AIMS: Disturbed blood flow at arterial branches and curvatures modulates endothelial function and predisposes the region to endothelial inflammation and subsequent development of atherosclerotic lesions. Activation of the endothelial Toll-like receptors (TLRs), in particular TLR4, contributes to vascular inflammation. Therefore, we investigate whether TLR4 can sense disturbed flow to mediate the subsequent endothelial inflammation. METHODS AND RESULTS: En face staining of endothelium revealed that TLR4 expression, activation, and its downstream inflammatory markers were elevated in mouse aortic arch compared to thoracic aorta, which were absent in Tlr4mut mice. Similar results were observed in the partial carotid ligation model where TLR4 signaling was activated in response to ligation-induced flow disturbance in mouse carotid arteries and such effect was attenuated in Tlr4mut mice. Disturbed flow in vitro increased TLR4 expression and activation in human endothelial cells and promoted monocyte-endothelial cell adhesion, which were inhibited in TLR4-knockdown endothelial cells. Among endogenous TLR4 ligands examined as candidate mediators of disturbed flow-induced TLR4 activation, fibronectin containing the extra domain A (FN-EDA) expressed by endothelial cells was increased by disturbed flow and revealed to directly interact and activate TLR4. CONCLUSIONS: Our findings demonstrate the indispensable role of TLR4 in disturbed flow-induced endothelial inflammation and pinpoint FN-EDA as the endogenous TLR4 activator in this scenario. This novel mechanism of vascular inflammation under disturbed flow condition may serve as a critical initiating step in atherogenesis.
