CBIO3.1: Cell signalling in cancer Flashcards
Give three examples of proteins activating mutations can occur in
Ras, PI3k, Akt
Give three examples of proteins loss of function of tumour suppressor mutations
Retinoblastoma, p53
How many Receptor tyrosine kinase (RTK) subfamilies are there in humans?
20
What is the first member of the RTK family to be discovered and is the most studied?
Epidermal growth factor receptor (EGFR, ErbB-1)
What does the ErbB family consist of?
Four highly related receptors:
- EGFR/ErbB-1
- ErbB-2 (HER2)
- ErbB-3 (HER3)
- ErbB-4 (HER4)
What are ErbB receptors activated by?
Growth factors of the epidermal growth family
What determines the binding specificity of the ErbB family?
Disulphide bonds
What structural motifs other than disulphide bonds do the ErbB family have?
- immunoglobulin-like domains
- heparin-binding sites
- glycosylation sites
How does ErbB act and how does it bind?
They can bind to each other in different combinations, in fact ErbB2 does not bind ligands itself but acts as a co-receptor for other members o the subfamily
Where is EGFR found? and what does its abnormal expression lead to?
in many different organs and its abnormal expression and/or activation is associated with many types of cancer
After cross-phosphorylation of RTKs has occurred, how do the cytoplasmic tails of RTKs behave?
As docking sites for specific intracellular, cytoplasmic proteins
What domain do signalling proteins that bind to the RTK family have?
SH2-domain
What does SH stand for?
Scr homology
What happens if multiple SH2-containing signalling proteins bind to an activated RTK?
This can promote simultaneous activation of several intracellular signalling pathways
Where do signalling proteins bind to on the activated RTK?
- PLC-y bind the activated RTK directly
- Others bind via adaptor proteins such as Shc and Grb
In addition to SH2, many signalling proteins bind to one or more ____ domains
SH3
What do SH3 domains bind to? What does this lead to?
Proline-rich sequences on other proteins, and then recruit them to the activated RTK to propagate the growth factor signal
What happens following SH3 recruitment to activated RTK?
Propagate the growth factor signal and affect alterations in gene transcription
When does signalling become most complex? Why?
As signals are transferred from the membrane to the nucleus, due to possible cross talk that can occur between different intermediates in numerous signalling pathways found within the cell
What is Shc?
Transforming protein 1, which is an adaptor protein involved in signal transduction/cell communication
What is Grb2?
Growth factor receptor-bound protein 2, an adaptor protein involved in signal transduction/cell communication
What are SH2 and SH3 domain-containing proteins, such as Grb2, that interact with activated RTKs are known as?
Adaptor proteins
What is the role of adaptor proteins?
Proteins that link the activated receptors to other parts of the signalling pathway, and don’t themselves have intrinsic signalling abilities
What is RAS?
Membrane-bound guanosine triphosphate (GTP)/guanosine diphosphate (GDP)-binding (G) protein that acts as a “molecular switch,” transforming signals from the cell membrane to the nucleus
What does the Grb2 adaptor protein recruit?
Sos to RTK
What does the Grb2 SH2 domain bind to?
A phosphotryosine docking site on the RTK
What part of the Grb2 recruits Sos?
The two SH3 domains
What is Sos?
A nucleotide exchange factor that swaps GTP for GDP on Ras
Where do the genetic mutations occur in the majority of cancer cases?
Mutations where the result is aberrant signalling that is able to escape the normal control mechanisms
What proteins from cell signalling pathways are often activated in cancer?
- Growth factor receptor tyrosine kinases (RTKs)
- Specific proteins affected include the epidermal growth factor receptor (EGFR)
- Small GTPases such as Ras
- Serine/threonine kinases Raf and Akt
- Cytoplasmic tyrosine kinases Src and Abl
- Lipid kinases (phosphoinositide 3-kinases, PI3Ks)
- Nuclear receptors such as the estrogen receptor, ER.
Other than the proteins involved in activating cell signalling being upregulated, what other errors can arise?
- downstream nuclear targets of signalling pathways
- transcription factors such Myc
- cell cycle effectors like the cyclins
What are the five scenarios that could result from mutation of a cell signalling pathway?
1) Metastasis
2) Angiogenesis
3) Proliferation
4) Apoptosis
5) Differentiation
What 2 properties of tumour suppressors make a mutation in them lead to cancer?
- pivotal in controlling cell proliferation and stress responses such as apoptosis and DNA damage repair
- act as negative regulators of cytoplasmic signalling
Give an example of a tumour suppressor that acts as a negative regulator of cytoplasmic signalling?
Lipid phosphatase PTEN is a negative regulator of the PI3K-Akt pathway.
What can some cell membrane receptors act as, other than receptors?
Enzymes
How can some receptors act as enzymes?
An extracellular, soluble signalling molecule binding to the membrane receptor triggers the receptor’s in-built enzyme activity
What are the largest class or receptors that can act as enzymes?
RTKs
What do the receptors that can act as enzymes, respond to in order to become enzymatic?
These receptors are bound by, and respond to, growth factors and other proteins that are present at low concentrations in their surrounding environment
What does binding of a signalling molecule to a membrane receptor do?
It causes adjacent/neighbouring RTKs to join with each other, forming what are known as cross-linked dimers
What does this cross-linking do?
It triggers the tyrosine kinase activity in the RTKs through cross-phosphorylation. What this means is that each RTK within the dimer phosphorylates several tyrosines on the other RTK partner.
How does binding of a growth factor such as EGF do to an RTK cause activation of Ras?
1) Guanine nucleotide exchange factor (GEF) aids the separation of GDP from Ras.
2) GTP then binds spontaneously to Ras, and GEF detaches, resulting in the active Ras · GTP form.
3) Finally hydrolysis of the bound GTP restores the inactive Ras · GDP form. This process is enhanced a hundred fold by GTPase-activating protein (GAP).
How do Grb2 and Sos provide a vital connection to the binding between EGF and RTK?
An SH2 domain in GRB2 binds to a specific phosphotyrosine residue in the activated receptor. Grb2 also contains two SH3 domains, which bind to and activate Sos. Grb2 thus functions as an adapter protein for the EGF receptor, linking it to Sos. Sos is a guanine nucleotide exchange protein (GEF), which converts inactive GDP-bound Ras (note that Ras is located in the membrane) to the active GTP-bound form.
Give 3 examples of proteins that RAS can activate
MAP kinase (MAPK)
PI3 kinase
Ral-GEF signalling pathways
What does MAP stand for?
Mitogen-activated protein (MAP) kinase cascade
How many serine-threonine kinases does the MAP pathway involve?
3
Explain the Ras-Raf pathway
Each of the three kinases in this pathway activates the next by phosphorylating it, as the third is the MAP kinase (in this example Erk1), the kinase that phosphorylates it is sometimes termed the MAP kinase kinase (MAPKK, in this example MEK) and the kinase that phosphorylates the MAP kinase kinase (Raf) is termed the MAP kinase kinase kinase or MAPKKK. As all three kinases in this pathway phosphorylate several different factors, the initial signal can be amplified at each step. The final enzyme in the pathway then phosphorylates one or more transcription regulators, thus altering gene transcription.
Give two examples of growth factors signal via MAPK pathways
- Nerve growth factor (NGF) - - Platelet-derived growth factor (PDGF)
Explain the PI3K pathway
Ras activates the enzyme phosphoinositide 3-kinase (PI3K) by binding to the Ras-binding domain (RBD) of PI3K and stimulating its catalytic domain. Activated PI3K then phosphorylates phosphatidylinositol 4,5-bisphosphate ((PI4,5)P2 or PIP2) to produce the second messenger, phosphatidylinositol 3,4,5-triphosphate ((PI(3,4,5)P3 or PIP3). oth PIP2 and PIP3 are phospholipids that reside in the cell membrane. The production of PIP3 activates numerous downstream pathways that control cellular functions critical to cancer such as cell proliferation and apoptosis.
How else can PI3K be activated other than by Ras?
PI3K can also be activated directly through RTKs such as PDGFR β, which contains a docking site for PI3K.
How does the production of PIP3 activate numerous downstream pathways that control cellular functions critical to cancer such as cell proliferation and apoptosis?
It does this by binding directly to proteins with PH domains, such as phosphoinositide-dependent kinase 1 (PDK1) and Akt (also known as protein kinase B or PKB). PIP3 brings PDK1 and Akt to the membrane, where PDK1 activates Akt by phosphorylation. Activated Akt then inhibits the proapoptotic Bcl-2 family members BAD and BAX, hence promotes cell survival.
What does Akt do?
Activated Akt then inhibits the proapoptotic Bcl-2 family members BAD and BAX, hence promotes cell survival.
What is a PH domain?
Pleckstrin homology domain is a protein domain found in many proteins involved in signalling or part of the cytoskeleton.
What cancers are caused by a PTEN mutation?
Glioblatoma, Ovarian carcinoma, Breast carcinoma, Endometrial carcinoma
Hepatocellular carcinoma
Melanoma, Lung carcinoma, Renal cell carcinoma, Thyroid carcinoma, Lymphoid, Prostate carcinoma, colon carcinoma
What is the first step that occurs when a growth factor/ligand receptor binds to its signal molecule?
Binding of signal molecule causes receptors to dimerise
What is the effect of phosphatase and tensing homolog (PTEN) dephosphorylating PIP3 to PIP2 in the P13K signalling pathway?
Repression of PI3K-dependent signalling
