Figure 6: Effects of concentration gradient of NNN (a), NNK (b), and BP (c) on the growth of cultured A549 cells after 24, 48, and 72 hr as evidenced by MTT assay. Serum-starved A549 cells (2000 cells/well) grown on 96-well tissue culture plates were either nontreated (0) or exposed to 0.025, 0.05, 0.1, 0.5, or 1 μg/mL NNN (a), NNK (b), and BP (c) for 24, 48, and 72 hr. After treatment, MTT cytotoxicity assay was performed. All data are depicted as the means ± SD for four independent experiments.