Roberto Bravo-Sagua

Tumor Suppression and Promotion by Autophagy

Autophagy is a highly regulated catabolic process that involves lysosomal degradation of proteins and organelles, mostly mitochondria, for the maintenance of cellular homeostasis and reduction of metabolic stress. Problems in the execution of this process are linked to different pathological conditions, such as neurodegeneration, aging, and cancer. Many of the proteins that regulate autophagy are either oncogenes or tumor suppressor proteins. Specifically, tumor suppressor genes that negatively regulate mTOR, such as PTEN, AMPK, LKB1, and TSC1/2 stimulate autophagy while, conversely, oncogenes that activate mTOR, such as class I PI3K, Ras, Rheb, and AKT, inhibit autophagy, suggesting that autophagy is a tumor suppressor mechanism. Consistent with this hypothesis, the inhibition of autophagy promotes oxidative stress, genomic instability, and tumorigenesis. Nevertheless, autophagy also functions as a cytoprotective mechanism under stress conditions, including hypoxia and nutrient starvation, that promotes tumor growth and resistance to chemotherapy in established tumors. Here, in this brief review, we will focus the discussion on this ambiguous role of autophagy in the development and progression of cancer.

Cell Death and Survival Through the Endoplasmic Reticulum-Mitochondrial Axis

The endoplasmic reticulum has a central role in biosynthesis of a variety of proteins and lipids. Mitochondria generate ATP, synthesize and process numerous metabolites, and are key regulators of cell death. The architectures of endoplasmic reticulum and mitochondria change continually via the process of membrane fusion, fission, elongation, degradation, and renewal. These structural changes correlate with important changes in organellar function. Both organelles are capable of moving along the cytoskeleton, thus changing their cellular distribution. Numerous studies have demonstrated coordination and communication between mitochondria and endoplasmic reticulum. A focal point for these interactions is a zone of close contact between them known as the mitochondrial-associated endoplasmic reticulum membrane (MAM), which serves as a signaling juncture that facilitates calcium and lipid transfer between organelles. Here we review the emerging data on how communication between endoplasmic reticulum and mitochondria can modulate organelle function and determine cellular fate.

Dinámica mitocondrial: un potencial nuevo blanco terapéutico para la insuficiencia cardiaca

Mitochondria are dynamic organelles able to vary their morphology between elongated interconnected mitochondrial networks and fragmented disconnected arrays, through events of mitochondrial fusion and fission, respectively. These events allow the transmission of signaling messengers and exchange of metabolites within the cell. They have also been implicated in a variety of biological processes including embryonic development, metabolism, apoptosis, and autophagy. Although the majority of these studies have been confined to noncardiac cells, emerging evidence suggests that changes in mitochondrial morphology could participate in cardiac development, the response to ischemia-reperfusion injury, heart failure, and diabetes mellitus. In this article, we review how the mitochondrial dynamics are altered in different cardiac pathologies, with special emphasis on heart failure, and how this knowledge may provide new therapeutic targets for treating cardiovascular diseases.

ER-to-mitochondria miscommunication and metabolic diseases

Eukaryotic cells contain a variety of subcellular organelles, each of which performs unique tasks. Thus follows that in order to coordinate these different intracellular functions, a highly dynamic system of communication must exist between the various compartments. Direct endoplasmic reticulum (ER)-mitochondria communication is facilitated by the physical interaction of their membranes in dedicated structural domains known as mitochondria-associated membranes (MAMs), which facilitate calcium (Ca(2+)) and lipid transfer between organelles and also act as platforms for signaling. Numerous studies have demonstrated the importance of MAM in ensuring correct function of both organelles, and recently MAMs have been implicated in the genesis of various human diseases. Here, we review the salient structural features of interorganellar communication via MAM and discuss the most common experimental techniques employed to assess functionality of these domains. Finally, we will highlight the contribution of MAM to a variety of cellular functions and consider the potential role of MAM in the genesis of metabolic diseases. In doing so, the importance for cell functions of maintaining appropriate communication between ER and mitochondria will be emphasized.

Dexamethasone-induced autophagy mediates muscle atrophy through mitochondrial clearance

Glucocorticoids, such as dexamethasone, enhance protein breakdown via ubiquitin–proteasome system. However, the role of autophagy in organelle and protein turnover in the glucocorticoid-dependent atrophy program remains unknown. Here, we show that dexamethasone stimulates an early activation of autophagy in L6 myotubes depending on protein kinase, AMPK, and glucocorticoid receptor activity. Dexamethasone increases expression of several autophagy genes, including ATG5, LC3, BECN1, and SQSTM1 and triggers AMPK-dependent mitochondrial fragmentation associated with increased DNM1L protein levels. This process is required for mitophagy induced by dexamethasone. Inhibition of mitochondrial fragmentation by Mdivi-1 results in disrupted dexamethasone-induced autophagy/mitophagy. Furthermore, Mdivi-1 increases the expression of genes associated with the atrophy program, suggesting that mitophagy may serve as part of the quality control process in dexamethasone-treated L6 myotubes. Collectively, these data suggest a novel role for dexamethasone-induced autophagy/mitophagy in the regulation of the muscle atrophy program.

Mitochondrial fragmentation impairs insulin-dependent glucose uptake by modulating Akt activity through mitochondrial Ca2+ uptake

Insulin is a major regulator of glucose metabolism, stimulating its mitochondrial oxidation in skeletal muscle cells. Mitochondria are dynamic organelles that can undergo structural remodeling in order to cope with these ever-changing metabolic demands. However, the process by which mitochondrial morphology impacts insulin signaling in the skeletal muscle cells remains uncertain. To address this question, we silenced the mitochondrial fusion proteins Mfn2 and Opa1 and assessed insulin-dependent responses in L6 rat skeletal muscle cells. We found that mitochondrial fragmentation attenuates insulin-stimulated Akt phosphorylation, glucose uptake and cell respiratory rate. Importantly, we found that insulin induces a transient rise in mitochondrial Ca(2+) uptake, which was attenuated by silencing Opa1 or Mfn2. Moreover, treatment with Ruthenium red, an inhibitor of mitochondrial Ca(2+) uptake, impairs Akt signaling without affecting mitochondrial dynamics. All together, these results suggest that control of mitochondrial Ca(2+) uptake by mitochondrial morphology is a key event for insulin-induced glucose uptake.

Defective insulin signaling and mitochondrial dynamics in diabetic cardiomyopathy.

Diabetic cardiomyopathy (DCM) is a common consequence of longstanding type 2 diabetes mellitus (T2DM) and encompasses structural, morphological, functional, and metabolic abnormalities in the heart. Myocardial energy metabolism depends on mitochondria, which must generate sufficient ATP to meet the high energy demands of the myocardium. Dysfunctional mitochondria are involved in the pathophysiology of diabetic heart disease. A large body of evidence implicates myocardial insulin resistance in the pathogenesis of DCM. Recent studies show that insulin signaling influences myocardial energy metabolism by impacting cardiomyocyte mitochondrial dynamics and function under physiological conditions. However, comprehensive understanding of molecular mechanisms linking insulin signaling and changes in the architecture of the mitochondrial network in diabetic cardiomyopathy is lacking. This review summarizes our current understanding of how defective insulin signaling impacts cardiac function in diabetic cardiomyopathy and discusses the potential role of mitochondrial dynamics.

Organelle communication: signaling crossroads between homeostasis and disease.

Cellular organelles do not function as isolated or static units, but rather form dynamic contacts between one another that can be modulated according to cellular needs. The physical interfaces between organelles are important for Ca2+ and lipid homeostasis, and serve as platforms for the control of many essential functions including metabolism, signaling, organelle integrity and execution of the apoptotic program. Emerging evidence also highlights the importance of organelle communication in disorders such as Alzheimers disease, pulmonary arterial hypertension, cancer, skeletal and cardiac muscle dysfunction. Here, we provide an overview of the current literature on organelle communication and the link to human pathologies.

Sarcoplasmic reticulum-mitochondria communication in cardiovascular pathophysiology

Repetitive, calcium-mediated contractile activity renders cardiomyocytes critically dependent on a sustained energy supply and adequate calcium buffering, both of which are provided by mitochondria. Moreover, in vascular smooth muscle cells, mitochondrial metabolism modulates cell growth and proliferation, whereas cytosolic calcium levels regulate the arterial vascular tone. Physical and functional communication between mitochondria and sarco/endoplasmic reticulum and balanced mitochondrial dynamics seem to have a critical role for optimal calcium transfer to mitochondria, which is crucial in calcium homeostasis and mitochondrial metabolism in both types of muscle cells. Moreover, mitochondrial dysfunction has been associated with myocardial damage and dysregulation of vascular smooth muscle proliferation. Therefore, sarco/endoplasmic reticulum–mitochondria coupling and mitochondrial dynamics are now viewed as relevant factors in the pathogenesis of cardiac and vascular diseases, including coronary artery disease, heart failure, and pulmonary arterial hypertension. In this Review, we summarize the evidence related to the role of sarco/endoplasmic reticulum–mitochondria communication in cardiac and vascular muscle physiology, with a focus on how perturbations contribute to the pathogenesis of cardiovascular disorders.

mTORC1 inhibitor rapamycin and ER stressor tunicamycin induce differential patterns of ER-mitochondria coupling

Efficient mitochondrial Ca2+ uptake takes place at contact points between the ER and mitochondria, and represents a key regulator of many cell functions. In a previous study with HeLa cells, we showed that ER-to-mitochondria Ca2+ transfer increases during the early phase of ER stress induced by tunicamycin as an adaptive response to stimulate mitochondrial bioenergetics. It remains unknown whether other types of stress signals trigger similar responses. Here we observed that rapamycin, which inhibits the nutrient-sensing complex mTORC1, increased ER-mitochondria coupling in HeLa cells to a similar extent as did tunicamycin. Interestingly, although global responses to both stressors were comparable, there were notable differences in the spatial distribution of such changes. While tunicamycin increased organelle proximity primarily in the perinuclear region, rapamycin increased organelle contacts throughout the entire cell. These differences were paralleled by dissimilar alterations in the distribution of regulatory proteins of the ER-mitochondria interface, heterogeneities in mitochondrial Ca2+ uptake, and the formation of domains within the mitochondrial network with varying mitochondrial transmembrane potential. Collectively, these data suggest that while increasing ER-mitochondria coupling appears to represent a general response to cell stress, the intracellular distribution of the associated responses needs to be tailored to meet specific cellular requirements.