The expression of
hypoxia-inducible factor (HIF) is influenced by
reactive oxygen species (ROS). Effect of
bilirubin on HIF-1 expression in proximal tubular cells was investigated under physiological
oxygen concentration, which is relative hypoxic condition mimicking
oxygen content in the medulla of renal tissue. The human kidney (HK2) cells were cultured in 5%
oxygen with or without
bilirubin. HIF-1α
protein expression was increased by
bilirubin treatment at 0.01-0.2 mg/dL concentration. The
messenger RNA expression of HIF-1α was increased by 1.69±0.05 folds in the cells cultured with 0.1 mg/dL
bilirubin, compared to the control cells. The inhibitors of PI3K/mTOR, PI3K/AKT, and ERK 1/2 pathways did not attenuate increased HIF-1α expression by
bilirubin. HIF-1α expression decreased by 10 µM exogenous
hydrogen peroxide (H2O2); scavenger of ROS with or without
bilirubin in the HK2 cells increased HIF-1α concentration more than that in the cells without
bilirubin. Exogenous H2O2 decreased the phosphorylation of P70S6
kinase, which was completely reversed by
bilirubin treatment. Knockdown of NOX4 gene by
small interfering RNA (
siRNA) increased HIF-1α
mRNA expression. In coonclusion,
bilirubin enhances HIF-1α transcription as well as the up-regulation of HIF-1α protein translation through the attenuation of ROS and subunits of
NADPH oxidase.