Myocardial Hypertrophic Remodeling and Impaired Left Ventricular Function in Mice with a Cardiac-Specific Deletion of Janus Kinase 2.

The Janus kinase (JAK) system is involved in numerous cell signaling processes and is highly expressed in cardiac tissue. The JAK isoform JAK2 is activated by numerous factors known to influence cardiac function and pathologic conditions. However, although abundant, the role of JAK2 in the regulation or maintenance of cardiac homeostasis remains poorly understood. Using the Cre-loxP system, we generated a cardiac-specific deletion of Jak2 in the mouse to assess the effect on cardiac function with animals followed up for a 4-month period after birth. These animals had marked mortality during this period, although at 4 months mortality in male mice (47%) was substantially higher compared with female mice (30%). Both male and female cardiac Jak2-deleted mice had hypertrophy, dilated cardiomyopathy, and severe left ventricular dysfunction, including a marked reduction in ejection fractions as assessed by serial echocardiography, although the responses in females were somewhat less severe. Defective cardiac function was associated with altered protein levels of sarcoplasmic reticulum calcium-regulatory proteins particularly in hearts from male mice that had depressed levels of SERCA2 and phosphorylated phospholamban. In contrast, SERCA2 was unchanged in hearts of female mice, whereas phosphorylated phospholamban was increased. Our findings suggest that cardiac JAK2 is critical for maintaining normal heart function, and its ablation produces a severe pathologic phenotype composed of myocardial remodeling, heart failure, and pronounced mortality.

The obesity-related peptide leptin sensitizes cardiac mitochondria to calcium-induced permeability transition pore opening and apoptosis.

The obesity-related 16 kDa peptide leptin is synthesized primarily in white adipocytes although its production has been reported in other tissues including the heart. There is emerging evidence that leptin may contribute to cardiac pathology especially that related to myocardial remodelling and heart failure. In view of the importance of mitochondria to these processes, the goal of the present study is to determine the effect of leptin on mitochondria permeability transition pore opening and the potential consequence in terms of development of apoptosis. Experiments were performed using neonatal rat ventricular myocytes exposed to 3.1 nM (50 ng/ml) leptin for 24 hours. Mitochondrial transition pore opening was analyzed as the capacity of mitochondria to retain the dye calcein-AM in presence of 200 µM CaCl2. Leptin significantly increased pore opening although the effect was markedly more pronounced in digitonin-permeabilized myocytes in the presence of calcium with both effects prevented by the transition pore inhibitor sanglifehrin A. These effects were associated with increased apoptosis as evidenced by increased TUNEL staining and caspase 3 activity, both of which were prevented by the transition pore inhibitor sanglifehrin A. Leptin enhanced Stat3 activation whereas a Stat 3 inhibitor peptide prevented leptin-induced mitochondrial transition pore opening as well as the hypertrophic and pro-apoptotic effects of the peptide. Inhibition of the RhoA/ROCK pathway prevented the hypertrophic response to leptin but had no effect on increased pore opening following leptin administration. We conclude that leptin can enhance calcium-mediated, Stat3-dependent pro-apoptotic effects as a result of increased mitochondrial transition pore opening and independently of its hypertrophic actions. Leptin may therefore contribute to mitochondrial dysfunction and the development of apoptosis in the diseased myocardium particularly under conditions of excessive intracellular calcium accumulation.

Ginseng reverses established cardiomyocyte hypertrophy and postmyocardial infarction-induced hypertrophy and heart failure.

BACKGROUND: A major challenge in the treatment of heart failure is the ability to reverse already-established myocardial remodeling and ventricular dysfunction, with few available pharmacological agents prescribed for the management of heart failure having demonstrated successful reversal of the remodeling and hypertrophic processes. North American ginseng (Panax quinquefolius) has previously been shown to effectively prevent cardiomyocyte hypertrophy and heart failure. Here, we determined whether North American ginseng can reverse established cardiomyocyte hypertrophy in cultured myocytes as well as hypertrophy and left ventricular dysfunction in experimental heart failure secondary to coronary artery occlusion. METHODS AND RESULTS: Ginseng was administered in drinking water (0.9 g/L) ad libitum to rats after 4 weeks of sustained coronary artery ligation when heart failure was established or to angiotensin II- (100 nmol/L), endothelin-1- (10 nmol/L), or phenylephrine- (10 µmol/L) induced hypertrophic cultured neonatal ventricular cardiomyocytes. Echocardiographic and catheter-based measurements of hemodynamic parameters 4 weeks after starting ginseng treatment (8 weeks postinfarction) revealed nearly complete reversibility of systolic and diastolic abnormalities. Similarly, ginseng administration to hypertrophic cardiomyocytes resulted in a complete reversal to a normal phenotype after 24 hours as determined by cell surface area and expression of molecular markers. The effects of ginseng both in vivo and in cultured cardiomyocytes were associated with reversal of calcineurin activation and reduced nuclear translocation of the transcription factor NFAT3 (nuclear factor of activated T cells 3) in cultured myocytes. Moreover, the beneficial effect of ginseng was associated with normalization in the gene expression of profibrotic markers, including collagen (I and III) and fibronectin. CONCLUSIONS: This study demonstrates a marked ability of ginseng to reverse cardiac hypertrophy, myocardial remodeling, and heart failure, which was associated with and likely mediated by reversal of calcineurin activation. Ginseng may offer a potentially effective approach to reverse the myocardial remodeling and heart failure processes, particularly in combination with other treatment modalities.

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.