Regulation of transforming growth factor-β1-dependent integrin β6 expression by p38 mitogen-activated protein kinase...

Bile duct epithelial cells (BDECs) contribute to liver fibrosis by expressing αVβ6 integrin, a critical activator of latent transforming growth factor β (TGF-β). β6 integrin (Itgβ6) mRNA induction and αVβ6 integrin expression in BDECs are partially TGF-β-dependent. However, the signaling pathways required for TGF-β-dependent Itgβ6 mRNA induction in BDECs are not known. We tested the hypothesis that the p38 mitogen-activated protein kinase (MAPK) signaling pathway contributes to TGF-β1 induction of Itgβ6 mRNA by activating SMAD and activator protein 1 (AP-1) transcription factors. Pretreatment of transformed human BDECs (MMNK-1 cells) with two different p38 MAPK inhibitors, but not a control compound, inhibited TGF-β1 induction of Itgβ6 mRNA. Inhibition of p38 also reduced TGF-β1 activation of a SMAD-dependent reporter construct. Expression of a dominant-negative SMAD3 (SMAD3ΔC) significantly reduced TGF-β1-induced Itgβ6 mRNA expression. Expression of JunB mRNA, but not other AP-1 proteins, increased in TGF-β1-treated MMNK-1 cells, and induction of JunB expression was p38-dependent. Consistent with a requirement for de novo induction of JunB protein, cycloheximide pretreatment inhibited TGF-β1 induction of Itgβ6 mRNA. Expression of a dominant-negative AP-1 mutant (TAM67) also inhibited TGF-β1 induction of Itgβ6 mRNA. Overall, the results suggest that p38 contributes to TGF-β1-induced Itgβ6 mRNA expression in MMNK-1 cells by regulating activation of both SMAD and AP-1 transcription factors.