VIBS 422 Lecture 11

How do endocrine disruptors act?

Directly—→ acts as hormone agonists/antagonists

Indirectly—→ alter hormone synthesis, transport, metabolism

What is a direct-acting EDC?

mimics or blocks hormones by binding receptors & altering signaling

What is an indirect-acting EDC?

alters hormone levels by affecting synthesis, secretion, transport, or metabolism

How do EDCs interact with hormone receptors?

• compete with natural ligands

• alter downstream signaling

• bind empty receptors —> agonist or antagonist effects

What happens when EDC replaces a natural hormone on a receptor?

altered gene signaling —> inappropriate biological response

5 key mechanisms of endocrine disruption?

• receptor activate (agonist)

• receptor blocking (antagonist)

• alter hormone metabolism

• change receptor numbers

• alter hormone synthesis

Where can EDCs bind estrogen receptors?

Membrane ER (mER)

Nuclear ER

What happens when EDC binds ER in the nulceus?

binds estrogen response element (ERE) —-? alters gene transcription

How do EDCs affect gene expression via ER?

recruit cofactors —→ RNA polymerase activation —→ inappropriate gene expression

What is receptor degradation in EDC signaling?

EDC-ER complexes bind proteasome —> receptor breakdown —→ decreased signaling

How do EDCs modify gene expression epigenetically?

• DNA methylation —> decreased expression

• Histone deacetylation —→ decreased expression

• Demethylation/acetylation —→ increased expression

Why is chromatin structure important?

• wrapped DNA = inactive

• unwrapped DNA = active transcription

Key domains of estrogen receptor (ER)?

• DNA binding domain (DBD)

• ligand binding domain (LBD)

• activation domains (AF-1, AF-2)

How does DDT act as an EDC?

bidns ERa & ERb —→ estrogenic activity

Effects of DDT on reproductive system?

• increased uterine cell proliferation

• impaired follicle development

Where is DDT stored in the body?

fat (adipose tissue)

What is the active metabolite of methoxychlor?

HPTE

How does HPTE act on estrogen receptors?

ERa agonist

ERb antagonist

What type of chemical is BPA?

industrial phenolic compound (plastic)

How does BPA act on receptors?

binds ERa, ERb (weakly)

stronger binding to GPR30 (membrane receptor)

What signaling pathways does BPA activate?

• cAMP

• MAPK

• PI3K

BPA cellular effects?

• ERK activation

• AKT activation

• Cell proliferation

Why is DES highly potent?

high ER affinity due to hydrophobic interactions

How does DES signal?

genomic (DNA transcription)

non-genomic (rapid signaling)

What activates AhR?

• dioxins

• PAHs (polycyclic aromatic hydrocarbons)

Where is AhR located before activation?

inactive cytosolic complex with Hsp90 & p23

What happens after AhR activation?

• moves to nucleus

• binds ARNT

• binds DNA (DREs)

• activates gene transcription

Key genes activated by AhR?

CYP enzymes (ex. CYP1A1)

How do AhR & ER interact?

• compete for coactivators

• AhR can inhibit ER signaling

• Can bind inhibitory DNA elements

Why is receptor “promiscuity” important?

receptors bind many different chemicals —→ widespread disruption

How do EDCs affect hormone levels?

• decreased synthesis

• decreased transport

• decreased binding proteins

• increased catabolism

Which enzymes are involved in hormone breakdown?

• CYP1A1

• CYP1A2

• CYP3A4

• CYP1B1

What is the effect of increased hormone catabolism?

reduced hormone signaling

Effect of chromium (Cr VI) on hormones?

increases steroid hormone catabolism —→ disrupts signaling

Major ways EDCs disrupt nuclear receptor signaling?

• agonism

• antagonism

• coactivator competition

• DNA binding interference

• cross-talk between receptors

What is coactivator competition?

EDCs compete for transcription cofactors —> disrupt normal gene expression

What is DNA-bidning competition?

EDCs prevent receptors from binding DNA —> block gene transcription