L18: Cancer 1 Flashcards
Characteristics of a cancer cell
- Uncontrolled cell growth
- Cell spreading
- Undifferentiated cells
- Cell signalling responses dysregulated
- part of a monoclonal tumour
Hyperplasia and dysplasia
- Hyperplasia: abnormally rapid growth
- Dysplasia: cells change form
Neoplasm
Precursor to cancer - not cancer until it breaks out of tissue of origin (no longer in situ).
Driver gene mutation
Directly/indirectly confers selective growth advantage to its cell
Passenger mutation
Has no direct/indirect effect on selective growth advantage of its cell (irrelevant to growth)
Positive and negative regulators of cell cycle
- Positive:
Classical oncogenes, telomerase, anti-apoptotic genes - Negative:
Classical tumour suppressor genes, indirectly acting tumour suppressor genes (e.g. in genome maintenance), apoptotic genes
Action of telomerase, consequence for overactivation
- Repairs ends of telomeres after division, which should shrink with every division
- Overactivation results in immortal cells as telomeres not shortening
Events of eukaryotic cell cycle
- S phase: DNA synth, creating 2 identical sister chromatids
- G2 phase: Gap phase for growth and preparation of sister chromatids. Mitotic spindle begins to form
- M phase: Inc. mitosis (nuclear envelope breaks down, chromosomes attached to spindle and sister chromatids pulled to opposite ends) and cytokinesis
- EITHER G1 phase: Growth before chromosome duplication
- OR G0 quiescence: exit form cell cycle
Key checkpoints of cell cycle
- G2/M transition: When all DNA is replicated, and environment is favourable for division
- Metaphase to anaphase transition: All chromosomes properly attached to spindle
- Restriction point: (START) Environment favourable for division
About cyclins and Cdks
- Cdks: Ser/Thr protein kinases
- No kinase activity unless bound to cyclin
- Constitutively expressed; governed by interactions with cyclin, which ebbs and flows
- A specific Cyclin-Cdk promotes expression of next cyclin (drives cell cycle progression)
- > 400 cyclin-Cdk substrates
Key cyclins
- Cyclin B: M phase
- Cyclin D: G1 phase
- Cyclin E: G1/S transition
- Cyclin A: S phase
Regulation mechanisms for Cdk activity
- Association with cyclins
- Phosphorylation (Thr160) by Cdk-activating kinases (CAKs)
- M-Cdk:
Phosphorylation by Wee1 kinase of Tyr 14 and 15 (inhibitory) - Inhibition by CDK inhibitor proteins (CDKIs)
Progression through the restriction point (signalling pathway involving G1 Cyclin-Cdks, Rb, Cdk 4 or 6, E2F, cyclin D)
Restriction Point
- In mammals, in G0 and early G1, transcription activator E2F is bound
to and inhibited by Rb protein - To enter cell cycle cyclin D associates with Cdk4 or Cdk6 – G1 Cdk
- G1 Cdk phosphorylates Rb - releases and activates E2F
- E2F activates transcription of genes required for G1/S transition and S phase (e.g. cyclins E and A, Cdk2, enzymes required for DNA synthesis)
(Mutations in Rb lead to inappropriate transcription and an increased risk of cancer)
- Progression form G2 to M (M-Cdk a.k.a. MPF actions)
- Activation of M-Cdk
Promoting G2/M:
- Induces spindle assembly
- Initiates chromosome assembly
- Ensures each sister chromatid attached to the opposite spindle pole
- Promotes breakdown of nuclear envelope
- Rearranges actin cytoskeleton
- Rearranges gogli apparatus
Activation of M-CDK:
-Tightly controlled
- Wee1 kinase inhibitory P
Why is protein degradation/proteolysis important in cell cycle progression?
How is this done by Cyclin-Cdks?
- Progression requires elimination of proteins from previous cell cycle stages (via ubiquitin-mediated proteolysis machinery)
- Cyclin-Cdks phosphorylate cell cycle regulators making them substrates for the SCF complex (ubiquitin ligase)
- Cyclin-Cdks also phosphorylate some ubiquitin ligases, activating them (Anaphase promoting complex or cyclosome, APC/C)
SCF (type of ubiquitin ligase)
- Essentially cleans up after each stage to allow cell cycle progression; requires targeting of waste proteins via phosphorylation
- p27 CKI targeted by G1/s and S phase activity
- Cyclin E also targeted
- Named for components (Skp1, Cullin, F-box protein which binds to substrate so is specific)
APC/C ubiquitin ligase
- Normally inactive; M-Cdk phosphorylation of APC/C promotes binding to Cdc20
- Subsequently, Cdc20 targets proteins for ubiquitination early in mitosis
- M-cyclin itself is a target for APC/C mediated ubiquitination
APC/C-Cdc20 and the Anaphase transition
- APC/C phosph. by M-Cdk allows binding to Cdc20
- APC/C-Cdc20 promotes degradation of securin activating separase
- Separase cleaves cohesin complex holding sister chromatids together, promoting transition to anaphase
APC/C-Cdh1 and exit from mitosis
- M-Cdk in decline; APC/C binds to Cdh1 instead of Cdc20
- Complex continues to target M-Cdk for destruction allowing exit form mitosis
- G1 cyclin-Cdk phosphorylates and inactivates Cdh1
- APC/C remains inactive until activated by association with Cdc20 in M phase
Checkpoint pathways
- G1 checkpoint (Restriction point)
- S phase checkpoint
- G2/M checkpoint
- Spindle checkpoint
Structure of a checkpoint pathway
- Damage-specific sensors bind to damaged DNA (e.g. RPA/MRN complex)
- Sensors activate transducers, which launch the damage response (ATR/ATM)
- Transducers activate effectors, which perform checkpoint functions (e.g. DNA repair proteins)
- Prolonged arrest leads to apoptosis in many multicellular eukaryotes
Defects in the spindle and cell cycle progression
- the mitotic spindle is also monitored for defects
- Lack of/incorrect attachment monitored for at spindle assembly checkpoint
- APC/C is the main effector, if incorrect, will be inactivated to prevent anaphase initiation and mitotic exit
Extrinsic regulation of cell division in normal cells
- GFs and mitogens promote passage through restriction point
- GFs stimulate incr. in cell size, reulated by TOR kinase (incr. rate of protein synthesis)
- Mitogens directly stimulate cell division by activating cyclin-Cdks through a signal cascade (e.g. Epithelial GF)
