In vitro degradation of extracellular matrix with Mr 52,000 cathepsin D secreted by breast cancer cells.

It has been proposed that proteases secreted by cancer cells facilitate metastasis by degrading extracellular matrix. Estrogen receptor-positive breast cancer cells secrete a Mr 52,000 pro-cath-D under estrogen stimulation, whereas this protease is produced constitutively by estrogen receptor-negative cancer cells. We report on the degradation in vitro of extracellular matrix by purified Mr 52,000 cathepsin D (cath-D) and by conditioned media prepared from different cell lines. The purified Mr 52,000 pro-cath-D was autoactivated at pH 4.5 into a Mr 51,000 cath-D and found to digest the extracellular matrix of endothelial bovine corneal cells labeled with [3H]proline or [35S]methionine. Culture medium conditioned by estrogen-treated MCF7 cells had a similar effect at pH 4.5 but not at pH 7.4. Matrix degradation was totally inhibited by pepstatin. Other breast cancer cells (BT20, MDA-MB231, T47D cells, etc.) and other cancer cells also secreted a pepstatin-sensitive proteinase able to degrade extracellular matrix. By contrast, the U2 variant of MCF7 cells, which lacks the Mr 52,000 cath-D gene, and the nontumoral epithelial mammary cells secreted a negligible amount of this proteinase. In all conditioned media, the pepstatin-dependent extracellular matrix degrading activity was highly correlated to the Mr 52,000 cath-D concentration measured by immunoenzymatic assay. We conclude that the Mr 52,000 cath-D is the major acidic protease secreted by mammary cancer cells. We suggest that this protease may degrade basement membrane and consequently facilitate tumor invasion when it is released in an acidic microenvironment.