Runx2 Regulates HIF1α-Induced Angiogenic Gene Expression

Objective: The processes of angiogenesis and bone formation are intimately related during skeletal development and regeneration. Hypoxia leads to stabilization of the transcription factor, hypoxia-inducible factor 1? (HIF1?) and increased expression of angiogenic targets including vascular endothelial growth factor (VEFG). Deficiency of the osteogenic transcription factor, Runx2, completely prevents vascular invasion, VEGF expression and differentiation of hypertrophic chondrocytes and osteoblasts, suggesting that Runx2 alone or together with HIF1? is able to regulate angiogenesis. The purpose of the present study was to investigate possible relationships between Runx2 and HIF1? in osteogenic cells.

Methods: HIF1? was stabilized by CoCl2 or gene delivery of a constitutive active HIF1? to the preosteoblast cell line, MC3T3-E1C4, or C3H10T1/2 mesenchymal cells. HIF1? and Runx2 transcriptional activities were measured using HRE and mOG2-luciferase reporter genes. Protein expression was measured by Western blot and ELISA (for VEGF).

Results: CoCl2 dose-dependently increased HIF-1?, angiogenic target gene expression and VEGF secretion in both cell lines. Runx2 over-expression was able to module CoCl2 activity with stimulatory or inhibitory effects seen depending on cell type.

Conclusion: This study suggests that Runx2 can modulate HIF1? responsiveness to enhance the angiogenic response in mature osteoblasts and inhibit it in progenitor cells.

The current findings reveal that osteogenic signaling can influence the activity of the angiogenic transcription factor, HIF1?, especially in the hypoxic conditions found after injury and during bone healing. Supported by NIH/NIDCR DE13386.