Protein C inhibitor is expressed in keratinocytes of human skin

Protein C inhibitor is a member of the serpin family that inhibits a variet y of serine proteases. Protein C inhibitor is present in numerous body flui ds and is produced in the liver and by various epithelial cells. To determi ne if this epithelial serpin is present in skin, immunohistochemical studie s were performed that showed strong staining for protein C inhibitor antige n in the epidermis, Protein C inhibitor mRNA was detected in the keratinocy te cell line HaCaT and the epidermoid carcinoma cell line A431 using revers e transcription-polymerase chain reaction suggesting that also in normal sk in protein C inhibitor is derived from keratinocytes, Conditioned media fro m these cell lines were analyzed on immunoblots, which revealed a protein C inhibitor-antigen band that comigrated with protein C inhibitor derived fr om the hepatoma cell line HepG2. Using an enzyme-linked immunosorbent assay specific for total protein C inhibitor antigen the accumulation of protein C inhibitor in the cell culture supernatants of HaCaT keratinocytes was fo und to be 0.3 ng per h per 1 million cells. This is similar to the amount o f plasminogen activator inhibitor-1 produced by these cells, which also pro duce tissue plasminogen activator and urokinase. Fluorescence-activated cel l sorter analysis revealed similar expression of intracellular protein C in hibitor antigen in proliferating and confluent HaCaT cells, These findings demonstrate that protein C inhibitor antigen is present in the normal epide rmis and that protein C inhibitor is constitutively expressed by keratinocy tes in culture. Therefore, protein C inhibitor may provide protease inhibit ory activity not only to internal, but also to the external surface of the body. Additionally, protein C inhibitor could contribute to the regulation of retinoid supply in the epidermis, as we have shown recently that retinoi c acid binds specifically to protein C inhibitor.