HIF1α-induced PDGFRβ signaling promotes developmental HSC production via IL-6 activation

Hematopoietic stem cells (HSCs) have the ability to both self-renew and differentiate each of the mature blood cell lineages and thereby reconstitute the entire blood system. Therefore, HSCs are therapeutically valuable for treatment of hematological malignances and bone marrow failure. We showed recently that transient glucose elevation elicited dose-dependent effects on HSCs through elevated metabolic activity and subsequent reactive oxygen species-mediated induction of Hypoxia-Inducible Factor 1α (Hif1α). Platelet-Derived Growth Factor B (pdgfb), a Hif1α-target, and its receptor, pdgfrb, were significantly upregulated in response to metabolic stimulation. Although the function of PDGF signaling is well established in vascular development, its role in hematopoiesis is less understood. Exposure to either a pan-PDGF inhibitor or a PDGFRβ-selective antagonist in the context of Hif1α stimulation blocked elevations in hematopoietic stem and progenitor cell (HSPC) formation as determined by runx1;cmyb whole-mount in situ hybridization (WISH) and HSPC-reporter flow cytometry analysis. Similar results were observed for morpholino (MO) knockdown of pdgfrb or dominant-negative pdgfrb expression, indicating that PDGFRβ signaling is a key downstream mediator of Hif1α-mediated induction of HSPCs. Notably, overexpression of Pdgfb ligand enhanced HSPC numbers in the aorta-gonado-mesonephros (AGM) at 36 hours postfertilization (hpf) and in the caudal hematopoietic tissue at 48 hpf. A survey of known PDGF-B/PDGFRβ regulatory targets by expression analysis revealed a significant increase in inflammatory intermediates, including Interleukin 6 (IL-6) and its receptor (IL-6R). MO-mediated knockdown of il6 or chemical inhibition of IL-6R antagonized the effect of Pdgfb overexpression. Furthermore, epistatic analysis of IL-6/IL-6R function confirmed activity downstream of Hif1α. Together, these findings define a Hif1α-regulated signaling axis mediated through PBFGB/PDGFRβ and IL-6/IL-6R that acts to control embryonic HSPC production.