Human airway trypsin-like protease (HAT) is released into saliva

We first discovered human airway trypsin-like protease (HAT) in human mucoid sputum. Precursor HAT (47 kDa), a cell surface type II transmembrane serine protease, is proteolyzed to mature HAT (27 kDa). Hitherto, HAT has not been detected in other biological fluids except for human sputum. We aimed to clarify whether human saliva contains mature HAT. Trypsin-like protease was isolated from saliva of healthy volunteers by a method adopted for isolation of HAT from sputum using Boc-Phe-Ser-Arg-MCA as the substrate. Biochemical properties of purified protease were similar to those of recombinant HAT (rHAT). HAT concentration in saliva was measured by ELISA, and immunoreactive HAT : total protein ratio (ng/mg) in saliva samples from healthy subjects was similar to that in mucoid sputum. RT-PCR showed that HAT mRNA was expressed in human gingival epithelial cells but not in gingival fibroblasts. Both indirect immunofluorescence and western blotting using monoclonal antibody for α-smooth muscle actin (α-SMA ; a myofibroblast marker) showed that HAT enhanced α-SMA fiber expression in gingival fibroblasts. These results indicate that both mucoid sputum and saliva from healthy subjects have similar concentrations of mature HAT, and HAT is related to certain physiological functions and pathological states of myofibroblasts in the oral cavity.