Reactive Oxygen Species Generated at Mitochondrial Complex III Stabilize Hypoxia-inducible Factor-1α during Hypoxia

Abstract

cells. Isolated mitochondria increase ROS generation during hypoxia, as does the bacterium Paracoccus denitrificans. These findings reveal that mitochondria-derived ROS are both required and sufficient to initiate HIF-1␣ stabilization during hypoxia.Hypoxia initiates transcription of a number of gene products that help to sustain the supply of O 2 to tissues and to enhance cell survival during severe O 2 deprivation. Gene products that augment O 2 supply at the tissue level include erythropoietin (Epo) 1 which increases the proliferation of erythrocytes, tyrosine hydroxylase which is necessary for the synthesis of the neurotransmitter dopamine in the carotid bodies, and the angiogenic factor VEGF which stimulates growth of new capillaries (1-3). At the cellular level, gene products that enhance survival during hypoxia include the glycolytic enzymes and the glucose transporters Glut1 and Glut3 (4). The induction of these genes is mediated by hypoxia-inducible factor-1 (HIF-1) (5-7), a heterodimeric transcription factor consisting of HIF-1␣ and the aryl hydrocarbon nuclear translocator (ARNT or HIF-1␤) subunits (7-9). The significance of HIF-1 in transcriptional regulation was recently demonstrated by the marked decrease in mRNA expression of VEGF and glycolytic enzymes seen during hypoxia in HIF-1␣-or ARNT-deficient murine embryonic stem cells (10 -12).The mechanism by which HIF-1 activation is initiated during hypoxia remains unclear. Both HIF-1␣ and ARNT mRNAs are constitutively expressed, indicating that functional activity of the HIF-1␣⅐ARNT complex is regulated by post-transcriptional events. ARNT levels are not significantly affected by [O 2 ], whereas HIF-1␣ protein is rapidly degraded under normoxic conditions by the ubiquitin-proteasome system (13,14). Hypoxia enhances HIF-1␣ protein levels by inhibiting its degradation, thereby allowing it to accumulate, to dimerize with ARNT, and to bind to the hypoxia-responsive element (HRE) in the promoter or enhancer regions of various genes. Thus, the functional HIF-1␣⅐ARNT complex is primarily regulated by the abundance of the HIF-1␣ subunit.Although much has been learned about the role of HIF-1 in controlling the expression of hypoxia-responsive genes, the underlying mechanism by which cells detect the decrease in [O 2 ] and initiate the stabilization of HIF-1␣ is not known. Presently, four diverse O 2 -sensing mechanisms have been proposed to mediate the transcriptional response to hypoxia (15). Two of these models postulate the involvement of an iron-containing unit in the form of either a heme group or an iron/sulfur cluster, which undergoes a change in activity during hypoxia that triggers the transcriptional response. These models are supported by the observation that cobaltous ions, or alternatively the iron chelator desferrioxamine (DFO), stabilize HIF-1␣ under normoxic conditions (16). However, no specific proteins with this role have been identified in mammalian systems. Two other models involve the generation of reactive oxygen species (ROS) by a f...

Cycling hypoxia and free radicals regulate angiogenesis and radiotherapy response

“…HIF1 has been referred to as the "master regulator of oxygen homeostasis" 7 , but it is regulated by many factors aside from hypoxia, including oncogenes, growth factors and free radicals [27][28][29] . Free radicals are chemical species that contain unpaired electrons; of particular importance here are oxygen-containing radicals, such as the superoxide anion (O 2 -).…”

“…Schumacker and Chandel have theorized that the oxygen sensor in cells is not the PHD but rather the formation of reactive oxygen species by mitochondria under hypoxia that lead to stabilization of HIF1α 28,30 . Evidence for this mechanism of stabilization of HIF1 is multifold.…”

“…Evidence for this mechanism of stabilization of HIF1 is multifold. First, ρ 0 cells, chemically depleted of functional mitochondria, cannot increase HIF1α levels in response to hypoxia 28 . Second, overexpression of catalase blocks hypoxic stabilization of HIF1α, whereas administration of H 2 O 2 to cells increases HIF1 expression 28 .…”

“…First, ρ 0 cells, chemically depleted of functional mitochondria, cannot increase HIF1α levels in response to hypoxia 28 . Second, overexpression of catalase blocks hypoxic stabilization of HIF1α, whereas administration of H 2 O 2 to cells increases HIF1 expression 28 . Recently, a genetic approach has been taken to further test the hypothesis that reactive oxygen species are important in hypoxic signalling for HIF1α stabilization and hypoxia signalling.…”

Relation of hypoxia inducible factor 1α and 2α in operable non-small cell lung cancer to angiogenic/molecular profile of tumours and survival

“…A large group of genes that includes erythropoietin (Goldberg et al, 1988), nitric oxide synthase (Palmer et al, 1998), tyrosine hydroxylase (Norris and Millhorn, 1995), VEGF (Forsythe et al, 1996;Ema et al, 1997), lactate dehydrogenase, haem oxygenase, transferin receptor and others are regulated by hypoxia (Blancher and Harris, 1998). Cells, whether normal or malignant, have the ability to 'sense' low oxygen conditions, probably via a haem flavo-oxido-reductase protein or even through hypoxia-stimulated release of reactive oxygen species from mitochondria (Chandel et al, 2000), which activates a signalling pathway for the expression of the hypoxia-regulated genes.…”