The Fifth Region of Epidermal Growth Factor like-Domain of Thrombomodulin Is Essential to Produce Cytoprotection

We previously found that the fourth and fifth regions of epidermal growth factor-like domain of thrombomodulin (TME45) protects vascular endothelial cells from calcineurin inhibitor-induced cell damage in association with activation of extracellular signal-regulated kinase (ERK)/myeloid cell leukemia-1 (Mcl-1) signal pathways via activated protein C (APC) independent manner (Arterioscler Thromb Vasc Biol. 2012;32:2259-70). The present study attempted to identify the minimum region of TME45 that is essential to mediate the cytoprotective function of thrombomodulin. The blocking antibody for TME5 but not for TME4 hampered the ability of TME45 to increase levels of Mcl-1 in human umbilical vein endothelial cells (HUVECs), indicating the important roles of TME5 in cytoprotection. We next divided TME5 into three parts and synthesized oligonucleotides for each region. None of the fragment was as potent as TME5 to induce Mcl-1 in HUVECs, suggesting that TME5 was essential for induction of Mcl-1. Importantly, TME5 was not able to produce APC. Furthermore, TME5 counteracted calcineurin inhibitor-induced vascular permeability and stimulated angiogenesis in vitro as well as in vivo. Taken together, TME5 may be useful to prevent or treat endothelial cell damage associated lethal complications developing after hematopoietic stem cell transplantation such as sinusoidal obstruction syndrome and thrombotic microangiopathy.