α5β1 integrin expression and luminal edge fibronectin matrix assembly by smooth muscle cells after arterial injury

Fibronectin is secreted from the cell as a soluble protein that must then polymerize to regulate cell function. To elucidate the process of fibronectin matrix assembly in vascular disease, we immunostained sections of balloon-injured rat carotid artery for the fibronectin-binding alpha 5 beta 1 integrin. Whereas alpha 5 beta 1 integrin was not evident in the normal carotid artery, its expression was induced after a vascular injury. By 14 days, the alpha 5 beta 1 integrin was localized exclusively to the less differentiated smooth muscle cells (SMCs) at the luminal surface of the neointima. Platelet-derived growth factor-BB, dominant in neointimal formation, selectively increased the expression of the alpha 5 beta 1 integrin by human SMCs in culture. To track the assembly of fibronectin fibers, fluorescence-labeled soluble fibronectin protomers were added to cultured SMCs and to fresh segments of normal and balloon-injured rat carotid arteries. Fibronectin fiber formation in cultured SMCs could be detected within 10 minutes, and was blocked by an RGD peptide, an anti-beta 1 integrin antibody, and an anti-alpha 5 beta 1 integrin antibody, but not by an anti-beta 3 integrin antibody. En face confocal microscopy of arterial segments revealed that soluble fibronectin had polymerized on the alpha 5 beta 1 integrin-expressing SMCs of the luminal surface of the injured arterial neointima, but not on the alpha 5 beta 1 integrin-negative neointimal SMCs below this or on the endothelial cells of uninjured arteries. Furthermore, in situ fibronectin assembly by the neointimal SMCs was inhibited by an RGD peptide and by an anti-beta 1 integrin antibody. These studies indicate that a subpopulation of SMCs in the repairing artery wall orchestrates integrin-mediated fibronectin assembly.