PML-RARα is a chimeric transcription factor deeply associated with acute promyelocytic leukemia (APL). PML-RARα plays an important role in the aberrant transcription repression on the target genes of wild type retinoic acid receptors (RARα). Pharmacological concentration of all-trans retinoic acid (ATRA) induces transcription de-repression on several target genes, and results in terminal differentiation of APL cells. However, the detailed mechanisms of transcription repression by PML-RARα in vivo are still unclear. Here we demonstrated that histone deacetylase 3 (HDAC3), one component of the N-CoR (nuclear receptor co-repressor)-TBL1/R1 (transducin beta-like protein 1/relating protein) transcription repressor protein complex, is a key regulator of the transcription repression by PML-RARα in vivo. Using immunoprecipitation (IP) assay, we first demonstrated that PML-RARα physically interacted with N-CoR/HDAC3 in vivo in the absence of ligand. The interaction was dissociated by adding ATRA in the dose dependent manner. Next we showed, using chromatin immunoprecipitation (ChIP) assay, that N-CoR/HDAC3 co-repressor complex was recruited to the endogenous target gene promoters (RARβ and CYP26) through PML-RARα. The neighboring histone H4 was de-acetylated and the gene expression was significantly repressed. When HDAC3 protein is knocked down by RNA interference in PML-RARα-presenting cells, the endogenous target gene expression was significantly activated. Almost the same results were also obtained when performing the luciferase reporter assay using RARβ and CYP26 promoter reporter vectors. Previously, we have shown that N-CoR-TBLR1 is recruited to the target gene promoter through PML-RARα in the absence of ligand, resulting in the transcription repression. Consistent with these data, it is strongly suggested that N-CoR/HDAC3/TBLR1 co-repressor complex is closely related to the aberrant transcription regulation by PML-RARα in APL cells. Furthermore, we also confirmed that PLZF-RARα, which is expressed in ATRA resistant APL cells, interacted with N-CoR/HDAC3/TBLR1 in ligand independent manner. These insights provide not only the basic mechanism of transcription repression by leukemia-related chimeric transcription factors, but also the new molecular targets for the transcription therapy for leukemia.

This work is supported by Grants-in-Aid from the Scientific Research of Education, Culture, Sports, Science and Technology, Japan, and the Public Trust Haraguchi Memorial Cancer Research Fund, Japan.

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