PLC-PRF-5 cells were transfected with GPC3, ΔGPC3, P26->30A, or Grb10 + GPC3, and stable clones were selected. Western blot analysis for ERK and phosphor ERK (pERK) was performed at 0, 5, 10, 15, 30, and 90 min after IGF-1 treatment. In cells expressing empty vector (Vector), the phosphorylation of ERK occurred shortly after IGF-1 stimulation (p < 0.001 at 15 min, T test) but decreased 15 min later. GPC3 overexpression resulted in exaggerated and prolonged ERK phosphorylation. ΔGPC3 and GPC3 mutant P26->30A did not show exaggerated ERK phosphorylation. The overexpression of Grb10 blunted IGF-1-induced ERK phosphorylation and blocked the effects of GPC3 overexpression. All experiments were duplicated.