Department of Medicine
Hematology / Oncology Division
Myeloid cells (granulocytes and monocytes) are critical for innate immunity and the inflammatory response. Differentiation of myeloid cells from hematopoietic stem cells is tightly regulated, and abnormalities in myeloid differentiation result in leukemia and myelodysplastic syndromes. Gene expression is tightly controlled during normal myeloid differentiation, and abnormalities of transcription factors underlie many forms of leukemia.1 Interests of the laboratory include the regulation of gene transcription in myeloid cells by the ets-related transcription factor, GA-Binding Protein (GABP), and the role of GABP in control of the cell cycle.
GABP is the only obligate multimeric member of the ets family of transcription factors. This tetrameric transcription factor includes two distinct proteins: GABPα binds to DNA through its ets domain, and it recruits the unrelated protein, GABPβ, which contains a transcription activation domain. GABPα also includes a Pointed domain, through which it binds the transcription co-activator, p300.2
GABP regulates the transcription of several important myeloid genes, including CD18 (β2 leukocyte integrin), α4 integrin, neutrophil elastase, and lysozyme. The promoter of the leukocyte integrin, CD18, is bound and activated by GABP in myeloid cells, and GABP is required for transcriptional activation of CD18 in response to retinoic acid.3,4 We identified an enhanceosome, which includes GABP, p300, and retinoic acid receptors, that forms on the CD18 promoter in the presence of retinoic acid; this is the first known retinoic acid-responsive enhanceosome.5 Because abnormalities of retinoic acid receptors cause acute promyelocytic leukemia, these observations suggest that GABP participates in the aberrant differentiation associated with certain forms of leukemia.
Absence of murine Gabpα causes early embryonic lethality, so we generated mice in which we can conditionally disrupt the encoding gene, Gabpa. Disruption of Gabpα dramatically reduces myeloid cells in the peripheral blood and bone marrow. Gabpα null cells contribute poorly to the myeloid compartment, and in vitro and in vivo studies suggest that GABP is required for both proliferation and differentiation of myeloid cells.
We generated mouse embryo fibroblasts (MEFs) from floxed Gabpa mice. In vitro disruption of Gabpa causes profound cell cycle arrest as the cells fail to enter S phase. Disruption of Gabpa reduces transcription of the genes that encode DNA Polymerase α and Thymidylate synthase, which are required for DNA synthesis, and Skp2, an E3 ubiquitin ligase that controls protein levels of the Cyclin dependent kinase inhibitors, p21 and p27. Serum stimulates transcription of Gabpa in growth-arrested cells, and expression of Gabpa is sufficient to induce cell cycle entry, even in the absence of serum stimulation. Unexpectedly, GABP regulates the cell cycle without altering expression of D-type cyclins, Cdks, Retinoblastoma (Rb) protein, and E2Fs. These findings indicate that GABP regulates a novel pathway to cell cycle entry that is independent of the canonical Cyclin D/Cdk/Rb/E2F pathway.6
- Rosmarin AG, Yang Z, Resendes KK. Transcriptional Regulation In Myelopoiesis: Hematopoietic Fate Choice, Myeloid Differentiation, And Leukemogenesis, Experimental Hematology, 33: 131-143, 2005.
- Rosmarin AG, Resendes KK, Yang Z, McMillan J, Fleming SL. GA Binding Protein (GABP) Transcription Factor: A Review - GABP as an Integrator of Intracellular Signaling and Protein-Protein Interactions. Blood Cells, Molecules, and Diseases 32: 143-154, 2004.
- Bush TS, St. Coeur M, Resendes KK, Rosmarin AG. GA Binding Protein (GABP) and Sp1 are required, along with Retinoid Receptors to mediate retinoic acid responsiveness of CD18 (β2 Leukocyte Integrin): a novel mechanism of transcriptional regulation in myeloid cells. Blood, 101:311-317, 2003.
- Resendes KK, Rosmarin AG. Sp1 Control of Gene Expression in Myeloid Cells. Critical Reviews in Eukaryotic Gene Expression. 14:171-181, 2004.
- Resendes KK, Rosmarin AG. GABP and p300 are essential components of a retinoic acid induced enhanceosome in myeloid cells. Molecular and Cellular Biology, 26:3060-3070, 2006.
- Yang Z-Y, Mott S, Rosmarin AG. The ets transcription factor GABP is required for cell cycle progression. Nature Cell Biology, 9:339-346, 2007.