Immunodetection of cathepsins b and l present in and secreted from human pre‐malignant and malignant colorectal tumour cell lines

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Abstract
Pre-malignant and malignant human colorectal tumour epithelial cell lines both secreted precursor forms of the 2 cysteine proteinases, cathepsins B and L. The amount of proteinases secreted by these cell lines varied according to the cell density. Comparison at similar cell densities showed that the pre-malignant, adenoma-derived cell line (PC/AA) secreted as much, or more, of both cathepsin B and L precursors as did the malignant, carcinoma-derived cell line (PC/JW/FI). However, mature forms of cathepsins B and L were detected in the culture media of only the carcinoma-derived cell line, thus indicating that the invasive potential of a tumour may be related to its ability to process extracellularly the secreted precursor enzyme to a mature and consequently active enzyme, rather than to the amount of proteinase synthesized and/or secreted. Similar results were obtained using 2 other epithelium-derived tumour cell lines, HT/29 (carcinoma) and SP/AN (adenoma). Immunolocation studies showed that cathepsin B was lysosomal while cathepsin L appeared to have a distribution more consistent with a plasma membrane association. Purified human cathepsins B and L (mature form) were capable of solubilizing an isolated basement membrane matrix (bovine anterior lens capsule) In vitro, thus indicating that the secreted mature enzymes and the membraneassociated cathepsin L could potentially degrade basal laminae or sub-endothelial basement membranes in vivo.