Xinhua Yan

Left Ventricular Hypertrophy in Ascending Aortic Stenosis Mice Anoikis and the Progression to Early Failure

BACKGROUND To determine potential mechanisms of the transition from hypertrophy to very early failure, we examined apoptosis in a model of ascending aortic stenosis (AS) in male FVB/n mice. METHODS AND RESULTS Compared with age-matched controls, 4-week and 7-week AS animals (n=12 to 16 per group) had increased ratios of left ventricular weight to body weight (4.7+/-0.7 versus 3.1+/-0.2 and 5. 7+/-0.4 versus 2.7+/-0.1 mg/g, respectively, P<0.05) with similar body weights. Myocyte width was also increased in 4-week and 7-week AS mice compared with controls (19.0+/-0.8 and 25.2+/-1.8 versus 14. 1+/-0.5 microm, respectively, P<0.01). By 7 weeks, AS myocytes displayed branching with distinct differences in intercalated disk size and staining for beta(1)-integrin on both cell surface and adjacent extracellular matrix. In vivo left ventricular systolic developed pressure per gram as well as endocardial fractional shortening were similar in 4-week AS and controls but depressed in 7-week AS mice. Myocyte apoptosis estimated by in situ nick end-labeling (TUNEL) was extremely rare in 4-week AS and control mice; however, a low prevalence of TUNEL-positive myocytes and DNA laddering were detected in 7-week AS mice. The specificity of TUNEL labeling was confirmed by in situ ligation of hairpin oligonucleotides. CONCLUSIONS Our findings indicate that myocyte apoptosis develops during the transition from hypertrophy to early failure in mice with chronic biomechanical stress and support the hypothesis that the disruption of normal myocyte anchorage to adjacent extracellular matrix and cells, a process called anoikis, may signal apoptosis.

Neuregulin in Cardiac Hypertrophy in Rats With Aortic Stenosis: Differential Expression of erbB2 and erbB4 Receptors

BACKGROUND Neuregulins are a family of peptide growth factors that promote cell growth and viability. The potential role of neuregulin-erbB signaling in hypertrophic growth and later failure in the adult heart in vivo is not known. METHODS AND RESULTS We used ribonuclease protection assays to quantify mRNA levels of neuregulin, erbB2, and erbB4 in left ventricular (LV) tissue and myocytes of normal rats and rats with aortic stenosis with pressure-overload hypertrophy 6 and 22 weeks after banding. At both stages of hypertrophy, Northern blot analyses of mRNA from LV myocytes showed upregulation of atrial natriuretic peptide, a molecular marker of hypertrophy (P<0.05). LV tissue neuregulin message levels were similar in animals with aortic stenosis compared with controls (P=NS) and were not detectable in myocytes. LV erbB2 and erbB4 message levels in LV tissue and myocytes were maintained during early compensatory hypertrophy in 6-week aortic stenosis animals compared with age-matched controls; in contrast, erbB2 and erbB4 message levels were depressed in 22-week aortic stenosis animals at the stage of early failure (both P<0.01 vs age-matched controls). Immunoblotting of erbB2 and erbB4 also showed normal protein levels in 6-week aortic stenosis animals compared with controls; however, erbB2 and erbB4 protein levels were depressed in 22-week aortic stenosis animals (48% decrease in erbB2, P<0.05, and 43% decrease in erbB4, P<0.01) relative to age-matched controls. CONCLUSIONS The neuregulin receptors erbB2 and erbB4 are downregulated at both the message and protein levels at the stage of early failure in animals with chronic hypertrophy secondary to aortic stenosis. These data suggest a role for disabled erbB receptor signaling in the transition from compensatory hypertrophy to failure.

Contractile reserve and intracellular calcium regulation in mouse myocytes from normal and hypertrophied failing hearts

Mouse myocyte contractility and the changes induced by pressure overload are not fully understood. We studied contractile reserve in isolated left ventricular myocytes from mice with ascending aortic stenosis (AS) during compensatory hypertrophy (4-week AS) and the later stage of early failure (7-week AS) and from control mice. Myocyte contraction and [Ca2+]i transients with fluo-3 were measured simultaneously. At baseline (0.5 Hz, 1.5 mmol/L [Ca2+]o, 25°C), the amplitude of myocyte shortening and peak-systolic [Ca2+]i in 7-week AS were not different from those of controls, whereas contraction, relaxation, and the decline of [Ca2+]i transients were slower. In response to the challenge of high [Ca2+]o, fractional cell shortening was severely depressed with reduced peak-systolic [Ca2+]i in 7-week AS compared with controls. In response to rapid pacing stimulation, cell shortening and peak-systolic [Ca2+]i increased in controls, but this response was depressed in 7-week AS. In contrast, the responses to both challenge with high [Ca2+]o and rapid pacing in 4-week AS were similar to those of controls. Although protein levels of Na+-Ca2+ exchanger were increased in both 4-week and 7-week AS, the ratio of SR Ca2+-ATPase to phospholamban protein levels was depressed in 7-week AS compared with controls but not in 4-week AS. This was associated with an impaired capacity to increase sarcoplasmic reticulum Ca2+ load during high work states in 7-week AS myocytes. In hypertrophied failing mouse myocytes, depressed contractile reserve is related to an impaired augmentation of systolic [Ca2+]i and SR Ca2+ load and simulates findings in human failing myocytes.

Neuregulin-1 attenuated doxorubicin-induced decrease in cardiac troponins

Neuregulin-1 (NRG1) is a potential therapeutic agent for the treatment of doxorubicin (Dox)-induced heart failure. NRG1, however, activates the erbB2 receptor, which is frequently overexpressed in breast cancers. It is, therefore, important to understand how NRG1, via erbB2, protects the heart against Dox cardiotoxicity. Here, we studied NRG1-erbB2 signaling in Dox-treated mice hearts and in isolated neonatal rat ventricular myocytes (NRVM). Male C57BL/6 mice were treated with recombinant NRG1 before and daily after a single dose of Dox. Cardiac function was determined by catheterization. Two-week survival was analyzed by the Kaplan-Meier method. Cardiac troponins [cardiac troponin I (cTnI) and cardiac troponin T (cTnT)] and phosphorylated Akt protein levels were determined in mice hearts and in NRVM by Western blot analysis. Activation of caspases and ubiquitinylation of troponins were determined in NRVM by caspase assay and immunoprecipitation. NRG1 significantly improved survival and cardiac function in Dox-treated mice. NRG1 reduced the decrease in cTnI, cTnT, and cardiac troponin C (cTnC) and maintained Akt phosphorylation in Dox-treated mice hearts. NRG1 reduced the decrease in cTnI and cTnT mRNA and proteins in Dox-treated NRVM. Inhibition of erbB2, phosphoinositide 3-kinase (PI3K), Akt, and mTOR blocked the protective effects of NRG1 on cTnI and cTnT in NRVM. NRG1 significantly reduced Dox-induced caspase activation, which degraded troponins, in NRVM. NRG1 reduced Dox-induced proteasome degradation of cTnI. NRG1 attenuates Dox-induced decrease in cardiac troponins by increasing transcription and translation and by inhibiting caspase activation and proteasome degradation of troponin proteins. NRG1 maintains cardiac troponins by the erbB2-PI3K pathway, which may lessen Dox-induced cardiac dysfunction.

Pressure overload-induced hypertrophy in transgenic mice selectively overexpressing AT2 receptors in ventricular myocytes

The role of the angiotensin II type 2 (AT2) receptor in cardiac hypertrophy remains controversial. We studied the effects of AT2 receptors on chronic pressure overload-induced cardiac hypertrophy in transgenic mice selectively overexpressing AT2 receptors in ventricular myocytes. Left ventricular (LV) hypertrophy was induced by ascending aorta banding (AS). Transgenic mice overexpressing AT2 (AT2TG-AS) and nontransgenic mice (NTG-AS) were studied after 70 days of aortic banding. Nonbanded NTG mice were used as controls. LV function was determined by catheterization via LV puncture and cardiac magnetic resonance imaging. LV myocyte diameter and interstitial collagen were determined by confocal microscopy. Atrial natriuretic polypeptide (ANP) and brain natriuretic peptide (BNP) were analyzed by Northern blot. Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2, inducible nitric oxide synthase (iNOS), endothelial NOS, ERK1/2, p70S6K, Src-homology 2 domain-containing protein tyrosine phosphatase-1, and protein serine/threonine phosphatase 2A were analyzed by Western blot. LV myocyte diameter and collagen were significantly reduced in AT2TG-AS compared with NTG-AS mice. LV anterior and posterior wall thickness were not different between AT2TG-AS and NTG-AS mice. LV systolic and diastolic dimensions were significantly higher in AT2TG-AS than in NTG-AS mice. LV systolic pressure and end-diastolic pressure were lower in AT2TG-AS than in NTG-AS mice. ANP, BNP, and SERCA2 were not different between AT2TG-AS and NTG-AS mice. Phospholamban (PLB) and the PLB-to-SERCA2 ratio were significantly higher in AT2TG-AS than in NTG-AS mice. iNOS was higher in AT2TG-AS than in NTG-AS mice but not significantly different. Our results indicate that AT2 receptor overexpression modified the pathological hypertrophic response to aortic banding in transgenic mice.

Clozapine-induced myocarditis: Role of catecholamines in a murine model

Clozapine, an atypical antipsychotic, is very effective in the treatment of resistant schizophrenia. However, cardiotoxicity of clozapine, particularly in young patients, has raised concerns about its safety. Increased catecholamines have been postulated to trigger an inflammatory response resulting in myocarditis, dilated cardiomyopathy, and death, although this has not yet been thoroughly studied. Here, we used the mouse to study whether clozapine administration could cause adverse myocarditis associated with an increase in catecholamines. Male Balb/C mice, age ~6 weeks, were administered 5, 10 or 25 mg/kg clozapine daily for 7 and 14 days; one group was administered 25 mg/kg clozapine plus 2 mg/kg propranolol for 14 days. Saline-treated mice served as controls. Heart sections were stained with hematoxylin and eosin for histopathological examination. Plasma catecholamines were measured with HPLC. Myocardial TNF-alpha concentrations were determined by ELISA. Histopathology of clozapine-treated mice showed a significant dose-related increase in myocardial inflammation that correlated with plasma catecholamine levels and release of TNF-alpha. Propranolol significantly attenuated these effects. A hypercatecholaminergic state induced by clozapine could explain the occurrence of myocarditis in some patients. Our data suggest that a beta-adrenergic blocking agent may be effective in reducing the incidence and severity of clozapine-induced myocarditis.

Breaking the 'harmony' of TNF-α signaling for cancer treatment.

Tumor necrosis factor-alpha (TNF-α) binds to two distinct receptors, TNFR1/p55 and TNFR2/p75. TNF-α is implicated in the processes of tumor growth, survival, differentiation, invasion, metastases, secretion of cytokines and pro-angiogenic factors. We have shown that TNFR2/p75 signaling promotes ischemia-induced angiogenesis via modulation of several angiogenic growth factors. We hypothesized that TNFR2/p75 may promote tumor growth and angiogenesis. Growth of mouse Lewis lung carcinoma (LLC1) and/or mouse melanoma B16 cell was evaluated in wild type (WT), p75 knockout (KO) and double p55KO/p75KO mouse tumor xenograft models. Compared with WT and p55KO/p75KO mice, growth of tumors in p75KO mice was significantly decreased (twofold) in both LLC and B16 tumors. Tumor growth inhibition was correlated with decreases in vascular endothelial growth factor (VEGF) expression and capillary density, as well as bone marrow-derived endothelial progenitor cells incorporation into the functional capillary network, and an increase in apoptotic cells in LLC xenografts. Gene array analysis of tumor tissues showed a decrease in gene expression in pathways that promote tumor angiogenesis and cell survival. Blocking p75 by short-hairpin RNA in cultured LLCs led to increases in TNF-mediated apoptosis, as well as decreases in the constitutive and TNF-mediated expression of angiogenic growth factors (VEGF, HGF, PLGF), and SDF-1α receptor CXCR4. In summary, p75 is essential for tumor angiogenesis and survival in highly vascularized murine lung tumor xenografts. Blocking p75 expression may lead to tumor regression. This may represent new and effective therapy against lung neoplasms and potentially tumors of other origin.

Improving murine embryonic stem cell differentiation into cardiomyocytes with neuregulin-1: differential expression of microRNA.

Identification of factors that direct embryonic stem (ES) cell (ESC) differentiation into functional cardiomyocytes is essential for successful use of ESC-based therapy for cardiac repair. Neuregulin-1 (NRG1) and microRNA play important roles in the cardiac differentiation of ESCs. Understanding how NRG1 regulates microRNA will provide new mechanistic insights into the role of NRG1 on ESCs. It may also lead to the discovery of novel microRNAs that are important for ESC cardiac differentiation. The objective of this study was to assess the microRNA expression profile during NRG1-induced ESC cardiac differentiation. Murine ESCs were incubated with a recombinant NRG1β or an inhibitor of ErbB2 or ErbB4 during hanging drop-induced cardiac differentiation. The expression of cardiac-specific markers and microRNAs was analyzed by RT-PCR and microRNA array, respectively. We found that the expression of NRG1 and the ErbB receptors was increased during hanging drop-induced cardiac differentiation of ESCs. NRG1 stimulation during a specific developmental window enhanced, while inhibition of the ErbB2 or ErbB4 receptor inhibited, cardiac differentiation of ESCs. NRG1 increased the expression of mmu-miR-296-3p and mmu-miR-200c*, and decreased mmu-miR-465b-5p. Inhibition of mmu-miR-296-3p or mmu-miR-200c* decreased, while inhibition of mmu-miR-465-5p increased, the differentiation of ESCs into the cardiac lineage. This is the first report demonstrating that microRNAs are differentially regulated by NRG1-ErbB signaling during cardiac differentiation of ESCs. This study has also identified new microRNAs that are important for ESC cardiac differentiation.

Cardiovascular Risks Associated with Low Dose Ionizing Particle Radiation

Previous epidemiologic data demonstrate that cardiovascular (CV) morbidity and mortality may occur decades after ionizing radiation exposure. With increased use of proton and carbon ion radiotherapy and concerns about space radiation exposures to astronauts on future long-duration exploration-type missions, the long-term effects and risks of low-dose charged particle irradiation on the CV system must be better appreciated. Here we report on the long-term effects of whole-body proton (1H; 0.5 Gy, 1 GeV) and iron ion (56Fe; 0.15 Gy, 1GeV/nucleon) irradiation with and without an acute myocardial ischemia (AMI) event in mice. We show that cardiac function of proton-irradiated mice initially improves at 1 month but declines by 10 months post-irradiation. In AMI-induced mice, prior proton irradiation improved cardiac function restoration and enhanced cardiac remodeling. This was associated with increased pro-survival gene expression in cardiac tissues. In contrast, cardiac function was significantly declined in 56Fe ion-irradiated mice at 1 and 3 months but recovered at 10 months. In addition, 56Fe ion-irradiation led to poorer cardiac function and more adverse remodeling in AMI-induced mice, and was associated with decreased angiogenesis and pro-survival factors in cardiac tissues at any time point examined up to 10 months. This is the first study reporting CV effects following low dose proton and iron ion irradiation during normal aging and post-AMI. Understanding the biological effects of charged particle radiation qualities on the CV system is necessary both for the mitigation of space exploration CV risks and for understanding of long-term CV effects following charged particle radiotherapy.

Cancer Therapy Targeting the HER2-PI3K Pathway: Potential Impact on the Heart

The HER2-PI3K pathway is the one of the most mutated pathways in cancer. Several drugs targeting the major kinases of this pathway have been approved by the Food and Drug Administration and many are being tested in clinical trials for the treatment of various cancers. However, the HER2-PI3K pathway is also pivotal for maintaining the physiological function of the heart, especially in the presence of cardiac stress. Clinical studies have shown that in patients treated with doxorubicin concurrently with Trastuzumab, a monoclonal antibody that blocks the HER2 receptor, the New York Heart Association class III/IV heart failure was significantly increased compared to those who were treated with doxorubicin alone (16 vs. 3%). Studies in transgenic mice have also shown that other key kinases of this pathway, such as PI3Kα, PDK1, Akt, and mTOR, are important for protecting the heart from ischemia-reperfusion and aortic stenosis induced cardiac dysfunction. Studies, however, have also shown that inhibition of PI3Kγ improve cardiac function of a failing heart. In addition, results from transgenic mouse models are not always consistent with the outcome of the pharmacological inhibition of this pathway. Here, we will review these findings and discuss how we can address the cardiac side-effects caused by inhibition of this important pathway in both cancer and cardiac biology.