Abstract-Shear stress regulates endothelial nitric oxide and superoxide (O 2 Ϫ· ) production, implicating the role of NADPH oxidase activity. It is unknown whether shear stress regulates the sources of reactive species production, consequent low-density lipoprotein (LDL) modification, and initiation of inflammatory events. Bovine aortic endothelial cells (BAECs) in the presence of 50 g/mL of native LDL were exposed to (1) pulsatile flow with a mean shear stress ( ave ) of 25 dyne/cm 2 and (2) oscillating flow at ave of 0. After 4 hours, aliquots of culture medium were collected for high-performance liquid chromatography analyses of electronegative LDL species, described as LDL Ϫ and LDL 2Ϫ . In response to oscillatory shear stress, gp91 phox mRNA expression was upregulated by 2.9Ϯ0.3-fold, and its homologue, Nox4, by 3.9Ϯ0.9-fold (PϽ0.05, nϭ4), with a corresponding increase in O 2 Ϫ· production rate. The proportion of LDL Ϫ and LDL 2Ϫ relative to static conditions increased by 67Ϯ17% and 30Ϯ7%, respectively, with the concomitant upregulation of monocyte chemoattractant protein-1 expression and increase in monocyte/BAEC binding (PϽ0.05, nϭ5). In contrast, pulsatile flow downregulated both gp91 phox and Nox4 mRNA expression (by 1.8Ϯ0.2-fold and 3.0Ϯ0.12-fold, respectively), with an accompanying reduction in O 2 Ϫ· production, reduction in the extent of LDL modification (51Ϯ12% for LDL Ϫ and 30Ϯ7% for LDL 2Ϫ ), and monocyte/BAEC binding. The flow-dependent LDL oxidation is determined in part by the NADPH oxidase activity. The formation of modified LDL via O 2 Ϫ· production may also affect the regulation of monocyte chemoattractant protein-1 expression and monocyte/BAEC binding.

Leukocyte recruitment to endothelial cells is a critical event in inflammatory responses. The spatial, temporal gradients of shear stress, topology, and outcome of cellular interactions that underlie these responses have so far been inferred from static imaging of tissue sections or studies of statically cultured cells. In this report, we developed micro-electromechanical systems (MEMS) sensors, comparable to a single endothelial cell (EC) in size, to link real-time shear stress with monocyte/EC binding kinetics in a complex flow environment, simulating the moving and unsteady separation point at the arterial bifurcation with high spatial and temporal resolution. In response to oscillatory shear stress (τ) at ± 2.6 dyn/cm 2 at a time-averaged shear stress (τ ave ) = 0 and 0.5 Hz, individual monocytes displayed unique to-and-fro trajectories undergoing rolling, binding, and dissociation with other monocyte, followed by solid adhesion on EC. Our study quantified individual monocyte/EC binding kinetics in terms of displacement and velocity profiles. Oscillatory flow induces up-regulation of adhesion molecules and cytokines to mediate monocyte/EC interactions over a dynamic range of shear stress ± 2.6 dyn/cm 2 (P= 0.50, n= 10).-Hsiai, T. K., Cho, S. K., Wong, P. K., Ing, M., Salazar, A., Sevanian, A., Navab, M., Demer, L. L., Ho, C.-M. Monocyte recruitment to endothelial cells in response to oscillatory shear stress. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptThe process of leukocyte adhesion to endothelial cells (EC) involves a complex balance of forces arising from hydrodynamic shear effects and the dynamics of leukocyte/ endothelial cell (EC) binding. The endothelium, which lines the inner lumen of blood vessel walls, is intimately involved in the recruitment of leukocytes fundamental to the initiation of immune responses (1, 2). Shear stress, the tangential drag force of blood passing along the surface of the endothelium (3), imparts profound effects on endothelial cell (EC) function (4-6).Around arterial bends and branches in which the inflammatory responses prevail, the fluid mechanical environment is distinct from the laminar pulsatile environment present in the long, straight sections of the vessel wall. Oscillatory flow with a time-averaged shear stress of 0, characteristic of reattachment points in the arterial branches, modulates the biological activities of EC (7-9). Leukocyte rolling, adhesion, and transmigration have been observed in response to inflammatory stimuli at the microvascular levels (10). What is warranted is a dynamic, high spatial and temporal resolution view of these events in a more complex physiologic flow environment. Therefore, we have developed specialized microelectromechanical systems (MEMS) sensors and cell tracking velocimetry to link spatial and temporal resolution of real-time shear stress with the monocyte/EC binding kinetics, simulating reattachment points at arterial bifurcations (11).No direct measurement has been achieved to resolve t...

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