Aryl hydrocarbon receptor
Encyclopedia
The Aryl hydrocarbon receptor (AhR or AHR) is a member of the family of basic-helix-loop-helix
transcription factor
s. AhR is a cytosolic transcription factor that is normally inactive, bound to several co-chaperone
s. Upon ligand
binding to chemicals such as 2,3,7,8-tetrachlorodibenzo-p-dioxin
(TCDD), the chaperones dissociate
resulting in AhR translocating into the nucleus
and dimer
izing with ARNT (AhR nuclear translocator)
, leading to changes in gene
transcription
.
The AhR protein
contains several domains critical for function and is classified as a member of the basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) family of transcription factors. The bHLH motif is located in the N-terminal of the protein and is a common entity in a variety of transcription factors. Members of the bHLH superfamily have two functionally distinctive and highly conserved domains. The first is the basic-region (b), which is involved in the binding of the transcription factor to DNA
. The second is the helix-loop-helix (HLH) region, which facilitates protein-protein interactions. Also contained with the AhR are two PAS domains, PAS-A and PAS-B, which are stretches of 200-350 amino acids that exhibit a high sequence homology to the protein domains that were originally found in the Drosophila
genes period (Per) and single-minded (Sim) and in AhR’s dimerization partner the aryl hydrocarbon receptor nuclear translocator
(ARNT). The PAS domains support specific secondary interactions with other PAS domain containing proteins, as is the case with AhR and ARNT, so that heterozygous and homozygous protein complexes can form. The ligand binding site of AhR is contained within the PAS-B domain and contains several conserved residues critical for ligand binding. Finally, a Q-rich domain is located in the C-terminal region of the protein and is involved in co-activator recruitment and transactivation.
s and biphenyls
) and polycyclic aromatic hydrocarbon
s (3-methylcholanthrene, benzo(a)pyrene, benzanthracenes
and benzoflavones
). However, recent work has focused on naturally occurring compounds with the hope of identifying an endogenous ligand.
Naturally occurring compounds that have been identified as ligands of Ahr include derivatives of tryptophan
such as indigo dye
and indirubin, tetrapyroles such as bilirubin
, the arachidonic acid
metabolites lipoxin A4
and prostaglandin G
, modified low-density lipoprotein and several dietary carotinoids. One assumption made in the search for an endogenous ligand is that the ligand will be a receptor agonist
. However, work by Savouret et al. has shown this may not be the case since their findings demonstrate that 7-ketocholesterol competitively inhibits Ahr signal transduction.
as an inactive protein
complex consisting of a dimer of Hsp90
, prostaglandin E synthase 3 (Ptges3, p23) and a single molecule of the immunophilin-like protein hepatitis B virus X-associated protein 2 (XAP2), which was previously identified as AhR interacting protein (AIP) and AhR-activated 9 (ARA9). The dimer of Hsp90, along with p23, has a multifunctional role in the protection of the receptor from proteolysis, constraining the receptor in a conformation receptive to ligand binding and preventing the premature binding of ARNT. XAP2 interacts with carboxyl-terminal of Hsp90 and binds to the AhR nuclear localization sequence (NLS) preventing the inappropriate trafficking of the receptor into the nucleus.
, spineless (ss) is necessary for development of the distal segments of the antenna and leg. Ss dimerizes with tango (tgo), which is the homolog to the mammalian Arnt, to initiate gene transcription. Evolution
of the receptor in vertebrates resulted in the ability to bind ligand. In developing vertebrates, Ahr seemingly plays a role in cellular proliferation and differentiation. Despite lacking a clear endogenous ligand, AHR appears to play a role in the differentiation of many developmental pathways, including T-cells , neurons , and hepatocytes . AhR has also been found to have an important function in hematopoietic stem cells: AhR antagonism promotes their self-renewal and ex-vivo expansion and is involved in megakaryocyte differentiation .
, CYP1A2
, CYP1B1
, NQO1, ALDH3A1, UGT1A2 and GSTA1. Presumably, vertebrates have this function to be able to detect a wide range of chemicals, indicated by the wide range of substrates Ahr is able to bind and facilitate their biotransformation
and elimination.
and CYP1B1
in several tissues. The second approach to toxicity is the result of aberrant changes in global gene transcription beyond those observed in the “Ahr gene battery.” These global changes in gene expression lead to adverse changes in cellular processes and function. Microarray analysis
has proved most beneficial in understanding and characterizing this response.
with RELA
, cyclin T1,SRC-1, retinoblastoma protein
, NRIP1
, estrogen receptor alpha
, NEDD8
and ARNTL
.
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...
transcription factor
Transcription 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. AhR is a cytosolic transcription factor that is normally inactive, bound to several co-chaperone
Co-chaperone
Co-chaperones are proteins that assist chaperones in protein folding and other functions.-List of Co-chaperones:*Aha1*auxilin*BAG1*CAIR-1/Bag-3*CDC37/p50*Chp1*Cysteine string protein *Cyp40*Djp1*DnaJ*E3/E4-ubiquitin ligase*FKBP52*GAK*GroES...
s. Upon ligand
Agonist
An agonist is a chemical that binds to a receptor of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance...
binding to chemicals such as 2,3,7,8-tetrachlorodibenzo-p-dioxin
2,3,7,8-Tetrachlorodibenzodioxin
2,3,7,8-Tetrachlorodibenzo-p-dioxin is a polychlorinated dibenzo-p-dioxin . It is the most potent compound of the series and became known as a contaminant in Agent Orange, a herbicide used in the Vietnam War, as well as the Seveso disaster...
(TCDD), the chaperones dissociate
Dissociation (chemistry)
Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...
resulting in AhR translocating into the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
and dimer
Protein dimer
In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids...
izing with ARNT (AhR 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,...
, leading to changes in gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
.
Protein functional domains
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
contains several domains critical for function and is classified as a member of the basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) family of transcription factors. The bHLH motif is located in the N-terminal of the protein and is a common entity in a variety of transcription factors. Members of the bHLH superfamily have two functionally distinctive and highly conserved domains. The first is the basic-region (b), which is involved in the binding of the transcription factor to DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
. The second is the helix-loop-helix (HLH) region, which facilitates protein-protein interactions. Also contained with the AhR are two PAS domains, PAS-A and PAS-B, which are stretches of 200-350 amino acids that exhibit a high sequence homology to the protein domains that were originally found in the Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
genes period (Per) and single-minded (Sim) and in AhR’s dimerization partner the 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,...
(ARNT). The PAS domains support specific secondary interactions with other PAS domain containing proteins, as is the case with AhR and ARNT, so that heterozygous and homozygous protein complexes can form. The ligand binding site of AhR is contained within the PAS-B domain and contains several conserved residues critical for ligand binding. Finally, a Q-rich domain is located in the C-terminal region of the protein and is involved in co-activator recruitment and transactivation.
Ligands
Ahr ligands have been generally classified into two categories, synthetic or naturally occurring. The first ligands to be discovered were synthetic and members of the halogenated aromatic hydrocarbons (polychlorinated dibenzodioxins, dibenzofuranDibenzofuran
Dibenzofuran, is a heterocyclic organic compound with the chemical structure shown at right. It is an aromatic compound that has two benzene rings fused to one furan ring in the middle. All of the numbered carbon atoms have a hydrogen atom bonded to each of them...
s and biphenyls
Polychlorinated biphenyl
Polychlorinated biphenyls are a class of organic compounds with 2 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. The chemical formula for PCBs is C12H10-xClx...
) and polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...
s (3-methylcholanthrene, benzo(a)pyrene, benzanthracenes
Tetracene
Tetracene, also called naphthacene, is a polycyclic aromatic hydrocarbon. It has the appearance of a pale orange powder. Tetracene is the four-ringed member of the series of acenes, the previous one being anthracene and the next one being pentacene.Tetracene is a molecular organic semiconductor,...
and benzoflavones
Beta-Naphthoflavone
beta-Naphthoflavone, also known as 5,6-benzoflavone, is a potent agonist of the aryl hydrocarbon receptor and as such is an inducer of such detoxification enzymes as cytochromes P450 and uridine 5'-diphospho-glucuronosyltransferases . β-Naphthoflavone is a putative chemopreventive agent....
). However, recent work has focused on naturally occurring compounds with the hope of identifying an endogenous ligand.
Naturally occurring compounds that have been identified as ligands of Ahr include derivatives of tryptophan
Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
such as indigo dye
Indigo dye
Indigo dye is an organic compound with a distinctive blue color . Historically, indigo was a natural dye extracted from plants, and this process was important economically because blue dyes were once rare. Nearly all indigo dye produced today — several thousand tons each year — is synthetic...
and indirubin, tetrapyroles such as bilirubin
Bilirubin
Bilirubin is the yellow breakdown product of normal heme catabolism. Heme is found in hemoglobin, a principal component of red blood cells. Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases...
, the arachidonic acid
Arachidonic acid
Arachidonic acid is a polyunsaturated omega-6 fatty acid 20:4.It is the counterpart to the saturated arachidic acid found in peanut oil, Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid 20:4(ω-6).It is the counterpart to the saturated arachidic acid found in peanut oil,...
metabolites lipoxin A4
Lipoxin
Lipoxins are a series of anti-inflammatory mediators. Lipoxins are short lived endogenously produced nonclassic eicosanoids whose appearance in inflammation signals the resolution of inflammation....
and prostaglandin G
Prostaglandin
A prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring....
, modified low-density lipoprotein and several dietary carotinoids. One assumption made in the search for an endogenous ligand is that the ligand will be a receptor agonist
Agonist
An agonist is a chemical that binds to a receptor of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance...
. However, work by Savouret et al. has shown this may not be the case since their findings demonstrate that 7-ketocholesterol competitively inhibits Ahr signal transduction.
Signaling pathway
Cytosolic complex
Non-ligand bound Ahr is retained in the cytoplasmCytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
as an inactive protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
complex consisting of a dimer of Hsp90
Hsp90
Hsp90 is a molecular chaperone and is one of the most abundant proteins expressed in cells. It is a member of the heat shock protein family, which is upregulated in response to stress...
, prostaglandin E synthase 3 (Ptges3, p23) and a single molecule of the immunophilin-like protein hepatitis B virus X-associated protein 2 (XAP2), which was previously identified as AhR interacting protein (AIP) and AhR-activated 9 (ARA9). The dimer of Hsp90, along with p23, has a multifunctional role in the protection of the receptor from proteolysis, constraining the receptor in a conformation receptive to ligand binding and preventing the premature binding of ARNT. XAP2 interacts with carboxyl-terminal of Hsp90 and binds to the AhR nuclear localization sequence (NLS) preventing the inappropriate trafficking of the receptor into the nucleus.
Receptor activation
Upon ligand binding to AhR, XAP2 is released resulting in exposure of the NLS, which is located in the bHLH region, leading to importation into the nucleus. It is presumed that once in the nucleus, Hsp90 dissociates exposing the two PAS domains allowing the binding of ARNT. The activated AhR/ARNT heterodimer complex is then capable of either directly and indirectly interacting with DNA by binding to recognition sequences located in the 5’- regulatory region of dioxin-responsive genes.DNA binding (xenobiotic response element - XRE)
The classical recognition motif of the AhR/ARNT complex, referred to as either the AhR-, dioxin- or xenobiotic- responsive element (AHRE, DRE or XRE), contains the core sequence 5’-GCGTG-3’ within the consensus sequence 5’-T/GNGCGTGA/CG/CA-3’ in the promoter region of AhR responsive genes. The AhR/ARNT heterodimer directly binds the AHRE/DRE/XRE core sequence in an asymmetric manner such that ARNT binds to 5’-GTG-3’ and AhR binding 5’-TC/TGC-3’. Recent research suggests that a second type of element termed AHRE-II, 5’-CATG(N6)C[T/A]TG-3’, is capable of indirectly acting with the AhR/ARNT complex. Regardless of the response element, the end result is a variety of differential changes in gene expression.Role in development
In terms of evolution, the oldest physiological role of Ahr is in development. Ahr is presumed to have evolved from invertebrates where it served a ligand-independent role in normal development processes. The Ahr homolog in DrosophilaDrosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
, spineless (ss) is necessary for development of the distal segments of the antenna and leg. Ss dimerizes with tango (tgo), which is the homolog to the mammalian Arnt, to initiate gene transcription. Evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of the receptor in vertebrates resulted in the ability to bind ligand. In developing vertebrates, Ahr seemingly plays a role in cellular proliferation and differentiation. Despite lacking a clear endogenous ligand, AHR appears to play a role in the differentiation of many developmental pathways, including T-cells , neurons , and hepatocytes . AhR has also been found to have an important function in hematopoietic stem cells: AhR antagonism promotes their self-renewal and ex-vivo expansion and is involved in megakaryocyte differentiation .
Adaptive response
The adaptive response is manifested as the induction of xenobiotic metabolizing enzymes. Evidence of this response was first observed from the induction of cytochrome P450, family 1, subfamily A, polypeptide 1 (Cyp1a1) resultant from TCDD exposure, which was determined to be directly related to activation of the Ahr signaling pathway. The search for other metabolizing genes induced by Ahr ligands, due to the presence of DREs, has led to the identification of an “Ahr gene battery” of Phase I and Phase II metabolizing enzymes consisting of CYP1A1CYP1A1
Cytochrome P450, family 1, subfamily A, polypeptide 1 is a protein that in humans is encoded by the CYP1A1 gene. The protein is a member of the cytochrome P450 superfamily of enzymes.-Function:...
, CYP1A2
CYP1A2
Cytochrome P450 1A2 , a member of the cytochrome P450 mixed-function oxidase system, is involved in the metabolism of xenobiotics in the body...
, CYP1B1
CYP1B1
Cytochrome P450 1B1 is an enzyme that in humans is encoded by the CYP1B1 gene.- Function :CYP1B1 belongs to the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids...
, NQO1, ALDH3A1, UGT1A2 and GSTA1. Presumably, vertebrates have this function to be able to detect a wide range of chemicals, indicated by the wide range of substrates Ahr is able to bind and facilitate their biotransformation
Biotransformation
Biotransformation is the chemical modification made by an organism on a chemical compound. If this modification ends in mineral compounds like CO2, NH4+, or H2O, the biotransformation is called mineralisation....
and elimination.
Toxic response
Extensions of the adaptive response are the toxic responses elicited by Ahr activation. Toxicity results from two different ways of Ahr signaling. The first is a function of the adaptive response in which the induction of metabolizing enzymes results in the production of toxic metabolites. For example, the polycyclic aromatic hydrocarbon benzo(a)pyrene (BaP), a ligand for Ahr, induces its own metabolism and bioactivation to a toxic metabolite via the induction of CYP1A1CYP1A1
Cytochrome P450, family 1, subfamily A, polypeptide 1 is a protein that in humans is encoded by the CYP1A1 gene. The protein is a member of the cytochrome P450 superfamily of enzymes.-Function:...
and CYP1B1
CYP1B1
Cytochrome P450 1B1 is an enzyme that in humans is encoded by the CYP1B1 gene.- Function :CYP1B1 belongs to the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids...
in several tissues. The second approach to toxicity is the result of aberrant changes in global gene transcription beyond those observed in the “Ahr gene battery.” These global changes in gene expression lead to adverse changes in cellular processes and function. Microarray analysis
DNA microarray
A DNA microarray is a collection of microscopic DNA spots attached to a solid surface. Scientists use DNA microarrays to measure the expression levels of large numbers of genes simultaneously or to genotype multiple regions of a genome...
has proved most beneficial in understanding and characterizing this response.
Protein-protein interactions
In addition to the protein interactions mentioned above, AhR has also been shown to interactProtein-protein interaction
Protein–protein interactions occur when two or more proteins bind together, often to carry out their biological function. Many of the most important molecular processes in the cell such as DNA replication are carried out by large molecular machines that are built from a large number of protein...
with RELA
RELA
Transcription factor p65 is a protein that in humans is encoded by the RELA gene.-Interactions:RELA has been shown to interact with NFKBIB, ETHE1, NFKBIE, RFC1, TRIB3, CREB binding protein, Neutrophil cytosolic factor 1, Glucocorticoid receptor, MTPN, BRCA1, C-Fos, POU2F1, BTRC, TATA-binding...
, cyclin T1,SRC-1, retinoblastoma protein
Retinoblastoma protein
The retinoblastoma protein is a tumor suppressor protein that is dysfunctional in the majority types of cancer. One highly studied function of pRb is to prevent excessive cell growth by inhibiting cell cycle progression until a cell is ready to divide...
, NRIP1
NRIP1
Nuclear receptor-interacting protein 1 is a protein that in humans is encoded by the NRIP1 gene.RIP140 has an important role in regulating lipid and glucose metabolism, and regulates gene expression in metabolic tissues including heart, skeletal muscle, and liver.Knockout mice that completely lack...
, estrogen receptor alpha
Estrogen receptor alpha
Estrogen receptor alpha , also known as NR3A1 , is a nuclear receptor that is activated by the sex hormone estrogen...
, NEDD8
NEDD8
NEDD8 is a protein that in humans is encoded by the NEDD8 gene. This ubiquitin-like protein , which becomes covalently conjugated to a limited number of cellular proteins in a manner analogous to ubiquitination. Human NEDD8 shares 60% amino acid sequence identity to ubiquitin...
and ARNTL
ARNTL
Aryl hydrocarbon receptor nuclear translocator-like, also known as ARNTL, Bmal1, or Mop3, is a gene which is associated with susceptibility to hypertension and type 2 diabetes.-Function:...
.