Filamin Associates with Smads and Regulates Transforming Growth Factor-β Signaling

Members of the Smad proteins transmit signals triggered by the ligands of transforming growth factor (TGF)-β superfamily. Ligand-activated receptors induce phosphorylation of so-called receptor-regulated Smads, which then accumulate in the nucleus to participate in target gene transcription, in collaboration with Smad-interacting proteins. We performed yeast two-hybrid screening and identified filamin, a cytoskeletal actin-binding protein 280, as a Smad5-interacting protein. Filamin was found to be associated not only with Smad5 but also with other Smad proteins, including TGF-β/activin receptor-regulated Smad2. TGF-β signaling was defective in filamin-deficient human melanoma cells M2 compared with a filamin-transfected subline A7, as determined by TGF-β-responsive reporter gene activation and Smad2 nuclear accumulation. M2 cells restored TGF-β responsiveness following transient transfection of full-length filamin encoding vector. The defective TGF-β signaling in M2 cells seemed to be due to impaired receptor-induced serine phosphorylation of Smad2. These results suggest that filamin plays an important role in Smad-mediated signaling.