Expression of cathepsin B in small cell lung carcinoma cells in relation to in vitro invasiveness

Cathepsin B expression and its relation to in vitro invasiveness were investigated in small cell lung carcinoma cells (OC-10 cells). A subclone derived from OC-10 cells (10N) was similar to the parental cells both in growth pattern and morphology. However, the in vitro invasive capacity of OC-10 cells was 4 times higher than that of 10N cells. Cathepsin B activities in the plasma membrane fraction and spent medium of OC-10 cells were 2-fold higher than those of 10N cells. The invasiveness of OC-10 cells was markedly blocked by the addition of cysteine proteinase inhibitors, E-64C or leupeptin, while treatment of 10N cells with 2% (v/v) DMSO resulted in a 2-fold increment both in invasive capacity and cathepsin B activity. Immunoblot analysis demonstrated that the intensity of the cathepsin B band from OC-10 or 10N cells was not remarkably different regardless of DMSO treatment. Although no significant correlation was observed between the biochemical activity and protein of cathepsin B, in vitro invasive capacity of OC-10 cells strongly correlated with cathepsin B activity. These results suggest that cathepsin B plays an important role in the invasiveness of human small cell lung carcinoma cells.