Effect of TGF-β1 on BMP-4 signaling pathway. (A) C2C12BRA and HepG2BRA cells were treated with 14 pM of rBMP-4 in combination with increasing concentrations of TGF-β1. After 15 h, BMP activity was assessed by measuring luciferase activity in the cell lysates. Background luciferase level in the control sample (no BMP) was subtracted from each of the experimental values. The results are expressed as relative luciferase units (RLU) (the activity with 14 pM BMP-4 alone equal to one). (B) TGF-β-responsive reporter cells (TMLC) were stimulated with 250 pg/ml of TGF-β1 and increasing concentrations of BMP-4. After 16 h, TGF-β1 activity was assessed by measuring luciferase activity in the cell lysates. Each point represents the mean ± SEM of triplicate wells from one representative experiment. Similar results were obtained in two separate experiments.