A combined computational and experimental approach reveals the structure of a C/EBPβ-Spi1 interaction required for IL1B gene transcription

We previously reported that transcription of the human IL1B gene, encoding the proinflammatory cytokine interleukin 1β, depends on long-distance chromatin looping that is stabilized by a mutual interaction between the DNA-binding domains (DBDs) of two transcription factors: Spi1 proto-oncogene at the promoter and CCAAT enhancer-binding protein (C/EBPβ) at a far-upstream enhancer. We have also reported that the C-terminal tail sequence beyond the C/EBPβ leucine zipper is critical for its association with Spi1 via an exposed residue (Arg-232) located within a pocket at one end of the Spi1 DNA-recognition helix. Here, combining in vitro interaction studies with computational docking and molecular dynamics of existing X-ray structures for the Spi1 and C/EBPβ DBDs, along with the C/EBPβ C-terminal tail sequence, we found that the tail sequence is intimately associated with Arg-232 of Spi1. The Arg-232 pocket was computationally screened for small-molecule binding aimed at IL1B transcription inhibition, yielding l-arginine, a known anti-inflammatory amino acid, revealing a potential for disrupting the C/EBPβ-Spi1 interaction. As evaluated by ChIP, cultured lipopolysaccharide (LPS)-activated THP-1 cells incubated with l-arginine had significantly decreased IL1B transcription and reduced C/EBPβ's association with Spi1 on the IL1B promoter. No significant change was observed in direct binding of either Spi1 or C/EBPβ to cognate DNA and in transcription of the C/EBPβ-dependent IL6 gene in the same cells. These results support the notion that disordered sequences extending from a leucine zipper can mediate protein-protein interactions and can serve as druggable targets for regulating gene promoter activity.

Regulation of the C/EBPα signaling pathway in acute myeloid leukemia (Review)

The transcription factor CCAAT/enhancer binding protein α (C/EBPα), as a critical regulator of myeloid development, directs granulocyte and monocyte differentiation. Various mechanisms have been identified to explain how C/EBPα functions in patients with acute myeloid leukemia (AML). C/EBPα expression is suppressed as a result of common leukemia-associated genetic and epigenetic alterations such as AML1-ETO, RARα-PLZF or gene promoter methylation. Recent data have shown that ubiquitination modification also contributes to its downregulation. In addition, 10-15% of patients with AML in an intermediate cytogenetic risk subgroup were characterized by mutations of the C/EBPα gene. As a transcription factor, C/EBPα can translocate into the nucleus and further regulate a variety of genes directly or indirectly, which are all key factors for cell differentiation. This review summarizes recent reports concerning the dysregulation of C/EBPα expression at various levels in human AML. The currently available data are persuasive evidence suggesting that impaired abnormal C/EBPα expression contributes to the development of AML, and restoration of C/EBPα expression as well as its function represents a promising target for novel therapeutic strategies in AML.

Identification of the C/EBPα C-terminal tail residues involved in the protein interaction with GABP and their potency in myeloid differentiation of K562 cells

The CCAAT/enhancer-binding protein α (C/EBPα) is the member of a family of related basic leucine zipper (bZIP) transcription factors and is critical for granulopoiesis. We previously demonstrated that C/EBPα interacts with the ETS domain of widely expressed GABPα, which leads to cooperative transcriptional activation of the myeloid-specific promoter for human FCAR encoding the Fc receptor for IgA (FcαR, CD89) in part by facilitating recruitment of C/EBPα to the promoter. The C/EBPα molecule contains transactivation domains (TADs) at its N-terminus and a DNA-binding and dimerization bZIP structure at its C-terminus. We demonstrate here that GABPα interacts with the last 18 residues of the C/EBPα C-terminus beyond the bZIP DNA-binding and dimerizing region. Deletion of this C-terminus resulted in loss of GABPα interaction but not affecting its DNA binding ability, indicating that it is not required for homodimer formation. Moreover, the C-terminus confers the ability to functionally synergize with GABP on a heterologous TAD when fused to the C-terminus of the VP16 TAD. We identified a three-amino acid stretch (amino acids 341-343) that is important for both functional and protein interactions with GABP. Ectopic expression in K562 cells of C/EBPα mutant incapable of interacting with GABPα does not induce expression of granulocytic differentiation markers including CD15, CD11b, GCSF-R and C/EBPε, and does not inhibit proliferation, whereas wild type does. These results demonstrate the functional importance of the C/EBPα C-terminus beyond the bZIP DNA-binding and dimerization region, which may mediate cooperative activation by C/EBPα and GABP of myeloid-specific genes involved in C/EBPα-dependent granulopoiesis.