The estrogen-regulated 52K-cathepsin-D in breast cancer: From biology to clinical applications

Abstract

We have studied estrogen-regulated proteins in an attempt to understand the mechanism by which estrogens stimulate cell proliferation and mammary carcinogenesis. In estrogen receptor positive human breast cancer celllines (MCF7, ZR75-1) estrogens specifically increase the production into the culture medium of a 52,000 daltons (52K) glycoprotein. Several high affinity monoclonal antibodies to the partially purified secretory 52K protein have allowed to purify to homogeneity this protein and its cellular processed products. The 52K protein has been identified as the secreted precursor of a cathepsin-D like protease bearing mannose-6-phosphate signals and routed to lysosomes via mannose-6-phosphate receptor. The protease is mitogenic in vitro on estrogen deprived MCF7 cells and is able to degrade basement membrane and proteoglycans following its activation. The cellular related proteins, as detected by immunohistochemistry and immunoassay are more concentrated in proliferative mammary ducts than in resting ducts and their concentration in breast cancer cytosol appears to be more correlated with lymph nodes invasion and disease free survival (with S. Thorpe, Copenhagen) than with the estrogen receptor (RE) level. The protein is also produced constitutively by RE-negative cell lines, while in some antiestrogen resistant variants, it becomes inducible by tamoxifen, contrary to the wild type MCF7 cells. Cloning of its cDNA in λgt11 has allowed to show that the mRNA is rapidly induced by estrogens and to sequence the protein and compare it to that of the normal human kidney cathepsin-D. We conclude that the estrogen-induced cathepsin-D like protease may have important autocrine and/or paracrine functions in stimulating the growth and invasion of hormone-dependent and independent breast cancers and may be useful as a tissue marker to predict high risk mastopathies and breast cancer invasiveness, In addition to other estrogen regulated growth factors, the precursor of lysosomal proteases has the potential to stimulate the proliferation and invasiveness of breast cancer cells as long as they are secreted in excess rather than being routed to lysosomes.