Reactive oxygen species and molecular regulation of renal oxygenation

It has been known since the 1940s that a gradient of renal oxygenation exists in the kidney with the lowest PO2 in the renal inner medulla under physiological conditions. Due to a low PO2 milieu in the renal medulla, the cells in this region are at constant risk of hypoxic injury. Although numerous studies have shown that renal medullary cells adapt well to low PO2, the precise mechanism mediating this adaptive response remains poorly understood. Recently, hypoxia-induced molecular adaptation in mammalian tissues or cells has been studied extensively and many studies have indicated that the molecular regulation of gene expression is importantly involved. This paper focuses on the role of a transcription factor, hypoxia-inducible factor-1 (HIF-1)-mediated molecular adaptation and explores the physiological relevance of molecular activation of HIF-1 and its target genes in the renal medulla. Given that this HIF-1-mediated action is associated with local redox status, evidence is presented to indicate that reactive oxygen species (ROS), especially superoxide (O-2(.-)) is importantly involved in HIF-1-mediated molecular adaptation in renal medullary cells. O-2(.-) degrades HIF-1alpha, an HIF-1 subunit, by activating ubiquitin-proteasome and thereby decreases the transcriptional activation of many oxygen-sensitive genes. This action of O-2(.-) disturbs renal medullary adaptation to low PO2 and produces renal medullary dysfunction, resulting in sodium retention and hypertension. This report also provides evidence indicating the primary source of O-2(.-), enzymatic pathways for O-2(.-) production and activating mechanism of O-2(.-) production in the kidney. It is concluded that HIF-1-mediated molecular adaptation to low PO2 is of importance in the regulation of renal medullary function and that ROS may target this HIF-1-mediated medullary adaptation to damage renal function.