Thrombomodulin, a functional surface protein on human keratinocytes, is regulated by retinoic acid

Thrombomodulin, a major anticoagulant proteoglycan of the endothelial cell membrane, is a thrombin receptor that acts as a cofactor for protein C activation. It has previously been shown that thrombomodulin, present in human epidermis and in lysates of cultured keratinocytes, is implicated in cellular differentiation during mouse fetal development. The role of retinoic acid in keratinocyte differentiation prompted us to study retinoic acid regulation of thrombomodulin expression in primary cultures of keratinocytes isolated from adult human skin, grown at low (undifferentiated keratinocytes) and normal calcium levels (differentiated keratinocytes). Thrombomodulin antigen levels and total and surface activities were measured in cultures without and with retinoic acid. Thrombomodulin mRNA visualized by in situ hybridization was quantified by computer-based image analysis. Functional thrombomodulin was expressed on the surface and in the cytoplasm of cultured human keratinocytes regardless of the calcium concentration. In contrast, retinoic acid induced significant increases in the total antigen level and in surface and intracellular thrombomodulin activities only in keratinocytes grown in a low-calcium medium. In these undifferentiated keratinocytes, quantification of mRNA transcripts showed a threefold increase after retinoic acid stimulation. Thus, functional thrombomodulin is a human keratinocyte surface protein whose expression is controlled through the keratinocyte differentiation program and is modulated in vitro by retinoic acid.