Shear stress mediated alterations in the expression of leukocyte adhesion receptors on human endothelial cells
Doctor of Philosophy
Endothelial cells line the inner walls of blood vessels and are constantly subjected to hemodynamic shear forces due to flowing blood. Both in vivo and in vitro studies have shown that local fluid shear stress may play an important role both in normal inflammatory responses as well as in the initial development and progression of atherosclerosis. This study describes their contribution in regulating the expression of adhesion molecules ICAM-1, VCAM-1 and E-selectin on endothelial cells, with the goal of understanding the link between local fluid mechanics, inflammatory response and atherogenesis. HUVEC monolayers exposed to different levels of controlled shear stress conditions were analyzed for the kinetics of expression of these adhesion molecules both in terms of gene transcription and surface protein expression. Results from the fluorescence analysis showed a transient increase in the surface expression of ICAM-1 upon exposure to shear stress (25 dynes/cm$\sp2$) for 12 hours that returned to basal levels within 24 hours. The messenger RNA (mRNA) levels for ICAM-1 also showed a transient increase after one to three hours of exposure to shear stress compared to matched control values. Subsequently, however, the mRNA levels decreased and within 6 hours dropped significantly below control values. VCAM-1 mRNA levels, however, decreased monotonically upon onset of flow at all values of shear stresses, and dropped well below basal levels within 6 hours. Neither molecule mRNA was present in detectable levels after 24 hours of flow. E-selectin mRNA levels appeared to be constant and comparable to control values for up to 6 hours after exposure to shear stress. These studies also demonstrate altered response of shear preconditioned endothelial cells to subsequent activation with inflammatory mediators such as IL-1$\beta$. Both ICAM-1 and VCAM-1 mRNA levels were comparable in the 4 and 6 hour IL-1$\beta$ treated cells under both flow and static conditions. However, ICAM-1 levels were higher after 24 hours of IL-1$\beta$ under flow compared to similarly activated static cells whereas VCAM-1 levels were lower. ICAM-1 surface expression was also significantly (3- to 4-fold, p $<$ 0.02) higher under these conditions, while VCAM-1 did not change as a result of flow preconditioning. The observed differences between static and pre-sheared cells discussed here suggests that in vitro studies involving vascular cells should mimic as closely as possible the representative fluid mechanical environment of the region they are modeling.