PML, a growth suppressor disrupted in acute promyelocytic leukemia

1994 ◽  
Vol 14 (10) ◽  
pp. 6858-6867
Author(s):  
Z M Mu ◽  
K V Chin ◽  
J H Liu ◽  
G Lozano ◽  
K S Chang
Keyword(s):  
Fusion Protein ◽  
Acute Promyelocytic Leukemia ◽  
Cellular Localization ◽  
Retinoic Acid Receptor ◽  
Critical Role ◽  
Promyelocytic Leukemia ◽  
Dominant Negative ◽  
Dominant Negative Inhibitor ◽  
Growth Suppressor

The nonrandom chromosomal translocation t(15;17)(q22;q21) in acute promyelocytic leukemia (APL) juxtaposes the genes for retinoic acid receptor alpha (RAR alpha) and the putative zinc finger transcription factor PML. The breakpoint site encodes fusion protein PML-RAR alpha, which is able to form a heterodimer with PML. It was hypothesized that PML-RAR alpha is a dominant negative inhibitor of PML. Inactivation of PML function in APL may play a critical role in APL pathogenesis. Our results demonstrated that PML, but not PML-RAR alpha, is a growth suppressor. This is supported by the following findings: (i) PML suppressed anchorage-independent growth of APL-derived NB4 cells on soft agar and tumorigenicity in nude mice, (ii) PML suppressed the oncogenic transformation of rat embryo fibroblasts by cooperative oncogenes, and (iii) PML suppressed transformation of NIH 3T3 cells by the activated neu oncogene. Cotransfection of PML with PML-RAR alpha resulted in a significant reduction in PML's transformation suppressor function in vivo, indicating that the fusion protein can be a dominant negative inhibitor of PML function in APL cells. This observation was further supported by the finding that cotransfection of PML and PML-RAR alpha resulted in altered normal cellular localization of PML. Our results also demonstrated that PML, but not PML-RAR alpha, is a promoter-specific transcription suppressor. Therefore, we hypothesized that disruption of the PML gene, a growth or transformation suppressor, by the t(15;17) translocation in APL is one of the critical events in leukemogenesis.

scholarly journals PML, a growth suppressor disrupted in acute promyelocytic leukemia.

1994 ◽  
Vol 14 (10) ◽  
pp. 6858-6867 ◽  
Author(s):  
Z M Mu ◽  
K V Chin ◽  
J H Liu ◽  
G Lozano ◽  
K S Chang
Keyword(s):  
Fusion Protein ◽  
Acute Promyelocytic Leukemia ◽  
Cellular Localization ◽  
Retinoic Acid Receptor ◽  
Critical Role ◽  
Promyelocytic Leukemia ◽  
Dominant Negative ◽  
Dominant Negative Inhibitor ◽  
Growth Suppressor

The nonrandom chromosomal translocation t(15;17)(q22;q21) in acute promyelocytic leukemia (APL) juxtaposes the genes for retinoic acid receptor alpha (RAR alpha) and the putative zinc finger transcription factor PML. The breakpoint site encodes fusion protein PML-RAR alpha, which is able to form a heterodimer with PML. It was hypothesized that PML-RAR alpha is a dominant negative inhibitor of PML. Inactivation of PML function in APL may play a critical role in APL pathogenesis. Our results demonstrated that PML, but not PML-RAR alpha, is a growth suppressor. This is supported by the following findings: (i) PML suppressed anchorage-independent growth of APL-derived NB4 cells on soft agar and tumorigenicity in nude mice, (ii) PML suppressed the oncogenic transformation of rat embryo fibroblasts by cooperative oncogenes, and (iii) PML suppressed transformation of NIH 3T3 cells by the activated neu oncogene. Cotransfection of PML with PML-RAR alpha resulted in a significant reduction in PML's transformation suppressor function in vivo, indicating that the fusion protein can be a dominant negative inhibitor of PML function in APL cells. This observation was further supported by the finding that cotransfection of PML and PML-RAR alpha resulted in altered normal cellular localization of PML. Our results also demonstrated that PML, but not PML-RAR alpha, is a promoter-specific transcription suppressor. Therefore, we hypothesized that disruption of the PML gene, a growth or transformation suppressor, by the t(15;17) translocation in APL is one of the critical events in leukemogenesis.

scholarly journals In vivo analysis of the role of aberrant histone deacetylase recruitment and RARα blockade in the pathogenesis of acute promyelocytic leukemia

2006 ◽  
Vol 203 (4) ◽  
pp. 821-828 ◽  
Author(s):  
Hiromichi Matsushita ◽  
Pier Paolo Scaglioni ◽  
Mantu Bhaumik ◽  
Eduardo M. Rego ◽  
Lu Fan Cai ◽  
...  
Keyword(s):  
Retinoic Acid ◽  
Fusion Protein ◽  
Acute Promyelocytic Leukemia ◽  
Histone Deacetylases ◽  
Retinoic Acid Receptor ◽  
Promyelocytic Leukemia ◽  
Dominant Negative

The promyelocytic leukemia–retinoic acid receptor α (PML-RARα) protein of acute promyelocytic leukemia (APL) is oncogenic in vivo. It has been hypothesized that the ability of PML-RARα to inhibit RARα function through PML-dependent aberrant recruitment of histone deacetylases (HDACs) and chromatin remodeling is the key initiating event for leukemogenesis. To elucidate the role of HDAC in this process, we have generated HDAC1–RARα fusion proteins and tested their activity and oncogenicity in vitro and in vivo in transgenic mice (TM). In parallel, we studied the in vivo leukemogenic potential of dominant negative (DN) and truncated RARα mutants, as well as that of PML-RARα mutants that are insensitive to retinoic acid. Surprisingly, although HDAC1-RARα did act as a bona fide DN RARα mutant in cellular in vitro and in cell culture, this fusion protein, as well as other DN RARα mutants, did not cause a block in myeloid differentiation in vivo in TM and were not leukemogenic. Comparative analysis of these TM and of TM/PML−/− and p53−/− compound mutants lends support to a model by which the RARα and PML blockade is necessary, but not sufficient, for leukemogenesis and the PML domain of the fusion protein provides unique functions that are required for leukemia initiation.

In Vivo Analysis of the Role of C/EBPα in Acute Promyelocytic Leukemia Genesis.

Blood ◽  
2006 ◽  
Vol 108 (11) ◽  
pp. 1937-1937
Author(s):  
Barbara A.A. Santana-Lemos ◽  
Florence Guibal ◽  
Maria-Carolina Pintao ◽  
Priscila S. Scheucher ◽  
Rodrigo S. Abreu-Lima ◽  
...  
Keyword(s):  
Gene Expression ◽  
Fusion Protein ◽  
Acute Promyelocytic Leukemia ◽  
Leucine Zipper ◽  
Promyelocytic Leukemia ◽  
Dominant Negative ◽  
Leukemic Cells ◽  
Spleen Cells ◽  
Specific Subset

Abstract Acute promyelocytic leukemia (APL) is characterized by the infiltration of bone marrow (BM) and peripheral blood (PB) by leukemic cells presenting a block of differentiation at the stage of promyelocytes. At the cytogenetic level, APL is associated with the t(15;17) which causes the fusion of two genes: Retinoic Receptor α (RARα) and Promyelocytic Leukemia (PML), on chromosomes 17 and 15, respectively. C/EBPα is a leucine zipper transcription factor essential to normal granulopoiesis. Mutations in C/EBPα gene are detected in 6–10% of Acute Myeloid Leukemia (AML) cases and C/EBPα activity is down-regulated by AML1-ETO fusion protein associated with FAB M2 subtype. We decided to test whether PML-RARα interferes with C/EBPα function, thus contributing to the blockage of differentiation characteristic of APL. We generated mutant mice expressing PML-RARα and haploinsufficient for C/EBPα (PR C/EBPα+/−) by crossing hCG PML-RARα transgenic mice (PR TM) and C/EBPα+/−. Leukemia was not detected in WT (n=415) and C/EBPα+/− (n=47) mice after 800 days of follow up. In contrast, 8.2% PR TM (19/233) and 14.9% (13/87) PR C/EBPα+/− mice developed a form of leukemia that closely resembled human APL and identical to that developed by PR TM. The leukemia-free survival was significantly shorter in PR C/EBPα+/− compared to PR TM (644.3 days; 95% Confidence Interval, 95%C.I.: 586.4 – 702.2 vs 718.4 days; 95%C.I.: 689.3 – 747.5, P=0.02). Both groups presented a long latency for the development of the disease with a mean age (95%C.I.) at diagnosis of 399.9 days (184–673) and 495.8 days (215–757) in PR and PR C/EBPα+/−, respectively. PR and PR C/EBPα+/− leukemic mice presented similar WBC counts (107.1 ± 82.65x103vs 63.55 ± 57.82x103cells/ml, P=0.26), hemoglobin concentrations (10.87 ± 3.69 vs 9.92 ± 2.39 g/dl, P =0.57) and platelet counts (283.8 ± 188.7x103vs 177.8 ± 149.5x103platelets/ml, P =0.24). In both groups, the leukemic cells resembled promyelocytes, and expressed the phenotype CD11b+ Gr1+ CD34± c-Kit+, which represented 46.65 ± 26.89%; 32.72 ± 15.16% and 1.91 ± 1.42% of the spleen cells from PR; PR C/EBPα+/− and WT, respectively. In order to isolate PML-RARα leukemic cells and their normal counterparts to gene expression assessment, BM samples from WT mice (pooled from 2 mice), non leukemic PR C/EBPα+/− mice (pooled from 2 mice), and spleen cells from WT and leukemic PR C/EBPα+/− mice were first submitted to red cells lyses, stained with previously conjugated antibodies. Cells expressing CD16/32, CD11b, c-kit and CD34, but neither CD3 nor CD45/B220 were isolated. These cells presented medium to large size with a granular cytoplasm. C/EBPα and PML-RARα expression was analyzed by qRT-PCR in the sorted cells. Compared to WT cells, promyelocytes from leukemic and non leukemic PR C/EBPα+/− mice expressed significantly less C/EBPα, with the lowest levels detected in leukemic samples. As expected, PML-RARα was not detected in WT samples. Comparison between cells suspensions containing similar numbers of promyelocytes revealed that PML-RARα expression was higher in leukemic compared to non leukemic PR C/EBPα+/− mice. In conclusion, our data strongly suggest that PML-RARα fusion protein acts as a dominant negative product on C/EBPα gene expression in a specific subset of early myeloid cells and contribute to the pathogenesis of APL.

scholarly journals The PML/RAR alpha oncoprotein is a direct molecular target of retinoic acid in acute promyelocytic leukemia cells

Blood ◽  
1996 ◽  
Vol 88 (8) ◽  
pp. 2826-2832 ◽  
Author(s):  
JV Raelson ◽  
C Nervi ◽  
A Rosenauer ◽  
L Benedetti ◽  
Y Monczak ◽  
...  
Keyword(s):  
Retinoic Acid ◽  
Fusion Protein ◽  
Cell Line ◽  
Acute Promyelocytic Leukemia ◽  
Promyelocytic Leukemia ◽  
Dominant Negative ◽  
Myeloid Differentiation ◽  
Wild Type ◽  
Retinoid Receptor ◽  
Western Blots

Acute promyelocytic leukemia (APL) is characterized by the translocation, t(15;17) and the expression of a PML/RAR alpha fusion protein that is diagnostic of the disease. There is evidence that PML/RAR alpha protein acts as a dominant negative inhibitor of normal retinoid receptor function and myeloid differentiation. We now show that the PML/RAR alpha fusion product is directly downregulated in response to retinoic acid (tRA) treatment in the human APL cell line, NB4. tRA treatment induces loss of PML/RAR alpha at the protein level but not at the level of mRNA, as determined by Northern blots, by Western blots, and by ligand binding assays and in binding to RA-responsive DNA elements. We present evidence that this regulation is posttranslational. This evidence suggests that tRA induces synthesis of a protein that selectively degrades PML/RAR alpha. We further show that this loss of PML/ RAR-alpha is not limited to the unique APL cell line. NB4, because PML/RAR alpha protein is selectively downregulated by tRA when expressed in the transfected myeloid cell line U937. The loss of PML/RAR alpha may be directly linked to tRA-induced differentiation, because in a retinoid-resistant subclone of NB4, tRA does not decrease PML/RAR alpha protein expression. In NB4 cells, the specific downregulation of the fusion protein decreases the ratio of PML/RAR alpha to wild-type RAR alpha. Because the ratio of expression of PML/RAR alpha to wild-type RAR alpha and PML may be important in maintaining the dominant negative block of myelocytic differentiation, these data suggest a molecular mechanism for restoration by tRA normal myeloid differentiation in APL cells.

AML-1/ETO fusion protein is a dominant negative inhibitor of transcriptional repression by the promyelocytic leukemia zinc finger protein

Blood ◽  
2000 ◽  
Vol 96 (12) ◽  
pp. 3939-3947 ◽  
Author(s):  
Ari Melnick ◽  
Graeme W. Carlile ◽  
Melanie J. McConnell ◽  
Adam Polinger ◽  
Scott W. Hiebert ◽  
...  
Keyword(s):  
Fusion Protein ◽  
Zinc Finger ◽  
Transcriptional Repression ◽  
Target Genes ◽  
Zinc Finger Protein ◽  
Promyelocytic Leukemia ◽  
Dominant Negative ◽  
Myelogenous Leukemia ◽  
Promyelocytic Leukemia Zinc Finger ◽  
Dominant Negative Inhibitor

Abstract The AML-1/ETO fusion protein, created by the (8;21) translocation in M2-type acute myelogenous leukemia (AML), is a dominant repressive form of AML-1. This effect is due to the ability of the ETO portion of the protein to recruit co-repressors to promoters of AML-1 target genes. The t(11;17)(q21;q23)-associated acute promyelocytic leukemia creates the promyelocytic leukemia zinc finger PLZFt/RARα fusion protein and, in a similar manner, inhibits RARα target gene expression and myeloid differentiation. PLZF is expressed in hematopoietic progenitors and functions as a growth suppressor by repressing cyclin A2 and other targets. ETO is a corepressor for PLZF and potentiates transcriptional repression by linking PLZF to a histone deacetylase-containing complex. In transiently transfected cells and in a cell line derived from a patient with t(8;21) leukemia, PLZF and AML-1/ETO formed a tight complex. In transient assays, AML-1/ETO blocked transcriptional repression by PLZF, even at substoichiometric levels relative to PLZF. This effect was dependent on the presence of the ETO zinc finger domain, which recruits corepressors, and could not be rescued by overexpression of co-repressors that normally enhance PLZF repression. AML-1/ETO also excluded PLZF from the nuclear matrix and reduced its ability to bind to its cognate DNA-binding site. Finally, ETO interacted with PLZF/RARα and enhanced its ability to repress through the RARE. These data show a link in the transcriptional pathways of M2 and M3 leukemia.

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

Blood ◽  
1997 ◽  
Vol 89 (12) ◽  
pp. 4282-4289 ◽  
Author(s):  
Wenlin Shao ◽  
Laura Benedetti ◽  
William W. Lamph ◽  
Clara Nervi ◽  
Wilson H. Miller
Keyword(s):  
Gene Expression ◽  
Retinoic Acid ◽  
Ligand Binding ◽  
Cell Line ◽  
Acute Promyelocytic Leukemia ◽  
Regulation Of Gene Expression ◽  
Promyelocytic Leukemia ◽  
Dominant Negative

Abstract 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.

scholarly journals PMLRARα binds to Fas and suppresses Fas-mediated apoptosis through recruiting c-FLIP in vivo

Blood ◽  
2011 ◽  
Vol 118 (11) ◽  
pp. 3107-3118 ◽  
Author(s):  
Rong-Hua Tao ◽  
Zuzana Berkova ◽  
Jillian F. Wise ◽  
Abdol-Hossein Rezaeian ◽  
Urszula Daniluk ◽  
...  
Keyword(s):  
Retinoic Acid Receptor ◽  
Lethal Dose ◽  
Direct Interaction ◽  
Promyelocytic Leukemia ◽  
Dominant Negative ◽  
Mass Spectrometry Analysis ◽  
Death Domain ◽  
Spectrometry Analysis ◽  
Fas Signaling

Abstract Defective Fas signaling leads to resistance to various anticancer therapies. Presence of potential inhibitors of Fas which could block Fas signaling can explain cancer cells resistance to apoptosis. We identified promyelocytic leukemia protein (PML) as a Fas-interacting protein using mass spectrometry analysis. The function of PML is blocked by its dominant-negative form PML–retinoic acid receptor α (PMLRARα). We found PMLRARα interaction with Fas in acute promyelocytic leukemia (APL)–derived cells and APL primary cells, and PML-Fas complexes in normal tissues. Binding of PMLRARα to Fas was mapped to the B-box domain of PML moiety and death domain of Fas. PMLRARα blockage of Fas apoptosis was demonstrated in U937/PR9 cells, human APL cells and transgenic mouse APL cells, in which PMLRARα recruited c-FLIPL/S and excluded procaspase 8 from Fas death signaling complex. PMLRARα expression in mice protected the mice against a lethal dose of agonistic anti-Fas antibody (P < .001) and the protected tissues contained Fas-PMLRARα-cFLIP complexes. Taken together, PMLRARα binds to Fas and blocks Fas-mediated apoptosis in APL by forming an apoptotic inhibitory complex with c-FLIP. The presence of PML-Fas complexes across different tissues implicates that PML functions in apoptosis regulation and tumor suppression are mediated by direct interaction with Fas.

scholarly journals Regulation of the expression of annexin VIII in acute promyelocytic leukemia

Blood ◽  
1994 ◽  
Vol 84 (1) ◽  
pp. 279-286 ◽  
Author(s):  
A Sarkar ◽  
P Yang ◽  
YH Fan ◽  
ZM Mu ◽  
R Hauptmann ◽  
...  
Keyword(s):  
Plasma Membrane ◽  
Acute Promyelocytic Leukemia ◽  
Cellular Localization ◽  
Signal Transduction Pathway ◽  
Promyelocytic Leukemia ◽  
Induced Differentiation ◽  
Acute Leukemias ◽  
Nb4 Cells

Annexin VIII is a calcium-dependent phospholipid-binding protein previously identified as a blood anticoagulant based on in vitro studies. However, the physiologic function of annexin VIII remains unknown. In acute promyelocytic leukemia (APL) the annexin VIII gene is highly expressed, but its expression is undetectable in the blasts of other acute leukemias. In the present investigation, we showed using the APL-derived NB4 cell line that expression of the annexin VIII gene is regulated at the transcription level during induced differentiation by all-trans retinoic acid (ATRA). The half-life of the annexin VIII mRNA is about 5 to 6 hours, as determined by using actinomycin D as a transcription inhibitor. Analysis of the expression of annexin VIII protein in NB4 cells and in APL samples showed a consistent expression of a predominant 36-kD protein and a weak 72-kD protein. After ATRA- induced differentiation of NB4 cells, the annexin VIII protein level reduced gradually, but a detectable level persisted even after 4 days of induction. Because annexin VIII mRNA becomes undetectable after 48 hours of ATRA induction, this result indicates that annexin VIII is a relatively stable protein. A multiple tissue Northern blot analysis was performed, and we found that annexin VIII is normally expressed in the placenta and the lung. Cellular localization of the annexin VIII protein was determined by immunofluorescence staining and subcellular fractionation. These results indicated that annexin VIII is predominantly localized to the plasma membrane. The annexin VIII is neither an extracellular protein nor associated with the cell surface suggesting that it does not play a role in blood coagulation in vivo. The plasma membrane localization and its property as a phospholipase inhibitor suggests that annexin VIII may have a role in the signal transduction pathway in the APL cells.

scholarly journals Regulation of the expression of annexin VIII in acute promyelocytic leukemia

Blood ◽  
1994 ◽  
Vol 84 (1) ◽  
pp. 279-286 ◽  
Author(s):  
A Sarkar ◽  
P Yang ◽  
YH Fan ◽  
ZM Mu ◽  
R Hauptmann ◽  
...  
Keyword(s):  
Plasma Membrane ◽  
Acute Promyelocytic Leukemia ◽  
Cellular Localization ◽  
Signal Transduction Pathway ◽  
Promyelocytic Leukemia ◽  
Induced Differentiation ◽  
Acute Leukemias ◽  
Nb4 Cells

Abstract Annexin VIII is a calcium-dependent phospholipid-binding protein previously identified as a blood anticoagulant based on in vitro studies. However, the physiologic function of annexin VIII remains unknown. In acute promyelocytic leukemia (APL) the annexin VIII gene is highly expressed, but its expression is undetectable in the blasts of other acute leukemias. In the present investigation, we showed using the APL-derived NB4 cell line that expression of the annexin VIII gene is regulated at the transcription level during induced differentiation by all-trans retinoic acid (ATRA). The half-life of the annexin VIII mRNA is about 5 to 6 hours, as determined by using actinomycin D as a transcription inhibitor. Analysis of the expression of annexin VIII protein in NB4 cells and in APL samples showed a consistent expression of a predominant 36-kD protein and a weak 72-kD protein. After ATRA- induced differentiation of NB4 cells, the annexin VIII protein level reduced gradually, but a detectable level persisted even after 4 days of induction. Because annexin VIII mRNA becomes undetectable after 48 hours of ATRA induction, this result indicates that annexin VIII is a relatively stable protein. A multiple tissue Northern blot analysis was performed, and we found that annexin VIII is normally expressed in the placenta and the lung. Cellular localization of the annexin VIII protein was determined by immunofluorescence staining and subcellular fractionation. These results indicated that annexin VIII is predominantly localized to the plasma membrane. The annexin VIII is neither an extracellular protein nor associated with the cell surface suggesting that it does not play a role in blood coagulation in vivo. The plasma membrane localization and its property as a phospholipase inhibitor suggests that annexin VIII may have a role in the signal transduction pathway in the APL cells.

The t(5;17) acute promyelocytic leukemia fusion protein NPM-RAR interacts with co-repressor and co-activator proteins and exhibits both positive and negative transcriptional properties

Blood ◽  
2000 ◽  
Vol 95 (8) ◽  
pp. 2683-2690 ◽  
Author(s):  
Robert L. Redner ◽  
J. Don Chen ◽  
Elizabeth A. Rush ◽  
Hui Li ◽  
Sheri L. Pollock
Keyword(s):  
Retinoic Acid ◽  
Acute Promyelocytic Leukemia ◽  
Transcriptional Activation ◽  
Rna Splicing ◽  
Fusion Proteins ◽  
Retinoic Acid Receptor ◽  
Promyelocytic Leukemia ◽  
Dominant Negative ◽  
Wild Type ◽  
Retinoic Acid Response Element

The t(5;17) variant of acute promyelocytic leukemia (APL) fuses the genes for nucleophosmin (NPM) and the retinoic acid receptor alpha (RAR). Two NPM-RAR molecules are expressed as a result of alternative RNA splicing. Both contain RAR sequences that encode the DNA binding, heterodimerization, and ligand activation domains of RAR. This study was designed to test the ability of these fusion proteins to act as transcriptional activators of retinoic acid responsive promoters. The NPM-RAR fusion proteins bind to retinoic acid response element sequences as either homodimers or as heterodimers with RXR. Transcription of retinoic acid–inducible promoters is activated by the fusion proteins in the presence of retinoic acid. The level of transactivation induced by the NPM-RAR fusions differs from the level of transactivation induced by wild-type RAR in both a promoter and cell specific fashion, and more closely parallels the pattern of activation of the PML-RAR fusion than wild-type RAR. In addition, NPM-RAR decreases basal transcription from some promoters and acts in a dominant-negative fashion when co-transfected with wild-type RAR. Both NPM-RAR and PML-RAR interact with the co-repressor protein SMRTe in a manner that is less sensitive than RAR to dissociation by retinoic acid. Retinoic acid induces binding of the co-activator protein RAC3. These data indicate that the NPM-RAR fusion proteins can modulate expression of retinoid-responsive genes in a positive or negative manner, depending on context of the promoter, and lend support to the hypothesis that aberrant transcriptional activation underlies the APL phenotype.

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