Regulation of the activity of matriptase on epithelial cell surfaces by a blood‐derived factor

Matriptase is an epithelial‐derived, integral membrane, trypsin‐like serine protease. We have shown previously that matriptase exists both in complexed and noncomplexed forms. We now show that the complexed matriptase is an activated, two‐chain form, which is inhibited in an acid‐sensitive, reversible manner through binding to its cognate, Kunitz‐type inhibitor, HAI‐1 (hepatocyte growth factor activator inhibitor‐1). Conversely, the majority of the noncomplexed matriptase is a single‐chain zymogen, which lacks binding affinity to HAI‐1, suggesting that matriptase, similar to most other serine proteases, is activated by proteolytic cleavage at a canonical activation motif. We have now generated mAbs specific for the conformational changes associated with the proteolytic activation of matriptase. Using these mAbs, which specifically recognize the two‐chain form of matriptase, we demonstrate that matriptase is transiently activated on 184A1N4 human mammary epithelial cell surfaces following their exposure to serum. The ability of serum to activate matriptase is highly conserved across reptilian, avian, and mammalian species. This serum‐dependent activation of matriptase on epithelial cell surfaces is followed by ectodomain shedding of both matriptase and its Kunitz‐type inhibitor.