NPNT is Expressed by Osteoblasts and Mediates Angiogenesis via the Activation of Extracellular Signal-regulated Kinase

Abstract

Angiogenesis plays an important role in bone development and remodeling and is mediated by a plethora of potential angiogenic factors. However, data regarding specific angiogenic factors that are secreted within the bone microenvironment to regulate osteoporosis is lacking. Here, we report that Nephronectin (NPNT), a member of the epidermal growth factor (EGF) repeat superfamily proteins and a homologue of EGFL6, is expressed in osteoblasts. Intriguingly, the gene expression of NPNT is reduced in the bone of C57BL/6J ovariectomised mice and in osteoporosis patients. In addition, the protein levels of NPNT and CD31 are also found to be reduced in the tibias of OVX mice. Exogenous addition of mouse recombinant NPNT on endothelial cells stimulates migration and tube-like structure formation in vitro. Furthermore, NPNT promotes angiogenesis in an ex vivo fetal mouse metatarsal angiogenesis assay. We show that NPNT stimulates the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 mitogen-activated kinase (MAPK) in endothelial cells. Inhibition of ERK1/2 impaired NPNT-induced endothelial cell migration, tube-like structure formation and angiogenesis. Taken together, these results demonstrate that NPNT is a paracrine angiogenic factor and may play a role in pathological osteoporosis. This may lead to new targets for treatment of bone diseases and injuries.

EGFL7: Master regulator of cancer pathogenesis, angiogenesis and an emerging mediator of bone homeostasis

“…Nephronectin are secreted factors that promote endothelial cell migration and vascular tubulogenesis in the bone microenvironment (Chim et al, 2015;Chim et al, 2011;Kuek et al, 2016). This is in line with other findings that EGFL7 is a potent angiogenic factor and plays a critical role in embryonic vascular organization (Schmidt et al, 2007).…”

Nephronectin is Correlated with Poor Prognosis in Breast Cancer and Promotes Metastasis via its Integrin-Binding Motifs

“…Approximately 10% of the tumor cells had a granular staining pattern of NPNT similar to that found in the human tumors. The granular NPNT staining pattern has also been shown to be present in mouse tibias [28] . Direct comparison between NPNT and the ECM-proteins Collagen V and Fibronectin in ECM-rich areas of MMTV-PyMT tumors showed some signs of extracellular presence of NPNT; however, this was not the major site of localization ( Supplementary Figure S2 B ).…”

Coupling factors and exosomal packaging microRNAs involved in the regulation of bone remodelling

“…In addition, EGFL6, Nephronectin (NPNT) and Cxcl9 are expressed by osteoblasts to regulate angiogenesis via a paracrine mechanism in the bone microenvironment (Chim et al ., ; Huang et al ., ; Kuek et al ., ). Interestingly, NPNT and CD31 expression is reduced in the bone of ovariectomised mice and patients with osteoporosis (Kuek et al ., ). It is likely that exosomal‐containing miRNA released from osteoclasts and osteoblasts could also regulate angiogenesis in the bone microenvironment.…”

“…Epidermal growth factor-like domain 7 (EGFL7) which is expressed in osteoclasts and osteoblasts is an antagonist to the Notch pathway (Schmidt et al, 2009), and regulates endothelial cell migration and angiogenesis (Chim et al, 2013(Chim et al, , 2015. In addition, EGFL6, Nephronectin (NPNT) and Cxcl9 are expressed by osteoblasts to regulate angiogenesis via a paracrine mechanism in the bone microenvironment (Chim et al, 2011;Huang et al, 2016;Kuek et al, 2016). Interestingly, NPNT and CD31 expression is reduced in the bone of ovariectomised mice and patients with osteoporosis .…”