Key Points• RUNX1 inhibits erythroid differentiation by downregulation of the erythroid gene expression program.• RUNX1 can act as an activator and repressor during megakaryocytic differentiation and counteracts the activity of TAL1.The activity of antagonizing transcription factors represents a mechanistic paradigm of bidirectional lineage-fate control during hematopoiesis. At the megakaryocytic/erythroid bifurcation, the crossantagonism of krueppel-like factor 1 (KLF1) and friend leukemia integration 1 (FLI1) has such a decisive role. However, how this antagonism is resolved during lineage specification is poorly understood. We found that runt-related transcription factor 1 (RUNX1) inhibits erythroid differentiation of murine megakaryocytic/erythroid progenitors and primary human CD34 1 progenitor cells. We show that RUNX1 represses the erythroid gene expression program during megakaryocytic differentiation by epigenetic repression of the erythroid master regulator KLF1. RUNX1 binding to the KLF1 locus is increased during megakaryocytic differentiation and counterbalances the activating role of T-cell acute lymphocytic leukemia 1 (TAL1). We found that corepressor recruitment by RUNX1 contributes to a block of the KLF1-dependent erythroid gene expression program.Our data indicate that the repressive function of RUNX1 influences the balance between erythroid and megakaryocytic differentiation by shifting the balance between KLF1 and FLI1 in the direction of FLI1. Taken together, we show that RUNX1 is a key player within a network of transcription factors that represses the erythroid gene expression program. (Blood. 2015;125(23):3570-3579) IntroductionThe hematopoietic system is in a constant process of cell proliferation, differentiation, and cell death. Progenitor cells produced by hematopoietic stem cells undergo a hierarchical progression in which the selfrenewal capability is lost and a specific lineage determination is adopted. [1][2][3] In this process, genes important for stem cell functions are downregulated and the expression of genes important for differentiation and cell type-specific functions is upregulated. Transcription factors initiate and maintain cell-specific expression by binding to regulatory sequences of target genes and by recruitment of generegulative complexes with DNA-and histone-modifying activity. These epigenetic modifications reorganize the chromatin locally and genome-wide to sustain a cell type-specific gene expression pattern. [4][5][6] Antagonizing transcription factors play an important role in the establishment of cell type-specific gene expression programs during hematopoietic differentiation. 7 At the megakaryocytic/erythroid bifurcation, the crossantagonism of the transcription factors krueppel-like factor 1 (KLF1) and friend leukemia integration 1 (FLI1) plays such a decisive role. 8,9 However, the mechanism of how this antagonism is resolved is poorly understood. During differentiation of common megakaryocyte/erythroid progenitor cells (MEPs) 10 toward the megakaryoc...

The functional capacity of the transcriptional regulatory CCAAT/enhancer-binding protein-␤ (C/EBP␤) is governed by protein interactions and post-translational protein modifications. In a proteome-wide interaction screen, the histone-lysine N-methyltransferase, H3 lysine 9-specific 3 (G9a), was found to directly interact with the C/EBP␤ transactivation domain (TAD). Binding between G9a and C/EBP␤ was confirmed by glutathione S-transferase pulldown and co-immunoprecipitation. Metabolic labeling showed that C/EBP␤ is post-translationally modified by methylation in vivo. A conserved lysine residue in the C/EBP␤ TAD served as a substrate for G9a-mediated methylation. G9a, but not a methyltransferase-defective G9a mutant, abrogated the transactivation potential of wild type C/EBP␤. A C/EBP␤ TAD mutant that contained a lysine-toalanine exchange was resistant to G9a-mediated inhibition. Moreover, the same mutation conferred super-activation of a chromatin-embedded, endogenous C/EBP␤ target gene. Our data identify C/EBP␤ as a direct substrate of G9a-mediated post-translational modification that alters the functional properties of C/EBP␤ during gene regulation.

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