Impact of different MDR mechanisms on the cytotoxicity of hellebrigenin. The different drug-resistant cancer cell models (as indicated) were treated for 72 hours with hellebrigenin (left panels) and with the respective drugs affected by the indicated resistance mechanisms as positive controls (right panels). Viability was determined using a 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay. Each assay was carried out in six replicates. The various cell lines used are A: the colon carcinoma cell line HCT-116 p53/wt and the p53 knock-out cell line HCT-116 p53/ko; B: the small cell lung carcinoma cell line GLC-4 and the ABCC1- and LRP-overexpressing subline GLC-4/ADR; C: the epidermal carcinoma-derived cell line KB-3-1 and the ABCB1-overexpressing subline KB-C-1; D: the ovarian carcinoma cell line A2780 and the cisplatin resistant subline A2780cis; E: the promyelocytic leukemia cell line HL60 and the mitoxantrone resistant subline HL60/mx. Cells were not starved during the experiments.