Degradation of filamin induces contraction of vascular smooth muscle cells in type-I collagen matrix honeycombs

Background: Dedifferentiated rabbit vascular smooth muscle cells (SMCs) exhibit similar features to differentiated SMCs when cultured in three-dimensional matrices of type-I collagen called "honeycombs," but the mechanism is unknown. The role of filamin, an actin-binding protein that links actin filaments in SMCs, was investigated. Methods: Filamin and other related proteins were detected by western blot analysis and immunofluorescence staining. Honeycomb size was measured to confirm the contraction of SMCs. Results: Full-length filamin was expressed in subconfluent SMCs cultured on plates; however, degradation of filamin, which might be regulated by calpain, was observed in confluent SMCs cultured on plates and in honeycombs. While filamin was co-localized with β-actin in subconfluent SMCs grown on plates, filamin was detected in the cytoplasm in SMCs cultured in honeycombs, and degraded filamin was mainly detected in the cytoplasmic fraction of these cells. In addition, β-actin expression was low in the cytoskeletal fraction of SMCs cultured in honeycombs compared with cells cultured on plates, and the size of the honeycombs used for culturing SMCs was significantly reduced. Conclusion: These data suggest that degradation of filamin in SMCs cultured in honeycombs induces structural weakness of β-non-muscle actin filaments, thereby permitting SMCs in honeycombs to achieve contractility.