Cathepsin B in tumors, normal tissue and isolated cells from the human lung

Human lung tumors of different histologic cell types and adjacent normal lung parenchyma, purified lung parenchyma, purified lung macrophages and three human lung-derived cell lines were investigated in an attempt to identify tumor specific premature and mature cathepsin B species. By polyacrylamide gel electrophoresis and immunoblotting techniques with specific antibodies we detected mature cathepsin B forms with molecular masses of 31 kDa and 32 kDa in tissues. Reductive conditions revealed in these populations single and double chain 31/32 kDa forms. The latter were recognized by their heavy part, the 26/27 kDa molecule forms. Qualitative differences in the pattern of cathepsin B species between tumor and corresponding normal material or between different histologies of lung tumors were not found. However, tumor material is considerably richer in cathepsin B activity than normal material. Isolated alveolar macrophage populations and established cell lines showed the same cathepsin B pattern as tissues. All these cells released cathepsin B proforms of 44 to 46 kDa into the culture medium, indicating that the release of pro-cathepsin B cannot be considered a tumor-specific mechanism. The secreted proforms were sensitive to in vitro activation by pepsin.