miR-150 regulates glucose utilization through targeting GLUT4 in insulin-resistant cardiomyocytes

MicroRNAs (miRNAs) play an important role in cardiac function and metabolism. However, whether they regulate insulin resistance (IR) of cardiomyocytes remains unclear. The aim of the present study was to shed light on this issue with a focus on miR-150. We found here that miR-150 level was elevated in myocardium of type 2 diabetes mellitus (T2DM) rat model and in insulin-resistant cardiomyocytes induced by high glucose (25 mM) and high insulin (1 μM). Deregulation of miR-150 downregulated the protein and mRNA levels of glucose transporter 4 (GLUT4) as assessed by western blot, real-time polymerase chain reaction (qPCR), and immunofluorescence assays. Overexpression of miR-150 inhibited glucose utilization in cardiomyocytes as detected by 2-deoxyglucose transport and glucose consumption assays. In contrast, knockdown of miR-150 significantly increased glucose uptake in cardiomyocytes. Moreover, GLUT4 translocation was increased after transfection of miR-150 inhibitor (AMO-150). Collectively, miR-150 reduced glucose utilization by directly decreasing the expression and translocation of GLUT4 in the cardiomyocytes with IR and therefore might be a new therapeutic target for metabolic diseases such as T2DM.

Up-regulation of miRNA-105 inhibits the progression of gastric carcinoma by directly targeting SOX9

OBJECTIVE

MicroRNAs (miRNAs) are involved in the tumorigenesis and progression of multiple tumor types and function as either tumor suppressor genes or oncogenes. This study was designed to investigate the functional behaviors and regulatory mechanisms of miR-105 in the progression of gastric carcinoma.

PATIENTS AND METHODS

24 pairs of patients with gastric carcinoma were enrolled in this study. The levels of miR-105 in gastric carcinoma tissues and cells were determined using quantitative Real Time-Polymerase Chain Reaction (qRT-PCR) assay. The biological functions of miR-105 in gastric carcinoma cell were detected by colony formation, transwell invasion and wound-healing assay. Luciferase activity assay and immunoblotting assay were applied to validate the direct target of miR-105. The expression of SRY-Box 9 (SOX9) was detected using immunofluorescence staining assay. Furthermore, the role of miR-105 on the growth of gastric carcinoma cell was examined in the established xenograft model. The role of miR-105 in the metastasis of gastric carcinoma cell in vivo, an experimental metastasis assay was performed.

RESULTS

Herein, we proved that miR-105 was down-regulated in gastric carcinoma specimens as well as gastric cancer cells. Up-regulation of miR-105 suppressed the colony formation and aggressiveness traits of gastric carcinoma cell lines BGC823 and SGC7901 in vitro. Furthermore, over-expression of miR-105 inhibited the tumor growth as well as lung metastasis of gastric carcinoma cell in vivo. Further investigation identified SOX9 was the target gene of miR-105 in gastric cancer and its expression was negatively associated with the expression of miR-105 in gastric carcinoma tissues. Finally, overexpression of SOX9 partially reversed the influence of miR-105 on the growth and aggressiveness of gastric carcinoma cell.

CONCLUSIONS

These results revealed the crucial role of miR-105 in the progression and metastasis of gastric carcinoma, which indicated the potential application of miR-105 in the treatment of gastric carcinoma.

Metformin Shortens Prolonged QT Interval in Diabetic Mice by Inhibiting L-Type Calcium Current: A Possible Therapeutic Approach

The incidence and mortality of cardiovascular disease in diabetic patients are 2-3 times higher than those in non-diabetic patients. Abnormal function of the L-type calcium channel in myocardial tissue might result in multiple cardiac disorders such as a prolonged QT interval. Therefore, QT prolongation is an independent risk factor of cardiovascular disease in patients with diabetes mellitus. Metformin, a hypoglycemic agent, is widely known to effectively reduce the occurrence of macrovascular diseases. The aim of the present study was to evaluate the effect of metformin on prolonged QT interval and to explore potential ionic mechanisms induced by diabetes. Diabetic mouse models were established with streptozotocin and an electrocardiogram was used to monitor the QT interval after 4 weeks of metformin treatment in each group. Action potential duration (APD) and L-type calcium current (I_Ca-L) were detected by patch-clamp in isolated mice ventricular cardiomyocytes and neonatal cardiomyocytes of mice. The expression levels of CACNA1C mRNA and Cav1.2 were measured by real-time PCR, western blot and immunofluorescence. A shortened QT interval was observed after 4 weeks of metformin treatment in diabetic mice. Patch-clamp results revealed that both APD and I_Ca-L were shortened in mouse cardiomyocytes. Furthermore, the expression levels of CACNA1C mRNA and Cav1.2 were decreased in the metformin group. The same results were also obtained in cultured neonatal mice cardiomyocytes. Overall, these results verify that metformin could shorten a prolonged QT interval by inhibiting the calcium current, suggesting that metformin may play a role in the electrophysiology underlying diabetic cardiopathy.

Topical GDF11 accelerates skin wound healing in both type 1 and 2 diabetic mouse models

This study aimed to investigate the role of truncated growth differentiation factor 11 (GDF11), in which the recognition site of Furin from wild-type GDF11 was deleted to enhance the cellular stability, in skin wound healing in the setting of diabetes mellitus (DM) and the underlying mechanisms. Our study found that both truncated and natural GDF11s effectively accelerated wound healing processes in both T1DM and T2DM mice with a potency compatible to PDGF, bFGF, and EGF, but being much higher than GDF8. At the cellular level, GDF11 stimulated the proliferation and suppressed HG-induced apoptosis of HSFs. Further study revealed that GDF11 activated the YAP-Smad2/3-CTGF fibrotic signaling pathway by reversing HG-induced upregulation of phosphorylated form of YAP (p-YAP), increases p-Smad2/3 levels, and restoring HG-induced repression of CTGF expression by GDF11. Overall, the study shows that both natural and truncated GDF11s promote the healing process of skin wound in mice of both T1DM and T2DM partly via stimulating dermal fibrosis via the YAP-Smad2/3-CTGF pathway, suggesting it a potential agent for treating skin wound in diabetic population.

Promising Treatment for Type 2 Diabetes: Fecal Microbiota Transplantation Reverses Insulin Resistance and Impaired Islets

Type 2 diabetes is a common metabolic disorder related to insulin resistance, or deficiency of insulin secretion, caused by decreased insulin sensitivity and destruction of islet structure and function. As the second human genome, the microbiota has been observed to have a growing relationship with diabetes in recent years. Microbiota imbalance has been hypothesized to be involved in the regulation of energy metabolism and the inflammatory immune response in diabetes. The present study aimed to investigate whether fecal microbiota transplantation (FMT) could alleviate the symptoms associated with type 2 diabetes. To this end, a type 2 diabetes mouse model was first established through the consumption of a high-fat diet combined with streptozotocin (100 mg/kg), and FMT was used to rebuild the gut microbiota of diabetic mice. Fasting blood glucose, oral glucose tolerance tests, and HbA1c levels were monitored, while the hypoglycemic effects of FMT were also observed. Insulin levels were tested by ELISA and related indexes such as HOMA-IR, HOMA-IS, and HOMA-β were calculated. We found that insulin resistance and pancreatic islet β-cells were improved after FMT treatment. Meanwhile, the markers of inflammation in the pancreatic tissue were detected by ELISA and immunohistochemistry, which indicated that inflammatory response decreased following FMT treatment. Furthermore, flow cytometry and western blot results revealed that FMT inhibited the β-cell apoptosis. Here, the effect of FMT on hypoglycemia in type 2 diabetes was addressed by improving insulin resistance and repairing impaired islets, thereby providing a potential treatment strategy for type 2 diabetes.

Fibroblast growth factor 21 inhibited ischemic arrhythmias via targeting miR-143/EGR1 axis

Ventricular arrhythmia is the most common cause of sudden cardiac death in patients with myocardial infarction (MI). Fibroblast growth factor 21 (FGF21) has been shown to play an important role in cardiovascular and metabolic diseases. However, the effects of FGF21 on ventricular arrhythmias following MI have not been addressed yet. The present study was conducted to investigate the pharmacological action of FGF21 on ventricular arrhythmias after MI. Adult male mice were administrated with or without recombinant human basic FGF21 (rhbFGF21), and the susceptibility to arrhythmias was assessed by programmed electrical stimulation and optical mapping techniques. Here, we found that rhbFGF21 administration reduced the occurrence of ventricular tachycardia (VT), improved epicardial conduction velocity and shorted action potential duration at 90% (APD90) in infarcted mouse hearts. Mechanistically, FGF21 may improve cardiac electrophysiological remodeling as characterized by the decrease of INa and IK1 current density in border zone of infarcted mouse hearts. Consistently, in vitro study also demonstrated that FGF21 may rescue oxidant stress-induced dysfunction of INa and IK1 currents in cultured ventricular myocytes. We further found that oxidant stress-induced down-regulation of early growth response protein 1 (EGR1) contributed to INa and IK1 reduction in post-infarcted hearts, and FGF21 may recruit EGR1 into the SCN5A and KCNJ2 promoter regions to up-regulate NaV1.5 and Kir2.1 expression at transcriptional level. Moreover, miR-143 was identified as upstream of EGR1 and mediated FGF21-induced EGR1 up-regulation in cardiomyocytes. Collectively, rhbFGF21 administration effectively suppressed ventricular arrhythmias in post-infarcted hearts by regulating miR-143-EGR1-NaV1.5/Kir2.1 axis, which provides novel therapeutic strategies for ischemic arrhythmias in clinics.

Systematic Analysis of Intestinal MicroRNAs Expression in HCC: Identification of Suitable Reference Genes in Fecal Samples

Hepatocellular carcinoma (HCC) is an extremely fatal malignancy. Intestinal microRNAs, which can be detected in fecal samples in humans may be involved in the pathological process of HCC. Therefore, screening for functional intestinal microRNAs in fecal samples and investigating their potential roles in the molecular progression of HCC are necessary. Quantitative real-time PCR (qRT-PCR) has been widely used in microRNA expression studies. However, few genes have been reported as reference genes for intestinal microRNAs in fecal samples. In order to obtain a more accurately analyzed intestinal microRNAs expression, we first searched for reliable reference genes for intestinal microRNAs expression normalization during qRT-PCR, using three software packages (GeNorm, NormFinder, and Bestkeeper). Next we screened and predicted the target genes of the differentially intestinal microRNAs of control and HCC mice through quantitative RT-PCR or miRtarBase. Finally, we also analyzed the mRNA targets for enrichment of Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways using the DAVID Bioinformatic Resources database. This study has successfully screened relatively suitable reference genes and we have discovered that the differential intestinal microRNAs play significant roles in the development of HCC. The top reference genes identified in this study could provide a theoretical foundation for the reasonable selection of a suitable reference gene. Furthermore, the detection of intestinal microRNAs expression may serve as a promising therapeutic target for the diagnosis and treatment of HCC.

Increase of late sodium current contributes to enhanced susceptibility to atrial fibrillation in diabetic mice

Studies demonstrated that the incidence of atrial fibrillation is significantly increased in patients with diabetes mellitus. Increase of late sodium current (I_NaL) has been associated with atrial arrhythmias. However, the role of I_NaL in the setting of atrial fibrillation in diabetes mellitus remained unknown. In this study, we investigated the alteration of I_NaL in the atria of diabetic mice and the therapeutic effect of its inhibitor (GS967) on the susceptibility of atrial fibrillation. The whole-cell patch-clamp technique was used to detect single cell electrical activities. The results showed that the density of I_NaL in diabetic cardiomyocytes was larger than that of the control cells at the holding potential of -100 mV. The action potential duration at both 50% and 90% repolarization, APD₅₀ and APD₉₀, respectively, was markedly increased in diabetic mice than in controls. GS967 application inhibited I_NaL and shortened APD of diabetic mice. High-frequency electrical stimuli were used to induce atrial arrhythmias. We found that the occurrence rate of atrial fibrillation was significantly increased in diabetic mice, which was alleviated by the administration of GS967. In GS967-treated diabetic mice, the I_NaL current density was reduced and APD was shortened. In conclusion, the susceptibility to atrial fibrillation was increased in diabetic mice, which is associated with the increased late sodium current and the consequent prolongation of action potential. Inhibition of I_NaL by GS967 is beneficial against the occurrence of atrial fibrillation in diabetic mice.

Long non-coding RNA CCRR controls cardiac conduction via regulating intercellular coupling

Long non-coding RNAs (lncRNAs) have emerged as a new class of gene expression regulators playing key roles in many biological and pathophysiological processes. Here, we identify cardiac conduction regulatory RNA (CCRR) as an antiarrhythmic lncRNA. CCRR is downregulated in a mouse model of heart failure (HF) and in patients with HF, and this downregulation slows cardiac conduction and enhances arrhythmogenicity. Moreover, CCRR silencing induces arrhythmias in healthy mice. CCRR overexpression eliminates these detrimental alterations. HF or CCRR knockdown causes destruction of intercalated discs and gap junctions to slow longitudinal cardiac conduction. CCRR overexpression improves cardiac conduction by blocking endocytic trafficking of connexin43 (Cx43) to prevent its degradation via binding to Cx43-interacting protein CIP85, whereas CCRR silence does the opposite. We identified the functional domain of CCRR, which can reproduce the functional roles and pertinent molecular events of full-length CCRR. Our study suggests CCRR replacement a potential therapeutic approach for pathological arrhythmias.

Downregulation of Long Non-Coding RNA Kcnq1ot1: An Important Mechanism of Arsenic Trioxide-Induced Long QT Syndrome

Background/Aims: Arsenic trioxide (ATO) is a known anti-acute promyelocytic leukemia (APL) reagent, whose clinical applications are limited by its serious cardiac toxicity and fatal adverse effects, such as sudden cardiac death resulting from long QT syndrome (LQTS). The mechanisms of cardiac arrhythmia due to ATO exposure still need to be elucidated. Long non-coding RNAs (lncRNAs) are emerging as major regulators of various pathophysiological processes. This study aimed to explore the involvement of lncRNAs in ATO-induced LQTS in vivo and in vitro. Methods: For in vivo experiments, mice were administered ATO through the tail vein. For in vitro experiments, ATO was added to the culture medium of primary cultured neonatal mouse cardiomyocytes. To evaluate the effect of lncRNA Kcnq1ot1, siRNA and lentivirus-shRNA were synthesized to knockdown lncRNA Kcnq1ot1. Results: After ATO treatment, the Kcnq1ot1 and Kcnq1 expression levels were down regulated. lncRNA Kcnq1ot1 knockdown prolonged the action potential duration (APD) in vitro and exerted LQTS in vivo. Correspondingly, Kcnq1 expression was decreased after silencing lncRNA Kcnq1ot1. However, the knockdown of Kcnq1 exerted no effect on lncRNA Kcnq1ot1 expression. Conclusions: To our knowledge, this report is the first to demonstrate that lncRNA Kcnq1ot1 downregulation is responsible for QT interval prolongation induced by ATO at least partially by repressing Kcnq1 expression. lncRNA Kcnq1ot1 has important pathophysiological functions in the heart and could become a novel antiarrhythmic target.

Suppression of microRNA-16 protects against acute myocardial infarction by reversing beta2-adrenergic receptor down-regulation in rats

microRNA-16 (miR-16) has been shown to be up-regulated in ischemic heart. Beta2-adrenoreceptor (β₂-AR) exerts cardioprotective property in ischemic injury. This study aims to determine the effect of miR-16 in cardiac injury in rats and the possible involvement of β₂-AR in this process. Acute myocardial infarction (AMI) model in rats was induced by ligation of left coronary artery. Neonatal rat ventricular cells (NRVCs) were cultured in vitro tests. The cardiomyocyte model of oxidative injury was mimicked by hydrogen peroxide. The expression of miR-16 was obviously up-regulated and β₂-AR was remarkably down-regulated in both AMI rats and NRVCs under oxidative stress. miR-16 over-expression in NRVCs reduced cell viability and increased apoptosis. Conversely, inhibition of endogenous miR-16 with its specific inhibitor reversed these changes. Over-expression of miR-16 using an miR-16 lentivirus in AMI rats markedly increased cardiac infarct area, lactate dehydrogenase and creatine kinase activity, and exacerbated cardiac dysfunction. Lentivirus-mediated knockdown of miR-16 alleviated acute cardiac injury. Moreover, miR-16 over-expression significantly suppressed β₂-AR protein expression in both cultured NRVCs and AMI rats, while inhibition of miR-16 displayed opposite effect on β₂-AR protein expression. Luciferase assay confirmed that miR-16 could directly target the 3′untranslated region of β₂-AR mRNA. miR-16 is detrimental to the infarct heart and suppression of miR-16 protects rat hearts from ischemic injury via up-regulating of β₂-AR by binding to the 3′untranslated region of β₂-AR gene. This study indicates that targeting miR-16/β₂-AR axis may be a promising strategy for ischemic heart disease.

Circulating long non-coding RNAs NRON and MHRT as novel predictive biomarkers of heart failure

This study sought to evaluate the potential of circulating long non-coding RNAs (lncRNAs) as biomarkers for heart failure (HF). We measured the circulating levels of 13 individual lncRNAs which are known to be relevant to cardiovascular disease in the plasma samples from 72 HF patients and 60 non-HF control participants using real-time reverse transcription-polymerase chain reaction (real-time RT-PCR) methods. We found that out of the 13 lncRNAs tested, non-coding repressor of NFAT (NRON) and myosin heavy-chain-associated RNA transcripts (MHRT) had significantly higher plasma levels in HF than in non-HF subjects: 3.17 ± 0.30 versus 1.0 ± 0.07 for NRON (P < 0.0001) and 1.66 ± 0.14 versus 1.0 ± 0.12 for MHRT (P < 0.0001). The area under the ROC curve was 0.865 for NRON and 0.702 for MHRT. Univariate and multivariate analyses identified NRON and MHRT as independent predictors for HF. Spearman's rank correlation analysis showed that NRON was negatively correlated with HDL and positively correlated with LDH, whereas MHRT was positively correlated with AST and LDH. Hence, elevation of circulating NRON and MHRT predicts HF and may be considered as novel biomarkers of HF.

MIAT Is a Pro-fibrotic Long Non-coding RNA Governing Cardiac Fibrosis in Post-infarct Myocardium

A long non-coding RNA (lncRNA), named myocardial infarction associated transcript (MIAT), has been documented to confer risk of myocardial infarction (MI). The aim of this study is to elucidate the pathophysiological role of MIAT in regulation of cardiac fibrosis. In a mouse model of MI, we found that MIAT was remarkably up-regulated, which was accompanied by cardiac interstitial fibrosis. MIAT up-regulation in MI was accompanied by deregulation of some fibrosis-related regulators: down-regulation of miR-24 and up-regulation of Furin and TGF-β1. Most notably, knockdown of endogenous MIAT by its siRNA reduced cardiac fibrosis and improved cardiac function and restored the deregulated expression of the fibrosis-related regulators. In cardiac fibroblasts treated with serum or angiotensin II, similar up-regulation of MIAT and down-regulation of miR-24 were consistently observed. These changes promoted fibroblasts proliferation and collagen accumulation, whereas knockdown of MIAT by siRNA or overexpression of miR-24 with its mimic abrogated the fibrogenesis. Our study therefore has identified MIAT as the first pro-fibrotic lncRNA in heart and unraveled the role of MIAT in the pathogenesis of MI. These findings also promise that normalization of MIAT level may prove to be a therapeutic option for the treatment of MI-induced cardiac fibrosis and the associated cardiac dysfunction.

Reciprocal Changes of Circulating Long Non-Coding RNAs ZFAS1 and CDR1AS Predict Acute Myocardial Infarction

This study sought to evaluate the potential of circulating long non-coding RNAs (lncRNAs) as biomarkers for acute myocardial infarction (AMI). We measured the circulating levels of 15 individual lncRNAs, known to be relevant to cardiovascular disease, using the whole blood samples collected from 103 AMI patients, 149 non-AMI subjects, and 95 healthy volunteers. We found that only two of them, Zinc finger antisense 1 (ZFAS1) and Cdr1 antisense (CDR1AS), showed significant differential expression between AMI patients and control subjects. Circulating level of ZFAS1 was significantly lower in AMI (0.74 ± 0.07) than in non-AMI subjects (1.0 ± 0.05, P < 0.0001), whereas CDR1AS showed the opposite changes with its blood level markedly higher in AMI (2.18 ± 0.24) than in non-AMI subjects (1.0 ± 0.05, P < 0.0001). When comparison was made between AMI and non-AMI, the area under ROC curve was 0.664 for ZFAS1 alone or 0.671 for CDR1AS alone, and 0.691 for ZFAS1 and CDR1AS combination. Univariate and multivariate analyses identified these two lncRNAs as independent predictors for AMI. Similar changes of circulating ZFAS1 and CDR1AS were consistently observed in an AMI mouse model. Reciprocal changes of circulating ZFAS1 and CDR1AS independently predict AMI and may be considered novel biomarkers of AMI.

Acetyl salicylic acid attenuates cardiac hypertrophy through Wnt signaling

Ventricular hypertrophy is a powerful and independent predictor of cardiovascular morbid events. The vascular properties of low-dose acetyl salicylic acid (aspirin) provide cardiovascular benefits through the irreversible inhibition of platelet cyclooxygenase 1; however, the possible anti-hypertrophic properties and potential mechanism of aspirin have not been investigated in detail. In this study, healthy wild-type male mice were randomly divided into three groups and subjected to transverse aortic constriction (TAC) or sham operation. The TAC-operated mice were treated with the human equivalent of low-dose aspirin (10 mg·kg(-1)·d(-1)); the remaining mice received an equal amount of phosphate buffered saline with 0.65% ethanol, which was used as a vehicle. A cardiomyocyte hypertrophy model induced by angiotensin II (10 nmol·L(-1)) was treated with the human equivalent of low (10 or 100 μmol·L(-1)) and high (1000 μmol·L(-1)) aspirin concentrations in plasma. Changes in the cardiac structure and function were assessed through echocardiography and transmission electron microscopy. Gene expression was determined through RT-PCR and western blot analysis. Results indicated that aspirin treatment abrogated the increased thickness of the left ventricular anterior and posterior walls, the swelling of mitochondria, and the increased surface area in in vivo and in vitro hypertrophy models. Aspirin also normalized the upregulated hypertrophic biomarkers, β-myosin heavy chain (β-MHC), atrial natriuretic peptide (ANP), and b-type natriuretic peptide (BNP). Aspirin efficiently reversed the upregulation of β-catenin and P-Akt expression and the TAC- or ANG II-induced downregulation of GSK-3β. Therefore, low-dose aspirin possesses significant anti-hypertrophic properties at clinically relevant concentrations for anti-thrombotic therapy. The downregulation of β-catenin and Akt may be the underlying signaling mechanism of the effects of aspirin.

Suppression of AKT expression by miR-153 produced anti-tumor activity in lung cancer

Lung cancer is one of the leading causes of cancer death worldwide. microRNAs have been shown to be a novel class of regulators in lung cancer. Here, we explored the role of miR-153 in the pathogenesis of lung cancer and its therapeutic potential. miR-153 was significantly decreased in lung cancer tissues than the adjacent tissues. The protein and mRNA levels of protein kinase B (AKT), which were shown to promote tumor growth, were both increased in lung cancer tissues than adjacent tissues. Overexpression of miR-153 significantly inhibited AKT protein expression, which were abrogated by co-transfection of AMO-153, the specific inhibitor of miR-153. Luciferase assay showed that transfection of miR-153 markedly suppressed the fluorescent intensity of chimeric vectors carrying the 3'UTR of AKT1, while produced no effect on the mutant construct, indicating that AKT is regulated by miR-153. Overexpression of miR-153 significantly inhibited the proliferation and migration, and promoted apoptosis of cultured lung cancer cells in vitro, and suppressed the growth of xenograft tumors in vivo. Interestingly, lung cancer cells with lower endogenous miR-153 expression are more sensitive to ectopic overexpressed miR-153. The IC50 of miR-153 on lung cancer cells is positive correlated with the endogenous miR-153 level, while negative correlated with AKT level. Knockdown of AKT expression suppressed lung cancer cell proliferation. In summary, miR-153 exerted anti-tumor activity in lung cancer by targeting on AKT. The sensitivity of lung cancer cells to miR-153 is determined by its endogenous miR-153 level.

miRNA in the regulation of ion channel/transporter expression

Ion channels and transporters are expressed in every living cell, where they participate in controlling a plethora of biological processes and physiological functions, such as excitation of cells in response to stimulation, electrical activities of cells, excitation-contraction coupling, cellular osmolarity, and even cell growth and death. Alterations of ion channels/transporters can have profound impacts on the cellular physiology associated with these proteins. Expression of ion channels/transporters is tightly regulated and expression deregulation can trigger abnormal processes, leading to pathogenesis, the channelopathies. While transcription factors play a critical role in controlling the transcriptome of ion channels/transporters at the transcriptional level by acting on the 5'-flanking region of the genes, microribonucleic acids (miRNAs), a newly discovered class of regulators in the gene network, are also crucial for expression regulation at the posttranscriptional level through binding to the 3'untranslated region of the genes. These small noncoding RNAs fine tune expression of genes involved in a wide variety of cellular processes. Recent studies revealed the role of miRNAs in regulating expression of ion channels/transporters and the associated physiological functions. miRNAs can target ion channel genes to alter cardiac excitability (conduction, repolarization, and automaticity) and affect arrhythmogenic potential of heart. They can modulate circadian rhythm, pain threshold, neuroadaptation to alcohol, brain edema, etc., through targeting ion channel genes in the neuronal systems. miRNAs can also control cell growth and tumorigenesis by acting on the relevant ion channel genes. Future studies are expected to rapidly increase to unravel a new repertoire of ion channels/transporters for miRNA regulation.

Overexpression of microRNA-1 causes atrioventricular block in rodents

The present study was designed to investigate whether microRNAs (miRNAs) are involved in atrioventricular block (AVB) in the setting of myocardial ischemia (MI). A cardiac-specific miR-1 transgenic (Tg) mouse model was successfully established for the first time in this study using microinjection. miR-1 level was measured by real-time qRT-PCR. Whole-cell patch clamp was employed to record L-type calcium current (I_Ca,L) and inward rectifier K⁺ current (I_K1). Expression of connexin 43 (Cx43) protein was determined by western blot analysis. Alternations of [Ca²⁺]_i was detected by laser scanning confocal microscopy in ventricular myocytes. The incidence of AVB was higher in miR-1 Tg mice than that in wild-type (WT) mice. The normalized peak current amplitude of I_Ca,L was lower in ventricular myocytes from miR-1 Tg mice as compared with WT mice. Similarly, the current density of I_K1 was decreased in miR-1 Tg mice than that in WT mice. Compared with WT mice, miR-1 Tg mice exhibited a significant decrease of the systolic [Ca²⁺]_i in ventricular myocytes but a prominent increase of the resting [Ca²⁺]_i. Moreover, Cx43 protein was downregulated in miR-1 Tg mice compared to that in WT mice. Administration of LNA-modified antimiR-1 reversed all the above changes. miR-1 overexpression may contribute to the increased susceptibility of the heart to AVB, which provides us novel insights into the molecular mechanisms underlying ischemic cardiac arrhythmias.

MicroRNA-26 governs profibrillatory inward-rectifier potassium current changes in atrial fibrillation

Atrial fibrillation (AF) is a highly prevalent arrhythmia with pronounced morbidity and mortality. Inward-rectifier K+ current (IK1) is believed to be an important regulator of reentrant-spiral dynamics and a major component of AF-related electrical remodeling. MicroRNA-26 (miR-26) is predicted to target the gene encoding KIR2.1, KCNJ2. We found that miR-26 was downregulated in atrial samples from AF animals and patients and this downregulation was accompanied by upregulation of IK1/KIR2.1 protein. miR-26 overexpression suppressed expression of KCNJ2/KIR2.1. In contrast, miR-26 knockdown, inhibition, or binding-site mutation enhanced KCNJ2/KIR2.1 expression, establishing KCNJ2 as a miR-26 target. Knockdown of endogenous miR-26 promoted AF in mice, whereas adenovirus-mediated expression of miR-26 reduced AF vulnerability. Kcnj2-specific miR-masks eliminated miR-26-mediated reductions in Kcnj2, abolishing miR-26's protective effects, while coinjection of a Kcnj2-specific miR-mimic prevented miR-26 knockdown-associated AF in mice. Nuclear factor of activated T cells (NFAT), a known actor in AF-associated remodeling, was found to negatively regulate miR-26 transcription. Our results demonstrate that miR-26 controls the expression of KCNJ2 and suggest that this downregulation may promote AF.

Potential therapeutic value of antioxidants for abnormal prolongation of QT interval and the associated arrhythmias in a rabbit model of diabetes

Abnormal QT prolongation is the major cardiac electrical disorder and a predictor of mortality in diabetic patients. Our previous studies suggest that dysfunction of delayed rectifier K(+) current (I(Kr)) is the main cause for the problem. Here we report the potential therapeutic role and mechanisms of vitamin E in the rabbit model of diabetes. The QT interval and action potential duration were considerably prolonged with frequent occurrence of ventricular tachyarrhythmias in diabetic rabbits. Administration of vitamin E corrected the abnormal QT prolongation and abolished the arrhythmic incidence. I(Kr) was found markedly reduced resulting in slowing of cardiac repolarization thereby QT prolongation in diabetic hearts. The diabetic depression of I(Kr) is primarily ascribed to oxidative damages to the cardiac membrane and proteins, as indicated by the overproduction of reactive oxygen species leading to severe lipid peroxidation and protein oxidation. Moreover, I(Kr) depression is most likely due to the dysfunction of HERG K(+) channel, the major subunit underlying native I(Kr), in response to oxidative stress, for peroxide anion-generating system produced similar depression of HERG channels. Vitamin E restored the depressed I(Kr) and HERG by its antioxidant actions which likely underlie its beneficial effects on diabetic QT prolongation and the associated arrhythmias. The data indicate that an antioxidant is sufficient for reversing the I(Kr)/I(HERG) dysfunction and the consequent electrical disorders in diabetic hearts. Our study also conceptually simplifies the complex nature of diabetic electrical disorders to primarily oxidative stress, and should stimulate interest in antioxidants as a therapeutic strategy for diabetic QT prolongation.

Transcriptional and post-transcriptional mechanisms for oncogenic overexpression of ether à go-go K+ channel

The human ether-à-go-go-1 (h-eag1) K⁺ channel is expressed in a variety of cell lines derived from human malignant tumors and in clinical samples of several different cancers, but is otherwise absent in normal tissues. It was found to be necessary for cell cycle progression and tumorigenesis. Specific inhibition of h-eag1 expression leads to inhibition of tumor cell proliferation. We report here that h-eag1 expression is controlled by the p53−miR-34−E2F1 pathway through a negative feed-forward mechanism. We first established E2F1 as a transactivator of h-eag1 gene through characterizing its promoter region. We then revealed that miR-34, a known transcriptional target of p53, is an important negative regulator of h-eag1 through dual mechanisms by directly repressing h-eag1 at the post-transcriptional level and indirectly silencing h-eag1 at the transcriptional level via repressing E2F1. There is a strong inverse relationship between the expression levels of miR-34 and h-eag1 protein. H-eag1antisense antagonized the growth-stimulating effects and the upregulation of h-eag1 expression in SHSY5Y cells, induced by knockdown of miR-34, E2F1 overexpression, or inhibition of p53 activity. Therefore, p53 negatively regulates h-eag1 expression by a negative feed-forward mechanism through the p53−miR-34−E2F1 pathway. Inactivation of p53 activity, as is the case in many cancers, can thus cause oncogenic overexpression of h-eag1 by relieving the negative feed-forward regulation. These findings not only help us understand the molecular mechanisms for oncogenic overexpression of h-eag1 in tumorigenesis but also uncover the cell-cycle regulation through the p53−miR-34−E2F1−h-eag1 pathway. Moreover, these findings place h-eag1 in the p53−miR-34−E2F1−h-eag1 pathway with h-eag as a terminal effecter component and with miR-34 (and E2F1) as a linker between p53 and h-eag1. Our study therefore fills the gap between p53 pathway and its cellular function mediated by h-eag1.

The concept of multiple-target anti-miRNA antisense oligonucleotide technology

The multiple-target AMO technology or MT-AMO technology is an innovative strategy, which confers on a single AMO fragment the capability of targeting multiple miRNAs. This modified AMO is single-stranded 2'-O-methyl-modified oligoribonucleotides carrying multiple AMO units, which are engineered into a single unit and are able to simultaneously silence multiple-target miRNAs or multiple miRNA seed families. Studies suggest that the MT-AMO is an improved approach for miRNA target finding and miRNA function validation; it not only enhances the effectiveness of targeting miRNAs but also confers diversity of actions. It has been successfully used to identify target genes and cellular function of several oncogenic miRNAs and of the muscle-specific miRNAs (Lu et al., Nucleic Acids Res 37:e24-e33, 2009). This novel strategy may find its broad application as a useful tool in miRNA research for exploring biological processes involving multiple miRNAs and multiple genes, and the potential as an miRNA therapy for human disease such as cancer and cardiac disorders. This technology was developed by my research laboratory in collaboration with Yang's group (Lu et al., Nucleic Acids Res 37:e24-e33, 2009), and it is similar but distinct from the miRNA Sponge technology developed by Sharp's laboratory in 2007 (Ebert et al., Nat Methods 4:721-726, 2007) and modified by Gentner et al. (Nat Methods 6:63-66, 2009).

The principles of MiRNA-masking antisense oligonucleotides technology

MiRNA-masking antisense oligonucleotides technology (miR-mask) is an anti-microRNA antisense oligodeoxyribonucleotide (AMO) approach of a different sort. A standard miR-mask is single-stranded 2'-O-methyl-modified oligoribonucleotide (or other chemically modified) that is a 22-nt antisense to a protein-coding mRNA as a target for an endogenous miRNA of interest. Instead of binding to the target miRNA like an AMO, an miR-mask does not directly interact with its target miRNA but binds to the binding site of that miRNA in the 3' UTR of the target mRNA by fully complementary mechanism. In this way, the miR-mask covers up the access of its target miRNA to the binding site so as to derepress its target gene (mRNA) via blocking the action of its target miRNA. The anti-miRNA action of an miR-mask is gene-specific because it is designed to be fully complementary to the target mRNA sequence of an miRNA. The anti-miRNA action of an miR-mask is miRNA-specific as well because it is designed to target the binding site of that particular miRNA. The miR-mask approach is a valuable supplement to the AMO technique; while AMO is indispensable for studying the overall function of an miRNA, the miR-mask might be more appropriate for studying the specific outcome of regulation of the target gene by the miRNA. This technology was first established by my research group in 2007 (Xiao et al., J Cell Physiol 212:285-292; Wang et al., J Mol Med 86:772-783, 2008) and a similar approach with the same concept was subsequently reported by Schier's laboratory (Choi et al., Science 318:271-274, 2007).

The guideline of the design and validation of MiRNA mimics

The miRNA mimic technology (miR-Mimic) is an innovative approach for gene silencing. This approach is to generate nonnatural double-stranded miRNA-like RNA fragments. Such an RNA fragment is designed to have its 5'-end bearing a partially complementary motif to the selected sequence in the 3'UTR unique to the target gene. Once introduced into cells, this RNA fragment, mimicking an endogenous miRNA, can bind specifically to its target gene and produce posttranscriptional repression, more specifically translational inhibition, of the gene. Unlike endogenous miRNAs, miR-Mimics act in a gene-specific fashion. The miR-Mimic approach belongs to the "miRNA-targeting" and "miRNA-gain-of-function" strategy and is primarily used as an exogenous tool to study gene function by targeting mRNA through miRNA-like actions in mammalian cells. The technology was developed by my research group (Department of Medicine, Montreal Heart Institute, University of Montreal) in 2007 (Xiao, et al. J Cell Physiol 212:285-292, 2007; Xiao et al. Nat Cell Biol, in review).

miR-605 joins p53 network to form a p53:miR-605:Mdm2 positive feedback loop in response to stress

In cancers with wild-type (WT) p53 status, the function of p53 is inhibited through direct interaction with Mdm2 oncoprotein, a negative feedback loop to limit the function of p53. In response to cellular stress, p53 escapes the p53:Mdm2 negative feedback to accumulate rapidly to induce cell cycle arrest and apoptosis. We demonstrate herein that an microRNA miR-605 is a new component in the p53 gene network, being transcriptionally activated by p53 and post-transcriptionally repressing Mdm2. Activation of p53 upregulated miR-605 via interacting with the promoter region of the gene. Overexpression of miR-605 directly decreased Mdm2 expression at the post-transcriptional level but indirectly increased the transcriptional activity of p53 on miR-34a via downregulating Mdm2; knockdown of miR-605 did the opposite. Mdm2 inhibitor upregulated expression of both miR-34a and miR-605, which was mitigated by p53 inhibitor. miR-605 preferentially induced apoptosis in WT p53-expressing cells, an effect abolished by p53 inhibition. These results indicate that miR-605 acts to interrupt p53:Mdm2 interaction to create a positive feedback loop aiding rapid accumulation of p53 to facilitate its function in response to stress.

MicroRNA-328 contributes to adverse electrical remodeling in atrial fibrillation

BACKGROUND

A characteristic of both clinical and experimental atrial fibrillation (AF) is atrial electric remodeling associated with profound reduction of L-type Ca(2+) current and shortening of the action potential duration. The possibility that microRNAs (miRNAs) may be involved in this process has not been tested. Accordingly, we assessed the potential role of miRNAs in regulating experimental AF.

METHODS AND RESULTS

The miRNA transcriptome was analyzed by microarray and verified by real-time reverse-transcription polymerase chain reaction with left atrial samples from dogs with AF established by right atrial tachypacing for 8 weeks and from human atrial samples from AF patients with rheumatic heart disease. miR-223, miR-328, and miR-664 were found to be upregulated by >2 fold, whereas miR-101, miR-320, and miR-499 were downregulated by at least 50%. In particular, miR-328 level was elevated by 3.9-fold in AF dogs and 3.5-fold in AF patients relative to non-AF subjects. Computational prediction identified CACNA1C and CACNB1, which encode cardiac L-type Ca(2+) channel α1c- and β1 subunits, respectively, as potential targets for miR-328. Forced expression of miR-328 through adenovirus infection in canine atrium and transgenic approach in mice recapitulated the phenotypes of AF, exemplified by enhanced AF vulnerability, diminished L-type Ca(2+) current, and shortened atrial action potential duration. Normalization of miR-328 level with antagomiR reversed the conditions, and genetic knockdown of endogenous miR-328 dampened AF vulnerability. CACNA1C and CACNB1 as the cognate target genes for miR-328 were confirmed by Western blot and luciferase activity assay showing the reciprocal relationship between the levels of miR-328 and L-type Ca(2+) channel protein subunits.

CONCLUSIONS

miR-328 contributes to the adverse atrial electric remodeling in AF through targeting L-type Ca(2+) channel genes. The study therefore uncovered a novel molecular mechanism for AF and indicated miR-328 as a potential therapeutic target for AF.

MicroRNAs and atrial fibrillation: new fundamentals

Atrial fibrillation (AF) is the most commonly encountered clinical arrhythmia associated with pronounced morbidity, mortality, and socio-economic burden. This pathological entity is associated with an altered expression profile of genes that are important for atrial function. MicroRNAs (miRNAs), a new class of non-coding mRNAs of around 22 nucleotides in length, have rapidly emerged as one of the key players in the gene expression regulatory network. The potential roles of miRNAs in controlling AF have recently been investigated. The studies have provided some promising results for our better understanding of the molecular mechanisms of AF. In this review article, we provide a synopsis of the studies linking miRNAs to cardiac excitability and other processes pertinent to AF. To introduce the main topic, we discuss basic knowledge about miRNA biology and our current understanding of mechanisms for AF. The most up-to-date research data on the possible roles of miRNAs in AF initiation and maintenance are presented, and the available experimental results on miRNA and AF are discussed. Some speculations pertinent to the subject are made. Finally, perspectives on future directions of research on miRNAs in AF are provided.

Differential Alterations of Receptor Densities of Three Muscarinic Acetylcholine Receptor Subtypes and Current Densities of the Corre-sponding K+ Channels in Canine Atria with Atrial Fibrillation Induced by Experimental Congestive Heart Failure

Parasympathetic tone and congestive heart failure (CHF) are two of promoting factors in initiation and perpetuation of atrial fibrillation (AF). Recent studies indicate co-existence of multiple muscarinic acetylcholine receptor subtypes (mAChRs) that mediate several distinct K+ currents in the heart; inward rectifier K+ current IKACh by the M2, and two delayed rectifier K+ currents IKM3 and IK4AP by the M3 and M4 receptors, respectively. We studied the alterations of atrial mAChRs and their coupled K+ channels in the setting of AF in dogs with ventricular tachypacing-induced CHF. Whole-patch-clamp recordings showed that the current densities of IKACh (induced by 1 mM acetylcholine) and IK4AP (induced by 1 mM 4-aminopyridine) were ñ45% and ñ55% lower, respectively, while that of IKM3 (induced by 10 mM choline) was ñ75% higher, at a plateau voltage of 0 mV in atrial myocytes from CHF than those from healthy hearts. In healthy hearts, IKACh comprised >60%, and IKM3 and IK4AP <30%, of the total outward K+ currents mediated by mAChRs at depolarized potentials (between -20 mV and +50 mV). In AF atria of CHF dogs, however, the contribution of IKM3 increased to approximately 50%, exceeding those of IKACh or IK4AP. Western blot analyses with atrial membrane protein samples indicated that receptor densities of the M2 and M4 subtypes decreased by approximately 33% and approximately 22%, respectively, whereas that of the M3 subtype increased by approximately 2.3 folds, in parallel to the alterations of the corresponding K+ currents. We conclude that differential alterations of mAChR subtypes underlie differential alterations of their coupled K+ channels in AF atria and these differential alterations may contribute to atrial remodeling in AF induced in the setting of CHF.

Regulation of human cardiac ion channel genes by microRNAs: theoretical perspective and pathophysiological implications

Excitability is a fundamental characteristic of cardiac cells, which is delicately determined by ion channel activities modulated by many factors. MicroRNA (miRNA) expression is dynamically regulated and altered miRNA expression can render expression deregulation of ion channel genes leading to channelopathies-arrhythmogenesis. Indeed, evidence has emerged indicating the crucial role of miRNAs in controlling cardiac excitability by regulating expression of ion channel genes at the post-transcriptional level. However, the very limited experimental data in the literature hinder our understanding of the role of miRNAs and the often one-to-one interaction between miRNA and ion-channel gene in the published studies also casts a doubt about fullness of our view. Unfortunately, currently available techniques do not permit thorough characterization of miRNA targeting; computational prediction programs remain the only source for rapid identification of a putative miRNA target in silico. We conducted a rationally designed bioinformatics analysis in conjunction with experimental approaches to identify the miRNAs from the currently available miRNA databases which have the potential to regulate human cardiac ion channel genes and to validate the analysis with several pathological settings associated with the deregulated miRNAs and ion channel genes in the heart. We established a matrix of miRNAs that are expressed in cardiac cells and have the potential to regulate the genes encoding cardiac ion channels and transporters. We were able to explain a particular ionic remodeling process in hypertrophy/heart failure, myocardial ischemia, or atrial fibrillation with the corresponding deregulated miRNAs under that pathological condition; the changes of miRNAs appear to have anti-correlation with the changes of many of the genes encoding cardiac ion channels under these situations. These results indicate that multiple miRNAs might be critically involved in the electrical/ionic remodeling processes of cardiac diseases through altering their expression in cardiac cells, which has not been uncovered by previous experimental studies.

MicroRNA: A matter of life or death

Progressive cell loss due to apoptosis is a pathological hallmark implicated in a wide spectrum of degenerative diseases such as heart disease, atherosclerotic arteries and hypertensive vessels, Alzheimer’s disease and other neurodegenerative disorders. Tremendous efforts have been made to improve our understanding of the molecular mechanisms and signaling pathways involved in apoptosistic cell death. Once ignored completely or overlooked as cellular detritus, microRNAs (miRNAs) that were discovered only a decade ago, have recently taken many by surprise. The importance of miRNAs has steadily gained appreciation and miRNA biology has exploded into a massive swell of interest with enormous range and potential in almost every biological discipline because of their widespread expression and diverse functions in both animals and humans. It has been established that miRNAs are critical regulators of apoptosis of various cell types. These small molecules act by repressing the expression of either the proapoptotic or antiapoptotic genes to produce antiapoptotic or proapoptotic effects. Appealing evidence has been accumulating for the involvement of miRNAs in human diseases associated with apoptotic cell death and the potential of miRNAs as novel therapeutic targets for the treatment of the diseases. This editorial aims to convey this message and to boost up the research interest by providing a timely, comprehensive overview on regulation of apoptosis by miRNAs and a synopsis on the pathophysiologic implications of this novel regulatory network based on the currently available data in the literature. It begins with a brief introduction to apoptosis and miRNAs, followed by the description of the fundamental aspects of miRNA biogenesis and action, and the role of miRNAs in regulating apoptosis of cancer cells and cardiovascular cells. Speculations on the development of miRNAs as potential therapeutic targets are also presented. Remarks are also provided to point out the unanswered questions and to outline the new directions for the future research of the field.

MicroRNAs and apoptosis: implications in the molecular therapy of human disease

1. MicroRNAs (miRNAs), the small non-coding RNAs of approximately 22 nucleotides, are now recognized as a very large family present throughout the genomes of plants and metazoans. These small transcripts modulate protein expression by binding to complementary or partially complementary target protein-coding mRNAs and targeting them for degradation or translational inhibition. 2. The discovery of miRNAs has revolutionized our understanding of the mechanisms that regulate gene expression, with the addition of an entirely novel level of regulatory control. Considerable information on miRNAs has been accumulated in this rapidly evolving research field. We now know that miRNAs play pivotal roles in diverse processes, such as development and differentiation, control of cell proliferation and death, stress response and metabolism. Indeed, aberrant miRNA expression has been documented in human disease as well as in animal models, with evidence for a causative role in tumourigenesis. 3. One of the most active fields of miRNA research is miRNA regulation of apoptosis, a programmed cell death implicated in many human diseases, such as cancer, Alzheimer's disease, hypertrophy and heart failure. Thus far, nearly 30 of 500 human miRNAs have been validated experimentally to regulate apoptosis; this number is likely to increase with future studies. 4. The present review provides a comprehensive summary and analysis of the currently available data, focusing on the transcriptional controls, target genes and signalling pathways linking the apoptosis-regulating miRNAs and apoptotic cell death.

Transcriptional control of pacemaker channel genes HCN2 and HCN4 by Sp1 and implications in re-expression of these genes in hypertrophied myocytes

Cardiac hypertrophy is characterized by electrical remolding with increased risk of arrhythmogenesis. Enhanced abnormal automaticity of ventricular cells may contribute to hypertrophic arrhythmias. The pacemaker current I(f), carried by the hyperpolarization-activated channels encoded mainly by the HCN2 and HCN4 genes in the heart, plays an important role in rhythmogenesis. Their expressions reportedly increase in hypertrophic and failing hearts, contributing to arrhythmogenesis under these conditions. However, how their expressions are controlled remained unclear. We performed a study to characterize the regulatory elements and transcriptional control of HCN2 and HCN4 genes. We identified the transcription start sites by 5'RACE and core promoter regions of these genes using luciferase reporter assay, and revealed the ubiquitous Sp1 protein as a common transactivator of HCN2 and HCN4 genes. We further unraveled robust increases in HCN2/HCN4 transcripts and protein levels, using real-time RT-PCR and Western blot analyses, in a rat model of left ventricular hypertrophy and in angiotensin II-induced neonatal ventricular hypertrophy. The upregulation of HCN2 and HCN4 transcription was accompanied by pronounced elevations of Sp1 and silencing of Sp1 by siRNA prevented overexpression of HCN2/HCN4 in hypertrophic cardiomyocytes. Our data indicate that Sp1 drives HCN2/HCN4 transcription and determines the functional level of HCN2/HCN4 mRNAs, and upregulation of Sp1 underlie the abnormal re-expression of HCN2/HCN4 genes in hypertrophied myocytes. This study also provides the first evidence for the role of Sp1 in the reactivation of 'fetal' cardiac genes, HCN2 and HCN4, in ventricular myocytes, and thereby in the pathological electrical remodeling in hypertrophied myocytes.

Funny Current Downregulation and Sinus Node Dysfunction Associated With Atrial Tachyarrhythmia

Background— Sinoatrial node (SAN) dysfunction is frequently associated with atrial tachyarrhythmias (ATs). Abnormalities in SAN pacemaker function after termination of ATs can cause syncope and require pacemaker implantation, but underlying mechanisms remain poorly understood. This study examined the hypothesis that ATs impair SAN function by altering ion channel expression.

Methods and Results— SAN tissues were obtained from 28 control dogs and 31 dogs with 7-day atrial tachypacing (400 bpm). Ionic currents were measured from single SAN cells with whole-cell patch-clamp techniques. Atrial tachypacing increased SAN recovery time in vivo by ≈70% ( P <0.01), a change which reflects impaired SAN function. In dogs that underwent atrial tachypacing, SAN mRNA expression (real-time reverse-transcription polymerase chain reaction) was reduced for hyperpolarization-activated cyclic nucleotide-gated subunits (HCN2 and HCN4) by >50% ( P <0.01) and for the β-subunit minK by ≈42% ( P <0.05). SAN transcript expression for the rapid delayed-rectifier ( I Kr ) α-subunit ERG, the slow delayed-rectifier ( I Ks ) α-subunit KvLQT1, the β-subunit MiRP1, the L-type ( I CaL ) and T-type ( I CaT ) Ca 2+ -current subunits Cav1.2 and Cav3.1, and the gap-junction subunit connexin 43 (were unaffected by atrial tachypacing. Atrial tachypacing reduced densities of the HCN-related funny current ( I f ) and I Ks by ≈48% ( P <0.001) and ≈34% ( P <0.01), respectively, with no change in voltage dependence or kinetics. I Kr , I CaL , and I CaT were unaffected. SAN cells lacked Ba 2+ -sensitive inward-rectifier currents, irrespective of AT. SAN action potential simulations that incorporated AT-induced alterations in I f accounted for slowing of periodicity, with no additional contribution from changes in I Ks .

Conclusions— AT downregulates SAN HCN2/4 and minK subunit expression, along with the corresponding currents I f and I Ks . Tachycardia-induced remodeling of SAN ion channel expression, particularly for the “pacemaker” subunit I f , may contribute to the clinically significant association between SAN dysfunction and supraventricular tachyarrhythmias.

A single anti-microRNA antisense oligodeoxyribonucleotide (AMO) targeting multiple microRNAs offers an improved approach for microRNA interference

Anti-miRNA antisense inhibitors (AMOs) have demonstrated their utility in miRNA research and potential in miRNA therapy. Here we report a modified AMO approach in which multiple antisense units are engineered into a single unit that is able to simultaneously silence multiple-target miRNAs, the multiple-target AMO or MTg-AMO. We validated the technique with two separate MTg-AMOs: anti-miR-21/anti-miR-155/anti-miR-17-5p and anti-miR-1/anti-miR-133. We first verified the ability of the MTg-AMOs to antagonize the repressive actions of their target miRNAs using luciferase reporter activity assays and to specifically knock down the levels of their target miRNAs using real-time RT-PCR methods. We then used the MTg-AMO approach to identify several tumor suppressors—TGFBI, APC and BCL2L11 as the target genes for oncogenic miR-21, miR-155 and miR-17-5p, respectively, and two cardiac ion channel genes HCN2 (encoding a subunit of cardiac pacemaker channel) and CACNA1C (encoding the α-subunit of cardiac L-type Ca²⁺ channel) for the muscle-specific miR-1 and miR-133. We further demonstrated that the MTg-AMO targeting miR-21, miR-155 and miR-17-5p produced a greater inhibitory effect on cancer cell growth, compared with the regular single-target AMOs. Moreover, while using the regular single-target AMOs excluded HCN2 as a target gene for either miR-1 or miR-133, the MTg-AMO approach is able to reveal HCN2 as the target for both miR-1 and miR-133. Our findings suggest the MTg-AMO as an improved approach for miRNA target finding and for studying function of miRNAs. This approach may find its broad application for exploring biological processes involving multiple miRNAs and multiple genes.

Feedback Remodeling of Cardiac Potassium Current Expression

Background— Inhibition of individual K + currents causes functionally based compensatory increases in other K + currents that minimize changes in action potential duration, a phenomenon known as repolarization reserve. The possibility that sustained K + channel inhibition may induce remodeling of ion current expression has not been tested. Accordingly, we assessed the effects of sustained inhibition of one K + current on various other cardiac ionic currents.

Methods and Results— Adult canine left ventricular cardiomyocytes were incubated in primary culture and paced at a physiological rate (1 Hz) for 24 hours in the presence or absence of the highly selective rapid delayed-rectifier K + current (I Kr ) blocker dofetilide (5 nmol/L). Sustained dofetilide exposure led to shortened action potential duration and increased repolarization reserve (manifested as a reduced action potential duration–prolonging response to I Kr blockade). These repolarization changes were accompanied by increased slow delayed-rectifier (I Ks ) density, whereas I Kr , transient-outward (I to ), inward-rectifier (I K1 ), L-type Ca 2+ (I CaL ), and late Na + current remained unchanged. The mRNA expression corresponding to KvLQT1 and minK (real-time polymerase chain reaction) was unchanged, but their protein expression (Western blot) was increased, suggesting posttranscriptional regulation. To analyze possible mechanisms, we quantified the muscle-specific microRNA subtypes miR -133a and miR -133b, which can posttranscriptionally regulate and repress KvLQT1 protein expression without affecting mRNA expression. The expression levels of miR -133a and miR -133b were significantly decreased in cells cultured in dofetilide compared with control, possibly accounting for KvLQT1 protein upregulation.

Conclusions— Sustained reductions in I Kr may lead to compensatory upregulation of I Ks through posttranscriptional upregulation of underlying subunits, likely mediated (at least partly) by microRNA changes. These results suggest that feedback control of ion channel expression may influence repolarization reserve.

Down-regulation of miR-1/miR-133 contributes to re-expression of pacemaker channel genes HCN2 and HCN4 in hypertrophic heart

Cardiac hypertrophy is characterized by electrical remolding with increased risk of arrhythmogenesis. Enhanced abnormal automaticity of ventricular cells contributes critically to hypertrophic arrhythmias. The pacemaker current I(f), carried by the hyperpolarization-activated channels encoded mainly by the HCN2 and HCN4 genes in the heart, plays an important role in determining cardiac automaticity. Their expressions reportedly increase in hypertrophic and failing hearts, contributing to arrhythmogenesis under these conditions. We performed a study on post-transcriptional regulation of expression of HCN2 and HCN4 genes by microRNAs. We experimentally established HCN2 as a target for repression by the muscle-specific microRNAs miR-1 and miR-133 and established HCN4 as a target for miR-1 only. We unraveled robust increases in HCN2 and HCN4 protein levels in a rat model of left ventricular hypertrophy and in angiotensin II-induced neonatal ventricular hypertrophy. The up-regulation of HCN2/HCN4 was accompanied by pronounced reduction of miR-1/miR-133 levels. Forced expression of miR-1/miR-133 by transfection prevented overexpression of HCN2/HCN4 in hypertrophic cardiomyocytes. The serum-responsive factor protein level was found significantly decreased in hypertrophic hearts, and silencing of this protein by RNA interference resulted in increased levels of miR-1/miR-133 and concomitant increases in HCN2 and HCN4 protein levels. We conclude that down-regulation of miR-1 and miR-133 expression contributes to re-expression of HCN2/HCN4 and thereby the electrical remodeling process in hypertrophic hearts. Our study also sheds new light on the cellular function and pathological role of miR-1/miR-133 in the heart.

Phospholipid Lysophosphatidylcholine as a Metabolic Trigger and HERG as an Ionic Pathway for Extracellular K+ Accumulation and “Short QT Syndrome” in Acute Myocardial Ischemia

The most profound abnormalities during acute myocardial ischemia are extracellular K(+) accumulation ([K(+)](o)- upward arrow) and shortening of action potential duration or QT interval (APD- downward arrow or QT- downward arrow), which are pivotal in the genesis of ischemic arrhythmias and sudden cardiac death. The ionic mechanisms however remained obscured. We performed studies in a rabbit model of acute global myocardial ischemia in order to explore ionic and metabolic mechanisms for ischemic [K(+)](o)- upward arrow and QT- downward arrow. Exogenous 1-palmitoyl-lysophosphatidylcholine (LPC-16) mimicked the low-perfusion ischemia to produce significant [K(+)](o)- upward arrow and QT- downward arrow. The [K(+)](o)- upward arrow and QT- downward arrow induced by either LPC-16 or ischemia were prevented by dofetilide, a blocker of rapid delayed rectifier K(+) current (I(Kr)), but not by blockers for other K(+) channels. Consistently, dofetilide efficiently abolished the ventricular tachy-arrhythmias induced by ischemia or LPC-16. LPC-16 remarkably shortened APD and enhanced the function of I(Kr) and HERG (the pore-forming subunit of I(Kr)). The effects of LPC-16 manifested with shorter APD (faster repolarization rate) and at more negative potential (membrane repolarization). Dofetilide abolished the I(Kr)/HERG enhancing and APD shortening effects of LPC-16. Our results suggest that LPC-16 accumulation/HERG enhancement may be a link between metabolic trigger and ionic pathway for ischemic [K(+)](o)- upward arrow and QTc- downward arrow. This represents the first documentation of I(Kr)/HERG as the ionic mechanism in ischemic [K(+)](o)- upward arrow and QTc- downward arrow. Inhibition of LPC-16 production and accumulation and/or of I(Kr)/HERG may be a promising therapeutic strategy to attenuate the incidence of lethal arrhythmias associated with ischemic heart disease.

JAK/STAT and PI3K/AKT pathways form a mutual transactivation loop and afford resistance to oxidative stress-induced apoptosis in cardiomyocytes

Cardiac tissues contain cells susceptible to and cells resistant to apoptosis, and this difference is important for normal morphogenesis during development and for abnormal loss of cells during pathogenesis such as myocardial infarction and heart failure. While efforts have been made to understand the cellular and intercellular events of apoptotic cells, the signaling mechanisms in cells surviving from apoptotic injuries have been overlooked. Understanding signal transduction processes in cells with apoptosis resistance is of crucial importance to develop better strategies of preserving post-mitotic cells. To this end, we performed studies in neonatal rat ventricular myocytes using oxidative stress (H(2)O(2)) as an apoptotic inducer. We identified a population of cells bearing higher resistance to apoptosis and found that the cells that survived from apoptotic insults had markedly higher levels of AKT and STAT3. Inhibition of AKT activity by a dominant negative AKT construct or by a PI3K inhibitor reduced active NF-kappaB and STAT3 expression without significantly altering the activity of the latter. Activation of AKT by a constitutively activated AKT construct caused the opposite effects. Direct activation of NF-kappaB also enhanced STAT3 expression, an effect abrogated by NF-kappaB inhibitor. On the other hand, knockdown of STAT3 by siRNA or inhibition of STAT3 activity by decoy oligodeoxynucleotides or by JAK2 inhibitor diminished AKT expression. In conclusion, cardiomyocytes possess an apoptosis-resistant property as a cytoprotection mechanism which is likely conferred by mutual transactivation between AKT/NF-kappaB and JAK2/STAT3, a novel crosstalk between the two signaling pathways within the networking governing the cell fate.