EFEMP1 binds to STEAP1 to promote osteosarcoma proliferation and invasion via the Wnt/β-catenin and TGF-β/Smad2/3 signal pathways

PURPOSE: To investigate the prognostic value and function of six-transmembrane epithelial antigen of prostate 1 (STEAP1) in osteosarcoma and determine whether EFEMP1 mediates its effects. METHODS: IHC (immunohistochemistry)/ICC (immunocytochemistry) in conjunction with RT-qPCR (quantitative real-time polymerase chain reaction) were employed to assess the expression of STEAP1 in paratumoral tissues, osteosarcoma, benign fibrous dysplasia, osteosarcoma cells, normal osteoblastic hFOB cells, as well as various invasive subclones. The association of STEAP1 with outcome was examined with Kaplan–Meier graph among the osteosarcoma population. The effects of the down-regulation and up-regulation of STEAP1 on the biological behavior of osteosarcoma cells were studied through in-vitro and in-vivo functional tests. RESULTS: Up-regulation of STEAP1 in the osteosarcoma tissues, whose correlations with the malignant osteosarcoma phenotype and the poor patient outcome were positive. In addition, STEAP1 induced the epithelial–mesenchymal transition (EMT) via the Wnt/β-catenin and TGF-β/Smad2/3 pathways and facilitated the osteosarcoma cell infiltration and migration. An increase or decrease in EFEMP1 expression directly promoted or inhibited the expression of STEAP1. In osteosarcoma cells overexpressing EFEMP1, STEAP1 knockdown significantly inhibited cell invasion, EMT process, and increased activity of Wnt/β-catenin and TGF-β/Smad2/3 signaling pathways. Although exogenous EFEMP1 could stimulate the Wnt/β-catenin and TGF-β/Smad2/3 pathways to promote the EMT, it had not effect on osteosarcoma cells with STEAP1 knockdown. Collectively, similar to EFEMP1, STEAP1 acted like an oncogene in the osteosarcoma progression. CONCLUSION: EFEMP1 enabled the Wnt/β-catenin and TGF-β/Smad2/3 axises initiation and EMT elicitation by targeting STEAP1, thereby facilitating the osteosarcoma cell infiltration and migration. These results are expected to contribute to the search for new targeted drugs able to effectively inhibit invasion and metastasis and improve prognosis in osteosarcoma.