Targeting promyelocytic leukemia protein: a means to regulating PML nuclear bodies

Novel Cell-Penetrating Peptides Derived From Scaffold-Attachment- Factor A Inhibits Cancer Cell Proliferation and Survival

Scaffold-attachment-factor A (SAFA) has important roles in many normal and pathologic cellular processes but the scope of its function in cancer cells is unknown. Here, we report dominant-negative activity of novel peptides derived from the SAP and RGG-domains of SAFA and their effects on proliferation, survival and the epigenetic landscape in a range of cancer cell types. The RGG-derived peptide dysregulates SAFA binding and regulation of alternatively spliced targets and decreases levels of key spliceosome proteins in a cell-type specific manner. In contrast, the SAP-derived peptide reduces active histone marks, promotes chromatin compaction, and activates the DNA damage response and cell death in a subset of cancer cell types. Our findings reveal an unprecedented function of SAFA-derived peptides in regulating diverse SAFA molecular functions as a tumor suppressive mechanism and demonstrate the potential therapeutic utility of SAFA-peptides in a wide range of cancer cells.

CRISPR-Mediated Programmable 3D Genome Positioning and Nuclear Organization

Programmable control of spatial genome organization is a powerful approach for studying how nuclear structure affects gene regulation and cellular function. Here, we develop a versatile CRISPR-genome organization (CRISPR-GO) system that can efficiently control the spatial positioning of genomic loci relative to specific nuclear compartments, including the nuclear periphery, Cajal bodies, and promyelocytic leukemia (PML) bodies. CRISPR-GO is chemically inducible and reversible, enabling interrogation of real-time dynamics of chromatin interactions with nuclear compartments in living cells. Inducible repositioning of genomic loci to the nuclear periphery allows for dissection of mitosis-dependent and -independent relocalization events and also for interrogation of the relationship between gene position and gene expression. CRISPR-GO mediates rapid de novo formation of Cajal bodies at desired chromatin loci and causes significant repression of endogenous gene expression over long distances (30-600 kb). The CRISPR-GO system offers a programmable platform to investigate large-scale spatial genome organization and function.

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.

Pharmacological inhibition of SUMO-1 with ginkgolic acid alleviates cardiac fibrosis induced by myocardial infarction in mice

BACKGROUND AND PURPOSE

Protein modification by small ubiquitin-like modifier (SUMO) plays a critical role in the pathogenesis of heart diseases. The present study was designed to determine whether ginkgolic acid (GA) as a SUMO-1 inhibitor exerts an inhibitory effect on cardiac fibrosis induced by myocardial infarction (MI).

EXPERIMENTAL APPROACH

GA was delivered by osmotic pumps in MI mice. Masson staining, electron microscopy (EM) and echocardiography were used to assess cardiac fibrosis, ultrastructure and function. Expression of SUMO-1, PML, TGF-β1 and Pin1 was measured with Western blot or Real-time PCR. Collagen content, cell viability and myofibroblast transformation were measured in neonatal mouse cardiac fibroblasts (NMCFs). Promyelocytic leukemia (PML) protein was over-expressed by plasmid transfection.

KEY RESULTS

GA improved cardiac fibrosis and dysfunction, and decreased SUMO-1 expression in MI mice. GA (>20 μM) inhibited NMCF viability in a dose-dependent manner. Nontoxic GA (10 μM) restrained angiotensin II (Ang II)-induced myofibroblast transformation and collagen production. GA also inhibited expression of TGF-β1 mRNA and protein in vitro and in vivo. GA suppressed PML SUMOylation and PML nuclear body (PML-NB) organization, and disrupted expression and recruitment of Pin1 (a positive regulator of TGF-β1 mRNA), whereas over-expression of PML reversed that.

CONCLUSIONS AND IMPLICATIONS

Inhibition of SUMO-1 by GA alleviated MI-induced heart dysfunction and fibrosis, and the SUMOylated PML/Pin1/TGF-β1 pathway is crucial for GA-inhibited cardiac fibrosis.

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.

Assembly of Epstein-Barr Virus Capsid in Promyelocytic Leukemia Nuclear Bodies

The Epstein-Barr virus (EBV) capsid contains a major capsid protein, VCA; two minor capsid proteins, BDLF1 and BORF1; and a small capsid protein, BFRF3. During the lytic cycle, these capsid proteins are synthesized and imported into the host nucleus for capsid assembly. This study finds that EBV capsid proteins colocalize with promyelocytic leukemia (PML) nuclear bodies (NBs) in P3HR1 cells during the viral lytic cycle, appearing as nuclear speckles under a confocal laser scanning microscope. In a glutathione S-transferase pulldown study, we show that BORF1 interacts with PML-NBs in vitro. BORF1 also colocalizes with PML-NBs in EBV-negative Akata cells after transfection and is responsible for bringing VCA and the VCA-BFRF3 complex from the cytoplasm to PML-NBs in the nucleus. Furthermore, BDLF1 is dispersed throughout the cell when expressed alone but colocalizes with PML-NBs when BORF1 is also present in the cell. In addition, this study finds that knockdown of PML expression by short hairpin RNA does not influence the intracellular levels of capsid proteins but reduces the number of viral particles produced by P3HR1 cells. Together, these results demonstrate that BORF1 plays a critical role in bringing capsid proteins to PML-NBs, which may likely be the assembly sites of EBV capsids. The mechanisms elucidated in this study are critical to understanding the process of EBV capsid assembly. IMPORTANCE Capsid assembly is an important event during the Epstein-Barr virus (EBV) lytic cycle, as this process is required for the production of virions. In this study, confocal microscopy revealed that the EBV capsid protein BORF1 interacts with promyelocytic leukemia (PML) nuclear bodies (NBs) in the host nucleus and is responsible for transporting the other EBV capsid proteins, including VCA, BDLF1, and BFRF3, to these subnuclear locations prior to initiation of capsid assembly. This study also found that knockdown of PML expression by short hairpin RNA significantly reduces EBV capsid assembly capabilities. This enhanced understanding of capsid assembly offers potential for the development of novel antiviral strategies and therapies that can prevent the propagation and spread of EBV.

Promyelocytic leukemia protein enhances apoptosis of gastric cancer cells through Yes-associated protein

It has been shown that Yes-associated protein (YAP) acts as a transcriptional co-activator to regulate p73-dependent apoptosis in response to DNA damage in some cell types, and promyelocytic leukemia (PML) protein is involved in the regulation loop through stabilization of YAP through sumoylation. Although YAP has been shown to be significantly upregulated in gastric cancer, whether the YAP/PML/p73 regulation loop also functions in gastric cancer is unknown. Here, we show significantly higher levels of YAP and significantly lower levels of PML in the gastric cancer specimen. Overexpression of YAP in gastric cancer cells significantly increased cell growth, but did not affect apoptosis. However, overexpression of PML in gastric cancer cells significantly increased cell apoptosis, resulting in decreases in cell growth, which seemed to require the presence of YAP. The effect of PML on apoptosis appeared to be conducted through p73-mediated modulation of apoptosis-associated genes, Bcl-2, Bak, and caspase9. Thus, our study suggests the presence of a YAP/PML/p73 regulatory loop in gastric cancer, and highlights PML as a promising tumor suppressor in gastric cancer through YAP-coordinated cancer cell apoptosis.

JC virus inclusions in progressive multifocal leukoencephalopathy: scaffolding promyelocytic leukemia nuclear bodies grow with cell cycle transition through an S-to-G2-like state in enlarging oligodendrocyte nuclei

In progressive multifocal leukoencephalopathy, JC virus–infected oligodendroglia display 2 distinct patterns of intranuclear viral inclusions: full inclusions in which progeny virions are present throughout enlarged nuclei and dot-shaped inclusions in which virions are clustered in subnuclear domains termed “promyelocytic leukemia nuclear bodies” (PML-NBs). Promyelocytic leukemia nuclear bodies may serve a scaffolding role in viral progeny production. We analyzed the formation process of intranuclear viral inclusions by morphometry and assessed PML-NB alterations in the brains of 2 patients with progressive multifocal leukoencephalopathy. By immunohistochemistry, proliferating cell nuclear antigen was most frequently detected in smaller nuclei; cyclin A was detected in larger nuclei. This suggests an S-to-G2 cell cycle transition in infected cells associated with nuclear enlargement. Sizes of PML-NBs were variable, but they were usually either small speckles 200 to 400 nm in diameter or distinct spherical shells with a diameter of 1 μm or more. By confocal microscopy, JC virus capsid proteins were associated with both small and large PML-NBs, but disruption of large PML-NBs was observed by ground-state depletion fluorescence nanoscopy. Clusters of progeny virions were also detected by electron microscopy. Our data suggest that, in progressive multifocal leukoencephalopathy, JC virus produces progeny virions in enlarging oligodendrocyte nuclei in association with growing PML-NBs and with cell cycle transition through an S-to-G2-like state.

Murine gammaherpesvirus 68 encodes a second PML-modifying protein

The ORF75c tegument protein of murine gammaherpesvirus 68 (MHV68) promotes the degradation of the antiviral promyelocytic leukemia (PML) protein. Surprisingly, MHV68 expressing a degradation-deficient ORF75c replicated in cell culture and in mice similar to the wild-type virus. However, in cells infected with this mutant virus, PML formed novel track-like structures that are induced by ORF61, the viral ribonucleotide reductase large subunit. These findings may explain why ORF75c mutant viruses unable to degrade PML had no demonstrable phenotype after infection.

Ablation of promyelocytic leukemia protein (PML) re-patterns energy balance and protects mice from obesity induced by a Western diet

The promyelocytic leukemia protein is a well known tumor suppressor, but its role in metabolism is largely unknown. Mice with a deletion in the gene for PML (KO mice) exhibit altered gene expression in liver, adipose tissue, and skeletal muscle, an accelerated rate of fatty acid metabolism, abnormal glucose metabolism, constitutive AMP-activating kinase (AMPK) activation, and insulin resistance in skeletal muscle. Last, an increased rate of energy expenditure protects PML KO mice from the effects of obesity induced by a Western diet. Collectively, our study uncovers a previously unappreciated role of PML in the regulation of metabolism and energy balance in mice.

The epigenetic regulator UHRF1 promotes ubiquitination-mediated degradation of the tumor-suppressor protein promyelocytic leukemia protein

The promyelocytic leukemia (PML) protein is a tumor suppressor originally identified in acute promyelocytic leukemia and implicated in tumorigenesis in multiple forms of cancer. Here, we demonstrate that the PML protein undergoes ubiquitination-mediated degradation facilitated by an E3 ligase UHRF1 (ubiquitin-like with PHD and RING finger domains 1), which is commonly upregulated in various human malignancies. Furthermore, UHRF1 negatively regulates PML protein accumulation in primary human umbilical vein endothelial cells (HUVECs), HEK 293 cells and cancer cells. Knockdown of UHRF1 upregulates whereas ectopic overexpression of UHRF1 downregulates protein abundance of endogenous or exogenous PML, doing so through its binding to the N-terminus of PML. Overexpression of wild-type UHRF1 shortens PML protein half-life and promotes PML polyubiquitination, whereas deletion of the RING domain or coexpression of the dominant-negative E2 ubiquitin-conjugating enzyme, E2D2, attenuates this modification to PML. Finally, knockdown of UHRF1 prolongs PML half-life and increases PML protein accumulation, yet inhibits cell migration and in vitro capillary tube formation, whereas co-knockdown of PML compromises this inhibitory effect. These findings suggest that UHRF1 promotes the turnover of PML protein, and thus targeting UHRF1 to restore PML-mediated tumor suppression represents a promising, novel, anticancer strategy.

Murine gammaherpesvirus 68 ORF75c contains ubiquitin E3 ligase activity and requires PML SUMOylation but not other known cellular PML regulators, CK2 and E6AP, to mediate PML degradation

All gammaherpsviruses encode at least one gene related to the cellular formylglycinamide ribonucleotide amidotransferase (FGARAT) enzyme but their biological roles are relatively unknown. The murine gammaherpesvirus 68 (MHV68) vFGARAT, ORF75c, mediates a proteasome-dependent degradation of the antiviral promyelocytic leukemia (PML) protein by an unknown mechanism, which is addressed in this study. We found that ORF75c interacts weakly with PML and SUMO-modified forms of PML are important for its degradation by ORF75c. ORF75c-mediated PML degradation was not dependent on two known cellular regulators of PML stability, Casein kinase II (CK2) and human papilloma virus E6-associated protein (E6AP). Finally, ORF75c had self-ubiquitination activity in vitro and its expression increased levels of ubiquitinated PML in transfected cells. Taken together, the evidence accumulated in this study provides new insights into the function of a vFGARAT and is consistent with a model in which ORF75c could mediate direct ubiquitination of PML resulting in its degradation by the proteasome.

Promyelocytic leukemia (PML) protein plays important roles in regulating cell adhesion, morphology, proliferation and migration

PML protein plays important roles in regulating cellular homeostasis. It forms PML nuclear bodies (PML-NBs) that act like nuclear relay stations and participate in many cellular functions. In this study, we have examined the proteome of mouse embryonic fibroblasts (MEFs) derived from normal (PML^+/+) and PML knockout (PML^−/−) mice. The aim was to identify proteins that were differentially expressed when MEFs were incapable of producing PML. Using comparative proteomics, total protein were extracted from PML^−/− and PML^+/+ MEFs, resolved by two dimensional electrophoresis (2-DE) gels and the differentially expressed proteins identified by LC-ESI-MS/MS. Nine proteins (PML, NDRG1, CACYBP, CFL1, RSU1, TRIO, CTRO, ANXA4 and UBE2M) were determined to be down-regulated in PML^−/− MEFs. In contrast, ten proteins (CIAPIN1, FAM50A, SUMO2 HSPB1 NSFL1C, PCBP2, YWHAG, STMN1, TPD52L2 and PDAP1) were found up-regulated. Many of these differentially expressed proteins play crucial roles in cell adhesion, migration, morphology and cytokinesis. The protein profiles explain why PML^−/− and PML^+/+ MEFs were morphologically different. In addition, we demonstrated PML^−/− MEFs were less adhesive, proliferated more extensively and migrated significantly slower than PML^+/+ MEFs. NDRG1, a protein that was down-regulated in PML^−/− MEFs, was selected for further investigation. We determined that silencing NDRG1expression in PML^+/+ MEFs increased cell proliferation and inhibited PML expression. Since NDRG expression was suppressed in PML^−/− MEFs, this may explain why these cells proliferate more extensively than PML^+/+ MEFs. Furthermore, silencing NDRG1expression also impaired TGF-β1 signaling by inhibiting SMAD3 phosphorylation.

Deregulation of epigenetic mechanisms by the hepatitis B virus X protein in hepatocarcinogenesis

This review focuses on the significance of deregulation of epigenetic mechanisms by the hepatitis B virus (HBV) X protein in hepatocarcinogenesis and HBV replication. Epigenetic mechanisms, DNA methylation, and specific histone modifications, e.g., trimethylation of H3 on lysine-27 or lysine-4, maintain ‘cellular memory’ by silencing expression of lineage-inducing factors in stem cells and conversely, of pluripotency factors in differentiated cells. The X protein has been reported to induce expression of DNA methyltransferases (DNMTs), likely promoting epigenetic changes during hepatocarcinogenesis. Furthermore, in cellular and animal models of X-mediated oncogenic transformation, protein levels of chromatin modifying proteins Suz12 and Znf198 are down-regulated. Suz12 is essential for the Polycomb Repressive Complex 2 (PRC2) mediating the repressive trimethylation of H3 on lysine-27 (H3K27me3). Znf198, stabilizes the LSD1-CoREST-HDAC complex that removes, via lysine demethylase1 (LSD1), the activating trimethylation of H3 on lysine-4 (H3K4me3). Down-regulation of Suz12 also occurs in liver tumors of woodchucks chronically infected by woodchuck hepatitis virus, an animal model recapitulating HBV-mediated hepatocarcinogenesis in humans. Significantly, subgroups of HBV-induced liver cancer re-express hepatoblast and fetal markers, and imprinted genes, suggesting hepatocyte reprogramming during oncogenic transformation. Lastly, down-regulation of Suz12 and Znf198 enhances HBV replication. Collectively, these observations suggest deregulation of epigenetic mechanisms by HBV X protein influences both the viral cycle and the host cell.

p53 regulates Period2 expression and the circadian clock

The mechanistic interconnectivity between circadian regulation and the genotoxic stress response remains poorly understood. Here we show that the expression of Period 2 (Per2), a circadian regulator, is directly regulated by p53 binding to a response element in the Per2 promoter. This p53 response element is evolutionarily conserved and overlaps with the E-Box element critical for BMAL1/CLOCK binding and its transcriptional activation of Per2 expression. Our studies reveal that p53 blocks BMAL1/CLOCK binding to the Per2 promoter, leading to repression of Per2 expression. In the suprachiasmatic nucleus (SCN), p53 expression and its binding to the Per2 promoter are under circadian control. Per2 expression in the SCN is altered by p53 deficiency or stabilization of p53 by Nutlin-3. Behaviourally, p53⁻/⁻ mice have a shorter period length that lacks stability, and they exhibit impaired photo-entrainment to a light pulse under a free-running state. Our studies demonstrate that p53 modulates mouse circadian behaviour.

PML tumor suppressor protein is required for HCV production

PML tumor suppressor protein, which forms discrete nuclear structures termed PML-nuclear bodies, has been associated with several cellular functions, including cell proliferation, apoptosis and antiviral defense. Recently, it was reported that the HCV core protein colocalizes with PML in PML-NBs and abrogates the PML function through interaction with PML. However, role(s) of PML in HCV life cycle is unknown. To test whether or not PML affects HCV life cycle, we examined the level of secreted HCV core and the infectivity of HCV in the culture supernatants as well as the level of HCV RNA in HuH-7-derived RSc cells, in which HCV-JFH1 can infect and efficiently replicate, stably expressing short hairpin RNA targeted to PML. In this context, the level of secreted HCV core and the infectivity in the supernatants from PML knockdown cells was remarkably reduced, whereas the level of HCV RNA in the PML knockdown cells was not significantly affected in spite of very effective knockdown of PML. In fact, we showed that PML is unrelated to HCV RNA replication using the subgenomic HCV-JFH1 replicon RNA, JRN/3-5B. Furthermore, the infectivity of HCV-like particle in the culture supernatants was significantly reduced in PML knockdown JRN/3-5B cells expressing core to NS2 coding region of HCV-JFH1 genome using the trans-packaging system. Finally, we also demonstrated that INI1 and DDX5, the PML-related proteins, are involved in HCV production. Taken together, these findings suggest that PML is required for HCV production.

Interplay between human cytomegalovirus and intrinsic/innate host responses: a complex bidirectional relationship

The interaction between human cytomegalovirus (HCMV) and its host is a complex process that begins with viral attachment and entry into host cells, culminating in the development of a specific adaptive response that clears the acute infection but fails to eradicate HCMV. We review the viral and cellular partners that mediate early host responses to HCMV with regard to the interaction between structural components of virions (viral glycoproteins) and cellular receptors (attachment/entry receptors, toll-like receptors, and other nucleic acid sensors) or intrinsic factors (PML, hDaxx, Sp100, viperin, interferon inducible protein 16), the reactions of innate immune cells (antigen presenting cells and natural killer cells), the numerous mechanisms of viral immunoevasion, and the potential exploitation of events that are associated with early phases of virus-host interplay as a therapeutic strategy.

Promyelocytic leukemia protein (PML) regulates endothelial cell network formation and migration in response to tumor necrosis factor α (TNFα) and interferon α (IFNα)

Promyelocytic leukemia protein (PML) is a tumor suppressor that is highly expressed in vascular endothelium and inflamed tissues, yet its role in inflammation-associated cytokine-regulated angiogenesis and underlying mechanism remains largely unclear. We show that tumor necrosis factor α (TNFα) and interferon α (IFNα) stimulate PML expression while suppressing EC network formation and migration, two key events during angiogenesis. By a knockdown approach, we demonstrate that PML is indispensable for TNFα- and IFNα-mediated inhibition of EC network formation. We further demonstrate that signal transducer and activator of transcription 1 (STAT1) binds PML promoter and that is an important regulator of PML expression. Knockdown of STAT1 reduces endogenous PML and blocks TNFα- and IFNα-induced PML accumulation and relieves TNFα- and IFNα-mediated inhibition of EC network formation. Our data also indicate that PML regulates EC migration, in part, by modulating expression of downstream genes, such as negatively regulating integrin β1 (ITGB1). In addition, knockdown of STAT1 or PML alleviates TNFα- and IFNα-mediated inhibition of ITGB1 expression. Antibody blockade demonstrates that ITGB1 is functionally important for PML- and STAT1-regulated EC migration. Taken together, our data provide novel mechanistic insights that PML functions as a negative regulator in EC network formation and migration.

Interferon-γ enhances promyelocytic leukemia protein expression in acute promyelocytic cells and cooperates with all-trans-retinoic acid to induce maturation of NB4 and NB4-R1 cells

In order to investigate the effect and mechanisms of interferon (IFN)-γ in combination with all-trans-retinoic acid (ATRA) on NB4 cells [ATRA-sensitive acute promyelocytic leukemia (APL) cell line] and NB4-R1 cells (ATRA-resistant APL cell line) and to search for a novel approach to solve the problem of ATRA resistance in APL, we initially treated NB4 and NB4-R1 cells with IFN-γ, ATRA and IFN-γ in combination with ATRA, respectively. The cell proliferation was then tested by MTT assay, and the cell differentiation was tested through light microscopy, by NBT test and flow cytometry (FCM). The expression of promyelocytic leukemia (PML) protein was observed by indirect immune fluorescent test. Results showed that ATRA inhibited the growth of NB4 cells, however, it could not inhibit the growth of NB4-R1 cells. IFN-γ inhibited the growth of both NB4 and NB4-R1 cells. Meanwhile, the growth inhibition effect of IFN-γ in combination with ATRA on both NB4 and NB4-R1 cells was significantly stronger than that of any single drug treatment. The results of the NBT reduction test and CD11b antigen detection by FCM indicated that IFN-γ induces the differentiation of NB4 and NB4-R1 cells to some extent. Moreover, the maturation degree of both NB4 and NB4-R1 cells induced by IFN-γ in combination with ATRA was more significant than that of IFN-γ or ATRA alone. After treatment with IFN-γ, the number of fluorescent particles in NB4 and NB4-R1 cell nuclei was higher than those in the control group, which indicated that IFN-γ may induce the expression of PML protein. Together, IFN-γ augments the proliferation inhibition effect of ATRA on NB4 and NB4-R1 cells through enhancing the expression of PML protein. IFN-γ in combination with ATRA not only strengthens the induction differentiation effect of ATRA on NB4 cells, but also can partially induce the maturation of NB4-R1 cells with ATRA resistance.

JC virus intranuclear inclusions associated with PML-NBs: analysis by electron microscopy and structured illumination microscopy

Progressive multifocal leukoencephalopathy is a fatal demyelinating disorder caused by JC virus infection. JC virus was recently found to target promyelocytic leukemia nuclear bodies (PML-NBs), punctuate domains in the nuclei. Thus, the virus progenies cluster in dots as intranuclear inclusions (ie, as dot-shaped inclusions). In the present study, both the viral major and minor capsid proteins were expressed from polycistronic expression vectors with a powerful promoter, and formation into virus-like particles (VLPs) was examined by electron microscopy. When the upstream regulatory sequence including the agnogene (nt 275 to 490) was present, capsid protein expression was suppressed, but numerous VLPs were efficiently formed with restricted accumulation to PML-NBs. VLPs were uniform, and the cells were severely degraded. In contrast, when the 5' terminus of the agnogene (nt 275 to 409; 135 bp) was deleted, capsid protein expression was markedly enhanced, but VLPs were more randomly produced in the nucleus outside of PML-NBs. VLPs were pleomorphic, and cell degradation was minimal. JC virus association with PML-NBs was confirmed in human brain tissues by structured illumination microscopy. PML-NBs were shaped in spherical shells, with viral capsid proteins circumscribing the surface. These findings indicate that PML-NBs are intranuclear locations for pathogenic JC virus proliferation. Either the agnogene or its product likely supports efficient progeny production at PML-NBs, leading to subsequent degeneration of host glial cells.

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

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

Mitogen-activated protein kinase extracellular signal-regulated kinase 2 phosphorylates and promotes Pin1 protein-dependent promyelocytic leukemia protein turnover

The promyelocytic leukemia (PML) protein is a tumor suppressor that has an important role in several cellular processes, including apoptosis, viral infection, DNA damage repair, cell cycle regulation, and senescence. PML is an essential component of sub-nuclear structures called PML nuclear bodies (NBs). Our laboratory has previously demonstrated that the peptidyl-prolyl cis-trans isomerase, Pin1, binds and targets PML for degradation in a phosphorylation-dependent manner. To further elucidate the mechanisms underlying Pin1-mediated PML degradation, we aimed to identify one or more factors that promote PML phosphorylation. Here we show that treatment with U0126, an inhibitor of the ERK2 upstream kinases MEK1/2, leads to an increase in PML protein accumulation and an inhibition of the interaction between Pin1 and PML in MDA-MB-231 breast cancer cells. Consistent with this observation, phosphorylated ERK2 partially co-localized with PML NBs. Although U0126 up-regulated exogenous wild-type PML levels, it did not have an effect on the steady-state level of a mutant form of PML that is defective in binding Pin1. In addition, exogenous wild-type, but not Pin1 binding-defective PML protein expression levels were decreased by overexpression of ERK2. In contrast, knockdown of ERK2 by siRNA resulted in an increase in PML protein levels and an increase in the formation of PML NBs. Using phospho-specific antibodies, we identified Ser-403 and Ser-505 as the ERK2 targets that promote Pin1-mediated PML degradation. Finally, we demonstrated that EGF induced activation of ERK and interaction between PML and phosphorylated ERK resulting in a decrease in PML protein levels. Taken together, our results support a model in which Pin1 promotes PML degradation in an ERK2-dependent manner.

The herpesvirus associated ubiquitin specific protease, USP7, is a negative regulator of PML proteins and PML nuclear bodies

The PML tumor suppressor is the founding component of the multiprotein nuclear structures known as PML nuclear bodies (PML-NBs), which control several cellular functions including apoptosis and antiviral effects. The ubiquitin specific protease USP7 (also called HAUSP) is known to associate with PML-NBs and to be a tight binding partner of two herpesvirus proteins that disrupt PML NBs. Here we investigated whether USP7 itself regulates PML-NBs. Silencing of USP7 was found to increase the number of PML-NBs, to increase the levels of PML protein and to inhibit PML polyubiquitylation in nasopharyngeal carcinoma cells. This effect of USP7 was independent of p53 as PML loss was observed in p53-null cells. PML-NBs disruption was induced by USP7 overexpression independently of its catalytic activity and was induced by either of the protein interaction domains of USP7, each of which localized to PML-NBs. USP7 also disrupted NBs formed from some single PML isoforms, most notably isoforms I and IV. CK2α and RNF4, which are known regulators of PML, were dispensable for USP7-associated PML-NB disruption. The results are consistent with a novel model of PML regulation where a deubiquitylase disrupts PML-NBs through recruitment of another cellular protein(s) to PML NBs, independently of its catalytic activity.

PML isoforms I and II participate in PML-dependent restriction of HSV-1 replication

Intrinsic antiviral resistance mediated by constitutively expressed cellular proteins is one arm of defence against virus infection. Promyelocytic leukaemia nuclear bodies (PML-NBs, also known as ND10) contribute to host restriction of herpes simplex virus type 1 (HSV-1) replication via mechanisms that are counteracted by viral regulatory protein ICP0. ND10 assembly is dependent on PML, which comprises several different isoforms, and depletion of all PML isoforms decreases cellular resistance to ICP0-null mutant HSV-1. We report that individual expression of PML isoforms I and II partially reverses the increase in ICP0-null mutant HSV-1 plaque formation that occurs in PML-depleted cells. This activity of PML isoform I is dependent on SUMO modification, its SUMO interaction motif (SIM), and each element of its TRIM domain. Detailed analysis revealed that the punctate foci formed by individual PML isoforms differ subtly from normal ND10 in terms of composition and/or Sp100 modification. Surprisingly, deletion of the SIM motif from PML isoform I resulted in increased colocalisation with other major ND10 components in cells lacking endogenous PML. Our observations suggest that complete functionality of PML is dependent on isoform-specific C-terminal sequences acting in concert.

The SUMO protease SENP6 is a direct regulator of PML nuclear bodies

We show that SUMO-specific protease SENP6 can cleave mixed SUMO-1 and SUMO-2/3 chains. Depletion of SENP6 results in accumulation of SUMO-2/3 and SUMO-1 conjugates in promyelocytic leukemia (PML) nuclear bodies. Inactivation of SENP6 results in its accumulation at the SUMO-2/3-rich core of PML nuclear bodies. Biochemical analysis indicates that SUMO-modified PML is a SENP6 substrate.

Promyelocytic leukemia protein (PML) is the core component of PML-nuclear bodies (PML NBs). The small ubiquitin-like modifier (SUMO) system (and, in particular, SUMOylation of PML) is a critical component in the formation and regulation of PML NBs. SUMO protease SENP6 has been shown previously to be specific for SUMO-2/3–modified substrates and shows preference for SUMO polymers. Here, we further investigate the substrate specificity of SENP6 and show that it is also capable of cleaving mixed chains of SUMO-1 and SUMO-2/3. Depletion of SENP6 results in accumulation of endogenous SUMO-2/3 and SUMO-1 conjugates, and immunofluorescence analysis shows accumulation of SUMO and PML in an increased number of PML NBs. Although SENP6 depletion drastically increases the size of PML NBs, the organizational structure of the body is not affected. Mutation of the catalytic cysteine of SENP6 results in its accumulation in PML NBs, and biochemical analysis indicates that SUMO-modified PML is a substrate of SENP6.

Blockade of the ubiquitin protease UBP43 destabilizes transcription factor PML/RARα and inhibits the growth of acute promyelocytic leukemia

More effective treatments for acute promyelocytic leukemia (APL) are needed. APL cell treatment with all-trans-retinoic acid (RA) degrades the chimeric, dominant-negative-acting transcription factor promyelocytic leukemia gene (PML)/RARα, which is generated in APL by chromosomal translocation. The E1-like ubiquitin-activating enzyme (UBE1L) associates with interferon-stimulated gene ISG15 that binds and represses PML/RARα protein. Ubiquitin protease UBP43/USP18 removes ISG15 from conjugated proteins. In this study, we explored how RA regulates UBP43 expression and the effects of UBP43 on PML/RARα stability and APL growth, apoptosis, or differentiation. RA treatment induced UBE1L, ISG15, and UBP43 expression in RA-sensitive but not RA-resistant APL cells. Similar in vivo findings were obtained in a transgenic mouse model of transplantable APL, and in the RA response of leukemic cells harvested directly from APL patients. UBP43 knockdown repressed PML/RARα protein levels and inhibited RA-sensitive or RA-resistant cell growth by destabilizing the PML domain of PML/RARα. This inhibitory effect promoted apoptosis but did not affect the RA differentiation response in these APL cells. In contrast, elevation of UBP43 expression stabilized PML/RARα protein and inhibited apoptosis. Taken together, our findings define the ubiquitin protease UBP43 as a novel candidate drug target for APL treatment.

Progressive multifocal leukoencephalopathy and promyelocytic leukemia nuclear bodies: a review of clinical, neuropathological, and virological aspects of JC virus-induced demyelinating disease

Progressive multifocal leukoencephalopathy is a fatal viral-induced demyelinating disease that was once rare but has become more prevalent today. Over the past decades, much has been learned about the disease from molecular study of the etiological agent of the disease, JC virus. Recently, promyelocytic leukemia nuclear bodies (PML-NBs), punctuate structures for important nuclear functions in eukaryotic cells, were identified as an intranuclear target of JC virus infection. Neuropathologically, JC virus-infected glial cells display diffuse amphophilic viral inclusions by hematoxylin–eosin staining (full inclusions), a diagnostic hallmark of this disease. Recent results using immunohistochemistry, however, revealed the presence of punctate viral inclusions preferentially located along the inner nuclear periphery (dot-shaped inclusions). Dot-shaped inclusions reflect the accumulation of viral progeny at PML-NBs, which may be disrupted after viral replication. Structural changes to PML-NBs have been reported for a variety of human diseases, including cancers and neurodegenerative disorders. Thus, PML-NBs may provide clues to the further pathogenesis of JC virus-induced demyelinating disease. Here, we review what we have learned since the disease entity establishment, including a look at recent progress in understanding the relationship between JC virus, etiology and PML-NBs.

Dysregulation of promyelocytic leukemia (PML) protein expression in preeclamptic placentae

Promyelocytic leukemia (PML) protein is a nucleoprotein that can regulate a variety of cellular stress responses. The aim of this study was to determine qualitative and quantitative changes in PML expression in preeclamptic placentae. Immunoblot, quantitative reverse transcription polymerase chain reaction (qRT-PCR), and immunohistochemistry techniques were used to determine PML gene expression and localization in normal (n = 6) and preeclamptic (n = 6) placentae and primary cells. Promyelocytic leukemia protein was immunolocalized within nuclei of villus mesenchyme, but largely absent in trophoblast nuclei, with a trend for increased PML reactivity in preeclamptic placenta. Immunoblot analyses of nuclear extracts confirmed relative increases (approximately 3-fold) of PML expression in preeclamptic placentae (P < .05). Conversely, less PML messenger RNA (mRNA; approximately 2-fold) was detected in preeclamptic versus normal placental samples. In vitro, PML expression could be increased by hypoxia in cultured endothelial cells but not trophoblast. Increased PML protein expression in preeclamptic villi suggests it could contribute to decreased vascularity and placental growth and/or function.