Reciprocal repression between Tax and Smad3 is mediated through competition for CBP/p300.

(A) Repression of Smad3-mediated transactivation by Tax is recovered by p300 or CBP. HepG2 cells were transfected with 100 ng p3TP-Lux, 100 ng Smad3 expression plasmid, 3 μg Tax expression plasmid, and 0.1, 0.5, or 2 μg p300 or CBP expression plasmid. Cells were harvested 24 hours after transfection, and a luciferase assay was performed. Luciferase activity is presented as fold induction relative to the basal level measured in cells transfected with p3TP-Lux alone. (B) Reciprocal repression between Tax and Smad3. HepG2 cells were transfected with 10 ng HTLV-I LTR-LUC plasmid in combination with 3 μg of either the Tax or Smad3 expression plasmid. Results shown are expressed as the fold activation of luciferase activity of cells transfected with the LTR-LUC alone. Data represent the mean ± SD from 3 separate experiments.