Ubiquitin ligase Cbl-b ubiquitinated and degradated IGF-IR protein after IGF-I treatment in MGC-803 cell. The serum-starved MGC-803 cell was pretreated with or without PS341 (5 nM) for 12 hours and then incubated with IGF-I for indicated times. (A) The expression of IGF-IR was analyzed by western blot. (B) The cell was treated with IGF-I for indicated times before immunoprecipitation with anti-Cbl-b antibody, Cbl-b and IGF-IR were analyzed by western blot. (C-D) The serum-starved MGC-803, ShRNA Cbl-b and NS Control cells were treated with IGF-I for indicated times. IGF-IR was immunoprecipitated and ubiquitin was analyzed by western blot. ShRNACbl-b, Cbl-b shRNA transfected; NS Control, Non-silencing controls.