Potentiation of Smad transactivation by Jun proteins during a combined treatment with epidermal growth factor and transforming growth factor-β in rat hepatocytes: Role of phosphatidylinositol 3-kinase-induced AP-1 activation

Cross-talk between Smad and mitogen-activated protein kinase pathways has been described recently, and evidence for Smad cooperation with AP-1 is emerging. Here we report that epidermal growth factor (EGF) potentializes transforming growth factor β (TGF-β)-induced Smad3 transactivation in rat hepatocytes, an effect abrogated by TAM-67, a dominant negative mutant of AP-1. Antisense transfection experiments indicated that c-Jun and JunB were involved in the synergistic effect, and endogenous c-Jun physically associated with Smad3 during a combined EGF/TGF-β treatment. We next investigated which signaling pathway transduced by EGF was responsible for the Jun-induced synergism. Whereas inhibition of JNK had no effect, inhibition of the phosphatidylinositol-3′ kinase (PI3-kinase) pathway by LY294002 or by expression of a dominant negative mutant of PI3-kinase reduced EGF/TGF-β-induced Smad3 transcriptional activity. Transfection of an activated Ras with a mutation enabling the activation of the PI3-kinase pathway alone mimicked the EGF/TGF-β potentiation of Smad3 transactivation, and TAM-67 abolished this effect, suggesting that the PI3-kinase pathway stimulates Smad3 via AP-1 stimulation. The EGF/TGF-β-induced activation of Smad3 correlated with PI3-kinase and p38-dependent but not JNK-dependent phosphorylation of c-Jun. Since potentiation of a Smad-binding element-driven gene was also induced by EGF/TGF-β treatment, this novel mechanism of Jun/Smad cooperation might be crucial for diversifying TGF-β responses.