Hypoxia inducible factors
Encyclopedia
Hypoxia-inducible factors (HIFs) are transcription factors that respond to changes in available oxygen
in the cellular environment, specifically, to decreases in oxygen, or hypoxia
.
(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:
. However, hypoxia promotes the formation of blood vessels
, and is important for the formation of a vascular system in embryo
s, and cancer tumors. The hypoxia in wound
s also promotes the migration of keratinocyte
s 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 bone
s. HIF plays a central role in the regulation of human metabolism.
residues by HIF prolyl-hydroxylase
s, 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.
Hypoxia also results in a buildup of succinate, due to inhibition of the electron transport chain in the mitochondria. The buildup of succinate further inhibits HIF prolyl-hydroxylase action, since it is an end-product of HIF hydroxylation.
In a similar manner, 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.
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.
s. The most notable of these include FibroGen's compounds FG-2216 and FG-4592, both are intended as orally acting drugs for use in the treatment of forms of anemia
. By inhibiting HIF prolyl-hydroxylase, the activity of HIF-1α in the bloodstream is prolonged, which results in an increase in endogenous production of erythropoetin. Both of these drugs made it through to phase II clinical trials, but these were suspended temporarily in May 2007 following the death of a trial participant from fulminant hepatitis
. However it is unclear whether this death was caused by FG-2216. The hold has been lifted in early 2008 as FDA has reviewed and approved a thorough response from FibroGen.
s. Consequently, 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. The results of a recently published study 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 are under investigation for anti-cancer effects, (e.g. phenethyl isothiocyanate (PEITC)).
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
in the cellular environment, specifically, to decreases in oxygen, or hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
.
Structure
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 translocatorAryl hydrocarbon receptor nuclear translocator
The ARNT gene encodes the aryl hydrocarbon receptor nuclear translocator protein that forms a complex with ligand-bound aryl hydrocarbon receptor , and is required for receptor function. The encoded protein has also been identified as the beta subunit of a heterodimeric transcription factor,...
(ARNT). HIF-1 belongs to the PER-ARNT-SIM
PAS domain
The PAS domain is a protein domain contained in many signaling proteins where it functions as a signal sensor. PAS domains are found in a large number of organisms from bacteria to humans...
(PAS) subfamily of the basic-helix-loop-helix
Basic-helix-loop-helix
A basic helix-loop-helix is a protein structural motif that characterizes a family of transcription factors.- Structure :The motif is characterized by two α-helices connected by a loop. In general, transcription factors including this domain are dimeric, each with one helix containing basic amino...
(bHLH) family of transcription factors. The alpha and beta subunit are similar in structure and both contain the following domains:
- N-terminus – a bHLH domain for DNA binding
- central region – Per-ARNT-SimPAS domainThe PAS domain is a protein domain contained in many signaling proteins where it functions as a signal sensor. PAS domains are found in a large number of organisms from bacteria to humans...
(PAS) domain which facilitates heterodimerization - C-terminus – recruits transcriptional coregulatoryTranscription coregulatorIn molecular biology and genetics, transcription coregulators are proteins that interact with transcription factors to either activate or repress the transcription of specific genes. Transcription coregulators that activate gene transcription are referred to as coactivators while those that...
proteins
Members
The following are members of the human HIF family:| member | gene | protein |
|---|---|---|
| HIF-1α HIF1A Hypoxia-inducible factor 1, alpha subunit , also known as HIF1A, is a protein that in humans is encoded by the HIF1A gene... |
hypoxia-inducible factor 1, alpha subunit HIF1A Hypoxia-inducible factor 1, alpha subunit , also known as HIF1A, is a protein that in humans is encoded by the HIF1A gene... |
|
| HIF-1β | aryl hydrocarbon receptor nuclear translocator Aryl hydrocarbon receptor nuclear translocator The ARNT gene encodes the aryl hydrocarbon receptor nuclear translocator protein that forms a complex with ligand-bound aryl hydrocarbon receptor , and is required for receptor function. The encoded protein has also been identified as the beta subunit of a heterodimeric transcription factor,... |
|
| HIF-2α | endothelial PAS domain protein 1 EPAS1 Endothelial PAS domain-containing protein 1 is a protein that in humans is encoded by the EPAS1 gene.- Function :This gene encodes a transcription factor involved in the induction of genes regulated by oxygen, which is induced as oxygen levels fall... |
|
| HIF-2β | aryl-hydrocarbon receptor nuclear translocator 2 ARNT2 Aryl hydrocarbon receptor nuclear translocator 2 is a protein that in humans is encoded by the ARNT2 gene.-Further reading:... |
|
| HIF-3α | hypoxia inducible factor 3, alpha subunit HIF3A Hypoxia-inducible factor 3 alpha is a protein that in humans is encoded by the HIF3A gene.-Further reading:... |
|
| HIF-3β | ARNT3 | aryl-hydrocarbon receptor nuclear translocator 3 |
Role
The HIF signaling cascade mediates the effects of hypoxia, the state of low oxygen concentration, on the cell. Hypoxia often keeps cells from differentiatingCellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
. However, hypoxia promotes the formation of blood vessels
Angiogenesis
Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for the formation of new blood...
, and is important for the formation of a vascular system in embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s, and cancer tumors. The hypoxia in wound
Wound
A wound is a type of injury in which skin is torn, cut or punctured , or where blunt force trauma causes a contusion . In pathology, it specifically refers to a sharp injury which damages the dermis of the skin.-Open:...
s also promotes the migration of keratinocyte
Keratinocyte
Keratinocytes are the predominant cell type in the epidermis, the outermost layer of the human skin, constituting 95% of the cells found there. Those keratinocytes found in the basal layer of the skin are sometimes referred to as "basal cells" or "basal keratinocytes"...
s and the restoration of the epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
.
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
Chondrocyte
Chondrocytes are the only cells found in cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans...
survival, allowing the cells to adapt to low-oxygen conditions within the growth plates
Endochondral ossification
Endochondral ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is created. Unlike intramembranous ossification, which is the other process by which bone tissue is created, cartilage is present during endochondral...
of bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s. HIF plays a central role in the regulation of human metabolism.
Mechanism
The alpha subunits of HIF are hydroxylated at conserved prolineProline
Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
residues by HIF prolyl-hydroxylase
EGLN2
Egl nine homolog 2 is a protein that in humans is encoded by the EGLN2 gene.-Further reading:...
s, allowing their recognition and ubiquitination by the VHL E3 ubiquitin ligase, which labels them for rapid degradation by the proteasome
Proteasome
Proteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
. This occurs only in normoxic conditions. In hypoxic conditions, HIF prolyl-hydroxylase is inhibited, since it utilizes oxygen as a cosubstrate.
Hypoxia also results in a buildup of succinate, due to inhibition of the electron transport chain in the mitochondria. The buildup of succinate further inhibits HIF prolyl-hydroxylase action, since it is an end-product of HIF hydroxylation.
In a similar manner, inhibition of electron transfer in the succinate dehydrogenase complex due to mutations in the SDHB
SDHB
Succinate dehydrogenase [ubiquinone] iron-sulfur subunit, mitochondrial also known as iron-sulfur subunit of complex II is a protein that in humans is encoded by the SDHB gene....
or SDHD
SDHD
Succinate dehydrogenase [ubiquinone] cytochrome b small subunit, mitochondrial , also known as succinate dehydrogenase complex subunit D , is a protein that in humans is encoded by the SDHD gene....
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
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
enzymes, which allow ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
synthesis in an oxygen-independent manner, and vascular endothelial growth factor
Vascular endothelial growth factor
Vascular endothelial growth factor is a signal protein produced by cells that stimulates vasculogenesis and angiogenesis. It is part of the system that restores the oxygen supply to tissues when blood circulation is inadequate....
(VEGF), which promotes angiogenesis
Angiogenesis
Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for the formation of new blood...
. HIF-1 acts by binding to HIF-responsive elements (HREs) in promoters that contain the sequence NCGTG.
It has been shown that muscle A kinase–anchoring protein
AKAP
The A-kinase anchor proteins are a group of structurally diverse proteins, which have the common function of binding to the regulatory subunit of protein kinase A and confining the holoenzyme to discrete locations within the cell. At least 20 AKAPs have been cloned. There are at least 50 members...
(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.
Anemia
Recently several drugs have been developed which act as selective HIF prolyl-hydroxylase inhibitorHIF prolyl-hydroxylase inhibitor
HIF prolyl-hydroxylase inhibitors are members of a class of new drugs that act on the hypoxia-inducible factor pathway. They are being investigated for treatment of anemia, chronic kidney disease, cancer, and for their effects on heart tissue and others....
s. The most notable of these include FibroGen's compounds FG-2216 and FG-4592, both are intended as orally acting drugs for use in the treatment of forms of anemia
Anemia
Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...
. By inhibiting HIF prolyl-hydroxylase, the activity of HIF-1α in the bloodstream is prolonged, which results in an increase in endogenous production of erythropoetin. Both of these drugs made it through to phase II clinical trials, but these were suspended temporarily in May 2007 following the death of a trial participant from fulminant hepatitis
Hepatitis
Hepatitis is a medical condition defined by the inflammation of the liver and characterized by the presence of inflammatory cells in the tissue of the organ. The name is from the Greek hepar , the root being hepat- , meaning liver, and suffix -itis, meaning "inflammation"...
. However it is unclear whether this death was caused by FG-2216. The hold has been lifted in early 2008 as FDA has reviewed and approved a thorough response from FibroGen.
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 factorTranscription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
s. Consequently, 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. The results of a recently published study have numerous implications for a number of pathologies where NF-κB and HIF-1 are deregulated, including rheumatoid arthritis
Rheumatoid arthritis
Rheumatoid arthritis is a chronic, systemic inflammatory disorder that may affect many tissues and organs, but principally attacks synovial joints. The process produces an inflammatory response of the synovium secondary to hyperplasia of synovial cells, excess synovial fluid, and the development...
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
Angiogenesis
Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for the formation of new blood...
required for cancer tumor growth, so HIF inhibitors are under investigation for anti-cancer effects, (e.g. phenethyl isothiocyanate (PEITC)).