HIF1 alpha Promotes BMP9-Mediated Osteoblastic Differentiation and Vascularization by Interacting with CBFA1

Bone morphogenetic protein 9 (BMP9) as the most potent osteogenic molecule which initiates the differentiation of stem cells into the osteoblast lineage and regulates angiogenesis, remains unclear how BMP9-regulated angiogenic signaling is coupled to the osteogenic pathway. Hypoxia-inducible factor 1 alpha (HIF1 alpha) is critical for vascularization and osteogenic differentiation and the CBFA1, known as runt-related transcription factor 2 (Runx2) which plays a regulatory role in osteogenesis. This study investigated the combined effect of HIF1 alpha and Runx2 on BMP9-induced osteogenic and angiogenic differentiation of the immortalized mouse embryonic fibroblasts (iMEFs). The effect of HIF1 alpha and Runx2 on the osteogenic and angiogenic differentiation of iMEFs was evaluated. The relationship between HIF1 alpha- and Runx2-mediated angiogenesis during BMP9-regulated osteogenic differentiation of iMEFs was evaluated by ChIP assays. We demonstrated that exogenous expression of HIF1 alpha and Runx2 is coupled to potentiate BMP9-induced osteogenic and angiogenic differentiation both in vitro and animal model. Chromatin immunoprecipitation assays (ChIP) showed that Runx2 is a downstream target of HIF1 alpha that regulates BMP9-mediated osteogenesis and angiogenic differentiation. Our findings reveal that HIF1 alpha immediately regulates Runx2 and may originate an essential regulatory thread to harmonize osteogenic and angiogenic differentiation in iMEFs, and this coupling between HIF1 alpha and Runx2 is essential for bone healing.