CTSK and breast cancer: In addition, we also demonstrated that nAS‐E regulated the expressions of nuclear receptor NFATc1 and c‐fos that controlled the expressions of bone resorption enzymes such as TRAP, V‐ATP and cathepsin K. We further determined the effects of nAS‐E on the proliferation, migration and apoptosis of MDA‐MB‐231 breast cancer cells by MTT assay, transwell assay and flow cytometric analysis.