MTORC1 regulates cardiac function and myocyte survival through 4E-BP1 inhibition in mice

Genome-wide translational reprogramming of genes important for myocyte functions in overload-induced heart failure

Genome-wide changes in gene translational efficiency during the development of heart failure are poorly understood. We tested the hypothesis that aberrant changes in translational efficiency of cardiac genes are associated with the development of myocyte decompensation in response to persistent stress stimuli. We demonstrated that chronic pressure overload in mice resulted in a genome-wide reprogramming of translational efficiency, with >50% of the translatome exhibiting decreased translational efficiencies during the transition from myocardial compensation to decompensation. Importantly, these translationally repressed genes included those involved in angiogenesis and energy metabolism. Moreover, we showed that the stress-induced translational reprogramming was accompanied by persistent activation of the eukaryotic initiation factor 2α (eIF2α)-mediated stress response pathway. Counteracting the endogenous eIF2α functions by cardiac-specific overexpression of an eIF2α-S51A mutant ameliorated the development of myocyte decompensation, with concomitant improvements in translation of cardiac functional genes and increases in angiogenic responses. These data suggest that the mismatch between transcription and translation of the cardiac genes with essential functions may represent a novel molecular mechanism underlying the development of myocyte decompensation in response to chronic stress stimuli, and the eIF2α pathway may be a viable therapeutic target for recovering the optimal translation of the repressed cardiac genes.

Endothelial EphB4 maintains vascular integrity and transport function in adult heart

The homeostasis of heart and other organs relies on the appropriate provision of nutrients and functional specialization of the local vasculature. Here, we have used mouse genetics, imaging and cell biology approaches to investigate how homeostasis in the adult heart is controlled by endothelial EphB4 and its ligand ephrin-B2, which are known regulators of vascular morphogenesis and arteriovenous differentiation during development. We show that inducible and endothelial cell-specific inactivation of Ephb4 in adult mice is compatible with survival, but leads to rupturing of cardiac capillaries, cardiomyocyte hypertrophy, and pathological cardiac remodeling. In contrast, EphB4 is not required for integrity and homeostasis of capillaries in skeletal muscle. Our analysis of mutant mice and cultured endothelial cells shows that EphB4 controls the function of caveolae, cell-cell adhesion under mechanical stress and lipid transport. We propose that EphB4 maintains critical functional properties of the adult cardiac vasculature and thereby prevents dilated cardiomyopathy-like defects.

Cardiac metabolomic profile of the naked mole-rat-glycogen to the rescue

The African naked mole-rat (Heterocephalus glaber) is unique among mammals, displaying extreme longevity, resistance to cardiovascular disease and an ability to survive long periods of extreme hypoxia. The metabolic adaptations required for resistance to hypoxia are hotly debated and a recent report provides evidence that they are able to switch from glucose to fructose driven glycolysis in the brain. However, other systemic alterations in their metabolism are largely unknown. In the current study, a semi-targeted high resolution ¹H magnetic resonance spectroscopy (MRS) metabolomics investigation was performed on cardiac tissue from the naked mole-rat (NMR) and wild-type C57/BL6 mice to better understand these adaptations. A range of metabolic differences was observed in the NMR including increased lactate, consistent with enhanced rates of glycolysis previously reported, increased glutathione, suggesting increased resistance to oxidative stress and decreased succinate/fumarate ratio suggesting reduced oxidative phosphorylation and ROS production. Surprisingly, the most significant difference was an elevation of glycogen stores and glucose-1-phosphate resulting from glycogen turnover, that were completely absent in the mouse heart and above the levels found in the mouse liver. Thus, we identified a range of metabolic adaptations in the NMR heart that are relevant to their ability to survive extreme environmental pressures and metabolic stress. Our study underscores the plasticity of energetic pathways and the need for compensatory strategies to adapt in response to the physiological and pathological stress including ageing and ischaemic heart pathologies.

GRP78 (Glucose-Regulated Protein of 78 kDa) Promotes Cardiomyocyte Growth Through Activation of GATA4 (GATA-Binding Protein 4)

The heart manifests hypertrophic growth in response to elevation of afterload pressure. Cardiac myocyte growth involves new protein synthesis and membrane expansion, of which a number of cellular quality control machineries are stimulated to maintain function and homeostasis. The unfolded protein response is potently induced during cardiac hypertrophy to enhance protein-folding capacity and eliminate terminally misfolded proteins. However, whether the unfolded protein response directly regulates cardiac myocyte growth remains to be fully determined. Here, we show that GRP78 (glucose-regulated protein of 78 kDa)-an endoplasmic reticulum-resident chaperone and a critical unfolded protein response regulator-is induced by cardiac hypertrophy. Importantly, overexpression of GRP78 in cardiomyocytes is sufficient to potentiate hypertrophic stimulus-triggered growth. At the in vivo level, TG (transgenic) hearts overexpressing GRP78 mount elevated hypertrophic growth in response to pressure overload. We went further to show that GRP78 increases GATA4 (GATA-binding protein 4) level, which may stimulate Anf (atrial natriuretic factor) expression and promote cardiac hypertrophic growth. Silencing of GATA4 in cultured neonatal rat ventricular myocytes significantly diminishes GRP78-mediated growth response. Our results, therefore, reveal that protein-folding chaperone GRP78 may directly enhance cardiomyocyte growth by stimulating cardiac-specific transcriptional factor GATA4.

Rapamycin regulates the balance between cardiomyocyte apoptosis and autophagy in chronic heart failure by inhibiting mTOR signaling

The progressive loss of cardiomyocytes caused by cell death leads to cardiac dysfunction and heart failure (HF). Rapamycin has been shown to be cardioprotective in pressure-overloaded and ischemic heart diseases by regulating the mechanistic target of rapamycin (mTOR) signaling network. However, the impact of rapamycin on cardiomyocyte death in chronic HF remains undetermined. Therefore, in the current study we addressed this issue using a rat myocardial infarction (MI)-induced chronic HF model induced by ligating the coronary artery. Following surgery, rats were randomly divided into six groups, including the sham-, vehicle- and rapamycin-operated groups, at 8 or 12 weeks post-MI. A period of 4 weeks after MI induction, the rats were treated with rapamycin (1.4 mg-kg-day) or vehicle for 4 weeks. Cardiac function was determined using echocardiography, the rats were subsequently euthanized and myocardial tissues were harvested for histological and biochemical analyses. In the cell culture experiments with H9c2 rat cardiomyocytes, apoptosis was induced using angiotensin II (100 nM; 24 h). Cardiomyocyte apoptosis and autophagy were assessed via measuring apoptosis- and autophagy-associated proteins. The activities of mTOR complex 1 (mTORC1) and mTORC2 were evaluated using the phosphorylation states of ribosomal S6 protein and Akt, respectively. The activity of the endoplasmic reticulum (ER) stress pathway was determined using the levels of GRP78, caspase-12, phospho-JNK and DDIT3. Echocardiographic and histological measurements indicated that rapamycin treatment improved cardiac function and inhibited cardiac remodeling at 8 weeks post-MI. Additionally, rapamycin prevented cardiomyocyte apoptosis and promoted autophagy at 8 weeks post-MI. Rapamycin treatment for 4 weeks inhibited the mTOR and ER stress pathways. Furthermore, rapamycin prevented angiotensin II-induced H9c2 cell apoptosis and promoted autophagy by inhibiting the mTORC1 and ER stress pathways. These results demonstrated that rapamycin reduced cardiomyocyte apoptosis and promoted cardiomyocyte autophagy, by regulating the crosstalk between the mTOR and ER stress pathways in chronic HF.

Differential effects of various genetic mouse models of the mechanistic target of rapamycin complex I inhibition on heart failure

Inhibition of mammalian target of rapamycin complex I (mTORC1) by rapamycin improves cardiac function in both aging and heart failure. While the protective mechanisms are not fully understood in mammals, they are presumably mediated through metabolic regulation and suppression of protein translation by reduced phosphorylation of 4EBP1, a target of mTORC1. Using transverse aortic constriction (TAC) and Gαq overexpression-induced heart failure models, we examined the effect of cardiac-specific heterozygous deletion (het) of Raptor, a component of mTORC1, and cardiac-specific transgenic overexpression of wild type or phosphorylation site mutant 4EBP1. In wild-type mice with TAC-induced heart failure, quantitative shotgun proteomics revealed decreased abundance of proteins of mitochondrial metabolism and increased abundance of proteins in oxidative stress response, ubiquitin, and other pathways. The Raptor het ameliorated both TAC- and Gαq overexpression-induced heart failure and the associated proteomic remodeling, especially those pathways involved in mitochondrial function, citric acid cycle, and ubiquitination. In contrast, transgenic overexpression of either wild type or mutant 4EBP1 aggravated TAC and Gαq, consistent with reduced adaptive hypertrophy by suppression of protein translation, in parallel with adverse remodeling of left ventricular proteomes. Partial mTORC1 inhibition by Raptor heterozygous deletion ameliorates heart failure and is associated with better preservation of the mitochondrial proteome; however, this effect does not appear to be mediated through suppression of protein translation by increased 4EBP1. Increased activity of 4EBP1 reduced adaptive hypertrophy and aggravated heart failure, suggesting that protein translation is essential for adaptive hypertrophy in pressure overload.

ATF6 Regulates Cardiac Hypertrophy by Transcriptional Induction of the mTORC1 Activator, Rheb

RATIONALE

Endoplasmic reticulum (ER) stress dysregulates ER proteostasis, which activates the transcription factor, ATF6 (activating transcription factor 6α), an inducer of genes that enhance protein folding and restore ER proteostasis. Because of increased protein synthesis, it is possible that protein folding and ER proteostasis are challenged during cardiac myocyte growth. However, it is not known whether ATF6 is activated, and if so, what its function is during hypertrophic growth of cardiac myocytes.

OBJECTIVE

To examine the activity and function of ATF6 during cardiac hypertrophy.

METHODS AND RESULTS

We found that ER stress and ATF6 were activated and ATF6 target genes were induced in mice subjected to an acute model of transverse aortic constriction, or to free-wheel exercise, both of which promote adaptive cardiac myocyte hypertrophy with preserved cardiac function. Cardiac myocyte-specific deletion of Atf6 (ATF6 cKO [conditional knockout]) blunted transverse aortic constriction and exercise-induced cardiac myocyte hypertrophy and impaired cardiac function, demonstrating a role for ATF6 in compensatory myocyte growth. Transcript profiling and chromatin immunoprecipitation identified RHEB (Ras homologue enriched in brain) as an ATF6 target gene in the heart. RHEB is an activator of mTORC1 (mammalian/mechanistic target of rapamycin complex 1), a major inducer of protein synthesis and subsequent cell growth. Both transverse aortic constriction and exercise upregulated RHEB, activated mTORC1, and induced cardiac hypertrophy in wild type mouse hearts but not in ATF6 cKO hearts. Mechanistically, knockdown of ATF6 in neonatal rat ventricular myocytes blocked phenylephrine- and IGF1 (insulin-like growth factor 1)-mediated RHEB induction, mTORC1 activation, and myocyte growth, all of which were restored by ectopic RHEB expression. Moreover, adeno-associated virus 9- RHEB restored cardiac growth to ATF6 cKO mice subjected to transverse aortic constriction. Finally, ATF6 induced RHEB in response to growth factors, but not in response to other activators of ATF6 that do not induce growth, indicating that ATF6 target gene induction is stress specific.

CONCLUSIONS

Compensatory cardiac hypertrophy activates ER stress and ATF6, which induces RHEB and activates mTORC1. Thus, ATF6 is a previously unrecognized link between growth stimuli and mTORC1-mediated cardiac growth.

mTOR as a Marker of Exercise and Fatigue in Octopus vulgaris Arm

Cephalopods are highly evolved marine invertebrates that colonized almost all the oceans of the world at all depths. This imposed the occurrence of several modifications of their brain and body whose muscle component represents the major constituent. Hence, studying their muscle physiology may give important hints in the context of animal biology and environmental adaptability. One major pathway involved in muscle metabolism in vertebrates is the evolutionary conserved mTOR-signaling cascade; however, its role in cephalopods has never been elucidated. mTOR is regulating cell growth and homeostasis in response to a wide range of cues such as nutrient availability, body temperature and locomotion. It forms two functionally heteromeric complexes, mTORC1 and mTORC2. mTORC1 regulates protein synthesis and degradation and, in skeletal muscles, its activation upon exercise induces muscle growth. In this work, we characterized Octopus vulgaris mTOR full sequence and functional domains; we found a high level of homology with vertebrates’ mTOR and the conservation of Ser²⁴⁴⁸ phosphorylation site required for mTORC1 activation. We then designed and tested an in vitro protocol of resistance exercise (RE) inducing fatigue in arm samples. We showed that, upon the establishment of fatigue, a transient increase in mTORC1 phosphorylation reaching a pick 30 min after exercise was induced. Our data indicate the activation of mTORC1 pathway in exercise paradigm and possibly in the regulation of energy homeostasis in octopus and suggest that mTORC1 activity can be used to monitor animal response to changes in physiological and ecological conditions and, more in general, the animal welfare.

Cardiac mTORC1 Dysregulation Impacts Stress Adaptation and Survival in Huntington's Disease

Huntington’s disease (HD) is a dominantly inherited neurological disorder caused by CAG-repeat expansion in exon 1 of Huntingtin (HTT). But in addition to the neurological disease, mutant HTT (mHTT), which is ubiquitously expressed, impairs other organ systems. Indeed, epidemiological and animal model studies suggest higher incidence of and mortality from heart disease in HD. Here, we show that the protein complex mTORC1 is dysregulated in two HD mouse models through a mechanism that requires intrinsic mHTT expression. Moreover, restoring cardiac mTORC1 activity with constitutively active Rheb prevents mortality and relieves the mHTT-induced block to hypertrophic adaptation to cardiac stress. Finally, we show that chronic mTORC1 dysregulation is due in part to mislocalization of endogenous Rheb. These data provide insight into the increased cardiac-related mortality of HD patients, with cardiac mHTT expression inhibiting mTORC1 activity, limiting heart growth, and decreasing the heart’s ability to compensate to chronic stress.

The effects of Tel2 on cardiomyocyte survival

AIMS

Overexpression of the mechanistic target of rapamycin (mTOR), a member of the PIKK (phosphoinositide kinase-related kinase) family, protects cardiomyocytes from cell death induced by pathological stimuli such as ischemia. We previously reported that posttranslational modification of mTOR plays an important role in regulating cardiac mTOR expression. The aim of this study was to see if Tel2 (telomere maintenance 2), a protein that regulates the abundance of PIKKs, confers similar cardioprotective effects as mTOR. Tel2 is not well-characterized in cardiomyocytes, therefore we examined the effects of Tel2 on cardiomyocyte viability under ischemic stress conditions.

MATERIALS AND METHODS

We overexpressed Tel2 or silenced Tel2 with siRNA in the HL-1 cardiomyocyte cell line to survey the effects of Tel2 overexpression and downregulation on cell survival during hypoxia. Adult mouse cardiomyocytes transfected with Tel2 adenoviruses were used to test whether Tel2 sufficiently prevented cardiomyocyte cell death against hydrogen peroxide (H₂O₂).

KEY FINDINGS

Overexpressing Tel2 increased mTOR expression with a concomitant increase in mTOR Complex 1 (mTORC1) and mTORC2 activity in HL-1 cells. Tel2 deletion decreased mTOR expression, and mTORC1 and mTORC2 activity accordingly. In both HL-1 cells and adult mouse cardiomyocytes, Tel2 overexpression protected cardiomyocytes under ischemic stress. These effects were mTOR-dependent, as mTOR inhibitors blunted the effects of Tel2. While gene silencing of Tel2 did not affect cell survival under normoxia, Tel2 silencing made cardiomyocytes more vulnerable to cell death under hypoxia.

SIGNIFICANCE

Upregulating Tel2 expression increases mTOR-mediated cardiomyocyte survival and targeting Tel2 could be another therapeutic strategy against ischemic heart disease.

Enhanced mTOR complex 1 signaling attenuates diabetic cardiac injury in OVE26 mice

The protein kinase mechanistic target of rapamycin (mTOR) performs diverse cellular functions through 2 distinct multiprotein complexes, mTOR complex (mTORC)1 and 2. Numerous studies using rapamycin, an mTORC1 inhibitor, have implicated a role for mTORC1 in several types of heart disease. People with diabetes are more susceptible to heart failure. mTORC1 activity is increased in the diabetic heart, but its functional significance remains controversial. To investigate the role of mTORC1 in the diabetic heart, we crossed OVE26 type 1 diabetic mice with transgenic mice expressing a constitutively active mTOR (mTORca) or kinase-dead mTOR (mTORkd) in the heart. The expression of mTORca or mTORkd affected only mTORC1 but not mTORC2 activities, with corresponding changes in the activities of autophagy, a cellular degradation pathway negatively regulated by mTORC1. Diabetic cardiac damage in OVE26 mice was dramatically reduced by mTORca but exacerbated by mTORkd expression as assessed by changes in cardiac function, oxidative stress, and myocyte apoptosis. These findings demonstrated that the enhanced mTORC1 signaling in the OVE26 diabetic heart was an adaptive response that limited cardiac dysfunction, suggesting that manipulations that enhance mTORC1 activity may reduce diabetic cardiac injury, in sharp contrast to the results previously obtained with rapamycin.-Xu, X., Kobayashi, S., Timm, D., Huang, Y., Zhao, F., Shou, W., Liang, Q. Enhanced mTOR complex 1 signaling attenuates diabetic cardiac injury in OVE26 mice.

TIP30 counteracts cardiac hypertrophy and failure by inhibiting translational elongation

Pathological cardiac overload induces myocardial protein synthesis and hypertrophy, which predisposes to heart failure. To inhibit hypertrophy therapeutically, the identification of negative regulators of cardiomyocyte protein synthesis is needed. Here, we identified the tumor suppressor protein TIP30 as novel inhibitor of cardiac hypertrophy and dysfunction. Reduced TIP30 levels in mice entailed exaggerated cardiac growth during experimental pressure overload, which was associated with cardiomyocyte cellular hypertrophy, increased myocardial protein synthesis, reduced capillary density, and left ventricular dysfunction. Pharmacological inhibition of protein synthesis improved these defects. Our results are relevant for human disease, since we found diminished cardiac TIP30 levels in samples from patients suffering from end‐stage heart failure or hypertrophic cardiomyopathy. Importantly, therapeutic overexpression of TIP30 in mouse hearts inhibited cardiac hypertrophy and improved left ventricular function during pressure overload and in cardiomyopathic mdx mice. Mechanistically, we identified a previously unknown anti‐hypertrophic mechanism, whereby TIP30 binds the eukaryotic elongation factor 1A (eEF1A) to prevent the interaction with its essential co‐factor eEF1B2 and translational elongation. Therefore, TIP30 could be a therapeutic target to counteract cardiac hypertrophy.

For Better or Worse: The Potential for Dose Limiting the On-Target Toxicity of PI 3-Kinase Inhibitors

The hyper-activation of the phosphoinositide (PI) 3-kinase signaling pathway is a hallmark of many cancers and overgrowth syndromes, and as a result, there has been intense interest in the development of drugs that target the various isoforms of PI 3-kinase. Given the key role PI 3-kinases play in many normal cell functions, there is significant potential for the disruption of essential cellular functions by PI 3-kinase inhibitors in normal tissues; so-called on-target drug toxicity. It is, therefore, no surprise that progress within the clinical development of PI 3-kinase inhibitors as single-agent anti-cancer therapies has been slowed by the difficulty of identifying a therapeutic window. The aim of this review is to place the cellular, tissue and whole-body effects of PI 3-kinase inhibition in the context of understanding the potential for dose limiting on-target toxicities and to introduce possible strategies to overcome these.

Novel Role for Pleckstrin Homology-Like Domain Family A, Member 3 in the Regulation of Pathological Cardiac Hypertrophy

Background Pleckstrin homology-like domain family A, member 3 (PHLDA3), a crucial member of the PHLDA family, is involved in tumor suppression, kidney injury, liver injury, and glucose metabolism. However, the role of PHLDA3 in pathological cardiac hypertrophy and heart failure remains unclear. Methods and Results In the present study, PHLDA3 expression was downregulated in hypertrophic murine hearts and angiotensin II-treated cardiomyocytes. Next, an in vitro study suggested, by using gain- and loss-of-function approaches, that PHLDA3 attenuates Ang II exposure-induced cardiomyocyte hypertrophy. Consistent with the cell phenotype, disruption of PHLDA3 aggravated the effects of pressure overload-induced pathological cardiac hypertrophy, fibrosis, and dysfunction. In contrast, PHLDA3 overexpression resulted in an attenuated hypertrophic phenotype. Molecular analysis revealed that PHLDA3 suppressed the activation of AKT-mTOR-GSK3β-P70S6K signaling in response to hypertrophic stress, and the blockage of AKT activation rescued these adverse pathological effects of PHLDA3 deficiency-induced by AB and Ang II, respectively, in vivo and in vitro. Conclusions Collectively, our data indicated that PHLDA3 could ameliorate pressure overload-induced cardiac remodeling mainly by blocking the AKT signaling pathway, suggesting that PHLDA3 may represent a therapeutic target for the treatment of pathological cardiac hypertrophy and heart failure.

Metallothionein Protects the Heart Against Myocardial Infarction via the mTORC2/FoxO3a/Bim Pathway

Aims: Cardiac-specific overexpression of metallothionein (MT) has been shown to be beneficial in ischemic heart disease, but the detailed mechanisms through which MT protects against myocardial infarction (MI) remain unknown. This study assessed the involvement of the mTORC2/FoxO3a/Bim pathway in the cardioprotective effects of MT. Results: MI was induced in wild-type (FVB) mice and in cardiac-specific MT-overexpressing transgenic (MT-TG) mice by ligation of the left anterior descending (LAD) coronary artery. Cardiac function was better; infarct size and cardiomyocyte apoptosis were lower in MT-TG mice than in FVB mice after MI. Moreover, MT-TG mice exhibited better phenotypes after LAD ligation than FVB mice treated with Mn(III)tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride (MnTMPyP; a reactive oxygen species [ROS] scavenger) and cardiac-specific catalase-overexpressing transgenic (CAT-TG) mice, which showed the same ROS levels as MT-TG mice after MI. Activation of mechanistic target of rapamycin complex 2 (mTORC2) was essential for the cardioprotective effects of MT against MI. In addition, MT attenuated the downregulation of phospho-FoxO3a after MI, inhibiting the expression of the apoptosis-associated gene Bim, located downstream of FoxO3a, and reducing the level of apoptosis after MI. To mimic ischemic-injured FVB and MT-TG mice in vitro, H9c2 and MT-overexpressing H9c2 (H9c2MT7) cardiomyocytes were subjected to oxygen and glucose deprivation, with the results being consistent with those obtained in vivo. Innovation and Conclusion: The cardioprotective effects of MT against MI are not entirely dependent upon its ability to eliminate ROS. Rather, MT overexpression mostly protects against MI through the mTORC2-FoxO3a-Bim pathway.

Low density lipoprotein receptor-related protein 1 regulates cardiac hypertrophy induced by pressure overload

BACKGROUND

Cardiac hypertrophy is associated with functional changes in cardiomyocytes, which often results in heart failure. The low-density lipoprotein receptor-related protein 1 (LRP1) is a large multifunctional endocytic receptor involved in many physiological and pathological processes. However, its function in the development of cardiac hypertrophy remains largely unclear.

METHODS

Adenoviral constructs were used for either overexpression or silencing of LRP1 in both in vitro and in vivo experiments. Cardiac function was measured using the Millar catheter.

RESULTS

LRP1 expression was upregulated in both transverse aortic constriction (TAC)-induced hypertrophic myocardium and catecholamine (phenylephrine (PE) and norepinephrine (NE))- and angiotensin II (AngII)-induced hypertrophic cardiomyocytes. In addition, cell surface area, protein/DNA ratio, and the mRNA levels of hypertrophic markers were significantly increased in LRP1-overexpressing cardiomyocytes without catecholamine stimulation. Conversely, LRP1 inhibition by LRP1-specific siRNA or a specific ligand-binding antagonist (RAP) significantly rescued hypertrophic effects in PE, NE, or AngII-induced cardiomyocytes. LRP1 overexpression induced PKCα, then activated ERK, resulting in cardiac hypertrophy with the downregulation of SERCA2a and calcium accumulation, which was successfully restored in both LRP1-silenced cardiomyocytes and TAC-induced hearts.

CONCLUSIONS

LRP1 regulates cardiac hypertrophy via the PKCα-ERK dependent signaling pathway resulting in the alteration of intracellular calcium levels, demonstrating that LRP1 might be a potential therapeutic target for cardiac hypertrophy.

Pathway analysis with genome-wide association study (GWAS) data detected the association of atrial fibrillation with the mTOR signaling pathway

Background

Genome-wide association studies (GWAS) have identified numerous loci associated with diseases and traits. However, the elucidation of disease mechanisms followed by drug development has remained a challenge owing to complex gene interactions. We performed pathway analysis with MAGENTA (Meta-Analysis Geneset Enrichment of variaNT Associations) to clarify the pathways in genetic background of AF.

Methods

The existing GWAS data were analyzed using MAGENTA. A microarray analysis was then performed for the identified pathways with human atrial tissues, followed by Gene-Set Enrichment Analysis (GSEA).

Results

MAGENTA identified two novel candidate pathways for AF pathogenesis, the CTCF (CCCTC-binding factor, p = 1.00 × 10⁻⁴, FDR q = 1.64 × 10⁻²) and mTOR pathways (mammalian target of rapamycin, p = 3.00 × 10⁻⁴, FDR q = 3.13 × 10⁻²). The microarray analysis with human atrial tissue using the GSEA indicated that the mTOR pathway was suppressed in AF cases compared with non-AF cases, validating the MAGENTA results, but not CTCF pathway.

Conclusions

MAGENTA identified a novel pathway, mTOR, followed by GSEA with human atrial tissue samples. mTOR pathway is a key interface that adapts the change of environments by pressure overload and metabolic perturbation. Our results indicate that the MTOR pathway is involved in the pathogenesis of AF, although the details of the basic mechanism remain unknown and further analysis for causal-relationship of mTOR pathway to AF is required. CTCF pathway is essential for construction of chromatin structure and transcriptional process. The gene-set components of CTCF overlap with those of mTOR in Biocarta.

New Insights Into the Role of mTOR Signaling in the Cardiovascular System

The mTOR (mechanistic target of rapamycin) is a master regulator of several crucial cellular processes, including protein synthesis, cellular growth, proliferation, autophagy, lysosomal function, and cell metabolism. mTOR interacts with specific adaptor proteins to form 2 multiprotein complexes, called mTORC1 (mTOR complex 1) and mTORC2 (mTOR complex 2). In the cardiovascular system, the mTOR pathway regulates both physiological and pathological processes in the heart. It is needed for embryonic cardiovascular development and for maintaining cardiac homeostasis in postnatal life. Studies involving mTOR loss-of-function models revealed that mTORC1 activation is indispensable for the development of adaptive cardiac hypertrophy in response to mechanical overload. mTORC2 is also required for normal cardiac physiology and ensures cardiomyocyte survival in response to pressure overload. However, partial genetic or pharmacological inhibition of mTORC1 reduces cardiac remodeling and heart failure in response to pressure overload and chronic myocardial infarction. In addition, mTORC1 blockade reduces cardiac derangements induced by genetic and metabolic disorders and has been reported to extend life span in mice. These studies suggest that pharmacological targeting of mTOR may represent a therapeutic strategy to confer cardioprotection, although clinical evidence in support of this notion is still scarce. This review summarizes and discusses the new evidence on the pathophysiological role of mTOR signaling in the cardiovascular system.

mTOR as a central regulator of lifespan and aging

The mammalian/mechanistic target of rapamycin (mTOR) is a key component of cellular metabolism that integrates nutrient sensing with cellular processes that fuel cell growth and proliferation. Although the involvement of the mTOR pathway in regulating life span and aging has been studied extensively in the last decade, the underpinning mechanisms remain elusive. In this review, we highlight the emerging insights that link mTOR to various processes related to aging, such as nutrient sensing, maintenance of proteostasis, autophagy, mitochondrial dysfunction, cellular senescence, and decline in stem cell function.

As time flies by: Investigating cardiac aging in the short-lived Drosophila model

Aging is associated with a decline in heart function across the tissue, cellular, and molecular levels. The risk of cardiovascular disease grows significantly over time, and as developed countries continue to see an increase in lifespan, the cost of cardiovascular healthcare for the elderly will undoubtedly rise. The molecular basis for cardiac function deterioration with age is multifaceted and not entirely clear, and there is a limit to what investigations can be performed on human subjects or mammalian models. Drosophila melanogaster has emerged as a useful model organism for studying aging in a short timeframe, benefitting from a suite of molecular and genetic tools and displaying highly conserved traits of cardiac senescence. Here, we discuss recent advances in our understanding of cardiac aging and how the fruit fly has aided in these developments.

Targeting mTOR suppressed colon cancer growth through 4EBP1/eIF4E/PUMA pathway

Colorectal cancer is the third most frequently diagnosed malignancies among both men and women, which has an increased mortality but a poor prognosis. Targeting mTOR becomes an effective approach that shows promising antitumor activities in various cancers including colonic carcinoma. However, the potential mechanism against colon cancer remains incompletely understood. Here, we demonstrated that the anti-cancer effect of AZD8055 and OSI-027 is at least in part modulated by the gradual process of apoptosis initiation, progressing from mTOR suppression, 4EBP1 dephosphorylation, or EZH2 suppression, thereby leading to PUMA-dependent apoptosis via the intrinsic mitochondrial pathway. Furthermore, AZD8055 inhibited colorectal cancer tumor growth in mice significantly. PUMA deletion caused resistance of dual mTOR inhibitors, suggesting PUMA mediated carcinogenesis in vitro and in vivo. Collectively, these findings established a vital status of PUMA in driving the antineoplastic efficacy of targeting mTOR by AZD8055 and OSI-027 and offered the rationales for the current clinical assessment.

Loss of SRSF3 in Cardiomyocytes Leads to Decapping of Contraction-Related mRNAs and Severe Systolic Dysfunction

RATIONALE

RBPs (RNA binding proteins) play critical roles in the cell by regulating mRNA transport, splicing, editing, and stability. The RBP SRSF3 (serine/arginine-rich splicing factor 3) is essential for blastocyst formation and for proper liver development and function. However, its role in the heart has not been explored.

OBJECTIVE

To investigate the role of SRSF3 in cardiac function.

METHODS AND RESULTS

Cardiac SRSF3 expression was high at mid gestation and decreased during late embryonic development. Mice lacking SRSF3 in the embryonic heart showed impaired cardiomyocyte proliferation and died in utero. In the adult heart, SRSF3 expression was reduced after myocardial infarction, suggesting a possible role in cardiac homeostasis. To determine the role of this RBP in the adult heart, we used an inducible, cardiomyocyte-specific SRSF3 knockout mouse model. After SRSF3 depletion in cardiomyocytes, mice developed severe systolic dysfunction that resulted in death within 8 days. RNA-Seq analysis revealed downregulation of mRNAs encoding sarcomeric and calcium handling proteins. Cardiomyocyte-specific SRSF3 knockout mice also showed evidence of alternative splicing of mTOR (mammalian target of rapamycin) mRNA, generating a shorter protein isoform lacking catalytic activity. This was associated with decreased phosphorylation of 4E-BP1 (eIF4E-binding protein 1), a protein that binds to eIF4E (eukaryotic translation initiation factor 4E) and prevents mRNA decapping. Consequently, we found increased decapping of mRNAs encoding proteins involved in cardiac contraction. Decapping was partially reversed by mTOR activation.

CONCLUSIONS

We show that cardiomyocyte-specific loss of SRSF3 expression results in decapping of critical mRNAs involved in cardiac contraction. The molecular mechanism underlying this effect likely involves the generation of a short mTOR isoform by alternative splicing, resulting in reduced 4E-BP1 phosphorylation. The identification of mRNA decapping as a mechanism of systolic heart failure may open the way to the development of urgently needed therapeutic tools.

Adenosine kinase attenuates cardiomyocyte microtubule stabilization and protects against pressure overload-induced hypertrophy and LV dysfunction

Adenosine exerts numerous protective actions in the heart, including attenuation of cardiac hypertrophy. Adenosine kinase (ADK) converts adenosine to adenosine monophosphate (AMP) and is the major route of myocardial adenosine metabolism, however, the impact of ADK activity on cardiac structure and function is unknown. To examine the role of ADK in cardiac homeostasis and adaptation to stress, conditional cardiomyocyte specific ADK knockout mice (cADK-/-) were produced using the MerCreMer-lox-P system. Within 4 weeks of ADK disruption, cADK-/- mice developed spontaneous hypertrophy and increased β-Myosin Heavy Chain expression without observable LV dysfunction. In response to 6 weeks moderate left ventricular pressure overload (transverse aortic constriction;TAC), wild type mice (WT) exhibited ~60% increase in ventricular ADK expression and developed LV hypertrophy with preserved LV function. In contrast, cADK-/- mice exhibited significantly greater LV hypertrophy and cardiac stress marker expression (atrial natrurietic peptide and β-Myosin Heavy Chain), LV dilation, reduced LV ejection fraction and increased pulmonary congestion. ADK disruption did not decrease protein methylation, inhibit AMPK, or worsen fibrosis, but was associated with persistently elevated mTORC1 and p44/42 ERK MAP kinase signaling and a striking increase in microtubule (MT) stabilization/detyrosination. In neonatal cardiomyocytes exposed to hypertrophic stress, 2-chloroadenosine (CADO) or adenosine treatment suppressed MT detyrosination, which was reversed by ADK inhibition with iodotubercidin or ABT-702. Conversely, adenoviral over-expression of ADK augmented CADO destabilization of MTs and potentiated CADO attenuation of cardiomyocyte hypertrophy. Together, these findings indicate a novel adenosine receptor-independent role for ADK-mediated adenosine metabolism in cardiomyocyte microtubule dynamics and protection against maladaptive hypertrophy.

Pressure overload inhibits glucocorticoid receptor transcriptional activity in cardiomyocytes and promotes pathological cardiac hypertrophy

Glucocorticoid receptor (GR) is abundantly expressed in cardiomyocytes. However, the role of GR in regulating cardiac hypertrophy and heart failure in response to pressure overload remains unclear. Cardiomyocyte-specific GR knockout (GRcKO) mice, mineralocorticoid receptor (MR) knockout (MRcKO), and GR and MR double KO (GRMRdcKO) mice were generated using the Cre-lox system. In response to pressure overload, GRcKO mice displayed worse cardiac remodeling compared to control (GR^f/f) mice, including a greater increase in heart weight to body weight ratio with a greater increase in cardiomyocytes size, a greater decline in left ventricular contractility, and higher reactivation of fetal genes. MRcKO mice showed a comparable degree of cardiac remodeling compared to control (MR^f/f) mice. The worse cardiac remodeling in pressure overloaded GRcKO mice is not due to compensatory activation of cardiomyocyte MR, since pressure overloaded GRMRdcKO mice displayed cardiac remodeling to the same extent as GRcKO mice. Pressure overload suppressed GR-target gene expression in the heart. Although plasma corticosterone levels and subcellular localization of GR (nuclear/cytoplasmic GR) were not changed, a chromatin immunoprecipitation assay revealed that GR recruitment onto the promoter of GR-target genes was significantly suppressed in response to pressure overload. Rescue of the expression of GR-target genes to the same extent as sham-operated hearts attenuated adverse cardiac remodeling in pressure-overloaded hearts. Thus, GR works as a repressor of adverse cardiac remodeling in response to pressure overload, but GR-mediated transcription is suppressed under pressure overload. Therapies that maintain GR-mediated transcription in cardiomyocytes under pressure overload can be a promising therapeutic strategy for heart failure.

Aldosterone Modulates the Mechanistic Target of Rapamycin Signaling in Male Mice

Both mechanistic target of rapamycin (mTOR) pathway and aldosterone are implicated in the development of cardiovascular and renal disease. However, the interaction between aldosterone and the mTOR pathway is unknown. We hypothesized the following: that (i) increased aldosterone will modulate the activity of the mTORC1 and mTORC2 molecular pathways in the heart and kidney; (ii) a physiologic increase in aldosterone will affect these pathways differently than a pathophysiologic one; and (iii) the changes in the mTOR level/activity will differ between the heart and kidney. In both kidney and heart tissues, phosphorylation of mTOR is significantly decreased when aldosterone levels are physiologically increased (by dietary sodium restriction), followed by a decrease in phosphorylated p70S6K1 in cardiac, but not renal, tissue. Sirtuin 1, an epigenetic modulator, is decreased in the heart but increased in the kidney. Conversely, pathophysiologic aldosterone levels (an infusion for 3 weeks) had divergent effects on phosphorylated mTOR and the downstream substrates of mTORC1 and mTORC2 in cardiac and renal tissues. Increased aldosterone levels significantly alter mTOR activity in the heart and kidney. In the kidney, substantial differences were noted if the increase was produced physiologically vs pathophysiologically, suggesting that mTOR activity, in part, may mediate aldosterone-induced renal damage. Thus, modulating mTOR activity may reduce aldosterone-dependent renal damage similar to mineralocorticoid receptor blockade but potentially with less adverse side effects.

The Cutting Edge: The Role of mTOR Signaling in Laminopathies

The mechanistic target of rapamycin (mTOR) is a ubiquitous serine/threonine kinase that regulates anabolic and catabolic processes, in response to environmental inputs. The existence of mTOR in numerous cell compartments explains its specific ability to sense stress, execute growth signals, and regulate autophagy. mTOR signaling deregulation is closely related to aging and age-related disorders, among which progeroid laminopathies represent genetically characterized clinical entities with well-defined phenotypes. These diseases are caused by LMNA mutations and feature altered bone turnover, metabolic dysregulation, and mild to severe segmental progeria. Different LMNA mutations cause muscular, adipose tissue and nerve pathologies in the absence of major systemic involvement. This review explores recent advances on mTOR involvement in progeroid and tissue-specific laminopathies. Indeed, hyper-activation of protein kinase B (AKT)/mTOR signaling has been demonstrated in muscular laminopathies, and rescue of mTOR-regulated pathways increases lifespan in animal models of Emery-Dreifuss muscular dystrophy. Further, rapamycin, the best known mTOR inhibitor, has been used to elicit autophagy and degradation of mutated lamin A or progerin in progeroid cells. This review focuses on mTOR-dependent pathogenetic events identified in Emery-Dreifuss muscular dystrophy, LMNA-related cardiomyopathies, Hutchinson-Gilford Progeria, mandibuloacral dysplasia, and type 2 familial partial lipodystrophy. Pharmacological application of mTOR inhibitors in view of therapeutic strategies is also discussed.

Snapshot: Implications for mTOR in Aging-related Ischemia/Reperfusion Injury

Aging may aggravate the damage and dysfunction of different components of multiorgan and thus increasing multiorgan ischemia/reperfusion (IR) injury. IR injury occurs in many organs and tissues, which is a major cause of morbidity and mortality worldwide. The kinase mammalian target of rapamycin (mTOR), an atypical serine/threonine protein kinase, involves in the pathophysiological process of IR injury. In this review, we first briefly introduce the molecular features of mTOR, the association between mTOR and aging, and especially its role on autophagy. Special focus is placed on the roles of mTOR during ischemic and IR injury. We then clarify the association between mTOR and conditioning phenomena. Following this background, we expand our discussion to potential future directions of research in this area. Collectively, information reviewed herein will serve as a comprehensive reference for the actions of mTOR in IR injury and may be significant for the design of future research and increase the potential of mTOR as a therapeutic target.

PKG1-modified TSC2 regulates mTORC1 activity to counter adverse cardiac stress

The mechanistic target of rapamycin complex-1 (mTORC1) coordinates regulation of growth, metabolism, protein synthesis, and autophagy¹. Its hyper-activation contributes to disease in many organs including the heart^1,2, though broad mTORC1 inhibition risks interference with its homeostatic roles. Tuberin (TSC2) is a GTPase-activating protein and prominent intrinsic regulator of mTORC1 by modulating Rheb (Ras homolog enriched in brain). TSC2 constitutively inhibits mTORC1, but this activity is modified by phosphorylation from multiple signaling kinases to in turn inhibit (AMPK, GSK3β) or stimulate (Akt, ERK, RSK-1) mTORC1 activity^3–9. Each kinase requires engagement of multiple serines, impeding analysis of their role in vivo. Here, we reveal phosphorylation or gain-or-loss of function mutations at either of two adjacent serines in TSC2 (S1365/1366 mouse; 1364/1365 human), with no prior known function, is sufficient to bi-directionally potently control growth-factor and hemodynamic-stress stimulated mTORC1 activity and consequent cell growth and autophagy. Basal mTORC1 activity, however, is unchanged. In heart, myocytes, and fibroblasts, phosphorylation occurs by protein kinase G (PKG), a primary effector of nitric oxide and natriuretic peptide signaling whose activation is protective against heart disease^10–13. PKG suppression of hypertrophy and stimulation of autophagy in myocytes requires TSC2 phosphorylation. Homozygous knock-in (KI) mice expressing a phospho-silenced TSC2 (S1365A) mutation develop far worse heart disease and mortality from sustained pressure-overload (PO) due to hyperactive mTORC1 that cannot be rescued by PKG stimulation. Heterozygote SA-KI are rescued, and KI-mice expressing a phospho-mimetic (S1365E) mutation are protected. Neither KI model alters resting mTORC1 activity. Thus, TSC2 phosphorylation is both required and sufficient for PKG-mediated cardiac protection against pressure-overload. These newly identified serines provide a genetic tool to bi-directionally regulate the amplitude of stress-stimulated mTORC1 activity.

Blossoming 20: The Energetic Regulator's Birthday Unveils its Versatility in Cardiac Diseases

The peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1α (PGC-1α) was first identified in 1998 as a PGC-1 family member that regulates adaptive thermogenesis and mitochondrial function following cold exposure in brown adipose tissue. The PGC-1 family has drawn widespread attention over the past two decades as the energetic regulator. We recently summarized a review regarding PGC-1 signaling pathway and its mechanisms in cardiac metabolism. In this review, we elaborate upon the PGC-1 signaling network and highlight the recent progress of its versatile roles in cardiac diseases, including myocardial hypertrophy, peripartum and diabetic cardiomyopathy, and heart failure. The information reviewed here may be useful in future studies, which may increase the potential of this energetic regulator as a therapeutic target.

Targeting Autophagy in Aging and Aging-Related Cardiovascular Diseases

Aging, an irreversible biological process, serves as an independent risk factor for chronic disease including cancer, pulmonary, neurodegenerative, and cardiovascular diseases. In particular, high morbidity and mortality have been associated with cardiovascular aging, but effective clinical therapeutic remedies are suboptimal for the ever-rising aging population. Recent evidence suggests a unique role for aberrant aggregate clearance and the protein quality control machinery - the process of autophagy - in shortened lifespan, compromised healthspan, and the onset and development of aging-associated cardiovascular diseases. Autophagy degrades and removes long-lived or damaged cellular organelles and proteins, the functions of which decline with advanced aging. Induction of autophagy using rapamycin, resveratrol, nicotinamide derivatives, metformin, urolithin A, or spermidine delays aging, prolongs lifespan, and improves cardiovascular function in aging. Given the ever-rising human lifespan and aging population as well as the prevalence of cardiovascular disease provoked by increased age, it is pertinent to understand the contribution and underlying mechanisms of autophagy and organelle-selective autophagy (e.g., mitophagy) in the regulation of lifespan, healthspan, and cardiovascular aging. Here we dissect the mechanism of action for autophagy failure in aging and discuss the potential rationale of targeting autophagy using pharmacological agents as new avenues in the combating of biological and cardiovascular aging.

Upregulation of C/EBPβ and TSC2 by an HDAC inhibitor CG200745 protects heart from DOCA-induced hypertrophy

Histone deacetylases (HDACs) are a vast family divided into four major classes: class I (1, 2, 3, and 8), class II (4, 5, 6, 7, 9 and 10), class III (sirtuin family) and class IV (HDAC11). HDAC inhibition attenuates cardiac hypertrophy through suppression of the mechanistic target of rapamycin complex1 (mTORC1) signaling. HDAC inhibitors upregulate the expression of tuberous sclerosis complex 2 (TSC2), an mTORC1 inhibitor. However, the molecular mechanism underlying HDAC inhibitor-mediated upregulation of TSC2 is unclear. We hypothesized that an HDAC inhibitor, CG200745 (CG), ameliorates cardiac hypertrophy through the inhibition of mTORC1 signaling by upregulating of the CCAAT/enhancer-binding protein-β (C/EBP-β)/TSC2 pathway. To establish a cardiac hypertrophy model, deoxycorticosterone acetate (DOCA, 40 mg/kg/wk) was subcutaneously injected for 4 weeks into Sprague-Dawley rats. All rats were unilaterally nephrectomized and had free access to drinking water containing 1% NaCl with or without CG of different concentrations. The expression level of TSC2 and C/EBP-β was measured by quantitative real-time PCR (qRT-PCR) and western blot analysis. Acetylation of C/EBP-β was analyzed by immunoprecipitation. The recruitment of C/EBP-β and polymerase II (Pol II) on TSC2 promoter region was analyzed by chromatin immunoprecipitation (ChIP). CG treatment increased the expression of TSC2. In addition, CG treated rats showed an increased in the expression and acetylation of C/EBP-β, owing to the increase in the recruitment of C/EBP-β and Pol II at Tsc2 gene promoter. Thus, CG ameliorates cardiac hypertrophy through the inhibition of mTORC1 signaling via upregulation of the C/EBP-β/TSC2 pathway in DOCA-induced hypertensive rats.

Atorvastatin, but not pravastatin, inhibits cardiac Akt/mTOR signaling and disturbs mitochondrial ultrastructure in cardiac myocytes

Statins, which reduce LDL-cholesterol by inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, are among the most widely prescribed drugs. Skeletal myopathy is a known statin-induced adverse effect associated with mitochondrial changes. We hypothesized that similar effects would occur in cardiac myocytes in a lipophilicity-dependent manner between 2 common statins: atorvastatin (lipophilic) and pravastatin (hydrophilic). Neonatal cardiac ventricular myocytes were treated with atorvastatin and pravastatin for 48 h. Both statins induced endoplasmic reticular (ER) stress, but only atorvastatin inhibited ERK1/2^T202/Y204, Akt^Ser473, and mammalian target of rapamycin signaling; reduced protein abundance of caveolin-1, dystrophin, epidermal growth factor receptor, and insulin receptor-β; decreased Ras homolog gene family member A activation; and induced apoptosis. In cardiomyocyte-equivalent HL-1 cells, atorvastatin, but not pravastatin, reduced mitochondrial oxygen consumption. When male mice underwent atorvastatin and pravastatin administration per os for up to 7 mo, only long-term atorvastatin, but not pravastatin, induced elevated serum creatine kinase; swollen, misaligned, size-variable, and disconnected cardiac mitochondria; alteration of ER structure; repression of mitochondria- and endoplasmic reticulum-related genes; and a 21% increase in mortality in cardiac-specific vinculin-knockout mice during the first 2 months of administration. To our knowledge, we are the first to demonstrate in vivo that long-term atorvastatin administration alters cardiac ultrastructure, a finding with important clinical implications.-Godoy, J. C., Niesman, I. R., Busija, A. R., Kassan, A., Schilling, J. M., Schwarz, A., Alvarez, E. A., Dalton, N. D., Drummond, J. C., Roth, D. M., Kararigas, G., Patel, H. H., Zemljic-Harpf, A. E. Atorvastatin, but not pravastatin, inhibits cardiac Akt/mTOR signaling and disturbs mitochondrial ultrastructure in cardiac myocytes.

Cardioprotective Effects of High-Density Lipoprotein Beyond its Anti-Atherogenic Action

High-density lipoprotein cholesterol (HDL-C) has been identified as a powerful independent negative predictor of cardiovascular disease. The beneficial effect of HDL is largely attributable to its key role in reverse cholesterol transport, whereby excess cholesterol in the peripheral tissues is transported to the liver, reducing the atherosclerotic burden. However, mounting evidence indicates that HDL also has pleiotropic properties, such as anti-inflammatory, anti-oxidative, and vasodilatory properties, which may contribute in reducing the incidence of heart failure. Actually, previous data from clinical and experimental studies have suggested that HDL exerts cardioprotective effects irrespective of the presence/absence of coronary artery disease. This review summarizes the currently available evidence regarding beneficial effects of HDL on the heart beyond its anti-atherogenic property. Understanding the mechanisms of cardiac protection by HDL will provide new insight into the underlying mechanism and therapeutic strategy for heart failure.

Emerging Role of mTOR Signaling-Related miRNAs in Cardiovascular Diseases

Mechanistic/mammalian target of rapamycin (mTOR), an atypical serine/threonine kinase of the phosphoinositide 3-kinase- (PI3K-) related kinase family, elicits a vital role in diverse cellular processes, including cellular growth, proliferation, survival, protein synthesis, autophagy, and metabolism. In the cardiovascular system, the mTOR signaling pathway integrates both intracellular and extracellular signals and serves as a central regulator of both physiological and pathological processes. MicroRNAs (miRs), a class of short noncoding RNA, are an emerging intricate posttranscriptional modulator of critical gene expression for the development and maintenance of homeostasis across a wide array of tissues, including the cardiovascular system. Over the last decade, numerous studies have revealed an interplay between miRNAs and the mTOR signaling circuit in the different cardiovascular pathophysiology, like myocardial infarction, hypertrophy, fibrosis, heart failure, arrhythmia, inflammation, and atherosclerosis. In this review, we provide a comprehensive state of the current knowledge regarding the mechanisms of interactions between the mTOR signaling pathway and miRs. We have also highlighted the latest advances on mTOR-targeted therapy in clinical trials and the new perspective therapeutic strategies with mTOR-targeting miRs in cardiovascular diseases.

Nitrite exerts antioxidant effects, inhibits the mTOR pathway and reverses hypertension-induced cardiac hypertrophy

Cardiac hypertrophy is a common consequence of chronic hypertension and leads to heart failure and premature death. The anion nitrite is now considered as a bioactive molecule able to exert beneficial cardiovascular effects. Previous results showed that nitrite attenuates hypertension-induced increases in reactive oxygen species (ROS) production in the vasculature. Whether antioxidant effects induced by nitrite block critical signaling pathways involved in cardiac hypertrophy induced by hypertension has not been determined yet. The Akt/mTOR signaling pathway is responsible to activate protein synthesis during cardiac remodeling and is activated by increased ROS production, which is commonly found in hypertension. Here, we investigated the effects of nitrite treatment on cardiac remodeling and activation of this hypertrophic signaling pathway in 2 kidney-1 clip (2K1C) hypertension. Sham and 2K1C rats were treated with oral nitrite at 1 or 15 mg/kg for four weeks. Nitrite treatment (15 mg/kg) reduced systolic blood pressure and decreased ROS production in the heart tissue from hypertensive rats. This nitrite dose also blunted hypertension-induced activation of mTOR pathway and cardiac hypertrophy. While the lower nitrite dose (1 mg/kg) did not affect blood pressure, it exerted antioxidant effects and tended to attenuate mTOR pathway activation and cardiac hypertrophy induced by hypertension. Our findings provide strong evidence that nitrite treatment decreases cardiac remodeling induced by hypertension as a result of its antioxidant effects and downregulation of mTOR signaling pathway. This study may help to establish nitrite as an effective therapy in hypertension-induced cardiac hypertrophic remodeling.

Cardiac Arrhythmias and Antiarrhythmic Drugs: An Autophagic Perspective

Degradation of cellular material by lysosomes is known as autophagy, and its main function is to maintain cellular homeostasis for growth, proliferation and survival of the cell. In recent years, research has focused on the characterization of autophagy pathways. Targeting of autophagy mediators has been described predominantly in cancer treatment, but also in neurological and cardiovascular diseases. Although the number of studies is still limited, there are indications that activity of autophagy pathways increases under arrhythmic conditions. Moreover, an increasing number of antiarrhythmic and non-cardiac drugs are found to affect autophagy pathways. We, therefore, suggest that future work should recognize the largely unaddressed effects of antiarrhythmic agents and other classes of drugs on autophagy pathway activation and inhibition.

Emerging roles of RNA-binding proteins in diabetes and their therapeutic potential in diabetic complications

Diabetes is a debilitating health care problem affecting 422 million people around the world. Diabetic patients suffer from multisystemic complications that can cause mortality and morbidity. Recent advancements in high-throughput next-generation RNA-sequencing and computational algorithms led to the discovery of aberrant posttranscriptional gene regulatory programs in diabetes. However, very little is known about how these regulatory programs are mis-regulated in diabetes. RNA-binding proteins (RBPs) are important regulators of posttranscriptional RNA networks, which are also dysregulated in diabetes. Human genetic studies provide new evidence that polymorphisms and mutations in RBPs are linked to diabetes. Therefore, we will discuss the emerging roles of RBPs in abnormal posttranscriptional gene expression in diabetes. Questions that will be addressed are: Which posttranscriptional mechanisms are disrupted in diabetes? Which RBPs are responsible for such changes under diabetic conditions? How are RBPs altered in diabetes? How does dysregulation of RBPs contribute to diabetes? Can we target RBPs using RNA-based methods to restore gene expression profiles in diabetic patients? Studying the evolving roles of RBPs in diabetes is critical not only for a comprehensive understanding of diabetes pathogenesis but also to design RNA-based therapeutic approaches for diabetic complications. WIREs RNA 2018, 9:e1459. doi: 10.1002/wrna.1459 This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > Splicing Regulation/Alternative Splicing Translation > Translation Regulation.

Metabolism in cardiomyopathy: every substrate matters

Cardiac metabolism is highly adaptive to changes in fuel availability and the energy demand of the heart. This metabolic flexibility is key for the heart to maintain its output during the development and in response to stress. Alterations in substrate preference have been observed in multiple disease states; a clear understanding of their impact on cardiac function in the long term is critical for the development of metabolic therapies. In addition, the contribution of cellular metabolism to growth, survival, and other signalling pathways through the generation of metabolic intermediates has been increasingly noted, adding another layer of complexity to the impact of metabolism on cardiac function. In a quest to understand the complexity of the cardiac metabolic network, genetic tools have been engaged to manipulate cardiac metabolism in a variety of mouse models. The ability to engineer cardiac metabolism in vivo has provided tremendous insights and brought about conceptual innovations. In this review, we will provide an overview of the cardiac metabolic network and highlight alterations observed during cardiac development and pathological hypertrophy. We will focus on consequences of altered substrate preference on cardiac response to chronic stresses through energy providing and non-energy providing pathways.

Novel obscurins mediate cardiomyocyte adhesion and size via the PI3K/AKT/mTOR signaling pathway

The intercalated disc of cardiac muscle embodies a highly-ordered, multifunctional network, essential for the synchronous contraction of the heart. Over 200 known proteins localize to the intercalated disc. The challenge now lies in their characterization as it relates to the coupling of neighboring cells and whole heart function. Using molecular, biochemical and imaging techniques, we characterized for the first time two small obscurin isoforms, obscurin-40 and obscurin-80, which are enriched at distinct locations of the intercalated disc. Both proteins bind specifically and directly to select phospholipids via their pleckstrin homology (PH) domain. Overexpression of either isoform or the PH-domain in cardiomyocytes results in decreased cell adhesion and size via reduced activation of the PI3K/AKT/mTOR pathway that is intimately linked to cardiac hypertrophy. In addition, obscurin-80 and obscurin-40 are significantly reduced in acute (myocardial infarction) and chronic (pressure overload) murine cardiac-stress models underscoring their key role in maintaining cardiac homeostasis. Our novel findings implicate small obscurins in the maintenance of cardiomyocyte size and coupling, and the development of heart failure by antagonizing the PI3K/AKT/mTOR pathway.

Thioredoxin-1 maintains mechanistic target of rapamycin (mTOR) function during oxidative stress in cardiomyocytes

Thioredoxin 1 (Trx1) is a 12-kDa oxidoreductase that catalyzes thiol-disulfide exchange reactions to reduce proteins with disulfide bonds. As such, Trx1 helps protect the heart against stresses, such as ischemia and pressure overload. Mechanistic target of rapamycin (mTOR) is a serine/threonine kinase that regulates cell growth, metabolism, and survival. We have shown previously that mTOR activity is increased in response to myocardial ischemia-reperfusion injury. However, whether Trx1 interacts with mTOR to preserve heart function remains unknown. Using a substrate-trapping mutant of Trx1 (Trx1C35S), we show here that mTOR is a direct interacting partner of Trx1 in the heart. In response to H₂O₂ treatment in cardiomyocytes, mTOR exhibited a high molecular weight shift in non-reducing SDS-PAGE in a 2-mercaptoethanol-sensitive manner, suggesting that mTOR is oxidized and forms disulfide bonds with itself or other proteins. The mTOR oxidation was accompanied by reduced phosphorylation of endogenous substrates, such as S6 kinase (S6K) and 4E-binding protein 1 (4E-BP1) in cardiomyocytes. Immune complex kinase assays disclosed that H₂O₂ treatment diminished mTOR kinase activity, indicating that mTOR is inhibited by oxidation. Of note, Trx1 overexpression attenuated both H₂O₂-mediated mTOR oxidation and inhibition, whereas Trx1 knockdown increased mTOR oxidation and inhibition. Moreover, Trx1 normalized H₂O₂-induced down-regulation of metabolic genes and stimulation of cell death, and an mTOR inhibitor abolished Trx1-mediated rescue of gene expression. H₂O₂-induced oxidation and inhibition of mTOR were attenuated when Cys-1483 of mTOR was mutated to phenylalanine. These results suggest that Trx1 protects cardiomyocytes against stress by reducing mTOR at Cys-1483, thereby preserving the activity of mTOR and inhibiting cell death.

High-density lipoprotein protects cardiomyocytes from oxidative stress via the PI3K/mTOR signaling pathway

Low levels of plasma high-density lipoprotein (HDL) cholesterol are associated with an increased risk of heart failure, regardless of the presence or absence of coronary artery disease. However, the direct effects of HDL on failing myocardium have not been fully elucidated. We found that HDL treatment resulted in improved cell viability in H9c2 cardiomyocytes under oxidative stress. This cardioprotective effect of HDL was regulated via the phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway. mTOR signaling promotes cell survival through the inactivation of the BCL2-associated agonist of cell death via phosphorylation of ribosomal protein S6 kinase. Modulation of cardiac PI3K/mTOR signaling by HDL could represent a novel therapeutic strategy for heart failure.

Prenatal Mechanistic Target of Rapamycin Complex 1 (m TORC1) Inhibition by Rapamycin Treatment of Pregnant Mice Causes Intrauterine Growth Restriction and Alters Postnatal Cardiac Growth, Morphology, and Function

Background

Fetal growth impacts cardiovascular health throughout postnatal life in humans. Various animal models of intrauterine growth restriction exhibit reduced heart size at birth, which negatively influences cardiac function in adulthood. The mechanistic target of rapamycin complex 1 (mTORC1) integrates nutrient and growth factor availability with cell growth, thereby regulating organ size. This study aimed at elucidating a possible involvement of mTORC1 in intrauterine growth restriction and prenatal heart growth.

Methods and Results

We inhibited mTORC1 in fetal mice by rapamycin treatment of pregnant dams in late gestation. Prenatal rapamycin treatment reduces mTORC1 activity in various organs at birth, which is fully restored by postnatal day 3. Rapamycin‐treated neonates exhibit a 16% reduction in body weight compared with vehicle‐treated controls. Heart weight decreases by 35%, resulting in a significantly reduced heart weight/body weight ratio, smaller left ventricular dimensions, and reduced cardiac output in rapamycin‐ versus vehicle‐treated mice at birth. Although proliferation rates in neonatal rapamycin‐treated hearts are unaffected, cardiomyocyte size is reduced, and apoptosis increased compared with vehicle‐treated neonates. Rapamycin‐treated mice exhibit postnatal catch‐up growth, but body weight and left ventricular mass remain reduced in adulthood. Prenatal mTORC1 inhibition causes a reduction in cardiomyocyte number in adult hearts compared with controls, which is partially compensated for by an increased cardiomyocyte volume, resulting in normal cardiac function without maladaptive left ventricular remodeling.

Conclusions

Prenatal rapamycin treatment of pregnant dams represents a new mouse model of intrauterine growth restriction and identifies an important role of mTORC1 in perinatal cardiac growth.

MTOR controls genesis and autophagy of GABAergic interneurons during brain development

Interneuron progenitors in the ganglionic eminence of the ventral telencephalon generate most cortical interneurons during brain development. However, the regulatory mechanism of interneuron progenitors remains poorly understood. Here, we show that MTOR (mechanistic target of rapamycin [serine/threonine kinase]) regulates proliferation and macroautophagy/autophagy of interneuron progenitors in the developing ventral telencephalon. To investigate the role of MTOR in interneuron progenitors, we conditionally deleted the Mtor gene in mouse interneuron progenitors and their progeny by using Tg(mI56i-cre,EGFP)1Kc/Dlx5/6-Cre-IRES-EGFP and Nkx2-1-Cre drivers. We found that Mtor deletion markedly reduced the number of interneurons in the cerebral cortex. However, relative positioning of cortical interneurons was normal, suggesting that disruption of progenitor self-renewal caused the decreased number of cortical interneurons in the Mtor-deleted brain. Indeed, Mtor-deleted interneuron progenitors showed abnormal proliferation and cell cycle progression. Additionally, we detected a significant activation of autophagy in Mtor-deleted brain. Our findings suggest that MTOR plays a critical role in the regulation of cortical interneuron number and autophagy in the developing brain.

The mTOR Signaling Pathway in Myocardial Dysfunction in Type 2 Diabetes Mellitus

PURPOSE OF REVIEW

T2DM (type 2 diabetes mellitus) is a risk factor for heart failure. The mTOR (mechanistic target of rapamycin) is a key mediator of the insulin signaling pathway. We will discuss the role of mTOR in myocardial dysfunction in T2DM.

RECENT FINDINGS

In T2DM, chronically activated mTOR induces multiple pathological events, including a negative feedback loop that suppresses IRS (insulin receptor substrate)-1. While short-term treatment with rapamycin, an mTOR inhibitor, is a promising strategy for cardiac diseases such as acute myocardial infarction and cardiac hypertrophy in T2DM, there are many concerns about chronic usage of rapamycin. Two mTOR complexes, mTORC1 and mTORC2, affect many molecules and processes via distinct signaling pathways that regulate cardiomyocyte function and survival. Understanding mechanisms underlying mTOR-mediated pathophysiological features in the heart is essential for developing effective therapies for cardiac diseases in the context of T2DM.

Insulin receptor substrate signaling controls cardiac energy metabolism and heart failure

The heart is an insulin-dependent and energy-consuming organ in which insulin and nutritional signaling integrates to the regulation of cardiac metabolism, growth and survival. Heart failure is highly associated with insulin resistance, and heart failure patients suffer from the cardiac energy deficiency and structural and functional dysfunction. Chronic pathological conditions, such as obesity and type 2 diabetes mellitus, involve various mechanisms in promoting heart failure by remodeling metabolic pathways, modulating cardiac energetics and impairing cardiac contractility. Recent studies demonstrated that insulin receptor substrates 1 and 2 (IRS-1,-2) are major mediators of both insulin and insulin-like growth factor-1 (IGF-1) signaling responsible for myocardial energetics, structure, function and organismal survival. Importantly, the insulin receptor substrates (IRS) play an important role in the activation of the phosphatidylinositide-3-dependent kinase (PI-3K) that controls Akt and Foxo1 signaling cascade, regulating the mitochondrial function, cardiac energy metabolism and the renin-angiotensin system. Dysregulation of this branch in signaling cascades by insulin resistance in the heart through the endocrine system promotes heart failure, providing a novel mechanism for diabetic cardiomyopathy. Therefore, targeting this branch of IRS→PI-3K→Foxo1 signaling cascade and associated pathways may provide a fundamental strategy for the therapeutic and nutritional development in control of metabolic and cardiovascular diseases. In this review, we focus on insulin signaling and resistance in the heart and the role energetics play in cardiac metabolism, structure and function.

Pharmacological Strategies to Retard Cardiovascular Aging

Aging is the major risk factor for cardiovascular diseases, which are the leading cause of death in the United States. Traditionally, the effort to prevent cardiovascular disease has been focused on addressing the conventional risk factors, including hypertension, hyperglycemia, hypercholesterolemia, and high circulating levels of triglycerides. However, recent preclinical studies have identified new approaches to combat cardiovascular disease. Calorie restriction has been reproducibly shown to prolong lifespan in various experimental model animals. This has led to the development of calorie restriction mimetics and other pharmacological interventions capable to delay age-related diseases. In this review, we will address the mechanistic effects of aging per se on the cardiovascular system and focus on the prolongevity benefits of various therapeutic strategies that support cardiovascular health.

Insights into the Mechanisms Involved in Protective Effects of VEGF-B in Dopaminergic Neurons

Vascular endothelial growth factor-B (VEGF-B), when initially discovered, was thought to be an angiogenic factor, due to its intimate sequence homology and receptor binding similarity to the prototype angiogenic factor, vascular endothelial growth factor-A (VEGF-A). Studies demonstrated that VEGF-B, unlike VEGF-A, did not play a significant role in angiogenesis or vascular permeability and has become an active area of interest because of its role as a survival factor in pathological processes in a multitude of systems, including the brain. By characterization of important downstream targets of VEGF-B that regulate different cellular processes in the nervous system and cardiovascular system, it may be possible to develop more effective clinical interventions in diseases such as Parkinson's disease (PD), Amyotrophic Lateral Sclerosis (ALS), and ischemic heart disease, which all share mitochondrial dysfunction as part of the disease. Here we summarize what is currently known about the mechanism of action of VEGF-B in pathological processes. We explore its potential as a homeostatic protective factor that improves mitochondrial function in the setting of cardiovascular and neurological disease, with a specific focus on dopaminergic neurons in Parkinson's disease.