Oxygen-sensitive reset of hypoxia-inducible factor transactivation response: Prolyl hydroxylases tune the biological normoxic set point

Cellular O-2 sensing enables physiological adjustinents to variations in tissue pO(2), Under basal conditions, cells are adjusted to an O-2 environment biologically read as normoxia. Any sharp departure from that state of normoxia triggers O-2-sensitive biological responses. The stabilization of hypoxia-inducible factor (HIF) signifies a robust biological readout of hypoxia. In the presence of sufficient O-2, HIF is hydroxylated and degraded. HIF protyl hydroxylation is catalyzed by prolyl hydroxylase isoenzymes PHD1, 2, and 3. Using HT22 neurons stably transfected with a HIF reporter construct, we tested a novel hypothesis postulating that biological cells are capable of resetting their normoxic set point by O-2-sensitive changes in PHD expression. Results of this study show that the pO(2) of the mouse brain cortex was 35 mm Hg or 5% O-2. Exposure of HT22, adjusted to growing in 20% O-2, to 5% O-2 resulted in HIF-driven transcription. However, cells adjusted to growing in 5% 02 did not report hypoxia. Cells adjusted to growing in 30% O-2 reported hypoxia when acutely exposed to room air culture conditions. When grown under high O-2 conditions, cells reset their normoxic set point upward by down-regulating the expression of PHD 1-3. When grown under low O-2 conditions, cells reset their normoxic set point downward by inducing the expression of PHD1-3. Exposure of mice in vivo to a hypoxic 10% O-2 environment lowered blood as well as brain pO(2), Such hypoxic exposure induced PHD1-3. Exposure of mice to a hyperoxic 50% 02 ambience repressed the expression of PHD1-3, indicating that O-2-sensitive regulation of PHD expression is effective in the brain in vivo. siRNA dependent knockdown of PHD expression revealed that O-2-Sensitive regulation of PHD may contribute to tuning the nonnoxic set point in biological cells. (c) 2006 Elsevier Inc. All rights reserved.