Glycolysis-Mediated Activation of v-ATPase by Nicotinamide Mononucleotide Ameliorates Lipid-Induced Cardiomyopathy by Repressing the CD36-TLR4 Axis

BACKGROUND

Chronic overconsumption of lipids followed by their excessive accumulation in the heart leads to cardiomyopathy. The cause of lipid-induced cardiomyopathy involves a pivotal role for the proton-pump vacuolar-type H+-ATPase (v-ATPase), which acidifies endosomes, and for lipid-transporter CD36, which is stored in acidified endosomes. During lipid overexposure, an increased influx of lipids into cardiomyocytes is sensed by v-ATPase, which then disassembles, causing endosomal de-acidification and expulsion of stored CD36 from the endosomes toward the sarcolemma. Once at the sarcolemma, CD36 not only increases lipid uptake but also interacts with inflammatory receptor TLR4 (Toll-like receptor 4), together resulting in lipid-induced insulin resistance, inflammation, fibrosis, and cardiac dysfunction. Strategies inducing v-ATPase reassembly, that is, to achieve CD36 reinternalization, may correct these maladaptive alterations. For this, we used NAD+ (nicotinamide adenine dinucleotide)-precursor nicotinamide mononucleotide (NMN), inducing v-ATPase reassembly by stimulating glycolytic enzymes to bind to v-ATPase.

METHODS

Rats/mice on cardiomyopathy-inducing high-fat diets were supplemented with NMN and for comparison with a cocktail of lysine/leucine/arginine (mTORC1 [mechanistic target of rapamycin complex 1]-mediated v-ATPase reassembly). We used the following methods: RNA sequencing, mRNA/protein expression analysis, immunofluorescence microscopy, (co)immunoprecipitation/proximity ligation assay (v-ATPase assembly), myocellular uptake of [3H]chloroquine (endosomal pH), and [14C]palmitate, targeted lipidomics, and echocardiography. To confirm the involvement of v-ATPase in the beneficial effects of both supplementations, mTORC1/v-ATPase inhibitors (rapamycin/bafilomycin A1) were administered. Additionally, 2 heart-specific v-ATPase-knockout mouse models (subunits V1G1/V0d2) were subjected to these measurements. Mechanisms were confirmed in pharmacologically/genetically manipulated cardiomyocyte models of lipid overload.

RESULTS

NMN successfully preserved endosomal acidification during myocardial lipid overload by maintaining v-ATPase activity and subsequently prevented CD36-mediated lipid accumulation, CD36-TLR4 interaction toward inflammation, fibrosis, cardiac dysfunction, and whole-body insulin resistance. Lipidomics revealed C18:1-enriched diacylglycerols as lipid class prominently increased by high-fat diet and subsequently reversed/preserved by lysine/leucine/arginine/NMN treatment. Studies with mTORC1/v-ATPase inhibitors and heart-specific v-ATPase-knockout mice further confirmed the pivotal roles of v-ATPase in these beneficial actions.

CONCLUSION

NMN preserves heart function during lipid overload by preventing v-ATPase disassembly.

J147 affects cognition and anxiety after surgery in Zucker rats

Vulnerable patients are at risk for neuroinflammation-mediated post-operative complications, including depression (POD) and cognitive dysfunction (POCD). Zucker rats, expressing multiple risk factors for post-operative complications in humans, may provide a clinically relevant model to study pathophysiology and explore potential interventions. J147, a newly developed anti-dementia drug, was shown to prevent POCD in young healthy rats, and improved early post-surgical recovery in Zucker rats. Aim of the present study was to investigate POCD and the therapeutic potential of J147 in male Zucker rats. Risk factors in the Zucker rat strain were evaluated by comparison with lean littermates. Zucker rats were subjected to major abdominal surgery. Acute J147 treatment was provided by a single iv injection (10 mg/kg) at the start of surgery, while chronic J147 treatment was provided in the food (aimed at 30 mg/kg/day), starting one week before surgery and up to end of protocol. Effects on behavior were assessed, and plasma, urine and brain tissue were collected and processed for immunohistochemistry and molecular analyses. Indeed, Zucker rats displayed increased risk factors for POCD, including obesity, high plasma triglycerides, low grade systemic inflammation, impaired spatial learning and decreased neurogenesis. Surgery in Zucker rats reduced exploration and increased anxiety in the Open Field test, impaired short-term spatial memory, induced a shift in circadian rhythm and increased plasma neutrophil gelatinase-associated lipocalin (NGAL), microglia activity in the CA1 and blood brain barrier leakage. Chronic, but not acute J147 treatment reduced anxiety in the Open Field test and protected against the spatial memory decline. Moreover, chronic J147 increased glucose sensitivity. Acute J147 treatment improved long-term spatial memory and reversed the circadian rhythm shift. No anti-inflammatory effects were seen for J147. Although Zucker rats displayed risk factors, surgery did not induce extensive POCD. However, increased anxiety may indicate POD. Treatment with J147 showed positive effects on behavioral and metabolic parameters, but did not affect (neuro)inflammation. The mixed effect of acute and chronic treatment may suggest a combination for optimal treatment.

Prevalence and Clinical Consequences of Multiple Pathogenic Variants in Dilated Cardiomyopathy

Background:

Dilated cardiomyopathy (DCM) was considered a monogenetic disease that can be caused by over 60 genes. Evidence suggests that the combination of multiple pathogenic variants leads to greater disease severity and earlier onset. So far, not much is known about the prevalence and disease course of multiple pathogenic variants in patients with DCM. To gain insight into these knowledge gaps, we (1) systematically collected clinical information from a well-characterized DCM cohort and (2) created a mouse model.

Methods:

Complete cardiac phenotyping and genotyping was performed in 685 patients with consecutive DCM. Compound heterozygous digenic (LMNA [lamin]/titin deletion A-band) with monogenic (LMNA/wild-type) and wild-type/wild-type mice were created and phenotypically followed over time.

Results:

One hundred thirty-one likely pathogenic/pathogenic (LP/P) variants in robust DCM-associated genes were found in 685 patients with DCM (19.1%) genotyped for the robust genes. Three of the 131 patients had a second LP/P variant (2.3%). These 3 patients had a comparable disease onset, disease severity, and clinical course to patients with DCM with one LP/P. The LMNA/Titin deletion A-band mice had no functional differences compared with the LMNA/wild-type mice after 40 weeks of follow-up, although RNA-sequencing suggests increased cardiac stress and sarcomere insufficiency in the LMNA/Titin deletion A-band mice.

Conclusions:

In this study population, 2.3% of patients with DCM with one LP/P also have a second LP/P in a different gene. Although the second LP/P does not seem to influence the disease course of DCM in patients and mice, the finding of a second LP/P can be of importance to their relatives.

Clustering of Cardiac Transcriptome Profiles Reveals Unique

Dilated cardiomyopathy is a heterogeneous disease characterized by multiple genetic and environmental etiologies. The majority of patients are treated the same despite these differences. The cardiac transcriptome provides information on the patient's pathophysiology, which allows targeted therapy. Using clustering techniques on data from the genotype, phenotype, and cardiac transcriptome of patients with early- and end-stage dilated cardiomyopathy, more homogeneous patient subgroups are identified based on shared underlying pathophysiology. Distinct patient subgroups are identified based on differences in protein quality control, cardiac metabolism, cardiomyocyte function, and inflammatory pathways. The identified pathways have the potential to guide future treatment and individualize patient care.

Characterization of cardiac metabolism in iPSC-derived cardiomyocytes: lessons from maturation and disease modeling

Background

Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) have emerged as a powerful tool for disease modeling, though their immature nature currently limits translation into clinical practice. Maturation strategies increasingly pay attention to cardiac metabolism because of its pivotal role in cardiomyocyte development and function. Moreover, aberrances in cardiac metabolism are central to the pathogenesis of cardiac disease. Thus, proper modeling of human cardiac disease warrants careful characterization of the metabolic properties of iPSC-CMs.

Methods

Here, we examined the effect of maturation protocols on healthy iPSC-CMs applied in 23 studies and compared fold changes in functional metabolic characteristics to assess the level of maturation. In addition, pathological metabolic remodeling was assessed in 13 iPSC-CM studies that focus on hypertrophic cardiomyopathy (HCM), which is characterized by abnormalities in metabolism.

Results

Matured iPSC-CMs were characterized by mitochondrial maturation, increased oxidative capacity and enhanced fatty acid use for energy production. HCM iPSC-CMs presented varying degrees of metabolic remodeling ranging from compensatory to energy depletion stages, likely due to the different types of mutations and clinical phenotypes modeled. HCM further displayed early onset hypertrophy, independent of the type of mutation or disease stage.

Conclusions

Maturation strategies improve the metabolic characteristics of iPSC-CMs, but not to the level of the adult heart. Therefore, a combination of maturation strategies might prove to be more effective. Due to early onset hypertrophy, HCM iPSC-CMs may be less suitable to detect early disease modifiers in HCM and might prove more useful to examine the effects of gene editing and new drugs in advanced disease stages. With this review, we provide an overview of the assays used for characterization of cardiac metabolism in iPSC-CMs and advise on which metabolic assays to include in future maturation and disease modeling studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03021-9.

Ketone Body Exposure of Cardiomyocytes Impairs Insulin Sensitivity and Contractile Function through Vacuolar-Type H+-ATPase Disassembly—Rescue by Specific Amino Acid Supplementation

The heart is metabolically flexible. Under physiological conditions, it mainly uses lipids and glucose as energy substrates. In uncontrolled diabetes, the heart switches towards predominant lipid utilization, which over time is detrimental to cardiac function. Additionally, diabetes is accompanied by high plasma ketone levels and increased utilization of energy provision. The administration of exogenous ketones is currently being investigated for the treatment of cardiovascular disease. Yet, it remains unclear whether increased cardiac ketone utilization is beneficial or detrimental to cardiac functioning. The mechanism of lipid-induced cardiac dysfunction includes disassembly of the endosomal proton pump (named vacuolar-type H+-ATPase; v-ATPase) as the main early onset event, followed by endosomal de-acidification/dysfunction. The de-acidified endosomes can no longer serve as a storage compartment for lipid transporter CD36, which then translocates to the sarcolemma to induce lipid accumulation, insulin resistance, and contractile dysfunction. Lipid-induced v-ATPase disassembly is counteracted by the supply of specific amino acids. Here, we tested the effect of ketone bodies on v-ATPase assembly status and regulation of lipid uptake in rodent/human cardiomyocytes. 3-β-hydroxybutyrate (3HB) exposure induced v-ATPase disassembly and the entire cascade of events leading to contractile dysfunction and insulin resistance, similar to conditions of lipid oversupply. Acetoacetate addition did not induce v-ATPase dysfunction. The negative effects of 3HB could be prevented by addition of specific amino acids. Hence, in sedentary/prediabetic subjects ketone bodies should be used with caution because of possible aggravation of cardiac insulin resistance and further loss of cardiac function. When these latter maladaptive conditions would occur, specific amino acids could potentially be a treatment option.

Titin truncating variant cardiomyopathy and related sarcomere insufficiency causes high energy demand resulting in mitochondrial dysfunction, autophagosome formation, and apoptosis

Background/Objectives

Titin truncating variants (TTNtv) are the most prevalent genetic cause of dilated cardiomyopathy (DCM), resulting in upregulation of cardiac transcripts of oxidative phosphorylation (1,2). However, the underlying molecular mechanism(s) and cellular consequences of these findings remain unknown.

Methods and results

To gain insight into the metabolic changes and cellular consequences of a TTNtv, metabolic, mitochondrial, and survival pathways were studied in human TTNtv DCM hearts and isolated cardiomyocytes of TTNtv mice. TTNtv resulted in a significant increase of cardiac transcripts of glycolysis, citric acid cycle, mitochondrial fission, autophagy, and apoptosis when comparing RNAseq in 24 TTNtv and 27 mutation-negative DCM cardiac biopsies. Furthermore, a decrease in the area of myofibrils in human TTNtv hearts (TTNtv vs. mutation-negative DCM: 46%, and 62%, P=0.001), and an increase of mitochondrial (49% and 31%, P=0,001) and autophagosome areas (4% and 2%, P=0.002) was observed using transmission electron microscopy (TEM). Similar patterns of cardiomyocyte disorganization and stress could be seen in TTNtv hearts of mice even without a phenotype. Additionally, observed swollen mitochondria by TEM and decreased quantity of OXPHOS proteins by immunoblotting in murine TTNtv hearts indicate mitochondrial stress. Mitochondrial oxygen consumption at baseline and the maximum respiration in TTNtv cardiomyocytes of mice increased by a factor of 1.8 and 1.5 respectively (both P≤0.05), compared to WT. Furthermore, palmitate oxidation in TTNtv cardiomyocytes increased by 1.3 fold (P=0.005) compared to WT mice, suggestive of increased energy demand in TTNtv.

Conclusion

Myofibrillar insufficiency in human TTNtv DCM augments the cardiac oxygen and energy consumption, leading to pronounced morphological and functional mitochondrial decompensation. Swelling, damage and fission of mitochondria is further characterized by autophagosome formation and increased apoptosis pathways in TTNtv hearts.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Double-Dose consortium by Dutch Cardiovascular Alliance (DCVA)

Guidelines on Models of Diabetic Heart Disease

Diabetes is a major risk factor for cardiovascular diseases, including diabetic cardiomyopathy, atherosclerosis, myocardial infarction, and heart failure. As cardiovascular disease represents the number one cause of death in people with diabetes, there has been a major emphasis on understanding the mechanisms by which diabetes promotes cardiovascular disease, and how antidiabetic therapies impact diabetic heart disease. With a wide array of models to study diabetes (both type 1 and type 2), the field has made major progress in answering these questions. However, each model has its own inherent limitations. Therefore, the purpose of this guidelines document is to provide the field with information on which aspects of cardiovascular disease in the human diabetic population are most accurately reproduced by the available models. This review aims to emphasize the advantages and disadvantages of each model, and to highlight the practical challenges and technical considerations involved. We will review the preclinical animal models of diabetes (based on their method of induction), appraise models of diabetes-related atherosclerosis and heart failure, and discuss in vitro models of diabetic heart disease. These guidelines will allow researchers to select the appropriate model of diabetic heart disease, depending on the specific research question being addressed.

Evaluation of the Interaction of Sex Hormones and Cardiovascular Function and Health

Purpose of Review

Sex hormones drive development and function of reproductive organs or the development of secondary sex characteristics but their effects on the cardiovascular system are poorly understood. In this review, we identify the gaps in our understanding of the interaction between sex hormones and the cardiovascular system.

Recent Findings

Studies are progressively elucidating molecular functions of sex hormones in specific cell types in parallel with the initiation of crucial large randomized controlled trials aimed at improving therapies for cardiovascular diseases (CVDs) associated with aberrant levels of sex hormones.

Summary

In contrast with historical assumptions, we now understand that men and women show different symptoms and progression of CVDs. Abnormal levels of sex hormones pose an independent risk for CVD, which is apparent in conditions like Klinefelter syndrome, androgen insensitivity syndrome, and menopause. Moreover, sex hormone–based therapies remain understudied and may not be beneficial for cardiovascular health.

CD36 (SR-B2) as master regulator of cellular fatty acid homeostasis

PURPOSE OF REVIEW

Transmembrane glycoprotein cluster of differentiation 36 (CD36) is a scavenger receptor class B protein (SR-B2) that serves various functions in lipid metabolism and signaling, in particular facilitating the cellular uptake of long-chain fatty acids. Recent studies have disclosed CD36 to play a prominent regulatory role in cellular fatty acid metabolism in both health and disease.

RECENT FINDINGS

The rate of cellular fatty acid uptake is short-term (i.e., minutes) regulated by the subcellular recycling of CD36 between endosomes and the plasma membrane. This recycling is governed by the activity of vacuolar-type H+-ATPase (v-ATPase) in the endosomal membrane via assembly and disassembly of two subcomplexes. The latter process is being influenced by metabolic substrates including fatty acids, glucose and specific amino acids, together resulting in a dynamic interplay to modify cellular substrate preference and uptake rates. Moreover, in cases of metabolic disease v-ATPase activity was found to be affected while interventions aimed at normalizing v-ATPase functioning had therapeutic potential.

SUMMARY

The emerging central role of CD36 in cellular lipid homeostasis and recently obtained molecular insight in the interplay among metabolic substrates indicate the applicability of CD36 as target for metabolic modulation therapy in disease. Experimental studies already have shown the feasibility of this approach.

Specific amino acid supplementation rescues the heart from lipid overload-induced insulin resistance and contractile dysfunction by targeting the endosomal mTOR-v-ATPase axis

Objective

The diabetic heart is characterized by extensive lipid accumulation which often leads to cardiac contractile dysfunction. The underlying mechanism involves a pivotal role for vacuolar-type H⁺-ATPase (v-ATPase, functioning as endosomal/lysosomal proton pump). Specifically, lipid oversupply to the heart causes disassembly of v-ATPase and endosomal deacidification. Endosomes are storage compartments for lipid transporter CD36. However, upon endosomal deacidification, CD36 is expelled to translocate to the sarcolemma, thereby inducing myocardial lipid accumulation, insulin resistance, and contractile dysfunction. Hence, the v-ATPase assembly may be a suitable target for ameliorating diabetic cardiomyopathy. Another function of v-ATPase involves the binding of anabolic master-regulator mTORC1 to endosomes, a prerequisite for the activation of mTORC1 by amino acids (AAs). We examined whether the relationship between v-ATPase and mTORC1 also operates reciprocally; specifically, whether AA induces v-ATPase reassembly in a mTORC1-dependent manner to prevent excess lipids from entering and damaging the heart.

Methods

Lipid overexposed rodent/human cardiomyocytes and high-fat diet-fed rats were treated with a specific cocktail of AAs (lysine/leucine/arginine). Then, v-ATPase assembly status/activity, cell surface CD36 content, myocellular lipid uptake/accumulation, insulin sensitivity, and contractile function were measured. To elucidate underlying mechanisms, specific gene knockdown was employed, followed by subcellular fractionation, and coimmunoprecipitation.

Results

In lipid-overexposed cardiomyocytes, lysine/leucine/arginine reinternalized CD36 to the endosomes, prevented/reversed lipid accumulation, preserved/restored insulin sensitivity, and contractile function. These beneficial AA actions required the mTORC1–v-ATPase axis, adaptor protein Ragulator, and endosomal/lysosomal AA transporter SLC38A9, indicating an endosome-centric inside-out AA sensing mechanism. In high-fat diet-fed rats, lysine/leucine/arginine had similar beneficial actions at the myocellular level as in vitro in lipid-overexposed cardiomyocytes and partially reversed cardiac hypertrophy.

Conclusion

Specific AAs acting through v-ATPase reassembly reduce cardiac lipid uptake raising the possibility for treatment in situations of lipid overload and associated insulin resistance.

• High physiological concentrations of specific AAs (K/L/R) activate v-ATPase.

• The KLR mix activates v-ATPase by mutually dependent activation of mTORC1.

• KLR-induced v-ATPase activation enables endosomes to retain lipid transporter CD36.

• KLR mends lipid-induced insulin resistance and cardiomyocytic contractile dysfunction.

• KLR reverses v-ATPase disassembly and cardiac hypertrophy in high-fat diet-fed rats.

CD36 as a target for metabolic modulation therapy in cardiac disease

Introduction: Disturbances in myocardial lipid metabolism are increasingly being recognized as drivers of the development and progression of heart disease. Therefore, there is a need for treatments that can directly target lipid metabolic defects in heart failure. The membrane-associated glycoprotein CD36 plays a pivotal role in governing myocardial lipid metabolism by mediating lipid signaling and facilitating the cellular uptake of long-chain fatty acids. Emerging evidence suggests that CD36 is a prominent target in the treatment of heart failure.Areas covered: This article provides an overview of the key role of CD36 for proper contractile functioning of a healthy heart, its implications in the development of cardiac disease (ischemia/reperfusion, cardiac hypertrophy, and diabetic cardiomyopathy), and its application as a target to normalize cardiac metabolism as part of so-called metabolic modulation therapy.Expert opinion: CD36 appears a promising and effective therapeutic target in the treatment of heart failure. Natural compounds and chemical agents known to alter the amount or subcellular distribution of CD36 or inhibit its functioning, should be evaluated for their potency to correct cardiac metabolism and cure heart disease.

Multiview deconvolution approximation multiphoton microscopy of tissues and zebrafish larvae

Imaging in three dimensions is necessary for thick tissues and small organisms. This is possible with tomographic optical microscopy techniques such as confocal, multiphoton and light sheet microscopy. All these techniques suffer from anisotropic resolution and limited penetration depth. In the past, Multiview microscopy-imaging the sample from different angles followed by 3D image reconstruction-was developed to address this issue for light sheet microscopy based on fluorescence signal. In this study we applied this methodology to accomplish Multiview imaging with multiphoton microscopy based on fluorescence and additionally second harmonic signal from myosin and collagen. It was shown that isotropic resolution was achieved, the entirety of the sample was visualized, and interference artifacts were suppressed allowing clear visualization of collagen fibrils and myofibrils. This method can be applied to any scanning microscopy technique without microscope modifications. It can be used for imaging tissue and whole mount small organisms such as heart tissue, and zebrafish larva in 3D, label-free or stained, with at least threefold axial resolution improvement which can be significant for the accurate quantification of small 3D structures.

Comparison of human and rodent cell models to study myocardial lipid-induced insulin resistance

Isolated or cultured cells have proven to be valuable model systems to investigate cellular (patho)biology and for screening of the efficacy of drugs or their possible side-effects. Pluripotent stem cells (PSC) can be readily obtained from healthy individuals as well as from diseased patients, and protocols have been developed to differentiate these cells into cardiomyocytes. Hence, these cellular models are moving center stage for a broader application. In this review, we focus on comparing mouse HL-1 cardiomyocytes, isolated adult rat cardiomyocytes, human embryonic stem cell-derived cardiomyocytes (hESC-CMs) and human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) for the study of metabolic aspects of cardiac functioning in health and disease. Various studies have reported that these cellular models are suitable for assessing substrate uptake and utilization, in that each display an adequate and similar response to physiological triggers, in particular the presence of insulin. Likewise, disease conditions, such as excess lipid supply, similarly affect each of these rodent and human cardiomyocyte models. It is concluded that PSC-CMs obtained from patients with cardiogenetic abnormalities are promising models to evaluate the functional consequence of gene variants with unknown significance.

Metabolic Interventions to Prevent Hypertrophy-Induced Alterations in Contractile Properties In Vitro

(1) Background: The exact mechanism(s) underlying pathological changes in a heart in transition to hypertrophy and failure are not yet fully understood. However, alterations in cardiac energy metabolism seem to be an important contributor. We characterized an in vitro model of adrenergic stimulation-induced cardiac hypertrophy for studying metabolic, structural, and functional changes over time. Accordingly, we investigated whether metabolic interventions prevent cardiac structural and functional changes; (2) Methods: Primary rat cardiomyocytes were treated with phenylephrine (PE) for 16 h, 24 h, or 48 h, whereafter hypertrophic marker expression, protein synthesis rate, glucose uptake, and contractile function were assessed; (3) Results: 24 h PE treatment increased expression of hypertrophic markers, phosphorylation of hypertrophy-related signaling kinases, protein synthesis, and glucose uptake. Importantly, the increased glucose uptake preceded structural and functional changes, suggesting a causal role for metabolism in the onset of PE-induced hypertrophy. Indeed, PE treatment in the presence of a PAN-Akt inhibitor or of a GLUT4 inhibitor dipyridamole prevented PE-induced increases in cellular glucose uptake and ameliorated PE-induced contractile alterations; (4) Conclusions: Pharmacological interventions, forcing substrate metabolism away from glucose utilization, improved contractile properties in PE-treated cardiomyocytes, suggesting that targeting glucose uptake, independent from protein synthesis, forms a promising strategy to prevent hypertrophy and hypertrophy-induced cardiac dysfunction.

Putative Role of Protein Palmitoylation in Cardiac Lipid-Induced Insulin Resistance

In the heart, inhibition of the insulin cascade following lipid overload is strongly associated with contractile dysfunction. The translocation of fatty acid transporter CD36 (SR-B2) from intracellular stores to the cell surface is a hallmark event in the lipid-overloaded heart, feeding forward to intracellular lipid accumulation. Yet, the molecular mechanisms by which intracellularly arrived lipids induce insulin resistance is ill-understood. Bioactive lipid metabolites (diacyl-glycerols, ceramides) are contributing factors but fail to correlate with the degree of cardiac insulin resistance in diabetic humans. This leaves room for other lipid-induced mechanisms involved in lipid-induced insulin resistance, including protein palmitoylation. Protein palmitoylation encompasses the reversible covalent attachment of palmitate moieties to cysteine residues and is governed by protein acyl-transferases and thioesterases. The function of palmitoylation is to provide proteins with proper spatiotemporal localization, thereby securing the correct unwinding of signaling pathways. In this review, we provide examples of palmitoylations of individual signaling proteins to discuss the emerging role of protein palmitoylation as a modulator of the insulin signaling cascade. Second, we speculate how protein hyper-palmitoylations (including that of CD36), as they occur during lipid oversupply, may lead to insulin resistance. Finally, we conclude that the protein palmitoylation machinery may offer novel targets to fight lipid-induced cardiomyopathy.

Phosphatidylinositol 4-kinase IIIβ mediates contraction-induced GLUT4 translocation and shows its anti-diabetic action in cardiomyocytes

In the diabetic heart, long-chain fatty acid (LCFA) uptake is increased at the expense of glucose uptake. This metabolic shift ultimately leads to insulin resistance and a reduced cardiac function. Therefore, signaling kinases that mediate glucose uptake without simultaneously stimulating LCFA uptake could be considered attractive anti-diabetic targets. Phosphatidylinositol-4-kinase-IIIβ (PI4KIIIβ) is a lipid kinase downstream of protein kinase D1 (PKD1) that mediates Golgi-to-plasma membrane vesicular trafficking in HeLa-cells. In this study, we evaluated whether PI4KIIIβ is involved in myocellular GLUT4 translocation induced by contraction or oligomycin (an F₁F₀-ATP synthase inhibitor that activates contraction-like signaling). Pharmacological targeting, with compound MI14, or genetic silencing of PI4KIIIβ inhibited contraction/oligomycin-stimulated GLUT4 translocation and glucose uptake in cardiomyocytes but did not affect CD36 translocation nor LCFA uptake. Addition of the PI4KIIIβ enzymatic reaction product phosphatidylinositol-4-phosphate restored oligomycin-stimulated glucose uptake in the presence of MI14. PI4KIIIβ activation by PKD1 involves Ser294 phosphorylation and altered its localization with unchanged enzymatic activity. Adenoviral PI4KIIIβ overexpression stimulated glucose uptake, but did not activate hypertrophic signaling, indicating that unlike PKD1, PI4KIIIβ is selectively involved in GLUT4 translocation. Finally, PI4KIIIβ overexpression prevented insulin resistance and contractile dysfunction in lipid-overexposed cardiomyocytes. Together, our studies identify PI4KIIIβ as positive and selective regulator of GLUT4 translocation in response to contraction-like signaling, suggesting PI4KIIIβ as a promising target to rescue defective glucose uptake in diabetics.

Pivotal role of membrane substrate transporters on the metabolic alterations in the pressure-overloaded heart

Cardiac pressure overload (PO), such as caused by aortic stenosis and systemic hypertension, commonly results in cardiac hypertrophy and may lead to the development of heart failure. PO-induced heart failure is among the leading causes of death worldwide, but its pathological origin remains poorly understood. Metabolic alterations are proposed to be an important contributor to PO-induced cardiac hypertrophy and failure. While the healthy adult heart mainly uses long-chain fatty acids (FAs) and glucose as substrates for energy metabolism and to a lesser extent alternative substrates, i.e. lactate, ketone bodies, and amino acids (AAs), the pressure-overloaded heart is characterized by a shift in energy metabolism towards a greater reliance on glycolysis and alternative substrates. A key-governing kinetic step of both FA and glucose fluxes is at the level of their substrate-specific membrane transporters. The relative presence of these transporters in the sarcolemma determines the cardiac substrate preference. Whether the cardiac utilization of alternative substrates is also governed by membrane transporters is not yet known. In this review, we discuss current insight into the role of membrane substrate transporters in the metabolic alterations occurring in the pressure-overloaded heart. Given the increasing evidence of a role for alternative substrates in these metabolic alterations, there is an urgent need to disclose the key-governing kinetic steps in their utilization as well. Taken together, membrane substrate transporters emerge as novel targets for metabolic interventions to prevent or treat PO-induced heart failure.

Understanding the distinct subcellular trafficking of CD36 and GLUT4 during the development of myocardial insulin resistance

CD36 and GLUT4 are the main cardiac trans-sarcolemmal transporters for long-chain fatty acids and glucose, respectively. Together they secure the majority of cardiac energy demands. Moreover, these transporters each represent key governing kinetic steps in cardiac fatty acid and glucose fluxes, thereby offering major sites of regulation. The underlying mechanism of this regulation involves a perpetual vesicle-mediated trafficking (recycling) of both transporters between intracellular stores (endosomes) and the cell surface. In the healthy heart, CD36 and GLUT4 translocation to the cell surface is under short-term control of the same physiological stimuli, most notably increased contraction and insulin secretion. However, under chronic lipid overload, a condition that accompanies a Western lifestyle, CD36 and GLUT4 recycling are affected distinctly, with CD36 being expelled to the sarcolemma while GLUT4 is imprisoned within the endosomes. Moreover, the increased CD36 translocation towards the cell surface is a key early step, setting the heart on a route towards insulin resistance and subsequent contractile dysfunction. Therefore, the proteins making up the trafficking machinery of CD36 need to be identified with special focus to the differences with the protein composition of the GLUT4 trafficking machinery. These proteins that are uniquely dedicated to either CD36 or GLUT4 traffic may offer targets to rectify aberrant substrate uptake seen in the lipid-overloaded heart. Specifically, CD36-dedicated trafficking regulators should be inhibited, whereas such GLUT4-dedicated proteins would need to be activated. Recent advances in the identification of CD36-dedicated trafficking proteins have disclosed the involvement of vacuolar-type H⁺-ATPase and of specific vesicle-associated membrane proteins (VAMPs). In this review, we summarize these recent findings and sketch a roadmap of CD36 and GLUT4 trafficking compatible with experimental findings.

Augmenting Vacuolar H+-ATPase Function Prevents Cardiomyocytes from Lipid-Overload Induced Dysfunction

The diabetic heart is characterized by a shift in substrate utilization from glucose to lipids, which may ultimately lead to contractile dysfunction. This substrate shift is facilitated by increased translocation of lipid transporter CD36 (SR-B2) from endosomes to the sarcolemma resulting in increased lipid uptake. We previously showed that endosomal retention of CD36 is dependent on the proper functioning of vacuolar H⁺-ATPase (v-ATPase). Excess lipids trigger CD36 translocation through inhibition of v-ATPase function. Conversely, in yeast, glucose availability is known to enhance v-ATPase function, allowing us to hypothesize that glucose availability, via v-ATPase, may internalize CD36 and restore contractile function in lipid-overloaded cardiomyocytes. Increased glucose availability was achieved through (a) high glucose (25 mM) addition to the culture medium or (b) adenoviral overexpression of protein kinase-D1 (a kinase mediating GLUT4 translocation). In HL-1 cardiomyocytes, adult rat and human cardiomyocytes cultured under high-lipid conditions, each treatment stimulated v-ATPase re-assembly, endosomal acidification, endosomal CD36 retention and prevented myocellular lipid accumulation. Additionally, these treatments preserved insulin-stimulated GLUT4 translocation and glucose uptake as well as contractile force. The present findings reveal v-ATPase functions as a key regulator of cardiomyocyte substrate preference and as a novel potential treatment approach for the diabetic heart.

CD36 (SR-B2) as a Target to Treat Lipid Overload-Induced Cardiac Dysfunction

The heart faces the challenge of adjusting the rate of fatty acid uptake to match myocardial demand for energy provision at any given moment, avoiding both too low uptake rates, which could elicit an energy deficit, and too high uptake rates, which pose the risk of excess lipid accumulation and lipotoxicity. The transmembrane glycoprotein cluster of differentiation 36 (CD36), a scavenger receptor (B2), serves many functions in lipid metabolism and signaling. In the heart, CD36 is the main sarcolemmal lipid transporter involved in the rate-limiting kinetic step in cardiac lipid utilization. The cellular fatty acid uptake rate is determined by the presence of CD36 at the cell surface, which is regulated by subcellular vesicular recycling from endosomes to the sarcolemma. CD36 has been implicated in dysregulated fatty acid and lipid metabolism in pathophysiological conditions, particularly high-fat diet-induced insulin resistance and diabetic cardiomyopathy. Thus, in conditions of chronic lipid overload, high levels of CD36 are moved to the sarcolemma, setting the heart on a route towards increased lipid uptake, excessive lipid accumulation, insulin resistance, and eventually contractile dysfunction. Insight into the subcellular trafficking machinery of CD36 will provide novel targets to treat the lipid-overloaded heart. A screen for CD36-dedicated trafficking proteins found that vacuolar-type H⁺-ATPase and specific vesicle-associated membrane proteins, among others, were uniquely involved in CD36 recycling. Preliminary data suggest that these proteins may offer clues on how to manipulate myocardial lipid uptake, and thus could be promising targets for metabolic intervention therapy to treat the failing heart.

Re-balancing cellular energy substrate metabolism to mend the failing heart

Fatty acids and glucose are the main substrates for myocardial energy provision. Under physiologic conditions, there is a distinct and finely tuned balance between the utilization of these substrates. Using the non-ischemic heart as an example, we discuss that upon stress this substrate balance is upset resulting in an over-reliance on either fatty acids or glucose, and that chronic fuel shifts towards a single type of substrate appear to be linked with cardiac dysfunction. These observations suggest that interventions aimed at re-balancing a tilted substrate preference towards an appropriate mix of substrates may result in restoration of cardiac contractile performance. Examples of manipulating cellular substrate uptake as a means to re-balance fuel supply, being associated with mended cardiac function underscore this concept. We also address the molecular mechanisms underlying the apparent need for a fatty acid-glucose fuel balance. We propose that re-balancing cellular fuel supply, in particular with respect to fatty acids and glucose, may be an effective strategy to treat the failing heart.

Increased cardiac fatty acid oxidation in a mouse model with decreased malonyl-CoA sensitivity of CPT1B

Aims

Mitochondrial fatty acid oxidation (FAO) is an important energy provider for cardiac work and changes in cardiac substrate preference are associated with different heart diseases. Carnitine palmitoyltransferase 1B (CPT1B) is thought to perform the rate limiting enzyme step in FAO and is inhibited by malonyl-CoA. The role of CPT1B in cardiac metabolism has been addressed by inhibiting or decreasing CPT1B protein or after modulation of tissue malonyl-CoA metabolism. We assessed the role of CPT1B malonyl-CoA sensitivity in cardiac metabolism.

Methods and results

We generated and characterized a knock in mouse model expressing the CPT1BE3A mutant enzyme, which has reduced sensitivity to malonyl-CoA. In isolated perfused hearts, FAO was 1.9-fold higher in Cpt1bE3A/E3A hearts compared with Cpt1bWT/WT hearts. Metabolomic, proteomic and transcriptomic analysis showed increased levels of malonylcarnitine, decreased concentration of CPT1B protein and a small but coordinated downregulation of the mRNA expression of genes involved in FAO in Cpt1bE3A/E3A hearts, all of which aim to limit FAO. In vivo assessment of cardiac function revealed only minor changes, cardiac hypertrophy was absent and histological analysis did not reveal fibrosis.

Conclusions

Malonyl-CoA-dependent inhibition of CPT1B plays a crucial role in regulating FAO rate in the heart. Chronic elevation of FAO has a relatively subtle impact on cardiac function at least under baseline conditions.

Microbial-Driven Butyrate Regulates Jejunal Homeostasis in Piglets During the Weaning Stage

Microbe-derived butyrate plays an important role in the gut health of young mammals during the weaning stage. A greater understanding of how butyrate regulates intestinal development is necessary for overcoming post-weaning diarrheal diseases. We aimed to investigate whether jejunal microbial metabolite butyrate modulates the apoptosis/proliferation balance and immune response in piglets during the post-weaning period of the first 3 weeks of life. On the one hand, during the first week post-weaning, the relative abundances of the dominant bacterial families Erysipelotrichaceae (P < 0.01) and Lachnospiraceae (P < 0.01) were increased, which induced decreases in both butyrate production (P < 0.05) and its receptor (G-protein coupled receptor 43) expression (P < 0.01). The resulting intestinal inflammation (inferred from increased TNF-α and IFN-γ expression) contributed to the onset of cell apoptosis and the inhibition of cell-proliferation along the crypt-villus axis, which were followed by impaired jejunal morphology (i.e., increased crypt-depth) (P < 0.05) and intestinal dysfunction (i.e., decreased creatine kinase, and lactate dehydrogenase) (P < 0.05). On the other hand, during the second week post-weaning, the relative abundances of Lactobacillaceae (P < 0.01) and Ruminococcaceae (P < 0.05) were increased. The increases were accompanied by increased butyrate production (P < 0.05) and its receptor expression (P < 0.01), leading to the inhibition of cell apoptosis and the stimulation of cell proliferation via decreased pro-inflammatory cytokines and thereby the improvement of intestinal development and function. Herein, this study demonstrates that microbial-driven butyrate might be a key modulator in the maintenance of intestinal homeostasis after weaning. The findings suggest that strategies to promote butyrate production can maintain the apoptosis/proliferation balance via minimizing intestinal inflammation, and thereby improving post-weaning jejunal adaptation toward gut health.

Acute and Chronic Effects of Protein Kinase-D Signaling on Cardiac Energy Metabolism

Protein kinase-D (PKD) is increasingly recognized as a key regulatory signaling hub in cardiac glucose uptake and also a major player in the development of hypertrophy. Glucose is one of the predominant energy substrates for the heart to support contraction. However, a cardiac substrate switch toward glucose over-usage is associated with the development of cardiac hypertrophy. Hence, regulation of PKD activity must be strictly coordinated. This review provides mechanistic insights into the acute and chronic regulatory functions of PKD signaling in the healthy and hypertrophied heart. First an overview of the activation pathways of PKD1, the most abundant isoform in the heart, is provided. Then the various regulatory roles of the PKD isoforms in the heart in relation to cardiac glucose and fatty acid metabolism, contraction, morphology, function, and the development of cardiac hypertrophy are described. Finally, these findings are integrated and the possibility of targeting this kinase as a novel strategy to combat cardiac diseases is discussed.

Diabetic db/db mice do not develop heart failure upon pressure overload: a longitudinal in vivo PET, MRI, and MRS study on cardiac metabolic, structural, and functional adaptations

Aims

Heart failure is associated with altered myocardial substrate metabolism and impaired cardiac energetics. Comorbidities like diabetes may influence the metabolic adaptations during heart failure development. We quantified to what extent changes in substrate preference, lipid accumulation, and energy status predict the longitudinal development of hypertrophy and failure in the non-diabetic and the diabetic heart.

Methods and results

Transverse aortic constriction (TAC) was performed in non-diabetic (db/+) and diabetic (db/db) mice to induce pressure overload. Magnetic resonance imaging, 31P magnetic resonance spectroscopy (MRS), 1H MRS, and 18F-fluorodeoxyglucose-positron emission tomography (PET) were applied to measure cardiac function, energy status, lipid content, and glucose uptake, respectively. In vivo measurements were complemented with ex vivo techniques of high-resolution respirometry, proteomics, and western blotting to elucidate the underlying molecular pathways. In non-diabetic mice, TAC induced progressive cardiac hypertrophy and dysfunction, which correlated with increased protein kinase D-1 (PKD1) phosphorylation and increased glucose uptake. These changes in glucose utilization preceded a reduction in cardiac energy status. At baseline, compared with non-diabetic mice, diabetic mice showed normal cardiac function, higher lipid content and mitochondrial capacity for fatty acid oxidation, and lower PKD1 phosphorylation, glucose uptake, and energetics. Interestingly, TAC affected cardiac function only mildly in diabetic mice, which was accompanied by normalization of phosphorylated PKD1, glucose uptake, and cardiac energy status.

Conclusion

The cardiac metabolic adaptations in diabetic mice seem to prevent the heart from failing upon pressure overload, suggesting that restoring the balance between glucose and fatty acid utilization is beneficial for cardiac function.

Human embryonic stem cell-derived cardiomyocytes as an in vitro model to study cardiac insulin resistance

Patients with type 2 diabetes (T2D) and/or insulin resistance (IR) have an increased risk for the development of heart failure (HF). Evidence indicates that this increased risk is linked to an altered cardiac substrate preference of the insulin resistant heart, which shifts from a balanced utilization of glucose and long-chain fatty acids (FAs) towards an almost complete reliance on FAs as main fuel source. This shift leads to a loss of endosomal proton pump activity and increased cardiac fat accumulation, which eventually triggers cardiac dysfunction. In this review, we describe the advantages and disadvantages of currently used in vitro models to study the underlying mechanism of IR-induced HF and provide insight into a human in vitro model: human embryonic stem cell-derived cardiomyocytes (hESC-CMs). Using functional metabolic assays we demonstrate that, similar to rodent studies, hESC-CMs subjected to 16h of high palmitate (HP) treatment develop the main features of IR, i.e., decreased insulin-stimulated glucose and FA uptake, as well as loss of endosomal acidification and insulin signaling. Taken together, these data propose that HP-treated hESC-CMs are a promising in vitro model of lipid overload-induced IR for further research into the underlying mechanism of cardiac IR and for identifying new pharmacological agents and therapeutic strategies. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.

Statins Promote Cardiac Infarct Healing by Modulating Endothelial Barrier Function Revealed by Contrast-Enhanced Magnetic Resonance Imaging

OBJECTIVE

The endothelium has a crucial role in wound healing, acting as a barrier to control transit of leukocytes. Endothelial barrier function is impaired in atherosclerosis preceding myocardial infarction (MI). Besides lowering lipids, statins modulate endothelial function. Here, we noninvasively tested whether statins affect permeability at the inflammatory (day 3) and the reparative (day 7) phase of infarct healing post-MI using contrast-enhanced cardiac magnetic resonance imaging (MRI).

APPROACH AND RESULTS

Noninvasive permeability mapping by MRI after MI in C57BL/6, atherosclerotic ApoE^-/-, and statin-treated ApoE^-/- mice was correlated to subsequent left ventricular outcome by structural and functional cardiac MRI. Ex vivo histology, flow cytometry, and quantitative polymerase chain reaction were performed on infarct regions. Increased vascular permeability at ApoE^-/- infarcts was observed compared with C57BL/6 infarcts, predicting enhanced left ventricular dilation at day 21 post-MI by MRI volumetry. Statin treatment improved vascular barrier function at ApoE^-/- infarcts, indicated by reduced permeability. The infarcted tissue of ApoE^-/- mice 3 days post-MI displayed an unbalanced Vegfa(vascular endothelial growth factor A)/Angpt1 (angiopoetin-1) expression ratio (explaining leakage-prone vessels), associated with higher amounts of CD45⁺ leukocytes and inflammatory LY6C^hi monocytes. Statins reversed the unbalanced Vegfa/Angpt1 expression, normalizing endothelial barrier function at the infarct and blocking the augmented recruitment of inflammatory leukocytes in statin-treated ApoE^-/- mice.

CONCLUSIONS

Statins lowered permeability and reduced the transit of unfavorable inflammatory leukocytes into the infarcted tissue, consequently improving left ventricular outcome.

Palmitate-Induced Vacuolar-Type H+-ATPase Inhibition Feeds Forward Into Insulin Resistance and Contractile Dysfunction

Dietary fat overconsumption leads to myocardial lipid accumulation through mechanisms that are incompletely resolved. Previously, we identified increased translocation of the fatty acid transporter CD36 from its endosomal storage compartment to the sarcolemma as the primary mechanism of excessive myocellular lipid import. Here, we show that increased CD36 translocation is caused by alkalinization of endosomes resulting from inhibition of proton pumping activity of vacuolar-type H⁺-ATPase (v-ATPase). Endosomal alkalinization was observed in hearts from rats fed a lard-based high-fat diet and in rodent and human cardiomyocytes upon palmitate overexposure, and appeared as an early lipid-induced event preceding the onset of insulin resistance. Either genetic or pharmacological inhibition of v-ATPase in cardiomyocytes exposed to low palmitate concentrations reduced insulin sensitivity and cardiomyocyte contractility, which was rescued by CD36 silencing. The mechanism of palmitate-induced v-ATPase inhibition involved its dissociation into two parts: the cytosolic V₁ and the integral membrane V₀ subcomplex. Interestingly, oleate also inhibits v-ATPase function, yielding triacylglycerol accumulation but not insulin resistance. In conclusion, lipid oversupply increases CD36-mediated lipid uptake that directly impairs v-ATPase function. This feeds forward to enhanced CD36 translocation and further increased lipid uptake. In the case of palmitate, its accelerated uptake ultimately precipitates into cardiac insulin resistance and contractile dysfunction.

Dietary nitrate does not reduce oxygen cost of exercise or improve muscle mitochondrial function in patients with mitochondrial myopathy

Muscle weakness and exercise intolerance negatively affect the quality of life of patients with mitochondrial myopathy. Short-term dietary nitrate supplementation has been shown to improve exercise performance and reduce oxygen cost of exercise in healthy humans and trained athletes. We investigated whether 1 wk of dietary inorganic nitrate supplementation decreases the oxygen cost of exercise and improves mitochondrial function in patients with mitochondrial myopathy. Ten patients with mitochondrial myopathy (40 ± 5 yr, maximal whole body oxygen uptake = 21.2 ± 3.2 ml·min−1·kg body wt−1, maximal work load = 122 ± 26 W) received 8.5 mg·kg body wt−1·day−1 inorganic nitrate (~7 mmol) for 8 days. Whole body oxygen consumption at 50% of the maximal work load, in vivo skeletal muscle oxidative capacity (evaluated from postexercise phosphocreatine recovery using 31P-magnetic resonance spectroscopy), and ex vivo mitochondrial oxidative capacity in permeabilized skinned muscle fibers (measured with high-resolution respirometry) were determined before and after nitrate supplementation. Despite a sixfold increase in plasma nitrate levels, nitrate supplementation did not affect whole body oxygen cost during submaximal exercise. Additionally, no beneficial effects of nitrate were found on in vivo or ex vivo muscle mitochondrial oxidative capacity. This is the first time that the therapeutic potential of dietary nitrate for patients with mitochondrial myopathy was evaluated. We conclude that 1 wk of dietary nitrate supplementation does not reduce oxygen cost of exercise or improve mitochondrial function in the group of patients tested.

2-Arachidonoylglycerol ameliorates inflammatory stress-induced insulin resistance in cardiomyocytes

Several studies have linked impaired glucose uptake and insulin resistance (IR) to functional impairment of the heart. Recently, endocannabinoids have been implicated in cardiovascular disease. However, the mechanisms involving endocannabinoid signaling, glucose uptake, and IR in cardiomyocytes are understudied. Here we report that the endocannabinoid 2-arachidonoylglycerol (2-AG), via stimulation of cannabinoid type 1 (CB1) receptor and Ca²⁺/calmodulin-dependent protein kinase β, activates AMP-activated kinase (AMPK), leading to increased glucose uptake. Interestingly, we have observed that the mRNA expression of CB1 and CB2 receptors was decreased in diabetic mice, indicating reduced endocannabinoid signaling in the diabetic heart. We further establish that TNFα induces IR in cardiomyocytes. Treatment with 2-AG suppresses TNFα-induced proinflammatory markers and improves IR and glucose uptake. Conversely, pharmacological inhibition or knockdown of AMPK attenuates the anti-inflammatory effect and reversal of IR elicited by 2-AG. Additionally, in human embryonic stem cell-derived cardiomyocytes challenged with TNFα or FFA, we demonstrate that 2-AG improves insulin sensitivity and glucose uptake. In conclusion, 2-AG abates inflammatory responses, increases glucose uptake, and overcomes IR in an AMPK-dependent manner in cardiomyocytes.

Post-translational modifications of CD36 (SR-B2): Implications for regulation of myocellular fatty acid uptake

The membrane-associated protein CD36, now officially designated as SR-B2, is present in various tissues and fulfills multiple cellular functions. In heart and muscle, CD36 is the main (long-chain) fatty acid transporter, regulating myocellular fatty acid uptake via its vesicle-mediated reversible trafficking (recycling) between intracellular membrane compartments and the cell surface. CD36 is subject to various types of post-translational modification. This review focusses on the role of these modifications in further regulation of myocellular fatty acid uptake. Glycosylation, ubiquitination and palmitoylation are involved in regulating CD36 stability, while phosphorylation at extracellular sites affect the rate of fatty acid uptake. In addition, CD36 modification by O-linked N-acetylglucosamine may regulate the translocation of CD36 from endosomes to the cell surface. Acetylation of CD36 has also been reported, but possible effects on CD36 expression and/or functioning have not yet been addressed. Taken together, CD36 is subject to a multitude of post-translational modifications of which their functional implications are beginning to be understood. Moreover, further investigations are needed to disclose whether these post-translational modifications play a role in altered fatty acid uptake rates seen in several pathologies of heart and muscle. This article is part of a special issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck and Jan F.C. Glatz.

Preservation of myocardial fatty acid oxidation prevents diastolic dysfunction in mice subjected to angiotensin II infusion

RATIONALE

Diastolic dysfunction is a common feature in many heart failure patients with preserved ejection fraction and has been associated with altered myocardial metabolism in hypertensive and diabetic patients. Therefore, metabolic interventions to improve diastolic function are warranted. In mice with a germline cardiac-specific deletion of acetyl CoA carboxylase 2 (ACC2), systolic dysfunction induced by pressure-overload was prevented by maintaining cardiac fatty acid oxidation (FAO). However, it has not been evaluated whether this strategy would prevent the development of diastolic dysfunction in the adult heart.

OBJECTIVE

To test the hypothesis that augmenting cardiac FAO is protective against angiotensin II (AngII)-induced diastolic dysfunction in an adult mouse heart.

METHODS AND RESULTS

We generated a mouse model to induce cardiac-specific deletion of ACC2 in adult mice. Tamoxifen treatment (20mg/kg/day for 5days) was sufficient to delete ACC2 protein and increase cardiac FAO by 50% in ACC2 flox/flox-MerCreMer⁺ mice (iKO). After 4weeks of AngII (1.1mg/kg/day), delivered by osmotic mini-pumps, iKO mice showed normalized E/E' and E'/A' ratios compared to AngII treated controls (CON). The prevention of diastolic dysfunction in iKO-AngII was accompanied by maintained FAO and reduced glycolysis and anaplerosis. Furthermore, iKO-AngII hearts had a~50% attenuation of cardiac hypertrophy and fibrosis compared to CON. In addition, maintenance of FAO in iKO hearts suppressed AngII-associated increases in oxidative stress and sustained mitochondrial respiratory complex activities.

CONCLUSION

These data demonstrate that impaired FAO is a contributor to the development of diastolic dysfunction induced by AngII. Maintenance of FAO in this model leads to an attenuation of hypertrophy, reduces fibrosis, suppresses increases in oxidative stress, and maintains mitochondrial function. Therefore, targeting mitochondrial FAO is a promising therapeutic strategy for the treatment of diastolic dysfunction.

A genistein-enriched diet neither improves skeletal muscle oxidative capacity nor prevents the transition towards advanced insulin resistance in ZDF rats

Genistein, a natural food compound mainly present in soybeans, is considered a potent antioxidant and to improve glucose homeostasis. However, its mechanism of action remains poorly understood. Here, we analyzed whether genistein could antagonize the progression of the hyperinsulinemic normoglycemic state (pre-diabetes) toward full-blown T2DM in Zucker Diabetic Fatty (ZDF) rats by decreasing mitochondrial oxidative stress and improving skeletal muscle oxidative capacity. Rats were assigned to three groups: (1) lean control (CNTL), (2) fa/fa CNTL, and (3) fa/fa genistein (GEN). GEN animals were subjected to a 0.02% (w/w) genistein-enriched diet for 8 weeks, whereas CNTL rats received a standard diet. We show that genistein did not affect the overall response to a glucose challenge in ZDF rats. In fact, genistein may exacerbate glucose intolerance as fasting glucose levels were significantly higher in fa/fa GEN (17.6 ± 0.7 mM) compared with fa/fa CNTL animals (14.9 ± 1.4 mM). Oxidative stress, established by electron spin resonance (ESR) spectroscopy, carbonylated protein content and UCP3 levels, remained unchanged upon dietary genistein supplementation. Furthermore, respirometry measurements revealed no effects of genistein on mitochondrial function. In conclusion, dietary genistein supplementation did not improve glucose homeostasis, alleviate oxidative stress, or augment skeletal muscle metabolism in ZDF rats.

Good and bad consequences of altered fatty acid metabolism in heart failure: evidence from mouse models

The shift in substrate preference away from fatty acid oxidation (FAO) towards increased glucose utilization in heart failure has long been interpreted as an oxygen-sparing mechanism. Inhibition of FAO has therefore evolved as an accepted approach to treat heart failure. However, recent data indicate that increased reliance on glucose might be detrimental rather than beneficial for the failing heart. This review discusses new insights into metabolic adaptations in heart failure. A particular focus lies on data obtained from mouse models with modulations of cardiac FA metabolism at different levels of the FA metabolic pathway and how these differently affect cardiac function. Based on studies in which these mouse models were exposed to ischaemic and non-ischaemic heart failure, we discuss whether and when modulations in FA metabolism are protective against heart failure.

Lack of UCP3 does not affect skeletal muscle mitochondrial function under lipid-challenged conditions, but leads to sudden cardiac death

UCP3's exact physiological function in lipid handling in skeletal and cardiac muscle remains unknown. Interestingly, etomoxir, a fat oxidation inhibitor and strong inducer of UCP3, is proposed for treating both diabetes and heart failure. We hypothesize that the upregulation of UCP3 upon etomoxir serves to protect mitochondria against lipotoxicity. To evaluate UCP3's role in skeletal muscle (skm) and heart under lipid-challenged conditions, the effect of UCP3 ablation was examined in a state of dysbalance between fat availability and oxidative capacity. Wild type (WT) and UCP3(-/-) mice were subjected to high-fat feeding for 14 days. From day 6 onwards, they were given either saline or etomoxir. Etomoxir treatment induced an increase in markers of lipotoxicity in skm compared to saline. This increase upon etomoxir was similar for both, WT and UCP3(-/-) mice, suggesting that UCP3 does not play a role in protection against lipotoxicity. Interestingly, we observed 25 % mortality in UCP3(-/-)s upon etomoxir administration vs. 11 % in WTs. This increased mortality in UCP3(-/-) compared to WT mice could not be explained by differences in cardiac lipotoxicity, apoptosis, fibrosis (histology, immunohistochemistry), oxidative capacity (respirometry) or function (echocardiography). Electrophysiology demonstrated, however, prolonged QRS and QTc intervals and greater susceptibility to ventricular tachycardia upon programmed electrical stimulation in etomoxir-treated UCP3(-/-)s versus WTs. Isoproterenol administration after pacing resulted in 75 % mortality in UCP3(-/-)s vs. 14 % in WTs. Our results argue against a protective role for UCP3 on skm metabolism under lipid overload, but suggest UCP3 to be crucial in prevention of arrhythmias upon lipid-challenged conditions.

High levels of whole-body energy expenditure are associated with a lower coupling of skeletal muscle mitochondria in C57Bl/6 mice

Considerable variation in energy expenditure is observed in C57Bl/6 mice on a high-fat diet. Because muscle tissue is a major determinant of whole-body energy expenditure, we set out to determine the variation in energy expenditure and its possible association with skeletal muscle mitochondrial function upon high-fat diet intervention. Metabolic cages using indirect calorimetry were used to assess whole-body energy metabolism in C57Bl/6 male mice during the first 3 days of high-fat diet intervention. Mice were grouped in a negative or positive residual nocturnal energy expenditure group after correction of total nocturnal energy expenditure for body mass by residual analysis. The positive residual energy expenditure group was characterized by higher uncorrected total nocturnal energy expenditure and food intake. On day 7, mitochondria were isolated from the skeletal muscle of the hind limb. Mitochondrial density was determined by mitochondrial protein content and did not differ between the positive and negative residual energy expenditure groups. Using high-resolution respirometry, mitochondrial oxidative function was assessed using various substrates. Mitochondria from the positive residual energy expenditure group were characterized by a lower adenosine diphosphate-stimulated respiration and lower respiratory control rates using palmitoyl-coenzyme A as substrate. These results indicate that reduced mitochondrial coupling is associated with positive residual energy expenditure and high rates of total energy expenditure in vivo.

Adaptations in mitochondrial function parallel, but fail to rescue, the transition to severe hyperglycemia and hyperinsulinemia: a study in Zucker diabetic fatty rats

Cross-sectional human studies have associated mitochondrial dysfunction to type 2 diabetes. We chose Zucker diabetic fatty (ZDF) rats as a model of progressive insulin resistance to examine whether intrinsic mitochondrial defects are required for development of type 2 diabetes. Muscle mitochondrial function was examined in 6-, 12-, and 19-week-old ZDF (fa/fa) and fa/+ control rats (n = 8-10 per group) using respirometry with pyruvate, glutamate, and palmitoyl-CoA as substrates. Six-week-old normoglycemic-hyperinsulinemic fa/fa rats had reduced mitochondrial fat oxidative capacity. Adenosine diphosphate (ADP)-driven state 3 and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP)-stimulated state uncoupled (state u) respiration on palmitoyl-CoA were lower compared to controls (62.3 +/- 9.5 vs. 119.1 +/- 13.8 and 87.8 +/- 13.3 vs. 141.9 +/- 14.3 nmol O(2)/mg/min.). Pyruvate oxidation in 6-week-old fa/fa rats was similar to controls. Remarkably, reduced fat oxidative capacity in 6-week-old fa/fa rats was compensated for by an adaptive increase in intrinsic mitochondrial function at week 12, which could not be maintained toward week 19 (140.9 +/- 11.2 and 57.7 +/- 9.8 nmol O(2)/mg/min, weeks 12 and 19, respectively), whereas hyperglycemia had developed (13.5 +/- 0.6 and 16.1 +/- 0.3 mmol/l, weeks 12 and 19, respectively). This mitochondrial adaptation failed to rescue the progressive development of insulin resistance in fa/fa rats. The transition of prediabetes state toward advanced hyperglycemia and hyperinsulinemia was accompanied by a blunted increase in uncoupling protein-3 (UCP3). Thus, in ZDF rats insulin resistance develops progressively in the absence of mitochondrial dysfunction. In fact, improved mitochondrial capacity in hyperinsulinemic hyperglycemic rats does not rescue the progression toward advanced stages of insulin resistance.

The effect of UCP3 overexpression on mitochondrial ROS production in skeletal muscle of young versus aged mice

Uncoupling protein 3 (UCP3) is suggested to protect mitochondria against aging and lipid-induced damage, possibly via modulation of reactive oxygen species (ROS) production. Here we show that mice overexpressing UCP3 (UCP3Tg) have a blunted age-induced increase in ROS production, assessed by electron spin resonance spectroscopy, but only after addition of 4-hydroxynonenal (4-HNE). Mitochondrial function, assessed by respirometry, on glycolytic substrate was lower in UCP3Tg mice compared to wild types, whereas this tended to be higher on fatty acids. State 4o respiration was higher in UCP3Tg animals. To conclude, UCP3 overexpression leads to increased state 4o respiration and, in presence of 4-HNE, blunts the age-induced increase in ROS production.

Mitochondrial function, content and ROS production in rat skeletal muscle: effect of high-fat feeding

A high intake of dietary fat has been suggested to diminish mitochondrial functioning in skeletal muscle, possibly attributing to muscular fat accumulation. Here we show however, that an 8-week high-fat dietary intervention did not affect intrinsic functioning of rat skeletal muscle mitochondria assessed by respirometry, neither on a carbohydrate- nor on a lipid-substrate. Interestingly, PPARGC1A protein increased by approximately 2-fold upon high-fat feeding and we observed inconsistent results on different markers of mitochondrial density. Mitochondrial ROS production, assessed by electron spin resonance spectroscopy remained unaffected. Intramyocellular lipid levels increased significantly illustrating that a reduced innate mitochondrial function is not a prerequisite for intra-muscular fat accumulation.

Mitochondrial uncoupling protein 3 and its role in cardiac- and skeletal muscle metabolism

Uncoupling protein 3 (UCP3), is primarily expressed in skeletal muscle mitochondria and has been suggested to be involved in mediating energy expenditure via uncoupling, hereby dissipating the mitochondrial proton gradient necessary for adenosine triphosphate (ATP) synthesis. Although some studies support a role for UCP3 in energy metabolism, other studies pointed towards a function in fatty acid metabolism. Thus, the protein is up regulated or high when fatty acid supply to the mitochondria exceeds the capacity to oxidize fatty acids and down regulated or low when oxidative capacity is high or improved. Irrespective of the exact operating mechanism, UCP3 seems to protect mitochondria against lipid-induced oxidative stress, which makes this protein a potential player in the development of type 2 diabetes mellitus. Next to skeletal muscle, UCP3 is also expressed in cardiac muscle where its role is relatively unexplored. Interestingly, energy deficiency in cardiac muscle is associated to heart failure and UCP3 might contribute to this energy deficiency. It has been suggested that UCP3 decreases energy status via uncoupling of mitochondrial respiration, but the available data does not provide a unified answer. In fact, the results obtained regarding cardiac UCP3 are very similar as in skeletal muscle, implying that its physiological function can be extrapolated. Therefore, cardiac UCP3 can just as well serve to protect the heart against lipid-induced oxidative stress, similar to the function described for skeletal muscle UCP3. The present review will deal with the available literature on both skeletal muscle- and cardiac UCP3 to elucidate its physiological function in these tissues.

A potential role for angiotensin II in obesity induced cardiac hypertrophy and ischaemic/reperfusion injury

BACKGROUND

The mechanisms for obesity induced myocardial remodelling and subsequent mechanical dysfunction are poorly understood. There is good evidence that angiotensin II and TNFalpha have strong growth promoting properties and are elevated with obesity. In addition, these two peptides may interact to exacerbate myocardial ischaemic/reperfusion injury.

HYPOTHESIS

Obesity increases systemic and myocardial renin-angiotensin system (RAS) activity and TNFalpha levels and contributes to obesity induced cardiac remodelling and ischaemic/reperfusion injury.

METHODS

Male Wistar rats were placed on a standard rat chow diet or cafeteria diet for 16 weeks. Two additional groups of rats received the respective diets and losartan (30 mg/ kg/d) in their drinking water. Hearts were perfused on the isolated working rat heart perfusion system and mechanical function was documented before and after 15 min normothermic total global ischaemia. Blood and myocardial samples were collected for angiotensin II, TNFalpha and NADPH oxidase activity determinations.

RESULTS

The rats on the cafeteria diet became obese compared to rats on the standard rat chow (438 +/- 5.9 g vs 393 +/- 7.3 g for control, p < 0.05). Obesity was associated with elevated serum angiotensin II (0.050 +/- 0.015 pmol/ml vs. 0.035 +/- 0.003 pmol/ml, p < 0.05) and TNFalpha levels (42.8 +/- 5.93 pg/ml vs. 13.18 +/- 2.50 pg/ml, p < 0.05), and increased heart to body weight ratios (3.1 +/- 0.04 mg/g vs. 2.8 +/- 0.03 mg/g, p < 0.05). Losartan had no effect on body weight but decreased basal myocardial angiotensin II and TNFAlpha levels as well as heart to body weight ratio in the obese and lean controls (2.5 +/- 0.05 mg/g and 2.6 +/- 0.04 mg/g relative to their controls, p < 0.05). Hearts from obese rats had lower reperfusion aortic outputs (AO) than their concurrent controls (18.42 +/- 1.17 ml/min vs. 27.8 +/- 0.83 ml/min, p < 0.05). Losartan improved aortic output recoveries in obese rats (23.0 +/- 1.71 ml/min, p < 0.05).

CONCLUSIONS

Obesity increased serum angiotensin II and TNFalpha levels, blood pressure, and heart weight to body weight ratios. These changes were associated with decreased basal and post-ischaemic myocardial mechanical function. Chronic AT(1) receptor antagonism prevented the adverse changes in heart weight, mechanical function and susceptibility to ischaemic/reperfusion injury. Although current data do not exclude additional mechanisms for obesity induced cardiac remodelling, they suggest that angiotensin II may contribute to obesity induced cardiac remodelling and ischaemic/reperfusion injury.