Lysophosphatidic acid increases the electrophysiological instability of adult rabbit ventricular myocardium by augmenting L-type calcium current

Lysophosphatidic Acid Is Associated With Cardiac Dysfunction and Hypertrophy by Suppressing Autophagy via the LPA3/AKT/mTOR Pathway

Background: Lysophosphatidic acid (LPA), as a phospholipid signal molecule, participates in the regulation of various biological functions. Our previous study demonstrated that LPA induces cardiomyocyte hypertrophy in vitro; however, the functional role of LPA in the post-infarct heart remains unknown. Growing evidence has demonstrated that autophagy is involved in regulation of cardiac hypertrophy. The aim of the current work was to investigate the effects of LPA on cardiac function and hypertrophy during myocardial infarction (MI) and determine the regulatory role of autophagy in LPA-induced cardiomyocyte hypertrophy.

Methods:In vivo experiments were conducted in Sprague-Dawley rats subjected to MI surgery or a sham operation, and rats with MI were assigned to receive an intraperitoneal injection of LPA (1 mg/kg) or vehicle for 5 weeks. The in vitro experiments were conducted in H9C2 cardiomyoblasts.

Results: LPA treatment aggravated cardiac dysfunction, increased cardiac hypertrophy, and reduced autophagy after MI in vivo. LPA suppressed autophagy activation, as indicated by a decreased LC3II-to-LC3I ratio, increased p62 expression, and reduced autophagosome formation in vitro. Rapamycin, an autophagy enhancer, attenuated LPA-induced autophagy inhibition and H9C2 cardiomyoblast hypertrophy, while autophagy inhibition with Beclin1 siRNA did not further enhance the hypertrophic response in LPA-treated cardiomyocytes. Moreover, we demonstrated that LPA suppressed autophagy through the AKT/mTOR signaling pathway because mTOR and PI3K inhibitors significantly prevented LPA-induced mTOR phosphorylation and autophagy inhibition. In addition, we found that knockdown of LPA3 alleviated LPA-mediated autophagy suppression in H9C2 cardiomyoblasts, suggesting that LPA suppresses autophagy through activation of the LPA3 and AKT/mTOR pathways.

Conclusion: These findings suggest that LPA plays an important role in mediating cardiac dysfunction and hypertrophy after a MI, and that LPA suppresses autophagy through activation of the LPA3 and AKT/mTOR pathways to induce cardiomyocyte hypertrophy.

Effect of lysophosphatidic acid on the immune inflammatory response and the connexin 43 protein in myocardial infarction

Lysophosphatidic acid (LPA) is an intermediate product of membrane phospholipid metabolism. Recently, LPA has gained attention for its involvement in the pathological processes of certain cardiovascular diseases. The aim of the present study was to clarify the association between the effect of LPA and the immune inflammatory response, and to investigate the effects of LPA on the protein expression levels of connexin 43 during myocardial infarction. Surface electrocardiograms of myocardial infarction rats and isolated rat heart tissue samples were obtained in order to determine the effect of LPA on the incidence of arrhythmia in rats that exhibited changes in immune status. The results demonstrated that the incidence of arrhythmia decreased when the rat immune systems were suppressed, and the incidence of arrhythmia increased when the rat immune systems were enhanced. The concentration levels of tumor necrosis factor (TNF)-α were determined by ELISA, and the results demonstrated that LPA induced T lymphocyte synthesis and TNF-α release. Using a patch-clamp technique, LPA was shown to increase the current amplitude of the voltage-dependent potassium channels (K_v) and calcium-activated potassium channels (K_Ca) in Jurkat T cells. The protein expression of connexin 43 (Cx43) was determined by immunohistochemical staining. The results indicated that LPA caused the degradation of Cx43 and decreased the expression of Cx43. This effect was associated with the immune status of the rats. There was a further decrease in Cx43 expression in the rats of the immune-enhanced group. To the best of our knowledge, these results provide the first evidence that LPA causes arrhythmia through the regulation of immune inflammatory cells and the decrease of Cx43 protein expression. The present study provided an experimental basis for the treatment of arrhythmia and may guide clinical care.

Ginsenoside Rg1 inhibits high-voltage-activated calcium channel currents in hippocampal neurons of beta-amyloid peptide-exposed rat brain slices

OBJECTIVE

To examine whether ginsenoside Rg1 (Rg1) inhibits the high-voltage-activated calcium currents (ICa,HVA) via mitogen-activated protein kinase (MAPK) in hippocampal neurons in rat brain slices exposed to beta-amyloid peptide 25-35 (Aβ25-35).

METHODS

An experimental Alzheimer disease (AD) model was prepared by exposure of rat brain slices to Aβ25-35 (10 µmol/L). After treatment with Rg1 (20 µmol/L), the ICa,HVA elicited in hippocampal neurons in these rat brain slices upon depolarization from-40 to 40 mV for 200 ms was recorded by a whole-cell patch clamp to analyze the changes in the peak current density, I-V curve, activation-V curve, and inactivation-V curve.

RESULTS

Exposure of rat brain slices to Aβ led to a significant increase in ICa,HVA, enhancement of the voltage sensitivity of channel activation, and reduction of the voltage sensitivity of channel inactivation in neurons in the hippocampus of rat brain slices. Rg1 treatment significantly inhibited these changes. These effects of Rg1 could be effectively inhibited by the MAPK inhibitor PD98059.

CONCLUSION

Rg1 can inhibit Ica,HVA via MAPK in hippocampal neurons in Aβ-exposed rat brain slices.

Different effects of lysophosphatidic acid on L-type calcium current in neonatal rat ventricular myocytes with and without H2O2 treatment

L-type calcium current (I(Ca-L)) alterations are implicated in various cardiac diseases, and the lysophosphatidic acid (LPA) level increases in several ischemic heart diseases. We investigated the effects of LPA on I(Ca-L) in normal and H2O2-treated neonatal rat ventricular myocytes. LPA treatment (24h) increased the action potential duration (APD) and I(Ca-L) in normal ventricular myocytes, but it decreased these parameters in H2O2-treated myocytes. LPA increased the single-channel open probability of L-type calcium channels in both normal and H2O2-treated myocytes. LPA activated calcineurin (CaN) and induced the cytoplasm-to-nucleus translocation of nuclear factor of activated T-cells (NFAT) in H2O2-treated cardiomyocytes. In H2O2-treated cardiomyocytes, LPA decreased Ca(v)1.2 mRNA and protein expression levels at 4 and 8h, respectively. A CaN inhibitor (FK-506) prevented LPA-induced APD, I(Ca-L), and Ca(v)1.2 mRNA and protein down-regulation. The LPA-induced I(Ca-L) increase in normal cardiomyocytes was CaN-NFAT signaling-independent, and LPA did not affect Ca(v)1.2 mRNA or protein expression. In conclusion, LPA increases the I(Ca-L) in normal ventricular myocytes by increasing the single-channel open probability of L-type calcium channels, and LPA decreases I(Ca-L) in H2O2-treated cardiomyocytes via the CaN-NFAT pathway.

Effects of deformation on transmural dispersion of repolarization using in silico models of human left ventricular wedge

Mechanical deformation affects the electrical activity of the heart through multiple feedback loops. The purpose of this work is to study the effect of deformation on transmural dispersion of repolarization and on surface electrograms using an in silico human ventricular wedge. To achieve this purpose, we developed a strongly coupled electromechanical cell model by coupling a human left ventricle electrophysiology model and an active contraction model reparameterized for human cells. This model was then embedded in tissue simulations on the basis of bidomain equations and nonlinear solid mechanics. The coupled model was used to evaluate effects of mechanical deformation on important features of repolarization and electrograms. Our results indicate an increase in the T-wave amplitude of the surface electrograms in simulations that account for the effects of cardiac deformation. This increased T-wave amplitude can be explained by changes to the coupling between neighboring myocytes, also known as electrotonic effect. The thickening of the ventricular wall during repolarization contributes to the decoupling of cells in the transmural direction, enhancing action potential heterogeneity and increasing both transmural repolarization dispersion and T-wave amplitude of surface electrograms. The simulations suggest that a considerable percentage of the T-wave amplitude (15%) may be related to cardiac deformation.

Reciprocal regulation of miR-23a and lysophosphatidic acid receptor signaling in cardiomyocyte hypertrophy

Earlier, our study demonstrated that lysophosphatidic acid (LPA) receptor mediated cardiomyocyte hypertrophy. However, the subtype-specific functions for LPA1 and LPA3 receptors in LPA-induced hypertrophy have not been distinguished. Growing evidence indicates that microRNAs (miRNAs) are involved in the pathogenesis of cardiac hypertrophy by down-regulating target molecules. The present work therefore aimed at elucidating the functions mediated by different subtypes of LPA receptors and investigating the modulatory role of miRNAs during LPA induced hypertrophy. Experiments were done with cultured neonatal rat cardiomyocytes (NRCMs) exposed to LPA and we showed that knockdown of LPA1 by small interfering RNA (siRNA) enhanced LPA-induced cardiomyocyte hypertrophy, whereas LPA3 silencing repressed hypertrophy. miR-23a, a pro-hypertrophic miRNA, was up-regulated by LPA in cardiomyocytes and its down-regulation reduced LPA-induced cardiomyocyte hypertrophy. Importantly, luciferase reporter assay confirmed LPA1 to be a target of miR-23a, indicating that miR-23a is involved in mediating the LPA-induced cardiomyocyte hypertrophy by targeting LPA1. In addition, knockdown of LPA3, but not LPA1, eliminated miR-23a elevation induced by LPA. And PI3K inhibitor, LY294002, effectively prevented LPA-induced miR-23a expression in cardiomyocytes, suggesting that LPA might induce miR-23a elevation by activating LPA3 and PI3K/AKT pathway. These findings identified opposite subtype-specific functions for LPA1 and LPA3 in mediating cardiomyocyte hypertrophy and indicated LPA1 to be a target of miR-23a, which discloses a link between miR-23a and the LPA receptor signaling in cardiomyocyte hypertrophy.