Abstract
Bone marrow suppression is an important cause of death in patients exposed to radiation or in cancer patients treated with conventional chemotherapeutic agents. Myeloablative treatments (i.e. 5-fluorouracil administration) lead to apoptosis of blood forming cells and to regression of blood vessels in bone marrow. It is well known that hematological recovery post-bone marrow insult depends on the capacity of hematopoietic stem cells to regenerate the entire hematopoietic system, however, the transcriptional machinery involved in the regeneration of sinusoidal blood vessels in bone marrow from endothelial progenitor cells is largely unknown. Endothelial cells express the Tie2 receptor tyrosine kinase (a.k.a. Tek), which is involved in the angiogenic remodeling and vessel stabilization. Gene targeting of Tie2 showed that it is not required for differentiation and proliferation of definitive hematopoietic lineages in the embryo although Tie2 is needed during postnatal bone marrow hematopoiesis. ELF is a subgroup of the ETS family of transcription factors composed by ELF1, ELF2 (a.k.a. NERF), ELF3, ELF4 (a.k.a. MEF) and ELF5. ELF1 and ELF2 have been shown to regulate Tie2 expression in vitro. Recently we showed that ELF4 modulates the exit of hematopoietic stem cells (HSC) from quiescence (Lacorazza et al., Cancer Cell2006, 9:175–187). Given the high homology between ELF1 and ELF4 and the same origin of HSC and endothelial progenitor cells, we hypothesize that ELF4 regulates proliferation and Tie2 expression of endothelial cells. We used a luciferase gene reporter system in COS-7 and HEK cells to examine the capacity of ELF proteins to activate Tie2. ELF4 is the strongest activator of Tie2 expression following the hierarchy ELF4>ELF1>ELF2 variant 1>ELF2 variant 2. Site directed mutagenesis of each of the five ETS-binding sites (EBS) present in the Tie2 promoter shows that ELF4 binds preferentially to EBS 1, 3 and 5. Binding of ELF4 to the Tie2 promoter was confirmed by chromatin immunoprecipitation and EMSA. Although Elf1 gene expression is essentially normal in Elf4−/− bone marrow cells collected after 5-FU treatment, we detected diminished Tie2 expression compared to Elf4+/+ bone marrow cells. The association of this effect to human endothelial cells derived from umbilical cord (HUVEC cells) was investigated. All-trans retinoic acid (ATRA) and vascular-endothelial growth factor (VEGF) induced ELF4 expression in HUVEC cells in a dose and time dependent manner which was followed by increased Tie2 expression, suggesting that expression of ELF4 is modulated by angiogenic signals. Moreover, endothelial cells treated with ATRA showed rapid wound colonization in a wound assay. Expression of the pan-endothelial marker MECA-32 was determined by immunohistochemistry to correlate Tie2 with the regeneration of blood vessels: myeloablated Elf4−/− femurs exhibited a reduction of MECA-32 positive arterioles. Finally, temporal and spatial expression of Tie2 during hematological recovery post ablation was measured in bone marrow using transgenic Tie2-LacZ mice crossed to Elf4−/− mice. Collectively, our data suggests that ELF4 regulates Tie2 expression in endothelial cells but most importantly their proliferative capacity in response to angiogenic signals.
Erythroid/Myeloid Progenitors and Hematopoietic Stem Cells Originate from Distinct Populations of Endothelial Cells
