The cysteine protease cathepsin B in cancer

Of the cysteine proteases, cathepsin B has been most often implicated in tumor progression. Accumulating evidence indicates that the regulation of cathepsin B and other cysteine proteases is under both transcriptional and posttranscriptional control. The elucidation of such control mechanisms may be important for therapeutic strategies aimed at reducing cathepsin B overexpression in tumors and other pathologies. Transcription initiation at more than one initiation site and/or the induction of alternative splicing of mRNA may lead to the production of transcripts that differ in their rate of translation or their stability. Hence, the amount of protein produced may not be proportional to the amount of mRNA observed in tissues. Transcripts lacking exon 3 would encode a truncated procathepsin B, a form predicted to be trafficked to the cytoplasm due to the absence of its signal peptide. Evidence of alterations in trafficking of cathepsin B and distribution of organelles labeling for cathepsin B to more peripheral, basolateral or cell membrane localizations is seen in bladder, breast, colon, prostate and thyroid carcinomas and in gliomas. These may reflect alterations in the cytoskeleton as similar changes in localization of cathepsin B accompany the rearrangement of cytoskeletal elements and the acquisition of an invasive phenotype in MCF-10A breast epithelial cell lines after transfection with oncogenic c-Ha-ins. Alterations in trafficking of cathepsin B and other cysteine proteases may thus be brought about by changes in signal transduction, induced by transfection with an activated oncogene. Signal-transduction pathways as well as the enzymes directly involved in the invasive degradation of basement membranes may therefore both be targets for therapeutic intervention. The results of recent studies suggest that cathepsins B and L are more stable in the extracellular environment than previously believed and so could play a significant extracellular role in invasive cancer.