resistant following treatment with high glucose (25mmol/L) and high insulin (1 mol/L). Furthermore, adipocytes were transfected with specific antisense oligonucleotides against miRNA-320 (anti-miR-320 oligo) and the effects on the development of insulin resistance were evaluated. 3. We identified 50 upregulated and 29 downregulated miRNAs in insulin-resistant (IR) adipocytes, including a 50-fold increase in miRNA-320 (miR-320) expression. Using bioinformatic techniques, the p85 subunit of phosphatidylinositol 3-kinase (PI3-K) was found to be a potential target of miR-320. In experiments with anti-miR-320 oligo, insulin sensitivity was increased in IR adipocytes, as evidenced by increases in p85 expression, phosphorylation of Akt and the protein expression of the glucose transporter GLUT-4, as well as insulin-stimulated glucose uptake. These beneficial effects of anti-miR-320 oligo were observed only in IR adipocytes and not in normal adipocytes. 4. In conclusion, the miRNA profile changes in IR adipocytes compared with normal 3T3-L1 adipocytes. Anti-miR-320 oligo was found to regulate insulin resistance in adipocytes by improving insulin–PI3-K signalling pathways. The findings provide information regarding a potentially new therapeutic strategy to control insulin resistance.

Differential expression of microRNAs in mouse liver under aberrant energy metabolic status

Despite years of effort, exact pathogenesis of nonalcoholic fatty liver disease (NAFLD) remains obscure. To gain an insight into the regulatory roles of microRNAs (miRNAs) in aberrant energy metabolic status and pathogenesis of NAFLD, we analyzed the expression of miRNAs in livers of ob/ob mice, streptozotocin (STZ)-induced type 1 diabetic mice, and normal C57BL/6 mice by miRNA microarray. Compared with normal C57BL/6 mice, ob/ob mice showed upregulation of eight miRNAs and downregulation of four miRNAs in fatty livers. Upregulation of miR-34a and downregulation of miR-122 was found in livers of STZ-induced diabetic mice. These results demonstrate that distinct miRNAs are strongly dysregulated in NAFLD and hyperglycemia. Comparison between miRNA expressions in livers of ob/ob mice and STZ-administered mice further revealed upregulation of four miRNAs and downregulation of two miRNAs in livers of ob/ob mice, indicating that these miRNAs may represent a molecular signature of NAFLD. A distinctive miRNA expression pattern was identified in ob/ob mouse liver, and hierarchical clustering of this pattern could clearly discriminate ob/ob mice from either normal C57BL/6 mice or STZ-administered mice. These findings suggest an important role of miRNAs in hepatic energy metabolism and implicate the participation of miRNAs in the pathophysiological processes of NAFLD.

Differential expression of microRNAs in mouse liver under aberrant energy metabolic status

Despite years of effort, exact pathogenesis of nonalcoholic fatty liver disease (NAFLD) remains obscure. To gain an insight into the regulatory roles of microRNAs (miRNAs) in aberrant energy metabolic status and pathogenesis of NAFLD, we analyzed the expression of miRNAs in livers of ob/ob mice, streptozotocin (STZ)-induced type 1 diabetic mice, and normal C57BL/6 mice by miRNA microarray. Compared with normal C57BL/6 mice, ob/ob mice showed upregulation of eight miRNAs and downregulation of four miRNAs in fatty livers. Upregulation of miR-34a and downregulation of miR-122 was found in livers of STZ-induced diabetic mice. These results demonstrate that distinct miRNAs are strongly dysregulated in NAFLD and hyperglycemia. Comparison between miRNA expressions in livers of ob/ob mice and STZ-administered mice further revealed upregulation of four miRNAs and downregulation of two miRNAs in livers of ob/ob mice, indicating that these miRNAs may represent a molecular signature of NAFLD. A distinctive miRNA expression pattern was identified in ob/ob mouse liver, and hierarchical clustering of this pattern could clearly discriminate ob/ob mice from either normal C57BL/6 mice or STZ-administered mice. These findings suggest an important role of miRNAs in hepatic energy metabolism and implicate the participation of miRNAs in the pathophysiological processes of NAFLD.

The identification of differentially expressed microRNA in osteoarthritic tissue that modulate the production of TNF-alpha and MMP13

OBJECTIVE

To identify differentially expressed microRNAs (miRNAs) in human osteoarthritic (OA) cartilage and bone tissue and to determine their relevance to chondrocyte function.

METHODS

Cartilage and bone was obtained from OA patients who underwent total knee joint replacement surgery or from post-mortem patients with no previous history of OA. MiRNA expression was quantified by real-time PCR (RT-PCR). Functional pathway analysis of miRNA was performed using Ingenuity Pathway Analysis. Primary chondrocytes were isolated by collagenase digestion and transfected with miRNA mimics and miRNA inhibitors using cationic lipid. Tumour Necrosis Factor-alpha (TNF-alpha) and Matrix metalloprotease 13 (MMP13) protein levels were measured by Enzyme-Linked ImmunoSorbent Assay (ELISA).

RESULTS

In total we identified 17 miRNA that showed greater than 4-fold differential expression between OA and normal cartilage, and 30 miRNA that showed greater than 4-fold differential expression in OA bone. Functional pathway analysis of the predicted gene targets for miR-9, miR-98, which were upregulated in both OA bone and cartilage tissue, and miR-146, which was downregulated in OA cartilage, suggested that these miRNA mediate inflammatory functions and pathways. Over-expression of miR-9, miR-98 or miR-146 in isolated human chondrocytes reduced interleukin-1 beta (IL-1 beta) induced TNF-alpha production. Furthermore, inhibition and over-expression of miR-9 modulated MMP13 secretion.

CONCLUSIONS

We have identified a number of differentially expressed miRNAs in late-stage human OA cartilage and bone. Functional analysis of miR-9, miR-98 and miR-146 in primary chondrocytes suggests a role in mediating the IL-1 beta induced production of TNF-alpha. MiR-9, upregulated in OA tissue, was found to inhibit secretion of the collagen type II-targeting metalloproteinase MMP13 in isolated human chondrocytes.

Identification of microRNAs controlling human ovarian cell steroidogenesis via a genome-scale screen

The aim of our studies was to identify miRNAs affecting the release of the major ovarian steroid hormones progestagen, androgen and estrogen by human ovarian cells. The effect of transfection of cultured primary ovarian granulosa cells with 80 different gene constructs encoding human pre-miRNAs on release of progesterone, testosterone and estradiol was evaluated by enzyme immunoassay. In addition, effect of two selected antisense constructs blocking corresponding miRNA on progesterone release was tested. Efficiency of transfection (incorporation transfection reagent) and silencing of marker substances (GAPDH mRNA, GAPDH and CREB-1) were validated by fluorescent microscopy, real-time reverse transcription-PCR analysis and immunocytochemical analysis. Thirty-six out of 80 tested miRNA constructs resulted in inhibition of progesterone release in granulosa cells, and 10 miRNAs promoted progesterone release. Transfected of cells with antisense constructs to two selected miRNAs blocking progesterone release induced increase in progesterone output. Fifty-seven miRNAs tested inhibited testosterone release, and only one miRNA enhanced testosterone output. Fifty-one miRNAs suppressed estradiol release, while none of the miRNAs tested stimulated it. This is the first demonstration that miRNAs can control reproductive functions resulting in enhanced or inhibited release of ovarian progestagen, androgen and estrogen. We hypothesize that such miRNA-mediated effects could be potentially used for regulation of reproductive processes, including fertility, and for treatment of reproductive and other steroid-dependent disorders.

Cluster analysis of rat pancreatic islet gene mRNA levels after culture in low-, intermediate- and high-glucose concentrations

AIMS/HYPOTHESIS

Survival and function of insulin-secreting pancreatic beta cells are markedly altered by changes in nutrient availability. In vitro, culture in 10 rather than 2 mmol/l glucose improves rodent beta cell survival and function, whereas glucose concentrations above 10 mmol/l are deleterious.

METHODS

To identify the mechanisms of such beta cell plasticity, we tested the effects of 18 h culture at 2, 5, 10 and 30 mmol/l glucose on the transcriptome of rat islets pre-cultured for 1 week at 10 mmol/l glucose using Affymetrix Rat 230 2.0 arrays.

RESULTS

Culture in either 2-5 or 30 mmol/l instead of 10 mmol/l glucose markedly impaired beta cell function, while little affecting cell survival. Of about 16,000 probe-sets reliably detected in islets, some 5,000 were significantly up- or downregulated at least 1.4-fold by glucose. Analysis of these probe-sets with GeneCluster software identified ten mRNA profiles with unidirectional up- or downregulation between 2 and 10, 2 and 30, 5 and 10, 5 and 30 or 10 and 30 mmol/l glucose. It also identified eight complex V-shaped or inverse V-shaped profiles with a nadir or peak level of expression in 5 or 10 mmol/l glucose. Analysis of genes belonging to these various clusters using Onto-express and GenMAPP software revealed several signalling and metabolic pathways that may contribute to induction of beta cell dysfunction and apoptosis after culture in low- or high- vs intermediate-glucose concentration.

CONCLUSIONS/INTERPRETATION

We have identified 18 distinct mRNA profiles of glucose-induced changes in islet gene mRNA levels that should help understand the mechanisms by which glucose affects beta cell survival and function under states of chronic hypo- or hyperglycaemia.

MicroRNA miR-7 is preferentially expressed in endocrine cells of the developing and adult human pancreas

MicroRNAs (miRNA) are small non-coding RNAs that inhibit gene expression through binding to complementary messenger RNA sequences. miRNAs have been predicted to target genes important for pancreas development, proper endocrine cell function and metabolism. We previously described that miRNA-7 (miR-7) was the most abundant and differentially expressed islet miRNA, with 200-fold higher expression in mature human islets than in acinar tissue. Here we have analyzed the temporal and spatial expression of miR-7 in human fetal pancreas from 8 to 22 weeks of gestational age (wga). Human fetal (8-22wga) and adult pancreases were processed for immunohistochemistry, in situ hybridization, and quantitative RT-PCR of miRNA and mRNA. miR-7 was expressed in the human developing pancreas from around 9wga and reached its maximum expression levels between 14 and 18wga, coinciding with the exponential increase of the pancreatic endocrine hormones. Throughout development miR-7 expression was preferentially localized to endocrine cells and its expression persisted in the adult pancreas. The present study provides a detailed analysis of the spatiotemporal expression of miR-7 in developing human pancreas. The specific localization of miR-7 expression to fetal and adult endocrine cells indicates a potential role for miR-7 in endocrine cell differentiation and/or function. Future functional studies of a potential role for miR-7 function in islet cell differentiation and physiology are likely to identify novel targets for the treatment of diabetes and will lead to the development of improved protocols for generating insulin-producing cells for cell replacement therapy.

Insulin producing cells derived from embryonic stem cells: are we there yet?

Derivation of insulin producing cells (IPCs) from embryonic stem (ES) cells provides a potentially innovative form of treatment for type 1 diabetes. Here, we discuss the current state of the art, unique challenges, and future directions on generating IPCs.

microRNA-449 is a putative regulator of choroid plexus development and function

microRNAs are short RNA molecules that are often expressed in specific tissues and regulate a variety of developmental processes. We used locked nucleic acid probes in in situ hybridisation reactions to study the distribution of microRNA-449 (miR449) during mouse embryonic development in order to obtain clues about its function/s. Between E9.75 and E11.5, miR449 was found to be expressed specifically in the developing roof plate of the fourth ventricle within the domain of roof plate marker, Lmx1a. From E12.5 onwards, this expression became restricted to the epithelial cell layer of the fourth ventricle choroid plexus. MiR449 also became detectable specifically in the choroid plexuses of the lateral and 3rd ventricles at E13.5 and E15.5, respectively. Northern blot analysis of adult brain also showed a selective and enriched expression in the choroid plexus tissue. Potential target genes regulated by miR449 were selected for experimental validation in luciferase-reporter assays and the transcription factor E2f5, which regulates CSF production, was verified as a miR449 target gene. Taken together, these findings suggest that miR449 has a specific role in the development and functioning of choroid plexuses.

MicroRNAs: key players in the immune system, differentiation, tumorigenesis and cell death

Micro (mi)RNAs are small, highly conserved noncoding RNAs that control gene expression post-transcriptionally either via the degradation of target mRNAs or the inhibition of protein translation. Each miRNA is believed to regulate the expression of multiple mRNA targets, and many miRNAs have been linked to the initiation and progression of human cancer. miRNAs control various activities of the immune system and different stages of hematopoietic development, and their misexpression is the cause of various blood malignancies. Certain miRNAs have oncogenic activities, whereas others have the potential to act as tumor suppressors. Because they control fundamental processes such as differentiation, cell growth and cell death, the study of the role of miRNAs in human neoplasms holds great promise for novel forms of therapy. Here, we summarize the role of miRNAs and their targets in contributing to human cancers and their function as regulators of apoptotic pathways and the immune system.

The pancreatic beta-cell: birth, life and death

Defective insulin secretion is a hallmark of all forms of diabetes. Whereas Type 1 diabetes has long been known to result from the immune-mediated destruction of beta-cells, Type 2 diabetes appears to involve both loss of beta-cell mass and glucose sensitivity in the face of extrapancreatic insulin resistance. We summarize here the proceedings of a Biochemical Society Focused Meeting, held at the St Thomas campus of King's College London in December 2007, which highlighted recent research advances targeting the beta-cell.

Type 2 diabetes: new genes, new understanding

Over the past two years, there has been a spectacular change in the capacity to identify common genetic variants that contribute to predisposition to complex multifactorial phenotypes such as type 2 diabetes (T2D). The principal advance has been the ability to undertake surveys of genome-wide association in large study samples. Through these and related efforts, approximately 20 common variants are now robustly implicated in T2D susceptibility. Current developments, for example in high-throughput resequencing, should help to provide a more comprehensive view of T2D susceptibility in the near future. Although additional investigation is needed to define the causal variants within these novel T2D-susceptibility regions, to understand disease mechanisms and to effect clinical translation, these findings are already highlighting the predominant contribution of defects in pancreatic beta-cell function to the development of T2D.

Expression of islet-specific microRNAs during human pancreatic development

During pancreatic islet development, sequential changes in gene expression are known to be necessary for efficient differentiation and function of the endocrine pancreas. Several studies till now have demonstrated that microRNAs (miRNAs), which regulate translation of gene transcripts, influence gene expression cascades involved in pancreas development. Some of these miRNAs; miR-7 and miR-375 have been known to be expressed at high levels in pancreas and are also known to be involved in Zebrafish pancreas development as well as insulin secretion in mice. We demonstrate here that 4 different islet-specific microRNAs (miR-7, miR-9, miR-375 and miR-376) are expressed at high levels during human pancreatic islet development. Of these, miR-375, is seen to be differentially expressed in human islet beta- as well as non-beta-cells. Though no significant difference in abundance of miR-375 was noted in either cell type, analysis of islet-specific miRNA and mRNA in single cells show that non-beta cells contain higher levels of miR-375. Our data demonstrate that miRNAs that are known to be regulated during Zebrafish pancreatic development may play similar role in human pancreatic islet development.

miR-375 targets 3'-phosphoinositide-dependent protein kinase-1 and regulates glucose-induced biological responses in pancreatic beta-cells

OBJECTIVE

MicroRNAs are short, noncoding RNAs that regulate gene expression. We hypothesized that the phosphatidylinositol 3-kinase (PI 3-kinase) cascade known to be important in beta-cell physiology could be regulated by microRNAs. Here, we focused on the pancreas-specific miR-375 as a potential regulator of its predicted target 3'-phosphoinositide-dependent protein kinase-1 (PDK1), and we analyzed its implication in the response of insulin-producing cells to elevation of glucose levels.

RESEARCH DESIGN AND METHODS

We used insulinoma-1E cells to analyze the effects of miR-375 on PDK1 protein level and downstream signaling using Western blotting, glucose-induced insulin gene expression using quantitative RT-PCR, and DNA synthesis by measuring thymidine incorporation. Moreover, we analyzed the effect of glucose on miR-375 expression in both INS-1E cells and primary rat islets. Finally, miR-375 expression in isolated islets was analyzed in diabetic Goto-Kakizaki (GK) rats.

RESULTS

We found that miR-375 directly targets PDK1 and reduces its protein level, resulting in decreased glucose-stimulatory action on insulin gene expression and DNA synthesis. Furthermore, glucose leads to a decrease in miR-375 precursor level and a concomitant increase in PDK1 protein. Importantly, regulation of miR-375 expression by glucose occurs in primary rat islets as well. Finally, miR-375 expression was found to be decreased in fed diabetic GK rat islets.

CONCLUSIONS

Our findings provide evidence for a role of a pancreatic-specific microRNA, miR-375, in the regulation of PDK1, a key molecule in PI 3-kinase signaling in pancreatic beta-cells. The effects of glucose on miR-375 are compatible with the idea that miR-375 is involved in glucose regulation of insulin gene expression and beta-cell growth.

Alterations in microRNA expression contribute to fatty acid-induced pancreatic beta-cell dysfunction

OBJECTIVE

Visceral obesity and elevated plasma free fatty acids are predisposing factors for type 2 diabetes. Chronic exposure to these lipids is detrimental for pancreatic beta-cells, resulting in reduced insulin content, defective insulin secretion, and apoptosis. We investigated the involvement in this phenomenon of microRNAs (miRNAs), a class of noncoding RNAs regulating gene expression by sequence-specific inhibition of mRNA translation.

RESEARCH DESIGN AND METHODS

We analyzed miRNA expression in insulin-secreting cell lines or pancreatic islets exposed to palmitate for 3 days and in islets from diabetic db/db mice. We studied the signaling pathways triggering the changes in miRNA expression and determined the impact of the miRNAs affected by palmitate on insulin secretion and apoptosis.

RESULTS

Prolonged exposure of the beta-cell line MIN6B1 and pancreatic islets to palmitate causes a time- and dose-dependent increase of miR34a and miR146. Elevated levels of these miRNAs are also observed in islets of diabetic db/db mice. miR34a rise is linked to activation of p53 and results in sensitization to apoptosis and impaired nutrient-induced secretion. The latter effect is associated with inhibition of the expression of vesicle-associated membrane protein 2, a key player in beta-cell exocytosis. Higher miR146 levels do not affect the capacity to release insulin but contribute to increased apoptosis. Treatment with oligonucleotides that block miR34a or miR146 activity partially protects palmitate-treated cells from apoptosis but is insufficient to restore normal secretion.

CONCLUSIONS

Our findings suggest that at least part of the detrimental effects of palmitate on beta-cells is caused by alterations in the level of specific miRNAs.

Molecular medicine of microRNAs: structure, function and implications for diabetes

MicroRNAs (miRNAs) are a family of endogenous small noncoding RNA molecules, of 19–28 nucleotides in length. In humans, up to 3% of all genes are estimated to encode these evolutionarily conserved sequences. miRNAs are thought to control expression of thousands of target mRNAs. Mammalian miRNAs generally negatively regulate gene expression by repressing translation, possibly through effects on mRNA stability and compartmentalisation, and/or the translation process itself. An extensive range of in silico and experimental techniques have been applied to our understanding of the occurrence and functional relevance of such sequences, and antisense technologies have been successfully used to control miRNA expression in vitro and in vivo. Interestingly, miRNAs have been identified in both normal and pathological conditions, including differentiation and development, metabolism, proliferation, cell death, viral infection and cancer. Of specific relevance and excitement to the area of diabetes research, miRNA regulation has been implicated in insulin secretion from pancreatic β-cells, diabetic heart conditions and nephropathy. Further analyses of miRNAs in vitro and in vivo will, undoubtedly, enable us determine their potential to be exploited as therapeutic targets in diabetes.

Development of type 2 diabetes following intrauterine growth retardation in rats is associated with progressive epigenetic silencing of Pdx1

Intrauterine growth retardation (IUGR) has been linked to the onset of diseases in adulthood, including type 2 diabetes, and has been proposed to result from altered gene regulation patterns due to epigenetic modifications of developmental genes. To determine whether epigenetic modifications may play a role in the development of adult diabetes following IUGR, we used a rodent model of IUGR that expresses lower levels of Pdx1, a pancreatic and duodenal homeobox 1 transcription factor critical for beta cell function and development, which develops diabetes in adulthood. We found that expression of Pdx1 was permanently reduced in IUGR beta cells and underwent epigenetic modifications throughout development. The fetal IUGR state was characterized by loss of USF-1 binding at the proximal promoter of Pdx1, recruitment of the histone deacetylase 1 (HDAC1) and the corepressor Sin3A, and deacetylation of histones H3 and H4. Following birth, histone 3 lysine 4 (H3K4) was demethylated and histone 3 lysine 9 (H3K9) was methylated. During the neonatal period, these epigenetic changes and the reduction in Pdx1 expression could be reversed by HDAC inhibition. After the onset of diabetes in adulthood, the CpG island in the proximal promoter was methylated, resulting in permanent silencing of the Pdx1 locus. These results provide insight into the development of type 2 diabetes following IUGR and we believe they are the first to describe the ontogeny of chromatin remodeling in vivo from the fetus to the onset of disease in adulthood.

Role of microRNAs in diabetes

Diabetes is one of the most common chronic diseases in the world. Multiple and complex factors including various genetic and physiological changes can lead to type 1 and type 2 diabetes. However, the major mechanisms underlying the pathogenesis of diabetes remain obscure. With the recent discovery of microRNAs (miRNAs), these small ribonucleotides have been implicated as new players in the pathogenesis of diabetes and diabetes-associated complications. MiRNAs have been shown to regulate insulin production, insulin secretion, and insulin action. This review summarizes the recent progress in the cutting-edge research of miRNAs involved in diabetes and diabetes related complications.