A biologic role of HIF-1 in the renal medulla

Background. Activation of hypoxia-inducible factor-1 (HIF-1) is the primary defensive mechanism against hypoxia. HIF-1 activation generally occurs in pathologic disruption of tissue oxygenation. However, a biologic role of HIF-1 in the medulla of the kidney, which is considered perpetually hypoxic under physiologic conditions due to its unique circulation, remains to be elucidated. Methods. The expression of HIF-1α was detected by immunohistochemical analysis. Functional studies of HIF in medulla were carried out by gene transfer of various plasmids by retrograde injection via ureter. Results. Our immunohistochemical analysis detected HIF-1α in the inner stripe and the inner medulla of normal rats. Water deprivation increased the number of HIF-1α-positive cells, which may be mediated by an increase in medullar work-load and a decrease in local blood flow. To perform functional studies, we performed gene transfer. Efficient expression of the transgene was confirmed using an enhanced green fluorescent protein (E-GFP) expressing vector. Our histologic and immunoblotting analysis detected the transgene product at the inner medulla and the inner stripe 48 hours after injection. Administration of negative-dominant HIF induced severe damage in the medulla of normal rats. In contrast, gene transfer of constitutively active HIF (HIF/VP16) induced expression of various HIF-regulated genes and protected the medulla against ischemic insults. Conclusion. Our studies demonstrated a crucial role of HIF in the renal medulla under normal and hypoxic circumstances.