INDUCES PECAM-1 PHOSPHORYLATION AND TRANSENDOTHELIAL MIGRATION OF MONOCYTES

Polymorphonuclear neutrophils (PMN) adhere to the vascular endothelium under hypoxic conditions, causing microvascular injury. The molecular mechanism of hypoxia-induced adhesion of PMN to and diapedesis throug h the vascular endothelium is poorly understood. We examined the effec ts of hypoxia on the transendothelial migration of monocytes. Exposure of human umbilical vein endothelial cells (HUVEC) cultured in Transwe ll chambers under low oxygen tension (3% O-2 compared with 21% O-2) re sulted in an increased rate of migration of both monocyte-like HL-60 c ells and human peripheral blood monocytes. Migration was inhibited by addition of an antibody to platelet endothelial cell adhesion molecule -1 (PECAM-1), a protein kinase C (PKC) inhibitor, or a platelet-activa ting factor (PAF)-receptor antagonist. In HUVEC, hypoxic conditions (1 , 3, 5, and 14% O-2) increased the phosphorylation of PECAM-1. The ext ent of phosphorylation of PECAM-1 was inversely related to the concent ration of oxygen to which HUVEC were exposed. Hypoxia-induced phosphor ylation of PECAM-1 was inhibited by either a PKC inhibitor or a PAF-re ceptor antagonist, indicating the involvement of hypoxia-induced relea se of PAF in both PKC activation and the concomitant phosphorylation o f PECAM-1. These results were substantiated by the finding that treatm ent of HUVEC with 100 nM PAF under normoxic conditions augmented 11.8- fold the phosphorylation of PECAM-1 and twofold increase in the transe ndothelial migration of monocyte-like HL-60 cells. We conclude that PA F, produced by cultured endothelial cells in response to hypoxia, acts in an autocrine fashion to activate PKC, causing PECAM-1 phosphorylat ion and thus the transendothelial migration of monocytes.