Abstract
The type 2 transmembrane serine protease matriptase is under tight control primarily by the actions of the integral membrane Kunitz-type serine protease inhibitor HAI-1. Growing evidence indicates that HAI-2 might also be involved in matriptase inhibition in some contexts. Here we showed that matriptase inhibition by HAI-2 depends on the subcellular localizations of HAI-2, and is observed in breast cancer cells but not in mammary epithelial cells. HAI-2 is co-expressed with matriptase in 21 out of 26 human epithelial and carcinoma cells examined. HAI-2 is also a potent matriptase inhibitor in solution, but in spite of this, HAI-2 inhibition of matriptase is not observed in all contexts where HAI-2 is expressed, unlike what is seen for HAI-1. Induction of matriptase zymogen activation in mammary epithelial cells results in the formation of matriptase-HAI-1 complexes, but matriptase-HAI-2 complexes are not observed. In breast cancer cells, however, in addition to the appearance of matriptase-HAI-1 complex, three different matriptase-HAI-2 complexes, are formed following the induction of matriptase activation. Immunofluorescent staining reveals that activated matriptase is focused at the cell-cell junctions upon the induction of matriptase zymogen activation in both mammary epithelial cells and breast cancer cells. HAI-2, in contrast, remains localized in vesicle/granule-like structures during matriptase zymogen activation in human mammary epithelial cells. In breast cancer cells, however, a proportion of the HAI-2 reaches the cell surface where it can gain access to and inhibit active matriptase. Collectively, these data suggest that matriptase inhibition by HAI-2 requires the translocation of HAI-2 to the cell surface, a process which is observed in some breast cancer cells but not in mammary epithelial cells.
Highlights
Interactions between a protease and a protease inhibitor that can be observed in solution may be irrelevant in whole cells and in vivo
While the role of HAI-1 in the control of HGF activator (HGFA) in vivo remains the subject of debate due to the expression of these proteins by different cell types with different subcellular localization, considerable evidence does indicate that the type 2 transmembrane serine protease matriptase is the genuine physiological target protease of HAI-1
Expression of matriptase is tightly correlated with HAI-1 expression, and to a lesser extent with HAI-2, in human epithelial and carcinoma cells
Summary
Interactions between a protease and a protease inhibitor that can be observed in solution may be irrelevant in whole cells and in vivo. The identification of artifactual protease-inhibitor complexes can be a pitfall of common experimental strategies; for example protease inhibitors identified using purified active protease in solution One example of this involves the protein products of the SPINT1 and SPINT2 genes, which encode two highly related, integral membrane, Kunitz-type serine protease inhibitors, named hepatocyte growth factor (HGF) activator inhibitor type (HAI)-1 and 2 [1,2]. As indicated by their nomenclature, HAI-1 and HAI-2 which are expressed predominantly by epithelial cells [3,4], have been shown to act against HGF activator (HGFA), a predominantly liver-derived, blood-borne serine protease [5]. In addition to being a potent matriptase inhibitor with a Ki of the order of nM [8] and the widespread co-expression of the inhibitor with matriptase in epithelial tissues [3,4,9], HAI-1 plays an important role in matriptase synthesis, intracellular trafficking and zymogen activation [10,11]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
