Experimental and clinical evidence has linked cathepsin B with tumour invasion and metastasis. Cathepsin B expression is increased in many human cancers at the mRNA, protein and activity levels. In addition, cathepsin B is frequently overexpressed in premalignant lesions, an observation that associates this protease with local invasive stages of cancer. Increased expression of cathepsin B in primary cancers, and especially in preneoplastic lesions, suggests that this enzyme might have pro-apoptotic features. Expression of cathepsin B is regulated at many different levels, from gene amplification, use of alternative promoters, increased transcription and alternative splicing, to increased stability and translatability of transcripts. During the transition to malignancy, a change in the localization of cathepsin B occurs, as demonstrated by the presence of cathepsin B-containing vesicles at the cell periphery and at the basal pole of polarized cells. Due to increased expression of cathepsin B and changes in intracellular trafficking, increased secretion of procathepsin B from tumours is observed. Active cathepsin B is also secreted from tumours, a mechanism likely to be facilitated by lysosomal exocytosis or extracellular processing by surface activators. Cathepsin B is localized to caveolae on the tumour surface, where binding to the annexin II heterotetramer occurs. Activation of cathepsin B on the cell surface leads to the regulation of downstream proteolytic cascade(s).
The Biochemical Society's Annual Symposium, Proteases and the Regulation of Biological Processes, was held at Imperial College, London in December 2002