Unit 2 • Lecture 2 — Estrogen Effects on DNA

ESTROGEN IN THE BODY
- Binds to variety of diff areas —> brain, heart, liver, the breast
Primary sites of synthesis
- Ovaries (major, cyclic production in females)
- Testes (minor baseline amounts in males)
Transport medium: bloodstream (carried on serum proteins such as sex-hormone-binding globulin; carrier names were implied, not listed)
Target tissues (examples explicitly cited): brain, breast, liver, heart, bone, uterine tissue, plus many others
Defining a “target cell”
- Must express an intracellular estrogen receptor (ER)
- Ligand–receptor specificity: no ER → no estrogen response
  - can only bind to a cell if cell has a receptor for that particular ligand

WORKSHEET 3 Q.1
- Estrogen is made in the gonads; large amounts in the ovaries & little amounts in the testes
  - travels through the body through bloodstream
  - target cell —> a cell that has an ESTROGEN RECEPTOR to bind estrogen
  - estrogen is steroid hormone —> it is nonpolar/hydrophobic
  - Cytoplasmic binding (TRIGGERED CHANGE IN GENE EXPRESSION)
    1. Estrogen (E) directly diffuses through plasma membrane
    2. finds receptors in cytoplasm; Binds to ERs in CYTOPLASM(note: many figures incorrectly start inside nucleus; author corrected this in diagram)
    3. receptor ligand complex travels into nucleus to influence transcription
      1. LIGAND RECEPTOR —> complex of estrogen & its receptor
    4. either an increased transcription of a gene or decreased; in this case inhibiting gene expression
- Nuclear translocation
  - E–ER complex undergoes conformational change → enters nucleus through nuclear pore
DNA interaction
- Complex binds to estrogen-response elements (EREs) in promoter / regulatory region of specific genes
- Acts as a transcriptional regulator
- ↑ transcription → gene activation
- ↓ transcription → gene repression (context-dependent co-factor availability)

ESTROGEN & CANCER:
- Primary roles are its developmental targets:
  - Breast and uterine tissue differentiation & maturation
    - helps w development of breasts
  - Stimulates cell growth & proliferation in breast/uterine epithelium
    - proliferation* = the process where cells increase in number through cell growth and division
    - too much estrogen = too much cell growth in proliferation = could cause breast or uterine cancer (depending on area)
  - Skeletal effects
    - Bone remodelling; maintains bone density
      - exposure to estrogen mimics causes too much cell growth & proliferation; leading to cancer, causing changes in bone (osteoporosis)
  - Cholesterol production in Liver
    - Regulates cholesterol synthesis in liver (excess signalling → potential ↑ serum cholesterol)

Definition: exogenous compounds that bind ER and trigger estrogen-like signalling
Environmental case study – mudsucker fish
- Normal: two discrete ovaries or testes
- Contaminated water: “ovotestes” (mixed gonadal tissue) due to chronic EDC exposure
- Consequences: sterility, abnormal gametes
Human relevance
- Unintended hormonal signalling → infertility, early puberty, hormone-dependent cancers, altered bone density, dyslipidaemia

DNA structure refresher (Unit 1 linkage)
- Nucleotide = sugar + phosphate + base (A,T,C,G)
- Complementary base pairing provides double-helix stability
Mutation = any change in nucleotide sequence (BASE substitution, insertion, deletion) —> CHANGE IN DNA
Mutagen = chemical or other substance that causes a mutation
- Radiation (e.g.
- UV → thymine dimers
- X-ray / γ-ray)
- Chemicals (carcinogens such as benzo[a]pyrene, nitrosamines, etc.)
- Some mutations arise spontaneously from replication errors
Mutations are the raw material of biodiversity & evolution but can impair health when they disrupt critical genes

Cancer occurs when a mutation affects cell division
requires a combo of approximately $6 \text{–} 9$ independent DNA mutations within the same cell lineage
- it’s not entirely impossible with 1 mutation, but it is highly unlikely compared to 6-9
- more common in older ppl due to taking a long time in someone's lifetime to have all of those mutations occur within the same cell or cell line.
Mutation sources
1. Inherited defective allele(s)
2. exposure to radiation/Radiation-induced DNA damage
3. Chemical mutagens/carcinogens
4. Spontaneous polymerase error; during DNA replication
- IF mutations from multiple sources build up & accumulate, we get abnormal cells that proliferate(divide) uncontrollably
Progression model
- normal cell —> starts to accumulate mutations → abnormal cell → body doesn’t respond to eliminating ab cells → uncontrolled proliferation → tumour (a massive abnormal cell)

Menstrual-cycle physiology
- Day-0 (menses) = low estrogen
- Peak around ovulation (≈ day 10) = high estrogen → normal proliferation of breast epithelium
- lowest at the end of menstrual cycle —> day 28
- Decline pre-menses → programmed cell death of surplus cells
Scenario with chronic EDC or elevated endogenous estrogen
- Breast cells (including any already-mutated precancerous clones) receive continuous proliferative signal
- Faster division of abnormal cells accelerates tumour formation
Key distinction
- Estrogen itself is not a mutagen — it does not create mutations
- It is a transcriptional activator that amplifies proliferation of cells that may already harbour mutations

Property	Mutagen	Estrogen/ER Complex
Primary action	Alters DNA sequence (damage → mutation)	Alters gene-expression level
Molecular target	DNA directly	DNA indirectly via receptor binding to EREs
Cancer link	Initiates transformation by generating oncogenic mutations	Promotes growth of existing abnormal (or normal) cells, accelerating tumour expansion
Example agent	UV radiation, aflatoxin	$\beta$-estradiol, BPA (mimic)

Diagram entire estrogen pathway: ovary → bloodstream → membrane diffusion → cytoplasmic ER → nucleus → ERE → altered transcription
Explain why ER-positive cells respond to estrogen while ER-negative do not
Compare membrane transport of estrogen (passive diffusion) versus hydrophilic molecules (glucose, amino acids → carrier‐mediated transport)
Differentiate mutagenic initiation of cancer from estrogen’s promotional role
Predict physiological/clinical outcomes of excess or deficient estrogen signalling (e.g.
osteoporosis, hypercholesterolaemia, breast/uterine cancer)

$6 \text{–} 9$ independent mutations usually required for full malignant transformation
Ovulatory estrogen peak ≈ $\text{day }10$ of $\approx 28\,\text{day}$ cycle
Estrogen classes: $E<em>1$ (estrone), $E</em>2$ (estradiol – biologically dominant), $E_3$ (estriol) — names occasionally appear in literature