Figure 5. Oxidative stress-induced activation of AP-1 in an ASK1-dependent manner induces TGF-β and galectin-9 expression. THP-1 cells were treated with the Toll-like receptor 4 (TLR4) ligand, high mobility group box 1 (HMGB1), for 24 h. TLR4 mediates activation of NADPH oxidase using myeloid differentiation factor 88 (MyD88), TLR4 TIR domain-associated protein (TIRAP) and Bruton’s tyrosine kinase (Btk). Activation of Btk by MyD88 and TIRAP leads to Btk-dependent phosphorylation of phospholipase C (PLC, mainly isoform 1γ), which triggers activation of protein kinase C alpha (PKCα). PKCα activates NADPH oxidase which produces superoxide anion radical, activating the ASK1 pathway and activation of AP-1 transcription factor. The scheme is shown in panel (A). THP-1 cells were exposed for 24 h to 1 μg/ml HMGB1 with or without pre-treatment with 30 μM DPI (NADPH oxidase inhibitor), 1 μM SR11302 (AP-1 inhibitor) or transfection with dominant-negative isoform of ASK1 (ΔN-ASK1). Levels of secreted TGF-β and galectin-9 were measured by ELISA (B). Data are shown as mean values ± SEM of four independent experiments. * - p < 0.05 and ** - p < 0.01 vs control.