Proteolytic cleavages in the extracellular domain of receptor tyrosine kinases by membrane-associated serine proteases

The epithelial extracellular membrane-associated serine proteases matriptase, hepsin, and prostasin are proteolytic modifying enzymes of the extracellular domain (ECD) of the epidermal growth factor receptor (EGFR). Matriptase also cleaves the ECD of the vascular endothelial growth factor receptor 2 (VEGFR2) and the angiopoietin receptor Tie2. In this study we tested the hypothesis that these serine proteases may cleave the ECD of additional receptor tyrosine kinases (RTKs). We co-expressed the proteases in an epithelial cell line with Her2, Her3, Her4, insulin receptor (INSR), insulin-like growth factor I receptor (IGF-1R), the platelet-derived growth factor receptors (PDGFRs) α and β, or nerve growth factor receptor A (TrkA). Western blot analysis was performed to detect the carboxyl-terminal fragments (CTFs) of the RTKs. Matriptase and hepsin were found to cleave the ECD of all RTKs tested, while TMPRSS6/matriptase-2 cleaves the ECD of Her4, INSR, and PDGFR α and β. Prostasin was able to cleave the ECD of Her3 and PDGFRα. Matriptase cleaves phosphorylated Her2 at Arg558 and Arg599 and the Arg599 cleavage produces a CTF not recognized by the monoclonal antibody trastuzumab/Herceptin. Her2 cleavages by matriptase can be inhibited by the hepatocyte growth factor activator inhibitor 1 (HAI-1) in the MDA-MB-231 human breast cancer cells. Matriptase silencing in the Her2, matriptase, and HAI-1 triple-positive SKBR3 human breast cancer cells enhanced Her2 protein down-regulation induced by a sustained exposure to phorbol 12-myristate 13-acetate (PMA), which down-regulated matriptase protein. The novel Her2 cleavage and expression regulation mechanisms mediated by matriptase may have potential impacts in Her2-targeting therapies.