Purification and characterization of a complex containing matriptase and aKunitz-type serine protease inhibitor from human milk

Matriptase, a trypsin-like serine protease with two potential regulatory mo dules (low density lipoprotein receptor and complement C1r/s domains), was initially purified from T-47D breast cancer cells, Given its plasma membran e localization, extracellular matrix-degrading activity, and expression by breast cancer cells, this protease may be involved in multiple aspects of b reast tumor progression, including cancer invasion. In breast cancer cells, matriptase was detected mainly as an uncomplexed form; however, low levels of matriptase were detected in complexes. In striking contrast, only the c omplexed matriptase was detected in human milk, The complexed matriptase ha s now been purified. Amino acid sequences obtained from the matriptase-asso ciated proteins reveal that they are fragments of a Kunitz-type serine prot ease inhibitor that was previously reported to be an inhibitor of the hepat ocyte growth factor activator. In addition, matriptase and its complexes we re detected in milk-derived, SV40 T-antigen-immortalized mammary luminal ep ithelial cell lines, but not in human foreskin fibroblasts or in HT-1080 fi brosarcoma cells. These results suggest that the milk-derived matriptase co mplexes are likely to be produced by the epithelial components of the lacta ting mammary gland in vivo and that the activity and function of matriptase may be differentially regulated by its cognate inhibitor, comparing breast cancer with the lactating mammary gland.