Cancer and the Cell Cycle : An overview

Ken Wu

Disclaimer

• This tutorial is a simple and conceptual guide to the cancer module and the cell cycle

• If there are any conflicts between my slides and the lecturers, THE LECTURER IS ALWAYS RIGHT…

• …maybe not always but they set your exams so if in doubt, refer back to their teaching

The Cell Cycle

• Most adult cells, without growth stimulus, will go into the G0 phase of the cell cycle

• However, when a growth factor binds to its receptor on the cell membrane, a cascade starts and the cell prepares to enter G1

Growth stimulus

• Growth factors– Epidermal Growth Factor (EGF)– Platelet-Derived Growth Factor (PDGF)

• Binds to Receptors Protein Tyrosine Kinase (RPTK)

Preparing the cascade

• Grb2 (adaptor protein) binds to phosphorylated tyrosine– Recruits SoS (Ras activating protein)

• SoS exchanges GDP for GTP– Activates Ras

• Ras must be membrane bound to be active

The ERK Cascade

• RAS– Raf• MEK

– ERK

• Causes gene expression changes via proteins such as c-Myc

Cyclin dependent kinases (Cdk)

• Cyclically activated protein kinases• Activation depends on– Cyclin – Cdk interaction– Phosphorylation

• Cyclins– Expressed at different points in cell cycle– Transiently expressed and degraded

Cyclin – Cdk interation

• Cdk 4,6 + cyclin D (up regulated by c-Myc)– G0 – G1

– Also stimulates cyclin E synthesis• Cdk 2 + cyclin E– S phase entry

• Cdk 2 + cyclin A– Metaphase of mitosis entry

• Cdk 1 + mitotic cyclin (cyclin B)– Promotes mitosis

Cell cycle timing and direction

• Due to sequentially active Cdk, and synthesis of Cdk for the next phase of cell cycle

Cyclin – Cdk function

• Phosphorylate pRb protein• Phosphorylated pRb ‘releases’ E2F

transcription factor• E2F is now free to facilitate gene transcription

pRb

Cdk4/6

pRbpRbPP

P

Cdk2 Cdk2E2FE2FD E A

PP

Cdk1TF B

MitosispRb

Cdk inhibition

• INK4 family– Inhibit Cdk 4,6– G1 phase inhibitors

• CIP/KIP family– Inhibit all Cdks– S phase inhibitors

• Degradation allows cell cycle progression

The Big Picture

• G0 + EGF– RAS, Raf, MEK, ERK, c-Myc

• Cyclin – Cdk– D + 4,6 (G0 – G1)

– E + 2 (G1 – S)– A + 2 (Metaphase)– B + 1 (Anaphase)

Cancer – when it goes wrong

• Overexpressed EGFR• Mutant RAS– Does not dephosphorylate GTP– Constantly bound to GTP thus constantly active

• Overexpressed c-Myc, cyclin D• Inactive pRB

Apoptosis vs Necrosis - basics

• Necrosis– Unregulated– Trauma, cellular disruption– Inflammatory response

• Apoptosis– Regulated– Controlled disassembly– No inflammatory response

Apoptosis vs necrosis - process• Necrosis

– Plasma membrane becomes permeable– Cell swelling– Membrane rupture– Protease autodigestion– Localised inflammation

• Apoptosis– Activate death pathway– Cell shrinkage– Nuclear condensation– DNA fragmentation– Apoptotic bodies– Macrophages

Caspases

• Activation– Proteolysis– Cascade

• Initiator caspases– CARD or DED domain

• Effector caspases

Caspase function

• Initiator caspase– Activation via proteolytic cleavage– Caspase cascade

• Effector caspase– Cleave and inactive proteins– Activate enzymes in apoptosis

Receptor mediated caspase activation (extrinsic pathway)

• Fas receptor– Fas – Fas ligand interation– Has DD intracellular domain

• Recruits FADD – DD of FADD attaches to DD of Fas

• DED domain of FADD interacts with DED domain of caspase• Recruits caspase 8

– Caspase cascade– Cleaves Bid – mitochondrial pathway

• Process inhibited by FLIP

DED DDFADD

DED DEDFLIP

Mitochondrial death pathway (intrinsic pathway)

• Loss of mitochondrial membrane potential– Releases cytochrome c + other factors

• Forms apoptosome complex– Apaf 1

• Binds to cytochrome c• CARD domain binds to CARD of caspase 9

– Caspase cascade

• Needs ATP– Therefore energy levels decide apoptosis vs necrosis

Apoptosis modulators

• Bcl – 2 family– Anti – apoptotic• Bcl – 2• Bcl –xL

– Pro – apoptotic• Bid• Bad• Bax

Mechanism of apoptosis modulation

• Growth factor presence– PI3 – K pathway• PKB/Akt production

– Inactivates Bad, caspase 9• Inhibited by PTEN

• Bax– Forms pore on mitochondrial matrix

Cancer – when it goes wrong

• Overexpressed Bcl – 2• Overexpressed PKB/Akt• Inactive PTEN

Any questions?

• Email me at ken.wu09@imperial.ac.uk• Visit the ICSM Year 1+2 past paper bank

Facebook group/the note bank on the ICSMSU website

• Good luck with exams next term!