HORMONES AND CANCER THE BASICS Flashcards
In cancer, what do hormones do?
Drive proliferation and hormone dependency
In post menopausal women, what 2 therapies can be used for oestrogen dependent cancers?
Most of the sex steroids are produced in the adrenal gland most menopausally. The most common sex steroid is DHEA sulfate and estrone sulfate which act as biological pools of oestrogen. Use sulfatase inhibition to stop conversion to oestrogen. Also aromatase inhibitors. sulphatase activity can be 50 times higher in breast cancer tissue
What treatment can be used for prostate cancer?
Prostate cancer cells undergo apoptosis in absence of androgen. LHRH agonists cause an initial increase in androgens but then a decrease as there is down sensitivity of the receptors
What can there be over production of with specific reference to breast cancer?
Growth factors such as the permanently activated ERBb2
What is a key function of the PI3 pathway in terms of the cell cycle?
Inactivate p53, a tumour suppressor that binds to MDM2
What is the most successful tyrosine kinase inhibitor?
Imatinib. An ATP analogue that binds to BCR-ABL. Displaces and competes with ATP stopping BCR from working. ABL is a tyrosine kinase involved with DNA repair
How to tyrosine kinase inhibitors generally work?
Generally they block receptor tyrosine autophosphorylation by competing for ATP binding sites and then induce dimerisation and internalisation of the receptor. Eg induce cell cycle arrest with up regulation of p27Kip1 (inhibitor of cyclin dependent kinases) to potentiate cytotoxin induced apoptosis and restore sensitivity to cytotoxic agents.
What are the three overall ways to treat cancer
Surgery, chemo and radiotherapy (chemo and radio do not kill cancer but damage cells to induce apoptosis)
Give any example of a chemotherapy
anthracylines (doxrubicin, epirubicin): disrupt DNA replication, •cyclophosphamide: disrupts DNA replication •5-Fluorouracil: inhibits DNA synthesis and repair •methotrexate: inhibits DNA synthesis and repair •mitomycin: cross-links DNA strands
What is the point of an adjuvant therapy?
Enhance sensitivity and prevent reoccurrence
How is a cancer mechanism linked to GF?
growth factors: over-production, e.g. of PDGF and TGFα, can lead to runaway positive feedback
Give an example of an intracellular mediator causing cancer
intracellular growth signal mediators: overproduction or unresponsiveness to inhibition, e.g. of ras and src
Give a list of hormone deprivation therapies
Gonadectomy, anti-oestrogens, SERMS, aromatase inhibitors, sulphatase inhibitors, LHRH agonists, antiandrogens
Why use SERMs over anti-oestrogens?
anti-estrogens such as faslodex completely block estrogen receptor which is associated with osteoporosis and menopausal symptoms but SERMs such as tamoxifen partially block the receptor so it has desired effects on bone and vasculature but blocks estrogen effects on cell division
In endometrial cancer, what stimulates proliferation?
The action of oestrogen without the opposing act of progesterone
Why might some cancers become resistant?
Typically hormone dependent cancer may be resistant to hormone deprivation (e.g ER –ve breast cancer in 1/3 of cases). Resistance may be due to epigenetic changes – upregulation of receptor cofactors or receptor mutations or may be due to activation of other growth factor pathways for example EGF may start to activate cell division.
How do you target growth factors? Give an example of one
Using monoclonal antibodies e.g. Herceptin which is an EGFR antibody it is also able to block erbB2
What is a disadvantage of blocking tyrosine kinase activity?
There are many side effects as cells are dependent on it for normal function
Give an example of some of the intracellular signalling targets of monoclonal antibodies
trametinib is a MAP kinase inhibitor, dabrafenib is a B-raf inhibitor. Tipifarnib is a farnesyl transferase inhibitor
Give specific examples of genome targetting
Killer T cells which were genetically modified to attack metastatic melanoma. Put genes coding for t cell receptors to tumour markers into a lymphocyte. Also replace any missing or altered genes (eg p53). Introduce genes into the tumour cell that convert a harmless pro-drug into its active form so normal body cells aren’t affected. Introduce genes which block angiogenesis (major rate limiting step in tumor growth)
How can the genome be targetted?
Antisense oligos: oligonucleotides that bind to and block complementary messenger or micro RNA. Ribozymes: RNAs with specific enzyme activity. Small-interfering RNAs (siRNAs): target specific mRNAs for degradation - used in an animal model against anti-apoptotic Bcl2
What are three forms of epigenetic changes?
DNA methylation, histone modification and altering microRNA
Describe what happens in DNA methylation?
DNA methyl transferase takes S-adenosyl methionine and transfers its methyl group onto a cytosine (only happens of cytosines adjacent to guanine) –> methylation pattern to the newly synthesized DNA strand
What is methylation associated with?
Methylation is associated with gene silencing in GENE PROMOTERS – makes histone tight and unable for genes to be transcribed
What are epigenetic changes?
Transmissable changes in gene expression that do not involve changes in the primary DNA sequence
How is imprinting carried out?
Through methylation
With respect to methylation, what can happen in cancer?
Hypomethylation can activate proto-oncogenes and can lead to chromosomal instability
What is hypermethylation linked to
Gene silencing, can also occur with tumour suppressors e.g. protein Retinoblastoma
How can methylation be targetted in cancer therapy? What is a potential problem?
Can inhibit DNA methyltransferase to regain tumour suppressor function. Hypomethylation is associated with long term gene instability. For eliminating hypermethylation: Antisense oligo nucleotides can eliminate DNA methyltransferase activity.
What is histone modification?
It is the process by which DNA methylation modulates gene transcription. Proteins that bind to methylated CpG recruit histone methyl transferase and histone deacetylases. Histone methylation packs the nucleosome to prevent gene transcription
Describe the epigenetic mechanism microRNA?
This NEGATIVELY regulates gene expression post transcriptionally and controls certain pathways by targeting oncogenes and TSG in cancer. Carcinogenic microRNA involvement has been found in relation to many cancer related genes and miRN profiling can improve cancer subtype profiling and prognosis.
Specific examples of micro RNA
Micro RNA 15 and 16 act as tumour suppressors by suppressing anti-apoptotic BCL2. MicroRNA21 suppressing PTEN.
What is an epigenetic tag?
Epigenetic “tags” (eg histone acetylation) sustain cell identity. Eg c-Myc→ major target in cancer promotion but is not druggable.
What are some of the acquired mutations?
Unrestrained cell growth, evasion of apoptosis, angiogenesis, immortalization and metastasis.
Overall what is hypermethylated and what is hypomethylated?
Oncogenes + chromosomal instability – hypomethylation
Tumour suppressor genes – hypermethylation
What can hypermethylation do?
‘lock’ differentiation-related genes in a silent state, resulting in differentiation arrest, expansion of progenitor cells and facilitation of tumour development
What are some of the epigenetic changes occurring in cancer?
- Global hypomethylation
- Promoter specific hypermethylation
- Histone deacetylation
- Global down regulation of microRNAs
What are some of the problems associated with epigenetic cancer targeting?
BUT, hypomethylation may promote long-term genomic instability
BUT, the same type of tumour may be caused by disruption of methylation at different gene loci
BUT, gene silencing may be re-established despite DNA
methyl-transferase inhibition as a result of other gene silencing mechanisms
BUT, HDAC inhibitors have high toxicity
What is an oncogene? Give an example
PROTO-ONCOGENES are genes for GROWTH FACTOR SIGNALLING so when mutated (one hit required) there is constitutive activation. C-erb, c-ras, c-src, c-myc and c-fos.
What are tumour suppressor genes? Example
are growth inhibitory factors so that when mutated (2 hits required) you get constitutive growth promotion signaling. P-Rb (suppresses progression through G1 phase), P53 (important for deciding DNA repair or apoptosis), APC, PTEN (inhibits PI£ kinase signaling) and BRCA 1 (promotes DNA repair).
What are the different broad types of growth factor?
They can either be ENDOcrine (E2, testosterone, IGF1, insulin and GH), PARAcrine/AUTOcrine (EGF, FGF, PDGF, VEGF, NGF and inflammatory cytokines) or INTRAcrine (Ras, PI3K and G protein).
How does intracellular communication generally occur?
Through the exchange of phosphates which is regulated by kinases
How do receptor tyrosine kinases work? Give an example specifically with EGFR.
Ligand binding causes AUTOphosphorylation of the intracellular portion and dimer or heterodimer formation. Then recruitment of various binding partners. Eg EGFR recruits adaptor protein SHC and GRB2. GRB2 mediates the interaction between SOS and Ras. Ras is a small guanosine triphosphate phosphatase that is activated by GTP and inactivated by GDP. SOS promotes dissociation of GDP from Ras and uptake of GTP activating it.
When can PIP3 be recruited?
At p21Ras through PI3 kinase. PIP3 can lead to the activation of AKT. Then mediates a cell survival signal as well as cell division allowing cells to progress into the S phase of the cell cycle
What does PTEN do?
A phosphatase that breaks down PIP3 and can thus inactivate AKT activity
What happens in apoptosis?
Programmed cell death (essential for development, maintenance and renewal of cells), highly regulated, no membrane rupture and ATP dependent.
Give examples of regulatory apoptosis in the body
For example ovarian cells atresia by apoptosis, lactating breast to return to normal state, sculpting of tissues during normal development, autoreactive t cells in thymus and virally infected cells.
What are the key events in apoptosis?
DNA fragmentation, chromatin condensation, breakdown of nuclear lamins, membrane enclosed apoptotic bodies form which then undergo phagocytosis, then caspases (exist as pro-caspases so DNA transcription not necessary) that then do proteolysis of key elements
What activates the extrinsic pathway? What is it also known as?
The death receptor pathway is activated by the binding of external ligands to cell surface death receptors (caspase 8+10), mainly viruses. Signalling via TNF family and Fas ligand. Balance between FADD and FLIP determines whether cell death proceeds.
What activates the intrinsic pathway?
2) INTRINSIC (MITOCHONDRIAL DEATH PATHWAY) is activated by internal signals (caspase 2 and 9). Cytochrome c is the main pro-apoptotic protein. APAF 1 is a cytoplasmic protein with caspase activation and recruitment domain that binds to cytochrome c. Controlled by BCL2 family and by IAPs (inhibitors of apoptosis) that inhibit conversion of procaspase 3 to caspase 3. SMAC/ DIABLO released from the mitochondria on initiation of apoptosis and inhibits IAPS
What is a telomere?
Protective non genomic strands of DNA at each end of the chromosome that shortens with cell division.
What eventually happens to telomeres?
After 50-60 divisions, SENESCENCE sets in with no more cell divisions but the cell remaining viable.
What can restore telomeres?
REVERSE TRANSCRIPTASE can restore telomeres but is switched off in most cells.
What does proto-oncogene activation do to telomeres?
- further cell divisions, driven by proto-oncogene activation completely erode the telomeres
- errors in DNA copying at the chromosome end then initiate apoptosis
- the human telomerase reverse transcriptase (hTERT) gene codes for the catalytic subunit of telomerase, which contains an RNA segment coding for telomere DNA
- Replicative immortality is achieved by hTERT re-expression, which occurs in 1 in 10 million cells
- telomerase is re-activated in >90% of cancers