Abstract 1918: A Fat1 Cadherin-Myocardin Regulatory Pathway Controls Smooth Muscle Cell Differentiation and Neointimal Formation in Response to Vascular Injury
Vascular smooth muscle cells (SMCs) play a central role in vascular remodeling in diseases such as atherosclerosis and restenosis. Vascular injury and mitogens induce robust expression of the atypical cadherin Fat1 in SMCs; paradoxically, Fat1 knockdown by siRNA enhances SMC proliferation in vitro, suggesting that Fat1 acts as a physiologic brake on SMC growth after vascular injury. In the present study, we assessed the effect of Fat1 on SMC differentiation and vascular remodeling in vivo. Knockdown of Fat1 reduced expression of SMC marker genes, including SM α-actin, SM22α, and SM myosin heavy chain, in both primary aortic SMCs and in 10T1/2 cells differentiated by overexpression of myocardin, a major regulator of SMC differentiation. Accordingly, we hypothesized that the Fat1 intracellular domain (Fat1ICD), which localizes to the cell nucleus after Fat1 cleavage, might affect differentiation directly. Indeed, Fat1ICD overexpression increased marker gene expression, and the Fat1ICD synergized with myocardin in a CArG- and E-box-dependent fashion to activate the SM α-actin promoter. Moreover, the Fat1ICD associated with myocardin, and was recruited to CArG- and E box-bearing sequences in native SMC marker gene promoters. To examine the role of Fat1 in vascular remodeling in vivo, we generated conditional (floxed) Fat1 mice, bred them with SM22α-Cre-recombinase transgenic mice, and analyzed the resultant SMC-targeted Fat1 knockout (Fat1smKO) mice in the carotid artery ligation injury model. Fat1 expression was strongly induced in the neointimal and medial layers of injured carotid arteries in wt mice, but not in those of Fat1smKO mice. Fat1smKO mice showed significantly larger neointimas at 14, 21 and 28 days after injury (area 2.97 ± 0.25-fold larger, P< .05 vs wt at 28 days), and markedly increased SMC proliferation in both neointimal and medial layers. Moreover, reinduction of SM α-actin and SM22α, after injury was delayed in Fat1smKO mice. Together, these studies show that Fat1 inhibits proliferation of SMCs, suppresses neointimal formation and promotes SMC re-differentiation during vascular remodeling in vivo, and frame a novel Fat1-myocardin signaling pathway that spans from the cell surface to the nucleus to govern SMC activity after injury.
This research has received full or partial funding support from the American Heart Association, Founders Affiliate (Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, Vermont).