Chronic NHE-1 blockade induces an antiapoptotic effect in the hypertrophied heart

Beneficial Consequences of One-Month Oral Treatment with Cannabis Oil on Cardiac Hypertrophy and the Mitochondrial Pool in Spontaneously Hypertensive Rats

Introduction: It has been demonstrated the dysregulation of the cardiac endocannabinoid system in cardiovascular diseases. Thus, the modulation of this system through the administration of phytocannabinoids present in medicinal cannabis oil (CO) emerges as a promising therapeutic approach. Furthermore, phytocannabinoids exhibit potent antioxidant properties, making them highly desirable in the treatment of cardiac pathologies, such as hypertension-induced cardiac hypertrophy (CH). Objective: To evaluate the effect of CO treatment on hypertrophy and mitochondrial status in spontaneously hypertensive rat (SHR) hearts. Methods: Three-month-old male SHR were randomly assigned to CO or olive oil (vehicle) oral treatment for 1 month. We evaluated cardiac mass and histology, mitochondrial dynamics, membrane potential, area and density, myocardial reactive oxygen species (ROS) production, superoxide dismutase (SOD), and citrate synthase (CS) activity and expression. Data are presented as mean ± SEM (n) and compared by t-test, or two-way ANOVA and Bonferroni post hoc test were used as appropriate. p < 0.05 was considered statistically significant. Results: CH was reduced by CO treatment, as indicated by the left ventricular weight/tibia length ratio, left ventricular mass index, myocyte cross-sectional area, and left ventricle collagen volume fraction. The ejection fraction was preserved in the CO-treated group despite the persistence of elevated systolic blood pressure and the reduction in CH. Mitochondrial membrane potential was improved and mitochondrial biogenesis, dynamics, area, and density were all increased by treatment. Moreover, the activity and expression of the CS were enhanced by treatment, whereas ROS production was decreased and the antioxidant activity of SOD increased by CO administration. Conclusion: Based on the mentioned results, we propose that 1-month oral treatment with CO is effective to reduce hypertrophy, improve the mitochondrial pool and increase the antioxidant capacity in SHR hearts.

Polyunsaturated ω3 fatty acids prevent the cardiac hypertrophy in hypertensive rats

It has been demonstrated that supplementation with the two main omega 3 polyunsaturated fatty acids (ω3 FAs), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA), leads to modifications in the cardiac physiology. ω3 FAs can affect the membrane's lipid composition, as well as proteins' location and/or function. The Na⁺/H⁺ exchanger (NHE1) is an integral membrane protein involved in the maintenance of intracellular pH and its hyperactivity has been associated with the development of various cardiovascular diseases such as cardiac hypertrophy. Our aim was to determine the effect of ω3 FAs on systolic blood pressure (SBP), lipid profiles, NHE1 activity, and cardiac function in spontaneously hypertensive rats (SHR) using Wistar rats (W) as normotensive control. After weaning, the rats received orally ω3 FAs (200 mg/kg body mass/day/ 4 months). We measured SBP, lipid profiles, and different echocardiography parameters, which were used to calculate cardiac hypertrophy index, systolic function, and ventricular geometry. The rats were sacrificed, and ventricular cardiomyocytes were obtained to measure NHE1 activity. While the treatment with ω3 FAs did not affect the SBP, lipid analysis of plasma revealed a significant decrease in omega-6/omega-3 ratio, correlated with a significant reduction in left ventricular mass index in SHR. The NHE1 activity was significantly higher in SHR compared with W. While in W the NHE1 activity was similar in both groups, a significant decrease in NHE1 activity was detected in SHRs supplemented with ω3 FAs, reaching values comparable with W. Altogether, these findings revealed that diet supplementation with ω3 FAs since early age prevents the development of cardiac hypertrophy in SHR, perhaps by decreasing NHE1 activity, without altering hemodynamic overload.

The HVCN1 voltage-gated proton channel contributes to pH regulation in canine ventricular myocytes

KEY POINTS

Intracellular pH (pH_i ) regulation is crucial for cardiac function, as acidification depresses contractility and causes arrhythmias. H⁺ ions are generated in cardiomyocytes from metabolic processes and particularly from CO₂ hydration, which has been shown to facilitate CO₂ -venting from mitochondria. Currently, the NHE1 Na⁺ /H⁺ exchanger is viewed as the dominant H⁺ -extrusion mechanism in cardiac muscle. We show that the HVCN1 voltage-gated proton channel is present and functional in canine ventricular myocytes, and that HVCN1 and NHE1 both contribute to pH_i regulation. HVCN1 provides an energetically-efficient mechanism of H⁺ -extrusion that would not cause Na⁺ -loading, which can cause pathology, and that could contribute to transport-mediated CO₂ disposal. These results provide a major advance in our understanding of pH_i regulation in cardiac muscle.

ABSTRACT

Regulation of intracellular pH (pH_i ) in cardiomyocytes is crucial for cardiac function; however, currently known mechanisms for direct or indirect extrusion of acid from cardiomyocytes seem insufficient for energetically-efficient extrusion of the massive H⁺ loads generated under in vivo conditions. In cardiomyocytes, voltage-sensitive H⁺ channel activity mediated by the HVCN1 proton channel would be a highly efficient means of disposing of H⁺ , while avoiding Na⁺ -loading, as occurs during direct acid extrusion via Na⁺ /H⁺ exchange or indirect acid extrusion via Na⁺ -HCO₃ ^- cotransport. PCR and immunoblotting demonstrated expression of HVCN1 mRNA and protein in canine heart. Patch clamp analysis of canine ventricular myocytes revealed a voltage-gated H⁺ current that was highly H⁺ -selective. The current was blocked by external Zn²⁺ and the HVCN1 blocker 5-chloro-2-guanidinobenzimidazole (ClGBI). Both the gating and Zn²⁺ blockade of the current were strongly influenced by the pH gradient across the membrane. All characteristics of the observed current were consistent with the known hallmarks of HVCN1-mediated H⁺ current. Inhibition of HVCN1 and the NHE1 Na⁺ /H⁺ exchanger, singly and in combination, showed that either mechanism is largely sufficient to maintain pH_i in beating cardiomyocytes, but that inhibition of both activities causes rapid acidification. These results show that HVCN1 is expressed in canine ventricular myocytes and provides a major H⁺ -extrusion activity, with a capacity similar to that of NHE1. In the beating heart in vivo, this activity would allow Na⁺ -independent extrusion of H⁺ during each action potential and, when functionally coupled with anion transport mechanisms, could facilitate transport-mediated CO₂ disposal. Abstract figure legend The HVCN1 proton channel is expressed in canine ventricular myocytes and contributes to H⁺ extrusion. This article is protected by copyright. All rights reserved.

Potential Therapeutic Effect of Citronellal on Diabetic Cardiomyopathy in Experimental Rats

Diabetic cardiomyopathy (DCM), a cardiovascular complication of patients with diabetes, is a special cardiomyopathy that is independent of coronary heart disease, hypertension, and valvular disease. Citronellal (CT) is a monoterpene compound generated by the secondary metabolism of plants. In this work, the therapeutic effect and mechanism of CT in DCM were investigated. Experimental diabetic rat models were constructed through a high-fat and high-carbohydrate diet combined with low-dosage streptozotocin (STZ) treatment. CT was intragastrically administered at the dosage of 150 mg/kg/day. The cardiac functions of the rats were evaluated via cardiac Doppler ultrasound. Changes in myocardial structure were analyzed through histopathology. Changes in the representative indices of oxidative stress, namely, superoxide dismutase (SOD) activity and malondialdehyde (MDA) content were detected on the basis of a biochemical test. Related protein levels were assayed via immunofluorescence and Western blot analyses. The DCM rats in the nontreatment group experienced diastolic and systolic dysfunctions, associated with myocardial hypertrophy, fibrosis, and cardiomyocyte apoptosis. Moreover, this condition was concurrent with metabolic disorders, the degradation of SOD activity in myocardial tissues, the increase in MDA content, the abnormal activation of sodium–hydrogen exchanger 1 (NHE1), and the aggravation of cell apoptosis (Bax levels were elevated, whereas Bcl-2 levels decreased). Myocardial hypertrophy, fibrosis, oxidative stress, and cell apoptosis were obviously inhibited after treatment with CT (150 mg/kg/day). The abnormal activation of NHE1 was recovered under the action of CT. Our study results showed that CT might play a protective role in the treatment of DCM by repressing the abnormal activation of NHE1.

Attenuating vascular stenosis-induced astrogliosis preserves white matter integrity and cognitive function

BACKGROUND

Chronic cerebral hypoperfusion (CCH) causes white matter damage and cognitive impairment, in which astrogliosis is the major pathology. However, underlying cellular mechanisms are not well defined. Activation of Na+/H+ exchanger-1 (NHE1) in reactive astrocytes causes astrocytic hypertrophy and swelling. In this study, we examined the role of NHE1 protein in astrogliosis, white matter demyelination, and cognitive function in a murine CCH model with bilateral carotid artery stenosis (BCAS).

METHODS

Sham, BCAS, or BCAS mice receiving vehicle or a selective NHE1 inhibitor HOE642 were monitored for changes of the regional cerebral blood flow and behavioral performance for 28 days. Ex vivo MRI-DTI was subsequently conducted to detect brain injury and demyelination. Astrogliosis and demyelination were further examined by immunofluorescence staining. Astrocytic transcriptional profiles were analyzed with bulk RNA-sequencing and RT-qPCR.

RESULTS

Chronic cerebral blood flow reduction and spatial working memory deficits were detected in the BCAS mice, along with significantly reduced mean fractional anisotropy (FA) values in the corpus callosum, external capsule, and hippocampus in MRI DTI analysis. Compared with the sham control mice, the BCAS mice displayed demyelination and axonal damage and increased GFAP+ astrocytes and Iba1+ microglia. Pharmacological inhibition of NHE1 protein with its inhibitor HOE642 prevented the BCAS-induced gliosis, damage of white matter tracts and hippocampus, and significantly improved cognitive performance. Transcriptome and immunostaining analysis further revealed that NHE1 inhibition specifically attenuated pro-inflammatory pathways and NADPH oxidase activation.

CONCLUSION

Our study demonstrates that NHE1 protein is involved in astrogliosis with pro-inflammatory transformation induced by CCH, and its blockade has potentials for reducing astrogliosis, demyelination, and cognitive impairment.

Myocardial Impact of NHE1 Regulation by Sildenafil

The cardiac Na⁺/H⁺ exchanger (NHE1) is a membrane glycoprotein fundamental for proper cell functioning due its multiple housekeeping tasks, including regulation of intracellular pH, Na⁺ concentration, and cell volume. In the heart, hyperactivation of NHE1 has been linked to the development of different pathologies. Several studies in animal models that reproduce the deleterious effects of ischemia/reperfusion injury or cardiac hypertrophy have conclusively demonstrated that NHE1 inhibition provides cardioprotection. Unfortunately, NHE1 inhibitors failed to reproduce these effects in the clinical arena. The reasons for those discrepancies are not apparent yet. However, a reasonable clue to consider would be that drugs that completely abolish the exchanger activity, including that its essential housekeeping function may not be the best therapeutic approach. Therefore, interventions tending to specifically reduce its hyperactive state without affecting its basal activity emerge as a novel potential gold standard. In this regard, a promising goal seems to be the modulation of the phosphorylation state of the cytosolic tail of the exchanger. Recent own experiments demonstrated that Sildenafil, a phosphodiesterase 5A inhibitor drug that has been widely used for the treatment of erectile dysfunction is able to decrease NHE1 phosphorylation, and hence reduce its hyperactivity. In connection, growing evidence demonstrates cardioprotective properties of Sildenafil against different cardiac pathologies, with the distinctive characteristic of directly affecting cardiac tissue without altering blood pressure. This mini-review was aimed to focus on the regulation of NHE1 activity by Sildenafil. For this purpose, experimental data reporting Sildenafil effects in different animal models of heart disease will be discussed.

Cardioprotective role of IGF-1 in the hypertrophied myocardium of the spontaneously hypertensive rats: A key effect on NHE-1 activity

AIM

Myocardial Na⁺/H⁺ exchanger-1 (NHE-1) hyperactivity and oxidative stress are interrelated phenomena playing pivotal roles in the development of pathological cardiac hypertrophy and heart failure. Exercise training is effective to convert pathological into physiological hypertrophy in the spontaneously hypertensive rats (SHR), and IGF-1-key humoral mediator of exercise training-inhibits myocardial NHE-1, at least in normotensive rats. Therefore, we hypothesize that IGF-1 by hampering NHE-1 hyperactivity and oxidative stress should exert a cardioprotective effect in the SHR.

METHODS

NHE-1 activity [proton efflux ( J H + ) mmol L^-1 min^-1], expression and phosphorylation; H₂O₂ production; superoxide dismutase (SOD) activity; contractility and calcium transients were measured in SHR hearts in the presence/absence of IGF-1.

RESULTS

IGF-1 significantly decreased NHE-1 activity ( J H + at pH_i 6.95: 1.39 ± 0.32, n = 9 vs C 3.27 ± 0.3, n = 20, P < .05); effect prevented by AG1024, an antagonist of IGF-1 receptor (2.7 ± 0.4, n = 7); by the PI3K inhibitor wortmannin (3.14 ± 0.41, n = 7); and the AKT inhibitor MK2206 (3.37 ± 0.43, n = 14). Moreover, IGF-1 exerted an antioxidant effect revealed by a significant reduction in H₂O₂ production accompanied by an increase in SOD activity. In addition, IGF-1 improved cardiomyocyte contractility as evidenced by an increase in sarcomere shortening and a decrease in the relaxation constant, underlined by an increase in the amplitude and rate of decay of the calcium transients.

CONCLUSION

IGF-1 exerts a cardioprotective role on the hypertrophied hearts of the SHR, in which the inhibition of NHE-1 hyperactivity, as well as the positive inotropic and antioxidant effects, emerges as key players.

Activation and Inhibition of Sodium-Hydrogen Exchanger Is a Mechanism That Links the Pathophysiology and Treatment of Diabetes Mellitus With That of Heart Failure

The mechanisms underlying the progression of diabetes mellitus and heart failure are closely intertwined, such that worsening of one condition is frequently accompanied by worsening of the other; the degree of clinical acceleration is marked when the 2 coexist. Activation of the sodium-hydrogen exchanger in the heart and vasculature (NHE1 isoform) and the kidneys (NHE3 isoform) may serve as a common mechanism that links both disorders and may underlie their interplay. Insulin insensitivity and adipokine abnormalities (the hallmarks of type 2 diabetes mellitus) are characteristic features of heart failure; conversely, neurohormonal systems activated in heart failure (norepinephrine, angiotensin II, aldosterone, and neprilysin) impair insulin sensitivity and contribute to microvascular disease in diabetes mellitus. Each of these neurohormonal derangements may act through increased activity of both NHE1 and NHE3. Drugs used to treat diabetes mellitus may favorably affect the pathophysiological mechanisms of heart failure by inhibiting either or both NHE isoforms, and drugs used to treat heart failure may have beneficial effects on glucose tolerance and the complications of diabetes mellitus by interfering with the actions of NHE1 and NHE3. The efficacy of NHE inhibitors on the risk of cardiovascular events may be enhanced when heart failure and glucose intolerance coexist and may be attenuated when drugs with NHE inhibitory actions are given concomitantly. Therefore, the sodium-hydrogen exchanger may play a central role in the interplay of diabetes mellitus and heart failure, contribute to the physiological and clinical progression of both diseases, and explain certain drug-drug and drug-disease interactions that have been reported in large-scale randomized clinical trials.

Down-regulation of Na+/H+ exchanger 1 by Toll-like receptor stimulation in macrophages

The role of Na⁺/H⁺ exchanger 1 (NHE1) in various cell types, including inflammatory cells, has been extensively studied. However, regulation of NHE1 protein level in activated inflammatory cells is yet to be characterized. In this study, we investigated whether Toll-like receptor (TLR) ligands can regulate NHE1 protein level in the mouse macrophage-like RAW 264 cell line. We found that lipopolysaccharide (LPS), a TLR4 ligand, lowered NHE1 level and activity in RAW 264 cells and in primary murine macrophages. Other TLR ligands, such as zymosan A and poly(I:C), also displayed reduced NHE1 level. LPS promoted NHE1 ubiquitination and reduced the expression of calcineurin homologous protein 1 (CHP1), a regulator of NHE1 activity and stability. These responses were inhibited by c-Jun N-terminal kinase (JNK) inhibitor SP600125 and dexamethasone. A proteasome inhibitor, but not caspase-3 or lysosomal inhibitors, blocked the LPS-induced NHE1 down-regulation. These results suggested that LPS promotes the degranulation of NHE1 mediated by the ubiquitin-proteasome system and CHP1 downregulation resulting from activation of JNK.

Dihydropyridines' metabolites-induced early apoptosis after myocardial infarction in rats; new outlook on preclinical study with M-2 and M-3

Our previous studies established cardio-protective effects of furnidipine and its active metabolites called M-2 and M-3. The aim of current research was to compare the effects of single oral pretreatment with 20 mg kg(-1) of M-2 and M-3 on mortality, different forms of arrhythmias, blood pressures parameters and ST-segment changes during occlusion (for 90 min) and reperfusion in the model of myocardial infarction in rats evoked by left anterior descending coronary artery occlusion. Additionally, the development of programmed cell death and biochemical parameters in blood serum were studied at 4th day after infarction. Furnidipines' metabolites effectively reduced mortality index while did not markedly influence on blood pressures parameters, arrhythmias, ST-segment changes as well as biochemical parameters. Intriguingly, programmed cell death study (TUNEL) showed distinct increase in the amount of apoptotic nuclei in post-infarcted myocardium, granulation tissue and what is more in arteriolar walls after M-2 and M-3 application. Moreover, M-2 turned out to be more powerful in stimulation of apoptosis in granulation tissue surrounding infarcted area whereas M-3 presented balanced profile in this matter. Taking into account that programmed cell death plays positive role in post-infarcted heart healing, M-2 presents itself as more attractive agent for oral pretreatment in early stages of ischemia by non-stable individuals due to its more specific action in stimulation repairing processes in granulation tissue as well as in arteriolar walls. While M-2 and M-3 are common metabolites present in degradation pathways of many widely used dihydropyridines in clinic, this key fact put the new outlook on understanding additional mechanism and effects of not only furnidipines' metabolites but also other dihydropyridines.

Physiological cardiac hypertrophy: critical role of AKT in the prevention of NHE-1 hyperactivity

BACKGROUND

The involvement of NHE-1 hyperactivity, critical for pathological cardiac hypertrophy (CH), in physiological CH has not been elucidated yet. Stimulation of NHE-1 increases intracellular Na(+) and Ca(2+) favouring calcineurin activation. Since myocardial stretch, an activator of NHE-1, is common to both types of CH, we speculate that NHE-1 hyperactivity may also happen in physiological CH. However, calcineurin activation is characteristic only for pathological hypertrophy. We hypothesize that an inhibitory AKT-dependent mechanism prevents NHE-1 hyperactivity in the setup of physiological CH.

METHODS

Physiological CH was induced in rats by swimming (90 min/day, 12 weeks) or in cultured isolated cardiomyocytes with IGF-1 (10 nmol/L).

RESULTS

Training induced eccentric CH development (left ventricular weight/tibial length: 22.0±0.3 vs. 24.3±0.7 mg/mm; myocyte cross sectional area: 100±3.2 vs. 117±4.1 %; sedentary (Sed) and swim-trained (Swim) respectively; p<0.05] with decreased myocardial stiffness and collagen deposition [1.7±0.05 % (Sed) vs. 1.4±0.09 % (Swim); p<0.05]. Increased phosphorylation of AKT, ERK1/2, p90(RSK) and NHE-1 at the consensus site for ERK1/2-p90(RSK) were detected in the hypertrophied hearts (P-AKT: 134±10 vs. 100±5; P-ERK1/2: 164±17 vs. 100±18; P-p90(RSK): 160±18 vs. 100±9; P-NHE-1 134±10 vs. 100±10; % in Swim vs. Sed respectively; p<0.05). No significant changes were detected neither in calcineurin activation [calcineurin Aβ 100±10 (Sed) vs. 96±12 (Swim)], nor NFAT nuclear translocation [100±3.11 (Sed) vs. 95±9.81 % (Swim)] nor NHE-1 expression [100±8.5 (Sed) vs. 95±6.7 % (Swim)]. Interestingly, the inhibitory phosphorylation of the NHE-1 consensus site for AKT was increased in the hypertrophied myocardium (151.6±19.4 (Swim) vs. 100±9.5 % (Sed); p<0.05). In isolated cardiomyocytes 24 hours IGF-1 increased cell area (114±1.3 %; p<0.05) and protein/DNA content (115±3.9 %, p<0.05), effects not abolished by NHE-1 inhibition with cariporide (114±3 and 117±4.4 %, respectively). IGF-1 significantly decreased NHE-1 activity during pHi recovery from sustained intracellular acidosis (JH+ at pHi 6.8: 4.08±0.74 and 9.09±1.21 mmol/L/min, IGF-1 vs. control; p<0.05), and abolished myocardial slow force response, the mechanical counterpart of stretch-induced NHE-1 activation.

CONCLUSIONS

NHE-1 hyperactivity seems not to be involved in physiological CH development, contrary to what characterizes pathological CH. We propose that AKT, through an inhibitory phosphorylation of the NHE-1, prevents its stretch-induced activation. This posttranslational modification emerges as an adaptive mechanism that avoids NHE-1 hyperactivity preserving its housekeeping functioning.

Cleavage and reduced CD36 ectodomain density on heart and spleen macrophages in the spontaneously hypertensive rat

Metabolic disease is accompanied by a range of cellular defects ("comorbidities") whose origin is uncertain. To investigate this pathophysiological phenomenon we used the Spontaneously Hypertensive Rat (SHR), which besides an elevated arterial blood pressure also has many other comorbidities, including a defective glucose and lipid metabolism. We have shown that this model of metabolic disease has elevated plasma matrix metalloproteinase (MMP) activity, which cleaves the extracellular domain of membrane receptors. We hypothesize here that the increased MMP activity also leads to abnormal cleavage of the scavenger receptor and fatty acid transporter CD36. To test this idea, chronic pharmaceutical MMP inhibition (CGS27023A) of the SHR and its normotensive control, the Wistar Kyoto Rat (WKY), was used to determine if inhibition of MMP activity serves to maintain CD36 receptor density and function. Surface density of CD36 on macrophages from the heart, spleen, and liver was determined in WKY, SHR, CGS-treated WKY (CGS WKY), and CGS-treated SHR (CGS SHR) by immunohistochemistry with an antibody against the CD36 ectodomain. The extracellular CD36 density was lower in SHR heart and spleen macrophages compared to that in the WKY. MMP inhibition by CGS served to restore the reduced CD36 density on SHR cardiac and splanchnic macrophages to levels of the WKY. To examine CD36 function, culture assays with murine macrophages (RAW 264.7) after incubation in fresh WKY or SHR plasma were used to test for adhesion of light-weight donor red blood cell (RBC) by CD36. This form of RBC adhesion to macrophages was reduced after incubation in SHR compared WKY plasma. Analysis of the supernatant macrophage media by Western blot shows a higher level of CD36 extracellular protein fragments following exposure to SHR plasma compared to WKY. MMP inhibition in the SHR plasma compared to untreated plasma, served to increase the RBC adhesion to macrophages and decrease the number of receptor fragments in the macrophage media. In conclusion, these studies bring to light that plasma in the SHR model of metabolic disease has an unchecked MMP degrading activity which causes cleavage of a variety of membrane receptors, including CD36, which attenuates several cellular functions typical for the metabolic disease, including RBC adhesion to the scavenger receptor CD36. In addition to other cell dysfunctions chronic MMP inhibition restores CD36 in the SHR.

RETRACTED: Probucol via inhibition of NHE1 attenuates LPS-accelerated atherosclerosis and promotes plaque stability in vivo

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the authors. Concern had previously been raised about the veracity of some of the data in Figure 6 which appears to contain similar experimental data to many other publications, described here: https://pubpeer.com/publications/72ACDBE9751C8FD443A223FEB806E8#. The corresponding author contacted the journal and acknowledged that some of the experiments were not conducted by the authors themselves and they no longer have confidence in the reliability of the results. The authors would like to apologize for any inconvenience caused. The Editor-in-Chief agreed to retract the article.

Intracellular Acid-extruding regulators and the effect of lipopolysaccharide in cultured human renal artery smooth muscle cells

Homeostasis of the intracellular pH (pHi) in mammalian cells plays a pivotal role in maintaining cell function. Thus far, the housekeeping Na(+)-H(+) exchanger (NHE) and the Na(+)-HCO3(-) co-transporter (NBC) have been confirmed in many mammalian cells as major acid extruders. However, the role of acid-extruding regulators in human renal artery smooth muscle cells (HRASMCs) remains unclear. It has been demonstrated that lipopolysaccharide (LPS)-induced vascular occlusion is associated with the apoptosis, activating calpain and increased [Ca(2+)]i that are related to NHE1 activity in endothelia cells. This study determines the acid-extruding mechanisms and the effect of LPS on the resting pHi and active acid extruders in cultured HRASMCs. The mechanism of pHi recovery from intracellular acidosis (induced by NH4Cl-prepulse) is determined using BCECF-fluorescence in cultured HRASMCs. It is seen that (a) the resting pHi is 7.19 ± 0.03 and 7.10 ± 0.02 for HEPES- and CO2/HCO3(-)- buffered solution, respectively; (b) apart from the housekeeping NHE1, another Na(+)-coupled HCO3(-) transporter i.e. NBC, functionally co-exists to achieve acid-equivalent extrusion; (c) three different isoforms of NBC: NBCn1 (SLC4A7; electroneutral), NBCe1 (SLC4A4; electrogenic) and NBCe2 (SLC4A5), are detected in protein/mRNA level; and (d) pHi and NHE protein expression/activity are significantly increased by LPS, in both a dose- and time- dependent manner, but NBCs protein expression is not. In conclusion, it is demonstrated, for the first time, that four pHi acid-extruding regulators: NHE1, NBCn1, NBCe1 and NBCe2, co-exist in cultured HRASMCs. LPS also increases cellular growth, pHi and NHE in a dose- and time-dependent manner.

Interactions between the regulatory subunit of type I protein kinase A and p90 ribosomal S6 kinase1 regulate cardiomyocyte apoptosis

Cardiomyocyte apoptosis contributes toward the loss of muscle mass in myocardial pathologies. Previous reports have implicated type I cAMP-dependent protein kinase (PKA) and p90 ribosomal S6 kinase (RSK) in cardiomyocyte apoptosis. However, the precise mechanisms and the isoform of RSK involved in this process remain undefined. Using adult rat ventricular myocytes and mouse-derived cardiac HL-1 cardiomyocytes, we demonstrate that hypoxia/reoxygenation (H/R)-induced apoptosis is accompanied by a decrease in the type I PKA regulatory subunit (PKARIα) and activation of RSK1. As previously described by us for other cell types, in cardiomyocytes, inactive RSK1 also interacts with PKARIα, whereas the active RSK1 interacts with the catalytic subunit of PKA. Additionally, small interfering (siRNA)-mediated silencing of PKARIα or disrupting the RSK1/PKARIα interactions with a small, cell-permeable peptide activates RSK1 and recapitulates the H/R-induced apoptosis. Inhibition of RSK1 or siRNA-mediated silencing of RSK1 attenuates H/R-induced apoptosis, demonstrating the role of RSK1 in cardiomyocyte apoptosis. Furthermore, silencing of RSK1 decreases the H/R-induced phosphorylation of sodium-hydrogen exchanger 1 (NHE1), and inhibition of NHE1 with 5'-N-ethyl-N-isopropyl-amiloride blocks H/R induced apoptosis, indicating the involvement of NHE1 in apoptosis. Overall, our findings demonstrate that H/R-mediated decrease in PKARIα protein levels leads to activation of RSK1, which via phosphorylation of NHE1 induces cardiomyocyte apoptosis.

Mitochondrial targets for arrhythmia suppression: is there a role for pharmacological intervention?

Mitochondrial dysfunction is a hallmark of common cardiovascular disorders, including ischemia-reperfusion injury, hypertrophy, heart failure, and diabetes mellitus. While the role of the mitochondrial network in regulating energy production and cell death pathways is well established, its active control of other critical cellular functions, including excitation-contraction coupling and excitability, is less understood. The purpose of this focused review article is to highlight the growing mechanistic link between mitochondrial dysfunction and arrhythmogenesis. The goal is not to provide a comprehensive listing of all factors by which mitochondrial bioenergetics and altered cellular redox status affect ion channel function but rather to focus on one central mechanism of arrhythmogenesis which arises from a mitochondrial origin. In doing so, we discuss the role of mitochondrial targets for suppressing arrhythmias through this mechanism.

H⁺-activated Na⁺ influx in the ventricular myocyte couples Ca²⁺-signalling to intracellular pH

Acid extrusion on Na(+)-coupled pH-regulatory proteins (pH-transporters), Na(+)/H(+) exchange (NHE1) and Na(+)-HCO3(-) co-transport (NBC), drives Na(+) influx into the ventricular myocyte. This H(+)-activated Na(+)-influx is acutely up-regulated at pHi<7.2, greatly exceeding Na(+)-efflux on the Na(+)/K(+) ATPase. It is spatially heterogeneous, due to the co-localisation of NHE1 protein (the dominant pH-transporter) with gap-junctions at intercalated discs. Overall Na(+)-influx via NBC is considerably lower, but much is co-localised with L-type Ca(2+)-channels in transverse-tubules. Through a functional coupling with Na(+)/Ca(2+) exchange (NCX), H(+)-activated Na(+)-influx increases sarcoplasmic-reticular Ca(2+)-loading and release during intracellular acidosis. This raises Ca(2+)-transient amplitude, rescuing it from direct H(+)-inhibition. Functional coupling is biochemically regulated and linked to membrane receptors, through effects on NHE1 and NBC. It requires adequate cytoplasmic Na(+)-mobility, as NHE1 and NCX are spatially separated (up to 60μm). The relevant functional NCX activity must be close to dyads, as it exerts no effect on bulk diastolic Ca(2+). H(+)-activated Na(+)-influx is up-regulated during ischaemia-reperfusion and some forms of maladaptive hypertrophy and heart failure. It is thus an attractive system for therapeutic manipulation. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Role of α-crystallin B as a regulatory switch in modulating cardiomyocyte apoptosis by mitochondria or endoplasmic reticulum during cardiac hypertrophy and myocardial infarction

Cardiac hypertrophy and myocardial infarction (MI) are two major causes of heart failure with different etiologies. However, the molecular mechanisms associated with these two diseases are not yet fully understood. So, this study was designed to decipher the process of cardiomyocyte apoptosis during cardiac hypertrophy and MI in vivo. Our study revealed that mitochondrial outer membrane channel protein voltage-dependent anion channel-1 (VDAC1) was upregulated exclusively during cardiac hypertrophy, whereas 78 kDa glucose-regulated protein (GRP78) was exclusively upregulated during MI, which is an important upstream regulator of the endoplasmic reticulum (ER) stress pathway. Further downstream analysis revealed that mitochondrial pathway of apoptosis is instrumental in case of hypertrophy, whereas ER stress-induced apoptosis is predominant during MI, which was confirmed by treatment with either siRNA against VDAC1 or ER stress inhibitor tauroursodeoxycholic acid (TUDCA). Very interestingly, our data also showed that the expression and interaction of small heat-shock protein α-crystallin B (CRYAB) with VDAC1 was much more pronounced during MI compared with either hypertrophy or control. The study demonstrated for the first time that two different organelles—mitochondria and ER have predominant roles in mediating cardiomyocyte death signaling during hypertrophy and MI, respectively, and activation of CRYAB acts as a molecular switch in bypassing mitochondrial pathway of apoptosis during MI.

Regulation of the cardiac Na⁺/H⁺ exchanger in health and disease

The Na(+) gradient produced across the cardiac sarcolemma by the ATP-dependent Na(+)-pump is a constant source of energy for Na(+)-dependent transporters. The plasma membrane Na(+)/H(+) exchanger (NHE) is one such secondary active transporter, regulating intracellular pH, Na(+) concentration, and cell volume. NHE1, the major isoform found in the heart, is activated in response to a variety of stimuli such as hormones and mechanical stress. This important characteristic of NHE1 is intimately linked to heart diseases, including maladaptive cardiac hypertrophy and subsequent heart failure, as well as acute ischemic-reperfusion injury. NHE1 activation results in elevation of pH and intracellular Na(+) concentration, which potentially enhance downstream signaling cascades in the myocardium. Therefore, in addition to determining the mechanism underlying regulation of NHE1 activity, it is important to understand how the ionic signal produced by NHE1 is transmitted to the downstream targets. Extensive studies have identified many accessory factors that interact with NHE1. Here, we have summarized the recent progress on understanding the molecular mechanism underlying NHE1 regulation and have shown a possible signaling pathway leading to cardiac remodeling, which is initiated from NHE1. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Amiloride attenuates lipopolysaccharide-accelerated atherosclerosis via inhibition of NHE1-dependent endothelial cell apoptosis

AIM

To investigate the effects of the potassium-sparing diuretic amiloride on endothelial cell apoptosis during lipopolysaccharide (LPS)-accelerated atherosclerosis.

METHODS

Human umbilical vein endothelial cells (HUVECs) were exposed to LPS (100 ng/mL) in the presence of drugs tested. The activity of Na(+)/H(+) exchanger 1 (NHE1) and calpain, intracellular free Ca(2+)level ([Ca(2+)](i)), as well as the expression of apoptosis-related proteins in the cells were measured. For in vivo study, ApoE-deficient (ApoE(-/-)) mice were fed high-fat diets with 0.5% (w/w) amiloride for 4 weeks and LPS (10 μg/mouse) infusion into caudal veins. Afterwards, atherosclerotic lesions, NHE1 activity and Bcl-2 expression in the aortic tissues were evaluated.

RESULTS

LPS treatment increased NHE1 activity and [Ca(2+)](i) in HUVECs in a time-dependent manner, which was associated with increased activity of the Ca(2+)-dependent protease calpain. Amiloride (1-10 μmol/L) significantly suppressed LPS-induced increases in NHE1 activity, [Ca(2+)](i). and calpain activity. In the presence of the Ca(2+) chelator BAPTA (0.5 mmol/L), LPS-induced increase of calpain activity was also abolished. In LPS-treated HUVECs, the expression of Bcl-2 protein was significantly decreased without altering its mRNA level. In the presence of amiloride (10 μmol/L) or the calpain inhibitor ZLLal (50 μmol/L), the down-regulation of Bcl-2 protein by LPS was blocked. LPS treatment did not alter the expression of Bax and Bak proteins in HUVECs. In the presence of amiloride, BAPTA or ZLLal, LPS-induced HUVEC apoptosis was significantly attenuated. In ApoE(-/-) mice, administration of amiloride significantly suppressed LPS-accelerated atherosclerosis and LPS-induced increase of NHE1 activity, and reversed LPS-induced down-regulation of Bcl-2 expression.

CONCLUSION

LPS stimulates NHE1 activity, increases [Ca(2+)](i), and activates calpain, which leads to endothelial cell apoptosis related to decreased Bcl-2 expression. Amiloride inhibits NHE1 activity, thus attenuates LPS-accelerated atherosclerosis in mice.

Lipopolysaccharide induces endothelial cell apoptosis via activation of Na(+)/H(+) exchanger 1 and calpain-dependent degradation of Bcl-2

The calcium-dependent protease calpain is involved in lipopolysaccharide (LPS)-induced endothelial injury. The activation of Na(+)/H(+) exchanger (NHE) is responsible to increase intracellular Ca(2+) (Ca(i)(2+)) in cardiovascular diseases. Here we hypothesized that activation of NHE mediates LPS-induced endothelial cell apoptosis via calcium-dependent calpain pathway. Our results revealed that LPS-induced increases in NHE activity are dependent on NHE1 in human umbilical vein endothelial cells (HUVECs). Treatment of HUVECs with LPS increased the NHE1 activity in a time-dependent manner associated with the increased Ca(i)(2+), which resulted in enhanced calpain activity as well as HUVECs apoptosis via NHE1-dependent degradation of Bcl-2.

The cardioprotection of simvastatin in reperfused swine hearts relates to the inhibition of myocardial edema by modulating aquaporins via the PKA pathway

BACKGROUND AND OBJECTIVE

Myocardial edema plays a role in myocardial no-reflow and infarction during ischemia and reperfusion. The effects of statins against no-reflow and infarction may relate to the inhibition of myocardial edema. The current study investigated the role of protein kinase A (PKA) in statin-reduced myocardial edema in reperfused swine hearts.

METHODS AND RESULTS

Minipigs were treated with simvastatin (SIM, 2mg/kg), SIM+H-89 (a PKA inhibitor, 1.0 μg/kg/min), or H-89 alone 1h before 90-min ischemia and 3-h reperfusion or sham operation. Ischemia or ischemia-reperfusion induced severe myocardial edema, PKA activation, and up-regulation of aquaporin-1, -4, -8, and -9 in the reflow and no-reflow myocardium. SIM pretreatment reduced the sizes of no-reflow and infarct areas by 18.5% and 11.1% (P<0.01), decreased water content in the left ventricle, reflow and no-reflow myocardium by 1.4%, 5.3%, and 4.3% (P<0.05), inhibited cardiomyocytes swelling in the reflow and no-reflow areas by 19.8% and 13.1% (P<0.01), suppressed mitochondrial water accumulation in the reflow and no-reflow areas by 49.0% and 35.9% (P<0.01), increased PKA activity (P<0.01), and blocked the up-regulation of aquaporin-1, -4, -8, and -9 in the reflow and no-reflow myocardium. However, these beneficial effects of SIM were partially abolished by inhibiting PKA with H-89.

CONCLUSIONS

The cardioprotective effects of acute SIM therapy against myocardial no-reflow and infarction relate to the reduction of myocardial edema by suppressing the expression of aquaporin-1, -4, -8, and -9 in a partially PKA-dependent manner.

OSR1-sensitive regulation of Na+/H+ exchanger activity in dendritic cells

The oxidative stress-responsive kinase 1 (OSR1) is activated by WNK (with no K kinases) and in turn stimulates the thiazide-sensitive Na-Cl cotransporter (NCC) and the furosemide-sensitive Na-K-2Cl cotransporter (NKCC), thus contributing to transport and cell volume regulation. Little is known about extrarenal functions of OSR1. The present study analyzed the impact of decreased OSR1 activity on the function of dendritic cells (DCs), antigen-presenting cells linking innate and adaptive immunity. DCs were cultured from bone marrow of heterozygous WNK-resistant OSR1 knockin mice (osr(KI)) and wild-type mice (osr(WT)). Cell volume was estimated from forward scatter in FACS analysis, ROS production from 2',7'-dichlorodihydrofluorescein-diacetate fluorescence, cytosolic pH (pH(i)) from 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein fluorescence, and Na(+)/H(+) exchanger activity from Na(+)-dependent realkalinization following ammonium pulse and migration utilizing transwell chambers. DCs expressed WNK1, WNK3, NCC, NKCC1, and OSR1. Phosphorylated NKCC1 was reduced in osr(KI) DCs. Cell volume and pH(i) were similar in osr(KI) and osr(WT) DCs, but Na(+)/H(+) exchanger activity and ROS production were higher in osr(KI) than in osr(WT) DCs. Before LPS treatment, migration was similar in osr(KI) and osr(WT) DCs. LPS (1 μg/ml), however, increased migration of osr(WT) DCs but not of osr(KI) DCs. Na(+)/H(+) exchanger 1 inhibitor cariporide (10 μM) decreased cell volume, intracellular reactive oxygen species (ROS) formation, Na(+)/H(+) exchanger activity, and pH(i) to a greater extent in osr(KI) than in osr(WT) DCs. LPS increased cell volume, Na(+)/H(+) exchanger activity, and ROS formation in osr(WT) DCs but not in osr(KI) DCs and blunted the difference between osr(KI) and osr(WT) DCs. Na(+)/H(+) exchanger activity in osr(WT) DCs was increased by the NKCC1 inhibitor furosemide (100 nM) to values similar to those in osr(KI) DCs. Oxidative stress (10 μM tert-butyl-hydroperoxide) increased Na(+)/H(+) exchanger activity in osr(WT) DCs but not in osr(KI) DCs and reversed the difference between genotypes. Cariporide virtually abrogated Na(+)/H(+) exchanger activity in both genotypes and blunted LPS-induced cell swelling and ROS formation in osr(WT) mice. In conclusion, partial OSR1 deficiency influences Na(+)/H(+) exchanger activity, ROS formation, and migration of dendritic cells.

Hypoxia-induced invadopodia formation involves activation of NHE-1 by the p90 ribosomal S6 kinase (p90RSK)

The hypoxic and acidic microenvironments in tumors are strongly associated with malignant progression and metastasis, and have thus become a central issue in tumor physiology and cancer treatment. Despite this, the molecular links between acidic pH- and hypoxia-mediated cell invasion/metastasis remain mostly unresolved. One of the mechanisms that tumor cells use for tissue invasion is the generation of invadopodia, which are actin-rich invasive plasma membrane protrusions that degrade the extracellular matrix. Here, we show that hypoxia stimulates the formation of invadopodia as well as the invasive ability of cancer cells. Inhibition or shRNA-based depletion of the Na(+)/H(+) exchanger NHE-1, along with intracellular pH monitoring by live-cell imaging, revealed that invadopodia formation is associated with alterations in cellular pH homeostasis, an event that involves activation of the Na(+)/H(+) exchange rate by NHE-1. Further characterization indicates that hypoxia triggered the activation of the p90 ribosomal S6 kinase (p90 RSK), which resulted in invadopodia formation and site-specific phosphorylation and activation of NHE-1. This study reveals an unsuspected role of p90RSK in tumor cell invasion and establishes p90RS kinase as a link between hypoxia and the acidic microenvironment of tumors.

Activated NHE1 is required to induce early cardiac hypertrophy in mice

The Na+/H+ exchanger isoform 1 (NHE1) has been implicated as being causal in cardiac hypertrophy and the protein level and activity are elevated in the diseased myocardium. However, it is unclear whether mere elevation of the protein is sufficient for cardiac pathology, or whether activation of the protein is required. In this study, we examined the comparative effects of elevation of wild type and activated NHE1. Two mouse transgenic models that expressed either a wild type NHE1 protein or an activated NHE1 protein were characterized. Expression of activated NHE1 caused significant increases in heart weight to body weight, apoptosis, cross-sectional area, interstitial fibrosis and decreased cardiac performance. Expression of wild type NHE1 caused a much milder pathology. When we examined 2 or 10-week-old mouse hearts, there was neither elevation of calcineurin levels nor increased phosphorylation of ERK or p38 in either NHE1 transgenic mouse line. Expression of activated NHE1 in intact mice caused an increased sensitivity to phenylephrine-induced hypertrophy. Our results show that expression of activated NHE1 promotes cardiac hypertrophy to a much greater degree than elevated levels of wild type NHE1 alone. In addition, expression of activated NHE1 promotes greater sensitivity to neurohormonal stimulation. The results suggest that activation of NHE1 is a key component that accentuates NHE1-induced myocardial pathology.