Expression of matrix metalloproteinase-9 in endothelial cells is differentially regulated by shear stress. Role of c-Myc.

Atherosclerotic plaques preferentially localize to areas of the vasculature with complex laminar or oscillatory blood flow. Prior data implicate matrix metalloproteinases (MMPs) in the initiation and progression of atherosclerotic lesions. In cultured endothelial cells, oscillatory but not unidirectional shear significantly increases MMP-9 mRNA as well as secretion of the MMP-9 protein (p < 0.05). In contrast, cell-associated protein levels of Tissue Inhibitor of MMP 1 (TIMP-1), an inhibitor of MMP-9, are insensitive to the shear regimen. To investigate transcriptional regulation of MMP-9 gene expression, we utilized retroviral-based reporter constructs containing different lengths of the human MMP-9 promoter. The activity of the full MMP-9 promoter is 3-fold higher (p < 0.05) in unidirectional shear compared with static conditions, and the activity is further increased approximately 10-fold by oscillatory shear (p < 0.01) over unidirectional flow. Our data identify a shear-sensitive binding site at -152 in the MMP-9 promoter. We show that the c-Myc transcription factor binds specifically to this site and that reporter constructs in which the c-Myc binding site was abolished lacked the shear responsiveness of native MMP-9 reporter constructs. Our results suggest that endothelial MMP-9 expression is flow-sensitive and is up-regulated by oscillatory flow via activation of c-Myc. This effect may contribute to the development and progression of atherosclerotic lesions in areas of vasculature that are subject to disturbed flow.