REGULATION OF CELL CYCLE

By, Chinmayi Upadhyaya 1st M.Pharm

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Coordination of cell divisionA multicellular organism needs to

coordinate cell division across different tissues & organs.◦Critical for normal growth,

development & maintenance. Coordinate timing of

cell division. Coordinate rates of

cell division . Not all cells can have the

same cell cycle.

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Overview of Cell Cycle :Two irreversible points in cell cycle

◦Replication of genetic material◦Separation of sister chromatids

centromere

sister chromatids

single-strandedchromosomes double-stranded

chromosomes

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Checkpoint control systemCheckpoints

◦Cell cycle controlled by STOP & GO chemical signals at critical points.

◦Signals indicate if key cellular

processes have been completed correctly.

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Regulation of the Cell Cycle:Cell Cycle Checkpoints

Differentiating cells

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Checkpoint control system

◦G1/S Can DNA synthesis

begin?◦G2/M

Has DNA synthesis been completed correctly?

Commitment to mitosis◦spindle checkpoint

Are all chromosomes attached to spindle?

Can sister chromatids separate correctly?

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G1/S checkpoint:G1/S checkpoint is most critical

◦Primary decision point “Restriction point”

◦If cell receives “GO” signal, it divides. Internal signals: cell growth (size), cell

nutrition. External signals: “growth factors”.• If cell does not receive

signal, it exits cycle & switches to G0 phase Non-dividing, working state

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G0 phase

MMitosis

G1Gap 1

G0Resting

G2Gap 2

SSynthesis

◦Non-dividing, differentiated state◦Most human cells in G0 phase

liver cells in G0, but can be “called back” to cell cycle by external cues

nerve & muscle cells highly specialized arrested in G0 & can never divide

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How do cells know when to divide? ◦Cell communication signals. Chemical signals in cytoplasm give cue.

Signals usually mean proteins.ActivatorsInhibitors

Activation of cell division

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Evidence that Cytoplasmic Signals Control the Cell Cycle

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“Go-ahead” signalsProtein signals that promote cell growth

& division.◦Internal signals “Promoting factors”

◦External signals “Growth factors”

Primary mechanism of control.◦Phosphorylation Kinase enzymes. Either activates or inactivates cell

signals.

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Cell Cycle signals Cell cycle controls

◦Cyclins Regulatory proteins.

◦Cdks Cyclin-dependent kinases. Phosphorylates cellular proteins. Activates or inactivates proteins.

◦Cdk-cyclin complex Triggers passage through different

stages of cell cycle.

activated Cdk

inactivated Cdk

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Cyclin & Cyclin-dependent kinases

CDKs & cyclin drive cell from one phase to next in cell cycle.

Proper regulation of cell cycle is so key to life that the genes for these regulatory proteins have been highly conserved through evolution.

The genes are basically the same in yeast, insects, plants & animals (including humans).

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CDKs are positively regulated by cyclins

A Cyclin promotes synthesis of the next cyclin that in turn, promotes destruction of the previous one.These regulatory activities are indirect.

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Mechanisms of CDKs regulation

1. Abundance of cyclins

2. CDK phosphorylation3. Binding to CKIs (inhibitory proteins)

CDK

Cyclin

active

p21+

inactive

CDK

Cyclin

p21

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Activating phosphorylation is catalyzed by Cdk-Activating Kinases (CAKs). However, they are abundant and not regulated

Cyclin

Cdk

Phosporylation of Thr by CAK

Substrate binding to the kinase

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2 4

3

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CDK-CYCLIN COMPLEXES Intrinsically inactive catalytic

subunit (CDK) .Positive regulatory subunit

(cyclin).Phosphorylation of conserved

threonine residue .

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CYCLIN D3 Cyclin D genes.Synthesised by mid-G1 phase and

maximum at G1/S boundary.Stimulated by growth factors.Promote catalytic activity of CDK4 and

CDK6.Major substrate is retinoblastoma

protein.Cells lacking pRb do not require cyclin D

activation.

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CYCLIN ESynthesis at late G1 phase.Complexes with CDK2 .

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NEGATIVE REGULATORS OF G1 PHASE PROGRESSION

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CYCLIN-DEPENDENT KINASE INHIBITORS

p21cip1

CIP/KIP p27kip1

p57kip2

p16INK4a

p15INK4b

INK4 p18INK4c

p19INK4d

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INK4 CDKISpecifically target CDK’s.INK4 proteins sequester cdk4/6 into

complexes liberating cip/kip proteins.

Ability to arrest in G1 dependent on intact pRb.

p16 and p19 alternative transcripts.

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Cip/Kip CDKIp21 inhibits cyclin/CDK function

through cell cycle.Universally inhibits cdks.Activated by p53 -critical in induction

of cell cycle arrest.

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External signalsGrowth factors

◦ Coordination between cells.◦ Protein signals released by body cells that

stimulate other cells to divide. Density-dependent inhibition

Crowded cells stop dividing. Each cell binds a bit of growth factor.

Not enough activator left to trigger division in any one cell.

Anchorage dependence To divide cells must be attached to a

substrate. “Touch sensor” receptors.

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E2F

nucleuscytoplasm

cell division

nuclear membrane

growth factor

protein kinase cascade

nuclear pore

chromosome

Cdkcell surfacereceptor

P

PP

P

P

E2FRb

Rb

Growth factor signals

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Example of a Growth FactorPlatelet Derived Growth Factor (PDGF)◦made by platelets in blood clots

◦binding of PDGF to cell receptors stimulates cell division in connective tissueheal wounds

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Growth Factors and CancerGrowth factors can create cancers

◦ Proto-oncogenes Normally activates cell division . Growth factor genes . Become oncogenes (cancer-causing) when mutated.

If switched “ON” can cause cancer. Example: RAS (activates cyclins)

◦ Tumor-suppressor genes Normally inhibits cell division. If switched “OFF” can cause cancer. Example: p53

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DNA damage is causedby heat, radiation, or chemicals.

p53 allows cellswith repairedDNA to divide.

Step 1

DNA damage iscaused by heat,radiation, or chemicals.

Step 1 Step 2

Damaged cells continue to divide.If other damage accumulates, thecell can turn cancerous.

Step 3p53 triggers the destruction of cells damaged beyond repair.

ABNORMAL p53

NORMAL p53

abnormalp53 protein

cancercell

Step 3The p53 protein fails to stopcell division and repair DNA.Cell divides without repair todamaged DNA.

Cell division stops, and p53 triggers enzymes to repair damaged region.

Step 2

DNA repair enzymep53protein p53

protein

p53 — master regulator gene

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G2/M DNA damage checkpointThe G2/M DNA damage

checkpoint prevents the cell from entering mitosis (M phase) if the genome is damaged.

It also checks if the cell is big enough (i.e. has the resources to undergo mitosis)

Almost exclusively, internally controlled

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M checkpointThe M checkpoint is where the

attachment of the spindle fibres to the centromeres is assessed.

Only if this is correct can mitosis proceed.

Failure to attach spindle fibres correctly would lead to failure to separate chromosomes

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Fusion of mitotic cell with cells in other stages of the cell-cycle causes chromosomes of the other cell to condense and the nucleus to breakdown.Mitotic cells must contain a mitosis -promoting factor (MPF) – produced at the end of G2?

MITOSIS PROMOTING FACTOR

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MPF may:• Phosphorylate Histone protein H1 to condense Chromatin. 

• Phosphorylate nuclear Lamins to break nuclear envelope. 

• Phosphorylate MAPs (Microtubule-Associated Proteins) resulting in the formation of spindle.

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Evidence for a Maturation (or Mitosis) Promoting Factor (MPF) –

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ProteinLevel

Time

cyclin A cyclin B

M M M

Remember?Cyclins must be removed for mitosis to be completed.

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Spindle Assembly Checkpoint Controls Metaphase Anaphase

MPF (+) anaphase promoting complex, which destroys:1. Securin, which allows

separin protease to cleave cohesin.

2. Mitotic cyclin, which causes loss of MPF activity, leading to chromosome decondensation and envelope reformation.

Mad2 signal stops coming from kinetochore MT’s once attached, also (+) APC.

(+)

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p53 MDM2

CELL CYCLE PROGRESSION

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p53 MDM2

CELL CYCLE ARREST

P

SERINE-15

ATM

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What causes these “hits”?

Mutations in cells can be triggered by UV radiation chemical

exposure radiation

exposure heat

cigarette smoke pollution age genetics

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TumorsMass of abnormal cells

◦ Benign tumor abnormal cells remain at original site as a

lump p53 has halted cell divisions

most do not cause serious problems &can be removed by surgery

◦ Malignant tumor cells leave original site

lose attachment to nearby cells carried by blood & lymph system to other tissues

start more tumors = metastasis impair functions of organs throughout body

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Traditional treatments for cancersTreatments target rapidly dividing cells

◦high-energy radiation kills rapidly dividing cells

◦chemotherapy stop DNA replication stop mitosis & cytokinesis stop blood vessel growth

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New “miracle drugs”Drugs targeting proteins (enzymes)

found only in cancer cells◦Gleevec

treatment for adult leukemia (CML)& stomach cancer (GIST)

1st successful drug targeting only cancer cells

Novartes

withoutGleevec

withGleevec

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References:Molecular biology of THE CELL.5TH

EDITION .PGno;:1060-80Jump up^ Malumbres, Marcos; Barbacid,

Mariano (March 2009). "Cell cycle, CDKs and cancer: a changing paradigm".Nature Reviews Cancer 9 (3): 153–166.doi:10.1038/nrc2602.

Jump up^ Bertoli, Cosetta; Skotheim, Jan M.; de Bruin, Robertus A. M. (23 July 2013). "Control of cell cycle transcription during G1 and S phases". Nature Reviews Molecular Cell Biology 14 (8): 518–528. doi:10.1038/nrm3629.

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