Lecture 7 - Cytoplasmic Signalling Circuitry Programs Flashcards
What do growth factors do in relation to cell signalling?
Growth factors induce signalling cascades
What are immediate early genes? What happened in the cyclohexamide cell experiment? What is cyclohexamide?
Immediate early genes are the first genes that are transcribed when the cell moves back into the cell cycle. This means that myc, other kinases etc… are already waiting in the wings with a whole mess of other genes waiting to be turned on at the drop of a hat. Cyclohexamide is a compound that prevents any new proteins of being produced, but immediate early genes are still induced in treated cells because transcription factors are already present (cyclohexamide can’t prevent them).
What are delayed early genes? What prevents some early genes from being expressed (how delayed early genes get their chance to shine)?
Delayed Early genes require their respective transcription factors to be synthesized first, but are then subsequently expressed. They succeed when early genes are blocked due to their mRNA binding with polyribosomes. As this initial pathway is halted, the necessary transcription factors are synthesized in order for the (delayed) early genes to become transcribed. Though the immediate early genes might be partially blocked, the immediate response still occurs.
In the receptor-Ras signalling cascade, describe which molecules interact in between Ras and the receptor as signal transducing proteins in order to send the signal downstream.
The receptor-Ras pathway is a highly conserved pathway. It begins with the receptor being phosphorylated which affect the localization of downstream signalling partners. The SH2 domain of the Shc molecule then binds to one of the phosphorous groups attached to the receptor. Similarly, the SH2 domain of Grb2 then subsequently binds to the phosphorylated Shc. The two SH3 domains of Grb2 then bind with the Sos (GEF) molecule which in turn cause the phorphorylation of GDP to GTP and the simultaneous activation of Ras.
What is the Ras signalling cycle/loop. Name the different roles of GEF and GAP proteins, as well as what occurs following an Ras-specific oncogenic mutation.
See Diagram in Notes. Ras is inactive when it is bound to GDP. Upstream signal stimulus and Ras activation is initiated by the GEF protein, which rips off GDP and allows GTP to bind in its place. When GTP binds to Ras, the Ras protein becomes activated and induces further downstream signalling. When it is time to turn this signalling off, GAP proteins trigger GTP hydrolysis where a phosphate group is lost and the GTP bound to Ras becomes a GDP group. Ras is then deactivated once again.
An oncogenic mutation prevents the active form of Ras (bound to GTP) from binding with GAP proteins. Ras is then trapped in the “activated state” and will continuously send downstream signals related to proliferation, cell division, growth etc…
Give some further details about the Ras Effector Loop. How to ligands feel about Ras GDP vs. Ras GTP etc…
Is major change required to prevent GTP hydrolysis? Expand on this material.
Ras is able to interact with a vast variety of ligands, most of which have a high affinity to Ras GTP, and a subsequent low affinity for Ras GDP. Also, it only requires a very tiny change in order to prevent Ras GTP hydrolysis by GAP proteins. This tiny change is quick, simple and easy and can occur in many places in order to turn Ras on, perpetually.
What are some of the downstream signalling pathways initiated by an activated Ras? List three and their respective effects.
1 - MAP Kinase Pathway: effects include proliferation, anchorage independence, loss of contact inhibition, cytoskeletal rearrangement.
2 - PI3K Pathway: stimulates apoptosis inhibition, proliferation, motility, angiogenesis
3 - RAI Pathway: cell motility, proliferation, metastasis
Describe the PI3K Pathway. List the final four molecules and their effects.
PI3K –> PIP3
PIP3 –> Akt/PKB or Rho-GEFs
Akt/PKB –> 1-4
1 - Bad: apoptosis inhibition
2 - mTOR: increased protein synthesis (growth)
3 - GSK-3beta: proliferation
4 - FOXO: apoptosis inhibition
What is the Jak-STAT Pathway? Describe the four proteins involved, how they interact, and what they produce.
The Jak-STAT pathway involves the activation of a cytokine receptor through the binding of a ligand. Following this, two attached proteins, Tyk2 and Jak1 are phosphorylated and simultaneously induce the phosphorylation of the two receptor tails with one phosphorous group each. Then, two proteins STAT 1 and STAT2 bind to each new phosphorous group and the translocate as a transcription factor to the nucleus. This pathway is involved in proliferation and anti-apoptosis.
It is a hematopoietic system: pertaining to the formation of blood or blood cells.
List the 4 roles that integrins play in/for cells. What do integrin signalling systems look like? Describe some of the components.
Role of integrins in cells: 1 - linkage of cell to ECM 2 - Monitoring connections to ECM 3 - Facilitating motility 4 - Sensing tension between the cell and the ECM
Integrin signalling systems are composed of the alpha (halved, fork-looking) and beta (spoon-looking) subunits that pierce the plasma membrane. The system eventually interacts with PI3K, Src, and Ras pathways through phosphorylation cascades.
What is anoikis?
Triggered cell death due to loss of attachment to solid substrate
Describe the Wnt-beta catenin signalling pathway, and mention the differences for when this pathway is compromised. What is this an example of?
Wnt ligand binds to LRP receptor & combines with Frizzled intramembrane protein to form a complex. This further influences Dishevelled protein, allowing the inner cell complex to activate GSK-3beta. This in turn activates beta-catenin which leads to degradation. Within the nucleus, Groucho and Tcf/Lef are paired on the genome.
When altered, the Dishevelled protein sticks to the Frizzled intramembrane protein and prevents axin from binding to the complex. This in turn prevents the activation of GSK-3beta. No degradation occurs and beta-catenin replaces groucho on the genome within the nucleus (binds to Tcf/Lef).
This is an example of G-protein coupled receptor signalling.
What are notch signalling pathways stimulated by and where are they commonly found to be altered (give an example of a medical affliction).
How do notch signalling pathways work? Describe the process briefly.
Notch signalling pathways are stimulated by oncogenic Ras and are commonly found to be constitutively active in leukemias.
Notch signalling pathways work when a Delta or jagged notch ligand bind to the notch and stimulates a cutting of the notch anchor. The inner tail end, having been cut at the inner surface of the plasma membrane, then enters the nucleus and binds to co-activators on the genome in order to activate certain genes.
What are patched-smoothened signalling pathways? What is the key term that we learn here? Where are these mutations commonly found?
How does the patched smoothened signalling pathway/hedgehog pathway work? Briefly describe the process.
Patched-smoothened signalling pathways, or hedgehog protein pathways are used primarily during embryonic development to influence cell specialization and growth.
Altering or turning on this pathway therefore results in increased growth, angiogenesis and metastasis.
Mutations in hedgehog pathways are commonly seen in cancers of the digestive tract.
The pathway works through a combination between two intramembrane proteins, patched and smooth. Once activated by the hedgehog ligand, the smoothened intramembrane protein stimulates the Gli protein (looks like a ball and stick) to enter the nucleus, bind to a co-activator, and initiate the transcription of certain genes.
What are TGF-beta signalling pathways related with? What do they promote in cancer cells?
TGF-beta inhibits proliferation in normal cells. Therefore, in cancer cells, it promotes invasiveness.
