Cathepsin B, thiols and cysteine protease inhibitors in squamous-cell lung cancer.

We investigated activities of the cysteine protease cathepsin B (CB; EC 3.4.22.1), the levels of reduced glutathione (GSH) and cysteine and the activity of gamma-glutamyltransferase (gamma-GT; EC 2.3.2.2) in squamous-cell lung carcinoma (SQCLC) and the lung parenchyma specimens from surgically treated patients. The basal CB activity, assayed in tissue extracts in the absence of exogenous activators, was significantly higher in SQCLC compared to the lung. The residual CB activity, remaining in tissue extracts after preincubation at 37 degrees C, was not any longer significantly different in SQCLC and the lungs. The inhibited CB activity, calculated as the difference between the basal and residual CB activities, was significantly higher in SQCLC compared to the lung. In the case of the cysteine protease cathepsin C (CC; EC 3.4.14.1), neither the basal nor the residual nor the inhibited CC activities in SQCLC and the lung were significantly different. Compared to CC, the powerfulness of endogenous cysteine protease inhibitors to inhibit CB was much higher in both SQCLC and the lung. The cysteine protease inhibitors from SQCLC and the lung which effectively inhibited CB could be related to the inhibitors with an apparent M(r) ranging from 10,000 to 30,000. Isoelectric focusing studies indicated significant differences in the progress of inhibition of the activity of CB isoforms in SQCLC and lung parenchyma extracts. The levels of both GSH and Cys were significantly higher in SQCLC compared to the lung and the level of GSH was significantly higher in Stage III tumors compared to Stage I tumors. The activity of gamma-GT was not significantly different in SQCLC and the lung but it was significantly higher in Stage I tumors compared to Stage III tumors and showed a significant negative correlation with GSH level in SQCLC. Dithiothreitol did not increase the basal activity of CB from SQCLC and the lung which indicates that reversibly oxidized forms of CB do not accumulate in the tumors and the lungs. The basal activity of CB from SQCLC and the lung was competitively inhibited by Cys. Moreover, increasing Cys concentrations had a modulatory effect on the basal activity of CB from SQCLC and the lung which was featured by Cys-induced inhibition of CB activity and by subsequent Cys-effected recovery of CB activity from its previous inhibition by Cys.