Cysteine Endopeptidases and Their Inhibitors in Malignant Progression of Rat Embryo Fibroblasts

Cathepsins B and L and their endogenous inhibitors were evaluated in rat embryo fibroblast lines which have been developed as a model system for the study of malignant progression and metastatic capability. Three groups of lines were analyzed: 1) immortalized/non-tumorigenic, 2) tumorigenic/metastatic lines transfected with c-Ha-ras, and 3) metastatic revertants transfected with c-Ha-ras + the El A region of adenovirus type 2. The metastatic revertants are tumorigenic, but non-metastatic. No correlation was seen between tumorigenicity and metastatic potential and the level of expression of cathepsin B or the subcellular distribution of cathepsins B and L. However, cathepsin L activity was increased 2-fold in the 4R metastatic line. Although transfection of aneuploid 3T3 fibroblasts with ras has been shown to increase the expression of cathepsin L and cathepsin B, transfection of the diploid rat embryo fibroblasts with ras did not correlate with increased expression of cathepsin L or cathepsin B. However, ras transfection of the rat embryo fibroblasts was associated with a significant (4-15-fold) decrease in the activity of heat-stable cysteine endopeptidase inhibitors. Thus, in tumorigenic rat embryo fibroblast lines, regulation of the activities of cysteine endopeptidases by their endogenous inhibitors may be compromised, resulting in increased effective activities of the cysteine endopeptidases.