Acquisition of invasive and metastatic properties by MCF-7/DOX cells. (A) The activity of MMP-2, MMP-9, and uPA in MCF-7 and MCF-7/DOX cells was determined by gelatin or fibrinogen/plasminogen zymography. (B) An invasion assay was performed with MCF-7 and MCF-7/DOX cells. Invasion of MCF-7 and MCF-7/DOX cells through a transwell membrane coated with Matrigel was compared. *P < 0.001 (t-test). (C) Postmortem examination of the lungs of mice that were killed 3 months after injection of MCF-7 or MCF-7/DOX cells. Lung tissues were fixed in Bouin's solution, and the gross tumor nodules were counted. *P < 0.001 (Chi-square test). (D) Representative photograph of lung tumors from nude mice injected with MCF-7/DOX cells. The arrow head indicates invading MCF-7/DOX cells in the lung of nude mice. Original magnification, × 100; Scale bar = 100 μm.