In vitro activation of pro-cathepsin B purified from ascitic fluid of ovarian carcinomas by serine proteinases was studied. Both elastase and cathepsin G from human leucocytes were found to be activators, on the basis of generation of cathepsin B activity and processing of the precursor. These results represent a new cooperative pathway between cancer cells and host cells. The urokinase-type plasminogen activator activated pro-cathepsin B faster than leucocyte proteinases. A new relationship is emerging between the cysteine proteinases and the plasmin-activation system. Both pathways suggest an important role of cathepsin B in the proteolytic cascade associated with tumour invasion.
