In line with these results, stimulation of OCI-AML3 cells with SDF1 but also with GM-CSF or TPO revealed that ERK phosphorylation is dampened by overexpression of RGS1 (Number 6C, Number S4 in File S1). Transforming Growth Element beta (TGF) pathway was observed within the hypoxia/HIF1/HIF2 transcriptomes. Probably one of the most significantly upregulated genes in both gene units was the cyclin dependent kinase inhibitor CDKN1C (p57kip2). Combined hypoxia treatment or HIF overexpression together with TGF stimulation resulted in enhanced manifestation of CDKN1C and enhanced cell cycle arrest within the CD34+/CD38? stem cell compartment. Interestingly, we observed that CD34+ cells cultured under hypoxic conditions secreted high levels of latent TGF, suggesting an auto- or paracrine part of TGF in the rules of quiescence of these cells. However, knockdown of SMAD4 could not save the hypoxia induced cell cycle arrest, arguing against direct effects of hypoxia-induced secreted TGF. Finally, the G-coupled receptor GTPase RGS1 was identified as a HIF-dependent hypoxia target that dampens SDF1-induced migration and transmission transduction in human being CD34+ stem/progenitor cells. Intro Hematopoietic stem cells (HSCs) reside within specialized hypoxic niches in the bone marrow microenvironment where they may be kept in a relative quiescent state , , , , , , . One of the important pathways triggered under low oxygen conditions is the Hypoxia-inducible element (HIF) pathway. HIF1 and HIF2 (EPAS1) act as oxygen detectors that are degraded under normoxic conditions but at lower oxygen levels HIF proteins are stabilized, translocate to the nucleus and initiate gene transcription , , . In well-oxygenated conditions HIFs are bound from the Von Hippel Lindau (VHL) tumor suppressor protein which recruits an ubiquitin ligase that focuses on these transcription factors for proteasomal degradation . VHL binding is definitely critically dependent on hydroxylation of proline residues in HIF1 (P405 and P564) and HIF2 (P405 and P531) . The oxygen-sensitive subunits of HIF1 or HIF2 can heterodimerize with the stable HIF1 (ARNT) subunit that collectively forms a basic helix-loop-helix-PAS (bHLH-PAS) transcriptional regulator that binds to the core sequence RCGTG termed the hypoxia response element (HRE) in promoters of presumed target genes , , , . Using murine knockout models it has been demonstrated that both HIF1 and HIF2 fulfill essential and at least in part nonoverlapping tasks in hematopoiesis. Conditional depletion of HIF1 resulted in loss of HSC quiescence Ubrogepant and loss of stem cell function when exposed to stress such as transplantation, myelo-suppression or upon ageing . Stabilization of HIF1, either by loss of VHL  or by using pharmacological inhibitors that target prolyl hydroxylases , resulted in improved HSC quiescence and improved hematopoietic recovery after myelosuppressive conditions. Historically, the influence of hypoxia within the behaviour of hematopoietic stem and progenitor cells has been analyzed in vitro by culturing murine and human being bone marrow cells under reduced oxygen tension. It was demonstrated that Ubrogepant murine bone marrow generated roughly two-fold more CFU-GM colonies when this assay was performed under reduced (5%) oxygen conditions , . Culturing murine or human being bone marrow cells for a limited period of time under 1% oxygen conditions was shown to result in a preservation of the progenitor-generating compartment as compared to normoxic conditions , . Furthermore, by using a transplantation model, it was demonstrated the repopulating activity of HSCs could be maintained and even expanded when cultured under reduced oxygen conditions , . Furthermore, it was demonstrated that long-term HSCs reside within the glycolysis-dependent subpopulation of the Ubrogepant bone marrow that display low mitochondrial Ubrogepant activity and communicate high levels Ubrogepant of HIF1 inside a Meis1-dependent manner . Besides a role in HSCs, both HIF1 and HIF2 also play important part during hematopoietic development and differentiation, most notably on erythropoiesis by controlling EPO levels . RGS1 is definitely a member of the R4 subgroup of RGS proteins, known for his or her ability to accelerate the hydrolysis of G-GTP to G-GDP, therefore dampening the activity of GPCR signaling , . Little is known about the specificity RETN of the different RGS users towards different GPCR signaling, but RGS1 has been reported to be active against SDF1-induced migration of B cells by inhibiting CXCR4-mediated signaling . Moreover, upregulation of RGS1 by MEIS1 and binding of MEIS1 to the promoter of RGS1 could suggest a role of RGS1 in the maintenance of HSCs . Despite the critical tasks of HIF1.