MMP9 and breast cancer: COX2 can enhance the permeability of BBB by up-regulating the expression of matrix metallopeptidase 1 (MMP1) [37], and integrin αvβ3 (encoded by ITGAV and ITGB3) facilitates breast cancer brain metastasis by mediating the adhesion between cancer cells and brain endothelium and up-regulating the expressions of MMP2 and MMP9 [41,42].