The t(15;17) translocation alters a nuclear body in a retinoic acid-reversible fashion

Heterogeneous nuclear expression of the promyelocytic leukemia (PML) protein in normal and neoplastic human tissues

The RING-finger promyelocytic leukemia (PML) protein is the product of the PML gene that fuses with the retinoic acid receptor-alpha gene in the t(15; 17) translocation of acute promyelocytic leukemia. Wild-type PML localizes in the nucleus with a typical speckled pattern that is a consequence of the concentration of the protein within discrete subnuclear domains known as nuclear bodies. Delocalization of PML from nuclear bodies has been documented in acute promyelocytic leukemia cells and suggested to contribute to leukemogenesis. In an attempt to get new insights into the function of the wild-type PML protein and to investigate whether it displays an altered expression pattern in neoplasms other than acute promyelocytic leukemia, we stained a large number of normal and neoplastic human tissues with a new murine monoclonal antibody (PG-M3) directed against the amino-terminal region of PML. As the PG-M3 epitope is partially resistant to fixatives, only cells that overexpress PML are detected by the antibody in microwave-heated paraffin sections. Among normal tissues, PML was characteristically up-regulated in activated epithelioid histiocytes and fibroblasts in a variety of pathological conditions, columnar epithelium in small active thyroid follicles, well differentiated foamy cells in the center of sebaceous glands, and hypersecretory endometria (Arias-Stella). Interferons, the PML of which is a primary target gene, and estrogens are likely to represent some of the cytokines and/or hormones that may be involved in the up-regulation of PML under these circumstances. In keeping with this concept, we found that PML is frequently overexpressed in Hodgkin and Reed-Sternberg cells of Hodgkin's disease, a tumor of cytokine-producing cells. Among solid tumors, overexpression of PML was frequently found in carcinomas of larynx and thyroid (papillary), epithelial thymomas, and Kaposi's sarcoma, whereas carcinomas of the lung, thyroid (follicular), breast, and colon were frequently negative or weakly PML+. We did not observe any changes in the levels of PML expression as the lesion progressed from benign dysplasia to carcinoma. Our immunohistological data are consistent with the hypothesized growth suppressor function of PML and strongly suggest that PML expression levels are likely to be modulated by a variety of stimuli, including cytokines and hormones.

Identification and characterization of a leukocyte-specific component of the nuclear body

The nuclear body (NB) is a cellular organelle that is involved in the pathogenesis of acute promyelocytic leukemia and viral infection. The NB is also a target of antibodies in the serum of patients with the autoimmune disease primary biliary cirrhosis. In this study, serum from a patient with primary biliary cirrhosis was used to identify a cDNA encoding a novel component of the NB, a 140-kDa protein designated Sp140. The predicted amino acid sequence of the amino-terminal portion of Sp140 was similar to Sp100, a previously identified NB protein. The carboxyl portion of Sp140 contained a zinc-finger domain and a bromodomain, motifs that are present in proteins regulating gene transcription. High levels of Sp140 mRNA were detected in human spleen and peripheral blood leukocytes, but not other human tissues. The level of SP140 mRNA in myeloid precursor cell lines HL60 and NB4 markedly increased in response to chemically induced cellular differentiation. Immunohistochemical techniques were used to demonstrate that SP140 localized to the NB in differentiated HL60 and NB4 cells. The location of Sp140 in the NB, and expression of this gene in cells involved in host defense, suggest that Sp140 may be involved in the pathogenesis of acute promyelocytic leukemia and viral infection.

The interferon (IFN)-stimulated gene Sp100 promoter contains an IFN-gamma activation site and an imperfect IFN-stimulated response element which mediate type I IFN inducibility

Expression of the nuclear domain-associated proteins Sp100, PML, and NDP52, is enhanced by interferons (IFNs) on the mRNA and protein level. Increase both of Sp100 and PML mRNA is due to enhanced transcription of the corresponding genes which occurs independently of cellular protein synthesis immediately upon IFN-beta addition. Here, we describe the molecular cloning and functional analysis of the Sp100 promoter. DNA sequence analysis revealed potential binding sites for several constitutive and IFN-inducible transcription factors. Consistent with the absence of a TATA box and an initiator element, several transcription initiation sites were found. Transient expression studies identified an imperfect IFN-stimulated response element within the first 100 nucleotides upstream of the major transcription start site. This element rendered a heterologous promoter IFN-beta-inducible and bound IFN-stimulated gene factor 2 strongly but IFN-stimulated gene factor 3 only weakly. An IFN-gamma activation site approximately 500 base pairs upstream of the IFN-stimulated response element was found to bind three IFN-alpha/beta activation factors upon IFN-beta induction and conferred both type I and type II IFN inducibility upon a heterologous promoter. These data demonstrate a novel arrangement of a nonoverlapping IFN-gamma activation site and an IFN-stimulated response element mediating type I IFN inducibility, previously not reported for other IFN-stimulable promoters.

The biologic function of PML and its role in acute promyelocytic leukemia

Patients with acute promyelocytic leukemia (APL) are characterized by the presence of a t(15;17) chromosomal translocation. The fusion protein PML-RAR alpha encoded from the breakpoint can form a heterodimer and acts as a dominant negative inhibitor against the normal function of PML. Recently we demonstrated that PML is a growth suppressor and transcription suppressor expressed in all cell lines tested. We also found that PML suppresses the clonogenicity and tumorigenicity of APL-derived NB4 cells, as well as the transformation of rat embryo fibroblasts by cooperative oncogenes and NIH/3T3 by neu. Overexpression of PML in human tumor cell lines induces a remarkable reduction in growth rate in vitro and in vivo. More recently, we have shown that PML is a phosphoprotein associated with the nuclear matrix and that its expression is cell cycle related. PML expression is altered during human oncogenesis, implying that PML may be an anti-oncogene involved not only in APL but also in other oncogenic events. Mutation analysis of the functional domains of PML demonstrated that its ability to form PML nuclear bodies or PODs (PML oncogenic domains) is essential for suppressing growth and transformation. In light of the above studies it appears that disruption of the normal function of PML plays a critical role in the pathogenesis of APL.

Effects on differentiation by the promyelocytic leukemia PML/RARalpha protein depend on the fusion of the PML protein dimerization and RARalpha DNA binding domains

The block of terminal differentiation is a prominent feature of acute promyelocytic leukemia (APL) and its release by retinoic acid correlates with disease remission. Expression of the APL-specific PML/RARalpha fusion protein in hematopoietic precursor cell lines blocks terminal differentiation, suggesting that PML/ RARalpha may have the same activity in APL blasts. We expressed different PML/RARalpha mutants in U937 and TF-1 cells and demonstrated that the integrity of the PML protein dimerization and RARalpha DNA binding domains is crucial for the differentiation block induced by PML/RARalpha, and that these domains exert their functions only within the context of the fusion protein. Analysis of the in vivo dimerization and cell localization properties of the PML/RARalpha mutants revealed that PML/RARalpha--PML and PML/RARalpha--RXR heterodimers are not necessary for PML/RARalpha activity on differentiation. We propose that a crucial mechanism underlying PML/RARalpha oncogenic activity is the deregulation of a transcription factor, RARalpha, through its fusion with the dimerization interface of another nuclear protein, PML.

Two distinct actions of retinoid-receptor ligands

Signalling by all-trans retinoic acid is mediated through RXR-RAR retinoid receptor heterodimers, in which RXR has been considered to act as a transcriptionally silent partner. However, we show here that in cultured NB4 (ref. 6) human acute promyelocytic leukaemia cells treated with either an RAR-alpha-selective agonist alone, or certain RAR-alpha antagonists in combination with an RXR agonist, receptor-DNA binding is induced in vivo, resulting in expression of the target genes of retinoic acid as well as acute promyelocytic leukaemia protein (PML) relocation to nuclear bodies and differentiation before apoptosis. These results indicate that RAR-alpha ligands can induce two separate events: one enables RXR-RAR-alpha heterodimers to bind to DNA in vivo and allows RXR agonists to act; the other induces transcriptional activity of RAR-alpha. The availability of receptor-specific synthetic retinoids that can induce distinct receptor functions has potential in extending the therapeutic repertoire of retinoids.

Exclusion of Int-6 from PML nuclear bodies by binding to the HTLV-I Tax oncoprotein

The Tax transactivator of the human T cell leukemia virus type I (HTLV-I) exhibits oncogenic properties. A screen for proteins interacting with Tax yielded a complementary DNA (cDNA) encoding the human Int-6 protein. In mice, the Int-6 gene can be converted into a putative dominant negative oncogene after retroviral insertion. Here, Int-6 was localized in the cell nucleus to give a speckled staining pattern superposed to that of the promyelocytic leukemia (PML) protein. The binding of Tax to Int-6 caused its redistribution from the nuclear domains to the cytoplasm. Thus, Int-6 is a component of the PML nuclear bodies and Tax disrupts its normal cellular localization by binding to it.

Functional compartmentalization of the nucleus

Recent applications of cell biology and molecular genetics have built an image of nuclear organization in which the molecular machines involved in transcription, RNA processing and replication assemble morphologically distinct nuclear organelles with defined functional properties. These observations indicate a very high level of structural organization for the various metabolic activities occurring within the nucleus. We discuss the possible existence of novel regulatory functions inherent to nuclear architecture itself.

A novel nuclear structure containing the survival of motor neurons protein

Spinal muscular atrophy (SMA) is a common, often fatal, autosomal recessive disease leading to progressive muscle wasting and paralysis as a result of degeneration of anterior horn cells of the spinal cord. A gene termed survival of motor neurons (SMN), at 5q13, has been identified as the determining gene of SMA (Lefebvre et al., 1995). The SMN gene is deleted in > 98% of SMA patients, but the function of the SMN protein is unknown. In searching for hnRNP-interacting proteins we found that SMN interacts with the RGG box region of hnRNP U, with itself, with fibrillarin and with several novel proteins. We have produced monoclonal antibodies to the SMN protein, and we report here on its striking cellular localization pattern. Immunolocalization studies using SMN monoclonal antibodies show several intense dots in HeLa cell nuclei. These structures are similar in number (2-6) and size (0.1-1.0 micron) to coiled bodies, and frequently are found near or associated with coiled bodies. We term these prominent nuclear structures gems, for Gemini of coiled bodies.

The RNA 3' cleavage factors CstF 64 kDa and CPSF 100 kDa are concentrated in nuclear domains closely associated with coiled bodies and newly synthesized RNA

The cleavage stimulation factor (CstF), and the cleavage and polyadenylation specificity factor (CPSF) are necessary for 3'-terminal processing of polyadenylated mRNAs. To study the distribution of 3' cleavage factors in the nuclei of human T24 cells, monoclonal antibodies against the CstF 64 kDa subunit and against the CPSF 100 kDa subunit were used for immunofluorescent labelling. CstF 64 kDa and CPSF 100 kDa were distributed in a fibrogranular pattern in the nucleoplasm and, in addition, were concentrated in 1-4 bright foci. Double immunofluorescence labelling experiments revealed that the foci either overlapped with, or resided next to, a coiled body. Inhibition of transcription with alpha-amanitin or 5,6-dichloro-beta-D-ribofuranosyl-benzimidazole (DRB) resulted in the complete co-localization of coiled bodies and foci containing 3' cleavage factors. Electron microscopy on immunogold double-labelled cells revealed that the foci represent compact spherical fibrous structures, we named 'cleavage bodies', intimately associated with coiled bodies. We found that approximately 20% of the cleavage bodies contained a high concentration of newly synthesized RNA, whereas coiled bodies were devoid of nascent RNA. Our results suggest that the cleavage bodies that contain RNA are those that are adjacent to a coiled body. These findings reveal a dynamic and transcription-dependent interaction between different subnuclear domains, and suggest a relationship between coiled bodies and specific transcripts.

A retrovirus carrying the promyelocyte-retinoic acid receptor PML-RARalpha fusion gene transforms haematopoietic progenitors in vitro and induces acute leukaemias

The promyelocyte (PML)-retinoic acid receptor alpha (RARalpha) fusion gene results from a t(15;17) chromosome translocation in acute promyelocytic leukaemia. We have analysed the oncogenic potential of the human fusion PML-RARalpha product in chicken using retrovirus vectors. We show that PML-RARalpha transforms very early haematopoietic progenitor cells in vitro and induces acute leukaemias. Neither PML nor RARalpha domains alone achieve such a transformation. The PML-RARalpha viruses recovered from the transformed cells carry two point mutations in the PML domain, one of which alters both the pattern of intracellular localization of the fusion protein and its functional interference with AP-1, thus defining an essential domain in PML for oncogenic transformation.

Interferon-modulated expression of genes encoding the nuclear-dot-associated proteins Sp100 and promyelocytic leukemia protein (PML)

Promyelocytic leukemia protein (PML) and Sp100 are transcription-regulatory proteins which colocalize in discrete nuclear dots and play a role in autoimmunity, oncogenesis and virus-host interaction. Interferons (IFNs) were shown previously to increase strongly the levels of Sp100 mRNA and protein. Here, we examined which mechanisms lead to upregulation of Sp100 gene expression and whether IFNs also increase expression of the promyelocytic leukemia (PML) gene. We found that both mRNA and protein levels of PML are also strongly upregulated by IFNs. In addition, new Sp100 and PML proteins were detected immunologically after IFN treatment of cells. Nuclear run-on analysis revealed protein-synthesis-independent, rapid IFN-enhanced transcription rates as well as synergistic activation of the Sp100 and PML genes by type-I and type-II IFNs. These data demonstrate that PML and Sp100 belong to the growing family of IFN-stimulated genes (ISGs) upregulated most likely by the transcription factor ISGF3, and indicate that IFNs also qualitatively alter the expression of these two genes.

Characterization of a helicase-like transcription factor involved in the expression of the human plasminogen activator inhibitor-1 gene

A 5.4-kb cDNA encoding the protein that binds to the B Box of the plasminogen activator inhibitor-1 (PAI-1) gene was isolated and sequenced. The protein, named helicase-like transcription factor (HLTF), contains a DNA-binding domain, a RING finger domain, and seven helicase domains and is homologous to SWI/SNF proteins. Two HLTF mRNAs of 5.5 and 4.5 kb were detected in most human tissues, a single gene was located on chromosome 3q24-25, and the protein was located in the nucleoplasm. Two HLTF proteins differing in translation start site (Met-1 or Met-123) were obtained by in vitro translation in reticulocyte lysate or by immunoprecipitation from HeLa cell nuclear extracts. In vitro transcription from the PAI-1 promoter in HeLa cell extracts was inhibited by HLTF antibodies and by the HLTF DNA binding domain. Over-expression of HLTF or HLTFMet123 produced a three-fold induction of PAI-1-LUC transient expression in HeLa cells. Mutation of the PAI-1 B Box led to an eight-fold reduction of basal PAI-1-LUC expression in these cell lines, but did not affect the four- to six-fold induction by phorbol esters.

The RING finger domain: a recent example of a sequence-structure family

In the past 18 months, two RING finger structures have been solved. They represent the first reported structures for this novel zinc-binding sequence motif. Both structures are significantly different from other zinc-binding domains, in terms of both their zinc-ligation scheme and their three-dimensional structures. The RING finger domain appears to be a convenient scaffold which can be altered to provide functional specificity in those proteins that contain the motif.

The Epstein-Barr virus-encoded nuclear antigen EBNA-5 accumulates in PML-containing bodies

EBNA-5 is one of the Epstein-Barr virus (EBV)-encoded nuclear proteins required for immortalization of human B lymphocytes. In the nuclei of EBV-transformed lymphoblastoid cell lines EBNA-5 is preferentially targetted to distinct nuclear foci. Previously we have shown (W.Q. Jiang, L. Szekely, V. Wendel-Hansen, N. Ringertz, G. Klein, and A. Rosen, Exp. Cell Res. 197:314-318, 1991) that the same foci also contained the retinoblastoma (Rb) protein. Using a similar double immunofluorescence technique, we now show that these foci colocalize with nuclear bodies positive for PML, the promyelocytic leukemia-associated protein. Artificial spreading of the chromatin by exposure to the forces of fluid surface tension disrupts this colocalization gradually, suggesting that the bodies consist of at least two subcomponents. Heat shock or metabolic stress induced by high cell density leads to the release of EBNA-5 from the PML-positive nuclear bodies and induces it to translocate to the nucleoli. In addition to their presence in nuclear bodies, both proteins are occasionally present in nuclear aggregates and doughnut-like structures in which PML is concentrated in an outer shell. Nuclear bodies with prominent PML staining are seen in resting B lymphocytes. This staining pattern does not change upon EBV infection. In freshly infected cells EBNA-5 antigens are first distributed throughout the nucleoplasm. After a few days intensely staining foci develop. These foci coincide with PML-positive nuclear bodies. At a later stage and in established lymphoblastoid cell lines EBNA-5 is almost exclusively present in the PML-positive nuclear foci. The colocalization is restricted to EBV-infected human lymphoblasts. The data presented indicate that the distinct EBNA-5 foci are not newly formed structures but the result of translocation of the viral protein to a specialized domain present already in the nuclei of uninfected cells.

Retinoic acid regulatory pathways, chromosomal translocations, and acute promyelocytic leukemia

Retinoic acids (RAs) exert a broad range of physiologic actions during embryonic development and adult life. Two families of RA receptors, retinoic acid receptor (RAR) and retinoid X receptor (RXR), have been identified. The therapeutic effect of all-trans-RA (ATRA) in induction of remission for acute promyelocytic leukemia (APL) has largely been proved, and this has, over the past 10 years, greatly stimulated research on oncogenesis and RA-regulated differentiation pathways. In APL, one of the RAR genes, RARA, is fused to PML in the great majority of patients as a result of the chromosomal translocation t(15; 17). However, a small subset of APL patients have a different fusion gene, PLZF-RARA, resulting from the variant translocation t(11;17). A third translocation, t(5;17), in which the NPM gene is fused to RARA, has been described. Current data suggest that PML-RAR alpha and PLZF-RAR alpha fusion receptors may play an important role in the development of APL and that PML-RAR alpha could be the target of ATRA differentiation therapy. Characterization of the genes regulated by retinoic acid may open up new prospects for an understanding of the mechanisms of ATRA differentiation therapy for APL and may help to extend the concept of cancer-targeting treatment to other types of leukemias or solid tumors.

GATA transcription factors associate with a novel class of nuclear bodies in erythroblasts and megakaryocytes

The nuclear distribution of GATA transcription factors in murine haemopoietic cells was examined by indirect immunofluorescence. Specific bright foci of GATA-1 fluorescence were observed in erythroleukaemia cells and primary murine erythroblasts and megakaryocytes, in addition to diffuse nucleoplasmic localization. These foci, which were preferentially found adjacent to nucleoli or at the nuclear periphery, did not represent sites of active transcription or binding of GATA-1 to consensus sites in the beta-globin loci. Immunoelectron microscopy demonstrated the presence of intensely labelled structures likely to represent the GATA-1 foci seen by immunofluorescence. The GATA-1 nuclear bodies differed from previously described nuclear structures and there was no co-localization with nuclear antigens involved in RNA processing or other ubiquitous (Spl, c-Jun and TBP) or haemopoietic (NF-E2) transcription factors. Interestingly, GATA-2 and GATA-3 proteins also localized to the same nuclear bodies in cell lines co-expressing GATA-1 and -2 or GATA-1 and -3 gene products. This pattern of distribution is, thus far, unique to the GATA transcription factors and suggests a protein-protein interaction with other components of the nuclear bodies via the GATA zinc finger domain.

Adenovirus replication is coupled with the dynamic properties of the PML nuclear structure

Wild-type PML and at least four other novel proteins are localized within discrete nuclear structures known as PODs. We demonstrate here that during adenovirus infection, immediate early viral proteins from the E1 and E4 transcription units associate with the POD, which in turn undergoes a dramatic morphological change. During this process, the auto-antigen Sp-100 and NDP55 but not PML, relocate from the POD to the viral inclusion bodies, the sites of adenovirus DNA replication and late RNA transcription. The E4-ORF3 11-kD protein alone will induce this reorganization and reciprocally, viruses carrying mutations in the E4-domain fail to do so. These same viral mutants are defective in viral replication as well as the accumulation of late viral mRNAs and host cell transcription shutoff. We show that interferon (INF) treatment enhances the expression of PML, reduces or blocks PODs reorganization, and inhibits BrdU incorporation into viral inclusion bodies. In addition, cell lines engineered to overexpress PML prevent PODs from viral-induced reorganization and block or severely delay adenovirus replication. These results suggest that viral replication relies on components of the POD and that the structure is a target of early viral proteins.

Analysis of the growth and transformation suppressor domains of promyelocytic leukemia gene, PML

The promyelocytic leukemia gene (PML) involved in the t(15;17) (q22;q12) translocation in acute promyelocytic leukemia is a growth suppressor. To elucidate the functional domains of PML, several mutants lacking the nuclear localization signal (PMLnls-), the dimerization domain (PMLdim-), the proline-rich domain at the N-terminal (PMLpro-), the proline-rich RING finger motif (PMLpr-), the proline-rich RING finger B-box-1 (PML-prb-), the serine-proline-rich domain at the C-terminal (PMLsp-), and the double mutant (PMLprb-nls-) have been constructed. Immunofluorescence staining of transiently transfected NIH3T3 cells demonstrated that the RING finger motif, dimerization domain, and nuclear localization signal are all required for the formation of PML oncogenic domains (PODs). Immunofluorescence staining of transiently transfected GM637D human fibroblasts indicated that expression of PMLprb-, PM-Lnls-, and PMLprb-nls- led to a significant reduction or, in some cases, complete elimination of PODs. PMLdim-, PMLnls-, PMLpr-, PMLprb-, and PMLprb-nls- mutants were found to lose their ability to suppress transformation of NIH3T3 cells by activated neu, while PMLpro- and PMLsp- mutants did not. These results suggest that the ability of PML to form a POD is essential for suppression of growth and transformation. Furthermore, since PMLprb-, PMLnls-, and PMLprb-nls- mutants could block the suppression effect of wild-type PML on transformation of NIH3T3 cells by the neu oncogene, these PML mutants are potential dominant negative inhibitors of PML. Our study also suggests that the RING finger motif may interact with other nuclear proteins.

Nuclear domains and the nuclear matrix

This overview describes the spatial distribution of several enzymatic machineries and functions in the interphase nucleus. Three general observations can be made. First, many components of the different nuclear machineries are distributed in the nucleus in a characteristic way for each component. They are often found concentrated in specific domains. Second, nuclear machineries for the synthesis and processing of RNA and DNA are associated with an insoluble nuclear structure, called nuclear matrix. Evidently, handling of DNA and RNA is done by immobilized enzyme systems. Finally, the nucleus seems to be divided in two major compartments. One is occupied by compact chromosomes, the other compartment is the space between the chromosomes. In the latter, transcription takes place at the surface of chromosomal domains and it houses the splicing machinery. The relevance of nuclear organization for efficient gene expression is discussed.

The dosage compensation regulators MLE, MSL-1 and MSL-2 are interdependent since early embryogenesis in Drosophila

We have analyzed the expression pattern and localization of MLE, MLS-1, MSL-2, and histone H4Ac16 during embryogenesis to determine when msl-dependent dosage compensation begins. Maternal MSL-1 and MLE are present in both sexes at fertilization. MSL-2 lacks a maternal component, and male-specific zygotic expression is detectable at the end of blastoderm. During germ band extension, MSL-1, MSL-2, MLE, and histone H4Ac16 display coincident sub-nuclear localization in male embryos. In embryos lacking one of the MSL proteins, the sub-nuclear localization of the other MSLs and of histone H4Ac16 is not detected. We conclude that the MSL proteins associate with the X chromosome and are interdependent since early embryogenesis.

Cloning of the murine homolog of the leukemia-associated PML gene

PML, a Ring-finger protein, participates in the disruption of normal myeloid differentiation when fused to the retinoic acid receptor alpha (RAR alpha) by the translocation between chromosomes (Chrs) 15 and 17 in acute promyelocytic leukemia (APL). As an initial step in the characterization of PML in species other than human, a murine cDNA clone of the PML gene was isolated and sequenced, and the intron/exon organization of the murine locus determined. The predicted amino acid sequence of the mouse PML protein shows 80% similarity to that of its human homolog. However, the mouse and human proteins show greater than 90% similarity in the proposed functional domains of the proteins. Despite its role in the etiology of APL, PML expression is not detectably altered during granulocytic differentiation in a murine in vitro system. Chromosomal localization of the Pml locus by somatic cell hybrids and by linkage analysis indicates that the gene maps to a region of mouse Chr 9 with known linkage homology to the region on human Chr 15q to which PML has been localized.

Nuclear ultrastructures associated with the RNA synthesis and processing

Relatively little is known about the spatial organization of RNA synthesis, processing, and transport in (mammalian) cell nuclei. This review summarizes results of electron microscopic mapping of RNA synthetic sites and macromolecules involved directly, or indirectly, in the metabolism of RNAs in somatic cell mammalian nuclei. Significance of these results will be discussed in the context of the molecular mechanisms underlying spatial arrangements of RNA metabolism.

Two nuclear dot-associated proteins, PML and Sp100, are often co-autoimmunogenic in patients with primary biliary cirrhosis

The nucleoproteins Sp100 and PML, the first an autoantigen predominant in patients with primary biliary cirrhosis (PBC) and the second a transformation and cell growth suppressing protein aberrantly expressed in promyelocytic leukaemia cells, were recently shown to colocalize in dot-like nuclear domains. Here we analysed whether PML, like Sp100, is also an autoantigen in patients with PBC and other autoimmune diseases, and wether both proteins interact directly. Testing sera from autoimmune patients using an immunoprecipitation assay with radiolabelled PML and an immunofluorescence assay based on a cell line overexpressing PML, autoantibodies (Aabs) against PML were found in the majority o anti-Sp100 Aab positive patients. Only very few patients with PBC or other autoimmune diseases contained anti-PML or anti-Sp100 Aabs exclusively. In contrast to Sp100, immunoreactivity of recombinant PML in immunoblots was only weak and was directed to one region. This suggests that anti-PML Aabs recognize fewer and preferentially conformation-dependent epitopes. In an immunoprecipitation assay using in vitro synthesized Sp100 and PML proteins and Abs to recombinant proteins, no direct interaction was observed. Taken together, these data indicate that Aabs against PML are as highly prevalent and specific for patients with PBC as those against Sp100. The colocalization of these autoantigens and the frequent co-occurrence of the corresponding Aabs might reflect an association of both proteins mediated by one or several other proteins.

The N-terminal part of TIF1, a putative mediator of the ligand-dependent activation function (AF-2) of nuclear receptors, is fused to B-raf in the oncogenic protein T18

Nuclear receptors (NRs) bound to response elements mediate the effects of cognate ligands on gene expression. Their ligand-dependent activation function, AF-2, presumably acts on the basal transcription machinery through intermediary proteins/mediators. We have isolated a mouse nuclear protein, TIF1, which enhances RXR and RAR AF-2 in yeast and interacts in a ligand-dependent manner with several NRs in yeast and mammalian cells, as well as in vitro. Remarkably, these interactions require the amino acids constituting the AF-2 activating domain conserved in all active NRs. Moreover, the oestrogen receptor (ER) AF-2 antagonist hydroxytamoxifen cannot promote ER-TIF1 interaction. We propose that TIF1, which contains several conserved domains found in transcriptional regulatory proteins, is a mediator of ligand-dependent AF-2. Interestingly, the TIF1 N-terminal moiety is fused to B-raf in the mouse oncoprotein T18.

The solution structure of the RING finger domain from the acute promyelocytic leukaemia proto-oncoprotein PML

Acute promyelocytic leukaemia (APL) has been ascribed to a chromosomal translocation event which results in a fusion protein comprising the PML protein and the retinoic acid receptor alpha. PML is normally a component of a nuclear multiprotein complex (termed ND10, Kr bodies, nuclear bodies, PML oncogenic domains or PODs) which is disrupted in the APL disease state. PML contains a number of characterized motifs including a Zn2+ binding domain called the RING or C3HC4 finger. Here we describe the solution structure of the PML RING finger as solved by 1H NMR methods at physiological pH with r.m.s. deviations for backbone atoms of 0.88 and 1.39 A for all atoms. Additional biophysical studies including CD and optical spectroscopy, show that the PML RING finger requires Zn2+ for autonomous folding and that cysteines are used in metal ligation. A comparison of the structure with the previously solved equine herpes virus IE110 RING finger, shows significant differences suggesting that the RING motif is structurally diverse. The role of the RING domain in PML nuclear body formation was tested in vivo, by using site-directed mutagenesis and immunofluorescence on transiently transfected NIH 3T3 cells. Independently mutating two pairs of cysteines in each of the Zn2+ binding sites prevents PML nuclear body formation, suggesting that a fully folded RING domain is necessary for this process. These results suggest that the PML RING domain is probably involved in protein-protein interactions, a feature which may be common to other RING finger domains.

Nuclear remodeling in response to steroid hormone action

Steroid and similar hormones comprise the broadest class of gene regulatory agents known, spanning vertebrates through the lower animals, and even fungi. Not unexpectedly, therefore, steroid receptors belong to an evolutionarily highly conserved family of proteins. After complexing with their cognate ligands, receptors interact with hormone response elements on target genes and modulate transcription. These actions are multifaceted and only partly understood, and include large-scale changes in the structure and molecular composition of the affected cell nuclei. This chapter examines steroid hormone action and the resultant nuclear remodeling from the following perspectives: (1) Where are the receptors located? (2) Which nuclear domains are most affected? (3) Are there extended or permanent nuclear changes? (4) What is the role of coiled bodies and similar structures in this regard? To address these and related questions, information is drawn from several sources, including vertebrates, insects, and malignant tissues. Entirely new data are presented as well as a review of the literature.

Acute promyelocytic leukemia: from clinic to molecular biology

Acute promyelocytic leukemia (APL) is a good model for studying the human malignancies in that up to 90% of APL patients can achieve complete remission (CR) with a differentiation inducer, all-trans retinoic acid (ATRA). APL is also associated with a specific chromosomal translocation t(15;17) which fuses the retinoic acid receptor alpha (RAR alpha) gene with a chromosome 15q locus, PML. Recently the RAR alpha and the PML gene structural alterations in t(15;17) have been characterized. The heterogeneity of the PML rearrangements juxtaposes different PML gene portions to the same set of RAR alpha exons, producing two major PML-RAR alpha fusion mRNA isoforms. A retrotranscriptase/polymerase chain reaction (RT-PCR) analysis of the fusion transcripts has been developed which allows the detection of minimal residual disease during the clinical remission of APL. Molecular study showed PML-RAR alpha can form heterodimers with wild-type PML and RXR. Recently, PML has been shown to be one of the components of a nuclear body, POD. In APL, the normal organization of POD is disrupted by PML-RAR alpha, whereas ATRA treatment in vivo and in vitro can induce a reorganization of this organelle. Cytogenetic and molecular study allowed a variant translocation t(11;17) being recently discovered in a small subset of APL. This time RAR alpha is fused to a new gene, PLZF, on chromosome 11q23. It has been shown that the PLZF-RAR alpha, like PML-RAR alpha, has a "dominant negative" effect on the wild-type RAR-RXR. Clinical data obtained from a group of t(11;17) APL patients showed that these respond poorly to ATRA and could be grouped in a special clinical syndrome within APL. The comparison of the biological activities mediated by PML-RAR alpha and PLZF-RAR alpha may give new insights into the pathogenesis as well as the mechanisms of ATRA-induced differentiation in APL.

Vitamin A, differentiation and cancer

Retinoids, which are derivatives of vitamin A, have a variety of effects on normal cellular differentiation and on the process of carcinogenesis. A number of novel endogenous retinol metabolites have been identified recently. The response of many cell types to retinoid treatment is mediated by retinoid receptors, and involves changes in gene expression, cell growth and cell differentiation. The gene encoding one of the retinoic acid receptors is disrupted by the chromosome translocations associated with acute promyelocytic leukemia, and the expression of another is altered in epithelial tumors; both of these findings have important implications for the use of retinoids as anti-carcinogenic agents. It has been demonstrated recently that certain homeobox genes are regulated by retinoids; these genes may also prove to be useful agents for anti-carcinogenic therapies.

HSV-1 IE protein Vmw110 causes redistribution of PML

Herpes simplex virus immediate-early protein Vmw110 is required for fully efficient viral gene expression and reactivation from latency. At early times of viral infection, Vmw110 localizes to discrete nuclear structures (known as ND10, PODs or Kr bodies) which contain several cellular proteins, including PML. Interestingly, the unregulated growth of promyelocytic leukaemia cells is correlated with disruption of the normal state of ND10. In this paper we show that: (i) Vmw110 affects the distribution of PML in the cell; (ii) Vmw110 proteins lacking a functional RING finger zinc-binding domain cause the production of striking abnormal cytoplasmic and nuclear structures, some of which contain PML and other ND10 antigens; (iii) a mutant form of Vmw110 which is confined to the cytoplasm appears to result in cytoplasmic PML in some cells; (iv) normal interaction with the nuclear structures requires the C-terminal portion of Vmw110; (v) the C-terminal portion of Vmw110, when linked to a heterologous protein, disrupts the normal distribution of PML. The results suggest that, in normal cells, the PML protein migrates between nucleus and cytoplasm. These observations present an unexpected link between processes involved in the control of cell growth and viral infection and latency.

White gene expression, repressive chromatin domains and homeotic gene regulation in Drosophila

The use of Drosophila chromosomal rearrangements and transposon constructs involving the white gene reveals the existence of repressive chromatin domains that can spread over considerable genomic distances. One such type of domain is found in heterochromatin and is responsible for classical position-effect variegation. Another type of repressive domain is established, beginning at specific sequences, by complexes of Polycomb Group proteins. Such complexes, which normally regulate the expression of many genes, including the homeotic loci, are responsible for silencing, white gene variegation, pairing-dependent effects and insertional targeting.

The POZ domain: a conserved protein-protein interaction motif

We describe a novel zinc finger protein, ZID (zinc finger protein with interaction domain). At its amino terminus ZID contains a 120-amino-acid conserved motif present in a large family of proteins that includes both the otherwise unrelated zinc finger proteins, such as Ttk, GAGA, and ZF5, and a group of poxvirus proteins: We therefore refer to this domain as the POZ (poxvirus and zinc finger) domain. The POZ domains of ZID, Ttk, and GAGA act to inhibit the interaction of their associated finger regions with DNA. This inhibitory effect is not dependent on interactions with other proteins and does not appear dependent on specific interactions between the POZ domain and the finger region. The POZ domain acts as a specific protein-protein interaction domain: The POZ domains of ZID and Ttk can interact with themselves but not with each other, POZ domains from ZF5, or the viral protein SalF17R. However, the POZ domain of GAGA can interact efficiently with the POZ domain of Ttk. In transfection experiments, the ZID POZ domain inhibits DNA binding in NIH-3T3 cells and appears to localize the protein to discrete regions of the nucleus. We discuss the implications of multimerization for the function of POZ domain proteins.