Lecture 8: Gene Regulation Flashcards
Regulation of Xenobiotic Metabolism
Induction of xenobiotic metabolism enzymes occurs at a genetic level.
- CYPs are induced by PAH (Polycyclic aromatic hydrocarbon), TCDD, PB (phenobarbital), etc.
- GSTs are induced by PAH, PB, beets, goitrin.
- GST/EH/quinone reductases are induced by electrophiles
Regulation of Gene Expression:
- Transcription (initiation, elongation)
- mRNA stability
- Enhancer (cis-acting element, response element): short DNA sequences that can bind to specific transcription factors that enhance the transcription levels of specific genes. Usually each gene is regulated by multiple enhancers, and each individual response may need coordinated efforts from multiple enhancers as well.
- Transcription regulator (trans-acting factor)
Transcription Factors Regulate the Transcription of Genes encoding Xenobiotic Metabolic Enzymes
- Aryl hydrocarbon receptor (AhR)
- NRF2
- Nuclear Receptors:
- Constitutive Androstane Receptor (CAR)
- Pregnane X receptor (PXR)/xenobiotic sensing nuclear receptor (SXR)
- Steroid receptor (glucocorticoid, estrogen..)
- Peroxisome proliferator activated receptor α, β/δ, γ
Mechanism of Induction of Certain CYPs: Aryl hydrocarbon receptor (AhR)
- AhR, a ligand-activated transcription factor, mediates xenobiotic signaling (Dioxin) to enhance the expression of target genes, including drug-metabolizing cytochrome P450s.
- Adaptive response against xenobiotics entering the our body. AhR activates transcription of genes encoding enzymes that can metabolize xenobiotics.
AhR mediated activation:
- Normally, AhR exists in a dormant state in the cytoplasm in association
with a complex of chaperones (HSP90, XAP2, and p23). Chaperone complex
induces AhR conformation that is optimal to bind to its ligands. - Upon ligand binding, AhR in the complex is activated by a conformation
change that results in dissociation of HSP90-chaperone complex. - The ligand-activated AhR forms a heterodimer with the closely related Arnt
Protein (aryl hydrocarbon receptor nuclear translocator). - This complex (xenobiotic-AhR/ARNT) binds the xenobiotic/dioxin
response element (XRE/DRE) in the CYP promoter region to activate gene
transcription. - XRE/DRE: core sequences 5’ GCGTG 3’ or T/GNGCGTGA/CG/CA
How to Introduce DNA into cells?
In aqueous solutions, cationic lipids form vesicles with a bilayer lipid sheet, known as liposomes. When liposomes encounter nucleic acids (plasmid-DNA) they re-form into nucleic acid lipid complexes called lipoplexes which can be actively taken up by eukaryotic cells by means of endocytosis. In this case, the lipoplex enters into the cell cytosol via the endosomes.
Other AhR-responsive genes
- CYP1A1, 1A2
- Glucuronosyl transferase
- Quinone reductase
- Glutathione S-transferase
- Epidermal growth factor
- Transformation growth factor
AhR
- Important adaptation against xenobiotic response by increasing enzymes to metabolize xenobiotics. Ahr has been shown to play a role in immune response and inhibits inflammation.
AhR: Carcinogens
-The constitutive expression of AhR induced tumors in the glandular part of the stomach and increased hepatocarcinogenesis in transgenic
B6C3F1 mice following a single injection of N-nitrosodiethylamine.
-AhR-null mice are resistant to benzo[a]pyrene-induced tumors.
benzo[a]pyrene is bioactivated by Cyp1A1 and Cyp1B1.
-the role of AhR in the development of naturally occurring tumors remains largely unknown.
AhR also Affects Reproduction and development
-The fertility of AhR-null females is reduced
-The litter size of AhR deficient mice was significantly decreased
compared to wild-type mice, and this resulted from impaired
folliculogenesis and ovulation in AhR deficient females
-In wild-type mice, exposure to TCDD during development induces
hydronephrosis, reduced kidney size, and some developmental renal
disorders. In contrast, AhR-deficient mice are completely resistant to
these TCDD-induced teratogenic effects
-vascular development; smaller liver size in AhR KO.
What is the endogenous ligand for AhR?
Antioxidant Response
It has become increasingly clear that the induction of several Phase II enzymes protects against various oxidative and electrophilic agents, thereby preventing cancer and other chronic diseases.
This is called the antioxidant response.
Induction of the antioxidant response is regulated by at least three
essential components:
1) Antioxidant response elements (AREs)
2) Nrf2
3) Keap1
Antioxidant response elements (AREs)
upstream regulatory sequences present on the responsive gene in either single or multiple copies. RTKAYNNNGCR (K=G/T, Y=C/T)
Nrf2
nuclear factor-erythroid 2-related factor 2, the principal basic leucine zipper (bZIP) transcription factor that heterodimerizes with members of the small Maf family of transcription factors and binds to the ARE, and recruits the general transcriptional machinery for expression of ARE-regulated genes.
Keap1
a cytosolic repressor protein that binds to Nrf2, retains it in the cytoplasm, and promotes its proteasomal degradation.
