Hormonal Dependent Cancer- Breast Cancer

Question 1

What is a hormone?

Answer 1

A hormone is a chemical messenger that is made by specialist cells, usually within an endocrine gland, and it is released into the bloodstream to have an effect in another part of the body.

Question 2

What are the three hormone groups?

Answer 2

They can be grouped into 3 main classes

- Steroids – lipid soluble small molecules    e.g. testosterone
- Peptide / proteins  e.g. insulin
- Modified amino acids / amine hormones     e.g. adrenaline

Question 3

How are steroid hormones synthesised?

Answer 3

All steroids are synthesised from cholesterol, either ingested or synthesised de novo in the body
Main corticosteroids and mineralocorticoids synthesised in the adrenal cortex
Androgenic and oestrogenic precursors released into the bloodstream
Androgens and oestrogens produced in the target tissues e.g. testes and ovaries then released into the bloodstream

Question 4

What are sex hormones?

Answer 4

These are responsible for the sexual dimorphism between males and females,
Also responsible for the development of the secondary sexual characteristics e.g. the growth spurt during puberty, body hair, gonadal development, voice change, breast growth and accessory organs of the reproductive organs e.g. the prostate in men.

Question 5

What are the effects of steroid hormones on males and females?

Answer 5

Steroid hormones work systemically, having effects on several tissues
These effects are:
- In females, oestrogen controls the menstrual cycle, and breast tissue development, fertility, and reproductive organ development, secondary sexual characteristics - body hair etc.
- In males, testosterone controls reproductive and supportive organs (prostate), development of sexual characteristics in men e.g. deepening of the voice, body hair etc

Question 6

How do steroid hormones act?

Answer 6

They are small lipophilic molecules, they can easily enter cells by passing through the plasma membrane.
Once the steroid hormones enter the cells, they bind to receptors.
These receptors are known as nuclear receptors – as they have their effects in the nucleus, however they may be found in the cytoplasm or nucleus initially.

Question 7

What is the steroid hormone receptor mechanism?

Answer 7

1. Steroid hormones cross into the cell cytoplasm where they will bind to their receptor
   
   2. Binding to the receptor causes a conformational change in the nuclear receptor, causing it to become activated (some nuclear receptor dimerise at this point)
   3. Nuclear receptors then translocate into the nucleus
   4. Nuclear receptors bind to specific DNA sequences called response elements located in the promoters of steroid responsive genes.
   5. Steroid responsive genes are switched on and upregulated.

Question 8

What domains do the nuclear receptors have?

Answer 8

Ligand binding domain (LBD)
- Binds specific steroid molecules with high affinity
DNA binding domain (DBD)
- Binds specific DNA sequences
Activation function domain (AF1 & 2)
- Recruits gene activation machinery, some receptors have a secondary AF2 domain towards the C-terminal
The same basic domains and structure are shared with many of the major nuclear receptors

Question 9

How does a ligand activate transcription factors?

Answer 9

1. Ligand binding to the ligand binding site causes a shift in an a-helix, activating the receptor.
   
   2. Receptor dimerises, moves into the nucleus and binds to specific DNA sequences
   3. Receptor then recruits DNA modifying enzymes e.g. histone deacetylases, other transcription factors and RNA polymerase to promoters of hormone responsive genes.

The DNA binding domain contains 2 zinc finger domains which are essential for sequence specific DNA binding
One is for specific DNA sequence binding and the other is for interaction with the DNA phosphate backbone

Question 10

What are Hormone Response Elements?

Answer 10

Hormone Response Elements are specific DNA sequences found in the promoters of hormone responsive genes.
Many are palindromic
AGGTCAnnnTGACCT- If they are specific to oestrogen they are called oestrogen response element
AGAACAnnnTGTTCT- Same for Glucocorticoid response element etc.

Question 11

What are the main steroid receptors and their ligands?

Answer 11

Name Abbreviation Ligands
Estrogen Receptor ER estradiol, estrone, estriol
(a and b isoforms)
Androgen Receptor AR androstenedione, testosterone, dihydrotestosterone
Progesterone Receptor PR progesterone, pregenolone
Glucocorticoid Receptor GR cortisol and cortisone
Mineralocorticoid Receptor MR aldosterone

We will deal mainly with

- ER & PR for breast cancer
- AR for prostate cancer

Question 12

What is the breast like anatomically?

Answer 12

The breast is an apocrine gland that produces the milk used to feed an infant
The breast is composed of glands and ducts, which produce the fatty breast milk.
The milk-producing part of the breast is organized into 15 to 20 sections, called lobes.
Within each lobe are smaller structures, called lobules, where milk is produced.
The milk travels through a network of tiny tubes called ducts. The ducts connect and come together into larger ducts, which eventually exit the skin in the nipple.

Question 13

What are the 3 different types of glands?

Answer 13

The mammary gland is a specialised type of exocrine gland called and apocrine gland.
Exocrine glands – secrete substances out onto a surface or cavity, via a ductal structure.
Endocrine glands – secrete substances directly into the bloodstream
Apocrine glands – are a specialised exocrine gland in which a part of the cells’ cytoplasm breaks off releasing the contents.

Question 14

What is the mammary gland tissue structure like?

Answer 14

The mammary epithelium consists of two cell compartments:

- Luminal – form a single layer of polarized epithelium around the ductal lumen, luminal cells produce milk during lactation.
- Basal – comprise of the cells that do not touch the lumen, basally oriented myoepithelial cells in contact with the basement membrane, have contractile function during lactation
- Although oversimplified, this constitutes the main mammary gland cell types.

Question 15

What are the ER functions in the functional breast?

Answer 15

Two major phases can be distinguished in mammary gland development:

- hormone-independent from embryonic development up to puberty
- hormone-dependent thereafter during puberty, menstrual cycle and pregnancy.

Question 16

What is the role of progesterone in the normal breast?

Answer 16

Oestrogen is primarily involved in the initial growth of breast cancer
The progesterone receptor gene is switched on by the oestrogen receptor
Progesterone increases the branching of the ducts
Prolonged progesterone receptor activity i.e. during pregnancy, leads to more side branching and lactogenic differentiation (together with prolactin hormone).

Question 17

What are the risk factors for breast cancer?

Answer 17

Age - The risk for breast cancer increases with age; most breast cancers are diagnosed after age 50.
Genetic mutations to certain genes, such as BRCA1 and BRCA2. Women who have inherited these genetic changes are at higher risk of breast and ovarian cancer.
Reproductive history. Early onset of menstrual cycle before 12yrs and starting menopause after 55yrs expose women longer to hormones.
Previous treatment using radiation therapy to the chest or breasts (e.g. treatment for lymphoma) before age 30 have a higher risk of getting breast cancer later in life.
Not being physically active increases the risk of breast cancer.
Being overweight or obese.
Taking hormones. Some forms of hormone replacement taken during menopause can raise risk for breast cancer when taken for more than five years.
Certain oral contraceptives (birth control pills) also have been found to raise breast cancer risk.
Reproductive history. Having the first pregnancy after age 30, not breastfeeding, and never having a full-term pregnancy can raise breast cancer risk.
Drinking alcohol. Risk for breast cancer increases with more alcohol.

Question 18

What is DCIS?

Answer 18

Breasts are made up of lobules (milk-producing glands) and ducts (tubes that carry milk to the nipple), which are surrounded by glandular, fibrous and fatty tissue.
When cancer cells develop within the ducts of the breast but remain within the ducts (‘in situ’), it is called DCIS.
The cancer cells have not yet developed the ability to spread outside these ducts into the surrounding breast tissue or to other parts of the body.

Question 19

What is LCIS?

Answer 19

Lobular breast carcinoma in situ (LCIS)
Lobular carcinoma in situ (LCIS) is an uncommon condition in which abnormal cells form in the milk glands (lobules) in the breast.
LCIS isn’t cancer.
- But being diagnosed with LCIS indicates that there could be an increased risk of developing breast cancer.

Question 20

How do ER+ and ER- breast cancers differ?

Answer 20

The majority of breast cancers arise from the luminal cells.
These cells express ER
The majority of breast cancers are ER+ve and have a good prognosis, however the remainder are ER-ve and have a relatively poor prognosis.
ER-ve breast cancers cannot be treated ‘hormonally’ and patients are given more conventional therapies.

Question 21

What is the role of ER in breast cancer?

Answer 21

In the normal breast the ER controls functions such as cell proliferation, development and differentiation in a highly controlled manner.
However, in breast cancer, the ER signalling pathway is subverted and becomes uncontrolled.
ER’s ability to bind DNA and open chromatin becomes hijacked and is used to transcribe many genes, non-coding RNAs and miRNAs.
ER then governs cancer cell proliferation, and controls and influences many hundreds of genes involved in metastasis, invasion and adhesion.

Question 22

What happens when oestrogen binds to its receptor?

Answer 22

Oestrogen binds to its receptor at the ligand binding site which causes the oestrogen receptor to dimerise and translocate into the nucleus where it binds DNA and recruits proteins for gene transcription such as Coactivator 1 and 2, chromatid modifiers and RNA Pol
AF1 and AF2 are activated and this triggers the full gene activation and drive cell growth forward in this case breast cancer cell growth

Question 23

What is fulvestrant?

Answer 23

Fulvestrant is an analogue of estradiol.
Fulvestrant competitively inhibits binding of estradiol to the ER, with a binding affinity that is 89% that of estradiol
Impairs receptor dimerisation and receptor translocation into the nucleus

Question 24

What does fulvestrant do?

Answer 24

Fulvestrant – ER binding impairs receptor dimerisation, and energy-dependent nucleo-cytoplasmic shuttling, thereby blocking nuclear localisation of the receptor.
Additionally, any fulvestrant – ER complex that enters the nucleus is transcriptionally inactive because both AF1 and AF2 are disabled.
The fulvestrant–ER complex is unstable, resulting in accelerated degradation of the ER protein.

Question 25

What is Tamoxifen?

Answer 25

Tamoxifen binds the ER at the ligand binding site.
Tamoxifen is a partial agonist but does not cause the full activation of ER.
It has a mixed activity – it activates ER in the uterus and liver, but acts as an antagonist in breast tissue.
Tamoxifen is a Selective Oestrogen Receptor Modulator (SERM).
Tamoxifen bound ER does not fold properly and the AF2 domains do not function

Question 26

How how does tamoxifen change oestradiol?

Answer 26

Normally oestradiol binds deep within a pocket in the receptor and is covered by a loop of protein chain, as shown in the upper illustration
This loop forms part of the activation signal that will stimulate growth in the cell (AF2).
Tamoxifen (in pink in the lower illustration) binds, the extra tail of the drug is too bulky and the receptor loop is not able to adopt its active conformation.

Question 27

How does tamoxifen act?

Answer 27

Tamoxifen binds to the ligand binding site and causes the oestrogen receptor to become partially active
The binding of tamoxifen causes the conformational change in the oestrogen receptor but causes only one of the activation domain to become active, AF1
The ER then remains partially inactive which stops activity within breast tissue but not in liver and uterus

Question 28

Where does oestrogen come after menopause?

Answer 28

When ovaries are no longer functional in postmenopausal women, potential sources of oestrogens come from the peripheral conversion of androgens by the aromatase enzyme.
This enzyme is present in multiple organs including adipose tissue, brain, blood vessels, skin, bone, endometrium, and breast tissue.
Androgens are hormones such as testosterone, or adrenal androgens such as androstenedione

Question 29

What are the different types of aromatase inhibitors?

Answer 29

Type 1 inhibitors, like exemestane (Aromasin ©), are androgen analogues and bind irreversibly to aromatase, so they are also called “suicide inhibitors”. The duration of inhibitory effect is primarily dependent on the rate of de novo synthesis of aromatase.

Type 2 inhibitors, like anastrozole (Arimidex ©), contain a functional group within the ring structure that binds the heme iron of the cytochrome P450, interfering with the hydroxylation reactions.