Cancer Bio – Lecture 7 – pRb and the Cell Cycle – Flashcards

A cycle of growth and division G1: As long as 12-15 hours S: DNA synthesis: 6-8 hrs G2: 3-5 hrs preparing for cell division M: Cell division: Mitosis and Cytokinesis --M and S phase must occur with high precision in order to maintain the integrity of the genome.

G1: DNA damage checkpoint: entrance into 'S' is blocked if genome is damaged S: DNA damage checkpoint: DNA replication halted if genome is damaged G2: Entrance into M is blocked if DNA replication is not completed M: Anaphase is blocked if chromatids are not properly assembled on mitotic spindle

A point of no return -time leading up to R point is a period during which cells are responsive to mitogenic GFs and to TGF-B -GFs need to be present for a cell to go through the R point -Going through the R point represents a commitment to complete a full cell cycle -The cell cycle can still halt due to checkpoint activation

Frog and Clam embryos -The fertilized Xenopus egg divides rapidly and synchronously -Frog egg extracts at a particular stage of the cell cycle can be prepared

Extracts from M-phase eggs can drive an oocyte into M phase -The oocyte is arrested at a stage equivalent to the G2 phase -The biochemical factor responsible for driving the oocyte into M phase was named MPF (=> Maturation Promoting Factor)

-Cyclin-dependent protein kinases are regulated by the accumulation and destruction of cyclins -The M-cycling is known as the cyclin B

Early to Late G1: D/CDK4/6 Late G1 to early S: E/CDK2 Early S to Mid S: A/CDK2 Mid S to Late G2: A/CDC2 G2 to M: B/CDC2 G0 to G1: C/CDK3 ***CDC2 = CDK1*** -Each cyclin-CDK dimer activates many targets that drive cell cycle progression

Cyclin-dependent Kinase

-The fluctuation in cyclin levels acts as a mechanism of CDK regulation; CDK phosphorylation represents another mechanism of CDK regulation. -The gradual accumulation followed by rapid destruction of cyclins ensures directional progression of the cell cycle

Levels of D-type cyclins do not display the same, well-programmed, fluctuations seen for other cyclins -Once the cell has passed through the R point, the cell cycle machinery is no longer responsive to extracellular signals -Rise and fall of cyclins (except cyclin D) is hardwired -Levels of D-type cyclins depend on the levels of mitogens/GFs -The three D cyclins function similarly, but their promoters respond to different pathways (see table 8.1)

represent another layer of control of cell cycle progression -Inhibitors of CDK4: specific to CDK4/6: p15, p16, p18, p19 --Inhibitors of all others: p57, p21, p27: ----More widely acting: they can inhibit all of the other cyclin/CDK complexes and they can also affect processes other than the cell cycle. ----p27Kip1 and p21Cip1, but not p57Kip2, have been shown to play a role in cancer development

-TGF-B activates a signaling pathway that antagonizes cell proliferation -TGF-B's growth-inhibitory capabilities are lost beyond the R point -Strongly affects cyclin D/CDK4/6 -weakly affects the other cyclin-CDK complexes

GF --> Akt/PKB --l P21 and P27 So, GF/Mitogen inhibit p21 and p27 (CKIs), which results in activation of the cyclin/CDK -opposite effect of TGF-B

p21 and p27 STIMULATE D-type cyclin/CDK complexes p21 and p27 INHBIT all other types of cyclin/CDK complexes

Hypo-phosphorylation from start of G1 to 'R' point Hyper-phosphorylation from 'R' point to end of M De-phosphorylation at end of M

un-P pRb and hypo-P pRb are ACTIVE hyper-P pRb are INACTIVE

-Active pRb sequesters E2Fs -Hyperphosphorylation of pRb releases E2Fs, which direct transcription of genes needed for cell cycle progression -Tumor virus oncoproteins (large T antigen, adenovirus E1A, papilloma virus E7) can associate with a binding pocket in pRb

-Rapid transitions involve self-reinforcing positive feedback loop to quickly activate controller and drive cycle forward -For example, small amount of cyclin E-CDK2 drives synthesis of more cyclin E

-Myc-Max induces expression of: • Activators of cycle progression (e.g., D2-CDK4) • Inhibitors of CKIs (e.g., Cul1) • E2Fs Myc-Miz-1 represses expression of: • CKIs (e.g., p21, p27) l l / Active Myc strongly favors pRb inactivation and cell cycle progression

-TGF-B: main growth-inhibitory signal for many cells => these cells must evade TGF-Bto become cancerous -TGF-B normally: • Triggers CKI expression • Activates repressors of Myc expression

Wnts, mitogens, cyclins D, E/CDK2, Myc, B-catenin

TGF-B, CKIs: p21, p15, p27; pRb, p16