Retinoids exert a variety of effects on both normal and malignant hematopoietic cells. To date, three different retinoic acid receptor (RAR) and retinoid X receptor (RXR) genes have been characterized, each encoding multiple N-terminal protein isoforms. RXRs serve as co-regulators for RARs, and many other nuclear receptors integrating different signalling pathways. All-trans-retinoic acid (ATRA) signaling pathway is of critical importance for optimal myelomonocytic differentiation and its disruption by translocations of the RARα gene leads to acute promyelocytic leukemia (APL). APL associated fusion oncoproteins, such as PML-RARα and PLZF-RARα, function through recruitment of histone deacetylases (HDACs) and DNA methyltransferases (DNMTs), thus promoting an inactive chromatin state and leading to repression of RARα target genes. Recently, we demonstrated that up-regulation of RARα2 expression by ATRA directly correlates with differentiation of APL and non-APL AML cells and that RARα2 transcription is silenced by DNA methylation in AML cell lines. Using primary AML samples as well as normal cord and peripheral blood derived cells representing different stages of myelomonocytic development we now show that expression of RARα2 increases with maturation of hematopietic cells. Expression of RARα1 on the other hand, which is transcribed from a distinct promoter, remains relatively constant throughout the different stages of myelomonocytic development. The levels of RARα1 expression in various primary AML cell types appear to be similar to those found in normal hematopietic cells. Consistent with data derived from AML cell lines, however, the RARα2 isoform is poorly expressed in all samples. Compared with CD34+/CD133+ or CD34+ progenitors, and more mature CD33+ myeloid cells, RARα2 is expressed at much lower levels in a variety of primary AML cells and its expression is not effectively induced by myelomonocytic growth factors and/or ATRA. Negatively acting epigenetic changes, such as DNA methylation, appear to be responsible for deregulated expression of RARα2 in AML cells, although their pattern and extent differs significantly between AML cell lines and primary AML samples. Taken together our data suggest that agents, which revert negatively acting epigenetic changes may restore expression of the RARα2 isoform in AML cells and render them more responsive to ATRA as well as other differentiation inducers.

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