Immunofluorescent studies on the occurrence of cathepsin B1 at tumor cell surfaces

The specific antibody against the lysosomal proteinase cathepsin B1 was isolated and labelled with FITC, in order to enable a direct immunofluorescent screening technique for the alleged localization of this enzyme. A number of normal cells, cultivated 3T3 fibroblasts, virus-transformed 3T3 lines and malignant cell lines were studied. Normal non-growing differentiated cells showed no receptor groups at their surface for this antibody, whilst all transformed and tumor line cells showed positive fluorescent staining at or near their surfaces. This reaction was most marked in premitotic and mitotic cells. The increased availability of cathepsin B1 at tumor cell surfaces seems to be correlated with an altered stability and function of the lysosomal system, associated with a release of lysosomal enzymes onto the cell surface and into the microenvironment of tumors. The implications for cellular interrelations and invasive growth of tumors are discussed. The functional significance of a lysosomal proteinase for the maintenance of cyclic growth in tumors is further correlated with current data from other authors.