Induction of ATF4 and M-CSF by hypoxia in HemSCs in vitro. (A-C) Hypoxia (5% O2) upregulated the mRNA expression of HIF-1α, ATF4, and M-CSF in HemSCs more significantly compared with HUVECs in vitro. (D) Hypoxia (5% O2) upregulated the protein expression of HIF-1α and ATF4 in HemSCs more significantly compared with HUVECs in vitro. Abbreviations: ATF4, activating transcription factor 4; M-CSF, macrophage colony-stimulating factor; HemSCs, hemangioma stem cells; HIF-1α, hypoxia inducible factor-1α; HUVECs, human umbilical vein endothelial cells. ****p<0.0001.