HIF-1α contributes to increased proliferation of cells at physiological oxygen tensions.

<p>(<b>A</b>) Western blot showing the protein levels of HIF-1α in HCT116 HIF^+/+ and HIF^−/−cultured either at 20% O₂ or under hypoxic stress (<0.1% O₂) for 16 hours. (<b>B</b>) Proliferation curves of HIF^+/+ and HIF^−/− HCT116 cells cultured at 20% or 5% O₂ from 2 to 8 days. Values represent ratio of cell numbers normalized to the initial seeded cells (10⁶). (<b>C</b>) Representative colony formation assay for HCT116 HIF^+/+ and HIF^−/−cultured at 20% or 5% O₂. 200 cells were seeded in each plate and 14 days later they were stained with Giemsa. Media was not changed during the process. (<b>D</b>) Percentage of EdU positive HCT116 HIF^+/+ and HIF^−/− cells as assessed by immunofluorescence (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097938#pone.0097938.s002" target="_blank">Figure S2</a>). Cells were incubated with EdU for 30 minutes in the corresponding oxygen tensions. Results represent means of two independent experiments. Two microscope fields were scored in each experiment. Error bars represent standard error. P value (unpaired t-test): 0.0127 (*), (<b>E</b>) Proposed model of the roles of HIF-1 at different oxygen concentrations.