AML-associated translocation products block vitamin D3-induced differentiation by sequestering the vitamin D3 receptor

Acute myeloid leukemia (AML)-associated chromosomal translocations result in formation of chimeric transcription factors, such as PML/RARα, PLZF/RARα, and AML-1/ETO, of which the components are involved in regulation of transcription by chromatin modeling through histone acetylation/deacetylation. The leukemic differentiation block is attributed to deregulated transcription caused by these chimeric fusion proteins, which aberrantly recruit histone-deacetylase (HDAC) activity. One essential differentiation pathway blocked by the leukemic fusion proteins is the vitamin (Vit) D₃ signaling. Here we investigated the mechanisms by which the leukemic fusion proteins interfere with VitD₃-induced differentiation. The VitD₃-receptor (VDR) is, like the retinoid receptors RAR, retinoid X receptor, and the thyroid hormone receptor (TR), a ligand-inducible transcription factor. In the absence of ligand, the transcriptional activity of TR and RAR is silenced by recruitment of HDAC activity through binding to corepressors. In the presence of ligand, TR and RAR activate transcription by releasing HDAC activity and by recruiting histoneacetyltransferase activity. Here we report that VDR binds corepressors in a ligand-dependent manner and that inhibition of HDAC activity increases VitD₃ sensitivity of HL-60 cells. Nevertheless, the inhibition of HDAC activity is unable to overcome the block of VitD₃-induced differentiation caused by PLZF/RARα expression. Here we demonstrate that the expression of the translocation products PML/RARα and PLZF/RARα impairs the localization of VDR in the nucleus by binding to VDR. Furthermore, the overexpression of VDR in U937 cells expressing AML-related translocation products completely abolishes the block of VltD₃-induced differentiation. Taken together these data indicate that the AML-associated translocation products block differentiation not only by interfering with chromatin-modeling but also by sequestering factors Involved in the differentiation signaling pathways, such as VDR in the VitD₃-induced differentiation.