Reduced levels of cathepsin D associated with tamoxifen resistance and estrogen independence in the ZR-75-1 human breast cancer cell line.

The expression and secretion of cathepsin D by ZR-75-1 human breast cancer cells, and tamoxifen-resistant (ZR-75-9a1) and estrogen-independent (ZR-PR-LT) variants was examined by electrophoresis of labeled proteins and Western blotting. Secreted proteins of 160 kDa, 52 kDa and 34 kDa were identified, and in ZR-75-1 cells, they were shown to be estrogen-inducible. Treatment of ZR-75-9a1 cells with 17 beta-estradiol (E2) and the progestin ORG 2058 increased secretion of the 52 kDa protein; ZR-PR-LT cells were unaffected. Western blotting showed that each cell line expressed high levels of the 52 kDa and 34 kDa forms of cathepsin D but that relatively little was being secreted. Each cell line secreted 52 kDa procathepsin D, but 34 kDa mature-cathepsin D was not detected as a secreted protein. ZR-75-1 cells expressed and secreted the highest levels of cathepsin D while ZR-75-9a1 cells expressed and secreted the least.