The ubiquitin-proteasome system in myocardial ischaemia and preconditioning

Phosphorylation Modifications Regulating Cardiac Protein Quality Control Mechanisms

Many forms of cardiac disease, including heart failure, present with inadequate protein quality control (PQC). Pathological conditions often involve impaired removal of terminally misfolded proteins. This results in the formation of large protein aggregates, which further reduce cellular viability and cardiac function. Cardiomyocytes have an intricately collaborative PQC system to minimize cellular proteotoxicity. Increased expression of chaperones or enhanced clearance of misfolded proteins either by the proteasome or lysosome has been demonstrated to attenuate disease pathogenesis, whereas reduced PQC exacerbates pathogenesis. Recent studies have revealed that phosphorylation of key proteins has a potent regulatory role, both promoting and hindering the PQC machinery. This review highlights the recent advances in phosphorylations regulating PQC, the impact in cardiac pathology, and the therapeutic opportunities presented by harnessing these modifications.

Targeting Protein Kinase G to Treat Cardiac Proteotoxicity

Impaired or insufficient protein kinase G (PKG) signaling and protein quality control (PQC) are hallmarks of most forms of cardiac disease, including heart failure. Their dysregulation has been shown to contribute to and exacerbate cardiac hypertrophy and remodeling, reduced cell survival and disease pathogenesis. Enhancement of PKG signaling and PQC are associated with improved cardiac function and survival in many pre-clinical models of heart disease. While many clinically used pharmacological approaches exist to stimulate PKG, there are no FDA-approved therapies to safely enhance cardiomyocyte PQC. The latter is predominantly due to our lack of knowledge and identification of proteins regulating cardiomyocyte PQC. Recently, multiple studies have demonstrated that PKG regulates PQC in the heart, both during physiological and pathological states. These studies tested already FDA-approved pharmacological therapies to activate PKG, which enhanced cardiomyocyte PQC and alleviated cardiac disease. This review examines the roles of PKG and PQC during disease pathogenesis and summarizes the experimental and clinical data supporting the utility of stimulating PKG to target cardiac proteotoxicity.

Deficiency of the Immunoproteasome LMP10 Subunit Attenuates Angiotensin II-Induced Cardiac Hypertrophic Remodeling via Autophagic Degradation of gp130 and IGF1R

Background/Aim

Hypertensive cardiac hypertrophy is the leading cause of cardiac remodeling and heart failure. We recently demonstrated that the immunoproteasome, an inducible form of the constitutive proteasome, plays a critical role in regulating cardiovascular diseases. However, the role of the immunoproteasome LMP10 (β2i) catalytic subunit in the regulation of angiotensin II (Ang II)-induced cardiac hypertrophic remodeling remains unclear.

Methods

Wild-type (WT) and LMP10 knockout (KO) mice were infused with Ang II 1,000 ng/kg/min for 2 weeks. Blood pressure was measured using a tail-cuff system. Cardiac function and hypertrophic remodeling were examined by echocardiography and histological staining. The expression levels of genes and proteins were examined with quantitative real-time PCR and immunoblotting analysis, respectively.

Results

LMP10 mRNA and protein expression was significantly increased in Ang II-stimulated hearts and primary cardiomyocytes. Moreover, Ang II infusion for 2 weeks increased systolic blood pressure, abnormal cardiac function, hypertrophy, fibrosis, and inflammation in WT mice, which were significantly reversed in KO mice. Moreover, a marked reduction in the protein levels of insulin growth factor-1 receptor (IGF1R), glycoprotein 130 (gp130), and phosphorylated AKT, mTOR, STAT3, and ERK1/2 and an increase in the LC3II/I ratio were also observed in LMP10 KO mice compared with WT mice after Ang II infusion. In vitro culture experiments confirmed that LMP10 knockdown activated autophagy and increased IGF1R and gp130 degradation, leading to the inhibition of cardiomyocyte hypertrophy. However, inhibiting autophagy with chloroquine reversed this effect.

Conclusion

The results of this study indicate that LMP10 KO attenuates Ang II-induced cardiac hypertrophic remodeling via the autophagy-dependent degradation of IGF1R and gp130, and suggests that LMP10 may be a novel therapeutic target for hypertrophic heart diseases.

Proteostasis in cardiac health and disease

The incidence and prevalence of cardiac diseases, which are the main cause of death worldwide, are likely to increase because of population ageing. Prevailing theories about the mechanisms of ageing feature the gradual derailment of cellular protein homeostasis (proteostasis) and loss of protein quality control as central factors. In the heart, loss of protein patency, owing to flaws in genetically-determined design or because of environmentally-induced 'wear and tear', can overwhelm protein quality control, thereby triggering derailment of proteostasis and contributing to cardiac ageing. Failure of protein quality control involves impairment of chaperones, ubiquitin-proteosomal systems, autophagy, and loss of sarcomeric and cytoskeletal proteins, all of which relate to induction of cardiomyocyte senescence. Targeting protein quality control to maintain cardiac proteostasis offers a novel therapeutic strategy to promote cardiac health and combat cardiac disease. Currently marketed drugs are available to explore this concept in the clinical setting.

Therapeutic Effects of Ischemic-Preconditioned Exosomes in Cardiovascular Diseases

Despite years of researches, cardiovascular disease (CVD) remains the most common cause of death around the world. Lots of studies showed that by pretreating with short nonfatal ischemia in in situ organ or distant organ, one could develop tolerance to the following fatal ischemia. The process is called ischemic preconditioning (IPC). IPC prepare the heart for damage by producing inflammatory signals, miRNA, neuro system stimulation and exosomes. Among them, exosomes have been gaining increasing interest since it is characterized by its capability to carry information and its specific ligand-receptor system. Here we will discuss IPC induced exosomes and its protective effects during ischemic heart disease.

Chapter Three - Ubiquitination and Protein Turnover of G-Protein-Coupled Receptor Kinases in GPCR Signaling and Cellular Regulation

G-protein-coupled receptors (GPCRs) are responsible for regulating a wide variety of physiological processes, and distinct mechanisms for GPCR inactivation exist to guarantee correct receptor functionality. One of the widely used mechanisms is receptor phosphorylation by specific G-protein-coupled receptor kinases (GRKs), leading to uncoupling from G proteins (desensitization) and receptor internalization. GRKs and β-arrestins also participate in the assembly of receptor-associated multimolecular complexes, thus initiating alternative G-protein-independent signaling events. In addition, the abundant GRK2 kinase has diverse "effector" functions in cellular migration, proliferation, and metabolism homeostasis by means of the phosphorylation or interaction with non-GPCR partners. Altered expression of GRKs (particularly of GRK2 and GRK5) occurs during pathological conditions characterized by impaired GPCR signaling including inflammatory syndromes, cardiovascular disease, and tumor contexts. It is increasingly appreciated that different pathways governing GRK protein stability play a role in the modulation of kinase levels in normal and pathological conditions. Thus, enhanced GRK2 degradation by the proteasome pathway occurs upon GPCR stimulation, what allows cellular adaptation to chronic stimulation in a physiological setting. β-arrestins participate in this process by facilitating GRK2 phosphorylation by different kinases and by recruiting diverse E3 ubiquitin ligase to the receptor complex. Different proteolytic systems (ubiquitin-proteasome, calpains), chaperone activities and signaling pathways influence the stability of GRKs in different ways, thus endowing specificity to GPCR regulation as protein turnover of GRKs can be differentially affected. Therefore, modulation of protein stability of GRKs emerges as a versatile mechanism for feedback regulation of GPCR signaling and basic cellular processes.

Quantitative Proteomic Analysis of Differentially Expressed Proteins and Downstream Signaling Pathways in Chronic Bladder Ischemia

PURPOSE

Growing evidence suggests that ischemia may contribute to aging associated bladder dysfunction and lower urinary tract symptoms. Our goal was to determine the effects of chronic ischemia on bladder proteomic profiles and characterize downstream signaling pathways.

MATERIALS AND METHODS

Bilateral iliac artery atherosclerosis and chronic bladder ischemia were created in male Sprague Dawley® rats. At 8 weeks cystometrograms were obtained. Ischemic and control bladder tissues were then processed for label-free quantitative proteomic analysis. GO (Gene Ontology) and IPA (Ingenuity® Pathway Analysis) software were used to classify altered proteins in bladder ischemia. Western blot was done to confirm differentially expressed proteins. Tissue structure was examined by transmission electron microscopy.

RESULTS

Chronic ischemia resulted in detrusor instability and noncompliance. Proteomic analysis revealed a total of 4,277 proteins in ischemic and 4,602 in control bladder tissues. In ischemic bladders 359 and 66 proteins were differentially expressed with a greater than twofold and fivefold change, respectively. On GO analysis differentially expressed proteins were associated with molecular signaling mechanisms underlying proteolysis and degenerative processes. Pathway and network analysis of ischemic tissues suggested that altered proteins are involved in ubiquitination, Nrf2 mediated oxidative stress response, cell death, glucose metabolism and cytoskeleton remodeling. Western blot verified changes in 4 representative proteins, including Nedd4l, Mpo, Ca3 and Fkbp5. Altered proteomic profile of the bladder was associated with widespread ultrastructural damage.

CONCLUSIONS

Alterations of bladder proteomic profiles in ischemia may provide new insight into molecular pathways underlying bladder dysfunction and lower urinary tract symptoms in pelvic atherosclerosis.

Integrated microRNA and mRNA responses to acute human left ventricular ischemia

MicroRNAs (miRNAs) play a significant role in ischemic heart disease. Animal models of left ventricular (LV) ischemia demonstrate a unique miRNA profile; however, these models have limitations in describing human disease. In this study, we performed next-generation miRNA and mRNA sequencing on LV tissue from nine patients undergoing cardiac surgery with cardiopulmonary bypass and cardioplegic arrest. Samples were obtained immediately after aortic cross clamping (baseline) and before aortic cross clamp removal (postischemic). Of 1,237 identified miRNAs, 21 were differentially expressed between baseline and postischemic LV samples including the upregulated miRNAs miR-339-5p and miR-483-3p and the downregulated miRNA miR-139-5p. Target prediction analysis of these miRNAs was integrated with mRNA expression from the same LV samples to identify anticorrelated miRNA-mRNA pairs. Gene enrichment studies of candidate mRNA targets demonstrated an association with cardiovascular disease, cell death, and metabolism. Therapeutics that intervene on these miRNAs and their downstream targets may lead to novel mechanisms of mitigating the damage caused by ischemic insults on the human heart.

Targeting the ubiquitin-proteasome system in heart disease: the basis for new therapeutic strategies

SIGNIFICANCE

Novel therapeutic strategies to treat heart failure are greatly needed. The ubiquitin-proteasome system (UPS) affects the structure and function of cardiac cells through targeted degradation of signaling and structural proteins. This review discusses both beneficial and detrimental consequences of modulating the UPS in the heart.

RECENT ADVANCES

Proteasome inhibitors were first used to test the role of the UPS in cardiac disease phenotypes, indicating therapeutic potential. In early cardiac remodeling and pathological hypertrophy with increased proteasome activities, proteasome inhibition prevented or restricted disease progression and contractile dysfunction. Conversely, enhancing proteasome activities by genetic manipulation, pharmacological intervention, or ischemic preconditioning also improved the outcome of cardiomyopathies and infarcted hearts with impaired cardiac and UPS function, which is, at least in part, caused by oxidative damage.

CRITICAL ISSUES

An understanding of the UPS status and the underlying mechanisms for its potential deregulation in cardiac disease is critical for targeted interventions. Several studies indicate that type and stage of cardiac disease influence the dynamics of UPS regulation in a nonlinear and multifactorial manner. Proteasome inhibitors targeting all proteasome complexes are associated with cardiotoxicity in humans. Furthermore, the type and dosage of proteasome inhibitor impact the pathogenesis in nonuniform ways.

FUTURE DIRECTIONS

Systematic analysis and targeting of individual UPS components with established and innovative tools will unravel and discriminate regulatory mechanisms that contribute to and protect against the progression of cardiac disease. Integrating this knowledge in drug design may reduce adverse effects on the heart as observed in patients treated with proteasome inhibitors against noncardiac diseases, especially cancer.

Fasting protects liver from ischemic injury through Sirt1-mediated downregulation of circulating HMGB1 in mice

BACKGROUND & AIMS

Fasting and calorie restriction are associated with a prolonged life span and an increased resistance to stress. The protective effects of fasting have been exploited for the mitigation of ischemic organ injury, yet the underlying mechanisms remain incompletely understood. Here, we investigated whether fasting protects liver against ischemia reperfusion (IR) through energy-preserving or anti-inflammatory mechanisms.

METHODS

Fasted C57BL6 mice were subjected to partial hepatic IR. Injury was assessed by liver enzymes and histology. Raw264-7 macrophage-like cells were investigated in vitro. Sirt1 and HMGB1 were inhibited using Ex527 and neutralizing antibodies, respectively.

RESULTS

Fasting for one, but not two or three days, protected from hepatic IR injury. None of the investigated energy parameters correlated with the protective effects. Instead, inflammatory responses were dampened in one-day-fasted mice and in starved macrophages. Fasting alone led to a reduction in circulating HMGB1 associated with cytoplasmic HMGB1 translocation, aggregate formation, and autophagy. Inhibition of autophagy re-elevated circulating HMGB1 and abolished protection in fasted mice, as did supplementation with HMGB1. In vitro, Sirt1 inhibition prevented HMGB1 translocation, leading to elevated HMGB1 in the supernatant. In vivo, Sirt1 inhibition abrogated the fasting-induced protection, but had no effect in the presence of neutralizing HMGB1 antibody.

CONCLUSIONS

Fasting for one day protects from hepatic IR injury via Sirt1-dependent downregulation of circulating HMGB1. The reduction in serum HMGB1 appears to be mediated by its engagement in the autophagic response. These findings integrate Sirt1, HMGB1, and autophagy into a common framework that underlies the anti-inflammatory properties of short-term fasting.

Regulation of cardiac proteasomes by ubiquitination, SUMOylation, and beyond

The ubiquitin-proteasome system (UPS) is the major intracellular degradation system, and its proper function is critical to the health and function of cardiac cells. Alterations in cardiac proteasomes have been linked to several pathological phenotypes, including cardiomyopathies, ischemia-reperfusion injury, heart failure, and hypertrophy. Defects in proteasome-dependent cellular protein homeostasis can be causal for the initiation and progression of certain cardiovascular diseases. Emerging evidence suggests that the UPS can specifically target proteins that govern pathological signaling pathways for degradation, thus altering downstream effectors and disease outcomes. Alterations in UPS-substrate interactions in disease occur, in part, due to direct modifications of 19S, 11S or 20S proteasome subunits. Post-translational modifications (PTMs) are one facet of this proteasomal regulation, with over 400 known phosphorylation sites, over 500 ubiquitination sites and 83 internal lysine acetylation sites, as well as multiple sites for caspase cleavage, glycosylation (such as O-GlcNAc modification), methylation, nitrosylation, oxidation, and SUMOylation. Changes in cardiac proteasome PTMs, which occur in ischemia and cardiomyopathies, are associated with changes in proteasome activity and proteasome assembly; however several features of this regulation remain to be explored. In this review, we focus on how some of the less common PTMs affect proteasome function and alter cellular protein homeostasis. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

Upregulation of proteasome activity rescues cardiomyocytes following pulse treatment with a proteasome inhibitor

This study examined the hypothesis that cardiomyocyte metabolism is inherently linked to the Ubiquitin Proteasome System. Rat neonatal ventricular cardiomyocytes were pulse-treated with 5 αM lactacystin for 30 min, resulting in 95% loss of proteasome activity, and then maintained in culture for up to 24 h. Pulse-treatment resulted in 36% decrease in cardiomyocyte mitochondrial reductase activity by 8 h which improved to 15% by 24 h. Bax proteins were increased 2.5-fold by 8 h but declined by 16 h. Similar effects were observed for ubiquitinated proteins suggesting recovery of proteasome function. Proteasome activity started to increase by 4 h and was back to baseline by 16 h. Multiple proteasome subunits, including α1, were upregulated with peak 2 to 2.5-fold increased protein levels at 8-16 h post-lactacystin which then declined. Incubating cardiomyocytes with 4 αM morpholino-antisense oligonucleotides to the α1-subunit for up to 24 h post-lactacystin diminished recovery of proteasome activity (45% at 24 h) and prevented the increase in α1 protein levels. Ubiquitinated proteins remained elevated and cardiomyocyte mitochondrial reductase activity was decreased 35% by 16 h. These results show that diminished function of the ubiquitin proteasome system decreases cardiomyocyte metabolism. If proteasome activity recovers, function improves, but preventing recovery diminishes metabolic function supporting the hypothesis that cardiomyocyte metabolism is inherently linked to the ubiquitin proteasome system.

Hypoxia-inducible factor-1α and vascular endothelial growth factor in the cardioprotective effects of intermittent hypoxia in rats

OBJECTIVE

This study investigated the effects of short-term intermittent hypoxia (IH) preconditioning on cardiac structure and function in rats and the influence of ischemia reperfusion (I/R) injury. Special attention was then paid to the involvement of hypoxia-inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF).

METHODS

Wistar rats were given IH treatment for 1, 7, 14, or 28 days. Some of them were thereafter subject to myocardial infarction surgery. Right ventricle systolic pressure (RVSP), myocardial capillary density (CD), and mRNA/protein expression of HIF-1α, VEGF, and Bcl-2 in rat myocardial tissue were determined. Apoptotic cell number was determined by TUNEL staining, and concentrations of malondialdehyde (MDA) and superoxide dismutase (SOD) were measured.

RESULTS

IH treatment for 1, 7, 14, and 28 days reduced the myocardial infarction size, whereas IH for 28 days increased the RVSP, ratio of right to left ventricle weight (RV/LV+S), and CD. IH up-regulated the mRNA and protein levels of HIF-1α, VEGF, and Bcl-2 both under normal and I/R conditions. The induced expression of HIF-1α and VEGF by IH reached a peak after 7 days of treatment. Moreover, IH for 28 days induced cardiomyocyte apoptosis, whereas prior treatment with IH for 1, 7, 14, and 28 days all markedly attenuated the apoptosis effected by the subsequent I/R injury. IH also decreased the concentrations of MDA but increased those of SOD in myocardial tissue of both in normal rats and following I/R.

CONCLUSIONS

The present study demonstrates that short-term IH protects the heart from I/R injury through inhibiting apoptosis and oxidative stress. The up-regulation of HIF-1α and VEGF by short-term IH may participate in the cardioprotective effect of IH.

Role of various proteases in cardiac remodeling and progression of heart failure

It is believed that cardiac remodeling due to geometric and structural changes is a major mechanism for the progression of heart failure in different pathologies including hypertension, hypertrophic cardiomyopathy, dilated cardiomyopathy, diabetic cardiomyopathy, and myocardial infarction. Increases in the activities of proteolytic enzymes such as matrix metalloproteinases, calpains, cathepsins, and caspases contribute to the process of cardiac remodeling. In addition to modifying the extracellular matrix, both matrix metalloproteinases and cathepsins have been shown to affect the activities of subcellular organelles in cardiomyocytes. The activation of calpains and caspases has been identified to induce subcellular remodeling in failing hearts. Proteolytic activities associated with different proteins including caspases, calpain, and the ubiquitin-proteasome system have been shown to be involved in cardiomyocyte apoptosis, which is an integral part of cardiac remodeling. This article discusses and compares how the activities of various proteases are involved in different cardiac abnormalities with respect to alterations in apoptotic pathways, cardiac remodeling, and cardiac dysfunction. An imbalance appears to occur between the activities of some proteases and their endogenous inhibitors in various types of hypertrophied and failing hearts, and this is likely to further accentuate subcellular remodeling and cardiac dysfunction. The importance of inhibiting the activities of both extracellular and intracellular proteases specific to distinct etiologies, in attenuating cardiac remodeling and apoptosis as well as biochemical changes of subcellular organelles, in heart failure has been emphasized. It is suggested that combination therapy to inhibit different proteases may prove useful for the treatment of heart failure.

Impaired assembly and post-translational regulation of 26S proteasome in human end-stage heart failure

BACKGROUND

This study examined the hypothesis that 26S proteasome dysfunction in human end-stage heart failure is associated with decreased docking of the 19S regulatory particle to the 20S proteasome. Previous studies have demonstrated that 26S proteasome activity is diminished in human end-stage heart failure associated with oxidation of the 19S regulatory particle Rpt5 subunit. Docking of the 19S regulatory particle to the 20S proteasome requires functional Rpt subunit ATPase activity and phosphorylation of the α-type subunits.

METHODS AND RESULTS

An enriched proteasome fraction was prepared from 7 human nonfailing and 10 failing heart explants. Native gel electrophoresis assessed docking of 19S to 20S proteasome revealing 3 proteasome populations (20S, 26S, and 30S proteasomes). In failing hearts, 30S proteasomes were significantly lower (P=0.048) by 37% suggesting diminished docking. Mass spectrometry-based phosphopeptide analysis demonstrated that the relative ratio of phosphorylated:non phosphorylated α7 subunit (serine250) of the 20S proteasome was significantly less (P=0.011) by almost 80% in failing hearts. Rpt ATPase activity was determined in the enriched fraction and after immunoprecipitation with an Rpt6 antibody. ATPase activity (ρmol PO4/μg protein per hour) of the total fraction was lowered from 291±97 to 194±27 and in the immunoprecipitated fraction from 42±12 to 3±2 (P=0.005) in failing hearts.

CONCLUSIONS

These studies suggest that diminished 26S activity in failing human hearts may be related to impaired docking of the 19S to the 20S as a result of decreased Rpt subunit ATPase activity and α7 subunit phosphorylation.

The ubiquitin proteasome system in human cardiomyopathies and heart failure

Maintenance of protein quality control is a critical function of the ubiquitin proteasome system (UPS). Evidence is rapidly mounting to link proteasome dysfunction with a multitude of cardiac diseases, including ischemia, reperfusion, atherosclerosis, hypertrophy, heart failure, and cardiomyopathies. Recent studies have demonstrated a remarkable level of complexity in the regulation of the UPS in the heart and suggest that our understanding of how UPS dysfunction might contribute to the pathophysiology of such a wide range of cardiac afflictions is still very limited. Whereas experimental systems, including animal models, are invaluable for exploring mechanisms and establishing pathogenicity of UPS dysfunction in cardiac disease, studies using human heart tissue provide a vital adjunct for establishing clinical relevance of experimental findings and promoting new hypotheses. Accordingly, this review will focus on UPS dysfunction in human dilated and hypertrophic cardiomyopathies and highlight areas rich for further study in this expanding field.

The ubiquitin proteasome system and myocardial ischemia

The ubiquitin proteasome system (UPS) has been the subject of intensive research over the past 20 years to define its role in normal physiology and in pathophysiology. Many of these studies have focused in on the cardiovascular system and have determined that the UPS becomes dysfunctional in several pathologies such as familial and idiopathic cardiomyopathies, atherosclerosis, and myocardial ischemia. This review presents a synopsis of the literature as it relates to the role of the UPS in myocardial ischemia. Studies have shown that the UPS is dysfunctional during myocardial ischemia, and recent studies have shed some light on possible mechanisms. Other studies have defined a role for the UPS in ischemic preconditioning which is best associated with myocardial ischemia and is thus presented here. Very recent studies have started to define roles for specific proteasome subunits and components of the ubiquitination machinery in various aspects of myocardial ischemia. Lastly, despite the evidence linking myocardial ischemia and proteasome dysfunction, there are continuing suggestions that proteasome inhibitors may be useful to mitigate ischemic injury. This review presents the rationale behind this and discusses both supportive and nonsupportive studies and presents possible future directions that may help in clarifying this controversy.

Identification of seven water-soluble non-storage proteins from pomegranate (Punica granatum Linn.) seeds

As pomegranate ( Punica granatum Linn.) processing is fast growing, the usage of pomegranate processing wastes containing seeds has been receiving great attention. The protein component accounts for 100–130 g/kg of the seeds in weight. However, so far, there is no information on the composition and function of the pomegranate seed proteins. In this study, a global view of water-soluble non-storage proteins isolated from mature pomegranate seeds were studied using two-dimensional polyacrylamide gel electrophoresis coupled with liquid chromatography–tandem mass spectrometry. With the two-dimensional polyacrylamide gel electrophoresis approach, over 120 protein spots were resolved, of which 7 abundant protein spots showing low molecular mass were identified. These identified proteins may be linked to seed development and metabolism, but more importantly, the occurrence of these proteins provides the possibility of conversion the pomegranate processing wastes into useful products or raw material for food industry.

Proteolytic Potential of the MSC Exosome Proteome: Implications for an Exosome-Mediated Delivery of Therapeutic Proteasome

Mesenchymal stem cells (MSCs) are used in many of the current stem cell-based clinical trials and their therapeutic efficacy has increasingly been attributed to secretion of paracrine factors. We have previously demonstrated that a therapeutic constituent of this secretion is exosome, a secreted bilipid membrane vesicle of ~50-100 nm with a complex cargo that is readily internalized by H9C2 cardiomyocytes. It reduces infarct size in a mouse model of myocardial ischemia/reperfusion (MI/R) injury. We postulate that this therapeutic efficacy is derived from the synergy of a select permutation of individual exosome components. To identify protein candidates in this permutation, the proteome was profiled and here we identified 20S proteasome as a protein candidate. Mass spectrometry analysis detected all seven α and seven β chains of the 20S proteasome, and also the three beta subunits of "immunoproteasome" with a very high confidence level. We demonstrated that a functional proteasome copurified with MSC exosomes with a density of 1.10-1.18 g/mL, and its presence correlated with a modest but significant reduction in oligomerized protein in a mouse model of myocardial infarction. Circulating proteasomes in human blood also copurified with exosomes. Therefore, 20S proteasome is a candidate exosome protein that could synergize with other constituents to ameliorate tissue damage.

Genetically induced moderate inhibition of the proteasome in cardiomyocytes exacerbates myocardial ischemia-reperfusion injury in mice

RATIONALE

Both cardiomyocyte-restricted proteasome functional enhancement and pharmacological proteasome inhibition (PSMI) were shown to attenuate myocardial ischemia/reperfusion (I/R) injury. The role of cardiac proteasome dysfunction during I/R and the perspective to diminish I/R injury by manipulating proteasome function remain unclear.

OBJECTIVES

We sought to determine proteasome adequacy in I/R hearts, create a mouse model of cardiomyocyte-restricted PSMI (CR-PSMI), and test CR-PSMI impact on I/R injury.

METHODS AND RESULTS

Myocardial I/R were modeled by ligation (30 minutes) and subsequent release of the left anterior descending artery in mice overexpressing GFPdgn, a validated surrogate proteasome substrate. At 24 hours of reperfusion, myocardial proteasome activities were significantly lower whereas total ubiquitin conjugates and GFPdgn protein levels were markedly higher in all regions of the I/R hearts than the sham controls, indicative of proteasome functional insufficiency. CR-PSMI in intact mice was achieved by transgenic (tg) overexpression of a peptidase-disabled mouse β5 subunit (T60A-β5) driven by an attenuated mouse mhc6 promoter. Overexpressed T60A-β5 can replace endogenous β5 and inhibits proteasome chymotrypsin-like activities in the heart. Mice with moderate CR-PSMI showed no abnormalities at the baseline but displayed markedly more pronounced structural and functional damage during I/R, compared with non-tg littermates. The exacerbation of I/R injury by moderate CR-PSMI was associated with significant increases in the protein level of PTEN and protein kinase Cδ (PKCδ), decreased Akt activation, and reduced PKCε.

CONCLUSIONS

Myocardial I/R causes proteasome functional insufficiency in cardiomyocytes and moderate CR-PSMI augments PTEN and PKCδ, suppresses Akt and PKCε, increases cardiomyocyte apoptosis, and aggravates I/R injury in mice.

Post-translational modification of cardiac proteasomes: functional delineation enabled by proteomics

Proteasomes are ubiquitously expressed multicatalytic complexes that serve as key regulators of protein homeostasis. There are several lines of evidence indicating that proteasomes exist in heterogeneous subpopulations in cardiac muscle, differentiated, in part, by post-translational modifications (PTMs). PTMs regulate numerous facets of proteasome function, including catalytic activities, complex assembly, interactions with associating partners, subcellular localization, substrate preference, and complex turnover. Classical technologies used to identify PTMs on proteasomes have lacked the ability to determine site specificity, quantify stoichiometry, and perform large-scale, multi-PTM analysis. Recent advancements in proteomic technologies have largely overcome these limitations. We present here a discussion on the importance of PTMs in modulating proteasome function in cardiac physiology and pathophysiology, followed by the presentation of a state-of-the-art proteomic workflow for identifying and quantifying PTMs of cardiac proteasomes.

The ubiquitin-proteasome system and cardiovascular disease

Over the past decade, the role of the ubiquitin-proteasome system (UPS) has been the subject of numerous studies to elucidate its role in cardiovascular physiology and pathophysiology. There have been many advances in this field including the use of proteomics to achieve a better understanding of how the cardiac proteasome is regulated. Moreover, improved methods for the assessment of UPS function and the development of genetic models to study the role of the UPS have led to the realization that often the function of this system deviates from the norm in many cardiovascular pathologies. Hence, dysfunction has been described in atherosclerosis, familial cardiac proteinopathies, idiopathic dilated cardiomyopathies, and myocardial ischemia. This has led to numerous studies of the ubiquitin protein (E3) ligases and their roles in cardiac physiology and pathophysiology. This has also led to the controversial proposition of treating atherosclerosis, cardiac hypertrophy, and myocardial ischemia with proteasome inhibitors. Furthering our knowledge of this system may help in the development of new UPS-based therapeutic modalities for mitigation of cardiovascular disease.

Heterogeneous cardiac proteasomes: mandated by diverse substrates?

Proteasome biology has taken central stage in cardiac physiology and pathophysiology. The molecular heterogeneity of proteasome subpopulations supports the specificity of proteasome function to degrade diverse substrate repertoires. Unveiling the dynamics of proteasome function should inspire new therapeutic strategies for combating cardiac disease.

Possible involvement of ubiquitin proteasome system and other proteases in acute and delayed aspects of ischemic preconditioning of brain in mice

The present study has been designed to investigate the potential role of ubiquitin proteasome system and other proteases in acute as well as delayed aspects of ischemic preconditioning induced reversal of ischemia-reperfusion injury in mouse brain. Bilateral carotid artery occlusion of 17 min followed by reperfusion for 24 h was employed in present study to produce ischemia and reperfusion induced cerebral injury in mice. Cerebral infarct size was measured using triphenyltetrazolium chloride staining. Memory was evaluated using elevated plus-maze test. Rota rod test was employed to assess motor incoordination. Bilateral carotid artery occlusion followed by reperfusion produced cerebral infarction and impaired memory and motor co-ordination. Three preceding episodes of bilateral carotid artery occlusion for 1 min and reperfusion of 1 min (ischemic preconditioning) both immediately before (for acute preconditioning) and 24 h before (for delayed preconditioning) global cerebral ischemia prevented markedly ischemia-reperfusion-induced cerebral injury as measured in terms of infarct size, loss of memory and motor coordination. Z-Leu-Leu-Phe-Chinese hamster ovary (CHO) (2 mg/kg, intraperitoneally (i.p.)), an inhibitor of ubiquitin proteasome system and other proteases attenuated the neuroprotective effect of both the acute as well as delayed ischemic preconditioning. It is concluded that the neuroprotective effect of both the acute as well as delayed phases of ischemic preconditioning may be due to the activation of ubiquitin proteasome system and other proteases.

The role of the proteasome in heart disease

Intensive investigations into the pathophysiological significance of the proteasome in the heart did not start until the beginning of the past decade but exciting progress has been made and summarized here as two fronts. First, strong evidence continues to emerge to support a novel hypothesis that proteasome functional insufficiency represents a common pathological phenomenon in a large subset of heart disease, compromises protein quality control in heart muscle cells, and thereby acts as a major pathogenic factor promoting the progression of the subset of heart disease to congestive heart failure. This front is represented by the studies on the ubiquitin-proteasome system (UPS) in cardiac proteinopathy, which have taken advantage of a transgenic mouse model expressing a fluorescence reporter for UPS proteolytic function. Second, pharmacological inhibition of the proteasome has been explored experimentally as a potential therapeutic strategy to intervene on some forms of heart disease, such as pressure-overload cardiac hypertrophy, viral myocarditis, and myocardial ischemic injury. Not only between the two fronts but also within each one, a multitude of inconsistencies and controversies remain to be explained and clarified. At present, the controversy perhaps reflects the sophistication of cardiac proteasomes in terms of the composition, assembly, and regulation, as well as the intricacy and diversity of heart disease in terms of its etiology and pathogenesis. A definitive role of altered proteasome function in the development of various forms of heart disease remains to be established. This article is part of a Special Issue entitled The 26S Proteasome: When degradation is just not enough!

Modulation of Na(+)-K(+)-ATPase cell surface abundance through structural determinants on the α1-subunit

Through their ion-pumping and non-ion-pumping functions, Na(+)-K(+)-ATPase protein complexes at the plasma membrane are critical to intracellular homeostasis and to the physiological and pharmacological actions of cardiotonic steroids. Alteration of the abundance of Na(+)-K(+)-ATPase units at the cell surface is one of the mechanisms for Na(+)-K(+)-ATPase regulation in health and diseases that has been closely examined over the past few decades. We here summarize these findings, with emphasis on studies that explicitly tested the involvement of defined regions or residues on the Na(+)-K(+)-ATPase α1 polypeptide. We also report new findings on the effect of manipulating Na(+)-K(+)-ATPase membrane abundance by targeting one of these defined regions: a dileucine motif of the form [D/E]XXXL[L/I]. In this study, opossum kidney cells stably expressing rat α1 Na(+)-K(+)-ATPase or a mutant where the motif was disrupted (α1-L499V) were exposed to 30 min of substrate/coverslip-induced-ischemia followed by reperfusion (I-R). Biotinylation studies suggested that I-R itself acted as an inducer of Na(+)-K(+)-ATPase internalization and that surface expression of the mutant was higher than the native Na(+)-K(+)-ATPase before and after ischemia. Annexin V/propidium iodide staining and lactate dehydrogenase release suggested that I-R injury was reduced in α1-L499V-expressing cells compared with α1-expressing cells. Hence, modulation of Na(+)-K(+)-ATPase cell surface abundance through structural determinants on the α-subunit is an important mechanism of regulation of cellular Na(+)-K(+)-ATPase in various physiological and pathophysiological conditions, with a significant impact on cell survival in face of an ischemic stress.

Autophagy and the ubiquitin-proteasome system in cardiac dysfunction

The ubiquitin-proteasome system (UPS) and autophagy are two major intracellular protein degradation pathways. The UPS mediates the removal of soluble abnormal proteins as well as the targeted degradation of most normal proteins that are no longer needed. Autophagy is generally responsible for bulky removal of defective organelles and for sequestering portions of cytoplasm for lysosomal degradation during starvation. Impaired or inadequate protein degradation in the heart is associated with and may be a major pathogenic factor for a wide variety of cardiac dysfunctions, while enhanced protein degradation is also implicated in the development of cardiac pathology. It was generally assumed that the UPS and autophagy serve distinct functions. Therefore, the functional roles of the UPS and autophagy in the hearts have been largely investigated separately. However, recent advances in understanding the shared mechanisms contributing to UPS alteration and the induction of autophagy have helped reveal the link and interplay between the two proteolytic systems in the heart. These links are exemplified by scenarios in which inadequate UPS proteolytic function leads to activation of autophagy, helping alleviate proteotoxic stress. It is becoming increasingly clear that a coordinated and complementary relationship between the two systems is critical to protect cells against stress. Several proteins including p62, NBR1, HDAC6, and co-chaperones appear to play an important role in harmonizing and mobilizing the consortium formed by the UPS and autophagy.

Stressing the ubiquitin-proteasome system

Unfolded and misfolded proteins are inherently toxic to cells and have to be quickly and efficiently eliminated before they intoxicate the intracellular environment. This is of particular importance during proteotoxic stress when, as a consequence of intrinsic or extrinsic factors, the levels of misfolded proteins are transiently or persistently elevated. To meet this demand, metazoan cells have developed specific protein quality control mechanisms that allow the identification and proper handling of non-native proteins. An important defence mechanism is the specific destruction of these proteins by the ubiquitin-proteasome system (UPS). A number of studies have shown that various proteotoxic stress conditions can cause functional impairment of the UPS resulting in cellular dysfunction and apoptosis. In this review, we will summarize our current understanding of proteotoxic stress-induced dysfunction of the UPS and some of its implications for human pathologies.