The malignant phenotype and cysteine proteinases.

Expression, membrane association and secretion of both cathepsin B and cathepsin L have been associated with the malignant phenotype of murine B16 melanomas. Only native forms of the two enzymes were found in the lysosomal fractions, whereas both native and latent forms were found in the membrane fractions. Brief exposure to acid pH induced secretion of only native forms of both cathepsin B and cathepsin L and a concomitant reduction in membrane-associated activities. Thus, the pericellular acidification associated with malignant tumors may provide optimal conditions for proteolysis by the cysteine proteinases in terms of their enhanced stability and their induced release in native forms not requiring proteolytic activation.