Skeletal muscle mitochondrial dysfunction precedes right ventricular impairment in experimental pulmonary hypertension

Reduced exercise capacity occurs before intrinsic skeletal muscle dysfunction in experimental rat models of pulmonary hypertension

Reduced exercise capacity in pulmonary hypertension (PH) significantly impacts quality of life. However, the cause of reduced exercise capacity in PH remains unclear. The objective of this study was to investigate whether intrinsic skeletal muscle changes are causative in reduced exercise capacity in PH using preclinical PH rat models with different PH severity. PH was induced in adult Sprague-Dawley (SD) or Fischer (CDF) rats with one dose of SU5416 (20 mg/kg) injection, followed by 3 weeks of hypoxia and additional 0-4 weeks of normoxia exposure. Control s rats were injected with vehicle and housed in normoxia. Echocardiography was performed to assess cardiac function. Exercise capacity was assessed by VO2 max. Skeletal muscle structural changes (atrophy, fiber type switching, and capillary density), mitochondrial function, isometric force, and fatigue profile were assessed. In SD rats, right ventricular systolic dysfunction is associated with reduced exercise capacity in PH rats at 7-week timepoint in comparison to control rats, while no changes were observed in skeletal muscle structure, mitochondrial function, isometric force, or fatigue profile. CDF rats at 4-week timepoint developed a more severe PH and, in addition to right ventricular dysfunction, the reduced exercise capacity in these rats is associated with skeletal muscle atrophy; however, mitochondrial function, isometric force, and fatigue profile in skeletal muscle remain unchanged. Our data suggest that cardiopulmonary impairments in PH are the primary cause of reduced exercise capacity, which occurs before intrinsic skeletal muscle dysfunction.

Targeting Mitochondrial Metabolic Dysfunction in Pulmonary Hypertension: Toward New Therapeutic Approaches?

Pulmonary arterial hypertension (PAH) is a rare disease characterized by pulmonary vascular remodeling leading to right heart failure and death. To date, despite the three therapeutic approaches targeting the three major endothelial dysfunction pathways based on the prostacyclin, nitric oxide/cyclic guanosine monophosphate, and endothelin pathways, PAH remains a serious disease. As such, new targets and therapeutic agents are needed. Mitochondrial metabolic dysfunction is one of the mechanisms involved in PAH pathogenesis in part through the induction of a Warburg metabolic state of enhanced glycolysis but also through the upregulation of glutaminolysis, tricarboxylic cycle and electron transport chain dysfunction, dysregulation of fatty acid oxidation or mitochondrial dynamics alterations. The aim of this review is to shed light on the main mitochondrial metabolic pathways involved in PAH and to provide an update on the resulting interesting potential therapeutic perspectives.

Skeletal muscle blood flow during exercise is reduced in a rat model of pulmonary hypertension

Pulmonary arterial hypertension (PAH) is characterized by exercise intolerance. Muscle blood flow may be reduced during exercise in PAH; however, this has not been directly measured. Therefore, we investigated blood flow during exercise in a rat model of monocrotaline (MCT)-induced pulmonary hypertension (PH). Male Sprague-Dawley rats (∼200 g) were injected with 60 mg/kg MCT (MCT, n = 23) and vehicle control (saline; CON, n = 16). Maximal rate of oxygen consumption (V̇o2max) and voluntary running were measured before PH induction. Right ventricle (RV) morphology and function were assessed via echocardiography and invasive hemodynamic measures. Treadmill running at 50% V̇o2max was performed by a subgroup of rats (MCT, n = 8; CON, n = 7). Injection of fluorescent microspheres determined muscle blood flow via photo spectroscopy. MCT demonstrated a severe phenotype via RV hypertrophy (Fulton index, 0.61 vs. 0.31; P < 0.001), high RV systolic pressure (51.5 vs. 22.4 mmHg; P < 0.001), and lower V̇o2max (53.2 vs. 71.8 mL·min-1·kg-1; P < 0.0001) compared with CON. Two-way ANOVA revealed exercising skeletal muscle blood flow relative to power output was reduced in MCT compared with CON (P < 0.001), and plasma lactate was increased in MCT (10.8 vs. 4.5 mmol/L; P = 0.002). Significant relationships between skeletal blood flow and blood lactate during exercise were observed for individual muscles (r = -0.58 to -0.74; P < 0.05). No differences in capillarization were identified. Skeletal muscle blood flow is significantly reduced in experimental PH. Reduced blood flow during exercise may be, at least in part, consequent to reduced exercise intensity in PH. This adds further evidence of peripheral muscle dysfunction and exercise intolerance in PAH.

Skeletal Muscle Dysfunction in Experimental Pulmonary Hypertension

Pulmonary arterial hypertension (PAH) is a serious, progressive, and often fatal disease that is in urgent need of improved therapies that treat it. One of the remaining therapeutic challenges is the increasingly recognized skeletal muscle dysfunction that interferes with exercise tolerance. Here we report that in the adult rat Sugen/hypoxia (SU/Hx) model of severe pulmonary hypertension (PH), there is highly significant, almost 50%, decrease in exercise endurance, and this is associated with a 25% increase in the abundance of type II muscle fiber markers, thick sarcomeric aggregates and an increase in the levels of FoxO1 in the soleus (a predominantly type I fiber muscle), with additional alterations in the transcriptomic profiles of the diaphragm (a mixed fiber muscle) and the extensor digitorum longus (a predominantly Type II fiber muscle). In addition, soleus atrophy may contribute to impaired exercise endurance. Studies in L6 rat myoblasts have showed that myotube differentiation is associated with increased FoxO1 levels and type II fiber markers, while the inhibition of FoxO1 leads to increased type I fiber markers. We conclude that the formation of aggregates and a FoxO1-mediated shift in the skeletal muscle fiber-type specification may underlie skeletal muscle dysfunction in an experimental study of PH.

SNPs for Genes Encoding the Mitochondrial Proteins Sirtuin3 and Uncoupling Protein 2 Are Associated With Disease Severity, Type 2 Diabetes, and Outcomes in Patients With Pulmonary Arterial Hypertension and This Is Recapitulated in a New Mouse Model Lacking Both Genes

Background

Isolated loss‐of‐function single nucleotide polymorphisms (SNPs) for SIRT3 (a mitochondrial deacetylase) and UCP2 (an atypical uncoupling protein enabling mitochondrial calcium entry) have been associated with both pulmonary arterial hypertension (PAH) and insulin resistance, but their collective role in animal models and patients is unknown.

Methods and Results

In a prospective cohort of patients with PAH (n=60), we measured SNPs for both SIRT3 and UCP2, along with several clinical features (including invasive hemodynamic data) and outcomes. We found SIRT3 and UCP2 SNPs often both in the same patient in a homozygous or heterozygous manner, correlating positively with PAH severity and associated with the presence of type 2 diabetes and 10‐year outcomes (death and transplantation). To explore this mechanistically, we generated double knockout mice for Sirt3 and Ucp2 and found increasing severity of PAH (mean pulmonary artery pressure, right ventricular hypertrophy/dilatation and extensive vascular remodeling, including inflammatory plexogenic lesions, in a gene dose‐dependent manner), along with insulin resistance, compared with wild‐type mice. The suppressed mitochondrial function (decreased respiration, increased mitochondrial membrane potential) in the double knockout pulmonary artery smooth muscle cells was associated with apoptosis resistance and increased proliferation, compared with wild‐type mice.

Conclusions

Our work supports the metabolic theory of PAH and shows that these mice exhibit spontaneous severe PAH (without environmental or chemical triggers) that mimics human PAH and may explain the findings in our patient cohort. Our study offers a new mouse model of PAH, with several features of human disease that are typically absent in other PAH mouse models.

Treatments for skeletal muscle abnormalities in heart failure: sodium-glucose transporter 2 and ketone bodies

Various skeletal muscle abnormalities are known to occur in heart failure (HF), and are closely associated with exercise intolerance. Particularly, abnormal energy metabolism caused by mitochondrial dysfunction in skeletal muscle is a cause of decreased endurance exercise capacity. However, to date, no specific drug treatment has been established for the skeletal muscle abnormalities and exercise intolerance occurring in HF patients. Sodium-glucose transporter 2 (SGLT2) inhibitors promote glucose excretion by suppressing glucose reabsorption in the renal tubules, which has a hypoglycemic effect independent of insulin secretion. Recently, large clinical trials have demonstrated that treatment with SGLT2 inhibitors suppresses cardiovascular events in patients who have HF with systolic dysfunction. Mechanisms of the therapeutic effects of SGLT2 inhibitors for HF have been suggested to be diuretic, suppression of neurohumoral factor activation, renal protection, and improvement of myocardial metabolism, but has not been clarified to date. SGLT2 inhibitors are known to increase blood ketone bodies. This suggests that they may improve the abnormal skeletal muscle metabolism in HF, i.e., improve fatty acid metabolism, suppress glycolysis, and utilize ketone bodies in mitochondrial energy production. Ultimately, they may improve aerobic metabolism in skeletal muscle, and suppress anaerobic metabolism and improve aerobic exercise capacity at the level of the anaerobic threshold. The potential actions of such SGLT2 inhibitors explain their effectiveness in HF, and may be candidates for new drug treatments aimed at improving exercise intolerance. In this review, we outlined the effects of SGLT2 inhibitors on skeletal muscle metabolism, with a particular focus on ketone metabolism.

Role of IL-33 receptor (ST2) deletion in diaphragm contractile and mitochondrial function in the Sugen5416/hypoxia model of pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease of the pulmonary vasculature that leads to right ventricular failure. Skeletal muscle maladaptations limit physical activity and may contribute to disease progression. The role of alarmin/inflammatory signaling in PAH respiratory muscle dysfunction is unknown. We hypothesized that diaphragm mitochondrial and contractile functions are impaired in SU5416/hypoxia-induced pulmonary hypertension due to increased systemic IL-33 signaling. We induced pulmonary hypertension in adult C57Bl/6 J (WT) and ST2 (IL1RL1) gene ablated mice by SU5416/hypoxia (SuHx). We measured diaphragm fiber mitochondrial respiration, inflammatory markers, and contractile function ex vivo. SuHx reduced coupled and uncoupled permeabilized myofiber respiration by ∼40 %. During coupled respiration with complex I substrates, ST2^-/- attenuated SuHx inhibition of mitochondrial respiration (genotype × treatment interaction F[1,67] = 3.3, p = 0.07, η² = 0.04). Flux control ratio and coupling efficiency were not affected by SuHx or genotype. A higher substrate control ratio for succinate was observed in SuHx fibers and attenuated in ST2^-/- fibers (F[1,67] = 5.3, p < 0.05, η² = 0.07). Diaphragm TNFα, but not IL-33 or NFkB, was increased in SuHx vs. DMSO in both genotypes (F[1,43] = 4.7, p < 0.05, η² = 0.1). Diaphragm force-frequency relationships were right-shifted in SuHx vs. WT (F[3,440] = 8.4, p < 0.05, η² = 0.0025). There was no effect of ST2^-/- on the force-frequency relationship. Force decay during a fatigue protocol at 100 Hz, but not at 40 Hz, was attenuated by SuHx vs. DMSO in both genotypes (F[1,41] = 5.6, p < 0.05, η² = 0.11). SuHx mice exhibit a modest compensation in diaphragm contractility and mitochondrial dysfunction during coupled respiration; the latter partially regulated through ST2 signaling.

Pulmonary arterial hypertension induces the release of circulating extracellular vesicles with oxidative content and alters redox and mitochondrial homeostasis in the brains of rats

Pulmonary arterial hypertension (PAH) is characterized by increased resistance of the pulmonary vasculature and afterload imposed on the right ventricle (RV). Two major contributors to the worsening of this disease are oxidative stress and mitochondrial impairment. This study aimed to explore the effects of monocrotaline (MCT)-induced PAH on redox and mitochondrial homeostasis in the RV and brain and how circulating extracellular vesicle (EV) signaling is related to these phenomena. Wistar rats were divided into control and MCT groups (60 mg/kg, intraperitoneal), and EVs were isolated from blood on the day of euthanasia (21 days after MCT injections). There was an oxidative imbalance in the RV, brain, and EVs of MCT rats. PAH impaired mitochondrial function in the RV, as seen by a decrease in the activities of mitochondrial complex II and citrate synthase and manganese superoxide dismutase (MnSOD) protein expression, but this function was preserved in the brain. The key regulators of mitochondrial biogenesis, namely, proliferator-activated receptor gamma coactivator 1-alpha and sirtuin 1, were poorly expressed in the EVs of MCT rats, and this result was positively correlated with MnSOD expression in the RV and negatively correlated with MnSOD expression in the brain. Based on these findings, we can conclude that the RV is severely impacted by the development of PAH, but this pathological injury may signal the release of circulating EVs that communicate with different organs, such as the brain, helping to prevent further damage through the upregulation of proteins involved in redox and mitochondrial function.

Oxygen sensing, mitochondrial biology and experimental therapeutics for pulmonary hypertension and cancer

The homeostatic oxygen sensing system (HOSS) optimizes systemic oxygen delivery. Specialized tissues utilize a conserved mitochondrial sensor, often involving NDUFS2 in complex I of the mitochondrial electron transport chain, as a site of pO₂-responsive production of reactive oxygen species (ROS). These ROS are converted to a diffusible signaling molecule, hydrogen peroxide (H₂O₂), by superoxide dismutase (SOD2). H₂O₂ exits the mitochondria and regulates ion channels and enzymes, altering plasma membrane potential, intracellular Ca²⁺ and Ca²⁺-sensitization and controlling acute, adaptive, responses to hypoxia that involve changes in ventilation, vascular tone and neurotransmitter release. Subversion of this O₂-sensing pathway creates a pseudohypoxic state that promotes disease progression in pulmonary arterial hypertension (PAH) and cancer. Pseudohypoxia is a state in which biochemical changes, normally associated with hypoxia, occur despite normal pO₂. Epigenetic silencing of SOD2 by DNA methylation alters H₂O₂ production, activating hypoxia-inducible factor 1α, thereby disrupting mitochondrial metabolism and dynamics, accelerating cell proliferation and inhibiting apoptosis. Other epigenetic mechanisms, including dysregulation of microRNAs (miR), increase pyruvate dehydrogenase kinase and pyruvate kinase muscle isoform 2 expression in both diseases, favoring uncoupled aerobic glycolysis. This Warburg metabolic shift also accelerates cell proliferation and impairs apoptosis. Disordered mitochondrial dynamics, usually increased mitotic fission and impaired fusion, promotes disease progression in PAH and cancer. Epigenetic upregulation of dynamin-related protein 1 (Drp1) and its binding partners, MiD49 and MiD51, contributes to the pathogenesis of PAH and cancer. Finally, dysregulation of intramitochondrial Ca²⁺, resulting from impaired mitochondrial calcium uniporter complex (MCUC) function, links abnormal mitochondrial metabolism and dynamics. MiR-mediated decreases in MCUC function reduce intramitochondrial Ca²⁺, promoting Warburg metabolism, whilst increasing cytosolic Ca²⁺, promoting fission. Epigenetically disordered mitochondrial O₂-sensing, metabolism, dynamics, and Ca²⁺ homeostasis offer new therapeutic targets for PAH and cancer. Promoting glucose oxidation, restoring the fission/fusion balance, and restoring mitochondrial calcium regulation are promising experimental therapeutic strategies.

Exercise intolerance in pulmonary arterial hypertension: insight into central and peripheral pathophysiological mechanisms

Exercise intolerance is a cardinal symptom of pulmonary arterial hypertension (PAH) and strongly impacts patients' quality of life (QoL). Although central cardiopulmonary impairments limit peak oxygen consumption (V' O2peak ) in patients with PAH, several peripheral abnormalities have been described over the recent decade as key determinants in exercise intolerance, including impaired skeletal muscle (SKM) morphology, convective O2 transport, capillarity and metabolism indicating that peripheral abnormalities play a greater role in limiting exercise capacity than previously thought. More recently, cerebrovascular alterations potentially contributing to exercise intolerance in patients with PAH were also documented. Currently, only cardiopulmonary rehabilitation has been shown to efficiently improve the peripheral components of exercise intolerance in patients with PAH. However, more extensive studies are needed to identify targeted interventions that would ultimately improve patients' exercise tolerance and QoL. The present review offers a broad and comprehensive analysis of the present literature about the complex mechanisms and their interactions limiting exercise in patients and suggests several gaps in knowledge that need to be addressed in the future for a better understanding of exercise intolerance in patients with PAH.

Mitochondrial dysfunction in lung ageing and disease

Mitochondrial biology has seen a surge in popularity in the past 5 years, with the emergence of numerous new avenues of exciting mitochondria-related research including immunometabolism, mitochondrial transplantation and mitochondria-microbe biology. Since the early 1960s mitochondrial dysfunction has been observed in cells of the lung in individuals and in experimental models of chronic and acute respiratory diseases. However, it is only in the past decade with the emergence of more sophisticated tools and methodologies that we are beginning to understand how this enigmatic organelle regulates cellular homeostasis and contributes to disease processes in the lung. In this review, we highlight the diverse role of mitochondria in individual lung cell populations and what happens when these essential organelles become dysfunctional with ageing and in acute and chronic lung disease. Although much remains to be uncovered, we also discuss potential targeted therapeutics for mitochondrial dysfunction in the ageing and diseased lung.

High-fat diet affects skeletal muscle mitochondria comparable to pressure overload-induced heart failure

In heart failure, high-fat diet (HFD) may exert beneficial effects on cardiac mitochondria and contractility. Skeletal muscle mitochondrial dysfunction in heart failure is associated with myopathy. However, it is not clear if HFD affects skeletal muscle mitochondria in heart failure as well. To induce heart failure, we used pressure overload (PO) in rats fed normal chow or HFD. Interfibrillar mitochondria (IFM) and subsarcolemmal mitochondria (SSM) from gastrocnemius were isolated and functionally characterized. With PO heart failure, maximal respiratory capacity was impaired in IFM but increased in SSM of gastrocnemius. Unexpectedly, HFD affected mitochondria comparably to PO. In combination, PO and HFD showed additive effects on mitochondrial subpopulations which were reflected by isolated complex activities. While PO impaired diastolic as well as systolic cardiac function and increased glucose tolerance, HFD did not affect cardiac function but decreased glucose tolerance. We conclude that HFD and PO heart failure have comparable effects leading to more severe impairment of IFM. Glucose tolerance seems not causally related to skeletal muscle mitochondrial dysfunction. The additive effects of HFD and PO may suggest accelerated skeletal muscle mitochondrial dysfunction when heart failure is accompanied with a diet containing high fat.

Mitochondria in the Pulmonary Vasculature in Health and Disease: Oxygen-Sensing, Metabolism, and Dynamics

In lung vascular cells, mitochondria serve a canonical metabolic role, governing energy homeostasis. In addition, mitochondria exist in dynamic networks, which serve noncanonical functions, including regulation of redox signaling, cell cycle, apoptosis, and mitochondrial quality control. Mitochondria in pulmonary artery smooth muscle cells (PASMC) are oxygen sensors and initiate hypoxic pulmonary vasoconstriction. Acquired dysfunction of mitochondrial metabolism and dynamics contribute to a cancer-like phenotype in pulmonary arterial hypertension (PAH). Acquired mitochondrial abnormalities, such as increased pyruvate dehydrogenase kinase (PDK) and pyruvate kinase muscle isoform 2 (PKM2) expression, which increase uncoupled glycolysis (the Warburg phenomenon), are implicated in PAH. Warburg metabolism sustains energy homeostasis by the inhibition of oxidative metabolism that reduces mitochondrial apoptosis, allowing unchecked cell accumulation. Warburg metabolism is initiated by the induction of a pseudohypoxic state, in which DNA methyltransferase (DNMT)-mediated changes in redox signaling cause normoxic activation of HIF-1α and increase PDK expression. Furthermore, mitochondrial division is coordinated with nuclear division through a process called mitotic fission. Increased mitotic fission in PAH, driven by increased fission and reduced fusion favors rapid cell cycle progression and apoptosis resistance. Downregulation of the mitochondrial calcium uniporter complex (MCUC) occurs in PAH and is one potential unifying mechanism linking Warburg metabolism and mitochondrial fission. Mitochondrial metabolic and dynamic disorders combine to promote the hyperproliferative, apoptosis-resistant, phenotype in PAH PASMC, endothelial cells, and fibroblasts. Understanding the molecular mechanism regulating mitochondrial metabolism and dynamics has permitted identification of new biomarkers, nuclear and CT imaging modalities, and new therapeutic targets for PAH. © 2020 American Physiological Society. Compr Physiol 10:713-765, 2020.

Skeletal and Respiratory Muscle Dysfunctions in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a rare disease, which leads to the progressive loss and remodeling of the pulmonary vessels, right heart failure, and death. Different clinical presentations can be responsible for such a bad prognosis disease and the underlying mechanisms still need to be further examined. Importantly, skeletal and respiratory muscle abnormalities largely contribute to the decreased quality of life and exercise intolerance observed in patients with PAH. At the systemic level, impaired oxygen supply through reduced cardiac output and respiratory muscle dysfunctions, which potentially result in hypoxemia, is associated with altered muscles vascularization, inflammation, enhanced catabolic pathways, and impaired oxygen use through mitochondrial dysfunctions that are likely participate in PAH-related myopathy. Sharing new insights into the pathological mechanisms of PAH might help stimulate specific research areas, improving the treatment and quality of life of PAH patients. Indeed, many of these muscular impairments are reversible, strongly supporting the development of effective preventive and/or therapeutic approaches, including mitochondrial protection and exercise training.

Metabolic Remodeling in the Pressure-Loaded Right Ventricle: Shifts in Glucose and Fatty Acid Metabolism-A Systematic Review and Meta-Analysis

Background

Right ventricular (RV) failure because of chronic pressure load is an important determinant of outcome in pulmonary hypertension. Progression towards RV failure is characterized by diastolic dysfunction, fibrosis and metabolic dysregulation. Metabolic modulation has been suggested as therapeutic option, yet, metabolic dysregulation may have various faces in different experimental models and disease severity. In this systematic review and meta‐analysis, we aimed to identify metabolic changes in the pressure loaded RV and formulate recommendations required to optimize translation between animal models and human disease.

Methods and Results

Medline and EMBASE were searched to identify original studies describing cardiac metabolic variables in the pressure loaded RV. We identified mostly rat‐models, inducing pressure load by hypoxia, Sugen‐hypoxia, monocrotaline (MCT), pulmonary artery banding (PAB) or strain (fawn hooded rats, FHR), and human studies. Meta‐analysis revealed increased Hedges’ g (effect size) of the gene expression of GLUT1 and HK1 and glycolytic flux. The expression of MCAD was uniformly decreased. Mitochondrial respiratory capacity and fatty acid uptake varied considerably between studies, yet there was a model effect in carbohydrate respiratory capacity in MCT‐rats.

Conclusions

This systematic review and meta‐analysis on metabolic remodeling in the pressure‐loaded RV showed a consistent increase in glucose uptake and glycolysis, strongly suggest a downregulation of beta‐oxidation, and showed divergent and model‐specific changes regarding fatty acid uptake and oxidative metabolism. To translate metabolic results from animal models to human disease, more extensive characterization, including function, and uniformity in methodology and studied variables, will be required.

Mitochondrial Dysfunction: Metabolic Drivers of Pulmonary Hypertension

Significance: Pulmonary hypertension (PH) is a progressive disease characterized by pulmonary vascular remodeling and lung vasculopathy. The disease displays progressive dyspnea, pulmonary artery uncoupling and right ventricular (RV) dysfunction. The overall survival rate is ranging from 28-72%. Recent Advances: The molecular events that promote the development of PH are complex and incompletely understood. Metabolic impairment has been proposed to contribute to the pathophysiology of PH with evidence for mitochondrial dysfunction involving the electron transport chain proteins, antioxidant enzymes, apoptosis regulators, and mitochondrial quality control. Critical Issues: It is vital to characterize the mechanisms by which mitochondrial dysfunction contribute to PH pathogenesis. This review focuses on the currently available publications that supports mitochondrial mechanisms in PH pathophysiology. Future Directions: Further studies of these metabolic mitochondrial alterations in PH could be viable targets of diagnostic and therapeutic intervention.

Pulmonary arterial hypertension and the potential roles of metallothioneins: A focused review

The pathophysiology of pulmonary arterial hypertension (PAH) is underlined by cell proliferation and vasoconstriction of pulmonary arterioles this involves multiple molecular factors or proteins, but it is not clear what the exact roles of these factors/proteins are. In addition, there may be other factors/proteins that have not been identified that contribute to PAH pathophysiology. Therefore, research has focused on investigating novel role players, in order to facilitate a better understanding of how PAH develop. Evidence suggest that mitochondrial regulators are key role players in PAH pathophysiology, but regulators that have not received sufficient attention in PAH are metallothioneins (MTs). In PAH patients, MT expression is elevated compared to healthy individuals, suggesting that MTs may be possible biomarkers. In other disease-models, MTs have been shown to regulate cell proliferation and vasoconstriction, processes that are instrumental in PAH pathophysiology. Due to the involvement of these processes in PAH pathophysiology and the ability of MTs to modulate them, this paper propose that cellular MTs may also play a role in PAH development. This paper suggests that PAH-research should perhaps begin to investigate the involvement of cellular MTs in the development of PAH.

Mitochondrial metabolism in pulmonary hypertension: beyond mountains there are mountains

Pulmonary hypertension (PH) is a heterogeneous and fatal disease of the lung vasculature, where metabolic and mitochondrial dysfunction may drive pathogenesis. Similar to the Warburg effect in cancer, a shift from mitochondrial oxidation to glycolysis occurs in diseased pulmonary vessels and the right ventricle. However, appreciation of metabolic events in PH beyond the Warburg effect is only just emerging. This Review discusses molecular, translational, and clinical concepts centered on the mitochondria and highlights promising, controversial, and challenging areas of investigation. If we can move beyond the "mountains" of obstacles in this field and elucidate these fundamental tenets of pulmonary vascular metabolism, such work has the potential to usher in much-needed diagnostic and therapeutic approaches for the mitochondrial and metabolic management of PH.

Epigenetics, inflammation and metabolism in right heart failure associated with pulmonary hypertension

Right ventricular failure (RVF) is the most important prognostic factor for both morbidity and mortality in pulmonary arterial hypertension (PAH), but also occurs in numerous other common diseases and conditions, including left ventricle dysfunction. RVF remains understudied compared with left ventricular failure (LVF). However, right and left ventricles have many differences at the morphological level or the embryologic origin, and respond differently to pressure overload. Therefore, knowledge from the left ventricle cannot be extrapolated to the right ventricle. Few studies have focused on the right ventricle and have permitted to increase our knowledge on the right ventricular-specific mechanisms driving decompensation. Here we review basic principles such as mechanisms accounting for right ventricle hypertrophy, dysfunction, and transition toward failure, with a focus on epigenetics, inflammatory, and metabolic processes.

Novel putative pharmacological therapies to protect the right ventricle in pulmonary hypertension: a review of current literature

Pulmonary hypertension (PH) is defined by elevated mean pulmonary artery pressure following the pathological remodelling of small pulmonary arteries. An increase in right ventricular (RV) afterload results in RV hypertrophy and RV failure. The pathophysiology of PH, and RV remodelling in particular, is not well understood, thus explaining, at least in part, why current PH therapies have a limited effect. Existing therapies mostly target the pulmonary circulation. Because the remodelled RV fails to support normal cardiac function, patients eventually succumb from RV failure. Developing novel therapies that directly target the function of the RV may therefore benefit patients with PH. In the past decade, several promising studies have investigated novel cardioprotective strategies in experimental models of PH. This review aims to comprehensively discuss and highlight these novel experimental approaches to confer, in the long-term, greater health benefit in patients with PH.

Downhill exercise training in monocrotaline-injected rats: Effects on echocardiographic and haemodynamic variables and survival

BACKGROUND

Eccentric exercise training has been shown to improve muscle force strength without excessive cardiovascular stress. Such an exercise modality deserves to be tested in pulmonary arterial hypertension.

AIM

We aimed to assess the effects of an eccentric training modality on cardiac function and survival in an experimental monocrotaline-induced model of pulmonary arterial hypertension with right ventricular dysfunction.

METHODS

Forty rats were randomly assigned to one of four groups: 40mg/kg monocrotaline-injected sedentary rats; 40mg/kg monocrotaline-injected eccentric-trained rats; sedentary control rats; or eccentric-trained control rats. Eccentric exercise training consisted of downhill running on a treadmill with a -15° slope for 30minutes, 5 days a week for 4 weeks. Training tolerance was assessed by echocardiography, right ventricle catheterization and the rats' maximal eccentric speed.

RESULTS

Survival in monocrotaline-injected eccentric-trained rats was not different from that in monocrotaline-injected sedentary rats. Monocrotaline-injected eccentric-trained rats tolerated this training modality well, and haemodynamic status did not deteriorate further compared with monocrotaline-injected sedentary rats. The eccentric maximal speed decline was less pronounced in trained compared with sedentary pulmonary arterial hypertension rats.

CONCLUSIONS

Eccentric exercise training had no detrimental effects on right heart pressure, cardiac function and survival in rats with stable monocrotaline-induced pulmonary hypertension.

Magnetic Resonance–Augmented Cardiopulmonary Exercise Testing

Background—

Conventional cardiopulmonary exercise testing can objectively measure exercise intolerance but cannot provide comprehensive evaluation of physiology. This requires additional assessment of cardiac output and arteriovenous oxygen content difference. We developed magnetic resonance (MR)–augmented cardiopulmonary exercise testing to achieve this goal and assessed children with right heart disease.

Methods and Results—

Healthy controls (n=10) and children with pulmonary arterial hypertension (PAH; n=10) and repaired tetralogy of Fallot (n=10) underwent MR-augmented cardiopulmonary exercise testing. All exercises were performed on an MR-compatible ergometer, and oxygen uptake was continuously acquired using a modified metabolic cart. Simultaneous cardiac output was measured using a real-time MR flow sequence and combined with oxygen uptake to calculate arteriovenous oxygen content difference. Peak oxygen uptake was significantly lower in the PAH group (12.6±1.31 mL/kg per minute; P =0.01) and trended toward lower in the tetralogy of Fallot group (13.5±1.29 mL/kg per minute; P =0.06) compared with controls (16.7±1.37 mL/kg per minute). Although tetralogy of Fallot patients had the largest increase in cardiac output, they had lower resting (3±1.2 L/min per m 2 ) and peak (5.3±1.2 L/min per m 2 ) values compared with controls (resting 4.3±1.2 L/min per m 2 and peak 6.6±1.2 L/min per m 2 ) and PAH patients (resting 4.5±1.1 L/min per m 2 and peak 5.9±1.1 L/min per m 2 ). Both the PAH and tetralogy of Fallot patients had blunted exercise–induced increases in arteriovenous oxygen content difference. However, only the PAH patients had significantly reduced peak values (6.9±1.3 mlO2/100 mL) compared with controls (8.4±1.4 mlO2/100 mL; P =0.005).

Conclusions—

MR-augmented cardiopulmonary exercise testing is feasible in both healthy children and children with cardiac disease. Using this novel technique, we have demonstrated abnormal exercise patterns in oxygen uptake, cardiac output, and arteriovenous oxygen content difference.

Time course of ubiquitin-proteasome and macroautophagy-lysosome pathways in skeletal muscle in rats with heart failure

Patients with heart failure have limited exercise capacity due to not only the myocardial dysfunction but also skeletal muscle atrophy. However, the mechanisms and time course of protein degradation in skeletal muscle during heart failure remain unclear, and there is no established standard treatment. The purpose of the present study was to investigate the time course of major protein degradation pathways in skeletal muscle during heart failure. Four-week-old male Wistar rats were randomly assigned to heart failure induced by monocrotaline or control groups. At 14 and 21 days after monocrotaline injection, the lungs, heart, and gastrocnemius and soleus muscles were removed and analyzed. There was no significant difference in body weight between the groups at 14 days after monocrotaline injection. Although there were no morphological changes in the skeletal muscle of the monocrotaline group at this time point, ubiquitin-proteasome and macroautophagylysosome pathways were activated in the monocrotaline group. Additionally, the pathways were less strongly activated in the soleus muscle than in the gastrocnemius muscle. These results suggest that physical exercise that shifts to slow muscle characteristics should begin when there is no indication of skeletal muscle atrophy to prevent exercise intolerance with heart failure.

SIRT3-AMP-Activated Protein Kinase Activation by Nitrite and Metformin Improves Hyperglycemia and Normalizes Pulmonary Hypertension Associated With Heart Failure With Preserved Ejection Fraction

BACKGROUND

Pulmonary hypertension associated with heart failure with preserved ejection fraction (PH-HFpEF) is an increasingly recognized clinical complication of metabolic syndrome. No adequate animal model of PH-HFpEF is available, and no effective therapies have been identified to date. A recent study suggested that dietary nitrate improves insulin resistance in endothelial nitric oxide synthase null mice, and multiple studies have reported that both nitrate and its active metabolite, nitrite, have therapeutic activity in preclinical models of pulmonary hypertension.

METHODS AND RESULTS

To evaluate the efficacy and mechanism of nitrite in metabolic syndrome associated with PH-HFpEF, we developed a 2-hit PH-HFpEF model in rats with multiple features of metabolic syndrome attributable to double-leptin receptor defect (obese ZSF1) with the combined treatment of vascular endothelial growth factor receptor blocker SU5416. Chronic oral nitrite treatment improved hyperglycemia in obese ZSF1 rats by a process that requires skeletal muscle SIRT3-AMPK-GLUT4 signaling. The glucose-lowering effect of nitrite was abolished in SIRT3-deficient human skeletal muscle cells, and in SIRT3 knockout mice fed a high-fat diet, as well. Skeletal muscle biopsies from humans with metabolic syndrome after 12 weeks of oral sodium nitrite and nitrate treatment (IND#115926) displayed increased activation of SIRT3 and AMP-activated protein kinase. Finally, early treatments with nitrite and metformin at the time of SU5416 injection reduced pulmonary pressures and vascular remodeling in the PH-HFpEF model with robust activation of skeletal muscle SIRT3 and AMP-activated protein kinase.

CONCLUSIONS

These studies validate a rodent model of metabolic syndrome and PH-HFpEF, suggesting a potential role of nitrite and metformin as a preventative treatment for this disease.

Skeletal Muscle Abnormalities in Heart Failure

Exercise capacity is lowered in patients with heart failure, which limits their daily activities and also reduces their quality of life. Furthermore, lowered exercise capacity has been well demonstrated to be closely related to the severity and prognosis of heart failure. Skeletal muscle abnormalities including abnormal energy metabolism, transition of myofibers from type I to type II, mitochondrial dysfunction, reduction in muscular strength, and muscle atrophy have been shown to play a central role in lowered exercise capacity. The skeletal muscle abnormalities can be classified into the following main types: 1) low endurance due to mitochondrial dysfunction; and 2) low muscle mass and muscle strength due to imbalance of protein synthesis and degradation. The molecular mechanisms of these skeletal muscle abnormalities have been studied mainly using animal models. The current review including our recent study will focus upon the skeletal muscle abnormalities in heart failure.

Pulmonary arterial hypertension-related myopathy: an overview of current data and future perspectives

BACKGROUND AND AIM

Exercise intolerance is one of the key features of pulmonary arterial hypertension (PAH). The main determinants of exercise impairment include hypoxemia, reduced right ventricular output, perfusion/ventilation mismatch, and weakness of skeletal and breathing muscles. The aim of the current review is to describe the findings in the existing literature about respiratory and muscle dysfunction in PAH. Animal and clinical studies regarding both respiratory and peripheral skeletal muscles and the effect of exercise training on muscle function in PAH patients are analyzed.

DATA SYNTHESIS

PAH myopathy is characterized by reduced skeletal muscle mass, reduced volitional and non-volitional contractility, reduced generated force, a fiber switch from type I to type II, increased protein degradation through ubiquitin-proteasome system (UPS) activation, reduced mitochondrial functioning, and impaired activation-contractility coupling. Increased inflammatory response, impaired anabolic signaling, hypoxemia, and abnormalities of mitochondrial function are involved in the pathophysiology of this process. Exercise training has been shown to improve exercise capacity, peak oxygen uptake, quality of life, and possibly clinical outcomes of PAH patients.

CONCLUSIONS

The skeletal muscles of PAH patients show a wide spectrum of cellular abnormalities that finally culminate in muscle atrophy and reduced contractility. Exercise training improves muscle function and bears a positive impact on the clinical outcomes of PAH patients.

Skeletal muscle proteomic signature and metabolic impairment in pulmonary hypertension

Exercise limitation comes from a close interaction between cardiovascular and skeletal muscle impairments. To better understand the implication of possible peripheral oxidative metabolism dysfunction, we studied the proteomic signature of skeletal muscle in pulmonary arterial hypertension (PAH). Eight idiopathic PAH patients and eight matched healthy sedentary subjects were evaluated for exercise capacity, skeletal muscle proteomic profile, metabolism, and mitochondrial function. Skeletal muscle proteins were extracted, and fractioned peptides were tagged using an iTRAQ protocol. Proteomic analyses have documented a total of 9 downregulated proteins in PAH skeletal muscles and 10 upregulated proteins compared to healthy subjects. Most of the downregulated proteins were related to mitochondrial structure and function. Focusing on skeletal muscle metabolism and mitochondrial health, PAH patients presented a decreased expression of oxidative enzymes (pyruvate dehydrogenase, p < 0.01) and an increased expression of glycolytic enzymes (lactate dehydrogenase activity, p < 0.05). These findings were supported by abnormal mitochondrial morphology on electronic microscopy, lower citrate synthase activity (p < 0.01) and lower expression of the transcription factor A of the mitochondria (p < 0.05), confirming a more glycolytic metabolism in PAH skeletal muscles. We provide evidences that impaired mitochondrial and metabolic functions found in the lungs and the right ventricle are also present in skeletal muscles of patients.

KEY MESSAGE

• Proteomic and metabolic analysis show abnormal oxidative metabolism in PAH skeletal muscle. • EM of PAH patients reveals abnormal mitochondrial structure and distribution. • Abnormal mitochondrial health and function contribute to exercise impairments of PAH. • PAH may be considered a vascular affliction of heart and lungs with major impact on peripheral muscles.

Introduction: oxidation and inflammation, a molecular link between non-communicable diseases

Non-communicable diseases are, by definition, those chronic diseases that are non-infectious and non-transmissible. The most common non-communicable diseases are obesity, diabetes, cancer, and cardiovascular, chronic respiratory and neurological diseases. Altogether, they are the commonest cause of death and disability in modern world. Recent investigations show that many of these diseases share common pathophysiological mechanisms and are, at least in part, different manifestations in different organs of similar molecular alterations. Mitochondrial alterations, oxidative stress and inflammation are inextricably linked and play major roles in the onset and development of non-communicable diseases. Therefore, it is conceivable that pharmacological or nutritional manipulation of oxidation and inflammation allows a significant decrease in the mortality and morbility associated to these diseases.

A miR-208-Mef2 axis drives the decompensation of right ventricular function in pulmonary hypertension

RATIONALE

Right ventricular (RV) failure is a major cause of morbidity and mortality in pulmonary hypertension, but its mechanism remains unknown. Myocyte enhancer factor 2 (Mef2) has been implicated in RV development, regulating metabolic, contractile, and angiogenic genes. Moreover, Mef2 regulates microRNAs that have emerged as important determinants of cardiac development and disease, but for which the role in RV is still unclear.

OBJECTIVE

We hypothesized a critical role of a Mef2-microRNAs axis in RV failure.

METHODS AND RESULTS

In a rat pulmonary hypertension model (monocrotaline), we studied RV free wall tissues from rats with normal, compensated, and decompensated RV hypertrophy, carefully defined based on clinically relevant parameters, including RV systolic and end-diastolic pressures, cardiac output, RV size, and morbidity. Mef2c expression was sharply increased in compensating phase of RVH tissues but was lost in decompensation phase of RVH. An unbiased screening of microRNAs in our model resulted to a short microRNA signature of decompensated RV failure, which included the myocardium-specific miR-208, which was progressively downregulated as RV failure progressed, in contrast to what is described in left ventricular failure. With mechanistic in vitro experiments using neonatal and adult RV cardiomyocytes, we showed that miR-208 inhibition, as well as tumor necrosis factor-α, activates the complex mediator of transcription 13/nuclear receptor corepressor 1 axis, which in turn promotes Mef2 inhibition, closing a self-limiting feedback loop, driving the transition from compensating phase of RVH toward decompensation phase of RVH. In our model, serum tumor necrosis factor-α levels progressively increased with time while serum miR-208 levels decreased, mirroring its levels in RV myocardium.

CONCLUSIONS

We describe an RV-specific mechanism for heart failure, which could potentially lead to new biomarkers and therapeutic targets.

MitoNEET-mediated effects on browning of white adipose tissue

MitoNEET is an outer mitochondrial membrane protein that, upon overexpression in white adipose tissue (WAT), exerts a positive impact on tissue expansion and whole-body lipid and carbohydrate homeostasis by altering mitochondrial matrix iron metabolism. Here we determine the key transcriptional events in subcutaneous WAT of mice in response to mitoNEET overexpression and a high-fat diet (HFD). Microarray analyses at key points during weight gain upon body-weight divergence with wild-type mice demonstrate that mitoNEET-enriched sWAT early on upregulates a browning signature program that limits WAT expansion in transgenic mice for a period of up to 12-weeks of HFD. This compensatory browning phenotype is subsequently lost, resulting in rapid WAT expansion and body-weight gain. Exposure to thermoneutral temperatures during HFD prompts weight gain significantly earlier. Similar WAT expansion is achieved upon infection with an adeno-associated virus expressing mitoNEET. Collectively, the mitoNEET enriched fat-pads feature a more vascularized, anti-inflammatory and less fibrotic environment.

Endothelial progenitor cells and pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease characterised by lung endothelial cell dysfunction and vascular remodelling. A number of studies now suggest that endothelial progenitor cells (EPCs) may induce neovascularisation and could be a promising approach for cell based therapy for PAH. On the contrary EPCs may contribute to pulmonary vascular remodelling, particularly in end-stage pulmonary disease. This review article will provide a brief summary of the relationship between PAH and EPCs, the application of the EPCs to PAH and highlight the potential clinical application of the EPCs cell therapy to PAH.

Mitochondrial dysfunction: a basic mechanism in inflammation-related non-communicable diseases and therapeutic opportunities

Obesity is not necessarily a predisposing factor for disease. It is the handling of fat and/or excessive energy intake that encompasses the linkage of inflammation, oxidation, and metabolism to the deleterious effects associated with the continuous excess of food ingestion. The roles of cytokines and insulin resistance in excessive energy intake have been studied extensively. Tobacco use and obesity accompanied by an unhealthy diet and physical inactivity are the main factors that underlie noncommunicable diseases. The implication is that the management of energy or food intake, which is the main role of mitochondria, is involved in the most common diseases. In this study, we highlight the importance of mitochondrial dysfunction in the mutual relationships between causative conditions. Mitochondria are highly dynamic organelles that fuse and divide in response to environmental stimuli, developmental status, and energy requirements. These organelles act to supply the cell with ATP and to synthesise key molecules in the processes of inflammation, oxidation, and metabolism. Therefore, energy sensors and management effectors are determinants in the course and development of diseases. Regulating mitochondrial function may require a multifaceted approach that includes drugs and plant-derived phenolic compounds with antioxidant and anti-inflammatory activities that improve mitochondrial biogenesis and act to modulate the AMPK/mTOR pathway.