Altered signalling

Question 1

short vs long term effects of xenobiotics

Answer 1

• Short term: acute toxicity
  • Immune systems –> immunotoxins: dioxins (TCDD), arsenic and methylmercury
  • Nervous system –> neurotoxins: PCBs, methylmercury, arsenic, lead and pesticides
  • Endocrine system –> endocrine disrupters: Bisphenol A, PCBs, pesticides, phthalates - interact with endocrine organs or mimic hormones
• Long term:
  • Carcinogens cause or promote cancer
  • Mutagens induce mutations, or changes, in the DNA molecules
  • Teratogens cause birth defects to a foetus

Question 2

dioxins

Answer 2

• Polyhalogenated aromatic hydrocarbons
• Environmental pollutants
• Fat soluble –> accumulates in fatty tissue of animals (fish)
• Biological response to Dioxins
  • Modulation of gene expression –> AhR
  • Modulation of steroid hormones and receptors
  • Modulation of growth factors and receptors

-	Adverse Health effects 
•	Cancer (IARC Class 1) 
•	Immunosuppression 
•	Reproductive effects --> act as hormones 
•	Developmental effects

Question 3

dioxin toxicity symptomes

Answer 3

• Acute symptoms / short term effects:
  • skin lesions and darkening; chloracne
  • liver dysfunction

-	Chronic symptoms / long term effects:
•	diabetes 
•	cancer 
•	birth defects - physical deformities, mental disabilities, etc. 
•	 infertility 

• Effects on babies and/or pregnant women
  • impairs nervous system development
  • impairs normal development
  • dissolves in breast milk
• Effects on children/adolescents
  • impaired immune function&raquo_space;> cancer
  • impaired endocrine function&raquo_space;> diabetes, infertility
  • impaired reproduction

Question 4

mechanism of action dioxins on AhR

Answer 4

• Toxicity of dioxins are mediated though their strong binding (agonists) to the aryl hydrocarbon receptor (AhR)
  1. TCDD is very lipophilic so easily enters the cytosol where it meets the nuclear receptor AhR
  2. The chaperones (2 HSP90s and XAP2) are kicked off AhR
  3. ARNT is bound
  4. This complex then promotes transcription in the nucleus of certain genes (including CYP1A1) by binding to the XRE-sequence

Question 5

cytochrome P450 system

Answer 5

• CYP catalyses a variety of reactions including epoxidation, N-dealkylation, O-dealkylation, S-oxidation and hydroxylation aiming to produce water soluble metabolites
• Most common reaction catalysed by P450s is a monooxygenase reaction –> insertion of one atom of oxygen into an organic substrate (RH) while the other oxygen atom is reduced to water
  • RH + O2 + NADPH + H+ → ROH + H2O + NADP+
• Complex supergene of 18 gene families: at least 40 enzymes expressed in human tissues
• CYP1, 2, 3, involved in detoxification of lipophilic, or nonpolar substances
• CYP1, 2 and 3 exert a major role in drug metabolism.
• Other CYP families involved in metabolism of endogenous substances, such as fatty acids, prostaglandins, steroids, and thyroid hormones

Question 6

xenobiotic metabolism

Answer 6

• Drugs and xenobiotics are usually lipophilic&raquo_space; cross membranes and enter cells
• Lipophilic compounds difficult to remove from body
• Enzymatic transformation of drugs into hydrophilic, inactive metabolites is necessary and happens in the liver in 2 phases
• Phase 1 reactions add or expose a functional group through oxidative reactions:
  • N-dealkylation; O-dealkylation; hydroxylation; N-oxidation; S-oxidation; deamination
• Phase 2 reactions are conjugations with
  • glucuronic acid; sulphate; acetate; amino acids; reduced glutathione

Question 7

AhR mechanism of action

Answer 7

• Roles of AhR in normal function
  • Target genes in detoxification of lipophilic compounds
  • Targets genes involved in tumour initiation/promotion
  • Targets genes involved in signalling pathways
  • Cell cycle and apoptosis
  • Developmental roles
  • Immune system function
  • Tissue remodelling
  • Cell growth and arresting effects  net effect??  depends on cell type, cell cycle and type of agonist
• CARCINOGENESIS
• AhR ligands are often in food –> flavonoids, carotenoids, indoles tryptophane (normal endogenous ligand)

Question 8

AhR and TME

Answer 8

• Stromal cells and immune cells make AhR agonists which influence the tumour micro-environment
• Immune cells use IDO-1 (enzyme) to produce IDO1s (AhR agonists) from tryptophane which leads to immunosuppression
• Ahr also mediates tumour growth and survival signals
• Also leads to metastasis since AhR activation leads to cell adhesion loss –> mesenchymal transition (increased expression of MMPs, SLUG and beta-catenenin and decreased experession of E-cadherin)

Question 9

BPA

Answer 9

• Bisphenol-a is a polycarbonate compound with two phenol functional groups. It is majorly used in the production of plastics as a hardener
• BPA is exposed by leaching from plastic containers
• Environmental concern
• Mimics the oestrogen hormone by attaching to the oestrogen receptor
• Endocrine disrupters can be agonists and overly induce hormonal effects or inhibit normal signalling by competing for the receptors (in this case the compounds would be a partial agonist or an antagonist)

Question 10

BPA mechanism of toxicity

Answer 10

• Bisphenol-A mimics the oestrogen hormone
  • Binds ER with moderate affinity
  • Many downstream effects
• Oestrogen Receptor α is activated by oestrogen and analogues (BPA)
  • Receptor located in cytosol –> after activation and dimerization acts as transcription factor by binding to HRE in genes

• Membrane oestrogen receptors (mERs) –> GPCR which alter cell signalling via modulation of intracellular signalling cascades
o MAPK
o GSK

• BPA also causes demethylation of DNA leading to inappropriate gene expression