Β-adrenergic receptor stimulation induces endoplasmic reticulum stress in adult cardiac myocytes: role in apoptosis

Adipocyte-released adipomes in Chagas cardiomyopathy: Impact on cardiac metabolic and immune regulation

Chronic Trypanosoma cruzi infection leads to Chagas cardiomyopathy (CCM), with varying manifestations such as inflammatory hypertrophic cardiomyopathy, arrhythmias, and dilated cardiomyopathy. The factors responsible for the increasing risk of progression to CCM are not fully understood. Previous studies link adipocyte loss to CCM progression, but the mechanism triggering CCM pathogenesis remains unexplored. Our study uncovers that T. cruzi infection triggers adipocyte apoptosis, leading to the release of extracellular vesicles named "adipomes". We developed an innovative method to isolate intact adipomes from infected mice's adipose tissue and plasma, showing they carry unique lipid cargoes. Large and Small adipomes, particularly plasma-derived infection-associated L-adipomes (P-ILA), regulate immunometabolic signaling and induce cardiomyopathy. P-ILA treatment induces hypertrophic cardiomyopathy in wild-type mice and worsens cardiomyopathy severity in post-acute-infected mice by regulating adipogenic/lipogenic and mitochondrial functions. These findings highlight adipomes' pivotal role in promoting inflammation and impairing myocardial function during cardiac remodeling in CD.

Timolol maleate, a β blocker eye drop, improved edema in a retinal vein occlusion model

Purpose

To investigate the therapeutic effects of eye drops, namely, timolol maleate, a β-adrenergic receptor antagonist, and latanoprost, a prostaglandin F2α analog, on retinal edema in a murine retinal vein occlusion (RVO) model.

Methods

An RVO model was established using laser-induced RVO in mice, which were administered timolol maleate and latanoprost eye drops several times after venous occlusion. Subsequently, the thickness of the inner nuclear layer (INL) and the expression levels of such genes as Vegf and Atf4, which are stress markers of the endoplasmic reticulum, were examined. Primary human cultured retinal microvascular endothelial cells (HRMECs) were treated with timolol under hypoxic conditions, after which the gene expression pattern was investigated. Importantly, an integrated stress response inhibitor (ISRIB) was used in the RVO model, he known ISRIB, which suppresses the expression of ATF4 in retinal edema.

Results

Increased INL thickness was suppressed by timolol eye drops, as were the expressions of Vegf and Atf4, in the RVO model. However, latanoprost eye drops did not induce any change in INL thickness. In HRMECs, hypoxic stress and serum deprivation increased the Vegf and Atf4 expressions; in response, treatment with timolol suppressed the Vegf expression. Furthermore, the ISRIB decreased the Vegf expression pattern and edema formation, which are associated with RVO.

Conclusions

These results indicate that timolol eye drops may be a potential option for RVO treatment.

Adrenergic Reprogramming of Preexisting Adipogenic Trajectories Steer Naïve Mural Cells Toward Beige Differentiation

In adult white adipose tissue, cold or β3-adrenoceptor activation promotes the appearance of thermogenic beige adipocytes. Our comprehensive single-cell analysis revealed that these cells arise through the reprogramming of existing adipogenic trajectories, rather than from a single precursor. These trajectories predominantly arise from SM22-expressing vascular mural progenitor cells. Central in this transition is the activation of Adrb3 in mature adipocytes, leading to subsequent upregulation of Adrb1 in primed progenitors. Under thermoneutral conditions, synergistic activation of both Adrb3 and Adrb1 recapitulates the pattern of cold-induced SM22+ cell recruitment. Lipolysis-derived eicosanoids, specifically docosahexaenoic acid (DHA) and arachidonic acid (AA) prime these processes and in vitro, were sufficient to recapitulate progenitor cells priming. Collectively, our findings provide a robust model for cold-induced beige adipogenesis, emphasizing a profound relationship between mature adipocytes and mural cells during cold acclimation, and revealing the metabolic potential of this unique cellular reservoir.

IL-6/gp130/STAT3 signaling contributed to the activation of the PERK arm of the unfolded protein response in response to chronic β-adrenergic stimulation

Prolonged activation of the PERK branch of the unfolded protein response (UPR) promotes cardiomyocytes apoptosis in response to chronic β-adrenergic stimulation. STAT3 plays a critical role in β-adrenergic functions in the heart. However, whether STAT3 contributed to β-adrenoceptor-mediated PERK activation and how β-adrenergic signaling activates STAT3 remains unclear. This study aimed to investigate whether STAT3-Y705 phosphorylation contributed to the PERK arm activation in cardiomyocytes and if IL-6/gp130 signaling was involved in the chronic β-AR-stimulation-induced STAT3 and PERK arm activation. We found that the PERK phosphorylation was positively associated with STAT3 activation. Wild-type STAT3 plasmids transfection activated the PERK/eIF2α/ATF4/CHOP pathway in cardiomyocytes while dominant negative Y705F STAT3 plasmids caused no obvious effect on PERK signaling. Stimulation with isoproterenol produced a significant increase in the level of IL-6 in the cardiomyocyte's supernatants, while IL-6 silence inhibited PERK phosphorylation but failed to attenuate STAT3 activation in response to isoproterenol stimulation. Gp130 silence attenuated isoproterenol-induced STAT3 activation and PERK phosphorylation. Inhibiting IL-6/gp130 pathway by bazedoxifene and inhibiting STAT3 by stattic both reversed isoproterenol-induced STAT3-Y705 phosphorylation, ROS production, PERK activation, IRE1α activation, and cardiomyocytes apoptosis in vitro. Bazedoxifene (5 mg/kg/day by oral gavage once a day) exhibited similar effect as carvedilol (10 mg/kg/day by oral gavage once a day) on attenuating chronic isoproterenol (30 mg/kg by abdominal injection once a day, 7 days) induced cardiac systolic dysfunction, cardiac hypertrophy and fibrosis in C57BL/6 mice. Meanwhile, bazedoxifene attenuates isoproterenol-induced STAT3-Y705 phosphorylation, PERK/eIF2α/ATF4/CHOP activation, IRE1α activation, and cardiomyocytes apoptosis to a similar extend as carvedilol in the cardiac tissue of mice. Our results showed that chronic β-adrenoceptor-mediated stimulation activated the STAT3 and PERK arm of the UPR at least partially via IL-6/gp130 pathway. Bazedoxifene has great potential to be used as an alternative to conventional β-blockers to attenuate β-adrenoceptor-mediated maladaptive UPR.

Dapagliflozin attenuates cardiac remodeling and dysfunction in rats with β-adrenergic receptor overactivation through restoring calcium handling and suppressing cardiomyocyte apoptosis

Background: Long-term β-adrenergic receptor (β-AR) activation can impair myocardial structure and function. Dapagliflozin (DAPA) has been reported to improve clinical prognosis in heart failure patients, whereas the exact mechanism remains unclear. Here, we investigated the effects of DAPA against β-AR overactivation toxicity and explored the underlying mechanism.Methods and Results: Rats were randomized to receive saline + placebo, isoproterenol (ISO, 5 mg/kg/day, intraperitoneally) + placebo, or ISO + DAPA (1 mg/kg/day, intragastrically) for 2-week. DAPA treatment improved cardiac function, alleviated myocardial fibrosis, prevented cardiomyocytes (CMs) apoptosis, and decreased the expression of ER stress-mediated apoptosis markers in ISO-treated hearts. In isolated CMs, 2-week ISO stimulation resulted in deteriorated kinetics of cellular contraction and relaxation, increased diastolic intracellular Ca2+ level and decay time constant of Ca2+ transient (CaT) but decreased CaT amplitude and sarcoplasmic reticulum (SR) Ca2+ level. However, DAPA treatment prevented abnormal Ca2+ handling and contractile dysfunction in CMs from ISO-treated hearts. Consistently, DAPA treatment upregulated the expression of SR Ca2+-ATPase protein and ryanodine receptor 2 (RyR2) but reduced the expression of phosphorylated-RyR2, Ca2+/calmodulin-dependent protein kinase II (CaMKII), and phosphorylated-CaMKII in ventricles from ISO-treated rats.Conclusion: DAPA prevented myocardial remodeling and cardiac dysfunction in rats with β-AR overactivation via restoring calcium handling and suppressing ER stress-related CMs apoptosis.

Mechanism of cis-Nerolidol-Induced Bladder Carcinoma Cell Death

Nerolidol is a naturally occurring sesquiterpene alcohol with multiple properties, including antioxidant, antibacterial, and antiparasitic activities. A few studies investigating the antitumor properties of nerolidol have shown positive results in both cell culture and mouse models. In this study, we investigated the antitumor mechanism of cis-nerolidol in bladder carcinoma cell lines. The results of our experiments on two bladder carcinoma cell lines revealed that nerolidol inhibited cell proliferation and induced two distinct cell death pathways. We confirmed that cis-nerolidol induces DNA damage and ER stress. A mechanistic study identified a common cAMP, Ca²⁺, and MAPK axis involved in signal propagation and amplification, leading to ER stress. Inhibition of any part of this signaling cascade prevented both cell death pathways. The two cell death mechanisms can be distinguished by the involvement of caspases. The early occurring cell death pathway is characterized by membrane blebbing and cell swelling followed by membrane rupture, which can be prevented by the inhibition of caspase activation. In the late cell death pathway, which was found to be caspase-independent, cytoplasmic vacuolization and changes in cell shape were observed. cis-Nerolidol shows promising antitumor activity through an unorthodox mechanism of action that could help target resistant forms of malignancies, such as bladder cancer.

ER stress and calcium-dependent arrhythmias

The sarcoplasmic reticulum (SR) plays the key role in cardiac function as the major source of Ca²⁺ that activates cardiomyocyte contractile machinery. Disturbances in finely-tuned SR Ca²⁺ release by SR Ca²⁺ channel ryanodine receptor (RyR2) and SR Ca²⁺ reuptake by SR Ca²⁺-ATPase (SERCa2a) not only impair contraction, but also contribute to cardiac arrhythmia trigger and reentry. Besides being the main Ca²⁺ storage organelle, SR in cardiomyocytes performs all the functions of endoplasmic reticulum (ER) in other cell types including protein synthesis, folding and degradation. In recent years ER stress has become recognized as an important contributing factor in many cardiac pathologies, including deadly ventricular arrhythmias. This brief review will therefore focus on ER stress mechanisms in the heart and how these changes can lead to pro-arrhythmic defects in SR Ca²⁺ handling machinery.

Metabolic Pathway of Cardiospecific Troponins: From Fundamental Aspects to Diagnostic Role (Comprehensive Review)

Many molecules of the human body perform key regulatory functions and are widely used as targets for the development of therapeutic drugs or as specific diagnostic markers. These molecules undergo a significant metabolic pathway, during which they are influenced by a number of factors (biological characteristics, hormones, enzymes, etc.) that can affect molecular metabolism and, as a consequence, the serum concentration or activity of these molecules. Among the most important molecules in the field of cardiology are the molecules of cardiospecific troponins (Tns), which regulate the processes of myocardial contraction/relaxation and are used as markers for the early diagnosis of ischemic necrosis of cardiomyocytes (CMC) in myocardial infarction (MI). The diagnostic value and diagnostic capabilities of cardiospecific Tns have changed significantly after the advent of new (highly sensitive (HS)) detection methods. Thus, early diagnostic algorithms of MI were approved for clinical practice, thanks to which the possibility of rapid diagnosis and determination of optimal tactics for managing patients with MI was opened. Relatively recently, promising directions have also been opened for the use of cardiospecific Tns as prognostic markers both at the early stages of the development of cardiovascular diseases (CVD) (arterial hypertension (AH), heart failure (HF), coronary heart disease (CHD), etc.), and in non-ischemic extra-cardiac pathologies that can negatively affect CMC (for example, sepsis, chronic kidney disease (CKD), chronic obstructive pulmonary disease (COPD), etc.). Recent studies have also shown that cardiospecific Tns are present not only in blood serum, but also in other biological fluids (urine, oral fluid, pericardial fluid, amniotic fluid). Thus, cardiospecific Tns have additional diagnostic capabilities. However, the fundamental aspects of the metabolic pathway of cardiospecific Tns are definitively unknown, in particular, specific mechanisms of release of Tns from CMC in non-ischemic extra-cardiac pathologies, mechanisms of circulation and elimination of Tns from the human body, mechanisms of transport of Tns to other biological fluids and factors that may affect these processes have not been established. In this comprehensive manuscript, all stages of the metabolic pathway are consistently and in detail considered, starting from release from CMC and ending with excretion (removal) from the human body. In addition, the possible diagnostic role of individual stages and mechanisms, influencing factors is analyzed and directions for further research in this area are noted.

Importance of Autophagy in Mediating Cellular Responses to Iron Overload in Cardiomyocytes

Both iron overload and deficiency can promote development of cardiomyopathy. Advances in our knowledge from recent research have indicated numerous potential cellular mechanisms. Regulation of myocardial autophagy by iron is of particular interest and will be reviewed here. Autophagy is already well established to play a significant role in regulating the development of heart failure. This review will focus on regulation of autophagy by iron, crosstalk between autophagy and other cellular process which have also already been implicated in heart failure (oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress, ferroptosis) and the therapeutic potential of targeting these interactions.

Lipid Dyshomeostasis and Inherited Cerebellar Ataxia

Cerebellar ataxia is a form of ataxia that originates from dysfunction of the cerebellum, but may involve additional neurological tissues. Its clinical symptoms are mainly characterized by the absence of voluntary muscle coordination and loss of control of movement with varying manifestations due to differences in severity, in the site of cerebellar damage and in the involvement of extracerebellar tissues. Cerebellar ataxia may be sporadic, acquired, and hereditary. Hereditary ataxia accounts for the majority of cases. Hereditary ataxia has been tentatively divided into several subtypes by scientists in the field, and nearly all of them remain incurable. This is mainly because the detailed mechanisms of these cerebellar disorders are incompletely understood. To precisely diagnose and treat these diseases, studies on their molecular mechanisms have been conducted extensively in the past. Accumulating evidence has demonstrated that some common pathogenic mechanisms exist within each subtype of inherited ataxia. However, no reports have indicated whether there is a common mechanism among the different subtypes of inherited cerebellar ataxia. In this review, we summarize the available references and databases on neurological disorders characterized by cerebellar ataxia and show that a subset of genes involved in lipid homeostasis form a new group that may cause ataxic disorders through a common mechanism. This common signaling pathway can provide a valuable reference for future diagnosis and treatment of ataxic disorders.

The Importance of Cardiac Troponin Metabolism in the Laboratory Diagnosis of Myocardial Infarction (Comprehensive Review)

The study of the metabolism of endogenous molecules is not only of great fundamental significance but also of high practical importance, since many molecules serve as drug targets and/or biomarkers for laboratory diagnostics of diseases. Thus, cardiac troponin molecules have long been used as the main biomarkers for confirmation of diagnosis of myocardial infarction, and with the introduction of high-sensitivity test methods, many of our ideas about metabolism of these cardiac markers have changed significantly. In clinical practice, there are opening new promising diagnostic capabilities of cardiac troponins, the understanding and justification of which are closely connected with the fundamental principles of the metabolism of these molecules. Our current knowledge about the metabolism of cardiac troponins is insufficient and extremely disconnected from various literary sources. Thus, many researchers do not sufficiently understand the potential importance of cardiac troponin metabolism in the laboratory diagnosis of myocardial infarction. The purpose of this comprehensive review is to systematize information about the metabolism of cardiac troponins and during the discussion to focus on the potential impact of cTns metabolism on the laboratory diagnosis of myocardial infarction. The format of this comprehensive review includes a sequential consideration and analysis of the stages of the metabolic pathway, starting from possible release mechanisms and ending with elimination mechanisms. This will allow doctors and researchers to understand the significant importance of cTns metabolism and its impact on the laboratory diagnosis of myocardial infarction.

Biology of Cardiac Troponins: Emphasis on Metabolism

Simple Summary

Cardiovascular diseases, including myocardial infarction, are among the most common diseases worldwide. Markers associated with the diagnosis of myocardial infarction have been in the spotlight for many years. The most commonly used markers of myocardial infarction are cardiac troponins. However, insufficient understanding of the biology and metabolism of cardiac troponins does not allow us to fully unlock the full diagnostic potential of these cardiomarkers. In this article, I summarized and discussed in detail the features of the metabolism of cardiac troponins. I conducted a comprehensive review of current literary sources and presented my point of view. The format of the manuscript includes a consistent description of the biology and stages of the metabolism of cardiac troponins, starting from the release and circulation, and ending with the features of elimination of cardiac troponins. The possible influence of the biology of cardiac troponins on the diagnostic value of cardiac troponins is analyzed. Based on the analysis of the literature, I found a close relationship between the diagnostic value of cardiac troponins and their biology/metabolism. Further research is needed to increase the diagnostic value of cardiac troponins, and to fully unlock their diagnostic potential.

Abstract

Understanding of the biology of endo- and exogenous molecules, in particular their metabolism, is not only of great theoretical importance, but also of high practical significance, since many molecules serve as drug targets or markers for the laboratory diagnostics of many human diseases. Thus, cardiac troponin (cTns) molecules have long been used as key markers for the confirmation of diagnosis of myocardial infarction (MI), and with the introduction of contemporary (high sensitivity) test methods, many of our concepts related to the biology of these cardiac markers have changed significantly. In current clinical practice, there are opening new promising diagnostic capabilities of cTns, the understanding and justification of which is closely connected with the theoretical principles of the metabolism of these molecules. However, today, the biology and metabolism of cTns have not been properly investigated; in particular, we do not know the precise mechanisms of release of these molecules from the myocardial cells (MCs) of healthy people and the mechanisms of circulation, and the elimination of cTns from the bloodstream. The main purpose of this manuscript is to systematize information about the biology of cTns, with an emphasis on the metabolism of cTns. The format of this paper, starting with the release of cTns in the blood and concluding with the metabolism/filtration of troponins, provides a comprehensive yet logically easy way for the readers to approach our current knowledge in the framework of understanding the basic mechanisms by which cTns are produced and processed. Conclusions. Based on the analysis of the current literature, the important role of biology and all stages of metabolism (release, circulation, removal) of cTns in laboratory diagnostics should be noted. It is necessary to continue studying the biology and metabolism of cTns, because this will improve the differential diagnosis of MI and i a new application of cTns immunoassays in current clinical practice.

Ero1α-Dependent ERp44 Dissociation From RyR2 Contributes to Cardiac Arrhythmia

BACKGROUND

Oxidative stress in cardiac disease promotes proarrhythmic disturbances in Ca²⁺ homeostasis, impairing luminal Ca²⁺ regulation of the sarcoplasmic reticulum (SR) Ca²⁺ release channel, the RyR2 (ryanodine receptor), and increasing channel activity. However, exact mechanisms underlying redox-mediated increase of RyR2 function in cardiac disease remain elusive. We tested whether the oxidoreductase family of proteins that dynamically regulate the oxidative environment within the SR are involved in this process.

METHODS

A rat model of hypertrophy induced by thoracic aortic banding (TAB) was used for ex vivo whole heart optical mapping and for Ca²⁺ and reactive oxygen species imaging in isolated ventricular myocytes (VMs).

RESULTS

The SR-targeted reactive oxygen species biosensor ERroGFP showed increased intra-SR oxidation in TAB VMs that was associated with increased expression of Ero1α (endoplasmic reticulum oxidoreductase 1 alpha). Pharmacological (EN460) or genetic Ero1α inhibition normalized SR redox state, increased Ca²⁺ transient amplitude and SR Ca²⁺ content, and reduced proarrhythmic spontaneous Ca²⁺ waves in TAB VMs under β-adrenergic stimulation (isoproterenol). Ero1α overexpression in Sham VMs had opposite effects. Ero1α inhibition attenuated Ca²⁺-dependent ventricular tachyarrhythmias in TAB hearts challenged with isoproterenol. Experiments in TAB VMs and human embryonic kidney 293 cells expressing human RyR2 revealed that an Ero1α-mediated increase in SR Ca²⁺-channel activity involves dissociation of intraluminal protein ERp44 (endoplasmic reticulum protein 44) from the RyR2 complex. Site-directed mutagenesis and molecular dynamics simulations demonstrated a novel redox-sensitive association of ERp44 with RyR2 mediated by intraluminal cysteine 4806. ERp44-RyR2 association in TAB VMs was restored by Ero1α inhibition, but not by reducing agent dithiothreitol, as hypo-oxidation precludes formation of covalent bond between RyR2 and ERp44.

CONCLUSIONS

A novel axis of intraluminal interaction between RyR2, ERp44, and Ero1α has been identified. Ero1α inhibition exhibits promising therapeutic potential by stabilizing RyR2-ERp44 complex, thereby reducing spontaneous Ca²⁺ release and Ca²⁺-dependent tachyarrhythmias in hypertrophic hearts, without causing hypo-oxidative stress in the SR.

Cardiac hypertrophy caused by hyperthyroidism in rats: the role of ATF-6 and TRPC1 channels

Hyperthyroidism influences the development of cardiac hypertrophy. Transient receptor potential canonical channels (TRPCs) and endoplasmic reticulum (ER) stress are regarded as critical pathways in cardiac hypertrophy. Hence, we aimed to identify the TRPCs associated with ER stress in hyperthyroidism-induced cardiac hypertrophy. Twenty adult Wistar albino male rats were used in the study. The control group was fed with standard food and tap water. The group with hyperthyroidism was also fed with standard rat food, along with tap water that contained 12 mg/L of thyroxine (T4) for 4 weeks. At the end of the fourth week, the serum-free triiodothyronine (T3), T4, and thyroid-stimulating hormone (TSH) levels of the groups were measured. The left ventricle of each rat was used for histochemistry, immunohistochemistry, Western blot, total antioxidant capacity (TAC), and total oxidant status (TOS) analysis. As per our results, activating transcription factor 6 (ATF-6), inositol-requiring kinase 1 (IRE-1), and TRPC1, which play a significant role in cardiac hypertrophy caused by hyperthyroidism, showed increased activation. Moreover, TOS and serum-free T3 levels increased, while TAC and TSH levels decreased. With the help of the literature review in our study, we could, for the first time, indicate that the increased activation of ATF-6, IRE-1, and TRPC1-induced deterioration of the Ca2+ ion balance leads to hypertrophy in hyperthyroidism due to heart failure.

Catecholamines Induce Endoplasmic Reticulum Stress Via Both Alpha and Beta Receptors

Severely burned patients suffer from a hypermetabolic syndrome that can last for years after the injury has resolved. The underlying cause of these metabolic alterations most likely involves the persistent elevated catecholamine levels that follow the surge induced by thermal injury. At the cellular level, endoplasmic reticulum (ER) stress in metabolic tissues is a hallmark observed in patients following burn injury and is associated with several detrimental effects. Therefore, ER stress could be the underlying cellular mechanism of persistent hypermetabolism in burned patients. Here, we show that catecholamines induce ER stress and that adreno-receptor blockers reduce stress responses in the HepG2 hepatocyte cell line. Our results also indicate that norepinephrine (NE) significantly induces ER stress in HepG2 cells and 3T3L1 mouse adipocytes. Furthermore, we demonstrate that the alpha-1 blocker, prazosin, and beta blocker, propranolol, block ER stress induced by NE. We also show that the effects of catecholamines in inducing ER stress are cell type-specific, as NE treatment failed to evoke ER stress in human fibroblasts. Thus, these findings reveal the mechanisms used by catecholamines to alter metabolism and suggest inhibition of the receptors utilized by these agents should be further explored as a potential target for the treatment of ER stress-mediated disease.

Adaptation to exercise-induced stress is not dependent on cardiomyocyte α1A-adrenergic receptors

The 'fight or flight' response to physiological stress involves sympathetic nervous system activation, catecholamine release and adrenergic receptor stimulation. In the heart, this induces positive inotropy, previously attributed to the β₁-adrenergic receptor subtype. However, the role of the α_1A-adrenergic receptor, which has been suggested to be protective in cardiac pathology, has not been investigated in the setting of physiological stress. To explore this, we developed a tamoxifen-inducible, cardiomyocyte-specific α_1A-adrenergic receptor knock-down mouse model, challenged mice to four weeks of endurance swim training and assessed cardiac outcomes. With 4-OH tamoxifen treatment, expression of the α_1A-adrenergic receptor was knocked down by 80-89%, without any compensatory changes in the expression of other adrenergic receptors, or changes to baseline cardiac structure and function. Swim training caused eccentric hypertrophy, regardless of genotype, demonstrated by an increase in heart weight/tibia length ratio (30% and 22% in vehicle- and tamoxifen-treated animals, respectively) and an increase in left ventricular end diastolic volume (30% and 24% in vehicle- and tamoxifen-treated animals, respectively) without any change in the wall thickness/chamber radius ratio. Consistent with physiological hypertrophy, there was no increase in fetal gene program (Myh7, Nppa, Nppb or Acta1) expression. In response to exercise-induced volume overload, stroke volume (39% and 30% in vehicle- and tamoxifen-treated animals, respectively), cardiac output/tibia length ratio (41% in vehicle-treated animals) and stroke work (61% and 33% in vehicle- and tamoxifen-treated animals, respectively) increased, regardless of genotype. These findings demonstrate that cardiomyocyte α_1A-adrenergic receptors are not necessary for cardiac adaptation to endurance exercise stress and their acute ablation is not deleterious.

Curcumin and cardiovascular diseases: Focus on cellular targets and cascades

Cardiovascular diseases (CVDs) are one of the leading causes of the most considerable mortality globally, and it has been tried to find the molecular mechanisms and design new drugs that triggered the molecular target. Curcumin is the main ingredient of Curcuma longa (turmeric) that has been used in traditional medicine for treating several diseases for years. Numerous investigations have indicated the beneficial effect of Curcumin in modulating multiple signaling pathways involved in oxidative stress, inflammation, apoptosis, and proliferation. The cardiovascular protective effects of Curcumin against CVDs have been indicated in several studies. In the current review study, we provided novel information on Curcumin's protective effects against various CVDs and potential molecular signaling targets of Curcumin. Nonetheless, more studies should be performed to discover the exact molecular target of Curcumin against CVDs.

Cardioprotective Potential of Exogenous Ubiquitin

Ischemic heart disease (IHD) accounts for the majority of heart disease-related deaths worldwide. Ubiquitin (UB), found in all eukaryotic cells, is a highly conserved low molecular weight (~8.5 kDa) protein. A well-known intracellular function of UB is to regulate protein turnover via the UB-proteasome system. UB is a normal constituent of plasma, and elevated levels of UB are observed in the serum of patients under a variety of pathological conditions. Recent studies provide evidence for cardioprotective potential of exogenous UB in the remodeling process of the heart in IHD, including effects on cardiac myocyte apoptosis, inflammatory response, and reorganization of the vasculature and extracellular matrix. This review summarizes functions of UB with an emphasis on the role of exogenous UB in myocardial remodeling in IHD.

Activation of ER stress signalling increases mortality after a major trauma

The endoplasmic reticulum (ER) adapts to stress by activating a signalling cascade known as the ER stress response. While ER stress signalling is a central component of the cellular defence against environmental insult, persistent activation is thought to contribute to the progression of various metabolic complications via loss of protein function and cell death. Despite its importance however, whether and how ER stress impacts morbidity and mortality in conditions of hypermetabolism remain unclear. In this study, we discovered that chronic ER stress response plays a role in mediating adverse outcomes that occur after major trauma. Using a murine model of thermal injury, we show that induction of ER stress with Tunicamycin not only increased mortality but also resulted in hepatic damage and hepatic steatosis. Importantly, post‐burn treatment with chaperone ER stress inhibitors attenuated hepatic ER stress and improved organ function following injury. Our study identifies ER stress as a potential hub of the signalling network affecting multiple aspects of metabolism after major trauma and as a novel potential molecular target to improve the clinical outcomes of severely burned patients.

Intermittent Hypoxia Triggers Early Cardiac Remodeling and Contractile Dysfunction in the Time-Course of Ischemic Cardiomyopathy in Rats

BACKGROUND Sleep-disordered breathing is associated with a poor prognosis (mortality) in patients with ischemic cardiomyopathy. The understanding of mechanisms linking intermittent hypoxia (IH), the key feature of sleep-disordered breathing, to ischemic cardiomyopathy progression is crucial for identifying specific actionable therapeutic targets. The aims of the present study were (1) to evaluate the impact of IH on the time course evolution of cardiac remodeling and contractile dysfunction in a rat model of ischemic cardiomyopathy; and (2) to determine the impact of IH on sympathetic activity, hypoxia inducible factor-1 activation, and endoplasmic reticulum stress in the time course of ischemic cardiomyopathy progression. METHODS AND RESULTS Ischemic cardiomyopathy was induced by a permanent ligature of the left coronary artery in male Wistar rats (rats with myocardial infarction). Rats with myocardial infarction were then exposed to either IH or normoxia for up to 12 weeks. Cardiac remodeling and function were analyzed by Sirius red and wheat germ agglutinin staining, ultrasonography, and cardiac catheterization. Sympathetic activity was evaluated by spectral analysis of blood pressure variability. Hypoxia-inducible factor-1α activation and burden of endoplasmic reticulum stress were characterized by Western blots. Long-term IH exposure precipitated cardiac remodeling (hypertrophy and interstitial fibrosis) and contractile dysfunction during the time course evolution of ischemic cardiomyopathy in rodents. Among associated mechanisms, we identified the early occurrence and persistence of sympathetic activation, associated with sustained hypoxia-inducible factor-1α expression and a delayed pro-apoptotic endoplasmic reticulum stress. CONCLUSIONS Our data provide the demonstration of the deleterious impact of IH on post-myocardial infarction remodeling and contractile dysfunction. Further studies are needed to evaluate whether targeting sympathetic nervous system or HIF-1 overactivities could limit these effects and improve management of coexisting ischemic cardiomyopathy and sleep-disordered breathing.

Curcumin attenuates hypoxia/reoxygenation‑induced myocardial injury

Curcumin (Cur) has been reported to function as an antioxidant and anti-inflammatory agent and to play a role in anti-atherosclerosis. The present study aimed to explore the protective effect of Cur on hypoxia/reoxygenation (H/R) injury. The morphological changes in H9c2 cardiomyocytes were observed under an inverted microscope. Cell viability was determined by Cell Counting Kit-8 (CCK-8). Lactate dehydrogenase (LDH) level, malondialdehyde (MDA) level and the antioxidant superoxide dismutase (SOD) activity were determined by corresponding kits. Apoptosis and reactive oxygen species (ROS) levels were determined by flow cytometry. Endoplasmic reticulum (ER) stress-related factors, which were examined by quantitative real-time polymerase chain reaction (qPCR) and western blot analysis, included 78-kDa glucose-regulated protein (GRP78) and C/EBP homologous protein (CHOP). Extracellular signal regulating kinase 1/2 (ERK1/2), p38, c-Jun NH2-terminal kinase (JNK) and the phosphorylation levels of key proteins in the mitogen-activated protein kinase (MAPK) signaling pathway were all determined by western blot analysis. Compared to the control group, the cell morphology of the H9c2 cells was obviously altered upon H/R. Cell viability was significantly decreased, while apoptosis was significantly increased by H/R. We also observed that the levels of LDH and MDA were elevated and the activity of SOD was decreased in the H/R group. Notably, LDH, MDA and SOD levels were reversed following treatment with Cur; while apoptosis and ROS levels in the H/R injury group were decreased by Cur. H/R injury-triggered ER stress and the MAPK signaling pathway were suppressed by Cur. These results demonstrated that Cur has a protective effect on cardiomyocytes via suppression of ER stress and the MAPK pathway.

Endoplasmic reticulum stress contributed to β1-adrenoceptor autoantibody-induced reduction of autophagy in cardiomyocytes

Autophagy reduction has been confirmed as an important mechanism in apoptosis induction. Our previous study showed that decreased autophagy induced by β1-adrenoceptor autoantibodies (β1-AAs) enhanced cardiomyocyte apoptosis and contributed to heart failure progression. Endoplasmic reticulum stress (ERS) is known to be an important mechanism in intracellular homeostasis and is closely related to autophagy. However, ERS in β1-AA-induced autophagy dysfunction of cardiomyocytes remains unclear. In this study, we used an active immunization rat model and H9c2 cardiomyocytes to study the role of ERS in β1-AA-induced autophagy. Results showed that prolonged action of β1-AAs significantly reduced the autophagy of myocardial tissues and H9c2 cardiomyocytes, and ERS and its related apoptotic pathways were significantly activated. Moreover, mRFP-GFP-LC3 double-labeled adenoviruses were used to detect cardiomyocyte autophagic flux to confirm that β1-AAs caused a significant decrease in autophagic flux in H9c2 cardiomyocytes. The ERS inhibitor, 4-phenylbutyrate (4-PBA), partially attenuated the β1-AA-induced reduction of cardiomyocyte autophagy, consistent with the effect of the mammalian target of rapamycin inhibitor rapamycin (Rapa). Compared to the pretreatment with 4-PBA or Rapa alone, pretreatment with the combination of 4-PBA and Rapa had a greater effect on attenuating the β1-AA-induced decrease in autophagy and β1-AA-induced apoptosis in cardiomyocytes. This study provides an experimental basis for the role of β1-AAs in the homeostatic maintenance of cardiomyocytes in patients with heart failure with respect to autophagy and ERS.

AQP4-knockout aggravation of isoprenaline-induced myocardial injury is mediated by p66Shc and endoplasmic reticulum stress

Aquaporin 4 (AQP4) is a type of water channel protein that maintains the water balance of cardiomyocytes. However, the physiological role of AQP4 in cardiovascular disease is poorly understood. We wanted to explore whether p66Shc and endoplasmic reticulum stress participates in AQP4 knockout (KO)-mediated cardiac injury. There were two types of mice: AQP4 knockout and wild-type mice. Each type was randomly divided into three groups: Control group, isoprenaline stimulation group (ISO, 1 mg/kg, s.c., 5 days), and apocynin treatment group (APO, 100 mg/kg, p.o., 3 days). H9c2 rat cardiomyocytes were cultured for RNA interference of AQP4. Results showed increased left ventricular weight index and more severe myocardial inflammation were induced in AQP4 knockout mice relative to wild-type mice, accompanied by significantly increased levels of the oxidative stress biomarkers MDA and NOX4. In addition, the expressions of p66Shc, ER stress markers PERK, GRP78 and CHOP and proinflammatory factors such as ET_A , IL6 and TNFα were upregulated in the myocardium of AQP4 knockout mice or AQP4 siRNA treated cardiomyocytes, whereas CASQ2 was downregulated. ISO stimulation aggravated these abnormalities, which were significantly attenuated by apocynin. This study showed that AQP4 knockout mice were susceptible to cardiac injury induced by ISO. The mechanism was closely connected with p66Shc and proinflammatory factors. Endoplasmic reticulum stress was also involved in the pathological process.

NF2 signaling pathway plays a pro-apoptotic role in β-adrenergic receptor stimulated cardiac myocyte apoptosis

β-adrenergic receptor (β-AR) stimulation induces cardiac myocyte apoptosis in vitro and in vivo. Neurofibromin 2 (NF2) is a member of the ezrin/radixin/moesin (ERM) family of proteins. Post-translational modifications such as phosphorylation and sumoylation affect NF2 activity, subcellular localization and function. Here, we tested the hypothesis that β-AR stimulation induces post-translational modifications of NF2, and NF2 plays a pro-apoptotic role in β-AR-stimulated myocyte apoptosis.

Endoplasmic reticulum stress in the heart: insights into mechanisms and drug targets

The endoplasmic reticulum (ER) serves several essential cellular functions including protein synthesis, protein folding, protein translocation, calcium homoeostasis and lipid biosynthesis. Physiological or pathological stimuli, which disrupt ER homoeostasis and disturb its functions, lead to an accumulation of misfolded and unfolded proteins, a condition referred to as ER stress. ER stress triggers the unfolded protein response to restore the homoeostasis of ER, through activating transcriptional and translational pathways. However, prolonged ER stress will lead to cell dysfunction and apoptosis. Recent evidence revealed that ER stress is involved in the development and progression of various heart diseases, such as cardiac hypertrophy, ischaemic heart diseases and heart failure. Therefore, improved understanding of the molecular mechanisms of ER stress in heart disease will help to investigate more potential targets for new therapeutic interventions and drug discovery.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

The Role of β-Adrenergic Overstimulation in the Early Stages of Renal Injury

BACKGROUND/AIMS

To assess the possible contribution of the β-adrenergic overstimulation in early stages of renal injury, the present study evaluated, in rats, the effects of the β-adrenoceptor agonist isoproterenol (ISO) on renal function and morphology, as well as the renal mRNA and protein expression of the NADPH oxidase isoform 4 (Nox 4) and subunit p22phox, endoplasmic reticulum (ER) stress, pro-inflammatory, pro-apoptotic and renin-angiotensin system (RAS) components.

METHODS

Wistar rats received ISO (0.3 mg.kg-1.day-1 s.c.) or vehicle (control) for eight days. At the end of the treatment, food and water intake, urine output and body weight gain were evaluated and renal function studies were performed. Renal tissue was used for the morphological, quantitative PCR and immunohistochemical studies.

RESULTS

ISO did not change metabolic parameters or urine output. However it induced a decrease in renal blood flow and an increase in the filtration fraction. These changes were accompanied by increased cortical mRNA and protein expression for the renal oxidative stress components including Nox 4 and p22phox; ER stress, pro-inflamatory, pro-apoptotic as well as RAS components. ISO also induced a significant increase in medullar renin protein expression.

CONCLUSION

These findings support relevant information regarding the contribution of specific β-adrenergic hyperactivity in early stage of renal injury, indicating the reactive oxygen species, ER stress and intrarenal RAS as important factors in this process.

SIRT1 protects the heart from ER stress-induced cell death through eIF2α deacetylation

Over the past decade, endoplasmic reticulum (ER) stress has emerged as an important mechanism involved in the pathogenesis of cardiovascular diseases including heart failure. Cardiac therapy based on ER stress modulation is viewed as a promising avenue toward effective therapies for the diseased heart. Here, we tested whether sirtuin-1 (SIRT1), a NAD⁺-dependent deacetylase, participates in modulating ER stress response in the heart. Using cardiomyocytes and adult-inducible SIRT1 knockout mice, we demonstrate that SIRT1 inhibition or deficiency increases ER stress-induced cardiac injury, whereas activation of SIRT1 by the SIRT1-activating compound STAC-3 is protective. Analysis of the expression of markers of the three main branches of the unfolded protein response (i.e., PERK/eIF2α, ATF6 and IRE1) showed that SIRT1 protects cardiomyocytes from ER stress-induced apoptosis by attenuating PERK/eIF2α pathway activation. We also present evidence that SIRT1 physically interacts with and deacetylates eIF2α. Mass spectrometry analysis identified lysines K141 and K143 as the acetylation sites on eIF2α targeted by SIRT1. Furthermore, mutation of K143 to arginine to mimic eIF2α deacetylation confers protection against ER stress-induced apoptosis. Collectively, our findings indicate that eIF2α deacetylation on lysine K143 by SIRT1 is a novel regulatory mechanism for protecting cardiac cells from ER stress and suggest that activation of SIRT1 has potential as a therapeutic approach to protect the heart against ER stress-induced injury.

High-intensity training reduces intermittent hypoxia-induced ER stress and myocardial infarct size

Chronic intermittent hypoxia (IH) is described as the major detrimental factor leading to cardiovascular morbimortality in obstructive sleep apnea (OSA) patients. OSA patients exhibit increased infarct size after a myocardial event, and previous animal studies have shown that chronic IH could be the main mechanism. Endoplasmic reticulum (ER) stress plays a major role in the pathophysiology of cardiovascular disease. High-intensity training (HIT) exerts beneficial effects on the cardiovascular system. Thus, we hypothesized that HIT could prevent IH-induced ER stress and the increase in infarct size. Male Wistar rats were exposed to 21 days of IH (21-5% fraction of inspired O2, 60-s cycle, 8 h/day) or normoxia. After 1 wk of IH alone, rats were submitted daily to both IH and HIT (2 × 24 min, 15-30m/min). Rat hearts were either rapidly frozen to evaluate ER stress by Western blot analysis or submitted to an ischemia-reperfusion protocol ex vivo (30 min of global ischemia/120 min of reperfusion). IH induced cardiac proapoptotic ER stress, characterized by increased expression of glucose-regulated protein kinase 78, phosphorylated protein kinase-like ER kinase, activating transcription factor 4, and C/EBP homologous protein. IH-induced myocardial apoptosis was confirmed by increased expression of cleaved caspase-3. These IH-associated proapoptotic alterations were associated with a significant increase in infarct size (35.4 ± 3.2% vs. 22.7 ± 1.7% of ventricles in IH + sedenary and normoxia + sedentary groups, respectively, P < 0.05). HIT prevented both the IH-induced proapoptotic ER stress and increased myocardial infarct size (28.8 ± 3.9% and 21.0 ± 5.1% in IH + HIT and normoxia + HIT groups, respectively, P = 0.28). In conclusion, these findings suggest that HIT could represent a preventive strategy to limit IH-induced myocardial ischemia-reperfusion damages in OSA patients.

Osteopontin: At the cross-roads of myocyte survival and myocardial function

Heart failure represents a major cause of morbidity and mortality in Western society. Cardiac myocyte loss due to apoptosis plays a significant role in the progression of heart failure. The extracellular matrix (ECM) maintains the structural integrity of the heart and allows the transmission of electrical and mechanical signals during cardiac contraction and relaxation. Matricellular proteins, a class of non-structural ECM proteins, play a significant role in ECM homeostasis and intracellular signaling via their interactions with cell surface receptors, structural proteins, and/or soluble extracellular factors such as growth factors and cytokines. Osteopontin (OPN), also called cytokine Eta-1, is a member of the matricellular protein family. The normal heart expresses low levels of OPN. However, OPN expression increases markedly under a variety of pathophysiological conditions of the heart. Many human and transgenic mouse studies provide evidence that increased OPN expression, specifically in myocytes, is associated with increased myocyte apoptosis and myocardial dysfunction. This review summarizes OPN expression in the heart, and its role in myocyte apoptosis and myocardial function.

β-Adrenergic receptor-mediated transactivation of epidermal growth factor receptor decreases cardiomyocyte apoptosis through differential subcellular activation of ERK1/2 and Akt

β-Adrenergic receptor (βAR)-mediated transactivation of epidermal growth factor receptor (EGFR) has been shown to relay pro-survival effects via unknown mechanisms. We hypothesized that acute βAR-mediated EGFR transactivation in the heart promotes differential subcellular activation of ERK1/2 and Akt, promoting cell survival through modulation of apoptosis. C57BL/6 mice underwent acute i.p. injection with isoproterenol (ISO)±AG 1478 (EGFR antagonist) to assess the impact of βAR-mediated EGFR transactivation on the phosphorylation of ERK1/2 (P-ERK1/2) and Akt (P-Akt) in distinct cardiac subcellular fractions. Increased P-ERK1/2 and P-Akt were observed in cytosolic, plasma membrane and nuclear fractions following ISO stimulation. Whereas the P-ERK1/2 response was EGFR-sensitive in all fractions, the P-Akt response was EGFR-sensitive only in the plasma membrane and nucleus, results confirmed in primary rat neonatal cardiomyocytes (RNCM). βAR-mediated EGFR-transactivation also decreased apoptosis in serum-depleted RNCM, as measured via TUNEL as well as caspase 3 activity/cleavage, which were sensitive to the inhibition of either ERK1/2 (PD184352) or Akt (LY-294002) signaling. Caspase 3 activity/cleavage was also sensitive to the inhibition of transcription, which, with an increase in nuclear P-ERK1/2 and P-Akt in response to ISO, suggested that βAR-mediated EGFR transactivation may regulate apoptotic gene transcription. An Apoptosis PCR Array identified tnfsf10 (TRAIL) to be altered by ISO in an EGFR-sensitive manner, results confirmed via RT-PCR and ELISA measurement of both membrane-bound and soluble cardiomyocyte TRAIL levels. βAR-mediated EGFR transactivation induces differential subcellular activation of ERK1/2 and Akt leading to increased cell survival through the modulation of caspase 3 activity and apoptotic gene expression in cardiomyocytes.

The eIF2B-interacting domain of RGS2 protects against GPCR agonist-induced hypertrophy in neonatal rat cardiomyocytes

The protective effect of Regulator of G protein Signaling 2 (RGS2) in cardiac hypertrophy is thought to occur through its ability to inhibit the chronic GPCR signaling that promotes pathogenic growth both in vivo and in cultured cardiomyocytes. However, RGS2 is known to have additional functions beyond its activity as a GTPase accelerating protein, such as the ability to bind to eukaryotic initiation factor, eIF2B, and inhibit protein synthesis. The RGS2 eIF2B-interacting domain (RGS2(eb)) was examined for its ability to regulate hypertrophy in neonatal ventricular myocytes. Both full-length RGS2 and RGS2(eb) were able to inhibit agonist-induced cardiomyocyte hypertrophy, but RGS2(eb) had no effect on receptor-mediated inositol phosphate production, cAMP production, or ERK 1/2 activation. These results suggest that the protective effects of RGS2 in cardiac hypertrophy may derive at least in part from its ability to govern protein synthesis.

Osteopontin stimulates apoptosis in adult cardiac myocytes via the involvement of CD44 receptors, mitochondrial death pathway, and endoplasmic reticulum stress

Increased osteopontin (OPN) expression associates with increased myocyte apoptosis and myocardial dysfunction. The objective of this study was to identify the receptor for OPN and get insight into the mechanism by which OPN induces cardiac myocyte apoptosis. Adult rat ventricular myocytes (ARVMs) and transgenic mice expressing OPN in a myocyte-specific manner were used for in vitro and in vivo studies. Treatment with purified OPN (20 nM) protein or adenoviral-mediated OPN expression induced apoptosis in ARVMs. OPN co-immunoprecipitated with CD44 receptors, not with β1 or β3 integrins. Proximity ligation assay confirmed interaction of OPN with CD44 receptors. Neutralizing anti-CD44 antibodies inhibited OPN-stimulated apoptosis. OPN activated JNKs and increased expression of Bax and levels of cytosolic cytochrome c, suggesting involvement of mitochondrial death pathway. OPN increased endoplasmic reticulum (ER) stress, as evidenced by increased expression of Gadd153 and activation of caspase-12. Inhibition of JNKs using SP600125 or ER stress using salubrinal or caspase-12 inhibitor significantly reduced OPN-stimulated apoptosis. Expression of OPN in adult mouse heart in myocyte-specific manner associated with decreased left ventricular function and increased myocyte apoptosis. In the heart, OPN expression increased JNKs and caspase-12 activities, and expression of Bax and Gadd153. Thus, OPN, acting via CD44 receptors, induces apoptosis in myocytes via the involvement of mitochondrial death pathway and ER stress.

Deficiency of ataxia telangiectasia mutated kinase modulates cardiac remodeling following myocardial infarction: involvement in fibrosis and apoptosis

Ataxia telangiectasia mutated kinase (ATM) is a cell cycle checkpoint protein activated in response to DNA damage. We recently reported that ATM plays a protective role in myocardial remodeling following β-adrenergic receptor stimulation. Here we investigated the role of ATM in cardiac remodeling using myocardial infarction (MI) as a model. Methods and Results: Left ventricular (LV) structure, function, apoptosis, fibrosis, and protein levels of apoptosis- and fibrosis-related proteins were examined in wild-type (WT) and ATM heterozygous knockout (hKO) mice 7 days post-MI. Infarct sizes were similar in both MI groups. However, infarct thickness was higher in hKO-MI group. Two dimensional M-mode echocardiography revealed decreased percent fractional shortening (%FS) and ejection fraction (EF) in both MI groups when compared to their respective sham groups. However, the decrease in %FS and EF was significantly greater in WT-MI vs hKO-MI. LV end systolic and diastolic diameters were greater in WT-MI vs hKO-MI. Fibrosis, apoptosis, and α-smooth muscle actin staining was significantly higher in hKO-MI vs WT-MI. MMP-2 protein levels and activity were increased to a similar extent in the infarct regions of both groups. MMP-9 protein levels were increased in the non-infarct region of WT-MI vs WT-sham. MMP-9 protein levels and activity were significantly lower in the infarct region of WT vs hKO. TIMP-2 protein levels similarly increased in both MI groups, whereas TIMP-4 protein levels were significantly lower in the infarct region of hKO group. Phosphorylation of p53 protein was higher, while protein levels of manganese superoxide dismutase were significantly lower in the infarct region of hKO vs WT. In vitro, inhibition of ATM using KU-55933 increased oxidative stress and apoptosis in cardiac myocytes.

Klotho suppresses cardiomyocyte apoptosis in mice with stress-induced cardiac injury via downregulation of endoplasmic reticulum stress

Cardiomyocyte apoptosis is a common pathological alteration in heart disease which results in systolic dysfunction or sudden death. Klotho is a novel anti-aging hormone. We tested the effects of klotho on cell apoptosis in isoproterenol-treated cardiomyocytes. In BALB/c mice, cardiac injury was induced by subcutaneous injection of isoproterenol (5 mg/kg, for 9 days, s.c.). Klotho (0.01 mg/kg, every other day for 4 days, i.p.) was administered to determine the changes in isoproterenol-induced apoptosis. Mouse heart was harvested at day 2, day 5, and day 9 after isoproterenol injection. Isoproterenol induced cardiac apoptosis and endoplasmic reticulum (ER) stress in a time-dependent manner. However, klotho partly reversed isoproterenol-induced cardiac apoptosis and ER stress. These same effects were observed in cultured cardiomyocytes. Furthermore, the results also showed that SB203580, a p38 inhibitor, and SP600125, a c-Jun NH2-terminal kinase (JNK) inhibitor, reduced cardiomyocyte apoptosis and ER stress, however, klotho suppressed isoproterenol-induced activation of p38 and JNK. Taken together, these results indicated that cardioprotection by klotho was related to the attenuation of ER stress and ER stress-induced apoptosis, at least partly, through suppressing activation of the p38 and JNK pathway.

ER stress-induced cell death mechanisms

The endoplasmic-reticulum (ER) stress response constitutes a cellular process that is triggered by a variety of conditions that disturb folding of proteins in the ER. Eukaryotic cells have developed an evolutionarily conserved adaptive mechanism, the unfolded protein response (UPR), which aims to clear unfolded proteins and restore ER homeostasis. In cases where ER stress cannot be reversed, cellular functions deteriorate, often leading to cell death. Accumulating evidence implicates ER stress-induced cellular dysfunction and cell death as major contributors to many diseases, making modulators of ER stress pathways potentially attractive targets for therapeutics discovery. Here, we summarize recent advances in understanding the diversity of molecular mechanisms that govern ER stress signaling in health and disease. This article is part of a Special Section entitled: Cell Death Pathways.

Xenobiotic perturbation of ER stress and the unfolded protein response

The proper folding, assembly, and maintenance of cellular proteins is a highly regulated process and is critical for cellular homeostasis. Multiple cellular compartments have adapted their own systems to ensure proper protein folding, and quality control mechanisms are in place to manage stress due to the accumulation of unfolded proteins. When the accumulation of unfolded proteins exceeds the capacity to restore homeostasis, these systems can result in a cell death response. Unfolded protein accumulation in the endoplasmic reticulum (ER) leads to ER stress with activation of the unfolded protein response (UPR) governed by the activating transcription factor 6 (ATF6), inositol requiring enzyme-1 (IRE1), and PKR-like endoplasmic reticulum kinase (PERK) signaling pathways. Many xenobiotics have been shown to influence ER stress and UPR signaling with either pro-survival or pro-death features. The ultimate outcome is dependent on many factors including the mechanism of action of the xenobiotic, concentration of xenobiotic, duration of exposure (acute vs. chronic), cell type affected, nutrient levels, oxidative stress, state of differentiation, and others. Assessing perturbations in activation or inhibition of ER stress and UPR signaling pathways are likely to be informative parameters to measure when analyzing mechanisms of action of xenobiotic-induced toxicity.

Salubrinal protects against tunicamycin and hypoxia induced cardiomyocyte apoptosis via the PERK-eIF2α signaling pathway

Objectives

This study examined the protective effect of salubrinal and the mechanism underlying this protection against tunicamycin (TM)- and hypoxia-induced apoptosis in rat cardiomyocytes.

Methods

Neonatal rat cardiomyocytes were cultured from the ventricles of 1-day-old Wistar rats. Cells were exposed to different concentrations of salubrinal (10, 20, and 40 µmol/L) for 30 min followed by TM treatment or hypoxia for 36 h. Apoptosis was measured by a multiparameter HCS (high content screening) apoptosis assay, TUNEL assay and flow cytometry. The phosphorylation of eukaryotic translation initiation factor 2 subunit alpha (eIF2α) and the expression of cleaved caspase-12 were determined by Western blotting. C/EBP homologous protein (CHOP) was detected by immunocytochemistry.

Results

HCS, TUNEL assays and flow cytometry showed that salubrinal protected cardiomyocytes against apoptosis induced by TM or hypoxia. Western blotting showed that salubrinal protected cardiomyocytes against apoptosis by inducing eIF2α phosphorylation and down-regulating the expression of the endoplasmic reticulum stress-mediated apoptotic proteins, CHOP and cleaved caspase-12.

Conclusions

Our study suggests that salubrinal protects rat cardiomyocytes against TM- or hypoxia-associated apoptosis via a mechanism involving the inhibition of ER stress-mediated apoptosis.

New concepts of endoplasmic reticulum function in the heart: programmed to conserve

Secreted and membrane proteins play critical roles in myocardial health and disease. Studies in non-myocytes have shown that the peri-nuclear ER is the site for synthesis, folding, and quality control of most secreted and membrane proteins, as well as a nexus of a signal transduction system, called the ER stress response, which informs the cell about the status of ER protein folding. Moreover, the dynamic physical and functional association of the ER with mitochondria is a key site responsible for integrating ER function and mitochondrial metabolism, but is only just beginning to be understood in the myocardium. Although a great deal is known about roles played by the sarcoplasmic reticulum (SR) in contractile calcium handling in the heart, little is known about the relative locations and functions of the peri-nuclear ER and the SR in terms of secreted and membrane protein synthesis and folding. In this review we will explore the current state of knowledge of the location of secreted and membrane protein synthesis, folding, and quality control machinery in cardiac myocytes, as well as our understanding of the functional consequences of ER stress and the unfolded protein response in the heart in terms of protein synthesis, cell growth, and metabolic regulation. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism".

Pharmacological modulation of the retinal unfolded protein response in Bardet-Biedl syndrome reduces apoptosis and preserves light detection ability

Ciliopathies, a class of rare genetic disorders, present often with retinal degeneration caused by protein transport defects between the inner segment and the outer segment of the photoreceptors. Bardet-Biedl syndrome is one such ciliopathy, genetically heterogeneous with 17 BBS genes identified to date, presenting early onset retinitis pigmentosa. By investigating BBS12-deprived retinal explants and the Bbs12(-/-) murine model, we show that the impaired intraciliary transport results in protein retention in the endoplasmic reticulum. The protein overload activates a proapoptotic unfolded protein response leading to a specific Caspase12-mediated death of the photoreceptors. Having identified a therapeutic window in the early postnatal retinal development and through optimized pharmacological modulation of the unfolded protein response, combining three specific compounds, namely valproic acid, guanabenz, and a specific Caspase12 inhibitor, achieved efficient photoreceptor protection, thereby maintaining light detection ability in vivo.