Youyi Zhang

Intermolecular failure of L-type Ca2+ channel and ryanodine receptor signaling in hypertrophy.

Pressure overload-induced hypertrophy is a key step leading to heart failure. The Ca(2+)-induced Ca(2+) release (CICR) process that governs cardiac contractility is defective in hypertrophy/heart failure, but the molecular mechanisms remain elusive. To examine the intermolecular aspects of CICR during hypertrophy, we utilized loose-patch confocal imaging to visualize the signaling between a single L-type Ca(2+) channel (LCC) and ryanodine receptors (RyRs) in aortic stenosis rat models of compensated (CHT) and decompensated (DHT) hypertrophy. We found that the LCC-RyR intermolecular coupling showed a 49% prolongation in coupling latency, a 47% decrease in chance of hit, and a 72% increase in chance of miss in DHT, demonstrating a state of intermolecular failure. Unexpectedly, these modifications also occurred robustly in CHT due at least partially to decreased expression of junctophilin, indicating that intermolecular failure occurs prior to cellular manifestations. As a result, cell-wide Ca(2+) release, visualized as Ca(2+) spikes, became desynchronized, which contrasted sharply with unaltered spike integrals and whole-cell Ca(2+) transients in CHT. These data suggested that, within a certain limit, termed the stability margin, mild intermolecular failure does not damage the cellular integrity of excitation-contraction coupling. Only when the modification steps beyond the stability margin does global failure occur. The discovery of hidden intermolecular failure in CHT has important clinical implications.

In Vivo Suppression of MicroRNA-24 Prevents the Transition Toward Decompensated Hypertrophy in Aortic-Constricted Mice

Rationale: During the transition from compensated hypertrophy to heart failure, the signaling between L-type Ca2+ channels in the cell membrane/T-tubules and ryanodine receptors in the sarcoplasmic reticulum becomes defective, partially because of the decreased expression of a T-tubule–sarcoplasmic reticulum anchoring protein, junctophilin-2. MicroRNA (miR)-24, a junctophilin-2 suppressing miR, is upregulated in hypertrophied and failing cardiomyocytes. Objective: To test whether miR-24 suppression can protect the structural and functional integrity of L-type Ca2+ channel–ryanodine receptor signaling in hypertrophied cardiomyocytes. Methods and Results: In vivo silencing of miR-24 by a specific antagomir in an aorta-constricted mouse model effectively prevented the degradation of heart contraction, but not ventricular hypertrophy. Electrophysiology and confocal imaging studies showed that antagomir treatment prevented the decreases in L-type Ca2+ channel–ryanodine receptor signaling fidelity/efficiency and whole-cell Ca2+ transients. Further studies showed that antagomir treatment stabilized junctophilin-2 expression and protected the ultrastructure of T-tubule–sarcoplasmic reticulum junctions from disruption. Conclusions: MiR-24 suppression prevented the transition from compensated hypertrophy to decompensated hypertrophy, providing a potential strategy for early treatment against heart failure.

Mir-24 Regulates Junctophilin-2 Expression in Cardiomyocytes

Rationale: Failing cardiomyocytes exhibit decreased efficiency of excitation-contraction (E-C) coupling. The downregulation of junctophilin-2 (JP2), a protein anchoring the sarcoplasmic reticulum to T-tubules, has been identified as a major mechanism underlying the defective E-C coupling. However, the regulatory mechanism of JP2 remains unknown. Objective: To determine whether microRNAs regulate JP2 expression. Methods and Results: Bioinformatic analysis predicted 2 potential binding sites of miR-24 in the 3′-untranslated regions of JP2 mRNA. Luciferase assays confirmed that miR-24 suppressed JP2 expression by binding to either of these sites. In the aortic stenosis model, miR-24 was upregulated in failing cardiomyocytes. Adenovirus-directed overexpression of miR-24 in cardiomyocytes decreased JP2 expression and reduced Ca2+ transient amplitude and E-C coupling gain. Conclusions: MiR-24–mediated suppression of JP2 expression provides a novel molecular mechanism for E-C coupling regulation in heart cells and suggests a new target against heart failure.

14-3-3 Proteins regulate glycogen synthase 3β phosphorylation and inhibit cardiomyocyte hypertrophy

14‐3‐3 Proteins are dimeric phophoserine‐binding molecules that participate in important cellular processes such as cell proliferation, cell‐cycle control and the stress response. In this work, we report that several isoforms of 14‐3‐3s are expressed in neonatal rat cardiomyocytes. To understand their function, we utilized a general 14‐3‐3 peptide inhibitor, R18, to disrupt 14‐3‐3 functions in cardiomyocytes. Cardiomyocytes infected with adenovirus‐expressing YFP‐R18 (AdR18) exhibited markedly increased protein synthesis and atrial natriuretic peptide production and potentiated the responses to norepinephrine stimulation. This response was blocked by the pretreatment with LY294002, a phosphoinositide 3‐kinase (PI3K) inhibitor. Consistent with a role of PI3K in the R18 effect, R18 induced phosphorylation of a protein cloned from the vakt oncogene of retrovirus AKT8 (Aktu2003–u2003also called protein kinase B, PKB) at Ser473 and glycogen synthase 3β (GSK3β) at Ser9, but not extracellular signal‐regulated kinase 1/2 (ERK1/2). AdR18‐induced PKB and GSK3β phosphorylation was completely blocked by LY294002. In addition, a member of the nuclear factor of activated T cells (NFAT) family, NFAT3, was converted into faster mobility forms and translocated into the nucleus upon the treatment of AdR18. These results suggest that 14‐3‐3s inhibits cardiomyocytes hypertrophy through regulation of the PI3K/PKB/GSK3β and NFAT pathway.

Alteration of α1-adrenoceptor subtypes in aortas of 12-month-old spontaneously hypertensive rats

Alterations in α1-adrenoceptor subtypes in aortas from 12-month-old spontaneously hypertensive rats (SHR) were studied in functional studies and RNase protection assays. The norepinephrine-induced contraction, including maximum response and pD2 values, was not significantly different between the SHR and age-matched Kyoto Wistar (WKY) rats. The pA2 values of the α1D-adrenoceptor subtype-selective antagonist BMY7378 (8-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-8-azaspiro(4.5)decane-7,9-dionedihydrochloride) were increased from 8.10±0.12 in WKY rats to 8.45±0.13 in SHR (P 0.05). Preincubation of preparations in 50 μM chloroethylclonidine for 30 min irreversibly inhibited the norepinephrine-induced response more profoundly in aortas from SHR than in aortas from WKY rats. The results of RNase protection assays showed that mRNAs for α1A- and α1B-adrenoceptor subtypes were decreased and that mRNA for the α1D-adrenoceptor subtype was increased in aortas from SHR compared with WKY rats. The results suggested that the α1A-adrenoceptor subtype was decreased and the α1D-adrenoceptor subtype was increased in aortas of 12-month-old SHR.

Transactivated EGFR mediates α1-AR-induced STAT3 activation and cardiac hypertrophy

α(1)-Adrenergic receptor (α(1)-AR) is a crucial mediator of cardiac hypertrophy. Although numerous intracellular pathways have been implicated in α(1)-AR-induced hypertrophy, its precise mechanism remains elusive. We aimed to determine whether α(1)-AR induces cardiac hypertrophy through a novel signaling pathway-α(1)-AR/epidermal growth factor receptor (EGFR)/signal transducer and activator of transcription 3 (STAT3). The activation of STAT3 by α(1)-AR was first demonstrated by tyrosine phosphorylation, nuclear translocation, DNA binding, and transcriptional activity in neonatal Sprague-Dawley rat cardiomyocytes. Activated STAT3 showed an essential role in α(1)-AR-induced cardiomyocyte hypertrophic growth, as assessed by treatment with STAT3 inhibitory peptide and lentivirus-STAT3 small interfering RNA. The results were further confirmed by in vivo experiments involving intraperitoneal injection of the STAT3 inhibitor WP1066 significantly inhibiting phenylephrine-infusion-induced heart hypertrophy in male C57BL/6 mice. Furthermore, the α(1)-AR-activated STAT3 was associated with transactivation of EGFR because inhibition of EGFR with the selective inhibitor AG1478 prevented α(1)-AR-induced STAT3 tyrosine phosphorylation and its transcriptional activity, as well as cardiac hypertrophy. In summary, these results suggest that α(1)-AR induces the activation of STAT3, mainly through transactivation of EGFR, which plays an important role in α(1)-AR-induced cardiac hypertrophy.

Gene expression profile of cardiomyocytes in hypertrophic heart induced by continuous norepinephrine infusion in the rats

Catecholamines play an important role in the development of cardiac hypertrophy. To observe cardiomyocyte-specific gene expression changes induced by catecholamines in vivo, left ventricular cardiomyocytes were isolated from male Sprague-Dawley rats after continuous infusion of norepinephrine (NE; 0.2 mg/kg per hour intravenously) for 0.5, 1, 2, 3 and 7 days. The gene expression profiles of these cells during different NE infusion stages were assessed by using a cDNA microarray, and the microarray data were further analyzed by a clustering method. Cardiac hypertrophy was induced upon continuous NE infusion, with the peak at 3 days. Meanwhile, manifest changes in gene expression profile within cardiomyocytes over the time course were revealed, most of the genes never having been reported to be involved in cardiac hypertrophy. The number of genes displaying differential expression also peaked at the middle stage of infusion (2–3 days), and the majority of the signaling molecules were found differentially expressed mainly at this stage, including phosphatidylinositol 3-kinase, calcium/calmodulin-dependent protein kinase II and non-receptor tyrosine kinases, etc. The tumor suppressor p53 was found up-regulated at very early (0.5 days) and late stages (7 days) of NE infusion. Self-organization clustering analysis revealed subsets of coordinate regulated genes. One set consisted of several enzymes involved in energy metabolism, including carnitine octanoyltransferase, ATP synthase subunit c, pancreatic lipase and glycogen phosphorylase, possessing a similar expression pattern with a rapidly elevated expression level at the early stage of NE infusion. This is the first study to provide transcriptional information for cardiomyocytes, a single cell type, in the heart during the development of cardiac hypertrophy in vivo, and may provide accurate clues to elaborate hypotheses about the evolution of this pathology.

Inhibition of the STAT3 signaling pathway is involved in the antitumor activity of cepharanthine in SaOS2 cells

Aim:To investigate the molecular mechanisms underlying the antitumor activity of cepharanthine (CEP), an alkaloid extracted from Stephania cepharantha Hayata.Methods:Human osteosarcoma cell line SaOS2 was used. MTT assay, Hoechst 33342 nuclear staining, flow cytometry, Western blotting and nude mouse xenografts of SaOS2 cells were applied to examine the antitumor activity of CEP in vitro and in vivo. The expression levels of STAT3 and its downstream signaling molecules were measured with Western blotting and immunochemistry analysis. The activity of STAT3 was detected based on the phosphorylation level of STAT3, luciferase gene reporter assay and translocation of STAT3 to the nucleus.Results:Treatment of SaOS2 cells with CEP (2.5–20xa0μmol/L) inhibited the cell growth in a concentration- and time-dependent manner. CEP (10xa0μmol/L) caused cell cycle arrest at G1 phase and induced apoptosis of SaOS2 cells. CEP (10 and 15xa0μmol/L) significantly decreased the expression of STAT3 in SaOS2 cells. Furthermore, CEP (5 and 10xa0μmol/L) significantly inhibited the expression of target genes of STAT3, including the anti-apoptotic gene Bcl-xL and the cell cycle regulators c-Myc and cyclin D1. In nude mouse xenografts of SaOS2 cells, CEP (20 mg·kg−1·d−1, ip for 19 d) significantly reduced the volume and weight of the tumor.Conclusion:Our findings suggest that inhibition of STAT3 signaling pathway is involved in the anti-tumor activity of CEP.

Activation of α1B-adrenoceptors contributes to intermittent hypobaric hypoxia-improved postischemic myocardial performance via inhibiting MMP-2 activation

Inhibition of matrix metalloproteinases-2 (MMP-2) activation renders cardioprotection from ischemia/reperfusion (I/R) injury; however, the signaling pathways involved have not been fully understood. Intermittent hypobaric hypoxia (IHH) has been shown to enhance myocardial tolerance to I/R injury via triggering intrinsic adaptive responses. Here we investigated whether IHH protects the heart against I/R injury via the regulation of MMP-2 and how the MMP-2 is regulated. IHH (Po2 = 84 mmHg, 4-h/day, 4 wk) improved postischemic myocardial contractile performance, lactate dehydrogenase (LDH) release, and infarct size in isolated perfused rat hearts. Moreover, IHH reversed I/R-induced MMP-2 activation and release, disorders in the levels of MMP-2 regulators, peroxynitrite (ONOO(-)) and tissue inhibitor of metalloproteinase-4 (TIMP-4), and loss of the MMP-2 targets α-actinin and troponin I. This protection was mimicked, but not augmented, by a MMP inhibitor doxycycline and lost by the α1-adrenoceptor (AR) antagonist prazosin. Furthermore, IHH increased myocardial α1A-AR and α1B-AR density but not α1D-AR after I/R. Concomitantly, IHH further enhanced the translocation of PKC epsilon (PKCε) and decreased the release of mitochondrial cytochrome c due to I/R via the activation of α1B-AR but not α1A-AR or α1D-AR. IHH-conferred cardioprotection in the postischemic contractile function, LDH release, MMP-2 activation, and nitrotyrosine as well as TIMP-4 contents were mimicked but not additive by α1-AR stimulation with phenylephrine and were abolished by an α1B-AR antagonist chloroethylclonidine and a PKCε inhibitor PKCε V1-2. These findings demonstrate that IHH exerts cardioprotection through attenuating excess ONOO(-) biosynthesis and TIMP-4 loss and sequential MMP-2 activation via the activation of α1B-AR/PKCε pathway.

Different roles of α1-adrenoceptor subtypes in mediating cardiomyocyte protein synthesis in neonatal rats

1.u2002Three different α1‐adrenoceptor subtypes, designated α1A, α1B and α1D, have been cloned and identified pharmacologically in cardiomyocytes. In vitro studies have suggested that α1‐adrenoceptors play an important role in facilitating cardiac hypertrophy. However, it remains controversial as to which subtype of α1‐adrenoceptors is involved in this response. In the present study, we investigated the different role of each α1‐adrenoceptor subtype in mediating cardiomyocyte protein synthesis, which is a most important characteristic of cardiac hypertrophy in cultured neonatal rat cardiomyocytes.