Hypoxia refers to a condition in which the body or a certain area of the body lacks sufficient oxygen supply at the tissue level. Hypoxia can be divided into systemic, affecting the entire body or local, affecting a region of the body. Although hypoxia is usually a pathological condition, changes in arterial oxygen concentration may be part of normal physiology, such as in hypoventilation training or strenuous exercise. Hypoxia Proteins, also known as Hypoxia-inducible factors (HIFs), are transcription factors that respond to reduced oxygen or hypoxia in the cellular environment.
HIF-1 originated from the increase in the number of red blood cells after the Viault alpine skiing in 1890. It was discovered by Semenza et al. in 1994 that hypoxia stimulates the expression of erythropoietin (EPO) gene in the kidney and is a DNA-binding protein widely present in chronic hypoxia. In the cell, the binding site is located in the first part of the 3' end enhancer of EPO and consists of 50 or so nucleotides. Therefore, named hypoxia inducing factor.
HIF-1 is thought to be a core regulator of hypoxia-producing genes and repairing the cellular oxygen environment. Oxygen directly interacts with oxygen receptors and is independent of mitochondrial respiration, keeping oxygen receptors in an inactive state; when oxygen is reduced, oxygen receptors activate a signal that leads to increased HIF-1/β expression via protein phosphorylation and/or redox. Reduced after reoxygenation. Heme-containing proteins are involved in oxygen sensing and signaling. Hypoxia or certain oxidative stress can provide information for hypoxia induction. Although all cells have the same pathway, the end point of response for each cell is different. Therefore, most cells induce glycolysis genes and various angiogenic factors during hypoxia, but the liver and kidney induce EPO. At present, little is known about the mechanism of hypoxia-induced gene expression, and the expression of HIF-1 is only a phenomenon in hypoxia. The “murderer” behind it remains to be ascertained. The gene expression induced thereby seems to be either to increase oxygen supply to hypoxic tissue or to reduce oxygen consumption and to maintain homeostasis.
Most respiratory oxygen species express the highly conserved transcriptional complex HIF-1, a heterodimer composed of alpha and beta subunits, a constitutively expressed aromatic hydrocarbon receptor nuclear transporter (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 subunits are structurally similar and each contain the following domains:
N-terminal – DNA-binding bHLH domain
Central region – each ARNT-Sim (PAS) domain promotes heterodimerization
C-terminal – recruitment of transcriptional regulatory proteins
Figure 1. Structure of a HIF-1a-pVHL-ElonginB-ElonginC Complex.
Hypoxia-inducible factor 1-alpha, also known as HIF-1-α, is a subunit of the heterodimeric transcription factor hypoxia inducible factor 1 (HIF-1) encoded by the HIF1A gene. It is a basic helix-loop-helix PAS domain containing proteins and is considered to be a major transcriptional regulator of hypoxic cellular and developmental responses. Hypoxia or genetic alternation leads to dysregulation and overexpression of HIF1A and is closely related to cancer biology and many other pathophysiology, particularly in terms of angiogenesis and angiogenesis, energy metabolism, cell survival and tumor invasion. Two other alternative transcripts encoding different isoforms have been identified.
Figure 2. Structure of the HIF1A protein. Based on PyMOL rendering of PDB 1h2k.
The ARNT gene encodes an aryl hydrocarbon receptor nuclear translocator that forms a complex with the ligand-bound aromatic hydrocarbon receptor (AhR) and is required for receptor function. The encoded protein was also identified as the beta subunit of the heterodimeric transcription factor beta hypoxia inducible factor 1 (HIF1). Three alternative splice variants encoding different isoforms have been described for this gene.
Figure 3. Structure of the ARNT protein. Based on PyMOL rendering of PDB 1x0o.
Endothelial PAS domain-containing protein 1 (EPAS1, also known as hypoxia inducible factor-2 alpha (HIF-2alpha)) is a protein encoded by the human EPAS1 gene. It is a hypoxia-inducible factor, a group of transcription factors involved in the physiological response of oxygen concentration. This gene is active under hypoxic conditions. It is also important for the development of the heart and the balance of catecholamines required to maintain heart protection. Mutations usually result in neuroendocrine tumors.
Figure 4. Structure of the EPAS1 protein. Based on PyMOL rendering of PDB 1p97.
The expression of HIF1α in hematopoietic stem cells explains the static properties of stem cells, which are maintained at a low rate of metabolism, thereby maintaining the efficacy of stem cells over the long-term life cycle of the organism. The HIF signaling cascade mediates the effects of hypoxia (low oxygen concentration state) on cells. Hypoxia often leaves cells undifferentiated. However, hypoxia promotes the formation of blood vessels and is important for the formation of vascular systems in embryos and tumors. Hypoxia in the wound also promotes migration of keratinocytes and recovery of the epithelium. Often, HIF is critical to development. In mammals, deletion of the HIF-1 gene results in perinatal death. HIF-1 has been shown to be critical for the survival of chondrocytes, which allows cells to adapt to hypoxic conditions in skeletal growth plates. HIF plays a central role in regulating human metabolism.
Inflammation and cancer
In other cases, contrary to the above-mentioned therapies, recent studies have shown that HIF induction under normoxic conditions may have a serious impact on diseases with chronic inflammatory components. It has also been shown that chronic inflammation is self-sustaining and deforms the microenvironment due to abnormally active transcription factors. As a result, changes in the balance of growth factors, chemokines, cytokines and ROS occur in the cellular environment, which in turn provides the growth and survival axes required for cancer and metastatic development from scratch. These results have many implications for many pathologies that cause NF-κB and HIF-1 to be out of control, such as rheumatoid arthritis and cancer. Therefore, it is believed that understanding the interaction between these two key transcription factors, NF-κB and HIF, will greatly enhance the drug development process.
1. Smith TG.; et al. The human side of hypoxia-inducible factor. British Journal of Haematology. 2008,141 (3): 325–34.
2. Wilkins SE.; et al. Targeting Protein-Protein Interactions in the HIF System. Human Genetics. ChemMedChem. 2016,11 (8): 773–86