title: Physical and functional interactions between Runx2 and HIF-1α induce vascular endothelial growth factor gene expression.
abstract: Angiogenesis and bone formation are intimately related processes. Hypoxia during early bone development stabilizes hypoxia-inducible factor-1α (HIF-1α) and increases angiogenic signals including vascular endothelial growth factor (VEGF). Furthermore, stabilization of HIF-1α by genetic or chemical means stimulates bone formation. On the other hand, deficiency of Runx2, a key osteogenic transcription factor, prevents vascular invasion of bone and VEGF expression. This study explores the possibility that HIF-1α and Runx2 interact to activate angiogenic signals. Runx2 over-expression in mesenchymal cells increased VEGF mRNA and protein under both normoxic and hypoxic conditions. In normoxia, Runx2 also dramatically increased HIF-1α protein. In all cases, the Runx2 response was inhibited by siRNA-mediated suppression of HIF-1α and completely blocked by the HIF-1α inhibitor, echinomycin. Similarly, treatment of preosteoblast cells with Runx2 siRNA reduced VEGF mRNA in normoxia or hypoxia. However, Runx2 is not essential for the HIF-1α response since VEGF is induced by hypoxia even in Runx2-null cells. Endogenous Runx2 and HIF-1α were colocalized to the nuclei of MC3T3-E1 preosteoblast cells. Moreover, HIF-1α and Runx2 physically interact using sites within the Runx2 RUNT domain. Chromatin immunoprecipitation also provided evidence for colocalization of Runx2 and HIF-1α on the VEGF promoter. In addition, Runx2 stimulated HIF-1α-dependent activation of an HRE-luciferase reporter gene without requiring a separate Runx2-binding enhancer. These studies indicate that Runx2 functions together with HIF-1α to stimulate angiogenic gene expression in bone cells and may in part explain the known requirement for Runx2 in bone vascularization.
authors.affiliation: Department of Periodontics & Oral Medicine and Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1078, USA.
authors.forename: Tae-Geon
authors.initials: TG
authors.lastname: Kwon
authors.forename: Xiang
authors.initials: X
authors.lastname: Zhao
authors.forename: Qian
authors.initials: Q
authors.lastname: Yang
authors.forename: Yan
authors.initials: Y
authors.lastname: Li
authors.forename: Chunxi
authors.initials: C
authors.lastname: Ge
authors.forename: Guisheng
authors.initials: G
authors.lastname: Zhao
authors.forename: Renny T
authors.initials: RT
authors.lastname: Franceschi
mesh_terms: D016475:3T3 Cells; D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D047369:Chromatin Immunoprecipitation; D050659:Core Binding Factor Alpha 1 Subunit / Q000502:physiology*; D017931:DNA Primers; D005786:Gene Expression Regulation / Q000502:physiology*; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000502:physiology*; D051379:Mice; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000235:genetics; D042461:Vascular Endothelial Growth Factor A / Q000235:genetics*
chemical_list: D050659:Core Binding Factor Alpha 1 Subunit; D017931:DNA Primers; C497443:Hif1a protein, mouse; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D012333:RNA, Messenger; C493716:Runx2 protein, mouse; D042461:Vascular Endothelial Growth Factor A