OSCILLATORY AND STEADY LAMINAR SHEAR-STRESS DIFFERENTIALLY AFFECT HUMAN ENDOTHELIAL REDOX STATE - ROLE OF A SUPEROXIDE-PRODUCING NADH OXIDASE

Atherosclerotic lesions are found opposite vascular flow dividers at s ites of low shear stress and oscillatory flow. Since endothelial proin flammatory genes prominent in lesions are regulated by oxidation-sensi tive transcriptional control mechanisms, we examined the redox state o f cultured human umbilical vein endothelial cells after either oscilla tory or steady laminar fluid shear stress. Endothelial oxidative stres s was assessed by measuring activity of the superoxide (O-2(.-))-produ cing NADH oxidase (a major source of reactive oxygen species in vascul ar cells), intracellular O-2(.-) levels, induction of the redox-sensit ive gene heme oxygenase-1 (HO-1), and abundance of Cu/Zn superoxide di smutase (Cu/Zn SOD), an antioxidant defense enzyme whose level of expr ession adapts to changes in oxidative stress. When cells were exposed to oscillatory shear (+/-5 dyne/cm(2), 1 Hz) for 1, 5, and 24 hours, N ADH oxidase activity and the amount of HO-1 progressively increased up to 174+/-16% (P<0.05) and 505+/-111% (P<0.05) versus static condition s, respectively, whereas levels of Cu/Zn SOD remained unchanged. This upregulation of HO-1 was completely blocked by the antioxidant N-acety lcysteine (NAC, 20 mmol/L). In contrast, steady laminar shear (5 dyne/ cm(2)) induced NADH oxidase activity and NAG-sensitive HO-1 mRNA expre ssion only at 1 and 5 hours, a transient response that returned toward baseline at 24 hours. Levels of Cu/Zn SOD mRNA and protein were incre ased after 24 hours of steady laminar shear. Furthermore, intracellula r O-2(.-), as measured by dihydroethidium fluorescence, was higher in cells exposed to oscillatory than to laminar shear. These data are con sistent with the hypothesis that continuous oscillatory shear causes a sustained activation of pro-oxidant processes resulting in redox-sens itive gene expression in human endothelial cells. Steady laminar shear stress initially activates these processes but appears to induce comp ensatory antioxidant defenses. We speculate that differences in endoth elial redox state, orchestrated by different regimens of shear stress, may contribute to the focal nature of atherosclerosis.