Regulation of hypoxia-inducible factor is preserved in the absence of a functioning mitochondrial respiratory chain

Hypoxia-inducible factor (HIF) mediates a large number of transcriptional r esponses to hypoxia and has an important role in processes that include ang iogenesis and erythropoiesis, The HIF DNA binding complex consists of 2 bas ic-helix-loop-helix PAS proteins designated alpha and beta subunits. Regula tion occurs principally through the alpha subunits, which are stabilized an d activated in hypoxia, Although substantial evidence implicates reactive o xygen species (ROS) in the regulatory process, the precise mechanisms remai n unclear. Mitochondria are an important source of ROS, and in one model it has been proposed that hypoxia increases the generation of ROS at complex III in the mitochondrion and that this signal acts through a transduction p athway to stabilize HIF-1 alpha and to activate HIF. To test this model the induction of the HIF-1 alpha subunit and the HIF target gene, glucose-tran sporter-1, was examined in a variety of mutant cells that lacked mitochondr ial DNA (rho0) or had other genetic defects in mitochondrial respiration. H IF induction by hypoxia was essentially normal in all cells tested. Hydroge n peroxide production was measured by the luminol/peroxidase method and fou nd to be reduced in rho0 versus wild-type cells and reduced by hypoxia in b oth rho0 and wild-type cells. Furthermore, concentrations of rotenone that maximally inhibited respiration did not affect HIF activation by hypoxia, T hese data do not support the model outlined above and indicate that a funct ional respiratory chain is not necessary for the regulation of HIF by oxyge n. (C) 2001 by The American Society of Hematology.