Secreted phosphoprotein 24 kD (Spp24) inhibits growth of human osteosarcoma through the BMP-2/Smad signaling pathway

Abstract

Background Autocrine stimulation of tumors cells is an important mechanism for the growth of skeletal tumors. In tumors that are sensitive, growth factor inhibitors can dramatically reduce tumor growth. In this study, we aimed to investigate the effects of Secreted phosphoprotein 24 kD (Spp24) on the growth of osteosarcoma cells induced by BMP-2 both in vitro and in vivo. Methods Two osteosarcoma cell lines, 143B and MG63, were used in this study. MTT assay, flow cytometry, wound-healing assay, Matrigel invasion assay and immunohistochemical staining were used to evaluate the effects of Spp24 inhibition on the osteosarcoma cells induced by BMP-2 in vitro. Subcutaneous and intratibial tumor models in nude mice were used to evaluate the effects of Spp24 in vivo. Quantitative real-time PCR, western blotting and co-immunoprecipitation were used to investigate the underlying mechanisms. Results Our study demonstrated that Spp24 can inhibit proliferation and promote apoptosis of osteosarcoma cells as confirmed by MTT assays and immunohistochemical staining. We found that BMP-2 increased the mobility and invasiveness of tumor cells in vitro whereas Spp24 inhibited both effects either in the presence or absence of exogenous BMP-2. Intracellular binding of Spp24 and BMP-2 was confirmed by co-immunoprecipitation. Mechanically, phosphorylation of Smad1/5/8 was enhanced by treatment with BMP-2 but was significantly inhibited by treatment with Spp24. Subcutaneous and intratibial tumor models in nude mice demonstrated that BMP-2 promoted osteosarcoma growth in vivo, while Spp24 significantly inhibited tumor growth and reduced osteolytic lesions in the tibia induced by BMP-2. Conclusions From this study, we concluded that the BMP-2/Smad signaling pathway is involved in the pathogenesis of osteosarcoma growth, and that Spp24 inhibits the growth of human osteosarcoma induced by BMP-2 both in vitro and in vivo. These results validate the potential of Spp24 as a therapeutic agent for the treatment of osteosarcoma.