Dipeptidyl peptidase IV, prolyl endopeptidase and cathepsin B activities in primary human lung tumours and lung parenchyma

Peptidases which are specific for proline residues have been described and include endopeptidases (post-proline cleaving enzyme and proline specific endopeptidase), N-terminal exopeptidases (post-proline dipeptidyl aminopeptidase, proline iminopeptidase, aminopeptidase P), C-terminal exopeptidases (prolylcarboxypeptidase, and carboxypeptidase P) and dipeptidases (prolyl dipeptidase and proline dipeptidase). The properties, distinguishing charcteristics, and possible significance of these proline specific endo- and exopeptidases are discussed. In addition, reference is made to a series of enzymes which can hydrolyze proline containing peptide bonds, but which are not specific for proline.

We demonstrated the cysteine proteinase cathepsin B in two human lung tumor cell lines by cytochemical and immunocytochemical methods. The cell lines were derived from a squamous cell carcinoma of the lung (HS-24) and a metastasis to the adrenal gland from an adenocarcinoma of the lung (SB-3). For comparison and control, normal human lung fibroblasts cells (Wi-38) were also investigated. Intracellular cathepsin B activity was detected in all three cell lines. SB-3 and the normal fibroblast cells showed almost equal cathepsin B activity, which was considerably stronger than that in the HS-24 cells. Specific inhibitors for cathepsin B (E64, leupeptin, antipain) suppressed its activity completely. Stefin A, the physiological cathepsin B inhibitor, was less effective; this might depend on its limited penetrability into living cells. Localization of the cathepsin B was performed by conventional immunofluorescence microscopy and laser scanning microscopy. With specific anti-cathepsin B antibodies, the enzyme was localized in HS-24, SB-3, and Wi-38 fibroblast cells within perinuclear granules representing the lysosomal compartment. In the SB-3 cells, we additionally localized a minor fraction of the enzyme bound to the plasma membrane in a speckled distribution, accessible to the antibodies from the outside. This direct demonstration of cathepsin B distribution supports biochemical data about the dual localization of the enzyme in tumor cells. It also supports the possibility of a direct involvement of cathepsin B in the degradation of the extracellular matrix, and thus a contribution of the enzyme in invasion and metastasis.