Proteolytic enzymes at various stages of oncogenic transformation of rat fibroblasts. I. Aspartyl and cysteine proteinases

Aspartyl and cysteine proteinases at distinct stages of carcinogenesis were analyzed in rat embryo fibroblasts, sequentially immortalized and transformed by 2 different genes: the early region of simian adenovirus SA7 and c‐Ha‐ros oncogene. The dynamics of expression and distribution of proteinases throughout the transformation process were examined. It was shown that in immortalized and transformed cells the activities of the aspartyl and cysteine proteinases were expressed to a variable degree and that the expression was dependent on cell‐propagation time in vitro . The increase in activity both of cathepsin‐D‐like aspartyl proteinase and of cathepsin‐L‐ and ‐B‐like cysteine proteinases in cell lysates was correlated with the stages of fibroblast transformation (immortalization and tumorigenic transformation). In all cell types the major part of cysteine proteinases was localized inside the cell, while the cathepsin‐D‐like proteinase was apparently predominant among secreted proteinases. The cathepsin‐L‐like proteinase accounts for the major part of the cysteine‐proteinase activity as measured by Z‐Phe‐Arg‐MCA hydrolysis. We suggest that considerable portions of the cathepsin‐D‐ and ‐L‐like proteinases in all cell lines studied are secreted as a complex with inhibitor(s) and that inhibitor expression plays an important role in regulating the activity of cathepsin‐D‐like proteinase at different stages of transformation. Cathepsin‐L‐like proteinase is probably secreted in the precursor form.