Molecular and functional interactions between apolipoprotein O and caveolin 3 in the heart : implication in the development of metabolic disorder-associated cardiomyopathy
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Abstract EN:
With the dramatic modification of the lifestyle in the last century, new pandemic public health issues have emerged. The increase in the prevalence of obesity and its strong association with cardiovascular diseases has aroused interest in the understanding of mechanisms linking metabolic disorders and cardiac dysfunctions. It has recently emerged that cardiac altered energy metabolism, lipotoxicity, insulin resistance and mitochondrial alterations are leading causes in the development of metabolic or diabetic cardiomyopathy. Through a functional genomics study aimed at identifying genes differentially regulated in the heart by obesity, we discovered a new apolipoprotein (ApoO) which is also overexpressed in the myocardium from diabetic patients. In attempt to uncover how changes in the expression of this protein relate to modifications of cardiac function, we used cardiac myoblasts, human heart samples a well as cardiac specific transgenic mouse lines constitutively expressing ApoO at physiological levels. We show that ApoO localizes within mitochondria and induces mitochondrial dysfunction in mouse and human heart. ApoO interacts with adenine nucleotide translocase (ANT) which is known as mitochondrial permeability transition pore (mtPTP) regulator. This interaction enhances mtPTP opening, thereby inducing "mild uncoupling". Consequently, mitochondrial respiration, oxidative phosphorylation and fatty acid metabolism are enhanced. This cascade of events generates a mitochondrial metabolic sink whereby cells accumulate lipids and lipotoxic byproducts leading to apoptosis, loss of cardiac cells and cardiomyopathy, mimicking the metabolic phenotype of the diabetic heart. As a spin-off of these observations, we proposed a model for the original molecular mechanisms accounting for ApoO induced mitochondrial dysfunction and lipotoxicity. Besides, we observed that ApoO expressing cardiomyocytes develop adaptive mechanisms to protect cells from the ApoO-induced excessive oxidative metabolism. As revealed in human auricular heart samples and expression database from human heart ventricles, Caveolin-3 (Cav-3) expression is positively correlated to ApoO levels. Cav-3, the main caveolin isoform in cardiac myocytes, is known to have scaffolding domains that anchor and regulate the function of proteins, thereby modulating a variety of cellular processes. These properties make Cav-3 as an actor for cardiac protection. Interestingly, ApoO-induced metabolic stress, both in mouse heart and in vitro in cardiac cells, leads to a rise in Cav-3 levels and its translocation to mitochondria where it interacts with ApoO, through a direct association between the C-terminal scaffolding domain of Cav-3 and the specific aromatic caveolin binding motif (CBM) of ApoO. Blue native polyacrylamide gel electrophoresis of mouse heart mitochondrial protein complexes reveals that ApoO and Cav-3 are present in the same macromolecular complex with known mtPTP regulators. We show that ApoO and Cav-3 interaction results in a protective effect through reduction of ApoO-induced mild uncoupling and consequently restoration of coupled respiration and reduction in apoptosis. Site-directed mutagenesis in ApoO CBM domain prevents its interaction with Cav-3 and led to a loss of Cav-3-mediated protection, as reflected by strongly enhanced uncoupling which is considered as one hallmark of mitochondrial dysfunctions. Therefore, the involvement of ApoO and Cav-3 in mitochondrial homeostasis may reveal novel strategies to control pathophysiological situations involving mitochondrial dysfunctions, such as metabolic disorders and cardiomyopathies.
Abstract FR:
L'apolipoprotéine O (ApoO) est une nouvelle apolipoprotéine connue pour être régulée positivement dans le cœur de l'obèse et du diabétique. Dans cette thèse, nous montrons que l'ApoO est une protéine mitochondriale qui interagit avec l'adénine nucléotide translocase (ANT), une protéine régulatrice du pore de perméabilité de transition mitochondrial (mtPTP). Ce pore, crucial pour le destin de la cellule, peut en situation d'ouverture permanente induire la mort cellulaire par apoptose ou nécrose. Toutefois, une ouverture transitoire du pore est associée à des régulations homéostatiques ioniques. Nous rapportons ici que l'ApoO par son interaction avec l'ANT, conduit à l'ouverture ménagée du mtPTP, induisant un découplage de la respiration mitochondriale, un shift métabolique de la cellule cardiaque vers l'entrée, l'oxydation et l'accumulation massive de lipides, conduisant in fine à la lipotoxicité cardiaque, à la dysfonction mitochondriale et à l'apoptose des cellules. Néanmoins, la fonctionnalité de l'ApoO apparaît régulée par la cavéoline 3 (Cav-3), une protéine bien connue pour son rôle dans l'homéostasie lipidique, dans la régulation des processus cellulaires et dans les mécanismes de cardioprotection. Nous montrons que la Cav-3 est augmentée par la surexpression de l'ApoO via une voie dépendante du cholestérol. Le stress métabolique induit par l'action de l'ApoO sur le mtPTP conduit à une translocation de Cav-3 à la mitochondrie où elle interagit avec l'ApoO. Cette interaction est cruciale pour le maintien du cycle ouverture-fermeture du mtPTP, car elle module l'interaction ApoO-ANT, rétablit une respiration couplée à la production ATP, permettant ainsi à la cellule cardiaque de retrouver un métabolisme efficace, de réduire la dysfonction mitochondriale et la mort cellulaire. Les rôles de l'ApoO et de la Cav-3 dans l'homéostasie mitochondriale suggèrent de nouvelles stratégies pour contrôler les situations pathophysiologiques associées aux dysfonctions mitochondriales, telles que les désordres métaboliques et les cardiomyopathies.