Presence of proteolytically processed keratins in the culture medium of MCF-7 cells.

MCF-7 human mammary epithelial cells are known to express three distinct cytoskeletal intermediate filament proteins designated numbers 8 (Mr 52,000), 18 (Mr 45,000), and 19 (Mr 40,000) keratin. Using a panel of monoclonal antibodies and peptide mapping, we have determined that MCF-7 cells release proteolytic derivatives of keratins 18 and 19 into the culture medium. These extracellular keratins consist of multiple isoforms (Mr 38,000-45,000, isoelectric point 5.0-5.2) on two-dimensional gels, many of which are smaller and more acidic than the predominant intracellular forms and appear to be associated together as a soluble, large molecular weight complex. These results may potentially be correlated with observations of keratin-like antigens in body fluids of cancer patients and the presence of antikeratin antibodies in individuals with certain autoimmune diseases and acute viral infections.