ABSTRACT
Although PML-enforced RARA homodimerization allows PML/RARA to bind DNA independently of its coreceptor RXR, the latter was identified within the PML/RARA complex. We demonstrate that a PML/RARA mutant defective for RXR binding fails to trigger APL development in transgenic mice, although it still transforms primary hematopoietic progenitors ex vivo. RXR enhances PML/RARA binding to DNA and is required for rexinoid-induced APL differentiation. In RA-treated PML/RARA-transformed cells, the absence of RXR binding results in monocytic, rather than granulocytic, differentiation. PML/RARA enhances posttranslational modifications of RXRA, including its sumoylation, suggesting that PML-bound sumoylation enzymes target RXRA and possibly other PML/RARA-bound chromatin proteins, further contributing to deregulated transcription. Thus, unexpectedly, RXR contributes to several critical aspects of in vivo transformation.
Subject(s)
Nuclear Proteins/physiology , Oncogenes , Receptors, Retinoic Acid/physiology , Retinoid X Receptors/physiology , Transcription Factors/physiology , Tumor Suppressor Proteins/physiology , Animals , Mice , Mice, Transgenic , Nuclear Proteins/genetics , Promyelocytic Leukemia Protein , Protein Processing, Post-Translational , Retinoic Acid Receptor alpha , Retinoid X Receptors/genetics , Transcription Factors/genetics , Tumor Suppressor Proteins/geneticsABSTRACT
The pathogenesis of acute promyelocytic leukemia involves the transcriptional repression of master genes of myeloid differentiation by the promyelocytic leukemia-retinoic acid receptor alpha (PML/RARA) oncogene. PML-enforced RARA homodimerization allows the tighter binding of corepressors, silencing RARA target genes. In addition, homodimerization dramatically extends the spectrum of DNA-binding sites of the fusion protein compared with those of normal RARA. Yet, any contribution of these two properties of PML/RARA to differentiation arrest and immortalization of primary mouse hematopoietic progenitors was unknown. We demonstrate that dimerization-induced silencing mediator of retinoid and thyroid receptors (SMRT)-enhanced binding and relaxed DNA-binding site specificity are both required for efficient immortalization. Thus, enforced RARA dimerization is critical not only for triggering transcriptional repression but also for extending the repertoire of target genes. Our studies exemplify how dimerization-induced gain of functions converts an unessential transcription factor into a dominant oncogenic protein.
Subject(s)
DNA/metabolism , Neoplasm Proteins/chemistry , Neoplasm Proteins/metabolism , Oncogene Proteins, Fusion/chemistry , Oncogene Proteins, Fusion/metabolism , Animals , Binding Sites , Carrier Proteins/genetics , Carrier Proteins/metabolism , Cell Transformation, Neoplastic , Cells, Cultured , Co-Repressor Proteins , Dimerization , Fibroblasts/cytology , Fibroblasts/metabolism , Gene Expression Regulation , Genes, Neoplasm , Hematopoietic Stem Cells/cytology , Hematopoietic Stem Cells/physiology , Humans , Intracellular Signaling Peptides and Proteins/genetics , Intracellular Signaling Peptides and Proteins/metabolism , Leukemia, Promyelocytic, Acute , Mice , Mice, Inbred C57BL , Molecular Chaperones , Mutation , Neoplasm Proteins/genetics , Nuclear Proteins/genetics , Nuclear Proteins/metabolism , Oncogene Proteins, Fusion/genetics , Phenotype , Retinoid X Receptors/genetics , Retinoid X Receptors/metabolismABSTRACT
Pathogenesis of acute promyelocytic leukemia (APL) has been proposed to involve transcriptional repression through enhanced corepressors binding onto RARA moieties of PML/RARA homodimers. Unexpectedly, we show that the K160 sumoylation site in the PML moiety of PML/RARA is required for efficient immortalization/differentiation arrest ex vivo, implying that RARA homodimerization is insufficient to fully immortalize primary hematopoietic progenitor cells. Similarly, PML/RARAK160R transgenic mice develop myeloproliferative syndromes, but never APL. The Daxx repressor no longer binds PML/RARAK160R, but fusion of these two proteins restores the differentiation block ex vivo. Thus, transcriptional repression dependent on a specific sumoylation site in PML is critical for the APL phenotype, while forced RARA dimerization could control expansion of the myeloid compartment.
