Binding of coronin 1B to TβRI negatively regulates the TGFβ1 signaling pathway

Coronin 1B is an actin-binding protein that regulates several actin-dependent cellular processes including migration and endocytosis. However, the role of coronin 1B in the tumor growth factor (TGF)β signaling pathway is largely unknown. Here, we investigated whether coronin 1B affects the TGFβ signaling cascade and found that coronin 1B negatively regulates the TGFβ signaling pathway. Immunoprecipitation and glutathione-S-transferase-pulldown assays revealed that coronin 1B directly associated with TGFβ receptor I (TβRI). Overexpression of coronin 1B inhibited the TGFβ1-induced interaction between TβRI and Smad2/3 in plasmid-transfected HEK293T cells. Coronin 1B was basally bound to TβRI in vascular smooth muscle cells (VSMCs), but TGFβ1 stimulation did not affect their association, suggesting constitutive binding between coronin 1B and TβRI. Overexpression of coronin 1B suppressed TGFβ1-induced activation of a Smad-binding element-luciferase reporter construct and a plasminogen activator inhibitor (PAI)-1 promoter-luciferase reporter construct in HEK293T cells. By contrast, depletion of coronin 1B by siRNA transfection increased TGFβ1-induced Smad2/3 phosphorylation and PAI-1 expression in VSMCs. These results suggest that coronin 1B regulates the TGFβ1 signaling cascade by constitutively interacting with TβRI and inhibiting the binding of Smad2/3 to TβRI in response to TGFβ1 stimulation.