It has been proposed that
proteases secreted by
cancer cells facilitate
tumor invasion and
metastasis by degrading the components of extracellular membranes. The lysosomal
cysteine protease cathepsin L is synthesized in large amounts and secreted by many malignantly transformed cells in culture. The secreted
protease is potent in degrading
collagen,
laminin,
elastin, and other structural
proteins of basement membranes. To determine whether human
cancers synthesize
cathepsin L, the expression of
cathepsin L in approximately 100 human
tumor samples was determined by quantitative
RNA slot blot analysis using a specific human
cathepsin L complementary DNA probe. Results of the present study suggest that
cancers in general express higher levels of
cathepsin L than do normal tissues. Kidney and
testicular tumors expressed the highest levels of
cathepsin L; non-
small cell carcinomas of the lung expressed the next highest levels; and most
cancers of the breast, ovary, colon, adrenal, bladder, prostate, and thyroid expressed elevated levels as well.
Cathepsin L may prove useful as a diagnostic or prognostic marker of human
malignancy.