23 - RTKs in Cancer

Question 1

What can multiple RTK inputs cause?

Answer 1

Multiple RTK inputs can cause cells to divide, replicate DNA, and undergo cytokinesis to create two new cells

Question 2

What do RTK inputs stimulate?

Answer 2

• They stimulate survival pathways through the activation on PI3K and PKB
• Stimulate cell differentiation which is important in specialisation of cells for example differentiation of blood cells into red and white

Question 3

What happens if you remove growth factor stimulation?

Answer 3

When you remove growth factor stimulation this causes cells to die which is also a regulated response, there are two types: apoptosis and necrosis

Question 4

What happens to RTKs during cancer?

Answer 4

What happens during cancer is there is hyperactivation of stimulation of RTKs, often in the absence of ligand
o So, in this case what happens is the cells lose their ability to die, we get uncontrolled promotion of cell survival, cells undifferentiate so they return to a simpler state and divide uncontrollably

Question 5

What research has been done in oncogenes and cell growth?

Answer 5

Considerable effort has been made to characterise the signal transduction pathways involved in the control of cell growth and proliferation by growth actor and oncogenes

Question 6

What has the study of cell transformation lead to?

Answer 6

The study of cell transformation by oncogenes has led to many advances in the elucidation of normal cell proliferation control pathways

Question 7

What work first suggested a link between growth factors and growth of cancer cells?

Answer 7

The first suggestion of a link between growth factors and growth of cancer cells came from work suggesting that transformed cells might synthesise and secrete growth factors which would act in an AUTOCRINE or PARACRINE fashion to stimulate growth

Question 8

What can mutations result in (relating to oncogenesis)?

Answer 8

There are normally growing cells but sometimes mutations occur that lead to aberrant cell proliferation
* So uncontrolled cell proliferation we get expansion of the population and they start to swamp the normally growing cells so they become the minor growing population

Question 9

What can be amongst the senescent cells?

Answer 9

Amongst the senescent cells we begin to generate cancer cells and they can proliferate
o At this point we have loss of tumour suppressors which normally control proliferation
o There is also expansion of malignant cells

Question 10

Timeline of the discovery of oncogenes

Answer 10

1916- Francis Peyton Rous
- Rous sarcoma virus
- Ground up cancerous tumour from a chicken and filtered out everything larger than a virus
- The resulting liquid produced cancer when injected into other chicken
1966- Rous shared the Nobel Prize for Physiology or Medicine for his work
1976- J. Michael “Moke” Bishop and Harold Varmus discover oncogenes
-Identified the src gene as a cancer-causing gene is Rous sarcoma virus
- Whose normal functions have gone awry
1989 - Bishop and Various win the Nobel Prize in Physiology or Medicine

Question 11

What was most important about the discovery of oncogenes?

Answer 11

More important than the technical details was the revolution this caused in thinking about cancers

Question 12

How many known oncogenes are there?

Answer 12

There are now over 80 known oncogenes that have been identified

Question 13

Quote by J.M. Bishop?

Answer 13

“If a cellular gene can cause cancer, then aberrations of normal cellular metabolism can cause cancer” -J.M. Bishop

Question 14

What are oncogenes?

Answer 14

Mutated versions of normal regulatory genes (proto-oncogenes)

Question 15

Where are oncogenes from?

Answer 15

From host genome (c-onc) or viruses (v-onc)

Question 16

Proto-oncogene activation to an oncogene can occur by:

Answer 16

o Point mutation
o Overexpression/ amplification
o Deletion of regulatory domains
o Gene fusion (chromosomal translocation)

Question 17

What is more dominant: an oncogene or porto-oncogene?

Answer 17

An oncogene is dominant over a proto-oncogene

Question 18

What are oncogenic targets?

Answer 18

Oncogenic targets are genes that regulate cell growth and proliferation (including apoptosis and cellular senescence)

Question 19

List the protein products of oncogenes (what can be mutated?)

Answer 19

• Growth factors (e.g., PDGF and EGF)
• Protein-tyrosine kinases (e.g., Src)
• Growth factor receptors (e.g., PDGF and EGF receptors)
• G proteins (e.g., Ras and Gs mutations)
• Cytoplasmic kinases (e.g., RAF)
• Survival proteins (e.g., PI3-kinase)
• Nuclear proteins (e.g., androgen receptor)

Question 20

What is the history of RTKs as oncogenes?

Answer 20

• RTKs have a long history of involvement in human cancer
• It was originally found that the protein sequence of the EGF receptor was closely related to the protein encoded by the transforming oncogene v-ERB-B of the avian erythroblastosis virus
• This suggests that the erythroblastosis virus has hijacked part of the EGF receptor involved in signal transduction after growth factor binding

Question 21

Structure of the EGF receptor and the v-Erb-B protein

Answer 21

• The EGF receptor contains two cysteine rich extracellular domains and a cytoplasmic tyrosine kinase domain
• The vErbB protein lacks most of the extracellular portion of the EGF receptor required for EGF- binding but possess all but the final 32 C-terminal cytoplasmic amino acids

Question 22

How many times has cancer been observed in RTKs?

Answer 22

Increased receptor signalling (cancer) has been observed in at least 10 (of 20) RTK familiar

Question 23

Why has cancer been observed in RTKs?

Answer 23

This is a result of CONSTITUTIVE ACTIVATION by:
o Receptor over-expression
o Somatic mutation (genetic alterations)
o Receptor amplification (autocrine/paracrine loops)

Question 24

Explain the different methods of constitutive activation

Answer 24

• MUTATION of the receptor to truncate it (point mutations) which lead to its activation
• OVER EXPRESSION of the receptor itself leading to receptor clustering and activation in the absence of ligand
• Autocrine growth factor loops, overproduction of ligand and they act on neighbouring cells to overstimulate transcription (RECEPTOR AMPLIFICATION)

Question 25

Strategies toward the prevention and interception of RTK signalling

Answer 25

• TKI
• MaB
• Immunotoxin
• Ligand toxin
• Monoclonal antibody approach
• HER2

Question 26

What is the TKI strategy?

Answer 26

One strategy is to generate small molecules that inhibit small molecule antagonists of the RTK activity
o Suppress downstream signalling

Question 27

What is the MaB strategy?

Answer 27

Monoclonal antibodies to the receptor extracellular domain have been generated , so the patient will be injected with these and they’ll interact with a receptor and have a number of effects e.g., blocking receptor activation

Question 28

What is the immunotoxin strategy?

Answer 28

These are injected into patients and the antibody will recognise the receptor and this will lead to an accumulation of the toxin at the cancer so it can be targeted directly to the cancer

Question 29

What is the ligand toxin strategy?

Answer 29

We could label a ligand like PDGF with a toxin and it would interact with PDGF receptors overexpressed on the cancer leading to poisoning of the cancer
* Then we can knock down transcription of target genes by antisense ODN

Question 30

What is the monoclonal antibody approach?

Answer 30

o An important example for such a scenario is HER2, also known as ErbB2, that belongs to the epidermal growth factor (EGF) receptor family of ETKs
o Overexpression and/or gene amplification of HER2 was found in various types of human cancers, especially in human breast and ovarian carcinomas
o Research has shown that HER2 positive breast cancers is a more aggressive disease with a greater likelihood of recurrence, a poorer prognosis and a decreased chance of survival compared with HER2 negative breast cancer

Question 31

What is the HER2 strategy?

Answer 31

o HER2 tumours can now be treated with Herceptin
o Trastuzumab (Herceptin) is the first humanized antibody approved for the treatment of HER2 positive metastatic breast cancer
o Herceptin is designed to target and block the function of HER2 receptor overexpression

Question 32

What RTK-based drugs have been discovered?

Answer 32

Since Herceptin was discovered, other methods to treat HER2 breast cancer have been developed for example a vaccine has been used in clinical trials
* The first successful small molecule treatment was a chemical called Gleevec, which blocks the tyrosine kinase activity of a chimeric protein

Question 33

Name some drugs used to treat HER2 cancer (RTK)

Answer 33

• Trastuzumab Herceptin
• BsAb 2B-1, NSC-673928
• APC8024

Question 34

Name some drugs used to treat EGRF cancer

Answer 34

• C225 Cetuximab
• MDX-447
• ABX-EGF
• ZD18539 Iressa
• DAB389EGF
• OSI-774 Tarceva

Question 35

What makes ERK ideal for the treatment of cancer?

Answer 35

ERKs involvement in cell proliferation, differentiation and apoptosis makes this pathway ideal for the treatment of cancer

Question 36

What drugs are currently in development?

Answer 36

o Ras inhibitors: antisense (phase I) and farnesyl transferase inhibitors (phase III)
o Raf inhibitors: kinase inhibitors (e.g., BAY43-9006 computes for ATP; phase III)
o MEK inhibitors: chemicals which lock MEK in inactive conformation (clinical trials unpublished)