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Post-Translational Modification of Cellular Proteins by Ubiquitin and Ubiquitin-Like Molecules: Role in Cellular Senescence and Ageing

,* , and .

* Corresponding Author: Johannes Grillari—Institute of Applied Microbiology, Department of Biotechnology, University for Natural Resources and Applied Life Sciences, Vienna, Muthgasse 18, A-1190 Vienna, Austria.

Protein Metabolism and Homeostasis in Aging edited by Nektarios Tavernarakis.
©2010 Landes Bioscience and Springer Science+Business Media.
Read this chapter in the Madame Curie Bioscience Database here.

Ubiquitination of endogenous proteins is one of the key regulatory steps that guides protein degradation through regulation of proteasome activity. During the last years evidence has accumulated that proteasome activity is decreased during the ageing process in various model systems and that these changes might be causally related to ageing and age-associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system of ubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination confers also functions different from targeting proteins for degradation.

Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, post-translational modifications of cellular proteins by covalent attachment of ubiquitin and ubiquitin-like proteins are involved in transcriptional regulation, receptor internalization, DNA repair, stabilization of protein complexes and autophagy. Here, we summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue ageing as well as in segmental progeroid syndromes and discuss potential causes and consequences for ageing.


Post-translational modifications of cellular proteins by ubiquitin and ubiquitin-like proteins are important regulatory events involved in several aspects of cellular physiology that have come into focus of ageing research recently. One of the most important machineries for degradation of endogenous proteins is the proteasome, whose activity decreases during the ageing process. Since several major age-related diseases have been characterized as protein degradation failures, most studies have aimed at elucidating the function of the proteasome and its activity in the course of cell, tissue and organism ageing and several excellent reviews are available in this regard1-4 as well as on age-associated neurodegenerative pathologies.5,6

However, little has been reported on age-associated modulation of the primary event, the selection of proteasome targets through polyubiquitination, a major regulatory step in this system resulting from the opposing activities of ubiquitin ligases and deubiquitinylases (DUB). In addition, during the last years it became quite clear that degradation is not the only possible consequence for proteins after modification by ubiquitin and ubiquitin-like proteins. Depending on the type of ubiquitination, it has been found as to have "noncanonical" functions like regulation of DNA repair (reviewed in ref. 7), transcriptional regulation,8 membrane trafficking,9 cell signalling10 and autophagy.11

In this review we provide some examples of the current research regarding activity and functions of the ubiquitin system with focus on ubiquitination during cellular ageing and ageing of tissues. Since, as stated above, many excellent reviews on changes of the proteasome during ageing exist, we refer the reader to those for detailed information. A contemporary review of this topic is also found in the chapter by Gonos et al in this book. Similarly, the recent tight links that have been found between aging and autophagy have been reviewed in detail and here in the chapter by G. Mariño et al. Changes in this major cellular pathway are of high interest since they will affect cellular behaviour within tissues and organisms and thus might influence the ageing process and might also be causally related to the ageing process or pathogenesis of ageing related diseases.

Ubiquitin Is Activated and Transferred by a Cascade of E1, E2 and E3 Enzymes

A sequence of reactions guarantees the correct selection of target proteins for ubiquitination. Most targets are ubiquitinated at internal lysine residues, but in some lysine-less proteins also N-terminal ubiquitination has been observed.12 The first enzyme in this cascade is E1 that activates ubiquitin under consumption of ATP by forming a thiol ester with the C-terminal glycine of ubiquitin. Then the ubiquitin moiety is transferred to an E2 enzyme by transesterification. The E2 enzyme in turn either transfers the ubiquitin to an E3 enzyme that is responsible for the transfer to a specific final target, or it ubiquitinates the target by direct interaction with an E3 enzyme (reviewed in refs. 13-15). In some cases, a novel class of E4 enzymes exists that is responsible for polyubiquitylation, as reviewed by ref. 16.

Specificity of target recognition is guaranteed in a sophisticated and economic way by up to ten E1 enzymes, up to 100 E2 enzymes and an estimated number of over 1000 E3 enzymes in human cells.17 Current estimations suggest that there are as many E3 enzymes encoded in the human genome as kinases, underlining the universal importance of the ubiquitin proteasome pathway. Therefore we also propose to use the term "ubiquitinome" to refer to the total of ubiquitinated proteins in analogy to the "phosphoproteome" and other well known-omes.

Alternative Linking of Polyubiquitin Chains Decides the Fate of the Target Protein

The "classical" signal for protein degradation is conferred by K48 linkage of several ubiquitin moieties to the target protein. At least 4 ubiquitins are necessary for efficient binding to the proteasome and for degradation.18 Including the regular K48, there are a total of 7 lysine residues in ubiquitin which can be used for linking poly-ubiquitin chains together. A recent report shows different reactivity of these individual residues: K48, K63 and K6 are approximately equally reactive followed by K33 and K11. The least reactive sites are K27 and K29.19

K63 linkage is involved in a variety of different cellular pathways like endocytosis of cell surface receptors,20 cell signalling,10 mitochondrial inheritance and morphogenesis,21 ribosomal function,22 DNA repair7 and activation of the NFκB signalling complex,23,24 whose inactivation is in turn guaranteed by deubiquitination;25 see also below).

Little is known about the function of K29 ubiquitin chains yet, but it seems to allow proteasomal degradation similar to K48.26 E2 and E3 enzymes that act together for this type of polyubiquitination have been identified as UbcH5A, a 120-kDa E2,27 as well as the HECT domain E3 KIAA10.28 Both proteins can also promote K48 chain assembly and at least for KIAA10 it has been shown that site selection is dependent on the ubiquitin.29

One of the few currently known E3 ligases that synthesize K11 linked polyubiquitin is CHIP, a "Really Interesting New Gene" (RING)-domain E3 ligase. Upon formation of a trimer consisting of HSc70, the cochaperone BAG-1 and CHIP, BAG-1 is strongly polyubiquitinated by a K11 ubiquitin chain. This leads to a degradation-independent association of the cochaperone with the proteasome, suggesting that ubiquitination of delivery factors might represent a novel mechanism to regulate protein sorting to the proteasome.30 Furthermore, CHIP seems by ubiquitination of tau to attenuate the formation of neurofibrillary tangles, which are a major diagnostic marker of Alzheimer's disease.31 K6 linkage has been observed to counteract proteasomal degradation as competitor.32 The last noncanonical form of ubiquitin attachment is monoubiquitination that is involved in DNA repair,7 receptor endocytosis33 and signalling.8

Ubiquitin is Expressed as Fusion Protein and Is Recycled by Ubiquitin Specific Hydrolases

Ubiquitin is not expressed from a single gene locus but either from multi-copy loci or as linear fusion to ribosomal subunits. Because of the latter, translation and maturation of the ribosome that synthesizes proteins simultaneously provides the signal to destroy them. Furthermore, steady state levels of free ubiquitin are also regulated by the recycling rate from ubiquitinated proteins that are degraded by the proteasome (reviewed by ref. 14). The proteins involved in providing free ubiquitin are categorized as deubiquitinating enzymes (DUBs). All of them are cysteine proteases that hydrolyze the amide bond immediately after the COOH-terminal Gly76. Two different classes of enzymes are discriminated: ubiquitin COOH-terminal hydrolyses (UCHs) and ubiquitin-specific proteases (UBPs). The discrimination is based on molecular weight and sequence similarity. While the smaller UCHs cut peptides and flexible small proteins, the larger UBPs are processing ubiquitinated proteins. Genome sequencing projects have identified many candidate deubiquitinating enzymes, making them the largest family of enzymes in the ubiquitin system.34

Ubiquitin-Like Proteins

In addition to ubiquitin, several ubiquitin-like molecules have been described in the past of which SUMO is the molecule that is most studied. Although SUMOylation affects a similar large number of substrates than ubiquitination, the pathway is much simpler and involves one E1 ligase, a single E2-ligase (UBC9) and only a few E3 ligases. UBC9 usually binds the substrates directly, but the E3 ligases specific for SUMOylation also seem to contribute to substrate specificity. SUMOylation often targets a lysine residue within a consensus sequence, but other lysine residues can be modified as well. In contrast to yeast, the vertebrate genomes encode four SUMO variants, referred to as SUMO-1, -2 ,-3 and -4. Both ubiquitin and SUMO can form polymodifier chains and then can be conjugated to lysine residues of an already substrate-conjugated molecule. In contrast to polyubiquitin chains, little is known about the general function of polySUMO chains. The situation is even more complex, since recently it was discovered that branched polyubiquitin modifications occur in vivo and that SUMO itself may be modified by ubiquitin, resulting in mixed ubiquitin/SUMO chains, although the relevance of these modifications still have to be demonstrated. Importantly, both ubiquitination and SUMOylation are reversible by specific hydrolases, which can remove the modifications (for review, see ref. 35).

Further ubiquitin-like modifiers the enzymology of which is highly similar to that of ubiqutin transfer are atg8 and atg12, necessary for autophagy (reviewed in ref. 36), Nedd837 and Isg15 (interferon stimulated gene 15), involved in anti-viral responses.38

Role of Ubiquitination, SUMOylation and ISG15 in Cellular Senescence

In 1961, Hayflick and Moorhead described that normal human cells, after a defined number of cell divisions, enter a phase of irreversible growth arrest termed replicative senescence.39 Since then replicative senescence has been widely studied and discussed for its value as model system for ageing research. Recently replicative senescence has been associated with cellular ageing and tumor suppression,40 since cellular senescence was shown to inhibit unwanted cell proliferation; moreover senescent cells in vivo might provide a microenvironment in tissues that favours proliferation of tumor cells.41 In addition, replicative senescent cells were observed in a large variety of tissues of aged mice42 and different human tissues such as skin,43,44 liver,45 kidney46,47 and vasculature48,49 and preliminary evidence suggests that they are implicated in ageing and ageing associated disease.

Several lines of evidence indicate that a prominent stimulus for entering the phase of replicative senescence is progressive telomere erosion due to the "end-replication problem".50 Critically short telomeres induce DNA damage response pathways and subsequent the induction of a permanent cell cycle arrest,51 which is executed by the p53/p21 or the p16INK4a pathway in a cell type specific manner (reviewed by refs. 52, 53). The hypothesis of telomere dependent induction of senescence is supported by the fact that overexpression of the catalytic subunit of human telomerase (hTERT) and subsequent telomere stabilization immortalizes a large variety of normal human cells.54,55

In addition to telomere erosion, other stimuli can induce (premature) cellular senescence, which in most cases is not linked to telomere shortening. Other events that lead to senescent-like phenotypes are oxidative stress,56 DNA damage57 and aberrant signalling (e.g., ras).58

A direct link of ubiquitination/SUMOylation and cellular senescence has been established by experiments regarding phenotypic consequences of the depletion of proteins involved in post-translational modification by ubiquitin and/or SUMO.

Furthermore, this link seems to be conserved during evolution, since direct interaction between replicative and chronological aging with the ubiquitin-dependent proteasome system have been identified by a systems biology approach.59

In human cells, it was shown that knockdown of ROC1, a key component of the SCF E3 ubiquitin ligases, inhibited the growth of multiple human cell lines by the induction of senescence and/or apoptosis.60 Similarly, inactivation of the VHL tumor suppressor gene, coding for a subunit of E3 ubiquitin ligase, induces a senescent-like phenotype in human cancer cell lines. This phenotype is independent of p53 and HIF-1, but dependent on the retinoblastoma protein.61 In mice, knock-out of Cdh1, an adaptor protein of the anaphase promoting complex (APC), which is an E3 ligase, induces premature replicative senescence in MEFs.62

Furthermore, upregulation of the ubiquitin ligase SMURF2 induced senescence in a variety of human cell types.63 However, the ability of SMURF2 to induce senescence did not require its ubiquitin ligase activity.64 With respect to SUMOylation, it was shown that overexpression of the E3 SUMO ligase PIASy in normal human fibroblasts recruits the p53 and Rb tumor suppressor pathways to provoke a senescence arrest. By contrast, in Rb-deficient fibroblasts, expression of PIASy leads to p53-dependent apoptosis and PIASy was shown to stimulate SUMOylation and transcriptional activity of p53.65 Interestingly PIASy has been shown to interact with pyruvate kinase M2 (M2-PK), a key regulatory enzyme controlling proliferation and tumorigenesis by a modulation of cellular metabolism. Previous evidence implicated M2PK as a regulator of senescence-associated growth arrest and senescent cells have been shown to accumulate preferentially the active tetrameric form of the enzyme.66

In addition, stable cell lines overexpressing processed forms of SUMO-2/3 (SUMO-2/3GG) showed a premature senescence phenotype. Both p53 and pRB were found to be modified by SUMO-2/3, suggesting that such modification of p53 and pRB may play roles in premature senescence and stress response,67 reviewed by ref. 68.

Other experiments have revealed an important role for the SUMO specific proteases SENP1, SENP2 and SENP7 in cellular senescence. Knockdown of these genes in human fibroblasts induced senescence and a global increase in SUMOylated proteins. The data suggest that SENP1 repression induces p53 mediated premature senescence and that SUMO proteases are required for the proliferation of normal human cells.69

How could ubiquitination or SUMOylation be involved in onset or bypass of senescence? The first approach to answer this question was to study global changes in the ubiquitinome during cellular ageing and was published by Pan and coworkers in 1993, reporting a decrease of free ubiquitin, while the pool of ubiquitinated proteins was increased in senescent fibroblasts. The most prominent ubiquitin conjugate was a discrete band of 55 kD, most probably the E2-55 ligase.70 The ubiquitin-proteasome system also seems to play a role in vascular senescence and atherosclerotic progression of elderly patients. Thus, atherosclerotic plaques from elderly patients had increased ubiquitin levels and reduced proteasome activity compared to controls, suggesting that reduction in the activity of the ubiquitin-proteasome system promotes vascular cell senescence, thereby contributing to the pathogenesis of human atherosclerosis.71 Experiments summarized below have identified several individual proteins, that might link the SUMO/ubiquitin system to cellular ageing (Fig. 1). Specifically it was shown, that key processes regulated by ubiquitination are telomere maintenance, DNA repair and cell cycle control, as is detailed in the following sections.

Figure 1.. Implications of the ubiquitin system on pathways that are involved in induction of replicative senescence.

Figure 1.

Implications of the ubiquitin system on pathways that are involved in induction of replicative senescence. See text for description. Reproduced with permission from Grillari J et al. Exp Gerontol 2006; 41(11):1067-79

Ubiquitin, Telomeres and Telomerase

Telomerase activity itself is regulated by ubiquitination. The catalyitic subunit hTERT directly interacts with the chaperone Hsp90.72 If this interaction is disrupted, e.g., by geldamycin, hTERT is targeted to ubiquitylation and subsequent degradation by the RING finger E3 ligase MKRN1. This leads to telomere shortening in cells with active telomerase. This finding was confirmed by overexpression of MKRN1 which as well results in shortened telomeres.73 Furthermore, the telomeric repeat binding factor 1 (TRF1), a negative regulator of telomere length,74 is targeted to ubiquitin dependent degradation. The responsible protein is Fbx4, an adaptor to Cul1 type E3 ligases, that upon overexpression is sufficient to stabilize telomeres, while its knock-down by siRNA leads to increased telomere shortening and to accelerated entry into senescence-like growth arrest.75 It was shown that RLIM, a Ring H2 zinc finger protein with intrinsic ubiquitin ligase activity interacts with TRF1 and increases its turnover by targeting it for degradation by the proteasome. Interestingly, this activity is independent of FBX4. Depletion of endogenous RLIM by shRNA mediated knockdown increased the level of TRF1 and led to telomere shortening and growth arrest.76 In fission yeast, PLI1B has been described as a SUMO E3 ligase, which upon mutation leads to deregulated homologous recombination and marked defects in chromosome segregation and centromere silencing grown, with a consistent increase in telomere length. It was shown that telomere lengthening induced by lack of SUMOylation was not due to unscheduled telomere recombination, instead SUMOylation was shown to increase telomerase activity.77 A role for SUMOylation of telomere binding proteins in alternative lengthening of telomeres (ALT) has also been demonstrated. The hallmark of ALT cells is the recruitment of telomeres to PML bodies. It was shown that the SUMO-ligase MNS21, part of the SMC5/6 complex, SUMOylates multiple telomere binding proteins, including TRF1 and TRF2, which prevents recruitment of telomeres to PML bodies. Depletion of FMC5/6 subunits by shRNA was shown to induce telomere shortening and senescence in ALT cells, suggesting that SUMOylation of telomere binding proteins facilitates telomere homologous recombination and elongation in ALT cells.78

Concerning ubiquitination, it was shown that the human orthologue of the yeast protein EST1B is a target for ubiquitin mediated degradation. EST1B was shown to bind histone deacetylase 8 (HDAC8) and phosphorylated HDAC8 inhibits degradation of HEST1B by the E3 ligase CHIP. Regulation of HEST1B protein stability by HDAC8 modulates the enzymatic activity of telomerase.79 Furthermore, poly-ubiquitin conjugates of yet unknown nature are found in PML bodies of senescent cells.80

A surprising link between another ubiquitin like modifier, ISG15 and telomere length has been observed only recently. The ISG15 gene is located in the subtelomeric region of chromosome 1 and its mRNA and protein expression levels are tightly correlated with telomere length. While telomere position effects have been found in yeast a couple of years ago,81 ISG15 is the first known human gene whose expression is regulated by telomere length.82

Regulation of DNA Repair and Growth Arrest in Cell Culture

The regulatory function of ubiquitination in DNA repair might be of high importance for ageing, not only in regard to replicative senescence, but also in regard to segmental progeroid syndromes whose symptoms are largely caused by defects in the DNA repair system.83 Recent data suggest that virtually all the major DNA repair pathways and DNA damage response mechanisms and checkpoint responses are regulated in some way by ubiquitination, SUMOylation or both. Strikingly, genes of the RAD6 damage tolerance pathway encode mostly enzymes involved in the ubiquitin pathway and the interstrand crosslink repair pathway linked to Fanconi anemia seems to consist largely of enzymes and substrates of the ubiquitin system (for recent review see ref. 35). DNA damage usually induces the activation of DNA repair or damage avoidance pathway and often a checkpoint response that triggers cell cycle arrest to allow time for the repair. This reaction, referred to as a DNA damage response (DDR), is typically initiated by proteins that recognize DNA lesions and is followed by the recruitment and activation of proteins that trigger checkpoint signaling or directly perform the necessary repair steps. This involves large protein assemblies, which are microscopically identifiable as repair foci. Once the DNA is repaired, the machinery needs to be disassembled and the DNA damage response turned off. It has been shown that both ubiquitination and SUMOylation are key steps controlling consecutive events required for the DNA damage response (for a more detailed description the reader is referred to 35). In the following section, individual components of this pathway,84 which are of particular importance for senescence and ageing, are addressed in detail.

Breast Cancer Associated Protein 1 (BRCA1)

Most combined familial breast and ovarian cancers and ~40% of familial breast cancer cases have been linked to mutations in BRCA1. A mouse model carrying a homozyogous mutant of Brca1Δ11 and haploidity for p53, shows p53-dependent senescence in mouse embryonic fibroblasts as well as a premature ageing syndrome at the organismal level.85 The heterodimer of BRCA1-BARD1 is one of the E3 enzymes that catalyzes autoubiquitination by K6 linkage in vitro and in vivo, that is not degraded by the 26S proteasome in vitro.86 Rather, it leads to 20-fold activation of its E3 ligase activity,87 as well as to relocalization to nuclear foci upon DNA damage.88

One target of BRCA1-BARD1 ubiquitination is nucleophosmin/B23,89 a nucleolar-cytoplasm shuttling protein that induces premature senescence upon overexpression.90 Another protein in this regulatory pathway is the central cell cycle regulator CDK2-cyclin E1 as a negative regulator of BRCA1-BARD1 activity.91 Since CDK2 protein levels and activity are markedly downregulated during replicative senescence of endothelial cells,92 it is intriguing to think that this might cause high BRCA1-BARD1 activity. As a consequence, this might lead to K6 polyubiquitination of nucleophosmin/B23, activating and most probably also stabilizing it, since K6 chains are also known to counteract ubiquitin dependent hydrolysis.32 At the end of this signalling high nucleophosmin levels might help to induce or at least to maintain the senescent phenotype by directly interacting with p53 and inducing p21.90

Another target of BRCA1-BARD1 E3 ligase activity is the histone H2AX, which is monoubiquitinated upon DNA damage and colocalizes with BRCA1 at foci of DNA damage.87 Formation of the phosphorylated H2AX (γH2AX) foci is also one of the hallmarks of replicative senescence in cell culture as reviewed by ref. 93 and also the phosphorylated form of H2AX colocalizes with BRCA1.94 Therefore, it seems plausible that these foci also contain the monoubiquitinated γH2AX, since phosphorylation is a often a signal for consecutive ubiquitination.95

Proliferating Cell Nuclear Antigen (PCNA)

Other proteins that are monoubiquitinated upon DNA damage are histone 2A (H2A)96 and proliferating cell nuclear antigen (PCNA). PCNA is involved in a large variety of DNA transactions (reviewed by ref. 97) like DNA replication, repair of interstrand cross links (ICL) and translesion synthesis.98 The E2 and E3 ligases transferring the monoubiquitin moietiy to PCNA are Rad6 and Rad18.99 Only the monoubiquitinated form of PCNA can physically interact with polymerase η (pol η)100 and this interaction is necessary to bypass pyrimidine dimers after UV damage in DNA replication.101 A clinical phenotype of mutations in this DNA repair function is known: mutations in pol η result in patients suffering from hypersensitivity to UV-light phenotypically related to the segmental progeroid syndrome xeroderma pigmentosa. Indeed this mutation has been classified to the XP variant (XPV) complementation group.102 Regulation of PCNA function by post-translational modification has been worked out recently and provides a striking example for the regulatory interplay between ubiquitin and SUMO modifications. PCNA homotrimeres act as processing factors for DNA polymerases and as a moving platform for factors that mediate replication linked functions, such as chromatin assembly or sister chromatid cohesion.103 Remarkably, PCNA can be modified on the same conserved lysine residue (Lys164) either by monoubiquitylation, Lys63 linked polyubiquitylation chains or by SUMOylation. Both ubiquitination and SUMOylation of PCNA occur in S-phase, but ubiquitination specifically occurs, when DNA is damaged. SUMOylation of PCNA predominantly on Lys164 attracts the antirecombinogenic helicase SRS2 to inhibit unwanted recombination during DNA synthesis. SUMOylation on Lys127 inhibits the interaction with certain PCNA binding proteins, such as Eco1. SUMOylation and ubiquitination are carried out by a complex system of conjugating enzymes, such as RAD15, RAD18, RAD6 and these modifications regulate the protein-protein interactions within DNA synthesis and DNA repair complexes. Because Lys164 of PCNA is a target for ubiquitin and SUMO, the two modifiers compete for the substrate. This has led to the development of a model for assembly and disassembly of PCNA multiprotein complexes, which is regulated by ubiquitination and SUMOylation, referred to as the ubiquitin/SUMO "switchboard".35

An additional link to ageing might be provided by PCNA's function in DNA replication that is inhibited by p21 binding in the early phase of replicative senescence104 reviewed by ref. 105. Since besides monoubiquitination, PCNA can be K63-polyubiquitinated as well as SUMOylated at the same lysine residue (K164), it is well possible that these modifications might as well be involved in inducing or maintaining the terminal growth arrest at the Hayflick limit.

Senescence Evasion Factor (SNEVPrp19/Pso4)

In a previous screening for differentially expressed genes after induction of replicative senescence we have identified the highly conserved protein SNEVPrp19/Pso4.106 Overexpression of SNEVPrp19/Pso4 increases the life span of human endothelial cells, which is correlated with higher resistance to stress and lower levels of DNA damage.107 Furthermore, SNEVPrp19/Pso4 is a U-box E3 ligase108 and interacts with the proteasome, by directly binding to the beta 7 subunit,109 while in mouse an interaction to the regulatory subunit SUG1 was reported.110 It is of note that the beta subunits β1, β2 and β5 of the proteasome are markedly downregulated in senescent fibroblasts111 and that overexpression of β5 is sufficient to confer higher stress resistance and survival to fibroblasts as well as a delay in onset of replicative senescence for 4-5 population doublings.112 In mouse, SNEV is essential for embryonic development, since its knock-out is lethal at the blastocyst stage.113 MEFs isolated from SNEV+/- heterozygous mice undergo senescence earlier than MEFs derived from litter mate controls.113 Furthermore, SNEV is necessary for hematopoietic progenitor cell self renewal.114 Although it is possible that SNEVPrp19/Pso4 and the proteasome act in the same pathway resulting in the observed increase of the life span, also different explanations for the underlying mechanisms in the case of SNEVPrp19/Pso4 are possible, since SNEVPrp19/Pso4 is also involved in DNA repair,115,116 specifically in DNA interstrand cross link repair, where SNEV acts in a complex containing WRN, the gene mutated in Werner syndrome.117 Although the DNA repair function seems to be as plausible a reason for life span extension in endothelial cells as the proteasome connection, it cannot be excluded that other functions like SNEVPrp19/Pso4's role in pre-mRNA splicing118 are involved, before the substrate of SNEVPrp19/Pso4 or the domains that are involved in DNA repair have been identified.

p53 Senescence and Tumor Suppressor Pathway


The regulation of p53 tumor suppressor protein plays a major role in signalling of DNA damage, protecting the integrity of the genome and is largely involved in ageing of cells and organisms.119,120 Excellent reviews are available on its highly complex and sophisticated regulation by the ubiquitin and sumoylation system and its multiple regulators Mdm2, vHL, ARF, ARF-BP1, HAUSP and MdmX to which readers are referred.121,122 Still, also ubiquitin independent degradation of p53 is reported.123


Besides the CDK inhibitor p16INK4A (see below), the INK4 locus codes for a second protein generated by alternative splicing, which leads to changes in the open reading frame, therefore this protein has been referred to as ARF (for alternative reading frame). Early reports suggested that ARF binds to p53 and stabilizes this tumor suppressor protein, however more recent data have identified that ARF can also promote SUMOylation of a variety of cellular target proteins, including MDM2.124,125 Similar to p16INK4A, ARF is N-terminally ubiquitinated and degraded in proteasomes. Recent studies have revealed an antagonism between SUMO deconjugating protease SENP3 and ARF, both of which interact with the abundant nucleolar protein nucleophosmin (NPM). It was shown that SENP3 and ARF antagonize each other's function in regulating the SUMOylation of target proteins, including NPM itself. The data suggest that the p53 independent tumor suppressor functions of ARF may be mediated by its ability to antagonize SENP3 thereby elevating cellular levels of SUMOylated proteins and inducing subsequent cell cycle arrest.126 ARF was also found to promote the polyubiquitination, through Lys63 of ubiquitin, of COMMD1, triggering this protein for proteasome dependent proteolysis. However, the enzymology of COMMD1 polyubiquitination needs to be clarified.127 One question of interest in regard to p53, p21 and p16 is if and which of the pathways to degrade or stabilize these key molecules are changed during the ageing process.

Cell Cycle Inhibitors are Mainly Regulated by Ubiquitination Followed by Degradation


One downstream target of p53, the cdk inhibitor, p21 (WAF1), which is necessary for induction of replicative senescence,128 can as well either be degraded in a ubiquitin independent way or can be regulated by SCF/Skp2 ubiquitin ligase activity,129 which attaches the ubiquitin its N-terminus. Recent data have revealed molecular mechanisms underlying ubiquitin-dependent degradation of p21. It was shown that p21 is degraded in mitosis subsequently to APC/C mediated ubiquitination, suggesting that degradation of p21 contributes to the full activation of CDK1 necessary for mitosis and prevents mitotic slippage during spindle checkpoint activation.130 p21 is also degraded during S-phase and in response to low doses of ultraviolet light and this involves PCNA, which promotes the ubiquitination and degradation of p21. This involves the ubiquitin ligase Cul4A-DDB1(Cdt2). It was suggested that the ubiquitin ligases CRL4 (Cdt2) and SCF are redundant with each other to promote the degradation of p21 during S-phase.131,132 p21 can also be degraded in a ubiquitin and ATP independent way in response to high doses of UV,133 a process that may involve the proteasome activator REGgamma.134 Ubiquitin dependent degradation of p21 can also be triggered by reactive oxygen species.135


p16(INK4a) encodes another cell cycle inhibitor, involved in induction of both replicative and oncogenic ras-induced senescence.57,136 It is ubiquitinated by N-terminal linkage, since p16 does not contain any lysine residues.12 The CDK inhibitor p19INK4D, which bears structural and functional similarity with p16, was shown to be a target for ubiquitin proteasome dependent degradation, which explains the oscillation of the protein, but not the mRNA, during a normal cell cycle. Ubiquitination of p19INK4D was dependent on the integrity of K62.137


Another important regulator of the cell cycle involved in Rb-mediated senescence is p27 that regulates cdk2 kinase activity.138 In senescent fibroblasts, p27 is stabilized by decrease in the F-box protein Skp2, which forms part of the SCF E3 ligase complex responsible for p27 ubiquitination and consequent degradation.139 Similarly, in cells of the Ewing sarcoma tumor family (EFT), depletion of the tumour-causing, rearranged EWS-Fli1 protein by siRNA elicits a senescent-like phenotype which is dependent on stability of p27, since Skp2 knockdown reversed the senescent phenotype.140

The stability of p27 is regulated at several levels, predominantly including two distinct pathways for ubiquitin proteasome mediated proteolysis. On the one hand, Serin10 phosphorylation increases in the early G1-phase of the cell cycle, allowing nuclear export of p27 and proteolysis of p27 in a KPC-mediated reaction. Secondly, phosphorylation of p27 at threonin187 occurs in S-phase, which triggers proteolysis through SCF ubiquitin ligase. Major pathways for p27 ubiquitination depend on KPC and SCF (SKP2) ubiquitin ligase complexes (reviewed in ref. 22).141

Deubiquitinating Enzymes and Their Influence on the Cell Cycle


An alternative possibility to stabilize p27 is its deubiquitination and indeed, the DUB UchL1/PGP9.5 was found to form a heterotrimeric complex with Jab1 and p27. This complex forms after serum restimulation of cells. In contrast, UchL1/PGP9.5 localizes to the perinuclear region and the cytoplasm in contact inhibited cells, but p27 remains nuclear,142 suggesting that it does not get deubiquitinated and therefore is not stabilized to induce cell cycle arrest under these conditions.

Furthermore, UchL1/PGP9.5 transcription is largely increased in pituitary glands of aged mice143 and has been implicated in neurodegeneration in Parkinson's, Alzheimer's and Huntington's disease patients. In all these diseases, reduced UCHL1 function may jeopardize the survival of central nervous system neurons.144

Potential Role of Other DUBs in Cellular Proliferation and Growth Arrest

No direct role of other DUBs in cellular ageing has yet been reported, however, several of them are involved in regulating cell cycle progression. DUB-1 for example inhibits the cell cycle in G1 phase, if expressed from an inducible promoter in the pre B cell line Ba/F3.145 Similarly, DUB-3, a cytokine-inducible deubiquitinating enzyme blocks proliferation in Ba/F3, but also in mouse embryonic fibroblasts upon overexpression.146 CYLD, a DUB that negatively affects NFκB signalling by removing K63-linked ubiquitin chains from tumour necrosis factor familiy members,147,148 inhibits proliferation of keratinocytes by deubiquinating Bcl-3.149 Furthermore, von Hippel Lindau (VHL) interacting deubiquitinating enzyme (VDU2) deubiquitinates and thus stabilizes hypoxia inducible factor 1 (HIF-1),150 which is causally involved in several types of cancer. In contrast, HIF-1 downregulation induces cellular senescence in endometrial cancer cells.151 In keeping with this, hypoxia has been reported to extend the replicative life span of rat aortic smooth muscle cells.152 Thus, deregulation of HIF-1 might contribute to cellular senescence or tumor pathogenesis. Since HIF-1 is also involved in the reduced response of aged organisms to hypoxic stress,153,154 its ubiquitination status might be of importance also during organismal ageing.

Signal Transduction, Receptor Endocytosis—EGFR and Ras

Epidermal growth factor receptor (EGFR) is one prominent example for the role of monoubiquitination in receptor endocytosis (reviewed by ref. 20). EGFR signalling is necessary for epithelial cell proliferation and differentiation155 and resistance to EGFR ligand signalling in senescent cells is well documented156 and is considered to be involved in slower wound healing in the elderly.

Upon monoubiquitination, EGFR is internalized and sorted to the lysosomes, where it is degraded.33 The ubiquitin ligases Cbl157 as well Sts2158 are involved in this pathway. Deubiquitination by UBPY/USP8 can antagonize this and slow down the EGFR internalization.159

The mechanisms leading to decreased EGFR signalling in senescent cells described so far include diminished EGFR mRNA transcription, enhanced phosphatase activitiy that dephosphorylates EGFR160 as well as altered endocytosis. Dependent on the dose of ligand EGFR is either internalized by ubiquitin-independent mechanism via caveolae at high EGF doses or by ubiquitin dependent clathrin-coated pits at lower doses.8 Both receptor internalization pathways are affected in senescent cells: Enhanced EGFR internalization by caveolin has been shown as a major mechanism to downregulate EGF signalling in senescent cells, since knockdown of caveolins led to restoration of EGF signalling in senescent fibroblasts and overexpression of caveolins in young cells led to a senescent-like phenotype.161,162 Similarly, the clathrin-dependent pathway is downregulated in senescent fibroblasts since one of the essential proteins, amphiphysin-1, is not internalized.163

One of the branches of EGFR signalling leads via ras to the MAP kinase pathway. Since constant signalling of ras, e.g., by overexpression of oncogenic ras (H-ras), leads to induction of premature senescence,58 also ubiquitin might be involved in its regulation. Indeed, mono or deubiquitination of H-ras is important for docking to endosomes and to activate the downstream MAPK/raf signal cascade.164

Ubiquitin-Dependent and Independent Mitochondrial Protein Quality Control

Ubiquitin-independent protein quality control of mitochondrial matrix proteins is well characterized and until recently the mitochondria were considered ubiquitination-free organelles. However, several recent studies indicate multiple roles of the ubiquitin proteasome pathway in the regulation and maintenance of mitochondrial integrity. Of particular interest is the finding of a mitochondrial ubiquitin dependent protein quality control, which shares similarity to the endoplasmic reticulum associated degradation (ERAD) pathway that acts to eliminate misfolded proteins from the lumen of the endoplasmic reticulum. Given the key role of mitochondrial functionality for healthy ageing and the role of mitochondrial dysfunction in various ageing processes, it can be expected that further insight into this field will elucidate new roles of ubiquitination in the ageing process.165

Ubiquitination in Tissues during Ageing

Several tissues are subject to intensive investigations in regard to the activity and to single components of the ubiquitin system. In the following section, a summary on the most prominent examples is presented (Fig. 2).

Figure 2.. Age related changes in the ubiquitin system affect several different tissues.

Figure 2.

Age related changes in the ubiquitin system affect several different tissues. Reproduced with permission from Grillari J et al. Exp Gerontol 2006; 41(11):1067-79

Ubiquitination in the Nervous System

Since ubiquitination and protein degradation failure are involved in the pathogenesis of several neurodegenerative diseases and has been extensively reviewed, the reader is referred to some of the excellent current reviews.2,6,166 Concerning Parkinson's disease some recent data shed new light on the role of ubiquitination in neuronal degeneration. Specifically Parkin, which is mutated in familial forms of PD, encodes a ubiquitin E3 ligase,167 the inactivation of which leads to a dysfunction of the ubiquitin proteasome system and to the accumulation of aggregated alpha synuclein in the cytosol of dopaminergic cells. Recent data suggest that parkin can also translocate to the nucleus upon DNA damage168 where it participates in the regulation of DNA repair, probably through an interaction with PCNA.169 Another gene that is relevant for Parkinson's disease codes for ubiquitin c-terminal hydrolase L1, which degrades the polyubiquitin chain and provides monoubiquitin to the cell. Similar to Parkin, dysfunction of UCHL1 leads to a dysfunction of the ubiquitin proteasome system and the accumulation of protein aggregates.170 Expression of a human Parkin missense mutant in Drosophila led to the degeneration of specific dopaminergic neuronal clusters and concommittant locomotor deficits that accelerated with age. These results provide in vivo evidence that Parkin mutants may directly exert neurotoxicity in vivo.171

Ubiquitination, Eye Diseases and Cataracts

Similar to neurodegenerative diseases, accumulation of damaged protein is also involved in the formation of cataracts, in which high molecular weight protein aggregates form as an ageing associated phenomenon.172 Since this might be related to the ubiquitinome of epithelial lens cells,173,174 several studies have explored the amount of ubiquitin conjugates, or the ubiquitination activity in either in vitro cultivated lens cells or in the epithelial lens tissue. Ageing and cellular maturation cause decreases in ubiquitination activity as well as in the amount of free and conjugated ubiquitin in bovine lens epithelial tissue.175 In response to oxidative stress the increase in ubiquitination activity was as well reduced.176 Furthermore, gene expression profiling in human age-related nuclear cataracts found, besides others, two ubiquitin-conjugating enzymes177 as well as an E3 ligase178 as downregulated in the aged tissue. Furthermore, an N-terminal protease-cleaved α-crystallin fragment which is increases during cataract formation is resistant against ubiquitin-proteasome mediated degradation.179

Taken together, this would suggest that the selection of degradation targets is decreased and that a slower turn-over of proteins might result in accumulation of damaged proteins and protein complexes. The role of the ubiquitin proteasome pathway in the length retinal cornea was recently reviewed.180 Recently it was shown, that inactivation of the ubiquitin c-terminal hydrolase 3 (UCH-L3) leads to retinal degeneration along with muscular degeneration. In a normal retina UCH-L3 was enriched in the photoreceptor inner segment and loss of UCH-L3 led to mitochondrial oxidative stress related photoreceptor cell apoptosis in a caspase independent manner.181 Proteins modified by a highly reactive lipid peroxide, 4-hydroxynonenal (HNE) are also ubiquitinated in cultured epithelial lens cells. Surprisingly, these damaged proteins are degraded in a novel ubiquitin dependent pathway that degrades them within the lysosomes.182 To what extent this pathway might be deregulated and contribute to accumulation of nonfunctional protein during ageing has not been investigated yet.

Ubiquitination in the Liver: Ageing and Calorie Restriction

Calorie restriction increases the life span of rodents183 and agents mimicking calorie restriction might be a target for anti-ageing drugs.184 Effect of ageing and calorie restriction on the ubiquitination potential in liver was first analysed by Scrofano and coworkers. They found increased levels of ubiquitin conjugates, correspondingly higher E1 and E2 activities in the livers of 23-months versus 6-months old mice.185 Calorie restriction, however, maintained a "young" phenotype also in this regard and prevented the increase. Surprisingly, the activity of ATP dependent and independent proteolysis did not change with age when using β-lactoglobulin and oxidized RNAse as model substrates.186 Similarly, in a study aiming at the validation of housekeeping genes for normalization of gene expression levels in the ageing liver of Fischer 344 rats, the amount of ubiquitin transcripts was found to change dramatically.187

In a consecutive study, the response to oxidative stress in livers of 23-months old Emory mice after injection of paraquat, a generator of superoxide radicals, was analysed.186 Again, calorie restricted animals showed significantly lower levels of E1 and E2 activity as well as of ubiquitin conjugates. After exposure to paraquat, however, E1, E2 activity was induced to the same high amounts observed in ad libitum fed mice and ubiquitin-conjugates were even significantly higher in the calorie restricted mice, suggesting an increased turnover of protein and thus improved cellular repair. However, it is not clear if and how these ubiquitinated proteins are degraded, since Davies and coworkers have shown that oxidatively damaged proteins are degraded by the 20S proteasome in a ubiquitin independent manner.188,189 Using lipopolysaccharides (LPS) as a stressor, calorie restriction also attenuated liver injury, did not increase a pro-inflammatory response by NFκB pathway and slightly influenced the mRNA levels of proteasome subunits like beta 2 and 3, as well as several ATPases of the proteasome lid in the liver of Fisher 344 rats.190

Additional experiments performed in mouse models suggest that dietary restriction significantly reduced age-related impairments in proteasome mediated protein degradation and reduced age-related increases in ubiquitinated, oxidized and SUMOylated protein in the heart. These results indicate that dietary restriction has many beneficial effects towards the ubiquitin proteasome system and suggest that a preservation of the UPP may be a potential mechanism, by which dietary restriction mediates beneficial effects on the cardiovascular system.191

Insulin-Like Growth Factor 1 (IGF-1) Signalling as Example of the Endocrine System

The highly conserved members of the IGF1 signalling pathway are implicated in regulating the life span of species as diverse as C. elegans, D. melanogaster and mouse.192 This pathway includes the hormone IGF-1 whose bio-availability is regulated by IGF1-binding proteins (IGFBPs) and signal transduction through the cell membrane by the IGF-1 receptor (IGFR), which forms dimers and shows tyrosine kinase activity upon ligand binding. One branch of the signalling then leads to the MAP kinase pathway via ras, another one via insulin receptor substrate 1 and 2 to akt, both branches specifically activating different genes in the nucleus.193

In regard to its regulation by the ubiquitin system, one of the IGFBPs, IGFBP3 has been found not only to be secreted, but also to be present in the nucleus,194 where it is subject to polyubiquitination and degradation by the proteasome.195 IGFBP3 internalization and translocation, however, might be independent from IGF-1.196 Thus, it is not clear to which extent IGF-1 signalling might be affected by ageing related changes of the ubiquitin system.

Ubiquitin System and the Ageing Muscle

Certainly related to the IGF signalling pathway, however, is the ubiquitination and down-regulation of IGFR by β-arrestin which acts as adaptor for Mdm2 as the specific E3 ligase.197 Furthermore, IGF-1 triggers ubiquitination and proteasomal degradation of its downstream signalling molecules insulin receptor substrate 1 (IRS-1),198 IRS-2199 and Akt.200 IGF-1 stimulates muscle growth by suppressing the atrophy related ubiquitin ligases atrogin-1 and muscle ring finger-1 (MuRF1).201 Similarly, it reduces the levels of free ubiquitin as well as that of various ubiquitin ligases.202 Decrease in the growth hormone/IGF-1 axis thus plays a major role in muscle wasting associated with diseases like diabetes,203 chronic heart failure204 and cancer.205

Sarcopenia, an ageing related form of muscle wasting,206 is also associated with decrease in IGF-1 signalling (reviewed by ref. 207), followed by significantly higher free ubiquitin amounts in human and rat skeletal muscle of aged as compared to young individuals. These high levels of free ubiquitin seem to be directly linked to sarcopenia, since direct injection of ubiquitin into young healthy rat muscle induces muscle degeneration.208,209 Although a different study does not report changes in total ubiquitin conjugate contents in ageing rat soleus muscle, a decline in mRNAs coding for E2(14K) and MuRF-1210 are observed. In any case, a direct link between loss of muscle protein mass due to changes is ubiquitination and ageing seems obvious.

Recent studies have highlighted a key role of two E3 ubiquitin ligases, namely atrogin-1/MAFbx and MuRF1 as major factors responsible for skeletal muscle atrophy. Both enzymes are upregulated in many conditions of atrophy and utilized for protein degradation during muscle atrophy. It was reported that this is not the case in age-related loss of muscle mass (sarcopenia). On the contrary, Atrogin-1/MAFbx and MuRF1 were found downregulated in skeletal muscle of 30-month-old rats, probably due to AKT (protein kinase B)-mediated inactivation as well as by MDM2 mediated degradation211 of the forkhead box O 4 (FOXO4) transcription factor. Dietary restriction was found to impede both sarcopenia as well as the effects of ageing on AKT phosphorylation, FOXO4 phosphorylation and Atrogin-1/MAFbx and MuRF1 transcript regulation. Hence, sarcopenia appears mechanistically different from acute atrophies induced by disuse, disease and denervation.212 It appears that a major age-dependent alteration in muscle proteolysis is a lack of responsiveness of the ubiquitin proteasome dependent proteolytic pathway to anabolic and catabolic stimuli (reviewed in ref. 213).

In mice overexpressing a mitochondrial T3 receptor, which acts as a mitochondrial transcription factor, a progressive decrease of mitochondrial DNA content was observed, which led to muscle atrophy probably through a stimulation of atrogin-1 and MURF1.214 In a separate study overexpression of MURF1 did not lead to elevated ubiquitination of myosins instead the mice displayed lower levels of several metabolic enzymes required for glycolysis and glycogen metabolism. Whereas these data suggest that MURF1 expression in skeletal muscle redirects glycogen synthesis to the liver, molecular mechanisms are not clear at the moment.215

Age Related Changes in the Blood and Ubiquitination

In erythrocytes, spectrins are membrane proteins responsible for shape and mechanical properties. α-spectrin is a chimeric E2/E3 enzyme,216 which is also ubiquitinated. A marked decrease in α-spectrin ubiquitination due to age-dependent changes in the erythrocyte membrane have been observed217 and might influence the stability and deformability as well as the oxygen transport properties, all of which are reported to change with age.218-220

Ageing associated changes in the behaviour of T-cells is also of major importance for organisms. One of the reasons of reduced defense by the ageing immune system is involution of the thymus and consequently, reduction of naïve T-cells.221,222 But also functional changes of T-lymphocytes in the elderly are observed, especially the NFκB pathway that leads to activation by cytokines and to inflammatory response of T-cells undergoes major changes in T-cells of the elderly reviewed by ref. 223. One of the underlying mechanisms is a change in proteasomal as well as in deubiquitination activity.224 Since various types of ubiquitination play a major regulatory part in at least 3 different steps of NFκB pathway,23 all of these might be of importance in regard to the ageing process.

Phosphorylation of inhibitors of NFκB (IκB) by a kinase complex termed IκB kinase (IKK) is the event that consequently allows K48 polyubiquitination and proteasomal degradation of IκB. This ultimately leads to activation of NFκB.225 At a different level, K63 polyubiquitin linkage is involved as well, since the regulatory subunit of IKK, NEMO, binds to K63 polyubiquitin chains. If the interaction site in NEMO is disrupted by point mutations, the activation of NFκB is inhibited.24 Finally, also monoubiquitination plays a role in the NFκB pathway. Site-specific monoubiquitination of IKK down regulates continuous signalling by pro-inflammatory cytokines as well as by the oncogenic viral protein Tax.226,227

The discovery that two other inhibitors of NFκB signaling, A20 and CYLD1, are deubiquitinating enzymes, has emphasized the importance of these modifications. From these studies a general model of activation of NFκB has emerged. Whereas the details of this regulation vary depending on the particular signal triggering the signaling events, the ubiquitin dependent regulation of the NFκB pathway consists of the following steps: receptor engagement activates members of the TRAF family of E3 ligases to assemble a K63 linked polyubiquitin chain, which recruits the TAK1 protein kinase and its substrate IKB kinase (IKK). The complex catalyzes the K63 polyubiquitination of NEMO, the regulatory γ-subunit of IKK. The phosphorylated and ubiquitinated IKK is now active to phosphorylate IkB resulting in its K48 polyubiquitination to trigger its degradation by the proteasome. Finally the removal of the K63 linked polyubiquitination by the DUB A20 downregulates the signaling response.228 In this way this pathway presents an excellent example, how subsequent ubiquitination and deubiquitination steps cooperate in the execution of a biological response such as NFκB activation.229

Besides its role in the decline of immune functions, NFκB deregulation is involved in loss of hearing and audionerve degeneration during ageing in a mouse model,229 its activity is dramatically increased in livers of old rats in comparison to those of young,230 while DNA binding activity of NFκB is significantly reduced in skeletal muscle of old rats.231 However, a direct role of ubiquitin dependent regulation of these processes has yet to be established.

Segmental Progeroid Syndromes and the Ubiquitin System

The ubiquitin system might also play a role in several segmental progeroid syndromes that are regarded as model systems for ageing research.232,233 Several of these syndromes are caused by mutations in DNA damage repair genes, which might provide one link to ageing as outlined above. A connection to the ubiquitin system is presented by interaction of these proteins with and their regulation by the ubiquitin system. Some examples are listed in the following.

Although not initially included in Martin's list of segmental progeroid syndromes, Fanconi Anemia (FA) is postulated to induce several progeroid features in patients (reviewed in ref. 234). Monoubiquitination plays a specially important role in FA,235 since the so far known catalytic activity of the FA complex is that of an E3 ligase. Mutations that interfere with monoubiquitination of one of the FA pathway proteins, FANCD2, lead to defects in homologous recombination and translesion synthesis resulting in increased chromosome instability and leukaemia236 and is classified as segmental progeroid syndrome.232 Furthermore, FA derived hematopoietic bone marrow cells undergo premature senescence upon hypoxia/reoxigenization stress treatment237 and bone marrow failure is one of the major causes of death in these patients at the age of around 16. The E3 ligase complex responsible for this ubiquitination contains 8 FA proteins so far.7 In addition, BRCA1-BARD1 can act as FANCD2 E3 ligase in vitro, however, its significance in vivo is not clear yet.238

Other examples of proteins mutated in progeroid syndromes and linked to the ubiquitin system comprise WRN, the DNA helicases that is mutated in Werner syndrome patients. WRN associates with VCP/p97, an AAA ATPase implicated in the ubiquitin/proteasome pathway.239,240 RECQL4, the dysfunctional protein in Rothmund-Thomson syndrome, interacts with UBR1 and UBR2, the E3 ligases of the N-end rule pathway but is not ubiquitinated nor degraded.241 The responsible mutation of Hutchinson-Gilford progeria has recently been identified to lie within lamin A/C242,243 and lamin A/C interacts with the sumoylation specific E2 ligase UBC9.244 Mutations in the sumoylation site of lamin A lead to cardiomyopathy.245

Ataxia telangiectasia (A-T) is due to lack of functional ATM kinase that responds to DNA damage and oxidative stress.246 ATM is furthermore essential in inducing growth arrest upon reaching critically short telomeres.247 Nonetheless, the absence of functional ATM in fibroblasts derived from A-T patients leads to a reduced life span in vitro.248 Lymphoblastoid A-T-cells have higher endogenous ubiquitin conjugate levels and enhanced ubiquitination activity, but a muted response to H2O2.249

Role of Ubiquitinylation and SUMOylation in Ageing of Short-Lived Model Organisms

Accumulating evidence, in particular in the nematode C. elegans, suggests that there is a role of ubiquitinylation for ageing also in short-lived model organisms. It has been shown that the transcription factor DAF16, which mediates signaling through the insulin IGF-like signaling pathway in C.elegans, is the target for ubiquitin mediated proteasomal degradation (reviewed by ref. 250). On the one hand, DAF16 is polyubiquitinated by a recently identified E3 ligase called RLE-1, which is the C. elegans orthologue of a murine E3 ubiquitin ligase called Roquin. DAF16 is driving the expression of genes favoring longevity and it was reported that RLE1 mutants are long-lived and that lifespan extension requires DAF16.251 On the other hand, DAF16 activity, but not expression level, is also affected by depletion of the C. elegans homologue of Cullin1 and Cullin1 depletion results in a specific reduction of the lifespan in IIS long-lived mutants, whereas it has little effect on the lifespan of normal worms. Since Cul1 depletion did not affect the level of DAF16, it was concluded that the Cullin based E3 ligase complex targets a so far unidentified repressor of DAF16, thereby explaining the negative effects on lifespan.252 In addition, it was shown that loss of the C. elegans homologue of the von Hippel-Lindau tumor suppressor gene VHL1, a Cullin based E3 ubiquitin ligase that negatively regulates the hypoxic response, increases the lifespan of C. elegans. It was also observed that hypoxia-inducible factor (HIF1) acts downstream of VHL1 to modulate ageing and proteotoxicity in C. elegans and that both genes control longevity by a mechanism distinct from both dietary restriction and insulin-like signaling.253

In mice, CHIP deficiency reduces the life span with accelerated age-related pathophysiology and premature senescence of in-vitro cultivated cells of these mice.254


The universal importance of the ubiquitin-proteasome system for molecular and cellular biology, as well as for medical sciences is still increasing and there are only few cellular pathways left that at one step or another are not regulated by ubiquitin. In regard to ageing of cells and tissues, the examples presented in this review (summarized in Fig. 3) suggest that deregulation and changes of the ubiquitinome might have vast implications for ageing of organisms as well as for ageing associated diseases, although in some cases direct evidence is still missing. In any case, further understanding of the influence of the ubiquitin system and its related molecules like SUMO, Nedd8, ISG15, atg8 or atg12 on the ageing process might help to identify targets for prevention of deleterious loss of cell and tissue function and pathogenesis of ageing related diseases.

Figure 3.. Overview on the so far known implications of ubiquitin in aging.

Figure 3.

Overview on the so far known implications of ubiquitin in aging.


This work was supported by NRN grant S093-B06 of the Austrian Science fund.


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