Characterization of Monoamine Oxidases in Mesenchymal Stem Cells: Role in Hydrogen Peroxide Generation and Serotonin-Dependent Apoptosis

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Dose-dependent activation of distinct hypertrophic pathways by serotonin in cardiac cells.

International audienceThere is substantial evidence supporting a hypertrophic action of serotonin [5-hydroxytryptamine (5-HT)] in cardiomyocytes. However, little is known about the mechanisms involved. We previously demonstrated that 5-HT-induced hypertrophy depends, in part, on the generation of reactive oxygen species by monoamine oxidase-A (MAO-A) (see Ref. 3). Cardiomyocytes express 5-HT(2) receptors, which may also participate in hypertrophy. Here, we analyzed the respective contribution of 5-HT(2) receptors and MAO-A in H9C2 cardiomyoblast hypertrophy. 5-HT induced a dose-dependent increase in [(3)H]leucine incorporation and stimulation of two markers of cardiac hypertrophy, ANF-luc and alphaSK-actin-luc reporter genes. Experiments using 1 microM 5-HT showed that hypertrophic response occurred independently from MAO-A. Using pharmacological inhibitors (M100907 and ketanserin), we identified a novel mechanism of action involving 5-HT(2A) receptors and requiring Ca(2+)/calcineurin/nuclear factor of activated T-cell activation. The activation of this hypertrophic pathway was fully prevented by 5-HT(2A) inhibitors and was unaffected by MAO inhibition. When 10 microM 5-HT was used, an additional hypertrophic response, prevented by the MAO inhibitors pargyline and RO 41-1049, was observed. Unlike the 5-HT(2A)-receptor-mediated H9C2 cell hypertrophy, MAO-A-dependent hypertrophic response required activation of extracellular-regulated kinases. In conclusion, our results show the existence of a dose-dependent shift of activation of distinct intracellular pathways involved in 5-HT-mediated hypertrophy of cardiac cells

Serotonin 5-HT2A receptor-mediated hypertrophy is negatively regulated by caveolin-3 in cardiomyoblasts and neonatal cardiomyocytes.

International audienceThe serotonin 5-HT(2A) receptor belongs to the G-protein-coupled receptors (GPCRs) superfamily and mediates the hypertrophic response to serotonin (5-HT) in cardiac myocytes. At present the regulatory mechanisms of 5-HT(2A) receptor-induced myocyte hypertrophy are not fully understood. The localization and the compartmentation of GPCRs within specialized membrane microdomains are known to modulate their signalling pathway. Therefore, we hypothesized that caveolae microdomains and caveolin-3, the predominant isoform of cardiac caveolae, might be regulators of 5-HT(2A) receptor signalling. We demonstrate that 5-HT(2A) receptors interact with caveolin-3 upon 5-HT stimulation and traffic into caveolae membrane microdomains. We provide evidence that caveolin-3 knockdown abolishes the redistribution of 5-HT(2A) receptors into caveolae and enhances 5-HT(2A) receptor-induced myocyte hypertrophic markers such as cell size, protein synthesis and ANF gene expression. Importantly, we demonstrate that caveolin-3 and caveolae structures are negative regulators of 5-HT(2A) receptor-induced nuclear factor of activated T cells (NFAT) transcriptional activation. Taken together, our data demonstrate that caveolin-3 and caveolae microdomains are important regulators of the hypertrophic response induced by 5-HT(2A) receptors. These findings thus open new insights to target heart hypertrophy under the enhanced serotonin system. This article is part of a Special Issue entitled "Local Signaling in Myocytes"

Genetic deletion of MAO-A promotes serotonin-dependent ventricular hypertrophy by pressure overload.

International audienceThe potential role of serotonin (5-HT) in cardiac function has generated much interest in recent years. In particular, the need for a tight regulation of 5-HT to maintain normal cardiovascular activity has been demonstrated in different experimental models. However, it remains unclear how increased levels of 5-HT could contribute to the development of cardiac hypertrophy. Availability of 5-HT depends on the mitochondrial enzyme monoamine oxidase A (MAO-A). Therefore, we investigated the consequences of MAO-A deletion on ventricular remodeling in the model of aortic banding in mice. At baseline, MAO-A deletion was associated with an increase in whole blood 5-HT (39.4+/-1.9 microM vs. 24.0+/-0.9 microM in KO and WT mice, respectively). Cardiac 5-HT(2A), but not 5-HT(2B) receptors were overexpressed in MAO-A KO mice, as demonstrated by real-time PCR and Western-blot experiments. After aortic banding, MAO-A KO mice demonstrated greater increase in heart wall thickness, heart to body weight ratios, cardiomyocyte cross-section areas, and myocardial fibrosis compared to WT. Exacerbation of hypertrophy in KO mice was associated with increased amounts of 5-HT in the heart. In order to determine the role of 5-HT and 5-HT(2A) receptors in ventricular remodeling in MAO-A KO mice, we administered the 5-HT(2A) receptor antagonists ketanserin (1 mg/kg/day) or M100907 (0.1 mg/kg/day) during 4 weeks of aortic banding. Chronic administration of these antagonists strongly prevented exacerbation of ventricular hypertrophy in MAO-A KO mice. These results show for the first time that regulation of peripheral 5-HT by MAO-A plays a role in ventricular remodeling via activation of 5-HT(2A) receptors

Mesenchymal Stem Cells Promote Matrix Metalloproteinase Secretion By Cardiac Fibroblasts And Reduce Cardiac Ventricular Fibrosis After Myocardial Infarction.

International audienceRecent studies showed that mesenchymal stem cells (MSCs) transplantation significantly decreased cardiac fibrosis. However, the mechanisms involved in these effects are still poorly understood. In this work, we investigated whether the antifibrotic properties of MSCs involve the regulation of matrix metalloproteinases (MMPs) and matrix metalloproteinase endogenous inhibitor (TIMPs) production by cardiac fibroblasts.In vitro experiments showed that conditioned medium from MSCs decreased viability, alpha-SMA expression and collagen secretion of cardiac fibroblasts. These effects were concomitant to the stimulation of MMP-2/MMP-9 activities and MT1-MMP expression. Experiments performed with fibroblasts from MMP2-/- mice demonstrated that MMP2 plays a preponderant role in preventing collagen accumulation upon incubation with conditioned-medium from MSCs. Interestingly, we found that MSC-conditioned medium also decreased the expression of TIMP2 in cardiac fibroblasts. In vivo studies showed that intracardiac injection of MSCs in a rat model of post-ischemic heart failure induced a significant decrease in ventricular fibrosis. This effect was associated with the improvement of morphological and functional cardiac parameters.In conclusion, we showed that MSCs modulate the phenotype of cardiac fibroblasts and their ability to degrade extracellular matrix. These properties of MSCs open new perspective for understanding of the mechanisms of action of MSCs and anticipate their potential therapeutic or side effects

p53-PGC-1α Pathway Mediates Oxidative Mitochondrial Damage and Cardiomyocyte Necrosis Induced by Monoamine Oxidase-A Upregulation: Role in Chronic Left Ventricular Dysfunction in Mice

International audienceAIMS:Oxidative stress and mitochondrial dysfunction participate together in the development of heart failure (HF). mRNA levels of monoamine oxidase-A (MAO-A), a mitochondrial enzyme that produces hydrogen peroxide (H(2)O(2)), increase in several models of cardiomyopathies. Therefore, we hypothesized that an increase in cardiac MAO-A could cause oxidative stress and mitochondrial damage, leading to cardiac dysfunction. In the present study, we evaluated the consequences of cardiac MAO-A augmentation on chronic oxidative damage, cardiomyocyte survival, and heart function, and identified the intracellular pathways involved.RESULTS:We generated transgenic (Tg) mice with cardiac-specific MAO-A overexpression. Tg mice displayed cardiac MAO-A activity levels similar to those found in HF and aging. As expected, Tg mice showed a significant decrease in the cardiac amounts of the MAO-A substrates serotonin and norepinephrine. This was associated with enhanced H(2)O(2) generation in situ and mitochondrial DNA oxidation. As a consequence, MAO-A Tg mice demonstrated progressive loss of cardiomyocytes by necrosis and ventricular failure, which were prevented by chronic treatment with the MAO-A inhibitor clorgyline and the antioxidant N-acetyl-cystein. Interestingly, Tg hearts exhibited p53 accumulation and downregulation of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), a master regulator of mitochondrial function. This was concomitant with cardiac mitochondrial ultrastructural defects and ATP depletion. In vitro, MAO-A adenovirus transduction of neonatal cardiomyocytes mimicked the results in MAO-A Tg mice, triggering oxidative stress-dependent p53 activation, leading to PGC-1α downregulation, mitochondrial impairment, and cardiomyocyte necrosis.INNOVATION AND CONCLUSION:We provide the first evidence that MAO-A upregulation in the heart causes oxidative mitochondrial damage, p53-dependent repression of PGC-1α, cardiomyocyte necrosis, and chronic ventricular dysfunction