Abstract 1654: Shear Stress Regulates Nuclear Receptor LXR in Endothelial Cells
A key feature of atherosclerosis is its focal distribution at bifurcations and curved regions, which is due largely, to the disturbed flow patterns in these areas. Nuclear receptor LXRs regulate a set of target genes that participate in lipid metabolism and reverse cholesterol transport. However, the correlation between shear stress and LXR-mediated cholesterol metabolism in artery wall is still unknown. We report here the regulation of LXR by fluid shear stress in cultured vascular endothelial cells (ECs) and in mouse model and the involved mechanism. Bovine aorta ECs were subjected to a steady laminar flow at 12 dyne/cm2 for 8 hr. The expressions of LXR and its target genes, including ABCA1, ABCG1, ApoE, and lipoprotein lipase (LPL), at both mRNA and protein levels, were increased by laminar flow compared with those under static conditions or low shear stress (3 dyne/cm2). ECs were transiently transfected with LXRE-luc and human ABCA1 promoter reporter construct and then subjected to the laminar flow for 8 hr. Luciferase activity assays revealed that laminar flow increased the activities of both LXRE and the ABCA1 promoter. Isolated mouse aorta and human umbilical artery were labeled with 3H-cholesterol to examine the ApoA-I-mediated cholesterol efflux and cholesterol retention in these arteries ex vivo. Treatment with LXR agonist caused an increase in ApoA-I-mediated 3H-cholesterol efflux into the media and a complementary decrease in the 3H-cholesterol retention in the arteries. Furthermore, C57/BL6 mice were treated with LXR ligand TO901317 for 3 days and the intima of thoracic aorta and aortic arch were then isolated. Western blotting showed that levels of LXRα and ABCA1 proteins in the thoracic aorta (under steady laminar flow) were higher than that in the aortic arch (under disturbed flow patterns). Although TO901317 increased the levels of LXR and ABCA1, their levels were still higher in the thoracic aorta, compared with those in aortic arch. Real-time PCR experiments showed that mRNA levels of LXR and LXR target genes in the thoracic aorta were also higher. Take together, our result demonstrate that laminar flow can enhance LXR in vitro and in vivo. This is a newly defined athero-protective effect of laminar flow exerting on the endothelium of the artery wall.