Shear stress, generated by flowing blood exerting friction on the vessel wall, represents a major contributor to vascular endothelial cell phenotype. The development of atherosclerosis tends to occur in vascular beds associated with low and reversing shear stress (atheroprone). In contrast, vascular beds exposed to relatively high and unidirectional shear stress tend to be resistant to atherosclerosis (atheroprotective). The identification of signaling pathways associated with atherogenic phenotypes presents an opportunity for the therapeutic intervention of a global health problem.

To study the effect of regional hemodynamics on endothelial cell biology, a quantitative description of the forces present in the human body is required. Phasecontrast magnetic resonance imaging was used in order to measure the blood velocity distribution of carotid bifurcations of healthy human subjects. Shear stress was then calculated throughout the cardiac cycle in regions predisposed and resistant to atherosclerosis development. Hemodynamic characteristics including time-average, minimum and maximum shear stresses were greater in atherosclerosis-resistant regions. Oscillatory shear index, a measure of direction changes of shear stress was greater in atheroprone vascular regions. In order to assess the role of individual frequency components to the shear stress signal, Fourier transform analysis was performed. A new parameter (the Harmonic Index) describing the relative contributions of time-varying and time-independent components of a shear stress signal was developed and found to be substantially higher in atheroprone regions compared to atheroprotected.

These quantitative shear stress profiles derived from the human vasculature were then employed to study their effects on endothelial cell biology and underlying mechanisms of atherosclerosis development. Using a cell culture system capable of accurately reproducing shear stress profiles, human endothelial cells were assessed for activation of the β-catenin/TCF signaling pathway. β-catenin localizes to the adherens junction of quiescent endothelial monolayers. In the presence of various stimuli including Wnt and growth factors, β-catenin accumulates in the nucleus and co-activates the TCF family of transcription factors. Application of atheroprone hemodynamic shear stress resulted in increased nuclear β-catenin and TCF transcriptional activation compared to atheroprotective shear stress. Further, vascular endothelium in atherosclerosis susceptible regions of the mouse aorta exhibits increased nuclear β-catenin prior to and during lesion development. Additionally, TCF-reporter mice exhibited higher TCF transcriptional activation in atheroprone regions compared to atheroprotected regions both prior to and during atherosclerosis development.

Several known regulators of β-catenin/TCF signaling were studied in the context of atherosclerosis. GSK-3P, a critical upstream inhibitor of TCF transcription activity was preferentially inactivated in endothelial cells within atherosclerotic lesions. PECAM-1 is a known mediator of many endothelial responses to shear stress. PECAM-1 was found to be required for β-catenin nuclear localization as well as GSK-3P inactivation both within atherosclerotic lesions and in cells exposed to atheroprone shear stress.

In order to assess the implications of TCF transcriptional activation in atherosclerosis development, a set of TCF-responsive genes was assessed for their regulation by atheroprone shear stress. Application of atheroprone shear stress induced the upregulation of several genes associated with atherosclerosis (IL-8, Fibronectin, CyclinD1). Shear stress-induced activation of these genes was abolished by overexpression of dominant-negative TCF (a construct that strongly inhibits activation of TCF-dependent transcription) and a β-catenin transcriptional repression fusion protein. Treatment with dominant-negative TCF also strongly inhibited NF-KB transcriptional activity in cells exposed to atheroprone shear stress. Finally, the role of histone acetyl transferases in hemodyanmically induced β-catenin/TCF gene transcription was assessed. Together, the data indicate that activation of β-catenin nuclear signaling is preferentially activated in the atheroprone vasculature by regional hemodynamics and contributes to atherogenic gene expression. This pathway may provide a novel therapeutic target for the treatment or prevention of atherosclerosis.