Angiopoietin-like protein 2 is a positive regulator of osteoblast differentiation

Several studies have reported that angiopoietin-like protein 2 (Angptl2) is expressed abundantly in adipocytes and is associated with adipose tissue inflammation. In the present study, we found that osteoblasts and mesenchymal stem cells also expressed Angptl2 at high levels. The aim of this study was to understand the role of Angptl2 in osteoblastic cell differentiation.

Angptl2 expression was examined during osteoblast and adipocyte differentiation. The role of Angptl2 on cell differentiation and associated signaling was analyzed by gene knockdown using Angptl2 small interfering ribonucleic acid (siRNA).

Angptl2 was highly expressed in MC3T3-E1 cells, ST2 cells and primary osteoblasts, but not in RAW264 cells. Inhibition of Angptl2 expression using siRNA markedly inhibited alkaline phosphatase (ALP) expression and osteoblastic differentiation in MC3T3-E1, ST2 cells and primary osteoblasts. Angptl2 siRNA also inhibited adipocyte differentiation in ST2 cells. Treatment of MC3T3-E1 cells with Angptl2 siRNA led to the down-regulation of the activities of several cell signaling pathways, including extracellular signal-regulated kinase (ERK), Jun amino-terminal kinase (JNK), Akt, and nuclear factor kappa B (NF-κB) signals. It also down-regulated the expression of Osterix, but not that of runt-related transcription factor 2 (Runx2), suggesting that Angptl2 is a positive activator of Osterix and its down-stream signals. Treatment of MC3T3-E1 cells with anti-Angptl2 antibodies suppressed ALP gene expression. In addition, treatment of Angptl2 siRNA-treated cells with culture supernatants of normal MC3T3-E1 cells restored ALP gene expression, indicating that Angptl2 acts in an autocrine manner.

The results suggest that Angptl2 is an autocrine positive regulator of cell differentiation. Thus, it is suggested that Angptl2 regulates not only adipose tissue metabolism but also bone metabolism.