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Physical and functional interactions between Runx2 and HIF-1α induce vascular endothelial growth factor gene expression

Authors

  • Tae-Geon Kwon,

    1. Department of Periodontics & Oral Medicine and Biological Chemistry, University of Michigan, Ann Arbor, Michigan
    2. Department of Oral & Maxillofacial Surgery, Kyungpook National University, Daegu, Republic of Korea
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  • Xiang Zhao,

    1. Department of Periodontics & Oral Medicine and Biological Chemistry, University of Michigan, Ann Arbor, Michigan
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  • Qian Yang,

    1. Department of Periodontics & Oral Medicine and Biological Chemistry, University of Michigan, Ann Arbor, Michigan
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  • Yan Li,

    1. Department of Periodontics & Oral Medicine and Biological Chemistry, University of Michigan, Ann Arbor, Michigan
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  • Chunxi Ge,

    1. Department of Periodontics & Oral Medicine and Biological Chemistry, University of Michigan, Ann Arbor, Michigan
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  • Guisheng Zhao,

    1. Department of Periodontics & Oral Medicine and Biological Chemistry, University of Michigan, Ann Arbor, Michigan
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  • Renny T. Franceschi

    Corresponding author
    1. Department of Periodontics & Oral Medicine and Biological Chemistry, University of Michigan, Ann Arbor, Michigan
    • Department of Periodontics and Oral Medicine, School of Dentistry, University of Michigan, 1011 N. University Ave, Ann Arbor, MI 48109-1078, USA.
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  • Tae-Geon Kwon and Xiang Zhao contributed equally to this study.

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. J. Cell. Biochem. 112: 3582–3593, 2011. © 2011 Wiley Periodicals, Inc.

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