| First Author | Zhu K | Year | 2013 |
| Journal | J Bone Miner Res | Volume | 28 |
| Issue | 9 | Pages | 1870-1884 |
| PubMed ID | 23649506 | Mgi Jnum | J:244618 |
| Mgi Id | MGI:5913397 | Doi | 10.1002/jbmr.1958 |
| Citation | Zhu K, et al. (2013) ATF4 promotes bone angiogenesis by increasing VEGF expression and release in the bone environment. J Bone Miner Res 28(9):1870-1884 |
| abstractText | Activating transcription factor 4 (ATF4) is a critical transcription factor for bone remodeling; however, its role in bone angiogenesis has not been established. Here we show that ablation of the Atf4 gene expression in mice severely impaired skeletal vasculature and reduced microvascular density of the bone associated with dramatically decreased expression of hypoxia-inducible factor 1alpha (HIF-1alpha) and vascular endothelial growth factor (VEGF) in osteoblasts located on bone surfaces. Results from in vivo studies revealed that hypoxia/reoxygenation induction of HIF-1alpha and VEGF expression leading to bone angiogenesis, a key adaptive response to hypoxic conditions, was severely compromised in mice lacking the Atf4 gene. Loss of ATF4 completely prevented endothelial sprouting from embryonic metatarsals, which was restored by addition of recombinant human VEGF protein. In vitro studies revealed that ATF4 promotion of HIF-1alpha and VEGF expression in osteoblasts was highly dependent upon the presence of hypoxia. ATF4 interacted with HIF-1alpha in hypoxic osteoblasts, and loss of ATF4 increased HIF-1alpha ubiquitination and reduced its protein stability without affecting HIF-1alpha mRNA stability and protein translation. Loss of ATF4 increased the binding of HIF-1alpha to prolyl hydroxylases, the enzymes that hydroxylate HIF-1a protein and promote its proteasomal degradation via the pVHL pathway. Furthermore, parathyroid hormone-related protein (PTHrP) and receptor activator of NF-kappaB ligand (RANKL), both well-known activators of osteoclasts, increased release of VEGF from the bone matrix and promoted angiogenesis through the protein kinase C- and ATF4-dependent activation of osteoclast differentiation and bone resorption. Thus, ATF4 is a new key regulator of the HIF/VEGF axis in osteoblasts in response to hypoxia and of VEGF release from bone matrix, two critical steps for bone angiogenesis. |
