HGF and cancer: In the pericancerous microenvironment, HGF is mainly secreted from stromal cells, including cancer-associated fibroblasts (CAF), as an inactive pro-form (pro-HGF) and proteolytically activated by the specific serine proteases, including HGF activator (HGFA, serum activator), hepsin and matriptase (the latter two are transmembrane proteases) [8,9,10].