Proteinase inhibitor 6 (PI-6) expression in human skin: Induction of PI-6 and a PI-6/proteinase complex during keratinocyte differentiation

Proteinase inhibitor 6 (PI-6) is a 42-kDa intracellular protein present in epithelial cells and endothelial cells. It is capable of inhibiting a numbe r of serine proteinases, including trypsin and chymotrypsin. In this study we examined PI-6 expression in human skin and its primary cell type, the ke ratinocyte. By immunohistochemical analysis, PI-6 staining is absent from t he basal cells, weak in the spinous layer, and strongest in the granulosa l ayer of human epidermis. Immunoblotting of cultured primary keratinocytes r evealed that PI-6 production increases 24-fold on differentiation. Analysis of an immortalized keratinocyte cell line, HaCat, showed a B-fold increase in PI-6 mRNA and a 7-fold increase in PI-6 protein upon differentiation, a nd indirect immunofluorescence revealed that this is due to an increase in the number of differentiated cells expressing high levels of PI-6. Of parti cular interest is the appearance of a preformed complex between PI-6 and an endogenous serine proteinase in differentiating HaCat cells, which was det ected by a monoclonal antibody demonstrated to preferentially recognize PI- 6 in complex with a proteinase. This identification of a PI-6/proteinase co mplex is the first example of a serpin bound to a proteinase in keratinocyt es. We postulate that a physiological role of PI-6 is to regulate a serine proteinase associated with keratinocyte differentiation. (C) 1998 Academic Press.