Pulsatile versus oscillatory shear stress regulates NADPH oxidase subunit expression -: Implication for native LDL oxidation

Shear stress regulates endothelial nitric oxide and superoxide (O-2(-.)) 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 mug/mL of native LDL were exposed to ( 1) pulsatile flow with a mean shear stress (tau(ave)) of 25 dyne/cm(2) and (2) oscillating flow at tau(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- and LDL2-. In response to oscillatory shear stress, gp91(phox) mRNA expression was upregulated by 2.9 +/- 0.3-fold, and its homologue, Nox4, by 3.9 +/- 0.9-fold ( P < 0.05, n = 4), with a corresponding increase in O-2(-.) production rate. The proportion of LDL- and LDL2- relative to static conditions increased by 67 +/- 17% and 30 +/- 7%, respectively, with the concomitant upregulation of monocyte chemoattractant protein-1 expression and increase in monocyte/BAEC binding ( P < 0.05, n = 5). In contrast, pulsatile flow downregulated both gp91phox and Nox4 mRNA expression ( by 1.8 +/- 0.2-fold and 3.0 +/- 0.12-fold, respectively), with an accompanying reduction in O-2(-.) production, reduction in the extent of LDL modification ( 51 +/- 12% for LDL- and 30 +/- 7% for LDL2-), 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(-.) production may also affect the regulation of monocyte chemoattractant protein-1 expression and monocyte/BAEC binding.