Role of PML and the PML-nuclear body in the control of programmed cell death

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.

Disordered regions mediate the interaction of p53 and MRE11

The genome is frequently targeted by genotoxic agents, resulting in the formation of DNA scars. However, cells employ diverse repair mechanisms to restore DNA integrity. Among these processes, the Mre11-Rad50-Nbs1 complex detects double-strand breaks (DSBs) and recruits DNA damage response proteins such as ataxia-telangiectasia-mutated (ATM) kinase to DNA damage sites. ATM phosphorylates the transactivation domain (TAD) of the p53 tumor suppressor, which in turn regulates DNA repair, growth arrest, apoptosis, and senescence following DNA damage. The disordered glycine-arginine-rich (GAR) domain of double-strand break protein MRE11 (MRE11GAR) and its methylation are important for DSB repair, and localization to Promyelocytic leukemia nuclear bodies (PML-NBs). There is preliminary evidence that p53, PML protein, and MRE11 might co-localize and interact at DSB sites. To uncover the molecular details of these interactions, we aimed to identify the domains mediating the p53-MRE11 interaction and to elucidate the regulation of the p53-MRE11 interaction by post-translational modifications (PTMs) through a combination of biophysical techniques. We discovered that, in vitro, p53 binds directly to MRE11GAR mainly through p53TAD2 and that phosphorylation further enhances this interaction. Furthermore, we found that MRE11GAR methylation still allows for binding to p53. Overall, we demonstrated that p53 and MRE11 interaction is facilitated by disordered regions. We provide for the first time insight into the molecular details of the p53-MRE11 complex formation and elucidate potential regulatory mechanisms that will promote our understanding of the DNA damage response. Our findings suggest that PTMs regulate the p53-MRE11 interaction and subsequently their colocalization to PML-NBs upon DNA damage.

Japanese encephalitis virus perturbs PML-nuclear bodies by engaging in interactions with distinct porcine PML isoforms

Promyelocytic leukemia (PML) protein constitutes an indispensable element within PML-nuclear bodies (PML-NBs), playing a pivotal role in the regulation of multiple cellular functions while coordinating the innate immune response against viral invasions. Simultaneously, numerous viruses elude immune detection by targeting PML-NBs. Japanese encephalitis virus (JEV) is a flavivirus that causes Japanese encephalitis, a severe neurological disease that affects humans and animals. However, the mechanism through which JEV evades immunity via PML-NBs has been scarcely investigated. In the present study, PK15 cells were infected with JEV, and the quantity of intracellular PML-NBs was enumerated. The immunofluorescence results indicated that the number of PML-NBs was significantly reduced in JEV antigen-positive cells compared to viral antigen-negative cells. Subsequently, ten JEV proteins were cloned and transfected into PK15 cells. The results revealed that JEV non-structural proteins, NS2B, NS3, NS4A, NS4B, and NS5, significantly diminished the quantity of PML-NBs. Co-transfection was performed with the five JEV proteins and various porcine PML isoforms. The results demonstrated that NS2B colocalized with PML4 and PML5, NS4A colocalized with PML1 and PML4, NS4B colocalized with PML1, PML3, PML4, and PML5, while NS3 and NS5 interacted with all five PML isoforms. Furthermore, ectopic expression of PML isoforms confirmed that PML1, PML3, PML4, and PML5 inhibited JEV replication. These findings suggest that JEV disrupts the structure of PML-NBs through interaction with PML isoforms, potentially leading to the attenuation of the host's antiviral immune response.

Downregulation of LRP/LR with siRNA inhibits several cancer hallmarks in lung cancer cells

The incidence and mortality rates of cancer are growing rapidly worldwide, with lung cancer being the most commonly occurring cancer in males. Human carcinomas circumvent the inhibitory pathways induced by DNA damage and senescence through the upregulation of telomerase activity. The 37 kDa/67 kDa laminin receptor (LRP/LR) is a cell surface receptor which plays a role in several cancer hallmarks, including metastasis, angiogenesis, cell viability maintenance, apoptotic evasion, and mediating telomerase activity. We have previously shown that the knockdown of LRP/LR with an LRP-specific siRNA significantly impedes adhesion and invasion, induces apoptosis, and inhibits telomerase activity in various cancer cell lines in vitro. Here, we investigated the effect of downregulating LRP/LR with LRP-specific siRNA in A549 lung cancer cells. Downregulation of LRP/LR resulted in a significant decrease in cell viability, migration potential, and telomerase activity, as well as a significant increase in apoptosis. Proteomic analysis further suggested the re-establishment of immune control over the lung cancer cells, a previously unidentified facet of LRP downregulation in cancer. Altogether, we suggest that targeting LRP/LR for downregulation may have therapeutic potential for inhibiting several cancer hallmarks.

Multimodal approach to characterize the tetrameric form of human PML-RBCC domain and ATO-mediated conformational changes

RING-B box-coiled coil (RBCC) domain of promyelocytic leukemia (PML) comprises a zinc finger motif that is targeted by arsenic trioxide (ATO) to treat acute promyelocytic leukemia (APL) pathogenesis. Preliminary evidence suggests that the PML-RBCC has different functional characteristics, but no structural details have been reported despite its importance in differential expression and cell-cycle regulation. Therefore, the recombinant h-PML-RBCC protein was purified to its homogeneity, and characterized for oligomeric behaviour which indicated that RBCC domain exists as a tetramer in solution. Furthermore, nano-DSF and circular-dichroism demonstrated that the tetrameric form preserves its native conformation along with thermal stability (Tm = 83.2 °C). In-silico-based PML-RBCC structure was used to perform the molecular dynamics simulation for 300 ns in the presence of zinc atoms, which demonstrated the differential dynamic of PML-RBCC tetrameric chains. MMPBSA analysis also indicated the role of hydrophobic interactions that favours stable tetrameric structure of PML-RBCC. ATO-induced secondary and tertiary structure changes were observed in PML-RBCC using circular dichroism and fluorescence spectroscopy. Dynamic light scattering and transmission electron microscopy revealed ATO-induced higher-order oligomerization and aggregation of PML-RBCC. The unique oligomeric nature of the h-PML-RBCC protein and its interactions with ATO will help to understand the mechanism of APL pathogenesis and drug resistance.

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.

Promyelocytic leukemia nuclear body-like structures can assemble in mouse oocytes

Promyelocytic leukemia (PML) nuclear bodies (PML-NBs), a class of membrane-less cellular organelles, participate in various biological activities. PML-NBs are known as the core-shell-type nuclear body, harboring ‘client’ proteins in their core. Although multiple membrane-less organelles work in the oocyte nucleus, PML-NBs have been predicted to be absent from oocytes. Here, we show that some well-known PML clients (but not endogenous PML) co-localized with small ubiquitin-related modifier (SUMO) protein in the nucleolus and peri-centromeric heterochromatin of maturing oocytes. In oocytes devoid of PML-NBs, endogenous PML protein localized in the vicinity of chromatin. During and after meiotic resumption, PML co-localized with SUMO gathering around chromosomes. To examine the benefit of the PML-NB-free intranuclear milieu in oocytes, we deliberately assembled PML-NBs by microinjecting human PML-encoding plasmids into oocytes. Under conditions of limited SUMO availability, assembled PML-NBs tended to cluster. Upon proteotoxic stress, SUMO delocalized from peri-centromeric heterochromatin and co-localized with SC35 (a marker of nuclear speckles)-positive large compartments, which was disturbed by pre-assembled PML-NBs. These observations suggest that the PML-NB-free intranuclear environment helps reserve SUMO for emergent responses by redirecting the flux of SUMO otherwise needed to maintain PML-NB dynamics.

Summary: PML-NB-free intranuclear environment in the oocyte helps reserve SUMO for emergent responses by redirecting the flux of SUMO otherwise needed to maintain PML-NB dynamics.

Almost famous: Human adenoviruses (and what they have taught us about cancer)

Papillomaviruses, polyomaviruses and adenoviruses are collectively categorized as the small DNA tumour viruses. Notably, human adenoviruses were the first human viruses demonstrated to be able to cause cancer, albeit in non-human animal models. Despite their long history, no human adenovirus is a known causative agent of human cancers, unlike a subset of their more famous cousins, including human papillomaviruses and human Merkel cell polyomavirus. Nevertheless, seminal research using human adenoviruses has been highly informative in understanding the basics of cell cycle control, gene expression, apoptosis and cell differentiation. This review highlights the contributions of human adenovirus research in advancing our knowledge of the molecular basis of cancer.

p53 regulated senescence mechanism and role of its modulators in age-related disorders

Multiple co-morbidities are associated with age, and there is a need for the broad-spectrum drug to prevent multiple regimens that may cause an adverse effect in the geriatric population. Cellular senescence is a primary mechanism for ageing in various tissues. p53, a tumor suppressor protein, plays a significant role in forming DNA damage foci and post different stress responses. DNA damage foci can be transient or persistent that can progress to DNA-SCARS inducing senescence. p53 also plays a role in apoptosis and negative regulation of SASP. Few upstream targets like FOXO4, MDM2, MDM4, USP7 control the availability of p53 for apoptosis. Hence, the senolytic therapies, modulating p53 upstream targets, can be a good approach for preventing age-related disorders. This review discusses the insights on the role of p53 in the formation of DNA-SCARS, various upstream target proteins, and pathways involved in p53 regulation. Further, the review aimed to include recently discovered small molecules acting on these upstream targets, and those can be modified using medicinal chemistry approaches to give successful senotherapeutics.

Tumor suppressors in acute myeloid leukemia

Acute myeloid leukemia (AML) is a very heterogeneous type of blood cancer, which presents with a high rate of mortality especially in elderly patients. Better understanding of critical players, such as molecules with tumor suppressive properties, may help to fine-tune disease classification and thereby treatment modalities for this detrimental disease. Here, we summarize well-known and established tumor suppressors as well as emerging tumor suppressors, including transcription factors (TCFs) and other transcriptional regulators, such as epigenetic modulators. In addition, we look into the versatile field of miRNAs also interfering with tumorigenesis and progression, which offer new possibilities in AML diagnosis, prognosis, and therapy.

Hypertriglyceridemia in Newly Diagnosed Acute Promyelocytic Leukemia

The primary aim of the present retrospective study was to investigate lipid profiles and kinetics in acute promyelocytic leukemia (APL) patients. We analyzed 402 newly diagnosed APL patients and 201 non-APL patients with acute myeloid leukemia (as control). Incidence of hypertriglyceridemia in APL patients and non-APL patients was 55.82% and 28.4% (p = 0.0003). The initial levels of triglycerides, total cholesterol, high-density lipoprotein cholesterol and low-density lipoprotein cholesterol were higher in APL patients than in control (all p < 0.0001). In APL patients, triglyceride levels were significantly increased during induction treatment with all-trans retinoic acid and arsenic. Multivariable analysis showed that age, being overweight (body mass index ≥25) and APL were independent risk factors for hypertriglyceridemia in all patients before treatment. High triglyceride levels were not significantly associated with disease-free survival or overall survival in the APL patients. In summary, in the current study triglyceride levels were significantly elevated in APL patients before treatment, and they increased during induction treatment, but there were no significant corresponding effects on survival.

Autophagic degradation of PML promotes susceptibility to HSV-1 by stress-induced corticosterone

Rationale: Herpes simplex virus type 1 (HSV-1) is a neurotropic virus that can cause a variety of clinical syndromes including mucocutaneous disease and HSV-1 encephalitis (HSE). Here, we characterize the molecular mechanisms underlying the susceptibility to HSV-1 under stressful conditions.

Methods: Restraint stress and corticosterone (CORT, a primary stress hormone) were respectively used to establish HSV-1 susceptible model in vivo and in vitro. Viral titers were determined by plaque assay. Western blotting, immunofluorescence, transmission electron microscopy (TEM), qRT-PCR, H&E staining, IHC staining and flow cytometry were employed to evaluate virus-related protein expressions and detect the activation of autophagy. Loss- and gain-function assays, co-immunoprecipitation (co-IP) technique and autophagy agonist/antagonist treatments were applied in mechanistic experiments.

Results: Restraint stress increased the susceptibility of mouse brain to HSV-1. Similarly, CORT treatment enhanced the susceptibility of neural cells to HSV-1. Furthermore, PML protein level in HSV-1 infected brain tissues and neural cells was remarkably decreased by stress treatment in vivo or CORT treatment in vitro, while its transcriptional level was not affected. Notably, a striking decline in protein expressions of ICP27 and gB was observed in PML-overexpressing cells, which was reversed by CORT treatment. By contrast, protein expression of gB was increased by knockdown with si-PML in virus-infected SH-SY5Y cells. We further discovered that CORT-driven PML degradation was dependent on the activation of autophagy in a ULK1-independent manner, rather than proteasome pathway. Bafilomycin A1 (BaF1) attenuated the augmentation effect of CORT on HSV-1 infection. The expressions of viral proteins were reduced in LC3-depleted cells, and the degradation of PML by CORT-induced autophagy was prevented in cells with LC3 knockdown by RNAi. Interestingly, PML was revealed to interact with the autophagic cargo receptor P62 and the autophagic effector protein LC3. Additionally, CORT failed to increase gB protein level when PML was silenced, providing direct evidence linking autophagic degradation of PML and CORT-induced virus susceptibility.

Conclusion: Our results revealed that restraint stress/CORT increased HSV-1 susceptibility by delivering PML into autolysosomes for degradation. The results obtained from in vitro and in vivo models not only demonstrated the adverse effects of stress on HSV-1 infection, but also systematically investigated the underlying molecular mechanisms. These discoveries broaden our understanding of the interplay between host and viruses, and a comprehensive understanding of the role of autophagy in viral infection will provide information for future development of innovative drugs against viral infection.

The Role of Apoptin in Chicken Anemia Virus Replication

Apoptin is the Vp3 protein of chicken anemia virus (CAV), which infects the thymocytes and erythroblasts in young chickens, causing chicken infectious anemia and immunosuppression. Apoptin is highly studied for its ability to selectively induce apoptosis in human tumor cells and, thus, is a protein of interest in anti-tumor therapy. CAV apoptin is known to localize to different subcellular compartments in transformed and non-transformed cells, depending on the DNA damage response, and the phosphorylation of several identified threonine residues. In addition, apoptin interacts with molecular machinery such as the anaphase promoting complex/cyclosome (APC/C) to inhibit the cell cycle and induce arrest in G2/M phase. While these functions of apoptin contribute to the tumor-selective effect of the protein, they also provide an important fundamental framework to apoptin’s role in viral infection, pathogenesis, and propagation. Here, we reviewed how the regulation, localization, and functions of apoptin contribute to the viral life cycle and postulated its importance in efficient replication of CAV. A model of the molecular biology of infection is critical to informing our understanding of CAV and other related animal viruses that threaten the agricultural industry.

Mechanisms of Host IFI16, PML, and Daxx Protein Restriction of Herpes Simplex Virus 1 Replication

The initial events after DNA virus infection involve a race between epigenetic silencing of the incoming viral DNA by host cell factors and expression of viral genes. Several host gene products, including the nuclear domain 10 (ND10) components PML (promyelocytic leukemia) and Daxx (death domain-associated protein 6), as well as IFI16 (interferon-inducible protein 16), have been shown to restrict herpes simplex virus 1 (HSV-1) replication. Whether IFI16 and ND10 components work together or separately to restrict HSV-1 replication is not known. To determine the combinatorial effects of IFI16 and ND10 proteins on viral infection, we depleted Daxx or PML in primary human foreskin fibroblasts (HFFs) in the presence or absence of IFI16. Daxx or IFI16 depletion resulted in higher ICP0 mutant viral yields, and the effects were additive. Surprisingly, small interfering RNA (siRNA) depletion of PML in the HFF cells led to decreased ICP0-null virus replication, while short hairpin RNA (shRNA) depletion led to increased ICP0-null virus replication, arguing that different PML isoforms or PML-related proteins may have restrictive or proviral functions. In normal human cells, viral DNA replication increases expression of all classes of HSV-1 genes. We observed that IFI16 repressed transcription from both parental and progeny DNA genomes. Taken together, our results show that the mechanisms of action of IFI16 and ND10 proteins are independent, at least in part, and that IFI16 exerts restrictive effects on both input and replicated viral genomes. These results raise the potential for distinct mechanisms of action of IFI16 on parental and progeny viral DNA molecules.IMPORTANCE Many human DNA viruses transcribe their genomes and replicate in the nucleus of a host cell, where they exploit the host cell nuclear machinery for their own replication. Host factors attempt to restrict viral replication by blocking such events, and viruses have evolved mechanisms to neutralize the host restriction factors. In this study, we provide information about the mechanisms of action of three host cell factors that restrict replication of herpes simplex virus (HSV). We found that these factors function independently and that one acts to restrict viral transcription from parental and progeny viral DNA genomes. These results provide new information about how cells counter DNA virus replication in the nucleus and provide possible approaches to enhance the ability of human cells to resist HSV infection.

PML: Regulation and multifaceted function beyond tumor suppression

Promyelocytic leukemia protein (PML) was originally identified as a fusion partner of retinoic acid receptor alpha in acute promyelocytic leukemia patients with the (15;17) chromosomal translocation, giving rise to PML–RARα and RARα–PML fusion proteins. A body of evidence indicated that PML possesses tumor suppressing activity by regulating apoptosis, cell cycle, senescence and DNA damage responses. PML is enriched in discrete nuclear substructures in mammalian cells with 0.2–1 μm diameter in size, referred to as alternately Kremer bodies, nuclear domain 10, PML oncogenic domains or PML nuclear bodies (NBs). Dysregulation of PML NB formation results in altered transcriptional regulation, protein modification, apoptosis and cellular senescence. In addition to PML NBs, PML is also present in nucleoplasm and cytoplasmic compartments, including the endoplasmic reticulum and mitochondria-associated membranes. The role of PML in tumor suppression has been extensively studied but increasing evidence indicates that PML also plays versatile roles in stem cell renewal, metabolism, inflammatory responses, neural function, mammary development and angiogenesis. In this review, we will briefly describe the known PML regulation and function and include new findings.

Characterization of Elements Regulating the Nuclear-to-Cytoplasmic Translocation of ICP0 in Late Herpes Simplex Virus 1 Infection

Infected cell protein 0 (ICP0) of herpes simplex virus 1 (HSV-1) is an immediate early protein containing a RING-type E3 ubiquitin ligase. It targets several host factors for proteasomal degradation and subsequently activates viral expression. ICP0 has a nuclear localization sequence and functions in the nucleus early during infection. However, later in infection, ICP0 is found solely in the cytoplasm. The molecular mechanism and biological function of the ICP0 nuclear-to-cytoplasmic translocation are not well understood. In this study, we sought to characterize elements important for this translocation. We found that (i) in human embryonic lung fibroblast (HEL) cells, ICP0 C-terminal residues 741 to 775 were necessary but not sufficient for the nuclear-to-cytoplasmic translocation; (ii) the loss of ICP0 E3 ubiquitin ligase activity, which led to defective viral replication in nonpermissive cells, also caused mutant ICP0 to be retained in the nucleus of HEL cells; (iii) in permissive U2OS cells, however, ICP0 lacking E3 ligase activity was translocated to the cytoplasm at a pace faster than that of wild-type ICP0, suggesting that nuclear retention of ICP0 occurs in an ICP0 E3 ligase-dependent manner; and (iv) the ICP0 C terminus and late viral proteins cooperate in order to overcome nuclear retention and stimulate ICP0 cytoplasmic translocation. Taken together, less ICP0 nuclear retention may contribute to the permissiveness of U2OS cells to HSV-1 in the absence of functional ICP0.IMPORTANCE A distinct characteristic for eukaryotes is the compartmentalization of cell metabolic pathways, which allows greater efficiency and specificity of cellular functions. ICP0 of HSV-1 is a multifunctional viral protein that travels through different compartments as infection progresses. Its main regulatory functions are carried out in the nucleus, but it is translocated to the cytoplasm late during HSV-1 infection. To understand the biological significance of cytoplasmic ICP0 in HSV-1 infection, we investigated the potential players involved in this nuclear-to-cytoplasmic translocation. We found that there is a nuclear retention force in an ICP0 E3 ubiquitin ligase-dependent manner. In addition, we identified the C terminus of ICP0 as a cis element cooperating with late viral proteins to overcome the nuclear retention and stimulate the nuclear-to-cytoplasmic translocation of ICP0.

Potential mechanisms facilitating herpesvirus-induced nuclear remodeling: how are herpesvirus capsids able to leave the nucleus?

Herpesviruses replicate their DNA, assemble and package their capsids in the host nucleus. How capsids transverse the nuclear space to reach nuclear egress sites at the inner nuclear membrane has been a matter of some debate. We recently showed that HSV-1 and pseudorabies virus rely on the large-scale remodeling of host chromatin to allow intranuclear capsids to cross the nucleoplasm by diffusion. Which molecular pathways induce large-scale chromatin remodeling is currently not known. In this perspective, we propose a four-step speculative model that bridges the gap between known virus–host interactions and large-scale chromatin remodeling. We hope that this hypothetical framework will be used as a basis to elucidate how herpesviruses remodel the host nucleus and enable capsids to escape the nucleus.

RXRα ligand Z-10 induces PML-RARα cleavage and APL cell apoptosis through disrupting PML-RARα/RXRα complex in a cAMP-independent manner

The major oncogenic driver of acute promyelocytic leukemia (APL) is the fusion protein PML-RARα originated from the chromosomal translocation t(15;17). All-trans retinoic acid (ATRA) and arsenic trioxide cure most patients by directly targeting PML-RARα. However, major issues including the resistance of ATRA and arsenic therapy still remain in APL clinical management. Here we showed that compound Z-10, a nitro-ligand of retinoid X receptor α (RXRα), strongly promoted the cAMP-independent apoptosis of both ATRA- sensitive and resistant NB4 cells via the induction of caspase-mediated PML-RARα degradation. RXRα was vital for the stability of both PML-RARα and RARα likely through the interactions. The binding of Z-10 to RXRα dramatically inhibited the interaction of RXRα with PML-RARα but not with RARα, leading to Z-10's selective induction of PML-RARα but not RARα degradation. Z-36 and Z-38, two derivatives of Z-10, had improved potency of inducing PML-RARα reduction and NB4 cell apoptosis. Hence, RXRα ligand Z-10 and its derivatives could target both ATRA- sensitive and resistant APL cells through their distinct acting mechanism, and are potential drug leads for APL treatment.

NPM1 Mutant Mediated PML Delocalization and Stabilization Enhances Autophagy and Cell Survival in Leukemic Cells

Accumulating evidence has defined nucleophosmin 1 (NPM1) mutation as a driver genetic event in acute myeloid leukemia (AML), whereas the pathogenesis of NPM1-mutated AML remains to be fully elucidated. In this study, we showed that mutant NPM1 elevated autophagic activity and autophagic activation contributed to leukemic cell survival in vitro. Meanwhile, we also found high expression of promyelocytic leukemia gene (PML) and its cytoplasmic dislocation in primary NPM1-mutated AML blasts and NPM1-mA positive OCI-AML3 cells. Mechanically, mutant NPM1 interacted with PML and mediated it delocalization as well as stabilization. Notably, NPM1-mA knockdown impaired autophagic activity, while induced expression of PML reversed this effect. Finally, we confirmed that PML modulated autophagic activity via AKT signal. These findings suggest that aberrant PML expression and autophagy are beneficial to the leukemic transformation driven by NPM1 mutations. This indicates an attractive therapeutic avenue for PML targeting and/or autophagy inhibition in the treatment of NPM1-mutated AML.

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.

Biomolecular condensates: organizers of cellular biochemistry

Biomolecular condensates are micron-scale compartments in eukaryotic cells that lack surrounding membranes but function to concentrate proteins and nucleic acids. These condensates are involved in diverse processes, including RNA metabolism, ribosome biogenesis, the DNA damage response and signal transduction. Recent studies have shown that liquid-liquid phase separation driven by multivalent macromolecular interactions is an important organizing principle for biomolecular condensates. With this physical framework, it is now possible to explain how the assembly, composition, physical properties and biochemical and cellular functions of these important structures are regulated.

Nuclear bodies reorganize during myogenesis in vitro and are differentially disrupted by expression of FSHD-associated DUX4

Background

Nuclear bodies, such as nucleoli, PML bodies, and SC35 speckles, are dynamic sub-nuclear structures that regulate multiple genetic and epigenetic processes. Additional regulation is provided by RNA/DNA handling proteins, notably TDP-43 and FUS, which have been linked to ALS pathology. Previous work showed that mouse cell line myotubes have fewer but larger nucleoli than myoblasts, and we had found that nuclear aggregation of TDP-43 in human myotubes was induced by expression of DUX4-FL, a transcription factor that is aberrantly expressed and causes pathology in facioscapulohumeral dystrophy (FSHD). However, questions remained about nuclear bodies in human myogenesis and in muscle disease.

Methods

We examined nucleoli, PML bodies, SC35 speckles, TDP-43, and FUS in myoblasts and myotubes derived from healthy donors and from patients with FSHD, laminin-alpha-2-deficiency (MDC1A), and alpha-sarcoglycan-deficiency (LGMD2D). We further examined how these nuclear bodies and proteins were affected by DUX4-FL expression.

Results

We found that nucleoli, PML bodies, and SC35 speckles reorganized during differentiation in vitro, with all three becoming less abundant in myotube vs. myoblast nuclei. In addition, though PML bodies did not change in size, both nucleoli and SC35 speckles were larger in myotube than myoblast nuclei. Similar patterns of nuclear body reorganization occurred in healthy control, MDC1A, and LGMD2D cultures, as well as in the large fraction of nuclei that did not show DUX4-FL expression in FSHD cultures. In contrast, nuclei that expressed endogenous or exogenous DUX4-FL, though retaining normal nucleoli, showed disrupted morphology of some PML bodies and most SC35 speckles and also co-aggregation of FUS with TDP-43.

Conclusions

Nucleoli, PML bodies, and SC35 speckles reorganize during human myotube formation in vitro. These nuclear body reorganizations are likely needed to carry out the distinct gene transcription and splicing patterns that are induced upon myotube formation. DUX4-FL-induced disruption of some PML bodies and most SC35 speckles, along with co-aggregation of TDP-43 and FUS, could contribute to pathogenesis in FSHD, perhaps by locally interfering with genetic and epigenetic regulation of gene expression in the small subset of nuclei that express high levels of DUX4-FL at any one time.

A Tale of Two PMLs: Elements Regulating a Differential Substrate Recognition by the ICP0 E3 Ubiquitin Ligase of Herpes Simplex Virus 1

Infected cell protein 0 (ICP0) of herpes simplex virus 1 (HSV-1) is an α gene product required for viral replication at low multiplicities of infection. Upon entry, nuclear domain 10 (ND10) converges at the incoming DNA and represses viral gene expression. ICP0 contains a RING-type E3 ubiquitin ligase that degrades the ND10 organizer PML and disperses ND10 to alleviate the repression. In the present study, we focused on understanding the regulation of ICP0 E3 ligase activity in the degradation of different ICP0 substrates. We report the following. (i) A SUMO interaction motif located at ICP0 residues 362 to 364 is required for the degradation of PML isoforms II, IV, and VI but not isoform I. This differentiation mechanism exists in both HEp-2 and U2OS cells, regardless of the cell's permissiveness to the ICP0-null virus. (ii) Physical interaction between SIM_362-364 and PML II is necessary but not sufficient for PML II degradation. Both proximal sequences surrounding SIM_362-364 and distal sequences located at the ICP0 C terminus enhance the degradation of PML II. (iii) The ICP0 C terminus is dispensable for PML I degradation. Instead, bipartite PML I binding domains located in the N-terminal half of ICP0 coordinate to promote the degradation of PML I. (iv) The stability of ICP0, but not its ND10 fusion ability, affects the rate of PML I degradation. Taken together, our results show that ICP0 uses at least two regulatory mechanisms to differentiate its substrates. The disparate recognition of the ICP0 E3 substrates may be related to the different roles these substrates may play in HSV-1 infection.

IMPORTANCE

Viruses have a limited genetic coding capacity but must encounter a multilayered comprehensive host defense. To establish a successful infection, viruses usually produce multifunctional proteins to coordinate the counteractions. Here we report that an HSV-1 protein, ICP0, can recognize individual host factors and target them differently for destruction. We identified elements that are important for the ICP0 E3 ubiquitin ligase to differentially recognize two of its substrates, PML I and PML II. This is the first study that has systematically investigated how ICP0 discriminates two similar molecules by very different mechanisms. This work lays the foundation for understanding the role of host defensive factors and the mechanisms viruses use to take advantage of some host proteins while destroying others.

Localized Movement and Levels of 53BP1 Protein Are Changed by γ-irradiation in PML Deficient Cells

We studied epigenetics, distribution pattern, kinetics, and diffusion of proteins recruited to spontaneous and γ-radiation-induced DNA lesions. We showed that PML deficiency leads to an increased number of DNA lesions, which was accompanied by changes in histone signature. In PML wt cells, we observed two mobile fractions of 53BP1 protein with distinct diffusion in spontaneous lesions. These protein fractions were not detected in PML-deficient cells, characterized by slow-diffusion of 53BP1. Single particle tracking analysis revealed limited local motion of 53BP1 foci in PML double null cells and local motion 53BP1 foci was even more reduced after γ-irradiation. However, radiation did not change co-localization between 53BP1 nuclear bodies and interchromatin granule-associated zones (IGAZs), nuclear speckles, or chromocenters. This newly observed interaction pattern imply that 53BP1 protein could be a part of not only DNA repair, but also process mediated via components accumulated in IGAZs, nuclear speckles, or paraspeckles. Together, PML deficiency affected local motion of 53BP1 nuclear bodies and changed composition and a number of irradiation-induced foci. J. Cell. Biochem. 117: 2583-2596, 2016. © 2016 Wiley Periodicals, Inc.

Intrinsic host restriction factors of human cytomegalovirus replication and mechanisms of viral escape

Before a pathogen even enters a cell, intrinsic immune defenses are active. This first-line defense is mediated by a variety of constitutively expressed cell proteins collectively termed “restriction factors” (RFs), and they form a vital element of the immune response to virus infections. Over time, however, viruses have evolved in a variety ways so that they are able to overcome these RF defenses via mechanisms that are specific for each virus. This review provides a summary of the universal characteristics of RFs, and goes on to focus on the strategies employed by some of the most important RFs in their attempt to control human cytomegalovirus (HCMV) infection. This is followed by a discussion of the counter-restriction mechanisms evolved by viruses to circumvent the host cell’s intrinsic immune defenses. RFs include nuclear proteins IFN-γ inducible protein 16 (IFI16) (a Pyrin/HIN domain protein), Sp100, promyelocytic leukemia, and hDaxx; the latter three being the keys elements of nuclear domain 10 (ND10). IFI16 inhibits the synthesis of virus DNA by down-regulating UL54 transcription - a gene encoding a CMV DNA polymerase; in response, the virus antagonizes IFI16 via a process involving viral proteins UL97 and pp65 (pUL83), which results in the mislocalizing of IFI16 into the cytoplasm. In contrast, viral regulatory proteins, including pp71 and IE1, seek to modify or disrupt the ND10 proteins and thus block or reverse their inhibitory effects upon virus replication. All in all, detailed knowledge of these HCMV counter-restriction mechanisms will be fundamental for the future development of new strategies for combating HCMV infection and for identifying novel therapeutic agents.

A comprehensive compilation of SUMO proteomics

Small ubiquitin-like modifiers (SUMOs) are essential for the regulation of several cellular processes and are potential therapeutic targets owing to their involvement in diseases such as cancer and Alzheimer disease. In the past decade, we have witnessed a rapid expansion of proteomic approaches for identifying sumoylated proteins, with recent advances in detecting site-specific sumoylation. In this Analysis, we combined all human SUMO proteomics data currently available into one cohesive database. We provide proteomic evidence for sumoylation of 3,617 proteins at 7,327 sumoylation sites, and insight into SUMO group modification by clustering the sumoylated proteins into functional networks. The data support sumoylation being a frequent protein modification (on par with other major protein modifications) with multiple nuclear functions, including in transcription, mRNA processing, DNA replication and the DNA-damage response.

Pias1 is essential for erythroid and vascular development in the mouse embryo

The protein inhibitor of activated STAT-1 (PIAS1) is one of the few known SUMO E3 ligases. PIAS1 has been implicated in several biological processes including repression of innate immunity and DNA repair. However, PIAS1 function during development and tissue differentiation has not been studied. Here, we report that Pias1 is required for proper embryonic development. Approximately 90% of Pias1 null embryos die in utero between E10.5 and E12.5. We found significant apoptosis within the yolk sac (YS) blood vessels and concomitant loss of red blood cells (RBCs) resulting in profound anemia. In addition, Pias1 loss impairs YS angiogenesis and results in defective capillary plexus formation and blood vessel occlusions. Moreover, heart development is impaired as a result of loss of myocardium muscle mass. Accordingly, we found that Pias1 expression in primary myoblasts enhances the induction of cardiac muscle genes MyoD, Myogenin and Myomaker. PIAS1 protein regulation of cardiac gene transcription is dependent on transcription factors Myocardin and Gata-4. Finally, endothelial cell specific inactivation of Pias1 in vivo impairs YS erythrogenesis, angiogenesis and recapitulates loss of myocardium muscle mass. However, these defects are not sufficient to recapitulate the lethal phenotype of Pias1 null embryos. These findings highlight Pias1 as an essential gene for YS erythropoiesis and vasculogenesis in vivo.

Role of ND10 nuclear bodies in the chromatin repression of HSV-1

Herpes simplex virus (HSV) is a neurotropic virus that establishes lifelong latent infection in human ganglion sensory neurons. This unique life cycle necessitates an intimate relation between the host defenses and virus counteractions over the long course of infection. Two important aspects of host anti-viral defense, nuclear substructure restriction and epigenetic chromatin regulation, have been intensively studied in the recent years. Upon viral DNA entering the nucleus, components of discrete nuclear bodies termed nuclear domain 10 (ND10), converge at viral DNA and place restrictions on viral gene expression. Meanwhile the infected cell mobilizes its histones and histone-associated repressors to force the viral DNA into nucleosome-like structures and also represses viral transcription. Both anti-viral strategies are negated by various HSV countermeasures. One HSV gene transactivator, infected cell protein 0 (ICP0), is a key player in antagonizing both the ND10 restriction and chromatin repression. On one hand, ICP0 uses its E3 ubiquitin ligase activity to target major ND10 components for proteasome-dependent degradation and thereafter disrupts the ND10 nuclear bodies. On the other hand, ICP0 participates in de-repressing the HSV chromatin by changing histone composition or modification and therefore activates viral transcription. Involvement of a single viral protein in two seemingly different pathways suggests that there is coordination in host anti-viral defense mechanisms and also cooperation in viral counteraction strategies. In this review, we summarize recent advances in understanding the role of chromatin regulation and ND10 dynamics in both lytic and latent HSV infection. We focus on the new observations showing that ND10 nuclear bodies play a critical role in cellular chromatin regulation. We intend to find the connections between the two major anti-viral defense pathways, chromatin remodeling and ND10 structure, in order to achieve a better understanding of how host orchestrates a concerted defense and how HSV adapts with and overcomes the host immunity.

Infected cell protein 0 functional domains and their coordination in herpes simplex virus replication

Herpes simplex virus 1 (HSV-1) is a ubiquitous human pathogen that establishes latent infection in ganglia neurons. Its unique life cycle requires a balanced “conquer and compromise” strategy to deal with the host anti-viral defenses. One of HSV-1 α (immediate early) gene products, infected cell protein 0 (ICP0), is a multifunctional protein that interacts with and modulates a wide range of cellular defensive pathways. These pathways may locate in different cell compartments, which then migrate or exchange factors upon stimulation, for the purpose of a concerted and effective defense. ICP0 is able to simultaneously attack multiple host pathways by either degrading key restrictive factors or modifying repressive complexes. This is a viral protein that contains an E3 ubiquitin ligase, translocates among different cell compartments and interacts with major defensive complexes. The multiple functional domains of ICP0 can work independently and at the same time coordinate with each other. Dissecting the functional domains of ICP0 and delineating the coordination of these domains will help us understand HSV-1 pathogenicity as well as host defense mechanisms. This article focuses on describing individual ICP0 domains, their biochemical properties and their implication in HSV-1 infection. By putting individual domain functions back into the picture of host anti-viral defense network, this review seeks to elaborate the complex interactions between HSV-1 and its host.

Mutant p53 Drives Cancer by Subverting Multiple Tumor Suppression Pathways

The tumor suppressor p53 normally acts as a brake to halt damaged cells from perpetrating their genetic errors into future generations. If p53 is disrupted by mutation, it may not only lose these corrective powers, but counterproductively acquire new capacities that drive cancer. A newly emerging manner in which mutant p53 executes its cancer promoting functions is by harnessing key proteins, which normally partner with its wild type, tumor-inhibiting counterpart. In association with the subverted activities of these protein partners, mutant p53 is empowered to act across multiple fundamental cellular pathways (regulating cell division and metabolism) and corrupt them to become cancer promoting.

The function, regulation and therapeutic implications of the tumor suppressor protein, PML

The tumor suppressor protein, promyelocytic leukemia protein (PML), was originally identified in acute promyelocytic leukemia due to a chromosomal translocation between chromosomes 15 and 17. PML is the core component of subnuclear structures called PML nuclear bodies (PML-NBs), which are disrupted in acute promyelocytic leukemia cells. PML plays important roles in cell cycle regulation, survival and apoptosis, and inactivation or down-regulation of PML is frequently found in cancer cells. More than 120 proteins have been experimentally identified to physically associate with PML, and most of them either transiently or constitutively co-localize with PML-NBs. These interactions are associated with many cellular processes, including cell cycle arrest, apoptosis, senescence, transcriptional regulation, DNA repair and intermediary metabolism. Importantly, PML inactivation in cancer cells can occur at the transcriptional-, translational- or post-translational- levels. However, only a few somatic mutations have been found in cancer cells. A better understanding of its regulation and its role in tumor suppression will provide potential therapeutic opportunities. In this review, we discuss the role of PML in multiple tumor suppression pathways and summarize the players and stimuli that control PML protein expression or subcellular distribution.

Translational control of PML contributes to TNFα-induced apoptosis of MCF7 breast cancer cells and decreased angiogenesis in HUVECs

The tumor suppressor protein promyelocytic leukemia (PML) is a key regulator of inflammatory responses and tumorigenesis and functions through the assembly of subnuclear structures known as PML nuclear bodies (NBs). The inflammation-related cytokine tumor necrosis factor-α (TNFα) is known to induce PML protein accumulation and PML NB formation that mediate TNFα-induced cell death in cancer cells and inhibition of migration and capillary tube formation in endothelial cells (ECs). In this study, we uncover a novel mechanism of PML gene regulation in which the p38 MAPK and its downstream kinase MAP kinase-activated protein kinase 1 (MNK1) mediate TNFα-induced PML protein accumulation and PML NB formation. The mechanism includes the presence of an internal ribosome entry site (IRES) found within the well-conserved 100 nucleotides upstream of the PML initiation codon. The activity of the PML IRES is induced by TNFα in a manner that involves MNK1 activation. It is proposed that the p38–MNK1–PML network regulates TNFα-induced apoptosis in breast cancer cells and TNFα-mediated inhibition of migration and capillary tube formation in ECs.

Cellular promyelocytic leukemia protein is an important dengue virus restriction factor

The intrinsic antiviral defense is based on cellular restriction factors that are constitutively expressed and, thus, active even before a pathogen enters the cell. The promyelocytic leukemia (PML) nuclear bodies (NBs) are discrete nuclear foci that contain several cellular proteins involved in intrinsic antiviral responses against a number of viruses. Accumulating reports have shown the importance of PML as a DNA virus restriction factor and how these pathogens evade this antiviral activity. However, very little information is available regarding the antiviral role of PML against RNA viruses. Dengue virus (DENV) is an RNA emerging mosquito-borne human pathogen affecting millions of individuals each year by causing severe and potentially fatal syndromes. Since no licensed antiviral drug against DENV infection is currently available, it is of great importance to understand the factors mediating intrinsic immunity that may lead to the development of new pharmacological agents that can boost their potency and thereby lead to treatments for this viral disease. In the present study, we investigated the in vitro antiviral role of PML in DENV-2 A549 infected cells.

Identification of three redundant segments responsible for herpes simplex virus 1 ICP0 to fuse with ND10 nuclear bodies

Infected cell protein 0 (ICP0) of herpes simplex virus 1 (HSV-1) is a key regulator in both lytic and latent infections. In lytic infection, an important early event is the colocalization of ICP0 to nuclear domain 10 (ND10), the discrete nuclear bodies that impose restrictions on viral expression. ICP0 contains an E3 ubiquitin ligase that degrades promyelocytic leukemia protein (PML) and Sp100, two major components of ND10, and disperses ND10 to alleviate repression. We previously reported that the association between ICP0 and ND10 is a dynamic process that includes three steps: adhesion, fusion, and retention. ICP0 residues 245 to 474, defined as ND10 entry signal (ND10-ES), is a region required for the fusion step. Without ND10-ES, ICP0 adheres at the ND10 surface but fails to enter. In the present study, we focus on characterizing ND10-ES. Here we report the following. (i) Fusion of ICP0 with ND10 relies on specific sequences located within ND10-ES. Replacement of ND10-ES by the corresponding region from ORF61 of varicella-zoster virus did not rescue ND10 fusion. (ii) Three tandem ND10 fusion segments (ND10-FS1, ND10-FS2, and ND10-FS3), encompassing 200 amino acids within ND10-ES, redundantly facilitate fusion. Each of the three segments is sufficient to independently drive the fusion process, but none of the segments by themselves are necessary for ND10 fusion. Only when all three segments are deleted is fusion blocked. (iii) The SUMO interaction motif located within ND10-FS2 is not required for ND10 fusion but is required for the complete degradation of PML, suggesting that PML degradation and ND10 fusion are regulated by different molecular mechanisms.

IMPORTANCE

ND10 nuclear bodies are part of the cell-intrinsic antiviral defenses that restrict viral gene expression upon virus infection. As a countermeasure, infected cell protein 0 (ICP0) of herpes simplex virus 1 (HSV-1) localizes to ND10s, degrades the ND10 organizer, and disperses ND10 components in order to alleviate repression. We studied the ICP0-ND10 association to delineate elements important for this dynamic interaction and to understand its role in viral replication and host defense. In this work, we show that ICP0 contains three redundant segments to ensure an effective mergence of ICP0 with ND10 nuclear bodies. This is the first study to systematically investigate ICP0 elements that are important for ICP0-ND10 fusion.

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.

Novel chemotherapeutic approaches in adult high-grade gliomas

Despite decades of advancing science and clinical trials, average survival remains dismal for individuals with high-grade gliomas. Our understanding of the genetic and molecular aberrations that contribute to the aggressive nature of these tumors is continually growing, as is our ability to target such specific traits. Herein, we review the major classes of such targeted therapies, as well as the relevant clinical trial outcomes regarding their efficacy.

Microtubule-associated protein 1 light chain 3 interacts with and contributes to growth inhibiting effect of PML

Previously we reported that the expression of promyelocytic leukemia (PML)-retinoic acid receptor alpha (RARα) fusion gene, which is caused by specific translocation (15;17) in acute promyelocytic leukemia, can enhance constitutive autophagic activity in leukemic and nonleukemic cells, and PML overexpression can sequestrate part of microtubule-associated protein light chain 3 (LC3) protein in PML nuclear bodies, suggesting that LC3 protein also distributes into nuclei although it is currently thought to function primarily in the cytoplasm, the site of autophagosomal formation. However, its potential significance of nucleoplasmic localizations remains greatly elusive. Here we demonstrate that PML interacts with LC3 in a cell type-independent manner as assessed by Co-IP assay and co-localization observation. Overexpressed PML significantly coprecipitates with endogenous and nuclear LC3 protein. Furthermore, a fraction of endogenous PML protein is found to be co-localized with LC3 protein under steady state condition, which is further enhanced by IFNα induction, indicating that PML up-regulation potentiates this interaction. Additionally, DsRed-PML associates with EGFP-LC3 during telophase and G1 phase but not in metaphase and anaphase. Two potential LC3-interacting region (LIR) motifs in PML are required for interaction of PML with LC3 while this association is independent of autophagic activity. Finally, we show that interaction between PML and LC3 contributes to cell growth inhibition function of PML. Considering that PML is an important tumor suppressor, we propose that nuclear portion of LC3 protein may associate with PML to control cell growth for prevention and inhibition of cancer occurrence and development.

Crystal structure of cytomegalovirus IE1 protein reveals targeting of TRIM family member PML via coiled-coil interactions

PML nuclear bodies (PML-NBs) are enigmatic structures of the cell nucleus that act as key mediators of intrinsic immunity against viral pathogens. PML itself is a member of the E3-ligase TRIM family of proteins that regulates a variety of innate immune signaling pathways. Consequently, viruses have evolved effector proteins to modify PML-NBs; however, little is known concerning structure-function relationships of viral antagonists. The herpesvirus human cytomegalovirus (HCMV) expresses the abundant immediate-early protein IE1 that colocalizes with PML-NBs and induces their dispersal, which correlates with the antagonization of NB-mediated intrinsic immunity. Here, we delineate the molecular basis for this antagonization by presenting the first crystal structure for the evolutionary conserved primate cytomegalovirus IE1 proteins. We show that IE1 consists of a globular core (IE1_CORE) flanked by intrinsically disordered regions. The 2.3 Å crystal structure of IE1_CORE displays an all α-helical, femur-shaped fold, which lacks overall fold similarity with known protein structures, but shares secondary structure features recently observed in the coiled-coil domain of TRIM proteins. Yeast two-hybrid and coimmunoprecipitation experiments demonstrate that IE1_CORE binds efficiently to the TRIM family member PML, and is able to induce PML deSUMOylation. Intriguingly, this results in the release of NB-associated proteins into the nucleoplasm, but not of PML itself. Importantly, we show that PML deSUMOylation by IE1_CORE is sufficient to antagonize PML-NB-instituted intrinsic immunity. Moreover, co-immunoprecipitation experiments demonstrate that IE1_CORE binds via the coiled-coil domain to PML and also interacts with TRIM5α We propose that IE1_CORE sequesters PML and possibly other TRIM family members via structural mimicry using an extended binding surface formed by the coiled-coil region. This mode of interaction might render the antagonizing activity less susceptible to mutational escape.

Research of the last few years has revealed that microbial infections are not only controlled by innate and adaptive immune mechanisms, but also by cellular restriction factors, which give cells the capacity to resist pathogens. PML nuclear bodies (PML-NBs) are dot-like nuclear structures representing multiprotein complexes that consist of the PML protein, a member of the TRIM family of proteins, as well as a multitude of additional regulatory factors. PML-NB components act as a barrier against many viral infections; however, viral antagonistic proteins have evolved to modify PML-NBs, thus abrogating this cellular defense. Here, we delineate the molecular basis for antagonization by the immediate-early protein IE1 of the herpesvirus human cytomegalovirus. We present the first crystal structure for the evolutionary conserved core domain (IE1_CORE) of primate cytomegalovirus IE1, which exhibits a novel, unusual fold. IE1_CORE modifies PML-NBs by releasing other PML-NB proteins into the nucleoplasm which is sufficient to antagonize intrinsic immunity. Importantly, IE1_CORE shares secondary structure features with the coiled-coil domain (CC) of TRIM factors, and we demonstrate strong binding of IE1 to the PML-CC. We propose that IE1_CORE sequesters PML and possibly other TRIM family members via an extended binding surface formed by the coiled-coil domain.

SCP phosphatases suppress renal cell carcinoma by stabilizing PML and inhibiting mTOR/HIF signaling

The tumor-suppressor protein promyelocytic leukemia (PML) is aberrantly degraded in multiple types of human cancers through mechanisms that are incompletely understood. Here, we show that the phosphatase SCP1 and its isoforms SCP2/3 dephosphorylate PML at S518, thereby blocking PML ubiquitination and degradation mediated by the prolyl isomerase Pin1 and the ubiquitin ligase KLHL20. Clinically, SCP1 and SCP3 are downregulated in clear cell renal cell carcinoma (ccRCC) and these events correlated with PMLS518 phosphorylation, PML turnover, and high-grade tumors. Restoring SCP1-mediated PML stabilization not only inhibited malignant features of ccRCC, including proliferation, migration, invasion, tumor growth, and tumor angiogenesis, but also suppressed the mTOR-HIF pathway. Furthermore, blocking PML degradation in ccRCC by SCP1 overexpression or Pin1 inhibition enhanced the tumor-suppressive effects of the mTOR inhibitor temsirolimus. Taken together, our results define a novel pathway of PML degradation in ccRCC that involves SCP downregulation, revealing contributions of this pathway to ccRCC progression and offering a mechanistic rationale for combination therapies that jointly target PML degradation and mTOR inhibition for ccRCC treatment.

Restriction factors against human CMV

Cellular proteins called 'restriction factors' (RFs) form an important component of the innate immune response to viral replication. However, viruses have learned how to antagonize RFs through mechanisms that are specific for each virus. Here, we summarize the general hallmarks of RFs before going on to discuss the specific strategies recruited by some key RFs that strive to hold human CMV (HCMV) infection back, as well as the counter-restriction mechanisms employed by the virus to overcome this innate defense. Such RFs include the cellular constituents of nuclear domain 10 (ND10), and IFI16, a nuclear member of the PYHIN protein family. Viral regulatory proteins, such as IE1 or pp71, try to oppose the ND10-induced blockade of virus replication by either modifying or disrupting this RF. IFI16, on the other hand, inhibits virus DNA synthesis by downregulating the transcription of viral gene UL54; the intruding virus attempts to antagonize IFI16 by mislocalizing it from the nucleus to the cytoplasm via the action of viral protein UL97. Finally, we consider how Viperin, a RF initially thought to inhibit HCMV maturation late during infection, has actually been demonstrated to enhance virus maturation by increasing lipid metabolism and enhancing virus envelopment.

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.

Acute promyelocytic leukemia: A model of molecular target based therapy

Leukemia, a group of hematological malignancies characterized by clonal expansion of hematopoietic cells with uncontrolled proliferation, decreased apoptosis and blocked differentiation, is one of the most notorious enemies of mankind which accounts for some 300,000 new cases and 222,000 deaths each year worldwide. Leukemia can be divided into acute or chronic, lymphoid or myeloid types, based on the disease progression and hematopoietic lineages involved 5. The responses of leukemia to therapies differ from one type or subtype to another. Hence, to improve the clinical outcome, the therapeutic strategies should be disease pathogenesis-based and individualized. The close collaboration between bench and bedside may not only shed new lights on leukemogenesis, gain insights into therapeutic mechanisms, but also provide opportunities for designing more rational therapies. The development of curative approaches for acute promyelocytic leukemia (APL) may serve as a paradigm.

Mutation of the LXCXE binding cleft of pRb facilitates transformation by ras in vitro but does not promote tumorigenesis in vivo

BACKGROUND

The Retinoblastoma protein (pRB) is a key tumor suppressor that is functionally inactivated in most cancers. pRB regulates the cell division cycle and cell cycle exit through protein-protein interactions mediated by its multiple binding interfaces. The LXCXE binding cleft region of pRB mediates interactions with cellular proteins that have chromatin regulatory functions. Chromatin regulation mediated by pRB is required for a stress responsive cell cycle arrest, including oncogene induced senescence. The in vivo role of chromatin regulation by pRB during senescence, and its relevance to cancer is not clear.

METHODOLOGY/PRINCIPAL FINDINGS

Using gene-targeted mice, uniquely defective for pRB mediated chromatin regulation, we investigated its role during transformation and tumor progression in response to activation of oncogenic ras. We report that the pRB(∆L) mutation confers susceptibility to escape from HrasV12 induced senescence and allows transformation in vitro, although these cells possess high levels of DNA damage. Intriguingly, LSL-Kras, Rb1 (∆L/∆L) mice show delayed lung tumor formation compared to controls. This is likely due to the increased apoptosis seen in the early hyperplastic lesions shortly following ras activation that inhibits tumor progression. Furthermore, DMBA treatment to induce sporadic ras mutations in other tissues also failed to reveal greater susceptibility to cancer in Rb1 (∆L/∆L) mice.

CONCLUSIONS/SIGNIFICANCE

Our data suggests that chromatin regulation by pRB can function to limit proliferation, but its loss fails to contribute to cancer susceptibility in ras driven tumor models because of elevated levels of DNA damage and apoptosis.

Interaction of herpes simplex virus ICP0 with ND10 bodies: a sequential process of adhesion, fusion, and retention

On entry into the nucleus, herpes simplex virus 1 (HSV-1) DNA localizes to nuclear bodies known as ND10. Gene repression imposed by ND10 is released by a viral protein, ICP0, via degradation of the ND10 constituents promyelocytic leukemia protein (PML) and Sp100 and the subsequent dispersal of ND10 bodies. In order to understand the dynamic interaction between ICP0 and ND10, we carried out deletion mapping to identify the domains of ICP0 responsible for its association with ND10. Here, we report the following. (i) An ND10 entry signal (ND10-ES), located between residues 245 and 474, is required for ICP0 to penetrate and fuse with ND10. ICP0 lacking ND10-ES adheres to the surface of ND10 but fails to enter. (ii) In the absence of ND10-ES, the E3 ubiquitin ligase of ICP0 facilitates the transient adhesion of the truncated ICP0 to the ND10 surface, whereas the presence of ND10-ES in ICP0 renders ND10 fusion regardless of the E3 ligase activity. (iii) The C terminus of ICP0 is required for retention of ICP0 in ND10 but plays no role in the recruitment process. (iv) The adverse effects of an inactive RING domain on viral replication are partially reversed by deleting either ND10-ES or the C-terminal retention domain, suggesting that additional ICP0 functions require the release of ICP0 from ND10. Based on these results, we conclude that association of ICP0 and ND10 is a dynamic process, in which three sequential steps--adhesion, fusion, and retention--are adopted to stabilize the interaction. A faithful execution of these steps defines the ultimate productivity of the virus.