Fog1 and Cebpa Are DNA Targets of GATA-1/PU.1 Antagonism during Leukemia Differentiation.

PU.1 and GATA1 are hematopoietic lineage-specific transcription factors that play key roles in normal myeloid and erythroid differentiation respectively. Inappropriate expression of PU.1 in proerythroblasts causes its binding to GATA-1 on DNA resulting in a block of erythroid differentiation and development of murine erythroleukemia (MEL) (Rekhtman 1999). Activation of a conditional transgene of GATA-1 (fused with the ligand binding domain of the estrogen receptor, ER) in MEL cells disrupts PU.1-mediated repression in chromatin leading to re-intiation of erythroid differentiation and cell cycle arrest (Choe 2003, Stopka 2005). In this study we show that MEL cells can also be induced to express myeloid differentiation programs upon PU.1-ER activation. Gene expression microarray analysis of GATA-1-ER MEL cells and PU.1-ER MEL cells treated with ER activators allowed us to identify mRNAs that are regulated by both GATA-1 and PU.1 including the set of GATA-1 targets repressed by PU.1, as well as the set of PU.1 targets repressed by GATA-1 in MEL cells. The targets of mutual antagonism of PU.1 and GATA-1 consisted of lineage specific transcription factors, differentiation markers and genes that cause cell cycle arrest and antiapoptotic regulators previously associated with myeloid and erythroid cell differentiation. To determine if PU.1 and GATA-1 directly regulate the lineage specific transcription factor genes, we performed chromatin immunoprecipitation (ChIP) and analyzed the ChIP samples on microarrays and by qPCR. We found that PU.1 and GATA-1 are localized near PU.1 binding sites in the genes for myeloid transcription factors Cebpa and Cbfb and near GATA-1 binding sites in the genes for erythroid transcription factors Fog1 and Nfe2. In addition, further ChIP experiments delineated chromatin architecture near the binding sites for PU.1 and GATA-1 including histone H3 content and acetylation of histone H3K9. We propose that the mutual antagonism of PU.1 and GATA-1 in inhibiting the respective differentiation programs is rendered through specific changes in chromatin structure around lineage specific transcription factors genes. These changes may contribute to the block to differentiation evident in leukemias.