The ubiquitin-proteasome system in cardiac physiology and pathology

Unlocking the secrets of Cardiac development and function: the critical role of FHL2

FHL2 (Four-and-a-half LIM domain protein 2) is a crucial factor involved in cardiac morphogenesis, the process by which the heart develops its complex structure. It is expressed in various tissues during embryonic development, including the developing heart, and has been shown to play important roles in cell proliferation, differentiation, and migration. FHL2 interacts with multiple proteins to regulate cardiac development as a coactivator or a corepressor. It is involved in cardiac specification and determination of cell fate, cardiomyocyte growth, cardiac remodeling, myofibrillogenesis, and the regulation of HERG channels. Targeting FHL2 has therapeutic implications as it could improve cardiac function, control arrhythmias, alleviate heart failure, and maintain cardiac integrity in various pathological conditions. The identification of FHL2 as a signature gene in atrial fibrillation suggests its potential as a diagnostic marker and therapeutic target for this common arrhythmia.

Rnd3 Ameliorates Diabetic Cardiac Microvascular Injury via Facilitating Trim40-Mediated Rock1 Ubiquitination

ARTICLE HIGHLIGHTS

Impaired cardiac microvascular function is a significant contributor to diabetic cardiomyopathy. Rnd3 expression is notably downregulated in cardiac microvascular endothelial cells under diabetic conditions. Rnd3 overexpression mitigates diabetic myocardial microvascular injury and improves cardiac function through the Rock1/myosin light chain signaling pathway. Rnd3 facilitates the recruitment and interaction with Trim40 to promote Rock1 ubiquitination, thereby preserving endothelial barrier integrity in the diabetic heart.

ProteotoxomiRs: Diagnostic and pathologic miRNA signatures for reductive stress induced proteotoxic heart disease

Proteotoxic stress progressively leads to irreversible cardiac abnormalities. Using a mouse model of reductive stress-induced proteotoxic cardiomyopathy, we identified novel microRNA signatures, termed "ProteotoxomiRs," which reflect stage-specific and transgene-specific responses to proteotoxic stress. Seven microRNAs were uniquely linked to the human mutant R120G-αB-Crystallin transgene, indicating their direct association with the pathogenic protein. Additionally, we uncovered two distinct microRNA profiles associated with the early (pre-onset) and late (cardiomyopathy/heart failure) stages of disease progression. Early-stage signatures primarily modulate signaling pathways essential for cardiac health, including mTOR and MAPK, while late-stage signatures reveal regulatory disruptions in calcium signaling and autophagy insufficiency, driving irreversible cardiac damage caused by reductive stress (RS) and proteotoxicity in transgenic mice. These findings reveal stage-specific miRNA biomarkers with potential diagnostic and prognostic value, offering new insights into the molecular underpinnings of proteotoxic cardiac disease. Moreover, our miRNA-mRNA interaction analysis uncovered potential targets unique to the transgene-specific, early, and late stages of the disease, including several promising druggable candidates, warranting further validation for translational applications.

Role for the F-box proteins in heart diseases

The maintenance of cardiac homeostasis necessitates proper protein turnover, which is regulated by the ubiquitin-proteasome system. F-box proteins are one type of E3 ubiquitin ligases, and accumulating evidence suggests that dysregulation of FBPs exacerbates heart diseases. Therefore, in this review, we summarized the F-box proteins present in the heart, which can be divided into three types based on their repeated sequences, namely FBXO (Fbxo32, Fbxo25, Fbxo44, Fbxo27 and Fbxo28), FBXW (Fbxw7 and Fbxw5), and FBXL (Fbxl1, Fbxl10, Fbxl16 and Fbxl2). Moreover, the physiological and pathological roles and the functional mechanisms of these F-box proteins were elucidated within the cardiac context, providing new theories and strategies for the prevention and treatment of heart diseases.

NEDD4L is a promoter for angiogenesis and cell proliferation in human umbilical vein endothelial cells

Dysregulated angiogenesis leads to neovascularization, which can promote or exacerbate various diseases. Previous studies have proved that NEDD4L plays an important role in hypertension and atherosclerosis. Hence, we hypothesized that NEDD4L may be a critical regulator of endothelial cell (EC) function. This study aimed to define the role of NEDD4L in regulating EC angiogenesis and elucidate their underlying mechanisms. Loss- and gain-of-function of NEDD4L detected the angiogenesis and mobility role in human umbilical vein endothelial cells (HUVECs) using Matrigel tube formation assay, cell proliferation and migration. Pharmacological pathway inhibitors and western blot were used to determine the underlying mechanism of NEDD4L-regulated endothelial functions. Knockdown of NEDD4L suppressed tube formation, cell proliferation and cell migration in HUVECs, whereas NEDD4L overexpression promoted these functions. Moreover, NEDD4L-regulated angiogenesis and cell progression are associated with the phosphorylation of Akt, Erk1/2 and eNOS and the expression of VEGFR2 and cyclin D1 and D3. Mechanically, further evidence was confirmed by using Akt blocker MK-2206, Erk1/2 blocker U0126 and eNOS blocker L-NAME. Overexpression NEDD4L-promoted angiogenesis, cell migration and cell proliferation were restrained by these inhibitors. In addition, overexpression NEDD4L-promoted cell cycle-related proteins cyclin D1 and D3 were also suppressed by Akt blocker MK-2206, Erk1/2 blocker U0126 and eNOS blocker L-NAME. Our results demonstrated a novel finding that NEDD4L promotes angiogenesis and cell progression by regulating the Akt/Erk/eNOS pathways.

Hypertrophic cardiomyopathy in MYBPC3 carriers in aging

Hypertrophic cardiomyopathy (HCM) is characterized by abnormal thickening of the myocardium, leading to arrhythmias, heart failure, and elevated risk of sudden cardiac death, particularly among the young. This inherited disease is predominantly caused by mutations in sarcomeric genes, among which those in the cardiac myosin binding protein-C3 (MYBPC3) gene are major contributors. HCM associated with MYBPC3 mutations usually presents in the elderly and ranges from asymptomatic to symptomatic forms, affecting numerous cardiac functions and presenting significant health risks with a spectrum of clinical manifestations. Regulation of MYBPC3 expression involves various transcriptional and translational mechanisms, yet the destiny of mutant MYBPC3 mRNA and protein in late-onset HCM remains unclear. Pathogenesis related to MYBPC3 mutations includes nonsense-mediated decay, alternative splicing, and ubiquitin-proteasome system events, leading to allelic imbalance and haploinsufficiency. Aging further exacerbates the severity of HCM in carriers of MYBPC3 mutations. Advancements in high-throughput omics techniques have identified crucial molecular events and regulatory disruptions in cardiomyocytes expressing MYBPC3 variants. This review assesses the pathogenic mechanisms that promote late-onset HCM through the lens of transcriptional, post-transcriptional, and post-translational modulation of MYBPC3, underscoring its significance in HCM across carriers. The review also evaluates the influence of aging on these processes and MYBPC3 levels during HCM pathogenesis in the elderly. While pinpointing targets for novel medical interventions to conserve cardiac function remains challenging, the emergence of personalized omics offers promising avenues for future HCM treatments, particularly for late-onset cases.

Paralogue-Specific Roles of SUMO1 and SUMO2/3 in Protein Quality Control and Associated Diseases

Small ubiquitin-related modifiers (SUMOs) function as post-translational protein modifications and regulate nearly every aspect of cellular function. While a single ubiquitin protein is expressed across eukaryotic organisms, multiple SUMO paralogues with distinct biomolecular properties have been identified in plants and vertebrates. Five SUMO paralogues have been characterized in humans, with SUMO1, SUMO2 and SUMO3 being the best studied. SUMO2 and SUMO3 share 97% protein sequence homology (and are thus referred to as SUMO2/3) but only 47% homology with SUMO1. To date, thousands of putative sumoylation substrates have been identified thanks to advanced proteomic techniques, but the identification of SUMO1- and SUMO2/3-specific modifications and their unique functions in physiology and pathology are not well understood. The SUMO2/3 paralogues play an important role in proteostasis, converging with ubiquitylation to mediate protein degradation. This function is achieved primarily through SUMO-targeted ubiquitin ligases (STUbLs), which preferentially bind and ubiquitylate poly-SUMO2/3 modified proteins. Effects of the SUMO1 paralogue on protein solubility and aggregation independent of STUbLs and proteasomal degradation have also been reported. Consistent with these functions, sumoylation is implicated in multiple human diseases associated with disturbed proteostasis, and a broad range of pathogenic proteins have been identified as SUMO1 and SUMO2/3 substrates. A better understanding of paralogue-specific functions of SUMO1 and SUMO2/3 in cellular protein quality control may therefore provide novel insights into disease pathogenesis and therapeutic innovation. This review summarizes current understandings of the roles of sumoylation in protein quality control and associated diseases, with a focus on the specific effects of SUMO1 and SUMO2/3 paralogues.

PSMA2 knockdown impacts expression of proteins involved in immune and cellular stress responses in human lung cells

Proteasome subunit alpha type-2 (PSMA2) is a critical component of the 20S proteasome, which is the core particle of the 26S proteasome complex and is involved in cellular protein quality control by recognizing and recycling defective proteins. PSMA2 expression dysregulation has been detected in different human diseases and viral infections. No study yet has reported PSMA2 knockdown (KD) effects on the cellular proteome.

METHODS

We used SOMAScan, an aptamer-based multiplexed technique, to measure >1300 human proteins to determine the impact of PSMA2 KD on A549 human lung epithelial cells.

RESULTS

PSMA2 KD resulted in significant dysregulation of 52 cellular proteins involved in different bio-functions, including cellular movement and development, cell death and survival, and cancer. The immune system and signal transduction were the most affected cellular functions. PSMA2 KD caused dysregulation of several signaling pathways involved in immune response, cytokine signaling, organismal growth and development, cellular stress and injury (including autophagy and unfolded protein response), and cancer responses.

CONCLUSIONS

In summary, this study helps us better understand the importance of PSMA2 in different cellular functions, signaling pathways, and human diseases.

Research progress of Nedd4L in cardiovascular diseases

Post-translational modifications (PTMs) are a covalent processing process of proteins after translation. Proteins are capable of playing their roles only after being modified, so as to maintain the normal physiological function of cells. As a key modification of protein post-translational modification, ubiquitination is an essential element, which forms an enzyme-linked reaction through ubiquitin-activating enzyme, ubiquitin binding enzyme, and ubiquitin ligase, aiming to regulate the expression level and function of cellular proteins. Nedd4 family is the largest group of ubiquitin ligases, including 9 members, such as Nedd4-1, Nedd4L (Nedd4-2), WWP1, WWP2, ITCH, etc. They could bind to substrate proteins through their WW domain and play a dominant role in the ubiquitination process, and then participate in various pathophysiological processes of cardiovascular diseases (such as hypertension, myocardial hypertrophy, heart failure, etc.). At present, the role of Nedd4L in the cardiovascular field is not fully understood. This review aims to summarize the progress and mechanism of Nedd4L in cardiovascular diseases, and provide potential perspective for the clinical treatment or prevention of related cardiovascular diseases by targeting Nedd4L.

The Case for, and Challenges of, Human Cardiac Tissue in Advancing Phosphoprotein Research

Cardiovascular disease (CVD) and stroke affect over 92 million Americans and account for nearly 1 out of 3 deaths in the US. The use of animal models in cardiovascular research has led to considerable advances in treatment and in our understanding of the pathophysiology of many CVDs. Still, animals may not fully recapitulate human disease states; species differences have long been postulated to be one of the main reasons for a failure of translation between animals and humans in drug discovery and development. Indeed, it has become increasingly clear over the past few decades that to answer certain biomedical questions, like the physiological mechanisms that go awry in many human CVDs, animal tissues may not always be the best option to use. While human cardiac tissue has long been used for laboratory research, published findings often contradict each other, leading to difficulties in interpretation. Current difficulties in utilizing human cardiac tissue include differences in acquisition time, varying tissue procurement protocols, and the struggle to define a human “control” sample. With the tremendous emphasis on translational research that continues to grow, research studies using human tissues are becoming more common. This mini review will discuss advantages, disadvantages, and considerations of using human cardiac tissue in the study of CVDs, paying specific attention to the study of phosphoproteins.

Coronary Large Conductance Ca2+-Activated K+ Channel Dysfunction in Diabetes Mellitus

Diabetes mellitus (DM) is an independent risk of macrovascular and microvascular complications, while cardiovascular diseases remain a leading cause of death in both men and women with diabetes. Large conductance Ca2+-activated K+ (BK) channels are abundantly expressed in arteries and are the key ionic determinant of vascular tone and organ perfusion. It is well established that the downregulation of vascular BK channel function with reduced BK channel protein expression and altered intrinsic BK channel biophysical properties is associated with diabetic vasculopathy. Recent efforts also showed that diabetes-associated changes in signaling pathways and transcriptional factors contribute to the downregulation of BK channel expression. This manuscript will review our current understandings on the molecular, physiological, and biophysical mechanisms that underlie coronary BK channelopathy in diabetes mellitus.

The Role of HECT-Type E3 Ligase in the Development of Cardiac Disease

Despite advances in medicine, cardiac disease remains an increasing health problem associated with a high mortality rate. Maladaptive cardiac remodeling, such as cardiac hypertrophy and fibrosis, is a risk factor for heart failure; therefore, it is critical to identify new therapeutic targets. Failing heart is reported to be associated with hyper-ubiquitylation and impairment of the ubiquitin–proteasome system, indicating an importance of ubiquitylation in the development of cardiac disease. Ubiquitylation is a post-translational modification that plays a pivotal role in protein function and degradation. In 1995, homologous to E6AP C-terminus (HECT) type E3 ligases were discovered. E3 ligases are key enzymes in ubiquitylation and are classified into three families: really interesting new genes (RING), HECT, and RING-between-RINGs (RBRs). Moreover, 28 HECT-type E3 ligases have been identified in human beings. It is well conserved in evolution and is characterized by the direct attachment of ubiquitin to substrates. HECT-type E3 ligase is reported to be involved in a wide range of human diseases and health. The role of HECT-type E3 ligases in the development of cardiac diseases has been uncovered in the last decade. There are only a few review articles summarizing recent advancements regarding HECT-type E3 ligase in the field of cardiac disease. This study focused on cardiac remodeling and described the role of HECT-type E3 ligases in the development of cardiac disease. Moreover, this study revealed that the current knowledge could be exploited for the development of new clinical therapies.

Mitochondrial Dysfunction Is a Common Denominator Linking Skeletal Muscle Wasting Due to Disease, Aging, and Prolonged Inactivity

Skeletal muscle is the most abundant tissue in the body and is required for numerous vital functions, including breathing and locomotion. Notably, deterioration of skeletal muscle mass is also highly correlated to mortality in patients suffering from chronic diseases (e.g., cancer). Numerous conditions can promote skeletal muscle wasting, including several chronic diseases, cancer chemotherapy, aging, and prolonged inactivity. Although the mechanisms responsible for this loss of muscle mass is multifactorial, mitochondrial dysfunction is predicted to be a major contributor to muscle wasting in various conditions. This systematic review will highlight the biochemical pathways that have been shown to link mitochondrial dysfunction to skeletal muscle wasting. Importantly, we will discuss the experimental evidence that connects mitochondrial dysfunction to muscle wasting in specific diseases (i.e., cancer and sepsis), aging, cancer chemotherapy, and prolonged muscle inactivity (e.g., limb immobilization). Finally, in hopes of stimulating future research, we conclude with a discussion of important future directions for research in the field of muscle wasting.

A novel LMNA nonsense mutation causes two distinct phenotypes of cardiomyopathy with high risk of sudden cardiac death in a large five-generation family

Aims

Characterization of the cardiac phenotype associated with the novel LMNA nonsense mutation c.544C>T, p.Q182*, which we have identified in a large five-generation family.

Methods and results

A family tree was constructed. Clinical data [arrhythmia, syncope, sudden cardiac death (SCD), New York Heart Association (NYHA) class] were collected from living and deceased family members. DNA of 23 living family members was analysed for mutations in LMNA. Additionally, dilated cardiomyopathy multi-gene-panel testing and whole exome sequencing were performed in some family members to identify potential phenotype-modifiers. In this five-generation family (n = 65), 17 SCDs occurred at 49.3 ± 10.0 years. Furthermore, we identified eight additional mutation-carriers, seven symptomatic (44 ± 13 years), and one asymptomatic (44 years). First signs of disease [sinus bradycardia with atrioventricular (AV)-block I°] occurred at 36.5 ± 8.1 years. Paroxysmal atrial fibrillation (AF) (onset at 41.8 ± 5.7 years) rapidly progressed to permanent AF (46.2 ± 9.8 years). Subsequently, AV-conduction worsened, syncope, pacemaker-dependence, and non-sustained ventricular tachycardia (43.3 ± 8.2 years) followed. Ventricular arrhythmia caused SCD in patients without implantable cardioverter-defibrillator (ICD). Patients protected by ICD developed rapidly progressive heart failure (45.2 ± 10.6 years). A different phenotype was seen in a sub-family in three patients with early onset of rapidly decompensating heart failure and only minor prior arrhythmia-related symptoms. One patient received high-urgency heart transplantation (HTX) at 32 years, while two died prior to HTX. One of them developed lethal peripartum-associated heart failure. Possible disease-modifiers were identified in this 'heart failure sub-family'.

Conclusion

The novel LMNA nonsense mutation c.544C>T causes a severe arrhythmogenic phenotype manifesting with high incidence of SCD in most patients; and in one sub-family, a distinct phenotype with fast progressing heart failure, indicating the need for early consideration of ICD-implantation and listing for heart-transplantation.

Mitochondrial quality control mechanisms as molecular targets in cardiac ageing

Cardiovascular disease is the leading cause of morbidity and mortality worldwide. Advancing age is a major risk factor for developing cardiovascular disease because of the lifelong exposure to cardiovascular risk factors and specific alterations affecting the heart and the vasculature during ageing. Indeed, the ageing heart is characterized by structural and functional changes that are caused by alterations in fundamental cardiomyocyte functions. In particular, the myocardium is heavily dependent on mitochondrial oxidative metabolism and is especially susceptible to mitochondrial dysfunction. Indeed, primary alterations in mitochondrial function, which are subsequently amplified by defective quality control mechanisms, are considered to be major contributing factors to cardiac senescence. In this Review, we discuss the mechanisms linking defective mitochondrial quality control mechanisms (that is, proteostasis, biogenesis, dynamics, and autophagy) to organelle dysfunction in the context of cardiac ageing. We also illustrate relevant molecular pathways that might be exploited for the prevention and treatment of age-related heart dysfunction.

Ubiquitin proteasome system (UPS) activation in the cardiac hypertrophy of hyperthyroidism

Ubiquitin proteasome system (UPS) is the main proteolytic pathway in eukaryotic cells. Changes in proteasome expression and activity have been associated to cardiovascular diseases as cardiac hypertrophy. Considering that cardiac hypertrophy is commonly associated to hyperthyroidism condition, the present study aimed to investigate the contribution of UPS in cardiac hypertrophy induced by thyroid hormones. Hyperthyroidism was induced in male Wistar rats by intraperitoneal injections of triiodothyronine (T3; 7  μg/100 g of body weight) for 7 days and confirmed by raised levels of total T3 and decreased levels of total T4. In addition, systolic blood pressure and heart rate were significantly increased in hyperthyroid group. Cardiac hypertrophy was confirmed in hyperthyroid group by increased heart weight/tibia length ratio and by increased α-MHC/β-MHC relative expression. Both catalytic (20SPT) and regulatory subunits (19SPT) of the constitutive proteasome were upregulated in hyperthyroid hearts. In addition, the transcripts that encode immunoproteasome subunits were also elevated. Furthermore, ATP-dependent chymotrypsin-like activity (26SPT) was significantly increased in hyperthyroid group. Despite the upregulation and activation of UPS in hyperthyroid hearts, the content of polyubiquitinated proteins was unaltered in relation to control. Together, these results evidence the activation of cardiac proteasome by thyroid hormones, which possibly contribute to the maintenance of protein quality control and regulation of cardiac hypertrophy in response to thyroid hormones.

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

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

F-box protein-32 down-regulates small-conductance calcium-activated potassium channel 2 in diabetic mouse atria

Diabetes mellitus (DM) is an independent risk factor for atrial fibrillation, but the underlying ionic mechanism for this association remains unclear. We recently reported that expression of the small-conductance calcium-activated potassium channel 2 (SK2, encoded by KCCN2) in atria from diabetic mice is significantly down-regulated, resulting in reduced SK currents in atrial myocytes from these mice. We also reported that the level of SK2 mRNA expression is not reduced in DM atria but that the ubiquitin-proteasome system (UPS), a major mechanism of intracellular protein degradation, is activated in vascular smooth muscle cells in DM. This suggests a possible role of the UPS in reduced SK currents. To test this possibility, we examined the role of the UPS in atrial SK2 down-regulation in DM. We found that a muscle-specific E3 ligase, F-box protein 32 (FBXO-32, also called atrogin-1), was significantly up-regulated in diabetic mouse atria. Enhanced FBXO-32 expression in atrial cells significantly reduced SK2 protein expression, and siRNA-mediated FBXO-32 knockdown increased SK2 protein expression. Furthermore, co-transfection of SK2 with FBXO-32 complementary DNA in HEK293 cells significantly reduced SK2 expression, whereas co-transfection with atrogin-1ΔF complementary DNA (a nonfunctional FBXO-32 variant in which the F-box domain is deleted) did not have any effects on SK2. These results indicate that FBXO-32 contributes to SK2 down-regulation and that the F-box domain is essential for FBXO-32 function. In conclusion, DM-induced SK2 channel down-regulation appears to be due to an FBXO-32-dependent increase in UPS-mediated SK2 protein degradation.

Molecular mechanisms of carfilzomib-induced cardiotoxicity in mice and the emerging cardioprotective role of metformin

Carfilzomib (Cfz), an irreversible proteasome inhibitor licensed for relapsed/refractory myeloma, is associated with cardiotoxicity in humans. We sought to establish the optimal protocol of Cfz-induced cardiac dysfunction, to investigate the underlying molecular-signaling and, based on the findings, to evaluate the cardioprotective potency of metformin (Met). Mice were randomized into protocols 1 and 2 (control and Cfz for 1 and 2 consecutive days, respectively); protocols 3 and 4 (control and alternate doses of Cfz for 6 and 14 days, respectively); protocols 5A and 5B (control and Cfz, intermittent doses on days 0, 1 [5A] and 0, 1, 7, and 8 [5B] for 13 days); protocols 6A and 6B (pharmacological intervention; control, Cfz, Cfz+Met and Met for 2 and 6 days, respectively); and protocol 7 (bortezomib). Cfz was administered at 8 mg/kg (IP) and Met at 140 mg/kg (per os). Cfz resulted in significant reduction of proteasomal activity in heart and peripheral blood mononuclear cells in all protocols except protocols 5A and 5B. Echocardiography demonstrated that Cfz led to a significant fractional shortening (FS) depression in protocols 2 and 3, a borderline dysfunction in protocols 1 and 4, and had no detrimental effect on protocols 5A and 5B. Molecular analysis revealed that Cfz inhibited AMPKα/mTORC1 pathways derived from increased PP2A activity in protocol 2, whereas it additionally inhibited phosphatidylinositol 3-kinase/Akt/endothelial nitric oxide synthase pathway in protocol 3. Coadministration of Met prevented Cfz-induced FS reduction and restored AMPKα phosphorylation and autophagic signaling. Conclusively, Cfz decreased left ventricular function through increased PP2A activity and inhibition of AMPKα and its downstream autophagic targets, whereas Met represents a novel promising intervention against Cfz-induced cardiotoxicity.

Comparative evaluation of CacyBP/SIP protein, β-catenin, and immunoproteasome subunit LMP7 in the heart of rats with hypertension of different etiology

Calcyclin-binding protein/Siah-1-interacting protein (CacyBP/SIP) is the recently discovered peptide, which participates in various intracellular processes. Recent reports indicated that CacyBP/SIP activates the ubiquitin ligases and promotes proteasomal degradation of proteins. One of the most important proteins degraded in CacyBP/SIP-dependent pathway is β-catenin. Considering the key importance of β-catenin in the functioning of the cardiovascular system and in the view of the close relationship between CacyBP/SIP, β-catenin, and proteasomal activity, we have decided to undertake research to identify and evaluate the distribution of CacyBP/SIP, β-catenin and the LMP7 subunit of the immunoproteasome in the heart of rats with hypertension of various etiology. The studies were carried out on the hearts of rats with spontaneous hypertension (SHR), renovascular hypertension, and DOCA-salt hypertension. The myocardial expression of CacyBP/SIP, β-catenin, and LMP7 was detected by immunohistochemistry using the EnVision method. The hypertension significantly increased the immunoreactivity to CacyBP/SIP and LMP-7, while weakening the β-catenin immunoreaction. The intensity of the observed changes depends on the type of hypertension. Our results show an innovative and important network of interactions between proteins potentially involved in the development and progression of heart problems in various types of hypertension. This report might contribute to deeper understanding of the role of the CacyBP/SIP protein, β-catenin, and immunoproteasomes in heart function, as well as to bringing new information concerning pathophysiologic mechanisms leading to cardiac dysfunction in the state of elevated blood pressure.

Impact statement

Despite extensive research into the pathogenesis of hypertension and disease-related end organ damage, the mechanisms leading to cardiac complications of hypertensive patients are still not fully elucidated. The aim of the presented research was immunodetection and evaluation of CacyBP/SIP, β-catenin, and proteasomes in the hearts of rats with hypertension of different etiology. Our results show an innovative and important network of interactions between proteins potentially involved in the development and progression of heart problems in various types of hypertension. This report might contribute to deeper understanding of the role of the CacyBP/SIP protein, β-catenin, and proteasomes in heart function. Our results might also bring new information concerning the intracellular processes and signal pathways involved in the regulation of cardiomyocytes functioning in hypertension state. In addition to cognitive significance, the results of presented studies may contribute to further successes in preventing and treatment of cardiac complications associated with hypertension.

Myosin storage myopathy mutations yield defective myosin filament assembly in vitro and disrupted myofibrillar structure and function in vivo

Myosin storage myopathy (MSM) is a congenital skeletal muscle disorder caused by missense mutations in the β-cardiac/slow skeletal muscle myosin heavy chain rod. It is characterized by subsarcolemmal accumulations of myosin that have a hyaline appearance. MSM mutations map near or within the assembly competence domain known to be crucial for thick filament formation. Drosophila MSM models were generated for comprehensive physiological, structural, and biochemical assessment of the mutations' consequences on muscle and myosin structure and function. L1793P, R1845W, and E1883K MSM mutant myosins were expressed in an indirect flight (IFM) and jump muscle myosin null background to study the effects of these variants without confounding influences from wild-type myosin. Mutant animals displayed highly compromised jump and flight ability, disrupted muscle proteostasis, and severely perturbed IFM structure. Electron microscopy revealed myofibrillar disarray and degeneration with hyaline-like inclusions. In vitro assembly assays demonstrated a decreased ability of mutant myosin to polymerize, with L1793P filaments exhibiting shorter lengths. In addition, limited proteolysis experiments showed a reduced stability of L1793P and E1883K filaments. We conclude that the disrupted hydropathy or charge of residues in the heptad repeat of the mutant myosin rods likely alters interactions that stabilize coiled-coil dimers and thick filaments, causing disruption in ordered myofibrillogenesis and/or myofibrillar integrity, and the consequent myosin aggregation. Our Drosophila models are the first to recapitulate the human MSM phenotype with ultrastructural inclusions, suggesting that the diminished ability of the mutant myosin to form stable thick filaments contributes to the dystrophic phenotype observed in afflicted subjects.

Phospholamban Is Downregulated by pVHL-Mediated Degradation through Oxidative Stress in Failing Heart

The E3 ubiquitin ligase, von Hippel–Lindau (VHL), regulates protein expression by polyubiquitination. Although the protein VHL (pVHL) was reported to be involved in the heart function, the underlying mechanism is unclear. Here, we show that pVHL was upregulated in hearts from two types of genetically dilated cardiomyopathy (DCM) mice models. In comparison with the wild-type mouse, both DCM mice models showed a significant reduction in the expression of phospholamban (PLN), a potent inhibitor of sarco(endo)plasmic reticulum Ca²⁺-ATPase, and enhanced interaction between pVHL and PLN. To clarify whether pVHL is involved in PLN degradation in failing hearts, we used carbonylcyanide m-chlorophenylhydrazone (CCCP), a mitochondrial membrane potential (MMP)-lowering reagent, to mimic the heart failure condition in PLN-expressing HEK293 cells and found that CCCP treatment resulted in PLN degradation and increased interaction between PLN and pVHL. However, these effects were reversed with the addition of N-acetyl-l-cysteine. Furthermore, the co-transfection of VHL and PLN in HEK293 cells decreased PLN expression under oxidative stress, whereas knockdown of VHL increased PLN expression both under normal and oxidative stress conditions. Together, we propose that oxidative stress upregulates pVHL expression to induce PLN degradation in failing hearts.

Regulation of vascular large-conductance calcium-activated potassium channels by Nrf2 signalling

BK channels are major ionic determinants of vasodilation. BK channel function is impaired in diabetic vessels due to accelerated proteolysis of its beta-1 (BK-β1) subunits in response to increased oxidative stress. The nuclear factor E2-related factor-2 (Nrf2) signalling pathway has emerged as a master regulator of cellular redox status, and we hypothesized that it plays a central role in regulating BK channel function in diabetic vessels. We found that Nrf2 expression was markedly reduced in db/db diabetic mouse aortas, and this was associated with significant downregulation of BK-β1. In addition, the muscle ring finger protein 1 (MuRF1), a known E-3 ligase targeting BK-β1 ubiquitination and proteasomal degradation, was significantly augmented. These findings were reproduced by knockdown of Nrf2 by siRNA in cultured human coronary artery smooth muscle cells. In contrast, adenoviral transfer of Nrf2 gene in these cells downregulated MuRF1 and upregulated BK-β1 expression. Activation of Nrf2 by dimethyl fumarate preserved BK-β1 expression and protected BK channel and vascular function in db/db coronary arteries. These results indicate that expression of BK-β1 is closely regulated by Nrf2 and vascular BK channel function can be restored by Nrf2 activation. Nrf2 should be considered a novel therapeutic target in the treatment of diabetic vasculopathy.

Cardio-Oncology: Cancer Therapy-related Cardiovascular Complications in a Molecular Targeted Era: New Concepts and Perspectives

Cardio-oncology is a medical discipline that identifies, prevents, and treats the cardiovascular complications related to cancer therapy. Due to the remarkable proliferation of new cancer therapies causing cardiovascular complications, such as hypertension, heart failure, vascular complications, and cardiac arrhythmia, we provide an extensive, comprehensive revision of the most up-to-date scientific information available on the cardiovascular complications associated with the use of newer, novel chemotherapeutic agents, including their reported incidence, suggested pathophysiology, clinical manifestations, potential treatment, and prevention. The authors consider this topic to be relevant for the clinicians since cardiovascular complications associated with the administration of recently approved drugs are relatively underappreciated. The purpose of this article is to provide a state-of-the-art review of cardiovascular complications associated with the use of newer, novel chemotherapeutic agents and targeted therapies, including their reported incidence, suggested pathophysiology, clinical manifestations, potential treatment, and prevention.  Ongoing efforts are needed to provide a better understanding of the frequency, mechanisms of disease, prevention, and treatment of cardiovascular complications induced by the newer, novel chemotherapeutic agents. Development of a cardio-oncology discipline is warranted in order to promote task forces aimed at the creation of oncology patient-centered guidelines for the detection, prevention, and treatment of potential cardiovascular side effects associated with newer cancer therapies.

Modest overexpression of FOXO maintains cardiac proteostasis and ameliorates age-associated functional decline

Heart performance declines with age. Impaired protein quality control (PQC), due to reduced ubiquitin‐proteasome system (UPS) activity, autophagic function, and/or chaperone‐mediated protein refolding, contributes to cardiac deterioration. The transcription factor FOXO participates in regulating genes involved in PQC, senescence, and numerous other processes. Here, a comprehensive approach, involving molecular genetics, novel assays to probe insect cardiac physiology, and bioinformatics, was utilized to investigate the influence of heart‐restricted manipulation of dFOXO expression in the rapidly aging Drosophila melanogaster model. Modest dFOXO overexpression was cardioprotective, ameliorating nonpathological functional decline with age. This was accompanied by increased expression of genes associated predominantly with the UPS, relative to other PQC components, which was validated by a significant decrease in ubiquitinated proteins. RNAi knockdown of UPS candidates accordingly compromised myocardial physiology in young flies. Conversely, excessive dFOXO overexpression or suppression proved detrimental to heart function and/or organismal development. This study highlights D. melanogaster as a model of cardiac aging and FOXO as a tightly regulated mediator of proteostasis and heart performance over time.

E3 ubiquitin ligase CHIP interacts with C-type lectin-like receptor CLEC-2 and promotes its ubiquitin-proteasome degradation

C-type lectin-like receptor 2 (CLEC-2) was originally identified as a member of non-classical C-type lectin-like receptors in platelets and immune cells. Activation of CLEC-2 is involved in thrombus formation, lymphatic/blood vessel separation, platelet-mediated tumor metastasis and immune response. Nevertheless, the regulation of CLEC-2 expression is little understood. In this study, we identified that the C terminus of Hsc70-interacting protein (CHIP) interacted with CLEC-2 by mass spectrometry analysis, and CHIP decreased the protein expression of CLEC-2 through lysine-48-linked ubiquitination and proteasomal degradation. Deleted and point mutation also revealed that CHIP controlled CLEC-2 protein expression via both tetratricopeptide repeats (TPR) domain and Ubox domain in a HSP70/90-independent manner. Moreover, reduced CHIP expression was associated with decreased CLEC-2 polyubiquitination and increased CLEC-2 protein levels in PMA-induced differentiation of THP-1 monocytes into macrophages. These results indicate that CLEC-2 is the target substrate of E3 ubiquitin ligase CHIP, and suggest that the CHIP/CLEC-2 axis may play an important role in the modulation of immune response.

The role of AMPK in cardiomyocyte health and survival

Cellular energy homeostasis is a fundamental process that governs the overall health of the cell and is paramount to cell survival. Central to this is the control of ATP generation and utilization, which is regulated by a complex myriad of enzymatic reactions controlling cellular metabolism. In the cardiomyocyte, ATP generated from substrate catabolism is used for numerous cellular processes including maintaining ionic homeostasis, cell repair, protein synthesis and turnover, organelle turnover, and contractile function. In many instances, cardiovascular disease is associated with impaired cardiac energetics and thus the signalling that regulates pathways involved in cardiomyocyte metabolism may be potential targets for pharmacotherapy designed to help treat cardiovascular disease. An important regulator of cardiomyocyte energy homeostasis is adenosine monophosphate-activated protein kinase (AMPK). AMPK is a serine-threonine kinase that functions primarily as a metabolic sensor to coordinate anabolic and catabolic activities in the cell via the phosphorylation of multiple proteins involved in metabolic pathways. In addition to the direct role that AMPK plays in the regulation of cardiomyocyte metabolism, AMPK can also either directly or indirectly influence other cellular processes such as regulating mitochondrial function, post-translation acetylation, autophagy, mitophagy, endoplasmic reticulum stress, and apoptosis. Thus, AMPK is implicated in the control of a wide variety of cellular processes that can influence cardiomyocyte health and survival. In this review, we will discuss the important role that AMPK plays in regulating cardiac metabolism, as well as the additional cellular processes that may contribute to cardiomyocyte function and survival in the healthy and the diseased heart. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan. F.C. Glatz.

Macromolecular Degradation Systems and Cardiovascular Aging

Aging-related cardiovascular diseases are a rapidly increasing problem worldwide. Cardiac aging demonstrates progressive decline of diastolic dysfunction of ventricle and increase in ventricular and arterial stiffness accompanied by increased fibrosis stimulated by angiotensin II and proinflammatory cytokines. Reactive oxygen species and multiple signaling pathways on cellular senescence play major roles in the process. Aging is also associated with an alteration in steady state of macromolecular dynamics including a dysfunction of protein synthesis and degradation. Currently, impaired macromolecular degradation is considered to be closely related to enhanced inflammation and be involved in the process and mechanism of cardiac aging. Herein, we review the role and mechanisms of the degradation system of intracellular macromolecules in the process and pathophysiology of cardiovascular aging.

Smad3 Couples Pak1 With the Antihypertrophic Pathway Through the E3 Ubiquitin Ligase, Fbxo32

Pathological cardiac hypertrophy is regarded as a critical intermediate step toward the development of heart failure. Many signal transduction cascades are demonstrated to dictate the induction and progression of pathological hypertrophy; however, our understanding in regulatory mechanisms responsible for the suppression of hypertrophy remains limited. In this study, we showed that exacerbated hypertrophy induced by pressure overload in cardiac-deleted Pak1 mice was attributable to a failure to upregulate the antihypertrophic E3 ligase, Fbxo32, responsible for targeting proteins for the ubiquitin-degradation pathway. Under pressure overload, cardiac overexpression of constitutively active Pak1 mice manifested strong resilience against pathological hypertrophic remodeling. Mechanistic studies demonstrated that subsequent to Pak1 activation, the binding of Smad3 on a critical singular AGAC -286 -binding site on the FBXO32 promoter was crucial for its transcriptional regulation. Pharmacological upregulation of Fbxo32 by Berberine ameliorated hypertrophic remodeling and improved cardiac performance in cardiac-deficient Pak1 mice under pressure overload. Our findings discover Smad3 and Fbxo32 as novel downstream components of the Pak1-dependent signaling pathway for the suppression of hypertrophy. This discovery opens a new venue for opportunities to identify novel targets for the management of cardiac hypertrophy.

The mammalian target of rapamycin modulates the immunoproteasome system in the heart

The mammalian target of rapamycin (mTOR) plays an important role in cardiac development and function. Inhibition of mTOR by rapamycin has been shown to attenuate pathological cardiac hypertrophy and improve the function of aging heart, accompanied by an inhibition of the cardiac proteasome activity. The current study aimed to determine the potential mechanism(s) by which mTOR inhibition modulates cardiac proteasome. Inhibition of mTOR by rapamycin was found to reduce primarily the immunoproteasome in both H9c2 cells in vitro and mouse heart in vivo, without significant effect on the constitutive proteasome and protein ubiquitination. Concurrent with the reduction of the immunoproteasome, rapamycin reduced two important inflammatory response pathways, the NF-κB and Stat3 signaling. In addition, rapamycin attenuated the induction of the immunoproteasome in H9c2 cells by inflammatory cytokines, including INFγ and TNFα, by suppressing NF-κB signaling. These data indicate that rapamycin indirectly modulated immunoproteasome through the suppression of inflammatory response pathways. Lastly, the role of the immunoproteasome during the development of cardiac hypertrophy was investigated. Administration of a specific inhibitor of the immunoproteasome ONX 0914 attenuated isoproterenol-induced cardiac hypertrophy, suggesting that the immunoproteasome may be involved in the development of cardiac hypertrophy and therefore could be a therapeutic target. In conclusion, rapamycin inhibits the immunoproteasome through its effect on the inflammatory signaling pathways and the immunoproteasome could be a potential therapeutic target for pathological cardiac hypertrophy.

Partial inhibition of the ubiquitin-proteasome system ameliorates cardiac dysfunction following ischemia-reperfusion in the presence of high glucose

Background

Acute hyperglycemia co-presenting with myocardial infarction (in diabetic and non-diabetic individuals) is often associated with a poor prognosis. Although acute hyperglycemia induces oxidative stress that can lead to dysregulation of the ubiquitin–proteasome system (UPS), it is unclear whether increased/decreased UPS is detrimental with ischemia–reperfusion under such conditions. As our earlier data implicated the UPS in cardiac damage, we hypothesized that its inhibition results in cardioprotection with ischemia–reperfusion performed under conditions that simulate acute hyperglycemia.

Methods

Ex vivo rat heart perfusions were performed with Krebs–Henseleit buffer containing 33 mM glucose vs. controls (11 mM glucose) for 60 min stabilization, followed by 20 min global ischemia and 60 min reperfusion ± 5 µM lactacystin and 10 µM MG-132, respectively. The UPS inhibitors were added during the first 20 min of the reperfusion phase and various cardiac functional parameters evaluated. In parallel experiments, infarct sizes were assessed following 20 min regional ischemia and 120 min reperfusion ± each of the respective UPS inhibitors (added during reperfusion). Heart tissues were collected and analyzed for markers of oxidative stress, UPS activation, inflammation and autophagy.

Results

The proteasome inhibitor doses and treatment duration here employed resulted in partial UPS inhibition during the reperfusion phase. Both lactacystin and MG-132 administration resulted in cardioprotection in our experimental system, with MG-132 showing a greater effect. The proteasome inhibitors also enhanced cardiac superoxide dismutase protein levels (SOD1, SOD2), attenuated pro-inflammatory effects and caused an upregulation of autophagic markers.

Conclusions

This study established that partial proteasome inhibition elicits cardioprotection in hearts exposed to ischemia–reperfusion with acute simulated hyperglycemia. These data reveal that protease inhibition triggered three major protective effects, i.e. (a) enhancing myocardial anti-oxidant defenses, (b) attenuating inflammation, and (c) increasing the autophagic response. Thus the UPS emerges as a unique therapeutic target for the treatment of ischemic heart disease under such conditions.

The ubiquitin-proteasome system and autophagy are defective in the taurine-deficient heart

Taurine depletion leads to impaired mitochondrial function, as characterized by reduced ATP production and elevated superoxide generation. These defects can fundamentally alter cardiomyocyte function and if left unchanged can result in cell death. To protect against these stresses, cardiomyocytes possess quality control processes, such as the ubiquitin-proteasome system (UPS) and autophagy, which can rejuvenate cells through the degradation of damaged proteins and organelles. Hence, the present study tested the hypothesis that reactive oxygen species generated by damaged mitochondria initiates UPS and autophagy in the taurine-deficient heart. Using transgenic mice lacking the taurine transporter (TauTKO) as a model of taurine deficiency, it was shown that the levels of ubiquitinated protein were elevated, an effect associated with a decrease in ATP-dependent 26S β5 proteasome activity. Treating the TauTKO mouse with the mitochondria-specific antioxidant, mitoTEMPO, largely abolished the increase in ubiquitinated protein content. The TauTKO heart was also associated with impaired autophagy, characterized by an increase in the initiator, Beclin-1, and autophagosome content, but a defect in the generation of active autophagolysosomes. Although mitoTEMPO treatment only restores the oxidative balance within the mitochondria, it appeared to completely disrupt the crosstalk between the damaged mitochondria and the quality control processes. Thus, mitochondrial oxidative stress is the main trigger initiating the quality control systems in the taurine-deficient heart. We conclude that the activation of the UPS and autophagy is another fundamental function of mitochondria.

Age influences inflammatory responses, hemodynamics, and cardiac proteasome activation during acute lung injury

BACKGROUND

Acute lung injury (ALI) is a significant source of morbidity and mortality in critically ill patients. Age is a major determinant of clinical outcome in ALI. The increased ALI-associated mortality in the older population suggests that there are age-dependent alterations in the responses to pulmonary challenge. The objective of this observational study was to evaluate age-dependent differences in the acute (within 6 hours) immunological and physiological responses of the heart and lung, to pulmonary challenge, that could result in increased severity.

METHODS

Male C57Bl/6 mice (young: 2-3 months, old: 18-20 months) were challenged intratracheally with cell wall components from Gram-positive bacteria (lipoteichoic acid and peptidoglycan). After 6 hours, both biochemical and physiological consequences of the challenge were assessed. Alveolar infiltration of inflammatory cells and protein, airspace and blood cytokines, cardiac function and myocardial proteasome activity were determined.

RESULTS

In young mice, there was a dose-dependent response to pulmonary challenge resulting in increased airspace neutrophil counts, lung permeability, and concentrations of cytokines in bronchoalveolar lavage fluid and plasma. A midrange dose was then selected to compare the responses in young and old animals. In comparison, the old animals displayed increased neutrophil accumulation in the airspaces, decreased arterial oxygen saturation, body temperatures, plasma cytokine concentrations, and a lack of myocardial proteasome response, following challenge.

CONCLUSIONS

Age-dependent differences in the onset of systemic response and in maintenance of vital functions, including temperature control, oxygen saturation, and myocardial proteasome activation, are evident. We believe a better understanding of these age-related consequences of ALI can lead to more appropriate treatments in the elderly patient population.

Progesterone modulates SERCA2a expression and function in rabbit cardiomyocytes

We recently showed that progesterone treatment abolished arrhythmias and sudden cardiac death in a transgenic rabbit model of long QT syndrome type 2 (LQT2). Moreover, levels of cardiac sarco(endo)plasmic reticulum Ca(2+)-ATPase type 2a (SERCA2a) were upregulated in LQT2 heart extracts. We hypothesized that progesterone treatment upregulated SERCA2a expression, thereby reducing Ca(2+)-dependent arrhythmias in LQT2 rabbits. We therefore investigated the effect of progesterone on SERCA2a regulation in isolated cardiomyocytes. Cardiomyocytes from neonatal (3- to 5-day-old) rabbits were isolated, cultured, and treated with progesterone and other pharmacological agents. Immunoblotting was performed on total cell lysates and sarcoplasmic reticulum-enriched membrane fractions for protein abundance, and mRNA transcripts were quantified using real-time PCR. The effect of progesterone on baseline Ca(2+) transients and Ca(2+) clearance was determined using digital imaging. Progesterone treatment increased the total pool of SERCA2a protein by slowing its degradation. Using various pharmacological inhibitors of degradation pathways, we showed that progesterone-associated degradation of SERCA2a involves ubiquitination, and progesterone significantly decreases the levels of ubiquitin-tagged SERCA2a polypeptides. Our digital imaging data revealed that progesterone significantly shortened the decay and duration of Ca(2+) transients. Progesterone treatment increases protein levels and activity of SERCA2a. Progesterone stabilizes SERCA2a, in part, by decreasing the ubiquitination level of SERCA2a polypeptides.

A design principle underlying the paradoxical roles of E3 ubiquitin ligases

E3 ubiquitin ligases are important cellular components that determine the specificity of proteolysis in the ubiquitin-proteasome system. However, an increasing number of studies have indicated that E3 ubiquitin ligases also participate in transcription. Intrigued by the apparently paradoxical functions of E3 ubiquitin ligases in both proteolysis and transcriptional activation, we investigated the underlying design principles using mathematical modeling. We found that the antagonistic functions integrated in E3 ubiquitin ligases can prevent any undesirable sustained activation of downstream genes when E3 ubiquitin ligases are destabilized by unexpected perturbations. Interestingly, this design principle of the system is similar to the operational principle of a safety interlock device in engineering systems, which prevents a system from abnormal operation unless stability is guaranteed.

Cardiac mitochondrial proteome dynamics with heavy water reveals stable rate of mitochondrial protein synthesis in heart failure despite decline in mitochondrial oxidative capacity

We recently developed a method to measure mitochondrial proteome dynamics with heavy water ((2)H2O)-based metabolic labeling and high resolution mass spectrometry. We reported the half-lives and synthesis rates of several proteins in the two cardiac mitochondrial subpopulations, subsarcolemmal and interfibrillar (SSM and IFM), in Sprague Dawley rats. In the present study, we tested the hypothesis that the mitochondrial protein synthesis rate is reduced in heart failure, with possible differential changes in SSM versus IFM. Six to seven week old male Sprague Dawley rats underwent transverse aortic constriction (TAC) and developed moderate heart failure after 22weeks. Heart failure and sham rats of the same age received heavy water (5% in drinking water) for up to 80days. Cardiac SSM and IFM were isolated from both groups and the proteins were separated by 1D gel electrophoresis. Heart failure reduced protein content and increased the turnover rate of several proteins involved in fatty acid oxidation, electron transport chain and ATP synthesis, while it decreased the turnover of other proteins, including pyruvate dehydrogenase subunit in IFM, but not in SSM. Because of these bidirectional changes, the average overall half-life of proteins was not altered by heart failure in both SSM and IFM. The kinetic measurements of individual mitochondrial proteins presented in this study may contribute to a better understanding of the mechanisms responsible for mitochondrial alterations in the failing heart.

Atrogin-1 deficiency promotes cardiomyopathy and premature death via impaired autophagy

Cardiomyocyte proteostasis is mediated by the ubiquitin/proteasome system (UPS) and autophagy/lysosome system and is fundamental for cardiac adaptation to both physiologic (e.g., exercise) and pathologic (e.g., pressure overload) stresses. Both the UPS and autophagy/lysosome system exhibit reduced efficiency as a consequence of aging, and dysfunction in these systems is associated with cardiomyopathies. The muscle-specific ubiquitin ligase atrogin-1 targets signaling proteins involved in cardiac hypertrophy for degradation. Here, using atrogin-1 KO mice in combination with in vivo pulsed stable isotope labeling of amino acids in cell culture proteomics and biochemical and cellular analyses, we identified charged multivesicular body protein 2B (CHMP2B), which is part of an endosomal sorting complex (ESCRT) required for autophagy, as a target of atrogin-1-mediated degradation. Mice lacking atrogin-1 failed to degrade CHMP2B, resulting in autophagy impairment, intracellular protein aggregate accumulation, unfolded protein response activation, and subsequent cardiomyocyte apoptosis, all of which increased progressively with age. Cellular proteostasis alterations resulted in cardiomyopathy characterized by myocardial remodeling with interstitial fibrosis, with reduced diastolic function and arrhythmias. CHMP2B downregulation in atrogin-1 KO mice restored autophagy and decreased proteotoxicity, thereby preventing cell death. These data indicate that atrogin-1 promotes cardiomyocyte health through mediating the interplay between UPS and autophagy/lysosome system and its alteration promotes development of cardiomyopathies.

Bortezomib protects from varicose-like venous remodeling

Despite the high prevalence of venous diseases that are associated with and based on the structural reorganization of the venous vessel wall, not much is known about their mechanistic causes. In this context, we demonstrated that the quantity of myocardin, a transcriptional regulator of the contractile and quiescent smooth muscle cell phenotype, was diminished in proliferating synthetic venous smooth muscle cells (VSMCs) of human and mouse varicose veins by 51 and 60%, respectively. On the basis of the relevance of proteasomal activity for such phenotypic changes, we hypothesized that the observed VSMC activation is attenuated by the proteasome inhibitor bortezomib. This drug fully abolished VSMC proliferation and loss of myocardin in perfused mouse veins and blocked VSMC invasion in collagen gels by almost 80%. In line with this, topical transdermal treatment with bortezomib diminished VSMC proliferation by 80%, rescued 90% of VSMC myocardin abundance, and inhibited varicose-like venous remodeling by 67 to 72% in a mouse model. Collectively, our data indicate that the proteasome plays a pivotal role in VSMC phenotype changes during venous remodeling processes. Its inhibition protects from varicose-like vein remodeling in mice and may thus serve as a putative therapeutic strategy to treat human varicose veins.

Diggin' on u(biquitin): a novel method for the identification of physiological E3 ubiquitin ligase substrates

The ubiquitin-proteasome system (UPS) plays a central role in maintaining protein homeostasis, emphasized by a myriad of diseases that are associated with altered UPS function such as cancer, muscle-wasting, and neurodegeneration. Protein ubiquitination plays a central role in both the promotion of proteasomal degradation as well as cellular signaling through regulation of the stability of transcription factors and other signaling molecules. Substrate-specificity is a critical regulatory step of ubiquitination and is mediated by ubiquitin ligases. Recent studies implicate ubiquitin ligases in multiple models of cardiac diseases such as cardiac hypertrophy, atrophy, and ischemia/reperfusion injury, both in a cardioprotective and maladaptive role. Therefore, identifying physiological substrates of cardiac ubiquitin ligases provides both mechanistic insights into heart disease as well as possible therapeutic targets. Current methods identifying substrates for ubiquitin ligases rely heavily upon non-physiologic in vitro methods, impeding the unbiased discovery of physiological substrates in relevant model systems. Here we describe a novel method for identifying ubiquitin ligase substrates utilizing tandem ubiquitin binding entities technology, two-dimensional differential in gel electrophoresis, and mass spectrometry, validated by the identification of both known and novel physiological substrates of the ubiquitin ligase MuRF1 in primary cardiomyocytes. This method can be applied to any ubiquitin ligase, both in normal and disease model systems, in order to identify relevant physiological substrates under various biological conditions, opening the door to a clearer mechanistic understanding of ubiquitin ligase function and broadening their potential as therapeutic targets.

Cardiac aging and insulin resistance: could insulin/insulin-like growth factor (IGF) signaling be used as a therapeutic target?

Intrinsic cardiac aging is an independent risk factor for cardiovascular disease and is associated with structural and functional changes that impede cardiac responses to stress and to cardio-protective mechanisms. Although systemic insulin resistance and the associated risk factors exacerbate cardiac aging, cardiac-specific insulin resistance without confounding systemic alterations, could prevent cardiac aging. Thus, strategies aimed to reduce insulin/insulin-like growth factor (IGF) signaling in the heart prevent cardiac aging in lower organisms and in mammals but the mechanisms underlying this protection are not fully understood. In this review, we describe the impact of aging on the cardiovascular system and discuss the mounting evidence that reduced insulin/IGF signaling in the heart could alleviate age-associated alterations and preserve cardiac performance.

Regulation of large conductance Ca2+-activated K+ (BK) channel β1 subunit expression by muscle RING finger protein 1 in diabetic vessels

The large conductance Ca(2+)-activated K(+) (BK) channel, expressed abundantly in vascular smooth muscle cells (SMCs), is a key determinant of vascular tone. BK channel activity is tightly regulated by its accessory β1 subunit (BK-β1). However, BK channel function is impaired in diabetic vessels by increased ubiquitin/proteasome-dependent BK-β1 protein degradation. Muscle RING finger protein 1 (MuRF1), a muscle-specific ubiquitin ligase, is implicated in many cardiac and skeletal muscle diseases. However, the role of MuRF1 in the regulation of vascular BK channel and coronary function has not been examined. In this study, we hypothesized that MuRF1 participated in BK-β1 proteolysis, leading to the down-regulation of BK channel activation and impaired coronary function in diabetes. Combining patch clamp and molecular biological approaches, we found that MuRF1 expression was enhanced, accompanied by reduced BK-β1 expression, in high glucose-cultured human coronary SMCs and in diabetic vessels. Knockdown of MuRF1 by siRNA in cultured human SMCs attenuated BK-β1 ubiquitination and increased BK-β1 expression, whereas adenoviral expression of MuRF1 in mouse coronary arteries reduced BK-β1 expression and diminished BK channel-mediated vasodilation. Physical interaction between the N terminus of BK-β1 and the coiled-coil domain of MuRF1 was demonstrated by pulldown assay. Moreover, MuRF1 expression was regulated by NF-κB. Most importantly, pharmacological inhibition of proteasome and NF-κB activities preserved BK-β1 expression and BK-channel-mediated coronary vasodilation in diabetic mice. Hence, our results provide the first evidence that the up-regulation of NF-κB-dependent MuRF1 expression is a novel mechanism that leads to BK channelopathy and vasculopathy in diabetes.