4-Hydroxytamoxifen inhibits K(+) currents in mouse ventricular myocytes

Inhibition of the unfolded protein response reduces arrhythmic risk after myocardial infarction

Ischemic cardiomyopathy is associated with an increased risk of sudden death, activation of the unfolded protein response (UPR), and reductions in multiple cardiac ion channels. When activated, the protein kinase-like ER kinase (PERK) branch of the UPR reduces protein translation and abundance. We hypothesized that PERK inhibition could prevent ion channel downregulation and reduce arrhythmic risk after myocardial infarct (MI). MI induced by coronary artery ligation resulted in mice exhibited reduced ion channel levels, ventricular tachycardia (VT), and prolonged corrected intervals between the Q and T waves of the ECGs (QTc). Protein levels of major cardiac ion channels were decreased. MI cardiomyocytes showed significantly prolonged action potential duration and decreased maximum upstroke velocity. Cardiac-specific PERK knockout (PERKKO) reduced electrical remodeling in response to MI with shortened QTc intervals, less VT episodes, and higher survival rates (P<0.05 vs. MI). Pharmacological PERK inhibition had similar effects. In conclusion, activated PERK during MI contributed to arrhythmic risk by downregulation of select cardiac ion channels. PERK inhibition prevented these changes and reduced arrhythmic risk. These results suggest that ion channel downregulation during MI is a fundamental arrhythmic mechanism and maintaining ion channel levels is antiarrhythmic.

IL-18 mediates sickle cell cardiomyopathy and ventricular arrhythmias

Previous reports indicate that IL18 is a novel candidate gene for diastolic dysfunction in sickle cell disease (SCD)-related cardiomyopathy. We hypothesize that interleukin-18 (IL-18) mediates the development of cardiomyopathy and ventricular tachycardia (VT) in SCD. Compared with control mice, a humanized mouse model of SCD exhibited increased cardiac fibrosis, prolonged duration of action potential, higher VT inducibility in vivo, higher cardiac NF-κB phosphorylation, and higher circulating IL-18 levels, as well as reduced voltage-gated potassium channel expression, which translates to reduced transient outward potassium current (Ito) in isolated cardiomyocytes. Administering IL-18 to isolated mouse hearts resulted in VT originating from the right ventricle and further reduced Ito in SCD mouse cardiomyocytes. Sustained IL-18 inhibition via IL-18-binding protein resulted in decreased cardiac fibrosis and NF-κB phosphorylation, improved diastolic function, normalized electrical remodeling, and attenuated IL-18-mediated VT in SCD mice. Patients with SCD and either myocardial fibrosis or increased QTc displayed greater IL18 gene expression in peripheral blood mononuclear cells (PBMCs), and QTc was strongly correlated with plasma IL-18 levels. PBMC-derived IL18 gene expression was increased in patients who did not survive compared with those who did. IL-18 is a mediator of sickle cell cardiomyopathy and VT in mice and a novel therapeutic target in patients at risk for sudden death.

The Risk of QTc-Interval Prolongation in Breast Cancer Patients Treated with Tamoxifen in Combination with Serotonin Reuptake Inhibitors

PURPOSE

Antidepressants like the serotonin reuptake inhibitors (SRIs) are often used concomitantly with tamoxifen (e.g. for treatment of depression). This may lead to an additional prolongation of the QTc-interval, with an increased risk of cardiac side effects. Therefore we investigated whether there is a drug-drug interaction between tamoxifen and SRIs resulting in a prolonged QTc-interval.

METHODS

Electrocardiograms (ECGs) of 100 patients were collected at steady state tamoxifen treatment, with or without concomitant SRI co-medication. QTc-interval was manually measured and calculated using the Fridericia formula. Primary outcome was difference in QTc-interval between tamoxifen monotherapy and tamoxifen concomitantly with an SRI.

RESULTS

The mean QTc-interval was 12.4 ms longer when tamoxifen was given concomitantly with an SRI (95% CI:1.8-23.1 ms; P = 0.023). Prolongation of the QTc-interval was particularly pronounced for paroxetine (17.2 ms; 95%CI:1.4-33.0 ms; P = 0.04), escitalopram (12.5 ms; 95%CI:4.4-20.6 ms; P < 0.01) and citalopram (20.7 ms; 95%CI:0.7-40.7 ms; P = 0.047), where other agents like venlafaxine did not seem to prolong the QTc-interval. None of the patients had a QTc-interval of >500 ms.

CONCLUSIONS

Concomitant use of tamoxifen and SRIs resulted in a significantly higher mean QTc-interval, which was especially the case for paroxetine, escitalopram and citalopram. When concomitant administration with an SRI is warranted venlafaxine is preferred.

Anticancer drugs-related QTc prolongation, torsade de pointes and sudden death: current evidence and future research perspectives

Anticancer drugs may have proarrhythmic effects including drug-induced QT interval prolongation, which is of particular importance because it can lead to a fatal polymorphic ventricular tachycardia termed torsade de pointes (TdP). QT interval prolongation and TdP are rare life-threatening untoward effects of anticancer therapy, particularly with arsenic trioxides and anthracyclines, and even some novel molecular targeted drugs touted as 'tumor specific'. Several factors that affect myocardial repolarization can further increase the risk of TdP. This article reviews the mechanism of QT interval prolongation, risk factors for TdP and the QT toxicity of anticancer drugs as well as its management. Specific attention should be paid to high-risk populations such as patients with underlying heart diseases, electrolyte imbalance and bradycardia. To minimize the occurrence of QT interval prolongation and TdP, it is advisable to conduct a careful risk factor assessment before antitumor therapy. To this end, several new biomarkers have been introduced to predict TdP triggering and recent studies have pointed out the potential clinical relevance of genetic testing.

Mitochondrial Ca2+ Influx Contributes to Arrhythmic Risk in Nonischemic Cardiomyopathy

BACKGROUND

Heart failure (HF) is associated with increased arrhythmia risk and triggered activity. Abnormal Ca2+ handling is thought to underlie triggered activity, and mitochondria participate in Ca2+ homeostasis.

METHODS AND RESULTS

A model of nonischemic HF was induced in C57BL/6 mice by hypertension. Computer simulations were performed using a mouse ventricular myocyte model of HF. Isoproterenol-induced premature ventricular contractions and ventricular fibrillation were more prevalent in nonischemic HF mice than sham controls. Isolated myopathic myocytes showed decreased cytoplasmic Ca2+ transients, increased mitochondrial Ca2+ transients, and increased action potential duration at 90% repolarization. The alteration of action potential duration at 90% repolarization was consistent with in vivo corrected QT prolongation and could be explained by augmented L-type Ca2+ currents, increased Na+-Ca2+ exchange currents, and decreased total K+ currents. Of myopathic ventricular myocytes, 66% showed early afterdepolarizations (EADs) compared with 17% of sham myocytes (P<0.05). Intracellular application of 1 μmol/L Ru360, a mitochondrial Ca2+ uniporter-specific antagonist, could reduce mitochondrial Ca2+ transients, decrease action potential duration at 90% repolarization, and ameliorate EADs. Furthermore, genetic knockdown of mitochondrial Ca2+ uniporters inhibited mitochondrial Ca2+ uptake, reduced Na+-Ca2+ exchange currents, decreased action potential duration at 90% repolarization, suppressed EADs, and reduced ventricular fibrillation in nonischemic HF mice. Computer simulations showed that EADs promoted by HF remodeling could be abolished by blocking either the mitochondrial Ca2+ uniporter or the L-type Ca2+ current, consistent with the experimental observations.

CONCLUSIONS

Mitochondrial Ca2+ handling plays an important role in EADs seen with nonischemic cardiomyopathy and may represent a therapeutic target to reduce arrhythmic risk in this condition.

Activation of the unfolded protein response downregulates cardiac ion channels in human induced pluripotent stem cell-derived cardiomyocytes

RATIONALE

Heart failure is characterized by electrical remodeling that contributes to arrhythmic risk. The unfolded protein response (UPR) is active in heart failure and can decrease protein levels by increasing mRNA decay, accelerating protein degradation, and inhibiting protein translation.

OBJECTIVE

Therefore, we investigated whether the UPR downregulated cardiac ion channels that may contribute to arrhythmogenic electrical remodeling.

METHODS

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were used to study cardiac ion channels. Action potentials (APs) and ion channel currents were measured by patch clamp recording. The mRNA and protein levels of channels and the UPR effectors were determined by quantitative RT-PCR and Western blotting. Tunicamycin (TM, 50 ng/mL and 5 μg/mL), GSK2606414 (GSK, 300 nmol/L), and 4μ8C (5 μmol/L) were utilized to activate the UPR, inhibit protein kinase-like ER kinase (PERK) and inositol-requiring protein-1 (IRE1), respectively.

RESULTS

TM-induced activation of the UPR caused significant prolongation of the AP duration (APD) and a reduction of the maximum upstroke velocity (dV/dt_max) of the AP phase 0 in both acute (20-24 h) and chronic treatment (6 days). These changes were explained by reductions in the sodium, L-type calcium, the transient outward and rapidly/slowly activating delayed rectifier potassium currents. Na_v1.5, Ca_v1.2, K_v4.3, and K_vLQT1 channels showed concomitant reductions in mRNA and protein levels under activated UPR. Inhibition of PERK or IRE1 shortened the APD and reinstated dV/dt_max. The PERK branch regulated Na_v1.5, K_v4.3, hERG, and K_vLQT1. The IRE1 branch regulated Na_v1.5, hERG, K_vLQT1, and Ca_v1.2.

CONCLUSIONS

Activated UPR downregulates all major cardiac ion currents and results in electrical remodeling in hiPSC-CMs. Both PERK and IRE1 branches downregulate Na_v1.5, hERG, and K_vLQT1. The PERK branch specifically downregulates K_v4.3, while the IRE1 branch downregulates Ca_v1.2. Therefore, the UPR contributed to electrical remodeling, and targeting the UPR might be anti-arrhythmic.

Rapid estrogen actions on ion channels: A survey in search for mechanisms

A survey of nearly two hundred reports shows that rapid estrogenic actions can be detected across a range of kinds of estrogens, a range of doses, on a wide range of tissue, cell and ion channel types. Striking is the fact that preparations of estrogenic agents that do not permeate the cell membrane almost always mimic the actions of the estrogenic agents that do permeate the membrane. All kinds of estrogens, ranging from natural ones, through receptor modulators, endocrine disruptors, phytoestrogens, agonists, and antagonists to novel G-1 and STX, have been reported to be effective. For actions on specific types of ion channels, the possibility of opposing actions, in different cases, is the rule, not the exception. With this variety there is no single, specific action mechanism for estrogens per se, although in some cases estrogens can act directly or via some signaling pathways to affect ion channels. We infer that estrogens can bind a large number of substrates/receptors at the membrane surface. As against the variety of subsequent routes of action, this initial step of the estrogen's binding action is the key.

Long‑term treatment of spontaneously hypertensive rats with losartan and molecular basis of modulating Ito of ventricular myocytes

The present study aimed to determine the effect of chronic treatment with losartan on transient outward potassium current (Ito) and the expression levels of potassium voltage-gated channel subfamily D members 2 and 3 (Kv4.2 and 3) and voltage-gated potassium channel-interacting protein 2 (KChIP2) in rats. Spontaneously hypertensive (SH) rats and Wistar-Kyoto (WKY) rats were used in the study. The rats were divided into lo-SH and SH groups and los-WKY and WKY groups, respectively. Ito was recorded and expression levels of Kv4.2, Kv4.3 and KChIP2 were measured by western blot analysis and quantitative polymerase chain reaction. Ito current density was smaller in SH compared with WKY, los-WKY and los-SH groups (P<0.01). Inactivation time constant of myocytes was larger in SH compared with WKY, los-WKY and los-SH groups (P<0.01). The mean levels of mRNA and protein of Kv4.2 and Kv4.3 were significantly lower in the SH compared with WKY, los-WKY and los‑SH groups in vivo and in vitro (P<0.01). The Pearson statistical test showed no correlation between the expression levels of Kv4.2, Kv4.3, KChIP2 and the changes in blood pressure in the losartan treatment group. In conclusion, chronic blockade of angiotensin II type 1 receptors with losartan reversed SH rats' electrical remodeling and shortened action potential duration, which was associated with an increase in Ito density as the expression levels of Kv4.2, Kv4.3 increased and the expression levels of KChIP2 decreased. However, the expression levels of Kv4.2, Kv4.3 and KChIP2 were not correlated with the change in blood pressure in the losartan treatment group. Losartan may decrease the inactivation time by increasing the expression of KChIP2.

Voltage-dependent open-channel block of G protein-gated inward-rectifying K(+) (GIRK) current in rat atrial myocytes by tamoxifen

Tamoxifen (Tmx) is a nonsteroidal selective estrogen receptor antagonist and is frequently used in the treatment and prevention of breast cancer. The compound and its metabolites have been reported to inhibit functions of different classes of membrane proteins, including various ion channels. For members of the inward-rectifying K(+) (Kir) channel family, interference of Tmx with binding of phosphatidylinositol 4,5-bisphosphate (PIP(2)) has been suggested as the mechanism underlying such inhibition. We have studied the inhibition of G protein-activated K(+) (GIRK) current by Tmx in isolated myocytes from hearts of adult rats using whole-cell voltage clamp and experimental conditions for measuring K(+) currents as inward currents (E (K) -50 mV; holding potential -90 mV). Extracellular Tmx reversibly inhibited GIRK current activated by acetylcholine (I (K(ACh))) with an EC(50) of 7.4 × 10(-7) M. This inhibition was composed of two components, a basal reduction in peak current and a block that required opening of channels by ACh. The open-channel block was partially relieved by depolarizing voltage steps in a voltage- and time-dependent fashion. A voltage-dependent open-channel block was not observed when I (K(ACh)) was measured as outward current (E (K) -90 mV; holding potential -40 mV). Intracellular application of Tmx via the patch clamp pipette at a concentration (7 × 10(-6) M) that caused a rapid inhibition of I (K(ACh)) upon extracellular application did not affect the current. Intracellular application of the H(2)O-soluble PIP(2) analog diC(8)-PIP(2) reduced the voltage-independent component of inhibition but had no effect on voltage-dependent open-channel block. The effects of 4-hydroxy-Tmx, a major active metabolite, tested at 2 × 10(-6) M, had effects on I (K(ACh)) analogous to those of Tmx. Inhibition of constitutive inward-rectifying K(+) current (I (K1)) in ventricular myocytes, carried by Kir2 complexes, by Tmx was devoid of a voltage-dependent component. This study suggests the voltage-dependent open-channel block of GIRK inward current as a novel mechanism of Tmx action.

Upregulation of ventricular potassium channels by chronic tamoxifen treatment

AIMS

Tamoxifen is a selective oestrogen receptor modulator widely used in the prevention and treatment of breast cancer. Women receiving long-term tamoxifen therapy do not experience cardiac arrhythmias although acute perfusion of tamoxifen has been shown to inhibit cardiac K(+) currents. This observation suggests that chronic tamoxifen treatment does not negatively modulate cardiac K(+) currents. Therefore, we investigated the chronic effects of tamoxifen on K(+) currents and channels in mouse and guinea pig ventricles.

METHODS AND RESULTS

Female mice and guinea pigs were treated with placebo or tamoxifen pellets for 60 days. Voltage-clamp experiments showed that the density of the Ca²(+)-independent transient outward (I(to)), the ultrarapid delayed rectifier (I(Kur)), the steady-state (I(ss)), and the inward rectifier (I(K1)) K(+) currents were increased in tamoxifen-treated mice ventricle. Western blot analysis revealed that protein expression of the underlying K(+) channels Kv4.3 (I(to)), Kv1.5 (I(Kur)), Kv2.1 (I(ss)), and Kir2.1 (I(K1)) were significantly higher in the ventricle of tamoxifen-treated mice. Protein expression of the K(+) channel subunits encoding I(Kr) and I(Ks) (ERG1, KCNQ1, and KCNE1) was also increased in tamoxifen-treated guinea pig ventricle.

CONCLUSION

Conditions with high oestrogen levels are associated with reduced K(+) currents. Thus, conceivably, tamoxifen might prevent the inhibitory effects of oestrogen on K(+) channels by blocking the oestrogen receptors, which would explain the reported increase in K(+) currents. These findings could contribute to explain the absence of cardiac arrhythmia with long-term tamoxifen therapy.