Hypoxic induction of vascular endothelial growth factor regulates murine hematopoietic stem cell function in the low-oxygenic niche

Abstract

Hypoxia is emerging as an important characteristic of the hematopoietic stem cell (HSC) niche, but the molecular mechanisms contributing to quiescence, self-renewal, and survival remain elusive. Vascular endothelial growth factor A (VEGFA) is a key regulator of angiogenesis and hematopoiesis. Its expression is commonly regulated by hypoxiainducible factors (HIF) that are functionally induced in low-oxygen conditions and that activate transcription by binding to hypoxia-response elements (HRE).Vegfa is indispensable for HSC survival, mediated by a cell-intrinsic, autocrine mechanism. We hypothesized that a hypoxic HSC microenvironment is required for maintenance or up-regulation of Vegfa expression in HSCs and therefore crucial for HSC survival. We have tested this hypothesis in the mouse model Vegfa ␦/␦ , where the HRE in the Vegfa promoter is mutated, preventing HIF binding. Vegfa expression was reduced in highly purified HSCs from Vegfa ␦/␦ mice, showing that HSCs reside in hypoxic areas. Loss of hypoxiaregulated Vegfa expression increases the numbers of phenotypically defined hematopoietic stem and progenitor cells. However, HSC function was clearly impaired when assessed in competitive transplantation assays. Our data provide further evidence that HSCs reside in a hypoxic microenvironment and demonstrate a novel way in which the hypoxic niche affects HSC fate, via the hypoxia-VEGFA axis. (Blood. 2011; 118(6):1534-1543) IntroductionThe process of blood formation originates with the hematopoietic stem cell (HSC) which is responsible for the life-long supply of mature blood cells through the unique ability to combine selfrenewal and differentiation. During the later stages of mammal development, HSCs are present in the fetal liver (FL) from where they seed the bone marrow (BM), which remains the main site of hematopoiesis throughout adult life. The HSC niche is a term used to describe the location and regulatory microenvironment of HSCs. In the BM, cellular HSC niche components include boneforming osteoblasts 1-2 and perivascular cells, [3][4] that through production of various secreted factors as well as through direct cell-cell interactions can affect HSC behavior. As for location it is thought that HSCs are present at a higher frequency at the endosteum close to the bone surface, because ex vivo labeled HSCs tend to localize to the endosteal areas of the BM. [5][6] However, within these areas HSCs are likely to associate with both osteoblastic and sinusoidal endothelial cells. 7 The FL HSC niche is much less defined than that in the BM, but a candidate FL niche component is a CD3 ϩ Ter119 Ϫ population that can provide HSC support. 8 Apart from HSC regulatory cues such as cell-cell contact and production of soluble factors, it is possible that variations in oxygen availability can affect HSCs, and it has been hypothesized that hypoxia is a characteristic of the HSC niche. 9 Oxygen levels are generally lower in BM compared with peripheral blood (PB). 10 Two studies have demonstrated that fractions of BM that w...

Extramedullary Hematopoiesis: A New Look at the Underlying Stem Cell Niche, Theories of Development, and Occurrence in Animals

“…85,90 Low perfusion and low oxygen tension in the endosteal region exist despite high vascularity, presumably because blood velocity in the endosteal sinusoids is one-tenth of that in central medullary capillaries. 59 Overall, cellular and molecular interactions within the osteoblastic niche appear to induce the quiescent microenvironment that supports longterm survival of HSCs.…”

Roles of the hypoxia response system in hematopoietic and leukemic stem cells

“…1). First, Vegfa(d/d) mouse, a mutant mouse model with a mutated HRE in VEGF promoter region, shows a defective HSC phenotype in vivo [30]. HRE is recognized and bound by HIF-1 complex for the transcriptional activation.…”

The Promise of Hematopoietic Stem Cell Therapy for Stroke: Are We There Yet?

“…In addition, Hooper et al (2009) showed that impaired VEGFR2 on the HSC also failed to engraft lethally irradiated mice (Hooper et al, 2009). Rehn et al (2011) recently futher confrimed that loss of VEGF expression in VEGF knockout mice increased impaired HSC which were not able to engraft secondary lethally irradiated mice (Rehn et al, 2011).…”