Hornerin expressed on endothelial cells via interacting with thrombomodulin modulates vascular inflammation and angiogenesis

Thrombomodulin is predominantly expressed on vascular endothelial cells and modulates endothelial cell functions by interacting with multiple ligands. The specific thrombomodulin receptor or cofactor active on the endothelial cell surface remains elusive. This study aims to identify interacting partners of thrombomodulin on endothelial cells. Here, using a liquid chromatograph-tandem mass spectrometer, hornerin was identified as a candidate protein. We then investigated hornerin protein and mRNA expression in endothelial cells. Hornerin protein was detected in the mouse endothelium of the aorta and lung. Both human- and mouse-cultured endothelial cells expressed hornerin mRNA and protein. Moreover, immunoprecipitation analysis suggested the direct protein interaction between thrombomodulin and hornerin. Lipopolysaccharides administration increased serum hornerin concentrations in mice and reduced hornerin protein levels on the surface of cultured endothelial cells as same as thrombomodulin protein. Thrombomodulin-targeting siRNA decreased not only thrombomodulin protein levels but also hornerin protein levels in cultured endothelial cells. Thrombomodulin- or hornerintargeting siRNA impaired tube formation and leukocyte adhesion to endothelial cells. Our findings reveal that hornerin is located on vascular endothelial cells in the presence of thrombomodulin and suggest that endothelial thrombomodulin and hornerin may interact, which may play an important role in endothelial cell functions such as vascular inflammation and angiogenesis.