Expression of nondegradable Skp2 and SnoN antagonizes TGF-β signaling. (A) Diagram of retroviral vector expressing nondegradable Skp2 and SnoN. (B) Protein levels of stably expressed wild-type Skp2 (a) and nondegradable Skp2 (b) in response to TGF-β signaling. (C) Protein levels of stably expressed wild-type SnoN and nondegradable SnoN in response to TGF-β signaling. (D) Growth inhibition assay. Mv1Lu cells stably expressing Skp2, Skp2Δdb, SnoN, and SnoNΔdb, respectively, and coexpressing Skp2Δdb and SnoNΔdb were incubated for 4 days with various concentrations of TGF-β1 as indicated. The growth of cells was quantified by cell counting and compared with the growth of unstimulated cells. WT, wild type.