flow-Mediated Atheroprotection

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Inflammation contributes at each stage in the development of clinically significant atherosclerosis. The initiation and progression of atherosclerosis is decreased in regions of steady flow associated with high laminar shear stress, compared to regions of turbulent and low flow. This finding has yielded the concept that "normal" flow is atheroprotective. Tissue culture and in vivo studies have shown that normal flow decreases oxidative stress by activating antioxidant mechanisms. The major hypothesis of this proposal is that normal flow promotes a reducing environment in endothelial cells that decreases inflammation and limits atherosclerosis. Our laboratory has focused on regulation of the mitogen activated protein kinases (MAPKs) by flow. MAPKs phosphorylate and activate transcription factors that induce expression of both pro- and anti-inflammatory molecules. In particular, apoptosis signal kinase-1 (ASK1) and its downstream effectors, c-Jun N-terminal kinase (JNK) and p38, are activated by almost all inflammatory cytokines. We believe that understanding the mechanisms by which flow regulates activation of ASKl-JNK-p38 will provide insight into the atheroprotective mechanisms induced by flow. We propose a model based on preliminary data that flow stimulates glucose 6-phosphate dehydrogenase (G6PD) which increases NADPH formation. NADPH increases the level of reduced glutathione (GSH) which maintains the key antioxidant molecules glutaredoxin (Grx) and thioredoxin (Trx) in reduced forms. Grx and Trx bind to ASK1 and keep ASK1 inactive. Flow also decreases expression of vitamin D3 upregulated protein (VDUP1) which is an endogenous inhibitor of Trx. To characterize the mechanisms by which flow inhibits ASK1 we will compare the effects of normal flow (shear stress = 12 dyn/cm2) and disturbed flow (low shear stress = 0.4 dyn/cm2 or oscillatory flow) on ASK1 activity basally and in response to tumor necrosis factor-alpha. Four aims are proposed. 1) To characterize the mechanisms by which flow activates G6PD based on mechanosensitive signaling pathways. 2) To determine how Trx inhibits ASK1 function based on the concept that flow maintains Trx in an active state. 3) To show that flow inhibits VDUP1 expression thereby increasing Trx activity and Trx binding to ASK 1. 4) To characterize the role of VDUP1 in atherosclerosis by evaluating the effect of VDUP 1 deficiency on the pathology of the LDL receptor deficient mouse. These studies should provide insight into mechanisms by which flow inhibits arterial inflammation and facilitate development of new therapeutic approaches to limit atherosclerosis.