Understanding proteasome assembly and regulation: importance to cardiovascular medicine

Myocardial Protection and Current Cancer Therapy: Two Opposite Targets with Inevitable Cost

Myocardial protection against ischemia/reperfusion injury (IRI) is mediated by various ligands, activating different cellular signaling cascades. These include classical cytosolic mediators such as cyclic-GMP (c-GMP), various kinases such as Phosphatydilinositol-3- (PI3K), Protein Kinase B (Akt), Mitogen-Activated-Protein- (MAPK) and AMP-activated (AMPK) kinases, transcription factors such as signal transducer and activator of transcription 3 (STAT3) and bioactive molecules such as vascular endothelial growth factor (VEGF). Most of the aforementioned signaling molecules constitute targets of anticancer therapy; as they are also involved in carcinogenesis, most of the current anti-neoplastic drugs lead to concomitant weakening or even complete abrogation of myocardial cell tolerance to ischemic or oxidative stress. Furthermore, many anti-neoplastic drugs may directly induce cardiotoxicity via their pharmacological effects, or indirectly via their cardiovascular side effects. The combination of direct drug cardiotoxicity, indirect cardiovascular side effects and neutralization of the cardioprotective defense mechanisms of the heart by prolonged cancer treatment may induce long-term ventricular dysfunction, or even clinically manifested heart failure. We present a narrative review of three therapeutic interventions, namely VEGF, proteasome and Immune Checkpoint inhibitors, having opposing effects on the same intracellular signal cascades thereby affecting the heart. Moreover, we herein comment on the current guidelines for managing cardiotoxicity in the clinical setting and on the role of cardiovascular confounders in cardiotoxicity.

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.

Ischemia/Reperfusion

Ischemic disorders, such as myocardial infarction, stroke, and peripheral vascular disease, are the most common causes of debilitating disease and death in westernized cultures. The extent of tissue injury relates directly to the extent of blood flow reduction and to the length of the ischemic period, which influence the levels to which cellular ATP and intracellular pH are reduced. By impairing ATPase-dependent ion transport, ischemia causes intracellular and mitochondrial calcium levels to increase (calcium overload). Cell volume regulatory mechanisms are also disrupted by the lack of ATP, which can induce lysis of organelle and plasma membranes. Reperfusion, although required to salvage oxygen-starved tissues, produces paradoxical tissue responses that fuel the production of reactive oxygen species (oxygen paradox), sequestration of proinflammatory immunocytes in ischemic tissues, endoplasmic reticulum stress, and development of postischemic capillary no-reflow, which amplify tissue injury. These pathologic events culminate in opening of mitochondrial permeability transition pores as a common end-effector of ischemia/reperfusion (I/R)-induced cell lysis and death. Emerging concepts include the influence of the intestinal microbiome, fetal programming, epigenetic changes, and microparticles in the pathogenesis of I/R. The overall goal of this review is to describe these and other mechanisms that contribute to I/R injury. Because so many different deleterious events participate in I/R, it is clear that therapeutic approaches will be effective only when multiple pathologic processes are targeted. In addition, the translational significance of I/R research will be enhanced by much wider use of animal models that incorporate the complicating effects of risk factors for cardiovascular disease. © 2017 American Physiological Society. Compr Physiol 7:113-170, 2017.

Autophagic-lysosomal inhibition compromises ubiquitin-proteasome system performance in a p62 dependent manner in cardiomyocytes

Intracellular protein degradation is primarily performed by the ubiquitin-proteasome system (UPS) and the autophagic-lysosomal pathway (ALP). The interplay between these two pathways has been rarely examined in intact animals and the mechanism underlying the interplay remains unclear. Hence, we sought to test in vivo and in vitro the impact of inhibition of the ALP on UPS proteolytic performance in cardiomyocytes and to explore the underlying mechanism. Transgenic mice ubiquitously expressing a surrogate UPS substrate (GFPdgn) were treated with bafilomycin-A1 (BFA) to inhibit the ALP. Myocardial and renal GFPdgn protein levels but not mRNA levels were increased at 24 hours but not 3 hours after the first injection of BFA. Myocardial protein abundance of key proteasome subunits and the activities of proteasomal peptidases were not discernibly altered by the treatment. In cultured neonatal rat ventricular myocytes (NRVMs), the surrogate UPS substrate GFPu and a control red fluorescence protein (RFP) were co-expressed to probe UPS performance. At 12 hours or 24 hours after ALP inhibition by 3-methyladenine (3-MA) or BFA, GFPu/RFP protein ratios and the protein half-life of GFPu were significantly increased, which is accompanied by increases in p62 proteins. Similar findings were obtained when ALP was inhibited genetically via silencing Atg7 or Rab7. ALP inhibition-induced increases in GFPu and p62 are co-localized in NRVMs. siRNA-mediated p62 knockdown prevented ALP inhibition from inducing GFPu accumulation in NRVMs. We conclude that in a p62-dependent fashion, ALP inhibition impairs cardiac UPS proteolytic performance in cardiomyocytes in vitro and in vivo.

Altered ubiquitin-proteasome signaling in right ventricular hypertrophy and failure

Alterations in the ubiquitin-proteasome system (UPS) have been described in left ventricular hypertrophy and failure, although results have been inconsistent. The role of the UPS in right ventricular (RV) hypertrophy (RVH) and RV failure (RVF) is unknown. Given the greater percent increase in RV mass associated with RV afterload stress, as present in many congenital heart lesions, we hypothesized that alterations in the UPS could play an important role in RVH/RVF. UPS expression and activity were measured in the RV from mice with RVH/RVF secondary to pulmonary artery constriction (PAC). Epoxomicin and MG132 were used to inhibit the proteasome, and overexpression of the 11S PA28α subunit was used to activate the proteasome. PAC mice developed RVH (109.3% increase in RV weight to body weight), RV dilation with septal shift, RV dysfunction, and clinical RVF. Proteasomal function (26S β₅ chymotrypsin-like activity) was decreased 26% (P < 0.05). Protein expression of 19S subunit Rpt5 (P < 0.05), UCHL1 deubiquitinase (P < 0.0001), and Smurf1 E3 ubiquitin ligase (P < 0.01) were increased, as were polyubiquitinated proteins (P < 0.05) and free-ubiquitins (P = 0.05). Pro-apoptotic Bax was increased (P < 0.0001), whereas anti-apoptotic Bcl-2 decreased (P < 0.05), resulting in a sixfold increase in the Bax/Bcl-2 ratio. Proteasomal inhibition did not accelerate RVF. However, proteasome enhancement by cardiac-specific proteasome overexpression partially improved survival. Proteasome activity is decreased in RVH/RVF, associated with upregulation of key UPS regulators and pro-apoptotic signaling. Enhancement of proteasome function partially attenuates RVF, suggesting that UPS dysfunction contributes to RVF.

Role of the ubiquitin proteasome system in the heart

Proper protein turnover is required for cardiac homeostasis and, accordingly, impaired proteasomal function appears to contribute to heart disease. Specific proteasomal degradation mechanisms underlying cardiovascular biology and disease have been identified, and such cellular pathways have been proposed to be targets of clinical relevance. This review summarizes the latest literature regarding the specific E3 ligases involved in heart biology, and the general ways that the proteasome regulates protein quality control in heart disease. The potential for therapeutic intervention in Ubiquitin Proteasome System function in heart disease is discussed.

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.

Daunorubicin therapy is associated with upregulation of E3 ubiquitin ligases in the heart

Daunorubicin (DNR) and doxorubicin (DOX) are two of the most effective anthracycline drugs known for the treatment of systemic neoplasms and solid tumors. However, their clinical use is hampered due to profound cardiotoxicity. The mechanism by which DNR injures the heart remains to be fully elucidated. Recent reports have indicated that DOX activates ubiquitin proteasome-mediated degradation of specific transcription factors; however, no reports exist on the effect of DNR on the E3 ubiquitin ligases, MURF-1 (muscle ring finger 1) and MAFbx (muscle atrophy F-box). The aim of this study was to investigate the effect of DNR treatment on the protein and organelle degradation systems in the heart and to elucidate some of the signalling mechanisms involved. Adult rats were divided into two groups where one group received six intraperitoneal injections of 2 mg/kg DNR on alternate days and the other group received saline injections as control. Hearts were excised and perfused on a working heart system the day after the last injection and freeze-clamped for biochemical analysis. DNR treatment significantly attenuated cardiac function and increased apoptosis in the heart. DNR-induced cardiac cytotoxicity was associated with upregulation of the E3 ligases, MURF-1 and MAFbx and also caused significant increases in two markers of autophagy, beclin-1 and LC3. These changes observed in the heart were also associated with attenuation of the phosphoinositide 3-kinase/Akt signalling pathway.

Tyrosine nitration of PA700 links proteasome activation to endothelial dysfunction in mouse models with cardiovascular risk factors

Oxidative stress is believed to cause endothelial dysfunction, an early event and a hallmark in cardiovascular diseases (CVD) including hypertension, diabetes, and dyslipidemia. However, the targets for oxidative stress-mediated endothelial dysfunction in CVD have not been completely elucidated. Here we report that 26S proteasome activation by peroxynitrite (ONOO⁻) is a common pathway for endothelial dysfunction in mouse models of diabetes, hypertension, and dyslipidemia. Endothelial function, assayed by acetylcholine-induced vasorelaxation, was impaired in parallel with significantly increased 26S proteasome activity in aortic homogenates from streptozotocin (STZ)-induced type I diabetic mice, angiotensin-infused hypertensive mice, and high fat-diets -fed LDL receptor knockout (LDLr^−/−) mice. The elevated 26S proteasome activities were accompanied by ONOO⁻-mediated PA700/S10B nitration and increased 26S proteasome assembly and caused accelerated degradation of molecules (such as GTPCH I and thioredoxin) essential to endothelial homeostasis. Pharmacological (administration of MG132) or genetic inhibition (siRNA knockdown of PA700/S10B) of the 26S proteasome blocked the degradation of the vascular protective molecules and ablated endothelial dysfunction induced by diabetes, hypertension, and western diet feeding. Taken together, these results suggest that 26S proteasome activation by ONOO⁻-induced PA700/S10B tyrosine nitration is a common route for endothelial dysfunction seen in mouse models of hypertension, diabetes, and dyslipidemia.

Cell biology of ischemia/reperfusion injury

Disorders characterized by ischemia/reperfusion (I/R), such as myocardial infarction, stroke, and peripheral vascular disease, continue to be among the most frequent causes of debilitating disease and death. Tissue injury and/or death occur as a result of the initial ischemic insult, which is determined primarily by the magnitude and duration of the interruption in the blood supply, and then subsequent damage induced by reperfusion. During prolonged ischemia, ATP levels and intracellular pH decrease as a result of anaerobic metabolism and lactate accumulation. As a consequence, ATPase-dependent ion transport mechanisms become dysfunctional, contributing to increased intracellular and mitochondrial calcium levels (calcium overload), cell swelling and rupture, and cell death by necrotic, necroptotic, apoptotic, and autophagic mechanisms. Although oxygen levels are restored upon reperfusion, a surge in the generation of reactive oxygen species occurs and proinflammatory neutrophils infiltrate ischemic tissues to exacerbate ischemic injury. The pathologic events induced by I/R orchestrate the opening of the mitochondrial permeability transition pore, which appears to represent a common end-effector of the pathologic events initiated by I/R. The aim of this treatise is to provide a comprehensive review of the mechanisms underlying the development of I/R injury, from which it should be apparent that a combination of molecular and cellular approaches targeting multiple pathologic processes to limit the extent of I/R injury must be adopted to enhance resistance to cell death and increase regenerative capacity in order to effect long-lasting repair of ischemic tissues.

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.

Inhibition of Ras-GTPase farnesylation and the ubiquitin-proteasome system or treatment with angiotensin-(1-7) attenuates spinal cord injury-induced cardiac dysfunction

Cardiovascular diseases are one of the principal causes of death and disability in people with spinal cord injury (SCI). The present study was designed to investigate if acute treatment with FPTIII (an inhibitor of Ras-GTPase farnesylation) or MG132 (an inhibitor of ubiquitin-proteasome pathway [UPS]) or administration of angiotensin-(1-7), also known as Ang-(1-7), (a known inhibitor of cardiac NF-kB) would be cardioprotective. The weight drop technique produced a consistent contusive injury of the spinal cord at the T13 segment. Hearts were isolated from rats either 6 months (SCI-6) or 12 months (SCI-12) after SCI. Hearts were perfused for 30 min and then subjected to 30 min ischemia followed by 30 min reperfusion (I/R). Recovery of cardiac function after I/R was measured as left ventricular developed pressure (P(max)) and coronary flow (CF). Drugs were given during perfusion before hearts were exposed to ischemia and reperfusion. Percent recovery (%R) in P(max) and CF in hearts from control animals were 48±6 and 50±5, respectively, whereas none of the hearts from animals with SCI recovered after 30 min of ischemia. Treatment with FPTIII, MG 132, or Ang-(1-7) before ischemia for 30 min led to significant recovery of heart function following ischemia in SCI-6 but not in SCI-12 animals. Thus we have shown that acute treatments with FPTIII, MG132, or Ang-(1-7) improve cardiac recovery following ischemic insult in animals with SCI and may represent novel therapeutic agents for preventing ischemia-induced cardiac dysfunction in patients with SCI.

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!

Enhancement of proteasome function by PA28&alpha; overexpression protects against oxidative stress

The principal function of the proteasome is targeted degradation of intracellular proteins. Proteasome dysfunction has been observed in experimental cardiomyopathies and implicated in human congestive heart failure. Measures to enhance proteasome proteolytic function are currently lacking but would be beneficial in testing the pathogenic role of proteasome dysfunction and could have significant therapeutic potential. The association of proteasome activator 28 (PA28) with the 20S proteasome may play a role in antigen processing. It is unclear, however, whether the PA28 plays any important role outside of antigen presentation, although up-regulation of PA28 has been observed in certain types of cardiomyopathy. Here, we show that PA28α overexpression (PA28αOE) stabilized PA28β, increased 11S proteasomes, and enhanced the degradation of a previously validated proteasome surrogate substrate (GFPu) in cultured neonatal rat cardiomyocytes. PA28αOE significantly attenuated H(2)O(2)-induced increases in the protein carbonyls and markedly suppressed apoptosis in cultured cardiomyocytes under basal conditions or when stressed by H(2)O(2). We conclude that PA28αOE is sufficient to up-regulate 11S proteasomes, enhance proteasome-mediated removal of misfolded and oxidized proteins, and protect against oxidative stress in cardiomyocytes, providing a highly sought means to increase proteasomal degradation of abnormal cellular proteins.

A role for the ubiquitin-proteasome system in activity-dependent presynaptic silencing

Chronic changes in electrical excitability profoundly affect synaptic transmission throughout the lifetime of a neuron. We have previously explored persistent presynaptic silencing, a form of synaptic depression at glutamate synapses produced by ongoing neuronal activity and by strong depolarization. Here we investigate the involvement of the ubiquitin-proteasome system (UPS) in the modulation of presynaptic function. We found that proteasome inhibition prevented the induction of persistent presynaptic silencing. Specifically, application of the proteasome inhibitor MG-132 (carbobenzoxy-L-leucyl-L-leucyl-L-leucinal) prevented decreases in the size of the readily releasable pool of vesicles and in the percentage of active synapses. Presynaptic silencing was accompanied by decreases in levels of the priming proteins Munc13-1 and Rim1. Importantly, overexpression of Rim1alpha prevented the induction of persistent presynaptic silencing. Furthermore, strong depolarization itself increased proteasome enzymatic activity measured in cell lysates. These results suggest that modulation of the UPS by electrical activity contributes to persistent presynaptic silencing by promoting the degradation of key presynaptic proteins.

Functional alterations of cardiac proteasomes under physiological and pathological conditions

The cardiac proteasome is a complex, heterogeneous, and dynamic organelle. Its function is regulated by its molecular organization, post-translational modifications, and associated partner proteins. Pressure overload, ischaemic heart disease, or genetic mutations in contractile proteins can cause heart failure, during which misfolded protein levels are elevated. At the same time, numerous interconnected signal transduction pathways are activated that may modulate any of the three proteasomal regulatory mechanisms mentioned above, resulting in functional changes in cardiac proteasomes. Many lines of evidence support the important role of the ubiquitin-proteasome system (UPS) in the development of heart diseases. Many researchers have focused on the UPS, applying new drug discovery methods not only in the field of cancer research but also in cardiovascular fields such as cardiac hypertrophy and ischaemic heart diseases. More understanding of UPS in the pathophysiology of heart diseases will lead to new routes for therapy.

Differential interferon responses enhance viral epitope generation by myocardial immunoproteasomes in murine enterovirus myocarditis

Murine models of coxsackievirus B3 (CVB3)-induced myocarditis mimic the divergent human disease course of cardiotropic viral infection, with host-specific outcomes ranging from complete recovery in resistant mice to chronic disease in susceptible hosts. To identify susceptibility factors that modulate the course of viral myocarditis, we show that type-I interferon (IFN) responses are considerably impaired in acute CVB3-induced myocarditis in susceptible mice, which have been linked to immunoproteasome (IP) formation. Here we report that in concurrence with distinctive type-I IFN kinetics, myocardial IP formation peaked early after infection in resistant mice and was postponed with maximum IP expression concomitant to massive inflammation and predominant type-II IFN responses in susceptible mice. IP activity is linked to a strong enhancement of antigenic viral peptide presentation. To investigate the impact of myocardial IPs in CVB3-induced myocarditis, we identified two novel CVB3 T cell epitopes, virus capsid protein 2 [285-293] and polymerase 3D [2170-2177]. Analysis of myocardial IPs in CVB3-induced myocarditis revealed that myocardial IP expression resulted in efficient epitope generation. As opposed to the susceptible host, myocardial IP expression at early stages of disease corresponded to enhanced CVB3 epitope generation in the hearts of resistant mice. We propose that this process may precondition the infected heart for adaptive immune responses. In conclusion, type-I IFN-induced myocardial IP activity at early stages coincides with less severe disease manifestation in CVB3-induced myocarditis.

Multiple cardiac proteasome subtypes differ in their susceptibility to proteasome inhibitors

AIMS

The proteasome is the proteolytically active core of the ubiquitin-proteasome system, which regulates vital processes and which can cause various diseases when it malfunctions. Therefore, the proteasome has become an attractive target for pharmaceutical interventions. Inhibition of the cardiac proteasome by specific proteasome inhibitors has been shown to attenuate cardiac hypertrophy and ischaemia reperfusion injury of the heart. We have resolved the cardiac proteasome into its subtypes and have addressed the key question of how proteasome inhibitors affect single cardiac proteasomal subtypes.

METHODS AND RESULTS

The 20S proteasome from rat heart was dissected into three different subpopulations (groups I-III), each comprising 4-7 different subtypes. The major group (group II) comprises standard proteasome subtypes; the two minor subpopulations (groups I and III) contain intermediate proteasome subtypes. All subtypes exhibit chymotrypsin-, trypsin-, and caspase-like activity but to different degrees. We have tested the effect of two common proteasome inhibitors on the chymotrypsin-like activity of all subtypes: 20-30 nmol/L MG132 caused 50% inhibition of all subtypes from groups I and II, whereas 100 nmol/L was necessary to affect group III subtypes to the same extent. However, another inhibitor, bortezomib (VELCADE), already used clinically, inhibited 50% of the activity of group III proteasome subtypes even below 20 nmol/L, a concentration showing almost no effect on group I and II proteasome subtypes. The caspase-like activity of group II proteasome subtypes was not affected by MG132 and was inhibited by bortezomib only at concentrations above 100 nmol/L.

CONCLUSION

These data show that different inhibitors have differential inhibitory effects on the various cardiac proteasome subtypes. Different cardiac subtypes are inhibited by the same dose of proteasome inhibitor to a different extent.

Targeting proteasomes for cardioprotection

The ubiquitin-proteasome system (UPS) plays a central role in intracellular protein degradation and regulates many cellular processes, including cell proliferation, inflammation, adaptation to stress, cell death, and the removal of damaged or misfolded proteins. Numerous studies have demonstrated that altered UPS function is involved in the pathogenesis of a wide range of cardiac diseases including hypertrophy and failure, myocardial ischemia, atherosclerosis, and diabetic cardiovascular disease. Impairment of proteasome function is a common feature of cardiac disease; however several studies have also demonstrated increased proteasome activity in models similar but not identical with those having decreased function. Recent studies have shown that use of proteasome inhibitors before or following production of the model of cardiac disease may confer cardioprotection under certain conditions.

A therapeutic dose of doxorubicin activates ubiquitin-proteasome system-mediated proteolysis by acting on both the ubiquitination apparatus and proteasome

The ubiquitin proteasome system (UPS) degrades abnormal proteins and most unneeded normal proteins, thereby playing a critical role in protein homeostasis in the cell. Proteasome inhibition is effective in treating certain forms of cancer, while UPS dysfunction is increasingly implicated in the pathogenesis of many severe and yet common diseases. It has been previously shown that doxorubicin (Dox) enhances the degradation of a UPS surrogate substrate in mouse hearts. To address the underlying mechanism, in the present study, we report that 1) Dox not only enhances the degradation of an exogenous UPS reporter (GFPu) but also antagonizes the proteasome inhibitor-induced accumulation of endogenous substrates (e.g., beta-catenin and c-Jun) of the UPS in cultured NIH 3T3 cells and cardiomyocytes; 2) Dox facilitates the in vitro degradation of GFPu and c-Jun by the reconstituted UPS via the enhancement of proteasomal function; 3) Dox at a therapeutically relevant dose directly stimulates the peptidase activities of purified 20S proteasomes; and 4) Dox increases, whereas proteasome inhibition decreases, E3 ligase COOH-terminus of heat shock protein cognate 70 in 3T3 cells via a posttranscriptional mechanism. These new findings suggest that Dox activates the UPS by acting directly on both the ubiquitination apparatus and proteasome.

Contrasting proteome biology and functional heterogeneity of the 20 S proteasome complexes in mammalian tissues

The 20 S proteasome complexes are major contributors to the intracellular protein degradation machinery in mammalian cells. Systematic administration of proteasome inhibitors to combat disease (e.g. cancer) has resulted in positive outcomes as well as adversary effects. The latter was attributed to, at least in part, a lack of understanding in the organ-specific responses to inhibitors and the potential diversity of proteomes of these complexes in different tissues. Accordingly, we conducted a proteomic study to characterize the 20 S proteasome complexes and their postulated organ-specific responses in the heart and liver. The cardiac and hepatic 20 S proteasomes were isolated from the same mouse strain with identical genetic background. We examined the molecular composition, complex assembly, post-translational modifications and associating partners of these proteasome complexes. Our results revealed an organ-specific molecular organization of the 20 S proteasomes with distinguished patterns of post-translational modifications as well as unique complex assembly characteristics. Furthermore, the proteome diversities are concomitant with a functional heterogeneity of the proteolytic patterns exhibited by these two organs. In particular, the heart and liver displayed distinct activity profiles to two proteasome inhibitors, epoxomicin and Z-Pro-Nle-Asp-H. Finally, the heart and liver demonstrated contrasting regulatory mechanisms from the associating partners of these proteasomes. The functional heterogeneity of the mammalian 20 S proteasome complexes underscores the concept of divergent proteomes among organs in the context of an identical genome.