Figure 2: ABCG2 promoter luciferase reporter gene assay showing that the promoter region harboring the HIF-1 𝛼 response element is required for the activation of ABCG2 in S1 human colon cancer cell line. Similar results were also obtained for HCT-116, which are shown in Supplementary Figure  1. (a) Schematic representations of the various 5′ deleted ABCG2-promoter constructs. The 5′-end of each of the constructs relative to the transcription start site (arrows) is indicated. The pGL3-basic (promoterless) vector, encoding firefly luciferase, was used to determine the basal levels. The last construct (−105/+396) does not contain the HIF-1 𝛼 response element. (b) ABCG2 reporter activity was measured in S1 cells transiently transfected with the various ABCG2 promoter constructs with or without 24 h pretreatment with 2-DG (20 mM) or hypoxia. The mean reporter activity ± SD (firefly/renilla luciferase units (RLU)) from three independent experiments is presented.