Abstract

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

General introduction

The presence of unfolded, misfolded, or otherwise abnormal proteins is a never-ending threat to cells.1 These proteins, which contain non-native structures and have a propensity to form insoluble protein aggregates, are inherently toxic.2 Several sources contribute to the pool of aggregation-prone proteins. First, the structure of proteins can be severely affected by the introduction of non-native modifications. Besides spontaneous denaturation, aberrant modifications such as oxidation, nitrosylation, and isomerization can distort the folding of proteins rendering them prone to aggregation. Secondly, proteins may fail to adopt their native conformation due to the absence of binding partners that are crucial for the formation of a multicomponent complex. Thirdly, proteins may be intrinsically misfolded or structurally unstable due to the presence of mutations, which is a prevalent factor in genetic diseases characterized by the presence of protein aggregates.3 These disorders, which are collectively referred to as ‘conformational diseases’,3 are a heterogeneous collection of familial and sporadic pathologies, including many neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and prion diseases,4 as well as type II diabetes mellitus5 and cardiac diseases.6

Although one may expect that old and ‘worn-out’ proteins are overrepresented among misfolded proteins, a number of studies suggest that a major fraction is derived from the pool of newly synthesized proteins.7,8 It has been estimated that as much as one-third of all proteins are destroyed within 10 min after synthesis.9 Although these polypeptides have been coined ‘DRiPs’ (for Defective Ribosomal Products),10 it is unclear whether they are truly inaccurately synthesized polypeptides or whether they represent a pool of nascent proteins that are degraded before they have been able to adopt their native conformation.11 Alternatively, these short-lived proteins may have been damaged directly after proper synthesis since newly synthesized proteins are particularly sensitive to proteotoxic stress conditions.12

As the generation of misfolded proteins is directly linked to protein synthesis and hence an inevitable part of life, this leaves the cell with little alternative than to cope with their presence and eliminate them in a timely fashion. The potential danger of aggregation-prone proteins can be countered by assisting proteins in their folding, preventing the exposure of non-native structures or by elimination of proteins.1 This process is facilitated by molecular chaperones, which assist in the de novo folding of nascent chains or refolding of damaged proteins.13 A general feature of chaperones is that they bind hydrophobic structures, thereby keeping non-native proteins in a soluble state preventing aggregation.

If a protein fails to fold properly, the only safe way to solve the potential threat permanently is through destruction. For this purpose, cells have developed an elaborate degradation system that can break down almost any polypeptide in a highly specific manner. The ubiquitin-proteasome system (UPS), which resides both in the nucleus and in the cytosol, consists of a large number of proteins that are dedicated to the identification, targeting, and destruction of proteins (Figure 1).14 The fact that this system can hydrolyse almost any polypeptide chain is accomplished through the engagement of three different proteases with complementary activities: chymotrypsin-like, trypsin-like, and caspase-like activities.15 These proteases are embedded in a large barrel-shaped proteolytic complex known as the proteasome. The proteolytic chamber of the 20S core particle of the proteasome can only be accessed through gated entrances on either side of the proteasome.16 It is this compartmentalized structure that safeguards the highly specific nature of the process and prevents non-specific protein degradation.17

Figure 1

The ubiquitin-proteasome system. Free ubiquitin is generated from processing of ubiquitin precursors or ubiquitin chains by DUBs. An enzymatic cascade, involving the E1, E2, and E3 enzymes, forms covalently conjugated ubiquitin chains on target proteins that are singled out by the presence of a degradation signal (degron). The ubiquitylated substrate is recognized by the proteasome, where also unfolding and deubiquitylation take place prior to hydrolysis in its interior chamber. For more details see text.

Figure 1

The ubiquitin-proteasome system. Free ubiquitin is generated from processing of ubiquitin precursors or ubiquitin chains by DUBs. An enzymatic cascade, involving the E1, E2, and E3 enzymes, forms covalently conjugated ubiquitin chains on target proteins that are singled out by the presence of a degradation signal (degron). The ubiquitylated substrate is recognized by the proteasome, where also unfolding and deubiquitylation take place prior to hydrolysis in its interior chamber. For more details see text.

Proteins are marked for degradation by the proteasome by the covalent linkage of a chain of ubiquitin proteins (Figure 1, left panel). An enzymatic cascade, involving the ubiquitin activase (E1), one of several ubiquitin conjugases (E2s), and one of the many different ubiquitin ligases (E3s), results in the covalent linkage of the C terminus of the 76 amino acid long ubiquitin polypeptide to the ε-NH2 group of an internal lysine residue of the substrate.18 Since ubiquitin contains seven lysine residues, which can each be targeted for ubiquitin conjugation (i.e. ubiquitylation), successive rounds can result in the formation of long ubiquitin chains. The lysine residue in the ubiquitin that is used for formation of the polyubiquitin chain determines the fate of the protein.19 Although lysine 48-linked polyubiquitin chains are the canonical targeting signal for proteasomal degradation, recent studies revealed that alternative chains can also facilitate ubiquitin-dependent proteasomal degradation.20–22 Polyubiquitylated proteins bind to specific ubiquitin-binding domains (UBDs) that are located in shuttling factors and the proteasome itself.23 Ubiquitylation is a reversible process and the chains can be disassembled to ubiquitin monomers by deubiquitylation enzymes (DUBs),24 a process that also takes place at the proteasome prior to degradation (Figure 1, right panel).25 Finally, the protein has to be unfolded and funnelled through the narrow entrance of the proteasome in order to be hydrolysed.26

The UPS is not only the final destination for proteins that fail the protein quality control in the nucleus and cytosol but also for misfolded proteins residing in the endoplasmic reticulum (ER), a process known as ER-associated degradation (ERAD).27 In addition to its task in protein quality control, which is undoubtedly of vital importance for cell survival, the UPS also plays an essential role in other cellular processes such as intracellular signalling, regulation of the cell cycle, and induction of apoptosis (Figure 2).14 Since the UPS is the only proteolytic machinery that facilitates processive degradation in the nuclear and cytosolic compartments in a swift and tightly regulated manner, it is not surprising that this system has taken a central position in both protein quality control and these regulatory processes. As discussed below, the prioritization of these divergent but essential processes may form an extra challenge under conditions of proteotoxic stress.

Figure 2

Protein quality control and regulation by the UPS. Proteasomal degradation of regulatory proteins as well as misfolded proteins is a continuously ongoing process, here indicated by red arrows. Extrinsic and intrinsic factors can cause stress that impairs the ability to degrade substrates whereupon potentially harmful protein accumulation takes place as indicated by grey arrows.

Figure 2

Protein quality control and regulation by the UPS. Proteasomal degradation of regulatory proteins as well as misfolded proteins is a continuously ongoing process, here indicated by red arrows. Extrinsic and intrinsic factors can cause stress that impairs the ability to degrade substrates whereupon potentially harmful protein accumulation takes place as indicated by grey arrows.

Protein quality control and the ubiquitin-proteasome system (UPS)

To ensure that proteins meet the demands for proper functioning, they are subject to a process referred to as protein quality control. One of the most important steps in this process is the recognition of non-native proteins before they encounter other non-native proteins and form insoluble aggregates.13 The potential damage to the cellular homeostasis can only be prevented or minimized by sending the terminally misfolded protein for destruction by the UPS in a timely fashion. At the same time, it is unavoidable that newly synthesized proteins require time to reach their folded state and, during this process, they are likely to expose aggregation-prone structures. Cells must therefore somehow balance the rigorous destruction of misfolded proteins with giving immature proteins sufficient time to reach their final conformation.

The protein quality control machinery has been studied in most detail in the ER.28 An additional challenge for the folding in this compartment is that many of these proteins are secretory and cell surface proteins, which will be exposed to the harsh extracellular milieu.29 Consequently, folding of these proteins is more complicated and different from folding in the cytosol and nucleus. Only properly folded proteins are allowed to reach their final destination, whereas unfolded or misfolded proteins are retained and subjected to additional rounds of folding cycles.28 Failure to fold properly will not only result in retention but eventually target the protein for ERAD starting with dislocating the substrate to the cytosol.27 During this stage, the cytosolic ubiquitylation machinery comes in play and specific ubiquitylation,30 deubiquitylation,31 and UBD-containing proteins32 reside at the ER/cytosol interface to regulate the retro-translocation from the ER to the cytosol and the subsequent degradation of these proteins. The urgent need to avoid excessive production of misfolded proteins in the ER is reflected in the unfolded protein response (UPR), which is an adaptive response triggered by the accumulation of non-native proteins in the ER. The UPR results in inhibition of translation, which reduces the ER load, and up-regulation of chaperones and ERAD components to promote folding and eventually degradation of the terminally misfolded proteins.33 Although less well understood, recent studies suggest that the UPS may also be involved in the degradation of proteins that fail the protein quality control in the matrix and inter-membrane space of the mitochondria, which would require retro-translocation of proteins analogous to ERAD.34

The cytosol is an important place for protein quality control since, together with the ER, it is the primary location for de novo protein synthesis.7,12 Unlike the situation in the ER, where the identification of misfolded proteins takes place in complete isolation of the UPS, components of the UPS can play a direct role in protein quality control in the cytosol and nucleus. Most notably are the interactions between chaperones and components of the ubiquitylation machinery, which can regulate whether unfolded proteins bound to chaperones are targeted for either refolding or degradation. This mechanism of cross-talk between the refolding and degradation machineries has been referred to as the protein triage.35 The co-chaperone CHIP (carboxyl terminus of Hsp70 interacting protein) illustrates the direct link between these systems in the cytosol.36 CHIP is a ubiquitin ligase that interacts with Hsp70 and Hsp90 facilitating ubiquitylation and degradation of non-native proteins bound to these molecular chaperones.37 It has indeed been shown that CHIP stimulates degradation of a diverse set of aggregation-prone proteins implicated in conformational diseases.38,39 Interestingly, CHIP also ubiquitylates Hsp70, in the absence of a non-native cargo protein, establishing a negative feedback loop that adjusts the levels of this chaperone to the amounts of cytosolic non-native proteins.40 Another interesting protein in this respect is BAG-1, which contains an internal ubiquitin-like domain and forms a trimeric complex together with Hsp70 and CHIP. BAG-1 stimulates the release of non-native proteins from Hsp70 leading to their proteasomal degradation.41 It should be noted that in addition to refolding and proteasomal degradation, the cytosolic compartment has at least two additional means to deal with misfolded proteins, namely the transient or permanent storage in intracellular inclusions42 and the degradation in the lysosomes through autophagy.43 These alternative solutions may be particularly important in the case of aggregated proteins that are resistant to proteasomal degradation.44

Little is known about protein quality control in the nucleus. The nuclear ubiquitin ligase San1 has recently been identified in the budding yeast Saccharomyces cerevisiae as a key player in nuclear protein quality control but no orthologs were found in metazoan cells.45

UPS impairment

It has been well documented that deficiencies in one of the two key players, i.e. ubiquitin or the proteasome, are related to proteotoxic stress. These conditions, which we will refer to as ‘ubiquitin stress’ and ‘proteasome stress’, respectively, have a great impact on the physiology of the cell and, unless timely corrected, can cause cellular dysfunction or death.

Ubiquitin stress

The role of ubiquitylation is not limited to the regulation of proteolysis but is also critical for numerous other cellular processes such as endocytosis,46 transcription,47 and DNA repair.48 It is noteworthy that in addition to targeting non-native proteins for proteasomal degradation, ubiquitylation is also involved in the formation of intracellular inclusions,49 as well as the clearance of aggregation-prone proteins through autophagy.50 Thus, ubiquitin appears to be implicated at various levels of the response to misfolded proteins.

The ubiquitin status in cells is highly dynamic since conjugation and de-conjugation are continuously ongoing processes. This means that the pool of free unconjugated ubiquitin is constantly replenished with ubiquitin molecules constitutively being released from substrates or newly synthesized ubiquitin precursors. Although the intracellular ubiquitin levels are relatively high, the pool of free unconjugated ubiquitin is surprisingly small.51 It appears that the vast majority of ubiquitin is either conjugated or in the process of being conjugated, leaving only a limited pool of free ubiquitin. This implies that an increase in the demands of ubiquitin for a specific ubiquitin-dependent process can dramatically change the homeostasis and affect other processes that are also mediated by ubiquitin conjugation. Proteotoxic stress increases the demand on ubiquitylation for the purpose of targeting misfolded proteins for destruction resulting in an accumulation of polyubiquitylated proteins at the expense of the levels of free ubiquitin.51 Accumulation of polyubiquitylated proteins during proteotoxic stress coincides with a reduction in the level of monoubiquitylated histone H2A,52,53 which is an important epigenetic marker for chromatin condensation and transcriptional regulation.54 This correlation appears to be merely a consequence of a direct competition between proteasome substrates and histones for the limited pool of free ubiquitin.51 Interestingly, proteotoxic stress-induced UPS impairment in thermally shocked cells can be directly attributed to a reduction in free ubiquitin levels since increasing the ubiquitin concentration prior to the proteotoxic insult prevents impairment of proteasomal degradation.55 Accordingly, the UPS dysfunction in thermally shocked cells is confined to ubiquitin-dependent substrates, whereas the degradation of substrates that are targeted independent of ubiquitin was not affected, consistent with ubiquitin being the limiting factor.55,56

Given the limited availability of free ubiquitin and the high demands during proteotoxic stress, it is not surprising that synthesis of ubiquitin is strongly induced as part of the heat shock response and that ubiquitin levels steadily increase during or in the aftermath of a proteotoxic insult.57 Besides transcriptional activation, the levels of free ubiquitin can also be increased by accelerating the disassembly of ubiquitin chains.58,59 In budding yeast, two additional mechanisms have been described for maintaining the ubiquitin equilibrium by these means. First, the DUB Ubp6, which associates with the proteasome and rescues ubiquitin by disassembling chains more efficiently prior to protein degradation, is induced during stress conditions.58 The neurological pathology that is observed in mice carrying recessive mutations in the murine ortholog Usp1460 can be rescued by overexpression of ubiquitin,61 which suggests an important role of this enzyme in maintaining the ubiquitin equilibrium also in mammalian cells. Secondly, the DUB inhibitor Rfu1 (Regulator of free ubiquitin chain-1) fine-tunes the generation of free monomeric ubiquitin in yeast through the regulation of DUBs that disassemble unanchored ubiquitin chains.59 The human genome does not contain genes that encode any known Rfu1 orthologs and it is unclear if a similar regulatory mechanism exists in human cells.

A special case regarding the role of ubiquitin in proteotoxic stress is the aberrant ubiquitin UBB+1, which has been found in neurological62 and non-neurological63–65 conformational diseases. The UBB+1 protein is derived from an erroneous transcriptional product from the ubiquitin B gene. It consists of the ubiquitin polypeptide with the last glycine residue substituted for a tyrosine followed by a 19 amino acid long C-terminal extension.62 The UBB+1 is polyubiquitylated and is refractory to DUBs that disassemble polyubiquitin chains.66 At low concentrations, it is degraded by the UPS, but at elevated levels, it resists degradation and causes a general UPS inhibition.67,68 Proteotoxic stress can affect the clearance of UBB+1,69 and it has been speculated that its presence in conformational diseases may be an indication that the affected cells have experienced episodes of dysfunctional UPS.70 The reason for the inefficient clearance of UBB+1 can be attributed to the limited length of the C-terminal extension, which is too short to be handled as an efficient proteasome substrate.71 Although UBB+1 exemplifies that ubiquitin itself can be a cause for UPS impairment, the molecular mechanism is unresolved and it is therefore not clear if UBB+1 causes ubiquitin stress (i.e. ubiquitin deficiency) or affects other events critical for proteasomal degradation.

Proteasome stress

Due to size constraints, unfolding of the substrate prior to translocation through the entrance of the proteasome is an absolute prerequisite for proteasomal degradation.72 The 19S regulatory particle, which docks to the entrance of the proteasome 20S core particle, contains a hexameric ring of AAA-ATPases with unfoldase activity.26 For efficient proteasomal degradation, substrates need a loosely folded initiation site.73 Recent studies suggest that a minimal length of 20–30 amino acids is required for efficient degradation,71,74 which may be needed to cover the physical distance from the folded polyubiquitylated protein to the AAA-ATPase responsible for unwinding the polypeptide chain.75 Although the diverse activities of the proteases residing in the proteasome enables the complex to hydrolyse virtually any polypeptide chain, protein unfolding can still be a major hindrance for efficient degradation. This may be problematic in particular for proteins that form tight non-native interactions as in the case of protein aggregates. Consistently, aggregated proteins can resist degradation by the UPS,44 and preferentially accumulate under conditions of a compromised UPS.55 Since both ubiquitin and proteasomes are present in the intracellular inclusions observed in conformational diseases, it has been proposed that this may be the result of futile attempts of the system to degrade these proteins, which in turn may impair the UPS.4

The presence of aggregation-prone proteins in cellular models can severely inhibit the functionality of the UPS causing a general impairment.76,77 In at least one conformational disorder, the UPS impairment has been directly linked to reduced proteasome activity. Namely, disease-associated prion protein induces the formation of intracellular inclusions,78 inhibits the proteolytic activity of the proteasome, and causes a general inhibition of ubiquitin-mediated degradation in both cellular and animal models for prion pathology.79 This suggests that a direct inhibition of the proteasome may be responsible for the UPS impairment in some conformational diseases. However, the absence of general UPS impairment has been reported for several other conformational diseases, suggesting that this is not a shared property for these disorders.80–83

Proteasome activity appears to be largely excessive during normal cellular conditions since the chymotrypsin-like activity has to be inhibited by as much as 80% before a functional impairment of the UPS occurs.76,84 The surplus activity may reflect the need of the cell to be able to respond instantly to conditions of proteotoxic stress without having to synthesize large amounts of new proteasomes. Cells that are encountering chronic proteotoxic stress conditions appear indeed to be more sensitive to proteasome inhibitors.85 This feature is likely to be a consequence of the fact that these cells utilize a larger proportion of their UPS capacity, which in turn decrease the threshold of the minimal proteasome activity required for cell viability.84

Even though the induction of genes encoding proteasome subunits is probably too slow to generate enough proteolytic capacity to avoid acute protein aggregation, increasing the proteasome pool may eventually help to relieve the burden on the UPS. The mechanism responsible for regulating proteasome levels during proteotoxic stress has been studied in most detail in budding yeast. The transcription factor Rpn4 is not only a general activator of genes encoding proteins involved in the UPS but is also a proteasome substrate.86 An insufficient proteasome capacity will cause a general accumulation of proteasome substrates including Rpn4, which will adjust the capacity of the UPS by a general induction of its components by activating promoters that contain a so-called PACE (Proteasome-Associated Control Element) sequence.87 Although no orthologs of Rpn4 have been found in the human genome, metazoan cells also compensate for reduced proteasome activity by increasing the levels of proteasome subunits.88,89 In addition to increased proteasome synthesis, cells enhance proteasome assembly by increasing the expression of the maturation factor POMP.90

Cells with compromised proteasome activity can restore proteolysis by increasing expression of other proteases, such as the tripeptidyl peptidase-II91,92 and DUBs,93 which by a poorly defined mechanism are able to allow cells to proliferate with minimal levels of proteasome activity. Cells that have activated these compensatory mechanisms displayed an increased resistance to proteasome inhibitor-induced apoptosis.93,94 To our knowledge, the only presently described regulatory system that directly anticipates the increased demand for proteasome activity during proteotoxic stress in mammalian cells is the transcriptional activator Nrf2, which induces expression of proteasome subunits in response to oxidative stress.95 Nrf2 is constitutively ubiquitylated and degraded by the proteasome but stabilized during oxidative stress when its ubiquitin ligase Keap1 is targeted for degradation instead.96 An insufficient capacity of the UPS in mammalian cells triggers a profound stress response through the activation of the heat shock factor 1, which in turn results in the induction of molecular chaperones that can assist the folding or prevent aggregation of non-native proteins.97

Other limiting factors

Besides ubiquitin and the proteasome, a large number of proteins are involved in the regulated destruction of aberrant proteins during proteotoxic stress. Consequently, UPS impairment may be caused by a severe overload or dysfunction of other proteins crucial for a functional UPS. In this respect, it is noteworthy that a recent study mechanistically linked the AAA-ATPase p97Ufd1/Npl4 complex, which is implicated in the degradation of ERAD substrates98,99 as well as some specific cytosolic substrates,100–102 for being a primary cause of aggregation-induced UPS impairment.103

UPS impairment and apoptosis

The involvement of the UPS in various cellular functions, including protein quality control as well as cycle regulation and apoptosis, involves a risk since overloading the UPS with defective proteins, which may occur during proteotoxic stress, can in turn potentially interfere with cell cycle progression and cell viability (Figure 2).104 It is well established that a general inhibition of the UPS causes cell cycle arrest followed by cell death, a feature that is exploited in the usage of proteasome inhibitors for the treatment of some malignancies.105 In cellular models, the presence of aggregation-prone proteins76,106 or other proteotoxic stress conditions55,69 can also cause impairment of the UPS and accumulation of pro-apoptotic proteasome substrates. It seems plausible that the induction of apoptosis under those conditions is, at least partly, caused by impairment of the UPS. However, it should be emphasized that these findings are mostly correlative and do not conclusively show that there is a casual relationship. Since proteotoxic stress has, besides impairment of the UPS, a plethora of additional effects on the physiology of cells, the induction of programmed cell death through stabilization of key regulators may be just one of several mechanisms by which these cells perish.

In the case of ER stress, a condition that also causes UPS dysfunction,69 cells activate the UPR to restore homeostasis, but if left unresolved, a specific pro-apoptotic programme will be triggered to eliminate affected cells.33 The transcriptional activator C/EBP homologous protein (CHOP) is an important regulator of the ER stress-induced apoptosis.107 Strikingly, inhibition of apoptosis by ablation of CHOP can reduce the symptoms in some mouse models for conformational diseases.108,109 Cells appear to have downstream effectors dedicated to the elimination of terminally stressed cells, with caspase 12110 and caspase 4111 being responsible for ER stress-induced apoptosis in murine and human cells, respectively. The presence of specific systems to directly link cellular stress to an apoptotic response underscores the importance to eradicate dysfunctional cells.

The occurrence of stress-induced UPS impairment in the context of human diseases has been generally considered as a negative factor that contributes to the pathology. Accordingly, restoration of UPS function may have a beneficial effect.1,4 It remains, however, possible that the stress-induced UPS impairment fulfils a protective function as a last resort to eliminate aggregation-prone proteins through the organized destruction of cells containing the rogue proteins. Although proliferating cells may be able to get rid of insoluble protein aggregates through asymmetrical division,112 induction of apoptosis may be the only way to eliminate post-mitotic cells that are affected by protein aggregation, a process that may be initiated by stress-induced UPS impairment. Notably, stress-induced UPS impairment may also be relevant for non-pathological processes. It has been proposed that the natural lifespan of activated B lymphocytes may be regulated by proteasome stress caused by the large production of non-native immunoglobulins that gradually overload of the UPS.85

The UPS in cardiac diseases

An increasing body of evidence implicates UPS dysfunction in cardiac diseases. Cardiopathies caused by misfolded proteins display the typical intracellular inclusions composed of amyloid-like deposits similar to those observed in neurodegenerative conformational diseases. A most striking example of the commonalities in conformational neurodegenerative and cardiac diseases is the observation that a mice expressing a prion protein lacking the glycophosphatydylinositol membrane anchor develops a cardiac disease with amyloidogenesis.113 A well-studied example of a conformational disease affecting the heart is the desmin-related myopathies, which are a group of congestive heart failure disorders characterized by the presence of misfolded desmin protein. These diseases can be caused by missense mutations in desmin114 or in the small heat shock protein αB-crystallin, required for proper folding of desmin.6,115 Similar to the situation in other conformational diseases,116,117 the presence of the aggregation-prone proteins causes a general impairment of the UPS, which is accompanied by changes in proteasome activity.118 The UPS dysfunction caused by mutant αB-crystallin could largely be prevented by overexpression of the small heat shock protein Hsp22 or Hsp25, presumably through the inhibition of the formation of toxic oligomers.119 Transgenic mice overexpressing a mutant αB-crystallin that is linked to a familial cardiomyopathy revealed that reductive stress caused by an increase in reductive molecules causes protein aggregation and contributes to the pathology suggesting a pivotal role for protein misfolding in this disease.120 Interestingly, the presence of the aberrant ubiquitin UBB+1, which can inhibit the UPS66,67 and may be an indicator for UPS dysfunction,70 has been reported in desmin-like pathologies.64

Another condition that is directly linked to protein misfolding is ischaemia both of the heart and the brain.121 A number of studies have shown that ischaemia causes a general activation of the UPR, suggesting that the cardiomyocytes are encountering ER stress under hypoxic conditions,122 a condition which is known to compromise the functional status of the UPS.69 It is not clear whether the induction of the UPR in cardiomyocytes contributes to the disease or plays a protective role. Indeed, inducing ER stress in cardiomyocytes prior to a hypoxic stress can have a protective role and improve survival.123 On the other hand, ER stress can result in activation of an apoptotic programme and this, at least in vitro, seems to contribute to cell death under hypoxic conditions.124 Ischaemic conditions result in accumulation of ubiquitylated and oxidized proteins and activate the UPR consistent with the presence of misfolded proteins and proteotoxic stress.125 Thus, it remains to be seen whether the UPR contributes to, or protects from, the molecular pathophysiology in ischaemia of the heart.

The proteasome inhibitor bortezomib (also known as Velcade™) has recently been approved for treatment of multiple myeloma.105 Although it remains unclear why malignant cells display increased sensitivity to proteasome inhibition,89 it appears that in the case of multiple myeloma, major contributing factors are inhibition of NF-κB activation and interference with the UPR.126 Although the side effects of bortezomib are relatively mild, it is noteworthy that congestive heart failure has been reported for several patients.127,128 It has been shown that proteasome inhibition induces ER stress in cardiomyocytes129 and that overexpression of ER chaperones can prevent proteasome inhibitor-induced apoptosis of cardiomyocytes, suggesting that ER stress may be implicated in this phenomenon.129 Moreover, ER stress results in the induction of an apoptotic programme in cardiomyocytes, which has been linked to heart failure.124 It has, however, also been shown that treatment of cardiomyocytes with low doses of proteasome inhibitors can enhance survival of cardiomyocytes under ischaemic conditions.130 This may be mediated through the activation of the heat shock response, which allows the cardiomyocytes to more promptly deal with misfolded proteins during stress conditions.

The possible implication of UPS dysfunction in cardiac diseases is intriguing and relevant for our understanding of the pathophysiology as well as for the development of therapies. In this review, we have discussed only a few examples of the link between proteotoxic stress, the UPS, and cardiac diseases as this topic is covered in other reviews in this issue.131,132 Although the intimate link between the UPS and muscle disease has gained much interest during the last few years, this is a topic that entered the stage two decades ago shortly after the discovery of the UPS. It is noteworthy that the first proteasome inhibitors, of which the first derivative recently found its way to the clinic as an anti-cancer drug, were originally developed with the aim to block muscle wasting (such as the commonly used peptide aldehyde proteasome inhibitor MG132, which stand for MyoGenic 132).133 More recently, the UPS was found to be important also for muscle development.102 Altogether a far more complicated picture is emerging with intracellular protein degradation by the UPS playing a central role in multiple pathways in the cardiomyocyte in health and disease.

Funding

The work is supported by the Swedish Research Council (N.P.D. and K.L.), Magn Bergvall's Foundation (K.L.), Karolinska Institutet (K.L.), the Swedish Cancer Society (N.P.D.), the Nordic Center of Excellence Neurodegeneration (N.P.D.), and the EU Network of Excellence Rubicon (N.P.D. and K.L.).

Acknowledgements

We thank Martijn Luijsterburg and Florian Salomons for critical reading of the manuscript and helpful suggestions.

Conflict of interest: none declared.

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Author notes

This article is part of the Spotlight Issue on: The Role of the Ubiquitin-Proteasome Pathway in Cardiovascular Disease