Bilirubin Inhibits the Transmigration of Monocytes Across Endothelial Cell Monolayers: Implications for Atherosclerosis Prevention

Abstract

Monocyte infiltration into the vascular endothelium, a critical early step in the formation of atherosclerotic plaques, is mediated primarily by the binding of α and β integrins to endothelial cell adhesion molecules Vascular Cell Adhesion Molecule-1 (VCAM-1) and Intercellular Adhesion Molecule-1 (ICAM-1). Our lab previously has shown that bilirubin, a potent, chain-breaking antioxidant, disrupts VCAM-1-dependent processes by scavenging reactive oxygen species (ROS) signaling intermediates. Therefore, we examined the effect of bilirubin on the transmigration of THP-1 human monocytic cells across TNF-α-activated human umbilical vein endothelial cell (HUVEC) monolayers. Bilirubin, at physiological concentrations (0 – 20 µM) was found to inhibit the movement of THP-1 cells across HUVEC monolayers in a dose-dependent manner. Bilirubin did not alter endothelial cell expression of VCAM-1 or ICAM-1 in response to TNF-α, nor did it inhibit the binding of THP-1 cells to HUVEC monolayers, supporting that bilirubin exerts its effect at the level of cellular transmigration. Studies employing blocking antibodies support that VCAM-1 is the principal adhesion molecule mediating monocyte transmigration. In light of epidemiological studies demonstrating an inverse association between serum bilirubin levels and the risk of cardiovascular disease, we speculate that bilirubin prevents plaque formation by inhibiting monocyte migration across the vascular endothelium through disruption of VCAM-1 signaling.