* L05 - Cell signaling and cancer

Question 1

What are the 4 main features of the intracellular signaling pathway?

Answer 1

STAR
Specificity-molecules and Ligands are specific
Transduction- of signals across the membrane
Amplification-of external signal within cell
Regulate cellular functions e.g. Differentiation and proliferation

Question 2

What type of molecules can interact with intracellular receptors? give an example

Answer 2

Small hydrophobic molecules can pass through the membrane and interact with the intracellular receptors. Such as steroids.
Hydrophillic signalling molecules have to interact with cell surface receptors.

Question 3

What receptor type is most associated with growth of cells?

Answer 3

Receptor tyrosine kinase

Question 4

What are the basic steps of signal transduction of an RTK?

Answer 4

Ligand binds to the surface receptor and causes dimerisation.
This leads to phosphorylation through the receptors cross phosphorylating one another.
This activates the integral tyrosine kinase
This allows for signal transduction because they phosphorylate downstream proteins.

Question 5

What are the difference between kinases and phosphatases?

Answer 5

Kinases are enzymes that add phosphate groups

Phosphatases are enzymes that remove phosphate groups.

Question 6

What is EGFR and what type of receptor is it?

Answer 6

It is epidermal growth factor receptor and is a tyrosine kinase receptor.

Question 7

What effect does binding of EGFR have on the cell?

Answer 7

It causes mitogenesis (proliferation) in epithelial cells.
As epithelial cells line the surface of organs and are exposed to carcinogens they may increase chances of developing cancer.

Question 8

What do VEGFRs have a role in?

Answer 8

Angiogenesis for tumors.

Release of VEGR and other angiogenic factors stimulates proliferation of endothelial cells.

Question 9

What is the ras cycle?

Answer 9

Ras is a small protein that effects downstream growth and proliferation.
It undergoes cycling between activated form (bound to GTP) and inactive form (bound to GDP)

Question 10

What causes activation of Ras?

Answer 10

Guanine nucleotide exchange factors (GEF), when phosphorylated, catalyse the removal of GDP and adding of GTP to ras. Hence this activate it. GEFs are the proteins sos and Grb2.

Question 11

What is the normal inactivated state of ras and how is it switched on?

Answer 11

It is normally bound by GDP in its inactivated state. GEF (sos + grb2) catalyse the removal of GDP and replace it with GTP which activates ras. To be swtiched off the GTP is converted back to GDP by GTPase. This is done by GTPase activating proteins.

Question 12

What occurs in oncogenic ras mutants that causes them to become cancerous?

Answer 12

The ability of GTPase is reduced resulting in the ras protein to remain bound to GTP in the active state. This leads to deregulated signalling that is oncogenic.

Question 13

What are the consequences of ras activation?

Answer 13

Ras activates MAP (mitogen activated protein) kinase via phosphorylation. This phosphorylates and activates Erk1 and Erk2 which enter the nucleus. These activate transcription factors that lead to changes in gene expression.

Question 14

In the map kinase pathway what are the order of activation after the ras proteins?

Answer 14

MAP-kinase-kinase-kinase (raf)
Activates MAP-kinase-kinase (mek)
Activates MAP-kinase (erk)

Question 15

Why is it hard to produce drugs to target ras?

Answer 15

The binding site is open and charged making it difficult to design drugs for.

Question 16

What is the difference between apoptosis and anoikis?

Answer 16

Apoptosis is a self destruct mechanism when stimulated by a particular trigger.
Anoikis is programmed cell death induced by anchorage dependent cells detaching from the surrounding ecm

Question 17

What are the three main types of survival factors that stop cells from going into apoptosis?

Answer 17

Mitogens - e.g. growth factors (EGF, VEGF), cytokines
Integrins (cell-ECM adhesion receptors)
Survival factors - e.g. IGF1 etc ligands that bind receptor tyrosine kinase.

Question 18

What is the molecule binding and the receptor for the first stage of the PI3K pathway?

Answer 18

EGF binds to EGFR causing dimerisation and activation of PI3K

Question 19

What is the role of PI3K?

Answer 19

It attaches phosphate groups (phospholylates) phospholipids, mainly PIP2.
This results in it being converted to PIP3

Question 20

What is the role of PTEN?

Answer 20

It converts PIP3 back to PIP2 by dephosphorylating it. This allows it to work as a tumour suppressor protein. This is because it stops the action of PIP3 which is to inhibit apoptosis downstream.

Question 21

What is the action of PIP3?

Answer 21

It leads to tethering of Akt which leads to cell survival by inhibiting apoptosis.
Akt also activates mTOR which stimulates protein synthesis and cell growth.
It also inactivates BAD which normally suppresses apoptosis.

Question 22

What is the second most mutated protein in cancer?

Answer 22

PTEN

Question 23

What is HER2 and what happens when it is overexpressed?

Answer 23

HER2 is a member of the EGFR family of receptor tyrosine kinases.
When overexpressed it can self activate and provide survival signals.
This commonly occurs in breast cancer.

Question 24

What is herceptin and how does it work?

Answer 24

It is a humanised anti-HER2 antibody. It works by targetting the HER2 oncoprotein.

Question 25

What is normal Abl signalling?

Answer 25

Normally the binding of Mitogens to the mitogen receptor results in activation of the Abl tyrosine kinase and the survival of the cell.

Question 26

What causes the formation of the BCR-abl gene?

Answer 26

A translocation between ch22 and ch9 leads to a BCR-Abl fusion (philadelphia chromosome)

Question 27

What is the consequence of of Bcr-Abl?

Answer 27

The BCR domain allows dimerisation and the Abl domain causes transactivation of tyrosine kinase domain.
This allows mitogenic signalling that is independent of growth factors.
This is associated with CML

Question 28

How is the BCR-Abl gene treated?

Answer 28

Imatinib (gleevec) acts as an inhibitor of the tyrosine kinase.
This is be occupying the ATP binding pocket and preventing substrate phophorylation by tyrosine kinases.

Question 29

Why is imatinib not always effective?

Answer 29

Often many signalling pathways are affected by the cancer and this only blocks one.