MMP9 and osteosarcoma: Functionally, enforced expression of circTCF25 promoted osteosarcoma cell proliferation, migration and invasion, accompanied by corresponding alterations of phenotype‐related proteins (upregulation of cyclin D1 and CDK6 for increased cell proliferation; upregulation of MMP2, MMP9 and Vimentin and downregulation of TIMP‐1 for enhanced cell migration and invasion).