Identification of miR-9 as a negative factor of insulin secretion from beta cells

MicroRNA is a novel class of small noncoding RNA that has been implicated in a variety of physiological and pathological processes, including glucose homeostasis and diabetes mellitus. So far, a few studies have reported that miRNAs may be an important regulator in glucose-stimulated insulin secretion (GSIS) pathway. However, the role of miRNAs in this process remains unclear. The levels of miRNAs in mouse islets and MIN6 cells were determined by quantitative RT-PCR. Concentration of insulin was determined by ELISA, and the expression of the target protein was determined with western blot assay. The overexpression and downregulation of miRNAs in MIN6 were conducted using cell transfection methods. And luciferase assay was used to measure the direct interaction between miRNAs and target messenger RNAs 3′UTR. miR-9 was screened out for it was downregulated under the effects of short-term high glucose, while long-term high glucose relatively increased miR-9 expression. The Stxbp1 expression was decreased with the overexpression of miR-9 in MIN6 cells and increased when miR-9 was downregulated. Moreover, it was verified by luciferase assay that miR-9 regulated Stxbp1 gene expression by directly targeting Stxbp1 messenger RNA 3′UTR. This study suggests that the pathway consisting of miR-9 and Stxbp1 plays a key role in β-cell function, thus contributing to the network of miRNA-insulin secretion and offering a new candidate for diabetes therapy.     

Kinetic analysis and design of experiments to identify the catalytic mechanism of the monocarboxylate transporter isoforms 4 and 1

Transport of lactate, pyruvate, and other monocarboxylates across the sarcolemma of skeletal and cardiac myocytes occurs via passive diffusion and by monocarboxylate transporter (MCT) mediated transport. The flux of lactate and protons through the MCT plays an important role in muscle energy metabolism during rest and exercise and in pH regulation during exercise. The MCT isoforms 1 and 4 are the major isoforms of this transporter in skeletal and cardiac muscle. The current consensus on the mechanism of these transporters, based on experimental measurements of labeled lactate fluxes, is that monocarboxylate-proton symport occurs via a rapid-equilibrium ordered mechanism with proton binding followed by monocarboxylate binding. This study tests ordered and random mechanisms by fitting experimental measurements of tracer exchange fluxes from MCT1 and MCT4 isoforms to theoretical predictions derived using relationships between one-way fluxes and thermodynamic forces. Analysis shows that: 1), the available kinetic data are insufficient to distinguish between a rapid-equilibrium ordered and a rapid-equilibrium random-binding model for MCT4; 2), MCT1 has a higher affinity to lactate than does MCT4; 3), the theoretical conditions for the so-called trans-acceleration phenomenon (e.g., increased tracer efflux from a vesicle caused by increased substrate concentration outside the vesicle) do not necessarily require the rate constant for the lactate and proton bound transporter to reorient across the membrane to be higher than that for the unbound transporter; and finally, 4), based on model analysis, additional experiments are proposed to be able to distinguish between ordered and random-binding mechanisms.     

Regulation of Cardiac Expression of the Diabetic Marker MicroRNA miR-29

Diabetes mellitus (DM) is an independent risk factor for heart disease and its underlying mechanisms are unclear. Increased expression of diabetic marker miR-29 family miRNAs (miR-29a, b and c) that suppress the pro-survival protein Myeloid Cell Leukemia 1(MCL-1) is reported in pancreatic β-cells in Type 1 DM. Whether an up-regulation of miR-29 family miRNAs and suppression of MCL-1 (dysregulation of miR-29-MCL-1 axis) occurs in diabetic heart is not known. This study tested the hypothesis that insulin regulates cardiac miR-29-MCL-1 axis and its dysregulation correlates with DM progression. In vitro studies with mouse cardiomyocyte HL-1 cells showed that insulin suppressed the expression of miR-29a, b and c and increased MCL-1 mRNA. Conversely, Rapamycin (Rap), a drug implicated in the new onset DM, increased the expression of miR-29a, b and c and suppressed MCL-1 and this effect was reversed by transfection with miR-29 inhibitors. Rap inhibited mammalian target of rapamycin complex 1 (mTORC1) signaling in HL-1 cells. Moreover, inhibition of either mTORC1 substrate S6K1 by PF-4708671, or eIF4E-induced translation by 4E1RCat suppressed MCL-1. We used Zucker diabetic fatty (ZDF) rat, a rodent model for DM, to test whether dysregulation of cardiac miR-29-MCL-1 axis correlates with DM progression. 11-week old ZDF rats exhibited significantly increased body weight, plasma glucose, insulin, cholesterol, triglycerides, body fat, heart weight, and decreased lean muscle mass compared to age-matched lean rats. Rap treatment (1.2 mg/kg/day, from 9-weeks to 15-weeks) significantly reduced plasma insulin, body weight and heart weight, and severely dysregulated cardiac miR-29-MCL1 axis in ZDF rats. Importantly, dysregulation of cardiac miR-29-MCL-1 axis in ZDF rat heart correlated with cardiac structural damage (disorganization or loss of myofibril bundles). We conclude that insulin and mTORC1 regulate cardiac miR-29-MCL-1 axis and its dysregulation caused by reduced insulin and mTORC1 inhibition increases the vulnerability of a diabetic heart to structural damage.     

Glucose transporter isotypes switch in T-antigen-transformed pancreatic beta cells growing in culture and in mice.

High-level expression of the low-Km glucose transporter isoform GLUT-1 is characteristic of many cultured tumor and oncogene-transformed cells. In this study, we tested whether induction of GLUT-1 occurs in tumors in vivo. Normal mouse beta islet cells express the high-Km (approximately 20 mM) glucose transporter isoform GLUT-2 but not the low-Km (1 to 3 mM) GLUT-1. In contrast, a beta cell line derived from an insulinoma arising in a transgenic mouse harboring an insulin-promoted simian virus 40 T-antigen oncogene (beta TC3) expressed very low levels of GLUT-2 but high levels of GLUT-1. GLUT-1 protein was not detectable on the plasma membrane of islets or tumors of the transgenic mice but was induced in high amounts when the tumor-derived beta TC3 cells were grown in tissue culture. GLUT-1 expression in secondary tumors formed after injection of beta TC3 cells into mice was reduced. Thus, high-level expression of GLUT-1 in these tumor cells is characteristic of culture conditions and is not induced by the oncogenic transformation; indeed, overnight culture of normal pancreatic islets causes induction of GLUT-1. We also investigated the relationship between expression of the different glucose transporter isoforms by islet and tumor cells and induction of insulin secretion by glucose. Prehyperplastic transgenic islet cells that expressed normal levels of GLUT-2 and no detectable GLUT-1 exhibited an increased sensitivity to glucose, as evidenced by maximal insulin secretion at lower glucose concentrations, compared with that exhibited by normal islets. Further, hyperplastic islets and primary and secondary tumors expressed low levels of GLUT-2 and no detectable GLUT-1 on the plasma membrane; these cells exhibited high basal insulin secretion and responded poorly to an increase in extracellular glucose. Thus, abnormal glucose-induced secretion of insulin in prehyperplastic islets in mice was independent of changes in GLUT-2 expression and did not require induction of GLUT-1 expression.     

Monocarboxylate transporter 1 promotes proliferation and invasion of renal cancer cells by mediating acetate transport

One hallmark of renal cell carcinoma (RCC) is metabolic reprogramming, which involves elevation of glycolysis and upregulation of lipid metabolism. However, the mechanism of metabolic reprogramming is incompletely understood. Monocarboxylate transporter 1 (MCT1) promotes transport for lactate and pyruvate, which are crucial for cell metabolism. The aim of present study was to investigate the function of MCT1 on RCC development and its mechanism on metabolic reprogramming. The results showed that MCT1 messenger RNA and protein levels significantly increased in cancer tissues of ccRCC compared to normal tissue. MCT1 was further found to mainly located in the cell membrane of RCC. The knockdown of MCT1 by RNAi significantly inhibited proliferation and migration of 786-O and ACHN cells. MCT1 also induced the expressions of proliferation marker Ki-67 and invasion marker SNAI1. Moreover, we also showed that acetate treatment could upregulate the expression of MCT1, but not other MCT isoforms. On the other hand, MCT1 was involved in acetate transport and intracellular histone acetylation. In summary, this study revealed that MCT1 is abnormally high in ccRCC and promotes cancer development. The regulatory effect of MCT1 on cell proliferation and invasion maybe mediated by acetate transport.     

Elimination of KATP channels in mouse islets results in elevated [U-13C]glucose metabolism, glutaminolysis, and pyruvate cycling but a decreased gamma-aminobutyric acid shunt

Pancreatic beta cells are hyper-responsive to amino acids but have decreased glucose sensitivity after deletion of the sulfonylurea receptor 1 (SUR1) both in man and mouse. It was hypothesized that these defects are the consequence of impaired integration of amino acid, glucose, and energy metabolism in beta cells. We used gas chromatography-mass spectrometry methodology to study intermediary metabolism of SUR1 knock-out (SUR1(-/-)) and control mouse islets with d-[U-(13)C]glucose as substrate and related the results to insulin secretion. The levels and isotope labeling of alanine, aspartate, glutamate, glutamine, and gamma-aminobutyric acid (GABA) served as indicators of intermediary metabolism. We found that the GABA shunt of SUR1(-/-) islets is blocked by about 75% and showed that this defect is due to decreased glutamate decarboxylase synthesis, probably caused by elevated free intracellular calcium. Glutaminolysis stimulated by the leucine analogue d,l-beta-2-amino-2-norbornane-carboxylic acid was, however, enhanced in SUR1(-/-) and glyburide-treated SUR1(+/+) islets. Glucose oxidation and pyruvate cycling was increased in SUR1(-/-) islets at low glucose but was the same as in controls at high glucose. Malic enzyme isoforms 1, 2, and 3, involved in pyruvate cycling, were all expressed in islets. High glucose lowered aspartate and stimulated glutamine synthesis similarly in controls and SUR1(-/-) islets. The data suggest that the interruption of the GABA shunt and the lack of glucose regulation of pyruvate cycling may cause the glucose insensitivity of the SUR1(-/-) islets but that enhanced basal pyruvate cycling, lowered GABA shunt flux, and enhanced glutaminolytic capacity may sensitize the beta cells to amino acid stimulation.     

Glucagon-like peptide-1 and islet lipolysis.

A role for glucagon-like peptide 1 (GLP-1) has been suggested in stimulating beta-cell lipolysis via elevation of cAMP and activation of protein kinase A, which in turn may activate hormone-sensitive lipase (HSL), thereby contributing to fatty acid generation (FFA) from intracellular triglyceride stores. FFAs may then be metabolized to a lipid signal, which is required for optimal glucose-stimulated insulin secretion. Since HSL is expressed in islet beta-cells, this effect could contribute to the stimulation of insulin secretion by GLP-1, provided that a lipid signal of importance for insulin secretion is generated. To examine this hypothesis, we have studied the acute effect of GLP-1 on isolated mouse islets from normal mice and from mice with high-fat diet induced insulin resistance. We found, however, that although GLP-1 (100 nM) markedly potentiated glucose-stimulated insulin secretion from islets of both feeding groups, the peptide was not able to stimulate islet palmitate oxidation or increase lipolysis measured as glycerol release. This indicates that a lipid signal does not contribute to the acute stimulation of insulin secretion by GLP-1. To test whether lipolysis might be involved in the islet effects of long-term GLP-1 action, mice from the two feeding groups were chronically treated with exendin-4, a peptide that lowers blood glucose by interacting with GLP-1 receptors, in order to stimulate insulin secretion, for 16 days before isolation of the islets. The insulinotropic effects of GLP-1 and forskolin were exaggerated in isolated islets from exendin-4 treated mice given a high-fat diet, with a augmented palmitate oxidation as well as islet lipolysis at high glucose levels in these islets. Exendin-4 treatment had less impact on mice fed a normal diet. From these results we conclude that while GLP-1 does not seem to induce beta-cell lipolysis acutely in mouse islets, the peptide affects beta-cell fat metabolism after long-term adaptation to GLP-1 receptor stimulation.     

miR-29b and miR-29c Are Involved in Toll-Like Receptor Control of Glucocorticoid-Induced Apoptosis in Human Plasmacytoid Dendritic Cells

Glucocorticoids (GCs) are frequently used to treat many of the acute disease manifestations associated with inflammatory and autoimmune disorders. However, Toll-like receptor (TLR) pathway-activated plasmacytoid dendritic cells (pDCs) are resistant to GC-induced apoptosis, which leads to the inefficiency of GCs in the treatment of type I interferon-related autoimmune diseases, such as systemic lupus erythematosus (SLE). Therefore, compounds promoting pDC apoptosis may be helpful for improving the efficacy of GCs. In this study, we performed screening to identify microRNAs (miRNAs) involved in TLR-inhibited GC-induced pDC apoptosis and found an array of miRNAs that may regulate pDC apoptosis. Among those demonstrating altered expression, 6 miRNAs were inhibited in TLR-activated pDCs. Bioinformatics analysis and functional studies indicated that miR-29b and miR-29c were 2 key miRNAs involved in TLR-inhibited GC-induced pDC apoptosis. Furthermore, both of these miRNAs promoted pDC apoptosis by directly targeting Mcl-1 and Bcl-2 in human primary pDCs. Our findings provide new targets that could improve the efficacy of GCs for the treatment of SLE.     

The malate-aspartate NADH shuttle member Aralar1 determines glucose metabolic fate, mitochondrial activity, and insulin secretion in beta cells

The NADH shuttle system, which transports reducing equivalents from the cytosol to the mitochondria, is essential for the coupling of glucose metabolism to insulin secretion in pancreatic beta cells. Aralar1 and citrin are two isoforms of the mitochondrial aspartate/glutamate carrier, one key constituent of the malate-aspartate NADH shuttle. Here, the effects of Aralar1 overexpression in INS-1E beta cells and isolated rat islets were investigated for the first time. We prepared a recombinant adenovirus encoding for human Aralar1 (AdCA-Aralar1), tagged with the small FLAG epitope. Transduction of INS-1E cells and isolated rat islets with AdCA-Aralar1 increased aralar1 protein levels and immunostaining revealed mitochondrial localization. Compared with control INS-1E cells, overexpression of Aralar1 potentiated metabolism secretion coupling stimulated by 15 mm glucose. In particular, there was an increase of NAD(P)H generation, of mitochondrial membrane hyperpolarization, ATP levels, glucose oxidation, and insulin secretion (+45%, p < 0.01). Remarkably, this was accompanied by reduced lactate production. Rat islets overexpressing Aralar1 secreted more insulin at 16.7 mm glucose (+65%, p < 0.05) compared with controls. These results show that aspartate-glutamate carrier capacity limits glucose-stimulated insulin secretion and that Aralar1 overexpression enhances mitochondrial metabolism.     

miR-124 Regulates the Expression of BACE1 in the Hippocampus Under Chronic Cerebral Hypoperfusion

Chronic cerebral hypoperfusion (CCH) is a high-risk factor of Alzheimer’s disease (AD). MicroRNAs (miRNAs) are ideal mediators of hypoxic stress responses to facilitate cellular adaptation to long-term hypoxia. MiR-124 is a kind of nervous system-specific miRNAs, and one of its target genes is β-site amyloid precursor protein cleaving enzyme 1 (BACE1). In the present study, miR-124 was found to be inhibited all the time from early to late stage of cerebral hypoxia accompanying with the upregulation of BACE1 protein and overproduction of amyloid-β (Aβ) in the hippocampus from cerebral hypoperfusion rat models. Meanwhile, Aβ could further enhance the expression of BACE1 protein due to the inhibition of miR-124. Thus, miR-124 was the key factor in this hypoxia/Aβ–miR-124–BACE1–Aβ cycle. The activation of EPAC-Rap1 pathway was involved in the inhibition of miR-124 in hippocampus under hypoxia or Aβ insult. Our data suggest that, as an endogenous regulator of BACE1 protein, miR-124 may play a role in AD onset induced by CCH.     

MicroRNA miR-124 regulates neurite outgrowth during neuronal differentiation

MicroRNAs (miRNAs) are small RNAs with diverse regulatory roles. The miR-124 miRNA is expressed in neurons in the developing and adult nervous system. Here we show that overexpression of miR-124 in differentiating mouse P19 cells promotes neurite outgrowth, while blocking miR-124 function delays neurite outgrowth and decreases acetylated α-tubulin. Altered neurite outgrowth also was observed in mouse primary cortical neurons when miR-124 expression was increased, or when miR-124 function was blocked. In uncommitted P19 cells, miR-124 expression led to disruption of actin filaments and stabilization of microtubules. Expression of miR-124 also decreased Cdc42 protein and affected the subcellular localization of Rac1, suggesting that miR-124 may act in part via alterations to members of the Rho GTPase family. Furthermore, constitutively active Cdc42 or Rac1 attenuated neurite outgrowth promoted by miR-124. To obtain a broader perspective, we identified mRNAs downregulated by miR-124 in P19 cells using microarrays. mRNAs for proteins involved in cytoskeletal regulation were enriched among mRNAs downregulated by miR-124. A miR-124 variant with an additional 5′ base failed to promote neurite outgrowth and downregulated substantially different mRNAs. These results indicate that miR-124 contributes to the control of neurite outgrowth during neuronal differentiation, possibly by regulation of the cytoskeleton.     

miR-222 and miR-29a contribute to the drug-resistance of breast cancer cells

Adriamycin (Adr) and docetaxel (Doc) are two chemotherapeutic agents commonly used in the treatment of breast cancer. However, patients with breast cancer who are treated by the drugs often develop resistance to them and some other drugs. Recently studies have shown that microRNAs (miRNAs, miRs) play an important role in drug-resistance. In present study, miRNA expression profiles of MCF-7/S and its two resistant variant MCF-7/Adr and MCF-7/Doc cells were analyzed using microarray and the results were confirmed by real-time quantitative polymerase chain reaction. Here, 183 differentially expressed miRNAs were identified in the two resistant sublines compared to MCF-7/S. Then, five up-regulated miRNAs (miR-100, miR-29a, miR-196a, miR-222 and miR-30a) in both MCF-7/Adr and MCF-7/Doc were selected to explore their roles in acquisition of drug-resistance using transfection experiment. The results showed that miR-222 and miR-29a mimics and inhibitors had partially changed the drug-resistance of breast cancer cells, which was also confirmed by apoptosis assay. Western blot results suggested that miR-222 and -29a could regulate the expression of PTEN, maybe through which the two miRNAs conferred Adr and Doc resistance in MCF-7 cells. Finally, pathway mapping tools were employed to further analyze signaling pathways affected by the two miRNAs. In summary, this study demonstrates that altered miRNA expression pattern is involved in acquiring resistance to Adr and Doc in breast cancer MCF-7 cells, and that there are some miRNAs who displayed consistent up- or down-regulated expression changes in the two resistant sublines. The most importance is that we identify two miRNAs (miR-222 and miR-29a) involved in drug-resistance, at least in part via targeting PTEN.     

miR-29a and miR-142-3p downregulation and diagnostic implication in human acute myeloid leukemia

Expression profiling of microRNAs (miRNAs) in most diseases might be popular and provide the possibility for diagnostic implication, but few studies have accurately quantified the expression level of dysregulated miRNAs in acute myeloid leukemia (AML). In this study, we analyzed the peripheral blood mononuclear cells (PBMCs) from 10 AML patients (subtypes M1 to M5) and six normal controls by miRNA microarray and identified several differentially expressed miRNAs. Among them miR-29a and miR-142-3p were selectively encountered in Northern blot analysis and their significantly decreased expression in AML was further confirmed. Quantitative real-time PCR in 52 primarily diagnosed AML patients and 100 normal controls not only verified the expression properties of these 2 miRNAs, but also established that the expression level of miR-142-3p and miR-29a in PBMCs could be used as novel diagnostic markers. A better diagnostic outcome was achieved by combining miR-29a and miR-142-3p with about 90% sensitivity, 100% specificity, and an area under the ROC curve (AUC) of 0.97. Our results provide insights into the involvement of miRNAs in leukemogenesis, and offer candidates for AML diagnosis and therapeutic strategy.     

miR-29ab1 Deficiency Identifies a Negative Feedback Loop Controlling Th1 Bias That Is Dysregulated in Multiple Sclerosis

Th cell programming and function is tightly regulated by complex biological networks to prevent excessive inflammatory responses and autoimmune disease. The importance of microRNAs (miRNAs) in this process is highlighted by the preferential Th1 polarization of Dicer-deficient T cells that lack miRNAs. Using genetic knockouts, we demonstrate that loss of endogenous miR-29, derived from the miR-29ab1 genomic cluster, results in unrestrained T-bet expression and IFN-γ production. miR-29b regulates T-bet and IFN-γ via a direct interaction with the 3' untranslated regions, and IFN-γ itself enhances miR-29b expression, establishing a novel regulatory feedback loop. miR-29b is increased in memory CD4(+) T cells from multiple sclerosis (MS) patients, which may reflect chronic Th1 inflammation. However, miR-29b levels decrease significantly upon T cell activation in MS patients, suggesting that this feedback loop is dysregulated in MS patients and may contribute to chronic inflammation. miR-29 thus serves as a novel regulator of Th1 differentiation, adding to the understanding of T cell-intrinsic regulatory mechanisms that maintain a balance between protective immunity and autoimmunity.     

Beta-cell specific deletion of Dicer1 leads to defective insulin secretion and diabetes mellitus

Mature microRNAs (miRNAs), derived through cleavage of pre-miRNAs by the Dicer1 enzyme, regulate protein expression in many cell-types including cells in the pancreatic islets of Langerhans. To investigate the importance of miRNAs in mouse insulin secreting β-cells, we have generated mice with a β-cells specific disruption of the Dicer1 gene using the Cre-lox system controlled by the rat insulin promoter (RIP). In contrast to their normoglycaemic control littermates (RIP-Cre(+/-) Dicer1(Δ/wt)), RIP-Cre(+/-)Dicer1(flox/flox) mice (RIP-Cre Dicer1(Δ/Δ)) developed progressive hyperglycaemia and full-blown diabetes mellitus in adulthood that recapitulated the natural history of the spontaneous disease in mice. Reduced insulin gene expression and concomitant reduced insulin secretion preceded the hyperglycaemic state and diabetes development. Immunohistochemical, flow cytometric and ultrastructural analyses revealed altered islet morphology, marked decreased β-cell mass, reduced numbers of granules within the β-cells and reduced granule docking in adult RIP-Cre Dicer1(Δ/Δ) mice. β-cell specific Dicer1 deletion did not appear to disrupt fetal and neonatal β-cell development as 2-week old RIP-Cre Dicer1(Δ/Δ) mice showed ultrastructurally normal β-cells and intact insulin secretion. In conclusion, we have demonstrated that a β-cell specific disruption of the miRNAs network, although allowing for apparently normal β-cell development, leads to progressive impairment of insulin secretion, glucose homeostasis and diabetes development.     

miR-29b,miR-205 and miR-221 Enhance Chemosensitivity to Gemcitabine in HuH28 Human Cholangiocarcinoma Cells

Background and Aims

Cholangiocarcinoma (CCA) is highly resistant to chemotherapy, including gemcitabine (Gem) treatment. MicroRNAs (miRNAs) are endogenous, non-coding, short RNAs that can regulate multiple genes expression. Some miRNAs play important roles in the chemosensitivity of tumors. Here, we examined the relationship between miRNA expression and the sensitivity of CCA cells to Gem.

Methods

Microarray analysis was used to determine the miRNA expression profiles of two CCA cell lines, HuH28 and HuCCT1. To determine the effect of candidate miRNAs on Gem sensitivity, expression of each candidate miRNA was modified via either transfection of a miRNA mimic or transfection of an anti-oligonucleotide. Ontology-based programs were used to identify potential target genes of candidate miRNAs that were confirmed to affect the Gem sensitivity of CCA cells.

Results

HuCCT1 cells were more sensitive to Gem than were HuH28 cells, and 18 miRNAs were differentially expressed whose ratios over ± 2log2 between HuH28 and HuCCT1. Among these 18 miRNAs, ectopic overexpression of each of three downregulated miRNAs in HuH28 (miR-29b, miR-205, miR-221) restored Gem sensitivity to HuH28. Suppression of one upregulated miRNA in HuH28, miR-125a-5p, inhibited HuH28 cell proliferation independently to Gem treatment. Selective siRNA-mediated downregulation of either of two software-predicted targets, PIK3R1 (target of miR-29b and miR-221) or MMP-2 (target of miR-29b), also conferred Gem sensitivity to HuH28.

Conclusions

miRNA expression profiling was used to identify key miRNAs that regulate Gem sensitivity in CCA cells, and software that predicts miRNA targets was used to identify promising target genes for anti-tumor therapies.