A Role for PML3 in Centrosome Duplication and Genome Stability

RNF20 Regulates Oocyte Meiotic Spindle Assembly by Recruiting TPM3 to Centromeres and Spindle Poles

Previously a ring finger protein 20 (RNF20) is found to be essential for meiotic recombination and mediates H2B ubiquitination during spermatogenesis. However, its role in meiotic division is still unknown. Here, it is shown that RNF20 is localized at both centromeres and spindle poles, and it is required for oocyte acentrosomal spindle organization and female fertility. RNF20-depleted oocytes exhibit severely abnormal spindle and chromosome misalignment caused by defective bipolar organization. Notably, it is found that the function of RNF20 in spindle assembly is not dependent on its E3 ligase activity. Instead, RNF20 regulates spindle assembly by recruiting tropomyosin3 (TPM3) to both centromeres and spindle poles with its coiled-coil motif. The RNF20-TPM3 interaction is essential for acentrosomal meiotic spindle assembly. Together, the studies uncover a novel function for RNF20 in mediating TPM3 recruitment to both centromeres and spindle poles during oocyte spindle assembly.

Keep Calm and Carry on with Extra Centrosomes

Simple Summary

Precise chromosome segregation during mitosis is a vital event orchestrated by formation of bipolar spindle poles. Supernumerary centrosomes, caused by centrosome amplification, deteriorates mitotic processes, resulting in segregation defects leading to chromosomal instability (CIN). Centrosome amplification is frequently observed in various types of cancer and considered as a significant contributor to destabilization of chromosomes. This review provides a comprehensive overview of causes and consequences of centrosome amplification thoroughly describing molecular mechanisms.

Abstract

Aberrations in the centrosome number and structure can readily be detected at all stages of tumor progression and are considered hallmarks of cancer. Centrosome anomalies are closely linked to chromosome instability and, therefore, are proposed to be one of the driving events of tumor formation and progression. This concept, first posited by Boveri over 100 years ago, has been an area of interest to cancer researchers. We have now begun to understand the processes by which these numerical and structural anomalies may lead to cancer, and vice-versa: how key events that occur during carcinogenesis could lead to amplification of centrosomes. Despite the proliferative advantages that having extra centrosomes may confer, their presence can also lead to loss of essential genetic material as a result of segregational errors and cancer cells must deal with these deadly consequences. Here, we review recent advances in the current literature describing the mechanisms by which cancer cells amplify their centrosomes and the methods they employ to tolerate the presence of these anomalies, focusing particularly on centrosomal clustering.

Acute Promyelocytic Leukemia: A Constellation of Molecular Events around a Single PML-RARA Fusion Gene

Although acute promyelocytic leukemia (APL) is one of the most characterized forms of acute myeloid leukemia (AML), the molecular mechanisms involved in the development and progression of this disease are still a matter of study. APL is defined by the PML-RARA rearrangement as a consequence of the translocation t(15;17)(q24;q21). However, this abnormality alone is not able to trigger the whole leukemic phenotype and secondary cooperating events might contribute to APL pathogenesis. Additional somatic mutations are known to occur recurrently in several genes, such as FLT3, WT1, NRAS and KRAS, whereas mutations in other common AML genes are rarely detected, resulting in a different molecular profile compared to other AML subtypes. How this mutational spectrum, including point mutations in the PML-RARA fusion gene, could contribute to the 10%–15% of relapsed or resistant APL patients is still unknown. Moreover, due to the uncertain impact of additional mutations on prognosis, the identification of the APL-specific genetic lesion is still the only method recommended in the routine evaluation/screening at diagnosis and for minimal residual disease (MRD) assessment. However, the gene expression profile of genes, such as ID1, BAALC, ERG, and KMT2E, once combined with the molecular events, might improve future prognostic models, allowing us to predict clinical outcomes and to categorize APL patients in different risk subsets, as recently reported. In this review, we will focus on the molecular characterization of APL patients at diagnosis, relapse and resistance, in both children and adults. We will also describe different standardized molecular approaches to study MRD, including those recently developed. Finally, we will discuss how novel molecular findings can improve the management of this disease.

E3 Ubiquitin Ligase TRIM Proteins, Cell Cycle and Mitosis

The cell cycle is a series of events by which cellular components are accurately segregated into daughter cells, principally controlled by the oscillating activities of cyclin-dependent kinases (CDKs) and their co-activators. In eukaryotes, DNA replication is confined to a discrete synthesis phase while chromosome segregation occurs during mitosis. During mitosis, the chromosomes are pulled into each of the two daughter cells by the coordination of spindle microtubules, kinetochores, centromeres, and chromatin. These four functional units tie chromosomes to the microtubules, send signals to the cells when the attachment is completed and the division can proceed, and withstand the force generated by pulling the chromosomes to either daughter cell. Protein ubiquitination is a post-translational modification that plays a central role in cellular homeostasis. E3 ubiquitin ligases mediate the transfer of ubiquitin to substrate proteins determining their fate. One of the largest subfamilies of E3 ubiquitin ligases is the family of the tripartite motif (TRIM) proteins, whose dysregulation is associated with a variety of cellular processes and directly involved in human diseases and cancer. In this review we summarize the current knowledge and emerging concepts about TRIMs and their contribution to the correct regulation of cell cycle, describing how TRIMs control the cell cycle transition phases and their involvement in the different functional units of the mitotic process, along with implications in cancer progression.

Promyelocytic leukemia protein (PML) controls breast cancer cell proliferation by modulating Forkhead transcription factors

The multitasking promyelocytic leukemia (PML) protein was originally recognized as a tumor‐suppressive factor, but more recent evidence has implicated PML in tumor cell prosurvival actions and poor patient prognosis in specific cancer settings. Here, we report that inducible PMLIV expression inhibits cell proliferation as well as self‐renewal and impairs cell cycle progression of breast cancer cell lines in a reversible manner. Transcriptomic profiling identified a large number of PML‐deregulated genes associated with various cell processes. Among them, cell cycle‐ and division‐related genes and their cognitive regulators are highly ranked. In this study, we focused on previously unknown PML targets, namely the Forkhead transcription factors. PML suppresses the Forkhead box subclass M1 (FOXM1) transcription factor at both the RNA and protein levels, along with many of its gene targets. We show that FOXM1 interacts with PMLIV primarily via its DNA‐binding domain and dynamically colocalizes in PML nuclear bodies. In parallel, PML modulates the activity of Forkhead box O3 (FOXO3), a factor opposing certain FOXM1 activities, to promote cell survival and stress resistance. Thus, PMLIV affects the balance of FOXO3 and FOXM1 transcriptional programs by acting on discrete gene subsets to favor both growth inhibition and survival. Interestingly, PMLIV‐specific knockdown mimicked ectopic expression vis‐à‐vis loss of proliferative ability and self‐renewal, but also led to loss of survival ability as shown by increased apoptosis. We propose that divergent or similar effects on cell physiology may be elicited by high or low PMLIV levels dictated by other concurrent genetic or epigenetic cancer cell states that may additionally account for its disparate effects in various cancer types.

Effects of the Mammalian Target of Rapamycin Inhibitor Everolimus on Hepatitis C Virus Replication In Vitro and In Vivo

BACKGROUND

The influence of immunosuppressants on hepatitis C virus (HCV) re-infection after liver transplantation, particularly mammalian target of rapamycin inhibitors, remains unclear. The aim of our study was to analyze the influence of everolimus (EVR) on HCV replication activity in the context of underlying molecular mechanisms, with focus on the pro-myelocytic leukemia protein (PML).

METHODS

HCV viral load was recorded in 40 patients with post-transplant HCV re-infection before and 8 weeks after introduction of EVR. An HCV cell culture replicon system for genotype (GT) 1b, GT2b, and GT3a was used to compare the influence of EVR on HCV replication for the respective genotypes in vitro. Fluorescence-activated cell-sorting analysis was used to test for effects on cell proliferation. PML expression was silenced with the use of small hairpin RNA constructs, and PML expression was quantified by means of quantitative real-time polymerase chain reaction.

RESULTS

In patients with HCV, the viral load of GT1a and GT1b was hardly affected by EVR, whereas the viral load was reduced in patients with GT2a (P ≤ .0001) or GT3 infection (P ≤ .05). In vitro EVR impairs HCV replication activity of GT2a and GT3a up to 60% (P ≤ .0005), whereas in GT1b cells, HCV replication activity is increased by 50% (P ≤ .005). Replicon cell viability was not impaired. HCV replication activity is impaired in the absence of PML, which can be reversed by overexpression of PML isoforms. Furthermore, in the absence of PML, the effect of EVR on HCV replication activity is nearly abrogated.

CONCLUSIONS

The mammalian target of rapamycin inhibitor EVR influences HCV replication via PML. The herein presented results suggest a genotype-dependent benefit for an EVR-based immunosuppressive regimen in patients with GT2a or GT3 re-infection after liver transplantation.

Functional Significance of Aurora Kinases-p53 Protein Family Interactions in Cancer

Aurora kinases play critical roles in regulating spindle assembly, chromosome segregation, and cytokinesis to ensure faithful segregation of chromosomes during mitotic cell division cycle. Molecular and cell biological studies have revealed that Aurora kinases, at physiological levels, orchestrate complex sequential cellular processes at distinct subcellular locations through functional interactions with its various substrates. Aberrant expression of Aurora kinases, on the other hand, cause defects in mitotic spindle assembly, checkpoint response activation, and chromosome segregation leading to chromosomal instability. Elevated expression of Aurora kinases correlating with chromosomal instability is frequently detected in human cancers. Recent genomic profiling of about 3000 human cancer tissue specimens to identify various oncogenic signatures in The Cancer Genome Atlas project has reported that recurrent amplification and overexpression of Aurora kinase-A characterize distinct subsets of human tumors across multiple cancer types. Besides the well-characterized canonical pathway interactions of Aurora kinases in regulating assembly of the mitotic apparatus and chromosome segregation, growing evidence also supports the notion that deregulated expression of Aurora kinases in non-canonical pathways drive transformation and genomic instability by antagonizing tumor suppressor and exacerbating oncogenic signaling through direct interactions with critical proteins. Aberrant expression of the Aurora kinases–p53 protein family signaling axes appears to be critical in the abrogation of p53 protein family mediated tumor suppressor pathways frequently deregulated during oncogenic transformation process. Recent findings reveal the existence of feedback regulatory loops in mRNA expression and protein stability of these protein families and their consequences on downstream effectors involved in diverse physiological functions, such as mitotic progression, checkpoint response pathways, as well as self-renewal and pluripotency in embryonic stem cells. While these investigations have focused on the functional consequences of Aurora kinase protein family interactions with wild-type p53 family proteins, those involving Aurora kinases and mutant p53 remain to be elucidated. This article presents a comprehensive review of studies on Aurora kinases–p53 protein family interactions along with a prospective view on the possible functional consequences of Aurora kinase–mutant p53 signaling pathways in tumor cells. Additionally, we also discuss therapeutic implications of these findings in Aurora kinases overexpressing subsets of human tumors.

Promyelocytic leukemia protein isoform II inhibits infection by human adenovirus type 5 through effects on HSP70 and the interferon response

Promyelocytic leukemia (PML) proteins have been implicated in antiviral responses but PML and associated proteins are also suggested to support virus replication. One isoform, PML-II, is required for efficient transcription of interferon and interferon-responsive genes. We therefore investigated the PML-II contribution to human adenovirus 5 (Ad5) infection, using shRNA-mediated knockdown. HelaΔII cells showed a 2-3-fold elevation in Ad5 yield, reflecting an increase in late gene expression. This increase was found to be due in part to the reduced innate immune response consequent upon PML-II depletion. However, the effect was minor because the viral E4 Orf3 protein targets and inactivates this PML-II function. The major benefit to Ad5 in HelaΔII cells was exerted via an increase in HSP70; depletion of HSP70 completely reversed this replicative advantage. Increased Ad5 late gene expression was not due either to the previously described inhibition of inflammatory responses by HSP70 or to effects of HSP70 on major late promoter or L4 promoter activity, but might be linked to an observed increase in E1B 55K, as this protein is known to be required for efficient late gene expression. The induction of HSP70 by PML-II removal was specific for the HSPA1B gene among the HSP70 gene family and thus was not the consequence of a general stress response. Taken together, these data show that PML-II, through its various actions, has an overall negative effect on the Ad5 lifecycle.

PML isoform II plays a critical role in nuclear lipid droplet formation

PML-II plays a critical role in generating nuclear lipid droplets, which are associated with promyelocytic leukemia nuclear bodies as well as with the extension of the inner nuclear membrane.

Lipid droplets (LDs) in the nucleus of hepatocyte-derived cell lines were found to be associated with premyelocytic leukemia (PML) nuclear bodies (NBs) and type I nucleoplasmic reticulum (NR) or the extension of the inner nuclear membrane. Knockdown of PML isoform II (PML-II) caused a significant decrease in both nuclear LDs and type I NR, whereas overexpression of PML-II increased both. Notably, these effects were evident only in limited types of cells, in which a moderate number of nuclear LDs exist intrinsically, and PML-II was targeted not only at PML NBs, but also at the nuclear envelope, excluding lamins and SUN proteins. Knockdown of SUN proteins induced a significant increase in the type I NR and nuclear LDs, but these effects were cancelled by simultaneous knockdown of PML-II. Nuclear LDs harbored diacylglycerol O-acyltransferase 2 and CTP:phosphocholine cytidylyltransferase α and incorporated newly synthesized lipid esters. These results corroborated that PML-II plays a critical role in generating nuclear LDs in specific cell types.

Induction of centrosome amplification by formaldehyde, but not hydroquinone, in human lymphoblastoid TK6 cells

Benzene and formaldehyde (FA) are important industrial chemicals and environmental pollutants that cause leukemia by inducing DNA damage and chromosome aberrations in hematopoietic stem cells (HSC), the target cells for leukemia. Our previous studies showed that workers exposed to benzene and FA exhibit increased levels of aneuploidy in their blood cells. As centrosome amplification is a common phenomenon in human cancers, including leukemia, and is associated with aneuploidy in carcinogenesis, we hypothesized that benzene and FA would induce centrosome amplification in vitro. We treated human lymphoblastoid TK6 cells with a range of concentrations of hydroquinone (HQ, a benzene metabolite) or FA for 24 h, allowed the cells to recover in fresh medium for 24 h, and examined centrosome amplification; chromosomal gain, loss, and breakage; and cytotoxicity. We included melphalan and etoposide, chemotherapeutic drugs that cause therapy-related acute myeloid leukemia and that have been shown to induce centrosome amplification as well as chromosomal aneuploidy and breakage, as positive controls. Melphalan and etoposide induced centrosome amplification and chromosome gain and breakage in a dose-dependent manner, at cytotoxic concentrations. HQ, though cytotoxic, did not induce centrosome amplification or any chromosomal aberration. FA-induced centrosome amplification and cytotoxicity, but did not induce chromosomal aberrations. Our data suggest, for the first time, that centrosome amplification is a potential mechanism underlying FA-induced leukemogenesis, but not benzene-induced leukemogenesis, as mediated through HQ. Future studies are needed to delineate the mechanisms of centrosome amplification and its association with DNA damage, chromosomal aneuploidy and carcinogenesis, following exposure to FA.

Promyelocytic Leukemia Protein Isoform II Promotes Transcription Factor Recruitment To Activate Interferon Beta and Interferon-Responsive Gene Expression

To trigger type I interferon (IFN) responses, pattern recognition receptors activate signaling cascades that lead to transcription of IFN and IFN-stimulated genes (ISGs). The promyelocytic leukemia (PML) protein has been implicated in these responses, although its role has not been defined. Here, we show that PML isoform II (PML-II) is specifically required for efficient induction of IFN-β transcription and of numerous ISGs, acting at the point of transcriptional complex assembly on target gene promoters. PML-II associated with specific transcription factors NF-κB and STAT1, as well as the coactivator CREB-binding protein (CBP), to facilitate transcriptional complex formation. The absence of PML-II substantially reduced binding of these factors and IFN regulatory factor 3 (IRF3) to IFN-β or ISGs promoters and sharply reduced gene activation. The unique C-terminal domain of PML-II was essential for its activity, while the N-terminal RBCC motif common to all PML isoforms was dispensable. We propose a model in which PML-II contributes to the transcription of multiple genes via the association of its C-terminal domain with relevant transcription complexes, which promotes the stable assembly of these complexes at promoters/enhancers of target genes, and that in this way PML-II plays a significant role in the development of type I IFN responses.

Inhibition of Sp1 functions by its sequestration into PML nuclear bodies

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

Liver kinase B1 regulates the centrosome via PLK1

Liver kinase B1 (LKB1) is a tumor suppressor mutationally inactivated in Peutz-Jeghers syndrome (PJS) and various sporadic cancers. Although LKB1 encodes a kinase that possesses multiple functions, no individual hypothesis posed to date has convincingly explained how loss of LKB1 contributes to carcinogenesis. In this report we demonstrated that LKB1 maintains genomic stability through the regulation of centrosome duplication. We found that LKB1 colocalized with centrosomal proteins and was situated in the mitotic spindle pole. LKB1 deficiency-induced centrosome amplification was independent of AMP-activated protein kinase (AMPK), a well-defined substrate of LKB1. Cells lacking LKB1 exhibited an increase in phosphorylated and total Polo-like kinase 1 (PLK-1), NIMA-related kinase 2 (NEK2), and ninein-like protein (NLP). Overexpression of active PLK1 (T210D) reversed the inhibition of LKB1 on centrosome amplification. In contrast, depletion of PLK1 with siRNA or suppression of PLK1 kinase activity with BTO-1 (5-Cyano-7-nitro-2-benzothiazolecarboxamide-3-oxide) abrogated LKB1 deficiency-induced centrosome amplification. We further characterized that LKB1 phosphorylated and activated AMPK-related kinase 5 (NUAK1 or ARK5) that in turn increased the phosphorylation of MYPT1, enhanced the binding between MYPT1-PP1 and PLK1, and conferred an effective dephosphorylation of PLK1. More importantly, we noted that LKB1-deficient cells exhibited multiple nuclear abnormalities, such as mitotic delay, binuclear, polylobed, grape, large, and micronuclear. Immediate depletion of LKB1 resulted in the accumulation of multiploidy cells. Expression of LKB1 is reversely correlated with the levels of PLK1 in human cancer tissues. Thus, we have uncovered a novel function of LKB1 in the maintenance of genomic stability through the regulation of centrosome mediated by PLK1.

Promyelocytic leukemia bodies tether to early endosomes during mitosis

During mitosis the nuclear envelope breaks down, leading to potential interactions between cytoplasmic and nuclear components. PML bodies are nuclear structures with tumor suppressor and antiviral functions. Early endosomes, on the other hand, are cytoplasmic vesicles involved in transport and growth factor signaling. Here we demonstrate that PML bodies form stable interactions with early endosomes immediately following entry into mitosis. The 2 compartments remain stably associated throughout mitosis and dissociate in the cytoplasm of newly divided daughter cells. We also show that a minor subset of PML bodies becomes anchored to the mitotic spindle poles during cell division. The study demonstrates a stable mitosis-specific interaction between a cytoplasmic and a nuclear compartment.

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

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

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

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

Differential Roles of PML Isoforms

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

Contribution of the C-terminal regions of promyelocytic leukemia protein (PML) isoforms II and V to PML nuclear body formation

Promyelocytic leukemia protein (PML) nuclear bodies are dynamic and heterogeneous nuclear protein complexes implicated in various important functions, most notably tumor suppression. PML is the structural component of PML nuclear bodies and has several nuclear splice isoforms that share a common N-terminal region but differ in their C termini. Previous studies have suggested that the coiled-coil motif within the N-terminal region is sufficient for PML nuclear body formation by mediating homo/multi-dimerization of PML molecules. However, it has not been investigated whether any of the C-terminal variants of PML may contribute to PML body assembly. Here we report that the unique C-terminal domains of PML-II and PML-V can target to PML-NBs independent of their N-terminal region. Strikingly, both domains can form nuclear bodies in the absence of endogenous PML. The C-terminal domain of PML-II interacts transiently with unknown binding sites at PML nuclear bodies, whereas the C-terminal domain of PML-V exhibits hyperstable binding to PML bodies via homo-dimerization. This strong interaction is mediated by a putative α-helix in the C terminus of PML-V. Moreover, nuclear bodies assembled from the C-terminal domain of PML-V also recruit additional PML body components, including Daxx and Sp100. These observations establish the C-terminal domain of PML-V as an additional important contributor to the assembly mechanism(s) of PML bodies.

PML regulates PER2 nuclear localization and circadian function

Studies have suggested that the clock regulator PER2 is a tumour suppressor. A cancer network involving PER2 raises the possibility that some tumour suppressors are directly involved in the mammalian clock. Here, we show that the tumour suppressor promyelocytic leukaemia (PML) protein is a circadian clock regulator and can physically interact with PER2. In the suprachiasmatic nucleus (SCN), PML expression and PML-PER2 interaction are under clock control. Loss of PML disrupts and dampens the expression of clock regulators Per2, Per1, Cry1, Bmal1 and Npas2. In the presence of PML and PER2, BMAL1/CLOCK-mediated transcription is enhanced. In Pml(-/-) SCN and mouse embryo fibroblast cells, the cellular distribution of PER2 is primarily perinuclear/cytoplasmic. PML is acetylated at K487 and its deacetylation by SIRT1 promotes PML control of PER2 nuclear localization. The circadian period of Pml(-/-) mice displays reduced precision and stability consistent with PML having a role in the mammalian clock mechanism.

PML nuclear bodies and other TRIM-defined subcellular compartments

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

Different expression of alternative lengthening of telomere (ALT)-associated proteins/mRNAs in osteosarcoma cell lines

Tumors, including osteosarcoma (OS), are capable of evading senescence and cell death, which is caused by telomere loss with cell division. Alternative lengthening of telomeres (ALT) is considered as the main telomere maintenance mechanism in OS. In this study, we investigated the expression of ALT-associated proteins and mRNAs in human OS cell lines. Western blotting was used to detect the protein expression in OS cell lines, while the expression of mRNA was determined by reverse-transcriptase PCR and quantitative real-time PCR analysis. Whole-genome expression arrays were used to analyze the expression of all the mRNAs involved in telomere maintenance mechanisms (TMMs) including human telomerase reverse transcriptase, promyelocytic leukemia proteins and other related proteins. OS and normal cell lines do not express telomerase reverse transcriptase (hTERT) as a key subunit of telomerase, although they show varying levels of ALT-associated proteins and mRNAs such as PML, Rad52, MRE11 and FEN1 by Western blotting and quantitative real-time PCR analysis. A number of mRNAs that play essential roles in ALT are expressed more in OS cell lines than in the osteoblast cell line, as shown by whole-genome expression arrays. In conclusion, OS cell lines maintain their telomere length primarily through the ALT mechanism. There are numerous other proteins that regulate this process in OS; therefore, anti-ALT therapy may be a more effective method to treat OS than anti-telomerase therapy.

Subcellular distribution of nuclear import-defective isoforms of the promyelocytic leukemia protein

Background

The promyelocytic leukemia (PML) protein participates in a number of cellular processes, including transcription regulation, apoptosis, differentiation, virus defense and genome maintenance. This protein is structurally organized into a tripartite motif (TRIM) at its N-terminus, a nuclear localization signal (NLS) at its central region and a C-terminus that varies between alternatively spliced isoforms. Most PML splice variants target the nucleus where they define sub-nuclear compartments termed PML nuclear bodies (PML NBs). However, PML variants that lack the NLS are also expressed, suggesting the existence of PML isoforms with cytoplasmic functions. In the present study we expressed PML isoforms with a mutated NLS in U2OS cells to identify potential cytoplasmic compartments targeted by this protein.

Results

Expression of NLS mutated PML isoforms in U2OS cells revealed that PML I targets early endosomes, PML II targets the inner nuclear membrane (partially due to an extra NLS at its C-terminus), and PML III, IV and V target late endosomes/lysosomes. Clustering of PML at all of these subcellular locations depended on a functional TRIM domain.

Conclusions

This study demonstrates the capacity of PML to form macromolecular protein assemblies at several different subcellular sites. Further, it emphasizes a role of the variable C-terminus in subcellular target selection and a general role of the N-terminal TRIM domain in promoting protein clustering.

Formation of extra centrosomal structures is dependent on beta-catenin

beta-Catenin has important roles in cell-cell adhesion and in the regulation of gene transcription. Mutations that stabilize beta-catenin are common in cancer, but it remains unclear how these mutations contribute to cancer progression. beta-Catenin is also a centrosomal component involved in centrosome separation. Centrosomes nucleate interphase microtubules and the bipolar mitotic spindle in normal cells, but their organization and function in human cancers are abnormal. Here, we show that expression of stabilized mutant beta-catenin, which mimics mutations found in cancer, results in extra non-microtubule nucleating structures that contain a subset of centrosome proteins including gamma-tubulin and centrin, but not polo-like kinase 4 (Plk4), SAS-6 or pericentrin. A transcriptionally inactive form of beta-catenin also gives rise to abnormal structures of centrosome proteins. HCT116 human colon cancer cell lines, from which the mutant beta-catenin allele has been deleted, have reduced numbers of cells with abnormal centrosome structures and S-phase-arrested, amplified centrosomes. RNAi-mediated depletion of beta-catenin from centrosomes inhibits S-phase-arrested amplification of centrosomes. These results indicate that beta-catenin is required for centrosome amplification, and mutations in beta-catenin might contribute to the formation of abnormal centrosomes observed in cancers.

PML3 interacts with TRF1 and is essential for ALT-associated PML bodies assembly in U2OS cells

Telomerase-negative cancer cells maintain their telomeres by a mechanism known as alternative lengthening of telomeres (ALT) and achieve unlimited replicative potential. A hallmark of ALT cells is the recruitment of telomeres to promyelocytic leukemia (PML) bodies and formation of ALT-associated PML bodies (APBs). Although the exact molecular mechanism of APBs assembly remains unclear, APBs assembly requires telomere and PML body-associated proteins, including TRF1 and PML. Here, we report that PML3, one of PML isoforms, is involved in APBs formation. As a new binding protein of TRF1 (telomeric repeat binding factor 1), PML3 directly interacts with TRF1 and recruits TRF1 to PML bodies in U2OS cells. More notably, depletion of PML3 by small interfering RNA does not affect PML bodies formation, but inhibits the recruitment of both TRF1 and TRF2 to APBs. Further study shows that the recruitment of TRF1 to APBs depends on its interaction with a specific PML3 isoform. Thus, the interaction of PML3 with TRF1 is isoform specific and likely to be essential for APBs assembly in U2OS cells.

Cell-cycle regulation and dynamics of cytoplasmic compartments containing the promyelocytic leukemia protein and nucleoporins

Nucleoporins and the promyelocytic leukemia protein (PML) represent structural entities of nuclear pore complexes and PML nuclear bodies, respectively. In addition, these proteins might function in a common biological mechanism, because at least two different nucleoporins, Nup98 and Nup214, as well as PML, can become aberrantly expressed as oncogenic fusion proteins in acute myeloid leukemia (AML) cells. Here we show that PML and nucleoporins become directed to common cytoplasmic compartments during the mitosis-to-G1 transition of the cell cycle. These protein assemblies, which we have termed CyPNs (cytoplasmic assemblies of PML and nucleoporins), move on the microtubular network and become stably connected to the nuclear membrane once contact with the nucleus has been made. The ability of PML to target CyPNs depends on its nuclear localization signal, and loss of PML causes an increase in cytoplasmic-bound versus nuclear-membrane-bound nucleoporins. CyPNs are also targeted by the acute promyelocytic leukemia (APL) fusion protein PML-RARalpha and can be readily detected within the APL cell line NB4. These results provide insight into a dynamic pool of cytoplasmic nucleoporins that form a complex with the tumor suppressor protein PML during the G1 phase of the cell cycle.

PML3 Orchestrates the Nuclear Dynamics and Function of TIP60

The promyelocytic leukemia (PML) protein is a major component to govern the PML nuclear body (NB) assembly and function. Although it is well defined that PML NB is a site recruiting sumoylated proteins, the mechanism by which PML protein regulates the process remains unclear. Here we show that PML3, a specific PML isoform, interacts with and recruits TIP60 to PML NBs. Our biochemical characterization demonstrates that PML3 physically interacts with TIP60 via its N-terminal 364 amino acids. Importantly, this portion of TIP60 is sufficient to target to the PML NBs, suggesting that PML3-TIP60 interaction is sufficient for targeting TIP60 to the NBs. The PML3-TIP60 interaction is specific, since the region of TIP60 binding is not conserved in other PML isoforms. The physical interaction between PML3 and TIP60 protects TIP60 from Mdm2-mediated degradation, suggesting that PML3 competes with MDM2 for binding to TIP60. Fluorescence recovery after photobleaching analysis indicates that the PML3-TIP60 interaction modulates the nuclear body distribution and mobility of TIP60. Conversely, the distribution and mobility of TIP60 are perturbed in PML3-deficient cells, accompanied by aberrations in DNA damage-repairing response. Thus, PML3 orchestrates the distribution, dynamics, and function of TIP60. Our findings suggest a novel regulatory mechanism by which the PML3 and TIP60 tumor suppressors cooperate to ensure genomic stability.

Dynamics of component exchange at PML nuclear bodies

PML nuclear bodies (NBs) are involved in the regulation of key nuclear pathways but their biochemical function in nuclear metabolism is unknown. In this study PML NB assembly dynamics were assessed by live cell imaging and mathematic modeling of its major component parts. We show that all six nuclear PML isoforms exhibit individual exchange rates at NBs and identify PML V as a scaffold subunit. SP100 exchanges at least five times faster at NBs than PML proteins. Turnover dynamics of PML and SP100 at NBs is modulated by SUMOylation. Exchange is not temperature-dependent but depletion of cellular ATP levels induces protein immobilization at NBs. The PML-RARalpha oncogene exhibits a strong NB retention effect on wild-type PML proteins. HIPK2 requires an active kinase for PML NB targeting and elevated levels of PML IV increase its residence time. DAXX and BLM turn over rapidly and completely at PML NBs within seconds. These findings provide a kinetics model for factor exchange at PML NBs and highlight potential mechanisms to regulate intranuclear trafficking of specific factors at these domains.

New insights into the role of PML in tumour suppression

The PML gene is involved in the t(15;17) translocation of acute promyelocytic leukaemia (APL), which generates the oncogenic fusion protein PML (promyelocytic leukaemia protein)-retinoic acid receptor alpha. The PML protein localises to a subnuclear structure called the PML nuclear domain (PML-ND), of which PML is the essential structural component. In APL, PML-NDs are disrupted, thus implicating these structures in the pathogenesis of this leukaemia. Unexpectedly, recent studies indicate that PML and the PML-ND play a tumour suppressive role in several different types of human neoplasms in addition to APL. Because of PML's extreme versatility and involvement in multiple cellular pathways, understanding the mechanisms underlying its function, and therefore role in tumour suppression, has been a challenging task. In this review, we attempt to critically appraise the more recent advances in this field and propose new avenues of investigation.

A role for cytoplasmic PML in cellular resistance to viral infection

PML gene was discovered as a fusion partner with retinoic acid receptor (RAR) α in the t(15:17) chromosomal translocation associated with acute promyelocytic leukemia (APL). Nuclear PML protein has been implicated in cell growth, tumor suppression, apoptosis, transcriptional regulation, chromatin remodeling, DNA repair, and anti-viral defense. The localization pattern of promyelocytic leukemia (PML) protein is drastically altered during viral infection. This alteration is traditionally viewed as a viral strategy to promote viral replication. Although multiple PML splice variants exist, we demonstrate that the ratio of a subset of cytoplasmic PML isoforms lacking exons 5 & 6 is enriched in cells exposed to herpes simplex virus-1 (HSV-1). In particular, we demonstrate that a PML isoform lacking exons 5 & 6, called PML Ib, mediates the intrinsic cellular defense against HSV-1 via the cytoplasmic sequestration of the infected cell protein (ICP) 0 of HSV-1. The results herein highlight the importance of cytoplasmic PML and call for an alternative, although not necessarily exclusive, interpretation regarding the redistribution of PML that is seen in virally infected cells.

Regulation of microtubule assembly and organization in mitosis by the AAA+ ATPase Pontin

To identify novel proteins important for microtubule assembly in mitosis, we have used a centrosome-based complementation assay to enrich for proteins with mitotic functions. An RNA interference (RNAi)-based screen of these proteins allowed us to uncover 13 novel mitotic regulators. We carried out in-depth analyses of one of these proteins, Pontin, which is known to have several functions in interphase, including chromatin remodeling, DNA repair, and transcription. We show that reduction of Pontin by RNAi resulted in defects in spindle assembly in Drosophila S2 cells and in several mammalian tissue culture cell lines. Further characterization of Pontin in Xenopus egg extracts demonstrates that Pontin interacts with the gamma tubulin ring complex (gamma-TuRC). Because depletion of Pontin leads to defects in the assembly and organization of microtubule arrays in egg extracts, our studies suggest that Pontin has a mitosis-specific function in regulating microtubule assembly.

Structure, dynamics and functions of promyelocytic leukaemia nuclear bodies

The promyelocytic leukaemia (PML) tumour suppressor protein epitomizes the PML-nuclear body (PML-NB) and is crucially required for the proper assembly of this macromolecular nuclear structure. Unlike other, more specialized subnuclear structures such as Cajal and Polycomb group bodies, PML-NBs are functionally promiscuous and have been implicated in the regulation of diverse cellular functions. PML-NBs are dynamic structures that favour the sequestration and release of proteins, mediate their post-translational modifications and promote specific nuclear events in response to various cellular stresses. Recent data suggest that PML-NBs may be heterogeneous in composition, mobility and function.

Lats2 is an essential mitotic regulator required for the coordination of cell division

Tumor suppressor Lats2 is a member of the conserved Dbf2 kinase family. It localizes to the centrosome and has been implicated in regulation of the cell cycle and apoptosis. However, the in vivo function of this kinase remains unclear. Here, we show that complete disruption of the gene encoding Lats2 in mice causes developmental defects in the nervous system and embryonic lethality. Furthermore, mutant cells derived from total LATS2-knock-out embryos exhibit mitotic defects including centrosome fragmentation and cytokinesis defects, followed by nuclear enlargement and multinucleation. We show that the Mob1 family, a regulator of mitotic exit, associates with Lats2 to induce its activation. We also show that the complete LATS2-knock-out cells exhibit an acceleration of exit from mitosis and marked down-regulation of critical mitotic regulators. These results suggest that Lats2 plays an essential mitotic role in coordinating accurate cytokinesis completion, governing the stabilization of other mitotic regulators.

Interferon regulatory factor-8 is indispensable for the expression of promyelocytic leukemia and the formation of nuclear bodies in myeloid cells

Interferon (IFN) regulatory factor-8 (IRF-8), previously known as ICSBP, is a myeloid cell essential transcription factor. Mice with null mutation in IRF-8 are defective in the ability of myeloid progenitor cells to mature toward macrophage lineage. Accordingly, these mice develop chronic myelogenous leukemia (CML). We demonstrate here that IRF-8 is an obligatory regulator of the promyelocytic leukemia (PML) gene in activated macrophages, leading to the expression of the PML-I isoform. This regulation is most effective together with two other transcription factors, IRF-1 and PU.1. PML is a tumor suppressor gene that serves as a scaffold protein for nuclear bodies. IRF-8 is not only essential for the IFN-gamma-induced expression of PML in activated macrophages but also for the formation of nuclear bodies. Reduced IRF-8 transcript levels were reported in CML patients, and a recovery to normal levels was observed in patients in remission following treatment with IFN-alpha. We demonstrate a significant correlation between the levels of IRF-8 and PML in these CML patients. Together, our results indicate that some of the myeloleukemia suppressor activities of IRF-8 are mediated through the regulation of PML. When IRF-8 levels are compromised, the reduced PML expression may lead to genome instability and eventually to the leukemic phenotype.

Characterization of endogenous human promyelocytic leukemia isoforms

Promyelocytic leukemia (PML) has been implicated in a variety of functions, including control of TP53 function and modulation of cellular senescence. Sumolated PML is the organizer of mature PML bodies, recruiting a variety of proteins onto these nuclear domains. The PML gene is predicted to encode a variety of protein isoforms. Overexpression of only one of them, PML-IV, promotes senescence in human diploid fibroblasts, whereas PML-III was proposed to specifically interact with the centrosome. We show that all PML isoform proteins are expressed in cell lines or primary cells. Unexpectedly, we found that PML-III, PML-IV, and PML-V are quantitatively minor isoforms compared with PML-I/II and could not confirm the centrosomal targeting of PML-III. Stable expression of each isoform, in a pml-null background, yields distinct subcellular localization patterns, suggesting that, like in other RBCC/TRIM proteins, the COOH-terminal domains of PML are involved in interactions with specific cellular components. Only the isoform-specific sequences of PML-I and PML-V are highly conserved between man and mouse. That PML-I contains all conserved exons and is more abundantly expressed than PML-IV suggests that it is a critical contributor to PML function(s).

Promyelocytic leukemia activates Chk2 by mediating Chk2 autophosphorylation

Chk2 is a kinase critical for DNA damage-induced apoptosis and is considered a tumor suppressor. Chk2 is essential for p53 transcriptional and apoptotic activities. Although mutations of p53 are present in more than half of all tumors, mutations of Chk2 in cancers are rare, suggesting that Chk2 may be inactivated by unknown alternative mechanisms. Here we elucidate one such alternative mechanism regulated by PML (promyelocytic leukemia) that is involved in acute promyelocytic leukemia (APL). Although p53-inactivating mutations are extremely rare in APL, t(15;17) chromosomal translocation which fuses retinoic acid receptor (RARalpha) to PML is almost always present in APL, while the other PML allele is intact. We demonstrate that PML interacts with Chk2 and activates Chk2 by mediating its autophosphorylation step, an essential step for Chk2 activity that occurs after phosphorylation by the upstream kinase ATM (ataxia telangiectasia-mutated). PML/RARalpha in APL suppresses Chk2 by dominantly inhibiting the auto-phosphorylation step, but inactivation of PML/RARalpha with alltrans retinoic acid (ATRA) restores Chk2 autophosphorylation and activity. Thus, by fusing PML with RARalpha, the APL cells appear to have achieved functional suppression of Chk2 compromising the Chk2-p53 apoptotic pathway.

Suppression of centrosome amplification after DNA damage depends on p27 accumulation

The centrosome plays a fundamental role in cell division, cell polarity, and cell cycle progression. Centrosome duplication is mainly controlled by cyclin-dependent kinase 2 (CDK2)/cyclin E and cyclin A complexes, which are inhibited by the CDK inhibitors p21Cip1 and p27Kip1. It is thought that abnormal activation of CDK2 induces centrosome amplification that is frequently observed in a wide range of aggressive tumors. We previously reported that overexpression of the oncogene MYCN leads to centrosome amplification after DNA damage in neuroblastoma cells. We here show that centrosome amplification after gamma-irradiation was caused by suppression of p27 expression in MYCN-overexpressing cells. We further show that p27-/- and p27+/- mouse embryonic fibroblasts and p27-silenced human cells exhibited a significant increase in centrosome amplification after DNA damage. Moreover, abnormal mitotic cells with amplified centrosomes were frequently observed in p27-silenced cells. In response to DNA damage, the level of p27 gradually increased in normal cells independently of the ataxia telangiectasia mutated/p53 pathway, whereas Skp2, an F-box protein component of an SCF ubiquitin ligase complex that targets p27, was reduced. Additionally, p27 levels in MYCN-overexpressing cells were restored by treatment with Skp2 small interfering RNA, indicating that down-regulation of p27 by MYCN was due to high expression of Skp2. These results suggest that the accumulation of p27 after DNA damage is required for suppression of centrosome amplification, thereby preventing chromosomal instability.

Interaction of the adenovirus type 5 E4 Orf3 protein with promyelocytic leukemia protein isoform II is required for ND10 disruption

Nuclear domain 10 (ND10s), or promyelocytic leukemia protein (PML) nuclear bodies, are spherical nuclear structures that require PML proteins for their formation. Many viruses target these structures during infection. The E4 Orf3 protein of adenovirus 5 (Ad5) rearranges ND10s, causing PML to colocalize with Orf3 in nuclear tracks or fibers. There are six different PML isoforms (I to VI) present at ND10s, all sharing a common N terminus but with structural differences at their C termini. In this study, PML II was the only one of these six isoforms that was found to interact directly and specifically with Ad5 E4 Orf3 in vitro and in vivo; these results define a new Orf3 activity. Three of a series of 18 mutant Orf3 proteins were unable to interact with PML II; these were also unable to cause ND10 rearrangement. Moreover, in PML-null cells that contained neoformed ND10s comprising a single PML isoform, only ND10s formed of PML II were rearranged by Orf3. These data show that the interaction between Orf3 and PML II is necessary for ND10 rearrangement to occur. Finally, Orf3 was shown to self-associate in vitro. This activity was absent in mutant Orf3 proteins that were unable to form tracks and to bind PML II. Thus, Orf3 oligomerization may mediate the formation of nuclear tracks in vivo and may also be important for PML II binding.

Myeloproliferative disorders: the centrosome connection

Some myeloproliferative disorders (MPD) result from a reciprocal translocation that involves the FGFR1 gene and a partner gene. The event creates a chimeric gene that encodes a fusion protein with constitutive FGFR1 tyrosine kinase activity. FGFR1-MPD is a rare disease, but its study may provide interesting clues on different processes such as cell signalling, oncogenesis and stem cell renewal. Some partners of FGFR1 are centrosomal proteins. The corresponding oncogenic fusion kinases are targeted to the centrosome. Constitutive phosphorylation at this site may perturbate centrosome function and the cell cycle. Direct attack at this small organelle may be an efficient way for oncogenes to alter regulation of signalling for proliferation and survival and get rid of checkpoints in cell cycle progression. The same effect might be triggered by other fusion kinases in other MPD and non-MPD malignancies.