TGF-β1 increases collagen type IV secretion in A549. A549 cells were incubated with several concentrations of TGF-β1 in the presence of 5 μg/ml of L-ascorbic acid for 72 h. Concentrations of collagen type IV in conditioned media were determined using ELISA. TGF-β1 increases collagen type IV in a concentration-dependent manner. Each bar was expressed as the mean ± SD of four independent experiments. ** P < 0.01.