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.

AIMS: 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.

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

The 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".

Mitochondrial proteomic approach reveals galectin-7 as a novel BCL-2 binding protein in human cells.

Although the anti-apoptotic activity of Bcl-2 has been extensively studied, its mode of action remains incompletely understood. Deciphering the network of Bcl-2 interacting factors is necessary to better understand the key function of Bcl-2 in apoptosis initiation. To identify novel Bcl-2 mitochondrial partners, we have combined a Bcl-2 immunocapture with a mass spectrometry analysis using highly pure mitochondrial fractions isolated from human cancer cells. We identified at high confidence 127 potential Bcl-2-interacting proteins. Gene ontology mining reveals enrichment for mitochondrial proteins, endoplasmic reticulum-associated proteins, and cytoskeleton-associated proteins. Importantly, we report the identification of galectin-7 (Gal7), a member of a family of ß-galactoside-binding lectins that was already known to exhibit a pro-apoptotic function, as a new mitochondrial Bcl-2 interacting partner. Our data further show that endogenous Bcl-2 coimmunoprecipitates with Gal7 and that recombinant Gal7 directly interacts with recombinant Bcl-2. A fraction of Gal7 is constitutively localized at mitochondria in a Bcl-2-dependent manner and sensitizes the mitochondria to the apoptotic signal. In addition, we show that the Bcl-2/Gal7 interaction is abolished following genotoxic stress. Taken together, our findings suggest that the binding of Gal7 to Bcl-2 may constitute a new target for enhancing the intrinsic apoptosis pathway.

The cell envelope structure and properties of Mycobacterium smegmatis mc(2)155: is there a clue for the unique transformability of the strain?

Mycobacterium smegmatis is often used as a surrogate host for pathogenic mycobacteria, especially since the isolation of the transformable smooth morphotype strain mc(2)155 from the isogenic rough wild-type strain ATCC 607. Biochemical analysis of the cell envelope components revealed a lack of polar glycolipids, namely the lipooligosaccharides and the polar subfamilies of glycopeptidolipids, in the mc(2)155 strain. In addition, the latter strain differs from its parent by the distribution of various species of glycolipids and phospholipids between the outermost and deeper layers of the cell envelope. The presence of filamentous and rope-like structures at the cell surface of mc(2)155 cells grown in complex media further supported an ultrastructural change in the cell envelope of the mutant. Importantly, a significantly more rapid uptake of the hydrophobic chenodeoxycholate was observed for the mutant compared to wild-type cells. Taken together, these data indicate that the nature of the surface-exposed and envelope constituents is crucial for the surface properties, cell wall permeability and bacterial phenotype, and suggest that the transformable character of the mc(2)155 strain may be in part explained by these profound modifications of its cell envelope.

Mycobacteria use their surface-exposed glycolipids to infect human macrophages through a receptor-dependent process.

Two subfamilies of the polar glycopeptidolipids (GPLs) located on the surface of Mycobacterium smegmatis, along with unknown phospholipids, were recently shown to participate in the nonopsonic phagocytosis of mycobacteria by human macrophages (Villeneuve, C., G. Etienne, V. Abadie, H. Montrozier, C. Bordier, F. Laval, M. Daffe, I. Maridonneau-Parini, and C. Astarie-Dequeker. 2003. Surface-exposed glycopeptidolipids of Mycobacterium smegmatis specifically inhibit the phagocytosis of mycobacteria by human macrophages. Identification of a novel family of glycopeptidolipids. J. Biol. Chem. 278: 51291-51300). As demonstrated herein, a phospholipid mixture that derived from the methanol-insoluble fraction inhibited the phagocytosis of M. smegmatis. Inhibition was essentially attributable to phosphatidylinositol mannosides (PIMs), namely PIM2 and PIM6, because the purified phosphatidylethanolamine, phosphatidylglycerol, and phosphatidylinositol were inactive. This was further confirmed using purified PIM2 and PIM6 from M. bovis BCG that decreased by half the internalization of M. smegmatis. Both compounds also inhibited the uptake of M. tuberculosis and M. avium but had no effect on the internalization of zymosan used as a control particle of the phagocytic process. When coated on latex beads, PIM2 and polar GPL (GPL III) favored the particle entry through complement receptor 3. GPL III, but not PIM2, also directed particle entry through the mannose receptor. Therefore, surface-exposed mycobacterial PIM and polar GPL participate in the receptor-dependent internalization of mycobacteria in human macrophages.

Surface-exposed glycopeptidolipids of Mycobacterium smegmatis specifically inhibit the phagocytosis of mycobacteria by human macrophages. Identification of a novel family of glycopeptidolipids.

Phagocytosis by macrophages represents the early step of the mycobacterial infection. It is governed both by the nature of the host receptors used and the ligands exposed on the bacteria. The outermost molecules of the nonpathogenic Mycobacterium smegmatis were extracted by a mechanical treatment and found to specifically and dose dependently inhibit the phagocytosis of both M. smegmatis and the opportunistic pathogen M. kansasii by human macrophages derived from monocytes. The inhibitory activity was attributed to surface lipids because it is extracted by chloroform and reduced by alkaline hydrolysis but not by protease treatment. Fractionation of surface lipids by adsorption chromatography indicated that the major inhibitory compounds consisted of phospholipids and glycopeptidolipids (GPLs). Mass spectrometry and nuclear magnetic resonance spectroscopy analyses, combined with chemical degradation methods, demonstrated the existence of a novel family of GPLs that consists of a core composed of the long-chain tripeptidyl amino-alcohol with a di-O-acetyl-6-deoxytalosyl unit substituting the allo-threoninyl residue and a 2-succinyl-3,4-di-O-CH3-rhamnosyl unit linked to the alaninol end of the molecules. These compounds, as well as diglycosylated GPLs at the alaninol end and de-O-acylated GPLs, but not the non-serovar-specific di-O-acetylated GPLs, inhibited the phagocytosis of M. smegmatis and M. avium by human macrophages at a few nanomolar concentration without affecting the rate of zymosan internalization. At micromolar concentrations, the native GPLs also inhibit the uptake of both M. tuberculosis and M. kansasii. De-O-acylation experiments established the critical roles of both the succinyl and acetyl substituents. Collectively, these data provide evidence that surface-exposed mycobacterial glycoconjugates are efficient competitors of the interaction between macrophages and mycobacteria and, as such, could represent pharmacological tools for the control of mycobacterial infections.

The impact of the absence of glycopeptidolipids on the ultrastructure, cell surface and cell wall properties, and phagocytosis of Mycobacterium smegmatis.

Glycopeptidolipids (GPLs) are a class of species- or type-specific mycobacterial lipids and major constituents of the cell envelopes of many non-tuberculous mycobacteria. To determine the function of GPLs in the physiology of these bacteria, a mutant of Mycobacterium smegmatis in which the gene encoding a mycobacterial nonribosomal peptide synthetase has been inactivated by transposon mutagenesis was analysed. Labelling experiments indicated that half of the bacterial GPLs were located on the cell surface and represented 85% of the surface-exposed lipids of the parent strain whereas the mutant was defective in the production of the GPLs. Compared to the parent smooth morphotype strain, the GPL-deficient mutant strain exhibited a rough colony morphology, an increase of the cell hydrophobicity and formed huge aggregates. As a consequence, the mutant cells were no longer able to bind ruthenium red, as observed by transmission electron microscopy. The altered surface properties of the mutant cells also affected the phagocytosis of individual bacilli by human monocyte-derived macrophages since mutant cells were internalized more rapidly than cells from the parent strain. Nevertheless, no specific release of surface constituents into the culture broth of the mutant was observed, indicating that the cell surface is composed of substances other than GPLs and that these are essential for maintaining the architecture of the outermost layer of the cell envelope. Importantly, the absence of these major extractable lipids of M. smegmatis from the mutant strain has a profound effect on the uptake of the hydrophobic chenodeoxycholate by cells, indicating that GPLs are involved in the cell wall permeability barrier of M. smegmatis. Altogether, these data showed that, in addition to being distinctive markers of numerous mycobacterial species, GPLs play a role in the bacterial phenotype, surface properties and cell wall permeability.