Hypoxia-inducible factor 1-alpha, also known as HIF-1-alpha, is a subunit of a heterodimeric transcription factor hypoxia-inducible factor 1 (HIF-1) that is encoded by the HIF1A gene. [5] [6] [7] The Nobel Prize in Physiology or Medicine 2019 was awarded for the discovery of HIF.
HIF1A is a basic helix-loop-helix PAS domain containing protein, and is considered as the master transcriptional regulator of cellular and developmental response to hypoxia. [8] [9] The dysregulation and overexpression of HIF1A by either hypoxia or genetic alternations have been heavily implicated in cancer biology, as well as a number of other pathophysiologies, specifically in areas of vascularization and angiogenesis, energy metabolism, cell survival, and tumor invasion. [7] [10] The presence of HIF1A in a hypoxic environment is required to push forward normal placental development in early gestation. [11] Two other alternative transcripts encoding different isoforms have been identified. [7]
HIF1 is a heterodimeric basic helix-loop-helix structure [12] that is composed of HIF1A, the alpha subunit (this protein), and the aryl hydrocarbon receptor nuclear translocator (Arnt), the beta subunit. HIF1A contains a basic helix-loop-helix domain near the C-terminal, followed by two distinct PAS (PER-ARNT-SIM) domains, and a PAC (PAS-associated C-terminal) domain. [8] [6] The HIF1A polypeptide also contains a nuclear localization signal motif, two transactivating domains CTAD and NTAD, and an intervening inhibitory domain (ID) that can repress the transcriptional activities of CTAD and NTAD. [13] There are a total of three HIF1A isoforms formed by alternative splicing, however isoform1 has been chosen as the canonical structure, and is the most extensively studied isoform in structure and function. [14] [15]
The human HIF1A gene encodes for the alpha subunit, HIF1A of the transcription factor hypoxia-inducible factor (HIF1). [16] Its protein expression level can be measured by antibodies against HIF-1-alpha through various biological detection methods including western blot or immunostaining. [17] HIF1A expression level is dependent on its GC-rich promoter activation. [18] In most cells, HIF1A gene is constitutively expressed in low levels under normoxic conditions, however, under hypoxia, HIF1A transcription is often significantly upregulated. [18] [19] [20] [21] [22] [23] Typically, oxygen-independent pathway regulates protein expression, and oxygen-dependent pathway regulates degradation. [10] In hypoxia-independent ways, HIF1A expression may be upregulated through a redox-sensitive mechanism. [24]
The transcription factor HIF-1 plays an important role in cellular response to systemic oxygen levels in mammals. [25] [26] HIF1A activity is regulated by a host of post-translational modifications: hydroxylation, acetylation, and phosphorylation. [27] HIF-1 is known to induce transcription of more than 60 genes, including VEGF and erythropoietin that are involved in biological processes such as angiogenesis and erythropoiesis, which assist in promoting and increasing oxygen delivery to hypoxic regions. [10] [28] [27] HIF-1 also induces transcription of genes involved in cell proliferation and survival, as well as glucose and iron metabolism. [27] In accordance with its dynamic biological role, HIF-1 responds to systemic oxygen levels by undergoing conformational changes, and associates with HRE regions of promoters of hypoxia-responsive genes to induce transcription. [29] [30] [31] [32] [33]
HIF1A stability, subcellular localization, as well as transcriptional activity are especially affected by oxygen level. The alpha subunit forms a heterodimer with the beta subunit. Under normoxic conditions, VHL-mediated ubiquitin protease pathway rapidly degrades HIF1A; however, under hypoxia, HIF1A protein degradation is prevented and HIF1A levels accumulate to associate with HIF1B to exert transcriptional roles on target genes [34] [35] Enzymes prolyl hydroxylase (PHD) and HIF prolyl hydroxylase (HPH) are involved in specific post-translational modification of HIF1A proline residues (P402 and P564 within the ODD domain), which allows for VHL association with HIF1A. [33] The enzymatic activity of oxygen sensor dioxygenase PHD is dependent on oxygen level as it requires oxygen as one of its main substrates to transfer to the proline residue of HIF1A. [30] [36] The hydroxylated proline residue of HIF1A is then recognized and buried in the hydrophobic core of von Hippel-Lindau tumor suppressor protein (VHL), which itself is part of a ubiquitin ligase enzyme. [37] [38] Once the hydrolylated HIF1A is buried in the VHL protein, VHL will transport it to a proteasome to digest and destroy HIF1A. This prevents HIF1A from entering into the cell nucleus to carry out the transcription of many different regulatory pathways. Many of these pathways are necessary for proper placental development in early gestation. Under normoxic conditions the HIF1A will be hydroxylated and destroyed, which leads to placental tissue necrosis, disorganization, and overgrowth. [39] [40] The hydroxylation of HIF1A proline residue also regulates its ability to associate with co-activators under hypoxia. [41] [42] Function of HIF1A gene can be effectively examined by siRNA knockdown based on an independent validation. [43]
In normal circumstances after injury HIF1A is degraded by prolyl hydroxylases (PHDs). In June 2015, scientists found that the continued up-regulation of HIF1A via PHD inhibitors regenerates lost or damaged tissue in mammals that have a repair response; and the continued down-regulation of HIF1A results in healing with a scarring response in mammals with a previous regenerative response to the loss of tissue. The act of regulating HIF1A can either turn off, or turn on the key processes of mammalian regeneration. [44] [45] One such regenerative process in which HIF1A is involved is peripheral nerve regeneration. Following axon injury, HIF1A activates VEGFA to promote regeneration and functional recovery. [46] [47] HIF1A also controls skin healing. [48] Researchers at the Stanford University School of Medicine demonstrated that HIF1A activation was able to prevent and treat chronic wounds in diabetic and aged mice. Not only did the wounds in the mice heal more quickly, but the quality of the new skin was even better than the original. [49] [50] [51] [52] Additionally the regenerative effect of HIF-1A modulation on aged skin cells was described [53] [54] and a rejuvenating effect on aged facial skin was demonstrated in patients. [55] HIF modulation has also been linked to a beneficial effect on hair loss. [56] The biotech company Tomorrowlabs GmbH, founded in Vienna in 2016 by the physician Dominik Duscher and pharmacologist Dominik Thor, makes use of this mechanism. [57] Based on the patent-pending HSF ("HIF strengthening factor") active ingredient, products have been developed that are supposed to promote skin and hair regeneration. [58] [59] [60] [61]
HIF1A abundance (and its subsequent activity) is regulated transcriptionally in an NF-κB-dependent manner. [62] [63] In addition, the coordinated activity of the prolyl hydroxylases (PHDs) maintain the appropriate balance of HIF1A protein in the post-translation phase. [64]
PHDs rely on iron among other molecules to hydroxylate HIF1A; as such, iron chelators such as desferrioxamine (DFO) have proven successful in HIF1A stabilization. [65] HBO (Hyperbaric oxygen therapy) and HIF1A imitators such as cobalt chloride have also been successfully utilized. [65]
Factors increasing HIF1A [66]
Factors decreasing HIF1A [66]
HIF1A is overexpressed in many human cancers. [67] [68] HIF1A overexpression is heavily implicated in promoting tumor growth and metastasis through its role in initiating angiogenesis and regulating cellular metabolism to overcome hypoxia. [69] Hypoxia promotes apoptosis in both normal and tumor cells. [70] However, hypoxic conditions in tumor microenvironment especially, along with accumulation of genetic alternations often contribute to HIF1A overexpression. [10]
Significant HIF1A expression has been noted in most solid tumors studied, which include cancers of the gastric, colon, breast, pancreas, kidneys, prostate, ovary, brain, and bladder. [71] [68] [67] Clinically, elevated HIF1A levels in a number of cancers, including cervical cancer, non-small-cell lung carcinoma, breast cancer (LV-positive and negative), oligodendroglioma, oropharyngeal cancer, ovarian cancer, endometrial cancer, esophageal cancer, head and neck cancer, and stomach cancer, have been associated with aggressive tumor progression, and thus has been implicated as a predictive and prognostic marker for resistance to radiation treatment, chemotherapy, and increased mortality. [10] [72] [73] [74] [75] [71] [76] HIF1A expression may also regulate breast tumor progression. Elevated HIF1A levels may be detected in early cancer development, and have been found in early ductal carcinoma in situ, a pre-invasive stage in breast cancer development, and is also associated with increased microvasculature density in tumor lesions. [77] Moreover, despite histologically-determined low-grade, lymph-node negative breast tumor in a subset of patients examined, detection of significant HIF1A expression was able to independently predict poor response to therapy. [69] Similar findings have been reported in brain cancer and ovarian cancer studies as well, and suggest at regulatory role of HIF1A in initiating angiogenesis through interactions with pro-angiogenic factors such as VEGF. [75] [78] Studies of glioblastoma multiforme show striking similarity between HIF1A expression pattern and that of VEGF gene transcription level. [79] [80] In addition, high-grade glioblastoma multiform tumors with high VEGF expression pattern, similar to breast cancer with HIF1A overexpression, display significant signs of tumor neovascularization. [81] This further suggests the regulatory role of HIF1A in promoting tumor progression, likely through hypoxia-induced VEGF expression pathways. [80] [71]
HIF1A overexpression in tumors may also occur in a hypoxia-independent pathway. In hemangioblastoma, HIF1A expression is found in most cells sampled from the well-vascularized tumor. [82] Although in both renal carcinoma and hemangioblastoma, the von Hippel-Lindau gene is inactivated, HIF1A is still expressed at high levels. [78] [82] [67] In addition to VEGF overexpression in response elevated HIF1A levels, the PI3K/AKT pathway is also involved in tumor growth. In prostate cancers, the commonly occurring PTEN mutation is associated with tumor progression toward aggressive stage, increased vascular density and angiogenesis. [83]
During hypoxia, tumor suppressor p53 overexpression may be associated with HIF1A-dependent pathway to initiate apoptosis. Moreover, p53-independent pathway may also induce apoptosis through the Bcl-2 pathway. [70] However, overexpression of HIF1A is cancer- and individual-specific, and depends on the accompanying genetic alternations and levels of pro- and anti-apoptotic factors present. One study on epithelial ovarian cancer shows HIF1A and nonfunctional tumor suppressor p53 is correlated with low levels of tumor cell apoptosis and poor prognosis. [75] Further, early-stage esophageal cancer patients with demonstrated overexpression of HIF1 and absence of BCL2 expression also failed photodynamic therapy. [84]
While research efforts to develop therapeutic drugs to target hypoxia-associated tumor cells have been ongoing for many years, there has not yet been any breakthrough that has shown selectivity and effectiveness at targeting HIF1A pathways to decrease tumor progression and angiogenesis. [85] Successful therapeutic approaches in the future may also be highly case-specific to particular cancers and individuals, and seem unlikely to be widely applicable due to the genetically heterogenous nature of the many cancer types and subtypes.
HIF1A has been shown to interact with:
Tumor hypoxia is the situation where tumor cells have been deprived of oxygen. As a tumor grows, it rapidly outgrows its blood supply, leaving portions of the tumor with regions where the oxygen concentration is significantly lower than in healthy tissues. Hypoxic microenvironments in solid tumors are a result of available oxygen being consumed within 70 to 150 μm of tumor vasculature by rapidly proliferating tumor cells thus limiting the amount of oxygen available to diffuse further into the tumor tissue. In order to support continuous growth and proliferation in challenging hypoxic environments, cancer cells are found to alter their metabolism. Furthermore, hypoxia is known to change cell behavior and is associated with extracellular matrix remodeling and increased migratory and metastatic behavior.
Hypoxia-inducible factors (HIFs) are transcription factors that respond to decreases in available oxygen in the cellular environment, or hypoxia. They also respond to instances of pseudohypoxia, such as thiamine deficiency. Both hypoxia and pseudohypoxia leads to impairment of adenosine triphosphate (ATP) production by the mitochondria.
A basic helix–loop–helix (bHLH) is a protein structural motif that characterizes one of the largest families of dimerizing transcription factors. The word "basic" does not refer to complexity but to the chemistry of the motif because transcription factors in general contain basic amino acid residues in order to facilitate DNA binding.
The study of the tumor metabolism, also known as tumor metabolome describes the different characteristic metabolic changes in tumor cells. The characteristic attributes of the tumor metabolome are high glycolytic enzyme activities, the expression of the pyruvate kinase isoenzyme type M2, increased channeling of glucose carbons into synthetic processes, such as nucleic acid, amino acid and phospholipid synthesis, a high rate of pyrimidine and purine de novo synthesis, a low ratio of Adenosine triphosphate and Guanosine triphosphate to Cytidine triphosphate and Uridine triphosphate, low Adenosine monophosphate levels, high glutaminolytic capacities, release of immunosuppressive substances and dependency on methionine.
The ARNT gene encodes the aryl hydrocarbon receptor nuclear translocator protein that forms a complex with ligand-bound aryl hydrocarbon receptor (AhR), and is required for receptor function. The encoded protein has also been identified as the beta subunit of a heterodimeric transcription factor, hypoxia-inducible factor 1 (HIF1). A t(1;12)(q21;p13) translocation, which results in a TEL–ARNT fusion protein, is associated with acute myeloblastic leukemia. Three alternatively spliced variants encoding different isoforms have been described for this gene.
The Von Hippel–Lindau tumor suppressor also known as pVHL is a protein that, in humans, is encoded by the VHL gene. Mutations of the VHL gene are associated with Von Hippel–Lindau disease, which is characterized by hemangioblastomas of the brain, spinal cord and retina. It is also associated with kidney and pancreatic lesions.
Temsirolimus, sold under the brand name Torisel, is an intravenous drug for the treatment of renal cell carcinoma (RCC), developed by Wyeth Pharmaceuticals and approved by the U.S. Food and Drug Administration (FDA) in May 2007, and was also approved by the European Medicines Agency (EMA) in November 2007. It is a derivative and prodrug of sirolimus.
HNF1 homeobox A, also known as HNF1A, is a human gene on chromosome 12. It is ubiquitously expressed in many tissues and cell types. The protein encoded by this gene is a transcription factor that is highly expressed in the liver and is involved in the regulation of the expression of several liver-specific genes. Mutations in the HNF1A gene have been known to cause diabetes. The HNF1A gene also contains a SNP associated with increased risk of coronary artery disease.
Metastasis-associated protein MTA1 is a protein that in humans is encoded by the MTA1 gene. MTA1 is the founding member of the MTA family of genes. MTA1 is primarily localized in the nucleus but also found to be distributed in the extra-nuclear compartments. MTA1 is a component of several chromatin remodeling complexes including the nucleosome remodeling and deacetylation complex (NuRD). MTA1 regulates gene expression by functioning as a coregulator to integrate DNA-interacting factors to gene activity. MTA1 participates in physiological functions in the normal and cancer cells. MTA1 is one of the most upregulated proteins in human cancer and associates with cancer progression, aggressive phenotypes, and poor prognosis of cancer patients.
Endothelial PAS domain-containing protein 1 is a protein that is encoded by the EPAS1 gene in mammals. It is a type of hypoxia-inducible factor, a group of transcription factors involved in the physiological response to oxygen concentration. The gene is active under hypoxic conditions. It is also important in the development of the heart, and for maintaining the catecholamine balance required for protection of the heart. Mutation often leads to neuroendocrine tumors.
Hypoxia-inducible factor 3 alpha is a protein that in humans is encoded by the HIF3A gene.
Egl nine homolog 2 is a protein that in humans is encoded by the EGLN2 gene. ELGN2 is an alpha-ketoglutarate-dependent hydroxylase, a superfamily of non-haem iron-containing proteins.
Hypoxia-inducible factor prolyl hydroxylase 2 (HIF-PH2), or prolyl hydroxylase domain-containing protein 2 (PHD2), is an enzyme encoded by the EGLN1 gene. It is also known as Egl nine homolog 1. PHD2 is a α-ketoglutarate/2-oxoglutarate-dependent hydroxylase, a superfamily non-haem iron-containing proteins. In humans, PHD2 is one of the three isoforms of hypoxia-inducible factor-proline dioxygenase, which is also known as HIF prolyl-hydroxylase.
Egl nine homolog 3 is a protein that in humans is encoded by the EGLN3 gene. ELGN3 is a member of the superfamily of alpha-ketoglutarate-dependent hydroxylases, which are non-haem iron-containing proteins.
Hypoxia-inducible factor 1-alpha inhibitor (FIH) is a protein that in humans is encoded by the HIF1AN gene.
Protein OS-9 is a protein that in humans is encoded by the OS9 gene.
"Basic helix-loop-helix family, member e41", or BHLHE41, is a gene that encodes a basic helix-loop-helix transcription factor repressor protein in various tissues of both humans and mice. It is also known as DEC2, hDEC2, and SHARP1, and was previously known as "basic helix-loop-helix domain containing, class B, 3", or BHLHB3. BHLHE41 is known for its role in the circadian molecular mechanisms that influence sleep quantity as well as its role in immune function and the maturation of T helper type 2 cell lineages associated with humoral immunity.
Diamine acetyltransferase 2 is an enzyme that in humans is encoded by the SAT2 gene. SAT2 maintains a key metabolic glutamine/glutamate balance underpinning retrograde signaling by dendritic release of the neurotransmitter glutamate.
In molecular biology, TAZ zinc finger domains are zinc-containing domains found in the homologous transcriptional co-activators CREB-binding protein (CBP) and the P300. CBP and P300 are histone acetyltransferases that catalyse the reversible acetylation of all four histones in nucleosomes, acting to regulate transcription via chromatin remodelling. These large nuclear proteins interact with numerous transcription factors and viral oncoproteins, including p53 tumour suppressor protein, E1A oncoprotein, MyoD, and GATA-1, and are involved in cell growth, differentiation and apoptosis. Both CBP and P300 have two copies of the TAZ domain, one in the N-terminal region, the other in the C-terminal region. The TAZ1 domain of CBP and P300 forms a complex with CITED2, inhibiting the activity of the hypoxia inducible factor (HIF-1alpha) and thereby attenuating the cellular response to low tissue oxygen concentration. Adaptation to hypoxia is mediated by transactivation of hypoxia-responsive genes by hypoxia-inducible factor-1 (HIF-1) in complex with the CBP and p300 transcriptional coactivators.
Hypoxia-inducible factor-asparagine dioxygenase (EC 1.14.11.30, HIF hydroxylase) is an enzyme with systematic name hypoxia-inducible factor-L-asparagine, 2-oxoglutarate:oxygen oxidoreductase (4-hydroxylating). This enzyme catalyses the following chemical reaction:
hypoxia-inducible factor-L-asparagine + 2-oxoglutarate + O2 hypoxia-inducible factor-(3S)-3-hydroxy-L-asparagine + succinate + CO2