Aryl-hydrocarbon Receptor: Signaling, Toxicity, and UDP
Transcript of Aryl-hydrocarbon Receptor: Signaling, Toxicity, and UDP
Aryl-hydrocarbon Receptor: Signaling, Toxicity, and UDP-
GlucuronosyltransferasesAnna Radominska-Pandya
Department of Biochemistry and Molecular Biology
University of Arkansas for Medical Sciences
Little Rock, Arkansas, US
October 2010; Gdansk University of Technology
Aryl-hydrocarbon Receptor (AhR)
• What is an ‘Aryl-hydrocarbon’?
– Aromatic hydrocarbon or arene
– “Cyclic molecule with characteristic electron density that is highly unsaturated and unexpectedly stable” (B&F, 1998)
• AhR is also know as the Dioxin Receptor
– Dioxin is the popular name for the family of chlorinated organic compounds consisting of:• Polychlorinated dibenzofurans (PCDFs)
• Polychlorinated dibenzodioxins (PCDDs)
– Dioxins have been shown to bioaccumulate in humans and wildlife due to their lipophilic properties.
-Aryl
Aryl-hydrocarbon Receptor (AhR)
• AhR was isolated in 1991
• Ligand-dependent transcription factor regulating gene expression
• Mediates toxicity of environmental contaminants exerting broad toxicological effects– Belongs to the basic helix-loop-helix/Per-Arnt-sim family
(bHLH/PAS) of chemosensors
– Highly conserved throughout evolution
– Expressed in:• Liver
• Central nervous system
• Reproductive organs
• Cardiovascular system
Aryl-hydrocarbon Receptor (AhR)
• Activated by a variety of environmental pollutants – Polycyclic aromatic hydrocarbons (PAHs)
• benzo[a]pyrene (BaP),
– Halogenated aromatic hydrocarbons (HAHs)• 2,3,7,8-tetrachlorobenzo-p-dioxin (TCDD)
– Polychlorinated biphenyls (PCBs)
• Natural endogenous ligands have not been identified– Potential candidates are:
• 7-keto-cholesterol
• Oxidative products of arachidonic acid
• Indols (products of the degradation of tryptophan),
• Retinoic acid
• Bilirubin
Exogenous ligand Structure example
Halogenated aromatic
hydrocarbons (HAH)TCDD
Polycyclic aromatic hydrocarbons
(PAH)
3-MC B[a]P
Flavonoids β-Naphthoflavone
Ligands for AhR
(Denison & Nagy, 2003)
Cl
ClO
OCl
Cl
CH3
O
O
Endogenous ligand Structure
2-(1’H-indole-3’-carbonyl)-thiazole-4-
carboxylic acid methyl ester
(Song et al., 2002) ITE
Tryptophan metabolites(?)
(Adachi et al., 2001)Indirubin
Bilirubin
Endogenous Ligands for AhR
NH O
CH3
HN
S
N O
O
O
AhR: Mechanisms of Toxicity
• Ligand biotransformation
• Induction/repression of genes
• Carcinogenesis
• Endocrine disruption
Ligand Biotransformation
• PAHs undergo oxygenation to generate diol epoxides or radical cations
B[a]P
(Cavalieri & Rogan, 1993)
PeroxidasesOxidants CYP1A1
B[a]P radical cation+
e-
OH
CYP1A1 and
epoxide hydroxylase
HO
O
B[a]P diol epoxide
DNA Adduct Formation
• Reactive electrophiles bind covalently to DNA
B[a]P radical cation
+
Guanine
HN
N
N
NH
O
H3C
..HN
N
N
NH
O
H3C
B[a]P-6-N7Gua
(Cavalieri & Rogan, 1993)
AhR and Carcinogenesis
• DNA adduct provides carcinogenesis initiation
• Activate protooncogenes genes
• Inactivate tumor suppressor genes
• DNA adduct formation suppressed in AhR-KO mice• non-AhR mediated DNA adduct formation
(Kondraganti, 2003)
2,3,7,8-tetrachlorobenzo-p-dioxin (TCDD)
• Most persistent and bioaccumulative environmental contaminant
• The most biologically active environmental contaminant
• The highest acute toxicity of any environmental contaminant
• Environmentally and biologically stable
• Mutagenic in vivo and classified as a tumor promoter
• Endocrine disruptor
Cl
ClO
OCl
Cl
HAH Mediated-Toxicity and Carcinogenesis
Dioxin exposure
Free Dioxin in tissues
Dioxin binding to AhR in tissues
AhR-Dioxin complex binding with DNA
Gene regulation
mRNA regulation
Protein synthesis
Biochemical alterations
Early cellular responses (cell growth stimulation)
Delayed tissue response (cancer)
Ligand-Activated AhR
• The biological activity of AhR ligands is mediated by AhR and may contribute to:– Malignant transformation – Carcinogenesis
– Arteriosclerosis – Immunosuppression
– Endocrine responses – Wasting
– Neurotoxicity – Reproductive toxicity
– Diminished viral resistance – Chloracne
Physiological Role of AhR
• Observations in mice lacking WT AhR
– Hepatic growth and development stunted (Schmidt et al. 1996)
– Cardiac hypertrophy in adult AhR null mice (Lund et al. 2003)
– Cell proliferation: • Angiotensin II and ET-1 implicated (Lund et al. 2003)
– Lack of teratogenic response to TCDD (Mimura et al. 1997)
Role of Chemoprotective Phytochemicals
Can be both AhR agonists and antagonists:
Resveratrol and Flavonoids
Resveratrol
• Structure
• Occurrence
– Grapes
– Wine
– Grape juice
– Peanuts
– Berries of Vaccinum species (blueberries, cranberries, etc.)
Flavonoids
• Basic Structure
• Structure of Isoflavonoid, Daidzein
• Occurrence– Soy Beans
– Red wine
– Berries of Vaccinum species (blueberries, cranberries, etc.)
• Biological Activities– Lower cholesterol
– Reduction of cardiac risk
– Decrease cancer incidence• Breast
• Prostate
– Increased longevity
O
O
O
OH
HO
Endocrine Disruption
• Crosstalk between AhR and ER
– Ligand-activated AhR-Arnt complex can activate unliganded ER to mimic estrogens
• Disruption of thyroid hormone
• Crosstalk between AhR and GR
Brosens & Parker (2003). Nature 423: 487.
AhR Pathway Participants
90
X XAP2 (hepatitis B virus X-associated protein)
HSP90 x 2 (90kDa heat shock protein)
L
L
Exogenous ligand
Endogenous ligand
AhR Arnt
XRE promoter gene
IC Initiation complex
Target DNA
AhR Signaling Pathway
XRE promoter gene
Translation
Increased expression CYP1A1 protein
Increased expression of other gene products
+
AhR/Arnt heterodimer
mRNA
IC
+
Xenobiotic Response Element (XRE)
• Dioxin response element
• AhR and Arnt binding to different parts of XRE
• XRE in enhancer in upstream regulatory sequence• Phase I enzymes – CYP 1A1 (eight XREs in enhancer)
• Phase II enzymes – UGTs
• AhRR
XRE gene
5’TNGC GTG3’
AhR
Arnt
(Whitlock, 1999)
Aryl hydrocarbon Receptor (AhR)
• Target Genes: Drug Metabolizing Enzymes
– CYP450s• Ex) 1A1, 1A2, and 1B1
– UGTs
– GSTs
• Induces the expression of growth factors and proto-oncogenes
• Many, many more…
Examples of AhR Target Genes in the Cardiovascular Domain
• Genes– iNOS– Endothelial NOS– Cyclooxygenase 2– PG H Synthase– PG endoperoxide synthase– 5-lipoxygenase– 15-lipoxygenase– CD23 p45– NF-kB p65– NF-kB p50– I kappa B alpha– Meloperoxidase
• Interlukines, Chemokines– IL7– RANTES– IL-1β
• Viral Receptors– CD4– CXCR4– Coxsackle-adenovirus receptor
• Membrane Receptors– Thromboxane A2 receptor– IL-2 receptor a – IL-2 receptor b– IL-8 receptor a – IL-8 receptor b– TNF receptor p75/80– Interferon gamma receptor II– Connexin 43– CD36– ICAM (CD54)– LDL receptor
• Viruses– Cytomegalovirus IE-1– HIV 1 isolate BRU (LAV-1)– HIV 2– Herpes virus HSV1– Hepatitis A virus– Adenovirus type 2– Adenovirus type 12– Adenovirus type 19A
Putative Interactions Between P450s and UGTs
• Evidence points to proteomic coupling between P450s (phase I oxidative enzymes) and UGTs (phase II conjugative enzymes).
• This coupling can be:A. Tight resulting in effective
detoxification
B. Loose resulting in toxicity.
C. Unbalanced resulting in toxicity.
• Figure modified from Nebert et al. JBC 2004