Increased persistent sodium current due to decreased PI3K signaling contributes to QT prolongation in the diabetic heart

Sudden cardiac death due to ventricular arrhythmia in diabetes mellitus: A bench to bedside review

Diabetes mellitus (DM) confers an increased risk of sudden cardiac death (SCD) independent of its associated cardiovascular comorbidities. DM induces adverse structural, electrophysiologic, and autonomic cardiac remodeling that can increase one's risk of ventricular arrhythmias and SCD. Although glycemic control and prevention of microvascular and macrovascular complications are cornerstones in the management of DM, they are not adequate for the prevention of SCD. In this narrative review, we describe the contribution of DM to the pathophysiologic mechanism of SCD beyond its role in atherosclerotic cardiovascular disease and heart failure. On the basis of this pathophysiologic framework, we outline potential preventive and therapeutic strategies to mitigate the risk of SCD in this population of high-risk patients.

Increased GIRK channel activity prevents arrhythmia in mice with heart failure by enhancing ventricular repolarization

Ventricular arrhythmia causing sudden cardiac death is the leading mode of death in patients with heart failure. Yet, the mechanisms that prevent ventricular arrhythmias in heart failure are not well characterized. Using a mouse model of heart failure created by transverse aorta constriction, we show that GIRK channel, an important regulator of cardiac action potentials, is constitutively active in failing ventricles in contrast to normal cells. Evidence is presented indicating that the tonic activation of M2 muscarinic acetylcholine receptors by endogenously released acetylcholine contributes to the constitutive GIRK activity. This constitutive GIRK activity prevents the action potential prolongation in heart failure ventricles. Consistently, GIRK channel blockade with tertiapin-Q induces QT interval prolongation and increases the incidence of arrhythmia in heart failure, but not in control mice. These results suggest that constitutive GIRK channels comprise a key mechanism to protect against arrhythmia by providing repolarizing currents in heart failure ventricles.

The association of QTc prolongation with cardiovascular events in cancer patients taking tyrosine kinase inhibitors (TKIs)

OBJECTIVE

To investigate the association between stages of QTc prolongation and the risk of cardiac events among patients on TKIs.

METHODS

This was a retrospective cohort study performed at an academic tertiary care center of cancer patients who were taking TKIs or not taking TKIs. Patients with two recorded ECGs between January 1, 2009, and December 31, 2019, were selected from an electronic database. The QTc duration > 450ms was determined as prolonged. The association between QTc prolongation progression and events of cardiovascular disease were compared.

RESULTS

This study included a total of 451 patients with 41.2% of patients taking TKIs. During a median follow up period of 3.1 years, 49.5% subjects developed CVD and 5.4% subjects suffered cardiac death in patient using TKIs (n = 186); the corresponding rates are 64.2% and 1.2% for patients not on TKIs (n = 265), respectively. Among patient on TKIs, 4.8% of subjects developed stroke, 20.4% of subjects suffered from heart failure (HF) and 24.2% of subjects had myocardial infarction (MI); corresponding incidence are 6.8%, 26.8% and 30.6% in non-TKIs. When patients were regrouped to TKIs versus non-TKIs with and without diabetes, there was no significant difference in the incidence of cardiac events among all groups. Adjusted Cox proportional hazards models were applied to estimate hazard ratios (HRs) with 95% confidence intervals (CIs). There is a significant increased risk of HF events (HR, 95% CI: 2.12, 1.36-3.32) and MI events (HR, 95% CI: 1.78, 1.16-2.73) during the 1st visit. There are also trends for an increased incidence of cardiac adverse events associated with QTc prolongation among patient with QTc > 450ms, however the difference is not statistically significant. Increased cardiac adverse events in patients with QTc prolongation were reproduced during the 2nd visit and the incidence of heart failure was significantly associated with QTc prolongation(HR, 95% CI: 2.94, 1.73-5.0).

CONCLUSION

There is a significant increased QTc prolongation in patients taking TKIs. QTc prolongation caused by TKIs is associated with an increased risk of cardiac events.

Caveolin-3 and Caveolae regulate ventricular repolarization

RATIONALE

Flask-shaped invaginations of the cardiomyocyte sarcolemma called caveolae require the structural protein caveolin-3 (Cav-3) and host a variety of ion channels, transporters, and signaling molecules. Reduced Cav-3 expression has been reported in models of heart failure, and variants in CAV3 have been associated with the inherited long-QT arrhythmia syndrome. Yet, it remains unclear whether alterations in Cav-3 levels alone are sufficient to drive aberrant repolarization and increased arrhythmia risk.

OBJECTIVE

To determine the impact of cardiac-specific Cav-3 ablation on the electrophysiological properties of the adult mouse heart.

METHODS AND RESULTS

Cardiac-specific, inducible Cav3 homozygous knockout (Cav-3KO) mice demonstrated a marked reduction in Cav-3 expression by Western blot and loss of caveolae by electron microscopy. However, there was no change in macroscopic cardiac structure or contractile function. The QTc interval was increased in Cav-3KO mice, and there was an increased propensity for ventricular arrhythmias. Ventricular myocytes isolated from Cav-3KO mice exhibited a prolonged action potential duration (APD) that was due to reductions in outward potassium currents (Ito, Iss) and changes in inward currents including slowed inactivation of ICa,L and increased INa,L. Mathematical modeling demonstrated that the changes in the studied ionic currents were adequate to explain the prolongation of the mouse ventricular action potential. Results from human iPSC-derived cardiomyocytes showed that shRNA knockdown of Cav-3 similarly prolonged APD.

CONCLUSION

We demonstrate that Cav-3 and caveolae regulate cardiac repolarization and arrhythmia risk via the integrated modulation of multiple ionic currents.

Transcriptional Dysregulation Underlies Both Monogenic Arrhythmia Syndrome and Common Modifiers of Cardiac Repolarization

BACKGROUND

Brugada syndrome (BrS) is an inherited arrhythmia syndrome caused by loss-of-function variants in the cardiac sodium channel gene SCN5A (sodium voltage-gated channel alpha subunit 5) in ≈20% of subjects. We identified a family with 4 individuals diagnosed with BrS harboring the rare G145R missense variant in the cardiac transcription factor TBX5 (T-box transcription factor 5) and no SCN5A variant.

METHODS

We generated induced pluripotent stem cells (iPSCs) from 2 members of a family carrying TBX5-G145R and diagnosed with Brugada syndrome. After differentiation to iPSC-derived cardiomyocytes (iPSC-CMs), electrophysiologic characteristics were assessed by voltage- and current-clamp experiments (n=9 to 21 cells per group) and transcriptional differences by RNA sequencing (n=3 samples per group), and compared with iPSC-CMs in which G145R was corrected by CRISPR/Cas9 approaches. The role of platelet-derived growth factor (PDGF)/phosphoinositide 3-kinase (PI3K) pathway was elucidated by small molecule perturbation. The rate-corrected QT (QTc) interval association with serum PDGF was tested in the Framingham Heart Study cohort (n=1893 individuals).

RESULTS

TBX5-G145R reduced transcriptional activity and caused multiple electrophysiologic abnormalities, including decreased peak and enhanced "late" cardiac sodium current (INa), which were entirely corrected by editing G145R to wild-type. Transcriptional profiling and functional assays in genome-unedited and -edited iPSC-CMs showed direct SCN5A down-regulation caused decreased peak INa, and that reduced PDGF receptor (PDGFRA [platelet-derived growth factor receptor α]) expression and blunted signal transduction to PI3K was implicated in enhanced late INa. Tbx5 regulation of the PDGF axis increased arrhythmia risk due to disruption of PDGF signaling and was conserved in murine model systems. PDGF receptor blockade markedly prolonged normal iPSC-CM action potentials and plasma levels of PDGF in the Framingham Heart Study were inversely correlated with the QTc interval (P<0.001).

CONCLUSIONS

These results not only establish decreased SCN5A transcription by the TBX5 variant as a cause of BrS, but also reveal a new general transcriptional mechanism of arrhythmogenesis of enhanced late sodium current caused by reduced PDGF receptor-mediated PI3K signaling.

Empagliflozin attenuates arrhythmogenesis in diabetic cardiomyopathy by normalizing intracellular Ca2+ handling in ventricular cardiomyocytes

Diabetic cardiomyopathy has been reported to increase the risk of fatal ventricular arrhythmia. The beneficial effects of the selective sodium-glucose cotransporter-2 inhibitor have not been fully examined in the context of antiarrhythmic therapy, especially its direct cardioprotective effects despite the negligible SGLT2 expression in cardiomyocytes. We aimed to examine the antiarrhythmic effects of empagliflozin (EMPA) treatment on diabetic cardiomyocytes, with a special focus on Ca²⁺ handling. We conducted echocardiography and hemodynamic studies and studied electrophysiology, Ca²⁺ handling, and protein expression in C57BLKS/J-lepr^db/db mice (db/db mice) and their nondiabetic lean heterozygous Lepr^db/+ littermates (db/⁺ mice). Preserved systolic function with diastolic dysfunction was observed in 16-wk-old db/db mice. During arrhythmia induction, db/db mice had significantly increased premature ventricular complexes (PVCs) than controls, which was attenuated by EMPA. In protein expression analyses, calmodulin-dependent protein kinase II (CaMKII) Thr287 autophosphorylation and CaMKII-dependent RyR2 phosphorylation (S2814) were significantly increased in diabetic hearts, which were inhibited by EMPA. In addition, global O-GlcNAcylation significantly decreased with EMPA treatment. Furthermore, EMPA significantly inhibited ventricular cardiomyocyte glucose uptake. Diabetic cardiomyocytes exhibited increased spontaneous Ca²⁺ events and decreased sarcoplasmic reticulum (SR) Ca²⁺ content, along with impaired Ca²⁺ transient, all of which normalized with EMPA treatment. Notably, most EMPA-induced improvements in Ca²⁺ handling were abolished by the addition of an O-GlcNAcase (OGA) inhibitor. In conclusion, EMPA attenuated ventricular arrhythmia inducibility by normalizing the intracellular Ca²⁺ handling, and we speculated that this effect was, at least partly, due to the inhibition of O-GlcNAcylation via the suppression of glucose uptake into cardiomyocytes.NEW & NOTEWORTHY SGLT2is are known to improve cardiovascular outcomes regardless of the presence of diabetes and decrease traditional cardiovascular risk factors. We demonstrated, for the first time, that EMPA inhibited PVCs by normalizing Ca²⁺ handling in diabetic mice. Our data suggest that the effects of SGLT2is on calcium handling may occur because of suppression of O-GlcNAcylation through inhibition of glucose uptake and not because of NHE inhibition, as previously suggested.

Inhibition of late sodium current via PI3K/Akt signaling prevents cellular remodeling in tachypacing-induced HL-1 atrial myocytes

An aberrant late sodium current (I_Na,Late) caused by a mutation in the cardiac sodium channel (Na_v1.5) has emerged as a contributor to electrical remodeling that causes susceptibility to atrial fibrillation (AF). Although downregulation of phosphoinositide 3-kinase (PI3K)/Akt signaling is associated with AF, the molecular mechanisms underlying the negative regulation of I_Na,Late in AF remain unclear, and potential therapeutic approaches are needed. In this work, we constructed a tachypacing-induced cellular model of AF by exposing HL-1 myocytes to rapid electrical stimulation (1.5 V/cm, 4 ms, 10 Hz) for 6 h. Then, we gathered data using confocal Ca²⁺ imaging, immunofluorescence, patch-clamp recordings, and immunoblots. The tachypacing cells displayed irregular Ca²⁺ release, delayed afterdepolarization, prolonged action potential duration, and reduced PI3K/Akt signaling compared with controls. Those detrimental effects were related to increased I_Na,Late and were significantly mediated by treatment with the I_Na,Late blocker ranolazine. Furthermore, decreased PI3K/Akt signaling via PI3K inhibition increased I_Na,Late and subsequent aberrant myocyte excitability, which were abolished by I_Na,Late inhibition, suggesting that PI3K/Akt signaling is responsible for regulating pathogenic I_Na,Late. These results indicate that PI3K/Akt signaling is critical for regulating I_Na,Late and electrical remodeling, supporting the use of PI3K/Akt-mediated I_Na,Late as a therapeutic target for AF.

Genetic analyses of the electrocardiographic QT interval and its components identify additional loci and pathways

The QT interval is an electrocardiographic measure representing the sum of ventricular depolarization and repolarization, estimated by QRS duration and JT interval, respectively. QT interval abnormalities are associated with potentially fatal ventricular arrhythmia. Using genome-wide multi-ancestry analyses (>250,000 individuals) we identify 177, 156 and 121 independent loci for QT, JT and QRS, respectively, including a male-specific X-chromosome locus. Using gene-based rare-variant methods, we identify associations with Mendelian disease genes. Enrichments are observed in established pathways for QT and JT, and previously unreported genes indicated in insulin-receptor signalling and cardiac energy metabolism. In contrast for QRS, connective tissue components and processes for cell growth and extracellular matrix interactions are significantly enriched. We demonstrate polygenic risk score associations with atrial fibrillation, conduction disease and sudden cardiac death. Prioritization of druggable genes highlight potential therapeutic targets for arrhythmia. Together, these results substantially advance our understanding of the genetic architecture of ventricular depolarization and repolarization.

Effects of high glucose and insulin on the electrophysiological properties of cardiomyocytes derived from human-induced pluripotent stem cells

OBJECTIVES

The risk of arrhythmia increases in diabetic patients. However, the effects of hyperglycemia and insulin therapy on the electrophysiological properties of human cardiomyocytes remain unclear. This study is to explore the effects of high glucose and insulin on the electrophysiological properties and arrhythmias of cardiomyocytes derived from human-induced pluripotent stem cells (hiPSC-CMs).

METHODS

Immunofluorescent staining and flow cytometry were used to analyze the purity of hiPSC-CMs generated from human skin fibroblasts of a healthy donor. The hiPSC-CMs were divided into 3 group (treated with normal medium, high glucose and insulin for 4 days): a control group (NM group, containing 5 mmol/L glucose), a high glucose group (HG group, containing 15 mmol/L glucose), and a high glucose combined with insulin (HG+INS group, containing 15 mmol/L glucose+100 mg/L insulin). Electrophysiological changes of hiPSC-CMs were detected by microelectrode array (MEA) before or after treatment with glucose and insulin, including beating rate (BR), field potential duration (FPD) (similar to QT interval in ECG), FPDc (FPD corrected by BR), spike amplitude and conduction velocity (CV). Effects of sotalol on electrophysiological properties and arrhythmias of hiPSC-CMs were also evaluated.

RESULTS

The expression of cardiac-specific marker of cardiac troponin T was high in the hiPSC-CMs. The purity of hiPSC-CMs was 99.06%. Compared with the NM group, BR was increased by (9.14±0.8)% in the HG group (P<0.01). After treatment with high glucose, FPD was prolonged from (460.4±9.0) ms to (587.6±23.7) ms in the HG group, while it was prolonged from (462.5±14.5) ms to (512.6±17.6) ms in the NM group. Compared with the NM group, FPD of hiPSC-CMs was prolonged by (16.8±1.4)% in the HG group (P<0.01). The FPDc of hiPSC-CMs was prolonged from (389.1±13.7) ms to (478.3±31.5) ms in the HG group, and that was prolonged from (387.7±21.6) ms to (422.6±32.9) ms in the NM group. Compared with the NM group, the FPDc of hiPSC-CMs was prolonged by (13.9±1.3)% in HG group (P<0.01). The spike amplitude and CV remained unchanged between the HG group and the NM group (P>0.05). Ten µmol/L of sotalol can induce significant arrhythmias from all wells in the HG group. After treatment with insulin and high glucose, compared with the HG group, BR was increased by (8.3±0.5)% in the HG+INS group (P<0.05). The FPD was prolonged from (463.4±9.7) ms to (532.6±12.8) ms in the HG+INS group, while it was prolonged from (460.4±9.0) ms to (587.6±23.7) ms in the HG group. Compared with the HG group, the FPD of hiPSC-CMs was shortened by (12.7±1.9)% in the HG+INS group (P<0.01). The FPDc of hiPSC-CMs was prolonged from (387.4±4.1) ms to (422.4±10.0) ms in the HG+INS group, and that was prolonged from (384.8±4.0) ms to (476.3±11.5) ms in HG group. Compared with the HG group, the FPDc of hiPSC-CMs was shortened by (14.7±1.1)% in HG group (P<0.01). After the insulin treatment, the spike amplitude of hiPSC-CMs was increased from (3.12±0.46) mV to (4.35±0.64) mV in the HG+INS group, while it was enhanced from (3.06±0.35) mV to (3.33±0.41) mV in the HG group. The spike amplitude of hiPSC-CMs was increased by (30.8±3.7)% in the HG+INS group compared with that in the HG group (P<0.05). The CV in the HG+INS group was increased from (0.23±0.08) mm/ms to (0.32±0.08) mm/ms after insulin treatment, which was increased from (0.21±0.04) mm/ms to (0.30±0.07) mm/ms in the HG group, but there was no significant difference in CV between the HG+INS group and the HG group (P>0.05). The induction experiment showed that 10 μmol/L of sotalol could prolong the FPDc of hiPSC-CMs by (78.9±11.6)% in the HG+INS group, but no arrhythmia was induced in each well.

CONCLUSIONS

High glucose can induce FPD/FPDc of hiPSC-CMs prolongation and increase the risk of arrhythmia induced by drugs. Insulin can reduce the FPD/FPDc prolongation and the risk of induced arrhythmia by high glucose.These results are important to understand the electrophysiological changes of the myocardium in diabetic patients and the impact of insulin therapy on its electrophysiology. Further study on the mechanism may provide new ideas and methods for the treatment of acquired and even inherited long QT syndrome.

Declining Levels and Bioavailability of IGF-I in Cardiovascular Aging Associate With QT Prolongation-Results From the 1946 British Birth Cohort

Background

As people age, circulating levels of insulin-like growth factors (IGFs) and IGF binding protein 3 (IGFBP-3) decline. In rat cardiomyocytes, IGF-I has been shown to regulate sarcolemmal potassium channel activity and late sodium current thus impacting cardiac repolarization and the heart rate-corrected QT (QTc). However, the relationship between IGFs and IGFBP-3 with the QTc interval in humans, is unknown.

Objectives

To examine the association of IGFs and IGFBP-3 with QTc interval in an older age population-based cohort.

Methods

Participants were from the 1946 Medical Research Council (MRC) National Survey of Health and Development (NSHD) British birth cohort. Biomarkers from blood samples at age 53 and 60-64 years (y, exposures) included IGF-I/II, IGFBP-3, IGF-I/IGFBP-3 ratio and the change (Δ) in marker levels between the 60-64 and 53y sampled timepoints. QTc (outcome) was recorded from electrocardiograms at the 60-64y timepoint. Generalized linear multivariable models with adjustments for relevant demographic and clinical factors, were used for complete-cases and repeated after multiple imputation.

Results

One thousand four hundred forty-eight participants were included (48.3% men; QTc mean 414 ms interquartile range 26 ms). Univariate analysis revealed an association between low IGF-I and IGF-I/IGFBP-3 ratio at 60-64y with QTc prolongation [respectively: β -0.30 ms/nmol/L, (95% confidence intervals -0.44, -0.17), p < 0.001; β-28.9 ms/unit (-41.93, -15.50), p < 0.001], but not with IGF-II or IGFBP-3. No association with QTc was found for IGF biomarkers sampled at 53y, however both ΔIGF-I and ΔIGF-I/IGFBP-3 ratio were negatively associated with QTc [β -0.04 ms/nmol/L (-0.08, -0.008), p = 0.019; β -2.44 ms/unit (-4.17, -0.67), p = 0.007] while ΔIGF-II and ΔIGFBP-3 showed no association. In fully adjusted complete case and imputed models (reporting latter) low IGF-I and IGF-I/IGFBP-3 ratio at 60-64y [β -0.21 ms/nmol/L (-0.39, -0.04), p = 0.017; β -20.14 ms/unit (-36.28, -3.99), p = 0.015], steeper decline in ΔIGF-I [β -0.05 ms/nmol/L/10 years (-0.10, -0.002), p = 0.042] and shallower rise in ΔIGF-I/IGFBP-3 ratio over a decade [β -2.16 ms/unit/10 years (-4.23, -0.09), p = 0.041], were all independently associated with QTc prolongation. Independent associations with QTc were also confirmed for other previously known covariates: female sex [β 9.65 ms (6.65, 12.65), p < 0.001], increased left ventricular mass [β 0.04 ms/g (0.02, 0.06), p < 0.001] and blood potassium levels [β -5.70 ms/mmol/L (-10.23, -1.18) p = 0.014].

Conclusion

Over a decade, in an older age population-based cohort, declining levels and bioavailability of IGF-I associate with prolongation of the QTc interval. As QTc prolongation associates with increased risk for sudden death even in apparently healthy people, further research into the antiarrhythmic effects of IGF-I on cardiomyocytes is warranted.

Knockout of interleukin-17A diminishes ventricular arrhythmia susceptibility in diabetic mice via inhibiting NF-κB-mediated electrical remodeling

Interleukin-17A (IL-17), a potent proinflammatory cytokine, has been shown to participate in cardiac electrical disorders. Diabetes mellitus is an independent risk factor for ventricular arrhythmia. In this study, we investigated the role of IL-17 in ventricular arrhythmia of diabetic mice. Diabetes was induced in both wild-type and IL-17 knockout mice by intraperitoneal injection of streptozotocin (STZ). High-frequency electrical stimuli were delivered into the right ventricle to induce ventricular arrhythmias. We showed that the occurrence rate of ventricular tachycardia was significantly increased in diabetic mice, which was attenuated by IL-17 knockout. We conducted optical mapping on perfused mouse hearts and found that cardiac conduction velocity (CV) was significantly decreased, and action potential duration (APD) was prolonged in diabetic mice, which were mitigated by IL-17 knockout. We performed whole-cell patch clamp recordings from isolated ventricular myocytes, and found that the densities of I_to, I_Na and I_Ca,L were reduced, the APDs at 50% and 90% repolarization were increased, and early afterdepolarization (EAD) was markedly increased in diabetic mice. These alterations were alleviated by the knockout of IL-17. Moreover, knockout of IL-17 alleviated the downregulation of Nav1.5 (the pore forming subunit of I_Na), Cav1.2 (the main component subunit of I_Ca,L) and KChIP2 (potassium voltage-gated channel interacting protein 2, the regulatory subunit of I_to) in the hearts of diabetic mice. The expression of NF-κB was significantly upregulated in the hearts of diabetic mice, which was suppressed by IL-17 knockout. In neonatal mouse ventricular myocytes, knockdown of NF-κB significantly increased the expression of Nav1.5, Cav1.2 and KChIP2. These results imply that IL-17 may represent a potential target for the development of agents against diabetes-related ventricular arrhythmias.

Common Ancestry-Specific Ion Channel Variants Predispose to Drug-Induced Arrhythmias

BACKGROUND

Multiple reports associate the cardiac sodium channel gene (SCN5A) variants S1103Y and R1193Q with type 3 congenital long QT syndrome and drug-induced long QT syndrome. These variants are too common in ancestral populations to be highly arrhythmogenic at baseline, however: S1103Y allele frequency is 8.1% in African Americans and R1193Q 6.1% in East Asians. R1193Q is known to increase late sodium current (I_Na-L) in cardiomyocytes derived from induced pluripotent stem cells but the role of these variants in modulating repolarization remains poorly understood.

METHODS

We determined the effect of S1103Y on QT intervals among African-American participants in a large electronic health record. Using cardiomyocytes derived from induced pluripotent stem cells carrying naturally occurring or genome-edited variants, we studied action potential durations (APDs) at baseline and after challenge with the repolarizing potassium current (I_Kr) blocker dofetilide and I_Na-L and I_Kr at baseline.

RESULTS

In 1479 African-American participants with no confounding medications or diagnoses of heart disease, QT intervals in S1103Y carriers was no different from that in noncarriers. Baseline APD was no different in cells expressing the Y allele (SY, YY cells) compared with isogenic cells with the reference allele (SS cells). However, I_Na-L was increased in SY and YY cells and the I_Na-L blocker GS967 shortened APD in SY/YY but not SS cells (P<0.001). I_Kr was increased almost 2-fold in SY/YY cells compared with SS cells (tail current: 0.66±0.1 versus 1.2±0.1 pA/pF; P<0.001). Dofetilide challenge prolonged APD at much lower concentrations in SY (4.1 nmol/L [interquartile range, 1.5-9.3]; n=11) and YY (4.2 nmol/L [1.7-5.0]; n=5) than in SS cells (249 nmol/L [22.3-2905]; n=14; P<0.001 and P<0.01, respectively) and elicited afterdepolarizations in 8/16 SY/YY cells but only in 1/14 SS cells. R1193Q cells similarly displayed no difference in baseline APD but increased I_Kr and increased dofetilide sensitivity.

CONCLUSIONS

These common ancestry-specific variants do not affect baseline repolarization, despite generating increased I_Na-L. We propose that increased I_Kr serves to maintain normal repolarization but increases the risk of manifest QT prolongation with I_Kr block in variant carriers. Our findings emphasize the need for inclusion of diverse populations in the study of adverse drug reactions.

Two-hit mechanism of cardiac arrhythmias in diabetic hyperglycaemia: reduced repolarization reserve, neurohormonal stimulation, and heart failure exacerbate susceptibility

AIMS

Diabetic hyperglycaemia is associated with increased arrhythmia risk. We aimed to investigate whether hyperglycaemia alone can be accountable for arrhythmias or whether it requires the presence of additional pathological factors.

METHODS AND RESULTS

Action potentials (APs) and arrhythmogenic spontaneous diastolic activities were measured in isolated murine ventricular, rabbit atrial, and ventricular myocytes acutely exposed to high glucose. Acute hyperglycaemia increased the short-term variability (STV) of action potential duration (APD), enhanced delayed afterdepolarizations, and the inducibility of APD alternans during tachypacing in both murine and rabbit atrial and ventricular myocytes. Hyperglycaemia also prolonged APD in mice and rabbit atrial cells but not in rabbit ventricular myocytes. However, rabbit ventricular APD was more strongly depressed by block of late Na+ current (INaL) during hyperglycaemia, consistent with elevated INaL in hyperglycaemia. All the above proarrhythmic glucose effects were Ca2+-dependent and abolished by CaMKII inhibition. Importantly, when the repolarization reserve was reduced by pharmacological inhibition of K+ channels (either Ito, IKr, IKs, or IK1) or hypokalaemia, acute hyperglycaemia further prolonged APD and further increased STV and alternans in rabbit ventricular myocytes. Likewise, when rabbit ventricular myocytes were pretreated with isoproterenol or angiotensin II, hyperglycaemia significantly prolonged APD, increased STV and promoted alternans. Moreover, acute hyperglycaemia markedly prolonged APD and further enhanced STV in failing rabbit ventricular myocytes.

CONCLUSION

We conclude that even though hyperglycaemia alone can enhance cellular proarrhythmic mechanisms, a second hit which reduces the repolarization reserve or stimulates G protein-coupled receptor signalling greatly exacerbates cardiac arrhythmogenesis in diabetic hyperglycaemia.

Hypernatremia and intercalated disc edema synergistically exacerbate long-QT syndrome type 3 phenotype

Cardiac voltage-gated sodium channel gain-of-function prolongs repolarization in the long-QT syndrome type 3 (LQT3). Previous studies suggest that narrowing the perinexus within the intercalated disc, leading to rapid sodium depletion, attenuates LQT3-associated action potential duration (APD) prolongation. However, it remains unknown whether extracellular sodium concentration modulates APD prolongation during sodium channel gain-of-function. We hypothesized that elevated extracellular sodium concentration and widened perinexus synergistically prolong APD in LQT3. LQT3 was induced with sea anemone toxin (ATXII) in Langendorff-perfused guinea pig hearts (n = 34). Sodium concentration was increased from 145 to 160 mM. Perinexal expansion was induced with mannitol or the sodium channel β1-subunit adhesion domain antagonist (βadp1). Epicardial ventricular action potentials were optically mapped. Individual and combined effects of varying clefts and sodium concentrations were simulated in a computational model. With ATXII, both mannitol and βadp1 significantly widened the perinexus and prolonged APD, respectively. The elevated sodium concentration alone significantly prolonged APD as well. Importantly, the combination of elevated sodium concentration and perinexal widening synergistically prolonged APD. Computational modeling results were consistent with animal experiments. Concurrently elevating extracellular sodium and increasing intercalated disc edema prolongs repolarization more than the individual interventions alone in LQT3. This synergistic effect suggests an important clinical implication that hypernatremia in the presence of cardiac edema can markedly increase LQT3-associated APD prolongation. Therefore, to our knowledge, this is the first study to provide evidence of a tractable and effective strategy to mitigate LQT3 phenotype by means of managing sodium levels and preventing cardiac edema in patients.NEW & NOTEWORTHY This is the first study to demonstrate that the long-QT syndrome type 3 (LQT3) phenotype can be exacerbated or concealed by regulating extracellular sodium concentrations and/or the intercalated disc separation. The animal experiments and computational modeling in the current study reveal a critically important clinical implication: sodium dysregulation in the presence of edema within the intercalated disc can markedly increase the risk of arrhythmia in LQT3. These findings strongly suggest that maintaining extracellular sodium within normal physiological limits may be an effective and inexpensive therapeutic option for patients with congenital or acquired sodium channel gain-of-function diseases.

The Risk of QTc Prolongation in Non-Diabetic and Diabetic Patients Taking Tyrosine Kinase Inhibitors (TKIs)- A Patient Safety Project at a Private Oncology Practice

Objective: To assess the prevalence of QTc prolongation in both non-diabetic and diabetic patients on TKIs. Some TKIs have been reported to cause QTc prolongation, which is prevalent in diabetes. However, there is no Risk Evaluation and Mitigation Strategy using series ECG to monitor those patients.

Methods: Patients taking TKIs, with two ECGs recorded between 1 January 2010 and 31 December 2017 were selected from the electronic database. The QTc duration >450 ms was determined as prolonged. Percentage of QTc prolongation on participants were compared using Chi-Square test.

Results: This study included 313 patients (age 66.1 ± 0.8 years and 57.5% are female) taking TKIs. In non-Diabetic patients, the prevalence of QTc prolongation is 19.1% (n = 253) before and 34.8% (n = 253) after treatment with TKIs (p < 0.001), respectively. In diabetic patients, the prevalence of QTc prolongation is 21.7% (n = 60) before and 40% (n = 60) after treatment with TKIs (p = 0.03), respectively. In addition, we examined the effect of modifying risk factors for cardiovascular disease (CVD) on the prevalence of QTc prolongation caused by TKIs. In non-diabetic patients, the prevalence of QTc prolongation is 33.3% (n = 57) before and 34.2% (n = 196) after risk factors modification (p = 0.91), respectively. In diabetic patients, the prevalence of QTc prolongation is 50% (n = 24) before and 33.3% (n = 36) after risk factors modification (p = 0.20), respectively.

Conclusion: Use of TKIs is associated with a significantly increased risk of QTc prolongation for patients, particularly when patients are diabetic. Modification of risk factors for CVD does not significantly affect the prevalence of QTc prolongation caused by TKIs.

Soluble Epoxide Hydrolase Inhibitors Regulate Ischemic Arrhythmia by Targeting MicroRNA-1

Background: Soluble epoxide hydrolase inhibitors (sEHis) inhibit the degradation of epoxyeicosatrienoic acids (EETs) in cells, and EETs have antiarrhythmic effects. Our previous experiments confirmed that t-AUCB, a preparation of sEHis, inhibited ischemic arrhythmia by negatively regulating microRNA-1 (miR-1), but its specific mechanism remained unclear. Aim: This study aimed to examine the role of serum response factor (SRF) and the PI3K/Akt/GSK3β pathway in t-AUCB-mediated regulation of miR-1 and the interaction between them. Methods/Results: We used SRF small interfering RNA (siSRF), SRF small hairpin (shSRF) RNA sequence adenovirus, PI3K/Akt/GSK3β pathway inhibitors, t-AUCB, and 14,15-EEZE (a preparation of EETs antagonists) to treat mouse cardiomyocytes overexpressing miR-1 and mice with myocardial infarction (MI). We found that silencing SRF attenuated the effects on miR-1 and its target genes KCNJ2 and GJA1 in the presence of t-AUCB, and inhibition of the PI3K/Akt/GSK3β pathway antagonized the effects of t-AUCB on miR-1, KCNJ2, and GJA1, which were associated with PI3Kα, Akt, and Gsk3β but not PI3Kβ or PI3Kγ. Moreover, the PI3K/Akt/GSK3β pathway was involved in the regulation of SRF by t-AUCB, and silencing SRF inhibited the t-AUCB-induced increases in Akt and Gsk3β phosphorylation. Conclusions: Both the SRF and the PI3K/Akt/GSK3β pathway are involved in the t-AUCB-mediated regulation of miR-1, and these factors interact with each other.

CaMKII Inhibition is a Novel Therapeutic Strategy to Prevent Diabetic Cardiomyopathy

Increasing prevalence of diabetes mellitus worldwide has pushed the complex disease state to the foreground of biomedical research, especially concerning its multifaceted impacts on the cardiovascular system. Current therapies for diabetic cardiomyopathy have had a positive impact, but with diabetic patients still suffering from a significantly greater burden of cardiac pathology compared to the general population, the need for novel therapeutic approaches is great. A new therapeutic target, calcium/calmodulin-dependent kinase II (CaMKII), has emerged as a potential treatment option for preventing cardiac dysfunction in the setting of diabetes. Within the last 10 years, new evidence has emerged describing the pathophysiological consequences of CaMKII activation in the diabetic heart, the mechanisms that underlie persistent CaMKII activation, and the protective effects of CaMKII inhibition to prevent diabetic cardiomyopathy. This review will examine recent evidence tying cardiac dysfunction in diabetes to CaMKII activation. It will then discuss the current understanding of the mechanisms by which CaMKII activity is enhanced during diabetes. Finally, it will examine the benefits of CaMKII inhibition to treat diabetic cardiomyopathy, including contractile dysfunction, heart failure with preserved ejection fraction, and arrhythmogenesis. We intend this review to serve as a critical examination of CaMKII inhibition as a therapeutic strategy, including potential drawbacks of this approach.

Electrical Features of the Diabetic Myocardium. Arrhythmic and Cardiovascular Safety Considerations in Diabetes

Diabetes is a chronic metabolic disease characterized by hyperglycemia in the absence of treatment. Among the diabetes-associated complications, cardiovascular disease is the major cause of mortality and morbidity in diabetic patients. Diabetes causes a complex myocardial dysfunction, referred as diabetic cardiomyopathy, which even in the absence of other cardiac risk factors results in abnormal diastolic and systolic function. Besides mechanical abnormalities, altered electrical function is another major feature of the diabetic myocardium. Both type 1 and type 2 diabetic patients often show cardiac electrical remodeling, mainly a prolonged ventricular repolarization visible in the electrocardiogram as a lengthening of the QT interval duration. The underlying mechanisms at the cellular level involve alterations on the expression and activity of several cardiac ion channels and their associated regulatory proteins. Consequent changes in sodium, calcium and potassium currents collectively lead to a delay in repolarization that can increase the risk of developing life-threatening ventricular arrhythmias and sudden death. QT duration correlates strongly with the risk of developing torsade de pointes, a form of ventricular tachycardia that can degenerate into ventricular fibrillation. Therefore, QT prolongation is a qualitative marker of proarrhythmic risk, and analysis of ventricular repolarization is therefore required for the approval of new drugs. To that end, the Thorough QT/QTc analysis evaluates QT interval prolongation to assess potential proarrhythmic effects. In addition, since diabetic patients have a higher risk to die from cardiovascular causes than individuals without diabetes, cardiovascular safety of the new antidiabetic drugs must be carefully evaluated in type 2 diabetic patients. These cardiovascular outcome trials reveal that some glucose-lowering drugs actually reduce cardiovascular risk. The mechanism of cardioprotection might involve a reduction of the risk of developing arrhythmia.

The Molecular Mechanisms of Cardiotoxicity Induced by HER2, VEGF, and Tyrosine Kinase Inhibitors: an Updated Review

AIM

In recent decades, there has been a revolutionary decrease in cancer-related mortality and an increase in survival due to the introduction of novel targeted drugs. Nevertheless, drugs targeting human epidermal growth factor receptor 2 (HER-2), angiogenesis, and other tyrosine kinases also come with unexpected cardiac side effects, including heart failure, hypertension, arterial thrombosis, and arrhythmias, and have mechanisms that are unlike those of classic chemotherapeutic agents. In addition, it is challenging to address some problems, as the existing guidelines need to be more specific, and further large-scale clinical trials and experimental studies are required to confirm the benefit of administering cardioprotective agents to patients treated with targeted therapies. Therefore, an improved understanding of cardiotoxicity becomes increasingly important to minimize the pernicious effects and maximize the beneficial effects of targeted agents.

METHODS

"Cardiotoxicity", "targeted drugs", "HER2", "trastuzumab", "angiogenesis inhibitor", "VEGF inhibitor" and "tyrosine kinase inhibitors" are used as keywords for article searches.

RESULTS

In this article, we report several targeted therapies that induce cardiotoxicity and update knowledge of the clinical evidence, molecular mechanisms, and management measures.

Cardiovascular toxicity of PI3Kα inhibitors

The phosphoinositide 3-kinases (PI3Ks) are a family of intracellular lipid kinases that phosphorylate the 3'-hydroxyl group of inositol membrane lipids, resulting in the production of phosphatidylinositol 3,4,5-trisphosphate from phosphatidylinositol 4,5-bisphosphate. This results in downstream effects, including cell growth, proliferation, and migration. The heart expresses three PI3K class I enzyme isoforms (α, β, and γ), and these enzymes play a role in cardiac cellular survival, myocardial hypertrophy, myocardial contractility, excitation, and mechanotransduction. The PI3K pathway is associated with various disease processes but is particularly important to human cancers since many gain-of-function mutations in this pathway occur in various cancers. Despite the development, testing, and regulatory approval of PI3K inhibitors in recent years, there are still significant challenges when creating and utilizing these drugs, including concerns of adverse effects on the heart. There is a growing body of evidence from preclinical studies revealing that PI3Ks play a crucial cardioprotective role, and thus inhibition of this pathway could lead to cardiac dysfunction, electrical remodeling, vascular damage, and ultimately, cardiovascular disease. This review will focus on PI3Kα, including the mechanisms underlying the adverse cardiovascular effects resulting from PI3Kα inhibition and the potential clinical implications of treating patients with these drugs, such as increased arrhythmia burden, biventricular cardiac dysfunction, and impaired recovery from cardiotoxicity. Recommendations for future directions for preclinical and clinical work are made, highlighting the possible role of PI3Kα inhibition in the progression of cancer-related cachexia and female sex and pre-existing comorbidities as independent risk factors for cardiac abnormalities after cancer treatment.

Diabetes-induced changes in cardiac voltage-gated ion channels

Diabetes mellitus affects the heart through various mechanisms such as microvascular defects, metabolic abnormalities, autonomic dysfunction and incompatible immune response. Furthermore, it can also cause functional and structural changes in the myocardium by a disease known as diabetic cardiomyopathy (DCM) in the absence of coronary artery disease. As DCM progresses it causes electrical remodeling of the heart, left ventricular dysfunction and heart failure. Electrophysiological changes in the diabetic heart contribute significantly to the incidence of arrhythmias and sudden cardiac death in diabetes mellitus patients. In recent studies, significant changes in repolarizing K⁺ currents, Na⁺ currents and L-type Ca²⁺ currents along with impaired Ca²⁺ homeostasis and defective contractile function have been identified in the diabetic heart. In addition, insulin levels and other trophic factors change significantly to maintain the ionic channel expression in diabetic patients. There are many diagnostic tools and management options for DCM, but it is difficult to detect its development and to effectively prevent its progress. In this review, diabetes-associated alterations in voltage-sensitive cardiac ion channels are comprehensively assessed to understand their potential role in the pathophysiology and pathogenesis of DCM.

Pathophysiology and therapeutic relevance of PI3K(p110α) protein in atrial fibrillation: A non-interventional molecular therapy strategy

Genetically modified animal studies have revealed specific expression patterns and unequivocal roles of class I PI3K isoenzymes. PI3K(p110α), a catalytic subunit of class I PI3Ks is ubiquitously expressed and is well characterised in the cardiovascular system. Given that genetic inhibition of PI3K(p110α) causes lethal phenotype embryonically, the catalytic subunit is critically important in housekeeping and biological processes. A growing number of studies underpin crucial roles of PI3K(p110α) in cell survival, proliferation, hypertrophy and arrhythmogenesis. While the studies provide great insights, the precise mechanisms involved in PI3K(p110α) hypofunction and atrial fibrillation (AF) are not fully known. AF is a well recognised clinical problem with significant management limitations. In this translational review, we attempted a narration of PI3K(p110α) hypofunction in the molecular basis of AF pathophysiology. We sought to cautiously highlight the relevance of this molecule in the therapeutic approaches for AF management per se (i.e without conditions associate with cell proliferation, like cancer), and in mitigating effects of clinical risk factors in atrial substrate formation leading to AF progression. We also considered PI3K(p110α) in AF gene association, with the aim of identifying mechanistic links between the ever increasingly well-defined genetic loci (regions and genes) and AF. Such mechanisms will aid in identifying new drug targets for arrhythmogenic substrate and AF.

Multiple daily insulin injections ameliorate QT interval by lowering blood glucose levels in patients with type 2 diabetes

BACKGROUND

A prolonged QT interval plays a causal role in fatal arrhythmia and is known to be a risk factor for sudden cardiac death. Although diabetic patients with microvascular complications tend to have a longer QT interval, the therapeutic effect of diabetes on the QT interval remains unclear. Here, we assessed the changes in QT interval in patients with type 2 diabetes (T2D) who received multiple daily insulin injections.

MATERIALS AND METHODS

Patients with T2D (n = 34) who were admitted to our hospital and initiated multiple daily insulin injections for glycemic control were enrolled in this study. Clinical measurements, including electrocardiogram, were taken on admission and discharge. The QT interval was measured manually in lead II on the electrocardiogram, and corrected QT interval (QTc) was calculated using Bazett's formula. The change in QTc (ΔQTc) during hospitalization (median, 15 days) and clinical parameters affecting ΔQTc were investigated.

RESULTS

QTc was shortened from 439 ± 24 to 427 ± 26 ms during hospitalization (p < 0.0001). ΔQTc was positively correlated with the changes in fasting plasma glucose (ΔFPG, r = 0.55, p = 0.0008) and glycated albumin (r = 0.38, p = 0.026) following insulin therapy, but not with the final dose of insulin (r = -0.20, p = 0.26). The multiple regression analyses revealed that ΔFPG was independently associated with ΔQTc.

CONCLUSIONS

Multiple daily insulin injections can ameliorate QT interval by lowering the blood glucose levels in T2D, suggesting that glycemic control is important for preventing patients with T2D from sudden cardiac death.

Mechanisms of ranolazine pretreatment in preventing ventricular tachyarrhythmias in diabetic db/db mice with acute regional ischemia-reperfusion injury

Studies have demonstrated that diabetic (db/db) mice have increased susceptibility to myocardial ischemia-reperfusion (IR) injury and ventricular tachyarrhythmias (VA). We aimed to investigate the antiarrhythmic and molecular mechanisms of ranolazine in db/db mouse hearts with acute IR injury. Ranolazine was administered for 1 week before coronary artery ligation. Diabetic db/db and control db/+ mice were divided into ranolazine-given and -nongiven groups. IR model was created by 15-min left coronary artery ligation and 10-min reperfusion. In vivo electrophysiological studies showed that the severity of VA inducibility was higher in db/db mice than control (db/ +) mice. Ranolazine suppressed the VA inducibility and severity. Optical mapping studies in Langendorff-perfused hearts showed that ranolazine significantly shortened action potential duration, Ca_i transient duration, Ca_i decay time, ameliorated conduction inhomogeneity, and suppressed arrhythmogenic alternans induction. Western blotting studies showed that the expression of pThr¹⁷-phospholamban, calsequestrin 2 and voltage-gated sodium channel in the IR zone was significantly downregulated in db/db mice, which was ameliorated with ranolazine pretreatment and might play a role in the anti-arrhythmic actions of ranolazine in db/db mouse hearts with IR injury.

Mechanisms of Chronic Metabolic Stress in Arrhythmias

Cardiac arrhythmias are responsible for many cardiovascular disease-related deaths worldwide. While arrhythmia pathogenesis is complex, there is increasing evidence for metabolic causes. Obesity, diabetes, and chronically consuming high-fat foods significantly increase the likelihood of developing arrhythmias. Although these correlations are well established, mechanistic explanations connecting a high-fat diet (HFD) to arrhythmogenesis are incomplete, although oxidative stress appears to be critical. This review investigates the metabolic changes that occur in obesity and after HFD. Potential therapies to prevent or treat arrhythmias are discussed, including antioxidants.

Electrical and structural remodeling contribute to atrial fibrillation in type 2 diabetic db/db mice

BACKGROUND

Atrial fibrillation (AF) is highly prevalent in diabetes mellitus (DM), yet the basis for this finding is poorly understood. Type 2 DM may be associated with unique patterns of atrial electrical and structural remodeling; however, this has not been investigated in detail.

OBJECTIVE

The purpose of this study was to investigate AF susceptibility and atrial electrical and structural remodeling in type 2 diabetic db/db mice.

METHODS

AF susceptibility and atrial function were assessed in male and female db/db mice and age-matched wildtype littermates. Electrophysiological studies were conducted in vivo using intracardiac electrophysiology and programmed stimulation. Atrial electrophysiology was also investigated in isolated atrial preparations using high-resolution optical mapping and in isolated atrial myocytes using patch-clamping. Molecular biology studies were performed using quantitative polymerase chain reaction and western blotting. Atrial fibrosis was assessed using histology.

RESULTS

db/db mice were highly susceptible to AF in association with reduced atrial conduction velocity, action potential duration prolongation, and increased heterogeneity in repolarization in left and right atria. In db/db mice, atrial K⁺ currents, including the transient outward current (I_to) and the ultrarapid delayed rectifier current (I_Kur), were reduced. The reduction in I_to occurred in association with reductions in Kcnd2 mRNA expression and K_V4.2 protein levels. The reduction in I_Kur was not related to gene or protein expression changes. Interstitial atrial fibrosis was increased in db/db mice.

CONCLUSION

Our study demonstrates that increased susceptibility to AF in db/db mice occurs in association with impaired electrical conduction as well as electrical and structural remodeling of the atria.

PI3Kα in cardioprotection: Cytoskeleton, late Na+ current, and mechanism of arrhythmias

PI 3-kinase α (PI3Kα) is a lipid kinase that converts phosphatidylinositol-4,5-bisphosphate (PIP2) to phosphatidylinositol-3,4,5-triphosphate (PIP3). PI3Kα regulates a variety of cellular processes such as nutrient sensing, cell cycle, migration, and others. Heightened activity of PI3Kα in many types of cancer made it a prime oncology drug target, but also raises concerns of possible adverse effects on the heart. Indeed, recent advances in preclinical models demonstrate an important role of PI3Kα in the control of cytoskeletal integrity, Na⁺ channel activity, cardioprotection, and prevention of arrhythmias.

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.

A current understanding of drug-induced QT prolongation and its implications for anticancer therapy

The QT interval, a global index of ventricular repolarization, varies among individuals and is influenced by diverse physiologic and pathophysiologic stimuli such as gender, age, heart rate, electrolyte concentrations, concomitant cardiac disease, and other diseases such as diabetes. Many drugs produce a small but reproducible effect on QT interval but in rare instances this is exaggerated and marked QT prolongation can provoke the polymorphic ventricular tachycardia 'torsades de pointes', which can cause syncope or sudden cardiac death. The generally accepted common mechanism whereby drugs prolong QT is block of a key repolarizing potassium current in heart, IKr, generated by expression of KCNH2, also known as HERG. Thus, evaluation of the potential that a new drug entity may cause torsades de pointes has relied on exposure of normal volunteers or patients to drug at usual and high concentrations, and on assessment of IKr block in vitro. More recent work, focusing on anticancer drugs with QT prolonging liability, is defining new pathways whereby drugs can prolong QT. Notably, the in vitro effects of some tyrosine kinase inhibitors to prolong cardiac action potentials (the cellular correlate of QT) can be rescued by intracellular phosphatidylinositol 3,4,5-trisphosphate, the downstream effector of phosphoinositide 3-kinase. This finding supports a role for inhibition of this enzyme, either directly or by inhibition of upstream kinases, to prolong QT through mechanisms that are being worked out, but include enhanced inward 'late' sodium current during the plateau of the action potential. The definition of non-IKr-dependent pathways to QT prolongation will be important for assessing risk, not only with anticancer therapies but also with other QT prolonging drugs and for generating a refined understanding how variable activity of intracellular signalling systems can modulate QT and associated arrhythmia risk.

From the gut to the heart: L. plantarum and inulin administration as a novel approach to control cardiac apoptosis via 5-HT2B and TrkB receptors in diabetes

BACKGROUND & AIMS

Type 2 diabetes mellitus, as a metabolic disorder, can lead to diabetic cardiomyopathy, identified by cardiomyocyte apoptosis and myocardial fibrosis. Brain-derived neurotrophic factor (BDNF) and serotonin are two neurotransmitters that can control cardiomyocyte apoptosis and myocardial fibrosis through their cardiac receptors. In the present study, we investigated the impacts of L. plantarum and inulin supplementation on the inhibition of cardiac apoptosis and fibrosis by modulating intestinal, serum, and cardiac levels of serotonin and BDNF as well as their cardiac receptors.

METHODS

Diabetes was induced by a high-fat diet and streptozotocin in male Wistar rats. Rats were divided into six groups and were supplemented with L. plantarum, inulin or their combination for 8 weeks. Finally, the rats were killed and levels of intestinal, serum, and cardiac parameters were evaluated.

RESULTS

Concurrent administration of L. plantarum and inulin caused a significant rise in the expression of cardiac serotonin and BDNF receptors (P < 0.001) as well as a significant fall in cardiac interstitial and perivascular fibrosis (P < 0.001, both) and apoptosis (P = 0.01). Moreover, there was a strong correlation of cardiac 5-Hydroxytryptamine 2B (5-HT2B) and tropomyosin receptor kinase B (TrkB) receptors with interstitial/perivascular fibrosis and apoptosis (P < 0.001, both).

CONCLUSIONS/INTERPRETATION

Results revealed beneficial effects of L. plantarum, inulin or their combination on intestinal, serum, and cardiac serotonin and BDNF accompanied by higher expression of their cardiac receptors and lower levels of cardiac apoptotic and fibrotic markers. It seems that L. plantarum and inulin supplementation could be considered as a novel adjunct therapy to reduce cardiac complications of type 2 diabetes mellitus.

Loss of insulin signaling may contribute to atrial fibrillation and atrial electrical remodeling in type 1 diabetes

Atrial fibrillation (AF) is prevalent in diabetes mellitus (DM); however, the basis for this is unknown. This study investigated AF susceptibility and atrial electrophysiology in type 1 diabetic Akita mice using in vivo intracardiac electrophysiology, high-resolution optical mapping in atrial preparations, and patch clamping in isolated atrial myocytes. qPCR and western blotting were used to assess ion channel expression. Akita mice were highly susceptible to AF in association with increased P-wave duration and slowed atrial conduction velocity. In a second model of type 1 DM, mice treated with streptozotocin (STZ) showed a similar increase in susceptibility to AF. Chronic insulin treatment reduced susceptibility and duration of AF and shortened P-wave duration in Akita mice. Atrial action potential (AP) morphology was altered in Akita mice due to a reduction in upstroke velocity and increases in AP duration. In Akita mice, atrial Na⁺ current (I_Na) and repolarizing K⁺ current (I_K) carried by voltage gated K⁺ (K_v1.5) channels were reduced. The reduction in I_Na occurred in association with reduced expression of SCN5a and voltage gated Na⁺ (Na_V1.5) channels as well as a shift in I_Na activation kinetics. Insulin potently and selectively increased I_Na in Akita mice without affecting I_K Chronic insulin treatment increased I_Na in association with increased expression of Na_V1.5. Acute insulin also increased I_Na, although to a smaller extent, due to enhanced insulin signaling via phosphatidylinositol 3,4,5-triphosphate (PIP₃). Our study reveals a critical, selective role for insulin in regulating atrial I_Na, which impacts susceptibility to AF in type 1 DM.

Gene therapy targeting cardiac phosphoinositide 3-kinase (p110α) attenuates cardiac remodeling in type 2 diabetes

Diabetic cardiomyopathy is a distinct form of heart disease that represents a major cause of death and disability in diabetic patients, particularly, the more prevalent type 2 diabetes patient population. In the current study, we investigated whether administration of recombinant adeno-associated viral vectors carrying a constitutively active phosphoinositide 3-kinase (PI3K)(p110α) construct (rAAV6-caPI3K) at a clinically relevant time point attenuates diabetic cardiomyopathy in a preclinical type 2 diabetes (T2D) model. T2D was induced by a combination of a high-fat diet (42% energy intake from lipid) and low-dose streptozotocin (three consecutive intraperitoneal injections of 55 mg/kg body wt), and confirmed by increased body weight, mild hyperglycemia, and impaired glucose tolerance (all P < 0.05 vs. nondiabetic mice). After 18 wk of untreated diabetes, impaired left ventricular (LV) systolic dysfunction was evident, as confirmed by reduced fractional shortening and velocity of circumferential fiber shortening (Vcf_c, all P < 0.01 vs. baseline measurement). A single tail vein injection of rAAV6-caPI3K gene therapy (2×10¹¹vector genomes) was then administered. Mice were followed for an additional 8 wk before end point. A single injection of cardiac targeted rAAV6-caPI3K attenuated diabetes-induced cardiac remodeling by limiting cardiac fibrosis (reduced interstitial and perivascular collagen deposition, P < 0.01 vs. T2D mice) and cardiomyocyte hypertrophy (reduced cardiomyocyte size and Nppa gene expression, P < 0.001 and P < 0.05 vs. T2D mice, respectively). The diabetes-induced LV systolic dysfunction was reversed with rAAV6-caPI3K, as demonstrated by improved fractional shortening and velocity of circumferential fiber shortening (all P < 0.05 vs pre-AAV measurement). This cardioprotection occurred in combination with reduced LV reactive oxygen species (P < 0.05 vs. T2D mice) and an associated decrease in markers of endoplasmic reticulum stress (reduced Grp94 and Chop, all P < 0.05 vs. T2D mice). Together, our findings demonstrate that a cardiac-selective increase in PI3K(p110α), via rAAV6-caPI3K, attenuates T2D-induced diabetic cardiomyopathy, providing proof of concept for potential translation to the clinic.NEW & NOTEWORTHY Diabetes remains a major cause of death and disability worldwide (and its resultant heart failure burden), despite current care. The lack of existing management of heart failure in the context of the poorer prognosis of concomitant diabetes represents an unmet clinical need. In the present study, we now demonstrate that delayed intervention with PI3K gene therapy (rAAV6-caPI3K), administered as a single dose in mice with preexisting type 2 diabetes, attenuates several characteristics of diabetic cardiomyopathy, including diabetes-induced impairments in cardiac remodeling, oxidative stress, and function. Our discovery here contributes to the previous body of work, suggesting the cardioprotective effects of PI3K(p110α) could be a novel therapeutic approach to reduce the progression to heart failure and death in diabetes-affected patients.

Altered mitochondrial metabolism in the insulin-resistant heart

Obesity-induced insulin resistance and type 2 diabetes mellitus can ultimately result in various complications, including diabetic cardiomyopathy. In this case, cardiac dysfunction is characterized by metabolic disturbances such as impaired glucose oxidation and an increased reliance on fatty acid (FA) oxidation. Mitochondrial dysfunction has often been associated with the altered metabolic function in the diabetic heart, and may result from FA-induced lipotoxicity and uncoupling of oxidative phosphorylation. In this review, we address the metabolic changes in the diabetic heart, focusing on the loss of metabolic flexibility and cardiac mitochondrial function. We consider the alterations observed in mitochondrial substrate utilization, bioenergetics and dynamics, and highlight new areas of research which may improve our understanding of the cause and effect of cardiac mitochondrial dysfunction in diabetes. Finally, we explore how lifestyle (nutrition and exercise) and pharmacological interventions can prevent and treat metabolic and mitochondrial dysfunction in diabetes.

Regulation of heart rate and the pacemaker current by phosphoinositide 3-kinase signaling

Heart rate is set by the specialized tissue of the sinoatrial node. Lin et al. demonstrate a novel role for phosphoinositide 3-kinase in regulating cardiac pacemaking currents independently of the autonomic nervous system, a finding with relevance for diabetes, heart disease, and cancer.

Heart rate in physiological conditions is set by the sinoatrial node (SN), the primary cardiac pacing tissue. Phosphoinositide 3-kinase (PI3K) signaling is a major regulatory pathway in all normal cells, and its dysregulation is prominent in diabetes, cancer, and heart failure. Here, we show that inhibition of PI3K slows the pacing rate of the SN in situ and in vitro and reduces the early slope of diastolic depolarization. Furthermore, inhibition of PI3K causes a negative shift in the voltage dependence of activation of the pacemaker current, I_F, while addition of its second messenger, phosphatidylinositol 3,4,5-trisphosphate, induces a positive shift. These shifts in the activation of I_F are independent of, and larger than, those induced by the autonomic nervous system. These results suggest that PI3K is an important regulator of heart rate, and perturbations in this signaling pathway may contribute to the development of arrhythmias.

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.

Inhibition of PI3Kinase-α is pro-arrhythmic and associated with enhanced late Na+ current, contractility, and Ca2+ release in murine hearts

BACKGROUND

Phosphoinositide 3-kinase α (PI3Kα) is a proto-oncogene with high activity in the heart. BYL719 (BYL) is a PI3Kα-selective small molecule inhibitor and a prospective drug for advanced solid tumors. We investigated whether acute pharmacological inhibition of PI3Kα has pro-arrhythmic effects.

METHODS & RESULTS

In isolated wild-type (WT) cardiomyocytes, pharmacological inhibition of PI3Kα (BYL719) increased contractility by 28%, Ca²⁺ release by 20%, and prolonged action potential (AP) repolarization by 10-15%. These effects of BYL719 were abolished by inhibition of reverse-mode Na⁺/Ca²⁺ exchanger (NCX) (KB-R7943) or by inhibition of late Na⁺ current (I_Na-L) (ranolazine). BYL719 had no effect on PI3Kα-deficient cardiomyocytes, suggesting BYL719 effects were PI3Kα-dependent and mediated via NCX and I_Na-L. I_Na-L was suppressed by activation of PI3Kα, application of exogenous intracellular PIP3, or ranolazine. Investigation of AP and Ca²⁺ release in whole heart preparations using epicardial optical mapping showed that inhibition of PI3Kα similarly led to prolongation of AP and enhancement of Ca²⁺ release. In hearts of PI3Kα-deficient mice, β-adrenergic stimulation in the presence of high Ca²⁺ concentrations and 12-Hz burst pacing led to delayed afterdepolarizations and ventricular fibrillation. In vivo, administration of BYL719 prolonged QT interval [QT_cF (Fridericia) increased by 15%] in WT, but not in PI3Kα-deficient mice.

CONCLUSIONS

Pharmacological inhibition of PI3Kα is arrhythmogenic due to activation of I_Na-L leading to increased sarcoplasmic reticulum Ca²⁺ load and prolonged QT interval. Therefore, monitoring of cardiac electrical activity in patients receiving PI3K inhibitors may provide further insights into the arrhythmogenic potential of PI3Ka inhibition.

Phosphorylation of cardiac voltage-gated sodium channel: Potential players with multiple dimensions

Cardiomyocytes are highly coordinated cells with multiple proteins organized in micro domains. Minor changes or interference in subcellular proteins can cause major disturbances in physiology. The cardiac sodium channel (Na_V1.5) is an important determinant of correct electrical activity in cardiomyocytes which are localized at intercalated discs, T‐tubules and lateral membranes in the form of a macromolecular complex with multiple interacting protein partners. The channel is tightly regulated by post‐translational modifications for smooth conduction and propagation of action potentials. Among regulatory mechanisms, phosphorylation is an enzymatic and reversible process which modulates Na_V1.5 channel function by attaching phosphate groups to serine, threonine or tyrosine residues. Phosphorylation of Na_V1.5 is implicated in both normal physiological and pathological processes and is carried out by multiple kinases. In this review, we discuss and summarize recent literature about the (a) structure of Na_V1.5 channel, (b) formation and subcellular localization of Na_V1.5 channel macromolecular complex, (c) post‐translational phosphorylation and regulation of Na_V1.5 channel, and (d) how these phosphorylation events of Na_V1.5 channel alter the biophysical properties and affect the channel during disease status. We expect, by reviewing these aspects will greatly improve our understanding of Na_V1.5 channel biology, physiology and pathology, which will also provide an insight into the mechanism of arrythmogenesis at molecular level.

The Association Between Diabetes Mellitus and Atrial Fibrillation: Clinical and Mechanistic Insights

A number of clinical studies have reported that diabetes mellitus (DM) is an independent risk factor for Atrial fibrillation (AF). After adjustment for other known risk factors including age, sex, and cardiovascular risk factors, DM remains a significant if modest risk factor for development of AF. The mechanisms underlying the increased susceptibility to AF in DM are incompletely understood, but are thought to involve electrical, structural, and autonomic remodeling in the atria. Electrical remodeling in DM may involve alterations in gap junction function that affect atrial conduction velocity due to changes in expression or localization of connexins. Electrical remodeling can also occur due to changes in atrial action potential morphology in association with changes in ionic currents, such as sodium or potassium currents, that can affect conduction velocity or susceptibility to triggered activity. Structural remodeling in DM results in atrial fibrosis, which can alter conduction patterns and susceptibility to re-entry in the atria. In addition, increases in atrial adipose tissue, especially in Type II DM, can lead to disruptions in atrial conduction velocity or conduction patterns that may affect arrhythmogenesis. Whether the insulin resistance in type II DM activates unique intracellular signaling pathways independent of obesity requires further investigation. In addition, the relationship between incident AF and glycemic control requires further study.

Disease Modifiers of Inherited SCN5A Channelopathy

To date, a large number of mutations in SCN5A, the gene encoding the pore-forming α-subunit of the primary cardiac Na⁺ channel (Na_V1.5), have been found in patients presenting with a wide range of ECG abnormalities and cardiac syndromes. Although these mutations all affect the same Na_V1.5 channel, the associated cardiac syndromes each display distinct phenotypical and biophysical characteristics. Variable disease expressivity has also been reported, where one particular mutation in SCN5A may lead to either one particular symptom, a range of various clinical signs, or no symptoms at all, even within one single family. Additionally, disease severity may vary considerably between patients carrying the same mutation. The exact reasons are unknown, but evidence is increasing that various cardiac and non-cardiac conditions can influence the expressivity and severity of inherited SCN5A channelopathies. In this review, we provide a summary of identified disease entities caused by SCN5A mutations, and give an overview of co-morbidities and other (non)-genetic factors which may modify SCN5A channelopathies. A comprehensive knowledge of these modulatory factors is not only essential for a complete understanding of the diverse clinical phenotypes associated with SCN5A mutations, but also for successful development of effective risk stratification and (alternative) treatment paradigms.

Inhibition of the α-Subunit of Phosphoinositide 3-Kinase in Heart Increases Late Sodium Current and Is Arrhythmogenic

Although inhibition of phosphoinositide 3-kinase (PI3K) is an emerging strategy in cancer therapy, we and others have reported that this action can also contribute to drug-induced QT prolongation and arrhythmias by increasing cardiac late sodium current (I_NaL). Previous studies in mice implicate the PI3K-α isoform in arrhythmia susceptibility. Here, we have determined the effects of new anticancer drugs targeting specific PI3K isoforms on I_NaL and action potentials (APs) in mouse cardiomyocytes and Chinese hamster ovary cells (CHO). Chronic exposure (10-100 nM; 5-48 hours) to PI3K-α-specific subunit inhibitors BYL710 (alpelisib) and A66 and a pan-PI3K inhibitor (BKM120) increased I_NaL in SCN5A-transfected CHO cells and mouse cardiomyocytes. The specific inhibitors (10-100 nM for 5 hours) markedly prolonged APs and generated triggered activity in mouse cardiomyocytes (9/12) but not in controls (0/6), and BKM120 caused similar effects (3/6). The inclusion of water-soluble PIP3, a downstream effector of the PI3K signaling pathway, in the pipette solution reversed these arrhythmogenic effects. By contrast, inhibition of PI3K-β, -γ, and -δ isoforms did not alter I_NaL or APs. We conclude that inhibition of cardiac PI3K-α is arrhythmogenic by increasing I_NaL and this effect is not seen with inhibition of other PI3K isoforms. These results highlight a mechanism underlying potential cardiotoxicity of PI3K-α inhibitors.

The Pattern of mRNA Expression Is Changed in Sinoatrial Node from Goto-Kakizaki Type 2 Diabetic Rat Heart

BACKGROUND

In vivo experiments in Goto-Kakizaki (GK) type 2 diabetic rats have demonstrated reductions in heart rate from a young age. The expression of genes encoding more than 70 proteins that are associated with the generation and conduction of electrical activity in the GK sinoatrial node (SAN) have been evaluated to further clarify the molecular basis of the low heart rate.

MATERIALS AND METHODS

Heart rate and expression of genes were evaluated with an extracellular electrode and real-time RT-PCR, respectively. Rats aged 12-13 months were employed in these experiments.

RESULTS

Isolated spontaneous heart rate was reduced in GK heart (161 ± 12 bpm) compared to controls (229 ± 11 bpm). There were many differences in expression of mRNA, and some of these differences were of particular interest. Compared to control SAN, expression of some genes were downregulated in GK-SAN: gap junction, Gja1 (Cx43), Gja5 (Cx40), Gjc1 (Cx45), and Gjd3 (Cx31.9); cell membrane transport, Trpc1 (TRPC1) and Trpc6 (TRPC6); hyperpolarization-activated cyclic nucleotide-gated channels, Hcn1 (HCN1) and Hcn4 (HCN4); calcium channels, Cacna1d (Ca_v1.3), Cacna1g (Ca_v3.1), Cacna1h (Ca_v3.2), Cacna2d1 (Ca_vα2δ1), Cacna2d3 (Ca_vα2δ3), and Cacng4 (Cav _γ 4); and potassium channels, Kcna2 (K_v1.2), Kcna4 (K_v1.4), Kcna5 (K_v1.5), Kcnb1 (K_v2.1), Kcnd3 (K_v4.3), Kcnj2 (K_ir2.1), Kcnk1 (TWIK1), Kcnk5 (K_2P5.1), Kcnk6 (TWIK2), and Kcnn2 (SK2) whilst others were upregulated in GK-SAN: Ryr2 (RYR2) and Nppb (BNP).

CONCLUSIONS

This study provides new insight into the changing expression of genes in the sinoatrial node of diabetic heart.

Necessity to evaluate PI3K/Akt signalling pathway in proarrhythmia

The incidence of QT prolongation and torsades de pointes is on the rise due to the use of cardiovascular and non-cardiovascular drugs. Robust efforts have been made and are still ongoing to understand the underlying mechanisms that can enhance or prevent the development of drug-induced proarrhythmia. A caveat in the use of antiarrhythmic drugs is the ability to obtain safe action potential prolongation therapeutic effects, through IKr blockade. This remains as yet completely unachievable, as blockers of the potassium channel have not provided complete safe measures. Because of this, efforts at understanding the mechanisms of proarrhythmia have continued. PI3K/Akt signalling pathway appears to possess some potential advantage in this regard because cardiomyocytes intracellular dialysis with phosphatidylinositol (3,4,5)-trisphosphate (PIP3) normalises ion channel alterations and eliminates proarrhythmic features. However, there is a conundrum. Increased activities of PIP3 signalling can enhance cell proliferation and survival, and reduced activities of PIP3 signalling can lead to proarrhythmia. PI3K inhibitors used in cancer treatment have been found to cause proarrhythmia, and represent a potential avenue for the research and evaluation of potential effectiveness of a battery of antiarrhythmic and cancer drugs that are either currently in use or in development. Despite this knowledge, limited information is available on PI3K/Akt signalling and arrhythmogenesis. This highlights the need to search for new ways to improve testing of antiarrhythmic drugs and increase our understanding in PI3K/Akt signalling and arrhythmogenesis.

Incidence of prolonged QTc and severe hypoglycemia in type 1 diabetes: the EURODIAB Prospective Complications Study

AIMS

To assess the independent role of severe hypoglycemia on 7-year cumulative incidence of prolonged QTc in a large cohort of patients with type 1 diabetes.

METHODS

People with type 1 diabetes recruited by the EURODIAB Prospective Complications Study who had normal QTc were examined at baseline and after 7 years with standardized methods (n = 1415; mean age ± SD 32.1 ± 9.6 years; diabetes duration 14.2 ± 8.8 years). Hypoglycemic episodes were assessed by a questionnaire. QTc was calculated according to Bazett's formula. In logistic regression analysis, we examined the role of severe hypoglycemia (none, 1-2, or 3 and more episodes/year) on the cumulative incidence of prolonged QTc, independently of age, sex, HbA1c, blood pressure, BMI, physical activity, distal symmetrical and autonomic neuropathy.

RESULTS

In total, 264/1415 (17%) patients had incident prolonged QTc. Compared to those with persistently normal QTc, a greater proportion of incident cases had 3 and more hypoglycemic episodes at baseline (16.3 vs 11.2%, p = 0.03) and after 7 years (15.2 vs 9.6%, p = 0.01). In logistic regression analysis, 3 or more episodes of severe hypoglycemia at baseline did not increase cumulative incidence of prolonged QTc (OR 1.34, 95% CI 0.88-2.03). By contrast, severe hypoglycemia at the follow-up examination was associated with higher incidence of QTc prolongation (OR 1.68, 1.09-2.58), which reverted to not significant after adjustment for diabetic neuropathy.

CONCLUSIONS

Severe hypoglycemia was not associated with incidence QTc prolongation in type 1 diabetic patients from the EURODIAB PCS.