miRNAs at the heart of the matter

Cardiovascular disease is among the main causes of morbidity and mortality in developed countries. The pathological process of the heart is associated with altered expression profile of genes that are important for cardiac function. MicroRNAs (miRNAs) have emerged as one of the central players of gene expression regulation. The implications of miRNAs in the pathological process of cardiovascular system have recently been recognized, representing the most rapidly evolving research field. Here, we summarize and analyze the currently available data from our own laboratory and other groups, providing a comprehensive overview of miRNA function in the heart, including a brief introduction of miRNA biology, expression profile of miRNAs in cardiac tissue, role of miRNAs in cardiac hypertrophy and heart failure, the arrhythmogenic potential of miRNAs, the involvement of miRNAs in vascular angiogenesis, and regulation of cardiomyocyte apoptosis by miRNAs. The target genes and signaling pathways linking the miRNAs to cardiovascular disease are highlighted. The applications of miRNA interference technologies for manipulating miRNA expression, stability, and function as new strategies for molecular therapy of human disease are evaluated. Finally, some specific issues related to future directions of the research on miRNAs relevant to cardiovascular disease are pinpointed and speculated.

The muscle-specific microRNAs miR-1 and miR-133 produce opposing effects on apoptosis by targeting HSP60, HSP70 and caspase-9 in cardiomyocytes

The microRNAs miR-1 and miR-133 are preferentially expressed in cardiac and skeletal muscles and have been shown to regulate differentiation and proliferation of these cells. We report here a novel aspect of cellular function of miR-1 and miR-133 regulation of cardiomyocyte apoptosis. miR-1 and miR-133 produced opposing effects on apoptosis, induced by oxidative stress in H9c2 rat ventricular cells, with miR-1 being pro-apoptotic and miR-133 being anti-apoptotic. miR-1 level was significantly increased in response to oxidative stress. We identified single target sites for miR-1 only, in the 3'-untranslated regions of the HSP60 and HSP70 genes, and multiple putative target sites for miR-133 throughout the sequence of the caspase-9 gene. miR-1 reduced the levels of HSP60 and HSP70 proteins without changing their transcript levels, whereas miR-133 did not affect HSP60 and HSP70 expression at all. By contrast, miR-133 repressed caspase-9 expression at both the protein and mRNA levels. The post-transcriptional repression of HSP60 and HSP70 and caspase-9 was further confirmed by luciferase reporter experiments. Our results indicate that miR-1 and miR-133 are involved in regulating cell fate with increased miR-1 and/or decreased miR-133 levels favoring apoptosis and decreased miR-1 and/or miR-133 levels favoring survival. Post-transcriptional repression of HSP60 and HSP70 by miR-1 and of caspase-9 by miR-133 contributes significantly to their opposing actions.

Genomic structure, transcriptional control, and tissue distribution ofHERG1andKCNQ1genes

The long QT syndrome genes human ether-a-go-go-related gene ( HERG1) and voltage-gated K+channel, KQT-like subfamily, member 1, gene ( KCNQ1), encoding K+channels critical to the repolarization rate and repolarization reserve in cardiac cells, and thereby the likelihood of arrhythmias, are both composed of two isoforms: HERG1a and HERG1b and KCNQ1a and KCNQ1b, respectively. Expression of these genes is dynamic, depending on the differentiation status and disease states. We identified their core promoter regions and transcription start sites. Our data suggest that HERG1a and HERG1b, and KCNQ1a and KCNQ1b, represent independent transcripts instead of being alternatively spliced variants of the same gene, for they each have their own transcription start sites and their own promoter regions. We obtained data pointing to the potential role of stimulating protein 1 (Sp1) in the transactivation of these genes. We compared expression profiling of these genes across a variety of human tissues. Consistent with the general lack of cis elements for cardiac-specific transcription factors and the presence of multiple sites for ubiquitous Sp1 sites in the core promoter regions of HERG1a/HERG1b and KCNQ1a/KCNQ1b genes, the transcripts demonstrated widespread distribution across a variety of human tissues. We further revealed that the mRNA levels of all HERG1 and KCNQ1 isoforms were asymmetrically distributed within the heart, being more abundant in the right atria and ventricles relative to the left atria and ventricles. These findings open up an opportunity for studying interventricular gradients of slow and rapid delayed rectifier K+current and of cardiac repolarization as well. Our study might help us understand the molecular mechanisms for arrhythmias since heterogeneity of ion channel activities is an important substrate for arrhythmogenesis.

Sphingolipid metabolite ceramide causes metabolic perturbation contributing to HERG K+ channel dysfunction

Ceramide, a sphingolipid metabolite, has emerged as a key second messenger molecule that mediates multiple cellular functions. Its de nova synthesis and accumulation in ischemic myocardium, congestive heart failure and diabetic cardiomyopathy is associated with the abnormalities such as abnormal QT prolongation and increased risk of arrhythmias. To investigate how ceramide is involved in modulating cardiac repolarization, we performed whole-cell patch-clamp studies on HERG current (I(HERG)), a critical determinant of cardiac repolarization, expressed in HEK293 cells. Acute application (superfusion for 25 min) of membrane permeable ceramide (C2, 5 microM) did not alter I(HERG). Prolonged incubation with C2 for 10 hrs caused pronounced I(HERG) inhibition in a concentration-dependent and voltage-independent fashion and positive shift of voltage-dependent HERG activation. The IC(50) for I(HERG) suppression was 19.5 microM. C2 did not affect the inactivation property and time-dependent kinetics of I(HERG). Similar effects were observed with production of endogenous ceramide catalyzed by sphingomyelinase. Tyrosine kinase inhibitors failed to reverse C2-induced suppression of HERG function, and PKA and PKC inhibitors only slightly reversed the I(HERG) depression. Western blotting and immunocytochemical analyses indicate that C2 does not alter HERG protein expression on the cytoplasmic membrane. The inhibitory effect of C2 on I(HERG) was reversed by antioxidants vitamin E or MnTBAP. C2 caused considerable production of intracellular reactive oxygen species (ROS), which was prevented by vitamin E or MnTBAP. We conclude that ceramide depresses I(HERG) mainly via ROS overproduction and ceramide-induced I(HERG) impairment may contribute to QT prolongation in prolonged myocardial ischemia, heart failure and diabetic cardiomyopathy.

Novel approaches for gene-specific interference via manipulating actions of microRNAs: examination on the pacemaker channel genes HCN2 and HCN4

Recent evidence has suggested microRNAs as viable therapeutic targets for a wide range of human disease. However, lack of gene-specificity of microRNA actions may hinder this application. Here we developed two new approaches, the gene-specific microRNA mimic and microRNA-masking antisense approaches, to explore the possibility of using microRNA's principle of actions in a gene-specific manner. We examined the value of these strategies as rational approaches to develop heart rate-reducing agents and "biological pacemakers" by manipulating the expression of the cardiac pacemaker channel genes HCN2 and HCN4. We showed that the gene-specific microRNA mimics, 22-nt RNAs designed to target the 3'untranslated regions (3'UTRs) of HCN2 and HCN4, respectively, were efficient in abrogating expression and function of HCN2 and HCN4. The gene-specific microRNA mimics repressed protein levels, accompanied by depressed f-channel conductance and the associated rhythmic activity, without affecting mRNA levels of HCN2 and HCN4. Meanwhile, we also designed the microRNA-masking antisense based on the miR-1 and miR-133 target sites in the 3'UTRs of HCN2 and HCN4 and found that these antisense oligodeoxynucleotides markedly enhanced HCN2/HCN4 expression and function, as reflected by increased protein levels of HCN2/HCN4 and If conductance, by removing the repression of HCN2/HCN4 expression induced by endogenous miR-1/miR-133. The experimental examination of these techniques and the resultant findings not only indicate feasibility of interfering miRNA action in a gene-specific fashion but also may provide a new research tool for studying function of miRNAs. The new approaches also have the potential of becoming alternative gene therapy strategies.

Transcriptional activation by stimulating protein 1 and post-transcriptional repression by muscle-specific microRNAs of IKs-encoding genes and potential implications in regional heterogeneity of their expressions

In cardiac cells, KCNQ1 assembles with KCNE1 and forms a channel complex constituting the slow delayed rectifier current I(Ks). Expression of KCNQ1 and KCNE1 are regionally heterogeneous and changes with pathological states of the heart. The aims of this study were to decipher the molecular mechanisms for transcriptional and post-transcriptional regulation expression of KCNQ1 and KCNE1 genes and to shed light on the molecular mechanisms for their spatial heterogeneity of distribution. We cloned the 5'-flanking region and identified the transcription start sites of the KCNQ1 gene. We characterized the core promoters of KCNQ1 and KCNE1 and revealed the stimulating protein (Sp1) as a common transactivator of KCNQ1 and KCNE1 by interacting with the Sp1 cis-acting elements in the core promoter regions of these genes. We also characterized the 3' untranslated regions (3'UTRs) of the genes and experimentally established KCNQ1 and KCNE1 as targets for repression by the muscle-specific microRNAs miR-133 and miR-1, respectively. We demonstrated spatial heterogeneity of KCNQ1 and KCNE1 distributions at three axes (interventricular, transmural and apical-basal) and disparity between mRNA and protein expressions of these genes. We also found characteristic regional differences of expressions of Sp1 and miR-1/miR-133 in the heart. Our study unraveled a novel aspect of the cellular function of miRNAs and suggests that the I(Ks)-encoding genes KCNQ1 and KCNE1 expressions are dynamically balanced by transcription factor regulation and miRNA repression. The heterogeneities of Sp1 and miR-1/miR-133 offer an explanation for the well-recognized regional differences and disparity between mRNA and protein expressions of KCNQ1 and KCNE1.

Ionic mechanisms underlying abnormal QT prolongation and the associated arrhythmias in diabetic rabbits: a role of rapid delayed rectifier K+ current

Abnormal QT prolongation with the associated arrhythmias is considered the major cardiac electrical disorder and a significant predictor of mortality in diabetic patients. The precise ionic mechanisms for diabetic QT prolongation remained unclear. We performed whole-cell patch-clamp studies in a rabbit model of alloxan-induced insulin-dependent diabetes mellitus. We demonstrated that heart rate-corrected QT interval and action potential duration (APD) were prolonged by approximately 20% with frequent occurrence of ventricular tachyarrhythmias. Several K(+) currents were found decreased in diabetic rabbits including transient outward K(+)current (I(to)) that was reduced by approximately 60%, rapid delayed rectifier K(+) current (I(Kr)) reduced by approximately 70% and slow delayed rectifier K(+) current (I(Ks)) reduced by approximately 40%. The time-dependent kinetics of these currents remained unaltered. The peak amplitude of L-type Ca% current (I(CaL)) was reduced by approximately 22% and the inactivation kinetics was slowed; the integration of these two effects yielded approximately 15% reduction of I(CaL). The inward rectifier K(+) current (I(K1)) and fast sodium current (I(Na)) were unaffected. Simulation with LabHEART, a computer model of rabbit ventricular action potentials, revealed that inhibition of I(to) or I(Ks) alone fails to alter APD whereas inhibition of I(Kr) alone results in 30% APD prolongation and inhibition of I(CaL) alone causes 10% APD shortening. Integration of changes of all these currents leads to approximately 20% APD lengthening. Protein levels of the pore-forming subunits for these ion channels were decreased to varying extents, as revealed by immunoblotting analysis. Our study represents the first documentation of I(Kr) channelopathy as the major ionic mechanism for diabetic QT prolongation.

Atrial tachycardia induces remodelling of muscarinic receptors and their coupled potassium currents in canine left atrial and pulmonary vein cardiomyocytes

BACKGROUND AND PURPOSE

Both parasympathetic tone and atrial tachycardia (AT) remodelling of ion channels play important roles in atrial fibrillation (AF) pathophysiology. Different muscarinic cholinergic receptor (mAChR) subtypes (M2, M3, M4) in atrial cardiomyocytes are coupled to distinct K+-currents (called IKM2, IKM3, IKM4, respectively). Pulmonary veins (PVs) are important in AF and differential cholinergic current responses are a potential underlying mechanism. This study investigated AT-induced remodelling of mAChR subtypes and K+-currents in left-atrial (LA) and PV cardiomyocytes.

EXPERIMENTAL APPROACH

Receptor expression was assayed by western blot. IKM2, IKM3 and IKM4 were recorded with whole-cell patch-clamp in LA and PV cardiomyocytes of nonpaced control dogs and dogs after 7 days of AT-pacing (400 bpm).

KEY RESULTS

Current densities of IKM2, IKM3 and IKM4 were significantly reduced by AT-pacing in LA and PV cardiomyocytes. PV cardiomyocyte current-voltage relations were similar to LA for all three cholinergic currents, both in control and AT remodelling. Membrane-protein expression levels corresponding to M2, M3 and M4 subtypes were decreased significantly (by about 50%) after AT pacing. Agonist concentration-response relations for all three currents were unaffected by AT pacing.

CONCLUSIONS AND IMPLICATIONS

AT downregulated all three mAChR-coupled K+-current subtypes, along with corresponding mAChR protein expression. These changes in cholinergic receptor-coupled function may play a role in AF pathophysiology. Cholinergic receptor-coupled K+-currents in PV cardiomyocytes were similar to those in LA under control and AT-pacing conditions, suggesting that differential cholinergic current properties do not explain the role of PVs in AF.

The muscle-specific microRNA miR-1 regulates cardiac arrhythmogenic potential by targeting GJA1 and KCNJ2

MicroRNAs (miRNAs) are endogenous noncoding RNAs, about 22 nucleotides in length, that mediate post-transcriptional gene silencing by annealing to inexactly complementary sequences in the 3'-untranslated regions of target mRNAs. Our current understanding of the functions of miRNAs relies mainly on their tissue-specific or developmental stage-dependent expression and their evolutionary conservation, and therefore is primarily limited to their involvement in developmental regulation and oncogenesis. Of more than 300 miRNAs that have been identified, miR-1 and miR-133 are considered to be muscle specific. Here we show that miR-1 is overexpressed in individuals with coronary artery disease, and that when overexpressed in normal or infarcted rat hearts, it exacerbates arrhythmogenesis. Elimination of miR-1 by an antisense inhibitor in infarcted rat hearts relieved arrhythmogenesis. miR-1 overexpression slowed conduction and depolarized the cytoplasmic membrane by post-transcriptionally repressing KCNJ2 (which encodes the K(+) channel subunit Kir2.1) and GJA1 (which encodes connexin 43), and this likely accounts at least in part for its arrhythmogenic potential. Thus, miR-1 may have important pathophysiological functions in the heart, and is a potential antiarrhythmic target.

MicroRNA miR-133 Represses HERG K+ Channel Expression Contributing to QT Prolongation in Diabetic Hearts

We have previously found that the ether-a-go-go related gene (ERG), a long QT syndrome gene encoding a key K(+) channel (I(Kr)) in cardiac cells, is severely depressed in its expression at the protein level but not at the mRNA level in diabetic subjects. The mechanisms underlying the disparate alterations of ERG protein and mRNA, however, remained unknown. We report here a remarkable overexpression of miR-133 in hearts from a rabbit model of diabetes, and in parallel the expression of serum response factor (SRF), which is known to be a transactivator of miR-133, was also robustly increased. Delivery of exogenous miR-133 into the rabbit myocytes and cell lines produced post-transcriptional repression of ERG, down-regulating ERG protein level without altering its transcript level and caused substantial depression of I(Kr), an effect abrogated by the miR-133 antisense inhibitor. Functional inhibition or gene silencing of SRF down-regulated miR-133 expression and increased I(Kr) density. Repression of ERG by miR-133 likely underlies the differential changes of ERG protein and transcript thereby depression of I(Kr), and contributes to repolarization slowing thereby QT prolongation and the associated arrhythmias, in diabetic hearts. Our study provided the first evidence for the pathological role of miR-133 in adult hearts and thus expanded our understanding of the cellular function and pathophysiological roles of miRNAs.

Retracted: Overexpression HERG K+ channel gene mediates cell-growth signals on activation of oncoproteins SP1 and NF-κB and inactivation of tumor suppressor Nkx3.1

The long QT syndrome gene human ether-a-go-go related gene (HERG) encodes a K(+) channel critical to cardiac repolarization. It peculiarly overexpresses in cancer cells of different histogenesis and promotes tumorigenesis. To decipher the molecular mechanisms for HERG overexpression, we identified and characterized the promoter region of the HERG gene, which contains cis-elements for multiple oncoproteins and tumor suppressors. Oncoprotein Sp1 was found to be essential to driving the HERG promoter thereby transcription. Another oncoprotein NF-kappaB transactivated, while tumor suppressor Nkx3.1 repressed HERG promoter activity and endogenous HERG transcription. Loss-of-function mutations in the corresponding cis-elements rendered a loss of the ability of the oncoproteins Sp1 and NF-kappaB to transactivate, and of the tumor repressor Nkx3.1 to repress, HERG transcription. Either activation of Sp1 and NF-kappaB or silencing of Nkx3.1 promoted tumor cell growth, and the effects were abrogated by HERG inhibition or knockdown, but facilitated by overexpression of HERG, indicating that HERG mediates the cell growth signals generated by activation of oncoproteins or inactivation of tumor suppressors. Binding of Sp1, NF-kappaB, and Nkx3.1 to their respective cis-elements in the HERG promoter in vitro and their presence on the HERG promoter in vivo were confirmed. Therefore, the HERG promoter region is characterized by multiple Sp1 binding sites that are responsible for transcription initiation of the HERG gene and by binding sites for multiple other oncogenes and tumor suppressor genes being important for regulating HERG expression. The HERG K(+) channel is likely a mediator of growth-promoting processes induced by oncoproteins and/or by silencing of tumor suppressors.

Function of cardiac M3receptors

1. Since the initial identification of the M3 subtype of muscarinic acetylcholine receptors (M3-mAChR) in the heart, there have been increasing interest and advances in studies on the pathophysiological roles of M3-mAChR in the heart. Recent studies from several laboratories have provided compelling and solid evidence in support of the important roles of M3-mAChR in regulation and maintenance of cardiac function and in generation and progression of heart disease as well. 2. The functions of the cardiac M3-mAChR revealed thus far include (i) M3-mAChR regulation of heart rate and cardiac repolarization, (ii) modulation of inotropic effects, (iii) cytoprotection against ischaemic injuries of myocardium, (iv) regulation of cell-to-cell communication, and (v) participation in generation and maintenance of atrial fibrillation. 3. Signal transduction mechanisms underlying these pathophysiological functions have also been studied, which have allowed us to get insight into the following mechanistic aspects. (i) M3-mAChR activates a delayed rectifying K+ current I(KM3) to participate in cardiac repolarization, negative chronotropic actions, and anti-dysrhythmic (suppresses ischaemic dysrhythmias) as well as pro-dysrhythmic (facilitates atrial fibrillation) actions. (ii) M3-mAChR interacts with gap-junctional channel connexin 43 to maintain cell-cell communication and excitation propagation. (iii) M3-mAChR regulates intracellular phosphoinositide hydrolysis to improve cardiac contraction and haemodynamic function. (iv) M3-mAChR activate anti-apoptotic signalling molecules, enhances endogenous antioxidant capacity, and diminishes intracellular Ca2+ overload, all of which contribute to protecting the heart against ischaemic injuries. 4. This article provides an overview of the potential roles of the M3-mAChR in parasympathetic control of heart function under normal physiological conditions and in the setting of a variety of pathological processes including heart failure, myocardial ischaemia and dysrhythmias.

A single decoy oligodeoxynucleotides targeting multiple oncoproteins produces strong anticancer effects

Cancer in general is a multifactorial process. Targeting a single factor may not be optimal in therapy, because single agents are limited by incomplete efficacy and dose-limiting adverse effects. Combination pharmacotherapy or "drug cocktail" therapy has value against many diseases, including cancers. We report an innovative decoy oligodeoxynucleotide (dODN) technology that we term complex decoy oligodeoxynucleotide (cdODNs) in which multiple cis elements are engineered into single dODNs attacking multiple target transcription factors, mimicking the drug cocktail approach. We designed dODNs targeting NF-kappaB, E2F, and Stat3 separately and a cdODN targeting NF-kappaB, E2F, and Stat3 concomitantly. We evaluated effects of this cdODN on expression of cancer-related genes, viability of human cancer cell lines, and in vivo tumor growth in nude mice. The cdODN targeting all NF-kappaB, E2F, and Stat3 together demonstrated enhancement of efficacy of more than 2-fold and increases in potency of 2 orders of magnitude compared with each of the dODNs or the combination of all three dODNs. The cdODN also showed earlier onset and longer-lasting action. Most strikingly, the cdODN acquired the ability to attack multiple molecules critical to cancer progression via multiple mechanisms, leading to elimination of regression. Real-time reverse transcription-polymerase chain reaction revealed that the cdODNs knocked down expression of the genes regulated by the target transcription factors. The cdODN strategy offers resourceful combinations of varying cis elements for concomitantly targeting multiple molecules in cancer biological processes and opens the door to "one-drug, multiple-target" therapy for a broad range of human cancers.

Sex-based transmural differences in cardiac repolarization and ionic-current properties in canine left ventricles

The female sex is associated with longer electrocardiographic QT intervals and increased proarrhythmic risks of QT-prolonging drugs. This study examined the hypothesis that sex differences in repolarization may be associated with differential transmural ion-current distribution. Whole cell patch-clamp and current-clamp were used to study ionic currents and action potentials (APs) in isolated canine left ventricular cells from epicardium, midmyocardium, and endocardium. No sex differences in AP duration (APD) were found in cells from epicardium versus endocardium. In midmyocardium, APD was significantly longer in female dogs (e.g., at 1 Hz, female vs. male: 288 ± 21 vs. 237 ± 8 ms; P < 0.05), resulting in greater transmural APD heterogeneity in females. No sex differences in inward rectifier K+ current ( IK1) were observed. Transient outward K+ current ( Ito) densities in epicardium and midmyocardium also showed no sex differences. In endocardium, female dogs had significantly smaller Ito (e.g., at +30 mV, female vs. male: 2.5 ± 0.2 vs. 3.5 ± 0.3 pA/pF; P < 0.05). Rapid delayed-rectifier K+ current ( IKr) density and activation voltage-dependence showed no sex differences. Female dogs had significantly larger slow delayed-rectifier K+ current ( IKs) in epicardium and endocardium (e.g., at +40 mV; tail densities, female vs. male; epicardium: 1.3 ± 0.1 vs. 0.8 ± 0.1 pA/pF; P < 0.001; endocardium: 1.2 ± 0.1 vs. 0.7 ± 0.1 pA/pF; P < 0.05), but there were no sex differences in midmyocardial IKs. Female dogs had larger L-type Ca2+ current ( ICa,L) densities in all layers than male dogs (e.g., at −20 mV, female vs. male, epicardium: −4.2 ± 0.4 vs. −3.2 ± 0.2 pA/pF; midmyocardium: −4.5 ± 0.5 vs. −3.3 ± 0.3 pA/pF; endocarium: −4.5 ± 0.4 vs. −3.2 ± 0.3 pA/pF; P < 0.05 for each). We conclude that there are sex-based transmural differences in ionic currents that may underlie sex differences in transmural cardiac repolarization.

Restoring depressed HERG K+ channel function as a mechanism for insulin treatment of abnormal QT prolongation and associated arrhythmias in diabetic rabbits

Abnormal QT prolongation (QT-P) in diabetic patients has become a nonnegligible clinical problem and has attracted increasing attention from basic scientists, because it increases the risk of lethal ventricular arrhythmias. Correction of QT-P may be an important measure in minimizing sudden cardiac death in diabetic patients. Here we report the efficacy of insulin in preventing QT-P and the associated arrhythmias and the mechanisms underlying the effects in a rabbit model of type 1 insulin-dependent diabetes mellitus (IDDM). The heart rate-corrected QT (QTc) interval and action potential duration were considerably prolonged, with frequent ventricular tachycardias. The rapid delayed rectifier K+ current (IKr) was markedly reduced in IDDM hearts, and hyperglycemia depressed the function of the human ether-a-go-go-related gene (HERG), which conducts IKr. The impairment was primarily ascribed to the enhanced oxidative damage to the myocardium, as indicated by the increased intracellular level of reactive oxygen species and simultaneously decreased endogenous antioxidant reserve and by the increased lipid peroxidation and protein oxidation. Moreover, IDDM or hyperglycemia resulted in downregulation of HERG protein level. Insulin restored the depressed IKr/HERG and prevented QTc/action potential duration prolongation and the associated arrhythmias, and the beneficial actions of insulin are partially due to its antioxidant ability. Our study represents the first documentation of oxidative stress as the major metabolic mechanism for HERG K+ dysfunction, which causes diabetic QT-P, and suggests IKr/HERG as a potential therapeutic target for treatment of the disorder.

Choline produces cytoprotective effects against ischemic myocardial injuries: evidence for the role of cardiac m3 subtype muscarinic acetylcholine receptors

BACKGROUND/AIMS

Accumulating evidence indicates the presence of functional M3 subtype of acetylcholine muscarinic receptors (M(3)-mAChR), in addition to the well-recognized M(2)-mAChR, in the heart of various species including man. However, the pathophysiological role of the cardiac M(3)-mAChR remain undefined. This study was designed to explore the possible role of M(3)-mAChR in cytoprotection of myocardial infarction and several related signaling pathways as potential mechanisms.

METHODS

Studies were performed in a rat model of myocardial infarction and in isolated myocytes.

RESULTS

We found that choline relieved myocardial injuries during ischemia or under oxidative stress, which was achieved by correcting hemodynamic impairment, diminishing ventricular arrhythmias and protecting cardiomyocytes from apoptotic death. The beneficial effects of choline were reversed by the M(3)-selective antagonists but not by the M(2)-selective antagonist. Choline/M(3)-mAChR activated several survival signaling molecules (antiapoptotic proteins Bcl-2 and ERKs), increased endogenous antioxidant reserve (SOD), and reduced apoptotic mediators (proapoptotic proteins Fas and p38 MAPK) and intracellular Ca2+ overload.

CONCLUSION

Choline improves cardiac function and reduces ischemic myocardial injuries via stimulating the cardiac M(3)-mAChRs which in turn result in alterations of multiple signaling pathways leading to cytoprotection. The findings suggest M(3)-mAChR as a new target for drug development for improving cardiac function and preventing cardiac injuries during ischemia/reperfusion.

Resveratrol, a natural ingredient of grape skin: antiarrhythmic efficacy and ionic mechanisms

Resveratrol has been demonstrated to produce a variety of biological actions. Accumulating line of evidence supported the view that resveratrol may exert protective effect on the cardiovascular system. The aim of the study was to assess the antiarrhythmic profile as well as electrophysiological properties of resveratrol. We observe the antiarrhythmic effect of resveratrol on aconitine induced rat arrhythmia, ouabain induced guinea pig arrhythmia, and coronary ligation induced rat arrhythmia animal models. Resveratrol significantly and dose-dependently increased the doses of aconitine and ouabain required to induce the arrhythmia indexes. In coronary ligation induced rat arrhythmia model, resveratrol shortened duration of arrhythmia, decreased incidence of ventricular tachycardia and mortality. Electrophysiological experiment revealed that resveratrol could shorten APD through inhibition of ICa and selective enhancement of IKs without an effect on IKr.

A Single Residue in the S6 Transmembrane Domain Governs the Differential Flecainide Sensitivity of Voltage-Gated Potassium Channels

Flecainide has been used to differentiate Kv4.2-based transient-outward K(+)-currents (flecainide-sensitive) from Kv1.4-based (flecainide-insensitive). We found that flecainide also inhibits ultrarapid delayed rectifier (I(Kur)) currents in Xenopus laevis oocytes carried by Kv3.1 subunits (IC(50), 28.3 +/- 1.3 microM) more strongly than Kv1.5 currents corresponding to human I(Kur) (IC(50), 237.1 +/- 6.2 microM). The present study examined molecular motifs underlying differential flecainide sensitivity. An initial chimeric approach pointed to a role for S6 and/or carboxyl-terminal sites in Kv3.1/Kv1.5 sensitivity differences. We then looked for homologous amino acid residues of the two sensitive subunits (Kv4.2 and Kv3.1) different from homologous residues for insensitive subunits (Kv1.4 and Kv1.5). Three candidate sites were identified: two in the S5-S6 linker and one in the S6 segment. Mutation of the proximal S5-S6 linker site failed to alter flecainide sensitivity. Mutation at the more distal site in Kv1.5 (V481L) modestly increased sensitivity, but the reciprocal Kv3.1 mutation (L401V) had no effect. S6 mutants caused marked changes: flecainide sensitivity decreased approximately 8-fold for Kv3.1 L422I (IC(50), 213 +/- 9 microM) and increased approximately 7-fold for Kv1.5 I502L (IC(50), 35.6 +/- 1.9 microM). Corresponding mutations reversed flecainide sensitivity of Kv1.4 and Kv4.2; L392I decreased Kv4.2 sensitivity by approximately 17-fold (IC(50) of 37.4 +/- 6.9 to 628 +/- 36 microM); I547L increased Kv1.4 sensitivity by approximately 15-fold (IC(50) of 706 +/- 37 to 40.9 +/- 7.3 microM). Our observations indicate that the flecainide sensitivity differences among these four voltage-gated K(+)-channels are determined by whether an isoleucine or a leucine is present at a specific amino acid location.

The Kv4.2 N-terminal restores fast inactivation and confers KChlP2 modulatory effects on N-terminal-deleted Kv1.4 channels

The N-terminal end of the subunits of the voltage-gated K+ channel Kv1.4 is essential for their rapid N-type inactivation, but removal of the entire Kv4.2 N-terminus slows inactivation only moderately. In this study, we investigated the effect of substituting the Kv4.2 N-terminal for that of Kv1.4 subunits. Despite the minor role of the Kv4.2 N-terminal in Kv4.2 inactivation and the limited degree of amino acid identity between Kv1.4 and Kv4.2 N-terminals, attachment of the Kv4.2 N-terminal to inactivation-deficient, N-terminal-deleted Kv1.4 subunits restored rapid inactivation. The Kv4.2 N-terminal/N-deleted Kv1.4 chimeric construct had inactivation kinetics like those of Kv4.2, inactivation voltage-dependence resembling Kv1.4 and recovery from inactivation substantially faster than wild-type Kv1.4. Acceleration of reactivation appeared to be due to the ability of chimeric channels to recover from inactivation without passing through the open state. Co-expression of wild-type Kv1.4 with the K+ channel interacting protein-2 (KChIP2) did not alter Kvl.4 properties, but co-expression of KChIP2 with Kv4.2 N-terminal/N-deleted Kv1.4 chimeric subunits significantly increased current expression and slowed inactivation without altering the rate of recovery from inactivation. We conclude that substitution of the Kv4.2 N-terminal for that of Kv1.4 transfers a variety of properties of Kv4.2, including inactivation time-dependence, accelerated recovery from inactivation and interaction with KChIP2, to Kv1.4, indicating the ability of Kvl.4 subunits to display these properties and the sufficiency of the Kv4.2 N-terminal to convey them.

Roles of K + channels in regulating tumour cell proliferation and apoptosis

K+ channels are a most diverse class of ion channels in the cytoplasmic membrane and are distributed widely in a variety of cells including cancer cells. Cell proliferation and apoptosis (programmed cell death or cell suicide) are two counterparts that share the responsibility for maintaining normal tissue homeostasis. Evidence has been accumulating from fundamental studies indicating that tumour cells possess various types of K+ channels, and that these K+ channels play important roles in regulating tumour cell proliferation and apoptosis, i.e. facilitating unlimited growth and promoting apoptotic death of tumour cells. The potential implications of K+ channels as a pharmacological target for cancer therapy and a biomarker for diagnosis of carcinogenesis are attracting increasing interest. This review aims to provide a comprehensive overview of current status of research on K+ channels/currents in tumour cells. Focus is placed on the roles of K+ channels/currents in regulating tumour cell proliferation and apoptosis. The possible mechanisms by which K+ channels affect tumour cell growth and death are discussed. Speculations are also made on the potential implications of regulation of tumour cell proliferation and apoptosis by K+ channels.

Functional M3 muscarinic acetylcholine receptors in mammalian hearts

In contrast to most peripheral tissues where multiple subtypes of muscarinic acetylcholine receptor (mAChR) coexist, with each of them playing its part in the orchestra of parasympathetic innervation, the myocardium has been traditionally considered to possess a single mAChR subtype. Although there is much evidence to support the notion that one receptor subtype (M2) orchestrates myocardial muscarinic transduction, there is emerging evidence that M1 and M3 receptors are also expressed and are of potential physiological, pathophysiological and pharmacological relevance. Clarifying this issue has a profound impact on our thinking about the cholinergic control of the heart function and disease and approaches to new drug development for the treatment of heart disease associated with parasympathetic dysfunction. This review article presents evidence for the presence of the M3 receptor subtype in the heart, and analyzes the controversial data from published pharmacological, functional and molecular studies. The potential roles of the M3 receptors, in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias, are discussed. On the basis of these considerations, we have made some proposals concerning the future of myocardial M3 receptor research.

HERG K+ Channel Conductance Promotes H2O2-Induced Apoptosis in HEK293 Cells: Cellular Mechanisms

The human ether-a-go-go-related gene (HERG) encodes a delayed rectifier K(+) channel, which is expressed in a variety of tissues and cells. Besides its well-recognized function in cellular electrophysiology, HERG channels have also been implicated in neuronal differentiation and cell cycle regulation. We have recently found that HERG regulates apoptosis. To elucidate the signaling pathways, we performed studies in HEK293 cells stably expressing HERG channels. ELISA was used to quantify DNA fragmentation, a biochemical hallmark of apoptosis. In HERG-transfected HEK cells, the degree of DNA fragmentation was found consistently higher (approximately 4-times) than in non-transfected cells. Correspondingly, remarkable activation of caspase 3, caspase 9 and cleavage of PARP were seen in HERG-expressing cells, which were otherwise minimal in non-transfected cells. Exposure of cells to H(2)O(2) (10 hrs) at concentrations up to 1 mM, which is known to induce apoptosis in a variety of cells, caused minimal DNA fragmentation in non-transfected cells. HERG expression facilitates DNA fragmentation induced by H(2)O(2) at a concentration-dependent fashion, starting at 200 microM and reaching maximum at 1 mM. Selective HERG channel inhibitors, dofetilide or E-4031 (5 microM) prevented DNA fragmentation. Inhibition of p38 by SB-203580 alleviated DNA-F and PD-98059, which inhibited activation of ERKs, nearly abolished DNA-F. Immunoblotting analysis demonstrated that p38, SAPKs and ERKs MAP kinases were all substantially activated (>10-fold higher) in HERG-expressing cells vs. non-transfected cells. Akt activity was approximately 4-fold lower in HERG cells vs. non-transfected cells in the absence of H(2)O(2) and was slightly increased (approximately 2-fold) after H(2)O(2) exposure. We conclude that HERG channels facilitate cellular DNA fragmentation in HEK cells via concomitant activation of MAP kinases and inactivation of Akt.

The M3 receptor-mediated K(+) current (IKM3), a G(q) protein-coupled K(+) channel

Stimulation of muscarinic acetylcholine receptors (mAChRs) can activate an inward rectifier K(+) current (I(KACh)), which is mediated by the M(2) subtype of mAChR in cardiac myocytes. Recently, a novel delayed rectifier-like K(+) current mediated by activation of the cardiac M(3) receptors (designated I(KM3)) was identified, which is distinct from I(KACh) and other known K(+) currents. While I(KACh) is known to be a G(i) protein-gated K(+) channel, the signal transduction mechanisms for I(KM3) activation remained unexplored. We studied I(KM3) with whole-cell patch clamp and macropatch clamp techniques. Whole cell I(KM3) activated by choline persisted with minimal rundown over 2 h in presence of internal GTP. When GTP was replaced by guanyl-5'-yl thiophosphate, I(KM3) demonstrated rapid and extensive rundown. While I(KACh) (induced by ACh) was markedly reduced in cells pretreated with pertussis toxin, I(KM3) was unaltered. Intracellular application of antibodies targeting alpha-subunit of G(i/o) protein suppressed I(KACh) without affecting I(KM3). Antibodies targeting the N and the C terminus, respectively, of G(q) protein alpha-subunit substantially depressed I(KM3) but failed to alter I(KACh). The antibody against beta-subunits of G proteins inhibited both I(KACh) and I(KM3). I(KM3) activated by choline in the cell-attached mode of macropatches persisted in the cell-free configuration. Application of purified G(q) protein alpha-subunit or betagamma-subunit of G proteins or guanosine 5'-O-(thiotriphosphate) to the internal solution activated I(KM3)-like currents in inside-out patches. Our findings revealed a novel aspect of receptor-channel signal transduction mechanisms, and I(KM3) represents the first G(q) protein-coupled K(+) channel. We propose that the G protein-coupled K(+) channel family could be divided into two subfamilies: G(i) protein-coupled K(+) channel subfamily and G(q) protein-coupled K(+) channel subfamily.

Impairment of HERG K(+) channel function by tumor necrosis factor-alpha: role of reactive oxygen species as a mediator

Congestive heart failure (CHF) is associated with susceptibility to lethal arrhythmias and typically increases levels of tumor necrosis factor-alpha (TNF-alpha) and its receptor, TNFR1. CHF down-regulates rapid delayed-rectifier K(+) current (I(Kr)) and delays cardiac repolarization. We studied the effects of TNF-alpha on cloned HERG K(+) channel (human ether-a-go-go-related gene) in HEK293 cells and native I(Kr) in canine cardiomyocytes with whole-cell patch clamp techniques. TNF-alpha consistently and reversibly decreased HERG current (I(HERG)). Effects of TNF-alpha were concentration-dependent, increased with longer incubation period, and occurred at clinically relevant concentrations. TNF-alpha had similar inhibitory effects on I(Kr) and markedly prolonged action potential duration (APD) in canine cardiomyocytes. Immunoblotting analysis demonstrated that HERG protein level was slightly higher in canine hearts with tachypacing-induced CHF than in healthy hearts, and TNF-alpha slightly increased HERG protein level in CHF but not in healthy hearts. In cells pretreated with the inhibitory anti-TNFR1 antibody, TNF-alpha lost its ability to suppress I(HERG), indicating a requirement of TNFR1 activation for HERG suppression. Vitamin E or MnTBAP (Mn(III) tetrakis(4-benzoic acid) porphyrin chloride), a superoxide dismutase mimic) prevented, whereas the superoxide anion generating system xanthine/xanthine oxidase mimicked, TNF-alpha-induced I(HERG) depression. TNF-alpha caused robust increases in intracellular reactive oxygen species, and vitamin E and MnTBAP abolished the increases, in both HEK293 cells and canine ventricular myocytes. We conclude that the TNF-alpha/TNFR1 system impairs HERG/I(Kr) function mainly by stimulating reactive oxygen species, particularly superoxide anion, but not by altering HERG expression; the effect may contribute to APD prolongation by TNF-alpha and may be a novel mechanism for electrophysiological abnormalities and sudden death in CHF.

Potential mechanisms for the enhancement of HERG K+ channel function by phospholipid metabolites

1. Phospholipid metabolites lysophospholipids cause extracellular K(+) accumulation and action potential shortening with increased risk of arrhythmias during myocardial ischemia. Here we studied effects of several lysophospholipids with different lengths of hydrocarbon chains and charged headgroups on HERG K(+) currents (I(HERG)) expressed in HEK293 cells and the potential mechanisms using whole-cell patch-clamp techniques. 2. Only the lipids with 16 hydrocarbons such as 1-palmitoyl-lysophosphatidylcholine (LPC-16) and 1-palmitoyl-lysophosphatidylglycerol (LPG-16) were found to produce significant enhancement of I(HERG) and negative shifts of HERG activation, although the voltage dependence of the effects was different between LPC-16 and LPG-16 which have differently charged headgroups. The lipid with 18 hydrocarbons modestly increased I(HERG). The lipids with 6 or 24 hydrocarbons had no effect or slightly decreased I(HERG). 3. Inhibition or activation of protein kinase C did not alter the effects of LPC-16 and LPG-16. Participation of phosphatidylinositol-4,5-bisphosphate in I(HERG) enhancement by LPC-16/LPG-16 was also excluded. 4. Vitamin E augmented the effects of LPC-16/LPG-16 whereas xanthine/xanthine oxidase reduced I(HERG): indicating that LPC-16/LPG-16 produced dual effects on I(HERG): direct enhancement of I(HERG) and indirect suppression via production of superoxide anion. 5. We conclude that enhancement of HERG function by lysophospholipids is specific to the lipids with 16-hydrocarbon chain structure and the pattern of voltage dependence is determined by the polar headgroups. The increase in I(HERG) is best described by direct interactions between lipid molecules and HERG proteins, which is consistent with lack of effects via membrane destabilization or modulation by intracellular signaling pathways.