Promyelocytic leukemia (PML) nuclear bodies are protein structures that do not accumulate RNA

The Rossmann fold of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a nuclear docking site for antisense oligonucleotides containing a TAAAT motif

The subcellular localisation of oligodeoxynucleotides (ODN) is a major limitation for their use against nuclear targets. In this study we demonstrate that an antisense ODN directed against cytosolic phospholipase A(2) (cPLA2) mRNA is efficiently taken up and accumulates in the nuclei of endothelial cells (HUVEC), human monocytes and HeLa cells. Gel shift experiments and incubation of cells with oligonucleotide derivatives show that the anti-cPLA2 oligo binds a 37 kDa protein in nuclear extracts. The TAAAT sequence was identified as the major binding motif for the nuclear protein in competition experiments with mutated ODNs. Modification of the AAA triplet resulted in an ODN which failed to localise in the nucleus. Moreover, inserting a TAAAT motif into an ODN localising in the cytosol did not modify its localisation. The 37 kDa protein was purified and identified after peptide sequencing as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). It was shown by confocal microscopy that GAPDH co-localises with anti-cPLA2 ODN in the nucleus and commercial GAPDH effectively binds the oligo. Competition experiments with increasing concentration of NAD(+) co-factor indicate that the GAPDH Rossmann fold is a docking site for antisense oligonucleotides containing a TAAAT motif.

Regulation of Pax3 transcriptional activity by SUMO-1-modified PML

Pax3 is an evolutionarily conserved transcription factor that plays a major role in a variety of developmental processes. Mutations in Pax3 lead to severe malformations as seen in human Waardenburg syndrome and in the Splotch mutant mice. The transcriptional activity of Pax3 was recently shown to be repressed by Daxx whereas the oncogenic fusion protein Pax3-FKHR is unresponsive to this repressive action. Here we demonstrate that Daxx-mediated repression of Pax3 can be inhibited by the nuclear body (NB)-associated protein PML. Interestingly, this suppression of Daxx properties correlates with its recruitment to the NBs. Factors such as arsenicals and interferons that enhance NB formation, trigger both the targeting of Daxx to these nuclear structures and the relief of the repressive activity of Daxx. Conversely, lack of structurally intact NBs profoundly impairs Pax3 transcriptional activity, likely by increasing the pool of available nucleoplasmic Daxx. Moreover, a PML mutant that can not be modified by the ubiquitin-related SUMO-1 modifier is no more able to interact with Daxx. Consistently, such a mutant fails both to inhibit the Daxx repressing effect on Pax3 and to induce its accumulation into the NBs. Taken together, these results argue that SUMO-1 modified PML can derepress Pax3 transcriptional activity through sequestration of the Daxx repressor into the NBs and suggest a role for these nuclear structures in the transcriptional control by Pax proteins. Oncogene (2001) 20, 1 - 9.

Effects of the acute myeloid leukemia--associated fusion proteins on nuclear architecture

Acute myeloid leukemias (AMLs) are consistently associated with chromosomal rearrangements that result in the generation of chimeric genes and fusion proteins. One of the two affected genes is frequently a transcription factor Involved in the regulation of hematopoletic differentiation. Recent findings suggest a common leukemogenic mechanism for the fused transcription factor: abnormal recruitment of histone deacetylase (HDAC)-containing complexes to its target promoters. Inhibition of HDAC enzymatic activity reverts the leukemic phenotype in vitro and therefore represents a plausible strategy for antileukemic therapy. In this review, we first briefly describe the molecular structure and mechanisms of the most frequent AML associated fusion proteins (RAR, MLL, and CBF fusions) and then summarize available knowledge about their effects on the nuclear architecture. We propose that alteration of nuclear compartmentalization might represent an additional common mechanism of leukemogenesis.

Compartmentalization of regulatory proteins in the cell nucleus

The cell nucleus is increasingly recognized as a spatially organized structure. In this review, the nature and controversies associated with nuclear compartmentalization are discussed. The relationship between nuclear structure and organization of proteins involved in the regulation of RNA polymerase II-transcribed genes is then discussed. Finally, very recent data on the mobility of these proteins within the cell nucleus is considered and their implications for regulation through compartmentalization of proteins and genomic DNA are discussed.

PML and COP1--two proteins with much in common

The recent discovery that the RING-finger domain is involved in mediating ubiquitin transfer from ubiquitin-conjugating enzymes to substrates have highlighted the importance of protein degradation through the ubiquitin-proteasome pathway in the regulation of different cellular processes. Two RING-finger-containing proteins, the promyelocytic leukemia protein (PML) from mammals and the constitutive photomorphogenic protein (COP1) from plants, show conspicuous similarities in their cellular distribution, dynamics and structure, indicating that they share a related function. Comparison of these two proteins suggests that they are involved in regulating the targeting of nuclear proteins to specific nuclear compartments for degradation through the ubiquitin-proteasome pathway.

RET finger protein is a transcriptional repressor and interacts with enhancer of polycomb that has dual transcriptional functions

RET finger protein (RFP) belongs to the large B-box RING finger protein family and is known to become oncogenic by fusion with RET tyrosine kinase. Although RFP is reported to be a nuclear protein that is present in the nuclear matrix, its function is largely unknown. Here we show that RFP interacts with Enhancer of Polycomb (EPC) and strongly represses the gene transcription. Yeast two-hybrid assays revealed that the coiled-coil domain of RFP was associated with the EPcA domain and the carboxyl-terminal region of EPC. In addition, both proteins were co-precipitated from the lysates of human cells and mostly colocalized in the nucleus. Using the luciferase reporter-gene assay, we found that they repress the gene transcription activity independent of the differences of enhancers and promoters used, although the repressive activity of RFP was much stronger than that of EPC. The coiled-coil domain of RFP and the carboxyl-terminal region of EPC were most important for the repressive activity of each protein, whereas the EPcA domain had the transcription activating ability that is unique as the Polycomb group protein function. These results suggested that RFP may be involved in the epigenetic gene silencing mechanism cooperating with Polycomb group proteins and that EPC is a unique molecule with both repressive and transactivating activities.

PML and the oncogenic nuclear domains in regulating transcriptional repression

Promyelocytic leukemia protein (PML) is a tumor suppressor involved in the t(15;17) translocation that causes acute promyelocytic leukemia (APL). PML is located at multiple nuclear domains known as PML oncogenic domains (PODs), whose structures are dynamically regulated and disrupted in t(15;17) APL cells. PML is involved in several important cellular processes; however, its exact function is unclear. Recently, a POD-associated protein was found to be transcriptional repressor, suggesting a new role for PODs in regulating transcriptional repression.

Mitotic transcription repression in vivo in the absence of nucleosomal chromatin condensation

All nuclear RNA synthesis is repressed during the mitotic phase of the cell cycle. In addition, RNA polymerase II (RNAP II), nascent RNA and many transcription factors disengage from DNA during mitosis. It has been proposed that mitotic transcription repression and disengagement of factors are due to either mitotic chromatin condensation or biochemical modifications to the transcription machinery. In this study, we investigate the requirement for chromatin condensation in establishing mitotic transcription repression and factor loss, by analyzing transcription and RNAP II localization in mitotic cells infected with herpes simplex virus type 1. We find that virus-infected cells enter mitosis and that mitotic viral DNA is maintained in a nucleosome-free and noncondensed state. Our data show that RNAP II transcription is repressed on cellular genes that are condensed into mitotic chromosomes and on viral genes that remain nucleosome free and noncondensed. Although RNAP II may interact indirectly with viral DNA during mitosis, it remains transcriptionally unengaged. This study demonstrates that mitotic repression of transcription and loss of transcription factors from mitotic DNA can occur independently of nucleosomal chromatin condensation.

CREB-binding protein (CBP)/p300 and RNA polymerase II colocalize in transcriptionally active domains in the nucleus

The spatial organization of transcription- associated proteins is an important control mechanism of eukaryotic gene expression. Here we analyzed the nuclear distribution of the transcriptional coactivators CREB-binding protein (CBP)/p300 in situ by confocal laser scanning microscopy, and in vivo complex formation by coimmunoprecipitation. A subpopulation of CBP and p300 is targeted to active sites of transcription and partially colocalizes with hyper- and hypophosphorylated RNA polymerase II (pol II) in discrete regions of variable size throughout the nucleus. However, the coactivators were found in tight association with hypophosphorylated, but not hyperphosphorylated pol II. Transcriptional inhibition induced a relocation of CBP/p300 and pol II into speckles. Moreover, double and triple immunofluorescence analyses revealed the presence of CBP, p300, and pol II in a subset of promyelocytic leukemia (PML) bodies. Our results provide evidence for a dynamic spacial link between coactivators of transcription and the basal transcription machinery in discrete nuclear domains dependent upon the transcriptional activity of the cell. The identification of pol II in CBP/PML-containing nuclear bodies supports the idea that transcription takes place at PML bodies.

The transcriptional role of PML and the nuclear body

The PML gene encodes a tumour suppressor protein associated with a distinct subnuclear domain, the nuclear body. Various functions have been attributed to the PML nuclear body, but its main biochemical role is still unclear. Recent findings indicate that PML is essential for the proper formation of the nuclear body and can act as a transcriptional co-factor. Here we summarize the current understanding of the biological functions of PML and the nuclear body, and discuss a role for these intra-nuclear structures in the regulation of transcription.