Insulin‐like growth factor‐1 signaling is responsible for cathepsin G‐induced aggregation of breast cancer MCF‐7 cells

Cathepsin G (CG), a neutrophil serine protease, induces cell migration and multicellular aggregation of human breast cancer MCF‐7 cells in a process that is dependent on E‐cadherin and CG enzymatic activity. While these tumor cell aggregates can cause tumor emboli that could represent intravascular growth and extravasation into the surrounding tissues, resulting in metastasis, the molecular mechanism underlying this process remains poorly characterized. In this study, we aimed to identify the signaling pathway that is triggered during CG‐mediated stimulation of cell aggregation. Screening of a library of compounds containing approximately 90 molecular‐targeting drugs revealed that this process was suppressed by the insulin‐like growth factor‐1 (IGF‐1) receptor (IGF‐1R)‐specific kinase inhibitor OSI‐906, as well as the multikinase inhibitors axitinib and sunitinib. Antibody array analysis, which is capable of detecting tyrosine phosphorylation of 49 distinct receptor tyrosine kinases, and the results of immunoprecipitation studies indicated that IGF‐1R is phosphorylated in response to CG treatment. Notably, IGF‐1R neutralization via treatment with a specific antibody or silencing of IGF‐1R expression through siRNA transfection suppressed cell aggregation. Furthermore, CG treatment of MCF‐7 cells resulted in increased release of IGF‐1 into the medium for 24 h, while antibody‐mediated IGF‐1 neutralization partially prevented CG‐induced cell aggregation. These results demonstrate that autocrine IGF‐1 signaling is partly responsible for the cell aggregation induced by CG.