Ischemia-reperfusion (IR)-induced kidney injury is a major clinical problem, but its underlying mechanisms remain unclear. The
transcription factor known as nuclear factor, erythroid 2-like 2 (NFE2L2 or Nrf2) is crucial for protection against oxidative stress generated by
pro-oxidant insults. We have previously shown that Nrf2 deficiency enhances susceptibility to IR-induced kidney injury in mice and that its upregulation is protective. Here, we examined Nrf2 target
antioxidant gene expression and the mechanisms of its activation in both human and murine kidney epithelia following acute (2 h) and chronic (12 h)
hypoxia and reoxygenation conditions. We found that acute
hypoxia modestly stimulates and chronic
hypoxia strongly stimulates Nrf2 putative target HMOX1 expression, but not that of other
antioxidant genes. Inhibition of AKT1/2 or ERK1/2 signaling blocked this induction; AKT1/2 but not ERK1/2 inhibition affected Nrf2 levels in basal and acute
hypoxia-reoxygenation states. Unexpectedly,
chromatin immunoprecipitation assays revealed reduced levels of Nrf2 binding at the distal AB1 and SX2 enhancers and proximal promoter of HMOX1 in acute
hypoxia, accompanied by diminished levels of nuclear Nrf2. In contrast, Nrf2 binding at the AB1 and SX2 enhancers significantly but differentially increased during chronic
hypoxia and reoxygenation, with reaccumulation of nuclear Nrf2 levels.
Small interfering-RNA-mediated Nrf2 depletion attenuated acute and chronic
hypoxia-inducible HMOX1 expression, and primary Nrf2-null kidney epithelia showed reduced levels of HMOX1 induction in response to both acute and chronic
hypoxia. Collectively, our data demonstrate that Nrf2 upregulates HMOX1 expression in kidney epithelia through a distinct mechanism during acute and chronic
hypoxia reoxygenation, and that both AKT1/2 and ERK1/2 signaling are required for this process.