Cell-derived Apolipoprotein E (ApoE) Particles Inhibit Vascular Cell Adhesion Molecule-1 (VCAM-1) Expression in Human Endothelial Cells

Anita K Stannard,David R Riddell,Sandra M Sacre,Aristides D Tagalakis,Claus Langer,Arnold Von Eckardstein,Paul Cullen,Takis Athanasopoulos,George Dickson,James S Owen

doi:10.1074/jbc.m104812200

Abstract

Sub-endothelial infiltration of monocytes occurs early in atherogenesis and is facilitated by cell adhesion molecules that are up-regulated on activated endothelium. Apolipoprotein E (apoE) helps protect against atherosclerosis, in part, because apoE particles secreted by macrophages have local beneficial effects at lesion sites. Here, we hypothesize that such protection includes anti-inflammatory actions and investigate whether cell-derived apoE can inhibit tumor necrosis factor-alpha-mediated up-regulation of vascular cell adhesion molecule-1 (VCAM-1) in human umbilical vein endothelial cells (HUVECs). Two models were used to mimic endothelial exposure to macrophage-derived apoE. In the first, HUVECs were transiently transfected to secrete apoE; VCAM-1 induction inversely correlated with secretion of apoE into the media (r = -0.76, p < 0.001). In the second, incubation of HUVECs with media from recombinant Chinese hamster ovary (CHO) cells expressing apoE (CHO(apoE)) also reduced VCAM-1 in a dose-dependent manner (r = -0.70, p < 0.001). Characterization of CHO(apoE) cell-derived apoE revealed several similarities to apoE particles secreted by human blood monocyte-derived macrophages. The suppression of endothelial activation by apoE most likely occurs via stimulation of endothelial nitric oxide synthase; apoE increased levels of intracellular nitric oxide and its surrogate marker, cyclic guanosine monophosphate, while the nitric oxide synthase inhibitor, ethyl-isothiourea, blocked its effect. We propose that apoE secreted locally at lesion sites by macrophages may be anti-inflammatory by stimulating endothelium to release NO and suppress VCAM-1 expression.

Highlights

Sub-endothelial infiltration of monocytes occurs early in atherogenesis and is facilitated by cell adhesion molecules that are up-regulated on activated endothelium
We propose that Apolipoprotein E (apoE) secreted locally at lesion sites by macrophages may be anti-inflammatory by stimulating endothelium to release NO and suppress vascular cell adhesion molecule-1 (VCAM-1) expression
This study provides the first evidence that cell-secreted apoE may be anti-inflammatory by suppressing VCAM-1 induction, most probably because endothelial cells are stimulated to release NO

Summary

Early in atherogenesis circulating monocytes are recruited to

§ Supported by British Heart Foundation Ph.D. Studentship (FS/95051). ApoE is abundant in atherosclerotic lesions, secreted by resident cholesterol-loaded macrophages [6]. ApoE-deficient mice have elevated VCAM-1 in aortic lesions [3], which enhances monocyte recruitment and adhesion [12], while apoE expression in the artery wall reduces early foam cell lesion formation [13]. These findings imply that apoE may influence early inflammatory responses by suppressing endothelial activation and CAM expression. We devise two models to mimic endothelial exposure to locally synthesized, cell-derived apoE and show, in both cases, that apoE limits cytokine-mediated VCAM-1 up-regulation in a dose-dependent manner

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION