Lysyl Oxidase Down-Regulates The Osteoblastic Potential of BMP9 Through Inhibiting HIF-1α/Wnt/β-Catenin Axis in Mesenchymal Stem Cells

Abstract

Background: Bone morphogenetic protein 9 (BMP9) is one of excellent osteogenic factors, but it can also initiate adipogenesis concomitantly. Thus, the osteogenic potential of BMP9 may be enhanced if the adipogenesis were reduced. It was reported that lysyl oxidase (Lox) may function as a critical switcher for adipogenesis. Up to date, the role of Lox in BMP9-induced osteoblastic differentiation remains unknown.Methods: The effect and possible mechanism of Lox on the osteogenic function of BMP9 were evaluated with RT-PCR, western blotting, immunofluorescent and histochemical staining. The same results were also confirmed with the in vivo BMP9-induced ectopic bone formation model.Results: The mRNA and protein of Lox are both detectable in progenitor cells, and it was increased by BMP9 in 3T3-L1 cells. BMP9-induced Runx2, OPN and mineralization were all enhanced by inhibiting or silencing Lox, but reduced by exogenous Lox. BMP9 increased the mRNA level of c-Myc, which was enhanced by inhibiting Lox, so did the protein level of β-catenin. Effects of Lox specific inhibitor on BMP9-induced Runx2, OPN and mineralization were reduced obviously by silencing β-catenin. HIF-1α was up-regulated by BMP9, which was enhanced by inhibiting or silencing Lox, but decreased by Lox over-expression. The effects of Lox specific inhibitor on increasing BMP9-induced osteogenic markers were reduced greatly by silencing HIF-1α. On the contrary, the inhibitory effect of Lox on BMP9-induced osteoblastic markers was almost abolished by HIF-1α over-expression. BMP9-induced bone formation was increased by silencing Lox or over-expressing HIF-1α. The effect of silencing Lox on potentiating BMP9-induced bone formation was attenuated by silencing HIF-1α. Lox specific inhibitor increased the level of β-catenin and decreased that of SOST, but these effects were almost reversed by silencing HIF-1α.Conclusions: Lox may reducing the osteoblastic-induction function of BMP9 through inhibiting Wnt/β-catenin signal via down-regulation of HIF-1α partly.