Immunological characterization of cell-surface and soluble forms of membrane type 1 matrix metalloproteinase in human breast cancer cells and in fibroblasts.

Membrane type (MT) 1 matrix metalloproteinase (MMP) activates progelatinase A (pro-MMP-2), a type IV collagenase, on the cell surface of tumors; however, its function in breast cancer progression and metastasis is not fully understood. To examine the expression of MT1-MMP in breast cancer cells and fibroblasts, a specific rabbit antibody (Ab) directed against a unique synthetic peptide derived from the human MT1-MMP catalytic domain was produced, purified, and characterized. This Ab is not likely to cross-react with MT2-, MT3-, or MT4-MMP or any other MMPs. MT1-MMP expression and pro-MMP-2 activation were stimulated by concanavalin A in two human breast carcinoma cell lines (BT549 and MDA-MB-231) and in normal human fetal-lung fibroblasts (HFL-1) and were slightly upregulated by breast cancer cell-fibroblast interactions. Both pro-MT1-MMP in plasma membrane (63.4 kDa) and the soluble forms of the enzyme in culture medium (57.6 and 25-30 kDa) were detected by immunoblot analysis, suggesting that cell-surface MT1-MMP exhibits an active conformation without the removal of its propeptide domain and that the mature enzyme is shed into the medium. In breast cancer cells, MT1-MMP and a recombinant catalytic domain of MT1-MMP were unable to activate pro-matrilysin, indicating that MT1-MMP is not a universal activator of all MMPs. MT1-MMP may play an important role in the invasive growth and spread of breast cancer cells by specifically activating pro-MMP-2 to cleave the connective-tissue barrier. Furthermore, use of the specific Ab may aid in the investigation of the role of MT1-MMP in human tumors.