Released in Response to Estrogen A Monoclonal Antibody to a Human Breast Tumor Protein

A panel of monoclonal antibodies was produced against cell surface antigens of the MCF-7 human breast cancer cell line. The monoclonal antibodies were selected by their ability to bind to live, intact MCF-7 cells in solid-phase radioimmunoassay, and to bind to human breast cancer cells in paraffin sections. One monoclonal antibody, designated UCD/AB 6.11, identified two cellular antigens and one extracellular antigen from MCF-7, and bound to 18 of 20 breast cancers in paraffin sections. The two cellular antigens were associated with M, 54,000 and M, 56,000 proteins, which could be identified in Western blots, and were localized to the cell surface by immune precipitation of lactoperoxidase-iodinated plasma membrane proteins. The extracellular antigen, a M, 52,000 protein, was slightly more acidic and was only found in the media from estrogen-stimulated cells. Estrogen did not appear to have an effect on the production of the cellassociated antigens, M, 54,000 and 56,000 proteins. We pos tulate that the cellular antigens are precursors to the secreted Mr 52,000 protein.