Integration of Oxygen Signaling at the Consensus HRE

The hypoxia-inducible factor 1 (HIF-1) was initially identified as a transcription factor that regulated erythropoietin gene expression in response to a decrease in oxygen availability in kidney tissue. Subsequently, a family of oxygen-dependent protein hydroxylases was found to regulate the abundance and activity of three oxygen-sensitive HIFα subunits, which, as part of the HIF heterodimer, regulated the transcription of at least 70 different effector genes. In addition to responding to a decrease in tissue oxygenation, HIF is proactively induced, even under normoxic conditions, in response to stimuli that lead to cell growth, ultimately leading to higher oxygen consumption. The growing cell thus profits from an anticipatory increase in HIF-dependent target gene expression. Growth stimuli–activated signaling pathways that influence the abundance and activity of HIFs include pathways in which kinases are activated and pathways in which reactive oxygen species are liberated. These pathways signal to the HIF protein hydroxylases, as well as to HIF itself, by means of covalent or redox modifications and protein-protein interactions. The final point of integration of all of these pathways is the hypoxia-response element (HRE) of effector genes. Here, we provide comprehensive compilations of the known growth stimuli that promote increases in HIF abundance, of protein-protein interactions involving HIF, and of the known HIF effector genes. The consensus HRE derived from a comparison of the HREs of these HIF effectors will be useful for identification of novel HIF target genes, design of oxygen-regulated gene therapy, and prediction of effects of future drugs targeting the HIF system. Oxygen availability regulates many physiological and pathophysiological processes, including embryonic development, high-altitude adaptation, wound healing, and inflammation, as well as contributing to the pathophysiology of ischemic diseases and cancer. Central to our understanding of these processes is an elucidation of the molecular mechanisms by which cells react and adapt to insufficient oxygen supply (hypoxia). The last few years have brought a wealth of novel insights into these processes. Oxygen-sensing protein hydroxylases have been discovered that regulate the abundance and activity of three hypoxia-inducible transcription factors (HIFs) and thereby the activity of at least 70 effector genes involved in hypoxic adaptation. In addition to the increase in HIF abundance in response to a decrease in tissue oxygenation, it became evident that HIF abundance is also proactively increased, even under normoxic conditions, in response to stimuli that lead to cell growth and thus ultimately require higher oxygen consumption. The growing cell thus profits from an anticipatory increase in HIF-dependent target gene expression. Growth stimuli–activated signaling pathways that influence the abundance and activity of HIFs include pathways that involve the activation of kinases and liberation of reactive oxygen species. All of these pathways converge at the hypoxia-response elements (HREs) of effector genes, to which the HIFs bind, thereby enabling HIF-dependent induction of gene expression.