C/EBPα induces PU.1 and interacts with AP-1 and NF-κB to regulate myeloid development
Introduction
The focus of this report is our laboratory's work on the role of C/EBPα as a regulator of myeloid (granulocyte versus monocyte) lineage commitment. A comprehensive overview of the regulation of myeloid development may be found in two published reviews [1], [2].
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Myeloid defects in mice with reduced or absent C/EBPα or PU.1
C/EBPα(−/−) newborn mice lack neutrophils and have reduced hepatic monocytes, and their marrow CFU-M numbers are decreased 2-fold [3]. Deletion of C/EBPα in the marrow of adult mice inhibits the common myeloid progenitor (CMP) to granulocyte-monocyte progenitor (GMP) transition [4]. Consistent with these findings, analysis of an independently derived C/EBPα(−/−) line demonstrates reduced CFU-GM, neutrophils, and monocyte/macrophages [5]. A dominant-inhibitory C/EBP, KRAB-C/EBPα-ER, inhibits
C/EBPα induces PU.1 gene transcription
C/EBPα-ER rapidly induces endogenous PU.1 mRNA in Ba/F3 or 32Dcl3 hematopoietic cell lines even in the presence of cycloheximide, but C/EBPα(L12V)-ER, which cannot heterodimerize or bind DNA due to mutation of the first two leucines of the leucine zipper (LZ) to valine, does not induce PU.1 mRNA [15], [16]. C/EBPα binds the murine PU.1 promoter in a chromosomal immunoprecipitation (ChIP) assay and binds the sequence TAGCGCAAG located at − 68 in the murine PU.1 promoter and conserved in the human
C/EBPα induces monocyte lineage commitment in cooperation with PU.1
To complement gene knockout loss-of-function studies, we have assessed the ability of exogenous C/EBPα to affect myeloid lineage commitment from murine marrow progenitors. As C/EBPα potently inhibits G1 to S cell cycle progression in myeloid cells via interaction with E2F1 and likely other mechanisms [18], [19], [20], [21], we developed an assay that minimizes biases due to lineage-specific cell cycle inhibition. One feature of our approach is use of C/EBPα-ER fusion proteins that remain
C/EBPα zippers with AP-1 proteins to favor monopoiesis
The initial suggestion that AP-1 proteins favor monocytic development comes from the finding that c-Jun or c-Fos can induce monocytic differentiation when expressed in myeloid cell lines and from the observation that phorbol esters rapidly induce AP-1 proteins and direct myeloid cell maturation to monocytes [27], [28], [29]. MafB can zipper with c-Fos, and c-Maf zippers with c-Jun, JunB or c-Fos [30]. MafB is expressed predominantly in monocytic cells within hematopoiesis, and MafB or c-Maf
C/EBPα interacts with NF-κB to regulate inflammation, survival and potentially myelopoiesis
C/EBPs and NF-κB cooperate to induce the IL-6, IL-8, G-CSF, serum amyloid, ICAM-1, superoxide dismutase and Mediterranean fever promoters during the inflammatory response [36], [37], [38], [39], [40]. The bZIP domain of C/EBPα or C/EBPβ interacts with NF-κB p50 or p65 [41], [42]. C/EBPα or C/EBPβ cooperates specifically with NF-κB p50 to activate the bcl-2 P2 promoter, leading to inhibition of apoptosis in myeloid or lymphoid cells. Endogenous C/EBPα preferentially interacts with endogenous
Summary and future directions
Onset of C/EBPα expression in multipotent stem/progenitors cells, such as the lymphoid-myeloid progenitor (LMMP) or CMP, may direct these cells to the GMP state, initiating myeloid development (Fig. 1). In this model, induction of PU.1 in cooperation with AP-1 and monocyte-specific cytokine signals directs monopoiesis, and C/EBPα and reduced PU.1 cooperate with other transcription factors to direct granulopoiesis. Many key questions remain. It will be of interest to identify monocytic genes
Acknowledgments
I thank the members of my laboratory and participants at the Myeloid Workshop for useful discussions. This work is supported by NIH grants HL082948 and CA098805. This paper is a summary of a presentation at the Seventh International Workshop on Molecular Aspects of Myeloid Stem Cell Development and Leukemia, Annapolis, Maryland, May 13–16, 2007, sponsored by The Leukemia and Lymphoma Society.
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