Hypoxia-inducible factor-1α signaling in aquaporin upregulation after traumatic brain injury

Previous studies have demonstrated that traumatic brain injury (TBI) causes brain edema via aquaporins (AQPs), the water-transporting proteins. In the present study, we determined the role of hypoxia inducible factor-1α (HIF-1α), which is a transcription factor in response to physiological hypoxia, in regulating expression of AQP4 and AQP9. Adult male Sprague-Dawley rats (400–425 g) received a closed head injury using the Marmarou weight drop model with a 450 g weight and survived for 1, 4, 24 and 48 h. Some animals were administered 30 min after injury with 2-methoxyestradiol (2ME2), a naturally occurring metabolite of estradiol which is known to post-transcriptionally down-regulate HIF-1α expression, and sacrificed 4 h after injury. Real-time PCR and Western blot were used, respectively, to detect gene and protein expressions of manganese superoxide dismutase (MnSOD, showing hypoxic stress), HIF-1α, AQP4, and AQP9. ANOVA analysis demonstrated a significant (p < 0.05) increase in gene expression of MnSOD, HIF-1α, AQP4, and AQP9, starting at 1 h after injury through 48 h. Western blot analysis further indicated a significant (p < 0.05) increase in protein expression of these molecules at the same time points. Pharmacological inhibition of HIF-1α by 2ME2 reduced the up-regulated levels of AQP4 and AQP9 after TBI. The present study suggests that hypoxic conditions determined by MnSOD expression after closed head injury contribute to HIF-1α expression. HIF-1α, in turn, up-regulates expression of AQP4 and AQP9. These results characterize the pathophysiological mechanisms, and suggest possible therapeutic targets for TBI patients.