Signalling Mechanisms Flashcards

Question

How is Wee1 then handled to allow for activation of Cdks?

Answer 1

Dephosphorylation of the inhibitor Wee1 phosphate • activates Cdk1 at the end of interphase Active MPF then phosphorylates Cdc25 to increase its activity • drives a +VE feedback • this helps drive mitosis as dephosphorylates Wee1 MPF = Mitosis Promoting Factor

Answer 2

MPF at the END of metaphase • phosphorylates key substrates in the mitotic process • puts mitosis on HOLD whilst substrates are phosphorylated One the kinetochores are attached, a signal is released • causes cyclin B to be degraded • this causes Cdk1 to become inactive • means the substrates are dephosphorylated • mitosis can now PROCEED

Answer 3

DIFFERENT cyclins & Cdks required at different stages of the cell cycle • e.g. G1/S = Cdk2, Cyclin E • e.g. S = Cdk2, Cyclin A Note how the same Cdk is bound BUT with a diff. cyclin • cyclin therefore changes the substrate specificity (to phosphorylate different substrates)

Answer 4

Growth factor stimulation of signalling pathways promotes G0 to G1 transition: • GF binds to the RPTK --> cascades via Ras • transcription factor phosphorylation --> expression of c-Myc • stimulate transcription of cyclin D Cyclin D activates Cdk4 and Cdk6 • goes on to synthesis of Cyclin E • this triggers the cell cycle

Answer 5

Each Cdk produced is involved in stimulating synthesis of the next Cdk by stimulating synthesis of genes required • e.g. Cyclin D/Cdk4/6 --> Cyclin E • gives direction and timing to cycle Cyclins are susceptible to degradation, hence cyclical activation

Answer 6

pRb - protein retinoblastoma pRb acts as a “brake” on the cell cycle. • Cdks phosphorylate & progressively inactivate pRb • Rb is a “tumour suppressor”. MoA: • In G0, Rb protein is resting with E2F transcription factors in the cell cytoplasm • Cdk4/6-cyclin D kinase phosphorylates Rb and E2F is released and binds to gene promoters to drive transcription

Answer 7

One target of E2F is the cyclin E, the cyclin required to continue cell cycle progression • Rb acts as a brake in the cell cycle by holding onto the E2F – this makes it a TUMOUR SUPRESSOR GENE. Many tumours have reduced levels of Rb protein so can’t regulate E2F

Answer 8

E2F regulate proto-oncogenes • includes Myc proteins Cyclin E is one of the FIRST targets of E2F --> pushes the cell cycle forward • Cyclin E --> binds to Cdk2 • “CyclinE-Cdk2 complex” phosphorylates Rb protein so another +ve feedback is initiated so more E2F is released by Rb

Answer 9

Myc turns on cyclin D which complexes with Cdk4/6… • Cyclins – D --> E --> A --> B. • Cdks – 4/6 --> 2 --> 2 --> 1. Ultimately leads to mitosis. • more and more pRb is phosphorylated to release more E2F (which is what initially creates cyclin E). • Cyclin A gene promoter is not activated until there is a HIGH concentration of E2F

Answer 10

Cdk Inhibitors Two families of Cdk-Is exist: • INK4 family - e.g. p15(INK4b) - G1-phase Cdk-I - Inhibits Cdk4/6 by displacing Cyclin D • CIP/KIP family - e.g. p21(C1P1) - S-phase Cdk-I - Inhibits all Cdks by binding to the Cdk/Cyc complex For the cell cycle to progress, these inhibitors need to be DEGRADED

Answer 11

TUMOUR SUPRESSOR GENES * Rb – inactivated in many cancers. * P27KIP1 – under expression correlates with poor prognosis

Answer 12

ONCOGENES * EGFR/HER2 – mutation activated in breast cancer – treated with Herceptin antibody (in HER2+ cancer) * Ras – mutation activated – treated with membrane attachment inhibitors * Cyclin D1 – overexpressed in 50% of breast cancer * B-Raf – mutation activated in melanomas – treat with kinase inhibitors * cMyc – overexpressed in many tumours