1 Prostaglandin D 2 strengthens human endothelial 2 barrier by activation of E-type receptor 4 3

Life-threatening inflammatory conditions such as acute respiratory distress syndrome or 12 sepsis often go hand in hand with severe vascular leakage. During inflammation, endothelial cell 13 integrity and intact barrier function are crucial to limit leukocyte and plasma extravasation. 14 Prostaglandin D2 (PGD2) is a potent inflammatory lipid mediator with vasoactive properties. It has 15 been suggested that PGD2 is involved in the regulation of endothelial barrier function; however, it 16 is unclear whether this is also true for primary human pulmonary microvascular endothelial cells. 17 Furthermore, as PGD2 is a highly promiscuous ligand, we set out to determine which receptors are 18 important in human pulmonary endothelial cells. In the current study, we found that PGD2 and the 19 DP1 agonist BW245c potently strengthened pulmonary and dermal microvascular endothelial cell 20 barrier function and protected against thrombin-induced barrier disruption. Yet surprisingly, these 21 effects were mediated only to a negligible extent via DP1 receptor activation. In contrast, we 22 observed that the EP4 receptor was most important and mediated the barrier enhancement by PGD2 23 and BW245c. These data demonstrate a novel mechanism by which PGD2 may modulate 24 inflammation and emphasizes the role of EP4 receptors in human endothelial cell function. 25