Transforming Growth Factor- β1 Signaling in Vascular Endothelial Cells Inhibits Hematopoietic Stem Cell Regeneration

Endothelial cells (EC) are known to be essential for hematopoietic regeneration; however, little is known about the pathways that regulate this activity. By modeling endothelial-dependent HSC interactions in vitro, we found that human umbilical vein endothelial cells (HUVEC) had a markedly reduced capacity to regenerate functional CD150+LSK cells (HSC) compared to other sources of arterial and venous EC. Transcriptional profiling revealed the overexpression of transforming growth factor- β1 (TGF-β1) in HUVEC and indicated that TGF-β1 driven transcriptional programs are highly active in these cells, a finding consistent with autocrine TGF-β1 signaling. Functional studies demonstrated that HSC regeneration by EC was potently inhibited by TGF-β1 and augmented by the ALK5 inhibitor SB431542, in a dose-dependent manner. Importantly, exposure of EC alone to TGF- β1 was sufficient to attenuate subsequent HSC self-renewal. Transcriptome analysis also identified hepatocyte growth factor (HGF) as a candidate EC-derived factor with the potential to enhance hematopoietic regeneration. HGF treatment of HUVEC activated endothelial Akt signaling and led to a >10-fold increase in HSC regeneration that could be blocked by the c-Met inhibitor PF04217903. HGF treatment also dramatically increased long-term multi-lineage hematopoiesis from HUVEC regenerated HSC. Our findings reveal a novel suppressive role for TGF-β1 in the vascular niche and demonstrate that EC-derived growth factors such as HGF have the potential to attenuate this suppression and significantly enhance HSC regeneration.