Nuclear Recptor-Mediated Toxicity

PAS domain receptors

1. Aryl hydrocarbon receptor (AHR)

2. Aryl hydrocarbon receptor repressor (AHRR)

3. Aryl hydrocarbon receptor nuclear translocator (ARNT or HIF-1β)

4. Hypoxia-inducible factor 1 (HIF-1α)

Steroid hormone receptors

1. Estrogen receptors (ERα and ERβ)

2. Glucocorticoid receptor (GR)

3. Progesterone receptor (PR)

4. Androgen receptor (AR)

5. Retinoic acid receptor (RARα, RARβ, and RARγ)

6. Thyroid receptor (TRα and TRβ)

7. Prognane X receptor (PXR)

8. Constitutively active receptor (CAR)

9. Peroxisome proliferator-activated receptor (PPARα, PPARβ, PPARγ)

10. Retinoid X receptor (RXRα, RXRβ, RXRγ)

11. Liver X receptor (LXRα, LXRβ)

12. Estrogen-related receptor (ERRα, ERRβ, ERRγ)

Steroid receptor hormone (SHR)

1. large family of receptors

2. always form dimers

3. heterodimers are usually paired with RXR and another SHR

4. contains a ligand-binding domain

5. contain a DNA-binding domain (Zn finger)

6. usually inactive, cytosolic proteins, HSP90 chaperone

ligand-activated transription factors

• N-terminal domina, transcriptional activation domain

• DNA-binding domain

• Hinge regions (allows dime to adopt different conformations)

• Ligand-binding domain (imparts uniqueness to each transcription factor)

membrane-bound steroid hormones

• GCPRs and RTKs

PAS domain proteins

• derived from PER (period circadian protein), ARNT (Ah receptor nuclear translocator protein), and SIM (single-minded protein)

• very important role in embryogenesis and differentiation

• includes AHR (aryl hydrocarbon receptor)

• appear across all kingdoms

Aryl hydrocarbon recptor (AHR)

1. cytosolic protein, HSP90 or ARA9 as chaperone

2. ligand binding by hydrocarbon causes AHR to shed its chaperone and translocate to the nucleus to form heterodimer with ARNT

3. downstream targets include Phase I enzymes (CYP1A1, CYP1A2, CYP1B1, and NQO) and Phase II enzymes (GST, UGT)

Nuclear receptor-mediated toxicity

• toxicity is characterized by tissue specificity (form of expression) and compound selectivity

• effects are tissue-specific and predictable

• increases in transactivation of specific genes can be demonstrates

• transcriptional responses occur rapidly

• often compounds reversibly bind to intracellular molecules

AHR-mediated toxicity of PCDDs, PCDFs, PCBs

• PCDD: polychlorinated dibenzodioxins

• PCDFs: polychlorinated dibenzofurans

• PCBs: polychlorinated biphenyls

• large group of environmental chemicals

  • PCDDs and PCDFs from chemical processes, PCBs produced for industrial purposes

• all are lipophilic with long biological half lives

• higher degree of chlorination, less easy to metabolically degrade

mechanisms of TCDD induced cancer

• comes from burning organic matter

• non-genotoxic

• two major tumor-promoting mechanisms

  • perturbation of the cell cycle: alters function of certain transcription factors and cyclin-dependent kinase inhibitors → cell proliferation

  • oxidative stress: activates CYP1a → induces oxidation of thionyl groups, increases calcium concentrations, productive of ROS and damaged DNA bases → acquisition of mutations

• toxicity primarily dependent on AHR and secondarily on CYP1

endocrine system - hypothalamus-pituitary axis

• hypothalamus releases gonadotropin-releasing, thyrotropin-releasing, and adrenocorticotropic-releasing hormones to pituitary gland

• pituitary gland releases gonadotropin to gonads, thyroid-dtimulating hormone to thyroid gland, and adrenocorticoptropin to adrenal gland

• gonads produce sex steroids → normal growth, sexual development, reproduction

• thyroid gland releases thyroxine → neural development, immune function, metabolism

• adrenal gland releases corticosteroids → stress response, immune suppression, enhanced blood sugar

endocrine system functions

1. metabolism and energy balance: regulate blood sugar and energy

2. growth and development

3. reproduction and sexual function

4. physiological homeostasis

5. sleep-wake cycle (melatonin from pineal gland)

6. stress response

endocrine tissues and functions

• pituitary gland: controls other endocrine glands

• thyroid gland: controls metabolism and calcium balance

• adrenal: stress response (adrenaline) and metabolism (cortisol)

• pancreas: blood sugar (insulin and glucagon)

• gonads: reproductive health and sexual development

• hypothalamus: connects endocrine to nervous systems

elements of the endocrine system

sender: sending cell

signal: hormones

nondestructive medium: serum and hormone binders

selective receiver: receptor protein

transducer: transducer proteins and secondary messengers

amplifier: transducer/effector enzymes

effector: effector proteins

response: secondary hormone release

endocrine disruptors

• environmental toxins that acts like hormones in the endocrine system and disrupt the physiological function of endogenous hormones

endocrine disrupting chemicals (EDCs) mechanisms of actions

1. mimicking natural hormones (agonists): BPA

2. blocking receptors (antagonists)

3. interfering with hormone synthesis and metabolism: phthalates, PCBs, alter speed of synthesis and metabolism. increased metabolism prevents negative feedback and can lead to chronic overproduction

4. targeting steroid hormone receptors: direct disruption of hypothalamic-pituitary-gonadal axis signaling

5. epigenetic changes: alter DNA methylation patterns

xeno-hormone

structurally mimics an endogenous hormone, binds to receptor and produces agonistic effect (enhancer)

anti-hormone

structurally mimics an endogenous hormone, binds to receptor and produces an antagonistic effect (blocker)

EDCs effect on reproduction

• female reproduction: impacts hypothalamic-pituitary-ovarian axis, affects oocyte development and pregnancy

  • polycystic overies, endometriosis, uterine fibrosis, miscarriage, preterm birth, low birth weight

• male reproduction: manifests as testicular dysgenesis syndrome (TDS)

  • semen quality decreased or developmental abnormalities (cryptochidism: failure of testes to descend to scrotum, hypospadias: misplacement of urethral opening)

EDCs effect on development

• early puberty (females), delayed puberty (both), alteration of reproductive organs (intersex phenotypes)

• examples of feminization:

  • testis contain eggs (fish), reduced sperm count and altered male behavior (mammals), feminize male embryos (birds)

• examples of masculinization:

  • intersex (snails, fish, reptiles)

EDCs effect on carcinogenesis

• cancer promotion and proliferation: enhancing growth of breast cancer cells and promoting tumor aggression

• hormonal disruption: EDCs act as estrogenic or adrogenic acgens and disrupt hormone synthesis, metabolism, and secretion

• epigenetic reprogramming: exposure to chemicals like BPA

• developmental sensitivity: prenatal or neonatal exposure has stronger link to tumor formation later in life

• tumor microenvironment alteration: modify environment surrounding tumor cells, facilitate immune evasion and angiogenesis

enstrogen receptor (ER)-mediated toxicity

• xenoestrogens: group of xenobiotics that can elicit an agonistic (enhancing) response mediated by estrogen receptor

  • includes DDT, hydroxy-PCB, alkylphenols, and BPA

• xenoestrogens related to increased incidences of hormonally controlled cancers

• estrogen receptor is promiscuous receptor (can bind a large variety of chemically unrelated ligands)

  • binding requires ring structure and usually a phenolic group in para and bulky hydrophin group in ortho

interaction between ER and AHR

• ER is localized to cell nucleus and inactive until ligand binding

• TCDD can regulate ER

  • absence of ER ligand: TCDD/AHR/ARNT complex binds to ER causing activation of ERAand activation of estrogen-dependent genes

  • presence of ER ligand: TCDD/AHR/ARNT complex can impair expression of estrogen-responsiv genes

androgen receptor (AR)-mediated toxicity

• AR is ligand-activated steroid hormone, cytosolic until translocation to nucleus

• ligands are testosterone and dihydrotestosterone

• mechasnism of toxicity for DDE (metabolite of DDT)

  • DDE is antiandrogen

  • binds to AR to create an unstable ligand/AR complex → altered expression of AR-regulated genes

  • complex is degraded in cytoplasm before nuclear localization and transcriptional activation

  • results in transcriptional repression of AR-mediated gene regulation

ER and AR-mediated toxicity

• BPA and nonylphenol can effect bot ER and AR

  • high estrogenic effects and antiandrogen effects

    • inhibit AR interaction with coactivator

    • inhibit androgen binding with AR

    • inhibit AR translocation

    • inhibit androgen-induced AR transcriptional activity

retinoic acid receptor (RAR

• partners with RXR for heterodimerization

• ligands are retinol and retinoic acid

• perturbations of RAR function are associated with testicular toxicity and embryotoxicity

• retinoic acid acts through RARs to disrupt development and cause severe birth defects

peroxisome proliferator-activated receptors (PPARs)

• ligand-dependent transcription factors (activated by fatty acids)

• form heterodimers with RXR

PPARα mechanism of action

• tumor-promoting

hepatocarcinogenesis in rodents

1. stimulate replicative DNA synthesis and liver growth

2. inhibit apoptosis

cyoprotection

1. not fully understood; pretreatment with drug clofibrate protects the liver from known hepatotoxic compounds

2. required for proper liver regeneration

3. suspected that clofibrate stimulates a mitogeneic response (stimulates cell division)

PPARγ-mediated toxicity

• adipose-selective nuclear receptor

• anti-tumor properties

• mediates and increase in lipid storage

• potential target for anti-cancer drugs

• adipogenesis-promoting effects in bone marrow

• development of fatty liver disease (excessive triglycerides in hepatocytes)

• also causes hepatic steatosis