Objective-The cysteine proteases, cathepsins, have been implicated in vascular remodeling and atherosclerosis, processes known to be regulated by shear stress. It is not known, however, whether shear regulates cathepsins. We examined the hypothesis that shear stress regulates cathepsin activity in endothelial cells. Methods and Results-Mouse aortic endothelial cells (MAECs) exposed to atheroprotective, unidirectional laminar shear (LS) degraded significantly less BODIPY-labeled elastin and gelatin in comparison to static and proatherogenic oscillatory shear (OS). The cathepsin inhibitor E64 also reduced this activity. Gelatin zymography showed that cathepsin activity of MAECs was blunted by LS exposure and by a cathepsin L inhibitor but not by cathepsin B and S inhibitors, whereas a cathepsin K inhibitor had a minor effect. Cathepsin L siRNA knocked down cathepsin L expression, gelatinase, and elastase activity in OS and static MAECs. A partial reduction of cathepsin B protein raised the possibility that the siRNA effect on the matrix protease activity could have been attributable to cathepsin L or B. Cathepsin B activity study using the synthetic peptide showed it was not regulated by shear. Key Words: shear stress Ⅲ cathepsin Ⅲ elastase Ⅲ gelatinase Ⅲ atherosclerosis V ascular endothelial cells are constantly exposed to fluid shear stress, the frictional force generated by blood flow over the vascular endothelium. The importance of shear stress in vascular biology and pathophysiology has been highlighted by the focal development patterns of atherosclerosis in hemodynamically defined regions. For example, the regions of branched and curved arteries exposed to disturbed flow conditions including oscillatory and low mean shear stresses (OS) correspond to atheroprone areas. In contrast, straight arteries exposed to pulsatile high levels of laminar shear stress (LS) are relatively well protected from atherosclerotic plaque development. 1 Changes in blood flow have been shown to be a critical factor inducing arterial remodeling. 1-7 Increases in arterial wall shear stress prevent vascular remodeling leading to thickening of the vascular wall and inflammation, 2 whereas decreases in arterial wall shear stress promote arterial remodeling and inflammation. 2,3 Additionally, low wall shear stress leads to degradation of the internal elastic lamina (IEL). 5 Despite these findings, the underlying mechanisms by which shear regulates proteases degrading vessel wall matrix and IEL are not well described. Although there is a report demonstrating that shear regulates matrix metalloproteases (MMPs) in endothelial cells, 8 it is not clear whether other proteases are also regulated by shear. Conclusions-TheseCathepsins are the papain family of cysteine proteases which degrade elastin in addition to collagen. 9 Unlike MMPs, the role for cathepsins in blood vessel remodeling and cardiovascular disease has been understudied until recently. Cathepsins K, L, and S, potent elastinolytic proteases, have been identified in atheroscleroti...

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