Structure, localization and transcriptional properties of two classes of retinoic acid receptor alpha fusion proteins in acute promyelocytic leukemia (APL): structural similarities with a new family of oncoproteins

A novel RING finger protein, BFP, predominantly expressed in the brain

RING finger is a variant zinc finger motif present in a new family of proteins including transcription regulators. A genomic DNA fragment containing RING finger motifs was identified by the polymerase chain reaction using degenerate primers. Using this fragment as a probe, we have isolated a novel cDNA from rat brain library. The predicted open reading frame contains a RING finger domain at its N-terminal portion. The corresponding transcript was detected predominantly in the brain and therefore was designated brain finger protein (bfp). An antibody raised against a recombinant bfp reveals the presence of the bfp in the brain. Interestingly, the bfp is induced during retinoic acid-mediated differentiation of P19 embryonal carcinoma cells into neural cells. These findings suggest the possible involvement of bfp in some aspects of neural cell regulation.

Gene rearrangements in the molecular pathogenesis of acute promyelocytic leukemia

Acute Promyelocytic Leukemia (APL) is a distinct subtype of myeloid leukemia that in the USA alone affects more than 3,000 individuals every year. APL is characterized by three distinct and unique features: i) the accumulation in the bone marrow of tumor cells with promyelocytic features; ii) the invariable association with specific translocations which always involve chromosome 17 and the Retinoic Acid Receptor alpha (RAR alpha) locus; iii) the exquisite sensitivity of APL blasts to the differentiating action of Retinoic Acid (RA). These features have led APL to become the paradigm for therapeutic approaches utilizing differentiating agents. The last 5 years have provided crucial insights into the molecular basis of APL. RAR alpha translocates in 99% of cases to a gene located on chromosome 15 that we initially named myl and subsequently has been called PML. In a few cases, RAR alpha variably translocates to chromosome 11 where it fuses to the PLZF gene or to a newly described partner, NuMA. In addition, RAR alpha is also found translocated to chromosome 5 where it fuses to the NPM gene. The cloning of variant translocations in APL and the comparative analysis of their associated products is crucial for the understanding of the molecular etiopathogenesis of the disease. The generation of animal models, i.e., transgenic mice expressing the fusion genes, will be instrumental in determining the precise contribution of these fusion genes to leukemogenesis. In fact, mice harboring a PML/RAR alpha transgene whose expression is specifically targeted to the myeloid-promyelocytic lineage develop acute myeloid leukemia with promyelocytic features. Moreover, the functional analysis of the various fusion proteins, as well as RAR alpha partners, is revealing striking common features beneath a misleading structural heterogeneity which unravels a possible unifying molecular mechanism towards APL leukemogenesis.

Acute promyelocytic leukemia: cellular and molecular basis of differentiation and apoptosis

Acute promyelocytic leukemia (APL) accounts for about 10% of all acute myeloid leukemias and is characterized by the chromosomal translocation t(15;17), which fuses the retinoic acid receptor (RAR) alpha gene to the promyelocytic leukemia (PML) gene. The PML-RAR alpha fusion gene plays an important role in leukemogenesis through antagonizing retinoic acid signalling and the regulatory pathways mediated by PML. APL is the first example of a human cancer that can be effectively treated with the differentiation inducer all-trans retinoic acid (ATRA). The therapeutic effect of ATRA in APL has been associated with the direct modulation of PML-RAR alpha, the restoration of the differentiation pathways regulated by wild-type RAR/retinoid X receptor heterodimer and PML. More recently, a second drug, arsenic trioxide (As2O3), has been discovered in China that also has a strong therapeutic effect against APL. As2O3 can induce clinical remission in de novo or relapsed APL patients and has no cross-resistance with ATRA. It has dual effects on APL cells: preferential apoptosis at high concentration (0.5-2 microM) and partial differentiation at low concentration (0.1-0.5 microM). Modulation and degradation of PML-RAR alpha proteins can be induced by As2O3 and probably contribute to these two effects. These studies lead to a model in which PML-RAR alpha could be the target of both ATRA differentiation therapy and As2O3 apoptosis therapy.

Leukemia-associated retinoic acid receptor alpha fusion partners, PML and PLZF, heterodimerize and colocalize to nuclear bodies

In acute promyelocytic leukemia (APL), the typical t(15;17) and the rare t(11;17) translocations express, respectively, the PML/RARalpha and PLZF/RARalpha fusion proteins (where RARalpha is retinoic acid receptor alpha). Herein, we demonstrate that the PLZF and PML proteins interact with each other and colocalize onto nuclear bodies (NBs). Furthermore, induction of PML expression by interferons leads to a recruitment of PLZF onto NBs without increase in the levels of the PLZF protein. PML/RARalpha and PLZF/RARalpha localize to the same microspeckled nuclear domains that appear to be common targets for the two fusion proteins in APL. Although PLZF/RARalpha does not affect the localization of PML, PML/RARalpha delocalizes the endogenous PLZF protein in t(15;17)-positive NB4 cells, pointing to a hierarchy in the nuclear targeting of these proteins. Thus, our results unify the molecular pathogenesis of APL with at least two different RARalpha gene translocations and stress the importance of alterations of PLZF and RARalpha nuclear localizations in this disease.

Sequence-specific DNA binding and transcriptional regulation by the promyelocytic leukemia zinc finger protein

Chromosomal translocation t(11;17)(q23;21) is associated with a retinoic acid-resistant form of acute promyelocytic leukemia. The translocation fuses the RARalpha gene to the PLZF gene, resulting in the formation of reciprocal fusion proteins, hypothesized to play prominent roles in leukemogenesis. Promyelocytic leukemia zinc finger (PLZF) encodes a transcription factor with nine Krüppel-like zinc fingers, seven of which are retained in the t(11;17) fusion protein RARalpha-PLZF. We identified a specific DNA-binding site for the PLZF protein and showed that PLZF binds to this site through its most carboxyl seven zinc fingers. In co-transfection experiments, PLZF repressed transcription through its cognate binding site. This repression function of PLZF was mapped to two regions on the protein, including the evolutionarily conserved POZ domain. In contrast, the RARalpha-PLZF protein activated transcription of a promoter containing a PLZF response element. These results suggest that RARalpha-PLZF, generated in acute promyelocytic leukemia, is an aberrant transcription factor that can deregulate the expression of PLZF target genes and contribute to leukemogenesis.

SMRT corepressor interacts with PLZF and with the PML-retinoic acid receptor alpha (RARalpha) and PLZF-RARalpha oncoproteins associated with acute promyelocytic leukemia

Retinoic acid receptors (RARs) are hormone-regulated transcription factors that control key aspects of normal differentiation. Aberrant RAR activity may be a causal factor in neoplasia. Human acute promyelocytic leukemia, for example, is tightly linked to chromosomal translocations that fuse novel amino acid sequences (denoted PML, PLZF, and NPM) to the DNA-binding and hormone-binding domains of RARalpha. The resulting chimeric receptors have unique transcriptional properties that may contribute to leukemogenesis. Normal RARs repress gene transcription by associating with ancillary factors denoted corepressors (also referred to as SMRT, N-CoR, TRAC, or RIP13). We report here that the PML-RARalpha and PLZF-RARalpha oncoproteins retain the ability of RARalpha to associate with corepressors, and that this corepressor association correlates with certain aspects of the leukemic phenotype. Unexpectedly, the PLZF moiety itself can interact with SMRT corepressor. This interaction with corepressor is mediated, in part, by a POZ motif within PLZF. Given the presence of POZ motifs in a number of known transcriptional repressors, similar interactions with SMRT may play a role in transcriptional silencing by a variety of both receptor and nonreceptor transcription factors.

Characterization of the Retinoid Binding Properties of the Major Fusion Products Present in Acute Promyelocytic Leukemia Cells

The bcr1- and bcr3- promyelocytic leukemia/retinoic acid receptor α (PML/RARα) are the two major fusion proteins expressed in acute promyelocytic leukemia (APL) patients. These proteins, which are present in different lengths of PML (amino acids 1-552 and 1-394, respectively), contain most of the functional domains of PML and RARα, bind all-trans-retinoic acid (t-RA), and act as t-RA–dependent transcription factors. T-RA is an effective inducer of clinical remission only in patients carrying the t(15; 17) and expressing the PML/RARα products. However, in APL patients achieving complete remission with t-RA therapy the bcr3-PML/RARα product has been found associated with a poorer prognosis than bcr1-PML/RARα. In the present study we have investigated the structural and functional properties of the bcr3-PML/RARα in comparison to the previously characterized bcr1-PML/RARα. In particular, we have measured the binding properties of the two endogenous ligands t-RA and 9-cis-RA to both of these isoforms. T-RA binding analysis of nuclear and cytosolic extracts prepared from bcr3-PML/RARα APL patients and from bcr3-PML/RARα COS-1 transfected cells indicates that this protein is present only as high-molecular-weight nuclear complexes. Using saturation binding assays and Scatchard analyses we found that t-RA binds with slightly less affinity to the bcr3-PML/RARα receptor than to bcr1-PML/RARα or RARα (Kd = 0.4 nmol/L, 0.13 nmol/L or 0.09 nmol/L, respectively). Moreover, two different high-affinity 9-cis-RA binding sites (Kd = 0.45 and 0.075 nmol/L) were detectable in the bcr3-PML/RARα product but not in the bcr1-PML/RARα product (Kd = 0.77 nmol/L). By competition binding experiments we showed that 9-cis-RA binds with higher specificity to the bcr3-PML/RARα isoform than to the bcr1-PML/RARα or RARα. Consistent with these data, the binding of 9-cis-RA to the bcr3-PML/RARα product resulted in increased transcriptional activation of the RA-responsive element (RARE) TRE, but not of the βRARE, in transiently transfected COS-1 cells. These results provide evidence indicating that preferential retinoid binding to the different PML/RARα products can be measured.

Common defects of different retinoic acid resistant promyelocytic leukemia cells are persistent telomerase activity and nuclear body disorganization

The acute promyelocytic leukemia (APL) t(15;17) rearrangement fuses the promyelocytic leukemia (PML) gene to the retinoic acid receptor-alpha (RAR alpha). There is expression of the chimeric transcript, PML/RAR alpha, in these APL cells. These clinical APL cases respond to the differentiation agent all-trans retinoic acid (ATRA) with complete but not durable remissions because ATRA resistance develops. The NB4 APL cell line expresses PML/RAR alpha and responds to the growth inhibitory and differentiation-inducing signals of ATRA. To identify mechanisms responsible for ATRA resistance in APL, ATRA-resistant NB4 cell lines were derived from parental NB4 cells using different strategies. These lines were resistant to the growth inhibition and differentiation effects of ATRA. ATRA-resistant cells were isolated as a de novo resistant line from parental NB4 cells (NB4-R1), following chemical mutagenization and selection in ATRA (NB4-R2), or after chronic selection in ATRA (NB4-R3). Common defects linked to this ATRA resistance were found. When cultured in ATRA, these resistant cells still express PML, RAR alpha, and PML/RAR alpha proteins. Sequence abnormalities were not detected in the RAR alpha DNA binding domains cloned from a representative RA-resistant NB4 line. In ATRA-sensitive but not ATRA-resistant NB4 cells, ATRA down-regulated retinoid X receptor-alpha (RXR alpha) expression, a known marker of ATRA response in parental NB4 cells. Notably, engineered overexpression of RXR alpha in ATRA-sensitive NB4 cells did not block ATRA-mediated growth suppression. ATRA treatment of these resistant NB4 lines did not signal a decline in telomerase activity or reorganization of PML-associated nuclear bodies, but both events occurred in ATRA-sensitive NB4 cells. These ATRA-resistant NB4 lines are not fully differentiation-defective, since monocytic maturation was induced following treatment with phorbol 12-myristate 13-acetate (PMA) and 1,25 dihydroxy vitamin D3 (vitamin D3). Notably, induced monocytic differentiation of these distinct ATRA-resistant APL lines markedly repressed telomerase activity. Thus, this study suggests that persistent telomerase activity and nuclear body disorganization are linked to ATRA resistance in APL.

Characterization of the Retinoid Binding Properties of the Major Fusion Products Present in Acute Promyelocytic Leukemia Cells

The bcr1- and bcr3- promyelocytic leukemia/retinoic acid receptor α (PML/RARα) are the two major fusion proteins expressed in acute promyelocytic leukemia (APL) patients. These proteins, which are present in different lengths of PML (amino acids 1-552 and 1-394, respectively), contain most of the functional domains of PML and RARα, bind all-trans-retinoic acid (t-RA), and act as t-RA–dependent transcription factors. T-RA is an effective inducer of clinical remission only in patients carrying the t(15; 17) and expressing the PML/RARα products. However, in APL patients achieving complete remission with t-RA therapy the bcr3-PML/RARα product has been found associated with a poorer prognosis than bcr1-PML/RARα. In the present study we have investigated the structural and functional properties of the bcr3-PML/RARα in comparison to the previously characterized bcr1-PML/RARα. In particular, we have measured the binding properties of the two endogenous ligands t-RA and 9-cis-RA to both of these isoforms. T-RA binding analysis of nuclear and cytosolic extracts prepared from bcr3-PML/RARα APL patients and from bcr3-PML/RARα COS-1 transfected cells indicates that this protein is present only as high-molecular-weight nuclear complexes. Using saturation binding assays and Scatchard analyses we found that t-RA binds with slightly less affinity to the bcr3-PML/RARα receptor than to bcr1-PML/RARα or RARα (Kd = 0.4 nmol/L, 0.13 nmol/L or 0.09 nmol/L, respectively). Moreover, two different high-affinity 9-cis-RA binding sites (Kd = 0.45 and 0.075 nmol/L) were detectable in the bcr3-PML/RARα product but not in the bcr1-PML/RARα product (Kd = 0.77 nmol/L). By competition binding experiments we showed that 9-cis-RA binds with higher specificity to the bcr3-PML/RARα isoform than to the bcr1-PML/RARα or RARα. Consistent with these data, the binding of 9-cis-RA to the bcr3-PML/RARα product resulted in increased transcriptional activation of the RA-responsive element (RARE) TRE, but not of the βRARE, in transiently transfected COS-1 cells. These results provide evidence indicating that preferential retinoid binding to the different PML/RARα products can be measured.

Multiple regulatory elements and binding proteins of the 5'-flanking region of the human estrogen-responsive finger protein (efp) gene

The efp, a member of the RING finger family, was previously identified as an estrogen responsive gene. Here, we characterized basal promoter of the human efp gene. Transcription initiation site was found at position -60 G relative to the site for translation initiation, and TATA motif was absent. Deletion and mutation analyses of the 5'-flanking region using MCF-7 breast cancer cells indicated that the sequences located at -137 to -72 had the promoter activity for which an E-box (CACGTG) element at -110 to -105 was essential. EMSA showed that USF-1 bound to the E-box and that a protein-DNA complex was formed at the positive regulatory region (-137 to -110). Moreover, a strong negative regulatory region was present in -235 to -174. These findings suggest that the human efp promoter is regulated by multiple elements and their interacting factors, and the E-box near the transcription initiation site is essential for the basal promoter activity.

RARα1/RARα2-PML mRNA Expression in Acute Promyelocytic Leukemia Cells: A Molecular and Laboratory-Clinical Correlative Study

In addition to the major fusion gene PML-RARα, the t(15; 17) in acute promyelocytic leukemia (APL) produces the reciprocal fusion gene RARα-PML. To determine the scope of RARα-containing mRNA expression in APL cells, we tested PML-RARα–positive APL cells for the presence of mRNAs initiated from two distinct RARα gene promoters, α1 and α2. From the normal allele, both RARα1 and RARα2 mRNAs were expressed in all APL cases (N = 24). From the translocated allele, RARα1-PML mRNA was expressed in 77% and RARα2-PML mRNA in 28% of cases (N = 98). RARα2-PML mRNA was not observed in the absence of RARα1-PML mRNA. There was no association between RARα1-PML or RARα2-PML mRNA expression and the type of PML-RARα mRNA formed by either 5′ or 3′ breaksites in the PML gene. RARα1-PML mRNAs and RARα2-PML mRNAs from 5′ PML breaksite cases coded for full-length RARα-PML proteins but RARα2-PML mRNAs from 3′ PML breaksite cases encoded a truncated RARα2 peptide. RARα1/α2-PML mRNA expression was not associated with differences in APL cell sensitivity to all-trans retinoic acid(tRA)-induced differentiation in vitro or in clinical outcome after tRA or chemotherapy induction therapy (protocol E2491). Our analysis indicated that RARα1/α2-PML mRNA expression markedly differs from normal RARα1/α2 mRNA expression, that the difference in RARα1-PML and RARα2-PML mRNA expression frequency is primarily related to the genomic separation of the RARα1 and RARα2 coding exons, and that variations in RARα1/α2-PML mRNA expression likely have no clinically relevant function in APL cells.

RARα1/RARα2-PML mRNA Expression in Acute Promyelocytic Leukemia Cells: A Molecular and Laboratory-Clinical Correlative Study

In addition to the major fusion gene PML-RARα, the t(15; 17) in acute promyelocytic leukemia (APL) produces the reciprocal fusion gene RARα-PML. To determine the scope of RARα-containing mRNA expression in APL cells, we tested PML-RARα–positive APL cells for the presence of mRNAs initiated from two distinct RARα gene promoters, α1 and α2. From the normal allele, both RARα1 and RARα2 mRNAs were expressed in all APL cases (N = 24). From the translocated allele, RARα1-PML mRNA was expressed in 77% and RARα2-PML mRNA in 28% of cases (N = 98). RARα2-PML mRNA was not observed in the absence of RARα1-PML mRNA. There was no association between RARα1-PML or RARα2-PML mRNA expression and the type of PML-RARα mRNA formed by either 5′ or 3′ breaksites in the PML gene. RARα1-PML mRNAs and RARα2-PML mRNAs from 5′ PML breaksite cases coded for full-length RARα-PML proteins but RARα2-PML mRNAs from 3′ PML breaksite cases encoded a truncated RARα2 peptide. RARα1/α2-PML mRNA expression was not associated with differences in APL cell sensitivity to all-trans retinoic acid(tRA)-induced differentiation in vitro or in clinical outcome after tRA or chemotherapy induction therapy (protocol E2491). Our analysis indicated that RARα1/α2-PML mRNA expression markedly differs from normal RARα1/α2 mRNA expression, that the difference in RARα1-PML and RARα2-PML mRNA expression frequency is primarily related to the genomic separation of the RARα1 and RARα2 coding exons, and that variations in RARα1/α2-PML mRNA expression likely have no clinically relevant function in APL cells.

A Retinoid-Resistant Acute Promyelocytic Leukemia Subclone Expresses a Dominant Negative PML-RARα Mutation

The unique t(15; 17) of acute promyelocytic leukemia (APL) fuses the PML gene with the retinoic acid receptor α (RARα) gene. Although retinoic acid (RA) inhibits cell growth and induces differentiation in human APL cells, resistance to RA develops both in vitro and in patients. We have developed RA-resistant subclones of the human APL cell line, NB4, whose nuclear extracts display altered RA binding. In the RA-resistant subclone, R4, we find an absence of ligand binding of PML-RARα associated with a point mutation changing a leucine to proline in the ligand-binding domain of the fusion PML-RARα protein. In contrast to mutations in RARα found in retinoid-resistant HL60 cells, in this NB4 subclone, the coexpressed RARα remains wild-type. In vitro expression of a cloned PML-RARα with the observed mutation in R4 confirms that this amino acid change causes the loss of ligand binding, but the mutant PML-RARα protein retains the ability to heterodimerize with RXRα and thus to bind to retinoid response elements (RAREs). This leads to a dominant negative block of transcription from RAREs that is dose-dependent and not relieved by RA. An unrearranged RARα engineered with this mutation also lost ligand binding and inhibited transcription in a dominant negative manner. We then found that the mutant PML-RARα selectively alters regulation of gene expression in the R4 cell line. R4 cells have lost retinoid-regulation of RXRα and RARβ and the RA-induced loss of PML-RARα protein seen in NB4 cells, but retain retinoid-induction of CD18 and CD38. Thus, the R4 cell line provides data supporting the presence of an RARα-mediated pathway that is independent from gene expression induced or repressed by PML-RARα. The high level of retinoid resistance in vitro and in vivo of cells from some relapsed APL patients suggests similar molecular changes may occur clinically.

The major immediate-early proteins IE1 and IE2 of human cytomegalovirus colocalize with and disrupt PML-associated nuclear bodies at very early times in infected permissive cells

The major immediate-early (MIE) gene products of human cytomegalovirus (HCMV) are nuclear phosphoproteins that are thought to play key roles in initiating lytic cycle gene regulation pathways. We have examined the intranuclear localization pattern of both the IE1 and IE2 proteins in virus-infected and DNA-transfected cells. When HCMV-infected human diploid fibroblast (HF) cells were stained with specific monoclonal antibodies, IE1 localized as a mixture of nuclear diffuse and punctate patterns at very early times (2 h) but changed to an exclusively nuclear diffuse pattern at later times. In contrast, IE2 was distributed predominantly in nuclear punctate structures continuously from 2 to at least 12 h after infection. These punctate structures resembled the preexisting PML-associated nuclear bodies (ND10 or PML oncogenic domains [PODs]) that are disrupted and dispersed by the IE110 protein as a very early event in herpes simplex virus (HSV) infection. However, HCMV differed from HSV by leading instead to a change in both the PML and SP100 protein distribution from punctate bodies to uniform diffuse patterns, a process that was complete in 50% of the cells at 2 h and in 90% of the cells by 4 h after infection. Confocal double-label indirect immunofluorescence assay analysis confirmed that both IE1 and IE2 colocalized transiently with PML in punctate bodies at very early times after infection. In transient expression assays, introduction of IE1-encoding plasmid DNA alone into Vero or HF cells produced the typical total redistribution of PML into a uniform nuclear diffuse pattern together with the IE1 protein, whereas introduction of IE2-encoding plasmid DNA alone resulted in stable colocalization of the IE2 protein with PML in the PODs. A truncated mutant form of IE1 gave large nuclear aggregates and failed to redistribute PML, and similarly a deleted mutant form of IE2 failed to colocalize with the punctate PML bodies, confirming the specificity of these effects. Furthermore, both Vero and U373 cell lines constitutively expressing IE1 also showed total PML relocalization together with the IE1 protein into a nuclear diffuse pattern, although a very small percentage of the cells which failed to express IE1 reverted to a punctate PML pattern. Finally, the PML redistribution activity of IE1 and the direct association of IE2 with PML punctate bodies were both confirmed by infection with E1A-negative recombinant adenovirus vectors expressing either IE1 or IE2 alone. These results confirm that transient colocalization with and disruption of PML-associated nuclear bodies by IE1 and continuous targeting to PML-associated nuclear bodies by IE2 are intrinsic properties of these two MIE regulatory proteins, which we suggest may represent critical initial events for efficient lytic cycle infection by HCMV.

Acute leukemia with promyelocytic features in PML/RARalpha transgenic mice

Acute promyelocytic leukemia (APL) is associated with reciprocal chromosomal translocations involving the retinoic acid receptor alpha (RARalpha) locus on chromosome 17. In the majority of cases, RARalpha translocates and fuses with the promyelocytic leukemia (PML) gene located on chromosome 15. The resulting fusion genes encode the two structurally unique PML/RARalpha and RARalpha/PML fusion proteins as well as aberrant PML gene products, the respective pathogenetic roles of which have not been elucidated. We have generated transgenic mice in which the PML/RARalpha fusion protein is specifically expressed in the myeloid-promyelocytic lineage. During their first year of life, all the PML/RARalpha transgenic mice have an abnormal hematopoiesis that can best be described as a myeloproliferative disorder. Between 12 and 14 months of age, 10% of them develop a form of acute leukemia with a differentiation block at the promyelocytic stage that closely mimics human APL even in its response to retinoic acid. Our results are conclusive in vivo evidence that PML/RARalpha plays a crucial role in the pathogenesis of APL.

Arsenic-induced PML targeting onto nuclear bodies: implications for the treatment of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is associated with the t(15;17) translocation, which generates a PML/RAR alpha fusion protein between PML, a growth suppressor localized on nuclear matrix-associated bodies, and RAR alpha, a nuclear receptor for retinoic acid (RA). PML/RAR alpha was proposed to block myeloid differentiation through inhibition of nuclear receptor response, as does a dominant negative RAR alpha mutant. In addition, in APL cells, PML/RAR alpha displaces PML and other nuclear body (NB) antigens onto nuclear microspeckles, likely resulting in the loss of PML and/or NB functions. RA leads to clinical remissions through induction of terminal differentiation, for which the respective contributions of RAR alpha (or PML/RAR alpha) activation, PML/RAR alpha degradation, and restoration of NB antigens localization are poorly determined. Arsenic trioxide also leads to remissions in APL patients, presumably through induction of apoptosis. We demonstrate that in non-APL cells, arsenic recruits the nucleoplasmic form of several NB antigens onto NB, but induces the degradation of PML only, identifying a powerful tool to approach NB function. In APL cells, arsenic targets PML and PML/RAR alpha onto NB and induces their degradation. Thus, RA and arsenic target RAR alpha and PML, respectively, but both induce the degradation of the PML/RAR alpha fusion protein, which should contribute to their therapeutic effects. The difference in the cellular events triggered by these two agents likely stems from RA-induced transcriptional activation and arsenic effects on NB proteins.

A PMLRARalpha transgene initiates murine acute promyelocytic leukemia

The malignant cells of acute promyelocytic leukemia (APL) contain a reciprocal chromosomal translocation that fuses the promyelocytic leukemia gene (PML) with the retinoic acid receptor alpha gene (RAR alpha). To test the hypothesis that the chimera PMLRAR alpha plays a role in leukemogenesis, we expressed a PMLRAR alpha cDNA in myeloid cells of transgenic mice. PMLRAR alpha transgenic mice exhibited impaired neutrophil maturation early in life, which progressed at a low frequency over the course of several months to overt APL. Both the preleukemic state and the leukemia could be transplanted to nontransgenic mice, and the transplanted preleukemia could progress to APL. The APL recapitulated features of the human disease, including a response to retinoic acid. Retinoic acid caused the leukemic cells to differentiate in vitro and in vivo, eliciting remissions of both the preleukemic state and APL in mice. Our results demonstrate that PMLRAR alpha impairs neutrophil differentiation and initiates the development of APL. The transgenic mice described here provide an apparently accurate model for human APL that includes clear evidence of tumor progression. The model should be useful for exploring the molecular pathogenesis of APL and the mechanisms of the therapeutic response to retinoic acid, as well as for preclinical studies of therapeutic regimens.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

Altered Myeloid Development and Acute Leukemia in Transgenic Mice Expressing PML-RARα Under Control of Cathepsin G Regulatory Sequences

Acute promyelocytic leukemia (APML) is characterized by abnormal myeloid development, resulting an accumulation of leukemic promyelocytes that are often highly sensitive to retinoic acid. A balanced t(15; 17) (q22; q21) reciprocal chromosomal translocation is found in approximately 90% of APML patients; this translocation fuses the PML gene on chromosome 15 to the retinoic acid receptor α (RARα) gene on chromosome 17, creating two novel fusion genes, PML-RARα and RARα-PML. The PML-RARα fusion gene product, which is expressed in virtually all patients with t(15; 17), is thought to play a direct role in the pathogenesis of APML. To determine whether PML-RARα is sufficient to cause APML in an animal model, we used the promyelocyte-specific targeting sequences of the human cathepsin G (hCG) gene to direct the expression of a PML-RARα cDNA to the early myeloid cells of transgenic mice. Mice expressing the hCG–PML-RARα transgene were found to have altered myeloid development that was characterized by increased percentages of immature and mature myeloid cells in the peripheral blood, bone marrow, and spleen. In addition, approximately 30% of transgene-expressing mice eventually developed acute myeloid leukemia after a long latent period. The splenic promyelocytes of mice with both the nonleukemic and leukemic phenotypes responded to all-trans retinoic acid (ATRA) treatment, which caused apoptosis of myeloid precursors. Although low-level expression of the hCG–PML-RARα transgene is not sufficient to directly cause acute myeloid leukemia in mice, its expression alters myeloid development, resulting in an accumulation of myeloid precursors that may be susceptible to cooperative transforming events.

Identification of Bmi1-interacting proteins as constituents of a multimeric mammalian polycomb complex

The Bmi1 gene has been identified as a mouse Polycomb group (Pc-G) gene implicated in the regulation of Hox gene expression. Here we describe the characterization of a Bmi binding protein Mph1, which shares similarity to Drosophila polyhomeotic. Coimmunoprecipitation experiments indicate that Bmi1 and Mph1, as well as the Mel18 and M33 proteins described previously, are constituents of a multimeric protein complex in mouse embryos and human cells. A central domain of Bmi1 interacts with the carboxyl terminus of Mph1, whereas a conserved alpha-helical domain in the Mph1 protein is required for its homodimerization. Transgenic mice overexpressing various mutant Bmi1 proteins demonstrate that the central domain of Bmil is required for the induction of anterior transformations of the axial skeleton. Bmi1, M33, and Mph1 show an overlapping speckled distribution in interphase nuclei. These data provide molecular evidence for the existence of a mammalian Polycomb complex.

Characterisation of the PML/RAR alpha rearrangement associated with t(15;17) acute promyelocytic leukaemia

The vast majority of cases of APL are associated with t(15; 17) leading to the formation of PML-RAR alpha, RAR alpha-PML and aberrant PML fusion products. PML-RAR alpha is invariably transcribed and is believed to mediate leukaemogenesis. PML was initially considered to be a transcription factor. However, characterisation of other RING finger containing proteins shows no direct evidence for DNA binding. The RING, B-box, and coiled-coil domains are more likely to represent sites of protein-protein interaction and may be critical for the stability of the multiprotein nuclear domains of which PML is an integral part. In APL the nuclear bodies become disrupted, presumably as a consequence of the presence of PML-RAR alpha and aberrant PML proteins that might render the structure unstable. PML-RAR alpha is capable of binding RXR and sequestering it into the disrupted nuclear domains. Sequestration of RXR would be expected to limit high affinity binding of VDR, TR and residual RARs to DNA response elements and might account for the block in myeloid differentiation at the promyelocyte stage that characterizes APL. Recently PML has been found to have growth suppressor/anti-oncogenic activity. It is unclear whether this is a property of PML itself or reflects a nonspecific function of the PML-associated nuclear domains. Hence the PML/RAR alpha rearrangement alone may be sufficient to cause APL. Abnormal PML function may prevent its growth-suppressor activity, leading to leukaemic transformation; concomitant disruption of retinoid pathways due to sequestration of RXR and/or an abnormal repertoire and character of response element activation mediated by the fusion protein, causing the block in myeloid differentiation (Fig. 3). Disruption of RAR alpha would be expected to account for the similar leukaemic phenotype associated with the t(5;17) and t(11;17) APL cytogenetic variants. Further characterisation of NPM and PLZF at the structural and functional level will determine whether PML and other proteins disrupted in APL associated translocations play an active or purely permissive role in leukaemogenesis and will help dissect the events leading to transformation from those causing blockade of myeloid differentiation and mediating the response to ATRA.

Transcriptional repression by RING finger protein TIF1 beta that interacts with the KRAB repressor domain of KOX1

Many of the vertebrate zinc finger factors of the Kruppel type (C2H2 zinc fingers) contain in their N-terminus a conserved sequence referred to as the KRAB (Kruppel-associated box) domain that, when tethered to DNA, efficiently represses transcription. Using the yeast two-hybrid system, we have isolated an 835 amino acid RING finger (C3HC4 zinc finger) protein, TIF1 beta (also named KAP-1), that specifically interacts with the KRAB domain of the human zinc finger factor KOX1/ZNF10. TIF1 beta, TIF1 alpha, PML and efp belong to a characteristic subgroup of RING finger proteins that contain one or two other Cys/His-rich clusters (B boxes) and a putative coiled-coil in addition to the classical C3HC4 RING finger motif (RBCC configuration). Like TIF1 alpha, TIF1 beta also contains an additional Cys/His cluster (PHD finger) and a bromo-related domain. When tethered to DNA, TIF1 beta can repress transcription in transiently transfected mammalian cells both from promoter-proximal and remote (enhancer) positions, similarly to the KRAB domain itself. We propose that TIF1 beta is a mediator of the transcriptional repression exerted by the KRAB domain.

PML-containing nuclear bodies: their spatial distribution in relation to other nuclear components

The PML protein is a human growth suppressor concentrated in 10 to 20 nuclear bodies per nucleus (PML bodies). Disruption of the PML gene has been shown to be related to acute promyelocytic leukaemia (APL). To obtain information about the function of PML bodies we have investigated the 3D-distribution of PML bodies in the nucleus of T24 cells and compared it with the spatial distribution of a variety of other nuclear components, using fluorescence dual-labeling immunocytochemistry and confocal microscopy. Results show that PML bodies are not enriched in nascent RNA, the splicing component U2-snRNP, or transcription factors (glucocorticoid receptor, TFIIH, and E2F). These results show that PML bodies are not prominent sites of RNA synthesis or RNA splicing. We found that a large fraction of PML bodies (50 to 80%) is closely associated with DNA replication domains during exclusively middle-late S-phase. Furthermore, in most cells that we analysed we found at least one PML body was tightly associated with a coiled body. In the APL cell line NB4, the PML gene is fused with the RAR alpha gene due to a chromosomal rearrangement. PML bodies have disappeared and the PML antigen, i.e., PML and the PML-RAR fusion protein, is dispersed in a punctated pattern throughout the nucleoplasm. We showed that in NB4 cells the sites that are rich in PML antigen significantly colocalize with sites at which nascent RNA accumulates. This suggests that, in contrast to non-APL cells, in NB4 cells the PML antigen is associated with sites of transcription. The implications of these findings for the function of PML bodies are consistent with the idea that PML bodies are associated with specific genomic loci.

ITA, a vertebrate homologue of IAP that is expressed in T lymphocytes

Apoptosis plays a crucial role in both the development and the control of the immune system. During T lymphocyte development, thymocytes undergo apoptosis as part of the process of elimination of self-reactive clones. Mature T cells also undergo apoptosis following antigen-stimulated proliferation as part of a mechanism that controls the immune response. Apoptosis also provides a defense mechanism against viruses whereby the rapid death of virus-infected cells reduces virus spread. Viruses, on the other hand, often express proteins that inhibit apoptosis of their host cells, thereby enhancing their infectivity. We have isolated a novel gene, ita (inhibitor of T cell apoptosis), which is a vertebrate homologue of the viral apoptosis inhibitor IAP. Expression of ita appears to be restricted to cells of the T lymphocyte lineage, and high levels of ita mRNA are induced within 4-8 hr of T cell activation. Immunohistologic studies show that medullary and cortical thymocytes express detectable levels of ITA. ITA is a 69 kDa protein that contains a C-terminal ring-finger motif that is found in several oncogenic proteins and N-terminal repeat elements that have only been reported in other apoptosis inhibitors. These findings suggest that ITA may play a role in controlling apoptosis in T cells.

Acute promyelocytic leukemia with t(15;16;17;19) and unusual fluorescence in situ hybridization pattern with PML and RARA probes

We report a 58-year-old female with typical morphological and clinical features of acute promyelocytic leukemia in whom a complex translocation involving chromosomes 15, 16, 17 and 19 was detected using conventional cytogenetics and fluorescence in situ hybridization (FISH) with chromosome specific paints. RARA-PML fusion was not evident by FISH, but the RARA signal was split in 74.5% of cells. GTL-banding and FISH with probes for PML, RARA and chromosome 15 specific paint raise the possibility of PML-RARA fusion on the abnormal chromosome 19 in the complex translocation. The unusual PML-RARA fusion may be related to this patient's poor response to induction therapy with all-trans-retinoic acid.

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.

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.

TIF2, a 160 kDa transcriptional mediator for the ligand-dependent activation function AF-2 of nuclear receptors

Nuclear receptors (NRs) act as ligand-inducible transcription factors which regulate the expression of target genes upon binding to cognate response elements. The ligand-dependent activity of the NR activation function AF-2 is believed to be mediated to the transcription machinery through transcriptional mediators/intermediary factors (TIFs). We report here the cloning of the 160 kDa human nuclear protein TIF2, which exhibits all properties expected for a mediator of AF-2: (i) it interacts in vivo with NRs in an agonist-dependent manner; (ii) it binds directly to the ligand-binding domains (LBDs) of NRs in an agonist- and AF-2-integrity-dependent manner in vitro; (iii) it harbours an autonomous transcriptional activation function; (iv) it relieves nuclear receptor autosquelching; and (v) it enhances the activity of some nuclear receptor AF-2s when overexpressed in mammalian cells. TIF2 exhibits partial sequence homology with the recently isolated steroid receptor coactivator SRC-1, indicating the existence of a novel gene family of nuclear receptor transcriptional mediators.

An Alu element in the myeloperoxidase promoter contains a composite SP1-thyroid hormone-retinoic acid response element

An Alu element preceding the myeloperoxidase gene (MPO) contains four hexamer motifs related to the consensus recognition sequence for nuclear hormone receptors (AGGTCA), arranged as direct repeats with spacing of 2, 4, and 2 nucleotides (DR-2-4-2). Gel shift experiments and transient transfection assays demonstrate that these sequences include binding sites for retinoic acid and thyroid hormone receptors and function in vivo to activate transcription of a chloramphenicol acetyltransferase reporter gene. The first DR-2 elements of the series do not bind known receptors but do bind the SP1 transcription factor. Two alleles of the MPO gene exist that differ at one position within this element, resulting in one allele with and one without a strong SP1 binding site. The element with the SP1 site activates transcription by 25-fold in transient transfection assays, while the alternative allele confers severalfold less transcriptional activity. Most cases of acute myelocytic leukemia are homozygous for the allele with the SP1 binding site, suggesting this element plays an important role in regulating the MPO gene in myeloid leukemias. This MPO-Alu is a representative of an Alu subclass numbering approximately 400,000 copies, suggesting many genes may be regulated by such elements.

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.

Immunohistochemical analysis of retinoic acid receptor-alpha in human breast tumors: retinoic acid receptor-alpha expression correlates with proliferative activity

Retinoids are known to prevent mammary carcinogenesis in rodents and inhibit the growth of human breast cancer cells in vitro. Previously we demonstrated that retinoid inhibition of proliferation of human breast cancer cell lines is largely mediated by retinoic acid receptor (RAR)-alpha. In this study we describe for the first time the histological distribution of RAR-alpha in 33 breast lesion specimens as determined by immunostaining with RAR-alpha antibody. Nuclear staining was observed in tumor tissue and normal portions of the breast samples. Connective tissue exhibited relative uniform staining, whereas a wide range of RAR-alpha expression was found in the epithelial tumor cells. RAR-alpha protein was expressed at significantly higher levels in tumors with greater proliferative activity as determined by immunostaining with Ki-67 antibody. This suggests that RAR-alpha expression may be altered with tumor progression. Although a positive correlation between RAR-alpha mRNA levels and estrogen receptor status of breast tumors has previously been documented, we did not find such a relationship at the protein level. As RAR-alpha plays a major role in retinoid-mediated growth inhibition of human breast cancer cell in vitro, our findings suggest that patients with highly proliferating tumors could be responsive to retinoid independently of their responsiveness to (anti)-estrogens.

Ligand-dependent interaction of nuclear receptors with potential transcriptional intermediary factors (mediators)

The activity of the ligand-inducible activation function 2 (AF-2) contained in the ligand binding domain (LBD) of nuclear receptors (NRs) is thought to be mediated by transcriptional intermediary factors (TIFs). We have recently reported the isolation and characterization of two novel mouse proteins, designated TIF1 and mSUG1, that interact in a ligand-dependent fashion with the LBD (region E) of several NRs in vivo as well as in vitro. Remarkably, these interactions require the conserved core motif of the AF-2 activating domain (AF-2 AD) and can be blocked by AF-2 antagonists. TIF1 and mSUG1 might therefore represent TIFs/mediators for the ligand-dependent AF-2 of NRs. By comparing the interaction properties of these two putative TIFs with different NRs including the oestrogen (ER), thyroid hormone (TR), vitamin D3 (VDR), retinoic acid (RAR alpha) and retinoid X (RXR) receptors, we demonstrate that: (i) RXR alpha efficiently interacts with TIF1, but not with mSUG1, whereas TR alpha interacts much more efficiently with mSUG1 than with TIF1, and RAR alpha, VDR and ER efficiently interact with both TIF1 and mSUG1; (ii) the amphipathic alpha helix core of AF-2 AD is differentially involved in the interactions of RAR alpha with TIF1 and mSUG1; and (iii) the AF-2 AD cores of RAR alpha and ER are similarly involved in their interaction with TIF1, but not with mSUG1. Thus the interaction interfaces between the various NRs and either TIF1 or mSUG1 may vary depending on the nature of both the receptor and the putative mediator of its AF-2 function. We discuss the possible roles of TIF1 and mSUG1 as mediators of the transcriptional activity of the AF-2 of NRs.

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.

Zinc fingers 1-7 of EVI1 fail to bind to the GATA motif by itself but require the core site GACAAGATA for binding

EVI1 is a zinc finger oncoprotein that binds via fingers 1-7 to the sequence GACAAGATAA. The target genes on which EVI1 acts are unknown. This binding motif overlaps with that for the GATA transcription factors, (T/A)GATA(A/G), and GATA-1 can bind to and activate transcription via a GACAAGATAA motif. The possibility has been raised that, when overexpressed in leukemogenesis, EVI1 may function by interfering with the differentiation-promoting action of GATA factors. To explore this, we have assessed the affinity of EVI1 for the GATA binding sites derived from erythroid-specific GATA-1 target genes, and found only low affinity interactions. We examined the contacts between EVI1 and DNA by methylation interference studies, which revealed extensive contacts between EVI1 and its binding site. The importance of the contacts for high affinity binding was shown by in vitro quantitative gel shift studies and in vivo cotransfection studies. To examine what types of sequences from mouse genomic DNA bind to EVI1, we isolated and sequenced five EVI1-binding fragments, and each showed the GACAAGATA site. The data presented contribute to our knowledge of the binding specificity of EVI1, and yield a clearer picture of what sequences can, and cannot, act as targets for EVI1 action.

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.

The architectural organization of nuclear metabolism

Nucleic acid metabolism is structurally organized in the nucleus. DNA replication and transcription have been localized to particular nuclear domains. Additional domains have been identified by their morphology or by their composition; for example, by their high concentration of factors involved in RNA splicing. The domain organization of the nucleus is maintained by the nuclear matrix, a nonchromatin nuclear scaffolding that holds most nuclear RNA and organizes chromatin into loops. The nuclear matrix is built on a network of highly branched core filaments that have an average diameter of 10 nm. Many of the intermediates and the regulatory and catalytic factors of nucleic acid metabolism are retained in nuclear matrix preparations, suggesting that nucleic acid synthesis and processing are structure-bound processes in cells. Tissue-specific and malignancy-induced variations in nuclear structure and metabolism may result from altered matrix architecture and composition.

Nuclear domain 10 (ND10) associated proteins are also present in nuclear bodies and redistribute to hundreds of nuclear sites after stress

The promyelocytic leukemia protein fused to the retinoic acid receptor alpha in t(15;17) acute promyelocytic leukemia, the primary biliary cirrhosis autoantigen, Sp100, as well as the incompletely characterized protein NDP55, are co-localized in specific immunohistochemically defined nuclear domains (ND10), which are potential equivalents of ultrastructurally defined nuclear bodies. We investigated whether the distribution of these proteins depends on environmental conditions and whether ND10 correlate with nuclear bodies. Certain nuclear bodies and ND10 react in a similar way and share antigens. Interferon exposure doubled the number of ND10 and increased the frequency of nuclear bodies, whereas herpes simplex virus infection or heat shock modify both. Redistribution of ND10-associated proteins to hundreds of small sites throughout the chromatin was inducible by stress in the form of heat shock and exposure to Cd++ ions. The change of distribution was rapid and independent of protein synthesis, and thus not part of the classical heat shock response. The very rapid redistribution of these proteins after heat shock, together with the development of ND10 upon interferon activation, raises the possibility that ND10 represent storage sites of certain matrix proteins readily accessible throughout the chromatin in response to stress or other effectors that induce global nuclear changes.

Molecular cloning, structure, and expression of mouse estrogen-responsive finger protein Efp. Co-localization with estrogen receptor mRNA in target organs

We have previously identified a human estrogen-responsive gene, efp (estrogen-responsive finger protein), which encodes a putative transcription regulator (Inoue, S., Orimo, A., Hosoi, T., Kondo, S., Toyoshima, H., Kondo, T., Ikegami, A., Ouchi, Y., Orimo, H., and Muramatsu, M. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 11117-11121). Here, we report isolation of mouse Efp cDNA and its structure containing three cysteine-rich domains (RING finger and B1 and B2 boxes), a coiled-coil domain, and a C-terminal domain. High levels of Efp mRNA were detected in uterus, ovary, and placenta by RNase protection assay. By in situ hybridization histochemistry the transcripts of efp were also detected in uterus, mammary gland, ovary, and brain, and the co-localization of Efp and estrogen receptor mRNA was particularly demonstrated in these female organs. Moreover, the level of Efp mRNA in uterus and brain, which are known as target organs for estrogen, was up-regulated in vivo by 17 beta-estradiol. Furthermore, both the Efp and estrogen receptor mRNA were stained in the brain vesicles of 11.5-day embryos by whole mount in situ hybridization. These findings raise the possibility that efp is an estrogen-responsive gene that mediates estrogen action in various target organs.

Alternative mechanisms of CAK assembly require an assembly factor or an activating kinase

We have cloned a mouse cDNA that encodes p36, a novel subunit of the CDK-activating kinase (CAK). p36 contains a C3HC4 zinc-binding domain or RING factor and is associated both with a TFIIH-bound form of CAK and with a free trimeric form. p36 promotes the assembly of CDK7 and cyclin H in vitro, stabilizing the transient CDK7-cyclin H complex. Stabilization and activation of CAK by p36 is independent of the phosphorylation state of T170, the conserved activating residue of CDK7. Assembly of active CDK7-cyclin H dimers can also occur through an alternative p36-independent pathway that requires phosphorylation of T170 by a CAK-activating kinase, or CAKAK. Thus, CDK7-cyclin H complex formation can be achieved by multiple mechanisms.

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.

Two forms of Xenopus nuclear factor 7 have overlapping spatial but different temporal patterns of expression during development

Xenopus nuclear factor 7 (xnf7) is a maternal gene product that functions in the determination of the dorsal-ventral body axis. We have cloned two xnf7 cDNAs, xnf7-O and xnf7-B, that have a different temporal pattern of expression. The cDNAs differ by 39 amino acid residues scattered throughout the molecule. Most of the changes were conservative in nature. Using gene specific probes we found that xnf7-O transcripts were abundant in oocytes and decreased until the neurula stage, after which they increased in abundance. Xnf7-B transcripts were in low abundance in oocytes and were expressed at high levels at the neurula stage and in adult brain. Both xnf7-O and xnf7-B transcripts at the neurula stage were localized in the dorsal region of the embryo, including the neural folds and somites. Xnf7 was not expressed in ventralized embryos that lacked dorsal structures, thereby substantiating its dorsal localization in the embryo. The promoter region of the xnf7-O gene does not possess a TATA box but does contain E2F, USF, Sp1-like and AP1 binding sites within the first 421 bp from the transcription initiation site. A 62 bp fragment of the xnf7-O promoter containing the Sp1-like and E2F sites can direct proper spatial expression of a transgene in embryos.