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

PML Nuclear bodies: the cancer connection and beyond

Promyelocytic leukemia (PML) nuclear bodies, membrane-less organelles in the nucleus, play a crucial role in cellular homeostasis. These dynamic structures result from the assembly of scaffolding PML proteins and various partners. Recent crystal structure analyses revealed essential self-interacting domains, while liquid-liquid phase separation contributes to their formation. PML bodies orchestrate post-translational modifications, particularly stress-induced SUMOylation, impacting target protein functions. Serving as hubs in multiple signaling pathways, they influence cellular processes like senescence. Dysregulation of PML expression contributes to diseases, including cancer, highlighting their significance. Therapeutically, PML bodies are promising targets, exemplified by successful acute promyelocytic leukemia treatment with arsenic trioxide and retinoic acid restoring PML bodies. Understanding their functions illuminates both normal and pathological cellular physiology, guiding potential therapies. This review explores recent advancements in PML body biogenesis, biochemical activity, and their evolving biological roles.

Unravelling the molecular interplay: SUMOylation, PML nuclear bodies and vascular cell activity in health and disease

In the seemingly well-researched field of vascular research, there are still many underestimated factors and molecular mechanisms. In recent years, SUMOylation has become increasingly important. SUMOylation is a post-translational modification in which small ubiquitin-related modifiers (SUMO) are covalently attached to target proteins. Sites where these SUMO modification processes take place in the cell nucleus are PML nuclear bodies (PML-NBs) - multiprotein complexes with their essential main component and organizer, the PML protein. PML and SUMO, either alone or as partners, influence a variety of cellular processes, including regulation of transcription, senescence, DNA damage response and defence against microorganisms, and are involved in innate immunity and inflammatory responses. They also play an important role in maintaining homeostasis in the vascular system and in pathological processes leading to the development and progression of cardiovascular diseases. This review summarizes information about the function of SUMO(ylation) and PML(-NBs) in the human vasculature from angiogenesis to disease and highlights their clinical potential as drug targets.

USP22 regulates APL differentiation via PML-RARα stabilization and IFN repression

Ubiquitin-specific peptidase 22 (USP22) is a deubiquitinating enzyme (DUB) that underlies tumorigenicity, proliferation, cell death and differentiation through deubiquitination of histone and non-histone targets. Ubiquitination determines stability, localization and functions of cell fate proteins and controls cell-protective signaling pathways to surveil cell cycle progression. In a variety of carcinomas, lymphomas and leukemias, ubiquitination regulates the tumor-suppressive functions of the promyelocytic leukemia protein (PML), but PML-specific DUBs, DUB-controlled PML ubiquitin sites and the functional consequences of PML (de)ubiquitination remain unclear. Here, we identify USP22 as regulator of PML and the oncogenic acute promyelocytic leukemia (APL) fusion PML-RARα protein stability and identify a destabilizing role of PML residue K394. Additionally, loss of USP22 upregulates interferon (IFN) and IFN-stimulated gene (ISG) expression in APL and induces PML-RARα stabilization and a potentiation of the cell-autonomous sensitivity towards all-trans retinoic acid (ATRA)-mediated differentiation. Our findings imply USP22-dependent surveillance of PML-RARα stability and IFN signaling as important regulator of APL pathogenesis, with implications for viral mimicry, differentiation and cell fate regulation in other leukemia subtypes.

Photobody formation spatially segregates two opposing phytochrome B signaling actions to titrate plant environmental responses

Photoactivation of the plant photoreceptor and thermosensor phytochrome B (PHYB) triggers its condensation into subnuclear photobodies (PBs). However, the function of PBs remains frustratingly elusive. Here, we found that PHYB recruits PHYTOCHROME-INTERACTING FACTOR5 (PIF5) to PBs. Surprisingly, PHYB exerts opposing roles in degrading and stabilizing PIF5. Perturbing PB size by overproducing PHYB provoked a biphasic PIF5 response: while a moderate increase in PHYB enhanced PIF5 degradation, further elevating the PHYB level stabilized PIF5 by retaining more of it in enlarged PBs. These results reveal a PB-mediated light and temperature sensing mechanism, in which PHYB condensation confers the co-occurrence and competition of two antagonistic phase-separated PHYB signaling actions-PIF5 stabilization in PBs and PIF5 degradation in the surrounding nucleoplasm-thereby enabling an environmentally-sensitive counterbalancing mechanism to titrate nucleoplasmic PIF5 and its transcriptional output. This PB-enabled signaling mechanism provides a framework for regulating a plethora of PHYB-interacting signaling molecules in diverse plant environmental responses.

On the Prevalence and Roles of Proteins Undergoing Liquid-Liquid Phase Separation in the Biogenesis of PML-Bodies

The formation and function of membrane-less organelles (MLOs) is one of the main driving forces in the molecular life of the cell. These processes are based on the separation of biopolymers into phases regulated by multiple specific and nonspecific inter- and intramolecular interactions. Among the realm of MLOs, a special place is taken by the promyelocytic leukemia nuclear bodies (PML-NBs or PML bodies), which are the intranuclear compartments involved in the regulation of cellular metabolism, transcription, the maintenance of genome stability, responses to viral infection, apoptosis, and tumor suppression. According to the accepted models, specific interactions, such as SUMO/SIM, the formation of disulfide bonds, etc., play a decisive role in the biogenesis of PML bodies. In this work, a number of bioinformatics approaches were used to study proteins found in the proteome of PML bodies for their tendency for spontaneous liquid-liquid phase separation (LLPS), which is usually caused by weak nonspecific interactions. A total of 205 proteins found in PML bodies have been identified. It has been suggested that UBC9, P53, HIPK2, and SUMO1 can be considered as the scaffold proteins of PML bodies. It was shown that more than half of the proteins in the analyzed proteome are capable of spontaneous LLPS, with 85% of the analyzed proteins being intrinsically disordered proteins (IDPs) and the remaining 15% being proteins with intrinsically disordered protein regions (IDPRs). About 44% of all proteins analyzed in this study contain SUMO binding sites and can potentially be SUMOylated. These data suggest that weak nonspecific interactions play a significantly larger role in the formation and biogenesis of PML bodies than previously expected.

The interactions between PML nuclear bodies and small and medium size DNA viruses

Promyelocytic leukemia nuclear bodies (PM NBs), often referred to as membraneless organelles, are dynamic macromolecular protein complexes composed of a PML protein core and other transient or permanent components. PML NBs have been shown to play a role in a wide variety of cellular processes. This review describes in detail the diverse and complex interactions between small and medium size DNA viruses and PML NBs that have been described to date. The PML NB components that interact with small and medium size DNA viruses include PML protein isoforms, ATRX/Daxx, Sp100, Sp110, HP1, and p53, among others. Interaction between viruses and components of these NBs can result in different outcomes, such as influencing viral genome expression and/or replication or impacting IFN-mediated or apoptotic cell responses to viral infection. We discuss how PML NB components abrogate the ability of adenoviruses or Hepatitis B virus to transcribe and/or replicate their genomes and how papillomaviruses use PML NBs and their components to promote their propagation. Interactions between polyomaviruses and PML NBs that are poorly understood but nevertheless suggest that the NBs can serve as scaffolds for viral replication or assembly are also presented. Furthermore, complex interactions between the HBx protein of hepadnaviruses and several PML NBs-associated proteins are also described. Finally, current but scarce information regarding the interactions of VP3/apoptin of the avian anellovirus with PML NBs is provided. Despite the considerable number of studies that have investigated the functions of the PML NBs in the context of viral infection, gaps in our understanding of the fine interactions between viruses and the very dynamic PML NBs remain. The complexity of the bodies is undoubtedly a great challenge that needs to be further addressed.

Regulating the p53 Tumor Suppressor Network at PML Biomolecular Condensates

By forming specific functional entities, nuclear biomolecular condensates play an important function in guiding biological processes. PML biomolecular condensates, also known as PML nuclear bodies (NBs), are macro-molecular sub-nuclear organelles involved in central biological processes, including anti-viral response and cell fate control upon genotoxic stress. PML condensate formation is stimulated upon cellular stress, and relies on protein-protein interactions establishing a PML protein meshwork capable of recruiting the tumor suppressor p53, along with numerous modifiers of p53, thus balancing p53 posttranslational modifications and activity. This stress-regulated process appears to be controlled by liquid-liquid phase separation (LLPS), which may facilitate regulated protein-unmixing of p53 and its regulators into PML nuclear condensates. In this review, we summarize and discuss the molecular mechanisms underlying PML nuclear condensate formation, and how these impact the biological function of p53 in driving the cell death and senescence responses. In addition, by using an in silico approach, we identify 299 proteins which share PML and p53 as binding partners, thus representing novel candidate proteins controlling p53 function and cell fate decision-making at the level of PML nuclear biocondensates.

Phase separation of p53 induced by its unstructured basic region and prevented by oncogenic mutations in tetramerization domain

Liquid-liquid phase separation (LLPS) drives the formation of extensive membrane-less compartments to regulate various cellular biological activities both physiologically and pathologically. It has been widely accepted that LLPS is closely related to amyloid diseases and increasing reports have linked this phenomenon to cancers. Mutations of tumor suppressor protein p53 exist in more than half of malignant tumors, making the protein vitally important in cancer research. Recently, p53 was reported to undergo phase separation, which may regulate the function of p53. The molecular mechanism of p53 phase separation and how this process relates to cancer remains largely unclear. Herein, we find that the disordered unstructured basic region (UBR) plays a crucial role in p53 LLPS, driven by electrostatic and hydrophobic interactions. Mutations in the tetramerization domain (TD) disrupt p53 phase separation by preventing the tetramer formation. Furthermore, our results have revealed that, in response to DNA damage in cell, the wild type (WT) p53 undergoes LLPS, while LLPS in oncogenic mutations is diminished or eliminated. The expression of the target gene of p53 decreased significantly with the mutations and cell survival increased with the mutations. Thus, we propose a novel mechanism of p53 carcinogenesis, whereby oncogenic mutations in TD impair the formation of p53 condensates, decreasing the activation of target genes and promoting cancer progression. This study helps to understand the behavior and function of p53 in a different aspect and may provide insights into cancer therapies targeting p53.

TRIM proteins in hepatocellular carcinoma

The tripartite motif (TRIM) protein family is a highly conserved group of E3 ligases with 77 members known in the human, most of which consist of a RING-finger domain, one or two B-box domains, and a coiled-coil domain. Generally, TRIM proteins function as E3 ligases to facilitate specific proteasomal degradation of target proteins. In addition, E3 ligase independent functions of TRIM protein were also reported. In hepatocellular carcinoma, expressions of TRIM proteins are both regulated by genetic and epigenetic mechanisms. TRIM proteins regulate multiple biological activities and signaling cascades. And TRIM proteins influence hallmarks of HCC. This review systematically demonstrates the versatile roles of TRIM proteins in HCC and helps us better understand the molecular mechanism of the development and progression of HCC.

Deciphering the acetylation code of p53 in transcription regulation and tumor suppression

Although it is well established that p53-mediated tumor suppression mainly acts through its ability in transcriptional regulation, the molecular mechanisms of this regulation are not completely understood. Among a number of regulatory modes, acetylation of p53 attracts great interests. p53 was one of the first non-histone proteins found to be functionally regulated by acetylation and deacetylation, and subsequent work has established that reversible acetylation is a general mechanism for regulation of non-histone proteins. Unlike other types of post-translational modifications occurred during stress responses, the role of p53 acetylation has been recently validated in vivo by using the knockin mice with both acetylation-defective and acetylation-mimicking p53 mutants. Here, we review the role of acetylation in p53-mediated activities, with a focus on which specific acetylation sites are critical for p53-dependent transcription regulation during tumor suppression and how acetylation of p53 recruits specific “readers” to execute its promoter-specific regulation of different targets. We also discuss the role of p53 acetylation in differentially regulating its classic activities in cell cycle arrest, senescence and apoptosis as well as newly identified unconventional functions such as cell metabolism and ferroptosis.

Drosophila p53 isoforms have overlapping and distinct functions in germline genome integrity and oocyte quality control

p53 gene family members in humans and other organisms encode a large number of protein isoforms whose functions are largely undefined. Using Drosophila as a model, we find that a p53B isoform is expressed predominantly in the germline where it colocalizes with p53A into subnuclear bodies. It is only p53A, however, that mediates the apoptotic response to ionizing radiation in the germline and soma. In contrast, p53A and p53B are both required for the normal repair of meiotic DNA breaks, an activity that is more crucial when meiotic recombination is defective. We find that in oocytes with persistent DNA breaks p53A is also required to activate a meiotic pachytene checkpoint. Our findings indicate that Drosophila p53 isoforms have DNA lesion and cell type-specific functions, with parallels to the functions of mammalian p53 family members in the genotoxic stress response and oocyte quality control.

Promyelocytic Leukemia Proteins Regulate Fanconi Anemia Gene Expression

Promyelocytic leukemia (PML) protein is the core component of subnuclear structures called PML nuclear bodies that are known to play important roles in cell survival, DNA damage responses, and DNA repair. Fanconi anemia (FA) proteins are required for repairing interstrand DNA crosslinks (ICLs). Here we report a novel role of PML proteins, regulating the ICL repair pathway. We found that depletion of the PML protein led to the significant reduction of damage-induced FANCD2 mono-ubiquitination and FANCD2 foci formation. Consistently, the cells treated with siRNA against PML showed enhanced sensitivity to a crosslinking agent, mitomycin C. Further studies showed that depletion of PML reduced the protein expression of FANCA, FANCG, and FANCD2 via reduced transcriptional activity. Interestingly, we observed that damage-induced CHK1 phosphorylation was severely impaired in cells with depleted PML, and we demonstrated that CHK1 regulates FANCA, FANCG, and FANCD2 transcription. Finally, we showed that inhibition of CHK1 phosphorylation further sensitized cancer cells to mitomycin C. Taken together, these findings suggest that the PML is critical for damage-induced CHK1 phosphorylation, which is important for FA gene expression and for repairing ICLs.

Emerging Roles of Liquid-Liquid Phase Separation in Cancer: From Protein Aggregation to Immune-Associated Signaling

Liquid-liquid Phase Separation (LLPS) of proteins and nucleic acids has emerged as a new paradigm in the study of cellular activities. It drives the formation of liquid-like condensates containing biomolecules in the absence of membrane structures in living cells. In addition, typical membrane-less condensates such as nuclear speckles, stress granules and cell signaling clusters play important roles in various cellular activities, including regulation of transcription, cellular stress response and signal transduction. Previous studies highlighted the biophysical and biochemical principles underlying the formation of these liquid condensates. The studies also showed how these principles determine the molecular properties, LLPS behavior, and composition of liquid condensates. While the basic rules driving LLPS are continuously being uncovered, their function in cellular activities is still unclear, especially within a pathological context. Therefore, the present review summarizes the recent progress made on the existing roles of LLPS in cancer, including cancer-related signaling pathways, transcription regulation and maintenance of genome stability. Additionally, the review briefly introduces the basic rules of LLPS, and cellular signaling that potentially plays a role in cancer, including pathways relevant to immune responses and autophagy.

Amyloid precursor protein elevates fusion of promyelocytic leukemia nuclear bodies in human hippocampal areas with high plaque load

The amyloid precursor protein (APP) is a type I transmembrane protein with unknown physiological function but potential impact in neurodegeneration. The current study demonstrates that APP signals to the nucleus causing the generation of aggregates consisting of its adapter protein FE65, the histone acetyltransferase TIP60 and the tumour suppressor proteins p53 and PML. APP C-terminal (APP-CT50) complexes co-localize and co-precipitate with p53 and PML. The PML nuclear body generation is induced and fusion occurs over time depending on APP signalling and STED imaging revealed active gene expression within the complex. We further show that the nuclear aggregates of APP-CT50 fragments together with PML and FE65 are present in the aged human brain but not in cerebral organoids differentiated from iPS cells. Notably, human Alzheimer’s disease brains reveal a highly significant reduction of these nuclear aggregates in areas with high plaque load compared to plaque-free areas of the same individual. Based on these results we conclude that APP-CT50 signalling to the nucleus takes place in the aged human brain and is involved in the pathophysiology of AD.

Regulation of p53 by E3s

More than 40 years of research on p53 have given us tremendous knowledge about this protein. Today we know that p53 plays a role in different biological processes such as proliferation, invasion, pluripotency, metabolism, cell cycle control, ROS (reactive oxygen species) production, apoptosis, inflammation and autophagy. In the nucleus, p53 functions as a bona-fide transcription factor which activates and represses transcription of a number of target genes. In the cytoplasm, p53 can interact with proteins of the apoptotic machinery and by this also induces cell death. Despite being so important for the fate of the cell, expression levels of p53 are kept low in unstressed cells and the protein is largely inactive. The reason for the low expression level is that p53 is efficiently degraded by the ubiquitin-proteasome system and the vast inactivity of the tumor suppressor protein under normal growth conditions is due to the absence of activating and the presence of inactivating posttranslational modifications. E3s are important enzymes for these processes as they decorate p53 with ubiquitin and small ubiquitin-like proteins and by this control p53 degradation, stability and its subcellular localization. In this review, we provide an overview about E3s that target p53 and discuss the connection between p53, E3s and tumorigenesis.

Tumor suppressor protein p53 expressed in yeast can remain diffuse, form a prion, or form unstable liquid-like droplets

Mutations in the p53 tumor suppressor are frequent causes of cancer. Because p53 aggregates appear in some tumor cells, it has been suggested that p53 could also cause cancer by forming self-replicating protein aggregates (prions). Here, using yeast, we show that transient p53 overexpression induced the formation of p53 prion aggregates that were transmitted for >100 generations, found in lysate pellets, stained with Thioflavin T, and transmitted by cytoplasmic transfer, or transfection with lysates of cells carrying the prion or with p53 amyloid peptide. As predicted for a prion, transient interruption of p53 expression caused permanent p53 prion loss. Importantly, p53 transcription factor activity was reduced by prion formation suggesting that prion aggregation could cause cancer. p53 has also been found in liquid-like nuclear droplets in animal cell culture. In yeast, we found that liquid-like p53 foci appear in response to stress and disappear with stress removal.

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A published yeast model of functional nuclear human p53 tumor suppressor was used

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Upon transient overexpression p53 loses its transcription function and aggregates

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These p53 aggregates are cytoplasmic and behave like stable heritable prions

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Stress induces p53 to form liquid-like droplets that are unstable and not prion-like

Double-edged role of PML nuclear bodies during human adenovirus infection

PML nuclear bodies are matrix-bound nuclear structures with a variety of functions in human cells. These nuclear domains are interferon regulated and play an essential role during virus infections involving accumulation of SUMO-dependent host and viral factors. PML-NBs are targeted and subsequently manipulated by adenoviral regulatory proteins, illustrating their crucial role during productive infection and virus-mediated oncogenic transformation. PML-NBs have a longstanding antiviral reputation; however, the genomes of Human Adenoviruses and initial sites of viral transcription/replication are found juxtaposed to these domains, resulting in a double-edged capacity of these nuclear multiprotein/multifunctional complexes. This enigma provides evidence that Human Adenoviruses selectively counteract antiviral responses, and simultaneously benefit from or even depend on proviral PML-NB associated components by active recruitment to PML track-like structures, that are induced during infection. Thereby, a positive microenvironment for adenoviral transcription and replication is created at these nuclear subdomains. Based on the available data, this review aims to provide a detailed overview of the current knowledge of Human Adenovirus crosstalk with nuclear PML body compartments as sites of SUMOylation processes in the host cells, evaluating the currently known principles and molecular mechanisms.

The SUMO-specific protease SENP1 deSUMOylates p53 and regulates its activity

The stability and activity of the p53 tumor suppressor protein are tightly regulated by various posttranslational modifications, including SUMOylation. p53 can be modified by both SUMO1 and SUMO2, although how SUMOylation regulates p53 activity is still obscure. Whether p53 activity is directly regulated by deSUMOylation is also unclear. Here, we show that SENP1, a SUMO-specific protease implicated in pro-oncogenic roles, is a p53 deSUMOylating enzyme. SENP1 interacts with p53 and deSUMOylates p53 in cells and in vitro. Knockdown of SENP1 markedly induced p53 transactivation activity. We further show that SENP1 depletion synergizes with DNA damage-inducing agent etoposide to induce p53 activation and the expression of p21, leading to synergistic growth inhibition of cancer cells. Our results reveal that SENP1 is a critical p53 deSUMOylating enzyme and a promising therapeutic target in wild-type p53 containing cancer cells.

[PML isoforms and TGF-β response]

PML/TRIM19 is the organizer of PML nuclear bodies (NB), a multiprotein complex associated to the nuclear matrix, which recruit a large number of proteins involved in various cellular processes. Alternative splicing from a single PML gene generates 6 nuclear PML isoforms (PMLI to PMLVI) and one cytoplasmic isoform, PMLVII. Murine PML-null primary cells are resistant to TGF-β-induced apoptosis. Cytoplasmic PML is an essential activator of TGF-β signaling by increasing the phosphorylation of transcription factors SMAD2/3 while nuclear PML plays a role in TGF-β-induced caspase 8 activation and apoptosis. TGF-β targets nuclear PML by inducing its conjugation to SUMO. In the nucleus, PML is mainly expressed in the nucleoplasm with a small fraction in the nuclear matrix. In response to TGF-β, PML and caspase 8 shift to the nuclear matrix, where both PML and caspase 8 colocalise within PML NBs. Here, we review the implication of cytoplasmic and nuclear PML isoforms in TGF-β response.

Liquid-like droplet formation by tumor suppressor p53 induced by multivalent electrostatic interactions between two disordered domains

Early in vivo studies demonstrated the involvement of a tumor-suppressing transcription factor, p53, into cellular droplets such as Cajal and promyelocytic leukemia protein bodies, suggesting that the liquid-liquid phase separation (LLPS) might be involved in the cellular functions of p53. To examine this possibility, we conducted extensive investigations on the droplet formation of p53 in vitro. First, p53 itself was found to form liquid-like droplets at neutral and slightly acidic pH and at low salt concentrations. Truncated p53 mutants modulated droplet formation, suggesting the importance of multivalent electrostatic interactions among the N-terminal and C-terminal domains. Second, FRET efficiency measurements for the dimer mutants of p53 revealed that distances between the core domains and between the C-terminal domains were modulated in an opposite manner within the droplets. Third, the molecular crowding agents were found to promote droplet formation, whereas ssDNA, dsDNA, and ATP, to suppress it. Finally, the p53 mutant mimicking posttranslational phosphorylation did not form the droplets. We conclude that p53 itself has a potential to form droplets that can be controlled by cellular molecules and by posttranslational modifications, suggesting that LLPS might be involved in p53 function.

Cell transformation by the adenovirus oncogenes E1 and E4

Extensive studies on viral-mediated oncogenic transformation by human adenoviruses have revealed much of our current understanding on the molecular mechanisms that are involved in the process. To date, these studies have shown that cell transformation is a multistep process regulated by the cooperation of several adenoviral gene products encoded in the early regions 1 (E1) and 4 (E4). Early region 1A immortalizes primary rodent cells, whereas co-expression of early region protein 1B induces full manifestation of the transformed phenotype. Beside E1 proteins, also some E4 proteins have partial transforming activities through regulating many cellular pathways. Here, we summarize recent data of how adenoviral oncoproteins may contribute to viral transformation and discuss the challenge of pinpointing the underlying mechanisms.

Complex interactomes and post-translational modifications of the regulatory proteins HABP4 and SERBP1 suggest pleiotropic cellular functions

The 57 kDa antigen recognized by the Ki-1 antibody, is also known as intracellular hyaluronic acid binding protein 4 and shares 40.7% identity and 67.4% similarity with serpin mRNA binding protein 1, which is also named CGI-55, or plasminogen activator inhibitor type-1-RNA binding protein-1, indicating that they might be paralog proteins, possibly with similar or redundant functions in human cells. Through the identification of their protein interactomes, both regulatory proteins have been functionally implicated in transcriptional regulation, mRNA metabolism, specifically RNA splicing, the regulation of mRNA stability, especially, in the context of the progesterone hormone response, and the DNA damage response. Both proteins also show a complex pattern of post-translational modifications, involving Ser/Thr phosphorylation, mainly through protein kinase C, arginine methylation and SUMOylation, suggesting that their functions and locations are highly regulated. Furthermore, they show a highly dynamic cellular localization pattern with localizations in both the cytoplasm and nucleus as well as punctuated localizations in both granular cytoplasmic protein bodies, upon stress, and nuclear splicing speckles. Several reports in the literature show altered expressions of both regulatory proteins in a series of cancers as well as mutations in their genes that may contribute to tumorigenesis. This review highlights important aspects of the structure, interactome, post-translational modifications, sub-cellular localization and function of both regulatory proteins and further discusses their possible functions and their potential as tumor markers in different cancer settings.

The Interplay Between the Genetic and Immune Landscapes of AML: Mechanisms and Implications for Risk Stratification and Therapy

AML holds a unique place in the history of immunotherapy by virtue of being among the first malignancies in which durable remissions were achieved with "adoptive immunotherapy," now known as allogeneic stem cell transplantation. The successful deployment of unselected adoptive cell therapy established AML as a disease responsive to immunomodulation. Classification systems for AML have been refined and expanded over the years in an effort to capture the variability of this heterogeneous disease and risk-stratify patients. Current systems increasingly incorporate information about cytogenetic alterations and genetic mutations. The advent of next generation sequencing technology has enabled the comprehensive identification of recurrent genetic mutations, many with predictive power. Recurrent genetic mutations found in AML have been intensely studied from a cell intrinsic perspective leading to the genesis of multiple, recently approved targeted therapies including IDH1/2-mutant inhibitors and FLT3-ITD/-TKD inhibitors. However, there is a paucity of data on the effects of these targeted agents on the leukemia microenvironment, including the immune system. Recently, the phenomenal success of checkpoint inhibitors and CAR-T cells has re-ignited interest in understanding the mechanisms leading to immune dysregulation and suppression in leukemia, with the objective of harnessing the power of the immune system via novel immunotherapeutics. A paradigm has emerged that places crosstalk with the immune system at the crux of any effective therapy. Ongoing research will reveal how AML genetics inform the composition of the immune microenvironment paving the way for personalized immunotherapy.

Grouper TRADD Mediates Innate Antiviral Immune Responses and Apoptosis Induced by Singapore Grouper Iridovirus (SGIV) Infection

Tumor necrosis factor (TNF) receptor type 1-associated DEATH domain protein (TRADD) is a TNFR1-associated signal transducer and an essential component of the TNFR1 complex that is involved in activating both apoptotic and nuclear factor (NF)-κB pathways as an adaptor. It also is required for TNFR-1-initiated neuronal apoptosis following in vitro infection with virus as an essential component of the antiviral response. To date, few studies have investigated the function of TRADD in lower vertebrates and its antiviral response to DNA virus infection. In the present study, a TRADD gene (named as EcTRADD) from the orange-spotted grouper (Epinephelus coioides) was cloned and characterized. The full-length cDNA of EcTRADD consists of 1,370 base pairs (bp) and contains a 44 bp 5′-terminal untranslated region (UTR), a 450 bp 3′-UTR including a poly (A) tail, and an 876 bp open reading frame encoding a putative 291 amino acid protein. EcTRADD has two conserved domains of N-terminal domain (TRADD-N) and a death domain (DD). EcTRADD was detected in all examined tissues. EcTRADD was up-regulated in the spleen after infection with Singapore grouper iridovirus (SGIV). Subcellular localization analysis revealed that EcTRADD and EcTRADD-DD exhibited a clear pattern of discrete and interconnecting cytoplasmic filaments resembling the death-effector filaments, while EcTRADD-N was observed in the cytoplasm. After infection with SGIV, EcTRADD, and EcTRADD-DD were transferred to the nucleus. Overexpression of EcTRADD and its domains inhibited replication of SGIV in vitro. Both EcTRADD and EcTRADD-DD induced the caspase-dependent apoptosis in control and infected cells, while EcTRADD-N inhibited the apoptosis. Additionally, EcTRADD and EcTRADD-DD significantly promoted activation of NF-κB and reporter gene p53, whereas EcTRADD-N had no significant effect on p53. The results may provide new insights into the role of fish TRADD in fish virus infection.

TGF-β induces PML SUMOylation, degradation and PML nuclear body disruption

ProMyelocytic Leukemia (PML) protein is essential for the formation of nuclear matrix-associated organelles named PML nuclear bodies (NBs) that act as a platform for post-translational modifications and protein degradation. PML NBs harbor transiently and permanently localized proteins and are associated with the regulation of several cellular functions including apoptosis. There are seven PML isoforms, six nuclear (PMLI-VI) and one cytoplasmic (PMLVII), which are encoded by a single gene via alternative RNA splicing. It has been reported that murine PML-null primary cells are resistant to TGF-β-induced apoptosis and that cytoplasmic PML is an essential activator of TGF-β signaling. The role and the fate of interferon (IFN)-enhanced PML NBs in response to TGF-β have not been investigated. Here we show that IFNα potentiated TGF-β-mediated apoptosis in human cells. IFNα or ectopic expression of PMLIV, but not of PMLIII, enhanced TGF-β-induced caspase 8 activation. In response to TGF-β, both PMLIII and PMLIV were conjugated to SUMO and shifted from the nucleoplasm to the nuclear matrix, however only PMLIV, via its specific C-terminal region, interacted with caspase 8 and recruited it within PML NBs. This process was followed by a caspase-dependent PML degradation and PML NB disruption. Taken together, these findings highlight the role of PML NBs in the enhancement by IFN of TGF-β-induced apoptosis and caspase 8 activation.

CRISPR-Cas9 Screening of Kaposi's Sarcoma-Associated Herpesvirus-Transformed Cells Identifies XPO1 as a Vulnerable Target of Cancer Cells

The abnormal proliferation of cancer cells is driven by deregulated oncogenes or tumor suppressors, among which the cancer-vulnerable genes are attractive therapeutic targets. Targeting mislocalization of oncogenes and tumor suppressors resulting from aberrant nuclear export is effective for inhibiting growth transformation of cancer cells. We performed a clustered regularly interspaced short palindromic repeat (CRISPR)-associated (Cas) screening in a unique model of matched primary and oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV)-transformed cells and identified genes that were growth promoting and growth suppressive for both types of cells, among which exportin XPO1 was demonstrated to be critical for the survival of transformed cells. Using XPO1 inhibitor KPT-8602 and by small interfering RNA (siRNA) knockdown, we confirmed the essential role of XPO1 in cell proliferation and growth transformation of KSHV-transformed cells and in cell lines of other cancers, including gastric cancer and liver cancer. XPO1 inhibition induced cell cycle arrest through p53 activation, but the mechanisms of p53 activation differed among the different types of cancer cells. p53 activation depended on the formation of promyelocytic leukemia (PML) nuclear bodies in gastric cancer and liver cancer cells. Mechanistically, XPO1 inhibition induced relocalization of autophagy adaptor protein p62 (SQSTM1), recruiting p53 for activation in PML nuclear bodies. Taken the data together, we have identified novel growth-promoting and growth-suppressive genes of primary and cancer cells and have demonstrated that XPO1 is a vulnerable target of cancer cells. XPO1 inhibition induces cell arrest through a novel PML- and p62-dependent mechanism of p53 activation in some types of cancer cells.IMPORTANCE Using a model of oncogenic virus KSHV-driven cellular transformation of primary cells, we have performed a genome-wide CRISPR-Cas9 screening to identify vulnerable genes of cancer cells. This screening is unique in that this virus-induced oncogenesis model does not depend on any cellular genetic alterations and has matched primary and KSHV-transformed cells, which are not available for similar screenings in other types of cancer. We have identified genes that are both growth promoting and growth suppressive in primary and transformed cells, some of which could represent novel proto-oncogenes and tumor suppressors. In particular, we have demonstrated that the exportin XPO1 is a critical factor for the survival of transformed cells. Using a XPO1 inhibitor (KPT-8602) and siRNA-mediated knockdown, we have confirmed the essential role of XPO1 in cell proliferation and in growth transformation of KSHV-transformed cells, as well as of gastric and liver cancer cells. XPO1 inhibition induces cell cycle arrest by activating p53, but the mechanisms of p53 activation differed among different types of cancer cells. p53 activation is dependent on the formation of PML nuclear bodies in gastric and liver cancer cells. Mechanistically, XPO1 inhibition induces relocalization of autophagy adaptor protein p62 (SQSTM1), recruiting p53 for activation in PML nuclear bodies. These results illustrate that XPO1 is a vulnerable target of cancer cells and reveal a novel mechanism for blocking cancer cell proliferation by XPO1 inhibition as well as a novel PML- and p62-mediated mechanism of p53 activation in some types of cancer cells.

FLT3-ITD impedes retinoic acid, but not arsenic, responses in murine acute promyelocytic leukemias

Acute promyelocytic leukemia (APL) is often associated with activating FLT3 signaling mutations. These are highly related to hyperleukocytosis, a major adverse risk factor with chemotherapy-based regimens. APL is a model for oncogene-targeted therapies: all-trans retinoic acid (ATRA) and arsenic both target and degrade its ProMyelocytic Leukemia/Retinoic Acid Receptor α (PML/RARA) driver. The combined ATRA/arsenic regimen now cures virtually all patients with standard-risk APL. Although FLT3-internal tandem duplication (ITD) was an adverse risk factor for historical ATRA/chemotherapy regimens, the molecular bases for this effect remain unknown. Using mouse APL models, we unexpectedly demonstrate that FLT3-ITD severely blunts ATRA response. Remarkably, although the transcriptional output of initial ATRA response is unaffected, ATRA-induced PML/RARA degradation is blunted, as is PML nuclear body reformation and activation of P53 signaling. Critically, the combination of ATRA and arsenic fully rescues therapeutic response in FLT3-ITD APLs, restoring PML/RARA degradation, PML nuclear body reformation, P53 activation, and APL eradication. Moreover, arsenic targeting of normal PML also contributes to APL response in vivo. These unexpected results explain the less favorable outcome of FLT3-ITD APLs with ATRA-based regimens, and stress the key role of PML nuclear bodies in APL eradication by the ATRA/arsenic combination.

EBNA1-targeted inhibitors: Novel approaches for the treatment of Epstein-Barr virus-associated cancers

Epstein-Barr virus (EBV) infects more than 90% of humans worldwide and establishes lifelong latent infection in the hosts. It is closely associated with endemic forms of a wide range of human cancers and directly contributes to the formation of some. Despite its critical role in cancer development, no EBV- or EBV latent protein-targeted therapy is available. The EBV-encoded latent protein, Epstein-Barr nuclear antigen 1 (EBNA1), is expressed in all EBV-associated tumors and acts as the only latent protein in some of these tumors. This versatile protein functions in the maintenance, replication, and segregation of the EBV genome and can therefore serve as an attractive therapeutic target to treat EBV-associated cancers. In the last decades, efforts have been made for designing specific EBNA1 inhibitors to decrease EBNA1 expression or interfere with EBNA1-dependent functions. In this review, we will briefly introduce the salient features of EBNA1, summarize its functional domains, and focus on the recent developments in the identification and design of EBNA1 inhibitors related to various EBNA1 domains as well as discuss their comparative merits.

Phase separated microenvironments inside the cell nucleus are linked to disease and regulate epigenetic state, transcription and RNA processing

Proteins and RNAs inside the cell nucleus are organized into distinct phases, also known as liquid-liquid phase separated (LLPS) droplet organelles or nuclear bodies. These regions exist within the spaces between chromatin-rich regions but their function is tightly linked to gene activity. They include major microscopically-observable structures such as the nucleolus, paraspeckle and Cajal body. The biochemical and assembly factors enriched inside these microenvironments regulate chromatin structure, transcription, and RNA processing, and other important cellular functions. Here, we describe published evidence that suggests nuclear bodies are bona fide LLPS droplet organelles and major regulators of the processes listed above. We also outline an updated "Supply or Sequester" model to describe nuclear body function, in which proteins or RNAs are supplied to surrounding genomic regions or sequestered away from their sites of activity. Finally, we describe recent evidence that suggests these microenvironments are both reflective and drivers of diverse pathophysiological states.

PMLIV overexpression promotes TGF-β-associated epithelial-mesenchymal transition and migration in MCF-7 cancer cells

The epithelial-mesenchymal transition (EMT) is a key event associated with metastasis and dissemination in breast tumor pathogenesis. Promyelocytic leukemia (PML) gene produces several isoforms due to alternative splicing; however, the biological function of each specific isoform has yet to be identified. In this study, we report a previously unknown role for PMLIV, the most intensely studied nuclear isoform, in transforming growth factor-β (TGF-β) signaling-associated EMT and migration in breast cancer. This study demonstrates that PMLIV overexpression promotes a more aggressive mesenchymal phenotype and increases the migration of MCF-7 cancer cells. This event is associated with activation of the TGF-β canonical signaling pathway through the induction of Smad2/3 phosphorylation and the translocation of phospho-Smad2/3 to the nucleus. In this study, we report a previously unknown role for PMLIV in TGF-β signaling-induced regulation of breast cancer-associated EMT and migration. Targeting this pathway may be therapeutically beneficial.

Quantitative SUMO proteomics reveals the modulation of several PML nuclear body associated proteins and an anti-senescence function of UBC9

Several regulators of SUMOylation have been previously linked to senescence but most targets of this modification in senescent cells remain unidentified. Using a two-step purification of a modified SUMO3, we profiled the SUMO proteome of senescent cells in a site-specific manner. We identified 25 SUMO sites on 23 proteins that were significantly regulated during senescence. Of note, most of these proteins were PML nuclear body (PML-NB) associated, which correlates with the increased number and size of PML-NBs observed in senescent cells. Interestingly, the sole SUMO E2 enzyme, UBC9, was more SUMOylated during senescence on its Lys-49. Functional studies of a UBC9 mutant at Lys-49 showed a decreased association to PML-NBs and the loss of UBC9’s ability to delay senescence. We thus propose both pro- and anti-senescence functions of protein SUMOylation.

PML nuclear bodies, membrane-less domains acting as ROS sensors?

PML Nuclear bodies (PML NBs) are spherical domains associated with a broad range of activities upon stress responses such as apoptosis, senescence DNA repair, epigenetic control, as well as control of oncogenesis. These bodies are considered as privileged sites for post-translational modifications, where sumoylation plays a key role. Here we summarize recent in vitro and in vivo findings on the link between PML NBs and ROS, in particular PML contributions to oxidative stress response. We discuss how it may regulate switch from cell protection against stress to cell arrest/cell death.

PML is a ROS sensor activating p53 upon oxidative stress

Promyelocytic leukemia (PML) nuclear bodies modulate several processes, including senescence or apoptosis. Niwa-Kawakita et al. demonstrate that PML regulates reactive oxygen species (ROS) homeostasis in vivo by coupling ROS to p53 signaling to enforce basal ROS protection and mediate their acute toxicity.

Promyelocytic leukemia (PML) nuclear bodies (NBs) recruit partner proteins, including p53 and its regulators, thereby controlling their abundance or function. Investigating arsenic sensitivity of acute promyelocytic leukemia, we proposed that PML oxidation promotes NB biogenesis. However, physiological links between PML and oxidative stress response in vivo remain unexplored. Here, we identify PML as a reactive oxygen species (ROS) sensor. Pml^−/− cells accumulate ROS, whereas PML expression decreases ROS levels. Unexpectedly, Pml^−/− embryos survive acute glutathione depletion. Moreover, Pml^−/− animals are resistant to acetaminophen hepatotoxicity or fasting-induced steatosis. Molecularly, Pml^−/− animals fail to properly activate oxidative stress–responsive p53 targets, whereas the NRF2 response is amplified and accelerated. Finally, in an oxidative stress–prone background, Pml^−/− animals display a longevity phenotype, likely reflecting decreased basal p53 activation. Thus, similar to p53, PML exerts basal antioxidant properties but also drives oxidative stress–induced changes in cell survival/proliferation or metabolism in vivo. Through NB biogenesis, PML therefore couples ROS sensing to p53 responses, shedding a new light on the role of PML in senescence or stem cell biology.

Sumoylation and Its Contribution to Cancer

Post-translational modifications play an important role in regulating protein activity by altering their functions. Sumoylation is a highly dynamic process which is tightly regulated by a fine balance between conjugating and deconjugating enzyme activities. It affects intracellular localization and their interaction with their binding partners, thereby changing gene expression. Consequently, these changes in turn affect signaling mechanisms that regulate many cellular functions, such as cell growth, proliferation, apoptosis , DNA repair , and cell survival. It is becoming apparent that deregulation in the SUMO pathway contributes to oncogenic transformation by affecting sumoylation/desumoylation of many oncoproteins and tumor suppressors. Loss of balance between sumoylation and desumoylation has been reported in a number of studies in a variety of disease types including cancer. This chapter summarizes the mechanisms and functions of the deregulated SUMO pathway affecting oncogenes and tumor suppressor genes.

A PML/Slit Axis Controls Physiological Cell Migration and Cancer Invasion in the CNS

Cell migration through the brain parenchyma underpins neurogenesis and glioblastoma (GBM) development. Since GBM cells and neuroblasts use the same migratory routes, mechanisms underlying migration during neurogenesis and brain cancer pathogenesis may be similar. Here, we identify a common pathway controlling cell migration in normal and neoplastic cells in the CNS. The nuclear scaffold protein promyelocytic leukemia (PML), a regulator of forebrain development, promotes neural progenitor/stem cell (NPC) and neuroblast migration in the adult mouse brain. The PML pro-migratory role is active also in transformed mouse NPCs and in human primary GBM cells. In both normal and neoplastic settings, PML controls cell migration via Polycomb repressive complex 2 (PRC2)-mediated repression of Slits, key regulators of axon guidance. Finally, a PML/SLIT1 axis regulates sensitivity to the PML-targeting drug arsenic trioxide in primary GBM cells. Taken together, these findings uncover a drug-targetable molecular axis controlling cell migration in both normal and neoplastic cells.

The functional roles of PML nuclear bodies in genome maintenance

In the nucleus, there are several membraneless structures called nuclear bodies. Among them, promyelocytic leukemia nuclear bodies (PML-NBs) are involved in multiple genome maintenance pathways including the DNA damage response, DNA repair, telomere homeostasis, and p53-associated apoptosis. In response to DNA damage, PML-NBs are coalesced and divided by a fission mechanism, thus increasing their number. PML-NBs also play a role in repairing DNA double-strand breaks (DSBs) by homologous recombination (HR). Clinically, the dominant negative PML-RARα fusion protein expressed in acute promyelocytic leukemia (APL) inhibits the transactivation of downstream factors and disrupts PML function, revealing the tumor suppressor role of PML-NBs. All-trans retinoic acid and arsenic trioxide treatment has been implemented for promyelocytic leukemia to target the PML-RARα fusion protein. PML-NBs are associated with various factors implicated in genome maintenance, and are found at the sites of DNA damage. Their interaction with proteins such as p53 indicates that PML-NBs may play a significant role in apoptosis and cancer. Decades of research have revealed the importance of PML-NBs in diverse cellular pathways, yet the underlying molecular mechanisms and exact functions of PML-NBs remain elusive. In this review, PML protein modifications and the functional relevance of PML-NB and its associated factors in genome maintenance will be discussed.

PML nuclear bodies contribute to the basal expression of the mTOR inhibitor DDIT4

The promyelocytic leukemia (PML) protein is an essential component of PML nuclear bodies (PML NBs) frequently lost in cancer. PML NBs coordinate chromosomal regions via modification of nuclear proteins that in turn may regulate genes in the vicinity of these bodies. However, few PML NB-associated genes have been identified. PML and PML NBs can also regulate mTOR and cell fate decisions in response to cellular stresses. We now demonstrate that PML depletion in U2OS cells or TERT-immortalized normal human diploid fibroblasts results in decreased expression of the mTOR inhibitor DDIT4 (REDD1). DNA and RNA immuno-FISH reveal that PML NBs are closely associated with actively transcribed DDIT4 loci, implicating these bodies in regulation of basal DDIT4 expression. Although PML silencing did reduce the sensitivity of U2OS cells to metabolic stress induced by metformin, PML loss did not inhibit the upregulation of DDIT4 in response to metformin, hypoxia-like (CoCl₂) or genotoxic stress. Analysis of publicly available cancer data also revealed a significant correlation between PML and DDIT4 expression in several cancer types (e.g. lung, breast, prostate). Thus, these findings uncover a novel mechanism by which PML loss may contribute to mTOR activation and cancer progression via dysregulation of basal DDIT4 gene expression.

Nuclear inclusion bodies of mutant and wild-type p53 in cancer: a hallmark of p53 inactivation and proteostasis remodelling by p53 aggregation

Although p53 protein aggregates have been observed in cancer cell lines and tumour tissue, their impact in cancer remains largely unknown. Here, we extensively screened for p53 aggregation phenotypes in tumour biopsies, and identified nuclear inclusion bodies (nIBs) of transcriptionally inactive mutant or wild-type p53 as the most frequent aggregation-like phenotype across six different cancer types. p53-positive nIBs co-stained with nuclear aggregation markers, and shared molecular hallmarks of nIBs commonly found in neurodegenerative disorders. In cell culture, tumour-associated stress was a strong inducer of p53 aggregation and nIB formation. This was most prominent for mutant p53, but could also be observed in wild-type p53 cell lines, for which nIB formation correlated with the loss of p53's transcriptional activity. Importantly, protein aggregation also fuelled the dysregulation of the proteostasis network in the tumour cell by inducing a hyperactivated, oncogenic heat-shock response, to which tumours are commonly addicted, and by overloading the proteasomal degradation system, an observation that was most pronounced for structurally destabilized mutant p53. Patients showing tumours with p53-positive nIBs suffered from a poor clinical outcome, similar to those with loss of p53 expression, and tumour biopsies showed a differential proteostatic expression profile associated with p53-positive nIBs. p53-positive nIBs therefore highlight a malignant state of the tumour that results from the interplay between (1) the functional inactivation of p53 through mutation and/or aggregation, and (2) microenvironmental stress, a combination that catalyses proteostatic dysregulation. This study highlights several unexpected clinical, biological and therapeutically unexplored parallels between cancer and neurodegeneration. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Promyelocytic Leukemia Protein, a Protein at the Crossroad of Oxidative Stress and Metabolism

SIGNIFICANCE

Cellular metabolic activity impacts the production of reactive oxygen species (ROS), both positively through mitochondrial oxidative processes and negatively by promoting the production of reducing agents (including NADPH and reduced glutathione). A defined metabolic state in cancer cells is critical for cell growth and long-term self-renewal, and such state is intrinsically associated with redox balance. Promyelocytic leukemia protein (PML) regulates several biological processes, at least in part, through its ability to control the assembly of PML nuclear bodies (PML NBs). Recent Advances: PML is oxidation-prone, and oxidative stress promotes NB biogenesis. These nuclear subdomains recruit many nuclear proteins and regulate their SUMOylation and other post-translational modifications. Some of these cargos-such as p53, SIRT1, AKT, and mammalian target of rapamycin (mTOR)-are key regulators of cell fate. PML was also recently shown to regulate oxidation.

CRITICAL ISSUES

While it was long considered primarily as a tumor suppressor protein, PML-regulated metabolic switch uncovered that this protein could promote survival and/or stemness of some normal or cancer cells. In this study, we review the recent findings on this multifunctional protein.

FUTURE DIRECTIONS

Studying PML scaffolding functions as well as its fine role in the activation of p53 or fatty acid oxidation will bring new insights in how PML could bridge oxidative stress, senescence, cell death, and metabolism. Antioxid. Redox Signal. 26, 432-444.

A platform for interrogating cancer-associated p53 alleles

p53 is the most frequently mutated gene in human cancer. Compelling evidence argues that full transformation involves loss of growth suppression encoded by wild-type p53 together with poorly understood oncogenic activity encoded by missense mutations. Furthermore, distinguishing disease alleles from natural polymorphisms is an important clinical challenge. To interrogate the genetic activity of human p53 variants, we leveraged the Drosophila model as an in vivo platform. We engineered strains that replace the fly p53 gene with human alleles, producing a collection of stocks that are, in effect, 'humanized' for p53 variants. Like the fly counterpart, human p53 transcriptionally activated a biosensor and induced apoptosis after DNA damage. However, all humanized strains representing common alleles found in cancer patients failed to complement in these assays. Surprisingly, stimulus-dependent activation of hp53 occurred without stabilization, demonstrating that these two processes can be uncoupled. Like its fly counterpart, hp53 formed prominent nuclear foci in germline cells but cancer-associated p53 variants did not. Moreover, these same mutant alleles disrupted hp53 foci and inhibited biosensor activity, suggesting that these properties are functionally linked. Together these findings establish a functional platform for interrogating human p53 alleles and suggest that simple phenotypes could be used to stratify disease variants.

The Role of Sumoylation in Senescence

Cellular senescence is a program initiated by many stress signals including aberrant activation of oncogenes, DNA damage, oxidative lesions and telomere attrition. Once engaged senescence irreversibly limits cellular proliferation and potently prevents tumor formation in vivo. The precise mechanisms driving the onset of senescence are still not completely defined, although the pRb and p53 tumor suppressor pathways converge with the SUMO cascade to regulate cellular senescence. Sumoylation translocates p53 to PML nuclear bodies where it can co-operate with many sumoylated co-factors in a program that activates pRb and favors senescence. Once activated pRb integrates various proteins, many of them sumoylated, into a repressor complex that inhibits the transcription of proliferation-promoting genes and initiates chromatin condensation. Sumoylation is required for heterochromatin formation during senescence and may act as a scaffold to stabilize the pRb repressor complex. Thus, SUMO is a critical component of a tumor-suppressor network that limits aberrant cell proliferation and tumorigenesis.

CACUL1/CAC1 attenuates p53 activity through PML post-translational modification

Promyelocytic leukaemia (PML) is a tumor suppressor protein covalently conjugated with SUMO family proteins, leading to the formation of PML nuclear bodies (NBs). PML-NBs provide a platform for efficient posttranslational modification of targets and protein-protein interaction, contributing to the adjustment of gene expression and chromatin integrity. Although PML SUMOylation is thought to play important roles in diverse cellular functions, the control mechanisms of adequate modification levels have remained unsolved. Here, we report that Cullin-related protein CACUL1/CAC1 (CACUL1) inhibits PML posttranslational modification. CACUL1 interacts with PML and suppresses PML SUMOylation, leading to the regulation of PML-NB size in the nucleus. We also found that Ubc9, a SUMO-conjugating enzyme, binds to CACUL1 and antagonizes the interaction between CACUL1 and PML. Furthermore, CACUL1 attenuates p53 transcriptional activity. These data suggest that CACUL1 is a novel regulator that negatively controls p53 activity through the regulation of PML SUMOylation.

Differential sub-nuclear distribution of hypoxia-inducible factors (HIF)-1 and -2 alpha impacts on their stability and mobility

Cellular adaptation to hypoxia occurs via a complex programme of gene expression mediated by the hypoxia-inducible factor (HIF). The oxygen labile alpha subunits, HIF-1α/-2α, form a heterodimeric transcription factor with HIF-1β and modulate gene expression. HIF-1α and HIF-2α possess similar domain structure and bind to the same consensus sequence. However, they have different oxygen-dependent stability and activate distinct genes. To better understand these differences, we used fluorescent microscopy to determine precise localization and dynamics. We observed a homogeneous distribution of HIF-1α in the nucleus, while HIF-2α localized into speckles. We demonstrated that the number, size and mobility of HIF-2α speckles were independent of cellular oxygenation and that HIF-2α molecules were capable of exchanging between the speckles and nucleoplasm in an oxygen-independent manner. The concentration of HIF-2α into speckles may explain its increased stability compared with HIF-1α and its slower mobility may offer a mechanism for gene specificity.

A novel single cell method to identify the genetic composition at a single nuclear body

Gene loci make specific associations with compartments of the nucleus (e.g. the nuclear envelope, nucleolus, and transcription factories) and this association may determine or reflect a mechanism of genetic control. With current methods, it is not possible to identify sets of genes that converge to form a “gene hub” as there is a reliance on loci-specific probes, or immunoprecipitation of a particular protein from bulk cells. We introduce a method that will allow for the identification of loci contained within the vicinity of a single nuclear body in a single cell. For the first time, we demonstrate that the DNA sequences originating from a single sub-nuclear structure in a single cell targeted by two-photon irradiation can be determined, and mapped to a particular locus. Its application to single PML nuclear bodies reveals ontologically related loci that frequently associate with each other and with PML bodies in a population of cells, and a possible nuclear body targeting role for specific transcription factor binding sites.

Down-regulation of LATS kinases alters p53 to promote cell migration

p53 is a pivotal tumor suppressor and a major barrier against cancer. We now report that silencing of the Hippo pathway tumor suppressors LATS1 and LATS2 in nontransformed mammary epithelial cells reduces p53 phosphorylation and increases its association with the p52 NF-κB subunit. Moreover, it partly shifts p53's conformation and transcriptional output toward a state resembling cancer-associated p53 mutants and endows p53 with the ability to promote cell migration. Notably, LATS1 and LATS2 are frequently down-regulated in breast cancer; we propose that such down-regulation might benefit cancer by converting p53 from a tumor suppressor into a tumor facilitator.

PML nuclear bodies: assembly and oxidative stress-sensitive sumoylation

PML Nuclear Bodies (NBs) have fascinated cell biologists due to their exquisitely dynamic nature and their involvement in human diseases, notably acute promyelocytic leukemia. NBs, as well as their master organizer--the PML protein--exhibit multiple connections with stress responses. Initially viewed as a tumor suppressor, PML recently re-emerged as a multifaceted protein, capable of controlling numerous aspects of cellular homeostasis. NBs recruit many functionally diverse proteins and function as stress-regulated sumoylation factories. SUMO-initiated partner retention can subsequently facilitate a variety of other post-translational modifications, as well as partner degradation. With this newly elucidated central role of stress-enhanced sumoylation, it should now be possible to build a working model for the different NB-regulated cellular activities. Moreover, pharmacological manipulation of NB formation by interferons or oxidants holds the promise of clearing many undesirable proteins for clinical management of malignant, viral or neurodegenerative diseases.

PML isoforms IV and V contribute to adenovirus-mediated oncogenic transformation by functionally inhibiting the tumor-suppressor p53

Although modulation of the cellular tumor-suppressor p53 is considered to have the major role in E1A/E1B-55K-mediated tumorigenesis, other promyelocytic leukemia nuclear body (PML-NB)/PML oncogenic domain (POD)-associated factors including SUMO, Mre11, Daxx, as well as the integrity of these nuclear bodies contribute to the transformation process. However, the biochemical consequences and oncogenic alterations of PML-associated E1B-55K by SUMO-dependent PML-IV and PML-V interaction have so far remained elusive. We performed mutational analysis to define a PML interaction motif within the E1B-55K polypeptide. Our results showed that E1B-55K/PML binding is not required for p53, Mre11 and Daxx interaction. We also observed that E1B-55K lacking subnuclear PML localization because of either PML-IV or PML-V-binding deficiency was no longer capable of mediating E1B-55K-dependent SUMOylation of p53, inhibition of p53-mediated transactivation or efficiently transforming primary rodent cells. These results together with the observation that E1B-55K-dependent SUMOylation of p53 is required for efficient cell transformation, provides evidence for the idea that the SUMO ligase activity of the E1B-55K viral oncoprotein is intimately linked to its growth-promoting oncogenic activities.