Lecture 7- General Pathways, Ras, PI3K

Question 1

1) Immediate early genes (function): What are they, how is their transcription induced even when cycloheximide is blocking transcription? Give one example of these IEGs

Answer 1

• First genes to be transcribed when serum is available
• transcribed anyways due to transcription factors within the cell itself
• Myc
• Most of them are transcription factors that help induce the next wave of gene transcription called the delayed early genes

Question 2

2) Describe how the Ras signaling pathway was initially discovered by those studying fruit flies. Make sure to include the terms sevenless, sos and GDPGTP.

Answer 2

• They discovered the receptor called sevenless which is homologues to EGF-R, this acts like a tyrosine kinase and sends the signal to a protein called son of seveless which acts like a GEF protein that causes G proteins like Ras to go from their GDP state to their active GTP state.

Question 3

3) How are signaling cascades controlled? What component(s) of the protein allows only certain proteins to interact with each other? Give an example.

Answer 3

• There are 3 domains in these proteins, the SH2 domain is the site that allows binding of specific proteins to the appropriate receptor. Eg: When the ligand binds to the receptor, it phosphorelates its tyrosine residue and attracts proteins containing the correct SH2 domain to bind to the receptor.

Question 4

4) What other function does SH2 domain have in Src?

Answer 4

• It keeps it in an inactive state by forming an INTRAmolecular bridge

Question 5

5) Name the entire signal cascade of the Ras pathway, include detail of what each pathway does, and how it may bind, etc.

Answer 5

Ligands, tyrosine kinase receptor, SHC and Grb2, Sos (GET), phosphorelates GDPGTP, Ras active.

• PI3K, PIP3, inhibits apoptosis, proliferation, cell growth, angiogenesis, motility
• Raf= protein synthesis and transcription
• Ral-GEF= metastisis, motility, proliferation

Question 6

6) What is the Ras effector loop? What happens to Ras when it is oncogenic?

Answer 6

• When Ras binds to GTP, the effector loops have higher affinity to bind to the specific ligands which are PI3K, Ral-GEF and Raf. When oncogenic, RAS cannot be hydrolyzed by GAP proteins and is always in the active GTP phase and never inactivated.

Question 7

7) Discribe how hyperactivity of PI3K and inactivity of pTEN may lead to cancer.

Answer 7

• Hyperactive PI3K causes production of PIP3 which recruits kinases such as AKT and Rho to the cell membrane that will inactivate Bad and GSK-3Beta and activate mTOR.
• If PTEN is inactive, it won’t stop PIP3

Question 8

8) Integrin signaling: Name 3 roles integrins play for cells.
What is anoikis? Why don’t cancerous cells undergo this?

Answer 8

• physically link cells to ECM, inform cells if tethering to certain ECM components have been achieved, facilitating movement.
• anoikis is apoptosis due to loss of connection to ECM, cancerous cells down regulate caspase-3 (pro-apoptotic factor) to prevent apoptosis.

Question 9

9) Give a secondary function for Beta-catenin

Answer 9

• anchorage for cadherins, when they break in cancer cells, b catenin used for proliferation (original function)

Question 10

10) What type of cancer are these pathways most commonly deregulated in?
- NF-KB
- Notched
- Patch-smoothened
- TGF-beta (promotes something)

Answer 10

• breast
• leukemia
• digestive
• invasiveness