Expression of serine protease inhibitors in epidermal keratinocytes is increased by calcium but not 1,25-dihydroxyvitamin D₃ or retinoic acid

Background: In human skin, the serine proteases kallikrein-related peptidase (KLK)5 and KLK7 degrade corneodesmosome proteins, leading to desquamation. Serine protease activity of the skin is tightly regulated by the interplay between such proteases and serine protease inhibitors, including lymphoepithelial Kazal-type related inhibitor (LEKTI), encoded by SPINK5; secretory leucocyte peptidase inhibitor (SLPI); and elafin. Expression of KLK5 and KLK7 is controlled and upregulated by stimulants such as calcium, 1,25-dihydroxyvitamin D₃ [1,25(OH)₂VD₃] and retinoic acid (RA). Objectives: To understand the effect of calcium, 1,25(OH)₂VD₃ and RA on the expression of serine protease inhibitors in epidermal keratinocytes. Methods: We stimulated normal human epidermal keratinocytes (NHEKs) with high calcium, 1,25(OH)₂VD₃ or RA, and then analysed the expression of serine protease inhibitors using quantitative real-time polymerase chain reaction, enzyme-linked immunosorbent assay and immunocytofluorescence. We also analysed trypsin- and chymotrypsin-like serine protease activities in stimulated NHEKs. Results: High calcium, but not 1,25(OH)₂VD₃ or RA, significantly induced the expression of LEKTI, SLPI and elafin at both transcript and protein levels in NHEKs. These inductions were time- and dose-dependent. The activities of trypsin- and chymotrypsin-like serine proteases were significantly up- and downregulated by high calcium, respectively, in NHEKs. Conclusions: High calcium, but not 1,25(OH)₂VD₃ or RA, increases the expression of serine protease inhibitors in epidermal keratinocytes. Our findings contribute to the understanding of the mechanisms by which serine protease activities are regulated by serine proteases and related inhibitors in epidermal keratinocytes.