Abstract

Atherosclerosis is a leading cause of medical complications and death in the developed world. While its progression is influenced by many systemic risk factors, atherosclerosis initiates at vessel branch points and curvatures, where shear stress is low and exhibits complex flow patterns. Endothelial cells in these areas show increased inflammatory gene expression, cytoskeletal rearrangement and apoptosis. In vivo, the extracellular matrix in areas of disturbed flow changes from a basement membrane of collagen and laminin, to one with increased fibronectin deposition. These matrix changes correlate with inflammatory gene expression, suggesting that certain shear stress responses are matrix-specific. The c-Jun NH₂-terminal kinase (JNK) is activated in response to shear stress. It is implicated in inflammation, cell migration and apoptosis, and feeding atheroprone mice the JNK inhibitor SP600125 inhibits atherosclerotic plaque development, making JNK an interesting candidate for mediating disease progression. However, both the mechanisms and role of shear-induced JNK activation, aside from its effects on inflammatory gene expression, remain elusive.

This first part of this study focuses on the pathway of JNK activation by shear stress. In arteries from wild-type and atheroprone ApoE^-/- mice, JNK activation localized to endothelial cells in areas of early atherogenesis that showed increased deposition of fibronectin. JNK activation by shear stress was matrix-specific, with enhanced activation on fibronectin compared to collagen I or matrigel, and involved new integrin binding to matrix. MKK4 and PAK were key upstream elements in the pathway. The second part of this study identifies a novel role for JNK in endothelial cell alignment in response to shear stress. Phosphorylated JNK localized to both the nucleus and focal adhesions in response to shear stress, and knockdown of JNK2 inhibited actin stress fiber alignment with shear. Cells on collagen I showed a late activation of JNK compared to cells on fibronectin, and in accordance with this observation, alignment was less efficient than in cells on fibronectin. Thus, JNK promotes alignment in response to high laminar shear, but under disturbed shear, where matrix remodeling occurs, its sustained activation contributes to the inflammatory processes associated with atherosclerotic development and progression.