Abstract 307: Fat1 Protocadherin Interacts with Atrophin-1 and Regulates Vascular Smooth Muscle Cell Growth and Migration
The significance of cadherin superfamily proteins in regulation of vascular smooth muscle cell (VSMC) activity after vascular injury is not well understood. We previously reported that expression of the protocadherin Fat1 by VSMCs increases robustly after arterial injury, and that Fat1 inhibits growth and facilitates VSMC migration, pointing to an important integrative role for Fat1 in active vascular remodeling. Recent studies in Drosophila have identified physical and genetic interactions between Fat and Atrophin, a potential nuclear receptor corepressor implicated in the control of cell polarity and migration. Interactions of mammalian Fat1 and Atrophins have not been reported; we hypothesized that such interactions might contribute to the phenotypic effects of Fat1 on VSMC activation. Western analysis confirmed Atrophin-1 expression in cultured VSMCs. Atrophin-1 levels were low in quiescent VSMCs, but increased strongly on cell cycle entry stimulated by serum, and in response to growth factors and cytokines such as angiotensin II, bFGF, IL-1beta, and PDGF-BB known to promote neointimal formation after vascular injury. This pattern of induction is very similar to that of Fat1, suggesting a functional relationship of Fat1 and Atrophin-1 in VSMC activities. By co-immunoprecipitation and Western analysis, we found a physical interaction of Atrophin-1 with the Fat1 cytoplasmic domain, and mapped the Fat1 sequences required for interaction to aa 4300–4400. Interestingly, this interaction region overlaps substantially with the highly conserved FC1 domain, which we previously described as the most important domain for Fat1 interaction with beta-catenin. This overlap raises the possibility that Atrophin-1 and beta-catenin compete for interaction with similar sequences in Fat1. Taken together, this interaction of Fat1 with Atrophin-1 identifies an alternative pathway by which Fat1 may affect cell polarity and migration, while its potential effects on the interaction of Fat1 and beta-catenin provide a link to growth regulation and canonical Wnt signaling.