Hypoxia-inducible factors (HIFs) are transcription factors that respond to decreases in available oxygen in the cellular environment, or hypoxia.
Most, if not all, oxygen-breathing species express the highly conserved transcriptional complex HIF-1, which is a heterodimer composed of an alpha and a beta subunit, the latter being a constitutively-expressed aryl hydrocarbon receptor nuclear translocator (ARNT). HIF-1 belongs to the PER-ARNT-SIM (PAS) subfamily of the basic helix-loop-helix (bHLH) family of transcription factors. The alpha and beta subunit are similar in structure and both contain the following domains:
Structure of a HIF-1a-pVHL-ElonginB-ElonginC Complex.
The following are members of the human HIF family:
||hypoxia-inducible factor 1, alpha subunit
||aryl hydrocarbon receptor nuclear translocator
||endothelial PAS domain protein 1
||aryl-hydrocarbon receptor nuclear translocator 2
||hypoxia inducible factor 3, alpha subunit
||aryl-hydrocarbon receptor nuclear translocator 3
The HIF signaling cascade mediates the effects of hypoxia, the state of low oxygen concentration, on the cell. Hypoxia often keeps cells from differentiating. However, hypoxia promotes the formation of blood vessels, and is important for the formation of a vascular system in embryos and tumors. The hypoxia in wounds also promotes the migration of keratinocytes and the restoration of the epithelium.
In general, HIFs are vital to development. In mammals, deletion of the HIF-1 genes results in perinatal death. HIF-1 has been shown to be vital to chondrocyte survival, allowing the cells to adapt to low-oxygen conditions within the growth plates of bones. HIF plays a central role in the regulation of human metabolism.
The alpha subunits of HIF are hydroxylated at conserved proline residues by HIF prolyl-hydroxylases, allowing their recognition and ubiquitination by the VHL E3 ubiquitin ligase, which labels them for rapid degradation by the proteasome. This occurs only in normoxic conditions. In hypoxic conditions, HIF prolyl-hydroxylase is inhibited, since it utilizes oxygen as a cosubstrate.
Inhibition of electron transfer in the succinate dehydrogenase complex due to mutations in the SDHB or SDHD genes can cause a build-up of succinate that inhibits HIF prolyl-hydroxylase, stabilizing HIF-1α. This is termed pseudohypoxia.
HIF-1, when stabilized by hypoxic conditions, upregulates several genes to promote survival in low-oxygen conditions. These include glycolysis enzymes, which allow ATP synthesis in an oxygen-independent manner, and vascular endothelial growth factor (VEGF), which promotes angiogenesis. HIF-1 acts by binding to HIF-responsive elements (HREs) in promoters that contain the sequence NCGTG (where N is either A or G).
It has been shown that muscle A kinase–anchoring protein (mAKAP) organized E3 ubiquitin ligases, affecting stability and positioning of HIF-1 inside its action site in the nucleus. Depletion of mAKAP or disruption of its targeting to the perinuclear (in cardiomyocytes) region altered the stability of HIF-1 and transcriptional activation of genes associated with hypoxia. Thus, "compartmentalization" of oxygen-sensitive signaling components may influence the hypoxic response.
The advanced knowledge of the molecular regulatory mechanisms of HIF1 activity under hypoxic conditions contrast sharply with the paucity of information on the mechanistic and functional aspects governing NF-κB-mediated HIF1 regulation under normoxic conditions. However, HIF-1α stabilization is also found in non-hypoxic conditions through an, until recently, unknown mechanism. It was shown that NF-κB (nuclear factor κB) is a direct modulator of HIF-1α expression in the presence of normal oxygen pressure. siRNA (small interfering RNA) studies for individual NF-κB members revealed differential effects on HIF-1α mRNA levels, indicating that NF-κB can regulate basal HIF-1α expression. Finally, it was shown that, when endogenous NF-κB is induced by TNFα (tumour necrosis factor α) treatment, HIF-1α levels also change in an NF-κB-dependent manner. HIF-1 and HIF-2 have different physiological roles. HIF-2 regulates erythropoietin production in adult life.
Repair or regeneration
In normal circumstances after injury HIF-1a is degraded by prolyl hydroxylases (PHDs). In June 2015, scientists found that the continued up-regulation of HIF-1a via PHD inhibitors regenerates lost or damaged tissue in mammals that have a repair response; and the continued down-regulation of Hif-1a results in healing with a scarring response in mammals with a previous regenerative response to the loss of tissue. The act of regulating HIF-1a can either turn off, or turn on the key process of mammalian regeneration.
As a therapeutic target
Recently, several drugs that act as selective HIF prolyl-hydroxylase inhibitors have been developed. The most notable compounds are: Roxadustat (FG-4592); Vadadustat (AKB-6548), Daprodustat (GSK1278863), Desidustat (ZYAN-1), and Molidustat (Bay 85-3934), all of which are intended as orally acting drugs for the treatment of anemia. Other significant compounds from this family, which are used in research but have not been developed for medical use in humans, include MK-8617, YC-1, IOX-2, 2-methoxyestradiol, GN-44028, AKB-4924, Bay 87-2243, FG-2216 and FG-4497. By inhibiting prolyl-hydroxylase enzyme, the stability of HIF-2α in the kidney is increased, which results in an increase in endogenous production of erythropoietin. Both FibroGen compounds made it through to Phase II clinical trials, but these were suspended temporarily in May 2007 following the death of a trial participant taking FG-2216 from fulminant hepatitis (liver failure), however it is unclear whether this death was actually caused by FG-2216. The hold on further testing of FG-4592 was lifted in early 2008, after the FDA reviewed and approved a thorough response from FibroGen. Roxadustat, vadadustat, daprodustat and molidustat have now all progressed through to Phase III clinical trials for treatment of renal anemia.
Inflammation and cancer
In other scenarios and in contrast to the therapy outlined above, recent research suggests that HIF induction in normoxia is likely to have serious consequences in disease settings with a chronic inflammatory component. It has also been shown that chronic inflammation is self-perpetuating and that it distorts the microenvironment as a result of aberrantly active transcription factors. As a consequence, alterations in growth factor, chemokine, cytokine, and ROS balance occur within the cellular milieu that in turn provide the axis of growth and survival needed for de novo development of cancer and metastasis. These results have numerous implications for a number of pathologies where NF-κB and HIF-1 are deregulated, including rheumatoid arthritis and cancer. Therefore, it is thought that understanding the cross-talk between these two key transcription factors, NF-κB and HIF, will greatly enhance the process of drug development.
HIF activity is involved in angiogenesis required for cancer tumor growth, so HIF inhibitors such as phenethyl isothiocyanate and Acriflavine are (since 2006) under investigation for anti-cancer effects.
Research conducted on mice suggests that stabilizing HIF using an HIF prolyl-hydroxylase inhibitor enhances hippocampal memory, likely by increasing erythropoietin expression. HIF pathway activators such as ML-228 may have neuroprotective effects and are of interest as potential treatments for stroke and spinal cord injury.
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