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

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Abstract
Hypoxia-inducible factor-1α (HIF-1α) is a transcription factor that regulates the oxygen-dependent expression of a number of genes. This transcription factor may contribute to the abundant expression of many genes in renal medullary cells that function normally under hypoxic conditions. The present study was designed to determine the characteristics of HIF-1α cDNA cloned from the rat kidney and the expression profile of HIF-1α in different kidney regions and to explore the mechanism activating or regulating HIF-1α expression in renal medullary cells. A 3,718-bp HIF-1α cDNA from the rat kidney was first cloned and sequenced using RT-PCR and TA cloning technique. It was found that 823 amino acids deduced from this renal HIF-1α cDNA had 99%, 96%, and 90% identity with rat, mouse, or human HIF-1α deposited in GenBank, respectively. The 3′-untranslated region of HIF-1α mRNA from the rat kidney contained seven AUUUA instability elements, five of which were found to be conserved among rat, mouse, and human HIF-1α. Northern blot analyses demonstrated a corticomedullary gradient of HIF-1α mRNA expression in the kidney, with the greatest abundance in the renal inner medulla. Western blot analyses also detected a higher HIF-1α protein level in the nuclear extracts from the renal medulla than the renal cortex. A classic loop diuretic, furosemide (10 mg/kg ip), markedly increased renal medullary Po2levels from 22.5 to 52.2 mmHg, which was accompanied by a significant reduction of HIF-1α transcripts in renal medullary tissue. In in vitro experiments, low Po2, but not elevated osmolarity, was found to significantly increase HIF-1α mRNA in renal medullary interstitial cells and inner medullary collecting duct cells. These results indicate that HIF-1α is more abundantly expressed in the renal medulla compared with the renal cortex. Increased abundance of HIF-1α mRNA in the renal medulla may represent an adaptive response of renal medullary cells to low Po2.