ATF4 promotes bone angiogenesis by increasing vegf expression and release in the bone environment

Abstract

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 1α (HIF-1α) and vascular endothelial growth factor (VEGF) in osteoblasts located on bone surfaces. Results from in vivo studies revealed that hypoxia/reoxygenation induction of HIF-1α 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-1α and VEGF expression in osteoblasts was highly dependent upon the presence of hypoxia. ATF4 interacted with HIF-1α in hypoxic osteoblasts and loss of ATF4 increased HIF-1α ubiquitination and reduced its protein stability without affecting HIF-1α mRNA stability and protein translation. Furthermore, parathyroid hormone-related protein (PTHrP) and receptor activator of NF-kappaB ligand (RANKL), both well-known activator 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 VEGF release from bone matrix, two critical steps for bone angiogenesis.

PKCδnull mutations in a mouse model of osteoarthritis alter osteoarthritic pain independently of joint pathology by augmenting NGF/TrkA-induced axonal outgrowth

“…22 The ATF4 expression level was significantly increased (p<0.05) in the joint synovium during OA progression at (4 and 8 weeks post-DMM) in PKCδ null mice ( figure 5E, F). Moreover, we found that introduction of siRNA for PKCδ into either human endothelial or synovial cells stimulated the expression of both ATF4 and VEGF (figure 5G-J).…”

Prolyl hydroxylase domain proteins regulate bone mass through their expression in osteoblasts

“…TRAP-positive multinucleated cells (MNCs) per well were counted using light microscopy. The TRAP staining technique for bone sections was previously described (Zhu et al, 2013). The osteoclast surface/bone surface (Oc.S/BS) and osteoclast numbers/bone perimeter (Oc.N/BPm) in both primary and secondary spongiosa of tibiae were measured using Image Pro Plus 7.0 software.…”

“…The PHD-HIF-α pathway is also involved in bone repair, angiogenesis, and osteogenesis (Wang et al, 2007;Wan et al, 2008;Zhu et al, 2013). Thoms et al identified PHD2 as the primary enzyme regulating HIF-2α stability in human chondrocytes.…”

Hypoxia‐induced miR‐497 decreases glioma cell sensitivity to TMZ by inhibiting apoptosis

“…These changes are carefully controlled by a large number of hypoxia-inducible genes [2]. Among these genes, hypoxia-inducible factor 1a (HIF1a) is the key transcription factor induced by hypoxia, increasing the expression of many genes that are involved in metastasis, tumor recurrence, and angiogenesis [3][4][5]. Hypoxia also promotes tumor cell resistance to apoptosis [6]; however, the detailed mechanism is not fully understood.microRNAs are small, non-coding, single-stranded RNAs that are 20-23 nucleotides in length and that suppress gene expression in a sequencespecific manner [7].…”