Oxygen-dependent expression of hypoxia-inducible factor-1 alpha in renal medullary cells of rats

Hypoxia-inducible factor-1 alpha (HIF-1 alpha) is a transcription factor th at regulates the oxygen-dependent expression of a number of genes. This tra nscription factor may contribute to the abundant expression of many genes i n renal medullary cells that function normally under hypoxic conditions. Th e present study was designed to determine the characteristics of HIF-1 alph a cDNA cloned from the rat kidney and the expression profile of HIF-1 alpha in different kidney regions and to explore the mechanism activating or reg ulating HIF-1 alpha expression in renal medullary cells. A 3,718-bp HIF-1 a lpha cDNA from the rat kidney was first cloned and sequenced using RT-PCR a nd TA cloning technique. It was found that 823 amino acids deduced from thi s renal HIF-1 alpha cDNA had 99%, 96%, and 90% identity with rat, mouse, or human HIF-1 alpha deposited in GenBank, respectively. The 3'-untranslated region of HIF-1 alpha mRNA from the rat kidney contained seven AUUUA instab ility elements, five of which were found to be conserved among rat, mouse, and human HIF-1 alpha. Northern blot analyses demonstrated a corticomedulla ry gradient of HIF-1 alpha mRNA expression in the kidney, with the greatest abundance in the renal inner medulla. Western blot analyses also detected a higher HIF-1 alpha protein level in the nuclear extracts from the renal m edulla than the renal cortex. A classic loop diuretic, furosemide (10 mg/kg ip), markedly increased renal medullary PO2 levels from 22.5 to 52.2 mmHg, which was accompanied by a significant reduction of HIF-1 alpha transcript s in renal medullary tissue. In in vitro experiments, low PO2, but not elev ated osmolarity, was found to significantly increase HIF-1 alpha mRNA in re nal medullary interstitial cells and inner medullary collecting duct cells. These results indicate that HIF-1 alpha is more abundantly expressed in th e renal medulla compared with the renal cortex. Increased abundance of HIF- 1 alpha mRNA in the renal medulla may represent an adaptive response of ren al medullary cells to low PO2.