Regulation of p53 activity in nuclear bodies by a specific PML isoform

Impact of the adenoviral E4 Orf3 protein on the activity and posttranslational modification of p53

Our previous studies have established that the p53 populations that accumulate in normal human cells exposed to etoposide or infected by an E1B 55-kDa protein-null mutant of human adenovirus type 5 carry a large number of posttranslational modifications at numerous residues (C. J. DeHart, J. S. Chahal, S. J. Flint, and D. H. Perlman, Mol Cell Proteomics 13:1-17, 2014, http://dx.doi.org/10.1074/mcp.M113.030254). In the absence of this E1B protein, the p53 transcriptional program is not induced, and it has been reported that the viral E4 Orf3 protein inactivates p53 (C. Soria, F. E. Estermann, K. C. Espantman, and C. C. O'Shea, Nature 466:1076-1081, 2010, http://dx.doi.org/10.1038/nature09307). As the latter protein disrupts nuclear Pml bodies, sites at which p53 is modified, we used mass spectrometry to catalogue the posttranscriptional modifications of the p53 population that accumulates when neither the E1B 55-kDa nor the E4 Orf3 protein is made in infected cells. Eighty-five residues carrying 163 modifications were identified. The overall patterns of posttranslational modification of this population and p53 present in cells infected by an E1B 55-kDa-null mutant were similar. The efficiencies with which the two forms of p53 bound to a consensus DNA recognition sequence could not be distinguished and were lower than that of transcriptionally active p53. The absence of the E4 Orf3 protein increased expression of several p53-responsive genes when the E1B protein was also absent from infected cells. However, expression of these genes did not attain the levels observed when p53 was activated in response to etoposide treatment and remained lower than those measured in mock-infected cells.

IMPORTANCE

The tumor suppressor p53, a master regulator of cellular responses to stress, is inactivated and destroyed in cells infected by species C human adenoviruses, such as type 5. It is targeted for proteasomal degradation by the action of a virus-specific E3 ubiquitin ligase that contains the viral E1B 55-kDa and E4 Orf6 proteins, while the E4 Orf3 protein has been reported to block its ability to stimulate expression of p53-dependent genes. The comparisons reported here of the posttranslational modifications and activities of p53 populations that accumulate in infected normal human cells in the absence of both mechanisms of inactivation or of only the E3 ligase revealed little impact of the E4 Orf3 protein. These observations indicate that E4 Orf3-dependent disruption of Pml bodies does not have a major effect on the pattern of p53 posttranslational modifications in adenovirus-infected cells. Furthermore, they suggest that one or more additional viral proteins contribute to blocking p53 activation and the consequences that are deleterious for viral reproduction, such as apoptosis or cell cycle arrest.

p53 Acetylation: Regulation and Consequences

Post-translational modifications of p53 are critical in modulating its tumor suppressive functions. Ubiquitylation, for example, plays a major role in dictating p53 stability, subcellular localization and transcriptional vs. non-transcriptional activities. Less is known about p53 acetylation. It has been shown to govern p53 transcriptional activity, selection of growth inhibitory vs. apoptotic gene targets, and biological outcomes in response to diverse cellular insults. Yet recent in vivo evidence from mouse models questions the importance of p53 acetylation (at least at certain sites) as well as canonical p53 functions (cell cycle arrest, senescence and apoptosis) to tumor suppression. This review discusses the cumulative findings regarding p53 acetylation, with a focus on the acetyltransferases that modify p53 and the mechanisms regulating their activity. We also evaluate what is known regarding the influence of other post-translational modifications of p53 on its acetylation, and conclude with the current outlook on how p53 acetylation affects tumor suppression. Due to redundancies in p53 control and growing understanding that individual modifications largely fine-tune p53 activity rather than switch it on or off, many questions still remain about the physiological importance of p53 acetylation to its role in preventing cancer.

The role of post-translational modifications in fine-tuning BLM helicase function during DNA repair

RecQ-like helicases are a highly conserved family of proteins which are critical for preserving genome integrity. Genome instability is considered a hallmark of cancer and mutations within three of the five human RECQ genes cause hereditary syndromes that are associated with cancer predisposition. The human RecQ-like helicase BLM has a central role in DNA damage signaling, repair, replication, and telomere maintenance. BLM and its budding yeast orthologue Sgs1 unwind double-stranded DNA intermediates. Intriguingly, BLM functions in both a pro- and anti-recombinogenic manner upon replicative damage, acting on similar substrates. Thus, BLM activity must be intricately controlled to prevent illegitimate recombination events that could have detrimental effects on genome integrity. In recent years it has become evident that post-translational modifications (PTMs) of BLM allow a fine-tuning of its function. To date, BLM phosphorylation, ubiquitination, and SUMOylation have been identified, in turn regulating its subcellular localization, protein-protein interactions, and protein stability. In this review, we will discuss the cellular context of when and how these different modifications of BLM occur. We will reflect on the current model of how PTMs control BLM function during DNA damage repair and compare this to what is known about post-translational regulation of the budding yeast orthologue Sgs1. Finally, we will provide an outlook toward future research, in particular to dissect the cross-talk between the individual PTMs on BLM.

The acute promyelocytic leukaemia success story: curing leukaemia through targeted therapies

The recent finding that almost all patients with acute promyelocytic leukaemia (APL) may be cured using a combination of retinoic acid (RA) and arsenic trioxide (As(2)O(3)) (N Engl J Med, 369, 2013 and 111) highlights the progress made in our understanding of APL pathogenesis and therapeutic approaches over the past 25 years. The study of APL has revealed many important lessons related to transcriptional control, nuclear organization, epigenetics and the role of proteolysis in biological control. Even more important has been the clinical demonstration that molecularly targeted therapy can eradicate disease.

Influence of ND10 components on epigenetic determinants of early KSHV latency establishment

We have previously demonstrated that acquisition of intricate patterns of activating (H3K4me3, H3K9/K14ac) and repressive (H3K27me3) histone modifications is a hallmark of KSHV latency establishment. The precise molecular mechanisms that shape the latent histone modification landscape, however, remain unknown. Promyelocytic leukemia nuclear bodies (PML-NB), also called nuclear domain 10 (ND10), have emerged as mediators of innate immune responses that can limit viral gene expression via chromatin based mechanisms. Consequently, although ND10 functions thus far have been almost exclusively investigated in models of productive herpesvirus infection, it has been proposed that they also may contribute to the establishment of viral latency. Here, we report the first systematic study of the role of ND10 during KSHV latency establishment, and link alterations in the subcellular distribution of ND10 components to a temporal analysis of histone modification acquisition and host cell gene expression during the early infection phase. Our study demonstrates that KSHV infection results in a transient interferon response that leads to induction of the ND10 components PML and Sp100, but that repression by ND10 bodies is unlikely to contribute to KSHV latency establishment. Instead, we uncover an unexpected role for soluble Sp100 protein, which is efficiently and permanently relocalized from nucleoplasmic and chromatin-associated fractions into the insoluble matrix. We show that LANA expression is sufficient to induce Sp100 relocalization, likely via mediating SUMOylation of Sp100. Furthermore, we demonstrate that depletion of soluble Sp100 occurs precisely when repressive H3K27me3 marks first accumulate on viral genomes, and that knock-down of Sp100 (but not PML or Daxx) facilitates H3K27me3 acquisition. Collectively, our data support a model in which non-ND10 resident Sp100 acts as a negative regulator of polycomb repressive complex-2 (PRC2) recruitment, and suggest that KSHV may actively escape ND10 silencing mechanisms to promote establishment of latent chromatin.

The tumor suppressor PML specifically accumulates at RPA/Rad51-containing DNA damage repair foci but is nonessential for DNA damage-induced fibroblast senescence

The PML tumor suppressor has been functionally implicated in DNA damage response and cellular senescence. Direct evidence for such a role based on PML knockdown or knockout approaches is still lacking. We have therefore analyzed the irradiation-induced DNA damage response and cellular senescence in human and mouse fibroblasts lacking PML. Our data show that PML nuclear bodies (NBs) nonrandomly associate with persistent DNA damage foci in unperturbed human skin and in high-dose-irradiated cell culture systems. PML bodies do not associate with transient γH2AX foci after low-dose gamma irradiation. Superresolution microscopy reveals that all PML bodies within a nucleus are engaged at Rad51- and RPA-containing repair foci during ongoing DNA repair. The lack of PML (i) does not majorly affect the DNA damage response, (ii) does not alter the efficiency of senescence induction after DNA damage, and (iii) does not affect the proliferative potential of primary mouse embryonic fibroblasts during serial passaging. Thus, while PML NBs specifically accumulate at Rad51/RPA-containing lesions and senescence-derived persistent DNA damage foci, they are not essential for DNA damage-induced and replicative senescence of human and murine fibroblasts.

Inhibition of Sp1 functions by its sequestration into PML nuclear bodies

Promyelocytic leukemia nuclear bodies (PML NBs) are comprised of PML and a striking variety of its associated proteins. Various cellular functions have been attributed to PML NBs, including the regulation of gene expression. We report here that induced expression of PML recruits Sp1 into PML NBs, leading to the reduction of Sp1 transactivation function. Specifically, Chromatin immunoprecipitation (ChIP) assay demonstrated that induced expression of PML significantly diminishes the amount of Sp1 binding to its target gene promoter, immunofluorescence staining showed dramatic increase in the co-localization between PML and Sp1 upon induction of PML expression, moreover, PML and Sp1 co-fractionated in the core nuclear matrix. Our study further showed that PML promotes SUMOylation of Sp1 in a RING-motif-dependent manner, SUMOylation of Sp1 facilitates physical interaction between Sp1 and PML and recruitment of Sp1 into the PML NBs, the SUMO binding motif of PML was also important for its interaction with Sp1. The results of this study demonstrate a novel mechanism by which PML regulates gene expression through sequestration of the transcription factor into PML NBs.

A Crowdsourced nucleus: understanding nuclear organization in terms of dynamically networked protein function

The spatial organization of the nucleus results in a compartmentalized structure that affects all aspects of nuclear function. This compartmentalization involves genome organization as well as the formation of nuclear bodies and plays a role in many functions, including gene regulation, genome stability, replication, and RNA processing. Here we review the recent findings associated with the spatial organization of the nucleus and reveal that a common theme for nuclear proteins is their ability to participate in a variety of functions and pathways. We consider this multiplicity of function in terms of Crowdsourcing, a recent phenomenon in the world of information technology, and suggest that this model provides a novel way to synthesize the many intersections between nuclear organization and function. This article is part of a Special Issue entitled: Chromatin and epigenetic regulation of animal development.

The p53-Mdm2 loop: a critical juncture of stress response

The presence of a functional p53 protein is a key factor for the proper suppression of cancer development. A loss of p53 activity, by mutations or inhibition, is often associated with human malignancies. The p53 protein integrates various stress signals into a growth restrictive cellular response. In this way, p53 eliminates cells with a potential to become cancerous. Being a powerful decision maker, it is imperative that p53 will be activated properly, efficiently and temporarily in response to stress. Equally important is that p53 activation will be extinguished upon recovery from stress, and that improper activation of p53 will be avoided. Failure to achieve these aims is likely to have catastrophic consequences for the organism. The machinery that governs this tight regulation is largely based on the major inhibitor of p53, Mdm2, which both blocks p53 activities and promotes its destabilization. The interplay between p53 and Mdm2 involves a complex network of positive and negative feedback loops. Relief from Mdm2 suppression is required for p53 to be stabilized and activated in response to stress. Protection from Mdm2 entails a concerted action of modifying enzymes and partner proteins. The association of p53 with the PML-nuclear bodies may provide an infrastructure in which this complex regulatory network can be orchestrated. In this chapter we use examples to illustrate the regulatory machinery that drives this network.

PML4 facilitates erythroid differentiation by enhancing the transcriptional activity of GATA-1

Promyelocytic leukemia protein (PML) has been implicated as a participant in multiple cellular processes including senescence, apoptosis, proliferation, and differentiation. Studies of PML function in hematopoietic differentiation previously focused principally on its myeloid activities and also indicated that PML is involved in erythroid colony formation. However, the exact role that PML plays in erythropoiesis is essentially unknown. In this report, we found that PML4, a specific PML isoform expressed in erythroid cells, promotes endogenous erythroid genes expression in K562 and primary human erythroid cells. We show that the PML4 effect is GATA binding protein 1 (GATA-1) dependent using GATA-1 knockout/rescued G1E/G1E-ER4 cells. PML4, but not other detected PML isoforms, directly interacts with GATA-1 and can recruit it into PML nuclear bodies. Furthermore, PML4 facilitates GATA-1 trans-activation activity in an interaction-dependent manner. Finally, we present evidence that PML4 enhances GATA-1 occupancy within the globin gene cluster and stimulates cooperation between GATA-1 and its coactivator p300. These results demonstrate that PML4 is an important regulator of GATA-1 and participates in erythroid differention by enhancing GATA-1 trans-activation activity.

PML isoforms in response to arsenic: high-resolution analysis of PML body structure and degradation

Arsenic is a clinically effective treatment for acute promyelocytic leukaemia (APL) in which the promyelocytic leukaemia (PML) protein is fused to retinoic receptor alpha (RARα). PML-RARα is degraded by the proteasome by a SUMO-dependent, ubiquitin-mediated pathway in response to arsenic treatment, curing the disease. Six major PML isoforms are expressed as a result of alternative splicing, each of which encodes a unique C-terminal region. Using a system in which only a single EYFP-linked PML isoform is expressed, we demonstrate that PMLI, PMLII and PMLVI accumulate in the cytoplasm following arsenic treatment, whereas PMLIII, PMLIV and PMLV do not. 3D structured illumination was used to obtain super-resolution images of PML bodies, revealing spherical shells of PML along with associated SUMO. Arsenic treatment results in dramatic isoform-specific changes to PML body ultrastructure. After extended arsenic treatment most PML isoforms are degraded, leaving SUMO at the core of the nuclear bodies. A high-content imaging assay identifies PMLV as the isoform most readily degraded following arsenic treatment, and PMLIV as relatively resistant to degradation. Immunoprecipitation analysis demonstrates that all PML isoforms are modified by SUMO and ubiquitin after arsenic treatment, and by using siRNA, we demonstrate that arsenic-induced degradation of all PML isoforms is dependent on the ubiquitin E3 ligase RNF4. Intriguingly, depletion of RNF4 results in marked accumulation of PMLV, suggesting that this isoform is an optimal substrate for RNF4. Thus the variable C-terminal domain influences the rate and location of degradation of PML isoforms following arsenic treatment.

The translation initiation factor 3 subunit eIF3K interacts with PML and associates with PML nuclear bodies

The promyelocytic leukemia protein (PML) is a tumor suppressor protein that regulates a variety of important cellular processes, including gene expression, DNA repair and cell fate decisions. Integral to its function is the ability of PML to form nuclear bodies (NBs) that serve as hubs for the interaction and modification of over 90 cellular proteins. There are seven canonical isoforms of PML, which encode diverse C-termini generated by alternative pre-mRNA splicing. Recruitment of specific cellular proteins to PML NBs is mediated by protein-protein interactions with individual PML isoforms. Using a yeast two-hybrid screen employing peptide sequences unique to PML isoform I (PML-I), we identified an interaction with the eukaryotic initiation factor 3 subunit K (eIF3K), and in the process identified a novel eIF3K isoform, which we term eIF3K-2. We further demonstrate that eIF3K and PML interact both in vitro via pull-down assays, as well as in vivo within human cells by co-immunoprecipitation and co-immunofluorescence. In addition, eIF3K isoform 2 (eIF3K-2) colocalizes to PML bodies, particularly those enriched in PML-I, while eIF3K isoform 1 associates poorly with PML NBs. Thus, we report eIF3K as the first known subunit of the eIF3 translation pre-initiation complex to interact directly with the PML protein, and provide data implicating alternative splicing of both PML and eIF3K as a possible regulatory mechanism for eIF3K localization at PML NBs.

RNF38 encodes a nuclear ubiquitin protein ligase that modifies p53

The RNF38 gene encodes a RING finger protein of unknown function. Here we demonstrate that RNF38 is a functional ubiquitin protein ligase (E3). We show that RNF38 isoform 1 is localized to the nucleus by a bipartite nuclear localization sequence (NLS). We confirm that RNF38 is a binding partner of p53 and demonstrate that RNF38 can ubiquitinate p53 in vitro and in vivo. Finally, we show that overexpression of RNF38 in HEK293T cells results in relocalization of p53 to discrete foci associated with PML nuclear bodies. These results suggest RNF38 is an E3 ubiquitin ligase that may play a role in regulating p53.

Is PML a Tumor Suppressor?

The role of the promyelocytic leukemia (PML) protein has been widely tested in many different contexts, as attested by the hundreds of papers present in the literature. In most of these studies, PML is regarded as a tumor suppressor, a notion on the whole accepted by the scientific community. In this review, we examine how the concept of tumor-suppressor gene has evolved until now and then systematically assess whether this assumption for PML is supported by unambiguous experimental evidence.

Identifying gene locus associations with promyelocytic leukemia nuclear bodies using immuno-TRAP

Important insights into nuclear function would arise if gene loci physically interacting with particular subnuclear domains could be readily identified. Immunofluorescence microscopy combined with fluorescence in situ hybridization (immuno-FISH), the method that would typically be used in such a study, is limited by spatial resolution and requires prior assumptions for selecting genes to probe. Our new technique, immuno-TRAP, overcomes these limitations. Using promyelocytic leukemia nuclear bodies (PML NBs) as a model, we used immuno-TRAP to determine if specific genes localize within molecular dimensions with these bodies. Although we confirmed a TP53 gene-PML NB association, immuno-TRAP allowed us to uncover novel locus-PML NB associations, including the ABCA7 and TFF1 loci and, most surprisingly, the PML locus itself. These associations were cell type specific and reflected the cell's physiological state. Combined with microarrays or deep sequencing, immuno-TRAP provides powerful opportunities for identifying gene locus associations with potentially any nuclear subcompartment.

Differential Roles of PML Isoforms

The tumor suppressor promyelocytic leukemia (PML) protein is fused to the retinoic acid receptor alpha in patients suffering from acute promyelocytic leukemia (APL). Treatment of APL patients with arsenic trioxide (As2O3) reverses the disease phenotype by a process involving the degradation of the fusion protein via its PML moiety. Several PML isoforms are generated from a single PML gene by alternative splicing. They share the same N-terminal region containing the RBCC/tripartite motif but differ in their C-terminal sequences. Recent studies of all the PML isoforms reveal the specific functions of each. Here, we review the nomenclature and structural organization of the PML isoforms in order to clarify the various designations and classifications found in different databases. The functions of the PML isoforms and their differential roles in antiviral defense also are reviewed. Finally, the key players involved in the degradation of the PML isoforms in response to As2O3 or other inducers are discussed.

Loss of PML cooperates with mutant p53 to drive more aggressive cancers in a gender-dependent manner

p53 mutations and downregulation of promyelocytic leukemia (PML) are common genetic alterations in human cancers. In healthy cells these two key tumor suppressors exist in a positive regulatory loop, promoting cell death and cellular senescence. However, the influence of their interplay on tumorigenesis has not been explored directly in vivo. The contribution of PML to mutant p53 driven cancer was evaluated in a mouse model harboring a p53 mutation (p53 (wild-type/R172H) ) that recapitulates a frequent p53 mutation (p53 (R175H) ) in human sporadic and Li-Fraumeni cancers. These mice with PML displayed perturbation of the hematopoietic compartment, manifested either as lymphoma or extramedullary hematopoiesis (EMH). EMH was associated with peripheral blood leucocytosis and macrocytic anemia, suggestive of myeloproliferative- myelodysplastic overlap. In contrast, a complete loss of PML from these mice resulted in a marked alteration in tumor profile. While the incidence of lymphomas was unaltered, EMH was not detected and the majority of mice succumbed to sarcomas. Further, males lacking PML exhibited a high incidence of soft tissue sarcomas and reduced survival, while females largely developed osteosarcomas, without impact on survival. Together, these findings demonstrate that PML is an important tumor suppressor dictating disease development in a pertinent mouse model of human cancer.  

KEY POINTS

(1) A mutant p53 allele disrupts hematopoiesis in mice, by promoting lymphomas and myeloproliferative / myelodysplastic overlap. (2) Coincidental p53 allele mutation and PML loss shifts the tumor profile toward sarcoma formation, which is paralleled in human leiomyosarcomas (indicated by immunohistochemistry; IHC).

PML-mediated signaling and its role in cancer stem cells

The promyelocytic leukemia (PML) protein, initially discovered as a part of the PML/retinoic acid receptor alpha fusion protein, has been found to be a critical player in oncogenesis and tumor progression. Multiple cellular activities, including DNA repair, alternative lengthening of telomeres, transcriptional control, apoptosis and senescence, are regulated by PML and its featured subcellular structure, the PML nuclear body. In correspondence with its role in many important life processes, PML mediates several complex downstream signaling pathways. The determinant function of PML in tumorigenesis and cancer progression raises the interest in its involvement in cancer stem cells (CSCs), a subpopulation of cancer cells that share properties with stem cells and are critical for tumor propagation. Recently, there are exciting discoveries concerning the requirement of PML in CSC maintenance. Growing evidences strongly suggest a positive role of PML in regulating CSCs in both hematopoietic cancers and solid tumors, whereas the underlying mechanisms may be different and remain elusive. Here we summarize and discuss the PML-mediated signaling pathways in cancers and their potential roles in regulating CSCs.

MOZ increases p53 acetylation and premature senescence through its complex formation with PML

Monocytic leukemia zinc finger (MOZ)/KAT6A is a MOZ, Ybf2/Sas3, Sas2, Tip60 (MYST)-type histone acetyltransferase that functions as a coactivator for acute myeloid leukemia 1 protein (AML1)- and Ets family transcription factor PU.1-dependent transcription. We previously reported that MOZ directly interacts with p53 and is essential for p53-dependent selective regulation of p21 expression. We show here that MOZ is an acetyltransferase of p53 at K120 and K382 and colocalizes with p53 in promyelocytic leukemia (PML) nuclear bodies following cellular stress. The MOZ-PML-p53 interaction enhances MOZ-mediated acetylation of p53, and this ternary complex enhances p53-dependent p21 expression. Moreover, we identified an Akt/protein kinase B recognition sequence in the PML-binding domain of MOZ protein. Akt-mediated phosphorylation of MOZ at T369 has a negative effect on complex formation between PML and MOZ. As a result of PML-mediated suppression of Akt, the increased PML-MOZ interaction enhances p21 expression and induces p53-dependent premature senescence upon forced PML expression. Our research demonstrates that MOZ controls p53 acetylation and transcriptional activity via association with PML.

Critical role of presenilin-dependent γ-secretase activity in DNA damage-induced promyelocytic leukemia protein expression and apoptosis

Promyelocytic leukemia (PML) is a major component of macromolecular multiprotein complexes called PML nuclear-bodies (PML-NBs). These PML-NBs recruit numerous proteins including CBP, p53 and HIPK2 in response to DNA damage, senescence and apoptosis. In this study, we investigated the effect of presenilin (PS), the main component of the γ-secretase complex, in PML/p53 expression and downstream consequences during DNA damage-induced cell death using camptothecin (CPT). We found that the loss of PS in PS knockout (KO) MEFs (mouse embryonic fibroblasts) results in severely blunted PML expression and attenuated cell death upon CPT exposure, a phenotype that is fully reversed by re-expression of PS1 in PS KO cells and recapitulated by γ-secretase inhibitors in hPS1 MEFs. Interestingly, the γ-secretase cleavage product, APP intracellular domain (AICD), together with Fe65-induced PML expression at the protein and transcriptional levels in PS KO cells. PML and p53 reciprocally positively regulated each other during CPT-induced DNA damage, both of which were dependent on PS. Finally, elevated levels of PML-NB, PML protein and PML mRNA were detected in the brain tissues from Alzheimer's disease (AD) patients, where γ-secretase activity is essential for pathogenesis. Our data provide for the first time, a critical role of the PS/AICD-PML/p53 pathway in DNA damage-induced apoptosis, and implicate this pathway in AD pathogenesis.

Post-translational modifications of PML: consequences and implications

The tumor suppressor promyelocytic leukemia protein (PML) predominantly resides in a structurally distinct sub-nuclear domain called PML nuclear bodies. Emerging evidences indicated that PML actively participates in many aspects of cellular processes, but the molecular mechanisms underlying PML regulation in response to stress and environmental cues are not complete. Post-translational modifications, such as SUMOylation, phosphorylation, acetylation, and ubiquitination of PML add a complex layer of regulation to the physiological function of PML. In this review, we discuss the fast-moving horizon of post-translational modifications targeting PML.

The adenoviral oncogene E1A-13S interacts with a specific isoform of the tumor suppressor PML to enhance viral transcription

PML nuclear bodies (PML NBs), also called ND10, are matrix-bound nuclear structures that have been implicated in a variety of functions, including DNA repair, transcriptional regulation, protein degradation, and tumor suppression. These domains are also known for their potential to mediate an intracellular defense mechanism against many virus types. This is likely why they are targeted and subsequently manipulated by numerous viral proteins. Paradoxically, the genomes of various DNA viruses become associated with PML NBs, and initial sites of viral transcription/replication centers are often juxtaposed to these domains. The question is why viruses start their transcription and replication next to their supposed antagonists. Here, we report that PML NBs are targeted by the adenoviral (Ad) transactivator protein E1A-13S. Alternatively spliced E1A isoforms (E1A-12S and E1A-13S) are the first proteins expressed upon Ad infection. E1A-13S is essential for activating viral transcription in the early phase of infection. Coimmunoprecipitation assays showed that E1A-13S preferentially interacts with only one (PML-II) of at least six nuclear human PML isoforms. Deletion mapping located the interaction site within E1A conserved region 3 (CR3), which was previously described as the transcription factor binding region of E1A-13S. Indeed, cooperation with PML-II enhanced E1A-mediated transcriptional activation, while deleting the SUMO-interacting motif (SIM) of PML proved even more effective. Our results suggest that in contrast to PML NB-associated antiviral defense, PML-II may help transactivate viral gene expression and therefore play a novel role in activating Ad transcription during the early viral life cycle.

Mutual regulation between SIAH2 and DYRK2 controls hypoxic and genotoxic signaling pathways

The ubiquitin E3 ligase SIAH2 is an important regulator of the hypoxic response as it leads to the ubiquitin/proteasomal degradation of prolyl hydroxylases such as PHD3, which in turn increases the stability of hypoxia-inducible factor (HIF)-1α. In the present study, we identify the serine/threonine kinase DYRK2 as SIAH2 interaction partner that phosphorylates SIAH2 at five residues (Ser16, Thr26, Ser28, Ser68, and Thr119). Phosphomimetic and phospho-mutant forms of SIAH2 exhibit different subcellular localizations and consequently change in PHD3 degrading activity. Accordingly, phosphorylated SIAH2 is more active than the wild-type E3 ligase and shows an increased ability to trigger the HIF-1α-mediated transcriptional response and angiogenesis. We also found that SIAH2 knockdown increases DYRK2 stability, whereas SIAH2 expression facilitates DYRK2 polyubiquitination and degradation. Hypoxic conditions cause a SIAH2-dependent DYRK2 polyubiquitination and degradation which ultimately also results in an impaired SIAH2 phosphorylation. Similarly, DYRK2-mediated phosphorylation of p53 at Ser46 is impaired under hypoxic conditions, suggesting a molecular mechanism underlying chemotherapy resistance in solid tumors.

Requirement of PML SUMO interacting motif for RNF4- or arsenic trioxide-induced degradation of nuclear PML isoforms

PML, the organizer of nuclear bodies (NBs), is expressed in several isoforms designated PMLI to VII which differ in their C-terminal region due to alternative splicing of a single gene. This variability is important for the function of the different PML isoforms. PML NB formation requires the covalent linkage of SUMO to PML. Arsenic trioxide (As₂O₃) enhances PML SUMOylation leading to an increase in PML NB size and promotes its interaction with RNF4, a poly-SUMO-dependent ubiquitin E3 ligase responsible for proteasome-mediated PML degradation. Furthermore, the presence of a bona fide SUMO Interacting Motif (SIM) within the C-terminal region of PML seems to be required for recruitment of other SUMOylated proteins within PML NBs. This motif is present in all PML isoforms, except in the nuclear PMLVI and in the cytoplasmic PMLVII. Using a bioluminescence resonance energy transfer (BRET) assay in living cells, we found that As₂O₃ enhanced the SUMOylation and interaction with RNF4 of nuclear PML isoforms (I to VI). In addition, among the nuclear PML isoforms, only the one lacking the SIM sequence, PMLVI, was resistant to As₂O₃-induced PML degradation. Similarly, mutation of the SIM in PMLIII abrogated its sensitivity to As₂O₃-induced degradation. PMLVI and PMLIII-SIM mutant still interacted with RNF4. However, their resistance to the degradation process was due to their inability to be polyubiquitinated and to recruit efficiently the 20S core and the β regulatory subunit of the 11S complex of the proteasome in PML NBs. Such resistance of PMLVI to As₂O₃-induced degradation was alleviated by overexpression of RNF4. Our results demonstrate that the SIM of PML is dispensable for PML SUMOylation and interaction with RNF4 but is required for efficient PML ubiquitination, recruitment of proteasome components within NBs and proteasome-dependent degradation of PML in response to As₂O₃.

Promyelocytic leukemia nuclear bodies support a late step in DNA double-strand break repair by homologous recombination

The PML protein and PML nuclear bodies (PML-NB) are implicated in multiple cellular functions relevant to tumor suppression, including DNA damage response. In most cases of acute promyelocytic leukemia, the PML and retinoic acid receptor alpha (RARA) genes are translocated, resulting in expression of oncogenic PML-RARα fusion proteins. PML-NB fail to form normally, and promyelocytes remain in an undifferentiated, abnormally proliferative state. We examined the involvement of PML protein and PML-NB in homologous recombinational repair (HRR) of chromosomal DNA double-strand breaks. Transient overexpression of wild-type PML protein isoforms produced hugely enlarged or aggregated PML-NB and reduced HRR by ~2-fold, suggesting that HRR depends to some extent upon normal PML-NB structure. Knockdown of PML by RNA interference sharply attenuated formation of PML-NB and reduced HRR by up to 20-fold. However, PML-knockdown cells showed apparently normal induction of H2AX phosphorylation and RAD51 foci after DNA damage by ionizing radiation. These findings indicate that early steps in HRR, including recognition of DNA double-strand breaks, initial processing of ends, and assembly of single-stranded DNA/RAD51 nucleoprotein filaments, do not depend upon PML-NB. The HRR deficit in PML-depleted cells thus reflects inhibition of later steps in the repair pathway. Expression of PML-RARα fusion proteins disrupted PML-NB structure and reduced HRR by up to 10-fold, raising the possibility that defective HRR and resulting genomic instability may figure in the pathogenesis, progression and relapse of acute promyelocytic leukemia.

Lens epithelium-derived growth factor deSumoylation by Sumo-specific protease-1 regulates its transcriptional activation of small heat shock protein and the cellular response

Lens epithelium-derived growth factor (LEDGF), a ubiquitously expressed nuclear protein, acts by interacting with DNA and protein and is involved in widely varying cellular functions. Despite its importance, the mechanism(s) that regulate naturally occurring LEDGF activity are unidentified. In the present study, we report that LEDGF is constitutively Sumoylated, and that the dynamical regulatory mechanism(s) (i.e. Sumoylation and deSumoylation) act as a molecular switch in modulating the DNA-binding and transcriptional activity of LEDGF with the functional consequences. Using bioinformatics analysis coupled with in vitro and in vivo Sumoylation assays, we found that lysine (K) 364 of LEDGF was Sumoylated, repressing its transcriptional activity. Conversely, mutation of K364 to arginine (R) or deSumoylation by small ubiquitin-like modifier (Sumo)-specific protease-1, a nuclear deSumoylase, enhanced the transactivation capacity of LEDGF and its cellular abundance. The enhancements were directly correlated with an increase in the DNA-binding activity and small heat shock protein transcription of LEDGF, whereas the process was reversed in cells overexpressing Sumo1. Interestingly, cells expressing Sumoylation-deficient pEGFP-K364R protein showed increased cellular survival compared to wild-type LEDGF protein. The findings provide insights into the regulation and regulatory functions of LEDGF in Sumoylation-dependent transcriptional control that may be essential for modifying the physiology of cells to maintain cellular homeostasis. These studies also provide new evidence of the important role of post-translational modification in controlling LEDGF function.

Functions of the Epstein-Barr virus EBNA1 protein in viral reactivation and lytic infection

EBNA1 is the only nuclear Epstein-Barr virus (EBV) protein expressed in both latent and lytic modes of infection. While EBNA1 is known to play several important roles in latent infection, the reason for its continued expression in lytic infection is unknown. Here we identified two roles for EBNA1 in the reactivation of latent EBV to the lytic cycle in epithelial cells. First, EBNA1 depletion in latently infected cells was shown to positively contribute to spontaneous EBV reactivation, showing that EBNA1 has a role in suppressing reactivation. Second, when the lytic cycle was induced, EBNA1 depletion decreased lytic gene expression and DNA amplification, showing that it positively contributed to lytic infection. Since we have previously shown that EBNA1 disrupts promyelocytic leukemia (PML) nuclear bodies, we investigated whether this function could account for the effects of EBNA1 on lytic infection by repeating the experiments with cells lacking PML proteins. In the absence of PML, EBNA1 did not promote lytic infection, indicating that the EBNA1-mediated PML disruption is responsible for promoting lytic infection. In keeping with this conclusion, PML silencing was found to be sufficient to induce the EBV lytic cycle. Finally, by generating cells with single PML isoforms, we showed that individual PML isoforms were sufficient to suppress EBV lytic reactivation, although PML isoform IV (PML IV) was ineffective because it was most efficiently degraded by EBNA1. Our results provide the first function for EBNA1 in lytic infection and show that EBNA1 interactions with PML IV lead to a loss of PML nuclear bodies (NBs) that promotes lytic infection.

β-catenin inhibits promyelocytic leukemia protein tumor suppressor function in colorectal cancer cells

BACKGROUND & AIMS

Loss of promyelocytic leukemia protein (PML) nuclear body (NB) formation has been reported in colorectal and other solid tumors. However, genetic alteration of PML is rarely observed in these tumors; the exact mechanisms that mediate loss of PML function are not known.

METHODS

We previously used a comprehensive shotgun mass spectrometry approach to identify PML as 1 of 70 proteins that coimmunoprecipitate with anti-T-cell factor 4 in DLD-1 and HCT116 colorectal cancer cell lines; we investigated the effects of altered β-catenin expression on PML function in these cells.

RESULTS

β-catenin specifically interacted with the product of PML transcript variant IV (PML-IV) through the armadillo repeat domain of β-catenin. Overexpression of β-catenin in colorectal cancer cells disrupted the subcellular compartmentalization of PML-IV, whereas knockdown of β-catenin restored formation of PML-NB. Modification of PML by the small ubiquitin-related modifier (SUMO) is required for proper assembly of PML-NB. β-catenin inhibited Ran-binding protein 2-mediated SUMOylation of PML-IV.

CONCLUSIONS

β-catenin interacts with PML isoform IV and disrupts PML-IV function and PML-NB formation by inhibiting Ran-binding protein 2-mediated SUMO modification of PML-IV. These findings indicate the involvement of a posttranslational mechanism in disruption of PML-NB organization in cancer cells and provide more information about the oncogenic functions of β-catenin.

PML nuclear bodies and other TRIM-defined subcellular compartments

Tripartite motif (TRIM) proteins are defined by their possession of a RING, B-box and predicted coiled coil (RBCC) domain. The coiled-coil region facilitates the oligomerisation of TRIMs and contributes to the formation of high molecular weight complexes that show interesting subcellular compartmentalisations and structures. TRIM protein compartments include both nuclear and cytoplasmic filaments and aggregates (bodies), as well as diffuse subcellular distributions. TRIM 19, otherwise known as promyelocytic leukaemia (PML) protein forms nuclear aggregates termed PML nuclear bodies (PML NBs), at which a number of functionally diverse proteins transiently or covalently associate. PML NBs are therefore implicated in a wide variety of cellular functions such as transcriptional regulation, viral response, apoptosis and nuclear protein storage.

MageA2 restrains cellular senescence by targeting the function of PMLIV/p53 axis at the PML-NBs

MAGE-A genes are a subfamily of the melanoma antigen genes (MAGEs), whose expression is restricted to tumor cells of different origin and normal tissues of the human germline. Although the specific function of individual MAGE-A proteins is being currently explored, compelling evidence suggest their involvement in the regulation of different pathways during tumor progression. We have previously reported that MageA2 binds histone deacetylase (HDAC)3 and represses p53-dependent apoptosis in response to chemotherapeutic drugs. The promyelocytic leukemia (PML) tumor suppressor is a regulator of p53 acetylation and function in cellular senescence. Here, we demonstrate that MageA2 interferes with p53 acetylation at PML-nuclear bodies (NBs) and with PMLIV-dependent activation of p53. Moreover, a fraction of MageA2 colocalizes with PML-NBs through direct association with PML, and decreases PMLIV sumoylation through an HDAC-dependent mechanism. This reduction in PML post-translational modification promotes defects in PML-NBs formation. Remarkably, we show that in human fibroblasts expressing RasV12 oncogene, MageA2 expression decreases cellular senescence and increases proliferation. These results correlate with a reduction in NBs number and an impaired p53 response. All these data suggest that MageA2, in addition to its anti-apoptotic effect, could have a novel role in the early progression to malignancy by interfering with PML/p53 function, thereby blocking the senescence program, a critical barrier against cell transformation.

Direct interaction between scaffolding proteins RACK1 and 14-3-3ζ regulates brain-derived neurotrophic factor (BDNF) transcription

RACK1 is a scaffolding protein that spatially and temporally regulates numerous signaling cascades. We previously found that activation of the cAMP signaling pathway induces the translocation of RACK1 to the nucleus. We further showed that nuclear RACK1 is required to promote the transcription of the brain-derived neurotrophic factor (BDNF). Here, we set out to elucidate the mechanism underlying cAMP-dependent RACK1 nuclear translocation and BDNF transcription. We identified the scaffolding protein 14-3-3ζ as a direct binding partner of RACK1. Moreover, we found that 14-3-3ζ was necessary for the cAMP-dependent translocation of RACK1 to the nucleus. We further observed that the disruption of RACK1/14-3-3ζ interaction with a peptide derived from the RACK1/14-3-3ζ binding site or shRNA-mediated 14-3-3ζ knockdown inhibited cAMP induction of BDNF transcription. Together, these data reveal that the function of nuclear RACK1 is mediated through its interaction with 14-3-3ζ. As RACK1 and 14-3-3ζ are two multifunctional scaffolding proteins that coordinate a wide variety of signaling events, their interaction is likely to regulate other essential cellular functions.

Contributions of the Epstein-Barr virus EBNA1 protein to gastric carcinoma

Approximately 10% of gastric carcinomas (GC) are comprised of cells latently infected with Epstein-Barr virus (EBV); however, the mechanism by which EBV contributes to the development of this malignancy is unclear. We have investigated the cellular effects of the only EBV nuclear protein expressed in GC, EBNA1, focusing on promyelocytic leukemia (PML) nuclear bodies (NBs), which play important roles in apoptosis, p53 activation, and tumor suppression. AGS GC cells infected with EBV were found to contain fewer PML NBs and less PML protein than the parental EBV-negative AGS cells, and these levels were restored by silencing EBNA1. Conversely, EBNA1 expression was sufficient to induce the loss of PML NBs and proteins in AGS cells. Consistent with PML functions, EBNA1 expression decreased p53 activation and apoptosis in response to DNA damage and resulted in increased cell survival. In addition, EBNA1 mutants unable to bind CK2 kinase or ubiquitin-specific protease 7 had decreased ability to induce PML loss and to interfere with p53 activation. PML levels in EBV-positive and EBV-negative GC biopsy specimens were then compared by immunohistochemistry. Consistent with the results in the AGS cells, EBV-positive tumors had significantly lower PML levels than EBV-negative tumors. The results indicate that EBV infection of GC cells leads to loss of PML NBs through the action of EBNA1, resulting in impaired responses to DNA damage and promotion of cell survival. Therefore, PML disruption by EBNA1 is one mechanism by which EBV may contribute to the development of gastric cancer.

Promyelocytic leukemia isoform IV confers resistance to encephalomyocarditis virus via the sequestration of 3D polymerase in nuclear bodies

Promyelocytic leukemia (PML) protein is the organizer of nuclear matrix-associated nuclear bodies (NBs), and its conjugation to the small ubiquitin-like modifier (SUMO) is required for the formation of these structures. Several alternatively spliced PML transcripts from a single PML gene lead to the production of seven PML isoforms (PML isoform I [PMLI] to VII [PMLVII]), which all share a N-terminal region that includes the RBCC (RING, B boxes, and a α-helical coiled-coil) motif but differ in the C-terminal region. This diversity of PML isoforms determines the specific functions of each isoform. There is increasing evidence implicating PML in host antiviral defense and suggesting various strategies involving PML to counteract viral production. We reported that mouse embryonic fibroblasts derived from PML knockout mice are more sensitive than wild-type cells to infection with encephalomyocarditis virus (EMCV). Here, we show that stable expression of PMLIV or PMLIVa inhibited viral replication and protein synthesis, leading to a substantial reduction of EMCV multiplication. This protective effect required PMLIV SUMOylation and was not observed with other nuclear PML isoforms (I, II, III, V, and VI) or with the cytoplasmic PMLVII. We demonstrated that only PMLIV interacted with EMCV 3D polymerase (3Dpol) and sequestered it within PML NBs. The C-terminal region specific to PMLIV was required for both interaction with 3Dpol and the antiviral properties. Also, depletion of PMLIV by RNA interference significantly boosted EMCV production in interferon-treated cells. These findings indicate the mechanism by which PML confers resistance to EMCV. They also reveal a new pathway mediating the antiviral activity of interferon against EMCV.

The PML nuclear bodies-associated protein TTRAP regulates ribosome biogenesis in nucleolar cavities upon proteasome inhibition

TRAF and TNF receptor-associated protein (TTRAP) is a multifunctional protein that can act in the nucleus as a 5'-tyrosyl DNA phosphodiesterase and in the cytoplasm as a regulator of cell signaling. In this paper we show that in response to proteasome inhibition TTRAP accumulates in nucleolar cavities in a promyelocytic leukemia protein-dependent manner. In the nucleolus, TTRAP contributes to control levels of ribosomal RNA precursor and processing intermediates, and this phenotype is independent from its 5'-tyrosyl DNA phosphodiesterase activity. Our findings suggest a previously unidentified function for TTRAP and nucleolar cavities in ribosome biogenesis under stress.

Adenovirus type 5 early region 1B 55K oncoprotein-dependent degradation of cellular factor Daxx is required for efficient transformation of primary rodent cells

Early region 1B 55K (E1B-55K) from adenovirus type 5 (Ad5) is a multifunctional regulator of lytic infection and contributes in vitro to complete cell transformation of primary rodent cells in combination with Ad5 E1A. Inhibition of p53 activated transcription plays a key role in processes by which E1B-55K executes its oncogenic potential. Nevertheless, additional functions of E1B-55K or further protein interactions with cellular factors of DNA repair, transcription, and apoptosis, including Mre11, PML, and Daxx, may also contribute to the transformation process. In line with previous results, we performed mutational analysis to define a Daxx interaction motif within the E1B-55K polypeptide. The results from these studies showed that E1B-55K/Daxx binding is not required for inhibition of p53-mediated transactivation or binding and degradation of cellular factors (p53/Mre11). Surprisingly, these mutants lost the ability to degrade Daxx and showed reduced transforming potential in primary rodent cells. In addition, we observed that E1B-55K lacking the SUMO-1 conjugation site (SCS/K104R) was sufficient for Daxx interaction but no longer capable of E1B-55K-dependent proteasomal degradation of the cellular factor Daxx. These results, together with the observation that E1B-55K SUMOylation is required for efficient transformation, provides evidence for the idea that SUMO-1-conjugated E1B-55K-mediated degradation of Daxx plays a key role in adenoviral oncogenic transformation. We assume that the viral protein contributes to cell transformation through the modulation of Daxx-dependent pathways. This further substantiates the assumption that further mechanisms for efficient transformation of primary cells can be separated from functions required for the inhibition of p53-stimulated transcription.

Daxx-beta and Daxx-gamma, two novel splice variants of the transcriptional co-repressor Daxx

Daxx is involved in transcriptional control and apoptosis. It comprises several domains, including a regulatory C terminus that is responsible for the interaction with numerous proteins such as p53, promyelocytic leukemia protein (PML), and Hsp27. Here, we describe the identification and characterization of two novel variants of Daxx termed Daxx-β and Daxx-γ, which are generated by alternative splicing. Alternative splicing results in a truncated regulatory C terminus in both proteins. As a consequence, Daxx-β and Daxx-γ show a markedly decreased affinity to PML, which in turn is associated with a different subnuclear localization of these proteins compared with Daxx. Although Daxx is localized mainly in PML-oncogenic domains (PODs) Daxx-β and Daxx-γ display a distinct distribution pattern. Furthermore, Daxx-β and Daxx-γ show a decreased affinity to p53 also due to the truncated C terminus. We provide evidence that the p53 recruitment into PODs is Daxx isoform-dependent. The decreased affinity of Daxx-β/-γ to p53 and PML results in a diffuse localization of p53 throughout the nucleus. In contrast to Daxx, Daxx-β and Daxx-γ are unable to repress p53-mediated transcription. Therefore, alternative splicing of Daxx might indicate an additional level in the cellular apoptosis network.

Move or die: the fate of the Tax oncoprotein of HTLV-1

The HTLV-1 Tax protein both activates viral replication and is involved in HTLV-1-mediated transformation of T lymphocytes. The transforming properties of Tax include altering the expression of select cellular genes via activation of cellular pathways and perturbation of both cell cycle control mechanisms and apoptotic signals. The recent discovery that Tax undergoes a hierarchical sequence of posttranslational modifications that control its intracellular localization provides provocative insights into the mechanisms regulating Tax transcriptional and transforming activities.

p53 and p21(Waf1) are recruited to distinct PML-containing nuclear foci in irradiated and Nutlin-3a-treated U2OS cells

Promyelocytic leukemia nuclear bodies (PML-NBs) are multiprotein complexes that include PML protein and localize in nuclear foci. PML-NBs are implicated in multiple stress responses, including apoptosis, DNA repair, and p53-dependent growth inhibition. ALT-associated PML bodies (APBs) are specialized PML-NBs that include telomere-repeat binding-factor TRF1 and are exclusively in telomerase-negative tumors where telomere length is maintained through alternative (ALT) recombination mechanisms. We compared cell-cycle and p53 responses in ALT-positive cancer cells (U2OS) exposed to ionizing radiation (IR) or the p53 stabilizer Nutlin-3a. Both IR and Nutlin-3a caused growth arrest and comparable induction of p53. However, p21, whose gene p53 activates, displayed biphasic induction following IR and monophasic induction following Nutlin-3a. p53 was recruited to PML-NBs 3-4 days after IR, approximately coincident with the secondary p21 increase. These p53/PML-NBs marked sites of apparently unrepaired DNA double-strand breaks (DSBs), identified by colocalization with phosphorylated histone H2AX. Both Nutlin-3a and IR caused a large increase in APBs that was dependent on p53 and p21 expression. Moreover, p21, and to a lesser extent p53, was recruited to APBs in a fraction of Nutlin-3a-treated cells. These data indicate (1) p53 is recruited to PML-NBs after IR that likely mark unrepaired DSBs, suggesting p53 may either be further activated at these sites and/or function in their repair; (2) p53-p21 pathway activation increases the percentage of APB-positive cells, (3) p21 and p53 are recruited to ALT-associated PML-NBs after Nutlin-3a treatment, suggesting that they may play a previously unrecognized role in telomere maintenance.

FAT10 modifies p53 and upregulates its transcriptional activity

FAT10, also known as diubiquitin, has been implicated in the regulation of diverse cellular processes, including mitosis, immune response, and apoptosis. We seek to identify FAT10-targeted proteins, an essential step in elucidating the physiological function of FAT10. To this end, human FAT10 or its non-conjugatable derivative, FAT10ΔGG, was overexpressed in HEK293 cells. We observed a number of high molecular weight FAT10 conjugates in cells expressing wild-type FAT10, but not in FAT10ΔGG. The FAT10 conjugates are inducible by TNF-α and accumulated significantly when cells were treated with proteasome inhibitor, MG132. Among them, tumor suppressor p53 was found to be FATylated. The p53 transcriptional activity was found to be substantially enhanced in FAT10-overexpressing cells. In addition, overexpressing FAT10 in HEK293 cells also reduced the population of p53 which cross reacted with monoclonal anti-p53 antibody, PAB240, known to recognize only the transcriptionally inactive p53. FAT10 in the nucleus was found co-localized with p53 and altered its subcellular compartmentalization. Furthermore, overexpressing FAT10 led to a reduction in the size of promyelocytic leukemia nuclear bodies (PML-NBs) and altered their distribution in the nucleus. Based on these observations, a potential mechanism which correlates FATylation of p53 to its translocation and transcriptional activation is discussed.

Regulation of E2Fs and senescence by PML nuclear bodies

The tumor suppressor PML (promyelocytic leukemia protein) regulates cellular senescence and terminal differentiation, two processes that implicate a permanent exit from the cell cycle. Here, we show that the mechanism by which PML induces a permanent cell cycle exit and activates p53 and senescence involves a recruitment of E2F transcription factors bound to their promoters and the retinoblastoma (Rb) proteins to PML nuclear bodies enriched in heterochromatin proteins and protein phosphatase 1α. Blocking the functions of the Rb protein family or adding back E2Fs to PML-expressing cells can rescue their defects in E2F-dependent gene expression and cell proliferation, inhibiting the senescent phenotype. In benign prostatic hyperplasia, a neoplastic disease that displays features of senescence, PML was found to be up-regulated and forming nuclear bodies. In contrast, PML bodies were rarely visualized in prostate cancers. The newly defined PML/Rb/E2F pathway may help to distinguish benign tumors from cancers, and suggest E2F target genes as potential targets to induce senescence in human tumors.

Role of promyelocytic leukemia protein in host antiviral defense

Several pathways have been implicated in the establishment of antiviral state in response to interferon (IFN), one of which implicates the promyelocytic leukemia (PML) protein. The PML gene has been discovered 20 years ago and has led to new insights into oncogenesis, apoptosis, cell senescence, and antiviral defense. PML is induced by IFN, leading to a marked increase of expression of PML isoforms and the number of PML nuclear bodies (NBs). PML is the organizer of the NBs that contains at least 2 permanent NB-associated proteins, the IFN-stimulated gene product Speckled protein of 100 kDa (Sp100) and death-associated dead protein (Daxx), as well as numerous other transient proteins recruited in these structures in response to different stimuli. Accumulating reports have implicated PML in host antiviral defense and revealed various strategies developed by viruses to disrupt PML NBs. This review will focus on the regulation of PML and the implication of PML NBs in conferring resistance to DNA and RNA viruses. The role of PML in mediating an IFN-induced antiviral state will also be discussed.

SIRT1 stabilizes PML promoting its sumoylation

SIRT1, the closest mammalian homolog of yeast Sir2, is an NAD(+)-dependent deacetylase with relevant functions in cancer, aging, and metabolism among other processes. SIRT1 has a diffuse nuclear localization but is recruited to the PML nuclear bodies (PML-NBs) after PML upregulation. However, the functions of SIRT1 in the PML-NBs are unknown. In this study we show that primary mouse embryo fibroblasts lacking SIRT1 contain reduced PML protein levels that are increased after reintroduction of SIRT1. In addition, overexpression of SIRT1 in HEK-293 cells increases the amount of PML protein whereas knockdown of SIRT1 reduces the size and number of PML-NBs and the levels of PML protein in HeLa cells. SIRT1 stimulates PML sumoylation in vitro and in vivo in a deacetylase-independent manner. Importantly, the absence of SIRT1 reduces the apoptotic response of vesicular stomatitis virus-infected cells and favors the extent of this PML-sensitive virus replication. These results show a novel function of SIRT1 in the control of PML and PML-NBs.

SUMO-specific protease 2 in Mdm2-mediated regulation of p53

Genetic analysis in mice has revealed a key genetic pathway, SUMO-specific protease 2 (SENP2)-Mdm2-p53, essential for trophoblast development. Targeted disruption of SENP2 impairs the G-S transition required for mitotic and endoreduplication cell cycles during the expansion of trophoblast stem cells and their differentiation into polyploidy cells, respectively. The disruption disturbed the subcellular distribution and SUMO modification of Mdm2, leading to interference with p53 degradation. Here, we further explore the mechanism underlying SENP2-mediated regulation of Mdm2 in p53-induced cellular stress. We identify a specific isoform of SENP2 necessary and sufficient to negatively regulate the p53-dependent transcription and its related stress responses. This isoform-specific effect is attributed to the differential compartmentalization of SENP2. SUMO conjugation of Mdm2 induces its co-localization and association with SENP2 in promyelocytic leukemia bodies. Biochemical studies show that SENP2 catalyzes the desumoylation process of Mdm2. SENP2-dependent regulation of Mdm2 is sensitive to its p53-binding activity. Our findings led us to propose a mechanism underlying the SENP2-mediated regulation of Mdm2 that is critical for genome integrity in p53-dependent stress responses.

The role of PML in the nervous system

The promyeloctic leukemia protein PML is a tumor suppressor that was originally identified due to its involvement in the (15;17) translocation of acute promyelocytic leukemia. While the majority of early research has focused upon the role of PML in the pathogenesis of leukemia, more recent evidence has identified important roles for PML in tissues outside the hemopoietic system, including the central nervous system (CNS). Here, we review recent literature on the role of PML in the CNS, with particular focus on the processes of neurodevelopment and neurodegeneration, and propose new lines of investigation.

PML positively regulates interferon gamma signaling

PML, also known as TRIM19, belongs to the family encoding a characteristic RBCC/TRIM motif comprising several cysteine-rich zinc-binding domains (RING and B-boxes) and a coiled-coil domain. The RBCC domain and the covalent modification of PML by the small ubiquitin-like modifier (SUMO) are required for PML localization within the nuclear bodies (NBs). Analysis of PML(-/-) mice provided evidence for a physiological role of PML in apoptosis. Cells derived from these mice are defective in the induction of apoptosis by interferon (IFN). PML is expressed as a family of cytoplasmic and nuclear isoforms (PML I-VII) as a result of alternative splicing. Herein, we show that overexpression of all nuclear PML isoforms (I-VI) in human cells increased IFNγ-induced STAT1 phosphorylation, resulting in higher binding of STAT1 to DNA, higher activation of IFN-stimulated genes (ISGs), and an increase in the expression of their products. These effects, observed with IFNγ and not IFNα, required PML localization in the nucleus as they were not observed with the cytoplasmic isoform PMLVIIb or the cytoplasmic variants of PMLIV. They also necessitated PML SUMOylation and its RING finger domain. Conversely, downregulation of PML by RNA interference was accompanied by decrease in IFNγ-induced STAT1 phosphorylation, STAT1 DNA binding, transcription of ISGs and in the expression of their products. In addition, IFNγ-mediated STAT1 DNA-binding activity was decreased in PML(-/-) MEFs compared with wild-type MEFs. Taken together these results demonstrate that PML functions as a positive regulator of IFNγ signaling.

Adenovirus E1B 55-kilodalton protein is a p53-SUMO1 E3 ligase that represses p53 and stimulates its nuclear export through interactions with promyelocytic leukemia nuclear bodies

Oncogenic transformation by adenovirus E1A and E1B-55K requires E1B-55K inhibition of p53 activity to prevent E1A-induced apoptosis. During viral infection, E1B-55K and E4orf6 substitute for the substrate-binding subunits of the host cell cullin 5 class of ubiquitin ligases, resulting in p53 polyubiquitinylation and proteasomal degradation. Here we show that E1B-55K alone also functions as an E3 SUMO1-p53 ligase. Fluorescence microscopy studies showed that E1B-55K alone, in the absence of other viral proteins, causes p53 to colocalize with E1B-55K in promyelocytic leukemia (PML) nuclear bodies, nuclear domains with a high concentration of sumoylated proteins. Photobleaching experiments with live cells revealed that E1B-55K tethering of p53 in PML nuclear bodies decreases the in vivo nuclear mobility of p53 nearly 2 orders of magnitude. E1B-55K-induced p53 sumoylation contributes to maximal inhibition of p53 function since mutation of the major p53 sumoylation site decreases E1B-55K-induced p53 sumoylation, tethering in PML nuclear bodies, and E1B-55K inhibition of p53 activity. Mutation of the E1B-55K sumoylation site greatly inhibits E1B-55K association with PML nuclear bodies and the p53 nuclear export to cytoplasmic aggresomes observed in E1A-E1B-transformed cells. Purified E1B-55K and p53 form high-molecular-weight complexes potentially through the formation of a network of E1B-55K dimers bound to the N termini of p53 tetramers. In support of this model, a p53 mutation that prevents tetramer formation greatly reduces E1B-55K-induced tethering in PML nuclear bodies and p53 nuclear export. These data indicate that E1B-55K's association with PML nuclear bodies inactivates p53 by first sequestering it in PML nuclear bodies and then greatly facilitating its nuclear export.

Pharmacological inhibition of BMK1 suppresses tumor growth through promyelocytic leukemia protein

BMK1 is activated by mitogens and oncogenic signals and, thus, is strongly implicated in tumorigenesis. We found that BMK1 interacted with promyelocytic leukemia protein (PML), and inhibited its tumor-suppressor function through phosphorylation. Furthermore, activated BMK1 notably inhibited PML-dependent activation of p21. To further investigate the BMK-mediated inhibition of the tumor suppressor activity of PML in tumor cells, we developed a small-molecule inhibitor of the kinase activity of BMK1, XMD8-92. Inhibition of BMK1 by XMD8-92 blocked tumor cell proliferation in vitro and significantly inhibited tumor growth in vivo by 95%, demonstrating the efficacy and tolerability of BMK1-targeted cancer treatment in animals.